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Highly efficient procedure for the synthesis of biodiesel from soybean oil using chloroaluminate ionic liquid as catalyst
Fuel 88 (2009) 613–616
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Highly efficient procedure for the synthesis of biodiesel from soybean oil using chloroaluminate ionic liquid as catalyst Xuezheng Liang a, Guozhen Gong a, Haihong Wu a, Jianguo Yang a,b,* a b
Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal University, 3663 Northzhongshan Road, Shanghai 200062, China Energy Institute, Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA
a r t i c l e
i n f o
Article history: Received 5 August 2008 Received in revised form 24 September 2008 Accepted 25 September 2008 Available online 20 October 2008
a b s t r a c t The novel efficient procedure has been developed for the synthesis of biodiesel. The chloroaluminate ionic liquid has been selected for the synthesis of biodiesel. The catalyst was very efficient for the reaction with the yield of 98.5% when the reaction was carried out under the conditions of [Et3NH]Cl–AlCl3 (x(AlCl3) = 0.7), soybean oil 5 g, methanol 2.33 g, 9 h, 70 °C. Operational simplicity, low cost of the catalyst used, high yields, no saponification and reusability are the key features of this methodology. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Biodiesel Heterogeneous catalysis Ionic liquid Chloroaluminate
1. Introduction Biodiesel is well known as the replacement for the traditional mineral diesel fuel [1]. Biodiesel is characterized by excellent properties as diesel engine fuels and thus can be used in compression–ignition (diesel) engines with little or no modifications [2]. During the combustion process, biodiesel caused less emissions of SOx, CO, unburnt hydrocarbons and particulate matter than fossil fuels [3]. The limited resource of the vegetable oils was the key problem that affects the development of the biodiesel. Biodiesel is generally produced through the transesterification of vegetable oils to methyl esters. The transesterification of vegetable oils is a catalytic transesterification reaction where a triglyceride reacts with methanol producing glycerine and a mixture of fatty acid esters. Transesterification can be catalyzed by both acidic and basic catalysts [4]. Acidic catalyst, such as sulfuric acid, slowly catalyzes triglyceride transesterification (48–96 h) even at reflux of methanol, and a high molar ratio of methanol to oil is needed (30–150:1, by mol) [5]. Alkaline metal hydroxides (e.g., KOH and NaOH) are preferred as the basic catalysts. However, in the alkaline metal hydroxide catalyzed transesterification, even
* Corresponding author. Address: Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal University, 3663 Northzhongshan Road, Shanghai 200062, China. Tel.: +86 21 62233512/+1 814 8656617; fax: +86 21 62233424/+1 814 8653075. E-mail addresses: [email protected], [email protected] (J. Yang). 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.09.024
if a water-free vegetable oil and methanol are used, a certain amount of water is produced from the reaction of the hydroxide with methanol [6]. The presence of water leads to the hydrolysis of the esters, and as a result, soap is formed. The formation of soap reduces the biodiesel yield and causes significant difficulty in the separation of ester and glycerol. Moreover, in this conventional homogeneous method removal of these catalysts after reaction is technically difficult and a large amount of wastewater was produced to separate and clean the catalyst and the products. Therefore, conventional homogeneous catalysts are expected to be replaced in the near future by environmentally friendly heterogeneous catalysts. The replacement of homogeneous catalysts by heterogeneous catalysts would have various advantages. Recently, many different heterogeneous catalysts such as CaO, Sr(NO3)2/ZnO, alkylguanidines, KI/Al2O3, of Na/NaOH/Al2O3, WO3/ZrO2, ionexchange resin enzymes, ionic liquids have been developed to catalyze the transesterification of vegetable oils with methanol [7–20]. Although, some of these methods have convenient protocols with good to high yields, majority of these methods suffer at least from one of the following disadvantages: low activity, use of solvents, high temperature, long reaction time, moisture sensitivity of the catalyst, high cost and high toxicity, and saponification. Aiming to high efficient and green procedure for the synthesis of biodiesel, we developed a protocol for the transesterification of vegetable oils using ionic liquid as catalyst. The result showed that the catalyst showed very high activity for the reactions with the yield over 98%.
