Propylsulfonic acid functionalized mesoporous silica catalysts for esterification of fatty acids

Abstract

The catalytic properties of 3 types of mesoporous silica SBA-15 (rope, rod and fiber), with 9.2nm or 12.1nm large mesopores, were examined with respect to their morphology and pore size. Commercially available Amberlyst-15 and the small pore sized MCM-41 were used for comparison. The catalysts were prepared by functionalization of the silica supports with propylsulfonic acid (Pr-SO3H) using post-synthesis grafting with 3-mercaptopropyltrimethoxysilane as a propyl-thiol precursor. All materials remained in a well-ordered hexagonal mesoporous structure after Pr-SO3H functionalization. The performance of the Pr-SO3H-functionalized mesoporous silicas was evaluated in terms of their catalytic activity in the esterification of oleic acid with short (methanol) and long (glycerol) chain alcohols, i.e., to test the effect of the pore size on the substrate conversion and product yield. The synthesized catalysts were highly active and the product composition could be tuned by selective choice of the mesopore size. The Pr-SO3H-functionalized rope-shaped SBA-15 gave the highest catalytic activity (in terms of the highest methyl oleate and triglyceride yields and oleic acid conversion level), which was higher than that obtained with the commercial Amberlyst-15 catalyst. A high acid amount, large specific surface area and a suitable pore size are the likely reasons for the high yield gained by Pr-SO3H-functionalized rope-shaped SBA-15 silica.