Studies in Surface Science and Catalysis 63 PREPARATION OF CATALYSTS V Scientific Bases for the Preparation of Heterogeneous Catalysts This Page Intentionally Left Blank Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Vol. 63 PREPARATION OF CATALYSTS V Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3-6,1990 Editors G. Poncelet Catalyse et Chimie des Materiaux Divises, Groupe de Physico-Chimie Minerale et de Catalyse, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium P. A. Jacobs Centrum voor Oppervlaktescheikunde en Colloidale Scheikunde, Katholieke Universiteit, Leuven, Heverlee, Belgium and P. Grange and B. Delmon Catalyse et Chimie des Materiaux Divises, Groupe de Physico-Chimie Minerale et de Catalyse, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium E LS EVI E R Amsterdam - Oxford - New York - Tokyo 1991 ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada. ELSEVIER SCIENCE PUBLISHING COMPANY INC 655. Avenue of the Americas New York, NY 10010. U.S.A. ISBN 0-444-886 16-8 Q Elsevier Science Publishers B V,, 199 1 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B V / Academic Publishing Division, P.O. 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This book is printed on acid-free paper. Printed in The Nerherlands V CONTENTS Organizing Committee Foreword Acknowledgements Financial Support Studies of unit operations in catalyst preparation Illustration of process scale-up in heterogeneous catalyst preparation I. Biay, G. Dessalces, C. Hypolite, F. Kolenda, J.P. Reymond Deposition precipitation onto pre-shaped carrier bodies. Possibilities and limitations K.P. de Jong Influence of the preparation procedure on the physical properties, surface acidity and dispersion of MoP/A1203 catalysts R. Prada Silvy, Y. Romero, J. Guaregua, R. Galiasso preparation of tailored catalysts with specific activity M. Piemontese, F. Tnfir6, A. Vaccari, E. Foresti, M. Gazzano Effect of preparation variables on catalytic behaviour of copper/zirconia catalysts for the synthesis of methanol from carbon dioxide R.A. Koeppel, A. Baiker, Ch. Schild, A. Wokaun Preparation of Tia-Al203 by impregnation with TiC4-CC4 Liu Yingjun, Zhang Qinpei, Zhu Yongfa, Gui Linlin, Tang Youqi Interactions of the impregnating solution with the support during the preparation of Rh/Ti@ catalysts R.J. Fenoglio, W. Alvarez, G.M. Nuiiez, D.E. Resasco Impregnation of controlled-porosity silica : Cu/Si@, Co/SiO2 and Cu-Co/SiO2. Investigation of the parameters affecting selectivity in CO hydrogenation M.A. Martin Luengo-Yates, Y. Wang, P.A. Sermon catalyzed by Cu-Al2O3 V. Di Castro, M. Gargano, N. Ravasio, M. Rossi for partial hydrogenation of alkynes Y. Nitta, Y. Hiramatsu, Y. Okamoto, T. Imanaka Z. Dziewiecki, E. Ozdoba preparation parameters A.F. da Silva Jr, V.M.M. Salim, M. Schmal, R. Frety Preparation and properties of a Wsilica and its comparison with Europt-1 S.D. Jackson, M.B.T. Keegan, G.D. McLellan, P.A. Meheux, R.B. Moyes, G. Webb, P.B. Wells, R. Whyman, J. Willis Synthesis of non-stoichiometric spinel-type phases : a key tool for the Selective hydrogenation of cyclododecatriene isomers to cyclododecene Preparation and characterization of highly selective Fe-Cu/Si& catalysts Some remarks on the preparation of Fe-WCa-Cr catalyst for styrene production Hydrogenation of 2-ethyl hexen-2-al on Ni/SiOz catalysts. Role of X XI XIII XTV 1 19 37 49 59 69 77 87 95 103 113 123 135 VI Factors analysis for mechanical strength in pelleting process of Fe-based high temperature shift catalyst Yongdan Li, Jiusheng Zhao, Liu Chang exmudates with large unimodal pore structure by low temperature hydrothermal treatment M. Absi-Halabi, A. Stanislaus, H. Al-Zaid Studies on pore size control of alumina : preparation of alumina catalyst Production of nickel-on-alumina catalysts from preshaped support bodies L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen, J.W. Geus Development of a methodology for investigating the adsorption of species containing catalytically active ions on the surface of industrial carriers N. Spanos, Ch. Kordulis, A. Lycourghiotis Scaling down of the calcination process for industrial catalyst manufacturing G. Groen, J. Ferment, M.J. Groeneveld, J. Decleer, A. Delva Hydrothermal sintering of the active phase in alumina supported fixed bed nickel catalysts during reduction E.K. Poels, J.G. Dekker, W.A. van Leeuwen Catalyst preparation via the sol-gel route The influence of silica support on polymerisation catalyst performance Preparation and catalytic effects of Ce0,-MOy-A12@ (M = Ba, La, Zr and C.E. Marsden Pr) by an improved sol gel method for automotive catalysts K. Masuda, M. Kawai, K. Kuno, N. Kachi, F. Mizukami Influence of preparation parameters on pore structure of silica gels prepared from tetraethoxy orthosilicate B. Handy, K.L. Walther, A. Wokaun, A. Baiker Preparation of catalysts from layered structures and pillaring of clays Aspects of the synthesis of aryl sulfonic acid h4ELS@ catalysts D.L. King, M.D. Cooper, W.A. Sanderson, Ch.M. Schramm, J.D. Fellmann G.A. Martin, M.C. Durupty, C. Mirodatos, N. Mouaddib, V. Pemchon S.A. Moya, A. Flores, M. Escudey Preparation of basic silicates and their use as supports or catalysts Soils as unusual catalysts Thermal stability, acidity and cracking properties of pillared rectorite catalysts M.L. Occelli Preparation and properties of large-pore RE/Al-pillared montmorillonite. A comparison of RE cations J. Sterte Preparation of pillared montmorillonite with enriched pillars E. Kikuchi, H. Seki, T. Matsuda 145 155 165 175 185 205 215 229 239 247 269 279 287 30 1 311 Intercalation of La203 and La2e-NiO oxidic species into montmorillonite layered structure A.K. Ladavos, P.J. Pomonis Mixed Al-Fe pillared laponites : preparation, characterization and catalytic properties in syngas conversion F. Bergaya, N. Hassoun, L. Gatineau, J. Barrault precursor on their structure and stability E.M. Farfan-Torres, 0. Dedeycker, P. Grange cationic species D. Tichit, Z. Mountassir, F. Figueras, A. Auroux Zirconium pillared clays. Influence of basic polymerization of the Control of the acidity of montmorillonites pillared by Al-hydroxy Preparation and modification of zeolite-based catalysts The chemistry of dealumination of faujasite zeolites with silicon tetrachloride Factors affecting the formation of extra-framework species and mesopores during dealumination of zeolite Y D. Goyvaerts, J.A. Martens, P.J. Grobet, P.A. Jacobs Dispersion of aggregated zeolites into small particles J. Kanai, N. Kawata Design and preparation of vanadium resistant FCC catalysts D.J. Rawlence, K. Gosling, L.H. Staal, A.P. Chapple Double substitution in silicalite by direct synthesis : a new route to crystalline porous bifunctional catalysts G. Bellussi, A. Carati, M.G. Clerici, A. Esposito Study on titanium silicalite synthesis M. Padovan, F. Genoni, G. Leofanti, G. Pemni, G. Trezza, A. Zecchina J.A. Martens, P.J. Grobet, P.A. Jacobs Treatment of galloalumino-silicate (ZSM-5 type zeolite) with KOH solution. Carbon supported catalysts Activated carbon from bituminous coal J.A. Pajares, J.J. Pis, A.B. Fuertes, J.B. Parra, M. Mahamud, A.J. PLrez Carbon-supported palladium catalysts. Some aspects of preparation in connection with the adsorption properties of the supports A.S. Lisitsyn, P.A. Simonov, A.A. Ketterling, V.A. Likholobov Preparation of palladium-copper catalysts of designed surface structure Zs. BodnAr, T. Mall&, S. Szab6, J. Petr6 Optimization and characterization of Pt-Fe alloys supported on charcoal P. Fouilloux, D. Goupil, B. Blanc, D. Richard Supported metallic catalysts achieved through graphite intercalation compounds F. Beguin, A. Messaoudi, A. Chafik, J. Barrault, R. Erre VII 319 329 337 345 355 38 1 397 407 42 1 43 1 439 449 459 469 479 VIII Prepantion of graphite-iron-potassium catalysts for ammonia synthesis K. Kalucki, A.W. Morawski Preparation of oxidation catalysts Synthesis of V-P-0 catalysts for oxidation of Q hydrocarbons V.A. Zazhigalov, G.A. Komashko, A.I. Pyatnitskaya, V.M. Belousov, J. Stoch, J. Haber grinding at ambient temperature Z. Sobalik, O.B. Lapina, V.M. Mastikhin Weijie Ji, Shikong Shen, Shuben Li, Hongli Wang Preparation of well dispersed vanadia catalysts by ultra-high intensity Dispersion and physico-chemical characterization of iron oxide on various supports The use of chelating agents for the preparation of iron oxide catalysts for the selective oxidation of hydrogen sulfide P.J. van den Brink, A. Scholten, A. van Wageningen, M.D.A. Lamers, A.J. van Dillen, J.W. Geus Preparation of oxidation catalysts with a controlled architecture Y.L. Xiong, L.T. Weng, B. Zhou, B. Yasse, E. Sham, L. Daza, F. Gil-Llambias, P. Ruiz, B. Delrnon M. Kotter, H.-G. Lintz, T. Turek C.S. Brooks Structure and selectivity changes in vanadia-titania-deNOx catalysts Binary oxide catalysts synthesized by sequential precipitation Zr@ as a support : oxidation of CO on CrO- Methane oxidative coupling by definite compounds (e.g. perovskite, cubic T. Yamaguchi, M. Tan-no, K. Tanabe or monoclinic structure, . . .) obtained by low temperature processes J.L. Rehspringer, P. Poix, A. Kaddouri, A. Kiennemann Novel and unusual preparation methods Preparation of strong alumina supports for fluidized bed catalysts M.N. Shepeleva, R.A. Shkrabina, Z.R. Ismagilov, V.B. Fenelonov Synthesis and regeneration of Raney catalysts by mechanochemical methods A.B. Fasman, S.D. Mikhailenko, O.T. Kalinina, E.Yu. Ivanov, G.V. Golubkova Controlled preparation of Raney Ni catalysts from Ni2Al3 base alloys. Structure and properties S. Hamar-Thibault, J. Gros, J.C. Joud, J. Masson, J.P. Damon, J.M. Bonnier Novel type of hydromating catalysts prepared through precipitation from homogeneous solution (PFHS) method K. Somasekhara Rao, V.V.D.N. Prasad, K.V.R. Chary, P. Kanta Rao 487 497 507 517 527 537 547 557 567 575 583 591 601 61 1 IX Preparation of manganese oxide catalysts using novel NHqMnO4 and manganese hydroxide precursors. Comparison of unsupported and alumina supported catalysts A.K.H. Nohman, D. Duprez, C. Kappenstein, S.A.A. Mansour, M.I. Zaki Influence of surface OH groups and traces of water vapor during the preparation of Ti@-Si@ samples A. Muiioz-Paez, G. Munuera Catalysts and preparation of new titanates R.G. Anthony, R.G. Dosch New methods of synthesis of highly dispersed silver catalysts N.E. Bogdanchikova, V.V. Tretyakov Preparation of high-surface-area V-Si-P oxide catalysts M. Ai Preparation of fine particles of ruthenium-alumina composite by mist reduction method H. Imai, J. Sekiguchi amination reactions J.L. Margitfalvi, S. Gobolos, E. Tilas, M. Hegediis Preparation of high surface area hydrogen-molybdenum bronze catalysts C. Hoang-Van, 0. Zegaoui, B. Pommier, P. Pichat New preparation of supported metals. Hydrogenation of nitriles M. Blanchard, J. Barrault, A. Derouault Preparation of highly dispersed gold on titanium and magnesium oxide S. Tsubota, M. Haruta, T. Kobayashi, A. Ueda, Y. Nakahara Preparation of monodisperse colloidal Pt-Re@ particles using Designed catalysts for hydrodechlorination, reduction and reductive microemulsions A. Claerbout, J. B.Nagy New organometallic active sites obtained by controlled surface reaction of organometallic complexes with supported metal particles B. Didillon, A. El Mansour, J.P. Candy, J.M. Basset, F. Le Peltier, J.P. Bournonville Conversion coatings on stainless steel as multipurpose catalysts L. Aries, A. Komla, J.P. Traverse Author Index Studies in Surface Science and Catalysis (other volumes in the series) 617 627 637 647 653 66 1 669 679 687 695 705 717 729 74 1 745 X ORGANIZING COMMITTEE President Prof. B. DELMON, Universitt Catholique de Louvah Executive Chairmen Dr P. GRANGE, Universitt Catholique de Louvain Prof. P.A. JACOBS, Katholieke Universiteit Leuven Dr G. PONCELET, Universitt Catholique de Louvain Dr P. RUIZ, Universitt Catholique de Louvain SCIENTIFIC COMMITTEE Dr D. ARNTZ, Degussa AG, Germany Dr J.L. CIHONSKI, Catalytica, U.S.A. Dr Ph. COURTY, Institut FranGais du Pttrole, France Prof. B. DELMON, Universitt Catholique de Louvain, Belgium Prof. E.G. DEROUANE, Facultts Universitaires N.-D. de la Paix, Belgium Dr T. EDMONDS, BP Research Centre, U.K. Dr J.W. GEUS, Rijksuniversiteit Utrecht, The Netherlands Dr P. GRANGE, Universitt Catholique de Louvain, Belgium Dr J. GROOTJANS, Labofina, Belgium Mr C. HAMON, Zeocat, France Dr H. HINNEKENS, Labofina, Belgium Prof. P.A. JACOBS, Katholieke Universiteit Leuven, Belgium Dr W.T. KOETSIER, Unilever Research Laboratonum, The Netherlands Dr 0. KRAUSE, Neste Oy, Finland Dr L. LEROT, Solvay & Cie, Belgium Dr G. MATHYS, Exxon Chemical International Inc., Belgium Dr T. MEURIS, Belgian Shell, Belgium Dr G. PONCELET, Universitt Catholique de Louvain, Belgium Dr L. PUPPE, Bayer AG, Germany Dr P. RUIZ, Universitk Catholique de Louvain, Belgium Dr P. SCHWARZ, Enichem SPA, Italy Dr M. TOKARZ, Eka Nobel AB, Sweden Dr D. VANDE POEL, Catalysts and Chemicals Europe, Belgium Mr A. VAN GIJSEL, UCB SA, Belgium Dr R. van HARDEVELD, DSM Research, The Netherlands Mr A. VASTEELS, Kemira SA, Belgium Dr D.E. WEBSTER, Johnson Matthey, U.K. XI FOREWORD The organizers are pleased to present the Proceedings of the Fifth International Symposium on These Proceedings the "Scientific Bases for the Preparation of Heterogeneous Catalysts". correspond to the fourth organized in Louvain-la-Neuve, the first having taken place in Brussels. Throughout the five symposia, held successively in 1975, 1978, 1982, 1986 and 1990, the organizers have not departed from their initial objectives, namely to bring together experts from both Industry and Universities in order to discuss the scientific problems involved in the preparation of heterogeneous catalysts, and to encourage, as much as possible, the presentation of research work on catalysts which are of real, industrial significance. Indeed, even if industrial researchers have easy access to the work carried out in university laboratories or research centers, the reverse is not always true. But this feedback is nonetheless indispensable, as the university staff is not always sufficiently aware of the needs of industry and of the problems encountered in the preparation of real catalysts, which correspond ultimately to the most challenging issues. This is one of the reasons why at least 50% of the members of the scientific committees have always come from industrial research and development organizations (at this symposium, 20 out of the 27 members came from industry). This major goal of linking Industry and Universities was partly fulfilled at the Fifth Symposium : indeed, out of the 338 participants, 182 belonged to industry. Although only 25 abstracts were submitted by industrial laboratories, the quality of the corresponding work was outstanding : 17 were selected by the Scientific committee. Another established highlight of these symposia is the reservation of a substantial part of the program to new developments in catalyst preparation, new preparation methods and new catalytic systems. Indeed, the fact that chemical reactions which were hardly conceivable a few years ago have now become possible through the development of appropriate catalytic systems proves that catalysis, like all industrial and academic activities, is in a constant state of progress. Because of the very large number of submitted abstracts (234), the unanimous wish expressed by the Scientific Committee to avoid parallel sessions, and the desire to accept the largest possible number of contributions which could be accomodated in a reasonable sized volume of the Proceedings, it was decided to organize a poster session, to suppress the half-day session devoted in previous symposia to normalization methods, and not to print the discussions. This decision allowed us to accept 70 papers, half presented orally, the other half as posters. In these Proceedings, the papers (including three extended communications) are grouped under the following headings : . Studies of unit operations in catalyst preparation (19) . Catalyst preparation via the sol-gel route (3 ) . Preparation of catalysts from layered structures and pillaring of clays (10) . Preparation and modification of zeolite-based catalysts (6) . Carbon supported catalysts (6) . Preparation of oxidation catalysts (9) . Novel and unusual preparation methods (17) XI1 Finally we would like to express special thanks to 26 industrial companies for their financial support, and especially to Catalysts and Chemicals Europe who generously provided the reception on the occasion of their 25th anniversary. The financial contribution of these companies permitted us to rearrange the budget. In this way, their support allowed several participants from countries with economical difficulties to benefit from financial aid so that they could attend the Symposium and present their communication. Prof. B. DELMON Dr P. GRANGE Prof. P.A. JACOBS Dr G. PONCELET XI11 ACKNOWLEDGEMENTS The Organizing Committee thanks Professor P. Macq, Rector of the Universite Catholique de Louvain, who allowed the Fifth International Symposium to be held in Louvain-la-Neuve. We also gratefully acknowledge the University Authorities for providing us with facilities, and in particular Dr L. Van Simaeys, Head of the Library of Sciences, who provided us with the lecture room where the Poster session was organized. The organizers also thank Professor V. Hanssens for his welcome address to the participants. At this Symposium, even more than in the previous ones, the members of the Scientific Committee were faced with a very difficult task in selecting the communications. They are all most sincerely thanked for the outstanding job which they accomplished. The Organizing Committee gratefully thanks the authors of the 240 submitted abstracts, those who contributed an oral or a poster presentation, as well as those whose contribution could not be selected, mainly because of the limitations of time and space. The Organizers are pleased to thank the authors of the stimulating extended communications, and in particular Dr J.P. Reymond, Dr G. Groen, Dr D.L. King and Dr K.P. de Jong for their excellent oral presentations. Sixteen people deserve special thanks for their performance as session chairmen during the symposium : Dr D. Arntz, Dr J. Cihonski, Prof. E. Derouane, Dr E.B.M. Doesburg, Prof. J. Geus, Dr C. Hamon, Mr K. Johansen, Dr G. Mathys, Prof. J. B.Nagy, Prof. J.T. Richardson, Dr D.S. Thakur, Dr D. Van de Poel, Dr D.E. Webster and Dr F. Wunde. The hostesses of the REUL (Relations Extkrieures de 1'Universitk de Louvain), and particularly Mrs F. Volon-Bex, are congratulated on their perfect achievement. We also want to extend our gratitude to Mr M. Van Windekens, of the "Service du Logement", for his dedication to the symposium, We also owe our particular thanks to the secretaries, F. Somers, M. Saenen and especially P. Theys who had the hidden part of the organization of the symposium in their charge, from its inception to its end. Finally, the Organizers want to mention in their acknowledgements all the people from the "Unit6 de Catalyse et Chimie des MatCriaux DivisCs" and the "Centrum voor Oppervlaktechemie, K.U. Leuven", who contributed to the success of the symposium, in particular : F. Bautista, N. Blangenois, R. Castillo, S. Colque, L. Daza, S. Giraldo, E. Lament, R. Maggi, H. Matralis, R. Molina, S. Moreno, E. and N. Paez, C. Papadopoulou, G. Pelgrims, E. Ponthieu, L. Portela, M. Remy, P. Ruiz, M. Ruwet, R. Sosa, M. Tielen, A. and M. Vieira Coelho, and L.T. Weng. XIV FINANCIAL SUPPORT The following companies agreed to provide financial support to the Fifth Symposium. The Organizers are grateful to them for their generosity. AKZQ Catalysts AUSIMONT CATALEZATORI BRITISH PETROLEUM International Ltd. CATALYSTS AND CHEMICALS EUROPE DEGUSSA AG DOW BENELUX B.V. DSM Research EKA NOBEL AB EXXON CHEMICALS INTERNATIONAL HALDOR TOPSOE A/S JOHNSON MATTHEY CHEMICALS Kontaktgruppe Forschungsfragen (CIBA-GEIGY AG, HOFFMANN-LA-ROCHE AG, LONZA AG, SANDOZ AG) LABOFLNA S.A. METALLURGIE HOBOKEN-OVERPELT MONSANTO EUROPE NORSK HYDRO PROCATALY SE REILLY CHEMICALS REPSOL PETROLEO TEXACO TOLSA UNION CHIMIQUE BELGE (UCB) The Netherlands IdY U.K. Belgium Germany The Netherlands The Netherlands Sweden Belgium Denmark U.K. Switzerland Belgium Belgium Belgium Norway France Belgium Spain U.S.A. Spain Belgium The Organizing Committee would like to especially thank CATALYSTS and CHEMICALS EUROPE for the reception which they generously offered on the occasion of their 25th anniversary. We are also grateful to ENGELHARD-DE MEERN (The Netherlands) for supplying the conference folders. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 1 ILLUSTRATION OF PROCESS SCALE-UP IN HETEROGENEOUS CATALYST PREPA- RATION I. BIAY, G. DESSALCES, C. HYPOLITE, F. KOLENDA* and J.P. REYMOND "Labomtoire GCnie de la Fabrication des Catalyseurs Hitiroghes" UM36 CNRS-IFP ; IFP-CEDI - BP 3 - 69390 VERNAISON (France) *Institut Franqais du Pitrole, IFP-CEDI - BP 3 - 69390 VERNAISON (France) SUMMARY The command of heterogeneous catalyst preparation and process scale-up from laboratory to in- dustrial plant requires research at a pilot scale. This paper presents the methodology followed to study catalyst preparation. The case of amorphous silica-alumina dried solids, involving a sol-gel route is here developped. Influence of operating variables on catalyst properties (i.e texture) at each stage of the process and scale effect are analyzed. Pilot plant runs point out operating problems, equipment effects on product specifications and simulate key steps in manufacturing process. INTRODUCTION The world market for heterogeneous catalysts is expected to reach 4 billion dollar global business (1). Despite a slow growth and small profit expectation, catalysts suppliers andusers arevery active in new catalyst developments. This can be explained by the strategic role a catalyst plays in production sales (2). New chemical products are requiring more elaborate catalysts. Heterogeneous catalysts can be compared to high performance materials which greatly enhance production process competitiveness andeconomics. A new catalyst can boostprocessefficiency andreduce production costsormakeitob- solete (3) In spite of numerous scientific studies (4-6), the industrial preparation of heterogeneous catalysts is regarded asempirical and still remains an "art" proceeding through a "know-how" jalously kept by the catalyst manufacturers. As aresult, it is crucial to master catalyst preparation and scale-up from the la- boratory to the manufacturingplant (7). Each step of theprocess ofpreparation must be characterized not only with physical and chemical techniques but also by using chemical engineering science (8). Physical (heat, mass and momentum transfers) and chemical phenomena (reactions, kinetics) of each unit operation are pointed out and studied using chemical engineering concepts (9). These determina- tions are performed at the laboratory and then reproduced on a pilot plant. The size of each element of thispilotplantmust bechoseninorder to takeintoaccount limitingeffectssuchaswalleffect orsizeef- fect(l0). Pilotplantpreparation using industrial equipmentis anecessaryintermediate steprequiredforasu- cessfull scale-up of the process. It offers the following assets. 2 I. Preparation of prototvpe catalvsts which will be as similar as possible to the industrial catalyst in terms of activity, selectivity, life time performance, shape and evaluation of the production cost. 2. Simulat ion of the kev sm in the manufacturing process and quantification of the influence of each step on the catalyst characteristics (porosity, surface area, chemical composition, bulk properties). 3. Establishment of extrapolation rules using chemical engineering concepts. 4. Definition of product suec ificatioqchart for each manufacturing step in order to assure catalyst per- formance and reproductibility. 5. Avoidance of process scale-uu pitfalls, such as mixing problems, raw material purity and availabili- ty, fluid rheology which can make fluid transport impossible, solid handling ... which cannot be clearly shown during the earIy phases of the catalyst development in the laboratory (7). Advantages and valuable information brought by pilot scale catalyst preparation are illustrated in this paper. Itrepresentspartofthe workcarried out by ourresearch p u p which associatesthe "Institut FranCais du P6trole" and the "Centre National de la Recherche Scientifique". I - PROCESS SCHEME In the field of heterogeneous catalysts, silica-alumina are widely used as acid supports or catalyst matrices. Their preparation involves a large number of unit operations, usualy present in catalyst ma- nufacturing processes, such as precipitation or gelation, filtration, drying, ion exchange ... These unit operations are gathered on figure 1. Figure 1 - Example of unit operations in catalyst elaboration . PRECIPITATION . GELATION . HYDROTHERMAL TREATMENT . SOLID-LIQUID SEPARATION .WASHING . IMPREGNATION . ION EXCHANGE . KNEADING, COATING . CRYSTALLIZATION . DRYING . CALCINING . GRINDING, SOLID CLASSIFICATION . SHAPING 3 Intensive use of fluid bed type reactors led us to study the preparation of solids used in suchcataly- tic reactors. In this paper we examine the preparation of spray-dried silica-alumina matrices at the la- boratory (several hundred of grams) and on a pilot plant (several tens of kilograms). The figure 2 shows the chosen process scheme. The first step of this industrial process is the silica hydrogel formation obtained by successive additions of reactants (batch operation) using a sol-gel technique. A dilute sodium silicate solution is partly neutralized by adding a sulfuric acid solution to form colloldal particles (sol state) which link together giving a tridimensional network (hydrogel state). Addition of aluminium sulphate followed by pH adjustment of the suspension with ammoniain- duces alumina precipitation and its incorporation with silica. A catalytically active phase or other so- lids such as clays can be added to the hydrogel. This hydrogel is then filtered, washed and repulped in Figure 2 - Catalyst elaboration process CLAYS SODIUM SILICATE ----- ACID 7 SILICA HYDROGEL I SILICA - ALUMINA HYDROGEL I I FILTRATION , WASHING SPRAY - DRYLNG IONIC EXCHANGES FINAL DRYING 5 water to make it pumpable. The suspension is spray-driedin order to produce spherical particles which are washed and dried. At the laboratory such a preparation takes place in a five liter s t i d glass vessel. This vessel is equipped with sensors (pH, temperature, stirrer torque) which allow the control of the gelation. Filtra- tions and washings are performed on a vacuum filter. The filter cake is repulped in water and the sus- pension is dried in a laboratory spray-dryer (3 kg/hr water evaporation rate). This dryer produces 20-30 micrometers diameter particles with 15-25 % moisture content. A scheme of the pilot plant unit dedicated to the preparation of the catalysts is shown on figure 3. The four first steps of the process (gelation, filtration, drying and washing) arerepresented. The final stage of the preparation which could involve drying alone or drying and calcination depending on the application, is not covered here. All the equipments are of industrial type. The size of each piece of equipment (agitators, filters, dryers, pumps ...) has been set by the need to extrapolate the manufactu- ring process to industrial scale. Gelation is performed in a stainless-steel reactor (1000 liters) equipped with sensors allowing us to follow the hydrogel formation (pH, temperature, stirrers rotary speed, po- wer consumption). The hydrogel is filteredon avacuumdrumfilter. The filter cake (= 15 % solidcontent) is repulped with water and the suspension is dried in a pilot spray-dryer (100 kghr water evaporation rate at 400°C). The spherical particles of 65 pm average diameter contain 15-25 % of residual water. Centri- fugal atomization is carried out using a vane atomizer wheel. The impureties contained in the spray dried particles (sulphates) are removed by successive filtra- tions and washings of aqueous suspensions of these particles. These steps are simultaneously opera- ted on a vacuum belt filter. The last step of the process consists in drying the washed particles at 350OC. II - EXPERIMENTAL METHODS 1. Analvtical methods Sampling procedures including pretreatments have been defined in order to obtain representative samples of solids, liquids and suspensions by means of grinder, spinning rifler, sampler probe.,.. 1.1. Chemical analvsis and moisture content Inductively coupled plasma (I.C.P.) and atomic absorption quantitative chemical analysis of Na, Si, Al, S and Nare achieved by a C.N.R.S. laboratory, the "Service CentraldAnalyses (S.C.A.)". Mass balances of all catalyts preparation steps are based on these chemical analysis. 6 1.2. Textural and momholoeical determinations Pore texture of a catalyst (i.e. pore size distribution P.S.D., total pore volume and surface of the pores) governs catalytic performance such as activity and selectivity, through diffusion of reactants and products in the pore system of the solid, density, mechanical strength and thermal stability (5, 1 1, 12). 1.2.1. Thermooorometry The hydrogel formation and the spray-drying steps are the key operations of the process in terms of influence on the catalyst properties (texture and morphology). It is important to measure the influence of operating variables for each step on the pore texture of the catalyst. The difficulty arises from the ne- cessity of measuring the porosity of an hydrogel (95 % water and 5 % solid) and of axemgel (dry solid particles) with the same technique. To our knowledge, thermoporometry is the only method which can apply (13,14). It is acalorimetric technique based on the measurement of the temperature of solidi- fication of aliquidconfined or divided into a porous texture. Pore diameters ranging from 2 to 150 nm (mesopores) can be measured. 1.2.2. Other textural methods The texture of solids is evaluated using well-known techniques : - structural density : helium picnometry - total pore volume of xerogels : high pressure mercury porosimeter - surface area and pore diameter distribution : nitrogen adsoption (B.E.T. and B.J.H. methods) - colloidal silica particle surface area as well as xerogel silica particle can be measured by the Sears' analytical method (15). Typical silica-alumina we producehave a specific area of 250-600 m2g-l, a skeletal density of 2,l- 2,4g. cm~3,aporevolumeof0,5-0,9cm3-g~1 (poresofdiameterlessthan7,5 pasdeterminedbymer- cury porosimetry). 1.2.3. Morphologv and size of particles Optical and scanningelectronic miscrocopies give information on the morphology and the size of solid particles. The photographies of figure 4 show that silica (photo a) and silica-alumina (photo b) particles we obtain are quite spherical. Silica-alumina particles seem to have arougher surface than si- lica particles. Size distribution of powdery raw materials,suspensions and xerogels are determined by laser dif- fraction (size range : 1,2-560 pn). Size measurements of powders are performed on either aqueous suspension and dry aerosols (interactions between water and dried particles can result in breakage of particles). 7 Figure 4 a) spray-dried silica b) Spray-dried silica-alumina Diameter of droplets generated by the cenb-ifugal atomizing device have been measured with this technique and results are discussed further in the text. 1.2.4. Other characterisations The knowledge of therheological behaviour of hydrogels is very important for pipe, pump and mi- xer sizing and design. It allows also a theoretical approach of internal structure of gels and interaction between gel particles. Hydrogel flow curves are established by means of a rotational rheometer. During the gelation vis- cosity evolution of the fluid is followed in the reactor by means of a torque sensor set up on the agitator shaft (laboratory reactor). Kinetic study of the filtration and establishment of mass balances during this unit operation allow the determination of the specific resistance, the compressibility and the filtrability coefficients of the filtration cakes (16). The optimisation of pilot filter operation can be. deduced from these determina- tions. Theresistance to attrition is an important characteristic of catalysts usedin fluid beds (5). This me- chanical property is evaluated with an air jet Gwyn-type-apparatus (17). 8 I11 - RESULTS AND DISCUSSIONS The process involves two key steps which need to be mastered: - the gelation step which gives to the catalyst its textural and catalytic characteristics - the spray-drying step which gives to the catalyst its morphology (shape and size of the particles). The results presented is this paper concern these two steps. Catalytic activity of the solids will not be described here. 1. Textural studv The influenceof operating variableson the silicaand silica-aluminatextures isquantified by means of thermoporometry. During the gelation step these variables are : . pH of the sol-gel transition . temperature of hydrogel formation . mixing . batch or continuous preparation . percentage of alumina . concentration of reactants . aging time of hydrogels. We also study the influence of drying on the textural characteristics of silica and silica-alumi- na. Figure 5 shows the differences observed between the textureof silica and silica-alumina hydrogels. Silica presents a very narrow pore size distribution (2-6 nm) with a median pore radius around 3 nm and pore volume of 0,5 cm3 g-l. The addition of alumina broadens the P.S.D. which ranges for silica- alumina from 2 to 20 nm and pore volume (1 - 1,4 cm3 g-'). The figure 6 illustrates the effect of spray- drying on the texture of silica and silica-alumina ( 25 % of alumina). Drying results in an important skrinkage and noticeable reduction of pore volume and median pore diameter for the two-types of hydrogels. The effect of drying is more drastic on the silica-aluminagel. Drying tends to minimize the influenceof each operating variables of the precipitation step. Nevertheless xerogels are similar to ini- tial hydrogels in terms of texture. 9 Figure 5 - Comparison of pore size distribution (cumulative curves) of silica and silica-alumina hydrogels I " . 0 5 10 15 20 R (nm.) Figure 6 - Effect of drying on pore size distribution of hydrogels - Si02 +A1203 hydrogel (I) - . - - Si02 hydrogel (2) -I-- 5 0 2 xerogel(3) 1 1400 1200 - 1000 - > 600 - 0 5 10 15 R (nm.) 3 10 The figure 7 shows the scale effect on the texture of hydrogels prepared at laboratory and on pilot plant. No differences can be observed on silica-alumina samples (curves 3 and 4). On the contrary P.S.D. of pilot plant silica sample is broader than P.S.D. of laboratory silica sample (curves 1 and 2). Differences in mixing efficiency of each reactor are probably the cause. Figure 7 - Comparison between laboratory and pilot plant preparation - SiO2 laboratory ( I ) - - - - Si02 pilot plant ( 2 ) - - - - SiO2lAUO3 laboratory (3) 0 5 10 15 20 R (nm.) TABLE 1 Effect of subhates on Dorous texture of silica gel gel particle pore pore radius filtration washing washing volume surface nm mm3.g-1 m2.g-1 Hydrogel X X 659 476 3,4 X X 236 282 2 2 Xerogels X X 233 237 2 2 X 137 117 2,1 11 TABLE 2 Effect of subhates on vorous texture of silica-alumina gel gel particle porous porous radius filtration washing washing volume surface nm -34-1 m2.g-1 Hydrogel X X 1561 601 4.4 X X 500 373 2 s Xerogel X X 267 239 2 3 X 345 308 2,1 Thermoporometry can indicate optimun way and timing to realize an operation along the manufac- turing process. An example is provided by theelimination of sulphates produced during silicaand alu- mina precipitations. Table 1 and 2 show that silica and silica-alumina textures are depending on the presence of sulphates during the drying. In tables 1 and 2, the crosses indicate the operations (column) realizedon samples (line). In each table comparison between the hydrogel andcorresponding xerogels shows the influence of drying. Comparison between xerogels shows the influence of the presence of sulphates during the drying. It has been checked that xerogel particles washing does not affect theirpo- rous texture. The texture of silica xerogels is detrimentally modified by the presence of sulphates (1 8). Sulphates cristallization anddeposition duringdryingdecrease the pore diameter and must be avoided. Sulphates must be washed out before the drying of silica hydrogels in order to preserve the hydrogel texture. The same observations can be made for silica-alumina (see table 2). 2. Influence of rheolotkal behavior of eel on meparation Simulation of catalyst preparation in the laboratory and then at a pilot plant are complementary. Pitfalls along the way can lead to unsucessfull scale-up. These problems can only be studied at the pilot plant where all the equipment used is of an industrial conception. One of the major problems encountered with hydrogels concerns their ability to be transfered from one vessel to another. These problems are not observed in the laboratory and can result in huge opera- ting difficulties in industrial plants if not studied carefully. Study of the hydrogel rheology is very im- portant for the design of agitators, pump selection and pipe sizing. The figure 8 shows hydrogel rheograms for silica and silica-alumina prepared at the pilot plant. Hydrogels are non-newtonian plastic fluids of Bingham type and can be described by the following re- lation: 12 Figure 8 - Rheograms of silica and silica-alumina hydrogels - SIUCA GEL 80 I60 240 320 4 SHEAR RATE(S-1) zc (yield stress value; Pa), the minimum stress to develop in order to establish flow, is acrucial cha- racteristic for scaling. +, is theplastic (orBingharn) viscosity (Pa.s) and y is the shear rate(s'*). A slight hysteresis can be observed, showing a slight thixotropy. AP As a result from these measurements the minimum pressure drop, (TImi* by unit lenght in a pipe of diameter D, needed to establish the flow can be calculated from AP Fluids after gelation have low stress values due to a low solid content (=5 %). After filtration and repulping, zc is noticeably modified and strongly related to the solid content in the hydrogel. The Bingharn viscosity remains small (few mPa.s). The effect of the solid content in silica-alumina hydrogels is illustrated on figure 9. The value of zc increases from 2 Pa to 20 Pa if the solid content in- creases from 73 to 15 %. After a certain level, an increase of 0,l percent in the solid content can cause a drastic change of zc which must be controlled during the repulping of the filtration cakes (figure 10). Pressure drop is affected in the same way and can reach high values, as shown on figure 10, depending 13 Figure 9 - Influence of water content of a silica-alumina hydrogei on its rheologkal properties - 84.89 % of wafer - - - - 8S.69 % of water 25 20 0 50.0 10.0 5.0 Q cr- w 4 4 I.0 9 0.5 rn 0. I ---- 86.23 90 ofwater -- 87.98 90 ofwater I I I 30 60 90 120 I50 SHEAR RATE(S-I) Figure 10 - Repulping test REPULPING TEST [3 RHEOLOGY MEASUREMENTS 14 on pipe diameter and pipe lay out. A bad control of the solid content can result in an inability to pump the fluid at the suction of the pump or no flow at all at the discharge side if pressure drops are exces- sive. Pilot plant can also avoid pitfalls in equipment selection. The two rheograms on figure 11 show how a screw type pump can affect the rheological properties of silica-alumina hydrogel. The shear stress imposed by the rotor-stator couple of the pump results in an increase of the yield value of the hydrogel. In the pump, the size of the particles which compose the hydrogel is decreased by a milling ef- fect and the forces of cohesion are increased. This can stop the flow if the pump characteristics have been underestimated. This results also in modification of the gel structure and ability to be spray dried. Figure 11 - Pump effect on the rheological properties of a silica-alumina hydrogel - SUCTION SIDE 3. Studv of the smav-drving The hydrogel suspension is atomized using centrifugal force into a hot air stream. The drying step is rather fast (few seconds to 30 seconds). Residual moisture of the spherical particles is still high (around 20 % wt). It is necessary to master the shape, the size andresistance to attrition of spherical par- ticles produced by the dryer. The properties of the solid are determined by the action of two phenome- na: - droplet formation, which depends on gel composition, viscosity, surface tension, density and the pul- verisation technique. 15 - drying phase which is the result of interactions between acontinuous phase (hot air stream) and adis- continuous, highly dispersed phase (droplets of hydrogel). The efficiency of the drying will depend on the droplet size, hydrogel properties (texture, composi- tion), hot air characteristics (temperature, flow rate) and air hydrodynamics in the drying chamber. It is important to understand and quantify these underlying phenomena in order to master spray drying process and its extrapolation. The centrifugal atomizing technique produces spherical particles with a size distribution between 20 to 150 microns as demonstrated on figure 4. Research is being conducted tocorrelate physico-chemical properties of the feed to droplet size dis- tribution at the exit of the atomizing device. These experiments are conducted in a mock-up. Atomi- zing wheel dimension and rotational speed can be adjusted in order to have a peripheral speed of ejec- tion varying between 50 and 100 m.s-l, common valuesencountered in industrial practice. Droplet si- zing is realized usinglight scattering technique. The laser beam goes through perpendicular to thedro- plet umbrella created by the turbine. The figure 12 shows droplet size distribution for water and hydrogel suspension. The atomizing speed is 80 m.s-l and the feed flowrate 3 kg.hr-l (curves 1 and 2). Curve 3 represents the particle size distribution of a xerogel, which is produced from the hydrogel of curve 1 dried in the laboratory spray dryer using the atomizing device described here. Figure 12 - Diameter distribution (cumulative curves) of droplets geuerated by atomizing vaned wheel 100 90 80 70 60 50 â¬3 20 10 0 5 10 50 100 500 DIAMETER (micron) 16 Droplet size distribution of atomized water and hydrogel are quite similar. These two-fluids (a newtonian one and anon-newtonian one) have same surface tension and, from ourresults, we think that their apparent viscosities arevery similar at the high yiel stress existing at the atomizing wheel periphe- ry. Comparison of curves 1 and 3 shows that drying does not affect droplet size distribution and that phenomena such as coalescence of droplet during drying are not very important for this kind of product. Correlations can be established between droplets and dried particle size distributions, for spray dryers of different sizes. CONCLUSION The formulation of an industrial catalyst depends on the choice of catalytic reaction and reactor de- sign (19). Once the formulation has been determined, the first step of the catalyst manufacture is the choice of the type of preparation. Laboratory experiments allow to specify influences of operating va- riables on the catalyst characteristics anddefine processoperation specifications. Pilot plantprepara- tion is the necessary and complementary step, devoted to check faisability and reproductibility of the chosen process using industrial types of equipments as well as pointing out scale effects and operating problems. This paper is an attempt to illustrate this methodology through the elaboration of silica-alu- mina microspheres usable in fluid bed reactors. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 i6 S. Wilkinson and D. Hunter, Chem. Week, 144, (1989), 24-40. L.L. Hegedus (Ed.), "Catalyst Design: Progress and Perspectives", J. Wiley and Sons, New- York, (1987), p 1-10. M.M. Van Kessel, R.H. Van Dongen and G.M.A. Chevalier, O.G.J., Feb. 16, (1987), p 55. B. Delmon and Coll. (Eds), "Preparation of Catalysts: Scientific Bases for the Preparation of He- terogeneous Catalysts"; vol. I, 11, 111 and IV, Elsevier, Amsterdam, 1975, 1979, 1983, 1987. J.F. Le Page and Coll. (Eds); "Catalyse de Contact : Conception et Mise en Oeuvre des Cataly- seurs Industriels", Editions Technip, Paris, 1978. D.L. T r i m , "Design of Industrial Catalysts", Chem. Eng. Monographs 11, Elsevier, Amsterdam, 1980. E.F. Sanders and E.J. Schlossmacher, in B.E. Leach (Ed.), "Applied Industrial Catalysis", Aca- demic Press, London, 1,(1983), pp. 31-40. P. Trambouze, J.P. Reymond, D. Vanhove and F. Kolenda, Information Chimie n0294, (1988), J.N. Fulton, Chem. Eng., July 7, (1986), pp. 59-63. P. Trambouze, Chem. Eng. Progr., 86, (1990) pp. 23-31. K.D. Ashley and W.B. Innes, Ind. Eng. Chem., 44, (1952), pp. 2857-2863. 0. Levenspiel (Ed.), "The chemical Reactor Omnibook", OSU Book Stores Inc., Corvallis, Oregon, (1984), ch.23. M. Brun, A. Lallemand, J.F. Quinson and Ch. Eyraud, Thermochimica Acta, 21, (1977), pp. 59- 88. J.F. Quinson, M. Brun, in K.K. Unger and Coll. (Eds), "Characterization of Porous solids", Elsevier, Amsterdam, (1988), p 307-315. G.W. Sears Jr., Anal. Chem., 28, (1956) pp. 1981-1983 P. Rivet, "Guide de la skparation liquide-solide", SociCtC Franqaise dz Filtration, Idexpo, Ed. Cachan, (1981 pp. 275-282. 17 17 18 19 J.E. Gwyn, A.I.Ch.E., 15, (1969) 35-39. M.E. Winyall, in B.E. Leach (Ed.), "Applied Industrial Catalysis", Academic Press, London, 3, R. Montarnal and J.F. Le Page, in B. Claude1 (Ed.), "La Catalyse au Laboratoire et dans 1'In- dustrie", Masson Paris, (1967), pp. 231-287. (1984), ch. 3, pp. 43-62. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 19 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands DEPOSITION PRECIPITATION ONTO PRE-SHAPED CARRIER BODIES. POSSIBILITIES AND LIMITATIONS K.P. de Jong Koninklijke/Shell-Laboratorium, Amsterdam (Shell Research B.V.) P.O. Box 3003, 1003 AA Amsterdam, The Netherlands. ABSTRACT are dealt with. Several features of deposition precipitation onto pre-shaped carrier bodies Firstly, the kinetics and mass transfer effects of catalyst synthesis via deposition precipitation onto pre-shaped and powder carriers have been studied under pseudo-stationary conditions. The precipitation of manganese hydroxide onto silica by urea hydrolysis has been used as a model reaction. The overall disappearance of manganese ions from the aqueous solution could be described as a first-order process. The rate-determining step for Mn deposition is related - as expected - to the urea hydrolysis. From the distribution of Mn over the silica granules after precipitation the rate constant for the surface deposition process has been determined. The latter process has a much higher rate constant than the rate-determining hydrolysis reactions. The surface reaction appears to determine the ultimate distribution by a combined process of reaction and diffusion. The consequences of this study for the viability of deposition precipitation onto pre-shaped carriers for practical application are addressed. Secondly, transient phenomena taking place initially during deposition precipitation onto large carrier particles have been considered. More specifi- cally, the occurrence of transient pH gradients over carrier bodies directly after liquid imbibition has been utilized to control the local rate of precipi- tation in carrier bodies. This could be effected by applying a precipitation reaction whose rate depends on the pn. It has been found that thereby non- uniform activity distributions can be realized in a controlled manner. The method seems to be especially useful to produce egg-yolk type catalysts by the application of redox reactions for ureciuitation. Several catalvst formulations _. have been prepared via this novel synthesis method, viz. Mo/Si02, Cu/A1203 and Ag/A1203. INTRODUCTION Catalyst synthesis by means of deposition precipitation comprises application of the active component onto an existing support via a chemical reaction. This reaction gives rise to the formation of an insoluble compound which involves the active element. The insoluble compound can be formed by an increase of the pH of the solution (hydroxide precipitation), a valence change of the element in question and the like. Under certain conditions, such as a suitable interaction between the compound and the carrier, the preparation 20 method has a number of unique features, e.g. a uniform distribution of the active component over the carrier surface even at high loadings. tion precipitation method has been known for quite some time; its history is summarized in Table 1. It has been extensively studied by Geus and co-workers The deposi- ~51. TABLE 1 History of deposition precipitation. Year Assignee/author Remarks Reference ~~~ ~~ ~ 1943 IG Farben Precipitation of (hydr)oxides, sulfides, 1 1967 Stamicarbon Precipitation with urea from homogeneous 2 selenides solution; powder carriers 1970 Unilever Precipitation with urea from concentrated 3 solutions; prevention of evaporation 1970 Stamicarbon Precipitation with reduction reaction from 4 homogeneous solutions; powder carriers 1983 Geus et al. Basic studies on deposition precipitation 5 with powder carriers 1988 Shell Controlled non-uniform activity distributions 6 with large, pre-shaped carriers The work of Geus et al. [5] on deposition precipitation has been directed towards the use of support materials consisting of small particles (powders, particle size smaller than say 1 pm). For practical applications, deposition of the active component onto powders implies that shaping of the small particles into larger bodies has to be done afterwards. The shaping procedure might invoke a number of possible disadvantages such as occlusion of the active component and a low mechanical strength. Therefore, the scope of deposition precipitation for the production of uniform metal distributions would be enhanced considerably if it were applicable with larger (pre-shaped) carrier particles. There are indications in the patent literature [ 3 ] that this is indeed possible. Recent publications [7,8], on the other hand, suggest that with medium-size silica particles (50-100 pm) precipitation of nickel hydroxide via urea hydrolysis gives rise to significantly enhanced, but uncontrolled concentrations of the active component at the outer surface of the silica particles. In this paper we report on some work undertaken to assess the scope cf deposition precipitation onto large, pre-shaped carrier particles (about 1 mm). 21 Specifically two aspects are addressed: (i) the kinetics and mass transfer effects under pseudo stationary conditions, and (ii) the utilization of a unique feature of precipitation onto large carrier particles, viz. transient concentration gradients occurring directly after liquid imbibition. The kinetics and mass transfer effects are studied with the often considered method of precipitation of metal hydroxydes via a controlled increase of the pH of the solution by the hydrolysis of urea [5,9]. The deposition of manganese hydroxide onto silica was chosen as a model reaction because of its relative simplicity (e.g. no formation of silicates, no formation of basic carbonates). The kinetics of the relevant reactions have been established and the mass transfer effects during precipitation assessed. Based on the quantitative data a model for the precipitation reaction is formulated. Implications for precipitation onto large carrier particles are addressed. In the course of our work on deposition precipitation onto pre-shaped carrier particles we have found that under certain circumstances the technique can also be used for realizing controlled non-uniform activity distributions over carrier bodies [ 6 ] . After careful consideration it turned out that the basic phenomenon underlying this finding is connected with transient pH gradients occurring in carrier particles directly after liquid imbibition. In case the rate of the precipitation reaction in question depends on the pH of the aqueous phase, the metal (oxide) distribution after precipitation will reflect the previous pH gradient. In this paper the principles of this new application of precipitation reactions in catalyst synthesis are elaborated. Furthermore, several examples of catalysts with their respective metal distributions are described. EXPERIMENTAL Precipitation of manganese onto silica (pseudo-stationarv) Precipitation of manganese hydroxide in the presence of silica was brought about by the hydrolysis of urea according to the overall reaction Mn2+ + CO(NH2)2 + 3 H20 __ > Mn(OH)2 + C02 + 2 NH4' To this end Mn(N03)2.6H20 was dissolved in 250 ml water; the solution was acidified with 1 ml of nitric acid ( 6 5 % ) and transferred to a double-walled, thermostatted vessel equipped with baffles and a stirrer. Five gram of silica was added to this solution in experiment U30 and U31, while experiment U33 was carried out without silica and experiment U34 with 10 g of silica. For further details on the concentrations of reactants the reader is referred to Table 2. After heating the mixture to 90 OC the urea was added at time zero (cf. Figs. 1-5). At that temperature the reaction was continued for several hours while nitrogen was bubbled through (to prevent oxidation of the di-valent 22 manganese precipitate) and the pH monitored continuously. At certain intervals of time a small sample (about 2 ml) of the liquid was withdrawn, filtered, weighed and acidified. The total manganese content of these samples was deter- mined using Atomic Absorption Spectrometry. From these data the concentration of manganese in the solution was calculated. TABLE 2 Experimental conditions and kinetics of the precipitation of manganese. kl is the first-order rate constant of the urea decompodition. kt is the first-order rate constant of the manganese precipitation. Exp. Carrier Temp. Mn2+ conc. Urea conc. kt *lo5 kl* lo5 ( O C ) (mmol .I-') (mmol. 1-1) (s-1) (S-5 U30 granulesa) 90.5 9.1 46 3.3 n. d U31 powderb) 90.5 9.1 46 3.3 3.8 u33 - 88.5 36 182 1.1 2.7 ~ 3 4 granulesa) 88.5 36 182 2.7 2.7 n.d. = not determined a) Grace Davison silica gel: 0.6-1.4 mm, pore volume 1.2 ml.g-', surface b) Degussa Aerosil: surface area (BET) 2 8 1 m2.g-'. area (BET) 310 rn2.g-'. Precipitation of molybdenum onto silica spheres (transient phenomenal As has been reported before [4], molybdenum can be deposited onto silica powder via the redox reaction: 4M004~- + 3N2Hq + 4H20 -> 4Mo(OH)3 + 3N2 + 8OH- A molybdate solution was prepared by dissolving the appropriate amount of (NH4)6M07024.4H20 and adding concentrated ammonia till pH=8.7. A hydrazine solution of similar pH was obtained by neutralizing N2H5QH with acetic acid. Further experimental details can be found in Table 3. The procedure of con- tacting the carrier with the solution is further described below. Experiment RK15 was done by first contacting the silica spheres with the Mo solution (0 OC), followed by addition of the cooled hydrazine solution. Subsequently, the mixture was rotated in a round-bottomed flask under nitrogen and heated slowly. In experiments RK29, FX33 and RK37 (Table 3) a mixed Mo/N2H4 solution was applied. This solution (0-10 OC) was circulated for several hours through a small vessel, which was maintained at higher temperatures and con- tained 25 g of silica spheres. Under these conditions M o was deposited exclusively in the inner part of the spheres except for catalyst RK33 where some precipitation in the liquid occurred. Catalyst RK77 (Table 3) was also prepared using a mixed solution. The reaction was observed to start at the 23 centre of the spheres and continued for 15 min at 0 OC. Subsequently, the reaction was 'quenched' by diluting the reaction mixture with water at 0 OC. TABLE 3 Synthesis conditions of Mo/Si02 catalysts. Cat. Si02 Soln. [Mo] [N2H4]/[Mo] pH Method T;y?fzk;;e Mob) (g)a) (ml) (g/l) (mol/mol) (%W) RK15 25 95 79 2.8 8.7 rotating 1 h at OC 16 flask 0 --* 60 OC in 20 h RK29 25 500 15.0 2.0 8.5 recircu- 19 h at 50 OC 6.7 lation RK33 25 1300 15.4 2.0 8.7 recircu- 40 min. at 1 OC 20.6 lation 1 6 h at 7 OC RK37 25 1300 15.4 2.0 9 . 0 recircu- 30 min at 2 OC 1.5 lation 1 22 h at 30 OC RK77 6 . 7 40 72 2.0 8.5 quench 15 min. at 0 OC 0.66 ~~~~ ~ ~ ~~ a) Si02 carrier used: 1.5 mm Shell silica spheres, pore volume b) Determined by X-ray fluorescence. 0 . 9 8 ml.g-l, surface area (BET) 263 m2.g-'. Precipitation of copDer and silver onto alumina (transient Dhenomena) Copper and silver were deposited onto alumina extrudates applying the reac- tions and conditions summarized in Table 4. The mixed solution of the metal salt and reducing agent was poured onto the extrudates at ambient conditions. It was observed visually that the precipitation started at, and was restricted to, the central part of the extrudates. After some time the reaction was 'quenched' by dilution with water. Catalyst characterization After drying in air the metal distribution was established using electron microprobe analysis. The scanning electron microscope used was a Jeol-35. A step size between 20 and 30 pm was applied. The metal distribution was deter- mined by carrying out a scan along the shortest line through the centre of the catalyst body. 24 TABLE 4 Synthesis conditions of Cu/y- A1203 and Ag/y- A1203 (egg - yolk) Catal. Formu- A1203 Soh. Reactants pH Condi- Reaction Metal lation tions load- (g)a) (ml) ing') RK129 Cu/A1203 10 64 2.7 ml N2H50H 2.4 2 h at 4Cu2++ N2Hq+2H20 1.8 %w solution 21 OC + 2Cu20J+N2 Cu (80 %w) + 8H+ 1.5 ml acetic acid (99.9 %w) 11.4 g C~(N03)2.3H20 After a spon- taneous reaction at 21 OC for 1.5 h and filtration the resulting clear solution was used for precipation onto alumina. RK130 Ag/A1203 10 41 5.0 g AgNO3 0.94 3 min. 2Ag+ + CH20 + 0.38 at H20+2AgJ %w Ag 21 OC + CHOOH + 2H+ 0.87 g forma- lin (37 %w '3320) nitric acid (65 %w) a) A1203 carrier used: AC300, 1.5 mm cylindrical extrudates, pore volume 0.68 m2.g-'. h) Determined by X-ray fluorescence. RESULTS AND DISCUSSION Kinetics and mass transfer effects under pseudo-stationarv conditions pH measurements. Figures 1 and 2 show the pH records as obtained during the four Mn precipitation experiments. In all experiments a rapid initial increase of the pH after addition of the urea at zero time is observed. This increase coincides with the neutralization of the nitric acid by the ammonium hydroxide generated via the hydrolysis of urea according to the equation kl CO(NH2)2 + 3 H 2 0 ~ > Cog + 2 NH4+ + 2 OH- (1) 25 The rate of urea decomposition has been calculated from the increase of the pH in the range pH 2-4. The rates obtained directly from the linearized pH curves have been interpreted using the first-order kinetics for the hydrolysis reaction which are clearly indicated in the literature [ l o ] . From the litera- ture mentioned it is furthermore apparent that the rate of hydrolysis of urea is independent of pH. The first-order rate constants kl calculated and summa- rized in Table 2 , therefore, are valid throughout the experiments. Note that the value of kl is not affected by the presence or the absence of the carrier. The higher value of kl obtained from experiment U31 is not caused by the nature of the silica carrier but by the fact that the temperature is slightly higher in experiment U31 than in experiments U33 and U34. The qualitative features of similar pH curves have already been described by Hermans et al. [9]. In the absence of a carrier (U33 in Fig. 2), after the initial rapid increase of the pH an overshoot develops after 1.5 to 2.0 hours. This overshoot is characteristic of the precipitation process being activated due to the required formation of nuclei of the solid manganese hydroxide phase. Following the nucleation phase (pH=6.6) the pH drops, which coincides with the growth of the nuclei formed. This process of growth takes place as a pseudo- stationary process at virtually constant pH (pH=6.3). In the presence of a carrier no overshoot o f the pH is observed and the pH level of the pseudo-stationary precipitation phase (pH=5.8) is lowered (Fig. 2 ) . In accordance with previous studies [ 9 ] these observations are ascribed to the smooth, non-activated nucleation at the carrier and to the stabilization of the MII(OH)~ phase by interaction with silica, respectively. The pH curves obtained with silica powder on the one hand and silica granules on the other (Fig. 1) do not display any relevant differences. The apparent difference of the plateau of the records (pH=6.1 with powder and pH=6.2 with granules) is within the experimental error of the pH measurements. PH 1 0 TIME, h PH 1 1 1 1 1 1 1 1 1 1 2 4 6 8 1 0 TIME, h Fig. 1. Records of pH as a function Fig. 2. Records of pH as a function of time for experiments U30 (granules) and U31 (powder). of time for experiments U33 (no carrier) and U34 (granules), 26 Removal of manganese from solution. The total concentration of manganese species dissolved in the aqueous phase as a function of time has been esta- blished in the experiments (results in Figs. 3 and 4 ) . From Fig. 3 it is apparent that the rate of removal of Mn from the solution is very much alike for the silica powder and the silica granules. The presence or absence of silica, however, has a large impact on the kinetics of the removal of Mn (Fig. 4 ) . In the absence of silica the precipitation process is slowed down considerably. Furthermore, the shape of the plots is different. The non- linear shape of the plots (Figs. 3 and 4 ) in the presence of silica suggests a reaction order higher than zero, whereas the linearity of the graph in the absence of silica (experiment U 3 3 , Fig. 4 ) suggests a zero-order process. In Fig. 5 we have gathered the experimental data on the total Mn concen- tration in the form of a first-order plot. The experiments carried out in the presence of silica ( U 3 0 , U 3 1 and U 3 4 ) give rise to a linear relationship in Fig. 5, which supports the idea that the overall precipitation process in the presence of silica can be described by first-order kinetics. For experiment U 3 3 (no carrier), too, a linear plot is obtained, but we hesitate to conclude from this that the precipitation kinetics are first-order in this case. This hesitation arises from (i) the low levels of manganese 'conversion' attained in experiment U 3 3 , and (ii) the linear relationship obtained in Fig. 4 for this experiment, which suggests zero-order kinetics. From the slopes of the fitted straight lines (Fig. 5) the first-order rate constants (kt) for the overall precipitation reaction of Mn from solution in the experiments were obtained and tabulated (Table 2 ) . 0 50 - 040 - 030L 0 20 TIME, h TIME. h Fig. 3 . Manganese concentration in the Fig. 4 . Manganese concentration in the liquid as a function of time for liquid as a function of time for experiments U 3 0 (granules) and U 3 1 experiments U 3 3 (no carrier) and U 3 4 (powder). (granules). Clearly, kt is much larger in the presence than in the absence of silica. From the kt values for experiments U30 and U31, it turns out that the size of the silica particles does not affect the overall kinetics of the precipitation process. The difference with respect to kt between experiments U30 and U 3 4 is caused by the higher rate of urea hydrolysis in experiment U30, which itself is brought about by the slightly higher temperature in that experiment. The four-fold difference in initial Mn concentration (experiment U30 versus U 3 4 ) giving rise to comparable values for kt - especially when the temperature difference is taken into account - further supports the overall precipitation being adequately described as a first-order process. Since the rate of Mn deposition is increased in the presence of silica and is not affected by the silica particle size, it is tempting to conclude that the precipitation process is not influenced by mass transfer effects. In the next section it will be demonstrated, however, that these effects still play an important role. From the rate constants and concentration data gathered in Table 2 it can be easily seen that the rate of urea hydrolysis (mol/s) exceeds the rate of manganese removal (mol/s) by a factor of 5 or more. This implies that the reaction between manganese ions and urea is not a stoichiometric one and it is proposed that part of the ammonium hydroxide formed (cf. reaction (1)) will Ln (C&) 0.8 7 - + - U30 - - U31 - 8 - u33 --4-- u34 0.6 - - 0.4 - - 0 2 - I I I I I I I I I I 0 2 4 6 8 10 TIME, h Fig. 5 . First-order plot of the rate of removal of manganese from solution. Go - manganese concentration at zero time C - actual manganese concentration 28 leave the solution as ammonia while the remaining part brings about the hydro- lysis reactions of the manganese species. The reactions can be written as follows: N H ~ + + OH- - > NH3 t + H20 (2) Mn2+ + OH- - > M~(oH)+ ( 3 ) Hereafter we will assume that as a first approach reactions (2) and ( 3 ) can be considered to be in equilibrium. Although the rate of hydrolysis of urea is larger than that of the removal of Mn, reaction (1) is the rate-determining step for the overall precipitation process. The âselectivityâ of the utilization of the generated ammonium hydro- xide is determined by âequilibriaâ (2) and ( 3 ) . It is expected that higher temperatures and/or a higher pH will favour (2) over ( 3 ) . This indeed explains that for experiment U33 the rate constant kt is lower than in the other cases, seeing that, as is shown in Fig. 2, the pH at which precipitation takes place is much higher. Deuosition reaction of manganese. The final reaction which leads to deposition of the partly hydrolysed Mn species onto the silica carrier is tentatively described as k4 Mn(OH)+ + SiOH + OH- - > SiO-Mn(0H) + H20 (4) The kinetics of reaction (4) cannot be assessed from the overall precipitation reaction since - as outlined above - the latter is dominated by the urea hydro- lysis in combination with the âselectivityâ determining reactions (2) and ( 3 ) . The kinetics of the consecutive reaction ( 4 ) could be found, however, from the resulting distribution of manganese over the silica granules. An example of such a distribution as obtained from electron microprobe analysis is shown in Fig. 6. The qualitative shape of the distribution indicates that reaction (4) is influenced by both kinetic and diffusion effects. In order to further extract quantitative information from the manganese distributions we have assumed that the kinetics of reaction ( 4 ) can be approached by a first-order dependence in manganese and a zero-order dependence for the other components. With these assumptions one can make use of the Thiele concept of diffusion- limited reactions [ll]. 29 O2 CONCENTRATION, C ( z / L l / C ( - 1 ) - . EXPERIMENT -MODEL - lob 0.8 0.6 1 I . 0 F 5. 6 . Distribution of manganese over a representative silica granule from experiment U 3 4 . The drawn lines have been calculated for different values for the Thiele modulus (@). We start with the quantitative determination of k4 by defining the Thiele modulus of reaction (4) as 0 = V/S *qkq/De (5 ) with V = (average) volume of the carrier particles S = (average) surface area of the carrier particles De = effective diffusivity of manganese species in the porous silica particles Furthermore we use the known relationship [ll] between the Thiele modulus and the concentration of manganese at the outer surface ( G o ) and that at the centre of the particles (Ci) written as Ci/Co = 1 / (cash 0) (6) From an average value of Ci/C0=0.32 obtained in experiment U 3 4 a value of +1.8 follows. By approaching the silica granules as cubes with a length of From an estimate of De (lo-' m2/s) according to relationships given by Perry [12] for the bulk diffusivity and by Satterfield [13 ] for the tortuosity of the carrier in combination with equation (5) it follows that k4=O.12 l/s. Comparison of this value with those of the rate constants reported in Table 1 shows that the consecutive precipitation reaction ( 4 ) is much more rapid than the rate-determining urea hydrolysis. 0.02 as determined for seven granules m we conclude that V/S=1.67*10-4 m. 30 Development of a semi-quantitative model. In this section we will briefly discuss a model which summarizes the above kinetic and mass-transfer effects. We distinguish three steps. viz. (i) the hydrolysis of urea, which leads to hydrolysis and precipitation of manganese, (ii) transport of (hydrolysed) manganese species from the bulk of the solution to the outer surface of the carrier particles, and (iii) reaction parallel with mass transfer inside the porous particles. These steps lead to the following expressions for the rate of removal of Mn from solution (Rt): Rt s * kl * C1 (7 ) Rt kl * ak * (âB - âB,i,l) S = selectivity defined as the rate of Mn hydrolysis over urea hydrolysis, mol(Mn)/mol(urea). kl = rate constant for urea hydrolysis (s-l) kl - mass transfer coefficient, liquid/solid (m/s) c1 - concentration of urea, mo1/m3 CB = concentration of Mn(OH)+ (-B) in liquid phase, mol/m3. CB,i,l = concentration of B at liquid side of interface between liquid and carrier particle, mol/m3. ak d = V/S, m. 11 - tanh(0)/0 (effectiveness factor). CB,i,s = concentration of B at solid/liquid interface = external carrier surface area, m2/m3. at solid side, mo1/m3. and realizing that âB,i,s = * âB,i,l with e = porosity of the carrier, m3/m3 the interfacial concentrations for B can be eliminated and equation (11) is obtained. S*Cl 4- CB Rt = l/kl + l/kp*ak + l/kq*ak*d*q*r With respect to the model summarized by equation (11) it is noted that a crude 31 approximation lies in the selectivity factor s (equation (7)) which describes the complex interplay between the urea hydrolysis, ammonia evaporation and manganese hydrolysis in a simplified manner. However, in the absence of more detailed data a more sophisticated description of the process cannot be vali- dated. The advantage of equation (11) is that it presents insight into the three main resistances for manganese deposition, viz. urea hydrolysis (l/kI), transport to the outer surface of the catalyst particles (l/kl*ak), and diffu- sion and reaction inside the particles (l/k&*ak*d*q*e). The first and the third resistance follow from this work and the second one can be estimated from the literature [14]. To a first approximation, for experiment U34 the three resistances as mentioned rate as follows: 90:1:8. In other words, the largest resistance is related to the urea hydrolysis, whereas the transport to the external surface causes a negligible resistance only. From equation (11) it can be easily shown that depending on the specific value of the selectivity parameter ( s ) the overall process of Mn removal from the solution can or cannot be described by a first-order process. In view of the importance of the surface deposition reaction for the distribution of the active components over the carrier particles, k4 has been determined for a number of catalytic components. The results shown in Table 5 indicate that the values for k4 can vary considerably; the distributions of the active component will vary correspondingly. TABLE 5 Rate constants for the surface precipitation reaction for several active components onto silica carriers. The precipitation reaction involves urea hydrolysis. Active metal Rate constant, k4 (s-l) Reference Mn Rh Ni 1.2 * 10-1 This work 3 . 6 * lo-* 15 9.0 * 10-3 16 Effects of transient concentration eradients A novel method to apply deposition precipitation which makes use of transient concentration gradients will be illustrated by the synthesis of egg- yolk type molybdenum-on-silica catalysts. Two methods have been elaborated to start and restrict the deposition of Mo at the centre of the silica spheres (cf. Table 3 ) . Note that in all experiments the volume of the solution exceeds the carrier pore volume considerably (wet 'impregnation'). The first method uses relatively dilute solutions and mild reaction conditions (pH, temperature) such that a reaction starts at the centre of the spheres and terminates spon- 32 taneously (typically after 1 h). This method was applied for experiments RK29 and RK37. The line scans for catalyst RK37 (Fig. 7 ) reveal - in accord with visual inspection of the dried catalyst - the Mo to be concentrated in the centre of the spheres (a so-called egg-yolk catalyst). Note the small variation of the metal distribution between the separate spheres (Fig. 7). The second method to control the local rate of deposition was investigated in experiment RK77 (Table 3 ) , where the reaction was allowed to continue for 1 5 min at 0 OC and then quenched by diluting the reaction mixture with water, also at 0 OC. From the line scan (Fig. 8) it is apparent that again an egg-yolk catalyst was obtained, although less pronounced than with RK37 (Fig. 7 ) . In future work it might be worthwhile to consider quenching the reaction by lowering the temperature of the reaction mixture. Fig. 7. Electron microprobe analysis Fig. 8 . Electron microprobe analysis of the molybdenum distribution over of the molybdenum distribution over a silica spheres of catalyst RK37 representative silica sphere of cata- (1.5 %w Mo). lyst RK77 ( 0 . 6 6 %w Mo). In Fig. 9 line scans of Mo/Si02 catalysts have been collected for different Mo loadings. Clearly, with increasing load the metal distribution shifts from convex to flat and, finally, to concave. In searching an explanation for the controlled precipitation reaction at the centre of the spheres, it is important to note that the reaction between Mo0h2- and N2Hh is strongly accelerated at reduced pH. This effect has been observed experimentally by us and is related to the liberation of hydroxyl ions during the reaction (cf. Experimental sectionj. Furthermore, we have observed that a more uniform Mo distribution was obtained when the reactants had contacted the carrier for an extended period under conditions which precluded any reaction. Based on the above observations, the controlled precipitation is ascribed to the existence of temporary concentration gradients inside the spheres. After imbibition with part of the solution containing the reactants, the 'acidic' 33 A ) 1 5 o/ow Mo (RK 3 7 ) B)6 .7° /owMo(RK 29) C ) 16 '/OW MO (RK 15) D) 21 o/ow Mo (RK 33) Fig. 9 . Microprobe analysis of the molybdenum distribution over silica spheres of Mo/Si02 catalysts with various metal loadings. silica support will give rise to a decrease of the pH of the penetrated solution in comparison to the pH of the 'excess' solution outside the spheres. This will lead to concave pH profiles within the spheres, which will exist typically for 1 h". This concave pH profile results in a convex Mo distribution as the rate of precipitation increases with decreasing pH. We now propose to generalize the above described phenomena as follows: - The combination of (i) a precipitation reaction proceeding via redox chemistry and liberating OH-(or H+) ions with (ii) the use of an acidic (or a basic) carrier may give rise to accelerated precipitation of the active component at the centre of the carrier body (egg-yolk distribution). - The combination of (i) a precipitation reaction proceeding via redox chemistry and liberating OH-(or H+) ions with (ii) the use of a basic (or an acidic) reacting carrier may give rise to accelerated precipitation of the active component at the edge of the carrier body (egg-shell distribution). Of course, this method may be even further generalized to other types of precipitation reactions and to concentration gradients other than that of H+. In this paper, however, we will restrict ourselves to the combination of redox reactions and pH gradients. * Typical times to efface concentration gradients by diffusion under these conditions range from 0.5 - 1.0 h related to the Fourier process in question [17]. Adsorption phenomena may even greatly prolonge these periods (18,191. 34 We note in passing that the question whether a carrier will react basic or acidic will depend both on the nature of the carrier and on the pH of the aqueous solution. Roughly speaking, one can say that if the pH of the solution is abovefielow the iso-electric point (IEP)' of the oxide in question, the carrier will react acidic/basic. Two precipitation reactions have been designed to check the generalization concerning the egg-yolk catalysts, viz. the liquid-phase reduction of Cu2+ and of Ag+ with N2H4 and CH20, respectively (for details see Table 4 ) . Both reactions generate H+ ions and it is predicted that a basic carrier, such as y-Al2O3 at sufficiently low pH, will lead to an egg-yolk distribution. Indeed, the line scans shown in Figs. 10 and 11 prove the metals to be concentrated in the inner part of the extrudates. Fig. 10 . Electron microprobe analysis of the copper distribution over an alumina extrudate of catalyst RK 129 (1.8 % m/m Cu). Fig. 11. Electron microprobe analysis of the silver distribution over an alumina extrudate of catalyst RK 130 ( 0 . 3 8 % m/m A g ) . The second generalization related to the egg-shell catalysts has been confirmed by repeating the copper precipitation under identical conditions with (acidic) silica spheres, Visual inspection of the precipitation process re- vealed the copper oxide to be concentrated at the outer surface of the spheres. Practical application of this method to produce egg-shell catalysts, however, involves the problem of precipitation in the liquid separate from the carrier. Therefore, the method seems to be especially suited for preparing egg-yolk type catalysts. GENERAL DISCUSSION AND CONCLUSIONS Kinetics and mass transfer effects (pseudo-stationarv conditions) Several conclusions for deposition precipitation onto pre-shaped carrier particles utilizing urea hydrolysis emerge from this study. Firstly, as # The IEP of 7 -A1203 is between 7 and 9 [ 20 ] while the IEP of Si02 has been reported to range from pH 0.5 to 3 . 7 [21]. 35 expected, the rate-determining step is connected with the urea hydrolysis. Secondly, the ultimate distribution of metal over the carrier body is dictated by the rapid consecutive precipitation reaction (rate constant kb) at the carrier surface. An important implication is that the rate of the urea hydro- lysis (and thus of the overall precipitation process) does not affect the metal distribution. Consequently, the rate may be much enhanced without affecting the distribution. Furthermore, it might be inferred that higher metal concentra- tions will not change the picture since we have indications that the processes are first-order in the metal concentration. In our laboratory [16] we have obtained evidence for the precipitation of nickel onto silica spheres that this is indeed the case. With respect to the viability of using deposition precipitation onto pre-shaped carriers for industrial applications the feasibility of the method is apparent from the work presented in this paper. Pore mouth plugging by deposition of the metal component will occur only with extremely rapid surface reactions. However, in general concentration gradients will result from applying this synthesis method with larger carrier particles. Whether these concentration gradients are acceptable or not depends on the catalyst and its specific application. Effects of transient concentration eradients A unique feature of deposition precipitation onto large carrier bodies has been found to be related to the occurrence of concentration gradients direct after imbibition with the reactant mixture. In general, when the iso-electric point of the carrier deviates from the pH of the solution, a pH gradient inside the carrier body will exist for some time (typically 1 h). By applying a preci- pitation reaction whose rates depend on the pH the local rate of precipitation in the carrier body may be controlled. It has been shown that deposition precipitation via redox chemistry can be used for the production of egg-yolk type catalysts. The synthesis of Mo/Si02, Cu/A1203 and Ag/A1203 catalysts displaying an egg-yolk type distribution has been described to illustrate this novel preparation route. ACKNOWLEDGEMENTS The skilful experimental assistance of Mr. E.J.G.M. Romers and Mr. R. van Kempen is gratefully acknowledged. Furthermore, the valuable review of this paper by Prof. Dr. J.A.R. van Veen is appreciated. 36 REFERENCES 1 German Patent No. 7 4 0 , 6 3 4 to IG Farben ( 1 9 4 3 ) . 2 Netherlands Patent Application 6 7 , 0 5 2 5 9 to Stamicarbon ( 1 9 6 7 ) . 3 US Patent 3 , 6 6 8 , 1 4 8 to Lever Brothers Company ( 1 9 7 0 ) . 4 Netherlands Patent Application 6 8 , 1 6 7 7 7 to Stamicarbon ( 1 9 7 0 ) . 5 J.W. Geus, in (B. Delmon et al., Eds.) Preparation of Catalysts 111, 6 European Patent Specification 2 5 8 , 9 4 2 ( 1 9 8 8 ) to S.1.R.M.-B.V. 7 M. Montes, Ch. Penneman de Bosscheyde, B.K. Hodnett, F. Delannay, P. 8 B. Delmon, Solid State Ionics 16 ( 1 9 8 5 ) 2 4 3 . 9 L.A.M. Hermans and J.W. Geus, in (B. Delmon et al., Eds.), Preparation of 10 R.C. Warner, J . Biological Chemistry USA 1 4 2 ( 1 9 4 2 ) 7 0 6 . 11 K.R. Westerterp, W.P.M. van Swaaij and A.A.C.M. Beenackers, Chemical Reactor Design and Operation, John Wiley, Chichester, 1 9 8 4 , p. 3 8 3 . 1 2 J .H. Perry, Chemical Engineersâ Handbook, 4th Edition, McGraw-Hill, 1 3 C.N. Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, 14 F. Kneule, Chemie-1ng.-Techn. 28 ( 1 9 5 6 ) 2 2 1 . 1 5 K.P. de Jong, J.H.E. Glezer, H.P.C.E. Kuijpers, A . Knoester and C.A. Emeis, 16 K.P. de Jong, unpublished results. 17 J. Crank, The Mathematics of Diffusion, Clarendon Press, Oxford, 1 9 7 5 , 18 P.B. Weisz, Trans. Faraday S O C . 6 3 ( 1 9 6 7 ) 1801. 1 9 M. Komiyama, Catal. Rev.-Sci. Eng. 27 ( 1 9 8 5 ) 3 4 1 . 20 J . P . Brunelle, Pure C Appl. Chem. 5 0 ( 1 9 7 8 ) 1211. 2 1 R.K. Iler, The Chemistry of Silica, John Wiley, New York, 1 9 7 9 , p. 188. Elsevier, Amsterdam, 1 9 8 3 , p. 1. Grange and B. Delmon, Appl. Catal. 12 ( 1 9 8 4 ) 3 0 9 . Catalysts 11, Elsevier, Amsterdam, 1 9 7 9 , p. 1 1 3 . 1 9 6 3 , pp. 14-23 . 1 9 8 0 , p. 3 3 6 . J . Catal., accepted for publication. p. 8 9 . G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 37 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands INFLUENCE O F THE PREPARATION PROCEDURE ON THE PHYSICAL PROPERTIES, SURFACE A C I D I T Y AND D I S P E R S I O N OF MoP/A1,0, CATALYSTS R. PRADA S I L V Y , Y. ROMERO, J . GUAREGUA, R. GALIASSO I N T E V E P S.A, Seccion de C a t a l i s i s ' A p l icada, Apdo. 76343, Caracas 1070-A, VENEZUELA. SUMMARY The i n f l u e n c e o f t h e p r e p a r a t i o n procedure on t h e phys i ca l p r o p e r t i e s , su r face a c i d i t y , d i s p e r s i o n and d i s t r i b u t i o n o f t h e supported phases i n MoP/A1,0, c a t a l y s t s i s here i n v e s t i g a t e d . The r e s u l t s i n d i c a t e t h a t t h e behav iour o f t h e molybdenum-phosphorus c a t a l y s t s depend s t r o n g l y on t h e impregnat ion sequence used (co impregnat ion o r doub le impregnat ion) . Coimpreg- n a t i o n procedure (Mo+P) a l l ows t h e o b t e n t i o n o f c a t a l y s t s w i t h l a r g e r Concent ra t ion o f a c i d s i t e s and su r face d i spe rs ion ! when compared t o consecut- i v e impregnat ion procedures (P->Mo, Mo->P). Th is sample a l s o .showed h i g h a c t i v i t y f o r t h e HDS, HDN and MHC reac t i ons . The d i f f e r e n c e s i n behav iour observed f o r t h e d i f f e r e n t samples c o u l d be ma in l y a t t r i b u t e d t o t h e ex i s tence o f d i f f e r e n t molybdenum d i s t r i b u t i o n phases, p robab ly Mo-P .he teropo l compounds a r e formed f o r t h e coimpregnated sample, . w h i l e t h e fo rma t ion o f b u l t MOO, species cou ld be f a v o r i z e d f o r t h e sample preimpregnated wJth phosphorus. I t was a l s o observed t h a t phosphorus produces t h e f o l l o w i n g e f f e c t s on alumina: i) i t improves t h e . s u r f a c e a c i d i t y , ii) i t inc rease t h e mechanical s t r e n g t h o f t h e ex t rudates , iii) i t decreases t h e su r face area o f t h e suppor t . We e x p l a i n our r e s u l t s on t h e b a s i s o f t h e mechanism proposed f o r t h e adso rp t i on o f molybdate. phosphate and phosphomolybdate compounds on alumina. INTRODUCTION Molybdenum supported alumina c a t a l y s t s promoted by meta ls o f t h e group V I I I ( N i , Co) a r e w ide ly used f o r hydroprocess ing heavy o i l s o r coa l de r i ved f u e l s . E f f o r t s have been made t o improve c a t a l y t i c a c t i v i t y by f i n d i n g appro- p r i a t e a d d i t i v e s (such as P, B, S i , T i , F , Ga, e tc . ) . , Phosphorus can be cons i - dered one o f t h e most e f f e c t i v e a d d i t i v e s o f t h e molybdenum supported alumina c a t a l y s t s . I n f a c t , i t appears as a component i n a number o f f o rmu la t i ons o f commercial h y d r o t r e a t i n g c a t a l y s t s . The p a t e n t l i t e r a t u r e c la ims t h a t i t s use p rov ides b e t t e r c a t a l y t i c performance, i n c r e a s i n g h y d r o d e s u l f u r i z a t i o n (HDS), hyd roden i t rogena t ion (HDN), hydrodemetal l i z a t i o n (HDM) , and improv ing t h e m i l d hydrocrack ing (MHC) a c t i v i t y ( r e f . 1-41. I t has been demonstrated t h a t phosphorus produces severa l e f f e c t s , such as: i) i t prov ides a more s t a b l e impregnat ing s o l u t i o n and thus b e t t e r d i s p e r s i o n o f t h e metal on t h e suppor t ( r e f . 1 , 4 ) , ii) i t i n h i b i t s t h e fo rma t ion o f Ni(Co)Al,O,-l ike species and enhances t h e f o r m a t i o n o f N i z t ' i o n s w h i c h a r e p r i m a r l y respons ib le f o r t h e fo rma t ion o f c a t a 1 . y t i c a l l y a c t i v e n i c k e l s u l f i d e o r NiMoS-l ike phase ( r e f . 3 ) , iii) i t a l t e r s t h e a c i d s t r e n g t h d i s t r i b u t i o n on alumina, t h e c o n c e n t r a t i o n o f medium a c i d s i t e s inc reases p r o g r e s s i v e l y w i t h i n c r e a s i n g phosphorus l e v e l 38 ( r e f . 5) and i v ) i t improves the thermal s t a b i l i t y o f gamma alumina w i t h respect t o s i n t e r i n g and and phase t r a n s i t i o n t o alpha alumina ( r e f . 5) . The benef ica l e f f e c t s o f phosphorus has s t imulated research on i t s i n f l uence on molybdena based ca ta l ys ts . However, most o f the above mentioned 1 i t e r a t u r e e s s e n t i a l l y focused on the i n f l uence o f phosphorus on the c a t a l y t i c p roper t i es o f the modi f ied system and i t s e f f e c t on the d ispers ion on the a c t i v e phase deposited on the alumina surface has no t y e t been invest igated. Also, very few works deal w i t h the i n f l uence o f the sequence o f phosphorus incorporat ion dur ing the preparat ion step o f the MoP/A1 ,O, ca ta l ys ts on the surface a c i d i t y , d ispers ion and d i s t r i b u t i o n o f the supported phases. A s the phosphate ions s t rong ly i n t e r a c t w i t h alumina, competing w i t h molybdate ions, a f a c t o r o f poss ib le importance i s the preparat ion procedure. Three d i f f e r e n t procedures can be fo l lowed . t o prepare a MoP/A1 ,03 c a t a l y s t . (1) P --> MO A1,0, -> (2) MO + P --> MOP/ A1,0, ( 3 ) Mo --> P This comun ica t i on i s p a r t o f a research program aimed a t a systematic i n v e s t i g a t i o n o f the preparat ion procedure o f MoP/A1,0, mild-hydrocracking ca ta l ys ts . Essen t ia l l y , we study the e f f e c t o f phosphorus i nco rpo ra t i on sequence on the s t a t e o f d ispers ion o f t he a c t i v e phase, surface a c i d i t y and physical proper t ies. For t h i s purpose, the samples were character ized using the fo l l ow ing physico-chemical techniques: BET surface area, mechanical strength, X-ray photoelect ron spectroscopy (XPS). scanning e lec t ron microscopy (SEM) , surface a c i d i t y determined by p y r i d i n e adsorption. EXPERIMENTAL Cata lyst preparat ion Three MoP/A1,0, ca ta l ys ts having constant molybdenum and phosphorus con- ten ts (10 w t % MOO, and 4.5 w t % P,O,) were prepared by e i t h e r consecutive (P->Mo o r Mo-->P) o r simultaneous (Mo+P) impregnation o f alumina extrudates (1/20 i nch length) w i t h aqueous so lut ions o f ammonium heptamolybdate and/or orthophosphoric acid. The alumina has a s p e c i f i c surface area o f 269 m2 /g and a pore volume of 0.69 cc/g. The impregnating so lut ions were adjusted t o pH = 1.5-2.0 by adding n i t r i c ac id before contact ing w i t h the support. The samples were d r i e d a t 373 K f o r 2 h and then ca lc ined i n two steps a t 623 K f o r 2 h and a t 773 K f o r 3 h. 39 Character izat ion Surface areas o f c a t a l y s t s were determined by the BET method from N, analyzer. An Erweka apparatus was used t o measure the s i z e crushing s t reng th o f c a t a l y s t extrudates. The method determines p a r t i c l e crushing s t reng th by measuring the fo rce i n k i lograms (k ) requ i red t o crush an ext rudate o f measur- ed s ize. A l a rge number o f extrudates (about f o r t y o f each sample) were t e s t - ed and the average value was establ ished. X-ray photoelect ron spectra (XPS) o f c a t a l y s t s were recorded us ing a Leybold Heraeus LHS-11 apparatus equipped w i t h a computer system, which a1 low- ed the determinat ion o f peak areas. The e x c i t a t i o n r a d i a t i o n was the Alka l i n e ( E = 1486 eV). A l l the samples were grounded and then pressed i n t o t h e sample h o l d e r s be fo re the analys is . Signals corresponding t o Cis. A l p p , A l Z s . M o ~ ~ , M o ~ ~ , P2p and P 2 s energy l e v e l s were recorded. The CIS energy l eve l (284.5 eV) was taken as a reference. Atomic surface concentrat ion o f supported elements was evaluated form the peak i n teg ra ted areas and the sensi- t i v i t y f ac to rs provided by the equipment manufacturer. across the t ransversa l sec t i on o f t he alumina extrudates, was obtained us ing the scanning e l e c t r o n microscopy technique (SEM). An ISI-60 apparatus equipped w i t h an energy d i spe rs i ve X-ray analyzer (Kevex 5-7000) was used f o r these measurements. Ca ta l ys t extrudates were mounted on an epoxy s l i d e and then pol ished before scanned under the e l e c t r o n beam. The a c i d i t y measurements were c a r r i e d ou t i n a Cahn 1000 e lect robalance us ing p y r i d i n e as probe molecule adsorbed on the c a t a l y s t surface. The i r r e v e r s i b l y adsorbed p y r i d i n e amounts were determined a t 273 K, 473 K and 573 K. Resul ts a re expressed as m o l o f p y r i d i n e i r r e v e r s i b l y adsorbed per surface area o f c a t a l y s t . The elemental p r o f i l e d i s t r i b u t i o n o f both molybdenum and phosphorus, RESULTS Surface areas and mechanical s t reng th S p e c i f i c surface areas corresponding t o MoP/A1 209 c a t a l y s t s prepared f o l l o w i n g d i f f e r e n t procedures are given i n Table 1. I t i s a l so repor ted i n the same t a b l e the values obtained f o r A1,0, support and f o r P/A1,0, and Mo/A1,0, samples. 40 TABLE 1 S p e c i f i c su r face areas and mechanical s t r e n g t h corresponding t o d i f f e r e n t p repara t i on procedures Sample Surface Area (m'/g) Crushing S t reng th ( kg /pes t l e ) A1 Z03 269 P / A 1 ,03 241 Mo/A1 ,O, 248 P->Mo 214 Mo->P 222 MotP 234 5.2 6.0 5.3 6.1 5.5 6.1 For bo th Mo/A1,0, and P/A1,0, samples, t he sur face area decreased i n approx imat ive ly , 8% and 11%, r e s p e c t i v e l y , a f t e r impregnat ion w i t h ammonium -heptamolybdate o r phosphor ic ac id , whereas f o r MoP/A1 ,03 c a t a l y s t s the l o s s i n sur face areas was more pronounced. One can observe t h a t t h e su r face area va lue ob ta ined f o r t h e sample prepared by coimpregnat ion i s s l i g h t l y h igher than t h a t observed f o r those samples prepared by consecut ive impregnat ion. A l l MoP/A1,0, c a t a l y s t s presented a pore volume i n the 0.52 - 0.55 cc /g range. Concerning t h e mechanical s t r e n g t h measurements, i t i s observed i n Table 1 t h a t phosphorus s l i g h t l y improves t h e mechanical s t reng th o f alumina ex t ru - dates, w h i l e molybdenum seems t o have no i n f l u e n c e on these p r o p e r t i e s . The p repara t i on procedures (P->Mo) and (Mo+P) produce c a t a l y s t s w i t h s imi 1 a r c rush ing s t r e n g t h values. X-ray pho toe lec t ron spectroscopy (XPS) The XPS r e s u l t s obta ined f o r P/A1 ,O,, Mo/A1 ,O, and MoP/A1 ,O, c a t a l y s t s , a re presented i n Table 2. The percentage o f sur face d i spe rs ion o f both Mo and P elements ( I e / I A l x 100) i s repo r ted as a f u n c t i o n o f c a t a l y s t p repara t i on procedure. When comparing the (Mo/A1) o r ( P / A l ) i n t e n s i t y r a t i o obta ined f o r Mo/A1,0, o r P/A1,0, s w p l e s w i t h t h a t o f MoP/A1,0, c a t a l y s t s , we can observe s t r i k i n g d i f f e r e n c e s i n t h e d i s p e r s i o n s t a t e o f t he supported species. Molyb- denum d i s p e r s i o n o f samples v a r i e s as f o l l o w s : whi le , i n t h e case o f t h e phosphorus d i spe rs ion , t he observed sequence i s as fo l l ows : Mo+P > P/A1,0, > Mo->P > P->Mo 41 MO MO -070 0 070 Radial Position (mm) Fig. 1. c a t a l y s t s prepared following d i f f e r e n t procedures. Electron Microprobe p r o f i l e of Mo and P corresponding t o MoP/A1,0, A t 373 K (weak acid s i t e s ) , the pyridine adsorbed amounts per surface area var ies as follows: Mo+P > P->Mo > Mo->P = A1,0, Whereas, f o r desorption temperatures of 473 K and 573 K (medium and strong acid s i t e s , respec t ive ly) , the observed sequence i s : 42 TABLE 2 XPS r e s u l t s cor respond ing t o t h e c a t a l y s t s prepared us ing d i f f e r e n t procedures. Sample 2P IMojp/ I A 1 I P 2 p / I A 1 2P _ _ P / A 1 *03 Mo/A1 ,O, 4.5 P->Mo 4 .1 Mo->P 4.4 Mo+P 4.8 2.7 _ _ 2.3 2.5 3.3 Scanning e l e c t r o n microscopy (SEM) F igu re 1 represents t h e p r o f i l e d i s t r i b u t i o n , across t h e t ransve rsa l sec- t i o n o f t h e alumina ex t rudates , b o t h f o r phosphorus and molybdenum elements, ob ta ined th rough SEM techn ique. For c a t a l y s t s p repared f o l l o w i n g t h e procedures (Mo+P) and (Mo->P), b o t h Mo and P elements a re d i s t r i b u t e d homogeneously i n t h e suppor t , whereas sample prepared accord ing t o procedure (P->Mo), c l e a r l y shows some he te rogene i t i es . S t reng th o f a c i d i t y Sur face a c i d i t y r e s u l t s , ob ta ined th rough p y r i d i n e adsorp t ion , correspon- d i n g t o A1,0, suppor t and d i f f e r e n t MoP/A1,0, c a t a l y s t s , a re p resented i n Table 3 as a f u n c t i o n o f t h e deso rp t i on temperature. D i f f e r e n c e s i n p y r i d i n e i r r e v e r s i b l y adsorbed amounts p e r su r face area u n i t can be observed i n t h e 373 -573 K temperature range, depending on t h e p r e p a r a t i o n method used. TABLE 3 I r r e v e r s i b l y a c i d i t y o f MoP/A1,0, prepared c a t a l y s t s ( m o l py r id ine lm ' ) x l o 3 Sample TEMPERATURE ( K ) 373 473 573 2'3 P->Mo Mo->P Mo+P 1.41 0.67 0.11 1.78 0.83 0.42 1.44 0.76 0.32 2.06 0.85 0.45 43 D I S C U S S I O N The above r e s u l t s show how a c i d i t y and su r face d i s p e r s i o n o f supported phases found i n MoP/A1,0, c a t a l y s t s can be s t r o n g l y i n f l u e n c e d by t h e prepara- t i o n procedure. Tables 2 and 3 i n d i c a t e t h a t co impregnat ion o f Mo and P a l l ows t h e o b t e n t i o n o f a c a t a l y s t w i t h l a r g e r concen t ra t i on o f a c i d s i t e s and su r face d i spe rs ion , i n comparison w i t h consecut ive impregnat ion procedures (P->Mo, Mo->P). Phosphorus seems t o improve t h e a c i d i t y s t r e n g t h as w e l l as t h e mechanical p r o p e r t i e s o f a lumina ex t ruda tes . However, t h i s a d d i t i v e s t r o n g l y a f f e c t s t h e s p e c i f i c su r face area o f t h e suppor t . L e t us d i v i d e t h e d i scuss ion o f ou r r e s u l t s i n two p a r t s : we s h a l l f i r s t d iscuss t h e r e s u l t s ob ta ined on t h e sample prepared by co impregnat ion (MotP) and subsequent ly, examine t h e behav iour o f those c a t a l y s t s p repared by conse- c u t i v e impregnat ions (P->Mo and Mo->P). P repara t i on Procedure (Mo+P) Table 2 shows t h a t co impregnat ion method improves t h e su r face d i s p e r s i o n o f b o t h molybdenum and phosphorus. Th is agrees w i t h recen t r e s u l t s ob ta ined by Atanasova e t a l . ( r e f . 6), who s tudy ing a s e r i e s o f NiMoP/Al,O, c a t a l y s t by XPS and c a t a l y t i c measurements, s t a t e d t h a t co impregnat ion method and a s u i t a b l e alumina c a r r i e r can l ead t o a b e t t e r d i s p e r s i o n o f a c t i v e components and consequent ly, t o an i nc rease i n HDS a c t i v i t y . We s h a l l a t tempt t o e x p l a i n now our r e s u l t s supported on fundamental s t u - d ies d e a l i n g w i t h adso rp t i on o f Mo and/or P on alumina. Le t us r e f e r t o the more impor tan t r e s u l t s o f these s tud ies . L i t e r a t u r e shows t h a t phosphorus inc reases t h e s o l u b i l i t y and s t a b i l i t y o f molybdenum s o l u t i o n s ( r e f . 1,4). Adsorp t ion s tud ies proposed t h a t when alumina i s coimpregnated w i t h s o l u t i o n s c o n t a i n i n g molybdate and phosphate, t h e r e i s a compe t i t i on between bo th i ons f o r t h e same adsorp t i on s i t e s (bas i c hydroxy l groups o f a lumina) , thus , t h e adsorbed phosphate i n h i b i t s t h e adso rp t i on o f rnolybdates (3 ,6-9) . However, most o f these s tud ies deal w i t h molybdate and phosphate adso rp t i on separa te l y w i t h o u t cons ide r ing t h e fo rma t ion o f phosphomolybdate compounds i n t h e impregnat ing s o l u t i o n . Recent ly , Cheng and Lu th ra ( r e f . 8 ) , u s i n g t h e NMR techn ique, s t u d i e d t h e adso rp t i on o f va r ious phosphomolybdate compounds on alumina spheres. Authors observed t h a t when phosphor ic a c i d i s added t o a s o l u t i o n c o n t a i n i n g amonium hep tam0 1 y b d a t e , p e t amol ybdodi phosphate compounds ( P , Mo 1 a re formed. For s o l u t i o n s c o n t a i n i n g P/Mo molar r a t i o h i g h e r than 0.4, amounts o f phosphorus remained i n fo rm o f phosphates. Th is suggests t h e ex i s tence o f a chemical e q u i l i b r i u m between bo th phosphate and molybdate i ons i n s o l u t i o n . 44 8 H * + 5 MOO:' 6 - + 2 HP0:- = P,Mo,O,, + 5H20 (2) Accord ing t o t h e above equat ion , decomposi t ion o f phosphomolybdate i n t o s imp le molybdate and phosphate cou ld be favoured by a r i s e o f s o l u t i o n pH, which would s h i f t t h e chemical e q u i l i b r i u m t o l e f t . , Indeed, t h i s behaviour was observed d u r i n g phosphomolybdate adso rp t i on on alumina. The inc rease i n pH o f t h e impregnat ing s o l u t i o n was a t t r i b u t e d t o water fo rma t ion d u r i n g i o n exchange r e a c t i o n . Lu th ra and Cheng ( r e f . 10) observed t h a t e q u i l i b r i u m between heptamolybdate Mo,O:i and molybdate MOO:- i ons i s a l s o a f f e c t e d by a r i s e i n pH. 6 - 2 - Mo,O,, + 4 H,O = 7 MOO, + 8 H (3) I n sho r t , research o f Cheng and Lu th ra c l e a r l y i l l u s t r a t e s t h a t t h e h i g h s o l u b i l i t y and s t a b i l i t y observed when phosphor ic a c i d i s added t o molybdenum i s ma in l y due t o t h e fo rma t ion o f 'phosphomolybdate compounds. These compounds a r e ve ry s e n s i t i v e t o changes i n t h e impregnat ing s o l u t i o n pH. The f a c t t h a t t h e r e a r e d i f f e r e n c e s i n a c i d s i t e s d i s t r i b u t i o n and su r face d i s p e r s i o n when u s i n g d i f f e r e n t procedures t o p repare a MoP/A1 ,O, c a t a l y s t , suggests t h a t n a t u r e and concen t ra t i on o f t h e o x i d i c supported phases p resen t i n these c a t a l y s t s a re d i f f e r e n t . Our hypothes is i s c o n s i s t e n t w i t h t h e r e s u l t s ob ta ined by o t h e r research- e r s ( r e f . 3, 6, 9, 111, who combining va r ious c h a r a c t e r i z a t i o n techniques, s t u d i e d t h e s t r u c t u r a l changes t h a t occur red when phosphorus i s used as an a d d i t i v e o f Mo/A1 ,O, c a t a l y s t s . Atanasova and Halachev ( r e f . ll), s tudy ing th rough I R spectroscopy t h e phases p resen t i n NiMoP/Al ,O, c a t a l y s t s , p repared by coimpregnat ion, observed bands cor respond ing t o A lPO, and t o a mixed A1-Mo and Ni-Mo-P he te ropo ly compounds. IR-bands cor respond ing t o b u l k MOO,, Al,(MoO,), and NiMoO, were n o t observed i n those samples. Authors observed t h a t h i g h phosphorus con ten t leads t o an i nc rease i n degree o f molybdenum p o l y m e r i z a t i o n and t o changes i n t h e r a t i o between t h e d i f f e r e n t types o f he te ropo ly compounds, Ni-Mo-P/Al-Mo r a t i o inc reases w i t h i n c r e a s i n g phosphorus load ings . Lopez Corder0 e t a l . ( r e f . 9) s t u d i e d by TPR and DRS t h e su r face d i s t r i b u t i o n o f molybdenum on two s e r i e s o f MoP/A1 ,O, c a t a l y s t s which were prepared u s i n g simul taneous (P+Mo) o r double impregnat ion (P->Mo) methods. I n c o n t r a s t w i t h t h e r e s u l t s ob ta ined by Atanasova and Halachev ( r e f . 111, t h e au thors observed t h e presence o f b u l k MOO, and a l s o smal l c l u s t e r s o f p o l y - molybdate m u l t i l a y e r s f o r bo th c a t a l y s t s e r i e s . The concen t ra t i on o f t h e l a t t e r specieswas more impor tan t f o r t h e preimpregnated samples. 45 Cons ider ing our r e s u l t s toge the r w i t h those found i n l i t e r a t u r e , two p o s s i b l e exp lana t ions can be proposed: The f i r s t exp lana t ion i s based on t h e mechanism proposed f o r molybdate o r phosphate adso rp t i on on alumina ( r e f . 121, because phosphomolybdate compounds were observed t o decompose i n t o these two species d u r i n g adsorp t ion . These s tud ies suggest t h a t phosphate as w e l l as molybdate i n t e r a c t f i r s t w i t h bas i c hydroxy l groups o f a lumina, genera t i ng a water molecule. Compet i t ion between phosphorus and molybdenum takes p lace . Phosphate i s adsorbed on alumina more r a p i d l y than molybdate i s adsorbed.However, t h e r a t e o f adsorp t - i o n o f bo th compounds depends on severa l f a c t o r s , such as: i) natu re and concen t ra t i on o f t h e i ons i n t h e impregnat ing s o l u t i o n , ii) pH o f s o l u t i o n , iii) type o f a lumina and i v ) adso rp t i on temperature. For each exchanged hydroxy l group by phosphor ic a c i d molecule, two new a c i d s i t e s a re c rea ted . Th is would e x p l a i n t h e inc reases i n a c i d s i t e s concen t ra t i on observed a f t e r phosphorus i n c o r p o r a t i o n i n t o alumina. Morales e t a l . ( r e f . 12) suggested t h a t when a l l bas i c hydroxy l groups a re t i t r a t e d , t h e a c i d hydroxy l groups beg in t o be t i t r a t e d and then a monolayer o f phosphate i s formed by f u r t h e r a d d i t i o n o f phosphor ic ac id . I n t e r a c t i o n s between ne ighbor ing adsorbed phos- phates cou ld occurs l ead ing t o t h e fo rma t ion o f po lymer ic phosphate cha ins . I n t h i s p a r t i c u l a r case, t h e au tho rs mentioned t h a t t h e number o f a c i d s i t e s remains almost cons tan t because t h e s u b s t i t u t i o n o f two a c i d hydroxy l groups o f a lumina would y i e l d two a c i d s i t e s assoc ia ted t o phosphorus. I n t h e compe t i t i ve system, we can propose t h a t bo th molybdate and phos- phate i ons cou ld be adsorbed i n ne ighbor ing s i t e s . Th is would imp ly t h a t adsorbed molybdates impede t h e po lymer i za t i on o f adsorbed phosphate and phosphates would produce the same e f f e c t on molybdates. The l a t t e r may e x p l a i n t h e h ighes t su r face d i s p e r s i o n o f bo th P and Mo observed f o r t h e co- impregnated sample. Since p o l y m e r i z a t i o n o f phosphorus as we1 1 as molybdenum was favoured by h i g h conten ts o f these elements, we cou ld suggest t h a t i n t e r - a c t i o n s between ne ighbor ing molybdate and phosphate adsorbed species may occur caus ing t h e fo rma t ion o f Mo-P he te ropo ly compounds a f t e r c a l c i n a t i o n . One may specu la te t h a t a c i d i t y produced f o r t h e l a t t e r compounds should be d i f f e r e n t f rom t h a t produced by phosphates on alumina. The second exp lana t ion would be t o cons ider t h a t phosphomolybdate remains s t a b l e d u r i n g impregnat ion . In t h i s case, t h e behav iour o f these compounds towards adso rp t i on would be d i f f e r e n t f r o m t h a t o f p h o s p h a t e a n d molybdate. One may specu la te t h a t Mo-P he te ropo ly compounds may be formed f rom t h e adsorbed phosphomolybdate a f t e r c a l c i n a t i o n . There fore , t h e h ighes t d i s p e r s i o n observ- ed f o r t h e coimpregnated sample may be a t t r i b u t e d t o t h e fo rma t ion o f Mo-P he teropo ly compounds, which were observed on coimpregnated samples i n a recen t study ( r e f . 11). Le t us now complement our d i scuss ion showing some impor tan t e f f e c t s 46 observed through scanning e l e c t r o n microscopy technique i n the analys is o f t he samples. Figure 1 shows t h a t both phosphorus and molybdenum are homogeneously d i s t r i b u t e d i n the coimpregnated sample. However, r e s u l t s obtained by Cheng and Luthra ( r e f . 8) showed an i n t e r e s t i n g chromatographic e f f e c t appearing du r ing adsorpt ion o f phosphomolybdate on alumina spheres, phosphorus was p r e f e r e n t i a l l y located a t the edge, wh i l e molybdenum was concentrated a t the center o f t he spheres. The l a t t e r observat ion makes ev ident the decomposition o f phosphomolybdate du r ing adsorpt ion and the compet i t ion o f both phosphate and molybdate ions f o r t he same adsorpt ion s i t e s . I n our op in ion, we a t t r i b u t e the d i f f e r e n t behaviour observed i n both s tud ies t o d i f f e rences i n preparat ion cond i t i ons o f the samples, as wel l as the type o f alumina employ- ed. Preparat ion Procedures (P->Mo) and (Mo->P) As shown i n Tables 1 and 3, phosphorus seems t o improve the mechanical p roper t i es as wel l as the a c i d i t y s t reng th o f alumina extrudates, However, t h i s a d d i t i v e s t rong ly a f f e c t s the spec i f i c surface area o f support. Our r e s u l t s are i n agreement w i t h those obtained by several i nves t i ga to rs ( r e f . 7,9,12). Lopez Cordero e t a l . ( r e f . 9) suggested t h a t l oss i n surface area o f alumina a f t e r phosphorus i nco rpo ra t i on i s probably due t o a co r ros i ve e f f e c t o f surface caused by phosphoric a c i d molecules o r t o a pore blockage by phosphate species. We have observed t h a t pores having an average diameter i n the 30-60 A' range were the most a f f e c t e d by phosphorus deposi t ion. An explanat ion o f these r e s u l t s could be the f a c t t h a t these pores could be s e l e c t i v e l y plugged by polymeric phosphate adsorbed species, However, we can no t d i sca rd a poss ib le co r ros i ve e f f e c t produced by the phosphoric ac id molecules on the alumina surface. An e l e c t r o n microscopy study o f t he P/A1,0, sample could reveal poss ib le morphological changes due t o phosphorus. Table 2 i nd i ca tes t h a t f o r t he sample prepared f o l l o w i n g procedure (P->Mo). t h e Mo d i s p e r s i o n ( I M o / I A 1 ) was a f f e c t e d by phosphorus i n c o r p a t i o n . The I P 2 p / I A 1 2 p i n t e n s i t y r a t i o a l so decreased w i t h respect t o the value obtained f o r P/A1,0, sample, a f t e r molybdenum deposi t ion. These r e s u l t s can be explained by the f a c t t h a t preimpregnat ion o f alumina w i t h phosphorus reduces the number o f s i t e s ava i l ab le f o r molybdate adsorpt ion. Therefore, changes i n d i spe rs ion and d i s t r i b u t i o n o f molybdenum species should be expected. I n t h i s case, one could suggest t h a t phosphorus promotes the formdt ion o f b u l k MOO, species, which i s i n agreement w i t h the r e s u l t s obta in- ed by Lopez Cordero e t a l . ( r e f . 9) . A p a r t o f t he l a t t e r species may be deposi ted on the AlPO, monolayer, which would exp la in the decreasing i n 3P 2P 47 phosphorus i n t e n s i t y r a t i o observed f o r t h i s sample. A d d i t i o n a l l y , t h e e l e c t r o n microscopy s tudy con f i rms t h e heterogeneous d i s t r i b u t i o n o f t h e molybdenum species when alumina i s preimpregnated w i t h phosphorus. A s i m i l a r s i t u a t i o n m igh t be expected when t h e c a r r i e r i s preimpregnated w i t h molybden- um. I n t h i s p a r t i c u l a r case, mechanical s t r e n g t h as w e l l as a c i d i t y p r o p e r t i e s shou ld n o t be improved by phosphorus. A c t i v i t y o f t h e (MotP) and (P->Mo) samples I n t h i s work we have eva lua ted t h e c a t a l y t i c p r o p e r t i e s o f t h e molybde- num-phosphorus c a t a l y s t s . Fo r t h i s p u r p o s e , we have p r e p a r e d t w o Ni-Mo-P/Al,O, samples f o l l o w i n g t h e i m p r e g n a t i o n sequences (Mo+P->Ni) and (P->Mo->Ni). These samples p resen t t h e same chemical compos i t ion (15 w t % MOO, , 7 .5 w t % P,O, and 5.0 w t % N i O ) . The c a t a l y t i c r e a c t i o n was c a r r i e d o u t i n a h i g h pressure f i x e d bed r e a c t o r u s i n g a vacuum g a s o i l under t y p i c a l m i l d - h y d r o c r a c k i n g c o n d i t i o n s (T = 653K, P = 5 MPa, LHSV = 0.65 l / h , H,/Hc = 600). We p r e s e n t i n Table 4 t h e a c t i v i t y r e s u l t s o f t h e NiMoP/Al,O, c a t a l y s t s . Both samples show comparable a c t i v i t y i n MHC. However, t h e HDS and HDN r e a c t i o n s a r e h ighe r f o r t h e coimpregnated sample. The same behav iour was observed by o t h e r researchers ( r e f . 13 and 14) . TABLE 4 A c t i v i t y o f t h e NiMoP/Al,O, samples SAMPLE (Mo+P-> N i l (P-> Mo-> N i ) % CONVERSION HDS HDN MHC 82 59 13 77 50 11 I n o rde r t o analyze t h e p o s s i b l e changes i n t h e molybdenum d i s t r i b u t i o n phases induced by phosphorus, we have c a r r i e d ou t I R measurements on bo th MoP/A1 ,O, samples b e f o r e n i c k e l i m p r e g n a t i o n . The r e s u l t s , which w i l l be pub l i shed elsewhere ( r e f . 15) , c o n f i r m t h e f a c t phosphorus mod i f y t h e molybdenum d i s t r i b u t i o n , depending on t h e p r e p a r a t i o n procedure fo l l owed . No evidences about t h e presence o f phosphomolybdate compoundswere observed f o r t h e coimpregnated sample. 48 To summarize. we may conclude t h a t The d i f f e r e n c e s i n a c t i v i t y observed f o r t he (MotP-, N i l and (P->Mo-> N i ) samples cou ld be associated t o changes induced by phosphorus on t h e su r face molybdenum d i s t r i b u t i o n phases. CONCLUSIONS I n the frame o f t he present work we conclude t h a t coimpregnat ion p r o c e d u r e i s t h e more a p p r o p r i a t e method o f p repar ing MoP/Al *03 c a t a l y s t s . Th is procedure a l l ows the o b t e n t i o n o f samples w i t h h ighe r d i spe rs ion , sur face a c i d i t y and c a t a l y t i c p r o p e r t i e s . Changes observed i n molybdenum d i s t r i b u t i o n phases caused by phosphorus depend on the impregnat ion sequence employed. Coimpregnation procedure would l ead t o t h e Mo-P heteropoly compound format ion, w h i l e preimpregnat ion w i t h phosphorus cou ld induce the b u l k MOO, format ion. I t was a l s o observed t h a t phosphorus produces the f o l l o w i n g e f f e c t s on alumina: i) i t improves the sur face a c i d i t y , ii) i t increases the mechanical s t reng th o f ext rudates, iii) i t decreases the sur face area o f support . REFERENCES L. Hi l fman, U.S. Patent 3.617.528 (1971). A. Morales, M.M. Ramirez de Agudelo, Appl. Cata l . 23 (1986) 23. K. G i s h t i , A. i a n n i b e l l o , S. Marengo, G. M o r e l l i , Appl. Cata l . 1 2 (1984) 381. G.A. Mickelson, U.S. Patent 3.749.663 (1973). S. S tan i s laus , A. Asi-Habi, C. Dolana, Appl. Catal . , 39 (1988) 239. P. Atanasova, T. Halachev, J. U c h y t i l , M. Kraus, Appl. Cata l . , 38 (1988), 235. D . Chadwick, D.W. Atch ison, R. B a d i l l a , L. Josefsson, " I n p repara t i on o f Ca ta l ys ts 111, (G. Poncelet , P. Grange, P. Jacobs eds). E l sev ie r , Amsterdan, p 323, 1983. W.C. Cheng, N.P. Luthra, J. Cata l . , 109, (1988), 163. R. Lopez Cordero. N. Esquive l , J . Lazaro, J.L.G. F i e r r o , A. Lopez Agudo., Appl . Catal . 48 (1989), 341. 10 N.P. Luthra, W.C., Cheng, J . Catal . , 107, (19871, 154. 11 P. Atanasova, T. Halachev, App. Catal ., 48, (19891, 295. 12 A. Morales, M.M. Ramirez de Agudelo, F. Hernandez, Appl. Catal . , 41 (1988), 13 J.L.G. F i e r r o , A. LBpez Agudo, N. Esquive l , R. L6pez Cordero, Appl. Cata l . 14 P. Atanasova, T. Halachev, J. U c h y t i l , M. Kraus, Appl . Ca ta l . 38 (1988) 235. 15 R . Prada S i l v y , Y . Romero, M. GonzBlez, t o be publ ished. 261. 48 (1989), 353. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 49 SYNTHESIS OF NON-STOICHIOMETRIC SPINEL-TYPE PHASES: A KEY TOOL FOR THE PREPARATION OF TAILORED CATALYSTS WITH SPECIFIC ACTIVITY 1 Michele Piemontese , Ferruccio Trifiro'l, Angelo Vaccaril, Elisabetta Foresti and Massimo Gazzano2 'Dipartimento di Chimica Industriale e dei Materiali, Viale del Risorgimento 4,40136 BOLOGNA Dipartimento di Chimica "G. Ciamician" and CSFM (CNR) , Via Selmi 2 , 40126 BOLOGNA (Italy). SUMMARY The preparation, stability and catalytic activity of non-stoichiometric spinel-type phases used in the synthesis of methanol were investigated as a function of the composition, heating temperature and atmosphere. It was shown that these phases formed mainly via amorphous chromates, especially for copper-rich catalysts. High activities in the synthesis of methanol were observed for zinc-rich samples (with a maximum for a catalyst in which 20% of the zinc ions were substituted by copper ions) and associated with the presence of a non-stoichiomemc spinel-type phase, stable also in the reaction conditions. On the other hand, the low activity of copper-rich catalysts was attributed to the instability of the spinel-type phase where much of the copper segregates into well crystallized metallic copper, with a further poisoning effect by zinc and cobalt. INTRODUCTION pa"). , The majority of catalytic devices used in the modem chemical industry (i.e., both usual heterogeneous catalysts and materials based on applications of the catalytic properties) are based on mixed oxides (1- 3). The synthesis of specific tailor-made mixed oxides able to perform complex functions is one of the most current topics in solid state chemistry (4). Non-stoichiometric Zn/Cr and Cu/Cr mixed oxides are one of the principal examples of these unusual solids. They have applications as both solid state gas sensors (5 ) and catalysts for hydrogenation reactions (of CO to methanol and/or methanol-higher alcohol mixtures, and of many organic molecules) (6-12). These systems have been widely investigated over the last few years, and results obtained show that their peculiar catalytic properties may be associated with the presence of non-stoichiometric phases (with a M2+/M3+ ratio higher than 0.5, M= metal), in which some of the zinc or copper ions are present in octahedral positions, i.e., with an unusual coordination. However, until now very few data have been reported regarding the changes in structure and reactivity as a function of the composition in ternary systems (for instance Cu/Zn/Cr). The aim of the research reported here was to study the changes in structure, stability and reactivity as a function of catalyst composition. In particular, attention was focused on the role of the Cu/Cu+Zn ratio and, for copper-rich catalysts, on the differences related to the partial substitution of copper by zinc or cobalt. In all samples the chromium content was 50% (as atomic ratio) in order to favour the formation of monophasic systems (13-16). EXPERIMENTAL The precursors with different atomic ratios were obtained by coprecipitation at pH= 8.0 k 0.1 50 Table 1. ComPosition of the catalyst investigated (as atom %) (Table 1). A solution of the nitrate salts of the elements was added to a continually stirred solution containing a Sample Cu:Zn:Co:Cr slight excess of NaHC03 at 333K. Subsequent filtration Cat A 0.0:50.0:0.0:50.0 was performed, followed by washing until the sodium 10~0~40~0~o~o~50~0 content was lower than 0.05% (as Na20). The precipitates Cat B were dried at 363K and heated at different temperatures Cat C 25.025.0:0.050.0 and in different atmospheres. Catalyst compositions were Cat 40â0:10.0:0*050â0 confirmed by atomic absorption using a Perkin Elmer Cat E 40.0:0.0:1~.0:50.0 mod 360 spectrophotometer. Cat F 50.0:0.0:0.0:50.0 The XRD analysis was carried out with a Philips PW1050/81 diffractometer controlled by a PW1710 unit, using Nickel-filtered CuKa radiation, h= 0.15418 nm (40kV, 40mA). The data were processed on a Olivetti M240 computer. The lattice constants were determined from diffractometric data by least squares refinements. The crystal sizes were determined by the Scherrer equation, using Warrenâs correction for instrumental line broadening. Possible contributions to the line broadening due to disorder effects and/or lattice strains were not taken into account. The quantitative analysis of oxide phases in the catalysts was carried out using the method suggested by Klug and Alexander (17). The cation distribution between tetrahedral and octahedral sites in the cubic spinel-type phase was evaluated as an extension of the Bertaut method (18,19) on the basis of the I400/I440 ratio. A C.Erba Sorptomatic 1826 apparatus with N2 adsorption was used to measure the surface area. IR spectra were recorded using the KBr disk technique and a Perkin Elmer 1700 Fourier-transform spectrometer. U.v.-visible diffuse reflectance @R) spectra were recorded using a Uvikon 860 spectrophotometer, equipped with an integrator sphere. The amounts of CuO and chromates were determined after extraction with NH4OH:NH4NO3 (1:l v/w) (20) at 61511x11 and 446nm, respectively, using a Uvikon 860 spectrophotometer; in the last case, the solutions were previously buffered at pH= 5.0 f 0.1 with concentrated CH3COOH. The catalytic tests were carried out in a copper-lined piston flow reactor, operating at 6.0MPa and 500-600K, using a GHSV= 16,000h-â and a H2:CO:C02= 65:32:3 (v/v). Before the catalytic tests, the catalysts were activated in-situ by hydrogen diluted in nitrogen; the hydrogen concentration and temperature were progressively increased during this pretreatment. Outlet gases were monitored on-line by gas-chromatography, while the liquid products were condensed under pressure in a cold trap at 253K during the time-on-stream (6h), then weighed and âanalyzed off- line by gas-chromatography . RESULTS AND DISCUSSION Nature of the precipitates The precipitates dried at 363K show XRD powder patterns typical of quasi-amorphous phases, identified as hydroxycarbonates on the basis of the IR spectra (21). Further information may be obtained on the basis of the values of empirical parameters A and B35, calculated from the DR spectra (22). All precipitates (Fig. 1) show similar values of the A parameter (related to the Cr-0 distance), while the B35 parameter (inversely proportional to the C3â- Cr3+ interaction) shows a minimum for the Cu/Cu+Zn= 0.5 ratio. Therefore, the same type of structure may be hypothesized for all precipitates, with an increase in surface crystallinity for Cat C. 51 I 18000 I I k 0 0.5 1 .o Cu/Cu+M(I I l Fig.l.Empirica1 parameters calculated from the DR spectra of precipitates dried at 363K: zinc (m.0); cobalt (0,0). that some amounts of residual Cr6' ions are present in these phases (8,15). Taking into account that copper and zinc may form mixed spinel-type phases (with cubic symmetry for high zinc contents) Nature of the samples heated at 653 K Figures 2a and b report the XRD powder patterns of the precipitates heated at 653K in air and in a reducing atmosphere (H2:N2= 10:90 v/v), respectively. Calcined samples (Fig. 2a) show the presence only of spinel-type phases, whose XRD patterns become more and more broad as the copper content increases. IR spectra confirm the presence, for all calcined samples, of spinel phases, and also show he presence of dichromate-type phases (25), the amounts of which increase with increasing copper content. In previous papers it was shown that non- stoichiometric Zn/Cr spinel-type phases formed by decomposition of amorphous chromates and (20,24), we may hypothesize the formation up to a ratio Cu/Cu+Zn= 0.5 of cubic non-stoichiometric spinel-type phases, containing both elements and characterized by an excess of bivalent ions. On the other hand, on the basis of the XRD spectra of Figure 2a, we cannot speculate about the number and/or nature of the phases present in the copper-rich catalysts. After the samples had been heated in an H m 2 atmosphere, the XRD powder patterns (Fig. 2b) again showed the presence only of spinel-type phases for Cu/Cu+Zn 20.5, while for the copper-rich samples the main phases present were Cu (Cat D and E) or CuO (Cat F). The lack of reoxidation for the metallic copper in Cat D and E, cannot be justified on the basis of differences of crystal size, but most probably can be attributed to the formation of copper-rich alloys at the surface of the particles. The presence of small amounts of zinc or cobalt does not modify the XRD powder pattern of the copper particles, but may strongly influence their physicochemical or catalytic properties (25-27). For all catalysts, the IR spectra show the presence, together with small amounts of residual carbonates, of the typical bands of spinels (even if not well resolved), except the Cu/Cr= 1.0 sample (Cat F) (Fig. 3) for which only I \ wavenumbers cn a broad peak at 554 cm-' is present in the low frequency Fig* 3. IR spectra Of Cat region, attributable to the overlapping of CuO and Cr203 absorptions (13,28). heated at 653K for 24h in air (a), N2 (b) and H m 2 (c). Cat E 60 50 40 30 221P D E l 60 50 40 30 2 W " , . , . I , , I I . 1 . I . I 60 50 40 30 60 50 40 30 221P 2+/" Fig. 2 . XRD powder patterns of the different catalysts heated at different temperatures and in different conditions. (a) 653K in air; (b) 653K in an H2/N2 (10:90 v/v) mixture; (c) 753K in air and (d) 853K in air. (& Tetragonal phase (ASTM 34-424); (A) ZnO (ASTM 5-664); (m) Cu (ASTM 4-836); (e) CUO (ASTM 5-661); (0) c0304 (ASTM 9-418); without symbol: cubic phase (ASTM 22-1 107 and/or 26-509) . 53 I I I I ~ ~~ zoo 0 H a N 2 M Zinc 1 AlrM 28°C 6 ~ ~ r M = C o b a l l j HUNZM=CobalI - 1 5 0 - ,,a - Table 2 Amounts of CuO (w/w %) (a) and of Cr(h2- (w/w %) (b ) extracted for the catalysts heated in different conditions and after catalytic tests of the synthesis of methanol. T, K Cat A Cat B Cat C Cat D Cat E Cat F a b a b a b a b a b a b 50 653K/air - 6.0 6.0 13.5 13.5 26.5 28.5 27.5 20.0 13.5 29.5 28.0 653K/N2 n.d. n.d. 5.5 2.5 n.d. n.d. 11.0 3.5 n.d. n.d. n.d. n.d. 653IVI32-N~ - 0.5 10.0 0.5 24.5 1.0 40.0 2.0 41.0 2.0 48.0 1.0 853K/air - 3.0 2.0 2.5 3.0 1.0 5.0 1.5 6.0 3.0 2.5 1.0 Afterreact. - 1.0 7.5 1.0 15.0 1.5 38.5 1.0 n.d. n.d. 47.5 1.5 cuomax* - 10.1 25.4 25.5 25.7 25.6 cuomax** - 10.1 25.4 40.8 41.2 51.1 - * calculated on the basis of a phase composition ZnCnO4 (or cOcn04) + CuCnO4 +CuO. **calculated on the basis of a phase composition CuO + ZnO (or COO) + Cr203. Further information on the effect of the heating conditions may be obtained from the values of surface area (Fig. 4) and the amounts of CuO and chromates extracted with NH40WNH4N03 solutions (Table 2). From Figure 4, it is possible to observe that the heating atmosphere has a small effect on the surface of the samples. In both cases, samples with large surface areas may be obtained, especially in the range 0s Cu/Cu+Zn 10.5. Furthermore, it is possible to observe that cobalt is a better physical promoter than zinc in both conditions. Table 2 shows that in the samples heated in a mixture of H m 2 practically all the Cu2+ ions present may be extracted. Therefore we may hypothesize that in these conditions copper gives rise to a separate phase, while the formation of spinel phases is due essentially only to zinc or cobalt, in 0 0.2 0 4 06 0 8 1 Cu / Cu + M(II) (atomic ratio) Fig.4. Surface area of the samples heated at 6S3K for 24h in air and H7h-7 . agreement with the IR data for Cat F (Fig. 3). However, it should be pointed out that copper containing phases were detected only for the copper-rich catalysts, while for Cu/Cu+Zn ratios 5 0.5 they escape XRD detection. Table 2 confirmes the presence in the calcined samples of increasing amounts of chromates, with a maximum for Cat F in which ca 43% of the total chromium is present as Cr6' ions. Up to a Cu/Cu+Zn ratios 0.5, the amount of CuO extracted is lower than both the theoretical value and the chromate content, and does not depend directly on the latter. This is particulary true for Cat B, taking into account also the values of the 3 samples heated at 653K in N2. Therefore, two Cu2+ containing fractions are present in these spinel-type phases, which show different .. ~ - - solubilities in the m40H/NH4N03 solution, but Table 3 Crystallographic data for the samples calcined at 753 and 853 K ~ - -~ -~~ - ~ ~-~~ _ _ ~ . . . - - ~~ Sample T , K Phase a, nm c,nm c.s.,nm cuoa C U O ~ ZnOa ZnOb X r XO xc CatA 753 853 CatB 753 853 CatC 753 853 CatD 753 853 CatE 753 853 CatF 753 853 CatF 753/N, 853/N, cubic + ZnO 0.8356(1) cubic + ZnO 0.8336(5) cubic 0.8342(2) cubic + CuO 0.8334(3) cubic + CuO 0.8344(5) cubic+ CuO 0.8325(1) cubic + CuO 0.8323(5) cubic + CuO 0.834(3) cubic + CuO 0.833(3) tetragonal + CuO 0.5969(4) i Co,04 cubic > tetragonal 0.832(4) + CUO tetragonal + CuO 0.6025(6) CuCr02 0.2977(6) CuCrOp 0.2975(2) - 7.5 13.0 7.5 12.0 6.5 69.0 7.0 10.0 7 .O 0.798(2) 28.0 4 25.8 14 25.5 - 10.1 - 15.5 2 10.1 - 15.5 3 25.4 - 18 25.4 - 15 25.5 - 25 25.5 - 12 25.7 - 27 25.7 - 0.246 0.246 0.242 0.266 0.249 0.253 0.243 0.242 0.23 1 n.d. 0.137 0.082 0.158 0.124 0.137 0.049 0.077 0.005 0.107 n.d. 7.5 0.780(3) 16.0 1.704(5) 15.0 1.7105(9) 22.0 13 25.6 - 22 25.6 - n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. ~~ 0.41 1 0.448 0.400 0.406 0.409 0.465 0.453 0.502 0.441 n.d. n.d. n.d. n.d. n.d. C.S. = crystal size of the spinel-type phase; (a) = amount detected; (b) = amount calculated on the basis of a phase composition ZnCr20, (or CoCr204) + CuCr,O, + CuO. xT, xc, xo = on the basis of structural formula (M2txT ) 'e'rohedro' (MZtxo, C?',c) 0 oclahedral 55 are both not detectable by XRD and IR analysis. On the other hand, the amount of CuO extracted in the copper-rich samples is mainly related to the presence of Cr6' ions (compare Cat D heated in air or N2). However, it must be noted that for all samples the amount of CuO overlaps that of chromates ( if both are expressed on an atom basis), indicating that also in these samples a consistent fraction of CuO ( more than 33% of the total CuO extracted) may be present as excess of Cu2+ ions inside a spinel-type phase. Unfortunately, no further support for this hypothesis may be obtained from XRD powder patterns. In recent papers (13,29) Bonnelle et al. have reported the formation, by heating at 643K in N2, of cubic non-stoichiometric spinel-type phases for copper chromite catalysts, with Cu/Cr ratios from 0.8 to 1.5. However it must be pointed out that these authors claimed as necessary for the stability of the spinel-type phases, the formation of consistent amounts of Cr6' ions (36% or more of the total chromium), during the controlled decomposition of the hydroxynitrate precursors. In our case, the Cu/Cr= 1.0 sample gives rise upon heating in N2, mainly to the formation of CuCro;! (Fig. 3), as c o n f i i e d also by XRD patterns, the amount of which increases with increasing temperature. This difference may be explained taking into account the different natures of the precursors; however, it must be pointed out that the amount of Cr6' ions present in Cat F calcined at 653K as well as its IR spectrum are similar to those reported in the literature (13,29). Therefore, for copper-rich compositions the formation of non-stoichiometric phases may be related mainly to a controlled oxidation of the precursor obtained by coprecipitation, while, on the contrary, this does not seem to be a key factor for the zinc-rich catalysts, in agreement with that previously reported for Zn/Cr catalysts (14). Furthermore, the data reported in this section, suggest that non-stoichiometric phases do not form by heating the precipitates in a reducing atmosphere. Thermal stabilitv of the samples. In order to test the thermal stability of the catalysts, the precipitates were calcined at 753 and 853K for 24h. The XRD powder patterns of these samples are shown in Fig. 2c and d, and the main Surface area (rn2/g) ,Ki= Zlnc 853K M= Zmc I5O t 1 0 0.2 0.4 06 0.8 1 Cu I Cu + M(II) (atomic ratio) XRD data are summarized in Table 3. A cubic spinel-type phase is the main component present in all samples calcined at 753K, also for the copper- rich catalysts, suggesting that in these samples it forms via the amorphous phases discussed in the previous section. Therefore a general mechanism of formation of the spinel-type phases by decomposition of chromate phases may be hypothesized. When the temperature is increased further to 853K3, the cubic phase is again detected, with the exception of Cat E and F, in which teuagonal CuCnO4 is present. With increasing calcination temperature, increased segregation of oxide phases takes place. However, it must be noted in regard mainly to the CuO, that ZnO is observed only for the binary Cat A. Fig. 5 . Surface area of the samples calcined Furthermore, CuO segregation is less marked for Cu/Cu+Zn I 0.5 ratios, while in the copper-rich at 753K and 853K. 56 catalysts the values detected at 853K approach the calculated ones. The high stability towards calcination temperature of Cat B is worth noting, in which only 20% of the zinc ions are substituted by copper ions. For this catalyst ZnO is never observed, whereas 20% CuO is detected only after calcination at the highest temperature. On the other hand, the partial substitution of copper ions with zinc ions stabilizes the cubic spinel-type phase, whereas for cobalt ions this effect is less marked probably because of their tendency to segregate as c0304. In the literature it is reported that the cubic CuCr204 phase is stable only at high temperatures (30). Furthermore, it is noteworthy that no formation of CuCrO:! was observed when Cat D was heated in N2 up to 853K. The increase in stability for the ternary catalysts also is reflected by the surface areas of the samples calcined at 753K (Fig. 5). After calcination at 853K all samples show a dramatic decrease in surface area and appreciable differences are no longer detected. The collapse of the catalyst structure is also responsible for the low values of CuO extracted (Table 2) , taking into account that a sample of CuO (E.Merck, Germany) calcined at 853K for 24h was fully soluble in the N H 4 0 W N 0 3 solution. As already mentioned the 14oO/I440 ratio may be assumed to be a measure of the distribution ratio between the occupancy of the tetrahedral and octahedral sites in the cubic cell of the spinel-type phase. According to Miller data of the octahedral site preference energies (31), it is assumed that the Cr3+ ions are all located in octahedral sites, whereas the Zn2+ and Cu2+ ions are present in both tetrahedral and octahedral sites, according to results previously reported for Zn/Cr catalysts (14-16). From Table 3, it is possible to observe that the tendency of the M2+ ions to be retained in the octahedral positions decreases with increasing calcination temperature and copper content, indicating that the CuO side phase detected may be mainly attributable to the segregation of Cu2+ ions present in octahedral positions of the spinel- type phase. The behaviour of Cat B is noteworthy; in this catalyst more M2+ ions tend to be retained in octahedral positions than is the case for ZdCr catalysts (16). Methanol productivity (g/h kg Cat.) Catalytic activitv in methanol svnthesis The catalytic activity in methanol synthesis of the catalysts investigated is reported in Figure 6 as a function of the Cu/Cu+Zn ratio (Cat E, containing cobalt, was practically inactive in the temperature range investigated). The progressive substitution of zinc ions with copper ions gives rise to considerable differences in the catalytic activity, as a function of the copper content. However, two general behaviours are found: 1) Up to a Cu/Cu+Zn ratio5 0.5, the presence of copper considerably increases the activity in methanol synthesis, with a very high selectivity in methanol (the main by-product being water). 2) For the highest ratio, a dramatic deactivation is observed, accompanied also by a change in selectivity, especially for Cat D, for which the formation of hydrocarbons (mainly methane) is also detected. On the other hand, Cat F shows the - 0 0.25 0.5 0.75 Cu / Cu + Zn (atomic ratio) Fig- 6. Catalytic activity as a function of the ' Cu/Cu+Zn ratio. 51 formation at T2 550K of dimethylether, with a corresponding decrease in the values of productivity of methanol. However, the value of productivity in methanol for Cat F at 540K is in very good agreement with that reported by Apai er al. (32). It must be pointed out that the main increase in catalytic activity takes place for Cat B, in which only 20% of zinc ions is substituted by copper ions. The methanol productivity of this catalyst is similar to the best values reported in the literature , if based on kg of catalyst (33- 34), but clearly better if calculated on the basis of kg of copper, thus indicating the formation of very active copper-containing centers. XRD powder patterns of the catalysts after reaction show the presence until a Cu/Cu+Zn ratios 0.5 only of cubic spinel-type phases, while for copper-rich samples the main phases identified are metallic copper (Cat E) and Cu20 (Cat F). Furthermore, from Table 2, it is possible to observe that for these last catalysts practically all the theoretical CuO is extracted by NH40WNH4N03, while for Cat B and C values very similar to those of the calcined samples are obtained. In a previous paper, it was shown that there is a rough correlation between the catalytic activity in methanol synthesis and the whole chemisorption activity towards CO (35). Therefore, the low catalytic activity of copper-rich catalysts may be attributed to the segregation of much of the copper as well crystallized metallic particles (36). The further decrease in activity for Cat D and, especially, Cat E is in agreement with the hypothesis of the formation of alloys at the surface of the copper particles (26,27), taking into account the poisoning effect of small amount of cobalt (37). On the other hand, the high activity of Cat C and, especially, Cat B may be correlated to the presence of a non-stoichiometric spinel-type phase, in which some copper and zinc can be found in octahedral positions or to an interaction between highly dispersed metallic copper formed in reducing conditions and the spinel-type phase. However, it must be pointed out that this copper fraction is so dispersed and stable that it is not detected even after the catalytic tests. The role of a not detectable fraction of copper has already been reported in the literature (20, 36). Furthermore, the activity of Cat B higher than that of Cat C (notwithstanding its slightly lower CO chemisorption capacity) suggests that an important role is played by the oxide matrix, very probably in the hydrogen activation step (38,39). CONCLUSION Non-stoichiometric spinel-type phases may be obtained mainly via amorphous chromates and their stability and reactivity are strongly influenced by the composition. Very stable spinel-type phases active in the synthesis of methanol may be obtained at low copper contents, while copper-rich catalysts show a considerable tendency for segregation of metallic copper with a considerable decrease in catalytic activity . However, these last catalysts, especially those containing cobalt or zinc, may be very active in the hydrogenation of 0x0-aldehydes (12). Furtehr work will be directed towards obtaining a better understanding of the nature of the active sites, taking into consideration that the reaction conditions adopted are quite different from those reported by other authors (10,ll). REFERENCES 1 J.J. Burton and R.L. Garten (Ed.s), Advanced Materials in Catalysis, Academic, N.Y., 1977. 2 O.T. Sorensen (Ed.), Non-Stoichiometric Oxides, Academic, N.Y., 1981. 3 H. Yanagida, Angew. Chem. (Engl. Ed.) 27 (1988) 1389-1392. 58 4 G. Centi, F. Trifiroâ and A. Vaccari, Chim. Ind. (Milan), 71 (1989) 57-62. 5 A. Jones, P. Mosely and B. Tofield, Chem. Brit. 8 (1987) 749-766. 6 E. Errani, F. Trifiroâ, A. Vaccari, M. Richter and G. Del Piero, Catal. Lett. 3 (1989) 65-72. 7 P. Courty, D. Durand, E. Freund and A. Sugier, J. Mol. Catal. 17 (1982) 241-254. 8 A. Riva, F. Trifiroâ, A. Vaccari, G. Busca, L. Mintchev, D. Sanfilippo and W. Manzatti, J. 9 G. Fornasari, S. Gusi, F. Trifiroâ and A. Vaccari, I&EC Res. 26 (1987) 1500-1505. 10 R. Bechara, G. 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Catal. 108 (1987) 491-494. 39 G. Busca, M.E. Pattuelli, F. Trifiroâ and A. Vaccari, in C. Morterra, A. Zecchina and G. Costa (Eds), Structure and Reactivity of Surfaces, Elsevier, Amsterdam, 1989,239-248. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 59 EFFECT OF PREPARATION VARIABLES ON CATALYTIC BEHAVIOUR OF COPPER/ZIRCONIA CATALYSTS FOR THE SYNTHESIS OF METHANOL FROM CARBON DIOXIDE R.A. KOEPPEL', A. BAIKER', Ch. SCHILD' and A. WOKAUN' 'Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, ETH - Zentrum, CH - 8092 Zurich (Switzerland) 2Physical Chemistry 11, University of Bayreuth, D-8580 Bayreuth (FRG) SUMMARY A series of copper-zirconia catalysts have been prepared by methods of sequential precipitation, coprecipitation and deposition precipitation. The influence of various pretreatments and of the copper/zirconla ratio on the structural and chemical properties of these samples are examined. High activity and selectivity of the catalysts is shown to be correlated to the presence of amorphous zirconia which is stabilized by copper ions. The results indicate that the structural and chemical properties of the support and particularly the interface copper/zirconia are most decisive in governing the catalytic properties of these methanol synthesis catalysts. INTRODUCTION The synthesis of methanol from syngas (CO/CO,/H,) using Cu/ZnO/Al,O, catalysts is a well established industrial process (ref. 1,2). More recently zirconia-supported copper catalysts were found to be active and selective for this reaction (refs. 3-9). Although methanol synthesis catalysts have been studied intensively for several years there is still much controversy about the nature of the active components and the reaction steps that take place on them. Many aspects of the reaction mechanism are still not fully understood and are the subject of an active debate. Several investigations (refs. 10,ll) have shown that over typical commercial catalysts practically all of the methanol is formed from CO, under industrial conditions and that support effects are minimal for these catalysts (ref. 12). Other workers reported a marked support effect for the synthesis of methanol over copper catalysts prepared by different methods (refs. 3,7,8,13) showing that the activity of supported copper catalysts depends strongly on both the choice of the support and the nature of the feedstock. The results suggest that more than one mechanism may lead to methanol. As the choice of the preparation method and also of the further thermal and chemical treatments control the behaviour of a catalytic system to a large extent, considerable differences are found in catalysts of the same nominal composition but prepared in different ways (ref. 6). The morphology of a catalytic system as well as the appearing crystallographic phases are determined by the method of preparation. 60 While the conventional methanol synthesis reaction from syngas has been studied intensively, little attention has been paid so far to the synthesis from CO, and H,. In the present work a series of Cu/ZrO, catalysts were prepared and tested for methanol synthesis from carbon dioxide and hydrogen. Special emphasis was devoted to the influence of the preparation variables on the structural, chemical and catalytic properties of the catalysts. EXPERIMENTAL Catalyst Preparation A first type of catalyst precursors was prepared by sequential precipitation at constant pH and temperature (Samples S). An aqueous solution of ZrO(CH,C00),-2H20 or ZrO(NO,), .2H20 (0.6 M) and an aqueous solution of sodium hydroxide/sodium formate (2 M each) were poured into two separate dropping funnels. The reagents were added dropwise with vigorous stirring into a Pyrex flask containing 250 ml deionised water at 363-368 K. The addition was adjusted to keep the pH constant at about 7. The precipitation was complete after 5 min and the precipitate was further aged for 15 min at the same temperature. An aqueous Cu(N0,),.3H20 or Cu(CH,COO),.H,O solution (1.5 M) was then added simultaneous- ly with the alkaline solution under the same conditions as described above. Finally the precipitate was aged for further 30 min in the mother-liquor at 368 K and then filtered using a G-4 glass filter. The residue was washed four times by redispersing it in 200 ml deionised water. After washing of the precipitate with 200 ml methanol the voluminous gel was dried I t 333 K in a vacuum drier (the vacuum was kept at 1.25.1U Pa by a small stream of air passing through the drier) for 15 h to yield a rigid solid. This material was crushed to a grain size of 50 - 150 pm using an agate pestle and mortar. Sample A was prepared in the same way except that zirconia was substituted by alumina. Pure zirconia was prepared analogously by precipitation of zirconyl nitrate. Sample C was made by coprecipitation instead of sequential precipitation. Sample H was prepared by the method of deposition precipitation using urea. A suitable amount of amorphous zirconia was suspended in deionised water. After the addition of copper(I1)nitrat and urea the temperature was brought to 363 K under constant stirring. The reaction was accompanied by a rise of the pH to a final value of 8. The final product was treated in the same way as the sequentially precipitated catalysts. Nitrate- and acetate-precursors of copper and zirconium, respectively, were used for preparations to avoid the presence of chloride species in the final catalysts. All chemicals used were of analytical grade. The dried precursors were studied in both the freshly prepared state and after calcination in air at appropriate temperatures. Catalyst characterization The catalysts were characterized by means of nitrogen adsorption, nitrous oxide titration, X-ray diffraction (XRD), thermal analysis (TG/DSC), temperature-programmed reduction 61 (TPR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Surface areas (SBm) were calculated using a value of 0.162 nm2 for the cross-sectional area of the nitrogen molecule (ref. 14). Pore size distributions were determined following the BJH- method (ref. 15) using the equation of Halsey (ref. 16). Nitrous oxide pulse experiments were carried out using the procedure to that reported by Evans et al (ref. 19). Samples were first reduced in a flux of 75 ml min-â 5% H,/Ar by heating at 5 K m i d from 373 K to 523 K. Subsequently the samples were hold at this temperature for 30 min and then exposed to a flux of 75 ml m i d pure hydrogen for 1 h at the same temperature. The hydrogen was purged with 50 ml m i d He at 523 K for 5 min. After cooling to 363 K under He, pulses of nitrous oxide (0.5 cm3) were injected. Copper metal surface areas were calculated assuming 1.46.1e9 copper atoms per mz (ref. 15). Back-titration of the oxidized copper surface was realized either by pulses of CO (0.5 cm3) at 423 K or by TPR- measurements starting at 263K. X-ray diffraction patterns were measured using a powder diffractometer (Philips PW 1700) and Cum-radiation. Mean crystallite sizes were estimated from the peak width at half maximum of the (111) reflection of Cu or CuO, respectively, using the Scherrer equation. The measured peak width was corrected for instrumental broadening using the function proposed by Warren (ref. 17). Thermogravimetric- (TG) and differential scanning calorimetric- (DSC) studies were carried out using a Mettler TA 2000C thermoanalyzer. Measurements were performed under air with a heating rate of 10 K min.â. The apparatus used for the TPR studies was described in an earlier report (ref. 18). TPR profiles were measured under the following conditions: heating rate 10 K mid , flow rate 75 ml m i d 5% H,/Ar. The conditions of the IR-measurements have been reported in detail elsewhere (ref. 20). Catalytic tests The apparatus used for the catalytic tests consisted of a continuous tubular fixed-bed reactor (8 mm id.) which was operated at 1.7 MPa. Details are reported in (ref. 9). The premixed gas contained 25 mol% CO, and 75 mol% H, and was fed from a high-pressure cylinder. The reaction flow rate was typically 90 ml mid (STP). All experiments were carried out in the temperature range 433 - 533 K using 1.0 g of catalyst (50 - 150 pm sieve fraction). The prereduction of the catalysts prior to the kinetic tests was performed according to the following procedure: heating to 473 K at a heating rate of 15 K mid in 1.25 vol% HJN, at a pressure of l@ Pa. The H,-concentration was then increased stepwise (30 min per step) in the sequence 2.5/5/10/20/50 to 100%. After replacement of H, with reactant gas the temperature was brought to 533K under a pressure of 1.7 MPa. RESULTS Influence of chemical comDosition Samples with more than 20 at% copper resulted in black precipitates which is indicative 62 for the formation of cupric oxide under hydrothermal conditions. DSC curves of the decomposition behaviour of the dried precipitates are shown in Fig. 1. The endotherm events between 323 and 423 K are due to the volatilization of physisorbed and crystalline water, respectively. The corresponding TG curves showed the same characteristics as found for a zirconia gel (ref. 21). Dehydration occurs primarily between 300 and 700 K with a final weight loss of about 10% of the initial sample weight. Pure zirconia (Fig. la) exhibits an exothermic peak at about 710 K, characteristic for zirconia prepared by wet-chemical routes. The so- called glow-exotherm is commonly associated with the transition of an X-ray amorphous zirconia phase into a crystalline modification of zirconia. The presence of copper shifts the exothermal crystallization peak to 823 K for sample Sl (10 at% Cu) and to 893 K for samples S3-S7 (30-70 at% Cu, Fig. 1). The exothermic peaks at 473 K and 550 K (Fig. lc) are associated with the presence of acetate in the precursor, as emerges from a comparison with the DSC curve of the pure nitrate precursor (Fig. If). Calcination at 623 Kin air for 3 h results in an acetate free catalyst (Fig. Id). The exothermic signals appearing at 480 and 613 K after exposing this catalyst to methanol synthesis v I 273 373 473 573 673 773 873 973 Temperamre (K) Fig. 1. DSC curves of samples a) ZrO, , 6) SI , c) SS, d) SS-6234 e) S5-623K after reaction, f) S5N. 3 373 473 573 673 773 873 Temprmfrrrc (IS) Fig. 2. Temper~tiire-progr~irt~r, icd rdiictiori profiles or samples o) S2, 1 ) ) S.7, C ) SS, (1) S5-623 R , e) S5-923/(, j) SSN. 63 conditions are accompanied by a small weight gain due to reoxidation of reduced copper species (Fig. le). TPR-profiles are shown in Fig. 2 for samples with various copper contents. Note that all copper of calcined samples is quantitatively reduced to cu" below 523 K in 5% H,/Ar. Increasing the calcination temperature results in a shift of the T,,-values of the reduction profiles to lower temperatures (Fig. 2d,e). The small peak at about 740 K occurring with all samples, except the one calcined at 923 K, was accompanied by a measurabIe exothermicity and is attributed to the crystallization of amorphous zirconia. Pure uncalcined zirconia support yielded also a small peak at 718 K, followed by two broad peaks at 793 and 913 K, while a calcined sample (723 K) did not show the crystallization-peak at 718 K. Uncalcined samples resulted in TPR profiles which depended on the nature of the precursor salts used for preparation. While pure nitrate precursors showed a single reduction peak at 500 K (Fig. 2f), samples prepared in the presence of acetate-ions resulted in TPR profiles consisting of three peaks at about 500,567 and 623 K (Fig. 2b,c). We conclude that the reduction peaks at 500 and 567 K are attributable to the reduction of CuO, whilst the peaks at 623 K are due to the reductive decomposition of acetate-modified zirconia. Calcination at 623 K led to the disappearence of the reduction peak at 623 K and to a shift of the peaks at 500 and 567 K to lower temperatures (Fig. 2d). Nitrogen adsorption/desorption isotherms were measured after drying/calcination as well as after methanol synthesis reaction. All the isotherms were of type IV (BDDT classification), indicating the presence of well developed mesoporous systems (ref. 14). The shape of the hysteresis loop changed from type H2 (IUPAC classification, ref. 14) for both samples dried at 393 K and calcined at 623 K, to type H1 for samples dried at 923 K. Note that all samples were degassed at 523 K. Mesopore size distributions calculated from the adsorption and desorption branch of the isotherms showed the presence of pore size maxima in the range 3 - 5 nm for all samples except for those calcined at 923 K which exhibited a shift of the maxima to 8 - 9 nm. The mesopore volume decreased simultaneously from about 0.20 cm'/g to 0.12 cm3/g. Calculated t-plots (ref. 22) indicated the presence of microporosity for uncalcined samples before and after methanol synthesis reaction. Surface areas for all samples following methanol synthesis are listed in Table 1. Influence of calcination temnerature The X-ray patterns of the uncalcined zirconia support (Fig. 3a) shows only two broad bands in the range of 20" to 40" and 40" to 75" for 26, indicating the presence of zirconia with very low degree of crystallinity. Calcination of the sample at 723 K for 3 h resulted in metastable, probably tetragonal ZrO, and some stable monoclinic ZrO,. Note that a distinction between tetragonal and cubic zirconia is not possible based on XRD data alone. However, based on thermodynamical arguments tetragonal ZrO, is more likely. Higher calcination temperatures lead to an increase of the fraction of the monoclinic phase (Fig. 3b shows a sample calcined at 770 K for 3 h in air) until an almost pure monoclinic phase is obtained after calcination at 970 K. 64 TABLEI Textural properties of the catalysts - Catalyst Precursor@â Composition T,, S B E T S C , dcu (nm) Cu/Zr (at%) (K) (mYguT) @â/SCAT) from N,O from XRD SI s2 s3 s4 s5 s7 S5-623 S5-923 S5N c5 H H-823 A fb) ac/ac ni/ni ac/ac ni/ni ni/ac ni/ni ac/ni 10/90 20/80 30/70 40/60 50/50 70/30 50/50 18/82 50/50 623 623 623 623 923 623 823 169 1.70 181 2.25 183 3.80 174 4.80 174 7.10 57 8.60 138 6.90 44 2.50 5.70 141 6.30 161 3.00 80 3.20 177 7.85 21.0 33.0 30.5 37.5 29.7 37.5 30.6 84.5 37.1 33.5 25.0 23.3 41.7 30.0 31.0 16.5 25.0 40.0 39.0 (a) ac = acetate, ni = nitrate. (b) zirconia was substituted by alumina Sample H, calcined at 823 K for 3 h, yielded XRD-patterns characteristic of crystalline CuO and tetragonal ZrO,. No indication for the presence of the monoclinic phase of zirconia could be found. The XRD-patterns of the dried sample S5 are shown in Fig. 3c and are indicative of copper oxide and amorphous zirconia. The CuO particles had a mean size of about 15 nm as estimated from the line broadening of the CuO (111) reflection. Calcination at 970 K for 3 h resulted in well crystallized tetragonal zirconia and in CuO-particles of about 29 nm mean diameter (Fig. 3d). No reflections of the monoclinic phase were found with this sample. The XRD patterns of a sample after use for methanol synthesis are shown in Fig. 3e. The occurence of reflections due to crystalline Cu,O beside of the reflections of crystalline copper particles can be explained by the exposure of the sample to air during its transfer from the reactor to the XRD-measurement. Zirconia existed as amorphous phase. Mean copper particle sizes calculated from the line broadening of the Cu (111) reflections and copper surface areas measured by N,O-titration are listed in Table 1. By using a half-sphere model the average copper particle sizes were calculated from the copper surface areas. Back titration using CO-pulses at 423 K yielded values for copper surface areas identical to those measured by N,O-pulses. No CO, could be detected in the effluent gas stream indicating that none of the adsorbed carbon monoxide was removed as carbon dioxide at 423 K in the case of zirconia containing catalysts. This contrasts the behaviour of alumina or silica supported copper catalysts where all CO evolved as CO,. Back titration using TPR resulted in reduction peaks between 333 and 373 K for all samples. A characteristic feature of all catalysts was the appearence of a broad, intense desorption peak in the temperature range 513 to 573 K. 65 Catalvtic behaviour Preliminary experiments with respect to possible influences caused by interparticle and intraparticle mass transfer limitations confirmed that such limitations could be ruled out under the conditions used in this study. The results of the CO, hydrogenation experiments over the different catalysts are summarized in Table 2 and reflect the steady-state behaviour of the catalysts at 493 K after 15 h on stream. Carbon containing products were only methanol and carbon monoxide for all catalysts. Acetate precursors yielded some ethanol in the initial stage of reaction at 533 K, probably due to the hydrogenolysis of acetate species in the acetate- modified oxides. No ethanol could be detected after calcination of these samples at 623 K. The catalytic behaviour of some catalysts was compared on the basis of uncalcined samples. Note that the decomposition of the precursor under reducing conditions resulted in improved catalytic properties compared to the oxidative decomposition (Table 2). The activities of the catalysts were also compared on the basis of turnover frequencies (TOF) which were calculated either as molecules of methanol formed or as molecules CO, reacted per copper surface atom and per second. h Ih I . 66 Prevalent surface species Diffuse reflectance FI IR spectroscopy was used to investigate the species prevailing on the catalyst surface under CO, hydrogenation conditions at 2.5 bar. Before the FTIR measure- ments, the catalysts were heated in a hydrogen flow to the desired reaction temperature. The measurements were started by switching from the hydrogen flow to a defined flow of CO, and H,. Subsequently the surface reactions were studied by recording time-resolved FI'IR spectra as shown in Fig. 4. The spectra indicate that surface formate is readily formed after the reactant feed has been switched on. After 10 minutes peaks grow at 1150, 1050 and around 2900 cm-' reflecting the formation of surface-bound formaldehyde, methylate and methanol species. The peaks due to methanol are enhanced if the flow is stopped and the surface is exposed to static conditions. Methanol formation is paralled by the observation of gas phase CO originating from the reverse water gas shift reaction. It is noteworthy to mention that in all FTIR measurements performed with either CO,/H, or CO/H, no conclusive correlation between the disappearance of the prevalent formate species and the formation of methanol could be observed. On the other hand, the appearance of gas phase methanol was always associated with the observation of some CO, as well as formaldehyde and methylate surface species (ref. 20). DISCUSSION An importand aspect of the work concerns the nature of the zirconia phase produced and its influence on the catalytic behaviour. The occurence of ethanol species in the initial stage qf rraction under methanol synthesis conditions as well as the DSC and TPR data indicate the formation of anion modified oxides from acetate precursors as proposed earlier by Yurieva (ref. 23) for other oxides. We conclude from measurements with sequentially precipitated catalysts (ex zirconyl acetate/copper nitrate) and from the occurence of cupric oxide in the preparation step that oxygen ions in the zirconia are partially substituted by acetate ions. It is interesting to note that calcination of copper containing catalysts results in the formation of metastable tetragonal zirconia, while calcination of pure zirconia leads to the formation of stable monoclinic ZrO,. Although, an oxidative thermal treatment (DSC under air) shows a marked influence on the crystallization temperature of the amorphous zirconia, under reducing conditions (TF'R) the same crystallization temperature is observed for both zirconia as well as copper containing samples. These results confirm that probably copper- ions are responsible for the stabilizing effect onto amorphous zirconia and that the transformation of these ions into metallic copper eliminates this effect. The activity measurements (TOF values) summarized in Table 2 indicate that the intrinsic activity of the catalysts decreases with increasing copper content. An increase in CO, conversion is accompanied by a concomitant decrease in methanol selectivity. Calcination of samples S5 and H at 923 and 823 K, respectively, resulted in the transformation of the initially amorphous zirconia to crystalline tetragonal zirconia. The most striking effect of the crystallization process was the collaps of the activity and selectivity of the catalysts which was accompanied by a simultaneous decrease in the BET-surface area. It is interesting to note that 67 TABLE 2 Catalytic properties of the catalysts Catalyst Precursor" Composition T,,, Conversion Selectivity TOF,,,, TOF,,, Cu/Zr (at%) (K) (%) to methanol (d) (s-') (%) ac/ac 10/90 623 3.8 73 Z24.103 9.17-10' 20/80 623 4.1 73 6.18 Z 48 S l 30/70 5.9 70 5.14 6.37 s2 40/60 5.9 70 3.89 5.04 s3 50/50 6.6 66 2.89 3.81 s4 s5 s7 ni/ni 70/30 623 8.1 61 2.52 3.86 S5-623 ac/ac 50/50 623 5.2 67 2.33 3.09 923 3.6 51 3.35 5.91 5.9 66 3.22 4.25 ni'ni C5 ni/ac 6.0 67 2.87 3.91 S5N ni/ni 18/82 623 4.6 69 4.81 6.29 823 2.8 49 1.90 3.59 H ff-823 S5-923 A (b) ac/ni 50/50 5.3 37 1.13 2.77 (a) ac = acetate, ni = nitrate. (b) zirconia was substituted by alumina. in the case of sample H the copper surface area remains constant, however the intrinsic activity decreases. As sample A (Cu/Al,O,) had about the same copper surface area as the zirconia based catalysts, we may conclude that the copper surface area alone cannot explain the catalytic behaviour of the catalysts. High activity and selectivity are related to the presence of amorphous zirconia. Owen et a1 (ref. 24) proposed that oxygen anion vacancies characteristic of the fluorit structure of zirconia could be important in the methanol synthesis reaction. The presence of anions like acetate, formate or nitrate in the precursors could result in a distortion of the oxide structure of zirconia and their removal may generate vacancies in the anion lattice which are known to stabilize the cubic structure of zirconia. The crystallization of the amorphous zirconia is likely to result in a drastic decrease of the copper/zirconia interfacial area which certainIy contributes to the loss of activity observed upon crystallization. Our investigations provide further support for the crucial role of the interfacial area in copper/zircania catalysts. Further work focusing on the structural and chemical properties of this interphase and its role in methanol synthesis is presently undertaken . As to the surface species observed by in situ FTIR measurements during methanol synthesis, the most striking result is that the surface concentration of the prevalent formate species seems not to be directly influenced by methanol formation. This behaviour will be discussed in detail elsewhere (ref. 20). CONCLUSIONS The present studies confirm that highly active and selective Cu/ZrO, catalysts can be 68 prepared by precipitation. The catalytic behaviour of these catalysts depends strongly on the structural and chemical properties of the zirconia support. High activity and selectivity are related to the presence of amorphous zirconia which is stabilized by copper ions. Crystalliza- tion of the amorphous zirconia by calcination in air at appropriate temperatures results in the formation of metastable tetragonal zirconia and leads to a drastic decrease of the catalytic activity and selectivity. The results indicate that the structural and chemical properties of the support and particularly of its interface with the copper-species play an important role in methanol synthesis from CO,/H,. On all catalysts surface formate was found as an abundant surface species. However, the appearance of methanol is not correlated with the disap- pearance of formate, but with a decrease in surface formaldehyde and methylate signals. ACKNOWLEDGEMENTS This work has been supported by grants of the "Swiss National Science Foundation" (2.102- 086), the "Bundesamt f i r Bildung und Wissenschaft" and the "Deutsche Forschungsgemein- schaft" (SFB 213). REFERENCES 1 J.C.J. Bart and R P A . Sneeden, Catal. Today, 2 (1987) 1-124. 2 G.C. Chinchen, P.J. Denny, J.R Jennings, M.S. Spencer and KC. Waugh, Appl. Catal., 3 B. Denfie and R P A . Sneeden, Appl. Catal., 28 (1986) 235-239. 4 Y. Amenomja, AppL Catal., 30 (1987) 57-68. 5 B. Pommier and S.J. Teichner, in M.J. Philips and M. Ternan (Eds.), Proc. 9th Int. Congr. Catal., Calgary, 1988, The Chemical Institute of Canada, Ottawa, 1988, Vo1.2, pp. 6 Y. Amenomiya, I. T. Emesh, K W Oliver and G. Pleizier, in M.J. Philips and M. Ternan (Eds.), Proc. 9th Int. Congr. Catal., Calgq , 1988, The Chemical Institute of Canada, Ottawa, 1988, Vo1.2, pp. 634-641. 7 G.J.J. Bartley and R Burch, Appl. Catal., 43 (1988) 141-153. 8 B. Denise, 0. Cheriji, M.M. Bettahar and R.P.A. Sneeden, Appl. Catal., 48 (1989) 365- 372. 9 D. Gasser and A. Baiker, Appl. Catal., 48 (1989) 279-294. 10 A.Ya Rozovskii, YaB. Kagan, G.I. Lin, E.K Slivinskii, S.M. Loktev, L.G. Liberov and A.N. Bashkirov, Kinet. Catal., 17 (1976) 1132-1138 (engl). I1 G.C. Chinchen, P.J. Denny, D.G. Parker, G.D. Short, D.A. whan, M.S. Spencer and XC. Waugh, Am. Chem. SOC. Div. Fuel Chem., 29 (1984) 178-188. 12 G.C. Chinchen, KC. Waugh and D.A. Whan, Appl. Catal., 25 (1986) 101-107. 13 L R Jennings and M.S. Spencer, in C. Morterra, A. Zecchina and G. Costa (Eds.), Structure and Reactivity of Surfaces, Elsevier, Amsterdam, 1989, pp. 515-524. 14 KS.W Sing, D.H. Everett, RA.W Haul, L. Moscou, RA. Pierotti, J. Rouquh-ol and T. Siemieniewska, Pure & Appl. Chem., 57 (1985) 603-619. 15 E.P. Barrett, L.S. Joyner and P.P. Halenda, J. Am. Chem. SOC., 73 (1951) 373-380. 16 G. Halsq, J. Chem. Phys., 16 (1948) 931-93% 17 B.E. Wmen, J. AppL Phys., 12 (1941) B75. 18 D. Monti and A. Baiker, J. Catal., 83 (1983) 323-335. 19 J.K Evans, M.S. Wainwright, A.J. Bridgewater and D.J. Young, Appl. Catal., 7 (1983) 75- 20 Ch. Schild, A. Wokaun and A. Baiker, submitted for publication. 21 P.D.L. Mercera, J.G. Van Ommen, E.B.M. Doesburg, A.J. Butggraaf and J.RH. Ross, Appl. Catal., 57 (1990) 127-148. 22 B. K Lippens and J.H. de Boer, J. CataL, 4 (1965) 319-323. 23 T.M. Yurieva, React. Kinet. Catal. Lett., 29 (1985) 49-54. 24 G. Owen, C.M. Hawks and D. Lloyd, AppL Catal., 58 (1990) 69-81. 36 (1988) 1-65. 61 0-61 7. 83. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation ofCata2ysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands 69 PREPARATION OF Ti0,-Al,O, BY IMPREGNATION WITH TiC1,-CCl, Liu Yingjun, Zhang Qinpei, Zhu Yongfa, Gui Linlin and Tang Youqi Institute of Physical Chemistry, Department of Chemistry Peking University, Beijing 100871 (China) ABSTRACT Catalyst carrier Ti0,-AI,O, was prepared through impregnation y-Al,O, with non-aqueous solution of TiCI, (e.g. carbon tetrachloride or acetone as solvent), followed by calcination at 550'c for 24 h. Series of Ti0,-AI,O, samples with various TiO, loadings have been characterized by XRD, XPS, TEM and HEED techniques. The maximum dispersion capacities for TiO, on y-Al,O, measured by XRD and XPS are 0.12g TiO,/ g y-AI,O, and 0.1 lg TiO, / g y-Al2O3, respectively. It was also verified by TEM and HEED techniques. This value illustrates that TiO, disperses on the surface of y-Al,O, as a submonolayer and the observed monolayer coverage for TiO, on y-Al,O, is 58% as com- pared with a close-packed monolayer model. INTRODUCTION With its special properties, TiO, attracts more attention recently (refs. 1-2). Especially, to be used for hydrodesulfurization (HDS) or hydrodenitrogenation (HDN) in the petrol- eum refining process (refs. 3-4), the character of the catalyst with TiO, carrier is superior to that with y-Al,O, carrier. However, TiO, is seldom used as a catalyst carrier in commercial process. The reason is that as compared with the widely used industrial catalyst carrier y-AI,O, it has two disadvantages, one is that its specific surface area is rather small, usually of 10 mz / g and can only reach some tens mz / g prepared in special ways, the other is its strength rather poor with its mechanical strength five times less than that of y-Al,O,. In order to improve the properties of TiO, as a carrier many authors have tried several pre- paring methods for modifying y-Al,O, with TiO,, such as coprecipitation (ref.5), impregnation with aqueous solution (ref.6), mixed gelatinization (ref.7) and kneading process (ref.8). This research prepared monolayer dispersion type Ti0,-AI,O, carrier with y-Al,O, as a matrix impregnated with non-aqueous solution of TiCI, (e.g. carbon tetrachloride or acetone as solvent) and characterized it with XRD, XPS, TEM and HEED techniques. 70 EXPERIMENTAL Sample preparation y-Al,O, supplied by Changling Oil Refinery of Chinese Petro-Chemical Corporation, was ground down into small particles. The 40-80 mesh of y-Al,O, particles was calcinated in a muffle furnace at 650C for 4h. After calcination the specific surface area of y-Al,O, is 205m2/ g. The samples with various titanium oxide content of Ti0,-AI,O, carrier were prepared by the following procedures: impregnation of y-Al,O, with TiCI,-CCI, (or TiCI,-CH,COCH,) solution; giving off the solvent; neutralization with 1 : 1 ammonia liquor; washing with dilute ammonia solution until no CI- to be detected, then calcination at 550'c for 24 h. X-ray diffraction The XRD analysis, involving qualitative and quantitative analysis, have been carried out on a home made BD-86 diffractometer, with Cu Ku radiation, Ni filter and a scintillation couter. The residual crystalline TiO, after being dispersed on y-Al,O, can be measured by the matrix-flushing method (refs. 9-10) for quantitative X-ray diffraction analysis and calculated from the equation ZI / I , = k x , / x , (1) where I, is the X-ray intensity of the peak (101) for TiO,; I, is the X-ray intensity of the peak (200) for the flushing agent, KCl; x l and x , are the weight fractions of TiO, to the flushing agent, KCI, respectively; the constant k is the reference intensities of TiO, to KCl. In this case, the value of k is 0.99, calculated from the Equation(1). Specific surface area Specific surface areas of y-Al,O, and Ti0,-AI,O, were determined with thermal desorption gas chromatography method. XPS measurements All XPS spectra were acquired by using a VG ESCA LAB 5 Spectrometer equipped with a 4025 Data System. An aluminum anode (A1 Ku 1486.6eV) operated at 12 kV and 40 mA was used. During recording spectrum, pressure inside the sample chamber was ca. 5 x 10-*m bar. Intensities of Ti2p and A12p photoelectron lines were measured by the peak area with 4025 Data System. Sample was well spread on one side of double-sided adhesive tape and fixed on the sample stub. The extent of carbon contamination on samples controlled to be nearly the same by monitoring the CIS peak area, and then the influence of carbon contami- nation can be neglected. 71 u 5 -m 0 . 4 0 4 0 A N m i Crd Z L 9 0.30 o'\ 4 - d 2 N ; 2 0 . 2 0 m a 0.10 0.00 0.00 0.10 0.20 0.30 0 . 4 0 0.50 T i 0 loading ( g / g Y - A 1 2 0 3 ) 2 Fig.1 Dispersion threshold of TiOz on y-Al20, by XRD quantitative extrapolation method (prepared with TiC1,- CCI, solution) N 3 d Y \ 3 0.20- .3 c li 0 . 0 0 0 . 2 0 0 . 4 0 0 . 6 0 T i O Z loading (g/g f - A 1 2 0 3 ) Fig.2 Relationship between ITizp / IAnP and TiO, loading of Ti0,-AI,O, (prepared with TiC1,- CCI, solution) 72 TEM and HEED measurements Transmission Electron Microscopy (TEM) image and High Energy Electron Diffraction (HEED) pattern were carried out with JEM-200CX Electron Microscope oper- ated at 200 kV. RESULTS AND DISCUSSION XRD quantitative measuremental results The intensities of TiO, (101) peak of series of Ti0,-AI,O, samples were measured by XRD quantitatively. With residual amount of TiO, crystallite in samples after calcination as ordinate and with loading of TiO, as abscissa, plot has been got in Fig.1. The plot is a broken line consisting of two straight lines and shows a threshold value. Before the thresh- old TiO, disperses on the y-Al,O, surface as a monolayer and no TiO, peaks were ob- served. After the threshold, the residual amount of TiO, is in crystalline form on y-AI,O, surface. There is a linear relationship between the residual amount of TiO, crystallite and the added loading of TiO, on y-Al,O,. As impregnation y-Al,O, with TiCI,-CCI, solution, from Fig. I , the maximum dispersion has been obtained as 0.12g TiO, / g y-Al,O,. In the case of preparing through impregnation y-Al,O, with TiCI,-CH,COCH, solution, the maximum dispersion is 0.1 Ig TiO, / g y-Al,O,. The specif- ic surface of y-Al,O, used is 205m2 / g. In terms of surface area, the dispersion threshold for TiO, on y-Al,O, is 5.6 x 104g TiO, / m2. We can estimate the utmost monolayer capacity by employing the simple close packed monolayer model. Assuming that 02-ions from TiO, form a close-packed layer on the sur- face of y-Al,O, and the Ti4+ ions occupy the interstices formed by 02-ions. Taking 1.40A as the radius of 02-ion, the surface area occupied by a TiOz "molecule" on the surface of y-A1203 can be calculated as 2 . 0 4 ~ 10-'9m2, in other words, forming each m2 close-packed monolayer of TiO, requires 9.7 x 104g TiO,, i.e. the monolayer capacity of TiO, on y-Al,O, should be 9.7 x 104g TiO,/ m2. Comparing the observed dispersion threshold for TiO, on y-Al,O,as mentioned above, thus the observed monolayer coverage for TiO, on y-Al,O, is 58%. This value illustrates that TiO, disperses on the surface of y-AI,O, as a part covered monolayer or a submonolayer, rather than as a full covered monolayer. XPS measuremental results The intensities ratio ITi2, / I,,,, of XPS for series of TiO, / y-Al,O, have been deter- mined (refs. 11-12). The relationship between ITi2, / I,,,, and TiO, loading is shown in Fig.2. The plot consists of two straight lines with different slope, and the amount of TiO, at intersection point happened to correspond with the value of monolayer dispersion threshold by XRD as mentioned above. When the TiO, loading is less than the threshold, the monolayer coverage increases with the increase of TiO, loading. Thus the intensity I,,, in- creases in proportion to the TiO, loading directly and the intensity I,,,, from matrix 73 Fig.3 TEM image of Ti02-A1203 Samples (prepared with TiCI,- CCI, solution) a. y-Al,03 b. Ti02 (Anatase) c. 0.061g Ti02 / g y-Al,O, d. 0.44g TiO, / g y-Al,O, 14 Fig.4 HEED patterns of Ti0,-AI,O, Samples (prepared with TiC1,- CC1,solution) a. y-A120, b. TiO, (Anatase) c. 0.06 1 g TiO, / g y-Al,O, d. 0.44g Ti02 / g 9-A120, 75 y-Al,O, remains unchanged essentially. As a result, intensity ratios ITiZp / IAIZP increase linearly with the increase of Ti02 1oading.When the TiO, loading is higher than the threshold, the residual crystalline TiO, appears and resides on the top layer of the sample. Also the intensity ratio ITiZp / I,,,, increases with the increase of TiO, loading. Since XPS is a surface sensitive technique, the photoelecton contribution to the peak of Ti2p depends on the inelastic mean free path. Therefore the intensity contributed by crystalline TiO, is much less than that by monolayer TiO, on the surface of y-Al,O, at the same amount. Although two lines from two sources in plot are present in linear way, the slopes of them are quite dif- ferent. In Fig.2 the loading amount of TiO, at intersecting point of two straight lines corre- sponds to the threshold of monolayer dispersion, 0.1 lg TiO, / g y-Al,O,. The threshold a o cords with the XRD quantitative measurement by extrapolation. The result shows that both methods, XPS intensity ratio method and XRD quantitative extrapolation method, are complementary to each other in determining the dispersion and studing the surface state of TiO, / y-Al,O,. TEM and HEED measuremental results The results of XRD quantitative measurement and XPS measurement were also veri- fied by TEM and HEED techniques. Fig.3 and Fig.4 only show the results for samples pre- pared with TiC14-CCI, solution omitting similar results with TiCI,-CH,COCH, solution. As the loading amount of TiO, on y-Al,O, is less than the dispersion threshold, the TEM image (see Fig.3~) and the HEED pattern (see Fig.&) show just like that of matrix y-A1,0, (see Fig.3a and Fig.4a). As the loading amount of TiO, on y-Al,O, is more than the dipsersion threshold, the TEM image (see Fig.3d) and the HEED pattern (see Fig.4d) show like that of y-Al,O, and additional crystalline TiO,. CONCLUSIONS The results mentioned above have illustrated that as the samples of Ti0,-Al,O,carrier were prepared with impregnation of TiCI, nonaqueous solution (carbon tetrachloride or acetone as solvent), the maximum dispersion capacities for TiO, on y-Al,O, is 5.6 x 104g TiO,/ mz with coverage of 58%, and TiO, with molecule^^ disperses on y-Al,O, surface in form of a submonolayer, rather than as a full covered monolayer. It is different from the preparation with impregnation of Ti(SO,), aqueous solution. In this case strong acidity of Ti(SOJ, solution (PH 1-2) would cause y-Al,O, partially solubilizing during impregnation and then depositing with Ti4+ on y-Al,O, to form Ti0,-AI,O, mixed gelatination. Thus, the method described in this paper may be employed as a method for monolyer modification of y-Al,O, with TiO,. 76 ACKNOWLEDGMENT The authors are grateful to the National Laboratory for Structural Chemistry of Unstable and Stable Species (Beijing, China) for financial support. REFERENCES 1 S.J. Tauster, S.C. FungandR.L. Garten, J.A.C.S., 1(1978), 170-175 2 R.T.K. Baker, E.B. Prestridge and R.L. Garten, J.Catal., 56(1979), 390-406 3 M. Takeuchi, S. Matsuda, H. Okada, H. Kawagoshi, F. Nakajima, Ger Offen 4 H.B. Jones, and R. Smith, US Pat 4206038 (1980) 5 E. Rodenas, T. Yamaguchi, H. Hattori and K. Tanabe, J. Catal., 69(1981), 6 G.B. McVicker, and J.J. Ziemiak, J. Catal., 95(1985), 473-481 7 Zhu Yongfa, Gui Linlin and Tang Youqi, CUIHUA XUEBAO (Journal of Catalysis. China), 10(1989), 118-122 8 Liu Yingjun, Luo Shengcheng, Gui Linlin, Annual Report of National Laboratory for Structural Chemistry of Unstable and Stable Species (Beijing, China), 1987, pp.61 9 F.H. Chung, J.Appl.Crst., 7(1974), 526-531 10 Liu Yingjun, Xie Youchang, Ming Jing, Liu Jun and Tang Youqi, CUIHUA 11 S.C. Fung, J.Catal., 58(1979), 454-469 12 Gui Linlin, Liu Yingjun Guo Qinlin, Huang Huizhong and Tang Youqi, Scientia Sinica (Series B), Chinese Ed., No.6 (1985), 509-517; English Ed., 28(1985), 2838231 (1979) 434-444 XUEBAO (Journal of Catalysis, China), 3(1982), 262-267 1233-1242 G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 77 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands INTERACTIONS OF THE IMPREGNATING SOLUTION WITH THE SUPPORT DURING THE PREPARATION OF Rh/Ti02 CATALYSTS R . J . FENOGLIO, W . ALVAREZ, G . M . N U N E Z , D . E . RESASCO INTEHA (Universidad Nacional d e Mar d e l Plata-CONICET) Juan B. J u s t o 4302, (7600) Har d e l P l a t a , Argen t ina SUHHARY We have analyzed t h e d i f f e r e n t phenomena o c c u r r i n g d u r i n g t h e impregnat ion of t i t an i a w i t h aqueous s o l u t i o n s of rhodium c h l o r i d e . Our r e s u l t s d e n o n s t r a t e t h a t t h e d e p o s i t i o n of t h e Rh p r e c u r s o r s on t i t a n i a , a t low pH v a l u e s ( a b o u t 3), i n v o l v e s t h r e e d i f f e r e n t p r o c e s s e s : a) a d s o r p t i o n of a n i o n i c species, b ) l i g a n d exchange r e a c t i o n and c ) a c i d a t t a c k of t h e s u p p o r t . INTRODUCTION The i n t e r a c t i o n s t a k i n g place between t h e m e t a l p r e c u r s o r s p e c i e s and t h e s u p p o r t d u r i n g impregnat ion p r o c e s s e s , may p l a y a d e c i s i v e r o l e i n d e t e r m i n i n g t h e r e s u l t i n g p r o p e r t i e s of t h e catalysts . I n t h e case of t h e impregnat ion of o x i d e s u p p o r t s w i t h aqueous s o l u t i o n s c o n t a i n i n g t h e metal p r e c u r s o r s , w e can i d e n t i f y s e v e r a l p o s s i b l e types of i n t e r a c t i o n s ( r e f . 1 and 2). F i r s t , w e can mention t h e a d s o r p t i o n of m e t a l p r e c u r s o r species on t h e o x i d e s u p p o r t ( r e f . 3), which has been a wide ly i n v e s t i g a t e d phenomenon. We can a l s o c o n s i d e r t h e p o s s i b i l i t y t h a t t h e adsorbed s p e c i e s may undergo a subsequen t s u r f a c e r e a c t i o n w i t h t h e s u p p o r t ( r e f . 4 ) . F i n a l l y , when t h e impregna t ing s o l u t i o n is a c i d i c , t h e r e e x i s t s t h e p o s s i b i l i t y t h a t t h e s u r f a c e of t h e s u p p o r t be a t t a c k e d by t h e impregnat ing s o l u t i o n c a u s i n g a partial d i s s o l u t i o n . T h i s a c i d a t t a c k w a s f i r s t proposed by S a n t a c e s a r i a e t a l . ( r e f . 5) f o r t h e impregnat ion of H2PtC16 on A1203 . More r e c e n t l y , it h a s a l s o been found t o occur on o t h e r systems ( re f . 6 ) . I t h a s been sugges t ed t h a t t h e a c i d a t t a c k m a y n o t o n l y i n v o l v e t h e par t ia l d i s s o l u t i o n of t h e s u p p o r t , b u t a l s o a subsequen t r e - d e p o s i t i o n ( re f . 7 ) . T h i s , i n turn,may r e s u l t i n a pa r t i a l coverage of t h e metal p r e c u r s o r s b y s u p p o r t e d species. These i d e a s m a y b e immediately a s s o c i a t e d w i t h t h e w e l l known SMSI e f f e c t ( r e f . 8 ) , which a l s o 78 involves a par t ia l coverage of t h e m e t a l by t h e s u p p o r t . However, whi le t h e SMSI effect r e q u i r e s a h igh tempera ture reduct ion s t ep t o become e v i d e n t , an a c i d a t t a c k t o t h e s u p p o r t fol lowed by re -depos i t ion m a y have similar effects after low temperature r e d u c t i o n . I n t h i s c o n t r i b u t i o n w e w i l l c o n s i d e r t h e s e t h r e e t y p e s of i n t e r a c t i o n s f o r t h e p a r t i c u l a r case of of Rh/TiOp c a t a l y s t s prepared by impregnation of T i 0 2 w i t h rhodium c h l o r i d e species from aqueous s o l u t i o n s . EXPERIMENTAL To s t u d y t h e phenomena a s s o c i a t e d w i t h t h e impregnat ion p r o c e s s involved i n t h e p r e p a r a t i o n of Rh/Ti02 catalysts, w e have analyzed t h e changes o c c u r r i n g i n t h e impregnat ing s o l u t i o n and t h e suppor t when 60 cc of a 3.0 x lo-â l4 aqueous s o l u t i o n of RhC13.3 H20 (from A l f a Products ) w a s placed i n c o n t a c t w i t h one gram of TiOz(Degussa P25. BET 55 m 2 / g ) used as r e c e i v e d . The rhodium c h l o r i d e complexes p r e s e n t i n s o l u t i o n , b e f o r e and after b e i n g contac ted w i t h Ti02,were c h a r a c t e r i z e d by UV-visible spec t roscopy. The e v o l u t i o n of p H af ter p l a c i n g t h e s u p p o r t i n c o n t a c t w i t h impregnat ing s o l u t i o n w a s c o n t i n u o u s l y monitored by a d i g i t a l pH-meter. The amount of Rh d e p o s i t e d on t h e c a t a l y s t and t h a t remaining i n s o l u t i o n d u r i n g t h e d i f f e r e n t stages of t h e p r e p a r a t i o n were determined by atomic a b s o r p t i o n . The f r e s h and reduced c a t a l y s t s w e r e f u r t h e r c h a r a c t e r i z e d by t ransmis ion e l e c t r o n microscopy (TEH) and X-ray d i f r a c t i o n ( X R D ) . RESULTS AND DISCUSSION The Rh (111) c h l o r i d e s y s t e m i n a c i d i c s o l u t i o n s can e x h i b i t a series of complexes of t h e type RhC1,(HzO)6-n +3-nwhich m a y v a r y from RhC16 3- t o Rh(H20)6 . These species can be e a s i l y c h a r a c t e r i z e d by UV-visible a b s o r p t i o n spec t roscopy. The spectra of chloro-complexes of Rh(I I1) e x h i b i t two m a x i m a i n t h e 300-600 nm r e g i o n a s c r i b e d t o d-d t r a n s i t i o n s and a very i n t e n s e charge t r a n s f e r band i n t h e s h o r t e r wavelength r e g i o n ( r e f . 9 ) . The p o s i t i o n of t h e bands depends on t h e number of c h l o r i d e and water l i g a n d s . When t h e number of w a t e r l i g a n d s i n c r e a s e s , t h e l i g a n d f i e l d t h e o r y p r e d i c t s t h a t t h e d-d t r a n s i t i o n m a x i m a must s h i f t t o s h o r t e r wavelengths . I n fact , it is exper imenta l ly observed t h a t 3+ 79 t h e y s h i f t from 412-518 nn f o r t h e hexachloro complex (n=6) t o 305-393 nm f o r t h e hexa-aquo complex (n=O). T h e r e f o r e , t h e n a t u r e of t h e species p r e s e n t i n s o l u t i o n can be, a t least q u a l i t a t i v e l y , e s t i m a t e d from t h e spectra. F i g u r e 1 shows a b s o r p t i o n spectra of t h e aqueous s o l u t i o n of RhC13 used i n our impregnat ion p r o c e s s . The p o s i t i o n of t h e m a x i m a f o r t h e f r e s h impregna t ing s o l u t i o n i n d i c a t e t h a t t h e impregnat ing s o l u t i o n h a s a h i g h c o n c e n t r a t i o n of a n i o n s w i t h 5 and 6 c h l o r i d e l i g a n d s . I n o r d e r t o s a t i s f y t h e m a s s and c h a r g e b a l a n c e s , t h e s e complexes must b e compensated by a comparable number of aquo complexes, a l t h o u g h t h e low p H v a l u e of t h e s o l u t i o n i n d i c a t e s t h a t a f r a c t i o n of t h e s e aquo complexes may have undergone h y d r o l y s i s forming Rh(OH)m(HgO)6-n n- complexes. I 300 400 500 600 nm F i g u r e 1. Absorpt ion spectra of rhodium c h l o r i d e s u p e r n a t a n t s o l u t i o n s a f t e r d i f f e r e n t t i m e s of c o n t a c t w i t h t i t a n i a . 80 The spectra of t h e s u p e r n a t a n t s o l u t i o n s r e s u l t i n g af ter t h e 5 min and 24 h c o n t a c t w i t h t h e T i 0 2 are a l s o shown i n F i g . A clear d e c r e a s e i n t h e c o n c e n t r a t i o n of a n i o n i c species compared t o t h e i n i t i a l s o l u t i o n is observed i n t h e s u p e r n a t a n t s o l u t i o n s , i n d i c a t i n g t h a t a large f r a c t i o n of t h e rhodium s p e c i e s have been adsorbed on t h e Ti02 s u p p o r t . T h i s a d s o r p t i o n p r o c e s s h a s been q u a n t i f i e d by a tomic a b s o r p t i o n . As shown i n Tab le 1, after 5 min c o n t a c t w i t h t h e s u p p o r t , abou t h a l f of t h e Rh i n i t i a l l y p r e s e n t i n s o l u t i o n h a s been removed, s u g g e s t i n g t h a t most of t h e a n i o n i c complexes have been adsorbed on t h e TiOi( - T h i s w a s indeed t h e expected behav io r of t h e s y s t e m . When T i02 is immersed i n an aqueous medium a t a p H v a l u e below its i s o - e l e c t r i c p o i n t , it becomes p o s i t i v e l y charged and a b l e t o adso rb n e g a t i v e l y charged species ( r e f . 3). However, as a l s o shown i n Tab le 1, a f t e r l onge r c o n t a c t t i m e s , t h e amount of Rh r e t a i n e d by t h e s u p p o r t unexpectedly d e c r e a s e d . I t w a s observed t h a t a f te r 24 h i n c o n t a c t w i t h t h e a c i d i c s o l u t i o n a lmos t 40 X of t h e Rh i n i t i a l l y d e p o s i t e d on t h e TiOg w a s l o s t from t h e s u r f a c e . 1. TABLE 1 Rh c o n c e n t r a t i o n i n s o l u t i o n and on t h e s u p p o r t af ter d i f f e r e n t c o n t a c t t i m e s w i t h T i O Z . SAMPLE Rh mmol/l w t % Rh i n t h e s o l u t i o n i n t h e catalyst f r e s h s o l u t i o n 3.0 5 min c o n t a c t 1.3 24 h c o n t a c t 1.9 - 1.1 0.7 To de te rmine t h e n a t u r e of t h e species d e p o s i t e d on t h e s u p p o r t w e have used d i f f u s e r e f l e c t a n c e spec t roscopy . A s i l l u s t r a t e d i n F i g . 