Esterification of fatty acids by new ionic liquids as acid catalysts

•Three ionic liquids as acid catalyst were synthesized and characterized.•The methyl oleate was achieved with 95% yield at 6 h and 323 K.•The lengthening alkyl chain of free fatty acid and alcohol decrease yield of product.

Three new highly acidic ionic liquids based on 4-sulfobenzyl imidazolium hydrogensulfate and with different length of the alkyl chain were screened as catalysts for the esterification of oleic acid with methanol. The ionic liquids were characterized by FT-IR, 1H NMR, 13C NMR and CHNS analyses. The reaction parameters, such as reaction temperature, molar ratio, catalyst loading, reaction time and stability were studied. The experimental results indicated that acidic ionic liquid with the longest alkyl chain performed the best activity, due to the less strict hindrance. Finally, the methyl oleate was produced with 95.0% yield at lower time (6 h) under the optimized reaction conditions. Furthermore, different alcohols and fatty acids were investigated and the results showed that the length of alkyl chain and degree of saturation of fatty acid affects the catalytic activity of the best catalyst.