Influence of different alkyl and carboxylate substituents on Sn(IV) organometallic catalysts during fatty acid methyl ester production

Catalysis Communications 58 (2015) 204–208 Contents lists available at ScienceDirect Catalysis Communications j ourna l homepage: www.e lsev ie r .com/ locate /catcom Short Communication Influence of different alkyl and carboxylate substituents on Sn(IV) organometallic catalysts during fatty acid methyl ester production Jhosianna P.V. da Silva, Yariadner C. Brito, Danielle M. de A. Fragoso, Paula R. Mendes, Ana Soraya L. Barbosa, Janaína H. Bortoluzzi, Mario R. Meneghetti, Simoni M.P. Meneghetti ⁎ Instituto de Química e Biotecnologia, Universidade Federal de Alagoas, Av. Lourival de Melo Mota, s/n°, Maceió, AL 57072-970, Brazil ⁎ Corresponding author. Tel.: +55 82 3214 1703; fax: E-mail address: [email protected] (S.M.P. Men http://dx.doi.org/10.1016/j.catcom.2014.09.010 1566-7367/© 2014 Elsevier B.V. All rights reserved. a b s t r a c t a r t i c l e i n f o Article history: Received 8 July 2014 Received in revised form 3 September 2014 Accepted 4 September 2014 Available online 16 September 2014 Keywords: Transesterification Esterification Fatty acid methyl esters Biodiesel Tin catalysts Substituents Sn(IV) complexes (dimethyltin dineodecanoate (1), dibutyltin dineodecanoate (2), dioctyltin dineodecanoate (3), dimethyltin diundec-10-enoate (4), dibutyltin diundec-10-enoate (5), tributyltin undec-10-enoate (6) and tributyltin undecanoate (7)) were tested as catalysts for transesterification or esterification in the presence of methanol, in order to investigate the effect of the substituents coordinated to the metal center. All complexes were active at relative high reaction temperatures, and their reactivity can be associated with the steric effects induced by the substituents (alkyls and carboxylates) at the metal center. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Oils and fats are sources of triacylglycerides and fatty acids and are used as raw materials for alkyl ester (biodiesel) production via transesterification or esterification in the presence of short chain alcohols. Among of the several kind of catalysts proposed for transesterification of triacylglycerides, themost employed are Brønsted bases, such as hydrox- ides and alkoxides of sodium or potassium [1]. These catalytic systems allow the production of biodiesel with high level of conversions and high selectivity for monoalkyl esters. However, industrial processes based on these catalysts incur high energy costs, mainly due to intrinsic difficulties of the process, such as the separation and purification of the producedbiodiesel. To reduce these drawbacks, high purity rawmaterials are normally required, reducing the occurrence of side reactions, such as saponification or hydrolysis. For esterification, Brønsted acid catalysts are frequently used, and normally associated to manipulation and corrosion problems [1]. In this context, several studies have focused on alternative catalysts for biodiesel production to mitigate these problems [1–8]. One trend is to develop catalytic systems exhibiting Lewis acid sites, and several promising results have been presented [9–11]. The main advantage displayed for this kind of catalytic systems are associated to theminimization of: i) corrosion, if compared toH2SO4 nor- mally employed as catalyst in esterification of fatty acids; ii) purification +55 82 3214 1384. eghetti). problems, commonly associated to the formation of soap in trans- esterification processes when NaOH, and NaOCH3 are employed as cata- lysts in the presence of oils with high content of free fatty acids; and iii) mainly, to the possibility of catalyst recovery, since materials with Lewis acid sites can be solids with low solubility in the reactionmedium. Some works have been already reported showing the efficiency of these catalysts in this reaction using diverse kinds of alcohols, like methanol, ethanol and even alcohols with long alkyl chains [10,12–16]. Tin(IV)-based homogeneous or heterogeneous catalytic systems in