Zeolite ionic clusters

It follows that through its chemical composition and geometry the zeolite host exerts a considerable influence over the structure and stability of ionic clusters in zeolites and does not act merely as a convenient inert host. In particular, the formation of ionic clusters is much more likely to occur in aluminosilicates with a high aluminium content and a sizable anionic framework charge than in neutral aluminophosphate molecular sieves. In fact, a significant proportion of zeolite ionic clusters are found in just three aluminosilicate frameworks, each of which contains the sodalite cage structural unit (Fig. 1). 

P.P. Anderson, Ionic Clusters in Zeolites. In Molecular Sieves. Vol. 3. Springer-Verlag, Berlin, Heidelberg, 2002, 307-338. 

M.W. Simon, J.C. Edwards, and S.L. Suib, Characterization and Catalytic Studies on Defect Sites Formed upon the Thermal Decomposition of Sodium Ionic Clusters in NaX Zeolite. J. Phys. Chem., 1995, 99, 4698-4709. 

B. Xu and L. Kevan, Formation of Silver Ionic Clusters and Silver Metal Particles in Zeolite Rho Studied by Electron Spin Resonance and Far-infrared Spectroscopies. J. Phys. Chem., 1991, 95, 1147-1151. 

EPR metal carbonyl clusters ionic clusters structure and environment of duster in zeolite Useful for diaraderization of spe-des having unpaired electrons due to its high sensitivity, very small percentages of total metal loading are measurable. 

X-ray diffraction crystallography can be used to determine not only the framework structures but also the locations of the isolated metal ions in the zeolites. The method is applicable when the metals are periodically isolated within the crystalline zeolite structure and not agglomerate outside of it. Although the method is, in principle, appropriate for the characterization of all types of metal clusters in zeolites, its use has been largely restricted to ionic clusters, [138, 139] which are beyond the scope of the present review. 

Ionic clusters in zeolite cages are stabilized by their ionic interactions with the zeolite framework. They have metal-metal distances intermediate between those in metals and those in metal oxides. The subject of ionic clusters in zeolites is dealt with only briefly here, with sodium clusters chosen as an example. 

Fig.1. a Sodalite cage (P-cage) structural unit as encountered in the three zeolite structures (b-d) most commonly found to host ionic clusters b sodahte (SOD), c zeolite A (LTA) and d zeolites X and Y (FAU). Vertices represent silicon or aluminium oxygen atoms (white in a) are omitted for clarity in b-d... 

A natural development of the ideas outlined above is to regard the dehydrated zeolite as a polar solid solvent and reactions (1) and (2) as dissolution processes. When Barrer and Whiteman [4] studied the reaction of mercury metal with a number of different ion-exchanged zeolites, they found that mercury uptake was limited in sodium-, calcium- and lead-exchanged forms and copious in silver- and mercury-exchanged forms, where reduction of the exchangeable cations Ag+ and Hg + (to Ag° or Hgl" ) by Hg° would be possible. The implication of these observations is that the ionization process outlined in reaction (1) is necessary for the reaction of elemental metals with zeolites - one of the main synthetic routes to ionic clusters in zeolites - to occur. Reactions (1) and (2), in fact, can usefully be regarded as models for the formation of ionic clusters in zeolites, and detailed calculations of solvation energies [1] can help... 

Although spectroscopic measurements such as UV/visible spectroscopy and solid state MAS NMR can provide valuable information about clusters in zeolites, specific information relating to cluster nuclearity and charge is in general not available without recourse to theoretical calculations. To date few, if any, ionic clusters have been positively identified in this way. 

Among three-dimensional framework hosts forming intercalation compounds, zeolites (see Section 1.5) have attracted considerable attention because of their technical importance (Derouane, 1982 Dyer, 1984). Intracrystalline voids in anhydrous zeolites provide a strongly polar environment that can be filled with polar or ionic species to increase the crystal energy. Treatment of zeolites (e.g. Na-Y zeolite) with vapours of sodium results in the formation of a red product consisting of Na " intercalated into the zeolite cavities (Thomas, 1983). The locations of these clusters inside the zeolites are not yet known, but they seem to be buried within the rather inaccessible cavities inside zeolites, with the result that ordinary solvents do not reach them. Such intercalation of ionic species in zeolites may have implications in nuclear-waste treatment and storage. 

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