Single-Crystalline MFI Zeolite with Sheet-Like Mesopores Layered along the a Axis

Article abstract of DOI:10.1002/chem.201804841

Designing a templating strategy for directing mesopore growth along different crystallographic directions is essential for fabricating two- or three-dimensional single-crystalline mesoporous zeolites. However, so far, mesopores formed in MFI zeolites by soft templates have mostly been generated by disrupting growth along the b axis; generating mesopores by disrupting growth along the a axis is rare. Herein, a single-crystalline mesoporous MFI zeolite (SCMMZ) with sheet-like mesopores layered along the a and b axes was synthesized using a triply branched surfactant with diquaternary ammonium groups connected to 1,3,5-triphenylbenzene by a six- and eight-carbon alkyl chain (TPB-6 and 8). The sheet-like mesopores were embedded in the MFI framework and were retained even after calcination. Molecular mechanics calculations provided evidence of low binding energy configurations of the surfactant that directed the growth of straight and zigzag channels along the b and a axes, respectively. The formation of nanosheets was attributed to the geometric matching of the arrangement of the aromatic groups to the zeolite framework.

Full text of DOI:10.1002/chem.201804841

A Journal of
Accepted Article
Title: Single-crystalline MFI Zeolite with Sheet-like Mesopores Layered
along the a Axis
Authors: Yunjuan Zhang, Xuefeng Shen, Zheng Gong, Lu Han, Huai
Sun, and Shunai Che
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
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To be cited as: Chem. Eur. J. 10.1002/chem.201804841
Link to VoR: http://dx.doi.org/10.1002/chem.201804841
Supported by

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COMMUNICATION
Single-crystalline MFI Zeolite with Sheet-like Mesopores Layered
along the a Axis
Yunjuan Zhang , Xuefeng Shen , Zheng Gong , Lu Han*, Huai Sun* and Shunai Che*
+
+
+
Abstract: Designing a templating strategy for directing mesopore
growth along different crystallographic directions is essential for
fabricating two- or three-dimensional single-crystalline mesoporous
zeolites. However, so far, the mesopores formed in MFI zeolites by
soft templates have mostly been generated by disrupting growth
along the b axis; generating mesopores by disrupting growth along
the a axis is rare. Here, a single-crystalline mesoporous MFI zeolite
along the b axis, and we used a cylindrical azobenzene assembly in the
hydrophobic part to generate quaternary, ordered, straight, square
channels framed by MFI thin sheets in the a-c planes and joined by 90°
rotations.[ We also found that an amphiphilic surfactant with three
diquaternary ammonium-terminated alkyl chains connected uniformly
to a benzene ring[13] can direct the formation of single-crystalline
12]
mesoporous ZSM-5 with
a sheet-like three-dimensional porous
(
SCMMZ) with sheet-like mesopores layered along the a and b axes
structure. The mesopores of all of these mesostructured MFI zeolites
only opened at the (010) facets because the growth was disrupted along
the b axis, which would limit the adsorption properties and shape
selectivity.
was synthesized using a triply branched surfactant with diquaternary
ammonium groups connected to 1,3,5-triphenylbenzene by a six-
and eight-carbon alkyl chain (TPB-6 and 8). The sheet-like
mesopores were embedded in the MFI framework and were retained
even after calcination. Molecular mechanics calculations provided
evidence of low binding energy configurations of the surfactant that
directed the growth of straight and zigzag channels along the b and
a axes, respectively. The formation of nanosheets was attributed to
the geometric matching of the arrangement of the aromatic groups to
the zeolite framework.
It should be noted that tuning the hydrophobic part of the surfactant
is the key factor for mesostructure formation and the structural
matching between the mesopores and zeolite framework. The aromatic
groups introduced into the hydrophobic tail of the surfactants not only
provide π-π interactions that can stabilize the mesostructures but can
also adjust their configuration to geometrically match the MFI zeolite
framework, forming a single-crystalline lamellar MFI mesostructure.
