Modulating the Spatial Configuration of Bromine in Zinc-Based Covalent Organic Frameworks for Boosting Cocatalyst-Free CO ₂ Cycloaddition

For the cycloaddition of CO2 to cyclic carbonates to become a practical chemical means of achieving carbon neutrality, the development of efficient and selective catalysts is essential yet highly challenging. Here, we rationally design two zinc-based covalent organic framework (COF) catalysts featuring zinc–nitrogen (Zn–N) active sites and different bromide site configurations via a simple solvothermal strategy. We systematically investigate the influence of bromide configurations of COFs on CO2 cycloaddition performance. Notably, the optimum Zn-COF-366@OC2Br achieves an excellent yield of over 99% in the cycloaddition of CO2 and propylene oxide (PO) to propylene carbonate (PC) under cocatalyst-free conditions. Density functional theory calculations reveal that the energy barriers for ring-opening and the CO2 attacking step on Zn-COF-366@OC2Br are much lower than those on referenced Zn-COF-366@Br, highlighting the crucial role of flexibility of bromine. This work offers an insight into the modular design of highly active catalysts for the important acid–base catalyzed reactions.