Types of Sol-Gel Processes

Alkoxides are mainly used as reagents in the sol-gel method, but inorganic or organic salts are also used. In this way, two types of sol-gel processes are developed in an alcoholic (organic) or an aqueous medium. 

There are two types of precursors inorganic precursors (metal salts) or metal-organic (alkoxide) precursors. It is not the aim of this chapter to give an overview on all materials made by sol-gel processing, but instead to discuss the chemistry of the precursors, including ways to influence their reactivity, and the basic chemistry of sol-gel processes. 

The characteristics of sol-gel processing allow the application of different strategies for the preparation of solid catalytic materials. A schematic representation is shown in Figure 5.2. The gelation of colloidal solution followed by desolvation of the obtained gel can be applied for the preparation of three types of materials ... 

The high compatibility of sol-gel materials with organic molecules offers the possibility to develop new types of hybrid materials and devices. The inclusion of fiillerenes and fullerene derivatives represents an interesting example of the potentiality of sol-gel processing in this field. Even if several potential applications have been shown for sol-gel-fullerene nanocomposites a larger extension of the basic research in the next future is expected to widen the perspectives of these materials. 

Figure 26-4. Schematic outline for processing of two different types of sol/gel coatings (a) aminosilane coated SiOz soVgels (b) amino-functionalized hybrid sol/gels.

Silica xerogel monoliths are the major class of sol-gel process materials and their fabrication procedures were described extensively in many reviews and papers [54-58]. The principal concepts of fabrication of silica xerogel monoliths have already been described in Section 10.2, and the effects of parameters such as metal alkoxide precursor types, type of catalyst used, H2O Si molar ratio R), steric effect of precursor ligand groups, and functionality of organically modified silanes have also been discussed. Important pioneering research on the influence of these parameters on xerogel fabrication was presented in Refs [9,59-63]. 

Very recently, Wilkes and co-workers 330,331) have reported the preparation and characterization of hybrid materials, which they named as ceramers . Ceramers were synthesized through the incorporation of polymeric or oligomeric PDMS into silicate glasses by a sol-gel process as shown in Reaction Scheme XXIV. During these reactions the variables such as the type of the alkoxide used, amount of water added, the pH value of the reaction system, and the reaction temperature, were carefully monitored. 

Summary Materials containing uniform, nanometer-sized metal particles homogeneously dispersed in a Si02 matrix, with a variable metal loading, were prepared by the sol-gel processing starting from metal salts, alkoxysilanes of the type X(CH2)nSi(OR)3 and, optionally, Si(OR)4. 

Our approach, to achieve a high dispersion of the metal compound while the oxide network is formed, is to employ metal complexes of the type LnM[X(CH2)3Si(OR)3]y in the sol-gel process [2]. The metal ions then cannot aggregate because of complexation, and the metal complexes cannot leach because they are linked to the oxidic support. These complexes are formed in situ on reaction of silanes of the type X(CH2)nSi(OR)3 with suitable metal salts. 

PDMS nanocomposites with layered mica-type silicates were also reported.374 A two-step sol-gel process of the in situ precipitation of silica led to the development of siloxane-based nanocomposites with particularly high transparencies.3 5 Some unusual nanocomposites prepared by threading polymer chains through zeolites, mesoporous silica, or silica nanotubes were reviewed.3 6 Poly(4-vinylpyridine) nanocross-linked by octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane was reported.377... 

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