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Adsorption, Ion Exchange, and Catalysis

2.1 DEFINITIONS

2.1.1 Adsorption

The term “sorption” is used to describe eery type of capture of a substance from the
v
external surface of solids, liquids, or mesomorphs as well as from the internal surface of
oulikides,
porous solids or liquids (Sk 1989). Depending on the type of bonding in ed, v olv
sorption can be classified as follo ws.
(a) Physical sorption . In physical sorption (or physisorption), no e xchange of electrons
is observed; rather intermolecular attractions between forable energy sites take place v a
,
and are therefore independent of the electronic properties of the molecules in olv ed. v
Physisorption is characterized by interaction energies comparable to heats of v aporization
(condensation). The adsorbate is held to the surface by relatiely weak van der W aals
v
forces and multiple layers may be formed with approximately the same heat of adsorption.
The heat of adsorption for physisorption is at most a few kcal/mole and therefore this type
of adsorption is stable only at temperatures below 150 °C.
(b) Chemical sorption . Chemical sorption (or chemisorption) in es an e olv v xchange of
electrons between specific surface sites and solute molecules, and as a result a chemical
bond is formed. Chemisorption is characterized by interaction energies between the sur-
face and adsorbate comparable to the strength of chemical bonds (tens of kcal/mol), and is
consequently much stronger and more stable at high temperatures than physisorption.
Generally, only a single molecular layer can be adsorbed.
(c) Electrostatic sorption (ion e e) xc hang . This is a term reserved for Coulomb attrac-
tive forces between ions and charged functional groups and is commonly classified as ion
exchange.
The most important characteristics of physical and chemical sorption are presented in
Table 2.1.
The term “adsorption” includes the uptak e of gaseous or liquid components of mixtures
from the external and/or internal surface of porous solids. In chemical engineering, adsorp-
tion is called the separation process during which specific components of one phase of a
fluid are transferred onto the surface of a solid adsorbent (McCabe et al ., 1993).

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