- Email: [email protected]
Modified bentonite as catalyst for esterification of oleic acid and ethanol
G Model
JTICE-867; No. of Pages 6 Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Modified bentonite as catalyst for esterification of oleic acid and ethanol Behzad Aghabarari a,*, Nasim Dorostkar b a b
Nanotechnology and Advanced Materials Division, Materials and Energy Research Center (MERC), PO box 14155-4777, Tehran, Iran Department of Chemistry, Isfahan University of Technology, Isfahan 8415683111, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 September 2013 Received in revised form 8 February 2014 Accepted 16 March 2014 Available online xxx
In this study, bentonite was modified with acidic ionic liquid (modified bentonite with ionic liquid 8B: MBIL8B) and characterized with XRD, BET, SEM and TGA analyses. The esterification of oleic acid with ethanol was studied over this modified bentonite and the effect of reaction temperature, type and amount of catalyst, molar ratio and reaction time was investigated. The results showed that the MBIL8B had higher catalytic activity than sulfuric acid and able to catalyze the esterification of oleic acid to its ethyl ester in 6.5 h with yield greater than 93%. Finally, we studied the catalytic activity of MBIL8B in the esterification of other fatty acids with different alcohols. ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Bentonite Nanopore Ionic liquid Esterification Biodiesel
1. Introduction Esterification reactions are widely used for the synthesis of raw materials for emulsifiers in foods, plastics, lubricants, paints, emollients in cosmetics products, and as basis materials in perfumes [1–3]. On the Other hand, nowadays the synthesis of biodiesel as green fuel from esterification of free fatty acids and the transesterification of vegetable oils and animal fats is of interest, because it has several environmental benefits such as it does not produce greenhouse effects and it is renewable energy supply [2–6]. Esterification of fatty acid is commercially homogeneous catalysis process where mineral acids catalyze this reaction. However, these mineral acids cannot be reused and have other drawbacks such as equipment corrosion, tedious workup procedure and environmental problems. The use of solid acid catalysts can reduce many of the technological and environmental problems inherent to mineral acids. Therefore, the usage of solid acid catalysts in the esterification of fatty acids increased rapidly in recent years [1,3]. These catalysts are favored by industries as recoverable and recyclable catalysts but they have some
* Corresponding author. Tel.: +98 2636280040; fax: +98 2636280030. E-mail addresses: [email protected], [email protected], [email protected] (B. Aghabarari).
disadvantages such as low activity and easy deactivation [7–9]. Therefore, usage of hybrid organic–inorganic materials as catalyst in the homogenous–heterogeneous catalysis process can overcome the problems of each system [10]. In recent decade, we witnessed a fast growth in the use of mineral clay as a catalyst because there are a plenty of clay minerals in the world with their great properties like mechanical and thermal stability, high surface area and ion exchange capacity. Surface modifications of clay have received attention because it allows the formation of new organoclays with desired properties. The negative charge in surface of bentonites silicate layers can make a sticky coating on polar compounds like ionic liquids and the interlayer cations of bentonite can be exchanged by various types of organic cations. In the literature there are some studies that have investigated properties of modified clay with ionic liquids and their applications [10–12]. Ionic liquids have a wide range of industrial applications due to their chemical and physical properties, such as negligible vapor pressure, adjustable hydrophilic/lipophilic properties, and thermal stability. For these reasons, researchers have reported some data on their utilization of Brønsted and Lewis acid ionic liquid in the esterification of fatty acids [8,13,14]. The modified clay with ionic liquid possess the advantages of both ionic liquids and solid supports and use of the organo-clays as catalysts in different reactions have other advantages such as easier separation, higher
http://dx.doi.org/10.1016/j.jtice.2014.03.006 1876-1070/ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006
G Model
JTICE-867; No. of Pages 6 B. Aghabarari, N. Dorostkar / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
2
Scheme 1. Synthesized ionic liquid.
activity, reusability and there is no impurity of catalyst in the product [10]. In continuation to our program for the development of acid catalyst, we used the best Brønsted acidic ionic liquid (Scheme 1) in our last project [13] for modification of Na-bentonite. The new organo-clay was characterized and used as catalyst in the esterification of oleic acid with ethanol. 2. Experimental 2.1. Chemicals and instruments All the chemicals (AR grade) were commercially available and used without further purification. X-ray diffraction (XRD) was measured on the X-Ray Diffractometer, (Bruker D8ADVANCE with Ni filtered Cu-Ka radiation). Scanning electron microscopy (SEM) images were taken with a Philips 30X, operating at 10 kV and with a beam current of 90 mA. A Micromeritics model Gemini 2360 surface area analyzer was used to measure the nitrogen adsorption isotherms of the samples at 77 K. The specific surface area, pore volume and pore diameter were determined from BET plots (p/ p8 = 0.05–0.95). Prior to the measurement, the samples were degassed at room temperature for 12–16 h in nitrogen flow. TGA was carried out using a BAHR-STA/TGA-503 at a heating rate of 10 8C/min in air atmosphere. All the products were identified by GC– MS-QP5050 SHIMADZU with CBP20 (25 m) capillary column and a FID detector. Also, after each experiment, the product was quantified by Agilent 6890N gas chromatograph equipped with a HP-50+ (60 m 0.25 mm 0.25 mm) capillary column and a FID