Synthesis of pyrrolidonium acidic ionic liquids and their catalytic activity for esterification of acetic acid and butanol

A series of Bronsted acidic ionic liquids, including 2-pyrrolidonium hydrogen sulfate ([Hnhp]HSO4), 1-methyl-2-pyrrolidonium hydrogen sulfate ([Hnmp]HSO4), 1-methylimidazolium hydrogen sulfate ([Hmim]HSO4), 1-(3-sulfonic acid) propyl-3-methylimidazolium hydrogen sulfate ([C(3)SO(3)Hmim]HSO4), and 1-(3-sulfonic acid) propyl-2-pyrrolidonium hydrogen sulfate ([C(3)SO(3)Hnhp]HSO4), were synthesized and characterized. The catalytic activity of the acidic ionic liquids for the esterification of acetic acid with n-butanol was measured. Under the optimized conditions of n(n-BuOH) : n(MeCO2H) : n([C(3)SO(3)Hnhp]HSO4) = 1.2 : 1 : 0.005, 120 degrees C, and 1 h, the yield of n-butyl acetate could be greater than 99%. The reaction system exhibited good biphasic behavior when the reaction was completed. There was no appreciable decrease in the yield of n-butyl acetate after the esterification was repeated 8 times. The miscibility of the five acidic ionic liquids with the related components of the reaction was investigated, and their biphasic effects on ester yields were examined. The structure-reactivity and Bronsted acidity-catalytic activity relationships were also studied. The acidity and miscibility of the acidic ionic liquids are the important factors relevant to the catalytic activity for the esterification reaction. The activity of the acidic ionic liquids is in good agreement with their acidity order.