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Transesterification of Vegetable Oil to Biodiesel over MgO-Functionalized Mesoporous Catalysts† Eugena Li and Victor Rudolph* Chemical Engineering Department, UniVersity of Queensland, St. Lucia, Queensland 4072, Australia ReceiVed May 28, 2007. ReVised Manuscript ReceiVed August 31, 2007
This study aims to report the preliminary results of the transesterification of blended vegetable oil with ethanol to produce biodiesel using different mesoporous silica loaded with MgO as solid base catalysts. Variables that might affect the catalytic activities, such as the type of silica supports (MCM-41, KIT-6, and SBA-15), precursor salts (magnesium acetate and nitrate), and loading methods (impregnation and in situ coating), were investigated. Among all of the catalysts tested in this study, MgO-impregnated SBA-15 exhibits the highest activity for the production of biodiesel, achieving a conversion as high as 96% within 5 h. While the effect of the type of host material used is inconclusive in this study, the catalyst activity appears to be related to its surface Mg concentration, which varies with the loading method when the same host material is used. The effect of the type of precursor salt used for catalyst functionalization, however, was found to be unimportant.
1. Introduction Depletion of the world petroleum reserves and increasing environmental concerns have stimulated the search for renewable fuels, such as biodiesel, in recent years. Because the productiondemand gap of fossil fuel is fluctuating worldwide, the risk in energy supply security increases, the price of conventional fossil fuel continues to rise, and the economies of importing nations suffer significant disruption. From an environmental perspective, combustion of petroleum fuels is a main contributor of an increasing global CO2 atmospheric concentration, a driver of global warming. These concerns have driven significant investment into alternative energy sources for internal combustion engines. Biodiesel, derived from renewable biological resources, such as vegetable oils and animal fats, presents an opportunity for use as a substitute for petroleum-based diesel because there is an economic window, it is renewable, biodegradable, and nontoxic, with an environmentally friendly emission profile. At present, the most common process for converting vegetable oils or animal fats into biodiesel fuel is transesterification, most often using a catalyst intermediary. Homogeneous catalysts include acids, such as sulfuric acid, hydrochloric acid, or sulfonic acid, and bases, such as potassium or sodium hydroxides or methoxides.1 Base-catalyzed transesterification has been most frequently used industrially, mainly because of its fast reaction rate and benign operating conditions. In contrast, acid-catalyzed transesterification has received less attention because it has a relatively slow reaction rate and requires more expensive materials for construction. Nevertheless, acid catalysts have the advantage of being insensitive to free fatty acids (FFAs), often present in feedstock oil, compared to the base-catalyzed system. More recently, there has been an increased research activity directed at the development of heterogeneous catalyst systems † Presented at the International Conference on Bioenergy Outlook 2007, Singapore, April 26–27, 2007. * To whom correspondence should be addressed. Telephone: +61-733654171. Fax: +61-7-33654199. E-mail:
[email protected]. (1) Vicente, G.; Martinez, M.; Aracil, J. Bioresour. Technol. 2004, 3, 297–305.
