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Biodiesel Synthesis by Simultaneous Esterification and Transesterification Using Oleophilic Acid Catalyst Yi-Shen Lien, Li-Shan Hsieh, and Jeffrey C. S. Wu* Department of Chemical Engineering, National Taiwan UniVersity, Taipei, Taiwan 10617
Solid-acid catalysts can perform transesterification and esterification simultaneously so that free fatty acids (FFAs) in oil can be converted into biodiesel to avoid the disposal of biomaterial. A carbon catalyst was prepared by pyrolyzing glucose at 400 °C under a N2 stream. The catalyst was further sulfated using concentrated sulfuric acid. Transesterification of soybean oil and methanol was carried out at 150 °C and 1.7 MPa in a pressurized autoclave. More than 90% biodiesel yield was achieved within 2 h with a molar ratio of methanol to soybean oil of 30:1. The total biodiesel yield for the mixture of soybean oil and palmitic acid decreased to 85% when 20 wt % palmitic acid was used. A rate equation based on the Langmuir-Hishelwood mechanism was established to describe the kinetic behavior of transesterification. The adsorption equilibrium constant of soybean oil was higher than those of the other species, implying an oleophilic surface of the sulfated carbon catalyst. Introduction Biodiesel is one of the most popular biofuels because it is completely compatible with fossil diesel and the synthesis process is commercially available. Biodiesel can be produced by the transesterification of oil under ambient pressure at around 60 °C as shown in eq 1. In the reaction, 1 mol of triglyceride reacts with 3 mol of methanol under the help of catalysts, such as NaOH. The main product is fatty acid methyl ester (FAME), known as biodiesel, and the byproduct is glycerol. Usually, excess methanol is applied to enhance the conversion of triglyceride. The separation of excess methanol is done simply by distillation, and methanol is recycled to the feed. Because FAME is immiscible with glycerol, the former can be recovered by decanting.1,2
The traditional synthesis of biodiesel used homogeneous catalyst, NaOH, in a liquid-phase reaction. Although this method is simple and fast, the neutralization/washing step requires the consumption of acid, and the generation of acid/base waste becomes an environmental problem. Furthermore, when raw oil contains high free fatty acid (FFA) content (>4%), FFAs must be removed before transesterification. Otherwise, the saponification of FFAs occurs and produces undesired soap.3 The base catalyst, NaOH, is also consumed or deactivated by the saponification as shown in eq 2.4 Meher et al. suggested an alternative way to solve the FFA problem by using an acid catalyst. The esterification of FFA by methanol can be carried out before the transesterification of oil as shown in eq 3.5 Thus, valuable FFAs can be completely converted to biodiesel without being wasted. * To whom correspondence should be addressed. Tel.: +886-2-23631994. Fax: +886-2-2362-3040. E-mail:
[email protected].
RCOOH + CH3OH f RCOOCH3 + H2O
(3)
Using heterogeneous catalysts, such as solid-acid or base catalysts, is an attractive route for biodiesel production because there is no aqueous waste. A process of biodiesel synthesis is shown in Figure 1. A direct transesterification can be performed when the FFA content is less than 4% in the oil feed. Alternatively, pre-esterification can be carried out by acid catalyst to convert FFAs (>4%) to FAME, followed by transesterification by a base catalyst. Thus, the preliminary separation of FFA in the oil feed is eliminated, and the yield of biodiesel is increased. However, the two-step process is rather complicated and can be further simplified. The esterification and transesterification can be carried out simultaneously in a one-step process by using acid catalyst.6,7 Even though the esterification of FFAs with methanol to FAME is fast on acid catalyst, the transesterification is very slow on a regular acid catalyst. Thus, a superacid catalyst is necessary to give a reasonable transesterification rate comparable to that on base catalyst.8 Our objective was to prepare a highly acidic carbon catalyst to replace the conventional NaOH process in biodiesel production.9 We followed the method reported by Toda et al. to
Figure 1. Two-step processes of biodiesel production.