detector. The tricaprylin was added as internal standard and parameters for the temperature program were: initial temperature = 1108C; final temperature = 270 8C (5 min), heating rate = 20 8C/min. 2.2. Synthesis of the catalyst 2.2.1. Synthesis of ionic liquid We used an acidic ionic liquid (IL8B) that was prepared according to reported method [13]. In a three-neck round bottom flask equipped with condenser, 100 mL of THF was stirred at 50 8C for 20 min with 150 mmol of K2CO3. To this suspension, 100 mmol of 1H-imidazole was added and then the mixture was refluxed for 2 h. Subsequently, 100 mmol) of benzyl bromide was added dropwise over a period of 60 min, and then the mixture was refluxed for 24 h. The solution was cooled to room temperature and about 40 mL of water was added. The aqueous layer was removed and extracted three times with dichloromethane. The combined organic layers were added to the THF solution, then dried over anhydrous sodium sulfate and the solvent was removed under vacuum. Then the synthesized 1-benzyl imidazole 20 mmol
was dissolved in 60 mL of acetonitrile in a three-neck round bottom flask and 10 mmol of 1,8-dibromobutane was added dropwise. The mixture was refluxed for 72 h. After cooling and filtration, the resulting ionic liquid (IL8A) was recrystallized twice in ethyl acetate (100 mL) and then dried under vacuum at 70 8C for 12 h. To the 10 g of concentrated H2SO4 (98%) that was previously cooled in an ice bath, IL8A 10 mmol was added slowly and mixed at this temperature for 8 h. After this time, the mixture was stirred for 24 h at 80 8C to form 3,30 -(octane-1,8-diyl)bis(4-sulfobenzyl-1Himidazol-3-ium) hydrogensulfate (IL8B). 2.2.2. Preparation of modified bentonites with IL8B ionic liquid The parent material, Salafchegan bentonite, had the following chemical composition (in wt%): SiO2 (65.04), Fe2O3 (1.67), MgO (1.87), Al2O3 (13.61), CaO (2.01), TiO2 (0.19), Na2O (2.26), K2O (0.75). Since the XRD analysis revealed the presence of significant amounts of quartz and feldspar, the raw material was purified using the following procedure. Bentonite (5% by mass), was dispersed in water under continuous stirring. After separation of quartz and feldspar, 1 M NaCl solution was added. The suspension was stirred overnight followed by decantation. The solid was washed until a negative test for chloride in solution was obtained. For the synthesis of modified bentonite, 1 g of bentonite was dispersed in 30 mL of absolute ethanol under vigorous stirring for 24 h. The suspension was heated to 60 8C, and a solution of the IL8B ionic liquid (50% excess ionic liquid, based on the cation exchange capacity of the Na-bentonite) in 20 mL absolute ethanol was added in small fractions. The stirring was continued for 12 h at 60 8C. The modified bentonite was then filtered and washed with ethanol (5 10 mL). The final product was dried at room temperature and then under vacuum at 80 8C for 5 h. The bentonite modified with IL8B ionic liquids was named MBIL8B. We used this modified bentonites as catalysts in the esterification of oleic acid with ethanol. 2.3. General procedure for synthesis of biodiesel The required amounts of oleic acid, ethanol and MBIL8B were added to a 50 mL round-bottom flask fitted with a reflux condenser. The esterification was typically allowed to proceed for 6 h with vigorous stirring on the IKA Magnetic hot plate stirrer and maintained at the desired temperature in oil bath. After the esterification reaction, the solid catalyst was easily separated from the reaction mixture by filtration, and washed with diethyl-ether to remove all the remaining reactants from the catalyst surface, dried in air and reused for the next experiment. The quantitative analysis of the product was carried out by GC using the internal standard. The yield of fatty acid ethyl esters was defined as the ratio of the weight of fatty acid ethyl esters determined by GC, to
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006
G Model
JTICE-867; No. of Pages 6 B. Aghabarari, N. Dorostkar / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
3
Fig. 1. XRD patterns of (a) parent bentonite (b) MBIL8B.
Table 1 BET surface area, pore volume and pore size of bentonite and MBIL8B. Entry
Specific surface area (m2/g)
Pore volume (cm3/g)
Pore size diameter (nm)
Bentonite MBIL8B
128 34
0.052 0.074
2.27 2.59
the weight of fatty acid ethyl esters must be produced in the reaction if the reaction was complete. 3. Characterization of catalysts The structure of modified bentonites was confirmed by XRD, BET, SEM and TGA analyses. 3.1. XRD Fig. 1 shows the XRD patterns of bentonite and modified bentonite with IL8B. The characteristic peak at 8.338 corresponds to d0 0 1 spacing of 10.6 A˚ in the case of parent bentonite. After modification of bentonite with IL8B ionic liquid, the basal spacing d0 0 1 shifted to 17.1 (2u = 5.098) for MBIL8B. This result indicated that the embedment of dicationic ionic liquids into the silicate layers enlarged the spacing. 3.2. BET and SEM Table 1 gives the BET specific surface area, pore volume and pore diameter data for the parent bentonite and the modified bentonite (MBIL8B). As expected, modification of bentonite with ionic liquid coverage decreased the specific surface area. But the presence of ionic liquid in the interlamellar region increase pore size from 2.27 nm to 2.59 nm. The scanning electron microscope images of the catalysts were analyzed (Fig. 2). It was found that the morphology of MBIL8B (Fig. 2b) are completely different from the unmodified bentonite (Fig. 2a), and modification had a strong influence on the particle structure. Also SEM images show that probably the presence of ionic liquids has influence on crystal growth during modification and provides highly crystalline and platelike particles with more pores in the surface of modified bentonites than parent bentonite. 3.3. TGA Thermal stability of the modified bentonite and unmodified bentonite could be inferred from their TGA thermograms shown in Fig. 3.
Fig. 2. SEM images of (a) parent bentonite (b) MBIL8B.
