Adsorption of 1-hexene on γ-alumina (110C)

Adsorption of 1-hexene on the gamma-alumina (110C) surface is investigated with semi-empirical (PM3) cluster calculations. It is found that on the AI-O terminated surface, H-abstraction to surface oxygen from the hexene allylic position is the most favorable reaction, and is facilitated by C-AI interaction. Hexene interactions with surface aluminum atoms are purely repulsive. Except for the pure H-abstraction, chemisorption occurs through interactions of C and H with surface oxygen atoms, which is typically an endothermic process but is most favorable when an H is abstracted from the hexene allylic position accompanied by the formation of a C-O bond, and becomes exothermic when there is an associated transfer of a surface H. On the oxygen-terminated surface, numerous different types of H atoms on hexene can be abstracted by surface oxygen when they come sufficiently close to the surface and these reactions are exothermic. The energy barriers to these different H-abstraction processes are all in the range 10-15 kcal/mol, with those for the dehydrogenation of a terminal H farthest from the double bond or an allylic H being slightly lower than those for abstracting H from other positions. The comparable energy barriers for abstraction of numerous different kinds of H provide a possible explanation for the experimentally observed plethora of products. (c) 2005 Elsevier B.V. All rights reserved.