Page 173 - Adsorbents fundamentals and applications

158 ZEOLITES AND MOLECULAR SIEVES

14-oxygen rings (Dessau et al., 1990) and Cloverite, a gallophosphate with a
20-oxygen ring and 8-oxygen ring dual pore system (Eastermann et al., 1991).
[AlO 4 ] provides acid sites (as Lewis acid, or Brønsted acid when OH is bonded)
for catalytic reactions. The addition of tetrahedra such as [TiO 4 ] provides oxida-
tion sites for redox reactions.
Types A, X, and Y remain the dominant zeolites and molecular sieves that are
in commercial use for adsorption and ion exchange. As the focus of this book
is on sorbents, these zeolites will be the main subject for discussion. The basic
principles on adsorption properties discussed below, however, are applicable to all
other zeolites and molecular sieves. Potentially interesting adsorption properties
of other zeolites and molecular sieves will also be included.

7.1. ZEOLITE TYPES A, X, AND Y
Unit cells of type A and type X zeolites are shown in Figure 7.1. The cations
are necessary to balance the electric charge of the aluminum atoms in AlO 2 ,
each having a net charge of −1. The water molecules can be removed with ease
upon heating and evacuation, leaving an almost unaltered aluminosilicate skeleton
with a void fraction between 0.2 and 0.5. The skeleton has a regular structure
of cages, which are interconnected by windows in each cage. The cages can
imbibe or occlude large amounts of guest molecules in place of water. The size
of the window apertures, which can be controlled by fixing the type and number
of cations, ranges from 3 to 8 ˚ A. The sorption may occur with great selectivity
because of the size of the aperture (and to a lesser extent due to the surface
property in the cages) — hence the name molecular sieve. The windows of type
A zeolite consist of 8-membered oxygen rings, or simply, 8-rings. Similarly, the
windows of type X zeolite are referred to as 12-ring, which remain the largest
windows in zeolites today.
The ratio of Si/Al in Type A zeolite is normally one, while those in types X
and Y are typically one to five. The aluminum atom can be removed and replaced
by silicon in some zeolites, thereby reducing the number of cations. The cations
can also be exchanged. The inner atoms in the windows are oxygen. The size
of the windows then depend on the number of oxygen atoms in the ring (4, 5,
6, 8, 10, or 12). The aperture size, as well as the adsorption properties, can be
modified further by the number and type of exchanged cations. A description of
the structures will be given for the zeolites, Type A and Types X and Y, important
in gas separation. As mentioned, these types have dominated the commercial use
of zeolites for gas separation and purification as well as ion exchange since
their invention.

7.1.1. Structure and Cation Sites of Type A Zeolite
The structural unit in Type A zeolite (Linde Type A or LTA), as well as in Types
X and Y (faujasite or FAU), is the truncated octahedron shown in Figure 7.1(a).
This unit is also called sodalite cage or beta cage, as sodalite is formed by directly