Hydrogenolysis of alkanes. Part 6.—Modification of supported ruthenium catalysts by vanadium pentaoxide

Abstract

To prepare series of supported Ru–V catalysts, V₂O₅ was first applied to the supports (SiO₂, γ-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 Ru° at 380–450 K. After a first high-temperature (758 K) reduction (HTR1), rates of n-butane hydrogenolysis decreases as the V₂O₅ content increases, the least active catalyst showing a rate ca. 10⁵ 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≈ 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.