CaO Catalysts

Aug 15, 2012 - 90°, with a step size of 0.02° and a scanning speed of 1.2° min. −1 .... 5c verify the existence of Ca, Cs, and F. The evidence pr...
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Transesterification of Soybean Oil Using CsF/CaO Catalysts Chin-Chia Liu,† Wen-Chang Lu,‡ and Ta-Jo Liu†,* †

Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan R.O.C. Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 31040, Taiwan R.O.C.

‡

S Supporting Information *

ABSTRACT: Fatty acid methyl ether (FAME) is a green biofuel that can be used as an alternative fuel to replace conventional petroleum. A novel solid base catalyst, CsF/CaO, was used to catalyze the transesterification of soybean oil to produce FAME. This catalyst showed strong basicity, resulting in high reactivity on transesterification. With an 8% catalyst dosage, 12:1 molar ratio of methanol to oil and 1% water content in oil, the FAME yield reached 98% at 65 °C in 1 h. Moreover, the catalyst also exhibited high resistance to water, which allowed more flexibility for biodiesel mass production. The transesterification of soybean oil followed a second-order reversible kinetic rate equation. The forward reaction rate constants and apparent activation energy were evaluated. Comparisons with the experimental data indicated that the kinetic model was suitable for predicting the yield of FAME. FAME yield reached about 70% at 75 °C in 2 h under conventional heating, with a 1200 rpm stirring rate and 4:1 molar ratio of methanol to oil. Recently, many researchers have focused on the use of calcium oxide base catalyst for transesterification owing to its high catalyst activity and reusable characteristic. For instance, Reddy et al.12 investigated using a nanocrystalline CaO as the catalyst, and the FAME yield reached 100% at room temperature. However, the reaction rate was slow, and 6−24 h were required to obtain a high FAME yield. Granados et al.8 reported the reutilization of CaO in the transesterification of sunflower oil without significant deactivation. Liu et al.13 revealed that the rate of CaO catalysis was accelerated if the oil feedstock contained less than 2.03% of water. Yan et al.4 developed a La2O3/CaO catalyst, which could tolerate high water and FFA in oil feedstock. However, the use of this catalyst is limited to the feed stock containing a high molar ratio of methanol to oil (20:1). The kinetics of homogeneous catalytic transesterification has been widely studied,14−17 but very few have reported kinetic studies on heterogeneous catalytic transesterification. Singh and Fernando18 proposed a simple first-order kinetic model for metal oxide catalyzed soybean oil transesterification. Dossin et al.19,20 developed a kinetic model based on the three-step Eley− Rideal mechanisms for the transesterification of ethyl acetate and triolein with methanol catalyzed by MgO. Wang and Yang21 reported the kinetics of the transesterification catalyzed by nanoMgO at high temperature. Kouzu et al.7 investigated the kinetics in CaO catalyzed transterification of soybean oil with methanol and showed that when the FAME yield increased, the reaction kinetics varied from zero-order to firstorder with respect to the triglyceride concentration. Finally, Yan et al.4 showed that the soybean oil transesterification

1. INTRODUCTION Biofuels have aroused much attention in the green-tech revolution owing to energy demand and climate change around the world. Biodiesel is a popular biofuel because of its nontoxicity and biodegradable nature.1 It can reduce the exhaust emission of COx and SOx from vehicles, thus easing the greenhouse effect in the environment.2,3 Notably, biodiesel is a good alternative to conventional fossil diesel.4 Nowadays, biodiesel is mainly produced by transesterification of vegetable oil or animal fat with methanol. In current industrial processes, the promotion of the transesterification reaction is achieved utilizing homogeneous base catalysts such as KOH and NaOH.4 However, most of these catalysts are corrosive. The separation of the dissolved catalysts is difficult and is not environmentally benign because of the large quantity of wastewater generated.5,6 Furthermore, the transesterification rate may be affected by the presence of free fatty acids (FFA) and water (FFA content >0.5%, water content >0.06%) in the oil feedstock. Traditional base catalysts may react with water and FFA in oil feedstock to form soap, which can significantly lower the yield of the transesterification product.4,7 Therefore, many researchers have attempted to develop new processes to produce biodiesel with heterogeneous base catalysts.7 Biodiesel production using solid base catalysts is an effective and viable technology because the catalysts can be easily separated from liquid products and reused with or without regeneration. Moreover, successful utilization of solid catalysts can overcome the economical and environmental problems, which are usually encountered in the homogeneous process.8 Ebiura et al.9 examined the K2CO3/Al2O3 catalyzed triolein transesterification at 60 °C, with a 25:1 molar ratio of methanol to oil, and 90% yield of fatty acid methyl ester (FAME) was achieved in an hour. Xie et al.10 studied the transesterification of soybean oil using KF/ZnO as a solid base catalyst at mild reaction conditions (65 °C, 1 atm). The FAME yield was 87% in 8 h. Furthermore, Versiu et al.11 tested the catalyst activity of CsF/Al2O3 in the transesterification of sunflower oil. The © 2012 American Chemical Society

