Fabrication of a core-shell MFI?TON material and its enhanced catalytic performance for toluene alkylation

Article abstract of DOI:10.1039/c9cy02133g

Core-shell MFI?TON composites were designed and synthesized as a highly shape-selective catalyst for toluene alkylation with methanol by passivating the nonselective acid sites and tuning the diffusion behavior. The synthesis parameters were comprehensively investigated, indicating the importance of the Si/Al ratio compatibility of the ZSM-5 and ZSM-22 components on the formation of a core-shell structure. The synthesis process was systemically traced, which allowed the formulation of a crystallization mechanism involving the oriented crystal growth and selective fusion steps during the secondary crystallization. As a result, the MFI zeolites as the core were fully covered by the TON zeolites as the shell, yielding spherical morphology. When applied to toluene alkylation with methanol, the core-shell MFI?TON composite exhibited significantly improved para-xylene selectivity in comparison with the original, unattached, and physically mixed catalysts. The enhanced catalytic behaviors of the core-shell MFI?TON composite could be ascribed to the effective suppression of para-xylene isomerization as a result of the passivated acid sites on the external surface and the improved diffusion time and distance for the intermediates inside the channels due to the unique structure. The synthesis method for the MFI?TON composite described herein may provide a generic platform for the design of core-shell zeolites with potentially broader applicability to other porous materials with advanced applications.

