Fe3O4@SiO2 nanoparticle-supported Co(III)-Salen composites as recyclable heterogeneous catalyst for the fixation of CO2

Magnetic nanoparticle-supported heterogeneous catalysts have been widely researched in green catalysis, due to their convenient magnetic separation and nanometer effects. Herein, a magnetic Co(III)-Salen heterogeneous catalyst with uniform spherical core-shell structure was synthesized for the chemical fixation of CO2 into cyclic carbonate based on the sustainable protocol development and green chemistry principles. Fe3O4@SiO2 nanoparticles with uniform spherical core-shell structure were prepared by the reverse microemulsion method, and Co(III)-Salen complexes were subsequently immobilized on the resulted Fe3O4@SiO2 nanoparticles via covalent bonding, leading to Fe3O4@SiO2@Co(III)-Salen nanocomposites with particle size of 29 nm and saturation magnetization value of 27.2 emu/g. The catalytic performance study indicated that the catalyst exhibited high yield of 99% with selectivity of 100% for the ring-opening addition reaction of CO2 and epoxide under solvent-free reaction condition. A plausible reaction mechanism was proposed that the Lewis acidic sites of Co(III)-Salen and nucleophilic bromine ion components played synergetic roles in promoting cycloaddition reactions. A stable geometry of the Co(III)-Salen complex and a detailed evidence for the reaction mechanism were provided by the density functional theory (DFT) calculations. Based on their excellent magnetic properties, the Co(III)-Salen nanocomposites could be easily recovered by using an external magnet and reused for 5 times without significant leaching of metal center and loss of catalytic activity.