2 , t h e i n t e n s i t y of t h e band co r re spond ing t o t h e sample c o n t a c t e d f o r 24 h w i t h t h e impregnat ing s o l u t i o n is s i g n i f i c a n t l y lower t h a n t h a t of t h e 5 min c o n t a c t . T h i s d i f f e r e n c e conf i rms t h e unexpected r e s u l t s o b t a i n e d by atomic a b s o r p t i o n a n a l y s i s of t h e s u p e r n a t a n t s o l u t i o n f o r t h e 24 h c o n t a c t r e p o r t e d i n Tab le 1. 81 3 80 430 480 530 580 nm F i g u r e 2. D i f f u s e r e f l e c t a n c e s p e c t r a of t i t a n i a samples a f t e r d i f f e r e n t c o n t a c t t i m e s wi th t h e rhodium c h l o r i d e s o l u t i o n The a n a l y s i s of t h e band i n t e n s i t i e s does n o t add much t o t h e r e s u l t s d i s c u s s e d above. But, an impor tan t conclus ion can be drawn from t h e a n a l y s i s of t h e p o s i t i o n of t h e m a x i m a . I t is observed t h a t when t h e c o n t a c t t i m e i n c r e a s e s a s h i f t t o lower wavelengths t a k e s p l a c e . A s d e s c r i b e d above, t h i s s h i f t would i n d i c a t e a s u b s t i t u t i o n of weaker l i g a n d s by s t r o n g e r l i g a n d s . Accordingly, w e could propose t h a t , af ter t h e i n i t i a l a d s o r p t i o n p r o c e s s , t h e adsorbed s p e c i e s undergo a l i g a n d exchange r e a c t i o n 82 on t h e s u r f a c e by which some c h l o r o l i g a n d s (weaker) are exchanged by s u r f a c e OH g roups ( s t r o n g e r ) . T h i s two-step p r o c e s s can be d e s c r i b e d i n t h e f o l l o w i n g scheme: a ) anion adsorption: Ti-OH + H+ + HLx - _ _ _ _ _ _ Ti-OH2+ HLx - b) T ligand exchange reaction: + Ti-OH _ _ _ _ _ _ Ti-OHZ+ HLX-* -0-Ti + HL - -OHZ+ HLx where L is t h e l i g a n d and H t h e c e n t r a l metal atom. 3.20 PH 3.10 I I 0 100 2 00 300 time (sec) F i g u r e 3. Evolu t ion of t h e p H of t h e 3 x lo-% rhodium c h l o r i d e s o l u t i o n af ter t h e a d d i t i o n of Ti02. 83 T h i s scheme would p r e d i c t t h a t t h e p H of t h e impregnat ing s o l u t i o n should v a r y d u r i n g t h e d e p o s i t i o n p r o c e s s . H e shou ld e x p e c t t h a t , i n i t i a l l y , as t h e an ion a d s o r p t i o n p roceeds , t h e p H should i n c r e a s e because H+ are consumed from t h e s o l u t i o n . But , a f te r a wh i l e , t h e l i g a n d exchange r e a c t i o n of t h e adsorbed species shou ld beg in and, consequen t ly , t h e p H shou ld d e c r e a s e as H+ are evolved from t h e s u p p o r t . H e have shown t h a t t h i s is indeed t h e case. U e have ana lyzed t h e e v o l u t i o n of t h e p H of t h e rhodium c h l o r i d e s o l u t i o n af ter t h e a d d i t i o n of T i O Z . A s shown i n F i g . 3, t h e p H i n i t i a l l y i n c r e a s e d r a p i d l y from an i n i t i a l v a l u e of 3.14, r e a c h i n g a maximum a f t e r a f e w s econds , and then d e c r e a s e d . By c o n t r a s t , when H C 1 s o l u t i o n s of t h e same i n i t i a l p H were p u t i n c o n t a c t w i t h T iOZ i n s t e a d of t h e rhodium s o l u t i o n s , no maximum w a s d e t e c t e d b u t a c o n t i n u o u s i n c r e a s e u n t i l a c o n s t a n t v a l u e w a s r eached . Likewise, when SiOz w a s used i n s t e a d of T i O Z a lmos t no change i n p H w a s d e t e c t e d . These o b s e r v a t i o n s demons t r a t e t h a t t h e appearance of m a x i m a i n t h e e v o l u t i o n of t h e p H is r e l a t e d t o p a r t i c u l a r i n t e r a c t i o n s between t h e Rh complexes and t h e T iOz s u p p o r t and n o t t o an artefact of t h e expe r imen ta l p rocedure . To e x p l a i n t h e pronounced d e c r e a s e i n t h e amount of Rh r e t a i n e d by t h e s u p p o r t as t h e t i m e of c o n t a c t w i t h t h e s o l u t i o n i n c r e a s e s , w e have cons ide red t h e p o s s i b i l i t y of an a c i d a t t a c k t o t h e s u p p o r t s u r f a c e . We s p e c u l a t e t h a t i f t h e s u r f a c e of t h e s u p p o r t is a t t a c k e d by t h e a c i d i c s o l u t i o n , a l o s s of adsorbed species can t a k e place d u r i n g t h a t p r o c e s s . H e have ana lyzed t h e s u r f a c e of t h e T iOZ by TEH t o i n v e s t i g a t e whether an a t t a c k is e v i d e n t . As i l l u s t r a t e d i n F i g . 4 , a TiOz sample which had been c o n t a c t e d w i t h t h e impregnat ing rhodium c h l o r i d e s o l u t i o n f o r 24 h e x h i b i t e d s u r f a c e f e a t u r e s t h a t d i d n o t appear e i t h e r on t h e f r e s h T i 0 2 n o r on t h e one which had o n l y been i n c o n t a c t w i t h t h e s o l u t i o n f o r 5 min. The most marked f e a t u r e of t h e samples t h a t showed e v i d e n c e s of a t t a c k w a s t h e p re sence of o v e r l a y e r s . These o v e r l a y e r s m a y be t h e r e s u l t of a par t ia l d i s s o l u t i o n of t h e T i 0 2 fol lowed by r e - d e p o s i t i o n . 84 F i g u r e 4. Transmission e l e c t r o n micrograph of t h e t i t a n i a s u p p o r t c o n t a c t e d w i t h t h e rhodium c h l o r i d e s o l u t i o n f o r 24 h and reduced a t 323K W e have a l s o analyzed by XRD t h e r u t i l e / a n a t a s e r a t i o i n t h e Ti02support , which is about 1/4 i n t h e o r i g i n a l samples. These r a t i o s were obta ined from t h e peak i n t e n s i t i e s corresponding t o t h e s p a c i n g s d=3.25, r u t i l e (100) and d=3.52, a n a t a s e (101). As shown i n T a b l e 2 t h i s r a t i o is s i g n i f i c a n t l y i n c r e a s e d f o r t h e sample which has been i n c o n t a c t w i t h t h e s o l u t i o n f o r 24 h. T h i s a n a t a s e - r u t i l e t r a n s f o r m a t i o n would agree w i t h t h e d i s s o l u t i o n / r e - d e p o s i t i o n hypothes is , s i n c e r u t i l e is t h e most s t a b l e form of T i 0 2 and its format ion could be e f f e c t e d d u r i n g t h e r e - d e p o s i t i o n process . 85 TABLE 2 Anatase- to- ru t i le t r a n s f o r m a t i o n e f f e c t e d by t h e a c i d a t t a c k as determined from XRD peak i n t e n s i t y , d=3.25 r u t i l e (1001, 6 ~ 3 . 5 2 a n a t a s e (101) . A s mentioned above, t h e r e - d e p o s i t i o n may r e s u l t i n a par t ia l cover ing of t h e metal p r e c u r s o r s . We have looked f o r another evidence of t h i s phenomenon by measuring hydrogen chemisorpt ion capacities of a Rh/Si02 c a t a l y s t , used as r e f e r e n c e , a f t e r b e i n g i n c o n t a c t w i t h T i 0 2 i n both, n e u t r a l and a c i d i c l i q u i d media. If a partial coverage of t h e Rh s u r f a c e by T i 0 2 species occurred , w e should expec t a d e c r e a s e i n t h e hydrogen chemisorpt ion capacity. A s shown i n Table 3, t h e r e w a s no much change i n t h e chemisorpt ion v a l u e s when t h e r e f e r e n c e catalyst w a s e i t h e r c o n t a c t e d w i t h Ti02 i n a non acidic medium o r t r e a t e d i n an a c i d i c medium wi thout T i 0 2 - However, t h e r e w a s a clear d e c r e a s e when it w a s contac ted w i t h a s l u r r y c o n t a i n i n g T i 0 2 and a HC1 s o l u t i o n ( p H = 3.14) . TABLE 3 Hydrogen chemisorpt ion d a t a obta ined on t h e same Rh/SiOz contac ted w i t h T i O z under d i f f e r e n t c o n d i t i o n s Sample H/Rh _ , c a t a l y s t .................................................................. 86 CONCLUSIONS W e conclude t h a t when T i 0 2 is placed i n con tac t with rhodium ch lo r ide s o l u t i o n s of low p H va lues , s e v e r a l phenomena, lead ing t o t h e depos i t i on of metal p recu r so r s on t h e T i 0 may occur: i) anion adsorption. A s t h e oxide particles become p o s i t i v e l y charged i n t h e low p H s o l u t i o n , t h e adsorp t ion of an ion ic species is r e a d i l y e f f e c t e d . 2 ii) ligand exchange reaction. A f t e r t h e adsorpt ion of t h e an ionic complexes, f u r t h e r anchoring t o t h e support t a k e s place by means of an exchange of ch loro l i gands by su r face OH groups. iii) acid attack. The a c i d i c s o l u t i o n a t t a c k s t h e su r face of t h e support causing a p a r t i a l d i s s o l u t i o n followed by t h e re-deposi t ion of oxide species. ACKNOWLEDGEMENTS acknowledged. F inanc ia l suppor t from CONICET of Argentina is g r a t e f u l l y REFERENCES K . Foger, i n Anderson J . R. , and Boudart M.(Eds.), Ca ta lys i s : Science and Technology, Springer , Ber l in , Haidelberg, N.York, 1984, Ch. 4, p. 227. M . Che and C.O. Bennet t , Adv. C a t a l . , 36 (1989) 55 J. Brunel le , i n Delnon e t a l . (Ed.) , Prepara t ion of Heterogen-Cata lys t s , Vol. 11, El sev ie r . Amsterdam, 1979. J.C. Summers and S . A . Ausen, J. Cata l . , 52 (1978) 455. E. Santacesar ia , S . C a r r a , and I. Adami, Ind. Eng. Chem. Prod. R e s . & Dev., 16 (1977) 41. W.J. van den Boogert, G. van de r L e e , H. Luo, V . Ponec, A p p l . Ca ta l . (1987) Y.J. Lin, R . J . Fenoglio, D.E. Resasco, G.L. Haller. i n Delmon e t a l . (Ed.) , Prepara t ion of Heterogeneous Ca ta lys t s , Vol. I V , E l sev ie r , Amsterdam, 1987, p.125. G . L . H a l l e r and D . E . Resasco, Adv. Ca ta l . , 36 (1989) 173. C . K - Jorgensen, Acta Chim. Scand., 10 (1956) 500. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 87 IMPREGNATION OF CONTROLLED-POROSITY SILICA: I N V E S T I G A T I O N OF THE PARAMETERS AFFECTING SELECTIVITY I N CO HYDROGENATION Cu/Si02, Co/SiO2 and Cu-Co/Si02 M.A. MARTIN LUENGO (YATES), Y. WANG and P.A. SERMON* S o l i d s and Surfaces Research Group, Department o f Chemistry, Brunel U n i v e r s i t y , Uxbridge, UB8 3PH, UK SUMMARY S i l i c a spheres o f c o n t r o l l e d pore s i z e (83% o f pores between 6-10nm i n r a d i u s ) were impregnated w i t h aqueous n i t r a t e s o l u t i o n s t o g i v e 8%Cu/SiO , 8%Co/Si02 and 8%Cu-4%Co/SiO . Temperature-programmed reduc t i on , microproge ana lys i s , and propane hyd ro teno lys i s a l l i n d i c a t e d t h a t t h e Cu and Co i n t h e b i m e t a l l i c c a t a l y s t were i n i n t i m a t e con tac t . hydrogenat ion r a t e o f CO convers ion inc reased i n t h e sequence: w h i l e s e l e c t i v i t y t o methine inc reased i n t h e sa&e sequence (and s e l e c t i v i t y t o methanol decreased). t he a b i l i t y o f Co t o d i s s o c i a t e CO and produce methane i s w e l l known. However, none o f these c a t a l y s t s produced s i g n i f i c a n t amounts o f e thano l . There fore t h e s tudy o f these i d e a l i s e d c a t a l y s t s suggests t h a t Cu-Co i n t e r a c t i o n s a r e n o t s u f f i c i e n t t o induce s e l e c t i v i t y t o e thano l i n C O hydrogenat ion a t 523K and 2MPa b u t must a l s o r e q u i r e an i n t e r a c t i o n w i t h a ZnO o r A1203 component i n t h e f u l l c a t a l y s t . I n t h e c a t a l y s i s o f C O Cu/SiO < Cu-Co/Si02,< Co/SiO These s e l e c t i v i t y t rends a r e t o be expected s ince INTRODUCTION A t t h e l a s t conference cons ide r ing s c i e n t i f i c bases f o r t h e p r e p a r a t i o n o f heterogeneous c a t a l y s t s some o f t h e p resen t au thors descr ibed ( r e f . 1) a s tudy o f t he parameters a f f e c t i n g t h e d i s p e r s i o n and l o c a t i o n o f P t d u r i n g impregnat ion o f a s i l i c a o f c o n t r o l l e d pore s i z e w i t h s o l u t i o n s o f h e x a c h l o r o p l a t i n i c a c i d . Using t h i s approach i t was p o s s i b l e t o d i f f e r e n t i a t e pore d iameter and s o l u t i o n tempera tu re -v i scos i t y e f f e c t s i n t h e p r e p a r a t i o n o f a model mono-metal l ic c a t a l y s t . Th i s approach has now been extended t o cons ider t h e importance o f Cu-Co i n t e r a c t i o n s i n s i l i c a - s u p p o r t e d CO hydrogenat ion c a t a l y s t s i n d e f i n i n g s e l e c t i v i t y t o e thano l . o f CO/H2 and CO/C02/H2 m ix tu res w i t h good s e l e c t i v i t y t o h i g h e r a l c o h o l s such as ethanol ( re f s . 2,3). such c a t a l y s t s ( r e f . 3 ) , b u t i t s a d d i t i o n t o t h e Cu-only c a t a l y s t inc reases alkane fo rmat ion . The p r e c i s e i n t e r a c t i o n of Cu and Co i n such c a t a l y s t s i s c e r t a i n l y unc lea r and hence t h e p resen t work was undertaken. However, i t i s Cu-Co heterogeneous c a t a l y s t s a r e o f some c u r r e n t ' i n t e r e s t i n t h e convers ion I t seems u n c e r t a i n whether t h e Co i s e n t i r e l y reduced i n 88 a p p r e c i a t e d t h a t t h e p r e s e n t c a t a l y s t s a r e f a r s i m p l e r t h a n a l k a l i - t r e a t e d Cu-Co/ZnO/A1203 samples w h i c h a t 563K, 6MPa and CO/H2=0.5 gave 21-24%CO c o n v e r s i o n w i t h 30% s e l e c t i v i t y t o e t h a n o l ( r e f . 3 ) . A g a i n s t t h i s i n t r i g u i n g backg round t h e p r e s e n t r e s u l t s a r e now r e p o r t e d . EXPERIMENTAL M a t e r i a l s A s i l i c a ( S h e l l I n t e r n a t i o n a l Chemical Company L t d . ) was chosen because o f i t s c o n t r o l l e d p o r o s i t y , homogene i t y and p u r i t y ; i t s p r o p e r t i e s have been d e s c r i b e d p r e v i o u s l y ( r e f . 1 ) b u t i t s m a i n c h a r a c t e r i s t i c s a r e g i v e n i n T a b l e 1 be low. TABLE 1 P r o p e r t i e s o f s i l i c a used p a r t i c l e s i z e (mm) 2.5 sN2 (m2 /g ) 21 1 % p o r e s i n t h e r a d i u s r a n g e 6-10nm 8 3 I t s t o t a l s u r f a c e a r e a was e s t i m a t e d f r o m a n a l y s i s o f N a d s o r p t i o n d a t a a t 77K assuming i t s m o l e c u l a r c r o s s - s e c t i o n a l a r e a was 0.162nm . The a d s o r p t i o n i s o t h e r m was o f t y p e I V ( r e f . 4 ) w i t h a h y s t e r e s i s l o o p t y p i c a l o f t h a t f o r a mesoporous m a t e r i a l ( i . e . t h a t d e s i g n a t e d t y p e H 1 ) ( r e f . 4 ) . Samples o f t h i s p r e - d r i e d s u p p o r t (239 ) were 75cm3 aqueous s o l u t i o n o f Cu o r Co o r Cu-Co n i t r a t e s o f a s u f f i c i e n t s t r e n g t h t o g i v e t h e c a t a l y s t c o m p o s i t i o n s be low: 5 8 % C u / s i l i c a 8 % C u - 4 % C o / s i l i c a 8 % C o / s i l i c a These were t h e n a g i t a t e d f o r lOmin, e v a p o r a t e d t o d ryness , d r i e d f o r 16h i n a i r a t 403K, and c a l c i n e d f o r 3h a t 523K i n a i r . Methods T o t a l s u r f a c e a r e a s were e s t i m a t e d f r o m N2 a d s o r p t i o n a t 77K i n a S o r p t y a p p a r a t u s ( C a r l o E r b a ) a f t e r o u t g a s s i n g a t 523K f o r 2h. samples was assessed i n a c o n v e n t i o n a l f l o w temperature-programmed r e d u c t i o n system ( r e f . 5 ) u s i n g 5%H2/Ar f l o w i n g a t 50cm3/min o v e r 50-100mg c a t a l y s t samples. The r a t e and s e l e c t i v i t y o f CO h y d r o g e n a t i o n was f o l l o w e d o v e r samples (0 .19 ) o f c a t a l y s t s ( w h i c h had been p r e t r e a t e d t o 573K i n 6%H2/N2 and t h e n c o o l e d t o 523K) i n CO/H2 ( = 2 ) f l o w i n g a t 20cm3/min a t 2MPa and 523K; o v e r a p e r i o d o f 3-4h p r o d u c t s were a n a l y s e d b y gas ch romatog raphy u s i n g gas ch romatog raphs f i t t e d w i t h F I D d e t e c t o r s and Porapak Q and T co lumns. h y d r o g e n o l y s i s o f p ropane a t 523K was a l s o d e t e r m i n e d c h r o m a t o g r a p h i c a l l y . The r e d u c i b i l i t y o f t h e The a c t i v i t y o f t h e c a t a l y s t s i n t h e The 89 c a t a l y t i c c o n d i t i o n s used were a s f o l l o w s : 0.29 samples p r e - r e d u c e d t o 573K i n H 2 were s u b j e c t e d t o a r e a c t a n t s t r e a m (140cm3/min a t 1 0 l k P a ) c o n s i s t i n g o f p ropane : N2:H2=10:30:100 mol r a t i o . a n a l y s e d i n a s c a n n i n g e l e c t r o n m i c r o s c o p e b y EDAX-microprobe methods f o r Cu and Co c o n c e n t r a t i o n p r o f i l e s . Reduced c a t a l y s t s were s e c t i o n e d and c r o s s - s e c t i o n s CHARACTERISATION The r e s u l t s o f BET a n a l y s i s o f N2 a d s o r p t i o n a t 7 7 K showed t h a t t h e t o t a l s u r f a c e a r e a o f t h e s u p p o r t (211m / g ) dec reased on i m p r e g n a t i o n w i t h t h e m e t a l n i t r a t e s and c a l c i n a t i o n t o t h e s u p p o r t e d m e t a l o x i d e s . C u 0 - C o O x / s i l i c a samples had t o t a l s u r f a c e a r e a s o f o n l y 187 and 160m / g r e s p e c t i v e l y and t h i s i s p r o b a b l y due t o p a r t i a l b l o c k i n g o f t h e s u p p o r t p o r e s b y t h e m e t a l o x i d e p a r t i c l e s . r a t e a t 533K ( s e e F i g . 1) w h i c h was s i g n i f i c a n t l y above t h a t f o r C u O / s i l i c a o r CuO and s i g n i f i c a n t l y be low t h a t f o r C o O x / s i l i c a o r COO. t h e Cu and Co phases were i n t i m a t e l y m i x e d on t h i s s i l i c a s u p p o r t u n d e r t h e p r e s e n t c o n d i t i o n s . t h e e x p e c t e d v a l u e i f CuO and COO were b e i n g reduced t o t h e z e r o - v a l e n t m e t a l s , b u t hyd rogen s p i l l o v e r may b e r e s p o n s i b l e f o r t h i s . CuO and Coox may n o t be s u r p r i s i n g i n t h e l i g h t o f t h e f a c t t h a t COO can a c c e p t up t o 25mol% CuO w i t h o u t s t r u c t u r a l change ( r e f . 6 ) . The i n t i m a c y o f Cu and Co phases was c o n f i r m e d b y m i c r o p r o b e a n a l y s i s o f t h e m e t a l c o n c e n t r a t i o n s i n p a r t i c l e c r o s s - s e c t i o n s ( see F i g . 2). These show some p r e f e r e n c e f o r b o t h m e t a l s a t t h e o u t e r edge o f t h e s u p p o r t p a r t i c l e s ( a s e x p e c t e d f r o m i m p r e g n a t i o n ( r e f . 1 ) b u t mos t i m p o r t a n t l y t h a t t h e two m e t a l s a r e l o c a t e d i n t h e same p a r t s o f t h e s u p p o r t p a r t i c l e s ; t h i s t h e n i s c o n s i s t e n t w i t h temperature-programmed r e d u c t i o n . 2 Thus t h e C u O / s i l i c a and 2 Cu0-CoOx /s i l i ca reduced i n temperature-programmed r e d u c t i o n w i t h a maximum T h i s sugges ted t h a t I n each case t h e hyd rogen consumpt ions were 34-71% above The i n t i m a t e m i x i n g o f F i g u r e 1 TPR o f 8%Cu-4%Co/Si02 90 F igu re 2 . Concent ra t ion p r o f i l e s o f Cu ( ) and Co ( 0 ) i n c ross -sec t i ons o f c a l c i n e d S i O 8%Cu-4%Co/Si02 ( b ) determined b y k c r o p r o b e a n a l y s i s o f sec t i oned c a t a l y s t p a r t i c l e s whose edge i s marked by arrows . ( a ) and The i d e n t i c a l depth o f p e n e t r a t i o n o f t h e two meta l s i n ( b ) suggest these w i l l be i n good con tac t on s i l i c a impregnated i n t h e manner descr ibed. 91 Alkane hydrogeno lys is i s a s u i t a b l e probe f o r t h e p resen t model c a t a l y s t s i n t h a t ze ro -va len t Cuo i s expected t o have low a c t i v i t y i n t h e reac t i on , w h i l e on t h e o t h e r hand ze ro -va len t Coo shou ld have h i g h a c t i v i t y i n t h e r e a c t i o n ( r e f . 7 ) . I f a b i m e t a l l i c phase e x i s t s a t t h e su r face o f t h e p a r t i c l e s i n t h e Cu-Co/s i l i ca sample then one would expec t some in te rmed ia te a c t i v i t y , un less the Cu was p resen t t o t h e e x t e n t t h a t Co ensembles were fragmented t o such a degree t h a t s t r u c t u r e - s e n s i t i v e hydrogeno lys is c o u l d n o t occur a t a s i g n i f i c a n t r a t e . Thus t h e hydrogeno lys is r e a c t i o n may i n d i c a t e t h e e x t e n t t o which Cu and Co phases a r e i n c o n t a c t i n t h i s c a t a l y s t . a c t i v i t i e s and s e l e c t i v i t i e s o f t h e p resen t c a t a l y s t s i n propane hydrogeno lys is a t 523K. i n t h e r e a c t i o n . As expected f o r t h e ze ro -va len t s t a t e s i l i c a - s u p p o r t e d Co - does show s u b s t a n t i a l a c t i v i t y ; f u r the rmore i t shows h i g h s e l e c t i v i t y t o t o t a l hydrogeno lys is t o methane. I f one looks a t t h e da ta f o r t h e b i m e t a l l i c c a t a l y s t , then i t i s c l e a r t h a t a c t i v i t y i s v e r y low i n t h i s reac t i on , which suggests t h a t t h e Cu and Co a r e i n i n t i m a t e c o n t a c t and t h a t t h e su r faces o f t h e b i m e t a l l i c p a r t i c l e s must be p r e f e r e n t i a l l y Cu ( w i t h few l a r g e Co ensembles). Th is i s c o n s i s t e n t w i t h X-ray pho toe lec t ron spectroscopy evidence ( r e f . 8 ) on s i n g l e c r y s t a l s . However, even w i t h t h e b i m e t a l l i c t h e r e i s some s e l e c t i v i t y t o t o t a l hydrogeno lys is t o methane. hydrogeno lys is a l l i n d i c a t e t h a t i n t h e impregnated b i m e t a l l i c c a t a l y s t t h e Cu and Co phases a r e i n i n t i m a t e con tac t ; t h i s i s i n t r i g u i n g g i ven t h e s imp le p r e p a r a t i v e method used. Table 2 g i ves t h e s teady -s ta te Th is shows t h a t s i l i c a - s u p p o r t e d Cuo ( l i k e S i02 a lone ) has no a c t i v i t y Thus temperature-programmed reduc t i on , EDAX-microprobe a n a l y s i s and a lkane TABLE 2 A c t i v i t i e s and s e l e c t i v i t i e s i n propane hydrogeno lys is a t 523K * r a t e c a t a l y s t % conv propane 'CH4 'C2H6 - 8%Cu/Si O 2 0 0 8%Cu-4%Co/Si02 0.53 2.51 0.25 0.66 8%Co/Si O 2 9.25 2.85 0.07 11.50 * r a t e o f propane convers ion i n mmol propane/g c a t / h TABLE 3 A c t i v i t i e s and s e l e c t i v i t i e s i n CO hydrogenat ion a t 523K * ca ta 1 y s t r a t e %CH4 %C2H6 %CH30H %C2H50H 8%Cu/Si02 6.18 45.5 7.8 33.8 5.3 8%Cu-4%Co/Si02 26.2 51.3 10.8 24.0 0.0 8%Co/Si02 43.4 75.8 5.5 8.8 1.1 (SiO2) (0.96) (40.8) (37 .7 ) * r a t e i n pmol /g ca t /m in 92 RESULTS OF CATALYSIS OF CO HYDROGENATION The r e s u l t s f o r t h e a c t i v i t i e s and s e l e c t i v i t i e s o f t h e c a t a l y s t s prepared here a r e shown i n Table 3; here p a r t i c u l a r a t t e n t i o n shou ld be g i ven t o t h e e f f e c t o f hav ing t h e ze ro -va len t Cu and Co i n c o n t a c t i n t h e b i m e t a l l i c sample. These da ta aga in r e l a t e t o s teady -s ta te c o n d i t i o n s a f t e r 3-4h r e a c t i o n t ime a t 523K and 2MPa. F i r s t , t h e o v e r a l l a c t i v i t i e s o f t h e c a t a l y s t s i n terms o f t h e r a t e o f CO convers ion i s a lmost zero f o r S i02 a lone and then increases as Co i s added t o t h e s i l i c a - s u p p o r t e d Cu and i s h i g h e s t f o r t h e C o / s i l i c a . There fore f o r t h e b i m e t a l l i c c a t a l y s t i t would seem t h a t t h e r e r e a l l y a r e p a r t i c l e s c o n t a i n i n g bo th meta ls , r a t h e r than some Co p a r t i c l e s and some Cu p a r t i c l e s . Again, t h i s i s i n t r g u i n g f o r such a s imp le c a t a l y s t p r e p a r a t i v e method. Second, as t h e Co con ten t inc reases t h e s e l e c t i v i t y t o alkanes inc reases and t h i s i s e s p e c i a l l y so f o r methane. Th i rd , as t h e Co con ten t inc reases so t h e s e l e c t i v i t y t o CH30H decreases. These t rends towards methane and away from methanol a r e t o be expected i n terms o f t he g r e a t e r a b i l i t y o f Co t o d i s s o c i a t e C O and t o f a c i l i t a t e t h e methanat ion r e a c t i o n ( r e f . 9 ) . c a t a l y s t s ; t h i s must be impor tan t i n terms o f mechanisms and s i t e s i n t h e Cu-Co sample. Fourth, t h e r e i s almost no evidence f o r e thano l p roduc t i on f o r any o f these D I S C U S S I O N AND CONCLUSIONS Temperature-programmed reduc t i on , EDAX-microprobe a n a l y s i s and a lkane hydrogeno lys is a l l suggest t h a t i n these model impregnated heterogeneous c a t a l y s t s Cu and Co, when bo th p resen t i n t h e s i l i c a pores, e x i s t as l a r g e l y b i m e t a l l i c p a r t i c l e s . r a d i a l p o s i t i o n i n t h e suppor t p a r t i c l e s ; t h i s i s t h e s u b j e c t o f f u r t h e r study. It i s an impor tan t f i n d i n g here t h a t such b i m e t a l l i c p a r t i c l e s (which a re p robab ly q u i t e w e l l reduced under c o n d i t i o n s o f p re t rea tmen t here) a r e not a c t i v e i n f a c i l i t a t i n g C O hydrogenat ion t o e thano l . Alkane hydrogeno lys i h i n t s a t Cu be ing p r e f e r e n t i a l l y a t t h e b i m e t a l l i c sur faces , b u t t h i s may n o t be t h e case under c o n d i t i o n s o f CO hydrogenat ion. Us ing Cu-Co/ZnO/A1203 ( i n t h e presence o f a l k a l i ) o r w i t h o t h e r suppor t i ng ox ides ( r e f . 10) may produce Cuo-Coxt p a i r s b y s t a b i l i s i n g Co i n p o s i t i v e o x i d a t i o n s t a t e s v i a i n t e r - c a l a t i o n o f Co c a t i o n s o r f o rma t ion o f a su r face s p i n e l . c e r t a i n l y show d i f f e r e n t p r o p e r t i e s f rom those seen here and m igh t w e l l be a c t i v e i n t h e s e l e c t i v e convers ion o f CO/H2 t o e thano l . I t may however be t h a t t h e i r compos i t ion v a r i e s w i t h These would For t h e moment, t he p resen t s tudy has shown t h a t Cuo-Coo on ' i n e r t ' 93 silica are not in themselves and in isolation from support or promoter effects sites of effective production of ethanol from CO/H2. shown unequivocally using the model impregnation approach used here. Further work in which ZnO, A1203 and other oxides are added is in hand. It i s hoped that the study o f such model and well-defined precursors and catalysts will allow the critical sites and interactions in such catalysts to be defined. This has been ACKNOWLEDGEMENTS The authors gratefully acknowledge SERC and Royal Society support of MAML(Y) and YW respectively during the course of this work. REFERENCES 1 M.A.Martin-Luengo, P.A.Sermon and K.S.W.Sing 'Preparation of Catalysts 2 J.G.Nunan, C.E.Bogdan, K.Klier, K.J.Smith, C.W.Young and R.G.Herman 3 P.Courty, D.Durand, E.Freund and A.Sugier J.Molec.Cata1. 17 (1982) 241; 4 K.S.W.Sing, D.H.Everett, R.A.W.Hau1, L.Moscou, R.A.Pierotti, J.Rouquero1 5 G.C.Bond and S.Namijo J.Cata1. 118 (1989) 507 6 C.Delorine Bull.Soc.Chim.Fr. Miner.Cryst. 81 (1958) 19 7 IV' ed. B.Delmon, P.Grange, P.A.Jacobs and G.Poncelet (1987) Elsevier p.29 J.Cata1. 116 (1989) 195 H.F.Hardman and R.Beach US patent (1978) 905,703 and T.Siernieniewska Pure Appl .Chem. 57 (1985) 603 S.A.Goddard, M.D.Amiridis, J.E.Rekoske, N.C.Martinez and J.A.Dumesic J.Cata1. 117 (1989) 155; Z.Paa1, P.Tetenyi and M.Dobrovolszky React. Kin.Catal.Lett. 37 (1988) 163 8 D. C hadwi c k (pri va te communi cat i on) 9 M.A.M.Luengo (Yates) and P.A.Sermon (unpublished results) 10 M.Mouaddib and V.Perrichon Proc. 9th. Intern. Cong. Catal. 2 (1988) 521; A.J.Marchi, J.di Comino and C.R.Apestiguia Proc. 9th. Intern. Cong. Catal. 2 (1988) 529; M.Mouaddib, V.Perrichon and M.Primet J.Chem.Soc.Faraday Trans. I 85 (1989) 3413 This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 95 SELECTIW HYDROGENATION OF CYCLODODECATRIENE ISOMERS TO CYCLODODE- CENE CATALYSED BY Cu-A1203. V. Di Castro', M. Garganoz, N. Ravasio2 and M. Rossi3 Dipartimento di Chimica - Universiti3 " La Sapienza" - P.le A. Moro, 5 - 00185 Roma - Italy C.N.R. Cenm di Studio sulle Metodologie Innovative di Sintesi Organiche - Via Amendola, 173 Centro C.N.R. and Dipartimento di Chimica Inorganica e Metallorganica - Universiti di Milano - - 70126 Bari - Italy Via Venezian, 21 - 20133 Milano - Italy SUMMARY The behaviour of supported copper catalysts has been investigated in the hydrogenation of cyclododecamene to cyclododecane. The activity of these catalysts, which is influenced by the nature of oxide support and Cu(0) surface dispersion, is lower than that obtainable with conventional heterogeneous noble metal systems, whereas the selectivity is higher. In particular, Cu/A1203, prepared by chemisorption of CU(NH~)~* on alumina and preactivated in H2 atmosphere, allows high yield of cyclododecene (94%). INTRODUCTION The selective hydrogenation of mixtures of cyclododecamene (CDT) isomers, easily available through cyclotrimerization of butadiene, represents a convenient route to cis- and trans-cyclododecene (CDE). These latter compounds are of great practical importance because can be employed to prepare compounds such as 1,12-dodecandioic acid, 1,12-diaminododecane, 12-amminododecanoic acid lactam, which are monomers for polyamide manufacture, and cyclododecanecarboxilic acid, a pesticide, and its esters used as plasticizers. Highly selective catalysts are requested because the by-products, cyclododecane (CDA) and cyclododecadienes (CDD), cannot be easily separated fiom cyclododecene. Many efforts have been recently done in order to achieve this goal and different homogeneous and heterogeneous catalysts have been tested. Best results have been obtained under homogeneous conditions by using transition metal complexes. In particular the use of Co or Ru complexes, at temperature ranging between 120 and 160°C. allows CDE yields greater than 98% (ref. 1,2). According to patent literature, heterogeneous catalysts for CDE production are based on 96 transition metals, particularly Ru, Pd, Ni, Co, dispersed on different materials such as functionalised polymers, A1203, SO2, zeolites, activated carbon, ecc. . The selectivity achieved by these catalysts is generally lower than that reported for homogeneous ones and usually CDE yields are less than 90%; only in few cases higher yields are reported, as in the case of a bimetallic Pd-Co system which produces CDE up to 95%(ref. 3). The use of copper based catalysts has been scarcely explored in this hydrogenation reaction, CDE yields never exceeding 83%(ref. 4). The high selectivity obtained during our studies on the reduction of polyenes (ref. 5-7) and alkynes (ref. 8) by using supported copper catalysts prompted us to investigate the selective hydrogenation of CDT to CDE. Preliminary results have been already claimed (ref. 9). In this paper the full investigation is reported. EXPERIMENTAL SECTION Materials - Cyclododecatriene (supplied by Fluka) was distilled, purified by filtration through activated alumina and stored under nitrogen atmosphere at OOC. According to GLC analysis, the distilled product was a mixture of isomers with the following composition : trans-trans-trans-CDT = 58.7% , cis-trans-trans-CDT = 34.7% , cis-cis-trans-CDT = 2.7%, unidentified = 3.9% (probably cis-cis-cis-CDT). All oxides and salts were reagent grade products and were used without further purification. Catalysts preparation - Chemisorption method: all catalysts were prepared following a procedure similar to that used by Koritala for the CuO/Si02 (ref. 10). Ammonia was added to an aqueous solution of C U ( N O ~ ) ~ . ~ H ~ O until complete dissolution of the initially precipitated Cu(OH), occurred. The oxide support (M&) was added to the solution; the slurry, under continuous stirring, was then slowly diluted with H20. The solid was separated by filtration, washed with H20, dried over night at 1 lOOC and lastly calcined at 35OOC for 3 hrs. Coprecipitation method: all catalysts were obtained by addition of NaHC03 to a solution containing C U ( N O ~ ) ~ . ~ H ~ O and the metal nitrate corresponding to the oxide support. The solid was washed with H20, in order to eliminate Na+, dried over night at llO°C, then calcined at 35OOC for 3 hrs. Catalysts reduction - The reduction was carried out in glass reactors provided with a side arm connected to a sampler. All the catalysts were heated for 1 hr under vacuum to 27OOC before reductive treatment. Reduction with H2 at 100 kPa was carried out either at constant temperature (270°C) or increasing the temperature, from 100 to 20O0C, at 2OC/min. Cu(0) specific area determination - The Cu(0) surface area was determined by using the 97 N20 chemisorption method (ref. 11). The measurements were carried out on samples of reduced catalysts by using a Carlo Erba Sorptomatic 1800 automatic gas burette . N20 (75 Wa) was introduced in the sampler and allowed to react for 0.5 hrs at 37OC. Then the gas in the sampler was analysed for N2 content. The Cu(0) specific area was computed by using a surface coverage factor ( moles of oxygen atoms per moles of surface Cu(0) atoms) 8 = 0.35 and a mean surface area for a copper site of 7.41 A2(ref. 11). Samples for ESCA determinations were stored under inert atmosphere in sealed vials and analysed with a VG ESCA 3 spectrometer employing an A1 K, source (hv = 1486.6 eV) (ref. 12). Hydrogenation reactions - These were performed in glass reactors or, when H2 pressure was greater than 100 Wa, in stainless AISI 316 autoclave (60 ml) . The rate of H2 uptake was measured by using a Brooks electronic mass flow meter connected by an AD interface to an IBM XT personal computer. Analysis - The products composition was determined by GLC with a 50 m x 0.2 mm Carbowax 20M capillary column using a Hewlett Packard 5880A gas chromatograph equipped with a flame ionization detector. N2 from N20 chemisorption was analysed by GC with a 3 m x 2 mm stainless steel column packed with Carbosieve S using a Carlo Erba Fractovap gas chromatograph provided with a thermal conductivity detector. RESULTS AND DISCUSSION The hydrogenation of CDT catalysed by copper dispersed on different oxides, in the initial step of the reaction, follows in all cases a first order kinetic respect to CDT concentration. However, the activities of the catalysts are quite different. As shown in table 1, in fact, initial turnover number ranging from 0 to137 h-' were obtained. In all experiment the CDE produced was a mixture of trans- (65%) and cis-isomers (35%); the cislrruns ratio was independent from the catalyst, indicating that the same hydrogenation mechanism was operating for all the active systems. The wide range of catalytic activities showed by different catalysts can be related both to a different dispersion of the active copper species on the surface of the catalytic material and to the different cooperative effect of oxide support. As reported in experimental section, all catalytic systems were prepared by reduction in hydrogen atmosphere of CuO/Mfly precursors; ESCA data (ref.5,6,12) showed that this pretreatment causes the almost quantitative reduction of copper to Cu(0). This species , as suggested in our previous work, plays an essential role in promoting the hydrogenation reaction, 98 particularly with cyclic polyenes (ref.5-7). TABLE 1 Hydrogenation of Cyclododecamene catalysed by Cu/M,Oy systemsa Catal? Pr.C %Cu Red.temp. Tvd nHdnCDT Reaction products (%moles) (OC) (h-9 CDE CDD CDA Cu/Alz03 I Cu/A1203 C Cu/Si02 I Cu/Cr203g C C a n 0 C C Cu/MnOz I I Cum203 I cu/Mgo I Othersh I 7.0 9.5 7.0 37 8.4 8.7 8.7 6.3 7.4 270 100-270 100-200 270 100-200 270 130 9 67 16 122 77 137 < 5 40 86 < 5 2.00 1.37 1.87 2.03 1.94 2.00 94.0 45.1 83.0 90.7 93.0 93.6 91.0 90.0 90.8 3.0 3.0 42.0 1.7 12.7 2.3 5.9 3.4 3.5 3.5 3.2 3.2 4.5 4.5 7.8 2.2 4.6 4.6 a P H ~ = 100 KPa ; T = 18OOC ; CDT/Cu molar ratio = 100. All system were derived by corresponding CuO/Mfly by reduction in H2 atmosphere. Preparation method, C = coprecipitation, I = chemisorption. Turnover number at the beginning of the reaction in moles of Hz consumed per mole of copper per hr. Moles of Hz consumed per mole of CDT whcll the reaction was stopped. 2 % ca. of by-products are f m e d during the reaction. g Derived by commercial copper chromite catalyst by reduction. prepared by reduction of CuO supported on TiO,, BeO, SnOz, ZrOz,CeOz, TazOs, ZnO, CdO, Crz03,Nb0. The values of specific Cu(0) surface area (SCU(O)), determined by means of NzO chemisorption, are reported in table 2 for different catalysts. A comparison between SCU(O) and tumover number indicates that the nature of oxide support has a predominant influence on the activity of the catalytic system. In fact, Cu/A1203 shows a higher tumover number respect to Cu/SiOz, Cu/ZnO, Cu/MgO in spite of a lower Scu(0). In particular, the best results were obtained using Cu/A1203. This catalyst, which was prepared reducing in hydrogen atmosphere a material obtained by chemisorption of Cu(NH3)4* on alumina, showed best selectivity (CDE yield = 94% when 2 moles of Hz were consumed per mole of CDT) and high activity (Tv = 130 h-l). Also interesting for their activity and selectivity are Cu/MnOz/ and C a n 0 which exhibited activity comparable with that of Cu/A1203 and selectivity only slightly lower. Cu/MgO, Cu/Y203 and Cu/Cr203 , although less active, allowed also a CDE yield 2 90%. By using Cu/Si02 we observed a gradual loss of activity which didn't allow the reaction to go to completion. This behaviour was probably due to the acidity of support ; the catalyst 99 surface being slowly poisoned by reaction by-product. The systems Cu/riOz, Cu/BeO, Cu/Sn02, CuiZrOz, Cu/CeOz, Cu/Ta205, C a n O , Cu/CdO, Cu/Cr,03 and Cu/NbO, prepared by the same method of Cu/AI,O,, were all almost inactive. The dependence of the activity from the nature of support indicates a strong interaction between copper and oxide support; this latter playing a role during the molecular Hz activation. This is in agreement with the literature concerning the interaction of H, with copper chromite and copper aluminate (ref. 13,14). However, for catalysts with the same support, SCU(O) strongly influences the catalytic activity: conditions that increase the Cu(0) dispersion also increase catalyst performance. Thus the higher turnover numbers observed with CuOEnO pre-reduced at low temperature (100-200 "C), respect to that of the same catalyst pre-reduced at high temperature (270 "C), is related to the change of SCU(O) . The growth of the copper particles on the surface and the consequent diminution of Cu(0) dispersion by increasing the pre-reduction temperature occur also for other supports (examples for Cu/A1203 and Cu/SiO, are reported in tab. 2). TABLE 2 Cu(0) specific surface area of CUFlxOy systemsa Catal. pr.b %Cu Red.temp. Sup.Cu(0)C Disper? ( " 0 (m2/gcu) Cu/A1203 Cu/A1203 Cu/SiOz Cu/SiO, CuEnO I I I C I I C C I I I I 7.0 9.5 7.0 8.4 7.8 7.4 7.9 100-200 270 350 270 100-200 270 100-200 100-270 270 100-200 100-200 91 64 45 7 180 210 74 135 11 76 17 100 diffusion of the copper in the bulk of oxide support which produces a very low dispersion of Cu(0). The method employed for supporting copper also influences Cu(0) dispersion and the consequent catalyst activity. Thus, in the case of C a n 0 and Cu/Cr203, the coprecipitation method produces active catalysts whereas the chemisorption method produces almost inactive materials. On the contrary, Cu/A1203 is best prepared with the chemisorption method (tab.1). The relation between preparation method and activity can be explained, once again, in term of Cu(0) dispersion (tab. 2). In our conditions, in fact, all systems with a Cu(0) dispersion below 0.02 were almost inactive regardless of the oxide support and preparation method. The relation between total copper concentration and activity was studied for the Cu/A1203 catalyst. In table 3 are reported the turnover number and SCU(O) value for a series of catalysts with copper content ranging from 3.1 to 14.5%. TABLE 3 Turnover numbersa and Cu(0) specific area for Cu/A1203b systems with a different Cu content % cu SCu(0) SCU(0)d TvSe Tv 3.1 141 139 540 108 4.7 105 88 670 101 6.5 95 93 858 110 8.4 85 900 100 9.8 65 53 720 65 14.5 47 49 595 42 = 300 kPa; T = 1WC. CDT/Cu molar ratio = 100. Re-reduction temp. = 100 - 2WC. Initial specific turnover in Derived by N20 chemisorption. Derived by ESCA method. moles of H2 consumed per mole of surface Cu(0) (derived by N20 chemisorption) per hr. In order to exclude the presence of chemisorbed hydrogen (ref 14), which produces overestimated values when the Cu(0) specific area is determined by N20 chemisorption, we have also determined these values from ESCA data. In particular we have derived specific surface area of Cu(0) from Cu 2p signal intensity ratio (ref. 12). The SCU(O) values derived by means the two different techniques show to be in agreement within a 15% (tab. 3, col. 2 and 3) and indicate that reduced catalyst doesn't contain significative amounts of chemisorbed hydrogen. From the data in table 3, a progressive decrease of SCU(O) is observed as copper loading increases. Since the temperature used for pre-reduction should avoid the diffusion of copper in the bulk (ref. 16), this trend indicate an increment of average size of Cu(0) crystallite (from 2 to 7 101 nm (ref. 11)). The growth of Cu(0) particles scarcely influences the turnover number referred to the total copper (Tv) for catalysts with a copper loading less than 8.4%. This behaviour is apparently in contrast with the great variation of SCU(O), but can be explained by considering that the specific surface turnover (TvS) does not show a constant value but a maximum at c.a. 8% (tab 3 col. 4). The lowering of TvS at the extremities of the concentration range can be due to either higher effect of poisoning (low copper loading) or a less favourable surface ratio between copper and alumina (high copper loading). Finally, with the aim to optimize the performance of the Cu/A1203 systems, we have studied the influence of the operative conditions on the reaction selectivity. TABLE 4 Hydrogenation of Cyclododecatriene catalysed by Cu/A1203 a Catal. TPC) P(kPa) Tv(h-') Reaction Products (mole%)c CDE CDD CDE 100 130 94 3 3 133 92 4 4 200 140 180 250 207 86 7 7 160 300 90 91 4.5 4.5 C~/A120, 180 7 C U O / A ~ ~ O ~ ~ 180 100 102 REFERENCES 1 A. Misono and I. Ogata, Bull. Chem. Soc. Jap., 40 (1967) 2718 2 D.R. Fahey, J. Org. Chem., 38(1) (1973) 80 3 K. Kihara, K. Katsuragawa and K. Yoshimitsu, Jap. Pat. 7479,990 - C.A. 83:27714m 4 N.V. Unilever Neth. Pat. 6,608,993 - C.A. 675393511 5 C. Fragale, M. Gargano, M. Rossi, J. Am. Oil Chem. Soc., 59 (1982) 465 6 C. Fragale, M. Gargano, N. Ravasio, M. Rossi, I. Santo, Inorg. Chim. Acta, 82 (1984) 157 7 C. Fragale, M. Gargano, N. Ravasio and M. Rossi, Gazz. Chim. Ital., I17 (1987) 43 8 M. Gargano, N. Ravasio. M. Rossi, I. Santo, La Chimica e l'Indusma, 694 (1987) 1. 9 C. Fragale, M. Gargano, M. Rossi, Italian Patent 24437 A/82 10 S. Koritala, J. Am. Oil Chem. Soc., 49 (1972) 83 11 T.J. Osinga, B.G. Linsen, W.P. van Beek, J. Catal., 7 12 V. Di Castro, C. Furlani, M. Gargano, N.Ravasio, M. Rossi., I. Electr. Spectr. (1990) in press 13 R. Bechara, G. Wrobel, M. Daage, J.P. Bonnelle. Appl. Catal., 16 (1985) 15 14 L. Jalowiecki, G. Wrobel, M. Daage, J.P. Bonnelle, J. Catal., 107 (1987) 375 15 V. Di Castro, C. Furlani, M. Gargano, M. Rossi, Appl. Surf. Science 28 (1987) 270 16 B.R. Strohmeier, D.E. Leyden, R.S. Field, D.M. Hercules, J. Catal., 94 (1985) 514. (1976) 277 G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catulysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in T h e Netherlands 103 PREPARATION AND CHARACTERIZATION OF HIGHLY SELECTIVE Fe-Cu/Si02 CATALYSTS FOR PARTIAL HYDROGENATION OF ALKYNES Yur iko NITTA, Yoshifumi HIRAMATSU, Yasuaki OKAMOTO and Toshinobu IMANAKA Department of Chemical E n g i n e e r i n g , F a c u l t y of E n g i n e e r i n g S c i e n c e , Osaka U n i v e r s i t y , Toyonaka, Osaka 560, J a p a n SUMMARY A h i g h l y s e l e c t i v e Fe-Cu/Si02 c a t a l y s t w i t h a n enhanced a c t i v i t y f o r p a r t i a l h y d r o g e n a t i o n of p h e n y l a c e t y l e n e w a s p r e p a r e d by c o p r e c i p i t a t i o n of i r o n and c o p p e r s u l f a t e s o v e r u l t r a f i n e s i l i c a g e l w i t h a n e x c e s s a m o u n t of s o d i u m c a r b o n a t e a t a l o w t e m p e r a t u r e . The effects o f t h e p r e c i p i t a t i o n v a r i a b l e s and t h e d o p i n g o f c o p p e r and s u l f a t e i o n s on t h e r e d u c t i o n b e h a v i o r , s u r f a c e states, and c a t a l y t i c p r o p e r t i e s of t h e p r e c u r s o r s a n d / o r r e d u c e d c a t a l y s t s were s t u d i e d by u s i n g TGA, XRD, and XPS. The p r e s e n c e of c o p p e r lowers t h e d e c o m p o s i t i o n and r e d u c t i o n t e m p e r a t u r e s of a-FeOOH t o a-Fe20g and Fe2+, t h u s d e c r e a s i n g t h e amount o f water produced a t h i g h e r t e m p e r a t u r e s and p r e v e n t i n g t h e s i n t e r i n g of i r o n m e t a l . The l o w p r e c i p i t a t i o n t e m p e r a t u r e and t h e u s e of metal s u l f a t e s are b o t h e f f e c t i v e t o p r o v i d e h i g h l y - d i s p e r s e d metal s p e c i e s i n t h e p r e c u r s o r , which r e s u l t i n a n i n c r e a s e i n t h e i r o n d i s p e r s i o n o f t h e r e d u c e d c a t a l y s t . INTRODUCTION I r o n catalysts show s p e c i f i c a c t i v i t y and s e l e c t i v i t y for t h e h y d r o g e n a t i o n of c a r b o n monoxide. For p a r t i a l h y d r o g e n a t i o n o f a l k y n e s , a n a l o g o u s l y p r e p a r e d i r o n catalysts e x h i b i t h i g h s e l e c t i v i t y (refs. 1-5), b u t e x t r e m e l y l o w a c t i v i t y , which h i n d e r s p r a c t i c a l u s a g e of i ron-based c a t a l y s t s . Development o f i r o n c a t a l y s t s w i t h h i g h a c t i v i t y for a l k y n e h y d r o g e n a t i o n is r e q u i r e d f o r i n d u s t r i a l a p p l i c a t i o n s . R e c e n t l y , w e have r e p o r t e d t h a t , i n t h e p r e p a r a t i o n of Fe /Si02 c a t a l y s t s , t h e u s e o f f e r r i c s u l f a t e , non-porous u l t r a f i n e s i l i ca g e l , and a n e x c e s s amount of Na2C03 r e s u l t s i n a n a c t i v e and s e l e c t i v e catalyst f o r t h e h y d r o g e n a t i o n o f p h e n y l a c e t y l e n e t o s t y r e n e under m i l d r e a c t i o n c o n d i t i o n s , and t h a t t h e s e l e c t i v i t y f o r s t y r e n e i n c r e a s e s w i t h i n c r e a s i n g d i s p e r s i o n of i r o n m e t a l ( r e f . 6 ) . I n o r d e r t o i n c r e a s e t h e d i s p e r s i o n of i r o n , and hence t o i n c r e a s e t h e s e l e c t i v i t y as w e l l as t h e a c t i v i t y o f t h e c a t a l y s t , t h e e f f e c t of c o p r e c i p i t a t i o n of d i f f e r e n t metals w a s examined. The a d d i t i o n of p r o p e r amount of copper w a s found t o b r i n g a b o u t r e m a r k a b l e i n c r e a s e s i n t h e a c t i v i t y and s e l e c t i v i t y of t h e r e s u l t i n g c a t a l y s t s ( r e f . 7 ) . Moreover , s i m u l t a n e o u s a d d i t i o n o f copper and s u l f a t e i o n s and a l o w p r e c i p i t a t i o n t e m p e r a t u r e a p p e a r e d e s s e n t i a l t o t h e p r e p a r a t i o n o f a h i g h l y a c t i v e and s e l e c t i v e c a t a l y s t . I n t h i s work, t h e e f f e c t s o f t h e p r e p a r a t i o n v a r i a b l e s and t h e doping o f copper and s u l f a t e i o n s on t h e p r o p e r t i e s o f F e / S i 0 2 (1:l) catalysts were s t u d i e d i n d e t a i l by u s i n g TGA, XRD, and XPS. 104 EXPERIMENTAL C a t a l y s t p r e p a r a t i o n C a t a l y s t p r e c u r s o r s were p r e p a r e d by a p r e c i p i t a t i o n method as f o l l o w s : A s o l u t i o n o f a n e x c e s s amount of Na2C03 (1.5-1.7 molar e q u i v a l e n t ) i n 25 m l of d i s t i l l e d water was added t o a n aqueous s u s p e n s i o n (150 m l ) c o n t a i n i n g 2 g of non-porous u l t r a f i n e s i l i c a g e l (Nippon A e r o s i l , Aeros i l -300 , 320 m'g-l), i r o n ( l I 1 ) s u l f a t e , and c o p p e r ( l 1 ) s u l f a t e ( t o t a l amount o f metal = 36 mmol) o v e r a p e r i o d of 1-2 min under v i g o r o u s s t i r r i n g . The p r e c i p i t a t i o n t e m p e r a t u r e w a s changed i n t h e r a n g e of 8 t o 8 0 ° C . The p r e c i p i t a t e w a s aged f o r 15 min u s u a l l y a t 7 5 4 0 ° C and f i l t e r e d , f o l l o w e d by washing t h r e e times w i t h h o t water a n d d r y i n g a t 110°C f o r 20 h . O t h e r d e t a i l s were i d e n t i c a l t o t h o s e d e s c r i b e d f o r the p r e p a r a t i o n o f s i l i c a - s u p p o r t e d b a s i c n i c k e l c a r b o n a t e ( r e f s . 8 , 9 ) . A f e w p r e c u r s o r s were p r e p a r e d by u s i n g i r o n ( I I 1 ) and c o p p e r ( I 1 ) n i t r a t e s f o r comparison. Fe-Cu/Si02 c a t a l y s t s (metal l o a d i n g = ca. 50 wt%) were p r e p a r e d by h e a t i n g d r i e d p r e c u r s o r s (1 g ) i n f l o w i n g hydrogen ( 8 1 h-l ) up t o 500°C a t a t e m p e r a t u r e ramp of 10°C min-' and h o l d i n g a t t h i s t e m p e r a t u r e for 1 h u n l e s s o t h e r w i s e n o t e d . C a t a l y s t s a r e d e n o t e d by t h e a t o m i c c o m p o s i t i o n a n d t h e p r e p a r a t i o n ( p r e c i p i t a t i o n and a g i n g ) t e m p e r a t u r e s such as F7C3-LH f o r t h e Fe-Cu(7:3)/Si02 c a t a l y s t p r e c i p i t a t e d a t a low t e m p e r a t u r e ( u s u a l l y , 20°C) and s u b s e q u e n t l y aged a t a h i g h t e m p e r a t u r e ( u s u a l l y , 75-80%). Hydrogenat ion The h y d r o g e n a t i o n o f p h e n y l a c e t y l e n e (0.6 m l ) w a s c a r r i e d o u t i n e t h a n o l (27 m l ) w i t h a f r e s h l y - r e d u c e d c a t a l y s t a t 60°C under a hydrogen p r e s s u r e of 1 MPa by u s i n g a g l a s s a u t o c l a v e e q u i p p e d w i t h a m a g n e t i c s t i r r i n g s y s t e m . The i n i t i a l h y d r o g e n a t i o n rates (Ro) and s t y r e n e s e l e c t i v i t i e s , d e f i n e d as t h e molar p e r c e n t a g e of s t y r e n e i n a l l t h e p r o d u c t s , a t 50% c o n v e r s i o n (S50) were o b t a i n e d from t h e GLC d a t a o f t h e p r o d u c t s a t d i f f e r e n t r e a c t i o n times. A n a l y s i s Powder X-ray d i f f r a c t i o n (XRD) p a t t e r n s o f t h e c a t a l y s t s s e p a r a t e d from t h e r e a c t i o n m i x t u r e w e r e measured u s i n g a Shimadzu VD-1 d i f f r a c t o m e t e r w i t h C u b r a d i a t i o n . T h e mean c r y s t a l l i t e s i z e (D,) o f i r o n metal i n a c a t a l y s t w a s c a l c u l a t e d f rom t h e half-maximum b r e a d t h o f t h e (110) peak of a-Fe after correc- t i o n f o r i n s t r u m e n t a l b roadening ( r e f . 10). The c r y s t a l l i t e s i z e d i s t r i b u t i o n s (CSD) of i r o n i n some c a t a l y s t s were o b t a i n e d a c c o r d i n g t o t h e F o u r i e r t r a n s f o r m method ( S t o k e s method) for X-ray l i n e p r o f i l e a n a l y s i s ( r e f . 11). Thermogravimet r ic a n a l y s e s (TGA) and d i f f e r e n t i a l t h e r m a l a n a l y s e s (DTA) of c a t a l y s t p r e c u r s o r s were c a r r i e d o u t w i t h a Shimadzu DT-30 t h e r m a l a n a l y z e r by h e a t i n g i n a stream of h y d r o g e n t o 800°C a t a of 1 0 ° C m i n - l u n l e s s o t h e r w i s e n o t e d . The d e g r e e o f r e d u c t i o n o f a catalyst w a s e s t i m a t e d from t h e r a t e 105 weight l o s s i n t h e r a n g e above 100°C on t h e b a s i s of t h e t h e o r e t i c a l w e i g h t l o s s f o r 100% r e d u c t i o n of t h e p r e c u r s o r . The X-ray p h o t o e l e c t r o n s p e c t r a (XPS) of some p r e c u r s o r s and f r e s h l y - r e d u c e d c a t a l y s t s were measured on a H i t a c h i 507 p h o t o e l e c t r o n s p e c t r o m e t e r u s i n g a n A 1 a n o d e . T h e s a m p l e w a s m o u n t e d o n a d o u b l e - s i d e d a d h e s i v e t a p e . B i n d i n g e n e r g i e s (BE) were r e f e r e n c e d t o t h e S i ( 2 p ) band a t 103.1 eV due t o t h e c a t a l y s t s u p p o r t . The peak areas were used f o r a q u a n t i t a t i v e s u r f a c e a n a l y s i s of t h e sample. D e t a i l e d p r o c e d u r e s have been d e s c r i b e d e l s e w h e r e ( r e f . 1 2 ) . RESULTS R e d u c t i o n b e h a v i o r of copper-promoted p r e c u r s o r s The d i f f e r e n t i a l t h e r m o g r a v i m e t r i c (DTG) p r o f i l e s i n F i g . 1 i n d i c a t e t h a t t h e r e d u c t i o n of Fe-Cu/Si02 p r e c u r s o r s v i r t u a l l y p r o c e e d s i n two s t e p s , v i z . , a s h a r p peak a t a b o u t 200°C and a broad peak a t a round 500OC. On t h e b a s i s of t h e s e p a r a t e DTG a n d DTA e x p e r i m e n t s i n H2 a n d N2 f o r C u / S i 0 2 a n d F e / S i 0 2 p r e c u r s o r s , it was concluded t h a t t h e s h a r p DTG peak a t around 200°C c o r r e s p o n d s t o a r a p i d w e i g h t l o s s due t o t h e r e d u c t i o n of Cu2+ t o Cuo s u p e r p o s e d by a broad peak due t o t h e decomposi t ion of a-FeOOH t o a-Fe203, and t h a t t h e broad peak a t a r o u n d 5 0 0 ° C c o r r e s p o n d s t o t h e r e d u c t i o n of i r o n o x i d e s o r s i l i ca t e t o metallic i r o n . T h i s is c o n s i s t e n t w i t h t h e r e s u l t s i n l i t e r a t u r e ( r e f s . 1 4 , 1 5 ) . T h e p e r c e n t a g e weight l o s s e s i n t h e r a n g e of 100-300 "C ( A , ) a n d a b o v e 3 O O O C ( A , ) a re l i s t e d i n T a b l e 1, t o g e t h e r w i t h t h e h y d r o g e n a t i o n r e s u l t s on t h e reduced c a t a l y s t s . I t i s notewor thy t h a t t h e s h a r p DTG peak due t o Cu2+ r e d u c t i o n s h i f t e d t o a lower t e m p e r a t u r e f o r t h e m o s t a c t i v e a n d s e l e c t i v e catalyst c o n t a i n i n g ca. 30% Cu. I n a d d i t i o n , t h e w e i g h t l o s s above 300 "C (A;) for t h e F7C3-HH catalyst w a s c l o s e t o a t h e o r e t i c a l v a l u e , 9.0%, f o r t h e comple te r e d u c t i o n of Fez+ t o F e o ( F e O - C u / S i 0 2 + F e - C u / S i 0 2 ) , w h e r e a s t h e w e i g h t l o s s e s f o r t h e o t h e r c a t a l y s t s w e r e l a r g e r t h a n those e x p e c t e d , i n d i c a t i n g t h a t p a r t of Fe3+ s p e c i e s still e x i s t s i n t h e s e c u 100% - c u 90% Cu 70% c u 20% c u 5% c u 0% I I I I 100 200 300 400 500 600 Tempera ture / "C F i g . 1. DTG-in-H2 p r o f i l e s of Fe-Cu/Si02 p r e c u r s o r s w i t h d i f f e r e n t c o n t e n t o f c o p p e r . 106 TABLE 1 E f f e c t s of copper c o n t e n t on t h e r e d u c t i o n b e h a v i o r of Fe-Cu/Si02 p r e c u r s o r s a and on t h e c a t a l y t i c p r o p e r t i e s a f t e r r e d u c t i o n a t 500°C f o r 1 h. r0 '50 Catalyst Content Tcu Weight l o s s c D.R.d Dce o f CU/% "C A,/% A2/% A2'/% % nm mmol-min-lg-l z F10-HH 0 F9.5CO. 5-HH 5 F9C1-HH 10 F8C2-HH 20 F7C3-HH 30 F6C4-HH 40 F5C5-HH 50 F4C6-HH 60 F3C7-HH 70 FlC9-HH 90 c10-HHg 100 227 200 185 183 183 184 189 190 212 190-220 10.1 16.8 18.6 13.1 10.9 12.5 14.2 10.3 12.0 15.7 8.9 10.6 15.8 7 . 8 9.2 16 .9 7.3 8.8 17.2 5.8 6.9 17 .8 5.6 6.9 18.6 5.4 6.6 22.8 4.6 6.0 22.9 1.9 2 .5 95.2 19 93.5 20 93 .4 17 93.1 15 98.2 12 88 .2 14 83.1 15 90.6 16 91.9 17 91.7 93.1 1 . 9 ~ 1 0 - ~ 96.5 3.3x10-' 9 7 . 3 4.9x10-' 97.6 7.3x10-' 98.9 1. 9x1OF1 99.5 1. 0x10-1 99.1 7.9x10-' 98.8 8 . 2 ~ 1 0 - ~ 93.3f 4 . 5 ~ 1 0 ~ ~ 89.5f 7 . 3 ~ 1 0 - ~ 98.4f 1.2x10-3 94.6f ~~~~~~ a Tempera tures of p r e c i p i t a t i o n and a g i n g are both 75°C. b Peak t e m p e r a t u r e f o r r e d u c t i o n of Cu*+spec ies . c P e r c e n t a g e weight l o s s e s d u r i n g 100-3OO0C( AI) and above 3OO0C(A2), and t h e d Degree o f r e d u c t i o n measured by TGA i n H2. e Mean c r y s t a l l i t e size of w F e . f S e l e c t i v i t y a t 10% c o n v e r s i o n . g Reduced at 300°C f o r 1 h i n s t e a d of 500'C. p e r c e n t a g e l o s s based on t h e weight a t 3OO0C(A;). TABLE 2 E f f e c t s of p r e c i p i t a t i o n v a r i a b l e s on t h e p r o p e r t i e s of Fe-Cu(7:3)/Si02 and Fe /Si02 p r e c u r s o r s and t h e reduced c a t a l y s t s . r0 '50 C a t a l y s t S t a r t i n g Temperature/OC S B E ~ Tcu D.R. Dc s a l t P p t . Aging m2g-l "C % nm mmol.min-lg-l % F7C3-HH F7C3-MH F7C3-LH F7C3-LHa F7C3-LHb F7C3-L H F7C3-LL. F7C3-HH(N) F7C3-LH( N) F10-HH F10-LH F 10-L ' L FlO-HH(N) S u l f a t e S u l f a t e S u l f a t e S u l f a t e S u l f a t e S u l f a t e S u l f a t e Nitrate Nitrate S u l f a t e S u l f a t e S u l f a t e Nitrate 81 55 20 20 20 8 20 80 20 75 20 9 75 81 80 80 80 80 80 20 80 80 75 83 9 75 463 43 1 535 485 46 2 437 484 428 833 508 408 183 98.2 183 97.9 172 98.7 100 100 170 97 .1 178 94.5 183 107 c a t a l y s t s a t 300°C. T h e s e r e d u c t i o n p r o c e s s e s were conf i rmed by XPS as s t a t e d below. The above r e s u l t s s u g g e s t t h a t t h e lower t h e r e d u c t i o n t e m p e r a t u r e s of Cu2+ a n d a-FeOOH a r e , t h e m o r e a c t i v e t h e reduced catalyst becomes. ~- E f f e c t s o f p r e c i p i t a t i o n v a r i a b l e s I n t h e c o u r s e o f t h e s t u d i e s o n p r e p a r a t i o n c o n d i t i o n s , i t was found t h a t t h e p r e c i p i t a t i o n t e m p e r a t u r e had a g r e a t e f f e c t on t h e c a t a l y t i c p r o p e r t i e s of t h e r e s u l t i n g ca ta lys t . The p r o p e r t i e s o f Fe- Cu/Si02 and F e / S i 0 2 p r e c u r s o r s p r e p a r e d a t d i f f e r e n t p r e c i p i t a t i o n a n d / o r a g i n g t e m p e r a t u r e s a r e l i s t e d i n T a b l e 2 , t o g e t h e r w i t h t h e d a t a f o r t h e c a t a l y s t s p r e p a r e d f rom metal n i t r a t e s . I r r e s p e c t i v e o f t h e s t a r t i n g s a l t , t h e F e - C u / S i 0 2 p r e c u r s o r s p r e c i p i t a t e d a t a round 2OoC had l a r g e r BET s u r f a c e a r e a s w i t h f l o s s y t e x t u r e and e x h i b i t e d h i g h e r h y d r o g e n a t i o n a c t i v i t i e s , w h e n r e d u c e d , t h a n t h o s e p r e c i p i t a t e d a t 55-80°C. W i t h t h e a g i n g p r o c e s s , t h e h i g h t e m p e r a t u r e (75-80°C) w a s p r e f e r a b l e . I n t h e case of t h e c o p p e r - f r e e c a t a l y s t s , however, t h e l o w p r e c i p i t a t i o n t e m p e r a t u r e h a d n e g a t i v e e f f e c t s on t h e r e d u c i b i l i t y of t h e p r e c u r s o r s and on t h e c a t a l y t i c p r o p e r t i e s . As c a n b e s e e n f r o m F i g . 2, t h e DTG peak a t a round 200°C f o r t h e most a c t i v e c a t a l y s t , F7C3-LH, s h i f t e d f u r t h e r t o a lower t e m p e r a t u r e t h a n t h a t f o r F7C3-HH. The DTG p r o f i l e s measured a f t e r h e a t i n g a t 3OOOC for 3 h i n a n N2 stream, which have 100 300 500 Tempera ture / "C F i g . 2. DTG p r o f i l e s of F7C3-LH and -HH p r e c u r s o r s : ( A ) a s pre- p a r e d , ( B ) a f t e r h e a t e d a t 300°C f o r 3 . h i n N2, and ( C ) measured i n N 2 . I .", i 1 F7C3-LH Dc / nm F i g . 3. CSDs of a-Fe i n F7C3-LH and F7C3-LH(N) ca ta lys t s . no s u p e r p o s i t i o n of t h e decomposi t ion p r o f i l e s of a-FeOOH, show more c l e a r l y t h a t t h e r e d u c t i o n of CuO i n F7C3-LH p r e c u r s o r o c c u r s a t a t e m p e r a t u r e 30-50°C lower t h a n t h a t i n F7C3-HH p r e c u r s o r . The d i f f e r e n c e i n t h e d e c o m p o s i t i o n t e m p e r a t u r e of a-FeCQH t o a-Fe203 w a s less t h a n 10°C between F7C3-HH and F7C3-LH a s r e v e a l e d i n t h e p r o f i l e s m e a s u r e d i n a n N 2 s tream. T h e r e f o r e , t h e 108 r e d u c i b i l i t y of Cu2+ i n t h e p r e c u r s o r seems t o have a s t r i k i n g e f f e c t on t h e c a t a l y t i c p r o p e r t i e s of t h e reduced c a t a l y s t . The e f f e c t of t h e s t a r t i n g sa l t is also s i g n i f i c a n t . The F7C3-LH catalyst p r e p a r e d f rom s u l f a t e s e x h i b i t e d a l m o s t f i v e times h i g h e r a c t i v i t y t h a n t h e c a t a l y s t p r e p a r e d from n i t r a t e s , F7C3-LH(N), a l t h o u g h t h e y had similar v a l u e s o f Dc. The CSDs of a-Fe i n t h e s e c a t a l y s t s are c o m p a r e d i n F i g . 3. E v i d e n t l y , t he p r e s e n c e o f v e r y small c r y s t a l l i t e s of i r o n e x p l a i n s t h e h i g h a c t i v i t y of t h e F7C3-LH c a t a l y s t . The i n c r e a s e d r e d u c t i o n t e m p e r a t u r e of Cu2' f o r F7C3-LH(N) r e n d e r s f u r t h e r e v i d e n c e t o t h e i m p o r t a n c e of t h e r e d u c i b i l i t y of Cu2+ i n t h e p r e c u r s o r . XPS s t u d i e s of v a r i o u s p r e c u r s o r s and reduced c a t a l y s t s _-_- The XPS p a r a m e t e r s f o r v a r i o u s p r e c u r s o r s and the c a t a l y s t s reduced a t 300°C f o r 0.5 h o r a t 500°C f o r 1 h are summarized i n T a b l e 3. The s p e c t r a of t h e Fe2p l e v e l f o r the F7C3-LH p r e c u r s o r and the r e d u c e d s a m p l e s are shown i n F ig .4 . TABLE 3 XPS b i n d i n g e n e r g i e s (BE), k i n e t i c e n e r g i e s (KE), and i n t e n s i t y ra t ios f o r Fe-Cu/Si02 p r e c u r s o r s and t h e reduced c a t a l y s t s . Catalyst R e d u c t i o n BE/eV KE/eV BE/eV ea 2 Temp/"C Time/h Fe2p3/, C ~ 2 p ~ / , ( S a t e ) ~ Cu(LMM) S2p S i S i S i F10-HH F9C1-HH F8C2-HH F7C3-HH F7C3-LH F7C3-LH(N) F6C4-HH F5C5-HH F3C7-HH FlC9-HH c10-HH F7C3-HH F 7 C 3-LH F10-HH F10-LH F10-HH F8C2-HH F7C3-HH F7C3-LH F7C3-LH(N) F5C5-HH F3C7-HH C10-HH 300 300 300 300 500 500 500 500 500 500 500 300 0.5 0.5 0.5 0.5 710.9 711.5 712.1 711.4 711.6 712.0 711.7 711.5 712.7 713.5 710.7 710.0 711.4 710.7 934.1 (942.4) 935.5 (943.2) 935.1 (943.1) 934.4 (942.2) 934.0 (941.9) 935.0 (943.1) 934.5 (942.5) 935.5 (943.2) 935.2 (942.7) 935.7 (943.6) 932.2 931.9 710.0 706.7 710.4 706.9 932.1 710.4 706.9 931.7 710.5 707.0 931.9 710.6 707.1 931.9 710.9 706.5 931.9 710.0 707.0 931.7 932.0 C 916.0 c 916.0 c 915.5 c 917.0 c 915.5 c 916.1 c 916.6 c 916.0 169.5 915.9 168.9 915.6 169.9 915.5 c 919.3 c c C C 918.9 c 919.0 c 919.1 c 919.5 c 919.0 c 919.3 161.6 919.1 169.0 2.31 4.74 0.48 5.09 1.01 5.93 1.62 8.86 2.34 8.20 1.72 4.79 2.29 4.16 3.39 3.47 4.37 0.12 2.36 4.86 0.16 12.69 0.25 5.72 1.75 5.67 0.54 5.82 5.21 0.91 1.61 0.30 2.00 0.43 2.84 0.79 2.94 0.58 1.61 0.82 2.87 1.23 0.04 2.60 0.10 a I n t e n s i t y r a t i o : FeZp, Cu2p3/2, S2p, and S i 2 p . b S a t e l l i t e peak. c Not d e t e c t e d . 109 The b i n d i n g e n e r g i e s (BE) o f t h e F e ( 2 p 3 / 2 ) l e v e l f o r t h e u n r e d u c e d p r e c u r s o r s w i t h t h e c o p p e r c o n t e n t less t h a n 50% were v e r y c l o s e t o t h a t f o r a-FeOOH, w h i l e t h e BEs f o r t h e p r e c u r s o r s c o n t a i n i n g 70 and 90% Cu were c l o s e t o t h a t f o r F e 2 ( S 0 4 ) 3 . The BE of t h e Cu(2p3/2) l e v e l and t h e k i n e t i c e n e r g y (KE) o f t h e Cu(LMM) A u g e r l e v e l f o r a l l p r e c u r s o r s a r e c o n s i s t e n t w i t h t h o s e f o r CuO ( r e f . 1 2 ) . With t h e c a t a l y s t s reduced a t 5OO0C f o r 1 h , t h e BE and KE v a l u e s i n d i c a t e t h a t i r o n e x i s t s as Fe2' and Feo r e g a r d l e s s o f t h e c o m p o s i t i o n , w h i l e copper e x i s t s o n l y as Cu metal. A s f o r t h e c a t a l y s t s p a r t i a l l y reduced a t 300°C f o r 0.5 h , t h e BEs of t h e Fe( 2p3/2) l e v e l i n d i c a t e t h a t i r o n e x i s t as Fe2+ i n F7C3-LH and as Fe2' + Fe3+ i n F7C3-HH c a t a l y s t s . T h i s i s i n good a g r e e m e n t w i t h t h e r e s u l t o f T G A e x p e r i m e n t s a s d e s c r i b e d a b o v e . T h e Cu(LMM) KE v a l u e s f o r t h e s e two catalysts show t h a t t h e o x i d a t i o n s t a t e o f c o p p e r a l s o d i f f e r s f rom e a c h o t h e r : Cuo f o r F 7 C 3 - L H a n d C u t f o r F 7 C 3 - H H c a t a l y s t s . The S ( 2 p ) s p e c t r a were d e t e c t e d o n l y i n t h e c a t a l y s t s c o n t a i n i n g more t h a n 60% Cu, as S042- i n t h e p r e c u r s o r s a n d a s m e t a l s u l f i d e s i n t h e r e d u c e d c a t a l y s t s . T h e r e f o r e , t h e e x t r e m e l y l o w a c t i v i t y o f t h e c a t a l y s t w i t h s u c h a h i g h c o n t e n t of copper (see T a b l e 1) can b e , a t l e a s t p a r t l y , a t t r i b u t e d t o t h e p o i s o n i n g e f f e c t of t h e remain ing s u l f u r . I n F i g . 5 , t h e F e 2 p / S i 2 p a n d Cu2p3/2/Si2p i n t e n s i t y r a t i o s f o r t h e \ \ I I I 750 740 730 720 710 700 BE/eV F i g . 4. Fe2p XP s p e c t r a f o r F7C3-LH. ( A ) P r e c u r s o r , ( B ) reduced a t 300°C f o r 0.5 h , and ( C ) reduced a t 500°C f o r 1 h . 10 I a N .r( m \ s a N 3 V a N .d m \ a N a L C u / ( F e t C u ) i n Bulk F i g . 5. Fe2p/Si2p (0) and Cu2p3/,/Si2p (A) XPS i n t e n s i t y r a t i o s f o r Fe-Cu/Si02 p r e c u r s o r s p r e c i p i t a t e d a t 75OC as a f u n c t i o n o f Cu c o n t e n t i n b u l k . f o r 1 h . (0) Fe2p/Si2p a f t e r reduced a t 5OO0C 110 p r e c u r s o r s p r e c i p i t a t e d a t 75OC are p l o t t e d a g a i n s t t h e b u l k c o m p o s i t i o n . The Fe2p/Si2p r a t io i n c r e a s e d s i g n i f i c a n t l y w i t h i n c r e a s i n g c o n t e n t of c o p p e r i n t h e bulk up t o 30%, which i m p l i e s t h e i n c r e a s e d d i s p e r s i o n o f i r o n i n t h e p r e s e n c e o f c o p p e r . I n t h e p r e c u r s o r o f t h e most a c t i v e F7C3-LH c a t a l y s t , f u r t h e r i n c r e a s e s i n t h e XPS ratios were o b s e r v e d as c i t e d i n T a b l e 3. DISCUSSION Hydrogenat ion r e s u l t s p r e s e n t e d i n T a b l e s 1 and 2 show t h a t t h e c a t a l y s t w i t h h i g h e r a c t i v i t y e x h i b i t s h i g h e r s e l e c t i v i t y . T h i s is i n c o n f o r m i t y w i t h o u r p r e v i o u s r e p o r t s ( r e f s . 6, 7 ) . The a c t i v i t y of t h e Fe-Cu(7:3) /Si02 c a t a l y s t p r e c i p i t a t e d a t around 2OoC by u s i n g m e t a l s u l f a t e s , F7C3-LH, w a s t h r e e o r d e r s h i g h e r t h a n t h a t o f a n o r d i n a r y F e / S i 0 2 c a t a l y s t p r e p a r e d f rom ferric n i t r a t e , FlO-HH(N). The h i g h s e l e c t i v i t y of t h e F7C3-LH c a t a l y s t w a s k e p t a lmost c o n s t a n t d u r i n g t h e r e a c t i o n and w a s 99% e v e n a t 98% c o n v e r s i o n . Although t h e r e d u c t i o n e x t e n t of i r o n i s u s u a l l y i n c r e a s e d by t h e p r e s e n c e of c o p p e r ( r e f s . 16, 1 7 ) , t h e r e i s no s i g n i f i c a n t d i f f e r e n c e i n t h e p e r c e n t a g e r e d u c t i o n s o f t h e catalysts under t h e p r e s e n t r e d u c t i o n c o n d i t i o n s , s u g g e s t i n g t h a t t h e d i f f e r e n c e i n a c t i v i t y i s n o t d u e s o l e l y t o t h e d i f f e r e n c e i n t h e r e d u c t i o n d e g r e e o f i r o n . It h a s been r e p o r t e d t h a t t h e water produced d u r i n g t h e H 2 - r e d u c t i o n s t r o n g l y a f f e c t s t h e d i s p e r s i o n o f i r o n metal , c a u s i n g s i n t e r i n g e s p e c i a l l y a t h i g h t e m p e r a t u r e s ( r e f s . 18, 19). The effects of r e d u c t i o n c o n d i t i o n s o n t h e c a t a l y t i c p r o p e r t i e s , shown i n T a b l e 2 , are c o n s i s t e n t w i t h t h e s e o b s e r v a t i o n s . When t h e F7C3-LH p r e c u r s o r w a s s l o w l y reduced a t a lower t e m p e r a t u r e for a l o n g e r time t h a n t h e u s u a l c o n d i t i o n s employed h e r e (50O0C, 1 h ) , a d e c r e a s e i n Dc and much improved a c t i v i t y w e r e o b t a i n e d , p r o b a b l y due t o e f f i c i e n t e l i m i n a t i o n o f water ( r e f . 7 ) . Our DTG e x p e r i m e n t s showed t h a t t h e r e d u c t i o n o f CuO and a-Fe00H i n t h e p r e c u r s o r of t h e a c t i v e catalysts c o n t a i n i n g ca. 30% Cu p r o c e e d s a t lower t e m p e r a t u r e s t h a n t h o s e i n t h e o t h e r c a t a l y s t s . I n l i n e w i t h t h i s , Wielers e t a l . h a v e v e r y r e c e n t l y r e p o r t e d t h a t copper fac i l i t a tes t h e r e d u c t i o n of Fe3+ t o Fe2+ as w e l l as t h e r e d u c t i o n t o Feo ( r e f . 20) . The i n c r e a s e i n t h e r e d u c i b i l i t y of Fe3+ i n t h e p r e s e n c e o f c o p p e r i s c l e a r l y e v i d e n c e d by t h e XPS r e s u l t s i n T a b l e 3. A c c o r d i n g l y , one o f t h e p r o m o t i o n a l e f f e c t s of c o p p e r c a n be e x p l a i n e d i n terms of t h e i n c r e a s e d r e d u c i b i l i t y of Fe3+: t h e amount of water produced a t h i g h t e m p e r a t u r e s becomes smaller i n t h e p r e s e n c e of c o p p e r , t h u s p r e v e n t i n g t h e s i n t e r i n g o f i r o n metal. A s for t h e effect of p r e c i p i t a t i o n t e m p e r a t u r e , it h a s been r e p o r t e d t h a t amorphous o r smaller p a r t i c l e s of a-FeOOH are s t a b l e i n c o l l o i d a l s o l u t i o n s a t t e m p e r a t u r e s below 55"C, w h i l e l a r g e (50-6Om) p a r t i c l e s of h e m a t i t e , cx-Fe203, are formed above 85 OC ( r e f . 21) . It seems l i k e l y t h a t smaller p a r t i c l e s of o x i d i z e d i r o n s p e c i e s i n t h e p r e c u r s o r are r e d u c e d more e a s i l y , r e s u l t i n g i n 111 smaller metal p a r t i c l e s . T h e h i g h BET s u r f a c e areas a n d t h e f l o s s y n a t u r e o f t h e p r e c u r s o r s p r e c i p i t a t e d a t t h e l o w t e m p e r a t u r e a r e a l s o f a v o r a b l e f o r e f f i c i e n t e l i m i n a t i o n o f w a t e r d u r i n g t h e r e d u c t i o n p r o c e s s . Moreover, t h e p r e c i p i t a t i o n a t l o w e r t e m p e r a t u r e s w i l l c a u s e s m a l l e r e x t e n t o f m e t a l - s u p p o r t i n t e r a c t i o n a n d l e a d t o i n c r e a s e d r e d u c i b i l i t y o f Cu2+ a n d Fe3+, a s e v i d e n c e d by t h e TGA and XPS r e s u l t s . The e x o t h e r m i c r e d u c t i o n of Cu2+ a t a lower t e m p e r a t u r e g r e a t l y f a v o r s t h e r e d u c t i o n of Fe3+ i n a n early s t a g e of t h e a c t i v a t i o n . Accord ingly , i t is e v i d e n t t h a t t h e p r e c i p i t a t i o n a t t h e l o w t e m p e r a t u r e p r o v i d e s t h e c a t a l y s t s w i t h h i g h d i s p e r s i o n o f 0.51- Fe2p / S i 2 p 1 F i g . 6. Dependence of t h e hydrogena- t i o n a c t i v i t y on t h e Fe2p/Si2p XPS i n t e n s i t y ra t ios f o r Fe-Cu(7:3)/Si02 p r e c u r s o r (0) and t h e reduced catalyst (0) p r e p a r e d f rom metal s u l f a t e s . b,& P r e p a r e d from metal n i t r a t e s . i r o n a n d h e n c e h i g h a c t i v i t y a n d s e l e c t i v i t y . As shown i n F i g . 6 , t h e XPS r e s u l t s s u p p o r t t h e i d e a t h a t t h e i n c r e a s e i n a c t i v i t y is a t t r i b u t e d t o a n improved d i s p e r s i o n o f i r o n . Concern ing t h e e f f e c t s of t h e s t a r t i n g sa l t , t h e F7C3-LH(N) p r e c u r s o r and c a t a l y s t , p r e p a r e d from metal n i t r a t e s , had r e l a t i v e l y l a r g e XPS i n t e n s i t y r a t i o s f o r s u r f a c e i r o n , a l t h o u g h t h e c a t a l y s t e x h i b i t e d much lower a c t i v i t y t h a n t h a t p r e p a r e d f rom s u l f a t e s . The CSD i n F i g . 3 showed t h e l a c k of small i r o n c rys t a l l i t e s i n t h e F7C3-LH(N) c a t a l y s t . These f i n d i n g s s u g g e s t t h a t l a r g e i r o n p a r t i c l e s i n F7C3-LH(N) e x i s t i n t h e o u t e r s u r f a c e , whereas copper p a r t i c l e s e x i s t i n t h e i n n e r s u r f a c e i n t e r a c t i n g w i t h t h e s u p p o r t , as is e v i d e n c e d by t h e i n c r e a s e d r e d u c t i o n t e m p e r a t u r e of Cu2+ f o r F7C3-LH(N) ( T a b l e 2 ) . Thus , t h e p r e s e n c e o f s u l f a t e i o n s i n p r e c i p i t a t i o n a p p e a r s c r u c i a l t o p r o v i d e w e l l - m i x e d s m a l l p a r t i c l e s o f i r o n a n d c o p p e r i n t h e p r e c u r s o r . F u r t h e r s t u d i e s are n e e d e d t o make c l ea r t h e r o l e of s u l f a t e i o n s i n t h e p r e c i p i t a t i o n p r o c e s s . CONCLUSIONS I n t h e p r e p a r a t i o n o f h i g h l y a c t i v e and s e l e c t i v e Fe-Cu/Si02 catalyst f o r t h e p a r t i a l h y d r o g e n a t i o n o f p h e n y l a c e t y l e n e , c o p r e c i p i t a t i o n of 70% Fe and 30% Cu, t h e u s e o f metal s u l f a t e s , a n d a r e l a t i v e l y l o w p r e c i p i t a t i o n t e m p e r a t u r e (a round 2OoC) were found t o be v e r y e f f e c t i v e . The a c t i v i t y of t h i s improved c a t a l y s t w a s t h r e e o r d e r s h i g h e r t h a n t h e o r d i n a l F e / S i 0 2 c a t a l y s t and t h e 112 s e l e c t i v i t y r e a c h e d 99% a t 98% c o n v e r s i o n . The p r e s e n c e of copper lowers t h e d e c o m p o s i t i o n and r e d u c t i o n t e m p e r a t u r e s of a-FeOOH toa-Fe203 and Fez+, t h u s d e c r e a s i n g t h e amount of water produced a t h i g h e r t e m p e r a t u r e s and p r e v e n t i n g t h e s i n t e r i n g of i r o n metal. The p r e c i p i - t a t i o n a t a low t e m p e r a t u r e p r o v i d e s t h e p r e c u r s o r s w i t h h i g h d i s p e r s i o n o f b o t h a-FeOOH and Cu2+ s p e c i e s , which r e s u l t i n t h e c a t a l y s t s w i t h h i g h l y - d i s p e r s e d i r o n metal. The u s e of metal s u l f a t e s a p p e a r s c r u c i a l t o p r o v i d e well-mixed small p a r t i c l e s of i r o n and c o p p e r i n t h e p r e c u r s o r . REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 R . P a u l and G . H i l l y s , B u l l . SOC. Chim. F r . [ 5 ] , 6 (1939) 218. A . F. Thompson and S. B. Wyat t , J. Am. Chem. SOC., 62 (1940) 2555. W. Reppe, Ann., 596 (1955) 38. S. T a i r a , B u l l . Chem. SOC. J p n . , 35 (1962) 840. R. S. Mann and K. C . Khulbe, Can. J. Chem., 45 (1967) 2755. Y . Nitta, S. Matsugi and T . Imanaka, Chem. E x p r e s s , 4 (1989) 547. Y . Ni t ta , S. Matsugi and T. Imanaka, Catal. L e t t . , i n p r e s s . Y . Nitta, F. S e k i n e , T . Imanaka and S. T e r a n i s h i , J. Catal . , 74 (1982) 382. Y. Ni t ta , T. Imanaka and S. T e r a n i s h i , J. Catal., 96 (1985) 429. B. E. Warren, J. Appl. P h y s . , 12 (1941) B75. A. R . S t o k e s , Proc . Phys. SOC. London, 61 (1948) 382. Y. Okamoto, K . Fukino , T. Imanaka and S. T e r a n i s h i , J. Phys. Chem., 87 (1983) 3740 and 3747. M. Rameswaran and C. H. Bartholomew, J. Catal., 117 (1989) 218. J . W. Geus, i n : G. P o n c e l e t , P. Grange and P . A. J a c o b s (Eds . ) , P r e p a r a t i o n of C a t a l y s t s 111, E l s e v i e r , Amsterdam, 1983, pp.1-33. A. F. H . Wielers, A . J. H . M. Kock, C . E. C . A. Hop, J. W. Geus and A. M. van d e r Kraan , J. Catal . , 117 (1989) 1. R . N. P e a s e , H. S. T a y l o r , J. Am. Chem. SOC., 43 (1921) 2179. Y. Murata and S. Kasaoka, Kogyo Kagaku Z a s s h i , 62 (1959) 1195. Y . Mura ta and S. Kasaoka, Kogyo Kagaku Z a s s h i , 61 (1958) 1440. E. R u c k e n s t e i n and X. D. Hu, J. Catal . , 100 (1986) 1. A. F. H . Wielers, C. E . C. A . Hop, J. van Bei jnum, A . M. van d e r Kraan, and J. W. Geus, J. Catal., 121 (1990) 364. J. W. Geus, Appl. Ca ta l . , 25 (1986) 313. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 113 SOME REMARKS ON THE PREPARATION OF Fe-K/Ca-Cr CATALYST FOR STYRENE PRODUCTION Z.DZIEWII$KI and E. OZDOBA Institute of Chemistry, Pedagogical University, 25-020 Kielce (Poland) SUMMARY Several title catalyst specimens with an identical Fe, K, Ca, and Cr content had been prepared from the different FeOOH or Fe203 phases, and then tested at the same conditions in dehydrogenation of ethylbenzene to styrene. An influence has been discussed of the kind and the contamination degree of FeOOH or Fe 0 on the catalyst properties. The most selective but moderately active catal&$ was obtained from slightly contaminated with C1 which contained tra- ces of cc-FeOOH phase. The less selective but more active catalyst was obtained from pure K-FeOOH phase. P-FeOOH INTRODUCTION Actually, iron oxide based catalysts are the most widespread contacts utili- zed in industrial dehydrogenation of ethylbenzene (EB) to styrene, and many propositions how to enhance their efficiency are still reported in the literatu- re (refs. 1-3). A majority of the propositions has concerned, however, promoters variation rather than proper selection of the main components of the initial mixture from which the catalyst is prepared. The latter problem seems to be of importance because of a consolidating opinion that just Fe atoms are included, apart from 0 and K atoms, into such proposed active phases of the catalyst as KFe02 (ref. 4 ) , K2Fe220j4 (ref. 5), or solid solution of potassium in y-Fe203 (ref. 6 ) . It is conceivable that at least some amount of those phases can be produced at the stage of heat treatment of the mixture from which the catalyst is obtained. The easiness of such a primary formation of the active phase will be conditioned by the kind, structure, and texture of the initial iron compo- unds used. It is also conceivable that the active centers are still created du- ring the EB dehydrogenation. A feasibility of their effective formation ought to depend on the kind of iron oxide matrix adjacent to the K atoms on the catalyst surface. It is evident that a type of the matrix formed will be, in turn, con- ditioned by the kind of initial iron compound used. The purpose of the present work was, therefore, to elucidate which one of the numerous FeOOH or Fe203 phases could be used as the most appropriate material for the production of such a title catalyst which could exhibit high selectivity and/or satisfactory activity in the EB dehydrogenation to styrene. It seemed a l s o interesting to check to what extent a possible contamination of the initial iron oxide hydroxides could affect activity or selectivity of the catalyst pre- pared from the contaminated initial iron compound. Sulphates or chlorides were 114 chosen as the contaminants, because j u s t these substances could be predominantly incorporated i n t o FeOOH or Fe203 dur ing t h e i r large-scale production from the so lu t i ons prev ious ly prepared by i r o n scrap d i s s o l u t i o n i n H2S04 or HC1. EXPERIMENTAL I n i t i a l i r o n compound preparat ion AnalaR reagents were used throughout a l l the procedures appl ied. To c o n t r o l surface area and p a r t i c l e dimensions o f the specimen obtained, the appropriate p r e c i p i t a t i o n , ageing, and c a l c i n a t i o n condi t ions had been chosen. Some speci- mens were purposely contaminated with C1- or 50:- ions. The degree o f the contamination was con to l l ed by a v a r i a t i o n o f the ageing and washing condi t ions. The f o l l o w i n g procedures were appl ied t o ob ta in the FeOOH and Fe203 specimens: (i) a-FeOOH. A s o l u t i o n o f Fe(N03)3.9 H20 (1 M) was mixed a t room tern- perature with the equivalent amount o f 3.5 M ammonia water, and the superna- t a n t l i q u i d was removed a f t e r some t ime from above the t rans ien t hydrated i r o n oxide s l u r r y (I), which was mixed i n t u r n w i t h approximately the same volume o f 3.5 M KOH s o l u t i o n and heated a t 60 OC under s t i r r i n g f o r 2 hr. So obtained a-FeOOH p r e c i p i t a t e (11) was washed with 0.01 M s o l u t i o n o f NH4N03+NH3H20 b u f f e r (pH.8.0) thoroughly, and d r i e d a t 105 OC i n a i r . The other specimens o f a-FeOOH t h a t were p a r t i a l l y contaminated with 50:- i ons were prepared ac- cord ing t o (refs. 7-8). (ii) j3-FeOOH. 0.25 mole o f Fe(NO3I3.9 H20 and 0 .1 mole o f NH4C1 had been d isso lved i n 1 mole o f urea was added t o t h i s so lu t i on . The s o l u t i o n was b o i l e d a t about 95 OC f o r 1 hr. and then subjected t o 24 hr. ageing a t the temperatures decreasing from 95 'C t o 20 OC. A f t e r supernatant l i q u i d decantation, the obtained p r e c i p i t a t e (111) was washed w i t h the above mentioned b u f f e r u n t i l no C1- i o n was detected with AgN03 s o l u t i o n i n the f i l t r a t e . More in tense washing was requi red, however, i f the intended C 1 - content i n d ry p-FeOOH ought no t t o exceed 1 w t . % . The obtained p r e c i p i t a - t e was d r i e d a t 60 OC under vacuum f o r 24 hr. The specimens o f P-FeOOH t h a t contained more C 1 were prepared from FeC13 s o l u t i o n by p r e c i p i t a t i o n with e i t h e r ammonium acetate ( r e f . 9) or pure water ( re f .10) used as hydro ly- z ing agents. 1 dm3 o f H20 ,and then (iii) y-FeOOH. The procedure repor ted i n (ref. 11) was appl ied t o ob ta in t h i s i r o n oxide hydroxide by means of the ox ida t i on w i t h bubbled a i r o f the sus- pension o f i r o n ( I 1 ) hydroxide, which had been obtained prev ious ly by the p r e c i - p i t a t i o n w i t h ammonia water a t oxygen-free atmosphere from FeS04 s o l u t i o n . ( i v ) G e 2 c 3 . Very f i n e c r y s t a l l i n e Fe203 powder was obtained by c a l - c i n a t i o n o f t he (I) p r e c i p i t a t e a t 350 OC f o r l hr. i n a i r . The c a l c i n a t i o n 0 o f the (11) p r e c i p i t a t e a t 400-550 C f o r 2-6 hr. y ie lded w e l l c r y s t a l - l i z e d cc-Fe203 wi th a lower s p e c i f i c surface area. 115 (v) y-Fe,03. At first, the obtained (11) precipitate had been dehydrated at 320 OC for 1 hr. to hematite, which was subsequently reduced at 360 OC with hydrogen f o r 1 hr. to magnetite. So obtained magnetite powder was in turn reoxidized at in air enriched with oxygen until pure maghemite phase of Fe203 was produced. 300 OC Catalyst preparation To avoid the possible masking effects of some commonly used activators, such as Mo, V, W , and particularly Ce oxides, the relatively simple composition was chosen for the catalyst specimens investigated. In addition, each of the cata- lyst specimen was prepared strictly in the same manner. Before the catalyst preparation Fe content (and K content, if needed) had been determined in FeOOH or Fe203 used actually for the catalyst preparation, and such an amount of this iron compound which contained of Fe was weig- hted. That amount of FeOOH or Fe203 was mixed with 28 g KOH ( o r less, if FeOOH or Fe203 used had already contained some K content), 25 g CaO, and 14 g K2Cr207 . Some amount of was added to those components to obtain, by a thorough kneading, a tough paste from which 4 mm diameter extrudates were ob- tained. The extrudates were, after drying at 105 OC and cutting into ca 6 mm length cylinders, calcined in two steps, at 350 OC and 550 OC respectively, for 4 hr. Both the cooling and the storage of so obtained specimens were accom- plished at H20 and C02 free atmosphere. Some part of the cylinders were crushed and sieved to obtain the catalyst fraction with 0.5 mm mean size grains. 220 g H20 Initial iron compounds and catalyst characterization Conventional analytical methods were applied to determine C1 (ref. 12) or S (ref. 13) content in the specimens obtained. BET method was used to determine specific surface areas of the specimens. X-ray powder diffraction patterns were recorded on a DRON 3 diffractometer with CuK, , FeKa , or MoKx radiation. The results were compared wlth the lite- rature data (ref. 14) to determine phase composition of the specimens investiga- ted. IR spectroscopy for some amorphous specimens was performed on BECKMANN IR Spectrometer. The obtained spectra were compared with those reported in the 11- terature (ref. 15). Catalyst testing in ethylbenzene dehydrogenation to styrene The measurements were carried out in a conventional fixed-bed flow reactor, which had been equipped with a feed-stream overheater and with a cooler for the products. The values of catalyst bed temperature, feed-stream composition and rate, and total gas pressure were controlled by means of additional attachments. 116 Before the main test, an influence of the possible diffuse retardation on the reaction rate had been established. A microreactor housing ca 10 cm3 of the catalyst was used for a relatively quick detection if any difference appear in the catalyst activity when the catalyst grains of different dimensions had been separately investigated in the reaction. Since no substantial difference had been found for such beds of the same catalyst in the microreactor experiments, the 4 mm diameter cylinders of the catalyst were then used in the main test, which was carried out as follows: portion of the catalyst was placed in the reactor, the bed being first heated to 420 OC with a nitrogen stream and then up to 530 OC with an overheated steam passed through the bed. The catalyst temperature was then raised to and the feed-stream was gradually enriched with ethylobenzene until its flow rate reached 7 5 cm EB/hr . Then, the reaction was studied under the following conditions: temperature, 590 OC, total pressure, ca 100 kPa, H20/EB molar ratio, 10, NTP space velocity, 10,000 hr-', overall reaction time 30 hr. Ouring each run, samples of the feed and product were taken at intervals and analyzed. An ELWRO N-504 gas chromatograph with a column packed with 5 % SP 1200/1.75 % Bentonite 34 on 100/120 Supelcoport was used to the feed and product analysis. Gaseous products were additionally analyzed, if needed, by means of the conventional chemical method. The results obtained at stationary state of the reaction, usually after 30 hr. , were taken for calculations. Styrene (STY) selectivity was calculated as lOO.(STY/(EB inlet - EB outlet)), styrene yield as 100.(STY/EB inlet), ethylbenzene conversion was calculated as 100.( (EB inlet - EB outlet)/EB inlet). The two latter magnitudes were arbi- trary assumed as a measure of the catalyst activity. A 200 cm3 590 OC 3 RESULTS AN0 DISCUSSION Initial iron compounds purity Of the compounds that were to be pure in intention, only cc-FeOOH, oc-Fe203, and y-Fe20j proved to be single-phase materials not contaminated with C 1 or S atoms. All the attempts to obtain non-contaminated single-phase P-FeOOH failed. This fact seems to be consistent with the supposition of many resear- chers that the existence of C1 (or theoretically F ~ too) atoms in P-FeOOH lattice is required necessarily for the stability of this compound to be secu- red. There is no uniformity of views, however, to what extent the chlorine con- tent could be lowered without a disturbance of p-FeOOH phase homogenity. Theoretically, the chlorine content could be calculated from various proposed "real" formulae, e.g. Fe403(OH)5C1 (ref. 16) or Fe8(0,0H)16C1 117 No references were found on the necessity of the presence of SO:- ions in 7-FeOOH lattice as a prerequisite of this phase stability. Nevertheless, no y-FeOOH specimen could be prepared without 5 contamination from FeS04 solu- tion, and an attempt to obtain chemically pure y-FeOOH from the other available iron salt solutions failed. Apparently, too strong inclusion or adsorption of the 50:- ions on y-FeOOH precipitate caused that even an intense washing of the precipitate was ineffective. Hence, we were forced to recognize the least contaminated specimens of the p-FeOOH or y-FeOOH phases as sufficiently "pure" for the aims of this work. Relationships between the catalyst and the initial iron compound properties (i) All the pure FeOOH phases calcined without a presence of other substances at 550 OC were always finally transformed into cc-Fe203 after 4 hr. calcination. In spite of this, some amount of Y-Fe203 was formed apart from cx-Fe203 in some catalysts when the mentioned FeOOH phases were calcined together with the other components of the initial mixture. Table 1 illustrates which Fe203 phases appeared in the catalyst obtained from different FeOOH phases o r their mixtures. The contribution of respective phases in the catalyst speci- mens was not quantitatively determined, but it might be approximately estimated. Mutual phase composition and surface area relations Fe20j TABLE 1 Effect of kind and contamination of FeOOH on selected properties of the catalyst FeOOH used for the catalyst preparation Type of the Contamina- Mean sursace Mean surgace Type of the phase used ting ele- area, (m /g) area, (m /g) Fe 0 phase Catalyst characterization ment, (%) foZnd OL ,(amorph) cc ,(fine cr) cc ,(needles) cc cc 13 ,(plates) p ,(amorph) p ,(needles) y ,(fine cr) 7 ,(fine cr) 7 9 % ~ + 2 l % p 6 2 % ~ + 38%p 4 8 % ~ . + 52%p 240 92 48 45 38 52 116 46 88 76 49 49 50 22 19 1 2 10 11 10 18 10 19 16 13 15 11 Relatively high content of maghemite in the catalyst prepared from Y-FeOOH was confirmed in two ways. First, the line with 2.52 d spacing value, was the most intense line in XRD patterns of the catalysts prepared from T-FeDOH, simi- larly as it was seen in XRD pattern of phase pure y-Fe203. Secondly, the extru- dates of these catalysts were easily attracted by a magnet, although a presence of no other paramagnetic materials could be detected in the extrudates. A lo - wer content of maghemite must have been present in the catalysts prepared from more contaminated with C1 specimens of f3-FeOOH phase (cf. Table 1). Only two we- ak lines, with 1.47 and 2.95 d spacing values, could be attributed to maghe- mite phase in XRD patterns of those catalysts. Even fine grains of those cata- lysts were not attracted by a magnet. It is conceivable, that two-step mechanism of type: (j3 or y )-FeOOH -y-FegD3 - oc-Fep03 ,with the second step inhibi- ted e.g. by contaminants, is valid here, but more detailed study is required to confirm or deny this supposition. It is worthy noticing that the catalysts in which only cc-Fe203 was formed, had been produced from a-FeOOH or pure P-FeOOH. Calcination of the initial mixture caused expected reduction in the specific surface areas. The reduction degrees seem not to be, however, in any relation with the kind or contamination degree of the FeOOH phase used. Nevertheless, the catalysts witii higher specific surface areas could be obtained from amorphous or fine-crystalline FeOOH specimens. (ii) Catalyst properties vs. kind and contamination dearee of FeOOH phase. Fig. 1 illustrates the values of EB conversion at 590 OC over the catalysts prepared from different pure phases of FeOOH or Fe203. As it is seen from the Figure, the catalysts prepared from iron oxide hydroxides exhibited more /-. v o\- c 0 ..-I ln Ll (u 1 C 0 0 m W 60 - 40 - 20 - P ;Y cc Y oc Fig. 1. EB conversion,(%), over the catalysts prepared either from FeOOH ( IZZZZZJ o r Fe20j ( EXZZ3 >. Kind of the phase is given under X axis 119 differentiated activity than those prepared from iron oxides introduced to the initial mixture from which the catalyst was then prepared. The differences observed in the catalyst specimen activity suggest that active centers of the catalyst were apparently formed from the introduced iron compo- und rather than from iron oxides formed ultimately in each catalyst specimen due to calcination. If the centers had been prepared from oxides formed during the calcination process, the observed differences would have been slighter, not gre- ater than the difference in activity of the catalysts prepared directly from E-Fe 0 and y-Fep03 phases. As it can be seen from the Figure, the catalyst pre- 2 3 pared from cc-Fe203 displays somewhat higher activity than that prepared from y-Fe203. This observation correlates with Subrt et al. opinion (ref. 18) that active phase KFe02 forms more easily from a-Fe203 than from y-Fe203. Changes in the catalysts selectivity are shown in Fig. 2. It is clearly seen from the Figure that the catalyst prepared from pure (in earlier mentioned sen- se) P-FeOOH phase exhibits much higher selectivity than the remainilcg catalysts investigated. On the other hand, the catalyst prepared from Y-FeOOH displays the lowest selectivity among the catalysts investigated. So conside- rable differencies in selectivity suggest that the kind of initial iron com- considerable t A v o\- .. x + 94 i cc P oc Y Fig. 2. Selectivity, (%I , of the catalysts prepared either from FeOOH (-1 or from FeZOj ( ) phases. Kind of the phase used is given under X axis pound used for t h e catalyst production can determine remarkably the catalyst selectivity. It is conceivable that a number of different phase features, such as e.g. FeOOH structure ability to be rearranged into somewhat differently distorted iron(II1) oxide matrices formed in the catalyst itself, or the abili- ty of different FeOOH phases to be transformed into the iron oxides with a so- mewhat different surface acidity o r to induce different pore structures in the catalyst, can affect the selectivity of the catalyst. 120 Apart from the kind of the initial phase used, its contamination can exert an influence on the catalyst selectivity and activity. Fig. 3 shows the changes in selectivity or activity values, if the same FeOOH phases, but with the different 32 o\* v u 30 -I a, ..-I x 28 x * v) 26 (a) I I I 1 2 3 Fig. 3. catalysts contamina 42 87 A v o\- .. 86 40 +J ..-I > .A 4 0 a, ffl 38 2 85 36 84 (b) I I I 1 2 3 36 95 94 33 S (a) or C 1 ( 0 ) content, (%) , in initial FeOOH Relatioship between the: (a) styrene yield, (b) selectivity of the prepared from: p-FeOOH contaminated with C1 ( -) or y-FeOOH ted with S (-+-m--), and the degree of contamination contamination degree were used for a preparation of the catalysts investigated. Although both chloride and sulphate ion contaminants undoubtedly affect the ca- talysts activity and selectivity, scale of their influence on these features can be different and depends mainly on kind and concentration of the contaminant The 50:- concentrations h i g h e r than 4 % but lesser than 10 + present in T-FeOOH, exert rather a weak influence on the selectivity, and almost none on the acivity of the catalyst prepared from y-FeOOH (cf. Fig. 3). From a practical point of view there is, therefore, no need to decrease too high 50;- content in the precipitated 7-FeOOH below 7 % . Theoretically, however, more thorough purification of 7-FeOOH can either substantially enhance activity of g-FeOOH based catalyst or can have no substantial effect on the activity. Since non-contaminated 7-FeOOH was so far not obtained, there was no opportunity to check which one of those two suppositions was true. For the catalysts based on contaminated P-FeOOH more substantial decrease in selectivity has been observed only when j3-FeOOH used was contaminated with more than 2 % C1 .If that oxide hydroxide was less contaminated, the lower decrease in activity, and particularly in selectivity, was observed with the C1 content increase. If the contamination degree did not exceed 0.7 % the selec- tivity decrease could be hardly observed. High selectivity of the catalyst based on the "pure" (i.e. least contaminated) P-FeOOH suggest that C1 incorpora- ted into (3-FeOOH lattice cannot affect strongly the catalyst selectivity. 121 I I I I , - (iii) Properties of the catalysts prepared from mixed FeOOH phases High selectivity of the catalyst prepared from phase pure (in earlier mentio- ned sense) p-FeOOH, and satisfactory activity of the catalyst prepared from phase pure cc-FeOOH, prompted us to study the properties of the catalysts prepa- red from a mixture of the mentioned phases. Some results of such a catalysts testing are shown in Fig. 4 . Apart from experimental curves shown in the Fi- n .\- v U 4 al .r( x al c al FI x + Lo 60 50 40 t Fig. 4. Activity (a) and selectivity (b) of the catalysts prepared from the mixture of cx-FeOOH and P-FeOOH pure phases. Full lines illustrate experimen- tally found relationship, broken lines show how these relationships would have run if additivity rule had been preserved for the features of the catalysts gure with full lines, theoretical runs, calculated under assumption that a rule of features additivity is valid for the catalysts prepared from phase mixtures, were also plotted in Fig. 4 with broken lines, It is evident from the Figure, that experimental values for selectivity are higher, if the P-FeOOH content is higher than 23 %, than the calculated values for the selectivity. It is worthy noticing that even high contribution of a-FeOOH , up t o 60 % , in the initial mixture of both considered phases practically does not decrease the high selec- tivity of the catalyst based on the "pure" P-FeOOH. Unfortunately, experimen- tal values of activity of the catalysts based on mixed FeOOH phases are always lower, regerdless of p-FeOOH phase contribution, than calculated values for predicted activity. Only small changes in activity are observed (cf. Fig. 4 a> when the P-FeOOH content in the initial mixture was increased. Rather considerable decrease in activity of the catalysts based on the mixed FeOOH phases is, of course, unfavourable for an attempt to enhance low activity of the catalyst based on "pure" P-FeOOH by addition of a-FeOOH phase to the initial mixture, o r to enhance selectivity of the catalyst based on s-FeOOH by addition of the inducing selectivity P-FeOOH phase. 122 CONCLUSIONS It has been stated in this work that the kind and contamination degree of the initial iron compound used for the Fe-K/Ca-Cr catalyst preparation, which can be used in ethylbenzene dehydrogenation to styrene, exerted rather substan- tial influence on the catalyst activity and selectivity in the mentioned reac- tion. Hence, a proper selection of the kind of initial iron compound, as well as the selection of appropriate conditions for these compounds preparation can substantially help us to enhance the catalyst activity and selectivity. The most effective catalyst was prepared from pure crystalline x-FeOOH, that had been previously obtained from pure iron nitrate solution by the described in this work precipitation method. An application of the solutions obtained by iron-scrap dissolution in H2S04 or HC1 is not reccornended for FeOOH or Fe203 preparation. To obtain the high selective catalyst , the P-FeOOH phase was used as the initial iron compound. Its contamination with C1 was so lowered as it was pos- sible by 24 hr. ageing and thorough washing of p -FeOOH precipitate. REFERENCES 1 4 5 6 7 8 9 10 11 12 13 14 15 J.L. Smith, 8.S. Masters and D.J. Smith, U.S. Patent, 4,467,046 K. Sarumaru, T. Iwakura, A. Watanabe and M. Mori, Japan Kokai, 85193,934 J. Kryska, J. Spevatek and M. Novotny, CS Patent, 230,464 (1986) T. Hirano, Appl. Catal., 26 (1986) 81-90 M. Muhler, R . SchlOgl and G. Ertl, Surf. Interface Anal., 12 (1988) 233-238 J. Koppe, I. Raphtel and P. Kraak, Chem. Techn. (Leipzig), 40 (1988) 81-83 G. Buxbaum and F. Hund, Ger. Offen. 2,556,406 (1977) A. Krause and A. Borkowska, Roczniki Chem., 29 (1955) 999-1006 S . Hirai, ,A. Matsumoto and K. Wakai, Japan Kokai, 78-88,698 (1978) J.M. Gonzalez-Calbet, M.A. Alario-Franco and M. Gayoso-Andrade, Inorg. Nucl. Chem., 43 (1981) 257-264 A. Solcovd, J. Subrt, F. Hanousek, P. Holba, V. Zapletal and J. Lipka, Silikaty, 24 (1980) 133-141 Polish Industrial Standards, Tests PN-75/C-04617 (1975) and PN-80/C-04617.04 (1980) f o r chlorine and its compounds determination Polish Industrial Standards, Test PN-74/C-04566 for sulphur and its compo- unds determination T.V. Kalinskaya, L.8. Lobanova, I.V. Pologih, Zhurn. Prikl. Xhim., ASTM 4-0755, 8-98, 13-157, 17-536, 24-72 40 (1982) 2463-2467 16 W. Feitknecht, R. Giovanoli, W. Michaelis and M. Mueller, Helv. Chim. Acta, 17 P. Keller, Neues Jahrb. Miner. Abh., 113 (1970) 29-49 18 56 (1973) 2847-2856 J. Subrt and J. Vins, Thermochim. Acta, 93 (1985) 489-492 G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 123 HYDROGENATION OF 2 ETHYL HEXEN-2-AL O N N i / S i O , CATALYSTS. ROLE OF PREPARATION PARAMETERS A.F. DA S ILVA JUNIOR1, V.M.M.SALIM1, M.SCHMAL1 a n d R . FRETYZ 'PEQ/COPPE/UFRJ, TELEFAX 5 5 2 1 - 2 9 0 6 6 8 6 - R i o d e J a n e i r o ( B r a z i l ) * I R C / C N R S , TELEFAX 7 8 8 9 4 7 6 9 - V i l l e u r b a n n e ( F r a n c e ) S U M M A R Y The i n f l u e n c e o f a m a c r o p o r o u s FC C e l i t e c a r r i e r a n d t h e p r e p a r a t i o n p a r a m e t e r s w e r e i n v e s t i g a t e d i n o r d e r t o o p t i m i z e p r e - p a r a t i o n a n d v e r i f y t h e i n t e r a c t i o n o f N i w i t h t h e s u p p o r t b y accomp l i sh ing t h e m o d i f i e d p r o p e r t i e s b y TPR, q u i m i s s o r p t i o n a n d t h e i r e f f e c t s on t h e h y d r o g e n a t i o n o f 2 - e t h y l - h e x e n - 2 - a l . P r e c i p l t a t i o n w i t h NaHCO,, F!H4HC0, and PIH,OH, w i t h o p t i m i z a t i o n o f ag ing t ime, a 1 l o w u s t o p r e p a r e c a t a l y s t s w i t h h i g h e r m e t a l l i c s u r f a c e a n d b e t t e r a c t i v i t y t h a n t h e i m p r e g n a t e d a n d c o m m e r c i a l N i C r , u s e d a s r e f e r - e n c e . The m o s t a c t i v e c a t a l y s t r e d u c e s t h e f o r m a t i o n o f h e a v i e r p r o d u c t s d u e b a s i c n a t u r e o f p r e c i p i t a t e d a g e n t . INTRODUCTION S i l i c a s u p p o r t e d n i c k e l c a t a l y s t s , w i t h c o n t r o l l e d p r o p e r t i e s a r e i m p o r t a n t i n p r a c t i c a l c a t a l y s i s , d u e t o t h e g r e a t n u m b e r o f r e a c t i o n s t h e y c a n p r o m o t e . T h e r e f o r e , t h e p r e p a r a t i o n a n d c h a r a c t e r j . z a t i o n o f s u c h c a t a l y s t s , e v e n i f w e l l d o c u m e n t e d ( I - 6 ) , remains a n a r e a w h e r e c o n s t a n t e f f o r t s a r e d o n e . F o r e x a m p l e , i n r e c e n t y e a r s , Hermans a n d Geus showed the i n t e r e s t o f homogeneous p r e c i p i t a t i o n ( 6 - 7 ) , M o n t e s e t a l . ( 8 - 9 ) analyzed t h e r e l a t i o n s h i p b e t w e e n t h e p r e p a r a t i o n m e t h o d a n d t h e i n t e n s i t y o f n i c k e l - s i l i c a i n t e r a c t i o n , a s w e l l 6 s t h e e x t e n t o f r e d u c t i o n a n d t h e m e t a l l i c a r e a . M o r e r e c e n t l y , M i l e e t a 1 ( 1 0 ) u s e d TPR t o r e v e a l v a r i o u s t y p e s o f m e t a l l i c n i c k e l in i m p r e g n a t e d N i / S i O , c a t a l y s t s a n d W i l s o n e t a 1 ( 1 1 ) d e t a i l e d t h e t e x t u r a l v a r i a t i o n s b r o u g h t a b o u t b y p r e c i p i t a t i o n w i t h u r e a o f N i c a t a l y s t s w i t h h i g h o r l o w N i l o a d i n g s . W i t h t h e a i m t o h y d r o g e n a t e e d i b l e o i l , we h a v e p r e p a r e d h i g h l y l o a d e d N i / S i O , c a t a l y s t s , b y a d e p o s i t i o n - p r e c i p i t a t i o n m e t h o d a n d showed t h a t t h e u s e o f FC C e l i t e , a s i l i c a s u p p o r t , t u r n e d t h e ca;a- l y s t s m o r e e f f i c i e n t t h a n t h e o n e s p r e p a r e d w i t h o t h e r C e l i t e m a t e r i a l s , e i t h e r w i t h l o w e r o r h i g h e r s u r f a c e a r e a ( 1 2 ) . H o w e v e r , w i t h h i g h N i l o a d i n g , i t i s d i f f i c u l t t o c o r r e l a t e t h e v a r i a t i o n s 124 o f m e t a l p r o p e r t i e s w i t h p o s s i b l e i n t e r a c t i o n s b e t w e e n t h e n i c k e l a n d t h e s i l i c a . The p r e s e n t w o r k s t a r t e d t o v e r i f y t h e i n f l u e n c e o f p r e p a r g t i o n p a r a m e t e r s a n d t h e e f f e c t on FC C e l i t e a s s u p p o r t on low loaded N i s a m p l e s . The c a t a l y s t s w e r e a l s o t e s t e d i n t h e l i q u i d p h a s e h y d r o g e n a t i o n o f 2 e t h y l 2 h e x e n a l ( E P A ) . This r e a c t i o n h a s b e e n p r e v i o u s l y s t u d i e d , b o t h i n t h e g a s (13-14) a n d l i q u i d p h a s e ( 1 5 - 1 7 ) a s t h e m a i n p r o d u c t s , 2 e t h y l hexanal (HAL) a n d 2 e t h y l h e x a n o l (HOL) h a v e some i n d u s t r i a l i n t e r e s t . HOL i s u s e d d u r i n g t h e p r o d u c t i o n o f d i o c t y l p h t a l a t e , f o r t h e m a n u f a c - t u r i n g o f P V C s o f t e n e r s , w h e r e a s t h e o c t a n o i c a c i d c a n b e o b t a i n e d b y o x i d a t i o n o f HAL. EXPERIMENTAL P r e p a r a t i o n o f c a t a l y s t s F o r t h e p r e p a r a t i o n , w e u s e d t h e FC C e l i t e ( M a n v i l l e ) , a S i O , s u p p o r t , w i t h 1 w t % A1,0,. T h i s i s a d i a t o m a c e o u s e a r t h , w i t h a t o t a l s u r f a c e a r e a o f 4 5 mYg a n d a p o r e v o l u m e o f 3 . 7 c m 3 / g . As d e d u c e d b y N, a d s o r p t i o n a t 77 K , t h e p u r e FC C e l i t e w i t h o n l y 0 , 1 c m 3 / g o f m e s o p o r e s v o l u m e a n d a n e g l i g e a b l e m i c r o p o r e s v o l u m e , i s m a i n l y a m a c r o p o r o u s s u p p o r t . Two g r o u p s o f c a t a l y s t s w e r e p r e p a r e d . I n t h e f i r s t o n e , t w o s e r i e s o f c a t a l y s t s w e r e o b t a i n e d b y depos i t i on -p rec ip i t a t i on (N i -D1 a n d N i - D 2 ) , u s i n g Ni(NO,), .6 H,O a s s t a r t i n g N i compound a n d NaHCO, a s p r e c i p i t a t i n g a g e n t . The FC C e l i t e ( 3 6 g ) was s u s p e n d e d i n aqueous s o l u t i o n o f Ni(NO,), ( 1 7 0 cm3, 0 , 4 ?I) a n d s u b m i t t e d t o c o n s t a n t s t i r r i n g ( 1 1 0 0 r p m ) . Aqueous s o l u t i o n o f NaHCO, ( 1 3 6 cm3, 0 .8 PI) was a d d e d t o t h e s u s p e n s i o n ( 1 ,7 c m 3 / m i n ) a t room temperature (298 K). The p r e c i p i t a t i o n o f t h e N i - D 1 s e r i e s was s t o p p e d f o r a N a / N i r a t i o e q u a l t o 2 , a n d t h e p r e c i p i t a t i o n o f N i - D , s e r i e s s t o p p e d f o r a N a / N i r a t i o o f 1 . 5 . The s l u r r y was t h e n m a i n t a i n e d u n d e r s t i r r i n g i n t h e r e a c t i o n med ium f o r d i f f e r e n t p e r i o d s o f t i m e , c a l l e d t h e r e a f t e r " a g i n g t i m e " . T h e n , t h e s u s p e n s i o n was f i l t e r e d , w a t e r w a s h e d a n d d r i e d f o r 2 3 h a t 3 8 3 K , b e f o r e b e i n g s u b m i t t e d t o a c a l c i n a t i o n u n d e r a i r , a t 723 K,6 h . The s e c o n d g r o u p o f c a t a l y s t s i n c l u d e d f o u r s a m p l e s , N i I was p r e p a r e d b y s i m p l e w e t p o i n t i m p r e g n a t i o n ; N i D H e N H , , N i D H o N H , a n d N iDHoNa w e r e p r e p a r e d a s d e s c r i b e d f o r d e p o s i t i o n - p r e c i p i t a t i o n p r o c e d u r e , u s i n g r e s p e c t i v e l y NH,nH, NH4HC0, a n d NaPCO,, a s p r e c i p i t a t i n g compounds andar , a g i n g t i m e o f 3 h ( o p t i m i z e d i n t h e f i r s t g r o u p o f c a t a l y s t s ) . D H e a n d D H o i n d j 125 c a t e t h a t t h e d e p o s i t i o n p r e c i p i t a t i o n p r o c e d u r e i s c o n s i d e r e d a s h e t e r o g e n e o u s a n d homogeneous , r e s p e c t i v e l y . A N i - C r / S i O , c a t a l y s t , u s e d i n d u s t r i a l l y f o r E P A h y d r o g e n a t i o n , was u s e d a s a r e f e r e n c e c a t a l y s t . C h a r a c t e r i z a t i o n R e d u c i b i l i t y a n d d i s p e r s i o n o f t h e c a t a l y s t s w e r e e s t i m a t e d t h r o u g h TPR i n s t r u m e n t s . T h e e q u i p m e n t i s s i m i l a r t o t h a t d e s c r i b e d i n ( 1 2 ) . The r e d u c t i o n was c o n d u c t e d w i t h a n A r g o n + 1 ,5% H, m i x - t u r e and h e a t i n g r a t e o f 8 K / m i n u p t o 8 2 3 K . The e n v o l v e d w a t e r was t rapped i n t o a 3 A z e o l i t e s l o c a t e d b e f o r e t h e t h e r m a l c o n d u c - t i v i t y c e l l . The d i s p e r s i o n o f t h e r e d u c e d N i was e s t i m a t e d , a t t h e e n d o f t h e TPR, b y q u i c k l y c o o l i n g down o r hea t ing up (11OK/min) t h e r e d u c e d s a m p l e , i n t h e a r g o n - h y d r o g e n m i x t u r e . T h e s e h i g h t e m - p e r a t u r e v a r i a t i o n s g e n e r a t e e i t h e r a n a d s o r p t i o n , o r a d e s o r p t i o n p e a k t h a t , a f t e r c o r r e c t i o n f o r f l o w r a t e v a r i a t i o n s , were c o n v e r t e d i n H, v o l u m e . F o r t h e d i s p e r s i o n e s t i m a t i o n , we assumed t h a t a t 8 2 3 K a n d a t r o o m t e m p e r a t u r e ( 2 9 8 K ) , hydrogen coverage was r e s p e c - t i v e l y c l o s e t o 0 a n d t o 1 . The r a t i o H / N i * , w i t h N i * b e i n g a s u r f a c e a t o m , was t a k e n e q u a l t o 1 . V a l u e s i s s u e d f r o m a d s o r p t i o n a n d d e s o r p t i o n p e a k s w e r e i n g o o d a g r e e m e n t . C a t a l y t i c m e a s u r e m e n t s The h y d r o g e n a t i o n o f E P A was p e r f o r m e d i n a s e m i - b a t c h r e a c t o r , a t a t m o s p h e r i c p r e s s u r e . Sample we igh t cor respond ing t o 0 ,09 g o f N i was r e d u c e d i n " s i t u " , a t 773 K f o r 1 7 h w i t h a n H, f l o w c l o s e t o 12 l / h . A t t h e e n d o f t h e r e d u c t i o n , t h e r e d u c e d c a t a l y s t was o u t g a s s e d w i t h N, f l o w a n d 150 m l E P A w e r e i n t r o d u c e d i n t o t h e r e a c t o r . N , b u b b l i n g was m a i n t a i n e d d u r i n g e s t a b l i s h m e n t o f t he reac t i on t e m p e r a t u r e . Then, a H, f l o w o f 205 l / h was a d d e d t h e m e c h a n i c a l s t i r r i n g a n d s t o p p i n g t h e N, f l o w . w e r e c o l l e c t e d p e r i o d i c a l l y z e d c h r o m a t o g r a p h i c a l l y RESULTS C a t a l y s t s p r e p a r a t i o n The f i n a l pH v a l u e s , a y t h r o u g h a s e a l e d w i t h a C a r b o w a x c o t h e end o f t h e a d d i t n e e d umn. on o f w h i l e s t a r t i n g L i q u i d s a m p l e s e v a l v e a n d a n a - t h e p r e c i p i t a t i n g a g e n t , w e r e r e s p e c t i v e l y 5 . 0 , 7 . 0 , 7 .0 a n d 8 .0 , f o r i m p r e g n a t i o n a n d d e p o s i t i o n - p r e c i p i t a t i o n w i t h NaHC03, NH4C03 a n d NH40H. The pH e v o l u t i o n , d u r i n g p r e c i p i t a t i o n w i t h NaHCO,, showed t h a t t h e t i t r a t i o n c u r v e f o r Ni(NO,), + FC C e l i t e was a l w a y s b e n e a t h t h e same c u r v e f o r p u r e Ni(NO,), . 126 22 e B r" n C a t a l y t i c C h a r a c t e r i z a t i o n T a b l e s 1 and 2 s u m m a r i z e some c h a r a c t e r i z a t i o n p a r a m e t e r s , f o r d i f f e r e n t c a t a l y s t s , a n d F i g u r e s 1, 2 a n d 3 g a v e r e d u c t i o n p r o f i l e s o f t y p i c a l c a t a l y s t s . A -_---.- Ni-4-3 o f a g i n g t i m e a t r o o m Ni-D2-, t e m p e r a t u r e i s g i v e n f o r N i - D 1 a n d N i - D Z c a t a l y s t s , s a m p l e s p r e c i p i t a t e d b y NaHCO,. I n p a r a 1 l e l , F i g s . 1 a n d 2 show :- ~ i\, i. ,-.. U. i - TABLE 1 I n f l u e n c e o f N i c o n t e n t a n d a g i n g t i m e on r e d u c i b i l i t y a n d m e t a l l i c a r e a f o r N i / C e l i t e c a t a l y s t s . C a t a l y s t N i / N a A g i n g T ime ( h ) % N i %Na a ( % ) S N ~ ( m z / g N i ) N i - D 1 - 0 N i - 0 1 - 2 N i - D 1 - 3 N i - D 2 - 0 N i - D 2 - 3 N i - D 2 - 6 N i - D 2 - 2 2 0 . 5 0 0 8 . 5 0 . 2 0 55 36 0.50 2 10.0 0.16 70 4 2 0 .50 3 1 0 . 0 0 .15 8 4 4 7 0 . 6 6 0 5.8 0.14 5 9 23 0.66 3 6 . 7 0.17 64 36 0.66 6 7.3 0 . 1 7 60 3 1 0 . 6 6 2 2 8 .5 0 . 1 5 6 8 33 127 The p r o p e r t i e s o f i m p r e g n a t e d a n d c o m m e r c i a l c a t a l y s t s a n d t h a t o f t h e d e p o s i t e d - p r e c i p i t a t e d o n e s , f o r a g i n g t i m e s o f 3 h, i s p r e s e n t e d i n T a b l e 2 . The c o r r e s p o n d i n g r e d u c t i o n p r o f i l e s a r e g i ven i n F i g u r e 3 . TABLE 2 I n f l u e n c e o f t h e p r e p a r a t i o n m e t h o d on t h e r e d u c t i b i l i t y a n d tex t ! r a l p r o p e r t i e s o f t h e s e c o n d s e t o f c a t a l y s t s . C a t a l y s t s BET ( m 2 / g ) Vp ( c m 3 / g ) % N i %Na a ( % ) SNi(mz/gNi) a d) N i I 33 1 .6 10.4 - 9 1 13 530 N i D H e N H , 30 2 .6 8 .4 - 96 3 8 180 N i D H o N H , 29 2 .6 8 .0 - 9 5 36 190 N i DHoNa 3 2 2.7 9.3 0 .18 9 1 53 1 3 0 N i - C r Com.(a) 11 0 . 7 9.3 - 8 3 1 4 480 a % C r = 1 . 7 % - NiDHeNH4 NiCr Gm NiO -.-1(-1- NiDHoNo N i W N H 4 N i I - - - - - - - . . . . . . . . . . . . . . . . . . . . . . . . . -. - - - - -. - T e m p e r a t u r e I K I Fig.3 - Influence of Preparation Methods on the TPR Profiles. 128 C a t a l y t i c A c t i v i t y P r e l i m i n a r y e x p e r i m e n t s , u s i n g t h e c a t a l y s t w i t h t h e h i g h e r m e t a l l i c a r e a , w e r e p e r f o r m e d i n o r d e r t o m i n i m i z e t h e d i f f u s i o n , v a r y i n g t h e g a s f l o w r a t e a n d t h e c a t a l y s t w e i g h t . Then , u s i n g 0 ,6 gNi/lEpA a n d a H, f l o w r a t e o f 2 0 5 l / h we cons t ruc ted r e a g e n t - p r o d u c t s d i s t r i b u t i o n curves, Fig. 4 - Typical distribution of reagent - products for catalyst NiDHoNa at 413 K . w i t h t ime, f o r t h e d i f f e r e n t t e m p e r a t u r e s ( 3 9 3 , 403 and 41 3 K ) . The v a r i a t i o n o f t h e m e t a l l i c a r e a b e i n g l a r g e r f o r t h e s e c o n d s e t o f c a t - a l y s t s , t h e c a t a - l y t i c m e a s u r e m e n t s w e r e 1 i m i t e d t o t h i s l a t t e r s e t . F i g . 4 shows a t y p i c a l c u r v e r e p r e s e n t a t i o n w h i c h re f e r s t o t h e consump t i o n o f E P A and t h e f o r m a t i o n o f p r o d - â¬PA Consumption - T 413 K u c t s u s i n g t h e c a t a l y s t N i DHoNa a t 100 413 K. From F igu re 5, 90 80 t h a t p r e s e n t s t h e 70 c o n s u m p t i o n o f E P A f o r d i f f e r e n t s c a t U A NiDHoNH4 A Ni I 8 60 4 NiDHeNH4 0 NiDHoNH4 a l y s t s , we c a l c u - = 50 L l a t e d t h e i n i t i a l r e a c t i o n r a t e a n d 40 z &? 30 t h e c o r r e s p o n d i n g 20 TON a t 4 1 3 K . These 10 v a l u e s a r e r e p o r t e d i n T a b l e 3 toge the r 0 0 1 2 3 4 5 6 7 w i t h a p p a r e n t act! v a t i o n e n e r g y . Reaction Time ( h ) Fig. 5 - Comparison of EPA Consumption for the second set of catalyst at 413 K. 129 TABLE 3 I n f l u e n c e o f t h e p r e p a r a t i o n m e t h o d on t h e a c t i v i t y . C a t a l y s t I n i t i a l r a t e * 1 0 2 s N i T O N ( s - ' ) E A t ( K J / m o l ) (mo l /gNi *min) ( m ' / g N i r e d ) ~~ N i DHoNa 10.1 53 1 . 2 6 9 N i DHoNH, 6 . 2 36 1 .1 52 N i I 1 . 2 1 3 0 .62 59 N i C r Com. 1 . 0 14 0 .44 NiDH,NH, 6 .8 3 8 1 . 2 - The m a i n p r o d u c t s o f t h e h y d r o g e n a t i o n o f E P A a r e t h e i n t e r m e - d i a t e HAL a n d t h e f i n a l p r o d u c t HOL. I n add i t i o r? t o i t we h a v e a l - r e a d y i d e n t i f i e d b y GC-mass s p e c t r o m e t r i c a p o s i t i o n i s o m e r o f E P A ( 2 b u t h y l b u t e n - 2 - a l , c a l l e d 1 S O M . E P A ) a n d some h e a v i e r com- p o u n d s l i k e 2 e t h y l h e x y l h e m i a c e t a l o f 2 e t h y l h e x a n a l a n d c o n - s e q u e n t l y b i s - ( 2 e t h y l h e x y l ) a c e t a l o f 2 e t h y l h e x a n a l (bo th c a l l e d S O M . ACT) . T a b l e 4 p r e s e n t s t h e a c t u a l s e l e c t i v i t i e s i n s t a n d a r d r e a c t i o n c o n d i t i o n s f o r a c o n s t a n t c o n v e r s i o n of 25% ( 4 1 3 K ) . H e a v i e r com- p o u n d s w e r e d e t e c t e d o n l y f o r h i g h e r c o n v e r s i o n s . I n the l a s t column o f T a b l e 4 we h a v e p r e s e n t e d t h e s e l e c t i v i t i e s o f t h e s e compounds f o r t o t a l c o n v e r s i o n o f E P A , f o r t h e m o s t a c t i v e c a t a l y s t s . TABLE 4 I n f l u e n c e o f t h e p r e p a r a t i o n m e t h o d i n s e l e c t i v i t y a t 4 1 3 K . S e l e c t i v i t y S~~~ S H O L 'ISOM.EPA ' S O M . A C T 'HAL 'HOL C o n v e r s i o n 25% 1 0 0 % C a t a l v s t N i I NiDH,NH, 8 5 . 6 1 .10 13 .5 8 8 . 7 0 . 5 5 1 0 . 8 - - 0 . 4 1 9 1 . 5 8.1 N i D H o N H 8 9 . 0 0 . 7 5 1 0 . 5 0 . 4 2 8 8 . 2 11 .4 NiDHoNa 8 7 . 5 0 . 4 3 1 2 . 0 0 .18 9 0 . 0 9 . 8 N i C r Corn. 9 0 . 0 1 . 3 0 8 . 8 - - - D I S C U S S I O N D u r i n g t h e i r s t u d y o n d e p o s i t i o n - p r e c i p i t a t i o n o f N i , u s i n g u r e a d e c o m p o s i t i o n o r c o n t r o l l e d NaOH a d d i t i o n , Hermans a n s Geus ( 6 - 7 ) a n a l y z e t h e i n t e r a c t i o n b e t w e e n N i i o n s a n d h i g h s u r f a c e a r e a s i l i c a s . B a s e d on v a r i o u s m e a s u r e m e n t s and i n p a r t i c u l a r f o l l o w i n g t h e e v o l u t i o n o f p H d u r i n g t h e p r e c i p i t a t i o n , t h e y c o n c l u d e d t h a t t h e i r e x p e r i m e n t a l c o n d i t i o n s a l l o w e d an homogeneous d e p o s i t i o n o f 130 N i p r e c u r s o r o n t o t h e s i l i c a s u p p o r t . I n t h e p r e s e n t c a s e , w i t h a m a c r o p o r o u s m i d d l e s u r f a c e a r e a s i l i c a a s t h e s u p p o r t a n d NaHC0,as t h e p r e c i p i t a n t , t h e pH v a r i a t i o n s , a r e s i m i l a r t o t h e o n e s p r e s e n t e d i n ( 6 ) . T h e r e f o r e , we t h i n k , i n a g r e e m e n t w i t h W i l s o n e t a1 ( I I ) , t h a t o u r N i / C e l i t e c a t a l y s t s a l s o h a v e a r a t h e r homogeneous d i s t r i b u t i o n o f N i o n t o t h e c a r r i e r . By i n c r e a s i n g t h e p r e c i p i t a t i o n t i m e d u r i n g t h e p r e p a r a t i o n o f N i c a t a l y s t s v i a u r e a d e c o m p o s i t i o n , R i c h a r d s o n e t a l . ( 1 8 ) showed t h a t t h e N i c o n t e n t i n c r e a s e s , l e a d i n g b o t h t o a n i n c r e a s e d mean N i p a r t i c l e s i z e a n d a d e c r e a s e o f t h e t e x t u r a l p a r a m e t e r s . The v a r i a t i o n s o f t h e t e x t u r a l p r o p e r t i e s b e t w e e n l o w a n d h i g h l o a d e d N i / s i l i c a c a t a l y s t s w e r e f u r t h e r c o n f i r m e d b y W i l s o n e t a 1 ( 1 1 ) . T e x t u r a l a n d m e t a l l i c a r e a m o d i f i c a t i o n s r e s u l t i n g f r o m a n inc rease i n t h e a g i n g t i m e , a f t e r p r e c i p i t a t i o n , w e r e a l s o o b s e r v e d o n N i / C e l i t e c a t a l y s t s , w i t h 5 0 w t % N i ( 1 9 ) . There fore , t he p r e p a r a t i o n m e t h o d o l o g y n o t o n l y c h a n g e s t h e p r o p e r t i e s o f m e t a l p a r t i c l e s , b u t c a n a l s o a l t e r t h e i r a c c e s s i b i l i t y , b y a l t e r a t i o n o f t h e s u p p o r t t e x t u r e . I n t h e p r e s e n t w o r k , u s i n g a c o n s t a n t p r e c i p i t a t i o n t i m e , we h a v e o b s e r v e d t h a t t h e TPR p r o f i l e s a n d t h e r e f o r e t h e e x t e n t o f re d u c t i o n , a s w e l l a s t h e m e t a l l i c a r e a a r e s l i g h t l y c h a n g e d w h e n t h e i l i c o n t e n t o r t h e a g i n g t i m e i s c h a n g e d . I n a g e n e r a l way, i n a g r e e m e n t w i t h known r e s u l t s ( l o ) , t h e TPR p r o f i l e s p r e s e n t t w o m a j o r r e d u c t i o n p e a k s , o n e a r o u n d 5 7 3 - 6 4 3 K a n d t h e o t h e r a r o u n d t o 7 7 3 - 8 2 3 K. The f i r s t o n e seems c h a r a c t e r i s t i c o f n o n - i n t e r a c t i n g N i s p e c i e s , w h e r e a s t h e s e c o n d o n e c a n b e c o n s i d e r e d a s a f i n g e r - p r i n t o f t h e N i - S i O , i n t e r a c t i o n . S o m e t i m e s , a l i m i t e d H, u p t a k e i s o b s e r v e d a t t 5 1 0 K a n d a t t r i b u t e d t o t h e e l i m i n a t i o n o f n o n s t o i c h i o m e t r i c o x y g e n . The n o n i n t e r a c t i n g N i seems p r e s e n t i n l a r g e r q u a n t i t y when t h e p e r c e n t N i i n c r e a s e s ( F i g . 1 ) a n d when t h e a g i n g t i m e i n c r e a s e s ( F i g . 2 ) . The c o n t r i b u t i o n o f t h i s n o n i n t e r a c t i n g N i t o t h e m e t a l l i c a r e a i s h o w e v e r l e s s c l e a r l y e s t a b l i s h e d , a s i n T a b l e 1 , t h e m e t a l l i c a r e a i n c r e a s e s s l i g h t l y w i t h t h e a g i n g t i m e , u p t o 3 h , b u t t e n d s t o l e v e l o f f o r e v e n d e c r e a s e f o r h i g h e r a g i n g t i m e s . When c o m p a r i n g i m p r e g n a t e d w i t h p r e c i p i t a t e d c a t a l y s t s ( F i g . 3 a n d T a b l e 2 ) , i t c a n b e s e e n c l e a r l y t h a t t h e i m p r e g n a t e d c a t a l y s t i s r i c h i n n o n i n t e r a c t i n g N i , s i t u a t i o n l e a d i n g t o a r a t h e r l o w m e t a l l i c a r e a . The p r e c i p i t a t e d c a t a l y s t s p r e s e n t a n o p p o s i t e s i t u a t i o n , a s m a j o r i t y o f N i i s i n t e r a c t i n g w i t h FC C e l i t e a n d 131 g e n e r a t e s a l a r g e r m e t a l l i c a r e a . The c a s e o f t h e c o m m e r c i a l c a t a l y s t i s m o r e c o m p l e x : t h e r e d u c t i o n i s i n h i b i t e d b u t t h e m e t a l l i c a r e a lessened b e c a u s e some C r i s p r o b a b l y c o v e r i n g a f r a c t i o n o f t h e m e t a l l i c p a r t i c l e s , l i m i t i n g t h e r e f o r e H , a d s o r p t i o n . The d i f f e r - e n c e s o f p r o p e r t i e s b e t w e e n t h e v a r i o u s p r e c i p i t a t e d c a t a l y s t s a r e v e r y l i m i t e d , c o m p a r e d t o t h e d i f f e r e n c e s n o t e d f o r i m p r e g n a t e d c a t a l y s t s , a s e x p e c t e d . T h e r e f o r e , t h e n a t u r e o f t h e p r e c i p i t a t i n g a g e n t i s o f s e c u n d a r y i m p o r t a n c e i n o u r p r e p a r a t i o n . T h i s s i t u a t i o n seems d u e t o v a r i o u s p a r a m e t e r s , among w h i c h t h e l o w temperature o f p r e c i p i t a t i o n , t h e h i g h s t i r r i n g d u r i n g t h e p r e p a r a t i o n a n d t h e r a t h e r l o w s u r f a c e a r e a o f t h e s u p p o r t c a n b e r e t a i n e d . The b e t t e r m e t a l l i c N i a r e a o b t a i n e d f o r N i DHoNa c a n b e r e l a t e d b o t h t o t h e l o w e r q u a n t i f y o f e a s i l y r e d u c e d N i a n d t o a m o r e homogeneous i n t e r a c t i o n b e t w e e n N i a n d t h e s i l i c a ( t h e r i g h t h a n d s i d e o f t h e h i g h t e m p e r a t u r e r e d u c t i o n p e a k i s n o t p r o n o u n c e d ) . T h i s s m a l l e r N i - S i O , i n t e r a c t i o n i s t e n t a t i v e l y a t t r i b u t e d t o t h e p r e s e n c e o f r e s i d u a l Na, a s n o c l e a r t e x t u r a l v a r i a t i o n was o b s e r v e d . F i n a l l y , i t i s i n t e r e s t i n g t o n o t e t h a t t h e t e x t u r a l p a r a m e t e r s d r o p i s more i n t e n s e f o r t h e i m p r e g n a t e d s a m p l e t h a n f o r t h e p r e c i p i t a t e d o n e s . C a t a l y t i c a c t i v i t y The h y d r o g e n a t i o n o f E P A c a n b e r e p r e s e n t e d b y t h e f o l l o w i n g scheme: +H, + H * 2 e t h y l h e x e n - 2 - a 1 ------+ 2 e t h y l h e x a n a l -> 2 e t h y l h e x a n o l Condensation Isomerization Condensation 2 e t h y l h e x i l h e m i a c e t a l o f 2 e t h y l h e x a n a l J 2 b u t h y l b u t e n - 2 - a 1 b i s ( 2 e t h y l 2 e t h y l W i t h t h e p r e s e n t c a t a l y s t s , t h e u n s a t u r a t e d a n e v e r o b s e r v e d . The c a t a l y t i c a c t i v i t y e x p r e s s e d s u m p t i o n o f E P A , a n d g i v e n i n T a b l e 3 , v a r i e s b y t h e t h r e e p r e c i p i t a t e d c a t a l y s t s b e i n g i n one g r h e x i l ) a c e t a l o f h e x a n a l c o h o l , HEOL, was t h r o u g h t h e c o n a f a c t o r o f 1 0 , u p a n d t h e i m p r e g n a t e d a n d c o m m e r c i a l o n e s b e i n g i n a n o t h e r g r o u p . H o w e v e r , w i t h t h e e x c e p t i o n o f t h e c o m m e r c i a l c a t a l y s t , f o r w h i c h t h e m e t a l l i c a r e a m e a s u r e m e n t i s p r o b a b l y n o t o p t i m i z e d , t h e TON ( s - ' ) f o r a l l c a t a l y s t s i s r a t h e r s i m i l a r , a s a r e t h e a p p a r e n t a c t i v a t i o n e n e r g i e s . T h i s means t h a t , i n a f i r s t a n a l y s i s , t h e E P A h y d r o g e n a t i o n , i n t h e 132 . l i q u i d p h a s e , c a n b e c o n s i d e r e d i n i t i a l l y a s i n s e n s i t i v e t o t h e s t r u c t u r e o f t h e c a t a l y s t s . C o n c e r n i n g t h e s e l e c t i v i t i e s , HAL a n d I S 0 E P A a p p e a r a s i n i t i a l p r o d u c t s , e v e n i f I S 0 E P A was n e v e r m e n t i o n e d , u p t o now, i n b a s i c s t u d i e s . W i t h t h e e x c e p t i o n o f t h e c o m m e r c i a l c a t a l y s t , mod i f ied b y C r a n d l e a d i n g b o t h t o a s l i g h t l y b e t t e r s e l e c t i v i t y i n HAL a n d a l o w e r s e l e c t i v i t y i n I S 0 E P A , a n d t h e i m p r e g n a t e d c a t a l y s t s h o w i n g a l a r g e r p r o d u c t i o n o f HOL, a l l o u r c a t a l y s t s p r e s e n t r a t h e r compa r a b l e s e l e c t i v i t i e s . H o w e v e r , l o o k i n g a t t h e l o w q u a n t i t i e s o f HOL i n i t i a l l y p r o d u c e d , i t a p p e a r s t h a t t h e l o w e r s e l e c t i v i t y i s ob ta ined f o r t h e m o r e a c t i v e c a t a l y s t s . As HOL i s t h e e n d p r o d u c t o f t h e h y d r o g e n a t i o n , o n l y t h e s u p p r e s s i o n o f HAL r e a d s o r p t i o n w o u l d b e a b l e t o s u p p r e s s t h e f o r m a t i o n o f HOL. A t 2 5 % c o n v e r s i o n , t h e s t r o n g e r E P A a n d I S 0 E P A a d s o r p t i o n s a r e p r o b a b l y l i m i t i n g n a t u - r a l l y t h e HAL a d s o r p t i o n . B u t , a t h i g h e r c o n v e r s i o n , when t h e coy! c e n t r a t i o n s o f E P A a n d I S 0 E P A a r e v e r y l o w , i t seems n e c e s s a r y t o h a v e a g o o d N i a c c e s s i b i l i t y ( i . e t h e h e l p o f t h e m a c r o p o r o u s s t r u c t u r e ) a n d a n e f f i c i e n t a g i t a t i o n t o l i m i t t h e HAL r e a d s o r p t i o n . I n f a c t , e v e n a t 1 0 0 % c o n v e r s i o n , t h e b e t t e r s e l e c t i v i t y i n HAL a r e f o r t h e c a t a l y s t s h a v i n g t h e l a r g e r s u r f a c e a r e a . I n c r e a s i n g t h e c o n v e r s i o n , t h e HAL s e l e c t i v i t y inc reases s l i g h t l y , p a s s e s t h r o u g h a maximum a n d t h e n d r o p s f o r c o n v e r s i o n s b e t w e e n 9 0 a n d 95%. I n p a r a l l e l , some h e a v y p r o d u c t s , among w h i c h a c e t a l s h a v e b e e n r e c o g n i z e d , a r e f o r m e d . I f t h e f o r m a t i o n o f t h e s e b y - p r o d u c t s i s c a t a l y t i c a l o r n o t i s n o t known, b u t i t i s i n t e r e s t i n g t o s e e t h a t p r e s e n c e o f a l a r g e r N i a r e a ( o r t h e p r e s e n c e o f Na i n t h e c a t a l y s t ) seems t o l i m i t t h e i r f o r m a t i o n . F u r t h e r w o r k i s n e e d e d t o t r y t o c o r r e l a t e c a t a l y s t s t r u c t u r e a n d f o r m u l a t i o n w i t h t h e f o r m ? t i o n o f t h e s e h e a v y compounds . CONCLUSIONS The p r e p a r a t i o n o f N i c a t a l y s t s on t h e FC C e l i t e , a S i O , m a c r o p o r o u s c a r r i e r , b y p r e c i p i t a t i o n a t r o o m t e m p e r a t u r e , f o l l o w e d b y some a g i n g , a l l o w s t h e o b t e n t i o n o f c a t a l y s t e a s y t o r e d u c o a n d p r e s e n t i n g a s a t i s f a c t o r y m e t a l l i c d i s p e r s i o n . The n a t u r e o f t h e p r e c i p i t a t i n g a g e n t i s n o t i m p o r t a n t a n d t h e m a c r o p o r o u s s t r u c t u r e o f t h e s u p p o r t i s m a i n t a i n e d , i n a l a r g e p a r t . Used i n t h e l i q u i d p h a s e h y d r o g e n a t i o n o f 2 e t h y l 2 h e x e n a l , t h e s e c a t a l y s t s a r e a s s e l e c t i v e a n d 3 t o 4 t i m e s m o r e a c t i v e than a c o m m e r c i a l N i - C r r e f e r e n c e c a t a l y s t . The f o r m a t i o n o f h e a v y b y - p r o d u c t s seems a l s o m a i n t a i n e d a t a n a c c e p t a b l e l e v e l . 133 REFERENCES 1 2 3 4 5 6 . 7 8 9 10 1 1 1 2 1 3 1 4 15 16 1 8 19 G . C . A . S c h u i t , L . L . v a n R e i j e n - A d v a n c e s i n C a t a l y s i s - V o l . 1 0 , A c a d e m i c P r e s s London /New Y o r k , 1 3 5 8 , p p . 2 4 2 - 2 6 7 . J.W.E. Coenen, B.G. L i n s e n - " P h y s i c a l a n d C h e m i c a l A s p e c t s o f Adsorbents a n d C a t a l y s t s , B . G . L i n s e n ( E d . ) , A c a d e m i c P r e s s , London/New Y o r k , 1 9 7 0 , pp . 4 7 2 - 5 2 5 . J.W.E. Coenen (G. P o n c e l e t , P . G r a n g e a n d P . A . J a c o b s , E d s . ) , P r o c . o f t h e 3 r d I n t e r n a t i o n a l C o n g r e s s on The S c i e n t i f i c B a s e s of C a t a l y s t P r e p a r a t i o n , E l s e v i e r , A m s t e r d a n , 1 9 7 9 , p p . 89-111. J.W.E. Coenen - I n d . Eng. Chem. F u n d . , 25 ( 1 9 8 6 ) 4 3 - 5 2 . P . T u r l i e r , H. P r a l i a u d , P . M o r a l , G . A . Y a r t i n , J.A. Da lmon - A p p l . C a t . 18 ( 1 9 8 5 ) 389 . A .J . v a n D i l l e n , J.W. Geus, L.A.M. Hermans, J . v a n d e r Mei jden, P r o c . S i x t h I n t . C o n g r e s s on C a t a l y s i s L o n d o n , 1 9 7 6 , p p . 6 7 7 - 6 8 5 . L.A.M. Hermans , J.W. Geus , P r o c . o f t h e 3 r d I n t . C o n g r e s s on t h e S c i e n t i f i c Bass o f C a t a l y s t P r e p a r a t i o n , E l s e v i e r , Amsterdan, 1979 , p p . 1 1 3 - 1 3 0 . M . M o n t e s , Ch. Penneman de Boss Cheyde , B.K. Hodnett , F. Delannay, P . G r a n g e , B. De lmon , - A p p l . C a t . , 1 2 ( 1 9 8 4 ) pp . 3 0 9 - 3 3 0 . M. M o n t e s , J.B. S o u p a r t , M . S a e d e l e e r , B . K . H o d n e t t , B . Delrnon - J . Chem. S . C . , F a r a d a y T r a n s . 8 0 ( 1 ) ( 1 9 8 0 ) , p p . 3 2 0 9 - 3 2 2 0 . B . M i l e , D . S t i r l i n g , M . A . Z a m m i t t , A . L o v e l l , M . Webb - J . C a t a l . , 114 ( 1 9 8 8 ) p p . 2 1 7 - 2 2 9 . M.F. W i l s o n , 0. A n t i n l u o m a , J.R. B rown - S y m p o s i u n on t h e P r e - p a r a t i o n a n d C h a r a c t e r i z a t i o n o f C a t a l y s t s P r e s e n t e d be fo re t h e D i v i s i o n o f P e t r o l e u m C h e m i s t r y , I n c . , - A m . Chem. S O C . - L o s A n g e l s M e e t i n g - 1 9 8 8 , pp . 6 6 9 - 6 7 5 . V .M.M. S a l i m , M . S c h m a l , R . F r e t y , M. R o d r i g u e s , M . C . S i l v e i r a - 5 0 S e m i n a r i o de C a t a l i s e - B r a s i l ( 1 9 8 9 ) , p p . 9 3 - 1 0 1 . C . P l i k l a s s o n , G . S m e d l e r , I n d . Eng. Chem. Res. 26 ( 1 9 8 8 ) 4 0 3 - 410 . J . S m e d l e r - I n d . Eng. Chem. Res, 27 ( 1 9 8 8 ) , 2 0 2 3 - 2 0 3 0 . E.F. Souza A g u i a r , M . Schmal - V I I S i m p 6 s i o I b e r o - A m e r i c a n 0 de C a t a l i s e - A r g e n t i n a , 1 9 8 0 . J . L . A l m e i d a , L . D i e g u e z , M. S c h m a l , I X S i m p 6 s i o I b e r o A m e r i c a n o de C a t a l i s e , P o r t u g a l , 1984 . L.R.R.Araujo, L.C. D i e g u e z , R . F r e t y , M . Schma l , X I Simp6sio I b e - r o A m e r i c a n 0 d e C a t a l i s e s , M e x i c o , 1 9 8 8 , p p . 1 1 9 5 . J . T . R i c h a r d s o n , R.J. Dubus, J.G. Crump, P . Desai, U. Osterwalder, T.S. C a l e - P r o c e d . o f t h 3 r d . I n t e r n a t i o n a l C o n g r e s s o n t h e S c i e n t i f i c Bases o f C a t a l y s t P r e p a r a t i o n , E l s e v i e r , A m s t e r d a n , V . M . M . S a l i m , M . A . D u a r t e , M . C . S i l v e i r a , R . F r e t y M.Schma1 - X I 1 S i m p o s i o I b e r o A m e r i c a n 0 de C a t a l i s e - B r a s i l - 1 9 9 0 . ( i n p r e s s . 1 9 7 9 , p p . 1 3 1 - 1 4 2 . This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 135 PREPARATION AND PROPERTIES OF A PT/SILICA AND ITS COMPARISON WITH EUROPT-1 S.D. JACKSON3, M.B.T. KEEGAN', G.D. McLELLANl, P.A. MEHEUX', R.B. MOYES', G. WEBB1, P.B. WELLS', R. WHYMAN4 AND J. WILLIS3 'Department of Chemistry, University of Glasgow, Glasgow, 612 8QQ, Scotland (UK 'School of Chemistry, University of Hull, Hull, HU6 7RX (UK) 31CI Chemicals and Polymers Ltd., The Catalysis Centre, Research and Technology Dept., PO Box 1, Billingham, Cleveland, TS23 1LB (UK) 41CI Chemicals and Polymers Ltd., Research and Technology Dept., PO Box 8, The Heath, Runcorn, Cheshire, WA7 4QD (UK) SUMMARY A 0.76% Pt/silica has been prepared by conventional impregnation from aqueous solution using hexachloroplatinum( IV) ions as the Pt source. characteristics and reactivity are compared with those of the standard reference catalyst EUROPT-1 (a 6.3 wt% silica-supported Pt) which was prepared by ion exchange using platinum( 1I)tetrammine ions as the Pt source. dispersion by HRTEM and particle morphology by EXAFS are reported. catalysts both showed high metal dispersion and comparable behaviour in oxygen chemisorption and butadiene hydrogenation, whereas they differed with respect to mean Pt particle size and showed different behaviours in carbon monoxide chemisorption, cyclopropane hydrogenolysis, and enantioselective methyl pyruvate hydrogenation. evaluation. Its adsorption Platinum The Clearly the choice of comparators is of importance in catalyst INTRODUCTION The relationship between catalyst preparation and performance is complex and curious. Catalyst characterisation provides the bridge in that preparation can be directed towards the attainment of certain structural and chemical characteristics, and attempts can be made to relate performance to those same characteristics. The authors are collaborating in an extensive joint study in which the preparation, characterisation and function of a range of supported platinum catalysts is being evaluated. In part, this study will compare platinum catalysts prepared using conventional supports (e.g. silica, alumina) with those using less conventional supports (e.g. molybdena) or those prepared by less conventional methods (e.g. metal vapour deposition). The restricted object of the present paper is to compare the conventional Pt/silica prepared within this programme with the standard reference silica-supported Pt codenamed EUROPT-1 for which full preparation and characterisation details have been published (refs. 1-5). 136 NOMENCLATURE Throughout this paper the silica-supported p characterised in this investigation is referred si 1 ica-supported platinum reference catalyst is atinum prepared and to as Pt/silica, whereas the referred to as EUROPT-1. EXPERIMENTAL METHODS Catalyst preparation is described in the next Section. Conventional means were used to obtain electron microscopic images of the platinum particles in each catalyst at a magnification of ca. 450,000~. EXAFS spectra of samples at the Pt L3-edge were obtained at the Synchrotron Radiation Source at the SERC Daresbury Laboratory. Powdered samples were reduced at 523 K and examined under pure hydrogen at ambient temperature in specially constructed glass cells fitted with mylar windows. Chemisorption studies were performed in a dynamic mode using a pulse-flow microadsorption apparatus. Samples of precursor (0.3-0.5 g) were reduced in s i t u in flowing 5% hydrogen in nitrogen by heating to 573 K at 7 K min-'. reduction the flow was changed to helium, the temperature held at 573 K until hydrogen elution ceased and then lowered to ambient. into the helium stream ahead of the adsorbent as pulses of known size (typically 0.05 cm3 at 0.101 MPa). between the peak obtained and a calibration peak. adsorption was 2 x 10l6 molecules. investigated at ambient temperature in a static system using pressures up to 533 Pa (ref. 6 ) . FTIR spectra of adsorbed-C0 were obtained in transmission at a resolution of 2 cm-' using a Nicolet 5DXC spectrometer with TGS detector. Hydrocarbon reactions were carried out in a static reactor (200 ml) attached to a conventional grease-free high vacuum apparatus. Samples of precursor were placed in the reactor, evacuated, and reduced in pure hydrogen at 523 K for 0.5 or 1.0 h. Reactants (cyclopropane, butadiene) were admitted to the catalysts in the order (i) hydrocarbon, (ii) hydrogen; pressure fall was measured by use of a pressure transducer, and analysis was by glc. hydrogenation was conducted in the liquid phase in a stirred glass reactor (Fischer Porter). 10 ml methyl pyruvate, 20 ml ethanol, and dihydroc inchonid ine- t reated catalyst (0.1 g EUROPT-1 or 1.0 g Pt/silica) were placed in the reactor, hydrogen was admitted and maintained constant at 10 bar pressure, and the contents stirred vigorously. After the required hydrogen uptake the product was filtered, distilled, and analysed by glc and polarimetry according to procedures published elsewhere (ref. 7) to determine the optical After Adsorbate was injected The amount adsorbed was determined from the difference The detection limit for Chemisorption of [14C] carbon monoxide was Enantioselective ester 137 yield in methyl lactate formation. THE CATALYSTS: PREPARATION, ANALYSIS, MICROSCOPIC AND SPECTROSCOPIC PROPERTIES support was M5 Cab-0-Sil silica (surface area 203 m2 g-l; zero pore volume; impurities, C1=540 f 50 ppm (by neutron activation) Na = Cu = 20 ppm (by A stock of the precursor to Pt/silica was prepared as follows. The chosen ICP-MS)). 9,76 g chloroplatinic acid (Johnson Matthey, platinum assay 41%) was dissolved in deionised water (0.60 1) in a five litre flask. Sil) was added and mixed until the suspension began to gel, at which point further deionised water (ca . 0.50 1) was added to promote mobility. process was repeated until 398 g of silica had been introduced; at that point the total volume of water added had become 2.5 1. The flask was then attached to a Buchi rotary evaporator and water slowly removed by maintaining the contents at 353 K under a partial pressure of dry nitrogen. yellow free-flowing powder was obtained. at 456, 374 and 264 nm consistent with the presence of chloroplatinate ions (ref. 8) and a further band at 206 nm which., is assigned to an 0-ligand to Pt charge transfer (ref. 9) and which indicates the presence of hydroxochloro- or oxochloro-platinum anions, e.g. [PtC150H]2-. The Pt-content of the precursor was 0.73 wt %. and 72.7 eV. Silica (M5 Cab-O- This After 48 h a pale Its uv-visible spectrum showed bands The Pt(4f7/2) binding energy measured on two occasions was 72.4 Temperature programmed reduction (TPR) of the precursor was examined over the range 180 to 773 K. feature was an asymmetric peak having a sharp maximum in the range 400 to 410 K. Reduced Pt/silica provided values for the Pt(4f712) binding energy of 71.2 and 71.4 eV, the Pt content was 0.76 wt% and the C1 content 610 f 60 ppm. Thus, the chlorine content o f the reduced P t l s i l i ca was no greater than that o f the original support material , within the f 10% experimental error inherent in neutron activation analysis. EUROPT-1 was prepared in a 6 kg. batch by Johnson Matthey Chemicals plc (ref. 2). Sorbsil AQ U30 silica was treated with Pt(NH3)4C12 and Pt(NH3)4(0H)2 at pH 8.9, filtered, washed until free of C1-, dried at 378 K and reduced at 673 K (ref. 2). The catalyst became oxidised by air before issue (refs. 3,lO). The total surface area of EUROPT-1 is 185 m2 g-', the Pt-content is 6.3 wt% and the impurities (in ppm) are: A1 = Ca = 500; Ti = Na = 400; Mg, 200; K , 150; Fe, 90; C1, < 50; Cr < 10. The Pt(4f7/2) binding energy in the re-reduced catalyst was measured in three laboratories at 71.3, 71.4, and 71.5 eV. The catalyst may be re-reduced in pure hydrogen without sintering at temperatures up to 673 K . TPR (this work) showed that re-reduction commenced at 248 K and was complete at 425 K , the maximum in the reduction profile occurring at 340 K; this concurs With a heating rate of 10 K min-' th'e major reduction 138 with a previous report (ref. 11). The platinum particle size distribution (PSD) in Pt/silica, measured by HRTEM was: ,< 1.0 nm, 60%; 1.0 - 1.6 nm, 18%; 1.6 - 2.2 nm, 14%; 2.2 - 2.8 nm 6%; 2.8 - 4.0 nm, 2%. It is probable that some of the smallest particles present escaped detection, and hence this distribution is to be regarded only as a guide to the very high dispersion (approaching 100%) of the platinum active phase in this catalyst. The platinum PSD in EUROPT-1 reduced below 673 K contains maximum in the distribution at 1.8 nm, 75% of the platinum particles are c 2nm in diameter, and the dispersion i s 60% (ref. 3) . been studied by EXAFS spectroscopy. for 80 min (Ptlsilica) or 60 min (EUROPT-1) and spectra were taken with the catalysts in a hydrogen atmosphere. The experimental and computed spectra for Pt/silica are shown in Fig. 1, that for EUROPT-1 and a Pt foil (for reference) The morphology of the platinum particles in Pt/silica and in EUROPT-1 has Catalysts were reduced in pure H2 at 573 K -8 - 0 200 400 600 800 eV Fig. 1. Pt L -edge EXAFS spectrum of 0.76 wt% Pt/silica. Full curve = experimental (unsmoothed) spectrum: dashed curve = theoretical spectrum. 3 were of comparable quality. in Table 1. together with the coordination numbers (CN) and Debye Waller factors (DWF) appropriate for each shell. Structural information from these spectra is shown The d-, JZd, /3d, and 2d spacings are observed in each spectrum The values of the coordination numbers provide 139 direct information concerning the likely morphology of the average platinum particle present in each catalyst. Fig. 2 shows three 14-atom clusters o f Pt atoms of which b and c possess coordination numbers in reasonable agreement with experiment (Table 2). Similar calculations show that an average particle in TABLE 1 Structural parameters* obtained by EXAFS spectroscopy Platinum foil Pt/silica EUROPT-1 Pt-Pt/nm CN DWF Pt-Pt/nm CN DWF Pt-Pt/nm CN DWF 0.277 12.0 0.010 0.276 4.4 0.012 0.276 5.5 0.013 0.392 6.1 0.014 0.391 1.6 0.011 0.391 2.1 0.018 0.481 21.9 0.016 0.477 1.3 0.009 0.477 3.8 0.017 0.547 14.0 0.012 0.543 2.1 0.010 0.543 3.9 0.015 * CN = Coordination number; DWF = Debye Waller factor TABLE 2 Structural parameters for proposed model particles a, b, c, and d o f Fig. 2 Pt/silica Coord. Nos EUROPT-1 Coord. Nos P t -P t /nm Expt a b C Expt d 0.276 4.4 4.1 4.4 5.1 5.5 5.5 0.391 1.6 0.0 0.9 1.1 2.1 0.7 0.477 1.3 2.7 2.1 1.0 3.8 3.7 0.543 2.1 2.3 1.7 1.1 3.9 3.4 a C Fig. 2. Model configurations for Pt particles in Pt/silica (a.6.c) and in EUROPT-1 (d). EUROPT-1 i s larger and is reasonably described by model d of Fig. 2. This model will be discussed in greater detail elsewhere. HRTEM and EXAFS each demonstrate that the average platinum particles in Pt/silica are very small (ca. 0.1 nm) and i n EUROPT-1 are larger ( c a . 0.2 nm). In addition the Pt particles i n each 140 catalyst appear to consist of a raft of (111)-structure, with some atoms present in a partial second layer. ADSORPTION PROPERTIES Isotherms for [14C]carbon monoxide adsorption at 298 K over the range 0 to 526 Pa were measured using reduced Pt/silica (0.268 g) and EUROPT-1 (0.150 9). The adsorption capacities of the two catalysts were similar, and each isotherm showed a primary and secondary region (ref. 12) the transition occurring at about 26 Pa. (Pt/silica) or 5% (EUROPT-1) of the adsorbed-C0. '658 Pa [12C]carbon monoxide resulted in 90% (Pt/silica) or 58% (EUROPT-1) removal of the remaining [14C]carbon monoxide. on Pt/silica contained one strong band at 2085 cm-l having a slight shoulder on the high frequency side, whereas that for EUROPT-1 contained three bands: 2075(s), 1849(w) and 1720(vw) cm-l attributed to linear, bridged and capped species (ref. 5). Clearly, from the exchange measurements, more CO is reversibly adsorbed on Pt/silica than on EUROPT-1, and this is consistent with the higher frequency observed for CO adsorbed in the linear form on Pt/silica i n comparison with that on EUROPT-1 which implies a stronger Pt-C bond in the latter system. Similarly, CO adsorption on Pt/silica measured by the pulse technique over the range 256 to 294 K is an activated process (to be published), whereas no such claim has yet been made for CO adsorption on EUROPT-1. In oxygen adsorption on Pt/silica at 273 K measured by the pulse technique, 5.73 x 10l8 molecules (9. cat.)-' were adsorbed at saturation which corresponds to an O:Pttotal ratio of 0.5:l.O. The corresponding stoichiometry on EUROPT-1 was 0.65:l.O (ref. 5). However, after allowance for differences in dispersion, O:PtSurf i s 0.5:l.O for Pt/silica and 0.9:l.O for EUROPT-1. Furthermore, there is evidence for bulk oxidation of EUROPT-1 by oxygen (ref. 5) . these results that adsorption of CO and perhaps that of O2 i s stronger on EUROPT-1 than on Pt/silica. Evacuation for 0.5 h at 298 K caused the desorption of 47% Subsequent equilibration with The FTIR spectrum of CO adsorbed Thus, although comparisons must be made with caution, there is a consensus in CATALYTIC PROPERTIES 1,3-Butadiene hydrogenation particular a greater than expected extent of 1:4-addition is indicative of the presence of electronegative contaminants ( S , C1) at the active sites (ref. 14). Hydrogenations were conducted at 290 K over Pt/silica and EUROPT-1, each reduced at 523 K and, for reference purposes, over an evaporated Pt film at 326 K (initial pressures: C4H6, 6.6 kPa; H2, 19.7 kPa; conversion, 20%). The kinetics and mechanism of this reaction are well understood (ref. 13); in Butene and 141 butane yields were about 67% and 33% respectively and the butene compositions were: Pt film Pt/silica EUROPT-1 1-butene 76 78 75 t-2-butene 18 14 18 c-2-butene 6 8 7 Pt/silica and EUROPT-1 surfaces behave similarly to that of the clean evaporated film confirming the absence of C1 in the neighbourhood of the Pt sites in the supported catalysts and showing that there is no particle size effect on selectivity in this reaction. Cyclopropane hydrogenolysis The failure of butadiene hydrogenation to distinguish between Pt/silica and EUROPT-1 may be related to the very strong adsorption of this hydrocarbon on Pt. We therefore examined the conversion of cyclopropane to propane, as this cyclic hydrocarbon is among the most weakly adsorbed of those that undergo hydrogen addition. Bond has reported that orders for this reaction vary with reactant pressures such that rate passes through a maximum with increasing pressure of either reactant; moreover this behaviour conforms to expectation based on Langmuir-Hinshelwood theory (ref. 16). Pt/silica and EUROPT-1, each reduced in pure hydrogen at 523 K for 1 h were used as catalysts for this reaction at 313 K ; the variation of rate with hydrogen pressure is shown in Fig. 3. The expected maxima were observed but the behaviour o f Pt/silica conforms to expectation based on Langmuir-Hinshelwood kinetics (eq. 1) whereas that of EUROPT-1 does not. (In eqn. 1 the symbols have their usual significance, r = kOC OH = k e c O l = kbcPc(bH1/nPHân)2/(1 + bcPc + bA/nPH1/n)3 ( 1 1 3 1 c = cyclopropane, n is defined by the process: 2H(ads)- H2(g) + n(vacant sites)). For Pt/silica the experimental points are well modelled by an equation having n = 1 (firm curve, Fig. 3a, k = 10.2, bH = 0.70, b, = 0.0.33) and n = 0.5. However, we have failed to model the behaviour of EUROPT-1 adequately. The dashed curve in Fig. 3b i s a poor fit (n = 1) and we note that only the sharp maximum is well modelled by the dotted curve (n = 2, for which there is no ready interpretation). Thus the surfaces of the small Pt particles in Pt/silica behave as an energetically homogeneous surface on the Langmuir model whereas those of the larger particles in EUROPT-1 do not. It is not clear whether failure in the latter case i s due to the larger average size of the particles or to their wider size distribution. 142 P4I T o r r 100 2 00 300 0 Pi;l Tor r 1 Fig. 3. Dependence o f initial rate, R/Torr min- , on initial hydrogen pressure, Pi/Torr, in the hydrogenolysis of cyclopropane to propane catalysed at 313 K by Pt/silica (a) and by EUROPT-1 (b). 133.3 Pa]. eqn. 1 (see text), and the dotted and dashed curves show the corresponding theoretical variations o f surface coverages. the experimental variation of rate, and the dotted and dashed curves represent predicted behaviour according to eqn.1 (see text). Po = 125 Torr. [ I Torr = In (a) the full curve represents a vakation of rate given by In (b) the full curve describes Asymmetric Hydrogenation Each catalyst has been modified by the deposition on its surface of cinchonidine(1) and used for the high pressure hydrogenation of methyl pyruvate, MeCOCODEae, to MeCH(0H)COOMe according to our variation of Orito's method (refs. 7, 17). The reaction provides an excess of R-(+)-lactate. optical yields (%R - %S) at 290 K and 25 to 50% conversion were 87% over Typical 143 ( I ) Cinchonidine. R = vinyl Dihydrocinchonidine. R = ethyl EUROPT-1 and 54% over Pt/silica. several L-shaped alkaloid molecules occurs on each platinum particle of EUROPT-1 leaving exposed shaped ensembles of platinum atoms most of which accommodate methyl pyruvate in the conformation which, on hydrogenation, gives R-(+)-methyl pyruvate (ref. 7). Pt/silica contains much smaller particles (Fig. 2) most of which may be unable to accommodate more than one alkaloid molecule; hence the steric situation is less well defined and the optical yield much lower. it might be argued that the typical 1 nm Pt particle in Pt/silica does not contribute to the optical selectivity but catalyses the formation of racemic We have proposed that ordered adsorption of Indeed, lactate, and that a smaller proportion of larger Pt optical yield observed. CONCLUSIONS 1. A Pt/silica has been prepared from a C1-contain particles produces the ng source in such a way that the Pt particles show no evidence of contamination by C1. 2. The platinum particles in this Pt/silica are structurally similar to, but smaller than, those in the standard reference catalyst EUROPT-1. 3 . (cyclopropane, CO) exhibit different characteristics when adsorbed on the Pt surfaces of these two catalysts, whereas substances that are strongly adsorbed (02, butadiene) show similar or identical behaviour. 4 . Where the adsorption of a molecular template on the Pt surface is required in order to induce enantioselectivity, the optical yield is diminished as size of the Pt particles approaches that of the template. Substances which are weakly or only moderately strongly adsorbed 144 ACKNOWLEDGEMENTS We thank SERC and ICI C & P Ltd for financial support in the context of a Cooperative Award. studentships to MBTK and PAM. SERC is also thanked for SRS beam time and for the award of REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12. 13 14 15 16 17 G.C. Bond and P.B. Wells, Applied Catal., 18 (1985) 221. G.C. Bond and P.B. Wells, Applied Catal., 18 (1985) 225. J.W. Geus and P.B. Wells, Applied Catal., 18 (1985) 231. A. Frennet and P.B. Wells, Applied Catal., 18 (1985) 243. P.B. Wells, Applied Catal., 18 (1985) 259. S. Kinnaird, G. Webb and G.C. Chinchen, J. Chem. SOC. Faraday I, 8 3 (1987) 3399. I.M. Sutherland, A. Ibbotson, R.B. Moyes and P.B. Wells, J. Catal., accepted for publication. C.K. Jorgensen, Acta Chem. Scand., 10 (1956) 518. D.L. Swihart and W.R. Mason, Inorg. Chem., 9 (1970) 1749. R.W. Joyner, J. Chem. SOC. Faraday Trans. I , 76 (1980) 357. G.C. Bond and M.R. Gelsthorpe, Applied Catal., 35 (1987) 169. J.U. Reid, S.J. Thomson and G. Webb, J. Catal. 29 (1973) 421. J.J. Phillipson, P.B. Wells and G.R. Wilson, J. Chem. SOC. (A), (1969) 1351. M. George, R.B. Moyes, D. Ramanarao and P.B. Wells, J. Catal., 52 (1978) 486. A.G. Burden, J. Grant, J. Martos, R.B. Moyes and P.B. Wells, Discuss. Faraday SOC., 72 (1981) 95. G.C. Bond and J. Turkevich, Trans. Faraday SOC., 50 (1954) 1335; G.C. Bond and J. Newham, Trans. Faraday SOC., 56 (1960) 1501. Y. Orito, S. Imai, and S. Niwa, Nipp. Kag. Kaishi, 8 (1979) 1118. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 145 FACTORS ANALYSIS FOR MECHANICAL STRENGTH IN PELLETING PROCESS OF Fe-BASED HIGB TEMPERATURE SHIFT CATALYST YONGDAN LI, JIUSHENG ZHAO and LIU CHANG Department of Chemical Engineering, Tianjin University, Tianjin 300072, (China ) ABSTRACT The pelleting process and the factors influencing mechanical strength for Fe-based catalyst have been investigated. The horizontal crushing strength data is used in the text, their mean value and their corresponding Weibull modulus are taken as parameters for analysis. It is found that the processing precision of the pelleting machine, the pretreatment of the powder and the pelleting conditions all have strong effect on the final strength of pellets. The experimental result shows that a potentiality of increasing in overall properties exists, which means a better quality catalyst could be expected. INTRODUCTION Formation of pellets is one of the important processes in catalyst preparation, which significantly affects the mechanical strength and the texture, on which the reliability of catalyst performance in industrial reactor and the effectiveness of pellet depend, even though it usually does not change the intrinsic catalytic activity apparently. The influence of pelleting conditions on the properties of catalyst has been studied by several authors (ref.l,2). Their publications stressed mainly on the effect of pelleting pressure. But actually in the process of pelleting, there are many factors which will influence the properties of the resulting pellets. In most cases these factors are interdependent on one another. Yet in general idea, they can be distinguished into three different categories, that is the processing precision of pelleting machine, the property and the treatment of the powder material and additives, and the processing pressure of pelletization. This paper will report some important factors and their effectiveness influencing both the mechanical strength and the overall properties of resulting pellets, and finally it will also tell us the potentiality of getting better catalyst as a whole. SAMPLES AND EXPERIMENTAL Samples The powder material of mixed oxides was obtained from industry made 146 X I I I I I 20 30 40 50 60 70 Degrees (28) Fig.1 The XRD diagram of the pre-pelleting mterial by coprecipitation process. It contains about 10wt% of Cr2O3, 90wt% of Fez03 and a small amount of volatile combines. The XRD pattern (Fig.1) shows r-Fe203 phase is the most significant one. Several brands of commercial Fe-based catalyst pellets with a length of 4-7mm and a diameter of 9mm are taken for comparison. Pelletinq The pelleting was performed by using a hydraulic press with a set of specially designed die and punch, from which air can be released easily. Experiment shows that the contactsurface of both the die and the punch must be extremely smooth, or else during ejection there would be a big shearing stress "u" (refer to Fig.2) developed at the edge plane of the pellet, which in turn will induce big flaws within the pellet; at some cases there was even no complete pellet made. The precision of filling weight is also very important for strength, because under a definite operating condition the stress distribution in pelleting within the pellet is sensitive to the filling weight. The set of die and punch was carefully machined so that a variety of the strength data with a close distribution could be obtained. The purpose of which is to guarantee the repeatability of data for analyzing different factors involved. The process is illustrated as in Fig.2. (a). Transfer the material into the cylindrical cavity of the die, and rest it on the upper side of the lower punch. (b). Compress the powder by lowering the upper punch. (c). Eject the pellet by the upper movement of lower punch. The pellets obtained in our laboratory under a normal processing condition with the industrial material possess a mean horizontal crushing strength ( H C S ) of 45.8 and a Weibull modulus of 10.1, which are much higher than those coming from the industry. All the pellets in this paper made in our laboratory have the same size, with a length of 6mm and a diameter of 9mm. 147 Fig.2 Pelleting process Measurement The pellet strength is characterized by the HCS value, which has been shown in accordance with tensile fracture stress and proved to follow Weibull distribution (ref.3) both theoretically and practically, with the probability of failure under certain radial compression force given by where p is the maximum stress, or in this paper, the maximum compression force, m is the Weibull modulus, is a size factor, F(p) is the probability of failure under p. The HCS data of solid catalyst scatters in a certain mode characterized by the different value of Weibull parameters m and p, indicating the discrepancy of preparation technology. We can see from equation (l), that higher m and lower means less scattering of the data. In this paper, both the mean value of HCS and the Weibull parameters of the catalyst strength were used in the analysis. The XRD pattern was taken by Rigaku 2038 X-ray diffractometer, Cu KO(, and the texture data was measured by AUTO PORE 9220 I[ porosimeter. RESULTS AND DISCUSSION Statistical result of commercial catalyst pellets Statistical results of some properties of several brand commercial catalyst pellets show that the scattering of length of different samples also follows Weibull distribution reflecting the precision of the pelleting machine and bears some relationship with the scattering of HCS data, as indicated in Fig.3 and Table 1. Sample 1 and 2 are two different brands of commercial catalyst, but were pelleted by same model of pelleting machine. Sample 3 and 4 are another two brands but were pelleted by another same model of machine. We can see the Weibull modulus of strength and length given by same model machine are very close. In our other paper, the relationship between the scattering 148 of strength and density was studied (ref.4) and shown the same trend. Scattering of strength, length and density in most cases display the performance of the pelletinq machine. h cl Q 6 0 .6 - Q I 0 10 20 30 40 0 10 20 30 40 50 3.04.05.06.03.04.05.06.07.0 HCS(kg/pellet) l ( m ) Fig.3 Strength and length distribution of commercial catalysts a & b: strength distribution of sample 3 & 1 in Table 1, c & d: length distribution of sample 3 & 1. Table 1. Weibull parameters of length and strength sample mS BS ml B1 S 1 3.59 4.75 x 10-6 11.8 1.40 x 27.8 2 3.41 4.16 x 11.5 3.25 x 34.2 3 6.23 1.96 10-9 43.2 2.10 10-30 24.9 4 6.44 1.98 x 46.9 2.20 10-32 31.3 Note: ms & ps - Weibull parameters of strength: - Weibull parameters of length: m1 s - ' "the most probable strength (HCS). Factors Influencing the Mechanical Strength of Pellet For simplicity and convenience, two sets of two level fractional factorial experiments were carried out. First Set Factorial Experiment FOUr factors, pelleting pressure, powder size distribution, water content, and graphite amount are taken into account. Table 2 gives the factors and levels of the experiment. The results and the experimental matrix as well as the comparative effectiveness are shown in table 3. From these two tables, we can see that all the factors are sensitive. The powder size distribution and water content are most effective. The 149 pressure has a negative effect, it means as the pressure increases, the strength decreases. Notice that the change of density with strength is not in a simple manner, which shows it may be possible to get good mechanical strength with comparatively low desity after appropriate treatment. Table 2. Factors and levels of first set factorial experiment factors Levels A(kbars) B(mesh ~ 0 1 % ) C(Wt%) D(Wt%) pelleting powder size graphite water pressure distribution content content 0 3 1 5 0.5 4 60-100, 10 0.5 14.5 < 100, 60 20-60, 30 60-100, 10 1 10.1 < 100, 30 20-60, 60 60-100, 10 < 100. 45 20-60, 45 0.75 11.76 Table 3. Results and effectiveness of factors experimental matrix results HCS (mean) Density (mean) No. D A B C (kg/pellet) (g/cmâ) - - 1 0 2 0 3 0 4 0 5 1 6 1 7 1 8 1 9 0 0 0 1 1 0 0 1 1 5 0 0 1 0 1 0 1 0 1 5 0 0 1 1 0 1 0 0 1 5 0 58.1 47.8 43.6 48.8 31.9 57.9 42.5 29.8 5 45.5 2.46 2.13 2.14 2.47 2.43 2.73 2.72 2.40 2.48 A B C D AD BD CD Eff. for HCS -3.22 -12.3 2.07 -12.3 -11.6 9.2 0 Eff. for Density 0.257 0 0.013 -0.31 0.017 0 -0.032 Second Set Factorial Experiment In this set, another 4 factors, 150 pelleting pressure, HNO3 concentration, r-A100H doping, and grinding time are examined. The factors and levels are shown in Table 4. Levels of E denotes same volume but different concentration of HNO3 was added into the material. F refers to doping with r-Al00II (below 300 mesh), and G stands for the time of hand grinding of the material before pelleting. Table 4. Factors and levels of second set experiment __ factors Levels A(kbars) E(N) pelleting HNO3 pressure conc F(Wt%) r-A100H cont G(min) grinding time 0 3 1 5 0.5 4 0 0 1 2 0.5 1 0 20 10 Table 5. Results and effectiveness of factors experimental matrix results HCS (mean) Density (mean) (kg/pellet) (g/cmâ) G A E F No. 0 0 0 0 0 1 0 1 0 0 1 1 1 0 0 1 0 1 1 1 0 1 1 1 0.5 0.5 0.5 0 1 1 0 1 0 0 1 0 . 5 54.6 69.3 54.2 41.6 103.2 71.7 42.1 79.0 63.8 2.22 2.22 2.19 2.09 2.56 2.50 2.44 2.44 2.52 A E F G AG FG Eff. for HCS 8.73 -20.6 8.29 20.1 20.4 -12.6 Eff. for Density 0.697 0.306 -0.041 0.039 0 0 The results and effectiveness are given in Table 5 . The factors in the second set have more striking effect on the mechanical strength, while the effect on density is still less sensitive. It is shown that by grinding and doping of r-AlOOH, the strength i s greatly enhanced, but impregnation 151 with HNO3 gives bad result. Effect of grinding shows the same effect as particle size distribution, that means smaller particle size is beneficial for strength. The highest HCS in Table 5 is 103.2, which is approximately 5 times as high as that of ordinary commercial catalyst, as its density is only 2.56, still in the range of commercial ones. The experiment shows that a great potentiallity in increasing mechanical strength is existing. Effect of Pelleting pressure Pressure on Density Normally as pressure increases, the mean strength of pellets increases, and the density increases too. Yet in our experiment, 60 50 - 4 c) .i .i 40 2 \ 2 30 - :: 20 X 10 ,>â /o-----â 0 1 2 3 4 5 Pelleting Pressure (kbars) Fi9.4 Dependence of strength and density on pelleting pressure pellets made directly from industrial powder without any pre-treatment shows some limitations of pelleting pressure on HCS and density as illustrated in Fi9.4. There appears upper limit both in strength and density between a certain range of pelleting pressure. Such fact suggests the incompressibility of mother crystals in pellets. As for the diffusion limiting water gas shift reaction (ref.51, a relatively high strength and a comparatively low density to enhance the effectiveness factor of the catalyst are preferred. Table 6. Effect of pelleting pressure on the pore structure of oxidized state ~~ P HCS S V R D m c 2 40.4 71.6 0.233 13 2.22 9.56 2.99 x 10-16 3 45.8 67.0 0.184 11 2.30 10.1 1.21 10-17 5 58.4 63.5 0.159 10 2.64 7.51 3.46 10-14 In the table, P(kbars): pelleting pressure, HCS (kg/pellet): mean horizontal crushing strength, S (m2/9): specific surface area, V (ml/g): porosity, R (nm): most probable pore diameter, D (g/cmâ): density, m and @ : the Weibull parameters. Pressure on Pore Structure and Activity Table 6 and 7 display the effect of pelleting pressure on the mechanical strength and the pore structure 152 of oxidized and reduced state catalysts respectively. The reduction was performed under an optimum condition for strength developed by a set of optimization experiments (ref.6). Table 7. Effect of pelleting pressure on the properties of reduced state of the catalyst P HCS S V R D m B A 2 35.5 70.6 0.287 16.3 2.10 2.93 1.99 x 57.3 3 63.0 65.0 0.244 14.4 2.28 12.8 6.60 x 51.2 5 58.5 60.1 0.189 12.6 2.46 4.60 4.89 10-9 46.3 A: the apparent activity of CO conversion in a microreactor under normal high temperature shift reaction condition (ref.6). From the above two tables, we noticed that all of the parameters changed in the same trend as pelleting pressure changed, except Weibull parameters m and p . They give optimal values at P=3 kbars, under which the most reliable catalyst in mechanical strength can be expected as shown in Fig.5. Both the HCS and the Weibull modulus of the sample pelleted at P=3 kbars is much superior to that of commerial catalysts (refer to Table 1). Meanwhile, its density and activity are quite acceptable. As a matter of fact the reliability or the probability of failure may be the most important in industrial point of view. 1.0 0.8 0.6 0.4 0.2 f a/ a' 1.0 0 .8 0.6 0.4 0.2 0 10 20 3 0 40 50 60 70 40 50 60 70 80 20 30 40 50 60 70 80 90 100 HCS (kg/pellet) Fig.5 HCS distribution of reduced state at different pelleting pressures a: 2 kbars, b: 3kbars. c: 5 kbars. REFERENCES 1 J. Uchytil, M. Kraus and P. Schneider, Influence of Pelleting Conditions on Catalyst Pore Structure and Effectiveness, Appl. Catal., 28 (1986) 13-14. 153 2 I. Brasoveanu, S.I. Blejoiu, A. Ssabo, P. Rotaru and I.V. Nicolescu, Structural Strains Appearing in the High Temperature Shift Conversion Fe-Cr Catalyst, Revue Roumaine de Chimie, 25(8) (1980) 1159-1169. 3 Yongdan Li, Liu Chang and Zhou Li, Measurement and Reliability Analysis of Mechanical Strength of Cylindrical Metallic Oxide Catalyst, Journal of Tianjin University, 1989 ( 3 ) 9-17. 4 Yongdan Li et al., Statistical Analysis for Mechanical Strength of Cylindrical Fe-based Catalyst. Journal of Fuel Chemistry and Technology, in press. 5 Hans Bohlbro, An Investigation on the Kinetics of the Conversion of Carbon Monoxide with Water Vapour over Iron Oxide Based Catalysts, Second Edition, Gjellerup, Copenhagen, 1969. 6 Yongdan Li et al., Factors Analysis on Mechanical Strength in Heating and Reduction of High Temperature Shift Catalyst by Dn Saturation Optimum Experimental, C1 Chemistry and Chemical Industry, in press. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 155 STUDIES ON PORE SIZE CONTROL OF ALWINA: PREPARATBN OF ALWINA CATALYST EXTRUDATES WITH LARGE UNMODAL PORE STRUCTURE BY LOW TEMPERATURE HYDROTHEWW TREATMENT M. Absl-Halabi, A. Stanislaus and H. Al-Zaid Petroleum Technology Department, Petroleum, Petrochemicals and Materials Division, Kuwait Institute for Scientlflc Research, P. 0. Box 24885, 13109 Safat, Kuwait ABSTRACT In the present work, the application of low temperature hydrothermal treatment method for preparing y-alumina supports with large monomodal pore size distribution has been investigated. Gamma alumina in the form extrudates was subjected to hydrothermal treatment in an autoclave for various durations in the temperature range 150-3OO0C. The effect of ammonia vapor during hydrothermal treatment was also studied. The treated catalysts were characterized for surface area, pore size distribution and mechanical strength. The samples were also examined by x-ray diffraction and transmission electron microscope. The results revealed that pores can be widened selectively with greater than 70% pore volume In the mesopore range 100-250 A' and with improved crushing strength by low temperature hydrothermal treatment (e.g. 150°C). Further shift of pore size to any desired larger range (250-500 Ao or 500-1500A') is possible by increase of temperature and treatment time. Ammonia was found to enhance the pore enlargement. The undesirable effects on loss of surface area and mechanical strength were signlficantly low and alumina phase transition was negligible in this process. INTRODUCTION Alumina has been widely used as a catalyst support in many catalytic processes of industrial importance. This is largely because it is reasonably stable, contains acidic and basic sites and can provide, through its different phases a wide range o f surface areas and porosities which are suitable for many catalytic applications. For some applications like hydroprocessing of petroleum residues and coal derived liquids it is desirable to have cata- lysts with wide pores since the large complex molecules present in heavy residues must have access to the active surface sites within the catalyst pellets through an appropriate pore network (1.2). A greater emphasis has been placed in recent years on the development of wide pore catalysts with small external dimensions to overcome the problem o f diffusion limi- tations (3,4,5.6). Catalysts with bimodal pore size distribution having mesopores and macro- pores in different proportions have been recommended to prevent rapid deactivation In resi- due hydroprocessing. Large pore unimodal catalysts have also been found to be more effective in residue hydroprocessing. 156 Pore size control of supported catalysts Is effected mainly by controlling the pore size of the support. Several methods have been described in the literature for controlling the pore size of alumina supports (7,8.9,10). Thermal sintering has been chosen as a meth- od for obtaining catalyst supports with monomodal pore size distribution In the mesopore or macropore (11,12,13) range. The use o f some inorganic salt additives and different atmos- pheres in enhancing the rate of sintering and lowering the calcination temperature to produce large pore alumina catalyst supports has also been explored (12.14.15). Thus, for example, by using Moo3 doped alumina, Tischer noticed that the sintering temperature could be lowered from 1000 to 700°C to produce a similar pore size distribution. Sintering In presence of steam has also been found to result in enlargement o f pores (7,11,13,16). However, in these studies temperatures as high as 700% have been found necessary to promote sintering and t o produce aluminas with desired large pore size. The large pore aiuminas prepared by the high temperature sintering process usually have reduced surface area and low mechanical strength. Further, part o f the y-alumina phase is converted to other types (6. 8 & a) of aluminas that are catalytically not as active as the y-aiuminas. These undesirable effects on surface area, mechanical strength and alumina phase transformation may probably be minimized if selective sintering of the nar- row pores could be achieved at lower temperatures. A major obJective of the present work was to investigate the influence o f low temp- erature hydrothermal treatment on pore size enlargement of y-alumina support. The material (y-alumina) In the form o f extrudates was subjected to hydrothermal treatment in an auto- clave in the presence of water vapor for various duration In the temperature range 150-300°C. The influence o f ammonia vapor during hydrothermal treatment was also investi- gated. The treated catalyst samples were characterized for surface area, pore volume, pore size distribution and mechanlcai strength. The samples were also examined by x-ray diffraction and transmission electron microscope to assess possible changes in the alumina phase and the extent of sintering. The results o f the studies reveal that low temperature hydrothermal treatment can be used for the preparation of alumina support extrudates with large unimodal pore structure by selective enlargement of pores. EXPERIMENTAL Alumina extrudates were prepared from Condea Chemie Pural SB boehmite gel by knead- ing and extrusion. Alumina paste suitable for extrusion was prepared by peptizing and kneading the alumina powder with the peptizing solution. In a typical experiment, 250 g of alumina powder and appropriate quantity o f peptizing agent were used for each batch. The peptizing agent(l.5X HN03) was added at a constant flow rate over 20 min duration with continuous mixing using a kneader (Linden Model D5277, Germany). The paste was extruded through 1.5 mm nozzles, then dlred at 110°C in an oven for 24 h. The dried extrudates were calcined under programmed temperature condltions (at 370'C for 2 h, 450°C for 1 h and 550'C for 2 h). A slngle screw type extruder model No. 157 250 (Netzsch, Germany) was used in making the extrudates. For hydrothermal treatment studies y-alumina extrudates prepared from Condea Pural SB alumina by the above proce- dure were used as starting material. A weighed portion o f the sample (about 10 g) was heated In an autoclave at temperatures ranging between 150 and 300'C. Two types of reagents, namely, water and ammonium hydroxide were used in the study. The treatment time and the ratio o f water t o alumina were also studied. A mercury porosimeter (Micromeritics model 9305) was used to determine pore size distribution. A Quantasorb adsorption unit (Quantachrome Corporation, USA) was used for BET surface area measurements. Pharma test model PTB 300 equipment was used to meas- ure the side crushing strength of the alumina extrudates. X-ray diffraction patterns were obtained using a Phillips PW 1410 x-ray spectrometer operated at 30 kV and 20 mA with C u Ka radiation. Transmission electron micrographs were made with JEM-1200 EX microscope. RESULTS AND DISCUSSION Gamma alumina in the form of 1.5 mm extrudates was subjected to hydrothermal treat- ment in an autoclave In the presence of water vapor, for different time periods in the temperature range 150-300°C. The effect of water t o alumina ratio and the presence of ammonia vapor during hydrothermal treatment in the same temperature range on the modifica- tion of pore size was also Investigated. The results are presented and discussed below. H f e c t o f treatment t i m e . Influence of the duration of heating on pore size distribution was investigated at a constant temperature . The results obtained at a con- stant temperature of 150°C for water t o alumina ratio 1:l (w/w) are presented in Table 1. Table 1. Effect of Hydrothermal Treatment Time on Pore Size Distribution (Reagent: water; Temp. 150°C) 60-1 00 100-250 250-500 500-1 500 1500-4000 4000-1 0000 10000-100000 > 100000 Tota I 0.405 0.025 0.006 0.004 0.001 0.002 0.000 0.005 0.448 0.408 0.03 0.008 0.003 0.002 0.001 0.000 0.007 0.459 0.230 0.287 0.01 0.006 0.004 0.001 0.000 0.012 0.55 0.124 0.377 0.008 0.005 0.006 0.000 0.000 0.007 0.527 0.430 0.022 0.010 0.004 0.0 0.0 0.0 0.004 0.47 It can be noticed that the amount of pores in the 60-100 A' diameter range progressively decreases y-7 L;, increasing time o f heating with a corresponding increase in the amount of 158 100-250 A' pores. Thus, the amount o f 60-100 A' pores is reduced from 90.4 t o 36.9% with an increase in the amount of 100-250 A' pores from 5.6 t o 59.7% when the heating time is increased from l h t o 4 h. Further increase o f heating duration to 8 h resulted in a further increase in the amount of 100-250 A" pores (from 59.7 t o 71.5%) with a corre- sponding decrease in the amount o f the 60-100 A" pores. Thermal treatment in the pres- ence o f water at a temperature o f 150°C, thus, increases Selectively the amount of 100-250 A' pores. A similar effect o f heating t i e on selectively increasing the amount of pores o f a particular diameter was also noticed for samples heated at higher temperatures. However, the pore size range that is widened or enlarged depends to a large extent on the temperature o f heating as shown below. Effect of temperature. The temperature o f hydrothermal treatment was varied between 150 and 300°C t o study its effect on pore size modification. Table 2 presents pore volume distribution data for samples heated at 150, 200 and 300°C for a fixed time of 8 h. It Is seen that temperature has a remarkable effect in widening the pores. Thus, a sample heated at 150°C for 8 h contains 23.5% and 71.5% o f the pore volume in pores of diameter 60-100 Ao and 100-250 A", respectively. In this sample, only about 5% of the total pore volume is contributed by pores larger than 250 Ao. Table 2. Effect o f Temperature on Pore Size Distribution of Alumina During Hydrothermal Treatment in Presence of Water for 8 h Pore Volume 60-1 00 100-250 250-500 500-1 500 1500-4000 4000-1 0000 10000-1 00000 > 100000 Tota i 0.124 0.377 0.008 0.005 0.006 0.000 0.000 0.007 0.527 23.5 7 1 . 5 1 . 5 1 .o 1 . 2 0.0 0.0 1 . 3 100 0.002 0.076 0.247 0.188 0.007 0.005 0.000 0.013 0.538 0 . 4 14.1 45 .9 34 .9 1 . 3 1 .o 0.0 2 . 4 100 0 .009 0.018 0.131 0 . 3 4 0.006 0.004 0.001 0.025 0.534 I . 7 3 . 4 24.5 63 .7 1 . I 0 .7 0 . 2 4 . 5 100 On increasing the temperature to 2OO0C, the pore size distribution pattern is altered. The amount of pores in the 250-500 Ao is increased from 1.5 t o 45.9%. Substantial increase (1.5 to 34.9%) is also noticed in the 500-1500 Ao dia. pores. Further enlargement of the pores with maximum pore volume (about 63.7%) in the 500-1500 A' dia. range is noticed with increase o f temperature to 300°C. 159 The results show that pores can be widened and pore size distribution in alumina sup- port can be shifted from the narrow pore size range (e.g. 60-100 Ao dia.) t o the larger range (e.g. 100-250 A', 250-500 A' or 500-1500 A' dia.) by increasing the temperature of hydrothermal treatment. Effect of the Alumina: Water Ratio. The effect o f the amount o f water used in hydrothermal treatment studies in modifying the pore size distribution was investi- gated by varying the amount of water between 10 and 40 mi for a given weight (10 g) of alumina at a fixed temperature and duration. The results for the experiments conducted at a constant temperature of 150°C for a fixed duration of 1 h are shown in Table 3. The data indicate that the amount of water or in other words, the ratio between the alumina and water, used in hydrothermal treatment has no significant influence on the pore size distribution. Similar observations were also made for the experiments conducted at higher temperatures. Table 3. Influence of the Amount o f Water Used for Hydrothermal Treatment on Pore Size Distribution of Alumina at 150°C for 1 h Pore Volume 10 m i H20 40 m i H20 -----______- ___---_____ Pore Dia ( A " ) m i g-l % m i g-l % 60-1 00 0.405 90.4 0.413 89.0 100-250 0.025 5.6 0.026 5.6 250-500 0.006 1.3 0.008 1.7 500-1 500 0.004 0.9 0.007 1.5 1500-4000 0.001 0.2 0.003 0.6 4000-1 0000 0.002 0.4 0.000 0.0 10000-1 00000 0.000 0.0 0.002 0.4 > 100000 0.005 1 . 1 0.005 1.1 Tota I 0.448 100 0.464 100 The results of the studies presented above clearly show that low temperature hydrothermal treatment can lead to widening of pores. The extent of pore enlargement is dependent on the treatment temperature and duration, but not on the amount of water. The exact type of chemical interaction or mechanism that leads to pore enlargement is not clearly understood. Transmission electron microscope examination of the hydrothermaliy treated samples showed a progressive increase in the alumina crystaiiite size with increasing treatment time (Fig. 1). X-ray diffraction analysis showed progressive narrowing of y-alumina peaks indi- cating increase of particle size. No peaks corresponding to other phases o f alumina were noticed. Since porosity originates from the volume of the space between the packed alumina particles, Increase in the partlcie size may be expected to result In pore enlargement. 160 Fig. 1 . TEN 01 sluminas t rai led with water a1 2 0 O O C lor 111 I hr . Ibl 2 hr and icl 8 hr Although the exact nature of chemical interactions that has resulted in particle size growth during thermal treatment in presence of water vapor at relatively low temperatures is not clear, It would be useful t o consider the following: Sintering of alumina generally requires material transport in the solid state. This may proceed via surface diffuslon or bulk diffusion (17.18). The surface diffusion is very responsive to the presence o f impuri- ties such as adsorbed gas (19.20) and ions (14,15,21). During the process o f heating of y-alumina in presence of water vapor, hydroxylation and dehydroxylation of alumina are pos- sible. This may enhance the mobility of the oxide and hydroxyl Ions on the alumina surface leading to acceleration of their surface diffusion, and thus may promote the rate of particle size growth. E f fec t of ammonia. The influence of ammonia on the modification of pore size distribution o f alumina during hydrothermal treatment was studied in the temperature range 150-300°C. Fig. 2 shows the effect of treatment time at a fixed temperature (150°C) on the pore size distribution of alumina. It is seen that the pore diameter is increased progressively when the heating duration is increased, as in the case o f hydrothermal treatment with water alone. A comparison o f the pore size distribution data (Table 4) of hydrothermally treated alumina in presence and absence o f ammonia indicates that ammonia has a promoting effect on pore enlargement. Ammonia is a basic gas and it may strongly enhance the rate of hydroxyiation of y-Ai203 by cleavage of the AI-0-AI bond. Such enhanced hydroxylation during the hydroth- ermal treatment may increase mobility of OH ions on alumina surface and enhance the rate of recrystallization and particle agglomeration and consequently lead to pore enlargement. 161 PORE DIAMETER (A) Fig. 2. The effect of hydrothermal treatment time at 1 5OoC in presence of NH7 on the pore size distribution of alumina extrudates. Table 4. Comparison of Pore Size Distribution Data o f Alumina Hydrothermally Treated in Presence and Absence of Ammonia at 300T Pore Volume 60-1 00 100-250 250-500 500-1 500 1500-4000 4000-1 0000 > 10000 Tota l ml /g 0.00 0.08 0.27 0.17 0.01 0.01 0.01 0.55 (%) (0.0) (14.5) (49.0) (31 .O) (1.8) (1.8) (1.8) ml/g (%) 0.0 (0.0) 0.02 (3.6) 0.10 (17.8) 0.41 (73.2) 0.01 (1.8) 0.01 (1.8) 0.01 (1.8) 0.56 ml/g (%I ml/g (%) 0.01 (1.8) 0.01 (1.7) 0.02 (3.6) 0.02 (3.4) 0.13 (23.6) 0.05 (8.5) 0.34 (61.8) 0.47 (79.7) 0.01 (1.8) 0.01 (1.7) 0.01 (1.8) 0.01 (1.7) 0.03 ( 5 . 5 ) 0.02 (3.4) 0.55 0.59 Ef fec t o f hydrothermal treatment on surface area and mechanlcal strength. The surface area of hydrorthermally treated catalyst samples are plotted in Fig. 3 as a function of treatment time. All samples show a decrease h surface area with increasing treatment time. However, the drop in surface area Is significantly high for sam- ples treated at higher temperatures (>150°C) for longer duration. This is not surprising in view of the presence of the large amount of macropores in these samples. The crushing strength of the samples show an interesting behavior (Fig. 4). For the alumina hydrothermally treated at 1 50aC, the crushing strength Increases progresslveiy with 162 300 I - 250 - E M . N - Q 200 4 w a? Q w 150- 0 2 5 100- vl 0 2 4 6 8 TIME (h) Fig. 3. Effect of hydrothermal treatment time and temperature on surface area of alumina extrudates. - 20 z25r r C I E l . . , . z o 0 2 4 6 8 TIME (h) Fig. 4. Influence of hydrothermal treatment time and temperature on side crush- ing strength of alumina extrudates. increasing duration of heating, although there is a considerable increase (about 65%) in the volume o f 100-250 Ao diameter pores. However, at higher temperatures, a reverse trend is noticed. A similar effect was also noticed for samples treated in the presence of ammonia. During hydrothermal treatment at moderate temperatures (e.g. about 150°C). rehydration of the y-alumina is possible. This may lead to the creation of chemical functions with hydroxyl groups which on further calcination may increase the cohesion and consequently increase the mechanical strength. In the case of of hydrothermal treatment at higher temperatures (e.g. 3OO0C), hydroxylation-dehydroxylation cycles leads to larger particles that are probably loosely packed. A weak cohesion between the alumina particles can result in weak mechani- cal resistance. Currently, the effects o f other reagents are being investigated and further experi- ments to obtain a better understanding of the mechanism through which pore widening takes place are being undertaken. ACKNOWLEDGEMENT The authors thank Dr. S. Mansour for the TEM work, and MS. K. Al-Dolama for her assistance in catalyst characterization. This is KlSR Publication No. 3401, Kuwait institute for Scientific Research, Kuwait. 163 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1 1 . 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. R. J. Quan, R. A. Ware, C. W. Hung and J. Wei. Advances in Chemical Engineering, 14(1988) 95. H. Toulhoat and J. C. Plumall. In "Catalysts in Petroleum Refining 1989". D. L. Trhm, S. Akashah, M. Absi-Halabi and A. Bishara (editors), Elsevier, Amsterdam, 1990. p. 463. C. T. Adams, A. A. Del Pagglo, H. Schaper, W. H. J. Stork and W. K. carbon Processing, September 1989, p. 57. J. Wei. In "Catalyst Design Progress and Perspectives", L. L. Hegedus (editor), John Wiiey and Sons, New York, 1988, p. 245. R. L. Howell, C. W. Hung, K. R. Gibson and H. C. Chen. Oil and Gas J., 83(1985) 121. K. Onuma. In "Preparation of Catalysts IV", B. Delmon. P. Grange, P. A. Jacobs and G. Poncelet (editors). Elsevier, Amsterdam, 1987. p. 543. V. J. Lostaglio and J. D. Carruthers, Chem. Eng. Progr. March 1986, p. 46. D. L. Trimm and A. 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Presiand. G. L. Price and D. L. Trimm. Progr. In Surf. Scl.. X1972) 63. N. A. Gjostein, in Surface and Interfaces, T. T. Burke, N. L. Reed and V. Weiss (edi- tors) Syracuse Univ. Press (1 967). H. Schaper, E. B. M. Doesburg and L. L. Van Reijen, Appl. Catal. 