Calcium Ethoxide as a Solid Base Catalyst for The

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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 biodieselwith 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 asBrunauer-Emmett-Teller surface area measurements, scanning electron micrographs, and particle sizedistribution 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 showedthat the optimum conditions are a 12:1 molar ratio of methanol to oil, the addition of 3% Ca(OCH 2CH3)2catalyst, 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 ethoxideis better than that of CaO. Besides, a 91.8% biodiesel yield was obtained when it catalyzed soybean oil tobiodiesel with ethanol.

1. Introduction

Fatty acid methyl esters are known as the sources of biodiesel,which is synthesized by the direct transesterification of vegetableoils with a short-chain alcohol in the presence of a catalyst.The transesterification reaction can be carried out using bothhomogeneous (acid or base) and heterogeneous (acid, base, orenzymatic) catalysts.1,2 Homogeneous base catalysts providemuch faster reaction rates than heterogeneous catalysts, but itis considerably more costly to separate homogeneous catalystsfrom the reaction mixture.3,4

Heterogeneous catalysis has many advantages, such as beingnoncorrosive, being environmentally benign, and presentingfewer disposal problems. These catalysts are also much easierto separate from liquid products, and they can be designed togive a higher activity and selectivity and to have longer catalystlifetimes. Many types of heterogeneous catalysts, such asalkaline earth metal oxides, anion exchange resins, and variousalkali metal compounds supported on alumina or zeolite, cancatalyze many types of chemical reactions, such as isomeriza-tion, aldol condensation, Knoevenagel condensation, Michaelcondensation, oxidation, and transesterification.58 In transes-

terification of vegetable oils to biodiesel, most supported alkalicatalysts and anion exchange resins exhibit a short catalystlifetime because the active ingredients are easily corroded bymethanol.9,10 Some researchers found that alkaline-earth oxidecompounds, such as CaO and SrO, have a slight solubility inmethanol and have good catalytic activity and a long catalystlifetime.11,12

Gryglewicz studied the alkaline-earth metal alkoxides ascatalysts for alcoholysis reactions in terms of the synthesis ofdi(2-ethylhexyl) adipate and an oligomeric ester of neopentylglycol and found that magnesium methoxide and calciumalkoxides appear to be active catalysts for the transesterifica-tion.13 Gryglewicz12 and Liu et al.14 studied calcium methoxideas a solid base catalyst to catalyze the transesterification ofsoybean oil to biodiesel and found that it has excellent catalyticactivity and a long catalyst lifetime. In this research, we studiedcalcium ethoxide as one of the alkaline-earth metal alkoxidecatalysts for the transesterification of soybean oil to biodiesel.

* To whom correspondence should be addressed. Telephone: +8610-62773017. 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 tooptimize biodiesel production. Ind. Crops Prod. 1998, 8, 2935.

(2) Freedamn, B.; Pryde, E. H.; Mounts, T. L. Variables affecting theyields of fatty esters from transesterified vegetable oils.J. Am. Oil Chem.Soc. 1984, 61, 16381643.

(3) Ma, F.; Hanna, M. A. Biodiesel production: a review.Biotechnol.Tech. 1999, 70, 115.

(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 usingheterogeneous base catalyst. Catal. Today 2004, 93, 315320.

(5) Schachter, Y.; Herman, P. Calcium-oxide-catalyzed reactions ofhydrocarbons and of alcohols. J. Catal. 1968, 11, 147158.

(6) Xie, W. L.; Peng, H.; Chen, L. G. Transesterification of soybean oilcatalyzed by potassium loaded on alumina as a solid-base catalyst. Appl.Catal., A 2006, 300, 6774.

(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, 213223.(8) Kabashima, H.; Katou, T.; Hattori, H. Conjugate addtion of methanol

to 3-buten-2-one over solid base catalysts. Appl. Catal., A2001,214, 121124.

(9) Ebiura, T.; Echizen, T.; Ishikawa, A.; Murai, K.; Baba, T. Selectivetransesterification of triolein with methanol to methyl oleate and glycerolusing alumina loaded with alkail metal salt as a soid-base catalyst. Appl.Catal., A 2005, 283, 111116.

