(629a) Predicting an Optimal Oxide/Metal Interface Catalyst for Hydrodeoxygenation Chemistry of Biomass Derivatives

Selective C-O cleavage is the most difficult chemical transformation en route to fuels production from biomass derivatives such as furfuryl alcohol. Metal oxide-metal interfaces have recently been used to manipulate catalytic selectivity in such multistep reactions, and also hinder non-selective decarbonylation (DCO, C-C activation) or aromatic ring hydrogenation. Palladium nanoparticles encapsulated by porous TiO₂ show high selectivity and activity towards hydrodeoxygenation (HDO) and minimal DCO [1]. Our interfacial model of TiO₂/Pd core-shell catalyst in the form of rutile TiO₂ (110) nanowire over Pd (111) provides qualitative mechanistic determination of the role of interfacial active sites towards deoxygenation and decarbonylation. A much-lowered barrier to HDO is obtained over TiO_2-x/Pdoxygen deficient interface model relative to supported Pd catalysts (0.20 eV barrier vs 0.95 eV on Pd (111)).

The farther objective is, however, to predict an optimal combination of oxides and metal catalyst with interfacial properties to match the combined hydrogenation and redox requirements of HDO. The search relies on optimising different descriptors that influence the synergy between the oxide and metals’ functionalities for the studied reaction. Since oxygen vacancies at the interface activate the C-O bond of the hydroxylated reactants with the alcohol group filling the vacancies in a reverse Mars-van Krevelen mechanism, the HDO activity is correlated with the reducibility of the oxide. The choice of the oxide/metal interface is also governed by tweaking the metal oxide and the metal properties in terms of descriptors like metal–oxygen bond strength, metal-carbon binding energy, metal’s workfunction or its relative gap with the d-band centre. For example, metal-carbon bond dictates the binding of the organic fragments after HDO, thereby driving the enthalpy of the reaction. Furthermore, the interface should be equally capable of activating hydrogen to provide a hydrogen environment and complete the final hydrogenation cycle to 2-methylfuran (a potential fuel additive). Herein, we use density functional theory (DFT) to study the elementary surface reactions of furfuryl alcohol over the oxide/metal interface model comprised of metals with varying carbon adsorption energies and oxides’ nanowire with varying reducibilities (or tuned by dopants). The mechanistic study is also extended to predict deoxygenation of other stringent oxygenates like m-cresol, phenols etc. with the inclusion of descriptors that account for structural differences and differing C-O bond enthalpies of these oxygenated species. This intends to establish more generic correlations aimed to optimise the overall HDO chemistry, hydrogen activation and subsequent hydrogenation of different oxygenated biomass.

1. Zhang, J., et al., Directing Reaction Pathways through Controlled Reactant Binding at Pd–TiO₂ Interfaces. Angewandte Chemie, 2017. 129(23): p. 6694-6698.