Esterification of lauric acid with butanol over mesoporous materials

Esterification of lauric acid with butanol over mesoporous materials

Renewable Energy 50 (2013) 585e589 Contents lists available at SciVerse ScienceDirect Renewable Energy journal homepage:

301KB Sizes 0 Downloads 23 Views

Renewable Energy 50 (2013) 585e589

Contents lists available at SciVerse ScienceDirect

Renewable Energy journal homepage:

Esterification of lauric acid with butanol over mesoporous materials Suellen D.T. Barros a, Aline V. Coelho a, Elizabeth R. Lachter a, *, Rosane A.S. San Gil a, Karim Dahmouche b, Maria Isabel Pais da Silva c, Andrea L.F. Souza a a

Instituto de Química, Universidade Federal do Rio de Janeiro – UFRJ, Ilha do Fundão, CT, Bloco-A sala 617, CEP 21949-900, Rio de Janeiro, Brazil Universidade Federal do Rio de Janeiro, Instituto de Macromoléculas Profa. Eloisa Mano, Rio de Janeiro, RJ, Brazil c Pontifícia Universidade Católica do Rio de Janeiro, Departamento de Química, Rua Marques de São Vicente 225, Gávea, CEP 22453-900, Rio de Janeiro, RJ, Brazil b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 September 2011 Accepted 30 June 2012 Available online 24 August 2012

Mesoporous silica (e.g. SBA-15) was evaluated in the esterification of lauric acid with butanol. ZnO/SBA15 and MgO/SBA-15 were synthesized and characterized using N2 adsorption/desorption isotherms at 77 K, 29Si magic angle spinning solid state nuclear magnetic resonance (29Si MAS NMR), X-ray diffraction (XRD) and Small-Angle X-ray scattering (SAXS). The nanostructural parameters obtained from SAXS analysis were very similar for all samples revealing that the impregnation method for incorporation of Zn and Mg in mesoporous silica did not affect its structure. Both catalysts ZnO/SBA-15 and MgO/SBA-15 were able to promote the esterification reaction of lauric acid with 1-butanol at mild reactions conditions. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Esterification Lauric acid Butanol Mesoporous materials MgO/SBA-15 ZnO/SBA-15

1. Introduction The esterification reaction of carboxylic acids with alcohols is a well known class of reaction in the liquid phase and of considerable industrial interest due to the great practical importance of organic esters. Recently, the acid catalyzed esterification of fatty acids has attracted great interest, as long carbon chain esters can be used as biofuels [1,2]. Generally, esters are commercially produced by using liquid acid catalysts such as sulfuric or hydrochloric acids. Homogeneous catalysts, though effective, lead to serious contamination problems as they are toxic, corrosive, and produce byproducts which are difficult to separate from the reaction medium which results in higher production costs. The removal, handling, and disposal of corrosive waste have motivated a number of studies concerning the development and application of heterogeneous catalyst systems [1,3,4]. However, the application of solid catalysts in liquid-phase reactions involving biodiesel production has been limited because of the poor reactant/catalyst contact from either pore diffusion limitations and or low active site availability for the catalytic reaction. The solid acids used for the production of biodiesel and fatty acids esterification are for example: the organic ion exchange resins, such as Amberlyst 15 [5e7], sulphonated

* Corresponding author. Tel.: þ1 55 2122550559; fax: þ1 55 2125627256. E-mail address: [email protected] (E.R. Lachter). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.

zirconia [7,8], zeolites [9] or heteropoly acids supported on silica (e.g. H3PW12O40; HPA/silica) [10,11]. However, they have shown some limitations in application to esterification due to the low thermal stability (Amberlyst 15, <140  C), resistance to mass transfer (zeolites), and loss of active sites in the presence of a polar support (HPA/silica) [9]. Mesostructured materials could be a good alternative for heterogeneous catalysis due to the presence of large pores and high thermal stability [9]. Mesoporous silica materials (e.g. SBA-15) functionalized with sulfonic groups (SBA-15/SO3H) and heteropoly acids (HPA) have been evaluated both for esterification reactions [9,12] and transesterification reactions [13] and presented good activity. MgO-impregnated SBA-15 was evaluated for the transesterification reaction of vegetable oil with ethanol for biodiesel production. The conversions were high but required temperatures greater than 200  C and high pressure [14]. Biodiesel can be produced either by transesterification reaction of vegetables oils and fats and esterification reaction of fatty acids [1]. The esterification of carboxylic acids by solid catalysts is very important, considering the low cost of feedstocks that present a high concentration of free fatty acids (FFA) and no formation of glycerin [1,15]. However the most of papers about, the esterification of fatty acids catalyzed by heterogeneous catalysts for biodiesel production report the use of high temperature and pressure [4]. The esterification of lauric acid with methanol catalyzed by CaO was studied at 120  C and the acid conversion was 60.8% [16]. A mineral clay was evaluated as catalysts for the esterification of lauric acid


