(254g) Dual Role of Reducible Transition Metal Oxide Promoters on Pt Catalysts for Phenolic Hydrodeoxygenation

Catalytic hydrodeoxygenation (HDO) of lignin-derived phenolics is a promising route towards sustainable aromatic feedstocks. Inverse catalysts consisting of transition metal oxide domains on top of metal nanoparticle surfaces have exhibited remarkable (>96%) selectivity towards deoxygenated aromatics [1]. Our recent work [2] clarified that for WO_x/Pt/SiO₂ catalysts, selectivity enhancement to aromatics is partly ascribed to the dynamic decoration by WO_x of Pt surface sites, thus suppressing undesired aromatic hydrogenation. However, the role of WO_x species in deoxygenation has remained unclear.

Here, we address this uncertainty through an MO_x/Pt/SiO₂ catalyst series (M = W, Mo, Nb, Ti) utilizing MO_xspecies with a broad range of chemical properties proposed to be relevant to HDO. By quantifying the extent of decoration by CO pulse chemisorption and relating this to reaction performance in the HDO of 4-propylphenol (PHE), we find that aromatic hydrogenation of the PHE substrate is correlated with the extent of MO_x decoration on top of Pt, regardless of MO_x identity. Consistent with our prior work, this indicates that suppression of aromatic hydrogenation is a steric effect, mediated through MO_x domains blocking Pt sites which would otherwise perform hydrogenation. On the other hand, direct deoxygenation to 4-propylbenzene (BNZ) exhibits a complex dependence on both MO_x identity and coverage.

By comparing characterization results (H₂-TPR, NH₃-TPD) to reaction performance, we find that neither oxygen vacancies nor acid sites explain observed deoxygenation activity. Instead, BNZ formation correlates well with the Lewis acidity of MO_x cations at the MO_x-Pt interface, analogous to trends known for CO_x hydrogenation. Thus, we propose that interaction between adsorbed intermediates and Lewis acidic MO_x-Pt interfaces promotes C-O bond cleavage, clarifying the dual role of MO_x in inverse catalysts for HDO.

References

[1] Wang, C. et al. ACS Catal. 2018, 8, 9, 7749-7759.

[2] Marlowe, J. et al. JACS. 2024, 146, 20, 13862-13874.