Received: May 3, 2012 Revised: August 14, 2012 Published: August 15, 2012 5400

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and vacuum-dried to recover the catalyst. FAME was mixed with a heptane solution containing methyl heptadecanoate (internal standard for gas chromatography (GC)). The resulting solution was analyzed using a gas chromatograph (Shimazu GC-14A) with a flame ionization detector (FID) and a capillary column (WBC Attachment ASSY 221-29992-91). Solution (1 μL) at 140 °C was withdrawn by a syringe for analysis.10 The remaining solution was maintained at 140 °C in the oven for 5 min, and the oven temperature was increased at a rate of 10 °C/min until 220 °C was reached. The sample was kept for another 3 min at the final oven temperature of 220 °C. Nitrogen was used as the carrier gas at a flow rate of 30 mL/ min. The injector and detector temperatures were set at 220 and 250 °C, respectively. The FAME content of the upper layer in the samples was determined according to the European procedure EN 14103. It was assumed that MA ≅ 3MD and the FAME yield (%) is calculated by

catalyzed by CaO modified with La2O3 could be assumed to be a pseudo-first-order reaction in excess of methanol.4 The objective of the present research was to develop a high performance solid catalyst, CsF/CaO, for transesterfication of soybean oil with methanol. Effects of reaction parameters such as the loading amount of CsF, catalyst dosage, and reaction temperature at 12:1 and 6:1 molar ratios of methanol to oil on the FAME yield were investigated. The resistance of CsF/CaO to water content in oil feedstock was also examined. Finally, a kinetic model for the formation of FAME was proposed.

2. EXPERIMENTAL SECTION 2.1. Materials. Soybean oil was purchased from a local supermarket. Methanol (99.9%) and n-heptane (analytic grade) were purchased from Merck (Darmstadt, Germany). Ca(OH)2 (industrial grade) of average size 200 μm was obtained from Shye Jyh Enterprise Co., Ltd. (Hsinchu, Taiwan). CsF was obtained from Aldrich (Buchs, Switzerland). Methyl heptadecanoate (internal standard for GC) was purchased from Fluka (Switzerland). 2.2. Catalyst Preparation and Characterization. CsF/ CaO were prepared by impregnation of Ca(OH)2 with aqueous solution of CsF. Three different amounts of CsF (0.57 g, 2.84 g, 5.1 g) were dissolved separately in 50 mL of water and impregnated onto 50 g of Ca(OH)2. The impregnated sample was dried overnight at 100 °C and then calcined at 600 °C for another 8 h. CaO was prepared by the direct calcination of Ca(OH)2 at 600 °C for 8 h. The basic strength of the catalysts was determined by the Hammett indicators listed in Table 1.10

FAME Yield (y , %)

indicator

basic color

acid color

pKBH

blue pink red orange

red transparent yellow yellow

7.2 9.3 15 18.4

(1-1)

=[3(NA0 − NA )MD]/(NA0MA )

(1-2)

=(NA0 − NA )/NA0

(1-3)

=(CA0 − CA )/CA0

(1-4)

