Synthesis of nanocrystalline MFI-zeolites with intracrystal mesopores and their application in fine chemical synthesis involving large molecules

Article abstract of DOI:10.1002/chem.200801113

Synthesis of nanocrystalline MFI-zeolite with intracrystal mesopores, to be used to control the size of the zeolite crystals, particle morphology, and mesoporodity, and their application in fine chemical synthesis involving large molecules, was determined. An increase in the amount of PrTES to 5.0 mol% led to spherical macroporous zeolite particles of 300 nm diameter. The mesopores show a pore size distribution in the range of 3-8 nm, while Al and Si MAS MR show that all the samples contain only tetrahedral aluminum sites. It is also proposed that the mesoporous-zeolite crystallization is mediated by the nanoscale segregation of organic-rich and organic-lean domains on the growing zeolite particles. The high catalytic activity of MFI-10PrTES is due to the formation of smaller zeolite crystallites and the presence of intracrystal mesopores.

Full text of DOI:10.1002/chem.200801113

COMMUNICATION
DOI: 10.1002/chem.200801113
Synthesis of Nanocrystalline MFI-Zeolites with Intracrystal Mesopores and
Their Application in Fine Chemical Synthesis Involving Large Molecules
A
H
R
U
G
[
a]
Rajendra Srivastava, Nobuhiro Iwasa, Shin-ichiro Fujita, and Masahiko Arai *
Heterogeneous catalysts are widely used in petroleum re-
finery industries and for the production of fine chemicals.
However, challenges remain to maximize their efficiency.
Microporous crystalline aluminosilicate, zeolites, are widely
used as shape-selective catalysts, ion-exchange materials and
rous zeolite single crystals that combine most of the desira-
ble catalytic properties of zeolites and mesoporous molecu-
lar sieves. The direct synthesis of a zeolite with tunable mes-
oporosity by using organosilane surfactants as a mesopore
[6]
director has very recently been reported. The resultant
zeolites are highly mesoporous, and the mesopore walls
show the characteristics of a fully crystalline zeolite frame-
work. The mesopore diameters are tunable by the chain
length of the surfactants used. This MFI-zeolite with a mes-
oporous/microporous hierarchical structure shows a remark-
ably improved resistance to the deactivation of catalytic ac-
[1]
adsorbents for organic compounds. Their superior perfor-
mance can often be attributed to the existence of a well-de-
fined system of micropores (size below 2 nm in diameter)
with uniform shape and size, typically of molecular dimen-
sions. However, for some applications, the presence of such
micropores alone can also result in an unacceptably slow
diffusion of reactants and products to and from the active
[7]
tivities in various reactions. In addition, the hierarchical
MFI-zeolite exhibits very high catalytic activity in the syn-
thesis of large molecules for which conventional zeolite ma-
terials are inactive and conventional mesoporous materials
are less active. Another approach to improve the catalytic
activity is to minimize the size of the zeolite crystals and
[2]
sites located inside the zeolite crystals. To overcome this
diffusion limitation, researchers have pursued several differ-
ent preparative strategies. One approach is to increase the
pore size of the zeolite. The mesoporous structure is very
promising for reactions involving large molecular species,
owing to the possibility of overcoming the diffusion limita-
tion in crystalline microporous zeolites (0.4–1.5 nm in pore
diameter). The most common and successful approach so
far is to introduce an additional system of mesopores (sizes
[8]
thereby shorten the diffusion path.
The present study is based on the second approach in
which the size of zeolite nanocrystals is reduced to 20 nm by
adding alkyltriethoxysilane into the conventional zeolite
synthesis gel. Crystal size of MFI-zeolite was effectively con-
trolled by the addition of alkyltriethoxysilanes (alkyl=
methyl, propyl and octyl) into a hydrothermal synthesis mix-
ture of conventional MFI-zeolite. The materials obtained ex-
hibit remarkably enhanced catalytic activities in several or-
ganic reactions involving large molecules. Here, we have
shown that the alkyltriethoxysilanes mediated synthesis
method can be effectively used to control the size of the
zeolite crystals, particle morphology, and mesoporosity.
First, a conventional MFI was hydrothermally prepared at
[3–7]
between 2 and 50 nm) into each individual zeolite crystal.
