Phenol hydroxyl-modified imine-based covalent organic frameworks for enhanced solar-driven generation of H2O2via hydrogen bonds

Abstract

Photosynthesis of H₂O₂ has been considered an eco-friendly strategy. However, the concentration of H₂O₂ reported in earlier studies is far from the industrial requirement. Herein, we present a strategy of employing phenolic hydroxyl-modified imine-based covalent organic frameworks (COFs) as catalysts for enhancing the photosynthesis of H₂O₂ in a benzyl alcohol (BA)/water system. H₂O₂ production rate was 19 times that by unmodified imine-based COFs, and the H₂O₂ concentration reached 380 mM with a record rate of 61.3 M h⁻¹ under simulated solar irradiation. In addition, the selective oxidation of BA into benzaldehyde was achieved, indicating the potential for industrial applications. The phenolic hydroxylic group played an important role, as indicated by the result of experiments and DFT calculations. First, intermolecular hydrogen bonding between the phenolic hydroxyl group and BA facilitated electron transfer, thereby lowering the energy barrier for H₂O₂ generation. Second, intramolecular hydrogen bonding between the phenolic hydroxyl group and imine increased the energy barriers of H₂O₂ decomposition and ensured catalyst stability. Overall, our research highlights the critical role of hydrogen bonding in the H atom of CNH in imine-based COFs in augmenting the photocatalytic activity.