Mesoporous MFI Zeolite with a 2D Square Structure Directed by Surfactants with an Azobenzene Tail Group

Article abstract of DOI:10.1002/chem.201800307

Mesoporous MFI zeolites (MMZs) have been constructed by using the surfactant-containing azobenzene segment in the hydrophobic tail. The cylindrical π–π stacking of azeobenzene groups is considered to be the key factor to form the ordered mesostructure thr

Full text of DOI:10.1002/chem.201800307

A Journal of
Accepted Article
Title: Mesoporous MFI Zeolite with Two-Dimensional Square Structure
Directed by Surfactants with Azobenzene Tail Group
Authors: Xuefeng Shen, Wenting Mao, Yanhang Ma, Honggen Peng,
Dongdong Xu, Peng Wu, Lu Han, and Shunai Che
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Mesoporous MFI Zeolite with Two-Dimensional Square Structure
Directed by Surfactants with Azobenzene Tail Group
+
[a]
+ [a]
[b]
[c]
[d]
[e]
Xuefeng Shen , Wenting Mao , Yanhang Ma, Honggen Peng, Dongdong Xu, Peng Wu, Lu
[a, f]
[a, f]
Han*
and Shunai Che*
+
These authors contributed equally to this work.
Abstract: Mesoporous MFI zeolites (MMZs) have been constructed
by using the surfactant containing azobenzene segment in the
hydrophobic tail. The cylindrical π-π stacking of azeobenzene
groups is considered to be the key factor to form the ordered
mesostructure through cooperative structural matching and the
rearrangement of MFI frameworks. The mesostructure has been
tuned from disordered hierarchical arrangement into an ordered
two-dimensional (2D) square p4mm structure by changing the
length of alkyl chain between diquaternary ammonium head group
and azobenzene group. The geometrically matching between MFI
zeolitic framework and the alkyl chain length plays an important role
on the construction of the crystallographically correlated
mesostructure with 2D-square ordering. A combination of X-ray
diffraction patterns and electron microscopy studies provides visible
evidence for the mesostructural transformation from short range
hexagonal or lamellar ordering to 2D square mesostructure.
Introduction
Zeolites, microporous crystalline aluminosilicates, are widely
used in the field of sorbents, ion-exchange, catalysts due to
their high surface area and adsorption capacity, high thermal
and hydrothermal stabilities, uniform and well-defined
[1]
micropores with excellent shape-selectivity in catalysis. The
sole presence of micropores (mostly 0.4-1.2 nm) limits their
applications in catalyzing organic molecules with large
[
1a, 2]
dimensions.
On the other hand, highly ordered mesoporous
[
a]
X. Shen, W. Mao, Prof. L. Han, Prof. S. Che,
School of Chemistry and Chemical Engineering, State Key
Laboratory of Metal Matrix Composites,
aluminosilicates with adjustable pore size from 1.5 to 15 nm
and high surface areas opened a new possibility for preparing
[
3]
Shanghai Jiao Tong University,
large molecule catalysts in the nanoporous space. However,
mesoporous aluminosilicates are disappointing for the actual
catalysis process due to the amorphous wall that led to the
poor hydrothermal stability and acidity. Synthesizing
mesoporous zeolites could overcome these disadvantages.
Until now, many synthesis methods have been reported, such
800 Dongchuan Road, Shanghai, 200240 (P.R. China)
E-mail: chesa@sjtu.edu.cn, chesa@tongji.edu.cn, luhan@tongji.edu.cn
Prof. Y. Ma,
School of Physical Science and Technology,
ShanghaiTech University,
[b]
[c]
[d]
100 Haike Road, Pudong, Shanghai, 201210 (P.R. China)
Prof. H. Peng,
College of Chemistry,
Nanchang University,
[4]
as post-synthetic treatment of bulk zeolites, assembly of
[
5]
[6]
zeolitic nanocrystals, and hard or soft template methods.