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2. Experimental
Table 1 The effect of the different Lewis acids of the catalyst on the reaction.
The methanol was purchased from Shanghai Chemicals Co. (>99%) and dried with 5A zeolite to remove the trace water. The soybean oil was obtained from the Shanghai Oil Plant. The oil was purified and dried carefully. The acid value of the oil was reduced to lower than 0.3 mg KOH/g and the water content below 10 ppm. The oil owned the average molecular weight of 850.
Entry
Anion
Yield (%)a,b
1 2 3 4 5
[Et3NH]Cl–AlCl3 [Et3NH]Cl–FeCl3 [Et3NH]Cl–ZnCl2 [Et3NH]Cl–SnCl4 [Et3NH]Cl–MgCl2
98.5 94.5 64.2 75.8 32.1
2.1. Synthesis of the catalyst The ionic liquids (ILs) were prepared by the standard procedures discussed in the literature [21]. The IL was prepared by mixing appropriate amount of metal chloride with the organic species while stirring at 90 °C under vacuum. All the operation should be carried out under the nitrogen atmosphere in the glove box. The care was taken to handle the moisture-related problems of the catalyst and the experiment was carried out in a glove box out of water and oxygen. The composition of metal chloride was based on the scale as: x(MXn) = n(MXn)/(n(MXn) + n[amine]X). 2.2. The procedure for the synthesis of biodiesel Typical procedure for the synthesis of biodiesel: Soybean oil (0.1 mol), methanol and the catalyst were mixed together in a three necked round bottomed flask equipped with a magnetic stirrer and a thermometer. The mixture was heated at 70 °C for the certain time as shown below. The process of the reaction was monitored by GC analysis of the small aliquots withdrawn at half an hour intervals. On completion, the excess methanol was distilled off under vacuum. After the products were centrifugated, it formed three phases, the upper layer was biodiesel, the middle layer was glycerol, and the lower layer was the catalyst. The biodiesel was collected for chromatographic analysis firstly. The IL in the residual was separated after removal of glycerol layer. The quantitative analysis of the extract solution was carried out on a temperature-programmed shimadzu (GC-14B) gas chromatograph using the inner standard. The fatty acid methyl esters yield in each experiment was calculated from its content in the composition as analyzed by GC. The yield was defined as a ratio of the weight of fatty acid methyl esters, determined by GC, to the weight of fatty acid methyl esters that the oil used in the reaction, which is given in theory.
3. Results and discussion 3.1. The effect of the different Lewis acids of the catalyst on the reaction It was known that the acidity of the catalyst was originated from the anion, so the effect of the different Lewis acids that formed anion part on the reaction was investigated first. The [Et3NH]Cl was chosen as the model amine that form the cation part for the catalyst (Table 1). The results showed that the Lewis acids had the key effect on the reaction. The Lewis acid such as Mg2+ and Zn2+ owned relatively low activity for the reaction with the low yield. The [Et3NH]Cl–AlCl3 showed the highest activities for the reaction with the yield of 98.5%, so AlCl3 was chosen for the reactions below. 3.2. The effect of the different amines of the catalyst on the reaction The catalyst was composed of anion and cation, so the effect of the amines that form the cation on the reaction was also observed (Table 2). The amines also had an important effect on the reaction.
a The reaction conditions: x(MCln) = 0.7, soybean oil 5 g, methanol 2.33 g, catalyst 5 mmol, 70 °C, 9 h. b The yield was calculated on GC using an internal standard.
