Page 214 - Adsorbents fundamentals and applications

PREPARATION OF THREE TYPES OF SORBENTS 199

Table 8.3. Cation exchange capacity of various zeolites
Zeolite Si/Al Ratio Meq/g (Anhydrous)

Chabazite 2 5
Mordenite 5 2.6
Erionite 3 3.8
Clinoptilolite 4.5 2.6
Zeolite A 1 7.0
Zeolite X 1.25 6.4
Zeolite Y 2.0 5.0
Breck, 1974, with permission.

In the work of Takahashi et al. (2001b), four as-received zeolites were
used: Na-type Y-zeolite (Si/Al = 2.43, or 56 Al atoms/unit cell), NH 4 -type Y-
Zeolite (Si/Al = 6, or 27.4 Al atoms/u.c.), H-type Y-zeolite (Si/Al = 15, 12 Al
atoms/u.c.), and H-type ultra-stable Y-zeolite (H-USY, Si/Al = 95, 0.98 Al
atoms/u.c.). All zeolites were in powder form (binderless). The zeolites were
ion-exchanged by using excess amounts (five-fold cation-exchange-capacity)
of 0.1 M AgNO 3 at room temperature for 24 h. Because Ag + has a higher
+
selectivity compared with Na , this procedure ensured 100% exchange for Na-Y
(Si/Al = 2.43) (Padin et al., 1999). The same procedure was applied to NH 4 -Y
(Si/Al = 6), H-Y (Si/Al = 15) and H-USY (Si/Al = 195). After the exchange,
the zeolite suspension was filtered and washed with copious amounts of deionized
water until no free ions were present in the filter water (i.e., no precipitation was
observed upon treatment with Cl ). The products were dried at room temperature
−
and atmospheric conditions in a dark area.
To prepare mixed-cation zeolites by partial exchange, information on the
ion-exchange equilibrium between Ag and Na is needed in order to control
+
+
the Ag -exchange ratio. Thus, the ion-exchange isotherm for Ag-Na-Y-zeolite
+
(Si/Al = 2.66) developed by Sherry (1966) was used by Takahashi et al. (2001b).
The Na + and Ag + contents in the aqueous solution (0.1 M total) were pre-
determined so the final ratio of Ag /(Ag + Na ) in the zeolite was 21 or 49%,
+
+
+
+
the desired ratios. These Ag -exchange ratios were chosen to match the Ag
+
contents (or Ag /unit cell) in the fully Ag + ion-exchanged Ag-Y(Si/Al = 15)
and Ag-Y(Si/Al = 6), respectively. NaNO 3 and AgNO 3 serve as the sources for
+
Na + and Ag . Five-fold CEC of Ag + was added in the solution for full ion
exchange. The Ag /Na + molar ratios in the solution were 0.04/0.96 for 21%
+
+
+
Ag -exchange and 0.17/0.83 for 49% Ag -exchange. The procedure for mixed
cations ion-exchange was exactly the same as the single ion-exchange procedure
described above.
+
Preparation of Cu -Zeolites. Cu -zeolites are of particular interest as sorbents
+
for π-complexation because Cu + can interact with CO and olefins (and other
+
ligands) more strongly than Ag (Huang et al., 1999a), and also for economic