Page 147 - Adsorbents fundamentals and applications

132 SILICA GEL, MCM, AND ACTIVATED ALUMINA

of silicic acid, and (2) aggregation of particles of colloidal silica. Silicic acid,
Si(OH) 4 , has a strong tendency to polymerize and form a network of siloxane (Si-
O-Si), leaving a minimum number of uncondensed Si-O-H groups. The pH value
is important in the polymerization process (Iler, 1979). The second route involves
coagulation of silica sols of rather uniform sizes. These submicrometer particles
can coagulate by van der Waals forces or by cations bridging as coagulants.
Commercially, silica is prepared through the first route 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:

Na 2 SiO 3 + 2HC1 + nH 2 O → 2NaC1 + SiO 2 •nH 2 O + H 2 O (6.1)
The hydrosol, on standing, polymerizes into a white jellylike precipitate, which
is silica gel. The resulting gel is washed, dried, and activated. Various silica gels
with a wide range of properties, such as surface area, pore volume, and strength,
can be made by varying the silica concentration, temperature, pH, and activation
temperature (Iler, 1979). Two common types of silica gel are known as regular-
density and low-density silica gels, although they have the same densities (true
2
and bulk). The regular-density gel has a surface area of 750–850 m /g and an
average pore diameter of 22–26 ˚ A, whereas the respective values for the low-
2
density gel are 300–350 m /g and 100–150 ˚ A.
The silica gel is amorphous. By use of high-resolution electron microscopy, it
is known that its amorphous framework is composed of small globular (primary)
particles having sizes of 10 to 20 ˚ A (Rouquerol et al., 1999). Still another route
for preparing silica gel involves reactions of silicon alkoxides with water, and a
wide variety of materials can be produced in this manner (Jones, 1989; Brinker
and Sherer, 1990). The processes based on this route are referred to as “sol-
gel” processing, and they offer many promising possibilities. For silica gel, the
reaction is
ROH
Si(OR) 4 + 2H 2 O −−−→ SiO 2 + 4ROH (6.2)

Silicic acids are also formed by hydrolysis:

Si OR + H 2 O = Si OH + ROH (6.3)

The silicic acids thus formed can then polymerize via:

Si OH + HO Si = Si O Si + H 2 O (6.4)

or

Si OH + RO Si = Si O Si + ROH (6.5)