Selective cleavage of C[sbnd]O bond in benzyl phenyl ether over Pd/AC at room temperature

Abstract

Selectively cleaving the CO bonds in lignin to produce aromatic products are challenging problems. Currently, catalytic hydrogenolysis of CO bonds in lignin requires harsh conditions. Herein, selectively cleaving CO bond of benzyl phenyl ether (BPE) was studied over Pd/AC, Ru/AC and Ni/AC. Among these catalysts, Pd/AC efficiently cleaved CO bond in BPE under super mild conditions (25 °C, 2 h and 0.1 MPa H2). The products were toluene and phenol without hydrogenation of the aromatic ring. The main competitive step of hydrogenolysis and hydrogenation is sensitive to reaction temperature over Pd catalyst. A mild temperature is preferred for hydrogenolysis, since the H species nearby the oxygen atom are more strongly adsorbed than nearby the benzene rings. With the different H-donor solvents, the highest conversion of BPE was obtained in isopropanol. Based on the experiments, the reactions occurred via cleavage of the Caliphatic-O bond in BPE and the synergetic effects between the H from absorbed H2 on the surface of the Pd and the intermediates radicals of benzyl and phenoxy should be the key points to such an optimum result.