The role of tellurium and antimony in [formula omitted] and [formula omitted] reforming catalysts

Abstract

PtTe Al 2O 3 and PtSb Al 2O 3 bimetallic catalysts were studied using the model n-hexane isomerization, aromatization, and hydrocracking reactions at 673 K and the cyclohexane dehydrogenation reaction at 573 K, under atmospheric pressure in a recirculation batch reactor. An increase in the selectivity for isomerization (and a corresponding decrease in hydrocracking selectivity) was found for all coimpregnated PtTe Al 2O 3 and PtSb Al 2O 3 catalysts compared to Pt Al 2O 3 . The change in selectivity can be caused either by electronic effects, which increase the specific activities for isomerization, or by geometric effects, which reduce the rate of hydrocracking. The results of the kinetic experiments, along with FTIR spectroscopy of adsorbed CO using the isotopic dilution method, suggest that electronic effects can be identified with Te Pt alloying in catalysts of low ( Te Pt < 0.06 ) Te content, while geometric effects predominate in catalyst formulations more concentrated in the post-transition metal. The dehydrogenation turnover frequencies were also enhanced at low Te Pt ratio; in this respect the dehydrogenation and isomerization reactions are similar. Direct six-member ring closure, rather than ring expansion by way of an adsorbed methylcyclopentane intermediate, accounted for most of the production of benzene. PtTe Al 2O 3 catalysts were also prepared by vapor deposition of Te onto Pt Al 2O 3 . These catalysts exhibited selectivities which were influenced by the type of carrier gas used to deposit the tellurium. A carrier gas containing n-hexane improved the selectivity relative to hydrogen or helium carrier gases. The mechanism of this selectivity effect is unknown but possibly due to carbon incorporation into a PtTe alloy.