The active sites on reducible metal oxides can include both Lewis and Brønsted acid as well as redox sites depending on the crystal phase and non-stoichiometry, resulting in their ability to catalyze a wide range of chemical reactions. Elucidating the nature and reactivity of these materials requires both experimental and computational approaches. A combination of DFT calculations and adsorption and microcalorimetry of probe molecules shows that monoclinic WO3 has strong Lewis sites, whereas hex-WO3 nanowires have Bronsted acidity. We developed kinetic models to describe conversion of glucose to fructose, lactic acid and formic acid over monoclinic WO3 and further dehydration of fructose to hydroxymethyl furfural via either hex-WO3 or homogeneous Brønsted catalysts (HCl). The coupling of homogeneous and heterogeneous catalysts in a one-pot biphasic (aqueous/organic) solvent system introduces possibilities to tune selectivity among several desirable products which have been suggested computationally by the Vlachos group with beta zeolites; however, the hydrophilic nature of WO3 is an advantage over zeolites for the adsorption of polar sugars and desorption of relatively non-polar HMF. Reduction of MoO3 and WO3 in hydrogen produces non-stoichiometric bronzes, HxWO3-z, which are active for both hydrogenation and hydrodeoxygenation. The interplay between redox and acid sites is illustrated by computational (PES based on DFT cluster calculations) and experimental work on the hydrodeoxygenation of acrolein to allyl alcohol and propene over MoO3 and WO3 via a Mars van Krevelen mechanism. Rapid oxygen vacancy diffusion adds considerable complexity to the development of microkinetic models that couple surface reactions with changes in bulk composition and electronic properties.

Effects of composition and phase on catalytic sites in reducible metal oxides

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