3. KINETIC MODELING The overall transesterification reaction equation can be presented as follows: (2) A + 3B ⇌ C + 3D where A is soybean oil, B is methanol, C is glycerol, and D is biodiesel. For modeling the process of CsF/CaO catalyzed transesterification, the following assumptions are made: (1) Soybean oil, methanol, and solid catalyst are perfectly mixed, and the whole reaction system can be considered to be pseudo-homogeneous. (2) The mixing is vigorous such that no phase separation occurs during the reaction. The external mass transfer resistance is assumed to be negligible. (3) The sizes of catalyst particles are small so that the internal mass transfer resistance is negligible. (4) Adsorption and desorption rates of the reaction mixture are fast enough, and the whole CsF/CaO catalyzed transesterification process is chemical reaction controlled. (5) The reaction kinetics is assumed to follow a pseudosecond-order reversible reaction with respect to both reactants and products. (6) The saponification and neutralization of free fatty acids are negligible during the reaction. With these assumptions, the rate equation for the disappearance of soybean oil can be written as −dCA /dt = −rA = k1CAC B − k 2CCC D (3)

Table 1. Properties of the Hammett Indicators bromothymol blue phenolphthalein 2,4-dinitroaniline 4-nitroaniline

=[A t − AEI /AEI](C EI × VEI/W )(WP/WO)100

It was measured by a titration method using benzoic acid.4 In the measurement, 0.2 g of catalyst, 9 mL of methanol, and 1 mL of indicator were mixed together and titrated with 0.02 N benzoic acid solution. The titration was terminated as soon as the color of the indicator changed. Powder X-ray diffraction analysis was carried out in Rigaku Ultima IV equipped with Cu Kα radiation. The patterns were recorded over a 2θ range of 5− 90°, with a step size of 0.02° and a scanning speed of 1.2° min−1. Scanning electron images and elemental chemical analysis were obtained on a Hitachi S-4800 scanning electron microscope equipped with an EDS detector (EDAX, U.S.A.) and the maximum voltage was 10 kV. 2.3. Experimental Procedure. Transesterification catalyzed by CsF/CaO was carried out in a 500 mL four-neck, round-bottomed glass flask equipped with a mechanical stirrer, a reflux condenser, a thermometer, and a sampling port. Soybean oil and catalysts were rigorously mixed together in a glass batch reactor, heated by a mantle heater. Methanol, preheated at a desired temperature, was then added to the mixture, and the reaction started. Samples (4 mL) were taken during the progress of the reaction, after immediately quenching and centrifuging (6000 rpm for 25 min),22 for analysis. The top layer was the desired product FAME, the middle layer was glycerol, and the bottom layer was a mixture of glycerol and catalyst. The bottom layer was removed, filtered

Since

5401

CA = CA0(1 − y)

(4-1)

C B = CA0(⌀ − 3y)

(4-2)

Cc = CA0y

(4-3) dx.doi.org/10.1021/ef300941w | Energy Fuels 2012, 26, 5400−5407

Energy & Fuels C D = 3CA0y

Article

(4-4)

Equation 3 can be transformed in terms of yield y: dy/dt = k1CA0[(1 − y)(⌀ − 3y) − 3y 2 /Ke]

(5) 23,24

Equation 5 was solved by a direct integration method. The forward reaction rate constant (k1) and the equilibrium constant (Ke) were obtained through a nonlinear least-squares regression method based on the plot of time vs FAME yield using the mathematic software MATLAB7.0.

4. RESULTS AND DISCUSSION 4.1. Heterogeneous Catalysis of CsF/CaO. The transesterification catalyzed by a solid catalyst is a three-phase system. To overcome the mass transfer resistance, the system must be well-mixed. In this study, 8 wt % of 0.5 mmol CsF/ CaO catalyst was dosed with the liquid reactants consisting of methanol and soybean oil at a molar ratio of 12:1. The reaction temperature was 65 °C, and the stirring speeds were set at 100, 200, 400, 600 rpm to examine the effect of stirring rate on the FAME yield. As seen in Figure 1, the yield becomes

Figure 2. CsF and CsF/CaO catalyzed transesterification. Reaction conditions: catalyst amount 8%, molar ratio of methanol to oil 12:1, reaction temperature 65 °C.

Figure 3. Transesterification with or without CsF/CaO. Reaction conditions: catalyst amount 8%, molar ratio of methanol to oil 12:1, reaction temperature 65 °C, CsF loading amount 0.5 mmol.

Figure 1. Effect of the stirring speed on the FAME yield. Reaction conditions: catalyst amount 8%, molar ratio of methanol to oil 12:1, reaction temperature 65 °C.

