Control of Crystal Size and Distribution of Zeolite A - ACS Publications

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MATERIALS AND INTERFACES Control of Crystal Size and Distribution of Zeolite A Tejinder Brar, Paul France, and Panagiotis G. Smirniotis* Chemical Engineering Department, University of Cincinnati, Cincinnati, Ohio, 45221-0171

In the present work, we have explored numerous ways of reducing the mean particle size of zeolite A, keeping the economic imperatives in mind. Efforts were directed toward reducing the crystallization time and having a narrower particle size distribution. In addition to the study of more conventional parameters, such as temperature, alkalinity, and water content, the effect of using microwaves, centrifuging, and ultrasonication was also explored. Highly localized temperatures of thousands of Kelvin and pressures of hundreds of atmospheres produced by ultrasonication were expected to disrupt the nucleation process in a manner that led to smaller crystals. Surprisingly, it was found that the application of ultrasonication did not lead to any decrease in particle size. The crystal morphologies obtained in the case of ultrasonication and stirring were completely different, which was rather unexpected. Subjecting the zeolite batch to short periods of microwave radiation led to a narrow particle size distribution with small crystal size. A direct correlation between the nominal SiO2/Al2O3 ratio and the particle size was observed. Decreasing SiO2/Al2O3 ratios led to a narrower particle size distribution. The yield dropped dramatically when the SiO2/Al2O3 ratio dropped below 1. The use of some of these parameters has a synergistic effect and can be coupled to finally obtain a mean crystal size of about 0.5 µm within a very reasonable time frame of 5-6 h, which we believe can be easily tailored for the commercial synthesis of zeolite A. X-ray diffraction (XRD), laser scattering particle size analysis, scanning electron microscopy (SEM), EDAX, and energy-dispersive XRF have been used to characterize the zeolites synthesized. 1. Introduction Improved kinetics of ion exchange and rates of diffusion in zeolites play a large part in practical applications. Small crystals provide the unique advantage of having a relatively high external surface area-to-volume ratio and reduced mass transfer resistance. These properties make zeolites desirable for many industrial catalytic, sorption, and ion exchange processes. At the same time, very small crystals of zeolites act as good precursor materials for the synthesis of continuous, highly oriented, and submicrometer thickness films.1 However, the zeolite A usually obtained exhibits a wide particle size distribution, ranging from one to several micrometers. The search for methods for the synthesis of small crystals with a controlled particle size remains an important goal.2 Various additives, such as TMA+ and TEA, have been tried in the past in an attempt increase the crystallization rate and to influence the crystal size distribution.3 Some researchers have also tried using small quantities of promoters such as PO43 that can increase the crystallization rate of zeolites.4 However, some of these additives are usually quite expensive and/or toxic and present their own set of environmental and economical problems. Also, most of the earlier studies have been directed toward a singlestep batch crystallization, which does not offer a very * Author to whom correspondence should be addressed. Ph.: (513)-556-1474. Fax: (513)-556-3473. E-mail: Panagiotis. [email protected].

high degree of flexibility. A greater understanding of the various aspects of the nucleation kinetics and crystal growth mechanism is needed. We have been successful in making zeolite A with a narrow size distribution and a maximum size as low as 1-2 µm. The time for batch synthesis of zeolite A ranged from 2 to 6 h. No nucleating agents were used, as a template-free synthesis is always desired, primarily because of the toxicity of the aforementioned material as well as the high costs involved, which would render the process economically unattractive. Here, we have relied mainly on the physical conditions and synthesis parameters to narrow the particle size distribution and achieve a mean particle size that falls in the submicron region. Moreover, control over the crystal morphology, spherical versus cubical crystals, was achieved. 2. Experimental Section 2.1. Synthesis. The zeolite A gels were prepared using sodium aluminum oxide (technical grade, Alfa Aesar), sodium metasilicate (SiO2, 44-47%, Aldrich) and sodium hydroxide. An example of the synthesis procedure is given below. Further variations from this basic synthesis step are provided in Table 1. NaOH (0.01 mol) was dissolved in 2.22 mol of distilled water. The resulting solution was distributed equally in two 150-mL polypropylene bottles. Sodium aluminum oxide (Na2O‚Al2O3, 0.02 mol) was added to one bottle, and sodium metasilicate (Na2O‚SiO2, 0.0365 mol) to the other. The capped bottles were shaken until the solu-

