Energy Fuels 2009, 23, 4630–4634 Published on Web 07/20/2009
: DOI:10.1021/ef9003736
Transesterification of Soybean Oil to Biodiesel over Heterogeneous Solid Base Catalyst Guangyuan Teng, Lijing Gao, Guomin Xiao,* and Hu Liu School of Chemistry and Chemical Engineering, Southeast University, 2 Southeast University Rd., Jiangning District, Nanjing 211189, People’s Republic of China Received April 27, 2009. Revised Manuscript Received June 24, 2009
Synthesis of biodiesel from soybean oil over solid base catalyst was investigated in this paper. In process of catalyst preparation, potassium fluoride was used as alkaline metal salt to load onto γ-Al2O3. Over this basic catalyst, transesterification reactions were carried out to prepare biodiesel using soybean oil and methanol as feedstock. According to experiments, the best reaction conditions were gotten as followed: load ratio was 72.68% (by weight), molar ratio was 12:1 (methanol/oil), reaction temperature was 338 K, mass of catalyst was 2% of oil (by weight), and reaction time was 3 h. Yield of biodiesel exceeds 99%. With X-ray diffraction (XRD) and DSC-TGA characterization, it was proved that new crystal phase synthesized by calcinations showed its favorable effect to the transesterification reaction. badly shortened the life span of fixings. Recently, studies of heterogeneous solid basic catalyst were becoming more and more attractive.10-13 Compare to others, solid basic catalyst had many strongpoints, such as excellent catalysis effects, easy to separate and clean, long usage life-span, simple to rebirth, and so on.14,15 Transesterification of soybean oil to produce biodiesel using loaded solid basic catalyst was investigated in this paper. As most common material of biodiesel, soybean oil was main materials of biodiesel in the world. Upper problems could be avoided efficiently using heterogeneous solid base as catalyst. In experiments, potassium fluoride was selected as active component to catalyze the reactions, while globular γ-Al2O3 was applied as carrier. Impregnation method was utilized in experiments of catalyst preparation. Then, five influence factors were investigated to get the best reaction conditions, which were load ratio, molar ratio of methanol to soybean oil, reaction temperature, catalyst concentration and reaction time.
1. Introduction Reserves of fossil fuels become less and less, however demand of energy increases faster and faster. Biodiesel is a promising nontoxic and biodegradable substitute of fossil diesel, which could be used directly or as diesel mixture in engines with little change. Furthermore, biodiesel produced from vegetable oil and animal fat promises its renewability. In addition, compared to fossil fuels, burning of biodiesel release much less SOx and NOx, which means biodiesel is environmentally friendly and harmless. Transesterification reactions of triglycerides (most components of oil and fat) and methanol or ethanol are the most important and common methods to produce biodiesel.1,2 Because of the strong alkaline, sodium and potassium hydroxides applied widely as basic catalyst in transesterification reactions to produce biodiesel currently.3-8 At the same time, removal of soluble alkaline hydroxide and separation and cleaning of product was water- and energy-wasted, accompanied with saponification reactions, which could decrease yield of biodiesel dramatically. Acid catalysts, such as sulfonic acid and hydrochloric acid, may dissolve this problem; however, relatively long time (48-96 h) and high molar ratio (30:1-150:1) was demanded,9 also it led to huge amount of wastewater. Besides, fearful and horrible corrosion of equipment took place by strong acidity of catalyst, which
2. Experimental Section 2.1. Material and Catalyst Preparation. Sinopharm Chemical Reagent Co., Ltd. produced methanol used in experiments. The reagent KF 3 2H2O was the product of Guangdong Guanghua Chemical Factory Co., Ltd. The spherical γ-Al2O3 was obtained from plant. The soybean oil was bought from market. Moreover, all the other used reagents were analytic grade. Impregnation method was selected to make loaded catalyst. Carrier γ-Al2O3 was activated at 673 K. After calming down, 100 g of activated γ-Al2O3 was immersed into 100 mL of KF solutions of different concentrations. After the mixture was preserved for 24 h, it was filtrated and the balls were washed
