Dodecasill

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... 

The gas hydrates occur in three types of structures, named type I, type n and type H. The type I has 512 and 51262 polyhedra associated by face-sharing in the ratio 1 3, whereas in type II, 512 and 5,264 polyhedra are combined by face-sharing in the ratio 2 1. These structures have cubic symmetry. The recently discovered type H [779] has hexagonal symmetry and is isostructural with the hexagonal clathrasil-dodecasil 7-H [780]. The host lattice contains 512, 435663 and a larger 51268 void which can accommodate larger organic molecules than the type II cubic structure. 

Gies H, Liebau F, Gerke H (1982) Dodecasils . A new series of polytype intercalation compounds of Si02. Angew Chem Int Ed 21 206 -207... 

Gerke H, Gies H, Liebau F (1982) Dodecasil 1H. Der einfachste Vertreter einer polytypen Reihe von Si02-Clathraten. Z Krist 159 52-54... 

DDR Deca-dodecasil 3R [Sii20O240]- R. R = 1 -aminoadamantane Sigma-1 (Si02/Al203 60)... 

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

In order to achieve high selectivities with thermostable zeolite-based membranes, zeolites can be choosen with pore apertures matching the kinetic diameters of the molecules to be separated. Moreover, the hydrophobicity of all-silica zeolites provides continuous separation, independently of traces of water in the gas streams applied. In the total spectrum of tectosilica(te)s there is only one all-silica 8-ring system Deca-dodecasil 3R (DD3R) but several all-silica 6-ring systems Table 6). 

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]. 

The first organometallic SDA that was successfully used for the crystallization of zeotypes was the cobalticinium cation. After an early patent report, Shepelev and Balkus and our group independently reported the synthesis of cobalticinium nonasils [41, 42]. In our syntheses, we observed in addition the formation of octadecasil, [42] dodecasil IH [42] and ZSM-48 [47]. The conditions under which these different products are formed are summarized in Table 1. 

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