Tetramethoxysilane hydrolysis

Further examination has shown that the acid content should be small in order for the solution to become spinnable in the course of hydrolysis and polycondensation. It has been found (4 ) that very large concentrations at more than 0.15 in the [HCl]/[Metal alkoxide] ratio of acid catalyst produce round-shaped particles in the tetra-ethoxysilane (7) and tetramethoxysilane solutions, and so no spinnability appears. 

The sol-gel process involves the preparation of inorganic matrices via three steps. Components of the sol-gel cocktail are the sol-gel precursor (e.g. tetramethoxysilane), water, a catalyst (acids or bases), the indicator chemistry and a solvent such as ethanol. Mixing these components causes hydrolysis of the ester, silanol-ester condensation, and silanol-silanol condensation of the precursors ... 

For silica, much of the work in this area was carried out in the 1980s and the reader is referred to the series of proceedings volumes from the Materials Research Society entitled Better Ceramics through Chemistry. 63 To illustrate typical precursor structures, and the role of reaction chemistry on precursor structure, Fig. 2.3 presents capillary gas chromatography results for the hydrolysis of tetramethoxysilane [TMOS Si(OCH3)4].72 Typical polymeric species formed (Fig. 2.3a) include linear and cyclic structures. 

Tang et al. used columns packed with a slurry of beads suspended in supercritical C02. This packed column was filled with a dilute sol solution prepared by hydrolysis and polycondensation of tetramethoxysilane and ethyltri-methoxysilane precursors. The column was dried using supercritical C02 and heated first to 120 °C for 5 h followed by another 5 h at a temperature of 250 °C [108-110]. Column efficiencies of 127,000 and 410,000 plates/m were reported... 

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]. 

By using these observed rate constants for hydrolysis and condensation, a detailed formalism has been developed that treats the various reactions at the functional-group level (7-9). The observed rate constants were reported (8) for the acid-catalyzed hydrolysis and condensation of monomeric species prepared from Si(OCH3)4 (tetramethoxysilane [TMOS]). In this chapter, we report the observed rate constants for condensation of the dimeric species (CH30)3Si-0-Si(0CH3)3. 

Scheme 1. Hydrolysis and condensation of a silicious acid precursor (tetramethoxysilane, TMOS)...

Citrate - phosphate buffer was prepared freshly by mbdng 500 mM sodium phosphate with 500 mM citric acid resulting in titration to appropriate pH. A stock solution of 20 mg/ml R5 peptide solution was prepared in citrate -phosphate buffer of appropriate pH for each experiment. Immediately prior to each experiment, a fresh solution of IM tetramethoxysilane (TMOS) was prepared via addition to 1 mM HCl. (The pre-hydrolysis method adopted for TMOS was the one described by Kroger, Sumper and co-workers ). 

Figure 2 shows the gel time as a function of pH for the solutions produced by hydrolysis of tetramethoxysilane. The overall pattern, with a maximum between pH 1 and 2, does not appear to be affected by the presence of methanol however comparison of ion-exchanged solutions with and without added methanol showed that the presence of methanol does increase individual gel times by 50% or more. 

Acid or base is used as a catalyst for the hydrolysis of silicon alkoxide. Acid or base with a mole ratio to alkoxide as low as 0.01 or less is usually enough for the catalytic effect. On the other hand, water with a mole ratio to alkoxide as high as 10 or more is used for the complete hydrolysis of alkoxide. We have found, however, that monolithic opaque gels composed of micrometer-sized silica particles can be prepared from tetramethoxysilane (TMOS) when... 

The Fe complex of 1 (R = -SO3H) has been incorporated into sol-gel silica materials by acidic or basic hydrolysis of tetramethoxysilane or methyltrimetho-xysilane [49]. For example, 24 mg of the phthalocyanine per milliliter water was added to a mixture of 7.38 mL tetramethoxysilane and 1.75 mL water. The Fe complex was incorporated in an aggregated form. The composite was tested as a catalyst in oxidation reactions. 

The advantages associated with the sol-gel process make it a suitable method for the preparation of engineering products [1, 2]. In most cases silica gels are prepared by hydrolysis of a mixture of silicic acid esters such as tetramethoxysilane (TMOS), alcohol (methanol), and water under acid or basic conditions. Different solvents are used to avoid the development of cracks during the drying step [3, 4]. Several investigations have focused on the solvent effects on the kinetics of the hydrolysis reaction and on the influence on pore size distribution and surface area [5,6]. 

Fig. 18.3. Porosity distribution in a silica gel obtained by alkoxide hydrolysis (TMOS tetramethoxysilane). Drying control chemical agent (DCCA) allows a modification of porosity distribution .

The groups of Veith, Braunstein, and Bedford reported the use of Pluronic P123 (a polymeric material) as template for the pore structure [52]. Hydrolysis and co-condensation of l,3-bis(3-trimethoxysilylpropyl)imidazolium chloride and tetramethoxysilane in the presence of these vesicles under acidic conditions, followed by immobilization of palladium precursors, resulted in a highly ordered mesoporous material with a two-dimensional hexagonal pore structure with palladium nanoparticles inside the pores. These palladium particles have been successfully utilized for the Suzuki cross-coupling of aryUiaUdes with aryl-boronic acids in water, as well as the aerobic oxidation of alcohols under oxygen and air atmospheres as shown in Scheme 4.10 [53]. 

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