The catalytic activities decreased with the length of the hydrocarbon chain of the amines for the steric hindrance. The aim of the transesterification was to reduce the viscosity of the vegetable oil. The long carbon chain made the mass transmission difficult in the reaction system, so the [Et3NH]Cl–AlCl3 was selected as the most efficient catalyst for the reaction. 3.3. The effect of x(AlCl3) of the catalyst [Et3NH]Cl–AlCl3 on the reaction The acidity of the catalyst was determined by the AlCl3 content, so the effect of the x(AlCl3) of the catalyst was discussed (Table 3). The results showed that the x(AlCl3) was very important for the reaction. It is known that the catalyst owned the acidity when the x(AlCl3) over 0.5 and the same trend can be obtained from the reaction results. The catalytic activity was very low when the x(AlCl3) below 0.5 and the yield achieved the maximum when the x(AlCl3) is 0.7, so the x(AlCl3) was chosen the value of 0.7 for the reactions below. 3.4. The effect of the molar ratio of the reactants on the reaction The molar ratio of the vegetable oil and methanol was also investigated (Table 4). The results showed the yield increased with the amount of the methanol and the peak value was achieved with the methanol of 2.33 g with the molar ratio of vegetable oil to
Table 2 The effect of the different amines of the catalyst on the reaction. Entry
Cation
Yield (%)a,b
6 7 8 9 10 11 12
[Et3NH]Cl–AlCl3 [mimH]Cl–AlCl3 [C16TA]Br–AlCl3 [C4min]Br–AlCl3 [C12min]Br–AlCl3 [C14min]Br–AlCl3 [C16min]Br–AlCl3
98.5 96.8 74.5 89.4 82.3 78.6 72.5
a The reaction conditions: x(MCln) = 0.7, soybean oil 5 g, methanol 2.33 g, catalyst 5 mmol, 70 °C, 9 h. b The yield was calculated on GC using an internal standard.
Table 3 The effect of the x(AlCl3) on the reaction. Entry
x(AlCl3)
Yield (%)a,b
13 14 15 16 17
0.3 0.5 0.6 0.7 0.8
15.2 35.5 82.9 98.5 98.4
a The reaction conditions: soybean oil 5 g, methanol 2.33 g, catalyst 5 mmol, 70 °C, 9 h. b The yield was calculated on GC using an internal standard.
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X. Liang et al. / Fuel 88 (2009) 613–616 Table 4 The effect of molar ratio on the reaction.
100
The amount of the methanol (g)
Yield (%)a,b
18 19 20 21 22
1.12 1.56 2.33 3.15 4.66
89.4 94.5 98.5 98.4 97.5
80
Yield/%
Entry
a The reaction conditions: [Et3NH]Cl–AlCl3 (x(AlCl3) = 0.7), soybean oil 5 g, catalyst 5 mmol, 70 °C, 9 h. b The yield was calculated on GC using an internal standard.
60
40
20
0 1
2
3
4
5
6
Run
methanol = 12. It is very useful for the reaction when increasing the amount of the methanol. Too much methanol may also cause the dilution effect and made the reaction undergo at the low temperature. 3.5. The effect of the reaction time on the reaction The effect of the reaction time on the yield was investigated (Fig. 1). It can be seen from Fig. 1 that the catalyst was efficient for the reaction with the yield over 98% 9 h later. The reaction went fast at first, and gradually reached the balance after 9 h. The yield changed little after 9 h, so 9 h was chosen for the optimum reaction time.
Fig. 2. The reuse of the catalyst.
Table 6 The comparison of different catalysts. Entry
Catalyst
Yield (%)a,b
28 29 30 31
IL H2SO4 H3PO4 PTSA
98.5 97.8 88.7 93.5
a The reaction conditions: catalyst 5 mmol, soybean oil 5 g, methanol 2.33 g, 9 h, 70 °C. b The yield was calculated on GC using an internal standard.
3.6. The effect of the catalyst concentration on the reaction The catalyst amount was also very important for the reaction (Table 5). There were not enough active sites for the reaction when the catalyst amount was too low while the reverse reaction took
3.7. The reuse of the catalyst One property of the catalyst is the reusability. On completion, the excess methanol was distilled off under vacuum. The catalyst was recovered by centrifugation and washed with ethyl acetate to remove the organic esters. The recovered activities were investigated carefully (Fig. 2). The results showed that the catalyst owned high stability during the reaction process and the yield remained unchanged even after the catalyst had been recycled for six times. Here the water content and the acidity of the oil had important effect on the reaction. When the raw oil with the acidity of 15 mg KOH/g and water content of 10,000 ppm was used in the reaction, the yield decreased to 50% with much soap formation during the reaction. Moreover, the catalyst decomposed in the condition, which made the reusability impossible.
10 0
80
Yield/%
place when too much catalyst was employed. So the optimal amount of the catalyst was 5 mmol.
60
40
20
0 2
4
6
8
10
12
3.8. The comparative study on the catalytic activities of the catalyst and other catalysts
Time, h Fig. 1. The effect of reaction time on the yield. (a) The reaction conditions: [Et3NH]Cl–AlCl3 (x(AlCl3) = 0.7), soybean oil 5 g, methanol 2.33 g, catalyst 5 mmol, 70 °C. (b) The yield was calculated on GC using an internal standard.
Table 5 The effect of the catalyst concentration on the reaction. Entry
Catalyst concentration (mmol)
Yield (%)a,b
23 24 25 26 27
1 3 5 7 9
88.7 94.1 98.5 98.6 98.5
a The reaction conditions: [Et3NH]Cl–AlCl3 (x(AlCl3) = 0.7), soybean oil 5 g, methanol 2.33 g, 9 h, 70 °C. b The yield was calculated on GC using an internal standard.
A comparative study on the catalytic activities of the ionic liquid with the reported catalysts was carried out (Table 6). From this study it can be concluded that the novel catalyst owned the comparative activity to the traditional homogeneous catalysts. It clearly shows that the novel catalyst should be considered as one of the best choice for the economically convenient, user-friendly catalyst and for the scaling up purpose. 4. Conclusion In conclusion, the ionic liquid with the composition of [Et3NH]Cl–AlCl3 (x(AlCl3) = 0.7) has been selected as the most efficient catalyst for the synthesis of biodiesel. The catalyst was very efficient for the reaction with the yield of 98.5% under the optimum reaction condition. The novel process owned many advantages such as operational simplicity, low cost of the catalyst used,
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high yields, no saponification, and reusability, which made it hold great potential for the green process. Acknowledgements This work was supported by National Key Project of Scientific and Technical Supporting Programs Funded by Ministry of Science and Technology of China (No. 2006BAE03B06), Shanghai Leading Academic Discipline Project, Project No. B409, and Shanghai International Cooperation of Science and Technology Project, Project No. 06SR07101. References [1] Lopez DE, Goodwin JG, Bruce DA, Furuta S. Esterification and transesterification using modified-zirconia catalysts. Appl Catal A: Gen 2008;339:76–83. [2] Martyanov IN, Sayari A. Comparative study of triglyceride transesterification in the presence of catalytic amounts of sodium, magnesium, and calcium methoxides. Appl Catal A: Gen 2008;339:45–52. [3] Dooley S, Curran HJ, Simmie JM. Autoignition measurements and a validated kinetic model for the biodiesel surrogate, methyl butanoate. Combust Flame 2008;153:2–32. [4] Valliyappan T, Bakhshi NN, Dalai AK. Pyrolysis of glycerol for the production of hydrogen or syngas. Bioresour Technol 2008;99:4476–83. [5] Ranganathan SV, Narasimhan SL, Muthukumar K. An overview of enzymatic production of biodiesel. Bioresour Technol 2008;99:3975–81. [6] Kaczmarek Z, Adamska E, Cegielska T, Szala L. The use of statistical methods to evaluate winter oilseed rape doubled haploids for industrial purposes. Ind Crop Prod 2008;27:348–53. [7] Welton T. Ionic liquids in catalysis. Coordin Chem Rev 2004;248:2459–77.
