Hydrogen Effect on Hydrocarbon Reactions over Bulk Tungsten Carbide

The reactions of five hydrocarbon molecules (n-hexane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, and methylcyclopentane) were studied as a function of hydrogen pressure ranging from 60 to 680 Torr over bulk tungsten carbide prepared by direct carburisation of WO3in a 20% CH4/H2flow. The sample was activated in H2at 1073 K for 5 h before the reactions. No significant deactivation was observed during reactions. Olefins represent an important group of the products. Increasing hydrogen pressure leads to a decrease of isomerisation rate inn-hexane, 3-methylpentane, and 2,2-dimethylbutane reactions, while a maximum rate was observed in 2,3-dimethylbutane reactions. Irrespective of the hydrogen pressure, the isomerisation rate is always larger than the rate of methylcyclopentane ring opening. The formation rate of the products resulting from single hydrogenolysis exhibits a maximum on increasing hydrogen pressure; the deethylation rate is faster than the demethylation and depropylation rates by a factor of 3 to 10 at the hydrogen pressure corresponding to the maximum rate. The exposure of the catalyst to oxygen leads to a fast decrease of the hydrogenolysis rate and a high selectivity to the isomerisation products inn-hexane reactions. The results confirm that the hydrogenolysis takes place on a tungsten carbide phase and the isomerisation on a tungsten oxycarbide or a tungsten oxide phase through a bond-shift mechanism.