Vinylene-Linked Covalent Organic Frameworks with Ammonium-Promoted-Proton Transfer for Photocatalysis of H2O2 Evolution.

Abstract

Covalent organic frameworks (COFs) have emerged as effective photocatalysts for the environmentally friendly synthesis of hydrogen peroxide (H₂O₂) through the oxygen reduction reaction (ORR) under solar sunlight. Besides electron transfer in an ORR process, proton transport also serves as an important role in promoting kinetic rate, which was majorly improved via modifying the chemical structures of COFs, but seldom to be explored through a simple additive composition. In work, we report the preparation of two new vinylene-linked COFs termed g-TDM-COF and g-TBD-COF, respectively, by Knoevenagel condensation of trimethylpyridine (TMP) and 2,5-Dimethoxyterephthalaldehyde (DMTP) or 3,3'-dimethoxy-[1,1'-biphenyl]-4,4'-dicarbaldehyde (DMBD). They were crystalized in a hexagonal lattice and adopting AA stacking modes. Their porous structures with high surface areas and micro-/nano-channels were revealed. The methoxyl substituents pended on and pyridine atoms embedded in the backbones of these COFs rendered them with hydrogen bond donating capabilities. Combined with their substantial semiconducting properties, the COFs enable photocatalysis of hydroperoxide (H₂O₂) production. Simply compositing these COFs with ammonium ions markedly improved the photoelectric properties, leading to an over eightfold enhancement of photocatalytic H₂O₂ production relative to the neat COFs, and an increase in apparent quantum yields (AQYs) from 0.70% to 4.22% at 500 nm. Such a phenomenon could be attributed to the efficient interaction of ammonium ions with the COFs via hydrogen-bond interaction, thus favorable for broadening light-harvesting, narrowing band gaps, and strengthening proton conductivity. As a consequence, their photocatalytic performance could be distinctly enhanced.