Calcium Ethoxide as a Solid Base Catalyst for the Transesterification of Soybean Oil to Biodiesel Xuejun Liu, Xianglan Piao, Yujun Wang,* and Shenlin Zhu State Key Laboratory of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China ReceiVed August 29, 2007. ReVised Manuscript ReceiVed NoVember 20, 2007 In this work, calcium ethoxide is proposed as a catalyst for the transesterification of soybean oil to biodiesel with methanol and ethanol. First, calcium ethoxide was synthesized through a calcium reaction with ethanol. Then, its physical and chemical characteristics were determined using instrumental methods such as Brunauer-Emmett-Teller surface area measurements, scanning electron micrographs, and particle size distribution measurements. The effects of the mass ratio of catalyst to oil, the molar ratio of methanol to oil, and the reaction temperature were studied to optimize the reaction conditions. The experimental results showed that the optimum conditions are a 12:1 molar ratio of methanol to oil, the addition of 3% Ca(OCH 2 CH 3 ) 2 catalyst, and a 65 °C reaction temperature. A 95.0% biodiesel yield was obtained within 1.5 h in these conditions, and the activation energy was 54 149 J/mol. It also indicated that the catalysis performance of calcium ethoxide is better than that of CaO. Besides, a 91.8% biodiesel yield was obtained when it catalyzed soybean oil to biodiesel with ethanol. 1. Introduction Fatty acid methyl esters are known as the sources of biodiesel, which is synthesized by the direct transesterification of vegetable oils with a short-chain alcohol in the presence of a catalyst. The transesterification reaction can be carried out using both homogeneous (acid or base) and heterogeneous (acid, base, or enzymatic) catalysts. 1,2 Homogeneous base catalysts provide much faster reaction rates than heterogeneous catalysts, but it is considerably more costly to separate homogeneous catalysts from the reaction mixture. 3,4 Heterogeneous catalysis has many advantages, such as being noncorrosive, being environmentally benign, and presenting fewer disposal problems. These catalysts are also much easier to separate from liquid products, and they can be designed to give a higher activity and selectivity and to have longer catalyst lifetimes. Many types of heterogeneous catalysts, such as alkaline earth metal oxides, anion exchange resins, and various alkali metal compounds supported on alumina or zeolite, can catalyze many types of chemical reactions, such as isomeriza- tion, aldol condensation, Knoevenagel condensation, Michael condensation, oxidation, and transesterification. 5–8 In transes- terification of vegetable oils to biodiesel, most supported alkali catalysts and anion exchange resins exhibit a short catalyst lifetime because the active ingredients are easily corroded by methanol. 9,10 Some researchers found that alkaline-earth oxide compounds, such as CaO and SrO, have a slight solubility in methanol and have good catalytic activity and a long catalyst lifetime. 11,12 Gryglewicz studied the alkaline-earth metal alkoxides as catalysts for alcoholysis reactions in terms of the synthesis of di(2-ethylhexyl) adipate and an oligomeric ester of neopentyl glycol and found that magnesium methoxide and calcium alkoxides appear to be active catalysts for the transesterifica- tion. 13 Gryglewicz 12 and Liu et al. 14 studied calcium methoxide as a solid base catalyst to catalyze the transesterification of soybean oil to biodiesel and found that it has excellent catalytic activity and a long catalyst lifetime. 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