Nucleation silicalite

For elevated gravity reactions, these yields increased to over 55% based on total silica in the initial reaction mixture. A discussion of how elevated gravity affects nucleation, growth, yield and crystal size of silicalite is presented. 

Normally, increase of the crystal growth rate caused by an increase of crystallization temperature is much higher than that caused by an increase of the nucleation rate. Thus, big crystals could be obtained at high temperature in a short crystallization time (e.g., NaX, Silicalite-I). Crystallization temperature can affect the morphology of the crystals as well because the activation energy of the crystal faces is related to crystallization temperature. 

The procedure of Zhdanov and Samulevich enables the calculation of isothermal nucleation rate profiles from determinations of growth rate and crystal size distribution [16,82]. Originally implemented in analyses of zeolite Na-A [83] and Na-X [82] crystallisation, the method has subsequently been applied to other zeolite systems, including silicalite [84,85]. If it is supposed that all the crystals in a batch have the same (known) growth rate behaviour, the total growth time of each crystal can be calculated. Assuming also that the nuclcation point for each crystal can be obtained by linear extrapolation to zero time, the nucleation profile for the whole batch can be determined from their final sizes. 

Similar results have been reported by other investigators (19,20). Secondly, the rates of nucleation and crystallization are lowered by the addition of sodium to the reaction mixture. This suggests that the presence of highly hydrophillic sodium cations may interfere with the formation of the hydrophobic silicalite structure. Thirdly, the incorporation of TPA into the silicalite structure is independent of hydroxide concentration and is near the theoretical limit of four molecules per unit cell as reported by Lok et al. (22). In fact, crystallization of silicalite ceases prematurely when the amount of TPA in the initial batch mixture falls below four per unit cell. Finally, results from this study support the crystallization mechanism proposed by van Santen et al. (7), and provide a reasonable explanation of the variations observed in crystal morphology with changes in reaction mixture alkalinity. 

The addition of small amounts of certain oxyanions such as phosphate, perchlorate, arsenate, chlorate, bromate ect. in the synthesis mixture of zeolites and their metallosilicate / silicate analogues significantly enhances the nucleation and crystallization rates, thereby reducing the overall crystallization time by as much as five times. Sensitivity enhanced high resolution liquid state Si and P NMR studies, using a specially designed probe, on low temperature (358 K) synthesis of Silicalite-1 in the presence of NaH2P04 as promoter indicate a catalytic role of the promoter. 

The Si NMR results provide molecular level evidence for the promoter (NaH2P04) induced nucleation in a clear liquid of Silicalite-1 synthesis mixture (composition is given Table 1), taken as representative simple (TEOS + TPAOH + H2O) system [6]. This sensitivity enhanced liquid state Si NMR studies afford monitoring the soluble silicate precursors (Q -Q ) [10] as a function of crystallization time spent during hydrothermal synthesis at 358 K. The relative concentration of Q - Q" silicate species when plotted against the crystallization time of Silicalite-1 in the presence and absence of promoter, the relative concentration of among total soluble Q - Q" silicate species decreases sharply at the onset of crystallization and vanishes after the completion of the crystallization [6], The corresponding crystallinity... 

For a better understanding of the role played by the NH4+ ions on the crystallization rate of 2SM-48 the ammonium contents was varied from 0 to 5 moles in the hydrogel of type 1 (Table 1 samples 3, 7 and 8). It can be seen that the crystallinity of ZSM-48 decreases with increasing NH4 content. Similar behaviour was observed for the nucleation and growth of ZSM-5 [19]. Dodwell et al. [12] also observed a decrease of the crystallization rates of ZSM-48 and EU-1 with increasing ammonium content. A decrease of the crystallization rates due to the presence of NH4 ions was also observed during the formation of silicalite-2 in presence of F" ions [26]. 

