Synthesis, Characterization of Novel Hexamethyleneiminium-based Ionic Liquids and their Application in Esterification

Three novel Bronsted acidic ionic liquids (ILs), 1-(3-sulfopropyl) hexamethyleneiminium hydrogen sulfate ([HMIPS]HSO4), 1-(3-sulfopropyl) hexamethyleneiminium tosilate ([HMIPS]OTs) and 1-hexamethyleneiminium hydrogen sulfate([HMI]HSO4), were synthesized and characterized by H-1 NMR, FT-IR and DSC-TG. Their catalytic activities for the esterification of citric acid with n-butanol were examined and compared with the imidazolium-based ILs. The results showed that the hexamethyleneiminium-based ILs exhibits excellent catalytic activity. By using [HMIPS]HSO4 as the catalyst, the yield of tributyl citrate reached 97.5%. Furthermore, the catalysts were reused at least 10 times without significant decrease in activity