Alkoxides silicon-based

Mixed metal alkoxide systems are also of interest as a means of creating additional hybrid systems. However, recognition of the large differences in their hydrolysis and condensation rates is crucial. For example, if titanium isopropoxide is made to react under the same conditions as might be used for TEOS, hydrolysis and condensation rapidly occur and lead to particulate rather than network formation of Ti02- Cocondensation with TEOS under these conditions does not occur because of the fast precipitation of the titanium dioxide species. Indeed, of the general metal alkoxides, those based on silicon tend to be more easily controlled because of their slower hydrolysis... 

Larso GL Silicon-Based Blocking Agents. In Suplement to the Gelest-Catalog (ABCR) Silicon, Germanium Tin Compounds, Metal Alkoxides and Metal Diketonates See http //www.gelest.com... 

Alkoxysilanes. Alkoxysilanes are important intermediates in the manufac-tnre of silicone-based materials. The prevalent method for forming the silyl ether linkage is alcoholysis of Si—Cl bonds (eq. 6. Removal of the concurrently formed HCl is typically accomplished by addition of tertiary amines (87,88). Often NaOR is nsed instead of the corresponding alcohol to give the less corrosive and more easily removed NaCl as by-prodnct (89,90). Use of alkyl orthoformates represents an alternative to alcohols or metal alkoxides (eq. 7) (91-93). 

Alkoxides based, for example, on aluminium, titanium, silicon or zirconium will all hydrolyse in aqueous media but it is the silicon alkoxides which are favoured in Europe due to their slower overall reaction rates. Although silicon based alkoxides will readily hydrolyse, condensation reactions leading to structure growth are relatively latent when compared with zirconium, titanium and aluminium. This gives the end user a significant degree of control. 

Further, as well as the use of silicon-based alkoxides to control reaction rates, it is possible to prepare the sol-gel system in alcohol rather than water water is then added to the system only when reaction (hydrolysis) is required. 

The sol-gel process can be performed with a variety of metal alkoxides. The most common metal alkoxides used in the sol-gel process are silicon-based alkoxides such as tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS). Silicon-based sol-gel process results in the formation of an inorganic polymer of silica, Si02, with tetrahedral imit cell geometry. A dried gel xerogel after firing at 700 °C will transform completely to fused sflica. Additional frequently used metal alkoxides for the sol-gel process are transition metal alkoxides such as Ti,... 

Employing silicon alkoxides, the hydrolysis has to be catalyzed by the addition of an acid or a base, and an excess of water is often used. Employing zirconium alkoxides, the hydrolysis reaction proceeds much faster than the condensation so that the product is obtained as a precipitate rather than a gel. 

Because of the role of precursor structure on film processing behavior (consolidation, densification, crystallization behavior), the reaction pathways are typically biased through the use of the catalyst, which is simply an acid or a base. This steers the reaction toward an electrophilic or nucleophilic attack of the M—OR bond.1,63 Hydrolysis sensitivity of singly or multiply hydrolyzed silicon alkoxides is also influenced by the catalyst, which contributes to the observed variations in oligomer length and structure. Figure 2.3b illustrates... 

OH ratio on the rate of crystallization and crystallite size investigated Prehydrolysis method. Synthesis using binary mixtures of tetrabutylphosphonium hydroxide and tetraethylphosphonium hydroxide instead of TPAOH as base and template TEOS and TBOT are sources of Si and Ti, respectively. Molar gel composition, SiO2 xTiO2 0.4 ( TEPOH + (1 — jd)TBPOH) 30H2O (x = 0-0.02) temperature = 443 K and synthesis time = 4 days Influence of nature of silicon and titanium alkoxides on the incorporation of Ti Wetness impregnation method... 

Silica-based materials obtained by the sol-gel process are perhaps the most promising class of functional materials capable to meet such a grand objective. In the sol-gel process liquid precursors such as silicon alkoxides are mixed and transformed into silica via hydrolytic polycondensation at room temperature. Called soft chemitry or chimie douce, this approach to the synthesis of glasses at room temperature and pressure and in biocompatible conditions (water, neutral pH) has been pioneered by Livage and Rouxel in the 1970s, and further developed by Sanchez, Avnir, Brinker and Ozin. 

Biological species such as enzymes, whole cells, antibodies and even bacteria can all be successfully entrapped in silica sol-gel matrices, often with enhancement of activity with respect to the free biologicals. In these cases, the process is adapted to eliminate toxic alcohols which are typically released in conventional sol-gel processes based on the hydrolysis of silicon alkoxides. Two such methods are the use of silicon alkoxide... 

