Halide-free ionic liquids with dual-regulation of anion nucleophilicity and basicity for the efficient CO2 conversion into cyclic carbonates

Abtract The conversion of CO₂ into cyclic carbonates is a feasible approach for CO 2 mitigation. The environmentally friendly halide-free catalysts, which could avoid the waste halide contamination and industrial equipment corrosion are highly concerned. In this study, a series of halide-free ionic liquids with dual regulation of anion basicity and nucleophilicity were designed to achieve high catalytic efficiency. Among the synthesized ionic liquids, the [BmmIm][PA] demonstrated superior catalytic efficiency, delivering 94.9 % product yield with 99.9 % selectivity under the optimized conditions (70 °C, 2 mol%, 3.5 h, 10 bar). According to the deep activation energies analysis, the superior catalytic efficiency of [BmmIm][PA] was attributed to the strong nucleophilicity of [PA] reducing the ring-opening activation energy and the mild basicity of [PA] reducing the ring-closing activation energy simultaneously. The lower reaction activation energy catalyzed by [BmmIm][PA] was further confirmed by kinetic experiments. Finally, by combining DFT calculations with IR, NMR spectroscopy, a detailed catalytic mechanism involving direct epoxides activation by [PA], CO 2 insertion and ring-closing was proposed. This work provided a more comprehensive understanding of halide-free catalysts and laid a theoretical foundation for the green and efficient conversion of CO 2 .