Zinc Powder as an Effective Reducing Reagent during Liquid-Phase Oxidation of Benzene to Phenol Using Molecular Oxygen over V-Substituted Heteropoly Acid Catalysts

Abstract

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.