7(1983) 211. Schiflett, Hydro- In "Catalyst Deactivation", B. Delmon and G. F. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 165 Production of Nickel-on-Alumina Catalysts from Preshaped Support Bodies L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen, and J.W. Geus State University of Utrecht, Department of Inorganic Chemistry, Croesestraat 77A, 3522 AD Utrecht, The Netherlands. SUMMARY To apply nickel uniformly into preshaped support bodies of a- and y-alumina, two procedures based on incipient wetness impregnation were investigated. The first one involved fixation of nickel ions by the alumina surface. Deposition-precipitation by hydrolysis of urea or nitrite within the pores of the support attached the impregnated nickel almost completely to the y-alumina; impregnation with nickel nitrate solution affixed a smaller fraction of the nickel. The a-alumina took up appreciably less nickel. According to the second procedure, a stable, high dispersion of nickel on a-alumina was obtained by precipitation of nickel magnesium oxalate within the pores. Decomposition leads to small nickel particles attached to magnesium oxide. Introduction The most straightforward procedure to apply a catalytically active component into preshaped sup- port bodies is incipient wetness impregnation with a solution of a precursor of the active component, followed by drying and thermal treatment. To distribute the active component uniformly over the entire support surface, the pore volume has to be filled completely by the impregnating solution. Oc- clusion of air and poor wetting of the support have to be avoided. Evacuation of the support bodies prior to impregnation improves the imbibition of the support considerably, especially when atmos- pheric pressure is readmitted after addition of the solution to the evacuated support. To obtain a uniform distribution of the active component, two additional obstacles must be envis- aged. Rapid extensive adsorption of the dissolved precursor to the support surface will cause deposi- tion to proceed mainly at the pore mouths. Lack of interaction with the support, on the other hand, causes the distribution of the active component to be affected by migration of the solution, which may occur during the drying stage that follows impregnation. Thus the precursor is deposited mainly where the solvent evaporates. An important objective of our study was therefore to avoid the critical character of drying by immobilizing the precursor before the drying stage. Migration of the liquid phase does not disturb the distribution of the precursor when the latter has been fixed to the support. This can be achieved by (deposition-) precipitation. Geus et al. most extensively described this method for suspensions of powdered support materials [ 11. The additional requirement imposed by preshaped support bodies is that the precursor as well as the precipitant have to be distributed uni- formly throughout the pore volume before the onset of precipitation. To this end we impregnated with solutions containing nitrite ions or urea, intended for in situ generation of hydroxyl ions. The reac- tions of urea and nitrite are, respectively: H2N(CO)NH;! + 3H20 -> 2NH4+ + C02(g) + 2OH- 3NOz- + H20 -> 2 N 0 (g) + N03- + 2OH' to beprevented: 2 N 0 + 0, -> 2 N 0 , ; 3 NO, + H20 --> 2HN0, + NO 166 To investigate the feasibility of the procedure outlined above, this papex will deal with application of nickel within a - and y-alumina support bodies. With a-alumina an optimum activity per unit catalyst mass is not secured merely by a uniform distribution of the active component: the low reactivity and smoothness of the a-alumina surface causes a deposited component to be liable to sintering. The advantage of wide-porous a-alumina, providing good transport facilities, can be combined with a high catalytically active surface area by application within the alumina bodies of small clusters of a second support stabilizing a high disper- sion of the active component. To this end nickel magnesium oxalate will be applied into a-alumina extrudates. Reportedly [2,3], mixed oxalates are excellent precursors for nickel catalysts. The mixed oxalate decomposes in inert atmosphere at about 350 O C to veq fine particles consisting of metallic nickel and magnesium oxide. [4,5]. However, the powder cannot readily be shaped to tablets without the (pyrophoric) nickel being oxidized. The applicability in fixed bed reactors can be improved appre- ciably when the mixed oxalate is incorporated into porous bodies of a support, such as a-alumina, and is decomposed in situ in the reactor. Since the oxalate precursor is insoluble, a special impregnation procedure is required. In separate experiments without a-alumina, the length of the induction period preceding precipitation of the mixed oxalate will be assessed. If the induction period is sufficiently long, the support bodies can be impreg- nated with a solution obtained by rapidly mixing solutions of oxalate ions, and of nickel and mag- nesium nitrate (co-impregnation). According to our first objective, the nickel is immobilized as the precipitating mixed oxalate before the drying stage. Alternatively, the support can be impregnated successively with the nickel-magnesium solution and the oxalate solution or vice versa, with an inter- mediate drying stage (two-step impregnation). Experimental: Materials: Engelhard De Meern B.V. (The Netherlands) provided y-alumina supports of various shapes (specific surface areas ranging from 200 to 240 m2/g, pore volumes from 0.38 to 1.10 ml/g) pro- duced from the same pseudo-boehmite, and a-alumina extrudates (9.1 m2/g, 0.55 ml/g). Gases. either high purity grade (quality 5.0) or purified over Linde molecular sieve 4A and reduced copper BTS-catalyst (BASF) to remove water and oxygen, respectively, were obtained from Hoekloos. -nation orocedure: Typically 1 to 5 gram of a support was impregnated to incipient wetness. In all cases, therefore, the volume of the impregnating solution was equal to the total pore volume of the support bodies. Prior to impregnation the support bodies were evacuated to a few mm Hg for at least 15 minutes to avoid occlusion of air. After the vessel had been closed, the impregnation solution was added from a syringe through a rubber septum. To allow distribution of the fluid throughout the support bodies, the vacuum was maintained for at least 10 minutes. Finally the vessel was opened to the atmosphere to apply additional force on the penetrated liquid. Imprematio n with urea or nitrite solutions: A slight excess of urea was added to a 0.88 M nickel nitrate solution. The solution of nickel nitrite (0.837 M) was prepared by combining solutions of barium nitrite and, in slight excess, nickel 167 sulphate and removing the barium sulphate by filtration. Alternatively, a combined solution of nickel nitrate and potassium nitrite was used. In both cases the nickel-to-nitrite ratio was determined by titration to be 0.60, theoretically sufficient to precipitate 56 % of the nickel as Ni(OH)2. After impregnation the vessel was shortly evacuated to avoid reaction of NO with 02, closed, and kept at 90 O C for either 3 or 20 hours in order to bring about hydrolysis of the urea or the nitrite. Evaporation of water was prevented. After the heating period the amount of nickel that had remained in solution was determined by crushing some of the wet impregnates, extracting nickel with distilled water, and determining the amount of nickel by atomic absorption spectrometry or complexometric titration. The remainder of the impregnates was dried in air at 120 OC. To assess the extent of fixation by the alumina without hydroxyl ions being generated, support bodies were impregnated with 0.938 M nickel nitrate solution only, and kept at 90 OC for 20 hours while evaporation of water was prevented. The nickel that was not bound by the alumina was deter- mined as described above. Preparation of unsupported o xalates and app lication within a-alumina: Nickel magnesium oxalates with Ni-Mg ratios of m,3.55, 1.04,0.37, and 0 were prepared. The precipitates were centrifugated and washed with distilled water. In the application of mixed oxalates into a-alumina bodies, a Ni-Mg ratio of 6 was chosen for the nitrate solutions in order to attain a suitable nickel weight loading. Procedures, concentrations, and sample codes are to be found in Table IT. In two-step impregnations hot solutions were used in order to obtain a maximum concentration of oxalate ions. The impregnates were kept at 70 OC for about 1 hour before being dried to achieve com- plete reaction to nickel magnesium oxalate. Drying was performed at 100 OC in air. Besides the samples that were used in TPH (see below) the oxalates were decomposed by heating in a nitrogen flow with 5 OC/min to 400 OC, which temperature was held for 2 hours. Characterization: All samples were examined with scanning and transmission electron microscopy (SEM and E M ) . For comparison support bodies impregnated with 0.938 M nickel nitrate solution, and dried subsequently in air at 90 or 120 O C or in vacuum at 20 OC were studied. The distribution of nickel throughout the support bodies was studied with a Cambridge Stereoscan 150 S scanning electron microscope equipped with detectors for secondary and backscattered electrons and with a Link AN 10000 X-ray analysis system with energy dispersive detector. Impregnated support bodies were split, and mounted on an aluminium stub with carbon glue. A carbon layer was vapor-deposited onto the samples to provide a conducting surface. TEM samples were made by ultrasonic treatment of ground impregnates suspended in alcohol, and spreading a droplet of the suspension on a holey carbon film. The samples were examined in a Philips EM 420 "EM using an accelerating voltage of 100 kV. The reducibility of the deposited nickel species was investigated with temperature-programmed reduction (TPR) performed in 10 % H2/Ar flow with a heating rate of 5 OClmin. Except the oxalate- based catalysts, the impregnates had previously been calcined in air for two hours at 450 OC. A cold trap containing dry ice retained the water produced. The following characterization techniques were only used with the oxalate-based catalysts. Oxalate decomposition was studied with temperature-programmed heating in helium (TPH) using the 168 same apparatus as with TPR. The temperature was raised with 5 OC/min to 500 OC. Mean nickel particle sizes were determined with hydrogen chemisorption, performed in a conven- tional glass apparatus. The samples, typically containing 0.05 g nickel, were decomposed either in vacuum or in a flow of nitrogen. Evacuation was performed at 22 OC to a final pressure of about 5. Pa. Small doses of hydrogen were admitted to the sample at time intervals of 20 minutes unless complete uptake was attained earlier. An adsorption isotherm was measured up to a pressure of about 10 kPa. Next the samples were treated in hydrogen at 500 OC for 2 hours, the hydrogen was removed at 300 OC, evacuation was continued at 22 OC, and another chemisorption measurement was performed. The hydrogen up- take attributed to a monolayer was obtained by extrapolating the isotherm to zero hydrogen pressure. Calculation of the nickel metal surface area was based on a Ni : Had ratio of 1, and a mean surface area per nickel atom of 6.5. Vibrating sample magnetomehy (VSM), a technique that was described in detail by Van Stiphout [6], was performed with the unsupported mixed oxalates, which were decomposed in a 10% H2/N2 flow at 400 OC. The magnetization of the sample was measured at 77 K as a function of the applied field strength (maximum 12 kOe). The size distribution of the nickel particles was obtained by fitting the thus measured magnetization curve with theoretical curves calculated for discrete particle sizes. Oxidation of methane, performed in an automated flow apparatus, was used as a test reaction for the stability of the nickel particles with respect to surface structure and sintering. 1.0 g of the sample A-Amox-2 (see Table II) was decomposed in a N2-flow and pretreated in a 10 % H2/He flow for one hour at 400 OC. The reaction feed, 1 mole % CH4 and 4 mole % 0, in He, passed through the cata- lyst bed at a space velocity of 4000 h-l. Methane conversion was measured with a Perkin Elmer 8500 gas chromatograph. As oxygen was present in excess, the nickel particles were completely oxidized. A measurement comprised a successive increase and decrease of the temperature between 350 and 750 OC in steps of 10 OC. Before each measurement the sample was reduced at 400 or at 850 OC. m2. Results and discussion Imuremation with urea or nitrite solutions: In Table I the extent to which nickel ions were attached to the alumina support is represented for various impregnations with and without urea or nitrite. Impregnation with nickel nitrate alone already leads to fiation of a considerable fraction of the nickel, especially at 90 OC; at 22 OC, the attachment is lower. Per unit surface area a-alumina takes up more nickel than y-alumina. The hydroxyl ions provided by hydrolizing urea or nitrite cause the fixation of nickel by y-alumina to be about complete, and by a-alumina to be much higher than with nickel nitrate alone. The distribution of nickel in the y-alumina supports was completely uniform, irrespective of the preparation technique and the drylng procedure. SEM showed a homogeneous concentration through- out the support bodies (figure 1). The degree of fiiation at the onset of drying appears to be of no im- portance. Probably, the pore structure of this y-alumina support prevents migration of the solution over macroscopic distances whatever the drying rate. Furthermore, it can be concluded that the pro- duced carbon dioxide or nitric oxide can be discharged without expelling the liquid from the pore sys- tem, which would result in deposition of the active component on the outer edge of the support body. 169 TABLE I Fixation of nickel ions in alumina supports impregnated to incipient wetness concentrations support (mole/l) heating time (h) 0 . 9 3 8 ?'-A1203 0 . 9 3 8 Y-A1203 0 . 9 3 8 a - A l 2 O 3 0 . 8 8 ; 0 . 9 5 a - A 1 2 0 3 0 . 8 3 7 * Y-A1203 0 . 8 3 7 ; 1 . 6 7 4 " "f-Al2O3 0 . 8 3 7 " cc-Al2O3 0 . 8 8 ; 0 . 9 5 Y-A1203 1 7 2 0 20 20 20 2 0 3 1 9 temperature fixation ( O C ) ( 2 ) 9 0 6 6 22 35 9 0 2 7 9 0 9 5 9 0 65 9 0 97 9 0 8 7 9 0 7 2 The N02- concentration was 1.4 mole/l On a small scale, as was revealed by E M , dried as well as calcined impregnates could not be distinguished from the fresh, unloaded alumina. Only upon reduction nickel particles developed (figure 2) . Complete reduction asks for heating in hydrogen to about 800 OC or prolonged treatment at a lower temperature, as follows from TPR. Equal particle sizes were found with different reduction procedures. The size of the nickel particles ranged for some preparations from 3 to 9 nm, and for other from 5 to 18 nm. The nickel particles were evenly distributed over all clusters of alumina needles, demonstrating uniformity on a small scale as well. The interaction of dissolved nickel with the alumina surface can be assessed more in detail by TPR (figure 3). Since all samples had been calcined at 450 OC, differences must be due to different conditions during impregnation and drying. From the profiles obtained from the samples impregnated with nickel nitrate solution it appears that interaction leads to nickel species that are difficult to reduce: the sample that was dried rapidly at 22 OC, thus minimalizing interaction, exhibits reduction at tem- peratures much lower than the sample kept for 17 hours at 90 OC. Compared to the latter, also the samples prepared with nitrite are more readily reduced, demonstrating that generation of hydroxyl ions diminishes the extent of interaction with the alumina. The effect is more pronounced when the production of hydroxyl ions proceeds rapidly, as with KNO,-Ni(NO&. De Bokx demonstrated that the interaction between y-alumina and dissolved nickel involves for- figure 1 : Typical backscattered electron image of a cross-section of a nickelly-alumina catalyst body; brightness indicates a high nickel concentration in the alumina matrix; a line profile of nickel is obtained by passing the electron beam along the projected straight line and recording the emitted Ni K u radiation. 170 mation of Feitknecht compounds ( Ni,A1y(OH-)2x-3y-z(N03-)z ) [7]. The mixed compound of nickel and aluminium is converted to a species upon calcination that requires a high reduction temperature, probably nickel aluminate. With y-alumina impregnated with nickel nitrate solution and kept at 90 O C , the anchoring reaction proceeds to a considerable extent, which is, however, limited by the decrease in pH brought about by the continuing hydrolysis of nickel ions. Generation of hydroxyl ions by hydrolysis of urea or nitrite compensates for the protons released, and allows fixation to proceed to completeness. On the other hand, rapid production of hydroxyl ions leads to precipitation of Ni(OH)2 less strongly interacting with the alumina surface, resulting in an easily reducible nickel species. This effect is already apparent from the better reducibility exhibited by the sample prepared with nickel nitrite, but is obviously present in the KN02-Ni(N03), impregnate, which displayed a high rate of decomposition of nitrite. The improved reducibility is an important advantage of the method of in situ generation of hydroxyl ions when large batches of catalyst are to be reduced. The incomplete fixation of nickel to a-alumina may be due to the limited rise in pH attainable with urea (caused by the NH4+ - NH3 -equilibrium [8]) and the insufficient amount of nitrite present. Alternatively, instead of the concentration of hydroxyl ions, the surface area of the alumina may be the restricting factor. Possibly a fraction of the nickel hydroxide particles is not attached to the alumina surface and can be removed with distilled water. Pretreatment of the alumina or application of larger amounts of urea or nitrite may be useful to achieve complete fixation. 50nm ,-, figure 2: TEM image of a typical nickelly-alumina catalyst after reduction figure 3: TPR profiles of y-alumina impregnates: _ - - Ni-nitrate .+..*..+. Ni-nitrite dried at 100 OC _ _ _ _ Ni-nitratelK-nitrite H2 -consumption per mmole Ni (arbitrary units) heated at 90 OC; Ni-nitrate, dried subsequently at 20 O C Unsumorted and a - m nâckel mayesium oxalates: Results of preparations of nickel magnesium oxalates without alumina and within a-alumina ex- trudates are represented in Table II. To apply nickel magnesium oxalate into support bodies by co- impregnation the induction period for nucleation has to be sufficient to distribute the solution through- out the support bodies. We established the induction period in the absence of alumina under various conditions. It decreases with increasing concentration, temperature, and acidity of the solution pro- duced by rapidly mixing a solution of magnesium and nickel nitrate with a solution supplying oxalate ions. Even when an induction pencd of half a minute is acceptable and a high Ni-Mg ratio is used, a 171 3.55 1.04 0.37 n.m. n.m. n.m. , n.m. nickel weight loading of only 1.5 % can be achieved in a support having a pore volume of 0.5 ml/g. Therefore we will focuss on two-step impregnation, which allows application of solutions of a con- centration corresponding to the solubility, resulting in a much higher nickel weight loading. From the data in Table II for unsupported oxalates, it is seen that with oxalic acid a considerable fraction of the magnesium is not taken up in the precipitate. Magnesium oxalate is soluble in acid solution. Ni-Mg ratios of unity and 0.25 in the solution result in ratios of 3.55 and 0.37, respectively, in the precipi- tate. To maintain the Ni-Mg ratio of the solution in the oxalates precipitating within the extrudates, ammonium oxalate is to be preferred over oxalic acid, in spite the higher solubility of the acid. When the a-alumina extrudates were firstly impregnated with a solution of ammonium oxalate and subsequently with the nickel magnesium nitrate solution ( A-Amox-2 ), a fairly homogeneous distribution was observed with X-ray analysis in SEM (the distribution of magnesium could not be esablished, as the energy of Mg Ka photons is too close to that of the abundant A1 Ka photons). The reverse impregnation order ( A-Ox-2 and A-Amox-1 ) led to an egg-white distribution: a relatively high amount of nickel was present in a narrow band inside the extrudates as a result of a combination of depletion and diffusion processes. Decomposition of the oxalates sets free carbon monoxide and dioxide. It turned out that the flow of gas evolved may displace the fine nickel-on-magnesia clusters within the pores of the a-alumina. Although the uniform distribution of A-Amox-2 was not affected significantly, the aforementioned egg-white distribution became more diffuse. Decomposition of the unsupported mixed oxalates leads to severe shrinkage and a change from pale green to black. In TEM, decomposition of separate platelets of mixed oxalate by the electron beam can be observed. Platelike structures, consisting of small nickel metal and magnesium oxide particles, remain (figure 4). Mostly, the clusters of closely packed particles are irregularly shaped (figure 5a) and separate nickel particles are only discernible in a dark field image (figure 5b). It is noted that in the dark field image only a small fraction of the nickel particles shows up. In fact nickel constitutes about 38 volume percent of the specimen shown. Obviously, a relatively small amount of magnesium oxide is effective in preventing the nickel particles from sintering. Mixed oxalates that are contained in a-alumina extrudates display the same structure of packed particles as is exhibited by the unsupported samples. TABLE I1 Unsupported and a-alumina supported nickel magnesium oxalates Code Ni/Mg 3.55 Ni/Mg 1.04 Ni/Mg 0.37 A-Ox-1 A-OX-2 A-Amox-1 A-Amox-2 I I I impregnation method Ni:Mg ratio concentrat ion1 particle (mole/ 1 ) size (nm) H - ~ M co-impregnation lSt Ni-Mg,Znd Ox lSt Ni-Mg,2nd Amox lSt Amox,Znd Ni-Mg 1.0 1.0 0.25 6.0 6.0 6.0 6.0 0.66* 0.62* 0.46* 2.3 0.25* 2.33 2.33 22 8.2 2.7 16.4 16.6 16.5 n.m. 13.2 7.9 7.6 n.m. n.m. n.m. n.m. * concentration after combination with Ni-Mg-nitrate solution Ox= oxalic acid; Amox= ammonium oxalate; A= a-alumina; n.m.= not measured chs= chemisorption 172 figure 4: TEM micrographs of nickel magnesium oxalate platelet before (a) and afler (b) decomposition 100 nm - a b figure 5: TEM micrographs of nickel magnesium oxalate NUMg 1.04: bright field (a) and dark field (b) image In figure 6 TPH profiles of unsupported oxalates with Ni-Mg ratios of 0, 0.37, 3.55, and m are shown. Pure nickel oxalate decomposed from 280 to 350 OC. The sample with a small amount of magnesium exhibited decomposition within about the same temperature range. At higher magnesium contents a shoulder at higher temperatures developed. This may be attributed to a separate phase of a higher magnesium content, but not to pure magnesium oxalate, since the latter decomposed at a higher temperature, viz., above 400 OC. TPR indicated that nickel was not completely converted to the metallic state; X-ray powder diffraction provided evidence for nickel carbide and, possibly, for nickel oxide, besides for the ex- pected nickel metal and magnesium oxide. Nickel carbide may have originated from disproportion- ation of carbon monoxide to carbon dioxide and carbon at the nickel metal surface. Since the main diffraction maxima of the four components coincide, neither their relative amounts nor their particle sizes, to be obtained from line broadening, can be properly estimated. Nickel metal mean particle sizes obtained from hydrogen chemisorption and vibrating sample magnetometry are given in table 11. The values mentioned for the VSM measurements have been deduced from the particle size distributions obtained. Samples that had only been decomposed con- sumed an extra amount of hydrogen with increasing hydrogen pressure, probably reflecting reaction of nickel carbide (and possibly oxide). Extrapolation to zero pressure, however, led to the same cal- 173 onidizeo 2 h (50 "C TPH '. /'? T( 3C) ?-I-. ' ' I 7 3 o 200 400 6 6 0 800 l o n o 200 300 400 500 figure 6: TPH profiles of unsupported oxalates; figure 7: TPR profiles of A-Amox-2; the detector signal is normalized with respect to the amount of oxalate ions Hp consumption per mmle Ni (arbitrairy units) culated nickel surface area as was found in the second measurement, after reduction in hydrogen. The results from the three unsupported samples demonstrate that the nickel particle size strongly depends on the Ni-Mg ratio in the original oxalate. The VSM results only qualitatively display this relation; only the value obtained for Ni/Mg 1.04 agrees well with the chemisorption measurements. Ni/Mg 0.37 and 3.55 definitely contain nickel particles exceeding the upper and lower limit, respectively, of the size range to which the VSM theory applies (2 to 15 nm) [6]. The nickel particle size measured in a-alumina impregnates was about 16.5 nm, irrespective of the impregnation procedure. Considering the high Ni-Mg ratio applied (6, and probably higher in case oxalic acid had been used as precipitant), this diameter is small compared to the unsupported oxalates (see table 11). Nevertheless, much smaller nickel particles are expected to result with a mixed oxalate of lower Ni-Mg ratio. Increasing the amount of magnesium in the impregnating solution, implying a lower attainable nickel weight loading, will therefore be more effective in obtaining a higher nickel surface area than increasing the nickel content by means of multiple impregnation steps. In figure 8 conversion plots are displayed for the oxidation of methane over A-Amox-2. It is noted that upon exposing the (completely reduced) catalyst to the reaction feed, which contains excess oxy- gen, the nickel is almost immediately converted to nickel oxide, the active phase in the oxidation of methane. The hysteresis between the curves at increasing and decreasing temperature indicates severe 120 ' conversion figure 8: Conversion of methane by A-Amox-2 increasing temperature : 80 decreasing temperature: - - - - - - run 1 : fresh sample, reduced at 400 OC, 2 h run 2: deactivated sample, reduced at 400 O C , 2 h run 3: deactivated sample, reduced at 850 OC, 1 h 60 - 40 : 300 4 0 0 500 6 0 0 7 0 0 8OC 174 deactivation. Comparison of TPR profiles from samples of the same catalyst that were heated in air to 450 or 850 OC (figure 7) indicates that a nickel species difficult to reduce is formed at elevated tem- perature. Figure 8 shows that reduction at 400 OC for two hours partly restores the activity. Reduction at 850 OC (one hour) brings the activity back to the original level, which implies, moreover, that the nickel particles did not suffer from sintering. Formation of mixed magnesium nickel oxide nicely explains the observations. Nickel and mag- nesium ions have about equal radii and may react readily to a mixed oxide of the same crystal struc- ture. Diffusion of magnesium into the nickel oxide may lead to an inactive nickel oxide surface, probably by diminishing the amount of excess surface oxygen, which is the reactive species in the oxidation of methane. Removal of the magnesium ions from the nickel oxide can only be achieved by reducing the nickel to the metallic state. Nickel metal nucleation will be increasingly difficult for lower Ni-Mg ratios in the oxide. The reducibility of the nickel species depends on the overall Ni-Mg ratio and the degree to which interdiffusion has proceeded. The reduction procedure at 450 âW is therefore sufficient to reduce the nickel oxide particles containing a low concentration of magnesium ions, while nickel that has diffused into the magnesium oxide requires a more severe reduction treatment. Conclusions: Fixation of nickel ions to the inner surface of y- and a-alumina support bodies, impregnated to incipient wetness, takes place to a considerable extent, especially at elevated temperature. In y-alumina fixation proceeds to completeness upon generating hydroxyl ions by in situ hydrolysis of urea or nitrite. The gases released are discharged smoothly, without expelling liquid from the pore system. This procedure leads to an improved reducibility compared to catalysts prepared by ordinary impreg- nation and successive drying. With a - a l u m i ~ the surface area appears to restrict the amount of nickel that can be anchored. In order to realize sufficient interaction with the support surface in wide porous catalyst bodies, s m a l l particles of nickel-on-magnesium oxide can be synthesized within a-alumina by decomposition of nickel magnesium oxalate. A uniform distribution of mixed oxalate in the extrudates was obtained by two successive impregnation steps, viz., with a hot concentrated solution of ammonium oxalate, and with a solution of nickel and magnesium nitrate. Application of a low nickel-magnesium ratio strongly enhances the dispersion of the nickel particles developing upon decomposition. Thus a lower nickel weight loading may exhibit a higher specific surface area. REFERENCES 1 J.W. Geus, in: Preparation of Catalysts 111, Scientific Bases for the Preparation of Heterogeneous Catalysts, B. Delmon, P. Grange, P. Jacobs, and G. Poncelet, Eds., Elsevier Amsterdam, 1982 2 W. Langenbeck, H. Dreyer, D. Nehring, and J. Welker, Z.anorg.allg.Chem. 281 (1 955) 90-98 3 V. DaneS and P.Jir6, Coll.Czechoslov.Chem.Comm. 21 (1 956) 765-767 4 M. Ralek and V. DaneH, Coll.Czechoslov.Chem.Comm. 24 (1959) 1908-1913 5 V. Ponec and V. DaneS, Coll.Czechoslov.Chem.Comm. 25 (1 960) 820-828 6 P.C.M. van Stiphout, Ph.D. thesis, University of Utrecht, Utrecht, The Netherlands, 1987 7 P.K. de Bokx, Ph.0. thesis, University of Utrecht, Utrecht, The Netherlands, 1985 8 L.A.M. Hermans and J.W. Geus, in: Preparation of Catalysts II, Scientific Bases for the Preparation of Heterogeneous Catalysts, B. Delmon, P. Grange, P. Jacobs, and G. Poncelet, Eds., Elsevier Amsterdam, 1979 G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 175 DEVELOPMENT OF A METHODOLOGY FOR INVESTIGATING THE ADSORPTION OF SPECIES CONTAINING CATALYTICALLY ACTIVE IONS ON THE SURFACE OF INDUSTRIAL CARRIERS N. SPANOS, CH. KORDULIS and A. LYCOURGHIOTIS* Department o f Chemistry-Research I n s t i t u t e o f Chemical Engineering and High Temperature Processes, P.O.Box 1239, U n i v e r s i t y Campus GR-26110, Patras, Greece. ABSTRACT A methodology f o r e l u c i d a t i n g the mechanism o f adso rp t i on on i n d u s t r i a l o x i d i c suppor ts o f species c o n t a i n i n g c a t a l y t i c a l l y a c t i v e i ons has been developed. This invo lves the c o r r e l a t i o n between the surface concentrat ion corresponding t o monolayer and t h e su r face groups o f o x i d i c supports, t h e combined use o f potent iometr ic t i t r a t i o n s and microelect rophores is which a l - lows the determinat ion of t he surface and e l e c t r o k i n e t i c charge dens i t i es , respec t i ve l y as we l l as the mathematical analys is o f t he isotherms obtained. The methodology a p p l i e d t o t h e adso rp t i on o f molybdates and tungs ta tes on y-alumina, t o the adsorption o f molybdates on t i t a n i a and t o the adsorption o f Co2+ and N i 2 + i ons on y-alumina l e d t o t h e f o l l o w i n g conclus ions: (i) Responsible f o r t h e c r e a t i o n o f adso rp t i on s i t e s f o r nega t i ve ( p o s i t i v e ) species are, mainly, the protonated (deprotonated) sur face hydroxyls o f t he o x i d i c supports. ( i i ) These species are adsorbed on e n e r g e t i c a l l y equiva lent s i t e s o f t h e Inne r Helmholtz Plane o f t h e double l a y e r around t h e y-Al,O, p a r t i c l e s suspended i n t h e aqueous medium. (iii) L a t e r a l i n t e r a c t i o n s are operational between the adsorbed species, t he magnitude o f which depends on the nature o f the support and the species t o be adsorbed. INTRODUCTION Although a r e l a t i v e l y l a r g e number o f supported c a t a l y s t s are prepared by adso rp t i on o f a species c o n t a i n i n g t h e a c t i v e i o n on t h e su r face o f an o x i d i c suppor t , e.g. y-Al,O,, SiO,, TiO,, s t u d i e s d e a l i n g w i t h c a t a l y s t s prepared by e q u i l i b r i u m adsorption fo l lowed by f i l t r a t i o n are r a t h e r scarce i n the l i t e r a t u r e [e .g. l -81. Th is i s probably t h e main reason f o r which a c l e a r methodology a l l o w i n g t h e e l u c i d a t i o n o f t h e mechanism o f adsorpt ion from aqueous suspensions has n o t y e t been es tab l i shed . Th is method should take i n t o account t h e u s u a l l y ignored ex is tence o f an e l e c t r i c a l double l a y e r around t h e suspended support p a r t i c l e . Moreover, t h i s method should enable us t o i n v e s t i g a t e t h e f o l l o w i n g p o i n t s : (i) Are the surface groups responsible f o r the c rea t i on o f so rp t i ve s i t e s , the neu t ra l or the * A l l correspondence t o t h i s Author. 176 charged hydroxyls? (ii) The p a r t o f the double l a y e r where the species are located, i . e . the surface o f the c a r r i e r , the Inner Helmholtz Plane ( I H P ) o r t he d i f f u s e p a r t o f the double l aye r . (iii) The nature o f adsorption, i . e . whether i t i s l o c a l i z e d o r nonlocal ized. ( i v ) The existence o f l a t e r a l i n - t e rac t i ons between the adsorbed species. The establ ishment o f such a methodology i s t h e purpose o f t h i s com- municat ion. To i l l u s t r a t e t h e proposed methodology we use severa l r e s u l t s taken from our recent adsorption s tud ies o f molybdates [9,10] and tungstates [ll] on y-alumina, o f molybdates on t i t a n i a [12] and o f Co2+ and N i 2 + on y-alumina [13]. EXPERIMENTAL The experimental method used t o determine the concentrat ion o f the ad- sorbed species, r(mo1 .m-'), a t a g i ven e q u i l i b r i u m concen t ra t i on o f t he species i n the suspension, Cq(mol .dm?), has been described elsewhere [9,10]. Poten t iomet r i c t i t r a t i o n s have been used t o determine t h e su r face charge densi ty , oo(pC.cm-2), i n the absence and presence o f t he species t o be adsorbed and t h e c o n c e n t r a t i o n o f t h e s u r f a c e groups [SOH: n e u t r a l hydroxy l s, SOH,': p rotonated hydroxy l s, SO-: deprotonated hydroxy l s ] . F u l l d e t a i l s have been g i ven elsewhere [14-17].Microelectrophoretic m o b i l i t y measurements were used t o determine t h e e l e c t r o k i n e t i c charge dens i t y , o,(pC.cm-'), i n t he absence and presence o f the species t o be adsorbed. F u l l d e t a i l s are g iven elsewhere [18]. DESCRIPTION OF THE METHODOLOGY On the nature o f the adsomt ion s i t e s . I t i s w e l l known t h a t t h e su r face o f t h e p a r t i c l e s o f a simple oxide l i k e y-Al,O, and SiO, i s genera l l y charged i n e l e c t r o l y t e so lu t i ons . The w e l l es tab l ished surface i o n i z a t i o n model [I91 describes q u i t e we l l the charging mechanism. This process may be represented as: K, K, SOH,' SOH + Hs+ SOH SO- + Hs+ Hs+, H i : denote hydrogen ions on the surface o f the support and i n the bu lk so lut ion, respect ively. 177 I n the case o f TiO,, which i s a m i x t u r e o f r u t i l e and anatase, t h e above e q u i l i b r i a should be w r i t t e n f o r each component[20]. I t i s obvious t h a t i n order t o i n v e s t i g a t e which o f t h e su r face groups i s mainly responsible f o r t he c rea t i on o f adsorption s i t e s , i t i s necessary t o c o r r e l a t e the surface concentrat ion o f the adsorbed species corresponding t o the p la teau o f the isotherm, rm (see f i g u r e 8) w i t h the concentrat ion o f t he d i f f e r e n t types o f groups. The l a t t e r may be r e g u l a t e d by doping the c a r r i e r o r by changing e i t h e r t h e pH o r t h e temperature o f t h e suspension [14-171. I f the species t o be adsorbed i s nega t i ve l y charged, l i k e Mo,Ot- o r WxOyz-, i t seems reasonable t o assume t h a t the neu t ra l o r the p o s i t i v e groups are responsib le f o r the c rea t i on o f adsorption s i t e s . This may be tes ted by p l o t t i n g r m t h e concen t ra t i on o f SOH,' o r SOH. A t y p i c a l example i s il- l u s t r a t e d i n f i g u r e l. S i m i l a r t rends were observed f o r t h e adso rp t i on o f WxOyz- ions on y-Al,O, [ll] as we l l as f o r the adsorption o f the MoxOyz- ions on TiO,, though i n the l a t t e r case, uo, was used instead o f the concentrat ion o f t he p o s i t i v e groups [ 1 2 ] . The above shows t h a t t h e p o s i t i v e groups are responsib le f o r the c rea t i on o f adsorption s i t e s f o r negat ive ions. The co r - responding p l o t s o f rm the nega t i ve l y charged and the neu t ra l surface R l O H / sites.nmF2 6.0 6 . 5 7 . 0 7 . 5 8 . 0 .05 0 . 5 5 2 . 0 5 1 . 5 5 2 . A~oH; / sites.nm-2 5 Fig. 1. Saturat ion surface Mo(V1) concentrat ion obtained a t var ious tempera- tu res (ref.10) as a func t i on o f the concentrat ion o f the protonated (curve a) and neu t ra l (curve b) surface hydroxyls regulated by vary ing the tempera- t u r e of the impregnating suspension o f t he y-Al,O, ( re f .16) . 1 78 -2 A l O H / sites.nm n 2 4 6 8 0 2 4 6 8 -2 A ~ O - / sites-nm Fig. 2. Saturation surface Ni2' concentration obtained for the system Ni2'/y- Al,O,-F at room temperature (ref.13) as a function of the concentration o f the negative (curve a) and neutral (curve b) surface hydroxyls regulated by varying the F- content (ref.15) SOLID SOLTIT ION I Shear Fiq. 3 . Structure of the solid-solution interface I according to the "triple layer model". up, ud and uek refer to the total charge from the surface o f the support t o the IHP, to the OHP, and t o the shear plane,respectively, 179 groups f o r t he Ni2 ' ions, shown i n f i g u r e 2, show t h a t t h e nega t i ve groups are ma in l y respons ib le f o r t h e c r e a t i o n o f adso rp t i on s i t e s f o r p o s i t i v e ions [13]. S i m i l a r r e s u l t s were obtained f o r the Co2+ ions. Par t o f t he double l a v e r where the adsorbed soecies are l oca ted -Qua l i t a t i ve aDDroach. Adopting t h e " t r i p l e l a y e r model" f o r t h e double l a y e r ( f i g . 3 ) t h ree p o s s i b i l i t i e s do e x i s t : the adsorbates may be l oca ted on the surface, on the I H P o r i n the d i f f u s e p a r t o f the double l aye r . I n the f i r s t case adsorption o f negative ions i s expected t o cause a decrease i n the surface charge den- s i t y , whereas i n the t h i r d case t h i s type o f adsorption would r e q u i r e pos i - t i v e e l e c t r o k i n e t i c charge d e n s i t y . F igures 4 and 5, and s i m i l a r ones ob- served f o r t h e WxOyZ-/y-A1203 and MoxOyZ-/TiO, systems, show t h a t l o c a t i o n o f the adsorbates on the surface o r i n the d i f f u s e p a r t o f t he double l a y e r i s precluded. Therefore, t he on ly p o s s i b i l i t y i s the adsorpt ion a t the IHP. I n fac t , i n t h a t case the negative ions are expected t o promote the appearence o f add i t i ona l SOH,' groups on the surface by forming i o n p a i r s [14 , 15, 171 and t h e r e f o r e t o increase the p o s i t i v e su r face charge d e n s i t y (F ig . 4 ) . Moreover, the presence o f negative ions a t the I H P i s i n agreement w i t h the negative e l e c t r o k i n e t i c charge dens i t y i n the pH range stud ied (F ig . 5). Based on r e s u l t s i l l u s t r a t e d i n f i g u r e s 6 and 7 and f o l l o w i n g the above reasoning we may conclude t h a t the p o s i t i v e ions are a l so l oca ted a t the I H P 6 0 0 300 N I E Y 4 \ 0" - 3 0 0 - 6 0 0 4 5 6 7 8 9 3 PH Fig. 4. Surface charge dens i t y o f y-A1203 as a f u n c t i o n o f pH o f the suspen- s i o n a t 25OC. (a) i n t h e presence o f MoxOyZ- i ons (ammonium heptamolybdate so lu t i on , C0=l*10-3 mol Mo(VI)/dm3, i o n i c strength, I = O . 1 mol/dm3 NH,N03), (b) i n the absence o f MoxO:' ions (0.1 mol/dm3 NH,NO, s o l u t i o n ) . 180 \ x 0" 0.5 0.0 - 0.5 - 1.0 Fig. 5. Electrokinetic charge density of y-A1 0 as a function of pH of the suspension at 25OC. (a) in the presence of Moxb:- ions (ammonium heptamolyb- date solution, C0=l*10-3 mol Mo(VI)/dm3, ionic strength, I=O.Ol mol/dm3 NH,NO,), (b) in the absence of MoxOt- ions (0.01 mol/dm3 NH,NO, solution). of the double layer. Useful information concerning the mechanism of adsorption may also be drawn from the form of the isotherms obtained. For the systems already men- tioned at various temperatures [lo] as well as for the systems N I 6 3.5 4.5 5.5 6.5 7.5 PH Fig. 6. Surface charge density of y-Al,O, as a function of pH of the suspen- sion at 25OC. (a) in the presence of Co2+ ions ( Co(N0,),.6H20 solution, C0=l*10-3 mol Co2+/dm3, ionic strength, I=O.1 mol/dm3 NH,NO, ), (b) in the ab- sence of Co2+ ions (0.1 mol/dm3 NH,NO, solution) . 181 N I 5 U 5 \ 2 0" 0.0 \ '0.5 -1.0 I 4 5 6 7 8 9 10 1 PH Fig. 7 . Electrokinetic charge density of y-A1 0 as a function of pH of the suspension at 25°C. (a) in the presence of Co2' ions ( Co(N03),.6H,0 solution, C0=l*10-4 mol Co2+/dm3, ionic strength, I=O.Ol mol/dm3 NH,NO,), (b) in the ab- sence of Co2+ ions (0.01 mol/dm3 NH,NO, solution). Moody-Al,O,-Na [lo], Co2+/y-Al,0,-F [13] and Ni2+/y-A1203-F [13] at room tem- perature, the isotherms may be classified as S and I type suggesting local- ized, Langmuir type, adsorption at the IHP with strong and weak lateral in- teractions, respectively [21,22]. Typical examples of the $ type isotherms obtained are illustrated in figure 8. Part of the double laver where the adsorbed sDecies are located-Ouantitative The next step is to analyse the isotherms obtained on the basis o f the following assumptions: (i) More than one kind of ions (e.g. MOO:-, Mo,O,,"-), are specifically adsorbed at the IHP, as it has been inferred above. (ii) The adsorbed ions are located on energetically equivalent sites as suggested from the Langmuirian shape of the isotherms. (iii) One specifically adsorbed ion, i , replaces one water molecule from the IHP [9]. Assuming no lateral interactions between the adsorbed species we may derive [9] the "Stern-Langmuir" equation Amroach. where rm represents the saturation surface concentration of the adsorbed species (maximum in the and S type isotherms). The constant K is given by eqn (2). 182 N ' E 10 - 4 5 \ - L 0.0 1 0.02 0. L."V - 3 c,, / mol.dm n nn 3 F ig . 8. Surface concen t ra t i on o f M o ( V 1 ) as a f u n c t i o n o f t h e e q u i l i b r i u m Mo(VI) concentrat ion a t var ious temperatures o f t he impregnating suspension o f t h e y-Al,O,. pH=5, 14 .1 M NH,NO,. 13 : 20°C, 0 : 3OoC, A : 45OC. K = I i[(ai/55.5)exp(-AGaads,i/RT)], ( 2 ) where ai and AGOads, , represent a c o e f f i c e n t (independent from the Cq but de- pendent on t h e temperature, pH and the na tu re o f t h e species i) and t h e standard f r e e energy o f adsorption f o r the i o n i, respec t i ve l y . Assuming l a t e r a l i n t e r a c t i o n s between t h e adsorbed species we may de r i ve [9 ] the "Stern-Langmui r-Fowl e r " equation where E i s t he energy o f the l a t e r a l i n te rac t i ons , i s given by eqn (4) : I K = Ii[(ai/55.5)exp(-ZiFUlg /RT-AGocsJRT)], ( 4 ) where Z i and Wg represent t h e charge o f t h e ith k i n d o f t h e i o n s t o be ad- sorbed and t h e p o t e n t i a l a t IHP, r e s p e c t i v e l y . Equations (1) and (3 ) describe a l so the adsorption o f one k ind o f species bu t i n t h i s case simpler expressions f o r the values o f K and are ava i l ab le [9 ] . It was found t h a t i n a l l cases studied, eqn(3) described b e t t e r t he ex- perimental r e s u l t s as compared t o eqn(1). This i nd i ca tes t h a t l a t e r a l i n t e r - act ions e x i s t between the adsorbed species. However, t he magnitude o f these i n t e r a c t i o n s depends on the k ind o f the support and the ions adsorbed (Table 1). With regard t o the support, i t may be observed t h a t t he "support- 183 adsorbed species interactions", estimated by the value of t, are stronger in the case of TiO,. This justifies the relatively weaker lateral interactions observed with this carrier in the case o f adsorption of the molybdates. Con- cerning the adsorbate phase, it may be suggested that the lateral interac- tions between the Co2+ or Ni2' ions are negligible, in comparison with those observed for MoxO:- and WxO:- in which an S type of isotherm was obtained. TABLE 1 Values of the lateral interactions energy (E) and the adsorption constant (K) determined for different catalytic systems at temperature 25T. No Catalytic System PH E / KJ.mol-' K / mol-'.dm3 - ~ 1 MoxOyz-/ TiO, 4.6 2.2 2392 2 MoXOt-/ y-Al,O, 5.0 10.5 116 3 WxOt-/ y-Al,O, 5.1 6.8 2610 4 CO'+ / y-Al,O, 4.5 .4 0.0 242 5 Ni2' / y-Al,O, 4.5 + 0.0 385 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. L.Wang and W.K.Hal1, J.Catal., 77(1982)232. S.Kasztelan, J.Grimblot, J.P.Bonnelle, E.Payen, H.Toulhoat and Y.Jacquin, Applied Catalysis, 7(1983) 91. C.V.Caceres, L.G.Fierro, A.L.Agudo, M.N.Blanko and H.J.Thomas, J.Catal., 95(1985)501. J.A.R. Van Veen, H.De Wit, C.A. Emein and P.A.J.M. Hendriks, J.Catal., 107(1987)579. P.A.J.M.Hendriks and J.A.R. Van Veen, Polyhedron, 5(1985)75. D.S.Kim, Y.Kurusu, I.E.Wachs, F.D. Hardcastle and K.Segawa, J.Catal., 120(1989)325. K.Y.S. Ng and E.Gulari, J.Catal., 92(1985) 340. J.P.Brunelle, Pure Appl.Chem., 50(1978)1211. N.Spanos, L.Vordonis, Ch.Kordulis and A. Lycourghiotis, J.Catal., in press. lO.N.Spanos, L.Vordonis, Ch.Kordulis, P.Koutsoukos and A.Lycourgiotis, 11. L.Karakonstadi s , Ch .Kordul is and A. Lycourghiot i s , in preparation. lE.N.Spanos, Ch.Kordulis and A.Lycourghiotis, in preparation. lS.N.Spanos, L.Vordonis, Ch.Kordu1 is, P.Koutsoukos and A. Lycourghiotis, in 14.L.Vordonis, P.G.Koutsoukos and A.Lycourghiotis, J.Catal., 98(1986)296. 15. L. Vordoni s , P. G. Koutsoukos and A. Lycourghiot i s , J. Catal . , 101 ( 1986) 186. 16.K.Akratopoulou, L.Vordonis and A.Lycourghiotis, J.Chem.Soc. Faraday Trans J.Cata1, in press. preparation. I . 82(1986)3697. 184 17.K.Akratopoulou, L.Vordonis and A.Lycourghiotis, J.Catal., 109(1988)41. 18.P.H.Wiersema, A.L.Loeb and J.Th.G.Overbeek, J. Colloid Interface Sci., 19.C.P.Huang and W.J.Stumm, J.Colloid Interf.Sci., 43(1973)409. 20.K.Ch.Akratopoulou, Ch.Kordulis and A.Lycourghiotis, submitted. El.J.Lyklema, in "Adsorption from solution at the solid/liquid Interface" EE.C.H.Giles, D.Smith and A.Huitson, J.Colloid Interface Sci., 47(1974)755. 22(1966)78. G.P.Parfitt and C.H.Rochester eds, Acad.Press, London, 1983 ch.5. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands 185 SCALING DOWN OF THE CALCINATION PROCESS FOR INDUSTRIAL CATALYST MANUFACTURING G. GROEN' , J . FERMENT', M. J . GROENEVELD' , J . DECLEER2 and A. DELVA2 lKONINKLIJKE/SHELL-LABORATORIUM, AMSTERDAM (Shell Research B.V. ) Badhuisweg 3 , 1031 CM Amsterdam, The Netherlands 2SHELL GHENT - CATALYST PRODUCT TECHNOLOGY, Passagierstraat 100, 9000 Gent, Belgium SUMMARY composition of the gas atmosphere in rotary kilns is described. The model is applied for scaling down the continuous commercial calcination of catalyst materials to batch calcination in laboratory rotary kilns as used in catalyst development work. The results of the model are compared with measurements carried out in the rotary kiln of the catalyst plant at Ghent, which produces y-alumina extrudates used as carrier for heterogeneous catalysts. The plug flow transport model assumed for the solids is confirmed by residence time distribu- tion measurements. The measured temperature profiles are in agreement with the calculated profiles after adjustment of the kinetic rate constants. A computer model for the calculation of the temperature/time profile and the INTRODUCTION Industrial catalyst manufacturing involves several process steps such as preparation and mixing of solutions or suspensions, crystallization, filtra- tion, washing, drying, mixing and kneading of powders, shaping, drying, calci- nation and impregnation. Catalyst recipes to be developed in the laboratory must be properly translatable to an industrial processing scheme, taking into account the equipment and instrumentation of an existing catalyst plant. Hence the translation problem is merely that o f scaling down rather than that of scaling up: in view of the possible commercialization of a catalyst recipe each process step in the laboratory needs to be carried out in a way representative of the commercial production. Tests on small-scale process steps are represen- tative if the process conditions and the scale applied are such that the target product properties are similar to those to be obtained in the commercial pro- cess step. Representative equipment and methods should already be used in the generally small-scale experiments in the initial phase of the laboratory recipe development, but often a larger scale is necessary. Muffle furnaces might be used in the laboratory, but it should be realized that these furnaces are usually not representative of commercial calcination in rotary kilns. Computer models simulating the performance of commercial equipment are therefore applied to an increasing extent to assist in scaling. This paper will discuss a compu- ter model for the calcination of catalyst materials in rotary kilns. The pur- 186 pose of such a model is, among other things, to calculate the temperature/time profile and gas composition in a commercial rotary kiln in order to be able to apply the same conditions (preferably batchwise) in a laboratory rotary kiln at the smallest conceivable scale (e.g. 50 ml catalyst). The model can also be used for translation from one type or size of commercial rotary kiln to another and for optimization of existing calcination practices. Also in the manufactur- ing of special catalysts (e.g. based on zeolites), calcination might be the critical factor for the performance of the final catalyst. Representative cal- cination experiments will therefore remain essential for successful commercial- ization. The scaling down of the calcination process for industrial catalyst manufac- turing requires knowledge of both the processing characteristics of the commer- cial rotary kiln and, for each different catalyst material, the physical and chemical processes taking place during the calcination. In this paper the ele- ments of the model will be described in more detail and the problems of its validation discussed. It should be realized that the model is still in the development phase. Therefore, the most important heat and mass transfer pheno- mena occurring in a rotary kiln must be described properly first. A description of the development of important catalytic properties such as surface chemistry, crystallinity, pore structure and metal dispersion is still beyond the scope of the present model. DESCRIPTION OF ROTARY KILNS Calcination can be carried out in various types of process equipment such as fixed beds, moving beds, fluid beds, tunnel kilns, moving belts and rotary kilns. Because of their versatility rotary kilns are widely used in the cata- lyst manufacturing industry both for calcination of zeolite powders and for calcination of shaped carriers and catalysts. Other types of equipment (e.g. moving belts) are used less frequently. Only the use of rotary kilns will be discussed. Various types of rotary kilns are found in commercial catalyst plants: (1) Directly gas-fired rotary kilns (Fig. 1); (2) Indirectly gas-fired rotary kilns e.g. for the production of powders or in cases where combustion flue gases are harmful to the catalyst materi- al ; ( 3 ) Electrically heated rotary kilns for small-scale production or for use in the laboratory; Rotary kilns are often equipped with internals such as longitudinal strips on the wall to prevent slipping of the solids bed over the tube wall and with solids flow restrictions such as dams and slotted diaphragms (Fig. 2). Flights for raining down of particles through the gas phase, as used in rotary driers 187 - SOLIDS FEED L INE - are not commonly used for calcination of catalyst materials. Commercial-scale rotary kilns are as a rule continuously operated. Rotary kilns in the labora- tory might be operated batchwise or continuously and are usually indirectly heated. PROPANE PRIMARY AIR SECONDARY AIR # EXTRUDATES FROM SIEVES EXTRUDATES TO HOPPER Fig. 1. Rotary kiln of the alumina catalyst plant at Ghent. - SOLIDS FLOW T Y P E II T Y P E I Fig. 2. Slotted diaphragms used in rotary kilns 188 DESCRIPTION OF COMPUTER MODEL The following elements are essential in any computer model of rotary kiln calcination: (1) A description of the transport of solids and gas through the rotary kiln; (2) A description of the heat-transfer processes taking place between solids, gas and walls, including heat losses from the outside walls of the kiln to the surroundings; ( 3 ) A description of the physical and chemical conversions with their heat effects, including mass transfer between the solids and bulk gas phases. The first two items are common elements for rotary kiln models and are discussed in many publications. A brief summary will be given and a few areas where uncertainties still persist will be indicated. The third item varies with the material to be calcined. A few general outlines will be given here. More details will be presented in subsequent sections. Gas and solids transuort Gas and solids are both assumed to flow in plug flow from one end of the kiln to the other, either cocurrently or countercurrently. Hence, neither of the two phases is assumed to mix in axial direction. For the gas phase this is justified because the gas flow in commercial roc:ry kilns is usually turbulent and the length-to-diameter ratio of rotary kilns is usually large (i.e. larger than 10). For these reasons the bulk gas phase is assumed to be completely mixed in a cross section of the kiln (except for thin film layers adjacent to the tube wall and the solids bed surface). The transport mechanism of the solids depends on the surface roughness o f the inside wall, the inside diameter of the kiln, the solids properties and the operating conditions of the kiln. Henein et al. [l] distinguish slipping, slumping, rolling, cascading, cataracting and centrifuging beds. The latter two are not relevant for catalyst calcination. Existence regions have been experi- mentally determined as a function of fill percentage (bed depth) and the Froude number (rotation speed) and have been founded with theoretical arguments [l]. The present model assumes a rolling bed, which has been confirmed for the rotary kiln of the catalyst plant at Ghent. The rolling bed is characterized by two regions (Fig. 3 ) : a thin layer of sliding or rolling particles at the bed surface, and the bulk part of the bed, in which the particles are stagnant with respect to the rotating tube wall. Differences in axial velocity between particles travelling only in the middle of the bed (i.e. small particles) and particles travelling only on the periphery of the bed (i.e. large particles) are averaged out for particles following random paths in the solids bed (see right-hand side of Fig. 3 ) . Plug flow is therefore justified for 189 non-segregating beds of particles and this has also been experimentally con- firmed, as shown in a subsequent section. SOL FE CROSS SECTION OF THE KILN I t CROSS SECTION A-A (VIEW OF BED SURFACE) INSIDE THE RED - SLIDING/ROLLING OVER THE BED SURFACE Fig. 3 . Particle trajectories in a rolling bed. In a cross section of the kiln, the rolling type of solids bed is assumed to be well mixed for heating up calculations and for kinetic calculations of relatively slow chemical or physical conversions in spite of the absence of relative motion in the stagnant part of the bed. This is justified because the Fourier time of a solids bed in commercial kilns is in the order of hours, while the cyclus time of a particle is in the order of seconds. Hence, bed heating occurs mainly by the continuous replacement of particles in the bed rather than by conduction. For a rolling bed, Saeman [2] has derived the following formula for the rate of volumetric solids transport: dh dz where qh = - is the slope of the bed surface with respect to the kiln axis, which makes Eq. (1) an ordinary differential equation with as boundary condition: h = h L at z = L ( 3 ) where hL is slightly larger than zero for rotary kilns without a flow constric- tion at the solids discharge end of the kiln or slightly larger than the dam height for kilns with a dam at the solids discharge end. Eq. (1) can easily be solved numerically taking into account possible bed volume changes as a conse- quence of chemical or physical conversions. For kilns with dams at several in- termediate locations, the integration can be carried out section by section in 190 upstream direction. For kilns with slotted diaphragms the solution procedure involves the evaluation of the solids flow characteristics of the slotted diaphragms as a function of the operating conditions: qSD = VSDN (a + f(Ah/h,,Fr,B-d)) ( 4 ) where the first term in braces a represents the "pumping" action of the slotted diaphragm and the second term the "levelling out" action as a function of the bed depth difference over the diaphragm, the Froude number and the dynamic angle of repose of the material corrected for the slope of the kiln. The solids flow characteristics have to be determined experimentally by testing of a (preferably full-scale) model of the slotted diaphragm or have to be derived from the overall solids hold-up characteristics of the kiln under various conditions. For rotary kilns without solids flow constrictions and with a large length-to-bed depth ratio (shallow beds) Eq. (1) can be considerably simpli- fied. Saeman [ 2 ] has derived for that case the following equation: He validated his model with experimental data of Sullivan et al.[3]. Description of heat transfer The heat transfer in the kiln is described in terms of enthalpy flows of the solids and gas phases and in terms of heat losses to the environment: d'a dz - 'loss _ _ 'st = 'gs + "us 'loss = 'gu - 'us - 'sb where f = +1 for flow in the positive z direction and E = -1 for flow in the negative z direction, T, = Ts for gas species produced in the solids bed and T, = T P, and 0, are the enthalpy flows of the gas and the solids, respectively and g is the heat lost to the surroundings of the kiln. The heat fluxes on the right- hand sides of Eqs. (6)-(10) are schematically shown in Fig. 4 , being a cross section of a directly fired rotary kiln. The present model considers only heat transfer in the radial direction. Axial radiative exchange is neglected, which for gas species consumed by the s o l i d s bed. g 191 is justified only for directly fired rotary kilns with a separate combustor. Figure 4 also shows that, for heat-transfer calculations, infinitesimally thin boundary layers in the solids bed are assumed, one adjacent to the solids bed surface and one adjacent to the wall. loss 0 Fig. 4 . Temperatures and heat flows in a kiln cross section. pgu and (pgs are the net heat fluxes flowing from the gas to the uncovered wall (that part of the wall in contact with the gas phase) and the solids bed surface, respectively. Both fluxes consist of a convective and a radiative part. pgu is the net radiative heat flux flowing from the uncovered wall to the solids bed surface. The radiative exchange in the gas space is evaluated with the methods described by Frisch and Jeschar [ 4 ] . The gas emissivity is calcu- lated with the method outlined by Leckner [ 5 ] . For the convective heat transfer from the gas phase to the solids bed and to the uncovered wall, the correlation for turbulent pipe flow as recommended by the VDI [ 6 ] is used. However, the heat transfer to the solids bed is multiplied by an enhancement factor based on the observations of Tscheng and Watkinson [ 7 ] , who report the coefficient for gas-to-bed heat transfer to be a factor of 10 higher than for the gas-to-wall heat transfer for gas flow in the laminar/turbulent transition region (Re = 2000 - 8 0 0 0 ) . Reich and Beer [ 8 ] report that rotation of the tube suppresses turbulent motion and therefore reduces the heat transfer coefficient. Their experiments show that this effect is significant only at very high rotational speeds (i.e. centrifuging beds). psb is the net heat flux flowing from the covered wall to the solids bed. The heat transfer coefficient is calculated 192 according to the procedure described by Schlunder [ 9 ] and Martin [lo] and includes a contribution by radiation not present in the model of Lybaert [ll]. The effect of heat capacity of the thick refractory wall on the heat trans- fer of directly fired rotary kilns is taken into account with the semi-empiri- cal expressions derived by Vaillant [12] for the evaluation of the temperatures of the covered and uncovered wall together with a heat balance over the wall for evaluation of the heat loss, 'ploss, to the environment. The convective and radiative heat losses from the outside kiln shell have been described by Kuhle [13]. DescriDtion of mass transfer Exchange of gas species occurs over the solids bedbulk gas interface as a consequence of chemical and physical conversions in the solids bed. This exchange consists of two parts: (1) a convective part due to a net production or net consumption of gas species by the solids bed; (2) a mass transfer part due to concentration differences between the gas in the solids bed and in the bulk gas. The mass transfer on the gas side of the interface is supposed to be determined by a (thin) layer adjacent to the interface and is described with a mass trans- fer coefficient, k. The calculations are simplified by defining an imaginary thin layer on the solids side of the interface, in which all diffusive resis- tance is concentrated. Consequently the concentrations inside the solids bed will be assumed constant. The mass transfer model is schematically depicted in Fig. 5 for the case of drying. The overall mass transfer rate for species i is described by: bnet,i = Xs,i 4~ + kov,i b (Cs,i - cg,i) (11) where 4 is taken positive in the direction from solids bed to bulk gas phase, b is the surface area of the solids bed per unit length of the kiln, and xi and ci are the mol fraction and the concentration of species i in the gas phase, respectively. Eq. (11) adds two unknowns to the mathematical model and hence we need an additional equation - which might be obtained by equating &,et,i to zero for a gas species not involved in chemical or physical conversions - a thermodynamic equilibrium equation, or a kinetic rate equation. An expression for the overall mass transfer coefficient in rotary kilns has not yet been published. Eq. (11) is merely used to check the maximum mass transfer rate over the solids bed/bulk gas interface, which might be limiting the rate of a chemi- cal or physical conversion in the solids bed. In that case kov,i equals ki, which can be obtained from the analogy with convective heat transfer. 193 INTERFACE BULK G A S PHASE c -I Fig. 5 . Schematic concentration profiles during drying in a rotary kiln CALCINATION OF PSEUDOBOEHMITE EXTRUDATES IN A DIRECTLY-GAS-FIRED ROTARY KILN Introduction The catalyst plant of Shell Ghent produces a variety of heterogeneous cata- lysts, many of which are supported on y-alumina extrudates of various quali- ties. The extrudates are manufactured by kneading and peptization of pseudo- boehmite powder with water and (in)organic aids to a paste, followed by extru- sion, drying, classification, longsbreaking and finally calcination. The rotary kiln in the alumina plant at Ghent (Fig. 1) was used for validation of the model. The measurements were carried out during normal commercial production of 1.5 nun y-alumina trilobes@ as a catalyst carrier. A mixture of two different commercial pseudoboehmites, PURAL SB - a high-density pseudoboehmite (700 kg/m3) from Condea Chemie GmbH -, and VERSAL 250, a low-density pseudo- boehmite (200 kg/m3) from Kaiser Chemicals - was used as starting material and organics were used as feeding aids. Five runs were carried out with different throughputs and kiln rotation speeds. Residence times and temperature and conversion profiles were measured for each run. The results will be presented after discussion of the dehydration of pseudoboehmite. Thermal dehvdration of pseudoboehmite Pseudoboehmite is a poorly crystallized form of boehmite, Al203.1 H20. It usually consists of agglomerates of platelets of very small size (nanometers) 194 but a fibrillar form has also been reported [14]. The amount of structural water varies from the stoichiometric amount of 1 mol H 2 0 per mol of A1203 to as much as 3.5 for almost amorphous boehmite. Reviews of pseudoboehmite and other alumina-related compounds have been given by Lippens and Steggerda [15] and by Misra [16]. The differential thermal gravimetry (DTG) curves of PURAL SB and VERSAL 250 are given in Fig. 6 . PURAL SB has been described earlier by Decleer [17]. The first endothermic peak is due to the desorption of water and the second due to the conversion of pseudoboehmite to 7-alumina. This conversion takes place over a broad temperature range, dependent on the crystallinity of the pseudo- boehmite. For nearly amorphous pseudoboehmite the conversion takes place at a temperature as low as 300 " C [18], while on the other hand the conversion of well crystallized boehmite takes place in a narrow temperature range between 450 and 580 " C [18]. Hence, each different type of pseudoboehmite requires a new evaluation of its thermal behaviour. 1 dw , -- Wo d t I I I 1 I 200 400 600 800 1000 TEMPERATURE, C Fig. 6. DTG curves of two commercial pseudoboehmites (4 "C/min in air) Considering the DTG analysis a kinetic model on the thermal dehydration of dried pseudoboehmite extrudates should include the following elements: 195 (1) Evaporation of physically adsorbed water left after removal of the bulk amount of water in the drying step. For the removal of the residual amount of moisture the proper adsorption isotherm for multilayer and possibly multicomponent ad/desorption should be taken into account; (2) Thermal decomposition of the pseudoboehmite including changes in particle diameter, porosity and surface area. The decomposition should distinguish between the loss of stoichiometric and the loss of excess water; (3) Thermal dehydroxylation of the alumina surface taking into account any physical adsorption or chemisorption equilibrium; (4) Thermal decomposition of additives; (5) Mass- and heat-transfer resistances within the extrudates; (6) The thermochemical data of all species involved and the adsorption and chemisorption heats of water. Some kinetic work has been carried out on the decomposition of (pseudo)- boehmite. Callister et al. [19] reported on the effect of the water pressure. Tsuchida et al. [20] reported on the effect of crystallite size and confirmed the effect of water pressure as determined by Callister et al. [19]. A consis- tent kinetic model for the decomposition of pseudoboehmite, which is applicable over the entire temperature range of interest in calcination (20 - 800 "C) and which takes into account the effects of particle size and water pressure is not available in the literature. Therefore the model of Leyko et al. [21] has been fitted to the data of Fig. 6 as a first approximation: y = WlXl + w2x2 + w3x1x2x3 (12) dxl/dt = exp(-56,5E6/R g T + 12.6057)(1-~1)~'~ dx2/dt = exp(-E2/R g T + ln(kr,2)) (1-~2)O.~/x2~.~ dx3/dt = exp(-31.OE6/R T - 3.0943)(1-x3)'.' (13) ( 1 4 ) (15) g with the constants as given in Table 1. The exponent of 0.14 in Eq. (14) of the original model of Leyko et al. [ 2 1 ] has been replaced by 0.6 to obtain a better fit with our pseudoboehmites. The constants for w1 given in Table 1 apply to the pseudoboehmite powders as received. In the dried extrudates, an initial XI was used corresponding to the measured loss-on-ignition (LOI) of the dried extrudates. It is assumed that the model derived from powder data is also applicable to the decomposition of pseudoboehmite in extrudates. The thermal decomposition of the organic peptization and feeding aids must also be considered since even the addition of as little as 1 %w organic acid may cause a potential adiabatic temperature rise in the order of 200 "C upon complete combustion inside the solids bed phase. However, prior to this the 196 organics may partly evaporate or decompose incompletely to gaseous products. It is also possible that during evaporation, organic decomposition products ignite upon their release from the solids bed such that diffusion flames can be obser- ved just beyond the bed surface. For the calculations presented here, it was assumed that part of the organics had been evaporated during drying and that the remainder (1.5 %w of dry product) could only be removed by thermal oxida- tive decomposition. TABLE 1 Constants fitted to the kinetic model of Leyko et al. [21], Eqs.(12)-(15). Pseudoboehmite w1 W2 w3 E2 ln(kr,2) PUPAL SB 0.056 0.158 0.024 130.OE6 14.8057 VERSAL 250 0.096 0.148 0.044 100.OE6 9.8057 Most thermochemical data in the present model have been taken from Barin et al. [ 2 2 ] . The data for 7-alumina have been taken from the JANAF tables [ 2 3 ] . No data are available for pseudoboehmite [24]. As an approximation, the thermo- chemical data of liquid water were added to the data of Haas et al. [ 2 5 ] for crystalline boehmite in proportion to the molar amount of "excess" water present in pseudoboehmite. Experimental The residence time distributions were measured by pulse injection of a sample of calcined extrudates labelled with technetium-99m. The movement of the injected sample was followed with one scintillation detector in the solids feed line and three around the chute at the solids discharge end of the kiln (Fig. 1). The amount of injected radio-active material was too small for its passage through the kiln to be followed with detectors along the outside wall of the kiln, as has been done for measurement of the solids transport in rotary kilns used for production of clinker [26,27]. The temperatures and conversions were measured either before or after the radio-active tracer experiments. Intermediate samples were taken from nozzles at one quarter, at half and at three quarters of the kiln length. The latter two nozzles were also used for measurement of the gas and solids temperatures with a specially constructed thermowell, which could be quickly inserted or removed during rotation of the kiln. Samples were also taken from the solids feed and from the cooler. Samples were analysed for LO1 and boehmite conversion using X-ray diffractometry. 197 L The solids outlet temperature was measured with a fixed thermocouple. Tem- peratures of the outside shell wall (the skin) were measured with a contact thermometer and with colour chalk. A shielded velocity thermocouple (suction pyrometer) was used for measurement of gas temperatures at the cold and hot ends of the kiln. Gas samples were taken from the cold (gas outlet) end of the kiln using a long sampling tube to minimize inclusion of false air from the kiln seals at the cold end. The sampled gas was analysed on line for oxygen to determine the total amount of excess combustion and false air sucked in from the hot end of the kiln. A . .. ~. Results of residence time distribution measurements One example of a measured residence time distribution is given in Fig. 7. The measured distributions were interpreted with a model of n ideal mixers in series [ 2 8 ] . The results are given in Table 2, together with the theoretical predictions using the rolling-bed model of Saeman [ 2 ] . The theoretical particle velocity has been calculated with E q . ( 5 ) . The agreement between theoretical and measured bed velocities is considered good for the deep bed runs. The larger discrepancies with the shallow bed depths can be explained by the presence of the longitudinal strips. Particles lifted by the strips are with- held longer from axial movement than particles in the bed. Figure 7. Residence time distribution measured for run no. 1. Table 2 also shows that the number of ideal mixers is so large that solids mixing in axial direction can be neglected. It appears that the number of mixers is of the same order of magnitude as the average number of times a particle rol ls down the surface of the solids bed. 198 This supports the assumption of the rolling-bed model that mixing occurs only in the rolling layer. TABLE 2 Comparison between measured and theoretical solids transport parameters Run Kiln Bed Average axial particle Number of Number of speed, height, velocity, m/h mixers particle falls # rpm cm measured theoretical # # 1 0 . 8 4 5 1 5 . 5 10 .00 9 . 7 8 1416 563 2 0 . 5 14.4 5 . 7 6 5.81 473 587 3 1 . 5 7 . 8 1 4 . 1 9 1 7 . 3 7 312 8 5 1 5 0 . 5 1 7 . 6 6 . 2 0 5 . 8 1 922 519 4 1 . 5 9 . 7 1 5 . 1 7 1 7 . 3 7 648 748 ComDarison of calculated and measured temperature and conversion Drofiles Calculated and measured temperature and conversion profiles of one run are compared in Fig. 8. The measured solids inlet and outlet temperatures of the rotary kiln were used as boundary conditions of Eq. (6) and ( 7 ) , since these values are more accurate than the measured gas inlet and outlet temperatures. Fig. 8a compares the results using the kinetic decomposition model derived from the thermal analysis of the pseudoboehmite powder, neglecting the organics con- tent of the extrudates and ignoring enhanced convective heat and mass transfer at the solids bedbulk gas interface. The calculated solids temperatures at half and at three quarters of the kiln length appear to be about 90 and 65 'C lower than the measured values, respectively. The measured gas temperature at the solids inlet is about 1 5 "C lower than the calculated value. The measured gas temperatures at half and three quarters of the kiln length are 40 and 30 " C lower than the calculated values, respectively, but are possibly too low due to radiation losses from the unshielded thermocouple tips. The measured gas tem- perature at the solids outlet side of the kiln is about 50 "C higher than the calculated value, which is ascribed to the fact that hot gases from the combus- tor and false air from the seals and the cooler have not yet been completely mixed at the solids outlet end o f the kiln. Measured and calculated skin temperatures agree within a few degrees. The small discontinuity in the calcu- lated skin temperature profile is due to the use of a better fire-resistant but less insulating brick lining in the hotter part of the kiln. The conversion of the pseudoboehmite starts earlier according to the simulation than has been measured, but completion of the conversion is predicted correctly. To improve the agreement of measured and calculated profiles, the convective heat and mass transfer coefficients were enhanced by a factor of 10, in accor- dance with the experimental data of Tscheng et al. water was replaced (at constant LOI) by acetic acid as a model compound for oxidation in the temperature range of 300-450 " C as reported by Abrams [14]. To simulate a possible retarding effect of steam on the decomposition of pseudo- boehmite [19,20] and a possible retarding effect of pressure flow limitation inside the extrudates, we lowered the reaction rate at low temperatures by increasing the activation energy to 390 MJ/kmol and correspondingly increasing the pre-exponential constant of Eq. that the measured conversion profile was matched. The results of the improved simulation are given in Fig. 8b, showing that the agreement between measured and calculated solids temperatures had significantly improved. Better agreement was also obtained for the skin temperatures, while the agreement of the gas temperatures had improved for the intermediate kiln locations but had deterio- rated on the inlet and outlet sides of the kiln. [ 7 ] . The physically adsorbed (14) (i.e. to log(kr,2) = 57.7422) such The oxygen consumption by the oxidation of the acetic acid was maximal at a solids temperature of about 440 " C , while the maximal mass transport over the gas film at this temperature was a factor of 10 higher. Hence, no oxygen mass transfer limitation occurred for the kinetics and mass transfer enhancement assumed. FURTHER DEVELOPMENT OF THE MODEL From Fig. 8 it is clear that the rotary kiln model cannot be firmly validated with the measurements in the catalyst plant at Ghent due to the absence of a consistent kinetic model and due to the lack of reliable thermo- chemical data on the conversion of pseudoboehmite to y-alumina. If the measured decomposition rate of pseudoboehmite is fitted by adjustment of the rate con- stants, then an acceptable agreement with the solids temperature profile is also obtained. Uncertainties exist on the enhancement of the coefficients for convective heat and mass transfer between the turbulent flowing gas and the solids bed. The convective heat and mass transfer is also enhanced by the presence of longitudinal strips on the inside wall of the kiln from which particles are falling through the gas phase from a maximum height corresponding with the angle of repose. Corrections can be made for this with the methods described for rotary driers [30,31], which are designed for the purpose of raining the solid particles through the gas phase. These corrections have not been made in the present model. More work is also required on the oxidative decomposition of organic additives present in the solids and their possible combustion at the surface of the solids bed. Residual organic species present in the calcined product might ultimately have a negative effect on the cataly- tic performance. 200 a. Before TEMPERATURE, OC 700 / CONVERSION, â10 7-1â: .. . RELATIVE DISTANCE FROM SOLIDS INLET parameter adjustment. TEMPERATURE, OC 800 CONVERSION, % p-E 20 I0 0 0 1/4 I /2 3/4 I RELATIVE DISTANCE FROM SOLIDS INLET b. After parameter adjustment. Fig. 8. Comparison between calculated and measured temperature and conversion profiles in rotary kiln of alumina plant at Ghent. Legend: *--.--.--. solids temperatures, -------- &------- gas temperatures, ---+--- skin temperatures, -3- pseudoboehmite conversion. 201 The model is now being used for scaiing down the calcination of various types of catalysts requiring different kinetic models. Comparisons are being made between products obtained at different scales of rotary kilns and guide- lines are being established for carrying out a representative calcination at the smallest conceivable scale. CONCLUDING REMARKS (1) A computer model has been developed for the calculation of temperature and conversion profiles in rotary kilns. (2) Residence time measurements confirm a plug-flow-type solids transport, which is consistent with the rolling-bed model of solids transport. (3) The average residence time in the rotary kiln of the catalyst plant at Ghent is satisfactorily predicted by Saeman's simplified solids transport equation for shallow beds (Eq. (5)). (4) The temperature profiles in the rotary kiln of the catalyst plant can be predicted reasonably well provided that a good kinetic model is available. ACKNOWLEDGEMENT Dr. R. Jacobs (Rijksuniversiteit Gent) is acknowledged for making available the university facilities for preparing the Tn-99m labelled samples. NOMENCLATURE b Chord in kiln cross section corresponding to solids bed surface, m c Concentration, kmol/m3 D Inside diameter of rotary kiln, m Deff Effective diffusion coefficient. m2/s E Fr g H h k F N q R Rg r0 S T t 'SD W X Y Z U . , Activation energy J/kmol Froude number = N D/g Gravity constant = 9.80665 m/s2 Enthalpy , J/kmol Bed height, m Mass transfer coefficient, kmol/(m2.s) Reaction rate constant Kiln length, m Kiln rotation speed, s-' Volumetric solids bed transport, m3/s Kiln radius, m Gas constant = 8314.3 J/(kmol.K) Distance between kiln axes and solids bed surface centre line, m Slope of the kiln with respect to the horizontal, rad Temperature, K Time, s Volume of solids bed sliced out per revolution by slotted diaphragm, m3 Weight fraction of mass intake Mol fraction in E q . (ll), otherwise conversion Weight loss as fraction of mass intake Distance from solids inlet, m Constant in Eq. ( 4 ) 2 202 6 8 Central bed angle, rad f @ Enthalpy flow, Watt 4 3 'p Net heat exchange per unit kiln length, W/m Layer thickness characteristic for diffusion resistance, m +1 (or -1) for flow in positive (or negative) direction Gas flow crossing the solids bed bulk gas interface, kmol/(m.s) Slope of the bed surface with the kiln axes, rad Subscripts a b g i L ov S SD T U X air bottom of solids bed gas index of gas species solids outlet side of kiln overall solids Slotted Diaphragm Total of gas species uncovered kiln wall either g or s REFERENCES 1 H. Henein, J.K. Brimacombe and A.P. Watkinson, Met. Trans., B 14B (1983) 6, 2 W.C. Saeman, Chem. Eng. Progr., 47 (1951) 10, p. 508. 3 J.D. Sullivan, C.G. Maier and O.C. Ralston, U.S. Bureau of Mines, Technical 4 V. Frisch and R. Jeschar, Zement-Kalk-Gips, 36 (1983) 10, p. 549 (in 5 B. Leckner, Combustion and Flame, 1 9 (1972) , p. 33. 6 VDI Warme Atlas, 4th edition 1984, VDI-Verlag GmbH, Dusseldorf. 7 S.H. Tscheng and A . P . Watkinson, Can. J. Chem. Eng., 57 (1979) 8 , p. 433. 8 G. Reich and H. Beer, Int. J. Heat Mass Transfer, 32 (1989) 3 , p. 551. 9 E.U. Schlunder, Chem. Ing. Technik, 53 (1981) 1 2 , p. 925 (in German). pp. 191-205 and pp. 207-220. Paper 384 (1927) . German). 10 H. Martin, Chem. Ing. Technik, 52 (1980) 3 , p. 199 (in German). 11 P. Lybaert, Int. J. Heat Mass Transfer, 30 (1987) 8 , p. 1663. 1 2 A. Vaillant, Ph.D. Thesis Columbia University, 1965, University Microfilms 1 3 W. Kuhle, Zement-Kalk-Gips, (1970) 6 , p. 263 (in German). 14 L . Abrams and M.J.D. Low, I&EC Product and Development, 8 (1969) 1, 15 B.C. Lippens and J.J. Steggerda, in B.G. Linsen (Ed.), Physical and Chemical International, Ann Arbor, Michigan, U.S.A., 1979. pp. 38-48. Aspects of Adsorbants and Catalysts, Academic Press, New York, 1970, pp. 171-211. 16 C. Misra, Industrial Alumina Chemicals, ACS Monograph 184 , American Chemical 17 J.G.M. Decleer, Bull. SOC. Chim. Belg., 98 (1989) 7 , p. 449. 18 R.C. MacKenzie and G. Berggren, in R.C. MacKenzie (Ed.), Differential Thermal Analysis, V o l . 1 , Academic Press, London, 1970, pp. 279-302. 19 W.D. Callister, Jr., I.B. Cutler and R.S. Gordon, J. her. Ceramic SOC., 49 20 T. Tsuchida, R. Furuichi and T. Ishii, Thermochim. Acta, 39 (1980) , 21 J . Leyko, M. Maciejewski and R. Szuniewicz, J. Therm. Anal., 17 (1979) , 22 I. Barin, 0. Knacke and 0. Kubaschewski, Thermochemical Properties of Society, Washington D.C., 1986. (1966) 8 , pp. 419-422. pp. 103-115. pp. 275-286. Inorganic Substances, Springer Verlag, Berlin, 1973. Supplement, 1977. 203 23 JANAF Thermochemical Tables, J. Phys. Chem. Ref. Data, 14 Suppl. (1985), 24 S.C. Carniglia, J. Amer. Ceramic SOC., 66 (1983) 7, p . 495. 25 J.L. Haas Jr., G.R. Robinson Jr. and B.S. Hemingway, J. Phys. Chem. Ref. 26 K. Akerman, Chem. Ing. Techn, 43 (1971) 22, p. 1204. 27 H. Costa and K. Petermann, Silikattechn., 10 (1959) 4, p. 209 and 10 (1959) 28 0. Levenspiel, Chemical Reaction Engineering, 2nd ed., John Wiley and Sons, 29 R.S.C. Rogers and R.P. Gardner, Powder Technology, 23 (1979), p . 159. 30 F.A. Kamke and J.B. Wilson, AIChEJ, 32 (1986) 2, p. 269. 31 H. Hirosue, Powder Technology, 59 (1989), p. 125. pp. 156-159. Data, 10 (1981) 3, pp. 575-665. 5, p. 253. Inc., New York, 1972, Chapter 9. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 205 HYDROTHERMAL SINTERING OF THE ACTIVE PHASE IN ALUMINA SUPPORTED FIXED BED NICKEL CATALYSTS DURING REDUCTION E.K. POELS, J.G. DEKKER and W.A. VAN LEEUWEN Unilever Research Laboratory, Vlaardingen, The Netherlands SUMMARY Sintering of the active phase of alumina supported fixed bed nickel catalysts due to the hydrothermal conditions present during reduction is investigated. Three mechanisms are proposed which could lead to lowering of the degree of metal dispersion on the support: a) Sintering of the support; b) Thermal sintering of nickel metal particles and c) Sintering of the Ni2â precursor prior to formation of metallic nickel. Three nickel catalysts have been prepared in order to test the relative contribution of the proposed mechanisms: Two catalysts by impregnation with nickel nitrate solution of a gamma alumina support susceptible to sintering and of a sinter stable alpha alumina support. The third catalyst was prepared via a novel route developed in our laboratory not using nitrate. The results obtained after various treatments show that sintering of the nickel metal crystallites and of the support play only a minor role. The predominant mechanism is hydrothermal sintering of the Ni2+ species present on the support prior to their reduction. The novel catalyst proved far more stable than the other two. This could partly be ascribed to the fact that nitric oxide vapours evolved with the nitrate prepared catalysts enhanced Niâ sintering considerably. INTRODUCTION A key step in the manufacturing route of nickel catalysts is the reduction by hydrogen at high temperature to produce small, stable crystallites of metallic nickel. It is well known in the literature that sintering of this active phase can occur as a consequence of the hydrothermal conditions present during reduction, leading to lowering of catalyst quality [l] . The purpose of this study has been to identify the main mechanism underlying the sintering phenomenon and to compare the behaviour of catalysts prepared by a new method developed in our laboratory with those obtained by more conventional procedures. Three mechanisms can be envisaged which could lead to lowering of the degree of metal dispersion on the support: 1. Sintering of the support. I t is possible that the support used, shows surface area loss due to the hydrothermal treatment during reduction, thus influencing metal support interaction and perhaps surface mobility of the active phase. 2. Sintering of the metal crystallites. Thermal sintering of nickel particles at high temperatures in dry hydrogen due to atomic migration or Ostwald ripening [2] and in a steam atmosphere (due to film formation) [ 3 ] have both been reported in the literature. Sintering of the N i Z + precursor prior to formation of metallic nickel. During calcination and reduction hydrothermal treatment of the NiZf species present occurs, perhaps with similar effects on the dispersion of such species as described above. 3 . The emphasis in the literature is clearly on the second mechanism which is widely studied for noble and base metal catalysts. In this paper the relative contribution of the above mechanisms during reduction of alumina supported fixed bed nickel catalysts at realistic conditions is investigated. For this purpose three nickel catalysts have been prepared containing similar nickel loadings: a. A catalyst prepared by impregnating gamma alumina with aqueous nickel nitrate solution in order to test the effect of support sintering on the metal dispersion. b. A catalyst prepared by impregnation of alpha alumina with aqueous nickel nitrate solution in order to test the effect of precursor and/or metal crystallite sintering with minimal support sintering. c. A catalyst prepared via a novel impregnation method developed in our laboratory not involving application of nickel salts derived from strong mineral acids on a special wide pore support. (In many petroleum processing applications these now commercialised catalysts show superior performance to currently available products). The three catalysts were subjected to various reduction and pretreatment conditions and subsequently their BET surface area, metal surface area, nickel crystallite size and degree of reduction were compared. EXPERIMENTAL Two catalysts were prepared by impregnation of the alumina support in question with an aqueous nickel nitrate solution. The concentration of this solution was such as to result in a metal loading of approximately 11 wt.% (prior to calcination or reduction) after saturation of the pore volume of the support by submersion and subsequent filtration. A commercial gamma-alumina was used for one catalyst; an alpha-alumina support was prepared from this carrier by calcination at 1110°C for 2 hours. The third catalyst was prepared following a commercially applied preparation method involving impregnation of a special wide pore alumina support with a solution not containing nickel salts derived from strong mineral acid. This procedure was carried out such as to obtain the same nickel loading as described above. The catalysts were dried at 120°C for 16 hours. 207 TABLE 1 Analysis data of the catalysts studied. code alumina reduct. Ni cont. S(BET) Ni surf.area Ni Diam. Deg.of red. support method (wt. X ) (m2/g) (m2/gNitOt) (m) ( X ) 1037 gamma stdd 10.7 228 126 2.5 74 1045 special stdd 11.6 97 158 2.2 79 1075 alpha super 10.1 193 2.0 90 1034 alpha stdd 10.2 64 160 2.2 80 1074 gamma super 11.8 158 1.9 69 1076 special super 12.0 193 1.9 86 The BET surface area of the catalyst samples was determined using nitrogen physisorption. Nickel surface area determination by hydrogen chemisorption; calculation of metal crystallite sizes and measurement of degree of reduction were conducted according to reference [4]. In some instances catalyst samples were passivated after reductive treatment in all-glass flow equipment using nitrogen containing ca.1 v01.Z 0, and subsequently transferred into the chemisorption cell. In other cases the treatment could be carried out directly in the H, chemisorption apparatus. Sample sizes for chemisorption measurements and treatments were approximately three grammes. Nickel contents were determined using x-ray fluorescence spectroscopy. The analysis data of the catalysts thus obtained are presented in table 1. Ni dispersion retention (X) 100 60 40 v. cat. Ni(N03)2/alphaA1203 (N03)2/gammaA1203 a b Fig. 1. C d procedure Reiention of nickel surface area for the three catalysts tested after: a ) Standard reduction; b) standard reduction followed by l h H , extra at reduction temp.; c) lh H/H,O extra at reduction temperature; d) heating up in N 2 / H 2 0 to reduction temperature prior to standard reduction. 208 Chemisorption results are given after a "standard" reduction procedure as well as after a "super" reduction involving extremely efficient removal of the water generated. Water contents of moist treating gases were always the saturation level at ambient temperature (ca. 2.4 vol.%) unless otherwise stated. RESULTS AND DISCUSSION In a first set of experiments the three catalysts were reduced using a "standard" set of conditions and also with prolonged exposure to hydrogen or moist hydrogen at the reduction temperature after the standard reduction. A third reduction was preceded by a treatment in moist nitrogen up to the reduction temperature (see figure 1). From this plot it is clear that the nickel surface area loss upon prolonged treatment in dry hydrogen at reduction temperature is quite small. A flow of 2 . 4 vol.% H,O in H, after standard reduction did not result in extreme nickel sintering either. A more substantial loss of nickel dispersion was observed upon pretreatment in moist nitrogen at elevated temperature. From nitrogen physisorptionmeasurements of the nitrate prepared gamma alumina based catalyst it is clear that only very limited BET surface area l o s s occurred upon the treatments carried out (figure 2). As a result one could already cautiously rule out sintering of the support as a decisive cause BET surface area retention (%) 110, 100 90 80 7 0 60 5n -- std std(duplo) H2+H20 support sup+H sup+H/HSO procedure Fig. 2. Retention of BET surface area of gamma alumina supported nickel catalyst and the bare carrier after: standard reduction; standard reduction (duplo) ; reduction followed by lh treatment in moist hydrogen at reduction temperature; the bare support; the support treated under reduction conditions; the support reduced and treated for lh in moist hydrogen, respectively. 209 for hydrothermal sintering during reduction. Metal crystallite growth is also less likely on the basis of the minimal effects of prolonged H, and H,/H,O treatments. In order to test the importance of the remaining mechanism: NiZr precursor sintering the unreduced alpha alumina supported and novel catalyst were subjected to increasingly severe calcination treatments. In figure 3 it can be seen that calcination in a rotary calciner at 250°C already results in some loss of metal dispersion in the conventionally prepared catalyst. Calcination at 350°C of a monolayer of catalyst yields a worse nickel surface area and, finally, calcination of a bed of catalyst a few centimeters deep in a narrow necked Erlen- Meyer flask (i.e. removal of moisture is very restricted) really destroys catalyst quality. The novel catalyst is in all cases much more sinter stable. The extent of sintering of the support was checked by treating the alpha alumina support in the same way as described above and subsequently measuring BET surface areas of both the maltreated bare support samples and the catalysts based on this carrier. Indeed modest support sintering had occurred in all cases as expected (figure 4 ) . What surface area loss is apparent in the catalyst samples in comparison with the support must probably be ascribed to a decrease in the contribution of the active phase to the BET area, as in maltreated gamma alumina supported catalysts (i.e. a much less sinter stable carrier) surface area loss was negligable (see figure 2) . In order to check whether at standard reduction conditions water removal is Ni dispersion retention (%) 100 80 60 40 20 0 none rotary monol. limited N2/H20 procedure Fig. 3 . Retention of nickel surface area of standard reduced alpha alumina supported catalyst and the novel catalyst after: just reduction; rotary calcination; calcination of a monolayer on gauze; calcination in an Erlen-Meyer flask; heating up in moist N, prior to reducticn, respectively. 210 BET surface area retention (%) I /! 110 100 90 80 7 0 60 port I std H2/H20 calc. procedure Fig. 4 . Retention of BET surface area of the alpha alumina supported catalyst and the bare carrier after: standard reduction conditions: reduction followed by If. extra treatment in moist hydrogen at reduction temperature; calcination i n an Erlen-Meyer flask prior to reduction, respectively. inefficient and therefore in itself causes sintering thus rendering these pre- sintered catalysts insensitive to subsequent maltreatment, the following experiments were conducted. A so-called "super" reduction was performed i .e. water removal was enhanced by applying low heating rate and very high hydrogen flow rate. The water content of moist gases (when applicable) was doubled compared to the standard reduction experiments by saturating the gas at 35°C (i.e. ca.5.0 vol.% H,O) instead of ambient temperature in order to further enhance the effects (from this moment on all moist treating gases were saturated with water at 35°C). All post- and pre-treatments of the catalysts were extended to two hours for the same reason. During the standard reduction experiments after moist nitrogen treatment at reduction temperature, the switch to hydrogen flow was made at this elevated temperature. It may be assumed that at this temperature where reduction rate is high a sudden large quantity of water is generated by this switch. To avoid this shock treatment of the catalyst the reactor was cooled to ambient after moist nitrogen treatment prior to the super reduction. The results, summarised in figure 5 are quite similar to those of the standard reduction experiments (figure 1) although initial nickel surface areas are higher than upon standard reduction (see table 1) illustrative of the effect we se'i out to study. Therefore, the above derived conclusions still hold. Another conclusion to be derived from 211 Ni dispersion retention (%) 40 20 L dev. cat. 03)2/alphaA1203 std std+H2 H2/H20 N2/H20 procedure Fig. 5. Retention of nickel surface area of all three catalysts studied after "super" reduction and various treatments. For treating conditions see figure 1 and the text. the standard and super reduction experiments is that the order of initial metal dispersion is: novel f: alpha n gamma (see table 1). And the order of sinter stability is novel n gamma > alpha (see figures 1 and 5 ) . The increased sinter stability of the gamma alumina catalyst compared to the alpha alumina case is probably due to a better metal support interaction of the first catalyst as is reflected by the degrees of reduction of the catalysts (table 1). In figure 6 it can be seen that the nickel surface area loss of the catalysr s upon pretreatment in moist nitrogen for the alpha alumina Supported sample is totally due to an increase in crystallite size. For the gamma alumina prepared catalyst a combination of crystallite sintering and decrease in degree of reduction, probably caused by surface spinel formation, is apparent. With the novel catalyst reasoning along the same lines, only a slight surface NiA1,0, spinel formation is underlying the minor surface area loss observed. Quite another question is why the nickel nitrate based catalysts are so much less sinter stable than their newly developed counterpart. The influence of NO, containing fumes generated by nitrate decomposition during heating was studied in the following way. 11 Grammes of the novel catalyst were placed in a narrow glass tube on top of the same amount of the gamma alumina supported nickel nitrate containing catalyst separated by a layer of quartz wool. The tube was then calcined at 350°C for 2 hours, It was established by thermo- 212 [ryst. size (nm) degr. of red. (%/ 100 5 90 4 80 3 70 2 60 I I I ' ' 50 std etd+H2 H2/H20 N2/H20 procedure Fig. 6. Nickel crystallite size and degree of reduction of all three catalysts studied after "super" reduction and the same treatments as in figure 5. + = Ni(NO,),/gamma A1,O3; * = Ni(N0,)Jalpha Al,O,; 0 = novel catalyst. gravimetric analysis that at this temperature the nitrate fully decomposed. The same experiment was repeated with two layers of the development catalyst i. e. not creating any nitrogen oxides. The results are given in figure 7. The sintering without the NO, is not extreme considering the limited exhaust of vapours in the test tube (compare e.g. the surface area loss for this catalyst upon calcination in figure 3 ; there the effect of calcination with hindered gas exhaust was 89 % dispersion retention). The sintering upon exposure to nitrogen oxide vapours in combination with moisture is clearly more pronounced confirming the negativz effect of these oxides on catalyst quality. In order to check this effect on the nitrate prepared alpha alumina supported catalyst two samples of this catalyst were calcined for two hours at 3 5 0 ° C in an excessively high nitrogen flow (ca. 1000 ml/min) . The nickel surface areas of the samples after "super" reduction and after a moist nitrogen treatment at elevated temperature followed by similar reduction (in the same cell) are given in figure 8 . For comparison the metal surface area of an uncalcined sample treated in the same way as the latter of the above described samples is included in the plot. It may be assumed that the two calcined catalysts were substantially free of nitrates when reduced, whereas during heat treatment of the uncalcined sample NO, must have been present. From the plot it can, again, be concluded that exposure of Ni2+ to moisture is causing a decrease in the reduced metal dispersion. Furthermore, although the precalcination in high nitrogen flow will limit the 213 water generated during reduction thus restricting sintering somewhat, the results seem to confirm that the combination of moisture with NO, fumes is even worse. In order to prove that gaseous decomposition products are not the cause of sintering of the novel catalysts as well, a commercially calcined catalyst from Crosfield was obtained containing 15 wt.% nickel (trade name: HTC 4 0 0 ) . Upon standard reduction the nickel surface area proved 170 m2/gNitOt (i.e. a better dispersion than the lab prepared catalysts after standard reduction at higher nickel loading!). Upon treatment inwet nitrogen at elevated temperature; cooling to ambient and finally standard reduction, the surface area retention was 71%, proving that NiO is also susceptible to hydrothermal sintering. For the alpha alumina catalyst (figure 8) when precalcined in a very high flow of dry nitrogen followed by treatment with moist nitrogen and intermediate cooling, upon subsequent reduction a metal surface area retention of circa 73% was observed. The same treatment applied to the novel catalyst resulted in a nickel surface area retention of 96%. Although it may be concluded from the above that NO, fumes are responsible for considerable enhancement of the hydrothermal sintering of the Ni2+ precursor salt and their absence is a major cause for the increased sinter stability of the newly developed catalyst, it is clear from this experiment, that this effect cannot entirely explain the stability of the nickel dispersion of the novel catalyst Ni dispersion retention (%) 7 ref -NOx treatment +NOx Fig. 7. Retention of nickel surface area after: "super" reduction; calcination of the novel catalyst in a test tube prior to reduction and; calcination of the novel catalyst in a tube on top of a nitrate prepared catalyst prior to reduction, respectively. 214 Ni dispersion retention (%) I ' O 0 I 80 0 L calc/super calc/N+H20 nocalc/N+HPO treatment Fig. 8 . Retention of nickel surface area of the "super" reduced alpha alumina supported .. catalyst after the following pretreatments: calcination in very high nitrogen flow; calcination in nitrogen flow followed by treatment in moist nitrogen and cooling to ambient; treatment in moist nitrogen and cooling to ambient without preceding calcination. CONCLUSIONS 1. The major mechanism leading to poor metal dispersion of alumina supported nickel catalysts is hydrothermal sintering of NiZ+ precursors prior to reduction. Sintering of the support or metal crystallites once formed are relatively unimportant. Nitrogen oxide vapours produced during high temperature treatment o f nickel nitrate prepared catalysts in combination with moisture greatly enhance sintering of the active nickel species. 2. 3 . Improved NiZ+ stability can be achieved using new catalyst preparation procedures. 4 . This study has provided valuable information in identifying a key aspect in the reduction step which must be controlled during manufacture in order to obtain optimum quality catalysts. REFERENCES 1. G . C . Chinchen, in J . R . Jennings (Ed.), Critical Reports on Applied Chemistry, Vol. 12, Selected Developments in Catalysts, Blackwell Scient. P u b l . , London, 1985 , p.2. 2. K.-T. Kim and S . - K . Ihm, J . Catal., 96 (1985) 12. 3 . E. Ruckenstein and X.D. Hu, J. Catal., 100 (1986) 1. 4 . a) J.W.E. Coenen, Ph.D. thesis, Technical University Delft, 1 9 5 8 . b) R.Z.C. vanMeerten, A.H.G.M. Beaumont, P.F.M.T. van Nisselrooij andJ.W.E. Coenen, Surf. Sci., 135 (1983) 565 . G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 215 C. E. Marsden, Crosfield chemicals, P. 0. Box 26, Warr-n, Erqlanfi. SOMMARY silica is an inprtant support for ethylene polymerisation catalysts. It plays an active role in the polymerisation precess and its structure and camposition influence the catalyst activity, polymer morphology and properties. PKC&UES for making a wide range of silica structures are illustrated and key steps in their preparation highlightel. structure on polymer properties is described, together with the role of heteroatanr; in modifying the silica structure and polymer properties. Ihe particular influence of support mrucrIoN In excess of 4 million tonnes of high density polyethylene are prcduad annually using Phillips type cr/SiOa catalysts in slurry and gas phase processes. catalyst in olefin polymerisation must fragment during the reaction (ref. 1). Failure to do so results in the pores b s a u n g * blocked with polymer and the reaction ceasing at very law produdivity. Since the catalyst is not rammed f m the final polymer during manufacture, failure to fragment properly can also lead to wfiesirably large pieces of material in the polymer merm it unsuitable for virtually all applications. the catalyst particles break up during polymerisation, they must also be sufficiently strong to resist attrition during prolorqed activation treabnent at high temperature in a fluidid bed reactor. properties is therefore required in the silica support. In cmplete contrast to m t other supported catalyst systems, the H a w e v a whilst it is essential that A delicate balance of In addition to the mecharu 'cal strength requirements hwever the role of the silica suppo~3 in influercw polymer characteristics is of key importance. It is clear that silica does more than act as an inert support and that aspeds of its structure, in particular porosity, influenoe the activity of the catalyst and control polymer structure in ternrs of molecular weight, molecular weight distrhkion and chain branching (ref. 2). These polymer stru- characteristics in turn dictate the application properties of the polymers, eg, melt flaw, melt elasticity, enviromental stress crack resistance, etc, and hence defh its suitability for specific uses. 216 SIILCA m- synthetic m@ous silicas may be arbitrarily divided into gels and precipitates, although in many cases it is not easy to classify the material on the basis of their intrinsic properties and division mre typically reflects the methcd of preparation. hard, dense materials with a well-defined pore structure, whereas precipitates were typically loose, voluminous materials with less easily defined structural characteristics. However, silica technolcgy has advanced to the point where both types of product can now be prepared f m routes previously considered to be either precipitate or gel manufacturing methcds. Kitherto silica gels have been regarded as rather Both gels and precipitates are carmmercially manufacture3 by neutralisation of scdium silicate with acid. dictated by the polymerisation rate/@ curve (ref. 3 ) . The rate passes through a maxinnrm at pH7 and control is difficult under these conditions. therefore cammercial silicas tend to be prepared at either low or high pi. Gels are conventionally prepared at lcw pH, high concentration and law temperature, whenxis precipitates are typically produced at high pi, lcwer concentrations, higher temperatures and often in the presence of added electrolyte. precipitates are prepared in batch stirred tanks. The silicas used in polymrisation catalysts tend to be derived fmm the gel route where the specific particle size and strength requirements are more easily obtained. typically have particle sizes of 20-9Op and 50-18Op respectively. silica products are generally less than 30p. must have a high dqree of t h d stability to withstand the high pretreatmnt temperatures required during activation of the catalyst (ref. 4). Amorphous silicas are metastable with respect to crystallisation and structural collapse, a process catalysed by alkali m e t a l ions (ref. 5). !the requisite law soda levels are more difficult to achieve with precipitated silicas as a consequence of the high preparation pH. surface areas in region 50-1000 m2g-l and pore volumes 0.4-3 ~3n~g-l. stages of gel preparation are illustrated in Fig 1. influence the structure of the final silica prcduct. Neutralisation of the soda causes the polymerisation of the silicate species to fonn particles w h i c h link to give chains and then networks resulting in the -ional hydrosel structure. The technology for manufacturing silicas is In practice Generally gel mufacture is a semi-continuow operation whereas The silicas needed for gas and slurry phase reactors precipitated Additionally the silicas used A wide range of silica structures can be prepared via the gel route with The main All the preparation steps sodium silicate and acid are mixed at pH 217 SODIUM SILJ.cATE SULFINRIC ACID High intensity mixing Gelation sodium sulphate renKNal and structural rearrangement Milling and size classification Fig. 1 schematic diagram of the stages involvfxl in preption of ~r/si02 ethylene polymerisation catalyst via gel route. m e gel time is a function of pH and concentration of sol and is also infbmxed by temperature, additives and/or impurities. imposeS limitations on the conditions under w h i c h hcnnogeneous hydmsels can be prepred . strength is the Si02 concentration of the hydrosol. are obtained with law Si02 comzentrations but this also leads to reduced hydmsel strength. Tnerefore, althouqh hydrosols can be prepard in the range 2-20 wt% Si02, hydrosels having Si02 contents of 17% are insufficiently strong to withstand subsequent processing stages in conventional manufacture and require special proc&wes. ?he washing stage is essential for removing the large amounts of by- prcduct scdium &phate f m the silica to ensure high thermal stability. standing and during washing the silica particles in the hydrogel continue to make further links resulting in a contraction of the hydrogel framework and expulsion of water (syneresis) . fundion of the hydrcgel Si02 conOentration (Fig. 2) and influences the strength and potential pore volume of the final xercqel . In practice this A key parameter in defining potential pore volume and hydmgel In general hiqh porosities Upon Ihe extent of this shrinkage appears to be a 218 SIO, content of hydrogel after washlng I 2 0 t I I I 10 I/ 1 Potentla1 i pore volume i ( cmS g- '1 4 I Q S102 content of hydrosol (a ; ) Fig. 2 Influence of hydrosel SiOz concentration on shrinkage during washing and potential pore volume of produd. xerogel prepared by dry- hydrogel at this stage typically has high surface area (-800 dg-l) and l m pore volume (0.4 ~m~9-l). are unsuitable for application in polymerisation catalysts, the mall pores becaming quickly blocked with polymer causing catalyst deactivation. once a hydrogel s- is formed it can be d f i e d in the wet state to enlarye the pore size and reduce the surface area. structural rearrangement, generally referrel to as aging, is carried cut under hydrcrthermal conditions. on the nature of the hydrogel, pH, temperature, time and presence of impurities. Fig. of pore volume as a function of aging time. Such Structures Hudever l k i s irreversible The extent to w h i c h the structure is modified depends 3 illustrates the decrease in surface area and devel-t 200 Lf 1 I , 01, 2 4 6 8 ' 0 Aglng tlrne (hrs) Fig. 3 Structut-al changes during aging. The water remaining in the hydrosel at this stage is an integral part of an equilibrium structure and the ~ ~ n n e r in which it is removed has a powerful 219 Hydrosel Si02 surface area content (%) (m2Q1) influence on the pore volme of the final dry xemgel. comonly used for drying : the direct removal of water a t high temperature or the displacemnt of water by a water soluble organic solvent. case the final pore volume depends on the relative rate of water removal f r m the hydrosel and the rates of structural rezrangement and siloxane bond formation. but clearly other factors including the Si02 content of the hydmsel and the degree of aging also influence the extent of shrinkage and consequent porosity. '&a distinct routes are In the f i rs t Rapid remnml of water results in the hiqher pore volume (Table 1) F a r e volume (cm3g-1) Slaw Fast solvent Theoretical Drying Drying Drying TABLE1 Pore volume of xerogel vs. nature of hydrogel and drying methcd. 30 30 15 15 600 350 900 400 0.7 1.5 1 . 9 2.33 1 . 3 1.85 1 .9 2.33 2.0 5.67 1 .10 3 .0 5.67 me second rcerte involving solvent exchaqe/azeatropic distillation minimises the degree of shrinkage by sqprt ing the hydrosel structure during water removal and lcwering the surface tension of the liquid phase. .%all amounts of wa te r remaining in the solvent can have a substantial negative influence on the final xemgel pore volume (Fig. 4 ) . Pore volume ( cm3 8- '1 I- 2 4 6 Resldual water In IPA (%) 1 L I--_ I 0 Fig. 4 Influence of residual w a t e r during solvent exchange on xerogel pore volume. 220 A model for the develapnent of the silica gel structure is illustrated i n Fig. 5. procedure outlined above. primary silica entities ( 2m) linked together in a * ional network flurounded by w a t e r . to a dense and tightly packed array of s m a l l silica units. ( 800 6g-1) derives fram the external surface of these units and the measured value agrees w e l l w i t h the calculated value for 2m particles i f due allowance is made for area lost through particle contacts. The measured porosity of the xerogel (0.4 cm3Q1) is in line with that for a Landom close packed array of silica particles. It is best appreciated by reference to the gel manufacturirKJ A t the hydmgel stage the structure -rises small Remavdl of this w a t e r t o form the xerogel causes callape The surface area Prlmarv Partlcle H ydros ol Unaged Hydrogel Xe ro gel (Hlgh SA, Low PV) Structural Rearrangement I Te r t I a r y Ag g r e g ate Secondary Partlcle (developed by clusterlng of Prlrnary Partlcles) Aged hydrogel Xe ro g e I (Low SA, Hlgh P V ) Fig. 5 W e 1 of silica gel structure. On aging, the evidence f m surface area and pore volume -ts and substantiated by electron microsaopy, suggests that the primary particles marrange t o form secondary subunits w h i c h have a size of about 10-20m11, and ultimately lose their identity. external surface area of the SeCondaLy units and fa l ls correspdingly to values of 250-350 m2q-l. network with sc~ne loCali& clustering into tertiary aggregates. increased aggregation generates a r e i n f o e structure within the hydrcgel which results in less shrinkage upon removal of water. generated as a function of the strudurdl marranganent via aging reflect this (Fig. 3 ) . U n d e r mnnal c i r c u i w w of gel preparation the secondary The surface area is now controlled by the The secondary units are also arranged in an open ' Ihis 7% higher pore volumes 221 particles retain their identity. However, if forcirq conditions are used for the hydrothermal aging step it is possible to llfusel' the secondary particles and the surface area reduces to 30-50 m2g-1. upon the packing and degree of aggregation of the secondary particles. obviously going to be influencd by process conditions, for example, and explains the strong dependency of porosity on drying technique. silica strength will also be dictated by the packing of these particles and hence, the structural breakdcrwn, for exan@le during polymerisation, will most easily take place via the large pores to leave tertiary aggregates of seconchy particles. The porosity of the aged silica structure is thus critically dependent ?his is Mower the CATALYST MANUFACIUFE Catalysts are prepared from silicas with the appropriate structure by deposition of chramium, typically 1 wt%, on the surface of the silica. low pore volume silicas ( 222 diluent (111'C for k c h t a n e ) and on the crystallinity of the polymer. that temperatwe, particle swelling and then agglomeration - with reduction in heat transfer leading to reactor fouling. in the reactor is of the order of 1-2 haxs. Beyond Catalyst residence time 'Ihe Unipol fluidisid bed gas phase process (ref. 6) utilises a continuous feed of catalyst and ethylene. moxaner but also as the fluidising gas and heat transfer medium for m i n g heat of reaction. fluidisid bed mterial. 3%. catalyst residence time about 3 t o 5 hours. t imes larger than itself such that the shape and particle s i z e distribution of the polymer particles r e f l ed t h e of the catalyst. catalyst particle is fragmented by the polymerisation process such that the silica is distributed haqemmsly -cut the polymer particle and is not easily detectable. estimated to be of the order of 100 w by pomsimetric charaderisation (mercwy intrusion) of the catalyst residue after polymer ashing (ref. 7) and from a conbination of electron m i v and catalyst productivity data (ref. polymerisation are the tertiary aggregates of secondary particles. these clusters w i l l be a direct function of the way in w h i c h the secondary particles have coordinated and packed w e t h e r during the manufacture of the silica. INFLUENCE OF SILCCA STRUCXURE ON POL- FR3PERTIFS (a) selectivity fie latter acts not only as the main reactant Moreover the product polyethylene also serves as the Conversion per pass thrcugh the reactor is akwt 2 to The reactor pressure is n o d l y abcut 20 a h , ten@era- 80 to 105'C and In both processes each catalyst particle creates a polymer particle many It is also faLlrd that the The ultimate s i ze of the silica fragments has been 8 ) . This value Suggests that the silica fragments produced during ?he s i ze of mlyethylenes, produced in a wide range of densities, with each density produced in a w i d e range of m e l t indices ( i n v d y related t o molecular w e i g h t ) , provide a b rad spedrum of available polymers w i t h particular properties for specific applications (ref. 2 ) . determined by: 1) molecularweight 2) molecular weight distribution (m) 3) degreeofchainbranching 4) distribution of chain branching ?3ese parameters can be altered to some extent by modifying the reactor conditions, i.e., ethylene concentration, tenperatum=, presence of ccplloly~ner and/or cocatalyst but the major influence is provided by the structure, ccmposition and activation of the catalyst. fie polymer properties are 223 For example, a strong correlation exists between the catalyst pore structure and the molecular wight of the polymer produced (ref. 7). ?he obsesved decrease in mlecular weight with increasing pore size is well established but is at variance with the trend expe&ed if kinetics were influenced by ethylene diffusion into the pores. for the w e d behavicur has not yet been proposed. A satisfactory explanation A trend of decreasing molecular weight with increasing catalyst activation tenperature (500 to 900°C) is also observd. changes in the porous structure of the catalyst since the pore size distribution remains relatively unchanged at these temperatures in silicas containing law concentrations of soda. extent of dehydmxylation of the silica surface. coordinate to the active (=r centres thus interfering with polymerisation or alternatively, the condensation of hydroxyls may result in st ra in within the silica surface hence modifying the environment of the active site (ref. 9 ) . (b) Activity ?his cannot be attributed to Instead it has been correlated with Residual hydmxyls may m e activity of the catalyst also a m to be a cxnnplex function of catalyst structure, camposition and activation temperatures. ?he structural parameters, surface area, pore volume and pore size distribution are interrelated and it is inpossible to vary any one of these in isolation. example, although there is evidence to suggest that higher surface area, at constant pore volume, results in imreasd catalyst activity (ref. 10) , the polymer produced has 1- melt index reflecting the shmltaneous change in pore size distribution towards smaller pore size. For camnemial catalysts typically contain 1 w t % chromium although the catalyst activity is virtually independent of chrmnium loading in the range 0.75 to 2 w t % (ref. 11). can be attributed to formation of bulk Cr2O3 during catalyst activation resulting in loss of catalyst surface area and porosity by pore blocking. the other hand insufficient chromium generates an hadequate number of sites to scavenge the poisons (e.g., H20, 02, CO, etc.) to which these catalysts are so sensitive. in polymerisation. (ref. 8) and other techniques (ref. 7) w e s t this to be as lm as 3 to 7% of the total. Exwss chromium leads to a loss of activity w h i c h On ~n fact only a small proportion of the chromium present is active Determination of the active sites using l4C0 radiolabelling INFLUENCE OF MODIFIERS Whilst a wide range of polyethylenes can be prepared using silicas of different structures this range can be substantially extend& and catalyst activity increased by the addition of modifiers. Such modifiers may be 224 inaorporated within the catalyst durhq silica gel fomtion as, for example, in the production of silica titania (ref. 12) and silica zirconia cogels (ref. 13) , or by surface inpregnation as with titanium (ref. 14) , fluoride (ref. 15) or aluminium (ref. 16) . characteristics either by influencing the active site directly as with titanium via the formation of titanyl ChraMte (ref. 17) or indirecuy by replacement of neigkbmrirq hydroxyl groups as with fluoride (ref. 18). molecular weight distribution and hi- melt index observed with titanium modified catalysts can be attributed to the participation of 2 types of sites, t h e frcnn the silyl duoaMte preausors also present on unodified catalysts, wether with additional sites originating from titanyl chromate. sites produce 1- molecular weight polyethylene resulting in a bimodal MWD (ref. 19) . TpR studies (ref. 20) of activated chromiq/silica catalysts lend support for the existence of two types of sites. % standard chrcanium silica catalyst shm one redudion peak with a maximum at 440°C whilst an additional peak of similar magnitude with a maxbmm at 420°C is recorded for the titanium modified catalysts. hence the structure of the activated catalyst. silicas made by certain routes can display uncharacteristically law thermal stability even in the presence of lcw soda concentrations (200 ppn) . be substantially improved by hp-egnation with relatively d l amounts of titanium as sham in Table 2. Similar results are obtained with zirconium. High pore volume silicas preprd by slightly different routes have substantially higher thermal stability. surface modifiers alter the catalyst activity and resultant polymer Thus the broadened 'Ihese latter Surface modifiers can also influence the thermal stability of silica and For example, high pore volume mese can TABLE2 Influence of titanium hpreqnation on thermal stability of a high pore volume silica. Ti(%) Ti02 (%) 0 0 0.38 0.63 0.60 1.00 1.56 2.60 5.28 8.8 Untreted Calcined 870'C/2 hrs Surface Fore surface Fore (m2g-l) (cm3Q1) area volume volume 434 2.99 311 2.13 4 09 3.00 387 2.42 397 2.94 363 2.49 4 12 2.88 383 2.65 4 14 2.66 347 2.31 225 In the pruduction of cogels for application as polymerisation catalyst supports, the modifier, eg, titanium, z b n i u n or aluminium, is typically present at corcentrations of 14 w t % as oxide during hydrosol preparation. presence of these species influences the resultant hydrogel structure and its subsequent aging kinetics. ?his is best illustrated by reference to the data in Table 3 showing the influence of different alumina contents on the unaged and aged gel structures. Clearly the presence of alumina reduces the rate at w h i c h structural modifications occur. Rdditionally, the presence of im,reasing amounts of cogelled alumina reduces the potential pore volume of a given silica system. with titanium and zirconium but these are much less pronounced. The similar trends in aging kinetics and pore volume have been abserved TABLE 3 Influence of alumina on structure and aging kinetics of silica gds. Silica Silica alumina silica alumina cogd cogel 1% A1203 4% A1203 Surface Pore Surface Pore Surface Pore area volume area volume area volume (m2g-I) (cm3g-1) (m2g-l) (cm3g-l) (m2g-l) (an3g-1) 0 4 8.0 8 8.0 10 8.5 ~ ~~~ 960 2.38 909 1.88 1010 0.82 498 2.91 636 2.44 865 1.84 466 2.97 583 2.38 787 1.95 387 2.84 553 2.12 The lower pore volumes obtained in the presence of modifiers makes it difficult to investigate the influence of modifier conoentration in a cogel indepndently of pore structure. Conway so despite shwing that the pore volumes and th-1 stabilities of the catalysts decrease with increasing cogel titanium content. These trends, in isolation, would be e x p e c k 3 to result in a decrease in melt index if behavicur pardlleled pure silica systems. titanium loading must therefore be a result of the titanium content of the cog&. (ref. 21) have atteqted to do The increased melt index obsenred with No data have been published whereby cogel and hprqnated catalysts with the same pore size distributions have been compared and hence the influence of cogelled vs imprqnated modifier cannot be absolutely differentiated. ~cwever it is not unreasonable to suggest that in addition to modifying the porous structure and thermal stability of the support, cogelation can also result in d i e modification of the active centres. distribution of titanium envisaged in the cogel structure vs hprqnat& Thus for titanium, the better 226 titanium may result in precursor sites of the type: I -Si-O I \ //O 0 cr 0 I / \ --Ti-O I Hence 3 possible types of precursor chrmate sites can be envisaged (the above together with the two already suggested for the titanium impregnated system) with the mixed site above predcaninating. sites of this type may be responsible for the narraw MWD and extremely high envirornnental stress crack resistance of polymers generated by this type of catalyst (ref. 7). coNcLus1oN ~lthough the cr/silica ethylene polymerisation catalysts have been used for more than 30 years, this deceptively siqle system still has many of its perfomwce attrihtes poorly unfierstocd. the catalyst fractures in use and that the structure of the support influences this, no direct correlation has been established between the details of catalyst fragnr=ntation behaviour and polymerisation performance. broad relationship between catalyst porosity and polymer structure has been clearly demonstrated lxlt again there is no detailed umkrstandbg of the mechanism whereby the pore size distrhtion of the catalyst can influence the polymerisation kinetics and hence modify the polymer MWD. two mles either pruviding different active centres and/or modifying the support s-. with superior properties continues to be a challenging task w h i c h myires the cambined expertise of the silica and polymer mnufactumrs to make progress. whilst it is well established that Similarly the Heteroatms can have Ihe balance between these is not well defined. clearly the quest to develop new catalysts capable of producing polymers - The author wishes to thank A. L. bell (Cmsfield Chemicals) and D. R. Ward (Unilever FG?s%w&, Port Sunlight Laboratory) for helpful discussion in preparing this manuscript. REFERENCES 1. 2. 3. M. P. WlXdel, Fracturing silica-based catalysts during ethylene polymerisation, J. Polym. Sci., l%lym. Chem. Ed., 19(1981) pp 1967-1976. M. P. %Dmiel, Contmlling polymer properties with the Phillips chrcanum catalysts, M. E3lg. &em. Res., 27 (1988) pp 1559-1564. R. K. Iler, ?he Chemistry of Silica, John Wiley and Sons, New York, 1979. 227 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. J. P. Hogan, D. D. Nortyood and C. A. Ayres, Phillips petroleum capmy loop reactor technology, J. Appl. polym. sci., Appl. polym. symp., 36 (1981) pp 49-60. S. KondD, F. Fujiwara and M. Muroya, The Effect of heat treatment of silica at high temperature, J. coll. and Interface sci., 55 (1976) pp 421- 430. Union Carbide corporation, Chemicals and Plastics Division, Gas-phase, high density polyethylene process, Chem. Eng., (1973) pp 72-73. M. P. &Daniel, supeortsa cbrcnnium catalysts for ethylene polymerisation, S. Wang, C. E. Marsden and P. J. T. Tait, Phillips-type polymerisation catalp. M. P. McDaniel and M. B. Welch, The activation of the Phillips polymerisatation catalyst 1. Influence of the hydmxyl population, J. Personal aamnunication. J. P. Hogan and D. R. Witt, Supportd cbrumium catalysts for ethylene polymerisation. Disproportionation. Joint Meet- of the ACS and Chem. SOC. Japan Honolulu, (1979) pp 377-387. R. E. Dietz, U.S. Patent 3,887,494 (1975). R. A. Dcanbro and W. KirCh, U.S. Paat 4,246,137 (1981). T. J. Pullukat and M. Shida, U.S. Patent 3,780,011 (1973). J. P. Hogan, U.S. Fatent 3,130, 188 (1964). S. J. Katzen and L. J. Rekers, U.S. Patent 4,100,104 (1978) T. J. Fullukat, R. E. Hoff and M. Shida, A chemical sturfy of themally activated chromic titanate on silica ethylene polymerisation catalysts, J. polym. Sci., Polym. chem. Ed., 18 (1980), pp 2857-2866. F. J. K a r o l , B. E. Wagner, I. J. mine, G. L. Goeke and A. Noshay, New catalysis and process for ethylene polymerisation catalysts, Polyolefins Proc. ACS Int. Symp. 1987 pp 337-354. M. A. Sutton, Studies of titanium mcdified chromia catalysts for olefin polymerisation. %.D. Thesis, (1981), University of Notthqham. C. E. Marsden, Unpublished d t s . S. J. Conway, J. W. Falconer, C. H. Rcchester and G. W. Dawns, a71-cwia/silica-titania cogel catalysts for ethene polymerisation, characteristics, J. am. Soc. Faraday Trans. I, 85(7) (1989) , pp 1841- Adv. Catal., 33 (1985) pp 47-98. Kinetic behaviour and active site determination, J. Mol. catal., mpress. catal., 82 (1983) pp 98-109. Award Symp. on Olefin polymerisation and Adv. polymer 1851. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation ofCutu2ysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 229 Preparation and catalytic effects of and Pr)by an improved sol gel method for automotive catalysts K .Masuda* ,M. Kawai* , K. Kuno' , N . Kachi** , F .Mizukami** CeOX-MOy-A1203(M=Ba,La,Zr *Central Engineering Laboratories, Nissan Motor Co.,Ltd., 1,Natsushima-cho, Yokosuga, Kanagawa 2 3 7 , Japan National Chemical Laboratory for Industry, 1-7 * * Higashi, Tsukuba, Ibaraki 305, Japan ABSTRACT CeOX-MOy-Al2O3 supports (M=Ba,La,Zr and Pr) were prepared by an improved sol gel method thermal stability of both the supports and the platinum catalysts was investigated. It was found that the addition of Ba, La, Zr and Pr improves the thermal stability of alumina and cerium oxide. However, as for the thermal stability of the platinum catalysts, Ba, La and Pr catalysts showed much higher thermal stability than an impregnated one, while the Zr catalyst showed lower thermal stability. The activity order was B a 7 L a 7 Pr> impregnated catalyst7Zr in terms of CO oxidation. and the influence of MOy on the INTRODUCTION Three way catalysts are generally used today for controlling exhaust emissions from automotive internal combustion engines. Cerium oxide is widely employed as an additive in three way catalysts because of its abilities to store oxygen and to improve dispersion of platinum.[l] One drawback of the three way catalyst is that it tends to give rise to thermal deactivation caused by crystallization of cerium oxide, sintering of platinum and A1203. [ 2 I It has been reported that the addition of Ba arid La improves the thermal stability of A1203[3,41 and that the sol 230 gel method, a chemical mixing technique,is effective in obtaining thermostable supports. [51 However, supports composed of a three component system(CeOX-MOy-Al2O3) have not yet been prepared by this method. Moreover, Pt supported catalysts have not been investigated sufficiently. We have newly prepared CeOX-MOy-Al2O3 supports using the sol gel method and carried out investigations on the thermal stability of both CeOX-MOy-Al2O3 supports and Pt supported catalysts. EXPERIMENTAL The sol gel method that was developed for preparing CeOX-MOy-Al2O3 supports consists of complexing ,gelation, drying and activation steps as shown in Fig 1. acac: acetylacetonate Ce(N03)3- 6 H 2 0 y Ba(acac)z.PHzO B a ( N 0 3 ) 3 - ~ H 2 0 Ce(acac),,.xHZO .-- drying - calcination AI(OR)3 M(aca c)z-2 H 2 0 hexylene glycol H 2 0 Fig.1 Support preparation procedure by an improved sol gel method The preparation of Ce0x-Ba0-A1203 is described below as typical example. First, 60.4 g of Al(O-SBu)3 were put in a 500 ml beaker. To this, 1.7 g of Ba(acac)2 2H20 and 25 g of hexylene glycol were added. The beaker was placed in an oil bath at 100 OC and stirred for 3 hours. Meanwhile, 1.75 g of Ce(N03) 6 H 2 0 were put in a 231 300 ml beaker. To this, 12 g of C2H50H were added and then 2.23 g of Ba(acac):! 2 H 2 0 were added. The beaker was put into an oil bath at 50 OC and stirred for 3 hours. Subsequently, the temperature of the oil bath was lowered to room temperature and the contents of the beaker were filtered. The filtered solution was added to the 500 ml beaker. The beaker was put into an oil bath at 100 O C and stirred for 3 hours, after which 17 g of water were added. The beaker was maintained at 100 OC for 10 hours to obtain a gel. The gel was transferred to an eggplant type flask and dried under a reduced pressure. The dried gel was heated at 250 OC for 3 hours and calcined at 450 O C for 4 hours and at 1000 OC for 3 hours to yield a pale yellow alumina powder. The platinum catalyst was prepared by a conventional impregnation technique using Pt(N03)2(NH3)2 as the Pt source and calcining Thermal deterioration of the supports was investigated by calcining them at 1000 OC for 3 hours and at 1000 OC for 24 hours. Thermal deterioration of the Pt supported catalysts was only examined by calcining them at 1000 OC for 4 hours. at 400 OC for 2 hours and for 0.5 hour in H2. CO oxidation was carried out in a fixed-bed-type apparatus with a continuous flow system at atmospheric pressure. The thermal stability of the supports was determined by the BET method and X-ray diffraction (XRD). The crystallite size of cerium oxide was measured by XRD using the small-angle-scattering method. Lattice parameters were also measured by XRD. The supports were also analyzed by X-ray photoelectrons using VG ESCA Lab MK T I . Platinum dispersion was measured with a Quatasorb, which uses the CO adsorption method. RESULTS AND D I S C U S S I O N As it has been reported that the addition of baryum i s the most effective way to inhibit the sintering of alumina, a series of CeOx-BaO-A1203 supports were synthesized by the sol gel method as shown in Fig. 1 and their 1wt% Pt catalysts were compared with the corresponding impregnated Ce0,-A1203 catalysts with respect to CO oxidation. 232 Fig.2 shows the relationship between the Ce content and the temperature of the 50% CO conversion with respect to the oxidation of lwt% Pt-CeOX-Ba0-A1203 catalysts and lwt% Pt-CeOX-A120S catalysts calcined at 1000 OC for 4 hours. For all Ce amounts, the ternary support catalyst showed higher activity than the corresponding binary support catalysts, the optimum concentration of the ternary catalyst was found to be 8 % . s z 0 cc W > z 0 0 s 0 0 300 v 400 0 In - 0 W cc 3 I- 2 5 200 w a + o 10 20 30 Ce CONTENT (wt%) Fig.2 CO oxidation performance as a function of cerium content Next, while the contents of Ce and alumina were kept at 8 and 83 wt% respectively, Ba in the ternary support was substitute for La, Zr and Pr which are known to form thermostable oxides on account of the ion radius and e1ectrj.c charge. Fig.3 shows the 233 I "" 80 -$ om- v Z m [r W O 0 0 L O - 0 2 0 - n CO oxidation reactions with lwt% Pt-CeOX-BaO-Al2O3. lwt% Pt-CeOX-La2O3-Al2O3 , lwt% Pt-Ce0x-ZrO2-Al2O3 and lwt% Pt-Ce0,-Pr203-A1203 catalysts calcined at 1000 OC for 4 hours. Higher activities were seen for the 1 wt% Pt-CeOX-BaO-Al2O3. lwt% Pt-CeOX-La2O3-Al2O3 and lwt% Pt-CeOX-Pr2O3-Al2O3 catalysts than for the impregnated lwt% Pt-CeOX-Al2O3, but the lwt% Pt-Ce0x-ZrO2-Al2O3 catalyst showed lower activity than the impregnated lwt% Pt-CeOX-Al2O3. - I 1 1 1 I v 150 200 250 300 350 400 450 TEMPERATURE ( " C ) Fig.3 Relationship between CO conversion and reaction temperature From the foregoing results, two interesting phenomena can be noted. One i s that high level of activity and the optimum Ce concentration for CO oxidation are obtained with the lwt% Pt-Ce0,-Ba0-A1203 catalyst; the other is that a low 234 level of activity for CO oxidation is obtained with the lwt% Pt-CeOx-Zr02-A1203 catalyst. The following investigations were carried out to elucidate the reasons for differences. Fig.4 shows the XRD patterns of both CeOX-Ba0-Al2O3 and Ce0,- A 1 2 0 3 calcined at 1000 OC for 24 hours . The alumina in the ternary support keeps its 6- -structure following calcination, whereas that in the binary support does not and changes from the r - to the O( -structure as a result of calcination. The crystallite s i z e of cerium oxide of both the Ce0x-Ba0-A1203 and Ce0,-A1203 were measured by XRD. The value of Ce0,-Ba0-A1203 I 0 2 0 4 0 6 0 8 0 CeOx-AI203 CeOx-BaO-Ai203 Fig.4 XRD pattern of Ce0,-Ba0-Al2O3 and CeOX-Al2O3 was 176 i, while the value of Ce0x-A1203 was 267 K. Results obtained with the BET method indicated that the surface area of Ce0,-Ba0-AL203 was 1 6 7 m 2 / g after calcination at 1000 OC for 3 hours, while the surface area of Ce0x-A1203 was 35 m2/g under the same conditions. These experimental results show that 235 Ce0x-Ba0-A1203 prepared by the sol gel method is thermostable and suitable for use as an automotive catalyst support. Fig.5 shows Ce XPS spectra of a series of Ce0,-Ba0-A1203 supports in comparison with the spectra of Ce02 and r l I I I I 9 1 0 9 0 0 8 9 0 8 8 0 8 7 0 B i n d i n g E n e r s . 7 L e v 3 Fig.5 XPS spectra showing the Ce 3d512 binding energies for the each Ce0,-Ba0-A1203 supports Ce2(S04)3 . The spectra of cerium oxide was characterized by two well-known peaks at 884.8 ev and 883.2 ev.[61 The 884.8 ev peak is attributable while the 883.2 ev peak is due t o Ce3+, 236 to Ce4+. A s the amount of Ce in the support increases, the 883.2 ev peak of Ce4+ increases and the 884.8 ev peak of Ce3+ decreases. At a Ce content of 8wt%, the amount of Ce4+ in CeOX-Ba0-A1203 becomes roughly equal to Fig.6 shows the relationship between the Ce content and Pt dispersion. The maximum in the Pt dispersion curve corresponds perfectly with the minimum in the 50% CO conversion curve. It is deduced that in the lwt% Pt-CeOx-Ba0-A1203 system, 8 w t % Ce content is the most suitable amount for promoting the redox interaction between Pt and Ce and inhibiting Pt sintering. that of Ce3+. E E 0.00 0 10 20 30 Ce CONTENT (%) F i g . 6 Pt dispersion as a function of cerium content The crystallite size of cerium oxide of the CeOX-ZrO2-Al2O3 support was 111 1 and the surface area of the support was 130 m2/g. These data were roughly equal to those of 237 CeOx-Ba0-A1203. Accordingly, from these results it was impossible to discern why the lwt% Pt-CeOX-ZrO2-Al2O3 catalysts have low activity. Therefore, the supports were investigated further using XRD. Table.1 shows the cerium oxide lattice constant of these supports. The cerium oxide lattice constants, except those of the Ce0,-Zr02-A1203, are unchanged or slightly larger than that of the original Ce02; those of CeOX-ZrO2-Al2O3 are smaller. Table 1 Cerium lattice constant of Ce0,-MOy-A1203 support support lattice constant (8) 1000"C/3h 10OO0C/24h CeOx-Al203 5.413 CeOx-Ba0-A1203 5.412 CeOX-La203-A1203 5.430 CeOX-ZrQ-A1203 5.371 5.419 5.41 1 5.425 5.373 Ce02: 5.41 1 (A) Ceo.75Zr0.7502: 5.349 (A) Ionradius: Ce l.O2(A) Ba 1.36(.&) Zr 0.79 (A) La 1.04 (A) These results suggest that a substantial solid solution occurs between the oxides of Ce and Zr with a small radius, resulting in the formation of a complex oxide. It is therefore concluded that Ce in C C O , - Z ~ ~ ~ - A ~ ~ O ~ does not have any effect on improving the dispersion of Pt. The physical properties of these catalysts are summarized in Table 2. It is seen that the Pt dispersion of the Pt-Ce0,-Zr02-A1203 catalyst is much lower than that of the others. 238 Table 2 Physical properties of CeOx-MOy-Al203 catalyst Catalyst BET surface area Cerium oxide cryatallite Pt dispersion of support (rndg) of support (A) (COPM) ~ ~ Pt-CeOx-BaO-AI2Q 154- 180 176 0.024-0.027 Pt-CeOx-La203-Al203 160 134 0.021 Pt-CeOx-Pd03-Al203 130 (125) 0.017 R-CeOx-Zr02-Al203 135 111 0.004 Pt-CeOx-Al203 267 CONCLUSION The addition of Ba, La and Pr to a two component system (CeOx-A1203) by the sol gel method improves the thermal stability of both the support and the platinum catalyst. Although the addition of Zr to the system by the same method improves the thermal stability of the support, it docs not improve the thermal stability of the platinum catalyst. The activity order was found to be BazLa7Pr7Zr in terms of CO oxidation. Consequently, the CeOx-Ba0-A1203 support prepared by the sol gel method is concluded to be one of the most thermostable ones for automotive catalysts. REFERENCES 1 H.S.Gandhj ,A.G.Piken,H.K.Stepien,M.Shelef,R.G.Delosh and M.E.Heyde, SAE Paper 770196, 1977 2 K.Iahara,K.Ohkubo and Y.Niura, 4th International Pacific Conference, Paper No.871192, 1987 3 H.Ohuchi,Y.Horio and N.Yamaki, Sekiyu Gakkaishi, l O ( 1 9 7 6 ) 53 4 F.Oudet,E.Bordes,P.Courtine,G.Maxant,C.Lambert and J.P.Guerlet, Catalysis and Automotive Pollution Contorol, Elsevier, Amsterdam, 1987 . pp .313-321 5 F.Mizukami,M.Wada,S.Niwa,M.Toba and K.Shimizu, Nippon Kagaku Kaishi, 9 ( 1 9 8 8 ) 1542 6 G.Praline,B.E.Koel,R.L.Hance,H.I.Lee and J.M.White, J. Electron Spectrosc.Relat.Phenom., Z l ( 1 9 8 0 ) 1 7 G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 239 INFLUENCE OF PREPARATION PARAMETERS ON PORE STRUCTURE OF SILICA GELS PREPARED FROM TETRAETHOXY ORTHOSILICATE B. Handyl, K. L. Walther2, A. Wokaunz, and A. Baikerl IDepartment of Industrial & Engineering Chemistry, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland 2Physical Chemistry 11, University of Bayreuth, D-8580 Bayreuth, FRG SUMMARY The influence of hydrolysis conditions (pH) and of the drying temperature on the structural properties of porous silica gels derived from tetraethoxy orthosilicate (TEOS) has been investigated using nitrogen adsorption and 29Si MAS-NMR. Acid- catalyzed hydrolysis produced silica gels containing mainly micropores. Upon drying a t higher temperature (870 K) the weakly cross-linked network collapsed due t o dehydration and further cross-linking. This is indicated by diminished pore volume and by a shift in relative number of SiO4 centers with 2- and 3-Si nearest neighbors to more 4-Si nearest neighbor centers. Additionally, the cross- polarization times T s i ~ increased with drying, indicating a decrease in hydroxyl content. The general isotherm shape was unchanged with drying. The highly- branched cluster aggregates formed during base-catalyzed hydrolysis of TEOS yield a dried gel with a mesoporous silica network. Cross polarization times T s i ~ for this gel are significantly shorter than for the gels prepared by acid hydrolysis, suggesting closer proximity of the Si centers to hydroxyl groups than in the latter, and/or maintenance of an abundance of hydroxyl groups despite the drying treatment. INTRODUCTION Gel-based catalysts have been commonly prepared from colloidal solutions derived from the peptidization of metal salts. Another sol-gel route involves the hydrolysis and gelation of metal alkoxides, which can be obtained in highly pure form [l-31. Sol formation proceeds via three reactions, namely, hydrolysis, polycondensation, and dissolution [4]. Initially, alkoxy groups on the parent molecule are sequentially displaced by hydroxy groups. As reaction progresses, the partially hydrolyzed species can react with each other as well as other molecules of varying degrees of hydrolysis, leading to molecular complexity. All three reactions are somewhat reversible, and the relative importance of each throughout the process is determined by solution pH, waterhlkoxide ratio, temperature, and the intrinsic reactivity of the precursor alkoxide [4-6]. For the hydrolysis and gelation of tetraethoxy orthosilicate (TEOS), under conditions of low pH and water content (i.e., pH I 2.5, H20/TEOS 541, a transparent sol of linear polymeric chains is produced, which forms a weakly cross-linked gel structure [4]. However, when hydrolysis is carried out in basic media and an excess of water, the sol consists of 240 highly-branched cluster aggregates of a colloidal nature [7]. These two condition sets represent two clearly different structural unit types. Further, intermediate sol structure forms may be realized by using conditions lying between the two conditions outlined above. Thus, control properties are available at the solution stage that are important for catalyst design and production such as porosity, chemical composition, strength, and shape, and show promise for the development of new metal oxide catalysts. Solid state 29Si NMR spectroscopy can be useful to probe the local environment of Si in the silica matrix [5]. Magic angle spinning (MAS) and 29Si-1H cross-polarization (CP) techniques are employed to resolve the isotropic lines corresponding to different silica sites and to increase the signal-to-noise level. The Si environments near surface hydroxyl groups are probed when both methods are used simultaneously since these groups are the only ligands in the silica gel network for which 1H protons are neighbors to 29% centers. EXPERIMENTAL Sample Preparation Four different types of silica were employed for study: A B C D 400 cm3 TEOS, 400 cm3 Ethanol, and 160 cm3 H20 (2x-distilled) were mixed and several drops 1N HC1 added to give a solution pH of 2-3. The mixture was continuously stirred for 2 hrs and the alcohol removed at 313 K under reduced pressure. The sol was diluted to 1000 cm3, allowed to stand for 24 hrs, and then dried on a rotating evaporator at 393 K. Several gms of Sample A were dried in a muffle oven at 870 K for 3 hrs. 20 cm3 of pH=9 ammonia water were vigorously stirred with 10 cm3 of TEOS at 343 K for 12 hrs. The sol was dried on a rotating evaporator at 393 K, then in a muffle oven at 870 K for 3 hrs. Aerosil 200, a commercial silica gel (Degussa, Inc.). The preparation involves flame hydroIysis of dry tetrachlorosilane into non-porous, spherical particles. Sample Analysis Surface area and pore size information were obtained from nitrogen adsorptioddesorption isotherms at 77 K, using a Micromeritics ASAP 2000 Analyzer. Prior to measurement, all samples were degassed to 0.1 Pa at elevated temperatures. The degassing temperature was 473 K for all samples except A, which was degassed at 393 K. BET areas were calculated assuming a cross- sectional area of 0.162 nm2 for the nitrogen molecule. Mesopore size distributions were calculated using the Barrett, Joyner, and Halenda (BJH) method, assuming a cylindrical pore model [9]. Assessments of microporosity were made from t-plot 241 constructions, using the Harkins-Jura correlation [lo] for t as a function of p/po. Parameters were fitted to a low-area, non-porous silica. NMR spectrometer (MSL 300, Bruker). The 29Si spectra were obtained at 60 MHz and recorded a t room temperature. Magic-angle sample spinning was routinely carried out a t 4 kHz with Kel-f rotors. The CP/MAS spectra were obtained under Hartmann-Hahn conditions ( w1= 3 x 105 rad s-l ) in single-contact experiments by using variable contact times from 1 to 50 ms, and a pulse repetition rate of 10 s. All chemical shifts are reported with respect to TMS. The NMR measurements were performed on a high resolution solid state RESULTS Surface Area and Pore Structure Isotherms and t-plots from the four samples are shown in Figs. 1 and 3, with calculated values for surface area and pore volume displayed with each isotherm. The isotherms of acid-hydrolyzed TEOS gels A and B are nearly identical in form, being essentially Type I, with some hysteresis in the 0.35 < p/po < 0.55 regions. The t-plots show linearity for p/po < 0.3 ( t -= 0.5 nm) and a downward slope a t higher pressures. The evidence suggests that A and B consist almost entirely of micropores (dpore 5 2 nm), although the intercepts obtained by extrapolation of the low-pressure data to zero pressure show that very little pore volume is attributable to âprimary micropore fillingâ [lo]. The micropores could thus be very narrow slit pores with spacings of 1-2 nm that would fill completely after only several adlayers. A small percentage of slit pores sufficiently large to fill by capillary condensation would be responsible for the hysteresis region. In contrast to A and B, the isotherms of samples C and D are of type IV and H1 hysteresis (using IUPAC convention [ll]). Analysis of the t-plots did not show evidence of microporosity in either sample, and St estimates of external surface areas are similar to the BET areas. Mesopore size distributions of C and D (Fig. 2) obtained from the desorption branches are unimodal, with most frequent pore diameters (cylindrical model) of 10 and 40 nm, respectively. In the case of D, this is compatible with the literature describing Aerosil as aggregates of near-spherical particles of 15-30 nm diameter [12], in which pores exist as neck and interstitial spaces. A similar pore structure may hold for C as well, although the corresponding particle sizes are probably smaller than in the Aerosil. 242 - 0) n' I- v) v 8 x v) > h 0) I- v) n' 8 v v) -0 m > FIG. FIG 200 I n S(BET) = 770 Vpore = 0.43 0.0 0.5 1 .o 200 - 0 0.0 0.5 1 .o . ' . . I . . * . 300 2oo[ 100 S(BET) = 530 Vpore = 0.31 V- 0.0 0.5 1 .o ' D . S(BET) = 197 J 1000 - 800 - Vpore = 1.4 I I) 600 - 400 - 200 - 0.0 0.5 1 .o Nitrogen isotherms of Samples A-D. %BET) in m2/g and Vpore, defined as the total pore volume at p/po=O.98, in cdg. 0 20 40 60 80 100 Pore Size Distributions for Samples C and D. Distributions were derived from the desorption branches of hysteresis. 243 500 400 300 200 100 0 h P a !- cn V V v) -CJ v 2 h m I- cn 0 0 v) -0 h v 2 " * - I ' - . . I . . B - - - - 0 0 0 . 0 - - /+. " " . I " " 1 . 0 0.5 1 .o 400 1 &) = 265 FIG. 3 t-plots of Samples A - D. S(t) values in m2/g. The corresponding isotherms are plotted in Figs. l.A-D. 29Si NMR Spectroscopy The observed MAS spectra were composed of three lines which can be assigned to three different structural units Q2, Q3, and Q4, corresponding to Si centers bridged via -0- linkages to two, three, and four Si nearest neighbors [8]. Typical isomer shift values were -91.4 ppm for Q2, -100.7 ppm for Q3, and -110.0 ppm for Q4. Typical linewidths were obtained as 350 Hz for Q2,480 Hz for Q3, and 600 Hz for Q4, respectively. The NMR data are tabulated in Table 1. When the CP-MAS technique is used, the relative strength of the Q2 and Q3 signals is enhanced, due to the presence of protons near these Si centers. Since the protons exist in hydroxyl groups on or near the surface, the method provides some information of Si sites in this region. to the 2% nuclei, the CP-MAS experiments have been performed with variable Hartmann-Hahn contact times. The theory and assumptions of the model In order to study the dynamics of the magnetization transfer from the protons 244 employed are described in detail in Ref [13]. Usually the cross polarization dynamics is described within the framework of spin thermodynamics [141, which predicts an exponential rise in S-spin magnetization in the rotating frame with contact time. The measured time evolution of the 29Si magnetization was characterized by an oscillatory contribution, with amplitudes that varied from site to site [13]. The oscillations observed in the experiments were analyzed in the framework of the model developed by Miiller et al. [E l , and a combined model formed to provide a description of the time dependence of the magnetization M(t) [13]. Omitting oscillatory contributions in the present discussion for brevity, the cross polarization dynamics is described by the equation: where M, = M( t+ -, TIp+ - ) is the maximum magnetization achievable in the absence of spin-lattice relaxation, h = TIs/rlp , Tip = spin-lattice relaxation time, TIS = spin-difision time from proton I-spin to 29Si S-spin (=TSiH in Table 1). Where no decrease in magnetization was observed, TI,, was set to infinity. Since the spin diffusion times can be related to the distance between the coupled spins [141, i.e., T 1 s - 1 ~ rIS-6, then these constants are useful for inferring distance relationships between the 29Si centers and 1H centers in the silica structure. The parameter values are summarized in Table 1, showing M, values for the three Qn structural units in samples A-D. Sample B is the only investigated system where a long time decrease of the 29Si magnetization could be observed. The values of the time constant T ip are finite for all sites in this sample. DISCUSSION The formation of clear, transparent gels as seen with A and B indicates the presence of weakly cross-linked polymeric chains, since they do not scatter light. The similarity in form of the isotherms for samples A and B would indicate similar structure. Thus, the primary difference between the two is the reduced surface area and pore volume of B. The base-catalyzed sample C exhibited features of a colloidal gel. Under basic conditions, gelation (network-forming) reactions are favored over the hydrolysis reactions and the resulting sol species are more cluster- like 141. The packing of the separate colloidal entities is more open than the interwoven, weakly cross-linked strands of the structure from acid hydrolysis. The cluster-cluster contact is also more mechanically rigid. Upon solvent removal, the capillary forces from the receding liquid collapse the weak structure. manifested by condensations between neighboring hydroxyl groups, and forming more siloxane bridges. This is quite clear from the Mo data (Table l), which show decreases in the relative amounts of Q2 and Q3 units and increases in the Q4 units, Drying of the acid-hydrolyzed gel A at 870 K leads to further dehydration, 245 TABLE 1 Parameters characterizing the 29Si NMR spectra of the investigated silica gels. Sample Structural Mo [%I Mo [%I TsiH type MAS CP/MAS [ms] acid-hydrolyzed Q2 7.3 7.9 5.0 36.0 52.4 6.8 56.7 39.7 15.0 390 K A TEOS, dried Q3 Q4 B acid-hydrolyzed Q2 2.6 5.2 8.3 18.0 38.8 8.8 79.4 56.0 16.4 870 K base-hydrolyzed Q2 0.7 6.2 0.8 14.0 35.0 1.0 85.3 58.8 12.2 870 K 3.9 9.9 0.5 17.0 39.7 0.6 79.0 50.4 1.2 TEOS, dried Q3 Q4 TEOS, dried Q3 Q4 Aerosil200, Q2 dried 393 K Q3 C D 134 i.e., increased cross-linking with drying temperature. The fraction of cross-linked Si centers is even higher in the base-hydrolyzed gel, dried at 870 K , although a low temperature dried sample is not available for comparison. The CP-MAS technique sheds light on the nature of the hydroxyl groups. At short contact times, Si atoms with directly bound hydroxyl groups (Q2 and Q3) have the largest intensity , whereas a t longer contact times, the Q4 signal also increases as Si nuclei which are removed at least four bonds from the nearest hydroxyl group become polarized. Thus, the values of TSiH increase from Q2 to Q4 . In the acid- hydrolyzed gels, the loss of pore volume is reflected in a significant increase of T s i ~ for all three Si structural units. In contrast to the acid-hydrolyzed gels, the base- hydrolyzed gel is characterized by very short TsiH constants. The constants are even shorter than those of the 393 K-dried acid gel, indicative of a large proton reservoir. Even though the degree of cross-linking, as reflected by the number of Q4 sites, is significantly higher than in the acid-catalyzed gels, the surface hydroxyl groups appear to be sufficiently close to promote fast cross polarization. The large difference between the TsiH values for Q4 (12 ms) on the one hand, and Q2 and Q3 (-1 ms) on the other hand is indicative of the differences between sites in the interior (Q4) and on the surface (Q2, Q3) of the compact aggregates formed by basic hydrolysis. 29Si parameter values for the Aerosil sample D indicate the high degree of cross-linking (high percentage of Q4), as expected of the high temperature flame hydrolysis that produces the non-microporous âbulk silicaâ spheres. With very 246 short cross-relaxation times, it resembles the base-hydrolyzed gel more closely than the acid-hydrolyzed gels. CONCLUSIONS tetraethoxy orthosilicate as a precursor. From 2% solid-state MAS NMR measurements, the relative abundance of the different Si sites (Q2, Q3, and Q4) has been determined. The degree of dehydration and cross-linking in the gels increases when the drying temperature is raised from 390 K to 870 K. At 870 K, the fraction of cross-linked Si centers is higher in the base-hydrolyzed than in the acid-hydrolyzed silica gel. In CP-MAS measurements, large differences in cross-polarization times TsiH exist between the acid and base-hydrolyzed gels. In an acidic medium, many of the surface hydroxyl groups are protonated and are desorbed as water upon drying a t 390 K. This leads to long TsjH times. Loss of water is greater at 870 K, reflected in even longer TSiH times. In contrast, hydrolysis in a basic medium results in stable clusters which apparently retain their surface hydroxyl groups upon drying at 870 K. Silica gels of different pore structure were prepared by the sol-gel route, using ACKNOWLEDGEMENTS Financial support of this work by ETH and the Deutsche Forschungsgemeinschaft (SFB 213) is kindly acknowledged. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. D. Ulrich, J. Noncrystalline Solids, lOO(1-3) (1988) 174 P.A. Haas, Chem. Engr. Prog., April 1989 B.E. Yoldas, J. Mater. Sci., 14 (1979) 1843 C.J. Brinker, J. Noncrystalline Solids, lOO(1-3) (1988) 31 M. Guglielmi and G. Carturan, J. Noncrystalline Solids, lOO(1-3) (1988) 16 C.J. Brinker and G.W. Scherer, in L.L. Hench and D.R. Ulrich (Eds.) Ultrastructure Processing of Ceramics, Glasses, and Composites, Wiley, New York, 1984, Chapter 5 Stober, W., A. Fink and E. Bohn, J Coll. and Int. Sci., 26 (1986) 62 G. Engelhardt, High Resolution Solid State NMR of Silicates and Zeolites, Wiley, New York, 1987, p. 76 E.P. Barrett, L.G. Joyner and P.P. Halenda, J. Am. Chem. SOC., 73 (1951) 373 P.J.M. Carrott and K.S.W. Sing, in K.K. Unger et al (Eds.) Characterization of Porous Solids, Elsevier, Amsterdam, 1988, pp. 77-87 K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure & Appl. Chem., Vol. 57, No.4, 1985, E. Wagner and H. Briinner, Angew. Chem., 72( 19-20) (1960) 744 K.L. Walther, A. Wokaun, and A. Baiker, submitted for publication M. Mehring, High Resolution NMR Spectroscopy in Solids, Springer, 1976, p. 138 L. Miiller, A. Kumar, T. Baumann, and R.R. Ernst, Phys. Rev. Lett., 32 (1974) 2402 pp. 603-619 G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 241 ASPECTS OF THE SYNTHESIS OF ARYL SULFONIC ACID M E L P CATALYSTS DAVID L. KING, MICHAEL D. COOPER, WILLIAM A. SANDERSON, CHARLES M. SCHRAMM, and ERE D. FELLMANN Catalytica, Inc., 430 Ferguson Drive, Mountain View, California 94043 (USA) ABSTRACT organichnorganic polymers. The organic functionality provided by the phosphonate largely determines the surface chemistry of these materials. Choosing phosphonates with an arylsulfonic acid function produces materials that are effective as acid catalysts. This paper describes synthetic procedures used to prepare arylsulfonic acid MELS and methods to increase and stabilize their surface area, porosity, and swelling properties. Acid catalysis by MELS is exemplified by the synthesis of methyl tertiary butyl ether. INTRODUCTION Tetravalent metal phosphonates (MELSB) are layered compounds that can be considered Tetravalent metal phosphonates, or MELS (for Molecularly Engineered Layered Structures), provide a novel class of materials that combine many of the properties of inorganic metal oxides with the organic functionality more commonly found in functionalized polymeric resins. Early development work on these materials was carried out by Alberti and co-workers [ref. 11 and Dines et al. [ref. 21. Synthesis and characterization of related zirconium phosphates that also contain phosphonate groups as pillars have been described by Clearfield [ref. 31. There is a substantial patent estate for tetravalent metal phosphonates, and exclusive rights to this estate are owned by Catalytica [ref. 41. examples of the synthesis of zirconium phosphonates containing pendant acid groups and their utilization as ion exchangers or as solid catalysts [refs. 5-81. The ability to incorporate specific pendant functional groups into MELS provides opportunities for the development of several classes of novel solid acid catalysts. This paper provides an introduction to MELS materials, a description of synthetic methods used to produce solid MELS catalysts containing pendant sulfonic acid groups, and a discussion of some of the physical and catalytic properties of these materials. Despite the significant work reported in the literature for these materials, there are few 248 STRUCTURAL CONSIDERATIONS The structure of alpha zirconium bis(monohydrogen orthophosphate), Zr(03POH)2.H20, has been determined by Troup and Clearfield from single crystal work [ref. 91. This structure forms the basis for the structures of zirconium and other metal phosphonates. As shown in fig. 1, zirconium phosphate is a two-dimensional layered structure, with an octahedral coordination of oxygen surrounding the metal ion and tetrahedral coordination surrounding phosphorus. Each Zr is connected to a neighboring Zr through an 0 - P - 0 linkage; the phosphorus is connected through three oxide linkages to three different Zr ions, with the remaining phosphorus bond projecting perpendicular to the plane of the layer. It is this last bond, which is P-OH in zirconium phosphate and P-R in zirconium phosphonate, that leads to the unusual and varied surface properties of the MELS compounds. OH OH I I OH H@ OH I 7.5 i I Fig. 1 . Structure of alpha zirconium phosphate (from ref. 2). (a) Side view; (b) top view. The structure shown in fig. 1 is idealized for a crystalline material. The ordered stacking of these layers occurs with a fixed interlayer spacing responsible for the characteristic large d-spaced line in the X-ray diffraction pattern. The d-spacing (7.5 A for a-zirconium phosphate) varies with the nature of the material, with smallest spacing achieved with a P-H (phosphite) group (5.7 A) and extending to greater than 35 8, for long-chain alkyl groups (>CI2). The d-spacing is consistent with that predicted by molecular models. 249 Materials with a less well-defined X-ray pattern are frequently formed during synthesis. These materials are semicrystalline and are likely to have similar structure but comprise much smaller crystallites that may stack irregularly. An example of diffraction patterns for crystalline and semicrystalline zirconium phenylphosphonate is shown in fig. 2. A key parameter in MELS is the lateral spacing within the layer between adjacent P groups, since this defines the density of groups on the surface. For zirconium phosphate, the minimum distance is 5.3 A, leading to an effective cross section of 24 A2 [ref. 101. This value probably holds true for zirconium phosphonate as well. The spacing is less for titanium phosphonates and greater for MELS derived from heavier metals such as thorium or uranium. In some cases where the phosphonate group is bulky, this difference in lateral P-to-P spacing may have a significant effect on whether a MELS can be formed. It may also explain the differences in rate of formation and ultimate crystallinity that can be obtained with certain bulky phosphonates and differing metals; heavy metals such as Th typically form more crystalline samples [refs. 11, 121. This difference in spacing can also serve to explain the benefits of adding a phosphate group or a second (less bulky) phosphonate group to obtain a more structurally stable material, described later. The P-to-P distance also defines the interlamellar void space created by materials in which the layers are linked by pillaring groups that spread apart adjacent layers to a fixed distance [ref. 131. SYNTHESIS OF MELS Svnthesis from Precursors The synthesis of metal phosphonates is typically achieved via a precipitation reaction, where two soluble precursors are mixed, typically in aqueous solution, to produce an insoluble product. Any tetravalent metal ion can be utilized that can accommodate an octahedral coordination environment, such as Zr, Ti, Sn, Ce, Th, U. Most reported work employs zirconium as the metal, due to its availability, easy formation of products, and moderate cost. Other metals may be chosen for specific properties; i.e., to provide larger intralayer spacing. For synthesis of zirconium phosphonates, typical precursors such as zirconium oxychloride, ZrOCI2.8H2O or zirconium sulfate, Zr(SO&. 4H20 can be used. synthesize a material having the required z r ( 0 ~ P R ) ~ stoichiometry. This is because the different ligating ability of zirconium by chloride, sulfate, and other anions influences the competition by the phosphate or phosphonate [-P(O)(OH)2] group for the metal ion. The phosphorus source is typically a phosphonic acid, delineated generically by (HO)zP(O)R, although analogs such as phosphonate diesters (R'OhP(0)R and dihalides X2P(O)R also may be utilized. Using diphosphonic acids (HO)2P(O)R'P(O)(OH)2 in the precipitation reaction results in pillared materials where the layers are attached at fixed distances [refs. 3, 11-13]. The source of the zirconium precursor can influence the time and conditions required to 250 PS 4100.0 2690 .0 - 1200.0- 9870.0- 8460.0- 7050.0- 5640.0- 4230.0- 2820.0 - 1410.0- 20 20 Fig. 2. X-ray diffraction patterns for (a) crystalline and (b) semicrystalline zirconium phenylphosphonate. 251 Crystalline Phases hours, followed by filtration or cenmfugation and washing with water to remove impurities such as residual chloride or sulfate. This material is typically amorphous or poorly crystalline as determined by X-ray diffraction. In the case of zirconium phosphate, more crystalline material can be prepared by reflux in excess phosphoric acid [ref. 141, or by using fluoride in the synthesis mixture [ref. 151. For MELS, using excess phosphonic acid does not appear to have similar effect in enhancing crystallinity, and the HF addition method generally is employed to provide a more crystalline material. It is thought that complexation of the metal ion by fluoride is responsible for a slow release of the metal ion to solution, enhancing formation of crystalline materials. Exceptions to this method of producing crystalline samples occur when the final solid is substantially water soluble, for example with sulfonic acid-based materials, described later. Such materials have defied attempts to prepare them in crystalline form. The initially precipitated semicrystalline product is generally refluxed for a period of several Syntheses Involving More Than One Functional Group One of the attractive features of the metal phosphonates is that they can be tailored for a specific application through modification of the organic functionality of the layer. More than one functional group can be added during the synthesis step to provide greater flexibility of surface properties such as hydrophobicity or hydrophilicity or to provide for more than one chemical function at the surface. MELS containing more than one functional group are typically prepared by simultaneous addition of two (or more) phosphonic acids to the initial synthesis mixture [ref. 161. Solids formed typically contain both functional groups in approximately the concentration given in the original synthesis mixture provided it is stoichiomemc (P/zr = 2), although quantification by methods such as NMR provides a useful confirmation. With such materials, the distribution of the two functions at the molecular level has not been established, but a random distribution seems likely. Attempts by Alberti [ref. 171 to produce crystalline solids containing two or more functional groups without phase segregation have met with some success. Supporting evidence comes predominantly from X-ray diffraction,which reveals that the composite solid contains neither of the single component phases. Alberti suggests that crystalline materials comprising two different functional groups may alternate between being enriched in one function on one side of the layer and enriched in the other function on the opposite side of the layer. Creating layered crystalline materials having a uniform spacing or regular distribution of dissimilar functional groups across the surface has remained an elusive goal. Actual concentrations of the two components within the final solid may differ substantially in the final crystalline product compared to the precursor mixture, if fluoride is used to enhance crystallinity and the solution is not taken to dryness. At high concentrations of both components, phase segregation may occur; a single phase appears to most readily f o m when one of the components is present in significantly higher concentration. 252 Characterization of Short -Ranee Order in MELS by So lid State NMR Techniaues crystalline samples; this provides incentive for preparation of such materials. However, many practically useful materials are X-ray amorphous. For such materials, solid state NMR is a very useful technique, because it provides information on short-range rather than long-range order. Combining the techniques of cross polarization (CP) and magic angle spinning (MAS) allows us to obtain spectra with excellent resolution of MELS materials. Both 3lP and l3C NMR are useful for characterizing MELS materials. These techniques include the ability to: Good quality X-ray diffraction data for solid metal phosphonates is typically obtained from 1. describe the bonding between phosphorus and the metal ion. The chemical shift of the phosphorus resonance allows one to distinguish between the free acid, a phosphonate salt, or a MEiLS. We have observed that the isotropic chemical shift of the phosphorus is approximately 30 ppm upfield in a MELS relative to the free acid. The sign of the phosphorus chemical shift anisotropy reverses, changing from negative to positive when going from the free acid to a MELS. The static (no MAS) 31P NMR spectra of the phosphate, hexyl, and phenyl MELS are shown in fig. 3 and clearly demonstrate the dramatic change in the spectrum associated with MELS formation. Additionally, the relaxation behavior and cross polarization efficiencies of these forms are quite different and can be used to further characterize these materials. 2. detect phosphorus-containing impurities in the structure. A phosphorus impurity in MELS may occur, for example, if the original phosphonate source is impure or when subsequent chemistry is carried out on the solid metal phosphonate. A common impurity is phosphate. 3 . detect and quantitate the concentrations of different phosphorus groups when more than one group is present or has been used in the synthesis. For accurate quantitation, cross polarization is not used, since it may result in incorrect intensities being measured. For quantitative measurements, spectra are obtained with MAS using only single pulse excitation. We have found that the concentration of phosphonate groups in the final material is not always the same as the concentration of the initial solution precursors. 4. utilize carbon NMR to verify the organic composition of the material. In the case of pendant groups such as aryl rings, carbon NMR can sometimes distinguish between ortho, meta, or para substitution and indicate mono, di- or m-substitution on the ring. In the case of the âone-potââ materials described later, NMR analysis gives unique evidence for the presence of both aryl and alkyl groups in the solid, leading to the elucidation of the structure. 253 A Acid Phosphate B C MELS I . d L I - L J _ I L _ L L L I . L _ . _ I - d L I L - I - L I L l - # L 80 80 4 0 20 0 -20 - 4 0 -60 -00 80 8 0 40 20 . 0 -20 -40 -60 -00 PrtA - l - - L - l - I - l - l - l - . L . l - l - u L - Hexyl M _ 1 I . I L L _ L L L I _ _ ( _ I - L I _ L I _ _ I _ _ I L 80 80 40 20 0 -20 - 4 0 -60 -80 80 60 4 0 20 0 -20 - 4 0 -60 -00 PPh4 PPhl Fig. 3. Static 3 l P NMR spectra comparing free phosphonic acid and corresponding zirconium phosphonate: (a) phosphate, P-OH; (b) hexyl, P - C ~ H I ~ ; (c) phenyl, P-C6H5. 254 SYNTHESIS OF ACIDIC MELS AND PRECURSORS Variation in Aciditv through Choice of Functional Group pendant R group. The inorganic "end member" of a series of acidic materials is zirconium phosphatean ion exchanger [ref. 181 and weak acid catalyst. The acidity of this group has been demonstrated by catalysis of reactions such as isomerization of cyclopropane and linear butenes [ref. 191, dehydration of cyclohexanol [ref. 201, and dehydration and decomposition of methanol and ethanol [ref. 211. Organic functions expand the range of acidity that can be incorporated into the structure from weakly acidic carboxylic acids to strongly acidic sulfonic acids to very strongly acidic perfluoroalkylsulfonic acids. Materials of each type have been prepared at Catalytica and evaluated for catalytic activity. Substantial and ongoing work has been done to prepare MELS acid catalysts containing arylsulfonic groups, some of which is described below. A wide range of acidic materials can be synthesized by the choice of the phosphonate Synthesis of SulfoDhenvlDhosDhonic Acid Zirconium sulfophenylphosphonate can be prepared by the reaction of a water soluble zirconium salt with sulfophenylphosphonic acid. This phosphonic acid has been described in patent literature [ref. 221, but there is no evidence of its actual synthesis. We found that sulfophenylphosphonic acid can be synthesized by the reaction of phenylphosphonic dichloride with ClS@H or SO3. The reaction of phenylphosphonic dichloride with excess ClS@H proceeds smoothly to m-sulfophenylphosphonic dichloride at 150 OC and to the phosphonic acid on subsequent hydrolysis. Purification of the acid requires removal of excess sulfate by barium precipitation, followed by ion exchange to remove excess barium. For a sulfate-free synthesis, we tried the direct sulfonation of phenylphosphonic acid with liquid S@. The sulfonation proceeds readily at 125 'C, with excess SO3 relative to stoichiometry. We found that 1:l ratios of S@:phenylphosphonic acid are insufficient for complete sulfonation, even under forcing conditions, due to the competitive formation of mixed anhydrides of sulfophenylphosphonic acid and SO3, depicted in fig. 4. This competitive formation results in a consumption of greater than 1 mole of S@ per mole of phenylphosphonic acid. These anhydrides are thermally stable but may be converted to sulfophenylphosphonic acid by hydrolysis; however, this method also requires sulfate removal from the final product. 0 0 0 II/ OH 0 0 Fig. 4. Picture of possible mixed anhydrides formed by SO3 treatment of phenylphosphonic acid. 255 The 31P and l3C NMR spectra of sulfophenylphosphonic acid is shown in fig. 5(a). The phosphonate 3lP resonance is typically downfield by 18 ppm relative to phosphate. The carbon distribution pattern is consistent with meta bonding of the sulfonic acid group relative to the phosphorus-bound carbon. A B Fig. 5. (a) 31P and 13C NMR spectra of sulfophenylphosphonic acid (in dimethylsulfoxide solvent); (b) 3lP and l3C NMR spectra of disulfophenylphosphonic acid (in dimethylsulfoxide solvent). 256 Svnthesis of DisulfoohenvlphosDhonic Acid and Related Acids from phenylphosphonic acid [ref. 231. Preferred preparation conditions use 70% oleum as the reagent at 250 'C. The 31P and 13C NMR spectrum of disulfophenylphosphonic acid is shown in fig. 5(b). Based on the l3C NMR spectrum, the substitution pattern on the aromatic ring is 1,3,5. The titration curve for this acid is shown in fig. 6, which depicts the anticipated 3:l strong- acidweak-acid distribution. Biphenyl p,p'-diphosphonic acid can also be sulfonated under forcing conditions to introduce a maximum of one SO3H group per aromatic ring. Double sulfonation on each ring may be inhibited by steric effects between groups on adjacent rings. Under more forcing sulfonation conditions, disulfophenyl phosphonic acid can be prepared Base Equivalents (arbitrary units) Fig. 6. Titration curve for disulfophenylphosphonic acid. Svnthesis of MELS Containing Aromatic Sulfonic Acid Groups (i) Svnthesis from solution Drecursors. The reaction of sulfophenylphosphonic acid with a water soluble zirconium salt such as zirconium oxychloride produces zirconium sulfophenyl- phosphonate [ref. 71. The material is sufficiently hydrophilic that a solid does not precipitate from aqueous solution. Evaporation of the solution to dryness produces a glassy solid which is shown by NMR to be MELS. The glassy solid has a very low surface area and is not useful practically as a catalyst unless it is allowed to swell in the solvent medium. This occurs readily in polar media 251 but finds practical limitations in fixed-bed or other reactor configurations where a dimensionally stable solid is necessary. Addition of a second phosphonate or a phosphate function to the solution containing the metal and sulfophenylphosphonic acid allows the preparation of an acidic solid that is recoverable and filterable. The mechanical integrity and surface area increases with the amount of the second function, at the expense of acid titer. (ii) Svnthesis bv sulfonation of zirconium phenvlphosphonatc. Zirconium sulfophenyl- phosphonate can also be prepared by direct sulfonation of zirconium phenylphosphonate [refs. 6,7]. Typical sulfonating agents include SO3, CIS03H, and oleum. Reaction of phenyl MELS in excess oleum (typically containing 1%24% SO3) at 60-70 OC results in complete mono- sulfonation of the aromatic rings. Under synthesis conditions the solid zirconium sulfophenyl- phosphonate MELS appear to dissolve. However, quenching the reaction mixture with water produces a recoverable solid. Upon further washing to remove entrained sulfuric acid and purify the product, loss of solid product is noted, indicating some level of solubility of the sulfonated material in water. By methods analogous to those described for the preparation of zirconium sulfophenylphosphonate from solution precursors, a mixed material can be prepared that maintains its mechanical integrity. This can be accomplished by sulfonation of a material containing two functions (e.g., phenyl and alkyl), or by addition of more zirconium and phosphonic (or phosphoric) acid to the synthesis mixture following the sulfonation and quenching steps. In general, the use of a second phosphonate moiety provides reduced solubility of the sulfonic acid species, at a loss of acid titer. Disulfophenylphosphonate may be used to increase the titer. CHARACTERIZATION OF ACIDIC MELS Titration of the phosphonic acid is generally a useful procedure for determining purity of the phosphonic acid and the resulting MELS. Sulfophenylphosphonic acid titrates as two strong acid equivalents and a single weak acid equivalent, producing a 2: 1 titration curve, or a 3: 1 curve in the case of disulfophenylphosphonic acid, as demonstrated in fig. 6. Impurities (e.g.. presence of phosphate or sulfate) produce departures from this curve. A titration of pure zirconium sulfo- phenylphosphonate produces a curve having a single break, with an equivalence point in good agreement with the theoretical value of 3.6 meq/g. If the titration is continued beyond the neutralization of the sulfonic acid sites, a flattening of the titration curve occurs, a result of decomposition of the MELS in alkaline solutions. Thermal stability of the sulfonic acid MELS can be obtained from thermogravimemc analysis under either air or nitrogen. A comparison of TGA curves under air for zirconium phenylphosphonate and zirconium sulfophenylphosphonate is provided in fig. 7. Phenyl MELS has high thermal stability, which increases further with increasing crystallinity of the sample. The 258 thermal stability of the sulfonic acid containing MELS is significantly lower. The decrease in thermal stability is due to the decomposition of the sulfonic acid moiety; nevertheless, the thermal stability of the sulfophenyl MELS significantly exceeds the thermal stability of arylsulfonic acid ion exchange resins. too 00 Re@: 20.00 deg/mm r 0 .- - 850081 75 38% 5 H 7000 A 5000 15000 25000 35000 45000 55000 85000 75000 85000 95000 105000 Temperature (C) MM 50.00 5000 150.00 250.00 350.00 450.00 550.00 650.00 750.00 850.00 950.00 lO50.00 Temperature (C) Fig. 7. Thermogravimetric analysis in air of (a) phenyl and (b) sulfophenyl MELS. Hydrothermal stability of the arylsulfonic acid MELS is of practical commercial interest. A significant drawback of the arylsulfonic acid ion exchange resins as catalysts is their relatively poor hydrothermal stability, which restricts the useful operating range of these materials for reactions in polar media (for example, olefin hydration reactions [ref. 243). A comparison of the hydrothermal stability of MELS and an ion exchange resin is provided in fig. 8. Samples of an arylsulfonic acid MELS and Amberlyst 15 were steamed at 200 O C for increasing amounts of time. Desulfonation of both materials was followed by integration of the resonances of the sulfonated and nonsulfonated aromatic carbons in their 13C CPMAS NMR spectra. The hydrothermal stability of the arylsulfonic acid MELS compound is clearly demonstrated. No desulfonation of the MELS compound is observed over a 30-hour time span, while the resin is more than 30% desulfonated. This greater stability is probably due to the inductive effects provided by the presence of the phosphorus group bound to the aromatic ring, and the meta positioning of the sulfonic acid group relative to the phosphonate group. The net result is a reduced tendency toward desulfonation in the presence of steam. 259 110 100 .- 9 $ .Y C 2 80 s 70 60 50 -t- Sulfophenyl MELS - -M- Arnberlyst 15 0 5 10 15 20 25 30 35 Hours at 200% (Steam) Fig. 8. Hydrothermal stability comparison of sulfophenyl MELS and Amberlyst 15 (see text). ONE-POT PREPARATION OF HIGH SURFACE AREA, POROUS MATERIALS CONTAINING SULFONIC ACID GROUPS One-Pot Svnthesis of MELS Containing Aromatic Functionality The synthesis of aryl sulfonic acid MELS derives ultimately from the precursor phenyl- phosphonic acid, which is combined with the metal ion either before or after sulfonation-to make the solid acid. Preparation of a pillared material containing sulfonic acid groups provides three- dimensional stability to the solid and reduces swelling in polar media. Preparation of this latter material requires a diphosphonic acid precursor, either aryl or alkyl. Aryl pillars are desirable due to their greater rigidity, thermal stability, and their potential to be sulfonated, thus adding to the acid titer. Phenylphosphonic acid is commercially available, albeit expense. (Po1y)aryl- diphosphonic acids are not readily available from commercial sources. Consequently, developing preparation methods that allow lower cost production of phenylphosphonic and related arylphosphonic and diphosphonic acids is desirable. Our approach was first to focus upon the synthesis of phenylphosphonic acid. This material is prepared from the reaction of benzene with PCl3 using a Friedel Crafts catalyst, typically AlCl3, followed by an oxidation and hydrolysis step. The synthetic sequence is shown in fig. 9(a). Although none of the steps require complicated or expensive reagents, the product phenylphosphonic acid is expensive in part due to the cost of purification and separation of the 260 AlC13 or its hydrolyzed product from the synthesis mixture. We postulated that known Friedel Crafts catalysts of alternate metal chlorides could be used in place of AlC13, preferably metal halides that could be converted into MELS . Examples of such Friedel Crafts catalysts include ZrC4 and Tick. Subsequent oxidation and hydrolysis steps could then follow to produce Zr or Ti phenylphosphonate, thereby eliminating the need to remove the catalyst from the mixture [ref. 251. The general scheme is depicted in fig. 9(b). - 0 AICI, Oxidation Hydrolysis 1 1 1 AIC13 removal PCI, . ZrCI, I 1. Oxidation 2. Hydrolysis I + HCI + HCI Fig. 9. (a) Synthesis of phenylphosphonic acid; (b) one-pot synthesis of zirconium phenylphosphonate. 