From Homogeneous to Heterogeneous Catalysts in Biodiesel

The cost of biodiesel could certainly be lowered by using a heterogeneous catalyst instead of a homogeneous one, resulting in a higher quality of este...
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Ind. Eng. Chem. Res. 2007, 46, 6379-6384

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APPLIED CHEMISTRY From Homogeneous to Heterogeneous Catalysts in Biodiesel Production M. Di Serio, M. Cozzolino, M. Giordano, R. Tesser, P. Patrono,† and E. Santacesaria* UniVersity of Naples “Federico II”-Department of Chemistry, Complesso UniVersitario Monte S. Angelo, Via Cintia 4, I-80126 Naples, Italy

The production of biodiesel as a fuel in diesel engines greatly increased in recent years and is expected to grow more and more in the near future. Increasing biodiesel consumption requires optimized production processes allowing high production capacities, simplified operations, high yields, and the use of more economic feedstocks such as waste oils and fats. However, the latter often contain large amounts of free fatty acids and cannot be processed with the commonly practiced technology based on the use of alkaline catalysts in the homogeneous phase that requires the use of highly refined oil as raw materials. Therefore, the development of processes for low-cost biodiesel production requires the individuation of heterogeneous catalysts that are very efficient in promoting the transesterification reaction also in the presence of free fatty acids and water, allowing the prompt separation of pure glycerol and not requiring expensive purification of this byproduct. In the present contribution, the performances of different heterogeneous catalysts are compared both in the absence and in the presence of free fatty acids. In some cases, the resistance of the catalysts to the presence of water and the eventual deactivating effects after the first use have also been tested. The catalysts considered are both basic and acidic in nature, such as hydrotalcite, MgO, TiO2 grafted on silica, vanadyl phosphate, and different metals-substituted vanadyl phosphate of the type Me(H2O)xVO1-xPO4‚2H2O, where Me is a trivalent cation such as Al, Ga, Fe, and Cr and where x ) 0.18-0.20. Finally, the understanding of the kinetic behavior of the most stable catalyst TiO2/SiO2 has been deepened. Introduction Biodiesel is a renewable diesel fuel, normally obtained by transesterification of highly refined vegetable oils using methanol and an alkaline catalyst (NaOH, KOH, or related alkoxides).1 Current biodiesel production is ∼2 Mt in Europe and ∼100 Kt in the United States, and the expected trend is faster growth.2 The increase in the production levels is due not only to the environmental and technological advantages of using biodiesel, i.e., reduction of overall CO2 emissions and increase of the lubricating properties of modern ultralow-sulfur diesel by the addition of low amounts of biodiesel (1-2%),3 but also to the political intervention with environmental laws that provide incentives for the use of biodiesel through tax reductions. These incentives are necessary because of the higher cost of biodiesel with respect to diesel from petroleum. As a consequence, many research groups are making great efforts to reduce the cost of biodiesel by improving the production processes. The main factor determining the cost of biodiesel is the price of the refined oil feedstock, which accounts for 88% of the total estimated production cost.2 Therefore, a great economic advantage could be achieved simply by using more economical feedstock such as waste fats and oils.1,4 The currently used technology based on the use of homogeneous alkaline catalysts for promoting the transesterification reaction requires refined oils containing not more than 0.5% of free fatty acids (FFAs) and anhydrous * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +39 081674027. Fax: +39 081674026. † CNR-IMIP Area della Ricerca di Roma, via Salaria, 00016 Monterotondo Scalo, Roma, Italia.

