(344b) Elucidating the Mechanism for the Catalytic Hydrodeoxygenation of Phenols
One aspect crucial to the design of effective catalysts is knowledge of the elementary reaction mechanism, which is difficult to divine from experiment alone. However, first principle modeling techniques can be used to address this knowledge gap. An area currently in need of such fundamental insight is the hydrodeoxygenation (HDO) of bio-oil to create useable biofuels. Recent work has shown that Fe-based bimetallic catalysts are highly active for the HDO of phenolic[1-3] and furanic compounds. In order to better design and optimize these bimetallic catalysts, we use density functional theory to quantify the metal-metal and surface-adsorbate interactions. Here, we determine the HDO mechanism for guaiacol on Pt (111) and phenol on Pd (111) and Fe (110). The results provide significant insight into the deoxygenation of phenols on promoted Fe bimetallic catalysts by elucidating the catalytic function of noble and base metal surfaces. This information will allow for the further tailoring of the catalyst surface for the promotion of the deoxygenation reaction.
Under ultra-high vacuum conditions, the deoxygenation of phenol was found to be highly endothermic on Pd (111) while the same deoxygenation reaction on Fe (110) was found to be exothermic, which explains why Pd catalysts perform poorly for the HDO of phenols[2, 3]. Further mechanistic studies on Fe (110) under ultra-high vacuum have shown that the most favorable reaction pathway occurs via the direct cleavage of the C-O bond. The endothermic nature of the deoxygenation reactions on Pd (111) extends to guaiacol on Pt (111). Recent work on the HDO mechanism for guaiacol on Pt (111) has shown that several mechanisms lead to the production of catechol, with the oxygen-removing reactions being highly unfavorable.[6, 7] However, this previous work is predominantly theoretical with only qualitative experimental support. Our work investigated the C 1s and O 1s core level binding energy changes that occur for the various stable intermediate species in the guaiacol HDO reactions and compared those theoretical results to temperature programmed x-ray photoelectron spectroscopy obtained for guaiacol on Pt (111). Using this combination of theory and experiment, we were able to identify that guaiacol first loses a methyl hydrogen before being demethylated, narrowing down the likely mechanisms proposed from theory alone. Our combined theoretical and experimental results show that catechol is the most likely HDO product at temperatures under 500 K.
While these ultra-high vacuum studies provide significant insight into the reactions occurring on the catalyst's surface under typical experimental conditions,[1-3] liquid bio-oil has a high concentration of water which can significantly affect the surface species and reaction mechanisms. In order to understand how water could affect the HDO mechanisms of phenol on Fe (110), we re-examined the HDO mechanisms theoretically under an aqueous environment and the results show that the presence of water only affects elementary reactions involving the movement of hydrogen, promoting hydrogenation and tautomerization mechanisms. Furthermore, the presence of hydroxyl on the Fe (110) surface was found to be crucial in hydrogenating the aromatic ring with the surface hydroxyl acting as Brønsted acid sites.
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