Alkoxides sol-gel route

Chervin CN, Clapsaddle BJ, Chiu HW, Gadi AE, Satcher JH Jr, Kauzlarich SM (2005) Aerogel Synthesis of Yttria-Stabilized Zirctmia by a Non-Alkoxide Sol-Gel Route Chem Mater 17 3345-3351 Gash AE, Pantoya M, Satcher JH, Simpson RL (2008) Nanostructured energetic materials aerogel thermite composites. Polymer Preprints (American Chemical Society, Division of Polymer Chemistry) 49 558-559... 

Suh DJ, Park T, Kim W, Hong 1 (2003) Synthesis of high-surface-atea ruthenium oxide aerogels by non-alkoxide sol-gel route. J Power Sources 117 1-6. 

Chervin CN, Clapsaddle BJ, Chiu HW, Gash AE, Sateher JH, Kauzlarich SM (2005) Aerogel synthesis of yttria-stabilized zirconia by a non-alkoxide sol-gel route. Chem Mater 17 3345 3351. 

Mentruit MP, Palomino GT, Arean CO (2001) The alkoxide sol-gel route to high-sraface-area metal oxides. Trend Inorg Chem 7 1-14... 

Mechanistic studies are particularly needed for the hydrolysis and polymerization reactions that occur in sol-gel processing. Currently, little is known about these reactions, even in simple systems. A short list of needs includes such rudimentary data as the kinetics of hydrolysis and polymerization of single alkoxide sol-gel systems and identification of the species present at various stages of gel polymerization. A study of the kinetics of hydrolysis and polymerization of double alkoxide sol-gel systems might lead to the production of more homogeneous ceramics by sol-gel routes. Another major area for exploration is the chemistry of sol-gel systems that might lead to nonoxide ceramics. 

Chemical and physical vapor deposition technique has been widely applied for the preparation of such photocatalytic thin films. Since these vapor methods need an instrumental setup which enables control of temperature and pressure, their initial and running costs are generally high and the size of substrate is limited. Spray method, in which titanium alkoxide and water is sprayed on a substrate heated at a desired temperature, affords Ti02 thin films.69) However, like the sol-gel route, the physical properties and photocatalytic activity of Ti02 strongly depend on many factors such as temperature of substrate, flow rate of carrier gas, and partial pressure of starting material in the system. 

The various steps in the sol-gel technique described above may or may not be strictly followed in practice. Thus, many complex metal oxides are prepared by a modified sol-gel route without actually preparing metal alkoxides. For example, a transition metal salt solution is converted into a gel by the addition of an appropriate organic reagent. In the case of cuprate superconductors, an equimolar proportion of citric acid is added to the solution of metal nitrates, followed by ethylene-diamine until the solution attains a pH of 6-6.5. The blue sol is concentrated to obtain the gel. The xerogel is obtained by heating at approximately 420 K. The xerogel is decomposed at an appropriate temperature to obtain the cuprate. 

The ability to form hybrid silica glasses under aqueous, room-temperature conditions (at which proteins and cells are active) opens up the possibility to extend sol-gel processing to the encapsulation of biologicals. However, as mentioned earlier, the traditional sol-gel route using silicon alkoxides has to be adapted to avoid excess alcohol as well as low pH conditions (see section 3.2.1). Moreover, because biospecies could be denatured by covalent bonding, most bioorganic-inorganic composites are Class 1 hybrids. 

Preparation of metal oxides by the sol—gel route proceeds through three basic steps (/) partial hydrolysis of metal alkoxides to form reactive monomers (2) the polycondensation of these monomers to form coUoid-like oligomers (sol formation) and (i) additional hydrolysis to promote polymerization and cross-linking leading to a three-dimensional matrix (gel formation). Although presented herein sequentially, these reactions occur simultaneously after the initial processing stage. 

Acosta et al. [71] described the nonhydrolytic sol-gel route for the preparation of oxides, in particular, for silica, alumina, silica-alumina, and titania. In a classical hydrolytic route, the M—O bond of the alkoxide is cleaved [Eq. (1)], whereas in the nonhydrolytic route, the O—C bond is cleaved. 

The discovery of high-77 superconductors in the late 1980s led to the need to produce homogeneous and high-purity complex metal oxide species, a need that could be met by the use of sol-gel techniques. The first sol-gel routes to yttrium barium copper oxide (YBCO) involved coprecipitation,222 but latterly, routes based on the hydrolysis and condensation of yttrium and barium alkoxides with copper methoxyethoxide have been developed. It is found that the hydrolysis ratio used in the production of the gel can affect the temperature of the onset of Tc and the sharpness of the transition to superconductivity.223... 

Hydrolytic and non-hydrolytic sol-gel routes are implemented to prepare various pure and silica-dispersed vanadium- or niobium-based oxide catalysts corresponding to the compositions Nb-V, Sb-V and Nb-V-M (M = Sb, Mo, Si). Starting reagents in the hydrolytic procedure are isopropanol solutions of the metal alkoxides. The non-hydrolytic route is based on reactions between metal and Si alkoxides and hexane suspensions of niobium(V) chloride. The catalysts are tested in propane oxidative dehydrogenation. NbVOs, SbV04 and Nb2Mo30n are the major crystalline phases detected in the fresh catalysts, but structural modifications are in some cases observed after the use in the catalytic tests. At 500 C, propane conversions of 30 % and selectivities to propene between 20 and 40 % are attained. When the space velocity is decreased, acrolein is in some cases found as by-product. 

Alkoxide sol-gel precursors can be converted to the same hydroxylated activated substances by nonhydrolytic routes. The thermal intermolecular hydroxylation was used as a first step for gelation of Zr02 at 200-300°C in a conventional reactor [23] or inside the channels of mesostmctured SBA-15 siKca [24]. Partially hydroxylated alkoxides can be synthesized by reactions between metal chlorides and alcohols with electron-donor substituents or reactions between basic alkoxides and carbonyl compounds like ketones [9]. 

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