The nucleation period for crystallization of colloidal TPA-silicalite-1 with varying silica source

The nucleation period for crystallization of colloidalTPA-silicalite-1 with varying silica source

Qinghua Li a, Boriana Mihailova b, Derek Creaser a, Johan Sterte a,*

a Department of Chemical Technology, Lule�a University of Technology, S971-87 Lule�a, Swedenb Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Rakovski Street 92, 1000 So®a, Bulgaria

Received 4 January 2000; received in revised form 8 May 2000; accepted 15 May 2000

Abstract

The e�ect of varying silica source on the nucleation and crystallization of TPA-silicalite-1 was investigated. A direct

experimental method, involving a two-stage varying-temperature synthesis, was used to determine the nucleation period

for colloidal crystals of TPA-silicalite-1 with di�erent silica sources, including tetraethoxysilane (TEOS) and amor-

phous silica (Ludox TM and Ludox LS). For syntheses performed at 60°C with TEOS as silica source, the duration of

the nucleation was about 72 h, and a very rapid increase in the crystal population occurred during the initial crys-

tallization time. However, with the amorphous silica sources (Ludox TM or Ludox LS), the duration of the nucleation

period was extended to about 120 h, and the nucleation pro®le consisted of a self-accelerating nucleation rate at the

beginning of the nucleation period. The two-stage synthesis method could be used to determine the nucleation pro®le

for the various silica sources. However, this technique overestimated the crystal concentration at the earliest stage of

nucleation with amorphous silica. The use of amorphous silica gave rise to a broader crystal size distribution compared

to that of TEOS. However, it was found that for both TEOS and amorphous silica the vast majority of the nucleation

occurred during an induction period when little or no crystal growth was observed. In addition, Raman spectroscopy

revealed structural di�erences between Ludox TM and Ludox LS which may account for di�erences in the nucleation

processes observed for these two amorphous silicas. Ó 2000 Elsevier Science B.V. All rights reserved.

Keywords: Nucleation; Two-stage synthesis; Varying-temperature synthesis; Induction period; TPA-silicalite-1; Silica source

1. Introduction

Considerable work has been carried out to in-vestigate the kinetics of nucleation and crystalgrowth of ZSM-5 or silicalite-1 crystals [1±12].Meanwhile, much e�ort has been devoted to de-veloping mathematical models to simulate the

qualitative features of zeolite nucleation andcrystal growth [3,12±18].

Recently, a direct experimental method, whichis a two-stage varying-temperature synthesis, wasdeveloped to determine the nucleation period andnucleation rate for zeolite crystallization [19,20].This method is based on the fact that the nucle-ation process can be e�ectively halted when thesynthesis temperature is abruptly increased duringthe course of crystallization. The crystals nucleatedat the initial lower synthesis temperature continueto grow after the temperature increase until the

Microporous and Mesoporous Materials 40 (2000) 53±62

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* Corresponding author. Tel.: +46-920-72314; fax: +46-9-

209-1199.

E-mail address: [email protected] (J. Sterte).

1387-1811/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.

PII: S13 8 7-1 8 11 (0 0 )0 02 4 2- 0

crystallization is completed. The use of thismethod for the crystallization of TPA-silicalite-1with a TEOS silica source indicated that the crystalpopulation, during the nucleation period, asymp-totically approached the ®nal crystal population.Also, most nucleation occurred before lineargrowth of crystals. Zhdanov and Samulevich [21]were the ®rst to report a more commonly used, butexperimentally less direct method for determiningthe nucleation pro®les for zeolite crystallization. Inthis method, the ®nal product crystal size distri-bution (CSD) and the linear growth rate of thelargest crystals is used to calculate the nucleationpro®le. It has been found that TPA-silicalite-1synthesis solutions, even before hydrothermaltreatment, contain sub-colloidal silica particleswith a short-range order and entrapped TPA�

cations and thus may act as crystal nuclei or theprecursors of nuclei [22±24]. In the light of these®ndings, the term ÔnucleationÕ in this context mayactually refer to when these nuclei or nuclei pre-cursors are activated for growth.