The parent bentonite shows two weight losses corresponding to (i) dehydration: desorption of internal and external water and (ii) dehydroxylation: the loss of crystal lattice structural water occurs around 650 8C. The intercalated ionic liquid increases thermal stability and the number of decomposition steps in the modified bentonite. The weight losses of the modified bentonite in the temperature range of dehydration are less than the parent bentonite. The first main degradation step for MBIL8B started at about 280 8C which corresponds to the ionic liquid combustion. These results might show that the p interactions between aromatic rings and the oxygen plane of the silicate layers have increased thermal stability of the modified bentonite [15]. In the elevated temperature (>450 8C), next stages of combustion attributed to oxidation of organic segments, whose oxidation has not been completed in the lower temperatures, such as aromatic rings. The quantities of ionic liquid on modified clay were calculated from the difference in the weight loss determined between unmodified clay and modified clay (MBIL8B) at 800 8C assuming that the weight losses in this temperature is due to the loss of ionic liquid, no adsorbed water. Based on this assumption, the amount of ionic liquid 8B on the modified bentonite (MBIL8B) was 16.37%. 4. Results and discussion 4.1. The effect of temperature Esterification of oleic acid with ethanol was carried out in the temperature range of 40–80 8C in steps of 208 for 6 h (Table 2). The molar ratio of ethanol (EA) to oleic acid (OA) was 2:1. Based on the weight of fatty acid a total of 5 wt% of the MBIL8B was used as the catalyst. Also we used H2SO4 as an industrial Brønsted acid catalyst
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006
G Model
JTICE-867; No. of Pages 6 B. Aghabarari, N. Dorostkar / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
4
Fig. 3. TGA curves of (a) parent bentonite (b) MBIL8B.
Table 2 Effect of reaction temperature and different catalysts on the esterification of oleic acid with ethanol. Entry
Catalyst
Temperature (8C)
Yield (%)
1 2 3 4 5 6 7 8 9
H2SO4 H2SO4 H2SO4 Bentonite Bentonite Bentonite MBIL8B MBIL8B MBIL8B
40 60 80 40 60 80 40 60 80
28.5 39.1 47.5 5.9 10.2 14.6 47.6 57.7 58.2
Reaction conditions: reaction temperature = 70 8C, EA/OA (molar ratio) = 2, reaction time = 2 h, amount of catalyst = 5% (based on the weight of fatty acid). Moreover, the results showed that the yield of esterification reaction over MBIL8B increased from 47.6 to 57.7% as the temperature increased from 40 to 60 8C and oleic acid conversion did not clearly increased above this temperature. Fig. 4. Effect of molar ratio on the esterification of oleic acid with ethanol.
and Na-bentonite for comparison with MBIL8B in this work. The results show that the incorporation of ionic liquid with bentonite enhances catalytic activity compared to the bentonite and H2SO4. According to TG analysis, the amount of ionic liquid in the MBIL8B was about 16.37%. Therefore, the esterification reaction can be happen over acid sites of both ionic liquid and bentonite. Clearly, the ionic liquid in the structure of MBIL8B has more contribution in the estrification reaction because the yield of reaction was improved from 14.6% for bentonite to 58.2% for modified bentonite. To compare the role of external and internal surfaces of MBIL8B in the esterification of oleic acid and ethanol, the structure of the oleic acid was optimised computationally using Gaussian 98 with the B3LYP/6-311G basis set. The distance between hydroxyl group and methyl group in oleic acid (18:1 cis-9) molecules was achieved lower than 1.3 nm. Therefore, it seems that the esterification of oleic acid and ethanol can be happen in internal surface of modified bentonite because the diameter size of nanopore (2.59 nm) in MBIL8B is bigger than size of oleic acid molecule (Table 1). 4.2. The effect of molar ratio and amount of the catalyst Fig. 4 shows the results of esterification of oleic acid with ethanol over MBIL8B catalyst using different ethanol (EA):oleic
acid (OA) molar ratios 1:1 to 4:1. The reaction was carried out at 60 8C for a period of 6 h. With increase of ethanol to oleic acid molar ratio, from 1:1 to 2:1, the ethyl ester yield increased from 40.5 to 57.5% that might be due to more accessibility of ethanol molecules for more attack to activated carbonyl group of oleic acid molecules over acidic site of the catalyst [1]. However, more increasing in molar ratio did not result in a major increase in the yield. The yield of oleic acid ethyl esters amplified with increase in amount of the catalyst from 1 to 12.5 wt% (Table 3), because the increase in amount of the catalyst to 12.5% provides a higher number of acid sites for activation of carbonyl groups of reactants [14,16]. The additional increase in amount of the catalyst to 15 wt%
Table 3 Effect of amount of catalyst on the esterification of oleic acid with ethanol. Entry
1
2
Amount of catalyst (%) Yield (%)
Blank
1
4.2
38.7
3 2.5 45.3
4 5 57.5
5 7.5 68.9
6
7
8
10
12.5
15
77.6
89.2
89.5
Reaction conditions: catalyst = MBIL8B, reaction temperature = 60 8C, EA/OA (molar ratio) = 2, reaction time = 6 h.
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006
G Model
JTICE-867; No. of Pages 6 B. Aghabarari, N. Dorostkar / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
5
Table 4 Effect of type of alcohol and fatty acid on the esterification reaction. Entry
Carboxylic acid
Alcohol
Yield (%)
1 2 3 4 5 6
Lauric acid (C12:0) Palmitic acid (C16:0) Stearic acid (C18:0) Oleic acid (C18:1) Oleic acid (C18:1) Oleic acid (C18:1)
Ethanol Ethanol Ethanol Methanol Ethanol n-Butanol
97.3 92.4 85.6 94.8 93.1 82.9
Reaction conditions: catalyst = MBIL8B, reaction temperature = 60 8C, EA/OA (molar ratio) = 2, reaction time = 6.5 h, catalyst weight = 12.5% (based on the weight of fatty acid).
Fig. 5. Effect of reaction time on the esterification of oleic acid with ethanol.
