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

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 prereduction 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 carboxylic acid reduction to alcohols revealed that at least 50% of the Re in the bimetallic catalyst participated in the catalytic reaction. The Pd is proposed to enhance the catalytic activity of the bimetallic catalyst by spilling over hydrogen that can partially reduce Re and react with surface intermediates