999 Xuefu Road, Nanchang, Jiangxi, 330031 (P.R. China)
Nevertheless, the direct synthesis of highly ordered
mesoporous zeolites is still a challenge in material science that
remains unresolved.
Prof. D. Xu,
School of Chemistry and Mateirals Science, Jiangsu Key Laboratory
of Biofunctional Materials,
Nanjing Normal University,
Quaternary ammonium salts have been widely used to
generate zeolite structures due to their distinct kinetic size and
Nanjing, 210023 (P.R. China)
[
7]
[
e]
f]
Prof. P. Wu,
steric configuration, which lead to their close fit to inorganic
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, School of Chemistry and Molecular Engineering,
East China Normal University,
[8]
framework. To combine the templating units for both zeolite
framework and the mesostructural ordering, Ryoo et al.
3663 North Zhongshan Road, Shanghai, 200062 (P.R. China)
designed
a
bifunctional surfactant and the lamellar
[
Prof. L. Han, Prof. S. Che,
mesostructured zeolites have been constructed by MFI
School of Chemical Science and Engineering,
Tongji University,
[9]
nanosheets with three pentasil layer thickness. Furthermore,
a gemini-type, multiple quaternary ammoniums surfactant has
been used to generate a hexagonally ordered mesoporous
1239 Siping Road, Shanghai, China, 200092 (P. R. China)
Supporting information for this article is given via a link at the end of
the document.
[9c]
molecular sieves with MFI-like frameworks. In our previous
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reports, we introduced aromatic groups into the hydrophobic tail
[10]
of the surfactants, the strong π-π interaction of which could
stabilized the lamellar micelle structure, and the configuration of
these aromatic could be adjusted to geometrically match the
MFI zeolitic framework to form lamellar mesostructural MFI
zeolites. The theoretical analysis indicates that the present of
both multiple quaternary ammoniums and aromatic group could
reduce binding energies of the mesostructured zeolite
synthesis system.
It is well known that intergrowth is a common phenomenon in
MFI zeolites, occasionally with the intergrowth with MEL-type
[11]
structure.
The unit cell parameters of MFI along a- and b-
axies (a = 20.07 Å, b = 19.92 Å) are almost the same, and both
the straight channels and zig-zag channels of MFI zeolite are
Figure 1. Chemical formulas of the surfactants with different carbon chain
length.
10-membered ring (10-MR). These structure characteristic of
MFI zeolite provide a fundamental possibility for an intergrowth
[
12]
relationship between the (h00) and (0k0) facets. Bolaform or
triply branched cationic surfactants were used to template
mesostructured MFI zeolite constructed by MFI nanosheets
joined with a 90° rotational boundary due to their special
branched configuration and the stronger π-π stacking around
Results and Discussion
[
10b,13]
the boundary joints.
hierarchical MFI zeolite with house-of-cards nanosheets with
perpendicular arrangement that lead to to nm
Recently, we introduced an azobenzene
Tsapatsis et al. reported the
2
7
[
11b]
mesopores.
segment into the hydrophobic tail of the amphiphilic surfactant,
leading to the MFI zeolite with 2D square mesostructure framed
by the intergrowth of MFI nanosheets expanding along the a-c
[
14]
plane.
The introduction of azobenzene segment in the
hydrophobic part of surfactant could induce the cylindrical-
assembly of surfactants, which could match the zeolite
framework to form ordered mesoporous zeolites with 2D square
mesostructure.
Figure 2. Low-angle (A) and high-angle (B) powder XRD patterns of calcined
Herein, we investigate the formation mechanism of the MMZ
templated by azobenzene-containing surfactants. To elucidate
the importance of the length of alkyl chain and the templating
effect with geometrical matching, the surfactants with different
alkyl chain length connecting the diquaternary ammonium
group and azobenzene segment were designed (Figure 1).
