Page 215 - Adsorbents fundamentals and applications

200 π-COMPLEXATION SORBENTS AND APPLICATIONS

+
reasons. However, direct ion exchange with Cu is not applicable because the
cuprous salts are water-insoluble. Also, the cuprous compounds are fairly readily
oxidized to cupric compounds in solution. Therefore, Cu 2+ is exchanged first,
+
followed by partial reduction of Cu 2+ to Cu . Two approaches can be taken
for the reduction step. The one studied most extensively involves the use of a
reducing gas. The other approach used more recently is “auto-reduction,” that is,
without a reducing gas.
◦
Cu(II)Y was first reduced with CO at 400 C to Cu(I)Y by Naccache and
Ben Taarit (1971), and the same procedure was subsequently used for preparing
Cu(I)Y by others (Chao and Lunsford, 1972; Huang and Vansant, 1973; Pearce,
1988). Huang (1973) reported that by pre-adsorbing ammonia, Cu(II)Y could be
reduced by CO completely to Cu(I)Y at substantially lower temperatures, as low
◦
as 100 C. Rabo et al. (1977) reported the preparation of Cu(I)-ZSM-5 zeolite by
a similar two-step process. In the reduction step, Rabo et al. (1977) reported that
a mixture of 3% H 2 O and 97% CO was the most effective reducing atmosphere
◦
and that the cuprous-form zeolite was obtained at 250–300 C. Reduction of
◦
Cu(II)Y to Cu(I)Y by ethylene at 1 atm and 150 C was reported by Cen (1989).
In the work of Takahashi et al. (2001a), Cu(II)Y was reduced completely to
◦
Cu(I)Y with 75% CO in He at 450 C and 12 h.
Auto-Reduction of Cu(II)Y to Cu(I)Y. Because Cu 2+ is exchanged as [Cu 2+
− +
OH ] , it is conceivable that it can be reduced in an inert atmosphere without the
use of a reducing gas. This has indeed been proven and, as mentioned, this process
is referred to as auto-reduction. Auto-reduction of Cu(II)-ZSM-5 to Cu(I)-ZSM-5
has been studied for the selective catalytic reduction of NO with hydrocarbon,
where Cu-ZSM-5 is a catalyst (Iwamoto and Hamada, 1991; Sarkany et al., 1992;
Larson et al., 1994; Shelef, M., 1994). More recently, Takahashi et al. (2001a)
successfully prepared Cu(I)Y sorbent by auto-reduction and heating Cu(II)Y at
◦
300–450 C (for 1 h). In the work of Takahashi et al. (2001a), NaY was first
exchanged to Cu(II)Y with 10-fold cation exchange capacity of 0.5 M Cu(NO 3 ) 2
at room temperature for 24 h. This procedure yielded 72% replacement of Na .
+
◦
The sample was subsequently heat-treated in He at 300–450 C. Analysis of the
◦
resulting sample heat-treated at 450 C showed that at least one-half of the copper
− +
2+
existed as [Cu OH ] . In a study of Cu(II)-ZSM-5 that was subjected to auto-
reduction, Larson et al. (1994) assumed that protons existed as the cations other
than [Cu OH ] .
− +
2+
The mechanism of auto-reduction of Cu 2+ in zeolites has been studied by a
number of groups that used Cu-ZSM-5. Two main mechanisms are
(1) Mechanism by Larson et al. (1994)
2+ − + +
[Cu OH ] ←−−( Cu + OH
2+ − + 2+ −
[Cu OH ] + OH ←−−( Cu O + H 2 O
2+ − + + 2+ −
2[Cu OH ] ←−−( Cu + Cu O + H 2 O