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

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.