Reactions of C3-Hydrocarbons on Pt/Alumina and Chromia/Alumina Catalysts

[摘要] The deactivation phenomena associated with the interaction of alkanes and alkynes with platinum and chromia catalysts has been investigated with a view to establishing conditions under which transhydrogenation could be effected. 2H- and 13C-stable isotope techniques were used to (a) establish the origins and fates of the carbon and hydrogen during the reactions and (b) obtain a detailed understanding of the reaction mechanisms, including the coking process. When propane or propyne or a 1:1 mixture of propane and propyne was passed over a Pt/alumina catalyst, in the temperature range 500°C - 600°C, it was found that the main reaction taking place was dissociation of the hydrocarbons and subsequent formation of a carbonaceous overlayer on the catalyst. Isotopic tracer studies revealed that, within the carbonaceous deposit, there was considerable mixing of both carbon and hydrogen. When unlabelled and perdeutero propane were passed over chromia/K/alumina, in the temperature range 500°C to 600°C, a significant hydrogen-deuterium isotope effect was observed. At 500°C this isotope effect was large enough to prevent dehydrogenation of perdeutero propane. Though still present at 550°C and 600°C, the isotope effect was less significant. The rate determining step of propane dehydrogenation on chromia/K/alumina was found to be C3H8 (ads) → C3H6 (ads) + H2 (ads). From results of experiments investigating the interaction of propane with chromia/K/alumina, a sequential model for dehydrogenation of propane was devised. During the interaction of propyne with chromia/K/alumina, in the temperature range 500°C to 600°C, the main reaction which took place was found to be dissociation of propyne. When a 1:1 mixture of propane and propyne was passed over chromia/K/alumina, in the temperature range 500°C to 600°C, transhydrogenation did in fact take place. The three types of reaction observed to occur on chromia/K/alumina, dehydrogenation, hydrogenation and transhydrogenation, were accompanied by some dissociation of the relevant hydrocarbon. This resulted in the formation of a carbonaceous overlayer on the surface of the catalyst. Again isotopic tracer techniques revealed that there was considerable isotopic mixing on the surface of the catalyst. On both platinum/alumina and chromia/K/alumina the amount of carbon which could be removed from the used catalyst, by high temperature treatment with dioxygen, was found to be dependent on the ratio of total carbon to total hydrogen lost to the catalyst during a reaction. The more hydrogen deficient was the carbonaceous overlayer the easier it was to remove carbon. It was concluded that, in terms of these results, chromia/K/alumina exhibits greater transhydrogenation activity than platinum/alumina.

[发布机构] University:University of Glasgow

[关键词] Organic chemistry, Inorganic chemistry, Physical chemistry