The Stober method is also known as a sol-gel method [44, 45], It was named after Stober who first reported the sol-gel synthesis of colloid silica particles in 1968 [45]. In a typical Stober method, silicon alkoxide precursors such as tetramethylorthosili-cate (TMOS) and tetraethylorthosihcate (TEOS), are hydrolyzed in a mixture of water and ethanol. This hydrolysis can be catalyzed by either an acid or a base. In sol-gel processes, an acidic catalyst is preferred to prepare gel structure and a basic catalyst is widely used to synthesize discrete silica nanoparticles. Usually ammonium hydroxide is used as the catalyst in a Stober synthesis. With vigorous stirring, condensation of hydrolyzed monomers is carried out for a certain reaction time period. The resultant silica particles have a nanometer to micrometer size range. 

The size of silica nanoparticles affects their physical, chemical, electronic, and optical properties. Proper size of silica nanoparticles is crucial for design of silica-based nanomaterials. In Stober methods, the size of silica nanoparticles is adjusted by changing the type of organic solvent, the amount of silicon alkoxide, and the... 

In 1968, Stober et al. (18) reported that, under basic conditions, the hydrolytic reaction of tetraethoxysilane (TEOS) in alcoholic solutions can be controlled to produce monodisperse spherical particles of amorphous silica. Details of this silicon alkoxide sol-gel process, based on homogeneous alcoholic solutions, are presented in Chapter 2.1. The first attempt to extend the alkoxide sol-gel process to microemul-sion systems was reported by Yanagi et al. in 1986 (19). Since then, additional contributions have appeared (20-53), as summarized in Table 2.2.1. In the microe-mulsion-mediated sol-gel process, the microheterogeneous nature (i.e., the polar-nonpolar character) of the microemulsion fluid phase permits the simultaneous solubilization of the relatively hydrophobic alkoxide precursor and the reactant water molecules. The alkoxide molecules encounter water molecules in the polar domains of the microemulsions, and, as illustrated schematically in Figure 2.2.1, the resulting hydrolysis and condensation reactions can lead to the formation of nanosize silica particles. 

Silica Nanoparticles. The base-catalyzed hydrolysis of silicon aikoxides in microemulsions produces nanoparticles (20-39). Aqueous ammonia has been used primarily as the base, with AOT and nonionic polyoxyethylene ethers as the main surfactants. Figure 2.2.6 presents a flow diagram for the synthesis of pure silica (23-32) the microemulsion is first prepared and then the alkoxide is added. As can be seen from Table 2.2.1, the microemulsions include the systems AOT/ isooctane/water/ammonia, AOT/toluene/water/ammonia, NP-5/cyclohexane/water/ ammonia, and NP-4/heptane/water/ammonia. Typical reaction times are l -5 days. Various modified silica nanoparticles have also been prepared, including hydropho-... 

Our synthesis is based on the hydrolysis of a silicon alkoxide (TEOS Si(OCH2CH3)4) in a diluted solution of nonionic polyethylene oxide-based surfactants. The hydrolysis is then induced by the addition of a small amount of sodium fluoride [5], Depending on the initial mixing conditions, the size of the solubilized objects leads to either a colorless or milky emulsion. Small particles ( 300 nm) with a 3D worm-hole porous structure or small hollow spheres with mesoporous walls, are usually obtained [6]. The synthesis we report herein after exhibits an apparently slight but actually drastic change in the preparation conditions. The main feature of this approach is an intermediate step that utilizes a mild acidity (pH 2 - 4), in which, prior to the reaction, a stable colorless microemulsion containing all reactants is... 

The silica gel is amorphous. Using high-resolution electron microscopy, it is known that its amorphous framework is made up of small globular (primary) particles having sizes of 10 to 20 A (Rouquerol, Rouquerol and Sing, 1999). An alternative route involves reactions of silicon alkoxides with water, and a wide variety of materials can be made this way (Jones 1989 Brinker and Sherer, 1990). The processes based on this route are referred to as sol-gel processing, and they offer many promising possibilities. For silica gel, the reaction is... 

Titanates are valuable in other paint applications. Corrosion-resistant coatings have been described for tinplate, steel, and aluminum (440—444). Incorporation of phosphoric acid or polyphosphates enhances the corrosion resistance. Because titanates promote hardening of epoxy resins, they are often used in epoxy-based paint (445). Silicones (polysiloxanes) are often cured by titanates. Pigments, eg, Ti02, Si02, Al O and Zr02, are frequently pretreated with titanates before incorporation into paints (441,446). In these applications, the Ti(OR)4 compounds are often mixed with Si(OR)4, Al(OR)3, Zr(OR)4, and other metal alkoxides (12). 

Organosilicon spirocyclic compounds consisting of two five-membered rings linked through the silicon atom, readily form rather stable complexes with bases the central silicon atom in such complexes is penta-coordinate. Thus, bis(ethylene-l,2-dihydroxy )silane forms adducts with alkali metal alkoxides 232.233) which event is not... 

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