Vaporized silica, pyrogenic powder

Four main types of porous silica adsorbents have been identified compacts of pyrogenic powders, precipitated silicas, silica gels, and zeolitic silicas. The importance of porosity relative to the adsorptive properties of each group is reviewed, with particular reference to the adsorption of nitrogen, argon, and water vapor. The differences in size and specificity of these adsorptive molecules may be exploited to explore the surface properties of each grade of silica. A notable feature cf Silicalite I, which is the best known of the zeolitic silicas, is its remarkable hydrophobic character. Furthermore, the uniform tubular pore structure of this microporous silica is responsible for other highly distinctive properties. 

Aerosils or pyrogenic silica. Powders made by condensing silica from the vapor phase at elevated temperature. The silica vapor is produced by a) direct volatilization of SiOj, (6) reduction of SiOj to volatile SiO which is reoxidized, (c) oxidation of volatile silicon compounds such as chloride or esters, or (d) vapor phase hydrolysis of SiF. ... 

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