Page 175 - Adsorbents fundamentals and applications

160 ZEOLITES AND MOLECULAR SIEVES

the commercial Type 4A zeolite, with an effective aperture size of 3.8 ˚ A. The
sodium form can be replaced by various other cations or by a hydrogen ion. The
+
+
commercial Type 3A zeolite is formed by exchanging Na with K , resulting in
+
a smaller effective aperture size (of 3.0 ˚ A) due to the larger K . The aperture size
+2
+
of the sodium form can also be increased by exchanging Na with Ca +2 or Mg ,
because 2 Na are replaced by one divalent cation. The form of the exchanged
+
Ca +2 or Mg +2 is Type 5A with rather unobstructed and larger apertures. The
unobstructed apertures of 5A have a size of 4.3 ˚ A (Breck, 1974).
A detailed discussion on the sites of important cations in zeolites A, X, Y,
chabazite (cage-type with 8-oxygen ring window) and heulandite (channel-type)
is given in 7.4.1.
7.1.2. Structure and Cation Sites of Types X And Y Zeolites
The skeletal structure of Types X and Y zeolites is the same as naturally occur-
ring faujasite. The sodalite units are linked through 6-member prisms, as shown
in the unit cell in Figure 7.1(c). Each unit cell contains 192 (Si, A1)O 4 tetrahedra.
The number of aluminum ions per unit cell varies from 96 to 77 (i.e., Si/Al = 1
to 1.5) for Type X zeolite, and from 76 to 48 (Si/Al = 1.5 to 3) for Type Y zeo-
lite. Loewenstein’s rule forbids the formation of Al-O-Al bridges (Loewenstein,
1954). Thus, the maximum number of Al corresponds to a Si/Al ratio of 1. Kuhl
(1987) reported a procedure for the synthesis of the low-silica X (LSX) zeolite
(with Si/Al = 1). At Si/Al > 3, they are named USY (i.e., ultra-stable Y). The
Si/Al ratio in the commercial USY zeolite can be very high, for example, 195.
The framework of faujasite has the largest central cavity pore volume of any
known zeolite, amounting to about 50% void fraction in the dehydrated form.
The free diameter of the central cavity is 13.7 ˚ A (Eulenberger et al., 1967). A
unit cell, when fully hydrated, contains approximately 235 water molecules, pri-
marily in the central cavity. The volume of the central cavity, however, accounts
for only a small fraction (1/5–1/8) of the pore volume of the unit cell since there
are portions of other central cavities from the neighboring unit cells, as well as
window spaces that are also contained in the same unit cell (see Table 5.9 of
Breck, 1974). The aperture is formed by the 12-member oxygen rings with a
free diameter of approximately 7.4 ˚ A. The size of the unobstructed 12-ring is
approximately 8.1 ˚ A (Breck, 1974).
Three major locations for the cations are indicated in Figure 7.1(e). The loca-
tions are center of the 6-member or hexagonal prism (I) and opposite to I and
located in the sodalite cage (I ), similar to I and I but further from the central

cavity (II and II ), and the 12-ring aperture (III and III ). The commercial 10X

+
zeolite contains Ca +2 as the major cation, and Na is the major cation for 13X
+2
+
zeolite. The distribution of Na ,K ,Ca , other cations, and H 2 O among the
+
sites in X and Y zeolites has been discussed in detail by Barrer (1978). Cation
sites for important cations in X and Y zeolites will be given in 7.4.1. The BET
surface area measured with N 2 for zeolites falls in the range between 500 and
2
800 m /g.