Our previous study of the nucleation period ofthe crystallization of TPA-silicalite-1 was based onthe use of a tetraethoxysilane (TEOS) silica source[12,19]. It is known that the use of di�erent silicasources can strongly in¯uence the nucleation andcrystallization kinetics not only for TPA-silicalite-1 [25], but also for many other zeolites [26±29].Amorphous silica sources are more commonlyused for zeolite syntheses and typically result in theformation of larger crystals with a broader CSD.The present study has two purposes: First, thetwo-stage varying-temperature synthesis method isused to explore the e�ect of silica source on thedynamics of nucleation and crystallization ofTPA-silicalite-1. Second, the reliability of the two-stage varying-temperature synthesis method fordetermining the nucleation pro®les of zeolitecrystallization is further critically examined andcompared to methods more commonly used in theliterature.

2. Experimental

The silica sources used were TEOS (>98% GC,Merck) or commercial colloidal silica sols: Ludox

TM (39.8 wt.% SiO2, 0.11 wt.% Na2O particle size22±24 nm, Dupont) or Ludox LS (30.8 wt.% SiO2,0.108 wt.% Na2O particle size 11±13 nm, Dupont).The alkali source was tetrapropylammonium hy-droxide ± TPAOH (1.0 M in water, 143 ppm Na,4200 ppm K, <10 ppm Al, Sigma). Other materialswere ethanol (>99.5%, Kemetyl, Sweden), 0.1 MNaOH solution (Eka Nobel AB, Sweden) anddouble-distilled water.

TPA-silicalite-1 was crystallized from synthesissolutions with a molar composition of 9 TPAOH:25SiO2:0.13Na2O:595H2O:100 ethanol. Note thatethanol is evolved from the hydrolysis of theTEOS silica source. The silica source was mixedwith the TPAOH and, if necessary, ethanol and/or0.1 M NaOH solution were added. The mixturewas shaken on a gyratory shaker at room tem-perature for 24 h before hydrothermal treatment,then heated at 60±100°C under re¯ux and withoutstirring. In some cases, during the course of crys-tallization the treatment temperature was rapidlyincreased from 60°C to 100°C, which will be re-ferred to as a two-stage synthesis [19].

Particle size measurements by dynamic lightscattering (DLS) were made for crystallizationswith all-silica sources. In some cases, DLS mea-surements were complemented by scanning elec-tron microscopy (SEM) (Philip XL 30). Ramanspectroscopy and X-ray di�raction (XRD) ana-lyses were performed on extracted powder samplesbefore and after calcination. Details regarding theprocedures for DLS, SEM and XRD analyseswere reported earlier [19]. Raman measurementswere performed with a Perkin Elmer PE 1700XNIR FT-Raman spectrometer equipped with anNd YAG laser operating at 1064 nm. The spectrawere collected at room temperature averaging1200 scans with a spectral resolution of 4 cmÿ1

using 0.6 W power of the incident light.

3. Results and discussion

3.1. E�ect of silica source in one-stage syntheses

A series of one-stage syntheses with di�erentsilica sources were performed in order to investi-

54 Q. Li et al. / Microporous and Mesoporous Materials 40 (2000) 53±62

gate the e�ect of the silica source on the crys-tallization process. The synthesis mixtures werecrystallized at 60°C and 100°C. XRD patternscon®rmed that all products were well crystallizedsilicalite-1. A previous study had revealed that ifall other conditions besides the nature of the silicasource are identical, the use of an amorphous silicasource in the synthesis of colloidal TPA-silicalite-1strongly in¯uences the outcome of the synthesis[25]. Fig. 1(a) and (b) show the crystal size as afunction of the crystallization time for the silicasources, TEOS and colloidal silica (Ludox TM andLudox LS) at 60°C and 100°C. Irrespective of

silica sources and reaction temperature, smallcolloidal silica particle populations were alwayspresent in each of the suspensions prior to hy-drothermal treatment (zero time). The particlesfrom TEOS with an average hydrodynamic di-ameter of 4 nm were sub-colloidal amorphoussilica particles [19] which have been found to haveslab shapes with dimensions of 1:3� 4� 4 nm3

