Nucleation and growth of zeolite

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. 

In this section, we detail our results on the nucleation and growth of zeolite crystals with Si/Al ratios between 1 and 2. Various perturbations, including the effects of reaction time, D20, CH30H and C2Hs0H on the zeolite process are examined. A narrow range of starting compositions and reaction conditions are chosen, so that the effects of the perturbations can be evaluated with a minimum set of variables. These results are discussed in the context of present theories of zeolite growth in the next section. 

In a word, in the solid-phase mechanism, it is believed that neither the dissolution of solid gel nor the direct involvement of the liquid phase happened for the nucleation and growth of zeolite crystals during the crystallization process of zeolites. The nucleation and growth of zeolite crystals came from the structural rearrangement of the framework of solid aluminosilicate gel under hydrothermal crystallization conditions. 

The solution-mediated transport mechanism has been extensively discussed in the literature. In the middle of the 1960s, according to their studies on the crystallization of zeolite A, Kerr and Ciric proposed the solution-mediated transport mechanism. They believed that the nucleation and growth of zeolite crystals happened in solution. The initial gel was partially or completely dissolved in the solution with the formation of active silicate and aluminate ions. These active silicate and aluminate ions could further form the structural units of zeolite crystal. 

R. Grizzetti and G. Artioli, Kinetics of Nucleation and Growth of Zeolite LTA from Clear Solution by in situ and ex situ XRPD. Microporous Mesoporous Mater., 2002, 54, 105-112. 

It is widely believed that the nucleation and growth of zeolites occur through the reactions of dissolved silicate and aluminosilicate anions (1.2). Supporting this view, several experimental studies have shown that the structure of dissolved silicate species can influence the structure of solid aluminosilicate intermediates present in the gel from which a zeolite may form (3-5 ). As a consequence, determination of the structure and distribution of such anionic species has become a subject of considerable interest. 

Epping, J.D. and Chmelka, B.F. 2006. Nucleation and growth of zeolites and inorganic mesoporous solids Molecular insights from magnetic resonance spectroscopy. Curr. Opin. Colloid Interface Sci. 11 81-117. 

Factorial experiments can successfully serve to determine significant synthesis parameters for aluminophosphates and zeolites. Future studies will also focus on the underlying mechanisms of the nucleation and growth of high-silica zeolites out of alkaline-free ammonia containing reaction mixtures. 

In this study, we have shown that both alcohol and D20 have an Important effect on the nucleation and crystal growth of zeolites with Si/Al ratios between 1-2. In the case of alcohol, the formation of large pore zeolites such as zeolites X or Y is markedly accelerated at low alcohol levels. We attribute this to a stabilization of the cation-water complex and structured H20 which act as templates. However, at high alcohol levels, the structure of water disintegrates and leads to the formation of more condensed zeolites such as sodalite or cancrinite. Synthesis of zeolite A in D20 is slower than that in water, which primarily arises from the primary and secondary isotope effect during the condensation polymerization reactions necessary for zeolite growth. 

The use of a law typical of the solid state to describe the growth of zeolites was probably suggested by the presence of an amorphous gel in the synthesis medium. The rearrangement of this phase to form a zeolite network has been a favored theory until researchers showed that zeolites can nucleate and grow directly from solutions free of suspended solids (9). 

Methods. The crystallization of silicoaluminate mixtures into zeolite omega in the temperature range 105-130°C was performed in the presence of a structure-directing mixture (SDM) (10,11). The method gives minimum overlap between the nucleation and growth steps as indicated by the very homogeneous distribution of size of the crystals in the final product. The use of kaolinite as the aluminium source presents two main advantages (10). First, the low solubility of the clay under the crystallization conditions prevents the formation of a gel. Second, under the low supersaturation levels achieved, secondary nucleation is hindered. 

The metastable form which preferentially crystallizes could then transform to a more stable phase (i.e. zeolite Y - zeolite X or zeolite A - hydroxysodalite). Nucleation and growth rates would then become the limiting factors in determining how long this would take. For example, when synthesis conditions are chosen to produce zeolite A, the rate of hydroxysodalite formation is dependent on five variables. These variables and their effect on the conversion of zeolite A to hydroxysodalite are as follows ... 

The most successful approach to control membrane formation involves segregation of the processes of crystal nucleation and growth [24]. The so-called ex situ or secondary (seeded) growth methods, unlike the direct synthesis procedures just discussed, include a first step in which a closely packed layer of colloidal zeolite crystals, synthesized homogenously, is deposited onto... 

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