Geoscience Reference
In-Depth Information
growth. There are many examples of solution-mediated transport in the crystalliza-
tion of aluminum-rich zeolites. Ueda et al.
[65]
have prepared zeolites Y, S (GME),
and P (GIS), which were synthesized from clear solutions, that is, without the
presence of hydrogel (solid phase). The solid-phase transformation mechanism sug-
gests that the solid hydrogel reorganizes in forming the zeolite structure. Xu et al.
[66]
have well illustrated this process by the synthesis of ZSM-35 (FER) and ZSM-5
from nonaqueous reaction mixtures. There are several such reports on the synthesis
of various zeolites using both of the proposed mechanisms of zeolite crystallization.
In some cases, zeolites like Z.Y were found to crystallize by a combination of the
two mechanisms. As a particular structure can be formed via different crystallization
processes, extreme caution is necessary when attempting to generalize conclusions
from one zeolite to a class of zeolites or molecular sieves. Here, the authors discuss
some of the important aspects/factors influencing zeolite synthesis. For the sake of
convenience, the authors deal with aluminosilicate zeolites followed by aluminopho-
sphate zeolites and the family of microporous compounds.
6.5.1 Molar Composition
The chemical composition of a synthesized hydrogel is expressed generally
in terms of an oxide formula, SiO
2
(P
2
O
5
), Al
2
O
3
,bMxO, cNyO, dR, and eH
2
O, in
which M and N stand for (alkali) metal ions and R for organic templates. The rela-
tive amount of Si, P, Al, M, N, and R is one of the key factors determining the
outcome of the crystallization. Next to the nature of the template used (inorganic
and organic cations), the ratios SiO
2
/Al
2
O
3
,P
2
O
5
/Al
2
O
3
,MxO(NyO)/SiO
2
,MxO
(NyO)/P
2
O
5
, R/SiO
2
, R/P
2
O
5
, and H
2
O/SiO
2
or H
2
O/P
2
O
5
can intervene during
zeolitization. Such a variation greatly influences the crystallization processes right
at the level of nucleation and crystallization kinetics, the nature of the crystalliza-
tion material, the lattice Al content and distribution, the crystal size, and morphol-
ogy
[67,68]
. For example, the synthesis of high-silica zeolites requires the addition
of organic molecules into the reaction mixture, an exception is ZSM-5 which can
be synthesized without the use of organic reagents in a very narrow range of Na
1
and Al
3
1
concentrations, and the reaction temperatures are normally higher
(
200
C) than those used to crystallize aluminum-rich zeolites
[69]
.
Similarly, aluminum-rich zeolites, e.g., Z.A, X, P, sodalite, chabazite (CHA), and
edingtonite (EDI), have pore volumes in the range
100
B
0.5 cm
3
void/cm
3
of
crystal and are formed near 100
C, whereas the alkali-metal cations in the synthesis
of zeolites act as the source of hydroxyl ions (alkali-metal hydroxides commonly
used in zeolites synthesis) and have a limited structure directing role as well
[70]
.
Recently, it has been observed that the cation
0.4
B
water complexes stabilize small
aluminosilicate anions that are responsible for forming unique zeolite structures
primarily through electrostatic and steric factors
[71]
.
Figure
6.7
shows
a
crystallization
diagram for
the
system
a
Al
2
O
3
a
SiO
2
a
Na
2
O
H
2
O for a temperature of 363 K and (Al
2
O
3
1
SiO
2
)/
H
2
O
0.0005. An interesting factor that can be seen from this diagram is that
the content of alkali has a much stronger influence on the kind of crystallizing
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