Hierarchical MFI Zeolites with a Single-Crystalline Sponge-Like Mesostructure

Article abstract of DOI:10.1002/chem.201804193

Single-crystalline sponge-like MFI mesoporous zeolites (SSMZs) have been synthesized by using bolaform surfactants with an axial chiral binaphthyl core in the hydrophobic tail and triquaternary ammonium head groups, as bifunctional organic structure-directing agents (OSDAs). By changing the length of alkyl chain between a triquaternary ammonium head group and a binaphthyl group from 4 to 10 carbons, SSMZs with high specific surface area (382–434 m² g^?1), abundant micropore-mesopore connectivity, and uniform mesopore diameter (4–10 nm) were obtained. OSDAs with an alkyl chain length of 11 and 12 carbons led to the formation of nanorod-constructed mesoporous MFI zeolites. A geometrical matching between the cylindrical arrangement of the binaphthyl groups and the zeolitic framework is speculated to be the key factor for the formation of mesoporous zeolites. The SSMZ zeolites, with abundant mesopores beneficial for the diffusion of reactants, exhibited significantly higher catalytic efficiencies than those of the conventional ZSM-5 with a microcrystal morphology (≈1.5 μm).

Full text of DOI:10.1002/chem.201804193

A Journal of
Accepted Article
Title: Hierarchical MFI Zeolites with A Single-Crystalline Sponge-like
Mesostructure
Authors: Yunjuan Zhang, Peng Luo, Hao Xu, Lu Han, Peng Wu, Huai
Sun, and Shunai Che
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Hierarchical MFI Zeolites with A Single-Crystalline Sponge-like
Mesostructure
Yunjuan Zhang,[a] Peng Luo,[b] Hao Xu,[b] Lu Han,* [a, c] Peng Wu,* [b] Huai Sun,* [a] and Shunai Che*[a,c]
Abstract: Single-crystalline sponge-like MFI mesoporous zeolites
(SSMZs) have been synthesized by using bolaform surfactants with
axial chiral binaphthyl core in the hydrophobic tail and triquaternary
ammonium head groups as bifunctional organic structure-directing
agents (OSDAs). By changing the length of alkyl chain between
triquaternary ammonium head group and binaphthyl group from 4 to
10 carbons, SSMZs with high specific surface area (382-434 m2/g),
abundant micropore-mesopore connectivity and uniform mesopore
diameter (4-10 nm) were obtained. While the OSDAs with an alkyl
chain length of 11 and 12 carbons led to the formation of nanorods-
using designed soft templates, in which the connection of the
multiple quaternary ammoniums head group to hydrophobic
alkyl chains is the principal factor for the effective structure-
direction of MFI zeolite and simultaneous formation of
a
mesoscale micellar structure. Xiao et al. synthesized single-
crystalline zeolites ZSM-5 with mesopores aligned along b-axis
using a designed cationic amphiphilic copolymer as a mesoscale
organic structure-directing agent (OSDA) and single-crystalline
zeolites beta with disordered mesopores using polymers as a
dual-functional SDA.[7h, 8] An development of the soft templating
methods has attempted by introducing benzene[7e-g] and
azobenzene[7k] to the hydrophobic centre of bifunctional OSDAs
in our previous works, which the single-crystalline mesoporous
zeolite nanosheets[7f, 7g] and single-crystalline mesoporous ZSM-
5 with three-dimensional pores[7e] have been synthesized by
benzene rings centered molecules with quaternary ammonium
head groups. The OSDAs used in those systems were
composed of aromatic hydrophobic centre which can self-
assemble through strong π-π interaction, while the quaternary
ammonium head groups direct the growth of MFI zeolites.
Recently, we successfully synthesized a hierarchical MFI zeolite
with a two-dimensional (2D) square mesostructure by using
rational designed templates with azobenzene segment in the
hydrophobic tail.[7m] It is worth noting that the surfactant-type
OSDAs with a chiral hydrophobic center would self-assemble to
form helical cylindrical assembly units and then lead to 2D or
three dimensional (3D) interrupted zeolite growth.
constructed mesoporous MFI zeolites.
A geometrical matching
between the cylindrical arrangement of binaphthyl groups and the
zeolitic framework is speculated to be the key factor for the formation
of mesoporous zeolites. The SSMZ zeolites, with abundant
mesopores beneficial for the diffusion of reactants, exhibited
significantly higher catalytic efficiencies than those of the
conventional ZSM-5 with a microcrystal morphology (~1.5 μm).
