Role of oxygen vacancy in spinel (FeCoNiCrMn)3O4 high entropy oxides prepared via two different methods for the selective C H bond oxidation of p-chlorotoluene

The selective CH bond oxidation of aromatic hydrocarbon is an interesting but challenging task, it is desirable to develop efficient heterogeneous non-noble metal catalyst for this reaction. Herein, two kinds of spinel (FeCoNiCrMn)₃O₄ high entropy oxides were fabricated via two different methods (i.e., c-FeCoNiCrMn, prepared by a co-precipitation method, and m-FeCoNiCrMn, prepared by physically mixing method). Different from traditional environmentally-unfriendly Co/Mn/Br system, the prepared catalysts were employed for the selective CH bond oxidation of p-chlorotoluene to p-chlorobenzaldehyde in a green approach. Compared to m-FeCoNiCrMn, c-FeCoNiCrMn have smaller particles size and larger specific surface area, which were related to the enhanced catalytic activity. More importantly, characterization results disclosed that abundant oxygen vacancies were formed over c-FeCoNiCrMn. Such a result facilitated the adsorption of p-chlorotoluene on the catalyst surface and promoted the formation of *ClPhCH₂O intermediate as well as the desired p-chlorobenzaldehyde, as revealed by DFT (Density functional theory) calculations. Besides, scavenger tests and EPR (Electron paramagnetic resonance) results indicated that hydroxyl radical derived from H₂O₂ homolysis was the main active oxidative species for this reaction. This work revealed the role of oxygen vacancy in spinel high entropy oxide and also demonstrated its promising application for the selective CH bond oxidation in an environmentally-benign approach.