Growth zeolite crystal

McNicol et al. (49) used luminescence and Raman spectroscopy to study structural and chemical aspects of gel growth of A and faujasite-type crystals. Their results are consistent with a solid-phase transformation of the solid amorphous network into zeolite crystals. Beard (50) used infrared spectroscopy to determine the size and structure of silicate species in solution in relationship to zeolite crystallization. 

Zhdanov (1) describes the mechanism of zeolite crystallization in terms of a quasiequilibrium between the solid and liquid phase in gels and emphasizes that the formation and growth of nuclei occurs in the liquid phase. 

Zeolite crystallization can be interpreted in terms of a ripening mechanism. The initially formed gel consists of amorphous dispersed particles of the order of 100-300 A in size. Growth of these particles to approximately 1000 A occurs during the induction period after which zeolite crystals appear imbedded in the amorphous gel matrix. This is especially evident in electron microscopic studies of gel solids (66,88). Ciric comments on the observation of growing crystals imbedded in gel particles which, as the crystals grow, tend to shrink together, resulting in coalescence (74)-... 

It was possible for two of the systems chosen that the nucleation and crystallization activation energies could be determined separately by distinguishing the induction period and crystal growth period in the overall crystallization process. Of the two hypotheses proposed for zeolite crystallization, in the gel phase or from the solution phase, the data support the latter hypothesis for crystal growth with the crystal-liquid surface enhancing the nucleation process in seeded systems. The precise mechanism of nucleation in unseeded systems remains to be determined. 

In Linde A and sodalite syntheses the signal grew to about 20 times its initial intensity. In other systems, such as faujasite, the increase was somewhat smaller. The increase seemed to depend upon the Si/Al ratio of the resultant zeolite crystals—i.e., the smallest increase occurred for mordenite crystallizations having an Si/Al ratio of 5 (for Linde A and sodalite Si/Al = 1). No Fe3+ phosphorescence was observed in the liquid phase of the gel. In three experiments carried out under identical conditions Fe3+ phosphorescence studies of the growth kinetics gave identical results (induction periods equal within 5%, Fe3+ intensity increase on crystallization equal within 10%). 

Figure 1.5 Schematic illustration of the synthesis principle for crystallization of mesoporous zeolite single crystals. The individual zeolite crystals partially encapsulate the nanotubes during growth. Selective removal of the nanotubes by combustion leads to formation of intracrystalline mesopores. Reproduced from Schmidt et alP51 by permission of American Chemical Society...

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. 

A physico-chemical basis for the critical size requirement has been described. There is evidence that chemical events rather than diffusion can govern subsequent linear growth of zeolite crystals. 

Over the past decades, considerable effort has been focussed on the evaluation of the various parameters that influence zeolite crystallization. (3j 4) More recently, spectroscopic techniques are being used to monitor zeolite growth. These studies include high resolution NMR spectroscopy of silicate and aluminate solutions.(5.6) solid state NMR spectroscopy of the amorphous phase(7.8) and Raman spectroscopic studies of solution and solid phases during crystallization.(9-141 The typical solutions present during zeolite growth contain various oligomeric silicate species, A1(0H)4 and minute amounts of aluminosilicate anions, because of... 

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. 

Initiation of Zeolite Crystallization. An induction period (tj) is always observed at zeolite crystallization, and is accepted to be the period necessary for the formation of zeolite nuclei (20). The inverse value of induction period, 1/t, is called the rate of nucleation. The crystallization curves were plotted as the degree of crystallization (estimated by the XRD method) versus synthesis time. The values of tj were obtained by extrapolating the time when crystal growth started. 

Synthesis procedures to obtain large zeolite crystals are well developed (1,2). In particular much attention has been paid to the synthesis of ZSM-5 crystals (3-6). Elongated prismatic (Fig. la) and cubic-shaped orthorhombic (Fig. lb) ZSM-5 crystals of sizes between 2-50 /tm were reported in the first recipes (7) in the patent literature. Later on, systematic studies have led to excellent synthesis prescriptions for the growth of large crystals of the prismatic (8) as well as of the orthorhombic form (9). The synthesis parameters which are dominant in the crystallization of pure ZSM-5 single crystals, are still under study (10,11). 

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... 

Finally, zeolite nanoparticles have been used as building blocks to construct hierarchical self-standing porous stmctures. For example, multilayers of colloidal zeolite crystals have been coated on polystyrene beads with a size of less than 10 p,m [271,272]. Also, silicalite-1 membranes with a thickness ranging from 20 to several millimeters and controlled mesoporosity [273] have been synthesized by the self-assembly of zeolite nanocrystals followed by high-pressure compression and controlled secondary crystal growth via microwave heating. These structures could be useful for separation and catalysis applications. 

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