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

Adsorption of 1-hexene on the γ-alumina (110C) surface is investigated with semi-empirical (PM3) cluster calculations. It is found that on the Al–O terminated surface, H-abstraction to surface oxygen from the hexene allylic position is the most favorable reaction, and is facilitated by C–Al 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.

The energy barriers for abstraction of H from various positions on hexene by an oxygen-terminated alumina surface are similar, a possible explanation for the experimentally observed plethora of reaction products. Figure optionsDownload as PowerPoint slide