Engineering Nitrogen-Coordinated Single-Atom Catalysts for Efficient CO2 Cycloaddition.

Developing highly efficient nonprecious heterogeneous catalysts for the cycloaddition of carbon dioxide (CO2) to cyclic carbonates is crucial but challenging in the modern chemical industry. Here, a facile and scalable molecules-confined pyrolysis approach is demonstrated for the synthesis of nitrogen-coordinated transition-metal (TM) single-atom catalysts (SACs). Moreover, the various coordination structures of metal centers and the forms of nitrogen species are successfully revealed. The designed TM SACs show excellent catalytic performance for the cycloaddition of CO2 with epoxides to cyclic carbonates under solvent-free mild conditions. Among them, the optimum Zn SAC with 13.2 wt.% Zn content achieves a >99% yield at 80 °C within 2 h for the cycloaddition of propylene oxide to propylene carbonate. The crucial comprehension of the relationship between performance and reaction mechanism over TM SACs with various metal centers and adjacent N species is further enhanced through experimental investigations and theoretical simulations. Significantly, the high density of Lewis acid-base sites (Zn and N species) can appropriately regulate the activation of epoxide and CO2 and the reaction energy of cycloaddition, respectively, thus improving the CO2 cycloaddition performance. This work provides a new insight into the design of highly active and stable SACs for efficient cycloaddition reactions.