Tetrabutylammonium Bromide/Triethanolamine Deep Eutectic Solvents with Double Hydrogen Bond as Efficient Catalysts for Fixation of Co2 in Cyclic Carbonates

Abstract

Propylene carbonate (PC) was successfully prepared by cycloaddition of CO2 to propylene oxide (PO) in tetrabutylammonium bromide (TBAB)–based deep eutectic solvents (DESs). The experimental parameters, i.e., the type of hydrogen bond donor, reaction temperature, reaction time, and CO2 pressure, were evaluated. Changing the reaction conditions significantly affected the PC yields. Use of a DES with triethanolamine (TEA) as the hydrogen bond donor significantly shortened the reaction time for cycloaddition of CO2 to PO. A 98% PC yield was obtained at 90 °C with a 1.0 MPa pressure of CO2 for 2 h. Density functional theory (DFT) calculations were used to verify the catalytic action of DES in CO2 cycloaddition reaction for the first time. DFT calculations indicated that this high catalytic activity mainly results from cooperative catalysis involving the double hydrogen bond in the DES and Br anions. A possible mechanism for the cycloaddition reaction catalyzed by a TBAB/TEA DES catalyst is proposed.

Keywords: Deep eutectic solvent, CO2 catalyst, cyclic carbonate, cycloaddition reaction, DFT