S.D.T. Barros et al. / Renewable Energy 50 (2013) 585e589

with methanol at 160  C and pressure of 9 bar. The authors found a lauric acid conversion of 63% [17]. The use of butanol prepared from an agricultural feedstock for the esterification reaction would result in a totally biorenewable process for the preparation of the biodiesel [18]. The choice of the fatty acid for this study relies on the fact that to some renewable feedstock as babassu coconut oil is rich in lauric acid. We didn’t find in the literature the use of SBA-15 modified with Zn and Mg for the esterification of lauric acid with butanol. In this study we present our results for the synthesis, characterization and catalytic activity of SBA-15-type materials modified with Mg and Zn. Additionally, the esterification of lauric acid, a model for the esterification of high molecular weight carboxylic acids present as impurities in biodiesel feedstock was evaluated by using butanol as biorenewable alcohol.

The reactions were performed at the reflux temperature of the mixture (118  C), with magnetic stirring for 6 h at atmospheric pressure. The crude product was extracted with hexane/water and analyzed by 1H NMR (DPX200 Bruker equipment, 5 mm dual H/C probe, 10 mg sample dissolved in 60 mL of CDCl3, ambient temperature). By using NMR analysis of 1H NMR spectra from fatty acids and the reaction product, it was possible to follow the reaction progress and calculate the ester content during the reaction. The relevant signals chosen for the calculation are those of the OCH2 groups (triplet at 3.7 ppm) and those corresponding to the CH2 groups a to carboxylic group (triplet at 2.2e2.4 ppm). The ester content for the esterification reactions was obtained directly from integration of the relevant signals.

2. Experimental

3.1. Properties of catalysts

2.1. Catalyst preparation

Textural properties and FRX analysis of catalysts are presented in Table 1. It can be seen that all materials presented large surface areas. The value found for SBA-15 was similar to the results found in the literature [21]. After impregnation with Mg and Zn the loss of surface area in relation to SBA-15 was 15 and 24%, respectively, for MgO/SBA-15 and Zn-SBA-15 in contrast with the results found by Sun et al. [22]. They observed that after impregnation of SBA-15 with Ca salts the surface area decrease from 584 m2 g1 to 227 m2 g1 due the loading of the salt in the support [22].

SBA-15 was prepared following the procedure described in the literature [19] from poly(ethylene glycol)-poly(propylene glycol)poly(ethylene glycol) a triblock copolymer (EO20-PO70-EO20, P123, Pluronic 123 from Aldrich) and tetraethylorthosilicate (TEOS). The final product was filtered, washed with water, dried at 80  C overnight and calcined at 550  C for 2 h. The Mg(OAc)2 and Zn(OAc)2 were selected as precursors for Mg and Zn elements respectively at a concentration of 10% w/w. The Mgo/SBA-15 and ZnO/SBA-15 were prepared using aqueous solutions of the magnesium or zinc acetate by incipient wetness impregnation of mesoporous silica, followed by calcination at 550  C for 2 h. 2.2. Catalyst characterization Textural properties of the catalysts were determined by nitrogen adsorption and desorption data on a volumetric apparatus ASAP 2010 (Micromeritics). The X-ray fluorescence (XRF) analysis was obtained in a Rigaku RIX 3100, Tube Rhodium (4 KW) Fluorescence Spectrometer. X-ray diffraction (XRD) patterns of catalysts were recorded in the 2q range 0.6e80 at a scan rate of 0.6 /min with a RIGAKU Ultimate IV X-ray diffractometer, using CuKa (l ¼ 1.54 Å) radiation. 29Si magic angle spinning solid state nuclear magnetic resonance (29Si MAS NMR) spectra were acquired on a Bruker DRX300 spectrometer, operating at 59.6 MHz, and equipped with 4 mm Bruker probe. Samples were spun at 5 KHz in 4 mm ZrO2 rotors, using caulinite (91.5 ppm) as secondary reference. Bloch decay pulse sequence with repetition times of 60 s was enough for quantitative measurements of distinct Si sites. The spectral simulations were carried out by using the DmFit program [20]. The SAXS measurements were performed at room temperature using the beam line of National Synchrotron Light Laboratory (LNLS), Campinas, Brazil. This beam line was equipped with an asymmetrically cut-and-bent silicon (111) monochromator that yielded a monochromatic (l ¼ 1.608 Å) and horizontally focused beam. The spectra were plot from scattering intensity I(q) as a function of modulus of scattering vector, q ¼ (4p/l) sin(q/2), where q is the scattering angle. The scattering intensity was normalized by subtracting background scattering. Each SAXS pattern corresponds to a data collection time of 300 s. 2.3. Esterification procedure A mixture of lauric acid (Vetec-Brazil, 20 mmol, 4 g), 1-butanol (Vetec- Brazil, 40 mmol, 17.76 g) and catalyst (5%w/w, 0.2 g) was added to a three-necked flask equipped with a reflux condenser.