These surfactants are donated as Cazo-m-6-6 (m denotes the
length of the alkyl chain, the first 6 is the carbon chain
connected two quaternary ammoniums and the second 6 is the
carbon chain tail of the outer quaternary ammonium). The only
difference of these surfactants is the length of the alkyl chain
connecting azobenzene group and diquaternary ammonium
group (Cazo-m-6-6, m= 6, 8, 10, 12). To understand the structural
evolution of the MMZs and the molecular interaction during the
synthesis, X-ray diffraction (XRD) patterns, scanning electron
microscopy (SEM), transmission electron microscopy (TEM)
studies and UV-Vis absorption spectroscopy of products with
different reaction times were investigated.
products MMZ-6 (a), MMZ-8 (b), MMZ-10 (c) and MMZ-12 (d) obtained by
surfactants
C
azo-6-6-6
,
C
azo-8-6-6
,
C
azo-10-6-6 and
C
azo-12-6-6
,
respectively
(
wavelength 1.5418 Å).
Figure 2 shows the XRD patterns of calcined samples,
namely MMZ-6, MMZ-8, MMZ-10 and MMZ-12 templated by
Cazo-6-6-6, Cazo-8-6-6, Cazo-10-6-6 and Cazo-12-6-6, respectively. MMZ-6
shows one broad reflection in the low-angle region centred at
θ = 1.2°, indicating the mesostructural ordering with the d-
2
spacing of 7.4 nm. For MMZ-8, the reflection is more obvious
with the same d-spacing. When the alkyl chain increased to
C10, a well-resolved reflection at 2θ of 1.6° with the d-spacing
of 5.6 nm can be recognized, suggesting the highly ordered
mesostructure compared to MMZ-6 and MMZ-8. For the MMZ-
12, the ordering of mesostructure decreased as nearly no
reflection appears in the low-angle region. As revealed by the
TEM observations (vide post), there is no clear correlation
between the d-spacing of the mesostructure and the length of
alkyl chain, which the reflections are mainly depending on the
ordering of the structure. The high-angle XRD patterns (Figure
2B) indicate all these samples possess the MFI structure with
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similar crystallinity and free of other phases. Most of the sharp
peaks are associated with the h0l reflections of the MFI
framework, which indicates the formation the layered MFI
nanosheets expanding along the ac plane.
The SEM images are consistent with structural features
discerned from their XRD patterns (Figure 3). All these samples
show the house-of-cards morphology. MMZ-6 has the
tetragonal shape with size of ~2 μm (Figure 3a
nanosheets of which joined with rotational boundary (Figure
). These MFI nanosheets of MMZ-6 oriented growth along
1
), MFI
3a
2
the central axis of tetragonal crystal, namely the c-axis of MFI
framework (vide post). The diameter of MMZ-8 is smaller than
MMZ-6 (Figure 3b
the thickness of nanosheets is much larger with some of the
MFI nanosheets randomly stacked together (Figure 3b ). There
is no obviously oriented growth of the MFI nanosheets in MMZ-
from the SEM images. The size of tetragonal particles of
MMZ-10 is almost the same as MMZ-8 (~1.5 μm). MMZ-10 has
the uniform morphology (Figure 3c ) that a tetragonal shape
with four curved surfaces meeting at the vertexes (Figure 3c ).
1
). The intergrowth of MMZ-8 is sparser, and
2
8
1
2
A four-fold order of rotation with the mirror planes inclined to
each other by 45°, indicating the 4mm point group symmetry.
There is no obvious MFI nanosheets viewed form the SEM
images of MMZ-10, which is distinguished from other samples.
Of note, the MFI nanosheets are ordered in the tetragonal
particles with an oriented growth along the c-axis of MFI
framework (vide post). MMZ-12 has a deformed rhombohedral
morphology, the size of which is smaller than other three
samples (Figure 3d ). The intergrowth of MMZ-12 is not
1
obvious, and the nanosheets of MMZ-12 is fragmentary with
smaller size.