In agreement with literature, alcohol with longer alkyl chain has less nucleophilic property for the attack to an activated carbonyl group of fatty acid on the surface of catalyst. Also we concluded that the decrease in the yield along with the increase in the size of fatty acid alkyl chain might be as a result of decrease in the rate of mass transfer on the surface of catalyst. In the case of fatty acids with equal number of carbon in the alkyl chain, the presence of double bond on the alkyl chain operates as a determinant factor on the reaction conversion and presumably the higher electronic interaction of double bond with aromatic ring of the ionic liquid on the surface of the catalyst can accelerate mass transfer to the acid sites. 5. Conclusion
Fig. 6. Catalyst recycling of esterification of oleic acid with ethanol using MBIL8B.
had poor influence on reaction conversion. Thus the ideal amount of the catalyst was 12.5 wt%. 4.3. The effect of reaction time The effect of reaction time on the esterification of oleic acid determined by using 12.5 wt% MBIL8B catalyst, ethanol:oleic acid molar ratio of 2, 60 8C, as reaction temperature. Fig. 5 shows that the yield improved from 35.5% after 1 h to a maximum of about 93.1% for a period of 6.5 h, and exhibits minor change after this time. 4.4. Reusability The catalytic stability of the MBIL8B catalyst in the esterification of oleic acid is shown in Fig. 6. As mention in Section 2, after separating of the catalyst from the reaction mixture, washing and drying, it was charged into a new reaction cycle at optimized condition. It was probably shown that the activity of the catalyst remained almost steady after eight run and the decrease in yield was around 13% (first run = 93.1%, seventh run = 80.3%). In addition, this result indicates that the MBIL8B catalyst is recyclable and has good properties for the fatty acid esterification reaction. 4.5. The effect of the type of alcohol or fatty acid The result listed in Table 4 show that the type of alcohol and fatty acid had influence on the yield of esterification over MBIL8B catalyst at optimum reaction condition. We observed that the yield reduced when the alkyl chain of alcohol or fatty acid was extended.
In this paper, we have synthesized a modified bentonite with highly acidic ionic liquid and characterized by XRD, BET, SEM and TGA analyses. In the esterification of oleic acid with ethanol, reaction conversion greater than 93% was observed by changing several reaction parameters, such as temperature, molar ratio, amount of the catalyst and reaction time over the MBIL8B as the best catalyst. The reaction rate of esterification over modified bentonite decreased when the length of the alkyl chain in the fatty acid and alcohol increased as well as steric hindrances. Also, increasing in saturation degree of fatty acid had the same effect on the rate of reaction. Therefore, an environmentally friendly catalyst was provided which shows good catalytic performance and efficient reusablability in the esterification of fatty acids with alcohols. Acknowledgment Thanks are due to the Research Council of Materials and Energy Research Center (MERC) for supporting of this work. References [1] 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 2008;347:133–41. [2] Carmo Jr AC, de Souza LKC, da Costa CEF, Longo E, Zamian JR, da Rocha Filho GN. Production of biodiesel by esterification of palmitic acid over mesoporous aluminosilicate Al-MCM-41. Fuel 2009;88:461–8. [3] Alonso DM, Granados ML, Mariscal R, Douhal A. Polarity of the acid chain of esters and transesterification activity of acid catalysts. J Catal 2009;262:18–26. [4] Balat M, Balat H. A critical review of bio-diesel as a vehicular fuel. Energy Convers Manage 2008;49:2727–41. [5] Zabeti M, Wan Daud WMA, Aroua MK. Activity of solid catalysts for biodiesel production: a review. Fuel Process Technol 2009;90:770–7. [6] Russbueldt BME, Hoelderich WF. New sulfonic acid ion-exchange resins for the preesterification of different oils and fats with high content of free fatty acids. Appl Catal A 2009;362:47–57. [7] Park JY, Kim DK, Lee JS. Esterification of free fatty acids using water-tolerable Amberlyst as a heterogeneous catalyst. Bioresour Technol 2010;101:S62–5. [8] Zhao Y, Long J, Deng F, Liu X, Li Z, Xia C, Peng J. Catalytic amounts of Brønsted acidic ionic liquids promoted esterification: study of acidity–activity relationship. Catal Commun 2009;10:732–6.
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006
G Model
JTICE-867; No. of Pages 6 6
B. Aghabarari, N. Dorostkar / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
[9] Li X, Eli W. A green approach for the synthesis of long chain aliphatic acid esters at room temperature. J Mol Catal A: Chem 2008;279:159–64. [10] Ratti R, Kaur S, Vaultier M, Singh V. Preparation, characterization and catalytic activity of MMT-clay exchanged sulphonic acid functionalized ionic liquid for transesterification of b-ketoesters. Catal Commun 2010;11:503–7. [11] Awad WH, Gilman JW, Nyden M, Harris Jr RH, Sutto TE, Callahan J, et al. Thermal degradation studies of alkyl-imidazolium salts and their application in nanocomposites. Thermochim Acta 2004;409:3–11. [12] Hyun N, Malhotra SV, Xanthos M. Modification of cationic nanoclays with ionic liquids. Microporous Mesoporous Mater 2006;96:29–35.
[13] Aghabarari B, Dorostkar N, Ghiaci M, Rahimi E, Martinez-Huerta MV. Esterification of fatty acids by new ionic liquids as acid catalysts. J Taiwan Inst Chem Eng 2013. http://dx.doi.org/10.1016/j.jtice.08.003. [14] Zhang L, Xian M, He Y, Li L, Yang J, Yu S, et al. A Brønsted acidic ionic liquid as an efficient and environmentally benign catalyst for biodiesel synthesis from free fatty acids and alcohols. Bioresour Technol 2009;100:4368–73. [15] Park J-Y, Kim D-K, Lee J-S. Esterification of free fatty acids using water-tolerable Amberlyst as a heterogeneous catalyst. Bioresour Technol 2010;101:S62–5. [16] Caetano CS, Guerreiro L, Fonseca IM, Ramos AM, Vital J, Castanheiro JE. Esterification of fatty acids to biodiesel over polymers with sulfonic acid groups. App Catal A 2009;359:41–6.
