Liquid-phase oxidation of benzene to phenol by copper catalysts in aqueous solvent with a high acetic acid concentration

Abstract

The liquid-phase catalytic oxidation of benzene was carried out under mild reaction conditions over Cu-impregnated zeolite catalysts (Cu/NaX, Cu/HX, Cu/NaY, Cu/HY, Cu/NaZSM-5, Cu/HZSM-5, Cu/Namordenite, Cu/Hmordenite, and Cu/Hbeta) using both molecular oxygen and ascorbic acid as the oxidant and the reducing reagents, respectively. Phenol was exclusively produced as the oxidation product and no other product was detected. Among the catalysts tested in this study, the Cu/HY catalyst had the highest activity for phenol formation. H-type zeolites were more effective supports for phenol formation than the corresponding Na-type ones. The yield of phenol was found to increase with the simultaneous increase in the amounts of both the supported Cu and the added ascorbic acid. The Cu species supported on HY began to elute as Cu2+ species in the solvent from the HY support with the initiation of the oxidation of benzene. The dissolved Cu2+ species were reduced to form precipitated Cu1+ species during the benzene oxidation. The homogeneous Cu species dissolved from the Cu/HY catalyst were thus suggested as mainly contributing to the phenol formation. The precipitated Cu1+ species that had no activity for the benzene oxidation were found to be regenerated in their oxidation activity toward phenol formation by calcination of the Cu1+ species in flowing air, although parts of the Cu species were found to be converted to aggregated CuO during the calcination.Key words: supported copper catalysts, zeolite, oxidation, benzene, phenol, oxygen, ascorbic acid.