Until now, only one or two benzene groups have been introduced.
Stronger π-π interactions and a more stable micellar structure can be
formed by introducing larger aromatic groups, which would lead to the
formation of mesoporous MFI zeolites with disrupted growth along
other axes.
Zeolites are widely employed in catalysis, separations and adsorption
because of their open crystal structure, strong acidity and ion-exchange
capacity.[1] However, the small size of their pores (less than ~0.8 nm)
and cavities (typically < 1.5 nm)[2] limit the diffusion of guest
molecules through the pore channels in the catalytic inorganic
framework. This disadvantage can be overcome by introducing
mesopores into zeolites using post-synthesis treatments,[4] soft
templates (e.g., small molecules,[5] surfactants,[6] mesoscale
Herein, triply branched surfactants in which diquaternary
ammoniums are connected to 1,3,5-triphenylbenzene by different
[3]
+
carbon chain lengths to form C
6
H
3
{[C
6
H
4
-O-C
n
H
2n-N (CH
3
)
2
-C
6
H
12
-
+
13][Br-]
N (CH
3
)
2 6
-C H
2
}
3
(denoted TPB-n, n = 4, 6, 8, 10) (Supporting
Information, Figure S1) were designed. The three branches are
preferentially stretched along three directions, in contrast to the bilayer
configuration that extends in one or two opposite directions. Strong π-π
interactions and a low stabilizing energy are achieved by introducing
the larger aromatic “core”. The different lengths of the alkyl chains of
TPB-n ensure that the surfactants match mesoporous MFI zeolites
[7]
[8]
polymers ) and hard templates (e.g., carbon replicas, polystyrene
[9]
spheres ). The soft-template method, which involves self-assembling
amphiphilic molecules and an inorganic source, is known to be the
most promising method. Particularly, quaternary ammonium salts have
been extensively used to generate numerous zeolite structures due to
[10]
their strong interactions with the inorganic framework.
(
-
MMZs) with different structures. The MMZs templated by TPB-4, -6,
8 and -10 were denoted MMZ-4, -6, -8 and -10, respectively. Single-
In the soft-template method, the templates not only direct the
synthesis of the zeolite but also disrupt its growth through their
hydrophobic parts, resulting in the generation of mesopores. Ryoo et al.
successfully synthesized MFI zeolite nanosheets by disrupting the
crystal growth along the b axis using a diquaternary ammonium-type
surfactant.[6, 11] We previously introduced aromatic groups into the
hydrophobic tail of amphiphilic molecules to synthesize a single-
crystalline architecture with mesostructured zeolite nanosheets layered
crystalline mesoporous MFI zeolite (SCMMZs) with sheet-like
mesopores layered along the a and b axes were fabricated with TPB-6
and -8.
The low-angle X-ray diffraction (XRD) pattern (Figure 1a) of
calcined MMZ-6 exhibits a broad peak at approximately 1.44°,
indicating the existence of less-ordered mesopores. Indexing this peak
to the first-order reflection of layered MFI nanosheets gives a
repeating unit of 6.1 nm. The mesostructure is retained even after the
removal of the templates, in contrast to lamellar mesostructured
zeolites that collapse to form polycrystals or single crystals.[6a, 12a] The
high-angle XRD pattern of MMZ-6 (Figure 1b) exhibits the same
reflections as that of conventional MFI, indicating that a high-quality
MFI zeolite structure is produced. SEM images (Figures 1c and 1d)
show that the sample consists of interconnected primary particles with
an irregular morphology and average size of a few microns (see also
the low-magnification SEM images in Figure S2). These particles are
composed of secondary particles that are compacted with thin sheet-
like particles with a thickness of ~5 nm.
[
*] Y. Zhang,[+] X. Shen,[+] Z. Gong,[+] Prof. Dr. L. Han, Prof. Dr. H. Sun, Prof.
