Tunable and functional phosphonium-based deep eutectic solvents for synthesizing of cyclic carbonates from CO2 and epoxides under mild conditions

Carbon dioxide (CO2) conversion into carbonates is one of the important ways to reduce carbon emissions. In this study, several phosphonium-based functional deep eutectic solvents (DESs) were prepared as catalysts for the synthesis of cyclic carbonates under mild conditions. The research showed that under mild conditions (60 °C, 0.8 MPa, 4 h, and 3.5 mol% catalyst loading), 1-ethylamine-3-butylphosphine bromide−diethylene glycol ([P4,4,4,2NH2][Br]-DEG) DES catalyst can achieve 96% propylene carbonate (PC) yield. In addition, the DES catalyst can be recovered by simple solvent extraction, and the PC yield does not decrease significantly after five cycles. Finally, in situ FT-IR and density functional theory (DFT) were used to provide a potential reaction mechanism for the cycloaddition reaction. Mechanism studies show that synergistic catalysis between quaternary phosphine ionic liquids (ILs) and ethylene glycol (EG)/DEG promotes CO2 conversion to cyclic carbonate.