Environmental Catalysts Based on Nanocrystalline Zeolites
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Zeolites are aluminosilicate molecular sieves with pores of molecular dimensions. Zeolites can be synthesized with a wide range of pore sizes and topologies and are used in applications such as catalysis, chemical separations, and as adsorbents and ion exchangers. The zeolite chemical composition, framework topology, and pore size can be varied to control reactivity and selectivity.
Although the pore dimensions of these materials are in the nanometer-size range, the zeolite crystals prepared through conventional syntheses are typically on the order of micrometers in diameter. Recently, however, there has been a great deal of interest in the synthesis of nanocrystalline zeolites, i.e., zeolites with discrete, uniform crystals with dimensions of less than 100 nm, and their unique properties relative to conventional micrometer-sized zeolite crystals. For some applications, it would be advantageous to employ much smaller nanometer-sized zeolite crystals in the range 10–100 nm. New synthetic methods have been developed that enable the selective and quantitative formation of small crystallites.
The impact of crystal size on zeolite properties can be profound. For example, Bein has studied thin films formed using nanometer-sized zeolites (10–100 nm crystallite size) and has found that “thin films can adsorb and desorb most vapors within a few seconds to minutes, often at room temperature, while the bulk materials usually require substantial heating (ca. 200–300 °C) to remove adsorbed vapors.” The facile desorption of products would be a distinct advantage in many gas phase heterogeneous processes involving zeolites, so nanotechnology has the potential to be utilized in many zeolite-based applications. The increased surface area of the small particles also provides these materials with a distinct advantage in heterogeneous catalysis. In addition, these nanometer-sized zeolites can then be assembled into nanostructures, such as thin films, fibers, or membranes. These zeolite nanostructures can potentially be used to control chemical reactivity on molecular length scales. The objectives of this paper are twofold. First, the synthesis of nanocrystalline zeolite particles and the self-assembly of these particles into thin films, fibers, and coatings are discussed. Second, potential applications of nanocrystalline zeolite materials in environmental catalysis are described.