Sterically regulated halogen-free bifunctional ionic liquids for catalytic conversion of CO₂ into cyclic carbonates

In the cycloaddition reaction of carbon dioxide (CO2) with epoxides, halogen catalysts lead to halogen loss and environmental pollution. It is necessary to design efficient and environmentally friendly catalysts. A series of halogen-free phosphonium ionic liquids (ILs) regulated by steric hindrance were designed and synthesized. It is further used as effective catalysts for the catalytic conversion of CO2 into cyclic carbonates. The effect of different catalyst structures on catalytic activity has been investigated. Amino group has been shown to contribute to increased activity, and its location determines the degree of increased activity. Among them, [(BuP)-P-4](2)[5-AMP] with meta-substitution showed the best catalytic activity, the conversion of propylene oxide (PO) was 96 % at the optimum conditions of 120 C-degrees, 2 MPa and 3 h. Based on characterization analysis and DFT calculations, the interaction of ILs with CO2 and PO were revealed. Finally, a plausible synergistic catalytic mechanism was proposed. The halogen-free, metal-free phosphonium ILs proposed in this paper provide a new insight on catalyst design and CO2 utilization.