10.1021/ie000748q CCC: $20.00 © 2001 American Chemical Society Published on Web 01/24/2001

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Table 1. Synthesis Conditions for the Various Runs Performed in the Present Study run 1 2 3 4 5 A B 6 7 8 9 10 11 12 13 14 a

Na2O/SiO2 (molar ratio)

SiO2/Al2O3 (molar ratio)

NaNO3 (mol)

NaOH (mol)

H2O (mol)

aging 28 °C (h)

time

temp (°C)

St/USa

1.75 2.33 3.5 7 1.75

2 1.33 1 0.5 2

0.02 0.04 0.06 -

0.01 0.01 0.01 0.01 0.01

2.2 2.2 2.2 2.2 2.2

1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75 1.75

2 2 2 2 2 2 2 2 2

-

0.01 0.01 0.01 0.01 0.005 0.01 0.015 0.01 0.01

2.2 2.2 2.2 2.2 1.1 2.2 3.3 2.2 2.2

4h -

4h 4h 4h 4h 3h 105 s 135 s 7h 4h 2.5 h 4h 4h 4h 4h 4h 4h

80 80 80 80 60 MWb MWb 60 80 100 80 80 80 80 80 80

no no no no no no no no no no no no no no Sta USa

ST/US ) stirring/ultrasonication. b MW ) microwave.

tions were clear to the naked eye. Afterward, the sodium metasilicate solution was quickly poured into the sodium aluminum oxide solution. A gel formed immediately, which was shaken until homogenized. Afterward, the gel was heated at temperatures ranging from 60 to 100 °C for 2-7 h with or without stirring. For the majority of the cases, the system was subjected to 80 °C for 4 h without agitation. This system was seen to start to separate into two phases after about 3 h, and the phase separation was complete at 4 h, indicating that complete crystallization had taken place. The specific conditions for each run are described in Table 1. Except for the case in which SiO2/Al2O3 ratio was varied, the yield was almost the same in all cases and varied between 80 and 85% of the theoretical yield. The variation in yield for the case of varying SiO2/Al2O3 ratio is shown in Figure 3. The products were washed with distilled water, filtered using 0.1-µm mixed cellulose ester membrane filters (Cole Parmer), and dried at 110 °C overnight. 2.2. Characterization. 2.2.1. X-ray Diffraction (XRD). X-ray diffraction on a Siemens model D500 diffractometer (Cu KR radiation) was employed for the identification of the synthesized zeolite after the sample had been dried overnight. The phase identification was done by comparison with the diffraction pattern of commercially available zeolite A. Scintag DMSNT ver. 132 software was used to quantify the crystallinity of the zeolite samples synthesized. Unless otherwise specified, the crystallinity of all of the samples was found to be close to 98%. 2.2.2. LASER Scattering Particle Size Analysis. The particle size distribution was determined with a laser scattering particle size distribution analyzer (Malvern Mastersizer S series). For this purpose, the wet method of particle size distribution analysis was used, and water was used as the medium for dispersion of the zeolite. The solution was ultrasonicated for 45 min in order to break down the flocculates before the run was performed. As claimed by the manufacturer, the instrument is accurate to within 5% of the median value. All samples were run twice to ensure the accuracy of the measurements. Variations were never more than 2% of the median value. 2.2.3. Scanning Electron Microscopy (SEM/EDAX). Scanning electron microscopy (SEM) pictures were taken using a Hitachi 2700 scanning electron microscope. Samples for SEM analysis were prepared by

