Biodiesel from Waste Cooking Oil via Heterogeneous Superacid

Application of an Efficient Nonconventional Heterogeneous Catalyst for Biodiesel Synthesis from Pongamia pinnata Oil. Y. C. Sharma , Bhaskar Singh and...
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Energy & Fuels 2009, 23, 569–572

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Biodiesel from Waste Cooking Oil via Heterogeneous Superacid Catalyst SO42-/ZrO2 Baosong Fu, Lijing Gao, Lei Niu, Reiping Wei, and Guomin Xiao* School of Chemistry and Chemical Engineering, Southeast UniVersity, Nanjing 211189, People’s Republic of China ReceiVed September 7, 2008. ReVised Manuscript ReceiVed October 21, 2008

The solid superacid catalyst SO42-/ZrO2 was prepared by impregnation and characterized by infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction. Their performances were evaluated by the transesterification reaction of waste cooking oil (WCO) with methanol. The influence of the load ratio of SO42-/ZrO2, molar ratio of methanol/waste cooking oil, catalyst amount, reaction temperature, and reaction time on biodiesel yield was investigated. Under a condition of methanol/WCO molar ratio of 9:1, a catalyst amount of 3 wt %, reaction time of 4 h, and reaction temperature of 120 °C, 93.6% of biodiesel yield was obtained.

1. Introduction The majority of the world energy needs are supplied from petrochemical sources, coal, and natural gas, with the exception of hydroelectricity and nuclear energy. These sources are finite and nonrenewable and will be consumed shortly at current usage rates.1 The high energy demand in the industrialized world and modern society as well as pollution problems caused as a result of the widespread use of fossil fuels make it increasingly necessary to develop the renewable energy sources of limitless duration and smaller environmental impact than the traditional ones.2 Fatty acids methyl/ethyl esters (FAMEs/FAEEs) derived from vegetable oils have proven promising enough to be called biodiesel. Biodiesel, considered as one of the most promising renewable fuels, shows many better characters than the fossil diesel, such as improved viscosity, volatility, and combustion behavior, and can be used in conventional diesel engines without significant modifications.3-5 It will demonstrate its vitality not only by mitigating the pressure caused by petrochemical sources in the relied energy crisis but also as an environmentally friendly and sustainable fuel. The most common way of producing biodiesel is by transesterification, which can be catalyzed by both acid and base. Using homogeneous acid or base as the catalysts to make biodiesel has already been applied in many plants around the world, but the drawbacks of this approach are obvious: too much wasted water accompanied with the product after the reaction, very difficult to recycle the catalyst, and soap is formed when using homogeneous base.6-8 Thus, a heterogeneous catalyst is appealed and gradually becomes dominating. * To whom correspondence should be addressed. Telephone: +86-2552090612. Fax: +86-25-52090612. E-mail: [email protected]. (1) Srivastava, A.; Prasad, R. Renewable Sustainable Energy ReV. 2000, 4, 111–133. (2) Meher, L. C.; Vidya Sagar, D.; Naik, S. N. Renewable Sustainable Energy ReV. 2006, 10, 248–268. (3) Cui, L. F.; Xiao, G. M.; Xu, B. Energy Fuels 2007, 21, 3740–3743. (4) Meher, L. C.; Dharmagadda, V. S. S.; Naik, S. N. Bioresour. Technol. 2006, 97, 1392–1397. (5) Leung, D. Y. C.; Guo, Y. Fuel Process. Technol. 2006, 87, 883– 890. (6) Freedman, B.; Butterfield, R. O.; Pryde, E. H. J. Am. Oil Chem. Soc. 2000, 77 (12), 1263–1267.