This is typically done by suitable post-treatments such as
[3]
[4]
steaming and chemical etching routes. Mesopores can be
introduced directly into the zeolite crystals, without a parti-
ally destructive post-treatment, simply by conducting the
crystallization in the presence of mesoporous carbon and
[5]
using carbon nanotubes as a hard template. The carbon
particles are encapsulated by the zeolite crystals during
growth. After complete crystallization, the carbon template
is easily removed by combustion to produce highly mesopo-
4
43 K by using tetraethylorthosilicate (TEOS) and tetrapro-
pylammonium hydroxide (TPAOH) (gel composition
TEOS/Al O /Na O/TPAOH/H O: 100:2.5:3.3:25:2500).
[
a] Dr. R. Srivastava, Dr. N. Iwasa, Dr. S.-i. Fujita, Prof. M. Arai
Division of Chemical Process Engineering
Graduate School of Engineering
Hokkaido University
2
3
2
2
Later, different zeolite samples were prepared with alkyl-
triethoxysilane (ATES) using different gel compositions
Sapporo, 060-8628 (Japan)
Fax : (+81)(0)11-706-6595
E-mail: marai@eng.hokudai.ac.jp
(
TEOS/ATES/Al O /Na O/TPAOH/H O:
2 3 2 2
1
00Àx:x:2.5:3.3:25:2500, x=0À15). The product was desig-
nated by MFI-xATES according to the number of moles (x)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801113.
of the alkyltriethoxysilane used. All the synthesized zeolites
Chem. Eur. J. 2008, 14, 9507 – 9511
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9507

had the MFI structure with high phase purity, as confirmed
by XRD patterns of Figure 1 (and Figure S1 in the Support-
ing Information). Zeolite particles with capsule-like crystal
Figure 1. XRD patterns of conventional MFI and MFI samples synthe-
sized using alkyltriethoxysilanes.
morphology were obtained when synthesis was performed
without any additive under the present conditions (Figur-
es 2a and 3a,b). The sample prepared with ATES has spheri-
cal/egg-shaped nanocrytalline morphology (Figure 2 and 3).
The amount of ATES was found to have a high impact on
the quality of the samples, as shown by scanning electron
microscopy (SEM) analysis (Figure 2). No improvement in
crystal morphology was observed when the sample (MFI-
Figure 2. SEM images of conventional MFI and MFI prepared with alkyl-
triethoxysilanes: a) conventional MFI, b) MFI-2.5PrTES, c) MFI-5PrTES,
d,e) MFI-10PrTES, f) MFI-15PrTES, g) MFI-10MeTES, and h) MFI-
2.5PrTES) was prepared using 2.5 mol% propyltriethoxysi-
lane (PrTES) with respect to TEOS (Figure 2 b). When the
amount of PrTES was increased to 5.0 mol%, then spherical
macroporous zeolite particles (MFI-5PrTES) of about
1
0OcTES.
300 nm diameter were observed (Figure 2c). At 10 mol%
loading of PrTES, the zeolite particles (MFI-10PrTES) ex-
hibit a round/egg shape with a nanoscale jagged surface.
The SEM and transmission electron microscopy (TEM)
images show that the round/egg shape particles were built
with an assembly of tiny crystallites about 20 nm in diameter
or less (Figure 2d, e and 3c,d). However, on further increase
in PrTES concentration to 15 mol%, we did not obtain any
MFI-zeolite (MFI-15PrTES) (Figure 2 f), as confirmed by
XRD (Figure S1 in the Supporting Information). We have
also studied the influence of hydrophobicity of ATES by
varying the alkyl chain length in ATES used in the synthesis.
If the synthesis was performed with 10 mol% methyltrie-
thoxysilane (MeTES), the product (MFI-10MeTES) showed
quite uniform, spherical particles that also looked like as-
sembled by nanosize crystallites similar to the sample MFI-
10PrTES. As seen in Figure 2(g), the primary building
blocks (crystallites) were somewhat bigger in MFI-
Figure 3. TEM images of a,b) conventional MFI and c,d) MFI-10PrTES.
