Geoscience Reference
In-Depth Information
introduced a new term, molecular sieve, to zeolites to describe porous materials
which can act as sieves on a molecular scale, which is not just limited to aluminosili-
cate zeolites but also to a large number of other zeolites such as aluminophosphate
zeolites, vanadophosphosphate zeolites, and titanophosphate zeolites. The zeolites
and related materials, like phosphate-based molecule sieves, are microporous solids
with a broad range of physicochemical properties, which are listed in
Table 6.2 [10]
.
The commercial success of molecular sieves is largely due to the continual discov-
ery of new materials whose diverse properties have allowed process improvements
and the development of new technologies. Thus, the ability to control the materials'
properties through synthetic efforts has been and will continue to be of paramount
importance. Davis and Lobo
[10]
,Feijenetal.
[11]
,Auerbachetal.
[12]
, Cundy and
Cox
[13]
,andYu
[14]
have reviewed and discussed all the major issues related to the
synthesis and recent applications of zeolites. Accordingly, the zeolites are no longer of
interest to only those who are working in ion exchange. Distinction between the
microscopic structure and the macroscopic properties of these materials allows one to
use molecular sieves to recognize, discriminate, and organize molecules with a
precision that can be
1
˚
. Also, it has expanded to other exciting areas such as
supramolecular catalysis, photochemistry, nanochemistry, and electrochemistry. In
this chapter, the major developments in the field of zeolites, covering aspects of their
crystallization mechanism and applications, are discussed.
,
6.3 Crystal Chemistry of Zeolites
There are 46 known zeolite minerals in nature and over 204 synthetic zeolites
known in the literature. The number of synthetic zeolites is increasing day by day,
particularly after the discovery of aluminophosphate zeolites by Wilson et al.
[15]
.
Despite the remarkable structural diversity in aluminophosphate zeolites, the prod-
uct compositions are very similar:
xR
Al
2
O
3
1
ð
1
:
0
0
:
2
Þ
P
2
O
5
1
yH
2
O
1
2
Table 6.2 Physicochemical Properties of Zeolites and Molecular Sieves
[10]
Property
Range
B
4
13
˚
Pore size
Pore shape
Circular, elliptical
Dimensionality of pore system
1D, 2D, 3D
Pore configuration
Channels, cages
Surface properties
Hydrophilic, hydrophobic (high silica)
Void volume
Less than
B
50%
Framework oxide composition
Si, Al, P, Ga, Ge, B, Be, Zn:minor
Ti, Fe, Co, Cr, V, Mg, Mn:minor
