Deep insight into the catalytic systems for CO2 conversion to cyclic carbonates: A hard and soft acid- base approach

In recent years, the conversion of CO₂ into value-added products has emerged as a promising strategy to mitigate greenhouse gas emissions and address global energy challenges. This review is focused on the promising area of the synthesis of cyclic carbonates by employing CO 2 as a building block. Indeed, cycloaddition of epoxides with CO 2 offers a 100 % atomic efficient process as well as widespread industrial applications of the obtained corresponding cyclic carbonate products. In this perspective, we have attempted to provide an overview of the reported chemical catalytic systems, classifying them based on hard and soft acid-base (HSAB) theory. The HSAB theory offers a fundamental understanding of the interactions between CO₂ (a hard acid or electrophile from its carbon atom), epoxides (as soft acids or electrophiles from their carbon atoms) with the catalytic systems, enabling the rational design of catalysts that efficiently activate and transform CO₂ under mild conditions. This review paper discusses various catalytic systems including homogeneous and heterogeneous ones, emphasizing their hardness and softness properties, donor-acceptor interactions, and structure-activity relationships. Special attention is given to the metal-organic frameworks, transition metal complexes, and hybrid systems, which demonstrate tuneable acid-base characteristics for enhanced CO₂ activation. By bridging the gap between theory and application, this review highlights how the HSAB framework can guide the development of next-generation catalysts for CO₂ transformation and utilization, paving the way toward sustainable energy and chemical production.