(100) facets of gamma-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 gamma-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 a parts per thousand currency sign 473 K displayed a desorption feature in the 523-533 K temperature range, while those calcined at T a parts per thousand yen 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 Bronsted acidic (similar to 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 gamma-Al2O3 and the number of ethylene molecules formed in the ethanol TPD experiments on samples calcined at T a parts per thousand yen 673 K was found. Titration of the penta-coordinate Al3+ sites on the (100) facets of gamma-Al2O3 by BaO completely eliminated the methanol dehydration reaction activity. These results demonstrate that in alcohol dehydration reactions on gamma-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 (Bronsted acid sites), depending on the calcination temperature