Alkoxysilanes precursors

The exact procedure for entrapment will depend on the nature of the catalyst and the required sol-gel, but for enzyme entrapment it generally involves the hydrolysis or partial hydrolysis of an alkoxysilane precursor, giving a precursor sol-gel solution, followed by addition of more solvent and of the catalyst. Figure 5.7 provides a simple... 

The alkoxysilane sol-gel process is an efficient method to prepare silica glass by the hydrolysis of alkoxysilane precursors and by subsequent con-... 

Precursor solutions for the phosphates are much easier to handle, since the pH is not extreme, all components are liquid, and typically no two phase systems which are difficult to mix are formed, as could happen with alkoxysilane precursors. The additional problems encountered when doing zeolite synthesis on miniature scale have been discussed by Newsam et al. [4], and include ... 

Inorganic phase - silica network, was formed by sol-gel process from alkoxysilane precursors - TEOS or alkoxysilane-endcapped prepolymer SED2000. This endcapped prepolymer was prepared by alkoxy functionalization of oligomer D2000 with (3-glycidyloxypropyl)trimethoxysilane (GTMS). 

The reaction mechanisms are the same for both silicate and alkoxysilane precursors. However, from a mechanistic point of view, reactions under acidic or basic conditions have to be considered separately. Under acidic conditions, that is, at a pH below the PZC, the oxygen atom of a =Si—0 , =Si—OH, or =Si—OR group is protonated in a rapid first step (Eq. (1.3)). A good leaving group (water or alcohol) is thus created. In addition, electron density is withdrawn from the central silicon atom, rendering it more electrophilic and thus more susceptible to attack by water (in hydrolysis reactions) or silanol groups (in condensation reactions). 

The self-assembly properties of alkoxysilane precursors were shown to generate organosilicas with different size and shape [55]. A tris-silylated urea-based compound was also studied upon hydrolysis-condensation it gave hybrid materials with different shapes such as films, spheres, or tubes. A self-templating process was also demonstrated using electron microscopy and X-ray diffraction. 

Table 6.1 Selected examples of commercially available functional alkoxysilane precursors.

A new route for the synthesis of mesoporous phenylene-silicas from allylo-rganosilane precursors instead of the existing alkoxysilane precursors, often difficult to find in high purity, was proposed in 2005 by Inagaki s group. The... 

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