esterification, transesterification and polycondensation reactions are fre- quently employed in the production of polymeric and intermediatemate- rials [17–19]. Recently, we have reported the potential for using these types of catalytic systems for biodiesel production via esterification, transesterification, and simultaneous esterification and transesterification reactions [9,10,12–16]. In the present study, the influence of substituents bearing Sn(IV) was investigated. As far aswe are aware, no information is available regarding the influence of the nature of the alkyl and carboxylate groups present on this class of catalysts on transesterification or esterification reactions. 2. Experimental 2.1. Materials Methanol and d4-methanol (analytical grade; Sigma-Aldrich) were stored over MgSO4 as a desiccant before use. Tricaprylin and caprylic acid were purchased from Sigma-Aldrich (analytical grade), sodium http://crossmark.crossref.org/dialog/?doi=10.1016/j.catcom.2014.09.010&domain=pdf http://dx.doi.org/10.1016/j.catcom.2014.09.010 mailto:[email protected] http://dx.doi.org/10.1016/j.catcom.2014.09.010 http://www.sciencedirect.com/science/journal/15667367 205J.P.V. da Silva et al. / Catalysis Communications 58 (2015) 204–208 hydroxide and sulfuric acid were acquired from VETEC (analytical grade) and soybean oil (commercial grade) was supplied by Bunge Alimentos (Brazil). All reagents were used as received. The free fatty acids found in the soybean oil (characterized as oleic acid content, in percentage, according to the AOCS official method Ca 5a-40 [20]) were equivalent to a total amount of 0.1%. The fatty acid mixture was obtain- ed through the saponification of soybean oil. The soybean fatty acid pro- file, was determined by gas chromatography (GC) using the AOCS official method Ce 1-62 and Ce 2-26 [20] and the result found (wt.%) is 11.5% of C16:0 (palmitic), 3.9% of C18:0 (stearic), 25.9% of C18:1 (oleic), 54.1% of C18:2 cis/cis 9,12 (linoleic), and 4.6% of C18:3 cis/cis/ cis 9,12,15 (linolenic). 2.2. Catalysts The complexes dimethyltin dineodecanoate (1), dibutyltin dineodecanoate (2), and dioctyltin dineodecanoate (3) were purchased from Gelest (Morrisville, PA, USA) and used as received without further purification. The reactions involving the synthesis of the organometallic Sn(IV) catalysts dimethyltin diundec-10-enoate (4), dibutyltin diundec-10-enoate (5), tributyltin undec-10-enoate (6), and tributyltin undecanoate (7) were carried out, according to previously described procedures [21]. The complexes were characterized by infrared absorp- tion spectroscopy using a Varianmodel IR 660 infrared spectrophotom- eter. The 1H-NMR (externally referenced to Si(CH3)4) and 119Sn-NMR (externally referenced to Sn(CH3)4) spectra were recorded on a Bruker FT instrument (Bruker Avance 400). Chemical shifts (δ) are expressed in ppm. Elemental analyses were made in a Perkin-Elmer CNH 2400. Dimethyltin diundec-10-enoate (4). Reaction yield: 70%. Anal. calc for C24H44O4Sn (515.31): C, 55.94; H, 8.61. Found: C, 54.67; H, 8.96. NMR 1H (CDCl3): δ 5.81 (m, =CH); 4.96 (m, =CH2); 2.35 (br sig, CH2COO); 2.03 (br sig, CH2CH=); 1.62 (br sig, CH2CH2COO); 1.31 (m, (CH2)5); 0.95 (m, SnCH3). RMN 13C (100 MHz, CDCl3, ppm): δ 184.13 (C=O); 139.16 (=CH); 114.15 (=CH2); 34.07 (CH2C=O); 33.78 (=CHCH2); 29.28 to 28.87 (=CHCH2(CH2)5); 25.32 (CH2CH2C=O); 4.22 (SnCH3). NMR 119Sn (149 MHz, CDCl3, ppm): δ -121,60. IR (KBr, cm−1): 3081 and 2976 (ν=CH); 2976, 2919, and 2852 (ν CH); 1734 (C=O); 1639 (C=C); 1553 and 1409 (νas/s COO); 1468 (δ CH2); 727 (ρas CH2); 992 and 903 (γ CH2=CH); 640 and 573 (νas/s Sn-C), 515 (ν Sn-O). Dibutyltin diundec-10-enoate (5). Reaction yield: 60%. Anal. calc for C30H56O4Sn (599.47): C, 60.11;H, 9.42. Found: C, 59.94; H, 9.28. NMR 1H (400MHz, CDCl3, ppm): δ 5.81 (m, 2H,=CH); 4.99 (m, 4H,=CH2); 2.36 (t, 4H, CH2COO, J = 7,40 Hz); 2.03 (q, 4H, CH2CH=, J = 7,07 Hz); 1.65 (m, 12H, CH2CH2COO + Sn(CH2)2); 1.37 (m, 4H, Sn(CH2)2CH2); 1.29 (m, 20H, (CH2)5); 0.91 (t, 6H, Sn(CH2)3CH3, J = 7.33 Hz). NMR 13C (100 MHz, CDCl3, ppm): δ 184.15 (C=O); 138.95 (=CH); 114.09 (=CH2); 34.08 (CH2C=O); 33.71 (=CHCH2); 29.25 to 28.83 (=CHCH2(CH2)5); 26.61 (SnCH2CH2); 26.24 (Sn(CH2)2CH2); 25.44 (SnCH2); 24.85 (CH2CH2C=O); 13.48 (Sn(CH2)3CH3). NMR 119Sn (149 MHz, CDCl3, ppm): δ -149.78. IR (KBr, cm−1): 3081 and 2966 (ν=CH); 2919, and 2852 (ν CH); 1738 (ν C=O); 1640 (ν C=C); 1595 and 1384 (νas/s COO); 1460 (δ CH2); (ρas CH2); 992 and 908 (γ CH2=CH) 669 and 592 (νas/s Sn-C), 525 (ν Sn-O). Tributyltin undec-10-enoate (6). Reaction yield: 60%. Anal. calc for C23H46O2Sn (473.32): C, 58.34; H, 9.80. Found: C, 58.28; H, 9.53. NMR 1H (400 MHz, CDCl3, ppm): δ 5.78 (m, 1H, =CH); 4.93 (m, 2H, =CH2); 2.28 (t, 2H, CH2COO, J = 7,10 Hz); 2.01 (q, 2H, CH2CH=, J = 7.06 Hz); 1,58 (m, 8H, CH2CH2COO + SnCH2); 1.28 (m, 22H, (CH2)5 + SnCH2(CH2)2); 0.89 (t, 9H, Sn(CH2)3CH3, J = 7.30 Hz). NMR 13C (100 MHz, CDCl3, ppm): δ 179.51 (C=O); 139.16 (=CH); 114.08 (=CH2); 34.88 (CH2C=O); 33.78 (=CHCH2); 29.34 to 28.90 (=CHCH2(CH2)5); 27.83 (SnCH2CH2); 27.02 (Sn(CH2)2CH2); 25.81 (CH2CH2C=O); 16.35 (SnCH2); 13.63 (Sn(CH2)3CH3). NMR 119Sn (149 MHz, CDCl3, ppm): δ 102.42. IR (KBr, cm−1): 3079 and 2956 (ν=CH);, 2922 and 2853 (ν CH); 1738 (C=O); 1640 (C=C); 1551 and 1399 (νas/s COO); 1460 (δ CH2); 986 and 903 (γ=CH); 707 (ρas CH2); 669 and 602 (ν as/s Sn-C); 515 (ν Sn-O). Tributyltin undecanoate (7). Reaction yield: 64%. Anal. calc for C23H48O2Sn (475.34): C, 58.12; H, 10.18. Found: C, 58.10; H, 10.18. 1H NMR (CD3OD): δ 2.30-1.14 (m, CH2), 0.90 (t, CH3, 3JH-H = 7.27 Hz). NMR 1H (400 MHz, CDCl3, ppm): δ 2.28 (t, 2H, CH2COO, J = 7.42 Hz); 1.61 (m, 8H, CH2CH2COO + SnCH2); 1.28 (m, 26H, (CH2)7 + SnCH2(CH2)2); 0.89 (m, 12H, CH3 FA + Sn(CH2)3CH3). NMR 13C (100 MHz, CDCl3, ppm): δ 179.60 (C=O); 34.92 (CH2C=O); 31.88 (CH3CH2CH2) 29.56 to 29.31 (CH3CH2CH2(CH2)5); 27.83 (Sn(CH2) 2CH2); 27.01 (SnCH2CH2); 25.83 (CH2CH2C=O); 16.36 (SnCH2); 14.02 (CH3 FA); 13.63 (Sn(CH2)3CH3). NMR 119Sn (149 MHz, CDCl3, ppm): δ 104.10. IR (KBr, cm−1 2959, 2923 (ν CH); 1740 (C=O); 1552 and 1396 (νas/s COO); 1458 (δ CH2); 797 (ρas CH2); 668 and 602 (ν as/s Sn-C); 521 (ν Sn-O). 2.3. Transesterification and esterification The transesterification reactions were carried out using an alcohol: oil:catalyst molar ratio of 400:100:1 at different temperatures (80, 120 or 140 °C) for 10 h using the following: (i) a 250-mL batch reactor equippedwith a bathwith a temperature controller, a reflux condenser, and a magnetic stirrer operating at 1000 rpm. In this case, the reactions were performed at the reflux temperature of the alcohol employed. (ii) A 200-mL batch stainless steel reactor coupled to a manometer, a tem- perature probe and a mechanical stirrer operating at 1000 rpm. After the reaction, the product was washed three times with distilled water, dried with MgSO4, and centrifuged. The yield of the transesterification reaction was determined by GC and expressed in terms of the percent- age of fatty acid methyl esters (% FAMEs) produced, considering percentage error of 2%. For the esterifications, the same 200-mL batch stainless steel reactor was adopted, also equipped with a temperature controller and a mag- netic stirrer (1000 rpm) was employed. The reaction conditions were as follows: an alcohol:oil:catalyst molar ratio of 400:100:1 and temper- atures of 120, 140 and 160 °C over 1 h, and the yield was calculated based on the diminishing acid index for products relative to the acid index for the initial fatty acid mixture, according to the AOCS Cd3d63 standard method [20]. Transesterification and esterification reactions were prepared in NMR tubes, and 1H and 119Sn NMR spectra were obtained (using Sn(CH3)4 as external standard). The solutionswere prepared as follows: (i) 0.020 g of (1) at room temperature in 0.75 mL of d4-methanol, (ii) 0.020 g of 1 in 0.75 mL of d4-methanol heated at 120 °C for 1 h, (iii) 0.020 g of 1, 0.096 g of tricaprylin in 0.75mL of d4-methanol heated at 120 °C for 1 h, and (iv) 0.020 g of 1, 0.0293 g of caprylic acid in 0.75mL of d4-methanol heated at 120 °C for 1 h. 3. Results and discussion Sn(IV) catalysts bearing different alkyl and carboxylate substituents were