Synergistic cooperation of bi-active hydrogen atoms in protic carboxyl imidazolium ionic liquids to push cycloaddition of CO2 under benign conditions

Abstract

Nine protic carboxyl imidazolium ionic liquids are synthesized. Then, they are employed to catalyze the chemical fixation of carbon dioxide (CO2) and propylene oxide leading to propylene carbonate in the absence of co-catalyst and organic solvent. HCPImBr presents the best catalytic activity with the product yield of 92% under reaction temperature 120 °C, CO2 initial pressure 1.5 MPa, catalyst amount 0.5 mol%, and reaction time 2.0 h. Even if the reaction temperature and CO2 initial pressure are decreased to 80 °C and 1.0 MPa, respectively, the 85% of product yield would be kept with the 1.0% catalyst dosage along with 12.0 h. With the exception of the most optimal reaction conditions, generality, and recyclability of HCPImBr are also investigated. More importantly, the reaction mechanism is investigated by the density functional theory, which is the first time to report the mechanism for protic carboxyl imidazolium ionic liquids. The catalytic activity of ionic liquids would be further improved with the reasonable combination of cation and anion.