Dehydrocyclization by platinum monofunctional catalysts

The competitive conversion of an equimolar mixture of n-C₈H ₁₈ and n-C₈H₁₈ at 482° C and 1 atm. over Pt-Mg(Al)O shows that there is a kinetic isotope effect with a value of 2, which falls between those of Pt-Sn-SiO₂ and Pt-SiO₂ catalysts. Correspondingly, the H/D exchange in the unconverted n-octane falls between those of Pt-Sn-SiO ₂ and Pt-SiO₂. Overall, the kinetic isotope effect shows that the breaking of the first carbon-hydrogen bond and the adsorption of the n-octane on Pt-Mg(Al)O is irreversible.; The dehydrocyclization of a mixture of D₂ and n-octane-d ₀ over Pt-SiO₂ at 482° C and 6.8 atm. pressure of D ₂ was carried out to study the pressure effect on the H/D exchange in the unconverted n-octane. H/D exchange occurred extensively both in the unconverted n-octane and in the aromatic products. The adsorption of n-octane on the surface of the Pt-SiO₂ catalyst changes from irreversible to reversible when the reaction conditions changes from 1 atm. to 6.8 atm. at 482° C.; The shape-selective dehydrocyclization of n-octane and methylheptane was carded out over Pt-ZSM-5 catalysts to study the effect of catalyst support on the selectivity towards different aromatic compounds. The dehydrocyclization products from n-octane and 3-methylheptane at 482° C and 1 atm. with a basic Pt-ZSM-5 catalyst shows a strong selectivity for the formation of ethylbenzene. For n-octane, the ethylbenzene: ortho-xylene ratio is about 8:1. For 3-methylheptane, the ratio of ethylbenzene: para-xylene: ortho-xylene is about 19: 3: 1.; If the dehydrocyclization on Pt-ZSM-5 catalyst is a diffusion-controlled reaction, one expects that the kinetic isotope effect for the equimolar conversion of n-octane-d₀ and n-octane-d₁₈ to be less than that obtained on a catalyst that does not have spatial restriction on the diffusion of the reactants and products such as Pt-SiO₂. The observed kinetic isotope effect is 1.9, which is lower than the number for Pt-Mg(Al)O and Pt-SiO ₂. The extensive H/D exchange in the unconverted n-octane from the conversion of an equimolar mixture of n-octane-d₀ and n-octane-d₁₈ shows there is multi-step adsorption of the reactants and products on their way out of the zeolite channels.