Many heterogeneous catalysts based on solid acid9 or base10,11 have been reported in the literature. A solid base catalyst (such as KF/γ-Al2O33,8 or Mg-Al hydrotalcite) shows a good performance during the transesterification process, with a high purity and yield of biodiesel product in a relatively short time;12,13 however, it is very sensitive to the purity of the reactants. Only well-refined vegetable oil with less than 0.5% free fatty acid (FFA) can be used in the process.14 In China, there is 4 million-800 million tons of waste cooking oil (WCO) containing more than 10% FFA every year, and the number is still increasing; therefore, the acid catalyst, which is more suitable for catalyzing this kind of oil, is welcomed. Usually, the acid-catalyzed reaction requires a longer reaction time and higher temperature than the alkali-catalyzed reaction, but the acid catalyst is more efficient when catalyzing the oil with more FFA. Also, from an economic angle, the acid-catalyzed procedure, being a one-step process, is more economical than the alkali-catalyzed process, which requires an extra step to convert FFAs to methyl esters, thus, avoiding soap formation.15-17 In this work, a kind of solid super catalyst sulfated zirconia was prepared, and its performance applying in transesterification of WCO with methanol was investigated. To solve the long (7) Edward, C.; Cirilo, N. H.; Genta, K.; Kenji, S.; Ayaaki, I. Process. Biochem. 2001, 37, 65–71. (8) Xu, B.; Xiao, G. M.; Cui, L. F. Energy Fuels 2007, 21, 3109–3112. (9) Garcia, C. M.; Teixeira, S.; Marciniuk, L. L.; Schuchardt, U. Bioresour. Technol. 2008, 99, 6608–6613. (10) Venkat Reddy, C. R.; Oshel, R.; Verkade, J. G. Energy Fuels 2006, 20, 1310–1314. (11) Di Serio, M.; Ledda, M.; Cozzolino, M.; Minutillo, G.; Tesser, R.; Santacesaria, E. Ind. Eng. Chem. Res. 2006, 45, 3009–3014. (12) Cantrell, D. G.; Gillie, L. J.; Lee, A. F.; Wilson, K. Appl. Catal., A 2005, 287, 183–190. (13) Liu, Y. J.; Lotero, E.; Goodwin, J. J. G.; Mo, X. H. Appl. Catal., A 2007, 331, 138–148. (14) Wang, Y.; Ou, S. Y.; Liu, P. Z.; Xue, F.; Tang, S. Z. J. Mol. Catal. A: Chem. 2006, 252, 107–112. (15) Zheng, S.; Kates, M.; Dube´, M. A.; McLeana, D. D. Biomass Bioenergy 2006, 30, 267–272. (16) Zhang, Y.; Dube´, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 89, 1–16. (17) Zhang, Y.; Dube´, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 90, 229–240.

10.1021/ef800751z CCC: $40.75  2009 American Chemical Society Published on Web 12/04/2008

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reaction time issue, we used an autoclave to guarantee a higher temperature and pressure. 2. Experimental Section 2.1. Materials. Zirconium(VI) oxychloride octahydrate (ZrOCl2 · 8H2O), sulfuric acid (98.0%), ammonia (25%-28%), and methanol (99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai Ying Xiang Biology Technology Co. Ltd., Nanjing Chemical Reagent Co., Ltd., and Nanjing Chemical Industry Corporation Chemical Plant, respectively. The sample of WCO was provided from a local company that collected WCO from restaurants by the authority of the local government. After removing the remaining food and water, the acid value of the WCO sample was tested to be 81.25 ( 0.031 mg of KOH/g. 2.2. Catalyst Preparation and Characterization. 2.2.1. Catalyst Preparation. The solid superacid catalyst was prepared by mixing two materials, A and solution B: A is zirconium hydroxide powder, and B is 1 mol/L H2SO4 solution. A was sulfated by impregnation with 20 mL of B per gram of A for 24 h, followed by drying for 24 h at 110 °C and calcination in air at 600 °C for 3 h to obtain soild superacid catalyst. The zirconia (A), considered as the support of the catalyst, was prepared by precipitation of zirconium oxychloride hydrate solution (mass ratio of ZrOCl2/H2O is 1:8) with ammonia at pH 9. After aging overnight at 100 °C, the resulting deposit was thoroughly washed with purified water until there was no Cl- left, detected with 0.5 mol L-1 AgNO3, dried at 110 °C, and then ground into powder under 100 mesh. 2.2.2. Catalyst Characterization. The catalyst was characterized by infrared analysis, powder X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Infrared spectroscopy was recorded on a NICOLET 5700 Fourier transform infrared (FTIR) spectrometer, with a resolution of 1 cm-1. XRD measurements were performed on a Rigaku D/max-A instrument, with a Cu KR radiation at 50 kV and 30 mA and a scan speed of 0.02°/min. The catalyst has also been tested with a SDT Q600 simultaneous differential scanning calorimitry (DSC)-TGA. Samples were heated from 50 to 800 °C at a scanning rate of 15 °C min-1 under a nitrogen atmosphere with a flow rate of 15 mL/min. 2.3. Transesterification. The transesterification reaction was carried out in a 250 mL autoclave, with a thermostat, mechanical stirring, and sampling outlet. A prescribed amount of WCO, methanol, and catalyst were put into autoclave, and temperature was raised to the preset degree while stirring. We considered the very beginning heating as time zero of this reaction. The catalyst was separated from the product mixture by filtration after the reaction when the autoclave was cooled to room temperature. Two phases were formed after excess methanol was removed. The upper phase consisted of methyl esters, and the lower phase was mainly glycerol. The procedure was duplicated under different reaction conditions: WCO/methanol mole ratio from 1:3 to 1:15, catalyst amount from 1 to 5 wt %, reaction temperature from 80 to 160 °C, and reaction time from 20 min to 3 h.

