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.

Although butadiene is currently a byproduct of naphtha cracking, interest in producing butadiene from biobased ethanol has increased because of the lower environmental impact of the ethanol to butadiene reaction. This work explores a multifunctional catalyst system composed of silica-supported Ag and ZrO2 used for the cascade reaction of ethanol to butadiene at 573 K. The Ag and ZrO2 components were synthesized on separate support particles, enabling the characterization of each component without interference from the other. High selectivity to butadiene (65%) at high ethanol conversion (75%) was achieved with an appropriate ratio of Ag and ZrO2 in the reactor. Silver catalyzed the initial dehydrogenation of ethanol to acetaldehyde, while ZrO2 catalyzed the C–C coupling and subsequent dehydration reactions. The silica-supported ZrO2 exhibited superior selectivity relative to bulk ZrO2 in the Ag-promoted ethanol to butadiene reaction. Results from Zr K-edge X-ray absorption spectroscopy and UV–vis spectroscopy showed that ZrO2 was highly dispersed on the silica support over a range of loadings. Infrared spectroscopy of adsorbed pyridine, CO, and CO2, and kinetics of probe reactions 1-butene double bond isomerization, 2-propanol decomposition, and ethanol hydrogenation of acetone were used to compare the acid–base nature and chemical reactivity of silica-supported ZrO2 to bulk ZrO2.

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The mechanism of ethanol upgrading to higher products is still under debate, especially regarding intermediate species and hydrogenation and dehydrogenation steps. In this work, we conducted a combined theoretical and experimental approach to contribute to this discussion. For such, detailed electronic structure density functional theory calculations (aiming at probing density of states, infrared spectra, geometric parameters, charge densities, and reaction energetics) and diffuse reflectance infrared Fourier transform spectroscopy experiments were carried out revealing the relevance of an appropriate combination of reactive surface sites to support the formation of several intermediates that are formed in the C–C coupling over MgO. The roles of Mg and O sites were also studied under an electronic perspective and different geometrical arrangements. We found that a kink configuration was the most adequate for ethanol to 1-butanol upgrading. Our calculations also gave us arguments to propose distinct reaction routes, whose mutual predominance would depend upon reaction temperature. At temperatures up to 573 K, the so-called β-route, which goes through scission of a Cβ–H bond and formation of an oxametallacycle-like intermediate, would dominate the coupling, whereas at higher temperatures, up to 673 K, a more usual Guerbet mechanism, via an aldol coupling step and then consecutive hydrogenations, would be expected. The theoretical conclusions were followed by a careful experimental strategy using sequential experimental planning techniques in order to estimate accurate parameters with the lowest possible experimental load. Information from these different sources was coupled to develop a mathematical model for the rate of the ethanol upgrading reaction, using a Langmuir–Hinshelwood–Hougen–Watson approach. The developed and statistically validated model adequately described the experimental data at 673 K and 1.1 bar total pressure for ethanol partial pressures in the range from 0 to 20 kPa.

Silica‐, titania‐, and zirconia‐supported tungsten oxide catalysts were synthesized by wetness impregnation techniques. When promoted with Pd, these materials catalyzed the reduction of propanoic acid to 1‐propanol at 433 K with a selectivity of up to 92 % (13.5 % conversion) in atmospheric pressure of H2. Over Pd‐promoted WOx/TiO2, the observed orders of reaction were 0.2 in H2 and 0.7 in propanoic acid, and the apparent activation energy was 54 kJ mol−1. In situ X‐ray absorption spectroscopy of Pd‐promoted WOx/SiO2 revealed a slight reduction of the W from +6 to an average oxidation state of about +5 during H2 treatment above 473 K. In situ infrared spectroscopy indicated the catalyst surface was covered mostly by propanoate species during reaction. For comparison, supported phosphotungstic acid was also evaluated as a catalyst under identical conditions, but the resulting high acidity of the catalyst was deleterious to alcohol selectivity.