A Mechanistic Study of Polyol Hydrodeoxygenation over a Bifunctional Pt-WOx/TiO2 Catalyst

In recent years, metal-metal oxide catalysts have proven to be robust catalysts for hydrodeoxygenation (HDO) of oxygenated compounds derived from biorenewable 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 (k(H)/k(D) = 0.84 +/- 0.11) disproves a potential direct C-O bond scission mechanism. The observation of nearly complete deuterium incorporation in both the alpha-C and the beta-C is inconsistent with the reverse Mars-van Krevelen mechanism and suggests an enol formation pathway. 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 H-2/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 clarification about 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.