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Title: Exchange reactions on zeolites and related catalysts
Author: McCosh, Robert
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1968
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The exchange of hydrocarbons with deuterium makes/useful test reactions for studying catalytic processes and finding out what first happens when the molecules are adsorbed on the catalyst surface. The object of the present study was to examine the catalytic activity of amorphous and crystalline aluminosilicates for these reactions in order to try to elucidate the nature of the adsorbed species and compare the results with those obtained for other classes of catalysts. The exchange reactions of benzene, toluene, and m- xylene with deuterium have been studied on silica -alumina and alumina catalysts. On these catalysts the hydrogen atoms in the ring positions reacted in preference to those attached to the side groups and there were no appreciable directing effects due to the side groups. The main conclusion drawn is that these catalysts are of an acid nature although there are some differences between the results and those for the homogeneous acid catalysed exchange. Reaction mechanisms are discussed with particular reference to a possible slow step involving surface hydroxyl groups. The activity of synthetic crystalline alumino- silicates (X -type zeolites) were examined in various ion exchanged forms for hydrogen -deuterium exchange and for the reactions of ethylene, propylene, 1- butene, isobutene, isobutane and m- xylene with D20. Most of the zeolites were poor catalysts for the hydrogen -deut(;rium exchange reaction but in some cases the activity could be increased by heating in hydrogen. For the exchange of hydrocarbons with D20 the order of activities of the catalysts were CeX, LaX > NiX, CoX, Cu;) CaX > NaX. On CeX and CoX the re activities of the hydrocarbons for exchange were isobutene 1- butene, m- xylene o and p ring hydrogens) propylene > m -xyi ene m -ring hydrogen > m- xylene side group hydrogens ) ethylene, isobutane. Subsidiary experiments were also carried out to determine the variation in catalytic activity with increasing amounts of D20 and varying degrees of ion exchange. The results were interpreted in terms of ionic mechanisms which produced heterolytic splitting of hydrogen and hydrocarbon molecules on adsorption. Correlations were found to exist between the reaction parameters for propylene exchange, the charge on the zeolite cations and the trends reported for surface acidities. From thermochemical data it was shown that the ease of exchange of the hydrocarbons paralleled the ease of formation of the corresponding alkyl carbonium ions. Zeolite catalysts show more resemblance to true acid catalysts than was found for reaction on silica- alumina and alumina with the possible exception of NiX which resembles pure metals and may carry out reactions via ràidical intermediates. The nature of the active site for hydrogen-deuterium exchange and for the reactions of propylene with D₂0 are discussed. For both reactions sites other than the cation seem to be important.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available