Zwitterionic Covalent Organic Frameworks as Catalysts for Hierarchical Reduction of CO 2 with Amine and Hydrosilane

Controllable hierarchical reduction of carbon dioxide (CO 2 ) 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 CO 2 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 CO 2, [BE] X % -TD-COFs present high activity in catalyzing reduction of CO 2 with amine and phenylsilane (PhSiH 3 ) 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 CO 2 with amines and PhSiH 3 to form C–N bonds as well as afford C +II, C 0, and C –II species efficiently and selectively.