(239e) Improved Catalytic Reactivity of PdFe Bimetallic Catalysts Towards the Deoxygenation of Phenolic Compounds: A Density Functional Theory Study

With an increasing world population along with the emergence of newly industrialized nations, the availability of fossil fuels will significantly decrease as current oil wells and fields are depleted. One of the most promising renewable energy alternatives to fossil fuels are biofuels because of their compositional similarity to fossil fuels and their ability to use the current transportation fuel infrastructure¹. The major problem facing biofuels is the high level of oxygen, up to 40 wt%, present in the crude bio-oil after pyrolysis which cause the resulting fuel to be corrosive and thermally unstable². To address this problem, a novel PdFe catalyst was recently developed for the purpose of removing the oxygen functional groups from the phenolic compounds found in crude bio-oil³. Experimentally, this bimetallic catalyst showed a significant catalytic improvement for the hydrodeoxygenation (HDO) of guaiacol to benzene over its monometallic components⁴. To gain a deeper understanding of the interactions occurring within the PdFe bimetallic catalyst and their effect on the HDO of phenolic compounds, the adsorption of benzene, phenol, and guaiacol on model Fe (110), Pd (111), and mixed PdFe surfaces along with the HDO reaction path for the conversion of phenol to benzene on the monometallic surfaces were examined using density functional theory (DFT). Our results show that while the adsorption strength for these aromatic species on the pure surfaces was nearly identical, the Fe surface distorted the C-O bonds to a greater degree than the Pd surface. When these compounds were adsorbed onto mixed PdFe surfaces, their adsorption was found to be weakened by the interaction with the surface Pd. In particular, we find that benzene is physisorbed when one monolayer of Pd is deposited on Fe(110) while it is chemisorbed on a pure Fe(110) surface.⁵  In addition to the adsorption studies, the reaction pathway and energy barriers for the HDO of phenol to benzene on the monometallic model surfaces were calculated. Preliminary results show that the energy barrier for the HDO reaction on the pure Fe surface is significantly reduced as compared to the pure Pd surface. Overall, these results strongly support the hypothesis that the Fe surface acts as the active catalytic surface.

  References

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2.   Mohan, D.; Pittman, C. U.; Steele, P. H., Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review. Energy & Fuels 2006, 20, 848-889.

3.   Sun, J.; Karim, A. M.; Zhang, H.; Kovarik, L. S.; Wei, Z.; Li, X.; Wang, Y. In Hydrodeoxygenation of Biomass-Derived Compounds to Biofuels, AIChE Annual Meeting, Pittsburgh, PA, Pittsburgh, PA, 2012.

4.   Sun, J.; Karim, A. M.; Zhang, H.; Kovarik, L.; Li, X.; Hensley, A. J.; McEwen, J.-S.; Wang, Y., Carbon Supported Bimetallic Pd-Fe Catalysts for Vapor-Phase Hydrodeoxygenation of Guaiacol. Washington State University: Journal of Catalysis (submitted), 2013.

5.   Hensley, A. J.; Zhang, R.; Wang, Y.; McEwen, J.-S., Energetic and Electronic Interactions between Benzene and PdFe Surfaces: A Density Functional Theory Study. Washington State University: (in preparation), 2013.