Site-Dependent Lewis Acidity of γ‑Al2O3 and Its Impact on Ethanol Dehydration and Etherification

• Published in: Journal of physical chemistry. C 2014-06, Vol.118 (24), p.12899-12907
• Main Authors: Jenness, Glen R, Christiansen, Matthew A, Caratzoulas, Stavros, Vlachos, Dionisios G, Gorte, Raymond J
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp5028349

We examine the heterogeneity of the Lewis acidity on the (100) and (110) facets of γ-Al2O3 by computing the binding energies of various oxygenates, in addition to the reaction barriers of dehydration and etherification reactions of ethanol. We show that the ethanol dehydration barrier is moderately affected by site heterogeneity (barriers between 1.2 and 1.6 eV); in contrast, a nearly 3-fold change in the ethanol etherification barrier is found among the various Al3+ sites. In order to rationalize these results, the s-conduction band mean of the Al3+ sites is introduced as a descriptor to characterize the ability to transfer electron charge from the adsorbate to the Lewis acid site. It is shown for the first time that this descriptor quantitatively correlates the oxygenate binding energies and the ethanol dehydration reaction barriers. However, for the ethanol etherification reactions the s-conduction band mean of the Al3+ sites describes barriers only qualitatively due to the bimolecular nature of this reaction, which results in a change in the nucleophilicity of the ethoxy species by a nearby adsorbed ethanol. As a result, the strength of the Lewis acid sites is not the only descriptor for etherification chemistry. Hydration of the (110) facet indicates an increase in Lewis acidity strength as described by the s-conduction band mean that results in stronger binding. However, this increase in Lewis acidity results in either a negligible change of the ethanol dehydration reaction barriers on some sites or an increase due to a reduction in the basicity of the adjacent oxygen by the dissociated water. Similarly, ethanol etherification is slowed down by the presence of water due primarily to the change in nucleophilicity of the ethoxy species. Overall, our results clearly indicate that while the binding energy is an excellent descriptor of Lewis acidity strength and dehydration chemistry on the clean alumina surfaces, cooperative phenomena (i.e., modulation of the nucleophilicity of the ethoxy by the nearby oxygen or water and the basicity of oxygen in the presence of water) are key issues that lead to a breakdown in the correlation between Lewis acid strength in terms of the binding energy or the s-conduction band mean and the reaction barriers.