3. Results and discussion

3.2. NMR analysis The 29Si MAS NMR spectra obtained for SBA-15, ZnO/SBA-15 and MgO/SBA-15 are shown in Fig. 1. The SBA-15 spectrum was featureless, due to the amorphous nature, in accordance with Hu et al. [23]. On the other hand after introduction of Zn and Mg the Q sites could be clearly seen: three peaks in the range 89 to 92 ppm, 102 to 105 ppm, and 106 to 116 ppm, due to Q2 [(HO)2-Si-(OSi)2 or (HO,M)-Si-(OSi)2], Q3 [(HO)-Si-(OSi)3 or (M)-Si-(OSi)3] and Q4 [Si-(OSi)4] sites, resp., M ¼ Zn or Mg. The spectra could be simulated and the relative amounts of silicon sites could be defined. The results revealed that whereas in SBA-15 the Q3 þ Q2/Q4 was 0.11, for the catalysts ZnSBA-15 and Mg-SBA-15 the Q3 þ Q2/Q4 ratio was 0.76, evidencing the incorporation of Zn and Mg into the SBA-15, through the formation of SieOeMg and SieOeZn on the surface of Si walls. 3.3. XRD analysis Fig. 2 shows the XRD patterns of SBA-15, ZnO/SBA-15 and MgO/ SBA-15 catalysts. Only the well-known broad peak centered around 22 associated with amorphous silica is observed in all the patterns. This result reveals the amorphous character of the studied samples at the local scale. The same results were found in the literature for the impregnated Mg-modified and Zn-modified SBA-15 supports and the authors have concluded that the absence of peaks was indicative of a high dispersion of these oxides over the support [21,24]. Table 1 Properties of SBA-15 catalysts. Catalyst

S (m2 g1)

V (cm3 g1)

MgO (%)

ZnO (%)

SBA-15 Mg-SBA-15 Zn-SBA-15

673 553 515

0.89 0.78 0.72

e 2.23 e

e e 1.59

S-Surface area determined by BET method. V-Pore Volume determined around saturation pressure.

S.D.T. Barros et al. / Renewable Energy 50 (2013) 585e589

Fig. 1. Observed (top) and simulated (down)


Si MAS NMR spectra of SBA-15, ZnO/SBA-15 and MgO/SBA-15.

3.4. SAXS analyses Fig. 3 shows the SAXS patterns of SBA-15, ZnO/SBA-15 and MgO/ SBA-15. For all samples the spectra are very similar and show the presence of the three reflections (100), (110) and (200) correpffiffiffi sponding to relative q-spacings of 1, 3 and 2. This feature reveals the existence of an hexagonally ordered structure at nanometric scale, attributed to an hexagonal arrangement of mesoporous silica-based cylinders. The SAXS results shown that these structures, which have been observed for SBA-15 samples prepared in a similar manner [19], also exist for ZnO/SBA-15 and MgO/SBA-15. This feature is consistent with the facts that the amount of metal ions is very low compared to silica and NMR results have shown that metallic species are bonded to SieO groups. The distance a between the centers of two cylinders (unit cell parameter) can be calculated from

