The distribution of electrolytes between aqueous solution and the interior of porous crystals has been investigated for three open framework zeolites - Linde Sieves X,Y and A. The study was limited to metal halides and isotherms were obtained at various temperatures. The systems investigated were, LiC1, LiBr, LiI in LiX, NaC1 in NaX, NaY and NaA, KC1 and KI in ice, CsC1 in CsX, and CaCl2 in CaX and CaA. Equilibrium was rapidly reached for the X zeolite but only slowly for the A form. Diffusion studies were made for NaC1 in NaA, CaC12 in CaA and CaBr2 in CaA. If the imbibed salt displaced any intracrystalline water the experimental method used could over-estimate the amount of salt included as the solution phase was examined. However, in the case of NaC1 in NaA, it was possible to measure the inclusion directly and these results were in good agreement with those obtained by the general method. Nevertheless, from a consideration of intracrystalline volumes the standard method may still give slightly large values at high inclusions for CaC1, and the lithium halides. A comparison between the various inclusion isotherms plotted against the solution concentration on a scale corresponding to similar amounts of lattice showed the inclusion process to be almost temperature independent, independent of the anion for the same cation and to vary considerably with the cation. For the X zeolite the preferences of salts for the crystal phase were in the order KC1-;>NaC1>CaC12:> CsCl> LiC1 at external solution concentrations below 5N with the positions of CsC1 and LiC1 reversed at higher concentrations. The curves all had an upward inflexion. Oa changing from Nall. to NaY, which has the same framework structure, almost the sa7le unit cell constant but a lower alumina and sodium ion content, the inclusion decreased considerably. For NaC1 in the three zeolites the oreference shown by the salt for the crystal phase decreased in the order NaX>NaA>NaY. The eouilibrium results for three salts NaC1, KC1 and CsC1 obeyed the Donnan relation almost exactly: In other cases the agreement was not so good but, considering the variations of the activity coefficients for these latter systems, it was still very reasonable. The diffusion studies in the A zeolite gave a linear inclusion plot against. The activation energies obtained were 26 7tical/gm mole for NaC1 in NaA, 25 K cal. for CaCl2 in CaA and 13 Kcal for Ca r2in CaA. The latter two values indicated that the barrier was not due to anion size. The order of salt nreference KC1> NaC1> LiC1 where the conditions were demonstrated to be particularly similar was shown to be in agreement with the Born model of ion transfer. The independence of the anion, the change observed between NaX and NaY and the drop in salt inclusion for CsX were explained in terms of interactions between the included cations and the lattice cation sites. The inclusion results were compared with similar results obtained for ion-exchange resins. The partial heat of salt inclusion was calculated. Free energy calculations showed that the inclusion of salt helped to stabilize the lattice, a conclusion of possible importance when considering the formation of these open frameworks in nature.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:602138 |
| Date | January 1962 |
| Creators | Walker, Anthony John |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10044/1/13597 |
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