Synthesis and characterization of zoned MFI films by seeded growth

Synthesis and characterization of zoned MFI films byseeded growth

Qinghua Li a, Jonas Hedlund a, Johan Sterte a,*,Derek Creaser b, Anton-Jan Bons c

a Division of Chemical Technology, Lule�aa University of Technology, S-971 87 Lule�aa, Swedenb Department of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96 G€ooteborg, Swedenc ExxonMobil Chemical Europe Inc., European Technology Center, Hermeslaan 2, B-1831 Machelen, Belgium

Received 18 March 2002; received in revised form 8 August 2002; accepted 8 August 2002

Abstract

Supported zoned and sandwiched MFI films were prepared by a two-step crystallization procedure, using seeds. In

this work, a zoned MFI film is defined as one assembled by crystals propagating from the support to the film top surface

with varying Al content along the length of the crystal. A sandwiched MFI film is referred to as one assembled by at

least two layers of crystals. Six types of films were prepared, both zoned and sandwiched, with a high or a low Al-

content in the ZSM-5 part and with varying order of the layers, i.e. ZSM-5 coated with silicalite-1 or vice versa. The

films were characterized by SEM and TEM. The Al-distribution was measured by cross-sectional EDS, and the pre-

ferred orientation of the crystals could be determined by XRD. Truly zoned films are obtained when the compositional

difference between the layers is relatively small, and the synthesis conditions are similar or when the first layer is sili-

calite-1. If the first layer is ZSM-5 and the synthesis conditions and/or the composition vary too much, a discontinuity

occurs at the interface between the layers, and sandwiched film results, where nucleation of the second layer is initiated

by secondary nucleation or by applying seeds.

� 2002 Elsevier Science Inc. All rights reserved.

Keywords: Zoned MFI films; Seeded method; Two-step crystallization; Sandwich film; Truly zoned MFI film

1. Introduction

ZSM-5 zeolite has a regular channel system, i.e.straight circular channels (5:4� 5:6 �AA2) intercon-necting with sinusoidal and elliptical channels

(5:1� 5:4 �AA2). Since the pore sizes are near the

dimensions of many commercially important or-

ganic molecules, one of the specific features of

ZSM-5 is its shape selectivity. ZSM-5 zeolite hasbeen widely utilized as a catalyst in the selective

synthesis of chemicals [1–4]. It is known [5] that

both acid sites located in the pore channels and

those located at the external surface of the crystals

can act as catalytically active centers. However,

the external surface of zeolites is accessible to all

molecules in a non-shape selective manner, which

degrades the shape selectivity of the catalyst.

Microporous and Mesoporous Materials 56 (2002) 291–302

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

91199.

E-mail addresses: [email protected], [email protected].

se (J. Sterte).

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PII: S1387-1811 (02 )00503-6

mail to: [email protected],

Various techniques have been applied to reduce or

deactivate the external surface sites of ZSM-5

catalysts to enhance the shape selectivity. Large

ZSM-5 crystals increase the ratio of internal se-

lective sites to external non-selective sites, while

enhancing the diffusional length that reduces cat-alytic activity [6–8]. The adsorption of a carboxy-

late of a non-monovalent metal on the external

surface of the zeolite may deactivate or poison the

active centers on the external surface of a zeolite

[9]. However, new metal or metal-oxide active

centers are constructed after calcination. The se-

lective coverage of the external surface of zeolites

with silicon alkoxides has been extensively in-vestigated for eliminating the external acidity of

ZSM-5. Chemical vapor deposition (CVD) of sil-

icon alkoxides is a useful and practical method

[6,10–13]. Si(OCH3)4 with a kinetic diameter of

8:9� 0:2 �AA, which is much larger than that of theHZSM-5 pore (5.4–5.6 �AA), cannot enter thechannels of ZSM-5. Upon hydrolysis, a silica layer

is formed, which narrows the pore-opening size,inactivates the external surface sites and enhances

the shape selectivity. For instance, the CVD of

tetraalkoxysilanes increased the para-isomer pro-

portion of the dialkylbenzene fraction by up to

98% in reactions such as the isomerization of xyl-

enes [11] and the disproportionation of toluene

[11,14] over HZSM-5. Unfortunately, the CVD

method may also reduce the catalytic activity [11].An elegant approach to eliminate the external acid

