Mononuclear, trinuclear, and metallic rhenium catalysts supported on magnesia: effects of structure on catalyst performance

Abstract

MgO-supported Re catalysts were prepared from (HRe(CO){sub 5}) and (H{sub 3}Re{sub 3}(CO){sub 12}) to give isolated Re subcarbonyls and ensembles of three Re subcarbonyls, respectively. The latter sample was reduced in H{sub 2} to give Re metal crystallites on the support. The reactivities of the various surface species were characterized by temperature-programmed reduction, temperature-programmed desorption, and wet chemical experiments. The samples were probed as catalysts for the structure-insensitive propene hydrogenation and the structure-sensitive cyclopropane isomerization/hydrogenolysis. The supported Re complexes formed from (HRe(CO){sub 5}) and MgO catalyze the alkene hydrogenation but not the C-C bond rupture. The ensembles of these complexes formed from (H{sub 3}Re{sub 3}(CO){sub 12}) catalyze both, in this respect resembling supported crystallites of Re metal partially poisoned with CO. It is inferred that neighboring metal centers are required for the C-C bond rupture, whereas isolated metal centers are active for the hydrogenation. The catalyst containing Re metal is also active for both of these conversions and is the only one of these samples that is active for the isomerization of cyclopropane to give propene.