Alkoxide tetraethyl orthosilicate

Of the available silicon alkoxides, tetraethyl orthosilicate (TEOS) appears to be the most popular. This is because it reacts more slowly with water than tetramethyl orthosilicate, comes to equilibrium as a complex silanol and in a partially hydrolyzed state is stable over longer periods of time. The clear TEOS liquid is the product of the reaction of SiCl4 with ethanol. The reaction produces HCl along with the ester Si(0C2H5)4 (1). This colorless liquid of a density of about 0.9 g/cm is easy to handle and through multiple distillation extremely pure. 

Various inorganic esters have been claimed as coupling agents for reinforced plastics, including aminobenzyl phosphonates, dicetylisopropylborate, alkoxy compounds of aluminum, zirconium and titanium, zircoaluminates, and numerous substituted titanates [1]. These metal alkoxides could function in a similar manner to the orthosilicates by reacting with hydroxylated substrates. Like the simple orthosilicates such as tetraethyl orthosilicate (TEOS), it is less evident how an-... 

The use of silica particles in bioapplications began with the publication by Stober et al. in 1968 on the preparation of monodisperse nanoparticles and microparticles from a silica alkoxide monomer (e.g., tetraethyl orthosilicate or TEOS). Subsequently, in the 1970s, silane modification techniques provided silica surface treatments that eliminated the nonspecific binding potential of raw silica for biomolecules (Regnier and Noel, 1976). Derivatization of silica with hydrophilic, hydroxylic silane compounds thoroughly passivated the surface and made possible the use of both porous and nonporous silica particles in all areas of bioapplications (Schiel et al., 2006). 

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... 

It is becoming more common to use metal-organic precursors for the production of Si02 because higher purity can be obtained. Commonly, silicon alkoxides are used as single-source precursors. The most common alkoxide precursor is tetraethyl orthosilicate or tetraethoxysilane (TEOS), which will react 700°C and 1 torr ... 

The sol-gel process is a novel technique for the preparation of various inorganic oxide glasses at low temperatures without melting.51 They are produced by condensation-polymerization of a suitable metal alkoxide. For instance, the sol-gel process of tetraethyl orthosilicate (TEOS) consists of the hydrolysis reaction... 

The chemistry involved in the formation of mesoporous silica thin films is qualitatively well understood. However, specific reaction mechanisms of the individual steps are still debated. In addition, owing to the complexity of the sol-gel reaction pathways and cooperative self-assembly, full kinetic models have not been developed. From the time of mixing, hydrolysis reactions, condensation reactions, protonation and deprotonation, dynamic exchange with solution nucleophiles, complexation with solution ions and surfactants, and self-assembly, all occur in parallel and are discussed here. Although the sol-gel reactions involved may be acid or base catalyzed, mesoporous silica film formation is carried out under acidic conditions, as silica species are metastable and the relative rates of hydrolysis and condensation reactions lead to interconnected structures as opposed to the stable sols produced at higher pH. Silicon alkoxides are the primary silica source (tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, etc.) and are abbreviated TMOS, TEOS, and TPOS, respectively. Starting from the alkoxide, Si(OR)4, in ROH and H2O solution, some of the general reactions are ... 

T3.8.2.3 Obtained in Microemulsion Ammonium hydroxide was added to a solution containing Igepal Co520 (surfactant) and cyclohexane, and the mixture was shaken to obtain a microemulsion. Solutions of zirconium n-propoxide in acetylacetone and n-butanol (l 0.5 3) and of tetraethyl orthosilicate in water and ethanol (1 1 4) were mixed for 3 h, and the mixture was added to the microemulsion with stirring, and hydrolyzed for 3 d. The final concentrations of alkoxides were 0.0075 M, the water surfactant molar ratio was 0.8, and the water alkoxide molar ratio was 10. The powder was washed with acetone and heated at 900°C for 2 h. 

The first method uses the controlled concurrent hydrolysis of tetraethoxytitanium (IV) and tetraethoxysilane. This procedure has been labeled the mixed alkoxide method. Frequently, the acronyms TET (tetraethyltitanate) and TEOS (tetraethyl orthosilicate or tetraethoxysilane) are used for the respective reactants these are derived from the alternative names tetraethyl titanate and tetraethylorthosilicate. In examples described in patents, the synthesis involves adding TET to TEOS and then combining the alkoxide mixture with an aqueous solution of a SDA, which is typically tetrapropylammonium hydroxide (TPAOH). The resulting precursor mixture is then heated to a temperature of 175 °C to initiate crystallization. Subsequent washing of the crystallized solid with water, drying, and air calcination produces framework titanium-containing silicalite. 

The preparation method, firstly reported by Taramasso et al. [10] in 1983, is based on the controlled hydrolysis of an aqueous solution containing tetraethyl-orthosilicate (TEOS), tetraethylorthotitanate (TEOT) and tetrapropylammonium hydroxide (TPAOH) as template. This procedure is known as the mixed alkoxide method. Another procedure has been proposed, known as the dissolved titanium method, which makes use of colloidal siUca instead of TEOS [11]. The mixed alkoxide method allows the best control of the synthesis parameters, assuring the crystallization of high quaUty products. 

The alkoxides, Si(OR) such as tetraethyl orthosilicate (TEOS), Si(OC2Hj) or tetra-methyl orthosilicate (TMOS), Si(OCHj) are the well started precursors in the sol-gel formation. Alkoxides are ideal chemical precursors for sol-gel synthesis because they react readily with water, in which the hydrolysis of Si atom takes place to form a hydroxytriethyl orthosilicate or a hydroxytrimethyl orthosilicate.The reaction is called hydrolysis, because a hydroxyl ion becomes attached to the silicon atom as follows ... 

Mullite formation from an all-alkoxide system was achieved by Schneider et al. (1992). In the method used by them, tetraethyl orthosilicate (TEOS) and aluminum butylate were mixed in the equivalent oxide ratio 72 wt% (60mol%) AI2O3 and 28 wt% (40mol%) SiOi as the standard conposition. 2-propanol was added to the mixture as the solvent, followed by the addition ofwater in drops to cause hydrolysis. The final product was a homogeneous white gel, which was dried at 110°C and calcined at different temperatures for 15 h ... 

The sol-gel reaction during the formation of silica particles in the multiple emulsion system started in the external oil phase containing the precursor alkoxide type (tetraethyl orthosilicate, TEOS), as shown in Figure 7.25. Under stirring, the TEOS molecules can penetrate the surfactant layer surrounding the aqueous phase, and then hydrolysis can start. As hydrolysis proceeds, the Si-OH based molecules diffuse and dissolve in the aqueous phase. A gel network is formed by condensation, yielding the insoluble hydrated silica encapsulating the retinol molecules. The water content in the multiple emulsion was demonstrated to impart the final shape and size distribution of the particles. 

The synthesis of a sol-gel follows three primary steps [1]. First, the transition metal monomers are hydrolyzed to form a tetrahydroxy alkoxide. Because most sol-gels are formed fi om silica monomers, the remainder of the discussion will focus on silica sol-gels. The hydrolysis step for silica sol-gels typically involves the use of either tetramethyl orthosilicate (TMOS) or tetraethyl... 

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