Energy & Fuels 2008, 22, 1313–1317
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Calcium Ethoxide as a Solid Base Catalyst for the Transesterification of Soybean Oil to Biodiesel Xuejun Liu, Xianglan Piao, Yujun Wang,* and Shenlin Zhu State Key Laboratory of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China ReceiVed August 29, 2007. ReVised Manuscript ReceiVed NoVember 20, 2007
In this work, calcium ethoxide is proposed as a catalyst for the transesterification of soybean oil to biodiesel with methanol and ethanol. First, calcium ethoxide was synthesized through a calcium reaction with ethanol. Then, its physical and chemical characteristics were determined using instrumental methods such as Brunauer-Emmett-Teller surface area measurements, scanning electron micrographs, and particle size distribution measurements. The effects of the mass ratio of catalyst to oil, the molar ratio of methanol to oil, and the reaction temperature were studied to optimize the reaction conditions. The experimental results showed that the optimum conditions are a 12:1 molar ratio of methanol to oil, the addition of 3% Ca(OCH2CH3)2 catalyst, and a 65 °C reaction temperature. A 95.0% biodiesel yield was obtained within 1.5 h in these conditions, and the activation energy was 54 149 J/mol. It also indicated that the catalysis performance of calcium ethoxide is better than that of CaO. Besides, a 91.8% biodiesel yield was obtained when it catalyzed soybean oil to biodiesel with ethanol.
1. Introduction Fatty acid methyl esters are known as the sources of biodiesel, which is synthesized by the direct transesterification of vegetable oils with a short-chain alcohol in the presence of a catalyst. The transesterification reaction can be carried out using both homogeneous (acid or base) and heterogeneous (acid, base, or enzymatic) catalysts.1,2 Homogeneous base catalysts provide much faster reaction rates than heterogeneous catalysts, but it is considerably more costly to separate homogeneous catalysts from the reaction mixture.3,4 Heterogeneous catalysis has many advantages, such as being noncorrosive, being environmentally benign, and presenting fewer disposal problems. These catalysts are also much easier to separate from liquid products, and they can be designed to give a higher activity and selectivity and to have longer catalyst lifetimes. Many types of heterogeneous catalysts, such as alkaline earth metal oxides, anion exchange resins, and various alkali metal compounds supported on alumina or zeolite, can catalyze many types of chemical reactions, such as isomerization, aldol condensation, Knoevenagel condensation, Michael condensation, oxidation, and transesterification.5–8 In transes* To whom correspondence should be addressed. Telephone: +861062773017. Fax: +8610-62770304. E-mail: wangyujun@ mail.tsinghua.edu.cn. (1) Vicent, G.; Coteron, A.; Martinez, M.; Aracil, J. Application of the factorial design of experiments and response surface methodology to optimize biodiesel production. Ind. Crops Prod. 1998, 8, 29–35. (2) Freedamn, B.; Pryde, E. H.; Mounts, T. L. Variables affecting the yields of fatty esters from transesterified vegetable oils. J. Am. Oil Chem. Soc. 1984, 61, 1638–1643. (3) Ma, F.; Hanna, M. A. Biodiesel production: a review. Biotechnol. Tech. 1999, 70, 1–15. (4) Kim, H. J.; Kang, B. S.; Kim, M. J.; Park, Y. M.; Kim, D. K.; Lee, J. S.; Lee, K. Y. Tranesterification of vegetable oil to biodiesel using heterogeneous base catalyst. Catal. Today 2004, 93, 315–320. (5) Schachter, Y.; Herman, P. Calcium-oxide-catalyzed reactions of hydrocarbons and of alcohols. J. Catal. 1968, 11, 147–158. (6) Xie, W. L.; Peng, H.; Chen, L. G. Transesterification of soybean oil catalyzed by potassium loaded on alumina as a solid-base catalyst. Appl. Catal., A 2006, 300, 67–74.
