Synergistic effect of carboxylmethyl group and adjacent methylene substitution in pyrazolium ionic liquid promote the conversion of CO2 under benign condition

Abstract

Abstract The carboxyl functionalized ionic liquids normally show better catalytic activity than the corresponding ionic liquids without functional group for the coupling reaction of carbon dioxide and epoxides. However, a long alkyl chain, at least propyl, is required as bridge between carboxyl group and N atom of aromatic heterocycle to ensure the catalytic activity. Inclusion of long alkyl chain indicates the low synthetic yield and harsh synthetic conditions. It is rarely reported that the carboxyl functionalized ionic liquid with methyl would present the high catalytic activity. With the help of ethyl substituted on the other N atom of pyrazolium ring, the carboxylmethyl pyrazolium ionic liquid (CMEPzBr) would present the excellent catalytic activity (yield >99%). Moreover, the product yield still keeps larger than 90% even under 70\u2009°C and 1.0\u2009MPa CO2 initial pressure. The role of methylene appeared on the near N atom is elucidated by density functional theory. The catalytic activity of carboxylmethyl imidazolium ionic liquid is not refined greatly when methylene is substituted on the other N atom of imidazolium ring. The optimal reaction conditions are investigated along with the recyclability and reusability.