[8] Ha SH, Lan MN, Lee SH, Hwang SM, Koo YM. Lipase-catalyzed biodiesel production from soybean oil in ionic liquids. Enzyme Microb Technol 2007;41:480–3. [9] Bélafi-Bakó K, Kovács F, Gubicza L, Hancsók J. Enzymatic biodiesel production from sunflower oil by Candida antarctica lipase in a solvent-free system. Biocatal Biotrans 2002;20:437–9. [10] Song ES, Lim J, Lee HS, Lee YW. Transesterification of RBD palm oil using supercritical methanol. J Supercrit Fluid 2008;44:356–63. [11] Demirbas A. Relationships derived from physical properties of vegetable oil and biodiesel fuels. Fuel 2008;87:1743–8. [12] Sharma YC, Singh B. Development of biodiesel from karanja, a tree found in rural India. Fuel 2008;87:1740–2. [13] Lima JRO, Brandaoda SR, Miranda ME, Rodarte MCV. Biodiesel of tucum oil, synthesized by methanolic and ethanolic routes. Fuel 2008;87:1718–23. [14] Srivastava PK, Verma M. Methyl ester of karanja oil as an alternative renewable source energy. Fuel 2008;87:1673–7. [15] Brioude MM, Guimaraes DH, Fiuza RP, Prado LA, Sanches A, Boaventura JS, et al. Synthesis and characterization of aliphatic polyesters from glycerol, byproduct of biodiesel production, and adipic acid. Mater Res 2007;10:335–9. [16] Roskilly AP, Nanda SK, Wang YD, Chirkowski J. The performance and the gaseous emissions of two small marine craft diesel engines fuelled with biodiesel. Appl Therm Eng 2008;28:872–80. [17] Franceschini G, Macchietto S. Anti-correlation approach to model-based experiment design: application to a biodiesel production process. Ind Eng Chem Res 2008;47:2331–48. [18] Tittabut T, Trakarnpruk W. Metal-loaded MgAl oxides for transesterification of glyceryl tributyrate and palm oil. Ind Eng Chem Res 2008;47:2176–81. [19] Zafiropoulos NA, Ngo HL, Foglia TA, Samulski ET, Lin W. Catalytic synthesis of biodiesel from high free fatty acid-containing feedstocks. Chem Commun 2007:3670–2. [20] Kawashima A, Matsubara K, Honda K. Development of heterogeneous base catalysts for biodiesel production. Bioresour Technol 2008;99:3439–43. [21] Appleby D, Husse CL, Sedden KR, Turp JE. Room-temperature ionic liquids as solvents for electronic absorption spectroscopy of halide complexes. Nature 1986;323:614–6.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Highly efficient procedure for the synthesis of biodiesel from soybean oil using chloroaluminate ionic liquid as catalyst Xuezheng Liang a, Guozhen Gong a, Haihong Wu a, Jianguo Yang a,b,* a b
Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal University, 3663 Northzhongshan Road, Shanghai 200062, China Energy Institute, Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA
a r t i c l e
i n f o
Article history: Received 5 August 2008 Received in revised form 24 September 2008 Accepted 25 September 2008 Available online 20 October 2008
a b s t r a c t The novel efficient procedure has been developed for the synthesis of biodiesel. The chloroaluminate ionic liquid has been selected for the synthesis of biodiesel. The catalyst was very efficient for the reaction with the yield of 98.5% when the reaction was carried out under the conditions of [Et3NH]Cl–AlCl3 (x(AlCl3) = 0.7), soybean oil 5 g, methanol 2.33 g, 9 h, 70 °C. Operational simplicity, low cost of the catalyst used, high yields, no saponification and reusability are the key features of this methodology. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Biodiesel Heterogeneous catalysis Ionic liquid Chloroaluminate
1. Introduction Biodiesel is well known as the replacement for the traditional mineral diesel fuel [1]. Biodiesel is characterized by excellent properties as diesel engine fuels and thus can be used in compression–ignition (diesel) engines with little or no modifications [2]. During the combustion process, biodiesel caused less emissions of SOx, CO, unburnt hydrocarbons and particulate matter than fossil fuels [3]. The limited resource of the vegetable oils was the key problem that affects the development of the biodiesel. Biodiesel is generally produced through the transesterification of vegetable oils to methyl esters. The transesterification of vegetable oils is a catalytic transesterification reaction where a triglyceride reacts with methanol producing glycerine and a mixture of fatty acid esters. Transesterification can be catalyzed by both acidic and basic catalysts [4]. Acidic catalyst, such as sulfuric acid, slowly catalyzes triglyceride transesterification (48–96 h) even at reflux of methanol, and a high molar ratio of methanol to oil is needed (30–150:1, by mol) [5]. Alkaline metal hydroxides (e.g., KOH and NaOH) are preferred as the basic catalysts. However, in the alkaline metal hydroxide catalyzed transesterification, even
