METAL-CATALYZED HYDROCARBON CONVERSION REACTIONS

Palladium-catalyzed acetylene trimerization and the metathesis of olef ins catalyzed by molybdenum and its oxides are investigated in ultrahi gh vacuum and at high pressures. Benzene is formed on Pd(111) by the r eaction between adsorbed acetylene and a surface C-4 metallocycle. The resulting benzene evolves in temperature-programmed desorption in two distinct states. The low-temperature state is proposed, following inv estigations of the desorption kinetics of benzene from Pd(111), to be due to tilted benzene formed on a sterically crowded surface, and the high-temperature state, to be due to flat-lying benzene. The low stead y-state benzene formation rate found at high pressures (similar to 1 a tm) is suggested to-be due to blocking of the surface by the formation of vinylidene species. Olefin metathesis is found to proceed in two d ifferent regimes: one below similar to 650 K which mimics supported mo lybdena catalysts and where MoO3 is the best catalyst, and another reg ion above this temperature, where the reaction proceeds with a high ac tivation energy (similar to 60 kcal/mol) and the most effective cataly st is MoO2. The latter kinetics resemble those found for metallic moly bdenum, where the reaction is proposed to proceed by a mechanism throu gh which alkenes dissociate and recombine on the surface. This reactio n is found to proceed in the presence of a thick carbonaceous layer. T he addition of hydrogen is found to increase the rate of both metathes is and cyclotrimerization even though neither of these reactions invol ves hydrogen directly. This is proposed to be due to the titration of carbonaceous species from the surface. Extension of these ideas to eth ylene hydrogenation, a reaction that does involve hydrogen, is success ful in rationalizing the kinetics behavior found in that case.