Synthesis silicate

The mesoporous ordered silicas of different type represent the new generation of materials with unique properties. The discovery of these materials became basis for creation of new catalysts, adsorbents, sensors and supporter for other molecules. The most important way of the modifying physical and chemical properties of mesopurous silicas consist in organic components incorporation on the silica surface as part of the silicate walls or their insertion within channels of the mesopores. This ensured that interest in synthesis and study of functionalized mesoporous materials shai ply grew. In spite of it, these materials are studied insufficiently. 

Vapour phase synthesis may be carried out by passing a mixture of acetylene and acetic acid through a reaction tube at 210-215°C. Typical catalysts for this reaction are cadmium acetate, zinc acetate and zinc silicate. The monomer in each of the above mentioned processes is purified by distillation. 

The synthesis of T8[OSiMe2H]8 was first reported by Hoebbel et al. from the reaction between TgLONMeJs and chlorodimethylsilane in dimethylformamide (DMF)/heptane (Table 18, entry 1). Other authors have modified the procedure by changing the solvents to tetrahydrofuran (THF) or hexane, or by using TslONMes (C2H40H)]g as the silicate anion source, but without significant improvement in the yield (Table 18). 

Carbohydrate-coupling or glycosylation, is a major synthesis method in carbohydrate preparation. Silver silicates and Ag(I)-exchanged zeolite A - so-called insoluble Ag(I) - have been advocated as promoting agents, applied in more than stoichiometric amount (Fig. 9). All hydroxyl groups except the attacking one are suitably protected. 

Giannelis, E.P. 1998. Polymer- layered silicate nanocomposites synthesis, properties and applications. Applied Organometallic Chemistry 12 675-680. 

Vaia, R.A., Ishii, H. and Giannelis, E.P. 1993. Synthesis and properties of two dimensional nanostructures by direct intercalation of polymer melts in layered silicates. Chemistry of Material 5 1694-1696. 

Zanetti, M., Camino, G., Thomann, R. and Mulhaupt, R. 2001. Synthesis and thermal behaviour of layered silicate-EVA nanocomposites. Polymer 42 4501- 4507. 

Monnier, A Schuth, F Huo, Q Kumar, D Margolese, D Maxwell, RS Smcky, GD Krish-namurty, M Petroff, P Firouzi, A Janicke, M ChmeUca, BE, Cooperative Formation of Inorganic-Organic Interfaces in the Synthesis of Silicate Mesostructures, Science 261, 1299, 1993. 

To overcome the limitations of natural zeolites a whole range of synthetic zeolites have been manufactured since the 1950s. These have tailored pore sizes and tuned acidities, as well as often incorporating other metal species. The basic synthesis involves mixing a source of silica, usually sodium silicate or colloidal Si02, with a source of alumina, often sodium aluminate, and a base such as sodium hydroxide. The mixture is heated at temperatures up to 200 °C under autogenous pressure for a period of a few days to a few weeks to allow crystallization of the zeolite. 

We have presented spinel synthesis because these systems were studied first and can be understood in a simple manner. Various silicate systems have also been studied and it has been determined that they are more representative of the general case involving solid state synthesis reactions than spinel. Let us examine the following simple silicate reaction ... 

In the late 1940s zeolites were synthesized according to the procedure shown in Fig. 3.24. First an amorphous alumino-silicate gel is formed. This process is completely analogous to the production of alumina and silica gels described before. Subsequently this gel is crystallized into zeolite. The preparation of zeolites has drawn tremendous attention of the scientific and industrial community. A wide variety of zeolites have been synthesized, and reproducible synthesis procedures have been reported (often in the patent literature). Natural zeolites also exist massive deposits have been discovered in many places in the world. 

A fascinating area is micellar autocatalysis reactions in which surfactant micelles catalyse the reaction by which the surfactant itself is synthesized. Thus synthesis of dimethyldoceylamino oxide (reaction between dimethyl dodecyl amine and H2O2) benefits from this strategy. Here an aqueous phase can be used and an organic solvent can be avoided. Synthesis of mesoporous molecular sieves benefit through micellar catalysis and silicate polymerization rates have been increased by a factor 2000 in the presence of cetyltrimethyl ammonium chloride (Rathman, 1996). 

A new route to alkyl silicates and organosiloxanes is described. This route has three steps. These are (1) the protonation of a silicate (obtained by collection, mining, or laboratory or commercial synthesis), (2) the esterification of the silicic acid formed by the protonation, and (3) the organodealkoxylation of the alkyl silicate resulting from the esterification. An important feature of this route is that it does not depend on elemental silicon. The route is illustrated with the synthesis of (EtO)4Si from 7-Ca2SiC>4,... 

Various methods can be used to obtain the silicates necessary for the step including small scale collection (e.g., dioptase), mining, synthesis from silica (e.g., Na4Ca4Sig0i8), and synthesis from other silicates (e.g., Ca3Si30g and Ca2ZnSi207). The results obtained so far indicate that the silicates best suited for use are often calcium silicates. 

The route also provides a satisfactory path to some alkyl silicates for which no other satisfactory routes are available. Some of these alkyl silicates may be of interest in the synthesis of ceramics by the sol-gel technique. 

Although a number of factors affect structural evolution in sol-gel processing, many of the observed trends can be explained on the basis of the stability of the M-O-M bond in its synthesis environment. Structures which form in solution are the products of a successive series of hydrolysis, condensation and restructuring reactions. M-0-M bonds which are unstable with respect to hydrolysis and alcoholysis are absent. In borates this criterion precludes gelation for systems with insufficient fractions of 4-coordinated borons. In silicates... 

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