Heterogenized Phosphonium Salt Catalysts for Cyclic Carbonate Production from CO 2 and Epoxides

Abstract

Building on our recent homogeneous catalyst system, this work explores heterogenization by formation of covalent organic frameworks to enhance durability. Four pyridyl-functionalized porous ionic polymers (PPIPs) bearing phosphonium salts were synthesized and systematically evaluated for the solvent-free cycloaddition of CO2 to epoxides. The para-substituted catalyst (4-PPIP) exhibited superior activity compared to ortho-and meta-isomers, achieving 90% yield of styrene carbonate over 24 h at 100 °C and 0.1 MPa CO2. Protonation of the pyridyl nitrogen (4-PPIP-H + ) enhanced catalytic efficiency 2.3-fold under catalyst-limited conditions while maintaining stability throughout extended operation. Optimization studies revealed that polymerization of the functionalized monomer outperforms the post-functionalization of the polymer. Under optimized conditions (120 °C, 6 h and 0.1 MPa CO2), 4-PPIP-H + achieved near-quantitative conversion for styrene oxide (98%) and for a range of terminal epoxides including bio-based terpenes. Catalyst recyclability was confirmed over 4 cycles. These findings establish structure-activity relationships for heterogeneous phosphonium catalysts and provide design principles for metalfree CO2 utilization systems.