Gelder, Elaine A.
The hydrogenation of nitrobenzene over metal catalysts.
PhD thesis, University of Glasgow.
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The catalytic hydrogenation of nitrobenzene is an industrially important reaction used in the commercial production of aniline for use in the polyurethane industry. A mechanism for the reaction was first proposed by Haber in 1898 and has been widely accepted despite never being fully delineated. During this study the nitrobenzene hydrogenation reaction was investigated, over a range of metal catalysts, to probe the mechanism of hydrogenation and catalyst deactivation.
Initial investigations over Pd/C catalysts revealed the reaction to be sensitive to the solvent and the nature of the carbon support. However more importantly it was shown that the first intermediate in Haber’s scheme, nitrosobenzene, could not act as an intermediate to nitrobenzene hydrogenation. As a result, a new reaction mechanism was proposed where the hydrogenation of nitrobenzene and nitrosobenzene proceed via separate mechanistic routes, linked by a common adsorbed intermediate; the surface concentration of this adsorbed species controls the hydrogenation pathway followed. Further investigation over Raney nickel suggests this mechanism to be valid over other metals and not specific to palladium.
A series of novel bimetallic catalysts were also prepared for use in this study. Characterisation of these catalysts was carried out to determine the nature of the metal-metal interaction on the surface. the evidence suggests mixed metal particles may have been formed on some catalysts.
The activity of these catalysts was found to be greatly enhanced following pre-treatment with water vapour in a hydrogen atmosphere. It was postulated that partial oxidation of the metal active sites was occurring and that these systems were more active due to the enhanced adsorption of nitrobenzene. The copper nickel/systems were found to show enhanced catalytic activity, whereas all systems containing cobalt displayed irreversible deactivation following water treatment, which was attributed to the formation of irreducible cobalt aluminium spinel from the CoO formed on the surface.
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