Heterogeneous acid conversion of a tricaprylin-palmitic acid mixture over Al-SBA-15 catalysts: Reaction study for biodiesel synthesis

Abstract In this work mesoporous Al-SBA-15 was synthesized by the sol-gel method and used as a solid acid catalyst for the conversion of a model mixture of tricaprylin with 20 wt% of palmitic acid at T = 175–215 °C. The incorporation of aluminum to the SBA-15 through the sol-gel method, gives rise to mesoporous materials with the typical hexagonal arrangement of SBA-15 (XRD). In addition, these materials possess Lewis and Bronsted acid sites (pyridine adsorption), large surface area and wide pore diameter (N 2 physisorption) being capable of catalyzing simultaneously transesterification of triglycerides (tricaprylin) and esterification of palmitic acid (up to 20 wt%) resulting in FAME yields of 40–87%. A detailed analysis of the reaction products indicates that the conversion of triglyceride to diglyceride is the limiting step during the acid transesterification over Al-SBA-15 catalysts, which results from the difficulty to protonate triglycerides followed by methanol addition. A simplified lumped first order reaction model reproduces well the experimental data for the acid transesterification of tricaprylin in the presence of a high content of free fatty acids (palmitic acid). The calculated activation energy was 64.7 kJ mol −1 which is in the range of 45–120 kJ mol −1 previously reported for acid catalysts.