THE SURFACE AND CATALYTIC CHEMISTRY OF OLEFIN METATHESIS CATALYZED BYMETALLIC AND OXYGEN-MODIFIED MOLYBDENUM

It is shown that model oxides grown on metallic substrates catalyze pr opylene metathesis to form ethylene and butene with an activity that m imics that of supported catalysts for reaction below similar to 650 K. Another regime is found above similar to 650 K where the reaction pro ceeds with a much higher activation energy of similar to 60 kcal/mol. Unfortunately, alkenes do not react to any detectable extent on the mo del oxide surfaces in ultrahigh vacuum. However, the high-temperature( > 650 K) metathesis rate is found to be effected by the presence of ox ygen overlayers, which also modify the chemistry of alkenes on Mo(100) in ultrahigh vacuum. Methane is formed in temperature-programmed deso rption following adsorption of alkenes (ethylene, propylene and 2-bute ne) on O/Mo(100). The only other products detected are ascribed to eit her hydrogenation reactions or to total thermal decomposition into car bon and hydrogen and it is proposed that alkenes dissociate via C=C bo nd cleavage forming carbenes which then hydrogenate to yield methane. This chemistry is in accord with that found catalytically at high temp eratures where the product distribution from the reaction of ethylene is well described by a Schulz-Florey distribution and the product dist ribution from propylene is well described by co-polymerization of carb enes and methyl carbenes. The reaction is also found to proceed in the presence of a carbonaceous layer which appears to consist of both ads orbed hydrocarbons and graphite. The adsorbed hydrocarbons are removed by hydrogen with fairly low activation energy (similar to 6.5 kcal/mo l) and it is also shown that the rate of olefin metathesis is accelera ted by the addition of hydrogen to the reaction mixture. Since this re action does not require hydrogen to proceed, this effect is ascribed t o a removal of these strongly bound hydrocarbons which increases the n umber of reactions sites on the surface. (C) 1998 Elsevier Science B.V .