[24]. They were formed after the hydrolysis of thereaction mixture and were present throughout theentire crystallization. However, the particle sizeprior to the hydrothermal treatment from LudoxTM and Ludox LS was much larger than thatfrom TEOS. Analysis of these suspensions showedthat the particles from Ludox TM and Ludox LSwere in the range of 26±30 and 15±19 nm, re-spectively, which was the size of the colloidal silicaparticles in the original Ludox TM and Ludox LSsuspensions according to the DLS measurements.These amorphous colloidal silica particles wereprobably stabilized by surface adsorbed TPA�

cations in the synthesis mixture. The very smallsub-colloidal silica particles seen in the TEOS solmay also have formed in the Ludox sols; however,the presence of the larger colloidal silica particleswould have made them undetectable by DLS. Thecolloidal silica particles in each of the synthesissolutions were detectable by DLS for only a lim-ited time after the appearance of the growingcrystal population. However, colloidal silica wasprobably present throughout the crystallizations,particularly since a constant linear growth rate wasobserved which suggested that the presence ofamorphous silica maintained a constant concen-tration of silica in solution for growth.

Table 1 lists the crystallization time, crystal size,yield, crystal concentration and growth rate forcrystallizations from synthesis sols with identicalcompositions except with di�erent silica sources.The use of an amorphous silica source led to alonger crystallization time, a higher growth rateand a larger crystal size in comparison with that ofTEOS, in agreement with the previous results [25].At 60°C with TEOS, smaller silicalite-1 crystalswere produced with an average size of 63 nmcompared to 160 nm from Ludox TM and 169 nmfrom Ludox LS. The same features were observedat 100°C, as shown in Table 2. Due to the same

Fig. 1. Average crystal size during the crystallization with

various silica sources at di�erent temperatures: (a) 60°C and (b)

100°C.

Q. Li et al. / Microporous and Mesoporous Materials 40 (2000) 53±62 55

molar composition for all syntheses, the yieldswith di�erent silica sources were about the same ata given temperature, i.e., 51±55% at 60°C and 60±62% at 100°C. Calculated from the ®nal crystalsize and yield, the crystal concentration nucleatedfrom TEOS was about 11 times more than thatfrom amorphous silica at 60°C and 17 times moreat 100°C. The much larger crystal concentrationwith TEOS results in its shorter crystallizationtime compared to the Ludox systems, because thecumulative surface area on which growth or soluteconsumption takes place is larger. This is despitethe fact that the inherent crystal growth rates forthe Ludox systems were greater than for TEOS.

The crystal size distributions were also consid-erably di�erent. SEM micrographs (Fig. 2) indi-cated that the CSD from Ludox TM and LS wasnoticeably broader than that obtained fromTEOS. By extrapolating the linear growth curve tothe crystallization time axis, it was found that theinduction period from amorphous silica was muchlonger than that from TEOS, as shown in Fig. 1.This trend was most clear at 60°C where for TEOSthe induction period was 78 h, whereas for LudoxLS, it was 105 h and, for Ludox TM, 128 h.

3.2. E�ect of silica source in two-stage syntheses

Two-stage varying-temperature syntheses wereused to determine the nucleation period from dif-ferent silica sources. Brie¯y, this synthesis tech-

nique involved starting a crystallization at 60°Cand then rapidly increasing the temperature to100°C to halt further nucleation [19]. Fig. 3 com-pares the average crystal sizes for the one-stagesyntheses during the course of crystallization at60°C and 100°C with those for a selection of thetwo-stage syntheses from various silica sources.For all silica sources, the linear growth rate of thecrystals rapidly increased after the temperatureincrease and was approximately the same as thatfor the one-stage synthesis at 100°C.

Fig. 4 shows the ®nal crystal size against theduration of the synthesis that started at 60°C.With the TEOS silica source, the ®nal crystal sizeabruptly decreased as the ®rst-stage duration waslengthened beyond zero time, i.e., a one-stagesynthesis at 100°C. On the other hand, with theamorphous silica Ludox TM and Ludox LS, ini-tially there was an insigni®cant decrease in the ®-nal crystal size. The inset diagram indicates thatfor a ®rst-stage duration less than 12 h, the ®nalcrystal size was constant. The reasons for theseobservations will be discussed further below. Be-yond about 12 h, the ®nal crystal sizes decreaseduntil they were approximately equal to those ob-tained for a complete synthesis at the initial tem-perature of 60°C.

Table 3 lists the yield and linear growth ratefrom di�erent silica sources for all two-stage syn-theses. Since the starting synthesis compositionswere identical, regardless of the type of silica

Table 1

Crystallization time, crystal size, yield, crystal concentration and growth rate from di�erent silica sources at 60°C

Silica source Crystallization

time (h)

Average crystal

size (nm)

Zeolite yield (%) Crystal concentration

(�1013 gÿ1 solÿ1)

Growth rate

(nm hÿ1)

TEOS 264 63 51.00 22.21 0.32

Ludox TM 384 160 54.30 1.91 0.70

Ludox LS 372 169 52.70 1.57 0.62

Table 2

Crystallization time, crystal size, yield, growth rate and crystal concentration from di�erent silica sources at 100°C

Silica source Crystallization

time (h)

Average crystal

size (nm)

Zeolite yield (%) Crystal concentration

(�1013 gÿ1 solÿ1)

Growth rate

(nmhÿ1)

TEOS 32 101 60.60 6.40 3.76

Ludox TM 56 286 62.00 0.38 5.76

Ludox LS 55 297 61.45 0.34 5.61


source, the ®nal yields were the same and werecloser to that obtained for a one-stage synthesis at100°C rather than to that at 60°C (Tables 1 and 2).Apparently, the yield of silicalite-1 is controlled bythe thermodynamic equilibrium at the ®nal syn-thesis temperature of 100°C. For all silica sources,the linear growth rates for two-stage syntheseswere the same and close to that for a one-stagesynthesis at 100°C, as shown in Fig. 3.

The crystal concentration, calculated from theyield and ®nal crystal size, is plotted in Fig. 5(a) as

a function of crystallization time. It has beenshown [19] that when TEOS is used as the silica

Fig. 2. SEM of the ultimate products at 100°C from various

silica sources: (a) Ludox TM, (b) Ludox LS and (c) TEOS.

Fig. 3. Comparison of one- and two-stage crystallizations

starting at 60°C and ending at 100°C with various silica sources:

(a) TEOS, (b) Ludox LS and (c) Ludox TM.


source, linear growth of the crystals begins justafter the temperature increase, during the low-temperature crystallization induction period,without a broadening in the CSD of the product.This indicates that nucleation is e�ectively haltedas a result of the temperature increase and thecrystals nucleated at the initial lower synthesistemperature only continue to grow linearly afterthe temperature increase. These general featureswere also observed for the amorphous silicasources. Thus, the plot in Fig. 5(a) in fact repre-sents the crystal concentration during the nucle-

ation period. The nucleation process was con-sidered to continue until the crystal concentrationwas una�ected by the temperature variations. Thecurves in Fig. 5(b) were calculated from the de-rivatives of the ®tted curves in Fig. 5(a), and thus,represent the nucleation pro®les for each silicasource. With the TEOS silica source, nucleationwas completed within 72 h and the rate of nucle-ation was initially high and decreased throughoutthe induction period. A high rate of nucleationproduced a large number of nuclei at the earlieststage of the nucleation period. For example, forthe two-stage synthesis in which the initial tem-perature of 60°C was maintained for only 2 h, thecrystal population was 8:69� 1013 gÿ1 solÿ1, whichwas already greater than the crystal population fora one-stage synthesis at 100°C, viz. 6:4� 1013

gÿ1 solÿ1. However, with the Ludox TM and LudoxLS silica sources, nucleation was not completeduntil after 120 h, and the nucleation pro®les weremore similar to those usually considered to applyto zeolite syntheses with a self-accelerating nucle-ation rate [30]. The rate of nucleation was initiallylow before increasing to reach a maximum, andthen gradually decreased during the course ofnucleation.

Presumably, the crystal concentration for theamorphous silica sources should have started from

Fig. 4. E�ect of the duration of the ®rst stage at 60°C on the

ultimate crystal size (inset shows enlargement of same data):

(a) Ludox TM, (b) Ludox LS and (c) TEOS.

Table 3

Yield and growth rate for two-stage syntheses with the ®rst stage at 60°C and the second stage at 100°C

Duration of the

®rst stage (h)

Ludox LS Ludox TM TEOS

Yield (%) Growth rate

(nm hÿ1)


(nm hÿ1)


(nm hÿ1)

0 61.45 5.38 62 5.66 60.60 3.76

2 64.20 3.43

3 59.80 5.17 61.90 5.21

6 61.40 6.04 61.10 5.01 62.10 2.96

9 63.00 3.26

12 59.70 5.70 62.30 6.22 62.60 4.01

24(1d) 60.34 5.12 60.22 5.59 61.00 3.21

48(2d) 60.85 5.69 59.94 6.06 63.90 3.87

60(2.5d) 60.30 4.98 62.80 5.67

72(3d) 62.90 5.65 61.10 5.93 60.20 3.75

96(4d) 64.10 5.98 62.70 5.74

120(5d) 63.90 6.12 63.80 6.13 61.70 3.20

144(6d) 61.70 5.31 63.90 5.80

168(7d) 61.00 6.05 64.10 6.11 60.80 3.44

216(9d) 61.30 5.59 62.40 5.97


zero in Fig. 5(a). However, this was not the case.Instead, the crystal concentration for less thanabout 12 h was the same as that for a one-stagesynthesis at 100°C (Table 2). The constant crystalconcentration is due to the fact that the crystal sizefor the Ludox systems with ®rst-stage durationsless than 12 h was constant as indicated in Fig. 4.With an amorphous silica source, the rate of nu-cleation during the ®rst 12 h of the crystallizationwas likely to be so low that the crystal concen-tration at 60°C was lower than that for a one-stagesynthesis at 100°C. Thus, after the temperatureincrease, continuing dissolution of the amorphoussilica most likely maintained the silica concentra-tion over the supersaturation concentration, sothat nucleation continued at 100°C until the crys-tal concentration reached that for a crystallizationentirely at 100°C. This resulted in the constantcrystal size and crystal concentration for Ludoxsystems at less than 12 h in Figs. 4 and 5, respec-tively. These results indicate that the two-stagesynthesis method should be used with caution fordetermining the nucleation pro®le. Apparently,there is a tendency for the crystal concentration to

be overestimated during the early stage of thenucleation period. Presumably, if a temperaturegreater than 100°C with a lower one-stage syn-thesis crystal concentration is used during thesecond stage, an improved accuracy in determiningthe early part of the nucleation pro®le can beachieved. Beyond about 12 h, the crystal concen-tration increased slowly and was greater than thatfor a one-stage synthesis at 100°C. During thislatter part of the nucleation period, nucleation wase�ectively halted by the temperature increase andthe nucleation pro®le could be accurately de-termined. The ®nal crystal concentration withamorphous silica sources was about 10 times lessthan that with TEOS.

As shown in Fig. 1(a), the induction period withamorphous silica was much longer than that withTEOS. At 60°C, with Ludox TM it was 105 h andwith Ludox LS it was 128 h, whereas with TEOS itwas only 78 h. It is likely that colloidal silicaparticles were stabilized by surface adsorbed TPA�

cations in the solution. These TPA� cations wereprobably not present in the internal structure ofthe amorphous silica due to their relative bulki-ness. The amorphous silica must thus depolymer-ize to produce soluble silica species that arearranged around TPA� cations to form inorganic±organic composite species which have beenproposed as nuclei or the origin of nuclei for TPA-silicalite-1 crystal growth [31]. Since all solutionswith di�erent silica sources were aged for 24 hprior to hydrothermal treatment, less of these nu-clei-forming species were formed in the solutionsfrom amorphous silicas compared to that in thesolution from TEOS, due to the lower concentra-tion of silica species in the amorphous silica solu-tions. Upon the onset of hydrothermal treatment,the amorphous silica depolymerized, catalyzed bythe alkalinity of the solutions, to form elementarysilica species. The elementary silica could partici-pate in the nucleation and growth of the zeolitewhen its concentration reaches a certain criticallevel. This additional step of equilibration of silicaspecies from amorphous silica likely extended theinduction period and led to the slower nucleationrate. This explanation for the di�erences in thenucleation processes is based on the conventionalview that supersaturation drives both nucleation

Fig. 5. E�ect of silica sources on the nucleation at 60°C: (a)

crystal concentration and (b) nucleation rate.


and growth. Unfortunately, it may not be a com-plete explanation of these complex processes, be-cause the higher concentration of silica in theTEOS solution did not apparently persist whenlinear growth of crystals was later observed, sincecrystals in the TEOS solution had a lower lineargrowth rate compared to the amorphous silicasolutions (Tables 1 and 2). Di�erences in the nu-cleation mechanisms may also contribute to thedi�ering nucleation rates between the systems.

The nucleation period was 72 h for TEOS and120 h for both Ludox TM and Ludox LS, whichwas similar in length to the induction periods foreach silica source. Thus, an interesting ®nding inthis work is that for both TEOS and amorphoussilicas, Ludox TM and Ludox LS, the vast ma-jority of the nucleation occurred during an in-duction period before linear crystal growthstarted. It was concluded in our previous work [19]that the particularly narrow crystal size distribu-tion using a TEOS silica source could be attributedto the fact that nucleation occurred during an in-duction period. However, this is apparently not aunique feature of crystallization from TEOS sinceeven here with amorphous silica sources, that gavea broader crystal size distribution, most nucleationstill occurred during an induction period. Thedi�erences in crystal size distribution would seemto con®rm that in fact some undetected non-lineargrowth occurs during the nucleation period with agradually increasing rate. With an amorphoussilica source, if the nucleation rate is self-acceler-ating, perhaps more nucleation tends to occur laterin the induction period when the growth rate ishigher which results in a broader CSD of the ®nalproducts. Alternatively, the broader CSD perhapscan be attributed to the greater complexity of thenucleation process with amorphous silica sourcesand the fact that both homogeneous and hetero-geneous nucleation may occur. This may lead toan increased size distribution in the actual nuclei.

Nucleation pro®les for zeolite crystallizationsare often calculated based on the ®nal productCSD and the linear growth rate [4,10,17,18,21]. Itis assumed that only the simultaneous processes ofcrystal nucleation and growth contribute to theCSD of the product. Here, we have shown by di-rect experimental measurements of the nucleation

pro®le that most of the nucleation occurs beforecrystal growth is detected, even with amorphoussilica sources. Measurements of the nucleationprocess were made here at 60°C, a particularly lowtemperature, and it can be seen in Fig. 1 that at ahigher temperature of 100°C the duration of theinduction period is shorter. However, even at ele-vated temperatures, the same processes presum-ably occur except with shorter time scales.

For both Ludox LS and Ludox TM silicasources, the crystallization time, yield and the rateof crystal growth were the same at a given tem-perature, as shown in Tables 1 and 2. The onlydi�erence is the crystal size which indicates thatwith Ludox TM slightly more crystals were nu-cleated than with Ludox LS (Fig. 5). Thus, theobserved di�erence in crystal size between thesetwo amorphous silica sources would seem to havestemmed from di�erences in their nucleation pro-cesses. According to XRD, both of these silicasources were amorphous, but their Raman spectra,as shown in Fig. 6, indicated some di�erences fromamorphous silica. The spectrum of vitreous silica ischaracterized by a peak at 492 cmÿ1 and a very

Fig. 6. Raman spectra of optically homogeneous vitreous silica,

Ludox TM silica, Lodox LS silica and TPA-silicalite-1 (MFI)

crystals.


broad band at 445 cmÿ1, whereas several di�er-entiated peaks at 351, 406, 436, 453 and 490 cmÿ1

existed in the spectra of Ludox particles, thus, in-dicating that the structure of the Ludox silicapossessed some degree of ordering. Also, in theRaman spectrum of Ludox LS, there was a peak at475 cmÿ1, which is absent in the spectrum of Lu-dox TM. The peaks at 436 and 453 cmÿ1 observedin the spectra of both Ludox silicas as well as theextra peak at 475 cmÿ1 detected only in the spec-trum of Ludox LS coincided in frequency withthree of the peaks observed in the spectrum ofsilicalite-1. It is known that the peak positions inthe range 300±650 cmÿ1 of the Raman spectra oftectosilicates correlate with the types of silicon±oxygen rings existing in the MFI structure [32±35].The rings of SiO4 tetrahedra represent the inter-mediate range order in the silicate structure.Therefore, one can conclude that the structure ofboth Ludox LS and Ludox TM contained silicon±oxygen rings typical of the MFI structure, but theintermediate range ordering of Ludox LS wasslightly closer to the MFI structure than that ofLudox TM due to an extra peak at 475 cmÿ1.

In a plausible model of nucleation with simi-larities to the autocatalytic nucleation model[4,36], the ®rst step of crystallization is nucleationat the boundary of the amorphous solid and theliquid phase. The TPA� cations (structure-direct-ing agent) ®rst adsorb on the surface of particles toform regions with ordered structure. These regionsoccur in/on the gel skeleton and may act as theorigins of crystal nuclei. As the amorphous phaseis dissolved, these potential nuclei are released intothe aqueous phase and start to grow. Because thestructure of Ludox LS silica was slightly closer tothe MFI structure and more ordered than that ofLudox TM, it may be thermodynamically morestable and/or the kinetics of its dissolution may beslower. This results in fewer nuclei ``released'' intothe liquid phase with the Ludox LS silica source.Another possibility is that the silica species in so-lution with the Ludox LS silica have a structurewhich is more favorable for incorporation intoexisting growing nuclei which causes a lower su-persaturation concentration to be reached andthus the formation of fewer nuclei. However, againthese possible di�erences in the silica concentra-

tions during nucleation apparently did not persistwhen linear growth of the crystals was later ob-served, since the linear growth rates for the Ludoxsystems were about the same (Tables 1 and 2).

In this present study, all solutions with di�erentsilica source were aged for 24 h at room temper-ature. Aging time with amorphous silica likelyin¯uences the nucleation and crystallization.

4. Conclusions

A two-stage varying-temperature synthesisprocedure was utilized successfully to determinethe duration of the nucleation period for thecrystallization of colloidal TPA-silicalite-1 withvarious silica sources, including TEOS and amor-phous colloidal silica (Ludox TM and Ludox LS).This synthesis procedure was also used to estimatethe crystal concentration during the nucleationperiod. The crystal concentration from the amor-phous silica sources tended to be overestimated atthe earliest stage of the nucleation period, becausethe minimum crystal concentration for two-stagesyntheses was equal to that for a one-stage syn-thesis at the elevated temperature used. At 60°C,with the TEOS silica source nucleation was com-pleted within 72 h and the crystal concentrationinitially increased rapidly during the nucleationperiod. However, with the amorphous silicasources (Ludox TM or Ludox LS), the duration ofthe nucleation period was about 120 h and thenucleation pro®le had a self-accelerating nu-cleation rate at the beginning of the nucleationperiod.

It was found that for both TEOS and amor-phous silica sources, the vast majority of the nu-cleation occurred during an induction periodbefore linear growth started. With TEOS, smallercrystals with a particularly narrow CSD were ob-tained, which could be attributed to the fact thatnucleation occurred during an induction period.However, even with amorphous silica sources thatproduced larger crystals with a broader CSD, mostnucleation occurred during an induction period.The broader CSD might have been due to the factthat larger portions of crystals were nucleated laterduring a longer nucleation period when undetected


crystal growth was higher. Alternatively, thegreater complexity of the nucleation processes withamorphous silica sources may simply have led to agreater variation in the size of the nuclei. In ad-dition, the more ordered structure of Ludox LSsilica compared to that of Ludox TM might haveslowed its dissolution and hindered nucleation,thus resulting in the crystallization of slightlylarger crystals with a lower concentration. Theresults of our continuing work focusing on thee�ects of room temperature aging on the nucle-ation rate will be the subject of a future publica-tion.

Acknowledgements

The authors are grateful for the support of theSwedish Research Council for Engineering Sci-ences (TFR).

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The nucleation period for crystallization of colloidal TPA-silicalite-1 with varying silica source