dispersing the zeolite sample on a carbon-coated sample holder, followed by gold sputtering at 45 µA for 70 s. All samples were analyzed using an accelerating voltage of 5 kV. EDAX was used to characterize the constituent element of the crystals for selected samples. 2.2.4. X-ray Fluoroscence (XRF). To determine the SiO2/Al2O3 ratio, an energy-dispersive XRF PV 9500 instrument was employed. The ground powder was placed in sample cups lined with Mylar films. A chromium X-ray source operating at 200 µV/20 µA was used. Commercially available zeolite A with a known SiO2/ Al2O3 ratio was taken as the standard. 3. Results and Discussion According to the classical theory of nucleation, it is well-known that, for nucleation to occur, the solution has to be in the labile zone, and afterward, as it drops to the metastable zone, only crystal growth and no nucleation can occur.5 However, it has been found that hardly any discernible changes in supersaturation occur during the initial part of the synthesis process when nucleation is predominant,6 and therefore, the classical theory of homogeneous nucleation fails to explain this feature of zeolite nucleation. Also, it has been shown that, for zeolite A, classical homogeneous theory predicts a very low rate of nucleation (10-7-540 nuclei cm-3 s-1) in the liquid phase, even at relatively high levels of supersaturation of the liquid phase.7 Hence, to explain this discrepancy, various other models, such as the solution-mediated transport mechanism,8 the hydrogel transformation mechanism,9 and the autocatalytic nucleation mechanism,10 have been proposed. Of these, the autocatalytic nucleation mechanism has attracted the most attention.11-15 Since its development, various mathematical analyses that consider the autocatalytic nucleation mechanism have been published.16-18 This theory is based on the fact that, even during very early times, there are very small crystalline domains that appear in the amorphous gel. These nuclei were supposed to be uniformly distributed in the gel; to lie dormant there; and subsequently, to be released into the liquid phase as the gel dissolves. However, it was later shown that homogeneous distribution is a special case, and the distribution was assumed to be inhomogeneous to explain the observed facts.17 As these nuclei are released into the liquid phase, they grow rapidly at the expense of the reactive species dissolved in the liquid

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Figure 1. Particle size distribution for zeolite synthesized at various SiO2/Al2O3 ratios at 80 °C for 4 h.

phase, which lowers the concentration of these reactive species in the liquid phase. This, in turn, accelerates the dissolution of the gel, leading to a further increase in the rate of dissolution of the gel and, consequently, an increase in the number of nuclei released. 3.1. Effect of SiO2/Al2O3. As can be seen from the particle size distributions and the scanning electron micrographs (SEMs), it is evident that there is a very direct correlation between the mode of the particle size distribution and the SiO2/Al2O3 ratio (Figures 1 and 2). The mean particle size decreases significantly as the nominal SiO2/Al2O3 ratio decreases from 2.0 to 0.5 (runs 1-4). It is also remarkable to note that the decrease in the nominal SiO2/Al2O3 ratio results in a significant narrowing of the particle size distribution. Sodium nitrate was added to compensate for the lost sodium, as sodium metasilicate was the precursor used for silica. Because silica is the limiting reagent here, a loss in yield with decreasing SiO2/Al2O3 ratio is to be expected and was indeed found, as shown in Figure 3. The yield divided by the theoretical yield has also been plotted and can be seen to be almost constant in Figure 3. XFS results indicated the Si/Al ratio of the final zeolite to be unity irrespective of the nominal SiO2/Al2O3 ratio of the gel used in the synthesis. XRD analysis indicated that all of the samples had high crystallinity. The observed facts can be explained in terms of an increased OH- to silica ratio as the amount of the latter is decreased. It has been hypothesized that increasing OH- ion concentrations leads to cleavage of the SiO2SiO2 bond. However, the silica-alumina bonds are much more resistant to cleavage under similar conditions and are stable once formed.19 It was postulated that reaction of the alumina species with the silica oligomeric species that are present in the gel at low OHconcentrations is responsible for an inhomogeneous distribution of nuclei.20 This leads to an early release of the nuclei in the solution where, according to autocatalytic theory, crystal growth takes place. Because equimolar quantities of alumina and silica species are present in zeolite A, this means that there is bound to be an excess of aluminum oxide in the liquid phase that reacts with the surface Si atoms, thus releasing structurally ordered units at the surface early into the solution. Both of these factors lead to the early release of the “nuclei” in the gel into the solution where they can grow, thus leading to comparatively larger crystals than if they had been released later. Also, the SEMs (Figure 2A,B) show that, at lower nominal SiO2/Al2O3 ratios, the particles are round, whereas they become more cubic in shape with increasing nominal ratio.

Figure 2. (A) SEM for zeolite synthesized at a SiO2/Al2O3 ratio of 0.5 at 80 °C for 4 h. (B) SEM for zeolite synthesized at a SiO2/ Al2O3 ratio of 2.0 at 80 °C for 4 h.

Figure 3. Net and percentage theoretical yield with varying SiO2/ Al2O3 ratio.