Introduction
Zeolites are inorganic crystals with regular microporous
structures commonly used as commercial adsorbents and
catalysts.[1] However, the larger mass transfer resistance and
long transmission path proposed by the small aperture of
micropore (< 2 nm) severely hampered their applications.[2]
Recently, great efforts have been devoted to the synthesis of
zeolites with mesopores, which greatly improved the mass
transportation and enlarged the external surface area.[3] So far,
the assembly of zeolitic nanocrystals,[4] demetallation,[5] hard[6]
and soft[7] templating methods have been studied extensively.
Among those, the soft templating method shows great
advantages as it can synthesize various mesoporous zeolites
through the versatile molecular self-assembly.
Here we attempt to build helical cylindrical assembly by
introducing axis chiral binaphthyl into the hydrophobic tail of the
surfactant with quaternary ammonium head groups to form
[C10H6-O-CnH2n-[N+(CH3)2-C6H12]m-1-N+(CH3)2-C6H13·mBr−]2
(denoted Bn-mN, see Table 1). It has been shown that the two
naphthyl groups of the binaphthyl molecule can rotate along the
C-C single bond and the dihedral angle between the
naphthalene groups depends on the solution system and
adjacent functional groups.[9] The binaphthyl-containing
Ryoo et al. have synthesized lamellar mesostructured
zeolites,[7b] polycrystalline zeolites nanosponges[7j-l] and
hexagonally ordered MFI zeolite-like mesoporous zeolites[7c] by
surfactants can adjust their spatial conformations to form a
10]
helical cylindrical assembly.[9b,
The quaternary ammonium
groups pendant on binaphthol can interact with the inorganic
source to form a zeolite framework. These assemblies would
disrupt the zeolite growth into bulk crystals and also adjust the
pitch length to geometrically match the MFI framework, leading
to the formation of single-crystalline sponge-like mesostructure.
[a]
[b]
[c]
Y. Zhang, Prof. L. Han, Prof. H. Sun, Prof. S. Che,
School of Chemistry and Chemical Engineering, State Key
Labortory of Metal Matrix Composites,
Shanghai Jiao Tong University,
800 Dongchuan Road, Shanghai, 200240 (P.R. China)
E-mail: chesa@sjtu.edu.cn; luhan@sjtu.edu.cn; huaisun@sjtu.edu.cn
P. Luo, Prof. H. Xu, Prof. P. Wu,
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, School of Chemistry and Molecular Engineering,
East China Normal University,
3663 North Zhongshan Road, Shanghai, 200062 (P.R. China),
E-mail: pwu@chem.ecnu.edu.cn
Prof. L. Han, Prof. S. Che,
Table 1. Multiquaternary ammonium surfactants containing binaphthyl tail
groups.
Surfactant structure
Notation
Bn-mN
n=4, 6, 8, 10, 11, 12.
m=1,2,3
OR
School of Chemical Science and Engineering
Tongji University
1239 Siping Road, Shanghai, China, 200092 (P. R. China),
RO
R=CnH2n-[N+(CH3)2-C6H12 m-1
]
-N+(CH3)2-C6H13
Supporting information for this article is given via a link at the end of the
document.
3Br-
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Results and Discussion
The surfactants with different alkyl chain length and
quaternary ammonium head groups were used to direct the
mesoporous zeolite structures. Bulk ZSM-5 zeolite crystals were
formed by the surfactants with one quaternary ammonium head
group (Bn-1N) and two quaternary ammonium head groups (B4-
2N)as OSDAs (see Figure S1-S3), while hybrid hierarchical ZSM-
5 zeolites with a crystalline core and nanosheet shell were
obtained with B6,8,10,11,12-2N as OSDAs (see Figure S4-S7).By
increasing the quaternary ammonium head groups of the
surfactants to three (Bn-3N), SSMZ and nanorods constructed
mesoporous MFI zeolites have been obtained. Figure 1 shows
low- and high-angle X-ray diffraction (XRD) patterns of calcined
.
Figure 1. Low- (a) and high-angle (b) XRD patterns of calcined mesoporous
MFI zeolites templated by B4-3N, B6-3N, B8-3N, B10-3N, B11-3N and B12-3N surfactants.
Figure 2. Low-magnification (a1, b1, c1, d1, e1 and f1) and corresponding high-magnification (a2, b2, c2, d2, e2 and f2) SEM images of calcined mesoporous MFI
zeolites templated by B4-3N, B6-3N, B8-3N, B10-3N, B11-3N and B12-3N surfactants, respectively.