investigated during themethanolysis of soybean oil to obtain fatty acid methyl esters (FAMEs). The aim of this work was to verify how im- portant is the influence of the nature of the substituents coordinated to the active center, Sn(IV). The chemical structures of the pre-catalyst complexes are presented in Fig. 1. All catalyst precursors bear four substituents, alkyl and carboxylated substituents, with general formula R4-nSn(OOCR)n, where n = 1 or 2. Normally, this kind of structures has the carboxylated moieties as bidentate-type substituents, interacting with the metal center via the oxygen atoms of the carboxylic group [22]. Also, the presence of this long carbon chains in all catalyst precursors allows good solubility of the complexes in the reaction mixture. More precisely, neodecanoate contains 10 carbon atoms with ramifications at the C-2; while undec- 10-enoate, and undecanoate contain 11 carbons in a linear structure, Sn O O O O Sn O O O O Dimethyltin dineodecanoate (1) Dimethyltin diundec-10-enoate (4) Sn O O O O Sn O O O O Dibutyltin dineodecanoate (2) Dibutyl diundec-10-enoate (5) Sn O O O O Sn OO Dioctyltin dineodecanoate (3) Tributyltin undec-10-enoate (6) Sn OO Tributyltin undecanoate (7) Fig. 1. Chemical structure and the names of the catalysts. 206 J.P.V. da Silva et al. / Catalysis Communications 58 (2015) 204–208 differing by the presence or not of an unsaturation at the C-10. For the alkyl substituents, methyl, n-butyl, and n-octyl were used. 3.1. Catalytic essays on transesterification The catalysts precursors 1, 2, and 3were tested in transesterification of soybean oil during 10 h, under reflux conditions, and in a batch stain- less steel reactor at 80 °C, using a methanol:oil:catalyst molar ratio of 400:100:1 (Fig. 2). The best yields for these three catalyst complexes were achieved when the reactions were carried out with the batch stainless steel reac- tor, since appropriate phase equilibrium inside the reactor [14] is established. For the following reactions at 120 and 140 °C (Fig. 3A and B, respectively), this system was employed, because using the reflux system, temperatures higher than boiling point of alcohol cannot be reached without compromise the reaction, since methanol can be removed from the reaction mixture at atmospheric pressure. An increase in the reaction temperature leads to an increase of the reaction yields, and the maximum yields are more quickly reached. For all temperatures studied, the catalyst 1wasmost active, with yields in FAMEs of 30%, 78%, and 95% at 80, 120, and 140 °C, respectively, after 3 h. Reports from literature point out that Sn(IV) complexes can exhibit better performances at higher reaction temperatures [12–16,23,24]. In order to compare these results with those obtained using a classical transesterification catalyst, NaOH was investigated also at 120 and 140 °C until 6 h, at the same molar ratio used for organotin complexes (Fig. 3A and B). The yields obtained are lower than those observed when complexes 1, 2 and 3 are used and it is important to highlight that there is no influence of temperature increase on the results. Apart of the temperature range adopted in this work, the general trend of catalyst reactivity is: 1 N 2 N 3. The mechanism accepted for transesterification and esterification reactions catalyzed by Lewis acids involves the formation of a Lewis acid–base complex through an inter- action between the acylglycerides or fatty acids at the metal center via the oxygen of the carbonyl group and/or of the alcohol [22,25]. Here, it is expected that the presence of different substituents on the metal center raises different electronic and steric effects that lead to different catalytic performance. Indeed, it seems that the trend of the catalytic activity of the former catalytic complexes (1 N 2 N 3) suggests a strong influence tied to the steric effect of the alkyl groups coordinated to the metal center (methyl b n-butyl b n-octyl), hampering the coordination of the substrate to themetal center. The existence of an electronic effect from the alkyl groups cannot be