Novel biomass-derived deep eutectic solvents promoted cycloaddition of CO2 with epoxides under mild and additive-free conditions

Chemical fixation of CO2 to produce high value-added chemicals is one of the important measures to effectively control carbon emission. In this paper, novel biomass-derived deep eutectic solvents (BDESs) with multiple active sites were developed as green media for chemical conversion of CO2 and epoxides into cyclic carbonates. The coupling of propylene oxide (PO) and CO2 was selected as a model reaction to evaluate the catalytic activity. By screening various BDESs structures and reaction parameters, the iodide choline/citric acid (ChI/CA) with a molar ratio of 2:1 was proved to be an efficient catalytic medium, and it could afford 98 % propylene carbonate (PC) yield with 99 % selectivity under mild (70 °C, 0.5 MPa, 3 h) and additive-free conditions. Moreover, the catalyst reusability and versatility were examined. The ChI/CA could be reused in up to five consecutive runs without loss of activity, and the ChI/CA showed excellent activity to various terminal epoxides/CO2 coupling reaction. Finally, the activation behaviors of the ChI/CA to the CO2 and epoxide were studied respectively, and a synergistic catalytic mechanism was provided.