Recent advances in cycloaddition of CO2 with epoxides: halogen-free catalysis and mechanistic insights

The atom-economical cycloaddition of CO2 with epoxides to synthesize cyclic carbonates is a promising route for valuable utilization of CO2. Halogenide such as alkali metal halides and quaternary ammonium salt have been developed as the efficient catalysts. However, the spilled halogen causes equipment corrosion and affects the product purity. To address these concerns, the halogen-free cycloaddition of CO2 with epoxides has always been desired. In this review, we systematically discussed the halogen-free catalysis for cycloaddition of CO2 with epoxides from the mechanistic insights, aiming to promote the development of efficient halogen-free catalysts. Two types of catalysts, i.e., alternatives of halogen nucleophiles for epoxide activation, and bifunctional catalysts with Lewis acid-base sites for synergistic activation of CO2 and epoxides are summarized and emphasized. Specially, metal oxides as the potential halogen-free catalysts are highlighted due to their flexible acid-base sites for synergistic activation of CO2 and epoxides, facile preparation, and low cost.