| Abstract |
Pt/alkali-beta catalysts with various alkali metals (Na, K, Rb, Cs) were prepared by the exchange of alkali-beta zeolites with Pt(NH3)(4)(2+) cations, followed by oxidation and reduction. The reduced Pt particles were characterized and the catalytic activities tested in the transformation of n-hexane. The steric constraints, induced by the increase in the size of the alkali, result in a decrease in both the Pt content and the cyclohexane adsorption capacity of the beta support. Furthermore, the alkali metal strongly affects the size of the Pt particles and their electronic behavior in the adsorption of CO, Smaller Pt particles are obtained in the presence of the heavier alkali metals, suggesting that the more basic beta supports stabilize the Pt species. The Pt dispersion in Pt/Cs beta is as high as that in a reference Pt/KL sample prepared under similar conditions, The very small Pt particles in these samples strongly favor aromatization reactions as well as the formation of methylcyclopentane among the C-6 isomers. Moreover, the few cracked products on these samples are related mainly to terminal hydrogenolysis promoted by the metallic sites. In contrast, hydrocracking also takes place on the Pt/Na beta and Pt/KL catalysts owing to their weak acidity. The latter catalysts strongly favor isomerization, independent of the Pt content. Finally, the similar behavior of the Pt/Cs beta and Pt/KL zeolites emphasizes the combined roles of the alkali and its porous environment in determining the acid-base properties of the zeolitic support, the stabilization of the Pt species, and the catalytic behavior. (C) 2000 Academic Press. |
