Date of Award

1988

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Geoffrey L. Price

Abstract

PtTe/Al$\sb2$O$\sb3$ and PtSb/Al$\sb2$O$\sb3$ bimetallics were characterized as possible reforming catalysts by kinetic studies, mechanistic investigations, and spectroscopic measurements. The results were compared with Pt/Al$\sb2$O$\sb3$ with the aim to understand the effects of tellurium and antimony on the platinum function. Coimpregnated PtTe/Al$\sb2$O$\sb3$ and PtSb/Al$\sb2$O$\sb3$ catalysts exhibited an increase in selectivity for the isomerization of n-hexane. This enhancement 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 carbon monoxide using the isotopic dilution method, suggest that electronic effects can be identified with Te/Pt alloying in catalysts of low (Te/Pt $<$ 0.06) tellurium content, while geometric effects predominate in catalyst formulations more concentrated in the post-transition metal for both PtTe/Al$\sb2$O$\sb3$ and PtSb/Al$\sb2$O$\sb3$. The cyclohexane 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. The isomerization reactions of carbon-13 labeled 2-methylpentane were used to determine the relative contributions of cyclic and bond shift mechanisms. n-Hexane was formed primarily by a C$\sb5$ cyclic mechanism on the platinum surface, while 3-methylpentane was produced by both cyclic and bond shift mechanisms. Isomerization via bond shifts increases when tellurium or antimony is added to Pt/Al$\sb2$O$\sb3$. The reaction of benzene and deuterium at 110 and 250$\sp\circ$C showed that carbon-hydrogen bond breaking/making is extremely fast. Since diffusion may affect the deuterium levels in the hydrocarbon molecules, no other firm conclusions may be drawn. PtTe/Al$\sb2$O$\sb3$ catalysts were also prepared by vapor deposition of tellurium onto Pt/Al$\sb2$O$\sb3$. 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.

Pages

225

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