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006
JTICE-867; No. of Pages 6 Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Modified bentonite as catalyst for esterification of oleic acid and ethanol Behzad Aghabarari a,*, Nasim Dorostkar b a b
Nanotechnology and Advanced Materials Division, Materials and Energy Research Center (MERC), PO box 14155-4777, Tehran, Iran Department of Chemistry, Isfahan University of Technology, Isfahan 8415683111, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 September 2013 Received in revised form 8 February 2014 Accepted 16 March 2014 Available online xxx
In this study, bentonite was modified with acidic ionic liquid (modified bentonite with ionic liquid 8B: MBIL8B) and characterized with XRD, BET, SEM and TGA analyses. The esterification of oleic acid with ethanol was studied over this modified bentonite and the effect of reaction temperature, type and amount of catalyst, molar ratio and reaction time was investigated. The results showed that the MBIL8B had higher catalytic activity than sulfuric acid and able to catalyze the esterification of oleic acid to its ethyl ester in 6.5 h with yield greater than 93%. Finally, we studied the catalytic activity of MBIL8B in the esterification of other fatty acids with different alcohols. ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Bentonite Nanopore Ionic liquid Esterification Biodiesel
1. Introduction Esterification reactions are widely used for the synthesis of raw materials for emulsifiers in foods, plastics, lubricants, paints, emollients in cosmetics products, and as basis materials in perfumes [1–3]. On the Other hand, nowadays the synthesis of biodiesel as green fuel from esterification of free fatty acids and the transesterification of vegetable oils and animal fats is of interest, because it has several environmental benefits such as it does not produce greenhouse effects and it is renewable energy supply [2–6]. Esterification of fatty acid is commercially homogeneous catalysis process where mineral acids catalyze this reaction. However, these mineral acids cannot be reused and have other drawbacks such as equipment corrosion, tedious workup procedure and environmental problems. The use of solid acid catalysts can reduce many of the technological and environmental problems inherent to mineral acids. Therefore, the usage of solid acid catalysts in the esterification of fatty acids increased rapidly in recent years [1,3]. These catalysts are favored by industries as recoverable and recyclable catalysts but they have some
* Corresponding author. Tel.: +98 2636280040; fax: +98 2636280030. E-mail addresses: [email protected], [email protected], [email protected] (B. Aghabarari).
disadvantages such as low activity and easy deactivation [7–9]. Therefore, usage of hybrid organic–inorganic materials as catalyst in the homogenous–heterogeneous catalysis process can overcome the problems of each system [10]. In recent decade, we witnessed a fast growth in the use of mineral clay as a catalyst because there are a plenty of clay minerals in the world with their great properties like mechanical and thermal stability, high surface area and ion exchange capacity. Surface modifications of clay have received attention because it allows the formation of new organoclays with desired properties. The negative charge in surface of bentonites silicate layers can make a sticky coating on polar compounds like ionic liquids and the interlayer cations of bentonite can be exchanged by various types of organic cations. In the literature there are some studies that have investigated properties of modified clay with ionic liquids and their applications [10–12]. Ionic liquids have a wide range of industrial applications due to their chemical and physical properties, such as negligible vapor pressure, adjustable hydrophilic/lipophilic properties, and thermal stability. For these reasons, researchers have reported some data on their utilization of Brønsted and Lewis acid ionic liquid in the esterification of fatty acids [8,13,14]. The modified clay with ionic liquid possess the advantages of both ionic liquids and solid supports and use of the organo-clays as catalysts in different reactions have other advantages such as easier separation, higher
http://dx.doi.org/10.1016/j.jtice.2014.03.006 1876-1070/ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006
G Model
JTICE-867; No. of Pages 6 B. Aghabarari, N. Dorostkar / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
2
Scheme 1. Synthesized ionic liquid.
activity, reusability and there is no impurity of catalyst in the product [10]. In continuation to our program for the development of acid catalyst, we used the best Brønsted acidic ionic liquid (Scheme 1) in our last project [13] for modification of Na-bentonite. The new organo-clay was characterized and used as catalyst in the esterification of oleic acid with ethanol. 2. Experimental 2.1. Chemicals and instruments All the chemicals (AR grade) were commercially available and used without further purification. X-ray diffraction (XRD) was measured on the X-Ray Diffractometer, (Bruker D8ADVANCE with Ni filtered Cu-Ka radiation). Scanning electron microscopy (SEM) images were taken with a Philips 30X, operating at 10 kV and with a beam current of 90 mA. A Micromeritics model Gemini 2360 surface area analyzer was used to measure the nitrogen adsorption isotherms of the samples at 77 K. The specific surface area, pore volume and pore diameter were determined from BET plots (p/ p8 = 0.05–0.95). Prior to the measurement, the samples were degassed at room temperature for 12–16 h in nitrogen flow. TGA was carried out using a BAHR-STA/TGA-503 at a heating rate of 10 8C/min in air atmosphere. All the products were identified by GC– MS-QP5050 SHIMADZU with CBP20 (25 m) capillary column and a FID detector. Also, after each experiment, the product was quantified by Agilent 6890N gas chromatograph equipped with a HP-50+ (60 m 0.25 mm 0.25 mm) capillary column and a FID detector. The tricaprylin was added as internal standard and parameters for the temperature program were: initial temperature = 1108C; final temperature = 270 8C (5 min), heating rate = 20 8C/min. 2.2. Synthesis of the catalyst 2.2.1. Synthesis of ionic liquid We used an acidic ionic liquid (IL8B) that was prepared according to reported method [13]. In a three-neck round bottom flask equipped with condenser, 100 mL of THF was stirred at 50 8C for 20 min with 150 mmol of K2CO3. To this suspension, 100 mmol of 1H-imidazole was added and then the mixture was refluxed for 2 h. Subsequently, 100 mmol) of benzyl bromide was added dropwise over a period of 60 min, and then the mixture was refluxed for 24 h. The solution was cooled to room temperature and about 40 mL of water was added. The aqueous layer was removed and extracted three times with dichloromethane. The combined organic layers were added to the THF solution, then dried over anhydrous sodium sulfate and the solvent was removed under vacuum. Then the synthesized 1-benzyl imidazole 20 mmol
was dissolved in 60 mL of acetonitrile in a three-neck round bottom flask and 10 mmol of 1,8-dibromobutane was added dropwise. The mixture was refluxed for 72 h. After cooling and filtration, the resulting ionic liquid (IL8A) was recrystallized twice in ethyl acetate (100 mL) and then dried under vacuum at 70 8C for 12 h. To the 10 g of concentrated H2SO4 (98%) that was previously cooled in an ice bath, IL8A 10 mmol was added slowly and mixed at this temperature for 8 h. After this time, the mixture was stirred for 24 h at 80 8C to form 3,30 -(octane-1,8-diyl)bis(4-sulfobenzyl-1Himidazol-3-ium) hydrogensulfate (IL8B). 2.2.2. Preparation of modified bentonites with IL8B ionic liquid The parent material, Salafchegan bentonite, had the following chemical composition (in wt%): SiO2 (65.04), Fe2O3 (1.67), MgO (1.87), Al2O3 (13.61), CaO (2.01), TiO2 (0.19), Na2O (2.26), K2O (0.75). Since the XRD analysis revealed the presence of significant amounts of quartz and feldspar, the raw material was purified using the following procedure. Bentonite (5% by mass), was dispersed in water under continuous stirring. After separation of quartz and feldspar, 1 M NaCl solution was added. The suspension was stirred overnight followed by decantation. The solid was washed until a negative test for chloride in solution was obtained. For the synthesis of modified bentonite, 1 g of bentonite was dispersed in 30 mL of absolute ethanol under vigorous stirring for 24 h. The suspension was heated to 60 8C, and a solution of the IL8B ionic liquid (50% excess ionic liquid, based on the cation exchange capacity of the Na-bentonite) in 20 mL absolute ethanol was added in small fractions. The stirring was continued for 12 h at 60 8C. The modified bentonite was then filtered and washed with ethanol (5 10 mL). The final product was dried at room temperature and then under vacuum at 80 8C for 5 h. The bentonite modified with IL8B ionic liquids was named MBIL8B. We used this modified bentonites as catalysts in the esterification of oleic acid with ethanol. 2.3. General procedure for synthesis of biodiesel The required amounts of oleic acid, ethanol and MBIL8B were added to a 50 mL round-bottom flask fitted with a reflux condenser. The esterification was typically allowed to proceed for 6 h with vigorous stirring on the IKA Magnetic hot plate stirrer and maintained at the desired temperature in oil bath. After the esterification reaction, the solid catalyst was easily separated from the reaction mixture by filtration, and washed with diethyl-ether to remove all the remaining reactants from the catalyst surface, dried in air and reused for the next experiment. The quantitative analysis of the product was carried out by GC using the internal standard. The yield of fatty acid ethyl esters was defined as the ratio of the weight of fatty acid ethyl esters determined by GC, to
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006
G Model
JTICE-867; No. of Pages 6 B. Aghabarari, N. Dorostkar / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
3
Fig. 1. XRD patterns of (a) parent bentonite (b) MBIL8B.
Table 1 BET surface area, pore volume and pore size of bentonite and MBIL8B. Entry
Specific surface area (m2/g)
Pore volume (cm3/g)
Pore size diameter (nm)
Bentonite MBIL8B
128 34
0.052 0.074
2.27 2.59
the weight of fatty acid ethyl esters must be produced in the reaction if the reaction was complete. 3. Characterization of catalysts The structure of modified bentonites was confirmed by XRD, BET, SEM and TGA analyses. 3.1. XRD Fig. 1 shows the XRD patterns of bentonite and modified bentonite with IL8B. The characteristic peak at 8.338 corresponds to d0 0 1 spacing of 10.6 A˚ in the case of parent bentonite. After modification of bentonite with IL8B ionic liquid, the basal spacing d0 0 1 shifted to 17.1 (2u = 5.098) for MBIL8B. This result indicated that the embedment of dicationic ionic liquids into the silicate layers enlarged the spacing. 3.2. BET and SEM Table 1 gives the BET specific surface area, pore volume and pore diameter data for the parent bentonite and the modified bentonite (MBIL8B). As expected, modification of bentonite with ionic liquid coverage decreased the specific surface area. But the presence of ionic liquid in the interlamellar region increase pore size from 2.27 nm to 2.59 nm. The scanning electron microscope images of the catalysts were analyzed (Fig. 2). It was found that the morphology of MBIL8B (Fig. 2b) are completely different from the unmodified bentonite (Fig. 2a), and modification had a strong influence on the particle structure. Also SEM images show that probably the presence of ionic liquids has influence on crystal growth during modification and provides highly crystalline and platelike particles with more pores in the surface of modified bentonites than parent bentonite. 3.3. TGA Thermal stability of the modified bentonite and unmodified bentonite could be inferred from their TGA thermograms shown in Fig. 3.
Fig. 2. SEM images of (a) parent bentonite (b) MBIL8B.