Table 2. CsF/CaO Reusability on Transesterification

independent of stirring speed at speeds higher than 200 rpm, suggesting negligible mass transfer resistance at high stirring speeds.15 Hence, the effects of other parameters on the FAME yield in subsequent experiments were investigated at a fixed stirring speed of 400 rpm. Kouzu et al.25 examined the heterogeneous transesterification reaction using CaO as the catalyst. In the present study, the catalyst employed was a mixture of CsF/CaO. To verify that the increase in the activity of CsF/CaO was due to the combination of the two compounds, and not CsF alone, a different reaction experiment was carried out using 8% CsF as the sole catalyst, and the FAME yields were observed at the same operating conditions. Figure 2 indicates that there was no FAME yield using CsF as the lone catalyst over the entire range of stirring speed. The experiment to test the catalytic activity of the fluid phase after the reaction by removing the solid CsF/ CaO catalyst was carried out. The results are shown in Figure 3; it is clear that, after 90 min, there was little FAME yield (%) in the liquid phase.26 Furthermore, Table 2 shows the FAME yield maintains at a very high level after repeated usage. For example, the yield is 89% when the catalyst is reused three times. The

repetitions

FAME yield (%)

fresh catalyst first reuse second reuse third reuse

95.6 94 90 89.7

small drop in yield over repeated usage is probably due to the loss of catalyst in the recovery process involving filtration and drying.7 The transesterification will reach the equilibrium state within 10 min, owing to high activity of the CsF/CaO catalyst. The turnover rate was estimated at the beginning and 5 min after the reaction. The turnover rates are shown in Table 3. The weight of the soybean oil is 250 g. The weight of 0.5 mmol CsF/CaO (CaO with 0.5 mmol CsF) is 20 g, and the amount of CsF is 0.01 mol. The volumes of the reactants are 0.4 L at a 12:1 molar ratio and 0.34 L at a 6:1 molar ratio of methanol to oil. The turnover rate at the beginning of the reaction is larger than that after 5 min because the system is well-mixed, even within a few seconds, and the concentrations of the reactants are high. The turnover rates also increase with the reaction 5402

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Table 3. Turnover Rates of the CsF/CaO methanol/ oila

temp. (°C)

turnover rateb (min−1)

turnover ratec (min−1)

12 12 12 6 6 6

60 50 45 60 50 45

17.63 6.742 4.502 10.120 3.644 2.981

0.560 1.371 1.4 1.023 1.165 1.127

a Molar ratio of methanol to oil. bCalculated at t = 0. cCalculated at t = 5 min.

temperature because high temperature favors the transesterification. 4.2. Catalyst Characterization and Activity. Figure 4 illustrates the XRD patterns for three different concentrations

Figure 4. XRD patterns of solid catalysts: (a) 0.1 mmol CsF/CaO, (b) 0.5 mmol CsF/CaO (c), 0.9 mmol CsF/CaO. (1) CaO, JCPDS file 77-2376; (2) Ca(OH)2, JCPDS file 84-1264; (3) CsCaF3, JCPDS file 77-1571.

of CsF/CaO catalysts: (a) 0.1 mmol CsF/CaO, (b) 0.5 mmol CsF/CaO, and (c) 0.9 mmol CsF/CaO. The peaks of Ca(OH)2 were observed because the experiment of the present study was performed under atmospheric environment. The XRD patterns of solid catalysts indicate the possible existence of CsCaF3 (see the Supporting Information). Comparing the SEM image of pure CaO (Figure 5a) and with those of 0.5 mmol CsF/CaO (Figures 5b and c), it is easy to see the cubiclike crystalline structure of CsCaF3. The results of the EDS analysis in Table 4 on the red square A of the catalyst in Figure 5c verify the existence of Ca, Cs, and F. The evidence provided by XRD patterns, SEM images, and EDS analysis together can verify the existence of CsCaF3. It is easier for CH3OH to be dissolved into CH3O− with CsCaF3 and the increasing amount of CH3O− will enhance the reaction with the carbonyl groups of the oil molecules.26,27 In addition, the catalytic ability and stable structure of CsCaF3 will produce the maximum FAME yield because the existence of CsCaF3 may slow down the phase transformation of the catalyst, this was supported by the study of Fan et al. on a similar KF/CaO system.27 The basic sites over heterogeneous catalysts are the active centers for transesterification.28,29 The more basic sites, the higher the activity of the catalysts. The basic sites are directly related to the

Figure 5. Scanning electron microscopy images (SEM images) of the catalyst: (a) pure CaO, (b) 0.5 mmol CsF/CaO, 5000×, (c) 0.5 mmol CsF/CaO, 10 000×.