sites is to prepare a zoned zeolite catalyst, i.e., to

prepare MFI crystals with an Al-rich core and an

Al-free shell with a continuous channel system

throughout the crystal. Materials that are claimed

to be zoned MFI have been investigated previously

by adsorption and catalytic testing [15–17]. It has

been shown that the para-isomer selectivity couldbe enhanced by an elimination of the external acid

sites without changing the pore mouth size and

with only a slightly lower activity [15]. However,

little characterization except catalysis and ad-

sorption has been carried out to determine

whether such materials are truly zoned, i.e. consist

of a continuously propagating channel system, or

sandwiched materials, i.e. with a discontinuouschannel system. Also, it is unclear whether sepa-

rate small silicalite-1 crystals are formed as a by-

product during hydrothermal synthesis of the sil-

icalite-1 shell. It is possible that the prepared ma-

terials consist of mixtures of partially zoned MFI

crystals and silicalite-1 crystals.

Various methods for the synthesis and charac-

terization of zeolite films and membranes havebeen reported in the literature [18–21]. For in-

stance, a seeding method [22,23], which consists of

the deposition of nanosized seed crystals on the

substrate, followed by a hydrothermal growth of

the seed crystals to form a film, has been proven to

be a particularly versatile method for the prepa-

ration of zeolite films and membranes [23–25].

Since the presence of the seed layer on the sub-strate bypasses the nucleation and prohibits or

limits the incorporation of newly formed crystals,

this method provides an improved flexibility for

the control of film microstructure.

This paper concerns the synthesis of various

types of zoned and sandwiched materials in a form

of relatively thick zeolite MFI films. The films

synthesized are well suited for characterization bySEM, EDS and transmission electron microscopy

(TEM). The preferred orientation of crystals can

also be evaluated by XRD. Furthermore, it is easy

to obtain a pure product since crystals nucleated in

the bulk of the synthesis mixture can simply be

removed by rinsing. These capabilities enable a

more detailed investigation of the structural con-

figuration of zoned zeolites compared to what hasthus far been possible. In addition, zeolite films and

membranes have many potential applications for

separations, catalytic reactions and as chemical

sensors etc. [26,27], and zoned zeolite films un-

doubtedly share or even enhance these possibilities.

In order to elucidate the effects of synthesis

composition and nucleation, we compared several

combinations of ZSM-5 and silicalite-1 films,grown from gels or clear liquids, and with and

without seeding in between the synthesis steps.

2. Experimental section

2.1. Seed adsorption

A TPA-silicalite-1 seed sol with an average

crystal size of 60 nm was prepared from a synthesis

292 Q. Li et al. / Microporous and Mesoporous Materials 56 (2002) 291–302

solution with a molar composition of 9TPAOH:

25SiO2:360H2O:100EtOH by two weeks of hy-

drothermal treatment at 60 �C. The seed crystalswere purified by repeated centrifugation, followed

by re-dispersion in double distilled water four

times. The alkali source was tetrapropylammo-nium hydroxide (TPAOH, 1.0 M aqueous

solution, Sigma), and the silica source was tetra-

ethoxysilane (TEOS, <98%, Merck). The final seedsol had a silicalite-1 concentration of 1.0 wt.% by

dry content and pH ¼ 10:0 was maintained by theaddition of ammonia.

Polished silicon (1 0 0) and quartz (0 0 0 1) sub-

strates were mounted in Teflon holders and rinsed.The substrates were surface charge reversed by a

treatment in a filtered solution of 0.4 wt.% cationic

polymer (Redifloc 4150, Eka Chemicals AB,

Sweden) in distilled water at pH ¼ 8:0. After thecharge reversal treatment, the substrates were im-

mersed in a filtered (0.2 lm) seed solution to ad-sorb the silicalite-1 seeds. The details regarding the

seed adsorption were reported earlier [23].

