Role of Tricoordinate Al Sites in CH3ReO3/Al2O3 Olefin Metathesis Catalysts

Re2O7 supported oil gamma-alumina is an alkene metathesis catalyst active at room temperature, compatible with functional groups, but the'exact,structures of the active sites are unknown. Using CH3ReO3/Al2O3 as a model fOr Re2O7/Al2O3, we show through a combination of reactivity studies, iri situ solid-state- NMR, and_ ail extensive series of DFT calculations, that mu-methylene: structures (Al-CH2-ReO3-Al) containing a Re=O bourittto a tricoordinated Al (Al-III) arid CH2 bound tt a four-coordinated Al (Al-IVb) are the precursors of the most active-sites for Olefin Metathesis. The resting state of,CH3ReO3/Al2O3 is a distribution of mu-methylene species formed by the activation of the C-H bond of CH3ReO3 on different surface Al-O sites. In situ reaction with ethylene results in the formation of =Re metallacyde intermediates,, which were studied in detail thrOugh a combination of solid'-state NiVIR eiperiments, using labeled ethylene, and DFT calculations. In particular, we were able to distinguish between metallacycles in TBP (trigonal-bipyramidal) and Sp (square pyramidal), geometry, the latter being inactive and detrimental to catalytic activity. The 'SP sites: are more likely to-be,fOrmeul on other Al sites (Al-IVa/Al-IVa). Experimentally, the activity of CH3ReO3/Al2O3 depends on the activation temperature of alumina; catalysts activated at or above 500 degrees C coritain more active sites than those activated at 300 degrees C. We shoW that the deperitlence of catalytic activity on the Al2O3 activation temperature is related to the quantity of available Alm-defect sites and adsorbed H2O.