* Corresponding author. Address: Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal University, 3663 Northzhongshan Road, Shanghai 200062, China. Tel.: +86 21 62233512/+1 814 8656617; fax: +86 21 62233424/+1 814 8653075. E-mail addresses: [email protected], [email protected] (J. Yang). 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.09.024
if a water-free vegetable oil and methanol are used, a certain amount of water is produced from the reaction of the hydroxide with methanol [6]. The presence of water leads to the hydrolysis of the esters, and as a result, soap is formed. The formation of soap reduces the biodiesel yield and causes significant difficulty in the separation of ester and glycerol. Moreover, in this conventional homogeneous method removal of these catalysts after reaction is technically difficult and a large amount of wastewater was produced to separate and clean the catalyst and the products. Therefore, conventional homogeneous catalysts are expected to be replaced in the near future by environmentally friendly heterogeneous catalysts. The replacement of homogeneous catalysts by heterogeneous catalysts would have various advantages. Recently, many different heterogeneous catalysts such as CaO, Sr(NO3)2/ZnO, alkylguanidines, KI/Al2O3, of Na/NaOH/Al2O3, WO3/ZrO2, ionexchange resin enzymes, ionic liquids have been developed to catalyze the transesterification of vegetable oils with methanol [7–20]. Although, some of these methods have convenient protocols with good to high yields, majority of these methods suffer at least from one of the following disadvantages: low activity, use of solvents, high temperature, long reaction time, moisture sensitivity of the catalyst, high cost and high toxicity, and saponification. Aiming to high efficient and green procedure for the synthesis of biodiesel, we developed a protocol for the transesterification of vegetable oils using ionic liquid as catalyst. The result showed that the catalyst showed very high activity for the reactions with the yield over 98%.
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X. Liang et al. / Fuel 88 (2009) 613–616
2. Experimental
Table 1 The effect of the different Lewis acids of the catalyst on the reaction.
The methanol was purchased from Shanghai Chemicals Co. (>99%) and dried with 5A zeolite to remove the trace water. The soybean oil was obtained from the Shanghai Oil Plant. The oil was purified and dried carefully. The acid value of the oil was reduced to lower than 0.3 mg KOH/g and the water content below 10 ppm. The oil owned the average molecular weight of 850.
Entry
Anion
Yield (%)a,b
1 2 3 4 5
[Et3NH]Cl–AlCl3 [Et3NH]Cl–FeCl3 [Et3NH]Cl–ZnCl2 [Et3NH]Cl–SnCl4 [Et3NH]Cl–MgCl2
98.5 94.5 64.2 75.8 32.1
2.1. Synthesis of the catalyst The ionic liquids (ILs) were prepared by the standard procedures discussed in the literature [21]. The IL was prepared by mixing appropriate amount of metal chloride with the organic species while stirring at 90 °C under vacuum. All the operation should be carried out under the nitrogen atmosphere in the glove box. The care was taken to handle the moisture-related problems of the catalyst and the experiment was carried out in a glove box out of water and oxygen. The composition of metal chloride was based on the scale as: x(MXn) = n(MXn)/(n(MXn) + n[amine]X). 2.2. The procedure for the synthesis of biodiesel Typical procedure for the synthesis of biodiesel: Soybean oil (0.1 mol), methanol and the catalyst were mixed together in a three necked round bottomed flask equipped with a magnetic stirrer and a thermometer. The mixture was heated at 70 °C for the certain time as shown below. The process of the reaction was monitored by GC analysis of the small aliquots withdrawn at half an hour intervals. On completion, the excess methanol was distilled off under vacuum. After the products were centrifugated, it formed three phases, the upper layer was biodiesel, the middle layer was glycerol, and the lower layer was the catalyst. The biodiesel was collected for chromatographic analysis firstly. The IL in the residual was separated after removal of glycerol layer. The quantitative analysis of the extract solution was carried out on a temperature-programmed shimadzu (GC-14B) gas chromatograph using the inner standard. The fatty acid methyl esters yield in each experiment was calculated from its content in the composition as analyzed by GC. The yield was defined as a ratio of the weight of fatty acid methyl esters, determined by GC, to the weight of fatty acid methyl esters that the oil used in the reaction, which is given in theory.