2.2. Growth of MFI films using a gel for ZSM-5

growth

Samples were prepared by a two-step crystalli-

zation procedure using seeded supports. In the first

step, a ZSM-5 film was synthesized by a hydro-

thermal treatment in a synthesis gel free from or-ganic template with a molar composition of

30Na2O:1Al2O3:103SiO2:4000H2O at 180 �C for

18 h [24,28]. The content of aluminum was high

(Si=Al ¼ 10) in this film [23]. Quartz supports wereused since they were not etched by the gel. In the

second step, a TPA-silicalite-1 film was grown

using a clear solution with a molar composition of

3TPAOH:25SiO2:1500H2O:100EtOH at 100 �C.After three days of crystallization, the sample was

rinsed with a 0.1 M ammonia solution and sub-

merged in a fresh synthesis solution with the same

molar composition for a further hydrothermaltreatment. This procedure was repeated four times,

so the total crystallization time was 12 d [28]. After

completion of the two-step crystallization, the

sample was rinsed with an 0.1 M ammonia solu-

tion to remove sediments and unreacted materials

adsorbed on the film. Table 1 shows the four types

of samples prepared. The sample GZ-S (ZSM-5

grown from a gel and then coated with silicalite-1)was prepared by first treating the seeded support in

the gel and then in the clear solution. The sample

was rinsed in 0.1 M ammonia in between the

treatments in gel and clear solution. The sample S-

GZ was prepared by first treating the support in

the clear solution and then in the gel. In order to

obtain a better appreciation of the growth mech-

anism, two sandwich films were prepared using thesame method as above, but with adsorption of

seed crystals between the growth of the first and

second film. These two samples are denoted GZ-

s-S and S-s-GZ.

2.3. Growth of MFI films using a clear solution for

ZSM-5 growth

The ZSM-5 film can be synthesized from both a

gel and a clear solution [29]. Compared to silica-

lite-1 (or ZSM-5) grown from a clear solution

the ZSM-5 films grown from a gel develop a

very different preferred crystal orientation due to

varying growth rates in a given crystallographic

Table 1

Summary of sample preparation

Sample Mixture used for ZSM-5 growth Film composition

Bottom layer Intermediate seeding Top layer

GZ-S Gel ZSM-5 No Silicalite-1

S-GZ Gel Silicalite-1 No ZSM-5

GZ-s-S Gel ZSM-5 Yes Silicalite-1

S-s-GZ Gel Silicalite-1 Yes ZSM-5

CZ-S Clear solution ZSM-5 No Silicalite-1

S-CZ Clear solution Silicalite-1 No ZSM-5

Q. Li et al. / Microporous and Mesoporous Materials 56 (2002) 291–302 293

direction [29]. This phenomenon can explain the

growth mechanism for the zoned films. Two types

of zoned MFI films could be prepared using a clear

solution for the ZSM-5 growth. In this case, it was

possible to use less expensive silicon wafers as

opposed to quartz since silicon (and quartz), isnot etched by the clear solution. In the prepara-

tion with a two-step crystallization procedure, a

clear solution with molar composition 3TPAOH:

25SiO2:0.25Al2O3:1500H2O:100EtOH:0.1Na2O at

100 �C was used for the ZSM-5 growth. The alu-minum source was aluminum isopropoxide (Ald-

rich). The total crystallization time was 12 d. Note

that this synthesis mixture and the conditions forthe hydrothermal treatment are very similar to

those for the silicalite-1 growth. Two types of films

were grown (see Table 1). The sample CZ-S (ZSM-

5 grown from a clear solution and then coated

with silicalite-1) was prepared by hydrothermal

treatment of the seeded support in the clear solu-

tion, rinsed and then treated in the silicalite-1

synthesis mixture. The sample S-CZ was preparedin a similar way, but the two synthesis mixtures

were applied in a reversed order.

2.4. Characterization

Dynamic light scattering was used to determine

the average crystal size in seed sols and in synthesis

solutions. The film morphology and thickness weredetermined by analysis with a Philips XL 30 SEM

equipped with a LaB6 emission source. The sam-

ples for SEM were sputtered with a thin gold or

carbon layer. Elementary analysis of carbon-

coated zeolite films was performed using an EDS-

system attached to the SEM. XRD-data were

collected on the film samples with a Siemens

D5000 powder diffractometer. The X-ray sourcewas a copper target running at 40 kV and 50 mA.

Diffraction angles between 22.5� and 25� 2h wereinvestigated in Bragg–Brentano geometry. The

film surface was oriented perpendicular to a plane

defined by the X-ray source, sample holder and

detector. The step size was 0.01� and the time perstep was 80 s. Cross-sectional samples for TEM

were prepared by focused ion beam (FIB) etching.Before FIB etching the top surface of the film was

coated with a protective layer of Pt. The samples

were studied in a Philips CM12T TEM operated at

120 kV.

