Controlled Pyrolysis of Bromine-Modulated Metal-Organic Framework for Solvent-Free and Co-Catalyst-Free Cycloaddition of CO2

The cycloaddition of CO₂ with epoxides to synthesize cyclic carbonates is a crucial but challenging process. In particular, the co-catalyst required for higher yields in this reaction inevitably dissolves in organic solvents, resulting in product impurities and solvent-related hazards. Therefore, a heterogeneous catalyst that delivers high yields under solvent-free and co-catalyst-free conditions is highly desirable. In this study, we propose a controlled pyrolysis strategy to develop a metal-organic framework-derived catalyst that innovatively incorporates bromine via a C−Br bond. EXAFS, FT-IR and XPS analyses reveal the co-presence of unsaturated Zn sites and C−Br bonds, which act as acid and base sites, respectively. The synergy between these sites prevents bromine leakage and facilitates the ring-opening of epoxides, thereby overcoming the barriers in the cycloaddition reaction. As a result, this catalyst achieves superior catalytic performance with over 99 % yield of propylene carbonates under solvent-free and co-catalyst-free conditions. The resulting propylene carbonate can be used directly as an electrolyte in battery applications without further purification. This work provides valuable insights into the modular design of heterogeneous catalysts for solvent-free and co-catalyst-free CO₂ cycloaddition reactions.