Redox properties of tungstated zirconia catalysts: Relevance to the activation of n-alkanes

Abstract

A tungstated zirconia catalyst containing approximately enough tungsten oxide to give a theoretical monolayer on the zirconia support was characterised by Raman spectroscopy, temperature-programmed reduction, EPR and IR spectroscopies. The data show that treatment of the catalyst in H₂ at temperatures as low as 470 K led to partial reduction; EPR spectra indicated the formation of W⁵⁺ centres in two locations: in the bulk of tungstate clusters dispersed on the zirconia surface and at the surfaces of these clusters and consequently accessible to gas-phase reactants. When 1% n-pentane in N₂ flowed through a bed of particles of the catalyst in an EPR tube at 523 K for 20 min, the following surface species formed: W⁵⁺ centres (shown by EPR); organic radicals (shown by EPR); and OH groups (shown by IR). The results indicate that n-pentane undergoes a homolytic C–H bond cleavage reaction such as the following

followed by one-electron transfer steps that yield surface W⁵⁺ ions, OH groups, and chain carriers in the catalytic isomerisation of n-pentane, inferred to be carbenium ions. These processes are considered to be non-catalytic redox initiation reactions that explain the promoter role of the tungsten in the catalyst.