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Effect of various functional groups on biodiesel synthesis from soybean oils by acidic ionic liquids
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Chinese Chemical Letters 23 (2012) 1107–1110 www.elsevier.com/locate/cclet
Effect of various functional groups on biodiesel synthesis from soybean oils by acidic ionic liquids Ming Ming Fan, Jing Jie Zhou, Qiu Ju Han, Ping Bo Zhang * The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China Received 19 April 2012 Available online 23 September 2012
Abstract Preparation of biodiesel from soybean oils catalyzed by five acidic ionic liquids with three cationic functional groups was investigated. The improvement of the catalytic activities was affected by various functional groups including pyridine group, Nmethylimidazole group, triethylamine group. Among them [C4SO3Hpy]HSO4 with pyridine group showed better catalytic activity with the biodiesel yield of 94.5%, and still yielded more than 90% after six successive uses. The possible mechanism was also discussed by two reaction paths in detail. # 2012 Ping Bo Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Biodiesel; Acidic ionic liquid; Soybean oils; Transesterification
Because of rapidly increasing crude oil prices, limited fossil fuels, and intensified environment pollution, it is increasingly necessary to develop alternative clean and renewable energy sources. Biodiesel has been proposed as a replacement for conventional fossil fuels partly [1–4]. Efforts have been made in the development of base catalysts for biodiesel synthesis, such as Li-doped MgO catalysts [5], KOH/Al2O3 and KOH/NaY catalysts [6], solid base Mg–Zr catalysts [7], KF/CaO–Fe3O4 catalysts [8] and so on. However, vegetable oils may contain small amounts of water and free fatty acids (FFA). For a basecatalyzed transesterification, the basic catalyst will react with the FFA to form soap which will lower the yield of the biodiesel. The water produced during the saponification reaction will lead to the hydrolysis of the esters to form more FFA [2]. Acidic catalysts, such as sulfuric acid and benzene sulfonic acid, are also usually used in transesterification reactions. It is well known that acidic catalysts are corrosive for equipment and are not easily recovered, which inevitably results in a series of environmental problems. Recently, ionic liquids have been reported as the promising catalysts and alternative solvents for transesterification reaction for biodiesel synthesis [9–14]. The ionic liquids have important advantages over the other catalysts. However, in the case of synthesis of biodiesel by transesterification, only a few ionic liquids were used as the catalysts [9,13,14]. So it turns out to be highly interesting to further explore other possible applications of the catalytically attractive acidic ionic liquids and study the effect of various functional
* Corresponding author. E-mail address: [email protected] (P.B. Zhang). 1001-8417/$ – see front matter # 2012 Ping Bo Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2012.07.009
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M.M. Fan et al. / Chinese Chemical Letters 23 (2012) 1107–1110
groups on biodiesel synthesis from soybean oils by acidic ionic liquids. The objective of this work is to find a better acidic ionic liquid catalyst for the biodiesel synthesis. 1. Experimental Acidic ionic liquid catalysts preparation: under vigorous stirring without solvent, 0.1 mol the pyridine (or Nmethylimidazole, triethylamine) and equal-mole alkyl sultone was mixed in the 100 mL flask at 80 8C for 4 h. Then the reaction mixture was filtered to get the white precipitate. The precipitate was washed with ethanol thrice and was dried at 65 8C for 5 h. To weight 0.05 mol the amphion prepared, equal-mole H2SO4 was mixed in the 100 mL flask at 80 8C for 5 h under vigorous stirring. Then the reaction mixture was filtered to get the precipitate, and the precipitate was washed with ethyl acetate thrice and was dried at 60 8C for one day. Catalytic performance: weighed amounts of soybean oils, methanol and ionic liquid prepared were added to a flask having a reflux condenser, and a magnetic stirring apparatus. In typical experiment, 5 wt.