(715a) 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.