Fragmentation engineering on the edge of hydroxy-functional COFs for the enhanced photocatalytic production of H₂O₂ and direct photo-oxidation of benzene to phenol in aqueous systems

Structural design is important and challenging for the fabrication of covalent organic frameworks (COFs) in the photocatalytic applications. Hydroxyl groups have been found to be the active sites for O₂ adsorption and promote the electron-donating conjugation effects, which favorable for the oxygen reduction reaction (ORR) in photocatalytic hydrogen peroxide (H₂O₂) evolution. This study reports the preparation of “fragmentized” COFs (T_xH_y-COFs) photocatalysts and their application in H₂O₂ synthesis, as well as the direct hydroxylation of benzene to phenol. The results demonstrate that the functional design of surface hydroxyl (–OH) modification on fragmentized T_xH_y-COFs was achieved by varying the feed ratio of TAPT and THTA monomers, which greatly enhances surface hydrophilicity and effectively improves the separation and transfer of photoexcited electrons and holes. Under visible light irradiation without sacrificial agents, the fragmentized T₁H_1.8-COF exhibits a boosting H₂O₂ yield of 2567 μmol g⁻¹h⁻¹, which is 4.2 times higher than the original T₁H₁-COF. Furthermore, the introduction of metal ions (Fe³⁺ etc.) into the COF framework through the impregnation method enables the in situ activation of H₂O₂, allowing the direct hydroxylation of benzene to phenol in aqueous systems, with yield of 9.4 % and selectivity of 99 %. The results of this work demonstrate that the prepared photocatalyst exhibits high stability and photocatalytic activity by structural design optimization, showing great potential for industrial applications in H₂O₂ production and direct photo-oxidation.