Quaternary Ammonium-Functionalized Mesoporous Covalent Organic Frameworks for Effective Catalytic CO₂ Cycloaddition

The cycloaddition reaction involving CO2 and epoxides to generate cyclic carbonates represents a viable method for CO2 utilization. However, the challenge still lies in catalyzing this reaction under solvent-free, cocatalyst-free, and mild conditions. In this study, we constructed a robust mesoporous covalent organic framework (COF) platform that features flexible quaternary ammonium groups covalently affixed inside the pore walls of these COFs. By meticulously adjusting the density of these quaternary ammonium entities, we struck a balance between the active site density and porosity of the COFs. This guaranteed access to the active sites, resulting in an effective heterogeneous catalyst. Utilizing the COF-50%QA (where QA denotes quaternary ammonium) catalyst under conditions of 100 degrees C and 0.1 MPa of CO2, the CO2-epoxide cycloaddition reaction demonstrated superior performance with an exceptional yield of 98% and a selectivity surpassing 99%. This work not only broadens the scope of COF-based heterogeneous catalysts for CO2 cycloaddition reactions but also offers valuable understanding into the strategy for design and structure-property relationships of such catalysts.