Reaction Kinetics and Mechanism for the Catalytic Reduction of Propionic Acid over Supported ReO _xPromoted by Pd

Silica- A nd titania-supported Pd, Re, and Pd-promoted Re catalysts were prepared by incipient wetness impregnation and characterized using X-ray diffraction and H2 chemisorption. The rate of catalytic reduction of propionic acid in H2 to predominantly form propanal and propanol over the Re-containing catalysts was insensitive to propionic acid pressure and 0.6 order in H2 pressure. The apparent activation barriers of propionic acid reduction over PdRe/SiO2 and PdRe/TiO2 were 60 and 75 kJ mol-1, respectively. An inverse kinetic isotope effect of 0.79 was observed for the reduction of propionic acid over Pd-promoted Re on both SiO2 and TiO2, and a normal kinetic isotope effect of 1.6 was observed for hydrogenation of propanal under similar conditions. A large reservoir of surface propoxy species that turned over very slowly on the SiO2-supported PdRe catalyst was identified by in situ infrared spectroscopy and transient kinetic analyses. This reservoir of propoxy species was not present on the TiO2-supported catalyst. Thus, turnover frequencies and coverages of reactive intermediates over Pd-promoted Re/TiO2 catalysts were probed by transient kinetic analysis, which revealed that less than 2% of the Re atoms in the material were associated with intermediates leading to products. Insights into the mechanism of propionic acid hydrogenolysis and the individual role of both ReOx and Pd were established using density functional theory calculations. Theoretical results suggest that the Re sites are covered with propionate intermediates and that hydrogenolysis proceeds with the initial rate-determining hydrogenation of propionic acid (CH3CH2COOH) to form a CH3CH2CH(OH)(ORe) diol-like intermediate that subsequently dehydroxylates/dehydrates to form propanal (CH3CH2CHO). Propanal can then be hydrogenated to yield propanol (CH3CH2CH2OH). Palladium facilitates the reaction as it readily dissociates dihydrogen to provide surface hydrides (that catalyze C-H bond formation reactions to produce the diol intermediate) and protons (Brønsted acid sites that spill over onto ReOx and catalyze the dehydration of the diol). The close proximity between Pd and ReOx is desired for facile C-H formation reactions to enable hydrogen to be transferred from Pd sites to vicinally bound oxygenates on Re sites. Langmuirian-microkinetic analyses of the theoretical results as well as kinetic isotope effect calculations on converged structures show reasonable consistency with experimental observations, supporting the proposed mechanism.