The Esterification of Oleic Acid Using Acidic Ionic Liquid Catalysts Immobilized on Silica Gel

Esterification of free fatty acid with methanol to biodiesel was investigated in a batch reactor using various solid acid catalysts, such as polymer cation-exchanged resins with sulfuric acid functional group(Amberlyst-15, Dowex 50Wx8), acidic ionic liquids (ILs)-modified silica gels respectively with -SO3 H and -SO2 Cl functional group (SiO2 -[ASBI][HSO4], SiO2 -[ASCBI][HSO 4]) and grafted silica gels respectively with -SO3 H and -SO2 Cl functional group (SiO2 -R-SO3 H, SiO2 -R-SO2 Cl). The effects of reaction time, temperature, reactant concentration(molar ratio of methanol to oleic acid), and catalyst amount were studied. Allylimidazolium-based ILs on modified silica gels were superior to other tested solid acid catalysts. Especially, the performance of SiO2-[ASBI][HSO4] (immobilized by grafting of 3-allyl-1-(4sulfobutyl) imidazolium hydrogen sulfate on silica gel) was better than that of a widely known Amberlyst-15 catalyst at the same reaction conditions. A high conversion yield of 96% was achieved in the esterification reaction of the simulated cooking oil at 353 K for 2 h. The high catalytic activity of SiO2 -[ASBI][HSO4] was attributed to the presence of strong Bronsted acid sites from the immobilized functional groups. The catalyst was recovered and the biodiesel product was separated by simple processes such as decantation and filtration.