Reduction of Propionic Acid over a Pd-Promoted ReOx/SiO2 Catalyst Probed by X-ray Absorption Spectroscopy and Transient Kinetic Analysis

Abstract

A Pd-promoted Re/SiO2 catalyst was prepared by sequential impregnation and compared to monometallic Pd/SiO2 and Re/SiO2. All samples were characterized by electron microscopy, H2 and CO chemisorption, H2 temperature-programmed reduction, and in-situ X-ray absorption spectroscopy at the Re LIII and Pd K-edges. The samples were also tested in the reduction of propionic acid to 1-propanol and propionaldehyde at 433 K in 0.1-0.2 MPa H2. Whereas monometallic Pd was inactive for carboxylic acid reduction, monometallic Re catalyzed aldehyde formation, but only after high-temperature pre-reduction that produced metallic Re. When Pd was present with Re in a bimetallic catalyst, Pd facilitated the reduction of Re in H2 to ~4+ oxidation state at modest temperatures, producing an active catalyst for the conversion of propionic acid to 1-propanol. Under the conditions of this study, the orders of reaction in propionic acid and H2 were approximately zero and one, respectively. Transient kinetic analysis of the carboxyl...