Fischer Esterification Promoted by Recyclable Brönsted Acidic Ionic Liquids

A series of Brönsted acidic ionic liquids based on 2-pyrrolidonium, 1-methylimidazolium, morpholinium and 4-methylmorpholinium cations ([Hnhp]+, [Hmim]+, [Hnhm]+ and [Hnmm]+) with anions HSO4ˉ and BF4ˉ, were synthesized. Their catalytic activities in the esterification of n-butyric acid with ethanol were studied and the relationships between catalysis performance and acidity were discussed. The optimum conditions of the esterification were determined as follows: 80 °C oil bath, n (n-butyric acid): n (ethanol): n ([Hnhp]HSO4) = 1:1.2:0.4 and t = 2 h, the yield of ethyl butyrate could reach 86%. There is no appreciable decrease of catalyst activity of [Hnhp]HSO4 after 5 cyclic repetitions.