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samples. Broad reflections were observed in the low-angle XRD
profile of the zeolites templated by B4, 6, 8, 10-3N, which indicate the
presence of mesoporous structures in these samples. No
reflection was observed in the low-angle XRD pattern of zeolites
of B12-3N, scattered nanorods with non-uniform diameters were
observed(Figure 2e2, f2).
Figure 3 shows transmission electron microscopy (TEM)
images of sliced thin sections of the calcined SSMZ sample
templated by B10-3N. As shown in the low-magnification TEM
images (Figure 3a1, 3b1 and 3c1), the presence of abundant
randomly distributed mesopores can be confirmed, in agreement
with the results obtained from XRD and SEM images. Figure 3a4,
3b4 and 3c4 show the selected-area electron diffraction (SAED)
patterns taken from the area of Figure 3a1, 3b1 and 3c1,
respectively, which correspond to [100], [010] and [001]
directions of MFI zeolite, revealing the single-crystalline nature
of the particles. Figure 3a2, 3b2 and 3c2 reveal the TEM images
of the typical MFI lattice with single crystal nature, which the
Fourier diffractograms (FDs) show sharp spots. As the white
contrast in the images corresponds to areas of low electron-
scattering density, namely, the mesopores are randomly
distributed in the whole particle. The high-resolution TEM
(HRTEM) images are shown in Figure 3a3, 3b3 and 3c3
respectively, the crystalline MFI lattice can be clearly observed
along with abundant mesopores. The mesoporous structure is
stable and does not collapse after calcination, which can be
attributed to the 3D mesoporous structure with continuously
grown zeolite framework. The samples templated by B4-3N, B6-3N
templated by B11,
implying the absence of ordered
12-3N
mesoporous structures in these samples. High-angle XRD
patterns show that the samples with ZSM-5 structure (MFI) were
produced. The reflections in the high-angle region reflect the
presence of reflections (hkl) with wide FWHM and low intensity,
corresponding to the crystal structure with limited repeating unit
(small size).[7c, 7g]
The scanning electron microscopy (SEM) images show that all
the samples templated by Bn-3N exhibit an octahedral-like
morphology (Figure 2a1, b1, c1, d1, e1, f1). High resolution SEM
image reveal that the samples templated by B4,
are
6, 8, 10-3N
composed of stacking of nonuniform nanoparticles with sponge-
like morphology. Abundant intraparticle mesopores with
a
diameter of 5-20 nm were observed and randomly distributed on
the crystal surface (Figure 2a2, b2, c2, d2). After ultrasound
treatment, the fracture of sample templated by B10-3N also shows
abundant mesopores randomly distributing inside the particles a
sponge-like feature (see Figure S8). The crystal surface
structure of the sample templated by B11-3N were composed of
crisscross-shaped nanorods with uniform diameter, while for that
Figure 3. Low- and high-magnification TEM images (a1-3, b1-3 and c1-3), and SAED patterns (a4, b4 and c4 corresponding a1, b1 and c1) taken along the [100], [010]
and [001] directions of sliced SSMZ templated by B10-3N, respectively.
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and B8-3N show similar structural feature as observed in that of
B10-3N (see Figure S9-S11). Furthermore, the TEM images of
sliced thin sections of the calcined samples templated by B11-3N
and B12-3N are shown in Figure S12, which contains the
presence of ink-bottle type mesopores due to the stacking of
nanoparticles.[12] The pore size distributions (Figure 4b)
calculated from desorption branches with BJH methods show
relatively uniform pore size distribution. The BET specific
surface area (Table 2) of these mesoporous ZSM-5 zeolites are
higher than that of the conventional ZSM-5 zeolites. In particular,
the mesopore volumes of these mesoporous ZSM-5 zeolites are
much higher than that of conventional ZSM-5, while the
micropore volume is comparable between these two kinds of
MFI zeolites. For samples templated by B11-3N and B12-3N, similar
microporous adsorption, type IV isotherms and type H3
hysteresis loops were observed. The type H3 hysteresis loop at
relative pressure in the range of 0.4-0.9 indicates the presence
of disordered lamellar or slit-type mesopores, which is consistent
with the TEM observations. In addition, samples templated by
configurations of the nanorods stacking. Particles with
a
rectangular cross section that constructed by crossed nanorods
are observed from the low-magnification TEM images. The voids
between the nanorods show long slits morphology. Due to the
dense arrangement of the nanorods and the overlap problem in
the TEM images, it is difficult to observe the nanorod stacking in
thick sections of the particles.
