Nucleation of zeolites

While actual chemical events involved in nucleation and crystal growth are not known a phenomenological treatment (gives some insight. Willard Gibbs (9J considered processes of phase separation of two extreme kinds. In the first, fluctuations in concentration occur which are minute in volume but large in extent of departure from the mean (the case of binodal phase separation). In the second the volume of the fluctuation is large but the deviation from the mean for the solution is minute (responsible for spinodal phase separation). In nucleation of zeolites one is conerned only with fluctuations of the first kind. 

Figure 5.44 Crystallization and nucleation of zeolite NaX. 1 crystal-growth curve 2 nucleation curve 3 crystallization curve. Reproduced with permission from [129], Copyright (1980) John Wiley Son, Ltd...

The reluctance of many systems to crystallize is well-kwown by experimentalists. Nucleation of zeolites is in no way an exception to this general rule. The... 

The addition of zeolite seed crystals to hydrothermal synthesis media have long been known to accelerate the crystallization process, and even direct the outcome of syntheses in certain circumstances. The mechanism by which this occurs has been shown to involve very small alumino-silicate fragments in the seed crystal sample, either actually adhering to the seed crystal surfaces, or simply co-existing in the sample. These initial-bred nuclei , as they have been labeled, do not appear to prohibit the nucleation of zeolite crystals which would form in their absence in some cases. However, there are several examples reported in the open Uterature in which the phase formed by the unseeded solution did not form when seeds of another crystalHne phase were added to the solution. An interpretation of these results is provided. 

Our group has recently observed (ref. 10) that often synthesis time of zeolites can be substantially shortened when microwave radiation is applied instead of conventional heating. Table 3 gives some examples. Also a narrow crystal size distribution is obtained in this way. The crystallization temperature is rapidly (1 min) reached by microwave heating, this may be a factor in homogeneous and essentially simultaneous nucleation. 

It is this large and continuous variability in bulk composition coupled with the fact that crystal structures may be different for the same anhydrous or hydrous bulk composition which makes zeolite identification so difficult (see Breck, 1970, for example and Deer, et al., Vol. 4, 1962). The factors determining which species of zeolite will crystallize are undoubtedly complex, involving such variables as the chemical activity of dissolved ionic species, crystal growth rate and ease of nucleation however, certain patterns of mineral paragensis can be discerned through a survey of the literature. 

The cation plays a prominent structure-directing role in zeolite crystallization. The unique structural characteristics of zeolite frameworks containing polyhedral cages (62, 63) have led to the postulate that the cation stabilizes the formation of structural subunits which are the precursors or nucleating species in crystallization. The many zeolite compositions and complex cation base systems studied allow a test of the structuredirecting role of the cation and the cation templating concept. Table I summarizes the cation base systems from which zeolites have been synthesized. The systems used before 1969 are indicated to illustrate the number and complexities of new cation systems investigated since that time. Table II presents a summary of zeolite framework structure types, the cation systems in which they have been formed, and a proposal for a cation specificity for the formation of each framework type. A similar... 

Because of the high surface free energy at the liquid-solid interface, it is suggested that the stages of nucleation, transport of species by surface diffusion, and crystallization occur at the interface in the boundary layer. Culfaz and Sand in this volume (48) propose a mechanism with nucleation at the solid-liquid interface. This mechanism should be most evident in more concentrated gel systems where interparticle contact is maximized for aggregation, coalescence, or ripening processes. The epitaxy observed by Kerr et al. (84) in cocrystallization of zeolites L, offretite, and erionite further supports a surface nucleation mechanism. 

Mechanism of Nucleation and Crystallization of Zeolites from Gels... 

Breck (1) was the first to investigate the reaction in the hydrothermal formation of zeolites. He found that there is always some delay before crystallization starts. This so-called induction period can be reduced by raising the temperature or alkalinity of the reaction batch (2). As Sand (8) reported in 1968 in connection with the formation of mordenite, the nature of the Si02 material also has a decisive influence on the reaction and the nature of the zeolite crystals. The induction period as a nucleation phase is discussed by Domine and Quobex (4) in connection with kinetic investigations relating to mordenite formation. 

The first hypothesis, proposed by Breck and Flanigen [52,55], to account for the crystallization of aluminosilicate zeolites affirms that it proceeds through the formation of the aluminosilicate gel or reaction mixture, and the nucleation and growth of zeolite crystals from the reaction mixture. This initial model has been almost abandoned, and replaced by the hypothesis of Barrer and others [53,55], In the framework of this hypothesis, it is assumed that the formation of zeolite crystals occurs in solution. Accordingly, in this model, the nucleation and growth of crystalline nuclei are a consequence of condensation reactions between soluble species, where the gel plays a limited role as a reservoir of matter. 

The explanation of some experimentally observed features of zeolite synthesis follows from the treatment. A distinction is made between zeolitic stabilisers and nucleation templates. 

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