Stephen T. Wilson
UOP Research Center
25 East Algonquin Road
Des Plaines, IL 60017-5016, USA
Zeolite crystallization is a very complex phenomena that cannot
be adequately described by the classical variables of reactant composition,
temperature, and pressure. Crystallization also involves polymerization-depolymerization,
solution-precipitation, nucleation-crystallization, and other complex phenomena
encountered in aqueous colloidal dispersions.
In the early descriptions of molecular sieve synthesis, the species that formed
the oxide framework (such as silicate, aluminate) were distinguished from extraframework
species, such as exchangeable cations (Na+, K+) and water.
In some cases the "exchangeable" cations were not completely exchangeable
without significant structural damage (such as K+ in OFF). Nevertheless,
the cations played an obvious role in the charge compensation of the alumina
tetrahedra in the product. Less obviously, the frequent association of certain
alkali cations with the presence of smaller cage structures led to the concept
of a mechanistic role that was designated "templating." Templates
were cationic species added to synthesis media to aid/guide in the polymerization/organization
of the anionic building blocks that form the framework.
In one of the first broad studies of zeolite crystallization in the presence
of mixed alkali-organic bases, Aiello and Barrer explained the observed structure
specificity of the mixed cations by introducing the concept of templating of
the different cage structures by the larger organic and smaller alkali cations.
[1] In particular, the quaternary organic tetramethylammonium (TMA) appeared
to play an important role in the formation of the OFF and MAZ structures, since
these structures were not observed in its absence. The authors suggested that
the TMA helped form the aluminosiicate precursor of the gmelinite cage building
unit that was common to both structures through a templating action. In a subsequent
study of the role of multiple cations, H. Khatami postulated that in the synthesis
of zeolites from systems containing ternary, quaternary, or higher numbers of
cations, "the zeolite framework structure is determined by one or at most
two cations depending on their type and size. Additional cations, should they
be included in the lattice, affect the zeolite properties but have minimal or
no influence on the structure topology." [2]
In 1973 Flanigen reviewed the concepts governing zeolite crystallization and
observed that cations play "a prominent structure-directing role in zeolite
crystallization. The unique structural characteristics of zeolite frameworks
containing polyhedral cages have led to the postulation that the cation stabilizes
the formation of structural subunits which are the precursors or nucleating
species in crystallization." [3,4,5] The many zeolite compositions and
complex cation base systems were compelling examples of the structure-directing
role of the cation and the cation "templating" concept. In this early
context "templating" and "structure-directing" were synonymous.
| Structure Type |
Polyhedral Building Units |
Synthesis Cations |
Cation Specificity |
| LTA |
D4R, sodal, α
|
Na, Na-TMA, Na-K, Na-Li |
Na |
| FAU |
D6R, sodal |
Na, Na-TMA, Na-K |
Na |
| KFI |
D6R, α |
Na-DDO, (Ba?) |
Na-DDO |
| GME |
D6R, gmel |
Na, Na-TMA |
Na |
| MAZ |
Gmel |
Na-TMA, Na-K-TMA, Na-Li-TMA |
Na-TMA |
| OFF |
D6R, gmel, canc |
K-TMA, K-Na-TMA |
K-TMA |
| ERI (with OFF) |
D6R, canc, (gmel) |
Na-K, Ba-TMA, Na-Rb, Na-TMA,
Na-K-TMA, Na-Li-TMA,
Na-K-BTMA |
Na-K, Na-Rb,
Na-TMA
Ba-TMA |
| LTL |
D6R, canc |
K, K-Na, K-DDO, K-Na-TMA, Ba, Ba-TMA |
K or Ba |
| CHA |
D6R |
Na, K, Na-K, Ba-K, Sr |
Na, K, or Sr |
D4R = double 4-ring, D6R = double 6-ring, sodal = socialite
cage, α = truncated cuboctahedron, gmel
= gmelinite cage, canc = cancrinite cage.
Table 1 shows some of the early cation/structure relationships. It is clear
from this table and other data that some structures exhibit a structural cation
specificity (such as LTA and FAU
for Na) while others do not (such as ERI, CHA).
Table 2 further delineates the relationship between building units and cations
in the early zeolite structures. The formation of a specific framework type
and a polyhedral building unit depends on one or at most two cation species.