to produce biodiesel. Their benefits include simplification of the separation and purification of the reaction products, easy reuse of the catalyst in the reactor, reduced corrosion problems, and possibly low sensitivity to FFAs and water. Heterogeneous catalysts that are reported in the literature to be active in biodiesel production through transesterification include enzymes,2–5 calcium carbonate,6 alkali-earth-metal compounds,7 sulphated zirconia,8 tin compounds supported in ion-exchange resins,9 alkylguanidines heterogenized on organic polymers,10 as well as zeolite and alumina loaded with alkali-metal salt.11–16 Recent studies have also found functionalized mesoporous silica, such as tin-oxide-modified mesoporous SBA-15,17 titanium-grafted mesoporous silica,18 and Mg-MCM-41,19 to be effective in (2) Shimada, Y.; Watanabe, Y.; Sugihara, A.; Tominaga, Y. J. Mol. Catal. B: Enzym. 2002, 3–5, 133–142. (3) Dossat, V.; Combes, D.; Marty, A. Enzyme Microb. Technol. 2002, 1, 90–94. (4) Nelson, L. A.; Foglia, T. A.; Marmer, W. N. J. Am. Oil Chem. Soc. 1996, 9, 1191–1195. (5) Watanabe, Y.; Shimada, Y.; Sugihara, A.; Noda, H.; Fukuda, H.; Tominaga, Y. J. Am. Oil Chem. Soc. 2000, 4, 355–360. (6) Suppes, G. J.; Bockwinkel, K.; Lucas, S.; Botts, J. B.; Mason, M. H.; Heppert, J. A. J. Am. Oil Chem. Soc. 2001, 2, 139–145. (7) Gryglewicz, S. Bioresour. Technol. 1999, 249–253. (8) Jitputti, J.; Kitiyanan, B.; Rangsunvigit, P.; Bunyakiat, K.; Attanatho, L.; Jenvanitpanjakul, P. Chem. Eng. J. 2006, 1, 61–66. (9) Abreu, F. R.; Alves, M. B.; Macedo, C. C. S.; Zara, L. F.; Suarez, P. A. Z. J. Mol. Catal. A: Chem. 2005, 1–2, 263–267. (10) Schuchardt, U.; Vargas, R. M.; Gelbard, G. J. Mol. Catal. A: Chem. 1996, 1, 37–44. (11) Xie, W.; Huang, X.; Li, H. Bioresour. Technol. 2007, 4, 936–939. (12) Kim, H. J.; Kang, B. S.; Kim, M. J.; Park, Y. M.; Kim, D. K.; Lee, J. S.; Lee, K. Y. Catal. Today 2004, 315–320. (13) Ebiura, T.; Echizen, T.; Ishikawa, A.; Murai, K.; Baba, T. Appl. Catal., A 2005, 1–2, 111–116. (14) Clacens, J.-M.; Genuit, D.; Veldurthy, B.; Bergeret, G.; Delmotte, L.; Garcia-Ruiz, A.; Figueras, F. Appl. Catal., B 2004, 2, 95–100. (15) Xie, W.; Peng, H.; Chen, L. Appl. Catal., A 2006, 1, 67–74. (16) Suppes, G. J.; Dasari, M. A.; Doskocil, E. J.; Mankidy, P. J.; Goff, M. J. Appl. Catal., A 2004, 2, 213–223. (17) Shah, P.; Ramaswamy, A. V.; Lazar, K.; Ramaswamy, V. Appl. Catal., A 2004, 1–2, 239–248. (18) Gaudino, M. C.; Valentin, R.; Brunel, D.; Fajula, F.; Quignard, F.; Riondel, A. Appl. Catal., A 2005, 2, 157–164. (19) Barrault, J.; Bancquart, S.; Pouilloux, Y. C. R. Acad. Sci., Ser. IIc: Chim. 2004, 6–7, 593–599.
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catalyzing various transesterification reactions. However, there was no report of base-functionalized mesoporous silica being used to catalyze the transesterification of vegetable oil with ethanol for biodiesel production in the literature. In this work, the catalytic activities of various MgOfunctionalized mesoporous materials in the transesterification of vegetable oil were compared. Mesoporous catalysts were chosen because their acidic/basic properties can be easily modified by varying their surface groups and composition, and they have uniform structures and high surface areas, which are ideal as catalysts for large organic molecules and guest–host chemical supports. 2. Experimental Section 2.1. Materials. Commercial-edible-grade blended vegetable oil was purchased from the supermarket. Ethanol of AR 99.5% grade used for the transesterification reactions, HCl (32% Puriss), KCl (AR), methanol (AR), Mg(CH3COO)2 · 4H2O, and Mg(NO3)2 · 6H2O of reagent grade used for catalyst synthesis were purchased from the Chemical Store at the University of Queensland. Tetraethylorthosilicate (TEOS) used as the silica source for synthesizing SBA-15 and KIT-6, amphiphilic triblock copolymer P123 (EO20PO70EO20), butanol of 99.4% purity, N-trimethylsilylN-methyl trifluoroacetamide (MSTFA) of reagent grade, as well as the heptane [g99%, capillary gas chromatography (GC)] used for sample preparation for GC analyses were all purchased from Sigma-Aldrich Pty Ltd. 2.2. Preparation of the Catalysts. Three types of mesoporous silica, MCM-41, SBA-15, and KIT-6, were prepared as the support material for subsequent MgO loading. While the MCM-41 was taken from a batch of the material that was synthesized at the University of Queensland, SBA-15 and KIT-6 were synthesized by adopting literature methods.20,21 They were each impregnated with MgO