Page 277 - Adsorbents fundamentals and applications

262 CARBON NANOTUBES, PILLARED CLAYS, AND POLYMERIC RESINS

Vansant, 1995) have similar shapes, however the equilibrium amounts are about
30–40% when compared to activated carbon. Occelli et al. (1985) studied the
adsorption of normal paraffins on Al-PILC. Adsorption of nitrogen, water, n-
butane, methanol, and neo-pentane on Al-PILC and Zr-PILC has been investi-
gated by Stacey (1988).
Several strategies for PILC synthesis have been learned about achieving strong
interactions with sorbate molecules. One needs small pore sizes and cations that
have high valences as well as small ionic radii. The latter is for increasing
the field-induced dipole, field-dipole, and field gradient-quadrupole interactions
(see Chapter 2 and Chapter 7). To increase the density of cations, a clay with
a high CEC should be used as the starting clay. Treatment with ammonia, after
PILC synthesis and before cation exchange, will increase the CEC. The interlayer
d-spacing is controlled mainly by the size of the polynuclear cation used for
pillaring. Zr tetramers yield the smallest free interlayer spacing. Furthermore, the
free interlayer spacing can be decreased by calcining at a higher temperature. For
example, the d 001 spacing of Al-PILC decreased by approximately 1 ˚ A when the
◦
calcination temperature was raised from 400 to 600 C (Cheng and Yang, 1997).

9.2.5. PILC and Acid-Treated Clay as Supports
In preparing catalysts and π-complexation sorbents, an inert support with a high
surface area is often needed. The supports are, however, not always “inert.” For
example, strong metal-support interactions are known in catalysis (Ruckenstein,
1987). The effects of different supports (silica vs. alumina) for π-complexation
by AgNO 3 have also been noted (Padin and Yang, 2000).
Pillared clays and acid-treated clays are two types of supports that have shown
interesting properties. They are discussed separately below.
Cheng and Yang (1995b) compared the adsorption properties for π-complexa-
tion of CuCl spread on two different supports, γ -Al 2 O 3 and TiO 2 -PILC. TiO 2 -
PILC was chosen because it has the largest pore dimensions among the different
PILCs. Both sorbents showed good olefin/paraffin selectivities. However, the
isotherms of olefins were more linear or steeper when TiO 2 -PILC was used as the
support. This effect is shown in Figure 9.22. Isotherm steepness is desirable since
the steep portion will yield a high working capacity for pressure swing adsorption.
Steepness of the isotherm beyond the “monolayer” region, that is, point “B”
or where the “knee” is located, is a direct reflection of the surface energy hetero-
geneity. Mathematically, the factor “s” in the Unilam (i.e., uniform Langmuir)
isotherm is an indicator of the steepness of this portion of the isotherm, and is
hence referred to as the heterogeneity parameter. The Unilam isotherm has been
discussed by Valenzuela and Myers (1989) and by Do (1998). It is given by:

+s
q m c + Pe
q = ln (9.5)
2s c + Pe −s
where q m is the “monolayer” amount adsorbed, P is the pressure, and c and s are
parameters. This isotherm is derived from the Langmuir isotherm by assuming