ignored; however this effect should not be a relevant, since the electronic effect induced by n-butyl and n-octyl groups is practically the same. In addition, the presence of three alkyl substituents instead of two was evaluated, and the results are also presented in Table 1 for 5 and 0 2 4 6 8 10 0 10 20 30 40 50 FA M Es (% ) Reaction time (h) (1) at reflux conditions (2) at reflux conditions (3) at reflux conditions (1) batch stainless steel reactor (2) batch stainless steel reactor (3) batch stainless steel reactor Fig. 2. Production of FAMEs (percentage yield) by transesterification in the presence of 1, 2, 3 and NaOH at different reaction times using the reactor operating at reflux condition and the batch stainless steel reactor (80 °C) (molar proportions of MeOH:TG: catalyst = 400:100:1). A B 0 10 20 30 40 50 60 70 80 90 100 FA M Es % Reaction time (h) (1) (2) (3) NaOH 0 2 4 6 8 10 0 2 4 6 8 10 0 10 20 30 40 50 60 70 80 90 100 FA M Es % Reaction time (h) (1) (2) (3) NaOH Fig. 3. Production of FAMEs (percentage yield) by transesterification in the presence of 1, 2, and 3 at different reaction times using the batch stainless steel reactor (A= 120 °C and B = 140 °C) (molar proportions of MeOH:TG:catalyst = 400:100:1). Table 1 Production of FAMEs (percentage yield) by transesterification in the presence of 1, 2, 4, 5, 6 and 7 at different reaction times using the batch stainless steel reactor at 120 °C (molar proportions of fatty MeOH:TG:catalyst = 400:100:1). FAMEs (wt.%) Time (h)↓ 1 2 4 5 6 7 0.25 31 25 49 30 16 21 0.5 43 34 59 40 29 25 0.75 53 41 64 49 36 36 1 55 47 67 56 42 40 2 59 58 78 66 63 68 4 78 66 83 74 76 84 5 81 67 88 78 80 - 6 83 74 92 79 81 - 7 85 78 98 86 85 - 8 89 80 98 88 89 - 9 93 82 96 87 90 - 10 97 85 96 90 90 - 207J.P.V. da Silva et al. / Catalysis Communications 58 (2015) 204–208 6 at 120 °C. The different numbers of butyl substituents coordinated to the metal center also exert an influence, predominantly at first hours of reaction (kinetic control). As expected, the low Lewis acidity of the metal center on R3Sn(OOCR) complexes with respect to the R2Sn(OOCR)2 ones explains the lower catalytic activity of complex 6 in comparison to 5 (electronic effects) [26]. Indeed, the higher electron de- ficiency on the metal center of R2Sn(OOCR)2 complexes is due to the presence of twowithdrawing groups, here the carboxylate substituents. In order to determine the occurrence of a possible steric influence of the carboxylic substituents on the catalytic performance of the Sn(IV) complexes, transesterifications were carried out under the same condi- tions (120 °C, alcohol:oil:catalyst molar ratio of 400:100:1, 10 h) with complexes 1, 2, 4, and 5 (Table 1). With this test, we can see that the presence of alkyl substituents on the α-C of the carboxylic substituents does not result in significant modification on the catalytic activity, at least at the reaction conditions employed. Finally, in order to verify if thepresence of a saturated or unsaturated carboxylated chain brings onmodifications in the catalytic properties of the Sn(IV) complexes, we compare the catalytic performances of complexes 6 and 7 (Table 1) at the same reaction conditions (120 °C, alcohol:oil:catalyst molar ratio of 400:100:1). Here, no significant influence can be attributed to the presence or not of the terminal unsaturation regarding catalytic reactivity because this modification is located away from the metal center and does not interfere with the coordination of the substrate on the catalyst. 3.2. Catalytic essays on esterification The esterification reactions of soybean FFAs with methanol were performed in the presence of the Sn(IV) complexes 4, 5 and 6 over 1 h at different temperatures (see Table 2). Unlike transesterifications, the esterifications can be self-catalyzed due to the presence of the Brønsted acid (the fatty acid itself) in the reaction media. Therefore, the catalytic activity of eachmetal complexmust be compared to a reaction conducted without a metal compound. Furthermore, results of esterification employing the standard cata- lyst H2SO4 for the same reaction are presented in Table 2, for compari- son; slightly higher yields than those obtained without catalyst were observed. Under typical reaction conditions, concentrations of 1% to 5% of H2SO4 (weight % relative to the substrate) at 200 to 250 °C are used, representing greater levels than those employed in this study [27]. Table 2 Production of FAMEs (percentage yield) by esterification using 4, 5, 6 and H2SO4 as catalysts at 1 h and temperatures of 120, 140 and 160 °C (MeOH:FFA:catalyst molar ratio of 400:100:1). FAMEs (wt.