(10) Veldurthy, B.; Clacens, J. M.; Figueras, F. Correlation betweenthe basicity of solid bases and their catalytic activity towards the synthesisof unsymmetrical organic carbonates. J. Catal. 2005, 229, 237242.

(11) Liu, X. J.; He, H. Y.; Wang, Y. J.; Zhu, S. L. Transesterificationof soybean oil to biodiesel using SrO as a solid base catalyst. Catal.Commun. 2007, 8, 11071111.

(12) Gryglewicz, S. Rapeseed oil methyl esters preparation usingheterogeneous catalysts. Bioresour. Technol. 1999, 70, 249253.

(13) Gryglewicz, S. Alkaline-earth metal compounds as alcoholysiscatalysts for ester oils synthesis.Appl. Catal., A 2000, 192, 2328.

(14) Liu, X. J.; He, H. Y.; Wang, Y. J.; Zhu, S. L. Calcium methoxideas a solid base catalyst for the transesterification of soybean oil to biodieselwith methanol. Fuel 2007, in press, available online 19 July 2007.

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10.1021/ef700518h CCC: $40.75 2008 American Chemical SocietyPublished on Web 01/31/2008


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The physical and chemical characterizations of calcium ethoxidewere analyzed with some instrumental methods. Then, theeffects of various reaction conditions on the biodiesel yieldswere investigated.

2. Experimental Section

2.1. Materials and Catalyst Preparation.Ca(OCH2CH3)2was

synthesized in a 100 mL glass reactor with a condenser. Themagnetic stirring rate was 800 rpm. The reaction procedure was asfollows: First, calcium was dispersed in ethanol under magneticstirring. Then, it was heated to 65 C by water circulation. Thereaction can be expressed by eq 1. After 8 h of reaction, ethanolwas first distilled off under vacuum. Then, the catalyst was driedin an oven at 105 C for 1 h.

Ca+ 2CH3CH2OH )65C

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% linolenicacid, and traces of other acids. Methanol was analytical reagentgrade and was purchased from Beihua Fine Chemical Co., Beijing.

Analytical reagents (e.g., standards for high performance liquidchromatography (HPLC)) were of high grade and were obtainedfrom Sigma Chemical Co. All other chemicals were analyticalreagents 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 distributionof Ca(OCH2CH3)2were measured with a Quantachrome Autosorb-1-C chemisorption-physisorption analyzer. A weighed sample ofthe catalyst was prepared by outgassing for 1.5 h at 423 K on thedegas port. Adsorption isotherms were generated by dosing nitrogenonto the catalyst in a bath of liquid nitrogen at approximately 77K. The BET surface area was calculated from the adsorptionbranches in the relative pressure range of 0.050.25 bar, and thetotal pore volume was evaluated at a relative pressure of about 0.99

bar. The pore size distribution was calculated from the desorptionbranches using the BarrettJoynerHalenda (BJH) method. Theparticle size distribution was measured using a Malvern MastersizerMICRO-PLUS laser particle size analyzer and evaluated by avolume concentration. An FTIR-8201 (PC) infrared spectropho-tometer was used to identify the surface group of the catalyst.Scanning electron microscopy (SEM) observations were performedon a Hitachi JEOL JSM 7401F microscope operating at 1.0 kV.Thermogravimetry (TG) was performed with a Netzsch TA-449CTG analyzer from 25 to 1000 C at a heating rate of 10 C/minunder air atmosphere. The solubility of the catalyst in methanoland ethanol was determined by measuring the calcium concentrationwith a HITACHI Z-5000 polarized zeeman atomic absorptionspectrophotometer.

2.3. Reaction Procedures.The transesterification reactions (eq2) 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 undermagnetic stirring. Then, the soybean oil was added into the mixtureand heated by water circulation. The amount of soybean oil was 28mL every time. After the reaction, the excess methanol was distilledoff under vacuum and the Ca(OCH2CH3)2 catalyst was separatedby centrifugation. After removal of the glycerol layer, the biodieselwas collected for chromatographic analysis.

