Photocatalytic oxidation of various C–H bonds by a polyoxometalate nanocluster-dominated compartmentalized metal–organic framework

Abstract

Photocatalytic oxidation of hydrocarbons to carboxylic acids is considered an attractive method. However, exploring active and stable photocatalysts remains a challenge. In this work, a new polyoxometalate-based metal–organic framework (POMOF), NiW₁₂-TPT, was assembled from an ‘electronic sponge’ functional metatungstate ([H₂W₁₂O₄₀]⁶⁻) and a photoactive triazine organic ligand 2,4,6-tri(2-pyridyl)-s-triazine (TPT) using a one-pot method. Molecular compartments were constructed by introducing POM anions into the mesopores of the Ni–TPT MOF, which not only inhibited the interaction between photooxidation and photoreduction sites but also promoted the long-range delocalization of photogenerated carriers. The ordered arrangement of electron donor–acceptor and ligand-cluster charge transfer (LCCT) excitations in NiW₁₂-TPT effectively promoted the separation of photoinduced electrons and holes and the orderly transfer of electrons, endowing NiW₁₂-TPT with excellent photocatalytic performance. NiW₁₂-TPT was applied in heterogeneous photocatalytic toluene oxidation, achieving a high conversion (>99%) and excellent selectivity (>99%) in the presence of water under light irradiation. This strategy is suitable for the oxidation of various hydrocarbons, including polysubstituted toluenes and heterocyclic aromatic hydrocarbons, thereby overcoming substrate limitations. Notably, the catalyst could be applied in gram-scale hydrocarbon oxidation. The mechanism study shows that the generated holes with high oxidizability on NiW₁₂-TPT can oxidize H₂O to H₂O₂, which further generates ˙OH upon irradiation by UV light, thereby driving the hydrocarbon oxidation process.