Tetraalkoxysilanes, hydrolysis

Part of this symposium was directed to the synthesis, properties and applications of inorganic and organometallic macromolecules with network structures. The section on organo-oxo macromolecules relevant to sol-gel processing addresses the interesting synthesis and challenging characterization efforts in this area. Brinker (p. 314) outlines the complex chemical and physical factors which affect network formation and structure resulting from the hydrolysis of a tetraalkoxysilane. 

Available literature on TBOS and TKEBS mainly focuses on their thermal properties [24]. Specific research work related to the transformation of these compounds under environmental conditions is limited, and biological degradation of these compounds has not been investigated [423]. However, numerous hydrolysis studies have been conducted on the lower homologues of the tetraalkoxysilanes such as tetramethoxysilane and tetraethoxysilane [229,423]. These compounds hydrolyze abiotically to give the corresponding alcohols and silicic acid [424]. 

Entrapment in polymeric matrices is a variation of noncovalent attachment where the support is instead generated in the presence of the enzyme. A particularly popular entrapment technique is sol-gel encapsulation, where the enzyme is trapped within an Si02 matrix formed by acid- or base-catalysed hydrolysis of tetraalkoxysilanes in the presence of enzyme. ° The technique can be tuned to provide the appropriate microenvironment for each enzyme in much the same way as can be done with other immobihzation methods. ... 

Hydrolysis of tetraalkoxysilanes in pure water is usually incomplete, but it is more effectively carried out in an alcohol-water mixture and can be catalyzed by H+ or weak bases such as ammonia. Polymerization occurs in the range pH 2-7 under neutral conditions, the rate of polymerization is limited by the slowness of hydrolysis, whereas in acidic media hydrolysis is complete before polymerization begins. The nanostructure of the gel (density of particles, fractal dimensions of particle clusters, and degree of cross-linking of particles to form a network) is controlled by these conditions. 

Sol-gel technology - [ALUMINUMCOMPOUNDS - ALUMINIUMOXIDE(ALUMINA) - ACTIVATED] (Vol 2) - [SOL-GEL TECHNOLOGY] (Vol 22) -hydrolysis of tetraalkoxysilanes [SILICON COMPOUNDS - SILICONESTERS] (Vol 22) -for thin films [IHIN FILMS - FILM FORMATION TECHNIQUES] (Vol 23) -htanates for [TITANIUM COMPOUNDS - ORGANIC] (Vol 24) -for vitreous silica [SILICA - VITREOUS SILICA] (Vol 21)... 

Non-chlorine methods that are based on obtaining basic organic compounds of silicon fr om artificial amorphous silica have many stages and are ineffective, as they use the silica obtained by hydrolysis of tetraalkoxysilane or silicon tetrachloride. So it became necessary to find methods of SOC synthesis based on natural amorphous silica. Such methods are currently being developed by some authors [1-3]. 

SCHEME 4.9 Hydrolysis-condensation processes for tetraalkoxysilane. (Reprinted from... 

The example is the reaction between tetraalkoxysilanes and water. This reaction in solution takes place in consecutive steps of hydrolysis,... 

According the method of Stober et al. [1] colloids with a narrow size distribution are prepared via ammonium hydroxide-catalyzed hydrolysis and condensation of tetraalkoxysilanes and titanium tetraethoxide. The particle size can also be adjusted by variation of several reaction parameters (monomer, NH3 and H2O concentration solvent temperature) and by controlled cocondensation. The last method enables the incorporation of chromophores in two different ways ... 

Two categories of silica are available as the basis for column materials. Type A silica has a lower concentration of silanol groups, while type B silica is fully hydroxylated. Type B silica, from hydrolysis of tetraalkoxysilanes, also contains less metal impurities. 

Silica is an attractive material for inorganic gel applications because it is non-toxic, non-flammable and easy to dispose of. Dry silica gels have a translucent, bluish appearance. For gel synthesis at lab-scale, a tetraalkoxysilane Si(OR)4, commonly tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS), dissolved in the respective alcohol ROH, acts as the silica source. When water is added, two chemical reactions occur the first, hydrolysis. 

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