Alkane metathesis by a tungsten carbyne complex grafted on gamma alumina: Is there a direct chemical role of the support?

The activity in alkane metathesis of tungsten complexes grafted on gamma alumina raises many questions regarding the structure of the active site and the reaction pathways. We examine the case of a perhydrocarbyl tungsten complex on alumina from periodic density functional theory calculations. The quantum chemistry exploration of energy pathways suggests two possible mechanisms for alkane metathesis. The first mechanism occurs on the grafted complex only but is significantly activated. The second mechanism shows that hydration defects on the alumina surface can be active in alkane dehydrogenation toward alkenes, whereas the tungsten complex efficiently catalyzes the subsequent olefin metathesis step. The calculations thus underscore the role of the alumina support in the mechanism.