261 Several approaches based on this synthetic theme have been pursued. The reaction of PCl3 with ZrC4 in excess benzene proceeds at 85 'C. Reaction progress is evidenced by evolving HCI. The product can be oxidized to the phosphonate precursor by various means; chlorosulfonic acid proved to be a convenient reagent for the oxidation. Subsequent hydrolysis of the rather glassy resulting solid produced zirconium phenylphosphonate. Equivalence of this material with the conventionally prepared analog was verified by l3C and 3lP MAS NMR. Use of T i c 4 as the catalyst for reaction of PCl3 with benzene, on the other hand, showed no evidence of reaction (no HC1 evolution), indicating that T i c 4 is insufficiently active as a Friedel Crafts catalyst for this reaction. Solvent-mediated One-Pot Svnthesis of Pillared MELS more tractable, since hydrolysis of the glassy material resulting from the oxidation step was difficult and time consuming and would not be practical in commercial synthesis. The 1,2- dichloroethane was found to be a useful solvent to dissolve the oxidized product. Hydrolysis then proceeded rapidly, necessitating the use of ice or water-ice mixtures to control temperature. We next explored using 1,2-dichloroethane solvent throughout the reaction; i.e., replacing excess PC13 or benzene previously used as solvent medium. We expected this to provide a more physically manageable reaction mixture and to allow the subsequent hydrolysis to proceed as readily as before. The reaction of benzene, PCl3. and ZrC4 proceeded readily in the presence of dichloroethane, but yields of product solids were greater than 100%. Characterization of the final material by l3C CPh4AS NMR produced an unexpected result. The spectrum of the product is shown in fig. 10(b); it is distinctly different from the spectrum of phenylphosphonate MELS shown in fig. lO(a). The resonance at 142 ppm indicates that the aromatic ring is disubstituted, and that alkyl (at ca. 30 ppm) and aryl carbon groups were observed. Elemental analysis of the final product verified the absence of chlorine, suggesting that the aliphatic structure was not due to entrained solvent. Thus, the alkyl groups must have derived from the solvent dichloroethane that participated in the reaction sequence and ended up as an allcyl bridge between aryl rings from adjacent layers. The NMR data are indicative of predominantly para substitution on the aromatic ring. Thus, during the PC13-benzene reaction, an additional reaction has occurred: the Friedel Crafts alkylation by the solvent to produce a pillared bibenzyl group. Analysis of the material by XRD showed the presence of a weak, high d-space line consistent with the spacing between the layers from an axyl-ethylene-aryl grouping. pillared materials where the interlayer spacings can be quite large. As an example, the 13C CPMAS NMR spectrum of the 10-carbon bridged, biphenyl-pillared MELS is shown in fig. 10(c). Again, XRD confirms a large d-spacing for this material. A xylyl-bridged, biphenyl-pillared MELS was similarly prepared from the corresponding dichloroxylene solvent. The simplified formulas for these three materials synthesized by the solvent-mediated one-pot method are shown in fig. 11. We then explored using a solvent to make the ZrC4-phenylphosphonate oxidation product Extension of this approach to other chlorinated solvents is possible and leads to a range of 262 A Fig, 10. l3C NMR of one-pot phenyl MEiLS prepared using various solvents during the Friedel Crafts synthesis: (a) benzene; (b) 1,2-dichloroethane; (c) 1,lO-dichlorodecane. 263 ethylene-bridged B Fig. 11. Formulas for pillared one-pot materials prepared using chlorinated solvents during the Friedel Crafts synthesis: (a) dichloroethane; (b) 1 ,lo-dichlorodecane; (c) 1,4-di(chloromethyI) benzene. u The surface area and pore volumes of fully pillared one-pot materials are relatively low, typically I 2 5 m2/g and 0.1 cm3/g, respectively, somewhat dependent on the particle size of the sample. The low pore volume is suggestive of a material that has low porosity due to the bulk and density of the pillaring groups. The interlamellar regions may not be readily accessible to solvent or reagent molecules. This was conf i ied by attempts at sulfonation of a fully pillared, bibenzyl one-pot material to generate an acid catalyst, which proved to be quite difficult using either C1S03H or SO3 as the sulfonating agent. Only about 50% of the aromatic rings were sulfonated, even at forcing conditions. pCl3 in the initial synthesis step (Friedel Crafts reaction of PC13 with benzene in chlorinated solvent) with the idea that upon final hydrolysis the product would contain both aryl-akylene-aryl pillars and smaller P-OH groups. This preliminary step would create interlamellar voids that would allow both better access of sulfonating reagents to the aryl pillaring groups during catalyst synthesis and also good access of chemical reagents during utilization of the material as a catalyst. The excess PCl3 approach proved successful. Both functional groups were present in the final material, as demonstrated by 3lP MAS NMR characterization. The material comprised P-OH and P-phenyl (as bibenzyl pillars) groups in a ratio of 3:1, and had a surface area of 150 m2/g and pore volume of 0.45 cm3/g, indicative of a more open structure. Complete monosulfonation of the As a practical approach to the problem of access to the interior layers, we utilized excess N & TABLE 1 Q, Comparative performance of arylsulfonic acid-based catalysts for MTBE synthesis. Acid Titer (mwg) 2.08 2.30 2.29 3.0 WHSV (MeOH) 100 33 100 33 100 33 100 33 Methanol Conversion (%) 52 72 21 65 4 12 30 76 Productivity, g MTBWg cat-h 143 65 58 59 11 10.9 83 69 Activity, m o l e MTBWmeq-s (x102) 21 9.9 7.9 8.1 1.5 1.5 8.7 7.3 265 aromatic rings in the structure was accomplished under standard sulfonation conditions by using SO3 in dichloroethane as the sulfonating agent. CATALYTIC PERFORMANCE OF ARYLSULFONIC ACID MELS We evaluated the catalytic performance of the arylsulfonic acid MELS in a number of reactions, including isomerization of butenes, MTBE synthesis, methanol dehydration, aromatic alkyl'ation, and MTBE cracking. An example of its utilization as a catalyst for MTBE synthesis follows. The synthesis of methyl tertiary butyl ether from methanol and isobutylene is a convenient reaction to study since catalysts are readily tested in a fmed bed at moderate temperatures and pressures [ref. 261. Since sulfonated ion exchange resins are the catalysts of choice for this reaction, use of this reaction provides a convenient comparison of the efficiency of the MELS catalysts relative to ion exchange resins [refs. 7,271. We chose three mixed MELS containing arylsulfonic acid pendant groups to examine the effect of differing second functionalities (-H, -OH, -CH3) on the catalytic properties of the final catalyst. All were prepared by the direct sulfonation of phenyl MELS with oleum, followed by addition of zirconium and the second phosphorus (R') group (H3P03. H3P04, and CH3H2PO3, respectively) to the quenched mixture in proportions to maintain the 2 1 phosphorus:zirconium stoichiometry and approximate 1:l ratios of arylsulfonic: R' functionality. The mixture was refluxed, and the solid was recovered, washed free of residual sulfuric acid, and dried. Prior to using these materials as catalysts for MTBE synthesis, the materials were subjected to extraction with hot methanol at 62 OC for 48 h to remove any residual soluble (phosphonic) acid species. Amberlyst 15 was pretreated by ion exchange with 1N HC1 followed by washing with distilled water and drying at 110 "C. a methano1:isobutylene molar feed ratio of 1.21. A small amount (1 mole %) of n-heptane was added to the feed as an internal standard. Analysis of the reactor effluent was carried out by gas chromatography. Under the reaction conditions, MTBE is virtually the only product observed; C4 olefin dimers are observed only in trace amounts. Due to difficulties in reliably quantitating methanol, formation of MTBE was used in the quantitation of catalyst activity and methanol conversion. The MTBE synthesis reaction was carried out in a fixed bed reactor at 60 OC, 110 psig, and Table 1 provides a comparison of the activity of the three MELS sulfonic acid catalysts and Amberlyst 15 sulfonic acid resin at two different space velocities. The activity observed with Amberlyst is in good agreement with published literature [ref. 281. It is clear that the activity of the sulfophenyl MELS containing the phosphate second function is more active than the catalyst containing the phosphite second group, and both are substantially more active than the catalyst containing the methyl group. Thus, the second functional group can affect overall catalytic performance. We believe that the differences in activity experienced between the different catalysts reflects both variations in the hydrophilicity and in the swelling properties of the catalyst. These 266 materials are typically low surface area, gellular solids in the dry state. Their ability to swell allows access of reactants to the internal sulfonic acid sites. The catalyst productivities for the phosphate-based MELS and the Amberlyst 15 on a per- gram catalyst basis are comparable at the lower space velocity, but activity and productivity is clearly greater for the phosphate-containing h4ELS at higher space velocity where methanol conversion is lower and the system is farther from thermodynamic equilibrium. The differences may reflect effects of a MTBE-rich versus methanol-rich reaction medium, the swelling nature of MELS versus the more rigid macroreticular resin, and differences in activity. It is informative to compare the activity of MELS with Amberlyst on a per-acid-site basis, also shown in Table 1. Assuming comparable site accessibility between the phosphate-containing MELS and Amberlyst 15, the sulfonic acid sites of the phosphate/sulfophenylphosphonate MELS catalyst demonstrate greater turnover rates than the acid sites of Amberlyst 15. 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Catal., 94 (1985) 491. T. N. Frianeza and A. Clearfield, J. Catal., 85 (1984) 398. S. Cheng, G. Z. Peng, and A. Clearfield, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 219. A. C. Mc Kinnis, U.S. Patent 2,776,985 (1957), assigned to Union Oil Co., California. W. A. Sanderson, PCT Patent Application WO 89/11485 (1989), assigned to Catalytica, Inc. R. L. Albright and I. J. Jakovac, Catalysis by Functionalized Porous Organic Polymers, Rohm and Haas Product Literature, 1985. W. A. Sanderson and D. L. King, patent pending. 267 26. F. Colombo, L. Con, L. Dalloro, and P. Delogu, Ind. Eng. Chem. Fundam., 22 (1983) 219. 27. P. M. Lang, F. Martinola, and S . Oeckl, Hydrocarbon Proc., Dec. (1985) 5 1 28. F. Ancilloti, M. M. Massi, and E. Pescarollo, J. Catal., 46 (1977) 49. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 269 PREPARATION OF BASIC SILICATES AND THEIR USE AS SUPPORTS OR CATALYSTS G.A.MARTIN, M.C. DURUPTY, C. MIRODATOS, N. MOUADDIB* and V. PERRICHON Institut de Recherches sur la Catalyse, C.N.R.S., Conventionng l'Universit6 Claude Bernard LYON I, 2 avenue Albert Einstein, 69626 - VILLEURBANNE C&dex - France ABSTRACT The utilization in catalysis of non-swelling basic Silicates such as chrysotile and talc are briefly reviewed. Their preparations by hydrothermal synthesis for new applications in catalysis are described. They lead to original Co-Cu catalysts for alcohols synthesis from syngas and to Li/MgO-Si02 systems active in the oxidative coupling o f methane. INTRODUCTION Basic silicates belong to the family of clay minerals which exist in the nature under numerous varieties (1). Their formation under hydrothermal conditions has long been studied by geologists and soil scientists. Concerning catalysis, there is now a renewed interest in the modified clay minerals and particularly in the pillared interlayer clays for oil cracking (refs. 2-6), but only a limited number of studies are available concerning the possible use of basic silicates for catalytic applications (ref. 7). Their structure derives from that of brucite Mg(OH)2 where OH groups are replaced by silica tetrahedras according to the following formulas : brucite chrysotile talc lizardite By isornorphous substitution, the Mg2+ ion can be replaced by other divalent ions such as Ni2+ and Co2+ or by monovalent cations such as Li'. In this latter case, the excess of charge must be compensated by an additional exchangeable cation located between the layers. Talc and chrysotile of cobalt or nickel were prepared in our laboratory as precursors of Ni and Co/SiO2 catalysts * Present address : E.P.F.L., Ecublens, CH-1015 LAUSANNE 270 leading to metal particles with, in some cases, interesting preferential crystallographic orientations (refs. 8-9). It has to be noted that most of the catalytic studies deal with Ni or Co chrysotile (refs. 10-11). The possibility of introducing two divalent cations could be envisaged in some particular cases to obtain precursors of alloys or bimetallic systems. For example, it could be interesting to prepare cobalt-copper catalysts for which the association of cobalt and copper is involved in the selective synthesis of C2+ alcohols from CO,H2 (refs. 12-13). Some of these silicates are present in the nature. However they often contain impurities which make them unsuitable for catalytic purposes, so that the synthetic way appears desirable to prepare these compounds. This paper gives the results we obtained on the preparation of magnesium basic silicates supports and their use in the CO hydrogenation or oxidative coupling of methane after deposition of cobalt-copper or lithium phases respectively. Attempts to synthesize cobalt-copper chrysotile in which the Co and Cu ions would replace part of the Mg ions are also reported. EXPERIMENTAL METHODS Hydrothermal synthesis were performed in a passivated stainless steel autoclave (PROLABO). In a typical preparation of a Mg chrysotile, a stoichiometric mixture of 3.17 g Si02 (Aerosil 200 Degussa) and 4.56 g Mg(OH)2 in suspension in 60 ml H20 was stirred vigorously with a mixer during 5 mn. The autoclave charged with the slurry was sealed and heated at 623 K for 120 h. After cooling, the product was washed and dried 24 h at 373 K. Mg(OH)2 was prepared by precipitation of MgC12 solution (0.25 N) by KOH (0.5 N). The precipitate was washed to eliminate chlorine ions and driedovernight at 353 K under vacuum. Solid products were examined by X-ray powder diffraction using the Cu K Q radiation. BET areas were determined by N2 volumetric adsorption at 77 K. Chemical analyses were performed by atomic absorption spectroscopy. PREPARATION OF MAGNESIUM TALC AND CHRYSOTILE SUPPORTS A first set of experiments was performed to synthesize the Mg talc by heating under hydrothermal conditions a mixture of Si02 and Mg(OH)2 in a ratio Si/Mg=2. An excess of silica compared to the normal stoichiometry of the talc (1.33) was used as a silica source in order to increase the rate of talc formation (ref. 14). Sampling of the products were effected after 24 h at temperatures 271 ranging between 313 and 623 K. They were studied by X-ray diffraction and their specific area was determined after 5 h desorption under vacuum at 423 K. The spectrum of the initial mixture shown in Fig. 1 contains the four main lines of brucite, the silica being amorphous. At temperatures in excess of 373 K, the Mg(OH)2 pattern disappears and broad lines are detected corresponding to a new phase. Increasing the temperature up to 623 K confirms the formation of magnesium talc (Fig. 1). I I 2 41 00 I I I I 200 4 0 0 600 R K ) Fig. 1. X-ray diagrams showing the formation of Mg talc as a function of the temperature of the hydrothermal treatment. Fig. 2. Evolution of the BET surface after the hydrothermal treatments at different temperatures. Fig. 2 shows the evolution of the BET area as a function of the preparation temperature. Between 295 and 353 K, there is an increase from 150 to 260 m2.g-I, phenomenon which can be attributed to hydration effects (ref. 15). A net gap is observed between 353 and 373 K, from 260 to 435 m2.g-l which confirms the brucite transformation in talc. For higher preparation temperatures, the specific area decreases continuously, inagree- ment with the better crystallization observed on the diagram. After one week at 623 K, the specific area is 132 m2. A new set of experiments between 295 and 623 K, with a ratio Si/Mg=l, shows also evidence of the disappearance of the brucite X-ray lines between 353 and 373 K. A mixture of phases was 272 obtained, mainly talc and a small fraction of chrysotile. However, even after 8 days at 623 K, the sample remained poorly crystallized. From several experiments at 623 K, it results that Mg chrysotile could be obtained as a single phase only when starting from the stoichiometric composition. Moreover, a vigourous mixing of the precursors for at least 5 mn with an homogenizer was necessary for the synthesis. Otherwise, even with a stoichiometric mixture, the synthesis resulted in a polyphasic system with chrysotile, talc and brucite. Thus it is possible to prepare either talc or chrysotile at temperatures as low as 373 K provided that the starting materials are in a stoichiometric ratio and well mixed. The specific areas are higher than 300 m2g-l. After the hydrothermal treatment at 623 K, the corresponding BET areas are between 90 and 110 m2g-l. PREPARATION OF Co-Cu/CHRYSOTILE CATALYSTS FOR THE ALCOHOLS SYNTHESIS FROM CO/H2 In the case of Co-Cu bimetallic catalysts which are known to be selective for the hydrogenation of CO into higher alcohols, it is thought that a key factor for the selectivity is the proximity of the cobalt and copper atoms. Such a mixture at the surface scale is made difficult by the poor bulk miscibility of the two constituants ( 10% Cu maximum in cobalt and 0.2% Co in copper). In order to realize such a mixed system with homogeneous composition of equivalent amounts of Co and Cu, we have investigated the possibilities of using the peculiar properties of the basic silicates. Preparation Two types of preparation were tried, exchange and hydrothermal synthesis. The cationic exchanges were realized under N2 flow on 2 g of Mg chrysotile of 113 m2g-l in suspension in 100 ml H20. The cobalt and copper were used as nitrate salts. The conditions and the results are summarized in Table 1. EC refers to co-exchange whereas ES corresponds to successive exchanges starting with cobalt first. In all cases, the resulting Co concentration was low (0.1-0.3%) and that of copper was one order of magnitude higher. In spite of this relatively low copper content, it was possible to observe by X-ray analysis, the presence of a well defined phase, the gerhardite Cu2(0H)3N03 together with the initial Mg chrysotile. The easy formation of this phase can be taken as an evidence of the great affinj.ty of the surface towards copper, 273 which may explain that the exchange occurs selectively with copper rather than with cobalt. TABLE 1 Cobalt and copper exchanges on Mg chrysotile Catalyst % co % cu X-Ray analysis EC2a 0.25 2.2 Ma chrvsotile EC1 0.1 3.9 Mg chrysotile + Cu2(0H)3N03 ESlb 0.16 2.1 Mg chrcsotile + Cu2(0H)3N03 ES2 0.29 2.8 Mg chrysotile + Cu2(OH)3N03 Exchange temp. : 323 K. Theoritical Cu and Co contents : 5% a Theoritical Cu and Co contents : 2.5 % Exchange temperature : 298 K. For the preparation of a mixed MgCoCu chrysotile, several attempts were made starting from different precursors salts, but always with the same ratio (Cu+Co+Mg)/Si=1.5 which corresponds to the stoichiometry of the chrysotile. Four samples were prepared as follows : - HS1 : starting from Co and Cu hydroxides - HS2 : as above, but with addition of Mg(OH)2 - HS3 : from CuC12-CoC12 and Mg(OH)2 with a low Cu percentage - HS4 : from the solution of the chlorides salts, precipitated in- situ by K2CO3 on silica before the hydrothermal treatment. The formation of chrysotile was never observed. However, evidence of Mg or Co talc could be detected with the characteristic line at 0.95 nm. In the absence of magnesium, a badly crystallized Co talc was obtained together with CuO. In the presence of Mg, the Mg talc structure seemed to be favoured but the degree of crystallization remained low. Consequently, the introduction of copper in the chrysotile stucture appears very difficult. This fact has already been pointed out by Wey et al. (ref. 16) and explained by a Jahn-Teller effect, which makes the structure distorted and creates an unstability for the whole crystal. Catalytic behaviour in the CO/H:! reaction a reduction at 573 K by H2 at atmospheric pressure. Table 2 gives some significant results corresponding to the activity after stabilization at 523 K, i.e. practically after 5 h on stream. For a better comparison, the rates are expressed for 1 g cobalt. For the catalysts prepared by exchange, the main common feature is the high methanol selectivity which can be attributed to the high coppsr surface oonoentration. The activities are low. All the cobalt-copper catalysts were tested under 10 bar after 214 TABLE 2 Catalytic properties of CoCu catalysts in the CO/H2 reaction. T=523 K ; P=10 bar : CO/H2=0.5 ; D=1.8 1.h-1 ; m=100 mg. Catalyst Activity Selectivity % mmole . Hydrocarbon Alcohols C2+0H h-l. g-ko C1-C6 Cl-C5 EC2 0.7 ia a2 a ES2 0.3 19 a1 3 HS1 4 38 62 24 HS2 25 78 22 9 HS3 ia 78 22 11 HS4 3 26 74 a Concerning the hydrothermal method, the activities are much higher and the best data for the higher alcohols selectivity are obtained with the hydroxides as precursors without Mg(OH)2 addition. PREPARATION OF Li/MAGNESIUM BASIC SILICATES CATALYSTS FOR METHANE OXIDATIVE DIMERIZATION The objective was to improve the design of catalytic phases active and stable for the oxidative coupling of methane into C2 hydrocarbons. Among the numerous and various formulas tested up to now, the most selective and productive catalysts are generally basic oxides promoted with alkali compounds, such as Li/MgO catalyst (ref.17). Such catalysts are however rather unstable in the severe conditions of the reaction (T>900K, reaction mixtures with H20, CO, C02, 02 and hydrocarbons), mainly due to i) the loss of alkali (vaporization, reaction with the tubular quartz reactor) and ii) the loss of surface area (sintering of the oxides) ( ref. 18 ) . Attempts have been made in this laboratory to stabilize the reference catalyst Li/MgO by means of hydrothermal synthesis, on the basis that adding silica to the magnesia structure could induce beneficial effects on Li content and surface area. Preparation of Li promoted magnesium silicates A series of magnesium silicates has been prepared according to the general recipe of hydrothermal synthesis, with 3Si/Mg ratios variing from 0 (brucite) to 4 (talc) with intermediate ratios corresponding to mixtures of brucite, chrysotile and talc structure as reported in table 3 . The various Mg silicates were then impregnated with lithium carbonate aiming at a constant atomic ratio Li/Mg (around 0.6).. An other sample of chrysotile (3Si/Mg=2) has been alkalized at a lower content (Li/Mg+Si= 0.1) 275 in order to test the stability of the Li content in comparison with Li/silica and Li/magnesia loaded with similar content of Li. Fig.3 reports the changes in alkali content as a function of the 02-treatment temperature. The alkali appears to be very stable on silica and chrysotile while a major loss of Li is observed on magnesia for T>900K. The different behavior between silica and magnesia supports has been explained in (ref.19) by considering that a stabilizing lithium silicate interface was formed between lithium and silica phases while no equivalent compound could be formed with the magnesia support. The stabilization of lithium which is observed on chrysotile (Fig.3, curve a) indicates therefore that the addition of silica to magnesia via the hydrothermal synthesis could allow interface Li silicate to develop, preventing the loss temperature. 0.15 chrysotile f 0.05 2 i t m M: 0 I I I I 0 500 7 0 0 900 1100 1 of alkali at high K ) Fig. 3. Changes in Li content vs temperature of treatment (flowing 02 for 15h) for a : Li/chrysotile ; b : Li/SiO2 : c : Li/MgO. In Fig.4 are depicted the changes in morphology which are observed by TEM on Li/chrysotile before and after activation and catalytic test. Additional informations on surface composition were provided by STEk analysis. Initially (Fig.4, a), the catalyst is formed with two distinct phases: well crystallized chrysotile-type flakes and Li2CO3 crystals. After calcination at 723K (Fig.4, b), the chrysotile phase displays the same overall structure, now covered with lithium carbonate but with a heterogeneous composition indicating a local demixion of the Mg and Si phases. This effect could correspond to the formation of some Li/Si compound as postulated above. Finally, after catalytic test (Fig.4, c), the chrysotile structure is collapsed and replaced by a clustering of large partlcules ( 5 0 to 200 nm), 276 mainly MgO coated with Li2CO3 (from XPS measurements) and very large rafts of segregated silica. This picture is close to what is observed with the reference Li/MgO sample, but the initial insertion of silica tends however to stabilize both the lithium content and the surface area (Table 3). Fig. 4 . Electron a : initial : micrographs of Li/chrysotile. b : after calcination at 723 K : c : after catalytic test ar 1023 K. Catalytic behavior of Li promoted magnesium silicates Table 3 gives some data concerning the catalytic activity and selectivity in methane oxidative coupling obtained with the Li/magnesium silicate series ( for more details see ref.20). It is mainly observed that: - for small amounts of silica added to the reference MgO , leading to mixture of magnesia and chrysotile, beneficial effects 277 such as surface stabilization and prevention of alkali loss induce an increase in the CH4 conversion with a high C2 selectivity. - at higher silica contents, the above positive effects are counterbalanced by the development of a silica and Li silicate surface, unfavorable towards selectivity and activity. As a matter of fact, acidic surfaces are unable to activate methane in these catalytic conditions and the preferential reaction of lithium with silica is likely to hinder the formation of the Li/MgO interface necessary for methane coupling. TABLE 3 Characterization data and catalytic properties of Li/MgO-SiO2 Initial Atomic ratio BET area CH4 couplingC structure 3Si/Mg Li/Mg before after activity C2 select. ( X R D ) ~ catalytic test (mmole ( % ) magnesia 0 0.63 50 tl 3 73 (m .g-l) h-l. g-l) magnesia+ chrysotile 1.2 0.63 110 3 15 70 chrysotile 2.3 0.59 130 6 20 46 talc 4.0 0.62 150 12 6 18 silica b 180 80 tl 8 aSuperimposed with the Li2CO3 structure bLi/Si = 0.11 CReaction carried out at 1023 K; PCH4'7.8 kPa; P02=4.6 kPa, total flow rate = 3.6 1.h-l. In conclusion, although the initial complex structures of magnesium silicates are destroyed in the severe reaction conditions of methane coupling, they may induce positive effects on catalytic performances, at least for reduced Si/Mg ratios. CONCLUSION The possible uses in catalysis of metal hydroxide silicates as supports or precursors of active phases were evaluated in two reactions, the synthesis of higher alcohols from syngas and the oxidative coupling of methane. Although the initial structure of the silicate is often destroyed during the activation step and converted into a mixture of phases, it may induce positive effects on the physicochemical and catalytic properties. Particularly, magnesium basic silicates show a better thermal stability compared to MgO and favour the stabilization of lithium during the CH4 oxidative coupling. Due to the uniformity of the surface hydroxyls groups, the exchange methods should be limited to monometallic exchanges. Finally, the preparation of basic silicates homogeneous in composition remains an open and promising domain. 278 ACKNOWLEDGEMENTS We wish to thank Mrs M.T. Gimenez for the X-Ray diffraction measurements and I. Mutin for the electron microscopy and STEM studies. Part of this work was supported by GDF. REFERENCES 1 F. Liebau, Structural Chemistry of Silicates, Springer-Verlag eds., Berlin, (1988) 213-231. 2 T.J. Pinnavaia, Science, 220 (1983) 365-371. 3 V.N. Parulekar and J.W. Hightower, Appl. Catal., 35 (1987) 249- 262. 4 S. Yamanaka and M. Hattori, Catalysis Today, 2 (1988) 261-270. 5 C.I. Warburton, Catalysis Today, 2 (1988) 271-280. 6 F. Figueras, Catal.Rev. Sci.Eng. 30 (1988) 457-499. 7 H.E. Swift in: J.J. Burton and R . L . Garten (Eds), Advanced Materials in Catalysis, Academic Press, London, 1977 p. 230. 8 G. Dalmai-Imelik, C . Leclercq and A. Maubert-Muguet, J. Solid State Chem., 16 (1976) 129-139. 9 J.A. Dalmon and G.A. Martin, C.R. Acad. Sci. Paris, 267C (1968) 610. 10 Y. Ono, N. Kikuchi and H. Watanabe, in: B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Eds), Preparation of Catalysts IV, Elsevier Science Publishers B.V., Amsterdam, 1987 pp. 519-528. 11 L. Bruce, H. McArthur and T.W. Turney, Proc.of the 12th Aust. Chem. Eng. Conf., Melbourne, Australia, (1984) pp. 649-654. 12 R.M. Baillard-Letournel, A.J. Gomez-Cobo, C . Mirodatos, M. Primet and J.A. Dalmon, Catal. Letters, 2 (1989) 149. 13 N. Mouaddib, V. Perrichon and M. Primet, J. Chem. SOC., Faraday Trans. I, 85 (1989) 3413-3424. 14 H. Muraishi and S . Kitihara, Proc. Int. Symp. on Hydrothermal reactions, (1982) pp. 377-392. 15 C. Sudhakar and M.A. Vannice, Appl. Catal. 14 (1985) 47-63. 16 R. Wey, B. Siffert and A. Wolf, Bull.Gr.Fr. des Argiles, 20 17 T. Ito, J.X. Wang, C.H. Lin and J.H. Lunsford, J.Am.Chem.Soc., 107 (1985) 5062. 18 C. Mirodatos, V. Perrichon, M.C. Durupty and P. Moral, in : B. Delmon and G.F. Froment (Eds), Catalyst Deactivation, Elsevier Science Publishers, Amsterdam, 1987 pp. 183-195. 19 V. Perrichon and M.C. Durupty, Appl. Catal., 42 (1988) 217. 20 G.A. Martin, P. Turlier, V. Ducarme, C. Mirodatos and M. Pinabiau, Catal. Today, 6 (1990) 373 (1968) 79-92. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 279 SOILS AS UNUSUAL CATALYSTS SERGIO A. MOYA, ANSELMO FLORES and MAURICIO ESCUDEY Departamento de Quimica, Facultad de Ciencia, Universidad de Santiago de Chile Casilla 5659, Correo 2, Santiago(Chi1e) ABSTRACT as catalysts for the water gas shift reaction. The system produced in alkaline media develops a catalytic activity which depends on the soil mineralogy, organic matter and iron oxide contents. The yield obtained for WGSR using the soils as catalyst is comparable to those produced by prepared catalysts supporting Fe203 on y-Al203. Samples from the B horizons of three profiles of Chilean soils were used INTRODUCTION During the last decades literature shows some attempts to apply clay fractions as catalysts for several reactions: cracking of petroleum products; formation of amino-acids and their polymerization into peptides; polymerization of benzene; polymerization of styrene ( 1 - 4 ) . However, there is no information about the possibility of using soils directly as catalysts. The iron oxides or iron derivatives with different mineralogy are common and significant components of many soils and are usually dispersed throughout the external soil matrix as a coating of the aluminosilicate core. Thus the iron oxides may have an important influence on the properties of the soil as potential catalysts for many reactions. In this way we have made pioneer works searching into this possibility. Recently we reported the use of volcanic-ash-derived-soil as iron oxide supported catalyts ( 5 ) . Continuing our studies, we report here the application of three Chilean soils as catalysts for the water gas shift reaction (WGSR), taking into account that the characteristic of Chilean soils derived from volcanic material is their large specific surface area and high iron oxides content (6). The used soils were characterized in terms of their chemical properties, X-ray powder diffraction, isoelectric points and MBssbauer spectroscopy which was used to identify and characterize Fe oxides in the soils. The influence of factors such as soil pretreatment, heating and pH in the catalytic run were also considered. EXPERIMENTAL Samples of the B horizons of Chilean soils (Collipulli, Osorno and San Patricio) derived from volcanic ashes ( 7 ) were used. Collipulli: A Xeric Pa- lehumult with a clay fraction dominated by halloysite (> 50%); minor 280 components ( >5%) a r e c h l o r i t e ; g i b b s i t e , g o e t h i t e , p l a g i o c l a s e , q u a r t z and a - c r i s t o b a l i t e . Osorno: A Typic Dys t randept w i t h a c l a y f r a c t i o n dominated by a l lophane ( >50%); minor components ( >5%) a r e f e r r i h y d r i t e , o rgano-a l lophanic complexes, h a l l o y s i t e , g i b b s i t e and a - c r i s t o b a l i t e . San P a t r i c i o : A Hydric Dystrandept w i t h a c l a y f r a c t i o n dominated by a l lophane (> 50%); minor components ( > 5 % ) are a - c r i s t o b a l i t e , g o e t h i t e , p l a g i o c l a s e and h a l l o y s i t e . Chemical a n a l y s i s w a s c a r r i e d o u t by atomic abso rp t ion spec t roscopy a f t e r d i s s o l u t i o n i n Te f lon bombs (8) and o rgan ic carbon w a s determined by a d ry combustion method. X-ray d i f f r a c t i o n s were c a r r i e d o u t on powdered samples i n a P h i l i p s No- The r e l c o in s t rumen t wi th Cu& r a d i a t i o n and a carbon c r y s t a l monochromator. s p e c i f i c s u r f a c e area w a s ob ta ined by a g r a v i m e t r i c method based on t h e r e t e n t i o n of e thy lene g l y c o l monoethyl e t h e r (EGME) ( 9 ) . wi th cons t an t a c c e l e r a t i o n a t room tempera ture . The source w a s 2.13 m C i 57C0 i n a Pd ma t r ix . The r epor t ed isomer s h i f t v a l u e s are r e l a t e d t o n a t u r a l i r o n (57Feo) . The d a t a were ad jus t ed w i t h an i t e r a t i v e l ea s t - squa res program which uses a Marquandt a lgo r i thm de-veloped i n t h e Mtlssbauer Spec t roscopy Labora tory of t h e Phys ic s Department of t h e Un ive r s i ty of Sant iago . The Melssbauer s p e c t r a were ob ta ined i n a convent iona l Aus t in spec t rometer E l e c t r o p h o r e t i c m o b i l i t i e s w e r e measured wi th a Zeta Meter (ZM 77) appa ra tus f i t t e d wi th an au tomat ic sample t r a n s f e r system. Samples of about 5 - 3 mg were suspended i n 100 m l of 10 M i o n i c s t r e n g t h s o l u t i o n s f i x e d w i t h K C 1 . The m o b i l i t i e s were averaged and t h e z e t a p o t e n t i a l (ZP) w a s c a l c u l a t e d us ing t h e Helmholtz-Smoluchowski equa t ion (10). A computer program i n BASIC language w a s employed t o o b t a i n t h e i . e . p . I n a l l exper iments doubly d i s t i l l e d water was used. P r i o r t o u s e diglyme and t h e o t h e r chemicals were p u r i f i e d accord ing t o procedures a l r e a d y r epor t ed i n t h e l i t e r a t u r e (11). The WGSR w a s c a r r i e d ou t under r e l a t i v e l y mi ld c o n d i t i o n s ( l O O ° C , Q 1 a t m CO (Matheson, 99.99%)). A Perk in - E l m e r model 8500 gas chromatograph provided wi th a GP-100 p r i n t e r w a s used f o r a n a l y s i s of gas mix tu res . A ca rbos i eve S-I1 column ( 3 mX2.4 mm, Supelco) w a s used t o ana lyze H2, CO and C02, employing H e as c a r r i e r gas . The a b s o l u t e y i e l d of hydrogen was determined by c a l i b r a t i o n of t h e GC u s ing known volumes of hydrogen. 281 TABLE 1 Chemical a n a l y s i s (w t%) , o rgan ic carbon con ten t (w t%) , i s o e l e c t r i c p o i n t (IEP i n pH u n i t s ) and s u r f a c e area (m'g-l) a s a f u n c t i o n of h e a t i n g f o r s o i l samples. Temperature Fe203 A1203 S i O 2 Organic IEP Sur face of h e a t i n g Carbon Area C o L l i p u l l i 124°C 13.8 19.8 46.7 0.6 2.8 155 350°C 14.1 21.0 50.0 0.2 2.9 135 600°C 14.3 23.6 51.0 0 .1 3.0 94 Osorno 124°C 9 .5 17.5 43.5 3.7 6.7 142 350°C 11.3 19.8 45.4 0.6 6.9 98 600°C 1 2 . 7 21.3 51.6 0 . 1 6.7 63 San P a t r i c i o 124'C 7 .3 1 2 . 1 35.6 13.2 3.2 118 350°C 10.0 18 .2 51.6 1 .6 6 .1 100 600°C 10.5 20.1 53.6 0.2 6.7 43 The a n a l y s i s of t h e g a s samples and t h e type of r e a c t o r used were s i m i l a r t o those a l r e a d y desc r ibed (12) . Typ ica l Fez03 suppor ted c a t a l y s t s were prepared by a w e t impregnat ion procedure (excess of s o l u t i o n ) of a S t r e a m s i eved t o 1 mm) u s ing a s o l u t i o n of Fe(N03)3*9H20. S o l u t i o n s of 11 w t % (g of Fez03 pe r 100 g of d r i e d y - ~ l 2 0 3 ) were prepared . The impregnated samples were d r i e d a t 100°C and 27 kN m-2 and f i n a l l y ca l c ined a t t h e tempera ture of t h e s o i l samples. A l l t he se prepared c a t a l y s t s were used t o c a t a l y z e t h e WGSR. RESULTS AND DISCUSSION y-Al203 (S BET = 188 m2 g-l and Osorno and San P a t r i c i o s o i l s a r e Andepts w i t h a h igh o r g a n i c matter con ten t dominated by v a r i a b l e s u r f a c e charge i n o r g a n i c components (Table 1 ) . On t h e o t h e r hand, C o l l i p u l l i s o i l i s an U l t i s o l w i t h low o rgan ic m a t t e r con ten t , dominated by c r y s t a l l i n e c l a y mine ra l s w i th l i t t l e o r no v a r i a b l e s u r f a c e charge . Andepts samples mineralogy is dominated by low c r y s t a l l i n i t y compound and hea t ing (600'C) was n o t observed t o have any e f f e c t on t h e c r y s t a l l i n i t y (F igu re 1) . 282 v) t t t z w z H P âLc 600 G h- - H P b-c 350 G H 1 H P c-c 290 G - H P 500 G I H H P, â2â 01 0 G 220 -I I i 124 7 - 35 $0 2âs 20 ;5 - 1 ~ ~ TWO THETA (DEGREES) Fig.1. X-ray diffraction for as a function of heating temperature for San Pa- tricio sample. The heating temperatures are shown at the right ( â C ) . The small peaks are attributed to halloysite (H), goethite (G), plagioclase (P) and a-cristobalite (a-C) above shown. The isoelectric point shows the influence of dominant surface sites. The organic matter has active sites with low pKa values, consequently soils with high organic matter content show low IEP value. In well crystallized aluminosilicates the structural charge is more important than the pH dependent surface charge and soils dominated by those compounds will show low IEP values. Poorly crystallized aluminosilicates and iron oxides show variable surface charge, active surface sites dominated by A1-OH and Fe-OH and consequently high IEP values (between 8 to 9). The water lost due to heating, results in a decrease of its IEP values. After the above discussion, the IEP of soils depends on the organic matter content, the mineralogy and the soil hydration. Non-allophanic soils (Collipulli) have a low IEP value due to the presence of more stable crystalline aluminosilicates and iron oxides which are more important than their low organic matter content (Collipulli soil has 0.6% of organic carbon), and little or no change of the IEP is observed with heating (Table 1). In allophanic soils (Osorno, San Patricio), the IEP depends on organic matter content; as organic matter content increases the IEP decreases. A s result of heating from 124OC to 60ODC, two different types of reactions occur, the gradual destruction of organic matter (dehydration, dehydrogenation, decarboxylation and oxidation reactions; Table 1, Figure 11, and the dehydration and dehydroxylation of inorganic compounds. In allophanic soils 283 with high organic matter content the IEP increases as result of exposure of A1-OH and Fe-OH active surface sites; conversely, in allophanic soils with low organic matter content a decrease of IEP is observed as a result of dehydration and dehydroxylation of A1-OH and Fe-OH active surface sites. In both cases, due to similar mineralogy of allophanic soils, a IEP about 6 to 7 is obtained. After heating (600°C) an IEP of 6.7 was obtained for Osorno and San Patricio soil samples (Table 1) . The Mtlssbauer spectra at room temperatures for the studied soils showed appreciable amounts of quadrupole doublet together with a six-line envelope. As expected from the volcanic origen of the soils, the presence of magnetite .is supported by the broad peaks each one comprising two unresolved peaks of the 12 peaks of the magnetite spectrum ( 1 3 ) . The unresolved peaks also show the presence of hematite and probably goethite or ferrihydrite in small amounts ( 1 4 ) , if heating is not higher than 350°C because at this temperature important changes on ferrihydrite crystallinity occur, as observed in differential scanning calorimety (15). .rl,,r.%rl 1 COLLIPULLI . 4lOy 0 1 2 3 4 22 26 26 T I M E ( h ) Fig.2. Hydrogen production as a function of the reaction time of the WGSR and temperature of the catalyst preparation (Collipulli). 284 Fig . 3 . Hydrogen p roduc t ion as a f u n c t i o n of t h e r e a c t i o n t ime of t h e WGSR and tempera ture of t h e c a t a l y s t p r e p a r a t i o n (Osorno). H, Produced ( u M o l e ) I SAN PATRlClO T I M E ( h ) Fig . 4. Hydrogen p roduc t ion as a f u n c t i o n of t h e r e a c t i o n t ime of t h e WGSR and tempera ture of t h e c a t a l y s t p r e p a r a t i o n (San P a t r i c i o ) . The s o i l s samples, s i eved a t 1 mm, were hea ted a t t h e fo l lowing tempera tures , 124"C, 220"C, 29O"C, 350°C, 410"C, 500°C and 600°C t o g r a d u a l l y d e s t r o y t h e o rgan ic m a t t e r and a f t e r t h i s t r ea tmen t they w e r e used d i r e c t l y as c a t a l y s t s . S o i l samples hea ted a t 124°C showed p r a c t i c a l l y no o r g a n i c m a t t e r d e s t r u c t i o n . Conversely, i n t hose s o i l samples hea ted a t 600°C a lmost a l l t h e o r g a n i c matter w a s des t royed (Table 1). Thus, t h e r e i s a r e l a t i o n s h i p between 285 the heating temperature, the remaining organic matter content, the IEP (Table 1) and the catalytic activity observed (Figs. 2 - 4 ) . As a result of heating, a shifting of the IEP is observed depending on organic matter content, its destruction, and reaction of inorganic components (as explained above). As the heating temperature increases, the catalytic activity increases too, due to the organic matter destruction. A maximum in catalytic activity is observed at about 500°C for Collipulli and Osorno soils and at about 410°C for San Patricio soil where the best ratio between organic matter destruction and inorganic components dehydroxylation is reached (5). If the heating temperature is increased, the reactions of dehydroxylation and crystallization of inorganic components are more important than the organic matter destruction and a significant decreases of surface area is observed (Table l ) , consequently in all samples, a lower catalytic activity is observed. The WGSR was performed in basic media and mild conditions (KOH, 100°C and 0.9 atm CO). Good correlation was obtained for the CO consumed in the reaction and the H2 produced. However, C 0 2 was always detected in lower quantities which can be attributed to adsorption occurring on the catalyst or to some reactions with aqueous hydroxide to produce probably carbonate or formate. The catalytic systems studied showed an increasing catalytic activity during the first hour of reaction. After that time the activity as can be seen in Figs. 2 and 3 decreased significantly for Collipulli and Osorno soils. However, San Patricio soil maintained a high level of hydrogen production for a long time. As shown in Fig. 4 there is a limited temperature to obtain the highest hydrogen production and when the heating temperature of a soil preparation rises the hydrogen production decreases rapidly. Control experiments indicate no reaction in the absence of the catalyst. The activity of the catalyst decreases in neutral media and no activity at all is shown in the absence of KOH, which is a clear indication that OH- play a key role in the mechanism of the WGSR, probably in the interaction of CO, metal surface and OH- to generate C02. In order to compare the soil samples used as catalysts, specific Fez03 supported catalysts on A1203 were prepared at the same temperature of the soils. When the WGSR is carried out us ing these catalysts a similar pattern to that of the soil samples was observed. Under the experimental conditions used, the yields obtained in the WGSR catalyzed by the soil samples were comparable to those produced by the supported catalysts prepared. The differences found in catalytic activities can be explained by the different forms that Fe takes in the samples after the heating. Thus the Mussbauer spectra show magnetite as the dominant component in Collipulli soil and hematite as the dominant component in Osorno and San Patricio soils. However, for the San Patricio soil, the Mussbauer spectra shows a doublet 286 which indicates the presence of a component with a higher content of Fe(II1) which would be responsible for the greater catalytic activity shown by this soil. ACKNOWLEDGMENTS The authors express their gratitude to Direcci6n de Investigaciones Cienti- ficas y Tecnol6gicas of the Universidad de Santiago de Chile and to Fondo Na- cional de Investigaciones Cientrficas y Tecnol6gicas for financial support (grants 0899-90 and 0039-89). REFERENCES 1 T.H. Milliken, G.A. Mills and A.G. Oblad, Trans. Faraday SOC. 8 (1950) 279. 2 D.H. Solomon, B.C. Loft and J.D. Swift, Clay Miner. 7 (1968) 399. 3 F. Stoessel, J.L. Guth and R. Wey, Clay Miner., 12 (1976) 255. 4 D. Njopwuo, G. Roques and R. Wandji, Clay Miner., 22 (1987) 145. 5 M. Escudey and S.A. Moya, Colloids and Surfaces, 37 (1989) 141. 6 M. Escudey and G.G. Galindo, Colloid Interface Sci., 93 (1983) 78. 7 A. Mella and A. Kuhne, In J. Tosso (Ed.), Suelos Volc6nicos de Chile, INIA, 8 B. Bernas, Anal. Chem., 40 (1968) 1682. 9 M.D. Heilman, D.L. Carter and C.L. Gonzzlez, 100 (1965) 409. 10 R.J. Hunter, Zeta Potential in Colloid Science: Principles and 11 Ch. Urgermann, V. Landis, S.A. Moya, H. Cohen, H. Walker, R.G. Pearson, 12 S . A . Moya, A. Mansilla and F.J. Gil, Bull. SOC. Chim. Bel., 97 (1988) 9. 13 H.P. Weber and S.S. Hafner., 2. Krist., B-133 (1971) 327. 14 J.M. Bigham, D.C. Golden, L.H. Bowen, S.W. Buol and S. B. Weed, Soil Sci. SOC. Am. J., 42 (1978) 816. 15 M. Escudey, Unpublished results. Santiago, 1988 p. 548. Applications, Academic Press, London, (1981) p. 59. R.G. Rinker and P.C. Ford, J. Am. Chem. SOC., 101 (1979) 5922. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 281 THERMAL STABILITY, ACIDITY AND CRACKING PROPERTIES OF PILLARED RECTORITE CATALYSTS M A R I O L. OCCELLI Unocal Corpora t ion , Science & Technology D i v i s i o n P. 0. Box 76, Brea, C a l i f o r n i a 92621 ABSTRACT X-ray d i f f r a c t i o n (XRD), p y r i d i n e chemisorp t ion , and m i c r o a c t i v i t y t e s t (MAT) r e s u l t s have been used t o c h a r a c t e r i z e a sample o f n a t u r a l r e c t o r i t e p i l l a r e d w i t h alumina c l u s t e r s . p roduc t was ob ta ined t h a t a f t e r d r y i n g a t 10O0C/10h had d(001) spac ing o f 28.7 A. a t 800°C/5h o r a f t e r steam ag ing a t 760°C/5h w i t h steam a t 1 atm. p i l l a r e d r e c t o r i t e s have thermal and hydrothermal s t a b i l i t y comparable t o t h a t o f z e o l i t e s w i t h t h e F a u j a s i t e s t r u c t u r e . Steam-aged p i l l a r e d r e c t o r i t e s , a t MAT c o n d i t i o n s , have c r a c k i n g a c t i v i t y s i m i l a r t o t h a t o f a commercial f l u i d i z e d c r a c k i n g c a t a l y s t (FCC) and can be regenera ted w i t h ease. w i l l have t o be d r a s t i c a l l y improved b e f o r e these c l a y s can 6ompe?e w i t h z e o l i t e s i n c rack ing c a t a l y s t p repara t i on . A f t e r r e a c t i o n w i t h c h l o r h y d r o l , a p i l l a r e d The p i l l a r e d r e c t o r i t e r e t a i n e d i t s s t r u c t u r e even a f t e r c a l c i n i n g i n a i r Thus, However, t h e i r coke and l i g h t gas (hc, CH ) s e l e c t i v i t y INTRODUCTION A l though p i l l a r e d c l a y s c o u l d genera te low c o s t f l u i d i z e d c r a c k i n g c a t a l y s t s (FCC) w i t h unique s e l e c t i v i t y p r o p e r t i e s , t hey have n o t y e t been accepted by t h e pe t ro leum i n d u s t r y . f i e l d t e s t these new c a t a l y s t s because, i n a d d i t i o n t o a h i g h tendency f o r coke genera t ion , t hey e x h i b i t hydrothermal s t a b i l i t y i n f e r i o r t o t h a t o f those z e o l i t e s used i n hydrocarbon convers ion processes. p r o p e r t i e s o f p i l l a r e d c l a y s have been reviewed elsewhere (1,2). sample o f n a t u r a l r e c t o r i t e w i t h po l yoxoca t ion o f aluminum o r z i rcon ium, i t i s p o s s i b l e t o o b t a i n p i l l a r e d c l a y w i t h hydrothermal s t a b i l i t y t y p i c a l of z e o l i t e s and z e o l i t e - c o n t a i n i n g f l u i d c r a c k i n g c a t a l y s t s ( 3 ) . i n t e r s t r a t i f i e d l aye red s i l i c a t e m ine ra l c o n s i s t i n g o f a r e g u l a r s t a c k i n g of m i c a - l i k e and m o n t m o r i l l o n i t e - l i k e l a y e r s ( 4 ) . w e l l as t h e i r s t a c k i n g sequence i s d i f f i c u l t t o p r e d i c t because o f t h e v a r i a t i o n found w i t h i n and between samples. Kodama ( 5 ) has r e p o r t e d t h a t t h e m i c a - l i k e l a y e r s a r e s i m i l a r t o pa ragon i te whereas t h e m o n t m o r i l l o n i t e - l i k e l a y e r s can have a b e i d e l l i t e cha rac te r . Probable s t a c k i n g sequences o f these In f a c t , r e f i n e r s ( t o da te ) have been r e l u c t a n t t o The physicochemical Recent ly , J. Guan and co-workers ( 3 ) have r e p o r t e d t h a t by p i l l a r i n g a R e c t o r i t e i s an The n a t u r e o f t h e l a y e r s as 288 two types o f e lementary l a y e r s have been s t u d i e d u s i n g XRD methods (6,7). A ( s i m p l i f i e d ) schematic r e p r e s e n t a t i o n o f t h e r e c t o r i t e (and r n o n t m o r i l l o n i t e ) s t r u c t u r e i s shown i n F ig . 1. 0 0 =< :.:: :-: T 0 T T 0 T F($ :.:: :.$ T 0 T F ig . 1. Schematic r e p r e s e n t a t i o n o f t h e m o n t m o r i l l o n i t e (M) and r e c t o r i t e (R) s t r u c t u r e . represented by t rapezo ids and rec tang les . ( f i x e d ) charge compensating c a t i o n s a r e represented by open and s o l i d c i r c l e s . The T-0-T 3 - laye rs sequence (T= te t rahedra l , O=octahedral) i s Exchangeable and non-exchangeable T h i s paper descr ibes t h e physicochemical p r o p e r t i e s o f n a t u r a l r e c t o r i t e samples p i l l a r e d w i t h aluminum ch lo rhyd rox ide (ACH) s o l u t i o n s c o n t a i n i n g (AlI3) t h e [Al130,(OH)~,(H20)12]+7 c a t i o n . S t a b i l i t y , a c i d i t y , c r a c k i n g a c t i v i t y and p roduc t s e l e c t i v i t i e s f rom gas o i l convers ion w i l l be compared t o those o f s i m i l a r l y p repared p i l l a r e d m o n t m o r i l l o n i t e c a t a l y s t s . EXPERIMENTAL Two r e c t o r i t e samples f rom Garland County, Arkansas were ob ta ined f rom t h e C lay M ine ra l S o c i e t y Repos i to ry . The samples c o n s i s t e d o f q u a r t z aggregates c o n t a i n i n g 10-20% r e c t o r i t e . P a r t i c l e s we igh ing about 259 were g e n t l y crushed, t r a n s f e r r e d t o a 1 l i t e r p l a s t i c beaker, and then d i spe rsed i n d i s t i l l e d water u s i n g a 3 m inu te u l t r a s o n i f i c a t i o n t rea tment . a l l owed t o s e t t l e f o r 10 minutes, a f t e r which t ime t h e c l a y - s i l t s l u r r y was decanted i n t o 250 m l c e n t r i f u g e b o t t l e s . minu tes a t 650 RPM u s i n g an I .E.C. Model K c e n t r i f u g e t o separa te t h e < 2 - m c lay . The coarse p a r t i c l e s were The samples were c e n t r i f u g e d f o r 5 The 289 another vessel by f l o c c u l a t i o n w i t h 0.5M MgC12. A f t e r m u l t i p l e washes w i t h d i s t i l l e d wa te r t o remove t h e 290 prepared by p r e s s i n g samples between 25 mm d iameter d i e f o r one minute a t .6,000 - 7,000 pound pressure . mounted i n an Abspec I n s t . Corp. #2000 o p t i c a l c e l l and degassed by h e a t i n g a t 200°C f o r 2h a t vacuo) i n t h e 200-500°C tempera ture range. r e g i o n were smoothed w i t h a f i v e p o i n t s Sav i tzky-Go lay a l g o r i t h m and b a s e l i n e s lope co r rec ted ; peak i n t e n s i t i e s were normal ized t o t h e sample dens i t y . P r i o r t o p y r i d i n e s o r p t i o n , t h e wafers were The py r id ine - loaded wafers were then heated ( i n t o r r . Spec t ra o f t h e 0-H s t r e t c h i n g RESULTS AND D I S C U S S I O N Thermai P r o p e r t i e s I r r e s p e c t i v e o f t h e i r i r o n conten t , t h e two r e c t o r i t e samples reac ted w i t h Ch io rhyd ro l t o fo rm a p i l l a r e d p roduc t t h a t a f t e r d r y i n g i n a i r a t 10O0C/10h had d(001) spac ing o f 28.7 A. d(001) va lue t o 28.0 A p robab ly due t o p a r t i a l dehydroxy la t i on o f t h e Al13- p i l l a r s , F ig . 2. These c a l c i n e d r e c t o r i t e s had BET su r face area i n t h e 160-200 0 C a l c i n a t i o n i n a i r a t 40OoC/10h reduced t h e 0 200 - 175 I . - ~. a 200 175 , 150 125 100 75 50 25 D ~ ~ ~ L - ~ ~ ' - ' J J ~ ~ K K , ,&/ < , 2 6 I 0 14 18 22 26 30 34 38 42 46 0 2 4 6 8 10 1 2 1 4 1 TWO.THETA (DEG) TWO.THETA (DEG) F ig . 2. X-ray d i f f r a c t o g r a m s o f a sample F ig . 3. X-ray d i f f r a c t o g r a m s o f a o f n a t u r a l r e c t o r i t e b e f o r e (A) and a f t e r sample o f M g - r e c t o r i t e b e f o r e (A) ( 6 ) b e n e f i c i a t i o n and p i l l a r i n g f o l l o w e d and a f t e r p i l l a r i n g w i t h c h l o r - by c a l c i n a t i o n a t : C ) 400°C i n a i r f o r hydro1 and h e a t i n g a t : B) 400°C/ 10h; 0) 1000"C/ lhr and E ) 1200"C/lh. 10h i n a i r ; C ) 800°C/5h i n a i r and D ) 760°C/5h w i t h steam a t 1 atm. 2 2 m /g range. The su r face area f rom mercury po ros ime t ry measurement was o n l y 6.2 m / g i n d i c a t i n g a w e l l o rdered p i l l a r e d s t r u c t u r e c h a r a c t e r i z e d by a l o n g range s t a c k i n g o f s i l i c a t e l a y e r s face- to - face . t h e 001 and 002 r e f l e c t i o n s (F ig . 2 ) suggest t h a t t h e mica- and m o n t m o r i l l o n i t e l i k e l a y e r s a r e p robab ly s tacked i n a nea r r e g u l a r manner i n a 1 : l r a t i o . The h i g h i n t e n s i t y and sharpness o f 291 P i l l a r e d r e c t o r i t e s have thermal and hydro thermal s t a b i l i t y much s u p e r i o r t o t h a t o f s i m i l a r l y prepared m o n t m o r i l l o n i t e s and h e c t o r i t e c a t a l y s t s . I n f a c t , ACH-rec tor i tes r e t a i n t h e i r p i l l a r e d s t r u c t u r e even a f t e r c a l c i n a t i o n i n a i r a t 800°C/5h o r a f t e r steam-aging w i t h 100% steam a t 760°C/5h, F ig . 3. temperature (800°C) c a l c i n a t i o n o r steaming has l i t t l e e f f e c t on t h e shape and i n t e n s i t y o f t h e c l a y 001 and 002 r e f l e c t i o n s , F i g . 3. va lue decreases t o 26.8 A owing t o t o t a l dehydroxy la t i on o f t h e p i l l a r s . i s l i t t l e apparent r e a c t i o n between t h e Al I3-pi l lars and steam; a f t e r steaming t h e ACH-rec tor i te d(001) was 26.5 A. when exposed t o steam a t 760°C, c o l l a p s e i n l e s s than 2h. The excep t iona l s t a b i l i t y o f ACH-rec tor i tes i s p robab ly due t o t h e robus t m i c a - l i k e l a y e r s p resen t between t h e expanded ( p i l l a r e d ) montmori l l o n i t e - l i k e l a y e r s , see F ig . 1. High A t 800°C t h e d(001) 0 There 0 I n c o n t r a s t , p i l l a r e d m o n t m o r i l l o n i t e s , The thermograv imet r ic (TGA) p r o f i l e i n F ig . 4A shows t h a t a f t e r l o s i n g about 1% sur face water , t h e a i r d r i e d pa ren t r e c t o r i t e we igh t remains e s s e n t i a l l y unchanged up t o about 400°C. an a d d i t i o n a l 5% we igh t loss. I n c o n t r a s t , t h e p i l l a r e d r e c t o r i t e s we igh t decreases mono ton ica l l y w i th temperature due t o l osses o f water sorbed on t h e e x t e r n a l su r face and i n t h e microspace generated by p i l l a r i n g , F ig . 4B. The Between 400°C and 800°C, dehydroxy la t i on induces h a t p i l l a r s dehydroxy la t i on occur m a i n l y t h e r e i s a 0.5-1.0% weigh t change ng f rom t h e c r y s t a l l a t t i c dehydroxyla- d e r i v a t i v e s o f t h e TGA curve suggest between 450°C and 560°C. Above 560°C a t t r i b u t e d t o removal o f water r e s u l t t i o n . t 100 200 300 400 500 600 700 800 TEMPERATURE ("C) c , 1 0 200 400 600 800 1000 1200 TEMPERATURE ("C) Fig . 4. Thermograv imet r ic curves o f a sample o f M g - r e c t o r i t e b e f o r e (A) and a f t e r (B) p i l l a r i n g w i t h c h l o r h y d r o l . F ig . 5. D i f f e r e n t i a l thermal a n a l y s i s curves o f a sample o f M g - r e c t o r i t e b e f o r e ( A ) and a f t e r p i l l a r i n g w i t h c h l o r h y d r o l . 292 The cor respond ing d i f f e r e n t i a l thermal a n a l y s i s (DTA) p r o f i l e o f t h e s t a r t i n g M g - r e c t o r i t e e x h i b i t s a weak and broad endotherm between 400°C and 700°C r e p r e s e n t i n g l a t t i c e dehydroxy la t ion , F ig . 5A. Near 850°C t h e r e i s t h e beg inn ing o f a second endotherm a t t r i b u t e d t o t h e c o l l a p s e o f t h e r e c t o r i t e s t r u c t u r e w i t h q u a r t z fo rmat ion . A f t e r endotherms w i t h peak minima near 100°C and 480°C r e p r e s e n t i n g wa te r l osses due t o sorbed wa te r and p i l l a r s dehydroxy- l a t i o n , t h e DTA cu rve o f t h e ACH-rec tor i te sample remains f a i r l y f e a t u r e l e s s up t o l O O O " C , F ig . 56. Then, near 1000°C t h e r e i s an exotherm f o l l o w e d by a sharp endotherm r e p r e s e n t i n g t h e c o l l a p s e o f t h e ACH-rec tor i te w i t h fo rma t ion o f m u l l i t e and q u a r t z toge the r w i t h an amorphous res idue and smal l amounts o f gamma-A1203, F ig . 2D. F ig . 2E. A t 1200°C a w e l l c r y s t a l l i z e d m u l l i t e phase i s formed, I n f r a r e d A n a l y s i s OH s t r e t c h i n g reg ion , F ig . 6A. a t t r i b u t e d t o s t r e t c h i n g v i b r a t i o n s o f OH groups i n t h e c l a y oc tahedra l l a y e r s (10) . However, when t h e degass-ing tempera ture i s p r o g r e s s i v e l y inc reased t o 500°C f rom 2OO"C, t h e band a t 3654 cm-l mono ton ica l l y decreased i n i n t e n s i t y owing t o dehydroxy la t i on r e a c t i o n s . a t t r i b u t e d t o OH groups assoc ia ted w i t h Si-OH-A1 l i nkages i n t h e c l a y t e t r a h e d r a l l a y e r r e s u l t i n g f rom s u b s t i t u t i o n o f S i w i t h A l . shou lder on t h e low frequency s i d e of t h i s band probab ly generated by pe r tu rbed OH v i b r a t i o n s induced by t h e presence o f charge compensating c a t i o n s on t h e s i l i c a t e l a y e r s (10). t h e OH-region s i m i l a r t o t h a t o f t h e pa ren t m a t e r i a l ; as b e f o r e t h e r e i s l i t t l e apparent i n t e r a c t i o n between these OH and p y r i d i n e , F ig . 6B. However, a f t e r p i l l a r i n g , dehydroxy la t i on o f t h e exposed l a y e r s becomes more f a c i l e and a l a r g e r e d u c t i o n i n i n t e n s i t y o f t h e h i g h f requency band occurs when t h e degassing tempera ture inc reases t o 400°C from 300"C, F ig . 6B. The shou lder on t h e low f requency s i d e o f t h e 3470 cm-l band becomes now more pronounced. be fore , these hyd roxy l s do n o t r e a c t w i t h p y r i d i n e suggest ing t h a t t hey a r e mos t l y assoc ia ted w i t h t h e m i c a - l i k e l a y e r s p resen t i n r e c t o r i t e . a t 3430 cm-l broadens w i t h i n c r e a s i n g degassing temperature and d isappears a t 400°C, F ig . 66. A t t h i s temperature, TGA and DTA r e s u l t s i n d i c a t e t h a t most o f t h e z e o l i t i c wa te r i n t h e p i l l a r e d s t r u c t u r e has been removed, F igs . 4,5. Thus, S i (1V)-0-A1 ( I V ) g roup ings a r e ( i n p a r t ) charge compensated a l s o by r e s i d u a l mono and d i v a l e n t c a t i o n s p resen t on t h e s i l i c a t e l a y e r s and t h e i n t e n s i t y o f t h e band near 3470 cm-' decreases, F ig . 68. L i k e b e i d e l l i t e (8,9), t h e pa ren t r e c t o r i t e s e x h i b i t two bands i n t h e The i n t e n s e band centered near 3654 cm-l i s When exposed t o p y r i d i n e , t h e r e i s no n o t i c e a b l e change i n t h i s band. The second, l e s s i n t e n s e band a t 3468 cm-l i s There i s a weak A f t e r p i l l a r i n g w i t h Ch lo rhyd ro l , t h e ACH- rec to r i t e g i ves an I R spectrum i n As The shou lder The h i g h thermal 293 c 4 U P 4 3 Y 0 U c 0 2 3 3700 3600 3500 3400 3300 3200 3800 3700 3600 3500 3400 3300 3200 3648 3 3700 36W 3500 3400 3330 3200 WAVENUMBERS (CM ~ 1) 3666 w 0 z m U a 5: a 3800 3700 3600 3500 3400 3300 3200 WAVENUMBERS (CM-') F ig . 6. a f t e r p i l l a r i n g w i t h c h l o r h y d r o l and h e a t i n g a t : 800°C/5h i n a i r and D) 760°C/5h w i t h steam a t 1 atm. d r i e d a t 200°C and then loaded w i t h p y r i d i n e and degassed a t : 300"C, d ) 400°C and e ) 500°C i n vacuo f o r 2 hours a t each temperature. s t a b i l i t y o f these hyd roxy l s has been a t t r i b u t e d t o t h e f a c t t h a t t h e charge compensating p r o t o n i s h e l d between an oxygen and an OH group on an A l (V1 ) i o n Hydroxyl abso rp t i on bands f o r a sample o f M g - r e c t o r i t e b e f o r e (A) and B ) 400°C/10h i n a i r , C) Samples ( a ) have been b ) 200°C, c ) (10). The TGA p r o f i l e i n F ig . 4 i n d i c a t e s t h a t a t 800°C we igh t losses due t o s t r u c t u r a l water removal a r e e s s e n t i a l l y completed; t h e r e s i d u a l hyd roxy l s g i v e two weak bands a t 3648 cm-l and 3473 cm-' c h a r a c t e r i z e d by a l a c k o f r e a c t i v i t y w i t h p y r i d i n e and decreased r e s i s t a n c e t o dehydroxy la t ion , F i g . 6C. A f t e r steaming, t h e ease o f dehydroxy la t i on o f t h e c l a y oc tahedra l l a y e r s i s s i m i l a r t o t h a t observed a f t e r c a l c i n a t i o n a t 800°C; a t 500°C t h e h i g h f requency band ( a t 3666 cm- ) d isappears, F ig . 6D. 1 Steaming d i d n o t change t h e 294 Y 0 z rn 8 U , , I , 18W ISSO 15W 1450 14W Y s &% 8 U 1WO 1550 1500 1450 1400 WAVENUMBERS (CM - I) Fig . 7. and a f t e r p i l l a r i n g w i t h c h l o r h y d r o l and h e a t i n g a t B) 400°C/10h i n a i r , C) 800°C/5h i n a i r and D) 760°C/5h w i t h steam. vacuo a t : temperature. I R spec t ra o f p y r i d i n e sorbed on a sample o f M g - r e c t o r i t e be fo re (A) Samples have been degassed i n a ) 200"C, b ) 300"C, c ) 400°C and d ) 500°C f o r two hours a t each r e a c t i v i t y o f these hyd roxy l s w i t h p y r i d i n e . d i s p l a c e d charge compensating c a t i o n s f rom A l (1V) i n t h e s i l i c a t e l a y e r w i t h fo rma t ion o f new S i ( 1 V ) - O H - A l ( 1 V ) groupings. I R spec t ra i n t h e 1400-1600 cm-l reg ion , ob ta ined by evacua t ing t h e p y r i d i n e loaded c a l c i n e d ACH-rec tor i tes i n t h e 200-500°C temperature range, a r e shown i n F igs . 7A-D. F ig . 7A. r e c t o r i t e sample g i v i n g an I R spectrum c o n t a i n i n g an i n tense band a t 1452 CITI-~ t y p i c a l o f p y r i d i n e coo rd ina ted t o Lewis ( L ) a c i d s i t e s (11 ) . cen tered near 1546 cm-' has been a t t r i b u t e d t o p y r i d i n i u m i o n s fo rmat ion , t h a t i s , t o t h e presence o f Brons ted ( B ) a c i d s i t e s (11) . has been a t t r i b u t e d t o t h e presence o f b o t h B and L a c i d s i t e s (11 ) . degassing a t 300, evidence o f Brons ted a c i d i t y i s l o s t . I n c o n t r a s t t o s i m i l a r l y p i l l a r e d m o n t m o r i l l o n i t e s , p y r i d i n e i s e s s e n t i a l l y removed f rom ACH-rec tor i te a f t e r degassing a t 50OoC/2h, F ig . 78. by a f a c t o r o f e i g h t and most o f t h e p y r i d i n e can be desorbed a t 300°C, F ig . 7C. The I R spec t ra i n F igu re 7D i n d i c a t e t h a t steaming ( a t 760°C/5h) inc reases t h e ACH-rec tor i te Brons ted t ype a c i d i t y . A l (1V) must be p resen t a l s o i n t h e m o n t m o r i l l o n i t e - l i k e l a y e r s and t h e r e f o r e new S i (1V)-OH-A1 ( I V ) g roup ings a r e formed when ACH-rec tor i te i s steam-aged. Steaming d r a s t i c a l l y reduced Lewis t y p e a c i d i t y as w e l l as a c i d s i t e s t reng th ; It i s b e l i e v e d t h a t steaming The p a r e n t r e c t o r i t e does n o t sorb s i g n i f i c a n t amounts o f p y r i d i n e , I n c o n t r a s t , p y r i d i n e i s r e a d i l y sorbed i n a s i m i l a r l y d r i e d ACH- The weak band The band near 1490 cm-' A f t e r I n t h e ACH- rec to r i t e sample c a l c i n e d i n a i r a t 800°C/5h, a c i d i t y i s reduced Some s u b s t i t u t i o n o f S i (1V) w i t h 295 a t 300°C most o f t h e p y r i d i n e desorbed f rom t h e c l a y sample, F ig . 7D. I n summary, l i k e ACH-montmori l loni tes, p i l l a r e d r e c t o r i t e con ta ins b o t h Brons ted and Lewis a c i d s i t e s , and a c i d i t y i s m o s t l y o f t h e Lewis type. A c i d s i t e d e n s i t y as w e l l as a c i d s i t e s t r e n g t h i n ACH-rec tor i te i s l e s s than t h a t observed i n s i m i l a r l y prepared p i l l a r e d m o n t m o r i l l o n i t e s . Gas O i l Crack ing i n t o aggregates o f f l a k e s resembl ing mica p a r t i c l e s . ob ta ined a l s o a f t e r d r y i n g t h e p roduc t o f t h e p i l l a r i n g r e a c t i o n . e v a l u a t i o n f o r gas o i l c r a c k i n g a c t i v i t y , t h e p i l l a r e d r e c t o r i t e samples were crushed and s i z e d i n t o 20x60 mesh ( f l a k e - l i k e ) p a r t i c l e s and ca l c ined . chemical compos i t ion o f these c l a y c a t a l y s t s i s g i ven i n Table I. A f t e r separa t i on f rom i t s q u a r t z m a t r i x , t h e b e n e f i c i a t e d r e c t o r i t e d r i e s S i m i l a r m a t e r i a l s a r e P r i o r t o Typ ica l The p a r e n t r e c t o r i t e i s e s s e n t i a l l y i n a c t i v e , Tab le 2. However, a f t e r r e a c t i n g w i t h Ch lo rhyd ro l and c a l c i n a t i o n i n a i r a t 40OoC/10h, a p i l l a r e d p roduc t w i t h c r a c k i n g a c t i v i t y t y p i c a l o f z e o l i t i c f l u i d c r a c k i n g c a t a l y s t (FCC) and o f s i m i l a r l y p i l l a r e d m o n t m o r i l l o n i t e s i s ob ta ined, Tables 2,3. m i c a - l i k e p a r t i c l e s have a b u l k d e n s i t y t h a t i s l e s s than 50% t h a t o f ACH- b e n t o n i t e g ranu les w i t h s i m i l a r s i ze , Tab le 2. Thus, f o r a g i v e n c a t / o i l r a t i o l onger o i l - c a t a l y s t c o n t a c t t imes a r e ob ta ined when c r a c k i n g gas o i l s a t MAT c o n d i t i o n s w i t h ACH-rec tor i tes . r e c t o r i t e w i t h o n l y 160-190 m /g su r face area can be as a c t i v e f o r gas o i l convers ion as p i l l a r e d b e n t o n i t e s w i t h su r face area near 300 m /g. Greater LCGO y i e l d s a r e ob ta ined by c r a c k i n g more o f t h e heavy s l u r r y o i l (SO) f r a c t i o n , Table 2. The coke make i s a lmost t w i c e as l a r g e as t h a t o f a z e o l i t i c FCC (Dav ison ' s GRZ-1), which i s m a i n l y t h e r e s u l t o f t h e c l a y s t rong Lewis t y p e a c i d i t y . enhanced by t h e presence o f i r o n . inc reases t o 0.151 f rom 0.146 when t h e t o t a l i r o n con ten t o f t h e p i l l a r e d r e c t o r i t e inc reases t o 1.34% Fe203 f rom 0.80% Fe203. c rack ing r e a c t i o n s thus i n c r e a s i n g H2, d r y gas, C3 and C4 genera t i on a t t h e expense o f g a s o l i n e y i e l d s , Table 2. d e t a i l s elsewhere (13). does n o t exceed 800°C. I n c o n t r a s t , t h e ACH-rec tor i tes under s tudy a f t e r c a l c i n a t i o n a t 8OO0C, r e t a i n t h e i r p i l l a r e d s t r u c t u r e (F ig . 3 ) and more than 90% o f t h e i r i n i t i a l su r face area. As a r e s u l t , a f t e r c a l c i n i n g a t 800°C/5h, o n l y minor changes i n t h e c r a c k i n g p r o p e r t i e s o f t h e two ACH-rec tor i tes a r e observed, Table 2. The As a r e s u l t , a mixed l a y e r c l a y such as 2 2 The ACH-rec tor i te p roduc t s e l e c t i v i t y i s t y p i c a l o f p i l l a r e d c l a y c a t a l y s t s . The ACH-rec tor i te tendency f o r h i g h coke make seems t o be I n f a c t , t h e c l a y coke/conversion r a t i o n I r o n ca ta l yzes secondary The e f f e c t s o f i r o n a r e desc r ibed i n P i l l a r e d m o n t m o r i l l o n i t e (and h e c t o r i t e s ) have thermal s t a b i l i t y i n a i r t h a t 296 TABLE 2 M i c r o a c t i v i t y Tes t (MAT) Resul ts f o r R e c t o r i t e s and Mon tmor i l l on i tes P i l l a r e d w i t h A1?03-Clusters and Calc ined i n A i r ACH-Bentonites ACH-Rectorites Calc ined a t Calc ined a t Mg - R e c t o r i t e 40OoC/10h 800" C/ 5 h 400°C/10h I r o n (%Fe 0 ) 1.34 0.69% 0.98% 0.69% 0.98% 0.3% 3.4% Conversioz f V % FF) 24.9 87.1 86.5 86.3 85.2 85.7 86.7 Gasol ine ( V % FF) 10.3 63.7 58.5 59.5 60.3 56.8 58.4 LCGO ( V % FF) 35.1 11.8 12.3 12.5 13.5 12.8 11.7 SO ( V % FF) 40.0 1.2 1.2 1.2 1.3 1.4 1.6 C ( V % FF) 0.5 1.3 ?= ( V % FF) 1.4 6.3 n-?4 ( v% FF) 0.2 0.6 C- ( V % FF) -1.2 3.0 CH: (Wt% FF) 0.69 1.24 H (SCF/BBL) 319 416 06y Gas (Wt% FF) 2.3 2.6 Coke ( W t % FF) 3.2 12.7 Co ke/Converf ion 0.128 0.146 BET S.A. (m /g) 10 182 Dens i t y (g /cc) 0.57 0.65 i - C 4 ( V % FF) 0.2 4.5 1.7 7.9 0.6 5.3 3.6 0.94 510 3.5 13.1 0.151 167 0.60 1.7 7.5 0.8 6.1 4.0 0.91 471 3.2 12.6 0.146 174 - 1.3 2.4 6.0 11.4 0.7 1.2 4.7 7.6 3.5 6.8 1.38 0.42 527 424 2.7 4.9 13.1 10.0 0.154 0.117 149 314 - 1.25 3.3 6.9 1.4 7.2 3.2 0.30 324 4.7 11.5 0.133 298 1.20 T y p i c a l l y , t he p i l l a r e d s t r u c t u r e o f mon tmor i l l on i tes can be dest royed I n a d d i t i o n t o a h i g h thermal s t a b i l i t y i n a i r , t h e ACH-rector i te under e i t h e r by hea t ing a t 675°C f o r 10h o r a t 730°C f o r 2h i n presence o f steam (12) . s tudy r e t a i n most o f t h e i r su r face and c rack ing p r o p e r t i e s even a f t e r steam- aging a t 760°C/5h w i t h 100% steam i n a f l u i d i z e d bed, Table 3. Steaming may have a f f e c t e d the i r o n d i s t r i b u t i o n ( m i g r a t i o n ) i n t h e c l a y s i l i c a t e l aye rs . As a r e s u l t , lower l i g h t gas y i e l d s a re obta ined and the coke/conversion r a t i o decreases t o 0.120 from 0.146 when t h e thermal t reatment i n a i r a t 800°C/5h i s rep laced by steam-aging a t 76OoC/5h, Table 3. In Table 3 i t i s shown t h a t a steam-aged (76OoC/5h) p i l l a r e d r e c t o r i t e has c rack ing a c t i v i t y comparable t o t h e one o f a s i m i l a r l y steam-aged commercial FCC (Davison 's GRZ-1) c o n t a i n i n g an est imated 35% c a l c i n e d ra re -ea r th exchanged z e o l i t e Y (CREY). types o f c a t a l y s t s have s i m i l a r gaso l i ne s e l e c t i v i t y . However, t he ACH- r e c t o r i t e o f f e r t h e advantage o f h ighe r LCGO y i e l d s by c rack ing more o f t he heavy hydrocarbons i n t h e s l u r r y o i l (SO) range, Table 3. The lower o l e f i n s make o f the z e o l i t i c FCC i s a t t r i b u t e d t o i t s h i g h ra re -ea r th c a t i o n s content which a re known t o f a v o r hydrogen t r a n s f e r reac t i ons , Table 3. Al though the two c a t a l y s t s have s i m i l a r BET su r face areas (161 m /g vs. 155 m /g) a f t e r steam-aging, a t t he same convers ion l e v e l (.85%) they e x h i b i t t o t a l l y d i f f e r e n t As p r e v i o u s l y repo r ted (12) , a t h i g h convers ion t h e two 2 2 297 TABLE 3 M i c r o a c t i v i t y Tes t Resu l t s f o r P i l l a r e d R e c t o r i t e C a t a l y s t s Con ta in ing 0.69% The z e o l i t i c c r a c k i n g c a t a l y s t and t h e p i l l a r e d c l a y s have been aged :E?Og'h ours a t 760°C w i t h 100% steam a t 1 atm. Reqenerated Zeol i t i c ACH-kectori t e s * C a t a l y s t R e c t o r i t e 760°C 815°C Crack ing ACH- Conversion ( V % FF) Gaso l ine ( V X FF) LCGO ( V % FF) SO ( V % FF) C ( V % FF) ?= ( V % FF) n-?4 ( v % FF) i - C 4 ( V % FF) C i ( V % FF) CH4 (Wt% FF) H (SCF/BBL) D?y Gas (Wt% FF) Coke (Wt% FF) Co ke/Convergion BET S.A. (m / g ) Dens i t y (g /cc ) 85.4 59.1 9.8 4.8 4.7 7.1 2.0 8.5 2.4 0.26 356 5.6 6.5 0.076 161 0.89 85.2 59.5 13.2 1.6 2.2 9.3 1.0 7.4 4.6 0.40 227 3.2 10.2 0.120 155 0.54 87.2 59.8 11.7 1.0 1.7 7.2 0.9 6.1 3.9 0.47 254 2.9 10.3 0.118 157 0.55 60.9 43.7 28.5 10.7 0.9 6.3 0.5 3.9 5.2 0.77 214 1.9 4.3 0.071 47 0.58 * Samples have been f i r s t heated i n a i r a t 700"C/ lh and then steam-aged i n a f l u i d i z e d bed f o r 5h w i t h 100% steam a t 1 atm a t t h e temperature i nd i ca ted . carbon s e l e c t i v i t i e s , Table 3. The ACH- rec to r i t e coke/conversion r a t i o o f 0.120 w i l l have t o be decreased by about 50% b e f o r e these m a t e r i a l s can be cons idered f o r i n t r o d u c t i o n i n t o a f l u i d i z e d c r a c k i n g u n i t (FCCU). Another d i s t i n g u i s h i n g f e a t u r e o f ACH- rec to r i t e c a t a l y s t s i s t h e ease w i t h which spent m a t e r i a l s can be regenerated. ACH-rec tor i te ( n o t shown) shows a 2.8 w t % l o s s between 450 and 750°C which i s w e l l i n agreement w i t h a 2.6% carbon con ten t de termined by chemical ana lys i s . The cor respond ing DTA p r o f i l e ( n o t shown) i s c h a r a c t e r i z e d by a b road exotherm between 450°C and 700°C w i t h peak maximum near 550°C. Thus, by c a l c i n i n g i n f l o w i n g a i r a t 700"C/lh, a ca rbon- f ree ACH- rec to r i t e w i th BET s u r f a c e area o f 153 m2/g was ob ta ined; su r face p r o p e r t i e s remained e s s e n t i a l l y unchanged a f t e r a second steam-aging (76OoC/5h) t rea tment . The r e t e n t i o n o f d-spacing and i n t e n s i t y o f t h e 001 and 002 r e f l e c t i o n s i n d i c a t e t h a t t h e o x i d a t i v e decomposi t ion o f carbonaceous depos i t s f o l l o w e d by steaming d i d n o t a f f e c t t h e ACH-rec tor i te p i l l a r e d s t r u c t u r e , F ig . 8. The steam-aging tempera ture had t o be inc reased t o 815°C ( f r o m 760°C) i n o r d e r t o a f f e c t t h e c a t a l y s t su r face area and c r y s t a l l i n i t y , F ig . 8C. The TGA p r o f i l e i n a i r o f a spent 298 0 2 4 6 8 10 12 14 16 18 TWO-THETA (DEG) Fig . 8. X-ray d i f f r a c t o g r a m s o f ACH-rec tor i te : A ) a f t e r steam-aging a t 760°C/5h. A f t e r MAT e v a l u a t i o n , t h e spent c a t a l y s t was regenera ted a t 700"C/lh i n a i r and steamed a t : B) 760°C and C) 815°C f o r 5h. SUMMARY Na tu ra l r e c t o r i t e s p i l l a r e d w i t h alumina c l u s t e r s have thermal as w e l l as hydrothermal s t a b i l i t y f a r s u p e r i o r t o t h a t o f s i m i l a r l y p repared r n o n t m o r i l l o n i t e c a t a l y s t s . w i t h t h e F a u j a s i t e s t r u c t u r e . i n d i c a t e d t h a t these m a t e r i a l s c o n t a i n b o t h B and L a c i d s i t e s and t h a t a t c r a c k i n g c o n d i t i o c s a c i d i t y i s e s s e n t i a l l y o f t h e L-type. ( 760°C/5h) p i 1 l a r e d r e c t o r i t e s a t MAT c o n d i t i o n s have c r a c k i n g a c t i v i t y comparable t o t h a t o f s i m i l a r l y steam-aged commercial FCC c o n t a i n i n g an es t ima ted 35% CREY. Coke s e l e c t i v i t y (as w e l l as p a r t i c l e d e n s i t y ) w i l l have t o be improved f o r ACH-rec tor i tes t o compete w i t h z e o l i t e - c o n t a i n i n g FCC. T h e i r s t a b i l i t y i s comparable t o t h a t o f z e o l i t e s P y r i d i n e chemisorp t ion exper iments have Steam-aged ACKNOWLEDGMENTS The many u s e f u l d i scuss ions and suppor t rece ived f rom t h e Unocal A n a l y t i c a l Department s t a f f a r e g r a t e f u l l y acknowledged. Spec ia l thanks a r e due t o M s . E. R i v e t t e , D r . R. M o r r i s , and D r . P. R i t z f o r X-ray, Thermal and Raman measurements. A l l exper imenta l work was performed by M r . R. O r t i z . 299 REFERENCES 1. M. L. O c c e l l i , i n "Keynotes i n Energy Related Cata lys is , " 2. F. Figueras, Cata l . Review 30, 1988, 3, 457. 3. J. Guan, E. Min and Z. Yu, i n U.S. Patent No. 4,757,040, 1981. 4. R. E. G r i m , i n "Clay Mineralogy," McGraw-Hill Co., 1968. 5. H. Kodama, Am. M i n e r a l o g i s t 51, 1966, 1035. 6. W. F. Bradley, Am. M i n e r a l o g i s t 35, 1950, 590. 7. M. Sato, K. Oinuma, and Kobayashi, Nature, Lond. 208, 1965. 8. A. Schutz and G. Poncelet , NATO Workshop on Chemical React ions i n Organic and Ino rgan ic Systems Proc., Reidel , 1985. 9. A. Schutz, 0. Plee, F. Borg, P. Jacobs, G. Poncelet , and J . J . F r i p i a t , Clays and Clay Min. 10. J. 0. Russel l and J. L. White, Clays and Clay Min., Proc. 14 th Conf., Pergamon Press, Oxford, 181, 1966. 11. E. P. Parry , J . Cata l . 2,371, 1963. 12. M. L. O c c e l l i , Ind. Eng. Chem. Prod. Res. Dev. 22, 1983, 553. 13. M. L. O c c e l l i , J . M. Stencel and F. Huggins ( i n p repara t i on ) . S. Ka l i agu ine Ed., E l s e v i e r , 1988, P. 101. This Page Intentionally Left Blank G . Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 301 PREPARATION AND PROPERTIES OF LARGE-PORE RE/AI-PILLARED MONTMORILLONITE. A COMPARISON OF RE-CATIONS J. STERTE Department of Engineering Chemistry I, Chalmers University of Technology, 412 96 Goteborg (Sweden) SUMMARY RE/Al-pillared montmorillonites were prepared by cation exchange of montmorillonite with hydrothermally t reated (10O-16O0C, 12-240 h) mixtures of aluminum chlorohydrate (ACH) and chlorides of La, Ce, Pr, Nd or a mixture of RE-cations. Large-pore RE/AI- pillared montmorillonites, characterized by basal spacings of about 26 a and surface areas of 300-550 m2/g, were formed from solutions containing La, C e or t he mixture of RE- cations, refluxed for several days or t reated in autoclaves at higher temperatures for shorter times. Pr and Nd containing solutions did not result in large-pore pillared products a f t e r t reatment at the conditions of this investigation. The large-pore RE/Al-pillared montmorillonites were found to be similar in elemental composition t o a conventional Al- pillared montmorillonite but thermally more stable, retaining a surface a rea of about 300 m2/g a f t e r exposure to 8OOOC for 3 h. INTRODUCTION In recent y e a r s g r e a t i n t e r e s t h a s beenfocussed on t h e p r e p a r a t i o n and t h e characterization of different types of pillared clays and also on possible applications for these materials primarily as catalysts or adsorbents. One of the most interesting potential applications for pillared clays is as active components in cracking catalyst formulations designed for cracking of heavy oil fractions. The commercial use of pillared clays for this application has, however, been limited by their lack of thermal and hydrothermal stability. Recently, McCauley (ref. I ) found tha t hydrothermally stable pillared smect i tes can be prepared by using hydrothermally t reated pillaring solutions, containing mixtures of aluminum chlorohydrate (ACH) and a ceriurn(II1) salt in t he preparation. Pillared smectites prepared from such solutions differ from conventional Al-pillared smectites in that the basal spacing as measured by X-ray diffraction analysis is considerably larger, i.e. 25-28 a compared with 18-20 A. This larger basal spacing is believed to be due to formation of a large Ce-bearing Al-polycation upon hydrothermal t reatment of the solution. For use as heavy oil cracking catalysts, this larger spacing is another advantage over conventional Al-pillared smectites in addition t o the improved hydrothermal stability. McCauley found that large-pore Ce/Al-pillared smect i tes can be prepared from solutions with Ce/AI molar ratios down to 1:52, hydrothermally t reated either by refluxing for several days or by t reatment in autoclaves at higher temperatures for shorter times. 302 NO systematic investigation on the dependence of properties of t he Ce/AI- pillared products upon different synthesis parameters was, however, reported. Although the work of McCauley is primarily concerned with the preparation of Ce/Al-pillared smectites, he claims that, in addition to Ce(III), other r a re ear th (RE) cations in admixture with ACH can be used for the preparation of large-pore pillared products. S t e r t e (ref. 2) investigated the effects of t ime and temperature of hydrothermal t reatment , Al-concentration, OH/Al-ratio of the ACH, and La/AI molar ratio of the solution on the formation of large pore La/Al-pillared montmorillonite. The present paper reports on the preparation and characterization of pillared montmorillonites prepared by cation exchange of t h e montmorillonite with hydrothermally t reated solutions containing ACH in admixture with chlorides of La, Ce, Pr, Nd, or a commercial mixture of RE-cations. EXPERIMENTAL Montmorillonite A Wyoming Na+-Ca2+-montmorillonite (commercial designation, Voiclay SPV 200) was obtained from the American Colloid Company. Impurity quartz was removed by fractionation using conventional sedimentation techniques. The 303 t he vigorously stirred dispersion. The resulting product was left in contac t with the solution for 1 h and then separated by centrifugation. The product was then washed by redispersing it in distilled water, and separated by centrifugation. This procedure was repeated until t h e supernatant was f r ee from chloride ions as determined by AgN03. Characterization of pillared products N2-adsorption-desorption isotherms were determined using a Digisorb 2600 surface- a rea , pore-volume analyzer (Micromeritics Instrument Corporation). The samples were first outgassed at 200OC for 3 h, and the isotherms were recorded at liquid nitrogen temperature. Surface a reas were calculated using t h e BET equation. X-ray diffraction analyses were performed either on non-oriented or oriented mounts. The XRD patterns were obtained on a Philips powder diffractometer using Ni-filtered, fine-focus CuKr-radiation. Thermal stability was investigated by exposing separa te samples t o temperatures in the range 20O0-80OOC for 3 h in air. Elemental analysis of t he pillared samples was carried out by a tomic absorption spectroscopy (AAS) employing LiB02-fusion (ref. 3). RESULTS AND DISCUSSION Reflux experiments A series of samples was prepared from t h e different RE/Al-solutions and f rom t h e reference ACH-solution, refluxed for 0-240 h. Fig. 1 shows the X-ray diffraction pa t te rns of samples prepared from La- and ACH-solutions refluxed for 48 and 120 h and from Ce-, Pr-, Nd-, and RE-solutions refluxed for 120 and 240 h. The reference sample prepared from the RE-free ACH-solution, refluxed for 48 h, shows a basal reflexion corresponding to a basal spacing of 19.4 A. This value is within the range, 18-20 A, usually observed for conventional Al-pillared smectites. Further refluxing of this solution up t o 120 h results in an increase in crystallinity of t he pillared product but does not significantly a f f ec t its basal spacing. The sample prepared from the La/Al-solution refluxed for 48 h shows a broad 001-reflexion corresponding t o a basal spacing of about 19.2 A. In t h e pa t te rn recorded for t he sample prepared from the same solution refluxed for 120 h, t he major basal spacing has shifted to about 26 A. This basal spacing is similar t o those observed by McCauley (ref. 1) for large-pore Ce/Al-pillared montmorillonite and f luorhectorite, i.e. 27.4 and 25.6 A, respectively. The 26 A peak s t a r t s to develop for samples prepared from this solution refluxed for 72 h, grows sharper and more intensive with increasing t ime of reflux up to about 96 h and then remains unaffected with increasing t ime of reflux within t h e t ime range investigated. The Ce/Al-solution resulted in large-pore pillared products similar t o those prepared from t h e La/Al-solution but required longer t imes of reflux, i.e. about 240 h, in order to produce such products. Although a broadening of the 001-peaks is seen for t h e Pr/AI- and Nd/Al-samples prepared from solutions refluxed for 240 h, no 304 Pr/AI ----Mu- 8 5 2 8 5 2 8 5 2 8 5 28 5 2 8 5 2 Fig. 1. X-ray diffraction pa t te rns of RE/Al-pillared montmorillonites prepared from refluxed solutions. Degrees 2 0 major changes in the basal spacings of pillared products prepared from refluxed Pr/AI- or Nd/Al-solutions were observed. The RE/Al-solution, containing a mixture of RE-chlorides, showed a behavior simliar to tha t of t h e Ce/Al-solution, i.e. about 240 h of reflux was required in order to obtain a large-pore pillared product. Autoclave experiments A series of samples was prepared from the different RE/Al-solutions and from the reference ACH-solution, t rea ted in autoclaves at tempera tures in t h e range 120-16OoC for 12-96 h. Fig. 2 shows t h e X-ray diffraction pa t te rns of samples prepared from La/Al-, Ce/Al-, and RE/Al-solutions t rea ted at 120OC for 12, 24, 48, and 96 h. For t h e samples prepared from the La/Al-solutions a 26 a spacing s t a r t s to develop a f t e r 24 h of treatment. Further t rea tment of this solution, up t o 96 h, results in products with sharper and more intensive basal reflexions, indicating increased crystallinity of t he products with increasing t ime of hydrothermal treatment. For t h e Ce/AI- and the RE/Al-solutions longer t imes of t rea tment were required in order t o obtain large-pore pillared products, which is consistent with the results obtained in t h e reflux experiments discussed above. All these solutions did, however, yield very crystalline products with basal spacings of 26 A a f t e r t r ea tmen t at 120OC for 96 h. In Fig. 3, t he X-ray diffraction pa t te rns of samples prepared from La/AI-, Ce/AI-, and 305 La/AI Ce/AI RE /A1 Degrees 28 Fig. 2. X-ray diffraction patterns of La/AI-, Ce/AI-, and RE/Al-pillared rnontrnorillonites prepared from solutions autoclaved at 12OoC for 12-96 h. La /A1 Ce/AI REIAI Degrees 2 8 Fig. 3. X-ray diffraction patterns of La/AI-, Ce/AI-, and RE/Al-pillared montmorillonites prepared from solutions autoclaved for 12 h at 120, 140, and 16Ooc. 306 120 140 160 TABLE 1 BET sur face a reas (m2/g) of RE/Al-pillared montmorillonites prepared from solutions hydrothermally t rea ted at 120-16OOC for 12-96 h. ~ 422 436 463 493 436 443 461 537 430 394 434 538 428 410 389 374 429 428 407 518 422 409 393 499 487 428 412 408 507 487 386 390 468 458 442 396 La/AI Ce/AI RE/AI temp.a t ime of t rea tment (h)a (OC) I 12 24 48 96 I 12 24 48 96 I 12 24 48 96 RE/Al-solutions t rea ted at 120, 140 and 160OC for 12 h a r e shown. After t rea tment at 12OoC, all solutions result in products with basal spacings of about 19 8. Trea tment at 14OOC results in a large-pore pillared product from t h e La/Al-solution but not so for t h e Ce/Al-and RE/Al-solutions. After t rea tment at 160OC for 12 h, all t h e solutions yielded highly crystalline large-pore pillared products. These results show t h a t t h e r a t e of formation of t h e large RE/Al-polycations, believed to be responsible for t h e 26 8 spacing of large-pore pillared products, increases with increasing tempera ture of hydrothermal treatment. Hydrothermal t rea tment of t h e solution for t imes longer than tha t required for t he formation of these species does, however, result in a decline in crystallinity of t he resulting products. Thus, solutions of La/AI, Ce/Al as well as RE/AI t rea ted at 16OOC for 96 h result in considerably less crystalline products in comparison with those obtained from the same solutions t rea ted at this tempera ture for 12 h. Pr/AI- and Nd/Al-solutions t rea ted at 12O-16O0C for 12-96 h did not yield large-pore pillared products. All samples prepared from these solutions showed basal spacings in the range 19-20 8 which is within the range characterist ic for conventional Al-pillared smectites. Autoclave t rea tment of RE-free ACH-solutions at 12O-16O0C for 12-96 h resulted in t h e formation of colloidal dispersions or gels of boehmite or pseudoboehmite. Table 1 shows t h e BET surface a reas of samples prepared from La/AI-, Ce/AI-, and RE/Al-solutions t rea ted 12O-16O0C for 12-96 h. Samples prepared from solutions t rea ted at 12OoC show an increase in surface a rea with increasing t ime of t rea tment for all th ree solutions. Increasing t ime of hydrothermal t rea tment at 14OoC appears t o result in somewhat increasing surface a reas for samples prepared from Ce/AI- and RE/Al-solutions and in decreasing surface a reas for those prepared from La/Al-solutions. For samples prepared from solutions t rea ted at 16OoC t h e surface a reas decrease with increasing t ime of hydrothermal t rea tment of t he solution for all th ree solutions. The results of t he surface a rea measurements a r e in good agreement with those obtained by X-ray 307 diffraction analysis. The large-pore pillared products, showing intense and sharp basal reflexions corresponding t o a basal spacing of 26 A, generally have surface areas in t h e vicinity of 500 m*/g. Fig. 3 shows the N2-adsorption-desorption isotherms recorded for t he start ing montmorillonite, an Al-pillared montmorillonite and a large-pore Ce/Al-pillared montmorillonite prepared from a Ce/Al-solution hydrothermally t reated at 12OoC for 96 h. The isotherm recorded for t he montmorillonite is of type I1 in the classification of Brunauer, Deming, and Teller, characterist ic of non-porous soiids. The isotherms recorded for t he pillared products can both be described as composite isotherms of t he type I1 isotherm of the montmorillonite and type I isotherms due t o adsorption in pores introduced by the pillaring procedure. The strong adsorption at low relative pressures, characterist ic for microporous materials, is, however, less pronounced for t he Ce/AI- pillared sample. This may be taken as a further indication of larger pores in this material compared with the Al-pillared montmorillonite. A further discussion of the adsorption properties of large-pore RE/Al-pillared montmorillonite in relation to the nature of the pores of this material is given in (ref. 2). Elemental analysis Table 2 shows elemental analyses of t he start ing montmorillonite, of a Al-pillared montmorillonite, and of samples prepared from the La/AI-, Ce/AI-, Pr/AI-, Nd/Ai-, and 160 0 LO 0 05 1.0 Welafive pressure, PIP, Fig. 4. Nitrogen adsorption-dessrpbion isotherms for staring monSrnorillonite (a), Al- pillared montrnorillonite (b) and large-pore Ce/Al-pillared morrtmorillonite (c). 308 metal oxide (wt %) TABLE 2 Elemental analysis of Na+-Ca2+-montmorillonite, Al-pillared and RE/Al-pillared montmorillonites. montmo- Al- La/AI- Ce/AI- Pr/AI- Nd/AI- RE/AI- rillonitea m0nt.b m0nt.c m0nt.c m0nt.c m0nt.c m0nt.c 56.7 19.6 3.84 3.02 0.8 0.39 1.94 13.1 99.4 45.6 24.4 3.27 1.99 0.2 0.40 0.24 22.4 98.5 44.3 25.4 3.20 2.24 0. I 0.30 0.25 22.2 98.0 44.6 24.6 2.89 2.00 0.0 0.36 0.40 26.4 101.2 46.2 24.4 2.89 2.12 0.0 0.36 0.40 23.4 99.8 aNa+-Ca2+-montmorillonite, Volclay SPV 200, used as start ing mater ia l preparations. 41.5 24.1 2.59 2.11 0.1 0.18 0.13 29.6 00.3 1 all 43.0 25.7 2.69 2.47 0.0 0.19 0.18 27.1 01.3 bAi-pillared montmorillonite prepared using a RE-free ACH-solution. CAll RE/Al-pillared montmorillonites were prepared from solutions t rea ted at 12OoC for 96 h. RE/Al-solutions t rea ted at 12OoC for 96 h. The analyses of the samples prepared from hydrothermally t rea ted solution differ very l i t t l e from each other and a r e also similar to tha t of t h e Al-pillared montmorillonite. This indicates a similar uptake of A1 by t h e montmorillonite from the different solutions and also a similar charge per A1 in t h e Al- species taken up from these solutions. These results a r e somewhat surprising considering the grea t structural differences between t h e samples observed by X-ray diffraction and by N2-adsorption-desorption analysis. Thermal stabil i ty Fig. 5 shows t h e X-ray powder diffraction pa t te rns of an Al-pillared montmorillonite and a Ce/Al-pillared sample prepared from a solution t rea ted at 12OoC for 96 h, both thermally t r ea t ed at 200 and 8OOOC for 3 h. The sur face a reas of the different samples a r e also given in this Figure. For t h e Al-pillared sample, t h e basal spacing decreases from about 18 8 a f t e r t rea tment at 2OOOC to about 16 8 a f t e r t rea tment at 800°C. The decrease in basal spacing is accompanied by a substantial decrease in surface area. For the Ce/Al-pillared montmorillonite, t he basal spacing is approximately the same, about 25 8, for samples t rea ted at 200 and 8OOOC. After t rea tment at 800°, t h e Ce/Al-pillared sample retains a considerable fraction of i t s original surface area, indicating a good thermal stability. I t is noticeable tha t t h e surface a rea of t h e Ce/Al-pillared montmorillonite t rea ted at 800°C is of t he same order as tha t of t h e Al-pillared one t rea ted at 200oC. La/AI- and RE/Al-pillared samples responded t o thermal t rea tment in a manner similar to tha t of Ce/Al-pillared ones while only minor improvents in thermal 309 Al. Ce/A I -u 8 5 2 8 5 2 Degrees 2 8 Fig. 5. X-ray powder diffraction pa t te rns and surface a reas of thermally t r ea t ed Al- pillared and large-pore Ce/Al-pillared montmorillonites. stability were observed for Pr/AI- and Nd/Al-pillared montmorillonites. Structure of large-pore RE/Al-pillared montmorillonite It is generally accepted t h a t t h e oligomer responible for t h e layer separation of conventional Al-pillared smect i tes is A11304(OH)247+ (ref. 4). The s t ruc ture of this cation is a cent ra l four-coordinated aluminum a tom surrounded by twelve A106-octahedra joined by common edges to form a Keggin-type structure, McCauley (ref. I) found tha t large-pore Ce/Al-pillared montmorillonites can be prepared from hydrothermally t rea ted solutions with Ce/Al-ratios down t o 1:52. This, in combination with t h e fact tha t t h e interlayer spacing of these materials is about twice tha t of conventional Al-pillared smectites, led him t o suggest t ha t t h e polymeric cation responsible for t he pillaring in large-pore pillared smect i tes is built up of four A113-units, linked together by a tetrahedrally bound cerium atom. McCauley has, however, not investigated the s t ruc ture of t he pillaring species further in order t o substantiate this suggestion and no such polymeric ion has so fa r been reported in the literature. Investigations of hydrothermally t rea ted mixtures of aluminum chlorohydrate and r a r e ear th salts a r e required in order t o establish t h e s t ruc ture of this polycation. CONCLUSION Large-pore pillared montmorillonites can be prepared from hydrothermally t r ea t ed mixtures of ACH and lanthanum, cerium or RE-chlorides. The hydrothermal t r ea tmen t can b e carried ou t e i ther at reflux conditions fo r several days or at higher tempera tures 310 and for shorter durations in an autoclave. More crystalline materials with higher surface a reas can be prepared from autoclaved solutions compared with refluxed ones. Praseodymium and neodymium chlorides in admixture with ACH did not produce large- pore pillared products a f t e r hydrothermal t rea tment at t h e conditions used in this study. The large-pore pillared montmorillonites a r e characterized by sur face a reas in t h e range 300-550 m2/g, basal spacings of about 26 A, and a good thermal stability (surface area about 300 m2/g even a f t e r exposure t o 8OO0C for 3 h). Large-pore RE/Al-pillared smect i tes is a novel type of zeolite-like materials which may be of in te res t as ac t ive components in ca ta lys t s for cracking of heavy oil fractions. Cata ly t ic cracking performance of RE/Al-montmorillonites, alone and in admixture with zeolite Y, is currently under investigation. ACKNOWLEDGMENTS The author thanks t h e Swedish Board for Technical Development (STU) for financial support of this project. Helpful advice from Prof. J-E. Ot te rs ted t and experimental help from Mr. Ying Zhong-Shu a r e greatly appreciated. REFERENCES 1 2 3 4 J.R. McCauley, Stable intercalated clays and preparation method, Int. Pat. Appl. PCT/US88/00567, March 4, 1988. J. S ter te , Preparation and properties of large-pore La/Al-pillared montmorillonite, submitted for publication. J.H. Medlin, N.H. Suhr, and J.B. Bodkin, Atomic absorption analysis of silicates employing LiB02 fusion, At. Absorpt. Newsl., 8 (1969) 25-29. D. Plee, F. Borg, L. Gatineau, and J.J. Fripiat , High-resolution solid-state 27Al and 29Si nuclear magnetic resonance study of pillared clays, J. Amer. Chem. Soc., 107 (1985) 2362. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 311 PREPARATION OF PILLARED MONTMORILLONITE WITH ENRICHED PILLARS E. K I K U C H I , H. SEKI, and T. MATSUDA Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169, Japan S U m A R Y Montmorillonite intercalated with Al-polyoxycations, which will be called pillared montmorillonite, had extremely small cation exchange capacity(CEC) compared with the original montmorillonite. The CEC of pillared montmorillonite changed a little by heat treatment in a stream of nitrogen. When treated in a stream of ammonia, however, it pronouncedly increased. Thus obtained pi1 lared montmorillonite was re-intercalated with Al-polyoxycations to give pillared montmorillonite having larger number of pillars. The catalytic activity of pillared montmorillonite for disproportionation of 1.2,4-trimethylbenzene(l,2,4-TrMB) decreased with increasing number of pillars, which favored the formation of 1,2,4,5- tetramethylbenzene(l,2,4,5-TeMB) with the smallest molecular dimension among TeMB isomers. These results can be interpreted by increased shape selective property by enrichment o f pillars. INTRODUCTION Intercalation of smectite clays with polyoxycations provides a new class of porous materials. Intercalated clays are called pillared clays and they have high thermal stability and large surface area. Vaughan and Lussier(ref. 1) were the first to point out the shape selective sorption property of pillared montmorillonite using various probe hydrocarbons. The shape selective catalysis in cracking of alkylbenzenes was demonstrated by Shabtai and co- workers(ref. 2). The pore opening of pillared clays, which plays an important role in shape selective catalysis, is determined both by the interlayer distance and by the density of pillar or the number o f pillars. The interlayer distance depends on the dimension of intercalating species. Considerable efforts have been undertaken to develop pillared clays with different interlayer distances. Indeed, pillared clays having the interlayer distance from 0.4 to 2.0 nm were prepared using various intercalating species(refs. 3-7). In contrast, there have been few studies concerning the control of pillar density or the number of pillars. We showed in the previous works(refs. 8-10) that pillared montmorillonite was active and selective for disproportionation of 1,2.4-TrMB to 1,2,4.5-TeMB. 312 and the selectivity for formation of 1,2,4,5-TeMB was affected by the interlayer distance of pillared montmorillonite. The purpose of this work is to describe the preparation of pillared montmorillonite having a variety number of pillars and to discuss the shape selective catalysis by pillared montmorillonite. Disproportionation of 1,2,4-TrMB was studied as a model reaction to characterize shape selective property. EXPERIMENTAL Preparation o f pillared montmorillonite Na-type montmorillonite with the cation exchange capacity of 1,19 meq./g was used in this study. A [Al1304(OH)24(H20)12]7t cation was prepared by addition of NaOH solution to AlC13 solution to yield OH/A1 molar ratio of 2.5. T h e i n t e r c a l a t i o n m e t h o d o f N a - m o n t m o r i l l o n i t e w i t h [A11~04(OH)24(H20)12]7+ was previously described in detail(ref. 11). The intercalated montmorillonite was treated at a desired temperature in the range of 373-673 K in a stream of nitrogen or ammonia. To remove ammonia adsorbed on the acid sites, the ammonia-treated sample was treated in a stream of nitrogen at 673 K for 1 h. Characterization of pillared montmorillonite The cation exchange capacity(CEC) of pillared montmorillonite was determined as follows. Sodium cations were introduced to pillared montmorillonite by exhaustive exchange with a 1 N solution of NaCl at 323 K. The sodium exchanged pillared montmorillonite was added to a 0.1 N solution of NH4C1 and stirred for 3 h at 323 K. After filtration and washing, the amount of sodium ions replaced by ammonium ions was measured using atomic absorption spectrometer. The number o f acid sites on pillared montmorillonite was determined by means of ammonia temperature-programmed desorption(TPD). In each TPD experiment, a sample of 0.5 g was treated in vacuo at 673 K for 1 h and then cooled to 373 K. Ammonia was adsorbed at 373 K for 30 min and evacuated for 30 min. This sample was heated from 373 to 973 K at a rate of 10 K/min and the desorbed ammonia was monitored by thermal conductivity detector. As water was simultaneously desorbed with ammonia above 673 K. the ammonia TPD spectrum was obtained by point-by point subtraction o f the water desorption spectrum obtained with the sample which had not adsorbed ammonia. Apparatus and procedures Catalytic studies were performed in a continuous flow reactor with a fixed bed of catalyst. The catalyst was packed in the reactor and was treated at 313 673 K for 1 h in a nitrogen atmosphere prior to reaction. Disproportionation of 1,2,4- TrMB was carried out at 473 K and atmospheric pressure. 112,4-TrMB was di 1 uted with nitrogen in a molar ratio of 1:9. L i q u i d p r o d u c t s w e r e collected in an ice trap every 10 min and were analyzed by means of gas chromatography using a f l a m e i o n i z a t i o n d e t e c t o r and a F F A P g l a s s capillary separation column with temperature-programmed heating from 333 to 443 K. 0.6 0:5 0.4 - I 4 g 0.3 0.2 E 0 W v 0.1 0 573 673 Temperature( K I Fig. 1. E f f e c t o f heat treatment on the CEC of p i l l a r e d montmor i l lon i te : 0 , i n nitrogen; A , i n ammonia. RESULTS AND DISCUSSION Ammonia treatment The density of pillars can be controlled by the number of cation exchangeable sites on montmori 1 loni te. It has been considered that heat treatment converts Al-polyoxycation of [All 304(OH)24(H20)1 217+ cation to A1203, which is represented as follows: [Al1304(OH)24(H20)12]7+ 6.5 A1203 t 7 H+ + 20.5 H20 If it is the case, pillared montmorillonite after heat treatment should have almost the same size of C E C with the original montmorillonite. Hence, pillared montmorillonite with larger number of pillars will be prepared by further introduction o f Al-polyoxycations to pillared montmorillonite. Figure 1 shows the C E C o f pillared montmorillonite treated in the range from 373 to 673 K in a nitrogen atmosphere. Pillared montmorillonite treated at 373 K exhibited extremely small C E C , indicating that the cation exchangeable sites on montmorillonite were completely occupied by [A11~04(OH)24(H20)12]7+ cations. The CEC of pillared montmorillonite had tendency to increase with the temperature of treatment, although it was smaller even after treatment at 673 K than the original montmorillonite. Pillared montmorillonite exhibited large CEC after treatment in a stream o f ammonia. When treated in ammonia at 373 K. pillared montmorillonite had small 314 ABLE 1 F a t a l y t i c a c t i v i t i e s o f p i l l a r e d m o n t m o r i l l o n i t e s C a t a l y s t PM PM( NH3) Na/PM Na/PM( NH3) CEC(meq.9-l) 0.15 0.61 0.15 0.61 % Conversion 32.7 36.3 17.5 0.2 Y i e l d (mol%) Xyl ene 14.6 15.8 8.4 0.1 TrMB 3.7 4.4 0.8 0 TeMB 14.1 16.0 8.3 0.1 PMB+HMB 0.3 0.3 0.1 0 PM, P i l l a r e d m o n t m o r i l l o n i t e t r e a t e d i n n i t rogen : PM(NH3), P i l l a r e d P i 1 l a r e d montmor i l l o n i t e t r e a t e d i n ammonia: Na/PM. Na-exchanged p i l l a r e d m o n t m o r i l l o n i t e . React ion cond i t i ons : temp., 473 K: W/F, 8000 g-cat.min/mol. CEC compared w i t h t h e o r i g i n a l m o n t m o r i l l o n i t e . However, t h e CEC increased w i t h i n c r e a s i n g t e m p e r a t u r e o f ammonia t r e a t m e n t , and i t r e a c h e d t o 0.61 meq./g a t 573 K. P i l l a r e d m o n t m o r i l l o n i t e had a su r face area o f 400 m2/g and gave d(001) r e f l e c t i o n a t d=1.85 nm even a f t e r ammonia t rea tmen t a t 573 K, i n d i c a t i n g t h a t t h e l a y e r s t r u c t u r e o f p i l l a r e d m o n t m o r i l l o n i t e was m a i n t a i n e d . T h e s e r e s u l t s i n d i c a t e t h a t d e c o m p o s i t i o n o f [A11304(OH)24(H20)12]7+ c a t i o n s i s acce le ra ted i n t h e presence o f ammonia. Table 1 summarizes t h e a c t i v i t y o f p i l l a r e d m o n t m o r i l l o n i t e c a t a l y s t s f o r convers ion o f 112.4-TrMB. The c a t a l y t i c a c t i v i t i e s were compared us ing da ta taken i n t h e i n i t i a l 10 min o f run. Ammonia t rea tment d i d n o t a f f e c t t h e a c t i v i t y o f p i l l a r e d m o n t m o r i l l o n i t e c a t a l y s t a t a l l i f adsorbed ammonia was subsequent ly removed i n n i t r o g e n t rea tmen t a t 673 K , a l though t h e CEC was changed. The observed c a t a l y t i c a c t i v i t i e s were i n f a i r agreement w i t h the r e s u l t s ob ta ined f rom ammonia-TPD exper iments. When sodium c a t i o n s were i n t roduced t o t h e c a t i o n exchangeable s i t e s , p i l l a r e d m o n t m o r i l l o n i t e t r e a t e d i n ammonia a t 573 K became i n a c t i v e , i n d i c a t i n g t h a t t h e c a t i o n exchangeable s i t e s ac ted as a c t i v e s i t e s . I n c o n t r a s t , p i l l a r e d m o n t m o r i l l o n i t e t r e a t e d i n n i t r o g e n was a c t i v e even a f t e r i n t r o d u c t i o n o f sodium ca t i ons , a l though t h e c a t a l y t i c a c t i v i t y was reduced. Thus, t h e t y p e o r na tu re o f a c i d s i t e s i s cons idered t o v a r y w i t h t h e atmosphere o f t reatment. R e - i n t e r c a l a t i o n R e - i n t e r c a l a t i o n o f [A11 ,04 (OH)24(H20)12 ]7+ c a t i o n s t o p i 1 l a r e d m o n t m o r i l l o n i t e t r e a t e d i n ammonia was i n v e s t i g a t e d t o p r e p a r e p i l l a r e d m o n t m o r i l l o n i t e hav ing l a r g e number o f p i l l a r s . F igu re 2 shows t h e number o f p i l l a r s i n t r o d u c e d as a f u n c t i o n o f t h e p e r i o d o f i o n e x c h a n g e a t 323 K. [Al1304(OH)24(H20)l,J7+ c a t i o n s were h a r d l y i n t roduced t o p i l l a r e d 315 0 3 6 0 3 6 9 Ion exchange time(h) Fig. 2. of ion exchange at 323 K: 0 , Na-montmorillonite; A , pillared montmorillonite in nitrogen; n, pillared montmorillonite treated in ammonia. Variation in the number of pillars as a function of the period rnontmorillonite treated at 673 K in nitrogen, due to the extremely small C E C . I n the case o f pillared montmorillonite treated in ammonia at 573 K, however, re-intercalation gave pillared montmorillonite with additional number of pillars. The interlayer distance, which was determined from d(001) reflection, did not change at all by the further i n t r o d u c t i o n o f [ A1 1 304(OH)24( HZO)~~]~' cations. As shown in Fig. 3, the number of pillars increased with the C E C of pillared montmorillonite used for re-intercalation, indicating that the polyoxycations had access to the f 1.3 c 0 0.2 0.4 0.6 Cation exchange capac i ty Fig. 3. Relation between the CEC o f pillared montmorillonite and the total number o f pillars. (meq , 9- 1 ) cation exchangeable sites on pillared montmorillonite. However, the number of pillars obtained was small compared with the value expected from the charge of intercalating species and the CEC of pillared montmorillonite. If all of the cation exchangeable sites are occupied by [Al,304(OH)24(H20)12]7+ cation, the 316 Number o f p i 1 lar x lO-*O(g-l) Fig. 4. Catalytic activity and acidity of pillared montmorillonite as a function o f the number of pillars. nrimber of pillars will increase from 1.1 x lo-'' to 1.7 x 10-20/g by re- intercalation of pillared montmorillonite having CEC of 0.61 meq./g. Thus, diffusion of [Al,304(OH)24(H20)12]7i into the interlayer space of pillared montmori 1 loni te seems to be 1 imi ted. Figure 4 shows the catalytic activities of pillared montmorillonites as a function o f the number o f pillars. The activity of pillared montmorillonite catalyst decreased with an increase in the number of pillars. As shown in this figure, the acidity o f pillared montmorillonite increased with the number of pillars. Hence, the decrease in catalytic activity with increased number o f pillars is not caused by reduced acidity. It has been reported(refs. 12-14) that pillars as well as clay sheets are responsible f o r acidity of pillared clays. The results obtained in the present study also indicate that acid sites exist on the pillars. The pore opening and the pore volume are reduced by introduction of excess number of pillars. These results lead us to conclude that the observed change in the catalytic activity is related to the shape selective property of pillared montmorillonite: diffusion o f 1,2,4-TrMB or formation of the intermediate for disproportionation is restricted by the limited interlayer space. Disproportionation o f 1,2,4-TrMB yields all the isomers of xylene and TeMB. 112,3,5-TeMB is thermodynamically most stable among TeMB 317 Number of pillar x 10-20(g-1) Fig. 5. pillars on pillared montmorillonite. The selectivity of 1,2,4,5-TeMB as a function o f the number of isomers. However, 1,2,4,5-TeMB has the smallest molecular dimension among TeMB isomers. It has been reported(ref. 8) that the cross-sectional dimension of the transition state leading to 1,2,4,5-TeMB is small compared with those which lead to 1,2,3,5- and 1,2,3,4-TeMB. The 1.2.4,5-TeMB selectivity expressed by the fraction of 1,2,4,5-TeMB in TeMB isomers is thus dependent on the shape selective property o f a catalyst. The 1,2,4.5-TeMB selectivity changed with the level of 1,2.4-TrMB conversion, due to isomerization o f 1.2.4.5-TeMB to thermodynamically more stable 1,2,3.5-TeMB. Hence, the 1.2,4,5-TeMB selectivity was compared using the data taken at a common conversion level of 10%. Figure 5 shows the selectivity o f pillared montmorillonite for formation of 1,2,4,5-TeMB as a function o f the number of pillars. The 1.2,4,5-TeMB selectivity increased with increasing number of pillars. Therefore, the high selectivity of pillared montmorillonite with enriched pillars for the formation of 1.2,4,5- TeMB is a result of space restriction by pillars. It is apparent from these results that the number of pillars can govern the shape selective catalysis on pillared montmorillonite. CONCLUSION Ammonia treatment promotes decomposition o f [ A ~ I ~ O ~ ( O H ) ~ ~ ( H ~ O ) ~ ~ ] ~ + to A1203 and consequently gives pillared montmorillonite with the large number of cation exchangeable sites. Re-intercalation of thus treated pillared montmorillonite gives pillared montmorillonite with an additional number of pillars. The number of pillar increases with increasing CEC o f pillared 318 m o n t m o r i l l o n i t e . I t i s a p p a r e n t f r o m t h e s e r e s u l t s t h a t [Al1304(OH),4(H20)12]7+ c a t i o n has access t o the c a t i o n exchangeable s i t e s on p i l l a r e d mon tmor i l l on i te , a l though d i f f u s i o n a l l i m i t a t i o n appears i n re- i n t e r c a l a t i o n . The c a t a l y t i c a c t i v i t y o f p i l l a r e d m o n t m o r i l l o n i t e decreases w i t h i nc reas ing number o f p i l l a r s , w h i l e the s e l e c t i v i t y f o r format ion o f 1,2.4,5-TeMB i s enhanced. These r e s u l t s l ead us t o conclude t h a t t he shape s e l e c t i v e c a t a l y s i s by p i l l a r e d m o n t m o r i l l o n i t e can be c o n t r o l l e d by the number o f p i 11 ars. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 D.E.W. Vaughan and R.J. Luss ier , i n "Proceedings, 5 t h I n t . Conf. on Z e o l i t e , Naples, 1980". Heyden, London, 1981, p.94. J. Shabtai, R. Lazar, and R.G. Luss ier , i n "Proceedings, 7 t h I n t . Congr. on Cata l . I Tokyo, 1980", E l sev ie r . Amsterdam, 1981, p.828. G.W. B r i n d l e y and R.E. Sempels, Clays Clay Miner., 12 (1977) 229. S. Yamanaka and G.W. Br ind ley, Clays Clay Miner., 27 (1979) 119. R. Burch and C. I . Warburton, J. Catal . , 97 (1986) 503. T.J. Pinnavaia, M. Tzou, and S.D. Landau, J. Am. Chem, SOC., 107 (1985) 4783. J. Ster te , Clays Clay Miner., 34 (1986) 658. E . Kikuchi , T. Matsuda, H. F u j i k i , and Y. Mor i ta . Appl. Catal., 11 (1984) 331, E. Kikuchi . T. Matsuda, J . Ueda. and Y. Mor i ta . Appl. Catal., 16 (1985) 401. T. Matsuda, M. Asanuma, and E. K ikuchi , Appl. Catal., 38 (1988) 289. T. Matsuda, H. Nagashima, and E. K ikuchi , Appl. Catal . , 45 (1988) 171. A.Schutz, D. Plee, F. Borg, P. Jacobs, G. Poncelet. and J.J. F r i p i a t , i n i n "Proceedings, I n t . Clay Conf., Denver, 1985". 1987, p.305. M.L. O c c e l l i and R.M. Tindwa, Clays C lay Miner., 31 (1983) 22. T. Mor i and K. Suzuki, Chem. Let t . , (1989) 2165. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 319 INTERCALATION OF La203 AND La2Og-NiO OXIDIC SPECIES INTO MONTMORILLONITE LAYERED STRUCTURE A.K.LADAVOS AND P.J.POMONIS Chemistry Department, University of Ioannina, Ioannina 45110 (Greece). SUMMARY Oxidic species La203 and La203+NiO have been intercalated into montmorillonite layered structure by carefully controlling the pH of the solution used and freeze-drying the obtained slurry. The materials thus obtained were examined by XRD at low angles and showed reflections at 28=4O and 28=8O corresponding to inter ayer distances of 25.68 and 12.88 respectively as compared to 9 . d of the original Na-montmorillonite. The thermal stability of the intercalated solids extends up to 300-350°C, temperature at which the pillared layers shrink to the d-space of the Na-montmo- rillonite. The specific surface area of the obtained materials dried at 2OO0C is 35m2.g-1 for the La-intercalated species and 19m2.g-l for the La-Ni-intercalated ones, as compared to 17m2.g-l for the Na-montmorillonite. The introduction of La and La-Ni into the layered smectite increase the acidity of the solid in a very impressive way as found by employing as a probe reaction the dehydration of iso-propanol. The surface titration of the solids with NH3 showed that the concentration of the acid sites is (1.3-6.1) sites/m2, depending on the intercalant and the temperature. INTRODUCTION It has been shown some time ago (refs.l.2) that layered aluminosilicates such as montmorillonite can be intercalated by means of different oxidic pillars (refs.3.4) Oxidic species which have been introduced into smectite layers include Fe203(refs.2,5) AI203(refs.2,6-8), Ti02(ref.3), ZrO2(refs.9,10), CuO(refs.l1,12) NiO(ref.4). Sn(ref.13) and a few other cations. Such materials bear some promises as industrial catalysts in petrochemical industry if they stabilize above 500°-600°C, temperature at which they usually collapse. Among the oxidic species for which, as far as we know, there are no reports for intercalation in the literature is the La2O3. This cation exchanged into zeolites is capable of water hydrolysis, the formed proton remaining quasi-free into the cage above to 5OK (ref.14). Therefore, its introduction into montmorillonite layers might result in similar interesting features 320 Apart from the introduction of La203 in pure form we also became interested in intercalated mixtures of La203 and NiO with the possible results to obtain small perovskite particles LaNiOg and/or La2Ni04 into the layers of smectite. In attempting the procedure of intercalation different authors usually follow one of two possible routes: The first one is by introducing the metallic cations into a complex, which is often polynuclear (ref.l5), and which is then intercalated into the smectite layers. It then leaves behind the oxidic pillars after thermal decomposition of its ligand. The second route is by attempting intercalation of the hydroxy-species of the metal, which results from a gradual addition of NaOH into the suspension of montmorillonite and the metal salt, often a chloride. Nevertheless, this last method is usually applied in a rather empirical way by means of a trial and error procedure (ref.4). In an attempt to rationalize this method we start from the principle that the hydroxy-species formed initially during alkalization of the solution and precipitated at large addition of alkali, appear in a gradually increasing size according to the scheme provided by the theory of crystallization (refs. 16.17): Cluster- embryo -j nucleus+crystal. In order for the intercalation of small hydroxy-particles to occur we should stay at the very left extreme of this sequence of precipitation. By using this method we succeeded in intercalating hydroxy-oxidic species of the cations La alone, or La plus Ni, into montmorillonite structure. The details of intercalation as well as the acidic and catalytic properties of the obtained solids is the subject of this work. MATERIALS AND METHODS OF INTERCALATION The montmorillonite used was of Wyoming origin (Wardâs International). Such materials usually possess an exchanged capacity around 100meq/100gr (refs.4,7). The pH of its 1% suspension was equal to 9.5. The amount of smectite used for intercalation was exchanged with Na+ by letting it in equilibrium with a large excess of 0.1M NaCl for two days under continuous stirring. The excess of NaCl was then removed by successive centrifugations and washings with distilled water. Finally the traces of C1- were removed by putting the slurry in a dialysis tube until the final testing with AgN03 was negative. 32 1 The lanthanum and nickel oxidic species for intercalation were originated from La(N03)3.6H20 and Ni(NO3)2.6H20 (Merck 2.a.) in solutions of 0.1M possessing pH equal to 4.48 and 5.61 correspondingly. The equimolecular mixture of them showed a pH equal to 4.83. After several trials and in order to decide about the optimum pH for intercalation we came to the conclusion that the potentially intercalable hydroxy-species of the two cations should have dimensions of a few A, probably less than 100A. Since the conventional optical or electrophoretic methods are not able to detect such species we reach the conclusion that a way to check approximately the size of the species was electrochemically by adjusting the pH in suitable limits. ml NaOH QOIM Fig.1. Titration curves for La(N03)3 0.1M ( A ) and La(N0313 O.lM-Ni(N03)20.1M ( B ) with NaOH 0.01M. The pH suitable for intercalation was chosen at the points a and b respectively. The pH suitable for intercalation was chosen from the titrating curves of La(N0313 0.1M and La(N03)3 O.lM-Ni(N03)z 0.1M with 0.01M NaOH (Fig.1). Namely the pH was chosen in the half way between the initial nitrate solutions, e.g. at no addition of NaOH, and the turning points of those curves, at which precipitation starts. The pH at those points corresponds to 5.5 for Lanthanum and 5.7 for Nickel and Lanthanum. The intercalation took place as follows: A volume, usually 100ml. of Na-montmorillonite suspension acidified with dilute HC1 322 down to the above noticed pH values, were mixed with excess solution of La(N03)3 0.2M or equimolecular mixture of La(N03)3 0.1M-Ni(N03)2 0.1M kept at the same pH values. The control of pH was made automatically by using a auto-burette-titrator of Radiometer Copenhagen. The mixture was left for equilibration for one day, centrifuged for separation of the slurry and washed by distilled water. The obtained product was then freeze-dried and examined by different techniques as described next. CHARACTERIZATION OF THE PRODUCTS XRD Studies The freeze-dried samples were characterized by XRD at low angles at different temperatures in order to investigate the d-spacing achieved by pillaring as well as the temperature of the collapse of the aluminosilicate layers. The examinations took place after heating the samples at 100, 150, 200, 25OoC and so on at atmospheric conditions. The X-rays system used was a Philips diffractometer with Fe-filtered Co Ka radiation. The samples were prepared by putting a few drops of the suspension on a glass slide and drying the sample. In this way the layered solid is settled along its z-axis. 10" 8" 4" 28 Fig.2. XRD patterns for the La-Montmorillonite ( A ) and La-Ni- temperatures. . I , I I ' ( B ) j I , 1 , I 1 I 10" 8" 4 O 28 products of intercalation of -montmorillonite ( B ) at different ;oooc 150°C 3 OO°C 250°C A s it can be seen from Fig.2 both La- and La-Ni-pillared 323 solids showed approximately similar behavior with two peaks at 28-40 and 28-80 up to 3OO0C approximately. Those values mean interlayer distances of 25.6% and 12.88, respectively and they probably correspond to first and second order reflections on the 001 and 002 crystal levels. Upon heating to 35OoC a broadening of the above peaks is apparent, due to the shrinking of the layers. The shrink becomes complete a 4OO0C for both species, with a peak at 2f3=10.5°, which corresponds to the interlayer distance d=9.88 of the parent montmorillonite. TG Studies The wet intercalated slurries of La-montmorillonite and La-Ni-montmnrillonite as well as of the hydroxides of the pillaring agents were examined thermogravimetrically in a Chyo thermogravimetric balance model TRDA3H under N2 flow. Two typical thermographs obtained are shown in fig.3 for the La(OH)3 and La-montmorillonite species. In those diagrams we observe that the dried at room temperatures La-montmorillonite loses around 14% of its weight between 45 and 108OC which should be due to water removal trapped between the aluminosilicate layers. The pure La(OH)3 gel shows at the same region a weight loss reaching about 6 . 5 % which should be due to adsorbed water. Next it loses water in two steps, one between 298 and 324OC (6.53% weight loss) and one between 388 and 467OC (12.2% weight loss). Those two steps should correspond to successive dehydrations of La(OH)3 according to the scheme Those dehydration reactions (1) and (2) correspond to theoretical weight losses of 4.73 and 9.46% respectively which are comparable with the experimental values. It is important to notice that at the temperature region where the first dehydroxylation is observed, e.g. around 3OO0C, the pillared material starts to shrink (see fig.2) and the shrinking is completed at 4OO0C where total dehydroxylation has taken place. 324 So it seems that the transformation of La-hydroxy- or La-oxy-hydroxy-species to oxidic material has destructive effect on the pillaring. DTA - La (OH$ TG- La-Moni O 1 x / I DTA- La-Mont 600 400 200 T/OC Fig.3. TG-DTA curves for the La-montmorillonite and La(OH13 materials. Surface area measurements The specific surface area of the freeze-dried samples of La-montmorillonite, La-Ni-montmorillonite as well as the original Na-montmorillonite were measured by the one-point-method in a Carlo Erba-Sorpty 1750 system by N2 adsorption at -196OC. The results are summarized in Table 1. It can be seen that the surface area of Na-montmorillonite increases substantially upon pillaring with La from 17.1 m2.g-1 to 35.4 m2.g-l. On the contrary the La-Ni-montmorillonite species showed a rather small increment of specific surface area as compared to Na-montmo- rillonite from 17.lm2.g-1 to 18.9 m2.g-1. Acidity measurements The surface acidity of the prepared pillared clays as well as the original Na-montmorillonite was checked by titration with NH3 at different temperatures (140-2OOoC) in a gas chromatographic column. A Varian 3700 GC equipped with a TCD and connected with a LDC Milton Roy CI-10 integrator was used for this purpose. An 325 amount of clay equal about to 150 mg was put in the GC column and degassed at 25OoC for 20h under helium flow of l2ml.min-I. Next the temperature was set to 2OO0C and injections of 0.2ml of gaseous NH3 (Merk Wasserfrei, 0.2% H2O) were done into the column up to complete saturation of the sample and total elution of the injected ammonia. The sum of the irreversibly adsorbed ammonia at this particular temperature was considered as corresponding to the total (Bronsted and Lewis) acid sites of the pillared clay. Next the temperature was lowered and the experiment was repeated at 18OoC, 16OoC and 14OoC. The results are noticed in Table 1. In the same table the enthalpies of adsorption of ammonia are also noticed calculated by plotting the retention volume of it at different temperatures versus 1000/T. Perfect straight lines (correlation coefficient r=0.99-1.00) were obtained from the slops of which the values of -AHads were calculated. TABLE 1. Specific surface areas (m2.g-l), surface acidity (molecules of NH3 adsorbed per m2 at different temperatures), and enthalpy of adsorption (kJ/mol) of NH3, degrees of conversion (x%) of iso-propanol per m2 at 18OoC and activation energies for the iso-propanol dehydration on Na-, La- and La-Ni-montmorillonite. Species ssa Acid sites x m-2 ( ~ 1 0 ~ ~ ) -AHadsNH3 x%/m2 Ea -- m2/g 2OO0C 18OoC 160°C 14OoC kJ/mol 18OoC kJ/mol ~ ~~ Na-Mont 17.1 1.3 1.9 2.2 2.6 11.8 0.036 128.8 La-Mont 35.4 2.4 3.1 3.5 3.6 15.7 0.062 64.8 La-Ni-Mont 18.9 4.2 4.8 5.6 6.1 18.8 0.126 85.0 CATALYTIC TESTS - ISOPROPANOL DECOMPOSITION To test the catalytic activity of the prepared samples we chose to study the decomposition of iso-propanol. This is a reaction examined in details in the past (refs.20-25) and it has been shown that it is catalyzed by acid surfaces. For the catalytic tests a bench scale flow reactor similar to that described elsewhere (refs.26, 27) was used. The reactor consisted of a silica tube of lcm diameter with a perforated glass bed onto 326 which 300 mg of the catalyst was put. The system was heated with a tubular furnace within k 2 O C . Analyses of the reactants and products were carried out using a GC system described in the previous section. The column for analysis was 2m stainless steel 1/8" containing 10% Carbowax 20M on Chromosorb W-HP 80-100mesh with He as carrier gas. The same gas was also flowing through a bubble bottle (30cm3.rnin'l) containing the iso-propanol whose vapours were driven then to the reactor. Under the experimental conditions the partial pressure of iso-propanol was 0.0373 atm. Measurements were taken between 150-200°C in ten degrees intervals and the % degree of conversion x was taken as a measure of catalytic activity (Table 1). During the conversion the only products detected were H20 and C3H6. The activation energies for the conversion was calculated from the equation of the plug flow reactor Fdx=Rdm, where F-feeding rate (moles.min-l), x the % degree of conversion, R-the rate of reaction and m-the mass of the catalyst used. Considering first order kinetics, R=kPisopropano~ as usually done by different workers for this reaction (refs. 20-25 ) and substituting the partial pressure with the degree of conversion as done in the past for similar cases (refs. 26-27) we obtain the log forms of the corresponding Arrhenius-type plots ln[(v+n)[-ln(1-x)]-(n-11x1 = 1nB-E/RT (3) where B=PtmA/F, Pt=total pressure in the reactor, A-the pre-exponential term of Arrhenius equation k=Aexp(-E,/RT), v-the ratio moles of the inert gas (He): moles of the reacting gas (iso-propanol vapours) and n-the number of molecules yielded by a decomposed iso-propanol molecule. In our case n=2 and v=18.34. Plots of the left part hand of the above equation versus 1000/T results in perfect straight lines from which the activation energies of the reaction were found and listed in Table 1. In the same table the degree of conversion, calculated per m2 is given for comparison for Na-, La- and La-Ni-montmorillonite at 18OoC. DISCUSSION From the results cited in Table 1 we observe that the introduction of La into montmorillonite increases its acidity substantially. This is apparent not only in the number but also 327 in the strength of the acid sites as estimated by the enthalpy of adsorption of NH3 on its surface. One interesting result is that the simultaneous introduction of La and Ni into montmorillonite increases furthermore the number as well as the strength of the acid sites for reasons which are not apparent. On the contrary the surface area nearly doubles upon the introduction of La from 17.1 to 35.4m2.g-I but the increment is much smaller, from 17.1 to 18.9 m2.g-I upon substitution of Na with La plus Ni. It may be that this difference is due to some kind of porosity differences of the two samples but this point needs further investigation. As expected, the catalytic activity for the isopropanol decomposition as estimated by the degree of conversion per m2 runs parallel with the surface acidity. Thus the La-Ni-species are more active as compared to La-montmorillonite. Somehow strangely the activation energies of the dehydration are minimum in La-species, but this may be due to adsorption or/and compensation effects. It may be useful at this point to compare those results with similar ones obtained on Aluminum-Aluminum phosphate-M203 (M=Cr, Fe) catalyst (ref.26). In such solids the activation energy for the iso-propanol dehydration was found 2-3 times higher, the degree of conversion per m2 1-2 orders of magnitude lower, and the number of acid sites as estimated by pyridine adsorption one order of magnitude less. So it seems that the La- or La-Ni-intercalated species are very strong acid catalysts. It is exactly their strong acidity which is most certainly the reason for the early shrink of the aluminosilicate layers at 350oC. This mechanism should take place through acid attact on the sensitive aluminum sites. Methods of stabilizing such structures to higher temperatures and at the same time keeping their acidity at high or rather controllable levels would be of interest. ACKNOWLEDGEMENT The authors gratefully acknowledge financial support from Aspropyrgos Refineries. REFERENCES 1 T.J.Pinnavaia, Science, 220 (1983) 365-371. 2 W.Y.Lee, R.H.Raythatha and B.J.Tatarchuk, J.Catalysis, 115 3 J.Starte, Clays Clay Miner., 34(6) (1986) 658-664. (1989) 159-179. 328 4 5 6 7 8 9 10 11 1 2 13 14 15 1 6 17 18 19 20 21 22 23 24 25 26 S.Yamanaka and G.W.Brindley, Clays Clay Miner., 2 6 ( 1 ) (1978) C.I.Warburton, Catalysis Today, 2 (1988) 271-280. S.S.Singh and H.Kodama, Clays Clay Miner., 3 6 ( 5 ) (1988) K.Suzuki, M.Horio and T.Mori, Mat.Res.Bull., 23 (1988) N.Lahav, U.Shani and J.Shabtai, Clays Clay Miner., 2 6 ( 2 ) R.Burch and C.I.Warburton, J.Catalysis, 97 (1986) 503-510. F.Figureas, A.Mattrod-Bashi. G.Fetter, A.Thrierr and J.V.Zanchetta, J.Catalysis, 119 (1989) 91-96. O.Braddel1, R.C.Barklie, D.H.Doff, N.H.J.Gangas and A.McKimm, Z.Phys.Chemie, N.F.. 151 (1987) 157-164 J.L.Burba I11 and J.L.McAtee, Jr., Clays Clay Miner., 25 D.Petridis, T.Bakas, A.Simopoulos and N.H.J.Gangas, Inorganic Chemistry, 28 (1989) 2439-2443. J.M.Thomas, Angew.Chem.Int.Ed.Eng1.. 27 (1988) 1673-1691 E.M.Farfan Torres, P.Grange and B.Delmon, in "Chemical Physics of Intercalation", Eds. A.P.Legrand and S.Frandois. NATO AS1 Series, Plenum Press, (1987) pp.489-495. W.L/McCabe and J.C.Smith, "Unit Operations of Chemical Engineering", Mc Graw Hill, New York, 2nd edition ( 1 9 6 7 ) . R.J.Hunter, "Zeta Potential in Colloid Science", Academic Press, London-New York-Toronto-Sydney-San Francisco, ( 1 9 8 1 ) . M.E.Winfield, Catalysis, Vo1.17, (ed.P.H.Emmett) Reinhold, (1960) p . 9 3 . V.A.Dzisko, M.S.Borisova, N.S.Kotsarenko and E.V.Kuznetsova, Kin.Kat. 3 (1962) 728-735. F.S.Stone and A.L.Agudo, 2.Phys.Chemie. N.F., 6 4 (1969) V.Movarek and M.Krauss, J.Catalysis, 87 (1984) 452-460. V.Movarek, React.Kin.Catal.Lett., 30 (1980) 71-75. G.I.Gulodets, N.V.Parlenko and A.I.Tripolskii, Kinetika i Kataliz, 2 7 ( 2 ) (1986) 346-351. D.Petrakis, P.J.Pomonis and A.T.Sdoukos, J.Chem.Soc.Faraday Trans.1, 85 (1989) 3173-3186. C.Kordulis, L.Vordonis, A.Lycourghiotis and P.J.Pomonis, J.Chem.Soc.Faraday Trans. I, 883 (1987) 627-634. D.Petrakis and P.J.Pomonis (in preparation) 21-24. 397-402. 1711-1718. (1978) 107-115. (1977) 113-118. 161-170. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 329 MIXED A1-Fe PILLARED LAPONITES : PREPARATION, CHARACTERIZATION AND CATALYTIC PROPERTIES IN SYNGAS CONVERSION F. BERGAYA' , N HASSOUN' , L. GATINEAU' and J. BARRAULT' 'Centre de Recherche sur les Solides a Organisation Cristalline Imparfaite, C.N.R.S., lB, rue de la Ferollerie, 45071 Orleans, France. 'Laboratoire de Catalyse en Chimie Organique, UA 350, C.N.R. S . , 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France. ABSTRACT were obtained by a simple method of preparation of PCL (Pillared Clay Laponite). Solutions of A1C13, FeC13 and NaOH were mixed with a clay suspension in adequate concentrations and kinetic conditions. A combination of techniques (chemical analysis, XRD, NMR and H'TPR) shows iron cations replace a few octahedral A1 in the AlI3- pillars. The mixed A1-Fe pillars present original catalytic properties with shape selectivity different from that of conventional iron catalysts. Mixed A1-Fe pillars INTRODUCTION The synthesis of pillared clays has been extensively studied since 1974 (Ref.1) but syntheses of mixed pillared clays are rather scarce. The present work reports the pillaring of laponite (synthetic hectorite) by mixed aluminium-iron pillars in view of their use as catalysts in the Fischer-Tropsch reaction. METHODS Chemical analyses of the principal elements of the pillared and unpillared clays were determined by X-ray fluorescence, except for Li ions which were analyzed by atomic absorption. XRD patterns of oriented films were obtained on a Siemens diffractometer (CuKor). NMR spectra of 27Al were obtained on a Bruker spectrometer (model MSL) at 94 MHz, with magic angle spinning. Temperature programmed reduction (TPR) by H2 (heating rate = 4°C per mn to 500°C) was performed in a glass reactor. The samples were always predried at 130°C for 12 h and the water was trapped during the reaction. 330 PREPARATION OF Al, Fe AND MIXED A1-Fe PILLARED LAPONITES FROM AN INITIAL Na LAPONITE The laponite (a synthetic trioctahedral smectite from Laporte Industrie) was purified by classical ion exchange with 1N NaC1, and its CEC (determined by EDA CuC12) is 94 meq/100 g of calcined clay. Different mixtures of an 0.1 M solution of A1C13 and of an 0.025 M solution of FeC13 were prepared, each corresponding to one of the following values of the ratio Fe/(Fe + Al) : 0, 0.1, 0.2, 0 . 3 , 0.5 and 1.0. Each of these solutions was added slowly and at a well-controlled rate, together with a 0.12 N NaOH solution. [OH/(Al + Fe) = 1.21 to a well-stirred 2% clay suspension (previously prepared by stirring 2 hours at 80°C) at 40°C until the ratio of concentration reached 30 meq (A1 + Fe) per gram of clay. The rate of addition was such that the pH of the suspension decreased progressively from u 9 to N 4 . After two days ageing at room temperature, the clear supernatant was eliminated remaining suspension was dyalized six times with 0.3 1 distilled water per gram of clay. After a further seven days ageing, the clear supernatant of the dyalized suspension was again eliminated and a part of the remaining suspension was used to prepare oriented films for XRD while the rest was finally lyophilized to obtain powder samples. and the RESULTS 1. Chemical analvsis The chemical analysis of the initial laponite is given in Table 1. The formula of the initial Na laponite is : and its molecular weight is 728.823 g. A s the clay is free of iron, all the iron found later corresponds to added iron. The contribution of the amount of aluminium initially present is taken into account when considering the total aluminium present in the samples prepared. The chemical analyses of these samples, as regards the major oxide SiOz, MgO, A1203 and FeZ03 are given in Table 2. 331 TABLE 1 Chemical analysis results and composition of the initial Na laponite based on these data. Oxygen Normalized to Correspon Oxides Weight (%) content, 22 oxygens ding weight in calcined clay in atoms Oxveens Metals in gram S i02 '3 MgO Liz 0 TiOZ Na2 0 K2 0 6 5 . 5 6 3 0 . 3 5 6 30 .299 0 . 7 3 8 0.076 2 . 8 6 4 0 . 1 0 2 2 . 1 8 2 0.010 0 . 7 5 2 0 . 0 2 5 0 . 0 0 2 0 . 0 4 6 0.001 1 5 . 9 0 6 0 . 0 7 3 5 . 4 8 2 0 . 1 8 2 0.014 0 . 3 3 5 0 . 0 0 7 7 . 9 5 3 4 7 7 . 8 9 6 0 . 0 4 9 2 . 4 9 8 5 . 4 8 2 2 2 0 . 9 7 9 0 . 3 6 4 5 . 4 3 8 0.007 0 . 5 5 9 0 . 6 7 1 2 0 . 7 9 4 0.014 0 . 6 5 9 9 9 . 9 9 8 3 . 0 1 8 2 1 . 9 9 9 7 2 8 , 8 2 3 TABLE 2 Chemical analysis results normalized relatively to the major oxides. The label of each sample refers to the composition of the mixed halides solution used in the preparation. S i02 6 8 . 1 4 5 9 . 4 3 5 5 . 0 0 5 1 . 4 1 4 7 . 7 2 4 3 . 9 2 3 8 . 8 6 A12 '3 '3 3 1 . 4 9 2 7 . 4 2 2 4 . 7 9 2 2 . 7 5 2 0 . 4 8 1 9 . 6 8 1 6 . 7 3 0 . 3 7 1 3 , 1 4 1 1 . 6 2 9 . 9 3 7 . 2 8 0 . 4 6 nd 8 . 5 9 1 5 . 9 1 2 4 . 5 1 3 5 . 9 4 44,40 nd : not determined 332 The ratio of the amounts of SiOz and MgO in the initial laponaite and in the pillared laponaites is not constant as shown in Table 3 . TABLE 3 Relative variations between the ratio : RSi - SiO, in initial laponite/SiO, in pillared laponite and Rns = MgO in initial lamonite/Mgo in pillared laponite with the increasing amount of initial FeC13 solutions. Lap All00 1. 14b4 1.148, 0. 001, Lap Al,, Felo 1,238, 1. 270, 0 . 0 3 1 5 Lap AlmFe2, 1. 325, 1. 3 8 4 , 0.059, Lap Fe3o 1.427, 1. 537, 0.109, Lap A150 Fe50 1.551, 1.600, 0 . 0 4 9 , * Lap Fe,M, 1.759, 1.889, 0 . 1 2 9 , * Lap A15,Fe5, presents an exception for AX that we do not explain. The increase in AX with increasing amount of FeC13 added initially can be explained by the fact that the solution becomes more acidic, which i) causes a release of a small amount of octahedral ions (Mg and Li) eliminated by dyalisis, and ii) probably attacks the edge of the tetrahedral sheet with removal of collofdal silica and destruction of a part of the layer which may therefore be different from one sample to the other. To compare the samples among themselves and taking the remaining sheets as reference we should take into account the collofdal silica which was not eliminated by dyalisis and which changed the weight of the pillared laponites. The quantities of A1203 and Fe203 retained by the clay and normalized with respect to the remaining sheets are shown in Table 4 . From these results, it is clear that iron is more retained by the clay than aluminium. Comparing the quantities introduced and those retained, show a greater selectivity for iron. 333 TABLE 4 % of oxides re ta ined by the remaining sheets % A1203 X Fe203 14.59 14.27 13.26 10.72 0.35 10.82 21.86 37.40 57.04 82.92 2 . XRD pa t te rns The XRD p a t t e r n s of the or ien ted f i lms show a 001 peak a t 1 7 A f o r the i r o n - f r e e sample (Lap All, ) and t h i s peak is broadened with increasing i r o n content i n the samples. The organizat ion i s improved by ageing the suspensions. After heat ing the samples t o 400°C, the 001 peak is a t about 16 aluminium p i l l a r e d c lay and a t about 2 2 A f o r i r o n p i l l a r e d clay proving the p i l l a r i n g occured. A f o r 3 . NMR spec t ra The NMR spectrum of the i r o n - f r e e sample confirms p i l l a r i n g by Al13 p i l l a r species as shown by Plee e t a l , 1985 (Ref. 2 ) with the te t rahedra l A 1 peak a t 62.9 ppm, and a r a t i o of in tegra ted surfaces of te t rahedra l A 1 peak over octahedral A 1 close t o 1 2 . Luckily, we obtained spec t ra from two o ther samples i n s p i t e the f a c t t h a t i r o n i s present (Lap A190Fe,o and Lap Al,,FeZo). Two observations can be made regarding these spec t ra . ( i ) The te t rahedra l and octahedral peaks previously observed with Lap A l l , a r e always present , but a l i t t l e s h i f t e d . A second, very broad octahedral A 1 peak appears. This broadness i s probably due t o the paramagnetic e f f e c t of the i r o n i n the neighbouring environment. ( i i ) The t o t a l amount of A 1 v i s i b l e by NMR decreases when the amount of Fe increases . Although it could be t h a t only the pure A l l , p i l l a r s could be observable, then may e x i s t some mixed p i l l a r s with Fe s u b s t i t u t i n g a p a r t o f the A 1 atoms, therefore not e a s i l y de tec tab le by N M R . This concept of an 334 isomorphic substitution suggested by the NMR results will be later confirmed in the discussion. 4. â¬i2m We have used this technique to characterize the state of iron at the start of the catalytic process. The results of the reduction give some important information. (i) In the case of samples rich in aluminium (until Lap Al,Fe,,), there are two species of iron, with only one species reducible by H2TPR. (ii) In the case of Fe rich samples, all the iron is reduced but at two different rates, supporting also the idea that two species of iron exist, one more easily accessible to H2 than the other. DISCUSSION Both NMR and XRD patterns confirm the presence of an : Alr304(OH),, ( H 2 0 ) 1 2 pillar. The data from chemical analysis allow us to calculate the charge and the pillar density. From the contribution of A1203 (14.59%) and knowing the formula weight (707.37 g), the CEC (0.685 charge/unit cell) of the initial laponite and the weight of the oxides pillar (6.5 A1203 - 662.74 g) we find : 14.59 x 707.37 100 x 662.74 - 0.16 pillar/unit cell 0.685 0.16 and - - 4 .&+/pillar. These results agree with previous results of All, pillared montmorillonites obtained by Plee, 1984 (Ref.3). The H2TPR results show that we have two species of Fe. From the NMR results, we can suppose that some octahedral A1 of the pillars are replaced by Fe. Within this hypothesis, we can calculate the amount of iron necessary to compensate for the decrease of A1 in the All, pillar for the different samples. For the first three samples, the calculated isomorphic ratio allowsus to determine the iron content needed to form an (Al13., Fe,) pillar and to deduce from the total iron retained, the iron content out of the pillar. This last amount corresponds exactly to the part of iron reduced by H2TPR, as shown in Table 5. 335 TABLE 5 Isomorphic ratio (IR) of A1 by Fe and out of pillars content of Fe determined from chemical analysis data. Comparison with iron content reduced by H2TPR. Total IR Total Out of pillar % Fe203 % A1203 (%) % Fe203 Fe203 Reduced by H2TPR Lap A & , Felo 1 4 . 2 7 2 . 1 9 1 0 . 8 2 1 0 . 3 2 1 0 . 4 5 Lap Al,Fe,, 1 3 . 2 6 9 .13 2 1 . 8 6 1 9 . 7 7 21 .02 Lap Al,oFe30 1 0 . 7 2 2 5 . 5 1 37 .40 3 1 . 3 5 30.86 Lap Al,, Fego 0 . 3 5 5 7 . 0 4 60.05 Lap Fe,w 8 2 . 9 2 8 5 . 5 2 The two last samples practically do not contain any aluminium and, consequently, no (A1Fe)13 pillars, but we have no information about the nature of any iron pillars, except that the iron pillars are probably reducible, and reduced later than the extra pillars iron. In summary, the combination of all these techniques supports the presence of : (i) Al,, pillared laponite (Lap All, ) (ii) mixed (A113.x Fe,) pillared laponites with Fe in isomorphic substitution of octahedral A1 of the pillars. (iii) another kind of oxyhydroxides of iron retained by the clay, perhaps out of the interlamellar space. CATALYTIC PROPERTIES IN SYNGAS CONVERSION When these samples were used in the (CO, H2) reaction, we observed catalytic properties quite different from those of conventional iron catalysts for the mixed (A1 Fe) samples. 1.The activities are significantly enhanced when the total iron content 336 increases ; the fact that the initial activation phase is faster when iron content increases could be related to iron located out of the pillars. 2 . For the selectivities, we can consider two successive steps (i) at the beginning of each catalytic test, the CH, selectivity is similar for all the samples, but the selectivity of ( C 6 - C e ) hydrocarbon increases with increasing iron, since the olefins decrease. (ii) After the stabilization phase (when the activityreaches a plateau), the CH, increases but both olefins and heavier hydrocarbons decrease considerably when the iron content increases. This is evidence for the importance o f t h e h y d r o g e n a t i n g p r o p e r t i e s o f t h e i r o n l o c a t e d o u t o f t h e p i l l a r s . 3 . Surprinsingly, the hydrocarbon distribution obtained with mixed pillared laponites does not follow a Schulz-Flory law. This selectivity deviation is comparable to the shape selectivity observed with zeolite encapsulated metal clusters in the conversion of syngas or methanol into light olefins as shown by Nazar et al, 1 9 8 3 (Ref. 4 ) . The different behaviour of the Lap Al,Fe,, sample at this stage of stabilization, where the shape selectivity desappears, indicates that this catalyst is at the limit of isomorphic substitution and is not entirely representative of mixed pillared laponites, but is similar to the pillared iron catalysts which act as conventional iron catalysts used at high temperature. For these last samples, CH4 is always the major product. These new catalytic properties observed in syngas synthesis, namely high olefins formation, shape selectivity and stability at relatively high temperature are probably due to the presence of mixed (Al-Fe),, pillars. REFERENCES 1 D.E.W. Vaughan in R. Burch (Ed). Pillared clays : a historical perspective, Vol 2 Elsevier. Pillared clays Catalysis today, 1 9 8 8 , p.188. 2 D. PlBe, F. Borg, L. Gatineau and 3.5. Fripiat. 3. Am. Chem. SOC. 3 D. PlBe. (1984) PhD thesis, Orldans, France. 4 L.F. Nazar, G.A. Ozin, F. Hugues, 3 . Godber and D. Rancourt.Angew. 107 ( 1 9 8 5 ) , pp. 2 3 6 2 - 2 3 6 9 . Chem. Int., ( 1 9 8 3 ) , 2 2 , pp. 6 2 4 - 6 2 5 and see also Angew. Chem. Suppl. pp. 8 9 8 - 9 1 9 . G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 337 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands ZIRCONIUM PILLARED CLAYS. INFLUENCE OF BASIC POLYMERIZATION OF THE PRECURSOR ON THEIR STRUCTURE AND STABILITY E.M. Farfan-Torres, 0. Dedeycker, P. Grange Catalyse et Chimie des Mattriaux Divists, Universitt Catholique de Louvain, Place Croix du Sud, 2 Bte 17,1348 Louvain-la-Neuve (Belgium) ABSTRACT Increasing the pH of the pillaring solution by addition of NaOH increases the thermal stability of Zr pillared clays. The improvement may be directly correlated to the amount of ZrO2 and to the density of the pillars. The strong interaction between the pillars and the clays promotes high acidity of this microporous solid. INTRODUCTION The experimental conditions of the hydrolysis and polymerization of Zirconyl chloride largely influences the thermal stability and texture of the 3-pillared montmorillonites. Strong acidity of zirconium solution as well as high temperature of the pillaring process bring about brittleness and even partial destruction of the clay (1). An increase of the pH of the pillaring suspension modifies the hydrolysis-polymerization of the precursor as well as the cationic exchange capacity (CEC) of the clay (2,3,4). Vaughan et a1 (5) modified the pH of the ZrOC12-montmorillonite suspension adding Nap203 solution. The other papers on Zr-pillared montmorillonites paid relatively little attention to this parameter (6- 11). In this work we present the influence of the concentration of NaOH, introduced in two different ways, on the thermal stability, porosity and acidity of Zr-pillared montmorillonite. The basic solution was added either in the ZrOC12 solution before pillaring (ex-situ) or after the Zr solution was contacted with the montmorillonite suspension (in-situ). EXPERIMENTAL RESULTS Table 1 reports the way of preparation of the different samples. Sample final pH NaOH (0.1 M) CEC Zro2 "ex-situ" "in-situ" (meq/lOOg) (wt %) EI-05 1 _ _ _ _ 68.7 16.20 EII-01 1.9 X 48 24.22 EII-02 1.9 X 46.7 23.00 EII-03 3.9 X 48.4 25.40 EII-04 3.9 X 40.5 30.46 Table 1. Zr pillared clays: pH during the pillaring process, cationic exchange capacity and concentration of ZrO2 after pillaring. 338 Si@ A1203 w Fez03 K20 CaO Na20 rn H20+ Montmorillonite (Weston L-Eccagum) was first exchanged with NaC1. After ageing for 30 days, acetone was added to the Na-clay in order to obtain a water-acetone ratio equal to 1/1. NaOH (0.1 M) was introduced either in the ZrOCl2 solution or in the ZrOC12-clay suspension. During the pillaring process, the suspension was stirred for 2 hrs at 4 0 O C . The clay was then washed up to constant conductivity of the solution. After freeze-drying the samples were calcined at different temperatures up to 6 0 0 O C . Four samples were prepared, modifying the pH of the pillaring solution and the way of introduction of the NaOH. In addition one Zr pillared clay was prepared without addition of base. 60.82 50.12 46.06 48.30 47.71 42.72 21.48 16.10 15.40 16.39 15.38 14.28 3.66 2.31 2.56 2.57 2.60 2.22 0.83 0.64 0.67 0.66 0.65 0.59 0.11 0.11 0.03 0.03 0.19 1.41 0.05 0.05 0.00 0.00 0.00 0.01 2.74 0.06 0.05 0.07 0.05 0.01 _ _ 16.20 24.22 23.00 25.40 30.46 10.31 14.40 9.57 8.98 8.02 8.16 The chemical composition of the Zr-pillared clays as well as the Na montmorillonite are reported in table 2. Silica, alumina and zirconia have been analyzed by X-Ray fluorescence; the other elements by atomic absorption spectroscopy after sulfofluorhydric dissolution of the clay (6). oxide % Na+ mont. EI-05 EII-01 EII-02 EII-03 EII-04 I The amount of ZrQ in the clay increases with the pH of the pillaring solution. From these two figures it can be seen that the solids synthetized at pH=1.9 are thermally stable. In addition, the diffraction lines are narrower and better defined than for the sample prepared without ammonia. For higher pH, the structural characteristics of the clay are different. Two diffraction lines at 19 %, and 12.6 A are observed on the uncalcined solid EII-03. This behaviour mdicates the presence of two different basal spacings. The interlayer distance observed for the EII-04 sample is 12.6 A whatever the thermal treatment. XRD of the samples calcined up to 600°C are presented in fig. 1. The specific surface area of the pillared montmorillonite prepared by "ex-situ" polymerisation of the Zr complex at pH=3.9, is much lower than that of the other clays. At 700°C this sample presents the same surface area as the sample prepared without NaOH. 339 10 2 10 2 10 2 10 2 10 2 28 Fig. 1. XRD spectra. Calcination temperature : (a) 25°C; (b) : 110°C; (c) : 200°C; (d) : 300°C; (c) : 400oc; (0 : 500°C; (g) : 600°C. The evolution of the basal spacing with the calcination temperature is plotted in fig. 2. t 0 200 LOO 600 800 T I°Cl Fig. 2. : Evolution of the basal spacing with the calcination temperature : EI-05; 0 : EII-01; R EII-02 A EII-03; A EII-04. The evolution of the specific surface area of the different samples with the calcination temperature is illustrated in fig. 3. 340 0 ' 0 200 LOO 600 800 T /"C/ Fig. 3. Specific surface area. 8 : EI-05; 0 : EII-01; I : EII-02; A : EII-03; A : EII-04. The total acidity of the solids was evaluated by Programmed Temperature Desorption (TPD) of NH3. For those experiments 0.10 g of samples seived (200 341 DISCUSSION Schofield has shown that the pH of the solution controls the amount of ammonium cations fixed on montmorillonite and kaolin (13). For a determined pH, the amount of potentially fixed ions is constant, this amount increases with the pH. Electrophoretic mobility measurements of Na- montmorillonite support this view. The electrophoretic mobility and the superficial charge are drastically decreased at low pH (1 342 pH = 1.9 L 00 oc __t pH = 3.9 "in situ" - pH = 3.9 "ex situ" - LOO*C x=EIZE---- 3zzZz Fig. 4. Schematic representation of the architecture of the Zr pillared clays. CONCLUSIONS The increase of the pH of the pillaring solution up to 1.9 increases the amount of Zr intercalated with the layer structure. In addition, this leads to a better distribution of the pillars which induces a high thermal stability and microporous structure of the system. This improvement could be directly correlated with the density of the pillars. In addition, the number of structural defects created by the acidic solution could decrease, inducing less possible proton migration towards the octahedral layer. The strong interaction between the pillars and the silica layers improves the stability and stabilizes the 2102 amorphous phase. In this way, the acidic properties of the Zr PILCs are improved as compared with the Na montmorillonite or the bulk ZQ oxide. When the pH of the pillaring solution is adjusted at 3.9,Zr(OH)4 precipitates. The addition of NaOH to the Zr-clay suspension led to a mixed system in which bulk Z a and Zr pillars are present. When the adjustment of the pH to 3.9 is done on the ZrOC12 solution, no pillars are formed and most of the zirconium could be deposited as bulk Z e . ACKNOWLEDGEMENTS The financial support of the SPPS (Services de Programmation de la Politique Scientifique, Belgium), is greatly acknowledged. E.M. Farafan-Torres thanks the CGRI (Commissariat GCntral aux Relations Internationales de la Communautt FranGaise de Belgique) for a grant. 343 REFERENCES (1) E.M. Farfan-Torres, E. Sham, P. Grange, Clays and Clay Minerals, submitted for publication. (2) W. Rausch, H.D. Bale, J. Chem. Phys., 40 (1964), 3891. (3) A. Clearfield, Inorg. Chem., 3 (1964), 146. (4) D. Plee, F. Borg, L. Gatineau, J.J. Fripiat, J. Am. Chem. SOC., 107 (1985), 146. (5) D.E.W. Vaughan, R.Y. Lussier, J.S. Magee, U.S. Patent (1979), 4,176,000. (6) S . Yamanaka, G.W. Brindley, Clay and Clay Minerals, 27 (1979), 119. (7) E. Kikuchi, R. Hamana, M. Nakano, M. Takemara, Y. Morita, J. Japan Petrol. Inst., 26 (8) G.M. Muha, P.A. Vaughan, J. Chem. Phys., 33 (1960), 194. (9) R. Burch, C.I. Warburton, J. Catal., 97 (1986), 503. (10) G.J.J. Bartley, Catal. Today, 2 (1988), 233. (11 ) M.L. Occelli, D.H. Finseth, J. Catal., 99 (1986), 316. (12) LA. Vomovitch, J. Louvier, J. Debras-Guedon, L'analyse des Silicates, (Herman, ed.) (1962). (13) R.K. Schofield, J. Soil Sci., 1 (1949), 1. (14) K. Shibata, T. Kiyoura, J. Kitagawa, T. Sumiyoshi, K. Tanabe, Bull. Chem. SOC. Jap., 46, (1983), 116. 1973, 2985. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 345 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands CONTROL OF THE ACIDITY OF MONTMORILLONITES PILLARED BY A1-HYDROXY CATIONIC SPECIES D. TICHIT', Z . MOUNTASSIRl, F. FIGUERAS' and A.AUROUX" Laboratoire de Chimie Organique Physique et Cinetique Chimique Appli- quees, URA 418 du CNRS, ENSCM, 8 rue de 1'Ecole Normale, 34053 Montpellier Cedex 2, France. " Institut de Recherches sup la Catalyse du CNRS, 2 avenue A.Einstein, F69626 Villeurbanne Cedex, France. SUMMARY The nature, number and strength of the acid sites of pillared clays have been changed by three independent methods. In the step of synthesis, the use of competitive ion exchange for the intercalation of the A1 hydroxy ca- tions promotes a stronger acidity, attributed as f o r pillared beidellite, to a better retention of their individuality of the isolated All, cations. On the pillared clay , the ion exchange of the residual cations by Ce3+, La3*, Ca2*, Mg"' o r Zn2* affect differently the acidities: Ce exchange stabilizes Bronsted acidity, whereas Zn"+ suppresses i t , but the total number of acid sites is little affected. Steaming increases first Bronsted acidity through hydrolysis of the pillars, then decreases the acid strength. All these modifications also affect the thermal stability of the material. No correlation exists between the acidity of the pillared clays and their acidity, which can be then easily changed in a wide range. INTRODUCTION Clays have long been used as catalysts due to their acidic nature after cation exchange o r acid treatments. For montmorillonite and hectorite the most used smectites both Lewis and Bronsted acid sites were found the origin of which are well established. Bronsted acidity essentially results from the dissociation of water molecules in the hydration shell of the interlayering ex- changeable cations. Lewis acid sites result from the low coordination of alu- minium and iron atoms at crystal edges. These clays catalyse many organic reactions (1) and constituted also the first generation of FCC catalysts. Their interest in this last field was recently renewed by the preparation of pillared interlayered clays (PILCs) . The swelling of the interlayer space by oxydes pillars yields a class of molecular sieves containing both Lewis and Bronsted sites after calcination up to 500°C and only of Lewis sites beyond ( 2 , 3 ) . It is generally admitted that the pillars are the source of the improvement of the Lewis acidity. Higher 346 acid strengths are reported for calcined A1 , Ti o r Zr PILCs than for Ni o r Cr analogs ( 4 , 5 ) . The number of acid sites generally increases with cross- linking (4 ) but decreases regularly with the calcination temperature of the PILCs. Lewis acid strengths as high as those in CeY zeolites were reported (2,3) for A1-PILC samples calcined at 500OC. The acidity is also dependant of the clay layer structure. The occurence of a bidimensionnal zeolite structure (6) in the case of beidellite is accompa- nied by an increase in the number and strength of acid sites comparatively to pillared smectites without tetrahedral substitutions. This is due to the existence of an acidic OH group bonding the pillar to an inverted tetrahedron of the layer and also to the better retention of the All, individual structure due to the more localized charge of the clay layers preventing their coales- cence ( 6 ) . The Bronsted acidity was shown to be mainly provided by the structural OH groups of the clay layers (4). The disapearance of the Bronsted acidity after calcination at relatively low temperature is in line with the more easy dehydroxylation observed for the PILC than for the parent clays. Dehydration and dehydroxylation of the pillars liberates protons. They could also enhance the Bronsted acidity. But this protons produced under calcination migrate to the negative sites of the octahedral layer of the mont- morillonite. This trapping does not give active Bronsted sites and destabilizes the structure ( 7 ) . The aim of our work was to show that the acidity of the aluminium pilla- red montmorillonite could be modified in several ways, at first by the prepa- ration method doing an exchange in competition of the A1,,0,0Hz4(Hz0)lz7' with NH,' ions and then by cationic exchanges o r steaming of the well stabi- lized structure of the calcined PILCs . EXPERIMENTAL Catalyst preparation The clay material used was a suspension of Volclay montmorillonite refi- ned by CECA (Honfleur-Prance) having particles smaller than 0.5 pm in size. The A1-PILCs were prepared by adding to the clay an oligomeric cationic species All,0,0H,,(H,0)l,7+ obtained by mixing two 0.2M solutions of A1C1,.6Hz0 and NaOH at ambiant temperature with a ratio OH/Al=2. The pH=4.2 of the solution was raised to pH=6 before the addition to the clay and a ratio of 5 mmole Al/g clay was used. The slurry was then aged for 3 hours at 80°C , filtered, washed until chlorine free and dried at 6OoC in air. A calcination of this intercalated clay above 5OO0C induces a stabilization of the porous network by dehydroxylation and evolution of the intercalated oligome- ric cations to oxyde pillars. 347 Ionic exchange by competition In a variant of this method the exchange of the parent clay by the alu- minium cations was performed in competition with NH,' ions. Solution with NH,'/Al molar ratio of 10 was used. No modification of the "7Al NMR spec- trum was evidenced. The preparation is then realized as precedently descri- bed, Ionic exchange of the calcined PILCs After pillaring and calcination the C .E .C of the clay is lost due to the migration of H+ to the octahedral layer ( 8 , 9 ) . A treatment of the PILCs cal- cined at 5OO0C using a 4.10-3 M solution of K,CO, at 8OoC restores the C.E.C (10). Exchanges were further realized with 3.75.10-3 M solutions of MgCl,.6H,O, Ca(N0,),.4H20 ,ZnCl, ,CeCl,.6H2O and LaCl,.GH,O. After wa- shing samples were dried at 6OoC. Steaming of the calcined PILCs Steamings at 55OOC and 65OOC under 1 atm of vapor pressure were per- formed during 17 hours on A1-PILCs samples prealably calcined at 68OoC. These temperatures are in the range of those encountered during regenera- tion in catalytic craking process. Characterizations of the pillared clays Acidic properties were analyzed by several complementary methods in order to obtain informations on the nature, number and strength of the acid sites. The adsorption of pyridine, used as probe molecule, was studied by IR spectroscopy on self-supported wafers obtained by pressing the PILCs into thin films. Lewis and Bronsted sites give characteristics peaks having well established positions around 1450 and 1550 cm-I respectively. Evacuations at increasing temperatures were useful to investigate the acid strength . Adsorption isotherms of NH, were studied by microcalorimetric method. Samples on powder form were evacuated at 500'c. The heat of adsorption in function of the quantity of NH, gives a comparison of the respective acidity of the PILCs. The distribution of acid strength are given plotting the diffe- rential heat of adsorption in function of the coverage. It gives the fraction of sites adsorbing NH, with a given energy. The amount of NH, adsorbed at 100°C on degassed sample, measured by thermogravimetric method give an evaluation of the number of acid sites. RESULTS Ionic exchange by competition In table 1 are reported the chemical compositions of the parent Volclay montmorillonite , of the PILCs prepared by a conventionnal method (Vc) o r by ion exchange in competition (Vc.e) . 348 TABLE 1 Chemical compositions in moles on a dry basis, of the original clay and of the pillared clays prepared from it. oxide Volclay vc Ve.c SiO, 66.6 66.6 66.6 A1,Os 20.6 51.6 46.6 Fe&, 4.6 4.8 4.8 MgO 2.9 - - montmorillonite A s expected with ion exchange in competition less aluminium is introdu- ced. With the hypothesis of A1 belonging to Al,,0,0H,,(H,0).,7' cations in their individual form, 327 and 274 meq/100 g clay were respectively retained. This discrepancy with the cation exchange capacity of the clay (80- 110 meq/100 g) is due to the oligomerisation of the All, cations at pH=6. IR spectroscopy characterisations of pyridine adsorption reveal no change in the nature of the acid sites between Vc and Ve.c samples. At the opposite the total number of acid sites evaluated by the calorimetric adsorption of NH, is slightly higher in sample Vc as evidenced on Fig.1. This agrees with the ge- nerally observed increase of acidity with the quantity of aluminium intercala- ted and consequently the number of pillars. The most different behaviour is observed in the acid strength distribution as shown on Fig.2. 7- 0 '2 E 2' Y C Q L 0 VI -0 0 0 0 0 5 0 & L L 2wL 100 1UI 200 300 N H3 od so r b e d (pmoles.cjl) Fig. 1. Differential heats of adsorption versus coverage for the adsopr- Acid sites of intermediate strength (60 kJ/mole ) are obtained on both samples prepared by the conventionnal o r by the competition method. Moreo- ver in this last case there is also an intense peak characteristic of strong acid sites with heats of adsorption around 120 kJ/mole. tion of NH, at 15OoC on samples (a) Vc and (0) Ve.c. 349 Cationic exchange of the PILCs Up to 60 meq/100g of cations could be retained by the PILC sample Vc calcined at 5OO0C (Vc-500) after the alkaline treatment 0 100 200 0 lo0 200 Heat OF adsorption(KJ.mole' ) Fig. 2. Distribution of acid strengths for samples Vc and Ve.c The IR spectra after pyridine adsorption of Vc-500 and of this sample respectively exchanged with rather quantities (42-47 meq/100 g) of divalent : Caz', Zn2' (Vc(Ca-47)-500, Vc(Zn-44)-500) and trivalent: Ce"', La"' cations (Vc(Ce-47)-500, Vc(La-47)-500) are reported on Fig.3. The pyridine was desorbed by steps every 100°C up to 400OC. The nature of the acid sites is influenced by the type of cations ex- changed. Both Lewis and Bronsted acidities are detected with Caz+, La"' and Ce"' as in Vc-500, but only Lewis acidity with Zn"'. The number of acid sites measured by thermodesorption of NH, is in all cases in the range of 1.7-2.4 pmole NH,/m2 and not correlated to the ionic size o r charge of the cations. Cationic exchange induces greatest change on the acid sites strength. Ca"' restores Bronsted acid sites still observable after pyridine desorption at 2OO0C, higher than on Vc-500 or Vc(La-47)-500 and Vc( Ce-47) -500. Whatever cation is exchanged, the intense peak at 1450 cm-1 reveals that the acidity is mainly of the Lewis type in accordance to what is observed on the parent non exchanged PILC Vc-500 but comparati- vely a general decrease of the strength is noted. This trend is much less developped with Ca"' where Lewis sites are still observable after desorption at 4OO0C than with La"' and Ce"' where a residual shoulder only remains. The decrease of total acidity is drastic with Zn"' cations. 350 Quo 1500 cr a WAVENUMBERS r - 1700 1500 WAV ENWBERS lâ Fig. 3. I.R. spectra of pyridine after degassing at increasing temperatures on samples : a : Vc-500 ; b : Vc(La-47)-500 ; c : Vc(Ce-47)-500 ; d : Vc(Ca-42)-500 and e : Vc(Zn-44)-500 Acidity of steamed Al-PILCs : IR pyridine adsorption studies show that the AL-PILC sample Vc calci- ned at 68OOC (Vc-680) has a strong Lewis acidity only (Fig.4). On this sample steamed at 55OOC (Fig.5) a strong Bronsted acidity is observed still detectable after evacuation at 480OC. Steaming at 65OoC reduces the strength of both Bronsted and Lewis acid sites,no more detectable after degassing at 3 O O O C . 351 WAVEN UM BE R Fig. 4 . I . R . spectra of pyridine on Vc-680 after degassing at increasing temperatures 1800 1600 1400 WAVENUMBER 1800 1600 1400 Fig. 5. I .R . spectra of pyridine on Vc-680 ; a : steamed at 50OOC ; b : steamed at 65OoC after degassing at increasing temperatures. 352 DISCUSSION Ion exchange is a fast process, which can then be limited by diffusion. Diffusional limitations in the course of the intercalation of the oligomeric alu- minium species, responsible of the inhomogeneous distribution of the pillars, are greatly reduced by competitive ion exchange with NH,'. A first conse- quence reported in a precedent paper (11) is an enhancement of the thermal stability of the PILC. A second one is a higher acidity induced by isolated All, pillars, which contain a tetrahedral aluminium cation. This tendency is similar to that observed in zeolites where dealumination increases the acid strength by reducing the number of neighbour A1 atoms. I t was reported elsewhere (12) that ion exchange increases the thermal stability of the PILCs. Removal of H' from the exchange sites not occupied by the pillars reduces the reactions of hydrolysis of the framework. Ce3' and La"' are more efficient in this purpose than Ca2' and Zn2'. The modifications of acidity are hardly correlated to those of thermal stability. Acid strengths are reduced by cationic exchange and some changes are observed in the nature of the sites but their number is not appreciably modified. These trends are in line with the hypothesis that acidity is mainly related to the aluminium sites of the pillars. Cationic exchange is able to restore a Bronsted acidity , resulting then from the hydrolysis of the cations. But a decrease of the Lewis acidity also happens due to the destabilization of the alumina pillars by formation of spinel like phases. The reactions of hydrolysis taking place during steaming liberate protons as evidenced by the appearance of a strong Bronsted acidity not observed on the sample just calcined. An other consequence is the decrease of the thermal stability due to the probable migration of the protons into the clay layer. The decrease of the total acidity after steaming is a general phenomenon also observed on commercial FCC catalysts, which need such treatment to mo- derate the catalytic activity. In the case of PILCs the increase of Bronsted acidity is perfectly related to the enhancement of cracking activity observed in microactivity tests ( 1 2 ) . It is interesting to notice that all three methods reported here to modify acidity also induce great changes in the thermal sta- bility. Competitive ion exchange, used in the synthesis of the pillared clay, leads to an important modification of the structure of the PILC. Ion exchange and steaming are "post synthesis" treatments which permit an adaptation of the acid strength distribution of the solid f o r a given catalytic application. 353 REFERENCES J . M . Adams, Applied Clay Science, 2 (1987) 309-342. M.L. Occelli and J.E. Lester, Ind. Eng. Chem. Prod. Res . Dev., 24 D . Tichit, F. Fajula, F. Figueras, J. Bousquet, and C. Gueguen, in t'Catalysis by Acids and Basest' ( B .Imelik, C. Naccache, G. Coudurier, Y . Ben Taarit and J . Vedrine, Eds), Elsevier, Stud. Surf. Sci. Catal. 20 (1985) 351. H . Ming-Yuan, L. Zhunghui and M. Enze, Catalysis Today, 2 (1988) 321-338. L. Zhon Ghui and S . Guida, in "Zeolites: Synthesis, Structure, Technology and Application" ( B . Drazj, S . Hocevar, and S . Pejovnik Eds), Elsevier, Stud.Surf.Sci.Cata1. 24 (1985) 493. D . Plee, F. Borg, L. Gatineau and J. J. Fripiat, J . Amer. Chem. Soc.,lO7 (1985) 2362-2369. D . Tichit, F. Fajula, F. Figueras, B . Ducourant, G . Mascherpa, C. Gueguen and J . Bousquet, Clays Clay Miner. ,36 (1988) 369-375. J . D . Russell and R . A . Frazer, Clays Clay Miner., 13 (1971) 55. S. Yariv and L. Heller-Kallai, Clays Clay Miner., 21 (1973) 199. D.E.W. Vaughan, R. J . Lussier and J.S. Magee, U.S.Patent 4.271.043 (1981). F. Figueras, Z. Klapyta, P. Massiani, Z . Mountassir, D. Tichit, F. Fajula, C . Gueguen, J. Bousquet and A. Auroux, Clays Clay Miner., D. Tichit, F. Fajula, F. Figueras, C. Gueguen and J. Bousquet,in "Proc. 9th Int. Cong. Catal.", Calgary 1988, vol 1 Chemical Institute Canada (1988), p 112. (1985) 27-32. 38 (1990) 257-264. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 355 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands THE CHEMISTRY OF THE DEALUMINATION OF FAUJASITE ZEOLITES WITH SIL ICON TETRACHLORIDE Johan A. MARTENS, P i e t J. GROBET and Peter A. JACOBS Department o f Surface Chemistry, K.U. Leuven, Kardinaal Merc ier laan 92, 6-3030 Heverlee, Belgium ABSTRACT L i t e r a t u r e data on t h e dealumination w i t h Sic14 o f X and Y type z e o l i t e s w i t h Na, L i , H, Ca o r La as charge compensating cat ions i s reviewed. The p roper t i es o f the f i n a l z e o l i t e products, i n p a r t i c u l a r t h e degree o f dealumination, t he c r y s t a l 1 i n i t y , mesoporosity, and the nature and content o f extra-framework aluminium and s i l i c o n species are expla ined based on the chemistry invo lved i n the d i f f e r e n t steps o f t he dealumination procedures. The r e a c t i v i t y towards Sic14 o f t h e framework aluminium atoms i s expla ined based on the l o c a t i o n o f the associated charge compensating ca t i ons over accessible and hidden s i t e s . INTRODUCTION The dealumination o f z e o l i t e Y has been stud ied i n t e n s i v e l y du r ing the past two decades, mainly f o r t he purpose o f developing new FCC ca ta l ys ts . The changes i n a c i d i t y upon dealumination o f z e o l i t e Y samples has received much a t t e n t i o n from the t h e o r e t i c a l p o i n t o f view ( refs .1-11) . The c a t a l y t i c consequences o f t he dep le t i on o f t he framework w i t h aluminium and, i n p a r t i c u l a r , the changes i n product s e l e c t i v i t y w i t h t h e framework aluminium ( A I F ) and extra-framework aluminium (AIEF) content o f t he z e o l i t e are extremely important i n FCC. Reviews on t h i s subject are ava i l ab le ( re fs .12-14) . Many dealumination reagents can be used i n c l u d i n g steam (deep bed ca l c ina t i on , self-steaming and steaming), aluminium sequestering agents (EDTA, ACAC, ...), mineral acids and s i l i c o n conta in ing reagents ((NH4)2SiF6, SiC14). The removal o f framework aluminium du r ing dealumination o f t e n co inc ides w i t h a number o f secondary e f f e c t s . Hydrolyzed extra-framework A1 -0 species (AlEF) may be formed, as we l l as e x t r a - l a t t i c e Si-A1 oxide species. Another phenomenon consis ts i n the generation o f l a t t i c e defects (hydroxyl nests) and the annealing o f them w i t h amorphous s i l i c a . Upon dealumination, t he z e o l i t e l a t t i c e con t rac ts wh i l e some times mesopores are formed. The z e o l i t e shows an increased hydrophobic i ty and increased thermal and hydrothermal s t a b i l i t y . General ly speaking, upon dealumination the number o f Brensted ac id s i t e s decreases upon dealumination, wh i l e the average s t reng th per s i t e increases. 356 This review focusses on the preparational aspects of dealuminated Y zeolites and will not treat the catalytic implications of the modification. The dealumination method using Sic14 was selected because with this reagent, in principle, Si atoms should be supplied rapidly and be easily substituted in the lattice, thus avoiding defect and mesopore formation. Perfect siliceous and low-alumina faujasites are expected to be formed. An attempt is made to rationalize the 1 iterature data on the dealumination methods using SiC14, to compare methods and product properties and to treat the chemistry involved in the different steps of the dealumination processes. CRYSTAL CHEMISTRY OF FAUJASITE ZEOLITES Silicon en aluminium local environments Although all tetrahedral sites (T sites) in the faujasite framework are topologically equivalent, in real zeolite Y crystals this equivalency applies only in exceptional cases, e.g. when all T sites are occupied by silicon atoms. According to the Lowenstein rule (ref.15) the first shell of T sites surrounding an aluminium atom is occupied by silicon atoms only. Differences in the local environment of aluminium atoms are, therefore, only found in the second shell of surrounding T atoms, which may contain silicon as well as aluminium atoms. A T atom in the faujasite structure has nine next nearest neighbor (NNN) T atoms. When considering the loop configuration of T sites (Fig.1) these nine NNN can be subdivided in three NNN atoms in adjacent four- rings, four NNN atoms involved in adjacent six-ring loops, and two NNN atoms in a twelve-ring loop. Minimum approach and maximum electrostatic interaction is achieved by siting the aluminium atoms diagonally across the four-rings of T sites. In principle, four types of aluminium atoms can be distinguished on this basis, having respectively 3 , 2, 1 or 0 aluminium atoms diagonally opposed to it. Aluminium atoms are expected to be more electrostatically vulnerable and more susceptible to dealumination when a higher number of diagonally opposed aluminium atoms occurs (ref.2). A further subdivision is possible according to the number of NNN aluminium atoms in the six-ring loops and the twelve-ring loop, where up to six aluminiums can be located. The latter aluminium atoms are at a longer distance from the central aluminium atom and the electrostatic interaction is weaker (Fig.1). In conclusion, although all aluminium (and silicon) atoms in zeolite Y crystals are equally accessible through the zeolite cavities, they are chemically not identical, and their reactivity towards dealumination reagents could be different. Unfortunately, no methods are actually available for determining directly the T atom environment in terms of NNN of an aluminium atom. Indeed, the different aluminium T sites are even not resolved in the 27Al resonance envelope, measured with the high-resolution solid state magic angle 357 spinning nuclear magnetic resonance (MASNMR) technique. F igure 1. S i t i n g o f the n ine next nearest neighbors (NNN) o f a T s i t e i n f a u j a s i t e . Figure 2 . S i t i n g o f charge-compensating cat ions i n f a u j a s i t e . 29Si MASNMR spectra o f z e o l i t e X and Y samples e x h i b i t a s p l i t t i n g i n t o f i v e types o f s i l i c o n atoms according t o the number, n, o f aluminium atoms i n the neighboring T s i t e s ( re f .16) . Each Si(nA1) s ignal represents many components w i t h d i f f e r e n t l o c a l environments due t o the many l o c a l geometries o f t he f i r s t and second s h e l l A1 neighbors ( r e f s . 17-19). A t present, i t i s no t poss ib le t o resolve these components i n d i v i d u a l l y . Nevertheless, a 29Si MASNMR spectrum as a means t o determine the o v e r a l l framework composit ion has proven t o be very use fu l . From the 29Si MASNMR measurements, attempts have been made t o deduce complete z e o l i t e s t ruc tu res (ref.20-24). I n s t ruc tu res w i t h h igh aluminium content, t he aluminium d i s t r i b u t i o n i s no t random. For most c r y s t a l compositions, however, more than one S i ,A1 o rde r ing scheme i s compatible w i t h the 29Si MASNMR r e s u l t s , espec ia l l y f o r S i / A l r a t i o s ranging f r o m 2.4 t o 5. For higher S i /A1 r a t i o s the d i s t r i b u t i o n of aluminium seems t o be disordered, although considerable s c a t t e r o f t he experimental d i s t r i b u t i o n s i s observed ( re f .11) . It should be stressed t h a t t he NMR technique prov ides an average measurement and t h a t i n case o f composit ional inhomogeneity, f requen t l y encountered i n dealuminated samples, the NMR r e s u l t s even can be misleading. 358 S i t i n q o f charqe comDensatinq ca t i ons Aluminium atoms i n an a lum inos i l i ca te z e o l i t e in t roduce a n e t negat ive l a t t i c e charge which has t o be neu t ra l i zed w i t h charge compensating cat ions. The ca t i ons are d i s t r i b u t e d over the hexagonal prisms, s o d a l i t e cages and supercages (Fig.2). Fo l lowing the c l a s s i c a l nomenclature ( re f .25) , S i t e I i s s i t u a t e d i n t h e cen t re o f the hexagonal prism. Cations a t S i t e I are co- ord inated t o s i x 03 oxygens. S i t e Iâ i s l oca ted i n the s o d a l i t e cage adjacent t o the center o f a s i x - r i n g belonging t o the hexagonal pr ism i n c lose contact w i t h th ree 03 oxygen atoms. S i t e I 1 i s l oca ted i n the supercage adjacent t o the center o f a s i x - r i n g . S i t e 11â i s l i n k e d t o the same s i x - r i n g from i n s i d e the s o d a l i t e cage. Cations i n s i t e I 1 and 11â are co-ord inated t o 02 oxygens. S i tes I11 and 111â are supercage s i t e s a t t he fou r - r i ngs , l i n k e d t o oxygens o f type 01 and 04. Cations a t S i t e I and I â are accessible on l y through s i x - r i n g windows and can be considered t o be hidden f o r guest molecules t h a t can n o t overcome the s i x - r i n g d i f f u s i o n a l b a r r i e r . The cat ions a t the o the r s i t e s are accessible through the supercages. It w i l l be i l l u s t r a t e d t h a t t he a c c e s s i b i l i t y o f the charge compensating cat ions r a t h e r than the NNN con f igu ra t i ons o f aluminium atoms p l a y a c r u c i a l r o l e i n the dealumination process w i t h SiC14. Depending on t h e i r nature and s i t i n g , t he charge-compensating cat ions may âpreventâ o r â f a c i l i t a t e â t he d i s lodg ing o f aluminium atoms from l a t t i c e pos i t i ons . DEALUMINATION OF FAUJASITE ZEOLITES WITH Sic14 The dealumination method using Sic14 was in t roduced i n 1980 by Beyer and Belenykaja ( re f .26) . It cons is t s e s s e n t i a l l y o f passing s i l i c o n t e t r a c h l o r i d e vapor through a bed o f anhydrous z e o l i t e a t e levated temperature. The reac t i on w i t h Sic14 o f a z e o l i t e w i t h charge-compensating ca t i ons o f type M w i t h charge n fo rma l l y has the f o l l o w i n g stoicheometry ( re f .26) : - Mi/,, [A102.(Si02)x] + Sic14 --> l / n MC1, + AlCl3 + [(Si02)x+1] Due t o t h e s t rong exothermic i ty o f Reaction ( l ) , t he dealumination w i t h Sic14 vapor has t o occur under h i g h l y c o n t r o l l e d cond i t i ons i n order t o ob ta in h i g h l y c r y s t a l l i n e dealuminated Y z e o l i t e s i n a reproducib le way ( re f .27) . A f t e r contact w i t h SiClq, t he reac to r should be f lushed w i t h i n e r t gas, and the z e o l i t e washed w i t h deionised water. Mineral a c i d i t y develops i n the wash water due t o the hyd ro l ys i s o f AlCl3 and/or AlCl4- complexes present i n the z e o l i t e sample ( r e f . 26). The methods described i n l i t e r a t u r e can be subdivided i n two categor ies, v i z . isothermal reac t i ons and temperature-programmed react ions, denoted f u r t h e r as methods A and B, respec t i ve l y . 359 Method A: Isothermal dealumination with Sic14 - In the isothermal dealumination, zeolite Y is contacted with Sic14 vapor at a reaction temperature (TR) for a given period o f time (tR), after which the reactor is flushed at a post-treatment temperature (Tp) for a given post- treatment time (tp). Table 1 provides an overview of the literature on zeolite Y samples dealuminated by method A. The reaction between zeolite Y and Sic14 is so strongly exothermic that it is often impossible to keep the reaction temperature constant when the sample is first contacted with Sic14 (refs.26- 28). A pronounced temporary temperature rise, ATR, is observed in the zeolite bed (Table 1). A variety of starting materials and reaction conditions have been used, resulting in materials with a broad variation in framework A1 content. Table 1. Dealumination of Y zeolites with Sic14 according to method A No. Starting T t~ ATR Tp tp AIF/UCb Ref. materiala (KY (min.) (K) (K) (min.) of product 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 NaY (55) NaY (55) NaY (56) NaY (56) NaY (56) NaY (55) NaY (55) NaY (55) NaY (55) NaY (56) NaY (55) NaY (55) NaY (55) NaY (56) NaY (55) LiY (56)e LiY(56)e LiY(56)e LiY (56)e HY (56) f HY(56)g HY(56)g 423 240 - 423 120 473 240 - 473 120 523 35 50 713 90 523 90 50 713 90 523 35 50 623 90 523 240 - 523 120 573 - 573 240 - 573 120 620 -C - 620 - 623 240 - 623 120 673 240 - 673 120 723 - 773 120 - 773 240 - 773 120 623 35 50 713 90 423 35 17 823 90 523 35 50 623 90 523 35 50 713 90 523 90 50 713 90 473 - - a43 - 623 35 28 713 90 523 35 30 713 90 34 33 22 22 26 22 29 9 31 20 6 4 15 ( 2 27 27 24 23 34 29 34 Od 33 33 28 33 34 33 35 33 33 34 36 33 28 28 28 28 28 28 28 37 28 28 ~ ~ ( 5 6 ) f a43 - - a43 - 1 37 a, AlF/UC in brackets; b, number of framework aluminium atoms per unit cell; c, between 60 and 120 minutes; d, after acid leaching; e, Nay, for 68% exchanged with Li+; f, containing 2% NapO; g, Nay, for 90% exchanged with NH4+ and deammoniated at 673 K . 360 Two-step dealumination mechanism. A study o f t he r e a c t i o n o f LiY w i t h Sic14 a t a r e a c t i o n temperature o f 423 K has allowed t o d i s t i n g u i s h between two r e a c t i o n steps (ref.29). A f t e r t he exotherm has passed through the z e o l i t e bed, one S i - C1 bond i n Sic14 i s broken, and a new S i - 0 bond i s formed together w i t h the c h l o r i d e s a l t o f t he corresponding charge-compensating cat ion: c1 c1 S i c1 c1 c1 \ / \ I / I 0. \ / . / \ / \ / / \ / \ / \ / \ S i MC1 / \ c1 c1 -> M i 0- S i A1 S i -A1 Reaction ( 2 ) was i n f e r r e d from the observation o f a 29Si MASNMR s ignal a t a chemical s h i f t o f ca. -45 ppm, ascribed t o framework-bound 'SiC1-j' species ( re f .29) . On dehydrated Nay, contacted w i t h Sic14 a t a temperature o f 323 K , a 29Si MASNMR s ignal a t -45 ppm i s observed next t o a resonance o f adsorbed Sic14 a t -20 ppm, and the s igna ls o f the framework s i l i c o n atoms ranging from -80 t o -120 ppm (Fig.3A). The 'SiC13' species are observed i n HY, LiY as we l l as NaY zeo l i t es , t r e a t e d w i t h Sic14 a t temperatures below 423 K ( re f .30) . A t t h i s stage o f t he r e a c t i o n o f Sic14 w i t h NaY and LiY, a l l aluminium i s s t i l l present i n te t rahedra l environment (Fig.3B, re f .29) . The 27Al resonance i s s u b s t a n t i a l l y broadened (Fig.3B). Th is could be p a r t i a l l y due t o the dehydrated s t a t e o f t he sample, bu t a l so t o the d i s t o r t i o n o f t he aluminium tetrahedron i n the presence o f t he Sic13 species. A f t e r contact o f z e o l i t e HY w i t h Sic14 a t 423 K , t he 27Al MASNMR resonance o f an important f r a c t i o n o f t he aluminium i s s h i f t e d t o 35 ppm (Fig.4). A chemical s h i f t o f 35 ppm o f the 27A l nucleus i n dealuminated f a u j a s i t e s has been ascribed t o pentaco-ordinated aluminium ( re f .31 ) o r d i s t o r t e d t e t r a h e d r a l l y co-ord inated aluminium ( ref .32) . I n t h i s instance, we are i n c l i n e d t o assign the 35 ppm s ignals t o d i s t o r t e d aluminium tet rahedra. Indeed, upon hydration, t he Sic13 species are hydrolysed and e s s e n t i a l l y non-dealuminated Y z e o l i t e s are recovered ( re f .30) . 361 B F igu re 3. 29Si MASNMR spectrum (A) and 27Al MASNMR spectrum ( B ) o f Nay, con tac ted w i t h S ic14 vapor a t a tempera ture o f 323 K. ATR was 40 K. A F i g u r e 4. 27Al MASNMR spectrum o f HY J/ \\ k., z e o l i t e , con tac ted w i t h S ic14 vapor a t a tempera ture o f 423 K. ATR was 38 K. -- 1 . . . . 1 . . . . 1 . . . . 1 . . . , t a a a FFM Upon h e a t i n g t h e z e o l i t e t o temperatures exceeding 423 K t h e s i l i c o n atoms o f t h e Sic13 spec ies a re i n s e r t e d i n t h e framework a f t e r removal o f t h e aluminium atoms ( re fs .29 ,30) : c1 c1 c1 S i MC1 I \ I / A1C13 MC1 / \ / \ / \ / \ AlC13 r e a c t s w i t h t h e c h l o r i d e s a l t o f t h e a l k a l i me ta l s and forms t h e cor respond ing a1 k a l i metal t e t r a c h l o r o a luminate complexes ( r e f .26). The aluminium atoms o f these complexes e x h i b i t a chemical s h i f t o f ca. 100 ppm (refs.29,33) w i t h respec t t o aqueous sodium a luminate . HC1 and AlC13 a r e formed d u r i n g dea lumina t ion o f HY z e o l i t e s ( re f .28 ) . When NaY i s con tac ted w i t h Sic14 a t TR = 423 K, t h e ove rhea t ing i s impor tan t and React ions (2) and (3) occur s imu l taneous ly ( r e f . 3 0 ) . Indeed, i n 362 such sample a weak 29Si MASNMR signal at -45 ppm is present together with a 27Al MASNMR resonance at ca. 100 ppm (Fig.5). A B Figure 5. 