Characterization of Re-Pd/SiO2 Catalysts for Hydrogenation of Stearic Acid

Silica-supported Re-Pd bimetallic catalysts (Re-Pd/SiO2) with a high molar ratio of Re/Pd, which were reported to be effective for selective hydrogenation of carboxylic acids to the corresponding fatty alcohols in 1,4-dioxane solvent, were characterized by means of X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and CO adsorption. Various kinds of Re species (hexagonal closed packing (HCP) and face-centered cubic (FCC) Re metals, Re3+, Re4+, and Re6+) were detected on the catalysts after reaction or reduction, and the ratio of these Re species was estimated by the combination of characterization results. The activity of these catalysts is sensitive to air because of the high oxophilicity of Re, and the catalysts must be handled without contact to air. Pd addition and catalyst activation method (liquid-phase reduction and gas-phase reduction) influenced the ratio of the Re species. Liquid-phase reduced Re-Pd/SiO2 (Re/Pd = 8), which is the most effective catalyst, has Pd-0, Re-0, and Ren+ (Re3+ and Re4+) species on the catalyst, and the metal surface (Pd-0, Re-0(HCP), Re-0(FCC)) is modified with Ren+ species. This structure will be responsible for the high hydrogenation activity. Combined with kinetic studies with Re-Pd/SiO2 (Re/Pd = 8) and Re/SiO2 catalysts, Pd plays a role in promoting the reduction and dispersion of Re species, as well as strengthening the interaction of stearic acid with the catalytic surface, and on the other hand, Ren+ plays a role in promoting the heterolytic dissociation of H-2.