based on literature methods.19 The in-situ-coated SBA15 catalysts were synthesized separately in a one-pot method according to procedures obtained from the literature.22 2.2.1. Synthesis of SBA-15 Support. SBA-15 was prepared by dissolving 1.6 g of EO20PO70EO20 and 2.2 g of KCl in 60 g of 2.0 M HCl at 38 ( 1 °C. To this solution, 4.2 g of TEOS was added under vigorous stirring. The final molar composition of the reactants was 1:0.02:1.5:6:166 TEOS/P123/KCl/HCl/H2O. After the mixture was stirred for 8 min, it was kept in static conditions at the same temperature for 1 day. Then, the mixture was transferred into an autoclave and heated at 100 °C for another 24 h. The solid products were collected by filtration and dried at room temperature. The resulting powders were calcined at 550 °C for 4 h. 2.2.2. Synthesis of KIT-6 Support. KIT-6 was synthesized by dissolving 6 g of P123 into 217 g of distilled water and 11.8 g of concentrated HCl (35%). To this, 6 g of butanol was added under stirring at 35 °C. After 1 h of stirring, 12.9 g of TEOS was added at 35 °C. The mixture was left to stir for 24 h at 35 °C. The molar ratio of the reactants was 1:0.017:1.31:1.83:195 TEOS/P123/BuOH/ HCl/H2O. The mixture was then heated for 24 h at 100 °C under static conditions in an autoclave. The solid product obtained was then filtered and dried at 100 °C without washing. The template was removed by extraction in an ethanol/HCl mixture, followed by calcination at 550 °C. 2.2.3. Impregnation of MgO onto Silica Support. A mixture containing 1.5 g of the freshly calcined mesoporous silica support, a calculated amount of salt [Mg(CH3COO)2 · 4H2O or Mg(NO3)2 · 6H2O based on a Si/Mg2+ ratio of 1:1], and 25 g of methanol was stirred at room temperature for 2 h. The solvent was then evaporated, and the solid was calcined at 450 °C for (20) Yu, C. Z.; Fan, J.; Tian, B. Z.; Zhao, D. Y.; Stucky, G. D. AdV. Mater. (Weinheim, Ger.) 2002, 23, 1742–1745. (21) Kleitz, F.; Choi, S. H.; Ryoo, R. Chem. Commun. 2003, 2136– 2137. (22) Wei, Y. L.; Wang, Y. M.; Zhu, J. H.; Wu, Z. Y. AdV. Mater. (Weinheim, Ger.) 2003, 22, 1943–1945.
Li et al. 16 h. The impregnated sample is denoted as X/MgO(Y), where X designates the mesoporous material (i.e., M for MCM-41, K for KIT-6, and S for SBA-15) and Y represents the anion of the precursor salt used for impregnation (i.e., A for acetate and N for nitrate). 2.2.4. In Situ Coating of MgO onto SBA-15. A total of 2 g of P123 was dissolved in 75 g of 1.6 M HCl. A calculated amount of inorganic salt [Mg(CH3COO)2 · 4H2O or Mg(NO3)2 · 6H2O based on a Si/Mg2+ ratio of 1:1] was added under stirring for 0.5 h. Then, 4.25 g of TEOS was added while stirring at 40 °C. The molar composition of the mixture was 1:0.02:0.6:6:192 TEOS/P123/MgOx/ HCl/H2O. The solution was stirred for 24 h at 40 °C and then held at 100 °C for another 24 h under static conditions. Finally, the liquid was evaporated under stirring at 80 °C, and the solid obtained was dried at 80 °C and calcined at 550 °C for 6 h to remove the template and form MgO. The in-situ-coated sample is denoted as MgO(Y)–S, where Y represents the anion of the precursor salt (A for acetate and N for nitrate). 2.3. Characterization Methods. The X-ray diffraction analysis (XRD) measurements were performed on a Bruker D8 advance X-ray diffractometer over a 2θ range of 0.50–5.00°, with a step size of 0.02°. The atomic concentrations on the catalyst surface were acquired using a Kratos Axis ULTRA X-ray photoelectron spectrometer incorporating a 165 mm hemispherical electron energy analyzer. The incident radiation was monochromatic Al X-rays (1486.6 eV) at 225 W (15 kV and 15 mA). Survey scans were taken at an analyzer pass energy of 160 eV and carried out over a 1200–0 eV binding energy range, with 1.0 eV steps and a dwell time of 100 ms. The surface area and porosity of the mesoporous silica were determined by N2 adsorption/desorption using Quantachrome NOVA 1200. Before each N2 adsorption/desorption analysis, the catalyst was heated under vacuum at 200 °C overnight in a