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Title: Chromia catalysed halogen exchange reactions of chlorofluoroethanes
Author: Rowley, Lyn
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1987
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The vapour phase catalytic fluorination of chloro-fluoroethanes, in which hexachloroethane is converted to chloropentafluoroethane using hydrogen fluoride, has been studied using [18F]-fluorine and [36Cl]-chlorine radiotracers . Uptake of [36Cl]-chlorine labelled hydrogen chloride at 623 K on pre-fluorinated chromia was smaller than that determined for [18F]-fluorine labelled hydrogen fluoride at the same temperature. On chromia which had not been pre-treated with HF, the uptake of [36Cl]-chlorine from H 36Cl was comparable with that determined for [18F]-fluorine from H18 F treatment. Two types of chlorine-containing species have been detected on the catalyst, compared with the three types of fluorine-containing species reported in a previous study. In the case of both catalyst-chloride and catalyst-fluoride species, one type of halide species was catalytically active towards the reactions of chloro- fluoroethanes and a second type was permanently bound to the catalyst. The reaction of [36Cl]-chlorine labelled 1,1-dichlorotetrafluoroethane at a temperature of 623 K and above, resulted in uptake of [36Cl]-chlorine by the catalyst in the same range as that previously determined for H36Cl. The behaviour of catalyst-[36Cl]-chlorine originating from H36Cl and [36Cl ]-CCl2FCF3 was indistinguishable. [36Cl]-Chlorine was incorporated in the products from the reaction of 1,1,2-trichlorotrifluoroethane or 1,2-dichlorotetrafluoroethane with the chlorinated catalyst. The results of a previous [18 F]-fluorine radiotracer study showed that catalyst-[18F]-fluorine was also incorporated in the reaction products under similar conditions. A halogen exchange model involving Cl-for-F and F-for-Cl exchange between the catalyst and reacting chlorofluoroethanes is proposed. Experiments using mixed HF, HCl, C2CI3F2 gas flows indicate that HCl had a mild deactivating effect on the fluorination reactions. The results of a diffuse reflectance spectroscopic study show that C-F and C-Cl bonds were affected by adsorption of CCI2FCCIF2 on to pre-fluorinated chromia at room temperature. Kinetic data were consistent with the reactions to form chlorinated and fluorinated products occurring on the catalyst surface. Experiments involving the reaction of isomer mixtures and radiochemical analysis of the catalyst provided no evidence for direct isomerisation in the case of C2CI2F4 or C2CI2F2. In common with the proposed dismutation reactions, isomerisation can be described by a series of halogen exchange reactions and this model alone can account for all the experimental observations in the vapour phase system.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available