2.4. Analysis.The biodiesel samples were analyzed in an HP

5890 gas chromatograph equipped with a flame ionization detectorand a capillary column HP-INNOWAX (30 m 0.15 mm 0.25m). Four microliters of the upper oil layer were dissolved in 300L of n-hexane and 100 L of the internal standard solutions(heptadecanoic acid methyl ester-n-hexane solution) for gaschromatography (GC) analysis. Samples (1 L) were injected by asampler 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 Cand held for 7.5 min. Nitrogen was used as the carrier gas at aflow rate of 2 mL/min measured at 20 C and as the detector makeup gas at a flow rate of 30 mL/min. The inlet pressure was 96.4kPa. The split ratio was 10:1. The injector temperature and detectortemperatures were 300 and 320 C, respectively.

The biodiesel yield in each experiment was calculated by thefollowing expression:

yield)mactual

mtheoretical 100%

Cesters n Vesters

moil 100%

Cesters n Voil

moil 100%

Cesters n

Foil 100%

where both mactual [g] and mtheoretical [g] are the masses of methylester;moil[g] is the mass of the vegetable oil that was used in thereaction; Cester [g/mL] is the mass concentration of methyl esterwhich was acquired by GC; n is the diluted multiple of methylester; Foil[g/mL] is the density of the vegetable oil; and Vesters[mL]and Voil [mL] are the volumes of crude ester layer and vegetableoil, respectively.

3. Results and Discussion

3.1. Characterizations of the Ca(OCH2CH3)2 Solid Base

Catalyst. The analyzed results indicate that calcium ethoxidepossesses a surface area of 15.02 m2/g and a total pore volumeof 0.100 cm3/g. It is favorable for use in a slurry reactor. Fig-ure 1 shows the SEM image of the Ca(OCH2CH3)2catalyst. 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

Figure 1. SEM image of Ca(OCH2CH3)2.

(2)

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large part of the surface area is occupied by pores of relativelylarge size between 30 and 100 nm.

Figure 3 shows the particle size distribution of theCa(OCH2CH3)2catalyst. It indicates that it has a broad particlesize distribution and that a large number of the catalyst particlesare within the size range of 1300 m; the remainder are withinthe range of 0.61 m. Particle size distribution can markedlyaffect the settling and filtering characteristics in a slurry reactor,and a size range of 5200 m is favorable.

Figure 4 shows the IR spectra of Ca(OCH2CH3)2. It can beseen that the important features appear in the C-H stretching(28003000 cm-1), -C-H (alkane) bending (1460 cm-1), and-C-O (primary alcohol) stretching (10501085 cm-1). The IR

peak between 2000 and 1500 cm-1

is characteristic of Cd

Obecause of the catalyst surface adsorbed CO2. Figure 5 showsthe TG and differential thermal analysis (DTA) thermogram ofthe Ca(OCH2CH3)2catalyst. It can be seen that Ca(OCH2CH3)2begins to decompose at about 350 C, and a clear exothermicpeak appears between 330 and 400 C. The IR spectrum ofCa(OCH2CH3)2, which was calcined under air at 350 C for1 h, is identical to the spectrum of CaCO3. It indicates that thedecomposition of Ca(OCH2CH3)2has formed calcium carbonate.Then, the calcium carbonate began to decompose, and thisappears in Figure 5 as a steep slope between 550 and 700 C.The results of the TG analysis indicates that the Ca(OCH2CH3)2catalyst is stable under 300 C.

The solubility of a catalyst in reactants is an important

characteristic of a solid catalyst. The reaction will be homoge-neous if the catalyst is soluble in reactants. Table 1 shows the

solubility of calcium ethoxide in methanol and ethanol atdifferent temperatures. The results indicate that the Ca2+

concentration increases with increasing temperature, and thesolubility in methanol is much lower than that of the calciummethoxide heterogeneous catalyst, which is about 0.04 wt %.14

Therefore, calcium ethoxide mostly acted as a heterogeneouscatalyst in the transesterification of vegetable oils to biodieselwith methanol or ethanol. The experimental results also indicatethat the biodiesel yield is proportional to the amount of catalyst.