3. Results and discussion 3.1. The effect of the different Lewis acids of the catalyst on the reaction It was known that the acidity of the catalyst was originated from the anion, so the effect of the different Lewis acids that formed anion part on the reaction was investigated first. The [Et3NH]Cl was chosen as the model amine that form the cation part for the catalyst (Table 1). The results showed that the Lewis acids had the key effect on the reaction. The Lewis acid such as Mg2+ and Zn2+ owned relatively low activity for the reaction with the low yield. The [Et3NH]Cl–AlCl3 showed the highest activities for the reaction with the yield of 98.5%, so AlCl3 was chosen for the reactions below. 3.2. The effect of the different amines of the catalyst on the reaction The catalyst was composed of anion and cation, so the effect of the amines that form the cation on the reaction was also observed (Table 2). The amines also had an important effect on the reaction.
a The reaction conditions: x(MCln) = 0.7, soybean oil 5 g, methanol 2.33 g, catalyst 5 mmol, 70 °C, 9 h. b The yield was calculated on GC using an internal standard.
The catalytic activities decreased with the length of the hydrocarbon chain of the amines for the steric hindrance. The aim of the transesterification was to reduce the viscosity of the vegetable oil. The long carbon chain made the mass transmission difficult in the reaction system, so the [Et3NH]Cl–AlCl3 was selected as the most efficient catalyst for the reaction. 3.3. The effect of x(AlCl3) of the catalyst [Et3NH]Cl–AlCl3 on the reaction The acidity of the catalyst was determined by the AlCl3 content, so the effect of the x(AlCl3) of the catalyst was discussed (Table 3). The results showed that the x(AlCl3) was very important for the reaction. It is known that the catalyst owned the acidity when the x(AlCl3) over 0.5 and the same trend can be obtained from the reaction results. The catalytic activity was very low when the x(AlCl3) below 0.5 and the yield achieved the maximum when the x(AlCl3) is 0.7, so the x(AlCl3) was chosen the value of 0.7 for the reactions below. 3.4. The effect of the molar ratio of the reactants on the reaction The molar ratio of the vegetable oil and methanol was also investigated (Table 4). The results showed the yield increased with the amount of the methanol and the peak value was achieved with the methanol of 2.33 g with the molar ratio of vegetable oil to
Table 2 The effect of the different amines of the catalyst on the reaction. Entry
Cation
Yield (%)a,b
6 7 8 9 10 11 12
[Et3NH]Cl–AlCl3 [mimH]Cl–AlCl3 [C16TA]Br–AlCl3 [C4min]Br–AlCl3 [C12min]Br–AlCl3 [C14min]Br–AlCl3 [C16min]Br–AlCl3
98.5 96.8 74.5 89.4 82.3 78.6 72.5
a The reaction conditions: x(MCln) = 0.7, soybean oil 5 g, methanol 2.33 g, catalyst 5 mmol, 70 °C, 9 h. b The yield was calculated on GC using an internal standard.
Table 3 The effect of the x(AlCl3) on the reaction. Entry
x(AlCl3)
Yield (%)a,b
13 14 15 16 17
0.3 0.5 0.6 0.7 0.8
15.2 35.5 82.9 98.5 98.4
a The reaction conditions: soybean oil 5 g, methanol 2.33 g, catalyst 5 mmol, 70 °C, 9 h. b The yield was calculated on GC using an internal standard.