terification of vegetable oils to biodiesel, most supported alkali catalysts and anion exchange resins exhibit a short catalyst lifetime because the active ingredients are easily corroded by methanol.9,10 Some researchers found that alkaline-earth oxide compounds, such as CaO and SrO, have a slight solubility in methanol and have good catalytic activity and a long catalyst lifetime.11,12 Gryglewicz studied the alkaline-earth metal alkoxides as catalysts for alcoholysis reactions in terms of the synthesis of di(2-ethylhexyl) adipate and an oligomeric ester of neopentyl glycol and found that magnesium methoxide and calcium alkoxides appear to be active catalysts for the transesterification.13 Gryglewicz12 and Liu et al.14 studied calcium methoxide as a solid base catalyst to catalyze the transesterification of soybean oil to biodiesel and found that it has excellent catalytic activity and a long catalyst lifetime. In this research, we studied calcium ethoxide as one of the alkaline-earth metal alkoxide catalysts for the transesterification of soybean oil to biodiesel. (7) Suppes, G. J.; Dasari, M. A.; Doskocil, E. J.; Mankidy, P. J.; Goff, M. J. Transesterification of soybean oil with zeolite and metal catalysts. Appl. Catal., A 2004, 257, 213–223. (8) Kabashima, H.; Katou, T.; Hattori, H. Conjugate addtion of methanol to 3-buten-2-one over solid base catalysts. Appl. Catal., A 2001, 214, 121– 124. (9) Ebiura, T.; Echizen, T.; Ishikawa, A.; Murai, K.; Baba, T. Selective transesterification of triolein with methanol to methyl oleate and glycerol using alumina loaded with alkail metal salt as a soid-base catalyst. Appl. Catal., A 2005, 283, 111–116. (10) Veldurthy, B.; Clacens, J. M.; Figueras, F. Correlation between the basicity of solid bases and their catalytic activity towards the synthesis of unsymmetrical organic carbonates. J. Catal. 2005, 229, 237–242. (11) Liu, X. J.; He, H. Y.; Wang, Y. J.; Zhu, S. L. Transesterification of soybean oil to biodiesel using SrO as a solid base catalyst. Catal. Commun. 2007, 8, 1107–1111. (12) Gryglewicz, S. Rapeseed oil methyl esters preparation using heterogeneous catalysts. Bioresour. Technol. 1999, 70, 249–253. (13) Gryglewicz, S. Alkaline-earth metal compounds as alcoholysis catalysts for ester oils synthesis. Appl. Catal., A 2000, 192, 23–28. (14) Liu, X. J.; He, H. Y.; Wang, Y. J.; Zhu, S. L. Calcium methoxide as a solid base catalyst for the transesterification of soybean oil to biodiesel with methanol. Fuel 2007, in press, available online 19 July 2007.
10.1021/ef700518h CCC: $40.75 2008 American Chemical Society Published on Web 01/31/2008
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Liu et al.
Figure 1. SEM image of Ca(OCH2CH3)2.
The physical and chemical characterizations of calcium ethoxide were analyzed with some instrumental methods. Then, the effects of various reaction conditions on the biodiesel yields were investigated. 2. Experimental Section 2.1. Materials and Catalyst Preparation. Ca(OCH2CH3)2 was synthesized in a 100 mL glass reactor with a condenser. The magnetic stirring rate was 800 rpm. The reaction procedure was as follows: First, calcium was dispersed in ethanol under magnetic stirring. Then, it was heated to 65 °C by water circulation. The reaction can be expressed by eq 1. After 8 h of reaction, ethanol was first distilled off under vacuum. Then, the catalyst was dried in an oven at 105 °C for 1 h. 65 °C
Ca + 2CH3CH2OH ) Ca(OCH2CH3)2 + H2v
(1)
Refined soybean oil was purchased from Tianjin Jiali Oil Plant. The fatty acid composition consisted of 12.5% palmitic acid, 5.2% stearic acid, 23.5% oleic acid, 47.8% linoleic acid, 10% linolenic acid, and traces of other acids. Methanol was analytical reagent grade and was purchased from Beihua Fine Chemical Co., Beijing. Analytical reagents (e.g., standards for high performance liquid chromatography (HPLC)) were of high grade and were obtained from Sigma Chemical Co. All other chemicals were analytical reagents and were purchased from Beihua Fine Chemical Co., Beijing. 