Geoscience Reference
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3. High pressures of water can modify phase equilibrium temperatures.
4. Water is a good solvent, a property that assists disintegration of solid components of a
mixture and facilitates their transport and mixing.
Water is important as a guest molecule in zeolite structures with relatively high
Al contents and consequently aqueous media favor their formation while salts have
a parallel role in the stabilization of sodalite and cancrinite. Highly siliceous zeo-
lites, such as silicalite and ZSM-5 (Zeolite Socony Mobil-5), which have zeolitic
structures, but contain very little Al III and M m 2 , are less hydrophilic than alumi-
nous zeolites and their structures can be stabilized by certain organic guest mole-
cules, notably amines, alcohols, and amino-alcohols. In general, the zeolitic water
can be removed leaving the unchanged hydrous zeolite. In hydrothermal systems,
the good solvent powers of water promote mixing, transport of materials, and facil-
itates nucleation and crystal growth. Water stabilizes zeolite structures by filling
the cavities and forming a type of solid solution. The stabilizing effect is such that
the porous aluminosilicates will not form in the absence of a guest molecule, which
may be a salt molecule as well as water. However, the water concentration or the
degree of dilution is of minor importance for the synthesis of ZSM-5, which can
crystallize out of gels with an extremely wide range of H 2 O/SiO 2 ratios (from 7 to
22). During 1970s, Flanigen and Patton [87] introduced a new route for zeolite syn-
thesis that involved the use of F 2 as the mineralizing agent. This method was fur-
ther developed by Guth et al. [88] . Replacement of OH 2 by F 2 allows the
crystallization of zeolites, pure-silica ZSM-5, MFI (Membrane Fouling Influence),
FER (iron-sulfur cluster), MTT (3-(4,5-Dimethylthiazole-2-yl)-2,5-diphenyltetrazo-
lium bromide, a yellow tetrazole), MTN (Metriol Trinitrate), and TON (Turnover
Number), at neutral or acidic conditions.
6.5.4 Temperature and Time
Temperature and time have a positive influence on the zeolite formation process,
which occurs over a considerable range of temperatures. Based on geological evi-
dence, an upper limit of 623 K has been suggested. Analcime has been obtained at
639 K, mordenite up to 703 K, clinoptilolite up to 643 K, and ferrierite at 648 K
[40] . A rise in temperature will increase both the nucleation rate and the linear
growth rate (expressed as K
crystal size); hence, the crystallin-
ity of the samples normally increases in time. As far as time is concerned, zeolite
synthesis is governed by the occurrence of successive phase transformations
(Ostwald rule of successive phase transformation). The thermodynamically least
favorable phase will crystallize first and will be successively replaced in time by
more stable phases [40] . The best example is the crystallization sequence of
amorphous
0.5
δ
l/
δ
t with l
5
5
Na-P (gismondine type).
The temperature, however, can also influence the type of product that has to be
crystallized. A rise in temperature leads to the crystallization of more dense
products as the fraction of water in the liquid phase, which has to stabilize the
porous products by filling the pores, will drop. Therefore, the existence of an upper
limit for the formation of zeolites is to be expected. The use of a nonvolatile pore
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