Quantachrome FloVac degasser to desorb any impurities that might have adsorbed on the solid during exposure to air. Chemisorption of acidic carbon dioxide was performed using an Autosorb-1-C chemisorption–physisorption analyzer (Quantachrome Instruments). Each catalyst was degassed under vacuum at 200 °C overnight in a Quantachrome FloVac degasser before analysis. The solid was then transferred to the sample cell of the analyzer, heated to 200 °C at a rate of 5 °C/min, and held at the same temperature where the CO2 adsorption was performed at two set pressure points, 5 and 760 mmHg. The amount of CO2 uptake per gram of solid at the different manifold pressures provides an indication of the relative basic strength (at low pressure) and number of basic sites (at high pressure) among the catalysts. Although the chemisorption of acidic CO2 does not quantitatively determine the number of basic sites in the catalysts, it provides a relative measure for a basicity comparison among the catalysts. 2.4. Catalytic Test. A total of 50 g of blended vegetable oil, 17 g of ethanol, and 1 g of catalyst were mixed in a Parr 4560 Mini Bench Top Reactor. The mixture was heated to 220 °C under continuous maximum stirring, held for 5 h, and then cooled to room temperature. Approximately 15% volume of distilled water was stirred into the mixture for washing. The washed mixture was then transferred into a decanter to separate the biodiesel from the water, catalyst, and the byproduct glycerol mixture. 2.5. Analytical Methods. A total of 100 ( 0.1 mg of the washed and separated biodiesel sample was weighed into a 10 mL septa vial, derivatized with methyl-N-trimethylsilyl triflouroacetamide (MSTFA), and diluted in heptane as described in American Society for Testing and Materials (ASTM) method D6584.23 The ester content of biodiesel samples was determined by a Varian CP 3900 gas chromatograph equipped with a flame ionization detector (FID), an on-column injector, and a CP7491 column (15 m long, 0.320 mm I.D.). The combined yield of the selected esters, ethyl palmitate, ethyl oleate, and ethyl stearate, which are most abundant in vegetable oils, was calculated in terms of grams of ester produced (23) D6584-00E01. Test Method for Determination of Free and Total Glycerin in B-100 Biodiesel Methyl Esters by Gas Chromatography in Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2006.
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Table 1. Physical Data for All Tested Catalysts (before and after Mg Loading)a catalyst MCM-41 KIT-6 SBA-15 M/MgO(A) K/MgO(A) S/MgO(A) S/MgO(N) MgO(A)–S MgO(N)–S
S (m2/g)
w (Å)
V (cc/g)
Mg (atomic %)
Pure Mesoporous Silica 1172 36.7 1.08 661 60.7 1.00 679 50.3 0.85 Mesoporous Silica Impregnated 391 27.0 112 46.8 252 37.6 315 25.7
with MgO 0.26 0.13 0.24 0.20
SBA-15 with in Situ Coating of MgO 531 69.6 0.92 524 69.4 0.91
10.96 8.44 15.41 15.26 4.58 1.45
a S is the surface area. w is the pore size. V is the pore volume. Mg is the magnesium atomic % on the surface of the catalyst.
per gram of vegetable oil used. These esters are convenient characteristic markers, because they were each individually calibrated against certified standards in the GC. The extent of conversion for each catalyst over the fixed reaction time was then determined, by comparing its combined ester yield with that of a complete reaction catalyzed by homogeneous KOH based on the equation: conversionx ) (BDx/BDKOH) × 100%, where conversionx is the percentage of the triglyceride markers converted to biodiesel (ethyl palmitate, ethyl oleate, and ethyl stearate) in a transesterification catalyzed by catalyst x, BDx is the combined mass of the selected esters produced per gram of oil used in the reaction catalyzed by catalyst x, and BDKOH is that of the KOH-catalyzed reaction. BDKOH was used as a benchmark for the expected yield of a complete reaction using the same type of feedstock, in which 100% conversion has been achieved; i.e., all of the triglycerides have been converted into esters and glycerol, in the 5 h transesterification.