3. Results and Discussion 3.1. Catalyst Characterization. From Figure 1, we can see bands at 1236, 1155, 1064, and 1008 cm-1, which are typically assigned for chelating bidentate sulfate ions coordinated to the zirconium cation. The spectrum also shows a weak band at 1438 cm-1, because of the stretching vibrations of the SdO bond. The observation of this band is evidence for the presence of the SO3 species, as reported by Sun and Li.18,19 Also, a strong (18) Sun, Y. Y.; Ma, S. Q.; Du, Y. C. J. Phys. Chem. B 2005, 109, 2567–2572.

Figure 1. Infrared spectroscopy of SO42-/ZrO2.

Figure 2. DSC-TGA curves for SO42-/ZrO2.

and broad band in the 3000-3600 cm-1 region is shown in the spectrum, assigned to physisorbed and coordinated water, accompanied by a broad band in 1637.3 cm-1, assigned to the bending mode (δ HOH) of coordinated water.20 The DSC scans of SO42-/ZrO2 are shown in Figure 2: A is the DSC curve, and B is the TGA curve. From the curve, we can see that the weight losses are concentrated in two ranges: 70-300 and 500-800 °C, which are related to the loss of water and sulfate mass, respectively. The first weight loss occurred at around 100 °C and was most likely due to the desorption of molecular water from the sample. As the temperature rose, there comes another weight loss, which contained two modes of sulfate loss. One corresponded to the sulfate transition at low temperature from the tetragonal phase to the monoclinic phase, which will combine fewer sulfates than former ones. The other corresponded to the decomposition of sulfate at high temperature.21,22 The XRD spectrum in Figure 3 shows the sample ZrO2 that we prepared under 600 °C, containing both tetragonal and monoclinic phases, which can be compared to the standard ZrO2 spectrum of ZrO2-81-1546 and ZrO2-1-750 in the database. Figure 4 shows that sulfate zirconia peaks appeared in catalysts A, B, and C during the brand of 10-22, compared to the ZrO2 sample, which does not have them. The SO42-/ZrO2 catalyst was almost amorphous under 400 °C. As the calcination temperature increased, the tetragonal phase in the metastable state appeared, followed by a more stable monoclinic phase under 600 °C. Also, the intensity of the tetragonal phase, which (19) Li, X. B.; Nagaoka, K.; Simon, L. J.; Olindo, R.; Lercher, J. A.; Hofmann, A.; Sauer, J. J. Am. Chem. Soc. 2005, 127, 16159–16166. (20) Ardizzone, S.; Bianchi, C. L.; Cappelletti, G.; Porta, F. J. Catal. 2004, 227, 470–478. (21) Ward, D. A.; Ko, E. I. J. Catal. 1994, 150, 18–33. (22) Almeida, R. M.; Noda, L. K.; Gonc¸alves, N. S.; Meneghetti, S. M.; Meneghetti, M. R. Appl. Catal., A 2008, 347, 100–105.

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Figure 3. XRD pattern of prepared ZrO2 with standard ZrO2.

Figure 6. Influence of the methanol/WCO molar ratio on the yield of FAME. Reaction conditions: T ) 120 °C, catalyst amount ) 3 wt %; reaction time ) 3 h; catalyst loaded mass ratio of SO42-/ZrO2 ) 0.864.

Figure 4. XRD spectrum of SO42-/ZrO2 with different calcination temperatures: (A) SO42-/ZrO2 under 400 °C, (B) SO42-/ZrO2 under 500 °C, (C) SO42-/ZrO2 under 600 °C, and (D) ZrO2 under 600 °C. Figure 7. Influence of the catalyst amount on the yield of FAME. Reaction conditions: T ) 120 °C; methanol/WCO ) 9:1; reaction time ) 3 h; catalyst loaded mass ratio of SO42-/ZrO2 ) 0.864.

Figure 5. Influence of the loaded mass ratio of SO42-/ZrO2 on the yield of FAME. Reaction conditions: T ) 120 °C; catalyst amount ) 3 wt %; reaction time ) 3 h; methanol/WCO ) 9:1.

is easier to combine with SO42-, increased when the temperature increased. However, a further heat treatment will accelerate the transition from the tetragonal phase to the monoclinic phase, which means some sulfate loss during this process.22,23 Therefore, SO42-/ZrO2 under 600 °C can give a stronger acid, followed by a better catalyzing performance. 3.2. Transesterification. The influence of different load ratios of SO42-/ZrO2 was investigated in Figure 5. As the mass ratio increased, the FAME yield also increased. Because the better result will be obtained with the higher mass ratio of SO42-/ ZrO2, the performance of this catalyst can be measured by the amount of SO42-, which combined with ZrO2. However, the higher the ratio of SO42-, the more easily it deliquates and, of (23) Miao, C. X.; Gao, Z. Mater. Chem. Phys. 1997, 50, 15–19.