conditions, because water favors the formation of FFAs by hydrolysis of triglycerides. FFA gives, with the alkaline catalysts, soaps that have a markedly reduced catalytic activity and allows production of emulsions between the obtained biodiesel and the byproduct glycerol, requiring a very a long settling time for the separation. Moreover, alkaline catalysts need to be neutralized with mineral acids, and this allows production of a dirty glycerol requiring an expensive procedure of purification. For all the mentioned reasons, more economical waste oils and fats cannot be directly processed with the most commonly practiced technology for biodiesel production, and methanol or ethanol containing moisture cannot be used, too.5 Preliminary purification and pretreatment stages are necessary.1,4,6 A useful alternative could be the use of catalysts that do not have the drawbacks typical of the homogeneous alkaline catalysts, as Di Serio et al.7 have recently shown by using homogeneous Lewis acid catalysts. In that paper, it was observed that the best catalysts were metal salts showing a moderate Lewis acid character, neither too strong nor too weak. This seems to also be a good suggestion for developing new heterogeneous catalysts. The cost of biodiesel could certainly be lowered by using a heterogeneous catalyst instead of a homogeneous one, resulting in a higher quality of esters and glycerol, which can be more easily and promptly separated. Glycerol does not need, in this case, expensive refining operations.3 Toward this purpose, recently announced was the construction in France of a new 160 000 t/y biodiesel plant based on the use of a heterogeneous catalyst developed by the French Institute of Petroleum (IFP).8 The catalyst proposed by the IFP is a Lewis acid catalyst based on a zinc compound (zinc aluminate).3 Many other heteroge-

10.1021/ie070663q CCC: $37.00 © 2007 American Chemical Society Published on Web 08/25/2007

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Table 1. Specific Surface Areas Determined by Using the BET Method catalyst (acronym) hydrotalcite (CHT) MgO TiO2/SiO2 (TS) VOPO4‚2 H2O (VOP) Al(H2O)]0.18VO0.82PO4‚2H2O (AlVOP) Ga(H2O)]0.18VO0.82PO4‚2H2O (GAVOP) Fe(H2O)]0.2VO0.8 PO4‚2H2O (FeVOP) Cr(H2O)]0.2VO0.8 PO4‚2H2O (CrVOP)

specific surface area (m2/g)

main property Basic Catalysts ratio Al/Mg ) 4.4 commercial (Merck Co)

144 36

Acid Catalysts TiO2 7.47% b.w coating silica support

neous catalysts, characterized by both acid and basic catalytic sites, have recently been proposed in the literature, but only a little information has been provided about the influence of the amount of FFA on the reaction.5,8-17 On the other hand, it is evident from all the published papers that to pass from the current technology based on the use of homogeneous alkaline catalysis to a new approach based on the use of heterogeneous catalysts is a fundamental goal for lower-cost biodiesel production. In the present paper, an in-depth study of the catalytic performances of different heterogeneous catalysts will be reported in order to evaluate the possibility of using them in the production of biodiesel, starting in particular from unrefined feedstocks characterized by the presence of high FFA and/or water concentrations. The investigated heterogeneous catalysts are both basic, such as hydrotalcites (CHT) and MgO,14,18 and acidic, such as titanium oxide supported on silica, TiO2/SiO2 (TS);13,19 vanadyl phosphate VOPO4‚2H2O (VOP);15,20 and metal-substituted vanadyl phosphate Me(H2O)xVO(1-x)PO4‚2H2O (MeVOPO, where Me can be a trivalent cation such as Al, Ga, Fe, and Cr and x ) 0.18-0.20.20 In some cases, a study on the stability of the tested catalysts in the reaction conditions will be reported. Although TS catalyst is not the most active catalyst, it has shown the greatest stability. For this reason, its kinetic behavior has been studied in greater detail. Experimental Section Materials. The soybean oil used was purchased in a local food store. The fatty acid composition of this oil was determined by gas chromatographic analysis and resulted in the following (w/w %): palmitic ) 11; stearic ) 4; oleic ) 23; linoleic ) 56; linolenic ) 5; and others ) 1. All other employed reagents (when not specified) were supplied by Aldrich and used without further purification treatment. Regarding the methods of catalysts preparation, we distinguish, first of all, between basic and acidic catalysts. Basic catalysts tested in this work were hydrotalcite (CHT) and MgO. The preparation method of hydrotalcite is the one reported in a previous work and related references.14 A commercial MgO catalyst, supplied by Merck, has been used in the tests. Let us consider now the method of preparation of the acidic catalysts. TiO2/SiO2 (TS) catalyst has been prepared by contacting the silica (Grace S432, specific surface area ) 320 m2/g), calcined at 500 °C for 8 h, with a solution of titanium tetraisopropoxide (Fluka) dissolved in dioxane at room temperature, as described elsewhere.21 Then, the obtained solid was filtered, washed with dioxane solvent, dried at 120 °C overnight, and calcined at 200 °C for 2 h and then at 500 °C for 2 h. As resulted by UV chemical analysis of titanium charged on the silica surface, the catalyst contained 7.47% by weight of TiO2, which corresponds to the monolayer surface coverage of silica by the grafted titanium alkoxide.21 VOPO4‚2 H2O (VOP) was prepared,

