Adsorption - Principles of Chemical Separations with Environmental Applications - page 189

7.5.3 Silica gel

Silica gel is one of the synthetic amorphous silicas. It is a rigid network of spherical
colloidal silica particles. It is often sold in two forms: regular density, which has a surface
area per unit mass range of 750-850 m 2

/ g (average pore diameter = 22-26 Å); and low

density, which has a surface area per unit mass of 300-350 m 2

=
100-150 Å). Silica gel is prepared by mixing a sodium silicate solution with a mineral acid
such as sulfuric or hydrochloric acid. The reaction produces a concentrated dispersion of
finely divided particles of hydrated SiO 2 , known as silica hydrosol or silicic acid. The
hydrosol, on standing, polymerizes into a white jelly-like precipitate, which is silica gel.
This gel is washed, dried and activated. Properties such as surface area per unit mass, pore
volume, and strength are varied by adjusting reaction conditions.
Silica gel is used for water removal applications. Regeneration is achieved by heat-
ing to approximately 150 ◦ C, as compared to 350 ◦ C for zeolites, where the heats
of adsorption for water are considerably higher. Zeolites, however, have the advan-
tage of higher water capacities at low relative pressures; hence they are used at high
temperatures.

/ g (average pore diameter

7.5.4 Zeolites

Zeolites are nanoporous oxide crystalline structures, typically aluminosilicates. The alu-
minum in the structure has a negative charge that must be balanced by a cation, M .
This ionic structure leads to the hydrophilicity of the zeolite. Silicalite, a pure silica
version, is charge neutral and hydrophobic. Zeolites have uniform pore sizes that typi-
cally range from 0.3 to 0.8 nm. The pore size and/or adsorption strength can be altered
by the type and number of cations present in the structure. The void fraction can be
as high as 0.5. Zeolites can selectively adsorb or reject molecules based on their size,
shape, or sorption strength. The molecular sieving effect is a common term associated
with zeolites, and refers to selectivity based on size or shape exclusion. Zeolites can
also provide separations based on competitive sorption. This situation can lead to re-
verse selectivity where a larger molecule can be selectively sorbed and separated from
a smaller molecule. For example, most zeolites are polar adsorbents and will preferen-
tially adsorb polar species (i.e., water) over non-polar species (organics) of comparable
size.
Separation can be based on the molecular-sieve effect and/or selective adsorption. These
separations are governed by several factors [7]:
1 The basic framework structure of the zeolite determines the pore size and the void
volume.
2 The exchange cations, in terms of their specific location in the structure, number density,
charge, and size, affect the molecular-sieve behavior and adsorption selectivity of the