Verified Syntheses of Zeolitic Materials

2nd Revised Edition

Templating in molecular sieve synthesis

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.


Table 1. Synthesis cation specificity for framework structures

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.

Table 2. Synthesis cation-building unit relationship

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 SDA‚s 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. SDA‚s 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
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[10] M. D. Greaves, C. Bambrough, S. I. Zones, K. J. Balkus, in Book of Abstracts, 2 18th ACS National Meeting, 1999
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[12] M. E. Davis, S. I . Zones, in Synthesis of Porous Materials, M. L Occelli, H. Kessler (eds.), Dekker, 1997, pp. 1-34
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[18] J. M. Thomas, D. W. Lewis, Zeitschrift fur Physikalische Chemie 197 (1996) 37
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