% of catalyst and weighed amounts of methanol and soybean oils (molar ratio of methanol to soybean oils 8:1) were introduced into the reactor. The reaction was carried out at 120 8C for 8 h. Then the reaction mixture became biphasic, the upper phase, which was mainly the desired biodiesel, could be isolated simply by decantation; the lower phase, ionic liquid, methanol and glycerol. Quantitative analysis of the reaction products in biodiesel synthesis was carried out on a gas chromatograph (GC) (GC-122, SPSIC) with a FID detector. The infrared measurements were carried out with an ABB Bomem FTLA2000-104 spectrometer, using KBr pellets. 2. Results and discussion The modification of various functional groups appeared to have a more significant influence on the catalytic performance of biodiesel synthesis from cottonseed oil by acidic ionic liquids [10]. At this point, five acidic ionic liquids including various functional groups were investigated as the possible catalysts for synthesis of biodiesel from soybean oil in the first place. As shown in Table 1, five acidic ionic liquids with various functional groups had shown different catalytic performance in a certain extent. When the number of carbon atom was same, the catalytic activity of ionic liquid was closely related with the functional group containing nitrogen element. The catalytic activities of ionic liquids with different functional groups were in the following order: pyridine > N-methy-limidazole > triethylamine. When the functional group was same, the catalytic activities of ionic liquids increased with the increase of the number of carbon atom. The [C4SO3Hpy]HSO4 catalyst had shown favorable catalytic properties, and the yield of biodiesel was about 94.5%. The comparison of ionic liquids and H2SO4 system on the catalytic performance for biodiesel synthesis was investigated. The traditional concentrated sulfuric acid had shown favorable catalytic properties with the biodiesel yield of about 97.3%, and it was even better than the ionic liquid catalyst [C4SO3Hpy]HSO4 in some sort (compared with 94.5%) under the same reaction conditions, but traditional H2SO4 was corrosive for equipments and was also not easily recovered, so H2SO4 was not the best choice. Based on the excellent catalytic activities of the above acidic ionic liquid catalysts, we could draw a conclusion that the acidic ionic liquids should be as the promising catalysts for transesterification reaction of biodiesel synthesis. The experiments were done to test the utility of [C4SO3Hpy]HSO4 catalyst for repeated use at a 5% catalyst to the reactants weight ratio, a temperature of 120 8C, a methanol–soybean oil ratio of 8:1 and a reaction time of 8 h. The Table 1 Results of the biodiesel synthesis using different acidic ionic liquid catalysts. Catalysts
Yield of biodiesel (%)
[C3SO3HEt3N]HSO4 [C3SO3Hmim]HSO4 [C3SO3Hpy]HSO4 [C4SO3Hmim]HSO4 [C4SO3Hpy]HSO4 H2SO4
60.1 66.8 87.7 71.3 94.5 97.3
Reaction conditions: reaction temperature = 120 8C, reaction time = 8 h, methanol: soybean ratio of 8:1, m(catalyst)/m(reactants) = 5 wt.%.
M.M. Fan et al. / Chinese Chemical Letters 23 (2012) 1107–1110
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Fig. 1. IR spectra of five ionic liquids ((A) [C3SO3HEt3N]HSO4, (B) [C3SO3Hmim]HSO4, (C) [C3SO3Hpy]HSO4, (D) [C4SO3Hmim]HSO4, and (E) [C4SO3Hpy]HSO4).
experimental results showed that the [C4SO3Hpy]HSO4 catalyst still yielded 90.1% after six successive uses. Therefore [C4SO3Hpy]HSO4 catalyst not only exhibited a favorable catalytic performance during the transesterification reaction but also had good utility for repeated use. The infrared spectra of the ionic liquids samples were shown in Fig. 1. From Fig. 1, the bands at 1030 and 1167 cm 1, which were attributed to the S O asymmetric and the symmetric stretching vibration of HSO4 group, were found to be almost coincident with the data reported [15–17]. It was inferred that the ionic liquids samples had the O O HO S O
HO
RCOOCH3
S O
O
N
(2)
Mechanism 1 O O HO S O
HO
CH3OH
O
S O
O
O
N
O
O S
HO
S O O N OH
CH2OOCR
(1)
CH2OCR
CHOOCR
(1)
CHOCR
OH
CH2OOCR
CH2OCR OH O
O HO O
O S
HO
S O
O
HO O
N
O S
O
Mechanism 2 HO O
O S
HO
S O
O
N
(2)
CH3OH
O
S O
O
N
RCOOCH3
Scheme 1. The proposed reaction mechanism of ionic liquid [C4SO3Hpy]HSO4 catalyst.