280 2.0 4.0 2.9 16.3 18.2

according to the literature, by refluxing a suspension of V2O5 in diluted phosphoric acid for 16 h and then calcined at 500 °C for 2 h (ref 15 and literature cited therein). Metal-substituted vanadyl phosphate Me(H2O)]xVO1-xO4‚2H2O (MeVOP) were prepared by adding to a refluxing suspension of V2O5 in diluted phosphoric acid the appropriate amount of the corresponding metal salt (ref 20 and literature cited therein). Textural analyses of the catalysts were carried out by using a Thermoquest Sorptomatic 1990 instrument (Fisons Instruments) and by determining the nitrogen adsorption/desorption isotherms at 77 K. The samples were thermally pretreated under vacuum overnight up to 473 K (heating rate ) 1 K/min). Specific surface areas were determined by using the BrunauerEmmett-Teller (BET) method. The obtained specific surface areas of the different tested catalysts are reported in Table 1. Catalytic Tests. The screening of the catalysts have been performed by using a series of 5-6 small stainless steel vial reactors. Both the reagents (oil (FFA ) 0.2% w/w) ) 2.0 g and methanol ) 0.88 g) and a defined amount of the catalyst (0.1 g) were introduced in each reactor. All the reactors were then heated in a ventilated oven. The temperature of the oven was initially fixed at 50 °C for 14 min and then increased at a rate of 20 °C/min until it reached the reaction temperature, at which temperature the samples have been kept for ∼1 h. At the end of the reaction, the samples were quenched by putting the vials in a cold bath. Experimental runs were also performed by adding water and/or oleic acid to the reactants. Oleic acid has been chosen as the test molecule for simulating the behavior of FFA. The FAME (fatty acid methyl esters) yields, in the catalytic tests, were determined by using the H NMR technique (Bruker 200 MHz),22 i.e., measuring the area of the H NMR signal related to methoxylic (A1) and methylenic groups (A2), respectively. In the case of neutral oil, the FAME yields can be calculated by using the following equation:

YFAME )

A1/3 A2/2

(1)

In the case of the TS catalyst, runs were also performed in a 1 dm3 autoclave reactor at 220 °C. These runs were performed by introducing the reactants and catalyst into the autoclave and heating until the fixed reaction temperature was achieved. Samples of the reaction mixture were withdrawn at different times and analyzed by both H NMR spectroscopy and titration. Results and Discussion A catalytic screening of all the catalysts mentioned above in the transesterification of a “neutral” soybean oil (FFA concentration ) 0.2% w/w), at 180 °C, has been performed. The obtained results are reported in Table 2. Since at high temperatures the stainless steel internal surface of the vials can catalyze

Ind. Eng. Chem. Res., Vol. 46, No. 20, 2007 6381 Table 2. Performances of Different Tested Catalysts catalyst