*To whom correspondence should be addressed. Telephone: þ8613605197225. Fax: þ86-25-52090612. E-mail:
[email protected]. (1) Furuta, S.; Matsuhashi, H.; Arata, K. Catal. Commun. 2004, 5, 721–723. (2) Lopez, D. E.; Goodwin, J. G.; Bruce, D. A. Jr.; Lotero, E. Appl. Catal. A-gen. 2005, 295, 97–105. (3) Sun, H.; Hu, K.; Lou, H.; Zhang, X. M. Energy Fuels 2008, 22, 2756–2760. (4) Antolin, G.; Tinaut, F. V.; Briceno, Y.; Castano, V.; Perez, C.; Ramirez, A. I. Bioresour. Technol. 2002, 83, 111–114. (5) Encinar, J. M.; Gonzalez, J. F.; Rodriguez, J. J.; Tejedor, A. Energy Fuels 2002, 16, 443–450. (6) Dmytryshyn, S. L.; Dalai, A. K.; Chaudhari, S. T.; Mishra, H. K.; Reaney, M. J. Bioresour. Technol. 2004, 92, 55–64. (7) Wang, L. Y.; He, H. Y.; Xie, Z. F.; Yang, J. C.; Zhu, S. L. Fuel Process. Technol. 2007, 88, 477–481. (8) Arzamendi, G.; Campo, I.; Arguinarena, E.; Sanchez, M.; Montes, M.; Gandia, L. M. Chem. Eng. J 2007, 134, 123–130. (9) Siler-Marinkovic, S.; Tomasevic, A. Fuel 1998, 77, 1389–1391. r 2009 American Chemical Society
(10) Xie, W. L.; Peng, H.; Chen, L. G. Appl. Catal. A-gen. 2006, 300, 67–74. (11) Liu, X. J.; He, H. Y.; Wang, Y. J.; Zhu, S. L.; Piao, X. L. Fuel 2008, 87, 216–221. (12) Kouzu, M.; Kasunao, T.; Tajika, M.; Sugimoto, Y.; Yamanaka, S.; Hidaka, J. Fuel 2008, 87, 2798–2806. (13) Garcia, C. M.; Teixeira, S.; Marciniuk, L. L.; Schuchardt, U. Bioresour. Technol. 2008, 99, 6608–6613. (14) Cui, L. F.; Xiao, G. M.; Xu, B.; Teng, G. Y. Energy Fuels 2007, 21, 3740–3743. (15) Xu, B.; Xiao, G. M.; Cui, L. F.; Wei, R. P.; Gao, L. J. Energy Fuels 2007, 21, 3109–3112.
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Energy Fuels 2009, 23, 4630–4634
: DOI:10.1021/ef9003736
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Figure 1. XRD spectrum of catalysts of KF/Al2O3; a: K3AlF6; b: Al2O3.
Figure 2. XRD spectrum of catalysts of different load ratio. 1, load ratio was 45.67%; 2, load ratio was 18.89%; a: K3AlF6; b: Al2O3.
with distilled water. Then the balls kept at normal temperature for 6-8 h. After that, γ-Al2O3 was taken into desiccator to dry and then underwent calcination at 873 K for 3 h. Solutions of different concentrations were prepared to make catalysts having different load ratios. 2.2. Catalyst Characterization. By a Japan D/max-5A mode diffract meter, X-ray investigation of powder of catalyst was performed. DSC-TGA spectrum of the catalyst also was tested. The samples were heated from 323 to 1173 K at a scanning rate of 15 K/min under a nitrogen atmosphere with a flow rate of 15 mL/min. 2.3. Transesterification Reaction. Experiments of transesterification reaction were finished in a 250 mL four-neck flask reactor. Methanol was added into the flask first. Soybean oil and catalyst were heated to setting temperature and then added into flask. The temperature of reactor with mixture inside was decreased to normal temperature as soon as possible once the reaction was finished. The mixture was filtered and taken to test the yield of methyl esters. First, the impact of load ratio was investigated while other influence factors were set invariable. Similarly, the next four factors to yield of product;molar ratio of methanol to soybean oil, reaction temperature, catalyst concentration (mass percentage of soybean oil), and reaction time;were studied one-byone. 2.4. Analysis. The yield of methyl ester of product was tested by gas chromatography equipped with a flame ionization detector, employing a 15 m long silica capillary column with an inner diameter of 0.32 mm. In GC patterns, content of all the fatty acid methyl esters was set, a. Similarly, content of monoglyceride, content of diglyceride and content of triglyceride (oil) was set as b, c, and d. In addition, regulation factors were set as fa, fb, fc, fd, respectively. Then the calculating equation was gotten as below, a fa yieldð%Þ ¼ ð1Þ a fa þb fb þc fc þd fd
Figure 3. DSC-TGA profiles of γ-Al2O3.