3. Results and Discussion 3.1. Catalyst Characterization. The Brunauer–Emmett– Teller (BET) surface area, the pore size calculated from the adsorption branch by the Barrett–Joyner–Halenda (BJH) model, and the pore volume of the calcined catalysts are presented in Table 1. The atomic concentrations of Mg, calculated using CasaXPS software and a linear baseline, are also included for the functionalized catalysts. The nitrogen adsorption–desorption isotherms obtained for all calcined mesoporous catalysts tested in this study are type IV with H1 hysteresis loops, indicative of large channel-like pores in a narrow range size (data not shown). There is a large difference in physical properties between the impregnated and in-situ-coated SBA-15 catalysts, which is likely to be a result of the different preparation techniques. According to Wang et al.,24 the magnesium species in the in-situ-coating method form a smooth MgOx layer on SBA-15, resulting in less-blocked mesopores than the impregnated catalysts. The X-ray photoelectron spectroscopy (XPS) results also show much lower Mg concentrations on the surface of the in-situ-coated SBA-15 catalysts compared to the impregnated ones. Consequently, the in-situ-coated catalysts possess much higher surface areas and pore volumes than the impregnated ones. However, this also suggests that less Mg would be available for the transesterification reactions catalyzed by the in-situ-coated catalysts. XRD results show that the mesostructure of all catalysts are maintained after MgO loading. For example, the XRD patterns (24) Wang, Y. M.; Wu, Z. Y.; Wei, Y. L.; Zhu, J. H. Microporous Mesoporous Mater. 2005, 1–3, 127–136.
Figure 1. Low-angle XRD patterns of calcined SBA-15, MgOimpregnated SBA-15, and SBA-15 with in situ coating of MgO.
of MgO-functionalized SBA-15 catalysts in Figure 1 all have three well-resolved peaks indexed as (100), (110), and (200) reflections corresponding to p6mm hexagonal symmetry, identical to that of SBA-15. This indicates that the addition of a controlled amount of precursor salts, either by impregnation or the in-situ-coating method, does not obviously change the mesoscopic order of the SBA-15 host. Scanning electron microscopy (SEM) images of SBA-15-supported catalysts in Figure 2 also confirm that the morphology of the composites have been maintained after the addition of MgO. However, the reflection (100) of the impregnated catalysts has a tendency to shift to a higher angle, indicating a lower d100 spacing with the introduction of Mg2+. Furthermore, the in-situ-coated catalysts have higher surface areas, pore sizes, as well as pore volumes compared to the impregnated catalysts (Table 1), suggesting that the in-situ-coating method can maintain the mesostructure of SBA-15 better while introducing the basicity into the mesoporous materials. On the other hand, the reflection (100) of the in-situ-coated catalyst has a lower intensity compared to the original SBA-15 and impregnated SBA-15, which could be due to its less regular morphology as also seen in Figure 2. According to Yu et al.,20 inorganic salts can be used to control the morphology and adjust the wall structure of mesoporous silica materials. The original SBA-15 and impregnated SBA-15 materials synthesized with KCl are highly ordered with high-yield rod-like morphology, whereas the in-situ-coated SBA-15 synthesized without KCl appears to have a slightly less ordered morphology, which is consistent with ref 20. 3.2. Screening of Catalyst. To perform the catalytic screening of different functionalized mesoporous catalysts in the vegetable oil transesterification reaction, their ester conversions, calculated from the combined ester yields of ethyl palmitate, oleate, and stearate, were compared. The exact same reaction conditions as previously described were employed in each experiment for direct comparisons among the catalysts. The ester conversions of the reactions catalyzed by pure silica were compared to those catalyzed by functionalized mesoporous silica. As summarized in Figure 3, the pure mesoporous silica without any loading showed little or no activity, with a
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Figure 2. SEM images of (A) SBA-15, (B) S/MgO(A), (C) MgO(A)–S.
Figure 3. Ester conversions of transesterification reactions.
conversion similar to that of the control experiment, where no catalyst was used. These outcomes were expected because pure silica materials have low intrinsic catalytic activity. However, when MgO was loaded onto the mesoporous materials, either by impregnation or in situ coating, the supported catalysts showed a significant increase in catalytic activities in terms of ester conversions. Among the tested catalysts, S/MgO(A) was found to be the most active under the specified conditions, achieving a 96% ester conversion. 3.3. Basicity Determination of the Catalysts. The activity of a base catalyst may be hypothesized to depend predominantly upon the number and strength of the basic sites. To gain some insight into this effect for the catalysts using different supports, their relative basicity was investigated using a CO2 chemisorption method. The amount of CO2 adsorbed per gram of catalyst at 5 and 760 mmHg provides an indication of the relative basic strength and total basic sites, respectively. Although the CO2 adsorption value does not reflect an actual quantitative basicity of the catalyst, it provides a common comparative measure of basicity among the various catalysts. It is observed in Figure 4 that K/MgO(A) has the strongest basic sites as well as the largest amount of basic sites when compared to M/MgO(A) and S/MgO(A). The correlation between the conversion and basicity of the catalysts is presented in Figure 5. It shows that the activity of a base catalyst does not predominantly depend upon its basic properties. Furthermore, no correlation is found between the activity of the catalysts and any particular one of their properties listed in Table 1. These results suggest that the activity of a base catalyst cannot be attributed to its basicity or any isolated
Figure 4. Basicity of MgO-impregnated catalysts with different silica hosts.