Table 4. EDS Analysis on the Surface of the Catalyst in Figure 5c element

wt %

at %

F Ca Cs

11.99 17.65 70.36

39.42 27.51 33.07

amount of active species, CsF, and alkaline earth metal oxide, CaO.11,30,31 Table 5 lists the basic strength and basicity of the solid catalysts. The basicity of catalysts increases with the loading amount of CsF, as well as the FAME yield. The 5403

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Table 5. Basic Strength, Basicity, and the FAME Yield Using Different Loading Amounts of CsF on CaO catalyst 0.1 mmol CsF/ CaO 0.5 mmol CsF/ CaO 0.9 mmol CsF/ CaO

basic strengtha (H_)

basicityb (mmol/g)

FAME yieldc (%)

15 < H_< 18.4

25.6

90

15 < H_< 18.4

28

95.6

15 < H_< 18.4

28.4

96.3

a

Observed by color change of the indicator. bMeasured by titration. Reaction conditions: catalyst amount 8%, methanol/oil 12:1, reaction temperature 65 °C. c

increases in both basicity and FAME slow down from 0.5 to 0.9 mmol CsF as the loading approaches its saturation. Moreover, Figure 6 shows that raising the loading amount of CsF from 0.5

Figure 7. Effect of catalyst dosage on the FAME yield. Reaction conditions: molar ratio of methanol to oil 12:1, reaction temperature 65 °C, CsF loading amount 0.5 mmol.

Figure 6. Effect of the CsF loading amount on the FAME yield. Reaction conditions: catalyst amount 8%, molar ratio of methanol to oil 12:1, reaction temperature 65 °C.

Figure 8. Effect of the reaction temperature on the FAME yield. Reaction conditions: molar ratio of methanol to oil 12:1, catalyst dosage 8%, CsF loading amount 0.5 mmol.

to 0.9 mmol increases the reaction rate only slightly. Hence, subsequent experiments were carried out at 0.5 mmol of CsF loading to study the effects of other reaction parameters. 4.3. Effect of Catalyst Dosage on FAME Yield. It has been reported that the catalyst dosage strongly influences the FAME yield in the transesterification using solid catalysts.4,12,22 The effects of different catalyst dosages (1−10%, referred to oil weight) were investigated with a 12:1 molar ratio of methanol to oil at 65 °C for 90 min. The results in Figure 7 indicate that the FAME yield is greatly improved by increasing the amount of CsF/CaO added until the catalyst dosage reaches 8%. Further increase in catalyst dosage only improves the FAME yield slightly. This could be due to the imperfect mixing of the three-phase system containing high solid contents, thus creating more mass transfer resistance.32 4.4. Effect of Reaction Temperature on FAME Yield. Figures 8 and 9 illustrate the time courses of the FAME yield at different reaction temperatures with two different molar ratios of methanol to oil (12:1, 6:1). Higher reaction temperature accelerates the reaction rate in transesterification because of the higher energy state of the molecules, resulting in more effective collisions.15 The influence of the methanol to oil molar ratio on the FAME yield appears to be relatively small compared to the reaction temperature. With the 12:1 molar ratio of methanol to oil, the FAME yield increases from 77.5% to 95.6% in 90 min over the temperature range 45−65 °C. At the same conditions,

Figure 9. Effect of the reaction temperature on the FAME yield. Reaction conditions: molar ratio of methanol to oil 6:1, catalyst dosage 8%, CsF loading amount 0.5 mmol.

the increase in the FAME yield for the 6:1 molar ratio of methanol to oil is only marginally lower than the 12:1 mixture, that is, from 70% to 94%. The rate of increase in the reaction rate and the FAME yield is faster at lower temperature and 5404

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constants (k1) and the equilibrium constants (Ke) were determined from the MATLAB software, as tabulated in Table 7. As shown in the table, the forward reaction rate

lower molar ratio of methanol to oil. This is because the strong basic property of CsF/CaO decreases the activation energy of transesterification. In 5 min, the FAME yield reached near 90% with a 12:1 molar ratio of methanol to oil and near 80% with a 6:1 molar ratio at both 60 and 65 °C. 4.5. Effect of Water Content in Oil on FAME Yield. Liu et al.13 found that the presence of water molecules could accelerate the transesterification reaction rate using CaO as the solid catalyst. OH groups are formed due to water absorption on the CaO surface. In order to study the effect of water content on the activity of CsF/CaO, transesterification reactions of oil feedstock containing 1%, 3%, and 5% of water with methanol were conducted. Figure 10 indicates that

