(100) facets of γ-Al₂O₃: The Active Surfaces for Alcohol Dehydration Reactions

Abstract

Temperature programmed desorption (TPD) of ethanol, as well as ethanol and methanol dehydration reactions were studied on γ-Al₂O₃ in order to identify the active catalytic sites for alcohol dehydration reactions. Two high temperature (>473 K) desorption features were observed following ethanol adsorption. Samples calcined at T ≤ 473 K displayed a desorption feature in the 523–533 K temperature range, while those calcined at T ≥ 673 K showed a single desorption feature at 498 K. These two high temperature desorption features correspond to the exclusive formation of ethylene on the Lewis (498 K) and Brønsted acidic (~525 K) sites. The amount of ethylene formed under conditions where the competition between water and ethanol for adsorption sites is minimized is identical over the two surfaces. Furthermore, a nearly 1-to-1 correlation between the number of under-coordinated Al³⁺ ions on the (100) facets of γ-Al₂O₃ and the number of ethylene molecules formed in the ethanol TPD experiments on samples calcined at T ≥ 673 K was found. Titration of the penta-coordinate Al³⁺ sites on the (100) facets of γ-Al₂O₃ by BaO completely eliminated the methanol dehydration reaction activity. These results demonstrate that in alcohol dehydration reactions on γ-Al₂O₃, the (100) facets are the active catalytic surfaces. The observed activities can be linked to the same Al³⁺ ions on both hydrated and dehydrated surfaces: penta-coordinate Al³⁺ ions (Lewis acid sites), and their corresponding –OH groups (Brønsted acid sites), depending on the calcination temperature.

Graphical Abstract

Temperature-programmed desorption of ethanol, as well as steady state dehydration reactions of ethanol and methanol, indicate that the (100) facets are the primary active surfaces of γ-Al₂O₃. The active centers on both the hydroxylated and dehydroxylated (100) facets are related to the coordinatively unsaturated Al³⁺ ions