M.M. Fan et al. / Chinese Chemical Letters 23 (2012) 1107–1110
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same anion group, which was the active group. The band at 2987 cm 1 of an ionic liquid could be interpreted as the C– H stretching vibration of N–CH2, and the 1489 cm 1 and 1400 cm 1 bands might be attributed to the CH2 and CH3 deformation vibration of N–CH2CH3. The bands at 3156, 3113 and 1570 cm 1 for B and D ionic liquids might be attributed to the stretching vibration of N–H, C–H and C N from imidazole ring, respectively. The bands at 3061, 1637 and 1498 cm 1 could refer to the stretching vibration of C–H, C N and C C from pyridine ring, respectively [15–17]. It could be speculated that these functional groups of ionic liquids clearly promoted the improvement of the catalytic activity. The proposed reaction mechanism of ionic liquid catalyst [C4SO3Hpy]HSO4 was illustrated in Scheme 1, based on the present experimental observations. As known, the H+ of Brønsted acid acted as the active center on biodiesel synthesis through transesterification of methanol [18]. In our case, the ionic liquid catalyst had two H+ of Brønsted acid. It was suggested that both H+ of Brønsted acid groups as active centers could interact with isolated electron pair at O atom in carbonyl group, which would promote two reaction paths. As shown in mechanism 1, the carbonyl group of the triglyceride was firstly attacked by H+ of cationic group of [C4SO3Hpy]HSO4 catalyst. During this process, an intermediate with the carbocation was produced. Further, intermediate product reacted rapidly with nucleophilic reagent (CH3OH) to produce methyl ester and diglyceride. Finally, the active species were reformed after H+ releasing. As a result, it was easy to understand that [C4SO3Hpy]HSO4 catalyst had good activity stability in biodiesel synthesis. In mechanism 2, H+ of anionic group of ionic liquid catalyst followed the same pathway of H+ of cationic group. The presence of both H+ of cationic group and anionic group not only provided two active species but also promoted the activity stability for the catalysts. 3. Conclusion The yields of the acidic ionic liquid samples were affected by various functional groups (such as amine, imidazole and pyridine groups) in preparation of biodiesel from soybean oils. The ionic liquids with pyridine group cloud be used as an effective catalyst for transesterification of soybean oil and methanol. The [C4SO3Hpy]HSO4 successfully catalyzed the transesterification, and yielded 94.5% methyl ester under the optimized conditions. The proposed mechanism of the synthesis of biodiesel was described by two reaction paths in detail. Acknowledgments The financial supports from the Specialized Research Fund for the Doctoral Program of Higher Education (New Teachers) (No. 20100093120003), and the Fundamental Research Funds for the Central Universities (No. JUSRP21112) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
S.P. Singh, D. Singh, Renew. Sustain. Energy Rev. 14 (2010) 200. D.Y.C. Leung, X. Wu, M.K.H. Leung, Appl. Energy 87 (2010) 1083. J.X. Wang, K.T. Chen, S.T. Huang, et al. Chin. Chem. Lett. 22 (2011) 1363. S. Zhou, L. Liu, B. Wang, et al. Chin. Chem. Lett. 23 (2012) 379. Z.Z. Wen, X.H. Yu, S.T. Tu, et al. Appl. Energy 87 (2010) 743. K. Noiroj, P. Intarapong, A. Luengnaruemitchai, et al. Renew. Energy 34 (2009) 1145. Y.S. Li, S.A. Lian, D.M. Tong, et al. Appl. Energy 88 (2011) 3313. S.Y. Hu, Y.P. Guan, Y. Wang, et al. Appl. Energy 88 (2011) 2685. H.B. Xing, T. Wang, Z.H. Zhou, et al. Ind. Eng. Chem. Res. 44 (2005) 4147. Q. Wu, H. Chen, M.H. Han, et al. Ind. Eng. Chem. Res. 46 (2007) 7955. B.A.D. Neto, M.B. Alves, A.A.M. Lapis, et al. J. Catal. 249 (2007) 154. X.Z. Liang, G.Z. Gong, H.H. Wu, et al. Fuel 88 (2009) 613. M.H. Han, W.L. Yi, Q. Wu, et al. Bioresour. Technol. 100 (2009) 2308. L. Zhang, M. Xian, Y.C. He, et al. Bioresour. Technol. 100 (2009) 4368. R.Y. Chen, Practical Guide to Infrared Spectrum, Tianjin Science Technology Press, Tianjin, 1992. Z.X. Zhang, F. Feng, T.J. Hang, Analysis of Organic Spectrum, People Health Press, Beijing, 1995. B.W. Wu, B. Zhang, X.Q. Yu, et al. Spectrosc. Spect. Anal. 26 (2006) 106. X.Y. Li, J.C. Jiang, K. Wang, et al. Biomass Chem. Eng. 44 (2010) 1.
