Orthosilicate reaction

Tetraethyl orthosilicate (104.2 g) and deuterium oxide (40 g) are stirred in a stoppered distillation flask at room temperature until a viscous, miscible solution is obtained (approximately 24 hr). (A small amount of acid catalyst, such as deuteriochloric acid, speeds up the reaction considerably. ) A distillation head and a receiver cooled in a dry ice-acetone bath are attached and the solution is distilled at 2 mm pressure until no further product is obtained. A 90% yield of ethanol-OD is obtained having an isotopic purity equal to that of the deuterium oxide used. ... 

MOCVD of Silica. Considerable work is being done with the MOCVD of silica, which is rapidly expanding in semiconductor processing. The most common MOCVD reaction is the decomposition of tetraethyl orthosilicate (TEOS), shown in simplified form as follows ]... 

A widely used glass is phosphosilicate (PSG), which is used extensively in semiconductor devices as a passivation and planarization coating for silicon wafers. It is deposited by CVD by the reaction of tetraethyl orthosilicate (TEOS) (C2H50)4Si, and trimethylphosphate PO(OCH3)3, in a molecular ratio corresponding to a concentration of 5 to 7% P. Deposition temperature is usually 700°C and pressure is 1 atm. 

Note carefully the sequence of intermediate reactions that have occurred, as depicted in this diagram. We see that the first solid state reaction involves formation of the orthosilicate. But this is very quickly transformed into meta- and pyro-sllicates. However, it is the metasilicate which predominates as the solid state reaction reaches its maturity. Finally, it predominates over all other forms of silicate present. 

Some non-silica sol-gel materials have also been developed to immobilize bioactive molecules for the construction of biosensors and to synthesize new catalysts for the functional devices. Liu et al. [33] proved that alumina sol-gel was a suitable matrix to improve the immobilization of tyrosinase for detection of trace phenols. Titania is another kind of non-silica material easily obtained from the sol-gel process [34, 35], Luckarift et al. [36] introduced a new method for enzyme immobilization in a bio-mimetic silica support. In this biosilicification process precipitation was catalyzed by the R5 peptide, the repeat unit of the silaffin, which was identified from the diatom Cylindrotheca fusiformis. During the enzyme immobilization in biosilicification the reaction mixture consisted of silicic acid (hydrolyzed tetramethyl orthosilicate) and R5 peptide and enzyme. In the process of precipitation the reaction enzyme was entrapped and nm-sized biosilica-immobilized spheres were formed. Carturan et al. [11] developed a biosil method for the encapsulation of plant and animal cells. 

Later, Thangaraj et al. (275,281) developed a novel, improved route (prehydrolysis method) for the preparation of good quality TS-1 samples. In this method the silica source (tetraethyl orthosilicate TEOS) in Ao-propanol was first hydrolyzed with 20% aqueous TPAOH solution prior to the (dropwise) addition of titanium butoxide in dry iso-propanol under vigorous stirring. Crystallization was done statically at 443 K for 1-5 days and the solid was calcined at 823 K for 10 h. The TS-1 samples thus obtained exhibited high catalytic activity in hydroxylation reactions. 

The 4,4 -dibromobiphenyl was used as the starting material The strategy taken for the synthesis of the organosilica was to carry out Grignard-type reaction to incorporate tetraethyl orthosilicate (TEOS) to 4,4 -dibromobiphei5rl to create 4,4 -Bis(triethoxysilyl)biphenyl. Materials from this procedure had been subjected to polymerization surfactant-mediated polymerization. The product obtained was a white powder aggregate. Shown in Table 47.1 are the formulations used for the samples and the heat treatment. 

Another challenge faced by sol-gel technologies involves controUing the dispersion of different metals within a mixed metal (e.g., sihcon and titanium) matrix. The solvolysis and condensation steps for metal alkoxide precursors involved in sol-gel reactions can be quite different from that of orthosilicates, which often leads to the loss of dispersion and formation of separate silica and other metal oxide domains [54]. 

Reaction with amorphous silicon at 900°C, catalyzed by steam produces cadmium orthosilicate, Cd2Si04. The same product also is obtained by reaction with sdica. Finely divided oxide reacts with dimethyl sulfate forming cadmium sulfate. Cadmium oxide, upon rapid heating with oxides of many other metals, such as iron, molybdenum, tungsten, titanium, tantalum, niobium, antimony, and arsenic, forms mixed oxides. For example, rapid heating with ferric oxide at 750°C produces cadmium ferrite, CdFe204 ... 

Reactions with alcohols yield esters of orthosilicic acid. For example, with ethanol the product is tetraethyl orthosilicate or tetraethoxysilane, Si(OC2H5)4 ... 

Lactone 86 undergoes an intramolecular conjugate addition reaction in the presence of cesium fluoride and tetraethyl orthosilicate to give furo[3,2- ]pyridine dione 87 (Equation 25). After 12 h, a 1 1 mixture of 3-ethyl epimers is found however, extending the reaction time to 72 h results in exclusively /3-ethyl isomer 87 in good yield <2005JOC384>. 

Silicates. Zinc orthosilicate Zn2Si04 Mn2 + can be produced by firing a mixture of ZnO, Si02, and MnC03 at 1200 °C for 2 h. Small quantities of MgF2 or ZnF2 accelerate the reaction [5.296]. 

In 1995, Tanev and Pinnavaia [1] have reported the synthesis of a new type of mesoporous molecular sieve designated as the hexagonal mesoporous silica (HMS). Instead of using the ionic inorganic precursor and surfactant as in the case of MCM-41 [2], HMS is manufactured by hydrolysis reaction between a neutral inorganic precursor, tetraethyl-orthosilicate (TEOS) and a neutral primary amine surfactant (8-18 carbons). HMS possesses numerous favourable characteristics, but, like MCM-41, its synthesis process can only be concluded by the removal of the surfactant. This was reportedly done either by calcination at 630°C or by warm ethanol extraction [1]. 

Olivine and other orthosilicates have been exposed to oxygen potential gradients in order to investigate the demixing of solid solutions and internal reactions (oxidation, decomposition). The corresponding formalism was outlined in Chapters 8 and... 

Ett SiO — The reaction of EuO and Si02 above 1200° C in a vacuum or under a hydrogen atmosphere results [328] in europium orthosilicate. This lemon yellow silicate is insoluble in water and is resistant towards... 

The formation of a sol-gel porous material is through a hydrolysis-polycondensation reaction. An example is given in equation 1 with the methoxide of silicon (tetramethyl-orthosilicate, TMOS), but many other alkoxides, aryl oxides and acyl oxides can be used, as well as Si—N and Si—Cl compounds. 

Starting in the 1950s a process was developed that leads from small-molecule silicon alkoxides such as tetraethoxysilane (tetraethyl orthosilicate), to organosiloxane oligomers and low polymers, and eventually to silica via a low temperature synthesis route.14 24 A simplified outline of the basic chemistry is shown in reactions (l)-(4), where R is an ethyl or higher alkyl unit. Any or all of the Si-OR bonds can be... 

One of the more recent studies by Zerfoss and Davis (46) reveals that the /3-orthosilicate of calcium can be stabilized by some foreign atoms which form defective solid solutions and by others which physically protect the crystal boundary from initiating the reaction. 

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