Cyano-Cyclotrimerization Strategy for Constructing Bi-Functional Acid-Base Sites in Covalent Organic Frameworks for Achieving Synergistic-Optimization of Catalytic Activity and Rapid-Recyclability in CO2 Cycloaddition

Abstract

For the cycloaddition of CO₂ to epoxides to be a viable non-redox CO₂ fixation pathway, it is crucial to develop active, stable, selective, metal-free, rapidly separable, and cost-effective catalysts. To this end, three novel catalysts are synthesized via cyano-trimerization reactions on the cyano-groups of the sp²-c linked COF─CN, using two cyano-monomers and the polymers of intrinsic microporosity (PIM-1). Among these, the powder catalysts (COF─CN─COOH, COF─CN─NH₂), featuring acidic hydrogen-bond donors (─COOH, ─NH₂) and basic sites (triazine ring), exhibited excellent catalytic performance in CO₂ cycloaddition reaction due to their customizable hydrogen-bond sites, high CO₂ affinity, and stability. Notably, COF─CN─COOH achieved a catalytic yield of 99.9% with selectivity exceeding 99%. The 20%COF─CN@sPIM-1 membrane catalyst, formed by covalent-connection between COF─CN and PIM-1, demonstrates good interfacial compatibility, facilitating easy recycling while maintaining excellent catalytic activity. Furthermore, density functional theory (DFT) studies on the hydrogen-bond promoted mechanism reveal that hydrogen bond donors (HBDs) can significantly reduce the activation energy. Therefore, this work introduces a novel trimerization reaction strategy utilizing cyano-groups on the sp²-c linked COF as reaction sites, establishing a unique acid-base synergistic-catalytic system and laying the foundation for the development of a membrane catalytic system that can be rapidly separated while exhibiting high activity and stability.