Page 268 - Adsorbents fundamentals and applications

PILLARED CLAYS 253

9.2. PILLARED CLAYS
Pillared interlayered clays (PILCs), or pillared clays, are a class of porous, high-
surface area, two-dimensional materials that have been studied extensively for
application as catalysts and, to a much lesser degree, as sorbents for gas separa-
tions. Comprehensive reviews on pillared clays and their catalytic properties are
available (Pinnavaia, 1983; Burch, 1987; Figueras, 1988; Butruille and Pinnavaia,
1996; Gil et al., 2000).
There have been only a few studies on the adsorption properties of pillared
clays, primarily since 1991 (Yang and Baksh, 1991). Some of the results have
been reviewed by Yang and Cheng (1995).
The microporosity, that is micropore-size distribution of pillared clays has been
correctly characterized only recently. For Al 2 O 3 -pillared clay, the main group of
pores have sizes centered around 5.5 ˚ A, with a minor group of pores centered
around 4 ˚ A (Gil and Montes, 1994; Hutson et al., 1998). These micropores can
be controlled and tailored (Hutson et al., 1998) and could have potential applica-
tions as molecular sieves. PILCs are relatively hydrophobic when compared with
zeolites, silica gel, and activated alumina (Malla and Komarneni, 1990). PILCs
and acid-treated clays have been found to be excellent supports for the synthe-
sis of π-complexation sorbents (Cheng and Yang, 1995b; Cho et al., 2001). In
addition, synthetic clays are already produced commercially. Laponite is a syn-
thetic hectorite, which along with montmorillonite (a member of smectite), can
be easily pillared (Yang and Cheng, 1995). The availability of inexpensive clays
of controlled quality is ensured by the availability of the synthetic clays. For
the reasons above, further investigations of PILCs as possible sorbents for gas
separation and purification are warranted.

9.2.1. Syntheses of PILCs
Clays are two-dimensional aluminosilicates with layer structures. Smectite is a
family of clays that has been most studied as precursors for PILCs. Each layer of
the smectite consists of three sub-layers: an alumina layer sandwiched between
two silica layers. Two layers of smectite are depicted in Figure 9.15. The silica
sub-layer is formed by a stable SiO 4 tetrahedral structural unit that has poly-
merized into a two-dimensional sheet. This occurs through sharing three of the
oxygens at the corners of the tetrahedra. Like tetrahedra, the alumina octahedra
can also polymerize in two dimensions to form a sheet; in this case, by sharing
four oxygens. However, the aluminum atom is often substituted by Mg and Fe,
referred to as isomorphous substitution. Substitution of the trivalent aluminum by
divalent Mg or Fe imparts a net charge to the clay layer. Likewise, substitution
of silicon by aluminum or magnesium by lithium could also impart a net charge.
In order to achieve electroneutrality, the net charges must be compensated by
+
+
2+
interlayer cations such as Na ,K ,and Ca . The interlayer cations are depicted
in Figure 9.15. These cations are exchangeable. Pillared clays are prepared by
exchanging the charge compensating cations between the clay layers with larger
inorganic hydroxy cations, which are polymeric or oligomeric hydroxy metal