Dr. S. Che
School of Chemistry and Chemical Engineering, State Key Laboratory of
Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan
Road, Shanghai 200240 (P.R. China)
E-mail: chesa@sjtu.edu.cn, huaisun@sjtu.edu.cn, luhan@sjtu.edu.cn
Prof. Dr. L. Han, Prof. Dr. S. Che
School of Chemical Science and Engineering, Tongji University, 1239
Siping Road, Shanghai 200092 (P.R. China)
E-mail: chesa@tongji.edu.cn, luhan@tongji.edu.cn
+] These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the
document.
[
2
The N adsorption-desorption isotherm of calcined MMZ-6 (Figure
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1
e) is type IV(a) and exhibits an H4-type hysteresis loop.[14] The
the mesoporous structures are associated with the MFI lattice fringes;
however, the pores do not have a well-defined shape. The mesopores in
the MFI framework observed along the a and b directions are short (<
50 nm) and embedded in the MFI framework, and they are retained
after calcination. Figure 2d shows a possible structural model of
calcined MMZ-6, in which mesopores with a diameter of ~2 nm are
formed by breaking the Si-O bond that located at the middle of the 10-
membered ring along both the a and b axes. The TEM image obtained
along the a axis (Supporting Information, Figure S3) shows that
mesopores are also formed by disrupting the growth of the MFI crystal
along the b axis. Therefore, MMZ-6 is a mixture of MMZ with layered
pores along both the a and b axes. As observed in several low-
magnification TEM images, the average thickness of the repeating unit
is estimated to be 5.6 nm, which is similar to the result obtained from
the XRD patterns (Supporting Information, Figure S4 and S5).
uptake step below P/P
structure.[15] The H4-type hysteresis loop at relative pressures P/P
.4-0.9 indicates the presence of slit-like mesopores. The pore size
distribution of MMZ-6 (Figure 1f) indicates an average pore size of
2.2 nm (obtained from adsorption branch), which is approximately
0
= 0.02 reveals the presence of a microporous
0
=
0
~
equal to the diameter of the slit-like mesopores observed in the TEM
images. The Brunauer–Emmett–Teller (BET) surface area, micropore
2
volume and mesopore volume of MMZ-6 are measured to be 621 m /g,
3
3
0
.09 cm /g and 0.45 cm /g, respectively (Supporting Information,
Table S1). The BET surface area and mesopore volumes of MMZ-6 are
much higher than those of conventional ZSM-5, indicating a large
number of mesopores in this material, whereas the micropore volumes
of these two zeolites are comparable.
Figure 2. a) Low-magnification TEM image and b) corresponding SAED
pattern obtained along the b axis of MMZ-6. c) Enlarged image of the area
marked by the white box in (a). d) Proposed structural model of the pores
layered along the a axis.
When the alkyl chain length C4 is used, a lamellar MMZ is
synthesized (Supporting Information, Figure S6 and Table S1). Two
well-resolved peaks at 2θ = 1.88° and 3.86° are indexed to the first-
and second-order reflections of lamellar MFI nanosheets with a d-
spacing of 4.7 nm, indicating a periodic interlamellar structural order.
The high-angle XRD pattern of MMZ-4 confirms that the sample has a
2
pure MFI zeolite structure. The N adsorption-desorption isotherm of
Figure 1. a) Low-angle and b) high-angle XRD patterns, c) low-magnification
calcined MMZ-4 is type-IV(a) and exhibits an H4-type hysteresis loop.
The BET surface area, micropore volume and mesopore volume of
MMZ-4 are measured to be 409 m /g, 0.10 cm /g and 0.21 cm /g,
respectively. The SEM images of calcined MMZ-4 show a house-of-
cards-like morphology, and the TEM images indicate that the lamellar
structure collapses after calcination.
When the length of the alkyl chains is increased to C8, SCMMZ is
also produced (Supporting Information, Figure S7). The XRD patterns
show the formation of a mesostructured MFI zeolite, and the SEM
images reveal that MMZ-8 has a spherical morphology. MMZ-8 has an
and d) high-magnification SEM images, e) N
and f) pore size distribution of calcined MMZ-6.
