Tetraethylorthosilicate hydrolysis

Some elucidation of the mechanism of elastomer reinforcement is being obtained by precipitating chemically-generated fillers into network structures rather than blending badly agglomerated filler particles into elastomers prior to their cross-linking. This has been done for a variety of fillers, for example, silica by hydrolysis of organosilicates, titania from titanates, alumina from aluminates, etc. [85-87], A typical, and important, reaction is the acid- or base-catalyzed hydrolysis of tetraethylorthosilicate ... 

Hybrid membranes composed of poly(vinyl alcohol) (PVA) and tetraethylorthosilicate (TEOS), synthetised via hydrolysis and a co-condensation reaction for the pervaporation separation of water-isopropanol mixtures has also been reported [32], These hybrid membranes show a significant improvement in the membrane performance for water-isopropanol mixture separation. The separation factor increased drastically upon increasing the crosslinking (TEOS) density due to a reduction of free volume and increased chain stiffness. However, the separation factor decreased drastically when PVA was crosslinked with the highest amount of TEOS (mass ratio of TEOS to PVA is 2 1). The highest separation selectivity is found to be 900 for PVA TEOS (1.5 1 w/w) at 30°C. For all membranes, the selectivity decreased drastically up to 20 mass % of water in the feed and then remained almost constant beyond 20 mass %, signifying that the separation selectivity is much influenced at lower composition of water in the feed. 

A recent paper [69] presents a new type of PVA hybrid membrane prepared by hydrolysis followed by condensation of a PVA and a tetraethylorthosilicate (TEOS) mixture, which shows a significant performace in water-acetic acid mixture separation. The highest separation selectivity (1116) with a flux of 3.33xft)"2 kg m"2 h"1 at 30 °C for 10% mass of water in the feed has ben obtained by using the membrane containing 1 2 mass ratio of PVA and TEOS. The performance of these membranes was explained on the basis of a reduction of free volume and a decrease of the hydrophylic character owning to the formation of... 

Silica gels may also be produced by the hydrolysis and polycondensation of silicon alkoxides, eg, tetraethylorthosilicate, Si(OC2H5)4. This is often... 

Modern Manufacturing Techniques. Manufacturing techniques for making bulk vitreous silica are for the most part improved variations of the historical processes. The main exception is the sol—gel process (see Sol-gel technology). All processes involve the fusion or viscous sintering of silica particles. The particles can be in the form of a loose powder or a porous preform. The powders can be made from natural quartz or from the decomposition of chemical precursors, such as silicon tetrachloride, and tetraethylorthosilicate (1 EOS). In some approaches, such as flame hydrolysis, the powder is produced and fused in a single step. The improvements made to these techniques deal mainly with the procedures used to prepare the powders, that is, to control purity and particle size, and the specific conditions under which the powders are consolidated. 

PCH materials offer new opportunities for the rational design of heterogeneous catalyst systems, because the pore size distributions are in the supermicropore to small mesopore range (14-25A) and chemical functionality (e.g., acidity) can be introduced by adjusting the composition of the layered silicate host. The approach to designing PCH materials is based on the use of intercalated quaternary ammonium cations and neutral amines as co-surfactants to direct the interlamellar hydrolysis and condensation polymerization of neutral inorganic precursor (for example, tetraethylorthosilicate, TEOS) within the galleries of an ionic lamellar solid. 

Apart from tetraethylorthosilicate (TEOS), the majority of the metal alkoxides have the following setbacks [40] high cost, unavailability, toxicity, and fast hydrolysis rate. Thus, in the synthesis of... 

Batch Composition for the Synthesis of the Silica Microspheres by Means of the Hydrolysis of Tetraethylorthosilicate... 

Figure 2 shows the rates of hydrolysis and condensation of tetraethylorthosilicate (TEOS), which is the most widely studied precursor, vary as a function of pH [5], Under acidic conditions hydrolysis occurs at... 

Figure 2. Schematic diagram showing the variation of hydrolysis and condensation rates of tetraethylorthosilicate with pH, and the effect of relative rate on gel structure.

The functional materials of SBA-15 type were prepared by co-hydrolysis and polycondensation of 1 and x equivalents of tetraethylorthosilicate (TEOS) in presence of P123 as structure directing agent. The materials obtained were called SBAla and lb for x=9 and 19 respectively. Our procedure was the following ... 

Nanosized FezOs-SiOz composites are considered to be promising optical and magnetic materials, catalysts, and magneto-resistive sensors. To obtain the above composites most of authors [1,2] used the combined hydrolysis of Fe salt and tetraethylorthosilicate (TEOS) or introduction of Fe salt into silica sol prepared by TEOS hydrolysis. 

Nanotubes of oxides of several transition metals, as well as of other metals, have been synthesized by employing different methodologies [24, 216-220]. Silica nanotubes were first produced as a spin-off product during the synthesis of spherical silica particles by the hydrolysis of tetraethylorthosilicate (TEOS) in a mixture of water, ammonia, ethanol and D,L-tartaric acid [216]. Since self-assembly reactions are not straightforward with respect to the desired product, particularly its morphology, templated reactions have been employed using carbon nanotubes to... 

Effects of composition ratios and process parameters in silica precursors The synthesis route of silica membranes is schematically given in the upper part of Fig. 8.25. Tetraethylorthosilicate (TEOS) is not hydrolysed directly in water. To obtain a better control, the hydrolysis and condensation reaction rates were decreased by first diluting the TEOS in alcohol (ethanol) and then adding to this mixture a water-acid (HNO3) mixture dropwise under vigorous stirring. The mixture was kept for 3 h at 86°C under reflux conditions. Note that even with this procedure locally and for short times a relative large water excess exists in the reaction zone. 

---