Highly efficient fixation of carbon dioxide to cyclic carbonates with new multi-hydroxyl bis-(quaternary ammonium) ionic liquids as metal-free catalysts under mild conditions

Abstract

A series of multi-hydroxyl bis-(quaternary ammonium) ionic liquids were prepared by a simple method, and used as bifunctional catalysts for the fixation of CO2 through the cycloaddition with epoxides in the absence of co-catalyst and solvent. All these ionic liquid compounds were proved to be efficient catalysts for the synthesis of cyclic carbonates from CO2 and epoxides in excellent yield and selectivity. Thanks to synergistic effects of hydroxyl groups and halogen anion, the cycloaddition reaction proceeded smoothly even at atmospheric pressure. The influences of the type of catalyst, catalyst loading, CO2 pressure, reaction time and temperature on the yields have been investigated in detail and the optimal conditions were screened as (120 °C, 2 MPa, 3 h, IL loading 0.25 mol%). Meanwhile, preliminary kinetic investigations were in progress by using four typical catalysts and clarified the activation energies (Ea) of cyclic carbonate formation (37.2 kJ/mol for IL1, 38.1 kJ/mol for IL3, 39.1 kJ/mol for IL5, and 58.9 kJ/mol for IL6), which agrees well with the catalytic activity. The catalyst system could be reused at least five consecutive times successfully. Notably, a high turnover frequency (TOF) value of (33,300 h−1) for IL1 was achieved with a high yield of 83.2% within 1 h via adjusting reaction variables. Finally, an inferred mechanism was presented according to the kinetic studies and experimental results.