Table 7. Forward Reaction Rate Constant (k1) and the equilibrium constant (Ke) at Different Reaction Conditions methanol/oila

temp. (°C)

12 12 12 6 6 6

60 50 45 60 50 45

a

k1 (L mol−1 min−1 g catalyst−1) 4.3466 1.6623 1.11 4.3273 1.5463 1.2652

× × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3

Ke 3.9414 1.1355 0.6058 6.1419 1.0776 0.8417

Molar ratio of methanol to oil.

constants increase with the reaction temperature at both 12:1 and 6:1 molar ratios of methanol to oil. This indicates that high temperature favors the transesterification reaction, which is in agreement with the findings of Freedman et al.14 and Noureddini and Zhu.15 The prediction of Ke may not be accurate because the phase equilibrium between gas and liquid affects the entire reaction system.33,34 Nonetheless, the variation of Ke with temperature is clearly noticed. The apparent forward activation energy (Ea) was estimated by the Arrhenius equation: k1 = A1 × e−Ea / RT

Figure 11 shows the plot of ln k1 versus 1/T at various temperatures and two different molar ratios of methanol to oil

Figure 10. Effect of water content in oil on the FAME yield. Reaction conditions: molar ratio of methanol to oil 12:1, reaction temperature 65 °C, catalyst dosage 8%, CsF loading amount 0.5 mmol.

the FAME yield reaches above 90% for 90 min with water content below 3%. It appears that there is an optimum water content for maximum FAME yield. For example, 1% water content results in a 98% FAME yield. In the presence of excess water, the product FAME would hydrolyze into carboxylic acid and alcohol, which would deactivate the CsF/CaO catalyst by reacting with it to form soap.13 The maximum FAME yield is only 77% when the water content is 5%. Water had been shown to have negative effects on the homogeneous base catalyzed transesterification and was limited to 0.06% or less in traditional processes.1,4 In the present study, the water content could be tolerated up to 3% in the CsF/CaO catalyzed process, allowing a more flexible operation for biodiesel production. 4.6. Kinetics of Transesterification. The experimental data including the reaction conditions summarized in Table 6 were evaluated using the proposed kinetic model in Section 3. The temperature dependencies of the forward reaction rate

Figure 11. ln k1 versus 1/T at different temperatures (60 °C, 50 °C, 45 °C) and two molar ratios of methanol to oil (M/O = 12:1, M/O= 6:1).

(M/O = 12:1, 6:1). A good linear correlation was obtained, with a correlation coefficient of 0.98. The forward activation energy, evaluated from the slope of the straight line, is 77 922 J mol−1, which is consistent with the results obtained by Noureddini and Zhu,15 ranging from 26 883 J mol−1 to 83 150 J mol −1. High activation energy means that the transesterification catalyzed by CsF/CaO is sensitive to temperature, and the reaction is kinetically controlled as reported by Noureddini and Zhu.15 Comparisons between the experimental data and the calculated values of the FAME yield are shown in Figure 12. The linear correlation coefficient was 0.9903, indicating the

Table 6. Reaction Conditions for the Kinetic Modeling

a

run

methanol/oila

temp. (°C)

catalyst amount (%)

1 2 3 4 5 6

12 12 12 6 6 6

60 50 45 60 50 45

8 8 8 8 8 8

(6)

Molar ratio of methanol to oil. 5405

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Figure 12. Comparison between the experimental and calculated FAME yields.

Figure 14. Experimental (symbols) and calculated (lines) values of the FAME yield as a function of time at different temperatures. Reaction conditions: molar ratio of methanol to oil 6:1, catalyst dosage 8%, CsF loading amount 0.5 mmol.

agreement is satisfactory. Further comparisons plotted in terms of yield versus time at different temperatures are illustrated in Figures 13 and 14. Hence, the proposed pseudo-homogeneous

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ASSOCIATED CONTENT

■

AUTHOR INFORMATION

S Supporting Information *

Additional figures. This information is available free of charge via Internet at http://pubs.acs.org/. Corresponding Author

*Telephone: +886-3-5723380. Fax: +886-3-5715408. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was initiated by the late Prof. Shyh-Jye Hwang. The authors acknowledge the financial support provided by the Bureau of Energy, MOEA, Taiwan, R.O.C. Valuable comments from Prof. Carlos Tiu, Monash University, Australia, are highly appreciated.