The TEM images observed on slice of the calcined MMZ
samples embedded in epoxy resin show that all these materials
have large amounts of mesopores framed by thin MFI
nanosheets. The Fourier diffractograms (FDs) show that the
intergrown perpendicular with each other, which is coincide with
nanosheets expand in ac plane of MFI framework and he XRD
1
patterns and SEM observations. Figure 4a shows the low-
magnification TEM image taken from the top view of the cross-
section of MMZ-6. From the high-resolution TEM (HRTEM)
image, it can be found the MFI nanosheets have the layered
stacking of MFI nanosheets consist of 4-6 pentasils layers,
which results in nonuniform mesopores with 1-2 pentasil layers
thickness. This nonuniformity leads to the broad peak in the
low-angle XRD profile. The diffusion of the diffraction spots in
the FD shows the bending and frustration of the intergrowth
Figure 3. Low-angle (A) and high-angle (B) powder XRD patterns of calcined
products MMZ-6 (a), MMZ-8 (b), MMZ-10 (c) and MMZ-12 (d) obtained by
surfactants
C
azo-6-6-6
,
C
azo-8-6-6
,
C
azo-10-6-6 and
C
azo-12-6-6
,
respectively
(
wavelength 1.5418 Å).
1
structure. Figure 4b shows the low-magnification TEM image
of the cross-section of MMZ-8, indicating the sparse intergrowth
structure with nonuniform layer thickness and disordered
mesopores, which is also clearly shown in Figure 4b . Some
2
that the ordered mesostructure can be only formed in MMZ-10,
as shown in Figure4c. The cross-sectional image shown in
MFI nanosheets collapsed after removal of surfactants due to
the large void among the nanosheets. The diffusion of the
diffraction spots in the FD pattern of MMZ-8 is more obvious
due to the irregular intergrowth structure. It is worthy to note
Figure 4c
straight 2D square channels. framed by MFI nanosheets are
observed in Figure 4c . The nanosheets consist of three
pentasil layers and the pore diameter are uniform with three
1
shows the tetragonal morphology. The ordered
2
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resulted in the well-resolved peak in the low-angle XRD pattern.
The ordering of mesostructure decreased when increasing the
alkyl chain from C10 to C12. Figure 4d
1
shows the low-
magnification TEM image of MMZ-12, showing the
mesostructures framed by MFI nanosheets, however, with
nonuniform thickness of the nanosheets with wide pore size
2
distributions (Figure 4d ).
2
Figure 5. N adsorption-desorption isotherms (a) and pore size distribution
curves (b) obtained from desorption branches of calcined mesoporous
zeolites shown in Figure 2.
Table 1. The pore properties of calcined MMZ-6, MMZ-8, MMZ-10 and MMZ-
12.
a
2
-1
b
3
-1
c
3
-1
Samples
S
BET (m g )
V
Micro (cm g )
V
Total (cm g )
MMZ-6
MMZ-8
551
595
604
540
0.16
0.17
0.16
0.12
0.44
0.54
0.53
0.80
MMZ-10
MMZ-12
[
a] BET surface area calculated from the adsorption data
0
obtained at P/P between 0.05 and 0.25. [b] Micropore
volume determined via the t-plot method. [c] Total pore
volume determined via the single-point method.
The N
in Figure 5a. All samples show the existence of micropores, as
evidenced by a sharp increasing adsorption capacity at P/P
2
adsorption-desorption data of the samples are shown
0
<
Figure 4. TEM images from the top view of the octahedron-like particles of
the MMZ-6 (a), MMZ-8 (b), MMZ-10 (c) and MMZ-12 (d). The corresponding
FDs were inserted in their HRTEM images. The white triangles indicate the
diffraction spots of mesostructural ordering.
