ZnCl₂/phosphonium halide: An efficient Lewis acid/base catalyst for the synthesis of cyclic carbonate

An efficient Lewis acid/base catalyst composed of ZnCl₂ and phosphonium halide ([PR₁R₂R₃R₄]⁺X^-; X = Cl, Br, I) was developed and showed high activity and selectivity for the coupling reaction of CO₂ and epoxide under the mild conditions. The effects of reaction temperature, CO₂ pressure, various compositions of the catalysts have been investigated systematically. It was found that a 96.0% conversion of propylene oxide (PO) and high turnover frequency (TOF) value (4718.4 h^-1) could be achieved in the presence of ZnCl₂/PPh₃C₆H₁₃Br (molar ratio = l:6) at a low constant pressure of CO₂ (1.5 MPa) and mild temperature (120 °C) without any organic solvents, the catalyst was also proved to be applicable to other terminal epoxides. Additionally, the catalyst could be reused with little loss of catalytic activity after five times.