29Si MASNMR spectrum (A) and z7Al MASNMR spectrum (B) of Nay, contacted with Sic14 vapor at a temperature of 423 K . ATR was 67 K. Oriqin of reaction inhibition at moderate TR temperatures. There is evidence that for TR < 523 K , a reaction front runs through the zeolite bed. Indeed, when during an experiment with NaY at 523 K , the Sic14 feed is interrupted before this front (or the reaction exotherm) has reached the exit of the reactor, a mixture of the original and the dealuminated zeolite is obtained (refs.27,28). When the reaction zone has passed once through the zeolite bed at a given TR, the zeolite has become unreactive towards Sic14 at this temperature, although the dealumination is mostly incomplete. An amount of 22 AlF/UC remains in NaY zeolite at a reaction temperature of 523 K and reaction times of 35, 90 and 240 minutes, respectively (Table 1, Nos. 3, 4 and 6). Prolongation of the contact time with Sic14 at a temperature of 523 K apparently does not further increase the degree of dealumination. A similar observation was made with LiY, for which at a TR of 523 K the degree of dealumination was not altered upon increasing the reaction time from 35 to 90 minutes (Table 1, Nos. 18 and 19). In another experiment (ref.301, LiY zeolite was contacted with Sic14 at TR = 423 K, post-treated at Tp = 763 K, and subsequently contacted a second time with Sic14 vapor at 423 K. During the second contact no exothermicitv was observed. The zeolite was purged with inert gas at a Tp of 623 K . The final zeolite Y product contained 27 aluminium atoms per unit cell, which is the same value as that obtained after the first treatment with Sic14 (Table 1, No.16). With HY zeolites 22 Al/UC are removed at a temperature of 473 K (Table 1, No. 20). The degree of dealumination seems to remain quite constant up to a TR value o f 623 K (Table 1, Nos.21 and 22). - 363 The participation of the charge compensating cations in the reaction of zeolite Y with Sic14 appears in Reaction (2). The cation distributions can offer a satisfactory explanation for the observed 1 imits to the dealumination process at moderate reaction temperatures (ref.28). The charge compensating cations are distributed over cation sites in supercages, sodalite cages and hexagonal prisms (Fig.2). Given the diameter of SiC14, which is 0.687 nm (ref.38), the molecule has access to the supercages only. About 30 to 33 sodium cations are located in the supercages of NaY zeolites (refs.40-42). The number of aluminium atoms that can be extracted at TR temperatures below 523 K seems to be restricted to these numbers (Table 1, Nos.1-6). For Li-exchanged NaY zeolites a similar distribution of the cations over accessible and hidden positions can be expected. The upper amount of ca. 33 aluminium atoms that can be removed at 523 K (Table 1, Nos.16-19) could correspond to the number of supercage Li and Na cations. For HY zeolites the availability of hidden protons can be derived from the infrared spectra in the OH reagion. Based on these infrared spectra, the ratio of the intensities of the high frequency to the low frequency OH band and, consequently, the proportion of accessible to hidden protons is ca. 0.6 (ref.43) and the lattice charge associated with 21 aluminium atoms should be neutral ised by accessible protons. This number corresponds to the number of aluminium atoms removed from the lattice at reaction temperatures between 423 and 623 K (Table 1, Nos.20-22). At moderate reaction temperatures, the number of aluminium atoms that can be replaced with silicon seems to be limited by the number of accessible charge-compensating cations. Consequently, the preparation of samples with aluminium contents, equal to the number of hidden charge-compensating cations should be highly reproducible. Apparently, the nature and consequently the distribution of the charge-compensating cations can be handled as an instument to tune the degree of dealumination. The dealumination with Sic14 at moderate temperatures offers the advantage that, in principle, very homogeneously dealuminated samples with well-defined AlF/UC contents can be obtained, provided high overheating temperatures are avoided. Dealumination at hiqh TR temeratures. At reaction temperatures between 573 and 773 K the behaviour of NaY is different. Under such conditions a wide variety of AlF/UC values were obtained (Table 2, Nos. 7-15 and 23). At high reaction temperatures, there does not seem to exist a limitation to the degree of dealumination. At temperatures exceeding 523 K , the less accessible sodium cations and associated framework aluminium atoms probably become reactive towards Sic14 possibly as a result of cation mobility and redistribution among hidden and accessible sites. Temperatures higher than 623 K seem to be necessary to activate aluminium atoms associated with hidden protons (Table 1, - Nos.22-23). 364 There ex i s t s , however, an upper l i m i t t o t he dealumination temperature ( re f .27) . Beyer e t a l . repor ted t h a t t he contact o f dehydrated NaY w i t h Sic14 a t temperatures over 750 K produces a v i o l e n t exothermic r e a c t i o n r e s u l t i n g i n the format ion o f an amorphous product ( re f .27) . On the contrary , Kl inowski e t a l . seemed t o succeed i n the preparat ion o f an â e s s e n t i a l l y aluminium-free f a u j a s i t e s t ruc tu re â from z e o l i t e Y by r e a c t i o n w i t h Sic14 vapor a t 833 K (ref.44). However, i t i s no t c l e a r whether temperature programming o r no t was used i n the l a t t e r case. Method 8: TemDerature Droqrammed dealumination w i t h Sic14 I n the work o f Beyer and Belenykaja ( re f .26) , t he contact o f t he z e o l i t e w i t h Sic14 vapor was s t a r t e d a t a f i r s t reac t i on temperature ( T R ~ ) , and the temperature was g radua l l y increased a t a r a t e (r ) t o a f i n a l reac t i on temperature ( T R ~ ) . I n the o r i g i n a l method T R ~ was 650 K, T R ~ was va r ied from 730 K t o 830 K and r was 4 K min-1 ( re f .26 ) . NaY z e o l i t e s were thus dealuminated t o conta in between 4 and 9 A1 atoms per u n i t c e l l . I n another study ( re f .27) , T R ~ was about 520 K. A temporary temperature r i s e between 30 and 70 K occurred dur ing the f i r s t minutes o f t he contact o f NaY z e o l i t e w i t h Sic14 vapor. Only when the reac to r temperature had reached i t s o r i g i n a l value o f 520 K, t he z e o l i t e bed was heated a t a r a t e o f 10 K min-1 t o T R ~ values between 600 K and 745 K. This way samples w i t h A1 contents between 16 and 2 A l / U C were produced (Table 2, Nos. 24-29). I n the same work (ref.27) i t was s ta ted t h a t i n order t o avoid s t r u c t u r a l damage t o the z e o l i t e , t he heat ing t o T R ~ should on l y be s t a r t e d when the exo the rm ic i t y i s over. I n view o f t he r e a c t i o n mechanism advanced e a r l i e r , t h i s means t h a t a simultaneous a t tack o f aluminium atoms associated w i t h hidden and accessible cat ions causes a too v i o l e n t reac t i on and, consequently, l a t t i c e des t ruc t i on and has t o be avoided. Table 2 shows t h a t t he degree o f dealumination depends mainly on T R ~ . It seems very d i f f i c u l t t o remove the l a s t framework aluminium atoms from NaY and consequently i t has been suggested t h a t t he dealumination o f NaY w i t h Sic14 i s a p r o d u c t - i n h i b i t e d r e a c t i o n (refs.27,45,51). According t o t h i s hypothesis, the p r e c i p i t a t i o n o f sodium t e t r a c h l o r o aluminate complexes i n s i d e the z e o l i t e pores terminates the progression o f t he dealumination r e a c t i o n since i t prevents Sic14 from f u r t h e r d i f f u s i n g i n t o the z e o l i t e c a v i t i e s . The decomposition temperature o f NaAlC14 i n z e o l i t e Y i s est imated t o occur a t ca. 780 K ( re f .27) . The use o f LiY z e o l i t e s o f f e r s the advantage t h a t t he LiAlC14 complexes v o l a t i z e and/or decompose already a t a temperature o f 733 K ( re f .45) . Complete dealumination o f L i Y can be achieved (Table 2, No.33) under condi t ions where 3 A lF /UC remain i n NaY (Table 2, No.29). A z e o l i t e Y product w i t h v i r t u a l l y no aluminium atoms l e f t i n t he framework was obtained from NaY a t 833 - 365 K (ref.44). Attempts to force the dealumination to completion resulted in amorphous products (ref.44). Table 2. Zeolite Y products obtained from temperature programmed reaction with Sic14 (method B ) No. Starting TR tR1 r TR t p t p AIF/UC AlEF/UC Ref. material (Kf (min.) (K/min) (Kf (min.) (min.) 24 NaY 521 -a 4 600 40 - 16 11 27 25 NaY 521 -a 4 675 40 - 7 8 27 26 NaY 529 -a 4 745 15 - 4 6 27 27 NaY 521 -a 4 720 40 - 3 1 27 28 NaY 521 -a 4 720 40 - 3 0.4 27 29 NaY 520 -a ? 733 150 - 3 5 45 30 31 32 33 34 35 36 37 38 39 40 41 42 HYe HYf HYf 423 - 423 - 423 - 520 -a 323 0 4 650 - 650 - 650 - 650 - 650 - 298 0 5 298 0 5 298 0 5 473 663 723 733 548 700 750 775 800 823 823 823 1023 240 120 43 240 120 6 240 120 2 150 - 0 10 0 120 38 120 - 16 120 - 7 120 - 6 60 - 4 ? - 6 180 60 5 22 180 60 2 32 720 60 1 4 46 46 46 45 47 48 48 47 48 49 50 50 50 a, no heating till end of exothermicity; b, Nay, exhanged for 9% with protons and 65% with Li+; c, Nay, exchanged for 62% with Li+; d, Nay, 62% exchanged with NH4+, and deammoniated at 650 K; e , containing 2% of Na20; f, Nay, fully exchanged with NH4+ and deammoniated. The results on HY zeolites, in which the deposition of chloro aluminium complexes doesnot occur, show that in this zeolite also the last aluminium atoms resist to dealumination (Table 2, Nos.39-42). A satisfactory explanation why in HY zeolites a few aluminium atoms per unit cell remain unreactive towards Sic14 is lacking for the moment. Dealumination of X-tvoe zeolites with Sic14 In a NaX zeolite sample with for instance 85 AlF/UC, about 40 sodium cations are located in accessible sites (ref.41). If the aluminium substitution is limited to 40 Al/UC, a zeolite with 45 AlF/UC would result. It is evident that such a faujasite-type zeolite will be destroyed by the strong acidity developed during the water washing and the hydrolysis of 40 NaAlC14 molecules per unit cell, not mentioning the hydrolysis of occluded SiC14. - 366 Beyer e t a l . found t h a t NaX reac ts w i t h Sic14 a t a temperature o f 480 K ( re f .27) . The Sic14 uptake agreed w i t h a removal o f 38 aluminium atoms per u n i t c e l l , which corresponds roughly t o the number o f accessible sodium cat ions. Sul ikowski e t a l . used the temperature programmed method t o dealuminate LiX and Lax samples w i t h Sic14 (Table 2). A t T R ~ values o f 473 K and 573 K, 44 A lF /UC were found i n the product. This corresponds t o a removal o f 33 Al/UC. Only a t T R ~ temperatures h igher than 673 K, t he dealumination proceeded f u r t h e r (Table 3, No.&), as could have been predicted. Under i d e n t i c a l r e a c t i o n condi t ions, a Lax sample i s l e s s dealuminated compared t o a L iX sample (Table 3, Nos.46 and 47). The amount o f accessible charge-compensation ca t i ons i n Lax i s indeed lower than i n LiX. I n a l l instances (Table 3), products w i t h poor c r y s t a l l i n i t y were obtained from z e o l i t e X (ref.27, 46). Table 3. Dealumination w i t h Sic14 o f X z e o l i t e s No. S t a r t i n g T i TR t.82 Tp t p AlF/UC Ref. 43 NaX(85) 480 - 47 27 44 LiX(79)a 423 473 240 473 120 44 46 45 LiX(79)a 423 573 240 573 120 44 46 46 LiX(79)a 423 673 240 673 120 31 46 47 LaX(79) 423 673 240 673 120 44 46 mater i a1 ( j (K! (min.) (K) (min.) The f a i l u r e t o prepare h i g h l y c r y s t a l l i n e dealuminated f a u j a s i t e s from X z e o l i t e s has l e d t o speculat ions i n l i t e r a t u r e . I n the work o f Beyer and Belenykaja, it was supposed t h a t t h e dealumination o f NaX f a i l s because the z e o l i t e framework i s sh ie lded from the a t tack o f Sic14 by t h e presence o f a h igh concentrat ion o f ' l a t t i c e ' cat ions, by which probably charge-compensating cat ions were meant ( re f .27) . Sul ikowski and Kl inowski suggested t h a t t h e s t r u c t u r a l damage i s the r e s u l t o f t he simultaneous removal o f t h ree aluminium atoms from t r i p l y occupied six-membered r i n g s (ref.46). However, t he behavior o f X z e o l i t e s f i t s p e r f e c t l y i n t o the p i c t u r e developed h igher f o r t he dealumination o f z e o l i t e Y samples w i t h SiC14. E l im ina t i on o f extra-framework aluminium A f t e r t he isomorphic s u b s t i t u t i o n (Reaction 3), t he aluminium i s present under t h e form o f H+ AlCl4- complexes, depending on t h e na tu re o f t he charge compensating cat ions, and t h e temperature. There are, i n p r i n c i p l e , two a l t e r n a t i v e methods t o remove the aluminium from the z e o l i t e c a v i t i e s , v i z . (1) 367 decomposition of the tetrachloro-aluminium complexes (in case of LiY and Nay) and desorption of AlCl3 and HC1 (in case of HY), or (2) hydrolysis of AlC13 and the tetrachloro-aluminium complexes and leaching of the dissolved aluminium from the zeolite pores. DecomDosition of tetrachloro-aluminium ComDlexes in inert atmosohere. Thermal decomposition of the a1 kal i metal tetrachloro-aluminium complexes and subsequent desorption of AlCl3 should lead to a zeolite sample with only metal chloride salt deposited in the pores. The latter could be removed by washing with water. Starting from HY zeolite, the washing operation would even not be necessary. In practice, however, the situation is more complex. The thermostability in inert atmosphere of NaY is much reduced after the treatment with Sic14 (ref.27). Structural collapse starts already at a temperature of 770 K (ref.27). This degradation of the framework was interpreted as a reaction of the zeolite framework with AlCl3 coming from partially dissociated tetrachloro aluminate complexes (ref.27). The 27Al resonance at 100 ppm in NaY samples disappears during post-treatments at 823 K and 923 K (ref. 28) but the amount of aluminium that desorbs from the zeolite bed at these temperatures is always low (ref.28). White fumes typical of AlCl3 vapor are not observed at the outlet of the dealumination reactor (ref.29). Heating of SiClq-treated NaY samples in inert atmosphere causes the formation of mesopores (ref.28). This mesopore formation is probably due to a more gentle attack of the framework by AlCl3 and/or tetrachloro aluminate complexes and can be considered to be an onset to amorphisation. It can be concluded that thermal elimination of the dislodged aluminium does not seem to be a successful approach with Nay. The higher volatility and the lower stability of lithium compared to sodium tetrachloro-aluminium complexes is reflected in the formation o f mesopores and amorphisation (ref.28). The mesopore volume in LiY zeolites is systematically higher than in NaY zeolites, post-treated at the same temperature (Fig.6). Post-treatment temperatures of 823 K can be applied to LiY without important damage to the zeolite lattice when a TR of 423 K is used (ref.28). LiY zeolites reacted with Sic14 at 523 K and 623 K become partially amorphous during a post- treatment at 823 K , while NaY retains a better crystallinity (ref.28). The decomposition of tetrachloro aluminate complexes in inert atmosphere is harmful t o the microporosity and crystallinity of LiY zeolites. The mesopore volume of HY zeolites is situated between that o f NaY and LiY zeolites treated at the same temperatures (Fig.6). No further data are actually available on the thermostability of SiClq-treated HY zeolites. 368 d- I m M1 8 5 7 5 6 5 B W i 5 5 3 0 4 5 * 3 5 o 2 5 PI 2 1 5 d 0 ec TR- 5 2 3 K 0 a 6 0 0 7 0 0 8 0 0 9 0 0 1000 (K) TP Figure 6. I n f l uence o f the post-treatment temperature on the formation o f mesopores i n Nay, HY and LiY z e o l i t e s contacted w i t h Sic14 a t a TR o f 423 K, 523 K and 623 K. The mesopore volume i s determined according t o re f .28. DecomDosition o f te t rachloro-a lumin ium comolexes i n the Dresence o f SiC14. The f i n a l temperature o f the Sic14 treatment used i n dealumination method-B sometimes exceeds the decomposition temperature o f t he ch lo ro aluminate complexes (Table 2 ) . I n such instances, AlC13 vapors are observed a t t he o u t l e t o f t he dealumination reac to r . A ' t h i c k whi te fume o f A lCl3 ' was observed when L i Y was t r e a t e d a t a T R ~ value o f 733 K ( re f .45) . The z e o l i t e re ta ined f u l l c r y s t a l l i n i t y . Wi th in the de tec t i on l i m i t s o f the 29Si MASNMR technique, the s u b s t i t u t i o n o f s i l i c o n f o r aluminium was complete, but even a f t e r washing a subs tan t i a l amount o f extra-framework aluminium remained i n the sample (Table 2, No.33). A wh i te vapor o f AlCl3 escaped from a NaY sample du r ing r e a c t i o n w i t h S i c 1 4 a t 833 K ( re f .44 ) . The f i n a l product was c r y s t a l l i n e and the z e o l i t e framework deeply dealuminated. The presence o f extra-framework aluminium i n the washed product was detected w i t h 27A l MASNMR ( re f .44) . Aparently, t he desorpt ion o f AlC13 i s f a c i l i t a t e d and the det r imenta l e f f e c t o f AlCl3 i s neu t ra l i sed i n presence o f SiC14. HY z e o l i t e samples stand very h igh reac t i on temperatures o f up t o 1023 K i n presence o f Sic14 (Table 2 ) . Even i n these instances, an important f r a c t i o n o f the dislodged aluminium i s no t evacuated from the sample (Table 2, No.42). 369 El im ina t i on o f dislodqed aluminium bv water washinq. I n prev ious sect ion i t was explained t h a t i t i s very d i f f i c u l t , i f n o t impossible, t o desorb thermal ly a l l aluminium c h l o r i d e species from the z e o l i t e pores. I n the washing step, t he A1 - C1 bonds are hydrolysed and s t rong mineral a c i d i t y develops i n the z e o l i t e pores. Hydroxide complexes o f aluminium are formed and t r a n s f e r r e d i n t o so lu t i on , depending on the a c i d i t y o f t he suspension. Pe r t i nen t data from the l i t e r a t u r e on the amount o f aluminium evacuated du r ing the washing o f SiC14- dealuminated samples are shown i n Fig.7. The amount o f aluminium t h a t was leached from the samples was ca l cu la ted from the o r i g i n a l and f i n a l aluminium content o f the samples, neg lec t i ng aluminium losses du r ing r e a c t i o n w i t h Sic14 and eventual post-treatments. I n each se r ies o f samples, t he amount o f aluminium t h a t i s removed increases w i t h increas ing degree o f dealumination (F ig .7) . Important dif ferences are found between the d i f f e r e n t se r ies o f samples (Fig.7). 5 0 40 3 0 2 0 10 0 ' 0 10 2 0 3 0 40 AIP / UC Figure 7. Number o f aluminium atoms leached per u n i t c e l l versus A lF /UC. A, HY, using TR values between 473 K and 843 K ( re f .37 ) ; B, Nay, samples Nos.24-27 o f Table 2; C, Nay, samples Nos. 2, 6, 8, 11 and 15 o f Table 1; D, Nay, using TR values between 620 K and 770 K ( re f .35 ) ; E, Nay, sample No. 6 o f Table 1 and data o f re f .28. I n the work o f Anderson e t a l . ( re f .33) the removal o f aluminium f r o m the samples was most e f f i c i e n t (Fig.7C). The samples were washed using a water/sample w t / w t r a t i o o f 200. Kubelkova e t a l . measured the pH i n the f i r s t suspension and found values o f 4.8, 2.5 and 2.8 a t AlF/UC values o f 30, 25 and 15, respec t i ve l y ( re f .35) . The leaching o f aluminium was most e f f i c i e n t i n the sample t h a t developed the s t rongest a c i d i t y (Fig.7D). I n the work o f Beyer e t 370 al. (ref.28) the water/sample wt/wt ratio was 32. Although less water was used than in ref.33, lower amounts of aluminium were removed (Fig.7, B compared to C). In the work of Sohn et al. (ref.37) an amount of 2 g of NH4NaY zeolite containing 2 wt % Na2O was reacted with Sic14 at TR values from 473 K to 843 K. After reaction at a TR value below 623 K, a post-treatment at 843 K was applied. The samples were washed with 2 liters of deionised water in a Buchner funnel. In this series of zeolites the aluminium removal is very inefficient (Fig.7A). HY samples probably develop weaker acidity, due to their lower AlCl3 content, and the water washing may, therefore, be less effective. Only small amounts of aluminium were evacuated from the SiClq-dealuminated NaY samples of Goyvaerts et al. (Fig.7E). These samples were washed using a very high water/sample wt/wt ratio of 570. In that work, the zeolite powder was poored into the large water volume and pH values measured (ref.28). The pH of the wash water was typically between 2.8 and 3.2 for samples with a high degree of crystallinity, dealuminated using moderate TR and Tp values. Samples treated at high temperatures which lost partially their crystallinity, developed weaker acidity . The importance of the washing procedure can be appreciated when considering the AlEF/UC content of the different samples of Fig.7, as shown in Fig.8. The washing procedure is critical especially for samples with A1F content between 10 and 20 AlF/UC, since a large variety of AlEF/UC values can be obtained (Fig.8). 3 5 3 0 2 5 2 0 1 5 10 5 0 0 5 1 0 1 5 2 0 2 5 3 0 3 5 A1' / UC Figure 8. AlEF/UC versus ALF/UC for the samples of Fig.7. 371 Framework dealumination du r inq water washinq. The a c i d i t y developed du r ing the hyd ro l ys i s o f t he t e t r a c h l o r o aluminate complexes can be responsib le f o r an a d i t i o n a l dealumination. Upon ac id leaching o f A1 from the l a t t i c e , i t can be expected t h a t a hydroxyl nest conta in ing 4 OH groups i s formed (ref.52). Z e o l i t e Y samples deeply dealuminated w i t h Sic14 develop a sharp hydroxyl v i b r a t i o n a t a s p e c i f i c wavenumber i n the range from 3730 t o 3750 cm-1 (ref.27,34,37,45,51,53). Besides t h i s assignment o f t h a t hydroxyl v i b r a t i o n a t these wavenumbers t o the 4 OH hydroxyl nests (ref.27,51) i t has a l so been a t t r i b u t e d t o c r y s t a l terminat ing s i l a n o l groups o r amorphous s i l i c a (ref.54). I n h i g h l y c r y s t a l l i n e samples the assignment t o hydroxyl nests was confirmed by the observation t h a t the band completely disappeared upon steaming (ref.27, 51). Addi t ional support comes from the observation t h a t i n t e n s i t y o f t he 3750 cm-1 band i n deeply dealuminated NaY z e o l i t e s i s h ighe r than i n L i Y samples i n which the te t rachloro-a lumin ium complexes were p a r t i a l l y decomposed (ref.45). 8 0 6 0 40 2 0 0 B. %T + C . %T T D. %T E, abs. A 1 .. 0 2 0 4 0 6 0 8 0 100 M CI % dealumination Figure 9. Re la t i ve i n t e n s i t y o f t he 3730-3750 cm-1 hydroxyl v i b r a t i o n i n SiC14- dealuminated Y z e o l i t e s . A, HY, us ing TR values between 473 K and 843 K, data from ref.37; B, Nay, samples Nos.24-26 o f Table 2, data from ref.27; C, Nay, Nos.6, 8 and 15 o f Table 1, data from ref.53; D, Nay, Nos.7 and 13 o f Table 1, data from ref .34; E, Nay, Nos.4 and 5 o f Table 1, data from ref.28. F, Nay, using TR values between 620 K and 770 K, data from ref .35. The r e l a t i v e i n t e n s i t y o f the 3730-3750 cm-1 hydroxyl v i b r a t i o n i s p l o t t e d against t h e degree o f dealumination Fig.9. I n t h e NaY z e o l i t e s , t he 3730-3750 cm-1 band remains a t the very low i n t e n s i t y l e v e l o f t he parent z e o l i t e up t o 50% dealumination and increases sharp ly a f t e r dealumination degrees h ighe r than 75%. The data obtained w i t h HY z e o l i t e s e x h i b i t more sca t te r . I n these samples 372 the water washing was rather inefficient (see higher) indicating that probably only weak acidity was developed in the pores. For these samples at least part of the intensity of the 3740 cm-1 vibration should be due to SiOH groups in mesopores and/or amorphous material, in agreement with the assignment of the authors (ref .37). Framework dealumination during water washing is probably not very important. From a thermogravimetric analysis of sample No. 27 of Table 2, having a degree of dealumination of 93%, Beyer et al. derived that the dealumination during water washing accounted for only 4 AlF/UC (ref -27). Extra-framework aluminium aradients. The surface of SiClq-dealuminated zeolite crystals is sometimes enriched with aluminium, as detected with X-ray Photoelectron Spectroscopy (ref .35) and Fast Atom Bombardment Mass Spectrometry (ref.36). In the samples of Kubelkova et al. the surface Si/A1 ratio did not exceed twice the bulk ratio (ref.35). van Broekhoven et al. prepared zeolite Y samples with 27 and 17 AlF/UC in which no surface enrichment was found (ref.55). From the available data it is not possible to derive whether the accumulation of aluminium at the surface takes place during the Sic14 treatment or i s the result of migration during washing. Acid leachina of extra-framework aluminium. Hey et al. studied the leaching of aluminium from an ammonium exchanged sample with AlF/UC and AlEF/UC values of 12 and 7, respectively (ref.56). This sample was obtained by method B using a zeolite with Na21(NH4)34(AlO2)55(SiO2)137 composition as starting material. The aluminium content of the framework was monitored with the wavenumber of the internal asymmetric TO4 stretch, and the extra-framework to framework aluminium ratio with 27Al MASNMR (Fig.10). The authors concluded from their data that aluminium is extracted from extra-structural positions rather than from the zeolite framework during pretreatment at pH values o f 1.3 and above. Fig. 10 shows that the framework looses ca. 3 AlF/UC. Treatment at pH values o f 0.20 and 0.0 results in important framework dealumination (ref.56). I Fi ure 10. Evolution o f of aqueous suspension (data from ref.56). A1 i! /UC and AlEF/UC with pH 0 1 2 P H 373 100 8 0 6 0 4 0 Nature of extra-framework aluminium sDecies. AlOH groups within the extra- framework species give rise to infrared bands at ca. 3700 cm-1 and ca. 3600 cm- 1 (ref.57). In steam-dealuminated Y zeolites, the AlOH groups giving rise to 3600 cm-1 vibrations are part of polymeric 0x0-hydroxy-aluminium species, those at 3700 cm-* belong to oligomeric species (ref.58). The intensity of the 3700 cm-1 vibration is always weak in SiClq-dealuminated samples (ref.27, 28, 34, 35, 45, 51, 53) indicating that oligomers are present in small amounts. The polymers are much more abundant judged on the intensity of the 3600 cm-1 band (ref.27,28,34,35), For two series of samples, the relative intensity of the 3600 cm-1 hydroxyl band is plotted in Fig.11 against the degree of dealumination. The 3600 cm-1 signal exhibits a maximium (Fig.11). This maximum in a series of samples washed according to the same procedure, can be explained if the formation of these polymeric species occurs in a specific range of acidities and if the generation of acidity increases with increasing degree of deal uminat ion. - - - - I u 0 0 \o m E I 4 1 ------- ' I : \ \ A \ "! I i I \ \ 0 5 0 6 0 7 0 8 0 90 100 6 Q d k % dealumination Figure 11. Relative intensity of the 3600 cm-1 hydroxyl band in SiC14- dealuminated NaY zeolites A, samples Nos.24-27 of Table 2, data from ref.27; B, data from ref.35. 27Al MASNMR signals observed in washed SiClq-dealuminated zeolite Y samples are listed in Table 4. The 27Al resonance of tetrahedral AIF is in the range from 60 to 50 ppm. The z7Al chemical shift of A1F in NaX and NaY zeolites is at 62.8 ppm (ref.59). Table 4 shows that the A1F resonance is gradually shifted 374 downfield at increasing degree of dealumination. This downfield shift should be due to the changes in the distribution of the NNN environments of aluminium. The 27A1 MASNMR signals in the chemical shift range from 4 to -1 ppm are ascribed to the AlEF species. Two different species can give rise to signals in this range of chemical shifts. A sharp signal at 0 ppm in hydrated samples is due to mobile monomeric aluminium cations, whereas a broad signal at the same field is ascribed to polymeric species (ref.60). Monomeric species were found only in deeply dealuminated samples (ref.44, 46) or in a sample treated with acid solutions at a pH of 0.22 (ref.56). It is evident that deeply dealuminated samples develop the strongest acidity during water washing. Part of the monomeric aluminium cations can be extracted from the samples by extensive washing (ref.44) or exchanged with NH4+ (ref.55). For several SiC14- deal uminated samples a good correspondence was found between the AlF/AlEF proportions, determined on one hand by 27Al MASNMR, and on the other hand by a combination of 29Si MASNMR and chemical analysis (ref.33, 55, 56), indicating that all the aluminium is NMR-visible. 27Al NMR-silent aluminium atoms are part of the oligomeric alumina species (ref.60). The absence of NMR-silent aluminium in SiClq-dealuminated samples is in agreement with the low intensity of the 3700 cm-1 hydroxyl vibration in these samples. As an illustration, Fig. 12 shows the 27Al MASNMR spectrum of a washed dealuminated NaY zeolite containing polymeric AIEF. 27Al MASNMR signals at ca.30 ppm due to distorted aluminium tetrahedra are found in samples that were dealuminated with Sic14 according to method A, using post-treatment temperatures of 713 K and higher (Table 4, Fig.13). It was explained higher that such a treatment is harmful to the microporosity and crystallinity of the sample. 1 , 1 4 1 1 1 . . . I . . . . 1 . . . A 1 aa a I aa 0 FFM PPM Figure 12. 27Al MASNMR spectrum of washed SiC14-dealuminated Nay. Dealumination described in the caption of Fig.5. values of 423 and 763 K, respectively. Figure 13. 27Al MASNMR spectrum of washed SiC14-dealuminated LiY, dealuminated by method A, using TR and Tp 375 Extra-framework s i l i c o n sDecies The s i l i c o n atoms i n s i l i c e o u s f a u j a s i t e s t ruc tu res e x h i b i t a chemical s h i f t o f ca. -100 and -107 ppm f o r t he S i F ( l A 1 ) and S iF (OA1) environments, respect ive ly . The presence o f amorphous s i l i c a i n the samples can be detected i n the 29Si MASNMR spectrum by broad s igna ls i n the range from -110 t o -112 ppm (refs.28,33,44). The c o n t r i b u t i o n o f t he S i F s igna ls t o the t o t a l resonance envelope has been used as an NMR measurement o f t he degree o f c r y s t a l l i n i t y ( re f .28) . The degree o f âshort-rangeâ c r y s t a l 1 i n i t y , measured by NMR, i s always h igher than the degree o f â long-rangeâ c r y s t a l l i n i t y , determined w i t h X-ray d i f f r a c t i o n ( re f .28) . The deconvolut ion o f a 29Si MASNMR spectrum o f t he dealuminated NaY sample No.3 o f Table 1, having a NMR and XRD c r y s t a l l i n i t y o f 91% and 83%, respec t i ve l y , i s shown i n Fig.14. I I , , , I , , , I , [ , , , , I , -3a -38 -108 - 1 1 a - 1 2 a F P Y Figure 14. Experimental (A) and deconvoluted (B) 29Si MASNMR spectrum o f sample 3 o f Table 1 . Mesopores I n theory, t h e Sic14 technique could o f f e r t he advantage t h a t t he s i l i c o n needed f o r t he r e c o n s t i t u t i o n o f t he framework does n o t have t o come from other pa r t s o f t he c r y s t a l thus avoiding the formation o f mesopores. The existence o f mesopores can r e a d i l y be detected by the presence o f hys te res i s i n the n i t rogen adsorption isotherms. Adsorption isotherms w i t h n o t more hys te res i s compared t o the parent sample have been repor ted f o r LiY samples w i t h 24 and 27 AlF/UC ( re f .28) , NaY samples w i t h 20, 22 and 25 AlF/UC ( re f .28) and w i t h 22, 24 and 33 AlF/UC ( re f .61) and f o r HY w i t h 29 and 34 AlF/UC (ref.28). I n these samples, 376 the amount of aluminium extraction from the lattice does not exceed the original number of supercage cations. These samples were obtained according to method A and have not been exposed to temperatures higher than 713 K. For deeply dealuminated samples, sorption isotherms of hydrocarbons are available (ref.26,61). Samples prepared by Method B with 6 and 8 Al/UC show a near rectilinear adsorption isotherm for hexane, butane and benzene. It was concluded that in these samples mesopores having radii in the range from 1.5 to 1.9 nm, typical of hydrothermally dealuminated samples were absent. These results do, however, not imply that the faujasite micropore system is intact (ref .61). CONCLUSIONS The reaction of Sic14 with LiY, NaY and HY zeolites at temperatures below 423 K leads to the formation of framework-bound 'SiC13' species and LiC1, NaY and HC1, respectively. The Sic13 species exhibit a 29Si MASNMR resonance at ca. -45 ppm. The 27Al MASNMR spectra of the zeolite samples at this stage of the reaction show the presence o f aluminium in distorted tetrahedral environments. Upon heating of the zeolites to temperatures exceeding 423 K, the Si atoms of the Sic13 species are inserted in the framework, while the A1 atoms are re1 eased as Al Cl3. The 1 atter i s transformed into a1 kal i metal tetrachl oro- aluminate complexes in presence of alkali metal chloride salts. Charge compensating cations located in the hexagonal prisms and the sodalite cages are hidden for the Sic14 molecules. At moderate reaction temperatures, the number of A1 atoms that can be replaced with Si atoms seems to limited to the number o f charge compensating cations located in the supercages. Consequently, the preparation of samples with aluminium contents, equal to the number of hidden charge-compensating cations is highly reproducible. The accessibility of the charge compensating cations rather than the T atom environment in terms of next nearest neighbors seems to determine the reactivity of framework A1 atoms. The cation distribution can be handled as a tool to govern the degree o f dealumination as illustrated LiY, NaY and HY samples. Highly crystalline mesopore-free NaY and LiY zeolites with 22 AlF/UC and HY zeolites with 30 AlF/UC can be prepared. The A1 atoms that are associated with less accessible charge compensating Na+ cations become reactive at temperatures above 523 K, possibly as a result of increased Na+ mobility and redistribution. Temperatures higher than 623 K are necessary in order to activate A1 atoms associated with hidden protons. A simultaneous attack of aluminium atoms associated with accessible and hidden cations has to be avoided since it results in lattice destruction. 377 The dealumination of X type zeolites with Sic14 occurs via the same mechanism as explained for Y type zeolites. Thermal elimination of dislodged aluminium in NaY zeolites results in the formation of mesopores and, in the most severe conditions, in amorphisation. The higher volatility of lithium compared to sodium tetrachloro-aluminium complexes is reflected in more extensive mesopore formation in LiY compared to NaY samples. The desorption of AlCl3 is facilitated and the detrimental effect of AlCl3 neutral ised when the tetrachloro-aluminate complexes are decomposed in the presence of SiC14. However, an important fraction of the dislodged aluminium always remains in the sample. Strong mineral acidity develops during the washing step due to the hydrolysis of A1-C1 bonds. The amount of aluminium that can be leached from the sample varies largely depending on the washing conditions. Framework dealumination during water washing is not very important, unless acidified wash water is used. A hydroxyl vibration in the range from 3730 to 3750 cm-l represents the 4 OH hydroxyl nests formed upon acid leaching of A1 atoms from the lattice. Part of the intensity of this hydroxyl band may in some samples be due to SiOH groups in mesopores and/or amorphous silica. The intensity of the AlOH vibration at 3700 cm-1 is always weak, indicating that oligomeric oxo- hydroxy-aluminium species are not abundant in samples dealuminated with SiC14. Judged on the intensity of the 3600 cm-1 vibration, the 0x0-hydroxy-aluminium species that are present in the samples are highly polymerised. For several Sic1 4-deal uminated samples a good correspondence was found between the AlF/AlEF proportions, determined on one hand by 27Al MASNMR, and on the other hand by a combination of 29Si MASNMR and chemical analysis. This indicates that all the aluminium is NMR-visible, which is in agreement with the presence of low amounts of NMR-silent oligomeric alumina species. Distorted alumina tetrahedra are found in samples of low crystallinity. Broad 29Si MASNMR signals in the range from -110 to -112 ppm represent amorphous silica. The contribution of this signal to the total 29Si resonance envelope can be used as a âshort-rangeâ crystallinity measurement. The short- range degree of cystallinity is always higher than the âlong-rangeâ crystallinity, determined with XRD. ACKNOWLEDGMENTS JAM and PJG acknowledge the Flemish National Fund for Scientific Research (NFWO) for Positions as Research Associate and Senior Research Associate, respectively. 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JACOBS Laboratorium voor Oppervlaktechemie, K.U. Leuven, Kardinaal Merc ier laan 92, 6-3030 Heverlee, Belgium SUMMARY The dealumination o f HY, L i Y and NaY z e o l i t e s w i t h Sic14 i s i n t e r p r e t e d as a two-step process. The f i r s t step i s the formation o f c h l o r i d e s a l t o f the charge compensating cat ions and o f Sic13 species probably bound t o the framework. This r e a c t i o n takes place a t r e a c t i o n temperatures as low as 423 K. The second step i s t he A l , S i exchange. Changes i n the degree o f dealumination between HY, LiY and NaY are t e n t a t i v e l y explained on t h e bas is o f ca t i on d i s t r i b u t i o n s . Cloro-aluminium complexes formed from AlC13 and the ch lo r i de s a l t du r ing the second step g i v e r i s e t o 27Al MASNMR resonances a t ca. 100 ppm. The micropore system can be preserved i f the a1 ka l i -ch loro-a lumin ium complexes are no t decomposed before washing. No add i t i ona l dealumination was observed dur ing washing. The ext racted aluminium can on ly be removed p a r t i a l l y from the samples. INTRODUCTION Dealumination o f z e o l i t e Y i s an essent ia l step i n the preparat ion o f FCC ca ta l ys ts . The dep le t i on o f t he z e o l i t e framework w i t h aluminium i s accompanied by t h e generation o f mesopores and extra-framework s i l i c o n and aluminium species, which may p lay important c a t a l y t i c ro les . Dealumination o f z e o l i t e Y w i t h Sic14 was f i r s t repor ted by Beyer and Belenykaya ( re f .1 ) . I n t h i s paper some more l i g h t i s shed on the chemistry o f t h i s dealumination reac t i on and on the mechanism o f generation o f mesopores and extra-framework s i l i c o n and aluminium species du r ing dealumination w i t h SiC14. EXPERIMENTAL Dealumination orocedure. NaY z e o l i t e from Ventron w i t h a Si/A1 r a t i o o f 2.4 was used as the s t a r t i n g ma te r ia l . Nay, i o n exchanged f o r 68% w i t h L i + i s denoted as LiY. Z e o l i t e HY was obtained from NaY a f t e r i o n exchange w i t h NH4+ f o r 90%. The z e o l i t e powders were compressed i n t o f lakes, crushed and sieved. 4 g o f the 0.25 t o 0.50 mm f r a c t i o n was loaded i n a ho r i zon ta l t u b u l a r quar tz reactor , w i t h an i n t e r n a l diameter o f 3 cm. The bed l e n g t h was ca. 1.5 cm. The temperature was monitored w i t h a thermocouple mounted i n the middle o f the bed. The dealumination procedure consis ted o f t h ree steps, v i z . dehydration, contact w i t h SiClq, and a post-treatment. Dehydration o f the z e o l i t e was done under a f l ow o f n i t rogen o f 60 m l per minute. The heat ing r a t e was 3.3 K per minute t i l l a f i n a l temperature o f 623 K, where the z e o l i t e was kept f o r 2 h. Then the temperature was adjusted t o the reac t i on temperature, TR, and the n i t rogen stream saturated w i t h Sic14 vapor. The molar N2/SiC14 r a t i o was equal t o 4 and the r e a c t i o n time, t R , t y p i c a l l y 35 minutes. Accordingly, ca. 25 mmol o f Sic14 382 were contacted with 4 g of zeolite. After a 20 minutes purge under nitrogen stream at TR, the temperature was increased with a rate of 3.3 K per minute to the post-treatment temperature, Tp. The duration of the post-treatment was at least 90 minutes. After cooling, 500 mg of the sample was retained for NMR measurements. The remaining sample was poored into 2 1 of deionized water. The sample was washed till the wash water was free of C1, and dried at a temperature of 423 K. The sample notation is followed by their TR and Tp temperatures (K) in brackets. MASNMR measurements. The MASNMR measurements were performed on a Bruker 400 MSl spectrometer with a magnetic field of 9.4 T. The 29Si MASNMR experiments were run at 79.5 MHz, with a pulse length of 4 p s , a pulse interval of 5 s, a spinning rate of 3 kHz and a number of 10,000 scans. For the determination of the degree of NMR crystallinity of a Sam le the sigal at -110 ppm was considered to be due to amorphous material. &A1 MASNMR was performed at 104.2 MHz, with a pulse length o f 0.6 p s , a radiofrequency field strength of 5 mT, a pulse interval of 0.1 s, usual1 a spinning frequency of 5 kHz and a number of scans of 3,000. The 29Si and &A1 spectra were deconvoluted into curves with Gaussian or Lorentz line shape using the Bruker GLINFIT program. The treatment of the sample with acetylaceton (acac) in order to visualize 27Al MASNMR-invisible aluminium was performed as described previously (ref . 2 ) . 27Al MASNMR spectra at a spinning rate of 15 kHz were recorded 24 h after the impregnation with acac, using a pulse interval of 1 s. - N2 adsomtion. Nitrogen adsorption-desorption isotherms were recorded with an ASAP 2400 instrument from Micromeritics. The samples were pretreated during 15 h at 573 K under vacuum (10 mPa). The isotherm was measured at 77 K. 160 measurements along the adsorption and desorption branch of the isotherms between P/Po values of 0.002 and 0.995 were recorded. The BET surface area was calculated from at least 10 adsorption measurements at P/P0 70 60 50 40 3 0 20 10 0 A T t 4 2 3 K t 523K ~ Liy + 423K t 523K 0 5 10 I5 20 25 3 0 0 t R(minutes) 4 0 A T 3 0 20 10 0 0 5 10 15 20 25 3 0 3 5 t R(minutes) 5 10 15 20 25 3 0 35 t R(minutes) Fig .1 . Overheat ing i n t h e m idd le o f t h e z e o l i t e bed d u r i n g r e a c t i o n w i t h SiC14. Tab le 1. I n f l u e n c e o f dea lumina t ion procedure on p r o p e r t i e s o f washed samples 383 b S t a r t i n g TR t~ Tp pHa SiF/AIF C r y s t a l l i n i t y (%) mesopore volumec,d m a t e r i a l (K) (min.) (K) NMR XRD (mm3 9-11 NaY 523 35 713 3.0 7.9 91 83 24 NaY 523 90 713 3.0 7.7 91 93 33 L iY 523 35 713 3.0 7 .0 97 35 L iY 523 90 713 3.0 7.5 90 58 25 a, o f t h e wash water ; b, determined w i t h 2 9 S i NMR; c, pores w i t h d iameters f rom 1.7 t o 50 nm; d, t h e pa ren t NaY sample has a mesopore volume o f 35 mm3 g-1. I n f l u e n c e o f r e a c t i o n and Dos t - t rea tmen t tempera tures 2 9 S i MASNMR s p e c t r a o f washed p roduc ts a r e shown i n F ig .2 . The resonance o f t h e SiF(OA1) environment i s observed a t ca. -107 ppm. Resonances cor respond ing t o S iF ( lA1 ) , SiF(2A1) and SiF(3A1) occur w i th an i n c r e a s i n g chemical s h i f t o f ca. 5 ppm f o r each e x t r a coo rd ina ted aluminium. A broad l i n e a t ca. -110 ppm c h a r a c t e r i s t i c o f a SiNF(OA1) environment ( r e f . 2 ) i s observed i n L i Y and NaY samples t r e a t e d a t t h e h i g h e s t Tp and/or TR tempera tures . The S iF /AIF r a t i o s , d e r i v e d f rom t h e r e l a t i v e i n t e n s i t y o f t h e S i F ( n A l ) resonances, t h e degree o f 384 c r y s t a l l i n i t y and the BET surface area o f the washed z e o l i t e products are given i n Table 2. The c r y s t a l l i n i t y and the BET surface area are lower when Tp i s increased a t a g iven TR (Table 2 ) . Post-treatment temperatures o f 823 K can be appl ied w i thou t important damage t o the z e o l i t e l a t t i c e when on ly a TR o f 423 K i s used. Dealuminated LiY samples w i t h h igh c r y s t a l l i n i t y have t y p i c a l l y a S i F / A I F r a t i o between 6.0 and 7.0 (Table 2). A s l i g h t l y h ighe r value (7.6) i s found i n the LiY(623-713) sample, which, however, t o a l a r g e extent i s amorphous (Table 2, Fig.2). Under comparable condi t ions, HY i s l e s s dealuminated compared t o L i Y , whereas NaY seems t o be dealuminated t o a h igher extent (Table 2 ) . (523-713) (523-823) (623-713) A I.I.l.L.l, -a0 -100 -120 PPM -- - 8 0 -100 -120 -80 -100 -120 PPM PPM HY - I . I . I . I I I . . - 80 -100 -120 -a0 -100 -120 PPM PPM Fig.2. 29Si MAS NMR spectra o f washed samples. 385 NaY n (523-623) (523-713) (523-823) (623-713) (623-823) --- -80 -1B0 -120 -80 -100 -120 -80 -100 -126 PPM PPM PPM Fig. 2. Continued. Generation o f mesooores Isotherms f o r t he adsorption and desorpt ion o f n i t rogen a t 77 K on the parent NaY sample and dealuminated Nay, LiY and HY are g iven i n Fig.3. The adsorption isotherms have a rectangular Langmuir shape, c h a r a c t e r i s t i c o f micropore adsorption. The adsorption and desorpt ion branches o f the isotherm on the parent NaY z e o l i t e coincide, i n d i c a t i n g t h a t mesopores are v i r t u a l l y absent, as expected. Hysteres is occurs w i t h SiClq t r e a t e d samples, t he importance o f i t depending on TR and Tp. The occurrence o f hys te res i s i nd i ca tes the presence o f a mesopore system. For a l l samples the desorpt ion branch j o i n s the adsorption branch again a f t e r a sudden decrease i n the amount adsorbed a t a r e l a t i v e pressure o f ca. 0.45. This has been explained as a t e n s i l e - s t r e n g t h e f f e c t being a t t he o r i g i n o f sudden desorpt ion o f n i t rogen from mesopores, connected t o the e x t e r i o r o f the adsorbent v i a openings smal ler than 5 nm ( re f .3 ) . It i s , therefore, impossible t o determine the average diameter o f the mesopores. Volumes o f mesopores w i t h diameters from 1.7 t o 50 nm, as de r i ved from the desorpt ion curves, are given i n Table 3. The mesopore volume o f the dealuminated HY, LiY and NaY z e o l i t e s w i t h a h igh c r y s t a l l i n i t y , except NaY w i t h S i F / A I F o f 15.6, doesnot exceed t h a t o f the parent NaY z e o l i t e , f o r which a value o f 35 mm3 g - l was determined (Table 3). For comparison, t he mesopore volume o f NH4Y z e o l i t e , dealuminated by se l f -s teaming a t 973 K and w i t h S i F / A l F r a t i o o f 5.5 i s 100 mm3 g-1. 386 H L i Na 623 - 713 a23 - 90 75 923 - 0 Table 2. I n f l uence o f TR and Tp on p roper t i es o f washed products c r y s t a l l i n i t y (%) according t o XRD H L i Na I H L i Na - ao a5 68 64 a3 63 45 53 - 0 - 10 4 1 - 623 I 713 023 I 977 - 6.0 6.5 5.7 7 .0 7.9 4.7 7.6 8.6 - ND ND = no t determined. Table 3. Volume o f pores w i t h diameters from 1.7 t o 50 nm (mm3 9 - l ) I H L i Na I H L i Na I H L i Na 623 - 27 24 713 29 35 a23 923 - 69 Table 4. pH o f t he wash water I H L i Na I H L i Na I H L i Na 623 713 923 a23 387 P o 140- f 200 C 180- Table 5. SiF/AIF r a t i o b e f o r e washing HY 623-713 I H L i Na I H L i Na I H L i Na 623 - 6.5 7 .1 - 713 - 5.9 6.8 8 .8 4.6 8 .4 ND 823 923 N o / 1 : - ND I ( ! 6.5 i 15.2 N i l : a 160- + 140- C > I 0.1 012 0.3 0.4 6.5 0:s 0:7 0!8 0.9 200 180 - 160 $ 140 P. ," 220 2 200 180 V n I- 200 180 C m 0 160 E 2 200 > 180 160 140 120 100 Relative pressure (P/Po) LiY 423-823 --L LiY 523-623 --/ LiY 523-713 LiY 623-713 T- I I I I 0.1 012 0.3 014 015 0.6 0.7 0.8 0.9 Relative pressure (P/Po) Fig.3. N i t r o g e n adsorpt ion-desorpt ion isotherms a t 77 K . 388 + ads, * des I ,Nay 423-823 180 -1 I 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Rela t ive pressure (P/Po) Fig.3. Continued. Extra-framework aluminium 27Al MASNMR spectra of HY, LiY and NaY samples before and after washing are shown in Fig.4A and B, respectively. Before washing, essentially two 27Al resonance lines are present in the samples. The signal at ca. 60 ppm represents AIF. The signal at ca. 100 ppm should be due to aluminium chloro complexes. The 100 ppm signal has disappeared in NaY and LiY samples which have been exposed to Tp temperatures o f 823 or 923 K . This is in agreement with observations in literature that Na(AlC14) and Li(A1Clq) complexes in zeolite Y decompose at temperatures of 780 K (ref.4) and 733 K (ref.5), respectively. z7Al MASNMR signals at ca. 100 ppm are not observed in HY samples (Fig.4A). Upon washing, the chloro aluminium complexes are hydrolysed. pH values of the wash water are given in Table 4. For samples which finally have a high degree of crystallinity and which were subjected to a post-treatment with Tp < 823 K, the pH of the wash water i s typically between 2.8 and 3.2. When Tp equals 823 K, LiY samples develop less acidity compared to Nay. Samples which have lost partially their crystallinity develop weak acidity. 389 A new signal at ca. 0 ppm appears in the spectra of washed samples (Fig.4B). This signal is due to AlNF in octahedral environment. For T p equal to 823 K NaY exhibits an additional 27Al resonance at ca. 35 ppm. This signal is typical o f distorted tetracoordinated A1 (ref.6) or pentacoordinated A1 (ref.7) in non- framework environments. SiF/AlF ratios o f the samples before washing determined by 29Si MASNMR are given in Table 5. The SiF/AIF ratios o f the HY, LiY and NaY samples before and after washing are very similar, indicating that no additional dealumination occurs during the washing step. Infrared spectra o f the dealuminated Nay, LiY and HY samples are shown in Fig.5. Besides the high-frequency band at ca. 3620 cm-1 and the low-frequency band at ca. 3550 cm-1 due to bridging hydroxyl groups, hydroxyl vibrations are observed at ca. 3740, 3670 and 3600 cm-1. The 3740 cm-1 band should be due to silanol groups. The intensity of this band increases when the sample contains a larger amount o f mesopores (Fig.6), indicating that these hydroxyls are located in the mesopores. Bands at 3600 and 3670 cm-1 are generally ascribed to hydroxyl groups on non-framework aluminium species. It is striking that the intensity o f the 3600 and 3670 cm-1 bands is more pronounced in NaY compared to LiY samples. HY LiY A (623-713) A - 100 0 FPM NaY (423-823 ) n (423-823) A I I I I I 1 I I I , , , I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 100 0 100 0 PPM PPM (423-923) (523-713) -- 100 0 100 0 PPM PPM Fig.4A. 27Al MASNMR spectra before washing. 390 Fig.4B. *'A1 MASNMR spectra after washing. (623-713) (523-713) 111111111 (523-623) (623-713) 100 0 PPM (523-713) (423-823) IIIIIIIlllllllllrllll 0 100 0 PPM NaY 111111111111111111111111111111 100 0 (523-623) (523-713) (623-713) 111111111111111 100 0 PPM PPM (42 3-823 ) 111111111111111111111111111111 lcl0 0 100 0 PPM PPM 39 1 0 3 6 0 0 3 2 0 0 llhl 1 0 3 6 0 0 3 2 0 0 I I I I I - 1 c m Fig.5. Hydroxyl spectra of the H-form of the dealuminated samples Quantification of the amount of framework and non-framework aluminium was done using the acac method and using the absolute intensity mode. The 27Al MASNMR spectra of the acac-treated samples run at a spinning rate of 15 kHz are shown in Fig.7. The gain in resolution by the higher spinning rate can be appreciated by comparison with the spectra of Fig.4 which were run at 5 kHz. The AIF and AINF content of some of the samples are given in Table 6. The dealuminated LiY samples contain a considerable amount of AINF even when post- treated at temperatures at which the lithium-choro-aluminium complexes are decomposed. Due to changes in molecular weight of the zeolite upon the different treatments it is not straightforward to compare the aluminium content of dealuminated samples with that of the parent zeolite. From the intensity of the 27Al signal it was estimated that between 20% and 30% of the aluminium is extracted from the zeolite pores in the washing step. 392 -. i " 0 * l- m k .4 .* I a a .4 I N a Y Fig.6. Intensity o f 3740 cm-l hydroxyl vibration against mesopore vol ume. 10 IS 10 I 0 3 0 6 0 90 1 2 0 1 5 0 3 -1) Mesopore volume (mm g L i Y (523-713) (523-713) AL I\ (423-823) --- 0 1°' PPM 100 0 100 0 PPM PPM Fig.7. 27Al MASNMR spectra o f acac-treated samples run at a spinning rate o f 15 kHz using the absolute intensity mode. NH4Y zeolite i s used as reference. Table 6. A1F and AlNF content (mmol 9-1) o f washed samples sample A1 F A1 NF NaY (423-823) 0.7 2.5 NaY(523-713) 1.2 2.3 LiY(423-823) 1.6 2.0 LiY (523-713) 1.4 1.7 393 Deal umination mechanism The overheating curves of Fig.1 and the experiments with shortened and prolonged tR show that a reaction zone progresses through the zeolite bed during contact with SiC14. Once the reaction zone has passed, the zeolite has become unreactive towards SiC14, although the zeolite is not yet completely dealuminated (Table 1). Apparently, the number of A1F sites that react with Sic14 at temperatures of 423 K and 523 K is limited. The dealumination reaction of NaY zeolite with Sic14 corresponds to the following stoichiometry (ref. 1): Na(A102(Si02),) + SiClq - -> NaAlC14 t (Si02),+1 It has been suggested that the sodium-aluminium-chloro complexes deposited in the zeolite pores protect the residual framework aluminium atoms from being further attacked by Sic14 (ref.4). However, product-inhibition can not explain the influence of the nature of the charge compensating cations on the extent of dealumination (Table 2). All the 1-atom positions in the faujasite structure are equivalent and there should be no discrimination on this basis. Since the cations take part in the reaction with Sic14 (Eq.l), the cation location could influence the rate of the isomorphic substitution reaction. The dealumination reaction could proceed via two consecutive reaction steps (ref.8). The first step could be the rupture of one Si - C1 bond and formation of a Si - 0 bond and a chloride salt of the charge compensating cation. This step is exothermic and can proceed at mild reaction temperatures (ref.8). The second step is the aluminium-silicon exchange with concomitant formation of a aluminium chloro complex. This conversion is also exothermic (ref.8) and can proceed during the post- treatment. 0- - - - > 0 - - - > 0 \ / \ / \ / / \ / \ / / \ / \ / \ / \ / \ / \ Si Si Si A1 Si A1 The cations, M, are distributed over cation sites located in the supercages, the sodalite cages and the hexagonal prisms of the faujasite structure (ref.9). The Sic14 molecule has access to the supercages only. When the number of supercage cations is compared to the number of A1 atoms that were actually 394 subst i tu ted, a s t r i k i n g correspondence i s found (Table 7). It expla ins why under g iven dealumination condi t ions the degree o f dealumination fo l l ows the order: NaY > LiY > HY (3) I n l i t e r a t u r e samples have been repor ted w i t h very h igh S i /A1 r a t i o s ( re f s . l , 4 and 5). Such samples can be obtained i f a f t e r the f i r s t step, which necessar i ly has t o be performed a t low r e a c t i o n temperature, t h e temperature i s g radua l l y increased i n presence o f Sic14 t o values o f 720 o r 730 K. Under such condi t ions the l e s s access ib le cat ions and associated framework aluminium atoms probably become reac t i ve . Table 7. Number o f accessible cat ionsa and o f aluminium atoms ext racted w i t h Sic14 from the l a t t i c e per u n i t c e l l NaX 40b 38e NaY 30c 30 - 34f 28 - 30f 21d 22 - 27f LiY HY a, t o t a l ca t i ons minus cat ions i n s i t e s I and 1â; b, data from ref.10; c, data from ref.11, d, ca l cu la ted from I R i n t e n s i t y r a t i o o f h igh frequency and low frequency hydroxyl v i b r a t i o n s from ref.12; e, ca l cu la ted from Sic14 uptake a t a temperature o f 480 K; f, ca lcu la ted from S i F / A I F r a t i o determined w i t h z 9 S i MASNMR (Table 2). ACKNOWLEDGMENTS PJG and JAM acknowledge the Flemish National Fund f o r S c i e n t i f i c Research f o r f e l l owsh ips as a Senior Research Associate and Research Associate, respec t i ve l y . Th is work has been sponsored by the Belgian Government i n the frame o f a concerted ac t i on on c a t a l y s i s . We are g r a t e f u l t o D r . 0. Anton and N.V. REDCO f o r t he n i t rogen adsorption measurements. REFERENCES l .H .K . Beyer and I .M. Belenykaja, Stud. Sur f . Sc i . Catal. 5 (1980) 203. 2. P.J. Grobet, H. Geerts, M. Tielen, J.A. Martens and P.A. Jacobs, Stud. Surf. 3.S. Gregg and K. Sing, Adsorption, Surface Area and Poros i ty , Academic Press, 4. H.K. Beyer, I .M . Belenykaja, F. Hange, M. Tielen, P.J. Grobet and P.A. 5. B. Sul ikowski, G. Borbely, H.K. Beyer, H.G. Karge and I . W . Mishin, J. Phys. 6.A. Samoson, E. Lippmaa, 6. Engelhardt, U. Lohse and H.-G. Jerschkewitz, Chem. 7. J.-P. Gilson, G.C. Edwards, A.W. Peters, K. Rajagopalan, R.F. Wormsbecher, 8. P. Fejes, I. K i r i c s i , I. Hannus, 6. Schobel, i n : D. K a l l o and Kh.M. Minachev Sci. Catal. 46 (1989) 721. London, 1982. Jacobs, J. Chem. SOC. Faraday Trans I , 81 (1985) 2889. Chem. 93 (1989) 3240. Phys. L e t t . 134 (1987) 589. T.G. Roberie and M.P. Shatlock, J. Chem. Commun. (1987) 91. (Eds.), Ca ta l ys i s on Zeol i tes, Akademiai Kiado, Budapest, 1988, p.205. 395 9.W.J. Mortier, Compilation of Extra-framework Sites in Zeolites, Butterworths Scientific l t d . , Guildford, 1982. 10.G. Eulenberger, D.P. Schoemaker and J.G. Keil, J. Phys. Chem. 71 (1967)1812. ll.T. Hseu, Ph.D. thesis, University of Washington, 1972, University Microfilms No. 73-13835, Ann Harbor, Michigan, U.S.A. 12.A. Corma, V . Fornes, J. Perez-Pariente, E. Sastre, J.A. Martens and P.A. Jacobs, in: W.H. Flank and T.E. Whyte (Eds.), Perspectives in Molecular Sieve Science, ACS Symp. Ser. 368, American Chemical Society, Washington, 1988, 555. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation ojCutulysts V 397 0 1991 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands TREATMENT OF GALMALUMINO-SILICATE (ZSM-5 TYPE ZEOLITE) WITH KOH SOLUTION. DISPERSION OF AGGREGATED ZEOLITES INTO SMALL PARTICLES J. KANA1 and N. KAWATA Central Research Laboratories, Idemitsu Kosan Co., Ltd., 1280 Kamiizumi, Sodegaura, Kimitsu, Chiba 299-02, Japan Oil (RAULO) , Japan. As a member of Research Association for Utilization of Light ABSTRACT We have studied about the treatment of aggregated galloalumino- silicate (ZSM-5 type zeolite) with KOH solution. The suitable KOH treatment of TPA-Si-A1-Ga is found to be a very valid preparation to disperse the aggregated zeolites into small particles and expose the external surface of unit crystals without changing other properties. Furthermore, the suitable KOH teratment of TPA- Si-Al-Ga increased the catalyst life in aromatization of n- hexane. INTRODUCTION ZSM-5 type zeolites have attracted considerable interest f o r unusual reaction selectivities (ref.1). Moreover, the stability of these zeolites to coke formation has been also attributed to the molecular shape selectivity (refs.1-3). Dejaifve et al. (ref.4) reported that during methanol conversion coke is formed on the external surface of ZSM-5 zeolite, and proposed that coking for H-ZSM-5 zeolite could be limited by increasing particle size. On the other hand, Bibby et al. (ref.5) reported that coke is predominantly formed inside the crystals, and proposed that catalysts requiring the least frequent regeneration will have a relatively low density of active sites and small particle size. Actually, it has been reported (ref.6) that the life of catalysts containing ZSM-5 for converting hydrocarbons increased with decreasing the size of the zeolite crystals. Synthesis of smaller crystals of ZSM-5 zeolite has been attempted (ref. 6) . However, these attempts often result in lack of crystallinity or aggregates of small particles. For aggregates of small particles, it is difficult to distinguish which size of unit crystal or secondary aggregated zeolite represents the size 398 of catalyst. Furthermore, different condition of synthesis also causes the change of other properties such as distribution of A1 or morphology (refs.7,8). We have studied about the treatment of aggregated galloalumino- silicate (ZSM-5 type zeolite) with KOH solution. The present paper will show that the suitable KOH treatment can disperse the aggregated zeolites into small particles without changing other properties. Moreover, the problem of representative particle size and the relation between catalist life and particle size will be discussed. EXPERIMENTAL Catalysts Galloalumino-silicate (Si-Al-Ga) (Si02/A1203/Ga203 molar ratio = 78/1/1.3) was synthesized using tetrapropyl ammonium bromide (TPA) by the method described in a patent (ref.9). The structure of synthesized material (TPA-Si-A1-Ga) was confirmed by X ray diffraction to be that of ZSM-5 having a high crystallinity. SEM analysis of the zeolite revealed aggregates of about 1 consisting of small crystals of about 0.1 - 0.05pm (Fig.2). Three kinds of proton form of Si-Al-Ga were obtained and studied. Firstly, after calcination of TPA-Si-A1-Ga at 823 K for 8h, the ammonium form of zeolite was obtained by exchanging twice with 1 M NH4N03 solution at 353 K for 3h. H-Si-A1-Ga was prepared by calcining it at 973 K for 4 h. Secondly, TPA-Si-A1-Ga were treated with 1 M KOH solution (10 g/g-zeolite) at 323, 333 and 343 K for 1 h, respectively. H-Si-A1-Ga(TPA,KOH) was prepared by treating it in the same manner as H-Si-Al-Ga. Lastly, after calcination of TPA-Si-A1-Ga at 823 K for 8h, Na-Si-Al-Ga was treated with 1 M KOH solution (10 g/g-zeolite) at 333 K for lh. H-Si-Al-Ga(Na,KOH) was prepared by exchanging twice with 1 M NH4N03 solution at 353 K for 3 h, followed by calcining at 973 K for 4 h. rm External surface area by benzene filling method External surface area was measured by the method of Murakami et al. (ref .lo). The zeolite sample was pretreated by calcining at 673 K in a flow of nitrogen, followed by keeping in the desiccator with benzene at room temperature for at least 10 h. The sample in a glass cell was connected to the flow-type BET apparatus, cooled to 195 K by dry ice-ethanol, and exposed to a 399 flow of N2 and He gas mixture (Nz content, 30 % ) . The sample was next chilled to 77 K to adsorb nitrogen, and then flashed by the dry ice-ethanol to desorb the nitrogen. The amount of nitrogen adsorbed was determined by measuring the amount of nitrogen desorbed. 2,l-dimethylbutane (2,Z-DMB) adsorption ability 2,2-DMB adsorption ability was measured by a pulse reactor (atmospheric pressure, 6 mm i.d. tube made of stainless steel) connected to an automated system of gas chromatographic analysis. 100 mg of 16-32 mesh particles was charged into the reactor. 2 ~ 1 of the mixture consisting of equivalent mole of n-hexane, 3- methylpentane and 2,2-DMB was injected into the reactor at room temperature, and the effluents were analyzed under the following conditions: hydrogen carrier flow rate = 22 ml min-â; and V.P.C. column = 3.5 m packing squarane. In all cases, n-hexane and 3- methylpentane were completely adsorbed and only 2,2-DMB passed through the catalyst bed. The 2,2-DMB adsorption ability was defined by the next formula. Adsorbed 2,Z-DMB moles Injected 2 , 2-DMB moles 2,2-DMB adsorption ability (%) = x 100 TPD measurement TPD spectra of ammonia were measured with a conventional TPD apparatus, and desorption was detected by a thermal conductivity detector. Zeolite sample (0.1 g) was evacuated in a quartz cell at 673 K for lh, exposed to ammonia used as probe base at room temperature for 15 min, then evacuated at room temperature. TPD measurements were made from room temperature to 823 K with a heating rate of 30 K minâl in a helium flow of 150 ml min-l. Catalytic reactions Reactions were carried out at 773 K and 1 atm in a quartz tublar microflow reactor containing 0.5 g of 16-32 mesh particles. After heating the zeolite to 773 K under nitrogen, n- hexane was fed at WHSV of 2 h-â. Analysis of the reaction products was carried out by on-line gas chromatography (ref.14). Conversion and yield were calculated on the carbon basis. Catalyst life was defined as the time retaining above 50 C-mol% of aromatics yield. 400 RESULTS Chemical analysis, BET total surface area, external surface area by benzene filling method and 2,2-DMB adsorption ability were measured about fresh catalysts of H-Si-A1-Gal H-Si-A1- Ga (TPA,KOH) and H-Si-A1-Ga (Na,KOH) . The results are shown in Table 1 & 2. TABLE 1 Chemical analysis of zeolites Si02/A120 SiO, /Ga20 Zeolite (mo 1 /mo 13 (mol /mo13 H-Si-A1-Ga 78 60 323 K 76 57 333 K 72 54 343 K 53 55 H-Si-Al-Ga(Na,KOH) 34 26 H-Si-A1-Ga(TPA,KOH) TABLE 2 Physicochemical properties of zeolites total surface external surface 2,2-DMB Zeolite area (m2/g) area (m2/g) adsorption ( % ) H-Si-Al-Ga 309 H-Si-A1-Ga(TPA,KOH) 323 K 355 333 K 360 343 K 377 H-Si-Al-Ga(Na.KOH1 343 20 43 57 61 79 60 83 87 88 6 In Fig.1, aromatics yield is plotted against time on stream. The catalyst life of H-Si-Al-Ga(TPA,KOH) treated at 323 and 333 K (61 and 66 h, respectively) were longer than that of H-Si-A1- Ga(52 h). On the other hand, the catalyst life of H-Si-A1- Ga(TPA,KOH) treated at 343 K (35 h) and H-Si-Al-Ga(Na,KOH) ( 2 h) were shorter than that of H-Si-A1-Ga. Reactions were stopped when aromatics yield became about 40 C- mol%. BET total surface area and coke content were also measured about used zeolites of H-Si-Al-Ga and H-Si-A1-Ga(TPA,KOH). Coke formation rate and total surface area loss per milligram of coke are shown in Table 3. Coke formation rate and total surface area 401 Time on stream ( h ) Ffg. 1. Aromatics yield as a function of time on stream. 0 , H- Si-A1-Ga; a, H-Si-A1-Ga(TPA,KOH) treated at 323 K: m , H-Si-A1- Ga(TPA,KOH) treated at 333 K; A, H-Si-A1-Ga(TPA,KOH) treated at 343 K; A , H-Si-Al-Ga(Na,KOH). TABLE 3 Aging property of zeolites ~~ Yea coke formation total surface Zeolite rate (mg/s h) loss / coke (m /mg) H-Si-A1-Ga 1.7 H-Si-A1-Ga (TPA, KOH) 323 K 1.7 1.2 1.2 333 K 1.8 1.0 343 K 3.4 1.1 loss per milligram of coke were defined as the followings. coke content of used zeolite operation time Coke formation rate = Total surface area loss / coke = surface area of fresh zeolite - that of used zeolite coke content of used zeolite DISCUSSION H-Si-A1-Ga(TPA, KOH) The time for filtration of TPA-Si-A1-Ga treated with KOH solution became longer than that of untreated one, indicating 402 that the KOH treatment of TPA-Si-A1-Ga dispersed,the aggregated zeolites into small particles. In order to confirm above assumption, the measurements of external surface area by benzene filling method and 2,2-DMB adsorption ability were carried out. From Table 2, it is found that both external surface area and 2,2-DMB adsorption ability of H-Si-Al-Ga(TPA,KOH) were more than those of H-Si-A1-Ga. Total surface area of H-Si-A1-Ga(TPA,KOH) were larger than that of H-Si-A1-Ga (Table 2), and all zeolites were comfinned by X ray diffraction to remain high crystallinities. Moreover, the results of SEM analysis of H-Si-Al-Ga (external surface area: 20 m2/g) and H-Si-A1-Ga(TPA,KOH) treated at 323 K (external surface area: 43 m2/g) are shown in Fig.2. SEM analysis revealed that the aggregated zeolites of about 1 P m were dispersed into small particles of about 0.3 p m by the treatment with KOH solution. These suggest that the increase of external surface is due to not noncrystallization of zeolite but dispersion of the aggregated zeolites into small particles. Fig. 2. Scanning electron micrographs of (A) H-Si-Al-Ga and (3) H-Si-Al-Ga(TPA,KOH) treated at 323 K. 403 Let us consider the relation between the size of apparent particle by SEM analysis and the external surface area. Assuming that the size of particle = R ( p m ) and the specific gravity = a the weight of 1 particle (9) = 4/3 Xa(R/2)3 x the external surface area of 1 particle (m2) = 4 7~(R/2)~ x then, the external surface area ( m 2 / g ) = 6/aR. (s/cm3) I TABLE 4 Relation between the size of particle (R) and the calculated external surface area calculated exte nal surface area (m /g) 5 Size of particle (R) ( P I 1.0 0.3 0.1 3.4 11 34 Assuming that a = 1.78 (g/cm3), the relation between the size of particle (R) and the calculated external surface area is shown in Table 4. The calculated external surface area of the particle size of 1.0 and 0 . 3 ~ are 3.4 and 11 m2/g, respectively. These values are much smaller than the actual external surface area of Si-Al-Ga having the same apparent particle size of the secondary aggregates (20 and 43 m2/g, respectively). On the other hand, the calculated external surface area of the unit crystal (whose size is 0.05-0.1 ,um) is between 34 and 67 m2/g, which is much larger than the actual external surface area of H-Si-A1-Ga (20 m2/g), and is equal to those of H-Si-A1-Ga(TPA,KOH) (from 43 to 61 From above results, it is suggested that unit crystals in aggregated as-synthesized Si-Al-Ga are not completely separated each other, and the considerable parts of external surface are useless. Si02/A1203 ratio of H-Si-A1-Ga(TPA,KOH) were smaller than that of H-Si-Al-Ga (Table l), indicating that some SiO, were dissolved from catalyst by the treatment with KOH solution. Then It is inferred that the KOH treatment can seperate unit crystals by dissolving some SiO, on their external surface and expose the external surface. From Fig.1, it is found that the KOH treatment at 323 and 333 K m2/s) - 404 increased the catalyst life. We reported (ref.12) that the change of acid density of zeolite or the amount of non-framework Ga species affects the catalyst life in aromatization of n-hexane over galloalumino-silicate. However, TPD spectra of ammonia of H- Si-A1-Ga(TPA,KOH) were compared with that of H-Si-A1-Gal and no change was observed in their TPD spectra. Moreover, the coke formation rate of H-Si-A1-Ga(TPA,KOH) treated at 323 and 333 K were the same with that of H-Si-A1-Ga (Table 3). Then it is suggested that the increase of catalyst life by the KOH treatment is not due to either the change of acid density of zeolite or the amount of non-framework Ga species. The reasons for the increase of catalyst life by the KOH treatment at 323 and 333 K are considered as following: (1) If the effectiveness factor of aggregated catalyst (H-Si- A1-Ga) is below 1.0 under aging, the dispersion of aggregates into small particles would increase the effectiveness factor of catalyst under aging and the catalyst life. (2) From Table 2, the increase of total surface area seems to be due to the increase of external surface. Furthermore, the ratio of increase of catalyst life is almost equal to the ratio of increase of total surface area. If the external surface has the same aging property with the internal surface, the catalyst life would depend on the total surface area when the effectiveness factor of catalyst is 1.0. The reason for the increase of catalyst life is not clear from the results of this experiment. However, whichever the reason, it is suggested that the KOH suitable treatment of aggregated zeolites will have more effect on the catalyst life in reactions which the effectiveness factor of catalyst is very small. On the other hand, as shown in Table 3, the rate of coke formation of H-Si-A1-Ga(TPA,KOH) treated at 343 K was faster than that of H-Si-A1-Ga, and consequently the catalyst life became shorter (Fig.1). Since Si02/A1203 ratio of H-Si-A1-Ga(TPA,KOH) decreased with the treated temperature (Table l), it is suggested that the excess treatment with KOH solution increased the acid density of external surface and consequently the coke formation rate on them. H-Si-A1-Ga [Na,KOH) As shown in Table 2, total surface area and external surface area of H-Si-Al-Ga (Na, KOH) were larger than those of H-Si-A1-Ga. 405 Furthermore, SEM analysis also revealed that the aggregated zeolites were dispersed into small particles. However, Si02/A1203 ratio of H-Si-Al-Ga(Na,KOH) was below half of that of H-Si-A1-Ga (Table 1) , and 2,2-DMB adsorption ability of H-Si-Al-Ga(Na,KOH) was much smaller than that of H-Si-Al-Ga (Table 2). The low 2,2-DMB adsorption ability seems to depend on more hydrophilic property of external surface of H-Si-A1- Ga (Na, KOH) than that of H-Si-Al-Ga. Furthermore, TPD spectrum of ammonia of H-Si-A1-Ga (Na, KOH) was considerably different from that of H-Si-A1-Ga (Fig.3), and the catalyst life of H-Si-A1- Ga(Na,KOH) was much shorter than that of H-Si-Al-Ga (Fig.1). 373 473 573 673 7 7 3 Tempemture ( K I Fig. 3. TPD spectra of ammonia of (-) H-Si-A1-Ga and ( - - - - - - 1 H-Si-Al-Ga (Na, KOH) . From above results, it is suggested that the dissolution of SiOz from external surface of H-Si-Al-Ga(Na,KOH) is more severe than that of H-Si-A1-Ga(TPA,KOH), and considerable SiO, was also dissolved from internal surface of H-Si-Al-Ga (Na, KOH) . The reason for the decrease of catalyst life of H-Si-Al-Ga(Na,KOH) would be due to the change of acidity of both internal and external surface. This showsthat the existence of TPA in the pore of zeolite is inevitable for the suitable and selective KOH treatment of external surface of zeolite. CONCLUSION (1) It is suggested that unit crystals of aggregated Si-Al-Ga are not completely seperated each other, and the considerable 406 parts of external surface are useless. (2) The suitable treatment of TPA-Si-A1-Ga with KOH solution is found to be a very valid preparation to disperse the aggregated Si-Al-Ga into small particles and expose the external surface of unit crystals without changing other properties. (3) The suitable KOH teratment of TPA-Si-A1-Ga increased the catalyst life, This is suggested to be due to the dispersion of the aggregated zeolites into small particles or the increase of external surface area. REFERENCES 1 P. Weisz, in T. Seiyama and K. Tanabe (Editors), Proceedings of the 7th International Congress on Catalysis, Tokyo, 1980, 2 L.D. Rollman and D.E. Walsh, J. Catal., 56 (1979) 139 3 L.D. Rollman and D.E. Walsh, J. Catal., 56 (1979) 195 4 P. Dejaifve, A. Auroux, P.C. Gravelle, J.C. Vedrine, Z. Gabelica and E.G. Derouane, 3. Catal., 70 (1981) 123 5 D.M. Bibby and C.G. Pope! J. Catal., 116 (1989) 407 6 C.J. Plank, E.J. Rosinski and A.B. Schwartz, U.S. Patent 3 , 926,782 7 E.G. Derouane, S. Detremmerie, 2. Gabelica and N. Blom, Appl. Catal., 1 (1981) 201 8 E.G. Derouane, J.P. Gilson, Z. Gabelica, C.M. Desfuquoit and J. Verbist, J. Catal., 71 (1981) 447 9 C.J. Frank, G.B. Patent 1,402,981 10 M. Inomata, M. Yamada, S. Okada, M. Niwa and Y. Murakami, J. Catal. , 100 (1986) 264 11 J. Kanai and N. Kawata, 3. Catal., 114 (1988) 284 12 J. Kanai and N. Kawata, Appl. Catal., in press P l G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 407 D. J. RAw"X1, K. a&, L. H. S l Y d a n d A . P. cHAppIE2 h f i e l d Catalysts, WarringtOn (U.K.) 2~nilever -, port sunlight ~aboratory, ~uarry 1l0ad ~ast, wington, W k r a l , Me.rseyside L63 3JW (U.K.) SUMMARY The addition of primary distillation residues to the Fee feedstock leads toinmeas& vanadium loadings on the catalyst and major problem with catalyst deactivation. The deactivation process is believed to involve the formation of low r,elting point V205-Na2O-Al203 phases which accelerate the normal zeolite dealurnination process. for the acidic V205 preserves catalyst activity without introduc ing undesirable properties. laboratory tests and a camnercial trial. Intamqting this precess by pruvidhg a suitable trap The efficiency of a strontium titanate trap is demonstrated in m m C N Transition metal contarmnan ' ts in the form of pozphyrin ccanplexes are present to varying extents in the majority of crude oils. primary refinery distillation processes consign a large proportion of these contarmnan ' tstothe distillation residues and yield gas oil distillates w h i c h are low in metals. Such gas oils have traditionally been the principal feed to fluid catalytic cracking (m) units, and although the metal contaminants slowly accumulate on the catalyst particles throughcplt their life, the equilibrium levels remain relatively low ( 408 ?he other important metal conbminant is vanadium. Although it exhibits distinct dehydrqenation activity, the major effect of vanadium is to cause marked and irreversible deactivation of the catalyst. ?his paper is concerned with the mecham 'sm of this deactivation pmcess and shews that, frm a lawwledge of the chemical and physical properties of the vanadium species, it is possible to design efficient trapping agents w h i c h can be incorporated into the catalyst particle. T h e effectiveness of this apprcad~ is demonstrated t h rcqh laboratory tests and confirmed by the results of full-scale trials. - Four series of catalysts were employed in this study. An w i m e n t d l rare-earth exchanged Y-zeolite (REHY) catalyst was used for the work on the mechanisn of vaMdium attack. Tim severities of anm~nium ion-excharqe gave catalysts with 0.16 and 0.55 wt% Na20. A further series of experimental RElIy catalysts was prepared for vanadium trap studies. (no trap) and. catalysts containing 0.485 moles of CaTiO3, SrTiO3 and &Ti03 per kg of dry catalyst. Finally, Cmsfield SLS-80 and SIS-SOW were used for detailed laboratory tests and SLS-100 and SLS-1OOW for cawmercial trials. vanadium m m tion lmis consisted of a control Experimntal catalysts used for mechanistic studies were irqxqnated with 0, 1000, 3000 and 5000 ppn vanadium by weiqht by applying aqueous oxalic acid solutions of d u m vanadate using incipient wetness techniques. Catalysts used for vandim trap studies were inprqnated with Xylem solutions of vanadyl naphthenate according to the commonly used Mitchell method (ref. 3). Catalysts were dried at 100°C and heated to 540°C for 3 hours prior to steam deactivation. catalyst Deactivation Experimental catdlysts were deactivated by heating in 100% steam for 5 hours at 788°C in a fluid bed reactor. of the equipment have been given elsewhere (ref. 4). Catalysts involved in trap studies w e r e Steam deactivated at 760°C using similar procedures. catalvst Evaluation Detailed procedures and a description Deactivated catalysts were tested according to the microactivity (W) test procedure describe2 previously (ref. 4). X-Rav Diffraction x-my pmder diffraction patterns w e r e used to assess the relative zeolite crystallhities of catalysts according to ASIM D-3906. cell size -ts were carried out using RSIM D3942. Surface Area crystdllographic unit surface area was derived fmm nitrugen adsorpticol/desorption kothems generated with a MiQomeritics AsAp 2400 instrument. Totdl surface areas 409 were calculated fram the adsorption isotherms at p/po 410 level of added vanadium. Bese results are shown graphically i n Figum 1 and Mate that the zeolite acanponent of the catalyst is the min focus of attack by vanadium w i t h only minor, but definite, Qmage being suffered by the matrix. Additional results shown in Fig. 1 co~lcern the hflm of the level of residual sodium i n the catalyst. it is clear that the sinailtaneous presence of vanadium and high levels of sodium grsatly increse the destmction of the zeolite. 'Ihe effect on the nntrix is less dxrious in these catalysts. 2oo r 0.16% No20 h ..-I XI < v) 50 - 0.55% No20 - VANADIUM (ppm) 0 2000 4000 6000 ( C ) 0 2000 4000 6000 VANADIUM (ppd 40 3 30 \ N E 0 a I v) " < 20 (bl \ 0.55% Na20 I I . I 0 2000 4000 6000 VANAD I UM (ppn) (d) 0 2000 4000 6000 VANADIUM (ppd Fig. 1. prvperties of stearn cbxtivated mm catalysts, (a) micmpore surface area, (b) mescpore surface area, (c) relative zeolite crystall inity, and (d) MAT conversion. effect of vanadium and sodium on the Fhysical and catalytic 411 studies of the distribution of metals in sectioned equilibrium catalyst particles have been carried out using imging secondary ion mass spedrcanetry (ref. 8). with minor penetration only behg evident in the oldest particles. contrast, vanadium spreads thrcughout the catalyst particle and, in addition, can m e between particles (ref. 7, 8 ) . The diffuse distribution of vanadium accOuntS for the extent of its destructive attack on the mysical and catalytic properties of the catalyst and, in turn, demands that a valid explanation of vanadium poisoning must include an urd- ' of its mobility. particles are unlikely to retain their identity for long. Organic fragments w i l l be remrved by thermal and catalytic processes within the riser and any remaw parts of the porphyrin skeleton w i l l be destroyed in the oxidizing atmosphere of the regenerator. 'Ihe most likely vanadium species to result is an oxide. that vanadium remains as V(IV) in the reducing atmosphere of the riser and is not reduced to V ( I I 1 ) . oxidized to the V(V) state. An examination of the melting points of the cannon vanadium oxides - V2O3 (1970'C), VO2 (1967°C) and V2O5 (680'C) indicates that V2O5 w i l l be a liquid mer regenerator conditions and suggests that it is a t this stage in the FCC cycle that vanadium is most likely to diffuse thrmgh the catalyst. An approximate idea of the rate of diffusion was dtained by heat- intimate mixhrres of V2O5 and catalyst particles in a i r at 700°C for various t i m e s . Follawhg rapid cooling, the particles w e r e embedded and sectioned, and the vanadium distribution mapped ushg EDX techniques. F'rom Fig. 2 it is clear that penetration of vanadium is rapid and virtually wmplete within 15 minutes - less than the time spent in the regenerator in a single FCC cycle. These have shown that nickel accumulates at the catalyst surface In The vanadium (IV) cmplexes w h i c h are init ially depcsited on the catalyst A recent study using x-ray akorbance spectroscapy (ref. 9) has sham Ixlring the regeneration cycle, haever, vanadium is (a) (b) (c) Fig. 2. Vanadium distribution maps of cross-sections of-6Op catalyst particles after heating w i t h V2O5 a t 700°C for (a) 5 m b , (b) 10 m b and (c) 15 m i n . 412 In the presence of soda, V2O5 forms mixed oxide phases (ref. 10) with melting points down to 525'C. This extends dawIlwards the tenpxature range for vanadium mability and such systems are likely to be responsible for the enhanced zeolite destruction found when vanadium and sodium are bath present. The loss of zeolite structure in the presence of vanadium is accclIIlpanied by a reduction in the average crystallcgramc Unit cell size as shown in Fig. 3 . accderat- the dealmination process in the zeolite. Furthrmore, a similar process occurring in the silica-alumina matrix cmpmenb is likely to account for the observed changes in the catalyst matrix. can also be b-t abcut by subjecting a catalyst to hi- teqeratums and/or pmlomgd steam treatment (ref. 4) w h i c h suggests that the principal action of vanadium is to accelerate the destructive changes that take place within the catalyst in normal use. ?his presents a direct indication that vanadium functions as a poison by such enhanced dealumination t 24.26 CI u) -I -J ' 24.24 c No20 c.( z 3 0 2000 4000 6000 VANADIUM (ppm) Fig. 3 . The effect of vanadium and scdium on the u n i t cell size of the zeolite ccgnponent of steam deactivated exprimental catalysts. Further examination of binary and ternary mixed oxide phase diagram (ref. 11, 12) shm thatV205 and the V205-Na20 system will dissolve alumina at temperatures greater than 610°C. increasing tenperatum of the melt and at 700°C in the v205-Al203-Na20 system up to 1 mole percent A1203 will be in solution. alumina will be deposited until the new equilibriun cgnposition is reached for the 1- temperature. catalyst moves frcnn the regenerator to the riser and back to the regenerator again, ample opportunity exists for the dissolution of alumina, its transport, and its deposition in other regions of the catalyst structure. The formation 'Ihe solubility of alumina haeases with If such a system is cooled, With the temperature cycling that occurs as the 413 of solid AlVO4 - the only stoichianetric c m p m d to form between V2O5 and A1203 (ref. 13) - is a possibility. This a u l d inhibit the movement of vanadium, however, its melting point of 695'C suggests that it would not be very effective, especially i n units processing high residue-containing feeds where regenerator temperatures tend to be significantly higher. TIE alumina dissolved i n the m e l t can, in principle, originate fmw various saun=es, e.g., z e o l i t e framework alminium, extra-framswork aluminium, and aluminium fmm the catalyst matrix. zeolite Y framework a t high temperatures, however, provides a pawerful driving force for the net flux of aluminium f m framework to mn-framework positions and offers a ready explanation for the preferential attack on the zeolite. proposed- 'a, based on accelerated dealmination, is sham schematically in Fig. 4. me intrinsic instability of the me + Na20- V 0 + H P Si Al Si * liq - - - - - - - - I zeolite framework ,610'C 2 Na20 - A1203- V 0 liq \ T i ?( / + - A I $ i + N a g - v205 I iq non-framework aluminium species 0 OH HO 0 dealuminated zeolite - - - - - - - - Fig. 4. shming the involvement of an Na20-V205 mel t in the dealumination process. prcp?osed mecharu 'sm for zeolite and m a t r i x destruction by vanadim VANADIUM TRAPPING SYSTENS Maintenance of catalytic activity is fundamental to the FCc process. In the presence of hi@ levels of vanadium the additional activity loss is nonmlly countered by increasing the active zeolite content of the catalyst inventory either by increasing the catalyst make-up rate or by incorporating higher levels of zeolite into the catalyst. be to innrbilize the vanadium species within the catalyst particle and thereby inhibit its destructive action. For such a trap t o be viable, a number of criteria have to be met : the trap must be catalytically inert, it m u s t be stable throughout the catalyst preparation processes, it shall be capable of reading rapidly arid irreversibly with vanadium species t o form high melting A more satisfactory approach wuld 414 ccsrpxxmds, and it nust be cost-effective in caparison w i t h the traditional replacanent and high level zeolite d e s . -1ex nust be stable d e r the thermal., hydrourermal and redox conditions prevailing i n the Fa3 unit. Furthermore, the hap-vanadium A S b r t U K J ' point for vanadium trap design is a recognition that V205 is an acidic oxide and hence w i l l react, at suitable tenperatures, with basic oxides to form mixed oxide systems. of basic oxide is limited. form d c s w i t h n-elting points in excess of 1500°C. Unfortunately, rare earth oxides exhibit undesirable catalytic activity. given by the alkaline earth oxides whi& react to form stable vanadates k u t are themselves incmptible with the acidic conditions experienced during conventional catalyst manufacture. hmever, i f the trap is f i r s t neutralized with an acidic oxide which is a weaker acid than V2O5. displaament t o form the oxide. catalytic properties. earths) and weakly acidic oxides (e.g. Si02, TiO2, ZrO2, Sn02, C02) can be envisaged as potential vanadium traps. taken into account, however, the list of practical traps becapnes much shorter. For exanple, scane combinations require very high temperatures for their synthesis, others give extremely hard materials which are difficult to process and scane are insufficiently stable in the catalyst slurry. Frcnn a rnrmber of trials, calcium, s t r o n t i u m and barium titanates have the best ccsnbination of properties. results Shawn in Table 1 indicate that strontium titanate (ref. 14) gives the greatest activity retention. In 0- to meet the above criteria, the choice For exanple, rare earth oxides react w i t h V2O5 to Further examples are The acid-base approach is still viable, In use, such a system would react with V2O5 by ' vanadate and liberate the weakly acidic It isessentl 'al that this weakly acidic oxide has no undesirable A nunber of ccpnbinations of basic oxide (e.g. W i n e earths, rare when dl1 the desirable prqerties are ccanparative vanadium trapping performance has been tested and the TABIE 1 Effect of traps on the retention of catalyst activity in the presence of 5000 ppn vanadium 63 80 90 85 a: activity retention = MAT amversion w i t h 5000 rxpn V x 100 MAT conversion with no added V 415 Conversion (wt%) Gasoline (wt%) Gasmline/cOnversion specific cokeb strontium has dso been sham to be superior for vanadium trapping to either calcium or barium when tested as carbonates in catalysts prepared with a neutral silica-alumina gel binder system (ref. 15). 67.6 68.3 49.7 50.0 0.736 0.733 1.21 1.23 MANUFACIURE OF VANADIUM 'I" CATALySrs A key advantage of strontium titanate is that its preferred form is a highly crystalline, acid stable, perovskite structure which is prepared conveniently by calcination of the relevant oxides or oxide precwsors. Wing manufacture for this purpose, a slight excess of titania ensures the absence of free strontia in the pruduct and also directs the reaction toward the ABo3 perovskite structure in preference to the less stable, strontia-rich phases, e.g. Sr3Ti207. crystalline strontium titmate has no pore structure and its effectiveness as a vanadium trap is reliant upon maximizing its external surface area. is achieved by reducing the average particle size of the trap, by milling and classification, to levels ccanparable with other catalyst ingredients. Thereafter, the desired level of trap is incorporated into a particular catalyst formulation through displacement of an equal weight of clay. ?his Addition of strontium titanate to Fa3 catalysts has no adverse effects on the physical properties - attrition resistance is not affect& and density is marginally inrreased (ref. 16). Furthermore, the results of the MAT evaluation of amnercial catalysts with and without trap, given in Table 2, indicate no detrimental effect either to catalytic activity or to key prcduct selectivities. TABLE2 Effect of SrTiO3 trap on catalyst performance mtalysta SIS-80 SIS-8otTRAp a: catalysts deactivated at 760'C/5 hoUrs/lOO% steam b: defined as catalytic coke/khetic conversion - see ref. 4 TRAPPX EFFI(IIENcy Trapping efficiency is assessed by the retention of catalytic performance In and by the degree of passivation of the vanadium dehydrogenation activity. both cases the magnitude of the effect is assessed against vanadium 416 concentration. SIS-80 catalyst to form SIS-8OW clearly has a pronounced and beneficial effect on activity retention. Fig. 5b w h i c h demonstrates that the trap is effective in pr0teCtb-q the zeolite and suggests that surface area can be used as an alternative methcd for Fig. 5a shws that the addition of strontiumtitanate to an 'Ihe parallel retention of surface area is Shawn in assessing vanadium tolerance. R E +J z u) m W > z 0 u 2 I- < 2: 50 ' 40 . h 3 z I- z W I- W e El < VI 2000 4000 6000 0 2000 4000 6000 VANADIUM (ppd 0 VANADIUM (ppd Fig. 5. carnnercial catalysts with and without a vanadium trap, (a) MAT conversion, and (b) surface area retention. 7% effect of vanadium on the properties of stem deactiMted Another key property of the trap is that the trapvanadium species must be catalytically inert. ' Ihis is demonstmted in Figs. 6a and 6b which shaw a dramatic reauction in MAT coke and hydrosen yields. %ese yields are related to d&ydroqenation activity and provide clear evidence for the passintion of vanadium catalytic activity. 'Ihe trap is least effective at law levels of vanadium as this catalyst itself has scnne inherent vanadium tolerance. greater, however, the trap is clearly beneficial and its comtration in the catalyst formulation can be varied to meet individual requhmmts. shcm that a law level of incorporation is effective up to 3,000 plan whilst the indications are that higher concentrations can tolerate over 5000 plan. At levels of 2000 plan vanadium or Fig. 7 - EYAIIJATION 'Ihe incorporation of a vanadium trap will increase catalyst costs, and to be viable, this cost has to be offset by reduced catalyst addition ana/or inprove3 selectivity. In turn, the corcentration of the trap in the catalyst 417 (b) 7 7 6 - 5 - 4 - 3 - 2 - w x 0 U 2 U u W L In rl 0 2000 4000 6000 VANAD I UM (ppn) 0.5 L 0. 4 ii t 4J 0.3 z W c) 0 0.2 I 0. 1 SLS-80 + TRAP Fig. 6. cammercial catalysts with and without a vanadium trap, (a) MAT specific coke yields and (b) MAT hydrcgen yield. 'Ihe effect of vanadium on the selectivities of steam deadivated 6 7l P w Y 0 U U - % U W a v) SLS-80 WITH INCREASING / TRAP LEVEL SLS-80 0 2000 4000 6000 VANADIUM (ppm) Fig. 7. deactivated carmnercial catalysts containing increasing levels of vanadium trap. The effect of vanadium on the M?!T specific coke yields of steam will be dictated by the level of vanadium in the feedstock to be processed and the acceptable catalyst make-up rate. European refinery, Crosfield SLS-1OOW replad SIS-100 in order to aope with an increased level of residue in the feed whilst maintaining, or even rducing the catalyst addition rate. In a trial recently cond~~ctd in a At the erd of the six mnth trial, vanadium levels 418 had increased by 33% to 4000 ppn yet the catalyst addition rate had decreased frum 3.1 to 2.0 tom per day. allcued the overall level of conversion to incrsase by 5 wt% of f e d . As can be seen f m Table 3, the greatest praportion of the came fran a greater utilization of feed (HCO). TABLE 3 In addition, the b q m ~ ~ & coke selectivity in conversion mmnercial t r i a l of vanadium trap catalyst. U n i t Y i e l d s (wt%) Conversion Coke HCO L1=0 Gasoline G a s + L p G catdlyst addition rate (W) -myst SIS-100 sLs-1oovr 62.5 67.5 5.5 5.4 16.5 12.5 21.0 20.0 44.5 47.0 12.5 15.1 3.1 2.0 axJcwSI0N The destructive effect of vanadium on FCC catalysts can be inhibited by L ~ e addition of a vanadium trawing system. Suitable trap can be designed using the principles of acid-base chemistry, but consideration must also be given to the stability of the trap during catalyst manufacture and the -t that the trap has no undesirable catalytic effects. Laboratory tests show that strorrtium titanate confers vanadium tolerance to a catalyst, markedly reducby zeolite destruction, preserving catalytic activity, and reducing coke and hydrcqen formation. of a successful ccarmercial t r ia l . These results agree w i t h the conclusions RlTmENm 1. G. H. Ble and D. L. McKay, Passivate M e t a l s in FCC Feeds, Hydmcahn 2. G. A. Mills, Aging of Cracking Catalysts. Less of Selectivity, I M . Elq. 3. B. R. Mitchell, Metal Contamination of Cracking Catalysts. I. Synthetic mpcesS, 56(9) (1977) 97-102. chem., 42 (1950) 182-187. &tals DepositiononFreshCatalysts, Ind. Erg. chem. Prod. Res. Dev., 19 (1980) 209-213. 4. D. J. Rawlence and K. Gosling, Fcc Catalyst -0- EvdluatiOn, -1. Cdtal . , 43 (1988) 213-237. 5. B. C. Lippens and J. H. de Boer, Studies on -re Systenrs in Catalysts, V. The t Mew, J. Catalysis, 4 (1965) 319-323. 6. E. P. l3arrett, L. G. Joyner, and P. P. Halenda, The Detennination of pore Volume and Distributions in ponxls substances. I. ccarprtations frcin Nitrogen Isathernrs, J. Amer. Cbem. Soc., 73 (1951) 373-380. 419 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. J. L. Palmer and E. B. Cornelius, Separating Equilibrium (3racking Catalyst into Activity Graded Fractions, -1. Catal. , 35 (1987) 217-235. E. L. Kugler and D. P. Leta, Nickel and Vanadium on Equilibrium (3racking catalysts by Imaging Seconlary Ion Mass spectrC4netryf J. Catal., 109 (1988) 387-395. G. L . Wollery, A. A. Chin, G. W. Kirker and A. â¬Iuss, X-Ray Absorption Study of Vanadium in Fluid cracking catdlysts, in: M. L. ocoelli (IM.), Fluid Catalytic Cracking, ACS Symposium Ser. 375, ACS, Washington, 1988, phase Diagram for Cerarm 'sts, G. smith (Ed.), Fig. 5075, The American Cexamic Society, Inc., 1981. Ref. 10, Fig. 5192. Ref. 10, Fig. 5332. Fhase Diagram for Ceranu 'sts, M. K. Reser (Ed.), Fig. 320, The American Ceramic Society, Irc., 1964. A. P. Chapple, Eur. Pat. -1. EP204543, Dec. 1986. E. L. Kugler, Eur. Pat. -1. EP209240, Jan. 1987. Crosfield Technical publication: Vanadium Trap (VT) Catalysts, Crosfield Catalysts, W a r r i n g t o n (U.K.), 1990. pp 215-228. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 42 1 DOUBLE SUBSTITUTION IN SILICALITE BY DIRECT SYNTHESIS: A NEW ROUTE TO CRYSTALLINE POROUS BIPUNCTION~ CATALYSTS 1 2 G. Bellussi , 'Eniricerche S.p.A., San Donato M.se (MI) - Italy A. Caratil, M.G. Clerici', A. Esposito Enichemsynthesis R&D, San Donato M.se (MI) - Italy Abstract We have synthesized silicalites containing both Ti and a trivalent element (Al, Ga, Fe) in lattice position. Syntheses are similar to that of TS-1. As we add increasing amounts of a trivalent metal nitrate to the reaction mixture, we obtain silicalites containing both titanium and the other metal. Titanium content decreases with increasing content of the trivalent metal. Physico-chemical analyses (e.g., XRD, FT-IR, EPR, MAS-NMR, exchange capacity) and oxidizing and acidic catalytic activities confirmed isomorphous substitution of both elements. Introduction Most of the interesting catalytic properties of zeolites are related to the presence of strong acid sites. Many processes have been developed taking advantage of the zeolites acidity and shape selectivity. At the end of the seventies, we have prepared a silicalite with titanium in lattice positions (ref.s 1,2). The discovery of TS-1 and its peculiar properties in oxidative processes with hydrogen peroxide started a completely new field of shape selective reactions (ref. 3 ) . Several patents claimed also the synthesis of zeolites containing both titanium and a trivalent element (ref.s 4 , s ) . However only in few of them proofs about the presence of both elements in lattice position are given (ref. 5 ) . The presence of Ti and A1 (or Ga, or Fe) in lattice position can give rise to catalysts possessing a good activity in both oxidizing and acid catalyzed reactions. In this paper we report the results of our studies about the synthesis and characterization of aluminum, gallium and iron containing titanium-silicalite. 422 Experimental Zeolite synthesis TS-1 is synthesized as described in example n. 1 of US Pat. n. 4 410 501. A solution of 1.5 g tetraethyltitanate (TET) in 45 g of tetraethylsilicate (TES) was poured in a 400 ml pyrex glass vessel containing 100 g of a 2 0 % aqueous solution of tetrapropylammonium hydroxide (TPA-OH). We kept the resulting mixture at 60 OC for 3 hours. Occasionally we added distilled water to replace the water lost by evaporation. Molar ratios in the final reaction mixture were: SiO /TiO2=32.I, TPA /Si02=0.46, H20/Si02=35. We heated the mixture in a 260 r n l stainless steel autoclave in an oven at 175 OC, under autogeneous pressure, without stirring, for 24 hours. We separated the crystalline product from the liquid by filtration. We washed than several times with water, dried 2 hours at 100 OC and finally we calcined it 5 hours at 550 OC in air. We prepared several TPA-OH solutions containing known amounts of NaOH to verify the influence of Na cations in the reaction mixture. In the synthesis of the Al(Ga, Fe)-TS-1 samples, we dissolved the required amount of trivalent metal nitrate in 20 g of ethyl alcohol and added to the TES before the TET addition. The Si02/M203 molar ratio in the reaction mixture was varied from 100 to 1000. A reference ZSM-5 sample was prepared as the A1-TS-1 except that TET was not added to the reaction mixture. The final product (used as standard for ZSM-5 type catalysts) had a molar ratio Si02/A1203=164. We used the following reagents: TES (Dynasil 40 from Dynamit Nobel), TET (purum from Fluka), 20 % w aqueous TPA-OH (from Enichemsynthesis, free from alkaline impurities ) , NaOH (RPE-ACS drops from C. Erba) , A1 ( NO3 ) - 9H20 (RPE-ACS from C. Erba), Ga(N03)3.8H20 (puriss. from Fluka), Fe(N03)3-9H20 (RPE-ACS from C. Erba). X-Ray analyses were made by step scanning procedure on a Philips powder diffractometer equipped with a pulse height analyzer. The CuK& radiation (A =1.54178 A ) was used. FT-IR spectra were collected on a Perkin-Elmer 1730 spectrometer using the KBr wafer tecnique. MAS-NMR analysis was performed on a Bruker CXP-300 spectrometer (7.0 magn. field). Samples were put in a Delrin-made sample-holder and rotated at 4 KHz. EPR measurements were carried out on a Varian E-109 spectrometer operating at the X band (9.5 GHz) with 100 KHz power modulation, + 2 + 423 15 using the strong pitch Varian (9=2.002, spin conc.=3xl0 spin/cm) as a reference. Catalytic activity test We evaluated both acid and oxidative (with H202) catalytic activities. The catalyst tested in these experiments had the following molar composition: Si02/Ti02= 43 for TS-1; Si02/A1203= 164 for the standard ZSM-5; Si02/Ti02= 45, Si02/A1203= 150 for A1-TS-1; Si02/Ti02= 43, Si02/Ga203= 294 for Ga-TS-1; SiO,/TiO,= 42, Si02/Fe203= 362 for Fe-TS-1. All samples were well crystallized. Epoxidation of 1-butene with H2g2 A solution was prepared by dissolving 8 g of 1-butene in 100 g of methanol. The latter was previously distilled and stored on 4A molecular sieves. In a typical run, 25.5 g of this solution stored at -20 OC was quickly transferred in a 150 ml glass reactor, weighed and then kept at 5f0.1 OC. When we reached 5 OC, we added benzene (0.177 g, as g.1.c. internal standard) and the required amount of 33% hydrogen peroxide. We removed an aliquot of solution and titrated it iodometrically to determine the hydrogen peroxide concentration. The reaction started when we added the catalyst (0.74 wt % ) to the stirred solution. Aliquots were removed at time intervals and analyzed by gas-cromatography and iodometric titration. G.1.c. analyses were performed on a Hewlett-Packard HP 5880 gascromatograph using a FID and a glass column (2.4mx4mm) containing Porapak PS as the stationary phase. Selectivities and yields are based on hydrogen peroxide. Hydroxilation of phenols A 250 ml flask containing 112.0 g of phenol, 20.8 g of acetone, 27.2 g of water and 5.6 g of catalyst was heated to reflux tempe- rature ( - 98'C ) under stirring. Then we added 16 g of H202 (60 % w/w) dropwise in 45 minutes. After 1 hour from the last addition all the hydrogen peroxide was converted. The analyses were performed by using a FID and a glass column containing SE-52. We weighed the tars after removing volatile materials on a BUCHI-GKR-50 evaporator. Oligomerization of 1-octene We placed 6 ml of 1-octene, freshly distilled and stored under nitrogen and 0.4 g of catalyst in a 35 ml glass pressure vessel. The slurry was heated at 165 OC under stirring for 2.5 hours. After cooling, the mixture was filtered and the solution was 424 analyzed by gas-cromatography on a glass column containing Porapak PS. Results and discussion In general alkali cations favor the insertion of aluminum in the zeolite framework. They are more suitable than TPA' ions because they are less inhibi- ted by steric hindrance. Un- fortunately, alkaline cations can interfere with the syn- thesis of Al( Ga, Fe )-TS-l since it has been reported that they prevent the frame- work incorporation of tita- nium in the TS-1 (ref. 6 ) . In our work we observed the same effect. By using the in- tensity of the TS-1 IR band at 970 cm-' as a probe test for the presence of framework 0 2 4 6 8 10 (a)ppm Na x Fig.1 Effect of the presence of Na on the Ti insertAon in the Si- licalite lattice. ( measured on the TPA-OH solution) titanium (ref. 21, we observed that adding increasing amount of NaOH to the reaction mixture, the 970 cm-' band intensity decreases (Figure 1) even if titanium is still present. The intensity of the 970 cm-l IR band is measured relative to the intensity of the band due to Si-0-Si symmetrical stretching at 800 cm-'. At higher Na content more titanium is present in the solid. At the same time XRD shows the presence of anatase. The zeolites synthesized in the presence of sodium have less oxidation activity than TS-1 made without Na. Potassium has a similar effect. Sodium in the reaction mixture during the synthesis of TS-1 or other titanium containing silicalites (Al-TS-1, Ga-TS-1, Fe-TS-1) have a small influence on the amount of titanium in the final product. However, it reduces the intensity of the 970 cm-I IR band. Preliminary characterization results of extraframework titanium indicate that before calcination, extralattice Ti is in the form of amorphous titanium-silicate. After calcination at 5 5 0 ° C amorphous Ti is partially or totally converted to anatase. This suggests that the presence of sodium in the reaction mixture at the synthesis conditions described above promotes the + 425 A 970â SiO /Ti02 â800 L I t 1 0.8- 40 0.4L I L A O 0 100 200 300 400 500 SiO, /A1 *O 140- 100- 60. 500 SiO f M 2 0 iooo 0 Fig. 2 A1 content vs. frame- work Ti (represented by the reiftive intensity of the 970 Fig. 3 Ti content vs. trivalent cm IR band) in A1-TS-1. element in Al(Ga, Fe)-TS-1. formation of insoluble titanium-silicate species which reduce the amount of Ti available for the formation of the TS-1 crystals. The absence of sodium is a critical condition for the synthesis of TS-1 type materials. When we add A1(N03)3 to the reaction mixture we make silicalites which contain both Ti and Al. The maximum amount of each element depends on the concentration of the other. As the amount of A1 increases above a certain critical level the amount of Ti starts to decrease (Figure 2). Above a Si02/A1203 molar ratio of 150 the Si02/Ti02 becomes about 45. This is the lowest Si02/Ti02 ratio we ever found in TS-1 (ref.2). For Si02/A1203 molar ratios below 1 5 0 the Si02/Ti02 is higher than 45. The intensity of the 970 cm-l IR band is proportional to the titanium content. It is similar to that found in TS-1 for Si02/A1203 ratios above 150. Ga and Fe behave in a similar way except that the critical SiO /Ga203 and Si02/Fe203 ratios are higher (-300, see Figure 3). The only crystalline phase observed during these experiments was a silicalite type with orthorhombic symmetry. The 27A1-MAS-NMR of A1-TS-1 after calcination in air shows a peak at - 5 4 ppm. This suggests the presence of aluminum in tetrahedral coordination. In the case of Ga-TS-1, the pattern of 71Ga was very broad. We could not tell from this whether Ga is in tetrahedral or octahedral configuration. 2 426 The EPR spectrum of Fe-TS-1 after calcination shows a clear signal at g=4.3 indicating the presence of tetracoordinated iron atoms in the lattice (ref.7). The exchange capacity has been determined at room temperature. We treated the silicalite samples with 0.1 N CsCl aqueous solutions. The exchange capacity increased with increasing trivalent element concentration (Figure 4). The differences among the three curves observed mainly in the left side of the diagram are probably due to the presence of different amounts of extrafra- mework metal oxides. The exchange capacity and the presence of a TABLE 1: Oligomerization of 1-octene F 5 Catalyst TS-1 Ref. ZSM-5 A1-TS-1 Ga-TS-1 Fe -TS - 1 100.0 49.0 67.0 59.3 89.7 C16H3 2 ( % I - 47.0 31.0 38.5 10.0 10. 5 0 0 1c 500Si0 2 f M 2 0 3 â 10 Fig.4 Exchange capacity vs. trivalent element content in Al(Ga, Fe)-TS-1. strong C24H48 ( % I - 3.9 1.3 1.5 0.2 IR band at 970 cm-l, indicate that both Ti and the trivalent element are in the silicalite frame- work. The amount of trivalent element and titanium that can be inserted into the silicalite lat- tice is limited. The relative aboun- dance of the two a- tomic species in the silicalite is related to the concentration of the two elements in the liquid phase in which the crystals are dispersed. Catalytic activity experiments further confirm that both Ti and Al(Ga,Fe) are in the lattice. Unlike ZSM-5 which has only acid activity and TS-1 which has only oxidation activity, A1-TS-1, Ga-TS-1, Fe-TS-1 are all active in acid catalyzed reactions and in oxidations in the presence of H202. Table 1 shows 1-octene oligomerization results. TS-1 is not active because it has no strong Bronsted acid sites. The other samples are active. 427 Xun 3. 1 2 3 4 5 TABLE 2: Epoxidation of 1-butene with H,O, a M/Kg TS-1 0.098 Catalyst H202 Ref.ZSM-5 0.122 A1-TS-1 0.109 Ga-TS-1 0.099 Fe-TS-1 0.105 Run n. 1 2 3 4 5 - t [ min) - 30 145 145 145 30 - Catalyst TS-1 Ref.ZSM-5 A1-TS-1 Ga-TS-1 Fe-TS-1 H2O2 conv. % 80.8 < 1 81.0 77.5 < 10 86 1 74 7 3 81 1.20 - 1.18 1.38 - L L Epox . Sel.% b Glyc . Sel. % 7 - 61 7 4 15 initial concentration of hydrogen peroxide. 1,2-dihydroxy-butane and its monomethylethers-poly- ethers have not been calculated. The activity of A1-TS-1 and Ga-TS-1 is comparable with that of ZSM-5. The activity of Fe-TS-1 is much lower than that of A1 and Ga-TS-1. This difference may be due to the number and/or the TABLE 3: Hydroxylation of phenol a H2°2 Yield % O/P ratio I tars 0 0.80 - 0.91 1.02 3 .7 H 0 Yield = (moles of diphenols produced/ mgl& of H202 converted) x 100. a strength of the a- cid sites created by lattice iron (Ref.8). Table 2 shows the results obtained in the epoxidation of 1-butene with H202 at 5 OC. Only sam- ples containing Ti have significant o- xidation activity. Acidity affects oxidation activity and selectivity. Glycol selectivity increases with decreasing the H202 conversion rate (Figure 5). The reason of the H202 conversion decrease may be that the slow diffusion of glycols from the Al(Ga)-TS-l channels hinders the diffusion of H202 and the olefins. ZSM-5 has also very low phenol hydroxylation activity (Table 3 ) . A1-TS-1 and Ga-TS-1 acid sites do not affect catalytic activity. The behaviour of these solids is comparable with that of TS-1. 428 The Fe-TS-1 is an exception. It makes high molecular weight products instead of diphenols and has an high activity toward the H202 decomposition to H20 and 02. A small amount of non framework iron may be responsible for this. .04 0 0 20 40 60min. 0 40 80 120 160 .04 0 0 40 80 120 160min. 0 12.5 25 37.5 50 I f H,O, 0 Epoxide AGlycol Fig. 5 Catalytic activity of TS-1 and Al(Ga, Fe)-TS-1 in the 1-butene oxidation. Conclusions We have synthesized silicalites containing both titanium and a trivalent element in the framework. Acidic form of Al(Ga, Fe)-TS-1 may catalyze both acidic and oxidation reactions. The presence of two different sites may modify selectivities. For example, in olefin oxidation acidity increases glycol selectivity. Ion exchange capacities of A1-TS-1, Ga-TS-1 and Fe-TS-1 increase with increasing lattice trivalent element concentration. This provide the possibility to introduce a transition element into a non-framework position by ion exchange. The product will contain two (or in the case of Fe-TS-1, three) transition elements. The presence of different sites inside the zeolitic channels make this class of catalysts suitable to inve- stigate about the synergism among them. The direct synthesis of bifunctional zeolitic catalysts creates new opportunities for the application of shape selective mate- rials in heterogeneous catalysis. 429 Aknowledgements The authors are indebted to Dr. R. Millini, Dr. A. Gervasini, Dr. L. Montanari for providing X-Ray diffraction, EPR, MAS-NMR respectively and to EN1 Companies: Eniricerche S.p.A., Enichemsynthesis S.p.A. and Snamprogetti S.p.A. for the permission to publish these data. References M. Taramasso, G. Perego and B. Notari, US Pat.n.4410501 (1983). a) G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo andA.. Esposito, Stud. on Surf..Sci. and Catal.., 2 (1986) ~.129. - - b) G. Bellussi, G. Perego, A. Esposito, C. Corno and F. Buonomo, Proc. of Fifth Italian Nat. Cong. on Catalysis, Universita degli Sudi di Cagliari Ed., (1986) p. 423. U. Romano, A. Esposito, F. Maspero and C. Neri, Proc. of Int. Symp. on "New development in Selective Oxidations", G. Centi and F. Trifir6 Eds, Rimini 1989, Preprints B1. a) H. Baltes, H. Litterer, I.E. Leupold and F. Wunder, EP Appl.n.77522 (1982). b) B.M.T. Lok, B.K. Marcus and E.M. Flanigen, EP Appl.n.179876 and EP Appl.n.181884 (1985). a) G. Bellussi, A. Giusti, A. Esposito and F. Buonomo, EP Appl.n.226257 (1988). b) G. Bellussi, M.G. Clerici, A. Giusti and F. Buonomo, EP Appl.n.226258 (1988). c) G. Bellussi, M.G. Clerici, A. Carati and A. Esposito, EP Appl.n.266825 (1988). J.El Hage-qi Asswad, J.B. Nagy, Z. Gabelica and E.G. Derouane, 8 Int. Zeolite Conf., Recent Research Reports, J.C. Jansen, L. Moscou, M.F.M. Post Eds, Amsterdam 1989, p. 475. a) B.D. McNicol and G.T. Pott, J. Catal., 25 (1972) p.223. b) E.G. Derouane, M. Mestalagh and L.J. Vielvoye, J. Catal., - 33 (1974) p.169. C.T.W. Chu and C.D. Chang, J. Phys. Chem., 89 (1985) p.1569. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 431 S T U D Y ON TI TAN1 UM S I L I CALI TE SYNTHESI S M. Padovan(a) F. Genoni(a) G. L e o f a n t i ( a ) , G. P e t r i n i ( a ) , G. Trezza(b) : A. Zecchina(c) : (a)Montedipe S . r . l . P i e t r o 50 2 0 0 2 1 Bo l l a t e (Milano), I t a l y . Montedipe S. r. 1. Centro Ricerche d i Marghera, Via d e l l a C h i m i - ca 5, 30175 Por to Marghera (Venezia), I t a l y . Dipart imento d i Chimica Inorganica Chimica Fisica e d e i Mate- Unith d i Ricerca d i Bol la te , Via S. r i a l i , V i a P. Giur ia 7, 10125 Torino, I t a l y . SUMMARY The s y n t h e s i s of T i s i l i c a l i t e w a s s t u d i e d i n a s e a l e d s i l i c a tube v i a a d r y impregnat ion of a preformed microspheroidal porous s i l i c a w i t h a s o l u t i o n conta in ing TPAOH and a s o l u b l e a l k y l t i t a - nate . I n few hours t h e c r y s t a l l i z a t i o n proceeds towards t h e f o r - mation of t h e z e o l i t e having s u b s t a n t i a l l y a l l T i inc luded i n t h e framework. Extra s t r u c t u r a l T i appears on ly i n long t i m e synthe- s i z e d products . INTRODUCTION During t h e l a s t pe r iod cons iderable a t t e n t i o n has been given t o i n v e s t i g a t i o n s on T i s i l i c a l i t e (TS) , a z e o l i t e e x h i b i t i n g very va luab le c a t a l y t i c p rope r t i e s f o r a v a r i e t y of r e a c t i o n s of i n d u s t r i a l i n t e r e s t , i n p a r t i c u l a r f o r cyclohexanone ammoximation, phenol hydroxyla t ion and o lephins epoxida t ion (refs. 1, 2 ) . Despi te t h a t , t h e syn thes i s of TS, l i k e fundamental a spec t s of i t s c h a r a c t e r i s t i c s and i t s c a t a l y t i c behaviour, has not been s tud ied . I n t h e p re sen t work, t h e c r y s t a l l i z a t i o n of t h e z e o l i t e s t r u c t u r e and t h e i n s e r t i o n of t i t an ium i n t o t h e framework w e r e i n v e s t i gated. EXPERIMENTAL Sample p repa ra t ion TS samples a t var ious syn thes i s t i m e were obta ined according t o t h e method desc r ibed i n r e f . 3. Dried microspheroidal s i l i c a (Grace S G 360) w a s impregnated up t o i n c i p i e n t wetness w i t h an aqueous s o l u t i o n conta in ing both t h e t i t a n i u m source and te t ra - propilammonium hydroxide (TPAOH). The s o l u t i o n w a s p repared i n a s t i r r e d v e s s e l under i n e r t atmosphere by adding T i isopropoxide 432 t o i sopropyl a lcohol followed by TPAOH aqueous so lu t ion , and sub- sequent hea t ing a t 353K t o remove t h e alcohol . Af t e r impregnation t h e mixture was d iv ided i n t o e i g h t pa r t s : t h e f i r s t seven w e r e kept a t 448K i n s e a l e d g l a s s tubes r e spec t ive ly 1, 2, 5, 7, 10, 15 and 20 hours. The products w e r e t hen f i l t e r e d , washed with water and d r i e d f o r 1 6 hours a t 353K. A f r a c t i o n of each sample was then ca l c ined a t 823K f o r 10 hours. These samples w e r e l a - b e l l e d as TS followed by a number represent ing t h e syn thes i s t i m e (TS1, TS2, TS5, e t c . ) . The e i g t h p a r t of t h e s t a r t i n g mixture was submit ted t o t h e same t reatment but t h e syn thes i s temperature kept a t 353K only f o r 1 hour. The product was l a b e l l e d a s TSO. Chemical ana lys i s The T i conten t of var ious samples was determined by XRF. The A1 content of s t a r t i n g s i l i c a , as measured by AAS, r e s u l t e d less than 200 kg/g. XRD - The X-ray powder d i f f r a c t i o n p a t t e r n s w e r e recorded with a P h i l i p s d i f f rac tometer equipped with a propor t iona l counter, by us ing a N i - f i l t e r e d CuK rad ia t ion . The samples w e r e examined without any previous pretreatment . The c r y s t a l l i n i t y degree was determined by a procedure developed i n our l a b o r a t o r i e s ( r e f . 4 ) . The method was based on t h e comparison between t h e i n t e g r a t e d i n - t e n s i t i e s of two d i f f e r e n t s p e c t r a l ranges, s p e c i f i c a l l y a f f ec t ed by t h e c r y s t a l l i n e and by t h e amorphous f r a c t i o n s of t h e s o l i d r e spec t ive ly . I n t h i s manner t h e necess i ty of e x t e r n a l s tandards, having known c r y s t a l l i n i t y , can be avoided. The c r y s t a l l i t e s i z e was determined from t h e ha l f width of t h e peaks by using t h e She r re r equat ion a f t e r co r rec t ing f o r t he K a l , a 2 double t and t h e ins t rumenta l broadening ( r e f . 5 ) . a TGA ( thermoqravimetr ic ana lys i s 1 on 393K d r i e d samples 20 mg of each sample w e r e heated a t t h e rate of 4 K min-l i n a flow of 50 cm3 min-l from r . t . TGA was c a r r i e d out us ing a Mettler TG 3000 equipment. About H e t o 1073K. TGA on samples wi th preadsorbed m-xylene Before t h e m-xylene adsorp t ion each sample was i n s e r t e d i n t o a ho lder connected t o a vacuum l i n e (u l t ima te vacuum mbar) and heated a t 573K f o r 2 hours. The holder was then cooled t o r. t . , 433 i s o l a t e d from t h e vacuum system and f i n a l l y a s l i g h t excess of l i q u i d m-xylene w a s i n j e c t e d i n t o it. Af te r 10 min. the w e t sample was removed, pu t i n t o t h e thermobalance and kept a t r . t . i n H e flow (50 cm3 min-l) for a t i m e long enough t o remove t h e most l i q u i d excess. i n a H e f low up t o 673K, recording t h e weight loss ( r e f . 6 ) . F i n a l l y t h e sample was heated a t 4 K min-' r a t e - N2 adsorp t ion Erba SORPTOMATIC 1900 on prev ios ly out gassed samples (573K, 10 hours, f i n a l vacuum mbar). The micropore volume was determined by as method ( r e f . 6 ) by using a s re ference t h e d a t a of nonporous hydroxylated s i l i c a of r e f . 6 and ex t r apo la t ing t h e l i n e a r mult i - l a y e r reg ion of t h e p l o t t o a s = O . The s lope of t h e obtained s t r a i g h t l i n e was then used t o c a l c u l a t e t h e e x t e r n a l su r f ace a rea of t h e c r y s t a l s or,more exac ly ,of t h e primary p a r t i c l e s i z e . The above methods have been repor ted wi th more d e t a i l s i n r e f . 6. N2 adso rp t ion isotherms a t 77K were measured us ing a C. UV-Vis DRS ( d i f f u s e r e f l e c t a n c e spectrometry) -1 Dif fuse r e f l e c t a n c e s p e c t r a over t h e range 12500-50000 cm w e r e obtained with a Perkin-Elmer LAMBDA 15 spectrophotometer equipped with a d i f f u s e r e f l e c t a n c e attachment us ing MgO as a re ference . A s p e c i a l l y designed quar tz c e l l ( o p t i c a l pa th 1 cm) allowed t h e connect ion t o a vacuum system (u l t ima te vacuum mbar) and i t s i n s e r t i o n i n an e l e c t r i c furnace. I n t h i s manner each sample was outgassed a t 393K before t h e measure i n o rde r t o e l imina te t h e adsorbed water. The Kubelka-Munk func t ion was used t o express t h e experimental da t a ( r e f . 8 ) . I R - DRS ( d i f f u s e r e f l e c t a n c e spectrometry) Di f fuse r e f l e c t a n c e s p e c t r a w e r e measured between 400 and 4000 cm us ing a Perkin-Elmer F T I R 1640 spectrophotometer equipped wi th a d i f f u s e r e f l e c t a n c e attachement. Before each measure t h e samples w e r e i n t i m a t e l y mixed with K B r and f i n e l y ground. L i k e i n t h e case of UV-Vis s p e c t r a t h e r e s u l t s w e r e expressed i n terms of Kubelka-Munk func t ion . - 1 DISCUSSION The whole s e t of r e s u l t s allowed t o s e p a r a t e t h e examination of t h e syn thes i s process i i , f ou r pa r t s : i ) t h e i n i t i a l r e a c t i o n of TPAOH with S i and/or T i compounds(TS0 sample), ii) t h e d i s so - 434 1 Wt. loss 461 K -- ..)..â .- ....--. -........ *â *,..âC ..* 638 K 7 - .P I I I L Oh l u t i o n of S i and T i compounds and t h e s t a r t i n g z e o l i t e formation (TS1 sample) , iii) t h e c r y s t a l l i z a t i o n of t h e major f r a c t i o n of t h e z e o l i t e s t r u c t u r e (TS2-TS10 samples) and t h e completion of t h e c r y s t a l l i z a t i o n and i v ) t h e f i n a l p r e c i p i t a t i o n of T i from t h e s o l u t i o n (TS15-TS20 samples) . TSO sample The s t a r t i n g ma te r i a l looked l i k e a d ry powder wi th t h e whole l i q u i d entrapped i n i t s pores, so t h a t p r a c t i c a l l y a l l t h e s o l i d could be recovered from t h e tube (see Fig. l a ) . I n t h i s i n i t i a l s t e p of t h e syn thes i s t h e r e a c t i o n of TPAOH wi th s i l i c a and o r t i t a n i a had a l ready taken p l ace so only a f r a c t i o n of organic base could be removed by washing. According t o t h a t , w e l l ev ident bands i n t h e 2800-3100 and 1300-1500 ranges ( r e f . 9), belonging t o templa t ing agent , w e r e de t ec t ed i n t h e I R spec t r a , as w e l l as a s t r o n g DTG peak a t 461K corresponding t o a 1 4 . 0 % weight loss (see Fig. 2). A t t h i s s t a g e t h e s o l i d was completely amorphous a s dem- o n s t r a t e d by: i ) t h e absence of any de tec t ab le peak i n XRD pa t - 60 Solid yield 400 t nm f i r i s t a l l i t e size 0 I - \ m2g1 1 \âExternal - surface area 0 5 10 15 20 0 Synthesis t ime ( h ) P ? 4 6 1 ~ 6 3 8 ~ DTG . : : : i . . : . : 0 . -TS1 400 . . * . .*-. TS 2C ... .._,.... ..*â Fig. 1. Yield and c h a r a c t e r i s t i c Fig. 2. Peaks of TPA decomposi- t r ends of syn thes i s mater ia l s . t i o n (DTG curves) and TG t r ends versus syn thes i s t i m e . 435 t e r n (see Fig. l c ) , ii) t h e absence of t h e 550 cm-' band i n I R s p e c t r a ( r e l a t e d t o f i v e membered r i n g s system c h a r a c t e r i s t i c of z e o l i t e framework r e f . 9, Fig. 3 ) and iii) t h e absence of t h e DTG peak a t 670K (see Fig. 2 ) . A s f a r as t h e s t a t e of T i was concerned, in format ion could be gained from t h e UV-Vis r e f l e c t a n c e spec t r a , where a broad band was observed a t about 43500 cm-' (see Fig. 4). A comparison wi th t h e known s p e c t r a ( r e f s . 10, 11) of TS, c o p r e c i p i t a t e d Ti02/Si02, and T i O Z allowed a few consid- e r a t i o n s t o be made. The framework t e t r a h e d r a l T i i n TS i s asso- c i a t e d wi th a s t r o n g band a t 48000 cm-' having a d i s t i n c t l i gand t o metal charge t r a n s f e r (c. t. ) cha rac t e r . The amorphous Ti02/Si02 c o p r e c i p i t a t e s a r e cha rac t e r i zed by a peak i n t h e U V - V i s , whose frequency v a r i e s wi th t h e Ti /S i r a t i o . A t very l o w Ti /S i r a t i o s t h e peak approaches t h e frequency found i n TS because t h e T i atoms become more and more i s o l a t e d and t e t r a h e d r a l and by i n - c r eas ing t h e Ti /S i r a t i o t h e T i atoms agglomerate ( v i a oxygen- b r idges ) g iv ing T i 0 2 c l u s t e r s of i n c r e a s i n g s i z e : t h e c. t. band undergoes a gradual r ed s h i f t towards t h e va lues t y p i c a l of a U 3 - 600 cm Re la t i ve intensity . 5 n- 0 I 970/550 I 5 10 15 20 Synthesis time(h) Fig. 3. I R peaks and r e l a t i v e i n t e n s i t y versus syn thes i s time. 1 I 1 DO0 30000 45000 c m-1 Fig. 4. D R S U V - V i s cha rac t e r i za - t i o n a t i n c r e a s i n g s y n t h e s i s t i m e . 436 ana ta se c r y s t a l s cha rac t e r i zed by an absorp t ion band a t 21500 cm (where T i i s i n oc tahedra l p o s i t i o n ) . On t h i s bas i s w e could conclude t h a t a t t h i s s t a g e of t h e p repa ra t ion very small c l u s t e r s of oc tahedra l T i O Z g ra f t ed on s i l i c a w e r e p resent . -1 TS1 sample During t h e f i r s t hour under hydrothermal condi t ions a p a r t i a l d i s s o l u t i o n of S i and T i compounds took place: ca l c ined TS1 sam- p l e represented 90% of t h e t o t a l ob ta inable s o l i d (S i02+Ti02) . This sample contained only 1% T i (see Fig. l b ) so meaning t h a t a T i r i c h phase had t o be p re sen t i n so lu t ion . A t t h i s s t a g e a par - t i a l c r y s t a l l i z a t i o n had a l ready taken p1ace: the z e o l i t e conten ts of TS1 sample w a s about 10%. The above conclusion was obtained from XRD pa t t e rn , I R band a t 550 cm-' on 823K ca lc ined sample and from TGA on 353K d r i e d sample.In p a r t i c u l a r t h e in spec t ion of DTG curve poin ted ou t t h e p a r t i a l s u b s t i t u t i o n of t h e 461K peak wi th a peak having a maximum a t 638K c h a r a c t e r i s t i c of both s i l i c a l i t e and TS (see Fig. 2 r e f . 2 ) . The agreement among t h e d a t a from t h e var ious techniques i n d i c a t e d t h a t a l s o a t low c r y s t a l l i n i t y de- gree w e l l formed c r y s t a l l i t e s predominated on small nuc le i not w e l l d e t e c t a b l e by XRD. The r e l a t i v e l y l o w c r y s t a l l i z a t i o n degree and consequent ly t h e l a r g e amount of t h e amorphous f r a c t i o n made d i f f i c u l t t o i n v e s t i g a t e t h e i n s e r t i o n of T i i n t h e growing frame work. A s a mat te r of f a c t t h e U V - V i s spectrum was dominated by a l a r g e band a t 38000 cm-' and t h e 48000 cm-' band of framework T i could not be de tec ted . A s i m i l a r conclusion could be der ived a l s o from t h e I R spec t r a . I n f a c t band (ass igned t o a s t r e t c h i n g mode of a [S i04 ] s t r u c t u r e bonded t o a framework T i , r e f . 10) does not s u b s t a n t i a l l y d i f f e r from t h a t 02 served on TSO sample. F i n a l l y T G curves of TS1 and TSO samples conta in ing preads orbed m-xyl ene w e r e a l s o subs t a n t i a l l y i ndi s ti n- guishable: i n p a r t i c u l a r t h e weight l o s s i n t h e temperature range t y p i c a l of TS ( r e f . 6 ) was not de t ec t ed on TSO and TS1. The whole observa t ions prev ious ly made suggest , a l though t h e s t a t e of T i i s unce r t a in , t h a t dur ing t h e f i r s t hours of hydrothermal condi t ions a formation of s i l i c a l i t e nuc le i has a l ready taken p lace . I t i s most no t i ceab le t h a t a s i m i l a r behaviour was found f o r t h e ZSM-5 syn thes i s by i n c i p i e n t wetness impregnation of s i l i c a ( r e f . 1 2 ) . t h e i n t e n s i t y of t h e 970 cm-I TS2-TS 10 samples On i n c r e s i n g t h e syn thes i s t i m e t h e y i e l d of s o l i d recovered 437 s t a n t . The above r e s u l t s l e t us t o conclude t h a t dur ing t h e main 0 - a t t h e end of each experiment increased. A t t h e same t i m e c r y s t a l l i z a t i o n proceeded, reaching about 60% a t 5 hours and about 95% a f t e r 10 hours. The measure of t h e z e o l i t e conten t by X R D , I R (550 cm band), TGA (638K DTG peak) and N2 adso rp t ion (see Fig. l c ) gave s imilar r e s u l t s po in t ing out t h e r e g u l a r development of t h e framework. The observed inc rease of c r y s t a l l i t e s i z e (see Fig. Id ) sugges ted a process of c r y s t a l growth r a t h e r t han a formation of new nucle i . Af t e r a minimum a t about 3 hours hydrothermal t r e a t i n g t h e T i conten t showed a t r end towards gradual i nc rease . I n a par- a l l e l way t h e band a t 48000 cm-' c h a r a c t e r i s t i c of framework T i , p rog res s ive ly s u b s t i t u t e d t h e about 40000 cm-I one. The bu i ld ing up of TS w a s a l s o supported by t h e p a r a l l e l i n t e n s i t y i n c r e a s e of t h e 970 cm-l band i n t h e I R spectrum and by t h e increment of m-xylene adsorp t ion (see Fig. 5 ) . These observa t ions allowed us t o suppose a mechanism i n which, a f t e r an i n i t i a l r a p i d d i s s o l u t i o n of t h e s u r f a c e T i compounds and a formation of s i l i c a l i t e nuc le i , T i w a s p rog res s ive ly recovered from t h e s o l u t i o n and incorpora ted i n t h e growing z e o l i t e c rys t a l s .The growth of t h e c r y s t a l s i n t h e 5-10 hours of hydrothermal t r e a t i n g i n t e r v a l occurred a t approxi- mately cons tan t T i concentrat ion. I n f a c t , a t l e a s t w i t h i n t h e experimental errors of t h e method, t h e r a t i o between t h e i n t e n - -1 s i t i e s of 970 cm band ( r e l a - t e d t o s t r u c t u r a l T i con ten t ) 'Y and 550 cm band ( r e l a t e d t o z e o l i t e con ten t ) w a s cons t an t . I t a i s most no t i ceab le t h a t . dur ing t h e s a m e i n t e r v a l a l s o t h e adsor 200- p t i o n capac i ty of m-xylene pe r -1 r r. -1 DTG .....TSO .-...., \ ** ............... ... - I 1 1 I u n i t channel volume remained COG I - TS20 t o 10 hours ) t h e changes i n Am mother s o l u t i o n due t o t h e zeolL E l 7-1 t e formation w e r e s m a l l enough .04 t o not induce composition changes i n t h e growing c r y s t a l s . .03 5 10 ' 5 - 20 Synthesis t ime hh) TS15 TS20 samples As r epor t ed above t h e sample Fig. 5. DTG curves and m-xylene adsorp t ion t rend . TSlO had 96% c r y s t a l l i n i t y . 438 Fur ther i n c r e a s e i n syn thes i s t i m e d i d not change t h e z e o l i t e con t e n t of t h e s o l i d while i t s y i e l d increased approaching 100%. Dug i n g t h i s s t a g e t h e amorphous s i l i c a has a l ready been completely consumed and t h e c r y s t a l l i z a t i o n could occur only a t expense of t h e so lu t ion . The T i conten t of t h e s o l i d f u r t h e r increased from TSlO t o TS20. Nevertheless t h e normalized i n t e n s i t i e s of t h e 970 cm band and t h e m-xylene adsorp t ion capac i ty of t h e s o l i d d i d not change sugges t ing t h e framework t i t an ium was about constant . The seeming con t r ad ic t ion between t h e above r e s u l t s could be expla ined on t h e bas i s of W-Vis da ta . I n f a c t an absorp t ion i n t h e 35000-37000 c m - l range developed dur ing t h e syn thes i s per iod under d iscuss ion . A s repor ted i n Ref. 6 t h i s absorp t ion can be r e l a t e d t o t h e formation of t i t a n i a or T i r i c h s i l i c a - t i t a n i a sol- i d s o l u t i o n , meaning t h a t i n t h e l a s t mother s o l u t i o n t h e remain- i n g T i was incorpora ted i n extra-framework oc tahedra l pos i t i ons . -1 CONCLUSIONS I n conclusion our method allowed t o e a s i l y o b t a i n pure and well c r y s t a l l i z e d T i s i l i c a l i t e samples by us ing t h e appropr i a t e syn thes i s t i m e i n o rde r t o avoid a r e s idua l amorphous conten t o r extra-framework T i . Fur ther work i s s t i l l i n progress i n order t o i n v e s t i g a t e t h e e f f e c t of T i concentrat ion, syn thes i s tempe- r a t u r e , TPAOH concent ra t ion and s i l i c a impur i t ies . REFERENCES P. Roff ia , G. Leofant i , A. Cesana, M. Mantegazza, M. Padovan, G. P e t r i n i , S. Tonti . P. Gervasut t i . N e w Developments i n S e l e c t i v e Oxidation, Elsevier , Amsterdam, 1990, pp 43-52. U. Romano, M. C l e r i c i , A. Esposito, F. Maspero, C. N e r i , i b i d . , pp 33-42. I t . Pat. 21511A/85. G. Carazzolo, F. Ga t t i , A. Ponzoni, M. So la r i , Montedipe I n t e r n a l Rept., 1982 H. P. Klug, L. E. Alexander, X Ray D i f f r a c t i o n Procedure f o r p o l y c r i s t a l l i n e and amorphous Mater ia ls , 2nd edn. , John Wiley & Sons Ltd. , Chichester , 1974. G. Leofant i , F. Genoni, M. Padovan, G. P e t r i n i , G. Trezza, A. Zecchina, Proc 2nd. IUPAC-Symposium on Charac te r i za t ion of Porous Sol ids , Alicante , Spain, May 6-9, 1990, i n press . S. J. Gregg, K. S. W. Sing, Adsorption, Surface Area and Poro- s i t y . , 2nd. edn., Academic Press , London, 1982. R. A. Schoonheydt i n F. Dalannay (Ed. ), Charac te r i za t ion of heterogeneous Cat., M. Dekker Inc . , New York, 1984, pp. 125-160. K. F. M. G. J. Scholle , W. S. Veeman. P. Frenken, G. P. M. van de r Velden., Appl. Cata l . , l â l , 1985, pp. 233-259. 10) M.R. Boccutx, K.M. Rao, A. Zecchina, G. Leofant i . , G.Pe t r in i . S t r u c t u r e and Reac t iv i ty of Surfaces , E l sev ie r , Amsterdam, 1989, pp 133-144. 11) G. P e t r i n i e t a l . , i n prepara t ion . 12) M. Padovan, G. Leofant i , M. So la r i , E. More t t i . , Zeol i tes , 4 , 1984, pp 295-289. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 439 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands A C T I V A T E D CARBON FROM B I T U M I N O U S COAL J..4. F a j a r e s , J . J . P i s , -4.B. F u e r t e s , J . B . P a r r a , M . Mahamud a n d A . J . Pe'rez I n s t i t u t o N a c i o n a l d e l Carbo'n, C . S . I . C . , A p t d o 7 3 , 33080 Oviedo, SPA IN ABSTRACT The effect, on the preparation of activated carbons, of coal preosidation, particle size and activating flow rate was studied. .4s starting material A high bituminous coal was used. Data on the progressive evolution of texture in oxidized coal samples, and in subsequent chars and activated material are presented. T h e flow rate of the activating agent ( C O , ) has a big influence on the development of the porous network o f the activated carbons. I N T R O D U C T I O N Activated carbon has traditionally been used as an adsorbent and catalyst (ref. 1 , 2 ) , especially in some gas-phase osidation reactions at low temperature. Its use in simultaneous SO, and NO, removal, H,S elimination from industrial gaseous effluents, and also it,s role as activator agent of chlorination and dehydrochlorination is wellknown (ref. 1 - 4 ) . Activated carbon, as a catalyst support is in increasing demand nowadays (ref 2 - 5 1 . Especially notable is its ability to control factors like shape, particle size (powdered and granular) and mechani-cal properties (soft, hard carbon). It also presents a good potential on pore size distribution and subsequent specific surface area (30 - 3000 m'g-'), together with a hydro(phi1)phobicity character and/or surface concentration of acid/basic centers. A l l these aspects make activated carbon a versatile material, with a range of properties adequate f o r supporting all types and amounts ofmetallicand osidic active elements. Examples of this is its use as support of zinc acetate in the industrial preparation of vynil-acetate polymer ( ref. 3 , 6 ) , its strong performance as support of noble metals, mainly for hydrogenation in fine chemical preparation processes (ref. 5 , 7 , 8 ) . A l s o notable is its large surface area, which can give a high metal dispersion with relatively weak metal/support interaction (ref. 3 ) , and the possibility of an 440 easy r e c o v e r y o f t h e a c t i v e component j u s t by b u r n i n g . A l l t h e s e f e a t u r e s d e t e r m i n e i t s i n t r o d u c t . i o n as a c a t a l y t s s u p p o r t e v e n i n s u c h t r a d i t i o n a l f i e l d s as SO, o x i d a t i o n ( r e f . 8 ) , h y d r o c r a c k i n g ( r e f . 9 ) a n d h y d r o d e s u l p h u r i s a t i o n ( r e f . 1 0 ) . G r a n u l a r a c t i v a t e d c a r b o n s ( G A C ) are made m a i n l y f r o m c o c o n u t s h e l l , pea t , a n d b i t u m i n o u s c o a l s . The u s e o f a n a b u n d a n t , c h e a p s o u r c e , i m p l i e s t , h a t c a r b o n e x - m i n e r a l c o a l s r e p r e s e n t more t h a n 60% o f GAC p r o d u c t . i o n ( r e f . 1 1 ) . D e t a i l s o f t h e m a n u f a c t u r i n g p r o c e s s haire u s u a l l y b e e n kept. a s a n i n d u s t r i a l s e c r e t . However , v a r i a b l e s l i k e c o a l r a n k , p a r t i c l e s i z e , o x i d a t i o n , f l o w r a t e o f r e a c t a n t s and y i e l d i n t h e a c t i v a t i o n s t e p , m u s t b e t a k e n i n t o a c c o u n t i n t a i l o r i n g a n a r t i v a t e d c a r b o n f o r a s p e c i f i c a p p l i c a t i o n . I n t h i s p a p e r , some o f t h e s e v a r i a b l e s are s t u d i e d f o r t h e p r e p a r a t i o n o f a c t i v a t e d c a r b o n s by a c t i v a t i o n o f a b i t u m i n o u s c o a l w i t h CO,. S p e c i a l e m p h a s i s h a s b e e n p u t m a i n l y o n t h e e f f e c t of coal o x i d a t . i o n and t h e f l . o w r a t e o f t h e a c t i v a t i n g a g e n t . From t h e r ' e s u l t s , t h a t were r e s t r i c t e d t o materials a c t i v a t e d t o a p p r o s i m a t e l y 52% b i i r n - o f f , g e n e r a l l i n e s may b e t r a c e d for d e s i g n o f a c t i v a t e d c a r b o n s wit.h a p a r t i c u l a r p o r e s i z e d i s t r i b u t i o n . D a t a o n t h e p r o g r e s s i v e e v o l u t , i o n o f t e x t u r e o f o x i d i z e d c o a l s , and s u b s e q u e n t c h a r s a n d a c t i v a t , e d p r o d u c t s are also p r e s e n t e d . EXPER I MENTAL A High B i t u m i n o u s C o a l f rom t h e Ma L u i s a m i n e , f r o m t h e C e n t r a l A s t u r i a n B a s i n h a s b e e n u s e d . The most i m p o r t a n t c h a r a c t e r i s t i c s o f t.he s t a r t i n g mater ia l are g i v e n i n T a b l e 1. The c o a l was g r o u n d and two s i z e f r a c t i o n s w e r e u s e d : t O . 1 2 5 - 0 . 4 2 5 mm ( P s e r i e s ) and t 1 . 0 0 - 3 . 0 0 mrn ( G s e r i e s ) . T.4RI.E 1 C h x r a c t r r i s t i c s o f t h e c o a l u s e d P r o s i m a t e a n a l y s i s ( % w t ) U l t i m a t e A n a l y s i s ( % w t , d a f ) Y o i s t u r e Ash V . M . C H N S 0 ( d i f f . ) 1 . 3 5 3 . 8 0 3 7 . 1 6 8 6 . 7 0 5 . 0 4 1 . 2 7 0 . 5 2 6 . 4 7 ( d r y ) ( d a f ) A r n u t e s t Maceral C o m p o s i t i o n F . S . I . T r , K T s , K T c , K b , % V i t . E x i . S e m i f . F u s . 8 622 692 738 1 7 9 6 5 . 2 1 0 . 6 7 . 4 1 3 . 6 F . S . I . : F r e e S w e l l i n g I n d e x 441 The oxidation of coal was carried out in an oven with forced circulation . For series P oxidation was carried out at 4 7 3 K for different periods of time between 0 and 24 hours. For series G runs were carried out at 5 1 3 K and oxidation times were between 21 and 72 hours. The pyrolysis of fresh coal and oxidized coal samples was performed under nitrogen at 1 1 2 3 Ii with a heating rate of about 60 K min-' and 5 min of soaking time. The activation was carried out with CO, in a vertical quartz reactor (I.D. 20 m m ) , at 1 1 2 3 K and two different CO, flow rates: 7 and 500 mL min". Gasification was carried out under isobaric conditions, at 1 0 2 . 7 kPa ( 7 7 0 mm H g ) untilapproximately 52% burn-off. Textural properties were obtained from measurement o f true (helium) and apparent (mercury) densities, total open pore v o l u m e s and pore volume distributions. For determination of the helium densities, a Micromeritics Autopycnometer 1 3 2 0 uas used. Apparent densities were determined in a Carlo Erba Macropore Unit 1 2 0 . The pore volume distributions were evaluated with a mercury porosimeter, Carlo Erba 2 0 0 0 . Specific surface areas were determined by physical adsorption in a Oninisorb 3 6 0 and a Sorptomatic Carlo Erba 1 9 0 0 . N , at 77 K and CO, at 2 7 3 K were used. We assumed a cross-section for a 0 . 1 8 7 nm2 for a molecule of CO,. 411 textural properties are expressed on a dry ash free basis (daf). molecule of N~ of 0.162 nm2 and of RESULTS AND DISCUSSION Oxidation Preoxidation of bituminous coal is a crucial step in the preparation of activated carbons. Oxidation produces a decrease in the caking properties, or even its total destruction (ref. 1 2 ) . I n fact, an important transformation in the chemical composition and in the porous structure of the coals (ref. 1 3 ) was produced. Some of the more important characteristics of oxidized coals are given in Table 2. The caking properties of samples decrease as a result of air oxidation, so a drastic reduction in free swelling index ( F S I ) is observed from 8 in fresh coal to 1 and 0 in oxidized samples. Likewise, an important decrease in carbon content, and a parallel increase in volatile matter and oxygen content,, mairil>- in this last element, is observed. Micropore surface areas f o r the fresh and preoxidized coals are 442 p r e s e n t e d i n T a b l e 2 . P r e o s i d a t i o n had l i t t l e e f f e c t on t h e CO, s u r f a c e areas o f coa l s a m p l e s . The e n h a n c e m e n t o f s u r f a c e area d u e t o p r e o x i d a t i o n is o f t h e same o r d e r as t h o s e o b t a i n e d by o t h e r a u t h o r s ( r e f . 1 4 ) when u s i n g s i m i l a r c o n d i t i o n s o f o x i d a t i o n . TABLE 2 A n a l y s e s o f t h e o x i d i z e d coa l s . C o a l O x i d a t i o n FSI V . M . C H N S O S D R s a m p l e c o n d i t i o n s ( % I ( % ) ( % ) ( % ) ( % ) ( % ) co, PO f r e s h 8 3 7 . 2 8 6 . 7 5 . 0 1 . 3 0 . 5 6 . 5 1 4 6 P6 4 7 3 K - 6 h 1 3 2 . 6 8 1 . 7 4 . 1 1 . 7 0 . 4 1 1 . 9 1 5 9 P18 4 7 3 K - 18 h 0 3 3 . 1 7 8 . 9 3 . 6 1 . 7 0 . 4 1 5 . 2 1 8 0 G24 5 4 3 K - 2 4 h 1 3 7 . 9 7 6 . 3 3 . 1 1 . 7 0 . 4 1 8 . 2 1 6 8 G4 8 5 4 3 K - 4 8 h 0 4 0 . 8 7 0 . 2 2 . 0 1 . 9 0 . 4 2 5 . 4 217 G72 5 4 3 K - 72 h 0 4 2 . 3 6 9 . 6 1 . 3 2 . 1 0 . 4 2 6 . 6 2 1 1 A l l r e s u l t s are e x p r e s s e d on a d . a . f . b a s i s . Pyrolysis C h a r s o b t a i n e d by p y r o l y s i s f r o m o x i d i z e d c o a l show a n i m p o r t a n t e n h a n c e m e n t i n s u r f a c e area as c a n b e s e e n i n F i g u r e 1. The d r a s t i c r e d u c t i o n i n p l a s t i c p r o p e r t i e s o f b i t u m i n o u s c o a l w h i c h o c c u r s as a r e s u l t o f o s i d a t i v e t r e a t m e n t , seems to b e t h e p r i n c i p a l c a u s e o f t h i s i n c r e a s e . In f a c t , as a c o n s e q u e n c e o f t h e p r e v i o u s d e s t r u c t i o n o f c a k i n g p r o p e r t i e s , a more o p e n p o r o u s s t r u c t u r e w a s p r o d u c e d d u r i n g t h e p y r o l y s i s s t e p ( r e f . 1 5 ) . C o a l o x i d a t i o n a f f e c t s t h e t e x t u r a l p r o p e r t i e s o f t h e s u b s e q u e n t c h a r s as shown i n T a b l e 3 . The i n c r e a s e i n CO, s u r f a c e area is b i g g e r t h a n t h a t o b t a i n e d by o t h e r s a u t h o r s ( r e f . 1 6 ) . TABLE 3 T e s t u r a l p r o p e r t i e s of t h e c h a r s o b t a i n e d R a w c o a l S area P o r o s i t y P o r e volume ( cm3g-') r a d i u s ( n m l > 5 0 3 . 7 / 5 0 c 3 . 7 s a m p l e ( 3 g - l ) ( % ) Total F . 0 1 9 6 1 6 . 5 111 4 3 1 5 3 P . 6 6 9 2 2 8 . 5 215 3 7 1 5 9 P . 1 8 6 1 6 2 9 . 6 229 34 5 6 1 G.24 426 2 6 . 7 2 0 1 3 8 0 62 G . 4 8 5 1 8 3 2 . 1 252 3 3 1 66 G.72 514 3 3 . 5 2 6 9 3 1 1 6 8 I n b o t h se r ies , powdered a n d g r a n u l a r mater ia l s , p r e o x i d a t i o n o f coal d e t e r m i n e s a bi.g i n c r e a s e i n t h e CO, s u r f a c e area of t h e c h a r s 443 o b t a i n e d , t h i s i n c r e a s e b e i n g more n o t a b l e i n t h e case o f powdered ma te r i a l , as c a n b e s e e n i n T a b l e 3 . V a l u e s o f N, s u r f a c e area ( r e f . 17) are v e r y much l o w e r t h a n t h o s e d e t e r m i n e d f r o m CO, a d s o r p t i o n , c o n f i r m i n g t h e r e l a t i v e i m p o r t a n c e of t h e m i c r o p o r e n e t w o r k i n t h e e v o l u t i o n o f t h e ma te r i a l . _____ A c t i v a t i o n D u r i n g t h e c h a r f o r m a t i o n p r o c e s s , i n t h e p y r o l y s i s s t e p , a p r i m a r y p o r e s t r u c t u r e is d e v e l o p e d . La te r , t h i s s t r u c t u r e l e a d s t o t h e d e v e l o p m e n t o f p o r o s j - t y d u r i n g g a s i f i c a t i o n i n t h e a c t i v a t i o n s t e p . T h i s i n c r e a s e i n p o r o s i t y a n d i n i t i a l p o r e s t r u c t u r e are s t r o n g l y i n f l u e n c e d by p r e v i o u s t r e a t m e n t o f t h e c a k i n g c o a l s , e . g . a i r o x i d a t i o n . F i g u r e 1 shows t h e e v o l u t i o n o f t h e CO, s u r f a c e area o f c h a r s a n d a c t i v a t e d c a r b o n s o b t a i n e d f r o m c o a l s a m p l e s o x i d i z e d t o d i f f e r e n t d e g r e e s . A l s o t h e e v o l u t i o n o f t h e s u r f a c e area o f o x i d i z e d c o a l s w i t h t h e t i m e o f o x i d a t i o n are p r e s e n t e d . A s c a n b e s e e n , c o a l o x i d a t i o n p r o d u c e s a b i g i n c r e a s e i n t h e s u r f a c e area o f c h a r s o b t a i n e d by p y r o l y s i s a n d i n t h a t o f t h e a c t i v a t e d c a r b o n s o b t a i n e d f r o m c h a r g a s i f i c a t i o n . D u r i n g g a s i f i c a t i o n a p r o g r e s s i v e e n l a r g e m e n t o f t h e p o r e s , p r e v i o u s l y formed i n t h e p y r o l y s i s s t e p , i s p r o d u c e d . The e v o l u t i o n o f p o r o s i t y i n c h a r s h a s a b i g e f f e c t on t h e t e x t u r a l p r o p e r t i e s o f t h e a c t i v a t e d ma te r i a l s . I n f a c t , as c a n be s e e n i n F i g u r e 1 , t h e g r e a t e r t h e s u r f a c e area o f t h e c h a r s , t h e g r e a t e r t h e s u r f a c e area o f t h e a c t i v a t e d c a r b o n s . In t h e p r e p a r a t i o n o f a c t i v a t e d c a r b o n s o f g r e a t i m p o r t a n c e i s t h e c o n t r o l o f o p e r a t i o n a l p a r a m e t e r s f o r t a i l o r i n g t h e i r t e x t u r e f o r s p e c i f i c a p p l i c a t i o n s . Flow r a t e o f o x i d i z i n g g a s a n d p a r t i c l e s i z e , t o g e t h e r w i t h c o a l p r e o x i d a t i o n , a r e two o f t h e p a r a m e t e r s t h a t c a n b e u s e d i n t h i s way. I n t h e c a r b o n - CO, r e a c t , i o n , t h e i n h i b i t o r y e f f e c t o f t h e CO p r o d u c e d c a n g i v e r i s e t o n o n - u n i f o r m g a s i f i c a t i o n , o f t h e p a r t i c l e . Rand a n d Marsh ( r e f . 1 8 ) s u g g e s t e d t h a t a n i n c r e a s e i n CO c o n c e n t r a - t i o n i n r e a c t a n t g a s r e s u l t s i n a n e n h a n c e m e n t o f t h e d e g r e e o f u n i f o r m i t y o f g a s i f i c a t i o n . I n o r d e r t o create d i f f e r e n t CO c o n c e n t r a t i o n s i n t h e v i c i n i t y o f t h e r e a c t i o n a r ea , a se r ies o f e x p e r i m e n t s w e r e p e r f o r m e d , i n w h i c h two d i f f e r e n t CO, f l o w ra tes , 7 a n d 500 m L min- l , w e r e u s e d . 444 0 V " 1 I 0 V 1.000 \ -⬠4 800 I I I I a a W 01 600 n a n s 0 n W W - K J 3 z LL w a a I I I I 1 I * I I A 0 COAL 0 0 0-O I I I 0 24 48 72 T I M E O F COAL OXIDATION, h I Figure 1. Variation of CO, surface area of coal, chars and activated carbons, with the time of coal oxidation. Coal particle size: tl.OO - 3.00 mm. Temperature of oxidation: 5 4 3 K . C O A L 500 mL C02/ rn in P R E O X I D A T I O N x x x X I +---I h $aiilRp >50 nm K l . x x x x x n 3 . 7 5 < R p < 5 C x x x x x m R p < 3.75 x x x x x _ _ _ _ 0 P O R E V O L U M E , mrn3/g Figure 2 . Pore volume distributions of activated carbons from oxidized coal samples. Figure 2 shows the evolution o f pore volume distributions, c:alculated from mercury porosimetry and helium densities, of chars obtained f rom oxidized coal and activated at the above mentioned CO, f l o w rat-es. In general, an important enhancement in pore volume can h? observed when coal. preoxidation is increased and when low f l o w 445 rates (high CO concentrations) are used. When low flow rates of CO, were used the increase of pore volume is especially noticeable for the pore volume contribution of pores with a radius smaller than 3.7 nm. These results are in agreement with our previous results (ref. 19) and with the studies of Rand and Marsh (ref. 18), who observed that a greater micropore volume is developed by gasification when a lower flow rate i s used, presumably because of the inhibiting effect of CO in the outermost section of the particle. As can be seen in Figure 2, low flow rates of C 0 2 give activated carbons with a bigger development of mesopores. This is of great importance in view of the eventual use of these materials (ref. 20). In fact, a well-balanced pore size distribution is essential in processes in which both surface area and the penetration o f reactive gases into the inner porosity of the particle are important. Textural parameters obtained in the analysis of adsorption isotherms of N, at 77 K and CO, at 273 K is shown in Table 4. \;ads is the volume adsorbed in the N, isotherm; S,,,, the BET especific surface area; W,, the micropore volume obtained from the CO, isotherm, and S,, the corresponding equivalent surface area of KO. TABLE 1 Textural properties of the active carbons obtained Active Vads S,,, W, SDR ( SB,T-SD, ) / % E T Carbon ( cm3g-') (m'g-' ) ( cm3g-') ( m 2 g - l ) % P . 0 S 0.167 302 0.114 300 + 1 P.OB 0.084 152 0.108 261 -8 7 P. 6.5 0.359 784 0.266 700 +I1 P.6B 0.175 454 0.218 574 -26 P.18S 0.446 979 0.321 815 t11 P.18B 0.244 597 0.239 630 - 6 G.24S 0.402 908 0.259 684 t25 G.24B 0 . 3 0 3 698 0.260 684 t 2 G.48S 0.610 1381 0.374 990 t28 G.48B 0.464 1116 0.365 964 + I 4 G.72S 0.555 1290 0.376 992 +23 G.72B 0.427 1048 0.398 1051 0 S : 7 ml CO, min-'; B: 500 ml CO, min-'. In general, adsorption capacit3 increased with coal oridatj.ori. I n fact, activated carboils prepared from the most oxidized coals and activated at lower flow rates, exhibit the highest adsorpt-ion capacity. Powdered materials, activated with high flow rates of CO,, give the largest differences between SB,,r and SDR. These b i q 446 differences suggest that gasifying gas gets into the small micropores of the particles in the activation steps. In powdered materials, when low flow rates of CO, are used, the values of S,,, are twice as big as those obtained at high flow rates. The difference between S,,, an S,,, that is practically zero in activated carbons obtained from fresh coal (P.OS), increases with raw coal oxidation. An enlargement in the diameter of micropores is observed when low flow rates of CO, are used. These results agree with these of Marsh (ref. 21) who maintains that the increase in (SBET-SDR) involves a growth in the diameter of micropores. The results obtained with granular materials (G series) are sigpificantly different. The volume of gas adsorbed in the isotherms of N, and CO, is higher than the corresponding volume of series P. Moreover in all the samples the difference ( S , , , - SDR) is positive, and approximately constant in the samples activated at low flow rates. In the samples G.24s, G . 4 8 ~ and G . 7 2 ~ this difference is indicative of condensation of N, in the micropores of activated carbons, and suggests the existence of supermicropores i n these samples. The volume of micropores, W,, is practically independent of the CO, activation flow rate. However, the S,,, values are always higher in the samples activated with a small flow rate of CO,, as can be seen in Table 4. These results suggest that, the higher the coal preosidat,ion and the lower the CO, activation flow rate, the bigger the development of porous texture. The right choice of coal oxidation and activating gas flow rate, can produce activated carbons with a suitable textural development. CONCLUSIONS The flow rate of the activating agent has a big influence on the development of the porous network of the activated carbons. High CO, flow rates give rise to materials with a wide pore size distribut,ion, while low CO, flow rates gives materials with a higher pore volume, a better developed microporosity and a corresponding higher surface area. Raw coal preoxidation always has a beneficial effect on the textural development of the activated carbons obtained, both in powdered and granular materials. Activated carbons with a suitable textural development can be obtained if the right choice of raw coal osidation and activating gas flow rate ratio is made. 447 ACKNOWLEDGEMENTS The a u t h o r s t h a n k F u n d a c i 6 n p a r a e l Foment0 d e l a I n v e s t i g a c i o n e n A s t u r i a s (FICYT) f o r f i n a n c i n g t h i s w o r k . A . J . P . a n d M . M . w i s h t o e x p r e s s t h e i r t h a n k s t o M . E . C . f o r F . P . I . g r a n t s . 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C a t a l y s i s , 8 0 ( 1 9 8 3 ) 3 8 6 . 11 R . E . K i r k a n d D . F . O t h m e r , E n c y c l o p e d i a o f C h e m i c a l ' T e c h n o l o g y , The I n t e r s c i e n c e E n c y c l o p e d i a , N e w Y o r k , 3rd. E d i t i o n , 1 9 8 1 , \ o l . 4 , p . 5 6 1 . 1 2 J . J . P i s , A . C a g i g a s , P . Sim6n a n d J . J . L o r e n z a n a , E f f e c t o f a e r i a l o x i d a t i o n o f c o k i n g c o a l s o n t h e t e c h n o l o g i c a l p r o p e r t i e s o f t h e r e s u l t i n g c o k e s . F u e l P r o c e s s i n g T e c h n o l . , 2 0 ( 1 9 8 8 ) 307-316. 1 3 M . M . L u d v i g , G . 1 , . Gard a n d P .H. E m m e t t , U s e o f c o t i t r ' o l l e d o x i d a t i o n t o i n c r e a s e t h e s u r f a c e area o f c o a l . A p p l i c a t i o n t o a b i t u m i n o u s a n d a s e m i - a n t h r a c i t e c o a l . F u e l , 62 ( 1 9 8 3 ) 1 3 9 3 - 1 3 9 6 . 14 D . J . Maloney a n d R . G . J e n k i n s , I n f l u e n c e o f c o a l p r e o s i d a t i o n a n d t h e r e l a t i o n b e t w e e n c h a r s t r u c t u r e arld g a s i f i c a t i o n p o t e n t i a l . F u e l , 64 ( 1 9 8 5 ) 1 4 1 5 - 1 4 2 2 . 1 5 J . J . P i s , J . A . P a j a r e s , A . B . F u e r t e s , M . f.fahamiid, J . R . Parra, A . J . P 6 r e z a n d B . R u i z , I n f l u e n c e o f c o a l o x i d a t i o n o n t h e p r e p a r a t i o n o f a c t , i v e c a r b o n p r e c u r s o r s . C a r b o n e 9 0 , P a r i s F r a n c e ( a c c e p t e d ) . 448 1 6 O.P. M a h a j a n , M. Komatsu a n d P . L . W a l k e r , Jr., L o w - t e m p e r a t u r e a i r o x i d a t i o n o f c a k i n g c o a l s . 1 . E f f e c t on s u b s e q u e n t r e a c t i v i t y o f c h a r s p r o d u c e d . F u e l , 59 ( 1 9 8 0 ) 3-10 . 1 7 J.A. P a j a r e s , J.J. P i s , A.B. F u e r t e s , A.J. P B r e z , M . Hahamud a n d J.B. P a r r a , I n f l u e n c e o f c o a l p r e o x i d a t i o n a n d r e a c t i v e g a s f l o w r a t e on t e x t u r a l p r o p e r t i e s o f a c t i v e c a r b o n s . COPS T I , A l i c a n t e , S p a i n , may 1990 ( a c c e p t e d ) . 1 8 B . Rand a n d H . M a r s h , The p r o c e s s o f a c t i v a t i o n o f c a r b o n s by g a s i f i c a t i o n w i t h C 0 , - I 1 1 U n i f o r m i t y o f g a s i f i c a t i o n . C a r b o n , 9 1 9 J.J. P i s , A . B . F u e r t e s , A . J . P B r e z , J.J. L o r e n z a n a , S . Mendioroz a n d J.A. P a j a r e s , M o d i f i c a t i o n o f t e x t u r a l p r o p e r t i e s o f S p a n i s h c o a l - d e r i v e d c h a r s by a c t i v a t i o n w i t h c a r b o n d i o x i d e . F u e l P r o c e s s i n g T e c h n o l . , 24 ( 1 9 9 0 ) 305-310. 20 T. Wignians, I n d u s t r i a l a s p e c t s o f p r o d u c t i o n a n d u s e o f a c t i v a t e d c a r b o n s . C a r b o n , 27 ( 1 9 8 9 ) 13-22 . 2 1 H . Marsh , A d s o r p t i o n m e t h o d s t o s t u d y m i c r o p o r o s i t y i n c o a l s a n d c a r b o n s - a c r i t i q u e . C a r b o n , 25 ( 1 9 8 7 ) 49-58. ( 1 9 7 1 ) 79-85. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 449 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands CARBON-SUPPORTED PALLADIUM CATALYSTS. SOME ASPECTS OF PREPARATION I N CONNECTION WITH THE ADSORPTION PROPERTIES OF THE SUPPORTS A.S. LISITSYN, P.A. SIMONOV, A.A. KEPTERLING and V.A. LIKHOLOBOV Institute of Catalysis, Novosibirsk 630090 (USSR) Abstract Adsorption of metal complexes on carbon supports can be accom- panied by ligand-exchange involving intrinsic n-fragments of carbon framework. The size of metal crystallites derived from carbon-adsorbed species does not correlate with the proportion of metal accessible to carbon monoxide and reagents of catalytic reaction: this suggests blocking a part of the metallic surface by walls of the support pores. Appropriate choice of the support, metal precursor and procedures at the deposition step makes it possible to eliminate or, at least. diminish such a detrimental effect and to control the size. shape, sites of location. distribution along the catalyst grain and, thus, catalytic properties of final metal crystallites. A strightforward use of carbon-adsorbed complexes as heterogenized catalysts is also discussed. IN!PRODUCTION Despite the wide application which supported metal catalysts have found in industry and fundamental researches, knowledge on their genesis is still rather limited and is especially poor f o r the catalysts based on carbon supports. In part it is caused by a secretive character of most works in the field of catalyst manufacture. In addition, carbon supports prepared from natural polymeric substances, such as coal or wood, were mainly used so far, with the texture, ash content and kind of admixtures being variable from batch to batch; this makes comparison and interpretation of published results difficult. This paper deals with properties of palladium catalysts on carbon supports and the results presented below concern processes taking part at the first stage of the catalyst preparation,namely, deposition of metal precursor onto support. When starting the work, we addressed question on C ) how strong the state of supported precursor can be reflected in catalytic properties of final samples, i t ) what factors are more and what less important in this respect and C C C ) how can one operate by these factors to achieve desirable catalytic properties. 450 0.,1 , . 0,7 , P/Ps 5 lo' t , A no. Support 1 PN 2 Eponitll3H 3 PME800 4 5550 5 5270 6 PM105 7 5450 Surface, sq.m./g BET t -me tho d 770 185 700 450 550* - 270 1 90 450 450 850* - 110* - * a single point determination Fig. 1. Nitrogen adsorption data (78 K) and representative t-plots for carbon supports used: (1.2) active carbon, (3.6) carbon black, (4.5.7) Sibunit: t statistical thickness of liquid nitrogen film on non-porous sorbents at relative presure p/p S - EXPERIMENTAL Commercial active carbons and carbon blacks and samples of recently developed Sibunit carbon [I 3 were tried as supports; their characteristics are indicated in Fig. 1. Unlike carbon blacks and, especially, active carbons which displayed a high proportion of micropores (t-plots [ 2 I give large positive intemepts on the adsorption axis), the Sibunit carbons used possessed well-developed mesoporous structure with a small (5270) and negligible (S450, S550) volume of micropores (the t-plots pass through the origin and specific surfaces derived from their slope appeared equal to BET values). Before use, supports were purified by treating with aqueous solutions of HC1 and H F . Adsorption measurements were performed using a W VIS spectro- meter providing sufficient time for equilibrium to be reached (20 hs as a rule) [3,41. Following drying in vacuum or in air, the samples were reduced (if necessary) in a flow of hydrogen (2000 h-', 25OoC, 2 h; as tested by XPS [5], these conditions are suffioient for metallic palladium to be formed in all cases) and flushed with nitrogen before contact with air. Measurements with CO chemisorption were performed by a pulse technique (H2 flow, -8OOC). Catalytic testings i n hydrogenation and vinyl-exchange reactions were carried out in absence of external diffusional limitations; technique has been described elsewhere [4,51. 451 Fig. 2. ( 8 ) Adsorption isotherms for benzene, naphthalene and phenanthrene (1 -3 ) . respectively, from methanol solution. (b) The same for from acetonitrile (2). ( C ) Effect of addition of HC1 ( 1 ) : &C1 T 2 ) and Na CO (3) on adsorption of palladium species from solution of H PdC1; s0.025 M, 0.5 mg-atom Pd/g sup. to be a maximum of possi- bge coverages ; in the cross-hatched region palladium oxide preci- pitates rapidly upon adding solution of sodium carbonate in absen- ce of carbon support). ,3270 (b1.c) and ,9450 (a.b2) as sorbents. ambient temperature. H PdC14 from aqueous solution ( 1 1 and (CH CN) PdCl 2 RESULTS AND DISCUSSION 1. AdBorption studies Adsorption measurements with organic complexes of palladium and detailed investigation of adsorption of %PdC14 from aqueous solutions have been performed earlier [3,4,63. Results of complementary study are presented in Fig. 2. It is comonly accepted that the adsorption on carbon materials results from either van der Vaals forces or reactions with the surface oxygen groups (ion/ligand exchange and hydrolysis). Not discussing this in detail, it is only expedient to point out the strong dispersive interaction which arises between substances of aromatic nature and aromatic fragments of the carbon support. Anchoring of corresponding complexes proved to be possible thereby even on a graphite basal plane ( [71 and references cited therein). The ligands carrying functional groups (for complexing metal ions) which are attached to condensed rings might be appropriate f o r anchoring on carbon supports by such a means. As follows from Fig. 2a, the strength of adsorption to be enhanced as the number of fused benzene rings in the aromatic %nchort* increases. Adsorption of palladium chlorides represents a more complex case. It has been found [3,61 that up till a half of a monolayer 452 coverage, palladium species is absorbed from solutions of â¬$PdC14 or corresponding salts almost completely. The process was accompanied by releasing chloride ions (the Cl/Pd ratio in the surface complex was caclulated to become equal two) and there were virtually no changes in the pH value for the solution. The adsorbed species could be removed from the support (with exception of ca. 10 $% which remained irreversibly adsorbed) when treating the samples with concentrated solution of HC1; at the same time, treatment with solutions of HN03 or HC104 (poor complexing agents towards palladium) appeared ineffective. The number of adsorption sites for both irreversible. strong and weak adsorption (the second part of monolayer) appeared proportional to specific surface of the supports. The adsorption constants for the sites of strong adsorption have also been determined and proved to be somewhat higher in the case of micropous carbons. The same was true for preliminary oxidized supports, but the number of the strong adsorption sites in this case decreased. All this evidences that adsorption of palladium chlorides may be considered as a coordination process which involves n-fragments of the carbon framework. The exact nature of such fragments could alter from support to support thus explaining difference in the strength of adsorption on different carbons. In particularly, the surface oxygen groups in preoxidized supports are not excluded to enhance stability of the surface complexes through increasing the back donation of electrons from palladium to olefinic moiety due to decrease in electron density on carbon matrix. If one considers adsorption of ( CH3CN),PdC12 from acetonitrile solution (Fig. 2b, curve 2), the much lower coverage attained in this case is obviously explained by the large ooncentration of free ligand; adsorption perhaps takes place on diene-like fragments only. It could also be expected that conversion of [PdCl4I2- into [Pd(OH)4]2- species occurring in alkaline solutions would lead to decrease of adsorption because [Pd(OH)412- is a poorer electrophile to form complexes with olefinic moieties. This did hold true (Fig. 2c, case ( 3 ) at sufficiently large quantity of additive) but one can reach the opposite effect if a d d m alkali in a small surplus only to that for a simple neutralization of H2PdC14. This phenomenon reflects formation of polynuclear Pd(I1) oxy/hydroxy species (colloids ) under such conditions. 453 0.4 .0.3 .0.2 -0.1 -1 1 2 3 4 5 K ~ I = l o3 b RATE W 4 0.8 1 2 4 6 Pd, w k % F i g . 3. (a.b) Mean diameters of palladium crystallites as determined by small-angle X-ray scattering method (a). fraction of palladium exposed (a) and turn over frequencies in cyclohexene hydrogenation (b) f o r catalysts on different supports (as designated in 318. 1 ; precursor in the state of strong adsorption, 1 pg-atom Pd/m : KQvalue for the constant of strong adsorption on the surface sites: CO/Pdsurf was assumed equal unity). (C) Total rate of cyclohexene hydrggenation on catalysts with progressively increased Pd loading (cm H /mg cat/min. for conditions see Table 2 1 ; for clarity. data on catalysts 1 and 3 are omitted). tion of H2PdC14. aging (20 8 ) . drying in vacuum (50-70 reduction in hydrogen (250 C) . Catalyst preparation: incipient wetness impregnationowith solu- C) and 2. Properties of reduced catalysts Data on the samples which were dried and reduced under similar conditions are collected in Fig. 3 and Table 1. SAXS data in Fig. 3a indicate tendency for the size of palladium crystallites to increase as the constant for strong adsorption of precursor decreases. Such a trend did find confirmation upon studying the samples with electron microscopy. The increase in metal particle size proved to be not accompanied, however, by decrease in fraction of the palladium exposed. Moreover, it was the medium-sized palladium particles on Sibunit carbons which displayed highest proportion of metal accessible to carbon monoxide (Fig. 3a). Being determined on the basis of the CO chemisorption data, specific activities of the samples in cyclohexene hydrogenation (reaction known to be structure-insensi- tive on metal catalysts [a ] ) did fall within a rather narrow interval (Fig. 3b). In view of these data it seems logical to suppose a partial blockade of the surface of small metal particles by walls of the support pores. Suggestion may also be made that the relationship between metal 454 particle size and Ks values reflects mainly dependence of the size on the roughness of the support surface (with what Ks values correlate) rather than on the strength of the precursor bonding to the surface. At least, the latter provides little influence on Catalytic properties of the samples. One can see in Fig. 3c that the increase in palladium loading on a support (when the sites of the irreversible, strong and weak adsorption are sequentially occupied) does not lead t o appreciable changes in activity per gram of palladium basis (the curves seem to be linear and pass through the origin or nearby). The blocking effect provides an easy explanation for recent data [51 on properties of the PWC catalysts which were prepared from different precursors. Palladium crystallites with a broad distribution in size (2-20 nm) were seen by electron microscopy after reduction of supported palladium chloride in alkaline liquid phase and medium-sized palladium particles (1-4 nm) upon thermal decomposition of Pd(0) complex with dibenzylidenacetone. Nevertheless, both catalysts appeared approximately twice as active as the catalyst which was prepared via reduction of adsorbed palladium chloride in hydrogen and contained palladium particles of 1-2 nm in size. Because those were Sibunit-based catalysts in Ref. 5, the results show possibility for the blocking effect to be displayed in the case of mesoporous supports as well. The different character of interaction of palladium complex with aromatic ligands with the carbon surface, in comparison with that for palladium chloride, presumably plays a role in determi- ning properties of the Pd(dba)2-derived catalyst. Richard et al. have reported that reduction of related platinum complex Pt (dba)2 resulted in plate-like particles on basal planes of graphite microcrystallites but usual platinum particles on the edge-planes were seen after reduction of supported platinum chloride [91. Specific features of palladium catalysts which pass alkaline treatment (procedure employed often in the catalyst preparation [lo] ) may be derived from the data Fn Table 1. Due to adsorption on carbon support, the hydroxy-species are stabilized against agglomeration (which occurs in solutions) and, so, still small particles of metallic palladium can be produced. TEM examination showed that Pd crystallites in catalysts 1 and 2 are indistinguishable in size (1-2 nm; 3-8 nm as predominant size in sample 3.1). Moreover, fraction of the palladium exposed proved to be substantially higher in catalysts 2 than in Catalysts 1 . And 455 Table 1. Catalytic properties of â¬$PdCl -derived catalysts (1 .O wt.8 of Pd deposition. on 5450 carbon) in respec4 to the mode of precursor a) No. Procedures ~ ~ b) co Activity data Pd c-hexeneC) nitrobenzenedl - 'loo W TON R W TON R 1 - 1 adsorption 45 24 3.8 3.5 27 4.5 2.5 1.2 i. w. impregnation 45 22 3.6 3.5 25 4.3 2 2.1 adsorption. then tre- 65 29 3.3 3.5 38 4.3 2.5 atment with alkali 2.2 as (2.1) but i.w.impr. 75 33 3.3 4 42 4.2 2.5 3.1 conversion into col- 45 19 3.1 1.7 40 6.6 1.2 loidal Pd oxide. then adsorption 3.2 as (3.1) but i.w.impr. 50 22 3.3 3 45 6.7 2 Adsorption via adding dropwise solution of H2PdC14 (1.1, 2.1 ) or colloidal species (3.1) to a stirred slurry of carbon support ( 1 g ) in 10 ml of water. Sodium carbonate as alkali agent (6/1 and 2/1 in cases 2 and 3. respectively). Following deposition step, washing with water (10 ml), dryoing in a i r (ambient temperature) and reduction in hydrogen (250 C 1 . and turn over numbers TON ( m o l %/&atom Pd exposed/s) for preli- minary milled catalysts (5-7 pm as a mean grain size.COULTER TAII countings); R ratio of activities f o r the milled and original not ground catalysts (70-100 wJ. a 3 bVelocity of hydrogen consumption w (cm /me; cat/min)u100 ( 2 1 0 Cethanol solution (0.5 MI. 0 C. 1 bar. dmethanol solution (0.2 M), 30° C. 1 bar. even catalyst 3.1 with larger Pd crystallites is not unfavourable in this respect as compared with catalysts 1 . So, the blocking effect is probably eliminated as the precursor is converted into hydroxy species. Secondly, these results show a small size of metal crystallites to be not necessary accompanied by a pronounced blocking effect. Note also that reduction of palladium acetate on a preliminary oxidized Sibunit carbon allowed the very small Pd particles t o be obtained [Ill, with catalytic activity of the sample in cyclohexene hydrogenation close to that for catalysts 2 in Table 1. There are probably a number of factors which determine whether there will be or not and in what extent the blocking effect displayed. Supposedly, among them are morphology and chemical state of the surface, chemical composition and dimentions of the metal precursor, capability for preliminary adsorbed species and metal atom/clusters formed to moving around the surface: so, 456 conditions of drying, reduction or other steps at catalyst preparation oan also provide their influence. When colloidal palladium oxide is prepared in advance (methods 3 in Table I), the enhanced capability of such species to adsorbing provides their deposition onto the exterior surface of catalyst grains. Diffusion limitations for rapid catalytic processes diminish thereby (cf. the R factors for different catalysts in Table 1 ). Naturally, the active component is forced to penetrate more towards the grain core if incipient wetness impregnation is used instead of adsorption (method 3.2). 3. Carbon-adsorbed complexes as heterogenized catalysts. Surprisingly, but despite great interest to immobilized metal complexes, as the systems combining merits of both homogeneous and heterogeneous catalysts, there were few detailed studies on carbon-anchored catalysts. Meanwhile, the latters seem to be most promising for bringing in commercial practice, the goal which in the case of supported complexes has so far been attained in very limited cases [71. In Fig. 4 one can find the selected data on properties of carbon-adsorbed complexes in the vinyl-exchange reaction, which allow discussing the factors one should take into account when preparing and using such catalysts. Fig. 4 (a,b) shows that the catalyst preparation might be reduced to the simplest of all possible ways, namely, tn sttu deposition of active component onto carbon support. Interestingly, even the complex nearly insoluble in the reaction mixture can be supported by such a technique (IC;PdCl4 as example; by itself, it displays no activity in trans-vinylation but its rapid transfer onto the support added results in the same reaction rate as on the complex supported in advance, see curve 2 in Fig. 4b). In view of the known sensitivity of trans-vinylation to steric factor, it seems somewhat surprising that the anchoring of Li2PdC14 leads to diminishing the activity by a factor of two only (Fig. 4a). Perhaps , those changes in chemical composition of the chloride complexes which took part upon anchoring provide promotion effect which compensates in part the detrimental effect of proximity of the active center to the support. When the chlorides were deposited from acetonitrile, their fixation supposedly took place on somewhat different surface sites, may be less hindered, which explains the enhanced catalytic activity in this case (cf. data in Fig. 4c (curve 2) and 4a,4b). Such a 457 50 100 m i n 50 100 m i n 10 30 w i n Fig. A. Trans-vinylation on carbon-anchored palladium chlorides (on the ordinate axis conversion of vinyl acetate into vinyl propionate: initial propionic acid/vinyl acetate ratio 10 v/v; static reactor. 50 OC. S450 as support. content of supported pal- ladium l wt.% (a.b.cl) and 0.5 wt.% (~21, formal concentration of palladium In the reaction mixture ca. 6 (a.b,cl) and 2 mg-atom/l). Catalysts: (a) Li2PdClA. as homogeneous ( 1 ) and C n eCtu supported ( 2 . succes- sive runs 1. (b) K2PdC1 , as adsorbed in advance from aqueous solution (1) and as loaAed as solid ( 2 ) ; in case (2) the arrow indicates moment when carbon support was added. ( C ) sPdC14/C. promotion with lithium propionate (0.4 mol/l) ( 1 ) and (CH3CN)2PdC12 as anchored from acetonitrile (2 1. trivial method as promotion with carboxylates may also be applied (Fig. 4c, curve 1). The catalysts with adsorbed palladium chlorides could be reused several times and elemental analysis showed that deactivation, if observed, should be assigned to other side-processes (e.g., reduction of Pd(I1) species) rather than palladium leaching. As far as complexes with aromatic ligands are concerned, which were anchored due to dispersive forces, carboxylic acid competed rather effectively with them f o r the adsorption sites; complexes which are insoluble, by themselves, should be used in this case to prevent leaching of palladium. It appeared also preferable to use complexes in which the central atom was brought out of the plane of aromatic anchor [41, but pronounoed decrease in activity has been observed (an order of magnitude) f o r phenanthroline complex phen.Pd(OAc)2 upon anchoring. There is still uncertainty, however, in the site of looation of active Pd species in the former case, whether it is bound to the aromatic ligand or to carbon framework of the support. 458 CONCLUSION The results of the present study show that it would be mistake t o consider carbon supports as an inert matrix and much attention should be payed to both physical and chemical characteristics of these supports. In particular, the very rich chemistry of the carbon surface provides conditions for strong anchoring of metal complexes by such a simple technique as adsorption and the heterogenized species might be used in liquid-phase catalytic . processes. If these species are taken as a precursor for metal crystallites, the state and properties of final metal particles can be influenced to a great extent by the nature of initial complex and technique of its deposition . ACKNOWLEDGEMENTS The authors are very thankful to Drs V.N.Kolomiichuk and A.L. Chuvilin f o r SAXS and TE?d data, Prof. V.B.Fenelonov and Mrs L.G. Okkel f o r nitrogen adsorption measurements, Mrs V.P.Melânikova and N.I.Gergert for experimental assistance and Dr G.V.Plaks3n for a gift of Sibunit carbons. REFERENCES 1 Yu.1. Yemakov, V.F. Surovikin, G.V. Plaksin, V.A. Semikolenov, V.A. Likholobov, A.L. Chuvilin and S.V. Bogdanov, React.Rlnet. 2 s.J. Greg and K.S.W. S i n g , Admrptton, Surpace Area and Poro- sity, 2nd edn., Academic Press, London, 1982. 3 P.A. Simonov, V.A. Semikolenov, V.A. Likholobov, A.I. Boronin and Yu.1. Yermakov, Izu. Ak.ud. SSSR, Ser. Khtm., (1988) 2719-2724. 4 A.A. Ketterl A.S. Lisitsgn, V.A. Likholobov, A.A. Gall and A. S . Trachum ,?â tnet. KataZ.. in press. 5 A.S. Lisitsyn, S.V. Gurevich, A.L. Chuvilin, A.I. Boronin, V.I. Bukhtiyarov and V.A. Likholobov, React. litnet. CataZ. Lett., 6 Yu.A. Ryndin, O.S. Alekseev, P.A. Simonov and V.A. Likholobov, 7 V.A. Likholobov and A.S. Lisitsyn, J . YendeZeev Soc. (USSR), 9 D. Richard, P. Gallezot, D. Neibecker and I. Tkatchenko, 10 A.B. Stiles, CataZyst Mmjâacture. Larboratory and CommerctaZ 1 1 S.V. Gurevich, P.A. Simonov, A.S. Lisitsgn, V.A. Likholobov, Catal. Lett., 33 (1987) 435-440. 38 (1989) 109-114. J . YOZ. cataz., 55 (igag) 109-125. 34 (1989) 340-348- 8 E.E. G O ~ O and Y. Boudart, J . Cataâl.. 52 (1979) 462-714. CataZ. Today, 6 (1989) 171-179. Preparat tons, Marcel Dekker, New York, 1 983. E.Y. Moroz, A.L. Chuvilin and V.N. Kolomiichuk, React. H i n e t . CataZ. Lett., 41 (1990) 211-216. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 459 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands PREPARATION OF PALLADIUM-COPPER CATALYSTS OF OESIGNEO SURFACE STRUCTURE ZS. BOONARl, T. WLLAT', S . SZAB6' and J. P E T R d 'Department of Organic Chemical Technology, Technical Universi ty of Budapest, H-1521 Budapest, Hungary 'Central Research I n s t i t u t e f o r Chemistry, Hungarian Academy of Sc iences , H-1525 Budapest, Hungary SUMMARY reduc t ion of Cu on to Pd or Pd/C. I n g e n e r a l , bulk Cu depos i t i on is k i n e t i c a l l y mre favourable , although adsorbed Cu (submonolayer) is mre s t a b l e . Various methods f o r t h e e l imina t ion of bulk metal depos i t i on , a r e discussed. Hydrogenation i n formic a c i d or ionadsorpt ion followed by hydrogenation a r e found t o be s u i t a b l e methods f o r p r a c t i c a l a p p l i c a t i o n . Pd/C c a t a l y s t s modified by bulk or adsorbed Cu, have d i f f e r e n t s e l e c t i v i t i e s i n t h e p a r t i a l r educ t ion of 4- ch lo rmi t robenzene . The2pepa ra t ion of Pd+Cu c a t a l y s t s has been s tud ied by consecu t ive INTRODUCTION Bimetallic c a t a l y s t s a r e g e n e r a l l y prepared by s inu l t aneous or consecut ive r educ t ion of corresponding p recu r so r s . In t h e case of s i rmltaneous r educ t ion t h e composition of t h e c a t a l y t i c a l l y a c t i v e su r face is d i f f i c u l t t o des ign owing t o the phenomenon of s eg rega t ion ( r e f s .1 -3 ) . By t h e method of consecu t ive r educ t ion - although not a s widely a p p l i e d - su r face s t r u c t u r e of t h e monometallic c a t a l y s t can be modified a s r e q u i r e d , by metal depos i t i on on to t h e s u r f a c e . Th i s may o f f e r an economical method for preparing small amounts of c a t a l y s t s i n f i n e chemical industry. According t o e a r l i e r i n v e s t i g a t i o n s (refs .4-5) i n t h e c a s e of a Pd+Cu c a t a l y s t prepared by s i rml t aneous r educ t ion , fou r d i f f e r e n t , Cu-containing phases may form: (i) bulk Cu, ( i i ) adsorbed C u , ( i i i ) d i so rde red a l l o y and ( i v ) ordered a l l o y (PdCuj) phases. I n t h e absence of C1- or Pd2+ i o n , no a l l o y phase formation occurs during consecu t ive r educ t ion ( r e f . 6 ) . In t h e l a t t e r c a s e bulk metal and adsorbed metal formations occur s inu l t aneous ly . Th i s phenomenon was observed e.g. i n t h e case of Cu d e p o s i t i o n by hydrogen r educ t ion from an aqueous s o l u t i o n of /CU(NH~)&/(OH)~ on to a Pd/C c a t a l y s t (Fig. 1). The doub le t i n t h e range Pd ( r e f .71 , whereas the peak around 0.55 V is due t o i o n i z a t i o n of t h e adsorbed Cu ( refs .6-7) . of 0.3-0,4 V p o i n t s t o bulk Cu depos i t i on on var ious c r y s t a l f a c e s of I n t e r a c t i o n between Cu atoms is c h a r a c t e r i s t i c of " t h r e e dimensional" bulk C u , whereas t h e c h a r a c t e r i s t i c f e a t u r e of "two-dimensional" adsorbed Cu is t h e 460 Fig.1. Potentiodynamic curve o f a carbon-supported Pd+Cu c a t a l y s t prepared by consecutive reduc t i on ( r e f . 4 ) i n t e r a c t i o n between Pd and Cu atoms (Fig. 2). I n the case o f b u l k deposi t ion, coverage o f the Pd c a t a l y s t i s uncertain, it i s g r e a t l y dependent on the exper i - mental parameters. With adsorbed metal deposit ion, coverage i s unambiguously due t o q u a n t i t a t i v e condi t ions. From c a t a l y t i c aspects l a t t e r s t ruc tu re seems t o be more favourable. I n t h i s paper, through the example o f Pd+Cu ca ta l ys ts , we wish t o present methods by which bu lk w t a l depos i t i on can be avoided, and b i m e t a l l i c c a t a l y s t s with we l l - designed uni form sur face s t ruc tu re can be at ta ined. The sur face s t r u c t u r e of b i m e t a l l i c c a t a l y s t s has been stud ied by an electrochemical p o l a r i z a t i o n (EP) method and c a t a l y t i c p roper t i es were analyzed by s e l e c t i v e reduc t i on of 4-chloronitrobenzene (CNB). a . 9 b . , Fig.2. Bulk (a) and adsorbed (b) metal on the surface of the c a t a l y s t : bas i c me ta l (Pd) metal (Cu) deposited by consecutive reduct ion EXPERIMENTAL A n a l y t i c a l grade reagents and d i s t i l l e d water ( t r i p l e d i s t i l l e d f o r e lec t ro - chemical p o l a r i z a t i o n ) were used. A f t e r hydrogen reduction, a l l c a t a l y s t s were washed with water i n hydrogen atmosphere and d r i e d i n a i r . Pd c a t a l y s t A: Palladium was prepared from PdCl2, v i a Pd(OH)2 by hydrogen reduc t i on ( r e f ,8). 461 Pd/C c a t a l y s t s El : The 10 w t % pd on ac t i va ted carbon was a commercial product ("Selcat Q", Finomvegyszer Szovetkezet, Hungary). I t was used a f t e r p u r i f i c a t i o n by d i l u t e aqueous H2S04 so lu t i on i n hydrogen atmosphere. C: The ca ta l ys t contained 11 w t % Pd on ac t i va ted carbon ("Carbo C Ext ra" , BET surface area: 930 m2g-l). 50 g carbon was impregnated with a s o l u t i o n o f 9.2 g PdC12 i n 20 cm 17 w t % HC1 and d r i e d a t 105OC. The powder was added t o a mixed s o l u t i o n o f 40 g NaHC03 i n 500 cm water a t 7OoC. A f t e r 2 h the s l u r r y was hydrogenated a t 3OoC f o r 1 h. The d ispers ion o f Pd was 0.16 by TEM measurement. Pd+Cu c a t a l y s t fo r EP 0: 0.5 g Pd(A) was prehydrogenated i n 20 cm3 water f o r 0.5 h. A s o l u t i o n o f 53 mg CuS04.5H20 i n 20 cm 85 w t % HCOOH was added a f t e r decantation and hydrogenation continued f o r 0.5 h. E: 0.5 g Pd/C (El) was prehydrogenated i n 20 crn water. A f t e r 0.5 h a s o l u t i o n o f 212 mg c i t r i c ac id and 79 mg CuS04.5H20 i n 20 cm3 water was added, and hydrogenation continued f o r 2 h. F: A s l u r r y o f 0.49 g Pd (A) and 0.45 g CuS04.5H20 i n 20 cm i n a i r f o r 1.5 h. The ca ta l ys t was f i l t e r e d o f f and washed Cu2+-free w i t h water. 3 Then the ca ta l ys t was hydrogenated i n 50 cm water f o r lh. (Pd+Cu)/C ca ta l ys ts f o r hydrogenation o f CNB I n a l l se r ies Pd/C (C) c a t a l y s t was used and Cu content o f the c a t a l y s t s was 30 and 60 a t % Cu/Pds. The 0 a t % Cu/Pds ca ta l ys ts were prepared s i m i l a r l y , b u t wi thout Cu, t o check the e f f e c t o f organic addi t ives. G: 2 . 5 g o f Pd/C was prehydrogenated i n 20 cm water, a t 25OC f o r 1 h. A 3 s o l u t i o n o f 0.11 g c i t r i c a c i d and an appropriate amount o f CuS04 i n 10 cm water was in jec ted and hydrogenation continued f o r lh . 3 3 H: 2.5 g Pd/C was prehydrogenated i n 20 cm water a t 25OC f o r 1 h. 40 cm HCOOH conta in ing an appropr ia te amount o f CuS04 was added a f t e r decantation hydrogenation continued a t O°C f o r 3 h. I: 2.5 g Pd/C i n 30 cm water con ta in ing an appropriate amount o f CuS04 were mixed f o r 1 h i n a i r , then 1 h i n hydrogen. E lect ron microscopy (TEM) Oispersion of the Pd/C (El) c a t a l y s t was measured by means o f a PHILIPS TEM 505 microscope. About 1000 p a r t i c l e s were counted and sized. The sur face mean diameter (d=gni . di /gni.di) was transformed t o approximate d i spe rs ion (0) by the fo l l ow ing equation ( r e f . 