Ab initio reaction path analysis of catalytic reactions: Benzene hydrogenation and cyclohexane dehydrogenation

Density functional theory calculations, in combination with experimental surface science data, were used to elucidate the reaction path of the more complex benzene hydrogenation and cyclohexane dehydrogenation mechanisms. The possible reaction paths using fundamental concepts introduced by Boudart were analyzed. In both cases, a dominant reaction path was identified out of the 180 competing pathways. Along the dominant path, the activation energies for every step were lower than for any of the alternative paths branching away from it. Cyclohexene and cyclohexadiene were not on the dominant reaction path and were at best minor byproducts during benzene hydrogenation. In contrast, the dominant reaction path for cyclohexane dehydrogenation passed through cyclohexene and cyclohexenyl. Cyclohexadiene might be a minor byproduct of cyclohexane dehydrogenation, but was not formed along the dominant path. For benzene hydrogenation, the fifth hydrogenation step had the highest activation energy and might be rate determining. Along the cyclohexane dehydrogenation path cyclohexane activation had the highest activation energy. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 10/30/2005-11/4/2005).