a ¼

 pffiffiffi 4p=q100 , 3


The thickness E of the cylinder walls can be estimated from

E ¼ a  2Ra



where Ra is the internal radius of the cylinders, which is related to the ratio d between the volume of surfactant used in the synthesis and the volume of the final dried sample through the relation

pffiffiffi Ra ¼ R d where R ¼ (a/2) being the external radius of the cylinders. Finally, the correlation length Lc associated to the extension of the ordered mesoporous domains can be calculated from the full weight at half maximum DL of the main reflection (100): Lc ¼ 4p/DL. Table 2 shows the parameters q100, a, R, Ra, E, DL and Lc for the three samples described above. The nanostructural parameters are very similar for all samples evidencing that the impregnation method for incorporation of Zn and Mg in mesoporous silica does not affect its structure at a nanometer scale as described in the literature [25]. Note that the pore diameter of the catalysts should be estimated from the internal diameter Da ¼ 2Ra of the cylinders calculated by SAXS. The value of around 28 nm for our samples is larger than that in the literature for SBA-15 [19], which is consistent with the lower specific surface measured by N2 adsorption (Table 1). The pore wall thickness for all samples is similar to those obtained by Fulvio et al. [19] for pure SBA-15, showing that incorporation of Zn and Mg in


100 S B A -1 5 Z n -S B A -1 5 M g -S B A -1 5


I(q ) (a rb .u n its )









0 ,5 0

0 ,7 5

1 ,0 0

1 ,2 5

200 1 ,5 0

1 ,7 5


q (n m ) Fig. 2. DRX spectra of SBA-15, ZnO/SBA-15 and MgO/SBA-15.

Fig. 3. SAXS spectra of SBA-15, ZnO/SBA-15 and MgO/SBA-15.

2 ,0 0


S.D.T. Barros et al. / Renewable Energy 50 (2013) 585e589

Table 2 Parameters q100, a, R, Ra, E, DL and Lc for the three studied samples. Sample

q100 (nm1)

a (nm)

R (nm)


Ra (nm)

E (nm)

DL (nm1)

Lc (nm)

SBA-15 Zn-SBA-15 Mg-SBA-15

0.71 0.71 0.71

30.6 30.6 30.6

15.3 15.3 15.3

0.88 0.88 0.88

14.3 14.3 14.3

2.0 2.0 2.0

0.120 0.136 0.136

105 92.3 92.3

oil with ethanol. They found that MgOeimpregnated SBA-15 presented best results for biodiesel production [14]. To the best of our knowledge this paper reports for the first time the esterification of lauric acid promoted by basic SBA-15 derived catalysts. We achieved an 81% yield in butyl ester under atmospheric pressure and reflux conditions. 4. Conclusions

mesoporous silica should not lead to a decrease in thermomechanical stability of the catalyst. By comparing the results of SAXS and 29Si MAS NMR it can be suggested that Zn and Mg are probably located on the surface of the silica walls, and not inside the pore space. 3.5. Esterification reactions The results of the esterification reaction of lauric acid with butanol are presented in Table 3. It can be seen that in the reaction performed without catalyst or by employing SBA-15 the conversion of lauric acid was very low, less than 25%. This result is similar to those reported by Albuquerque et al. [26] and Sun et al. [22]. Compared to the autocatalyzed reactions, both MgO/SBA-15 and ZnO/SBA-15 presented better yields for esterification reaction despite the low concentration of metals in the support. Our results obtained for the esterification were compared to other catalysts reported in literature (Table 3). MgO/SBA-15 gave similar value for lauric acid conversion that obtained in the homogeneous and heterogeneous catalysis [27,28] and superior to that obtained in the heterogeneous catalysis with a zeolite (ZSM-5) [29], CaO [16] and a mineral clay [17]. The mechanism proposed for the esterification reactions in the presence of MgO/SBA-15 considered the alkoxide anion as the intermediate. In the first step alkoxide anion is formed on the surface of MgO/SBA-15. This anion attacks the carbonyl carbon atom of the fatty acid to form a tetrahedral intermediate. In the last step the ester, a hydrophobic molecule [13] leaves the catalyst surface due to the hydrophilic character of the MgO/SBA-15. Antunes et al. [30] evaluated the catalytic activity of bulk MgO and ZnO on the transesterification of soybean oil with methanol, in an autoclave at 130  C, and the best results were achieved with MgO, whose greater basicity than ZnO was confirmed through the 4-hydroxy-4-methyl-2-pentanone retroaldolization, a model reaction for basic site evaluation. Liu et al. evaluated the basicity of MgO-impregnated different mesoporous silica by CO2 chemisorption and the catalytic activity in the transeterification of vegetable Table 3 Conversion of lauric acid with 1-butanol in the presence of different catalystsa. Catalyst

