HYDROGENOLYSIS OF ALKANES .6. MODIFICATION OF SUPPORTED RUTHENIUM CATALYSTS BY VANADIUM PENTAOXIDE

To prepare series of supported Ru-V catalysts, V2O5 was first applied to the supports (SiO2, gamma-Al2O3 and TiO2) by impregnation and calcination; this was followed by a second impregnation with aqueous RuCl3 solution. Temperature-programmed reduction showed that the V2O5 is reduced to at least the V3+ state immediately after reduction of RuCl3 to Ru0 at 380-450 K. After a first high-temperature (758 K) reduction (HTR1), rates of n-butane hydrogenolysis decrease as the V2O5 content increases, the least active catalyst showing a rate ca. 10(5) times smaller than that of the corresponding V2O5-free catalyst. Oxidation and low-temperature (433 K) reduction (LTR) largely restore the activities, which are again suppressed by a second reduction at 758 K (HTR2). These effects are ascribed to a 'strong metal-support interaction' involving V3+ species. Very significant changes in product selectivities accompany the changes in activity; V3+-modified catalysts show high ethane selectivities (S2 almost-equal-to 1.0-1.45), but especially after HTR1 they do not alter progressively as the V2O5 content is raised. It appears that Ru particles are either entirely deactivated or exist in a partially modified condition (state A). A second condition (state B) is recognised after LTR with Ru-V/SiO2 and Ru-V/TiO2, but this may not be caused by V3+. HTR2 of Ru-V/SiO2 and Ru-V/Al2O3 produces catalysts of very low activity. It is thought that V3+ species decorate the surface of the Ru particles, simultaneously decreasing both the number of active centres and their average size; the latter effect encourages the chemisorption of n-butane in a manner favourable to central C-C bond breaking.