Geoscience Reference
In-Depth Information
morphology, shape, size, and crystallinity of the particles or materials as the OH
2
anions fulfill the crucial role of mineralizing agents. The pH is influenced by the
reactants and their concentrations/ratios, followed by temperature and time.
Furthermore, with the introduction of organics, the pH changes rapidly in the
system; hence, pH is the key parameter in determining the crystallization rate. An
increase in OH
2
concentration will generally bring about an accelerated crystal
growth and a shortened induction period before viable nuclei are formed.
In zeolite synthesis, the pH of the alkaline solution is usually between 8 and 12.
The major role of pH is to bring the Si and Al oxides or hydroxides into solution at
an adequate rate. It is well known that OH
2
is a powerful mineralizer. Water as a
mineralizer has been discussed in detail in Chapter 4. Water has much increased
the coefficient of expansion under hydrothermal conditions. The PVT measure-
ments for water and other solvents are rather rare. The PVT data for water up to
1000
C and 10 Kbar are known accurately within 1% error
[85]
. Figure 4.10 shows
the temperature
density diagram of water, with pressure as a parameter
[85]
.Ata
very high PT condition (1000
C and 100 kbar), water is completely dissociated
into H
3
O
1
and OH
2
behaving like a molten salt, and has a higher density of the
order of 1.7
1.9 g/cm
3
.
The OH
2
ion is a good complexing agent, which can bring amphoteric oxides and
hydroxides into solution. An increase in pH will accelerate crystal growth and shorten
the induction period (period before formation of viable nuclei) by an enhanced reac-
tant concentration
[89,92]
. For example, the values shown for Linde zeolite, Na-A,
(
Table 6.15
) indicate that the nucleation rate dN/dt, for both coefficients A and E
increase strongly with increasing pH. The solubility of silica increases nearly expo-
nentially with the concentration of alkali and, in the resultant mixture, a range of
silicate anions may appear with various degrees of oligomerization. In the case of
alumina at high pH minimal oligomerization, there is always the dominant anion,
Al
Þ
4
:
The high pH causes supersaturation of silicate and aluminate and the forma-
tion of a large number of nuclei. The growth of these nuclei proceeds until the alumi-
num in the gel is exhausted so that the alkaline media enable the ready mixing of
reactants and also facilitate nucleation and crystal growth. Lechert
[93]
has given the
importance of pH values in the crystallization of zeolites.
ð
OH
6.5.6 Structure-Directing and Composition-Determining
Species (Templating)
Templating has become an important tool for material preparation and processing.
Both organic and inorganic agents can be used as templating agents. Generally speak-
ing, this processing route uses channels, layers, and cavity spaces of nanoporous host
structures to grow nanoscale objects
[94]
. In some aspects, templating is related to bio-
mimetics as the controlled nucleation, and growth of inorganic phases in the organic
host is an important step of biomineralization
[95]
. There exists a variety of organic
and inorganic host materials with 1D, 2D, and 3D structures as reviewed in Ref.
94
.
The concept of templating in molecular sieve synthesis is not new.