9 ) : 0=2.5.a-d-1, where "a" i s t he l a t t i c e constant o f Pd. Electrochemical p o l a r i z a t i o n (EP) The P t sheet bottomed c e l l and p o l a r i z a t i o n method have been d i s c r i b e d ( r e f .lo). Potentiodynamic p o l a r i z a t i o n was c a r r i e d out i n 0.5 M H2S04 suppor t ing e l e c t r o l y t e , i n n i t rogen atmosphere. Anodic sweep commenced from 0.03 V with 3 3 3 3 3 water was mixed 3 85 w t % and 3 3 2 462 1 mvs-l sweep ra te , up t o 1.0 V. Hydrogenation of 4-chloronitrobenzene (CNB) 30-100 mg c a t a l y s t was prehydrogenated i n 10 em e t h y l acetate f o r 0.5 h. 9.5 rnm1 CNB dissolved i n 20 cm e t h y l acetate was i n j e c t e d and hydrogen consumption was measured (24OC, 1 bar). The products r e r e analyzed by GLC. 3 3 RESULTS AND DISCUSSION Preparation o f Pd+Cu .cata lysts au Over the past 25 years, metal adsorption has been a popular t o p i c i n the f i e l d of e l e c t r o c a t a l y s i s (refs.11-13). Ev iden t l y , therefore, the methods elaborated f o r adsorbed metal deposi t ion i nvo l ve mainly electrochemical procedures. I n the fo l lowing, we wish t o g i ve a b r i e f summary o f the various p o s s i b i l i t i e s , w i t h spec ia l s t ress on methods s u i t a b l e f o r i n d u s t r i a l r e a l i z a t i o n . Since only procedures app l i cab le i n the l i q u i d phase are o f p r a c t i c a l s ign i f icance, these methods w i l l be described below. I n the deposi t ion o f adsorbed metal, the phenomenon termed "underpotent ia l deposit ion" can be u t i l i z e d . By th is theory, d i f f e r e n t metals a t p o t e n t i a l s p o s i t i v e from t h e i r reve rs ib le Nerst p o t e n t i a l w i l l discharge and adsorb on other metal surfaces. The adsorbed metal can be e a s i l y separated from the bu lk metal by an EP method (Fig. 1). Thermodynamically, the bas is o f the preparat ion of a ca ta l ys t m d i f i e d by adsorbed meta l i s t h a t the e lect rode p o t e n t i a l o f t he c a t a l y s t should f a l l between the p o t e n t i a l s of bu lk and adsorbed meta l (Fig. 3). (E.g. i n hydrogen atmosphere, t he e lect rode p o t e n t i a l o f Pd can be de f i ned as a reve rs ib le H/H+ electrode). E cat E 'Hebulk /Men* 'Mead Inen+ Fig.3. E lect rode p o t e n t i a l s i n the deposktion o f adsorbed metal I. The simplest way f o r th , is i s t o c o n t r o l the p o t e n t i a l of t h e Pd c a t a l y s t by a po ten t i os ta t , a method genera l ly app l i ed i n electrochemistry, n o t p r a c t i c a l , however, for the preparat ion of l a r g e r amounts of cata lysts . 11. Another s o l u t i o n may be the adjustment of t he p o t e n t i a l by redox Systems. The cheapest and purest reducing agent i s hydrogen. The p o t e n t i a l of a Pd 463 c a t a l y s t (as a H/H+ electrode) and of a metal/metal i o n system t o be reduced can be determined by the fo l l ow ing equations: E = EoMe + (RT/nF) I n a E Mebulk ( 2 ) The e lec t rode p o t e n t i a l o f an adsorbed metal/metal i o n system can be ca l cu la ted from the work func t i on d i f f e r e n c i e s o f base and adsorbed metals (ref.111, o r der ived from p o l a r i z a t i o n curves. On the bas i s o f the above equations, by appropr ia te choice of the metal i o n concentrat ion and pH values, the p o t e n t i a l cond i t i ons given i n F ig . 3 can be at ta ined. I n t h i s way, i n aqueous a c i d i c medium, a number o f metals with standard e lec t rode p o t e n t i a l s negat ive t o hydrogen (Table 1.) are capable o f adsorpt ion on Pd wi thout bu lk metal deposi t ion, e.g. Pb or Cd (ref.15). TABLE 1 Standard e lect rode p o t e n t i a l s o f Me/&"+ systems ( r e f . 14) System E0 System E0 /v/ /v/ Pb/Pb2+ -0.13 H/H+ pH = 0 0.00 Cd/Cd2+ -0.40 pH = 7 -0.41 T U T ~ + -0.34 pH = 14 -0.82 Sn/Sn2+ -0.14 cu/cu2+ 0.34 s n + / s n + 0.15 cu /cu+ 0.52 cu+/cu2+ 0.16 I n the case o f Cu, however, which i s a metal with an e lec t rode p o t e n t i a l p o s i t i v e t o hydrogen, b u l k metal deposi t ion i n aqueous medium cannot be avoided. By changing the pH value, Cu2+ concentrat ion, temperature, and H2 pressure, no r e a l i s t i c values prevent ing bu lk metal deposi t ion can be a t ta ined . 1x1. A much mre f e a s i b l e so lu t i on f o r adsorbed Cu depos i t i on i s t o carry out reduc t i on i n a so lvent o ther than water. I n formic ac id the standard e lect rode P o t e n t i a l of the cU/cU2+ system r e l a t e d t o the hydrogen e lec t rode i s Eo= -0.14 v ( r e f .16). I n the potentiodynamic curve o f c a t a l y s t 0 prepared i n formic ac id by hydrogen reduct ion (Fig.4), the maximum (0.4-0.6 V ) der ived from the i o n i z a t i o n o f adsorbed Cu can only be observed. As could be expected, t he re i s no bu lk Cu ,on the surface. The two inseparable peaks o f adsorbed Cu a re due t o Cu adsorpt ion on d i f f e r e n t c r y s t a l faces. I n the range o f 0-0.25 V , t h e i o n i z a t i o n 464 1,mA 1.6 1,4 12 1.0. 4 8 0,6 0,4 0 2 '- peak of d i s so lved hydrogen and t h e adsorbed hydrogen, discerned a s a s h o u l d e r , appear , whereas above 0.7 V a wave c h a r a c t e r i s t i c of oxygen adso rp t ion may be observed ( r e f . 17) . I .mA/ Fig.5. Potentiodynamic cu rve of carbon-supported Pd+Cu c a t a l y s t E reduced i n t h e ppesence of c i t r i c a c i d by H~ (m.2 mg, v = l mV5-l) Fig.4. Potentiodynamic cu rve of c a t a l y s t D reduced i n formic a c i d by H2 (m=2 mg, v = l mvs-l) 0 0,2 0,4 0,6 0,8 E,V IV. In aqueous s o l u t i o n a t room temperature under atmospheric p re s su re , t h e p o t e n t i a l of Pd c a t a l y s t is a f u n c t i o n of pH, with a maxirmm value of 0 V. With s t rong complexing agen t s t h e e l e c t r o d e p o t e n t i a l of Cu can be s h i f t e d towards a negat ive d i r e c t i o n , t hus t h e r e l a t i o n s h i p of t he e l e c t r o d e p o t e n t i a l s d e p i c t e d i n Fig. 3 may be a t t a i n e d . We t r i e d ou t a p p l i c a t i o n of Cu complexes with EDTA, /)-alanine, c i t r i c a c i d and KCN. Bulk Cu deposi ton could not be avoided, however, i n any of t h e s e c a s e s . An i n t e r e s t i n g phenomenon was observed (F ig . 5) i n t h e case of c a t a l y s t E prepared i n t h e presence of c i t r ic a c i d by hydrogen reduct ion, where only bulk Cu formation occurred (0.25-0.35 V) . A p o s s i b l e explanat ion f o r t h e formation of bulk Cu may be t h a t a f t e r depos i t i on of t h e Cu atoms on t h e s u r f a c e , t h e rest of t h e atoms depos i t onto t h e Cu atoms owing t o 2+ 465 t h e s t r o n g adso rp t ion of c i t r ic a c i d on Pd. The compe t i t i ve adso rp t ion of ci tr ic a c i d and o t h e r a d d i t i v e s is u t i l i z e d a l s o i n t h e p r e p a r a t i o n of "egg s h e l l " o r "yolk"-type A1203 supported P t c a t a l y s t s ( r e f .18). A s p e c i a l ca se of r educ t ion i n H2 atmosphere is t h e metal adso rp t ion v i a i o n i z a t i o n of preadsorbed hydrogen ( r e f .13 ) . The method may be i n t e r p r e t e d by t h e r educ t ion of metal i ons by m a n s of a c a l c u l a t e d amount of reducing agen t (adsorbed hydrogen). This is a method f o r t h e p r e p a r a t i o n of Pt+Xad c a t a l y s t s , where x= Cu,Ag,Bi,Au (ref .13) . Unfortunately, i n t h e c a s e of Pd c a t a l y s t s , which con ta in high amounts of d i s so lved hydrogen, t h i s method is not app l i cab le . V. VI. According t o t h e r e l a t i o n s h i p shown i n Fig.3 , t h e p o t e n t i a l of t h e c a t a l y s t can be ad jus t ed not only with hydrogen b u t a l s o by means of redox systems. The e l e c t r o d e p o t e n t i a l s of a number of o r g a n i c redox systems have been determined ( r e f s . 14,16) . The use of d i f f e r e n t s u b s t i t u t e d quinone-hydroquinone type systems is e s p e c i a l l y favoured. Unfortunately, however, by a p p l i c a t i o n of t h e s e systems the s u r f a c e of t h e c a t a l y s t becomes contaminated, which r ende r s EP i n v e s t i g a t i o n s impossible. A s p e c i a l ca se is when the adsorbable metal ion of v a r i a b l e valence is a t t h e same t i m e t h e redox agent . In t h i s way, through d i s p r o p o r t i o n a t i o n of Sn2+ a c a t a l y s t modified by adsorbed Sn can be obtained ( r e f . 1 3 ) . The procedure is r a t h e r simple: t h e c a t a l y s t (poss ib ly oxidized on t h e s u r f a c e ) is shaken toge the r with an aqueous s o l u t i o n of Sn2+ s a l t . I n t h i s c a s e the p o t e n t i a l of t h e P t c a t a l y s t is determined by t h e Sn2+/Sn4+ system, whose e l e c t r o d e p o t e n t i a l is p o s i t i v e t o t h e Snbulk/Sn system, bu t nega t ive t o t h e Snad/Sn system (Table 1.). With Cu+ a s p recu r so r t h i s method is n o t a p p l i c a b l e , owing t o t h e bulk depos i t i on of Cu. VII. I n c o n t r a s t t o t h i s , t h e ion adso rp t ion method, where t h e s t r o n g l y adsorbed metal ion is reduced (by hydrogen), is no t based on e lec t rochemica l p r i n c i p l e s . Th i s procedure can be app l i ed , however, on ly i f t h e coverage of Pd by t h e s t r o n g l y adsorbed metal i o n s is below monolayer, and t h e excess metal i ons can be e a s i l y removed (e.g. washed o u t ) . The p recu r so r is, t h u s , a metal i on bound t o Pd by s t r o n g adsorpt ion. A s had been v e r i f i e d e a r l i e r ( r e f . l 9 ) , Pd poisoned by aqueous l e a d a c e t a t e , termed as t h e L ind la r c a t a l y s t ( r e f .20 ) , is a c t u a l l y Pd modified by Pdad. C a r e l e s s p repa ra t ion (e.g. incomplete removal of e x c e s s Pb ) may l e a d t o bulk l ead depos i t i on , which decreases a c t i v i t y and s e l e c t i v i t y a s well. ion adso rp t ion . On t h e su r face of Pd only adsorbed Cu (0.45-0.65 V) and hydrogen adsorbed on free Pd atoms (0.15-0.3 V) can be de tec t ed . With t h e assumption of Cuad/Pdskl and Had/PdsGl r a t i o s , oCu can be determined from t h e a r e a s below t h e curve ( r e f s .21 ,22 ) . Prel iminary design of t h i s va lue (here: 0.12) is no t p o s s i b l e as it is dependent on t h e parameters of excess Cu2+ removal (washing). 2+ 2+ 2+ Fig.6. shows t h e EP curve of a Pd+Cu c a t a l y s t prepared by t h e method of 466 1,mAI Fig.6. Potentiodynamic curve of Pd+Cu c a t a l y s t F prepared by hydrogen reduct ion a f t e r i on adsorpt ion (m=2 mg, v = l mvs ) -1 0 0,2 0.4 0,6 0,8 E,V U n c e r t a i n t i e s a r i s i n g from c a t a l y s t washing may be avoided by adsorpt ion i n the presence of a c a l c u l a t e d amount of metal ions and r e d u c t i o n , omi t t i ng t h e washing s t e p ( s e e c a t a l y s t I). The b e n e f i t of t h e method is t h a t by a r e l a t i v e l y s imple and cheap procedure, a s u r f a c e f r e e from c a t a l y s t poisons can be a t t a i n e d . P a r t i a l r educ t ion o f 4-chloronitrobenzene (CNB) Pd c a t a l y s t s m d i f i e d by adsorbed Cu as desc r ibed above can be prepared by hydrogen r educ t ion i n formic a c i d medium (H) ( r e f .23) by ion adsorpt ion, followed by hydrogenation (I) . In t h e fol lowing, catalyt ic p r o p e r t i e s of t hese c a t a l y s t s (G) a r e compared. As a test r eac t ion , we have chosen t h e well-known procedure of p a r t i a l r educ t ion of CN8 ( r e f .24). I n i n d u s t r i a l procedures , reduct ion is oene ra l lv c a r r i e d out in a c i d i c mrlitrm. n r hv r eo lec inn Prl hv P t . which is wore and those containing only bulk Cu prepared i n c i t r ic a c i d medium exoensive htit has hinher s e l e c t i v i t v . ,- _ _ _ _ -__- - - - r - - - - -- I n s tudy ing t h e above r eec t ion , we have found t h a t a n i l i n e (A) is c o n t i n u a l l y formed on both Pd/C and (Pd+Cu)/C c a t a l y s t s , and r educ t ion of t h e n i t r o group and hydro-dehalogenation t a k e p l a c e s imultaneously. I n t h e fol lowing, t h e amount of by-product t o determine t h e s e l e c t i v i t y of t h e c a t a l y s t . measurable a t 100 % conversion of E, w i l l be given The d a t a of Pd/C (C) and (Pd+Cu)/C (G,H,I) c a t a l y s t s a r e given i n Table 2. It has been found t h a t the s e l e c t i v i t y of 0 a t % Cu/Pds c a t a l y s t s , prepared a s r e f e r e n c e m a t e r i a l t o c l a r i f y t h e e f f e c t of a d d i t i v e s , d i f f e r s from t h e s e l e c t i v i t y of Pd/C (C) c a t a l y s t when an a c i d i c "add i t ive" ( c i t r i c ac id or formic a c i d ) was app l i ed . The i n h i b i t i n g e f f e c t of v a r i o u s ac ids on hydrodehalogenation has been descr ibed ( r e f . 2 4 ) , and we have a l s o found t h a t by a d d i t i o n of a c e t i c a c i d t o the e t h y l a c e t a t e s o l u t i o n , t h e r a t e of A formation c a n be reduced. 467 TABLE 2. S e l e c t i v e r educ t ion of CNB wi th carbon supported Pd+Cu c a t a l y s t s A a t % , C a t a l y s t s c u - c u - C - 0 7.2 - 0 3.1 G bulk 30 3.5 bulk 60 2.8 - 0 6.8 I adsorbed 30 5 .5 adsorbed 60 2 .2 - 0 3 .8 H adsorbed 30 2.8 adsorbed 60 0 . 1 PdS structure Modif icat ion of t h e s u r f a c e by bulk Cu depos i t i on caused p r a c t i c a l l y no change i n t he s e l e c t i v i t y . Th i s is due t o the f a c t t h a t even a t a r a t i o 60 a t % Cu/Pds, a small p ropor t ion of su r face Pd atcuns a r e covered by i n a c t i v e C U (see Fig.2) . I t sh lou ld a l s o be taken i n t o cons ide ra t ion t h a t t h e d i scha rge of Cu2+ and hydrogen i o n i z a t i o n may be l o c a l l y separated ( r e f .25 ) . Thus Cubulk pay a l s o depos i t onto carbon support of good conduct ivi ty . (One can conclude t o @ cu from t h e amount of adsorbed hydrogen. ) In t h e case of c a t a l y s t s modified by adsorbed C u , t h e r a t i o of Cu/PdS means a l s o Pd coverage. I n both series of c a t a l y s t s (I and H), t h e amount of A dec reases with t h e i n c r e a s e of t h e amount of i n a c t i v e Cu. As a result of t he j o i n t e f f e c t of Cuad and a c i d ( s e r i e s H), 4-ch lo raan i l ine (CA)can be prepared i n 99.9 % p u r i t y . (React ion r a t e is then approximately fou r times a s low as t h e r a t e a t t a i n e d with r e f e r e n c e c a t a l y s t C.) I n our op in ion , t h e i n c r e a s e i n s e l e c t i v i t y caused by adsorbed Cu may be explained by geometr ic e f f e c t s . By coverage of t h e a c t i v e Pd s u r f a c e with i n a c t i v e Cu, t h e number and s i z e of a c t i v e s i t e "ensembles" d e c r e a s e , which - beyond a c e r t a i n l i m i t - w i l l l e a d t o s i g n i f i c a n t change i n t h e s e l e c t i v i t y (ref .26). On t h i s b a s i s , reducing t h e amount of &would be p o s s i b l e a l s o by bulk Cu depos i t i on a t higher Cu/Pds r a t i o . This cannot be r e a l i z e d , however, owing t o t h e unce r t a in ty of t h e geometry of Cu depos i t s (and t h u s , Pd coverage) which is dependent on t h e k i n e t i c f a c t o r s of deposi t ion. Of REFERENCES 1 R. A . Van Santen and W . M. S a c h t l e r . A theorv of s u r f a c e enrichment i n ordered a l l o y s , J . C a t a l . , 33 (1774) 202-207. . C a t a l . , 37 (1975) 106-113. 2 3. 3 . Burton, E. Hymn and 0 . G . Fedak, Surface s e g r e g a t i o n i n a l l o y s , J. 468 7 8 9 10 11 12 13 1 4 15 M. 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B r e i t e r , D i s s o l u t i o n and adsorpt ion o f hydrogen a t smooth Pd wi res a t p o t e n t i a l s o f t he alpha phase. I n f l uence o f e l e c t r o l y t e , J . Elect roanal . Chem., 90 (1978) 425-430. 22 I. Bakos and S. Szabo, Determination o f t he surface area o f Pd c a t a l y s t s by underpotent ia l deposi t ion o f copper, React.Kinet. Catal. L e t t . , 4 1 (1990) 53-57. 23 T. Ma l l s t , Z s . Bodnhr, S. Szabb, J . Petro and P. Szab6, Methods f o r t he preparat ion o f Pd-Cu ca ta l ys ts , Hung. Patent Appl. 2693 (1988) 24 M. Fe re i fe lde r , P r a c t i c a l c a t a l y t i c hydrogenation, W i l l ey - In te rsc i . New York, 1971. 25 S . Szab6 and F. Nagy, I n v e s t i g a t i o n of s i l v e r , b isnuth and copper adsorpt ion v i a the i o n i z a t i o n of hydrogen adsorbed on p l a t i n i z e d p l a t i n i u m i n pe rch lo r i c ac id s o l u t i o n J. Elect roanal . Chem., 70 (1976) 357-364. 26 V . Pone;, S e l e c t i v i t y i n c a t a l y s i s by a l l oys , Catal. Rev.-Sci. Eng., 11 H2S04, (1975) 41-70. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 469 OPTIMIZATION AND CHARACTERIZATION OF Pt-Fe ALLOYS SUPPORTED ON CHARCOAL P. FOUILLOUXI, D. GOUPILâ, B. BLANC2 and D. RICHARDâ IUnite Mixte CNRS-Rh6ne Poulenc, 24 Avenue Jean Jaures, 69151 Decines-Charpieu CBdex BP166 (France) 21nstitut de Recherche sur la Catalyse, 2 Avenue Einstein, 69626 Villeurbanne Cedex (France) SUMMARY made by means of an empirical method. The carbon support was washed, oxydized in a liquid phase and thermally desorbed before its impregnation. The metals precursors were reduced by hydrogen. The reduced metals were characterized by electron microscopy, magnetization measurements and X Ray diffraction. The two metals are alloyed under the form of finely divided particles. The charcoal supported Pt-Fe catalysts are very active and selective in hydrogenation of cinnamaldehyde to cinnamyl alcohol. The optimization of Pt-Fe catalysts supported on charcoal was INTRODUCTION The platinum-iron alloys described in this work were prepared for the selective hydrogenation of alpha-beta unsaturated aldehydes on the background of Adam*s work published on this topic (ref. 1). This author showed that an addition of FeC1, to an hydrogenation solution containing cinnamaldehyde was able to produce cinnamyl alcohol with a good selectivity. More recently (ref. 2), the addition of FeCl,, SnC1, and GeC1, to platinum catalysts has also proved to be effective in this hydrogenation. Under the reaction conditions, we donât know whether the cationic iron, added as a selectivity promoter, will be reduced or not to the metallic state in the presence of platinum particles and hydrogen pressure. This observation led us to experiment the properties of a bimetallic Pt-Fe under the form of an alloy if possible (ref. 3 ) . Charcoal was chosen as a support because of its good resistance to corrosion. Platinum-iron catalysts supported on carbon were first prepared by Bartholomew and Boudart (ref. 4). For this purpose, they oxidised the carbon surface by partial burning and then the carbon was impregnated by the metals salts. This gaseous phase oxidation is not easy to extrapolate to a larger scale and we describe here a safer method to preDare well dispersed Pt-Fe alloys supported on charcoal. 470 PREPARATION OF THE Pt-Fe/C CATALYSTS .Cinnamaldehyde hvdroaena tion as a auide for catalvs ts Drenaration During the hydrogenation of cinnamaldehyde, hydrogen can be added to carbon-carbon double bond or to the carbonyl group: Cinnam- aldehyde Cinnamyl alcohol A CHO A The reaction vessel was a stainless steel autoclave with a PTFE coating and a magnetically coupled stirrer. The solvent - a mixture of 50ml isopropanol, lOml water and 2.5ml of 0.1M NaOAc - was injected in the autoclave with 0.69 catalyst and preheated at the reaction temperature (60'C). Cinnamaldehyde was introduced into the catalyst suspension after the temperature had reached the desired value. Liquid samples were periodically withdrawn and analysed by gas chromatography in the aim to determine the initial hydrogenation rate V, and the selectivity in cinnamyl alcohol (ref. 3 ) . Pretreatment of the charcoaL A commercial activated carbon black (505 from Carbonisation et Charbons Actifs) with a high specific surface area was chosen. Originally it contains 11.5wt% ashes with a non negligible amount of iron (ref. 5). For this reason, we washed the crude charcoal with hydrochloric acid which is a good complexing agent for iron ions. Washing with HNO, gives similar results (Table I) but nitrous evolution is produced and we preferred HC1 washing. The second stage of the support pretreatment was an oxidation with sodium hypochlorite following a well known process (7). The charcoal is added slowly to a concentrated solution of hypochlorite. The oxidation reaction is very exothermic and the slurry must be cooled in ice. This oxidation produces superficial groups on the carbon surface which prevent the metallic precursor 471 salts from migrating during hydrogen reduction. This favours the formation of a more divided supported metal (ref. 4). TABLE I Composition (wt%) of the charcoal before and after washing. Element Non washed Charcoal Charcoal Charcoal charcoal washed washed washed HCL 2N HNO, 2N NaClO Ashes 11.5 Fe 0.121 P 2.18 Ca 1.36 K 1.008 sio, 5.852 2.35 0.02 0.074 0.096 0.465 1.484 2.2 0.054 0.032 - 3.15 0.056 0.1 - ImDreanation and reduction of the Pt-Fe/ catalvsts The oxidized charcoal is filtered, dried at 120'C in an oven and finally desorbed at 430'C in flowing nitrogen. The pretreated TABLE I1 Optimization of the preparation parameters on a 30at% Pt-Fe/C. Drying of impregnated charcoal Reduct ion V,.103(mole.mn-1.g-1) Undr lh 2h 2H 15h lh lh lh 15h 15h 15h .ied He He He He He He He He He He 120'c 120'c 120'c 120'c 12O0C 120-c 12O0C 12O0C 12O0C 300'C H, 430°C H, 430'C H, 430'C H, 430'C H, 430°C H, 350°C H, 430'C H, 500°C 0.6 1.4 1.5 1.0 3.5 1.1 1.4 2.0 6h H, 500'C 1.6 6h H, 430'C 3.2 6h H, 400'C 3.3 472 0 50 1 100 kt 150 Figure 1. Surface distribution of pure platinum particles. 5.0 2.5 0 0 25 50 75 Particles diameter (A) 200 Particles diameter ( A j Figure 2. Surface distribution of Pt-Fe/C with 45.5at% Fe. 473 carbon powder is then put into a solution of H,PtCl, and Fe(NO,):,, n H,O in a mixture of benzene and ethanol. The liquid of the slurry is removed under vacuum in a rotatory evaporator. We determined the best conditions for the catalyst preparation on the background of activity measurements (Table 11). A mid iron content (30at%) in the Pt-Fe catalyst was taken to exemplify the influence of preparation parameters. Before reduction, the powder must be dried at low temperature under neutral gas flow to remove the remainder of the solvent and the catalytic activities of table I1 show that the best conditions are a heating temperature of 120'C during 15 hours. The most effective catalyst was obtained after a reduction in hydrogen at 430'C during 6 hours. This last process was followed for the preparation of all the platinum-iron catalysts. We prepared catalysts of a total metal loading of 5% and with iron to total metal percentages varying by steps of 10at% from 10 to 70at% . The most interesting compositions for the activity and the selectivity in hydrogenation of cingamaldehyde lying between 10 and 50 at%, we devoted our whole attention to this composition range. CHARACTERIZATION OF THE Pt-Fe/C CATALYSTS Metallic disDersion of the catalvsts The high surface area of the charcoal used in this work prevented us from using gas chemisorption to determine metallic dispersion of our catalysts. We used an electron microscope (Jeol JEM 100 CX) to investigate the geometric appearance of the particles. At first sight, the micrographs show that in the pure platinum catalyst the metal crystallites form two populations whereas in the bimetallic preparations the size is much more uniform. A statistical determination of particle size was made from electron micrographs. The results of these measurements are plotted on figure 1 for the monometallic supported platinum powder and on figure 2 for a bimetallic containing 45.5at% iron. The size distribution diagramms confirm that the pure platinum particles have a large binodal size distribution around 90 and 200A. The particles diameter for the bimetallic is narrower and lies about 25A. A third determination for a powder containing 18.7at% iron showed that the distribution presents a unique maximum at 45A. Thus the presence of iron in the bimetallic catalyst plays a 474 promoting role on the metallic dispersion and on the homogeneity of the particle size. Overall and local comvosition of the catalvsts Our catalysts were analysed by chemical means which give the overall composition. The sample were dissolved in aqua regia and the support was eliminated by concentrated nitric and perchloric acids. The elements were then titrated in solution by classical means. The local analysis is much more interesting to see whether the distribution of the elements in the catalyst grain is homogeneous or not. We performed these measures by means of X-Ray emission spectroscopy in a STEM apparatus (Vacuum Generator HB5) where the excited area has a diameter of 10 angstroms. It is possible to analyse the tiny metal particles individually. TABLE I11 Overall composition of two Pt-Fe catalysts and composition of metallic particles of different size (compositions in at%). Analyzed Chemical analysis X Ray emission analysis area or particle Pt Fe Pt Fe Catalyst I overall 20A part. 30A part. 60A part. Catalyst11 overall 20A part. 30A part. 50A part. 81.3 18.7 78 77 84 82 54.5 45.5 52 53 54 57 22 23 16 18 48 47 46 43 The results of table I11 allow us to compare the data obtained by chemical analysis to those given by X Ray emisssion for global compositions. The agreement between the two methods is satisfying. The physical method is able to go further in the knowledge of the samples: it detects a uniform composition for a given bimetallic catalyst - within the precision limits - whatever the composition and the metal particle size. This allows to rule out a surface enrichment in one of the two metals because it should involve a parallel enrichment for the small particles. 476 200 150 100 50 - Pâ (m,.) After ref.ll (0) After ref.12 (0) After ref.13 z 3 (0) Our results . ,r . ./ I= .3 0 /. .iâ 3â - 3 s-. m a, e F 0 .3 - 5 Fe (at%) U v) m - ;--ge,3 /d 0 âo oo o s d , 0 20 40 60 80 100 Figure 5. Saturation magnetization of the reduced catalyst as a function of its composition. Fe composition (at%) Figure 6. curie temperature of the reduced catalysts in function of their composition. 477 Crvstal structure of Pt-Fe particles X Ray diffraction was performed on three samples by Debye- Scherrer method in a controlled atmosphere camera. The powder was prereduced in situ under hydrogen before each experiment. The diffraction pattern presented a background due to the charcoal and additional enlarged lines produced by the metallic particles. None of them were those of pure platinum or pure iron. The diffraction diagram was in agreement with a cfc structure. The calculated lattice parameters were plotted in figure 3 and compared with those of bulk alloys (ref. 8) of the same composition. The agreement was considered as satisfying. This gives us a first proof that the supported metal should be under the form of an alloy. The electron beam wasfocussedon individual particles of the bimetallic catalysts. We obtained diffraction patterns of monocrystals which is a proof that each metallic particle is a monocrystal but we were not able to have a sufficient precision on the lattice spacings to make a difference between pure platinum and bimetallic particles. Maanetic measurements The first study of Pt-Fe/C by magnetic measurements was made by Bartholomew and Boudart (ref.9). In a first stage we used saturation magnetization measurements after heating the supported catalysts precursors after heating under hydrogen to precise the reduction conditions. The results are plotted on figure 4: the sample shows a diamagnetic behaviour of the carbon support before hydrogen reduction (negative slope of magnetization curve). Heating the sample by steps produces an increase in magnetization up to 400'C where the sample is completely reduced. A further exposition to air lowers the magnetisation but rereduction is performed at low temperature. This phenomenon was also observed by other authors with Pt-Fe/SiO, (ref.9-10). The magnetization data obtained on reduced samples proved to be very similar to those of bulk alloys (figure 5) . The second type of magnetic investigation was made by Curie temperature determinations in a thermomagnetic balance (ref.11). The Curie temperatures of the catalysts were also very near to those of bulk alloys (figure 6). DISCUSSION-CONCLUSION The Pt-Fe/C catalysts prepared in this work were optimized due to a test system. An iron content of 20at% gives catalyst activities 100 times higher than pure platinum and selectivities 478 of 90% in our test reaction (ref.3). The preparation method is very convenient and easy to perform with a very simple equipment. characterization of the most effective bimetallics by various physical methods leads us to the conclusion that platinum is alloyed to iron to form monocrystalline particles. Iron plays the role of a texture promoter. Platinum promotes the reduction of iron to metallic state during the initial preparation and after subsequent exposition to air so that the alloy could be formed by simple action of hydrogen at low temperature before hydrogenation reactions. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ~~ ~ W.F. Tuley and R. Adams, J.A.C.S.,45(1925)3061. S . Galvagno, A. Donato, G. Neri, R. Pietropaolo and D. Pietropaolo, J. Mol. Catal., 49(1989)223-32. D. Goupil, P. Fouilloux and R. Maurel, React. Kinet. Catal. Lett., 35(1987)185-193. C.H. Bartholomew and M. Boudart, J. of Cata1.,25(1972)173. D. Goupil, These de Doctorat Universite de Lyon, "9086, (1986). D. Richard and P. Gallezot,in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Eds.), Preparation of catalysts, Elsevier, Amsterdam, 1987, pp.71-81. J.B. DONNET, F. HUEBER, C. REITZER, J. ODOUX and G. RIESS, Bull. SOC. Chim. France, (1962)1727-35. W.B. Pearson, Handbook of spacings and Structures of metals and alloys, Pergamon Press, (1964)651. C.H. Bartholomew and M. Boudart, J. of Cata1.,29(1973)278 L. Guczi, Catal. Rew. Sci. Eng., 23(1981)329. M. Fallot, Ann. Phys., 10(1938)291. G.E. Bacon and J. Crangle, Proc. Roy. SOC., A272(1963)387. J. Crangle and W.R. Scott, J. of Appl. Phys.,36(1965)921. A. Kussmann and G. Rittberg, Z. Meta11.,41(1950)470. V. Perrichon, J.P. Candy and P. Fouilloux,,Progress in vacuum microbalance techniques, Heyden and Sons, London, 3(1975)18. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 479 SUPPORTED METALLIC CATALYSTS ACHIEVED THROUGH GRAPHITE INTERCALATION COMPOUNDS F. BEGUINl, A. MESSAOUDIl, A. CHAFIK2, J. BARRAULT2 and R. ERRE' 'C.R.S.O.C.I. - C.N.R.S., 1B rue de la FCrollerie, 45071 OrlCans CCdex 02, France. 2Catalyse en Chimie Organique, UnkersitC de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers CCdex, France. SUMMARY New preparations of supported metallic catalysts (Fe, Co, Ni) using a graphite intercalation compound as precursor were described. The graphite-MClx derivatives were reduced by otassium and metallic particles were obtained within a graphite matrix. The reduction ot!a MCl, salt or the decomposition of a metallocene lead to the formation of clusters at the edge of the graphite flakes. Only graphite samples with low specific area were used to obtain the intercalated precursors . Consequently, these materials do not present a very high activity in the hydrogenation of carbon monoxide. However, a marked selectivity to alkenes formation was observed with the graphite supports of the largest specific area. INTRODUCTION The research and the preparation of heterogeneous catalysts were largely developed during the last years. We present here the results of studies for the valorization of C1 molecules (carbon monoxide, methane ...) and the development of new processes in fine chemistry. Initially, catalysts were not specifically designed with respect to the complex reactions to which they were applied or as regards high selectivity. Recently, significant advances were obtained in preparing largely multifunctional catalysts, e.g. useful in the selective conversion of syngas to light olefins or alcohols or in hydrofunctionalization reactions, for which several kinds of reactional centers are necessary in the catalyst. For applications in fine chemistry processes, one must develop new selective catalysts taking into account the particular conditions of their use : presence of solvent, of heteroatoms (0, N, S, halogen ...) in the reagents and consequently a possible formation of products (H20, NH3, H2S, HX ...) which are well known to modify the classical catalysts. For these reasons,we were interested in a new synthesis of metals dispersed on carbon. Indeed, this support allows us to obtain high-performance catalysts selective either in gas or liquid phases. For the preparation of such catalysts one must take into account the classical concepts of heterogeneous catalysis : geometric and electronic effects, SMSI, effect of a promoting agent ... Inhomogeneous M catalysts are generally obtained by impregnation of a support with a metallic salt dissolved in aqueous solution, then reduced by hydrogen. With 480 graphite as support, a homogeneous dispersion of the metal could be obtained from its intercalation compound, as stated by Volpin (ref.1) to occur with the reduction of the binary graphite MC?, compounds (where M is a transition metal). Braga (ref.2) later claimed that he obtained intercalated metals by reduction of a metallic halide MCl, with the binary Kc8. More recently, Inagaki (ref.3) repeated Braga's work with CoC12 and found that the reaction gives a ferromagnetic ternary graphite-Co-THF compound. However, some authors claimed that whatever the process, a part of the metal is included and not intercalated (ref.4,S). In this paper, we corroborate that, starting either from donor of from acceptor GIC, reduction only yields metallic particles, even under mild conditions. Accordingly, processes based on the chemical reduction of acceptor graphite-MCl, compounds or on the reaction of KC8 with metallic halides or metallocenes constitue a good method for the synthesis of metallic catalysts supported on graphite. These new compounds were tested in the hydrogenation of carbon monoxide. RESULTS - DISCUSSION Reaction of graphite MC1, - (M = Ni,Co.Fe) with metallic potassium Second stage G-MCl, binaries were prepared from the direct reaction of a metallic chloride (FeC13, CoC12, NiC12) with natural graphite under a pressure of chlorine (ref. 6). The G-MCl, compound was then allowed to react with potassium under vacuum at 300°C for about two days. According to some authors, the alkali metal can intercalate the second stage acceptor compound to give a heterostructure (ref.7, 8). We have however found that MC!, and K, located in adjacent domains, are very reactive even at this relatively low temperature, and that the following reaction occurs spontaneously : The X-ray diffraction diagrams mainly show two sets of lines due to KCl and metallic clusters. After washing with a water/ethanol mixture (l/l), the lines of KC1 are still present, indicating that the products of the reaction are included in the graphite matrix. X.P.S. analysis shows that the main contribution of the 2p core level of the M element is due to a metallic state M" (80%). Compared to the chemical analysis (Table l), the C/M ratio obtained by X.P.S. shows a very weak concentration of the element M at the surface of graphite except with the G - FeC13 sample in which a part of the iron has migrated to the surface during the reduction process. In all cases, chemical analysis and X.P.S. reveal a K/C1 ratio greater that one due to the intercalation of excess potassium in the graphite freed by the reaction. Transmission electron microscopy shows a rather narrow and homogeneous distribution of particles sizes (about 30 nm). After exposure to air, the metal in only slightly oxidized, indicating that the particles are protected by the lattice or/and covered by the KC1 produced during the reaction. 481 TABLE 1 Atomic ratios given by chemical analysis (C.A.) and X.P.S. on the products found after reduction of G-CoC12, G-NiC12, G-FeC13 by potassium at 300" C. c / c 1 K/C1 C/M Sample C.A. X.P.S. C.A. X.P.S. C.A. X.P.S. G-CoC12 3.99 3.3 1.3 1.9 8.1 44.8 G-NiC12 5.09 1.6 1.4 1.4 10.6 120.0 G-FeC13 5.33 1.1 1.2 1.1 12.2 10.5 For catalytic applications, an exfoliation of the host lattice would be essential. To explain this reaction, we propose the following mechanism : the first step is the formation of a biintercalation compound with distinct islands of K and MCl,, since, in the pleated layered structure, the reagents occupy all the interlayered spaces of the host lattice ; then, even at low temperature, the reagents can react together to give metal clusters. Reduction of metallic halides bv K Q - The KC8 binary is prepared by the direct reaction of potassium on graphite under vacuum (ref. 9). Due to the electronic transfer to the graphene plane, this compound is a strong reducing agent. If it is allowed to react with a metallic halide MClX (M = Fe,Co,Ni) dissolved in tetrahydrofuran (THF), the reaction occurs spontaneously at room temperature and leads to the formation of metal M" dispersed on graphite : Three phases were identified by X-ray diffraction : graphite, KCI and the M" metallic species. The X.P.S. spectra of the reaction product show that the 2p core level of M may be attributed essentially to a metallic state (70 to 80%). However X.P.S. analysis reveals a strong concentration of the elements which constitue the KX salt on the surface. In Table 2, the atomic ratios given by chemical analysis are compared to the quantitative results deduced from X.P.S. spectra. The C/K ratio obtained by X.P.S. is always lower than the one given by chemical analysis. The K/X ratio is close to 1, confirming the existence of KCl. The KCl can be completely eliminated by washing with a carefully degased water/ethanol (1/1) mixture, but this treatment which can be used with Ni or Co, is not applicable to the iron derivatives as the metal reacts to give an oxihydroxide FeOOH which transforms to Fe2O3 under vacuum. All three M species are oxidized after exposure to air, proving that the metallic clusters are easily attacked by any reagent. Moreover transmission electronic micrographs show that the metallic particles are localized at the edge of the graphite flakes. 482 TABLE 2 Atomic ratios given by chemical analysis (C.A.) and X.P.S. for the products formed by reduction of some salts in THF solution by Kc8. Salt C/K K/X C/M C.A. X.P.S. C.A. X.P.S. C.A. X.P.S. coc12 7.9 1.9 1.0 1.1 14.0 15.9 NiBr2 8.5 1.5 0.8 1.3 12.1 8.0 FeC12 9.6 1.0 1.1 1.1 15.8 16.0 FeC13 18.6 1.0 0.8 1.0 36.4 12.0 DecomDosition of a metallocene bv KC8 The main disadvantage of the previous reaction (2) is the simultaneous formation of KCl which partly inhibits the properties of the metallic element. We have found that under appropriate conditions, the reaction of KC8 with metallocenes (Fe(C5Hg)z and Ni (CgH5)2) dissolved in dimethoxyethane (DME) gives soluble by-products, permitting the preparation of "fine M / C by the following reaction : - The results of chemical analyses are given in Table 3. For the reaction with Fe(CgHg)2, chemical analysis shows an important concentration of residual potassium. In the case of nickelocene, only traces of potassium were detected and two phases were observed by X-ray diffraction : pure metallic nickel Ni" and graphite. TABLE 3 Chemical analysis of the products obtained by the decomposition of metallocenes in DME by KC8. Metallocene T C C K M H X Global Formula 20 69.9 1.1 15.2 1.2 87.4 C22.5Ko.iNilH4.5 60 63.2 1.5 19.4 1.7 85.8 C25.9K0.1NiiHg.i Ni(C5H5)2 Fe(C5H5)2 60 71.0 9.1 10.5 0.5 91.1 C31Ki.2FeiH2.7 483 The intensity and the binding energy of the 2p core level of Ni confirm the metallic state. With ferrocene, reaction (3) is not complete because this molecule is stabler than nikelocene : X-ray diffraction clearly proves this results, with small peaks due to metallic iron and others attributed to a high stage intercalation compound. Due to its aromatic character, ferrocene probably first intercalates and reacts in the interlayer space. We tested this hypothesis by studying the direct reaction of KC8 with Fe(CgHg)2, with no solvent. Chemical analysis gives a global formula C28.3K2.3FeH10, equivalent to C ~ ~ . ~ K ~ J ( F ~ C ~ O H ~ O ) . Thus, approximately two KCs molecules react with one Fe(CgHg)2. X.P.S. analysis shows that the iron particles are essentially in the metallic state Fe" : the total decomposition of ferrocene by KCg would occur along the following reaction path : and the 001 X-ray lines with an identity period I, = 12.3 A could correspond to a second stage K2(CgHg)2C16 ternary phase with iron supported by the graphite matrix. At present, we are however unable to explain the exact nature of the intercalated compound found which may contain a polymerization product of the cyclopentadienyl groups. Hvdrogenation of carbon monoxide For better appreciation of the performances, the new catalysts were tested in the hydrogenation of carbon monoxide. In view of the results presented above, and taking into account the size and distribution of the metallic particles as determined by X-ray diffraction and Transmission Electron Microscopy (ref. lo), only the Co products prepared according to reaction (2) were studied. In fact, the best results were obtained with cobalt using the impregnation method (ref. 11). The distribution of the metal and the occlusion and migration of superficial species, as well as their reduction will greatly depend on the nature of the carbon used for the preparation of KCg. To prepare the supported cobalt, three different carbon materials were chosen : - Ceylon graphite (O.3m2g-') - Graphitized carbon black (Le Carbone Lorraine, Srn2.g-l) - Lonza graphite (reference HSAG, 300m2.gb1) The results are given in table 4. The nature of the carbonaceous support and probably its specific area have a determining effect on the catalytic properties of these solids. In the same experimental conditions, the catalyst from Lonza graphite is 60 times more active than the catalyst from Ceylon graphite. Moreover, the selectivity of Co/Lonza graphite is very different from the other solids as it gives a large amount of hydrocarbons and particularly olefins. 484 TABLE 4 Influence of the carbonaceous support on the properties of the Co/C catalysts used in the hydrogenation of carbon monoxide. P = 1 bar, H2/Co = 1, Total flow 3.6 1.h-I. The solids are reduced by hydrogen at 400°C before the reaction. Support Ceylon Graphitized Lo ma Graphite graphite carbon black Reaction temperature Activity (x ld) mole h-l g-lCat Selectivity (%) co2 CHq c2Hq c2Hg c3Hg (âC) c3H8 C4H8 C4H10 c 6 - c10 C5H10 300 300 0.08 1 100 92 5.1 1.6 1.2 280 1.8 28.7 21.3 4.3 1.2 8.1 5.6 0.7 6.0 24.0 300 5 Selectivity comparable to 280°C This behaviour is in good agreement with the structural observations on Co/Lonza graphite indicating well distributed metallic particles localized at the edge of the graphite flakes (ref. 10). With the other supports and owing to the lower specific area, the number of reactional centers at the edge of the flakes is smaller and the particle size larger than in the case of Lorna graphite. A comparison with the selectivity of a catalyst prepared by impregnation of a Lonza graphite with cobalt nitrate and reduced by hydrogen is given in table 5, its total activity being 8.6 x l o 3 mole.h-l.G-l cat. TABLE 5 Properties of a Co catalyst prepared by impregnation of Lorna graphite with cobalt nitrate and reduced by hydrogen at 410°C. Co 5% / Lonza graphite HSAG 300 (3O0m2.g-l) Temperature of the reaction 240°C. Selectivity c o 2 CHq c 2 c 3 c4 c 5 - c 9 (%> 8.5 53 5 5.6 5.5 21.5 485 Taking into account the differences in the temperatures of reaction, it appears that the impregnated catalyst is 20 to 50 times more active than the solid prepared from KC8. Moreover, our catalyst gives a larger amount of superior hydrocarbons. This behaviour could be due to the presence of potassium as many studies show that an alkali element favors chain growth (ref. 12). Nevertheless this catalyst is particularly selective for alkenes formation (= 30%). CONCLUSION The three reactions studied in this paper always lead to metallic clusters supported by the graphite matrix in the conditions of our experiments. Depending on the initial binary, the particles are either at the surface or included.Best activity was observed with the catalysts prepared from KC8. However, for the materials prepared from Graphite-MCI,, improved activity could be obtained after previous exfoliation of the graphitic matrix. The relatively low activity of supported cobalt obtained from an intercalation compound could be due to the low specific area of the support or the presence of KCl. Some trials on washed samples or on specimens prepared as in reaction (3) could give more information. The most striking fact with our products a particular selectivity to alkene formation with a support of relatively high specific area. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 M.E. Volpin, Yu.N. Novikov, N.D. Lapkina, V.I.Kasatochkin, Yu.T. Struchkov, M.E. Kazakov, R.A. Stukan, V.A.Povitskij, Yu.S. Karimov and A.V. Zvarikina, J. Amer. Chem. SOC., 97 : 12 (1975) 3366. P. Braga, A. Ri amonti, D. Savoia, C. Trombini and A. Umani-Ronchi, J.C.S. Chem. Comm., 8978) 927. M. Inagaki, Y . Shiwashi and Y. Maeda, J. Chim. Phys., 81 (1984) 847. G. Bewer, N. Wichmann and H.P. Boehm, Mat. Sci. and Eng., 31 (1977) 73-76. H. Schafer-Stahl, J.C.S. Dalton (1981) 328. S. Flandrois, J.M. Masson, J.C. Rouillon, J. Gaultier and C. Hauw, Synth. Met., 3 (1981) 1. G. Furdin, L. Hachim, D. GuCrard, A. HCrold, C.R. Acad. Sci., 3 0 1 (1985) 579. R. Erre, F. Bkguin, D. GuCrard, S. Flandrois, Proc. 4th International Carbon Conference, Baden-Baden (1986 516. A. HCrold, Bull. SOC. Chim. Fr., t1955) 999. F. BCguin, A. Messaoudi and R. Erre, submitted to Carbon and A. Messaoudi, Ph. D. Thesis, OrlCans, France (1989). A. Chafik, Ph. D. Thesis, Poitiers, France (1988). J. Abbot, N.J. Clark and B.G. Baker, Appl. Cat., 26 (1986) 141. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 487 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands PREPARATION OF GRAPHITE-IRON-POTASSIUM CATALYSTS FOR AMMONIA SYNTHESIS K. KALUCKI and A.W.MORAWSK1 Department of Foundations of Chemical Technology, Institute of Chemical Technology, Technical University of Szczecin, ul. Pulaskiego 10, 70-322 Szczecin (Poland) SUMMARY The preparation of graphite-Fe-K catalysts for ammonia synthesis was described. The process was realized in steps: 1) intercalation of iron (111) chloride into graphite; 2) reduction of obtained FeC13-GIC; 3) re- moval of a-iron and 4) intercalation of potassium into Fe-graphite pre- cursor. The influence of both iron and potassium on activity in ammonia synthesis was studied. Preparation o f catalyst in a large scale has been performed. The preparation method for these catalysts is proposed. INTRODUCTION Trying to prepare very active catalysts, many attempts have been devoted to the systems derived from so-called graphite intercalation compounds (GICs) (refs. 1-4). 2 Graphite possesses a layered structure, consisting of planes of sp - -hybridized benzenoid carbon. The weak 1-bonds between the carbon planes can easily be broken to allow insertion of guest atomic or molecular layers between them without disrupting the layered topology of graphite. In chemical sense, the intercalation of graphite is analogous to the for- mation of an "infinite" ion of aromatic molecules. The distance between the carbon layers is increased by the thickness of the intercalated layer. The catalytic formation of ammonia from hydrogen and nitrogen over alkali metal-graphite-transition metal chloride complexes has been discovered by Ichikawa et al. (ref.5). Several studies on the formation of ammonia on GICs catalysts have been carried out in recent years (refs. 6-10). The above mentioned studies proved that the FeCl GlCs are good precursors to highly active catalysts made by double step reduction followed by HC1 treatment. In spite of considerable amount of work performed in this field, the identity of the catalytically active species has not yet been established. The created iron is rather Fe(0) produced by reduction of Fe(I1) with potassium and located in the graphite matrix. Such active iron is not readily accessible to the HC1 (ref. 10). 3- 488 In the present work we report experimental data from studies on the influence of both the iron and the potassium contents in catalyst on activity in ammonia synthesis. We also performed the preparation of catalyst in large laboratory scale (ca. 100 g). As a consequence of this research we have proposed the technology of pre- paration of graphite-fe-K catalyst for ammonia synthesis. EXPERIMENTAL Catalyst preparation The FeC13-GICs samples were prepared according to the commonly used method. Powdered natural Sri Lanka graphite with a particle size of 1-20 microns in thickness and 30-100 microns in width was used in this study (Fig.1.). The mixture of powdered graphite and pure anhydrous ferric chloride (Riedel) was heated at a temperature of 300 OC for 24 h. The in- tercalation reaction was carried out with different amounts o t FeC13, controlled by means of the ratio of solid graphite to solid FeC13 in starting mixture (ref.11). Fig.1. Photomicrograph of natural Sri Lanka graphite. The products of intercalation were freed from excess metal chloride by washing with an aqueous solution of HCl(l:l), filtering, washing with distilled water, and drying overnight at 110 OC. The reduced samples of precursors were obtained by polythermal reduction of the GICs with a mixture of nitrogen and hydrogen (1:3) under atmospheric pressure at temperatures from 150 OC to 300 OC with a 489 heating step of 25 OC per 24 h, and from step of 50 OC per 24 h followed by further reduction at 625 OC for 5 days with a space velocity of gases of ca. 1000 h-l (ref.12). 300 OC to 625 OC with a heating The a-iron which partially appeared during the reduction process was removed by an aqueous solution of HCl(1:l). Sa_mpl_es _characteri Eat i on Both the precursors and the catalysts have been characterized by X-ray fluorescence spectroscopy (XRFS), Mu), and scanning electron micro- scopy (SEM) techniques. The following apparatus was used for the measurements: X-ray fluore- scence spectrometer VRA-30 (GDR) for XRFS, Universal Roentgen-Diffracto- meter HZG-4 (GDR) for XRD, and BS 300 scanning microscope (Czechoslo- vakia) for SEM. Catalyst- activities Both the activation of Fe-graphite precursors and the activity measurements were performed in flow reactor (Fig. 2). 8 Fig. 2. Presssure reactor for both activation and activity measurements. 1,7, - thermocouples; 2 - inlet gas; 3 - outlet gas; 4 - thermocouple wall 5 - precurosr of catalyst; 6 - potassium; 8 - position regulator; 9 - outlet to vacuum. The metallic potassium was introduced into the reduced precursors of GICs by vapour deposition at a temperature reaction of 350 O C under a pressure 490 of about 6 Pa. The potassium introduction process was controlled by the time of reaction. (s.v.) of 30000 h-'. RESULTS AND DISCUSSION The activity of samples was studied at 10 MPa with space velocity The results of both composition and activity of used precursors of catalysts are summarized in Table 1. TABLE 1 Composition and activity of used precursors of catalysts. t=350 OC; S.V. 30000 h-l; p = 10 MPa. Sample Denotation Cl/Fe C/Fe K/Fe X NH3 FeC13-GIC A 2.99 53.15 - - B 2.98 17.96 - - C 2.95 14.15 - - FeC 13-GIC Ax 0.37 53.15 - - (reduced) BR 0.32 18.97 - CR 0.30 14.54 - - - FeC13-GIC ART 0.70 80.28 - 0.03 (reduced, BRT 0.73 43.15 - 0.04 HC1 treated) CRT 0.73 33.20 - 0.04 - - - K-graphite 3.99 0.25 The polythermal reduction of FeCl3-G1C in a stream of 3H2+N2 which was proposed by the authors of this work, leads to formation of non-intercalated iron and the various phases of intercalated iron with mixing of the stages of iron-GICs (ref.12). during each treatment (intercalation, reduction and HC1 treatment of reduced samples) with intensity depending on iron concentration. The method of reduction prevented evaporation of FeC13-GIC but did not prevent partial de-intercalation of FeCl during reduction. The de-intercalated FeC13 was reduced to the a-Pe phase (Fig.3). The amount of a-Fe that collects on the surface of the carbon is dependent on the total iron concentration. The a-Pe consists of metal clusters which can be removed from the carbon surface by acid treatment (compare Fig. 3 and Fig.4 ). The reduction of FeCl -GIC was incomplete and ca. 0.7 - 0.3 chlorine atom per atom of iron remained (Tab. 1). The remaining iron-graphite precursors contain a phase probably with a mixture of stages that are temperature resistant up to 625 OC and are stable in aqueous HC1. presumably The free graphite phase was also present 3 3 Both a-iron supported on particles of graphite and iron encapsulated in graphite were practically inactive in ammonia synthesis reaction at 491 temperature of 350 OC (ref.13 and Tab. 1) even at presence of KON (ref . 13). Fig. 3. Photomicrograph of FeCI3- -GIC (1 stage ) reduced. Fig.4. Photomicrograph of FeC13- -GIC (1 stage) reduced and HC1 treated. The iron encapsulated in graphite can be a good precursor to produce highly active catalyst at mild conditions after activation with metallic potassium. The effect of composition on ammonia yield of studied catalysts is presented in Table 2. The influence of both the iron and the potassium contents on activity in ammonia synthesis was calculated using multiple regression. The response function is given as equation (1): y = 0.07502 x1 t 0.329993 x2 t where : y - represents yield of ammonia x - represents content wt.-X of 1 0.00378 x12 - 0.021718 x22 t 0 . 0 2 8 8 4 ~ ~ ~ ~ v01.-X at 35OoC, p=10 MPa,s.v. 30000 h-l iron in Fe-graphite precursor 492 x - represents molar ratio potassium per iron in catalyst 2 The data computed according to the relationship (1) are presented in Fig. 5. TABLE 2 Composition and activity of studied catalysts. 350 OC, s.v.30000 h-l,10 MPa Sample of catalyst Cl/Fe C/Fe C/K K/Fe X NH3 ~ ~ ~ ARTK (11.07 X K) 0.66 80.53 24.10 3.33 1.53 ARTK (14.85 X K) 0.68 80.44 17.20 4.60 1.70 ARTK (28.0 X K) 0.65 80.45 7.74 10.38 1.70 ARTK (37.8 X K) 0.68 80.51 4.95 16.20 1.65 BRTK ( 1.69 X K) 0.69 BRTK (13.53 X K) 0.65 BRTK (17.39 X K) 0.68 BRTK (25.51 % K) 0.69 BRTK (38.4 X K) 0.66 ~ ~~ CRTK ( 0.48 X K) 0.77 CRTK (19.16 X K) 0.75 CRTK (32.1 X K) 0.74 CRTK (39.0 % K) 0.75 43.29 43.22 43.27 43.19 43.59 32.9 33.03 32.99 34.65 164.3 18.02 13.41 8.23 4.11 557.6 11.38 5.70 4.41 0.26 2.39 3.23 5.24 10.59 0.06 2.90 5.76 7.85 0.68 2.19 2.50 2.89 3.08 0.55 3.23 4.17 4.22 Fig.5. Response plot of predicted activities of graphite-Fe-K catalysts. t = 350 OC; p = 10 m a ; S.V. 30000 h- . As it is shown in Fig. 5 the maximum of activity was reached at molar ratio K/Fe ca. 5 - 6. The further loading of potassium to Fe-graphite precursor did not influence the activity. 493 4 CL m 30' 20° 10° 28 rl 0 0 B E v) m 0 0 30' 20° loo 2e 70' 60' 50' 40' 30' 20° 28 goo 80° Pig.6. Diffractograms of samples in each step preparation. a) FeC13-GIC, MoK, radiation b) reduced FeC13-GIC, MoK radiation cj passivated graphite-Fe-potassiu catalyst, CoKa radiation a 494 In Figures 6a,6b,6c are given the diffractograms of samples at each step of catalyst preparation,in large laboratory scale (ca. 100 8). The starting intercalation compounds (Fig.6) forms mixture of FeC13-GIC 1 stage with dl = 936 pm and small amount of FeC12-GIC 1 stage with dl = 960 pm and free graphite. The total molar ratio C/Fe was 8.13. After reduction such FeC13-GIC (Fig. 6b) the dominant phases were Fe-GlC 1 stage with d = 581 p and graphite (d=335 pm). The total molar ratios were: C/Fe=8.07; Cl/Fe=0.88. 1 Pig. 7 . Photomicrograph of passivated graphite-Fe-K catalyst. The graphite-Pe-K catalyst (Fig. 6c) forms mixture of KC1, a-Fe. gra- phite and new periodic phase with d = 1224 p. The sandwiched structure of catalyst is very clearlydemonstrated in Fig. 7. 1 âhe described catalyst exhibits higher activity as compared to typical iron industrial catalysts (Fig. 8 ) particulary at lower temperatures. Proposed technology of preparation Based on the experimental work outlined above and our earlier works, an integrated preparation process is proposed. Procedure for preparation of paphite-Fe-K catalyst is illustrated by diagram given in Fig. 9. 495 1.3 1.5 1.7 1.9 1000/T Fig. 8 . Ln(k) versus temperature of graphite-Fe-K catalyst ( x ) and iron industrial catalyst ( 0 ) . Reaction rate constant âkâ in kgNH3MFâaoa5/(kgkat. h) . S.V. 100000 h-l, p = 10 MPa. intercalation of FeC13 into graphite 3 reduction of FeCl -graphite 1 I removal of a-iron I intercalation of potassium into Fe-graphite I 1 final catalyst Fig. 9. Proposed technology of preparation of graphite-Fe-K catalyst. CONCLUSIONS A novel graphite-Pe-potassium catalyst system can be made by intercalation of graphite. The iron encapsulated in graphite i s a good precursor to produce highly 496 active catalysts after activation with metallic potassium. The metallic potassium is capable of getting into the graphite matrix of Fe-graphite presursor and of creating a donor type compound. The presence of sandwiched phase has been found in catalyst. The described graphite-Fe-K catalyst exhibits higher activity as compared to typical iron industrial catalyst. As a consequence of our work, a method for preparing these catalysts is proposed. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 L.B.Ebert, J. Mol. Catal., 15 (1982) 275-296. M.A.M.Boersma, Catalytic properties of graphite intercalation compounds, in: J.J.Burton and R.L.Garten, Advanced Materials in Catalysis, Academic Press, New York, San Francisco, London 1977, pp. 67-99. W.Setton, Synth. Metals 23 (1988) 467-473. K.Aika, T.Yamaguchi and T.Onishi, Appl. Catal., 23 (1986) 129-137. M.Ichikawa, T.Kondo, K.Kawase, M.Sudo, T.Onishi and K.Tamaru, J.C.S.Chem.Comm., No 3 (1972) 176-177. K.Kalucki, W.Arabczyk and A.W.Morawski, Stud.Surf.Sci.Catal., 7 (1981) Ju.N.Novikov and M.E.Volpin, Physica B+C, 105 (1981) 471-477. K.Kalucki and A.W.Morawski, Graphite intercalation compounds of iron as catalysts for low temperature ammonia synthesis, in: Proc. Inter. Carbon Conference, Baden-Baden, FRG, June 30 - July 4, 1986, Arbeitskreis Kohlenstoff Der Deutchen Keramischen Gesellschaft E.V., K.Kalucki and A.W.Morawski, lron intercalated in graphite as catalysts for ammonia synthesis, in: Proc.Inter. Conference on Carbon, Newcastle upon Tyne, England, Sept. 18-23, 1988, Publlished by IOP Publishing A.W.Morawski, K.Kalucki, A.Pron, Z.Kucharski, M.tukasiak and J.Suwalski, Reactivityof Solids, 7 (1989) 199-205. K.Kalucki and A.W.Morawski, Reactivity of Solids, 4 (1987) 269-273. K.Kalucki and A.W.Morawski, Reactivity of Solids, 6 (1988) 29 - 38. K.Kafucki and A.W.Morawski, Synth. Metals, 34 (1990) 713-718. 1496-1497. 1986, Qp. 531-532. Ltd, 1988, pp. 215-217. G. Poncelet., P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 497 SYNTHESIS OF V-P-0 CATALYSTS FOR OXIDATION OF C4 HYDROCARBONS V.A. ZAZHIGALOV, G.A. KOMASHKO, A.I. PYATNITSKAYA, V.M. BELOUSOV, J.ST0CH' and J.HABER' L.V.Pisarzhevsky I n s t i t u t e of Physical Chemistry of t h e Ukrainian Academy of Sciences, Kiev-28 CUkraina, USSR> ' Ins t i tute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30-239 SUMMARY Based on t h e application of several techniques, t h e effect of preparat ion conditions on t h e ac t iv i ty and select ivi ty of promoted V-P-0 catalysts in t h e n-butane oxidation t o maleic anhydride is described. The type of solvent used, t h e t i m e of synthesis, conditions of shape-forming operation and composition of t h e gas mixture f o r act ivat ion are considered. INTRODUCTION In t h e synthesis of a V-P-0 c a t a l y s t , t h r e e basic s t e p s may be distinguished: - preparat ion of t h e precursor and its separat ion; - shape-forming operation; - act ivat ion of t h e precursor including its transformation in to t h e ac t ive component. Each of t h e m may influence the ca t a ly t i c properties of t he f i n a l c a t a l y s t . In t h e present paper t h e importance of parameters of t hese s t e p s in t h e synthesis of metal-promoted V-P-0 ca ta lys t s is discussed for the i r p roper t ies in par t ia l oxidation of C4 hydrocarbons. EXPERIMENTAL T h e V-P-0 c a t a l y s t w e r e synthesized by the reaction b e t w e e n V205 and H3P04 dissolved in butanol E l l , which w a s carr ied o u t in a 500 dm3 autoclave equipped with a s t i r r e r . The progress in t h e reaction w a s followed by extract ing samples of t h e react ing mixture s m a l l enough (200 cm > not t o pe r tu rb its composition. The product of t h e react ion w a s f i l t e red , dried in vacuum and heated a t 350OC. The solid obtained w a s pressed, ground and sieved, t h e f rac t ion 0.25-0.50 mm being taken f o r catalyt ic s tudies . 3 498 In order t o study t h e influence of t h e shape-forming s tep , two methods of shaping w e r e used pressing of t h e powder with a r o t a r y p re s s PTM-41 or extruding of t h e w e t pulp. Tablets with 5 = 4.8 mm and L = 5-6 nun or rings with t h e ou te r R = 4.8 nun and inner R = 1.2 nun w e r e obtained. The activation s t e p w a s studied with t ab le t s obtained by extruding. They w e r e activated by heating in t h e stream of gas m i x t u r e s of different compositions in t h e conditions near t o those used in t h e catalytic experiments and then rapidly cooled t o room temperature in helium. C a t a l y t i c properties w e r e determined in a quartz flow reac tor of t h e Temkina-Kulkova type, and t h e variations of these properties in t h e course of t h e reaction w e r e followed in the pulse reac tor 121. The reacting mixture contained 1.45-1.50 vol.% of C4H10 in air. S t ruc tura l ana lys i s w a s carried out by X - r a y diffraction in t h e w a y described in C31. FT-IR spec t r a w e r e recorded with t h e Brucker IFS-113V spectrometer with t h e resolution of 4 cm-'. The samples w e r e pressed with KBr in to thin w a f e r s . D e t a i l s of t h e experimental procedure used in scanning electron microscope < S E M > and photoelectron spectroscopy CXPS> are described in 131. The pore s t r u c t u r e w a s determined with a Carlo Erba 2000 porosimeter. The thermogravimetric analysis w a s carried out with t h e help of a Paulik-Paulik-Erdey Q-1500 D derivatograph in t h e helium atmosphere. RESULTS AND DISCUSSION Precursor Eynthesis A f t e r mixing of all components in t h e autoclave, temperature w a s r ised t o 104OC, a t which t h e synthesis w a s carried out. Results of s t ruc tu ra l ana lys i s revealed t h a t after 3 hours of t h e synthesis a set of lines charac te r i s t ic f o r VOHP04.0.5H20 14,51 appeared < m o s t intense lines are given in Table I>. H o w e v e r , o ther lines a t 0.438, 0.340 and 0.288 nm with t h e in tes i ty r a t i o 1:0.85:0.63 w e r e also present. Since these values are near t o t h e reference values of V205 C61 they m a y be assigned t o V205. The in tens i t ies of t he VOHP04.0.5H20 lines in respec t t o t h e standard, increase with t i m e of t h e synthesis also w e a k 499 bands a t 505, 995 and 1025 cm-' are present . They can be assigned t o V205 171. A s indicated by t h e data given in Table 2, t h e catalysts obtained f r o m t h i s m a s s are not select ive in t h e n-butane oxidation. TABLE 1 X - r a y phase control of t h e c a t a l y s t synthesis Time of synthesis, hours 3 6 15 d , - I / Io , X d , nm I/Io, X d , nm I / I o , X __ -- - -- - 0.570 65 0.570 70 0.571 75 0.451 40 0.451 42 0.450 44 0.367 33 0.367 35 0.367 36 0.328 40 0.328 50 0.328 49 0.293 75 0.292 79 0 .293 84 The in tens i ty of 0.293 line charac te r ic t ic f o r t h e VOHP04.1/2H20 phase w a s taken as 100%. In order to confirm t h i s conclusion t h e following experiments w e r e set up. The sample of V205 was devided in to t h r e e d i f fe ren t granulometric fractions (wgt %> : A: 500 4u J. (arb.) A B ,n - 2 - - 1 -* 40 2- 0 11, 0 100 200 0 100 200 Fig.1. FEM microprobe depth profiles of t h e c1 b C L, mm V20s granule showing contents of V and P C2>. Synthesis 15 and 25 CB> hours, drying a t 380 K . heating 0 hrs a t 620 K and 690 K Cb>. obtained from t h e f rac t ion of low dispersion even after 50 min of synthesis show inferior catalytic activity. SEM microprobe study revealed CFig.1) t h a t large grains of V205 have non-uniform distribution of phosphorous along the i r cross-section. The surface is enriched w h i l e bulk is almost free of phosphorous. I t can be concluded t h a t V-P-0 compound is formed only in t h e surface layer whose thicknees grows with synthesis time. On heating, t he compound zone migrates toward a grain. liberating free,, amorphous V205 a t the surface, as it is evidenced by the FT-IR data. T h e process is accompanied by a deterioration of catalytic properties. According t o our data, pm. For grains of t h i s size, t h e synthesis t i m e after which an effective c a t a l y s t is obtained amounts t o 10-14 hours. For larger g r a i n s t h e t i m e is considerably longer. t he V205 grains should not be larger than 120-150 The e f f e c t of organic solvents on surface morfology of t h e precursor has been studied earlier 181. In t h e present study more solvents have been t e s t e d . For all t h e solvents a product w a s VOHP04.0 .5H20. In t he thermogram of t h i s compound K51 t w o endo-effects are present. The second one corresponds t o t h e formation of X-ray-amorphous 501 CVO>2P207 phase according t o t h e scheme: VOHP04.0.5H20 --> CVOHP04> --> CVO>2P207. The egzo-effect observed after an increase of t e m p e r a t u r e corresponds t o its crystal l izat ion. A s is s e e n i n Table 3. t h e t e m p e r a t u r e of t h e beginning of CYO>2P207 formation and crystal l izat ion are a f f e c t e d by t h e so lvent na ture . I t is worth t o note , that t h i s t e m p e r a t u r e is connected also with p r o p e r t i e s of t h e ca ta lys t : t h e higher t h e c rys ta l l iza t ion tempera ture . t h e higher t e m p e r a t u r e of t h e c a t a l y s t opera t ion and l o w e r its select ivi ty . A l m o s t all t h e c a t a l y s t s after work are composed of t h e CVO>2P207 phase with m o s t i n t e n s e l ine 0.313 nm. Exceptionally, c a t a l y s t s R-7 and R-8 have m o s t i n t e n s e l ines 0.387 and 0.313 (82 % and 50% respectively). TABLE 3 E f f e c t of s o l v e n t n a t u r e Sign Solvent TGA effect, OC C a t a l y s i s , T = 2.4 s endoth. egzoth . TR, OC X , % S(MA>, % R-1 i-C4H90H 375 R-2 2-CqH90H 390 R-3 n-C4H90H 380 R-4 t -C4H90H 420 R-5 n-C3H70H 385 R-6 i-C3H70H 412 R-7 i-CSHllOH 415 R-8 y-Butlac* 400 y-butyrolactone 480 500 480 530 470 515 525 500 415 430 410 465 415 435 465 440 91 90 88 79 93 91 92 92 65 60 64 59 62 55 53 56 C a t a l v s t shape-forming Tha data presented i n Table 4 show t h a t conditions of shaping affect t h e ca ta ly t ic ac t iv i ty . If powders are pressed as cylinders, t h e finished c a t a l y s t h a s t h e u n s a t i s f a c t o r y pore s t r u c t u r e (preparat ion No 1 and 2). Since t h e r e a c t i o n of butane oxidation r u n s in t h e diffusion-controlled range, t h e se lec t iv i ty for maleic anhydride is lowered. Calculations show that e f f e c t i v e depth of n-C4H10 penet ra t ion is I = 1.0-1.2 mm Csee also [91>. For t h i s reason t h e "death zone" can be produced E101, where t h e hydrocarbon concentrat ion is equal t o zero . An e f f e c t i v e depth of oxygen p e n e t r a t i o n is much l a r g e r (about 5 mm>. A s t h e consequence of t h a t 502 a two-phase s y s t e m is formed after t h e catalyst act ivat ion : t h e ou te r (surface ring> region containing 2P207 and t h e inner one with c-VOP04 CFig.2). The last compound is unwanted in t h e par t ia l oxidation of n-C4HI0 because it oxidizes a w a y maleic anhydride C31. TABLE 4 E f f e c t of t h e shape-forming on p r o p e r t i e s of t h e c a t a l y s t No Method Shape Additions Mechan. Pore volume Catalysis 4OO0C s t r e n Z V ( t ~ t > ~ V>5Onm T = 1 . 8 s 3â kg/cm c m /kg X < C 4 > ,% S C M A ) ,% 1) 2 > 1 A C 2 A C 3 A c 4 A D 5 A D G A C 7 A D 8 B C P B C 10 B c 12 B D 13 B D 14 B C 15 B C 11 B n graphi te graphi te w a t e r PEO PVA w a t e r PEO PVA 6N4 120 0.32 0.07 53 0.36 0.17 I7 0.34 0.24 105 0.38 0.10 38 0.36 0.21 61 0.34 0.14 46 0.35 0.19 43 0.28 0.18 52 0.32 0.20 50 0.35 0.23 64 0.30 0.12 69 0.28 0.15 67 0.33 0.17 23 0.38 0.29 19 0.44 0.37 86 82 85 84 85 83 82 80 83 79 81 81 80 54 58 42 51 60 59 64 53 64 66 65 66 70 67 6P 12 16 I> A - pressing, B - extruding, 2> C - flat cylinders, D - r ings 3> PEO - polyethylen oxide, PVA - polyvinyl alcohol F i g 2 Micrograph of the ca+alyst cross-section 503 If t h e forming pressure is lowered, quant i ty of t h e t r a n s p o r t pores increases (samples No 1-a>, t h e e f f e c t i v e depth of n-C4H10 penetrat ion also increases and select ivi ty becomes higher. But a t t h e s a m e t i m e a mechanical s t r e n g t h of t h e ca ta lys t is lowered, what is an unwanted e f f ec t . The inner region can be mechanically eliminated when flat cylinders are changed for pel le ts with w a l l height 1.8 mm +A1' 517.7 134.0 531.8 1 : 1.7 : 6.6 533.6 HZ 517.7 134.0 532.2 1 : 1.9 : 6.6 135.1 534.2 C4H10 C20>+A 517.7 134.0 532.0 1 : 1.8 : 6.5 534.2 A i r 517.7 133.8 531.6 1 : 1.8 : 7.2 O2 +N 517.9 134 531.6 1 : 1.8 : 6.8 519.1 533.5 533.4 Binding energy (BE> V2p 517.7 e V corresponds t o V. The second value for t h e 01s peak with higher BE is due t o sur face groups containing H or C . The precursor conten t 0.1 at%. "Conversion of n-butane < 60 %. A - a i r . Shape-forming by extrusion is t h e most prospective method. I t gives a product with la rger proportion of t r a n s p o r t pores which brings an increase of t h e selectivity. The catalyst formed as r ings 504 also in t h i s case shows b e t t e r catalytic properties, though such catalyst has lower content of pores with diameter > 50 nm, because of s m a l l e r addition of w a t e r during the forming operation. An addition of polyethylenoxide CPEO) and polyvinyl alcohol does not improve the properties with an exception of s m a l l increase in porosity of t he tablets. U r e a and urotropin (samples No 14 and 15) increase significantly the content of t ransport pores,, however the catalytic properties of t he samples remain unsatisfactory. This may be caused by t h e presence of nitrogen-containing groups (effect of NH3 l ike C111> or a f i l m of coke deposite. The XPS study revealed both the presence of NHx or NO groups and significantly higher content of surface carbon. s C a t a l v s t activation The effect of t h e composition of activating gas m i x t u r e s on the phase composition of t he catalyst and their catalytic properties has already been considered in our earlier paper C121. It w a s shown t h a t t h e presence of a reducing agent (including the reaction product) leeds t o formation of a metallic phase of a promotor which in tu rn is adversly affecting the process of n-C4H10 partial oxidation. A reduction of t he promotor t a k e s place without change of t he vanadium C+4> valency, as i t w a s proved by XPS (Table 5). An excess of oxidant during the activation leeds t o oxidation of vanadium also without change of t he promotor valency x f 3 0 m v1 Fig.3. R a t e of butane oxidation (1, and s e l e c t i v i t y t o m a l e i c anhydride C 2 > as a function of WO>2P207 concentration 505 B e t t e r r e s u l t s w e r e obtained by ac t iva t ion with r e a c t i v e mixtures; the resu l t ing c a t a l y s t contained 2P207 formation f r o m VOHP04.1/2H20 i n the precursor . Promotor phosphate is already p r e s e n t i n t h e in i t ia l c a t a l y s t . Fig.3 shows t h e dependence between t h e rate of n-butane oxidation, t h e se lec t iv i ty of m a l e i c anhydride formation (pulse technique, 2-nd pulse) and t h e 2Fâ207 conten t . A signif icant se lec t iv i ty growth corresponding t o t h e increase of t h i s phase c o n t e n t is c l e a r l y visible TABLE 6 E f f e c t of t h e n-butane c o n t e n t on t h e rate of CVO>2P207 f o r m a t i o n P 0 after ac t iva t ion , % 2 2 7 Butane v0l.X 6 h r s 24 h r s 48 h r s 96 h r s 150 h r s 2.1 55 73 a6 92 100 1.7 53 72 82 88 93 1.5 50 74 ao 90 95 1 . 1 30 48 58 70 83 A possible oxidation of l o w e r c a t a l y s t zone i n t h e fixed-bed r e a c t o r w a s controlled by t h e s tudy of a n effect of l o w butane concent ra t ions on t h e vanadium valency during t h e act ivat ion. The experiments w e r e performed i n t h e d i f f e r e n t i a l reactor. From t h e d a t a i n Table 7 it is s e e n t h a t C4HI0 concent ra t ion should n o t be lower t h a n 0.5 vol.% and should be i n t h e range 0.7-0.55 v01.Z. This condition determines t h e ul t imate conversion of a hydrocarbon during t h e ac t iva t ion by t h e reactive mixture. TABLE 7 E f f e c t of t h e n-butane c o n t e n t on t h e s u r f a c e vanadium state Binding energ ies of V2p3/2 photoelectrons i n eV ~ ~~ Butane i n a i r , Vol.% Time of t h e a c t i v a t i o n , hours 24 72 96 1.5 0.7 0.5 0.3 0.15 517.7 517.6 517.7 517.7 517.7 517.8 517.7 517.8 517.8 517.9 597.6 517.6 519.0 517.7, 519.0 517.7, 519.0 506 Careful consideration of each s t a g e of t h e s y n t h e s i s creates t h e o p p o r t u n i t y to select conditions of the preparation of h i g h l y - e f f e c t i v e c a t a l y s t s for the oxidation of n-butane to m a l e i c a n h y d r i d e . REFERENCES I 2 3 4 5 6 7 8. 9. 10 11. 12 V.A.Zazhigalov, and V.M.Belousov in C a t a l y s i s and P r o g r e s s in Chemica l E n g i n e e r i n g CRuss) , Novosib i rsk , 1984, p 218. V.A.Zazhigalov, Yu.P.Zaitzev, V.M.Belousov, M. W o l f and N. Wustneck , React.Kinet.Catal.Letters, 24 (1984) 375. V.A.Zazhigalov, V.M.Belousov, G.A.Komashko, A.I .Pyatni tzkaya, Yu.N.Merkureva, A Z P o n y a k e v i c h , J.Stoch and J . H a b e r , Proc.9t.h 1 n t e r n a t . C o n g r . o n Catal., Calgary, vo1.4 (1988>, p.1546. J.W. Johnson, D.C. Johnaon, A. J. Jacobson and J.F.Brody, J.Am.Chem. SOC., 106 (1984) 8123. V.A.Zazhigalov, V . M . B e l o u s o v , H.Ludwig, G.A.Koma9hko and A.I. P y a t n i t z k a y a , Ukr.Khim.Zhurn., 54 (1988) 35. A.S.T.M. 1967. L. A b e l l o , E.Husson, Y R e p e l i n and Q. L u c a z e a u , J.SoLid State H.S.Horowitz, C.M.Biackst.one, A . W . S l e i g h t and G.Teufer , AppLCata lys i s , 38 (1988) 193. V.A.Zazhigalov, V.M.Belousov, A. I .Pya tn i tzkaya G.A.Komashko, A.V.Chkarin and L.S.Khuzhakaeva, Zh.Prikl.Khim., 61 C 1 9 8 8 > 101. M.I.Temkin, Kinet.Katal., 16 C1975> 504. V.A.Zazhigalov, V.M.Belousov, N.D.Konovalova, Yu.N.Merkurieva, A. I .Pya tn i tzkaya and G.A.Komashko, React.Kinet.Catal.Letters, 38 C19893 147. V.A.Zazhigalov, V.M.Belousov, A. I .Pya tn i tzkaya G.A.Komashko, Yu.N.Merkureva and J.Stoch, in G C e n t i and F . T r i f i r o (Ed.> New D e v e l o p m e n t s in Selective Oxida t ion , Univ.Bologna, Bologna, 1989, P r e p r . C 8. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 507 PREPARATI ON OF WELL DISPERSED VANADI A C A r a Y s T s BY ULTRA-HI GH I NTENSI yry GRI NDI NG AT AMBI ENT TEMPERATURE 2 2 2. S O B A L f K ' , O . B . LAF'INA and V.M. M A S T I K H I N ' I n s t i t u t e of Inorganic C h e m i s t r y , C z e c h o s l o v a k A c a d e m y of Sc iences , M a j a k o v s k g h o 24, 160 00 Prague C C z e c h o s l o v a k i a 3 . ' I n s t i t u t e of C a t a l y s i s , Sciences, N o v o s i b i r s k 630 090 CU. S. S. R . 3 Siber ian Branch of t h e USSR A c a d e m y of SUMMARY V a n a d i u m oxide supported catalysts have been prepared by t h e u l t r a - h i g h i n t e n s i t y g r i n d i n g of m i x t u r e s of V 2 0 5 w i t h oxidic s u p p o r t s a t t h e a m b i e n t t e m p e r a t u r e . T P R , U V / V I S d i f f u s i o n ref lectance, 'H NMR and ' lV NMR m e a s u r e m e n t s for t h e V 0 bo th anatase and r u t i l e s h o w t h a t t h e vanadia layer on t g e prepared m a t e r i a l is w e l l - sp read and t h a t t h e r e i s a s i g n i f i c a n t i n t e r a c t i o n w i t h t h e support prepared by t h i s m e t h o d have ca ta ly t ic a c t i v i t y is N O -NH3 reac t ion c o m p a r a b l e t o t h e catalysts prepared by i m p r e g n a t i o n . - A 1 0 and V O5 - Ti02 ' 2 5 . 2 5 The V 0 -Ti02 s a m p l e s I NTRODUCTI ON V a n a d i u m oxide c a t a l y s t s supported on a1 umi na or ti t a n i u m dioxide have received m u c h a t t e n t i o n both as catalysts for sel ec t i ve oxidation of or gani c c o m p o u n d s and for catal y t i c r e d u c t i o n of NO by a m m o n i a . V a r i o u s m e t h o d s for preparation of s u c h ca ta lys t s have been suggested i n t h e l i t e r a t u r e , one of t h e latest r e v i e w s on t h e s u b j e c t w a s g iven by B o s c h and J a n s s e n Cref . 11. The i n t e r a c t i o n b e t w e e n t h e vanadia and t h e support is m o s t l y regarded a p r e r e q u i s i t e i n t h e reac t ion a c t i v i t y . X A s reported previous ly t h e u l t r a - - h i g h i n t e n s i t y g r i n d i n g C U H I G - t r e a t m e n t 3 of Vg05 w i t h A1203 or TiO, a t a m b i e n t t e m p e r a t u r e w a s f ' o u n d t o c a u s e a vanadia-support i n t e r a c t i o n and a change of iranadi u m C V> e n v i r o n m e n t s C r e f . 23. The a i m of t h i s w o r k w a s t o e x a m i n e by d i f f e r e n t t e c h n i q u e s m o r e deeply t h e vanadia-support i n t e r a c t i o n d u r i n g t h e UHIG t r e a t m e n t i n order t o ob ta in m o r e c o m p l e t e p i c t u r e of t h e process and t o d e t e r m i n e c a t a l y t i c a c t i v i t y of t h e r e s u l t i n g m a t e r i a l . 508 EX PER1 MENTAL Mater i a1 s 2 The Ti02 C r u t i l e ; 3 m /g 3 and V20s w e r e c o m m e r c i a l p r o d u c t s of p .a . q u a l i t y , y A1203 CGOST 8136-85; 220 m /g3. Ti02 Canatase; 81 m /g3 w a s p repared by p r e c i p i t a t i o n f r o m TiC14 s o l u t i o n wi th NH OH. d r y i n g a t l l O ° C and c a l c i n a t i o n a t 5OO0C for 4 hours . Methods. Sampl es pr epar a t i on 2 2 4 Four methods are used t o p r e p a r e t h e vanadia catalysts. Ci3 UHIG t r e a t m e n t A mechanical mix ture of catalyst components w a s t r e a t e d for up t o 20 min i n a m i l l where t h e m a t e r i a l w a s d i s i n t e g r a t e d by m e t a l l i c s p h e r e s CO. 6 cm diameter3 i n c o n t a i n e r s which r o t a t e d round t h e c e n t r e of t h e m i l l and a t t h e s a m e t i m e revolved i n a p l a n e t - l i k e movement round its own axis. I n t h i s arrangement t h e s p h e r e s a c q u i r e d an a c c e l e r a t i o n of about 20 g. C i i 3 C a l c i n a t i o n of s i m p l e mixtures A mechanical mix ture of catalyst components w a s c a l c i n e d for 1 4 h a t 500OC. C i i i 3 W e t impregnat ion A1203 or Ti02 w a s impregnated by a s o l u t i o n of vanadyl The samples w e r e d r i e d a t l l O ° C f o r 2 h and t h e n c a l c i n e d cxals+.e. i n a i r f o r 3 h a t 5 0 0 O C . Civl G r a f t i n g Samples w e r e p repared by i n t e r a c t i o n of V 0 C l 3 vapour a t d r y The N2 c a r r i e r h y d r o l y s i s w a s t h e n c a r r i e d o u t a t t h e s a m e t empera ture i n wet a i r . C a t a l v s t c h a r a c t e r i z a t i o n g a s wi th t h e s u p p o r t samples c a l c i n e d a t 20OOC. The EET s u r f a c e measurements of t h e c a t a l y s t s w e r e performed by t h e D i g i s o r b 2600 a p p a r a t u s , mercury poros imet ry by means of t h e Auto-Pore 9200 C M i c r o m e r i t i c s , USA3 a p p a r a t u s . The s o l i d - s t a t e pro ton NMR s p e c t r a wi th M A S C magic-angle s p i n n i n g method> t e c h n i q u e have been recorded on a Bruker CXP 300 spec t rometer . The experimental parameters and t h e d e t a i l s of vacuum p r e t r e a t m e n t of samples have been d e s c r i b e d p r e v i o u s l y Cref . 33 . D i f f u s e r e f l e c t a n c e measurements w e r e performed by UV/VIS spec t rometer Shimadzu MPS 2000 wi th d i f f u s e r e f l e c t a n c e a t tachment i n t h e wavelength range of 250-700 nm u s i n g MgO as t h e r e f e r e n c e . The 'lV NMR s p e c t r a were o b t a i n e d on a Bruker MSL-400 spec t rometer as d e s c r i b e d p r e v i o u s l y Cref . 23. 509 TPR Ctemperature programmed reduct ion3 exper iments w e r e performed i n a convent iona l f l o w a p p a r a t u s wi th thermal c o n d u c t i v i t y d e t e c t o r u s i n g a h e a t i n g r a t e of 20 K min and a f l o w . The amount of sample used w a s of 80 m l min of 1 0 % H2 i n N chosen so t h a t t h e amount of vanadium i n t h e reactor i n m o s t experiments w a s about 5 mg. A l l measurements w e r e done on samples a f t e r a t least 2 hours of c a l c i n a t i o n a t 500 C i n a i r . -1 -1 0 C a t a l y t i c tests w e r e c a r r i e d o u t i n an i n t e g r a l i so thermal reactor. The f l o w ra te of t h e r e a c t i o n mixture CO.40% of NO, 0.40% of NH3, 3% of 02, n i t r o g e n as t h e carrier gas3 w a s 30 N 1 h per gram of sample. Reagent and r e a c t i o n products w e r e ana lyzed us ing NO/NO chemi 1 umi nescence a n a l yzer 951 A C B e c k man3. -1 X RESULTS AND DISCUSSION C a t a1 ysts Char act er i z a t i on Specific surface The u n t r e a t e d pure s u p p o r t s or mixtures wi th V205 are not much i n f l u e n c e d by c a l c i n a t i o n a t 500°C wi th e x c e p t i o n of V 0 -Ti02 mi x t u r es where s o m e si n t e r i ng w a s observed. 2 5 Marked d e c r e a s e of t h e s p e c i f i c s u r f a c e of b o t h a n a t a s e and alumina w a s caused by t h e U H I G t r e a t m e n t . The d e c r e a s e i s even higher i n case of V 0 -Ti02 Canatase3 mixtures . 2 5 Tab. 1 S p e c i f i c s u r f a c e of vanadia samples v 0 - Al2O3 2 5 v2°5 S. m . g w t % 2 -1 - [ a 1 21 8 - Cbl 112 2. s 142 5.0 155 7.5 103 25.0 46 2 2 -1 V 0 - T i 0 2 5 '2OS S, m . g wt% - [a1 81 - [ b l 69 1 .5 27 3.0 31 7.0 34 12.0 12 a A1203; UHIG-treated A1203. a n a t a s e ; U H I G t r e a t e d a a n a t a s e . The p o r o s i t y measurements proved t h a t p r e f e r e n t i a l l y pores of small diameters are f i l l e d or s i n t e r d u r i n g c a l c i n a t i o n of UHIG-pretreated V 0 0 i O 2 samples . ' H NHR measurements 2 5 The o r i g i n a l a lumina e x h i b i t s l i n e s w i t h a chemical s h i f t of 6 = -0.6 ppm, which belongs t o t h e b a s i c OH-groups and l i n e s with 510 chemical s h i f t of about 3.0 ppm belonging t o t h e m o r e a c i d i c OH-groups . The spectrum of o r i g i n a l T i 0 con ta ins l i n e s with 6 = 1.5. 3.6 and 6.7 belonging t o d i f f e r e n t OH bands on t h e s u r f a c e C s e e Fig. 1 a . d . The whole q u a n t i t y of protons a t t h e alumina and ana ta se w a s found about g-land I d 0 g , r e spec t ive ly . 2 20 -1 3.0 I 1 I I I I I I - 40 2 0 0 -20 PPD 3.6 I I I I I I I 40 20 0 -20 PPm Fig .1 . 'H NMR MAS UHI G t r e a t e d vanadi a ana ta se Cd> . % C a > , T i 0 C c > and t h e 2 s p e c t r a of A l mixtures C 7 2% of V 0 3 with alumina C b> or 2 5 A t samples with 7 w t X of V205 on both suppor ts t h e spectrum i n t e n s i t y decreased C s e e Fig. I b . d > , t h e m o s t prominent decrease w a s i n d i c a t e d f o r m o r e ba s i c OH-groups on alumina C 6 = -0.6 p p d or ana ta se Cb 1 .5 and 3.6 p p d . Gn alumina sample with 25 wt% of V205 t h i s band even f u l l y disappeared. The o ther OH bands on both support a r e less inf luenced by t h e process . Diffuse ref lectcrnce spectra The p o s i t i o n of t h e high i n t e n s i t y charge- t ransfer band of V5+ i o n s C d o > is s t r o n g l y inf luenced by t h e number of l i gands i n t h e environment of t h e c e n t r a l i ons Cref. 41. The V5' i o n s i n octahedral coord ina t ion g ives CT band a t 400 t o 480 nm region. By 511 decreas ing t h e coord ina t ion number of t h e c e n t r a l i on and forming a t e t r a h e d r a l coord ina t ion t h i s band s h i f t s towards t h e higher energy region C < 350 nml. The r e f l e c t a n c e spectrum of t h e pure V20s does not p r a c t i c a l l y changed by t h e UHIG C s e e Fig. 23 t hus i n d i c a t i n g no change of t h e former oc tahedra ly coord ina t ion of t h e V5+ i o n dur ing t h e t reatment . a 700 600 500 400 300 200 wavelength (nm) F ig .2 . Reflectance W M S s p e c t r a of V mixtures C 7% wt% of V 0 > before C cl and a f t e r U H I G Lregt%e:g?al. Spec t ra of pure V20S2b%ore C d3 and a f t e r 20 min of UHIG t rea tment Cbl. The decrease of t h e coord ina t ion number of t h e vanadium cen t r a l i on and formation probably of a t e t r a h e d r a l y coordinated V5+ 2 3 or TiOa w a s i nd ica t ed by a marked s h i f t of t h e absorp t ion towards t h e higher energy reg ion , C s e e Fig. 2a> where r e s u l t s on V205-A1 0 2 3 mixtures are presented . 5f v NMR spec tra spec ie s dur ing t h e WIG treatment of V205 mixtures with A l 0 The assignment of t h e s i g n a l s w e r e made on t h e b a s i s of t h e 512 d a t a o b t a i n e d i n p r e v i o u s papers Cref. 5.61. Ve05 with an o c t a h e d r a l vanadia environment e x h i b i t s a l i n e with an axial a n i s o t r o p y of t h e chemical s h i f t t e n s o r C d l = -310 ppm, 6,, = - 1270 p p d wi th s m a l l peaks due t o t h e f i r s t order quadrupole effects C s e e F ig . 3a3. I . I . l . 1 . 0 -500 -1000 -1500 0 -500 -1000 -1500 PPm PPm Fig. 3. 51V NMR s p e c t r a of V 0 alumina C b l C 5 V 0 > after c a l c i n a t i o n a t 520°C i n a i r . d u r i n g i n c r e a s i n g t i m e of &?G t r e a t m e n t Ce-93. Pure V205 af te r W I G t r e a t m e n t Cd3. C a > . and its mixtures with w t % of $ Q I2,âand T i s C c 3 C a n a t a s e . 3 wt% of 2 5 5 1 ~ NMR s p e c t r a of v - a n a t a s e mixtures c 5 wt% v205> After c a l c i n a t i o n of u n t r e a t e d V 0 -A1203 or V 0 -Ti.O 2 5 2 5 2 mixtures a t about, 510 OC for 1 4 hours t h e bulk of V205 remains unreac ted CFig. 3 b.c1. The l o w i n t e n s i t y l i n e a t about -570 ppm. can be a t t r i b u t e d t o p a r t i a l format ion of a s p e c i e s wi th V i n d i s t o r t e d t e t r a h e d r a l environment. During t h e UHIG t r e a t m e n t of t h e pure V205 C s e e F ig . 3d3 o n l y t h e local V environment is d i s t o r t e d whi le t h e g e n e r a l crystal s t r u c t u r e of Ve05 is r e t a i n e d . Much m o r e profound i n t e r a c t i o n can be i n d i c a t e d from t h e i r NMR s p e c t r a d u r i n g t h e t r e a t m e n t of mixtures of vanadia wi th t h e o x i d i c 513 suppor ts C s e e Fig. 3e-gl. I t r e s u l t s i n gradual disappearance of t h e s igna l a t -310 ppm from Ve05 and p a r a l l e l formation of a new spec ie s with NMR s igna l having a chemical s h i f t i n t h e range of -500 t o -700 ppm. Most probably a l l t h i s new l i n e s can be a t t r i b u t e d t o vanadi um atoms havi ng a tetr ahedr a1 environment with oxygen atoms CRef. 2 .3 . These new forms ev i dent1 y prevai 1 i n ramp1 es prepared by UHIG t reatment on a l l t h r e e suppor ts used C s e e Fig. 4 3 . 51V NMR s p e c t r a of t h e samples t r e a t e d a t 5OO0C a f t e r UHIG procedure a r e s i m i l a r t o those ob ta i ned f o r vanadi a suppor t e d I t should be noted t h a t c a t a l y s t s prepared by conventional impregnation- c a l c i n a t i o n procedure Cref. 2.5.63 . - 3 ~ I + L A 0 -500 -1000 -1500 PPm Fig. 4 5 1 ~ NMR s p e c t r a of Ca3, and its mixtures with a1 umina C b3 C 2 5 w t % of V g 2 , Iga?ase Cc3 C 1 2 wt% of V $ 2 , r u t i l e Cd3 C 7.5 wt% of V 0 3 , a f t e r 20 min of UHIG treatment . and 2 5 Temperature progrcunmed reduction The reduct ion of t h e bulk VzOs procceds i n a number of success ive s t e p s , t h e f i r s t peak corresponds t o t h e reduct ion of V 0 t o VsO13 Cref. 7>. For supported samples p o s i t i o n of t h e f i r s t peak s h i f t s markedly t o l o w e r temperature Cref . 83. I t should be supposed t h a t t h i s technique could d i sc r imina te between t h e unsupported bulk V205 and V20s i n a d i s t i n c t state of i n t e r a c t i o n with t h e suppor t . 2 5 On c a t a l y s t s prepared by impregnation or g r a f t i n g t h e vanadia- support i n t e r a c t i o n is r e f l e c t e d by a decrease of t h e pos i t i on of t h e f i r s t maximum for about 100°C C s e e Fig. 5a ,b ,g3 i f compared 514 with t h e bulk V20s. The UHIG t reatment of t h e V20s a lone has p r a c t i c a l l y no e f f e c t on t h e p o s i t i o n of t h e f i r s t maximum C s e e Fig. S j 3 . As a r e s u l t of c a l c i n a t i o n of un t rea ted mixtures of V205 with M203 or TiO, Canatase> a small reduct ion peak a t lower temperature emerged C s e e Fig. 5d.e) . Nevertheless m o s t of t h e reduct ion still proceeds a t temper a t ur es cha rac t e r i sti c of unsuppor t e d V205. L O O 400 600 800 t O , C 200 400 600 8 0 0 200 400 600 800 tO,C tO,C Fig .5 . before Cdl and a f t e r UHIS ?re, ment Cc>; sample prepared by impregnation C 9 . 3 VeOsl Ca3 . TPR p r o f i l e s for C& -Ti0 Canatase3 mixtures C 5 wt% ' fa%> before Ce> and a f t e r WIG Ereatment Cf l . ; C g > sample prepared by impregnation C 5.7 A% of V 0 >. TPR p r o f i l e s f o r V - r u t i l e mixtures C 7.5 wt% of V 0 3 before Ci3 and a f t e r WIG2 3rea tment C h> ; pure V20s before CkT 2nd a f t e r UHI G t rea tment C j l . TPR p r o f i l e s f o r V 0 -Al f 3 m i x t u r e s with C 5 w t % of VzOEsl wt% of V20s3 C b3 or by g r a f t i n g C7.0 w t % of 2 5 N o decrease of t h e reduct ion temperature w a s i nd ica t ed for r u t i l e mixtures C s e e Fig. 5 i l . With all suppor ts s t u d i e d t h e U H I G t rea tment r e s u l t e d i n t h e c h a r a c t e r i s t i c s h i f t of t h e f i r s t TPR peak t o lower temperature typ ica l of d i s t i n c t vanadia i n t e r a c t i o n with t h e support Csee Fig. 5 c , f , h l . Activity at NO-NH3 reaction Reported i n Fig. 6 a r e c a t a l y t i c a c t i v i t i e s of ana ta se or 515 r u t i l e supported samples i n s e l e c t i v e catalytic reduct ion of NO by ammonia. The a c t i v i t i e s obtained f o r t h e WHIG-treated samples are comparable a t t h e whole temperature reg ion t o catalysts prepared by impregnation. The unt rea ted samples have a t low temperatures i nf er i or acti v i t y . I I I I L I I I I I I I 200 250 300 350 400 450 t, OC Fig. 6 C a t a l y t i c tests on un t r ea t ed mixtures of ana ta se w i t h 5 w t % C 0 3 or 12 w t % of V 0 C 0 3 a f t e r c a l c i n a t i o n and s a m p l e s prepared by t h e WIG treafment of anatase with 5 w t % C 0 3 r u t i l e with 1 2 wt% of V205 Ca3. 5 or Model f o r t h e c a t a l y s t prepared by t h e U H I G t reatment This w o r k shows t h a t i t is poss ib l e t o prepare w e l l sp read vanadium oxide by t h e UHIG t reatment of mixtures of Vz05 with Ti02 and A1203 a t ambient temperatures . The t rea tment can provoke i n some cases s i n t e r i n g of t h e s u r f a c e a t subsequent hea t ing of t h e mixture. W e be l i eve t h a t t h e concept of s t i m u l a t i o n of s o l i d - s t a t e processes by energy i n t e n s i v e gr inding would be useful i n understanding t h e r e s u l t s C s e e r e f . 113. I t should be supposed t h a t mechanical deformation and consequent short- t ime temperature e l e v a t i o n a t t h e con tac t p o i n t s of d i r e c t impact of t h e s o l i d mater ia l aga ins t t h e spheres acce le ra t ed d i f f u s i o n of vanadia i n a way s imi l a r t o t h e e f f e c t of prolonged hea t ing Cref .91. bu t t h e 516 e x t e n t of t h e p r o c e s s is h i g h e r . I n ana logy t o impregnated o x i d i c sys t ems C r e f . 101, mainly t h e b a s i c OH-groups are invo lved i n t h e p r o c e s s of vanadia s p r e a d i n g . R e s u l t s of U V / V I S r e f l e c t a n c e and 51V NMR expe r imen t s are c o n s i s t e n t w i t h t h e concept of t r a n s f o r m a t i o n of t h e o r i g i n a l c r y s t a l l i n e V205 w i th square-pyramidal environment i n t o a form of o x i d e l a y e r w i t h predominant ly t e t r a h e d r a l c o o r d i n a t i o n d u r i n g t h e U H I G t r e a t m e n t . The i n t e r a c t i o n of vanad ia wi th t h e s u p p o r t a t t h e r e s u l t i n g o x i d e l a y e r b r i n g s abou t a d e c r e a s e of t h e t e m p e r a t u r e of r e d u c t i o n s i m i l a r l y as i n t h e impregnated or g r a f t e d c a t a l y s t s . I n comparison t o h igh - t empera tu re s p r e a d i n g , where o n l y p a r t of V205 is i n v o l v e d , t h e WHIG t r e a t m e n t produced vanad ia l a y e r i n a broad c o n c e n t r a t i o n r ange . The vanadia-support i n t e r a c t i o n caused by t h e WHIG t r e a t m e n t r e s u l t e d i n t h e V 0 /Ti02 c a t a l y s t s w i th a c t i v i t y i n NO-NH3 r e a c t i o n comparable t o c a t a l y s t s p repa red by o t h e r methods. I t i s remarkab le t h a t beyond t h e a c t i v e s u p p o r t s f o r which h i g h t e m p e r a t u r e s p r e a d i n g s h o u l d be supposed C r e f . 91 r a t h e r good a c t i v i t y w a s found a lso for r u t i l e s u p p o r t e d samples . 2 5 ACKNOWLEDGEMENT The a u t h o r s are t h a n k f u l t o M s . 0. N . Novgorodova, M r . A. V. Nosov, and M r . B. P. Zo lo tovsk i j C I n s t i t u t e of C a t a l y s i s , Novosibirsk) f o r p r o v i d i n g t h e c a t a l y s t s p r e p a r a t i o n . REFERENCES 1 H. Bosch, F. J a n s s e n , C a t a l . Today 2 C 1 9 8 8 1 369. 2 â7. S o b a l i k I 0. B. Lapina, 0. N. Novgorodova and V. M. Mas t ikh in , 3 K . I . Zamaraev and V . M . Mas t ikh in , C o l l o i d s and S u r f a c e , 12 4 G. L i schke , W. Hanke, H.-G. J e r s c h k e w i t z and G. Ohlmann, 5 0. B. Lapina, A. V. Simakov, V. M. Mast ikhin, S. A. Veniaminov and 6 H. E c k e r t and I . E . Wachs, J . Phys. Chem., 93 C 1 9 8 9 1 6796. 7 H. Borch, B. J . Kip, J . G. van Ommen and P. J . Gellingâs, J . Chem. Soc., Faraday Trans. 1, 80 C19841 2479. 8 H . Bosch and P. J . S i n o t , J . Chem. Soc. , Faraday Trans 1 , 8 5 , 1425 C1989I. 9 J . Haber, T. Machej and 1. Czeppe, S u r f a c e . S c i . , 151 C19853 301. 10 B.M. Reddy, V . M . Mast ikhin. i n M . J . P h i l i p s and M. Ternana CEds. I , Proc. 9 t h I n t . Congr. C a t a l . , Calga ry , 1988, vol . 1 , p. 82. 11 K. Tkdtovb, Mechanical A c t i v a t i o n of Mine ra l s , Developments i n Mineral p r o c e s s i n g , V o l . 11, D. W.Feurstenau CEd. I , E l s e v i e r , Amsterdam, 1989. Appl . C a t a l . , 63 C 1 9 9 0 2 191. C 19842 401. J . C a t a l . 91 CIS853 54. A. A. Shub in , J . M o l . C a t a l . , 50 C 1 9 8 9 1 55. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 517 DISPERSION AND PHYSICO-CHEMICAL CHARACTERIZATION OF IRON OXIDE ON VARIOUS SUPPORTS Weijie Ji, Shikong Shen, Shuben Li and Hongli Wang Lanzhou Institute of Chemical Physics, 730000, PR China ABSTRACT A somewhat slow but controllable impregnation method with the dipping of support into an excess of aqueous solution of (NH4)-j[Fe(C204)3].xH20 is adopted to prepare well-dispersed or monolayer-type femc oxide on various supports. Other methods with the same and other conditions are also used to make a comparison among them. The influences on the adsorption process are investigated and the suitable conditions are determined for the different supported systems. Extensive characterization has been done on the physico-chemical properties of these systems. The catalytic performances for different test reactions are also carried out on the various supported systems with distinctly different dispersions. INTRODUCTION Much attention has been devoted in recent literature to the phenomenon of interaction between oxides (refs. 1-3). The supported oxides in the form of three-dimensional crystallites whose properties are similar to bulk crystals, do not interact saongly with the support. However, when the oxide is dispersed in monolayer on the oxidic supports, the interaction between them becomes strong and the properties of such monolayer of the oxide differ from those of bulk oxide. Growth of three dimensional crystallites occurs only after a substantial fraction of the surface is covered by the monolayer and this is often the case for the oxides of 0, Mo, W, V, Re and Ni. Extensive studies have been devoted to these systems. For other systems, however, the state of dispersion is complicated, depending on the nature of the support, the preparative method and the conditions used in such process. In this paper, we tried to prepare well-dispersed iron oxide supported on various supports by different methods and investigate the physico-chemical properties of these iron oxides. EXPERIMENTAL PreDaration of catalvsts The supports employed in this study are y-Al2O3 (225 m2/g), MgO (23 m2/g), Zr02 (16 m2/g), Ti02 (43 m2/g) and Z n O Ti02 (anatase) was prepared by hydrolysis of T i c k . The other chemicals used are commercial products of analytical grade. Three methods were used to prepare the supported oxides. (I) Method of dipping into an excess of aqueous solution of (NH4)3[Fe(C204)3].xH20. An aqueous solution of the chosen compound with the proper concentration is added to the support particles under continuous stimng at room temperature or at 323 K. The dipping process is controlled by the contact time, pH of the aqueous solution and temperature. * Supported by Chinese National Natural Scientific Fundation. 518 (11) Incipient wetness impregnation or dipping in an excess of aqueous solution of iron nitrate. The incipient wetness method was performed with a series of aqueous solutions of iron nitrate. The supported samples were also prepared by the dipping method into an excess of aqueous solution of low concentration. (111) Batch adsorption at high temperature (ref. 4). After adsorption, the particles were After impregnation or adsorption, all the samples were dried and calcined in air at 373 K for 2 carefully washed and the remaining solution was removed by filtration. h, and 773 K for 2.5 h, respectively. Characterization The loadings of Fe were determined by atomic absorption spectroscopy or chemical analysis. BET surface areas were measured by N2 adsorption at 77 K. XRD measurements were operated on a Rigaku/D/MAX-RB X-ray diffractometer with Cu Ka. ESR spectra were recorded at room temperature on a E-115 spectrometer operated at band frequency with 100 KHz field modulation, IR absorption spectra were recorded on diffuse reflectance attachment of a Nicolet IODX FTIR spectrometer. Laser-Raman spectra were taken on SPEX 1403, and 5145 A emission lines were used for excitation. UV-Vis diffuse reflectance spectra were recorded in the wavelength range 250-850 nm (Shimadzu UV-365) using MgO (S) as a reference. XPS results were obtained on Perkin-Elmer PHI-550 with Mg cathode (320mw). The intensities of the peaks were referred to the area of the peaks including the satellite peaks for Fe2p levels. TPR experiments were carried out at the temperatures in the range of RT to 973 K. Before reduction, the samples were pretreated in oxygen flow at 773 K for 0.5 h. The test reactions were pulse reactions of CO oxidation and oxidative dehydrogenation of butene to butadiene with or without gaseous oxygen supply. Continuous flow reaction of CO hydrogenation was also used to test the catalytic activity for those samples with different dispersions. The syngas was a 2.8:l HdCO mixture and a flow of 50 mVmin was used. RESULTS AND DISCUSSION Preparation of catalysts Van Ommen and coworkers (4) have developed a method for the preparation of monolayer- type supported samples of femc oxide. However, this method has some limitations especially when using basic oxide supports. In the present approach, we med to use a new method, which requires only simple chemicals and operating conditions. When an oxide particle is brought in contact with an aqueous solution, surface polarization will occur. The sign and extent of surface charging will be determined by the isoelecmc point of the oxide (IEPS) and the pH of the aqueous solution. If the Fe3+ complex with negative charge is to be adsorbed in a monolayer, a positively polarized support surface is obviously required. With this consideration, (NH&[Fe(C204)3] .xH2O was chosen as raw material. Another reason for this choice is that the decomposition product in air of this compound is only ferric oxide. Because all the supports used in the experiment whose IEPS is above 5 and pH of this complex salt 519 dds ~ = 2 3 4mnio:/L t = 2 h * â¬1 2 3 c = 1 1 . 7 mmo 1 / 1 :(d)r , - ( b ' ) t ( h ) t=2h c ( m r i o l / l ) 0 . 3 0 . 2 0 . 4 0.3 0.4 0.3 support surfaces will be positively charged under these conditions and adsorption of the complex with negative charge through electrostatic forces is expected to occur. On the other hand, the pH has an additional effect determining the stability of a particular complex. At high pH, the hydrolysis becomes a noticeable side reaction. Fig. l(a) illustrates the influence of the pH of the aqueous solution on the adsorbed amount of Fe and the dispersion. 1,' ' 1 8 . ( on, en t 1 a t i o n o f a d s o r h e d a m o u n t amount is quite small. This is against the prediction of the electrical double layer o f Fe. I g d l u m i n d , d t RT . model. It should be noted that foreign ions can modify the IEPS and lead to competititve adsorption. So it is believed that the competitive adsorption of C2O4' is more favorable under this low pH condition. Adsorbed amount of Fe is increased with increasing the pH of solution. However, above p H 4 , the disintegration of the complex by the formation of a surface complex accompanied by substitution of C2O4= ligands or by hydrolysis on the camer surface becomes significant. Successive depositions on the covered surface rather than the free A1203 surface certainly make it difficult to obtain a "monolayer" type dispersion. Heating the solution may be helpful for the diffusion of the complex along the pore. In Fig. 1, the influences on the adsorbed amount of Fe by the concentration of solution and contact time are also indicated. As there is a relationship between pH and concentration of the solution, the pH will decrease and change in a narrow range during the dipping process when the concentration is relatively high. The adsorption of the complex in solution is in turn controlled by dipping. When the contact time was prolonged, the Fe loading increased slowly and continuously at the inital stage. Again, this increase is connected with the change of pH in solution. As the contact time changes from 0.5 h to 5 h, the final pH of the solution varies from 4 to 6. Therefore, the good dispersion of femc oxide cannot be obtained if the contact time is longer than one hour when the initial pH of the solution is relatively high and further increased during the dipping process. From our experiments, the suitable preparation conditions may be summarized as follows : (1) For y-Al2O3, the pH should be controlled in the range of 2.0-3.0, the dipping time is about 1 h and the concentration of the solution can be varied in a relatively wide range. (2) For Ti02 and ZQ2, because of their low values of IEPS, the pH of the solution is better adjusted at around 1.5 with the solution of H2C204.2H20. The solution temperature is maintained 520 at 323-328 K to accelerate the adsorption process and the contact time is prolonged to three hours. (3) For MgO and ZnO carriers, because of their basicity, especially for MgO, the pH of the solution should be below 2 so as to avoid the rapid disintegration of the complex. Moreover, a vigourous stirring is necessary. It seems that the most important factor in this case is the contact time. Usually, it must not be longer than two minutes. The third method is satisfactory for preparing monolayer-type materials except for the MgO and ZnO supports. The second method, however, is not suitable to get the uniformly dispersed samples in general. Characterization X-ray diffraction (XRD) shows no iron-containing crystalline phase for the samples prepared by the first and third methods. For the samples prepared by the second method, when the Fe loading is small, no XRD pattern can be attributed to the presence of iron-containing phase. However, as the Fe loading is increased above 3 wt%, the XRD pattern of a-Fe2O3 crystallite begins to appear. This loading is much lower than the threshold reported by Xie et al. (ref. 5). It is perhaps caused by the different preparation conditions used. Disappearance of the XRD patterns is probably caused by several reasons. So, we cannot be sure as to what the actual structure of the dispersed femc ions is, if judged only by the XRD results. F i g . 2. KS4 s p p c t ra o f v a r i o u s well-dispeised s u p p o r t e d s a m p l e s . The ESR results of the iron-containing specimens on the different supports give rise to rather different spectra, mainly depending on the location of the ferric ions and the interaction between them. As shown in Fig. 2, the iron-containing specimens on y-alumina, anatase, and ZrOz prepared by the first and third methods generate ESR lines at g14.28 and g2=2.0- 2.3. The first signal is typical of high spin ferric ions in sites with rhombic symmetry. This signal is assigned to femc ions in rhombically distorted sites which may be tetrahedral or octahedral and "isolated" (ref. 6). The presence and the amount of this kind of ferric ions strongly depend on the nature of the support. In fact, on MgO and ZnO, whatever the preparative methods used, this signal is always very weak indicating that the Fe3+ ions do not exist in the "isolated" state. The signal at g=2.0-2.3 for the different kinds of supported samples varies noticeably; the shape. and linewidth seem to be practically dependent on the preparative method and Fe content. This signal is assigned to the Fe3+ ions in the 521 nearest neighboring sites with strong coupling or only in the neighboring sites with a weak coupling. If the concentration of Fe on MgO is very low, the signal centered at g=2 almost disappears. For the femc ions on ZrO2, when the concentration of Fe is in the submonolayer range, an intense signal at g=4.28 and a very weak and broad signal centered at g=2 can be observed. As the concentration is increased near to full monolayer coverage, the intensity of the weak signal increases while the intensity of the signal at g=4.28 decreases significantly. l.FA(11)2.1 2. FA(I1I)l 5 3. FA(I)1.7 4. F Z r (III)0.4 3 n o 5 0 0 7 0 0 / n p F i g . 3 . I J V - V i s DPS o f s a ~ p l e s s i i p p o r t e d o n d i f f e r e n t c r l ~ I i e r s . A few UV-Vis diffuse reflectance spectra are shown in Fig. 3. In the range 250-550 nm, the absorption bands are described as charge transfer bands (CT bands) (ref. 7). This process is rather band- to-band transition. The CT bands arising from the well-dispersed ferric ions shift of about 60 nm to the high wave numbers as compared with the crystallites of iron oxide. This shift is probably caused by the following reasons. The large distortions in symmetry at the surface increase the energy separation of the d orbitals in the cation. The ionic surfaces could reduce the ionic charges of the surface Fe3+ ions, and make the energy of the surface orbitals shift downward and enhance the covalence of the cation-anion bond. In addition to the situations mentioned above, two other causes may be responsible for the blue shift of the CT bands. One is the substitution of surface ions by Fe3+ ions and localization in these relatively constricted sites. The other is the occupancy of the femc ions in smaller interstices such as the tetrahedral holes on the support surfaces. For ferric ions on alumina, both processes could happen but substitution process may be not so important on Ti02 and ZrO2 because of difference in valence. For femc ions on MgO, they may be restricted to the surface or form a surface chemical compound. Because all the octahedral holes are already occupied by the Mg2+ ions, the substitution could take place only in the octahedral sites and the occupancy in the tetrahedral holes. It is to be pointed out that the structural environment of the femc ions in the samples prepared by dipping into an excess of aqueous solution of iron nitrate always shows the characteristics of the "cluster" or microcrystalline MgFe204, which evidently differs from the behaviour of the ferric ions in the samples prepared by 522 the other two methods. The ferric ions have penetrated into MgO to a deeper level due to this preparation method. For ZrO2 support, there are roomier interstitial sites on it; the occupancy of these holes seems to be reasonable and can result in the red shift of the bands. This is the case for the sample with the near full monolayer coverage. When the Fe loading is in the submonolayer range, a large part of ferric ions become "isolated", and the CT bands shift to the short wavelengths. When the supported samples contain microcrystalline iron oxide, the spectra change noticeably. The positions of bands or edges appear in the long wavelengths, showing apparently the bulk characteris tics. FTIR-DRS shows no characteristic infrared absorption of crystalline iron oxide in the supported samples prepared by the first and third methods below the monolayer coverage. An attempt was also made to study the Laser-Raman spectra of those samples of well-disersed ferric ions in the range 100-1OOO cm-1 with or without grinding the samples into fine powder. Generally, the Raman spectra are ill-defined because of broadening, and the lines are very weak. For the samples prepared by method (l), only an intense phosphorescence peak arising from Fe can be observed as the samples were ground into powder. Almost the same results can be obtained without grinding the samples by using back-scattering. For the samples prepared by method (3), very broad and weak lines centered at around 800 cm-1 can be found using 90°-scattering and are probably due to the surface femc ions. The line at about 192 cm-l is almost certainly due to the presence of a two dimensional phase of Fe2O3, which exists in Zr02-supported sample with nearly full monolayer coverage. ( k ' e / A 1 ) 0 . 2 0 . 1 0 . 0 2 4 6 8 1 0 1 2 ( F e / A l ) , x I V 2 F i g . 4 . P h o t o e l e c t r o n i c r - e s p o n s e ( F e / h l ) , p s v s . t h e molar r a t i o ( F c / A l ) , i n t h e s t i p p l e s e r - i e s p r - p a r e d h y n e t h o d ( I 1 ) . XPS measurements are illustrated in Fig. 4. In order to get more accurate results the peak area sensitivity factors are used instead of peak height sensitivity factors. The results indicate that there is no threshold existing in the sample series prepared by incipient wetness method with aqueous solution of nitrate. The dispersion state of femc ions on alumina is complicated with increasing the Fe loading. The formation of "clusters" or crystallites of iron oxide with a broad size distribution and the solubility of ferric ions into the support lattice produce a changeable X P S response with increasing Fe loadings. In a word, a nonlinear relationship suggests a nonunifonnity in alumina coverage by iron species on alumina. TPR results of various supported samples indicated that the Fe3+ ions which are well- dispersed at the support surfaces (on alumina, MgO, Ti@ and ZnO) cannot be reduced beyond the Fe2+ state. The stability is thought to be caused by the formation of surface compounds. The femc ions at the surface of MgO and ZnO are reduced with difficulty to Fe2+ as compared with the other 523 21, - 0 6\0 0 y20 + - n . .i - r 2 n o C 6 0 i 50 Y .3 3 ; 3 0 ' - a 0 20 10 0 supported systems. In the samples supported on Z d 2 , small amount of well-dispersed femc ions can be reduced to Fe through the formation of an intermediate surface compound and the reduction of the femc ions is relatively easier. F A ( I ) l . 7 F W ( I ) 0 . 5 F Z r ( I J I ) 0 . 4 FA( ) 1 0 . 5 F T ( I I 1 ) I . n d t' 1 I I I I I . , l , l . I h I I * I I a 1 I I , I I I , I 1 1 . I I I . I I I . c - d - - . I l l I I I I I P u 1 s e CATALYTIC PERFORMANCES 1. Pulse reacb 'on of CO oxidation CO oxidation with the alumina-supported catalysts with different loadings and dispersions indicates that the catalytic activity seems to be independent of the dispersion of the femc ions on the support at high reaction temperature 693 K and with the supply of 02 . In this case, 0 2 must be highly activated and the density of this active oxygen is higher at the catalyst surfaces, and the adsorbed CO is rapidly attacked. When the temperature is decreased to 623 K or when no 0 2 is supplied at 693 K, the activity is obviously connected to the loading and dispersion. The activation of 0 2 by monolayer-dispersed Fe3+ ions becomes weaker as compared with that by crystallites of iron oxide. On the other hand, surface oxygens coordinated to "monolayer"-type Fe3+ are relatively inactive and not easily removed by CO. The amount of this kind of surface oxygens is much smaller on the monolayer materials. 2. Pulse reaction of OXD of butene to butadiene The results are shown in Fig. 5. Generally, the monolayer catalysts have a certain activity either with or without the supply of oxygen. It thus reveals that only one atomic layer of femc ions is needed for this reaction. However, the oxidation-reduction properties of the surface femc ions strongly determine their activity. For instance, the MgO- and ZnO-supported samples have a low activity due to the difficult reduction of femc ions. The relatively superior performance of the F i g . 5 . P r o d u c t d i s t r i h u t i o n o f 2-C4â¬18 p u l s o r . o a c t i o n o n t h e v a r i o u . ; s u p p o r t e d s a m p l e s w i t h d i f f e r e n t d i s p e r s i o n . (a). 1 - C / , 1 1 ~ ~ , ( h ) . r e s i d . , ( c ) . C O X a n d ( d ) . (:4Fiy ( T = 6 4 3 Y ) . 524 monolayer catalyst on y-alumina as compared with the other supported systems may arise from the structural factor. Because y-alumina has a spinel structure as that of y-Fe2O3, localization of ferric ions on the surface interstices makes the surface properties much similar to those of y-Fe2O3 rather than those of a-Fe203. The amount of COX product formed in OXD reaction is small on monolayer materials as compared with that for crystallites of Fe2O3 in high loading supported samples. The results indicate that the metal-oxygen bond strength in these samples may be responsible for the low combustion activity. Because of the presence of "isolated' Fe3+ ions, it can be expected that the density of lattice oxygens coordinated with this kind of Fe3+ ions is low on the surface. Therefore, it is not favorable for the combustion which requires a relatively high density of lattice oxygens in local environment. For the high loading samples supported on y-alumina, high yield of butadiene was obtained as reported earlier (ref. 8). The reason for this stimulation of the formation of butadiene is not yet clear. When an excess of these active lattice oxygen atoms which could result in combustion was removed in the first pulse, it helped the formation of butadiene in the second pulse on high loading sample. However, regeneration of surface active sites in the supported crystallites of iron oxide by diffusion of lattice oxygen is not very rapid but faster than that in monolayer catalysts. 3. CO hvdroeenation Dwyer and coworkers (ref. 9) have found that the catalytic activity of an iron foil is increased ten times when the foil is preoxidized in dry oxygen before exposure to a CO/H2 mixture. Reymond et al. (ref. 10) have reported that unsupported a-Fe2O3 is more active in the CO+H2 conversion than a prereduced oxide. However, our results do not agree with these observations. Because all the catalysts are pretreated in 0 2 for 0.5 h and then in He for 0.5 h at 560 K, respectively, before introduction of the syngas, an oxidized state of the catalysts can be expected. Surprisingly, no CHq is produced for either monolayer materials on various supports or supported crystallites of ferric oxide. This inactivity must be connected with the fact that the active sitedphases are absent on the catalyst surfaces under the reaction conditions. Of course, the low H2/CO ratio used in our experiment may cause a deposition of inactive carbon on the surfaces, which is at least a part of the reason for the inactivity. Temperature programmed reaction of the syngas indicates that no C& was observed until the temperature was raised to about 823 K for the supported crystallites of ferric oxide. In this case a part of ferric ions must be reduced to Fe which is responsible for CH4 production. Under the same conditions, no C& product could be found even when the temperature was raised to 873 K for the monolayer catalysts. Because the femc ions in the monolayer samples resist the reduction of Fe3+ beyond the Fez+ state, this further suggests that the well-dispersed Fe3+ ions including Fe2+ ions are not active. The superior catalytic activity of the unreduced a-Fe2O3 catalyst is due to the formation of very small crystallites of a-Fe and X-iron carbide which did not form in most cases in our experiments (ref. 11). 525 Conclusion Different methods were used to prepare well-dispersed or monolayer type materials on various carriers. A desirable dispersion can be obtained by the first method developed in our experiments as long as certain conditions are carefully chosen and controlled for the different systems. 'The location, coordination and M - 0 bond strength of those surface-dispersed ferric ions are strongly determined by the nature of the supports and the preparative methods. Significant changes in oxidation-reduction properties and catalytic performances for the most well-dispersed or monolayer dispersed samples suggest that there is a strong interaction between the support and supported component. REFERENCES 1 2 3 4 G.C. Bond, Famday Discuss. Chem. SOC., 87 (1989). J. Haber, Pure Appl. Chem., 56 (1984) 1163. Z. Iwasawa, Adv. Catal., 35 (1987) 187. J.G. Van Ommen, H. Bosch, P.J. Gellings and J.R.H. Ross, in Preparation of Catalysts IV (B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet, eds.) Elsevier, Amsterdam, 1987, p. 151. Xie Youchang, Xu Xianping, Zhao Biying, Tand Youqi and Wu Gongbao, Proc. 4th Nat. congr. Catal., Teijing, China, 1988, Vol. 2 (1988) 1-E-29. 5 6 (a) D. Cordischi, M.L. Jacono, G. Minelli and P. Porta, J. Catal., 102 (1986) 1. (b) G.T. Pott and B.D. Mchicol, Discuss. Faraday SOC., 87 (1971) 121. K. Klier, Catal. Rev., 1 (1968) 207. E. Rodenas, T. Lizuka, H. Katsumata and K. Tanabe, React. Kinet. Catal. Lett., 19 (1982) 341. D.J. Dwyer and G.A. Somorjai, J. Catal., 52 (1978) 291. J.P. Reymond, P. Meriaudeau and S.J. Teichner, J. Catal., 75 (1982) 32. R.A. Dictor and A.T. Bell, J. Catal., 97 (1986) 121. 7 8 9 10 11 This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation ofCata2ysts V 0 1991 Elsevier Science Publishers B.V.. Amsterdam -Printed in The Netherlands 527 The use of chelating agents for the preparation of iron oxide catalysts for the selective oxidation of hydrogen sulfide. P.J. van den Brink, A. Scholten, A. van Wageningen, M.D.A. Lamers, A.J. van Dillen, J.W. Geus Deparmenr of Inorganic Chemistry, University of Utrecht, Croesestraat 77A, 3522 AD Utrecht. The Netherlands. Iron oxide-on-silica catalysts have been prepared for the selective oxidation of hydrogen sulfide to elemental sulfur. Preshaped silica extrudates (Aerosil 0x50) were impregnated with aqueous solutions containing different precursors. The precursors used are ammonium iron(II1) EDTA, ammonium iron(II1) citrate, iron(1II) gluconate, iron(1II) chloride, iron(III) nitrate, and iron(II1) sulfate. After drying and heating in air the resulting iron oxide on silica catalysts were characterized using TEM, SEM, Light Microscopy, XRD, DRETS, and Temperature-Programmed Reduction. Moreover, the catalytic properties in the selective oxidation of H2S were tested. It is found that catalysts prepared from precursors that do not easily crystallize, such as the above mentioned chelated iron compounds, contain high quantities of small, highly active iron oxide particles (2-5 nm). However, catalysts prepared with precursors that crystallize readily, like most of the simple iron salts, contain large iron oxide particles ( ~ 2 0 nm). Catalysts containing small iron oxide particles exhibit higher activities and selectivities. INTRODUCTION Oil refineries and natural gas plants often produce large amounts of hydrogen sulfide. The process utilized most for converting the hydrogen sulfide into elemental sulfur is the Claus process. In this process one third of the hydrogen sulfide is first combusted with molecular oxygen to sulfur dioxide: H2S + + SO2 + H2O In the subsequent thermal and catalytic stages of the Claus process, the remaining part of the H2S reacts with the S@ to elemental sulfur: 2 H2S + S@ 3 1 S , + 2 H20 Due to the unfavorable equilibrium, high levels of H2S conversion are possible only by removing the sulfur. A Claus plant therefore consists of a burner and two or three additional catalytic converters with intermediate sulfur condensers. Still 3 to 5% of the H2S has finally not reacted to sulfur. At the University of Utrecht a new catalytic process has been developed to oxidize these low concentrations of H2S selectively to sulfur without establishing the equilibrium of reaction (2). The newly developed process involves direct oxidation of hydrogen sulfide to elemental sulfur [l]: In order to obtain high sulfur yields in excess of oxygen, the following three reactions leading to SO2 and, hence, affecting the selectivity adversely, have to be inhibited or at least minimized: 528 I) (sequential) oxidation of elemental sulfur in excess of oxygen: IS" + 0 2 + so2 (4) 11) direct (parallel) oxidation of H2S according to reaction (1); In) establishment of the equilibrium of reaction (2). Because Claw tail gas contains large amounts of water vapour (up to 30%), the equilibrium involves appreciable concentrations of H2S and SO2 (2). By using an iron oxide (Fe2O3) precursor a high selectivity in the oxidation of H2S can be obtained. Under reaction conditions the iron oxide reacts to iron(I1)sulfate (FeS04). On iron(II)sulfate reaction (3) proceeds much faster than reactions (l), (2) and (4). To minimize sequential oxidation and establishment of the equilibrium (3), the transport within the catalyst bodies must proceed fast, which calls for a low Thiele-modulus [2]. Therefore a catalyst of a high porosity and wide pores and, thus, a low surface area is required. With this low surface area a high activity and stability, required with industrial catalysts, can only be achieved provided the iron sulfate and, thus, the iron oxide is highly dispersed on a support. The interaction with the support prevents sintering. A suitable support is silica, which has a low activity for the reactions (l), (2) and (4). Production of supported catalysts from pre-shaped bodies of the support is technically attractive. However, to achieve a uniform distribution of the active component throughout the support by impregnation of pre-shaped supports of a low specific surface area having wide pores is difficult. Crystallization of the active precursor during drying of the impregnated support will lead to small particles enclosing narrow pores. Because capillary pressures in wide pores are substantially less than in narrow pores, transport of the impregnated liquid to narrow pores within clusters of small crystallites of the precursor proceeds during drying. This will result in clusters of small particles of the active precursors not uniformly distributed within the bodies of the support. Clusters of small active particles are liable to sintering and thus to deactivation and the narrow pores within the clusters can lead to a drop in selectivity. The crystallization of the dissolved precursor and thus the formation of clusters of small crystallites strongly depends on the nature of the dissolved compound. The choice of the proper precursor will therefore be essential to obtain a highly dispersed and active catalyst. It has been established that with supports containing wide pores high dispersion of the active material can be obtained when the carrier is impregnated with a complex of the cation of the active compound with a chelating agent [3,4]. Using an iron EDTA complex [5] , iron oxide on alumina catalysts for the selective oxidation of hydrogen sulfide have already been prepared. In this paper the effect of the nature of the dissolved precursor on the final distribution of the active component over silica extrudates will be investigated extensively. The precursors investigated can roughly be divided into two classes ,viz., iron chelates and simple iron salts. Table I surveys the precursors investigated. With iron EDTA solutions the effect of the pH of the impregnated solution (5.3 ,7.1, 8.5 and 10.0) was studied. At low pH values the FeEDTA- anion predominates, while at higher pH levels FeEDTAOH2- or FeEDTA(OH)$ is mainly present. The catalysts were characterized using Light Microscopy (LM),Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Diffuse Reflection Infra Red Fourier Transform Spectroscopy (DRIFTS), and Temperature-Programmed Reduction (TPR) The effects of the preparation on the catalytic properties in the H2S oxidatiotl were also assessed. 529 EXPERIMENTAL Preparation: Silica extrudates (about 1/2" x 1/12") were made by extrusion, drying and heating of a aqueous paste of Aerosil OX50 (Degussa) powder. This powder consists of relative large spheres (diameter 20 - 80 nm) produced by flame hydrolysis. The pore volume of the extrudates was 0.8 ml/g (E = a%), the average pore radius 35 nm, and the specific surface area (BET) 44 m2/g. Incipient wetness impregnation of the silica extrudates with aqueous solutions of the iron containing precursors resulted in a number of catalysts. The precursors were ammonium iron(I1I) EDTA, iron(II1) citrate, iron(II1) gluconate, ammonium iron(II1) citrate, iron(II1) chloride, iron(II1) nitrate, and iron(II1) sulfate. The concentration of the solutions of the precursors in distillated water was adjusted to result with the known pore volume (0.8 ml/g) in the desired loading of 5 wt% Fe2O3. The pH value of the different ammonium iron EDTA solutions was adjusted with ammonia. The extrudates were evacuated for at least 15 minutes before being impregnated. Subsequently the solution was admitted to the evacuated support. All impregnates were dried for two hours at room temperature in a stream of air. Subsequently the impregnated supports were kept in air successively for at least three hours at 60 O C , for at least three hours at 12OoC, and for three hours at SOO'C. TABLE I Survey of the precursors used for the impregnation Ikon chelate: PH colour solution 1 ammonium iron(II1) EDTAa) NH&EDTA.lSH20 5.3 green 7.1 blood red 8.5 blood red 10.0 blood red I ammonium iron(II1) citrate W F e ciuate b) 4.3 yellow-green Iiron(11I) gluconate. Fe ghlCOMte c, 7.0 deep brown Iron salt: iron@) chloride FeC13.6H20 1.1 orange-yellow iron(III) nitrate Fe(NO3)3.9H20 1 .o orange-yellow iron(III) sulfate Fe2(S04)3'5H20 1.3 pale yellow a) ethylene dkmine tetra acetate b) Fe content 15.2% c) Fe content 10% Light Microscopy: The extrudates were sliced radially or longitudinal and smoothed to a thickness of 0.5 mm. The samples were examined with a Leitz light microscope.. The extrudates were soaked in an immersion oil (Leitz, n=1.52) to make the silica (n about 1.5) transparent. Both bright field (transmitted-light), and dark field (incident-light, illumination angle 60') illumination was used. Transmission Electron MicroscoD v; The samples of the catalysts were ground in a mortar and ultrasonically dispersed into ethanol. The suspension was brought onto a holey carbon film supported by a copper grid. The samples were examined in a Philips EM 420 transmission electron microscope. The accelerating voltage was 120 kV. Scanning Electron Microscopv: The catalyst extrudates were broken and examined in a Jeol JSM- 840A scanning electron microscope at an accelerating voltage of 25 kV. To image the iron containing species more clearly, a back-scattered electron detector was used. 530 x-rav Diffraction; The extrudates were ground and pressed into a sample holder. The samples were examined in a Philips diffractometer o 531 The selectivities (table III) were taken at low conversions. Re-exponential factors (b) of reaction (3) were calculated assuming first order kinetics { 3 } . The activation energy used (85 kT/mole) had previously been determined by measuring different iron oxide catalysts. RESULTS and DISCUSSION Figure 1 shows transmission electron micrographs of catalysts prepared using different compounds. Table 111 contains the particle diameters of the different catalysts. It is apparent that preparation with chelated iron compounds leads to small iron oxide particles (2 to 5 nm). Zron citrate impregnation resulted in small iron oxide particles uniformly distributed over the silica. The catalysts prepared by impregnation with iron gluconate contained small iron oxide particles not intimately contacting the support. The results of impregnation with iron EDTA depended on the pH value of the impregnating solution. Higher pH values (7.1, 8.5, and 10.0) provided small iron oxide particles well dispersed over the silica. Lower pH values (5.3) led to larger particles (>lo nm) situated in between the silica spheres of the support. Impregnation with iron nitrate and iron sulfate, gave rise to large clusters (size > 200 nm) present locally within the silica spheres. Impregnation with a solution of iron chloride resulted in even larger clusters of iron oxide particles, vu., about 1 Frn in size. - 50 nm Figure 1 Transmission electron micrographs of catalysts prepared with different precursors a) NH4Fe citrate, d) Fe gluconate, d) Fe(N03)3, e) FeC13, Magnification 1350000~. b) NH4FeEDTA (pH S.3), c) NH4FeEDTA @H 8.S), 532 Using back-scattered electrons SEM can provide information about the distribution of the iron species deposited within the extrudates on a larger scale. Figure 2 shows back-scattered electron images of the differently prepared catalysts. It can be seen that impregnation with iron citrate and iron gluconate leads to a very uniform distribution of iron within the extrudates. With catalysts prepared with iron EDTA of a pH of 5.3, the distribution is less homogeneous. Featherlike regions can be seen of a higher concentration of iron. Impregnation with an iron nitrate solution leads to iron-rich areas of about 200 nm, and with an iron chloride solution of about 1 pm, which agrees nicely with the observations in the TEM. - 5 wm Figure 2 Back-scatter scanning electron micrographs of catalysts prepared with different precursors a) W F e citrate, d) Fe gluconate, d) Fef.N03)3, e) FeCl3, Magnification 1500~. b) WFeEDTA (pH 5.3), c) WFeEDTA @H 8.5), It is interesting that the distribution of the iron species within the extrudates can be determined very easily within the light microscope. Since LM does not call for expensive equipment and can be fairly easily performed, light-microscopical characterization of catalysts is very attractive. As indicated in table 11, the catalysts showed a remarkable difference in color after the thermal pretreatment. Catalysts prepared with iron chloride as precursor showed an inhomogeneously distributed dark red purple- blue color, while catalysts made from chelated iron compounds were pale yellow-red. Differences in optical behaviour were also observed after impregnating with the immersion oil and examining the extrudates within the light microscope. Catalyst extrudates prepared from citrate or gluconate became transparent / opalescent red, whereas extrudates prepared from iron nitrate were intransparent red- orange. At higher magnifications clusters were seen in the extrudates prepared with iron chloride. 533 The large differences between the four catalysts prepared with iron EDTA solutions of different pH values were striking. The catalyst prepared with a basic iron EDTA solution (pH 8.5) was homogeneously colored as the catalysts prepared with the other chelating agents, whereas the catalyst prepared with iron EDTA solutions of a lower pH (pH 5.3,7.1) showed an eggshell distribution with featherlike structures "entering" the extrudate (figure 3). The dependence of the color and transparency of the catalysts on the particle size of the iron oxide can be attributed to Rayleigh scattering. Relatively large particles have a high scattering intensity and show a low transparency, whereas small particles show a high transparency. The results obtained by LM agree with those obtained by "EM and SEM. It can therefore be concluded that LM provides reliable information about the particle size and the distribution of the material within a silica support. TABLE II Summary of the Light Microscopy results Precursor PH colour catalyst without oil with oil homogeneity NH4FeEDTAl SH20 5.3 red mans. red linmans. yellow eggshell 7.1 red & yellow trans. redhtrans. yellow eggshell 8.5 yellow-red trans. red homogeneous 10.0 yellow-red intrans. yellow homogeneous NH4Fe citrate 4.3 yellow-red trans. red homogeneous Fe gluconate 7.0 yellow-red trans. red homogeneous FeC13.6H20 1.1 red & purple-blue intrans. red inhomogeneous Fe(N03)3,9H20 1.0 strong red slightly trans. red homogeneous Fe2(S04)35H20 1.3 white trans. white homogeneous 0 . 2 m Light microscope pictures of a catalyst extrudate prepared with NH4FeEDTA a) pH = 5.3, b) pH = 7.1, c) pH = 8.5 Figure 3 XRD also showed large differences between the different catalysts (figure 4). Catalysts prepared from organic chelating compounds were found to contain mainly maghemite (y-FezO3) while catalysts prepared from nitrate and chloride only contain hematite (a-Fe2O3). This was confirmed by the magnetic properties of the catalysts. During thermal decomposition in air the iron moieties are 534 reduced to magnetite (Fe304) by the organic chelating agents [8,9]. After the reducing species are decomposed, oxygen oxidizes the magnetite to maghemite (y-Fe203). The catalyst prepared with iron(II1)sulfate only contains dehydrated iron(III)sulfate. Apparently the temperature used was not high enough to decompose the iron sulfate. XRD also exhibited large differences in the shape and the height of the diffraction maxima indicating a wide variation in crystallinity and particle size of the catalysts. Peaks due to large particles are higher and narrower than those due to small particles. The calculated particle sizes (table 111), however, only give information about the weight-mean particle size, and are thus dominated by the largest particles. Fmm the XRD patterns, it can be concluded that there is a large influence of the pH of the iron EDTA solution on the particle size. When impregnating with solutions of a low pH (5.3,7.1) more relatively large maghemite particles were formed. When solutions of a higher pH are used, the amount of large particles decreases. Fg(S04)3 FeC13 FeWW3 WFeEDTA (5.3) WFeEDTA (7.1) WFeEDTA (8.5) NmFeEDTA (10.0) Fe gluconate NHqFe citrate I I I I I I 90° 75O 600 4 5 O 300 1 5 O e 28 Figure 4 XRD patterns of catalysts prepared with different precmars.(Ka1+=1.9373 A) DRIFTS proved to be an appropriate technique to determine the bare part of the silica surface. It is found that the silica is most completely covered by the iron oxide, when iron citrate is used as a precursor (table DI). Very distinct is the difference between catalysts prepared from the chelating iron compounds and from the simple iron salts. Also the inffuence of the pH of the Fe EDTA solution on the coverage of the silica can be seen. 535 The fraction of the iron oxide intimately contacting the silica obtained with TF'R is given in table III. Very interesting is high extent of reduction of the catalysts prepared with iron gluconate and iron EDTA of a pH of 5.3. The small fraction of iron oxide strongly interacting with the silica was also evident from TEM mimgraphs. TABLE III Summary of the results of E M , XRD, DRIFTS , TF'R and activity measurements Precursor pH TEM XRD*) DRIFTS TPR Activity Selectivity diameter (nm) coverage (%) F (%) k,(xlO*s-l) % WFeEDTA.lSH20 5.3 5-25 23 a) 28 19 10.0 97.6 7.1 2-25 23 a) 29 62 9.1 97.6 8.5 2- 16 16 a) 33 51 10.0 97.7 10.0 5-20 21 a) 35 38 10.0 97.3 Fe gluconate 7.0 2-6 6.6 a) 32 26 11.2 96.8 w e cifrate 4.3 2-5 5.5 a) 41 82 13.8 97.8 FeC13.6H20 1.1 10-200 sob) 19 29 0.60 73.1 FeW03)39H20 1 .o 10-20 16b) 7 14 2.90 92.7 Fe2(S04)35HzO 1.3 10-35 34c) - - 1.38 95.7 *)XRD peaks used for calculation: a) (311) y-Fe203, b) (104) a-Fe203, c) (113) Fe2(SO4)3. The above data show a very good agreement between the results of the different characterization techniques. Generally the selective oxidation of hydrogen sulfide reflects nicely the structure of the catalysts as evident from the characterization. Catalysts prepared from chelated iron compounds exhibited a high activity and selectivity. Catalysts prepared from simple salts, on the other hand, exhibited a low activity. Interesting is the high activity of the catalyst prepared from iron EDTA of pH 5.3. Although most of the active material was less well dispersed, the activity still was high. The activity apparently was determined by the relative small fraction of well dispersed iron oxide particles. The high activity of the catalyst prepared from iron gluconate is also noteworthy. However, the activity of this catalyst continuously decreased pointing to sintering of the iron sulfate not interacting with the silica. In summary it can be concluded that catalysts containing small particles of iron oxide highly dispersed on the silica surface show the best performance in the selective oxidation of H2S. Impregnation with solutions of iron chelates, especially of iron citrate, provides the best results. With iron EDTA solutions, different particle sizes and distributions of the precursor after drying were found with solutions of a different pH. The effect of the pH can be attributed to a different crystallization of the precursor during drying. At low pH values iron EDTA easily crystallizes as m e E D T A , where at high pH values badly crystallizing anions are present in the solution. With iron EDTA at low pH values the solubility of NH$eEDTA is low, therefore saturation and primary crystallization already takes place at the outer surface of the catalyst. The narrow pores within the small crystallites of the initially crystallized precursor cause migration of the remaining solution to the external edge of the extrudates. Subsequent drying leads to an eggshell distribution. Initial nucleation of small crystallites of the precursor can also proceed in the exterior macropores of the extrudates. 536 Migration of the liquid to the initial clusters of crystallites leads to the featherlike structures observed in the extrudates impregnated with iron EDTA (pH 5.3) (figure 3). When the solubility is high (e.g. with iron nitrate and iron chloride), the evaporation zone moves back into the extrudates. Eventually the solution breaks up into micro domains [10,11]. When the crystallization takes place in these micro domains inhomogeneously distributed clusters result as shown in figure 1 and 2. When crystallization does not proceed the precursor can coat the surface of the carrier without causing redistribution of the impregnated solution. This results in a higher dispersion. CONCLUSIONS The results demonstrate that precursors, which do not easily crystallize, such as ammonium iron EDTA, ammonium iron citrate, and iron gluconate provide the best results. During drying, crystallization does not proceed readily; the precursor can fairly completely cover the surface of the carrier with an amorphous layer. During calcination highly dispersed iron(1II) oxide results. When the precursor crystallizes to small crystallites, micropores are generated that withdraw the solution from the pores of the support. With wide-porous supports nucleation of small crystallites can easily lead to an inhomogeneous distribution of the precursor. Therefore impregnation with solutions of chelated iron compounds leads to highly dispersed active components and thus to highly active catalysts. These results could not be obtained by impregnation with simple iron salts. ACKNOWLEDGEMENT acknowledged for the financial support. The authors wish to thank R. Zefrin for the SEM. Also VEG-GASINSTITUUT is greatly REFERENCES 1. C.N. Satterfield, Mass transfer in heterogeneous catalysis, M.1.T Press, Cambridge, (1970). 2. J.A. Lagas, J. Borsboom, P.H. Berben, "SUPERCLAUS - The answer to Claus plant limitations", 38th Canadian chemical engineering conference, Edmonton, Canada, (1988). 3. G.R. Meima, B.G. Dekker, A.J. v. Dillen, J.W. Geus e.a., in Preparation of Catalysts IV, B. Delmon, P. Grange, P.A. Jacobs, and G. Poncelet (Eds.), Elsevier, Amsterdam, 83, (1987). 4. A.Q.M. Boon, Ph.D.Thesis in preparation, University of Utrecht, (1990). 5. P.H. Berben, P.J. van den Brink, M.J. Kappers and J.W. Geus. Preprints of the IUPAC- Symposium on Characterisation of Porous Solids, Dechema, 224, (1987). 6. E.Vogt, Ph.D.Thesis, University of Utrecht, (1988). 7. W.J.J van der Wal, Ph.D.Thesis, University of Utrecht, (1987). 8 . M.Booy, T.W. Swaddle. Can. J. Chem., 56, 402, (1978). 9. M.A. Blesa, E. MatijeviC, Advances in Colloid Interface Science, 29, 173-221, (1989). 10. T.M.Shaw, Phys. Rev. Lett., 59 (15), 1671-74, (1987). 11. V.B. Fenelonov, A.V. Neimark, L.I. Kheifez, in Preparation of Catalysis 11, B. Delmon, P. Grange, P.A. Jacobs, and G. Poncelet (Eds.), Elsevier, Amsterdam, 233, (1978). G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 537 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands PREPARATION OF OXIDATION CATALYSTS WITH A CONTROLLED ARCHITECTURE Y.L. XIONG', L.T. WENG, B. ZHOU2, B. YASSE, E. SHAM, L. DAZA3, F. GIL- LLAMBIAS4, P. RUIZ and B. DELMON Unit6 de Catalyse et Chimie des MatCriaux DivisCs, UniversitC Catholique de Louvain, 1 Place Croix du Sud, 1348 Louvain-la-Neuve, Belgium on leave from Xiamen University, Xiamen, China on leave from Instituto de Catalisis y Petroleoquimica, CSIC, Spain Departamento de Quimica, Facultad de Ciencias, USACH, Santiago, Chile 2 presently: Dalian Institute of Chemical Physics, Academy of Sciences, Dalian, China ABSTRACT' We present examples of a method for improving the activity of two-phase catalysts where a remote control operates. The method is based on the combination of special impregnation methods with the detachment and fragmentation of the impregnated precursor when it transforms to the supported oxide. MoO3, Sn02, and Fe2(Mo04)3 were impregnated by quantities of Sb204 equal to those necessary for forming one or a few monolayers. Solutions of antimony chlorides in CHC13, or antimony butoxide in isobutanol were used. The well dispersed precursor fragmented to small crystallites during low temperature calcination or during catalytic work. This method produced catalysts which exhibited activities and/or selectivities superior to those of mechanical mixtures of separately prepared phases in spite of the fact that the quantity of activating oxide or promoter used (Sb2O4) was 6 to 75 times less than in the mechanical mixture. INTRODUCTION This work deals with methods for preparing catalysts made of two or several phases. In all catalysts composed of two (A and B) or several phases, the size of the particles of A and B, the number and the nature of the A-B contacts are critical. Making such catalysts is a challenge. A quite common type of catalysts nude of two phases corresponds to supported catalysts, where the active phase is dispersed on the surface of a carrier. In that case, the carrier may be inert. The problem of catalyst preparation can become more complicated when both phases play an active role, namely there is a cmmeration between phases A and B. This is the case we shall examine here. A well-known cooperation effect is due to bifunctional catalysis: in catalytic reforming, both Pt and alumina carry out part of the catalytic work. Several other types of cooperation have been mentioned in the last years, e.g. between catalysts within zeolitic structures and another component (1-3) or between separate oxide phases in oxidation or related reactions (4-12). Explanations for these cooperations often rest on spill over processes (13-15). The present article will focus on catalysts composed of two (or several) phases active in selective oxidation or oxidative dehydrogenation. Oxidation catalysts are often much more active and selective when they contain several phases. We have shown that a very frequent explanation for that is the occurrence of a "remote 538 control". One phase is able to activate molecular oxygen forming a mobile oxygen species (spill- over). These spill-over oxygen flow onto the surface of other oxide phases, where they create (or regenerate) active sites able to produce the selective products (14,15). A schematic representation of this mechanism is given in Fig. 1. Our results rest, on the one hand, on the study of a model reaction (oxygen aided dehydration of N-ethyl formamide (5,6,14,15,16)) which allowed the identification of catalytic sites created through the remote control (namely : Bronsted acidic sites), and, on the other hand, on the oxidation of isobutene to methacrolein and the oxidative dehydrogenation of 1-butene to butadiene. In these studies, the catalysts have been prepared by mechanically mixing oxides prepared separately (about 40 different mixtures were studied). When a remote control operates, spill-over oxygen must flow easily from one phase to the other. The number of contacts between the small domains of different chemical composition should thus be large and the quality of the contacts excellent for permitting an easy "jump" of spill over species from one phase to the other. The formation of the spill-over species depends on the surface area developed by the phase dissociating oxygen (e.g. Sb2O4). The number of sites to create on the catalytic phase (e.g. MoO3) depends on the surface area of that last phase. A second very important parameter is thus the phase A/phase B surface area ratio. In general, conventional methods cannot allow to achieve these goals. Mixing already formed solids is not very efficient, because the starting solids may have formed aggregates which cannot be easily dispersed: grinding or the use of ultra sound very rarely permit to reduce the size of the aggregates to below 0.5pm. Mixing intimately already formed gels turns out to be nearly as difficult, although technological developments may lead to better results in the near future. In principle, a general method could be used. This would be to start from finely dispersed, non-aggregated precipitates and to mix them energetically at a pH such that the surfaces of each of the phases to be mixed possess opposite electric charges. A modification of the method would be to use micromicelles instead of precipitated particles. Unfortunately, very little has been done in this direction. Another method for obtaining finely interdispersed phases can be employed when the phases to mix exhibit little chemical affinity for each other. The method consists in starting from a homogeneous precursor (e.g. salts of organic acids) incorporating all the elements and letting the to the desired phases during a controlled decomposition to oxides. This can be achieved, for example, for producing a finely interdispersed mixture of BiP04 and MoO3, which have no affinity for each other (17). It is easy to prepare the bismuth salt of phosphomolybdic acid. Upon decomposition, this gives the desired mixture (18). The catalyst is extremely active. Still another method rests on the fact that some oxides may have (at least under certain oxido- reduction conditions) very little affinity with the support of other oxides (19). In principle, one can select a precursor of the oxide to be impregnated, and adapt the impregnation conditions so that the precursor spreads perfectly all around the surface of the other. One can speculate that, if decomposition is made carefully, the continuous layer of precursor will fragment to very tiny oxide particles, thus giving a highly dispersed phase in good contact with the other oxide. 539 ISOBUTENE J\uy â¬IN Phase A Holacular 02 Uokular ISOBUTENE ME THACROLEIN 02 Figure 1. Schematic representation of Remote Control Mechanism. Selective active sites ("imgated' by spill over oxygen) - non selective sites ("non irrigated) Os.o Spill over oxygen This is the strategy that we have adopted in the present work. The objective is to show that active and selective catalysts for the oxidation of isobutene to methacrolein can be produced by this type of preparation method. Three oxide phases have been chosen as supports: Sn02, Moo3 and Fe2 (Mo04)3. These phases are very active in the oxidation of isobutene, but not very selective in the formation of methacrolein. These three phases have a very low (if any) chemical affinity for antimony oxide. This last oxide (Sb2O4) is very active for the dissociation of oxygen. Important synergetic effects have been observed in biphasic catalysts formed by mixtures of the above-mentioned oxides, prepared separately, with Sb2O4. A remote control operates in these systems (14,16,20,22). The phases selected as "supports" have been impregnated with solutions of antimony salts in amounts theoretically necessary to form coverages of 0.5 to a few monolayers of Sb204 over their surface. EXPERIMENTAL, 1. Preparation of catalysts a) Preparation of supports pH=7.5 followed by drying at 110°C for 16h and calcination at 600°C for 8h. i) Sn@ was prepared by precipitation of an aqueous solution of SnC12.2H20 with NH3 at 540 ii) Moo3 was obtained by thermal decomposition of (NH&M07024 . 4H20 in air at 500°C iii) Fe2(Mo04)3 was prepared by mixing a solution of Fe(N03)3 in aqueous citric acid with a solution of (N@)6Mo7024 . 4H20 in water at pH 1-1.5, followed by evaporation and drying at 110°C and calcination at 5OOoC/20h. b) Impregnation of supports The quantity necessary for forming a monolayer of S h O 4 over the supports was estimated from the size of the unit cell of Sb2O4 (0.16 nm2, calculated from its structure (21)) and the surface area of the supports: Sn02 = 9.0 m2 , g-1; Moo3 = 2.0 m2 . g-l and Fe2(Mo04)3 = 2.5 m2 . g-l. The calculated amount of Sb2O4 to form a monolayer represents 4% wt for Sn02, 0.64% wt for Moo3 and 1 % wt for Fe2(Mo04)3. Two types of solution of antimony salts were used: in one case a chloride solution in CHC13 and in the other an organic solution of the alcoxide. Additional solvent was used in order to homogenize the impregnated ions over all the surface. In the present case, the detachment of the impregnated layer of precursor for forming dispersed oxide crystallites on the surface of the support is made thanks to calcination. The details for each of the system are as follows: for 20h. i) Procedure for impregnation of S n q and MoO3. A solution containing Sb+3 and Sb+5 (Sb+3/Sb+5 = 1/1) was prepared from SbCl3 and SbC15. CHC13 was used as solvent. The support powder was immersed in the necessary amount of this solution in a rotavapor, with the addition of 250 ml CHC13. Evaporation was done under reduced presbure. The solvent was removed slowly. The powder so obtained was washed with an ammonia solution in order to eliminate C1- and finally dried at 1 10°C overnight. Impregnated SnO2 was not calcined. Impregnated Moo3 was calcined at 45OOC for lh. Samples with quantities of Sb+3 and Sb+s necessary to form 0.25, 1 and 2 monolayers were prepared for Sn02. They are denoted as 1/4Sb/Sn02, 1Sb/Sn02 and 2Sb/Sn02. Samples of Moo3 containing the amounts of Sb+3 and Sb+s necessary to form 0.5, 1 and 4 monolayers were prepared and denoted as l/2Sb/MoO3, lSb/MoO3 and 4sb/Moo3. ii) Procedure for impregnation of Fe2(Mo04)3. The impregnating solution was composed of antimony butoxide (Sb(OCqHg)3) in isobutanol. This solution was added in the powder support. Water was added dropwise under agitation.at anibiant temperature. A gel was formed. The solvent was evaporated slowly in a rotavapor. The catalyst obtained was dried overnight at 110°C and calcinated at 400°C for 2h. Samples with quantities of antimony necessary to form 1, 3 and 6 monolayers of Sb204 were prepared. They are denoted 1Sb/Fe2(Mo04)3, 3Sb/Fez(Mo04)3,6Sb/Fe2(Mo04)3. 2. Characterization techniques Samples were characterized before and after BET surface area measurements, XRD, XPS, ISS, electron microscopy , Mossbauer spectroscopy, ESR and electrophoretic migration. The measurement procedures are described elsewhere (16,20,22). 541 Sample Before reaction 2.0 M a 3 1/2S b/MoO3 2.0 1 S b/Mo@ 2.0 4s b/Moo3 2.1 Sn02 9.0 1/4S b/SnO2 8.9 1Sb/SnO2 9.1 2 s b/Sn02 9.8 FeAMo04h 2.57 1 S b/FeAMo04)3 1.91 3Sb/Fe2(MoO4)3 2.10 6s b/FeAMo04)3 2.33 1 S b/FedMo04)3 3.14 (in addition calcined at 5W0/2h I 3. Catalyric activity Isobutene oxidation was carried out in a continuous flow, fixed bed reactor (diameter 8 mm). 800 mg of catalysts (particle diameter = 500-800 pm) were used. The temperature of reaction was 400°C and the composition of the reactant gases: isobutylene/Ofl2 = 1/2/7 with a total gas flow of 30 mllmin. After reaction 2.0 2.0 2.0 2.1 9.0 9.8 9.8 10.7 2.47 2.58 3.07 3.11 2.85 RESULTS More details concerning the characterization and the catalytic activity results are given in references 16,20 and 22. 1. Surface area When the oxides supports are impregnated, the BET surface area increases slightly. After the catalytic reaction, the surface area increases subtantially. (The same effect occurs after a simple calcination in the case of the SnO2 and Fe2(Mo04)3 samples). As an example, BET surface areas of pure and impregnated supports are presented in Table 1. Table 1 . BET surface area (m2 g-') for pure and impregnated supports 2. X ray diffraction When the amount of Sb ions is low, only lines characteristics of the support were observed. When the Sb ions amount was higher, lines characteristics of Sbz04 were detected. N o changes were observed after reaction. No new peak appeared. 3. XPS and ISS The binding energies observed in XPS correspond to the values characteristic of pure Sn02, For impregnated supports, the intensity of the Sb XPS signals increases when the amount of For all supports, the Sb signals decrease after reaction, in particular when the Sb content is Sb2O4, Moo3 and Fe2(Mo04)3. These values do not change after reaction. Sb deposited increases. low. In some cases, calcination alone brings about a decrease of Sb signal. The changes of the ISS signals indicate the same evolution as those of XPS. 542 4. Electron microscopy and analytical electron microscopy the concentration of Sb increases, isolated particles of antimony are observed. When the concentration of Sb is low, only the signals of the metal supports are observed. AS 5. Electrophoretic migration measurements Sn02 or Fe2(MoO4)3 (23). The zero points of charge (ZPC) are characteristics of S b O 4 oxides on the surface of the 6. Mossbauer spectroscopy impregnation of antimony ions over the supports or the catalytic reaction. The spectra are characteristic of Sn02 and Fez(Mo04)3. No changes are observed with the 7. ESR A very weak ESR signal corresponding to the dissolution of Sb+5 in SnOZ is observed. This A summary of the characterization results is presented in Table 2. signal decreases after reaction. Table 2 Summary of the characterization results of the impregnated oxides supports using different physico-chemical techniques. A minus ' I - " sign means that the contaminating layer decreases or disappears and the tendency to form two separate phases predominates. A zero "0" sign means that the technique was not used. Methods XRD XPS BE intensity ISS AEM TEM SEM Mossbauer Zeta potential ESR 8. Catalytic activity An example of typical results obtained is presented in Table 3. Results obtained for pure supports are indicated in parenthesis. The catalytic properties of pure oxide supports (indicated in parentheses) are greatly improved by the addition of Sb ions. Compared with pure support, the methacrolein yield and selectivity increase when Sb ions are deposited on its surface. This effect is more pronounced as the amount deposited increases. At the same time, the total conversion decreases. 543 r Sample c (%I y (%I s lSb/MoO3 23.3 (40.0) 7.0 (6.0) 30 (15) 1Sb/SnO2 26 (58.0) 7.0 (1.0) 27 (3.0) 2s b/S n02 42 (58.0) 17.0 (1.0) 40 (3.0) lSb/Fez(Mo04)3 15.2 (51.4) 10.4 (5.5) 68 (10.7) 3Sb/Fe2(Mo04)3 22.3 (51.4) 11.8 (5.5) 53 (10.7) SAMPLE SBET m2.g-1 C(%) Y(%) S(%) M 0 0 3 + S ~ 0 4 (50% of Sb204) 2.0 26.8 7.5 28.0 lSb/MoOg (0.64 wt % of Sb204) 2.0 23.3 7.0 30.0 Sn02+Sb204 (50% of Sb2O4) 9.2 20.0 5.0 25.0 1Sb/SnO2 (4 wt % Sb2O4) 9.8 26.0 7.0 27.0 Fe2(Mo04)3+Sb04 (50% of SbO4) 2.8 10.3 3.0 29.0 . l S b/Fe2(MoO& ( 1 wt % S b204) 2.6 15.2 10.4 68.0 The deposition of small amounts of Sb ions on the surface of the supports modifies considerably their catalytic properties. When compared with pure supports, the yield and the selectivity in methacrolein are significantly increased. In the case of Sn@, this effect is dramatic. It suffices that an amount of Sb ions equivalent to that necessary to form one monolayer be present for observing an increase of 700% in the yield. The selectivity is increased from 3 to 25%. (The two effects together bring about a diminution of the conversion to half the value observed for pure S n e ) . In order to emphasize the effect of using the impregnation method, the results concerning catalytic activity observed for mechanical mixtures (50% of each oxide, prepared separately. (16,20, 22)) and the corresponding supports impregnated with a quantity of Sb2O4 equal to that necessary to form one monolayer, are compared in Table 4. The impregnated catalysts present, in spite of the small Sb content, activities comparable to those observed in the mechanical mixtures. The selectivity andlor the yield in methacrolein are better for the impregnated supports. The effect is particularly important in the case of iron molybdate. The surface area developped by antimony oxide over the surface of the support can be calculated by the difference, after reaction, between the surface area of the impregnated support and that of the pure support. For example, in the case of 1 Sb/Sn02, the antimony oxide develops a total Table 4. Comparison of the catalytic activity between mechanical mixtures and catalysts impregnated by a quantity of Sb2O4 equal to that necessary to form one monlayer (composition in wt%). 544 surface area of 0.8 m2, Table 1. On the contrary, the total surface area of Sb204 forming the mechanical mixtures (50% in weight) is 1 m2, Table 4 (surface area of separately prepared Sb204 is 2 m2/g). This means that only 4 wt % of SkO4 in the impregnated catalysts develop a surface area similar to that of Sb2O4 in mechanical mixtures. In other words, in the impregnated catalysts, the specific surface area by gram of Sb2O4 is 20 m2/g, which is 10 times greater than the specific area of SbzO4 in the mechanical mixtures. From Table 4, we conclude that only 4 wt% of Sb2O4 in the impregnated catalyst activates, at least, twice the number of active centers on SnO2 supports compared with the SnO;? activated in mechanical mixtures. Similar or more significant results can be obtained for other amounts of antimony impregnated or other oxide supports. I Mechanical mixture Impregnated support Figure 2. Comparison of the architecture of mechanical mixtures and impregnated catalysts. A : B : S k O 4 active support (MoO3, SnO2 or Fe2(MoO4)3) selective sites. Irrigated and protected by oxygen spill over The impregnation method improves significantly the selectivity and the yield in methacrolein, as we have discussed above. This means that this method not only allows the formation of a supported phase with a high specific surface area, but at the same time, that it makes that the small crystallites formed on the surface of the support be highly dispersed, thus increasing the number and the quality of the contacts between the two phases. This particular architecture of the catalysts allows the small crystallites of antimony to irrigate with great efficiency the active supports with spill-over oxygen, thus improving their catalytic properties, and consequently a higher surface area of the support is activated (or protected) by the Sb2O4. Fig. 2 shows the architectures of both oxides in mechanical mixtures and in the impregnated catalysts. Our approach to maximizing the interaction by remote control between two oxide phases has thus been to use two phenomena, the perfect spreading of the precursor of the activating (controlling) oxide on the surface of the potentially active phase (the support), and the detachment and fragmentation of the thus formed "envelope" to produce tiny crystallites in contact with the support. It is necessary to obtain two opposite effects with the same partners (Sb on the one hand and the potentially active oxide on the other hand), first a very good wetting, and in the second stage, a detachment. It is clear that the second stage depends on the particular couple of oxides investigated: they must "dislike" each other, namely be unable to form mixed oxides or solid solutions (at least 545 dissolve in each other only in very small proportion, as is the case with the Sb204-Sn@ system) and possess no mutual surface affinity. But the first stage, namely impregnation with the precursor, can be controlled by carefully selecting the impregnating salt or compound (e.g. alcoxy) and using adequate solvents, including "unconventional" organic solvents. Our work shows that, by the impregnation method, active and selective catalysts can be obtained for the oxidation of isobutene to methacrolein. The optimization of the properties of the active phase of the supports (textural: specific surface area, chemical: dopes, etc.) and of the supported phases (surface area, ability for oxygen activation, etc.) coupled with an adequate strategy in the impregnation procedure, can allow to optimize the cooperative effects between the phases to get very perfomant catalysts. In the work presented here, the potentially active phase is selected as a support, and the activating phase is dispersed on it. The other situation is also possible, namely to use the activating phase as a support, and to disperse the potentially active phase on it (19). We have achieved this in several cases (16b, 22, 20). ACKNOWLEGMENTS The financial support of the Service de Programmation de la Politique Scientifique SPPS (P.R.), the Commission of the European Communities (L.D., Y.L.X.), the Universitk Catholique de Louvain (L.T.W., B.Z.) are gratefully acknowledged. We also acknowledge the contribution of Dr. Jean Ladrikre for Mossbauer Spectroscopy (UnitC de Chimie Inorganique et NuclCaire, U.C.L.) and of Dr. Patrick Bertrand for ISS (UnitC de Physico-Chimie et de Physique des MatCriaux, U.C.L.). We are indebted to Mr. M. Genet for his constructive discussion and comments on the XPS measurements. REFERENCES 1 2 3 4 5 6 7 N.S. Gnep, M.L. Martin de Armando, M. Guisnet, in G.M. Pajonk, S.J. Teichner, J.E. Germain, (eds), Spill-over of Adsorbed Species, Elsevier, Amsterdam, 1983, p. 309. K.H. Steinberg, V. Mroczek, F. Roessner, in K.H. Steinberg (ed), 2nd Conference on Spillover, K. Marx Universitat, Leipzig, 1989, p. 150. R. Le Van Mao, L. Dufresne, Appl. Catal., 52( 1989), 1. L.T. Weng, Y.L. Xiong, P. Ruiz and B. Delmon. Tokyo Conference Catal. Sci. Technol., Tokyo, 1990. B. Zhou, S. Ceckiewicz, B. Delmon, J. Phys. Chem., 91(1987), 5061. S. Ceckiewicz, B. Delmon, J. Catal., 108(1987), 294. L.T. Weng, B. Zhou, B. Yasse, B. Doumain, P. Ruiz and B. Delmon, in M.J. Philips and M. Ternan (eds), Proc. 9th Inter. Congress Catal., vol. 4, 1609, Calgary, Canada, Chemical Institute of Canada, Ottawa (1988). 8 F.Y. Qiu, L.T. Weng, E. Sham, P. Ruiz and B. Delmon, in K.H. Steinberg (ed.), Proc. 2nd Internatioal Conf. Spillover, Leipzig, 1989, p. 136. 9 F.Y. Qiu, L.T. Weng, E. Sham, P. Ruiz and B. Delmon, Appl. Catal., 51(1989), 235. 10 a) L.T. Weng, E. Sham, B. Doumain, P. Ruiz and B. Delmon, Proc. New Devel. in Selective Oxidation, I. 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Catal., 97(1986), 312. 18 M.V.E. Rodriguez, B. Delmon, J.P. Damon, in T. Seiyama, K. Tanabe (eds), Proc. 7th Intern. Confer. on Catalysis, Kodansha and Elsevier, Tokyo and Amsterdam, 1981, 1141. 19 B. Delmon, J. Mol. Catal., 1990, in press. 20 Y. Xiong, L. Daza, P. Bemand, P. Ruiz and B. Delmon, submitted for publication. 21 P.S. Gopalakrishonan and H. Manohar, Cryst. Struct. Comm., 4(1975), 203. 22 L.T. Weng, P. Ruiz and B. Delmon, submitted for publication. 23 F. Gil-Llambias, Y.L. Xiong, L.T. Weng, B. Zhou, P. Ruiz and B. Delmon, submitted for publication. K. Marx Universitat, Leipzig, 1989, p. 204. 1(1987), 181. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 547 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands STRUCTURE AND SELECTIVITY CHANGES IN VANADIA-TITANIA-DENOX CATALYSTS M. KOTTER, H . 4 . LINTZ and T.TUREK Institut fur Chemische Verfahrenstechnik der Universitat Karlsruhe (TH), D-7500 Karlsruhe, F.R.G. SUMMARY The rapid evaporation of thin liquid films of precursor solutions containing vanadium and titanium followed by calcination is an adequate method to prepare active and selective catalysts for the SCR process. Catalysts useful for operation at temperatures above 350 OC can be obtained by modifying this preparation method. Addition of sulfuric acid to the precursor solution leads to high surface area catalysts with a low level of vanadia species weakly interacting with the titania carrier, if the vanadia loading does not exceed 10 wt%. Catalytic tests demonstrate that sulfate stabilization increases NO conversion while NzO formation is suppressed. The addition of tungsten is equally beneficial for high temperature operation of the catalyst. Quantitative information about activity and selectivity of the developed catalysts is obtained by rate measurements in a gradientless system. INTRODUCTION A new NO, abatement strategy proposes the use of the Ljungstrom heat exchanger of the power plant as a chemical reactor [I]. In that case the total gas flow, the residence time and the temperature profile inside the heat exchanger/reactor are given a priori and the catalyst has to be adapted t o those prefixed conditions. We need special highly active catalysts for a broad range of temperatures ( 200 to 400 "C). The usual catalysts in SCR processes consist of mixed transition metal olddes. Normally titania is taken as support, among the additional compounds W03, MOOS and especially VzO5 seem to be the most frequently used [2]. The nature of the active vanadia species and its interaction with the underlying titania have been discussed controversially. However, there is now general agreement that thin layers of vanadium oxide on anatase are required as active and selective catalysts in several important industrial processes [3-71. Based on these findings a new preparation method had been introduced leading to highly active and selective catalysts [8]. It is the aim of the present study to extend the operational range of those catalysts to temperatures above 400 "C. This requires both sufficient stability of the carrier and a strong vanadia-titania interaction t o resist in that temperature range. 548 EXPERIMENTAL Preparation of catalvsts All preparations start with an aqueous solution of titanium oxychloride (Kronos Titan; 43 wt% calculated as TiC14). To obtain vanadia-titania catalysts ammonium vanadate is dissolved in that standard solution. After 1 h stirring the dark brown solution shows a pH of about 6, indicating the presence of decavanadate ions [Q]. In the case of quaternary oxides an aqueous solution of ammonium metatungstate is added. Stabilized titanium dioxide carriers or catalysts may be obtained by addition of concentrated sulfuric acid t o the corresponding solutions. Thin layers of the as-prepared solutions are dried and calcined in a preheated oven for 1 h in the temperature range of 250 to 450 OC. The samples reach the calcination temperature within 5 min at 450 OC and 20 min at 250 OC. Characterization of catalvsts The total vanadium content of the catalysts is analyzed manganometrically after dissolution in hot concentrated sulfuric acid. The quantity of vanadia which is weakly interacting with the titania carrier is determined by treatment with 0.3 m ammonia solution followed by manganometric titration [lo]. In order to determine the sulfate content of the stabilized catalysts powdered samples are suspended in water. The acidic suspension is titrated with sodium hydroxide solution up to pH 7. The SO3 content is calculated based on the following stoichiometry: T i ( S 0 4 ) ~ + 4 NaOH - Ti(0H)d + 2 NazSO4 (1) Surface areas are determined by nitrogen adsorption using a standard volumetric BET apparatus. X-ray diffraction (XRD) is performed by use of a Seifert diffractometer with CuKa radiation. Porosity is measured with a Carlo-Erba mercury porosimeter. Egg shell type catalysts are used in the catalytic test measurements. Thin layers of the active compounds have been fixed on nonporous steatite pellets (2 - 3 mm diameter) using a colloidal silica solution (DuPont Ludox A S 4 0 ) as a binder. The tests are performed in an integral flow reactor using a standard gas mixture containing 4% 02, 1000 ppm NO and 1200 ppm NH3 in nitrogen. The gas stream passes over 5 g catalyst with 3 wt% of the active compound. The space velocity, calculated with respect to the total catalyst volume including the inert carrier, is maintained at 29000 h-l. Rate measurements Rate measurements are carried out in a gradientless recirculation system at a flow rate of 33.3 cc/min (STP) and a reflux ratio of 14. In that case the powdered catalyst is fixed on 549 both sides of ceramic platelets to simulate the flow in the Ljungstrom heat exchanger. The reacting system is unambiguously described by three linearly independent equations: NH3 + NO + 0.25 0 2 --+ Nz + 1.5HzO NH3 + + 1.25 02 - NO + 1.5HzO NH, + NO + 0.75 0 2 -t NzO + 1.5HzO The corresponding rates r = L.$ m,i m are related to total catalyst weight. The extent of reaction ti is determined by mass balancing the open system in steady state. The concentrations of ammonia, nitric oxide, nitrous oxide and for verification nitrogen dioxide are measured by non-dispersive infrared spectroscopy. Oxygen is determined by use of a magnetic device. RESULTS AND DISCUSSION Figure 1 illustrates the effect of different oven temperatures on the preparation of a 20 wt % V Z O ~ on Ti02 catalyst. I I 1 I I 250 300 350 400 450 T/ "C . 0.5 - 0 FIG.l different oven temperatures BET surface area (0) and fraction of soluble vanadia (0) after 1 h calcining at 550 The quality of the catalysts is controlled by measuring its surface area and the amount of vanadia species weakly interacting with the underlying anatase and therefore being soluble in ammonia solution. At temperatures less than about 350 OC high surface areas correspond to low amounts of soluble vanadia, the strongly interacting species are favoured. This is consistent with geometrical considerations [ll] estimating the vanadia content necessary to form an ideal monolayer of VzOs on anatase. The results indicate a strong correlation between the surface area and the different vanadia species. Operation of the as-prepared catalysts at temperatures above 350 OC is not possible. Changes of structure and morphology occur, causing a severe damage of activity and selectivity. A rapid sintering of the anatase carrier is leading to the formation of soluble, crystalline vanadia species the appearance of which is strongly promoting the formation of NzO. This has been reported elsewhere in detail [8]. Those results show that the lack of temperature stability is mainly due to the sintering of the anatase carrier which is additionally enhanced by the presence of vanadia in excess of 10 wt%. rutile or crystalline V205 have been observed. In all cases examination by XRD merely indicates the presence of anatase, no traces of Stabilized Carriers and Catalvsts Stabilization of titania and of catalysts with vanadia contents less than 10 wt % is achieved by adding sulfuric acid to the solutions. Evaporation and calcination at 350 OC forms Ti02 (anatase), vanadium and vanadium-tungsten containing catalysts with lower BET surface area (40 - 65 ma/g) than the values reported above. However, the calcination at 450 O C , originally proposed as a test for thermal stability and normally accompanied by loss of surface area due to the sintering of the anatase carrier, now increases the surface area and forms stable carriers and catalysts with surface areas in the range of 80 to 90 m2/g. In Table 1 surface areas and the fractions of soluble vanadia are given for three different catalysts. The results obtained with a sulphur-free sample are compared to those measured with two stabilized catalysts the nominal sulphur to titanium ratio of which is equal to nS/nTi = 0.28. The 9 wt % vanadia reference sample shows a drastic decrease in surface area and the corresponding increase of the fraction of soluble vanadia. In the case of the stabilized samples a growth of surface area is observed but the content of soluble vanadia is not changed in a significant way. 551 TABLE 1 Characteristics of fresh (1 h at 350 "C) and calcined catalysts m2 fresh 115 64 54 'BET/, 24 h at 450 'C 54 80 93 mv,sol fresh 0.25 0.43 0.32 m v 24 h a t 450 "C 0.60 0.36 0.37 The evolution of surface area with calcination time (Fig. 2) shows quite similar patterns for the stabilized pure titania and the vanadia titania sample. There is no interference of the vanadia loading with the thermal processes occurring. Ti02 0 9 wt% v20, 0 10 20 FIG.2 Evolution of surface area with cal- cination time. I / ' weight loss 11."7...1 ns/nTi = 0.28 P O a content L 0 10 20 FIG.3 Weight loss and corresponding SOX contents as a function of calcination time at 450 OC. Our interpretation of the observed phenomena in the case of sulphur containing catalysts is based on the findings reported in Figure 3. We suppose the formation of a titanium sulfate intermediate during evaporation and calcination at 350 "C, even if the real nature of the species could not be confirmed. The only species identified by XRD throughout 552 the thermal treatment has been the anatase form of titania. Nevertheless the existence of different Ti (IV) sulfate species is known from the literature [12]. The sulfate intermediate is decomposed in course of the calcination. This leads to an increase in surface area accompanied by a significant weight loss and a simultaneous decrease of the SO3 content determined by titration as shown in Fig. 3. The structure changes are completed after about 10 h. The remaining catalyst has a high surface area and contains still about 2 wt % SOB. The measured pore volumes for the 9 wt % catalyst are shown in Figure 4. Calcination causes only a slight increase of mean pore diameter but significant growth of the total pore volume. Very similar results are obtained in the case of pure titania and of tungsten containing catalysts. 0.2 1 10 100 1000 10000 pore radius / nm FIG.4 Pore volumes for 9 wt% Vz05 catalyst (ns/nTi=0.28) before (0) and after 24 h calcining at 450 OC (0). Catalytic Tests In Figs. 5 and 6 the formation of NzO as a sensitive measure of catalyst damage [8] is reported for several samples. Fig. 5 shows the strong negative effect of calcining at 450 OC for a 20 wt% VzO5; catalyst (nS/nTi = 0). 553 300 g 200- a \ 0, 0" 100- 0 500 400 2 300 20 wt% V& E 9 200 \ 0 100 - FIG.5 NzO formation for samples calcined at 450 0C (0 1 h; o 24 h) FIG.6 NzO formation for catalysts 24 h calcined at 450 OC I I I LEGEND (FIGS. 6 and 7) (0) 20 wt% V 2 0 5 (.) 9 wt%Vz05 } nS/nTi = 0 8 wt% V z O 5 (0) 9 wt% V z O 5 ('1 16 wt% XOQ nS/nTi = 0.28 1 0 2 $! 8 z" C 0.5 I 24 h at 450 "C 0- 200 250 300 350 400 T / "C FIG.7 NO conversion for samples 24 h calcined at 450 OC 554 Crystalline vanadia species are formed which promote the NzO formation. Figs. 6 and 7 demonstrate how the catalytic behaviour of catalysts calcined 24 h at 450 OC is improved at temperatures above 300 OC by (a) reducing the vanadia loading and (b) stabilization with sulfuric acid. The stabilized samples exhibit the lowest level of NzO formation while NO conversion (Fig.7) is significantly higher than with the 9 wt% and 20 wt% catalysts prepared without addition of sulfuric acid. RATE MEASUREMENTS The tests at constant space velocity allow a fast catalytic screening but give no detailed information about the selectivity behaviour because the catalysts are operated at different conversions. Rate measurements using a gradientless recirculation system provide the only means to quantify activity and selectivity of catalysts properly. Stationary values of the three reaction rates to rm,3 are measured isosystatically as a function of either the NO- or the NH3-concentration. Typical results are shown in Fig. 8. The catalyst chosen is a 20 wt% vanadia titania ternary oxide after calcination at 450 OC for 1 h to give detectable levels of the side reactions r,,Z and rm,3. Nevertheless the rate of the main reaction remains two orders of magnitude higher, demonstrating the high selectivity even in the case of a thermally deteriorated catalyst. lo4 10" 1 o - ~ N H ~ concentration / rnol*P FIG.8 Reaction rates for a 20 wt% catalyst I h calcined at 450 "C 555 Its concentration dependence can be quantified as follows with for the example shown. The influence of the oxygen concentration has not been investigated as it is known to be negligible under technically relevant conditions [13]. The values of the parameters indicated above are not free from the influence of inner transport phenomena. Variation of the thickness of the catalyst layer (lcat) has shown, that this remains true down to a thickness of only 40 p, thus demonstrating the high intrinsic activity of the catalyst. CONCLUSIONS The results show that rapid drying followed by calcination of the precursor solutions gives an adequate method to prepare active and selective catalysts for the SCR process. The catalytic layer in the Ljungstrom reactor should be structured with respect to the temperature profile. In the low temperature domain (below 35OoC) a ternary oxide with a high vanadia content (20 wt%) gives the best performance. To be useful in the high temperature region the vanadia content has to be reduced and the carrier must be stabilized by addition of sulfuric acid to the precursor solutions. The catalytic tests clearly show that sulfate stabilization causes a significant increase in the NO conversion accompanied by the suppression of the NzO formation. The performance of the high temperature catalyst may be improved further by addition of tungsten oxide. Rate measurements in a gradientless recirculation system emphasize the high activities of the catalysts obtained in that way. The use of unusual characteristic lengths of the catalyst layer is necessary to prevent the interference of inner pore diffusion with the chemical kinetics. ACKNOWLEDGEMENTS This work was financially supported by P.E.F. (Projekt Europiiisches Forschungszentrum fur Massnahmen zur Luftreinhaltung), project 87/002/3. Aid through the Fonds der Chemie is equally acknowledged. 556 LITERATURE M. Kotter, H.-G. Lintz, Entropie, 137 (1987), 109. H. Bosch, F.J.J.G. Janssen, Catal. Today., 2 (1988), 369. G.C. Bond, J.Sarkany, G.D. Parfitt, J. Catal., 57 (1979), 476. R.Y. Saleh, I.E. Wachs, S.S. Chan, C.C. Chersich, J.Catal., 98 (1986), 102. H. Bosch, F.J.J.G. Janssen, F.M.G. van den Kerkhof, J. Odenziel, J.G. van Ommen, J.R.H. Ross, Appl. Catal., 25 (1986), 239. A. Baiker, P. Dollenmeier, M. Glinski, A. Reller, Appl. Catal., 35 (1987), 351. F. Cavani, E. Foresti, F. Trifiro, G. Busca, J. Catal., 106 (1987), 251. M. Kotter, H.-G. Lintz, T. Turek, D.L. Trimm, Appl. Catal., 52 (1989), 225. R.J.H. Clark, Vanadium, in J.C. Bailar, A.F. Trotman-Dickenson (eds.) Comprehensive Inorganic Chemistry, Vol. 3, Pergamon Press, Oxford 1973, p. 520. S. Yoshida, T. Iguchi, S. Ishida and K. Tarama, Bull. Chem. SOC. Jpn., 45 (1972), 376. F. Roozeboom, M. C. Mittelmei jer-Hazeleger , J. A.Mouli jn, J .Medema, V.H.J. de Beer, P.J. Gellings, J.Phys.Chem., 84 (1980), 2783. Gmelin, Handbook of Inorganic Chemistry, 8th edn., Titanium, Vol.1, Verlag Chemie GmbH, Weinheim, 1951, pp.345-351. . M. Inomata, A. Miyamoto, Y.Murakami, J . Catal., 62(1980), 140. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands 557 BINARY OXIDE CATALYSTS SYNTHESIZED BY SEQUENTIAL PRECIPITATION C . S . BROOKS Recycle Meta ls , 4 1 Baldwin Lane, Glas tonhury , CT 06033, U.S.A. SUMMARY Cu/Al, Cu/Cr, Ni/Fe and Cu/Fe have been conducted i n t h e presence of o x a l i c a c i d wi th aluminum, chrominum o r f e r r i c hydroxide as t h e f i r s t s t a g e and ad- s o r p t i o n / p r e c i p i t a t i o n of copper o r n i c k e l as t h e second s t a g e . It has been demonstrated t h a t a f t e r a i r c a l c i n e ( 2 5 0 - 3 5 6 C ) enhanced d i s p e r s i o n and ca t a - l y t i c a c t i v i t y f o r t h e room tempera tu re decomposition of hydrogen pe rox ide and o x i d a t i o n of henzaldehyde are provided by t h e b i n a r y ox ides , no tab ly Cu/Al, Cu/Fe and Ni/Fe. INTRODUCTION I n t h i s i n v e s t i g a t i o n an e v a l u a t i o n was conducted of how adso rp t ion -coprec ip i - t a t i o n can h e used t o b e s t advantage w i t h mixed metal hydroxides t o r e a l i z e h igh d i s p e r s i o n and homogeneity. The b i n a r y systems cons idered c o n s i s t e d of combinations such as Cu/A1, N i / A l , Cu/Fe, Ni/Fe and CulCr. The hydroxide of Al, C r o r Fe i s p r e c i p i t a t e d i n a modera te ly a c i d pH r ange t o become t h e sup- p o r t f o r t h e second m e t a l , Cu o r N i , t o b e adsorbed-ion exchanged. The f r e s h l y p r e c i p i t a t e d A1,Cror Fe hydroxide i s an e s p e c i a l l y a c t i v e s u b s t r a t e f o r i o n ex- change. F u r t h e r examination w a s made of modifying t h e adso rp t ion -coprec ip i t a - t i o n p rocess by us ing a n i o n i c a g e n t s such a s a c e t i c a c i d , c i t r i c a c i d , e t h y l e n e diamine t e t r a a c e t i c a c i d (EDTA) and o x a l i c a c i d complexed w i t h t h e Cu and N i c a t i o n s . Sequen t i a l p r e c i p i t a t i o n of mixed hydroxides o f t h e b ina ry systems, N i / A l , The hydroxide c o p r e c i p i t a t e s were a i r c a l c i n e d a t 250 o r 350'C. For s e l e c t - ed sys tems BET s u r f a c e a r e a s and carbon monoxide chemisorp t ion measurements were made. Xray d i f f r a c t i o n measurements w e r e made on s e l e c t e d samples t o c h a r a c t e r i z e c r y s t a l l i n i t y . C a t a l y t i c performance tests were conducted a t 20" C f o r hydrogen peroxide decomposition and benzaldehyde o x i d a t i o n by hydro- gen peroxide . Seve ra l i n v e s t i g a t i o n (1-5) of t h e p r e p a r a t i o n of suppor ted c a t a l y s t s have used s p e c i a l a d s o r p t i o n c o n d i t i o n s f o r a n i o n i c and c a t i o n i c P t s p e c i e s in - vo lv ing t h e i o n exchange c h a r a c t e r i s t i c s of t h e alumina o r s i l i c a suppor t and conduct ing adso rp t ion on t h e a c i d pH s i d e of t h e zero p o i n t of change (ZPC). The novel a spec t of t h e p re sen t approach is t h e u s e of a f r e s h l y p r e c i p i t a t e d hydroxide such as aluminurn,chromium o r f e r r i c hydroxide i n s t e a d of a c a l c i n e d , s t a b i l i z e d ox ide as t h e suppor t . P r i o r a t t e n t i o n has been g iven t o t h e a d s o r p t i o n of m e t a l c a t ion -an ion ic complexes on meta l ox ides and hydroxides bu t r a r e l y f o r t h e purpose of synthe- 558 s i z i n g c o p r e c i p i t a t e d c a t a l y s t p recu r so r s . have involved t h e use of metal o rgan ic complexes such as c i t r a t e s , o x a l a t e s , e t c . which a r e subsequent ly decomposed t o provide enhanced d i s p e r s i o n and homogeneity f o r t h e ox ide c a t a l y s t p recu r so r . anionic-metal complexes f o r s e q u e n t i a l p r e c i p i t a t i o n is considered novel . A number of s t u d i e s (4 ,5-13) However, t h e p re sen t u s e of EXPERIMENTAL PROCEDURES Binary systems synthesized cons i s t ed of Cu/Fe, Ni/Fe, Cu/A1 and N i / A l and Cu/Cr f o r 4-10 w t pe rcen t Cu o r N i i n t h e ca l c ined mixed oxide. Anionic complexing agen t s a c e t i c , c i t r i c and o x a l i c a c i d s and EDTA were used i n molar r a t i o s of 1:l wi th t h e i n i t i a l copper o r n i c k e l . Two s t a g e p r e c i p i t a t i o n s w e r e used s t a r t i n g wi th an i n i t i a l formation of aluminum, chromium o r f e r r i c hydroxide o r Fe c h l o r i d e . In t h e second s t a g e aqueous s o l u t i o n s of Cu s u l f a t e o r N i n i t r a t e were mixed wi th t h e i n i t i a l p r e c i p i t a t e w i th o r without the presence of a 1:l mole r a t i o of s e l e c t e d a n i o n i c complexing agen t s t o complete t h e precip- i t a t i o n . A second mode of c o p r e c i p i t a t i o n used w a s t o preadsorb o x a l i c a c i d on t h e i n i t i a l l y p r e c i p i t a t e d A1,CrorFe hydroxide. by a d d i t i o n of NaOH t o an aqueous s o l u t i o n of A 1 n i t r a t e , C r n i t r a t e The c o p r e c i p i t a t e s were sepa ra t ed by f i l t r a t i o n , a i r d r i e d and ca l c ined a t 250 or 350" C i n p r e p a r a t i o n f o r c h a r a c t e r i z a t i o n and c a t a l y s t performance tests. The m e t a l con ten t s of t h e c a l c i n e d c o p r e c i p i t a t e s w e r e c a l c u l a t e d from t h e r e s i d u a l m e t a l con ten t s of t h e equ i l ib r ium f i l t r a t e s as determined by atomic a b s o r p t i o n (Griswold and Fuss Environmental Labora to r i e s , Manchester, CT). BET n i t r o g e n s u r f a c e areas w e r e measured by S t r u c t u r e Probe, F a i r f i e l d , CT . Addit ional BET s u r f a c e s and CO chemisorpt ion measurements w e r e made by Porous M a t e r i a l s , I n c . , I t h a c a , W . Xray d i f f r a c t i o n ana lyses were made f o r s e l e c t e d samples t o c h a r a c t e r i z e t h e c r y s t a l l i n i t y of t h e ca l c ined c o p r e c i p i t a t e s . C a t a l y t i c performance tests a t 2 O o C cons i s t ed hydrogen peroxide decom- p o s i t i o n and t h e ox ida t ion of benzaldehyde t o benzoic ac id bypass inghydrogenper - ox ide (3% aq . so ln . ) through a bed of ca l c ined c o p r e c i p i t a t e (0 .1 t o 0.2 gm.) supported on a coa r se g l a s s f r i t and measuring t h e ra te of oxygen evo lu t ion . The benzaldehyde ox ida t ion w a s conducted by vigouously mixing 1 0 m l of a xylene s o l u t i o n of benzaldehyde(20wtX)with50ml o f hydrogen peroxide (3%) i n the presence of 0.1-0.2 gm. of ca l c ined c o p r e c i p i t a t e w i t h N a l a u r y l s u l f a t e (50 ppm) s u f a c t a n t p re sen t t o f a c i l i t a t e phase mixing. Benzoic a c i d product ion w a s ea t ab l i shed by t i t r a t i o n wi th NaOH a f t e r 30-60 m i n r e a c t i o n t i m e . Ben- zaldehyde /H202 o x i d a t i o n tests w e r e a l s o conducted by passing 10 m l of so lu - t i o n of 6 w t % benzaldehyde i n aqueous methanol (24 v o l . pe rcen t ) through a bed of 0 . l g of ca l c ined c o p r e c i p i t a t e supported on a g l a s s f r i t and conducting a NaOH t i t r a t i o n of the f i l t r a t e . 