Adsorption at Silica, Alumina, and Related Surfaces

Abstract

A main step in many industrial processes is often found in the separation and purification of products. Adsorption technology has an important role in the removal of undesired components, for instance, wastewater and air streams. There are many materials, natural or synthesized, that have sorption capacity. The most important commercial adsorbents are silica gel, activated alumina, activated carbon, and molecular sieve zeolites. The applications for these adsorbents depend on their intrinsic sorptive properties. Silica gel and activated alumina are used for the drying of gases, organic solvents, and transformer oils. Applications of zeolites are distinct from other adsorbents in that for each type, there is no distribution of pore size because the crystal lattice into which the adsorbate molecules can or cannot enter is precisely uniform. For this reason, zeolites are capable of separating effectively on the basis of molecular size. They are used, among others, for the separation of oxygen and argon, the separation of normal from branched paraffins, the drying of gases, and for the removal of water from azeotropic mixtures. Surface polarity corresponds to affinity with polar substances such as water or alcohols. Polar adsorbents are thus called ‘‘hydrophilic,’’ and aluminosilicates, such as zeolites, porous alumina, silica gel, or silica–alumina, are examples of adsorbents of this type. On the other hand, nonpolar adsorbents are generally ‘‘hydrophobic’’ such as silicalite and a new related material-type group called silsesquioxane-derived. These adsorbents have more affinity with oil or hydrocarbons than water. The basic objective of this article is to describe the different adsorption types of some classes of porous materials containing Si and Al such as silica, alumina, zeolite, and, more recently, silsesquioxane-derived. Special attention is given to matrix effect phenomena of compounds on the organoand inorganofunctionalized silica gel surface.