course, the more difficult to conserve it. Therefore, we choose the catalyst that loaded a mass ratio of SO42-/ZrO2 at 0.864 as the optimal catalyst in the following study. Figure 6 shows the influence of the molar ratio of methanol/ WCO on the FAME yield. The methanol/WCO molar ratio is one of the most important parameters influencing FAME yield. The transesterification is an equilibrium reaction, and an excess methanol will push the reaction forward to increase FAME yield. Therefore, a large range of methanol/WCO was investigated. As the molar ratio of methanol/WCO increased, the FAME yield increased and obtained its highest value at 12:1. Above 12:1 molar ratio, the yield dropped. Because the molar ratio increased from 9:1 to 12:1, the FAME yield did not increase obviously. Considering the economic factor, we selected 9:1 molar ratio as the optimum condition. Figure 7 gives the FAME yield trend with different catalyst amounts in this transesterification. The FAME yield increased when more catalyst was added. The yield went up fast from the catalyst amount of 1-3 wt %; however, above 3 wt %, it did not rise dramatically. Therefore, 3 wt % was considered the optimum catalyst amount. The reaction temperature was investigated at 80, 100, 120, 140, and 160 °C in Figure 8, which shows that the FAME yield increased as the reaction temperature went up. Because transesterification is a reaction with less thermal effects, a higher temperature does not greatly impact the balance point of the reaction. The rising of the FAME yield could be contributed to the fact that a high temperature greatly accelerates the reaction. From the figure, when the temperature rose above 120 °C, the

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Figure 8. Influence of the temperature on the yield of FAME. Reaction conditions: methanol/WCO ) 9:1; reaction time ) 3 h; catalyst amount ) 3 wt %; catalyst loaded mass ratio of SO42-/ZrO2 ) 0.864.

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atmosphere, while a higher reaction temperature can accelerate the reaction and shorten the contact time to obtain the same yield. From Figure 9, to obtain the FAME yield around 80%, only 2 h was needed at 120 °C, while 11 h was needed at 65 °C, and when the reaction at 120 °C extended to 5 h, the FAME yield could reach 95.2%. Considering that the FAME yield did not rise dramatically when the reaction time extended from 4 to 5 h (93.2 and 95.2%, respectively), 4 h would be the optimal reaction time for 120 °C. 3.3. Testing for Leaching of Sulfonic Groups. In principal, water can capture sulfonic groups from the surface and hydrolyze them to give homogeneous H2SO4, and this can easily happen in an aqueous phase.24 However, Kiss et al. suggest that sulfated zirconia is not deactivated by leaching of sulfated groups when a small amount of water is present in the organic phase.25 Therefore, we test the catalyst to see if sulfonic groups are leaching in the product of FAME after reaction. The presence of sulfate ions in solution was checked with BaCl2 and KOH titration, and no sulfate ions were found. 4. Conclusions

Figure 9. Influence of the reaction time on the yield of FAME. Reaction conditions for B: T ) 120 °C; methanol/WCO ) 9:1; catalyst amount ) 3 wt %; catalyst loaded mass ratio of SO42-/ZrO2 ) 0.864. Reaction conditions for C: T ) 65 °C; methanol/WCO ) 9:1; catalyst amount ) 3 wt %; catalyst loaded mass ratio of SO42-/ZrO2 ) 0.864.

FAME yield did not increase apparently; therefore, 120 °C should be the optimal temperature to this system. Figure 9 reveals the influence of the contact time on the final FAME yield, which increased as the reaction continued. Transesterification catalyzed by an acid catalyst usually needed a longer reaction time to obtain a high conversion under

The solid superacid catalyst SO42-/ZrO2 is applicable for the system of methanol with WCO, which contains a high percentage of FFA, and with its efficiency, a high FAME yield is obtained in a relative short time. The heterogeneous superacid catalyst can avoid the water-washing step, which is used in a traditional homogeneous catalyzing method. Also, it is very easy to separate and recycle. With the catalyst that loaded a mass ratio of SO42-/ZrO2 at 0.864, under the optimal reaction conditions: methanol/WCO molar ratio of 9:1, catalyst amount of 3%, reaction time of 4 h, and reaction temperature of 120 °C, the biodiesel yield of the methanol transesterification with WCO could reach 93.2%. Acknowledgment. We thank the Program for Hi-Tech Research of the Jiangsu Province (BG2006034) for financial support. EF800751Z (24) Omota, F.; Dimian, A. C.; Bliek, A. Chem. Eng. Sci. 2003, 58, 3175–3185. (25) Kiss, A. A.; Dimian, A. C.; Rothenberg, G. AdV. Synth. Catal. 2006, 348, 75–81.