2.2. Apparatus and Procedure. The Brunauer-Emmett-Teller (BET) surface area, total pore volume, and pore size distribution of Ca(OCH2CH3)2 were measured with a Quantachrome Autosorb1-C chemisorption-physisorption analyzer. A weighed sample of the catalyst was prepared by outgassing for 1.5 h at 423 K on the degas port. Adsorption isotherms were generated by dosing nitrogen onto the catalyst in a bath of liquid nitrogen at approximately 77 K. The BET surface area was calculated from the adsorption branches in the relative pressure range of 0.05–0.25 bar, and the total pore volume was evaluated at a relative pressure of about 0.99 bar. The pore size distribution was calculated from the desorption branches using the Barrett–Joyner–Halenda (BJH) method. The particle size distribution was measured using a Malvern Mastersizer MICRO-PLUS laser particle size analyzer and evaluated by a volume concentration. An FTIR-8201 (PC) infrared spectrophotometer was used to identify the surface group of the catalyst. Scanning electron microscopy (SEM) observations were performed on a Hitachi JEOL JSM 7401F microscope operating at 1.0 kV. Thermogravimetry (TG) was performed with a Netzsch TA-449C TG analyzer from 25 to 1000 °C at a heating rate of 10 °C/min under air atmosphere. The solubility of the catalyst in methanol and ethanol was determined by measuring the calcium concentration with a HITACHI Z-5000 polarized zeeman atomic absorption spectrophotometer. 2.3. Reaction Procedures. The transesterification reactions (eq 2) were carried out in a 100 mL glass reactor with a condenser. The magnetic stirring rate was 800 rpm. The reaction procedure
was as follows: First, the catalyst was dispersed in methanol under magnetic stirring. Then, the soybean oil was added into the mixture and heated by water circulation. The amount of soybean oil was 28 mL every time. After the reaction, the excess methanol was distilled off under vacuum and the Ca(OCH2CH3)2 catalyst was separated by centrifugation. After removal of the glycerol layer, the biodiesel was collected for chromatographic analysis. 2.4. Analysis. The biodiesel samples were analyzed in an HP
(2)
5890 gas chromatograph equipped with a flame ionization detector and a capillary column HP-INNOWAX (30 m × 0.15 mm × 0.25 µm). Four microliters of the upper oil layer were dissolved in 300 µL of n-hexane and 100 µL of the internal standard solutions (heptadecanoic acid methyl ester-n-hexane solution) for gas chromatography (GC) analysis. Samples (1 µL) were injected by a sampler at an oven temperature of 220 °C. After an isothermal period of 4 min, the GC oven was heated at 10 °C/min to 230 °C and held for 7.5 min. Nitrogen was used as the carrier gas at a flow rate of 2 mL/min measured at 20 °C and as the detector make up gas at a flow rate of 30 mL/min. The inlet pressure was 96.4 kPa. The split ratio was 10:1. The injector temperature and detector temperatures were 300 and 320 °C, respectively. The biodiesel yield in each experiment was calculated by the following expression: yield )
mactual Cesters × n × Vesters × 100% ≈ × 100% ≈ mtheoretical moil Cesters × n × Voil Cesters × n × 100% ≈ × 100% moil Foil
where both mactual [g] and mtheoretical [g] are the masses of methyl ester; moil [g] is the mass of the vegetable oil that was used in the reaction; Cester [g/mL] is the mass concentration of methyl ester which was acquired by GC; n is the diluted multiple of methyl ester; Foil [g/mL] is the density of the vegetable oil; and Vesters [mL] and Voil [mL] are the volumes of crude ester layer and vegetable oil, respectively.
3. Results and Discussion 3.1. Characterizations of the Ca(OCH2CH3)2 Solid Base Catalyst. The analyzed results indicate that calcium ethoxide possesses a surface area of 15.02 m2/g and a total pore volume of 0.100 cm3/g. It is favorable for use in a slurry reactor. Figure 1 shows the SEM image of the Ca(OCH2CH3)2 catalyst. It shows that the surfaces comprise a large number of small pores. Figure 2 shows the pore size distribution. It can be seen that a
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Energy & Fuels, Vol. 22, No. 2, 2008 1315
Figure 2. Pore size distribution of Ca(OCH2CH3)2. Figure 4. IR pattern of Ca(OCH2CH3)2.
Figure 3. Particle size distribution of Ca(OCH2CH3)2.