3. Results and discussion

3.1. Seeding

Fig. 1(a) and (b) show the SEM images of

adsorbed 60 nm seeds on quartz and silicon sub-

strates, respectively. The seeds formed approxi-

mately a monolayer on both substrates. Since the

seeding density affects the preferred orientation of

the crystals in the film [29], it is important tocompare these two supports with a similar seeding.

In this case, the preferred orientations are closely

related to the hydrothermal treatment since the

Fig. 1. SEM top view images of adsorbed 60 nm seeds on quartz (a), silicon (b).


seeding density was similar, independent of the

support type.

3.2. Growth of the GZ-S film

Fig. 2(a) and (b) show the top- and side-viewimages of a ZSM-5 film (GZ) grown from the gel

on a seeded quartz support. It is a continuous film

consisting of well-intergrown crystals of ZSM-5

with very well developed crystal faces. The size of

the crystals exposed at the surface was up to 1000

nm and the film thickness was �3400 nm, inagreement with previous results [23]. Fig. 2(c) and

(d) show top- and side-view images of a TPA-sil-icalite-1 film (S) grown directly on a seeded quartz

support. The crystals exposed at the surface were

smooth, and their size was up to 800 nm. The film

was continuous with a constant thickness of �2800nm. Fig. 2(e) and (f) show the top- and side-view

images of the GZ-S film. After the second crys-

tallization step, the morphology of the film had

been changed completely. The surface of the filmno longer consisted of well-developed crystals as

for the ZSM-5 film, but became smoother and

similar to that for the TPA-silicalite-1 film. The

size of individual crystals exposed at the surface is

larger than observed for the silicalite-1 film and the

surface is more uneven, which may be explained by

the rough surface of the ZSM-5 film as opposed to

the smooth quartz wafer. Fig. 2(f) shows that thethickness of the film was �6000 nm, which agreedwell with the sum of the film thickness of the in-

dividual ZSM-5 and TPA-silicalite-1 films. A clear

border between the two different layers is visible in

the sample GZ-S (indicated by the arrow in Fig.

2(f)) and the columns are not continuous across

the boundary. This indicates that the silicalite-1

layer has grown from crystals that nucleated ontop of the ZSM-5 film. A larger distance between

these nuclei than the distance between the seed

crystals used for growth of the silicalite-1 film

(shown in Fig. 2(c)), may explain the larger lateral

crystal size observed in Fig. 2(e).

An EDS line scan was carried out on the sample

GZ-S. The Si/Al ratio is about 10 for the ZSM-5

part [24], while no aluminum can be detected in thesilicalite-1 part. Fig. 3 shows the Al–Ka and Si–Kasignals starting from a point in Fig. 2(f) on the

quartz substrate. The marker in Fig. 2(f) indicates

the position of the EDS line scan. For the first lmof the scan on the quartz support, the Si signal was

higher than on the film due to the higher density

and the stopping power of quartz, compared to the

porous zeolite. The signal eventually dropped tozero when the electron beam was scanning the

vacuum. In a range of the scan length from 1 to 4

lm, over the ZSM-5 film, the Al signal was higher.The Al signal decreased abruptly and remained

constant at the continuum level, corresponding to

zero Al-content, over the TPA-silicalite-1 film.

Diffractograms (a)–(c) in Fig. 4 show the XRD

patterns of the ZSM-5 film, the TPA-silicalite-1film and the GZ-S sample, respectively. In agree-

ment with previous findings [28,29], for the ZSM-5

film the (1 3 3) reflection was dominant (Fig. 4(a)),

whereas for the TPA-silicalite-1 film the (3 0 3)

reflection was significant (Fig. 4(b)) as expected for

such films with a thickness of 2800 nm [28]. For

sample GZ-S, two (3 0 3) reflections were observed,

as shown in Fig. 4(c). One was at 23.86� and theother was at 23.98�. Since the unit cell of ZSM-5 islarger than that of TPA-silicalite-1, the former

reflection is attributed to the ZSM-5 crystals and

the latter is derived from the TPA-silicalite-1

crystals, which is clear when comparing patterns

(a), (b) and (c). In an analogous way, the diffrac-

tion pattern of the zoned film has two (1 3 3),

(5 0 1) and (0 5 1) reflections. This figure showsthat the silicalite-1 part of the GZ-S film is mainly

(3 0 3) oriented like the pure silicalite-1 film, since

the (3 0 3) reflection is the dominating one among

the reflections from the silicalite-1 part of the

zoned film. From these XRD observations it

can be concluded that the crystal orientations in

the two layers are different, supporting the sug-

gestion from SEM observations that the silicalite-1layer has re-nucleated on the surface of the ZSM-5

film.