3.2.Crystallization Time and Temperature Effects. Whereas nucleation dominates in the initial stages of the synthesis process, crystal growth occurs throughout the synthesis. However, the rate of gel dissolution, the number and distribution of nuclei in the gel, and the crystal growth rate determine the final particle size distribution (PSD), all of which in turn depend on other factors such as temperature, alkalinity,

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Figure 6. Particle size distributions for zeolite A synthesized at different temperatures. Figure 4. XRD of zeolite A after (A) 3 h at 60 °C in the oven and (B) 3 h at 60 °C in the oven and subsequent treatment by microwave radiation for 105 s.

Figure 5. Particle size distribution for zeolite A synthesized with and without microwave heating: (a) 60 °C, 3 h, subsequent application of microwave radiation for 105 s; (b) 60 °C, 3 h, subsequent application of microwave radiation for 135 s; (c) 80 °C, 4 h, no microwave heating.

etc. To obtain a narrow particle size distribution with a small mean particle size, which is very important for numerous applications, the emphasis should be on the simultaneous release of the gel nuclei and the interrupted growth of the crystals once released. The idea behind this scheme is to allow the amorphous gel to produce sufficient nuclei at lower temperatures where crystal growth rate is slow and then give a burst of high energy that leads to rapid crystallization of these nuclei, leaving them little time to grow. To investigate this assumption, the zeolite system was subjected to microwave heating (runs 5A and 5B). There was little crystallization after 3 h at 60 °C in the oven (Figure 4). After this, the batch was removed and put into a microwave oven, where rapid crystallization along with a narrow particle size distribution was seen after application of microwave radiation for 105 s (Figure 5). There was only a single phase before the application of the microwave radiation, but the system was seen to separate into two phases after the application of the microwave radiation. No separation was seen to occur before 90 s. The energy of nucleation (about 15 kJ/mol) is much less than the energy of crystal growth (about 60 kJ/ mol).21 Hence, whereas lower temperatures favor nucleation, at relatively higher temperatures, crystal growth surpasses nucleation. It has already been reported in patented literature that leaving zeolite A at room temperature for long periods of time22 leads to a narrow

Figure 7. (A) SEM for zeolite A synthesis at 60 °C. (B) SEM for zeolite A synthesized at 100 °C.

PSD with small particle sizes of up to 0.4 µm. However, such lengths of time are clearly not feasible for operations on an industrial scale, and they need to be curtailed to more practical levels. Hence, an optimum balance between the crystal size and the time of operation must be determined. The effect of low temperature on nucleation is evident from the Figures 6 and 7A,B (runs 6-8). Aging at room temperature for even 4 h substantially reduces the growth of the nuclei that are released initially into the solution, and a narrower particle size distribution is observed (Figure 8, run 9). Although the gel dissolution rate is also reduced at lower temperature, crystal

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Figure 8. Particle size distribution for zeolite A synthesized with and without aging for 4 h at room temperature.

Figure 9. Particle size distribution for zeolite A subjected to various degrees of supersaturation.

growth decreases more rapidly with temperature than gel dissolution. This would be true only if the energy of gel dissolution is lesser than the energy of crystal growth, which has been corroborated in a recent paper.21 In that work, the energy of gel dissolution was calculated to be about 16 kJ/mol whereas the energy for crystal growth was estimated at about 60 kJ/mol.19 If the opposite trend were true, one would see a narrower PSD with increasing temperature. However, this is not the case, as can be seen in Figure 6. Temperature also affects the morphology of the final zeolite product obtained. At lower temperatures, the crystals are round, whereas at higher temperatures, they are cubic (Figure 7A,B). 3.3. Water Content. With decreasing water content, the nucleation rate increases exponentially and, consequently, more sharply than the low-order power law increase of the growth rate.21 To maintain the same alkalinity, NaOH was varied in the same ratio as the water content, and the pH noted in all cases was 13.7. This is illustrated by Figure 9 (runs 10-12). It has been shown that the linear growth rate is independent of the crystal size.18 As noted earlier, according to autocatalytic theory, growth of the crystals takes place in the solution. Also, the concentrations of alumina and silica species in solution do not change dramatically, thus making the growth of crystal dimensions a linear function of time.20 Taking all of these factors together, it is obvious that larger crystals have been obtained from nuclei that have been released earlier in the synthesis and vice versa for the smaller ones. It is well-known that higher supersaturation leads to an increase in nucleation sites. However, how it affects the release of gel nuclei is interesting. It is obvious from Figure 9 that the water

