Sol route

Alumina is much more adapted to the particulate sol route while silica and titania are more flexible in that they can lead to both particulate and polymeric gels as the membrane precursors. This is particularly true with the silica system. Hydrolysis and condensation reactions in silica systems are known to be slower than most other alkoxides. Their reactions, therefore, are easier to control. The fact that it is much easier to prepare alumina sol-gel membranes than their titania equivalents may suggest that the plate-shaped particles like alumina can be more readily processed to form defect-free membranes than spherical particles such as titania [van Praag et al., 1989]. 

Porous membranes have been prepared by the sol-gel process from a variety of metal oxides and composite oxides. The sol-gel process is divided into two main routes the polymeric sol-gel route and the colloidal sol-gel route [3]. A metal alkoxide or inorganic salt is hydrolyzed and a simultaneous condensation reaction occurs to form polymeric or colloidal sols. In the colloidal sol route. 

Microporous (< 2 nm) silica adsorbents are prepared by the sol-gel method following a polymeric sol route. The adsorbents have a large surface area (600-900 m /g). Doping alumina in silica adsorbents improves their thermal and hydrothermal stability. These silica adsorbents can be safely used at temperatures lower than 200°C. 

TABLE 11-1. PREPARATION CONDITIONS OF SELECTED METAL SULFIDE SOLS (ROUTE C)... 

Haber J A, Gunda N V and Buhro WE 1998 Nanostruoture by design solution phase prooessing routes to nanoorystalline metals, oeramios, intermetallios and oomposites J. Aerosol Sol. 29 637... 

S.3.2 Sol-Gel Encapsulation of Reactive Species Another new and attractive route for tailoring electrode surfaces involves the low-temperature encapsulation of recognition species within sol-gel films (41,42). Such ceramic films are prepared by the hydrolysis of an alkoxide precursor such as, Si(OCH3)4 under acidic or basic condensation, followed by polycondensation of the hydroxylated monomer to form a three-dimensional interconnected porous network. The resulting porous glass-like material can physically retain the desired modifier but permits its interaction with the analyte that diffuses into the matrix. Besides their ability to entrap the modifier, sol-gel processes offer tunability of the physical characteristics... 

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. 

There are various routes to synthesize rubber-based organic-inorganic hybrid composites through sol-gel process. These are mentioned in brief below. 

There are a variety of routes currently utilized to fabricate a wide range of hollow capsules of various compositions. Among the more traditional methods are nozzle reactor processes, emnlsion/phase-separation procednres (often combined with sol-gel processing), and sacrificial core techniques [78], Self-assembly is an elegant and attractive approach for the preparation of hollow capsules. Vesicles [79,80], dendrimers [81,82], and block hollow copolymer spheres [83,84] are all examples of self-assembled hollow containers that are promising for the encapsnlation of various materials. 

Silica is the support of choice for catalysts used in processes operated at relatively low temperatures (below about 300 °C), such as hydrogenations, polymerizations or some oxidations. Its properties, such as pore size, particle size and surface area are easy to adjust to meet the specific requirements of particular applications. Compared with alumina, silica possesses lower thermal stability, and its propensity to form volatile hydroxides in steam at elevated temperatures also limits its applicability as a support. Most silica supports are made by one of two different preparation routes sol-gel precipitation to produce silica xerogels and flame hydrolysis to give so-called fumed silica. 

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