The area ratio of the (3 0 3) and (1 3 3) reflec-

tions of the pure silicalite-1 film and the silicalite-1

fraction of the zoned film are roughly 20 and 4,

respectively. This indicates that the crystallo-

graphically preferred orientation in the silicalite-1

film on ZSM-5 is weaker than in the single silica-lite-1 film, which may be related to the roughness

of the ZSM-5 film surface as compared to the


Fig. 2. SEM images of the films. Top- and side-view images of the ZSM-5 film (a) and (b), top and side-view images of the TPA-

silicalite-1 film (c) and (d), top and side-view images of the GZ-S film (e) and (f) and top and side-view images of the GZ-s-S film (g)

and (h).


quartz support, or to the larger distance between

nuclei and seeds, as discussed above.The fact that there are two peaks of each re-

flection shows that the mean crystal size is large

enough to give rise to narrow peaks which is not

the case for sample GZ-s-S, see below.

3.2.1. Comparison with various types of MFI films

The so-called ‘‘sandwich’’ MFI film (GZ-s-S)

was prepared to gain a better understanding of the

effect of the preferred orientation of the first layer

crystals on the second layer for the GZ-S film. Its

preparation involved the adsorption of 60 nm

seeds onto the surface of the ZSM-5 film prior to

the synthesis of the second layer of TPA-silicalite-

1. Fig. 2(g) and (h) show the top- and side-viewimages of the sample GZ-s-S. The size of the

crystals exposed at the surface reached about 900

nm, similar to what was observed for the pure

silicalite-1 film. The film had a constant total

thickness of �6100 nm, similar to that for the GZ-S film. A clear border between two different layers

appears in the GZ-s-S film, similar to what was

observed for GZ-S. The columnar grain mor-phology does not extend across the film interface.

Thus, the silicalite-1 film with a thickness of 2800

nm is most likely grown directly from the seeds

adsorbed on the surface of the ZSM-5 film, instead

of from the crystals in the ZSM-5 film. The first

layer of ZSM-5 on quartz thus seemed to only play

the role of a substrate for the second layer. The

roughness of the ZSM-5 film surface (as comparedto the quartz support) may explain the observed

rougher surface and the broader crystal size dis-

tribution on the top of the GZ-s-S film.

Fig. 4(d) shows the XRD pattern for the GZ-s-S

film. The broad (5 0 1) and (3 0 3) peaks, which

are attributed to the TPA-silicalite-1 crystals, are

dominant. A very broad (1 3 3) reflection, which is

assigned to the overlapping (1 3 3) and (1 3 3*) re-flections from the TPA-silicalite-1 crystals in the

upper film and the ZSM-5 crystals in the lower

film, is observed. This reflection is broad probably

since the individual (1 3 3*) and (1 3 3) reflections

are broader due to the smaller mean crystal size in

the sandwich sample as opposed to the GZ-S film,

as discussed above. The SEM microstructure and

the XRD pattern of sample GZ-s-S, with inten-tional seeding in between the layers, is essentially

the same as those for GZ-S, confirming the hy-

pothesis that the silicalite-1 layer re-nucleates on

top of the ZSM-5 layer.

It was reported that the S-GZ film (ZSM-5

covering silicalite-1) was compositionally zoned

and appeared to consist of continuously propa-

gating crystals throughout the film [28]. In thepresent work, the sandwich film (S-s-GZ) was

also synthesized to understand whether the seed

Fig. 3. Si–Ka and Al–Ka signals of the EDS line scan indicatedby the line in Fig. 2(f), the scan starts at the position of the dot.

The Al–Ka signal was multiplied by 6.

Fig. 4. XRD patterns of the ZSM-5 film (a), the TPA-silicalite-

1 film (b), the GZ-S film (c) and the GZ-s-S film (d).


adsorption prior to the crystallization of the top

film could prevent the continued growth of the

crystals of the bottom film. The data are easier tointerpret in these cases, since the first film, silica-

lite-1, has a smooth surface. Fig. 5 shows the SEM

images of the samples S-s-GZ and S-GZ. Clearly,

the size of crystals at the surface of the sample S-s-

GZ (Fig. 5(a)) was smaller than that of the sample

S-GZ (Fig. 5(c)) and similar to that for the ZSM-5

film grown directly on a quartz support (Fig. 2(a)).

