Enhanced Hydrodeoxygenation of m-Cresol over Bimetallic Pt–Mo Catalysts through an Oxophilic Metal-Induced Tautomerization Pathway

Abstract

Supported bimetallic catalysts consisting of a noble metal (e.g., Pt) and an oxophilic metal (e.g., Mo) have received considerable attention for the hydrodeoxygenation of oxygenated aromatic compounds produced from biomass fast pyrolysis. Here, we report that PtMo can catalyze m-cresol deoxygenation via a pathway involving an initial tautomerization step. In contrast, the dominant mechanism on monometallic Pt/Al2O3 was found to be sequential Pt-catalyzed ring hydrogenation followed by dehydration on the support. Bimetallic Pt10Mo1 and Pt1Mo1 catalysts were found to produce the completely hydrogenated and deoxygenated product, methylcyclohexane (MCH), with much higher yields than monometallic Pt catalysts with comparable metal loadings and surface areas. Over an inert carbon support, MCH formation was found to be slow over monometallic Pt catalysts, while deoxygenation was significant for PtMo catalysts even in the absence of an acidic support material. Experimental studies of m-cresol deoxygenation together with density functional theory calculations indicated that Mo sites on the PtMo bimetallic surface dramatically lower the barrier for m-cresol tautomerization and subsequent deoxygenation. The accessibility of this pathway arises from the increased interaction between the oxygen of m-cresol and the Mo sites in the Pt surface. This interaction significantly alters the configuration of the precursor and transition states for tautomerization. A suite of catalyst characterization techniques including X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR) indicate that Mo was present in a reduced state on the bimetallic surface under conditions relevant for reaction. Overall, these results suggest that the use of bifunctional metal catalysts can result in new reaction pathways that are unfavorable on monometallic noble metal catalysts.

Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena‐Based Catalysts

“…It has been reported that reduced Mo 4+ sites play an important role in the adsorption of hydrogen, whereas Mo 5+ species are important for the adsorption of oxygenated substrates . In our work, we suggest that oxygen vacancies (i.e., reduced MoO x species: Mo 4+ and Mo 5+ ), where the reactants can be activated, are created primarily upon reduction . The UV/Vis spectra of fresh and reduced catalysts validate the generation of oxygen defects on the catalytic surface (Figure S6), as the relatively increased absorption in the visible region ( λ ≈400–800 nm) indicates the presence of oxygen vacancies .…”

“…[50] In our work, we suggest that oxygen vacancies (i.e.,r educed MoO x species:M o 4+ + and Mo 5+ + ), where the reac-tants can be activated, are created primarily upon reduction. [51] The UV/Vis spectra of fresh and reduced catalysts validate the generation of oxygen defects on the catalytic surface (Figure S6), as the relativelyi ncreased absorption in the visible region( l % 400-800 nm) indicates the presence of oxygen vacancies. [52] There are three different types of surface oxygen centers that can be reduced to create oxygen vacancies;t erminal (i.e.,l inked to one Mo atom), asymmetric bridging (i.e.,l ocated asymmetrically between two Mo atoms), and symmetric bridging (i.e.,l ocated symmetrically between two Mo atoms).…”

Directing Reaction Pathways through Controlled Reactant Binding at Pd–TiO2 Interfaces

“…[36][37][38] As with the model reactants, we observed high HDO selectivity using the Pd@TiO 2 -WI ( Figure 6). [36][37][38] As with the model reactants, we observed high HDO selectivity using the Pd@TiO 2 -WI ( Figure 6).…”

Zirconium Phosphate Heterostructures as Catalyst Support in Hydrodeoxygenation Reactions

“…In a later study, the presence of Mo(V) species on the surface of the catalyst was found to act as a Lewis acid site that weakened the C-O bond upon adsorption of the molecule on the active site [25]. The influence of Mo in HDO reactions indicates that it increases the selectivity toward deoxygenated products shown by PtMo catalyst with respect to the monometallic one, where the Mo seems to introduce a new preferred pathway by changing the kinetic barriers [26]. So these studies suggest that molybdenum-based catalysts have unique properties that make them suitable for use as HDO catalysts.…”