2
adsorption-desorption isotherm
2
3
3
Meso- and microstructural information about MMZ-6 was obtained
by transmission electron microscopy (TEM) observations of a slice of
calcined MMZ-6 embedded in epoxy resin. The low-magnification
TEM image (Figures 2a) shows the presence of sheet-like mesopores
with a thickness of ~2 nm and length of 5-50 nm. The corresponding
selected-area electron diffraction (SAED) pattern (Figure 2b) reveals a
single-crystalline nature. Based on the indexing of the SAED pattern,
the mesopores are layered along the [100] direction of the MFI
framework, which disrupts the crystal growth along the a axis. The
high-resolution TEM (HRTEM) image (Figure 2c) shows that many of
2
N adsorption-desorption isotherm (type IV(a)), hysteresis loop (H4-
2
3
type), surface area (590 m /g), mesopore volume (0.44 cm /g),
3
micropore volume (0.10 cm /g) and pore diameter (∼2.3 nm, obtained
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from adsorption branch) similar to those of MMZ-6 (Supporting
Information, Figure S7 and Table S1). The SAED patterns of thin
To understand the formation mechanism and the relation between
the mesostructure formation and the alkyl chain length of the
surfactants, the relative stabilizing energies of the mesoporous MFI
zeolite with different configurations of TPB-6 (Figure 3) were
calculated using molecular dynamics (MD) simulations. The
simulation model of MFI was constructed based on the pore size
distribution of MMZ-6, which exhibits a broad peak centred at ~2.2 nm.
For the single-crystalline MFI structure, the size of the mesopores must
be an integral multiple of the length of half the unit cell along the a or
slices of calcined MMZ-8 indicate
a
single-crystalline nature
Supporting Information, Figure S8). Mesopores stacked along the
100] and [010] directions of the MFI framework are clearly observed,
(
[
demonstrating that the growth of the MFI crystal along the a and b
axes is disrupted, which is similar to the results for MMZ-6.
Increasing the length of the alkyl chains to C10 results in MFI
nanosheets with a rotationally and sequentially intergrown structure
(
Supporting Information, Figure S9 and Table S1). A broad paeks
b axis. Three structural models for SCMMZ were proposed
centered at 1.5° is observed in the low-angle XRD pattern indicating
the existence of less-ordered mesopores, whereas the high-angle XRD
(Supporting Information, Figure S10), and model ii, which has a well-
matched pore size, is the most appropriate structure. The results of the
MD simulations are summarized in Table S2. The stabilizing energies
of 3*straight and 3*zigzag are higher than those of the other two
models (Figure 3). It can be concluded that the preferred arrangements
for a single surfactant molecule in the synthesis of MMZ-6 are
1*straight+2*zigzag and 2*straight+1*zigzag, because it is then
patterns indicate that high-quality ZSM-5 is obtained. The N
2
adsorption–desorption isotherm of calcined MMZ-10 is type IV(a) with
an H4-type hysteresis loop. The pore size distribution reveals a
uniform mesopore size of approximately 2.6 nm (obtained from
adsorption branch). The BET surface area, micropore volume and
2
3
3
mesopore volume of MMZ-10 are 642 m /g, 0.11 cm /g and 0.37 cm /g, oriented to direct the formation of SCMMZ by disrupting the growth of
respectively. The SEM images show that MMZ-10 has an oval
morphology with a size of ~300 nm. Both the atomic-scale crystalline
structure of MFI and the mesostructure of MMZ-10 can be observed in
the TEM image. The four diffraction spots (indicated by white
triangles) correspond to the similar periodicity of sheet-like mesopores
along two perpendicular axes. The possible formation mechanism of
MMZ-10 might be the overgrowth of the (100) faces on the (010) faces
via the c axis, with the zigzag channels of one nanosheet being
interconnected with the straight channels of the other nanosheets. The
connectivity at the intersections could be due to the formation of new
Si-O-Si bonds.
the zeolite along both the a and b axes. Considering the aggregation
behaviour of the surfactants, the 3*straight and 3*zigzag
configurations facilitate the self-assembly of the templates in a more
relaxed and symmetrical way and exhibit a suitable structural fit with
the arrangement of the mesopores and zeolite framework, directing the
growth of SCMMZ with sheet-like mesopores.