Figure 10. Particle size distribution for zeolite A subjected to centrifugation for 1000 rpm for 1 h with one-half of its water content subsequently removed as compared to a system with no centrifuging or water removal.

content strongly influences the final distribution. Apparently, more nucleation sites mean that fewer nutrients are available in the solution that can be utilized for later growth. However, it seems that the increasing water concentration does not influence the rate of release of nuclei into the solution, as the distributions remain normal in all cases, with no tails on either side. Another method of achieving supersaturation is to centrifuge the system. The water was seen to leave as a separate phase after the gel had been centrifuged for 1 h at 1000 rpm. One-half of the water was removed from the system, and afterward, the system was subjected to heating at 100 °C at 3 h. Little difference can be seen in the crystal sizes of the systems that underwent no centrifuging and maintained their original water contents and the system that was subjected to centrifuging for 1 h at 1000 rpm and had one-half of its water subsequently removed (Figure 10). By comparison with Figure 9, it is obvious that preparing a gel in a system that initially has one-half the original water content is quite different from preparing a gel in a system from which the water is later removed. The reason for this could be the “memory effect” cited in various publications,24,25 according to which it has been suggested that the particle size distribution of the crystalline end product formed by the hydrothermal treatment of gels prepared in the same way (constant number and distribution of nuclei) does not depend on the crystallization conditions. It might very well be asked why such as memory effect does not arise in the case when microwave radiation is used. When microwave radiation is used, the nucleation of new particles is vastly overshadowed by the growth of preexisting ones, thus leading to very fast crystallization because the energy of nucleation is far less than the energy of crystal growth, as mentioned before.21 Thus, not only the process of crystallization but also the process of nucleation has been changed. In centrifugation however, although the water content has been changed, this does not influence the characteristics of the gel that is seen to form immediately on mixing the sodium metasilicate and sodium aluminate solutions, as described in the Experimental Section. The additional water removed on centrifuging the system does contain aluminate and silicate species needed for crystal growth, but that does not influence the nucleation process taking place as a result of the rearrangement of the silicate and aluminate species, which

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primarily takes place inside the gel only. The reason is that, although removing water from the system might reduce the total number of nutrients, that does not reduce their concentrations, and the driving force, i.e., the concentration difference, remains unaffected, thereby leaving crystal growth unaffected. Thus, the memory effect in no way precludes the possibility of obtaining smaller crystals via an “interrupted growth” mechanism. 3.4. Agitation. Agitation in various forms can, in principle, significantly influence the final particle size distribution. In viscous systems, stirring leads to improved mass transfer and, consequently, bigger crystals.23 In an attempt to control the particle size distribution, we decided to use ultrasonics to influence the nucleation process. It is well-known that ultrasonication creates localized pressures of hundreds of atmospheres and temperatures of thousands of Kelvin.26 The ultrasound used was produced by a UWR ultrasonic processor at an amplitude of ∼40%, corresponding to a power input of 100-110 W at a frequency of 20 kHz. Under these conditions, one would expect that the nucleation process can be altered, i.e., that one can disturb the nucleation process and thus create more nuclei, leading to smaller-sized crystals. It was thought that this might help in achieving smaller zeolite particles, but apparently, it did not affect the final size distribution. This observation regarding the use of ultrasound is rather surprising. Also, the type of agitation (stirring versus ultrasonication) does not seem to be having much of a difference (runs 13-14), as the SEMs (Figure 11A,B) show very similarly sized crystals although the morphology varies greatly in the two cases. The crystals are rounded in the case of stirring, whereas one sees perfect cubes when ultrasonication is used. It can be seen that there are many small entities attached to the surface of the zeolite crystals in both cases, much more so in the case of the stirred batches. The small particles seen attached to the surface in both cases are small zeolite crystals. Indirect evidence in support of this fact was obtained by focusing EDAX at many of these small crystals and finding that the ratios obtained for Si/Al and Na/Al were quite similar to those for the regular zeolite A crystals. This indicates that these particles are indeed zeolite A. As seen from the SEMs, these particles seem to have settled down on the crystals rather than to have grown out of the crystal itself. The fact that they are much more prevalent in the case of stirring than ultrasonication is due to the fact that stirring is a much more vigorous type of agitation than ultrasonication on the macroscopic scale. Because ultrasonication is just another form of mixing, it was expected that similar results regarding morphology would be obtained. A slightly larger crystal size is obtained in the case of stirred systems as compared to nonagitated ones (Figure 12). In nonagitated systems, the crystals are allowed to settle, where they can be effectively shielded from any further growth by other crystals falling on top of them, which it is not the case for agitated systems. Another reason for this could be the Ostwald ripening of particles, which is known to be more prominent in stirred systems than nonagitated ones. It is well-known that the high surface energies associated with smaller crystals increase their solubilities according to Kelvin’s equation, as presented below.27