10.1021/ie901496h 2010 American Chemical Society Published on Web 02/02/2010
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synthesize sulfated carbon-base catalysts. The carbon support was prepared by the carbonization of glucose to form small polycyclic aromatic carbon rings that provided anchoring points for sulfonite groups (-SO3H), supplied by sulfuric acid. In addition, the carbon support is oleophilic, which means oil can be easily adsorbed on the catalyst surface as compared with the conventional hydrophilic oxide supports. Thus, the rate of transesterification can be enhanced as a result of increasing oil concentration on the catalyst surface. Experimental Section The carbon support was prepared by the pyrolysis of glucose. Twenty grams of glucose was pyrolyzed at 400 °C for 15 h under a N2 flow. After the carbon had been pulverized, 200 mL of 98% sulfuric acid was added, and the mixture was heated at 150 °C for 15 h under a N2 atmosphere. The sulfated carbon was washed with 1 L of deionized water at 80 °C for 30 min and then separated by centrifugation. The washing process was repeated seven times to remove residual sulfuric ion.10 The surface area of catalyst was measured by Ar adsorption and calculated by the Brunauer-Emmett-Teller (BET) method. The strength of acidity was examined using color-producing indicators, anthraquinone (Ho ) -8.2) and p-nitrotoluene (Ho ) -11.35). Because the coloration of the indicator cannot be observed by visual inspection due to the black carbon, the color change of indicator was determined by a diffuse reflectance UV-vis spectrometer (Varian Cary 100). Prior to the UV-vis measurement, the catalyst (0.2 g) was mixed with BaSO4 (1.0 g, a reference material for UV-vis spectroscopy) and heated at 80 °C for 1 h to remove any adsorbed moisture. The acidic density of the catalyst was measured by acid-base back-titration using NaOH and HCl aqueous solutions. The detailed procedure for acidity measurement was reported in the literature.11 Soybean oil (Uni-President Co., Yungkang City, Taiwan) was purchased from the supermarket and used without pretreatment. The transesterification of soybean oil was carried out in a pressurized autoclave (model 4560, Parr) under 1.7 MPa pressure at 130-150 °C for 6 h. Approximately 100 g of soybean oil was charged into the autoclave for each batch reaction. The molar ratio of methanol to soybean oil ranged from 6 to 30. The catalyst loading was 1-3 wt % soybean oil. The influence of FFAs was studied by adding palmitic acid into the soybean oil at concentrations ranging from 5 to 20 wt %. The products were analyzed by a gas chromatograph (HP-6890, Agilent) equipped with a 30-m-long HP-Innowax column and a flame ionization detector (FID). A 0.5-mL aliquot of sample was withdrawn and diluted with 25 mL of isopropanol for gas chromatography (GC) analysis each hour during the reaction. The biodiesel yield is defined as the amount of FAMEs formed in the transesterification reaction. The peak areas of FAMEs were lumped as total FAME. Therefore, the biodiesel yield was calculated as the weight percentage of total FAMEs relative to soybean oil.
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Table 1. Acidic Density of Catalysts under Different Conditions conditions fresh catalyst used for three consecutive transesterifications at 60 °C catalyst treated at 150 °C under ambient pressure catalyst treated at 150 °C under a pressure of 1.7 MPa
acidic density (mmol of SO3H/g) 2.73 2.51 2.66 1.18
were tested in glycerol solution under two conditions. The acidic loss was not serious at high temperature and ambient pressure (Table 1). However, a significant acidity loss was found under a high pressure of 1.7 MPa and at a temperature of 150 °C (see Table 1). The catalyst was heated at 150 °C for 15 h under ambient pressure (0.1 MPa) in the preparation procedure. Therefore, the catalyst should be stable at the reaction temperature of 150 °C. However, the leaching of SO3H groups could be significant under harsh conditions of high pressure (i.e., 1.7 MPa). A similar acidity loss due to SO3H leaching was also observed on sulfated carbon catalyst by Mo et al.9 Figure 2 shows the correlation between FAME (i.e., biodiesel) yield and reaction time for different catalyst loadings at 150 °C. A FAME yield of higher than 90% was obtained within 2 h of reaction time for 3 wt % catalyst loading. It is noted that the production rate of FAME decreased with decreasing catalyst loading. Nevertheless, near 90% FAME yield was still achieved in 6 h for 1 wt % catalyst loading. Figure 3 shows the influence of the methanol/oil ratio on the correlation between FAME and reaction time for 1 wt % catalyst loading at 150 °C. Apparently, the FAME yield decreased with decreasing ratio of methanol to oil. Theoretically, 3 mol of methanol is sufficient to convert 1 mol of triglyceride as shown in eq 1. However, because methanol and soybean oil are not miscible, a lower-thanexpected conversion rate resulted. To increase the utilization of oil, a much higher methanol-to-oil ratio is usually used to obtain a better dispersion, thus increasing the conversion of oil. Afterward, methanol can be easily separated by vaporization for recycling. The temperature effect on transesterification is shown in Figure 4. The FAME yield increased with temperature, as expected. The temperature was limited to 150 °C because the vapor pressure of methanol at this temperature is 1.4 MPa and it is under such a vapor pressure that liquid-phase methanol could be maintained in the reactor. Figure 5 shows the conversion of pure palmitic acid in esterification using 1 wt % catalyst loading at 60 °C. Note that,
Results and Discussion The specific surface area of the carbon support was 0.31 m2/g as determined by Ar adsorption. The carbon support was nonporous with a mean particle diameter of 38.7 µm. The acidic densities of sulfated catalysts are listed in Table 1. Fresh catalyst had an acidic density of 2.73 mmol of SO3H/g. The acidic density of the catalyst was only slightly reduced after three consecutive transesterifications at 60 °C. To examine the loss of acidic density at high temperature and pressure, the catalysts
Figure 2. Effect of catalyst loading on FAME yield, for methanol/oil ) 30:1, at 150 °C.