Figure 4a shows the N2 adsorption/desorption isotherms of the
calcined SSMZ and nanorods constructed mesoporous ZSM-5.
Samples templated by B4-3N, B6-3N, B8-3N, B10-3N exhibit type IV
isotherms and a type H2 hysteresis loops. The adsorption
amount showed a steep increase in the region of 0<P/P0<0.01,
implicating the adsorption of the micropores,[11] while hysteresis
loops close to H2 type in the region of 0.4<P/P0<0.8 indicate the
B11-3N,
possess much higher BET surface area and
12-3N
mesoporous pore volume than that of SSMZ samples (Table 2).
Figure 4. Nitrogen adsorption-desorption isotherms (a) and pore size distribution curves (b) of calcined mesoporous MFI zeolites templated by B4-3N, B6-3N, B8-3N
,
B
10-3N, B11-3N and B12-3N surfactants.
Table 2. The pore properties of calcined zeolites templated by Bn-3N
.
Surfactants
B4-3N
Zeolites materials
SBET(m2/g)[a]
382
Pore diameterb]
Vmicro(cm3/g) [c]
0.10
Vtotal(cm3/g) [d]
0.25
3.7
5.6
5.9
6.6
-
B6-3N
428
0.10
0.32
SSMZ
B8-3N
434
0.10
0.35
B10-3N
B11-3N
B12-3N
427
0.11
0.34
484
0.11
0.40
Nanorods constructed
mesoporous zeolites
530
-
0.10
0.49
[a] Given by BET method. [b] The pore size distributions were converted from the desorption (samples templated by B4-3N, B6-3N, B8-3N, and B10-3N,) or adsorption
(samples templated by B11-3N and B12-3N) branch according to the BJH algorithm. [c] Determined by t-plot method. [d] Obtained by the single-point method.
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To confirm the π-π stacking arrangement of the template
molecules in the synthesis system, we measured UV-Vis and
DRUV adsorption spectra of the B10-3N template in aqueous form
and in as-prepared SSMZ (Figure 5a). In a dilute water solution,
B10-3N molecules are isolated from each other, resulting in
absorption bands from the molecule at 230, 287 and 337 nm. It
was reported that the band at 230 nm corresponds to a long axis
of a naphthyl group, while the band at 287 nm corresponds to its
short axis[9b] and the crucial electron transition feature of the
band at 337 nm is due to the fact that the orbitals involved are
mainly localized on two naphthalene moieties with π-π*
character.[13] Templates in SSMZ showed an obvious red shift,
indicating the long-range ordered assembly of binaphthyl core
through π-π interaction. The peaks at 230 and 287 nm shifted to
257 and 297 nm for SSMZ, respectively, indicating that the
status of original conjugated templates have changed during the
zeolites growing process. The peak at 337 nm shifted to 343 nm
for SSMZ due to the π-π interaction of the assembled templates
in the zeolites. The displacement of the absorption peak is due
to the energy transition from the π-HOMO to the π*-LUMO
decreases as two π−π molecular orbitals from the aromatic
groups overlap, which usually leads to a red shift of the
absorption bands.[13] The mirror imaged CD signals of as-
prepared samples templated by enantiomeric S-B10-3N and R-B10-
were obtained in the corresponding UV/Vis absorption region
3N
(Figure S13), which indicates the helical assembly of surfactants.
The integrity of template molecule was confirmed by 13C NMR
spectroscopy, while the content of template was obtained from
thermogravimetric analysis (TGA) (see Figure S14, S15 and
Table S1). The N/Si ratio of 0.051 obtained from TGA is
relatively high compared to the reported value of the
conventional MFI zeolites[14], since partial ammonium groups in
the mesoporous zeolites did not direct the formation of the
zeolite framework and thus did not occupy the micropores of
Figure 5. Schematic representation of the formation of SSMZ. UV-DR spectra (a) of template molecules in a diluted water solution (black line, i), and in as-
prepared SSMZ solid (red line, ii). Schematic diagram of the formation of SSMZ (b, c), diquaternary ammonium groups of the template direct the formation of the
MFI framework while one quaternary ammonium stabilises the negative charges of the zeolite framework viewed along the [001] and [010] directions, respectively.
Structural model (d) of the arrangement of binaphthyl groups along a tilted (101) plane (The cylinder represents parts of the mesopores in the SSMZ).