This suggests that the growth of ZSM-5 in thesecond layer of sample S-s-GZ occurs from the

seeds adsorbed on the bottom layer rather than as

a continued growth of the silicalite-1 crystals in

this layer and that the sample S-GZ is truly zoned

[28]. The total film thickness of both samples was

�6100 nm. Similar to the sample GZ-s-S, a borderbetween two layers for the sample S-s-GZ is clearly

visible due to the existence of the seeds (Fig. 5(b)).For the sample S-GZ (Fig. 5(d)), the film border

is almost invisible and the continuous columnar

grains extend throughout the film.

XRD patterns for the latter two samples are

shown in Fig. 6. The (3 0 3) reflections dominate,

Fig. 5. SEM images of the films. Top- and side-view images of the S-s-GZ film (a) and (b) and top and side-view images of the S-GZ

film (c) and (d).


1 film (b), the S-GZ film (c) and the S-s-GZ film (d).


and the (1 3 3) reflection is almost absent in the

pattern from the zoned film S-GZ. The crystals in

the ZSM-5 part of the sample S-GZ are thus ori-

ented in the same way as the crystals in the un-

derlying silicalite-1 film, which indicates that this

sample is truly zoned. The (3 0 3) reflection is split,as a consequence of the different lattice parameters

of ZSM-5 and silicalite-1.

For the sandwich sample S-s-GZ, however, the

diffraction pattern is very different. It is roughly

the sum of patterns (a) and (b) in Fig. 6, which

shows that the ZSM-5 film has grown without the

influence of the underlying silicalite-1 film, i.e. the

sample is a sandwich film. This sample showssingle peaks for (3 0 3) (at the position typical for

silicalite-1, and (1 3 3) (at the position typical for

ZSM-5 in this study) indicating a strong and in-

dependent preferred orientation of the crystals in

each layer.

3.3. Growth of MFI films using clear solutions

The ZSM-5 films can be grown either from clear

solutions or gels. The preferred orientation of MFI

film is influenced by some important factors such

as the amount and size of seeds, the film thickness

and the hydrothermal treatment conditions [29,

30]. The latter factor affects the growth rates in a

given crystallographic direction. If a clear solu-tion is used for the ZSM-5 growth, the preferred

Fig. 7. SEM images of the films. Top- and side-view images of the ZSM-5 film (a) and (b), top- and side-view images of the CZ-S film

(c) and (d), and top- and side-view images of the S-CZ film (e) and (f).


orientation of the crystals in the ZSM-5 film be-

comes similar to that of the crystals in a silicalite-1

film, especially if the molar compositions of the

mixtures are very similar.

Fig. 7(a) and (b) show the top- and side-view

images of the ZSM-5 film grown directly on asilicon wafer. Compared to the SEM images ob-

tained from the gel synthesis (Fig. 2(a) and (b)),

the surface of the ZSM-5 film synthesized from

a clear solution was completely different. A very

smooth surface was formed, the size of lateral

crystals was about 300–700 nm, and the film

thickness was �2800 nm. Chemical analysis of thepurified crystals formed in the ZSM-5 synthesismixture showed that the Si/Al ratio was 90, which

was considerably higher than for the film obtained

from a gel. The silicalite-1 films grown on silicon

wafers appear to be identical to those grown on

quartz wafers (Fig. 2(c) and (d)). Fig. 7(c) and (d)

show the top- and side-view images of the CZ-S

film. An intergrown polycrystalline film with col-

umnar grains and small surface roughness wasformed. The size of surface crystals increased and

ranged from 700 to 1300 nm. The film thickness

was about 5600 nm, which was rather close to the

sum of the film thicknesses of the TPA-silicalite-1

and the ZSM-5 films. No border between the two

layers was observed. Similar results were found for

the S-CZ film (Fig. 7(e) and (f)).

