A hydroxyl-assisted ionic bifunctional hyper-crosslinked polymer as an efficient and recyclable heterogeneous catalyst for CO2 cycloaddition

Abstract

With the growing urgency to mitigate global warming and utilize CO₂ as a renewable C1 resource, developing efficient catalysts for CO₂ conversion into high-value chemicals has become critical. Herein, we report novel hydroxyl-assisted ionic bifunctional hyper-crosslinked polymers (HCPs-HQ) synthesized via Friedel–Crafts alkylation, which serve as highly efficient and recyclable heterogeneous catalysts for the cycloaddition of CO₂ with epoxides to produce cyclic carbonates. Comprehensive characterization studies (FT-IR, XPS, XRD, TGA, SEM, TEM, and N₂ adsorption–desorption) confirm that HCPs-HQ feature a hierarchical porous structure with a high specific surface area (up to 820 m² g⁻¹) and abundant active sites, including hydroxyl groups and quaternary ammonium salts. Under mild conditions (100 °C, 0.1 MPa CO₂, 20 h), HCPs-HQ achieve near-quantitative yields (up to 99%) for various epoxides, with an exceptional turnover number (TON) of up to 3241. Notably, the catalysts exhibit excellent recyclability over 6 cycles without significant activity loss. Mechanistic studies via control experiments and density functional theory (DFT) calculations reveal that hydroxyl groups reduce the activation energy of epoxide ring-opening (0.68 eV vs. 1.32 eV for hydroxyl-deficient analogs) through hydrogen bonding, while quaternary ammonium salts activate CO₂ via Lewis base interactions, synergistically promoting the reaction. These metal-, solvent-, and cocatalyst-free systems not only provide a sustainable strategy for CO₂ utilization but also highlight the critical role of hydroxyl-ionic synergy in enhancing catalytic efficiency, holding great promise for green chemistry and carbon neutrality.