Geoscience Reference
In-Depth Information
Figure 6.7 Crystallization diagram for the
system Na
2
O
a
Al
2
O
3
a
SiO
2
a
H
2
O for a
temperature of 363 K and (Al
2
O
3
1
SiO
2
)/
H
2
O
5
0.0005
[28]
.
2
Chabazite
gmelinite
1
P1 +
4A
0
0
5
10
SiO
2
/Al
2
O
3
Table 6.12 Primary Influence of the Compositions of Reaction Mixture
[72]
Mole Ration
Primary Influence
SiO
2
/Al
2
O
3
Framework composition
H
2
O/SiO
2
Rate, crystallization mechanism
OH
2
/SiO
2
Silicate molecular weight, OH
2
concentration
Na
1
/SiO
2
Structure, cation distribution
R
4
N
1
/SiO
2
Framework aluminum content
zeolite than the SiO
2
/Al
2
O
3
ratio. The primary influences of compositions of
reaction mixture are given in
Table 6.12 [72]
.
The earliest synthetic zeolites were essentially Al-rich or had a low Si to Al
ratio and their Al concentrations were equivalent
Al
linkages have not been observed in zeolites
[73]
. Loewenstein
[74]
first rationalized
the absence of Al
to Si; however, Al
a
O
a
Al linkages in tectoaluminosilicates on the basis of clusters
of negative charges. Thus, if Al atoms do not have Al atoms in their second coordi-
nation sphere, then Si/Al1. Low-silica zeolites are defined as having 1Si/Al
a
O
a
2,
,
while intermediate Si/Al zeolites as having 2
Si/Al5.
Table 6.13
gives a number
of molecular sieve structures and representative materials according to their Si/Al
ratio. In the late 1960s and early 1970s, high-silica zeolites with Si/Al
,
5 were
synthesized. As the Si/Al ratio increases, the properties of zeolites are significantly
altered. At higher Si/Al ratios, the crystals are hydrophobic. In the early 1980s,
silica-free molecular sieves were based on the aluminophosphate compositions
[50,75
.
77]
. The role of organics and their concentrations are discussed in detail
separately in the forthcoming sections.
6.5.2 The Aging of Hydrogel
After the preparation of the precursor or the hydrogel, using desired ratio of the
starting materials as explained in the previous section, the hydrogel is usually kept
for a certain period of time below the crystallization temperature. This aging