Figure 11. (A) SEM for zeolite A synthesis subjected to stirring (1000 rpm) at 80 °C for 4 h. (B) SEM for zeolite A synthesis subjected to ultrasonication at 80 °C for 4 h.

Figure 12. Particle size distributions of zeolite A system subjected to stirring as compared to those of nonagitated systems at 80 °C for 4 h.

[c*c ] ) 4Mγσ FRTL

ln

(1)

where L is the crystal size, c* is the equilibrium solubility for crystal size L, c is the equilibrium solubility for a large crystal, M is the molecular weight, γ is the activity coefficient, σ is the interfacial tension, F is the crystal density, R is the gas constant, and T is the absolute temperature. This effect is prominent only for crystal sizes less than 1 µm in size.27 It is obvious that when the aim is to synthesize submicron crystals, this effect cannot be

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altogether ignored. The general solubility of the smaller crystals induces a flux of matter toward the larger ones, so that the smaller crystals gradually disappear. However, as noted before, for the time periods involved in our case, it is important only for stirred systems. Another side effect of Ostwald ripening stems from the fact that, if a microscopic crystal of zeolite A is now added to the system, thermodynamics dictates that it will not dissolve. As a matter of fact, dissolved particles will deposit on the added crystal until the surrounding local solution reaches the equilibrium solubility of large crystals. This might serve to explain why a smaller number of nuclei are seen to form in systems with seeding and why crystallization takes lesser time in seeded systems.23 4. Conclusions The effect of various parameters and synthesis methods on the particle size distribution of zeolite A was analyzed. It is apparent that the rate of gel dissolution, the distribution of the germ nuclei, and the crystallization time play a significant role in determining the PSD and the size of the crystals. These factors are, in turn, influenced by physical conditions such as temperature, water content, etc. Ultrasonication, rather surprisingly, did not have much of an impact on the particle size distribution. Centrifuging the system also did not have much on an effect, which can be ascribed to the memory effects of the gel. Subjecting zeolite A to microwave radiation yielded a narrow particle size distribution. A direct correlation between the SiO2/Al2O3 ratio and the particle size distribution was seen. However, it was also noticed that the yield decreased with decreasing ratio. Agitation promotes the formation of larger crystals, although the type of agitation, namely, stirring versus ultrasonication leads to completely different morphologies. Aging leads to a substantial decrease in the spread of particle size, which can be seen as a consequence of the relatively low energy of nucleation as compared to the energy of crystal growth. Literature Cited (1) Boudreau, L. C.; Kuck, J. A.; Tsapatsis, M. Deposition of oriented Zeolite A films: In situ and secondary growth. J. Membr. Sci. 1999, 152, 41. (2) Qiu, S.; Yu, J.; Zho, G.; Terasaki, O.; Nozue, Y.; Pang, W.; Xu, R. Strategies for the Synthesis of Large Zeolite Crystals. Microporous Mesoporous Mater. 1998, 21, 245. (3) Zhu, G.; Qiu, S.; Yu, J.; Gao, F.; Xiao, F.; Xu, R.; Sakamoto, Y.; Terasaki, O. Synthesis of Zeolite LTA Single Crystals of Macro to Nanometer Size. In Proceedings of the 12th International Zeolite Conference; Tracey, M. M. J., Marcus, B. K., Bischer, M. E., Higgins, J. B., Eds; Materials Research Society: Warrendale, PA, 1999. (4) Kumar, R.; Bhaumik, A.; Ahedi, R. K.; Ganapathy, S. Promoter-Induced Enhancement of the Crystallization Rate of Zeolites and Related Molecular Sieves. Nature 1996, 381, 298.