Al2O3 had happened when they were calcined under the temperature of 873 K. As a new substance, K3AlF6 was thought to be an effective ingredient to the transesterification reaction. Therefore, the calcination process was very important to the catalysts in which K3AlF6 was brought out. XRD spectra of different load ratios of catalysts were shown in Figure 2. As was shown, K3AlF6 peaks of catalyst of 45.67% load ratio were stronger than those of catalyst with a loading of 18.89%. It was a good explanation for why different load ratios had different catalysis effect. Therefore, it was determined that the corking catalysis effect of KF/γAl2O3 was due to K3AlF6, which was formed in the process of calcinations. DSC-TGA profiles of γ-Al2O3 and KF/γ-Al2O3 are shown in Figures 3 and 4, respectively. As is shown in Figure 3, weight losses were concentrated in the three temperature ranges of 363-413, 673-733, and 773-913 K. However, weight losses were symmetrically scattered in the range of 363-913 K in Figure 4. It was result of interaction of carrier and KF in the process of calcining. It also could be seen in Figure 4 that heat was absorbed at about 1123 K, although without weight loss. Therefore, a new phase took place in the interaction of KF with Al2O3. 3.2. Transesterification Reaction. Effect of calcination temperatures on yield of FAMEs (fatty acid methyl esters) is shown in Figure 5. Apparently, the yield was just 13% when calcination temperature was 573 K, which was very low. From 573 to 873 K of calcination temperature, yield of FAMEs increased drastically. It reached 68% when the temperature was 873 K. Calcination under 873 K was
3. Results and Discussion 3.1. Catalyst Characterization. Figure 1 showed XRD of the catalysts that were calcined and those that were not. It was apparent that Al2O3 diffraction peaks (peak b) of uncalcined catalyst were relatively bigger than the calcined ones. Moreover, a new kind of diffraction peak emerged after calcinations, which was thought to be K3AlF6.14-16 Thus, it could be considered that the interaction of KF and (16) Ando, T.; Clark, J. H.; Cork, D. G. Tetrahedron Lett. 1987, 28, 1421.
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: DOI:10.1021/ef9003736
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Figure 7. Effect of molar ratio of methanol/oil on the yield of FAMEs. Reaction conditions: load ratio was 72.68%; temperature was 328 K; catalyst concentration was 2%; and time was 3 h.
Figure 4. DSC-TGA profiles of KF/γ-Al2O3.
Figure 5. Effect of calcinations temperatures of KF/Al2O3 on the yield of FAMEs. Reaction conditions: molar ratio of methanol/oil was 9:1; temperature was 328 K; catalyst concentration was 2%; and time was 3 h.
Figure 8. Effect of reaction temperature on the yield of FAMEs. Reaction conditions: load ratio was 72.68%; molar ratio was 12:1; catalyst concentration was 2%; and time was 3 h.
Figure 9. Effect of catalyst concentration on the yield of FAMEs. Reaction conditions: load ratio was 72.68%; molar ratio was 12:1; temperature was 328 K; and time was 3 h.
Figure 6. Effect of mass ratio of KF/Al2O3 on the yield of FAMEs. Reaction conditions: molar ratio of methanol/oil was 9:1; temperature was 328 K; catalyst concentration was 2%; and time was 3 h.
thought to be the key and the best temperature to form effective component. Thus, 873 K was thought the ideal calcinations temperature. In addition, all the catalyst was calcinated under 873 K. Effect of the load ratio of KF/Al2O3 is shown in Figure 6. As it could be seen, the yield of FAMEs increased dramatically with load ratio increasing. The yield reached 94.33% when the load ratio was 72.68%. After the ratio was over 72.68%, the yield decreased. More basic centers formed with
loaded KF increasing; this brought stronger catalytic activity before load ratio was 72.68%. However, after load ratio being 72.68%, an excessive quantity of KF covered the surface of balls of γ-Al2O3, which reduced the amount of naked basic centers. Decrease of basic cores lowered the catalysis effect. Therefore, the yield of FAMEs declined. Thus, 72.68% was thought the best load ratio. Figure 7 shows the effect of molar ratio of methanol/oil on the yield of FAMEs. From 3:1 to 12:1 of molar ratio of 4632
Energy Fuels 2009, 23, 4630–4634
: DOI:10.1021/ef9003736
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methanol to oil, yield of FAMEs increased. The maximum FAMEs yield was 98.2%, when molar ratio was 12:1. After the molar ratio was over 12:1, the FAMEs yield commenced to descend tempestuously with molar ratio mounting up. This might be because from 3:1 to 12:1 of molar ratio, increase of methanol engendered augment of FAMEs yield. However, with molar ratio amounting up, higher molar ratio lowered the relative concentration of soybean oil in mixture. Low concentration of soybean oil brought out low reaction rate, which also changed the equilibrium of reaction. Thus, 12:1 was accepted as the best molar ratio. In Figure 8, it is shown that effect of reaction temperature on the yield of FAMEs. Obviously, yield of FAMEs increased as temperature rose. The FAMEs yield was just about 88% when reaction temperature was 308 K, and it even reached 99.13% when the temperature was 338 K. The yield under reaction temperature of 343 K was just a little higher than the value under 338 K. Higher temperature made molecules move more actively, which increased the collision probability of molecules of