3.2. Reaction Results.3.2.1. Effect of Mass Ratio of Cata-

lyst to Oil on Biodiesel Yield.The mass ratio of Ca(OCH2CH3)2to soybean oil was varied within the range of 0.25-4.0%. Thebiodiesel yield increased with increasing Ca(OCH2CH3)2, anda 95.0% biodiesel yield was obtained by adding 4.0%

Ca(OCH2CH3)2(Figure 6). Therefore, with the addition of morecatalyst, there was also the faster rate at which the reaction

Figure 2. Pore size distribution of Ca(OCH2CH3)2.

Figure 3. Particle size distribution of Ca(OCH2CH3)2.

Figure 4.IR pattern of Ca(OCH2CH3)2.

Figure 5. TG/DTA thermogram of Ca(OCH2CH3)2.

Table 1. Ca2+ Concentration (ppm) in Methanol and Ethanol atDifferent Temperatures

temperature

solvent 20 C 30 C 40 C 50 C 60 C 65 C

methanol 1 2 9 12 14 36ethanol 2 3 5 9 12 38

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equilibrium was reached because of the increase in the totalnumber of available active catalytic sites for the reaction.However, when the catalyst amount exceeded 3.0%, there waslittle impact on the biodiesel yield by increasing Ca(OCH2CH3)2.The biodiesel yield is determined by the surface reaction andthe mass transfer. In this reaction, the optimum addition ofcatalyst is 3.0% by weight of oil. Gryglewicz obtained a 93.0%biodiesel yield at 2.5 h using CaO powder as a solid basecatalyst. 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 3M methanol/1 M triglyceride. Excess methanol was used in thisstudy to obtain a higher biodiesel yield. The results are shownin Figure 7. It indicates that the fast reaction rate was obtainedat a high molar ratio. The biodiesel yield was only 70.0% at3 h of reaction when the molar ratio of methanol to oil was3:1. However, the biodiesel yields all exceeded 93.0% whenthe molar ratios were higher than 6:1. Considering both thebiodiesel yield and the saving methanol amount, the optimummolar 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 thebiodiesel yield because the intrinsic rate constants are strongfunctions of temperature. Figure 8 shows the effect of thereaction temperature on the biodiesel yield. It indicates that thereaction rate was higher at high temperature than at low

temperature. The biodiesel yield was only 29.9% at 30 C after3 h of reaction, and it reached to 93.2% at 65 C after 1.5 h.Therefore, the optimum reaction temperature for the transes-terification of soybean oil to biodiesel is 65 C.

In excess of methanol, the transesterification is a pseudo-first-order reaction. The overall rate equation (k) can be given as eq 3.

k)-ln(1- )/t (3)

where is the biodiesel yield. The average overall reaction rateconstant at different temperatures can be calculated accordingto the above experimental data. Besides, the overall reactionrate constant has a relationship with temperature as follows:

lnk)-Ea

RT+C (4)

whereEais the activation energy, R is the gas constant (J mol-1

K-1),Tis the absolute temperature, and Cis a constant. Figure9 gives the plot of ln kversus 1/T for the transesterification

reaction. Linear regression analysis of these data gives a slopeof-6513.855 with a correlation coefficient of-0.996 93. From

Figure 6.Effect of the mass ratio of Ca(OCH2CH3)2to oil on biodieselyield. Methanol/oil molar ratio, 12:1; reaction temperature, 65 C.

Figure 7. Effect of the molar ratio of methanol to oil on biodieselyield. Ca(OCH2CH3)2/oil mass ratio, 3.0%; reaction temperature, 65C.

Figure8. Effectofreactiontemperatureon biodieselyield.Ca(OCH2CH3)2/oil mass ratio, 3.0%; methanol/oil molar ratio, 12:1.

Figure 9. Plot of ln kvs 1/T for the transesterification reaction.

Table 2. Transesterification of Soybean Oil to Biodiesel withEthanol at Different Temperaturesa

biodiesel yield (%)

temperature 0.5 h 1 h 1.5 h 2 h 2.5 h 3 h

75 C 27.5 40.0 52.3 64.2 81.1 91.870 C 3.5 13.3 17.5 21.2 27.6 32.965 C 0.1 1.1 1.9 2.9 5.7 8.5

a Catalyst/oil weight ratio, 3%; ethanol/oil molar ratio, 12:1.

