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Theoretical Investigation of Solvent Effects on the Hydrodeoxygenation of Propionic Acid over a Ni(111) Catalyst Model

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journal contribution
posted on 22.07.2020, 10:29 by Mehdi Zare, Rajadurai Vijay Solomon, Wenqiang Yang, Adam Yonge, Andreas Heyden
The effect of two solvents, liquid water and 1,4-dioxane, has been studied from first-principles on the hydrodeoxygenation of propionic acid over a Ni(111) catalyst surface model. A mean-field microkinetic model was developed to investigate these effects at a temperature of 473 K. Under all reaction conditions, a decarbonylation mechanism is favored significantly over a decarboxylation pathway. Although no significant solvent effects were observed on the decarbonylation rate, a substantial solvent stabilization of two key surface intermediates in the decarboxylation mechanism, CH3CCOO and CH3CHCOO, leads to a notable increase of the decarboxylation rate by 2 orders of magnitude in liquid water and by 1 order of magnitude in liquid 1,4-dioxane. Furthermore, a significant solvent stabilization of the transition state of C–H bond cleavage of the α-carbon of CH3CHCO, relative to the stabilization of the C–C bond cleavage of the α-carbon of CH3CHCO, leads to a change in dominant pathway in the liquid phase environments. Finally, a sensitivity analysis shows that the C–OH bond cleavage of propionic acid and C–C bond cleavage of the α-carbon of CH3CHCO are the most rate controlling states in the gas phase. In contrast, in solvents the dehydrogenation of CH3CHCO becomes the most influential step. This shift in rate controlling state is attributed to the solvent effect on the dehydrogenation of CH3CHCO, which is facilitated in the aqueous phase. Overall, it is likely that the investigated (111) facet of Ni is not active for the hydrodeoxygenation of propionic acid in either the gas or the liquid phase and other Ni facets or phases must be responsible for the experimentally observed kinetics.