9508
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Chem. Eur. J. 2008, 14, 9507 – 9511

Synthesis of Nanocrystalline MFI-Zeolites
COMMUNICATION
1
1
0MeTES than in MFI-10PrTES. The TEM image of MFI-
0PrTES clearly showed a nanocrystal-mesopore alternating
Supporting Information). The temperature-programmed de-
sorption (TPD) of NH was measured to characterize the
3
framework structure (Figure 3c,d). If the sample (MFI-
0OcTES) was prepared with 10 mol% octyltriethoxysilane,
the size of the crystal decreased, but no morphological
changes were observed (Figure 2 h).
acidity of the MFI samples. The results obtained are given
in Figure S4 and Table S1 of the Supporting Information.
1
[9]
A previous study based on biominerallization processes
[10]
and some successful synthesis works suggest that organic
molecules that have a strong interaction with the growing
crystal surface can effectively modulate the crystallization
process of inorganic materials. Therefore, we have chosen
some commercially available alkyltriethoxysilane molecules
to enhance the interaction with growing zeolite crystals. The
idea behind choosing such al-
The N -adsoption results reveal that external surface area
2
and pore volume increase proportionally with the concentra-
tion of ATES (Table 1). The N -adsoption isotherm for
2
MFI-10PrTES showed a typical type-IV isotherm similar to
the mesoporous silica materials (Figure 4). The major differ-
kyltriethoxysilane molecules is
that it contains a hydrolysable
alkoxysilyl moiety. It is expect-
ed that it could strongly inter-
act with growing crystal do-
mains through the formation
of covalent bonds with other
SiO and Al O sources using
Table 1. Textural characteristics of MFI and MFI prepared with alkyltriethoxysilane.
[
a]
Sample
Si/Al
Total surface
External surface
Micropore volume
Total pore
2
À1
2
À1 [b]
À1 [b]
À1
area [m g
]
area [m g
]
A
H
R
U
G
]
volume [ccg
]
MFI
24.0
–
–
26.3
29.2
33.4
304
318
387
526
486
388
53
62
127
262
171
125
0.124
0.130
0.132
0.120
0.125
0.134
0.211
0.219
0.295
0.427
0.343
0.254
MFI-2.5PrTES
MFI-5PrTES
MFI-10PrTES
MFI-10MeTES
MFI-10OcTES
2
2
3
the alkoxysilyl moiety. The
present SEM, TEM and N -ad-
2
[
a] Si/Al ratio was obtained by ICP analysis; [b] calculated by t-plot method.
sorption studies show that the
zeolite samples prepared with
PrTES and MeTES have a
ence of this isotherm from that of conventional MFI is a dis-
spherical/egg-shape morphology, which is built with nano-
sized zeolite crystals and that mesopores are present in the
void space between nanocrystals inside the zeolite particle.
Such a nanocrystal-mesopore alternating-framework struc-
ture is not likely to be formed if the distribution of the or-
ganic groups is homogeneous throughout the entire zeolite
particle. Therefore, it may be proposed that the mesopo-
rous-zeolite crystallization was mediated by the nanoscale
segregation of organic-rich and organic-lean domains on the
growing zeolite particles. The alkylalkoxysilanes contain
only three hydrolyzable moieties (with one hydrophobic
alkyl group), which is disadvantageous for the formation of
tinct increase of N adsorption in the region 0.4<P/P <0.9,
2
o
which is interpreted as capillary condensation in mesopore
void spaces. The mesopores show a pore size distribution in
the range of 3–8 nm. As evidenced from the SEM and TEM
images, the mesopores correspond to the void space be-
tween nanosized crystal domains inside a zeolite particle.
27
29
Al and Si MAS NMR results show that all the samples
contain only tetrahedral aluminium sites and most of the Si
is in Si(0Al) environment. Si(1Al) species were also detect-
ed in some quantity in each sample (Figure S2 and S3 in the
A
C
H
T
R
E
U
N
G
ACHTREUNG
extended tetrahedral SiO linkages. Consequently, the zeo-
2
lite growth might be significantly retarded at the organic
and inorganic interfaces, resulting in the formation of nano-
crystal-mesopore alternating frameworks. The surface area
of the obtained samples follows the order MFI-10PrTES>
MFI-10MeTES>MFI-10OcTES. This clearly indicates that
moderate length alkyl-chain-containing organosilanes have a
strong interaction with the growing crystals and, hence, ef-
fectively modulate the zeolite crystallization to form high
surface area nanosized zeolite nanocrystals. The long-alkyl-
chain hydrophobic ATES (octyltriethoxysilane) was found
to be less effective, as it may be segregated from the alumi-
nosilicate domain and unable to modulate the crystallization
process by participating in crystal growth.