Figure 4. a) UV-vis spectra of the dilute aqueous solution of the as-
synthesized template (black line) and the as-prepared MMZ-6 (red line). b-d)
Packing configurations of the surfactants in the as-prepared zeolite viewed
1 2
along the a, b and c axes. d , d ) Configurations of two adjacent templates
viewed along the c axis.
UV-vis spectroscopy was used to investigate the stereoregularity of
the TPB-6 molecules (Figure 4a). The UV-vis absorption spectrum of
the template molecules (TPB-6) in dilute water clearly exhibits two
absorption bands (198 nm and 274 nm), which are ascribed to the π-π*
transitions of the aromatic moieties.[16] In the solid crystalline state in
MMZ-6, the template molecules are more densely packed, leading to
obvious π-π stacking due to the overlap of the π molecular orbitals of
the aromatic groups. The energy of the electron transition from the π
highest occupied molecular orbital to the π* lowest unoccupied
molecular orbital decreases, which leads to a red shift (213 nm and 291
Figure 3. Stabilizing energy calculations of four simulation models of the TPB-
6-directed MMZ-6 system. a) The three branches are loaded in the zigzag
channels (3*zigzag), disrupting the a axis. b) The three branches of TPB-6 are
loaded in the straight channels (3*straight), disrupting the b axis. c) One
branch of TPB-6 is loaded in a straight channel, while the other two branches
are loaded in zigzag channels (1*straight+2*zigzag), disrupting both the a and
b axes. d) Two branches of TPB-6 are loaded in the straight channels, while
one branch is loaded in a zigzag channel (2*straight+1*zigzag), disrupting
both the a and b axes.
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nm) relative to the bands of TPB-6 in a dilute aqueous solution. These
results provide evidence for the π-π interactions and stacking of TPB-6
in MMZ-6.[17]
triply branched quaternary ammonium surfactants with different alkyl
chain lengths are used as templates. For the synthesis of SCMMZ, one
branch of TPB-6 or TPB-8 is loaded in the straight channel, whereas
the other two branches are loaded in the zigzag channels of MFI. The
key elements enabling the synthesis of SCMMZ, in which mesopores
are formed by disrupting crystalline growth along the a axis, are the
special configuration of TPB-6 and TPB-8 with the quaternary
ammoniums located in the zigzag channels to direct the synthesis of
the zeolite and the geometric matching between the assembled
surfactants and the zeolite framework. After removing the surfactants,
the mesostructures of MMZ-6, MMZ-8 and MMZ-10 are retained,
whereas the lamellar mesostructure of MMZ-4 collapses. The proposed
mechanism provides a new route for the generation of hierarchical
zeolites with different mesostructures.
The intact configuration of TPB-6 template without decomposition
in MMZ-6 was confirmed by 13C nuclear magnetic resonance (NMR)
spectra (see Figure S12). The thermogravimetric analysis (see Figure
S13) combined with elemental analysis data (ratio of N/Si = 0.043)
indicate that not only that all micropores were directed by the
quaternary ammonium groups, but also that one template corresponds
to three micropores. The dual stabilization effects of the diquaternary
ammonium head groups and the strong π−π interactions in the
hydrophobic core may energetically favour the formation of micellar-
structured zeolites, which can further facilitate the self-assembly of
templates with aromatic moieties to geometrically match the zeolitic
frameworks. Thus, a geometric matching model of the zeolite structure
and the cooperative packing of the TPB-6 molecules was proposed
based on the corresponding arrangements of the triply branched
templates in the MFI framework along the a and b axes. As mentioned
previously, model ii is the most suitable structural model of MMZ-6
Acknowledgements
We thank Prof. Osamu Terasaki for his guidance on TEM operation
and analysis.