615
X. Liang et al. / Fuel 88 (2009) 613–616 Table 4 The effect of molar ratio on the reaction.
100
The amount of the methanol (g)
Yield (%)a,b
18 19 20 21 22
1.12 1.56 2.33 3.15 4.66
89.4 94.5 98.5 98.4 97.5
80
Yield/%
Entry
a The reaction conditions: [Et3NH]Cl–AlCl3 (x(AlCl3) = 0.7), soybean oil 5 g, catalyst 5 mmol, 70 °C, 9 h. b The yield was calculated on GC using an internal standard.
60
40
20
0 1
2
3
4
5
6
Run
methanol = 12. It is very useful for the reaction when increasing the amount of the methanol. Too much methanol may also cause the dilution effect and made the reaction undergo at the low temperature. 3.5. The effect of the reaction time on the reaction The effect of the reaction time on the yield was investigated (Fig. 1). It can be seen from Fig. 1 that the catalyst was efficient for the reaction with the yield over 98% 9 h later. The reaction went fast at first, and gradually reached the balance after 9 h. The yield changed little after 9 h, so 9 h was chosen for the optimum reaction time.
Fig. 2. The reuse of the catalyst.
Table 6 The comparison of different catalysts. Entry
Catalyst
Yield (%)a,b
28 29 30 31
IL H2SO4 H3PO4 PTSA
98.5 97.8 88.7 93.5
a The reaction conditions: catalyst 5 mmol, soybean oil 5 g, methanol 2.33 g, 9 h, 70 °C. b The yield was calculated on GC using an internal standard.
3.6. The effect of the catalyst concentration on the reaction The catalyst amount was also very important for the reaction (Table 5). There were not enough active sites for the reaction when the catalyst amount was too low while the reverse reaction took
3.7. The reuse of the catalyst One property of the catalyst is the reusability. On completion, the excess methanol was distilled off under vacuum. The catalyst was recovered by centrifugation and washed with ethyl acetate to remove the organic esters. The recovered activities were investigated carefully (Fig. 2). The results showed that the catalyst owned high stability during the reaction process and the yield remained unchanged even after the catalyst had been recycled for six times. Here the water content and the acidity of the oil had important effect on the reaction. When the raw oil with the acidity of 15 mg KOH/g and water content of 10,000 ppm was used in the reaction, the yield decreased to 50% with much soap formation during the reaction. Moreover, the catalyst decomposed in the condition, which made the reusability impossible.
10 0
80
Yield/%
place when too much catalyst was employed. So the optimal amount of the catalyst was 5 mmol.
60
40
20
0 2
4
6
8
10
12
3.8. The comparative study on the catalytic activities of the catalyst and other catalysts
Time, h Fig. 1. The effect of reaction time on the yield. (a) The reaction conditions: [Et3NH]Cl–AlCl3 (x(AlCl3) = 0.7), soybean oil 5 g, methanol 2.33 g, catalyst 5 mmol, 70 °C. (b) The yield was calculated on GC using an internal standard.
Table 5 The effect of the catalyst concentration on the reaction. Entry
Catalyst concentration (mmol)
Yield (%)a,b
23 24 25 26 27
1 3 5 7 9
88.7 94.1 98.5 98.6 98.5
a The reaction conditions: [Et3NH]Cl–AlCl3 (x(AlCl3) = 0.7), soybean oil 5 g, methanol 2.33 g, 9 h, 70 °C. b The yield was calculated on GC using an internal standard.
A comparative study on the catalytic activities of the ionic liquid with the reported catalysts was carried out (Table 6). From this study it can be concluded that the novel catalyst owned the comparative activity to the traditional homogeneous catalysts. It clearly shows that the novel catalyst should be considered as one of the best choice for the economically convenient, user-friendly catalyst and for the scaling up purpose. 4. Conclusion In conclusion, the ionic liquid with the composition of [Et3NH]Cl–AlCl3 (x(AlCl3) = 0.7) has been selected as the most efficient catalyst for the synthesis of biodiesel. The catalyst was very efficient for the reaction with the yield of 98.5% under the optimum reaction condition. The novel process owned many advantages such as operational simplicity, low cost of the catalyst used,
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