Covalent Organic Frameworks for Metal-Free Photocatalytic Cyclohexane

In the process of photocatalytic oxidation of cyclohexane for cyclohexanol and cyclohexanone, the use of nonmetallic catalysts can reduce the adverse effects associated with metal leaching from metal catalysts on the properties of nylon. Here, we investigate covalent organic frameworks (COFs) with tunable structures and abundant active sites for the photocatalytic oxidation of cyclohexane and evaluate various COFs. Among them, COFs containing biphenyl groups and triazine units exhibited significantly higher catalytic activity, yielding up to four times more of the target product compared to C₃N₄ and outperforming other COFs as well. This improved efficiency is attributed to the extended conjugation length of the biphenyl groups, which enhances the separation of the photogenerated charge carriers. Furthermore, radical trapping experiments confirmed that h⁺ and ^•O₂^- are the primary active species in the reaction. This research marks the first application of COFs in the photocatalytic oxidation of cyclohexane, thus broadening the potential uses of nonmetallic catalysts in this area.