559 RESULTS AND DISCUSSION C o p r e c i p i t a t e s w i th Aluminum Hydroxide One series of N i / A 1 b i n a r y hydroxide c o p r e c i p i t a t e s w a s p repared w i t h a n i n i t i a l atomic r a t i o of 1: 1 N i / A l M t h n i c k e l e q u i l i b r a t e d w i t h a n i o n i c a g e n t s a c e t i c a c i d o r c i t r i c a c i d o r EDTA i n a molecular r a t i o 1:l and mixed w i t h t h e i n i t i a l l y p r e c i p i t a t e d A 1 hydroxide . I n t h i s sys tem s e q u e s t r a t i o n of t h e N i i n s o l u t i o n occurred u n t i l a pH of 10-12 w a s a t t a i n e d p rec lud ing a s t a g e d co- p r e c i p i t a t i o n i n an a c i d regime. A second series o f N i / A l b ina ry hydroxide c o p r e c i p i t a t e s u s ing a lower i n i t i a l a tomic r a t i o of 0 .5 f o r N i / A l w a s pre- pared i n t h e presence of a 1:l molecular r a t i o of c i t r i c a c i d o r o x a l i c a c i d . (Table 1.). I n t h i s c a s e N i l oad ings i n t h e range of 4.0-4.3 w t . % w e r e ob ta ined a t pH v a l u e s of 10.0 and 7.5 r e s p e c t i v e l y bu t no improvement i n t h e s t a t e of d i s p e r s i o n as i n d i c a t e d by t h e BET a r e a s of t h e p r e c i p i t a t e s ca l c ined a t 35OoC was obta ined . A series of s e q u e n t i a l c o p r e c i p i t a t i o n s was conducted a l s o w i t h Cu/A1 i n a n i n i t i a l atomic r a t i o of 1:l and w i t h a molecu la r r a t i o o f 1 : l w i t h c i t r i c a c i d o r o x a l i c a c i d (Table 1). In t h e s e systems Cu load ings i n t h e range of . 3.9-5.8 wt.% were ob ta ined a t pH v a l u e s of 4.0 and 7 . 4 r e s p e c t i v e l y . I n t h i s sequence t h e presence of c i t r i c a c i d r e s u l t e d i n a decreased s ta te of d i s p e r s i o n as i n d i c a t e d by t h e BET a rea of t h e c o p r e c i p i t a t e ca l c ined a t 350°C when com- pared wi th t h e r e f e r e n c e sys tem w i t h no a n i o n i c p r e s e n t . On the o t h e r hand t h e system prepared wi th t h e o x a l i c a c i d p re sen t preadsorbed a n t h e i n i t i a l l y pre- c i p i t a t e d Al hydroxide provided an o r d e r of magnitude i n c r e a s e i n t h e d i s p e r s i o n a s i n d i c a t e d by t h e BET area of t h e ca l c ined oxide . TABLE 1 Binary N i / A 1 , Cu/A1 c a t a l y s t p r e c u r s o r s 2 nd BET pH FOR METAL AREA BINARY PRECIPITATION ANIONIC W T O/e m 2/g N i /Al 12.0 None 4.4(Ni) 4.8 N V A I 1 0 . 0 C I TRlC ACID 4.3 (Nil 4 . 3 N i/A I 7 . 5 OXALIC ACID 4 . 0 ( N i ) 2.9 C u /A1 8.0 None 5.6(Cu) 11.0 Cu/Al 4.0 C I T R I C ACID 3.9(Cu) 1.4 Cu/AI 7 . 4 OXALIC ACID 5.8 (Cu) 112.0 560 C o p r e c i p i t a t e s w i t h F e r r i c Hydroxide A series of Ni /Fe b ina ry hydroxide c o p r e c i p i t a t e s w e r e p repared i n a s i m i - l a r manner, i n t h e presence of the a n i o n i c a g e n t s a c e t i c a c i d o r c i t r i c a c i d o r EDTA i n a molecular r a t i o of I:1 w i t h t h e N i p r i o r t o mixing w i t h t h e i n i t i a l l y p r e c i p i t a t e s Fe hydroxide. range 5.5-6.0 i n t h e presence of a c e t i c a c i d o r EDTA bu t s e q u e s t r a t i o n of t h e N i occur red w i t h t h e c i t r i c a c i d . E f f i c i e n t c o p r e c i p i t a t i o n was ob ta ined on ly i n t h e pH range 10-13 f o r a l l t h r e e a n i o n i c s p rec lud ing e f f i c i e n t s t aged p r e c i p i t a - t i o n i n an a c i d regime. b i n a r y hydroxide c o p r e c i p i t a t e s prepared i n t h e p re sence of t h e s e t h r e e a n i o n i c complexing agen t s . A second series of b i n a r y hydroxide c o p r e c i p i t a t e s u s ing a lower i n i t i a l a tomic r a t i o of 0.5 f o r Ni/Fe was prepared i n t h e presence of a 1:l molecular r a t i o of o x a l i c a c i d t o N i (Table 2 ) . The r e s u l t a n t N i l oad ing inc reased from 10.5 t o 21 .0 w t . % . The BET area of t h i s c o p r e c i p i t a t e ca l c ined a t 350° C inc reased from 35 t o 7 4 sq.m.per gm. i n d i c a t i n g a d i s t i n c t improve- ment i n t h e d i s p e r s i o n f o r t h e ox ide prepared i n t h e presence o f o x a l i c a c i d . A s i m i l a r sequence of b i n a r y hydroxide c o p r e c i p i t a t e s u s ing a n i n i t i a l P a r t i a l c o p r e c i p i t a t i o n was ob ta ined i n t h e pH The same r e s u l t s were ob ta ined f o r a series of Cu/Fe a tomic r a t i o of 0.5 f o r Cu/Fe w a s prepared i n t h e presence of a 1:1 molecular r a t i o of o x a l i c a c i d t o Cu (Table 2 ) . I n t h i s case t h e Cu load ing inc reased from 1 .9 t o 23.0 w t . % , a f a c t o r g r e a t e r t han 10. For t h i s sys tem t h e BET a r e a inc reased from 25 t o 78 sq.m.per gm f o r t h e ca l c ined p r e c i p i t a t e i n d i c a t i n g a n even g r e a t e r improvement i n d i s p e r s i o n . TABLE 2 Binary Ni /Fe , Cu/Fe c a t a l y s t p r e c u r s o r s 2 nd BET pH FOR METAL AREA B I N ARY PRECl PlTAT ION AN IONIC WT O/o m 2/g N i /Fe 6.0 None lO.S(Ni) 35 Ni/ Fe 4 .0 OXALIC ACID 21 .O(Ni) 74 Cu/Fe 4 .0 None I . 9 (CU) 25 Cu/Fe 6.0 OXALIC ACID 23.0(Cu) 78 561 C o p r e c i p i t a t e s w i t h Preadsorbed Oxa l i c Acid Seve ra l a d d i t i o n a l c o p r e c i p i t a t e s w e r e p repared wi th N i / A l , Cu/A1 and Cu/Cr i n which s e q u e n t i a l p r e c i p i t a t i o n w a s conducted f o r atomic r a t i o s of 0.1-0.2 wi th o x a l i c p re sen t p r i o r t o a d d i t i o n of t h e N i o r Cu t o t h e A 1 o r C r hydroxide. The r e s u l t s f o r t h e s e c a t a l y s t s a r e summarized i n Tab les 3 CATALYTIC PERFORMANCE The ca l c ined c o p r e c i p i t a t e s (250 " C ) w e r e eva lua ted f o r c a t a l y t i c per- formance a t 20" C f o r two r e a c t i o n s , hydrogen peroxide decomposition and ben- zaldehyde o x i d a t i o n t o benzoic a c i d by hydrogen peroxide (F igs . 1-3). Cu/A1 c a t a l y s t w i t h 3 . 9 w t . % Cu load ing prepared i n t h e presence of c i t r i c a c i d and ca l c ined a t 250'C provided an i n c r e a s e i n t h e hydrogen pe rox ide decomposi- t i o n k i n e t i c s g r e a t e r t han an o r d e r of magnitude and an i n c r e a s e of over two o r d e r s of magnitude f o r t h e system prepared i n t h e presence of o x a l i c a c i d The (F ig . 1 ) . IC R mmoles 0 2 g cat.min. I .c I NO ANIONIC CITRIC OXALIC ACID ACID FIG. 1 Hydrogen peroxide decomposition a t 20°C ove r Cu/A1 Binary oxide c a t a l y s t s 562 This l a t t e r system shows a n improvement i n c a t a l y t i c performance cor responding t o t h e i n c r e a s e i n t h e BET s u r f a c e area of a n o r d e r of magnitude. The i n c r e a s e i n t h e hydrogen peroxide decomposition k i n e t i c s runs coun te r t o t h e d e c r e a s e i n t h e BET s u r f a c e area. The i n c r e a s e of an o r d e r of magnitude f o r t h e hydrogen pe rox ide decomposi- t i o n k i n e t i c s f o r t h e b i n a r y Ni/Fe and Cu/Fe c a t a l y s t s c o r r e l a t e s w e l l w i th t h e doubl ing of t h e BET s u r f a c e areas (Fig . 2 ) . I N i NO ANIONIC OXALIC ACID FIG. 2 Hydrogen pe rox ide decomposition a t 20°C over Ni/Fe and Cu/Fe b i n a r y meta l ox ide c a t a l y s t s The r a t e s f o r benzaldehyde o x i d a t i o n t o benzoic a c i d a t 20' C are an o r d e r of magnitude lower than t h e hydrogen pe rox ide decomposition rates on t h e s e same c a t a l y s t s f o r N i / A 1 and Cu/A1 ca l c ined p r e c i p i t a t e s and provided on ly modest i n c r e a s e s f o r t h e systems prepared i n t h e presence of o x a l i c a c i d (F ig . 3 ) . 563 1.0 mmoles Benzoic Acid g cat. min. c u o+--c" R- .01 NO ANIONIC OXALIC ACID F I G . 3 Oxidat ion of Benzaldehyde a t 20'C t o benzoic a c i d ove r N i / A 1 and Cu/A1 b i n a r y ox ide c a t a l y s t s Add i t iona l c a t a l y t i c a c t i v i t y tes ts and base adso rp t ion c a p a c i t i e s were determined f o r N i / A l , Cu/Al and Cu/Cr b ina ry c o p r e c i p i t a t e s wi thout and wi th preadsorbed o x a l i c a c i d ( s t o i c h i o m e t r i c w i th t h e i n i t i a l N i o r Cu added i n t h e second s t a g e (Table 3 ) . The Cu/A1 c o p r e c i p i t a t e s w i th preadsorbed o x a l i c a c i d provided t h e most s i g n i f i c a n t improvement f o r H202 decomposition a t room tempera ture . A l l t h e c o p r e c i p i t a t e s , w i thou t o r w i t h preadsorbed o x a l i c a c i d f e l l w i t h i n a r e l a t i v e l y narrow range of 0.41-1.18 maq. a c i d pe r gram of ca t a - l y s t pe r minute f o r benzaldehyde o x i d a t i o n a t room t empera ture . These r e a c t i o n r a t e s (Table 3) determined by pass ing t h e benzaldehyde r e a c t i o n s o l u t i o n through t h e c a t a l y s t bed are a n o r d e r of magnitude h ighe r t han ob ta ined f o r t h e o x i d a t i o n rates ob ta ined wi th t h e c a t a l y s t suspended i n a s t i r r e d r e a c t i o n mix- t u r e (F ig . 3) and are cons idered t h e more a c c u r a t e measure of c a t a l y t i c a c t i v - i t y . 564 TABLE 3 C a t a l y t i c a c t i v i t y f o r hydrogen peroxide decomposition and benzaldehyde o x i d a t i o n and b a s e a d s o r p t i o n c a p a c i t y WALYST beta1 2Preadsorw 3 ~ 2 ~ 2 âBenzaldehyde 5Base Pdsorp . N i / C u O x a l i c Acid Decoripn. Oxidation NH40I-âmeq/qcat wt.% 0 zmles nxq acid 7- gcat.min. qcat.min. 2 mtal Qlemisorb, Ni/Al-18 N i /Al-2 4 Cu/Al-l Cu/Al-3 CWAl-6 Cu/Cr--18 Cu/Cr-20 A 4.4Ni No 8.3Ni Y e s A 5.6Cu No A 5.8Cu Y e s A 5.6Cu Y e s A 5 . 7 a No A 9.6Cu Y e s 0.024 0.41 0 .4 0i1.0 0.048 1.01 1.9 0 1 .2 0.0048 0.63 1.7 0 0 . 8 1.24 1.18 2.2 0 1.1 1.99 0.85 1 . 6 o 1.1 0.54 0.67 0 .3 - 0.96 0.48 1 . 8 - IWE3: 1 ) Based on w e i g h t after 1 hr. air calcine at 250 OC ( A ) or 35OOC (A). 2 ) 1:l stoickiometric w i t h N i or Cu added to Al or C r hydroxide. 3) Based on 0 evolution f rom 3 w t . % aq H 0 passed through 0 . l g of catalysz supported on porous glas; Lit. 4 ) Based on meg. of N/10 LhOH titer for benzaldehyde 6 w t . % i n H202 (3 s t . % ) and. CH30H (24 v o l . % ) solution passed through 0 . l g of catalyst supported on porous glass frit. solution pass& through 0.lg catalyst supprted on porous glass frit after initial drying 1 hr. 15OOC ( 0 ) and f o l l o w i n q in i t ia l NH40H exposure and. drying 1 hr. 15OOC. 6) C r ( V I ) appeared in sscond NH40H filtrate. 5 ) Based on meq. of N/10 â¬?a titer f r o m 5 m l of N/10 W40H Base a d s o r p t i o n c a p a c i t i e s determined by pass ing t h e N/lO NH OH through t h e c a t a l y s t bed i n d i c a t e s i g n i f i c a n t enhancement i n a l l cases where t h e CO- p r e c i p i t a t e s w e r e p repared i n t h e presence of preadsorbed o x a l i c a c i d . Approx- imate ly one h a l f t o two t h i r d s of t h e b a s e a d s o r p t i o n c a p a c i t i e s f o r ad- sorbed NH40Hare a t t r i b u t e d t o chemisorp t ion (not desorbed a f t e r 1 h r . 150°C h e a t i n g ) f o r t h e Cu/A1 system wi thout and w i t h o x a l i c a c i d and f o r t h e N i / A l w i th preadsorbed o x a l i c a c i d . a s d i s t i n g u i s h e d from vapor phase NH a d s o r p t i o n , may have some s e r i o u s l i m i t a - 4 It is recognized t h a t NH OH s o l u t i o n a d s o r p t i o n , 4 3 565 t i o n s f o r c h a r a c t e r i z a t i o n of t h e b a s e a d s o r p t i o n s i tes on t h e s e o x i d e s (14) . The chemisorbed b a s e a d s o r p t i o n c a p a c i t i e s a r e c o n s i d e r e d t o h a v e more l e g i t - imacy b u t no c o r r e l a t i o n s are o f f e r e d pending more thorough c h a r a c t e r i z a t i o n and o p t i m i z a t i o n of t h e c a t a l y s t c o m p o s i t i o n s . CONCLUSION It h a s been demonst ra ted t h a t s e q u e n t i a l p r e c i p i t a t i o n i n a m o d e r a t e l y a c i d pH r a n g e f o r t h e b i n a r y m e t a l sys tems Cu/Al, Cu/Cr, Cu/Fe and Ni /Fe i n t h e p r e s e n c e of o x a l i c a c i d w i t h aluminum o r f e r r i c h y d r o x i d e a s t h e f i r s t s t a g e and adsorption/precipitation of copper o r n i c k e l a s t h e second s t a g e p r o v i d e s metal o x i d e s ( a f t e r 250-350' C a i r c a l c i n e ) w i t h c o n s i d e r a b l e en- hancement i n d i s p e r s i o n and i n c a t a l y t i c a c t i v i t y , n o t a b l e f o r Cu/Al, f o r t h e room t e m p e r a t u r e decomposi t ion of hydrogen p e r o x i d e and benza ldehyde oxida- t i o n by hydrogen p e r o x i d e . REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 1 3 14 J . P . B r u n e l l e , P r e p a r a t i o n of C a t a l y s t s I1 Proceed . 2nd I n t . Symp. Louvain- la-Neuve S e p t . 4-7, 1978, Eds. B. Delmon, P. 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J a c o b s , E l s e v i e r , Amsterdam (1983), p . 1 Ch. S i v a r a s and P. K a n t a r a o , Applied C a t a l y s i s , 45 , (1988) , p.103 Y. Teraoka , Hua-Min-Zhang, N. Yamazoe, Proceed . 9 t h I n t . Congress on C a t a l y s t s , J u n e 26-July 1, C a l g a r y , Canada, Eds. , M . J . P h i l l i p s and M. Ternan , Chemical I n s t . of Canada, O t t a w a (1988) , p.1984 M.F. Wilson, 0. Antinluoma, J . R . Brown, Am. Chem. SOC. P e t . Div. R e p r i n t from Symp. on P r e p a r a t i o n and C h a r a c t e r i z a t i o n of C a t a l y s t s , S e p t . 25-30, N a t l . Mtg., Los Angeles , CA, Vol. 3 3 ( 4 ) , (1988) , p.669 J . M . Jehng and I . E . Wachs, Am. Chem. SOC. , P e t . Div. R e p r i n t from Symp. on New C a t a l y t i c Materials and T e c h n i q u e s , S e p t . 10-15, N a t l . Mtg. M i a m i , F1, Vol . 3 4 ( 3 ) , (1989), p.546 K. Tanabe, S o l i d Acids and Bases , Chapter 3 , Academic P r e s s , New York,(1970) This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 567 ZrO2 AS A SUPPORT : OXIDATION OF CO ON CrOx/ZrOZ T. YAMAGUCHI, M. TAN-NO and K. TANABE Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060 (Japan) SUMMARY Supported CrOx catalyst was prepared from Cr(C0) , (NH4)2Cr04 and Cr(N0 )3 by depositing on to and SiO2. Amount of frOx deposited was O.? - 1.8 mmol/g (0.7 - 13.4 wt8as Crp03). Catalytic activity was evaluated by the oxidation of CO. Catalytic activity of Zr02-supported CrOx was 20 times higher than that of Si02-supported one at the lower loading range. Physicochemical properties of resulting samples were examined by means of XRD. ESR, and XANES. I n conclusion, CrOx dispersed on SiO is i n the form of tetrahedral coordination with three dimensional cryst5 growth and hence lower number of catalytically active sites, while CrOx is finely dispersed or two dimensionally dispersed on ZrOz support with a square pyramidal coordination and hence the higher effective surface area. ZrO INTRODUCTION Structure, electronic states, and binding states of supported metal oxides are considered to be i n different states compared with bulk metal oxides because of their interaction with supports. These factors may affect strongly the catalytic performances. Supported Cr oxides are being widely used for the polymerization of ethylene, the hydrogenation of alkene, oxidation reactions and so on, and are one of the important catalysts in industry. Cr/Si02 is one o f the typical catalyst in supported Cr catalysts family, and the many works have been reported o n the nature of active site for the polymerization of ethylene (refs.1-3). Cr/A1203 as well as Cr/Si02 is also investigated in the sight of alcohol decomposition (refs. 4,5). Most of works on the supported Cr catalysts i s concentrated to use SiO2 or A1203. and only a little work was found for the use of other supports such as Ti02 and Zr02 (refs. 6 , 7 ) . Zirconium dioxide shows specific catalytic actions for the cleavage of a C- H bond (ref.8) and the hydrogenation of buta-1,3-diene by a molecular hydrogen and hydrogen donor molecules such as cyclohexadiene (refs.9-11) and high selectivities for the formation of 1-olefins from secondary alcohols (ref.12) and of isobutane in CO + HZ reaction (ref.13). Recently decomposition of triethylamine to yield acetonitrile, in which both dealkylation by acidic sites and dehydrogenation by basic sites were involved, was reported (ref.14). These characteristic behaviors of Zr02 are considered due to the acid-base 568 b i f u n c t i o n a l c a t a l y s i s . Z i r c o n i u m d i o x i d e i s n o t o n l y an i n t e r e s t i n g o x i d e c a t a l y s t , b u t i s becoming even more impor tan t as a c a t a l y s t support . For ins tance, t h e Rh supported on ZrO2 e x h i b i t s h i g h e r c a t a l y t i c a c t i v i t y f o r t h e hydrogenat ion o f CO and C02 compared w i t h t h a t supported on A1203, S iOz , e tc . ( re fs . l5 ,16) . I n p a r t l c u l a r , t h e Rh/Zr02 c a t a l y s t shows t h e h i g h e s t a c t i v i t y f o r t h e hydrogenat ion o f C02 ( re f .15) . Such a s p e c i f i c c a t a l y t i c p r o p e r t i e s has a l s o been repo r ted over Rh carbony l c l u s t e r s impregnated on to Z r -con ta in ing s i l i c a ( r e f s . 17,18). T h i s paper dea ls w i t h t h e p r e p a r a t i o n and t h e c h a r a c t e r i z a t i o n o f C r ox ide supported on Zr02 by t h e o x i d a t i o n r e a c t i o n o f CO on t h e one hand and t h e s t r u c t u r a l i n v e s t i g a t i o n by XRD, XANES, and ESR on t h e o the r . EXPERIMENTAL Supported CrOx c a t a l y s t s were prepared f rom (NH4)2Cr04 by d e p o s i t i n g on t o Zr02 and S i O z and c a l c i n i n g a t 773 K f o r 3 h. C r ( C 0 ) G and Cr(N03)3 were a l s o used as a s t a r t i n g m a t e r i a l . Amount o f CrOx depos i ted was 0.1 -1.8 mmol/g (0.7 - 13.4 w t % as Ct-203). C a t a l y t i c a c t i v i t y was eva lua ted by t h e o x i d a t i o n o f CO ( 4 5 T o r r ) w i t h 10 ( o r 90) T o r r o f 02 a t 473 K and by 60 min r e a c t i o n by us ing a c losed r e c i r c u l a t i o n r e a c t o r . Physicochemical p r o p e r t i e s o f t h e samples were examined by means o f XRD. ESR. and X A N E S . A q u a n t i t a t i v e a n a l y s i s o f Cr2O3 phase was p e r f o r m e d on Rigaku-Denki D-9C X-ray d i f f r a c t o m e t e r us ing CaF2 as an i n t e r n a l standard. Amounts of C r loaded were measured on P h i l l i p s PW-1404 X-ray f luorescence spectrometer. ESR spectrum was ob ta ined by us ing Var ian E-3 Spectrometer a t room tempera ture o r l i q u i d n i t r o g e n temperature. X-ray abso rp t i on exper iments i n t h e t ransmiss ion mode were c a r r i e d o u t on EXAFS f a c i l i t i e s i n s t a l l e d a t BLlOB a t t h e Photon Fac to ry i n Tsukuba, Japan. RESULTS AND DISCUSSION S t a r t i n q m a t e r i a l and c a t a l y t i c performance E f f e c t o f s t a r t i n g m a t e r i a l on t h e CO o x i d a t i o n was examined by u s i n g Cr(C0)6, (NH4)2Cr04 and Cr(N03)3. Table 1 summarizes t h e r e s u l t s . I n o x i d i z e d s t a t e , t h e r e i s l i t t l e d i f f e r e n c e i n c a t a l y t i c a c t i v i t y even t h o u g h t h e d i f f e r e n t s t a r t i n g m a t e r i a l s were employed. I n p a r t i a l l y reduced s ta tes , t h e r e i s a s l i g h t d i f f e r e n c e when S i02 was used as a support . S i02 i s r e l a t i v e l y i n e r t suppor t and t h e i n t e r a c t i o n w i t h CrOx may be weak. R e v e r s i b i l i t y between C r 6 + and C r 3 ' may be more p ronounced i n C r ( N 0 3 ) 3 - d e r i v e d c a t a l y s t t h a n (NH4)2CrO4-der ived one. No d i f f e r e n c e was f o u n d on t h e Z r 0 2 - s u p p o r t e d c a t a l y s t s . Thus i t can be concluded t h a t t h e s t a t e o f C r O x depos i ted on Zr02 569 TABLE 1 E f f e c t o f s t a r t i n g m a t e r i a l and suppor t on CO o x i d a t i o n c a t a l y s t C r loaded s t a r t i n g m a t e r i a l COP y i e l d/pmo 1 a ) /mmol g-1 02 oxidizedb)CO reducedb) a Y i e l d a f t e r 60 min r e a c t i o n a t 473 K. Ox ida t i on o r r e d u c t i o n a t 773 K. i s a lmost independent o f t h e s t a r t i n g m a t e r i a l s i n t h i s range. Herea f te r s t a r t i n g m a t e r i a l was f i x e d t o use (NHq)$r04. E f f e c t o f r e d u c t i o n on CO o x i d a t i o n An e f f e c t o f r e d u c t i o n on C O o x i d a t i o n was examined b y u s i n g t h e sample w i t h 0.2 mmol-Cr/g and i s shown i n F ig . 1. CO r e d u c t i o n a t 573 K r e s u l t e d i n t h e i nc rease i n t h e c a t a l y t i c a c t i v i t y r e g a r d l e s s t h e c a t a l y s t s ; c o p r e c i p i t a t e d Cr203-Zr02 c a t a l y s t showed an i n t e r m e d i a t e a c t i v i t y w i t h a s i m i l a r enhancement by t h e reduc t i on . A f u r t h e r i nc rease i n c a t a l y t i c a c t i v i t y was observed by t h e 773 K reduc t i on . The c a t a l y t i c a c t i v i t y of reduced C r O x / Z r 0 2 i s s t i l l h i g h e r t h a n t h a t o f CrOx/Si02. An enhancement i n t h e c a t a l y t i c a c t i v i t y b y t h e r e d u c t i o n may be ob ta ined by t h e i n c r e a s e i n t h e n u m b e r o f 0.4 a: oxdn. ( S O O T ) b: redn. (3OO'C) c : redn. (5OO'C) x a 0.3 E E - T3 W - .- 0.2 5 0.1 Fig. 1. E f f e c t o f CO r e d u c t i o n on c a t a l y t i c a c t i v i t y . ( a ) 0 o x i d a t i o n a t 773 K. ( b ) C 6 r e d u c t i o n a t 573 K. ( c ) CO r e d u c t i o n a t 773 K. C r con ten t = 0.2 mrnol/g. 570 catalytically active sites or by the reduction to lower oxidation states. Effect of loading amount on catalytic activity Since the surface area o f supports is limited, amount of deposited material also affects catalytic performances as well as s u p p o r t i t s e l f and s t a r t i n g materials. Figure 2 compares the changes of catalytic activities obtained over (NH4)pCrOq-derived Cr oxide supported on SiO2 and ZrOE when the amount o f Cr loaded was changed. It is interesting to note that the catalytic activity o f supported CrOx varies with the 0 0.5 10 1.5 . 2 .o C r content I rnrnolg-1 Fig. 2. Catalytic activity vs. Cr content in CO oxidation. amount of Cr loaded on both CrOx/Zr02 and CrOx/SiOZ but in a different manner. On CrOx/ZrOp, the catalytic activity first increases steeply until the amount o f Cr loaded reaches t o 0.5 mmol/g and then the slope becomes lower. On the other hand, oxidation activity on CrOx/Si02 was very low at low Cr content but became high at higher Cr content. Catalytic activity o f Zr02- supported CrOx was 20 times higher than that of Si02-supported one at the lower loading range. Even at the highest amount of loading, the catalytic activity o f CrOx/Zr02 was still 7 times higher than that of CrOx/Si02. The activity per a Cr atom on CrOx/Si02 was 0.3 below 0.5 mmol-Cr/g and this increased to 0.6 above this amount, while that o f CrOx/Zr02 was 7 .5 below 0.5 mmol-Cr/g and decreased to that of CrOx/Si02. Reaction kinetics also depends on Cr content and support. On CrOx/SiOZ. zero-th order with / < I C r l S i O 7 / 0 C r content I mrnoig-1 Fig. 3. Change of XRD intensity vs. Cr content. 571 respect to oxygen was found, while on CrOx/Zr02, a half order and zero-th order kinetics were observed below and above 0.5 mmol-Cr/g, respectively. These findings suggest that the state of Cr species varies with not only the amount of Cr dispersed, but also the support. Crystallization and loadinq amount Crystal structure and its development upon the amount of loading were investigated by XRD. When Si02 was used as a support no diffraction line was observed below 0.5 mmol-Cr/g. Diffractions from Cr2O3 starts to appear at 0.5 mmol-Cr/g and the intensity was increased by the increase of Cr content. No phase such as CrpO5 and Cr20 other than Cr2O3 was found. On a Zr02 support, no phase other than tetragonal and monoclinic Zr02 was found below 0.5 mmol-Cr/g. A Cr2O3 phase develops above this amount. Normalized intensities by using CaF2 as an internal standard were measured and plotted against Cr contents. Figure 3 illustrates the change of the normalized intensities obtained for Zr02 and SiO2 supports. This clearly indicates that the development of Cr203 phase is more pronounced on a SiO2 support than on Zr02. Thus a likely conclusion is that a raft-like phase of Cr2O3 grows preferably on a Zr02 support while a three-dimensional crystals tend to grow on SiO2. If we consider the dispersion simply based on the surface area (SiOz : 299 m2/g, Zr02 ; 50 m2/g), an i l l - dispersed state may be expected on Z r 0 2 . B u t i t w a s n o t t r u e . Aggregation took place more easily on a S i O 2 surface than on Zr02. A s p e c i f i c i n t e r a c t i o n may be expected on Zr02 surface as in the case of perovskite (ref.19). In Fig.4, catalytic activities a r e p l o t t e d a g a i n s t X R D intensities of CrOx/ZrOp and of CrOx/Si02. On CrOx/ZrOZ, a sharp increase in catalytic activity was found at the ill-crystallized or well-dispersed phase , while the activity of CrOx/Si02 was kept low -0- C r l Z r 0 2 --C C r l S i 0 2 0 0 0.5 1 .o XRD intensity / a.u Fig. 4. Catalytic activity vs. extent of crystallization. 572 even at highly crystallized state. (a) 36G T h l s s u g g e s t s t h a t a l o w e r A -4k c a t a l y t i c a c t i v i t y o v e r t h e ~,=1986,, g,=1 978 C r O x / S i 0 2 c a t a l y s t could be postulated by the lower surface area available. State at lower amount of loading ESR spectroscopic examination may be helpful t o u n d e r s t a n d valence states and coordination of metal cations microscopically. Electronic state of Cr cations, especially at the lower amount of loading was examined by means of ESR s p e c t r o s c o p y . F i g u r e 5 i l l u s t r a t e s t h e ESR s p e c t r a obtained at room temperature for Fig. 5. ESR spectra of CrOx/SiOz(a) oxidized Cr0x/SiO2(Fig.5a) and and CrOx/ZrOp(b) in oxidized state. Cr content = 0.2 mmol/g. CrOx/ZrOz(Fig.5b). Cr5+ exhibited on CrOx/Si02 (g,=1.986, g,,=1.905, iili=i45G). A H at 77 K was 173G and the intensity ratio was 20, though Curie's law predicts this ratio should be 4. Adsorption o f water resulted in the narrowing in AH (g,=1.978, g,,=l.958, AH=36G); no temperature dependency was observed in A H . Thus it was concluded that the Cr species on Siop was tetrahedrally coordinated Cr5+. On Zr02 support, Cr species also exhibited as Cr5+ ( g = 1 . 9 7 1 , AH=49G); no temperature dependency was found. Adsorption of water reduces line width to 39G but there was no temperature dependency. T h i s clearly indicates that Cr species on Zr02 is in the form of Cr5+ in a square pyramidal coordination. CO reduction of CrOx/Si02 and CrOx/Zr02 at 773 K resulted in a complete removal o f Cr5+ species. Though Cr3+ was found (g=1.97, A H = 8206) on CrOx/Si02, no signal was obtained on CrOx/Zr02 after reduction. Cr species can be reduced regardless the coordination structures, but square pyramidally coordinated Cr species are reduced more easily and deeply. Temperature- programmed-reduction of CrOx/Si02 and CrOx/Zr02 also supported this observation. Thus CrOx on SiO2 was in the form o f tetrahedral coordination while that on Zr02 was in a well-dispersed square pyramidal coordination. Cr K-edge XANES of CrOx/Si02 (0.2 mmol/g) clearly shows the pre-edge peak of 1s-3d transition, which indicates the existence o f tetrahedrally 573 coordinated Cr6+. This observation supports that Cr ions on Si02 was i n t h e f o r m o f t e t r a h e d r a l coordination. In conclusion, Cr2O3 crystals grow on both ZrO2 and SiO2 when the amount of Cr loaded was increased. However, there is a difference in crystal growth on these supports as illustrated in F i g . 6 . CrOx is f i n e l y d i s p e r s e d o r t w o dimensionally dispersed on Zr02 support with a square pyramidal coordination at lower Cr content and grows to Cr2O3 at higher Cr content, while CrOx dispersed on Si02 is i n the form of tetrahedral coordination with three dimensional crystal growth regardless the amount o f Cr loaded. It has been reported that terminal oxygens CrOx on Zro2 and si02- bound to tetrahedrally coordinated Cr are less active (ref. 20). Thus, t h e l o w e r a c t i v i t y toward C O oxidation on CrOx/Si02 can be interpreted in terms of the lower surface area and the tetrahedral coordination of Cr ions. The higher activity on CrOx/Zr02. on the other hand, is based on the higher effective surface area and the square pyramidal coordination. Fig. 6. Model of crystal growth of REFERENCES 1. M.P.McDanie1. Adv. Catal., 33 (1985) 47. 2. K.G.Miesserov. J. Catal.. 22 (1971) 40. 3. D.L.Myers and J.H.Lunsford. J . Catal., 92 (1985) 260. 4. E.M.Ezzo, N.A.Yousef and H.S.Maznar, Surf. Technol., 14 (1981) 65. 5. M.Richter, E.Alsdorf, R.Fricke, K.Jancke and G.Ohlmann. Appl. Catal., 24 6. F.D.Hardcastle and 1.E.Wachs. J. Mol. Catal., 46 (1988) 173. 7. A.Cimino, D.Cordischi, S.D.Rossi, G.Ferraris, D.Gazzoli, V. Indovina, (1986) 117. G.Minelli, M.Occhiuzzi and M.Valigi. Proc. 9th Intern. Congr. 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Catal., 76 (1982) 1. 17. M.Ichikawa, M.Sekizawa, K.Shikakura and M.Kawai. J . Mol. Catal., 11 (1981) 167. 18. T.M.Salama and T.Yamaguchi. Proc. I n t e r n . Symp. Acid-Base Catal.. Hokkaido U n i v e r s i t y , Sapporo, Kodansha S c i e n t i f i c , Tokyo, 1989. 19. N.Mizuno, H .Fu j i i and M.Misono, "Shokubai" (Ca ta l ys t ) , 30 (1988) 392. 20. Y.Iwasawa. Y.Sasaki and S.Ogasawara, J. Mol. Catal., 16 (1982) 27. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 5 75 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands Methane oxidative coupling by definite compounds( e.g. perovskite, cubic or monoclinic structure, . . . ) obtained by low temperature processes J. L. Rehspringer' , P.Poix A. Kaddouri2, A. Kiennemann2 -l GMI, IPCMS, EHICS, lrue Blaise Pascal Strasbourg 67008. -2 Laboratoire de chimie organique appliquee EHICS, lrue Blaise Pascal Strasbourg 67008. Abstract. We investigate the catalytic properties of several rare earth definite compounds ( e.g. perovskite, cubic or monoclinic structure,...). We notice that the nature of the rare earth oxygen environment is in relation with the selectivity. Coulombian energy computations show clearly these relationships. Introduction. Since the last ten years we observe an increase of researches concerning the production of chemicals and liquid transportation fuels using basically methane rather than syngas. The conversion of natural gas into methanol, ethylene or ethane is one of the solutions retained. The coupling oxidative reaction of methane into C2 hydrocarbon seems to be the most promising way. Since the first works of Lunsford (l), Keller and Bahsin (2),a large scale catalyst screening test and evaluation exercises have been carried out. But the diversity of the experimental works does'nt permit to draw any broad conclusions concerning catalyst activity and selectivity. Four catalytic systems can be considered: oxides from IIA group, rare earth oxides, transition metal oxides, oxides from IIIA,IVA and VA ( 3 ) . All these oxides are in fact doped by alcaline using the impregnation process. It has been recently discovered a new group formed by definite compounds from the former oxides. We can notice that the greater activity of lithium oxide deposited on magnesium oxide in comparison with sodium oxide on MgO has been assumed by the fact that Li+ and Mg" have close ionic distances and thus lithium oxide can easily form s o l i d solutions with MgO ( 4 ) . The studies of reaction mechanisms have shown clearly the presence of surface species such as 0-, 02- or 02*- as mediators of the creation of methyls radicals and the possibility to have sites or species which are specific for selective or non selective oxidation. For all these facts we try to bind the reactivity and the selectivity of several rare earthdefinite compounds with their crystallographic structure and peculiarlywith the bond energy of the rare earth-oxygen bond. We have prepared and tested rather different compounds such as: cubic perovskite, pyrochlore and garnet compounds . A \ Experimental I I\ A.Catalytic compound preparations. i) LnLiO2 (Ln= La, Nd, Sm) The catalyst are prepared from aqueous solution of rare earth nitrates, lithium carbonate or lithium hydroxide. The solids are obtained by evaporationtodrynessat 110"C-120°Cofthesolution the rare earth is precipited as oxalate by oxalic acid (pH = 2) or as hydroxide by ammonia (pH - 9 ) . After heat-treatment at 750°C for 24h we obtain the definite compounds asshownin figure 1. or suspension in which Figure 1: X-ray pattern of LaLi02 structure. ii) Lay03 We disolve log La(N03)3,6H20 and 8g of Y(N03)3,4H$ in distilled water at room temperature. We introduce 75 ml of aqueous solution of ammonia. The 577 precipitation occurs and the white suspension is filtered and washed with pure water. A 15 hours drying treatment is carried out and the resulting solid is heat-treated at 680°C for two hours under atmospheric conditions. X-Rays analysis (figure 2) shows the presence of bixbyite structure. A heat-treatment at 1000°C brings the compound to the perovskite structure. 2s"' 20 Figure 2: X-ray pattern of Lay03 I ( 1 1 LaY03 b i x b y i t e ( 2 ) LaY03 p e r o v s k i t e iii) Pyrochlore compounds, A2B2O7. For these preparation we use a sol-gel process. 5g of La203 are dissolved in propionic acid and we introduce 12,88g of zirconium propylate. A clear solution is obtained after stiring. The solution is brought at acid boiling point and the exces of organic acid is removed. We obtain a transluscent solid which is calcined at 725°C for two hours. X-rays analysis (figure 3) on the final compounds show the presence of pyrochlore type compounds. 1 - Figure 3 : X-ray pattern of La2Zr207 pyrochlore compound. 578 B. Catalytic activity. The activity and the selectivity of the various samples are determined in a fixed bed quarz reactor (6.6 mm I.D.) under the following conditions: inlet temperature - 600-750°C; feed gas pressures: 0.133 atm CH4, 0.0665 atm 02 and 0.8 atm He; gas flow: 4.5 l/h catalyst(STP); weight of catalyst- 0.67g; ratio CH4/O2=2 ( 2CH4 + 02 --> C2H4 + 2H20). Methane conversion is calculated as: moles of transformed CHq/rnoles of initial CH4 *loo. Selectivity in product i is defined as moles of transformed CH4 in product i / moles of transformed CH4 * 100; yield in product i: conversion * selectivity * 100. Results and discussion. i) LnLiO2 compounds. Table 1 shows the influence of the preparation method on C2 selectivity thought X-ray analysis show the same crystallographic structures before and after reaction. But we notice a relationship between specific area and selectivity in C2 for SmLi02aswellas f o r NdLi02. By FT-IR we see a very weak carbonate band. Catalyst preparation Samarium hydroxyde + Li20 " oxalate + Li2CO3 I' nitrate + Li2CO3 oxalate + LiOH Neodymium oxalate + Li2CO3 nitrate + Li2CO3 oxalate + LiOH surface area (m2/d 3.6 4.2 4.8 5.2 0.6 4.0 5.75 C2 selectivity% 4.4 14 24.6 28.7 14.2 27.1 38 Table 1: Relationship between catalyst preparation, surface area and selectivity. id Lay03 compounds. This compound can take two crystallographic structures, bixbyite and perovskite. In the bixbyite structure the lathanum and the yttrium anions are both placed in a mean eight-fold oxygen environment and in perovskite structure the lanthanum cation is strictly placed in a 12-fold oxygen environment and thus yttrium in a 6-fold environment. For a low temperature process we reach firstly the bixbyite structure and the perovskite 579 structure appears for a heat-treatment at 1000°C. But this fact leads also to specific surface changes as we can see in table 2. before after 22 14 23 .6 11 .2 6.3 4 . 9 6 . 8 3 . 9 catalyst conversion %CH4 %02 26.9 97 .8 24 .8 99 .2 selectivity % C2H4 C2H6 C2 C02 CO 1 2 . 3 11 .2 23.5 6 1 . 2 15.3 8 6 . 3 1 4 . 3 75.7 10.0 0.001 0 . 4 0.4 84.8 1 4 . 7 5.7 1 2 . 3 1 8 . 0 75.3 6 . 6 Table 2 : Relation ships between structure and catalytic datas for Lay03 compounds, In the case Lay03 with perovskite structure,the C2 selectivity is close or equal to zero for a CH4 conversion rate similar to the one obtained with bixbyite structure catalyst. In comparison the hexagonal oxides, La203 and Y2O3, with a same oxygen environment for the cationas in the bixbiyte structure, have close catalytic capabilities. A s a general rule,the perovskites are well known for their total oxidative capabilities into CO and C02 for CH4 gas ( 6 ) . A s an example,the SmAlO3 perovskite ( conversion 21 .38 , selectivity 6 .6%) is in agreement with last results. These results show that the metal-oxygen distance and thus the metal-oxygen bond energy can be the driving data of the oxidative coupling of methane. iii)pyrochlore compounds. Three kinds of pyrochlore have been prepared and tested: Ln2Zr207, Ln2Sn207, Ln2Ti207. In the pyrochlore structure both cations have a six- fold oxygen environment. Table 3 gives the catalytic conversion and reactivity for samarium, gadolinium and europium pyrochlores. We notice that the C2 yield results of these three pyrochlores follow the decreasing S o Zr> Ti order. Assuming the oxygen lablity near the rare earth cation energy of the rare earth bond has been computed using the metal-oxygen distances determined by P. Poix (9 ) and the partial charges of ions are determined by M.Henry'smethod. Thus the energy of the bond is computed by conventionnal formulas: facilitate the formation of 0 - , 02- and 02*- species, the E - p * q ' / r conversion C H ~ (%) 31.9 40.4 28.4 27.9 32.5 30 29.8 35.7 27.7 r - cation-oxygen invariant length q - partial charge of the cation q' - partial charge of the anion selectivity (a C2H4 C2H6 13 9 31.2 17.5 4.5 5.5 10.2 7.5 30.7 12.1 2.9 3.3 14.5 9.2 25.2 16.3 4.2 4.5 rield C2(% 7 19.7 2.8 4.9 13.9 1.9 7 14.8 2.4 surf ce area be for($ Ig&fter ?? 8 2.3 5 10.5 2.5 7.0 2.7 5.4 6.9 7.2 1.6 4.3 9.3 1.4 6.9 2.55 4.9 6.5 Table 3: Catalytic results of rare-earth pyrochlores compounds. If we take in account these results, for a close rare earth-oxygen energy and an increasing Sn-0, Zr-0 or Ti-0 bond energy, we notice that the increase of bond energy follow the decrease of C2 yield (table 4 ) . Thus,we computed the energy data These results show clearly that catalytic conversion and selectivity are also related to rare earth bond energy but also to its direct oxygen environment. for the previous catalytic systems (table 5). conversion(%) CH4 31.9 40.4 28.4 27.9 32.5 30 29.8 35.7 27.7 yield c2 % 7 19.7 2.8 4.9 13.9 1.9 7 14.8 2.4 Table 4: Relationship between bond energy bond Ln-0 0.1401 0.1352 0.1400 0.1294 0.1248 0.1293 0.1540 0.1488 0.1539 nergy B - 0 0.1428 0.0637 0.1537 0.1427 0.0643 0.1536 0.1427 0.0647 0.1536 snd catalytic properties 581 catalyst LaLi02 LaNa02 La203 Lay03 bix La2Zr207 NdLiO2 NdNaO2 Nd203 Nd2Zr207 SmLi02 SmNa02 Sm203 Sm2Zr2O7 oxygen coordination 7 7 6-8 6-8 8 7 7 6-8 8 7 7 6-8 8 conversion CH4 % 17.7 30.3 27. 28.6 31.5 30.4 36.8 28.6 33.3 31.9 30.4 25.5 31.9 selectivity c2 % 42.9 32.4 23.5 14.3 11.4 38 33.9 31 3.6 28.7 31.6 25.1 7.03 bond energq 0.1212 0.1229 0.1375 0.1378 0.1386 0.1231 0.1380 0.1398 0.1402 0.1229 0.1199 0.1402 0.1401 Table 5: Relationship between rare earth bond energy and catalytic properties. Conclusion. As a conclusion the methane oxidative coupling performances of some definite compounds obtained by low temperature processes are studied. The results show that: -the nature of the starting salts highly affects the selectivity -a 12 fold oxygen environment of the cation decreases drastically the selectivity (perovskite structure) -for the oxygen environment of 6 to 9,a relation can be found between coulombian bond energy of the cations and a high C2 hydrocarbon selectivity. This could be interpreted as an enhanced mobility of oxygens in the neighbourhood of the rare earth cation. References. /1/ T. Ito, J.X. Wang, C.H. Lin and J.H. Lunsford, J.A.C.S. 107 (1985) 5062. /2/G.E. Keller and M.M. Bahsin, J. Catal, 73 9 (1982) 582 /3/ G.J. Hutchings, M.S. Scurell, J.R. Woodhouse,J. Chem. SOC. rev. 18(1987) 251. /4 / N. Yamagata, K. Tamaka, S. Susaki and S. Okazoti, Chem. lett (1987) 81. /5/ A. Kaddouri, R. Kieffer, A. Kiennemann, J.L. Rehspringer, Appl . catal 51 (1989) L1. /6/ Zhen Kaiji, Liujian and Bi Yingi, Cata. lett. 1 (1989) 299. /7/ P. Poix, C.R. Acad. Sciences, 270 1852-1853 (1970). /8/ M. Henry, thesis universite P. et M. Curie, Paris (1988). G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 583 PREPARATION OF STRONG ALUMINA SUPPORTS FOR FLUIDIZED BED CATALYSTS M.N. Shepeleva, R.A. Shkrabina, Z.R. Ismagilov and V.B. Fenelonov Institute of Catalysis, Siberian Branch of the USSR Academy of Sciences, Pr. Akademika Lavrentieva, 5, Novosibirsk 630090 (U.S.S.R.) SUMMARY Physico-chemical processes o c c u r i n g i n alumina granu les moulded by t h e Hydrocarbon-Ammonia Granu la t i on Method have been i n v e s t i g a t e d ; op t ima l r e a l i s a t i o n c o n d i t i o n s o f main t e c h n o l o g i c a l s teps have been e s t a b l i s h e d . As a r e s u l t , h i g h l y s t r o n g spher i c alumina granu les w i t h developed spec i - f i c a rea and p o r o s i t y , a p p l i c a b l e as a suppor t f o r f l u i d i z e d bed c a t a l y s t , have been ob ta ined. INTRODUCTION Many modern technological processes are supplied with energy by combustion of organic fuels. Economically effective and ecologically clean installations with fluidized bed of the catalyst for flameless combustion were being deve- loped within the last years. Highly effective apparatuses of this type are the Catalytic Heat Generators (CHG) developed by the Institute of Catalysis (Sibe- rian Branch of the USSR Ac. Sci.) [1,2] . Catalysts in the course of work in CHG are subjected to at least three kinds of influence: chemical, thermal and mechanical. These factors are interconnect- ed, they complete and magnify the action of each other, destroying catalyst granules. The structural-mechanical properties of the supported catalysts are known to be determined at great extent by the properties of a support. The object of this work is the investigation of production conditions of a spheric J' -alumina,whichpossesses ahigh mechanical strength as well as highly developed surface area and porosity. It is known, that the preparation conditions and granulation method influen- ce the product characteristics. Recently the Hydrocarbon-Ammonia Granula- tion Method (HAG method) possessing high productivity and facility of techno- logical parameters regulation has received wide distribution. The distinctive feature of the method is the alumina chemical treatment at several stages, which allows to alter the texture of the initial substance in the necessary di- rection. In this respect the HAG method has advantages in comparison with the widely used method of mechanical moulding. 584 EXPERIMENTAL METHODS Argon thermal desorption was used for the determination of the specific area A. The pore volume V and the pore size (r, nm) distribution were determined by mercury porometer "Porosizer-2300" from the "Micromeretiks" company (USA). Sear- ching for a test of the granules shows the following pe- culiarities. Granules work in the CHG fluidized bed is complicated by chemical and thermal factors; granules are subjected not only to the surface friction, but at great extent to impact loadings, and gradual increase of the internal structural macro- and microtensions. The crushing test comprising granules press between two parallel plates has given satisfactory correlation with real speed of granules destruction in the CHG. Therefore, the sample strength was characte- rized by the average S minimal S . and maximal S values of the individual av' min max granules crushing pressure S. (MPa) in a series of 30 granules. Smin and Smax were calculated from 5 minimal and maximal values of Si. P mechanical properties The operating time of a catalyst support has been established to be more than 0.5 year for samples with S 3 25 MPa and Smin3 7 MPa. av RESULTS AND DISCUSSION Raw Material Preparation Pseudoboehmite aluminium hydroxides are usually used as a raw material in the HAG method. We have previously investigated the aluminium hydroxides obtained by gibbsite dissolution in the alkaly and deposition at pH 8.5-9.0 by nitric acid [5J . It was shown in [5-71 that the conditions of aluminium hydroxide synthesis determine the morphology and the structural type of the particles-", as well as the nature of binding between primary particles. A s it was shown by physico-chemical methods, the size of particles obtained from hydroxides synthe- sized at T 6 4OoC ly enough aggregates can exceed 100 nm. The bonds between the particles in such aggregates are mostly of Van-der-Vaals nature. Therefore, the acid treatment at the initial stage of peptization leads to the formation of a disordered system of fine needles and fibres. The dispersion of such mass in ammonia solution leads to rapid coagulation, fine particles of aluminium hydroxide (- 3-4 nm) being densely packed. Granules of aluminium hydroxide formed in these conditions have fine porous monodisperse structure. After calcination, the alumina with 3 A Z= 250 m2/g, Vp = 0.3-0.4 cm / g , Sav 3 25 MPa is obtained. does not exceed 10 nm and the size of the secondary crumb- Preparation of these hydroxides is connected,however, with certain difficul- , structures are divided into two main types: coagu- * 'According to Rebinder [8] lative, in which ion-solvate shell on the particle contact places is preserved and crystallizative with point or phase contact between primary particles. 585 ties, for example, at the stages of washing off alkaline metal ions and fil- tration. That is the reason that monodisperse hydroxides are not widely used. Hydroxides obtained by precipitation at T f 4OoC o r mixtures of precipitates obtained at high and low temperatures are used in many researches [9,10] . These hydroxides have contacts between the primary particles of both types; an extent of aggregates packing changes at the next technological stages is deter- mined by their number ratio. In the systems with phase contacts particles, the peptization does not lead to the aggregates destruction. Macro- pores preservation between the remained aggregates leads to the formation of low- strength alumina granules. Because of this, from each hydroxide obtained as in between the [9,lO] , one can prepare alumina granules with strength not exceeding a certain limiting value, unless special technological methods (e.g., high temperature calcination, additive incorporation, etc.) are used. A s is shown in the usually applied aluminium hydroxides, the values of Sav of the obtained gra- nules do not exceed 12 MPa. Application of these alumina granules in CHG is not effective, therefore we tried to change the structural type of hydroxide in or- der to strengthen the final alumina granules. Mechanical activation is known to be one of the ways to increase the solid reactivity. [5] , for The object of o u r investigation was aluminium hydroxide containing equal mix- between the sodium aluminate and nit- ture of deposits obtained by interaction ric acid at pH 8.7 and temperatures of 20 and 100°C. The phase composition of this hydroxide corresponds to the pseudoboehmite with the range of coherent dis- sipation llO°C are 230 m /g and 0.27 cm /g respectively, 12% of total pore volume is the volume of macropores (r > l o 0 nm). 12 nm. Specific area and total pore volume of the sample dried at 2 3 The radius distributions of pore volume of aluminium hydroxide before and after the treatment in various mills are shown in Fig. 1. It is seen that the treatment in a disk mill does not allow to destroy the secondary aggregates of hydroxide. Large pores are also preserved in the final alumina. The macroporosi- ty of alumina could be removed by grinding intensification, which also increases Sav and Smin substantially, rises the bulk density surface area A ( see Table in Fig. 1 ) . and slightly decreasesthe We have given in [11] the results of physico-chemical investigation of alumi- nium hydroxide grinding products. It was shown that the main result of grinding is connected not only with the destruction of the initial aggregates of alumi- nium hydroxyde, agulative contacts. mary partjkles. but also with an exchange of strong phase contacts by weak co- This does not practically change the structure of the pri- Peptization Stage Liquid mass capable t o flow freely from the moulding device spinnerets, is 586 0.4 0.3 Q, \ I+) E 0 0.2 >â 0. I ~~ No- d, 9 -_s,_arPa--- A , A , mkm av min g/ omâ 18 /g 1 - 4 2 0.69 250 2 >I00 5 2 0.70 270 3 25 31 12 0.84 240 4 10 35 17 0.84 220 Fig. 1. Pore volume radius distribution for initial (1) and grinded (2-4) alu- minium hydroxides. Mills: 2 - disk; 3 - ball; 4 - jet. The most abundant parti- cle size, d mkm: 2 - over 100; 3 - 25; 4 - 10. Pâ obtained at this stage by acid treatment of aluminium hydroxide. It should be noted, that basic aluminium salts show thixotropic properties and, therefore, it is necessary to adjust the mass preparation conditions to establish thixotro- pic setting time (s) long enough for a free mass flow along the pipelines. The mass rheological characteristics and properties of the final alumina granules are strongly dependent on the mass preparation conditions, particu- larly, on the mass maturation time (m). During the mass maturation the he- terogeneous system obtained as a result of the component mixing, becomes homo- geneous due to the precipitate swelling. âtâ (m) is influenced by the tempera- ture in reactor - plastificator. It was shown, that at T = 20-25OC L (m) makes up 1.0-1.5 days. The temperature rise leads to a substantial speeding up of the mass maturation ( (m) = 4-10 hrs). However, the alumina granules obtained from such masses possess not only the increased strength (Sav = 30-40 MPa) but also fine porosity, which complicates the drying and calcination stages and decreas- es the granules water stability. + r- L (m) also depends on the amount of acid added, M(a), g-m/g-m of alumina. / If M(a) equals to 0.06-0.08 and the solid phase concentration is 25-30%, L (m) makes up 1.0-1.5 days as necessary. The decrease of M(a) leads to the increase of (m) and vice versa. Besides, M(a) influences the time of mass thixotropic setting. A s it was shown in our experiments, 5 (s) should be within the range of 15-60 min. 587 We have established the dependence of the optimal value of M(a) at which opt z (s) and < (m) are within the required limits on the aluminium hydroxide pro- perties and mass preparation conditions. The formation of the stable disperse system from the structurated precipita- te or gel is known to be caused by the formation of the double electric layer on particles surface. Let us consider now the peptizator distribution in the bulk of aluminium hydroxide. The concentration of peptizator added after mixing with aluminium hydroxide is: 3 where m V are mass (kg) and volume (m ) of the peptizator, respectively; Wo is moisture content evaluated by drying at l l O ° C (kg H20 / kg A1203); m is mass concentration of A1 0 in hydroxide (kg); is liquid phase density (kg/ pep' Pep 2 3 3 m ). The mass balance equation calculated on the basis of A1203 is the following: 2 where d fic area of A 1 0 termicellar liquid of hydroxide (kg/m ). is specific sorption of substance - peptizator (kg/m ) ; A is speci- 2 (m / g ) ; C is equilibrium peptizator concentration in the in- 3 2 3 P In the left part of eq. (2) the first component expresses the liquid con- sumption for sorption and chemical interaction with aluminium hydroxide partic- les, the second component - for creation of equilibrium concentration of pepti- zator in intermicellar liquid. The value of M(a ) can be expressed as follows opt where M vely . and MA1203 are molecular masses of peptizator and A1203, respecti- Pep The formula ( 3 ) can be simplified by taking into consideration the follow- ing: (1) while using the nitric acid, the ratio M Al / Mpep is equal to 1.619; (2) A values of oxides and hydroxides of pseudoboeh&& structure (A ) are close enough; (3) the value of V /m is small. The formula (3) after simplification looks like: h Pep 588 1; 24 A M (a,opt) 4 â 0.9 E 7 0.84 L) 1 - 250 I I 0.06 0.08 M (ai Fig. 2. Dependence of the average strength Sav, the bulk density A specific area A of alumina with alumina concentration 28% and the granules on the M(a) value for the plastificated mass + According to the experimental data, = ( 4 âI 1) kg/m2; C = (14 - 2) P 3 kg/m . S o , if the aluminium hydroxide is treated to M(a) = M(a ) , the time needed opt for mass maturation and the time of mass thixotropic which establishes high mechanical properties of alumina granules. As it can be seen from Fig. 2, the acid treatment of hydroxide up to M(a ) does not change the alumina granules characteristics. setting are achieved, opt Sphere Formation and Coagulation The preparation of spheric alumina granules occurs in the column with two liquid layers: the upper layer is hydrocarbon, the lower one is a coagulant S O - lution. In the upper layer of hydrophobic liquid the mass drop is subjected to the surface tension forces which tighten dening and structure formation. The ammonia solution is usually used as a coagulant. For high strength alumina complete interaction between mass and coagulant. It was shown that while mould- ing of masses with alumina it into the sphere. The granules har- proceeds in the course of a coagulant diffusion into the volume of sphere granules preparation it is necessary to establish the concentration being less than 25% and M(a)& 0.1 589 granules hardening is completed in 30-40 s in 16-19% ammonia solution. If M(a) is increased, it is necessary to increase the contact time up to 60-80 s. and to rise the concentration of the ammonia solution. Granules Thermal Treatment The granules drying was carried out by several ways: in air, in a drying box, in aggregates with moving ribbon and heaters over it. The experimental results are presented in Table 1. TABLE 1 The influence of drying conditions on alumina granules characteristics Drving of Granules Alumina Characteristics < - Method T (OC) z (h) A(mL/g) Vp(cm3/g) Sav(MPa) Smin(MPa) Smax(MPa) ~~ ~~~ in air 20 48 240 0.31 41.8 20.7 59.2 in a drying box 110 4 240 0.27 23.5 6.8 38.9 under heat- ers 200 0.5 2 40 0.25 22.8 6.0 39.3 the same 40-200 0.5 240 0.28 31.3 12.8 45.3 It is seen that the drying mode does not practically affect the specific area A and pore volume V but it influences strongly the granule strength. This signi- ficant strength with an increase of residual microtensions with the rise of drying speed. So ra- pid drying under heaters at T >lOO°C leads to the decrease of S est granules are obtained while drying in air. This widely recommended method cannot be applied to continuous technology. It was found that lowering of the initial temperature and its gradual increase lead to the increase of S and 'mine P' change at the unchanged porosity could be probably explained . The strong- av av The thermal treatment at T >200°C strengthens the granules due to the transfer of coagulative structure to the crystallization one. The calcination of granules with bidisperse difficulties. Calcination of fine porous samples is more complicated. It was shown that the reduction of temperature rise and granule bed height the increase of calcination time and temperature (up to 75OoC) facilitate the slow moisture removal, granules shrinking, fine pore sintering. As a result, si- multaneous rise of V or wide porous structure does not present great as well as and Sav is observed, the specific area slightly drops. P CONCLUSIONS The investigation of the physico-chemical processes taking place at the main 590 stages of alumina method enables us to deve- lop the scientific background of preparation of granules with different charac- teristics, to establish the optimal conditions for each stage, to improve the process apparatus. It was shown that the preparation of strong alumina gra- nules requires the directed conducting of all the technological steps. moulding by hydrocarbon-ammonia Direct interrelation between the properties of the initial aluminium hydro- granules was xides and structural-mechanical properties of the final alumina established. Preparation of fibrous pseudoboehmite in mild precipitation condi- tions allows to prepare from it the fine porous strong alumina granules. For- mation of pseudoboehmite in the form of well crystallized needles and plates unable to react with acid-peptizator requires the introduction of the inten- sive grinding into the technological process. The investigation of peptization process shows that this stage breaks ground for textural and mechanical characteristics of the final alumina lid phase concentration, nature and amount of acid-peptizator determine the rheo- logical properties of mass and extent of dispersion of secondary aggregates of aluminium hydroxide. The properties of substance-coagulant and the residential time of granules in ammonia solution influence hardening. The conditions of granule thermal treatment allow to increase the amount of contacts between particles and aggregates in granule, strengthen these contacts due to transformation of coagulative type into the phase granules. So- the completion of coagulative one. Alumina granules prepared by the developed method posess the specific area and porosity necessary for the incorporation of the required amount of the ac- tive component and can be applied as a support for fluidized bed catalysts. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 G.K. Boreskov, E.A. Levitskii and Z.R. Ismagilov. Zh. Vsesouznogo Khim. Ob- shchestva, 29 (1984) 379-385. Z.R. Ismagilov, in: D.N. Saraf and D. Kunzry (eds.) Proc. Intern. Conf. on Advances in Chem. Eng., Kanpur, January 4-6, 1989, Tata McGrow Hill Publ. Co. Ltd, New Delhi, 1989, pp. 310-315. USA Patent 2805206 (1953). Ya.R. Katsobashvili and N.S. Kurkova, Zh. Priklad. Khim. 39 (1966) 2424- 2429. M.N. Shepeleva, V.B. Fenelonov, R.A. Shkrabina and E.M. Moroz, Kinet. Katal. M.N. Shepeleva, R.A. Shkrabina, L.G. Okkel, V.I. Zaikovskii, V.B. Fenelo- nov and Z.R. Ismagilov, Kinet. Katal. 29 (1988) 195-200. Z.R. Ismagilov, M.N. Shepeleva, R.A. Shkrabina and V.B. Fenelonov, Appl. Catal (in press). P.A. Rebinder, Physical and Chemical Mechanisms of Dispersed Structures, Nauka, Moskva, 1966, p .3 . M.D. Efros, A.V. Tabulina and N.V. Ermolenko, Izv. Akad. Nauk BSSR, 1 E.A. Vlasov, I.A. Rizak and E.A. Levitskii, Kinet. Katal., 5 (1972) 1311- 1314. M.N. Shepeleva, Z.R. Ismagilov, R.A. Shkrabina, E.M. Moroz, V.B. Fenelonov and V.I. Zaikovskii, Kinet. Katal.(in press). 27 (1986) 1202-1207. (1971) 9-13. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Prepamtion of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 591 SYNTHESIS AND REGENERATION OF RANEY CATALYSTS BY MECHANOCHEMICAL METHODS A. B. FASMAN 1, S. D . MIKHAI LENKO~ , 0. T . KALININA' , E . ~ u . IVANOV~, G . v . GOLUBKOVA~ Institute of Organic Ca tal ysys and Electrochemistry, 142, K.Marx st., 480100,Alma-Ata,USSR 21nstitute of Solid State Chemistry, 18, Derzhavina st. I 630091.Novosibirsk, USSR SUMMARY The effect of mechanical alloying (MA) on the structure of Raney catalysts and their activity and selectivity in liquid-phase hydrogenation reactions has been studied. The data illustrating a possibility of the method developed by the authors f o r mechanoche- mica1 (MC) regeneration of Raney catalysts irreversibly deactivated in hydrogenation process are given. INTRODUCTION Raney catalysts are widely used in industry due to their high ac- tivity, technological ability and relatively low cost (ref.1). As a rule they are made by leaching a non-noble component from pyro- metallurgical alloys (PM). The MC method for synthesis of alloys from initial component powders has a number of advantages over the traditional PM ones. With lower energy expenses it allows within one stage to produce alloy powders that form skeleton catalysts in a wider concentration range due to a higher efficiency while reac- ting with alkali (ref.2). At the same time the conditions of MA become the parameters influencing the properties of catalysts. The object of the present work is to study the effect of the MA conditions on the structure of alloys and Raney Ni-catalysts made from them as well as on adsorption properties, activity and selec- tivity of catalysts in liquid-phase hydrogenation reactions. EFFECT OF PREPARATION CONDITION ON THE FORMATION MECHANISM OF MA ALLOYS AND THEIR STRUCTURE BEFORE AND AFTER LEACHING MA elloy structure Initial alloys were made in a planetarium-type ball mill and a attritor as in (refs.3,4). Use was made of commercial carbonyl ni- ckel and aluminium powders. The phase composition was analysed by the X-ray diffraction method using CuK,,~emission. It had been found before (ref.4) that in the attritor with an uncooled case the MA 592 was characterized by a latent period during which local heating co- uld result in A1 melting. Then the exothermic effect initiates a reaction which proceeds very fast. The composition of its products differ but little from the equilibrium one. At the same time the mechanism of MA alloy formation in a cooled planetarium mill is close to a diffusion type, whereas the phase composition is far from an equilibrium one. Table 1 gives phase composition of alloys produced in these mills. TABLE 1 Effect of preparation conditions on the MA alloy phase composition Charge Duration Phase composition composition MA, Planetarium ball mill min Before annealing After Attri tor 17 A183 30 Al+NiA13 A1+NiAlj Al+NiA13 i25A1?5 30 NiA13+Ni2A13 NiAl3 Ni2A13 +A1 5 Ni+Al - Ni+A1 30 Ni2A13 Ni2A13 +NiA13 N i 2A13 + N i A 13 Ni32A168 5 NiZA13 +Ni+Al - 30 NiAl Ni2A13 i35 A '65 Ni+A1 Ni2A13 i42 A158 20 NiAl NiAl NigAlg +NiA1 i52 5 NiAl+Ni+Al - Ni+A1 20 NiAl NiAl NiAl Of importance is, probably, the fact that the reaction proceeds in an open apparatus (an attritor) and it has outside characteristics it is accompanied by a puff, and the activation was ceased right after its proceeding. A planetarium mill is a close-type apparatus so the control like that is impossible, that is why structural changes can occur after the reaction. Structures Ni2A13 and NiAl are close and the former can be produ- ced from the latter by replacing one third of nickel atoms for de- finitely-ordered vacancies. Their disordering that take place with MA leads to formation of solid solutions on the NiAl basis. Thus the NiAl homogeneity range is widened from equilibrium 45-60% to metastable 35-60% Ni. This supposition was verified by means of the X-ray method for radial distribution of atoms (ref.5). The MA Ni35Aluproduced in a planetarium mill during 60 min has a diffraction pattern that corresponds to the BCC-structure of NiA1. 593 The positions of coordination maxima on the radial distribution curve (r.d.c-1 correspond to BCC-structure of NiAl too, but there ore deviations from theoretical values as to the distribution of their areas. Table 2 gives the relationships betwen the areas of experimental r.d.c. maxima P e and theoretical P t for a model rela- ting to a non-distorted structure NiAl (Ni/A1=50/50) and for a mo- del, describing a solid solution on the NiAl-basis (Ni/A1=35/65). It is seen that the second model fully corresponds to experiment. Hence MA alloys are, indeed, able to produce solid solutions on the NiAl-basis losing up to 40% of nickel atoms. TABLE 2 Structural parameters of MA Ni35A165 as compared to NiAl structure models Rt, Re, P e / P t (Ni/Al=50/50 ) P,/Pt( Ni/A1=35/65) 2.50 2.50 0.549 4.08 4.05 0.422 5.78 5.72 0.471 6.29 6.32 0.448 7.08 7.00 0.368 1.018 1.125 1.089 1.019 0.979 Structure of Raney nickel catalysts from MA alloys Raney catalysts from Ni-A1 MA alloys possess structural peculia- rities. Leaching of A1 from NisAk5 -Ni50A15~, as a rule does not lead to any changes of diffraction pattern though from 60 to 20% A1 is removed. It should be noted that the PM NiAl does not react with alkali. Evidently the defect structure of MA NiAl facilitates A1 extraction. From Table 3 it is seen that leaching does not result in changes of MA alloy srtuctural parameters (a borderline composi- tion is taken as an example, Ni35A16s). TABLE 3 Structural parameters of MA Ni35A165 before and after leaching ~ ~ 0 0 Sample Lattice parameter.A Particle size,A C . 103 Initial alloy 2.860-0.001 120 Catalyst 2.860-0.002 110 7.94 8.77 On the diffractogram one can see only NiAl lines and a very weak diffused maximum that canbe attributedto FCC-Ni. However, calcula- tions of r.d.c. show that other components make contributions too. Taking account of the fact that the leaching degree was, In this 594 case, 55% by technique (ref.6). a difference curve ([cat.]-0.45* *[alloy]) was drawn. The calculation results correlate to the pat- tern of FCC-Ni on whose background there are maxima of NiO. Table 4 shows P e and P t of r.d.c. peaks for models built up in supposition of 100 and 40% content of Ni in a leached sample. The second model well agrees with experiment, though one can notice a greater lowering of coordination numbers with an increase of R. It is, evidently. due to a high dispersity of the nickel. When use is made of the regularities established in (ref.7) then from the slope of the P,/P+=f(R) the size of Ni particles as 595 600 F E 7: -200 ⬠c l-4 \ d 3 t=10 min - - - "4 t=20 min ./ t=40 min --- 20 30 40 20 30 40 20 30 40 Ni content in initial alloy, 8 Fig. 1. The relation between the catalyst activity in phenylacety- lene hydrogenation and MA duration and charge composition. 0 , e - hydrogenation of >C=C< bond A,A- hydrogenation of -C=C- bond -- fresh MA alloy -- - 6-month stored MA alloy conditions). The least stable in storage were catalysts made from MA alloys with a high A1 content and t=10 min. With all t the Ni*A&j&atalyst was the most stable one. To explain the observed facts it is necessary to consider the structure both of the initial and leached MA alloys. It was shown previously that their phase content is determined not only by the charge content but by the time of MA (ref.8). The general growth of activity with an increase of t from 10 to 20 min is due to more complete reaction of MA and transfer of the rest of non-reacted Ni into leachable compound. This is, partly, an explanation of the growth of catalyst activity with an increase of t up to 40 min. But in this case more important is the growth of the degree of alloy nonequilibrium at the expense of lattice distortions. No other fac- tors can explain, for example, the activity growth of catalyst from MA NiaA160 the phase content of which after t=20 min does not change. These defects are likely to affect the catalyst structure. The long life-time of defects is seen from the fact that even after 6 month storage the activity of t=40 min catalysts is higher than that of catalysts made of fresh alloys with t=20 min. 596 Fig. 2. DSC-curves of MA N i X A k 1- fresh ( 4 . 4 8 kJ/mol) 2- 6-month stored 1 I I I I I (1.95 kJ/mol) 100 300 500 T,OC The decrease of catalyst activity with alloy storage is well cor- related with the results of differential scanning calorimetry. From Fig.2 it follows that the DSC-curves recorded on DSC-111 for fresh and aged alloys obey the same rules. However, in the first case the store of excess energy is much higher. Apparently, a high- er activity of MA catalysts as compared to PM ones can be explained by a high degree o f disordering. With all t the catalysts from Ni35AlG5 were most active. As to PM catalysts their activity is increased up to 75% A1 (ref.9). As shown above the composition NisA165 is related by a solid solution on the NiAl-basis with a maximum possible deficit of Ni atoms. Al- loys with a higher Ni content are close to equilibrium NiA1, where- as with the smaller one they have other phases (NiA13 , NizA13). Apparently, the highest activity of these catalysts is due to the fact that as a result of MA N&Ak leaching the largest quantity of most dispersive Ni is formed, which is fixed on unleached NiAl par- ticles. The thing is that richer nickel alloys are leached to a less extent and contain less Ni skeleton phase. Whereas Ni2A13 and NiA13 leaching yields are not so fine particles of an active metal. Selectivity of MA catalysts and their adsomtion vroverties MA catalysts are more often of a higher selectivity then PM ones. In hydrogenation of phenylacetylene into styrene and of styrene in- to ethylbenzene,the relationship of rates varies from 1.8-2.0 for t=10 min to 2.3-3.5 for alloys with t=20 min and thus selectivity reaches 90-93%. With PM catalysts from alloys of the same dispersi- vity (5-8&) selectivity is 8045%. Catalyst selectivity is closely connected with adsorption proper- ties which were studied in this work by the method of hydrogen TPD. Fig.3 presents TPD-curves of catalysts from fresh and 6-month sto- red MA alloys. The surface of a catalyst from fresh Niq~Al~jo is the most energy-homogeneous. It adsorbs, mainly, weakly bound hydrogen 597 139 I 11 I / I I 100 300 100 300 100 300T,"G Fig. 3. TPD-curves of MA catalysts. - - fresh alloy --- 6-month stored alloy that is unable to displace styrene from the surface, which is the reason of its lowest selectivity -75%. The most selective catalyst (S=93%) is the one from fresh MA Ni35A165 , the TPD curve of which is moredisplaced to the high-tempereture range. It is of interest that the arm in the low-temperature range belongs only to NiA13 catalyst, that contains nickel in a less dispersive state than that made from WiA1. Its selectivity S=87%. Fig.3 shows that storage, practically, does not change the TPD- curve of a NiqoAl60 catalyst. Its selectivity does not change eith- er. It is of interest to compare the selectivity of catalysts from MA alloys after 6-month storage in another model reaction - that of hexenehydrogenation. Table 5 presents results for MA catalysts and for the sake of comparison for PM ones of the same dispersiveness and of coarser dispersion. This comparison allows to find out whe- ther the high activity of MA catalysts is due to their high disper- siveness or it is aconsequence of their microstructure. It follows from the table the MA catalyst activity is 2-3 times higher, which favours the second supposition. Their selectivity is also much higher than that of coarse-dispersion PM catalysts and differs but little from highly dispersed ones. The relatively low activity of the latter can be due to partial oxidation of alloys dissipated in an air separator. It is known that even introduction of oxide-form- ing additions or redox treatment (refs.lO,ll),suppressing migration of the >C=C< bond along the carbon chain affects but little the isomerization ability of Raney catalysts. 1n.this case the coeffi- cient of isomerization for all samples differed little indeed.A de- crease of the migration coefficient is due to an increase of the 598 TABLE 5 The activity (W ml H2/min*g) and migration (F,) and isomerisation (Fc) coefficients of MA and PM catalysts in hexen-1 hydrogenation. Ni content MA (5 -8JA) PM (5 -8f i ) PM ( 4 0 - 6 0 s ) in alloy.% W Fm F; W Fm FL W Fm Fi 25 584 0.27 0.63 113 0.25 0.61 248 0.77 0.72 35 814 0.44 0.65 176 0.36 0.65 202 0.78 0.72 40 290 0.30 0.67 - 114 0.58 0.73 - - surface adsorption heterogeneity and a rise of the fraction of strongly bound hydrogen. This is verified by comparison of the TPD-curves which with PM catalysts (5-8&) have a broad arm in the range of low temperatures. Thus, a high-adsorption potential of MA catalysts influences their increased selectivity in hydrogenation of different comp- ounds. In their turn, specific adsorption properties of MA cata- lysts are due to peculiarities of their structure,which can be ima- gined as a sinter of high-dispersion microcrystals of an active pha- se that are fixed on particles of an unleached initial aluminide. REGENERATION OF SPENT CATALYSTS VIA MA The MA method can be used for making new Raney catalysts from spent and deactivated ones in industrial processes. Utilization and regeneration of the latter is an important problem in economy and ecology which has not been settled as yet. There are big difficul- ties in the remelting of powders wich is associated with their easy oxidizability leading to big losses (up to 40%) of metallic nickel (ref.12). Experiments on MA alloy preparation using deactivated Raney Ni and A1 powder as initial components have shown that in this case leachable alloys are formed. Table 6 presents the activity of ske- leton catalysts from such alloys in hydrogenation of some of un- saturated compounds. Completely deactivated catalysts that were us- ed for hydrogenation of some of organic compounds, were taken as Ni components in MA. A3 seen from Table 6 the regenerated cata- lysts are, as a rule, more active than both the PM Ni-Raney and the MA ones for making of which commercial nickel was used. The nature of this effect requires further investigations; however, it is pos- sible to assume that in the burn-out of organic residues on the 599 TABLE 6 Activity the hydrogenation of different compounds ( t=4OoC) (ml H2/min-g) of Raney Nickel from MA and PM Ni35A165 in Reaction PM catalyst MA catalyst MA catalyst ( 5 - ~ J A ) (commercial Ni) (spent Raney Ni) Potassium maleate 70 Phenylacetylene 140 Nitrobenzene 100 Hexen-1 ( t=20°C) 200 100 160 480 a i o 160 700 220 1000 surface of catalysts they start interacting with it, modifying them in a specific way. Thus, MA can become a promising efficient method for utilization of production waste in catalytic processes and at the same time a a way to increase their activity. CONCLUSION Thus, mechanical alloying can be considered as a promising al- ternative for the pyrometallurgical method of producing initial al- loys for Raney catalysts. The MA performance conditions made it possible to influence the structure of initial alloys and, mainly to obtain nonequilibrium solid solutions on the NiAl basis the lea- ching of which as compared to PM ones yields more active and selec- tive catalysts in a number of processes. Besides, MA can be the basis of a few-operation technology for regeneration of Raney-Ni deactivated in industrial processes. The leaching of MA alloys pro- duced from spent Raney catalysts yields catalysts that are superior in activity to those from PM and even analogous MA alloys on the basis of commercial Ni powder. ACKNOWLEDGMENTS The authors are very thankful to Dr.E.V.Leongard for carrying out experiments on the temperature-programmed desorption of hydro- gen, to A.K.Dzhunusov. who took part in investigation of samples by the r.d.a. method and to Professor E.M.Moroz for useful discussion of the results of the r.d.a. experiments. REFERENCES 1 E-1-Gildebrandt and A.B.Fasman, Skeleton catalysts in organic chemistry, Nauka. Alma-Ata, 1982 (in Russian). E-Ivanov, T-Grigorieva, G.Golubkova, V-Boldyrev, A.B.Fasman, S.D.Mikhailenko. 0.T.Kalinina. Raney nickel catalysts from me- chanical Ni-A1 alloys, Materials Letters 7(1-2) (1988) 55-56 E.Ivanov, T-Grigorieva, G.Golubkova, V-Boldyrev, A.B.Fasman, S.D.Mikhailenko, O.T.Kalinina, Synthesis of Ni aluminides by chanical alloying, Materials Letters 7(1-2) (1988) 51-54 S.D.Mikhailenko, B.F.Petrov, 0.T.Kalinina. A.B.Fasman, Nickel aluminide mechanochemical synthesis mechanism, Powder metallur- gy,lO (1989) 44-48 (in Russian). K.G.Rikhter, X-ray analysis of amorphous catalysts by r.d.a. me- thod, in: Rentgenografiya katalizatorov, Nauka, Novosibirsk, 1977,pp.5-40. E.M.Moroz, Development of X-ray methods for the investigation of fine-dispersive systems, Doctor's thesis, Novosibirsk. 1989. V.N.Kolomiichuk, On the correctness of quantitative charcteris- tics of catalyst structure obtained from r.d.a. curves, in: Rentgenografya katalizatorov, Nauka. Novosibirsk, 1977, pp.67-70 E.Yu.Ivanov. T.F.Grigorieva, G.V.Golubkova, V.V.Boldyrev,A.B.Fa- sman, S.D.Mikhailenko, O.T.Kalinina, Mechanochemical synthesis of nickel aluminide, Izvestiya SO AN SSSR (ser khim), 19(6) (1988) 80-83. V.I.Vorobieva, V.M.Safronov. G.A.Pushkarieva, A.B.Fasman, Phe- nylacetylene hydrogenations on the Raney Ni from Ni-A1 alloys of different composition and dispersion, Vestnik AN KazSSR, 4(1987) 54-58. 10 A,B.Fasman, T.A.Khodareva, S.D.Mikhailenko, E.V.Leongard, A-1-Lyashenko, The effect of preparation condition on structure and properties of modified Raney Ni catalysts,Proc.II All Union Seminar on Scientific basis of catalyst preparation, Minsk, Sep- tember 25-28, 1989, Nauka, Minsk, 1989, p.295. 11 T.A.Khodareva. E.V.Leongard, S.D.Mikhailenko, Raney Ni transfor- mation under the influence of thermal treatment in redox media, in: Science and technology problems of catalysis, Nauka. Novosi- birsk,1989, p.101 (in Russian). 12 A.I.Kryagova, A new merhod for spent Raney Ni catalyst regenera- tion, Trudy LVMI, 5 (1956) 85-90. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 601 CONTROLLED PREPARATION OF RANEY Ni CATALYSTS FROM Ni,A13 BASE ALLOYS - STRUCTURE AND PROPERTIES. S . HAMAR-THIBAULTâ, J. GROSI, J. C . JOUD1, J. MASSONZ, J.P. DAMON2 and J.M. BONNIERâ 1.N.P.Grenoble - L.T.P.C.M. (CNRS-UA29), Universitg Joseph Fourier, L.E.D.S.S. (CNRS-UA332), BP 75, 38402 Saint Martin dâH8res-Cedex FRANCE. BP 53X, 38041 Saint Martin dâH8res-Cedex FRANCE. SUMMARY Physicochemical properties (metallic surface area, total surface area) of Raney Nickel catalysts prepared from well defined precursor alloys were related to the metallurgical structures of these alloys. The microstructure of the catalysts was correlated with the physicochemical characteristics and their activities f o r hydrogenation of acetophenone in the liquid phase. INTRODUCTION Raney Nickel catalysts are extensively used industrially and in laboratories in hydrogenation, hydrogenolysis and other reactions. NiR were prepared by removing A1 from Al-rich A1-Ni alloys in alkali solutions. The residue consists of small Ni particles connected in a porous agglomerate with small amounts of A1 in a metallic state and also in an oxidized state as alumina (1). Although Raney catalysts have been used for a long time, the knowledge of the influence of the metallurgical structure of the precursor alloy upon catalytic properties is limited. Moreover, catalytic properties can be strongly modified by metallic addition (2-4). This work is part of a systematic study on the influence of metallurgical parameters on the catalytic properties of doped and undoped Raney Nickel catalysts. Different types of structures were obtained with different solidificationtechniques.Besides c o n v e n t i o n a l solidification, rapid quenching from high temperature was used in order to obtain supersaturation of the dopant in the phases. In order to be able to precise the influence of the microstructures of the precursor alloys, these alloys were well characterized before they were turned into catalysts. 602 EXPERIMENTAL PreDaration of the alloys and catalysts. Conventional solidification, annealing at high temperature and rapid quenching from the melt ( /A) were used to obtain precursor alloys exhibiting well defined microstructures. The rapid quenching was performed under helium atmosphere. The temperature of the melt before ejection was kept at about 156OOC. The influence of the ejection pressure and the rolling velocity on the microstructure have been reported previously (5,6). Undoped and doped Ni,.,M,Al, base alloys were tested (M = Cr, Cu) . Ni catalysts were prepared from powdered alloys by treating twice 2 hours in a boiling 6N sodium hydroxide solution. The samples were then washed by NaOH solutions of decreasing concentrations and carefully washed in water and in an appropriate solvent before use. Characterization techniaueg. The bulk composition of the catalysts was determined by chemical analysis. All compositions were expressed as atomic ratios. The total surface area was determined by adsorption of N2 at 77K and the nickel surface area by reactive adsorption of 3- methylthiophene in the liquid phase as described previously (7). Transmission electron microscopy observations (TEM-JEMZOOCX), of Ni catalysts were performed on samples prepared by two different ways: the first as just described, the second from bulk alloys. For the first samples, a suspension of Ni catalyst in alcohol was deposited on a copper microgrid and dried in the specimen introduction chamber to avoid any contact with air. Samples prepared from bulk samples were first electrolytically thinned with an acid solution at room temperature then observed directly after Al-leaching. Al-leaching was sufficient to obtain thin observable regions. Quantitative microanalyses were obtained on a STEM-VG.HB5 which allows a high resolution in EDX and EELS microanalyses (lateral resolution of 1.5 nm at the sample level). Some XPS and Auger examinations were equally performed on the precursor alloys and on the catalysts. Catalytic tests. The catalysts were tested in the hydrogenation of acetophenone with reference to a Ni-A1 (50.50wt%) catalyst. Hydrogenation in 603 cyclohexane solution at 353K was carried out in a 250ml static reactor under a constant hydrogen pressure (0.9MPa) with a constant initial acetophenone concentration (0.3mol.l-1) and at a stirring speed of 1800rpm so that the diffusional limitation did not affect the reaction. RESULTS Microstructure of mecursor allovs. Raney Ni catalysts were usually prepared from Ni-A1 50.50wt% alloys. According to the A1-Ni binary phase diagram, these alloys contain the different binary phases formed during solidification (Ni2A13, NiA1, and the eutectic Al/Al,Ni). were different but the solidification began with the Ni2A1, phase. For Cu as-cast alloys, the solidification path was similar to the undoped Ni$13 alloy and ended with the NiA1, phase in agreement with the A1-Ni phase diagram (8). In the case of Cr-doped alloys, only 0.8at%Cr was solubilized in the primary phase, therefore the solidification ended with a Cr-rich binary compound with a composition around A18(CrNi)5. After annealing at high temperature, Cu-doped Ni,Al, was homogeneous but for the Cr-doped alloy, a Cr-A1- rich phase remained. The composition of this phase was analysed after 17 days at 950°C. It corresponds to Cr,~l,,Ni5. It had only be possible to solubilize 1.5at% Cr in the Ni$l, primary phase. The solidification paths for Cu and Cr as-cast Ni2A1, alloys In p-crystallized alloys, a typical microstructure was observed. Grains showed a dendritic structure with a central part constituted of the NiAl phase surrounded by large domains of the Ni2A1, phase. The composition of the interdendritic groove depended on the dopant (5). In the case of Cr-doped 1.1 alloys, only Cr segregation was detected at fine-scale observations. Phvsicochemical characteristics of Ranev Nickel catalysts. Table 1 summarizes physicochemical characteristics of doped and Total and metallic surface areas of catalysts prepared from commercial or undoped Ni,Al, as-cast alloys were almost the same. A slight decrease was observed with the Cu-doped alloy. On the contrary, total and metallic surface area increased in the presence of Cr. undoped catalysts with different microstructures. 604 Catalysts prepared from p alloys had smaller surface areas than those prepared from as-cast alloys; this was observed for both doped and undoped alloys. The reduction of surface area was the most significant in the case of the Cr-doped p alloys, wherethe metallic surface area was only 45% of that of the catalyst issued from the Cr-doped as-cast alloy. For Cu and undoped alloys, the metallic surface area remained about 80%. No significant influence on surface areas was observed by annealing the precursor alloys. TABLE 1 Physicochemical characteristics of the catalysts. Alloys 'I------ I Ni-A1 50.50 Ni2A13 L 1 Nil. gcUO. lA13 as cast I f l annealed I F I annealed Catalysts total meta 11 i c Al/Ni M/Ni Surface m2g-1 Composition % at ~ 80 60 ~ 10 1 I 80 64 66 53 26 32 69 49 59 39 75 54 53 48 62 4.9 5 . 1 4.9 1 2 0 75 72 33 120 73 48 75 63 5.4 3 4 In the precursor alloy, M/Ni = 5at%. Using Ni,Al, as precursor alloy favored A1 retention. In the presence of a dopant, A1 retention increased; moreover this phenomena was enhanced by the microstructure. Al/Ni ratio was respectively 0.48 to 0.75 in catalysts prepared from Cr-doped as- cast and ,LI alloys. The dopant in the catalyst remained at the same level as in the precursor alloy. During alkali leaching, dopant loss was small: 7 for Cu and 15% for Cr. Auger and XPS analyses indicated that Ni and A1 were only in the metallic state in all types of catalysts (9 -11) . These results contrasted with the work of Okamoto et a1 (12) who mentioned the presence of both metallic and oxidic Al. Cu remained in a metallic state in agreement with previous results (13). The surface and the bulk composition were the same (Cu/Ni surface ratios were 3.5 and 605 4.5at% in catalysts prepared from as-cast and p alloys respectively. depending on the microstructure of the precursor alloy. In catalysts prepared from as-cast and p Cr-doped alloy, the surface ratio Cr/Ni were respectively around 0.70 and 0.50. A1 concentration was smaller on the surface than in the bulk. On the contrary large oxidized Cr1Ir segregation was observed Microstructure of Ranev Nickel catalysts. By leaching out the alloys, the catalysts obtained were formed of different typesof agglomerates, the composition of which depended on the dopant and the microstructure of the precursor alloy. Inside each agglomerate, the composition was generally homogeneous. In commercial Raney Nickel catalysts, the observations showed that the Al/Ni ratio in the majority of Ni agglomerates varied from 0.07 to 0.12 whichconcords with the chemical analyses. However, some Ni agglomerates had a much higher (0.20) or much lower (0.03) Al/Ni ratio.These results indicated that the composition of the Ni agglomerates corresponds to precursor phases in agreement with the known leachability of the binary phases where NiA1, retained after leaching about 5at% of A1 and Ni,Al, over 23at%. Fig. 1. Bright field image of a Ni-agglomerate prepared from as- cast Cr-doped alloys: selected diffraction area and EDX spectrum. When the doped as-cast alloys were turned into Ni catalysts, the residual A1 content depended also on the nature of the 606 different phases present in the precursor alloys. EDX microanalyses performed on the agglomerate showed that the ratios Al/Ni (0 .22) and Cr/Ni (0.08) were in good agreement with the chemical analyses (fig.1). These values indicated that a large number of the agglomerates were formed from leaching out the primary Ni,Al, phase. However, some Cr-rich agglomerates were also observed in these catalysts. They were formed from the Cr-A1 rich phase previously ment ioned in this Cr doped as-cast alloy. Catalysts prepared from /I-alloys appeared as Ni crystallites supported on a NiAl core not completely leached. This result is in agreement with the known leachability of the different A1-Ni binary phases. The leachability decreases from NiA1, to Ni,A13 and is very slow for NiA1. The NiAl core not completely leached could explain the large A1 amount in Ni catalysts prepared from p- alloys. As shown on catalyst obtained directly after leaching a bulk sample (fig.2), the A1 level was low on the external part of the leached zone and high in the center. Fig. 2. Bright field image of bulk p Cr-doped alloys after leaching. EDX spectra showing the inhomogeneity of A1 composition. In these catalysts, all prepared from the Ni2A1, base alloys, the edges of the Ni-agglomerates were thin enough for 200kV e1ect:rons to transmit as shown in fig.1. Intensity enhancements on ring patterns indicated the presence o f a mosaic of Ni crystallites with preferential orientations due to the orientation relationships between Ni2A1,-NiA1 and NiAl-Ni cells (14). Catalvtic wroperties of catalysts Drewared from Ni,Al, allovs. acetophenone (AC) led to phenyl 1-ethanol (PE). Secondary As with other types of Ni catalysts (15,16), hydrogenation of 607 Alloys Ni2A13 P Nil. gcUO. lA13 as cast p hom. Nil. gCrO. lA13 as cast P reactions such as hydrogenation of the aromatic ring and hydrogenolysis of the hydroxy group gave by-products: methylcyclohexylketone (MCC), 1-cyclohexylethanol (CE) and ethylbenzene (EB). Table 2 gives the initial rates for acetophenone hydrogenation on the different catalyst. The selectivity is illustred by the maximum of MCC and PE. % MCCmax %%ax g-l "AC m-2 Catalysts Ni2-3 5.6 8.6 6.5 79 pNi2-3 4.4 8.3 5.2 84 Ni-Cu 3.7 7.0 5.5 79 pNi-Cu 3.7 9.5 5.1 81 hNi-Cu 3.2 5.9 4.0 82.5 Ni-Cr 4.8 6.8 0.8 83 pNi-Cr 4.6 13.9 1.5 90 vOAC : ini mmol .mix'. %MCC,,, and tial hydrogenation rate of acetophenone, expressed in g-1 and mmol.min-~.m-~,i10+2. %PE,,, : maximum yield in MCC and PE. Catalysts either doped or undoped had the same degree of activity expressed per m2 of Ni. Nevertheless, Cu slightly reduced the activity without modifing the selectivity (79% in PE). The addition of Cr did not really alter the activity but improved the selectivity (from 79 to 83% in PE) by reducting notably the ring hydrogenation: the maximum percentage in MCC decreasedfrom 6.5 to 0.8. We have shown in previous papers that Cr can increase the acetophenone hydrogenation activity and also the PE selectivity, these improvements being in relation to the amount of Cr added to Ni (7,9). The Cr content of the catalyst presented in this work (Cr/Ni = 5at%) corresponded to an effect only on the selectivity. We noted different catalytic properties according to the precursor used, as-cast or p-crystallized alloys, these effects being enhanced by the presence of a dopant. By p-crystallization of the undoped alloy, the activity (mz) of the resulting catalysts, remained unchanged but the selectivity in PE increased from 79 to 608 84%. The same behaviour was observed in the presence of Cu. In the case of the Cr-doped catalyst, besides a similar effect of the precursor alloy microstructure on the selectivity (an increase in PE from 83 to go%), the catalyst was twice as active per m2 of Ni surface as the catalyst obtained from the as-cast alloy. We only tested the annealed Nil,gCuo.lA13 alloy. Indeed, only this alloy became homogeneous by annealing as-cast alloy at high temperature; the Cr-doped alloy remained heterogeneous as shown previously. The homogeneisation of the precursor alloy gave a catalyst with only a slightly improved selectivity in PE. DISCUSSION As previously reported, physicochemical characteristics of catalysts prepared from undoped or Cu-doped alloys were rather similar. For both samples, p-crystallizationcauses a similar effect: reduction of total and metallic surface areas (15-20%) and an increase of the A 1 content ( 2 0 - 3 0 % ) . From Auger and XPS analyses, Cu was found in a metallic state and seemed substituted at random in the Ni lattice. The same effect on catalytic properties by p-crystallization of the precursor alloy, observed with the undoped and Cu-doped catalysts, could be explained by the similarity of the microstructure of both types of samples. In Cr-doped catalysts prepared from as-cast alloys, Cr was oxidized and moreover segregated to the surface. The best selectivity in PE of these samples was recently attributed to the surface oxidized chromium (10-15). But an other point must be mentioned. In the three cases (undoped, Cu and Cr-doped catalysts), a similar behaviour was observed concerning the influence of the microstructure. The yield in PE (%PE,,,) was always higher in catalysts prepared from p-alloys than from as-cast alloys. The increase of selectivity observed in p-systems seems due to a significant decrease in by-product formation (essentially EB) as seen in fig.3 which illustrates results obtained on Cr-doped as-cast and p-precursor alloys. In p-crystallized alloys, the grains appeared with a dendritic shape: a NiAl core surrounded with large domains of the Ni@, phase, the interdendritic groove being constitued of Al-rich phases. The p-crystallization stabilized an architecture with internal phases poorer in A1 than the external phases. After 609 alkali leaching, these different phases gave Raney catalysts with different residual A1 content. So, ,u-crystallization produced catalysts with increasing A1 contents from the core outwards. The A 1 content at the surface is then probably different between catalysts issued from as-cast or p-crystallized alloys and this difference can explain the variation in selectivity. Fig.3. Hydrogenation of acetophenone: scheme of reaction and products distribution as a function of time. With a dopant, the respective size of the different zones in the ,u-crystallized alloys changes as well as the distribution of the dopant. It is not the case for Cu, so the difference in selectivity between the samples issued from the as-cast and the p- crystallized alloys was in the same order as the one obtained with the undoped catalysts. In the case of Cr-doped p-alloys, chromium was segregated in the interdendritic zone and no Cr-rich phases such as A1,(CrNi)5 were observed as in the as-cast alloy. The p- crystallization of Cr-doped alloy modified both the A 1 and the Cr distribution. Therefore, it is not surprising that not only the activity, but also the selectivity were modified. Another hypothesis could explain the modification in selectivity occurringwith catalysts originating from p- crystallized alloys: the sensitivity of hydrogenolysis to the structure of the catalysts. As shown in fig.3, the increase in PE selectivity between catalysts issued from as-cast and ,u-crystallized alloys originates principally from a decrease of the hydrogenolysis. We have shown that alkali-leaching is not anarchic; the crystallographic orientation of Ni-crystallites is direcly related to the 610 orientation of the precursor phase (6). Then Raney Nickel catalyst keeps the memory of the metallurgical and crystallographic structures of the precursor alloy. As hydrogenolysis is a demanding reaction, sensitive to the structure of the catalyst, we can presume that this reaction will be dependent on the precursor. ACKNOWLEDGEMENT XPS analyses were performed in the "Laboratoire de Catalyse" of Louvain la Neuve. This study was supported by the CNRS (Chemical ATP 904332) and conducted in a Stimulation Plane of the CEE (Codest Program). REFERENCE 1 P. FOUILL0UX.- Appl. Catal., 8, (1983) 1-42. 2 M.S. WAINWRIGHT and R.B. ANDERSON - 3 J.M. BONNIER, J.P. DAMON and J. MASSON - 4 S.R. MONTGOMERY - Catalysis of Organic Reactions, W.R. Moser ed, M. Dekker, New York (1981) 383. 5 J. GROS, S. HAMAR-THIBAULT and J.C. JOUD - J. Mat. Science, 24, (1989) 2987-2998. 6 J. GROS, S. HAMAR-THIBAULT and J.C. JOUD - Surface and Interface Analysis, 11, (1988) 611-616. 7 S. SANE, J.M. BONNIER, J.P.DAMON and J. MASSON.- Appl. Catal., 9, (1984) 69-83. 8 M. HANSEN and A. ANDERKO - Constitution of binary Alloys. New York, Mac Graw Hill (1958). 9 C KORDULIS, B. DOUMAIN, J. DAMON, J. MASSON, J.L. DALLON and F. DELANNAY - Bull. SOC. Chim. Belge, 94, (1987) 371-377. J. Chem. Phys., 84, (1987) 889-894. Appl. Catal., 4, (1982) 169-180 . J.C.S. Faraday I, 76, (1980) 998-1012. J. Catal., 93, (1985) 55-67. J. Catal., 64 (1980) 124-131. Appl. Catal., 42, (1988) 285-297 . 10 J.M. BONNIER, J.P. DAMON, B. DELMON, B. DOUMAIN and J. MASSON - 11 F. DELANNAY, J.P. DAMON, J. MASSON and B. DELMON - 12 Y. OKAMOTO, Y. NITA, I. IMANAKA and S. TERANISHI - 13 V. BIRKENSTOCK, R. HOLM, B. REINFANDT and S. STORP - 14 F. DELANNAY - Reactivity of Solids, 2, (1986) 235-243. 15 T. KOSCIELSKI, J.M. BONNIER, J.P. DAMON and J. MASSON - 16 J.M. BONNIER, J. COURT, P. WIERZCHOWSKI and Appl. Catal., 43, (1989) 91-99. S. HAMAR-THIBAULT- Appl. Catal., 53 (1989) 217-231. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 611 NOVEL TYPE O F HYDROTREATING CATALYSTS PREPARED THROUGH PRECIPI- TATION FROM HOMOGENEOUS SOLUTION (PFHS) METHOD KAZA SOMASEKHARA RAO'I, V.V.D.N. PRASADI, K.V.R. CHARY' and P. KANTA 'Chemistry Depar tment , Andhra University, P.G. Extension C e n t r e , Nuzvid - 521 201, A.P., India 2Catalysis Sect ion, Indian Ins t i tu te of Chemical Technology, Hyderabad 500 007, A.P., India R A O ~ SUMMARY y -Alumina supported unpromoted ca ta lys t molybdenum sulphide and promoted c a t a l y s t s cobal t sulphide - molybdenum sulphide; nickel sulphide - molybdenum sulphide w e r e prepared by Prec ip i ta t ion From Homogeneous Solution (PFHS) t e c h - nique using t h ioace tamide hydrolysis in a single s tep. Oxygen chemisorpt ion s tudies , hydrodesulphurisation (HDS) and hydrogenat ion (HYD) s tudies w e r e made for t h e s e ca ta lys t s . These catalysts do not need pre-sulphidation prior to HDS reaction. INTRODUCTION Willard and Tang (ref. 1) utilised t h e technique for t h e precipi ta t ion of basic aluminium sulphate by t h e control led hydrolysis of urea t o yield ammonia and cal led it a s Precipi ta t ion From Homogeneous Solution (PFHS) which is t h e basis for deve- lopment . Since t h e n a la rge number of methods w e r e developed and t h e y w e r e reviewed (refs. 2-4). Anion re lease technique involves t h e re lease of anion in solution so a s t o prec ip i ta te m e t a l ions present through control led hydrolysis. Thioace tamide hydrolysis was used t o prec ip i ta te molybdenum sulphide (ref . 5) and nickel su l - phide (ref . 6 ) . Recent ly PFHS method has been ident i f ied a s a good means of making b e t t e r c a t a l y s t s (refs. 7-10). Hydrot rea t ing of petroleum crudes and coa l der ived liquids is a n industrial c a t a l y t i c process. The commonly employed c a t a l y s t s during hydrodesulphurisat ion (HDS) a r e MoS2 or WS2 promoted with cobal t or nickel on a high s u r f a c e a r e a gamma alumina. Usually t h e s e c a t a l y s t s a r e prepared by t h e impregnat ion of alumina support using aqueous solution containing ammonium molybdate and n i t r a t e s of cobal t or molybdenum followed by calcinat ion a t higher t e m p e r a t u r e (500°C). Single s t e p sulphide ca ta lys t p repara t ion method is not repor ted so f a r . In present work, we report new t y p e of hydrot rea t ing catalysts consisting in MoS2 promoted with C o or Ni prepared by PFHS method. The ac t iv i t ies of t h e c a t a l y s t s w e r e eva lua ted for hydrodesulphurisat ion (HDS) of th iophene and hydrogena - t i o n (HYD) of cyclohexene. A comparison of t h e per formance of t h e s e c a t a l y s t s has been made with commerc ia l hydro t rea t ing ca ta lys t s . 612 REAGENTS AND APPARATUS Molybdenum t r iox ide (Moo3), cobal t n i t r a t e ( C o ( N 0 ) .6H20), nickel n i t r a t e (Ni(N03)2 .6H20, t h i o a c e t a m i d e (CH3CSNH2), urea (NH2CONH2), (all f rom LOBA Chemie), n i t r ic acid (BDH), cyclohexene (Fluka), th iophene (Kodak) w e r e a l l of analyt ical reagent grade. y-A1203 (Harshaw, S.A. 234 m2g-â, P.V., 0.65 ml g - ). A convent ional high vacuum glass sys tem was used t o measure t h e BET sur face a r e a s by ni t rogen (0.162 nm ) absorbed a t -196°C. X-ray d i f f rac tograms were recorded on a Philips pW 1051 d i f f rac tometer . 3 2 1 2 EXPERIMENTAL Prepara t ion of Molybdenum Sulphide An aqueous solution of IOOml containing lOml of Moo3 (O.lM), I g urea, 0.75ml concent ra ted n i t r ic ac id and 30ml of t h i o a c e t a m i d e (0.135M) placed in a 250ml conical flask. The f lask was covered with rubber cork and t h e conten ts in t h e f lask w e r e hea ted on a w a t e r ba th (90-95°C) for about 3 hours by in te rmi t ten t s t i r r ing. A f t e r t h e precipi ta t ion was c o m p l e t e (pH 2.0 t o 3.0) it was f i l t e red , washed with distilled w a t e r and dr ied a t 110°C for I hour. Prepara t ion of Cobalt Sulphide An aqueous solution of lOOml containing lOml of cobal t n i t r a t e (O.Iml), 5 g urea, I m l n i t r ic ac id (0.75N) and 30ml of t h i o a c e t a m i d e solution (0.133M) placed in a 250ml conical flask. The f lask was covered with rubber cork and t h e c o n t e n t s in t h e f lask w e r e h e a t e d on a w a t e r ba th (90-95OC) for about 2- hours by in te rmi t ten t stirring. Af te r t h e precipi ta t ion w a s c o m p l e t e pH (7.5 t o 8.5), it was f i l t e red , washed with distilled w a t e r and dr ied a t 110°C for 1 hour. Prepara t ion of Nickel Sulphide I 2 An aqueous solution of IOOml containing lOml of nickel n i t r a t e (O.IM), 5 g urea, l m l n i t r ic ac id (0.75N) and 50ml of t h i o a c e t a m i d e solution (0.133M) placed in a 250ml conical flask. The f lask was covered with rubber cork and t h e conten ts in f lask w e r e hea ted on a w a t e r ba th for about 2- hours. Af te r t h e precipi ta t ion was comple te (pH 7-8). It was f i l t e red , washed with distilled w a t e r and dr ied a t 110°C for about 1 hour. Prepara t ion of Y-Al2O3 supported Molybdenum Sulphide 1 2 Suspend 4.7, 2.3, 1.5, 1.1, 0.86, 0.70, 0.54 grams of Y-A1203 in a n aqueous solution of IOOml in a 250ml conical f lask containing lOml Moo3 (0.lM) solution, I g urea, 0.75ml conc. n i t r ic acid and 30ml of t h i o a c e t a m i d e (0.133M) for obtaining 2,4,6,8,10,12,15 percentages of Mo/Y-A1203 c a t a l y s t s respect ively. The conten ts of t h e f lask w e r e hea ted t o 90-95°C f o r about 3 hours with in te rmi t ten t s t i r r ing. The resul tant solids w e r e f i l t e red , washed and dr ied a t llO°C. 613 Prepara t ion of Co-Mo/y-AlzOs C a t a l y s t s Suspend 1.905, 1.41, 1.12 g of 8% MoS2/y-A1203 in a n aqueous solution of IOOml in a 250ml conical f lask containing lOml cobal t n i t r a t e solution ( O . l M ) , 5 g urea, I m l , 0.75 N H N 0 3 and 30ml t h i o a c e t a m i d e (0.133M) for obtaining 3 , 4 , 5 percentages of CoS2 on 8% MoS2/y-Al2O3 c a t a l y s t s respect ively. The c o n t e n t s of t h e f lask w e r e hea ted t o 90-95°C for about 3 hours with in te rmi t ten t s t i r r ing. The resul tant solids w e r e f i l t e red , washed and dr ied a t 110°C. Prepara t ion of Ni-Mo/y-AlzOs Cata lys t s Suspend 1.89, 1.41, 1.12g of 8% MoS /y-A1 0 in aqueous solution of IOOml in a 250ml conical f lask containing lOml of nickel n i t r a t e (O.IM) solution, 5 g urea, I m l , 0.75 n i t r ic ac id and 30ml t h i o a c e t a m i d e (0.133M) for obtaining 3,4,5 percen- t a g e s of NiS2 on 8% MoS2/y-Al2O3 c a t a l y s t s respect ively. The c o n t e n t s of t h e flask w e r e hea ted t o 90-95°C for about 3 hours with in te rmi t ten t s t i r r ing. The resul t ing solids w e r e f i l t e red , washed and dr ied a t 110°C for one hour. 2 2 3 ACTIVITY MEASUREMENTS A d i f fe ren t ia l flow microreac tor , opera t ing under normal a t mospheric pressure and interfaced t o a gas chromatograph by a six-way gas-sampling valve, was used t o measure t h e ac t iv i t ies of t h e ca ta lys t . In a typ ica l experiment ca 0.3g of c a t a - lyst sample was secured be tween t w o plugs of pyrex glass wool inside t h e glass r e a c t o r (pyrex glass tube , 0.5cm i.d.). The reac t ion t e m p e r a t u r e w a s adjusted t o 400°C for th iophene HDS and for cyclohexene HYU. The ca ta lys t was c o n t a c t e d with t h e reac t ion mixture , which consis ted of a s t r e a m of hydrogen s a t u r a t e d with th iophene or cyclohexene a t 25°C. The par t ia l pressures of th iophene and cyc lo- hexene w e r e 80.0 and 85.0 Torr respect ively. Al l r a t e s w e r e measured under s teady- s t a t e conditions. ANALYSIS The HDS product of th iophene was butane and was analysed by gas c h r o m a t o - graphy with t h e help of a 2 m stainless-s teel column packed with 10% OV-17, main- ta ined a t 100°C. Cyclohexane was t h e only product found for t h e HYD of cyclo- hexene under t h e exper imenta l condi t ions and was analysed by 20% PEG-1500 (2m column maintained a t 90°C). A c a r r i e r gas (nitrogen) flow of 40 Cm3 min-l and a n FID w e r e used in both cases . CHEMISORPTION MEASUREMENTS A convent ional high-vacuum sys tem was used. In a typ ica l experiment ca . 0.5g of ca ta lys t was placed in t h e ca ta lys t c h a m b e r and sys tem was evacuated a t 400°C for 2 hours a t l o 4 Torr. The ca ta lys t c h a m b e r was t h e n cooled t o -78°C by dry- ice + a c e t o n e ba th and t h e evacuat ion was cont inued a t t h i s t e m p e r a t u r e for 15 min. Oxygen f rom a reservoir , connec ted t o a high vacuum manifold, 614 was al lowed t o e n t e r t h e ca ta lys t chamber with known dead space. An initial quick fal l in t h e pressure was followed by a levelling off within ca. 10 min. and t h e equilibrium pressure was noted. This process was repea ted with d i f fe ren t ini t ia l pressures and t h e f i rs t adsorpt ion isotherm, represent ing both t h e chemisorbed and physisorbed oxygen, was generated. Af te r t h i s t h e ca ta lys t was evacuated a t -78°C for 17 hour a t Torr t o remove t h e physisorbed oxygen and t h e second isotherm represent ing only t h e physisorbed oxygen, was genera ted in a n ident ical manner. From t h e s e t w o l inear and paral le l i so therms t h e amount of chemisorbed oxygen was de te rmined by t h e method dof Parekh and Weller (ref. 11). I A f t e r t h e chernisorption experiment t h e BET sur face a r e a of t h e ca ta lys t was de te rmined a t -196°C. RESULTS AND DISCUSSIONS Prec ip i ta tes of molybdenum sulphide, cobal t sulphide, nickel sulphide formed w e r e found t o b e quant i ta t ive. Oxygen Chemisorpt ion Oxygen chemisorpt ion exper iments w e r e car r ied out a t -78°C for d i f fe ren t composi t ions of unpromoted MoS2/y-A1203 and promoted Co-Mo/y-Al2O3, Ni-Mo/AI2O3 c a t a l y s t s and t h e resu l t s w e r e given in Table 1. BET s u r f a c e a r e a resul ts w e r e also repor ted . TABLE 1 Composi t ion, oxygen uptake and BET sur face a r e a s of various c a t a l y s t s Composi t ion (wt .%)a Oxygen uptake BET sur face p mol g-' ca ta lys t area a f te r Mo c o Ni LTOC m2g-' Cata lys t 1. 2. 3 . 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 2 4 6 8 10 12 15 8 8 8 8 8 8 .. .. .. 3 4 5 .. .. .. .. .. .. .. 3 4 5 5.30 8.50 12.00 15.00 12.20 10.00 4.50 20.16 24.00 26.19 19.82 22.34 24.17 214 206 203 200 200 190 I87 I47 141 130 137 134 125 aThe ba lance w a s y-Al2O3. The BET s u r f a c e a r e a of t h e alumina (Harshaw Al- I I I -61E, S.A. 234 m 2 -I g ) which w a s used t o prepare ca ta lys t s . 615 From t h e resu l t s of t h e t a b l e , it was observed t h a t oxygen chemisorpt ion increases linearly in t h e c a s e of unpromoted ca ta lys t a s a funct ion of M o loading upto 8% (w/w) and t h e n decreases with higher M o content . This is probably due t o t h e increase in s i z e of t h e individual MoS crystal l i tes . This 8% level corresponds t o a t ta inment of a monolayer of MoS2 on t h e alumina surface. The dispersion of MoS2 (O/Mox100) i s found t o be 0.036, X-ray d i f f rac t ion (XRD) resu l t s indicates t h a t no XRD peaks corresponding t o MoS2 w e r e observed for high Mo-loading. This, in turn, indicates t h a t molybdenum sulphide is present in highly dispersed and amorphous s t a t e on t h e sur face of y-A1203. In t h e c a s e of promoted c a t a l y s t s t h e oxygen uptake values w e r e higher for 8:5 Mo-Co/y-A1 0 and 8:5 Mo-Ni/y- A1203. BET s u r f a c e a r e a of t h e unpromoted Mo c a t a l y s t s decreased with increasing Mo loading and in t h e promoted c a t a l y s t s decreased with increasing promoted load- ings. HDS and HYD Act iv i t ies 2 2 3 HDS ac t iv i ty of t h e c a t a l y s t s repor ted a s t h e steadystate r a t e of HDS of thiophene and HYD ac t iv i ty of cyclohexene a r e repor ted in Table-2. TABLE 2 HDS and HYD ac t iv i t ies of various c a t a l y s t s a t 400°C Cata lys t HDS ac t iv i ty HYD ac t iv i ty 1. 2. 3. 4. 5. 6. I . 8. 9. 10. 11. 12. 13. Harshaw (HT.400) Ket jenf ine - 124 Harshaw (HT.500) Ket jenf ine - 802 I I .44 13.57 27.00 32.00 27.00 25.00 13.37 34.00 42.04 48.92 33.67 40.00 46.24 25.74 24.74 21.30 22.32 9.1 14.2 16.5 23.0 19.6 19.2 17.6 25.2 27.4 29.0 24.7 28.5 29.9 43.5 44.1 47.0 35.3 Composi t ions of 1-13 c a t a l y s t s a r e given in t h e Table-I . The resul ts of Table 2 show t h a t t h e HDS ac t iv i ty is maximum for unpromoted 8% Mo loading on y-A1 0 and 8:5 Mo-Co and 8:5 Mo-Ni ca ta lys t s . These resul ts c o r r e l a t e with t h e oxygen chemisorpt ion values. HDS ac t iv i ty oi t h e c a t a l y s t s prepared by PFHS was higher when compared t o commerc ia l ca ta lys t s . It i s a l so of in te res t to n o t e t h a t HDS ac t iv i t ies of cobal t and nickel promoted c a t a l y s t s appear t o b e higher than those of unpromoted catalysts . Therefore it appears t h a t 2 3 616 t h e role of promoter is mainly t o increase t h e intrinsic activity of t h e HDS sites and not to increase t h e number of ac t ive si tes responsible for HDS of thiophene. It is generally accepted tha t co-ordinatively unnaturated Mo ions (CUS) on sulphided ca ta lys t s a r e t h e ac t ive si tes for hydrodesulphurisation and hydrogenolysis react ions and tha t these a r e located on MoS2 a s a patchy - monolayer on t h e surface of alumina support (refs. 12, 13). HYD activity decreases for unpromoted and promoted catalysts. HYD activity appears t o be a function of only t h e extensive property (i.e. t h e no. of sites) and only due to dispersion e f fec t . Conclusion The ca ta lys t s prepared by PFHS method a r e not required t o sulphide prior t o HDS reaction. These sulphide catalysts can be prepared in a single step. These ca ta lys t s have higher HDS activity than t h e ca ta lys t s prepared by other met hods. Thus PFHS method is found t o be a novel method for preparing highly ac t ive hydro- t rea t ing catalysts. ACKNOWLEDGEMENT The authors thank t h e Council of Scientific and Industrial Research, New Delhi for the i r financial support to KSR and for awarding fellowship t o V.V.D.N. Prasad. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. H.H. Willard and N.K. Tang, J. Amer. Chem. SOC., 59 (1937) 1190. P.F.S. Cartweight, E.J. Newrnan and D.W. Wilson, Analyst., Rev., Vol. 92, No. 1100 (1967) pp. 663. Kaza Somasekhara Rao, U. lvturalikrishna and V.G. Vaidya, Quarterly Chemistry Rev., Vol. I , No. 2 (1985) 134-150. Kaza Somasekhara Rao, Acta Ciencia Indica, Vol. XIlc, No. 3 (1986) 122. F. Burriel - Marti and A.M. Vidan, Anal. Chim. Acta. 26 (1962) 163. D.H. Klein, D.G. Pe ters and E.H. Swift, Talanta, 12 (1965) 357. J.A. Van Dillen, J.W. Gevs., L.A.M. Hermans and J. Vander Meijden, in Proc. 6th Int. Congr. Catal., Edn., G.C. Bond, P.B. Wells and F.C. Tompkins, The Chemical Society, London, 1976, (1977), p. 677. H. Sehapper, E.B.M. Duisburg, J.M.C. Quantel and L.L. Van Reijen, in Prepara- t ion of Catalysts 111, Eds. G. Poncelet, P. Grange and P. Jacobe, Elsevier Amsterdam, 1983 p. 301. Ch. Sivaraj, B. Prabhakara Reddy, 8. Rama Rao and P. Kanta Rao, Applied Catal., 24 (1986) 25. Vemulapalli Prasad, Komanduri Chary, Kaza Somasekhara Rao and Panja Kanta Rao, J. Chem. SOC., CHEM COMMUN, 22 (1989) 1747. B.S. Parekh and S.W. Weller, J. Catal., 47 (1977) 100. N.K. Nag, J. Catal., 92 (1985) 432. W.S. Millman and W.K. Hall, J. Catal., 59 (1979) 311. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands 617 PREPARATION OF MANGANESE OXIDE CATALYSTS USING NOVEL NH4MnO4 AND MANGANESE HYDROXIDE PRECURSORS. COMPARISON OF UNSUPPORTED AND ALUMINA SUPPORTED CATALYSTS A.K.H. N O H M A N ~ ~ ~ , D. DUPREZ~, c. KAPPENSTEIN~, S.A.A. MANSOUR' AND M.I. ZAKI' l chemist ry Department, Facul ty o f Science, Min ia Un ive rs i t y , El-Minia, EGYPT 2Catalyse en Chimie Organique, Faculte des Sciences de P o i t i e r s , FRANCE. SUHMARY Unsupported and a1 umina supported manganese oxide c a t a l y s t s were prepared using manganese n i t r a t e , manganese hydroxide and ammonium permanganate. They were bu lk and surface character ized by thermal analysis, X-ray d i f f r a c t i o n , d i f f u s e ref lectance, I R and photoelectron spectroscopy, SBET and TPR. Moreover H202 decomposition and CO ox ida t i on were used as t e s t react ions. The most a c t i v e supported ca ta l ys ts are the manganese hydroxide coated samples which show a r- Mn 03 phase. For the ammonium permanganate-based ca ta l ys ts a s t rong i n t e r a c t i o n w i t h the c a r r i e r was evidenced. INTRODUCTION Studies concerning supported manganese oxides are relatively scarce, despite their potential activity in oxidation reactions. For example manganese oxide catalysts are very active for CO oxidation, particularly when they are promoted with CuO or COO (1). They are also used for methanol oxidation and ethylene hydrogenation (2). These catalysts were first prepared and investigated by Selwood et a1 ( 3 ) by impregnating manganese (11) nitrate onto high surf ace area alumina and then thermally decomposed. Thereafter, these catalysts were characterized by several techniques (4-9). Baltanas et a1 (2) prepared these catalysts by impregnating manganese nitrate onto alumina and in situ precipitation of manganese hydroxide by ammonia solution. The aim of the present study was to prepare various series of bulk and alumina-supported manganese oxide catalysts. Novel NH4Mn04 and manganese hydroxide precursors as well as the conventional manganese nitrate were used in hope of defining the impacts of the precursors and the support on physico-chemical characteristics and activity of the final catalysts. KMn04 was previously used by Cavallaro et a1 ( 8 ) but with little information. We have taken NH4Mn04 for our investigation to avoid 618 more complications arising from the presence of potassium. By adopting a coating procedure in case of manganese hydroxide, we aimed to have surface layers of manganese oxide on alumina which may be more easily detectable than in the case of the two previous precursors. EXPWIWENTAt Materials High surface area r-alumina (214 m2 g-l) was obtained by slow gel formation between ammonia and aqueous aluminum nitrate solutions, followed by decantation, drying (12OOC; 4 days) and calcination (450'C: air, 5 h; oxygen, 2 h). Three precursors were used to prepare the various series of unsupported and supported manganese oxide catalysts: (i) manganese (11) nitrate (W), (ii) ammonium permanganate NH4Mn04 (m) synthesized by a metathetical reaction between NH4C1 and KMn04 (lo), and (iii) manganese hydroxide coat (*A) obtained by slow addition of Mn(N03)2 6H2O solution to aqueous ammonia followed by filtration and drying Torr, 25'C). The unsupported catalysts were obtained by calcination of the and &!G precursors at 150', 300' and 6OO0C for 5 h in air. The products thus obtained are designated by formula like Hn2[1501 or MnC13001. In the case of precursor, crystals of NH4Mn04 were first slowly decomposed at 12OOC for 2 h in air prior to calcination. The product is then calcined as above: the calcination products are denoted like Mn7f1501. In case of manganous nitrate and ammonium permanganate, the supported catalysts were obtained by impregnation from aqueous solutions of various concentrations. The loading level for the cationic adsorption of Mn2+ remains low (0.13, 0.3 and 0.6 wt%-Mn) whereas for MnO4- the impregnation leads to higher values (0.5, 0.9 and 1.7 wt%-Mn). For the manganese hydroxide precipitate, a coating procedure was carried out by formation of the precipitate in presence of the carrier, to provide the loading levels of 0 . 4 , 4.1 and 6.8 wt%-Mn. All these samples were subsequently filtered, dried (25'C and loq2 torr) and then calcined (150, 300 or 600-C). They are denoted like v, where x gives the Mn loading. 619 Characterization techniaues The various samples of unsupported and supported manganese oxide catalysts were subjected to a range of physical and chemical characterization methods, so as to examine their surface as well as bulk properties, and hence the effect of the preparation variables. For the bulk properties the following techniques were used : - thermogravimetric and differential thermal analysis (TGA and DTA), Shimadzu apparatus type DT-30 H, heating rate 10'C min-l, reference a-A1 203 ; with microcomputer attachment, Cu Ka radiation (1.5418 A ) ; - infrared spectroscopy (IR), Perkin-Elmer recording spectrophotometer (Model 580 B), KBr pellets : - and temperature programmed reduction (TPR) in H2: pulses of H2 (0.285 cm3) being injected every other minute from ambient temperature to 500 'C (4 C min-l ) . On the other hand, the following methods were employed as surface characterizing techniques: - surface area measurements (BET) by low temperature nitrogen adsorption method; - diffuse reflectance spectroscopy (DRS), Beckmann 5240 spectrometer equipped with an integrating sphere and coupled to an HP 9816 microcomputer; dehydrated BaS04 was used as a standard for all the spectral regions (250 - 2500 nm); - X-ray photoelectron spectroscopy (XPS), Riber spectrometer , A1 Ka source (1488.6 eV), reference C l s at 285 eV.. - X-ray diffraction (XRD), Siemens D 500 diffractometer Moreover, and in order to reveal the effect of preparation variables on the redox activity of these catalysts, two model reactions were studied: (i) H202 decomposition in aqueous solution and (ii) CO oxidation in transient flow, carried out with the same chromatographic apparatus as for TPR measurements. The latter technique leads to the determination of the oxygen storage capa- city (OSC) of the catalyst: pulses of CO were injected every other minute at 3OO0C on a sample predosed with O2 pulses at 300'C . RESULTS AND DISCUSSION Bulk characterization A part of the catalysts used are listed in Table 1. TGA and DTA results of precursor indicated that this material commences decomposition at 80-C to give Mn02 which leads to 620 Mn2(150) (300) (600) 0.6Mn2(150) (300) (600) MnC(150) (300) (600) a-Mn2O3 upon calcination at 600'C, in agreement with previous results (11). XRD data and IR findings confirmed these results (table 1). For the supported catalysts, the thermal behavior was not the same, indicating a probable interaction of manganese nitrate with r-A1203 (6) due to the very low load of the samples. No detectable thermal events have been evidenced, indicating that the surface species do not change upon heating, also reflected by the pale brown color exhibited by all the samples. However TPR profiles are similar for calcination temperature 150 and 30OOC but different for 6OO0C. (Fig.1). The first peak of the TPR curves (= 350'C) in the case of the samples calcined at 150 and 300'C can be attributed to the reduction of adsorbed nitrate ions. It was shown previously that NO3- ions adsorbed on A1203 reduced quantitatively into N2 during TPR, thus requiring 5H/N03- for their reduction to be completed (12). However even at 15OOC the content of residual nitrate is low, typically of the order of 30% of the initial loading associated with Mn. At higher calcination temperature, these ions are decomposed. NS â¬5-Mn02 + few a-Mn2Og 10 NS R-Mn02 + few a-Mn203 11 NS a-Mn203 12 S Only r-A1203j 139 S Only r-A1203, 149 S Only r-A1203; 180 NS r-Mn203 22 NS r - ~ n ~ o ~ + ~r-1~0~ ? 28 NS a-Mn2O.3 24 TABLE 1: crystalline phases, surface area, oxygen storage capacity and kinetic rate constant for the decomposition (9-1 catalyst) . r-Mn203 + F - A ~ ~ o ~ r-Mn2O3 + ~ ~ 1 ~ 0 3 r - ~ n ~ o ~ + ~ ~ 1 ~ 0 3 a-Mn203 +few MnOl.88 MnOle88 + a-Mn203 MnOl 88 + a-Mn20 Only I'-Al2O3{ Only r-A1203, Only r-A1203; more * crystallize2 of H20i 174 183 188 60 131 82 149 130 147 S S S NS NS NS S S S - ISC, 300'C lcrnol 0 9-3 178 86 79 115 83 39 -- 907 480 -- 25 0 4.3 3.2 2.4 0.03: 0.10 0.13 10.7 11.3 10.2 18.3 17.6 10.8 11.0 24.5 22.2 10.2 2.1 0.52 a) NS: non supported: S: supported. 621 C L1 t 1 C t 1 0 li 0 w ._ s UNSUPPORTED 200 400 600 T("C) I I I I I I - Fig.1: TPR p r o f i l e s . Surface area in mmol H 4 - l Fig.2: thermal ana lys i s o f some sarrlpl e s . 622 For the unsupported samples, the thermal analysis curves (Fig.2) and the XRD indicate that this precursor is most probably changed from the hydrated r-Mn203 to the a-Mn2O3 form above 40OOC. The presence of nitrate ions up to 300'C was evidenced by IR bands at 1385 cm-I (Fig.3) and by the different exothermic peaks of the DTA curve (Fig.2). This is in agreement with the TPR profiles (Fig.1) showing a first reduction peak between 300 and 340'C which disappears for MnCf600). sample. TPR profiles for Mn reduction (>400'C) for MnCf1501 and MnC16001 display a small difference which can be associated with the change from the r to the Q form of Mn2O3. This behavior was modified on coating the carrier, since up to 600'C the surface species of the supported samples remains r-Mn203 (Table 1 and Fig.4). The TPR curves show that the content of NO3- is higher for 6.8HnCf150). than for 6.8MnCf6001 whereas the profiles at higher reduction temperature remain the same. Accordingly, the bulk phases of manganese oxide in these supported catalysts are detectable and the calcination temperature does not markedly affect either the crystalline or the chemical nature of these species, whereas the unsupported catalyst calcined at 600'C was markedly affected. Hence the interaction with the support may play an important role. In case of &Q unsupported and supported samples, bulk phases of manganese oxides were expected to be different owing to the different mode of decomposition and subsequent calcination. This was proved to be the case through the different results obtained for these catalysts. No bulk phases of manganese oxides were detectable for the supported catalysts by XRD, due to the low Mn loading. The possibility of strong interaction with the support is evident for the catalysts calcined at 600'C (cf. TPR profiles in fig.1). Surface character ization Surface characteristics of these catalysts are reflected on their SBET, DRS and XPS results. The surface areas of and unsupported samples are lower than the corresponding knx catalysts. This may be attributed to the differences in the porosity despite the similarity of the chemical nature of these different samples, as pointed out in their bulk characterization. On the other hand, the drop of surface area of all the supported catalysts relative to the support (214 m2 g-1) , is most likely due 623 3': 'r 3 v ) . 5=a a- * != I--! 1450 1050 650 75c Fig.3: IR-spectra of MnC f o r d i f f e r e n t ca lc ina t ion temperatures. r e f . : ?-?In 0 2 3 Fig.4: X-ray data f o r alumina, coated sample and d i f fe rence spectrun. The 1 ines correspond t o the reference compounds Y-Al 203 and Y-Mn203. 1 . F .5 .4 .2 0 10 5 0 DIF-KUB 0.6 :ln2(600) 461 700 1000 0. 9 Mn7 \ Fig.5: OR d i f fe rence spectra o f some samples obtained by substract ion of t h e spectrum of Y - A ~ 233. 624 Mn Samples 2P1/2 2P3/2 3P to the formation of a manganese oxide phase for 6.8MnC samples or to a blockage of the micropores for 0.9Mn7 and 0.6Mn2 samples DRS results can give information on surface species of Mn present in supported samples. The difference spectra (Fig.5) of 0.9Mn7 samples show that the surface species at 600'C are different from those formed at lower calcination temperatures, in accordance with the TPR curves of the corresponding samples (Fig.1). Moreover the surface species at 6OO0C are comparable to those of 0.6Mn21600LI displaying the same band position (= 460 nm). Thus the surface species are probably the same as in the case of 0.9Mn7f6001 sample,the activities becoming similar for the two catalysts. From the XPS data given in table 2, the variations of the Mn2p3/2 binding energy can be associated with the oxidation state of manganese (9,,14,15). Thus for the mechanical mixture Mn3O4 + A1203, this value (640.4 eV) corresponds to the presence of Mn(I1) and Mn(II1). In the case of 4.1NnC and 0.5Mn7 samples the oxidation state of Mn is higher probably between I11 and IV, and decreases slightly after calcination for 4.lMnC. The Mn/A1 ratio for the mechanical mixture is in agreement with the value calculated from the composition of the mixture (0.042). For 4.lMnC samples this ratio is higher than the calculated Mn/A1 ratio (0.04), reflecting the partial coating of the alumina surface, and for 0.5Mn7 the values correspond to a good dispersion of manganese on the surface of the carrier (calculated Mn/A1 ratio : 0.0047). TABLE 2: XPS data, surface area ratio and kinetic rate constant for the decomposition of H707. (13). 0 Is I I Binding Energy IIEbll eV fO. 2 4.1MnC(RT) 4.1MnC(600) 0.5Mn7(RT) 0.5Mn7(600) 653.3 641.7 48.7 531.3 652.7 641.2 48.5 531.2 653.7 642.0 48.7 531.3 653.6 641.9 48.5 531.2 Mn3O4 + A1203 651.9 640.4 48.5 531.2 (5 wt%-Mn) 1 1 1 1 0.069 0.060 0.024 0.025 0.12 0.10 0.043 0.044 I t I Area ratio O/Al 1.76 1.79 1.76 1.80 1.77 I C, 30°C ;-lg-1 8.1 6.2 7.0 0.15 625 Activity The rate constant values K~~~~ obtained at 3Q°C for the catalyzed decomposition of H202 as well as the values of OSC at 3QO'C are reported in table 1. These values are clearly correlated despite the fact that one of them is performed in aqueous solution, whereas the other is carried out in the gas phase Concerning the supported catalysts, 6.8MnC are the most active samples whereas the activity of the 0.6Mn2 samples for both reaction remains very low. The catalytic activities cannot be correlated with the values of SBETl except for the supported 0.6Mn2 serie. As a rule, when the atomic surface ratio Mn/O increases, the activity of the corresponding catalysts increases (compare 4.1MnC and 0.5Mn7 series, Table 2 ) . Accordingly one may conclude that a samples contain more surface manganese species with more surface active oxygen as indicated from OSC, which can initiate the decomposition of H202. For both and XNnC series the rise of the calcination temperature results in a decrease of the catalytic activity, this being more pronounced â¬or the former. This can be attributed to the loss of surface hydroxyl groups and probably of surface active oxygen upon calcination although the manganese oxide phase remains the same (6.8MnC series). Similar effects were already stated on Mn02 (16). On the contrary, for 0.6Nn2 supported samples, with lower values of the kinetic rate constant, the catalytic activity increases with the calcination temperature . This can be correlated with the increase of the surface area and a possible explanation is the migration of manganese leading to a better dispersion. In the case of the unsupported samples the series displays the highest activity in correlation with higher surface areas. The variation in the activity of these catalysts reflects the role of the stoichiometry and crystalline modification of the manganese oxides. CONCLUSION The most active unsupported catalysts for both model reactions are the permanganate-based samples m, after calcination at 300 or 60Q'C; these samples display the highest surface area and correspond to the highest oxidation number of manganese. On the contrary, the supported catalyst samples m, prepared with the same precursor, exhibit a drastic drop in the catalytic activity 626 despite an equally high surface area. The TPR measurements showed these supported samples to be difficult to reduce after calcination at 600°C, thus suggesting a strong interaction with the support. For the supported samples the use of the coating technique, with the hydroxide precursor, leads to the most active catalysts and manganese oxide phases were XRD detectable. Moreover, for this precursor, supported 6.8MnC and unsupported samples show comparable activities, in relation with the dispersion effect of the support. In the case of the supported catalysts prepared with manganese (11) nitrate, higher loadings of manganese are recommended, in order to verify the influence of the calcination temperature; this needs to change the impregnation procedure which presently limits the loading level. ACKNOhlLEDGJiXENT We thank very g r a t e f u l l y Prof . J.F. Hemidy (Univ. o f Caen) and Prof . G . Perot (Univ. o f P o i t i e r s ) f o r DRS data, XPS measurements and valuable discussions. A.K.H. Nohman thanks apprec iate ly the Egyptian Government f o r the grant given t o him. REFWENCES 1 W.B. Innes, i n : P.H. Emmett (Ed.), Catalysts, Vol. 2, Reinhold, New-York, 2 M.A. Baltanas, A.B. S t i l e s and J.R. Katzer, Appl. Catal., 28 (1986) 13-33. 3 P.W. Selwood, T.E. Moore, M.El l is , J. Amer. Chem. SOC., 73 (1949) 693. 4 G.T. Po t t and B.D. McNicol, Discuss. Faraday SOC., 52 (1971) 121-131. 5 M. Lo Jacono, M. Schiavel lo and G. Mercati , Gazz. Chim. I t a l . , 105 (1975) 6 L. Burlamacchi and P.L. V i l l a , React. K ine t . Catal. L e t t . , 3 (1975) 199-204. 7 M. Lo Jacono and M. Schiavel lo i n : B. Delmon, P.A. Jacobs and G. Poncelet (Ed.), Preparation o f Cata lysts I, Elsev ier , Amsterdam, 1976, pp. 474-487. 8 S. Cavallaro,.N. Bertuccio, P. Antonucci, N. Giordano and G.C. Bart, J. Catal. , 73 (1982) 337-348. 9 B.R. Strohmeier and D.M. Hercules, J. Phys. Chem., 88 (1984) 4922. 1955, Ch.1. 1165-1176. 10 L.L. Bircumshaw and F.M. Taylor, J. Chem. SOC., (1950) 3674. 11 R.D.W. Kemmit, in: J.C. B a i l a r (Ed.), Comprehensive Inorganic Chemistry, Vol. 3, Pergamon Press, New York, 1973, Ch.37, p.771. 12 0. Duprez and S . Kacimi, personal communication. 13 D. Dol l imore and J. Pearce, Powder Technology, 25 (1980) 71-78. 14 C.D. Wagner, W.M. Riggs, L.E. Davis and J.F. Moulder; "Handbook o f X-Ray 15 M. Lenglet, A. D'Huysser, J. Kasperek, 3. P. Bonnelle and J. Durr, Mat. Res. 16 S.B. Kanungo, J. Cata l . , 58 (1979) 419-435. Photoelectron Spectroscopy", Eds. G.E. Mullenberg, 1979.. Bu l l . , 20 (1985) 745-757. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 627 INFLUENCE OF SURFACE OH GROUPS AND TRACES OF WATER VAPOR DURING THE PREPARATION OF Ti02-Si02 SAMPLES A. MUBOZ-PAEZ and G. MUNUERA Dept. of Inorganic Chemistry and Instituto de Ciencia de Materiales (UNSE-CSIC) P.0.Box 1115, 41071 Sevilla SPAIN SUMMARY TiOz-SiOz samples have been prepared by impregnation of silica support with n-hexane solutions of titanium alcoholate controlling the hydration/ hydroxylation degree of the silica surface. Once calcined, the samples were characterized by IR, XRD, SEM/EDAX and XAS. The mechanism proposed for the decomposition of the alcoholate involves reaction with adsorbed molecular water in a first step, followed by anchoring by reaction with acid OH- groups. The amorphoustitaniumoxide obtained after calcination shows a layered open structure of clusters formed by a few octahedra sharing edges and corners. INTRODUCTION Titania has been widely used as support in metal catalyst due to its ability to modify the catalytic properties of the metal (ref. 1). As a consequence, the study of the interactions taking place at the metal-titania interface has attracted the interest of several research groups (ref. 2). Nevertheless it is very difficult to obtain high surface area titania (>lo0 m2/g), and while studying the metal-titania interactions it is difficult to get information from the support because the bulk properties of the Ti02 mask those of the surface, the unique part of the titaniaaffectedby the metal. To overcome both problems, inert oxides like silica, have been used as support to obtain high surface area dispersed titania, by grafting to the SiO, support (refs. 3,4)throughthe impregnation from n-hexane solutions of Ti alcoxides, that by hydrolysis and calcination would produce the final coated Ti02-Si02 powders. EXPERIMENTAL Preparation af catalysts The surface oxide was prepared by impregnation of silica Aerosil-200 (SBE,=200 m2/g), with a n-hexane solution of a Ti-alcoholate (tetraisopropyl- titanate, Ti(OPrâ)4 from Tilcom, 16.9% Ti). The TiOz percentage (by weight) used has been c.a. 12%. This value correspondsroughly to the amount required to form a monolayer of titania on this type of silica (14.7%, 5.5 Ti/nm2) and 628 is close to the amount needed to allow the grafting of each Ti atom to one hydroxyl group of the silica surface (13 %, 5 OH/nm2) (ref. 5). In the standard procedure, the desired amount o f Ti(OPri)4 was dissolved in dried n-hexane (25 ml/g of silica in Methods 1-3 and 6 ml/g of silica in Method 4) and the solution was allowed to react with the surface of the silica for several hours. After that, the solvent was removed at room temperature by flowing Nz, and the sample heated in N 2 up to 673K. Subsequently the solids were calcined i n air at 873K. Four differents methods were used as follows: Method 1. Reaction under Nz at 300K for 20h of undried SiOz. Method 2. Reaction under Nz at 300K for 20h of Si02 dried at Method 3. Reaction under Nz at 350K for 5h of SiOz dried at 440K for 2h, cooling down to room temperature, filtering and washing with n-hexane and subsequently with water. Method 4. Incipient impregnation at 300K in the air of undried silica with a n-hexane solution of Ti Tests were made along the preparation (Methods 1 to 3) by gas chromatography to detect i-PrOH in the liquid phase as well as the presence of Ti(OPrâ)4 by hydrolysis in aliquots of the liquid phase. Characterization of solids 388K for 2h. IR spectra were carried out at 300K on a wafer of the sample mounted in a cell that allows in situ thermal treatments under controlled atmospheres up to 773K, using a Perkin-Elmer 684 spectrometer fitted to a 3600 data station. X-ray diffractograms were recorded in a Phillips 1730 diffractometer and scanning electron micrographs using an IS1 microscope model SS-40, with an energy dispersive X-ray analyzer (EDAX) KEVEX, model 8000 fitted to it. XAS experiments were performed on the EXAFS station 8.1 in the Synchrotron Radiation Source at Daresbury Laboratories with ring energies of 2 GeV and ring currents of 250 mA. The EXAFS spectrum was recorded at 140K in an âin situâ cel1,wherethe sample was placed after being pressed with BN into a wafer with an absorbance ( p x ) of 2.5 at the Titanium K-edge assuring an optimum signal to noise ratio. Data analysis was carried out by fitting in k- and R-space using the phase and amplitude corrected Fourier transforms to identify the different contributions (ref. 6). Phase shift functions and backscattering amplitudes were obtained from reference compounds. RESULTS Assuming that the two reactions taking place during the decomposition of the alcoholate to TiOZ are grafting through OH- groups on the surface of the silica and hydrolysis of the Ti-alcoholate by water to produce colloidal 629 particles (refs. 3 , 4 ) , the only competitor to decompose the alcoholate would be the water vapor from moisture. Thus, we have used several preparation methods in which moisture was carefully avoided. Therefore, in the first case (Method l ) , the grafting would involve only OH- groups and/or water molecules adsorbed on the silica surface. Taking into account that water physisorbed on the surface of the silica could produce mainly ungraphted titania, we have carried out a second preparation method (Method 2) in which adsorbed water was avoided by submitting the silica to a previous outgassing treatment at 388K that would remove at least physisorbed water. A new method (Method 3 ) was designed in which all molecular water was removed and, considering that the hydrolysis process could be very slow at room temperature in such extremely dry conditions, the reaction temperature was raised up to the boiling point of the n-hexane. In this case, after five hours of reaction the liquid phase, still containing Ti(OPrâ)4, was filtered off and the sample was thoroughly washed with n-hexane, to remove the unreacted alcoholate, and then with water to get a complete hydrolysis of the grafted alcoholate. During the washing with water, formation of a thin, opaque white layer was clearly observed which, unlike the transparent silica, remained stuck on the surface of the filter. This white coating, presumably TiOz, once dried, calcined and weighed,turn out to be c.a. 50% of the total amount of titanium oxide that should be formed by decomposition of all the alcoholate employed in this preparation. Control test during the preparation in Methods 1 and 2 showed a complete hydrolysis of Ti(OPri)4 at the end of the reaction time (20h), while in Method 3 the liquid still contained the alcoholate in spite of the more drastic thermal conditions used in this case. Finally, a fourth type of preparation was carried out consisting in the well known incipient impregnation method, using the n-hexane solution of Ti (OPr i , and the si 1 ica support without any drying pretreatment. In principle, the degree of success of the anchoring process in our preparations could be followed by checking the changes in the concentration o f one of the reactants (i.e.surface OH-/HzO at the silica support) using I R spectroscopy, since silica aerosil shows a characteristic sharp band at 3750 cm due to basic free hydroxyls together with a broader band due to more acidic OH- groups at 3680 cm-â (ref. 7). So, their reaction can be followed by changes in their intensities as previously observed in similar preparations (ref. 8). Thus, figure 1 shows IR spectra o f the SiOp support and of a sample prepared by method 1 containing only 1% Ti02 before submitting the samples to any thermal or outgassing treatment. The unique change observed after the addition of the alcoholate is the decrease of the intensity in the range - 1 630 100. %A 50 0 a I \ I ' I \, 1% Ti0 S i O 2 2 I \\, . '..% I Fig. 1. I R spectra in the OH stretching region of a sample l%TiOz-SiOz (a) and of si 1 ica support (b) registered in the atmosphere (solid line) and after outgassing at 673K for 2h. b I \ I \ --I 3000 4000 cm 3000 -1 4000 cm 3700-3550 cm-', where the bands due to the more acidic OH- groups and/or molecular water appear, what suggests that these species are those mainly involved in the interaction of the alcoholate with the SiOz support. The I R spectra in the same figure, recorded after outgassing at 673K to remove the water readsorbed upon exposure to air, clearly show the decrease in the intensity of the band at 3680 cm-' assigned to the more acidic OH- groups of the silica, thus suggesting their participation in the decomposition of the alcoholate. Figure 2 shows I R spectra in the range 2900-3400 cm-' of the samples prepared with c.a. 12% Ti02 by the four methods (spectra have been normalized using u s i - o at 1830 cm-' from the bulk of the silica, to make them comparable). Except for the sample prepared by method 3 , in all other cases the intensity o f the band at 3750 cm -1 remains nearly unchanged with respect to that of the silica support pretreated under similar conditions, while changes in intensity and/or position are observed in the band at 3680 cm-' in all the preparation methods. The increase in intensity of the IR bands in the 0-H stretching region in sample prepared by Method 3 is probably related with the final washing with water used in this method. phases formed after calcination by decomposition of the hydrolyzed alcoholate. Only small shoulders appear in sample 1, 2 and 3 in the position o f the most XRD was used to check the crystallinity of the titanium oxide 63 1 8 b m I h 0 Ti0 S i O -4 2 0 - c - - - c - - t - - + -1 cm Fig. 2 . IR spectra in the range 3900-3400 cm-' of the silica support and the samples 12%TiOz-SiO2 prepared by the four methods outgassed at 673K for 2h. intense peak of anatase, while no peaks were visible at the positions of the most intense diffraction lines of anatase, rutile or brookite in sample 4. Nevertheless, when the alcoholate was hydrolyzed with water in the absence of silica and calcined under similar conditions, strong peaks appear in the positions of the most intense diffractions of anatase, thus indicating that the hydrolysis o f the pure alcoholate produces crystalline phases. Analysis o f the samples using SEM/EDAX was carried out to examine thehomogeneityof the titanium distribution on the TiOz-Si02 samples, and the homogeneity in grain shape and size. Thus, in sample prepared by Method 2, (using dried Si02) the grains have angular shapes and the local concentration of Ti changes drastically when going from one grain to another. The changes are less drastic, although still remarkable in sample 1, that shows round grains. In sample 3 the particle size was bigger than in the other cases, and the existence of different types of particles (opaque and transparent) could be seen without the aid of the microscope. The most homogeneous sample, considering grain shape and size, as well as titanium dispersion was sample 4, that has a very homogeneous spongy appearance with constant concentration of T i in all the grains. From the previous results, we deduced that method 4 is the best one, so this sample was studied by XAS to get a deeper insight into the structure around Ti ions. The XANES region of this sample has been plotted in figure 3 , where the corresponding spectra o f anatase and rutile, measured as a 632 C D 0.1 h ' " 0 Lr -0.1 J I I I 1 I -20 0 E(eV) 20 40 Fig. 3 . Ti k-edge XANES spectra of TiOz rutile (a), TiOz anatase (b) 12%TiOz-SiOz prepared by method 4. Fig. 4. Ti k-edge, Fourier transforp of the EXAFS syectrum of sample lZ%TiOz-SiOz prepared by method 4. (k , Ak=3.12-11.00 A- ) . Arrows indicate the ranges for Fourier filtering used during the data analysis. reference, have been included as well. In addition to the round shape of features C and D, typical of amorphous compounds (ref. 9), it has to be pointed out the appearance of the triplet A,,A2,A3 characterisitic of octahedral symmetry (ref. 10) that indicates that the absorbing atom is six fold coordinated. Nevertheless, there is a remarkable change in the intensity ratio between peaks AZ and AB, that is close to 1 in anatase or rutile and close to 0.5 in sample 4 where it shows a shape similar to the spectra of uncalcined TiOz colloids prepared from hydrolyzed Ti(OPr')4 (ref. 11). A similar shape has been observed in the spectra of titania-silica glasses prepared by gelation in air of Ti and Si alcoxides by Emili et a1 (ref. 9) , who have assigned it to the existence of Ti ions in tetrahedral environment. The Fourier Transform o f the EXAFS signal yields the radial distribution function shown in figure 4, where we can see A due to backscattering from the first shell of oxygen atoms. For higher distances there is a drop in intensity that, in principle, could be assigned to the lack of higher coordination shells. Nevertheless no good fit could be obtained with only one or two shells. So, we have performed the data analysis shell by shell, doing inverse Fourier Transforms of increasing ranges, shown an intense peak at around 1.7 633 1 Shel 1 TABLE 1 TiOz-Si02-4 N R(A) Ao2(AZ) - - 5.8 1.93 0.011 1.0 3.09 -0.005 6.7 3.78 0.03 7.2 4.37 0.06 3.3 5.34 0.00 ~ Shel 1 Ti-Ol Ti-Ti Ti-02 Ti-Ti2 Ti-03 Ti-Ti3 I Anatase NxR(A) 4x1.93 2x1.98 4x3.04 8x3.86 4~3.78 8x4.25 8x4.27 4x4.75 ax4. 85 number octahedr lS' q r d by the arrows in figure 4. We started the analysis considering the basic octahedra of anatase Ti06 Afterwards, we expanded the range up to 3 A, and included a shell Ti-Ti. When the fitting range was expanded to 4.1 A , two new Ti-0 bonds were required to reach a good fit. Finally, to reach the final values the range for the Fourier filtering was 0.16-5.4 A requiring the inclusion of a new Ti-0 bond at 5.3 A . The parameters of this fit are summarized in Table 1, that includes the number of neighboring atoms, N, the absorbing atom-neighbor distance, R, the Debye-Waller factor, Ao', related to static and thermal disorder, as well as the structural parameters of crystalline anatase appearing in a cluster of 4 octahedra (ref. 12). RTi-O= 1.95 A (fit range 0.16- 2.3 A). A plot of the raw data and the best fit in k and R space for the wider range has been included in Fig 5. The first peak in the Fourier transform may be attributed to the six Ti-Ol bonds of the basic octahedron, as already predicted from the XANES data. The distance is the same that the short bond of the distorted octahedra in anatase. The peaks between 3 and 6 A are a complex result of the overlap of four different features. The first one, Ti-Ti at 3.09 A, is very similar to the distance observed in anatase between two octahedra sharing edges (3.04 A), while the next one, Ti-02 at 3.78 A, is very close to the distance of the oxygen atoms in the second octahedron (3.86 A). The shell Ti-03 would correspond to oxygen atoms in a third octahedron in an anatase-like structure. The shell Ti-04 has no correspondance in a cluster of anatase structure including four octahedra. In relation with the similarities with the anatase strucutre in the other four shell, it has to be pointed out the low coordination number of the Ti-Ti bond at 3.09 A , as well as the lack 634 Fig. 5 . Ti k-edge EXAFS spectrum and Fourier transform (kl, Ak=3.5-10.5 A - ' ) of the raw data (solid line) and best fit (dotted line) of sample 12%TiOz-Sioz prepared by method 4. of Ti-Ti bonds for higher shells. Both facts indicate that only small clusters of TiOs octahedra are present on the SiOz support. DISCUSSION Formation in our conditions of colloidal particles of Ti02 grafted to the high surface area SiOz can be assumed t o occur according to one of the two following schemes: Scheme 1 Ti (OR) + - Si-OHb __ > (-Si-O)n-Ti(OR)4-n+ n ROH (1) (-Si-O)n-Ti(OR)4-n + (4-n)HzO - >(-Si-O)n-Ti(OH)4-n+(4-n)ROH (2) Scheme 2 Ti (OR) + 4 HZOads ------ > Ti (OH)4 + 4 ROH (3) - Si-OHa + Ti(OH)4 > Si-O-Ti(OH)3 + H20 (4) where -Si-OHa and -Si-OHb stand for basic and acid OH- groups at the SiOz surface, Ti (OR)4 for Ti(OPri)4 monomers and HZOads for physisorbed/chemisorbed water. In the first case, grafting should involve in a first step the more basic OH- groups of the silica through a hydrophylic attack, and in second step hydrolysis by reaction with adsorbed water or moisture. According to scheme 2, hydrolysis of the alcoholate by adsorbed water at the support is postulated, leading t o T i hydroxide colloidal particles in a first step, which must be followed by anchoring to the SiOz surface through reaction with more acidic OH- groups, a process that should be enhanced by the final thermal treatment during the calcination used in the preparation of the a SiOz samples. 635 IR data in figures1 and 2 suggest that Scheme 2 (hydrolysis by adsorbed water followed by grafting) is the most likely in the conditions used in our preparative work, since the band at 3750 cm-â, due to more basic OH- is not modified during the whole process. Moreover, changes in the band at 3680 cm-â due to more acidic hydroxyls, can be explained by assuming that the grafting involves this type of hydroxyls of the silica surface. It is worth noting that preparation by Method 3 , where adsorbed water and probably part of the acidic OH- groups have been removed from the S i 0 2 support before reaction, only allows ca. 50% reaction of the Ti-(OPr1)4 in spite of the presence of all the basic OH- groups. This fact again excludes these groups from the process (reaction (1)). In fact, hydrolysis of the alcoholate remaining at the SiO surface in this case only occurs by washing with water what probably also produces breaking of siloxane bridges at the Si02 surface, (partially dehydroxylated) as detected by the much larger intensities of the IR bands for this sample in figure 2. If we assume Scheme 2 , grafted colloidal titania particles, similar to those obtained from simple hydrolysis of Ti-(OPrâ)4 with water, should be obtained and therefore their structure should not be very different from that recently proposed by Leaustic et a1 (ref.11). In fact, the XANES spectrum of our sample is very similar to the spectrum recorded by these authors for such colloidal particles. However, there are big differences in the EXAFS region that can be explained by the smaller size of the titania particles obtained in our system. Moreover, after heating at 373K, these authors obtain crystalline anatase, as previously did Kozlowski et al.(ref. 12) and Reichmann et a1 (ref. 3) during the preparation of similar systems, while the crystalline structure of anatase could not be detected by XRD in our samples, even after calcination at 873K thus implying that the layered open structure, remains stabilized on the surface of the silica. groups, 2 It is not surprising that the best preparation method for this type of ultradispersed Ti02-SiOz systems was the incipient impregnation, since in this conditions the lack o f an excess of solvent will probably prevent the growth of the original nuclei t o bigger colloidal particles. Additionally, this method has the advantage that it is the easiest and provides and well dispersed amorphous samples. The analysis of the XAS spectrum of this sample is far from easy. Thus, although the XANES region of titanium oxides (anatase and rutile) has been the object o f several experimental and theoretical studies (refs. 9-14) the definitive explanation of all the features appearing in this region has not been given yet. Nevertheless, by comparing it with the spectra of previously studied compounds, we can use this region of the spectrum as a finger print. homogeneous 636 Thus, from the comparison with the Ti k-edge XANES spectra of several alcoholates previously measured (ref. 14), we can discard the presence of tetrahedral or square planar geometry around the Ti centers, as well as the long range order typical of crystalline structures, like anatase, rutile or brookite (refs. 12,13), confirming in this way the conclusions reached by XRD. The EXAFS results point to the existence of a phase similar to anatase but, since the distances Ti-Ti2 and Ti-Ti3 are missing, the coordination numbers for Ti-Til, Ti-02 and Ti-03 are very small, and there is a new distance Ti-04 above 5A, it seems that the new structure is more open and has grown in two dimensions. The parameters obtained are compatible with a structure similar to the Ti02-B, proposed by Brohan et al. (ref.15) and more recently by Reichmann and Bell (ref.16) as a precursor o f anatase in the decomposition of TiC14. In conclusion, incipient impregnation of SiOz with a n-hexane solution of Ti(OPr')4 leads to TiOz coated material with an extremely high dispersion where very small clusters of Ti06 octahedra (probably 3-4 octahedra sharing edges and corners) are formed. The process involves hydrolysis by physisorbed/chemisorbed water followed by anchoring during calcination. ACKNOWLEDGEMENTS. The authors wish to thank Prof. D.C.Koningsberger for the use o f his EXAFS analysis programs, CICYT and Junta de Andalucia for financial support, and the staff in the SRS (Daresbury lab., SERC) for help during the XAS measurements. REFERENCES i G.C Bond and R.Burch, Catalysis (Specialist Periodical Report).Chem.Soc., 2 K.Foger. Catalysis,Science and Technol. 6 (1984) 227-305. 3 M.G.Reichmann and A.T.Bel1, Appl.Catal., 32 (1987) 315-326. 4 C.Morrison and J.Kiwi, J.Chem.Soc.,Faraday Trans.1, 85(5) (1989) 1043-1048. 5 J.B.Peri and A.Hensley, J.Phys.Chem., 72 (1968) 2926 6 J.B.A.D van Zon, D.C.Koningsberger, H.F.J. van't Blik, and D.E.Sayers, 7 J.B.Peri, Catalysis,Science and Technol., 5 (1984) 171-220. 8 E.T.C. Vogt, M.de Boer, A.J. van Dillen, and J.W.Geus, Appl. Catal., 9 M.Emili, L.Incoccia, S.Mobilio, G. Fagherazzi, and M.Guglielmi, 10 L.A.Grunes, Phys. Rev. 8, 27(4) (1983) 2111-2131. 11 A.Leaustic, F.Babonneau and J.Livage, Chem. Mat., 1 (1989) 248-252. 12 R.Kozlowski, R.F.Pettifer, J.M.Thomas, J.Phys.Chem., 87 (1983) 5172-5176. 13 G.A.Waychunas, J.de Physique Colloque C8, 47(12) (1986) 841-844. 14.F.Babonneau, S.Doeuff, A.Leaustic, C.Sanchez C.Cartier, and M.Verdaguer, 15 L.Brohan, A.Verbaere, M.Tourneaux and G.Demazeau, Mat.Res.Bul1 ., 16 M.G.Reichmann and A.T.Bel1, Langmuir, 3 (1987) 111-116. 6 (1983) 27-60. J.Chem.Phys., 82 (1985) 5742-5754. 40 (1988) 255-275. J.Non Crys.Solids, 74 (1985) 129-146. Inorg.Chem., 27 (1987) 3166-3172. 17 (1982) 355. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 637 CATALYSTS AND PREPARATION OF NEW TlTANATES R. G. ANTHONY' and R. G. DOSCIf 'Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122 (USA) 'Sandia National Laboratories, Div. 6211, Albuquerque, NM 87185 (U.S.A.) SUMMARY A series of new crystalline titanates ((3) are shown to have considerable potential as catalysts supports. For Pd supported catalysts, the catalytic activity for pyrene hydrogenation was substaniatially different depending on the type of CT, and one was substantially more active than Pd on hydrous titanium oxide (HTO). For 1-hexene hydrogenation the activities of the new CTs were approximately the same as for the hydrous metal oxide supports. Stereochemical effects, such as shape selective catalysis, appears to be Occurring when pyrene is hydrogenated. INTRODUCTION Hydrous titanium oxides (HTO) have been shown to be excellent supports for Co, Mo, Ni, Pd, or vanadia for hydrogenation and oxidation reactions (ref. 1, 2,3,4,5). Specifically, Dosch et al. (ref. 1, 2, 3) have used ion exchanged techniques to prepare Co-HTO, Mo/Ni-HTO, and Pd-HTO to produce Fischer-Tropsch catalysts and coal liquefaction catalysts. Gruber (ref. 5) has used HTO as a support for vanadia in the selective catalytic reduction of NO with ammonia in the presence of oxygen. A unique feature has been the high activities and surface areas which can be obtained by using the HTO based catalysts. To increase the activity of metals or metal oxides supported on hydrous titanium oxide and to increase our ability to tailordesign catalysts, the synthesis of new crystalline titanates was initiated. The basis for synthesis of the new crystalline titanates was to modify the procedures developed at Sandia for preparing the hydrous metal oxides, and to utilize techniques for synthesis of zeolites and pillaring of layered materials (ref. 6-13). After several attempts in which anatase titania was synthesized, new crystalline titanates with d-spacings of 1.0, 1.17, and 1.6 nm were synthesized. This paper reports on the chemicals used in the synthesis, propeaies of the titanates and catalytic activities of the titanates when used as supports for Pd for the hydrogenation of pyrene and of 1-hexene. 638 EXPERTMENTAL Praaration The chemicals used in the preparation of the titanates were tetraisopropyl titanate, an aqueous solution of NaOH, a solution of tetramethylammonium hydroxide in methanol, Al(NO3),*9&O, tetrapropylammonium chloride, and tetrapentyl ammonium chloride. These chemicals were mixed in an appropriate manner to produce in most cases a white precipitate. In some cases, depending on the solution composition, a crystalline titanate formed at room temperature. The solution (slurry) mixture was divided and charged to 3/4 inch Swagelok tees, which were placed in an oven with the temperature set below 180 T. Each tee was removed at a specified time and rapidly cooled to room temperature. The contents were fiitered in a Buchner funnel, washed with acetone, and air dried. Samples were ion exchanged with HCl and then Pd2+ to prepare catalysts for evaluation of the activity for hydrogenation of pyrene and 1-hexene. The new titanates were also pillared by ion exchange with an aluminum solution prepared from "microdry." The pH of the solution corresponded to the expected formation of the Al,," Keggin ion. The catalysts were characterize by using XRD, Raman, FT-IR, BET for surface areas and pore size distributions, TGA, DSC, and AA. RESULTS AND DISCUSSION Five new types of crystalline titanates were prepared (XRD patterns are shown in Figure 1 .). A room temperature titanate (Type 1) was unstable except in the mother liquor or in isopropanol, but it had a f i i t reflection at a d-spacing of approximately 1.0 nm. The XRD pattern suggests a poorly crystalline material. However, these crystals were easily seen in an optical microscope, and they had a needle type of morphology. The next two types (Types 2 & 3) had d-spacings of approximately 1.0 nm for the f i t reflection, however the remaining portions of the XRD pattems had slight differences. Type 2 contains aluminum, whereas, Type 3 contains only titanium, sodium, organic cations, and oxygen. Even though hydrothermal synthesis was used in the preparation of both types of titanates, Type 3 was prepared from the amorphous hydrous titanium oxide. The next group of titanates were classified as Types 4a and 4b because of the differences in surface areas, reactivity, and synthesis conditions. The d-spacing of the first reflection of this group was 1.17 nm and the rest of the patterns were essentially identical. The Type 5 of titanate had a d-spacing of 1.6 nm, but the material showed a mixture of anatase and the new layered titanate. 639 Figure 1. Comparison of X-Ray Diffraction Patterns of New Crystalline Titanates with Each Other and With Anatase riania 640 Surface Areas and Pore Size Distributions Surface areas and pore size distributions of selected samples (Figure 2) were determined by BET using a Micromeritics Digisorb 2600. The samples were degassed at 150 "C prior to the nitrogen sorption experiments. Typical sorption curyes for layered materials were obtained. Type 2 titanate had a pore volume of 0.79 cc/g, pore sizes up to 50 nm, and were bimodal with peaks at ca. 5nm and 10 nm. Examination of the cumulative pore volume plots (not shown) and comparing the result with the total pore volume suggested possibility of pores less than 1.5 nm. The surface area of Type 2 titames was almost twice the surface areas of the Type 2,3, and 4 titanates. The Type 4 titanates (d-spacing=1.17 nm) had surface areas of 94 to 133 mz/g and with bimodal distributions (Figure 2). The pore sizes are in the range of 1.8 to 5.0 nm with peaks at 2 and 4 nm. Pore volumes of these samples are low being 0.069 and 0.056 cc/g. We interpret these low pore volumes to indicate that the space between the layers are fiied. Elemental analysis and a carbon balance indicated a mixture of tetramethylammonium and tetrapropylammonium cations occupy the space between the layers with 25% tetrapropyl ammonium and 75% tetramethyl ammonium ions. Ion Exchange and Catalytic Activity The Type 1 titanates were equilibrated in solutions containing a two-fold or more excess of H', N i o , or V(V) ions based on the ion exchange capacity. These materials had surface areas of 377, 373, and 232 m2/g, respectively, and pore volumes of 0.52, 0.42, and 0.43 g/cc, respectively, after outgassing at 300 "C. Under similar conditions sodium hydrous titanium oxide has a surface area of 41 mz/g and pore volume of 0.14 cc/g. In addition, the pore size distributions for these exchanged samples were bimodal and trirnodal. Whereas, bdrous sodium titanium oxide (HTO) is unimodal. The Type 1 crystals were used to prepare a Ni-Mo catalyst by ion exchanging with ammonium heptamolybdate at a pH of 3, rinsed, fidtered, and dried, reslurried in deionized water and ion exchanged with Ni(NO,), at a pH of 6, rinsed with acetone, dried, and then acidified with HC1 to a pH of 3 for removal of the sodium ion. The catalyst was amorphous after calcining at 300 "C, but the catalytic activity as measured by the hydrogenation of pyrene was sisnifcantly greater than catalysts prepared from the amorphous HTO. Types 1, 2, 3, and 4 were ion exchanged with a PdCL, solution to obtain a Pd loading of approximately 1%. As prepared (Ap), the final pH of the solution was greater than 10. After fiitering and drying, the catalysts were acidified (AC) with sulfuric acid to a pH of 3.5 to 4. Pd was also loaded onto anatase, amorphous titanium oxides, N%3T, Na,,,,T, and an amorphous titanium-silicon oxide, Ng,T-Si, and catalytic activity was tested for comparison with the 641 DIFFERENTIAL PORE VOLUME PLOT (DESORPTION) PORE DIAMETER, A DIFFERENTIAL PORE VOLUME PLOT (DESORPTION) PORE DIAMETER, A PORE DIAMETER, A Figure 2. Pore Size Distributions for Types 2, 3, 3-Al-Pillared (After Calcining), and Type 4 Crystalline Titanates 642 activity obtained with the new CTs. The activities for pyrene hydrogenation is measured by zero-order rate constants k, with the units of mg pyrene hydrogenated/(sec-g Pd). For 1-hexene hydrogenation, a first order rate constant was used. 'Ihe results of these test are reported in Table 1. The CT Catalysts, except for IJT TP4a&b AC, had activities greater than the Pd-HTO catalysts, i.e. Nao,T AC and N%,T AC. Type 2 acidified (Cr TP2 AC) had an activity more than twice that of the Nao,T AC and almost twice the activity of the N%,,,T AC. The reason for the significant increase in activity of the IJT TP2 AC (Type 2) is unknown, but it could be due to the ordering introduced by crystallization of the support. It might also be due to the fact that an aluminum cation was used in the synthesis of the titanate. Table 1 Evaluation of catalvtic activity: Test reactions-Hydrogenation of pyrene and 1- hexene: Reaction conditions-100 "C. charge pressure- 100 psig @ 22°C. Sample Pd in k' (pyrene) d (1-hexene) cr. Wt.% 1Nn s) ID. CTTP1 AC? 0.74 CTTP2AC 0.78 CTTP3 AC 1.36 CTTP4aAC 0.87 CTTP4b AC 0.56 CTTP4bAp" 0.55 Anatase AC 0.83 N%,T AC 0.63 N%.3T AC 0.53 N%,TSi(Pd#3) 0.55 380 -- 610 59 348 -- 82 -- 24 34 15 46 170 _- 199 56 329 -- 577 -- 1) Units are mg of pyrene hydrogenated&econd gram of Pd). 2) AC refers to acidified after preparation. Non acidified catalysts were significantly less active than the acidifkd catalysts. fl refers to crystalline titanate. Tp1 is Type 1 titanate. 3) First order rate constant for hexene hydrogenation, l/(second gram of Pd). 4) AP refers to as prepared prior to acidification. Of particular interest is that the Pd#3, an amorphous titanium silicon oxide, had an activity within 10% of the C T "2 AC. If use of procedures similar to those used to make the new crystalline titanates resulted in the synthesis of a crystalline Na,,sTSi material, the potential exists for preparing catalysts with activities significantly greater than the HTOSi supported catalysts. Thermopravimetric and Differential Scanning Calorimetry Studies TGA and DSC experiments were conducted on the Type 4 titanates. Heating rates for the 643 TGA studies were 5 T/min and for the the DSC experiments the heating rates were 10 "/min. The first weight loss, approximately 5 to 896, occurred below 100 "C, and is probably due to loss of water. Very little weight loss occurs up to 200 "C and then a fairly rapid loss of weight occurred up to a temperature of 400 "C. The TGA in air and nitrogen differ slightly but show the same general trend. Total weight loss is approximately 18 to 20% in nitrogen and air, respectively. The DSC's were conducted at 10 "C/min in the presence of nitrogen and air. A strong exotherm occurred in air over the temperahue range of 240 to 360 with the peak at 320 "C, which was probably due to the combustion of the organic template. A second exotherm occurred with a peak at 450 C, which was probably due to a phase transformation. In nitrogen no reactions appear to have occurred except for a peak at 400 "C which is probably due to a phase transformation. Surprisingly, no endotherm occurred due to the pyrolysis of the organic template. Infrared and Raman Smctra Infrared and Raman Spectra were obtained on selected samples of these new titanates. However, additional work is required to interpret these spectra. Bridged and non bridged oxygens are evident in the Raman spectra (ref. 2, 3). The IR spectra are diffuse reflectance spectra and illustrate the incorporation of the tetraalkylammonia cation. Somewhat surprising was that some spectra did not have bands in the 3900-4500 crri' which would be typical of the quaternary ammonium. Also, the spectra substantiate the differences in the five types of new titanates. Figure 3 illustrates typical spectra obtained in these studies. PilliXhlg Types 2,3,4 and 5 titanates were ion exchanged with a solution of alumjnum ions produced by dissolution of "microdry" aluminum hydroxy chloride. The solution should have contained cations of (AlI3O, (OH), * 12 &O)'*, the aluminum Keggin ion. The exchange was conducted for 2 h and the pH controlled to 4.9. The amount of aluminum exchanged into the crystalline titanates is given in Table 2. Table 2 Extent of ion exchange Type 2 3 4 5 Wt.6 Al 2.1 16.8 4.8 17.4 XRD patterns were obtained on the resulting samples before and after calcining at 300 "C for 1 hour. The XRD patterns (Figure 4) for Types 2 and 3, show that the layered structure is I RAMAN SPECTRA OF NEW CRYSTALLINE TITANATES Figure 3. Raman and I R Spectra of New C r y s t a l l i n e Titmates 645 P-Type 2-Al-Pillared Before 0 Ca lc in ing m * N Type 2-Al-Pillared A f t e r - Two-Theta (degrees) Figure 4. X-Ray Diffraction Patterns for Aluminum Pillared Crystalline Titanate Before and After Calcining at 300°C for 1 hour. 646 retained after exchange and heating. The d-spacings after heating were 0.99 and 0.94. Hence, a slight shrinkage in the d-spacing occurred, and the loss in crystallinity may be due to pillars being in an amorphous state. Anatase was formed from Types 4 and 5 titanates, after calcining. Prior to the incorporation of the aluminum Keggin ion, the crystalline titanates became amorphous between 200 and 300 "C, and on further heating anatase was formed. Figure 2 shows the pore size distributions for Type 3 titanate as prepared and after ion exchange with the aluminum Keggin ion and then calcined for 1 hour at 300 "C. The sorption curves (not shown) for Type 3-Al-pillared titanate showed the characteristics of layered and possibly pillared materials. There is, however, a loss of surface area and pore volume, apparently due to heating at 300 T. CONCLUSIONS The results presented above clearly indicate the potential of these new titanates as catalysts and physical and catalytic supports, and the need for further work on the synthesis characterization. Acknowledgment The majority of the work reported above was conducted at the Sandia National Laboratories while Professor Anthony was on "an academic study leave from Texas A&M University" i.e. a sabbatical. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 R. G. Dosch, H. P. Stephens, and F. V. Stohl, U.S.Patent No. 4,511,455 (April 16, 1985). R. G. Dosch, H. P. Stephens, F. V. Stohl, B. C. Bunker, and C. H. F. Peden, Hydrous Metal OxideSupporkd Catalysts: Part I. A Review of Preparation Chemistry and Physical and Chemical Properties, SAND89-2399, Sandia National Laboratories, 1990. R. G. Dosch, H. P. Stephens, and F. V. Stohl, Hydrous Metal Oxide-Supported Catalysts: Part II. A Review of Catalytic Properties and Applications, SAND89-2400, Sandia National Laboratories, 1990. H. P. Stephens, R. G. Dosch and F. V. Stohl, Ind. & Engr. Chem. Prod. Res.& Dev., 24 Gruber, K. A., "The Selective Catalytic Reduction of Nitric Oxide With Ammonia in the Presence of Oxygen", M. S. Thesis, Chem. Eng. Dept., Texas A&M University, College Station, TX, (August 1989) Thesis Advisor: R. G. Anthony. A. Clearfiild, Chem. Rev., 88 (1988) 125-148. A. Clearfiild and A. Lehto, J. of Solid State Chemistry, 73 J. Lehto, Sodium Titanate for Solidification of Radioactive Wastes- Prepaiation, Structure and Ion Exchange Properties, Academic Dissertation, Report Series in Radio chemistry, (5/1987), University of Helsinki, Finland. J. Lehto and A. Clearfield, A., J. Radioanal. Nucl. Chem., Letter, 118 No.1 (1987) 1-13. J.M. Adams, Awl. Clay Sci., 2 (1987) 309-342. F. Figueras, Catal. Rev.Sci. Eng., 30(3) (1988) 457499 . T. J. Pinnavaia, Science, 220-No. 1595 (1983) 365-371. (1985) 15-19. (1988) 98-106. 13 D. E. W. Vaughan, Catalysis Today, 2 (1988) 187-198. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 647 NEW METHODS OF SYNTHESIS OF HIGHLY DISPERSED SILVER CATALYSTS N.E. Bogdanchikova and V.V. Tretyakov Institute of Catalysis, 630090 Novosibirsk (USSR) ABSTRACT Original methods of synthesis of highly dispersed silver catalysts (based upon the application of strong reducing properties of electrons solvated in soldium-ammonia solutions, the adsorption- contact method of drying and a weak solubility in nitric acid of the Si02-supported small silver clusters) allowed us to synthesize Si02-supported silver particles of sizes less than 6 nm. It makes possible some unusual catalytic and other physico-chemical properties of these particles to be discovered. INTRODUCTION It is well known that metal particles less than 6 nm in size may essentially differ from the bulk metal in adsorption, catalytic and other physico-chemical properties; structural sensitivity of catalytic reactions are exhibited in this range of particle sizes. For the investigation of size effects in catalysis, the most suitable support for metal particles is Si@, since its interaction with them is negligible as compared with other supports. It is widely believed that regardless of the preparation method, the most probable minimum size of Ag particles supported on Si02 is 6 nm, which is due to a weak interaction of Ag with the support [l]. In fact, one cannot find in literature any description of Ag samples supported on pure Si02 with reliably established size of Ag particles less than 6 nm. In this work we have succeeded in synthesizing such supported silver samples using the following new methods. METHODS Ag dispersion was determined by various methods : size of coherent scattering region (average size d,) was obtained by X-ray method; surface average size ds - by adsorption method; size distribution of Af particles was obtained by the method of small angle X-ray scattering (SAXS) and TEM. The two last methods were applied to determine the most probable size of Ag particles d,. The specific surface S,, of silver blacks was determined by the BET method through the adsorption of N2. The specific surface of supported Ag catalysts was calculated from the data on chemisorption of 0 2 . Size distribution of Ag particles was obtained by transmission electron microscope JEM-100 CX and the SAXS method (KRM-1 apparatus). An average value of the regions of coherent X-ray scattering was determined through the widening of X-ray lines registered with a DRON type apparatus. Spectra of diffused reflection were recorded by the Shimadzu UV-300 spectrometer. 648 RESULTS AND DISCUSSION Pure Si02 (the moulded aerosil of the "A-175'' brand, specific surface - 235 m2/g, pore volume - 1 cm3/g, dominant radius of pores > 4 nm) was chosen as support. Method 1 For the preparation of the silver samples, hydrogen, borohydride, formaldehyde, hydrazine are usually used as reducers. One of the conditions preventing silver crystallites from growing, is a high rate of reduction of Ag+ cations. As a rule, the higher the rate of reduction, the higher the value of the redox potential of the reducer. Therefore it was of interest for the preparation of highly dispersed Ag particles to use one of the strongest reducers - solutions of alkaline and alkaline earth metals in liquid ammonia, where the electron solvation was performed by ammonia [2] Na + (m + n) NH3 -+ Na+ (NH3)m + e (NH3),,. To prepare the silver catalysts, we used the strong reducing properties of the electrons solvated in ammonia [3] We flowed up the AgN03 solution in liquid ammonia to that of metal sodium (at 239.5 K). Darkening of the solution was therewith observed as a result of formation of the highly dispersed metallic silver. The obained compound was divided into two parts, one of which was thoroughly mixed with Si02. Both portions were kept in the air up to the complete evaporation of ammonia. The next step was washing of the obtained samples from the sodium cations with distilled water : the sample supported on SiOz was washed on buchner funnel, and the non-supported silver was washed by centrifugation. Resulting from centrifugation, silver blacks settled at the bottom, while - 4% of the whole silver was still in solution in the form of colloidal particles (Table 1, sample 1). Silver black (sample 2) and supported silver (sample 4) were dried in the air at room temperature. Silver black was also prepared by application of some other succession of the solutions being flowed to : the sodium solution in liquid ammonia was flowed to the solution of AgN03 in liquid ammonia. Flakes of silver black, settling at the bottom of a glass, were therewith formed. Formation of the colloidal particles was not observed in this case. Washing out the sodium ions was performed by decantation. Afterwards, the sample was dried in air at room temperature (sample 3). The second supported catalyst was prepared in conditions favouring the formation of ultrafine Ag particles. For this, the aerosil was suspended in the AgN03 solution in liquid ammonia. This mixture was flowed to the sodium solution in liquid ammonia. Inasmuch as in the water solutions there is adsorption of the ammonia complex of silver nitrate on Si@ [4], it might have been expected that in the liquid ammonia there would also be an adsorption. This is the circumstance hindering growth of particles on the support surface. Sample 5 obtained by such a way, was washed by decantation and dried in air at room temperature. Next, two cycles of treatment for the sample 5 were performed with 02 and H2 at 473 K, pressure of gases < 800 Pa, the duration of the treatment being - 5 days (sample 6). While preparing the samples, the initial concentration of the AgN03 and Ag+ (NH3)m + e (NH3)n -+ Ago (NH3)m+n. 649 TABLE 1 : Characteristics of catalysts* A g .Average s i z e o f A g c r y s t a l l i t e s ( n m ) Sample ( w t . % l a b s o r b . S - r a v TEY d a t a s.41s** d c d,. s i z e ranse d _ d n d a t a d a t a d a t a 1 Colloid s i l v e r - 2 S i l l - e r b l a c k 9 9 . 8 3 S i l v e r b l a c k 9 9 . 8 4 A:/SiO? 1 3 . 0 5 Ag/SiO; i . 4 s o l u t i n n 6 A s / S i @ ; 1 . 4 7 B g / S i O , 2 . 4 8 .Ag/SiOi; 2 . 0 9 A g / S i O ; 2 . 1 - - - 2-50 7 5 . 2 50 - 2 4 8 . 4 2 0 - 4 4 . 0 1 5 2 - 2 0 - a m . 0 . 5 - 6 3 . 7 1 4 1 - 6 0 3 . 5 a m . 1 -6 4 . 0 a m . 1 -6 - 9 . 0 1 - 1 0 1 2 - - 3 . 3 3 0 . 8 1 1 . 0 7 3 . 0 2 . 0 5 . 0 - - - - - - * a m . - a m o r p h o u s ; d , d , d - s u r f a c e a i â e r a q e s i z e , vo lume a n d t h e m o s t p r o b g b l e v s i z e P o f Aq p a r t i c l e s c o r r e s p o n d i n g l r . * * P a r t i c l e s i z e s w e r e a l e r a g e d i n t h e r a n g e 0 . 5 - 3 0 nm. one Fig. 1 . TEM photographs of silver samples : a - 1 (Ag colloid), b - 8 (AdSi02). sodium in liquid ammonia were in the range from 0.01 to 0.90 M. The characteristics of the catalysts calculated from the data of the different physico-chemical methods are given in Table 1. X-ray data conform to the adsorption data for sample 2 (particle sizes 650 are 50 and 75 nm, respectively) and represent for sample 3, the average size one order less, which is obviously stipulated by the fact that silver particles (- 250 nm in size) of sample 3 are agglomerates of microcrystals of a lesser size (- 20 nm). Grinding this sample in a mortar, unlike sample 2, led to the increase of S,, by the factor of 1.5, while the size obtained by the X-ray method was the same. These microcrystals in the agglomerates are possibly less stably connected and for this reason, the agglomerates may be destroyed upon grinding. The microphotograph of silver colloid made approximately a month after it was prepared is given in Fig. la. The size distributions of the colloid silver particles (sample 1) and of the freshly prepared supported silver particles (sample 4) are different. First, smaller particles are formed (dp = 3 nm), which are "conserved" on a support; the colloid particles which are enough stable in time, are coarser (dp = 12 nm). The projection of colloid particles on a surface is chiefly hexagonal. The most possible size of the colloid silver particles has not practically changed after two years of ageing. The role of a stabilizer in this colloidal solution may be possibly performed by Na+. The TEM patterns of the freshly prepared specimen point to particles I 1 nm in size to be present, which is conf i ied by the SAXS method. The second examination of the sample kept in air did not allow to determine these particles on a support, which may be connected with a decrease of contrast of representation due to oxidation of silver particles in air. This is proved by the data of diffuse reflectance electron spectroscopy. The data of Table 1 indicate that the synthesis of Ag samples from silver cation reduction with electrons solvated in liquid ammonia allows to obtain stable colloidal solution of highly dispersed silver particles of 2-50 nm in size (dp = 3 nm), silver blacks with specific surface of 2.3 and 7.6 m2/g, and supported silver samples with a great contribution of particles < 6 nm in size. Method 2 In this case, we used the traditional method of impregnation, carried out in conditions leading to the formation of highly dispersed Ag particles on the support surface : (1) samples were prepared with a low content of Ag (-2 wt.%); (2) Ag was supported by adsorption on Si02 surface of the ammonia complex of the diluted silver nitrate solutions. In this case, the formation of the supported particles at the later stages of the sample preparation was mainly performed from the adsorbed silver complex. Conhibution of this complex being in volume of support pores was practically excluded. ( 3 ) samples with supported silver complex were dried by the method of sublimation or by the adsorption-contact method which preserved the uniformity of adsorbed silver complex distribution on the support surface. This contributed to the obtention of a more homogeneous distribution of metal particles after subsequent reduction. The application of the adsorption-contact drying method for the preparation of the supported metal catalysts has not been found in literature. For the drying by sublimation, a wet sample was introduced in an ampoule and immediately frozen in liquid nitrogen. Evacuation under vaccum was carried out by keeping the sample temperature lower than 268 K (sample 7). Sample 8 was dried by the adsorption-contact method developed in the Institute of Catalysis (Novosibirsk, USSR). This method is based on the contact of dehydrated desiccant with the grains of the catalyst impregnated with the solution of the active 651 component. For the security of transfer of the solvent (water) through the gas phase, a definite amount of desiccant was taken. It was calculated supposing a monolayer coverage of adsorbed water on its surface. y-Al2O3 (Ssp - 255 m2/g, dominant pore radius < 4 nm) was used as a desiccant. For the calculation it was assumed that 8 Fmol/m2 or more of water was needed to form a monolayer. The mixture of the desiccant and the wet sample was thoroughly shaken in a flask for a few minutes. The sample dried in this way was separated from the wet desiccant by means of a sieve. As we used the dilute solution of the ammonia complex, only a small part of it (I 3 wt. % of the complex kept by the support in an adsorbed state) was transferred to the desiccant. Sample 9 was dried in a cabinet drier for 6 h at 385 K. Drying the samples in a cabinet drier, in contrast with the adsorption-contact method and that of sublimation, is probably favourable to the formation of larger aggregates from the complex salt molecules adsorbed on support surface. It is accompanied by the partial decomposition of the salt into silver oxide or even (according to X-ray data), into metallic Ag. This is indicated by electron spectra of diffused reflection of the samples dried by these methods. The dried samples were reduced by H2 in a flowing-circulating installation for 6 h at 473 K with freezing of water in getters cooled by liquid nitrogen. From the data given in Table 1 and Fig. 1 b, the application of the adsorption-contact drying and the method of solvent sublimation allow to obtain the sample where the particle size of Ag is dominantly 2-3 nm. A narrower particle size distribution was therefrom reached, as compared with the analogous samples of silver catalysts prepared according to the usual method of drying (sample 9). TEM data indicate that increasing the Ag content up to 10 wt. % leads to the increase of the part of coarser metal particles (more than 10 nm in size), but the change of the position of the maximum of the silver particle distribution is negligible. The use of the adsorption-contact drying and the sublimation method leads to the obtention of Ag particles on a support with a practically equal particle size distribution. However, the adsorption- contact drying has some advantages, as it is less time-consuming (usually a few minutes) than drying by sublimation (pumping out lasts for about 20 h) and does not require special equipment (vacuum system). Method 3 This method is based on the isolation of "pure" small Ag clusters which form a rather strong bond with Si02. It was found that these clusters were obtained by heating in a water bath the Si02- supported samples with the most probable Ag particle size of 3 nm (samples 7 and 9 obtained by Method 2). The heating was done in a 13% HNO3 solution during 12 h. Further, the samples were washed from HNO3 with distilled water and dried in air under an IR lamp for about 30 minutes. The analogous acid treatment of Si02-supported samples containing large Ag crystallites, leads to complete removal of Ag from the support surface. Inasmuch as Ag content in the samples treated with acid was low (less than 0.1 wt.%), the application of the usual methods for studying the structure of supported metals (X-ray, TEM, etc.. .) 652 was impeded. Therefore, for the investigation of the properties of these clusters, the method of diffuse reflectance electron spectroscopy was used and found to be highly sensitive. Silver supported samples 7 and 9, not treated with HNO3 solutions, were dark brown, which was observed as structureless absorption in the whole region of the spectrum. The influence of HNO3 solution resulted in a decrease of the intensity of the sample colouring, and two absorption bands (a.b.) could be observed in the spectrum : at 320 and 400-440 nm. As the time of treatment was increased, the relative intensity of 320 nm a.b. was also increased. Increasing the treatment time up to 13 h led to the disappearance of the a.b. at 400 nm, and only the a.b. at 320 nm was observed. The absorption band at 400 nm is attributed to the surface plasma resonance of conduction electrons in the small metal Ag particles. The change of such a resonance with decreasing size of Ag particles in a photo-sensitive glass obtained in [5] was analogous to that observed in our work : increasing the treatment time of the sample with acid led to an increase of the relative intensity of the 320 nm a.b. and a decrease of the 400 nm a.b.. For Ag particles of 2.3 nm, only the 400 nm a.b. was recorded [5], but for 1 nm particles, two a.b. at 320 and 400 nm were observed. This allows to suppose that in our case the a.b. of 320 nm refers to small Ag clusters, the size of which is not more than 1 nm. Recent investigations of highly dispersed silver supported on aluminosilicate catalysts made it possible to assign the 320 nm a.b. observed to Ag clusters - 1 nm in size. It was found that the samples obtained by Method 1 containing Ag particles of about 3 nm in size, exhibited noticeable activity towards ethylene chemisorption and homoexchange of ethylene, different from pure bulk Ag catalysts. At the same time they were less active than large Ag crystals towards processes occurring with participation of oxygen (02 adsorption, interaction of adsorbed oxygen with H2, C02 adsorption on oxidized Ag surface, complete and selective catalytic oxidation of ethylene) [6]. X-ray photoelectron and Auger electron spectroscopy data suggested that the observed size effect was due to changes in electronic properties of metal silver [7]. The Ag samples obtained by Method 1 and Method 3 containing Ag particles of 1 nm in size, possessed unusual optical properties and abnormal features with respect to redox reactions [8]. Thus, using the original methods, Si@-supported silver particles of less than 6 nm in size were synthesized. This made it possible to discover unusual catalytic and some other physico- chemical properties of these particles. REFERENCES 1 2 3 4 5 6 7 8 K.P. Jong and J.W. Geus, Appl. Catal., 4 (1) (1982) 41-51. G. Thompson, Electrons in Liquid Ammonia, Mir, Moscow, 1979. G.W. Watt, J. Chem. Education, 34 (11) (1957) 538-541. M. Jarjoui, B. Maraweck, P.C. Gravelle and S.J. Teichner, J. Chim. Phys., 75 (11-12) (1978) L. Genzel, T.P. Martin and U. Kreibig, Z. Physik 8 , 21 (4) (1975) 339-346. N.E. Bogdanchikova, D.A. Bulushev, Yu.D. Pankratyev, E.A. Paukshtis and A.V. Khasin, Kinet. Katal., 31 (1) (1990) 151-157. N.E. Bogdanchikova, A.I. Boronin, V.I. Buktiarov, V.I. Zaikovskii, S.V. Bogdanov and A.V. Khasin, Kinet. Katal., 31 (1) (1990) 145-150. N.E. Bogdanchikova, M.N. Dulin, A.A. Davydov and V.F. Anufrienko, React. Kinet. Catal. 