FAME yields, % Basic Catalysts

CHT MgO

92 75 Acid Catalysts

TS VOP AlVOP GaVOP FeVOP CrVOP no catalyst

62.0 78.0 79.0 81.5 69.4 72.2 8-11

Table 3. Performances of Different Catalysts Used for Treating a Neutral Soybean Oil Containing About 10% of Oleic Acid in Both Transesterification and Esterification Reactions catalyst

FAME yields

FFA conversion

CHT MgO

Basic Catalysts 80.3 76.0

76.2 62.0

TS VOP AlVOP GaVOP FeVOP CrVOP no catalyst

Acid Catalysts 40.0 71.3 57.5 57.1 61.9 61.9 25.0

77.0 74.6 74.2 64.7 66.2 67.4 35.0

the transesterification reaction,23 different runs without catalyst were performed, obtaining yields in the range 8-11%. As can be seen, all the tested catalysts are more or less active in promoting the transesterification reaction. The hydrotalcite gives the highest yields, while the TS catalyst gives the lowest one. The VOP catalyst series gives, as MgO, yields in the range of 70-80%. The performances of VOP catalysts cannot be strictly related to the specific surface area, because the activity depends on the concentration of the acid sites with a definite strength on the catalyst surface and this concentration can change widely for the different examined catalysts. As a matter of fact, very strong acid sites are not active in the transesterification reaction,7,13 and the FeVOP catalyst has, for example, a concentration of strong acid sites higher than those for the AlVOP and GaVOP catalysts.24 In Table 3, the results obtained for the runs performed with the same catalysts in the presence of ∼10% by weight (b.w.) of oleic acid are reported. It can be pointed out, in this case, that the transesterification reaction, in the absence of catalyst, is accelerated by the presence of FFA. Moreover, apparently almost all the catalysts are poorly affected by the presence of oleic acid, and all resulted active in promoting esterification. However, it must be pointed out that basic catalysts, in particular MgO, despite their high performances, are subjected to a leaching effect in the presence of FFA, giving place to Mgsoaps and to a final opalescent product. Basic catalysts have also been tested in the presence of water, giving again high FAME yields but promoting, in the meantime, the hydrolysis of triglycerides to FFA. All these aspects show that basic heterogeneous catalysts such as MgO and CHT are very promising substitutes of the alkaline homogeneous catalysts for treating neutral refined oils. The acid catalysts do not show leaching effects, and as will be seen, the presence of water can give a moderate deactivation, normally because of the interaction of this molecule with the Lewis acid sites on the surface.

Figure 1. Percentage of FAME yields obtained in several cases, such as (a) without catalyst; (b) MgO; (c) TS; (d) VOP; and (e) AlVOP. For cases b-e, catalytic results are referred to both fresh and used catalysts in the reaction with neutral soybean oil (“neutral” oil ) 2.0 g, methanol ) 0.88 g, catalyst ) 0.1 g, T ) 180 °C).

Figure 2. Influence of water on the transesterification performances of TS and AlVOP catalysts (oil ) 2.0 g, methanol ) 0.88 g, catalyst ) 0.1 g, T ) 180 °C).

At the end of the reaction test, the catalysts were separated from reagents and products by centrifugation. Some of the most representative catalysts have then been submitted to reuse, in the same reaction conditions of the first runs, in order to evaluate an eventual deactivation effect. The obtained results are reported in Figure 1. From these results, it is interesting to note that VOP and AlVOP catalysts are subjected to a strong deactivation effect when reused. As shown in a recent work by Di Serio et al.,15 the deactivation of VOP-type catalysts is due to their reduction with methanol. At the investigated temperature (180 °C), a slow process of reduction of the vanadium species (V5+ f V3+) on the catalyst surface occurs during the run, while methanol is oxidized to formaldehyde. The activity of the catalyst can be restored easily and completely by calcination.15 This means that the use of VOP and MeVOP catalysts in biodiesel production is subordinated to the catalyst regeneration. This seems to be the only drawback of these very promising catalysts that, despite their very low specific surface areas, show high activities, that is, very high turnover numbers. Besides, it is interesting to observe that AlVOP catalyst is poorly affected by the presence of water, as can be appreciated in Figure 2, where FAME yields are reported as a function of the water initially added to methanol. In Figure 3a, the behaviors of different selected catalysts tested with oil containing about 10% b.w. of oleic acid after reuse are reported. From this figure, it results that MgO and TS are poorly affected by the presence of FFA in the catalyst

