Alloyed molybdenum enables efficient alcohol hydrodeoxygenation with supported bimetallic catalysts

Abstract

Bimetallic heterogeneous catalysts combining group 9 metals (Rh, Ir) or group 10 metals (Ni, Pd, Pt) with Mo on a silica-based support have been synthesized via surface organometallic chemistry and assessed in their catalytic activity for the hydrodeoxygenation (HDO) of alcohols with particular emphasis on the structural evolution of the catalysts and the role of Mo. The investigation was conducted with an air-free approach to isolate any sample alterations exclusively to those caused by the reaction. Structural analysis was performed using a combination of (S)TEM, IR, and XAS. It was found that Ir–Mo/SiO₂, Rh–Mo/SiO₂, and Pt–Mo/SiO₂ display high activity for primary, secondary, and tertiary alcohol deoxygenation, while Pd–Mo/SiO₂ selectively catalyses tertiary alcohol deoxygenation. Other combinations as well as the corresponding monometallic materials do not display the same activity. X-ray absorption spectroscopy confirmed metallic states for M (M = Ni, Rh, Pd, Ir, or Pt), while Mo K-edge XANES showed varying amounts of Mo(0), Mo(IV) and Mo(VI) depending on the metal counterpart in fresh materials, and indicated complete conversion of Mo(VI) to lower oxidation states (IV and 0) during the reaction. For Rh, Pd, Ir, and Pt, alloy formation (M–Mo) was identified via M–Mo paths in EXAFS and supported by CO-IR spectroscopy. In contrast to Ir, Rh, and Pt, where some Mo(0) is present at the nanoparticle surface, Pd–Mo forms an alloy but likely retains Mo in the nanoparticle core, as suggested by CO-IR spectroscopy and CO-chemisorption. Reactivity studies suggest that tertiary alcohols primarily undergo dehydration–hydrogenation, evidenced by olefin formation with MoO_x/SiO₂, as well as Ir/SiO₂ and Ir–Mo/SiO₂ under inert conditions. In contrast, primary and secondary alcohols follow a different mechanism, correlated with the presence of metallic Mo species on the nanoparticle surface, highlighting their role in C–O bond activation. These findings provide new insights into the structure–activity relationships of Mo-based bimetallic catalysts, underscoring the influence of Mo in different oxidation states and strong substrate dependence on mechanistic pathways.