Sodalite cage structures

Zeolites have also been recently studied using molecular mechanics simulation (Mabilia et al., 1987) with stretching and bending force constants and partial atomic charges taken from ab initio calculations. These calculations indicate that the sodalite cage structure is stabilized by replacement of Si by A1 and by local concentration of Al for a given Si Al ratio, but these results seem to be inconsistent with experiment. 

Zeolite structures based on the sodalite cage structural unit (the vertices of the rings and polyhedra are occupied by Si or Al atoms and the lines represent the oxygen bridges). 

The crystal structures of 4 ammonium exchanged, heat-treated faujasites were determined from x-ray powder data. Structure I, often called decationated Y, has lost 15 framework aluminum atoms and 21 framework 0(1) atoms (bridging oxygen atoms) per unit cell, and 15 Al(OH)2+ ions are present in the sodalite cages. Structure 11, called ammonium-aluminum Y hydrate, shows a complete rehydroxyla-tion of the vacant 0(1) positions. Structure III, called ultrastable Y, shows the same 15 framework aluminum atoms absent, and the removal of 25 0(3) and 13 0(h) framework oxygen atoms. Structure TV, which is a repetitive exchanged and heat-treated version of Structure 111, has a mean Si-O bond length of 1.610 A, which indicates that little framework aluminum is present. 

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). 

Figure 11. The truncated octahedron building block (also termed sodalite cage,f or p-cage ) (a) tetrahedral atoms (usually Si or Al) are located at the corners of the polygons with oxygen atoms halfway between them. Illustration of the linkage, through double four-membered rings, of two truncated octahedra (b) and the structure of zeolite-A (c).

Another difficulty is that if three prisms are connected in a particular way to form part of a single sodalite cage, six nearest neighbors of the fourth prism are determined. In accordance with Lowensteins rule this may restrict the structure of the fourth prism. Lowensteins rule means that there are constraints involved in the ways the prisms can connect that are not considered here. However, this will not restrict the possible distributions of a large enough crystal and should not affect the results of this work. 

The ammonia is released and the protons remain in the zeolite, which then can be used as acidic catalysts. Applying this method, all extra-framework cations can be replaced by protons. Protonated zeolites with a low Si/Al ratio are not very stable. Their framework structure decomposes even upon moderate thermal treatment [8-10], A framework stabilization of Zeolite X or Y can be achieved by introducing rare earth (RE) cations in the Sodalite cages of these zeolites. Acidic sites are obtained by exchanging the zeolites with RE cations and subsequent heat treatment. During the heating, protons are formed due to the autoprotolysis of water molecules in the presence of the RE cations as follows ... 

Figure 2.10 Framework structure for FAU zeolite formed by linking sodalite cages through double six-rings.

Since the main peaks are 3.25A, 4.2-4.6A and 7.0A, the most reasonable unit structure of the sodium aluminosilicate hydrogel is the 4-member chain illustrated in Fig, 11. Many 4-member chains easily form sodalite cages as shown in Fig. 12. In the case of a simpler silica structure, the double 4-member ring (cube) was recognized by Sakka et al C5). 

The crystal structure of the faujasite is built up by linking the sodalite cages tetrahedrally through their hexagonal faces forming connecting hexagonal prisms. 

Research in zeolites has also branched out to try to prepare new materials by incorporating various molecules and ions in the cages of these microporous and mesoporous structures. An early example of this was the preparation of the pigment ultramarine used in many paints and colourants. It is based on the zeolite sodalite (SOD) structure and contains 83 ions trapped in the cages this is the same anion found in the mineral lapis lazuli, to which it imparts the beautiful deep blue colour. Treatment of zeolites such as Na-zeolite Y with sodium vapour traps Na4 ions in the cavities, which impart a deep red colour. 

In the last few years, computer graphics with colour display are being more commonly used not only to visualize complex structures better, but also to examine unusual structural features, defects and transformations as well as reactions. In Fig. 1.45, we show the presence of a Nal" cluster within the sodalite cage of zeolite Y as depicted by computer graphics the cluster fits well within the cavity bounded by the van der Waals surface (net) of the framework atoms. The immense power of computer graphics has been exploited widely in recent years. Structural transitions in solids and sorbate dynamics in zeolites are typical areas where computer simulation and graphics have been used (Ramdas et al., 1984 Rao et al., 1992). 

The crystal structure of Pd. h Y zeolite was determined before and after hydrogen reduction at different temperatures. When the zeolite is evacuated at 600°C, Pd2+ ions are mainly found to occupy SI sites within the sodalite cages. Hydrogen adsorption at 25° C results in a complete withdrawal of Pd2+from SI sites. This displacement out of cation sites is attributed to the reduction Pd2+ — Pd(0) consistent with hydrogen volumetric measurements. Reduced palladium remains atomically dispersed inside the sodalite cages up to about 200° C. Between 200 and 800° C, Pd 0) atoms migrate toward the outer surface of the zeolite where they agglomerate into 20-A diameter crystallites. 

Hydrogen Reduction at 25°C (Sample AH 25). Adsorption at room temperature of carefully dried hydrogen on sample A produces considerable change of line intensities, and structure results show that cations undergo a complete redistribution within the sodalite cage. Hence, 1.5 Pd2+ out of... 

Cis structure, where CO groups occupy one axial and one equatorial position, would present two bands this configuration is the only one which agrees with all experimental observations. The two CO s are free to vibrate in the sodalite cage and point towards the hexagonal windows of the supercages. 

Three questions concerning ultrastabilization remain outstanding. They regard the precise mechanism of A1 removal, the nature of the intermediate defect structure (both are depicted schematically in Fig. 38), and the origin of the silicon needed for framework reconstruction. Gas sorption studies (172) indicate that materials prepared in a manner similar to that for sample 4 in ref. 163 (see above) contain a secondary mesopore system with pore radii in the range IS-19 A, suggesting that tetrahedral sites are reconstituted with silicon that, contrary to earlier speculations, does not come only from the surface or from amorphous parts of the sample, but also from its bulk, which may involve the elimination of the entire sodalite cages. 

Another example of the sensitivity of 13C MAS NMR to zeolite structure is the work of Jarman and Melchior (331) who could distinguish between TMA+ cations trapped in the ol and / (sodalite) cages in zeolite A structure in the course of crystallization from a precursor gel. If, therefore, the zeolite which is to be analyzed by 13C NMR, so as to evaluate the extent of intergrowth or variable cage environments is synthesized using TMA + cations as templates, then this method, as we show below, seems viable. 

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