Crystallization from clear solutions

Samples. TS-1 samples were synthesized by modifications of the preparation methods reported in the patent literatures (6,7). As shown in Table I, samples TS-1(A) and TS-l(B) were crystallized from clear solutions prepared by mixing titanium... 

K.A. Carlsson, J. Warzywoda, and A. Sacco, Jr., Modeling of Silicalite Crystallization from Clear Solution. Stud. Surf. Sci. Catal., 2001, 135 (Zeolites and Mesoporous Materials at the Dawn of the 21st Century), 255-262. 

P-16 - Modeling of silicaiite crystallization from clear solution... 

A mathematical model of Silicaiite crystallization from clear solution was developed and solved numerically. Crystallization was considered to be solution-mediated, and to involve three categories of silica species and three steps activation, nucleation, and crystal growth. Nucleation was represented by a condensation reaction between the hydroxyl groups present on the surface of 10 nm amorphous silica particles and the soluble silica species, producing activated complexes, which transform to crystalline nuclei. Both nucleation and crystal growth were considered to be reaction controlled. The model simulated well the in situ experimental data. This suggests that the hypothesized nucleation mechanism can be used to qualitatively describe the nucleation event during clear solution Silicaiite crystallization. 

P-29 - Study of zeolite A crystallization from clear solution by hydrothermal synthesis and population balance simulation... 

Table 7. Comparable studies from the literature, regarding the crystallization of ultra-small silicalite-1 crystals from clear solutions [59,60] ...

Primary nucleation is the classical form of nucleation. It occurs mainly at high levels of supersaturation and is thus most prevalent during unseeded crystallization or precipitation. This mode of nucleation may be subdivided into either homogeneous viz. spontaneously from clear solution, or heterogeneous viz. in the presence of dust particles in suspension, or solid surfaces. 

The structures of the solids isolated from strongly alkaline solutions cast a shadow of doubt over the simple model in which the only species formed are Be(OH)3 and BelOH) . Of course what crystallizes from a solution may well be a species with low equilibrium concentration as the process of crystallization is driven by the insolubility of the product. Nevertheless it is clear that relatively little effort has... 

This observation is in accordance with the phenomena of the crystallization in the resolution operation mentioned above in the following points. There are no clear, definite critical supersaturations above which nucleation of D-threonine occurs. Ohtsuki (2), however, reported supersolubility curve for this system, who gave the value of the supersaturation width At=7 C at 50 C. Their definition of the metastablllty was that no nucleation of the enantiomer other than seeded one was observed for two hours of resolution experiments. According to this definition, the supersolubility can be determined to lie somewhere between At=8 and 5 C from the present experimental data, this being in agreement with his result. If the crystallization proceeds further, however, D-threonine crystals may start to crystallize from the solution even if the initial supersaturation is 5 C. In this sense it is no longer the metastablllty limit. 

W. Pang, S. Qiu, and Q. Kan, Synthesis and Characterization of FAPO-5 Crystallized from Clear Homogeneous Solutions. Stud. Surf. Sci. Catal., 1989, 49A, 281-289. 

Powerful evidence for the liquid-phase mechanism comes from the direct crystallization of zeolite from clear solution. In the early 1980s, Koizumi and coworkers carried out an extensive study on this topic. They directly synthesized analcime, hydroxysodalite, zeolite B, mordenite, zeolite P, faujasite,[30] erionite, and potassium chabazite from clear solution. Pang et al. directly crystallized zeolite A[31] and FAPO-5[32] from clear solution as well. The study on the direct synthesis of faujasite [30, 33] from clear solution will be elaborated below. 

Figure 5.17 Crystallization field of zeolite from clear solution. Reproduced with permission from [301. Copyright (1984) Elsevier...

In the past 10 years, many characterization techniques have been used in the in situ study of the formation mechanism of zeolite from clear solution. For example, Honssian et al. studied the TPA-silicalite-1 system by using the small-angle X-ray scattering technique [34] Carlsson et al. performed a modeling study on the crystallization of silicalite-I from a liquid-phase system [35] Smaihi and colleagues applied in situ 27A1-,... 

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