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Nucleation and Crystal Growth of Zeolite Systems

Zeolites are hydrated natural and synthetic three-dimensional microporous crystalline solids with well-defined structures containing AlO4 and SiO4 tetrahedra linked through the common oxygen atoms in a regular framework. Chemical composition of zeolites may be expressed by a general formula: M(x/m)[(AlO2)x(SiO2)y]&#8226 ; z H2O, where n is the charge of cation M, and the values of x, y and z depend on the type of zeolite. Central atoms (Al, Si) of the primary building block tetrahedron of the zeolite can be replaced by a large number of trivalent (B, Fe, Ga, Cr) or tetravalent (Ge, Ti, Zr) atoms. The properties of zeolites such as ion-exchange, inter-crystalline pores that discriminate between molecules of different dimension, strong acidic sites and active hosts for metal catalyzed reactions have earned them extensive industrial uses, and fundamental zeolite research has also become an area of great interest. The interest in use of zeolites as catalysts, adsorbents and detergent builders has grown in the past three decades. Millions of tones of zeolite A are used annually in detergents manufacturing and thousands of tones of zeolite catalysts, molecular sieves and adsorbents are used annually in the petroleum processing alone, whereas the uses in other areas are still in a stage of infancy. However, until recently, zeolite synthesis has mainly been an empirical science in which a large number of experiments were performed to systematically change crystallization parameters and analyze their influence on the properties of final crystalline product(s). Thus, investigation on molecular level of complex processes which determine nucleation and crystal growth responsible for the synthesis of desired type(s) of zeolite(s) and of factors influencing these processes are of great importance for: 1) the accumulation of new fundamental knowledge about nucleation and crystal growth of zeolites, and 2) improvement of the synthesis procedures of known types of zeolites in order to obtain pure products of desired properties, and possible creation of new zeolitic structures of predictable characteristics.

Zeolites, Nucleation, Crystal growth, Critical processes, Kinetics, Mechanisms

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