The parent bentonite shows two weight losses corresponding to (i) dehydration: desorption of internal and external water and (ii) dehydroxylation: the loss of crystal lattice structural water occurs around 650 8C. The intercalated ionic liquid increases thermal stability and the number of decomposition steps in the modified bentonite. The weight losses of the modified bentonite in the temperature range of dehydration are less than the parent bentonite. The first main degradation step for MBIL8B started at about 280 8C which corresponds to the ionic liquid combustion. These results might show that the p interactions between aromatic rings and the oxygen plane of the silicate layers have increased thermal stability of the modified bentonite [15]. In the elevated temperature (>450 8C), next stages of combustion attributed to oxidation of organic segments, whose oxidation has not been completed in the lower temperatures, such as aromatic rings. The quantities of ionic liquid on modified clay were calculated from the difference in the weight loss determined between unmodified clay and modified clay (MBIL8B) at 800 8C assuming that the weight losses in this temperature is due to the loss of ionic liquid, no adsorbed water. Based on this assumption, the amount of ionic liquid 8B on the modified bentonite (MBIL8B) was 16.37%. 4. Results and discussion 4.1. The effect of temperature Esterification of oleic acid with ethanol was carried out in the temperature range of 40–80 8C in steps of 208 for 6 h (Table 2). The molar ratio of ethanol (EA) to oleic acid (OA) was 2:1. Based on the weight of fatty acid a total of 5 wt% of the MBIL8B was used as the catalyst. Also we used H2SO4 as an industrial Brønsted acid catalyst
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006
G Model
JTICE-867; No. of Pages 6 B. Aghabarari, N. Dorostkar / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
4
Fig. 3. TGA curves of (a) parent bentonite (b) MBIL8B.
Table 2 Effect of reaction temperature and different catalysts on the esterification of oleic acid with ethanol. Entry
Catalyst
Temperature (8C)
Yield (%)
1 2 3 4 5 6 7 8 9
H2SO4 H2SO4 H2SO4 Bentonite Bentonite Bentonite MBIL8B MBIL8B MBIL8B
40 60 80 40 60 80 40 60 80
28.5 39.1 47.5 5.9 10.2 14.6 47.6 57.7 58.2
Reaction conditions: reaction temperature = 70 8C, EA/OA (molar ratio) = 2, reaction time = 2 h, amount of catalyst = 5% (based on the weight of fatty acid). Moreover, the results showed that the yield of esterification reaction over MBIL8B increased from 47.6 to 57.7% as the temperature increased from 40 to 60 8C and oleic acid conversion did not clearly increased above this temperature. Fig. 4. Effect of molar ratio on the esterification of oleic acid with ethanol.
and Na-bentonite for comparison with MBIL8B in this work. The results show that the incorporation of ionic liquid with bentonite enhances catalytic activity compared to the bentonite and H2SO4. According to TG analysis, the amount of ionic liquid in the MBIL8B was about 16.37%. Therefore, the esterification reaction can be happen over acid sites of both ionic liquid and bentonite. Clearly, the ionic liquid in the structure of MBIL8B has more contribution in the estrification reaction because the yield of reaction was improved from 14.6% for bentonite to 58.2% for modified bentonite. To compare the role of external and internal surfaces of MBIL8B in the esterification of oleic acid and ethanol, the structure of the oleic acid was optimised computationally using Gaussian 98 with the B3LYP/6-311G basis set. The distance between hydroxyl group and methyl group in oleic acid (18:1 cis-9) molecules was achieved lower than 1.3 nm. Therefore, it seems that the esterification of oleic acid and ethanol can be happen in internal surface of modified bentonite because the diameter size of nanopore (2.59 nm) in MBIL8B is bigger than size of oleic acid molecule (Table 1). 4.2. The effect of molar ratio and amount of the catalyst Fig. 4 shows the results of esterification of oleic acid with ethanol over MBIL8B catalyst using different ethanol (EA):oleic
acid (OA) molar ratios 1:1 to 4:1. The reaction was carried out at 60 8C for a period of 6 h. With increase of ethanol to oleic acid molar ratio, from 1:1 to 2:1, the ethyl ester yield increased from 40.5 to 57.5% that might be due to more accessibility of ethanol molecules for more attack to activated carbonyl group of oleic acid molecules over acidic site of the catalyst [1]. However, more increasing in molar ratio did not result in a major increase in the yield. The yield of oleic acid ethyl esters amplified with increase in amount of the catalyst from 1 to 12.5 wt% (Table 3), because the increase in amount of the catalyst to 12.5% provides a higher number of acid sites for activation of carbonyl groups of reactants [14,16]. The additional increase in amount of the catalyst to 15 wt%
Table 3 Effect of amount of catalyst on the esterification of oleic acid with ethanol. Entry
1
2
Amount of catalyst (%) Yield (%)
Blank
1
4.2
38.7
3 2.5 45.3
4 5 57.5
5 7.5 68.9
6
7
8
10
12.5
15
77.6
89.2
89.5
Reaction conditions: catalyst = MBIL8B, reaction temperature = 60 8C, EA/OA (molar ratio) = 2, reaction time = 6 h.
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006
G Model
JTICE-867; No. of Pages 6 B. Aghabarari, N. Dorostkar / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
5
Table 4 Effect of type of alcohol and fatty acid on the esterification reaction. Entry
Carboxylic acid
Alcohol
Yield (%)
1 2 3 4 5 6
Lauric acid (C12:0) Palmitic acid (C16:0) Stearic acid (C18:0) Oleic acid (C18:1) Oleic acid (C18:1) Oleic acid (C18:1)
Ethanol Ethanol Ethanol Methanol Ethanol n-Butanol
97.3 92.4 85.6 94.8 93.1 82.9
Reaction conditions: catalyst = MBIL8B, reaction temperature = 60 8C, EA/OA (molar ratio) = 2, reaction time = 6.5 h, catalyst weight = 12.5% (based on the weight of fatty acid).
Fig. 5. Effect of reaction time on the esterification of oleic acid with ethanol.