oil and methanol, then accelerated the reaction more fleetly and more easily. Therefore, yield of FAMEs increased with temperature, as Figure 8 shows. However, much higher temperature was not suggested. Because the boiling point of methanol was 337.7 K, methanol volatilized and became less involved when temperature was greater than 338 K. Thus, 338 K was selected as the best reaction temperature here. Effect of catalyst concentration on the yield is shown in Figure 9. Yield of FAMEs was just about 73% when concentration was 1%, and it increased hugely when catalyst concentration rose to 2%. It may be that 1% catalyst gave not enough active basic centers, which are an essential component of the reaction. When catalyst concentration was 2%, the basic centers were relatively much more than that of the 1% concentration’s to the reaction. However, when the concentration increased from 2 to 6%, yield increased slightly. Likely, catalyst of 2% concentration
supply relatively enough basic centers, which meant basic active centers were saturated by reactants of triglyceride and methanol. More catalyst gave more basic cores, but they were not very efficient to the reaction. Therefore, increase of catalyst could not amount up yield dramatically when it was from 2 to 6%, as Figure 9 shows. As shown in Figure 10, yield of FAMEs rose with reaction time increasing. Obviously, addition of time prolonged the contact time of reactants with catalyst. There was little reaction between soybean oil and methanol without the catalyst. Both oil and methanol must diffuse to the inner and outer surface of catalyst if the reaction took place. More time made reaction of transesterification moved to the reaction balance. Then, more triglycerides were transformed into methyl fatty acid. This was why FAMEs was heightened with reaction time increasing. On the other hand, the increase of yield of FAMEs became more and more slight when reaction time was increased from 2 to 6 h, as has been shown. This was because the effect of the increase of reaction time was smaller and smaller while the reaction became more and more balanceable. 3.3. Comparison. Comparison of yields of different oils under their respective optimum conditions catalyzed by KF/Al2O3 is shown in Table 1. According to the results, biodiesel catalyzed by KF/Al2O3 from soybean oil had the better yield, which was over 99% (more than the yield of cottonseed oil and palm oil 90%). Meanwhile, less catalyst was used. Therefore, it could be thought that preparation of biodiesel from soybean oil was more effective. Comparison of preparation of biodiesel from soybean oil catalyzed by KF/Al2O3 and NaOH under same reaction condition is shown in Table 1. Yields of both of these experiments were over 99%. Thus, this biodiesel yield was accepted. 3.4. Repeated Experiments. Effect of repeated time of catalyst on biodiesel yield is shown in Figure 11. The first time, yield of biodiesel under the best conditions could reach over 99%. From the second time then, yield was just 90.1%, which decreased by near 10%. After the fifth time, the yield
Figure 10. Effect of reaction time on the yield of FAMEs. Reaction conditions: load ratio was 72.68%; molar ratio was 12:1; temperature was 328 K; and catalyst concentration was 2%.
Figure 11. Effect of repeated time on the yield of FAMEs. Reaction conditions: load ratio was 72.68%; molar ratio was 12:1; temperature was 338 K; and catalyst concentration was 2%.
Table 1. Comparison of Yields of Different Oil under Their Respective Optimum Conditions oils
cottonseed oil
palm oil
soybean oil
soybean oil
catalyst load ratio molar ratio temperature catalyst mass (by weight of oil) reaction time yields
KF/Al2O3 50.36% 12:1 338 K 4% 3h >90%
KF/Al2O3 33.1% 12:1 338 K 4% 3h >90%
KF/Al2O3 72.68% 12:1 338 K 2% 3h >99%
NaOH
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Energy Fuels 2009, 23, 4630–4634
: DOI:10.1021/ef9003736
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decreased drastically. Thus, loss of catalyst was very serious. Glycerol covering the surface of catalyst was thought to be the reason for activity loss. To keep the activity is next important work. 3.5. Discussion of Effect of Water and Acid. The mechanism of this methanolysis reaction was methanol and oil were absorbed onto the surface of catalyst, where the transesterification reaction occurred. Different from homogeneous basic catalyst, such as KOH and NaOH, the main effective ingredient of catalyst could not dissolve in the water and interact with fatty acids. Therefore, the catalyst was not very sensitive to water and acid. The occurrence of a decrease of catalysis activity was because the basic cores of the catalyst were covered, but not with water and acid.
4. Conclusions KF/γ-Al2O3 was an active catalyst to prepare biodiesel from soybean oil. The optimal conditions of transesterification reaction were as followed: the load ratio was 72.68% (KF to γ-Al2O3, wt/wt), molar ratio was 12:1(mol/mol), reaction temperature was 338 K, catalyst concentration was 2% of oil by weight, and reaction time was 3 h. Under these conditions, yield of FAMEs could reach over 99%. Acknowledgment. The authors are grateful to National High Technology Research and Development Program of China (No. 2009AA03Z222 and No. 2009AA05Z437) and “Six Talents Pinnacle Program” (No. 2008028) of Jiangsu Province of China for financial support.
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