Since similar synthesis systems were used for thepreparation of TPA-silicalite-1 and ZSM-5 films,

the preferred orientation of crystals was the same,

as confirmed by XRD. For the ZSM-5 (Fig. 8(a))

and TPA-silicalite-1 films (Fig. 8(b)), the (3 0 3)

reflection was dominant in both patterns. In ad-

dition, the location of the reflection for these two

samples was the same at 23.98� due to the lowaluminum content in the ZSM-5 film (Si/Al ratioof 90). Fig. 7(c)–(f) clearly show that the second

crystallization step only increased the film thick-

ness and extended the single grains along the film

thickness. This has also been verified by TEM, as

shown in Fig. 9. The columnar crystals clearly

extend across the ZSM-5-silicalite-1 boundary

without any disruption. The continuity of the bend

contours indicates that there is no strain at theinterface. Therefore, for both the zoned S-CZ and

CZ-S films, an increased ordering and strongerpreferred orientation of the crystals should be

observed by XRD. Fig. 8(c) and (d) show that the


1 film (b), the CZ-S film (c) and the S-CZ film (d).

Fig. 9. Cross-sectional TEM images (bright field image) of the

CZ-S film. Some minor etching of the support is visible.


(3 0 3) reflection became even more dominant for

the zoned films, which confirms this case. It can

thus be concluded that the individual crystals in

the films S-CZ and CZ-S are truly zoned and a

continuous channel system is expected to propa-

gate across the interface of the zoned regions.Whether a truly zoned or sandwiched film

forms is probably due to competition between

nucleation and crystal growth processes. Which

process predominates, i.e., nucleation of new crys-

tals or continued growth of existing crystals at

the film surface, depends on their relative rates

and whether a state of supersaturation can be

maintained at the film surface–solution interface.The GZ-S sample was sandwiched because the

rate of nucleation in the TPA-silicalite-1 synthesis

solution/film interface was greater than the rate

of growth of the ZSM-5 (high-alumina) exposed

crystal faces. This is not surprising since it is well

known that colloidal zeolite synthesis solutions,

particularly in this case with TEOS as a silica

source [31], have typically considerably highernucleation rates than gel solutions. By contrast,

the S-GZ sample was truly zoned, because the

degree of supersaturation and rate of nucleation in

the gel solution were not high enough to overcome

the growth rate of the exposed silicalite-1 crystals.

Both bilayered films produced from clear solutions

(S-CZ and CZ-S) were zoned since the growth rate

of existing crystals was always higher than nucle-ation at the surface. This is probably due to the

similarities in the synthesis solutions for both the

ZSM-5 and silicalite-1 layers.

4. Conclusions

A method for the preparation of relatively thickzoned MFI films has been developed. Single crys-

tal quartz and silicon substrates were first seeded

with colloidal TPA-silicalite-1 crystals and a two-

step crystallization procedure was then utilized to

prepare different types of zeolite films.

Four types of films (Sample GZ-S, S-GZ, GZ-s-

S and S-s-GZ) were synthesized on seeded quartz

substrates, a ZSM-5 film with high Al-content wassynthesized from a gel, and the TPA-silicalite-1

was grown from a clear synthesis mixture. In all

cases except S-GZ, SEM and XRD showed that

the second layer crystals re-nucleated at the sur-

face of the first layer, either spontaneously or

through the addition of seed crystals. Thus, the

first film on the quartz seemed to only play the role

of substrate for the second layer. The only excep-tion was S-GZ, which was zoned. When the syn-

thesis conditions used for growth of the two films

differ significantly, it thus seems that it is possible

to grow zoned films when the first layer is silicalite-

1, but not vice versa. This result is probably due to

the very different competing rates of nucleation of

new crystals and growth of existing crystals for the

different synthesis solutions. Two types of zonedfilms (sample CZ-S and S-CZ) were synthesized on

silicon substrates, where clear synthesis mixtures

with similar composition were used for the prep-

aration of both films. ZSM-5 has a significantly

lower Al content (Si=Al ¼ 90) than the GZ sam-ples described above (Si=Al ¼ 10). SEM and TEM

results clearly showed that the second crystalliza-

tion increased the film thickness and extended thecrystals continuously along the film. XRD pat-

terns indicated that the preferred crystal orienta-

tion for both zoned films CZ-S and S-CZ was

more intensified than that for the ZSM-5 and

TPA-silicalite-1 films grown from clear solutions.

This indicated that the crystals in the films grown

from clear solutions, with a smaller difference in

composition, were truly zoned.

Acknowledgement

The authors are grateful for the support of the

Swedish Research Council for Engineering Sci-

ences (TFR).

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Documents

Synthesis and characterization of zoned MFI films by seeded growth