N-doping of β-ketoenamine based covalent organic frameworks for catalytic conversion of CO2 to cyclic carbonate and Knoevenagel condensation

Covalent organic frameworks (COFs) based on β-ketoenamine structure are obtained through the irreversible enol-to-keto tautomerization, resulting in a highly stable and crystalline skeleton structure. Herein, N-doped β-ketoenamine based COFs (TP-BD-COF, TP-PD-COF, TP-PMD-COF) were successfully constructed between C2-symmetric diamine monomers containing different amounts of nitrogen atoms (0 N, 1 N and 2 N) and C3-symmetric 1,3,5-Triformylphloroglucinol (TP) monomer. The as-prepared COFs containing both the –NH– unit and pyridine nitrogen atom on the backbone of COFs could provide hydrogen bond and Lewis base double active sites for catalytic application. Studies showed that TP-BD-COF containing 0 N atom exhibited highest catalytic efficiency (99 %) for the cycloaddition reaction between carbon dioxide and styrene oxide, while TP-PD-COF (1 N) and TP-PMD-COF (2 N) showed the catalytic efficiency only with 87 % and 85 %, respectively. However, for Knoevenagel condensation of benzaldehyde and malononitrile, the order of catalytic efficiency is TP-PD-COF (99 %) > TP-PMD-COF (92 %) > TP-BD-COF (70 %), the recycled number and the possible catalytic mechanism were also investigated. This research provides theoretical guidance for the rational construction of non-metallic heterogeneous catalysts based on β-ketoenamine COFs.