The cation specificity is strong for the α-cage, sodalite cage, gmelinite
cage and D4R unit. It is weak for the D6R unit. In some cases the cation (hydrated
or anhydrous) is observed to fit nicely into the building unit; in others, multiple
cations are present.
| Building Unit |
Structure-types containing
building unit |
Cation specificity for building
unit |
|
α
|
LTA, KFI
|
Na
|
| Sodalite |
LTA, FAU
|
Na or TMA
|
| Gmelinite |
GME, OFF, MAZ
|
Na or TMA
|
| Cancrinite |
ERI, OFF, LTL
|
K, Ba, or Rb
|
| D4R |
LTA
|
Na
|
| D6R |
FAU, KFI, CHA. GME, ERI/OFF, LTL
|
Na, K, Sr, Ba
|
In 1983 Lok, et al, reviewed the role of organic molecules in molecular sieve
synthesis at a time when most new structures were the result of synthesis in the
presence of different quaternary ammonium cations or amines. [6] The charge distribution
and the size and geometric shape of the template were invoked to explain structure
direction. These authors also addressed the persistent issue of template specificity.
It was already known that in some instances: 1) one template can give rise to
several different structures, 2) many templates can yield the same structure,
and 3) some structures require the presence of a particular template. The solution
to this dilemma was the interplay of templating and "gel chemistry"
where "gel chemistry" represents all the other reaction parameters governing
the gel, such as oxide composition, temperature, time, reagent type, pH. In a
real sense the template was a necessary but not a sufficient condition for structure
formation.
As larger and more complex organic cations were employed, the resulting structures
had the organic species filling not just cages, but channels (such as 1,8-diaminooctane
in ZSM-48) and channel intersections (such as tetrapropyl-ammonium in MFI). The
ability of quaternary ammonium polymers to influence and to direct crystallization
of zeolites was described by Rollmann et al. for a series of 1,4-diazabicyclo[2.2.2]octane-based
polyelectrolytes. Examples are given in which the polyelectrolytes force crystallization
of the large-pore zeolite mordenite where the small-pore species analcite would
otherwise have resulted. Polymeric cations prevented stacking faults in a synthetic
gmelinite, faults which had hitherto restricted access to the 12-ring channels
of both natural and synthetic samples of this zeolite. [7]
In addition to the more numerous and readily available quaternary ammonium cations,
other classes of nitrogen-free, cationic templates have also been used successfully.
Complexes of alkali cations with crown ethers have been exploited to make novel
structures (such as EMT) and known structures with novel compositions (such as
FAU, KFI, RHO). [8] Balkus found that a variety of stable, cationic, substituted
metallocenes could also be used to template novel structures. [9,10]
More recently, there have been several reviews that address structure-direction
in high SiO2 zeolite crystallization. The concepts of structure-directing
agent (SDA) and pore filling agent have been used to describe the relationship
between the relatively hydrophobic organic cations and the oxide lattice with
little or no framework charge. Davis and Zones and their coworkers [11,12] have
attempted to correlate the ability of an organic cation for structure-direction
in zeolite synthesis with its hydrophobicity and rigidity. The hydrophobicity
of a variety of SDAâs was evaluated by measuring the phase transfer behavior of
the iodide form from H2O to CHCI3. The rigidity was evaluated
from the number of tertiary and quaternary connectivities. SDAâs with intermediate
hydrophobicity were found to be most useful for high-SiO2 molecular
sieve synthesis. In terms of SDA geometry, a bulky, rigid molecule with limited
conformational variability tends to template a single structure. The use of relatively
flexible molecules with a minimum diameter of~5Å gives more than one molecular
sieve, depending on the gel chemistry. Zones and coworkers have designed and prepared
numerous bulky, rigid templates and used them to synthesize a great variety of
new high-SiO2 molecular sieves.
Lobo, et al., have also reviewed the concepts of structure-direction in the synthesis
of clathrasils and high-silica zeolites with emphasis on the energetic interactions
between the organic guest and the inorganic framework. [13] The effects of size,
geometry, and the chemical nature of the organic structure-directing agent on
the crystalline structures that are formed were discussed beginning with clathrasils
and ending with 12-ring zeolites with 3-dimensional pore systems. The application
of structure-directing concepts is described using the syntheses of ZSM-18 and
SSZ-26 as examples, and the control over long-range order in zeolites by structure-directing
effects is illustrated by the purposeful variation of the stacking probability
of SSZ-33/CIT-1 and FAU/EMT intergrowths.
Although amines have been used to successfully synthesize some high silica zeolites,
they have been particularly effective in the synthesis of crystalline aluminophosphate-based
molecular sieves and the concept of templating persisted. A wide variety of neutral
AlPO4 molecular sieve frameworks have been prepared using quaternary
ammonium cations or amines, where these organic species occupy cages, channels,
and intersections. Additional AlPO-based compositions and structures resulted
when the framework incorporation of one or more additional elements (such as Si,
Mg, Mn, Fe, Go, Zn) generated negative charge. [14] Cationic templates were still
the norm, since the organic amines were predominately protonated in the more acidic
synthesis media. Most of the AlPO-based molecular sieves were prepared without
the need for any alkali or alkaline earth cation. In these systems the structure-directing
role of the template is dominated by stereospecific space-filling and stoichiometry
between the template and the framework, and is influenced to a lesser extent by
framework charge compensation. With the synthesis of alkali-free forms of the
CHA, ERI, FAU, SOD, and LTA structure-types, it was also clear that many structure-cation
specificities are likely to be framework composition dependent.
