Page 25 - Adsorption Technology & Design, Elsevier (1998)

22 Adsorbents

2.5 POLYMERIC ADSORBENTS

A broad range of synthetic, non-ionic polymers is available particularly for
analytical chromatography applications. For preparative and industrial
uses, commercially available resins in bead form (typically 0.5 mm dia-
meter) are based usually on co-polymers of styrene/divinyl benzene and
acrylic acid esters/divinyl benzene and have a range of surface polarities.
The relevant monomers are emulsion polymerized in the presence of a
solvent which dissolves the monomers but which is a poor swelling agent for
the polymer. This creates the polymer matrix. Surface areas may range up to
750 m2/g.
Selective adsorption properties are obtained from the structure, control-
led distribution of pore sizes, high surface areas and chemical nature of the
matrix. Applications include the recovery of a wide range of solutes from the
aqueous phase, including phenol, benzene, toluene, chlorinated organics,
PCBs, pesticides, antibiotics, acetone, ethanol, detergents, emulsifiers,
dyes, steroids, amino acids, etc. Regeneration may be effected by a variety
of methods which include steam desorption, solvent elution, pH change and
chemical extraction.

2.6 SILICA GEL

Silica gel is a partially dehydrated polymeric form of colloidal silicic acid
with the formula SiO2.nH20. This amorphous material comprises spherical
particles 2-20 nm in size which aggregate to form the adsorbent with pore
sizes in the range 6-25 nm. Surface areas are in the range 100-850 m2/g,
depending on whether the gel is low density or regular density. The surface
comprises mainly SiOH and SiOSi groups and, being polar, it can be used to
adsorb water, alcohols, phenols, amines, etc. by hydrogen bonding mechan-
isms. Other commercial applications include the separation of aromatics
from paraffins and the chromatographic separation of organic molecules.
At low temperatures the ultimate capacity of silica gel for water is higher
than the capacity on alumina or zeolites. At low humidity, however, the
capacity of silica gel for moisture is less than that of a zeolitic desiccant. On
the other hand, silica gel is more easily regenerated by heating to 150~ than
zeolitic materials which need to be heated to about 350~ Silica gel
therefore tends to be used for drying applications in which high capacity is
required at low temperature and moderate water vapour pressures. The
heat of adsorption of water vapour is about 45 kJ/mol. Silica gel may lose
activity through polymerization which involves the surface hydroxyl groups.
Typical properties of adsorbent grade silica gel are summarized in Table 2.3.