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Title: Kinetic studies of some reactions catalysed by sheet silicates
Author: McInnes, M. G.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1987
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It is now known that cation-exchanged layered or pillared sheet alumino-silicates are powerful catalysts for a wide range of organic reactions. The mechanistic details, however, are sparse and even less is known in quantitative terms of the competitive intercalation of the reactant species. This thesis describes an attempt to expand knowledge in these areas via a detailed study of the very facile reaction of liquid mixtures of trimethylorthoformate with acetone at sub-ambient temperatures, and of direct studies of the intercalation of the reactants and inert solvents. The kinetic behaviour of the reaction system, supported by thermogravimetric data, indicates that the basic process occurring is the bimolecular interlamellar attack of orthoformate on protonated acetone and an acceptable mechanistic model for this has been developed. However, an equally acceptable (in mechanistic terms) scheme, based on protonation of the orthoformate, can be written and there is indication that this may operate at the highest orthoformate/acetone ratios employed. The reaction is, thus, complex. The kinetic data obtained can be further interpreted, on a bimolecular model, to provide an estimate of the relative concentrations of the reactants in the interlamellar (one-layer) reaction zone over a range of supernatant liquid compositions. They indicate that an equimolar mixture of protonated acetone and orthoformate in the reaction zone corresponds to a threefold excess of acetone in the supernatant liquid. Direct studies of competitive intercalation provide results which establish the occurrence of considerable external as well as internal sorption. For example, the total liquid uptake of reactants could be as much as 1.7 ml/g of solid, a value some six times greater than the maximum interlamellar volume of the 'three-layer' sheet silicate. Correspondingly, the solid was visibly colloidally dispersed as a weak (thixotropic) gel in some cases. Combining both sets of results leads one to the view that, in the reaction conditions employed, the supernatant liquid and the one-layer interlamellar reaction zone mixture are each in equilibrium with a (micellar) reactant mixture. This conclusion is consistent with the observations and also with the observation by others that reaction rates in such systems change markedly when vigorous stirring liquifies the thixotropic gel. Further, in the interlamellar space a majority of the acetone remains unprotonated. The kinetic features of the reaction have been largely clarified but, evidently, a more general understanding of the processes concerned must await further detailed elucidation of the physical state of such systems.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available