In agreement with literature, alcohol with longer alkyl chain has less nucleophilic property for the attack to an activated carbonyl group of fatty acid on the surface of catalyst. Also we concluded that the decrease in the yield along with the increase in the size of fatty acid alkyl chain might be as a result of decrease in the rate of mass transfer on the surface of catalyst. In the case of fatty acids with equal number of carbon in the alkyl chain, the presence of double bond on the alkyl chain operates as a determinant factor on the reaction conversion and presumably the higher electronic interaction of double bond with aromatic ring of the ionic liquid on the surface of the catalyst can accelerate mass transfer to the acid sites. 5. Conclusion
Fig. 6. Catalyst recycling of esterification of oleic acid with ethanol using MBIL8B.
had poor influence on reaction conversion. Thus the ideal amount of the catalyst was 12.5 wt%. 4.3. The effect of reaction time The effect of reaction time on the esterification of oleic acid determined by using 12.5 wt% MBIL8B catalyst, ethanol:oleic acid molar ratio of 2, 60 8C, as reaction temperature. Fig. 5 shows that the yield improved from 35.5% after 1 h to a maximum of about 93.1% for a period of 6.5 h, and exhibits minor change after this time. 4.4. Reusability The catalytic stability of the MBIL8B catalyst in the esterification of oleic acid is shown in Fig. 6. As mention in Section 2, after separating of the catalyst from the reaction mixture, washing and drying, it was charged into a new reaction cycle at optimized condition. It was probably shown that the activity of the catalyst remained almost steady after eight run and the decrease in yield was around 13% (first run = 93.1%, seventh run = 80.3%). In addition, this result indicates that the MBIL8B catalyst is recyclable and has good properties for the fatty acid esterification reaction. 4.5. The effect of the type of alcohol or fatty acid The result listed in Table 4 show that the type of alcohol and fatty acid had influence on the yield of esterification over MBIL8B catalyst at optimum reaction condition. We observed that the yield reduced when the alkyl chain of alcohol or fatty acid was extended.
In this paper, we have synthesized a modified bentonite with highly acidic ionic liquid and characterized by XRD, BET, SEM and TGA analyses. In the esterification of oleic acid with ethanol, reaction conversion greater than 93% was observed by changing several reaction parameters, such as temperature, molar ratio, amount of the catalyst and reaction time over the MBIL8B as the best catalyst. The reaction rate of esterification over modified bentonite decreased when the length of the alkyl chain in the fatty acid and alcohol increased as well as steric hindrances. Also, increasing in saturation degree of fatty acid had the same effect on the rate of reaction. Therefore, an environmentally friendly catalyst was provided which shows good catalytic performance and efficient reusablability in the esterification of fatty acids with alcohols. Acknowledgment Thanks are due to the Research Council of Materials and Energy Research Center (MERC) for supporting of this work. References [1] 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 2008;347:133–41. [2] Carmo Jr AC, de Souza LKC, da Costa CEF, Longo E, Zamian JR, da Rocha Filho GN. Production of biodiesel by esterification of palmitic acid over mesoporous aluminosilicate Al-MCM-41. Fuel 2009;88:461–8. [3] Alonso DM, Granados ML, Mariscal R, Douhal A. Polarity of the acid chain of esters and transesterification activity of acid catalysts. J Catal 2009;262:18–26. [4] Balat M, Balat H. A critical review of bio-diesel as a vehicular fuel. Energy Convers Manage 2008;49:2727–41. [5] Zabeti M, Wan Daud WMA, Aroua MK. Activity of solid catalysts for biodiesel production: a review. Fuel Process Technol 2009;90:770–7. [6] Russbueldt BME, Hoelderich WF. New sulfonic acid ion-exchange resins for the preesterification of different oils and fats with high content of free fatty acids. Appl Catal A 2009;362:47–57. [7] Park JY, Kim DK, Lee JS. Esterification of free fatty acids using water-tolerable Amberlyst as a heterogeneous catalyst. Bioresour Technol 2010;101:S62–5. [8] Zhao Y, Long J, Deng F, Liu X, Li Z, Xia C, Peng J. Catalytic amounts of Brønsted acidic ionic liquids promoted esterification: study of acidity–activity relationship. Catal Commun 2009;10:732–6.
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006
G Model
JTICE-867; No. of Pages 6 6
B. Aghabarari, N. Dorostkar / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx
[9] Li X, Eli W. A green approach for the synthesis of long chain aliphatic acid esters at room temperature. J Mol Catal A: Chem 2008;279:159–64. [10] Ratti R, Kaur S, Vaultier M, Singh V. Preparation, characterization and catalytic activity of MMT-clay exchanged sulphonic acid functionalized ionic liquid for transesterification of b-ketoesters. Catal Commun 2010;11:503–7. [11] Awad WH, Gilman JW, Nyden M, Harris Jr RH, Sutto TE, Callahan J, et al. Thermal degradation studies of alkyl-imidazolium salts and their application in nanocomposites. Thermochim Acta 2004;409:3–11. [12] Hyun N, Malhotra SV, Xanthos M. Modification of cationic nanoclays with ionic liquids. Microporous Mesoporous Mater 2006;96:29–35.
[13] Aghabarari B, Dorostkar N, Ghiaci M, Rahimi E, Martinez-Huerta MV. Esterification of fatty acids by new ionic liquids as acid catalysts. J Taiwan Inst Chem Eng 2013. http://dx.doi.org/10.1016/j.jtice.08.003. [14] Zhang L, Xian M, He Y, Li L, Yang J, Yu S, et al. A Brønsted acidic ionic liquid as an efficient and environmentally benign catalyst for biodiesel synthesis from free fatty acids and alcohols. Bioresour Technol 2009;100:4368–73. [15] Park J-Y, Kim D-K, Lee J-S. Esterification of free fatty acids using water-tolerable Amberlyst as a heterogeneous catalyst. Bioresour Technol 2010;101:S62–5. [16] Caetano CS, Guerreiro L, Fonseca IM, Ramos AM, Vital J, Castanheiro JE. Esterification of fatty acids to biodiesel over polymers with sulfonic acid groups. App Catal A 2009;359:41–6.
Please cite this article in press as: Aghabarari B, Dorostkar N. Modified bentonite as catalyst for esterification of oleic acid and ethanol. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.03.006