Cage structures silicates

In principle, all the kinetic concepts of intercalation introduced for layer-structured silicates hold for zeolites as well. Swelling, of course, is not found because of the rigidity of the three dimensional frame. The practical importance of zeolites as molecular sieves, cation exchangers, and catalysts (cracking and hydrocracking in petroleum industry) is enormous. Molecular shape-selective transport (large differences in diffusivities) and micro-environmental catalysis (in cages and channels)... 

The following review is concerned with the synthetic and structural chemistry of molecular alumo-siloxanes, which combine in a molecular entity the elements aluminum and silicon connected by oxygen. They may be regarded as molecular counterparts of alumo-silicates, which have attracted considerable attention owing to their solid-state cage structures (see for example zeolites).1 3 Numerous applications have been found for these solid-state materials for instance the holes and pores can be used in different separation techniques.4,5 Recently the channel and pore structures of zeolites and other porous materials have been used as templates for nano-structured materials and for catalytical purposes.6 9... 

Walawalkar. M. G. Horchler, S. Dietrich. S. Chakraborty. D. Roesky. H. W. Schaefer. M. Schmidt H.-G. Sheldrick. G. M. Murugavel, R. Novel organic-soluble molecular titanophospho-nates with cage structures comparable to titanium-containing silicates. Orpanometa///cs 1998, 17, 2865-2868. 

Several types of nano-reinforcements (layered-silicates, cage-structured silicone, and carbon nanotubes) have been successfully added to polymer matrices in order to improve fiber properties, such as tensile strength, dyeing ability, fire performance, shielding, and electrical conductivity [3,12,14-18]. 

Less is known about the other cubic structure, SBA-1 (Pm3m) [25]. This material was first synthesized in the siliceous form in acidic media using larger head group cationic surfactants such as alkyltriethylammonium salts. SBA-1 has been proposed to have a large cage structure with a surface area > 1000 m g for a sample having a pore diameter of 20 A [33]. 

As in the case of zeolite, the mechanism of action looks similar. No direct comparison can be made because MMT is a layered silicate compared to the cage structure of zeolite, and also because the carbonization agent is no longer a polyol but a char-forming polymer (PA6). Nevertheless, the main conclusion we can draw is that the action of the synergist (nanoclay or zeolite) is to stabilize in a first step the carbonaceous structure forming aluminophosphates and silicophosphates. With the nanoclay, this effect is only effective up to 310°C, whereas it is still efficient at 560°C with zeolite. To keep its protection efficient at high temperatures, the nanoclay permits the formation of protective ceramiclike material after collapse of the phosphocarbonaceous structure. Note that we did not detect any specific influence of the surfactant of the nanoclays, probably because of its low amount in the formulation. 

We further illustrate the approach by reference to studies (331) of two related zeolite structures zeolite ZK-4 (isostructural with Linde A) and the highly siliceous analogue of sodalite known as TMA-sodalite. As was shown earlier (Sections III,A and III,D), the structure of zeolite A consists of a cubic array of / -cages linked through double four-membered rings so as to form larger polyhedral a-cages. The sodalite structure (Fig. 7) consists of a dense,... 

The zwitterion X5-spirosilicate bis[2,3-naphthalendiolato][2-(dimethylammonio)phe-nyl]silicate (56 isolated as 57 = 56-1/2 MeCN) was synthesized by reaction of [2-(dimethylamino)phenyl]dimethoxyorganosilanes with 2,3-dihydroxynaphthalene in acetonitrile at room temperature. Reaction of 57 or of [2-(dimethylamino)phenyl]trimetho-xysilane with water in acetonitrile yielded the cage-like silasesquioxane 58 (R = 2 — Me2NCgH4). The crystal structures of 57 and 58 were studied by X-ray diffraction. In addition, 57 and 58 were characterized by solid-state 29Si CPMAS NMR142 (Figure 34). 

The first crystal structure evidence of a molecular cage entity within an alumo-silicon oxide was reported in 1987 with tetrakis(tertamethylammonium)alumino-silicate.53 Although there is no organic ligand on the silicon atom, the compound contains a separated silsesquioxane-like Si4Al4012 unit (half of the silicon atoms of the silsesquioxane being substituted by aluminum atoms) with all silicon and... 

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