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Figure 5. Arrhenius type plot for a pseudo-first-order kinetic constant derived from the data of Figure 4 (oil ) 2.0 g, methanol ) 0.88 g, catalyst (TS) ) 0.1 g).

Figure 3. (a) FAME yields (%) and (b) residual FFA concentrations obtained both without catalyst and in the presence of fresh and used catalysts. Runs performed in the presence of oleic acid dissolved in the neutral soybean oil (FFA ) 10% w/w, oil ) 2.0 g, methanol ) 0.88 g, catalyst ) 0.1 g, T ) 180 °C).

presence of different amounts of water were performed. The obtained results are reported in Figure 2. From this figure, it appears evident that water has a strong deactivating effect on TS catalyst. These results are in agreement with data observed for homogeneous Lewis acid catalysts by Di Serio et al.7 and with the data reported by Hillion and Le Pennec.25 related to a heterogeneous Lewis acid catalyst (zinc aluminate). Evidently, water negatively interacts with Lewis acid sites. Some runs performed by using refined oil, at different temperatures, were then performed in order to estimate the activation energy of the reaction (see Figure 4). Because the reaction was performed in the presence of a high excess of methanol, a pseudo-first-order kinetic law can be considered as follows,

r ) kCG

(2)

where CG is the concentration of glyceride groups. Relation 2 can also be written by introducing the glyceride groups conversion (XG),

r ) kC 0GXG

(3)

where C0G is the initial concentration of glyceride groups. As for nonacid oil, the yield (Y) corresponds to the glyceride groups conversion; relation 3 can also be written as

r ) kC 0GY Figure 4. Influence of the temperature reaction on the TS catalyst activity (FAME yield) (oil ) 2.0 g, methanol ) 0.88 g, catalyst (TS) ) 0.1 g).

reuse, while VOP and AlVOP confirmed the already observed deactivation phenomenon due to the vanadium reduction on the surface. In Figure 3b, the residual FFA concentrations for the same catalysts of Figure 3a are reported. From the observation of this figure, we can conclude that TS is the most stable catalyst also for the esterification when the catalyst is reused. We can conclude from these preliminary data that TS catalyst appears suitable for promoting the transesterification reaction of oils containing high FFA concentrations, and on the basis of these promising results, we decided to deepen the investigation about other catalytic performances of the TS catalyst. The deactivation shown by the TS catalyst, in the presence of FFA, is probably due to the formation of water during the esterification reaction. In order to investigate this aspect, runs in the

(4)

The kinetic constant can then be calculated at each temperature by integration of eq 4:

k ) (1/t) ln[1/(1 - Y)]

(5)

In this evaluation, the short transient evolution of the temperature has been neglected. The obtained kinetic constants have been arranged in the Arrhenius plot of Figure 5. Figure 4 shows the necessity, for TS catalyst, to operate at high temperatures in order to obtain reaction rates suitable for an industrial application. This aspect is also confirmed by the data reported in Figure 5, from which an activation energy of ∼100 kJ/mol can be roughly estimated. The performances of the TS catalyst have then been confirmed also with a run performed in an autoclave (oil/methanol ) 1/1 and catalyst/oil ) 1/100 by weight; temperature 220 °C). In Figure 6, the obtained results for the transesterification

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Figure 6. Run performed in autoclave on neutral soybean oil by using TS catalyst (oil charged ) 180 g, catalyst TS/oil ) 1/100 w/w, methanol/oil ) 1/1 w/w).

