Silicates framework structures

Figure 23. The uranyl silicate framework structures in Na2[(U02XSi04)] (a) and RbNa[(U02)(Si206)](H20) (b).

Si 19f Location of fluoride anion in silicate-framework structure octadecasil 34... 

With the exception of the mineral melanophlogite, all porosils are synthetic products. All porosils were directly synthesized with structure-directing agents (SDA) as templates, in general, organic molecules. There are a few more cases where postsynthesis treatment of alu-minosilicate zeolites leads to the complete dealumina-tion of silicate framework structures, yielding all-silica... 

NMR AND RAMAN SPECTROSCOPIC INVESTIGATIONS OF SILICATE FRAMEWORK STRUCTURES... 

Figure 1. Scheme for the liquid crystalline templating mechanism proposed by Kresge et al 1 for synthesis of mesoporous silica MCM-41. Formation of a hexagonal array of cylindrical micelles possibly mediated by silicate anions followed by condensation of the silicate anions from the silicate source (tetraethylorthosilicate) leads to templated framework structure. Calcination or extraction of the template produces hexagonally ordered mesoporous silica. 

Zeolitic structures with pore sizes of 2000 to 10000 pm are known as mesoporous solids, and can be formed by a method known as liquid crystal templating (LCT). The combination of a suitable cationic surfactant together with silicate anions form arrays of rod-like surfactant micelles (Figure 3.7) surrounded by a polymeric siliceous framework. On calcination the mesoporous structure is formed. 

We saw that zeolite A has a Si/Al ratio of 1. Some zeolites have quite high Si/Al ratios zeolite ZK-4 (LTA), with the same framework structure as zeolite A, has a ratio of 2.5. Many of the new synthetic zeolites that have been developed for catalysis are highly siliceous ZSM-5 (MFI) can have a Si/Al ratio which lies between 20 and oo (the latter, called silicalite (see Section 7.2.2) being virtually pure Si02) this far outstrips the ratio of... 

Mixed framework structures containing octahedra and tetrahedra. Silicates,... 

A number of techniques have been employed to model the framework structure of silica and zeolites (Catlow Cormack, 1987). Early attempts at calculating the lattice energy of a silicate assumed only electrostatic interactions. These calculations were of limited use since the short-range interactions had been ignored. The short-range terms are generally modelled in terms of the Buckingham potential,... 

The Hb NMR spectrum of this sample contains a single narrow resonance centered at -3.2 ppm, which is characteristic of boron in a tetrahedral coordination environment in the framework structure. The Si nmr spectra of a synthetically prepared siliceous mordenite with the same Si/Al ratio is shown in Figure 8. No CP resonances are present, Which indicates that hydroxyl nest concentration in this material is very low compared to the acid treated sample. These data confirm that hydroxyl nests, generated by the removal of A1 from the zeolite structure, are reactive sites for isomorphous substitution. Aluminum deficient, preformed zeolites which do not contain hydroxyl nests, i.e. synthetically prepared samples, do not undergo isomorphous substitution when treated in a similar fashion. 

The recent descriptions of the ALPO-n, SAPO-n and MeAPO-n families of microporous materials illustrate that hydrothermal syntheses can afford a wide and diverse range of four-coordinate framework structures based on nearregular tetrahedra [1,2]. As building blocks, octahedra and tetrahedra can also be combined, in various proportions, into a variety of structure types [3,4]. Reflecting the conditions used for conventional synthesis [3,4], most of these structures are condensed, with little accessible pore volume. There are, however, examples of both synthetic [5-7] and natural materials [8-11] that have microporous crystalline structures. Further, the formation chemistry of silicates and aluminosilicates [12,13] illustrates that the more open structures are generally produced under relatively mild conditions. Open octahedral-tetrahedral structures with large pore systems might therefore also be accessible under appropriate low temperature hydrothermal conditions. 

Coordination polymers, open-framework materials, and hybrid compounds built up with various anions have been described in the literature. The most common anions employed in open-framework structures are silicates and phosphates.1-3 Metal carboxylates with a variety of structures and dimensionalities have also been described in the recent literature.4 5 In recent years, other oxyanions such as sulfate, selenate, selenite, and tellurite have also been employed to design these structures.6,7 Surprisingly, coordination polymers... 

In the previous three sections, we have not attempted to differentiate between dense and open hybrid framework structures. As with inorganic silicates and aluminosilicates, there is no fundamental chemical difference between the dense and open hybrid structures, though their properties and applications are often quite distinct. There are also differences in the synthetic strategies that are needed to synthesize them, and in section 4 we shall discuss some of the reaction... 

The polymorphic forms of ice illustrate the structural variety possible for the hydrogen-bonded polymers of four-coordinated water. Topologically, the ices belong to the same class of 4 2 framework structures as the silicates [746]. It is not surprising, therefore, that each of the ice polymorphs has its silica analog... 

---