Page 172 - Adsorbents fundamentals and applications

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ZEOLITES AND MOLECULAR

SIEVES

Zeolites are crystalline aluminosilicates of alkali or alkali earth elements, such
as sodium, potassium, and calcium, and are represented by the chemical compo-
sition:
M x/n [(A1O 2 ) x (SiO 2 ) y ] · zH 2 O
where x and y are integers with y/x equal to or greater than 1, n is the valence
of cation M,and z is the number of water molecules in each unit cell.The
primary structural units of zeolites are the tetrahedra of silicon and aluminum,
SiO 4 and A1O 4 . These units are assembled into secondary polyhedral building
units such as cubes, hexagonal prisms, octahedra, and truncated octahedra. The
silicon and aluminum atoms, located at the corners of the polyhedra, are joined
by a shared oxygen. The final zeolite structure consists of assemblages of the
secondary units in a regular three-dimensional crystalline framework. The tetra-
hedra can be arranged in numerous ways, resulting in the possibility of some 800
crystalline structures, less than 200 of which have been found in natural deposits
or synthesized in laboratories around the world (Thompson, 1998).
Substitution (i.e., isomorphous substitution) of other elements for Al and/or Si
in the zeolite framework can yield myriad molecular sieves (which are formally
not zeolites). However, the main interest for synthesizing these new molecular
sieve materials has been in catalysis for developing (1) large pores or channels
and (2) catalytic sites other than acid sites. The largest windows in zeolites are
the 12-membered oxygen rings (with an unobstructed diameter of 8.1 ˚ A), which
do not admit large hydrocarbon molecules that are of interest to the petrochemical
industry. Since the development of VPI-5 (Davis et al., 1988), a crystalline alu-
minophosphate with unidimensional channels formed by 18-member oxygen rings
(with a free diameter of 12.5 ˚ A), a number of large-pore molecular sieves have
been synthesized (Chen et al., 1994). These include AlPO 4 -8, which contains

Adsorbents: Fundamentals and Applications, Edited By Ralph T. Yang
ISBN 0-471-29741-0 Copyright  2003 John Wiley & Sons, Inc.

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