0.02. The calcined MMZ-6, MMZ-8 and MMZ-10 exhibit type-IV
isotherms with H4-type hysteresis loop. The hysteresis loops
[15]
of MMZ-6 and MMZ-8 at relative pressures in the range of P/P
0.5-0.9 indicate the mesopores with wide pore size
0
=
distributions. Compared with MMZ-6 and MMZ-8, MMZ-10
shows a higher uptake and the hysteresis loop in the range of
pentasil layers thickness. The rotational boundary of the joint
MFI nanosheets is almost all 90°, connected with each with
new Si-O-Si bonds between two related hydroxyl groups from
P/P
0
= 0.4-0.6 indicates the presence of abundant uniform
adsorption-desorption isotherms of MMZ-12
mesopores. The N
2
[
10a]
exhibit a type-IV isotherm and a H3-type hysteresis loop, which
shows this sample has disordered lamellar or slit pores. The
pore size distribution of the samples obtained from adsorption
ac and bc plane of MFI framework, respectively.
A detailed
structural study of MMZ-10 is shown in our previous
[
14]
publication.
The ordering of mesostructure of MMZ-10
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branches are shown in Figure 5b, the MMZ-6 shows mesopore
diameter of ~2.2 nm, which is agree with the HRTEM
observations. The MMZ-8 shows a wide pore size distribution,
indicating the nonuniform mesopores in the structure. The
MMZ-10 shows the uniform mesopores with pore size of ~3 nm.
The MMZ-12 shows no obvious pore size distribution peaks,
indicating the nonuniform mesopores. All the samples exhibit
transformation.
Simultaneously,
high-angle
reflections
corresponding to the MFI zeolite structure appeared. As the
product crystallized to 24 h, the first-order reflection shifted to a
2θ of 1.59°, which is same to the final product (4 days) with a d-
spacing of 5.6 nm, and the high-angle reflections became
stronger, suggesting an improved mesostructured ordering and
an increased crystallinity. A crystallization period of 24 h under
the present synthesis conditions is sufficient for the complete
transformation from an amorphous aluminosilicate framework
to MFI zeolite with 2D square mesostructure. Further
2
-1
BET surface area larger than 500 m g (Table 1). MMZ-10 has
the highest BET surface area among these four materials dues
to the ordered mesostructure. The MMZ-12 has the highest
mesopore volume, suggesting the existence of large
mesopores due to the nonuniform stacking of the MFI
nanosheets.
hydrothermal treatment (e.g.,
significant changes in the mesostructure, which only increases
the zeolitic crystallinity.
4 days) did not result in
The structural transformation of the products was observed in
SEM images (Figure 7). At the initial stage of crystallization (1-3
h), an undefined morphology was observed, which was
probably amorphous aluminosilicate aggregates. The initial
tetragonal morphology was first observed at 4 h with a particle
size of ~300 nm. As the crystal morphology is commensurate
with the structure, the appearance of the tetragonal morphology
indicates the beginning of the crystallization process at
atomistic scale. More tetragonal particles with a size of 400 nm
formed at 6 h, but still there is much amorphous aluminosilicate
aggregates, which results in no high-angle reflections
corresponding to the MFI zeolite structure in the high-angle
XRD pattern. Prolonging the crystallization process to 9 h,
almost no amorphous aluminosilicate aggregates existed, the
high-angle XRD pattern of which showed reflections of MFI
framework. Finally, uniform tetragonal particles were formed at
24 h upon the completion of the silica condensation and
crystallization. It is worthy to note that the average particle size
of the final product was ~1.5 μm, which is same as the final
product after 4 days, suggesting the sample grew from the
initially formed particles and almost no significant changes of
morphology from 24h.
Figure 6. Low-angle (a) and high-angle (b) powder XRD patterns of as-
synthesized products obtained by surfactant
crystallization time at 423 K.
