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

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.