Chinese Chemical Letters 23 (2012) 1107–1110 www.elsevier.com/locate/cclet
Effect of various functional groups on biodiesel synthesis from soybean oils by acidic ionic liquids Ming Ming Fan, Jing Jie Zhou, Qiu Ju Han, Ping Bo Zhang * The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China Received 19 April 2012 Available online 23 September 2012
Abstract Preparation of biodiesel from soybean oils catalyzed by five acidic ionic liquids with three cationic functional groups was investigated. The improvement of the catalytic activities was affected by various functional groups including pyridine group, Nmethylimidazole group, triethylamine group. Among them [C4SO3Hpy]HSO4 with pyridine group showed better catalytic activity with the biodiesel yield of 94.5%, and still yielded more than 90% after six successive uses. The possible mechanism was also discussed by two reaction paths in detail. # 2012 Ping Bo Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Biodiesel; Acidic ionic liquid; Soybean oils; Transesterification
Because of rapidly increasing crude oil prices, limited fossil fuels, and intensified environment pollution, it is increasingly necessary to develop alternative clean and renewable energy sources. Biodiesel has been proposed as a replacement for conventional fossil fuels partly [1–4]. Efforts have been made in the development of base catalysts for biodiesel synthesis, such as Li-doped MgO catalysts [5], KOH/Al2O3 and KOH/NaY catalysts [6], solid base Mg–Zr catalysts [7], KF/CaO–Fe3O4 catalysts [8] and so on. However, vegetable oils may contain small amounts of water and free fatty acids (FFA). For a basecatalyzed transesterification, the basic catalyst will react with the FFA to form soap which will lower the yield of the biodiesel. The water produced during the saponification reaction will lead to the hydrolysis of the esters to form more FFA [2]. Acidic catalysts, such as sulfuric acid and benzene sulfonic acid, are also usually used in transesterification reactions. It is well known that acidic catalysts are corrosive for equipment and are not easily recovered, which inevitably results in a series of environmental problems. Recently, ionic liquids have been reported as the promising catalysts and alternative solvents for transesterification reaction for biodiesel synthesis [9–14]. The ionic liquids have important advantages over the other catalysts. However, in the case of synthesis of biodiesel by transesterification, only a few ionic liquids were used as the catalysts [9,13,14]. So it turns out to be highly interesting to further explore other possible applications of the catalytically attractive acidic ionic liquids and study the effect of various functional
* Corresponding author. E-mail address: [email protected] (P.B. Zhang). 1001-8417/$ – see front matter # 2012 Ping Bo Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2012.07.009
1108
M.M. Fan et al. / Chinese Chemical Letters 23 (2012) 1107–1110
groups on biodiesel synthesis from soybean oils by acidic ionic liquids. The objective of this work is to find a better acidic ionic liquid catalyst for the biodiesel synthesis. 1. Experimental Acidic ionic liquid catalysts preparation: under vigorous stirring without solvent, 0.1 mol the pyridine (or Nmethylimidazole, triethylamine) and equal-mole alkyl sultone was mixed in the 100 mL flask at 80 8C for 4 h. Then the reaction mixture was filtered to get the white precipitate. The precipitate was washed with ethanol thrice and was dried at 65 8C for 5 h. To weight 0.05 mol the amphion prepared, equal-mole H2SO4 was mixed in the 100 mL flask at 80 8C for 5 h under vigorous stirring. Then the reaction mixture was filtered to get the precipitate, and the precipitate was washed with ethyl acetate thrice and was dried at 60 8C for one day. Catalytic performance: weighed amounts of soybean oils, methanol and ionic liquid prepared were added to a flask having a reflux condenser, and a magnetic stirring apparatus. In typical experiment, 5 wt.