The catalytic activities of the zeolite samples prepared
with ATES were compared with the conventional MFI-zeo-
lite. Test reactions used included alkylation, condensation
and protection reactions (Scheme 1). Using these reactions
several synthetic intermediates such as flavanone/chal-
2
Figure 4. N isotherms of conventional MFI and MFI prepared with alkyl-
triethoxysilanes (inset: pore size distribution of the corresponding sam-
ples).
[11]
[12]
cone, 7-hydroxy-4-methylcoumarin and diphenyl meth-
Chem. Eur. J. 2008, 14, 9507 – 9511
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9509

M. Arai et al.
those active sites and the distribution of them in the meso-
pores and micropores. However, it may be reasonable to
assume that one of important factors responsible for the dif-
ferences in the catalytic activity (Table 2) is improved mass
transport, owing to the presence of intracrystal mesopores.
To confirm this aspect, the catalytic activity was examined
for protection reaction (Entry 4, Table 2) involving reactants
with small molecular dimensions (i.e., methanol and cyclo-
hexanone). In this case, the catalytic activity was found to
be very similar for all the catalysts investigated in this study
(
Table 2). The high catalytic activity of MFI-10PrTES in all
the cases may be a result of the formation of smaller zeolite
crystallites and the presence of intracrystal mesopores be-
tween them, which will shorten the diffusion path and exhib-
it facile diffusion of product molecules.
In summary, the present study proposes a new method of
controlling the nano-crystal size and mesoporosity, by the
simple addition of alkyl-alkoxysilanes into conventional zeo-
lite-synthesis composition. The resultant zeolite products
have intracrystal mesoporosity and can exhibit significantly
enhanced catalytic activities for the synthesis of large mole-
cules that cannot enter zeolite micropores. Propyltriethoxy-
silane is more effective for the generation of mesoporosity
than methyltriethoxysilane and octyltriethoxysilane. It is
proposed that the mesoporosity could be generated accord-
ing to the nanoscale alternation of optimum hydrophobic,
organic-rich domains and less hydrophobic inorganic do-
mains, during the crystal growth. Further studies are in prog-
ress in our laboratory.
Scheme 1. Catalytic reactions investigated in this study.
[
13]
ane can be synthesized, which have important relevance
in chemical industries. The synthesis of 7-hydroxy-4-methyl-
coumarin takes place by means of the Pechmann reaction.
The reaction proceeds through trans-esterification and intra-
molecular hydroxyalkylation, followed by dehydration. The
synthesis of flavanone occurs through the reaction between
2
’-hydroxyacetophenone and benzaldehyde. The first step
yields 2’-hydroxychalcone through Claisen–Schmidt conden-
sation. Flavanone is obtained by intramolecular cyclisation
of the 2’-hydroxychalcone. The reaction results obtained are
summarized in Table 2. For the reactions (1)–(3) (Scheme 1)
involving large molecules, the zeolite samples synthesized
with ATES were found to be highly active compared to con-
ventional MFI-zeolite. In these reactions, the MFI-10PrTES
sample was significantly more active than the other catalysts
Experimental Section
In a typical synthesis of MFI-zeolite, alkylalkoxysilane (ATES, A=
propyl, methyl or octyl, purchased from Aldrich) was mixed with tetra-
propylammonium hydroxide (20% TPAOH aq. solution, TCI, Japan) and
sodium aluminate solution. The initial mixture was stirred for 15 min at
98 K, until it became a clear solution. Tetraethylorthosilicate (TEOS,
Aldrich) was added into the solution and stirring was continued for 6 h.
The molar composition of the gel mixture was TEOS/ATES/Al /Na O/
TPAOH/H
O, 100Àx:x:2.5:3.3:25:2500 (x=2.5–15). This mixture was
transferred to a Teflon-lined autoclave and hydrothermally heated at
43 K for 3 d under static conditions. The final product was filtered,
2
(
Table 2). The total amount of acid sites characterized by
2
O
3
2
the TPD of NH did not correlate with the catalytic activity
3
2
and so some particular acid sites should be labeled working
active sites, and would depend on the reactions examined.