large part of the surface area is occupied by pores of relatively large size between 30 and 100 nm. Figure 3 shows the particle size distribution of the Ca(OCH2CH3)2 catalyst. It indicates that it has a broad particle size distribution and that a large number of the catalyst particles are within the size range of 1–300 µm; the remainder are within the range of 0.6–1 µm. Particle size distribution can markedly affect the settling and filtering characteristics in a slurry reactor, and a size range of 5–200 µm is favorable. Figure 4 shows the IR spectra of Ca(OCH2CH3)2. It can be seen that the important features appear in the C-H stretching (2800–3000 cm-1), -C-H (alkane) bending (1460 cm-1), and -C-O (primary alcohol) stretching (1050–1085 cm-1). The IR peak between 2000 and 1500 cm-1 is characteristic of CdO because of the catalyst surface adsorbed CO2. Figure 5 shows the TG and differential thermal analysis (DTA) thermogram of the Ca(OCH2CH3)2 catalyst. It can be seen that Ca(OCH2CH3)2 begins to decompose at about 350 °C, and a clear exothermic peak appears between 330 and 400 °C. The IR spectrum of Ca(OCH2CH3)2, which was calcined under air at 350 °C for 1 h, is identical to the spectrum of CaCO3. It indicates that the decomposition of Ca(OCH2CH3)2 has formed calcium carbonate. Then, the calcium carbonate began to decompose, and this appears in Figure 5 as a steep slope between 550 and 700 °C. The results of the TG analysis indicates that the Ca(OCH2CH3)2 catalyst is stable under 300 °C. The solubility of a catalyst in reactants is an important characteristic of a solid catalyst. The reaction will be homogeneous if the catalyst is soluble in reactants. Table 1 shows the
Figure 5. TG/DTA thermogram of Ca(OCH2CH3)2. Table 1. Ca2+ Concentration (ppm) in Methanol and Ethanol at Different Temperatures temperature solvent
20 °C
30 °C
40 °C
50 °C
60 °C
65 °C
methanol ethanol
1 2
2 3
9 5
12 9
14 12
36 38
solubility of calcium ethoxide in methanol and ethanol at different temperatures. The results indicate that the Ca2+ concentration increases with increasing temperature, and the solubility in methanol is much lower than that of the calcium methoxide heterogeneous catalyst, which is about 0.04 wt %.14 Therefore, calcium ethoxide mostly acted as a heterogeneous catalyst in the transesterification of vegetable oils to biodiesel with methanol or ethanol. The experimental results also indicate that the biodiesel yield is proportional to the amount of catalyst. 3.2. Reaction Results. 3.2.1. Effect of Mass Ratio of Catalyst to Oil on Biodiesel Yield. The mass ratio of Ca(OCH2CH3)2 to soybean oil was varied within the range of 0.25-4.0%. The biodiesel yield increased with increasing Ca(OCH2CH3)2, and a 95.0% biodiesel yield was obtained by adding 4.0% Ca(OCH2CH3)2 (Figure 6). Therefore, with the addition of more catalyst, there was also the faster rate at which the reaction
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equilibrium was reached because of the increase in the total number of available active catalytic sites for the reaction. However, when the catalyst amount exceeded 3.0%, there was little impact on the biodiesel yield by increasing Ca(OCH2CH3)2. The biodiesel yield is determined by the surface reaction and the mass transfer. In this reaction, the optimum addition of catalyst is 3.0% by weight of oil. Gryglewicz obtained a 93.0% biodiesel yield at 2.5 h using CaO powder as a solid base catalyst. Therefore, the catalytic activity of calcium ethoxide is better than that of CaO. 3.2.2. Effect of the Molar Ratio of Methanol to Oil on Biodiesel Yield. The stoichiometry of this reaction requires 3 M methanol/1 M triglyceride. Excess methanol was used in this study to obtain a higher biodiesel yield. The results are shown in Figure 7. It indicates that the fast reaction rate was obtained at a high molar ratio. The biodiesel yield was only 70.0% at 3 h of reaction when the molar ratio of methanol to oil was 3:1. However, the biodiesel yields all exceeded 93.0% when the molar ratios were higher than 6:1. Considering both the biodiesel yield and the saving methanol amount, the optimum molar ratio of methanol to oil is 12:1. 3.2.3. Effect of Reaction Temperature on Biodiesel Yield. Reaction temperature can influence the reaction rate and the biodiesel yield because the intrinsic rate constants are strong functions of temperature. Figure 8 shows the effect of the reaction temperature on the biodiesel yield. It indicates that the reaction rate was higher at high temperature than at low
Liu et al.
Figure 8. Effect of reaction temperature on biodiesel yield. Ca(OCH2CH3)2/ oil mass ratio, 3.0%; methanol/oil molar ratio, 12:1.
Figure 9. Plot of ln k vs 1/T for the transesterification reaction. Table 2. Transesterification of Soybean Oil to Biodiesel with Ethanol at Different Temperaturesa biodiesel yield (%) temperature
0.5 h
1h
1.5 h
2h
2.5 h
3h
75 °C 70 °C 65 °C
27.5 3.5 0.1
40.0 13.3 1.1
52.3 17.5 1.9
64.2 21.2 2.9
81.1 27.6 5.7
91.8 32.9 8.5
a
Figure 6. Effect of the mass ratio of Ca(OCH2CH3)2 to oil on biodiesel yield. Methanol/oil molar ratio, 12:1; reaction temperature, 65 °C.
