Density Functional Theory-Computed Mechanisms of Ethylene
and Diethyl Ether Formation from Ethanol on γ‑Al<sub>2</sub>O<sub>3</sub>(100)
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Abstract
Multiple
potential active sites on the surface of γ-Al<sub>2</sub>O<sub>3</sub> have led to debate about the role of Lewis and/or
Brønsted acidity in reactions of ethanol, while mechanistic insights
into competitive production of ethylene and diethyl ether are scarce.
In this study, elementary adsorption and reaction mechanisms for ethanol
dehydration and etherification are studied on the γ-Al<sub>2</sub>O<sub>3</sub>(100) surface using density functional theory calculations.
The O atom of adsorbed ethanol interacts strongly with surface Al
(Lewis acid) sites, while adsorption is weak on Brønsted (surface
H) and surface O sites. Water, a byproduct of both ethylene and diethyl
ether formation, competes with ethanol for adsorption sites. Multiple
pathways for ethylene formation from ethanol are explored, and a concerted
Lewis-catalyzed elimination (E2) mechanism is found to be the energetically
preferred pathway, with a barrier of <i>E</i><sub>a</sub> = 37 kcal/mol at the most stable site. Diethyl ether formation mechanisms
presented for the first time on γ-Al<sub>2</sub>O<sub>3</sub> indicate that the most favorable pathways involve Lewis-catalyzed
S<sub>N</sub>2 reactions (<i>E</i><sub>a</sub> = 35 kcal/mol).
Additional novel mechanisms for diethyl ether decomposition to ethylene
are reported. Brønsted-catalyzed mechanisms for ethylene and
ether formation are not favorable on the (100) facet because of weak
adsorption on Brønsted sites. These results explain multiple
experimental observations, including the competition between ethylene
and diethyl ether formation on alumina surfaces