Hydrogenation of dicarboxylic acids to diols over Re-Pd catalysts

A Re-Pd/SiO2 (Re/Pd = 8) catalyst was applied to hydrogenation of dicarboxylic acids (succinic acid, glutaric acid and adipic acid) to diols. In the hydrogenation of dicarboxylic acids, ex situ liquid-phase (in only 1,4-dioxane solvent) reduced Re-Pd/SiO2 showed much higher activity than in situ liquid-phase (in the mixture of dicarboxylic acid and 1,4-dioxane) and gas-phase reduced ones, in which the in situ liquid-phase reduced catalyst has been reported to show good activity in the hydrogenation of monocarboxylic acids. High diol yields (71-89%) were achieved in the hydrogenation of dicarboxylic acids on the ex situ liquid-phase reduced catalyst at 413 K. Lactones and hydroxycarboxylic acids were first formed as intermediates in the reaction of C4-C5 and >= C6 dicarboxylic acids, respectively. Characterization using XRD, XPS and XAS indicates that ex situ liquid-phase reduced catalysts with high activity contains comparable amounts of Re-0 and Ren+ species, both of which have been reported to be necessary for good performance. The amount of Ren+ species on the in situ liquid-phase reduced catalysts is much larger than that of surface Re-0 species. This result suggests that the presence of dicarboxylic acids suppresses the reduction of Re species to Re-0 on the calcined catalysts while that of monocarboxylic acids does not, which leads to the low activity in the hydrogenation of dicarboxylic acids on in situ liquid-phase reduced catalysts.