Dodecasil 3C

It has been recognized for some time that SiOj and H2O show a similar tendency to form tetrahedrally linked framework structures (Kamb, 1965). The mineral melanoph-logite is isostructural with cubic gas hydrate I, while the synthetic clathrasil, dodecasil- iC is isostructural with cubic gas hydrate II (Gies et ai, 1981,1982). A section of dodecasil-3C is shown in Fig. 1.41. 

Figure 1.41 The pentagonododecahedron and a section of a layer of linked pentagonododecahedra of dodecasil-3C the unit cell is also indicated. (After Gies et ai, 1982.)...

Preparation of Pyridine Dodecasil-3C. A 2.2 M aqueous HF solution was prepared by dilution of 8.84 mL of 49 wt% HF solution with 108 mL of deionized water. A... 

Figure 12 The clathrate-type structure of dodecasil-3C (MTN) (see Table 23)...

There has not been a great deal of work in utilizing the zeolite-organic nanocomposites directly in materials applications. Recently pyridine (C5HnN)-incorpo-rated ZSM-39 and Dodecasil-3C zeolites have been synthesized" and these... 

Classical SDAs such as alkylamines or alkylammonium cations, which were mentioned in the second part of this article, do not possess special properties which would make them of interest for the construction of materials. Aromatic molecules, in contrast, can be equipped with many different properties. For example, they can act as chromophores, may possess large hyperpolarizabilities (for NLO applications such as SHG, cf the pNA molecule) or stabilize special electronic structures like radicals. However, aromatic molecules are notoriously poor SDAs. One of the few exceptions is pyridine. The hydrothermal treatment of a pyridine - HF - H2O silica solution at 190°C, for instance, results in the formation of large crystals of dodecasil 3C, a clathrasil with small cages. These crystals, which can grow to millimeter size, are acentric at room temperature and exhibit SHG, although the effect is only weak [37]. 

Another important factor influencing the produet in clathrasil synthesis is the active concentration of the SDA. For some SDA, high concentrations lead to frameworks with higher densities of framework-speeifie eages. Pyrrolidine, for example, gives siliea sodalite (2.9 eages per 1000 A) at high concentrations of SDA. whereas RUB-10 (2.0 per 1000 A ) and dodecasil 3C (1.1 per 1000 A ) form at lower eoncentrations. 

The dodecasil-3C is a non-zeolitic tectosilicate. It is a Si02 polymorph consisting of a unit cell of 34 TO2 tetrahedra, that encloses cavities (figure 3a). The system is of interest because it is the only low density SiO2-structure for which elasticity constants have been measured. 

Figure 3. Structure of shell model relaxed dodecasil-3C [8].

Figure 4. Vibrational mode in dodecasil-3C with an imaginary frequency [8].

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