Conversion of CO2 with epoxides to cyclic carbonates catalyzed by amino acid ionic liquids at room temperature

Abstract

CO2 utilization plays an important role in rational use of carbon resources and reduction of carbon emissions. It is still a challenge to achieve efficient CO2 utilization by amino acid ionic liquids (AAILs) under room temperature and atmospheric pressure with cocatalyst- and solvent-free conditions. Herein, the newly developed AAILs including 1,1,3,3-tetramethylguanidine cation ([HTMG]+) together with both amino acid and halogen as anions, were simply synthesized by neutralization reaction. Under mild conditions (30 °C, 20 h and 1 MPa CO2), the cycloaddition reaction of CO2 and propylene oxide could be efficiently catalyzed by [HTMG][His][I] with 99 % propylene carbonate yield and selectivity in the absence of additional solvent and cocatalyst. Notably, even at room temperature and atmospheric pressure for 72 h, [HTMG][His][I] also displayed 96 % yield and 99 % selectivity due to the cooperation of [HTMG]+, [His]2– and [I]–. Meanwhile, [HTMG][His][I] performed excellent universality of various epoxides and reusability. Based on the characterization results of 1H and 13C NMR, it was verified that propylene oxide and CO2 were simultaneously activated by the functional groups in [HTMG][His][I]. Hence, the reaction mechanism for CO2 cycloaddition with epoxide catalyzed by [HTMG][His][I] was proposed. This work provided a simple strategy to prepare sustainable AAILs for efficient CO2 utilization under very mild conditions of room temperature and atmospheric pressure (even the low CO2 concentration at simulated flue gases).