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

Surface ReOx Sites on Al2O3 and Their Molecular Structure–Reactivity Relationships for Olefin Metathesis

“…13 Because this (110) termination shows two structures with very similar energy, calculations suggest that the supported catalyst may be a mixture of different binding modes of the ReO 4 on γ-alumina, most likely having the surface (110_1) and (110_2) structures. It should, however, be noted that species of similar geometry can significantly differ in their stability, depending on their specific location on the surface (for instance, compare 110_1 and 110_9).…”

“…The two previous structural assignments of surface trioxo (O) 3 ReO and penta-coordinated dioxo (O) 2 ReO 3 were based on fitting of the EXAFS radial distributions of the oxygen atoms surrounding the Re 7+ site. 12,13 The current EXAFS analysis indicates that it is not possible to determine the exact number of ReO bonds with EXAFS alone since both the ReO and Re−O bonds have similar bond lengths that complicates analysis. In addition, EXAFS analysis, which averages over the two surface ReO 4 sites on Al 2 O 3 , further complicates molecular structural analysis.…”

“…5−12 The nature of the surface Re 7+ oxide species on alumina has recently come into question because of reported contradictory conclusions. 12,13 Bare et al concluded that isolated trioxo (O) 3 Re−O−Al species are present for calcined supported rhenia/alumina catalysts 12 mainly from EXAFS analysis. In contrast, Scott et al postulated isolated penta-coordinated dioxo (O) 2 Re(−O-Al) 3 from XANES/ EXAFS measurements and DFT calculations employing cluster models.…”

“…In contrast, Scott et al postulated isolated penta-coordinated dioxo (O) 2 Re(−O-Al) 3 from XANES/ EXAFS measurements and DFT calculations employing cluster models. 13 The precise coordination of the isolated surface Re 7+ oxide species on alumina is of great importance to understand the remarkable low-temperature olefin metathesis activity of this catalyst system. Indeed, it was shown for alumina supported Mo-oxo-carbene reactive intermediate complexes that the strain induced by multiple interactions with the support can strongly enhance the catalytic properties.…”

Single atom catalysts on amorphous supports: A quenched disorder perspective

“…We and others have used small cluster models containing specific ring sizes to model amorphous silica and silica-alumina. 3,[42][43][44][45][46][47] Other investigators have melted, quenched, and cut silicas and then capped them with hydroxyl groups to model the hydroxyl-terminated surfaces of amorphous silica. 48,49 Some investigators have used crystalline structures like zeolites 44 or β-cristobalite 50,51 to model amorphous silica-alumina and silica, respectively.…”