The liquid-phase oxidation of benzene was carried out in aqueous acetic acid solvent over V-substituted heteropolyacids (V−HPAs) using molecular O2 and ascorbic acid as the oxidant and reducing reagent, respectively. Phenol was exclusively obtained as the oxygenation product. The elimination of the V species from the V−HPA (Keggin structure) catalyst into the reaction solvent during the benzene oxidation reaction was largely inhibited by ion-exchanging the proton of the V−HPA catalyst with Cs+. The main active species were assumed to be the V species anchored in the HPA. The influences of the reaction temperature, the concentration of acetic acid in the aqueous solvent, and the reaction pressure on the yield of phenol were investigated to obtain the optimal reactions condition for phenol formation. The reuse of the V−HPA catalyst caused gradual deactivation for phenol formation, despite the retention of the structure of the V−HPA catalyst. The deactivation was suggested to be due to the reduction of the V species in the V−HPA catalyst on the basis of the diffuse reflection spectra of the used catalysts.

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The liquid-phase oxidation of benzene to phenol in an aqueous acetic acid solvent over a V-substituted heteropolyacid catalyst was attempted using zinc powder as the reducing reagent and gaseous oxygen as the oxidant. Phenol, an oxygenation product, was selectively obtained. Zinc powder, together with the use of gaseous oxygen as the oxidant, was found to be an effective reducing reagent for phenol formation. The influences of the phenol yield on the reaction temperature, the amount of zinc powder, the concentration of acetic acid, and the amount of the V-substituted heteropolyacid catalyst were investigated. The yield of phenol had a maximum value using an aqueous solvent of acetic acid of about 60 vol %. The reuse of the used V-substituted heteropolyacid catalyst caused a decrease in the phenol yield. The reduction of part of the V species in the used V-substituted heteropolyacid catalyst was suggested to be a cause of the deactivation of the phenol formation based on the observation of the diffuse reflectance spectra of the fresh and used catalysts. The generation of hydrogen peroxide (H2O2) was confirmed in the aqueous acetic acid solvent in the presence of both zinc powder and gaseous oxygen. The dependence of the amount of generated H2O2 on the concentration of acetic acid in the aqueous solvent was found to be fairly well correlated with that of the yield of phenol on the concentration of acetic acid.