Water-assisted selective hydrodeoxygenation of phenol to benzene over the Ru composite catalyst in the biphasic process

Abstract

Aromatic hydrocarbons as a liquid fuel have unique combustion properties (high volume energy density, etc.). Thus preserving the aromatic rings while selectively cleaving the C–O bonds in the hydrodeoxygenation of lignin derived substituted phenols without additional consumption of H₂ is of crucial importance. In this regard, the composite materials, which are used as the catalyst support, were prepared through the hybridization of Nb₂O₅ (niobium oxide) with MC (micro–mesoporous carbon). Ru NPs (nanoparticles) were loaded on the Nb₂O₅-MC composite supports by the incipient wetness impregnation method. And then the as-synthesized catalysts were characterized by various techniques, including XRD, SEM, TEM, BET, NH₃-TPD, XPS, etc. Under mild conditions (200–250 °C and 2.0–10 bar H₂), the Ru/Nb₂O₅-MC catalyst was proved to be highly effective for the hydrodeoxygenation (HDO) of phenol. With regard to achieving a high selectivity to benzene, the biphasic catalytic process in the decalin/water mixed solvent was superior to the monophasic processes in either the decalin or water solvent under the same conditions. Water acted as a co-solvent that prevented the occurrence of side reactions and promoted the catalytic C–O bond scission of phenol. The synergistic effect of the biphasic solvents (decalin and water) and the Ru/Nb₂O₅-MC composite catalysts, which might stabilize the emulsions and decrease the activation energy of HDO, was investigated. Meanwhile, other probe reactions were conducted to elucidate the mechanism of the HDO of phenol. The application of the efficient biphasic catalytic process may provide a promising approach for improving lignin valorization.