Adsorbents and Adsorption Isotherms

Molecules adsorb on virtually all surfaces. The amount they adsorb is roughly proportional to the amount of the surface. As a result, commercial adsorbents are highly porous, with surface areas typically of several hundred square meters per gram. Some specialized adsorbents have surface areas as high as 3000 m /g. 

In discussing these highly porous materials, we have two goals. First, we must describe what the materials actually are. Though there are a very wide variety of adsorbents, this description must be brief to be within the scope of the book. Second, we are interested in the isotherm, in how the amount adsorbed varies with the concentration in solution. Both adsorbents and isotherms are discussed in the following paragraphs. 

Inorganic materials vary widely. Activated alumina, which has a polar surface, is used largely as a dessicant. It is also used for laboratory-scale chromatography. Sihca gel, consisting of amorphous silicon dioxide, is also used as a dessicant. Clays are used as inexpensive adsorbents for some petroleum-based applications, they have in the past been used once and discarded. Fuller s earth is used to purify oils, an echo of its original purpose to adsorb lanolin from fleece. 

The most important class of inorganic adsorbents is the zeolites, a subclass of molecular sieves. These are crystalline aluminosilicates with specific pore sizes located within small crystals. Two common classes have simple cubic crystals (type A) or body-centered cubic crystals (type X). Sometimes, the type is assigned a number equal to a nominal pore size in the crystals. For example, zeolite 5A with a nominal 5 A pore size is used to separate normal from branched paraffins. 

The isotherms for these various materials are shown schematically in Fig. 15.2-1. Any isotherm with a downward curvature is referred to as favorable, and any isotherm with an upward curvature is referred to as unfavorable. These terms imply that adsorption will frequently be used to capture small amounts of solutes from dilute solutions. A highly favorable isotherm will be especially effective in dilute solutions, whereas a highly unfavorable isotherm will be particularly ineffective under those conditions. While these terms are useful, one must remember that an isotherm that is strongly favorable for adsorption will be strongly unfavorable when it is time to elute the adsorbed species. Such elution is necessary if the adsorbent is to be reused. 

---