Impact of Adsorption Configurations on Alcohol Dehydration over Alumina Catalysts
Rui Wang, Zhenchao Zhao, Pan Gao, Kuizhi Chen, Zhehong Gan, Qiang Fu, Guangjin Hou
Abstract
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.