Synthesis and catalysis performance in esterification of novel hexamethyleneiminium-based ionic liquid

Two novel Br?nsted acidic ionic liquids,1-(3-sulfopropyl)hexamethyleneiminium hydrogen sulfate(HSO4)and 1-(3-sulfopropyl)hexamethyleneiminium tosilate(OTs)were prepared using hexamethyleneimine and 1,3-propane sultone,etc.They were characterized by 1H NMR,FT-IR,LC-ESI-MS and DSC-TG.The results proved that the ionic liquids synthesized contain a hexamethyleneimine with seven member ring and possess stronger Br?nsted acidity,decomposition temperayure are over 260 ℃ and the acidity of aqueous solution of HSO4 is similar to that of aqueous solution with the same concentration of sulfuric acid.Their catalytic activities for the esterification of citric acid with n-butanol were examined and compared with the imidazolium-based ionic liquids.The results showed that HSO4 exhibits excellent catalytic activity,the conversion of citric acid was 98.3% and the yield of tributyl citrate reached 97.5% under the conditions of n(citric acid)∶n(n-butyl alcohol)∶n(HSO4)=5∶1∶0.02,110—130 ℃ and reflux 2 h.Furthermore,The activity of HSO4 catalyst reused at least 10 times was not significantly decreased.