Geoscience Reference
In-Depth Information
6.6 Crystal Growth
The synthesis of zeolites at temperatures around 100
C usually produces crystals
in the size range 0.1
m. In applications as catalysts, sorbents, and ion exchan-
gers, this size range is fully suitable. The small size indicates that once nucleation
begins there is a rapid formation of numerous tiny particles all competing for the
available chemical nutrients. In order to carry out the single crystal X-ray diffrac-
tion studies, zeolites should be 100
10
μ
m, or even more, in diameter. Similarly, for
other examinations, like molecule and ion-selective membranes or other electro-
chemical studies, bigger crystals are required. Therefore, crystal growth of zeolites
can be both challenging and interesting as well. Natural zeolites occur as large
single crystals; however, these are often impure, rare, and require conditions that
cannot be reproduced in the laboratory
[49]
. Large zeolite crystals are difficult to
produce in the laboratory by conventional hydrothermal synthesis where crystal
formation is controlled to a large extent by the solubility of the reagent gel parti-
cles, the rate of generation of nucleation centers, and the solubility of the resulting
products
[64]
. The low solubility of the crystallization products leads to a high
degree of supersaturation, causing high nucleation leading to the formation of small
crystals. A careful monitoring of the reactants, dilution temperature, agitation, and
crystallization time has yielded some large crystals of some zeolite types.
Similarly, the use of slow-release complexing agents seeding sub- and supragravity
conditions to increase crystal size has met with limited success. There are several
works on the crystal growth of zeolites.
Crystal growth succeeds nucleation, i.e., the nuclei will grow by addition or
condensation of precursor species toward full-grown crystals. The autocatalytic
nature of the first stage of the crystallization reflects the self-accelerating behavior
of a crystallization process.
Crystal size can be influenced by experimental factors which alter the numbers
of viable nuclei that develop so that if these numbers are reduced a greater share of
the chemical nutrients is available for each growing crystal. The use of seed crys-
tals can be similarly employed in the case of zeolites, Na-Y and Na-X. The smaller
the number of seeds involved, the larger the average size of the final crystals
because a larger amount of chemical nutrient is available for each crystal before
these nutrients are exhausted. Ghobarkar
[117]
, Ghobarkar and Schaf
[118
μ
120]
,
Kuperman et al.
[121]
, Mel'nikov et al.
[122]
, and Demianets et al.
[123]
have
carried out extensive work on hydrothermal crystal growth of several zeolites and
studied their morphology in detail.
Figures 6.20
j
show the characteristic
photographs of various zeolite crystals obtained under hydrothermal conditions.
Kuperman et al.
[121]
have used a nonaqueous solvent, a mineralizing agent,
reagent quantities of water, an optional organic template, silica or silica and
alumina, or alumina and phosphorous, and oxide sources to obtain giant crystals of
zeolites and molecular sieves
[121]
. A close examination of the reactant products,
under a polarizing microscope, did not show any twining. Ghobarkar and Schaf
[119,120]
have carried out hydrothermal experiments and found that at 1 kbar
6.23a
