Palladium-based bifunctional membrane reactor for one-step conversion of benzene to phenol and cyclohexanone

On the one-step hydroxylation of benzene to phenol over a Pd membrane reactor, the bifunctional effects between Pd membrane and metal particles were investigated. In this reaction system, the active oxygen species are formed by the reaction with the permeated hydrogen and adsorbed oxygen over Pd membrane. The active oxygen species are reacted with benzene and directly converts into phenol. For improving this system, various active metals were loaded on the α-Al2O3 porous tube which was the substrate of the thin Pd membrane. The loading of noble metals resulted in the enhancement of the activity of side reactions, i.e., complete oxidation and hydrogenation, and a decrease in the hydroxylation activity. The negative effect on hydroxylation was due to the imbalance between the catalytic activities of highly dispersed noble metal particles and the Pd membrane, which has a relatively small surface area. In contrast, the loading of Cu suppressed complete oxidation and enhanced the hydroxylation activity. This effect was mainly due to the increase in the hydrogenation activity without significant acceleration of oxidation. In addition, the potential for the selective conversion of benzene to cyclohexanone by this reactor was discussed.