Among the goals of zeolite science are the understanding of zeolite shape selectivity
and the synthesis of new zeolite structures with desirable properties. Harris
and coworkers have combined computer simulation of host/guest interactions and
experimental data to understand how organic templates interact with zeolite structures.
[15] In the cases of ZSM-5, ZSM-11 and SSZ-33 they found that modeling can reveal
details of the complex interaction of templates with zeolite frameworks and con-ectly
predict which silicate structure a template will make. They also observed that
in the NON and CHA structure types, the zeolite template interaction energies
con-elate extremely well with experimental crystallization times, indicating participation
of the template in the rate determining step of crystallization. [16]
A combination of several computer modeling techniques has been applied to investigate
the ability of organic molecules to template microporous materials. [17] The efficacy
of a template was rationalized in terms of the energetics of the host-template
interactions. The calculated geometries of the template/framework combinations
are in excellent agreement with the experimental structural data. The procedures
used can successfully identify optimum templates for a given host. These results
suggest that in the future it may be possible to design a theoretical zeolite
structure and design a template to make it, all by computer simulation.
Catlow and his collaborators have recently achieved half the objective by using
de novo design techniques (available in the computer code ZEBEDDE) to predict
that 4-piperidinopiperidine would produce a previously known structure, an aluminophosphate
molecular sieve with the CHA structure. The authors demonstrated the successful
application of computer aided materials design, through the identification of
critical parameters in the synthesis and the computational design of a suitable
template, and then used the template to synthesize a microporous material possessing
predefined structural and physical properties. [18,19] The material was structurally
characterized using a combination of diffraction and EXAFS techniques. [20] It
now only remains to design a theoretical structure and the template to make it,
then use that template to make the targetstructure.
Although most templating or structure-direction has been attributed to cations,
the fluoride anion has also shown a templating effect in molecular sieve synthesis.
[21] Fluoride has long been known to have a mineralizing (or mobilizing) effect
in synthesis. This effect applies especially to silica-based materials which may
thus be prepared in media with pH lower than 10-11. The structure-directing effect
was found when F- enabled the formation of three entirely novel materials
(the silica form of the AST structure type, LTA-GaPO4 and CLO-GaPO4).
In each of these structures F- is incorporated into D4R units of the
material and contributes strongly to the stabilization of the structure, as do
the organic template molecules. This effect is chiefly observed when the framework
has no charge or if its charges are auto-compensated. F- synthesis
has been most recently exploited by Coma and coworkers to make a variety of pure
SiO2 structures. [22]
In many of these structures the fluoride appears to be bonded to the SiO2
lattice imparting a temporary negative charge to balance the cationic template
charge. [23] Thermal treatment removes both F and organic.In summary, alkali cations
are most effective in templating low Si/Al zeolites from basic media. Quaternary
ammonium cations are best at templating medium to high Si/Al zeolites and AlPO-based
molecular sieves. Amines have been used to template AlPO-based molecular sieves
and high-Si zeolites, and it is believed that the effective form of the amine
is certainly the protonated form in AlPO-based synthesis and probably the protonated
form even at the higher pH range typical of the high Si zeolites. Quaternary ammonium
cations and amines have been very effective templates for phosphate-based structures,
in general. The effectiveness, variety, availability, stability, and cost of the
nitrogen-based cations as templates has not yet been rivaled.
References
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[2] H. Khatami, in Proceedings of Third International Conf. on Molecular Sieves,
J. B. Uytterhoeven (ed.) Leuven Univ. Press, 1973, pp. 167-173
[3] E M. Flanigen, in Adv. Chem. Ser. 121, R. F. Gould (ed.), 1973, p. 119
[4] R M. Barrer in Molecular Sieves, Soc, Chem. Ind., London, 1968, pp. 39-46
[5] W. M. Meier In Molecular Sieves, Soc. Chem. Ind., London, 1968, pp. 10-27
[6] B. M. Lok, T. R Cannan, C. A. Mesina, Zeolites 3 (1983) 282
[7] R H. Daniels, G. T. Ken-, L D. Rollmann, J. Am. Chem. Soc. 100, (1978) 3097
[8] T. Chatelain, J. Patarin, F. Brendle, F. Dougnier, J.-L Guth, P. Schulz,
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[15] T. V. Harris, Y. Nakagawa, D. S. Santilli, S. I. Zones, Book of
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