Quaternary Ammonium Salt Anchored on CuO Flowers as Organic–Inorganic Hybrid Catalyst for Fixation of CO into Cyclic Carbonates

The selective fixation of CO and epoxides into cycliccarbonates offers an atom-economical approach towards promising utilization of anthropogenic CO gas. This work demonstrates the development of recyclable organic–inorganic hybrid TBAI@CuO catalyst containing quaternary ammonium (TBAI) salt as active organic groups and CuO flowers as the inorganic component. Initially, CuO flowers was synthesized using a simple oxidation-precipitation method followed by immobilization of TBAI groups using impregnation method. The impregnated TBAI groups functioned as the active basic centers, whereas CuO flowers with effective surface area provided heterogeneity, Lewis acidic sites, facilitated better dispersion and strong interaction of TBAI. Resultantly, hybrid TBAI@CuO catalyst consisted of dual-active sites which played a decisive role in manipulating catalytic activity without requirement of a co-catalyst. With styrene oxide and CO as model substrates, the present catalyst system delivered 95% conversion and 97% selectivity towards styrene carbonate under solvent-free and milder reaction conditions when compared to other single metal oxide catalysts. The homogeneously dispersed TBAI groups in strong interaction with CuO flowers provided simultaneous access to both Lewis acidic as well as basic sites and hence resulted in superior catalytic activity. Meanwhile, hybrid TBAI@CuO catalyst showed catalytic adequacy for various terminal and internal epoxides. This catalytic system also presented good reusability without prominent loss in activity and no leaching of active TBAI groups. Based on characterization results, a plausible reaction mechanism was predicted to support the cycloaddition reaction with hybrid TBAI@CuO catalyst. It was proposed that inorganic Lewis acidic CuO activated the epoxide whereas the immobilized TBAI groups activated the inert CO molecule and stabilized the epoxide after ring opening. Therefore, the CuO flowers acting in concert with the immobilized TBAI groups showed exceptional acid–base cooperativity and provided high selectivity towards cyclic carbonates. Graphical Abstract: The hybrid TBAI@CuO catalyst in this effort bridges the gap between homogeneous and heterogeneous catalysis for sustainable transformation of CO2 and epoxides to cyclic carbonates under solvent-free and comparatively mild reaction conditions (Figure presented.).