Reaction Kinetics and Mechanism for the Catalytic Reduction of Propionic Acid over Supported ReO_x Promoted by Pd

Abstract

Silica-and titania-supported Pd, Re, and Pdpromoted Re catalysts were prepared by incipient wetness impregnation and characterized using X-ray diffraction and H 2 chemisorption. The rate of catalytic reduction of propionic acid in H 2 to predominantly form propanal and propanol over the Recontaining catalysts was insensitive to propionic acid pressure and 0.6 order in H 2 pressure. The apparent activation barriers of propionic acid reduction over PdRe/SiO 2 and PdRe/TiO 2 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 SiO 2 and TiO 2 , 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 SiO 2 -supported PdRe catalyst was identified by in situ infrared spectroscopy and transient kinetic analyses. This reservoir of propoxy species was not present on the TiO 2 -supported catalyst. Thus, turnover frequencies and coverages of reactive intermediates over Pd-promoted Re/TiO 2 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 ReO x 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 (CH 3 CH 2 COOH) to form a CH 3 CH 2 CH(OH)(ORe) diol-like intermediate that subsequently dehydroxylates/dehydrates to form propanal (CH 3 CH 2 CHO). Propanal can then be hydrogenated to yield propanol (CH 3 CH 2 CH 2 OH). 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 ReO x and catalyze the dehydration of the diol). The close proximity between Pd and ReO x is desired for facile C−H formation reactions to enable hydrogen to be transferred from Pd sites to vicinally bound oxygenates on Re sites. Langmuirianmicrokinetic 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.

Advances in Understanding the Selective Hydrogenolysis of Biomass Derivatives

“…For example, computational studies for hydrogenolysis reactions of carboxylic acids and esters propose these reactions involve kinetically relevant C–O bond cleavage within dehydrogenated intermediates, , which would likely result in rates that are inhibited by increasing hydrogen pressures. Yet, experimental evidence shows consistently that C–O bond cleavage rates in acids and esters increase with hydrogen pressure. , These positive hydrogen pressure dependencies suggest kinetically relevant hydrogen addition to an oxygenate (or a fragment of the original molecule) or C–O bond cleavage within partially hydrogenated intermediates . Beyond reactions of acids and esters, multiple discrepancies remain among descriptions for polyol hydrogenolysis reactions.…”

“…162 Recent findings bring further insight into the origins of the promotional effects of Re for hydrogenolysis of carboxylic acids, as demonstrated through the combination of kinetic measurements, in situ infrared spectroscopy, and DFT to investigate the reaction mechanism and site requirements for the hydrogenolysis of propionic acid over PdRe/SiO 2 and PdRe/TiO 2 catalysts. 156 Rates of C−O bond hydrogenolysis within propionic acid to yield propanal increase with a sublinear dependence on hydrogen pressure and are nearly independent of acid pressure (-r propionic acid ∼ [H 2 ] 0.6 [C 3 H 6 O 2 ] 0.1 ) over PdRe catalysts (0.15−0.55 kPa C 3 H 6 O 2 , 25−100 kPa H 2 , 433 K). Substitution of H 2 with D 2 leads to greater rates of propionic acid hydrogenation and exhibit an inverse kinetic isotope effect (k H /k D = 0.79 over PdRe/SiO 2 and PdRe/TiO 2 ) at standard conditions (0.5 kPa C 3 H 6 O 2 , 100 kPa H 2 or D 2 , 433 K).…”

“…The pathways that cleave C–C bonds produce alkanes with one less carbon than the original alkyl fragment and release carbon dioxide (CO 2 ) (Scheme B) or CO (Scheme C). Short-chain carboxylic acids, such as acetic acid , and propionic acid, − are frequently used model reactants within experimental and computational kinetic studies because of their relative simplicity and abundance in biomass pyrolysis oils . Longer-chain carboxylic acids derived from triglyceride hydrolysis are deoxygenated for production of long-chain hydrocarbons as drop-in fuels for gasoline and jet fuel. , …”

“…Neurock et al compared E a for acetic acid hydrogenolysis by DFT over Pd(111), Re(0001), and a pseudomorphic monolayer of Pd(111) on Re(0001) and found that the addition of metallic Re to Pd decreases the E a for C–OH cleavage . Recent findings bring further insight into the origins of the promotional effects of Re for hydrogenolysis of carboxylic acids, as demonstrated through the combination of kinetic measurements, in situ infrared spectroscopy, and DFT to investigate the reaction mechanism and site requirements for the hydrogenolysis of propionic acid over PdRe/SiO 2 and PdRe/TiO 2 catalysts . Rates of C–O bond hydrogenolysis within propionic acid to yield propanal increase with a sublinear dependence on hydrogen pressure and are nearly independent of acid pressure (- r propionic acid ∼ [H 2 ] 0.6 [C 3 H 6 O 2 ] 0.1 ) over PdRe catalysts (0.15–0.55 kPa C 3 H 6 O 2 , 25–100 kPa H 2 , 433 K).…”

Rhenium – A Tuneable Player in Tailored Hydrogenation Catalysis

“…The as‐formed aldehyde can undergo hydrogenation to yield the correspondent alcohol. In the presence of a second metal, such as palladium or platinum, the second metal facilitates the reaction as it readily dissociates H 2 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) [189,190] . A strong synergy effect was also observed between Fe and Re for the hydrogenation of levulinic acid in water under mild conditions.…”