Hydrodeoxygenation of phenolic compounds is a required step in most upgrading strategies for conversion of biomass-derived feedstocks into useful fuels and chemicals. In this presentation, we analyze experimental and theoretical results of the conversion of m-cresol, guaiacol, and anisole over a series of metal catalysts (Pt, Ru, Fe, Ni, etc.). For example, the conversion of m-cresol over Pt, Fe, and Ru/SiO2 catalysts show very different product distributions, when the reaction is conducted at similarly low conversions and the same operating conditions. While ring hydrogenation to methylcyclohexanone is dominant over Pt, deoxygenation to toluene prevails over Ru and Fe. In addition, C-C cleavage to C1-C5 hydrocarbons also occur on Ru. To understand this contrasting behavior, the conversion of m-cresol over the Pt(111), Fe(110), and Ru(0001) surfaces was analyzed using DFT.  These results show that the direct dehydroxylation of m-cresol is unfavorable over Pt. By contrast, the ring hydrogenation path towards methylcyclohexanone is significantly easy over this surface, in agreement with the experimental observations, which show that methylcyclohexanone is dominant over Pt, even at low conversions. By contrast, the direct dehydroxylation of m-cresol becomes more favorable over the more oxophilic Ru and Fe surfaces, also validated by experimental data that show higher selectivity to the deoxygenation product, toluene. The conversion of the methoxy group is also severely influenced by the oxophilicity of the metal surface. That is, while over Pt, the initial conversion of anisole produces phenol as primary product, leading to benzene as secondary product; this is not the case on the more oxophilic Fe surface. Over this surface, even at the lowest conversions measured, the primary product is benzene. Analysis of the gas phase indicates that only a fraction of the C-O cleavage occurs at the aromatic C-O bond, while a comparable fraction occurs at the aliphatic C-O bond, producing methane as a product. The lack of phenol in the products was interpreted in terms of DFT calculations which demonstrate that while a phenoxy species is formed on the Fe(110) surface, the energy barrier for the hydrogenation of this species is very high. Therefore, phenol is not produced since phenoxy tends to undergo DDO. The opposite behavior is obtained with Pt over which the hydrogenation of the phenoxy species is much easier, leading to phenol and methane, as observed experimentally.

Reaction pathways for phenolics hydrodeoxygenation governed by the oxophilicity of the metal catalysts

251st National Meeting (2016)

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