The Role of Platinum in Hexane Isomerization over Pt/FeOy/WOx/ZrO2

Abstract

Hexane isomerization was investigated in Pt-free and Pt-containing highly acidic iron oxide/tungsten oxide zirconia (FeOy/WOx/ZrO2) catalysts. It has been found that hexane isomerization on WOx/ZrO2_based catalysts takes place mainly on monofunctional acid sites via intermolecular hydride transfer reactions between neutral molecules and carbenium ions. The highly acidic character of WOx/ZrO2 is such that hexane isomerization does not proceed via the classical dual functional mechanism. These results are consistent with those obtained in strong acid systems, and contrast those of zeolite-based catalysts, where the presence of metal is required to achieve high conversions. The relative rate of hydrogen transfer and the thermodynamic limitation imposed on the hexane isomers dictate the product distribution. The presence of Pt is found to specifically facilitate the approach to equilibrium of the 2,2-dimethylbutane isomer. This isomer is shown to form from the most sterically-hindered carbenium ion intermediate. The results suggest that Pt-catalyzed hydrogen activation generates substantial amounts of surface atomic hydrogen which readily undergoes hydrogen transfer with this hindered intermediate, which would otherwise undergo very slow hydride transfer with neutral molecules. Our results also suggest that Pt-catalyzed hydrogen activation may play a more classical role in keeping the surface concentration of hexyl cations low, and consequently secondary reactions, oligomerization/cracking, that give rise to cracked products, are inhibited. The results of deuterium exchange measurements in D2 at low conversion, are best explained by invoking both acid- and metal-facilitated deuteration pathways. The presence of Pt results in a moderately higher level of deuterium exchange in the isohexane products, but a large increase in the deuteration of the normal hexane starting material. At ∼20% conversion, nearly all of the unreacted normal hexane molecules contain at least one deuterium atom over Pt/FeOy/WOx/ZrO2, while only 4–5% of the hexane is deuterated over Pt-free FeOy/WOx/ZrO2. These results can be attributed to branched isomer exchange with deuterated Brønsted sites, supplemented by non-isomer-selective C–H bond activation by Pt.