%) T (°C)↓ Without catalyst H2SO4 4 5 6 120 17 19 33 23 23 140 29 36 69 47 39 160 46 51 71 67 59 208 J.P.V. da Silva et al. / Catalysis Communications 58 (2015) 204–208 Here again, the steric effect induced by larger alkyl substituents on Sn(IV) is confirmed (compare reactions employing complexes 4 and 5). In this case, also a reduction on the catalytic activity is observed when the methyl substituents are substituted by the n-butyl ones. Nev- ertheless, the influence of the steric hindrance on esterification is less pronounced if compared to transesterification. This must occur due to the less molecular volume of the substrate (here, a fatty acid instead of a triacylglycerides). Also, at higher temperatures we confirmed the lower activity of the triorganotin carboxylate complex 6 in comparison to diorganotin carboxylate one 5 [26]. Furthermore, in order to verify if the catalysts aremodified in the re- action medium, in the presence of methanol and the other reagents, transesterification and esterification reactions were carried out in NMR tubes, in the presence of catalyst 1. For that, 1H and 119Sn NMR spectra were obtained for: (i) 1 at room temperature in d4-methanol; (ii) 1 in d4-methanol after the solution be heated at 120 °C for 1 h; (iii) 1 and tricaprylin in d4-methanol after the reaction medium be heated at 120 °C for 1 h (transesterification); and (iv) 1 and caprylic acid in d4-methanol after the reaction medium be heated at 120 °C for 1 h (esterification). The analysis of 1H NMR spectra attests that the transesterification and esterification reactions take place, as expected. In all cases there is no significant modification of the chemical shift for 119Sn NMR and the signals appear between −230 and −235 ppm (see Figs. S1 to S4 in Supplementary data). These results suggest that there is nomodification of the nature of the substituents, i.e. there is any significant reaction re- distribution of the substituent of the tin complex with methanol under the conditions studied. It is important to mention that Me2Sn(OMe)2 typically presents 119Sn chemical shifts around −126 ppm [26]. This result also suggests that the catalyst 1 acts as net Lewis acid system as discussed before, and for analogy the other catalysts, studied here, can follow the same behavior. 4. Conclusions In this work,we verified that the complexes testedweremore active at relatively high reaction temperatures.Moreover, we evaluate the ste- ric effect of different alkyl and carboxylate substituents on the catalytic activity of Sn(IV) complexes in transesterification and esterification re- actions. Itwas clearly determined that hindered alkyl and, in low extent, carboxylic substituents reduce the catalytic activity of the Sn(IV) com- plexes. Also, we evaluate the catalytic activity of tri and diorganotin carboxylate complexes, and as expected, diorganotin carboxylate complexes are more active since the metal center presents higher Lewis acidity. Acknowledgments The authors thank FINEP, CAPES, CNPq, and the INCT-Catálise for financial support. JPVS, YCB, DMAF, PRM and ASLB express their appre- ciation for fellowships granted by CAPES and CNPq. SMPM and MRM thank CNPq for research fellowships. Appendix A. 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Introduction 2. Experimental 2.1. Materials 2.2. Catalysts 2.3. Transesterification and esterification 3. Results and discussion 3.1. Catalytic essays on transesterification 3.2. Catalytic essays on esterification 4. Conclusions Acknowledgments Appendix A. Supplementary data References