Relating the Structure of Bimetallic Catalysts to Performance in the Hydrogenolysis and Hydrogenation of Biomass-Derived Intermediates

The addition of a second metal to supported metallic nanoparticles produces a range of unique catalytic properties by altering the structural and chemical properties of the catalytic surface. Elucidating the structure and function of supported bimetallic nanoparticles is therefore crucial to enable the rational design of new and improved catalysts for the production of fuels and chemicals from molecular-derivatives of renewable biomass.

In this work, the presence of copper on titania-supported Ru nanoparticles was observed to increase the selectivity of glycerol hydrogenolysis to 1,2-propanediol. Characterization of catalysts by H2 chemisorption, X-ray diffraction, scanning transmission electron microscopy and X-ray photoelectron spectroscopy suggested that both Ru and Cu were present on the surface of the titania-supported bimetallic nanoparticles. By normalizing conversion rate to the number of surface Ru atoms, the presence of Cu was shown to decrease the rate of glycerol hydrogenolysis slightly while increasing selectivity to 1,2-propanediol from 48% on monometallic Ru to over 90% on Ru-Cu. Copper likely diluted large Ru ensembles that are responsible for C-C bond cleavage leading to ethylene glycol instead of the desired 1,2-propanediol. The presence of Cu also inhibited the deactivation exhibited by monometallic Ru presumably by limiting the formation of strongly adsorbed CxHy species produced on large Ru ensembles.

The addition of Re to supported Pt catalysts promoted the rate of glycerol hydrogenolysis to propanediols in liquid water by more than an order of magnitude and shifted the initial product selectivity from predominantly 1,2-propanediol to a mixture of 1,2 and 1,3-propanediols. A combination of conventional and near-ambient-pressure X-ray photoelectron spectroscopy revealed that a range of Re oxidation states, as well as metallic Pt, were present on the surface of the Pt-Re catalysts after reduction in H2 at 393 and 493 K, even though reduction of Re was enhanced by the Pt.

Infrared spectroscopy of adsorbed pyridine and the aqueous-phase hydrolysis of propyl acetate were used to identify and characterize Brønsted acid sites present on the Pt-Re bimetallic catalyst after reduction in H2 at 473 K. Normalizing the rate of propyl acetate hydrolysis over Pt-Re to the rate in aqueous HCl, enabled the quantification of approximately 6% of the Re atoms that formed catalytically-active Brønsted acid sites.

Given the observation of both reduced Pt sites and Brønsted acid sites on the bifunctional Pt-Re catalyst, a proposed mechanism of glycerol hydrogenolysis involving acid-catalyzed dehydration followed by Pt-catalyzed hydrogenation of the unsaturated intermediate was supported by the negative influence of added base, a primary kinetic isotope effect with deuterated glycerol, an inverse isotope effect with dideuterium gas, and the observed orders of reaction with respect to glycerol and dihydrogen.

The inverse kinetic isotope effect obtained with dideuterium gas indicated that hydrogenation was not a kinetically relevant step for glycerol hydrogenolysis over Pt-Re. To confirm the kinetic insignificance of C=C and C=O bond hydrogenation in the glycerol hydrogenolysis mechanism, the low temperature hydrogenation of methyl vinyl ketone, crotonaldehyde, 2-butanone, and butanal was performed over Pt-Re. The presence of Re on Pt decreased the rate of C=C bond hydrogenation but enhanced the rate of C=O bond hydrogenation. Because the rates of double bond hydrogenation were higher than the rate of glycerol hydrogenolysis, hydrogenation is most likely a kinetically insignificant component of the hydrogenolysis reaction path.