(391c) Leveraging the Interplay of Structure and Support Effects to Engineer Active and Selective MoOx Catalysts for the Hydrodeoxygenation of Lignin

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 by-products 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 is primarily a result of hydrogenolysis of C-O bonds rather than direct hydrogenation of alkenes. Here, we evaluate how modulating the properties of the molybdenum active site could ameliorate this pathway for acetone hydrodeoxygenation (HDO) by synthesizing a library of catalysts across an array of metal oxide supports (Al₂O₃, SiO₂, TiO₂, and ZrO₂) at incremental MoO_x surface densities to produce a varying degree of MoO_x structures (i.e., monomeric, oligomeric, and bulk phase). The underlying MoO_x structure and active site for each catalyst were probed through a suite of characterization techniques, including ICP-OES, N₂ physisorption, XRD, Raman, UV-vis spectroscopy, and oxygen chemisorption. Consistent with previous results for the HDO of lignin models anisole and m-cresol, the per-site acetone HDO activity exhibited a linear increase with MoO_x surface density on the low-electronegative ZrO₂ supports and a decrease with support cation electronegativity for high-surface density catalysts (~4.5 Mo atoms/nm²). However, the broader scope of this study revealed that the support and structure effects are not isolated, as the optimal MoO_x structure for HDO activity is a function of the underlying support electronegativity (Figure 1a). This result implies that oligomerized MoO_x structures and the support cation work in parallel to deplete electrons from active Mo centers. Critically, 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(Figure 1b), with Al₂O₃ providing the minimal relative alkane selectivity (~4%).