Study on preparation of cyclohexanol from lignin-derived phenolic compounds catalyzed by metal oxide-loaded ruthenium

Hydrodeoxygenation of lignin bio-oil to prepare liquid fuels is a very promising route. In this paper, a series of catalysts (Ru/CeO₂, Ru/Nb₂O₅, Ru/ZrO₂, Ru/Al₂O₃ and Ru/CeO_x) supported on metal oxides were prepared by incipient wetness impregnation method, which were used to study the upgrading and hydrogenation of lignin-derived phenolic compounds phenol to cyclohexanol. By means of X-ray crystal diffraction (XRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS), the structure and physical and chemical characteristics of the prepared catalyst were characterized. It was found that the oxygen vacancies contained in Ru/CeO_x could adsorb the raw materials with oxygen groups well, which was beneficial to the efficient hydrogenation of phenol. At the same time, XPS showed that the effective active centers in Ru/CeO_x, RuO₂ and Ru⁰, were active sites for catalytic hydrogenation. Therefore, the combined action of oxygen vacancies and metal active sites made the catalyst have good hydrogenation activity. The effects of reaction temperature, pressure and time on hydrogenation were also investigated. It was found that the catalyst could completely convert phenol at a mild temperature (140 ℃) and the yield of cyclohexanol was 90.2%. The cycle characteristics of the catalyst were investigated, and it was found that the catalyst still showed excellent hydrogenation activity after being recycled for 4 times. At the same time, the intermediate products in the hydrogenation process were detected by GC-MS, and then the reaction path of phenol hydrogenation process was deduced.