Time (h)

Conv. (%)

No SBA-15 ZnO/SBA-15 MgO/SBA-15 H2SO4 PTSAc 30%HPW/MCM-41d HZSM-5e

6 6 6 6 4 4 4 4

22 22 65 81 93b 87b 88b 39.1f

a Conditions: temperature 118  C, at atmospheric pressure, molar ratio alcohol/ acid ¼ 10/1, catalysts-5%w/w in relation to the acid. b Values obtained from literature by homogeneous and heterogeneous catalysts catalysis [27]. c PTSA- p-toluenesulfonic acid. d HPW- 12-tunstophsphoric acid; heterogeneous catalysts. e HZSM-5 zeolite type catalyst. f Values obtained from literature by heterogeneous catalysis [29].

Catalysts with Zn and Mg linked to SBA-15 surface have been successfully prepared and characterized. The nanostructural parameters obtained by SAXS analyses are very similar for all samples evidencing that the impregnation method for incorporation of Zn and Mg in mesoporous silica does not affect its structure. The catalysts were able to promote the esterification of lauric acid with 1-butanol giving good yields at ambient pressure. Acknowledgments The authors are grateful to ANP, FAPERJ and CNPq for financial support, to the Laboratório Nacional de Luz Síncrotron (LNLS, Campinas, Brazil) for the SAXS measurements and Prof. Simon Garden for the English revision. References [1] Lotero E, Liu Y, Lopez DE, Suwannakarn K, Bruce DA, Goodwin Jr JG. Synthesis of biodiesel via acid catalysis. Ind Eng Chem Res 2005;44:5353e63. [2] Rodrigues Jr JA, Cardoso FP, Lachter ER, Estevão LRM, Nascimento RSV. Correlating chemical structure and physical properties of vegetable oil esters. J Am Oil Chem Soc 2006;83:353e7. [3] Hoydonckx HE, De Vos DE, Chavan SA, Jacobs PA. Esterification and transesterification of renewable chemicals. Top Catal 2004;27:83e96. [4] Serio MDi, Tesser R, Pwengmel L, Santacesaria E. Heterogeneous catalysts for biodiesel production. Energy Fuels 2008;22:207e17. [5] Lopez DE, Goodwin Jr JG, Bruce DA, Lotero E. Transesterification of triacetin with methanol on solid acid and base catalysts. Appl Catal A Gen 2005;295: 97e105. [6] Rezende SM, Reis MC, Reid MG, Silva PL, Coutinho FMB, San Gil RAS, et al. Transesterification of vegetable oils promoted by poly(styrenedivinylbenzene) and poly(divinylbenzene). Appl Catal A Gen 2008;349: 198e203. [7] Kiss AA, Dimian AC, Rothemberg G. Solid acid catalysts for biodiesel production e-towards sustainable energy. Adv Synth Catal 2006;348:75e81. [8] Ni J, Meunier FC. Esterification of free fatty acids in sunflower oil over solid acid catalysts using batch and fixed bed-reactors. Appl Catal A Gen 2007;333: 122e30. [9] Mbaraka IK, Shanks BH. Conversion of oils and fats using advanced mesoporous heterogeneous catalysts. J Am Oil Chem Soc 2006;83:79e91. [10] Morin P, Sapaly G, Rocha MGC, de Oliveira PGP, Gonzalez WA, Sales EA, et al. Transesterification of rapeseed oil with ethanol: I. Catalysis with homogeneous Keggin heteropolyacids. Appl Catal A Gen 2007;330:69e76. [11] Melero JÁ, Iglesias J, Morales G. Heterogeneous acid catalysts for biodiesel production: current status and future challenges. Green Chem 2009;11: 1285e308. [12] Melero JA, Bautista LF, Morales G, Iglesias J, Briones D. Biodiesel production with heterogeneous sulfonic acid-functionalized mesostructured catalysts. Energy Fuels 2009;23:539e47. [13] Liu H, Xue N, Peng L, Guo X, Ding W, Chen Y. The hydrophilic/hydrophobic effect of porous solid acid catalysts on mixed liquid phase reaction of esterification. Catal Comm 2009;10:1734e7. [14] Li E, Rudolph V. Biodiesel production with heterogeneous sulfonic acidfunctionalized mesostructured catalysts. Energy Fuels 2008;22:145e9. [15] Melero JA, Bautista LF, Morales G, Iglesias J, Sánchez-Vásquez R. Biodiesel production from crude palm oil using sulfonic acid-modified mesostructured catalysts. Chem Eng J 2010;161:323e39. [16] Lisboa FS, Arizaga GGC, Wypych F. Esterification of free fatty acyds using layred carboxylates and hydroxide salts as catalysts. Top Catal 2011;8-9: 474e81. [17] Zatta L, Gardolinski JEC, Wypych F. Raw halloysite as reusable heterogeneous catalyst for esterification of lauric acid. Appl Clay Sci 2011;51:165e9. [18] Leadbeater NE, Barnard TM, Stencel L. Batch and continuous-flow preparation of biodiesel derived from butanol and facilitated by microwave heating. Energy Fuels 2008;22:2005e8. [19] Fulvio PF, Pikus S, Jaroniec M. Short-time synthesis of SBA-15 using various silica sources. J Colloid Interface Sci 2005;287:717e20.

