Identification of Sabatier Descriptors for Hydrodeoxygenation Activity and Selectivity on Supported Molybdenum Oxide Catalysts

Abstract

While molybdenum oxide shows promise in deoxygenating lignin monomers to petrochemically relevant aromatics and alkenes, its current applicability is hindered by its tendency to oversaturate the aliphatic byproducts to alkanes, limiting the ability of the product stream to be directly integrated into the existing infrastructure. Previously, detailed kinetic experiments indicated that this parasitic alkane pathway can result from competitive C–O hydrogenolysis during deoxygenation rather than direct hydrogenation of alkenes. Here, we evaluate how modulating the properties of the molybdenum active site could ameliorate this pathway for short-chain (<C₅) carbonyl hydrodeoxygenation (HDO) by synthesizing a library of catalysts across an array of metal oxide supports (Al₂O₃, Nb₂O₅, SiO₂, TiO₂, and ZrO₂) at incremental MoO_x surface densities to alter the degree of two-dimensional MoO_x oligomerization. The study reveals that MoO_x structure sensitivity for oxygen removal highly depends on the supporting metal oxide. Notably, the electronegativity of the support and the MoO_x structure alter the electronic density of the average Mo active site (as quantified by the terminal Mo=O bond Raman shift) in parallel, leading to a Sabatier relationship between oxygen adsorption strength and the overall rate of oxygen removal. Conversely, this combinatorial MoO_x structure/support effect does not apply to the selectivity between the alkane and alkene products. Instead, the support appears to be the primary driver of the competitive hydrogenolysis pathway, with the support’s point of zero charge (PZC) being an apparent Sabatier descriptor for the relative alkane selectivity, implying the bridging Mo–O-support bond as the hydrogenolysis site. Interestingly, the Sabatier optimum for the hydrogenolysis pathway is reactant dependent as a shift to stronger binding on higher PZC supports occurs for molecules with less stable intermediates like the more lignin-relevant aldehyde molecules.