Special CPM Seminar

Elucidating the Mechanism for the Catalytic Hydrodeoxygenation of Phenols

Jean-Sabin McEwen

Gene and Linda Voiland School of Chemical Engineering and Bioengineering
Washington State University

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 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 present a study of the conversion of phenol to benzene on Fe (110) and Pd (111). We studied five different mechanisms which fell under the three typical HDO mechanism categories: hydrogenation, where the hybridization of the oxygen bonded carbon is altered from sp2 to sp3 prior to oxygen removal; direct deoxygenation, where the oxygen group is removed without altering the carbon backbone; and tautomerization, where the phenol converts to its respective ketone before being hydrogenated and the oxygen group removed. Under ultra-high vacuum (UHV) conditions, the deoxygenation of phenol was found to be highly unfavorable on Pd (111) while on Fe (110), all mechanisms were exothermic and the direct deoxygenation mechanism was found to be most favorable. While these UHV studies provide significant insight into the reactions occurring on the catalyst surface under typical experimental conditions, 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 under an aqueous environment and the results show that the presence of water only significantly affects elementary reactions involving the movement of hydrogen, promoting the 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 Br�nsted acid sites. 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, as well as the effect of water on the Fe surface and HDO mechanism. This information will allow for the further tailoring of the catalyst surface for the promotion of the deoxygenation reaction.