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zeolites. To investigate the formation mechanism and the
relationship between the surfactants and the derived SSMZ, the
relative binding energies between the templates with different
configurations and the mesoporous MFI were calculated using.
molecular dynamics (MD) simulations. The calculation indicates
that the binding energies of 2*zig-zag-s (two branches of B10-3N
loading in zig-zag channels of MFI zeolite with same direction)
and 2*zig-zag-o (two branches of B10-3N loading in straight
channels with opposite direction) are the lowest. The 2*straight-s
(two branches of B10-3N loading in straight channels with same
direction) shows slightly increased binding energy, while the
values of 2*straight-o (two branches of B10-3N loading in zig-zag
channels with opposite direction) and 1*straight+1*zig-zag (one
branch of B10-3N loading in zig-zag channels) are much higher
(Table S2 and Figure S16). Considering the helical cylindrical
assembly of surfactants, the conformation of 2*zig-zag-o will
lead to the fact that one alkyl chain would be too stretched while
the other chain is highly relaxed in the structure, which would be
energetically unfavorable than that of 2*zig-zag-s and 2*
straight-s. Therefore, it can be concluded that the arrangements
of 2*zig-zag-s and 2* straight-s are preferential in the synthesis
of SSMZ. These results are in accordance with the experimental
results that the surfactants can disrupt the growth of zeolite MFI
along a- and b-axis simultaneously, which provided the evidence
for the geometric matching between the templates and the
zeolites.
Based on the surfactant geometry and the zeolite structure, it
can be speculated that the mesostructure is formed by
cylindrically assembly units of the surfactant, and the structural
matching between the mesostructure and zeolite framework is
the key factor for the synthesis of SSMZ. The two naphthol
groups of the axis chiral OSDAs are connected by one C-C
single chain, which rotate easily and form a scissor-like spatial
conformation.[9b, 9c] The scissor-like binaphthyl unit with a non-
coplanar molecular conformation can pack into helical cylindrical
assembled micelles in a cooperative manner with the inorganic
species.[10, 15]. Therefore, we propose a geometrical matching
model between the arrangement of the assembly units and the
zeolite structure with the cooperative self-assembly process
(Figure 5b-d). Presumably, each straight or sinusoidal channel
of five pentasil layers contains four quaternary groups belonging
two opposite stretching hydrophilic chains of one template
molecule (Figure 5b, 5c). Four quaternary groups of the B10-3N
stay inside the channel of the framework directing the zeolite
growth and the other two locate in the micelle stabilizing the
(010) and (100) surface, which is in accordance with the N/Si
ratio (0.051) obtained by TGA. To illustrate the assembly
behavior of the templates, the structural model of the
arrangement of the binaphthyl groups along a tilted (101) plane
is presented in Figure 5d. Naphthol groups of the template
molecules aligned parallel to [001] axis, while triquaternary
groups stay in the corresponding channels. The distance
between adjacent two naphthol groups is ~3.3 Å, which is within
the range of the theoretical distance of π-π stacking.[16]
Therefore, the structural matching of the assembly unit and the
sponge-like mesoporous structures in MFI can be established. It
is speculated that π-π stacking exists between the adjacent
naphthol groups and those in neighboring pitch, which effectively
stabilizes the helical structure. By increasing the length of alkyl
chain between triquaternary ammonium head group and
binaphthyl group to 11 and 12, mesoporous zeolites with
nanorods stacking were formed, which may go through a
surface-to-core growth process as reported in other nanorods
zeolites formation.[17] The template molecules with long chain
may have difficulties in keeping its helical assembled micelle
aggregation, which results in relatively loosely distribution on the
surface of the primary zeolite crystal and direct the MFI zeolites
growth from surface to core. The different assembly behaviors of
binaphthyl centred surfactants may derive from the structural
features of the axial chiral molecule. The helical assembly of the
binaphthyl group can only be formed with certain attached
groups, such as azobenzene[9b], poly(aryl carbonates)[10a] and
stilbenes[10b]
.
While other attached groups, such as
anthracene[15] and tetrathiafulvalene[13] do not exhibit helically
self-assembly properties. Taking the structural feature of the
binaphthyl group into consideration, we speculate that the
suitable length of alkyl chain is the key point to direct the SSMZ,
which tend to form helically assembly units and geometrically
match the MFI zeolite framework.