Catalyst/oil weight ratio, 3%; ethanol/oil molar ratio, 12:1.
temperature. The biodiesel yield was only 29.9% at 30 °C after 3 h of reaction, and it reached to 93.2% at 65 °C after 1.5 h. Therefore, the optimum reaction temperature for the transesterification of soybean oil to biodiesel is 65 °C. In excess of methanol, the transesterification is a pseudo-firstorder reaction. The overall rate equation (k) can be given as eq 3. k ) -ln(1 - η)/t
(3)
where η is the biodiesel yield. The average overall reaction rate constant at different temperatures can be calculated according to the above experimental data. Besides, the overall reaction rate constant has a relationship with temperature as follows: ln k ) -
Figure 7. Effect of the molar ratio of methanol to oil on biodiesel yield. Ca(OCH2CH3)2/oil mass ratio, 3.0%; reaction temperature, 65 °C.
Ea +C RT
(4)
where Ea is the activation energy, R is the gas constant (J mol-1 K-1), T is the absolute temperature, and C is a constant. Figure 9 gives the plot of ln k versus 1/T for the transesterification reaction. Linear regression analysis of these data gives a slope of -6513.855 with a correlation coefficient of -0.996 93. From
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Figure 10. Reaction mechanism for the transesterification of triglyceride with ethanol over the Ca(O CH2CH3)2 catalyst, where R1, R2, and R3 represent the long chain alkyl group.
the plot of ln k versus 1/T, the slope is equal to (-Ea/R). Thus, a value for Ea of 54 149 J/mol (12 954 cal/mol) was calculated for the reaction. It indicates that the experimental value of the activation energy in this study is consistent with that reported in the literature for this transesterification using homogeneous catalysts, such as NaOH, KOH, NaOCH3, NaOBu, H2SO4, and so on.15–17 Noureddini reported activation energies for the reaction involved in the transesterification of soybean oil to be in the range of 6400-20 000 cal/mol, and Bernard and co-workers reported these in the range of 8000–20 000 cal/mol. 3.2.4. Transesterification of Soybean Oil to Biodiesel with Ethanol. Biodiesel can be produced by the transesterification of soybean oil with ethanol. The experimental results are shown in Table 2. It indicates that the reaction rate was slow at low temperature. It cost 3 h to reach a 91.8% biodiesel yield at 75 °C. Therefore, calcium ethoxide can also catalyze the transesterification soybean oil to biodiesel with ethanol. It has strong basicity and can catalyze many transesterification reactions. Some researchers have proposed some possible mechanisms in chemical reactions over solid base catalysts.5,11–13,18,19 When calcium ethoxide is used as a solid base catalyst, the catalysis (15) Darnoko, D.; Cheryan, M. Kinetics of palm oil transesterification in a batch reactor. J. Am. Oil Chem. Soc. 2000, 77, 1263–1267. (16) Noureddini, H.; Zhu, D. Kinetics of Transesterification of soybean oil. J. Am. Oil Chem. Soc. 1997, 74, 1457–1463. (17) Bernard, F.; Royden, O. B.; Evereff, H. P. Transesterification kinetics of soybean oil. J. Am. Oil Chem. Soc. 1986, 63, 1375–1380. (18) Dossin, T. F.; Reyniers, M. F.; Marin, G. B. Kinetics of heterogeneously MgO-catalyzed transesterification. Appl. Catal., B 2006, 61, 35– 45. (19) Hideto, T.; Fuyuki, Y.; Hideshi, H.; Hideaki, K. Self-condensation of n-butyraldehyde over solid base catalysts. J. Catal. 1994, 148, 759– 770.
mechanism could be assumed to be one where that the catalytic reactions take place on the surface of calcium ethoxide. The proposed mechanism of the transesterification reaction by calcium methoxide with ethanol is given in Figure 10. Alcohol and triglyceride are adsorbed on two neighboring free catalytic sites (O- and Ca+). The surface O- extracts an H+, and Ca+ adsorbs CH3CH2O- from alcohol. The adsorbed triglyceride forms a surface intermediate with the catalyst. The two neighboring adsorbed species react to result in the formation of a fatty acid methyl and a diglyceride. The diglyceride reacts with alcohol along similar processes on the surface of the catalyst to form glycerol and biodiesel. 4. Conclusions From the experimental results, it can be seen that Ca(OCH2CH3)2 has excellent catalytic abilities as a solid base catalyst. It has a moderate surface area, a relatively broader particle size distribution, and a better low solubility in methanol and ethanol. When it catalyzes the transesterification of soybean oil to biodiesel with methanol, the optimal conditions are a 12:1 molar ratio of methanol to oil, the addition of 3.0% Ca(OCH2CH3)2 catalyst, a 65 °C reaction temperature, and about 1.5 h of reaction time. The reactions are completed under mild temperature and pressure conditions. This catalyst can also be applied in other chemical reactions as a solid base catalyst. Acknowledgment. The authors would like to thank Professor Yigui Li for his kind help. This work was supported by the National Basic Research Program (973 Plan, No. 2007CB714302). EF700518H