(5) Tavare, N. S. Industrial Crystallization: Process Simulation Analysis and Design; Plenum Press: New York, 1995. (6) Antonic, T.; Subotic, B. Influence of Gel Properties on the Crystallization of Zeolites. Part 1: Influence of Alkalinity during Gel Preparation on the Kinetics of Nucleation of Zeolite A. Zeolites 1997, 18, 291. (7) Bronic, J.; Subotic, B. Role of Homogeneous Nucleation in Primary Zeolite Particles. Microporous Mater. 1995, 4, 239. (8) Ueda, S.; Kageyama, N.; Koizumi, M. Proceedings of the 6th International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butterworths: Markham, Ontario, Canada, 1984; p 905. (9) Xu, W.; Li, J.; Li., W.; Zhang, H.; Liang, B. Zeolites 1989, 9, 468. (10) Subotic, B.; Graovac, A.; Sekovanic, L. Proceedings of the 5th International Zeolite Conference; Sersale, R., Collela, C., Aiello, R., Eds.; 1980; p 54. (11) Subotic, B.; Graovac, A. Stud. Surf. Sci. Catal. 1985, 24, 199. (12) Bronic, J.; Subotic, B.; Smit, I.; Despotvic, L. A. Stud. Surf. Sci. Catal. 1987, 37, 107. (13) Subotic, B. ACS Symposium Series 398; Occeli, M. L., Robson, H. E. Eds.; American Chemical Society: Washington, D.C., 1989; p 110. (14) Katovic, A.; Subotic, B.; Smit, I.; Despotivic, L. A.; Curic, M. ACS Symposium Series 398; Occeli, M. L., Robson, H. E., Eds.; American Chemical Society: Washington, D.C., 1989; p 124. (15) Subotic, B.; Bronic, J. Proceedings of the 9th International Zeolite Conference; Van Ballmoos, R., Higgins J. B., Treacy, M. M. J., Eds.; Butterworth-Heinemann, Boston, MA, 1993; p 321. (16) Warzywoda, J.; Thompson, R. W. Zeolites 1992, 12, 837. (17) Gonthier, S.; Gora, L.; Guray, I.; Thompson, R. W. Zeolites 1993, 13, 414. (18) Sheikh, A. Y.; Jones, A.G. Population Balance Modeling of Particle Formation during the Chemical Synthesis of Zeolite Crystals: Assessment of Hydrothermal Precipitation Kinetics. Zeolites 1996, 16, 164. (19) Gora, L.; Streletzky, K.; Thompson, R. W.; Phillipes, G. D. J. Study of the Effects of Initial-Bred Nuclei on Zeolite NaA Crystallization by Quasi-Elastic Light Scattering Spectroscopy and Electron Microscopy. Zeolites 1997, 19, 98. (20) Warzywoda, J.; Thompson, R. W. Zeolites 1991, 11, 577. (21) Subotic, B.; Bronic, J.; Anotonic, T. In Proceedings of the 12th International Zeolite Conference, Tracey, M. M. J., Marcus, B. K., Bischer, M. E., Higgins J. B., Eds; Materials Research Society: Warrendale, PA, 1999. (22) Tatsuo, Y.; Shinji, U.; Kazumi, I.; Yasuo, K. Submicron A Type Zeolite and Production thereof. Jpn. Patent 1153514A, 1989. (23) Renzo, F. Di. Zeolites as Tailor-Made Catalysts: Control of the Crystal Size. Catal. Today 1998, 41, 37. (24) Antonic, T.; Subotic, B. Croat. Chem. Acta 1998, 71, 929. (25) Bronic, J.; Subotic, B.; Skreblin, Investigation of the Influence of Seeding on the Crystallization of Zeolite A in the Membrane-Type Reactor. Microporous Mesoporous Mater. 1999, 28, 73. (26) Suslick, K. S. UltrasoundsIts Chemical, Physical and Bilogical Effects; VCH: New York, 1988. (27) Daniel, J. C., Audebert, R. Soft Matter Physics; Springer: New York, 1995.

Received for review August 14, 2000 Revised manuscript received December 22, 2000 Accepted December 22, 2000 IE000748Q