Interactions Involving Lewis Acidic Aluminum Sites in Oxide-Supported Perrhenate Catalysts

In oxide-supported perrhenate catalysts, isolated Re(VII) becomes anchored to the support at surface hydroxyl sites. However, oxides that also possess Lewis acidity offer the possibility of additional bonding interactions with perrhenate, which may account for their enhanced activity, for example, in olefin metathesis. Evidence for such interactions was sought by X-ray absorption spectroscopy at the Re L 1 - and L 3 -edges, in combination with DFT modeling. Only perrhenate grafted onto silica using an anhydrous method shows a simple ≡SiOReO 3 structure, with three equivalent Re═O multiple bonds. Silica-supported perrhenate prepared by aqueous impregnation is not grafted, but interacts only weakly with the silica support as a tetrahedral ReO 4 – ion. On silica–alumina and γ-alumina, the anchored perrhenates also interact with adjacent Lewis acid sites. In particular, a terminal oxo ligand interacts with a coordinatively unsaturated Al atom, creating a well-defined Re–Al EXAFS path. In addition, an oxygen atom from the support is coordinated to Re, increasing the metal coordination number from four to five. Simulated vibrational spectra of DFT-optimized models indicate that these pentacoordinate Re sites are consistent with reported Raman spectra for perrhenate supported on γ-alumina.