Figure 13. Experimental (symbols) and calculated (lines) values of the FAME yield as a function of time at different temperatures. Reaction conditions: molar ratio of methanol to oil 12:1, catalyst dosage 8%, CsF loading amount 0.5 mmol.

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reversible second-order rate equation is a reasonable kinetic model that can be used to predict the FAME yield, at least qualitatively, for CsF/CaO catalyzed transesterification of soybean oil with methanol.

5. CONCLUSIONS Solid catalyst CaO combined with CsF exhibits a high catalyst activity and water resistance (3% in oil feedstock) in the transesterification of soybean oil with methanol. It provides a more flexible operational condition for biodiesel mass production. With 8% catalyst dosage, 12:1 molar ratio of methanol to oil, and 1% water content in oil, the FAME yield reaches 98% at 65 °C in 1 h. The CsF/CaO catalyzed transesterification follows pseudo-second-order reversible reaction kinetics. Good agreement between the predicted and experimental values suggests that this model can be used to predict the FAME yield from the transesterification of soybean oil with methanol. 5406

NOMENCLATURE At = peak area of methyl ester including internal standard (C14−C22:1), eq 1-1 A1 = pre-exponential factor, eq 6 AEI = peak area of methyl heptadecanoate, eq 1-1 CA = concentration of soybean oil (mol/L), eq 3 CB = concentration of methanol (mol/L), eq 3 CC = concentration of glycerol (mol/L), eq 3 CD = concentration of biodiesel (mol/L), eq 3 CA0 = initial concentration of soybean oil (mol/L), eq 1-4 CEI = concentration of methyl heptadecanoate (10 g/L) eq 1-1 Ea = forward activation energy (J/mol) eq 6 k1 = forward rate constant (L/(mol min g catalyst)) eq 3 k2 = backward rate constant (L/(mol min g catalyst)) eq 3 L = liter Ke = equilibrium constant, eq 5 MA = molecular weight of soybean oil (g) MD = molecular weight of biodiesel (g), and MA ≅ 3 × MD NA = moles of A after reaction (mole), eq 1-2 NA0 = initial moles of A (mole), eq 1-2 rA = reaction rate based on soybean oil (mol/(L min)), eq 3 dx.doi.org/10.1021/ef300941w | Energy Fuels 2012, 26, 5400−5407

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(29) Van Gerpen, J. Fuel Process. Technol. 2005, 86, 1097−1107. (30) Clacens, J. M.; Genuit, D.; Veldurthy, B.; Bergeret, G.; Delmotte, L.; Garcia-Ruiz, A.; Figueras, F. Appl. Catal., B 2004, 53, 95−100. (31) Veldurthy, B.; Clacens, J. M.; Figueras, F. J. Catal. 2005, 229, 237−242. (32) Kim, H. K.; Kang, B. S.; Kim, M. J.; Park, Y. M.; Kim, D. K.; Lee, J. S.; Lee, K. Y. Catal. Today 2004, 93−95, 315−320. (33) Andreatta, A. E.; Casas, L. M.; Hegel, P.; Bottini, S. B.; Brignole, E. A. Ind. Eng. Chem. Res. 2008, 47, 5157−5164. (34) Barbosa, D.; Doherty, M. F. Chem. Eng. Sci. 1988, 43, 529−540.

R = gas constant (8.314 J/(mol k)), eq 6 t = reaction time (min), eq 3 T = absolute temperature (K), eq 6 VEI = volume of methyl heptadecanoate solution (5 mL), eq 1-1 W = weight of the upper layer of samples (250 mg), eq 1-1 WO = total weight of oil feedstock (g), eq 1-1 WP = total weight of the product (g), eq 1-1 y = FAME yield, eq 5 Greek Letters

ϕ = molar ratio of methanol to oil, eq 4-2

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dx.doi.org/10.1021/ef300941w | Energy Fuels 2012, 26, 5400−5407