Sulfur poisoning and regeneration of palladium-based catalysts. Part 1.—Dehydrogenation of cyclohexane on Pd/Al2O3and Pd/SiO2–Al2O3catalysts

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Abstract
The catalytic activity of palladium deposited on alumina and silica–alumina for the dehydrogenation of cyclo-hexane was studied. The introduction of sulfur-containing compounds, thiophene or hydrogen sulfide into the feed, was investigated. The Pd/Al2O3 catalyst was less active than platinum deposited on alumina for cyclohexane aromatization in the absence of any poisoning additive. In addition, the thio-tolerance is smaller for palladium than for platinum. The decontamination by the sulfur-free feed was also less efficient in the case of alumina supported palladium. However, regeneration under pure hydrogen was more effective for the Pd/Al2O3 sample than for the Pt/Al2O3 one. When palladium was supported on silica–alumina instead of alumina, the deactivation by coking was more intense for sulfur-free reactants. Nevertheless the thio-tolerance was strongly improved in comparison with the Pd/Al2O3 catalyst. In contrast to iridium and platinum supported on silica–alumina, previously studied under similar conditions in the literature, the removal of thiophene or hydrogen sulfide from the feed allowed a total recovery of the catalytic activity of the Pd/SiO2–Al2O3 sample. The sample regeneration was observed after treatment under flowing hydrogen. Such behaviour is discussed in terms of changes of coke precursor reactivity in the presence of adsorbed sulfur.