Catalytic properties of SSMZ zeolites. Firstly, to investigate
the catalytic performance for the reactants with relatively small
molecular sizes, the SSMZ zeolite was compared with
conventional ZSM-5 in the methanol to propylene (MTP)
reaction (Figure 6a). The Si/Al ratio of SSMZ zeolite was
adjusted to 104, which is generally considered to be suitable for
MTP. The MTP reaction includes an initial step from methanol
dehydration to dimethyl ether (DME) and then the subsequent
step of producing ethylene, propylene and other small
hydrocarbons such as alkanes and aromatics.[18] The SSMZ
zeolite showed a greatly prolonged lifetime than the ZSM-5 bulk
crystals, which should be attributed to the mesoporosity in the
former catalyst. The selectivity of these intermediate products is
intensively related to the catalyst structure, the acidity and the
operational conditions. SSMZ shows a high selectivity for
propylene (44.6%) in comparison with that of the conventional
ZSM-5 (see Figure S17 for textural properties) and a high
propylene to ethylene (P/E) ratio (see Table S3 for the product
distribution of methanol to propene reaction), which were
predominately attributed to its optimal combination of acidity and
mesopores.
Secondly, the catalytic activities of SSMZ and conventional
bulk ZSM-5 were tested for the acetalization of benzaldehyde
with pentaerythritol to compare their performance in converting
bulky molecules (Figure 6b, and Figure S18 for textural
properties). In this case, the diffusion of the large molecules,
such as the diacetal product, determine the reaction rate. The
SSMZ showed distinctly increased catalytic activities compared
with those of the conventional ZSM-5 (Figure 6b, and Figure
S18 and S19). This result is due to the existence of the
abundant mesopores and larger specific surface area of the
SSMZ.
Thirdly, the gas-phase cracking of 1,3,5-triisopropylbenzene
(TIPB) was also carried out to investigate the possibility of
mesoporous ZSM-5 zeolites for processing bulky molecules
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Figure 6. Catalytic efficiency of SSMZ and conventional ZSM-5. Conversion and selectivity for methanol to propene and the ratio of propylene to ethylene (P/E
ratio) (a). Conversion of the acetalization of benzaldehyde with pentaerythritol (b). Conversion and selectivity of cracking of 1,3,5-triisopropylbenzene (c).
(Figure 6c, Figure S18 and S20), as this reaction occurs
predominately on the external surface of ZSM-5 as TIPB is too
large to enter its micropores. The main products were
diisopropylbenzene (DIPB) isomers, isopropylbenzene (IPB) and
benzene (BZ), formed by successive cracking. The catalytic
activity for TIPB cracking reflects the Brønsted acidity solely
presenting on the external surfaces of the ZSM-5. The SSMZ
shows a high catalytic activity compared to the bulk conventional
ZSM-5, which clearly demonstrates the catalytic advantage of
SSMZ with hierarchically mesoporous structure to bulky
molecules.[19]
chain. Various mesoporous zeolite with different structures can
be tuned by different length of alkyl chain between triquaternary
ammonium head groups. It has been demonstrated that different
self-assembly behavior of the binaphthyl-type templates play an
important role in the structural formation, which direct the
nanosponge and nanorods constructed mesoporous zeolites,
respectively. This finding would provide new strategies to
synthesize highly ordered mesoporous zeolites by using
delicately designed templates.
The SSMZ ZSM-5 with a larger surface area and similar
micropore volume to conventional ZSM-5 exhibits excellent
catalytic activity in both small and relatively large molecule
reaction. The improved performance of SSMZ would be
attributed to the facile diffusion through the mesopores in SSMZ,
strong acidities of their crystalline zeolitic frameworks, and high
concentrations of surface acid sites that are accessible to
organic substrates particularly with bulky molecular dimensions.