% of catalyst and weighed amounts of methanol and soybean oils (molar ratio of methanol to soybean oils 8:1) were introduced into the reactor. The reaction was carried out at 120 8C for 8 h. Then the reaction mixture became biphasic, the upper phase, which was mainly the desired biodiesel, could be isolated simply by decantation; the lower phase, ionic liquid, methanol and glycerol. Quantitative analysis of the reaction products in biodiesel synthesis was carried out on a gas chromatograph (GC) (GC-122, SPSIC) with a FID detector. The infrared measurements were carried out with an ABB Bomem FTLA2000-104 spectrometer, using KBr pellets. 2. Results and discussion The modification of various functional groups appeared to have a more significant influence on the catalytic performance of biodiesel synthesis from cottonseed oil by acidic ionic liquids [10]. At this point, five acidic ionic liquids including various functional groups were investigated as the possible catalysts for synthesis of biodiesel from soybean oil in the first place. As shown in Table 1, five acidic ionic liquids with various functional groups had shown different catalytic performance in a certain extent. When the number of carbon atom was same, the catalytic activity of ionic liquid was closely related with the functional group containing nitrogen element. The catalytic activities of ionic liquids with different functional groups were in the following order: pyridine > N-methy-limidazole > triethylamine. When the functional group was same, the catalytic activities of ionic liquids increased with the increase of the number of carbon atom. The [C4SO3Hpy]HSO4 catalyst had shown favorable catalytic properties, and the yield of biodiesel was about 94.5%. The comparison of ionic liquids and H2SO4 system on the catalytic performance for biodiesel synthesis was investigated. The traditional concentrated sulfuric acid had shown favorable catalytic properties with the biodiesel yield of about 97.3%, and it was even better than the ionic liquid catalyst [C4SO3Hpy]HSO4 in some sort (compared with 94.5%) under the same reaction conditions, but traditional H2SO4 was corrosive for equipments and was also not easily recovered, so H2SO4 was not the best choice. Based on the excellent catalytic activities of the above acidic ionic liquid catalysts, we could draw a conclusion that the acidic ionic liquids should be as the promising catalysts for transesterification reaction of biodiesel synthesis. The experiments were done to test the utility of [C4SO3Hpy]HSO4 catalyst for repeated use at a 5% catalyst to the reactants weight ratio, a temperature of 120 8C, a methanol–soybean oil ratio of 8:1 and a reaction time of 8 h. The Table 1 Results of the biodiesel synthesis using different acidic ionic liquid catalysts. Catalysts
Yield of biodiesel (%)
[C3SO3HEt3N]HSO4 [C3SO3Hmim]HSO4 [C3SO3Hpy]HSO4 [C4SO3Hmim]HSO4 [C4SO3Hpy]HSO4 H2SO4
60.1 66.8 87.7 71.3 94.5 97.3
Reaction conditions: reaction temperature = 120 8C, reaction time = 8 h, methanol: soybean ratio of 8:1, m(catalyst)/m(reactants) = 5 wt.%.
M.M. Fan et al. / Chinese Chemical Letters 23 (2012) 1107–1110
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Fig. 1. IR spectra of five ionic liquids ((A) [C3SO3HEt3N]HSO4, (B) [C3SO3Hmim]HSO4, (C) [C3SO3Hpy]HSO4, (D) [C4SO3Hmim]HSO4, and (E) [C4SO3Hpy]HSO4).
experimental results showed that the [C4SO3Hpy]HSO4 catalyst still yielded 90.1% after six successive uses. Therefore [C4SO3Hpy]HSO4 catalyst not only exhibited a favorable catalytic performance during the transesterification reaction but also had good utility for repeated use. The infrared spectra of the ionic liquids samples were shown in Fig. 1. From Fig. 1, the bands at 1030 and 1167 cm 1, which were attributed to the S O asymmetric and the symmetric stretching vibration of HSO4 group, were found to be almost coincident with the data reported [15–17]. It was inferred that the ionic liquids samples had the O O HO S O
HO
RCOOCH3
S O
O
N
(2)
Mechanism 1 O O HO S O
HO
CH3OH
O
S O
O
O
N
O
O S
HO
S O O N OH
CH2OOCR
(1)
CH2OCR
CHOOCR
(1)
CHOCR
OH
CH2OOCR
CH2OCR OH O
O HO O
O S
HO
S O
O
HO O
N
O S
O
Mechanism 2 HO O
O S
HO
S O
O
N
(2)
CH3OH
O
S O
O
N
RCOOCH3
Scheme 1. The proposed reaction mechanism of ionic liquid [C4SO3Hpy]HSO4 catalyst.
