Page 275 - Adsorbents fundamentals and applications

260 CARBON NANOTUBES, PILLARED CLAYS, AND POLYMERIC RESINS

Thus, the original CEC is now taken up by the protons. Some or most of
these protons migrate, at the calcination temperature, into the octahedral sheet
of the clay, and toward the incompletely neutralized hydroxyl groups that are
coordinated to magnesium, aluminum or other octahedral structural metal atoms.
The migration of the cations into the octahedral layer is basically responsible
for the CEC loss of the calcined product because these protons are no longer
accessible for ion exchange.
It has been reported that approximately 80% of the initial CEC of the clay
could be restored by treating the PILC with a base, such as ammonia, potassium
carbonate or alkali solutions (Vaughan et al., 1981; Molinard et al., 1994a, b;
Cheng and Yang, 1995a; Li et al., 1996).
The ammonia treatment can be accomplished by PILC exposure to a small
partial pressure of ammonia at room temperature. For example, Molinard et al.
(1994b) evacuated a desiccator that contained both the PILC and a beaker of
ammonium solution. They were able to restore 80% of the CEC in 10 min. Their
IR spectra showed the formation of the ammonium ions on the PILC. Thus,
the restoration was apparently accomplished by retrieving, using ammonia, the
proton from the octahedral layer to the surfaces in order to form ammonium ion.
Restoration of CEC by treatment with alkali solutions (such as NaOH) is also
possible, but not well understood. Li et al. (1996) observed structural collapse
of the PILC after such treatment. They also reported less restoration than that
by ammonia treatment. The structural collapse was due to attack on the alumina
pillars. Cheng and Yang (1995) have proposed the formation of −OH groups on
the pillars where the proton is exchangeable.

9.2.4. Adsorption Properties
Although PILCs are aluminosilicates with cations, they are considerably less
hydrophilic than zeolites and commercial desiccants. Earlier studies by Malla
and Komarneni (1990) and by Yamanaka et al. (1990) indicated hydrophobicity
of the PILCs. The capacity for water was increased by introducing Ca 2+ into
the interlayer spacing (Malla and Komarneni, 1990). The isotherms for water
vapor on various PILCs are compared with that for activated carbon and 5A
zeolite in Figure 9.21. The lack of a strong affinity for water is an advantage for
applications.
Like zeolites, PILCs also show selectivity for N 2 over O 2 . However, their
capacities were substantially lower than that of zeolites. Cheng and Yang (1995a)
corrected the earlier results of Baksh and Yang (1992) on the isotherms of cation
exchanged PILCs for N 2 over O 2 . In the work of Yang and Cheng (1995), the clay
with the highest CEC, Arizona bentonite, was used as the starting clay. The PILC
with the smallest pore sizes, Zr-PILC, was prepared from the Arizona bentonite.
The smaller pores would provide the strongest force fields. After pillaring with
ZrO 2 , the sample was subjected to CEC restoration by treatment with ammonia.
+
The resulting PILC was subsequently ion exchanged with alkali cations (Li ,
+
+
Na ,K ,Rb ,Cs ). The adsorption capacities for N 2 and O 2 on these ion
+
+