THE METATHESIS OF PROPYLENE CATALYZED BY MODEL OXIDES STUDIED USING AHIGH-PRESSURE REACTOR INCORPORATED INTO AN ULTRAHIGH-VACUUM CHAMBER

The olefin metathesis activity of a series of model catalysts consisti ng of metallic molybdenum, MoO2, or MoO3 is investigated using a combi ned high-pressure reactor/ultrahigh vacuum apparatus. The model cataly sts are analyzed using X-ray and ultraviolet photoelectron, Raman, and Auger spectroscopies and it is shown that MoO2 is the most active abo ve similar to 650 K. Reaction in this temperature regime proceeds with a high activation energy (similar to 60 kcal/mol). Another, lower-act ivation-energy (similar to 6 kcal/mol) regime is encountered for react ion below similar to 650 K and it is suggested that both reactions pro ceed simultaneously, Ln addition, the absolute rate and selectivity fo r the reaction on MoO2 below 650 K are similar to those found for high loadings of supported molybdenum oxide, suggesting that an oxide laye r provides a good model catalyst for this reaction. Pt is also found t hat reaction proceeds in the presence of a thick carbonaceous layer wh ere the thickness of this layer decreases with increasing oxidation st ate, A Raman analysis of the surface of the catalyst following reactio n indicates that this layer consists predominantly of hydrocarbon spec ies, although a small amount of graphitic carbon is detected. (C) 1997 Academic Press.