The Reduction of Carboxylic Acids to Alcohols over Palladium-Promoted Rhenium and Tungsten Oxides

The direct catalytic reduction of carboxylic acids is an important reaction in the conversion of biomass-derived chemicals to useful products. The combination of metal function and a supported metal oxide can produce a catalyst that is capable of performing this reaction. The performance of the resulting catalyst depends strongly on the selection of both the metal and the metal oxide. Recent studies using metal-promoted rhenium oxide catalysts to reduce carboxylic acids to alcohols demonstrated activity at significantly lower H2 pressures than would be required using traditional copper-based catalysts. The active state of metal-promoted rhenium catalysts used for the direct reduction of carboxylic acids remains elusive, which has hindered the development of a mechanistic understanding of these types of catalysts.
In this dissertation, palladium-promoted rhenium oxide (Pd-promoted Re) catalysts supported on SiO2 were synthesized using a stepwise incipient wetness impregnation method. The SiO2-supported, Pd-promoted Re catalysts (Pd/SiO2, Re/SiO2, and PdRe/SiO2) were characterized using electron microscopy, X-ray absorption spectroscopy (XAS), and probe molecule adsorption (H2, CO, and N2). Palladium supported on SiO2 was not active for the reduction of propionic acid to oxygenates, while supported Re/SiO2 was minimally active for the production of a small amount of a mixture of propanal and 1-propanol after pre-reduction of the Re/SiO2 in H2 at elevated temperature. Promotion of Re/SiO2 with Pd formed an active catalyst for propionic acid reduction without pretreatment, and resulted in the reduction of the Re component of the catalyst to an average oxidation state of 4+ as determined by XAS. Transient kinetic analysis was used to determine the turnover frequency and coverage of reactive intermediates leading to propionic acid reduction products over PdRe/SiO2, and results suggested that rapid turnover of intermediates leading to aldehydes was followed by very slow turnover of intermediates leading to alcohols on the catalyst surface.
Palladium-promoted Re/TiO2 catalysts were also synthesized using a stepwise incipient wetness impregnation method. Reaction kinetics studies of the reduction of propionic acid over PdRe supported on SiO2 and TiO2 found a 0.6 order in H2 and a 0.1-0.2 order in propionic acid pressure. The reduction of propionic acid in D2 resulted in an inverse kinetic isotope effect of 0.79, and cofeeding water had no effect on the steady-state reduction of propionic acid. On the other hand, cofeeding water did result in a transient change in the rate of 1-propanol formation over PdRe/SiO2, which was studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Results from DRIFTS revealed significant coverage of the SiO2 support with alkoxy species, which could be removed from the SiO2 surface by water or propionic acid. These propoxy species could not be detected on PdRe/TiO2 under similar conditions. Transient kinetic analysis revealed similar turnover of intermediates leading to propanal and 1-propanol on the PdRe/TiO2 surface. These results were consistent with a 2-step cascade reaction, whereby reduction of propionic acid led to the formation of propanal, which was subsequently hydrogenated to form 1-propanol.
Finally, Pd-promoted tungsten oxide (PdW) and phosphotungstic acid (PdPTA) were supported on SiO2, TiO2, and ZrO2 using wetness impregnation techniques. These catalysts demonstrated higher rates of propionic acid reduction than the previously studied Pd-promoted Re catalysts under similar conditions, and gave similar product distributions. Catalytic activity and product selectivity depended on the support, the active tungsten phase, and the pretreatment conditions. The reason for enhanced activity over Re-based catalysts and the role of acid site type and density in these catalysts warrants further study.