ABSTRACT
Zeolite synthesis occurs by a hydrothermal process with reagents being a silica source, an alumina source, a mineralizing agent such as OH− or F−, and, for higher Si/Al ratio zeolites, organic molecules as structure-directing agents. The role of inorganic metal cations, such as Na+ or K+, is quite profound. A schematic of the zeolite growth process is shown in Fig. 1(1). The complexity of the process, including the presence of numerous soluble species, an amorphous phase, polymerization and depolymerization reactions, makes the synthesis susceptible to physical effects such as stirring, aging, and order of reagent addition (2). Several independent processes are occurring in the medium, including nucleation of various structures, crystallization as well as dissolution of metastable phases. It is commonly observed that the conversion of the composition (gel or solution) to crystals is quite rapid once the crystallization process gets started. This suggests that nucleation is the rate-limiting step and is consistent with studies that report addition of seed crystals decrease the induction time (2). Tezak suggested several decades ago that rather than viewing the synthesis process as nucleation and crystallization, at least four subsystems be considered: (a) formation of simple and polymeric
aluminosilicates, (b) aggregation of these complexes to form embryo, (c) nucleation as aggregate formation with a well ordered core and micelle formation (primary particles), and (d) aggregation of primary particles via oriented aggregation (3). Flanigen has elaborated on these concepts further (4).
