(173a) Effects of Domain Size and Support Composition on the Reactivity and Reducibility of Oxide-Supported Tungsten Oxide Clusters

Carboxylic acids are building block chemicals that can be converted into a variety of high value products. Undesired carboxylic acids produced from pyrolysis processes and high fatty acid composition in seed oils can be reduced to fatty alcohols with applications in detergents, lubricants, and high-performance oils for jet engines. Hence, cost-effective conversion of carboxylic acids is desirable. Supported metal oxides have been widely studied as a catalyst for biomass conversion. Although Pd-promoted ReO_x has been found to be an efficient catalyst for reduction of carboxylic acids to alcohols [1], the high volatility and cost of ReO_x make it difficult to scale for large industrial processes. Here, we explore the reactivity and reducibility of supported tungsten oxides, a low cost and high stability alternative, to understand its applicability as a catalyst for conversion of carboxylic acids.

We construct molecular models of different size WO_x clusters on two different supports - a reducible metal oxide, Titania, and an irreducible metal oxide, Silica. To understand the behavior of the tungsten oxide catalyst for reduction of acids using H₂, which dissociates on Pd and spills over to the WO_x, we used Density Functional Theory calculations to compare the acidity, reducibility, and thermodynamic stability of monomers, dimers, and monolayers of WO_x on each support and compare them to the properties of bulk WO₃. Using a global optimization scheme, we determined the most thermodynamically stable structure for different cluster sizes and stoichiometries. Our results indicate that smaller sizes of WO_x clusters require significantly more energy to be reduced than the WO₃ bulk, have higher acidity (evaluated by deprotonation energies), and computed band gaps for monomers range from 4.9 to 5.7 eV, consistent with experimental reports [2]. Our results are consistent with temperature programmed reduction experiments performed on variable loadings of WO_x supported on Silica and Titania.