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the plot of ln kversus 1/T, the slope is equal to (-Ea/R). Thus,a value forEaof 54 149 J/mol (12 954 cal/mol) was calculatedfor the reaction. It indicates that the experimental value of theactivation energy in this study is consistent with that reportedin the literature for this transesterification using homogeneouscatalysts, such as NaOH, KOH, NaOCH3, NaOBu, H2SO4, andso on.1517 Noureddini reported activation energies for thereaction involved in the transesterification of soybean oil to bein the range of 6400-20 000 cal/mol, and Bernard andco-workers reported these in the range of 800020 000 cal/mol.

3.2.4. Transesterification of Soybean Oil to Biodiesel with

Ethanol. Biodiesel can be produced by the transesterificationof soybean oil with ethanol. The experimental results are shownin Table 2. It indicates that the reaction rate was slow at lowtemperature. It cost 3 h to reach a 91.8% biodiesel yield at 75C. Therefore, calcium ethoxide can also catalyze the transes-terification soybean oil to biodiesel with ethanol. It has strongbasicity and can catalyze many transesterification reactions.

Some researchers have proposed some possible mechanisms inchemical reactions over solid base catalysts.5,1113,18,19 Whencalcium ethoxide is used as a solid base catalyst, the catalysis

mechanism could be assumed to be one where that the catalyticreactions take place on the surface of calcium ethoxide. Theproposed mechanism of the transesterification reaction bycalcium methoxide with ethanol is given in Figure 10. Alcoholand triglyceride are adsorbed on two neighboring free catalyticsites (O- and Ca+). The surface O- extracts an H+, and Ca+

adsorbs CH3CH2O- from alcohol. The adsorbed triglycerideforms a surface intermediate with the catalyst. The twoneighboring adsorbed species react to result in the formationof a fatty acid methyl and a diglyceride. The diglyceride reactswith alcohol along similar processes on the surface of thecatalyst to form glycerol and biodiesel.

4. Conclusions

From the experimental results, it can be seen thatCa(OCH2CH3)2has excellent catalytic abilities as a solid basecatalyst. It has a moderate surface area, a relatively broader

particle size distribution, and a better low solubility in methanoland ethanol. When it catalyzes the transesterification of soybeanoil to biodiesel with methanol, the optimal conditions are a 12:1molar ratio of methanol to oil, the addition of 3.0%Ca(OCH2CH3)2catalyst, a 65 C reaction temperature, and about1.5 h of reaction time. The reactions are completed under mildtemperature and pressure conditions. This catalyst can also beapplied in other chemical reactions as a solid base catalyst.

Acknowledgment. The authors would like to thank ProfessorYigui Li for his kind help. This work was supported by the NationalBasic Research Program (973 Plan, No. 2007CB714302).

EF700518H

(15) Darnoko, D.; Cheryan, M. Kinetics of palm oil transesterificationin a batch reactor. J. Am. Oil Chem. Soc. 2000, 77, 12631267.

(16) Noureddini, H.; Zhu, D. Kinetics of Transesterification of soybeanoil. J. Am. Oil Chem. Soc. 1997, 74, 14571463.

(17) Bernard, F.; Royden, O. B.; Evereff, H. P. Transesterificationkinetics of soybean oil. J. Am. Oil Chem. Soc. 1986, 63, 13751380.

(18) Dossin, T. F.; Reyniers, M. F.; Marin, G. B. Kinetics of hetero-geneously MgO-catalyzed transesterification.Appl. Catal., B2006,61, 3545.

(19) Hideto, T.; Fuyuki, Y.; Hideshi, H.; Hideaki, K. Self-condensationof n-butyraldehyde over solid base catalysts. J. Catal. 1994, 148, 759770.

Figure 10. Reaction mechanism for the transesterification of triglyceride with ethanol over the Ca(O CH2CH3)2 catalyst, where R1, R2, and R3represent the long chain alkyl group.

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Calcium Ethoxide as a Solid Base Catalyst for The