Experimental Section
Synthesis of binaphthyl core surfactants: The surfactants used in this
work were synthesised according to the methods reported in previous
report[20], their structures are listed in Table 1. In a typical synthesis, (S)-
(-)-1,1'-bi-2,2'-naphthol (1.43 g, 5 mmol) and 1,10-dibromodecane (30 g,
100 mmol) were mixed in 100 mL of acetone containing 12.42 g (90
mmol) of K2CO3 of and refluxed under N2 atmosphere for 24 h. The
product was filtered to remove K2CO3 and vapored to obtain a light
yellow viscous crude product. The crude product was chromatographed
on a silica gel column (petroleum ether/ethyl acetate, 100:1). The pure
intermediate B10 was obtained as colourless oil (68% yield). Then 2.6 g
(3.6 mmol) of B10 and 12.4 g (72 mmol) of N,N,N',N'-tetramethyl-1,6-
hexanediamine were mixed in 150 mL of acetonitrile and refluxed at
83 ℃ for 24 h. After cooling to room temperature, the intermediate B10-1
was filtered, washed with diethyl ether, and dried in a vacuum oven
Conclusions
We have synthesized a series of SSMZ and nanorods
constructed mesoporous zeolites by the templates composed of
an axis chirality hydrophobic centre and two hydrophilic alkyl
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overnight (3.4 g, 91%). Then 3.4 g (3.2 mmol) of B10-1 and 16.6 g (68
mmol) of 1,6-dibromohexane were mixed in 150 mL of acetonitrile and
refluxed at 83 ℃ for 24 h. After cooling to room temperature, the
intermediate B10-2 was filtered, washed with diethyl ether, and dried in a
vacuum oven overnight (4.3 g, 98%). Then 4.3 g (3.1 mmol) of B10-2 and
1.2 g (9.3 mmol) of N,N-dimethylhexylamine were mixed in 150 mL of
acetonitrile and refluxed at 83 ℃ for 24 h. After cooling to room
temperature, the product B10-3N was filtered, washed with diethyl ether,
and dried in a vacuum oven overnight (5.5 g, 98%). The expected
structures of the final organic products were confirmed by 1H NMR. The
other surfactants were synthesised in similar process but with different
raw materials.
recorded on a PerkinElmer Lambda 20 UV/vis spectrometer, whereas the
UV/vis absorption spectrum of the filter was measured with a Shimadzu
UV-2450 spectrophotometer. The absorbance spectra were obtained
from the reflectance spectra through Kubelka-Munk transformation. The
1H NMR spectra were recorded on an AVANCE III 500 (500 MHz)
spectrometer with chemical shifts reported in parts per million relative to
the residual deuterated solvent and the internal standard
tetramethylsilane (TMS)
Catalytic reactions
Methanol to propene (MTP) reaction: MTP reactions were performed in
a fixed-bed quartz reactor over the catalyst at 450 ℃ ,atmospheric
pressure, and a WHSV of methanol of 8 h-1. All samples were calcined
under nitrogen flow at 550 ℃ for 1 h before commencing the MTP
reaction. The reactor temperature was decreased to 450 ℃, where
methanol was fed to the reactor together with carrier gas. The effluent
products from the reactor were maintained at 140 ℃ and analysed by an
online gas chromatograph (GC) equipped with a Plot-Q capillary column
and a FID detector. In the reaction, dimethyl ether (DME) was considered
as reactant in the calculation.
Synthesis of mesoporous and conventional ZSM-5: The mesoporous
zeolites were prepared by hydrothermal synthesis using different Bn-mN as
templates or OSDAs. In a typical run, the mesoporous zeolites were
synthesised at an original gel composition of 1 B10-3N: 50 SiO2: 12.4
Na2O: 0.5 Al2O3: 200 EtOH: 4000 H2O. Tetraethyl orthosilicate (TEOS)
was used as a silica source, while ethanol (EtOH), with a stoichiometric
amount of four times, came from TEOS hydrolysis. To be specific, 0.18 g
surfactant, 0.0082 g sodium aluminate and 0.09 g sodium hydroxide
were added to 7.2 g deionized water to form a homogeneous gel. When
the gel was well mixed after stirring for 30 min, 1.04 g TEOS was quickly
added to the gel. Then the gel was stirred for another 5 h in water bath
pot at 60 ℃. Next, the mixture was transferred to a Teflon-lined stainless
steel. Then, the autoclave was tumbled at 413 K and 40 rpm to carry out
the crystallization for 4 days. Finally, the zeolite products were filtered,
washed with distilled water, and dried at 373 K overnight. Other samples
were synthesised accordingly, with different surfactants and molar ratio.
Before examing catalytic properties, the zeolites were ion-exchanged
with 2M NH4Cl for 2 h at 353 K and then converted into proton form by
calcination in air at 823 K for 6 h. The whole ion-exchanged process
repeated twice to obtain the final product for catalytic reaction.
