Radiotracer Studies of Heterogeneous Lewis Acid-Base Reactions

Dixon, Kim Whitfield (1986) Radiotracer Studies of Heterogeneous Lewis Acid-Base Reactions. PhD thesis, University of Glasgow.

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Caesium fluoride has been widely used as a catalyst in a whole range of synthetic reactions. Mechanisms for these and related reactions are usually considered to involve intermediates related to complex fluoro-anions. The aim of this work was to study the reactions between CsF, a solid Lewis base and several gaseous Lewis acids together with the reactions between the solid Lewis acids A1F3, NbF5 and B-UF5 with SF4, a Lewis base. This was done in an attempt to determine whether or not molecular analogies are useful in understanding the reactions which occur at metal halide surfaces. Lewis acid base reactions between the above noted fluorides have been studied under heterogeneous conditions at room temperature, using the radiotracers 14C, 18F and 35S. Surface adsorption of volatile acids or bases is always observed and in many systems additional bulk reactions occur. Treatment of CsF with (CF3)2 CO in the presence of MeCN followed by thermal decomposition of the 1:1 adduct formed is one of the methods used to activate CsF for use as a heterogeneous catalyst or reagent. In this work 85Kr adsorption at activated and untreated CsF has been used together with the reactions of both types of CsF with radiochemically labelled Lewis acids in order to quantify the activation effect. Activated CsF has a B.E.T. surface area in the range 3.01 - 2.08 m2 g-1 (95% confidence limits), compared with 0.31 - 0.19m2 g-1 (95% confidence limits) for untreated CsF. Chemical analyses of the CsF after activation shows that a small amount of the -OCF(CF3)2 anion is retained. Reaction results also suggest that activation of CsF produces a porous solid. Room temperature 18F exchange between activated CsF and BF2 18F or AsF4 18F is not observed, but reaction to give BF3 18F - or AsF4 18F - is rapid, and, in the case of BF3 18F, complete. Uptake of AsF4 18F by CsF is smaller due to sintering and due to the constraints upon adsorption of gas imposed by the pore structure. CsBF4 prepared by this means undergoes no observable room temperature 18F exchange with BF2 18F, but 18F exchange between BF3 and BF4- in MeCN is rapid and complete. 18F exchange is observed between F 18FCO and activated CsF, together with the formation of F2 18F CO-. Experiments carried out using 14 F2 14CO show that there is more than one adsorbed species present both on the surface of the CsF and in the bulk of the solid. No 18F exchange is observed between BF2 18F or AsF4 18F and untreated CsF. The uptakes of gas are far smaller reflecting the smaller surface area. The extent of these reactions is comparable to that between activated CsF and the weaker Lewis acid SF3 18F. No room temperature 18F exchange is observed but 18F and 35S experiments enable surface adsorption and bulk reaction to be differentiated. 15% of the surface species are strongly adsorbed SF5- anions. The remainder are weakly adsorbed SF4 molecules. Similar Lewis acid base reactions occur between untreated CsF and SF4, but due to the smaller surface area only the bulk reactions can be conveniently followed. 18F exchange is observed when SF3 18F interacts with NbF5 or AlF3 although experiments with 35SF4 show that adsorption is weak. Surface species are only observed in the presence of a pressure 35 of 35SF4, with all surface activity being removed on removal of the gas. The reaction of 35SF4 with B -UF5 results in strong adsorption. Previous work has shown that 18F exchange is observed between SF3 18F and B - UF5 at room temperature. The reaction between CO2 and CsF has been the subject of much debate in the literature. In an effort to clarify the situation a radiotracer study was undertaken. The reaction between CO2 and activated CsF is barely detectable but experiments carried out using 14CO2 confirm that uptake of gas occurs and that more than one adsorbed species is present on the surface of the CsF. The results obtained in this work show that molecular analogies can be very useful in describing the reactions which occur at metal halide surfaces.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry
Date of Award: 1986
Depositing User: Enlighten Team
Unique ID: glathesis:1986-77368
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 09:10
Last Modified: 14 Jan 2020 09:10

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