(734f) Role of the Support and Reaction Conditions on the Vapor-Phase Deoxygenation of M-Cresol over Pt/C and Pt/TiO2 Catalysts

1. Introduction

Biomass deconstruction using fast pyrolysis offers a promising route for the production of renewable bio-oil. However, the high oxygen content of bio-oil contributes to a number of undesirable characteristics, and bio-oil must be upgraded before it is suitable for use as a drop-in transportation fuel or blendstock.¹Ex-situ catalytic fast pyrolysis (CFP) provides a route for bio-oil upgrading in which pyrolysis vapors are catalytically deoxygenated in the presence of hydrogen prior to condensation.^2-4 Within our group, we are exploring a dual fixed-bed ex-situ CFP reactor system because it offers greater catalyst flexibility over a fluidized bed system, thus providing greater control over chemistry and product composition.³ In the first fixed-bed reactor, we are specifically targeting the deoxygenation of recalcitrant phenolic compounds. However, a clear understanding of the mechanistic details for the deoxygenation of these compounds has yet to be achieved, and questions remain about the role of the support (especially for reducible metal oxides) and effect of reaction conditions. To probe these questions, the vapor-phase deoxygenation of m-cresol, a model compound representative of the lignin-derived components contained within bio-oil, was investigated over Pt/C and Pt/TiO₂ catalysts using a combination of experimental and computational techniques.⁵

 2. Experimental

Carbon- and TiO₂-supported Pt were prepared from Pt(NH₃)₄(NO₃)₂ via standard incipient-wetness impregnation methods. Each catalyst was characterized using powder X-ray diffraction, NH₃ temperature programmed desorption, inductively coupled plasma optical emission spectroscopy, transmission election microscopy, N₂ physisorption, and CO pulse chemisorption. The performance of each catalyst was evaluated in a packed bed reactor under conditions representative of hydrotreating (HT; 250 °C, 2.0 MPa) and ex-situ CFP (CFP; 350 °C, 0.5 MPa). In both cases, a 8:1 molar ratio of molecular hydrogen to m-cresol was maintained for the duration of the reaction period. Quantitative analysis was carried out using gas chromatographs equipped with flame ionization and thermal conductivity detectors, which had been calibrated with standards of known concentrations. In all cases, the mass balance closure was > 90%. Calculations were performed using the Vienna Ab initio Simulation Package (VASP) 5.3. The periodic density functional calculations used the projector augmented wave (PAW) potentials and an energy cutoff of 400 eV for the plane wave basis set for all calculations. The Perdew-Burke-Ernzerhof (PBE) generalized gradient corrected functional was used for all periodic calculations with Monkhorst-Pack 5×5×1 k-point sampling for metals and 3×3×1 k-point sampling for metal oxides.

 3. Results and discussion

Toluene was the dominant product for both catalysts under ex-situ CFP conditions, with higher toluene selectivity observed over Pt/TiO2 as compared to Pt/C. 3-methylcyclohexanone, 3-methylcyclohexanol, and 3-methylcyclohexane were also observed over each catalyst. Under HT conditions, the production of 3-methylcyclohexanone and 3-methylcyclohexanol was highly favorable over both catalysts. Regardless of condition, Pt/TiO2 exhibited a TOF (metal site basis) that was more than double that of Pt/C.

The energetics of ring-hydrogenation (HDO), direct deoxygenation (DDO), and tautomerization (TAU) mechanisms were calculated over hydrogen-covered Pt(111) and oxygen vacancies on the surface of TiO₂(101). The results show that ring-hydrogenation to 3-methylcyclohexanone and 3-methylcyclohexanol is the most energetically favorable pathway over Pt(111). Over TiO₂(101), tautomerization and direct deoxygenation to toluene were identified as possible additional routes, although with slightly higher barriers. These calculations are consistent with the experimental results in which Pt/TiO₂ was shown to be more active on a metal site basis and exhibited comparatively high selectivity to toluene at 350 °C relative to Pt/C. Based on these findings, it is likely that the reactivity of Pt/TiO₂ and Pt/C is driven by the active phase at 250 °C while contributions from the TiO₂ support enhance deoxygenation at 350 °C.

 4. Conclusions

The activity and selectivity observed during the deoxygenation of m-cresol was shown to be dependent on the choice of support and reaction conditions. Pt/TiO₂ was more active on a Pt site basis and exhibited higher selectivity to toluene than Pt/C. Computational modeling of m-cresol over Pt(111) andTiO₂(101) surfaces was used to develop a reaction mechanism that describes deoxygenation pathways over the active phase and identifies additional pathways that deoxygenate m-cresol to toluene over oxygen vacancies on TiO₂. These results suggest that synergistic effects between hydrogenation catalysts and reducible metal oxide supports provide access to additional deoxygenation pathways that are not accessible to the active phase alone. The insight gained from this study aids in the identification of fundamental catalyst requirements for deoxygenation reactions and informs catalyst development for the production of advanced hydrocarbon fuels from the pyrolysis of biomass.

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