M.M. Fan et al. / Chinese Chemical Letters 23 (2012) 1107–1110
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same anion group, which was the active group. The band at 2987 cm 1 of an ionic liquid could be interpreted as the C– H stretching vibration of N–CH2, and the 1489 cm 1 and 1400 cm 1 bands might be attributed to the CH2 and CH3 deformation vibration of N–CH2CH3. The bands at 3156, 3113 and 1570 cm 1 for B and D ionic liquids might be attributed to the stretching vibration of N–H, C–H and C N from imidazole ring, respectively. The bands at 3061, 1637 and 1498 cm 1 could refer to the stretching vibration of C–H, C N and C C from pyridine ring, respectively [15–17]. It could be speculated that these functional groups of ionic liquids clearly promoted the improvement of the catalytic activity. The proposed reaction mechanism of ionic liquid catalyst [C4SO3Hpy]HSO4 was illustrated in Scheme 1, based on the present experimental observations. As known, the H+ of Brønsted acid acted as the active center on biodiesel synthesis through transesterification of methanol [18]. In our case, the ionic liquid catalyst had two H+ of Brønsted acid. It was suggested that both H+ of Brønsted acid groups as active centers could interact with isolated electron pair at O atom in carbonyl group, which would promote two reaction paths. As shown in mechanism 1, the carbonyl group of the triglyceride was firstly attacked by H+ of cationic group of [C4SO3Hpy]HSO4 catalyst. During this process, an intermediate with the carbocation was produced. Further, intermediate product reacted rapidly with nucleophilic reagent (CH3OH) to produce methyl ester and diglyceride. Finally, the active species were reformed after H+ releasing. As a result, it was easy to understand that [C4SO3Hpy]HSO4 catalyst had good activity stability in biodiesel synthesis. In mechanism 2, H+ of anionic group of ionic liquid catalyst followed the same pathway of H+ of cationic group. The presence of both H+ of cationic group and anionic group not only provided two active species but also promoted the activity stability for the catalysts. 3. Conclusion The yields of the acidic ionic liquid samples were affected by various functional groups (such as amine, imidazole and pyridine groups) in preparation of biodiesel from soybean oils. The ionic liquids with pyridine group cloud be used as an effective catalyst for transesterification of soybean oil and methanol. The [C4SO3Hpy]HSO4 successfully catalyzed the transesterification, and yielded 94.5% methyl ester under the optimized conditions. The proposed mechanism of the synthesis of biodiesel was described by two reaction paths in detail. Acknowledgments The financial supports from the Specialized Research Fund for the Doctoral Program of Higher Education (New Teachers) (No. 20100093120003), and the Fundamental Research Funds for the Central Universities (No. JUSRP21112) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
S.P. Singh, D. Singh, Renew. Sustain. Energy Rev. 14 (2010) 200. D.Y.C. Leung, X. Wu, M.K.H. Leung, Appl. Energy 87 (2010) 1083. J.X. Wang, K.T. Chen, S.T. Huang, et al. Chin. Chem. Lett. 22 (2011) 1363. S. Zhou, L. Liu, B. Wang, et al. Chin. Chem. Lett. 23 (2012) 379. Z.Z. Wen, X.H. Yu, S.T. Tu, et al. Appl. Energy 87 (2010) 743. K. Noiroj, P. Intarapong, A. Luengnaruemitchai, et al. Renew. Energy 34 (2009) 1145. Y.S. Li, S.A. Lian, D.M. Tong, et al. Appl. Energy 88 (2011) 3313. S.Y. Hu, Y.P. Guan, Y. Wang, et al. Appl. Energy 88 (2011) 2685. H.B. Xing, T. Wang, Z.H. Zhou, et al. Ind. Eng. Chem. Res. 44 (2005) 4147. Q. Wu, H. Chen, M.H. Han, et al. Ind. Eng. Chem. Res. 46 (2007) 7955. B.A.D. Neto, M.B. Alves, A.A.M. Lapis, et al. J. Catal. 249 (2007) 154. X.Z. Liang, G.Z. Gong, H.H. Wu, et al. Fuel 88 (2009) 613. M.H. Han, W.L. Yi, Q. Wu, et al. Bioresour. Technol. 100 (2009) 2308. L. Zhang, M. Xian, Y.C. He, et al. Bioresour. Technol. 100 (2009) 4368. R.Y. Chen, Practical Guide to Infrared Spectrum, Tianjin Science Technology Press, Tianjin, 1992. Z.X. Zhang, F. Feng, T.J. Hang, Analysis of Organic Spectrum, People Health Press, Beijing, 1995. B.W. Wu, B. Zhang, X.Q. Yu, et al. Spectrosc. Spect. Anal. 26 (2006) 106. X.Y. Li, J.C. Jiang, K. Wang, et al. Biomass Chem. Eng. 44 (2010) 1.