Novel Brønsted-acidic ionic liquids for esterifications

Several water-stable Brønsted-acidic task-specific ionic liquids (TSILs) with an alkane sulfonic acid group in a pyridinium cation were synthesized. These TSILs show good catalytic activity to esterification reactions of benzoic acid with methanol, ethanol, and butanol. The catalytic activity of each TSIL is dependent on its anion. The experimental results indicate that the increasing of the anion's Brønsted acidity improves the catalytic activity of the TSIL. Furthermore, the high degree of partial immisciblity of TSILs with the produced esters facilitates the esterification reaction equilibrium, shifting it to the product side. The TSIL with [HSO ₄]^- as the anion shows the best catalytic activity and biphasic behavior between the TSIL and the produced esters. The produced esters could be separated by decantation, and the TSILs could be reused after the removal of water.