1060- 1068. Lett., 41 (1) (1990) 73-78. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 653 PREPARATION OF HIGH-SURFACE-AREA V-Si-P O X I D E CATALYSTS M. A i Research l a b o r a t o r y o f Resources U t i l i z a t i o n , Tokyo I n s t i t u t e o f Technology, 4259 Nagatsuta. Midor i -ku, Yokohama 227 (Japan) ABSTRACT ox ides on t h e i r c a t a l y t i c performance i n t h e vapor-phase a l d o l condensat ion o f p r o p i o n i c a c i d w i t h formaldehyde t o fo rm m e t h a c r y l i c a c i d were s tud ied . The presence o f bo th vanadyl pyrophosphate and l a r g e su r face area was found t o be requ i red t o achieve a good c a t a l y t i c performance. Phosphorus serves t o form and s t a b i l i z e vanadyl pyrophosphate which i s be l i eved t o be a c t i v e si tes and s i l i c o n serves t o produce a l a r g e su r face area and t o mod i f y t h e vanadyl pyo- phosphate. The presence o f l a c t i c a c i d i s i nd i spensab le t o produce a l a r g e su r face area when t h e S i / V atomic r a t i o i s i n t h e range o f 1 t o 4. The e f f e c t s o f t h e compos i t ion and t h e methods o f p repar ing V-S i -P t e r n a r y INTRODUCTION V-P b i n a r y ox ide c o n s i s t i n g o f vanadyl pyrophosphate, (VO)2P207, i s a un ique c a t a l y s t possessing an e x c e l l e n t s e l e c t i v i t y i n o x i d a t i o n o f butene and n-bu- tane t o ma le i c anhydride. Fur ther , t h i s ox ide i s known t o be e f f e c t i v e a l s o as a c a t a l y s t f o r a vapor-phase a l d o l condensat ion o f a c e t i c a c i d and p r o p i o n i c a c i d w i t h formaldehyde (HCHO) t o form a c r y l i c a c i d and m e t h a c r y l i c acid, respec t i v e l y [ l -31. CH3COOH + HCHO - CH2=CHCOOH + H20 CH3CH2COOH + HCHO - CH2=C(CH3)COOH + H20 I t was found t h a t t h e combina t ion of t i t a n i u m phosphate, which has a smal l excess o f phosphorus w i t h respec t t o s t o i c h i o m e t r i c t i t a n i u m pyrophosphate, TiP207, w i t h (VO)2P207 b r i n g s about an enhanced c a t a l y t i c performance i n t h e r e a c t i o n o f a c e t i c a c i d and r e l a t e d compounds w i t h HCHO [4-71. However, t h e combinat ion o f t i t a n i u m phosphate does n o t improve t h e performance i n t h e r e a c t i o n o f p r o p i o n i c a c i d and r e l a t e d compound w i t h HCHO 14.81. More r e c e n t l y , i t has a l s o found t h a t V-Si-P t e r n a r y ox ides e x h i b i t t h e most p romis ing c a t a l y t i c performance i n t h e r e a c t i o n o f p r o p i o n i c a c i d and r e l a t e d compounds w i t h HCHO [8,9]. The o b j e c t of t h e p resen t s tudy i s t o o b t a i n high-surface-area V-Si-P ox ide c a t a l y s t s w i t h a h i g h s e l e c t i v i t y i n t h e fo rma t ion o f m e t h a c r y l i c a c i d and methacry la tes . t h e a c t i v i t y and s e l e c t i v i t y and t h e e f f e c t o f l a c t i c a c i d used i n p repar ing The s t r e s s i s p laced on t h e e f f e c t o f t h e V-Si -P compos i t ion on 654 c a t a l y s t s , s i n c e i t has been r e p o r t e d t h a t homogeneous m i x t u r e o f metal ox ides (amorphous) w i t h a l a r g e su r face area can be ob ta ined by us ing a hydroxy carbo- x y l i c a c i d as a complex-making agent [ l O , l l ] . EXPERIMENTAL C a t a l y s t s As t h e sources o f vanadium, s i l i c o n , and phosphorus, NH4V03, c o l l o i d a l s i l i c a "Snowtex 0" (Nissan Chem. Ind.) c o n t a i n i n g 20% Si02. and 85% H3P04 were used. Unless i n d i c a t e d o therw ise , NH4V03 (20 t o 60 g ) was d i sso l ved i n a h o t 2+ water c o n t a i n i n g about 20 m l o f l a c t i c ac id , y i e l d i n g a b l u e s o l u t i o n o f VO . It was then mixed w i t h t h e r e q u i r e d amounts o f 85%H3P04 and t h e c o l l o i d a l s i l i c a . Excess water was evaporated w i t h s t i r r i n g i n a h o t a i r cu r ren t . The ob ta ined cake was d r i e d i n an oven g r a d u a l l y h e a t i n g f rom 50 t o 200°C f o r 6 h. The r e s u l t i n g s o l i d was ground and s ieved t o g e t a 8- t o 20-mesh s i z e p o r t i o n . It was c a l c i n e d f i n a l l y a t 450°C f o r 6 h i n a stream o f a i r . Procedures f o r t h e a l d o l condensat ion The r e a c t i o n o f p r o p i o n i c a c i d and HCHO was c a r r i e d o u t w i t h a cont inuous- f l o w system. t e d v e r t i c a l l y and immersed i n a l e a d bath. N i t rogen was f e d i n f rom t h e t o p o f t h e r e a c t o r a t a f i x e d r a t e o f 140 ml/min ( a t 20°C) as t h e c a r r i e r o r t h e d i l u e n t , and a m i x t u r e o f t r i o x a n e [(HCH0)3] and p r o p i o n i c a c i d was in t roduced i n t o t h e p rehea t ing s e c t i o n o f t h e r e a c t o r by means o f an i n j e c t i o n s y r i n g e pump. The feed r a t e s o f p r o p i o n i c acid, HCHO, and n i t r o g e n were 33.6. 16.8, and 350 mmol/h, r e s p e c t i v e l y . descr ibed p r e v i o u s l y [3.4.9]. (moles o f m e t h a c r y l i c ac id ) / (mo les o f HCHO fed). The r e a c t o r was made o f a s t e e l tube (50 cm X 1.8 cm I .D.) moun- The o t h e r procedures were t h e same as those The y i e l d (mol-%) was d e f i n e d as 100 t imes C h a r a c t e r i z a t i o n o f c a t a l y s t s The su r face areas o f t h e c a t a l y s t s were measured by t h e BET method us ing The average o x i d a t i o n numbers o f vanadium n i t r o g e n as adsorbate a t -196°C. i ons i n t h e c a t a l y s t s were determined by t h e redox t i t r a t i o n method desr ibed p r e v i o u s l y [12-141. RESULTS AND DISCUSSION E f f e c t o f t h e V-Si-P compos i t ion The e f f e c t s o f t h e compos i t ion o f t h e V-Si-P t e r n a r y ox ides was s t u d i e d by changing bo th t h e s i l i c o n and phosphorus contents; V/Si/P atomic r a t i o = l / x / y , where x and y were changed. p r o p i o n i c a c i d w i t h HCHO was conducted over 20 g p o r t i o n s of seven s e r i e s o f The vapor-phase a l d o l condensat ion o f 655 b Catalyst V-Si-P = I - X - Y 1 2 3 4 5 Y uv 1 2 3 4 5 Y Fig . 1. maximum y i e l d o f m e t h a c r y l i c ac id . E f f e c t o f t h e compos i t ion o f t h e V-Si-P t e r n a r y ox ide c a t a l y s t s on t h e c a t a l y s t s a t temperatures f rom 270 t o 330°C. inc reased as t h e temperature was ra i sed , passed th rough a broad maximum, and then decreased, The maximum y i e l d s a r e shown i n F ig . 1 as a f u n c t i o n o f t h e phosphorus conten t , y. There e x i s t s an op t ima l con ten t o f phosphorus which inc reases as t h e s i l i c o n con ten t increases; t h e h i g h e s t y i e l d s a r e ob ta ined w i t h t h e V / S i / P atomic r a t i o o f 1 / x / [2 + (0.1-0.2)x], f o r example, V / S i / P = 1/1/2.1. 1/2/2.7, 1/4/2.4, 1/8/2.8, 1/16/3.3, 1/32/3.8, and 1/50/4.5 oxides. Poss ib ly , a p a r t of phosphorus i s i n t e r a c t e d w i t h s i l i c a , as a r e s u l t s , a smal l excess o f phosphorus w i t h respec t t o s t o i c h i o m e t r i c (VO)2P207 i s r e q u i r e d t o s t a b i l i z e t h e (VO)2P207 species. The one-pass y i e l d o f m e t h a c r y l i c a c i d reached 55 mol-% on HCHO b a s i s a t t h e p r o p i o n i c acid/HCHO molar r a t i o o f 2 The s p e c i f i c su r face areas o f t h e seven s e r i e s o f c a t a l y s t s were measured The y i e l d o f m e t h a c r y l i c a c i d by t h e BET method. The r e s u l t s a re shown i n Fig. 2. The su r face area decreases as t h e phosphorus con ten t increases, w h i l e i t increases markedly as t h e s i l i c o n con ten t increases. F igu re 3 shows t h e average o x i d a t i o n numbers o f vanadium i o n s i n t h e V-Si-P ox ide c a t a l y s t s . increased. It should be no ted t h a t a good performance i n t h e a l d o l condensa- t i o n i s achieved w i t h t h e c a t a l y s t i n which t h e o x i d a t i o n number o f vanadium ions i s around 4.0, rega rd less o f t h e con ten t o f s i l i c o n . These f i n d i n g s suggest t h a t t h e a c t i v e s i t e s i s asc r ibed t o (VO)2P2O7 s i m i l a r t o t h e case of The o x i d a t i o n number decreased as t h e phosphorus con ten t 656 160 140 - UI â 1 2 0 E v 100 80 60 ITJ Catalyst V- Si- P = 1 - X - Y X = l 6 â., Y Fig . 2. E f f e c t o f t h e compos i t ion o f t h e V-Si-P ox ides on t h e su r face area. o x i d a t i o n o f n-butane t o ma le i c anhydr ide [15,16], and t h a t t h e presence o f an excess o f phosphorus i s r e q u i r e d t o s t a b i l i z e t h e (VO)2P207 species. E f f e c t o f t h e l a c t i c a c i d used i n p repar inq t h e c a t a l y s t s The e f f e c t s o f t h e methods o f p repar ing V-S i -P t e r n a r y ox ide c a t a l y s t s were s tud ied : t h e V/Si/P compos i t ions were chosen so as t o g e t a good c a t a l y t i c performance bas ing on t h e r e s u l t s ob ta ined i n t h e preced ing s e c t i o n (Fig. 1). The c a t a l y s t s were prepared i n t h e presence o f l a c t i c Se r ies A c a t a l y s t s : a c i d and t h e procedures were desc r ibed i n t h e Exper imental sec t i on . Se r ies B c a t a l y s t s : The c a t a l y s t s were prepared i n t h e presence o f o x a l i c ac id . c i e n t t o d i s s o l v e t h e NH4V03, y i e l d i n g a s o l u t i o n o f VO dures were t h e same as those f o r t h e Ser ies A c a t a l y s t s . NH4V03 was added t o a h o t water c o n t a i n i n g o x a l i c a c i d i n amounts s u f f i - 2+ . The o t h e r proce- Se r ies C c a t a l y s t s : The c a t a l y s t s were prepared i n t h e presence o f e thy lene g l y c o l . The o t h e r procedures were t h e same as those f o r t h e Ser ies B c a t a l y s t s . NH4V03 was d i s s o l v e d i n a h o t water c o n t a i n i n g e thy lene g l y c o l [5-71. Ser ies N c a t a l y s t s : The c a t a l y s t s were prepared i n t h e absence o f an 657 4.81 catalyst: v - s i - P = I - X - Y 4.6 > *- 4.4 t 0 4.2 I C C 0 3.8 3 . 4 ' ' I I I I I I I I L 1 2 3 4 5 Y Fig . 3. t i o n numbers o f vanadium ions . E f f e c t o f t h e compos i t ion o f t h e V-Si-P ox ides on t h e average ox ida- o rgan ic so lvent . NH VO was d i sso l ved i n a warm water c o n t a i n i n g t h e requ i red amount of H3P04. procedures were t h e same as those f o r t h e Ser ies A c a t a l y s t s . 4 3 Then, i t was mixed w i t h t h e c o l l o i d a l s i l i c a . The o t h e r Se r ies S c a t a l y s t s : The c a t a l y s t s were prepared i n a non-aqueous medium; i n i s o b u t y l a lcoho l -benzy l a l coho l medium, accord ing t o t h e method o f Katsumoto and Marquis [15] . As an index of t h e a c t i v i t y f o r t h e a l d o l condensat ion, t h e y i e l d s (mol-%) o f m e t h a c r y l i c a c i d ob ta ined under t h e c o n d i t i o n s descr ibed i n t h e Exper imental s e c t i o n were measured f o r f o u r d i f f e r e n t amounts o f each c a t a l y s t . The su r face areas and t h e average o x i d a t i o n numbers o f vanadium ions i n t h e f r e s h c a t a l y s t s were a l s o determined. The r e s u l t s a re g i ven i n Table 1. The r e s u l t s may be summarized as fo l l ows . (1) V-P b i n a r y ox ides w i t h o u t s i l i c a : The su r face areas o f t h e ox ides pre- 2 pared i n an aqueous medium a r e i n t h e range o f 2 t o 4 m /g, w h i l e those pre- pared i n an i s o b u t y l a lcoho l -benzy l a l coho l reach o f l a c t i c a c i d and e thy lene g l y c o l i n an aqueous medium does n o t serve f o r i nc reas ing t h e su r face area. 20 t o 30 m2/g. The a d d i t i o n (2) 1 ,< S i / V < 4 oxides: (3) S i / V > 8 oxides: The e f f e c t o f l a c t i c a c i d added i n an aqueous medium i s very c l e a r , w h i l e t h a t o f e thy lene g l y c o l i s smal l . The su r face areas o f t h e ox ides a r e h i g h rega rd less o f t h e a d d i t i o n of l a c t i c a c i d i n an aqueous medium. (4 ) The su r face areas o f t h e ox ides i nc rease as t h e con ten t o f s i l i c o n i n - creases. 658 TABLE 1 Comparison o f t h e V-Si-P t e r n a r y ox ide c a t a l y s t s prepared by d i f f e r e n t methods C a t a l y s t Sur face O x i d a t i o n Y i e l d (mol-%) o f MAA" r a t i o (m2/g> v i o n s 20 5 1.2 0.6 V/Si/P Method area number o f Amount o f c a t a l y s t used (9) A 2.5 4.1 10.4 B 3.0 4.4 10.0 l / O l l . l c 2.2 4.1 8.2 N 3.9 4.4 16.4 S 23.0 4.0 40.8 33.8 18.2 11.4 A 10.4 4.1 52.0 40.2 20.0 13.2 2.4 3.9 10.6 11112.1 A 34.0 3.9 53.2 42.0 21.3 12.0 B 2.3 3.9 20.6 N 5.1 4.0 30.1 17.4 A 36.0 3.9 54.7 40.8 19.3 14.4 8.6 3.1 11.7 9.5 11212.2 c 2.2 3.8 10.0 1/4/2.4 A 51.4 4.0 52.2 44.2 21.3 14.8 11812.8 B 40.0 3.8 51.2 41.4 20.2 15.4 N 19.8 4.0 21.4 21.0 A 86. 3.9 53.5 50.5 27.1 19.6 73. 3.9 52.2 49.5 30.5 20.0 111613.2 A 132. 4.1 53.5 48.8 30.0 20.2 116. 4.2 52.2 47.2 32.2 23.0 113213.6 * One-pass y i e l d o f m e t h a c r y l i c a c i d on HCHO b a s i s a t t h e p r o p i o n i c acid/HCHO molar r a t i o o f 2 and 320°C. (5 ) However, it l e v e l s o f f a t S i / V atomic r a t i o = 16, suggest ing t h a t t h e su r face area measured does n o t rep resen t t h e amount o f a c t i v e s i t e s when t h e s i l i c o n con ten t i s h igh ; S i / V 3 16. C a t a l y t i c a c t i v i t y inc reases as t h e su r face area o f c a t a l y s t increases. ( 6 ) The average o x i d a t i o n numbers o f vanadium i o n s i n t h e f r e s h c a t a l y s t s a r e about 4.0, rega rd less o f t h e d i f f e r e n c e i n t h e method o f p repar ing c a t a l y s t . ( 7 ) The maximum y i e l d s o f m e t h a c r y l i c acid, t h a t i s . t h e y i e l d s w i t h 20 g p o r t i o n s o f c a t a l y s t s , a r e cons tan t a t about 53 t o 54 mol-%, over t h e V-Si-P t e r n a r y ox ide c a t a l y s t s w i t h a l a r g e su r face area. D iscuss ion As i s seen i n Figs. 1 - 3, t h e maximum y i e l d s of m e t h a c r y l i c a c i d ob ta ined w i t h t h e V-Si-P t e r n a r y ox ide c a t a l y s t s a r e t h e same l e v e l : 52 t o 55 mol-%, and t h e maximum y i e l d s a r e ob ta ined w i t h t h e c a t a l y s t s possessing t h e average o x i - d a t i o n numbers o f vanadium ions o f about 4.0. suggest ing t h a t t h e a c t i v e s i t e s 659 a r e r e l a t e d t o (VO)2P207 species, V-Si-P t e r n a r y ox ides a r e c l e a r l y h i g h e r than those ob ta ined w i t h t h e V-P b i n a r y ox ides c o n s i s t i n g o f (VO)2P207- (VO)2P207 mod i f i ed by s i l i c o n phosphate i s more s u i t a b l e than pure (VO) P 0 2 2 7 as a c t i v e s i t e s f o r t h e r e a c t i o n o f p r o p i o n i c a c i d w i t h HCHO. a c i d and base p r o p e r t i e s were checked i n d i r e c t l y by t h e c a t a l y t i c a c t i v i t i e s f o r dehydra t i on o f 2-propanol and dehydrogenat ion of acetaldehyde, r e s p e c t i v e l y However, a c l e a r d i f f e r e n c e i n t h e c a t a l y t i c a c t i v i t y f o r these r e a c t i o n s was n o t observed between t h e V-Si-P and V-P ox ides . governed by a more s u b t l e d i s t i n c t i o n i n t h e acid-base p r o p e r t i e s . As has a l ready been known, i t i s hard t o p repare V-P b i n a r y ox ides w i t h a However, t h e y i e l d s ob ta ined w i t h t h e Th is leads us t o cons ide r t h a t t h e Therefore, t h e Poss ib ly , t h e y i e l d may be h i g h surface area i n an aqueous medium, even i n t h e presence o f an o rgan ic compound such as l a c t i c a c i d and o x a l i c ac id . On t h e o t h e r hand, i n t h e case of V-Si-P t e r n a r y oxides, i t i s p o s s i b l e t o produce a l a r g e su r face area i n an aqueous medium. presence o f l a c t i c a c i d i s ind ispensab le . n o t e f fec t i ve . Poss ib ly , t h e presence o f c a r b o x y l i c group i s r e q u i r e d as a complex-making agent [10.11]. w h i l e o x a l i c a c i d i s uns tab le and i s decomposed d u r i n g t h e d r y i n g of c a t a l y s t p recursors . Si/Va 8, ox ides w i t h a h i g h surface area can be ob ta ined w i t h o u t us ing l a c t i c ac id . Vanadium phosphate and s i l i c o n phosphate can i n t e r a c t s u f f i c i e n t l y w i t h each o t h e r even i n t h e absence o f a complex-making agent. When t h e con ten t o f s i l i c o n i s n o t high: 1 4 S i / V ,C4, t h e O x a l i c a c i d and e thy lene g l y c o l a r e When t h e con ten t o f s i l i c o n i s high, I n conc lus ion , we would l i k e t o cons ide r t h a t phosphorus serves t o fo rm and s t a b i l i z e (VO)2P207 species, and t h a t s i l i c o n serves t o mod i f y t h e (VO)2P207 and a l s o t o enhance l a r g e l y t h e su r face area. a complex-making agent i s i nd i spensab le t o g e t a l a r g e su r face area when t h e con ten t o f s i l i c o n i s n o t high. The presence o f l a c t i c a c i d as REFERENCES 1 R.A. Schneider (Chevron Res. Co.). U.S. Pa ten t 4 165 438, 1979. 2 M. A i , J. Catal . . 107 (1987) 201-208. 3 M. A i . Appl. Catal., 36 (1987) 221-230. 4 M. A i . i n : M.J. P h i l l i p s and M. 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Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 661 PREPARATION OF FINE PARTICLES OF RUTHENIUM-ALUMINA COMPOSITE BY MIST REDUCTION METHOD H. IMAI and J. SEKIGUCHI Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Midori-ku, Yokohama, 227 (Japan) SUMMARY Fine particles of ruthenium-alumina composite (Ru = 2.3 wt8) were prepared by reduction of a mist of a mixed solution sf ruthenium chloride and aluminum nitrate. A mist of the mixed solution was treated in a hydrogen stream through three furnaces, successively. Temperatures of the furnaces were adjusted for evaporation of water, hydrogenolysis of the mixed metal salts, and reduction of the particles, respectively. Amorphous ruthenium clusters dispersed in an amorphous alumina particle were prepared by this method. The particles were porous and spherical with a narrow particle size distribution (average diameter=0.64 am). The diameter of the ruthenium clusters was less than 2 nm. The IR band of linearly adsorbed carbon monoxide shifted to high frequency side compared to that on an impregnated catalyst. The catalyst obtained by the direct synthesis showed much higher activity for benzene hydrogenation than that prepared by reduction of the mixed oxide. INTRODUCTION Supported metal catalysts have been prepared usually by the impregnation method. More finely dispersed metal catalysts were reported to be prepared by superficial reduction of dilute mixed metal oxide solid solutions (ref. 1). The dispersion of metal may be improved by direct synthesis of metal-oxide composite, because the chance of the aggregation of metal atoms is diminished in the direct synthesis. Moreover, clusters with a different structure may be obtained by the direct synthesis because the conditions of cluster formation are different. In this paper, we report a direct preparation method of ruthenium-alumina composite by reduction of the mist of a mixed solution of ruthenium chloride and aluminum nitrate. The essential features of the method are as follows. A mist of the mixed solution is generated into a stream of hydrogen by a supersonic atomizer, and treated successively through three furnaces. Tempera- tures of the furnaces are adjusted for evaporation of water, hydrogenolysis of the mixed metal salts, and further reduction of the particles, respectively. This method has the common advantages of solution methods for preparation of fine particles, i.e., better homogeneity and better purity of the material, lower temperature of preparation, precise control of the composition and so on. 662 Moreover, this method can control more precisely the structure and the texture without coalescence of particles, because the temperature of individual process is controlled independently (ref. 2 ) . METHODS Materials Ruthenium (111) chloride (99.9%), aluminum nitrate ( S grade) and benzene ( S grade) were obtained from Wako Pure Chemical. Hydrogen was obtained from Nippon Oxygen Co. and used through a dry ice trap. Carbon monoxide (99.5%) was obtained from Takachiho Chemical, and purified by distillation at liquid nitrogen temperature. Preparation of composite particles Figure 1 shows the schematic diagram of the apparatus. A mist of a mixed solution (5wt8) of RuC13 and Al(NO3I3 was generated into a stream hydrogen by a supersonic atomizer ( 6 ) , the diameter of the droplet being ca. 5 w. The mist was treated successively through three furnaces (7-9). Temperatures of the furnaces were adjusted for evaporation of water (443 K), hydrogenolysis of the mixed metal salts (573 K), and further reduction of the particles (773 K), respectively. The flow rate was controlled at 1 l/min by a control valve (13). It should be noted that the retention time of the mist in the constant temperature portion (*5 K) of the furnaces is ca. 10 S. The particles were collected by a glass filter (10) at 393 K. A gas handling valve (1) was provided to replace hydrogen in the apparatus with nitrogen when the filter is of r 0 n 17 Fig. 1. Schematic diagram of apparatus. 1: Gas handling valve, 2: Safety valve, 3: Reservoir, 4-5: Valve, 6: Supersonic atomizer, 7-9: Furnace, 10: Filter, 11: Tail gas treatment System, 12: Flow meter, 13: Control valve, 14: Pump. 663 exchanged. The exit gas from the filter was passed through a tail gas treatment system (11) to remove water, nitrogen dioxide etc. before entering into a flow meter (12). The powders were pressed into tablets, crushed and sized (32-60 mesh) for use in the measurements. A reference catalyst was prepared by reduction (at 673 K) of a ruthenium aluminum mixed oxide which had been prepared in the same way in the stream of air with the same raw materials. Physico-chemical characterization Temperature-programmed reduction (TPR) measurement was carried out by a volumetric method with a constant pressure gas circulation system of ca. 200 ml. Liquid nitrogen traps were placed before and after a quartz measuring tube to remove the water formed. After evacuation of sample at 513 K for 30 min, the temperature of the sample was decreased to room temperature, and a constant pressure of hydrogen was admitted to the system. Then the temperature was increased at a rate of 5.3 K/min. Infrared (IR) spectrum of adsorbed carbon monoxide was recorded with a JASCO FT/IR-3 Fourier-transform IR spectrometer. The construction of the vacuum IR cell used for the measurements was similar to that reported by Peri and Hannan (ref. 3). The sample was pressed into a thin self-supporting wafer and pretreated in the cell. After reduction with hydrogen at 673 K for 2 h, the sample was evacuated at the same temperature for 30 min and the temperature of the sample was lowered to room temperature for adsorption of carbon monoxide. The adsorption was carried out at a constant pressure of 40 Tor+ for 15 min at room temperature. Carbon monoxide in the gas phase was evacuated at room temperature for 5 min before IR measurement. The spectrum taken before carbon monoxide adsorption was used as the background spectrum. Specific surface areas were measured by the BET method with adsorption of nitrogen at 77 K. A JEOL JSM-T2OO scanning electron microscope (SEM), a JEOL JEM-2000EX transmission electron microscope (TEM) and a Rigaku Denki powder X-ray diffractometer with nickel filtered CuKd radiation were used for characterization of the samples. Catalytic activity The catalytic activity for benzene hydrogenation was measured by a flow method at 423 K. A given amount of the sample was packed in a Pyrex reactor. After pretreatment with hydrogen at 673 K for 16 h, the temperature of the sample was lowered to the reaction temperature in the hydrogen stream, and the hydrogen gas containing 5.25 Torr of benzene vapor was flowed through the packed bed at a rate of 30 ml/min. The products were analyzed by gas chromatography with a 2-m ethylene glycol adipate/Chromosorb W column. 664 Pig. 2. SEM micrograph of ruthenium-alumina composite powder. 20 w 10 0 0 Diameter ( p m ) ~ ~ i g . 3. Particle size distribution. 665 RESULTS AND DISCUSSION Characterization of particles The particles of ruthenium-alumina composite are spherical as shown in Fig. 2. The particle size distribution is narrow with an average diameter of 0.64 wn, as shown in Fig. 3. A cumulative surface area of 1.9 m 2 / g is obtained from the distribution curve. Comparison between the cumulative and BET ( 5 . 4 m2/g) surface areas suggests that the particles are porous. The ruthenium content of the sample was determined to be 2.33 wt8 by a chemical analysis. Result of TPR measurement is shown in Fig. 4, together with the result with the mixed oxide. With the mixed oxide sample ( b ) which had been prepared in air stream, consumption of hydrogen started at about 400 K and completed at about 570 K. This indicates that the reduction of ruthenium oxide takes place in this temperature range. With the ruthenium-alumina composite sample (a), on the other hand, consumption of hydrogen started at about 520 K , and the amount of hydrogen consumed below 570 K is much smaller than that of the mixed oxide. This indicates that most of the ruthenium ions were reduced in this sample. Figure 5 shows the XRD spectra of the composite particles. The XRD spectrum of the original sample (a) shows no crystalline peak; peaks of neither ruthenium metal nor ruthenium dioxide were detected. However, crystalline peaks of ruthenium metal, ruthenium dioxide and d-alumina appeared after the sainple was heated in the stream of helium up to 1273 K at a rate of 5 K/min (b). 8 1 ,............ * * b .... ** * ... a ___. .* , _: , 500 700 T e m p e r a t u r e ( K ) Fig. 4. TPR measurements. Pi12 = 201 Torr, Rate of temperature increase = 5 . 3 K/min, I 0 This 0 Fig. 5 . XRD saectra. (a) Original sample, (b) Treated in helium up to 1 2 7 3 :. ( a ) Obtained by direct synthesis, a: Ru, 0: Xu02, 0 : K - A l z O j (b) Mixed oxide sample. 666 indicates that the original sample was amorphous, and that the size of the ruthenium clusters increased by sintering during the high temperature treatment. Figure 6 shows the TEM micrograph of 3 particles, the diameters of which are 125, 580 and 900 nm, respectively. Various sizes of spots are observed in the particles. Spots larger than 10 nm may be ascribed to the porous structure of the particles, but smaller spots may be caused by aggregation of ruthenium metal atoms because the BET surface area is only about 3 times as large as the cumulative surface area. The micrograph shows that the size of the ruthenium clusters depends on the size of the particle; ruthenium clusters of small size ( < ca. 1 nm) are observed in the smallest particle. This suggests that the hydrogenolysis process in a particle is greatly influenced by the size of the particle. In the medium-sized particle which is the representative particle in the powder, the size of ruthenium clusters is less than about 2 nm. Both of the ruthenium clusters and alumina are amorphous because no electron diffraction Fig I. 6. TEM micrograph. 667 pattern was observed. Amorphous ruthenium clusters in amorphous alumina may be prepared because the fine droplets were rapidly dried and then rapidly reduced by hydrogen. The adsorption of carbon monoxide on supported ruthenium has been extensively studied by IR spectroscopy (ref. 4 ) . General agreement exists on the presence of three IR bands. The LF band at 1990-2030 cm-l is assigned to the vibration of carbon monoxide linearly bonded on ruthenium crystallites. The bands at 2080 and 2140 cm-l correspond to the vibrations of a multicarbonyl. In a recent investigation (ref. 5 ) , this species was shown to be a tricarbonyl associated with Ru2+ cations bonded directly to the support. Figure 7 shows the IR spectrum of carbon monoxide adsorbed on the ruthenium-alumina composite. Three absorption bands are observed at 2040, 2080 and 2140 cm-l, although the bands assigned to the vibrations of multicarbonyl are small. Comparison with the result on the impregnated Ru/A1203 reduced at the same temperature (ref. 4) reveals that the LF band on the present sample shifts to high frequency side. No clear interpretation can be made at present, but the amorphous nature of the present sample may be one of the reasons because the electronic structure and surface density of adsorbed carbon monoxide may be different. v 0 Fig. 7. IR spectrum of adsorbed carbon monoxide. x " 2 a 0 30 60 90 120150180 Time ( m i n ) Fig. 8. Catalytic activity for benzene hydroganation. Reduction temperature = 673 K, Reaction temperature = 423 I:. (a) Obtained by direct synthesis, (b) Wixed oxide sample. 668 Catalytic activity Catalytic activities for benzene hydrogenation were measured at 423 K. The reaction product was cyclohexane, and no other product was detected in the reaction condition studied. The catalytic activity of the mixed oxide, BET surface area of which was 4.1 m2/g, was also measured after reduction at 673 K for 16 h. The activities of both samples decrease a little with reaction time as shown in Fig. 8. The catalyst obtained by the direct synthesis (a) shows much higher (about 8 times) activity than that prepared by reduction of the mixed oxide (b). REFERENCES 1 J. G. Highfield, A. Bossi and F. S. Stone, Proc. 3rd Intern. Symp. on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, 1982, B5. 2 H. Imai and F. Orito, Nippon Kagaku Kaishi, (1984) 851-855. 3 J. B. Peri and R. B. Hannan, J. Phys. Chem., 64 ( 1 9 6 0 ) 1526-1530. 4 F. Solymosi and J. Rasko, J. Catal., 115 (1989) 107-119; and references there in. 5 G. H. Yokomizo, C. Louis and A. T. Bell, J. Catal., 120 (1989) 1-14. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 669 DESIGNED CATALYSTS FOR HYDRODECHLORINATION, REDUCTION AND REDUCTIVE AM I MAT I 0 N RE ACT I 0 N S J.L.Margitfalvi, S.GbbGlBs, E.Tdlas and M.Hegediis Central Research Institute for Chemistry o f the Hungarian Academy of Sciences, 1525 Budapest, POB 17, Hungary SUMMARY for hydrodechlorination o f chlorobenezne and f o r conversion of 4-chloro-Z-nitroaniline, in which both hydrodechlorination and r e - duction steps are involved. Best results were obtained on catalysts containing palladium in ionic form. F o r reductive amination o f ace- tone a skeletal nickel catalyst and its tin modified version was designed. On thesecatalysts the ratio of primary to secondary amines could be controlled and the formation o f isopropyl alcohol was strongly suppressed. Palladium containing alumina supported catalysts were designed INTRODUCTION There are different approaches in catalyst design[l]. In our ap- proach the design of an active and selective catalyst for a given process is based on (i) the primary knowledge of the reaction net- work and the reaction mechanism and (ii) the use of different types of Controlled Surface Reactions (CSRs) to introduce and stabilize either the active component or the added modifier of the given ca- talyst in the required form and environment. In this work results obtained in :wo case studies will be given and discussed: design of catalysts (i) for hydrodechlorination o f chlorobenzene and conversion o f 4-chloro-2-nitroaniline (CNA) to orthophenylendiamine (OPDA) and f o r (ii) reductive amination o f acetone. CATALYST DESIGN - Oesign o f catalyst for hydrodehalogenGion and for reduction of the nitro group. The dehalogenation of aryl halides can be carried out in stoichio- metric or catalytic reactions in the presence of bases. Reducing agents as LiA1H4 o r NaBH4 are used in the stoichiometric reactions 1 2 1 . The reaction can be carried out in the presence of homogeneous and heterogeneous catalyst under condition o f transfer hydrogenation 670 [ 3 , 4 ] o r i n t he p r e s e n c e o f gas phase h y d r o g e n [ 5 , 6 1 . The d e s i g n o f c a t a l y s t s f o r h y d r o d e h a l o g e n a t i o n i s based on t h e mechan ism o f o x i d a t i v e a d d i t i o n o f a r y l h a l i d e s t o d8 o r dâ t r a n - s i t i o n m e t a l s ( r e a c t i o n (111 . ( 1 ) +n+2 A r X + M+âLm - A r - K LmX I t has a l s o been p r o p o s e d t h a t r e a c t i o n (11 i s t h e r a t e l i m i t i n g s t e p i n h y d r o d e h a l o g e n a t i o n o f a r y l h a l i d e s i n t h e p r e s e n c e o f ho - mogeneous and m e t a l l o c o m p l e x c a t a l y s t s [ 7 , 8 ] . Based on t h i s know- l e d g e i t has been s u g g e s t e d t h a t t h e i n t r o d u c t i o n o f p a l l a d i u m i n t o t h e s u p p o r t n o t i n m e t a l l i c b u t i n i o n i c f o r m s h o u l d i n c r e a s e t h e r a t e o f h y d r o d e h a l o g e n a t i o n p r o v i d e d t h e mode o f s t a b i l i z a t i o n o f t h e i o n i c f o r m o f p a l l a d i u m c a n b e f o u n d . I n t h e p r e p a r a t i o n o f h y d r o d e c h l o r i n a t i o n c a t a l y s t t h e f o l l o w i n g s u r f a c e r e a c t i o n s a r e i n v o l v e d : 3-OH + C 4 H g L i - 3 O L i (I) + âqHâ10 (2) n ( 1 ) +PdC12 ___b g( -O)nPdC12-n + n L i C l ( 3 ) a ( - O ) n P d C 1 2 - n + H2 - ( - O I n P d + ( 2 - n l H C 1 (4) R e a c t i o n ( 2 1 and ( 3 ) has been w i d e l y u s e d f o r t h e p r e p a r a t i o n o f s i l i c a s u p p o r t e d m e t a l l o c o m p l e x c a t a l y s t s [ 9 , 1 0 ] . C h a r a c t e r i s t i c f e a t u r e o f t h e c a t a l y s t s p r e p a r e d i n t h i s way i s t h e p r e s e n c e o f a n c h o r e d i o n i c p a l l a d i u m on t h e a l u m i n a s u p p o r t . D e s i g n o f c a t a l y s t f o r r e d u c t i v e a m i n a t i o n o f a c e t o n e . R e a c t i o n s i n v o l v e d i n r e d u c t i v e a m i n a t i o n o f a c e t o n e a r e g i v e n i n Scheme I. CH3COCH3 CH3CHNH2CH3 -HZO (CH3CHCH3) 2NH + NH3, +H2 +(CH31 ZC0,+H2 CH3CHOHCH3 Scheme 1 . F o r r e d u c t i v e a m i n a t i o n o f a c e t o n e w i t h ammonia t w o t y p e s of c a - t a l y s t s were d e s i g n e d : ( i ) s k e l e t a l N i c a t a l y s t p r e p a r e d f o r m a N i - A 1 a l l o y and ( i ) i t s t i n m o d i f i e d v e r s i o n s . B o t h t y p e s o f c a t a - l y s t s w e r e used i n a c o n t i n u o u s f l o w gas phase r e a c t o r . T h e r e - q u i r e m e n t s f o r t h e s e c a t a l y s t s were as f o l l o w s : h i g h t h e r m a l and m e c h a n i c a l s t a b i l i t y , h i g h r a t e s f o r t h e f o r m a t i o n o f b o t h p r i m a r y and s e c o n d a r y amines and s u p p r e s s i o n o f t h e f o r m a t i o n o f i s o p r o p y l a l c o h o l f r o m a c e t o n e . Upon p r e p a r i n g t h e s k e l e t a l N i c a t a l y s t s t h e h i g h a c t i v i t y t o - wards t h e f o r m a t i o n o f b o t h p r i m a r y and s e c o n d a r y a m i n e s r e q u i r e s o p t i m a l i z a t i o n o f t h e l e a c h i n g p r o c e s s and u s i n g a t h e r m a l t r e - 671 a t m e n t p r o c e d u r e a b o v e 200OC. I t h a s b e e n d e m o n s t r a t e d e a r l i e r t h a t a c i d - b a s i s c a t a l y s t s can b e i n v o l v e d i n t h e f o r m a t i o n o f s e c o n d a r y a m i n e s [ 1 0 , 1 1 ] . B a s e d o n t h e a b o v e k n o w l e d g e t h e p r e p a r a t i o n o f t h e s k e l e t a l c a t a l y s t was a i m e d t o c r e a t e n o t o n l y h i g h l y a c t i v e m e t a l - l i c s i t e s b u t t o f o r m t h e p r e c u r s o r o f s i t e s r e q u i r e d for t h e c o n - d e n s a t i o n s t e p ( s e e Scheme I.). T h e f o r m a t i o n o f i s o p r o p y l a l c o h o l c o u l d b e s t r o n g l y d e c r e a s e d by s e l e c t i v e p o i s o n i n g o f s i t e s r e s p o n s i b l e f o r t h e h y d r o g e n a t i o n o f t h e c a r b o n y l g r o u p . The s e l e c t i v i t y o f t h e s k e l e t a l n i c k e l c a t a - l y s t t o w a r d s t h e f o r m a t i o n o f i s o p r o p y l a l c o h o l w a s c o n t r o l l e d v i a s e l e c t i v e p o i s o n i n g o f t h e n i c k e l s i t e s b y t i n u s i n g CSRs b e t w e e n a d s o r b e d h y d r o g e n on t h e n i c k e l s i t e s a n d t i n a l k y l s w i t h g e n e r a l f o r m u l a o f SnR t h e a b o v e CSRs h a s b e e n d i s c u s s e d f o r P t / A 1 2 0 3 [ 1 2 ] . E l n . D e t a i l s on s u r f a c e c h e m i s t r y i n v o l v e d i n ( 4 - n ) EXPERIMENTAL C a t a l y s t p r e p a r a t i o n [ i ) S o l v e n t s u s e d w e r e c a r e f u l l y d r i e d a n d d e o x y g e n a t e d . T h e a l u m i n a s u p p o r t w a s t r e a t e d i n vacuum a t S X I O - ~ b a r i n t h e t e m p e r a t u r e r a n g e o f 150- SOOOC. R e a c t i o n s (2) a n d ( 3 ) were c a r r i e d o u t i n n - h e x a n e a n d a c e - t o n e , r e s p e c t i v e l y . A f t e r c o m p l e t i o n o f r e a c t i o n (2) a n d ( 3 ) a w a s h i n g p r o c e d u r e w a s u s e d t o r e m o v e u n r e a c t e d bu:y?lithiurn(Euiij o r PdC12 , r e s p e c t i v e l y . T h e f i n a l s t e p o f t h e c a t a l y s t p r e p a r a t i o n was a h e a t t r e a t m e n t i n n i t r o g e n or h y d r o g e n a t m o s p h e r e i n t h e t e m p r e - r a t u r e r a n g e o f 15O-20O0C. F u r t h e r d e t a i l s o n c a t a l y s t p r e p a r a t i o n w i l l b e g i v e n i n R e s u l t s a n d D i s c u s s i o n . [ T i ) P r e p a r a t i o n o f c a t a l y s t s f o r r e d u c t i v e a m i n a t i o n . G r a n u l a r s k e l e t a l n i c k e l c a t a l y s t w i t h p a r t i c l e s i z e o f 3 - 5 mm was p r e p a r e d by l e a c h i n g a N i - A 1 a l l o y c o n t a i n i n g 50 w t % n i c k e l . H a l f o f t h e a m o u n t o f a l u m i n a was l e a c h e d o u t w i t h 3 w t % NaOH-wate r s o l u t i o n a t 50'C f o r 12 h o u r s . A f t e r l e a c h i n g t h e c a t a l y s t was w a s h e d w i t h d i s t i l l e d w a t e r a n d w a s k e p t u n d e r a n a q u e o u s s o l u t i o n h a v i n g pH=9. P r i o r t o t h e m o d i f i c a t i o n w i t h t i n t h e c a t a l y s t w a s d r i e d i n f l o w i n g n i t r o g e n a t 1 2 O o C f o r 4 h o u r s . A f t e r d r y i n g t h e c a t a l y s t was t r e a t e d i n h y d r o g e n a t 200 or 300'C f o r 2 h o u r s f o l l o w e d b y c o o l i n g t o room t e m p e r a t u r e i n h y d r o g e n . T h e m o d i f i c a t i o n o f t h e c a t a l y s t w i t h t i n a l k y l c o m p o u n d s was c a r r i e d o u t a t 5OoC u s i n g 20 g o f g r a n u l a r s a m p l e a n d 1 0 0 c m 3 o f b e n z e n e s o l v e n t . D e c o m p o s i - t i o n o f s u r f a c e c o m p l e x f o r m e d i n t h e r e a c t i o n o f t i n a l k y l s w i t h h y d r o g e n a d s o r b e d on n i c k e l w a s p e r f o r m e d i n h y d r o g e n us ing a hea t ing 672 r a t e o f 2'C/rnin a n d a f i n a l t e m p e r a t u r e o f 25OoC. F u r t h e r d e t a i l s on t h e p r e p a r a t i o n w i l l b e g i v e n i n R e s u l t s a n d D i s c u s s i o n . C a t a l y s t c h a r a c t e r i z a t i o n T h e p h a s e c o m p o s i t i o n o f s k e l e t a l n i c k e l c a t a l y s t s w a s s t u d i e d by u s i n g a P h i l l i p s 1 7 0 0 p o w d e r d i f f r a c t o m e t e r e q u i p p e d w i t h a g r a p h i t e m o n o c h r o m a t o r a n d CuK, r a d i a t i o n . E5R s p e c t r a were r e c o r d - e d a t 2 O o C u s i n g a J E O L J E S - F E 3 X s p e c t r o m e t e r . X P S m e a s u r e m e n t s were t a k e n by u s i n g a V G ESCA 3 s p e c t r o m e t e r w i t h a n a l u m i n i u m Ka r a d i a t i o n s o u r c e . A l l b i n d i n g e n e r g i e s were r e f e r r e d t o t h e A12p l i n e ( B t = 7 4 . 7 eV). C a t a l y t i c r e a c t i o n s T h e h y d r o d e c h l o r i n a t i o n o f c h l o r o b e n z e n e , a n d c o n v e r s i o n 4 - c h l o - r o - 2 - n i t r o - a n i l i n e t o o r t h o p h e n y l e n e d i a m i n e w a s c a r r i e d o u t u n d e r d i f f e r e n t r e a c t i o n c o n d i t i o n s u s i n g s t i r r e d t a n k a n d t r i c k l e bed r e a c t o r s i n t h e p r e s s u r e r a n g e o f 1 - 7 0 b a r . T h e r e d u c t i v e a m i n a t i o n o f a c e t o n e was s t u d i e d i n a c o n t i n u o u s f l o w g a s p h a s e r e a c t o r a t 2 0 - 5 0 b a r a n d 160-2OO0C. B o t h i n t h e h y d r o d e h a l o g e n a t i o n a n d r e d u c - t i v e a m i n a t i o n t h e r e a c t i o n p r o d u c t s were a n a l y s e d b y g a s c h r o r n a t o - g r a p h y . RESULTS AND DISCUSSION P r e p a r a t i o n o f c a t a l y s t s f o r h y d r o d e h a l o g e n a t i o n ( i ) S t u d y o f s u r f a c e r e a c t i o n s ( 2 ) a n d ( 3 ) . E x p e r i m e n t a l v a r i a b - l e s u s e d i n c a t a l y s t p r e p a r a t i o n were a s f o l l o w s : t e m p e r a t u r e o f d e h y d r o x y l a t i o n , a m o u n t o f E u L i u s e d , t e m p e r a t u r e a n d d u r a t i o n o f r e a c t i o n s ( 2 ) a n d (31, mode o f r e m o v a l o f B u L i , c o n d i t i o n o f t h e f i n a l t r e a t m e n t . P r e f e r e n c e w a s g i v e n for e x p e r i m e n t a l c o n d i t i o n s r e s u l t i n g i n h i g h p a l l a d i u m l o a d w i t h a v o i d i n g r e d u c t i o n o f PdC12 t o m e t a l l i c p a l l a d i u m . C o n d i t i o n s o f t h e p r e p a r a t i o n a n d p r o p e r t i e s o f c a t a l y s t s p r e p a r e d a r e s u m m a r i z e d i n T a b l e 1. I n r e a c t i o n ( 2 ) e x c e s s E u L i w a s u s e d . W a s h i n g a n d e x t r a c t i o n w i t h n - h e x a n e a p p e a r e d t o b e t h e m o s t e f f e c t i v e mode f o r t h e r e m o v - a l o f u n r e a c t e d BuLi a d s o r b e d o n t o t h e A 1 2 0 3 . D e c o m p o s i t i o n o f t h e u n r e a c t e d E u L i by h e a t t r e a t m e n t r e s u l t e d i n r e d u c t i o n o f PdC12 t o m e t a l l i c p a l l a d i u m c o m p a r e c a t a l y s t s N o 2 a n d 4 . U n d e r 50°C t h e r a t e o f s u r f a c e r e a c t i o n ( 3 ) w a s v e r y l o w . A t h i g h e r t e m p e r a t u r e a n d upon i n c r e a s i n g t h e d u r a t i o n o f r e a c t i o n ( 3 ) was o b s e r v e d ( s e e No 3 a n d 7 ) . T h e f o r m a t i o n o f m e t a l l i c p a l l a - r e d u c t i o n o f PdClZ 673 T a b l e 1 C o n d i t i o n s o f t h e p r e p a r a t i o n o f i o n i c p a l l a d i u m c a t a l y s t s . X P S d a t a T a R e a c t i o n R e a c t i o n C o n c e n t r a t i o n ( 2 ) ( 3 ) W % P d 3 d 5 / 2 b i n d i n g FWHM No (OC) B u L i b time, t i m e , P,l2O3 m i n Pd Li ene rgy ,e \ i m i n 1 2 0 0 1 . 5 9 0 3 0 0 . 5 1 1 . 1 0 . 3 2 1 5 0 2 . 3 61? 60 0 . 3 1 1 . 2 n . a . 3 1 5 0 2 . 3 6 0 1 4 4 0 0 . 6 1 1 . 2 n . a . 4C 1 5 0 2 . 3 6 0 6 0 0 . 4 6 1 . 2 n . a . 5 150 2 . 3 6 0 1 3 5 0 . 5 5 1 . 6 n . a . 6 d 1 5 0 2 . 3 6 0 1 3 5 0 . 5 5 1 . 6 n . a . 7 150 1 . 5 60 9 0 0 0 . 7 0 0 . 4 0 . 5 8 250 2 . 3 85 6 0 0 . 2 4 0 . 4 n . a . 9 200 2 . 3 85 6 0 0 . 3 5 0 . 7 n . a . 1 G 1 5 0 2 . 3 85 60 0 . 4 9 0 . 6 n . a . Ile 1 5 0 2 . 5 85 6 0 0 . 9 2 1 .0 n . a . 3 3 6 . 6 4 .0 3 3 6 . 6 3 .7 3 3 5 . 3 3 . 8 3 3 5 . 5 3 . 2 3 3 6 . 3 4 .3 3 3 5 . 6 3 . 3 3 3 4 . 9 2 . 8 n . a . n . a . n .a . n . a . T e m p e r a t u r e o f d e h y d r o x y l a t i o n G i v e n i n mmol/g A f t e r r e a c t i o n 12 ) t r e a t m e n t a t 15OoC f o r 1 h o u r a t I x I O - ~ b a r A f t e r r e a c t i o n ( 3 ) t r e a t m e n t a t 4OO0C f o r 2 h o u r s i n H2 P a r t i c l e size t 0 . 0 4 5 mm, i n o t h e r s a m p l e s : 0 . 3 1 - 0 . 6 3 mm. d i u m was a l s o o b s e r v e d i n t h e p r e s e n c e o f s m a l l amount o f w a t e r i n t r o d u c e d i n t o t h e a c e t o n e t o i n c r e a s e t h e s o l u b i l i t y o f PdC12. ESR s i g n a l w i t h g = 2 . 0 0 4 was d e t e c t e d i n c a t a l y s t s c o n t a i n i n g i o n i c Fd . T h i s s i g n a l was v e r y s t a b l e n o changes i n t h e g v a l i i e was o b s e r v e d a f t e r h e a t i n g i n n i t r o g e n o r h y d r o g e n a t 200°C. No E S R s i g n a l was d e t e c t e d on c a t a l y s t s p r e p a r e d b y c o n v e n t i o n a l t e c h n i q u e o r on l i t h i a t e d a l u m i n a . The o b s e r v e d ESR s i g n a l was a t t r i b u t e d t o a f r e e e l e c t r o n o r i g i n a t e d f r o m e l e c t r o n i c i n t e r a c t i o n b e t w e e n i o n - i c p a l l a d i u m and t h e a l u m i n a s u p p o r t [ 1 3 ] . X P S r e s u l t s a r e g i v e n i n T a b l e 1 . The b i n d i n g e n e r g i e s a r o u n d 3 3 5 . 0 and 3 3 6 . 7 eV w e r e as- s i g n e d t o m e t a l l i c and i o n i c p a l l a d i u m , r e s p e c t i v e l y [ 1 4 1 . C a t a l y s t c o n t a i n i n g i o n i c p a l l a d i u m had a r e l a t i v e l y b r o a d p e a k w i t h FWHM a r o u n d 4 . 0 eV, whereas s a m p l e c o n t a i n i n g m e t a l l i c p a l l a d i u m had a n a r r o w peak w i t h FWHM a r o u n d 3 . 0 e V . X P S measuremen ts s t r o n g l y i n - d i c a t e c t h a t a n c h o r e d i o n i c p a l l a d i u m i s s t a b l e up t o 200'C even i n h y d r o g s n a tmosphere , however , h e a t i n g a t 400'C r e s u l t e d i n r e d u c - t i o n o f t h e i o n i c s p e c i e s t o m e t a l l i c one. ( i i j C h a r a c t e r i z a t i o n o f c a t a l y s t s b y ESR and XPS. A n a r r o w P r e p a r a t i o n o f c a t a l y s t s f o r r e d u c t i v e a m i n a t i o n ( i 1 P r e p a r a t i o n a n d c h a r a c t e r i za t i o n o f s k e 1 e t a 1 n i c k e 1 ca ta l ys ts . -- 674 I n t h e l e a c h i n g p r o c e s s d i l u t e d NaOH was u s e d . O n l y h a l f o f t h e a l u m i n i u m i n t h e a l l o y was l e a c h e d o u t , I n t h i s way t h e h i g h mecha- n i c a l s t a b i l i t y o f t h e a l l o y c o u l d be m a i n t a i n e d . The c o n d i t i o n o f l e a c h i n g was f a v o u r a b l e f o r t h e f o r m a t i o n o f oxygen c o n t a i n i n g s u r - f a c e s p e c i e s o f a l u m i n i u m . T h e A 1 a n d N i c o n t e n t o f t h e c a t a l y s t s was 22 and 54 w % , r e s p e c t i v e l y , X R O m e a s u r e m e n t s p e r f o r m e d on t h e t h e r m a l l y t r e a t e d s k e l e t a l n i c k e l c a t a l y s t i n d i c a t e d t h e p r e s e n c e o f m e t a l l i c N i , A 1 3 N i 2 , A l N i , N i O , A 1 ( O H 1 3 and AlO(OH) p h a s e s . f a c e r e a c t i o n be tween h y d r o g e n a d s o r b e d on n i c k e l and t i n a l k y l compounds h a v e been u s e d for t h e m o d i f i c a t i o n o f s k e l e t a l n i c k e l by t i n . S u r f a c e r e a c t i o n s i n v o l v e d i n t h e m o d i f i c a t i o n c a n be w r i t t e n a s f o l l o w s : ( i i ) M o d i f i c a t i o n o f t h e s k e l e t a l n i c k e l c a t a l y s t w i t h t i n . S u r - xNiHa + SnRnC14-n Nix-SnRn-xC14-n (I) + xRH (5) Nix-SnRn-xC14-n & Nix-Sn + (n-x)RH + (4-n)HC1 (6) A R e s u l t s o b t a i n e d upon s t u d y i n g s u r f a c e r e a c t i o n ( 5 ) and (6) are summar ized i n T a b l e 2. T a b l e 2 S t u d y o f s u r f a c e r e a c t i o n s i n v o l v e d i n t h e m o d i f i c a t i o n o f s k e l e t a l r l i r l - e l c a t a l y s t by t i n . a T i n precursor I n i t i a l con- compo und cent ra t ion mnal .dm -3 Surface reac t ion (5) number of R reacted r a t e o f t i n anchoring mo1.dm m i n x10 - 3 . - 1 - 7 ( X I 1. 2. 3 . 4. 5. 6. 7. 8. 0.6 3.0 9.1 3.0 3.0 3.0 3.0 3.0 2.96 3.00 3.33 2.33 3.76 3.91 3.23 1.09 0.17 0.53 1.85 0.26 0.47 7.67 0.61 29.79 a b d C S t a n d a r d e x p e r i m e n t a l c o n d i t i o n s : t e m p e r a t u r e o f H2 t r e a t m e n t : 30OoC; c o o l i n g i n H 2 ; s o l v e n t used i n r e a c t i o n (5): b e n z e n e A f t e r H 2 t r e a t m e n t c o o l i n g i n N, H, t r e a t m e n t a t 200OC R e a c t i o n ( 5 1 i n n - h e x a n e R e a c t i o n (5) was v e r y s e l e c t i v e , o n l y s a t u r a t e d h y d r o c a r b o n s were d e t e c t e d . The i n i t i a l r a t e of s u r f a c e r e a c t i o n 1 5 ) s t r o n g l y d e p e n - d e d on t h e i n i t i a l c o n c e n t r a t i o n o f t h e t h i s p r e c u r s o r compound. S i g n i f i c a n t i n c r e a s e i n t h e i n i t i a l r a t e was o b s e r v e d w h e n r e a c t i o n 675 ( 5 1 was c a r r i e d out i n n-hexane and i n s t e a d o f t i n t e t r a a l k y l s compound w i t h g e n e r a l f o r m u l a o f SnRZCIZ was u s e d a s t i n p r e c u r s o r . C h a r a c - t e r i s t i c f e a t u r e o f s k e l e t a l n i c k e l i s , t h a t c o n t r a r y t o o u r e a r l - i e r f i n d i n g s 1 1 2 1 , more t h a n one a l k y l g r o u p has been l o s t i n r e a c - t i o n ( 5 ) . C a t a l y t i c r e a c t i o n s ( i ) H y d r o d e c h l o r i n a t i o n o f c h l o r o b e n z e n e . H y d r o d e c h l o r i n a t i o n o f c h l o r o b e n z e n e has been u s e d as a t e s t r e a c t i o n t o compare t h e h y d r o d e c h l o r i n a t i o n a c t i v i t y o f t h e c a t a l y s t s p r e p a r e d . R e s u l t s o b - t a i n e d upon s t u d y i n g t w o c a t a l y s t s : PdM and N O 1 a r e g i v e n i n T a b l e 3 . These d a t a r e v e a l s t h a t i n t h e t e m p e r a t u r e r a n g e o f 2O-7O0C c a - t a l y s t p r e p a r e d b y a n c h o r i n g ( c a t a l y s t N O 1 1 h a s h i g h e r h y d r o d e c h l o - r i n a t i o n a c t i v i t y t h a n c a t a l y s t c o n t a i n i n g m e t a l l i c P d (PdM) . T a b l e 3 H y d r o d e c h l o r i n a t i o n o f c h l o r o b e n z e n e i n s t i r r e d t a n k r e a c t o r a C a t a l y s t T e m p e r a t u r e I n i t i a l r a t e - 1 - 1 (OC) mrno1.s . g Pd PdM 2 0 1.93 N O 1 20 5.33 PdM 7 0 4.53 NO1 7 0 1 2 . 9 0 __-__ 'Amount o f c a t a l y s t : 0 .3 g, 3 mrnol c h l o r o b e n z e n e i n 2 0 cm3 e t h a n o l ( i i l C o n v e r s i o n o f 4 - c h l o r o - 2 - n i t r o a n i l i n e t o o r t h o p h e n y l e n e d i - a m i n e . T y p i c a l k i n e t i c c u r v e s o f t h e f o r m a t i o n o f OPOA and CPDA a r e shown i n F i g . 1. U n d e r g i v e n e x p e r i m e n t a l c o n d i t i o n t h e f o r m a t i o n o f o r t h o n i t r o a n i l i n e (ONA) was n e g l i g i b l e . T h i s f a c t i n d i c a t e d t h a t n o t t h e r e d u c t i o n b u t t h e h y d r o d e c h l o r i n a t i o n s t e p is t h e r a t e li- m i t i n g one i n t h e f o r m a t i o n o f OPDA. Upon i n c r e a s i n g t h e p a l l a d i u m c o n t e n t o f t h e c a t a l y s t a s t r o n g i n c r e a s e i n t h e i n i t i a l r a t e o f f o r m a t i o n of OPDA was o b s e r v e d . I t can a l s o be seen t h a t t h e l o w e r t h e i n i t i a l r a t e o f t h e f o r m a t i o n o f CPDA t h e h i g h e r is t h e t o t a l y i e l d o f OPDA. _ _ ~ ~ ~ Upon i n t r o d u c t i o n s m a l l amount o f m e t a l l i c p a l l a d i u m i n t o t h i s t y p e o f c a t a l y s t s i g n i f i c a n t d e c r e a s e i n t h e i n i t i a l h y d r o d e c h l o r i - n a t i o n a c t i v i t y was o b s e r v e d . The r e s u l t s a r e shown i n F i g . 2 . I n t h e s e c a t a l y s t s t h e i n t r o d u c t i o n o f m e t a l l i c p a l l a d i u m i n 0 . 0 5 and 0 . 1 w % was c a r r i e d o u t p r i o r t o t h e l i t h i a t i o n s t e p [ r e a c t i o n ( 2 1 1 . The c o n v e r s i o n o f CNA was a l so i nves t i ga ted i n a t r i c k l e bed r e a c t o r . 676 aJ a a 0 U 0 30 60 90 120 40t 0 30 60 90 120 time, rnin time, min F i g . 1 . I n f l u e n c e o f t h e p a l l a d i u m c o n t e n t on t h e OPDA a n d C P D A y i e l d s . S t i r r e d t a n k r e a c t o r ; NH3-H20 (50-50%), 300 cm3; T : 95OC; : 30 b a r ; C N A : 30 g ; a m o u n t o f c a t a l y s t : 3 . 2 g ; p a l l a d i u m c o n - ( w % ) : 0 - 0 . 2 4 , x - 0 . 3 5 ; a - 0 . 4 9 i c a t a l y s t s No8,9 and ?O, r e spec t ive ly ) , . 20 40 60 80 time, rnin F i g . 2 . I n f l u e n c e o f t h e p r e s e n c e o f m e t a l l i c p a l l a d i u m on t h e f o r - m a t i o n o f O P D A f r o m C N A . S t i r r e d t a n k r e a c t o r ; i - p r o p a n o l - w a t e r ( 9 0 : l O l 100 c m 3 , T : 6 O o C ; P : 1 b a r ; C N A : 5 g , a m o u n t o f c a t a l y s t : 0 . 4 g ; a m o u n t o f Pd a n c h o r e d : 0 . 9 5 w % ; m e t a l l i c p a l l a d i u m c o n t e n t ( w % l : rn - 0 . 0 ; 0 - 0 . 0 5 ; n - 0 . 1 0 . F i g . 3 . T e m p e r a t u r e d e p e n d e n c e o f t h e p r o d u c t f o r m a t i o n f r o m C N A i n t r i c k l e b e d r e a c t o r . P : 2 b a r ; a t h a n o l ( 5 % C N A ) ; l i q u i d f l o w r a t e : 0.74 c r n 3 / m i n ; g a s f l o w r a t e : 1 1 0 cm3/min ; a m o u n t o f c a t a l y s t : 5 g ; c a t a l y s t : No1 ( s e e T a b l e 2). T h e r e q u i r e m e n t f o r t h i s s t u d y was t o o b t a i n h i g h O P D A s e l e c t i v i t - i e s a t c o m p l e t e c o n v e r s i o n o f C N A . R e s u l t s o b t a i n e d i n t h i s s e r i e s o f exper iments are shown i n F i g . 3 . U n d e r r e l a t i v e l y low t e m p e r a t u r e a n d low h y d r o g e n p r e s s u r e h i g h c o n v e r s i o n o f C N A w a s o b t a i n e d . Upon i n c r e a s i n g t h e r e a c t i o n t e m p e r a t u r e t h e s e l e c t i v i t y o f t h e O P D A 677 T a b l e 4 R e d u c t i v e a m i n a t i o n o f a c e t o n e on t i n m o d i f i e d n i c k e l c a t a l y s t s . a Sn C1 Conversion s e 1 e c t i v i t i e s,b % C a t a l y s t s wt% wt% I IPA OIPA IPPA IPAL N i 0 0 99.3 83 .7 8.6 0 .5 7 .3 Ni-SnEtqC 0.082 0 99.2 65.2 24.2 1 . 7 6.0 Ni-SnBu - Id 0 .025 0 98 .3 74.7 20.6 0 .6 4.0 Ni-SnRuR-2' 0.076 0 98.5 70.9 20.7 2.9 5.5 Mi-SnBz:ClZd" 0.30 0.17 95.3 87.0 0 . 5 4.2 (1 Ni-SnEt,CIZC 0.12 0.07 38.2 12 .6 12.6 1 .4 4.0 a amount of c a t a l y s t = 20 g , P = 0.5 MPa, WHSV = 0.8 h - l , m o l a r r a t i o H 2 : N H 3 : A C = 2 : 4 : 1 , r e a c t i o n t e m p e r a t u r e 200OC; b a s e d o n t h e c o n t e n t o f (CH3-CH-Ct3 m o i e t i e s i n t h e p r o d u c t s ; a n d d p r i o r t o m o d i f i c a t i o n t h e c a t a l y s t p r e t r e a t m e n t i n H, was c a r r i e d o u t a t 3 0 0 a n d 2 0 0 ° C , r e s p E c t i v e l y ; i n s t e a d o f b e n z e n e a c e t o n e was u s e d a s a o l v e n t i n r e a c t i o n 1 1 ) . f o r m a t i o n i n c r e a s e d w i t h p a r a l l e l d e c r e a s e o f t h e s e l e c t i v i t y of CPDA. U n d e r g i v e n e x p e r i m e n t a l c o n d i t i o n O N A was d e t e c t e d o n l y i n t r a c e a m o u n t . 9 5 %. Upon f u r t h e r i n c r e a s e o f t h e r e a c t i o n t e m p e r a t u r e t h e s e l e c - t i v i t y o f OPOA s t r o n g l y d e c r e a s e d d u e t o t h e f o r m a t i o n o f d i f f e r e n t c o n d e n s a t i o n p r o d u c t s . A t 9O0C t h e s e l e c t i v i t y o f t h e OPOA f o r m a t i o n was A l l o f t h e s e r e s u l t s s t r o n g l y i n c i c a t e t h a t upon i n t r o d u c i n g i o - n i c p a l l a d i u m i n t o a l u m i n a i t i s p o s s i b l e t o o b t a i n a c a t a l y s t , i n w h i c h t h e h y d r o d e c h l o r i n a t i o n a c t i v i t y i s s i g n i f i c a n t l y h i g h e r t h a n t h e a c t i v i t y f o r r e d u c t i o n o f t h e n i t r o g r o u p . ( i i i ) R e d u c t i v e a m i n a t i o n o f a c e t o n e . R e s u l t s o b t a i n e d i n r e d u c - t i v e a m i n a t i o n o f a c e t o n e a r e g i v e n i n T a b l e 4 . C h a r a c t e r i s t i c f e a - t u r e o f r e d u c t i v e a m i n a t i o n r e a c t i o n s i s t h e s t r o n g c o n t r o l o f t h e p r o d u c t d i s t r i b u t i o n by t h e r m o d y n a m i c s . R e s u l t s g i v e n i n T a b l e 4 . r e v e a l s t h e s e l e c t i v i t y c o n t r o l by t i n m o d i f i c a t i o n . T h e i n t r o d u c - t i o n o f v e r y s m a l l a m o u n t o f t i n i n t o t h e s k e l e t a l n i c k e l c a t a l y s t f r o m t i n t e t r a a l k y l s r e s u l t e d i n a s t r o n g d e c r e a s e i n t h e IPA/OIPA r a t i o , I P A a n d O I P A : i s o p r o p y l a r n i n e a n d d i i s o p r o p y l a m i n e , r e s p e c t - i v e l y ] . T h e s e l e c t i v i t y o f t h e i s o p r o p a n o l ( I P A L : f o r m a t i o n showed a l s o a s m a l l d e c r e a s e . H o w e v e r , n o c o r r e l a t i o n h a s b e e n f o u n d b e t w e e n t h e t i n c o n t e n t a n d t h e IPA/DIPA r a t i o . Upon u s i n g t i n p r e - c u r s o r compound w i t h g e n e r a l f o r m u l a o f SnR2C12 t h e I P A / O I P A r a t i o was o n l y s l i g h t l y a l t e r e d b u t s i g n i f i c a n t d e c r e a s e w a s o b t a i n e d i n t h e s e l e c t i v i t y o f t h e i s o p r o p a n o l . C a t a l y s t m o d i f i e d w i t h SnBz2C12 678 (B = benzyll has the lowest selectivity for the formation of IPAL. On this catalyst sites responsible for hydrogenation both of the carbonyl group and the double bound of the isopropylidene-isopropyl- amine (IPPAI are strongly poisoned. These results strongly indicate that upon using CSRs for selec- tive poisoning of nickel b y tin the selectivity of the reductive amination o f acetone can be controlled and undesired side reaction can be suppressed. CONCLUSIONS Results obtained in this study reveals that catalyst design based on (il t h e primary knowledge of the reaction network and the mechanism of reactions involved in it and (ii) the application of Controlled Surface Reactions in catalyst preparation and rnodifica- tion can be used to obtain highly active and selective catalysts for different organic reactions taking place in the presence of hydrogen. REFERENCES 1 L.L.Hegedis (Editor), Catalyst Design,Progress and Perspectives, 2 A.R.Pinder. Synthesis 1980, 425. 3 G.Bringer and I.J.Nestrick, Chemical Reviews, 74 (19741 567. 4 T.Okamoto and S.Oka, Bull.Soc.Chim.Jpn. 54 (19811 1265. 5 P.Dini, J.C.J.Bart and N.Giordano, J.Chem.Soc. Perkin II., 1975, John Wiley and Sons, New York, 1987. 1479. 6 B.Coq, G.Ferrat and F.Figueras, J.Catal., 101 (1986) 434 7 J.K.Stille, K.S.Y,Lau, J.Am.Chem.Soc., 98 (1976) 5841. 8 C.Z.Sharf, A.S.Gurovets, I.B.Slinjakova, L.P.Finn, L.H.F and V.N.Krutii, 1zv.Akad.Nauk SSSR, Ser.Khirn., 1980, 114 9 Yu.I.Yermakov and V.A.Likholobov, Kinetika i Kataliz., 2 1208. 10 A.Le Bris, G.Lefebvre and F.Coussernant, Bull.Soc.Chim.. p . 1360, 1584. eidlin (19801 9 6 4 , I 1 V .V.Antonova, T. I. Ovchinnikova, B. F .Ustavshikov and V . K . 12 J.Margitfalvi, E.Tdlas and S.GBbElEs, Catal.Today. 6 (1989) 73. 13 P.A.Berger and J.F.Roth, J.Cata1. 4 ( 1 9 6 5 1 717. 14 Y.Shen, S.Wang and K.Huang, Appl.Catal., 57 (1990) 55. Promonenkov, Zh.Org.Khiâm., 16 (19801 547. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 679 PREPARATION OF HIGH SURFACE AREA HYDROGEN MOLYBDENUM BRONZE CATALYSTS C. HOANG-VAN*, 0. ZEGAOUI, B. POMMIER and P. PICHAT URA au CNRS Photocatalyse, Catalyse et Environnement, Ecole Centrale de Lyon, B.P. 163 - 69131 Ecully Cedex (France) SUMMARY Highly divided hydrogen molybdenum bronzes HxMo03 (0 < x 4 2) have been prepared from ultra fine orthorhombic MOO3 powders obtained in a flame reactor. The so-called hydrogen spillover process has been used for the preparation of Pt/HxMoO3 bronzes or that of neat H1.6Mo03 by momentarily contacting MoO3 with a Pt/Al2O3 catalyst in the presence of H2. Neat HxMo03 bronzes (with x 4 0.9) could also be obtained by use of an alcohol as a source of hydrogen atoms, either in the dark or under UV-illumination. The high surface area hydrogen-molybdenum bronzes thus obtained are potential catalysts for several types of reactions. INTRODUCIION Hydrogen bronzes are insertion compounds of atomic hydrogen in oxides (or chalcogenides) in which there is no formal chemical bond between the anion of the host lamce and the inserted element (ref. 1). These compounds have been used as catalysts for alkene hydrogenations and practical applications in that direction have been suggested (ref. 2). Any process in which atomic hydrogen is generated, e.g. nascent hydrogen from Zn and HCl, electrochemical reduction of H+, hydrogen plasma, can lead to the formation of a hydrogen bronze. For catalytic purposes, the so-called hydrogen spillover process is the preferred route (refs 3 and 4). In that case, small particles of Pt or Pd (metallic activators) are dispersed on the oxide surface by impregnation and hydrogen bronzes are formed when molecular hydrogen is brought into contact with the solid. The presence of Pt or Pd particles does not allow one to easily discriminate between the catalytic activity of the bronzes and that of the metal. Recently, however, one of us (ref. 5 ) has succeeded in preparing the bronze H1.jMOO3 without metal particles deposited on its surface and has definitely shown that H1.WoO3 is a catalyst for the hydrogenation of ethylene by molecular hydrogen in the absence of any metallic activator. In this paper we describe methods for the preparation of finely divided bronzes HxMo03, either neat or coated with Pt particles. Unlike most of the previous-works in which hydrogen molybdenum bronzes were elaborated from single crystals or from powders of Mo03 of low surface area (a few rn2g1), the high division state of the materials used in this study makes the bronzes particularly suitable for catalytic applications. 680 EXPERIMENTAL 1 -mtl âon of M a Ultra fine MOO3 particles were prepared in a flame reactor which has already been described (ref. 6). It should just be recalled that the oxide aerosol is generated from the vapor of a volatile metallic compound (Mo02C12 or Moc15) which is injected into the burner fed with hydrogen and oxygen and then decomposed in the flame either by hydrolysis or by oxidation. The oxide issued from the flame reactor was treated at 673 K in air for 24 h (standard treatment) in order to eliminate residual chlorine and to transform all the M a into the orthorhombic phase (see below). 2 - Preparation of bronzes Pt/HxMo03 bronzes were prepared by impregnation of MoO3, previously submitted to the standard treatment, with a solution of H2PtC4j (analytical grade) whose concentration was adjusted in order to obtain Pt contents in the range 0.1 - 1 wt 76. Evaporation of the solvent was carried out at 343 K under continuous stirring and the powder was then dried at 373 K overnight. To achieve the decompsition of H2PtC16, the impregnated oxide was heated under vacuum at 473 K for 2 h. This procedure is similar to that used by Marcq et al. (ref. 2). The temperature was then adjusted to that of the formation of the bronze and the powder was exposed to H2 in a volumetric apparatus until saturation of Mo03 with inserted hydrogen. Neat HxMo03 bronzes The bronze H1.6Mo03 was obtained by contacting Mo03 with Pt/Al2O3 in a hydrogen atmosphere at 433 K for 24 h. The Pt/A1203 was then removed from the reactor by a windlass device (ref. 7). The bronze H0.34Mo03 was prepared in a dynamic differential reactor by exposing Mo03 to a flow of allylic alcohol (20 Torr) and H2 (740 Torr) at 323 K for 20 h. This bronze could also be obtained at room temperature in a static photoreactor by UV-illuminating MoO3 suspensions in a liquid alcohol (such as methanol or 2-propanol). Addition of Ti@ to the MoO3 suspensions allowed the formation of H0.9M0@. 3 - Other exuerimental techniaues Surface areas were measured in a dynamic chromatography system (ref. 8) using N2 at 77 K. X-ray diffraction patterns were obtained with a Siemens diffractometer (Kristalloflex D500) using Preparation of Pt/HxMoOg bronzes and hydrogen absorption measurements were performed in a C u G radiation filtered through nickel. conventional Pyrex glass volumetric apparatus. 681 RESULTS AND DISCUSSION 1 -preDar;ltl âon of M a The surface area and morphology of MoO3 are controlled by the temperature of the flame, the concentration of MoCl5 injected as a vapor into the burner and the residence time of this vapor in the flame. This is illustrated by the results reported in Table 1 and by electron micrographs of Fig. 1 and L. TABLE 1 Surface areas of MoO3 samples prepared in a flame reactor M a - A 1200 3600 20 82 20 M a - B 1900 1 1 4 0 24 50 40 MoO3-C 2400 1140 20 49 34 M a - D 1900 1140 161 17 16 (a) : Temperature of the flame. [ 0 2 m 2 ] = 1 to 1.5 (b) : Total flow rate (c) : MoCl5 mass velocity (d) : after a standard treatment at 673 K for 24 h. Fig. 1 - Electron micrograph of MoO3-B Fig. 2 - Electron micrograph of MoO3-D 682 For low residence time and small concentration of MoCl5 vapor in a cold flame (T = 1200 K), the sample obtained (Mo03-A) exhibits the largest surface area (line 1. Table 1). Samples prepared in hot flames (T 3 1900 K) present surface areas which strongly depend on the mass velocity of MoCl5 The micrographs of M a - B and M a - D samples are presented on Fig. 1 and 2, respectively. Figure 1 shows particles of different sizes. Some of them are elongated plaques of 50 to 300 nm in length, whereas the others are very small (< 10 nm in diameter). By contrast, the micrograph of the low surface area sample (fig. 2) exhibits large particles (ca. 100 nm in diamter) in various shapes (squares. rectangles, ovals) and some very small particles (< 10 nm in diameter). When molybdenum chloride enters the flame, it reacts with water or oxygen leading to the formation of very small initial particles of M a by nucleation from the vapor phase. Following the nucleation, the growth process takes place in the hot zone either by the diffusion of the condensing species to the particle surface and condensation on this surface or by the collision between initial particles and coalescence of those particles. In both cases, the size of the oxide increases with the partial pressure of the chloride and the residence time of the "active species" in the flame which are respectively controlled by the mass velocity of MoCl5 and by the total flow rate of gases. The residence time is also affected by the temperature of the flame which determines the reaction hot zone. Therefore, high surface area MOO3 powders are preferentially obtained with a cold flame, at low MoC15 mass velocity and at high total flow rate. However, the surface area of samples issued from the flame reactor greatly decreases as a result of the standard treatment. The larger the initial surface area, the more important its decrease (Table 1, columns 5 and 6). This is particularly obvious for the Mo03-A sample prepared in a cold flame since its surface area is reduced more than fourfold. The samples directly issued from the flame reactor are composed of two crystalline phases : the orthorhombic phase and a metastable polymorphic h-Mo03 phase described by Kihlborg (ref. 9). This metastable phase is completely transformed into the orthorhombic phase as a result of the standard treatment. In this work, hydrogen bronzes were prepared from MOO3 having retained a large surface area (2 30 m2g-1) after the standard treatment. (c~mpare M a - D with M a - B and M a - C ) . 2-&pgaa 'on of bronzes PtiHXMo03 The insertion of hydrogen within finely divided M a 3 coated with 0.1 to 1 wt Z Pt leads to the formation of bronzes HxMo03 whose composition depends upon the reduction temperature and duration. The bronzes thus formed have been characterized by volumetric measurements and by XRD analyses. The results obtained for a 0.2 7% Pt-Mo03 catalyst are summarized in table 2. 683 TABLE 2 Formation of bronzes 0.2 8 â¬'t/I-IxM@ 263 298 298 323 433 473 573 30 Ho.34Md3 120 Ho.9Md3 400 1.44 m.9M003 + H l . m d 3 240 1.60 H0.9Md3 + H l . m d 3 100 1.67 H0.9Md3 + H1.6Md3 150 1.86 amorphous 800 2.10 amorphous * : x values determined volumetrically at equilibrium compositions. Initial H2 pressure = 760 Tom. > In z w I- c z i b l 10 20 30 40 50 60 TWO-THETA (DEGREES) Fig. 3 - X-ray diffractograms of 0.2 % Pt/HxMoOg reduced at 323 K (A), 433 K (B), 473 K (C) and 573 K @). Reduction duration for each sample is indicated in Table 2. In diffractograms A and B, the main peaks are those of H0.9M003 ; the other peaks (indicated by arrows) belong to the H 1 .6M003 phase. 684 The X-ray diffractograms of samples reduced at 323 K (A) and at 433 K (B) show the main peaks of the H0.9Mo03 phase together with some other peaks that may be indexed into the Hl.gMo03 phase. For those samples, hydrogen contents determined by volumetric measurements correspond to x values ranging from 1.44 to 1.67 (Table 2, column 3). Therefore, discrepancies exist between volumetric measurements and XRD analyses for bronzes reduced at temperatures in the range 298 - 433 K. This can be explained by the unstability of H1.6MoO3 bronze with respect to oxidation at ambient temperature whereas for low hydrogen contents (xd 0.9) the process is very slow (ref. 10). A partial transformation of H1@lm into m.9MoO3 is very likely to occur because of exposure to air during the transfer to the X-ray diffiactometer. This is corroborated by the color change in samples from bordeaux-red to dark blue. Reduction at temperatures above ca. 433 K allows the formation of large hydrogen content bronzes (x = 1.86 - 2.10, Table 2, column 3) that are amorphous with respect to XRD (fig. 3, spectres C and D). The surface area of crystalline bronzes is about 30 m2g-1 whereas the amorphous sample reduced at 573 K (last line) exhibits a surface area of 20 m2g-l . Electron micrographs of the bronzes Pt/HxMo03 show the presence of small Pt particles of 1 to 3 nm in diameter, homogeneously dispersed on the support. 3 - F'reparan 'on of neat HrMo03 We have used three methods to prepare molybdenum bronzes without deposited metal. In one of the methods, MoO3 is contacted with Pt/Al2O3 and exposed to H2 at 433 K for 24 h in a "reactor with an elevator" already described (ref. 7). The Pt/Al2O3 is then removed from the reactor by a windlass device (ref. 7). The bronze H1.6Mo03 thus obtained has about the same surface area as that of the MOO3 host sample. In an other method, finely divided MoO3 is placed in a dynamic differential reactor under a flow of allylic alcohol (20 Torr) and H2 (740 Torr) at 323 K for 20 h ; H0.34Mo03 is formed. If the carrier gas in the flow is N2 instead of H2, the same bronze is produced at ca. 373 K only. In both cases, the bronze obtained has almost the same surface area as that of the starting oxide. Thus, MoOg in a fmely divided state is capable of extracting hydrogen atoms from the allylic alcohol molecule at low temperatures. As yet, we have no clear explanation for the beneficial effect of H2 in this process. Another possibility of preparing neat HxMoOg bronzes at low temperature results from the photosentive properties of highly divided MoO3. Indeed, upon UV-illumination at room temperature a suspension of MoOg in methanol or in 2-propanol turned rapidly to a deep dark blue color. The X- ray diffractograms of samples show the presence of m.34MoO3. By use of a mixture of MoO3 and T i02 instead of MoO3 alone, the bronze H0.9M003 is formed as indicated by the X-ray diffractogram of the centrifugated solid and by its deep slate-blue color. The bronzes formed under UV-illumination retain the high division state of the starting materials. 685 4 - Potential catalytic applications The bronzes Pt/HxMo03 may be considered to some extent as bimetallic catalysts (ref. 2). The Pt metal particles on the surface may be expected to interact with the modified oxide and, under reaction conditions, fast spillover/reverx spillover of hydrogen between metal and support is expected. Those remarkable characteristics suggest the possibility of unusual interesting catalytic applications, particularly for very finely divided bronzes. Indeed, current investigations in our laboratory show that they are good and stable catalysts for the hydrogenation of various compounds (ethylene, acetylene, allylic alcohol, carbon monoxide, etc.). In the case of neat HxMo03 bronzes, one of us has shown, for the first time, that Hi.jMoO3 can catalyse the hydrogenation of ethylene by molecular hydrogen without the presence of metallic particles (ref. 5). The bronze H1.6Mo03 is therefore capable of activating molecular hydrogen without modifications of its structure under the reaction conditions. The initial rate of hydrogenation of ethylene at 433 K on H1.6Mo03 was close to that previously observed on 0.5 % Pt/H1.6MoO3 (ref. 2). The bronze H09Mo03 was observed to be active in the isomerization of methylcyclopropane at 353 K (ref. 11). Finally, preliminary experiments show that cinnamaldehyde can be photocatalytically hydrogenated into cinnamyl alcohol by a mechanism of hydrogen transfer from an alcohol, via a bronze HxMo03, to cinnamaldehyde. Under the same conditions, Pt/Ti02, used as a reference bifunctional photocatalyst, leads to the saturation of the C=C bond. CONCLUSION Various methods of synthetizing finely divided bronzes H x M a with or without the presence of metallic activator particles are available. The starting molybdenum trioxide can be prepared in a flame reactor, under conditions where nucleation and growth processes of the oxide particles in the hot zone of the flame are optimized in order to obtain high surface areas (2 30 m2g-I) and a stable structure (orthothombic phase) after a standard dechlorination treatment of Mo03. The hydrogen spillover process can be used in the prepration of either Pt/HxMo03 or H1.@4o03 (by momentarily contacting Mo03 with PdAl2O3 in the presence of H2 in this latter case). Neat HxMoOg (with x 6 0.9) bronzes can be obtained by using an alcohol as a hydrogen source either in the dark (allylic alcohol, H2, 323 K or N2, 373 K) or under UV-illumination (methanol or 2- propanol, room temperature). In this latter case, a mixture of M d 3 and Ti@ allows the formation of Hr3.9Mo03 instead of HO.34M003, which indicates interparticle hydrogen atom transfer. types of reactions. The finely divided hydrogen molybdenum bronzes thus prepared are potential catalysts for several 686 REFERENCES 1 2 3 4 5 6 7 D. Tinet, H. Estrade-Szwarckopf and J.J. Fripiat, Bul. SOC. Fr. Phys., 42 (1981) 28. J.P. Marcq, X. Wispenninckx, G. Poncelet, D. Keravis and J.J. Fripiat, J. Catal., 73 (1982) 309 and references cited therein. P.A. Sermon and G.C. Bond, Catal. Rev., 8 (1973) 211. S.D. Jackson, B.J. Brandreth and D. Winstanley, Appl. Catal., 27 (1986) 325. R. Benali, C. Hoang-Van and P. Vergnon, Bull. SOC. Chim. Fr., (1985) 417. M. Formenti, F. Juillet, P. Meriaudeau, S.J. Teichner and P. Vergnon, J. Colloid Interf. Sci., 39 (1972) 79. D. Maret, G.M. Pajonk and S.J. Teichner, in G.M. Pajonk, S.J. Teichner and J.E. Germain (Eds), Proc. Int. Symp. Spillover of Adsorbed Species, Lyon-Villeurbanne, September 12- 16, 1983, Elsevier, Amsterdam. 1983, p. 215. B. P o d e r , F. Juillet and S.J. Teichner, Bull. SOC. Chim. Fr., (1972) 1268. L. Kihlborg, Acta. Chem. Scand., 13 (1959) 954. J.J. Birtill and P.G. Dickds, f.?hâ¬H !&ate Chem., 29 (1979) 367. 8 9 10 11 Inpreparation. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 687 NEW PREPARATION OF SUPPORTED METALS. HYDROGENATION OF NITRILES M. BLANCHARD, 3. BARRAULT and A. DEROUAULT Ldborato i re de Catalyse en Chimie Organique URA CNRS 350 40, Avenue du Recteur Pineau 86022 POITIERS Ckdex (France) SUMMARY An a l k y l aluminium i s g ra f ted on a s o l i d by r e a c t i o n w i t h the hydroxy surface groups. By add i t i on o f a s o l u t i o n o f a metal s a l t , a surface reac- t i o n occurs which produces the metal and an aluminium s a l t . This alumi- nium s a l t i s hydrolysed t o alumina. The f i n a l s t a t e i s a supported metal surrounded by surface dlumina. Nickel , coba l t , copper c a t a l y s t s supported on oxides o r ac t i va ted carbon are prepared fo l l ow ing t h i s procedure and are used f o r the l i q u i d phase hydrogenation o f long chain n i t r i l e s . Their a c t i v i t i e s and selec- t i v i t i e s towards the formation o f amines (primary, secondary, t e r t i a r y ) dre compared w i t h the one obtained w i t h conventional ca ta l ys ts . INTRODUCTION During an e a r l i e r i n v e s t i g a t i o n i n t o CO hydrogenation (1,2) we were l e d t o prepare h igh l y d i v ided metals i n dn organic so lvent i n order t o ob ta in a s l u r r y which had t o be s tab le wi thout any annealing o f the meta l p a r t i c l e s . For t h i s purpose a m e t a l l i c s a l t i s reduced a t room temperature by an aluminium d k y l i n homogeneous phase, and the s l u r r y i s formed by heat ing t h i s s o l u t i o n up t o 2OO0C under a stream o f synthesis gas : the water which forms, i n s i t u , by the reduct ion o f the cdrbon monoxide i s used f o r the hyd ro l ys i s o f the aluminium s a l t i n t o dlumina ( 3 ) . We wish t o r e p o r t t h a t a s i m i l a r procedure can be used t o prepare supported ca ta l ys ts . One o f the advdntages o f these supported c a t a l y s t s i s t h e i r a b i l i t y t o s e t t l e very e a s i l y from the l i q u i d phase, and another one i s the p o s s i b i l i t y o f changing the support and therefore the metal d ispers ion, the metal-support i n t e r a c t i o n s and the acid-base p roper t i es o f the ca ta l ys t . Long chain n i t r i l e s were chosen as model compounds because t h e i r reduct ion leads t o primary, secondary or t e r t i a r y f a t t y amines. I t i s thus i n t e r e s t i n g t o study the s e l e c t i v i t y o f the c a t a l y s t s towards the production o f one c lass o f these compounds. EXPERIMENTAL 11 C a t a l y s t p r e p a r a t i o n I n d t y p i c a l exper iment, d sample o f powdered alumind (Rhone Poulenc GFSC 59, p a r t i c l e s average didmeter 0.1 mm) i s d r i e d under vacuum d t room temperdture d u r i n g 7 hours. I t i s then t r d n s f e r r e d , under dn A r atmosphere i n the r e a c t i o n f l a s k dnd 120 m l o f d ry benzene dre added. 2.5 m l o f A1Et3 a re i n t roduced s l o w l y w h i l e the suspension i s coo led d t 0°C. The redc- t i o n w i t h the su r fdce OH groups occurs immedidtety and ds sooti dS the d ischdrge O f gdS Stops, (ethdne, e thy lene ) d S o l u t i o n O f d ry CO (acac)2 i n benzene ( l g Co and 100 m l benzene) i s added t o the s l u r r y which t u r n s t o b l a c k immediately. Th i s r e a c t i o n i s f o l l owed by the d d d i t i o n o f 2.59 o f bu tad iene i n 10 m i o f benzene. The s l u r r y i s then t r d n s f e r r e d i n t o the hydrogenat ion r e a c t o r by medns o f dn d i r - t i g h t sy r i nge . Dur ing a l l these opera t i ons the apparatus are mdin td ined under an dtmosphere o f i n e r t gas ( A r ) . Th i s gds i s then rep ldced by d stream o f syn thes i s gas (CO:H2=1:2), 60 m l o f dodecane are added and the benzene is d i s t i l l e d . The temperature i s p r o g r e s s i v e l y inc reased (12OC per hou r ) up t o 190°C and as soon d s the produc ts o f t h e r e d u c t i o n o f CO appear i n t h e gas phase (ma in l y CH4) t h e syn thes i s gas i s rep ldced by pure hydrogen dnd the s l u r r y i s kep t d t t h i s temperature d u r i n g 15 hours. A f t e r t h i s p re t rea tment , t h e temperdture i s b rought down t o the d e s i r e d exper imen td l va lue . The sdme procedure i s used f o r t he p r e p d r d t i o n o f o t h e r meta ls (N i , C u ) d ispersed on t h e same alumina dnd o f c o b a l t d i spe rsed on d i f f e r e n t suppor ts : S i l i c d - A l u m i n a (KET3EN LA 3P), z i n c ox ide (CRAM) dnd cdrbon (LONZA-HSAG3OO). 21 C d t d l y t i c hydrogenat ion A l l t h e r e a c t i o n s were C a r r i e d o u t i n d 250 m l s t a t i c r e d c t o r under dtmospher ic p ressure w i t h d cont inuous f l o w o f hydrogen and a t temperatures rang ing f rom 50°C t o 120°C i n dn apparatus descr ibed p r e v i o u s l y ( 3 ) . A c t i v i t y dnd s e l e c t i v i t y va lues were ob ta ined by gds phase chromdto- graphy a n a l y s i s o f t he s o l u t i o n on a Cp S i l 5 c a p i l l a r y column. 689 RESULTS The c a t a l y t i c hydrogenat ion o f l o n g c h d i n n i t r i l e s was c d r r i e d o u t i n l i q u i d phase on va r ious c a t a l y s t s i n o rde r t o check t h e i r a c t i v i t y dnd t h e i r s e l e c t i v i t y towards t h e p r o d u c t i o n o f m i n e s . Besides t h e fundd- menta l aspect o f t h i s r e d c t i o n i t i s wor thwh i l e t o o b t a i n d c d t d l y s t se lec - t i v e f o r one c l a s s o f m i n e (p r imary , seconddry or t e r t i a r y ) because these compounds a re i n v o l v e d i n the p r e p a r a t i o n o f va ludb le p roduc ts . 11 E f f e c t o f t h e method o f p repdrd t i on . I n t h e f i g u r e 2 t h e r e s u l t s o f t h e r e d u c t i o n o f CI1HZ3CN are r e p o r t e d f o r two c o b d l t c a t a l y s t s a t 12Ooc. One i s prepared by a conven t iona l method which i n v o l v e s t h e impregnat ion o f an alumina suppor t w i t h a s o l u t i o n o f c o b d l t n i t r a t e fo l l owed by d r e d u c t i o n w i t h hydrogen a t 400OC. The o t h e r i s p repdred as p r e v i o u s l y descr ibed. 80- 60.. 40.- 20- 5 5 10 15 Time (hrs) Fig. 1 O u r c a t a l y s t i s l e s s a c t i v e f o r t h e p r o d u c t i o n o f t h e p r imary m i n e , b u t i t i s more s e l e c t i v e f o r t h e p roduc t i on o f t h e seconddry m i n e . Th is f i r s t r e s u l t shows t h a t t h e conven t iona l c a t a l y s t i s d good reduc ing dgent ; the o t h e r type o f c o b a l t c a t a l y s t i s l e s s a c t i v e f o r t he r e d u c t i o n 690 o f t h e - C Z N t r i p l e bond b u t more e f f i c i e n t f o r t h e r e a c t i o n o f t h e imine R-CH=NH w i t h t h e pr imary m i n e R-CH2NH2 which produces t h e seconddry amine (R-CH2)2NH. Morever t h e t e r t i a r y amine is n e v e r o b s e r v e d w i t h t h i s l a s t C d t d l y S t . 2/ E f f e c t of t h e s u p p o r t . The method p r e v i o u s l y d e s c r i b e d f o r t h e s u r f a c e r e d u c t i o n o f C o ( a c a c ) 2 was used f o r t h e p r e p a r a t i o n o f v a r i o u s s u p p o r t e d c a t a l y s t s . I n t h e f i g u r e 2 , t h e i r s e l e c t i v i t y towards t h e p r o d u c t i o n o f t h e s e c o n d a r y m i n e is p l o t t e d v s t h e time o f r e a c t i o n . These a r e p r a c t i c a l l y t h e same a t 12OoC, e x c e p t f o r t h e one which is p r e p d r e d on ZnO. I t is i n t e r e s t i n g t o n o t e t h a t t h i s s u p p o r t is a l s o t h e less a c i d i c one and t h e r e f o r e is n o t t h e b e s t f o r t h e r e a c t i o n between t h e i m i n e and t h e pr imary dmine. Y % (R-CH,),NH Y 1 % (R-CH,),NH l oo~L - = r/ec + - - < 80. .- - - - Co-Si0,-Al,O, co-c Co- ZnO +- Co-AI,O, .- rt 5 10 15 20 T i m (hn) Fig. 2 rt 5 10 15 20 T i m (hn) Fig. 2 69 1 3 / E f f e c t o f metal The a c t i v i t i e s and s e l e c t i v i t i e s of t h r e e metals are r e p o r t e d i n t h e t a b l e s 1 a n d 2. T h e s e c d t d l y s t s dre less d c t i v e t h d n t h e c o n v e n t i o n d l one b u t t h e o r d e r o f r e a c t i v i t y is t h e same. I t is w o r t h m e n t i o n i n g t h a t t h e s e n i c k e l dnd c o b d l t c a t a l y s t s a re more s e l e c t i v e t o w d r d s t h e p r o d u c t i o n o f s e c o n d a r y d m i n e s t h d n t h e c o n v e n t i o n d l n i c k e l a n d c o b d l t c d t d l y s t s p r e p d r e d by i m p r e g n d t i o n . T h i s means t h a t t h e c o n d e n s a t i o n o f t h e p r i m d r y m i n e w i t h t h e i m i n e a n d t h e s u b s e q u e n t h y d r o g e n d t i o n o f t h e d d d u c t dre f d s t e r t h a n t h e r e d u c t i o n of t h e n i t r i l e . I n c o m p d r i s o n w i t h n i c k e l and c o b a l t , c o p p e r shows a c o n s i d e r d b l y l o w e r c d t d l y t i c a c t i v i t y i n t h e h y d r o g e n d t i o n o f n i t r i l e s a n d t h e d i f f e r e n c e s a re much more i m p o r t a n t t h a n w i t h c o n v e n t i o n d l c d t d l y s t s (N i /Co /Cu ; 20/ 1 0 / 1 (4,5). M o r e o v e r , e v e n w i t h c o p p e r c d t d l y s t s , t h e t e r t i d r y drnine is n o t f o r m e d . TABLE 1 E f f e c t o f t h e m e t a l on t h e h y d r o g e n a t i o n o f l d u r o n i t r i l e T = 12OoC, pH2 z 1 atm. TABLE 2 I n i t i d l h y d r o g e n a t i o n r a t e o f l d u r o n i t r i l e on metal - Al2o3 c a t d l y s t s (mole h-â . g -1 4 x 1 0 ) T = 120°C, pH2 = 1 d t m . ~~ N i co c u 1 0 0 50 1 692 DISCUSSION O f RESULTS As f a r ds the p r e p d r d t i o n o f the c d t d l y s t i s concerned the f o l l o w i n g su r fdce r e a c t i o n s may occur, by dndlogy w i t h those which hdve been s t u d i e d when the r e d u c t i o n i s c a r r i e d o u t i n homogeneous phdse. The f i r s t s t e p i s t h e r e a c t i o n o f t he d l k y l a luminium w i th the su r fdce -OH groups o f t he s o l i d suppor t (A1203, Si02-A1203, ZnO, C) : E t + A1Et3 ->-O-Al ' + EtH 2 5 0 C ( s u r f . ) ' E t -OHsur f . Th is i s d w e l l known process which i s used f o r t he t i t r d t i o n o f su r - fdce hydroxy groups o f s o l i d s . The second s t e p i s t h e r e d u c t i o n o f Co(dcac)Z by the g r d f t e d reduc ing dgent. I t i s supposed thd t t h i s r e a c t i o n occurs i n the v i c i n i t y o f t he A 1 dtom. f rom p r e l i m i n d r y r e p o r t (3,6) i t i s known t h d t t h i s r e d u c t i o n g i v e s r i s e t o ve ry s m a l l p d r t i c l e s o f c o b a l t , t oge the r w i t h the fo rmd t ion o f Al(dCdC)3 dnd the d ischdrge O f ethdne dnd e thy lene. The t h i r d s t e p i s t he pe t red tment ( d c t i v a t i o n ) o f the c d t d l y s t d t 200°C w i t h syngds ( C O , ZH2). Dur ing t h i s p re t red tment , which l d s t s dbout 15h, t he syngds produces hydrocarbons (C,,, C2, C3 .... ) and &. This w d t r r i s used f o r t he h y d r o l y s i s o f t h e su r fdce Al(dCdC)3. The c d t d l y s t d f t e r t h i s p re t red tmen t i s d suppor ted metd l , surrounded by dlumind ds i t dppedrs from X-ray spec t ra . ( 7 ) AS f d r dS C d t d l y t i C p r o p e r t i e s d re concerned, n i c k e l and Coba l t C d t d - l y s t s rd f l k among those most o f t e n desc r ibed dnd used i n the p roduc t i on o f p r imdry dmines from h i g h e r f d t t y dc ids v i a the hydrogendt ion o f t he cor respond ing n i t r i l e s . I t wds a l s o dssumed i n p rev ious papers t h d t when n i c k e l is depos i ted on d suppor t , o n l y t h e degre o f n i c k e l d i s p e r s i o n i s d f f e c t e d dnd the re i s no SUbStdnt id l mOdif iCdt iOn o f t h e s e l e c t i v i t y ( 5 ) . The r e s u l t s p resented i n t h i s pdper show t h d t b o t h the suppor t and the me td l dnCh0ring l e d d t o impor tdn t chdnges i n n i t r i l e , hydrogen d c t i v d t i o n dnd consequent ly i n d c t i v i t y and s e l e c t i v i t y . I f the r e s u l t s c o n f i r m thdt the seconddry m i n e s e l e c t i v i t y deCredSeS when changing the metd l ( cu > N i > Co), never the less dn impor tan t R NH s e l e c t i v i t y i s r d p i d l y ob ta ined w i t h n i c k e l c a t a l y s t s (see t d b l e 1). Then i t appears t h d t hydrogendt ion proper - t i e s o f me td l s a re m o d i f i e d by alumina spec ies formed d u r i n g syngds d c t i v d - t i o n s tep . The p o s i t i o n dnd t h e l o c d l d e n s i t y o f such spec ies depend upon the i n i t i a l hyd roxy l groups s t r e n g t h dnd r e p d r t i t i o n . Accord ing t o the pdthwdy proposed i n i t i a l l y by BRAUN ( 8 ) dnd s t u d i e d more r e c e n t l y by GREENFIELD (Y), BAIKER (10 ) . 2 H2 > RCH2NH2 RCN f RCH = NH ~ HZ RCH = NH + RCH2NH2 the i nc redse o f seconddry dmine s e l e c t i v i t y shows t h d t t h e r d t e o f redc- t i o n o f t h e i m i n e w i t h t h e pr imdry m i n e ( s t e p 8 ) is h i g h e r than t h e hydro- gena t ion r a t e o f t h e im ine t o p r imdry dmine which i s q u i t e unusual f o r n i c k e l dnd c o b d l t c a t a l y s t s . The dhSenCe o f t e r t i a r y m i n e s which are formed i n t h e r e a c t i o n between p r imdry im ine and secondary m i n e is a l s o unexpected. CH-R RCH = NH + (RCH2)2NH 7 RCH - N ( E ) I ' CH2R NH2 I (RCH2)3N < HZ [RCH=CHN(CH2R)2] and we d re now s tud i yng t h i s p e c u l i a r p r o p e r t y . REFERENCES 1 2 3 4 5 6 7 8 9 10 M. Blanchdrd, D. Vdnhove, F. P e t i t dnd A . Mor t reux , 3. Chem. SOC. Chem. Comm., 1980, 908. D. Vdnhove, M. Bldnchdrd, F. P e t i t dnd A . Mor t reux , Nouv. Jou rnd l Chimie, 1981, 5-4, 205. c. Bechardergue-Lahiche, 5. M d i l l e , P. Cdnesson, M. Bldnchdrd and D. VdnhOVe, P r e p a r a t i o n o f c a t a l y s t s g, 8. Delmon e t d l E d i t o r E l s e v i e r , Amsterddm, 1987, 31, 725. 3. Pdsek, N. K O S t O V d dnd B. Dvordk, C o l l e c t . Czech. Chem. Comm. 1981, 46, 1011. 3. V o l f and 3. Pdsek, C d t a l y t i c hydrogendt ion , L. Cerveny E d i t o r , E l s e v i e r , Amsterdam, 1988, 27, 105. 3. Goma, C. Kdppenstein, B u l l . SOC. Chim. F r . , 1988, 621. H. Derule, Ph. D. Thesis, P o i t i e r s (1989) 3. Brdun, G. B l e s s i n g and F. Lohe l , Chem. Ber., 1923, 36, 1988. H. G r e e n f i e l d . I nd . Eng. Chem., Prod. Res. Develop., 1967, 6, 142. A . Bdiker , 3 . K i j e n s k i , Cd ta l . Rev. Sc i . Eng. 1985, 27-4, 653. This Page Intentionally Left Blank G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 695 PREPARATION OF HIGHLY DISPERSED GOLD ON TITANIUM AND MAGNESIUM OXIDE Susumu TSUBOTA, Masatake HARUTA, Tetsuhiko KOBAYASHI, Government Industrial Research Institute of Osaka Midorigaoka I, IKEDA 563, Japan Atsushi UEDA, and Yoshiko NAKAHARA ABSTRACT Gold c o u l d be highly d i s p e r s e d o n t i t a n i u m o x i d e and magnesium oxide in their aqueous dispersion containing Mg citrate. The mean diameter of gold particles are smaller than 5nm. These gold catalysts are active for the oxidation of CO even at a temperature below 0°C. On magnesia support, Mg citrate acts not as a reducing agent but a s a sticking agent which blocks the coagulation of gold particles. On titania support dispersed in neutral solution Mg2+ ions instead of citrate ions are mainly adsorbed. It is likely that Mg2+ ion suppresses the transformation of amorphous titania to anatase during calcination and prevent gold particles from coagulation caused by earthquake effect. INTRODUCTION Gold has been regarded as catalytically far less active than platinum-group metals. This is because of its chemically inert character and of low dispersion in supported catalysts. We have recently reported that through coprecipitation gold particles smaller than 10 nm can be highly dispersed on c0304, ' a -Fe203, Ni01-3), and Be(OH)24). These gold catalysts are active in the oxidation of CO at a temperature a s low a s -7OOC. However, coprecipitation is valid only for a selected group of metal oxides as mentioned above, because the precipitation rates of support metal hydroxide and gold hydroxides and their affinity might determine in the dispersion of gold. This paper deals with the methods for supporting gold in a highly dispersed state on pre-formed Ti02 and MgO powder, on w h i c h u l t r a f i n e gold p a r t i c l e s have been d i f f i c u l t to be supported by the conventional methods. EXPERIMENTAL Preparation of gold catalysts The following materials were used for catalyst supports; magnesia (Ube Industries,Ltd.; crystalline small particles 696 p r e p a r e d by v a p o r m e t h o d ; B E T = 1 4 0 m 2 / g ) , t i t a n i a - A ( I d e m i t s u K o s a n C o . ; a m o r p h o u s d r i e d a t 1 2 0 ° C ; B E T = l 1 0 m 2 / g ) , a n d t i t a n i a - B ( J R C - T I 0 4 ; a n a t a s e ; B E T = 4 0 m 2 / g ) . E a c h o f t h e s e s u p p o r t s w a s d i s p e r s e d i n a n a q u e o u s s o l u t i o n o f HAuC14. T h e pH o f t i t a n i a d i s p e r s i o n w a s a d j u s t e d t o 7 . 0 w i t h N a 2 C O 3 , w h i l e t h e pH f o r m a g n e s i a d i s p e r s i o n , w h i c h was n o t i n t e n t i o n a l l y a d j u s t e d , was n a t u r a l l y s e t t l e d a t a r o u n d 9 . 6 . T h e a q u e o u s d i s p e r s i o n s were s t i r r e d f o r 2 h r s a f t e r t h e a d d i t i o n o f a v a r i e t y o f r e a g e n t s ( c i t r a t e s o f Mg, Na, o r NH4, o r HCHO; 2 . 5 m o l / A u f o r m a g n e s i a , a n d 6 . O m o l I A u f o r t i t a n i a ) . T h e s e p r e c u r s o r s were w a s h e d w i t h d i s t i l l e d wa te r a n d t h e n f i l t e r e d . T h e c a k e was v a c u u m d r i e d a n d c a l c i n e d i n a i r f o r 5 h r s a t 4 0 0 ° C a n d 2 5 0 ° C f o r T i 0 2 a n d MgO, r e s p e c t i v e l y . T h e g o l d c o n t e n t o f t h e s e c a t a l y s t s t h u s o b t a i n e d were 1 a t . % ( A u / T i ) i n A u l t l t a n i a a n d 2 a t . Z (Au/Mg) i n A u l m a g n e s i a . C a t a l y t i c A c t i v i t y m e a s u r e m e n t s T h e a c t i v i t i e s o f t h e g o l d c a t a l y s t s were m e a s u r e d i n t h e o x i d a t i o n o f CO o r H2. E x p e r i m e n t s were c a r r i e d o u t i n a s m a l l f i x e d b e d r e a c t o r w i t h 0 . 1 0 g o f c a t a l y s t s t h a t h a d p a s s e d t h r o u g h 70 a n d 1 2 0 m e s h s i e v e s . A s t a n d a r d g a s o f 1 . 0 v o l . % H2 o r CO h s l a n c e d w i t h a i r t o 1 atm was p a s s e d t h r o u g h t h e c a t a l y s t b e d a t a f l o w r a t e o f 3 3 m l I r n i n . T h e c o n v e r s i o n o f C O a n d H2 w a s d e t e r m i n e d t h r o u g h g a s c h r o m a t o g r a p h i c a n a l y s e s ( G - 2 8 0 0 , Y a n a g i m o t o Co. L t d . ) o f e f f l u e n t f r o m t h e r e a c t o r . C h a r a c t e r i z a t i o n o f C a t a l y s t s T h e s t r u c t u r e s o f t h e g o l d c a t a l y s t s were o b s e r v e d u s i n g a H i t a c h i H-9000 e l e c t r o n m i c r o s c o p e o p e r a t e d a t 300 kV. X - r a y d i f f r a c t i o n ( X R D ) a n a l y s i s w a s m a d e b y u s i n g a R a d - B s y s t e m ( R i g a k u D e n k i C o . L t d . ) . I n f r a r e d s p e c t r a w e r e t a k e n w i t h a N i c o l e t 20-SXC s p e c t r o m e t e r . F o r t h e I R a n a l y s i s , e a c h s a m p l e was m i x e d w i t h K B r ( 2 w t . % f o r m a g n e s i a ; 10 w t . % f o r t i t a n i a ) , a n d p r e s s e d i n t o a t h i n wafe r . D i f f e r e n t i a l t h e r m a l a n a l y s i s (DTA) was m a d e b y u s i n g a S S C - 5 2 0 0 t h e r m a l a n a l y z e r ( S e i k o D e n s h i K o g y o C o . L t d . ) . X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y ( X P S ) was m e a s u r e d w i t h a SSX-100 s p e c t r o m e t e r ( S u r f a c e S c i e n c e L a b o r a t o r i e s , I n c . ) . RESULTS G o l d s u p p o r t e d o n m a g n e s i a T a b l e 1 s h o w s t h e c a t a l y t i c a c t i v i t i e s o f A u / m a g n e s i a p r e p a r e d w i t h d i f f e r e n t a d d i t i v e s . I t w a s f o u n d t h a t c a t a l y t i c 697 activities were enhanced by the addition o f Mg citrate. When Mg citrate was added into the suspension before the addition of HAuC14, the activity enhancement could not be observed. The use of Na citrate or HCHO caused lower catalytic activity. The pH of the suspension during the preparation, usually 9.6, was increased to 11 when Na citrate was added. The addition of HCHO to the suspension produced a purple color, which indicated the reduction of Au3+ to colloidal gold. Figure 1 shows the XRD patterns of Au/magnesia catalysts, where the presence of Mg(OH)2, not MgO, are evidenced. The starting material, MgO, changed to Mg(OH)2 by hydration in the aqueous suspension. From the width of the XRD peak of Au(200), the particle size of gold is calculated as about 14nm for Au/MgO prepared without additives, and this value is in good agreement with 10 nm determined by TEM observations. On the other hand, in the catalyst prepared with the addition of Mg citrate, gold particles smaller than 3 nm are observed by TEM. Although such very small particles of gold did not show the diffraction peak in XRD, the presence of metallic gold was confirmed by the binding energy of 84.2 eV for the XPS peak of Au4f5/2. The catalyst prepared with the addition of HCHO contained only large gold particles (more than 20nm, by TEM observation). F i g u r e 2 s h o w s t h e I R s p e c t r a o f t h e p r e c u r s o r o f Au/magnesia before calcination. Without Mg citrate, the IR absorptions of surface H20 and MgC03 are observed at 1638cm-1 and 1449cmV1, respectively. In the case of the precursor prepared with Mg citrate, other absorptions are detected at 1595cm-l, 1 4 2 3 ~ m - l ~ 1 2 6 3 ~ m - ~ , 1083cm-l, and 1 0 6 1 ~ m - ~ . These absorption bands coincide with those obtained for pure Mg citrate powder. TABLE 1 Catalytic activity of Au/magnesia prepared with various additives. Additives Cat a1 y tic activity CO conv.,% T1/2[H2],'C none 10 >200 Mg ct. 100 67 Na ct. 5 >200 HCHO 0 >200 CO conv.:CO conversion at -7OOC T1/2:temperature for 50% conversion ct.:citrate 698 20 Fig. 1. XRD patterns of Au/magnesia. (a)prepared with Mg citrate; (b)prepared without Mg citrate. 0 I I I I 2000 1800 1600 1400 1200 1000 WAVENUMBER (cmâ) Fig. 2. IR spectra of Admagnesia before calcination. (a)prepared with Mg citrate; (b)prepared without Mg citrate. (resolution 4cm-1; accumulation lootimes) 699 Gold supported on titania In Table 2, the effect of the addition of Mg citrate on the catalytic activity is compared o n the two different types of Ti02 supports. W h i l e the (amorphous) is enhanced (anatase) shows a high addition o f Mg citrate. Figure 3 shows TEM When Mg citrate is added c a t a l y t i c a c t i v i t y o f Au/titania-A by use of Mg citrate, Au/titania-B catalytic activity regardless of the photographs o f Au/titania catalysts. in the dispersion, the gold particles are highly dispersed on titania-A (the average particle size of gold is about 4nm), and gold particles become larger without Mg citrate. In the case of titania-B, however, the small gold particles are highly dispersed even when Mg citrate was not used. The IR absorption spectra of the precursor o f Au/titania prepared with Mg citrate are shown in Fig. 4. The adsorption band at 1 4 0 0 ~ m - ~ on titania-A might correspond to the adsorbed citrate species. Compared with the case of Au/Mg(OH)2, however, t h e a m o u n t of c i t r a t e s p e c i e s i s much l e s s o n t h e t i t a n i a support. TABLE 2 Catalytic activity of Au/titania prepared with and without Mg citrate, (Comparison of two different titania supports). Catalytic activity Titania Support Addition of Mg citrate none , c Titania-A(amorphous) < O 25 35 139 Titania-B(anatase) < O 35 700 Fig. 3. TEM photograph of Au/titania prepared with or without Mg citrate. (a)with Mg citrate; (b)without Mg citrate, on titania-A (amorphous): (c)with Mg citrate; (d)without Mg citrate, on titania-B (anatase). 701 I I I I 2000 1800 1600 1400 1200 1C WAVENUMBER (cm-1) 0 Fig. 4 . IR spectra of Adtitania prepared with Mg citrate before calcination. (a)on titania-A; (b)on titania-B. (resolution 4cm-I; accumulation 100times) -1 anatase (101) ,.OK- AuiTi (Blank) h,,[CO] 35 "C 20 20 2 8 (4 (c AuiTi Na-cit Tvz[CO] 23pC AuiTi Mg-at T,,[CO] < 0°C 2% ! i I I 5.0m 2.5K F i g . 5. XRD patterns of Au/titania-A prepared with various additives. (a)without any reagent(b1ank); (b)with Mg citrate; (c)with Na citrate, (d)with Mg(N03)~. 702 Table 3 shows the catalytic activities of Au/titania-A prepared w i t h a variety of additives. T h e a p p r e c i a b l e enhancement o f the catalytic activity is observed not only through the addition o f Mg citrate but also through Mg(N03)z addition. Other citrates bring about a slight increase in activity. Similarly to the case o f the magnesia support, HCHO causes the reduction of Au3+ in the suspension of titania-A giving a poor catalytic activity. F i g u r e 5 s h o w s t h e X R D p a t t e r n s o f f o u r k i n d s of Au/titania. The amorphous titania-A is transformed into anatase by calcination, and Mg2+ seems t o suppress this crystallization. The catalytic activity tends to become low with an increase in crystallinity of the support. Figure 6 shows DTA curves for t h e precursors of Au/titania-A before calcination. There is an exothermic peak at around 4 6 0 ° C i n e a c h s i g n a l . T h e s e p e a k s c o r r e s p o n d s t o t h e transformation from amorphous titania t o anatase. It is clear that t h e a d d i t i o n of Mg2+ s h i f t s t h e t e m p e r a t u r e f o r the crystallization toward higher temperature. DISCUSSION It has been demonstrated that Mg citrate plays an important r o l e in the preparation of highly dispersed gold catalysts with Mg(0H)z and Ti02 as supports. Fig. 6. DTA curves for Au/titania-A before calcination. (a)without any reagent(b1ank); (b)with Na citrate; (c)with Mg(N03)2, (d)with Mg citrate. (heating rate : 5'C/min in air). 703 H o w e v e r , i t h a s a p p e a r e d t h a t t h e r o l e o f Mg c i t r a t e i s d i f f e r e n t b e t w e e n T i 0 2 a n d Mg(OH)2. G o l d s u p p o r t e d o n m a g n e s i a T h e pH o f t h e s u s p e n s i o n o f m a g n e s i a i s 9 . 6 d u r i n g t h e p r e p a r a t i o n . A t s u c h a pH r e g i o n , A u C 1 4 - s h o u l d b e s u f f i c i e n t l y h y d r o l y z e d i n t o A u ( O H ) 3 j u d g i n g f r o m t h e s t a b i l i t y o f g o l d s p e c i e s i n a q u e o u s s o l u t i o n s 5 ) . T h e n t h e h y d r o x i d e o f Au m i g h t b e d e p o s i t e d o n t h e s u r f a c e o f M g ( O H ) 2 b e f o r e t h e a d d i t i v e s a r e i n t r o d u c e d i n t o t h e s u s p e n s i o n . S i n c e t h e p o i n t o f z e r o c h a r g e ( P Z C ) o f M g ( O H ) 2 a p p e a r s a t pH = 1 2 6 ) , t h e p o s i t i v e l y c h a r g e d s u r f a c e i s s u i t a b l e f o r t h e a d s o r p t i o n o f a n i o n s s u c h a s c i t r a t e i o n , a s o b s e r v e d i n t h e I R s p e c t r u m . T h e r e d u c t i o n o f Au3+ by HCHO i n t h e s u s p e n s i o n m a d e t h e Au p a r t i c l e s l a r g e a n d l o w e r e d t h e c a t a l y t i c a c t i v i t y ( T a b l e s 1 a n d 3 ) , a n d t h e r e f o r e , t h e r e d u c i n g p o w e r o f c i t r a t e i o n s e e m s n o t t o b e r e l a t e d t o t h e h i g h a c t i v i t y . T h e a d s o r b e d c i t r a t e i o n i s c o n s i d e r e d t o a c t a s a s t i c k i n g r e a g e n t w h i c h c a n b l o c k t h e c o a g u l a t i o n o f g o l d s p e c i e s i n t h e s u s p e n s i o n a n d / o r d u r i n g c a l c i n a t i o n . A s p e c u l a t e d b e h a v i o r o f c i t r a t e i o n i s i l l u s t r a t e d i n F i g . 7 . T h e pH o f t h e s u s p e n s i o n c o n t a i n i n g Na c i t r a t e w a s 11 a n d c l o s e t o t h e PZC o f Mg(OH)2. S i n c e t h e e f f e c t i v e a d s o r p t i o n o f c i t r a t e i o n i s n o t e x p e c t e d a t s u c h a pH r e g i o n , t h e e n h a n c e m e n t o f t h e c a t a l y t i c a c t i v i t y m i g h t n o t b e o b s e r v e d i n Au/Mg(OH)2 p r e p a r e d w i t h Na c i t r a t e . ' MgZt 4 H 2 C 0 0 - C C 0 H ) C O f ' dH2COO- Mg citrate F i g . 7. S p e c u l a t e d b e h a v i o r of Mg c i t r a t e i n aqueous d i s p e r s i o n . 704 Gold supported on titania Since the pH of the suspension of titania is adjusted to 7.0, Au species to be deposited on the support might be Au(OH)3 as in the case of Au/Mg(OH)2. However, since the surface of titania in the suspension was proved to be negatively charged, as was reasonable from the PZC of Ti02 at pH = 4 -66), cations such as Mg2+ should be more easily adsorbed on titania than citrate anion. Figures 4 and 5 show that the addition of Mg citrate or Mg(N03)~ enhances the catalytic activity of Au/titania-A and suppresses the crystal growth o f anatase. On the other hand, when anatase support, titania-B, was used as a starting support, no appreciable effect of Mg citrate addition was observed. The liability of the surface of the carrier will accelerate the coagulation of supported species due to the so called "earthquake effect". The presence of Mg2+ prevents amorphous titania-A from crystallization and therefore from the "earthquake effect". On the other hand, since anatase support has already a crystalline s t r u c t u r e , t h e r e i s n o a p p r e c i a b l e e f f e c t of Mg c i t r a t e addition on the dispersion of gold. A small amount of citrate species is detectable on titania- A in Fig. 6. The citrates other than Mg salt might have also a c e r t a i n e f f e c t o n the i n c r e a s e i n c a t a l y t i c a c t i v i t y of Au/titania-A. It is probable that citrate ions in Au/titania-A play similar role to that in Au/Mg(OH)2. Acknowledgment We would like to thank Mr. M. Genet (UCL, Belgium) for the analyses of X-ray photoelectron spectroscopy. REFERENCES 1) M. Haruta, N. Yamada, T. Kobayashi, and S.Iijima, J.Catal., 115, 301-309 (1989). 2) M. H a r u t a , H . K a g e y a m a , N. K a m i j o , T. K o b a y a s h i , and 3) M.Haruta, T. K o b a y a s h i , S. I i j i m a , and F. D e l l a n n a y , 4 ) M. Haruta, K.Saika, T. Kobayashi, S. Tsubota, and Y. Nakahara, 5) R . J . P u d d e p h a t t , T h e C h e m i s t r y of G o l d , E l s e v i e r , 6) G . A. Parks, Chem. Rev. 65, 177-198 (1965). F. Delannay, Stud. Surf. Sci. Catal. 44, 33-42 (1988). Proc. 9th Intern. Congr. Catal., 3, 1206-1213 (1988). Chem. Express, 3, 159-162 (1988). Amsterdam, 1978, p.91. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 705 PREPARATION OF MONODISPERSE COLLOIDAL R-Re02 PARTICLES USING MICROEMULSIONS A. CLAERBOUT and J. B.NAGY Laboratoire de Catalyse, Facultes Universitaires Notre-Dame de la Paix, 61, rue de Bruxelles, B-5000 Namur (Belgium) ABSTRACT Two microemulsions composed of Cetyltrimethylammonium bromide- Hexanol-Water or Pentaethyleneglycoldodecylether-Hexane-Water were used to obtain monodisperse Pt, Re02 and Pt-Re02 particles by reducing K2PtC14, H2PtC16 and NaReOq with hydrazine. The aggregation of the particles is governed by the surface charge of the particles and by the adsorption of the surfactant molecules on the monodisperse particles. The dependance of the particle size on the precursor concentration is explained by the specific location of the precursor salt in the inner water cores of the microemulsion. INTRODUCTION Monodisperse particles present the advantage of uniform active site distribution and can be considered as models for heterogeneous catalytic reactions. Monodisperse metals, metal oxides or metal borides can now be easily obtained using microemulsions, vesicles, polymers or normal micelles (refs. 1-4). Microemulsions were used to obtain monodisperse particles of platinum (refs. 5-7), palladium (refs. 5,6), rhodium (refs. 5,6), iridium (ref. 5) and gold (ref. 8) by reducing the precursor metal ions with hydrogen, hydrazine, sodium borohydride or solvated electrons. Monodisperse nickel boride (refs. 1,9-12), cobalt boride (refs. 1,10,13-17), nickel-cobalt boride (refs. 1 ,lO,lS-17), and mixtures of iron boride and iron oxides (refs. 1,18) were prepared by sodium borohydride reduction of the precursor metal ions. Iron oxides (ref. 19), magnetite (ref. 20), calcium carbonate (ref. 21) and silver chloride (ref. 22) were obtained by precipitation reactions. On the other hand, bimetallic catalysts (ref. 23) including Pt-Re pair become important in catalysis, because the activity and the selectivity of the two metals are strongly influenced by their dispersion in the alloyed particles. More recently, dispersed metals deposited on dispersed oxides have been reported to possess a rather high activity (ref. 24). 706 x CTAB y Hexenol z n,o moles of N m hms In the present paper, we report the formation of monodisperse Pt, Re02 and Pt-Re02 particles using microemulsions. The precursor ions are dissolved in most of the cases in the dispersed water phase ( inner water cores ), the organic medium forming the continuous phase. The so-obtained catalysts can be used for hydrogenation and/or CO oxidation reactions. EXPERIMENTAL M a t e r i a l s The commercial products PEGDE (Fluka, > 98 %), CTAB (Serva, 99 Yo), n- hexane (Merck,P.A), hydrazine hydrate (Fluka, > 99 Yo), sodium perrhenate (Alfa, 99,95 Yo), potassium tetrachloroplatinate (Alfa, 99,9 O/.) and hexachloroplatinic acid (Alfa,P.A) were used without further purification. Preparation of the particles The monodisperse Pt particles were prepared by reducing with hydrazine at room temperature K2PtC14 or H2PtC16 dissolved in two microemulsions : cetyltrimethylammonium bromide (CTAB) 30 wt Yo - Hexanol 50 % - Water 20 Yo and pentaethyleneglycol dodecylether (PEGDE) 9.5 % - Hexane 90 % - Water 0.5 %. The monodisperse Re02 particles were prepared only in the second microemulsion by reducing NaRe04 with hydrazine. The Pt-Re02 systems were obtained from a constant metal ion concentration (0.1 molal) varying the [Pt]/([Pt]+[Re]) ratio from 0 to 1. Fig.1 illustrates the preparation scheme of the catalysts (ref. 9). The synthesis was carried out in a glove box under argon atmosphere to prevent oxidation of the particles. Fig. 1. Experimental procedure for the preparation of monodisperse particles. 707 Electron microscopy The average size of the particles was measured using a Philips EM 301 electron microscope in the transmission mode. For these measurements the particles were dispersed in n-butanol using ultrasound and deposited on grids covered with Formvar. RESULTS AND DISCUSSION Monodisperse platinum particles from CTAB-Hexanol- Water microemulsion using H2PtC16 and K2PtC14 The monodisperse Pt particles prepared from H2PtC16 dissolved in the CTAB-Hexanol-Water microemulsion have an average size of 40f5 A and their size is not dependent on the H2PtC16 concentration (ref. 7). The aqueous solution of hydrazine containing a ten-fold molar excess of hydrazine with respect to H2PtC16 had an initial pH = 10. The metal particle precursor, in this case, is dissolved in both the dispersed inner water core and the continuous organic (or hexanol) phases. It has been supposed, that the nucleation occurs in both phases, and hence, the particle size is only dependent on their stabilization by the adsorbed surfactant molecules (refs. 1,7). This is verified in the concentration range of H2PtC16 from 5 x 10-3 to 5 x 10-2 molal with respect to water. If K2PtC14 is used, instead, as particle precursor (for the same hydrazine to K2PtCI4 excess), a complex behaviour is observed as a function of the pH. In the low pH region (1 < pH c 4), no Pt particles could be obtained. At 5 c pH c 8 , dispersed Pt particles are formed but the reduction could not be carried out until completion during 24 h. For the high pH region (pH > 9) complete reduction of the Pt-salt occurs, but the so-obtained particles are highly aggregated. It is clear that the surface charge does influence the aggregation of the metal particles. In addition, the adsorption of the surfactant molecules, also pH dependent, can also greatly influence the particle aggregation (refs. 25, 26). Monodisperse platinum particles from PEGDE-Hexane- Water microemulsion In order to avoid the latter phenomenon, a neutral surfactant PEGDE is used to form the microemulsion of composition PEGDE 9.5 % - Hexane 90 Yo - Water 0.5 Yo. Only K2PtC14 as precursor salt is tested, however, because it is insoluble in the organic medium. Its concentration was varied from 1 x 10-3 to 3 x 10-1 molal with respect to water. 708 [K2PtC14] n PtC142- rnolal/H20 a,b 0.001 0.54 0.01 5.4 0.05 27.0 0.1 54.0 0.3 162.0 Fig. 2 illustrates the Pt particles size obtained at high and low initial K2PtC14 concentrations. The particles are not aggregated and their size is quite uniform. m d Wt W Nn Nn/NM PI( Fx102 k = l ( A ) x103(g) ~1019(g) XIO-16 x i 0 2 C d ,c d 1 5 f 3 0.98 0.38 2.6 0.47 0.238 2.0 25-+3 9.8 1.75 5.6 0.99 0.995 1.0 5 0 f 5 49.0 1 4 3.5 0.63 1 0.6 90k10 98.0 81.9 1.2 0.22 1 0.2 130-+15 294.0 2 4 7 1.2 0.22 1 0.2 Fig. 2. TEM photographs of monodisperse Pt particles prepared from PEGDE 9.5% - Hexane 90% - Water 0.5% containing 0.1 (A) or 001 (B) molal K2PtC14 Table 1 and Fig. 3 show the variation of the size as a function of K2PtC14 concentration. The standard deviation is small in all cases studied. The particle size increases monotonously with increasing K2 PtC14 concentration and approaches a plateau at high concentrations. This behaviour seems to be different from those previously observed for the Pt particles formation using H2PtC16 dissolved in CTAB-Hexanol-Water and for the Ni2B particles obtained from the same microemulsion. In the first case, a constant particle size is obtained irrespective of the initial H2PtC16 concentration (refs. 1,7), while in the second case a minimum was observed in the curve particle size of Ni2B as a function of NiC12 concentration (refs. 1,9,10). 709 0 0.1 0.2 0.3 [KzPtClq] M I H20 Fig. 3. Variation of the Pt particles size as a function of the initial K2PtC14 concentration with respect to water The observed minimum in the curve was adequately explained, provided a critical number of Ni(ll) ions is assumed for the formation of one nucleus. This number was determined to be equal to 2 for the formation of Ni2B and C02B (refs. 1,9,10,15,16). The number of nuclei (Nn) formed by inner water core (NM) is determined as follows Wt Nn =-- W where Wt is the total weight of the catalysts prepared per kilogram of microemulsion and w is the weight of one particle. Knowing the volumic mass and the size of the particles, w is easily computed (refs. 1,10,17). Table 1 shows the different values obtained for different Pt concentrations. The average number of inner water core per kilogram of microemulsion is computed from the total volume of water (Vt) and the size of the inner water core (rM) (refs. 1,10,17,27) : V t 4/3 n: ( rM)3 NM = 710 In the present case, the literature value of ca. 60 8, is taken as the mean radius of the inner water core, a value which was obtained from light diffusion measurements (refs. 28, 29). Finally from the total K2PtC14 concentration, the number of Pt(ll) per water core (n P t C l ~ l ~ - ) is easily calculated (Table 1). As the distribution of the PtC142-ions in the microemulsion follows the Poisson statistics, the probability to have k Pt atoms per water core (pk) is given by : where k is one integer and h = n PtC142- m Table 2 shows the comptuded values for z p k . The initial value of k is equal to 1 because it is necessary to have,at least, one PtCl42-ion in one water core to obtain the formation of one surfactant stabilized Pt atom, which is considered as the nucleus of the Pt particle. k= 1 Indeed, it can be shown, that : m where F is a scaling factor and x p k gives the probability to have one or more k= 1 PtC142- ions per inner water core. Nn hnn 00 Figure 4 shows the variation of Z p k and of - as a function of K2PtC14 k= 1 concentration. 711 0 . 9 0 . 1 0 . 2 0 . 3 0 . 4 KptClponcentrntlon (rnolallwster) Nn NM m Fig.4. Variation of x p k and - as a function of K2PtC14 concentration. k= 1 For low initial K2PtC14 concentration (up to 0.01 molal with respect to water),- increases as well as C p k as a function of Pt concentration. This behaviour was already found in the case of the Ni2B and Co2B Nn particles (refs. 1,10,17). However, for higher Pt-concentrations, the - NM ratio decreases, leading to larger particles. Note, that this ratio is between 10-3 and 10-2, showing that every hundred or every thousand of inner water cores leads to the formation of Pt particles. This is also expressed by the scaling factor F, where a maximum variations of ten fold is observed. If however, a critical number of initial PtC142- ions higher than 1 is supposed, the variation of the so-computed F becomes larger. This analysis reinforces the hypothesis, that one surfactant-stabilized Pt atom is able to initiate the final Pt particle. m Nn NM k=l For higher initial K2PtC14 concentrat ion , the number of nuclei per inner water core decreases. This behaviour was not observed previously for Ni2B and C02B particles. It is not clear, at present, why this reduction of - occurs in the PEGDE-Hexane-Water microemulsion. A more systematic study is necessary to shed some light on the influence of the nature of the surfactant molecules, the mobility of the interface and the influence of hydrazine concentrations. Nn NM 712 [NaReOq] molal/H20 0.01 0.05 0.1 0.3 1 .o Monodisperse Re02 particles The monodisperse Re02 particles were obtained by reducing NaReOq with hydrazine in the system PEGDE-Hexane-Water. The presence of Re02 is confirmed by XPS experiments. Fig. 5 shows the monodisperse Re02 particles for two different initial N a Re04 concentrations. d 0 ( A ) ( A ) 1 8 2 27 3 3 1 4 42 5 5 5 6 Fig. 5. TEM photographs of monodisperse Re02 particles from PEGDE 9.5%- Hexane 90%-Water 0.5% containing 0.3 (A) or 0.005 (B) molal of NaReO4 Table 2 and Fig. 6 illustrate the variation of the particle size as a function of NaReOq concentrations. a.PEGDE 9,5 wt % - n-Hexane 90 % - H20 0.5 %. 713 60 70 c - 50 5 k Z 40 30 i m E a $! 20 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 [NaReOgl M / H20 Fig. 6. Variation of the Re02 particles size as a function of initial NaReOq concentration Once again the size of the monodisperse particles approaches a plateau for high NaReOq concentrations and this behaviour is quite similar to that of the Pt particles. However, a similar quantitative analysis for the Re02particles could not be carried out because NaReOq is only partially reduced in our experimental conditions (refs. 30,31). Monodisperse Pt-Re02 particles Monodisperse Pt-Re02particles were prepared from the PEGDE-Hexane -Water rnicroernulsion using a total ion concentration [K2PtC14]+[NaRe04] = 0.10 molal with respect to water. The monodispersity of the particles is illustrated in Fig. 7. Fig.7. TEM photographs of monodisperse Pt-Re02 particles prepared from PEGDE 9.5 wt % - n-Hexane 90 % - H 2 0 0,5 % containing [K2PtC14] + [NaRe04] = 0.10 rnolal with respect to water 714 mole fraction x of K2PtC14 Table 3 and Fig. 8 show the variation of the particle size as a function of the mole fraction x of KzPtCI4. d ( A ) P t Re@ Pt-Re02 f c ) ( C ) TABLE 3: Variation of the monodisperse Pt-Re02 particles size as a function of the mole fraction (x) of K2PtC14alb 0 0.16 0.33 0.5 0.66 0.8 30 31+3 30 - 29 25+3 - 35 - 28 24+3 50 .., 27 27f3 70 - 22 25+2 - 80 - 20 38+4 a. b. C. PEGDE 9.5 wt % - n-Hexane 90 % - H20 0.5% [K2PtC14] + [NaRe04] = 0.10 molal with respect to water. Hypothetical particle size estimated in the case where the system would contain pure Pt or Re02 particles. 5 0- 0 0.2 0.4 0.6 0.8 1 Molar ratio (x ) Fig. 8. Variation of the Pt-Re02 particles size as a function of mole fraction x of KzPtC14 ([K2PtC14] + [NaReOs] = 0.10 molal with respect to water) It is surprising, that up to x = 0.7, the diameter of the particles remains quasi constant and is close to that of the pure Re02 particles. For higher initial K2PtC14, the diameter of the particles increases monotonously to reach that of the pure Pt particles. The quasi constancy of the particle diameter for low K2PtC14 concentration suggests, that in that 715 region of concentration, the Pt is dispersed on the Re02 particles. Indeed, the slight decrease of the size could be due to the decrease of the particle size of the Re02 particles as it can be seen on the Fig. 5. For high K2PtC14 content, the reverse situation could occur, i.e. the dispersion of Re02 particles on the larger Pt particles. This hypothesis will be later checked by STEM measurements. All these results are different from those one could expect on the basis of a mechanical mixture. Indeed, in that case a bimodal distribution is expected at least for x 2 0.5, based on the different size of the separate Pt and Re02 particles. Table 3 also includes the hypothetical separate particles estimated from Figs 4 and 5. This comparison makes clear, that the presence of Re02 induces a higher dispersion of the Pt-Re02 particles for x > 0.7. Presently, experiments are carried out to deposit these particles on a support and their stabilisation is systematically studied to prevent them from sintering. CONCLUSIONS Monodisperse Pt, Re02 and Pt-Re02 particles are easily prepared by reducing with hydrazine the precusor salts dissolved in the inner water cores of PEGDE -Hexane - Water microemulsions. The Pt and Re02 particles size increases with increasing precursor salt concentration and approaches a plateau at high concentrations. At high initial Pt concentration in the K2PtC14 - NaReOq system, the presence of Re02 deposited on the Pt particles seems to impede the increase of Pt - particles size. REFERENCES 1 . J. B.Nagy, E.G. Derouane, N. Lufimpadio, I. Ravet and J.P. Verfaillie in K.L. Mittal (Ed), Surfactants in Solution, Vol 10, 2 . J.H. Fendler, Chem.Rev., 87 (1987) 877-899. 3 . M. Haruta and B. Delmon, J.Chem.Phys., 83 (1986) 859-868. 4 . T. Sugimoto, Adv.Colloid Interface Sci., 28 (1987) 65-108. 5 . M. Boutonnet, J. Kizling, P. Stenius and G. Maire, Colloids and Surfaces3 6 . M. Boutonnet, J. Kizling, V. Mintsa-Eya , A. Choplin, R. Touroude, G. Maire and P. Stenius, J.Catal., 103 (1987) 95-104. 7 . A. Whatelet, Memoire de Licence, Facultes Universitaires, Namur, Belgium, 1984. 8 . K. Kurihara, J. Kizling, P. Stenius and J.H. Fendler, J.Am.Chem.Soc., 105 9 . J.B. Nagy, A. Gourgue and E.G. Derouane, Stud.Surf.Sci.Catal., 16 (1983) Plenum, New-York, 1989, pp. 1-43. (1982) 209-225. (1 983) 2574-2579. 193-202. 716 10. J.B. Nagy, Colloids and Surfaces,35 (1989) 201-220. 11. D. Rosier, J.L. Dallons, G. Jannes and J.P. Puttemans, Acta Chim. Hung.124 (1987) 57-64. 12. G. Jannes, J.P. Puttemans and P. Vanderwegen, Catalysis Today, 5 13. I. Ravet, N.B. Lufimpadio, A. Gourgue and J.B. Nagy, Acta Chim.Hung.,llS 14. I . Ravet, A. Gourgue,Z. Gabelica and J.B. Nagy, Proc.8th Int.Congress on (1 989) 265-272. (1 985) 155-1 66. Catalysis, Berlin West, July 2-6, 1984, Vol IV, Verlag Chemie, Weinheim-Basel, 1984, pp. 871 -878. 15; I. Ravet, J.B. Nagy and E.G. Derouane, Stud.Surf.Sci.Catal., 31 (1987) 16. I. Ravet, A. Gourgue, and J.B. Nagy in K.L. Mittal and P. Bothorel (Ed), Surfactants in Solution, Vo1.5, Plenum, New-York, 1987, pp. 697-71 2. 17. J.B. Nagy, I. Bodart-Ravet and E.G. Derouane, Faraday Discuss. ChemSoc., 18. N.B. Lufimpadio, J.B. Nagy and E.G Derouane, in K.L. Mittal and B. Lindman (Eds), Surfactants in Solution, Vol. 3, Plenum, New-York, 1983, 19. V.R. Palkar, M.S. Multani and P. Ayyub, in K.L. Mittal (Ed.), Surfactants in Solution, Vol 10, Plenum, New-York, 1989, pp. 293-295. 20. M. Gobe, K. Kon-no,K. Kandori and A. Kitahara, J.Colloid Interface Sci., 21. K. Kandori, K. Kon-no, A. Kitahara, M. Fujiwara and T. Tamaru, in K.L. Mittal (Ed), Surfactants in Solution, Vol.10, Plenum, New-York, 1989, 22. R. Leung, M.J. Hou,C. Manohar,D.O. Shah and P.W. Chun, in D.O.Shah (Ed.),Macro- and Microemulsions, ACS Symposium Series 272, American Chemical Society, Washington D.C.,1985, pp. 325-344. 23. H. Charcosset, Int. Chem.lng., 23 (1983) 187-212. 24. A. Baiker, Faraday Discuss. Chem. SOC., 87 (1989) 239-251. 25. J. Kiwi, K. Kalyasundaram and M. Gratzel, Stuctrure and Bonding , Springer, Berlin, 1982, pp. 39-1 25 26. I . Bodart-Ravet, Ph.D Thesis, Namur, 1988. 27. J.B. Nagy, I. Bodart - Ravet, E.G. Derouane, A. Gourgue and J.P. Verfaillie, Colloids Surfaces, 36 (1989) 229-261. 28. S. Friberg and I . Lapczynska, Progr. Colloid and Polymer Sci., 56 (1976) 29. S. Friberg, I. Lapczynska and G. Gillberg, J. Colloid Interface Sci, 56 30. P. Dormont, Memoire de licence, Namur 1990. 31. A. Claerbout, Ph. D. thesis, Namur, in preparation. 505-51 7 . 87 (1989) 189-198. pp.1483-1493 93 (1983) 253-263. pp. 253-262. 16-20. (1 976),19-32. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 717 1991 Elsevier Science Publishers R.V., Amsterdam - Printed in The Netherlands NEW ORGANOMETALLIC ACTIVE S I T E S OBTAINED BY CONTROLLED SURFACE REACTION OF ORGANOMETALLIC COMPLEXES WITH SUPPORTED METAL PARTICLES B. DIDILLON (11, A. EL MANSOUR (11, J.P. CANDY ( 1 1 , ,J.M. BASSET ( l ) , F. LE PELTIER (21, and J.P. BOURNONVILLE ( 2 ) ( 1 ) I R C , 2 avenue A l b e r t E i n s t e i n , 69626 Vi l leurbanne, FRANCE ( 2 ) IFP, BP 311, 92506 Rueil-Malmaison cPdex, FRANCE ABSTRACT The c o n t r o l l e d su r face r e a c t i o n o f an o rganometa l l i c compound, such as t e t r a b u t y l t i n , w i t h hydrogen covered rhodium p a r t i c l e s supported on s i l i c a leads t o a ve ry w e l l def ined s u p e r f i c i a l o rganometa l l i c species. These s u p e r f i c i a l organometal l i c species are cha rac te r i zed by rhod ium- t i n bonds and con ta ins b u t y l r a d i c a l s s t i l l l i n k e d t o t i n atoms. As t h e temperature o f t h e hydrogen thermal t reatment i s increased, b u t y l r a d i c a l s a re p r o g r e s s i v e l y e l im ina ted l ead ing a t l a s t t o t h e fo rma t ion o f b u l k y rhod ium- t i n a l l o y p a r t i c l e s . The presence o f t hese b u t y l r a d i c a l s a t t h e su r face o f t h e m e t a l l i c p a r t i c l e s induces an i nc rease n o t o n l y i n s e l e c t i v i t y b u t a l s o i n a c t i v i t y rega rd ing t h e hydrogenat ion o f t h e carbonyl f u n c t i o n o f an unsaturated aldehyde such as c i t r a l . INTRODUCTION Supported group V I I I meta ls c a t a l y s t s are a c t i v e t o promote numerous r e a c t i o n s b u t they a r e n ' t enough s e l e c t i v e , ma in l y when p o l y f u n c t i o n n a l subs t ra tes have t o be t ransformed ( 1 ) . The c o n t r o l l e d a d d i t i o n o f a m e t a l l i c promotor, v i a t h e r e a c t i o n o f organometa l l ic t i n compounds w i t h a group V I I I metal supported on s i l i c a can lead t o a new genera t i on o f b i m e t a l l i c c a t a l y s t s , which e x h i b i t unusual a c t i v i t y and s e l e c t i v i t y (2 -4) . For example, b i m e t a l l i c c a t a l y s t s Rh-Sn/Si02 (5,6) and more r e c e n t l y Ni-Sn/Si02 and Ru-Sn/Si02 (7 -9) present h i g h s e l e c t i v i t y and a c t i v i t y i n t h e hydrogenat ion o f e t h y l - a c e t a t e t o ethanol . I n t e r e s t i n g l y , t i n add i t i on not on l y suppresses the m u l t i p l e hydrogenolysis of C-C and C-0 bonds, but a l so enhances the r a t e o f alcohol formation. This improvement i n s e l e c t i v i t y can be i n te rp re ted by t he s u p e r f i c i a l d i l u t i o n of group V I I I metal by i n a c t i v e metal atoms o f t i n (10) . Otherwise, t o account f o r t he increase i n a c t i v i t y , t h e mod i f i ca t i on o f t h e nature o f t he a c t i v e s i t e has been considered, bu t no t p rec i se l y described (11) . Furthermore, i f the o v e r a l l composition o f t he m e t a l l i c p a r t i c l e s i s mastered and known, the misunderstanding o f t he s u p e r f i c i a l s t ruc tu re and o f t h e nature o f t he a c t i v e phase hinders t o e t a b l i s h accurate r e l a t i o n s w i t h t h e c a t a l y t i c p roper t i es . I n t h i s paper we in tend t o show how the reac t i on between an organometal l ic complex ( t e t r a b u t y l t i n ) and the sur face o f supported m e t a l l i c p a r t i c l e s (Rh/Si02) proceeds, as we l l as the evo lu t i on of t he precursor species i n the course o f thermal ac t i va t i on . A l l t he successive stages o f t he genesis o f t he a c t i v e phase have been studied. This knowledge allowed t o s t a b i l i z e very we l l def ined s u p e r f i c i a l organometal l ic complexes RhSn(n-C4H9)x which have been character ized by temperature programmed r e a c t i o n (T.P.R. 1, i n f r a - r e d spectroscopy and e l e c t r o n microscopy. The hydrogenation o f unsatured aldehydes has been used as t e s t react ion. EXPERIMENTAL 2 S i l i c a (Degussa Aeros i l , 200 m /g) was used as the support ma te r ia l . Conventional e lec t ron microscopy (CTEM) was performed on a Jeol 100 C X microscope. It was used t o determine the p a r t i c l e s s i ze o f both supported monometall ic and b i m e t a l l i c ca ta l ys ts . Scanning Transmission Elect ron Microscope (STEM) HB 5 f rom Vacuum Generator was used t o character ize the b i m e t a l l i c ca ta l ys ts . I n f r a r e d spectra were obtained w i t h a N i c o l e t 10 MX-1 Four ie r t ransform instrument. It was used t o character ize the a l k y l groups evo lu t i on on t he surface dur ing reac t i on between Rh/Si02 and Sn(n-C4H9)4. Preparation o f t he monometall ic ca ta l ys ts Rhodium supported on s i l i c a i s prepared by c a t i o n i c exchange between (RhC1(NH3)5)2C ions and surface ( $ S i - O ) - (NHq)' groups i n amnonia s o l u t i o n a t pH 10. The surface complex obtained by t h i s route, (sSiO-)2(RhC1(NH3)5)2+ i s decomposed by c a l c i n a t i o n a t 573 K i n f l ow ing d r y a i r and then reduced i n f l ow ing hydrogen a t 573 K. The treatment w i t h d r y a i r a t 300 K gives c a t a l y s t A which contains 1 w t % o f rhodium. 719 Prepara t i on o f t h e b i m e t a l l i c spec ies I n t e r a c t i o n between Sn(n-CqHg)4 and Rh/Si02 c a t a l y s t was performed i n a c losed vessel . A g i ven amount ( t y p i c a l y 0,3 g ) of o x i d i z e d monometa l l i c sample A Rh203/Si02 i s reduced under hydrogen a t 623 K ( c a t a l y s t 6 ) and i s o l a t e d under 20 KPa hydrogen a t room temperature. The requested amount o f t e t r a b u t y l t i n Sn(n-C4H9)4 (Sn/Rh = 1 ) i s then i n t r o d u c e d w i t h o u t any s o l v e n t and t h e vessel i s heated by inc rements of 50 K up t o 573 K. For each temperature, ma in ta ined f o r a p e r i o d of 30 inn, q u a n t i t a t i v e a n a l y s i s o f t h e gas phase i s c a r r i e d o u t by vo lumet ry and mass spec t romet ry . The butane amount evo lved g i ves access by d i f f e r e n c e t o t h e number of b u t y l groups s t a y i n g on t h e sur face . A b lank exper iment c a r r i e d o u t on s i l i c a i n d i c a t e s t h a t i n s i m i l a r exper imenta l c o n d i t i o n s no r e a c t i o n occurs between Sn(n-C4H9)4 and t h e sur face . C a t a l y t i c t e s t s Hydrogenat ion o f c i t r a l i s performed i n au toc lave a t 340K i n l i q u i d phase under hydrogen. The c a t a l y s t i s i n t roduced i n t h e au toc lave under argon, w i t h o u t con tac t w i t h a i r . The argon i s removed by f l o w i n g hydrogen, then a s o l u t i o n o f 0.9 m l o f c i t r a l and 0 .4 m l o f t e t radecane i n 10 m l o f n-heptane i s i n t roduced under hydrogen. The hydrogen pressure i s r a i s e d t o 7.6 MPa. The k i n e t i c s o f t h e r e a c t i o n i s f o l l o w e d by chromatographic a n a l y s i s o f t h e l i q u i d phase. RESULTS C h a r a c t e r i z a t i o n o f t h e c a t a l y s t s The m e t a l l i c phase of t h e monometal l ic c a t a l y s t p recu rso r ( c a t a l y s t A ) has been cha rac te r i zed . The average p a r t i c l e s i z e and t h e d i s t r i b u t i o n o f t h e p a r t i c l e s i z e has been determined by E l e c t r o n Microscopy ( C T E M ) . As r e p o r t e d i n t h e f i g u r e l A , t h e d i s t r i b u t i o n o f t h e p a r t i c l e s i z e i s narrow, i n t h e range of 1-2 nm, wi th an average p a r t i c l e s i z e c l o s e t o 1,5 nm. Th is va lue i s i n good agreement w i t h t h e chemisorp t ion r e s u l t s a l ready pub l i shed ( 1 2 ) . 720 100 150 140 120 a80 m 100 z 2 60 cn 80 z - 50 i= YI U J 40 20 0 z 2 0 U 1 1 8 2 2 5 3 3 5 4 4 . 6 PARTICLES SIZE (am) F i g u r e 1 : P a r t i c l e s s i z e d i s t r i b u t i o n o f c a t a l y s t s A (Rh/Si02) (A) and C2 (RhSn(n-C4Hg)2/Si02) ( 2 ) . The temperature c o n t r o l l e d r e a c t i o n o f t e t r a b u t y l t i n w i t h rhodium p a r t i c l e s , a l lowed us t o f o l l o w t h e e v o l u t i o n o f t h e n a t u r e o f t h e s u p e r f i c i a l o rganob ime ta l l i c complex as shown i n f i g u r e 2 . C,/Sa c E 5 4- H 3 - a 2- 1- 0 rn a m 373 473 573 TEMPERATURE (I0 F i g u r e 2 : Butane e v o l u t i o n d u r i n g temperature c o n t r o l l e d i n t e r a c t i o n between Sn ( n-C4Hg) and Rh-H/Si 02. Below 323 K no r e a c t i o n occurs between t e t r a b u t y l t i n and hydrogen adsorbed on rhodium p a r t i c l e s . A t 323 K t e t r a b u t y l t i n begins t o r e a c t w i t h adsorbed hydrogen. Th is r e a c t i o n leads t o t h e f o r m a t i o n o f rhod ium- t i n bond and t o t h e evolvement o f one molecu le o f butane. A t t h i s stage 3 b u t y l groups remain l i n k e d t o t h e t i n atom ( c a t a l y s t Cl). A second b u t y l group i s removed a t 373 K ( c a t a l y s t C2). The fo rmu la o f t h e s u p e r f i c i a l o rganob ime ta l l i c spec ies o f t h e c a t a l y s t C2 can be descr ibed as f o l l o w i n g : Si02- RhS - Sn (nC4HgI2 721 The removal o f t h e two remaining groups, a f t e r hea t ing up t o 473 K , l ead t o t h e fo rma t ion o f a rhodium t i n b i m e t a l l i c p a r t i c l e s ( c a t a l y s t D ) . D i f f e r e n t samples (A,C2, D ) have been i s o l a t e d and cha rac te r i zed by i n f r a - r e d spectroscopy and e l e c t r o n microscopy (CTEM and STEM) ( f i g u r e s 3 and 4 ) . I 3200 2800 2400 2000 1800 1600 1400 1200 WAVENUMBER (em-') F igu re 3 : I n f r a r e d spec t ra o f Sn(n-C4H9I4 (A), Rh-H/Si02 (13). RhSn(n-C4H9)2/Si02 (C), RhSn/Si02 ( D ) . The f i g u r e 3 presents t h e i n f r a - r e d spec t ra o f : - t e t r a b u t y l t i n ( f i g u r e 3A) - Rh-H/Si02 - c a t a l y s t A ( f i g u r e 36) - Rh-Sn(nC4H9)2/Si02 - c a t a l y s t C2 ( f i g u r e 3C) - Rh-Sn/Si02 - c a t a l y s t D ( f i g u r e 3D). On t h e spectrum o f t h e pure t e t r a b u t y l t i n , t h e t y p i c a l wavenumbers o f V(C-H) and 8 (C-H) band o f b u t y l groups are e a s i l y i d e n t i f i e d i n t h e range 2800-3000 cm-' and i n t h e range 1200-1600 cm-'. Whi le no such bands a re detected on t h e spec t ra o f c a t a l y s t s A and D, t h e y are detected on t h e spectrum o f c a t a l y s t C 2 . These observat ions con f i rm t h a t t h e rhodium p a r t i c l e s are covered by d i b u t y l t i n f ragment a f t e r r e a c t i o n o f t e t r a b u t y l t i n w i t h rhodium p a r t i c l e s a t 373 K under hydrogen. 722 The c a t a l y s t C2 has a l so been character ized by e l e c t r o n microscopy (CTEM, STEM). The t i n anchoring on rhodium p a r t i c l e s broadens the p a r t i c l e s i ze d i s t r i b u t i o n and s h i f t s t h e mean p a r t i c l e s i ze towards higher p a r t i c l e s i ze i n comparison w i t h the monometall ic rhodium c a t a l y s t : from 1-5 nm t o 2.0 nm ( f i g u r e 1B). Moreover the shape o f the p a r t i c l e s have changed from spher ica l (monometall ic rhodium p a r t i c l e s ) t o f l a t t e r and bordered-less-contrasted p a r t i c l e s (Rh-Sn(nC4H9)2/Si 02). The STEM analys is ( f i g u r e 4) i nd i ca ted t h a t t i n i s never alone on t h e c a r r i e r : i t i s always associated w i t h the rhodium. The s ignals corresponding kcV Figure 4 : STEM analys is o f c a t a l y s t C2 RhSn(n-C4H9)2/Si02. C a t a l y t i c p roper t i es A l l t he ca ta l ys ts have been tes ted i n the s e l e c t i v e hydrogenation o f c i t r a l . This molecule s u i t s very we l l t o the study o f t h e i n f l uence o f t he nature and t h e s t r u c t u r e o f t h e a c t i v e phase on i t s c a t a l y t i c proper t ies, because i t inc ludes th ree k inds o f unsaturations : ( 1 ) an aldehydic funct ion, (2) a conjugated o l e f i n i c bond and (3 ) an i s o l a t e d o l e f i n i c bond. Moreover, rhodium o r p la t inum supported on s i l i c a are n o t se lec t i ve f o r t he hydrogenation o f c i t r a l t o d i o l e f i n i c a lcohols (geranio l and n e r o l ) (13, 14). The o v e r a l l r e a c t i o n path f o r reduct ion o f c i t r a l t rans i s represented i n f i g u r e 5. Depending on t h e s e l e c t i v i t y o f t he f i r s t hydrogenation step, three 723 d i f f e r e n t p roduc ts cou ld be ob ta ined : g e r a n i o l , t h e d imety l -3 ,7 octene 2 a1 and t h e c i t r o n e l 1 a1 . trans DIMETHYL-3.7 DIMETHYL-3,7 OCTENE-2 AL OCTANAL CITRAL TRANS GERANIOL CITRONELLAL A+B A b0 DIMETHYL 3,7 trans DIMETHY L - 3,7 OCTENE-2 OL / OCTANoL A+C C CITRONELLOL F i g u r e 5 : React ion scheme o f c i t r a l ( t r a n s ) c a t a l y t i c hydrogenat ion . I n t h e f i g u r e s 6A, 68 and 6C, t h e v a r i a t i o n s o f t h e concen t ra t i ons o f t h e d i f f e r e n t p roduc ts , r e s u l t i n g f rom t h e c i t r a l hydrogenat ion, as t h e r e a c t i o n proceeds, a re r e p o r t e d f o r t h e c a t a l y s t A, D and C2. 724 0.30- 0.25 - A 0,05 5 10 15 TIME (hours) F i g u r e 6 : E v o l u t i o n o f t h e produc ts concen t ra t i on d u r i n g c i t r a l hydrogenat ion ca ta l ysed by Rh/Si02 ( A ) , RhSn/Si02 ( B ) and RhSn(n-C4H9)2/Si02 (C). Hydrogen pressure = 7.6 MPa, T = 340 K, Rh/C i t ra l = 0,005. 725 C a t a l y s t A (Rh/SiOp). The con jugated o l e f i n i c bond i s hydrogenated t o g i v e t h e c i t r o n e l l a l (d imethyl-3,7, octene 6 - a l l . Then, t h e i s o l a t e d o l e f i n i c bond o f t h e c i t r o n e l l a l i s p a r t i a l l y and s l o w l y hydrogenated t o g i v e t h e sa tu red aldehyde (dimethyl-3,7 o c t a n a l ) . The fo rma t ion o f t h e sa tu red a l coho l i s v e r y l ow whatever t h e convers ion of t h e d i o l e f i n i c aldehyde. C a t a l y s t 0 (RhSn/Si02). A t f i r s t , con jugated o l e f i n i c band and carbony l group a re hydrogenated i n p a r a l l e l t o g i v e c i t r o n e l l a l and d i o l e f i n i c a l coho l , which appear as p r imary p roduc ts . Then, they a r e hydrogenated ma in l y i n t o o l e f i n i c a l coho l c i t r o n e l l o l (d imethyl-3,7 octene-6 011, t h e sa tu red a l coho l (d imethyl-3,7, oc tano l 1 ) be ing de tec ted i n a smal l amount. When t h e c i t r a l i s f u l l y consumed, t h e c i t r o n e l l o l (d imethy l -3 ,7 octene-6 01) hydrogenat ion beg ins l e a d i n g t o an i nc rease i n t h e r a t e o f fo rmat ion o f t h e sa tu ra ted a l coho l . L a s t l y , o n l y two produc ts a re de tec ted a f t e r 20 hours o f r e a c t i o n : t h e d imethy l -3 ,7 oc tano l 1 (80 % ) and t h e c i t r o n e l l o l (20 % ) . C a t a l y s t C2 (RhSn(n-C4H9)2/Si02). I n t h i s case, t h e a ldehyd ic f u n c t i o n hydrogenat ion i s ve ry s e l e c t i v e even a t v e r y h igh conversion. A ve ry smal l amount o f c i t r o n e l l a l i s de tec ted i n t h e e a r l y t i m e o f r e a c t i o n and d isappears as t h e r e a c t i o n proceeds. No produc t f rom c i t r o n e l l a l hydrogenat ion has been de tec ted owing t o t h e accuracy o f t h e a n a l y s i s . DISCUSSION The c o n t r o l l e d m o d i f i c a t i o n o f t h e monometa l l i c supported rhodium phase has a s t rong i n f l u e n c e on t h e c a t a l y t i c p r o p e r t i e s , a c t i v i t y and s e l e c t i v i t y : - Rhodium a lone a f f e c t s ma in l y t h e o l e f i n i c bonds and a t f i r s t t h e con jugated one. - The rhodium t i n a l l o y fo rma t ion i nc reases t h e r a t e o f c i t r a l t rans fo rma t ion . The two con jugated unsatura ted carbon-carbon and carbon-oxygen bonds are f i r s t l y a f f e c t e d : t h e carbon-carbon one be ing more f a s t l y hydrogenated. When one o f t h e con jugated unsatured carbon-carbon and carbon-oxygen bonds has been hydrogenated, t h e remain ing one i s e a s i l y 726 hydrogenated. Then, t h e l a s t carbon-carbon o l e f i n i c bond i s hydrogenated. - The s u p e r f i c i a l o rganob ime ta l l i c complex a f f e c t s o n l y t h e ca rbony l group lead ing t o s e l e c t i v i t y as h i g h as 96 % f o r g e r a n i o l and n e r o l p roduc t ion , when t h e c i t r a l i s f u l l y converted. Th is c o n t r o l l e d m o d i f i c a t i o n o f t h e s u p e r f i c i a l compos i t ion o f t h e m e t a l l i c a c t i v e phase a l l ows t o master t h e s e l e c t i v i t y i n t h e hydrogenat ion of m u l t i f u n c t i o n n a l compounds. I n t h e case o f c i t r a l , h i g h s e l e c t i v i t i e s c o u l d be reached i n t h e p roduc t i on o f : - C i t r o n e l l a 1 when supported rhodium a lone i s used as c a t a l y s t . - C i t r o n e l l o l (3,7 d imethy l octene-6 01) when supported rhodium t i n a l l o y i s used as c a t a l y s t . - Geran io l and n e r o l when t h e s u p e r f i c i a l o r g a n o b i m e t a l l i c complex i s t h e a c t i v e species. Moreover t h e s t a b i l i t y o f t h e s u p e r f i c i a l o r g a n o b i m e t a l l i c complex has been checked. A f t e r r e a c t i o n , t h e two b u t y l groups a re s t i l l p resent : a thermal t rea tment o f t h e used c a t a l y s t , under f l o w i n g hydrogen, up t o 523 K leads t o t h e removal o f two b u t y l groups. The presence o f t i n e i t h e r i n t h e rhodium t i n a l l o y o r i n t h e s u p e r f i c i a l o rganob ime ta l l i c complex a l l ows t h e carbony l f u n c t i o n hydrogenat ion. Tin, which can be cons idered as e l e c t r o p h i l i c , cou ld induce a s p e c i f i c adso rp t i on o f t h e unsatura ted aldehyde by i t s a ldehyd ic group [15), as shown i n t h e f o l l o w i n g scheme f o r t h e o rganob ime ta l l i c spec ies : SCHEME This e l ect roph (16) . l e e f f e c t o f t i n i s w e l l known i n c o o r d i n a t i o n chem s t r y The d i f f e r e n t c a t a l y t i c behaviour between rhodium t i n a l l o y and rhodium t i n (n-C4Hg)2 complex can be i n t e r p r e t e d by a s p e c i f i c po isoning e f f e c t of t i n accord ing t o t h e na tu re o f t h e s u p e r f i c i a l s t r u c t u r e . The t i n po i son ing e f f e c t on t h e hydrogenat ion o f unsaturated carbon-carbon bonds has been w i d e l y proved (17) . I n t h e case o f rhodium t i n a l l o y , t h e t i n d i f f u s i o n i n s i d e t h e m e t a l l i c p a r t i c l e r e s t o r e s s u p e r f i c i a l rhodium atoms ensembles which a re ab le t o hydrogenate carbon-carbon o l e f i n i c bonds. I n t h e case o f t h e o rganob ime ta l l i c s u p e r f i c i a l complex, t h e remain ing b u t y l groups s t a b i l i z e t h e t i n atoms on t h e m e t a l l i c p a r t i c l e s sur face. Then, t h e rhodium p a r t i c l e s coverage by d i b u t y l t i n fragments, i n h i b i t s f u l l y t h e hydrogenat ion o f unsaturated carbon-carbon bonds. Moreover, i t i s p o s s i b l e t h a t b u t y l groups cou ld ac t as an "organic molecular s ieve" , t hus coe rc ing t h e molecular d i f f u s i o n o f t h e reagents t o t h e a c t i v e s i t e s . CONCLUSION The knowledge o f a l l t h e stages o f t h e m o d i f i c a t i o n o f a supported m e t a l l i c phase leads t o t h e genesis o f p e r f e c t l y d e f i n e d s u p e r f i c i a l complex. As a f u n c t i o n o f t h e thermal a c t i v a t i o n procedure, t h e na tu re o f t h i s s u p e r f i c i a l complex i s va ry ing . This e v o l u t i o n o f t h e s u p e r f i c i a l na tu re and s t r u c t u r e o f t h e a c t i v e phase s t r o n g l y af fects t h e c a t a l y t i c p r o p e r t i e s . Moreover, as f a r as we know, i t i s t h e f i r s t t i m e t h a t n o t o n l y t h e na tu re o f a supported o rganob ime ta l l i c a c t i v e species has been cha rac te r i zed but , above a l l , i t s presence has been c o r r e l a t e d w i t h a s t rong increase i n t h e s e l e c t i v i t y f o r a g i ven r e a c t i o n . F i n a l l y , these r e s u l t s i l l u s t r a t e t h e p o s s i b i l i t i e s o f t h e " ta i lor -made" supported m e t a l l i c c a t a l y s i s . REFERENCES 1 G. CORDIER, Y. COLLEUILLE and P. FOUILLOUX. "Catalyse par l e s metaux", CNRS Ed. 1984, Pa r i s . 2 Y . I . YERMAKOV, B.N. KUZNETSOV and V.A. ZAKHAROV. " C a t a l y t i c hydrogenat ion, s tud ies i n su r face sc E lsev ie r , Amsterdam, 27, 459, 1986. 3 J . MARGITFALVI, S . SZmO and F. NAGY. " C a t a l y t i c hydrogenat ion, s tud ies i n su r face sc E lsev ie r , Amsterdam, 27, 373, 1986. ence and c a t a l y s i s " , ence and c a t a l y s i s " , 4 US Patent 4.380.673. US Patent 4.456.775. US Patent 4.504.593. US Patent 4.628.130. 5 Ch. TRAVERS, t h e s i s ENSPM, P a r i s 1982. 6 Ch. TRAVERS, J.P. BOURNONVILLE and G. MARTINO. "Proc. o f 8 t h I n t e r n a t i o n a l Congress on Ca ta l ys i s " , B e r l i n , West Germany, j u l y 2-6, 1984, Ver lag Chemie Ed. I V , 891-902. 7 O.A. FERRETTI, t h e s i s ENSPM, P a r i s 1986. 8 O.A. FERRETTI, J.P. BOURNONVILLE, J.P. CANDY and G. MARTINO. To be submit ted. 9 P. LOUESSARD, thes i s , Lyon, 1988. 10 A. EL MANSOUR, J.P. CANDY, J.P. BOURNONVILLE, O.A. FERRETTI and J.M. BASSET. Angew. Chem. I n t . Ed. Engl . 28 ( 3 1 , 347, 1989. J.M. BASSET and G. MARTINO. 11 J.P. CANDY, O.A. FERRETTI, G. MABILON, J.P. BOURNONVILLE, A. EL MANSOUR, J. Catal . , 112, 210, 1988. J.M. BASSET and G. MARTINO. 12 J.P. CANDY, O.A. FERRETTI, G. MABILON, J.P. BOURNONVILLE, A. EL MANSOUR, J. Catal . , 112, 201, 1988. " C a t a l y t i c hydrogenat ion i n organic synthes is" , Academic Press, New-York 72, 1980. J. Chem. S O C . Chem. Comm., 1729, 1986. J. Catal . , 102, 190, 1986. J. Am. Chem. SOC., 102, 5112, 1980. M. DEKKER, New-York, 20, 109, 1985. 13 P.N. RYLANDER. 14 S. GALVAGNO, Z. POLTAREWSKI, A. DONATO, G. N E R I and R. PIETROPAOLO. 15 Z. POLTAREWSKI, S. GALVAGNO, R. PIETROPAOLO and P. SAITI. 16 F. CORREA, R. NAKAMURA, R.E. STIMSON, R.L. BURWELL Jr and D.F. SCHRIVER. 17 J . BARBIER, i n "Deac t i va t i on and Poisoning o f Ca ta l ys ts " . G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 729 CONVERSION COATINGS ON STAINLESS STEEL AS MULTIPURPOSE CATALYSTS L. ARIES, A. KOMLA and J.P. TRAVERSE Laboratoire de Recherche sur 1âEnergie Universitk Paul Sabatier 31062 TOULOUSE Cedex - FRANCE - ABSTRACT alloys, supported catalysts can be prepared in one main step from the substrate which furnishes constitutive elements of the coating. The conversion coating is a microporous physically and chemically heterogenous medium with a fractal structure. Variations of the compositional and textural properties are studied against nature of the substrate, duration of the chemical treatment and cgFdi- tions of thermal or chemical post-oxidation treatment. Through a chemical treatmeht of iron-chromium based I. INTRODUCTION An original method for catalyst preparation has been developed. Applications have been found in hydrogenation, coal hydro-liquefaction and automobile emission control ( 1 ) , ( 2 ) . The process involves either anodic oxidation or chemical treatment of iron-chromium based alloys. Supported catalyst can be prepared is one main step from the substrate which furnishes constitutive elements of the coating. Powder catalyst can be obtained from such a coating by separating it from substrate. The paper focusses on preparational aspects. Our purpose is to describe and discuss more particularly the chemical process. 11. BASIS OF WORKING PROCESS AND EXPERIMENTAL Austenitic and ferritic steel sheets or turnings were used. The metal substrate can also be in the form of a conventional ca- talyst (rings, beads, foam, etc . . . ) The compositions are given in Table I. In the chemical treatment the surfaces were prepared by sim- ple dippin2 of the steel into a bath. One of the main conditions of the treatment is the fitting of .the electrode potential of the sample to the value of the natural corrosion potential of the steel ( E o c ) in the active state (Fis.1). This potential must be lower than the primary passive potential of the steel ( E p ) . It is then necessary to control the surface potential during the treat- ment having previously determined the electrochemical characte- 730 ristics of the interface metal solution, by means of polarization curves. TABLE I. Chemical composition of stainless steels (weight per cent) [dc"l ct i i ty : passivity . . . -. . . 4 eoc ; Ep Er, 0 500 Fig.1. Typical anodic polarization curve of stainless steels in sulfuric acid solutions. EOC : natural corrosion potential, Ep : passivity potential, Er : rupture potential. For some alloys, this condition of potential is naturally fulfilled for the treatment baths used. Generally, the potential can be adjusted to the required value by cathodic activation of the surface in the treatment bath with the help of a current ge- nerator and counter electrode playing the part of anode. The ope- ration time was in the region of one minute. The exact time de- pends on the initial oxidation state of the surface to be treated. The coatings were prepared in an acid bath with suitable additives, particularly substances containing chalcogenides. Sulphur seems to give the best results, and it is preferable to put sodium sulphide or sodiumthiosulphate in the bath. It is possible to use very different acids such as sulphuric acid, ni- tric acid and hydrochloric acid. It can be profitable to add a corrosion inhibitor specific to the alloy and the treatment'bath to further control the thick- ness of the coating. This effect can be correlated with the electro-chemical behaviour of the steel in the treatment bath. The plotted polarization curves of the steel show that the addi- 731 Steel tion of propargyl alcohol to the bath brings as large decrease of current density in the active domain. This influence of propargyl alcohol on the anodic behaviour of the steel is characteristic of a corrosion inhibitor effect.' The presence of propargyl alcehel reduces the aggressiveness of the bath and leads to a decreasi in the coat thickness. The compositions of baths used in this part of the study are given in table 11. Table 11. Typical elaboration conditions Sulfuric acid Sulfured species Propargyl alcohol vol % moI 1-1 moI 1-1 NazS203.5H20 Austenitic 1 4 1cd 1 Na2S203.5H20 After the preparation of the conversion coating, the samples were washed with water. They were then dried in an oven at SO'C or dried in ambient air for about 10 minutes. After rinsing, in some cases, the coatings were subjected to chemical oxidation treatment in an aqueous bath or to heat oxidation treatment in air. The bath temperature was generally in the range 4 5 to 60°C. The most easily changeableparameter is the duration of treatment, which was therefore used to modify the characteristics of the coatings. It was varied from a few minutes to up to an hour. Surface characterisation was achieved with different methods : microscopy (SEM and high voltage microscopy), secondary ion mass spectroscopy (SIMS), electron spectroscopy for chemical analysis ( X . P . S . ) . Textural properties were analysed with the followin$ methods : B.E.T., microscopy, impedance electrode mesurements, voltametry . 111. RESULTS AND DISCUSSION 111.1 GENERAL CHARACTERISTICS OF THE CATALYST The conversion coating prepared by the process described is a microporous medium of thickness between 100 nm and few p m . It is composed of three types of particles : metal crystallites of about 100 nm diameter, crystallites of metal compounds, mainly oxides, of about 100 nm diameter and microparticles in a size range of a few nm to 50 nm. It is, in fact, a heterogenous, po- rous medium with a random texture. Its characteristics seem to 5 4 10-3 Na2S.9H20 1.25 103 8.5 732 lead to a fractal type structure. The range of internal similari- ty can, in our coatings, cover a scale of characteristic lengths from a nanometer to several tens of micrometers. Chemically, the microporous material is composed of a mixtu- re of oxides - and in less proportion of sulphides - of the main elements present in thevsubstrate and of the alloy itself. Mino- rity elements are also present as dopants. There is a composition gradient from the support up to the surface where the metallic element are entirely in the combined state. In general by adjus- ting the nature of the substrate alloy and the conditions of treatment it is possible to modify the characteristics of the catalysts. The conversion coating can be subjected to oxidation treat- ments which modify its composition. Heat treatment was performed at temperatures between 150 and 600'C. Oxidation of the layer was also carried out by sAbjecting it to the action of an oxygenated aqueous bath. The chemical modifications are, in this case, res- tricted to the layer itself. 111.2 CHEMICAL COMPOSITION : INFLUENCE OF THE PREPARATION CONDITIONS 111.2.1. Influence of the nature of the substrate. We present the results for thin films prepared from two ty- pical substrates : ferritic and austenitic steel (see table I). The different analyses of the conversion coatings reveal their complex nature. On the one hand we can identifly strata which have a difference in the cohesion and in chemical composi- tions ; on the other hand there are numerous chemical compounds present in various crystallization states. 111.2.1.1. Thin films on ferritic steel Fig.2 shows a tentative phase representation of the typical coating drawn from all the analytical techniques used ( 3 1 . The width of the domain of given phases, at a given deph, is propor- tional to the ratio of the number of metal atoms present in these phases to the total number (only for the main compounds). This scheme allows the relative importance of components to be shown at various depths. There are five domains, and it is possible to distinguish 3 zones according to the depth : the superficial film ( A ) , the external ( B ) and the internal ( C ) zones which together form the deep zone. The thickness of the superficial film may be estimated at 20 nm or thereabout. Its adhesion to the coating is quite weak. The thickness of the deep zone is in the region of 135 nm. This zone is quite adherent. 111.2.1.2. Thin film on austenitic steel The distribution profiles of the elements obtained by SIYS shows that the treatment leads to an enrichment of the layer in nickel. From XPS analysis, the chromium included in the compounds is shokn to be in the form of oxides or hydroxide and the nickel 733 in the form of sulphate, *stalphide and hydroxide. SIMS analysis in the absence of oxygen indicates that the layer is composed of se- veral sublayers corresponding to different degrees of oxidstion of the metallic elements. In the surface layer, iron, chrdmium and nickel are present in their highest state of oxidation. In the first sub-layer C.he oxides contain Fez+ and the levels of Crs+ and Ni2+ are lower than at the surface. In the second sub- layer the metallic phase becomes increasingly preponderant. S ,' COATING P U Ill T C ' STEEL ATOMIC PROPORTION Fiq.2. Representation of the composition of the typical selective coating : the scheme gives the atomic proportion against the depth. Atomic proportion is the ratio between the number of metal atoms in the different compounds to the total number of metal atoms. (A):the superficial film, (B):the external zone (deep zone), (C):the internal Bone (deep zone), 1:domain of Fe3+ and Cr3+ oxide and hydroxide, 1I:domain of C r 3 + substituted magnetite, 1II:domain of metallic iron and chromium (alloy), 1V:domain of metal sulphate(s), V : domain of metal sulphide(s). 111.2.2. Influence of the duration of treatment The variation of treatment time leads, of course, to varia- tion of the thickness of the layer. A more detailed study of the growth of these layers has already been made (4). For example, the distribution profiles (SIMS) of the various components which make up the conversion coating after various treatment times are shown in fiq 3 with the coating steel inter- face at the origin. The ionic intensity of sulphur and oxygen al- ways steadily decreases from the coating surface to metal substrate. However the sulphur concentration reaches a steady mi- nimum much more quickly than oxygen at any time. The chemical treatment of 26CNb17 steel which contains litt- le nickel, leads to enrichment in this element. The proportion of iron and chromium metal in the whole of the conversion coating decreases throughout treatment, indicating an increase in the al- l oy oxidation. In the course of treatment the coating becomes ri- 734 ' = i g . 3 . SINS intensity vs. sputtering time for Fe' , C r ' , Xi+, 0- , S - and C- for the preparation times of 2min, 5min, l0min and 20 rnin. During the treatment the deep zone rapidly thickens, whereas the surface zone remains nearly constant at about 20 nm. A s re- gards the deep zone the distribution of the elements at a given distance from the substrate seems to be independent of the treat- ment duration. The very thin surface layer composition, including the coating solution interface, depends on the duration of the treatment. The oxygen ionic intensity is, at the beginning of treatment, particulary high whereas that of sulphur varies little. 111.2.3. Influence of oxidation treatment 111.2.3.1. Thermal oxidation : ferritic steels The thermal oxidation of conversion coatings on austenitic and ferritic steels has been the object of several publications ( 5 ) . We shall give a brief outline here of the main results ob- 735 tained with ferritic steels. The modifications brought about are different according to wether the oxidation is carried out in air o r under a low oxygen pressure (p02=2.10-2Pa). Up to 400 to 5 0 0 ' C in air, oxidation is limited to the con- version coating itself. It mainly affects the major component i.e substituted magnetite. The oxidation of substituted magnetite du- ring drying leads to the formation of the substituted aFezO3 phase which remains a minor phase. From 15O'C the substituted ma- gnetite becomes transformed according to the reaction : 2Fe2+ (Fe3+z-yCr3+y)04 t 1/202 - > aFez03 t 2(Fei-~Cr~ )203 The phase (Fei-xCrx)203, identified at the surface, is in fact doped by the minority elements from the metal substrate (e.g. the ions Nb5+and Si4+). It presents interestina semi- Above 500'C, oxygen diffuses into the substrate and the ap- pearance of phases such as FeCrz04 is noted. Thermal oxidation not only brings about the major che'mical modifications mentioned but also minor chemical modifications which have important ef- fects on the catalytic activity : possible elimination of certain sulphur-containing compounds, enrichement of the oxidized com- pounds in elements already present in the substrate. Also,'we note crystallisation of the amorphous phases and a modification of particle size. 111.2.3.2. Chemical oxidation : austenitic steels Coatings with characteristics close to those given above, are oxidized in aqueous medium containing Hz02. The oxidation is restricted to the coating. After treatment a 2-layer organisation remains but the total disappearance of iron in the surface layer was noted. Throughout the coating there is a strong decrease in the proportion of sulphides (and sulphates at the surface) as well in that of elements in the metallic state. Ni (OH12 on the other hand increases. The thickness of the coating is hardly modified. It retains good adherence and presents good physicoche- mica1 stability. conducting properties (6). 9 111.3. TEXTURE : IYFLUENCE OF THE PREPARATIOB CONDITIONS 111.3.1. Influence of the type of substrate Here, we compare the thin coatings corresponding to the com- position analyses given in section 111.2.1. The coatings on aus- tenitic steel were thicker than on ferritic steel(about 2 5 % thicker). They were also rougher (Ra=O.45pm) than on ferritic steel (Ra=O.Zym) (steel Z6CBb17). Unlike on ferritic steels, the distribution of the asperities is close to being Gaussian (Fiz 4 ) . Observations with scanning electron microscope show that in all cases the same type of irrezularity can be observed at diffe- rent scales. The coatings therefore present a fractal nature. The diagrams of electrochemical impedance of the coatinzs 736 present a capacitive ar2 characteristic of the charge transfer process at the electrode solution interface and at the very high frequencies, a domain which we attribute to a process of diffu- sion into the pores (fig. 5). L ~ Z m i F i g . 4 . S u r f a c e p r o f i l e of a t y p i c a l c o a t i n g on Z 8 C 1 7 s t a i n l e s s s t e e l . 3,955 H 9960 Hz 0 h\J0 15 2 0 ' 25 F i g . 5 . Impedance d i a g r a m o f a t y p i c a l c o a t i n g on Z 8 C 1 7 s t a i n l e s s s t e e l . E l e c t r o l y t e NazSOI 0 . 1 ?I a t 2 0 ° C . E = - 1 . 2 V / e c s . F r e q u e n c y i s i n Hz The c a p a c i t i v e a r c i s n o t c e n t r e d on t h e r e a l a x i s : t h e an- g l e o f r o t a t i o n 8 , o f t h e a rc a r o u n d i t s h i g h f r e q u e n c y l i m i t d o e s n o t d e p e n d on t h e a p p l i e d e l e c t r o d e p o t e n t i a l . The h i g h f r e - q u e n c y l i m i t , which g i v e s t h e r e s i s t a n c e p e r u n i t area o f t h e e l e c t r o l y t e , R E , is i n d e p e n d e n t o f t h e imposed p o t e n t i a l : RE = 5 +- 1 c m - 2 . T h i s f r e q u e n c y d i s p e r s i o n , a (a=1-28/~), known s i n c e 737 the works of Cole and Cole on dielectrics ( 7 ) indicates the tex- -. tural complexity of this type of coating (physical and/or chemi- cal heterogeneity). It is shown that the transfer arc, whzch is centered in the case of a flat smooth interface, is subjected, through creation of porosity and/or roughness, to rotation around its high-frequency limits. This difference from a smooth interfa- ce is due to the distribution of the response time constant of the system i.e. the'distribution of the current according to a scale law : the electrochemical impedance follows a relationship of the type Z = (jw)-.. To interpret any correlation which may exist between the particular physicochemical texture of certain interfaces and the dispersion factor, a, various authors have introduced a non- dimensional parameter, df,which is prepresentative of the diffe- rence from the ideal s'tuation of a perfectly smooth and homoge- neous surface. F o r an interface presenting internal similarity df should be identified to its fractal dimension. Several largely debated relationships have been proposed to determine df from the angle of rotation, 8 , o r from the dispersion parameter, a, of the capacitive arc. In the present case, we evaluated the complex texture of this type of material by the value of df obtained by the relationship proposed by Le Mehautd et al. (8) for sinkered powder electrodes (eg. sintered nickel) : df = ltl/a. The conversion coatings studied present fractal dimensions which, although close, remain distinct : df = 2 . 2 6 to 2 . 2 7 2 0 . 0 2 for ferritic steel coatings and df 2 . 2 0 ?: 0 . 0 1 5 for austenitic steel coatings. For coatings on both types of steel the shape of the diagram at very high frequency indicated the existence of a process of diffusion into the pores of the coating but the curves have a completely different appearance suggesting the existence of cy- lindrical pores for ferritic coatings and spherical pores for austenitic steels. The geometrical characteristics of the pores can be calculated by means of simplification hypotheses from the characteristic frequencies. The radius of the cylindrical pores (ferritic steels) is from about 10 to 2 0 nm and that of the sphe- rical pores (austenitic steel) about 5 nm. Overall porosity is difficult to evaluate (thin coatins on relatively thick substrate). Indirect measurements suggest values of between 25 and 50% . Measurement of the specific area by the BET method is problematic for the same reasons. Values of about 800m2/m2 were obtained for coatings on alloy ZaC17 compared to 260mz/m2 on the austenitic steel Z3CN1810. 111.3.2. Influence of the duration of treatment As seen before, the thickness of the coating increases v e r y rapidly during the first minutes of treatment, then it becomes steady and proportional to time. The various parameters described above are also modified with time of treatment. It is particular- ly interesting to observe the variation of the fractal dimension (fig 6 ) At the start of treatment the value of df is the same for the three types of steel studied ; this value is characteristic 738 o f t h e o r i g i n a l s tee l s u r ' f a c e and o f i t s o x i d e l a y e r t h a t is f o r - med n a t u r a l l y i n a i r . For v e r y s h o r t t r e a t m e n t t i m e s d i decre-ases and t h e s u r f a c e o f t h e material t e n d s towards a n " i d e a l " s t a t e w i t h a smooth , homogeneous a s p e c t (df = 2 ) . A s t h e t r e a t m e n t t i m e becomes t i m e g r e a t e r t h a 9 4 m i n u t e s , d i i n c r e a s e s and t e n d s t o - wards a maximum v a l u e wich is dependen t on t h e n a t u r e o f t h e s t a r t i n g material : 2 . 2 6 f o r f e r r i t i c s teels a n d 2 . 1 9 f o r a u s t e - n i t i c s t e e l . x 0 5 10 15 20 25 30 35 F i 9 . 6 . V a r i a t i o n o f df a g a i n s t t r e a t m e n t t i m e . (~):Z8C17, (+):Z6CNb17, (*):Z3CN18-10 The s p e c i f i c s u r f a c e area o f t h e c o a t i n g on f e r r i t i c s t e e l w a s s t u d i e d a g a i n s t t r e a t m e n t t i m e by c y c l i c v o l t a m e t r y . The re- s u l t s were conf i rmed by BET measurement. The re w a s a r a p i d i n - c r e a s e i n t h e s p e c i f i c s u r f a c e area from t h e f i r s t m i n u t e s of t r e a t m e n t which l e v e l l e d o u t f o r l o n g e r t r e a t m e n t t i m e s ( > 30 m i n ) . iII.3.3. INFLUENCE OF OXIDATIOK TREXTYENTS 111.3.3.1. H e a t o x i d a t i o n I t s h o u l d f i r s t be n o t e d t h a t t h e d r y i n g c o n d i t i o n s modify t h e parameters s t u d i e d . Drying i n ambien t a i r l e a d s t o q u i t e d i f - f e r e n c e s i n c e r t a i n parameters compared t o t h o s e o b t a i n e d a f t e r d r y i n g a t 9 0 ' i n a i r . A l s o , o x i d a t i o n induced v a r i a t i o n s of t h i c k n e s s , wh ich , from a c e r t a i n t e m p e r a t u r e , a l s o depended on t h e t r e a t m e n t t i m e ( 9 ) . W i t h i n t h e l i m i t s o f t h e c o n d i t i o n s u s e d , o x i d a t i o n t r e a t - ments d i d n o t erase t h e f r a c t a l c h a r a c t e r i s t i c o f t h e mater ia l . The f r a c t a l d imens ion , however, - d e t e r m i n e d from t h e impedance 739 diagrams - was modified. 'Table I11 gives the values obtained for the thermal treatment in air at various times. TABLE 111. Influence of the heat treatment at 400'C in air on the fractal dimensions of the conversion coatings. 2.20 f 0.015 2233 * 0.015 Concerning the porosity, the impedance diagrams show a sphe- ricalization of pore shape : the linear very high frequency range - when it exits - tends towards a pseudo arc. In general, heat treatment in air brings about moreclosed pore shapes. The geome- trical characteristics were evaluated in -certain cases. Concerning the specific surface area measurements carried out by the BET method on conversion layers oxidized by heat at 4OO'C in the air show that lower specific surface areas are obtained. For example, for coatings on austenic steel it is about half : 170 m 2 / m z for oxidized layers against 360 m 2 / m 2 f o r refere-cce layers. 111.3.3.2 Chemical oxidation : layers on austenitic steel The thickness of the layers does not vary appreciably. This is not true for the fractal dimension which vary appreciably with the duration of treatment. The appearance of the very high fre- quency impedance diagrams shows that the treatment greatly modi- fies the porosity of the coating. It seems that the pore shape changes f r o m closed to more open ( cylindrical pores) during the fi,rst minutes ot the treatment but drying in the oven then causes a reverse effect. The specific surface area is seen to be much greater for the oxidized layers compared to the original coatinz. References 1 L.ARIES and J.P.TRAVERSE, Fr. Pat. n"86.18124 (1986) 2 L.ARIES and J.P.TRAVERSE, ~ r . pat. n'88.08102 (1988) PCT/FR 11-89 00295 3 L.ARIES, P.FORT, J.A.FLORES and J.P.TRAVERSE S o l . Energy Materials, 1 4 , (1986), 143-159 4 L.ARIES, D.FRAYSSE, R.CALSOU and J.P. TRAVERSE Thin Solid Fi., 151, (1987), 413-128 5 L.ARIES, R.CALSOC', J.A. FLORES and J.P. TRAVERSE J.X?icrosc. Spectrosc. Electron. 11,(1989), 41-53 ( 6 ) L.ARIES, J.ROY and J.P.TRAVERSE Interfinish PIRIS ( 7 ) R.S.COLE, R.H.COLE, J.Chem. Phy, 9, 1941, 311-351 (8) X.LE YEHAUTE, G.CREPY, Solid State Ionics, 9-10,17,1983 (9) L.I\RIES, Y.EL BAKKOURI, J.ROY and J.P. TRAVERSE, R CALSOU and 1988. Proc. vol I1 713-720 R.SEMPERE. Thin Solid Fi., to be published. This Page Intentionally Left Blank 741 AUTHOR INDEX Absi-Halabi M. Ai M. Alvarez W. Al-Zaid H. Anthony R.G. Aries L. Auroux A. Baiker A. Barrault J. Basset J.M. Beguin F. Bellussi G. Belousov V.M. Bergaya F. Biay I. Blanc B. Blanchard M. B.Nagy J. Bodnir Zs. Bogdanchikova N.E. Bolt P.H. Bonnier J.M. Bournonville J.P. Brooks C.S. Candy J.P. Carati A. Chafik A. Chang Liu Chapple A.P. Chary K.V.R. Claerbout A. Clerici M.G. Cooper M.D. Damon J.P. da Silva Jr A.F. 155 653 77 155 637 729 345 59,239 329,479, 687 717 479 42 1 497 329 1 469 687 705 4.59 647 165 60 1 7 17 557 717 42 1 479 145 407 61 1 705 42 1 247 601 123 Daza L. Decleer J. Dedeycker 0. de Jong K.P. Dekker J.G. Delmon B. Delva A. Derouault A. Dessalces G. Di Castro V. Didillon B. Dosch R.G. Duprez D. Durupty M.C. Dziewiecki Z. El Mansour A: Erre R. Escudey M. Esposito A. Farfan-Torres E.M. Fasman A.B. Fellmann J.D. Fenelonov V.B. Fenoglio R.J. Ferment J. Figueras F. Flores A. Foresti E. Fouilloux P. Frety R. Fuertes A.B. Galiasso R. Gargano M. Gatineau L. Gazzano M. 537 185 337 19 205 537 185 687 1 95 717 637 617 269 113 717 479 279 42 1 337 591 247 503 77 185 345 279 49 469 123 439 37 95 329 49 742 Genoni F. Geus J.W. Gil-Llarnbias F. Gobolos S. Golubkova G.V. Gosling K. Goupil D. Goyvaerts D. Grange P. Grobet P.J. Groen G. Groeneveld M.J. Gros J. Guaregua J. Gui Linlin Haber J. Hamar-Thibault S. Handy B. Haruta M. Hassoun N. Hegediis M. Hiramatsu Y. Hoang-Van C. Hypolite C. lmai H. Imanaka T. Ismagilov Z.R. Ivanov E.Yu. Jackson S.D. Jacobs P.A. Ji Weijie Joud J.C. Kachi N. Kaddouri A. Kalinina O.T. Kalucki K. 43 1 165,527 537 669 59 1 407 469 381 337 355,381 185 185 601 37 69 497 601 239 695 329 669 103 679 1 66 1 103 583 59 1 135 355, 301 517 601 229 575 591 487 Kanai J. Kanta Rao P. Kappenstein C. Kawai M. Kawata N. Keegan M.B.T. Ketterling A.A. Kiennemann A. Kikuchi E. King D.L. Knijff L.M. Kobayashi T. Koeppel R.A. Kolenda F. Komashko G.A. Komla A. Kordulis Ch. Kotter M. Kuno K. Ladavos A.K. Lamers M.D.A. Lapina O.B. Leofanti G. Le Peltier F. Likholobov V.A. Lintz H.-G. Li Shuben Li Yongdan Lisitsyn A.S. Liu Yingjun Lycourghiotis A. Mahamud M. Mallk T. Mans0urS.A.A. Margitfalvi J.L. Marsden C.E. Martens J.A. Martin G.A. 397 61 1 617 229 397 135 449 575 311 247 165 695 59 1 497 729 175 547 229 319 527 507 43 1 717 449 547 5 17 145 449 69 175 439 459 617 669 215 355, 381 269 743 Martin Luengo-Yates M.A. Masson J. Mastikhin V.M. Masuda K. Matsuda T. McLellan G.D. Meheux P.A. Messaoudi A. Mikhailenko S.D. Mirodatos C. Mizukami F. Morawski A.W. Mouaddib N. Mountassir Z . Moya S.A. Moyes R.B. Muiioz-Paez A. Munuera G. 87 60 1 507 229 311 135 135 479 59 1 269 229 487 269 345 279 135 627 627 Nakahara Y. 103, 695 Nitta Y. 103 Nohman A.K.H. 617 Nuiiez G.M. 77 Occelli M.L. Okamoto Y. Odoba E. Padovan M. Pajares J.A. Pama J.B. PCrez A.J. Perrichon V. Petrini G. Petr6 J. Pichat P. Piemontese M. Pis J.J. Poels E.K. Poix P. 287 103 113 43 1 439 439 439 269 43 1 459 679 49 439 205 575 Pommier B. Pomonis P.J. Prada Silvy R. Prasad V.V.D.N. Pyatnitskaya A.I. Ravasio N. Rawlence D.J. Rehspringer J.L. Resasco D.E. Reymond J.P. Richard D. Romero Y. Rossi M. Ruiz P. Salim V.M.M. Sanderson W.A. Schild Ch. Schmal M. Scholten A. Schramm Ch.M. Seki H. Sekiguchi J. Sermon P.A. Sham E. Shen Shikong Shepeleva M.N. Shkrabina R.A. Simonov P.A. Sobalik 2. Somasekhara Rao K. Spanos N. Staal L.H. Stanislaus A. Sterte J. Stoch J. Szab6 S. Taas E. 679 319 37 61 1 497 95 407 575 77 1 469 37 95 537 123 247 59 123 527 247 311 66 1 87 537 5 17 583 583 449 507 61 1 175 407 155 301 497 459 669 744 Tanabe K. Tang Youqi Tan-no M. Tichit D. Traverse J.P. Tretyakov V.V. Trezza G. Trifiro F. Tsubota S . Turek T. Ueda A. Vaccari A. van den Brink P.J. van Dillen A.J. van Leeuwen W.A. van Wageningen A. van Yperen R. Walther K.L. Wang Hongli Wang Y. Webb G. Wells P.B. Weng L.T. Whyman R. Willis J. Wokaun A. Xiong Y.L. Yamaguchi T. Yasse B. Zaki M.I. Zazhigalov V.A. Zecchina A. Zegaoui 0. Zhang Qinpei 567 69 567 345 729 647 43 1 49 695 547 695 49 527 165,527 205 527 165 239 517 87 135 135 537 135 135 59,239 537 5 67 537 617 497 43 1 679 69 Zhao Jiusheng Zhou B. Zhu Yongfa 145 537 69 745 STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universitb Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1 Volume 2 Volume 3 Volume 4 Volume 5 Volume 6 Volume 7 Volume 8 Volume 9 Volume 10 Preparation of Catalysts I . Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14- 17, 1975 edited by 6. Delmon. P.A. Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7,1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci6t6 de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9- 1 1,1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13- 75,1980 edited by B. Delmon and G.F. Froment New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July 4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. 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Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-1 9, 1989 edited by J. Klinowski and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm. S. Akashah, M. Absi-Halabi and A. Bishara 748 Volume 54 Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura Volume 55 New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F. Trifiro Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 23-25, 1989 edited by T. Kelli and K. Soga Surface Analysis edited by J.L.G. Fierro Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Volume 59 Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd International Symposium, Poitiers, October 2-5, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Volume 60 Chemistry of Microporous Crystals. Proceedings of the International Symposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba and T. Tatsumi Volume 6 1 Natural Gas Conversion. Proceedings of the Natural Gas Conversion Symposium, edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II. Proceedings of the IUPAC Symposium (COPS II), Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Volume 63 Preparation of Catalysts V. Proceedings of the Fifth International Symposium on the Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la- Neuve, September 3-6, 1990 edited by G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Volume 578 Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Oslo, August 12-17, 1990 Also Available Preparation of Catalysts I, II, 111 and IV Scientific Bases for the Preparation of Heterogeneous Catalysts Preparation of Catalysts I Proceedings of the International Symposium, Brussels, Belgium, October 14-17,1975 editedby 8. Delmon, P.A. Jacobs andG. Poncelet Studies in Surface Science and Catalysis, Vol. 1 1976 3rd repr. 1987 xvi + 706 pages ISBN 0-444-41428-2 "...very useful and full of latest information on preparation of Catalysts. Technical Books Review Preparation of Catalysts II Proceedings of the 2nd International Symposium, Louvain-la-Neuve, September 4-7, 1978 editedby B. Delmon, P. Grange, P. Jacobs andG. Poncelet Studies in Surface Science and Catalysis, Vol. 3 1979 2nd repr. 1987 iv + 762 pages ISBN 0-444-41 733-8 Preparation of Catalysts 111 Proceedings of the 3rd International Symposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Studies in Surface Science and Catalysis, Vol. 16 "...essential reading for anyone concerned with the preparation or investigation of catalysts. It is well up to the high standard set by earlier volumes in this series and is likely to be a useful source of information for many readers. " Applied Catalysis Preparation of Catalysts IV Proceedings of the 4th International Symposium, Louvain-la-Neuve, Sep- tember 1-4,1986 edited by 8. Delmon, P. Grange, PA. Jacobs and G. Poncelet Studies in Surface Science and Catalysis, Vol. 31 1983 xvi + 854 pages 0-444-42184-X 1987 xviii + 868 pages ISBN 0-444-41428-2 For details write to: Elsevier Science Publishers P.O. Box 330,1000 AH Amsterdam, The Netherlands This Page Intentionally Left Blank Preparation of Catalysts V Copyright Page Contents Organizing Committee Foreword Acknowledgements Financial Support Section I: Studies of unit operations in catalyst preparation Chapter 1. Illustration of process scale-up in heterogeneous catalyst preparation Chapter 2. Deposition precipitation onto pre-shaped carrier bodies. Possibilities and limitations Chapter 3. Influence of the preparation procedure on the physical properties, surface acidity and dispersion of MoP/Al2O3 catalysts Chapter 4. Synthesis of non-stoichiometric spinel-type phases : a key tool for the preparation of tailored catalysts with specific activity Chapter 5. Effect of preparation variables on catalytic behaviour of copper/zirconia catalysts for the synthesis of methanol from carbon dioxide Chapter 6. Preparation of TiO2âAl2O3 by impregnation with TICl4âCCl4 Chapter 7. Interactions of the impregnating solution with the support during the preparation of Rh/TiO2 catalysts Chapter 8. Impregnation of controlled-porosity silica : Cu/SiO2, Co/SiO2 and Cu-Co/SiO2. Investigation of the parameters affecting selectivity in CO hydrogenation Chapter 9. Selective hydrogenation of cyclododecatriene isomers to cyclododecenecatalyzed by Cu-Al2O3 Chapter 10. Preparation and characterization of highly selective FeâCu/SiO2 catalysts for partial hydrogenation of alkynes Chapter 11. Some remarks on the preparation of Fe-K/Ca-Cr catalyst for styrene production Chapter 12. Hydrogenation of 2-ethyl hexen-2-al on Ni/SiO2 catalysts. Role of for partial hydrogenation of alkynes Chapter 13. Preparation and properties of a PT/Silica and its comparison with Europt-1 Chapter 14. Factors analysis for mechanical strength in pelleting process of Fe-based high temperature shift catalyst Chapter 15. Studies on pore size control of alumina : preparation of alumina catalyst exmudates with large unimodal pore structure by low temperature hydrothermal treatment Chapter 16. Production of nickel-on-alumina catalysts from preshaped support bodies Chapter 17. Development of a methodology for investigating the adsorption of species containing catalytically active ions on the surface of industrial carriers Chapter 18. Scaling down of the calcination process for industrial catalyst manufacturing Chapter 19. Hydrothermal sintering of the active phase in alumina supported fixed bed nickel catalysts during reduction Section II: Catalyst preparation via the sol-gel route Chapter 20. The influence of silica support on polymerisation catalyst performance Chapter 21. Preparation and catalytic effects of CeOx-MOy-Al2O3 (M = Ba, La, Zr and Pr) by an improved sol gel method for automotive catalysts Chapter 22. Influence of preparation parameters on pore structure of silica gels prepared from tetraethoxy orthosilicate Section III: Preparation of catalysts from layered structures and pillaring of clays Chapter 23. Aspects of the synthesis of aryl sulfonic acid MELS® catalysts Chapter 24. Preparation of basic silicates and their use as supports or catalysts Chapter 25. Soils as unusual catalysts Chapter 26. Thermal stability, acidity and cracking properties of pillared rectorite catalysts Chapter 27. Preparation and properties of large-pore RE/Al-pillared montmorillonite. A comparison of RE-cations Chapter 28. Preparation of pillared montmorillonite with enriched pillars Chapter 29. Intercalation of La2O3 and La2O3-NiO oxidic species into montmorillonite layered structure Chapter 30. Mixed Al-Fe pillared laponites : preparation, characterization and catalytic properties in syngas conversion Chapter 31. Zirconium pillared clays. Influence of basic polymerization of the precursor on their structure and stability Chapter 32. Control of the acidity of montmorillonites pillared by Al-hydroxy cationic species Section IV: Preparation and modification of zeolite-based catalysts Chapter 33. The chemistry of dealumination of faujasite zeolites with silicon tetrachloride Chapter 34. Factors affecting the formation of extra-framework species and mesopores during dealumination of zeolite Y Chapter 35. Treatment of galloalumino-silicate (ZSM-5 type zeolite) with KOH solution. Dispersion of aggregated zeolites into small particles Chapter 36. Design and preparation of vanadium resistant FCC catalysts Chapter 37. Double substitution in silicalite by direct synthesis : a new route to crystalline porous bifunctional catalysts Chapter 38. Study on titanium silicalite synthesis SectionV: Carbon supported catalysts Chapter 39. Activated carbon from bituminous coal Chapter 40. Carbon-supported palladium catalysts. Some aspects of preparation in connection with the adsorption properties of the supports Chapter 41. Preparation of palladium-copper catalysts of designed surface structure Chapter 42. Optimization and Characterization of Pt-Fe alloys supported on charcoal Chapter 43. Supported metallic catalysts achieved through graphite intercalation compounds Chapter 44. Preparation of graphite-iron-potassium catalysts for ammonia synthesis Section VI: Preparation of oxidation catalysts Chapter 45. Synthesis of V-P-O catalysts for oxidation of C4 hydrocarbons Chapter 46. Preparation of well dispersed vanadia catalysts by ultra-high intensity grinding at ambient temperature Chapter 47. Dispersion and physico-chemical characterization of iron oxide on various supports Chapter 48. The use of chelating agents for the preparation of iron oxide catalysts for the selective oxidation of hydrogen sulfide Chapter 49. Preparation of oxidation catalysts with a controlled architecture Chapter 50. Structure and selectivity changes in vanadia-titania-deNOx catalysts Chapter 51. Binary oxide catalysts synthesized by sequential precipitation Chapter 52. ZrO2 as a support : oxidation of CO on CrOx/ZrO2 Chapter 53. Methane oxidative coupling by definite compounds (e.g. perovskite, cubic or monoclinic structure, . . .) obtained by low temperature processes Section VII: Novel and unusual preparation methods Chapter 54. Preparation of strong alumina supports for fluidized bed catalysts Chapter 55. Synthesis and regeneration of Raney catalysts by mechanochemical methods Chapter 56. Controlled preparation of Raney Ni catalysts from Ni2Al3 base alloys - Structure and properties Chapter 57. Novel type of hydrotreating catalysts prepared through precipitation from homogeneous solution (PFHS) method Chapter 58. Preparation of manganese oxide catalysts using novel NH4MnO4 and manganese hydroxide precursors. Comparison of unsupported and alumina supported catalysts Chapter 59. Influence of surface OH groups and traces of water vapor during the preparation of TiO2-SiO2 samples Chapter 60. Catalysts and preparation of new titanates Chapter 61. New methods of synthesis of highly dispersed silver catalysts Chapter 62. Preparation of high-surface-area V-Si-P oxide catalysts Chapter 63. Preparation of fine particles of ruthenium-alumina composite by mist reduction method Chapter 64. Designed catalysts for hydrodechlorination, reduction and reductive amination reactions Chapter 65. Preparation of high surface area hydrogen-molybdenum bronze catalysts Chapter 66. New preparation of supported metals. Hydrogenation of nitriles Chapter 67. Preparation of highly dispersed gold on titanium and magnesium oxide Chapter 68. Preparation of monodisperse colloidal Pt-ReO2 particles using microemulsions Chapter 69. New organometallic active sites obtained by controlled surface reaction of organometallic complexes with supported metal particles Chapter 70. Conversion coatings on stainless steel as multipurpose catalysts Author Index Studies in Surface Science and Catalysis (other volumes in the series)