Silicates 18-membered ring structures

It is possible to obtain such silicates as solids by evaporation to dryness of the organic quaternary ammonium silicate aqueous solutions. In this case, however, the process is very slow and accordingly a problem of impurity incorporation arises. It was reported that silicate solids consisting of the double four-membered ring structure were abruptly separated out after an exothermal reaction on stirring the mixture of (2-hydroxyethyl)trimethylammonium hydroxide aqueous solution and tetraethoxysilane (26). 

The silicate anion with a double five-membered ring structure is mainly formed as a crystalline solid from the tetra-n-butylammonium (N+(n-C Ho) ) silicate solutions whose N/Si ratios range from 0.78 to 1.0 (20,21). Pyridinium ions are also effective in forming silicate anions with cage-like structures (27). 

SisOoQCNCCHs s, which has the double four-membered ring structure in the silicate skeleton (-97.7 or -99.3 ppm) (38,39). One large peak assigned to SigOogCSiCCHg Jg appears on the gas chromatogram of the solid trimethylsilylated by the method of Gotz and Masson. These indicate that the solid consists only of the cubic octameric silicate structure. 

The Si04 tetrahedra can be arranged into several silicate units, e.g. squares, six-or eight-membered rings, called secondary building blocks. Zeolite structures are then built up by joining a selection of building blocks into periodic structures. 

Another important silicate structural type is based on a six-membered ring that contains alternating Si and O atoms and has the formula Si30 96A... 

Fig. 19. The structure or ZSM-S/silicate (a) Secondary building units (indicated by bold lines) each composed of 12 tetrahedral atoms are linked into chains, one of which is shown in the c direction (b) The chains are interlinked to form a three-dimensional framework in which there are 10-membered ring openings (5.5 A in diameter) running in the [010] direction. In this portion of the ac structural projection, O denotes a tetrahedral site.

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,... 

Because the structure of silicates oligomers tend to be as condensed as possible, structures with long linear chains and large cyclic rings are less plausible. On the basis of this statement, structures containing three-membered rings and threefold substituted Si sites appear the most likely candidates. Therefore, the most probable structure of the analysed species is 7e. In combination with two other samples... 

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