Zwitterionic Covalent Organic Frameworks as Catalysts for Hierarchical Reduction of CO₂ with Amine and Hydrosilane

Abstract

Controllable hierarchical reduction of carbon dioxide (CO₂) to selectively afford versatile chemicals with specific carbon oxidation state is important but still remains a huge challenge to be realized. Here, we report new zwitterionic covalent organic frameworks ([BE] _X%-TD-COFs), prepared by introducing betaine groups (BE) onto the channel walls of presynthesized frameworks via pore surface engineering methodology, as the heterogeneous organocatalysts for CO₂ reduction. The adjustable density of immobilized BE groups as well as good preservation of crystallinity and porosity inherited from their parent COFs endow [BE] _X%-TD-COFs with highly ordered catalytic site distribution and one-dimensional mass transport pathway in favor of catalysis. By controlling the reaction temperature and amount of CO₂, [BE] _X%-TD-COFs present high activity in catalyzing reduction of CO₂ with amine and phenylsilane (PhSiH₃) to produce formamides, aminals, and methylamines, respectively, with high yield and selectivity. Furthermore, high stability and insolubility bring excellent reusability to [BE]_X%-TD-COFs with well-maintained catalytic performance after four cycles of use. Notably, this is a novel example that COFs are developed as heterogeneous catalysts for hierarchical two-, four-, and six-electron reduction of CO₂ with amines and PhSiH₃ to form C-N bonds as well as afford C^+II, C⁰, and C^-II species efficiently and selectively.