characteristic alone but is likely to be determined by the combined effects of the various attributes of the support material. 3.4. Influence of Catalyst Preparation Methods. Because S/MgO(A) demonstrated the highest catalytic activity, SBA-15 was thus selected to further investigate how the type of precursor salt used for MgO loading and the loading method, i.e., impregnation versus in situ coating, affect their catalytic activities. Conversions of vegetable oil over these catalysts are presented in Table 2. Similar catalytic activities were achieved whether magnesium acetate or magnesium nitrate was used as the precursor salt. On the other hand, the MgO-impregnated SBA-15 demonstrated
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(R2 ) 0.99) is found between the activity of a catalyst and its surface Mg concentration. S/MgO(A) and S/MgO(N) have much higher surface Mg concentrations than MgO(A)–S and MgO(N)–S (Table 1), and consequently, the impregnated catalysts have more MgO available for the catalysis, resulting in higher conversions. Therefore, when the same host material is used, the activity of the catalyst can be attributed to its surface Mg concentration, which is determined by the catalyst-loading method, whereas the effects of basicity, surface area, and porosity appear to be less important in this case. 4. Conclusions
Figure 5. Correlation between the basicity of the catalyst and ester conversion (2, CO2 adsorption at 5 mmHg; 9, CO2 adsorption at 760 mmHg). Table 2. Conversion of Vegetable Oil with MgO-Functionalized SBA-15 Catalysts catalyst
conversion (%)
S/MgO(A) S/MgO(N) MgO(A)–S MgO(N)–S
96 93 78 72
Figure 6. Correlations of the surface Mg concentration and basicity with conversion (9, surface Mg concentration; 2, CO2 adsorption at 5 mmHg; [, CO2 adsorption at 760 mmHg).
relatively higher catalytic activities than the in-situ-coated ones, even though the latter had higher surface areas and pores. Correlations of conversion with the surface Mg concentration and basicity are presented in Figure 6. Again, the activity does not seem to depend upon the basic properties of the catalysts, although a weak correlation between the activity and the number of strong basic sites is visible. In contrast, a strong correlation
This study compared the catalytic activity of MgO-functionalized mesoporous materials in the transesterification of blended vegetable oil with ethanol. When the reaction was carried out at 220 °C for 5 h under continuous stirring in a batch reactor, SBA-15 impregnated with MgO was found to be the most effective among all catalysts tested in this study, reaching a high conversion of 96%. The effect of the type of host material used in this study is inconclusive. A comparison between the different supports used for MgO impregnation shows that the activity of a catalyst does not depend solely upon its basic properties or any isolated determining factors, such as surface area, porosity, and surface Mg concentration. This suggests the activity of a catalyst to be a result of the combined effects of the multiple attributes of its host material. While the type of precursor salt used for MgO loading showed little effect on the extent of conversion, the activities of the SBA-15 catalysts seem to be closely related to their surface Mg concentrations. Loading Mg onto SBA-15 by in situ coating was found to be less effective than impregnation, because the surface Mg concentrations of the in-situ-coated catalysts were an order of magnitude lower than that of the impregnated ones. The surface Mg concentration determines the amount of catalysts available for catalyzing the transesterification reaction, which appears to dominate over other physical properties of the host material. As a result, the in-situ-coated SBA-15 catalysts were less catalytically active than impregnated SBA-15, even though they had larger surface areas, pore volumes, and pore sizes. In conclusion, SBA-15 impregnated with MgO is a very promising catalyst for the transesterification of vegetable oil for biodiesel production and has potential as a heterogeneous catalyst for such reactions. Further studies are required to investigate the kinetics of the reaction and optimize the conditions. Acknowledgment. This work has been funded by the ARC Linkage Grants Program with contributions from industrial sponsor IOR Energy Pty Ltd. The authors thank Dr. Chengzhong Yu from Fudan University for his valuable advice and Dr. Adrienne Chandler-Temple from the Centre for Microscopy and Microanalysis of the University of Queensland for her assistance with catalyst characterization. EF700290U