S.D.T. Barros et al. / Renewable Energy 50 (2013) 585e589 [20] Massiot D, Fayon F, Capron M, King I, Le Calve S, Alonso B, et al. Modelling one- and two-dimensional solid-state NMR spectra. Magn Reson Chem 2002;40:70e6. [21] Vizcaíno AJ, Carrero A, Calles JÁ. Ethanol steam reforming on Mg- and Camodified CueNi/SBA-15 catalysts. Catal Today 2009;146:63e70. [22] Sun H, Han J, Ding Y, Li W, Duan J, Chen P, et al. One-pot synthesized mesoporous Ca/SBA-15 solid base for transesterification of sunflower oil with methanol. Appl Catal A Gen 2010;390:26e34. [23] Hu W, Luo Q, Su Y, Chen L, Yue Y, Ye C, et al. Acid sites in mesoporous Al-SBA15 material as revealed by solid-state NMR spectroscopy. Microporous Mesoporous Mater 2006;92:22e30. [24] Du J, Xu H, Shen J, Huang J, Shen W, Zhao D. Catalytic dehydrogenation and cracking of industrial dipentene over M/SBA-15 (M ¼ Al, Zn) catalysts. Appl Catal A Gen 2005;296:186e93. [25] Mihai GD, Meynem V, Mertens M, Bilba N, Cool P, Vansant EF. ZnO nanoparticles supported on mesoporous MCM-41 and SBA-15: a comparative physicochemical and photocatalytic study. J Mater Sci 2010;45:5786e94.


[26] Albuquerque MCG, Urbistondo IJ, González JS, Robles JMM, Tost RM, Castellón ER, et al. CaO supported on mesoporous silicas as basic catalysts for transesterification reactions. Appl Catal A Gen 2008;334:35e43. [27] Juan JC, Zhang J, Yarmoa MA. 12-Tungstophosphoric acid supported on MCM41 for esterification of fatty acid under solvent-free condition. J Mol Catal A 2007;267:265e71. [28] Hanh HD, Dong NT, Okitsu K, Nishimura R, Maeda Y. Biodiesel production by esterification of oleic acid with short-chain alcohols under ultrasonic irradiation condition. Renewable Energy 2009;34:780e3. [29] Juan JC, Zhang J, Yarmo MA. Study of catalysts comprising zirconium sulfate supported on a mesoporous molecular sieve HMS for esterification of fatty acids under solvent-free condition. Appl Catal A Gen 2008;347: 133e41. [30] Antunes WM, Veloso CO, Henriques CA. Transesterification of soybean oil with methanol catalyzed by basic solids. Catal Today 2008;133-35: 548e54.