ABSTRACT
Compacts of Pyrogenic Powders . . . . . . . . . . . . . . . 590
Precipitated Silicas . . . . . . . . . . . . . . . . . . . . . . . . . . 590
Silica Gels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591
Zeolitic Silicas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592
Ordered Mesoporous Silicas . . . . . . . . . . . . . . . . . . 593
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594
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 of 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.
