(375g) Elucidating the Mechanism of Phenol Deoxygenation to Benzene on an Fe Surface: A Detailed Mechanistic Study from First Principles
The production of directly applicable liquid fuels from renewable resources is an area of intense research as we rush to find more sustainable energy sources than traditional fossil fuels. One promising technique is the fast pyrolysis of biomass to bio-oils as this technique is one of the simplest and most cost-effective methods for producing crude bio-oil and it can use the entirety of the biomass feedstock . A major obstacle to the widespread use of these bio-oils is the high level of oxygen, up to 50 wt%, contained within the bio-oils in the form of acetic acid, furans, ketones and phenolics . These oxygenates cause the bio-oils to be corrosive, thermally unstable, and highly viscous . In order to produce usable bio-oils using fast pyrolysis, it is necessary to further upgrade the bio-oils by decreasing the oxygen content.
Bimetallic PdFe catalysts have been shown to be highly active towards the hydrodeoxygenation of the phenolics found in crude bio-oil and our previous work has shown that the active catalytic surface is likely the exposed Fe surface with the surface Pd playing a support role during reaction [4, 5]. We have further investigated Pd’s support role and the nature of the Pd-Fe synergy by studying the reducibility of Fe2O3 with and without surface Pd in a combined experimental and theoretical study . This work showed that the Pd stabilizes the metallic Fe surface by a partial donation of electrons to the surface Fe, making the surface electrons more delocalized. With additional experimental studies, we were able to propose a possible mechanism to account for the high activity and HDO selectivity in Pd-Fe catalysts, which included Pd assisted H2 dissociation, Pd facilitated stabilization of metallic Fe surface, and Pd enhanced product desorption .
In order to better understand and control the catalytic deoxygenation reaction of phenolics, the detailed surface reaction mechanisms must be known. Using density functional theory, we have examined five distinct reaction pathways for the deoxygenation of phenol on the Fe (110) and Pd (111) surfaces so as to identify the most likely deoxygenation mechanism. For each elementary step in each mechanism, we have calculated the transition state structures, stable reaction intermediate structures, activation energies, reaction energies, and rate constants using transition state theory. A comparison of the resulting energies of reaction on the Pd (111) and Fe (110) surfaces show that the deoxygenation of phenol is highly endothermic on Pd (111) while the same reactions are exothermic on Fe (110). Work performed by Wang et al. has shown that the activation energy barrier can be linearly related to the reaction energy. This suggests that the deoxygenation reaction is energetically unfavorable on Pd (111) for all mechanisms. The mechanistic studies on Fe (110) have shown that the most favorable reaction pathway occurs via the direct cleavage of the C-O bond, as the partial hydrogenation of the aromatic ring is significantly unfavorable. These results provide significant insight into the deoxygenation of phenolic compounds on promoted Fe bimetallic catalysts and allow for the further tailoring of the catalyst surface for the promotion of the deoxygenation reaction.
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