Solgel Route Based on the Hydrolysis-Condensation of Metal Alkoxides

3 Solgel Route Based on the Hydrolysis-Condensation of Metal Alkoxides 

Another route for the production of materials involves the reaction of hydrolysis-condensation of metal alkoxides with water. We study here the important case of amorphous silica synthesis. In this case [38,39,44], silicic acid is first produced by the hydrolysis of a silicon alkoxide, formally a silicic acid ether. The silicic acids consequently formed can either undergo self-condensation, or condensation with the alkoxide. The global reaction continues as a condensation polymerization to form high molecular weight polysilicates. These polysilicates then connect together to form a network, whose pores are filled with solvent molecules, that is, a gel is formed [45], 

The solgel procedure, as was previously recognized, is a method for solid materials synthesis, carried out in a liquid at a low temperature (typically T 100°C). The synthesized inorganic solids are formed by chemical transformation of chemical solutes termed precursors [39], More specifically 

To better understand this solgel procedure, the synthesis of silica microspheres using the Stobe-Fink-Bohn (SFB) method [46] and a modification of the SFB method [47-49] are excellent examples. The SFB method consists of the hydrolysis of TEOS (Si(C2H50)4) in ethanol, methanol, //-propanol, or //-butanol in the presence of ammonia as a catalyst [46], In Table 3.3, the batch composition for the synthesis of silica microspheres, by means of TEOS in an alcohol (methanol or isopropanol) in the presence of ammonia as a catalyst, with and without double-distilled water (DDW) in the synthesis media, is presented [47], 

The source materials for the synthesis are TEOS, DDW, methanol (MeOH), isopropanol, and ammonium hydroxide (40 wt % NH4OH in water). The batch preparation, to carry out the synthesis following the recipes shown in Table 3.1, was as follows [46-49]  

---