Figure 7. Runs performed in autoclave (methanol/oil ) 1 w/w) without or in presence of catalyst (catalyst TS/oil ) 1/100) with soybean oil and addition of oleic acid (10% b.w.). Temperature heating profile is the same as that of Figure 6.

reaction are reported as FAME yield as a function of time. As can be seen, a FAME yield of ∼90% was obtained in ∼150 min. These results are comparable with those of other catalyst systems reported in the patent literature (see, for example, the patent by Stern et al.26). The effect of a high concentration of FFA was also tested in the autoclave, performing runs both with TS and without catalyst and using acid oil (FFA ≈ 10% w/w) at 220 °C. The obtained results are reported in Figure 7. Figure 7 confirms that TS is also an esterification catalyst, although sensitive to the water formed during the reaction. As a matter of fact, the FFA concentration was lowered from ∼10% to 0.5% in 150-200 min, while for the uncatalysed reaction, the residual acid concentration after the same time was 4-5%. Even if TS is partially deactivated in the presence of FFA, the final yield of the run performed in the presence of FFA reached a satisfactory value of ∼70%. In the view of an industrial application, another stage is necessary for completing the reaction. Conclusions Different heterogeneous catalysts, both basic and acidic, have been tested in the transesterification reaction of neutral soybean oil with methanol with the aim to find alternative routes to biodiesel production. Because the main objective was to open the possibility of producing biodiesel from cheaper raw materials

such as waste oils and fats, runs have also been performed in the presence of FFA, choosing oleic acid as a model molecule dissolved in the neutral soybean oil for simulating the behavior of FFA. All the proven catalysts, MgO, CHT, TS, VOP, and MeVOPs, have shown satisfactory activities in the transesterification reaction. Therefore, these catalysts represent possible alternatives to the currently used homogeneous alkaline catalysts with some significant advantages, such as the simplest realization of continuous reactors, the obtainment of a cleaner glycerol, and the absence of both the alkaline catalyst neutralization step and the necessity to replace the consumed catalyst. The same catalysts have also been proven, as mentioned before, in the treatment of oils containing FFA and/or water. Surprisingly, all the catalysts also promoted the esterification of the oleic acid in the reaction conditions, and almost all have been poorly affected by the presence of oleic acid. MgO and CHT have shown some leaching effect with the formation of MgO soaps. Therefore, despite the high activities shown by these two catalysts, they are less suitable to be used in the presence of FFA. Different acid catalysts have also been submitted to a reuse test. This test evidenced that VOP and MeVOP deactivate during the catalytic test for the reduction of vanadium from V5+ to V3+. The activity can be completely restored by calcinations. It is interesting to observe that AlVOP catalyst is poorly affected by the presence of water. However, we can conclude that the industrial use of vanadium-based catalysts is conditioned by the necessity of a frequent catalyst regeneration. Finally, TS catalyst, although less active than the other ones, is more stable and has also shown good activity in promoting the esterification reaction. It shows partial deactivation only in the presence of water, and the deactivation shown in the transesterification performed in the presence of oleic acid is due to the water formed as a consequence of the esterification reaction. Runs performed in an autoclave have shown that TS could be used in an industrial process using oils with high FFA concentration by adopting two reaction stages. After the first stage, the ester phase is separated by the obtained glycerol, methanol, and produced water. Then this ester phase can be sent to a second reaction stage to complete the transesterification of the residual glycerides. Acknowledgment Thanks are due to ASER s.r.l. and to CN/ASIA-PRO-ECO/ 11 Project 109087 for financial support. Literature Cited (1) Ma, F.; Hanna, M. A. Biodiesel production: A review. Bioresour. Technol. 1999, 70, 1. (2) Haas, M. J.; McAloon, A. J.; Yee, W. C.; Foglia, T. A. A Process Model to Estimate Biodiesel Production Costs. Bioresour. Technol. 2006, 97, 671. (3) Bournay, L.; Casanave, D.; Delfort, B.; Hillion, G.; Chodorge, J. A. New heterogeneous process for biodiesel production: A way to improve the quality and the value of the crude glycerin produced by biodiesel plants. Catal. Today 2005, 106, 190. (4) Ramadhas, A. S.; Jayaraj, S.; Muralledharan, C. Bioresour. Technol. 2002, 85, 253. (5) Eibura, T.; Echizen, T.; Ishikawa, A.; Murai, K.; Baba, T. Selective transestrification of triolein with methanol to methyl oleate and glycerol using alumina loaded with alkali metal salt as solid-base catalysts. Appl. Catal., A 2005, 283, 111. (6) Jeromin, L., Peukert, E., Wollomann, G. U.S. Patent No 4,698,186, 1987. (7) (a) Di Serio, M.; Tesser, R.; Dimiccoli, M.; Cammarota, F.; Nastasi, M.; Santacesaria, E. Synthesis of biodiesel via homogeneous Lewis acid catalyst. J. Mol. Catal., A 2005, 239, 111. (b) Siano, D.; Di Serio, M.;

