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ScienceWeek
MATERIALS SCIENCE: ON THE SYNTHESIS OF ZEOLITES
The following points are made by H. Lee et al (Nature 2003 425:385):
1) Zeolites are mainly used for the adsorption and separation of ions and small molecules, and as heterogeneous catalysts. More recently, these materials are receiving attention in other applications, such as medical diagnosis and as components in electronic devices(1). Modern synthetic methodologies for preparing zeolites and zeolite-like materials typically involve the use of organic molecules that direct the assembly pathway and ultimately fill the pore space(2-5). Removal of these enclathrated species normally requires high temperature combustion that destroys this high cost component, and the associated energy release in combination with the formed water can be extremely detrimental to the inorganic structure.
2) The elimination of high temperature treatments for SDA removal from crystalline structures is very desirable for many reasons in addition to the loss of the expensive SDA. For example, zeolite films that are used as molecular sieving membranes are susceptible to cracking by high temperature treatments for SDA removal because of mechanical stresses placed on the membrane by thermal expansion mismatches with supporting substrates. Newer, molecular sieve low dielectric components may also benefit from the ability to remove the "guest" organic molecule using only moderate heating, rather than combustion. This is because they require air to fill the microporous space in order to achieve their desired properties and will be components of devices that may not be compatible with high temperature processing steps(1).
3) Ordered, mesoporous materials can be prepared using organic components such as surfactants that can form organized aggregates that contain large numbers of molecules that ultimately fill the pore space of the as-synthesized solids. Unlike crystalline microporous materials, the mesoporous solids allow for the extraction of the organic structure-directing components. This is due to the fact that the individual molecules that form the assembled structure-directing components are held together by weak forces that are easily disrupted and are each sufficiently small to be removed through the relatively large mesopores. Additionally, the interaction energies between the organic and inorganic fractions in the ordered, mesoporous materials are not as large as with microporous materials, where interactions between the SDA molecules and the framework can be quite strong.
4) The authors report a synthetic methodology that avoids zeolite synthesis difficulties by creating organic structure-directing agents (SDAs) that can be disassembled within the zeolite pore space to allow removal of their fragments for possible use again by reassembly. The methodology is demonstrated for the synthesis of zeolite ZSM-5 using a SDA that contains a cyclic ketal group that is removed from the SDA while it is inside the zeolite without destruction of the inorganic framework. The authors suggest this approach should be applicable to the synthesis of a wide variety of inorganic and organometallic structures.
References (abridged):
1. Davis, M. E. Ordered porous materials for emerging applications. Nature 417, 813-821 (2002)
2. Freyhardt, C. C., Tsapatsis, M., Lobo, R. F., Balkus, K. J. & Davis, M. E. A high-silica zeolite with a 14-tetrahedral-atom pore opening. Nature 381, 295-298 (1996)
3. Wagner, P. et al. CIT-5: A high-silica zeolite with 14-ring pores. Chem. Commun. 2179-2180 (1997)
4. Wagner, P. et al. Guest/host relationships in the synthesis of the novel cage-based zeolites SSZ-35, SSZ-36, and SSZ-39. J. Am. Chem. Soc. 122, 263-273 (2000)
5. Wright, P. A. et al. Cation-directed syntheses of novel zeolite-like metalloaluminophosphates STA-6 and STA-7 in the presence of azamacrocycle templates. J. Chem. Soc. Dalton Trans. 1243-1248 (2000)
Nature http://www.nature.com/nature
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ON ZEOLITES
Science 2003 300:438
The following points are made by Mark E. Davis:
1) Zeolites and zeolite-like materials -- crystalline solids containing a large number of uniformly sized pores with diameters of < 2 nm -- can discriminate between molecules that can enter the pore space and those that cannot on the basis of their size and shape. Zeolites have historically been used as catalysts, ion exchangers, and adsorbents. Emerging areas of application -- for example, as magnetic resonance imaging contrast agents and low-k dielectrics for use as electrical insulators -- show great promise. Supported zeolite membranes are also of interest for future applications. They could function as true molecular recognition and separation barriers and allow for efficient separation of nitrogen from methane and carbon dioxide from air. Such membranes may also provide for the high-temperature separation of molecules, even in environments containing corrosive chemicals. This discrimination between molecules could be coupled to catalytic conversion and/or adsorption, yielding highly specific catalytic reactors and chemical sensors. These and other new technologies will require the preparation of highly selective and permeable molecular sieve films.
2) Lai et al (Science 2003 300:456) report the synthesis of zeolite membranes with high selectivity and high permeance. The authors created an oriented film of the siliceous zeolite ZSM-5 that enabled excellent separation of para-xylene from ortho-xylene --with separation factors in the hundreds -- while maintaining high permeance of the para-xylene. This separation is particularly difficult, because ortho-xylene is less than 0.1 nm larger than para-xylene. It is this small difference in molecular size that is recognized by the ZSM-5 pore. The membranes minimize all transport pathways other than through the zeolite crystals. The permeance is high because the membranes are only ~1 micron thick and are aligned such that the molecules traverse the film through straight pores oriented perpendicular to the membrane surface.
Science http://www.sciencemag.org
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ZEOLITES
The following points are made by X. Rozanska et al (J. Am. Chem. Soc. 2001 123:7655):
1) Zeolites are natural or synthetic silicon oxide crystals composed of a network of SiO(sub4) tetrahedral units that link together by sharing oxygen atoms. The potential energy surface of the zeolite Si-O-Si angles is rather flat, which explains the large variety of accessible micropore structures that can be formed by zeolites. With their well-defined microporous structures, zeolites are interesting agents for separation purposes. Moreover, their mechanical and thermal properties have given them an important position in heterogeneous catalysis. In addition, zeolite-catalyzed reactions often display high product-selectivity, the selectivity based on the zeolite microporous structure producing various consequences in the course of a reaction.
2) Concerning reactant selectivity, for example, since the zeolite micropore channels have a well-defined diameter, reactants larger than this diameter cannot enter the micropores in order to react, and reactants smaller than the micropore will be the only reactants involved in the reaction. Once a reactant molecule has adsorbed within the zeolite mouth, it must diffuse toward the reactions sites, and this diffusion can be highly dependent on the size and shape of the zeolite micropores as well as on the size of the reactants or products. After reaction has occurred the products must diffuse away from the micropores. These selective properties are not only valid in the case of zeolite catalysts, but also in the case of zeolite molecular sieves.
J. Am. Chem. Soc. http://pubs.acs.org/JACS
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