Page 216 - Adsorbents fundamentals and applications

PREPARATION OF THREE TYPES OF SORBENTS 201

(2) Mechanism by others (Iwamoto and Hamada, 1991; Sarkany et al., 1992;
Valyon and Hall, 1993)

2+ − + 2+
2[Cu OH ] ←−−( [CuOCu] + H 2 O
+
[CuOCu] 2+ ←−−( 2Cu + 1/2O 2
It appeared that the second mechanism was appropriate for the auto-reduction
of Cu(II)Y to Cu(I)Y (Takahashi et al., 2001a).

8.1.3. Ion-Exchanged Resins

Many macroreticular polymeric resins are available commercially. A series of
polystyrene cross-linked with divinyl benzene is also available commercially
(Albright, 1986). These resins are functionalized to form cation-exchange or
anion-exchange resins (Albright, 1986). The functional group for cation exchange
+
is the sulfonate group in the form of C 6 H 5 SO 3 . Usually the cation is H (in
−
the amberlyst resins) or Na + (in the DOWEX resins). These resins have high
cation-exchange capacities (in the range of 4–5 meq/g), and the cations can be
exchanged readily.
The resins are known to be hydrophobic as well as lyophobic, that is, with
a low affinity for hydrocarbons. Although heavy hydrocarbons have high polar-
izabilities, the lyophobicity results from the weakness of electric fields on the
surfaces of the polymeric resins. This property makes the resin an ideal candi-
date for modification as highly selective sorbents for separation and purification.
+
After modification by ion exchange with a cation such as Ag , the sorbent will
be highly selective for hydrocarbons that have π-electrons, while little of the
hydrocarbons without π-electrons will be adsorbed.
Figure 8.2 shows the isotherms of ethane and ethylene on the cation exchange
resin Amberlyst 15 (Yang and Kikkinides, 1995). Because of the lyophobicity of
the resin, the amounts adsorbed were considerably lower than those on all other
commercial sorbents, such as activated carbon, silica gel, and zeolites. However,
+
upon ion exchange of H by Ag , the amount adsorbed of ethylene exhibited a
+
dramatic sevenfold increase, due to π-complexation between ethylene and Ag ,
+
while the adsorption of ethane was unaffected.
The procedure of sample preparation is given next (Yang and Kikkinides,
1995). Amberlyst 15 (from Rohm & Haas Company) was used as the cation
exchange resin. It contained 20% divinylbenzene and was available as spher-
ical beads in the size range of 16–50 U.S. mesh. The BET surface area was
2
55 m /g, and the cation exchange capacity was 4.7 meq/g. Its average pore diam-
eter was given as 24 nm (Albright, 1986). Prior to ion exchange, the sample was
washed successively with de-ionized water and methanol, followed by drying in
◦
air at 100 C for 2 h. The sample was ready for ion exchange. The exchange
was performed with a dilute (0.014 N) solution of AgNO 3 at room tempera-
ture. After repeated exchanges, the resin was subjected to successive washing
◦
with de-ionized water and methanol, followed by air drying at 100 C. Methanol