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Tesser, R.; Dimiccoli, M.; Cammarota, F.; Santacesaria, E.; Siano, L.; Nastasi. PCT Int. Appl. WO 2006006033 A1 20060119, 2006. (8) Ondrey, G. Biodiesel production using a heterogeneous catalysts. Chem. Eng. 2004, 10, 13. (9) Leclercq, E.; Finiels, A.; Moreau, C. Transesterification of rapeseed oil in the presence of basic zeolites and related solid catalysts. J. Am. Oil Chem. Soc. 2001, 78, 1161. (10) Furuta, S.; Matsuhashi, H.; Arata, K. Biodiesel fuel production with solid superacid catalysis in fixed bed reactor under atmospheric pressure. Catal. Commun. 2004, 5, 712. (11) Mazzocchia, C.; Modica, G.; Kaddouri, A.; Nannicini, R. Fatty acid methyl esters synthesis from triglycerides over heterogeneous catalysts in the presence of microwaves. C. R. Chim. 2004, 7, 601. (12) Suppes, G. J.; Dasari, M. A.; Doskocil, E. J.; Mankidy, P. J.; Goff, M. J. T. Transesterification of soybean oil with zeolite and metal catalysts. Appl. Catal., A 2004, 257, 213. (13) (a) Bonelli, B.; Cozzolino, M.; Tesser, R.; Di Serio, M.; Piumetti, M.; Garrone, E.; Santacesaria, E. Study of the surface acidity of TiO2/SiO2 catalysts by means of FTIR measurements of CO and NH3 adsorption. J. Catal. 2007, 246, 293. (b) Cozzolino, M.; Tesser, R.; Di Serio, M.; Ledda, M.; Minutillo, G.; Santacearia, E. Preparation, characterization and catalytic performances of highly dispersed supported TiO2/SiO2 catalysts in biodiesel production. Stud. Surf. Sci. Catal. 2006, 162, 299. (14) Di Serio, M.; Ledda, M.; Cozzolino, M.; Minutillo, G.; Tesser, R.; Santacesaria, E. Transesterification of Soybean Oil to Biodiesel by Using Heterogeneous Basic Catalysts. Ind. Eng. Chem. Res. 2006, 45, 3009. (15) Di Serio, M.; Cozzolino, M.; Tesser, R.; Patrono, P.; Pinzari, F.; Bonelli, B.; Santacesaria, E. Vanadyl phosphate catalysts in biodiesel production. Appl. Catal., A 2007, 320, 1. (16) Xie, W.; Peng, H.; Chen, L. Calcined Mg-Al hydrotalcites as solid base catalysts for methanolysis of soybean oil. J. Mol. Catal., A 2006, 246, 24.

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ReceiVed for reView May 10, 2007 ReVised manuscript receiVed July 19, 2007 Accepted July 21, 2007 IE070663Q