Cazo-10-6-6 for various
Representative TEM images (Figure 8) of the products
obtained at different crystallization period distinctly illustrated
the transformation process from the amorphous aluminosilicate
to crystalline zeolite. The selected-area electron diffraction
The morphological and structural evolution of MMZ-10. To
investigate the morphological and structural evolution of MMZ-
10, the crystallization process of MMZ-10 was investigated as a
function of reaction time. After being mixed at 333 K with
vigorous magnetic stirring for 3 h, the original silica gel was
evenly divided into identical Teflon-lined stainless-steel
autoclaves, and the zeolite powder was collected after different
crystallization periods (1 h-24 h). Figure 6 shows the low- and
high-angle powder XRD patterns of the products collected
during the crystallization periods. The low-angle XRD pattern of
the products before 6 h shows a strong reflection with a 2θ of
(
SAED) pattern of the samples crystallized for 1-3 h shows a
ring pattern corresponding to a uniform d-spacing of ~4 nm,
and the HRTEM images of the sample in this stage show the
contrast of disordered mesostructures (Figure 8a). No
crystalline phase or lamellar structure was observed at this
stage, which the HRTEM and the corresponding FD are
consistent with this result. Therefore, the low-angle reflections
observed in the XRD pattern may not be the lamellar phase but
rather due to the disordered packing of the micelles of uniform
size. The micelles can be cylindrical, and the assemblies of the
micelles may have short range hexagonally or lamellar ordering,
which were marked by circles and rectangle. A particle with an
early tetragonal morphology was discovered at approximately 4
h (Figure 8b). The weak diffraction spots in the SAED pattern
suggest the appearance of atomistic crystalline structure. The
HRTEM shows partially crystallized lattice fringes with a sheet-
2.3° and a broad second-order reflection centred at 4.5°. The
two reflections with a d-spacing ratio of 2 : 1 probably indicate
the formation of lamellar mesostructures. However, other 2D-
and 3D- mesostructures cannot be excluded. No reflections
were detected in the high-angle region before 6h. As the
product crystallized for 9 h, the first-order reflection at a 2θ of
2.3° shifted to a lower angle, while the second-order reflection
disappeared, suggesting structural rearrangement or
a
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Figure 7. Representative SEM images of the as-synthesized products obtained by surfactant Cazo-10-6-6 for various crystallization times at 423 K for 1 h (a), 3 h (b),
h (c), 5 h (d), 6 h (e), 9 h (f), 12 h (g), 24 h (h).
4
like arrangement along one direction while the other part of the
particle remains disordered, indicating the coexistence of
amorphous silica and crystals, and the FD in the HRTEM
shows the same diffraction pattern. Then, the SAED pattern
taken from the sample crystallized for 5 h shows that the
diffraction spots of the MFI crystal become clearer as the
crystallization continues (Figure 8c). The TEM image shows
enlarged particles with a tetragonal morphology and the
HRTEM shows crystalline nanosheets in 90° rotation
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orientations. At 6 h, the SAED pattern shows the four-fold
mesostructural ordering (indicated by white triangles) with the
highly crystallized MFI lattice (Figure 8d). The HRTEM image
taken from the cross-sections of tetragonal particles indicates
the much higher crystallinity of the zeolite structure and good
consistency with the structural matching of the 2D-square
mesostructures with 90° rotational boundaries. The FD pattern
shows more clear diffraction spots of MFI framework.
When the silica source is added to the surfactant solution, the
quaternary ammonium sites of the surfactant interact with the
silica species through electrostatic force and π-π interactions in
the hydrophobic part. The self-assembly of the surfactant/silica
species forms a cylindrically assembled mesostructure in a
cooperative manner. Some of short range hexagonally or
lamellar ordering was generated. The short range of these
ordering may resulted in two reflections with a d-spacing ratio of
2 : 1 were appeared in the low-angle XRD patterns of the
products sampled before 3 h. At this stage of the reaction, the
framework is amorphous aluminosilicate, as shown in the XRD
patterns. (ii) Nucleation and crystallization. With the ageing
process, the quaternary ammonium groups pendant on the
azobenzene lead to the gradual formation of crystalline zeolite
UV-vis spectroscopy was used to investigate the
stereoregularity of Cazo-10-6-6 molecules (Figure 9). The UV-Vis
absorption spectra of template molecules (Cazo-10-6-6) in dilute
water clearly show two absorption bands, which are ascribed to
the π in transition (348 nm) and the n-π transition (440 nm) of
[
16]
the azobenzene moieties, respectively. The UV-Vis absorption
spectra of all as-synthesized products obtained after different
crystallization periods (1 h - 24 h) show three characteristic
bands at 348, 376 and 440 nm. The increasing intensity of
absorption bands at 376 nm and 440 nm during this
crystallization period is due to the close and regular packing of
the surfactants, which led to obvious π-π interactions between
the azobenzene groups. Then the corresponding absorption
bands shifted toward longer wavelengths (red-shift). The
appearance of a new absorption band at 376 nm is due to the π-
π interaction, which shift approximately 28 nm compared to the
pure surfactant in dilute water (from 348 nm to 376 nm).
