(157a) Supported Pt-Re Catalyst for Selective Hydrogenolysis and Hydrogenation Reactions
The addition of Re to a supported Pt catalyst promotes the rate of glycerol hydrogenolysis to propanediols in liquid water by more than an order of magnitude and shifts the initial product selectivity from predominantly terminal hydrogenolysis to internal hydrogenolysis. In this work, conventional XPS and near ambient pressure XPS were used to prove that a range of Re oxidation states was present on the Pt-Re catalysts after reduction in H2 at 393 and 493 K, even though reduction of Re is enhanced by the Pt. The DRIFTS of adsorbed pyridine and the aqueous phase hydrolysis of propyl acetate were used to identify and quantify Brønsted acid sites that were presumably associated with the Re component of the bimetallic catalyst. The mechanism of glycerol hydrogenolysis is proposed to involve acid-catalyzed dehydration followed by Pt-catalyzed hydrogenation of the unsaturated intermediate. This proposed mechanism for glycerol hydrogenolysis is supported by kinetic results after selective neutralization of the acid sites, observation of a primary kinetic isotope effect with deuterated glycerol, and measurement of the orders of reaction with respect to glycerol and dihydrogen. A Pt-Re sample also catalyzed the hydrogenation of C=C and C=O bonds in methyl vinyl ketone, crotonaldehyde, 2-butanone, and butanal in liquid water. The relative rates of C=C and C=O bond hydrogenation over Pt-Re were significantly faster than C-O bond hydrogenolysis, supporting the mechanistic hypothesis that a hydrogenation step in the glycerol hydrogenolysis path is kinetically irrelevant. Interestingly, oxophilic Re can be used to promote the Pt-catalyzed hydrogenation of carbonyl groups in multifunctional molecules.