A Step Toward Sustainability: Mechanisms of Hydrodeoxygenation Reactions over a Metal-Metal Oxide Catalyst

Abstract

In recent years, metal-metal oxide catalysts have proven to be robust catalysts for hydrodeoxygenation (HDO) of oxygenated compounds derived from bio-renewable feedstocks to value-added products. Herein, the conversion of 1,2,6-hexanetriol (1,2,6-HT) to 1,6-hexanediol (1,6-HD) in aqueous media over a Pt-WOx/TiO2 catalyst is examined via isotope incorporation in HDO of a model compound, 1,2-pentanediol (1,2-PD). Absence of a primary kinetic isotope effect (kH/kD = 0.84 ± 0.11) disproves a possible direct C‒O scission mechanism. The observation of nearly complete deuterium incorporation in both the α-C and the β-C is inconsistent with the reverse Mars-van Krevelen mechanism and suggests an enol formation pathway that has not been proposed for HDO reactions of this type until now. Evidence consistent with the intermediacy of an oxocarbenium ion as a minor contributor has also been observed. In drawing the conclusions, it was necessary to characterize the facile isotope exchange between surface activated hydrogen and the water solvent. Hydrogenation of a water soluble olefin, tetra(ethylene glycol) diacrylate (TEGDA) in H2/D2O revealed predominant incorporation of deuterium instead of hydrogen in the reduced product, confirming the rapid exchange of surface activated hydrogen. The methods used in this study provide a new perspective for a reaction mechanism currently under debate, and these findings can be applied to other systems involving HDO of linear polyols over metal-metal oxide catalysts, improving catalyst design and utilization of sustainable feedstocks.