Sol-gel hydrolysis

Complexation of (124) and (125) with [ Rh(COD)Cl 2] in the presence of Si(OEt)4, followed by sol-gel hydrolysis condensation, afforded new catalytic chiral hybrid material. The catalytic activities and selectivities of these solid materials have been studied in the asymmetric hydro-gen-transfer reduction of prochiral ketones and compared to that of the homogeneous rhodium complexes containing the same ligands (124) and (125) 307... 

Ba-hexaaluminate Sol-gel hydrolysis of organic precursors A1 alkoxy-ester and Ba acetate 1200-1300 Ba-P-Al203, no surface area Ceramic meterials 19... 

It is also possible to form reinforcing filler particles in situ, for example by the sol-gel hydrolysis and condensation of precursors such as organosilicates.82 This is one of the topics covered in Chapter 8 on composite materials in general. 

A relatively new area that involves silicon-containing materials is the synthesis of ultrastructure materials, that is materials in which structure can be controlled at the level of around 100 A. An example of such a synthesis is the sol-gel hydrolysis of alkoxysilanes (organosilicates) to give silica, Si02.1-14 The reaction is complicated, involving polymerization and branching, but a typical overall reaction may be written... 

Samples of mesoporous titanium dioxide were synthesized via sol-gel hydrolysis of Ti(IV) tetrabutoxide in 1-butanol by atmospheric moisture in the presence of a structure-directing template dibenzo-18-crown-6 ether. After completion of the sol-gel transformation and ageing of the precipitate, the mother liquor with the precipitate was exposed to hydrothermal treatment (HTT) at 100 - 175 °C for 24 h. Then, the precipitate was separated, dried, and calcinated on air at 350 - 500 °C for 4 hours. 

The design of the interstices filling in colloidal crystals with appropriate media and subsequently fluid-solid transformation is central to the whole synthesis. Fluid precursors in the voids of crystal arrays can solidify by polymerization and sol-gel hydrolysis. More recently, many methods have been developed including salt precipitation and chemical conversion, chemical vapor deposition (CVD), spraying techniques (spray pyrolysis, ion spraying, and laser spraying), nanocrystal deposition and sintering, oxide and salt reduction, electrodeposition, and electroless deposition. 

Poly(w-carboranyl-siloxane) elastomers containing a mixture of di-methyl-and methyl(phenyl)-silyl units were synthesised using the Ferric Chloride catalyzed condensation reaction between di-chloro-di-organosilane and 1,7-bis(di-methyl(methoxy)silyl)-/w-carborane. Silica Nano tubes synthesised using i 7situ growth of DL Tartaric acid nanocrystals and concurrent sol-gel hydrolysis and condensation of silane. 

The supports were synthesised by sol-gel hydrolysis of aluminium trisecbutoxide (Aldrich) and magnesium nitrate (Mallinckrodt) [4]. The resulting gels were dried at 120°C for 8 h to eliminate the excess alcohol then calcined in flowing air at 600°C for 8 h. 

Fig. 8-4 Molecular imprinting in bulk silica gel a sol-gel hydrolysis and polycondensation (pH 2.0), b capping of surface silanols...

At low pH, acid catalysis promotes hydrolysis but hinders both condensation and dissolution reactions, " leading to small and homogeneous particles. Base catalysis of sol-gel hydrolysis and condensation reactions, in contrast, promotes fast condensation and dissolution. This leads to the production of an inhomogeneous system due to rapid condensation of the hydrolyzed precursor monomers and to dense silica particles formed by the ripening of aggregates during the collision of droplets. As a result, the microparticles show essentially no porosity, with the particles being stabilized by a water/surfactant layer on the particle surface that prevents particle precipitation. ... 

An elegant example that illustrates the enormous potential of this area is that provided by the use of the hydrophilic polyether domains of phase-separated polyis-oprene-b-poly(ethylene oxide) (PI-b-PEO) as a reaction medium for the sol-gel hydrolysis of silicon and aluminum alkoxides [62]. The resulting structures can, for example, be subsequently dispersed in a solvent and consist of crosslinked silica/alumi-na/PEO nano-objects solubilized by the polyisoprene chains (Fig. 1.8). 

Fig. 1.8 Nano-objects with controlled shape and size from block copolymer mesophases At the top left, phase-separated PI-b-PEO is shown, where the spheres consist of the PEO block. Subsequent dispersion and sol-gel hydrolysis of silicon and aluminum alkoxides in the PEO block leads to swelling of this block and, if desired, morphological transitions. Dissolution of the PI block in a selective solvent leads to hai nano-objects consisting of cross-linked silica/alumina/PEO (Adapted from [62]).

Figure 5 Preparation procedures used to create imprinted silica comprising (a) sol-gel hydrolysis and condensation catalyzed by HCl and (b) carbamate deprotection with trimethyl-silyliodide in acetonitrile.

The sol-gel hydrolysis and condensation process can easily take place within an acrylic resin matrix. The reaction described in Scheme 1 takes place within an acrylate matrix, allowing for low viscosity (800 to 3000 mPa s) formulations suitable for coating applications. The low viscosity of these compositions is surprising, as fumed silica (Aerosil ) is often used for rheological purposes, and increases the viscosity of the system upon addition of several percent. In these systems fumed silica is a major component (Fig. 5). 

Sol-gel hydrolysis and condensation can be carried out within a polymeric matrix to generate particles of the ceramic material, typically with an average diameter of a few hundred angstroms. Silica has been... 

An early conjecture of Dislich in this direction is noteworthy. The extraordinary homogeneity of the final ceramic product obtained by the sol-gel hydrolysis of a mixtnre of alkoxides of various metals in alcoholic solution, led him to suggest that in addition to the advantage of much more efficient mixing at the molecular level in the initial precnrsor solntion, some sort of complexation reactions must be occurring amongst the alkoxides of various metals in the initial stages ... 

The sol-gel hydrolysis of alkoxysUanes, the most intensively explored one, basically proceeds in two steps. The first step is the hydrolytic replacement of alkoxy groups, OR, by hydroxyl groups, OH, shown schematically in Equation (1.1) for the first alkoxy group ... 

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