(
Figures 4b-4d). The TPB-6 molecules stack in the b-c plane of the
MFI framework, and the angle between the aromatic group and b-c
plane is ~23° or 157° (Figure 4b). The horizontal distance between two
adjacent surfactants is ~10 Å, and the perpendicular distance is 3.8
Å.[18] The aromatic groups of the TPB-6 molecules stack in the b-c
plane of the MFI framework, forming a stable network through π−π
stacking with an slipped configuration, which results in sheet-like
micelles (Figure 4d). The diquaternary ammonium head groups
tethered on triply branches of the template broke the extension of the
nanosheets assembly along the a and b axes, which led to the formation
of SCMMZ with sheet-like mesopores layered along the a and b axes.
It is demonstrated that the length of the alkyl chain between the
diquaternary ammoniums and aromatic group plays an important role
in the construction of the crystallographically correlated mesostructure.
The relative stabilizing energies of the mesoporous MFI zeolite with
different configurations of TPB-4 and TPB-10 were also calculated
using MD simulations (Supporting Information, Table S2). The
diameter of model ii (Supporting Information, Figure S10) is also
suitable for the size of TPB-4; however, it is too small for TPB-10.
Because the most common pore size of MMZ-10 is 2.6 nm, a new
structural model in which the diameter of the mesopore is ~3 nm was
built for TPB-10 (model iii, Supporting Information, Figure S10).
For TPB-4, the stabilizing energy of 1*straight+2*zigzag is the
lowest, followed by that of 2*straight+1*zigzag and 3*zigzag. It can be
concluded that when TPB-4 is loaded in the mesostructured MFI, a
single TPB-4 molecule tends to stretch along the a-c or b-c plane of the
MFI structure. However, C4 (Supporting Information, Figure S11) is
just long enough to reach from the aromatic segment to the zeolite
framework (~2 nm), which is too stretched and energetically
unfavourable for the aggregation of the surfactants that direct the
formation of SCMMZ. Therefore, TPB-4 only directs the synthesis of
lamellar mesostructured MFI zeolites, because no substantial stretching
is required in this system. When loading TPB-10 in the SCMMZ, the
This work was supported by the National Natural Science Foundation
of China (21533002, 21471099 and 21571128), National Key R&D
Program of China (2016YFC0205900), National Excellent Doctoral
Dissertation of P.R. China (201454) and Shanghai Rising-Star Program
(
17QA1401700).
Conflict of interest
The authors declare no conflict of interest.
Keywords: MFI • mesoporous zeolite • cationic surfactant • self-
assembly • transmission electron microscopy
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is
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preferentially synthesized by disrupting crystalline growth along the b
axis. Thus, it is concluded that the TPB-10 molecules stack in the a-c
plane of the MFI framework, disrupting the growth of the MFI crystal
along the b axis and directing the synthesis of an intergrown structure.
In summary, four different triply branched quaternary ammonium
surfactants that can readily self-assemble and whose aromatic groups
have a highly ordered orientation due to π-π stacking were designed.
Lamellar mesoporous MFI, SCMMZ and MFI nanosheets with a
rotationally and sequentially intergrown structure are obtained when
[
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A single-crystalline mesoporous MFI
zeolite with sheet-like mesopores
layered along the a and b axes was
Y. Zhang, X. Shen, Z. Gong, L. Han*,
H. Sun and S. Che*
templated by
surfactant
a
with
triply branched
diquaternary
Page No. - Page No.
Single-crystalline MFI Zeolite with
Sheet-like Mesopores Layered along
the a Axis
ammonium groups connected to
,3,5-triphenylbenzene by six-
1
a
carbon alkyl chain. The sheet-like
mesopores were embedded in the
MFI framework and were retained
even after calcination.
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