(403d) Nature of ReOx Sites during Olefin Metathesis By Supported ReOx/Al2O3 Catalysts

The nature of surface ReO_x sites on Al₂O₃ is poorly udnerstood, especially during olefin metathesis, because of the absence of reported in situ and operando spectroscopy studies. The initial structure of dehydrated surface ReO_x species (e.g., trioxo, dioxo or mono-oxo) prior to exposure to olefins was determined with in situ time-resolved isotopic ¹⁶O-¹⁸O Raman spectroscopy and DFT calculations. Two distinct surface dioxo (O=)₂ReO₂ species were found to be present on the alumina support and anchored at basic and acidic hydroxyl sites. The surface dioxo (O=)₂ReO₂ species on the acidic surface hydroxyl sites preferentially interact with olefins, while the surface dioxo (O=)₂ReO₂ species on basic sites could not be activated by olefins. During activation, the initial reaction oprducts of CH₃CHO and HCHO appeared indicating oxygen removal from the surface rhenia species. Corresponding in situ Raman spectroscopy indicated loss of the Re=O oxo oxygen and in situ UV-vis spectroscopy showed reduction of surface rhenia from Re⁺⁷ to Re⁺⁵. The partially reduced surface rhenia species, however, are oxidized back to Re⁺⁷ by formation of surface Re=CH₂ and Re=CHCH₃ carbene species, the reactive surface intermediates. These in situ and operando spectroscopy studies revela the dynamic nature of surface ReO_x species during the olefin metathesis reaction.