A comparison between thermal and microwave syntheses of colloidal silicalite-1 has provided a clear demonstration of the separate contributions of room-temperature ageing, heating rate and synthesis temperature to the nucleation process. At ageing times up to about 50 days, the product crystal size obtainable from a single synthesis composition is sensitive to reaction temperature and heating rate. After this time, ageing-generated proto-nuclei are so numerous that the normal self-nucleation of the reaction mixture is suppressed and the product crystal size is independent of reaction conditions. There is a limit to the number of crystals which can be nucleated and this is an intrinsic property of the system. 

P-40 - High-resolution solid state MAS NMR studies on the role of promoter (phosphate) in the nucleation and crystallization of Silicalite -1 (Si-MFI)... 

Use of microemulsions has been proposed for the synthesis of zeolites, their confined spaces acting as nanoreactors for growth [146]. The concept works with branched chain surfactant molecules at low temperature (368 K for 96 h), yielding silicalite-1 (MFI) crystals with narrow sizes tunable between 240 and 540 nm. Salt content is the morphology-determining parameter, which is consistent with the salt screening of the surfactant-electrostatic forces [147]. The zeolite does not nucleate in the microemulsion, and not before amorphous silica formed in the microemulsion separates from this medium. Whereas in conventional silicalite-1 synthesis conditions (433 K), the morphology is only sensitive to the electrostatic forces between the silicate and the surfactant... 

Schoeman, B. J. 1998. Analysis of the nucleation and growth of TPA-silicalite-1 at elevated temperatures with the emphasis on colloidal stability. Microporous and Mesoporous Materials 22, no. 1-3 9-22. 

Fig. 3. a Predicted (curve) and experimental (points) values of the nucleations rate vs. time. Theoretical values based the classical homogeneous nucleation model and the population balance model developed in [36]. Data for silicalite synthesis replotted from [32]. b Predicted (curve) and experimental (points) values of the per cent zeolite in the solid phase. Model calculations and data from same sources as a... 

Gundy et al. [7] also proposed that silicalite nucleation occurred on, or in, amorphous gel rafts. The evidence for their proposed mechanism was the observation that samples taken at early times contained a proportion of amorphous material, and that optical and electron microscopy indicated a close association of new zeolite crystals and these amorphous particles. The authors concluded that the initial nucleation period was due to a heterogeneous nucleation mechanism, and arose from the presence of macroscopic or colloidal particles in the solution. Nucleation was thought to be a surface-facilitated phenomenon. While their proposed mechanism appears to be slightly different than that of Doktor et al. [57, 58], it nonetheless involved the participation of extraneous material. 

Fig. 7. Schematic illustration of the model for nucleation of silicalite from clear synthesis mixtures a TPA-silicate clusters in solution, b primary fractal aggregates formed from the TPA-silicate clusters, c densification of the fractd aggregates from b above, d combination of densified aggregates into a second fractal aggregate structure, and e densification of the second fractal aggregates followed by crystal growth. Figure redrawn with permission from [58]...

Fig. 21 displays the PSD curves (by number) of the 260 nm sdicalite-l seed crystals (Fig. 21A) and that of the ZSM-5 zeolites obtained using same type of seeds (Fig. 21B). It can be observed that both curves possess almost the same shape and trend. The only difference is the size of crystals. Such phenomenon, although simple, clearly revealed the fact that the total number of crystals during the process of crystallization remains imchanged. It can be deduced that the nucleation process is completely suppressed under the current synthesis condition (SDA-free system with the presence of seed crystals), further proves the only occurrence of the growth of ZSM-5 zeolites on the surface of silicalite-1 seeds.

The same dynamic experiments were performed with a powder ZSM-5, Si/Al=36. This zeolite docs not give a transition. The same trends were observed. The dynamics of the adsorption experiment are important in determining pore volume however, pore size is relatively insensitive to experimental dynamics. However, we found that with this zeolite, equilibration times of 15-30 minutes produce identical isotherms. Phase relaxation in Uiis solid is apparently faster than in silicalite. The presence of significant aluminum in the framework may increase relaxation (perhaps by providing more nucleation sites) and help stabilize the sorbed phase. This would be consistent with the influence of aluminum on hysteresis. 

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