Non-framework cations

Materials with very high Si/Al ratios (tending to infinity) are called all-silica molecular sieves, zeosils or porosils. If any aluminium is present, non-framework cations such as alkaline or alkaline earth metals... 

A new family of crystalline molecular sieves, 2) having aluminophosphate frameworks was synthesized. Strict alternation of A1 and P on the tetrahedral nodes yields neutral oxygens in contrast to the aluminosilicate zeolites, and non-framework cations are not needed for charge balance. Whereas a microporous silica (silicalite, 3 ) with neutral oxygens is hydrophobic, the aluminophosphate sieves are moderatley hydrophilic. 

So far, we have not considered the role of the non-framework cations, M. Since the... 

Hydrocarbon sorbate vibrations. IINS spectra have been recorded for a number of simple sorbate molecules within aluminosilicate zeolites, including hydrogen in A (40, 41). acetylene in X (4, ethylene in A (42) and X (44-46). and p-xylene (42) in X type materials. In addition to intramolecular modes, where interaction between the sorbate and the non-framework cations is strong (for example in the ethylene - silver zeolite A system (42)), vibrational transitions associated with sorbate motion with respect to the zeolite s internal surface can be observed. The latter modes, and the dependence of their frequencies on loading, structure and composition are of particular interest as they convey detailed information about the character of the zeolite - sorbate... 

Transition metals exchanged into Y-zeolite offer a basis upon which to build an understanding of the important parameters involved in designing zeolitic redox catalysts. Y-zeolite was chosen for this study because it is the most thoroughly characterized catalytic zeolite. Thus, one can address such questions as what non-framework cation sites are occupied, whether the cations move between sites, whether interactions between the cations themselves are important and how these factors relate to the kinetics of catalytic reactions. 

The positions of non-framework cations in aluminosilicate zeolites can control or fine-tune their sorptive and catalytic properties. Measurement, however, requires careful and usually protracted analyses of accurate single crystal or powder dif action data. In cases for which extensive experimental data are available, statistical mechanics analyses can yield insight into relative site energies [53-55] etirlier analyses have also attempted to quantify the relative importance of short and long-range interactions in controlling site occupancy patterns [56]. Earlier atomistic simulations in this area [57-62] had mixed results. Recent developments in methods and interatomic potentials have allowed non-framework cation positions to be simulated based solely on a knowledge of the framework structure in zeolite systems for which validatory experimental data are available [113]. 

Planar faults are common in zeolites and related crystalline microporous solids. These can influence the sorptive characteristics in any one of several ways (i) they can have little influence on the overall accessibility or capacity, but alter the pore architecture, accessibility or difiusional constraints (ii) they can reduce the limiting dimensions of pore windows while leaving the tot pore volume unaffected (iii) they can block channels. Pores or pore access can also be blocked by detrital material such as alumina extracted from the framework, coke or sintered metal catalyst particles, immobile organic molecules or non-framework cations in blocking positions. 

The properties of these continua have not been explored to date. They may be metallic, and transitions in properties might occur as a function of temperature. They may be viewed as metal allotropes filled with electrons to a level far below their Fermi level. In addition, these "metals" would be very much effected by the electric fields of the zeolite framework. The mean charge on each non-framework cation in the structure can be varied by changing the composition of the zeolite framework, and by varying the extent of sorption. 

Chapter 4 by H. Forster is devoted to the potential of and achievements obtained by electron spectroscopy in the field of molecular sieves. This contribution comprises, in a rather detailed manner, the theoretical fimdamentals and principles, the experimental techniques, as well as a wealth of applications and results obtained. Results are, e.g., reported on the characterization of zeolites as hosts, guest species contained in zeolite structures, framework and non-framework cations, and zeoHtic acidity. 

Theoretical studies on zeolites have concentrated mainly on three aspects - the preferred sites for non-framework cations and for the aluminium framework ions, the acidity for different... 

Infrared spectroscopy is a widely available technique and has been applied extensively in the study of microporous solids. Using Fourier Transform analysis, sensitive detectors and operating either in transmission or in diffuse reflectance (DRIFT) mode, powders can give spectra with high resolution and sensitivity. The method is most valuable when analysing the interaction of molecules with adsorption sites (acid or base) - this is described in Chapters 7 and 8. It does give some structural insights, however, for example on the environment of protons and on the presence of framework and non-framework cations. 

Neal SL, Mackie RL, Larsen JC, Schrrltz A (2000) Variatiotts in the electrical conductivity of the upper mantle beneath North America and the Pacific Ocean. J Geophys Res, Sohd Earth 105 8229-8242 Nevirts D, Spera FJ (1998) Molecttlar dynamics simrtlations of molten CaAl2Si20g Dependence of structure and properties on pressttre. Am Mineral 83 1220-1230 Newsam JM, Freeman CM, Gorman AM, Vessal B (1996) Simrrlating non-framework cation location in alttminosilicate zeolites. Chem Comm 16 1945-1946... 

To better understand non-framework cation mixing in Ca-Mg aluminosilicate glasses, 0 MAS and 3QMAS NMR studies were done on glasses in the Mg3Al2Si30i2-Ca3Al2Si30 system. ... 

Gallium has been successfully introduced into munerous zeolite frameworks (Beta, MFI, offretite, faujasite,. ..). The Ga " " ions in zeolites can occupy tetrahedral framework sites (T) and non-framework cationic positions. The isomorphous substitution of gallium into aluminosilicate zeolites results in modified acidity and subsequently modified catalytic activity such as enhanced selectivity towards aromatic hydrocarbons. 

Both the high-quartz and the keatite structure contain more than one possible site for the non-framework cations. Size and oxygen coordination preferences of the cations determine site occupations, which in turn exert significant influences on the framework geometry and thermal expansion. 

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