Page 155 - Adsorbents fundamentals and applications

140 SILICA GEL, MCM, AND ACTIVATED ALUMINA

non-silica materials with structures similar to MCM-41 can be synthesized (Ying,
et al., 1999; Sun and Ying, 1997).
The unique feature of MCM-41 is the large uniform pore structure (and hence
3
a remarkably large pore volume >0.7 cm /g). This unique feature has made
these materials promising for both adsorption and catalysis (as discussed by Ying
et al., 1999; Ravikovitch et al., 1998). However, this unique feature may not be
attractive for gas adsorption because the interaction potentials are not enhanced
within the pores, as discussed earlier. Consequently, relatively few studies have
been made on MCM-41 as an adsorbent.
In order to obtain useful adsorption properties, MCM-41 needs to be modified
in either surface chemistry or pore structure. Modification of silica surfaces will
be discussed further in the next section. The same techniques of modification have
been used for MCM-41. The reported modifications have been based mainly on
the use of reactive silanes that contain organic groups (such as alkyls) and chlo-
ride. The formed MCM-41 has hydroxyl groups, and by reacting the hydroxyl
group with silanes, that is, silanation, pore sizes can be reduced by the grafted
silanes (Feng et al., 1997; Jaroniec et al., 1998). The grafting can be accom-
plished by many other compounds such as metal alkoxides and halides; hence
the chemistry of the surface can be altered or functionalized (Moller and Bein,
1998; Jaroniec et al., 1998). Functionalization of the surface can be also accom-
plished before the final calcination step by directly displacing the surfactants
with reactive silanes (Antochsuk and Jaroniec, 2000). A technique that reduces
only the pore size at the opening regions has been suggested (Zhao et al., 1999)
by grafting a controlled number of layers of silica in the opening regions. This
procedure was performed by displacing the surfactant in the opening regions by
H , followed by silanation with Si(OEt) 4 and hydrolysis, and finally calcination.
+
Only a few promising applications of the MCM-41 and its modified forms
have been reported. Izumi and co-workers have reported the use of MCM-41 for
VOC (Izumi, 1996) and SO 2 removal (Teraoka et al., 2000) by taking advantage
of the weak bonds with the surface, hence ease in desorption. They also reported
a low-temperature synthesis route for MCM-41 at very low pH (<1), which is a
possible low-cost method. Feng et al. (1997) grafted the MCM-41 with a silane-
containing thiol (−SH) group and produced a sorbent that is highly selective for
binding heavy metal ions such as mercury, silver, and lead from wastewaters. The
sorbent is also regenerable with HCl. Because of the large arrays of functionalities
and pore sizes that can be achieved with the MCM-41 material, unique adsorption
properties will no doubt be obtained. However, due to the cost, its potential use
appears to be limited to specialized applications.
The syntheses of a large number of templated, mesoporous materials other
than silica have also been reported. These materials are mainly of interest to
catalysis and hence will be described only briefly here. A good summary of
their synthesis routes and factors involved in the syntheses has been given by
Ruckenstein and co-worker (Ruckenstein and Chao, 2001; Chao and Ruckenstein,
2002a and 2002b). Metals/elements such as Al, B, Ti, Zr, V, Pd, and Mn can
be introduced by either grafting or isomorphous substitution onto the framework