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 20wt% 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 Brönsted acid sites (pyridine adsorption), large surface area and wide pore diameter (N2 physisorption) being capable of catalyzing simultaneously transesterification of triglycerides (tricaprylin) and esterification of palmitic acid (up to 20wt%) 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 64.7kJmol−1 which is in the range of 45–120kJmol−1 previously reported for acid catalysts.