Selective catalytic oxidation of benzene to phenol by a vanadium oxide nanorod (Vnr) catalyst in CH3CN using H2O2(aq) and pyrazine-2-carboxylic acid (PCA)

Abstract

Vanadium based catalysts are known to be highly efficient for oxidation reactions using O₂ and/or H₂O₂. In this study, we developed a vanadium catalyst that can be prepared by a reaction of VCl₃ in acetonitrile with the addition of 35% hydrogen peroxide (H₂O₂). The catalyst was further characterized by a variety of methods including ⁵¹V NMR, XRD, XPS, XAS, EA, ICP-AES, BET, SEM and TEM. Based on the results obtained, the catalyst was identified as a vanadium oxide material (V_nr) with the morphology and shape of a nanorod with dimensions of ∼40 nm × 450 nm (width × length). The porosity of a material can be essential for small molecule activation on a heterogeneous surface. This material exhibits exceptional catalytic performance for the selective oxidation of benzene to phenol (PhOH) at room temperature using H₂O₂ as an oxidant in acetonitrile. The addition of pyrazine-2-carboxylic acid (PCA) as a co-catalyst can efficiently enhance the catalytic activity with a percentage selectivity of 87–99% phenol, producing p-benzoquinone (p-BQ) as a minor product. With the deployment of H₂¹⁸O₂, we observed the product phenol to be highly enriched by ¹⁸O, indicating that the hydroperoxo-vanadium intermediate is essential for its catalytic conversion. Together with its high NIH shift ratio (80% deuterium remained) observed with the mechanistic probe of 4-[²H_0,1]toluene, the results suggested that the catalytic oxidation reaction mediated by V_nr may involve metal based oxygenated intermediates for its highly selective catalytic oxidation of benzene to phenol.