Page 156 - Adsorbents fundamentals and applications

CHEMICAL MODIFICATION OF SILICAS AND MOLECULAR IMPRINTING 141

of the mesosilicates. Direct synthesis based on templating appears to be a more
attractive route. Thus, pure oxides, such as those of Pb, Sb, Ti, Nb, Ta, Al,
Mn, Ce, Ha, Zr, V, and W, and composites, such as V-Mg-O, Al-B-O, Al-
P-O, V-P-O, Zr-P-O, and Al-P-V-O, have been prepared by the direct route.
With V-Mg-O used as an example (Ruckenstein and Chao, 2001; Chao and
Ruckenstein, 2002a and 2002b), the crystallinity as well as the morphology of
the mesoporous product depend on the template surfactant, V/Mg ratio, and pH.
Interestingly, fibrous and lamellar structures could be formed depending on the
synthesis conditions.

6.5. CHEMICAL MODIFICATION OF SILICAS AND MOLECULAR
IMPRINTING
Silicas are strong, mesoporous, and — more importantly — abundant in surface
hydroxyl groups. These properties make them ideally suited for chemical graft-
ing. The grafted sorbents are widely used for a number of applications. The most
important application is for the stationary phases in a variety of chromatogra-
phies. The grafted silicas are also promising sorbents for selective adsorption.
An example will be given for the potential use of a grafted silica as a selective
sorbent for acid gas removal. The most commonly used grafting compound is
3-aminopropyltriethoxysilane:

(CH 3 CH 2 O) 3 SiCH 2 CH 2 CH 2 NH 2

Upon refluxing of silicas with a solution of the silane at mild temperatures
◦
(e.g., 70 C), the surface is grafted by the following reaction:
C 2 H 5 O
OH
+ C 2 H 5 O SiCH 2 CH 2 CH 2 NH 2
OH
C 2 H 5 O

O(C 2 H 5 )
O
2C 2 H 5 OH +
SiCH 2 CH 2 CH 2 NH 2
O

A large number of silanes carrying different functional groups have been used
as the grafting agents. Due to the high separation power of chromatography, any
small differences in interactions with the sorbent by the different components in
the mixture will lead to resolution. New stationary phases and new applications
are being developed rapidly, particularly for bioseparations (Belter et al., 1988;
Garcia et al., 1999). Some of the silanes used for grafting are shown in Table 6.2.