Sol-gel formation

Scheme 5.6-1 Preparation of supported Lewis acidic chloroaluminate ionic liquids catalysts by impregnation with [BMIM][(AICl3)jfCl] (top), by grafting of l-(triethoxysilylpropyl)-3-methylimidazolium chloride followed by AICI3 addition (middle), and by sol-gel formation followed byAICis addition (bottom) [73].

Both tetraethoxysUane (TEOS) and tetramethoxysilane (TMOS) are used as precursors for sol-gel formation. The hydrolysis reactirm generally takes place in a water-alcohol mixture and is usually catalyzed either by acid or by basic aqueous solutions. 

In any case, it is clear that, for its own nature, sol-gel material is suitable for the formation of many different composite materials [207] inorganic and organic polymers, as well as many different kinds of nano-objects possessing effective electrocatalytic properties, can find stable inclusion into a host matrix possessing all the characteristics previously listed. The most simple approach consists of adding the filler to the siliceous matrix after sol-gel formation. However, composite materials can also be obtained by synthesizing the sol-gel film in the presence of the precursor of the filler, e.g., a monomer [227] or a metal complex [228]. The synthesis of the composite material is finalized in a subsequent step, e.g. by a chemical oxidation or reduction, leading to polymer chains or metal nanoparticles, respectively, included inside the sol-gel matrix. 

The alkoxides, Si(OR) such as tetraethyl orthosilicate (TEOS), Si(OC2Hj) or tetra-methyl orthosilicate (TMOS), Si(OCHj) are the well started precursors in the sol-gel formation. Alkoxides are ideal chemical precursors for sol-gel synthesis because they react readily with water, in which the hydrolysis of Si atom takes place to form a hydroxytriethyl orthosilicate or a hydroxytrimethyl orthosilicate.The reaction is called hydrolysis, because a hydroxyl ion becomes attached to the silicon atom as follows ... 

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