Tuesday, November 10, 2015: 4:15 PM
355E (Salt Palace Convention Center)
The nature of surface ReOx 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 ReOx 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 ReOx species during the olefin metathesis reaction.