Stable Engineered Trimetallic Oxide Scaffold as a Catalyst for Enhanced Solvent-Free Conversion of CO2 into Value-Added Products

Abstract

This work demonstrates the development of mesoporous trimetallic oxide scaffolds (TOSs) as heterogeneous catalysts for the solvent-free transformation of CO2 into various value-added products. The TOS catalyst was prepared using a solution-combustion protocol as a time- and energy-saving method using cobalt, magnesium, and cerium metal nitrate salts as precursors and ethylene glycol as the fuel system and compared with their monometallic counterparts. Characterization suggested strong metal–metal and metal–oxygen interactions in the 3D-interconnected hierarchical porous network, which resulted in substantial alteration in the electronic, structural, and physicochemical properties. This resulted in an appreciable surface area, acid–base cooperative sites, and a larger pore volume in the catalyst. Thereafter, the CoMgCe-TOS catalyst was first used for the solvent-free cyclization of o-phenylenediamines and CO2 to produce benzimidazoles. In the presence of dimethylamine borane as a reductant, benzimidazoles were obtained in 94% yield at 100 °C under pressurized conditions, along with good recyclability for 12 cycles. It was established that selectivity toward benzimidazole improved upon the incorporation of basic metals (Mg and Ce), with Co aiding in the formation of cooperative Lewis acid–base sites. A plausible mechanism was also predicted, wherein the overall conversion of reactants for the N-formylation step was influenced by the number and strength of basic sites, and the cyclization step for selectively obtaining benzimidazole was affected by the acidic site strength. Besides the production of benzimidazole, the catalyst was also highly active for the synthesis of cyclic carbonates using epoxides and CO2 under optimized reaction conditions. This work therefore provides a new and greener route for the synthesis of benzimidazoles and organic carbonates, which can be easily adapted for scale-up applications.