Enhancing Brønsted acid site activity of ion exchanged montmorillonite by microwave irradiation for ester synthesis

Esterification reactions over Al ^{3 +} montmorillonite as solid acid catalyst under microwave irradiation are reported. The activity of Al ³⁺ montmorillonite catalyst for esterification of butyl alcohol with different alkanoic acids like ethanoic acid, butanoic acid, hexanoic acid, octanoic acid and decanoic acid has been studied both under microwave and conventional heating in the presence and absence of solvent. The highest conversion of 80% butyl acetate was obtained for the molar ratio1:2 (acid:alcohol), 0.5 g of catalyst at 413 K in 30 min under microwave irradiation conditions . In order to understand the type of effect responsible for lower yield of longer chain alkanoic acids, esterification of benzoic acid with p-cresol was carried out over metal ion exchanged montmorillonite catalysts under solvent less conditions using microwave irradiation. These results have been compared with different metal ion exchanged montmorillonite and ZSM-5 zeolite catalyzed esterification. Among the catalysts studied, Al ³⁺ exchanged montmorillonite showed higher activity than the solid acids like ZSM-5 and other metal ion exchanged montmorillonite. This observation confirmed the absence of steric constraint aspect on modified montmorillonite during esterification. The influence of molar ratio of reactants, time, temperature and catalyst amount on the esterification was investigated under microwave irradiation. Microwave assisted synthesis has the potential to develop into a cost efficient route for ester synthesis.