Hydrogenolysis of Propionic Acid to 1-Propanol Using a Bimetallic Pd-Re/SiO2 Catalyst

The hydrogenolysis of carboxylic acids to more reduced compounds like alcohols and aldehydes is an important transformation in the production of biomass derived chemicals and fuels. Fatty alcohols, for example, can be derived from the hydrogenolysis of biologically produced fatty acids for use in plasticizers, lubricants, detergents, and other bulk chemicals. In this work, the structure of a SiO₂ supported 1 wt% Pd, 8 wt% Re catalyst was investigated using in-situ X-ray absorption spectroscopy (XAS), electron microscopy, steady-state reaction kinetics, and steady-state isotopic transient kinetic analysis (SSITKA) for the gas-phase hydrogenolysis of propionic acid. Reactions were carried out at 413-453 K under 0-3 atm H₂ and 0-0.05 atm propionic acid in a packed-bed flow reactor. The fresh catalyst consisted of highly-dispersed Re particles ~1nm in size, while the Pd particles were somewhat larger, 3-4 nm in size. Reaction conditions did not significantly affect the particle size. The catalyst demonstrated 80-90% selectivity to 1-propanol, with minor amounts of propionaldehyde, ethane, and propane. The XAS near-edge spectra demonstrated that Re was partially reduced to an intermediate oxide between Re⁺³-Re⁺⁵ during pretreatment and gas-phase reaction. Analysis of the transient kinetic response suggested that a majority of Re on the catalyst is active for the hydrogenolysis reaction, and that intermediate Re oxides are active for the hydrogenolysis of carboxylic acids to alcohols. The hydrogenolysis reaction demonstrated a first-order rate dependence on H₂, and a zeroth-order dependence on propionic acid. An inverse kinetic isotope effect of 0.7 was observed when D₂ was substituted for H₂ during the reaction, suggesting the rate limiting step is unlikely to be a hydrogen addition step.