Page 279 - Adsorbents fundamentals and applications

264 CARBON NANOTUBES, PILLARED CLAYS, AND POLYMERIC RESINS

The chemical and structural evolutions of montmorillonite during acid treat-
ment have been studied more recently by Rhodes and Brown (1993) and by
Kumar et al. (1995). Kumar et al. (1995) followed the chemical changes and
changes in pore structure after treatments of the clay with sulfuric acid at con-
◦
centrations from 1 to 8 N at 80 C for 2 h. After treatments with the acid at 1 and
2 N concentrations, alkali cations and Fe 3+ in the tetrahedral layer were removed,
2
while the surface area increased to 138 m /g. At higher concentrations, Mg 2+ as
well as Al 3+ started being removed, with large gains in surface area, sometimes
2
exceeding 300 m /g. The alumina octahedral layer was severely attacked only
2
at concentrations >5N. The surface area remained at 370 m /g after treatments
with higher concentrations. The pore-size distribution was also followed in their
work (Kumar et al., 1995). Pores with dimensions of 2–4 nm were developed
after treatment at low concentrations. Mesoporosity, with pore dimensions in the
4–10 nm range, was developed at high concentrations.
Using acid-treated clay as the support, Cho and co-workers (Cho et al., 2001;
Choudary et al., 2002) developed an excellent sorbent for olefin/paraffin separa-
tion by spreading AgNO 3 .Their C 2 H 4 /C 2 H 6 isotherms on this sorbent are shown
in Figure 9.23. The steep isotherms clearly reflect surface-energy heterogeneity.
This sorbent and a comparison of it with other π-complexation sorbents will be
discussed further in Chapter 10.

9.3. POLYMERIC RESINS

A broad range of synthetic, non-ionic polymers are available for use as sorbents,
ion exchangers, and chromatographic column packings. The technology of design-
ing and building porosity into polymers was accomplished in the late 1950’s and
early 1960’s (Kunin et al., 1962; Millar et al., 1963; Albright, 1986). These macro-
porous polymers are termed macroreticular polymers. Building porosity can be
accomplished by emulsion polymerization of the monomers in the presence of
a solvent which dissolves the monomers but which is a poor swelling agent for
the polymer. Although macroreticular polymers of acrylates and methacrylates are
available, most commercial macroreticular polymers are based on styrene cross-
linked by divinylbenzene (DVB). Over the years, these styrene/DVB copolymers
have been produced with a wide range of porosities and macropore sizes.
The macroporous polymeric resins can be further reacted to attach functional
groups to the benzene rings to generate functionalities for ion exchange. The
resulting polymers are ion exchange resins. For example, polystyrene can be
−
+
sulfonated by sulfuric acid resulting in an −SO 3 H group attached to the ben-
zene ring, and the proton can be easily exchanged with other cations. Likewise,
attaching ammonium or amine groups results in anion exchange resins. These
polymeric resins and the functional groups are illustrated in Figure 9.24.
More recently, carbonaceous polymeric sorbents have been developed by par-
tially pyrolyzing the styrene/DVB polymers and their sulfonated forms (Neely