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Figure 3. Effect of methanol-to-oil ratio on FAME yield, for 1 wt % catalyst, at 150 °C.
Figure 6. Conversion of palmitic acid to biodiesel in soybean oil, for molar methanol/oil ) 30:1 and 1 wt % catalyst, at 150 °C.
Figure 4. Effect of temperature on FAME yield, for 1 wt % catalyst and methanol/oil ) 30:1.
Figure 7. Effect of palmitic acid on biodiesel yield, for molar methanol/oil ) 30:1 and 1 wt % catalyst, at 150 °C.
Figure 5. Esterification of pure palmitic acid at 60 °C, for methanol/palmitic acid ) 30:1 and 1 wt % catalyst.
at a low temperature of 60 °C, a conversion of near 60% can be achieved in less than 4 h. However, the transesterification of soybean oil was negligible at 60 °C in our experiment (not
shown), which justifies the conclusion that the rate of esterification must be much higher than the rate of transesterification using a solid-acid catalyst. Figure 6 shows the esterification conversion of palmitic acid mixed with soybean oil at 150 °C. As shown in Figure 6, the conversion achieves 100% in less than 1 h for 5-20 wt % palmitic acid in soybean oil. The rate of esterification is much higher than the rate of transesterification in the mixture, which suggests that palmitic acid is easily adsorbed and reacts quickly on acid sites. Figure 7 shows the total FAME yield of the mixture of palmitic acid and soybean oil at 150 °C. The total FAME yield is the sum of palmitic and soybean oil, indicating that the simultaneous esterification of FFAs and transesterification of oil can be carried out on the solid-acid catalyst. Apparently, no significant hindrance in the reaction rate is observed even at a high content of palmitic acid in soybean oil. A Langmuir-Hinlshelwood rate equation was derived based on simple elemental steps including adsorption, surface reaction, and desorption. Both soybean oil and methanol adsorbed on the same active sites. The rate-limiting step was assumed to be the surface reaction. The rate constant and equilibrium constants
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a
Table 2. Kinetic Constants of Rate Equation temperature (°C)
k (mol-1 L gcat-1 h-1)
K
K1 (mol-1 L)
K2 (mol-1 L)
K3 (mol-1 L)
K4 (mol-1 L)
R2
130 140 150
1.20 1.28 1.38
4.17 5.04 5.14
9.71 8.64 6.83
0.06 0.03 0.01
1.34 0.70 0.46
0.64 0.48 0.23
0.91 0.93 0.97
a Where k is the rate constant; K is the reaction equilibrium constant; and K1, K2, K3, and K4 are the adsorption equilibrium constants of soybean oil, methanol, FAME, and glycerol, respectively.
in eq 4 are listed in Table 2 and were estimated by nonlinear regression based on experimental data.
