Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.796678
Title: Dynamic studies of chlorofluoroethanes at halogenated surfaces
Author: McGeough, Margaret M.
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1991
Availability of Full Text:
Access from EThOS:
Access from Institution:
Abstract:
In 1950 William Miller and coworkers studied the isomerisation of CCl2FCCIF2 in the presence of aluminium(III) chloride under reflux conditions. The products they obtained were CCl2FCClF2, CCl3CF3, CCl3CClF2 and C2Cl6. When the reaction was repeated using [36Cl]-chlorine labelled aluminium (III) chloride, only the chlorinated products CCI3CCIF2 and C2Cl6 contained the [36Cl]-chlorine isotope. There was no radiolabel observed in the isomeric product CCl3CF3. On the basis of these results an intramolecular mechanism was proposed to account for the isomerisation of CCl2FCClF2 in the presence of aluminium(III) chloride. Recent work involving studies of the catalytic fluorination and chlorination of gaseous chlorofluoroethanes using HF-fluorinated chromia catalysts has cast doubt on the importance of an intramolecular isomerisation mechanism. Studies using [36Cl]-chlorine and [18F]-fluorine have indicated that isomer-isation may occur as a result of a series of halogen exchange reactions. The present work is aimed at resolving this problem and re-examines Miller's original work under a variety of conditions. The surfaces chosen for this study were aluminium(III) chloride, aluminium(III) chloride pretreated with CH3CCl3, aluminium(III) chloride supported on y-alumina and [CCl4]-chlorinated gamma-alumina. All of these solids exhibit varying degrees of Lewis acidity. Aluminium(III) chloride pretreated with CH3CCl3 and aluminium (III) chloride supported on gamma-alumina were chosen for study to compare the results with untreated aluminium(III) chloride. [CCl4]-Chlorinated gamma-alumina was chosen for study because, like aluminium(III) chloride, it will dehydrochlorinate CH3CCl3 at room temperature but the reaction time for the alumina based catalyst is longer. All reactions were carried out at room temperature. The products from the reaction between CCl2FCCIF2 and aluminium (III) chloride were CCl2FCCIF2, the isomer CCl3CF3, a fluorinated product CCl2FCF3 and the chlorinated products CCI3CCIF2, CCl3CCl2F and C2Cl6 . A quantity of material was also retained on the solid surface. This was always inversely proportional to the quantity of CCl3CF3 produced. Identical results were obtained from reactions between CCl2FCClF2 and aluminium(III) chloride pretreated with CCl2FCCIF2. This suggested that either (i) the retained material did not affect the reaction sites on aluminium(III) chloride or (ii) the surface formed by the retained material was responsible for the observed reactions. Information regarding the composition and reactivity of the retained material was obtained from [36Cl]-chlorine radio-labelling experiments. Reaction of [36Cl]-CCl2FCClF2 with aluminium(III) chloride resulted in the incorporation of the [36Cl]-chlorine radiolabel on the surface. Further reaction of this solid, with gaseous CCl2FCClF2, resulted in a decrease of the surface count rate. This is believed to be due to chlorine self-absorption caused by the build-up of retained material on the solid surface. When aluminium(III) chloride pretreated with [36 Cl]-CCl2FCCIF2 was left in a reaction vessel, after a series of reactions with CCl2FCClF2, the surface count rate increased and a build up of vapour was observed. The increase in surface count rate was due to the desorption of material from the solid surface thus reducing chlorine self absorption. The desorbed material gave rise to the increase in vapour pressure observed. The vapour was identified by i. r. spectroscopy as mainly CCl2FCClF2 but some CCl3CF3 was also present. Some radioactivity was detected in the gas phase but it was too small to be determined precisely. Reactions between CCl2FCClF2 and [36Cl]-chlorine labelled aluminium(III) chloride were analysed by radio-g. c. The elutant fraction corresponding to CCl3CF2 did not contain any [36Cl]-chlorine isotope which agreed with Miller's original observation. Infrared studies of the vapour phase during the course of reaction between CCl2FCClF2 and aluminium(III) chloride showed that the presence of moisture inhibited the isomerisation mechanism. In a virtually moisture free environment the gaseous CCl2FCClF2 was consumed before the end of the 24 h reaction period. When moisture was present, CCl2FCClF2 remained in the vapour for the duration of the reaction. Kinetic data obtained from i. r. studies revealed that the production of CCl3CF3 did not occur via first or second order processes. Further information concerning the nature of the adsorbed species was obtained from 27Al and 13C MAS solid state n. m. r. studies. The 27Al n. m. r. spectrum obtained from aluminium(III) chloride after reaction with CCl2FCCIF2 was very similar to that for pure aluminium chloride. This suggested that adsorption process does not result in a change in the surface structure.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.796678  DOI: Not available
Share: