Synthesis and characterization of colloidal zoned MFI crystals

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Microporous and Mesoporous Materials 78 (2005) 1–10

Synthesis and characterization of colloidal zoned MFI crystals

Qinghua Li a, Zheng Wang a, Jonas Hedlund a,*, Derek Creaser b, Hong Zhang c,Xiaodong Zou c, Anton-Jan Bons d

a Division of Chemical Technology, Lulea University of Technology, SE-971 87 Lulea, Swedenb Department of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96 Goteborg, Sweden

c Structural Chemistry, Stockholm University, S-106 91 Stockholm, Swedend ExxonMobil Chemical R&D, European Technology Center, Hermeslaan 2, B-1831 Machelen, Belgium

Received 4 June 2003; received in revised form 25 August 2004; accepted 2 September 2004

Available online 21 November 2004

Abstract

Colloidal zoned MFI crystals, i.e., continuous crystals with a compositional gradient resulting in a ZSM-5 core covered with a

silicalite-1 shell, were synthesized by the addition of ZSM-5 seeds to a silicalite-1 synthesis solution. The effect of the surface alu-

minum content of the ZSM-5 crystals on the synthesis of the zoned MFI crystals was investigated using SEM, TEM, XPS and

XRD. An acid treatment of the ZSM-5 seeds, which removed some of the aluminum at the surface, proved favorable for the syn-

thesis of zoned MFI crystals.

� 2004 Elsevier Inc. All rights reserved.

Keywords: Zoned MFI crystals; ZSM-5 core; Silicalite-1 shell; Acid leaching

1. Introduction

HZSM-5 is used as a shape selective catalyst in

hydrocarbon conversions, e.g., alkylations and isomer-

izations of aromatics [1–3]. To maximize the activity

of such a catalyst it may be desirable to use small crys-

tals and thereby minimize the intra-pore mass transport

resistance. This is particularly the case if intra-pore masstransfer resistance is controlling the rate of a catalytic

reaction. Unfortunately, small crystals increase the frac-

tion of non-shape selective sites at the external surface of

the zeolite. This may decrease the shape selectivity of the

catalyst [4,5]. Another factor is that deactivation due to

deposition of carbonaceous material tends to be more

severe with larger zeolite crystals [6], because pore

blockage is more detrimental with longer diffusion

1387-1811/$ - see front matter � 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2004.09.010

* Corresponding author. Tel.: +46 920 492105; fax: +46 920 491199.

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

paths. Thus, in many cases an optimal HZSM-5 catalyst

may consist of nano-sized or colloidal crystals with a

thin skin of silica or even better silicalite-1 to eliminate

external surface activity. This silica skin can be provided

by covering the external surface with silicon alkoxides,

using chemical vapor deposition (CVD) [7–9]. After sev-

eral deposition–calcination cycles, a thin silica layer cov-

ering ZSM-5 is obtained, which completely inactivatesthe external surface acidity and thereby enhances the

catalyst selectivity. However, this method may narrow

the pore opening and decrease the catalyst activity

[10]. A reduction in the contribution of active sites at

the external surface can also be achieved by coating

the crystals with an aluminum-free shell having the same

structure as the core. This can be realized by first synthe-

sizing ZSM-5 and then altering the synthesis solution toone without aluminum during hydrothermal treatment,

as claimed by Rollmann [11]. Alternatively, smaller sili-

calite-1 crystals, possibly bound by intergrowth, can

cover larger ZSM-5 crystals [12]. Although successful

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2 Q. Li et al. / Microporous and Mesoporous Materials 78 (2005) 1–10

synthesis of so-called zoned MFI crystals have been

claimed in the literature, little characterization other

than sorption and catalysis experiments has been carried

out to investigate the structural configuration of such

materials [13–15]. In addition, it is unclear whether the

final product consists of polycrystalline aggregates ortruly discrete compositionally zoned crystals. Moreover,

no clear evidence whether small silicalite-1 crystals are

formed as a by-product in the final product has been

presented. It is quite possible that the prepared materials

consist of mixtures of partially zoned MFI crystals and

silicalite-1 crystals derived from secondary nucleation.

Recently, zoned MFI films, i.e., a silicalite-1 film cov-

ering a ZSM-5 film, were synthesized by a methodemploying seed crystals [16]. The reason for the prepara-

tion of zoned MFI materials in the form of films rather

than in the form of powders was in this case that the

interface between the films can be directly investigated

by SEM, EDS and TEM. The preferred orientation of

the crystals could also be evaluated by XRD. It was

found that truly zoned MFI films, i.e., with a continuous

channel system at the boundary, could be obtainedwhen the compositional difference between the two lay-

ers (ZSM-5 and silicalite-1) was relatively small and the

synthesis conditions were similar. However, if a ZSM-5

film with high aluminum content was grown from a gel

and then coated with silicalite-1 grown from a clear

solution, a discontinuity occurred at the interface due

to crystals re-nucleating on the surface of the first layer.

Since the aluminum content in the ZSM-5 film synthe-sized from a gel was much higher than that from a clear

solution, a higher content of aluminum appeared unfa-

vorable for the synthesis of a truly zoned MFI film. This

may have resulted because continued growth of alumi-

num rich ZSM-5 crystals in the aluminum-free synthesis

solution was less favorable than nucleation of new silica-

lite-1 crystals. It was thus assumed that reduction of the

aluminum content at the external surface of ZSM-5could be a key requirement to synthesize truly zoned

MFI materials. Selective dealumination of the external

surface has been accomplished by the use of acids or

complexing agents [17–19]. For instance, Apelian et al.

[18] made use of dicarboxylic acids to remove frame-

work aluminum at the external surface of organic-con-

taining zeolites. This treatment resulted in a reduction

of surface acidity without a significant reduction in theoverall activity.

The aim of the work reported here was to synthesize

colloidal zoned MFI crystals, which comprise ZSM-5

cores and silicalite-1 shells, from clear solutions. To

investigate the effect of aluminum at the surface of the

ZSM-5 crystals on the synthesis of zoned MFI crystals,

zoned crystals were grown with or without an intermedi-

ate dealumination step. The ZSM-5 seeds used as thecore were dealuminated by acid leaching in a HCl

solution.

2. Experimental

2.1. Materials

The silicon source used was TEOS (<98%, GC,

Merck) and the aluminum source was aluminumisopropylate (Sigma–Aldrich). Tetrapropylammonium

hydroxide (TPAOH, 2.0M in water, Sigma) was used

as the structure-directing agent. The sodium source

was 1.0M NaOH solution (Eka Chemcials, Sweden).

Double distilled water was used in all cases.

2.2. Synthesis of zoned MFI crystals

Colloidal ZSM-5 crystals (cores) were prepared

from a clear solution with a molar composition of

5TPAOH:0.25Al2O3:25SiO2:480H2O:100EtOH:0.1

Na2O. The synthesis mixture was heated in an oil bath

at 100 �C under reflux and without stirring for 48h.

After crystallization, the sol was purified by repeating

centrifugation at 12,500g for 30min and re-dispersion

four times. Half of the purified ZSM-5 crystals wereacid treated in a 0.1M HCl solution at 50 �C for 12h

with stirring. A concentrated sol containing ZSM-5

crystals was added to dilute acid. The dry content of

the ZSM-5 seeds in the 0.10M HCl solution was

1.0wt%. After acid treatment, the ZSM-5 crystals were

rinsed by repeated centrifugation and re-dispersion in

distilled water four times. Both acid treated and un-

treated ZSM-5 crystals were stored as sols withpH = 10 adjusted by the addition of ammonia. The

dry content was about 25.0wt%. Note that the cores

were never dried and always stored as sols. For the

preparation of the zoned MFI crystals, both acid trea-

ted and untreated ZSM-5 crystals were used as seeds

for further growth in a TPA–silicalite-1 solution with

a molar composition of 3TPAOH:25SiO2:1500H2O:

100EtOH. ZSM-5 seeds were hydrothermally treatedin the silicalite-1 synthesis solution at 100 �C with stir-

ring for a certain time. The ZSM-5 seeds were added in

the form of a sol to the silicalite-1 synthesis mixture.

The dry content of seeds in the solution was 2.0wt%.

For crystallization periods less than 16h, products

were extracted and purified by the same treatment as

used for the ZSM-5 seeds. In the case 24h of crystalli-

zation, the samples were centrifuged at 4500g for5min. The supernatant was discarded and the pellet

was recovered in order to separate the larger zoned

crystals from the small TPA–silicalite-1 crystals that

nucleated and grew in the silicalite-1 solution (see fur-

ther Section 3). Growth of silicalite-1 seeds in a silica-

lite-1 synthesis mixture was also investigated as a

control experiment. For this purpose, TPA–silicalite-1

crystals (cores) were synthesized in a mixture with amolar composition of 5TPAOH:25SiO2:1400H2O:

100EtOH at 100 �C for 3 days.

Q. Li et al. / Microporous and Mesoporous Materials 78 (2005) 1–10 3

2.3. Characterization

The particle size was measured using a Brookhaven

Instrument BI200SM Dynamic light scattering system

(DLS). A Philips XL 30 scanning electron microscope

(SEM) equipped with a LaB6 emission source was usedto measure the particle size and to record the crystal

morphology. A Philips CM12-T transmission electron

microscope (TEM) operated at 120kV was used for

investigation of samples prior to and after acid leaching.

Zoned samples were studied using a JEOL JEM-3010

TEM operated at 300kV with a KeenView Slow scan

CCD-camera. In both cases, samples were dispersed in

ethanol and a drop of the suspension was transferredto a Cu grid with holey carbon foil and the sample

was dried. A Simens D5000 X-ray diffractometer

(XRD) using CuKa radiation was used to determine

the phase and crystallite size of the powder samples. A

least squares model was used for refinement of the

XRD patterns in the 2h range 22.5–25.0�. A Pearson

VII line shape function with an exponent of 1.426 was

used for all Ka1/2 doublets. The error between refinedand experimental data is given by the reliability index

(RI), which is defined as:

Fig. 1. SEM micrographs of ZSM-5 cores without acid treatment (a) and M

16h (c) and 24h (d).

RI ¼P

j I0 � IC jP

I0

The crystallite size was determined using the Scherrer

equation with a constant of unity and an instrumentalbroadening of 0.047� (determined on quartz powder).The silicon and aluminum contents in bulk crystals were

determined by inductively coupled plasma atomic emis-

sion spectroscopy (ICP–AES). X-ray photoelectron

spectroscopy (XPS) was performed to analyze the Si/

Al ratio at the surface of the particles. The XPS signal

emanates from the surface to a depth of 5–6nm. Zeolite

powders for XPS measurements were pressed in amolybdenum holder with a 5mm diameter trough of

variable volume. All XPS spectra were recorded with a

KRATOS Axis Ultra electron spectrometer using a

monochromated AlKa source operated at 225W. To

compensate for the surface charging, a low energy elec-

tron gun was used. Wide spectra (pass energy 160eV)

and spectra of individual photoelectron lines (pass en-

ergy 20eV) were acquired. The spectra processing wereaccomplished with the KRATOS software. The binding

energy (BE) scale was referenced to the C 1s line of ali-

phatic carbon set at 285.0eV.

FI particles after treating the cores in a silicalite-1 solution for 14h (b),


3. Results and discussion

3.1. ZSM-5 cores

ZSM-5 cores with an average size of 180nm were syn-

thesized from a clear solution at 100 �C, as shown in Fig.1(a). The crystal size distribution (CSD) ranged from

about 100 to 240nm. The bulk Si/Al ratio of the crystals

was 45, determined by ICP–AES. According to XPS, the

Si/Al ratio was 11 at the surface of the crystals. There-

fore, the aluminum in the ZSM-5 cores was enriched

at the surface. Similar results have been reported in

Fig. 3. SEM micrographs of original TPA–silicalite-1 seeds (a) and

Fig. 2. Average particle size of TPA–silicalite-1 determined by DLS

during crystallization at 100�C in the presence or absence of ZSM-5

seeds.

the literature, especially when TPA+ was used as the

structure-directing agent [20,21].

3.2. Growth of zoned MFI particles without acid

leaching of the ZSM-5 cores

Purified ZSM-5 cores were hydrothermally treated in

a TPA–silicalite-1 solution at 100 �C. Prior to 12h ofcrystallization, no growth of the cores or new crystals

was observed. However, beyond 12h of crystallization,

some changes occurred. Fig. 1(b)–(d) show SEM images

of the final products after crystallization in the TPA–sil-

icalite-1 solution for 14h, 16h and 24h, respectively.

After crystallization for 12h (not shown), the ZSM-5cores had aggregated to some extent. A few small crys-

tals with a size of about 30nm were observed but no

growth of the cores occurred. The degree of the aggrega-

tion of crystals increased and some of the ZSM-5 seeds

seemed to dissolve or separate when the crystallization

time was extended to 14h (see Fig. 1(b)). Also, small

crystals in the size range of about 30–50nm were found

again. After 16h of crystallization some of the aggre-gated particles had grown further achieving a size rang-

ing from about 250 to 400nm. On the other hand, some

crystals with a size of about 180nm still existed, which

could be the original ZSM-5 seeds and even smaller

after growth in a silicalite-1 solution for 4h (b) and 10h (c).


crystals with a size of about 40nm again coexisted in the

final product (see Fig. 1(c)). After hydrothermal treat-

ment in the silicalite-1 solution for 24h, small crystals

with an average size of 75nm were detected along with

larger aggregates with an average size of 450nm. The

small crystals were separated from this product bydecanting the supernatant after centrifugation at 4500g

for 5min. Fig. 1(d) shows the purified product. Com-

pared to the original ZSM-5 seeds, the particle size

had increased from 180nm to 450nm after 24h of crys-

tallization in the silicalite-1 solution. However, SEM

also showed that the particles were aggregates rather

than separate crystals. Unfortunately, the SEM images

cannot show whether the final products contained a sil-icalite-1 shell or if they simply formed due to the aggre-

gation of seeds in the silicalite-1 solution.

Fig. 2 shows the TPA–silicalite-1 particle size moni-

tored by DLS during the course of crystallization at

100 �C in the presence and absence of ZSM-5 seeds. Inthe absence of seeds, the population of silicalite-1 crys-

tals grew with a mean rate of increase in particle diam-

eter of 15nm/h until growth ceased after 72h. In thepresence of ZSM-5 seeds, the silicalite-1 population

grew slower, with a mean increase in particle diameter

of 4nm/h. Note that two populations were present in

this case, ZSM-5 and silicalite-1, with a distinct size dif-

ference. This difference in growth rate shows that the

crystal growth of silicalite-1 was reduced due to the pres-

ence of the ZSM-5 seeds. The formation of the polycrys-

talline aggregates consumed nutrient and hindered thegrowth of silicalite-1. Thus, the polycrystalline aggre-

gates in the final product were possibly covered with sil-

icalite-1.

3.3. Growth of silicalite-1 cores

In order to elucidate the effect of the aluminum con-

centration at the surface of seeds on the formation of thezoned MFI crystals, a control experiment was studied

involving the further growth of silicalite-1 cores in a sil-

icalite-1 synthesis solution. The crystal size of the silica-

lite-1 seeds shown in Fig. 3(a) was about 340nm

according to DLS. These seeds were added to a fresh sil-

icalite-1 synthesis solution, in the same way as described

above. Fig. 3(b) and (c) show the products after crystal-

lization for 4h and 10h, respectively. Apparently, thesilicalite-1 cores already grew in the early stage of the

treatment without an appreciable induction time. After

4h of crystallization, the individual crystal size increased

to about 400nm according to DLS and the morphology

of the crystals remained unchanged. The size of the sili-

calite-1 crystals increased to about 460nm (DLS) after a

crystallization time of 10h. Meanwhile, smaller crystals

with a size of about 20nm appeared, which probablywere new silicalite-1 crystals that nucleated. The mean

growth rate of the silicalite-1 seeds during the first 10h

of crystallization was about 12nm/h, similar to that of

silicalite-1 crystals without the addition of ZSM-5 seeds.

Nutrient in the solution was probably mostly consumed

by the continued growth of the silicalite-1 seeds instead

of by the formation of new silicalite-1 crystals. Thus, the

presence of silicalite-1 seeds inhibited the nucleation andgrowth of new silicalite-1 crystals. It, therefore, seems

that the presence of aluminum on the surface of ZSM-

5 cores was unfavorable for the formation of zoned

MFI crystals. This may be related to observations that

the TPA+ ion predominantly interacts with silicate spe-

cies and not aluminosilicate [22,23]. Thus, dealumina-

tion of the surface of ZSM-5 cores prior to growth of

a silicalite-1 shell could enable the synthesis of the singlezoned MFI crystals.

3.4. Growth of zoned MFI crystals after acid leaching

of ZSM-5 cores

Aluminum can be removed from the zeolite lattice

with acid [17–19]. In the present work, purified ZSM-5

cores were treated in a 0.10M HCl solution at 50 �Cfor 12h. XPS measurement and chemical analysis indi-

cated that the Si/Al ratios on the surface and in the bulk

were 22 and 67, respectively, after acid leaching. This

should be compared to the untreated ZSM-5 seeds that

had a surface Si/Al ratio of 11 and a bulk Si/Al ratio of

45. A comparison of Figs. 1(a) and 5(a) does not reveal

any change of the crystal size or morphology due to the

acid treatment. TEM images of ZSM-5 cores prior toand after acid leaching are shown in Fig. 4. Both sam-

ples give lattice resolution at the edge of the crystals,

proving that the surface is crystalline. There is no evi-

dence of any amorphous coating on the samples (at a

scale of 0.5nm). Furthermore, XRD data recorded prior

to and after acid leaching is identical (not shown). If it is

assumed that the ZSM-5 crystals are spherical with a

diameter of 180nm and consist of a core with a diameterof 168nm and a skin with a thickness of 6nm (the pen-

etration depth of XPS) the Si/Al ratio of this hypothet-

ical core is 132 prior to leaching. In other words, the Al

content is 12 times higher in the skin than in the core

prior to acid leaching. After acid leaching, the Si/Al

ratio is 122 in the hypothetical core, i.e., almost the same

as before leaching. It thus seems that the acid leaching

procedure does not affect the core of the crystals, onlythe surface. This is probably due to the fact that the inte-

rior of the crystals is not accessible to the acid in the

presence of template molecules. However, the aluminum

content of the hypothetical skin was reduced to half by

the acid leaching procedure.

A series of syntheses were performed by the addition

of surface dealuminated ZSM-5 cores to a silicalite-1

synthesis solution. Fig. 5 shows that the growth ofthe dealuminated seed crystals shown in (a) was com-

pletely different compared to the untreated seeds. The

Fig. 4. TEM images of ZMS-5 cores before acid leaching (a,c) and after acid leaching (b,d).


morphology and size of crystals started to change even

early in the crystallization. Fig. 5(b)–(f) show the final

products obtained from the silicalite-1 solution at vary-

ing synthesis times. After 6h crystallization, the average

crystal size increased to 240nm and the surface of theparticles became rough (see Fig. 5(b)). Small crystallites

appeared to be growing on the surface of the seeds

rather than simply attached to the crystal surface. Fig.

5(c) shows the SEM image of the product after 8h of

crystallization. The crystals had reached a size of

290nm and the surface of the crystals became smoother.

With a further increase in the crystallization time, the

crystal size increased gradually and the morphology ofthe crystals resembled well-crystallized MFI. After 14h

of crystallization, crystals with a size of 420nm were ob-

tained. There were very few individual small crystals

present during growth (see Fig. 5(c)–(e)). After crystalli-

zation for 24 h, small crystals with a size of 60nm were

detected in addition to the larger zoned crystals. By

decanting the supernatant after centrifugation at 4500g

for 5min, a pure product of crystals with a size of500nm was obtained, as shown in Fig. 5(f). Acid leach-

ing of the ZSM-5 seeds thus appeared to allow the for-

mation of discrete zoned MFI crystals rather than

polycrystalline aggregates. Chemical analysis showed

that the Si/Al ratio in the bulk of the zoned MFI crystals

after 24h of crystallization was about 120, while the Al

content on the surface of the crystals was below the XPS

detection limit. Thus, it could be deduced that the sur-face of the zoned MFI crystals after 24h of crystalliza-

tion was Al free or contained only a minor amount of

Al. DLS measurements showed that the growth rate

for acid leached ZSM-5 cores was similar to the growth

rate of silicalite-1 cores. Presumably, it is possible to

completely inhibit the formation of the individual small

silicalite-1 crystals during the formation of zoned MFI

crystals by controlling the amount of the seeds added

and the composition of the synthesis solution used, sothat the silicate solution will serve only as a reservoir

to supply the growth of the zoned crystals.

The SEM images in Fig. 5 indicate that the crystal-

lites in the acid leached ZSM-5 cores grew in the silica-

lite-1 solution. However, it is inconclusive whether these

crystals consist of a continuous channel system through-

out the core and shell. In order to investigate this fur-

ther, XRD measurements were carried out. XRDanalysis of the 2h range from 5–40� showed that allmaterials prepared in the present work were pure MFI

without amorphous material. Fig. 6(a) shows the exper-

imental XRD pattern from non-calcined ZSM-5 cores in

the region about 22.5–25.0� 2h, where the strongest MFIreflections are located. The reflections are broad due to

the small crystallite size. The patterns from the sample

prepared by treating ZSM-5 cores directly (without sur-face dealumination) in a silicalite-1 synthesis mixture for

24h is shown in (b) and the corresponding sample pre-

pared employing surface dealumination is shown in

(c). These samples were obtained by collecting the pellet

after 5min centrifugation at 4500g. The XRD data in

Fig. 6 was refined using two models. The first model

was as simple as possible with five reflections. The sec-

ond model was based on 10 reflections, i.e., each peakwas modeled as two overlapping reflections, one for

ZSM-5 and one for silcalite-1. ZSM-5 has a larger unit

cell than silicalite-1 and the ZSM-5 reflections should

thus be shifted towards lower 2h, which justifies this

Fig. 5. SEMmicrographs of acid treated ZSM-5 seeds (a) and zoned MFI particles after crystallization in a silicalite-1 solution for 6h (b), 8h (c), 12h

(d), 14h (e) and 24h (f).


model. For each reflection, the peak position, FWHM

and intensity were fitted. The background intensity

was fitted at the beginning and end of the 2h range

and was assumed varying linearly in the range. The first

model thus had 17 (3 · 5 + 2) and the second model had32 (3 · 10 + 2) adjustable parameters. Table 1 shows thefull width at half maximum (FWHM) and the corre-

sponding crystallite size for all reflections in Fig. 6 for

the two models. Fig. 7 shows experimental and fitted

data for selected samples. The results from the model

based on 5 reflections are discussed first. The reliability

index (RI) is 3% and 4% for pattern (a) and (b) in Fig. 6,

which shows that the fit was good, while a somewhat

worse fit was obtained for pattern (c) with RI = 6%,

which indicates that this model not can describe the

experimental data fully. The model cannot predict the

asymmetrical peak shape observed experimentally for

the grown cores, since symmetrical peaks are used inthe model. However, for this model, the FWHM of all

reflections but 051 follow the same trend. The grown

ZSM-5 cores prepared without surface dealumination

has the widest reflections and thus the smallest average

crystallite size. The cores grown using surface dealumi-

nation has the narrowest reflections and thus the largest

average crystallite size. In other words, the average crys-

tallite size decreases when treating ZSM-5 cores directly

Fig. 6. XRD patterns of ZSM-5 cores (a), the product after 24h

treatment of cores (not acid leached) in a silicalite-1 synthesis mixture

(b) and the product after 24h treatment of acid leached cores in a

silicalite-1 synthesis mixture (c).


in a silicalite-1 synthesis mixture, but increases when the

cores are surface dealuminated and then treated in the

silicalite-1 synthesis mixture. An increased average crys-tallite size is a strong evidence for at least a partial suc-

cessful zoning. A somewhat different trend is observed

by analysis of the 051 reflection. The width of this

reflection is smaller than the other reflections, corre-

sponding to larger crystallite size. The width of the

051 reflection is decreasing slightly after treatment of

cores in silicalialite-1 synthesis mixture, even without

dealumination, but the width of the reflection decreasesmore if the cores are surface dealuminated. It should be

noted that the average crystallite size derived from the

051 reflection approximates the average thickness of

the crystallites in the b direction and that the 501 reflec-

tion approximates the thickness in the a direction. The

crystallite size reported in Table 1 is smaller than the ob-

served crystal size by SEM.

The model based on 10 reflections was applied to thegrown cores, comprised of both ZSM-5 and silicalite-1.

Table 1

Full width at half maximum (FWHM) and corresponding crystallite size for

Reflection 501 051 151 303

Sample FWHM (�)(model with 5 reflections)

ZSM-5 cores 0.123 0.090 0.126 0.101

Grown ZSM-5 cores 0.161 0.085 0.157 0.117

Grown dealuminated ZSM-5 cores 0.107 0.068 0.101 0.087

FWHM (�)(model with 10 reflections)

Grown ZSM-5 cores 0.174 0.085 0.185 0.126

0.077 0.070 0.105 0.071

Grown dealuminated ZSM-5 cores 0.116 0.073 0.112 0.089

0.059 0.047 0.062 0.063

For the model based on 10 reflections, the FWHM and crystallite size for th

This model gives better (lower RI) fit since more adjust-

able parameters are used. For all peaks, this model re-

sulted in a pair of reflections; one broad reflection

shifted towards lower 2h (most likely from ZSM-5)

and a narrow reflection (most likely from silicalite-1).

The reflections were separated about 0.044� (average).This model predicts the observed asymmetrical peak

shape well, due to the combination of a broad and a nar-

row reflection. As opposed to the 5-reflection model, this

model gives information on the size of both the ZSM-5

and silcalite-1 crystallites, not only the average crystal-

lite size. This model shows that the ZSM-5 cores shrink

if they are treated directly in the silicalite-1 synthesis

mixture; compare for instance the crystallite size derivedfrom the 501 reflection for the ZSM-5 cores (97nm)

with the size of the ZSM-5 part of the grown ZSM-5

crystallites (62nm). Etching of the ZSM-5 cores in the

silicalite-1 synthesis mixture may cause a decreased crys-

tallite size. As for the first model, a somewhat different

trend is observed for the 051 reflection. This reflection

shows that the ZSM-5 cores grow slightly (in the b direc-

tion) even without acid leaching. However, all reflec-tions show that dealuminated ZSM-5 cores grow

slightly upon treatment in the silicalite-1 synthesis mix-

ture. For instance, the crystallite size (051 reflection)

of the ZSM-5 part increases from 158nm to 236nm

upon treatment in the silicalite-1 synthesis mixture after

surface dealumination. According to this model, the sil-

icalite-1 part of the grown cores (with and without acid

leaching) is comprised of large crystallites, especially forsurface dealuminated ZSM-5 cores. The crystallite size

derived from the 10-reflection model agrees reasonably

with SEM observations. In summary, both models show

that acid leaching is beneficial for growth of ZSM-5

cores in a silicalite-1 synthesis mixture.

In order to prove that these particles are zoned and

consist of continuous channel systems throughout the

core and shell, selected area electron diffraction (SAED)patterns and high resolution electron microscopy

the XRD data in Fig. 6

133 501 051 151 303 133 RI (%)

Crystallite size (nm)

(model with 5 reflections)

0.113 97 158 94 129 110 3.2

0.137 68 172 70 104 84 3.7

0.095 118 278 130 168 144 6.2

Crystallite size (nm)

(model with 10 reflections)

0.156 62 174 57 94 71 1.8

0.083 210 261 122 252 181

0.100 105 236 111 160 132 2.5

0.055 435 – 366 347 596

e ZSM-5 part (at lower 2h) is given first for each reflection.

Fig. 7. Experimental XRD data, fitted peaks, background and sum

curve for selected samples.

Fig. 8. A TEM image of acid leached ZSM-5 cores grown 24h in a

silicalite-1 synthesis mixture (a). A SAED pattern from the particle

pointed by an arrow in (a). The SAED was taken along the [011]

direction. No splitting of the diffraction spots was observed (b). A

HREM image of the marked area in (a) taken along the [011] direction

showing a continuous lattice of the crystal. The results from (b) and (c)

indicate that the particle is a zoned single crystal of ZSM-5 and

silicalite-1 (c).


(HREM) images were taken from the sample with acid

leached ZSM-5 cores grown for 24h in a silicalite-1 syn-

thesis mixture. TEM images (Fig. 8(a)) show similar

crystal sizes as those in the corresponding SEM image

(Fig. 5(f)). The large crystal indicated by an arrow in

Fig. 8(a) is oriented along the [110] direction, shown

by the SAED pattern in (b). No splitting of the diffrac-

tion spots is observed, indicating that the core and theshell have the same orientation and form perfect inter-

growth. The HREM image of the area marked in Fig.

8(a) is shown in (c). No discontinuity is observed, fur-

ther confirming that the particle is a single crystal of

zoned ZSM-5 and silicalite-1.

4. Conclusions

ZSM-5 crystals were used as seeds to synthesize

zoned MFI crystals in a silicalite-1 synthesis solution.

Without dealumination of the ZSM-5 cores by acid

treatment, polycrystalline aggregates resulted. Smaller

silicalite-1 crystals were also formed due to secondary

nucleation. A comparison of silicalite-1 crystal growth

rates by DLS analysis indicated that the polycrystalline

aggregates contained silicalite-1. After acid treatment of

the ZSM-5 seeds, discrete zoned MFI crystals grew in

the silicalite-1 solution, in a similar fashion as that ob-served for silicalite-1 crystallization in the absence of

seeds. Secondary nucleation of new silicalite-1 crystals

was inhibited by the addition of the acid treated seeds.

By controlling the synthesis time, single zoned MFI

crystals with varying silicalite-1 shell thickness could

be synthesized.

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Documents

Synthesis and characterization of colloidal zoned MFI crystals