Hydrogenation of Arenes over Catalysts that Combine a Metal Phase and a Grafted Metal Complex: Role of the Single-Site Catalyst

In comparison with the separate components, the combination on the same support material of dispersed palladium nanoparticles and grafted molecular rhodium complexes has provided evidence of improved activity in the hydrogenation of arenes. It has been proposed that the catalytic efficiency is occasioned by a hydrogen-spillover process that would enhance specifically the hydrogenation activity of the molecular catalyst. A recent study of the hydrogenation of arenes with a catalyst obtained by silica sol-gel co-entrapment of metallic palladium and [Rh(cod)(m-Cl)]2 disagreed with the hydrogen spill-over hypothesis and suggested the action of both metals, between which a kind of synergism exists .2 No explanation on the nature of this synergism was provided, however. In this paper, we provide a rationale for this synergic effect, which involves partial inhibition by the tethered molecular complex of cyclic diene dehydrogenation occurring over the supported Pd(0) nanoparticles.