The
dehydration of alcohols to value-added chemicals using alumina
catalysts has been industrialized decades ago; however, despite years
of fundamental research, the molecular-level understanding of the
reaction process is still lacking. Herein, the dehydration reactions
of ethanol on two representative aluminas, i.e.,
γ-Al2O3 and five-coordinated Al-rich Al2O3-nanosheet (Al2O3-NS),
are comparatively investigated by a combination of solid-state NMR
spectroscopy and reaction evaluations. NMR results reveal that the
presence of hydroxyl groups nearby the Lewis acid sites (LASs) leads
to different ethanol adsorption modes. The surface hydroxyls nearby
LASs interfere with ethanol adsorption, which causes the methyl group
to move away from the alumina surface and hinders the elimination
of β-H, resulting in low reaction activity. In addition, the
desorption property of surface hydroxyls will affect the dehydration
reaction as they participate in the catalytic reaction cycles. Such
effects may also explain the lower reaction activities for diethyl
ether to ethylene compared with ethanol. Although both four- and five-coordinated
Als are the possible active sites for ethanol dehydration, the reactivity
of alcohol dehydration may also be substantially affected by the local
environment of undercoordinated Als besides coordination numbers.
These new insights demonstrate that the host–guest interaction
regulated by the local environment of active sites plays an important
role in the catalytic reaction.