Sulfonic acid functionalized mesoporous SBA-15 catalysts for biodiesel production

Sulfonic acid functionalized mesoporous SBA-15 catalysts have been studied in the microwave-assisted transesterification of soybean oil with 1-butanol in order to produce low freezing point biodiesel. Small-angle powder XRD and nitrogen adsorption analysis revealed the formation of mesoporous materials with high surface area and uniform porosity. Catalytic activity was found to be largely dependent on the acid strength rather than the number of acid sites. Propyl-SO3H and arene-SO3H functionalized SBA-15 catalysts showed high activity and stability in transesterification reactions whereas perfluoro-SO3H functionalized SBA-15 catalyst exhibited a complete loss of activity after recycling due to complete leaching of perfluoro-SO3H groups in the reaction medium. The results over arene-SO3H functionalized catalysts showed that the optimal loading of arene-SO3H groups is approximately 15% (molar percent of functional group to total silicon) and the optimal oil to alcohol molar ratio is 1:6. Increases in catalyst to oil weight ratio and reaction temperature significantly enhance the reaction rates. Simultaneous transesterification and esterification of soybean oil with up to 20 wt% oleic acid over arene-SO3H functionalized catalyst indicated that this catalyst could be a promising candidate for processing low quality feedstocks containing high fraction of free fatty acids. Further modification of Ar-SO3H SBA-15 catalyst with hydrophobic groups did not significantly improve its catalytic performance.