(
)
CFCG K rate ) (1 + K1CO + K2CM + K3CF + K4CG)2 k COCM -
(4)
In eq 4, CO, CM, CF, and CG are the concentrations of soybean, methanol, FAME, and glycerol, respectively. k is the rate constant; K is the reaction equilibrium constant; and K1, K2, K3, and K4 are the adsorption equilibrium constants of soybean oil, methanol, FAME, and glycerol, respectively. The correlation coefficients (R2) in Table 2 are all close to 1, meaning good statistical consistency of experimental data. The enthalpy of transesterification was calculated to be 14.9 kcal/ mol from chemical equilibrium constants (K) at temperatures ranging from 130 to 150 °C, indicating an endothermic reaction. The adsorption equilibrium constant of soybean oil, K1, was found to range from 9.71 to 6.83 at temperatures of 130-150 °C. The value was much higher than those of the other three adsorption equilibrium constants for methanol (K2), FAME (K3), and glycerol (K4), indicating a strong affinity of oil on the catalyst. This implies an oleophilic property of the surface of carbon-based catalyst. Moreover, the byproduct, hydrophilic glycerol, could also be desorbed quickly from the catalyst surface so that more active sites could be made available for oil molecules. Therefore, the rate of transesterification was enhanced by the high concentration of oil as well as the high turnover of active sites on the catalyst surface. Conclusions The conventional alkali aqueous-phase process has several drawbacks for biodiesel production, such as the generation of liquid acid/base waste in the neutralization step. In addition, FFAs must be removed before the transesterification of oil because FFAs consume base catalyst (e.g., NaOH) as a result of saponification. Solid-acid catalyst can be easily separated from the products and reused without generating aqueous acid/ base waste. Extra energy can also be saved on product separation by using a solid catalyst as compared with the aqueous NaOH process. An additional advantage of solid-acid catalyst is to achieve esterification simultaneously, thus eliminating the preliminary separation of FFAs from the oil feed. The disposal of FFAs is not only expensive but also a loss of biomaterial. In this study, oleophilic acidic carbon catalysts were prepared and
successfully applied to the synthesis of biodiesel. A >90% yield of FAME from soybean oil was achieved within 2 h at 1.7 MPa and 150 °C. The simultaneous process of esterification and transesterification will greatly improve the efficiency of biodiesel production when waste cooking oil and animal fat are used as the raw materials. Unlike in the aqueous NaOH batch process, a solid-acid catalyst can also be used in a packed-bed reactor so that the continuous production of biodiesel is feasible. Acknowledgment Financial support by the Ministry of Economic Affairs, Taiwan, under Grant 98-EC-17-A-09-S1-019 is gratefully acknowledged. Literature Cited (1) Ma, F. R.; Hanna, M. A. Biodiesel production: A review. Bioresour. Technol. 1999, 70 (1), 1. (2) Gerpen, J. V. Biodiesel processing and production. Fuel Process. Technol. 2005, 86 (10), 1097. (3) Freedman, B.; Pryde, E.; Mounts, T. Variables affecting the yields of fatty esters from transesterified vegetable oils. J. Am. Oil Chem. Soc. 1984, 61 (10), 1638. (4) Lotero, E.; Liu, Y. J.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.; Goodwin, J. G. Synthesis of biodiesel via acid catalysis. Ind. Eng. Chem. Res. 2005, 44 (14), 5353. (5) Meher, L. C.; Sagar, D. V.; Naik, S. N. Technical aspects of biodiesel production by transesterificationsA review. Renewable Sustainable Energy ReV. 2006, 10 (3), 248. (6) Luˆpez, D. E.; Goodwin, J. G., Jr.; Bruce, D. A.; Furuta, S. Esterification and transesterification using modified-zirconia catalysts. Appl. Catal. A: Gen. 2008, 339 (1), 76. (7) Yan, S. L.; Salley, S. O.; Ng, K. Y. S. Simultaneous transesterification and esterification of unrefined or waste oils over ZnO-La2O3 catalysts. Appl. Catal. A: Gen. 2009, 353 (2), 203. (8) Furuta, S.; Matsuhashi, H.; Arata, K. Biodiesel fuel production with solid superacid catalysis in fixed bed reactor under atmospheric pressure. Catal. Commun. 2004, 5 (12), 721. (9) Mo, X.; Lopez, D. E.; Suwannakarn, K.; Liu, Y.; Lotero, E.; Goodwin, J. G.; Lu, C. Q. Activation and deactivation characteristics of sulfonated carbon catalysts. J. Catal. 2008, 254 (2), 332. (10) Toda, M.; Takagaki, A.; Okamura, M.; Kondo, J. N.; Hayashi, S.; Domen, K.; Hara, M. Green chemistrysBiodiesel made with sugar catalyst. Nature 2005, 438 (7065), 178. (11) Takagaki, A.; Toda, M.; Okamura, M.; Kondo, J. N.; Hayashi, S.; Domen, K.; Hara, M. Esterification of higher fatty acids by a novel strong solid acid. Catal. Today 2006, 116 (2), 157.
ReceiVed for reView September 23, 2009 ReVised manuscript receiVed January 19, 2010 Accepted January 23, 2010 IE901496H