At present we do not know the details of the acidity of
4
washed with deionised water, and dried at 373 K. The organic template
and additives were removed by calci-
nations at 823 K for 4 h under flowing
Table 2. Catalytic applications of conventional MFI and MFI samples synthesized using different alkyltrie-
thoxysilanes.
air. The product was designated by
MFI-xPrTES according to the
number of moles (x) of the alkyltrie-
thoxysilane. For catalytic applications,
Reactions
MFI
MFI-10MeTES
MFI-10PrTES
MFI-10OcTES
5.2
(
1)
benzyl chloride conversion [%]
1.6
99:1
8.5
7.5
65:35
50.2
12.7
92:8
27.8
16.5
65:35
57.8
33.9
89:11
50.7
23.8
63:37
53.7
the calcined materials were ex-
mono-alkylated/di-alkylated product
7-hydroxy-4-methylcoumarin [%]
2’-hydroxyacetophenone conversion [%]
flavanone/chalcone selectivity
A
H
R
U
G
+
changed with NH
4
three times using
(
(
2)
3)
18.6
12.7
63:37
53.2
1
NH
molar NH
4
NO
3
solution. Then, the
+
4
exchanged-zeolite was calcined
again at 823 K to obtain the protonic
form of the zeolite.
(
4)
1,1-dimethoxycyclohexane yield [%]
Reaction conditions: Methyl substituted diphenylmethane synthesis: toluene (50 mmol); benzyl chloride
5 mmol); catalysts (100 mg); temperature (413 K), runtime (2 h). 7-Hydroxy-4-methylcoumarin synthesis: re-
sorcinol (5 mmol); ethylacetoactetate (7.5 mmol); catalyst (100 mg); temperature (423 K); runtime (4 h). Fla-
vonone/chalcone synthesis: 2’-hydroxyacetophenone (7 mmol); benzaldehyde (7 mmol); catalyst (100 mg);
temperature (423 K); runtime (8 h); Protection reaction: cyclohexanone (5 mmol); methanol (50 mmol); cata-
lyst (50 mg); temperature (298 K); runtime (2 h).
X-ray diffraction (XRD) patterns of
the solid samples prepared were re-
corded in the 2q range of 5–708 with
a scan speed of 28min by using a
Rigaku X-ray diffractometer using
(
À1
CuKa radiation (l=0.1542 nm, 40 kV,
9510
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Synthesis of Nanocrystalline MFI-Zeolites
COMMUNICATION
2
0 mA) and a proportional counter detector. Nitrogen adsorption meas-
Keywords: hydrothermal synthesis · mesoporous zeolite ·
MFI zeolite · morphology · nanocrystalline zeolite
urements at 77 K were performed by using a NOVA 1000 series Quan-
tachrome Instruments volumetric adsorption analyzer. The samples were
out-gassed at 573 K for 4 h before the adsorption measurement. The spe-
cific surface area was determined by the BETmethod using the data
0
points of P/P in the range of about 0.05–0.3. The pore size distribution
was estimated by using the Barret–Joyner–Halenda (BJH) model. Scan-
ning electron microscopy (SEM) images were obtained by means of a
A
C
H
T
R
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Hitachi S-4300 F instrument. Small amount of sample was mounted on
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a typical experiment, 250 mg of sample was placed in a U-shaped, flow-
through, quartz sample tube. Before the TPD experiments, the catalyst
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À1
was pretreated in He (50 cm min ) at 823 K for 1 h. After cooling down
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were dissolved by adding a small amount of acetone to the reaction mix-
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tion, cyclohexanone (0.98 g, 10 mmol) and methanol (3.2 g, 100 mmol)
were charged with catalyst (0.1 g) in a 25 mL round bottom flask. The re-
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were identified by using a GC-MS (Shimadzu GCMS-QP5050) and au-
thentic samples obtained from Aldrich. Quantitative determinations were
based on the measured response factors of the reactants and products.
2
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The authors wish to express their thanks to Japan Society for the Promo-
tion of Science for JSPS postdoctoral fellowship for R.S. (P07130) and
for Grant-in-Aid for JSPS fellows (19·07130). The authors are thankful to
Mr. Y. Nodasaka for TEM measurements and Mr. Yamada for NMR
analysis.
Received: June 9, 2008
Published online: September 12, 2008
Chem. Eur. J. 2008, 14, 9507 – 9511
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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