Density Functional Theory-Computed Mechanisms of Ethylene and Diethyl Ether Formation from Ethanol on γ-Al2O3(100)

Multiple potential active sites on the surface of gamma-Al2O3 have led to debate about the role of Lewis and/or Bronsted 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 gamma-Al2O3(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 Bronsted (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 E-a = 37 kcal/mol at the most stable site. Diethyl ether formation mechanisms presented for the first time on gamma-Al2O3 indicate that the most favorable pathways involve Lewis-catalyzed S(N)2 reactions (E-a = 35 kcal/mol). Additional novel mechanisms for diethyl ether decomposition to ethylene are reported. Bronsted-catalyzed mechanisms for ethylene and ether formation are not favorable on the (100) facet because of weak adsorption on Bronsted sites. These results explain multiple experimental observations, including the competition between ethylene and diethyl ether formation on alumina surfaces.