Therefore, the azobenzene group of the surfactant is trans-form
and kept intact in the whole present synthetic process.
[1c,17]
frameworks
(4 h). The nucleation and crystal growth at the
beginning might be homogeneous, so the initially formed
morphology is isotropic. (iii) Cooperative structural matching and
rearrangement. With further ageing, one growth direction
becomes dominant in the particles. As the structure of the MFI
crystal and the possible MFI nanosheets are connected to each
o
other with a 90 rotational angle, the MFI crystal grew gradually
with an intersectional a/b-axis and a unique common c-axis,
leading to the elongated tetragonal morphology. The pre-formed
disordered mesostructure was rearranged and transformed into
the cylindrical assembly-units to match the mesostructural
ordering and the MFI sheet, forming the new 2D-square lattice.
This process is consistent with the shift and the decrease in
intensity of the low-angle XRD and the appearance of the high-
angle reflections (6-9 h). (iv) Further crystallization and growth.
From the above results, a mechanism is proposed based on
the organic-inorganic interaction and the structural transition by
geometrical matching mechanism. (i) Mesostructure formation.
Figure 8. Representative TEM images of the as-synthesized products obtained at 423 K with various crystallization times of 3 h (a), 4 h (b), 5 h (c) and 6 h (d).
The SAED patterns and FDs were inserted in the corresponding TEM and HRTEM images, respectively.
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[
18]
interaction energy and minimizing the excluded volume.
However, in a few cases, the 2D square packing have been
[
19]
found in the liquid crystal phases by T-shape molecules and in
[
20]
DNA condensed phases.
It has been reported that achiral or chiral compounds bearing
a relatively bulky head group and a hydrocarbon tail can be
organized into chiral supramolecular assemblies by
spontaneous symmetry breaking. The trans-azobenzene unit
has
a planar molecular configuration, which favours the
cooperative packing in a helical sense due to the strong
intermolecular interactions facilitated by the π-π stacking
interactions and the steric hindrance triggered by the
overcrowded alignment of the adjacent bulky headgroups
(
diquaternary
ammoniums
in
azobenzene-containing
[
21]
surfactant).
The azobenzene-containing compounds tend to
assemble into a helical sense due to the characteristic of the
azobenzene segment. Therefore, a helix cylindroid micelle with
1
2 surfactant molecules in one pitch can be expected (Figure
[
14]
S1a). The 12 terminal quaternary ammoniums would direct 12
straight channels surrounding the helical azobenzene cylinder to
form the 2D square intergrown MFI frameworks (Figure S1b),
[
14]
and other 12 quaternary ammoniums are at the pore opening of
the 10-MR to stabilize the (010) surface. The diameter of the
central cylinder is ~1.6 nm. The quaternary ammoniums tethered
on the azobenzene with different largest length of ~0.7 nm (Cazo-
6
-6-6), ~1.0 nm (Cazo-8-6-6), ~1.2 nm (Cazo-10-6-6) and ~1.5 nm (Cazo-
1
2-6-6) (Figure S1b). The π-π stacking of azobenzene segments
not only exists between the adjacent azobenzenes in one circle
but also those in adjacent azobenzenes in adjacent circles,
which could effectively stabilize the helical structure.
Figure 9. Representative UV-vis spectra of the as-synthesized products for
various crystallization times at 423K.