Acetalization of benzaldehyde with pentaerythritol：Acetalization of
benzaldehyde with pentaerythritol reactions were carried out under slurry
phase reaction conditions using a conventional oil bath attached with
condenser. A typical reaction mixture in a 25 mL round bottle flask
contained benzaldehyde (20 mmol), pentaerythritol (10 mmol), toluene (8
mmol) and a freshly activated catalyst (50 mg). Activation of the catalyst
was done by calcination at 550 ℃ for 5 h. The reaction was carried out at
120 ℃ for 5 h. After cooling to room temperature, the flask was washed
several times by acetone. All the mixture was collected and separated by
centrifugation. The liquid mixture was analysed on a Shimadzu GC-2014
gas chromatograph using an Rtx®-Wax capillary column with FID
detector. The conversion was calculated based on benzaldehyde using
the internal standard method, where toluene was applied as the internal
standard. The formation of diacetals of pentaerythritol was confirmed by
the GC-MS.
Conventional ZSM-5 with bulk crystals was synthesised as control
through colloidal seed assisting method using tetraethylammonium
bromide as OSDA as in the previous reports.[21]
Characterizations: The synthesised surfactants were analysed by 1H
nuclear magnetic resonance (NMR) and UV/visible (UV/vis)
spectroscopies, whereas X-ray diffraction, scanning electron microscopy
(SEM), high-resolution transmission electron microscopy (HRTEM), and
N2 adsorption/desorption analysis were applied to the characterization of
synthesised zeolites. Powder XRD patterns were recorded on a Rigaku
X-ray diffractometer (D/MAX-2200/PC) equipped with Cu Kα radiation (40
kV, 30 mA, λ = 1.5418 Å) at a rate 1°/min in a 2θ range of 0.8-5° and 5-
40°. Observation via scanning electron microscopy (SEM) was performed
with a JEOL JSM-7800 Prime microscope operating at 0.5 kV with a
stage bias of -5 kV, using upper electron detector with 4 mm working
distance. Observation via transmission electron microscopy (TEM) was
performed with a JEOL JEM-2100 microscope operated at 200 kV (Cs
1.0 mm, point resolution of 2.3 Å). Images were recorded with a
TENGRA CCD camera (2304 × 2304 pixels with a 2:1 fiber-optical taper
and an effective pixel size of 18 μm2) at 30000-500000 magnification
under low-dose conditions. The specific surface area, pore volume, and
pore size distribution were analysed with N2 adsorption/desorption
isotherms, which were measured at 77 K using an ASAP 2010 M+C
analyser. Prior to the adsorption measurements, all the samples were
degassed under a vacuum for 3 h at 423 K. The surface area was
calculated from the desorption branch in the P/P0 range from 0.05 to
0.30 using the Brunauer-Emmett-Teller (BET) equation. The pore size
distributions were calculated from the entire adsorption or desorption
branch according to the Barrett-Joyner-Halenda (BJH) algorithm for
approximation. The diffuse-reflectance UV/vis (DRUV) spectra were
Cracking of 1,3,5-triisopropylbenzene：The cracking reaction of 1,3,5-
triiaopropylbenzene (TIPB) was conducted using a continuous system in
fixed-bed quartz reactor (i.d.8 mm) under atmospheric pressure with
nitrogen as a carrier gas. The catalyst was first activated at 823 K in a N2
flow rate of 30 mL min-1 for 1 h, and then decreased to the reaction
temperature (573 K), where the TIPB was fed into the reactor at a rate of
1.7 mL h-1. The typical catalyst loading and carrier gas flow were 0.2 g
and 30 mL min-1, respectively, which corresponded to a weight hourly
space velocity (WHSV) of 7.18 h-1 for TIPB. The liquid product was
collected with an ice-water trap and then analysed on A Shimadzu 14B
gas chromatograph equipped with FFAP capillary column and FID
detector.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21533002, 21571128) and the National
Excellent Doctoral Dissertation of PR China (201454).
Keywords: MFI Zeolite, hierarchical structure, sponge-like
structure, single-crystal.
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Entry for the Table of Contents (Please choose one layout)
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Single-crystalline sponge-like MFI
mesoporous zeolites (SSMZs) have
been synthesized by using bolaform
surfactants with axial chiral
Yunjuan Zhang,[a] Peng Luo,[b] Hao
Xu,[b] Lu Han, * [a, c] Peng Wu, * [b] Huai
Sun, * [a] and Shunai Che *[a,c]
Page No. – Page No.
binaphthyl core in the hydrophobic
tail and triquaternary ammonium
head groups as bifunctional organic
structure-directing agents (OSDAs).
Hierarchical MFI Zeolites with A
Single-Crystalline Sponge-like
Mesostructure
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