Page 305 - Adsorbents fundamentals and applications

290 SORBENTS FOR APPLICATIONS

provided for increased capacity over that of Li-X while maintaining some degree
of the advantageous isotherm linearity seen with Li-X.
Ab initio molecular orbital calculations showed the adsorption of nitrogen
was enhanced by weak chemical interaction (through a classical π-complexation
+
bond) with the Ag cation on the zeolite framework (Chen and Yang, 1996).
Numerous attempts have been made to reduce transition metal ions in zeo-
lites in order to form highly dispersed metallic clusters for use as catalysts.
These attempts have typically been completed via treatment at elevated tem-
peratures and/or in reducing atmospheres (e.g., sodium vapor, hydrogen gas,
carbon monoxide gas). However, color changes upon vacuum dehydration of
silver-exchanged A-type zeolites were found to be related to the formation of
metallic clusters within the sodalite cage or the 6-prism of the zeolite (Kim and
Seff, 1978a, 1978b; Jacobs et al., 1979). Using volumetric sorption techniques
and temperature-programmed desorption, Jacobs et al. (1979) could relate these
color changes to an autoreductive process involving framework oxygen. Autore-
duction is the reduction of the transition metal ion and the oxidation of water or
lattice oxygen. This has been observed for both Ag and Cu 2+ ions in zeolites
+
A, X, and Y. Autoreduction of Cu 2+ is discussed in Chapter 8. Autoreduction
of Ag + has been shown to occur by two mechanisms in two clearly defined
temperature regions (Jacobs et al., 1979; Baker et al., 1985):
◦
(i) autoreduction in the presence of zeolite water (25–250 C)
0
−
+
2(Ag − Z − O ) + H 2 O → 2Ag + (1/2)O 2 + 2Z − OH (10.2)
◦
(ii) autoreduction by oxygen from the zeolite lattice (127–380 C)
0
−
−
+
2(Ag − Z − O ) → 2Ag + (1/2)O 2 + Z − O + Z + (10.3)
Kim and Seff (1978a, 1978b) proposed the formation of octahedral hexas-
6
ilver metal clusters stabilized by coordination to six silver ions ((Ag ) 6 (Ag) )
+
from X-ray structural determinations of a dehydrated silver-exchanged zeolite A.
However, Jacobs et al. (1979) suggested that the formation of such large metal
clusters is improbable because color changes are seen even at low temperatures.
Also, low silver loadings where extensive migration of neutral silver atoms and
subsequent sintering into Ag 6 metal clusters are highly unlikely. Alternatively,
Jacobs et al. (1979) suggested, based on structural studies of Ag-A zeolites, the
0
+
+
formation of linear (Ag 3 ) 2+ charged clusters (Ag − Ag − Ag ) upon thermal
dehydration of the zeolite.
The location of the extraframework silver in relation to the aluminosilicate
framework has primary importance for elucidating the effect of silver clustering
on the adsorptive characteristics of the zeolite. This is not a trivial endeavor.
+
Numerous studies have been undertaken to identify the location of Ag ions and
Ag-clusters in argentiferous zeolites. These have mostly been for Ag-A and have
included X-ray diffraction methods (Kim and Seff, 1978a; 1978b; Gellens et al.,
1981a; 1981b) and far-infrared spectroscopy (Ozin et al., 1984; Baker et al.,