[14]
On the detailed calculation of the square mesopores with
three pentasil layers space, the space from the azobenzene
segment to the framework needs to be in a range of 0.8-1.3 nm
considering the shortest length to reach the framework
perpendicularly and longest distance through the diagnal,
Finally, the products were formed as
a result of the
crystallization process and the crystal growth (> 9 h). As the MFI
growth along the c-axis direction perpendicular to the a-b plane
is independent, which lead the final product to show a typical
tetragonal morphology with an elongated shape.
Based on the experimental results and the discussions above,
it can be considered that the mesostructure was formed by the
cylindrical-assembly-units and the structural matching between
the mesostructure and zeolite are the key factors for the
synthesis of ordered MMZ. Among the four surfactants, highly
ordered MMZ can be only formed by Cazo-10-6-6, while other
surfactants only gave random intergrowth structures. It can be
concluded that the alkyl chain length of the surfactant plays an
important role in the mesostructure formation. This can be
(
(
Figure S1b). Due to the apporoprate chain length of Cazo-10-6-6
~1.2 nm) and its flexiblility, its configuration can be adjusted to
fit the requirement of the square mesopore (Figure S1b and e).
Nevertheless, the chain length of Cazo-6-6-6 (~0.7 nm) (Figure S1c)
and Cazo-8-6-6 (~1.0 nm) (Figure S1d), are too short while that of
Cazo-12-6-6 (~1.5 nm) (Figure S1f) is too long to fit the structural
intergrowth. Thus, the strict geometrical matching is essential for
the ordered mesostructural formation.
explained in terms of geometric matching between the Conclusions
assembly-units and MFI frameworks. In the intergrowth MFI
zeolite, the (100) plane overgrown on the (010) plane due to the
similar unit cell parameters of MFI along a- and b-axis (a = 20.07
Å, b = 19.92 Å) and both 10-MR pores of straight channels and
By incorporating an azobenzene segment into the
hydrophobic tail of the diquaternary ammonium surfactant,
hierarchical MFI zeolite with 2D square mesostructure was
successfully synthesized. Compared to the conventional cationic
surfactants, the presence of azobenzene not only provided π-π
stacking interactions to stabilize the micelles of surfactants, but
also formed the cylindrical-assembly-units to match with the MFI
zeolite framework. By controlling the alkyl chain length, the
geometrical matching between cylindrical micelles and MFI
framework gave rise to ordered arrangement of alternating MFI
[11,12]
zig-zag channels of MFI zeolite.
The d-spacing of 2D square
MFI mesostructure is 5.9 nm (Figure S1a), and the
corresponding mesopore with three pentasil layers space is ~3
nm. To direct this 2D square MFI mesostructure, these
azobenzene-containing surfactants would be assembled into a
square columnar packing, which is very rare to generated due to
the long-range ordering is always formed by maximizing the
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Chemistry - A European Journal
10.1002/chem.201800307
nanosheets through a mesostructural transformation and the
crystallization process afterwards. We hole our findings will open
up new possibilities for the elaborate fabrication of three-
dimensionally ordered mesoporous zeolites.
Keywords: Mesoporous zeolite • MFI zeolite • Surfactant self-
assembly • Azobenzene group • Geometrical matching
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21533002, 21471099, 21571128), the
National Key R&D Program of China (2016YFC0205900), the
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The mesostructure has been tuned
from disordered hierarchical
Xuefeng Shen, Wenting Mao,
Yanhang Ma, Honggen Peng,
Dongdong Xu, Peng Wu, Lu Han* and
Shunai Che*
arrangement into an ordered 2D
square p4mm structure by changing
the length of alkyl chain between
diquaternary ammonium head group
and azobenzene group. Visible
evidence for the mesostructural
transformation from short range
hexagonal or lamellar ordering to 2D
square mesostructure is shown.
Page No. – Page No.
Mesoporous MFI Zeolite with Two-
Dimensional Square Structure
Directed by Surfactants with
Azobenzene Tail Group
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