Full text of DOI:10.1039/c9cy02133g

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Fabrication of a core–shell MFI@TON material and
its enhanced catalytic performance for toluene
Cite this: DOI: 10.1039/c9cy02133g
alkylation†
ab
Suyao Liu,
Huaike Zhang,ab Huimin Chen,ac Zhiqiang Chen,ac Liwei Zhang,ac
ab
ab
ab
Jie Ren,
Xiaodong Wen,
Yong Yangab and Yong-Wang Li
*
*
Core–shell MFI@TON composites were designed and synthesized as a highly shape-selective catalyst for
toluene alkylation with methanol by passivating the nonselective acid sites and tuning the diffusion
behavior. The synthesis parameters were comprehensively investigated, indicating the importance of the
Si/Al ratio compatibility of the ZSM-5 and ZSM-22 components on the formation of a core–shell structure.
The synthesis process was systemically traced, which allowed the formulation of a crystallization
mechanism involving the oriented crystal growth and selective fusion steps during the secondary
crystallization. As a result, the MFI zeolites as the core were fully covered by the TON zeolites as the shell,
yielding spherical morphology. When applied to toluene alkylation with methanol, the core–shell MFI@TON
composite exhibited significantly improved para-xylene selectivity in comparison with the original,
unattached, and physically mixed catalysts. The enhanced catalytic behaviors of the core–shell MFI@TON
composite could be ascribed to the effective suppression of para-xylene isomerization as a result of the
passivated acid sites on the external surface and the improved diffusion time and distance for the
intermediates inside the channels due to the unique structure. The synthesis method for the MFI@TON
composite described herein may provide a generic platform for the design of core–shell zeolites with
potentially broader applicability to other porous materials with advanced applications.
Received 22nd October 2019,
Accepted 15th January 2020
DOI: 10.1039/c9cy02133g
rsc.li/catalysis
of the acid sites is considered to be a very important factor
affecting the catalytic performance.9 A typical acid-catalyzed
1. Introduction
Microporous zeolites with high surface area and well-defined
pore dimensions have been widely used in the field of shape-
selective catalysis, adsorption, and sensor technologies.1,2 In
particular, the unique pore structure and strong acidity of
zeolites lead to a wide range of applications as heterogeneous
catalysts in petrochemistry and renewable energy areas.3,4
Cations (H+) acting as the Brønsted acid sites in
aluminosilicate zeolites are located at the oxygen atoms
between the bridge Si and Al atoms in the crystalline
framework, forming catalytically active sites.5,6 The acid sites
located within the confined channels of zeolites ensure their
prefect shape-selectivity, while acid sites on the external
surface of zeolites always lead to a reduction in the product
selectivity and confinement effects.7,8 Therefore, the location
reaction is toluene alkylation with methanol to obtain
p-xylene, which is an important intermediate to produce
terephthalic acid and downstream polyesters.10,11
The selective production of p-xylene via the alkylation of
toluene with methanol using various zeolite catalysts has been
intensively investigated.12,13 Among them, TON-type ZSM-22
zeolites have been effectively recognized as a shape-selective
catalyst for toluene alkylation to p-xylene.14,15 The high para-
selectivity of ZSM-22 zeolites in this reaction mainly results
from the one-dimensional 10-membered ring channels (0.45 ×
0.55 nm) that run along the longest dimension of the crystals
(the crystallographic c-axis direction). However, the
distributions of acid sites for ZSM-22 zeolites are nonuniform
due to the fact that the crystals have aluminum concentrated
on the external surfaces.16 Hence, many modification methods
have been developed to eliminate the acid sites on the external
surface.17–19 However, it is often difficult to control such
technologies that require complicated operations and post-
treatments, invariably leading to decreases in framework
stability and shape-selectivity. Moreover, diffusion plays an
important role in the transport of xylenes from the channels
due to the fact that the diffusion coefficient of p-xylene is much
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, PR China. E-mail: renjie@sxicc.ac.cn,
ywl@sxicc.ac.cn
b National Energy Research Centre for Clean Fuels, Synfuels China Co., Ltd.,
Beijing 101400, P. R. China
c University of Chinese Academy of Sciences, Beijing 100049, P. R. China
† Electronic supplementary information (ESI) available: XRD, SEM, TEM, N2-
adsorption–desorption, NH3-TPD, Py-IR. See DOI: 10.1039/c9cy02133g
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higher than those of o-xylene and m-xylene.20,21 Therefore, if
the diffusion resistance of the formed p-xylene inside the ZSM-
22 zeolite improves, the isomerization of p-xylene into these
two isomers can be inhibited, consequently leading to
enhanced para-xylene selectivity.
2.2 Synthesis
2.2.1 Synthesis of core ZSM-5 zeolites. ZSM-5 zeolites
(about 10 μm) with different Si/Al ratios used as the core of
the composites were synthesized via a laboratory recipe by
adding a solution of Al2IJSO4)3, NaOH, and n-butylamine (BTA)
accompanied with deionized water to the solution of sodium
silicate under stirring. The resultant homogeneous gel with a
molar composition of 12Na2O : 1Al2O3 : (80/250)SiO2 : 100BTA :
1400H2O was transferred into a stainless steel autoclave and
subjected to hydrothermal treatment at 180 °C for 48 h in an
oven under static conditions and autogenous pressure. After
this hydrothermal treatment, the solid product was recovered
by filtration, washed, dried at 120 °C overnight, and calcined at
550 °C for 10 h to remove the organic template. The dried and
calcined MFI zeolites were named SDA-MFI and Na-MFI,
respectively.
2.2.2 Synthesis of ZSM-22 nanorods. In the present work,
1,6-diaminohexane (DAH) was used as the organic template for
the synthesis of ZSM-22 zeolites. ZSM-22 zeolite nanorods were
prepared via the hydrothermal treatment of aluminosilicate
solution with the following chemical composition: SiO2 :
0.12K2O : 0.0067Al2O3 : 0.3DAH : 40H2O. The synthesis processes
were performed at 160 °C for 48 h in autoclaves under stirring
at autogenous pressures. The solid products were recovered by
filtration, extensively washed with deionized water, and dried
at 120 °C. Finally, the organic template in the zeolites was
removed by high-temperature calcination in air at 550 °C for
10 h. The dried and calcined ZSM-22 zeolites were named as
SDA-TON and K-TON, respectively.
At this point, if the one-dimensional straight channels of the
ZSM-22 zeolite can be perpendicular to a certain material as the
core leading to the formation of a spherical morphology with
largely exposed pore mouths, increasing the diffusion
resistance and passivating the acidity on the external surface of
ZSM-22 zeolites can be simultaneously resolved. The
hydrothermal synthesis of core–shell zeolites has been
demonstrated for a wide range of inorganic materials.22 Many
core–shell zeolite–zeolite composites possessing cores and
shells of different structural types have already been reported,
such as MOR@MFI,23 BEA@MFI,24 FAU/BEA,25 and
MFI@BEA.26 The shell layer can always be formed by the
secondary growth of the preliminarily adsorbed seeds on the
core crystals. During the adsorption step, the organic molecules
and surfactants, such as polyIJdiallyl dimethylammonium
chloride), are used. However, the high cost of the polycation
agent and rigorous procedures may limit the large-scale
applications and commercial development of such zeolite
composites. Moreover, the key factor in the fabrication of core–
shell zeolite composites is that the zeolites are metastable
materials that can be easily transformed into other porous or
nonporous materials under hydrothermal conditions.27 It is
well known that both ZSM-22 and ZSM-5 zeolites have a 10-
membered ring, and ZSM-5 zeolites are the most commonly
occurring impurities during the crystallization procedure of
ZSM-22 zeolites.28,29 Combining ZSM-5 and ZSM-22 zeolites as
a core–shell material not only meets the aforementioned
requirements and circumvents the side-effects of zeolite surface
modification, but it also provides a deeper understanding into
the crystallization mechanism of ZSM-22 zeolites.
These Na-MFI and K-TON zeolites were converted into
their protonic forms by ion exchange with NH4NO3, drying,
and calcination at 550 °C for 4 h. These post-treatment steps
were repeated three times. The corresponding protonic
zeolites were denoted as MFI and TON.
2.2.3 Synthesis of the core–shell materials. The schematic
diagram for the preparation process of the seed is shown in
Fig. 1. Firstly, the big MFI crystal was hydrothermally treated
in an aqueous solution of DAH (0.2 mol L−1) for 12 h at 50
°C. This treated MFI was filtered and redispersed in distilled
water and then a precalculated amount of SDA-TON crystals
was adsorbed. Then, the mixture was subjected to ultrasonic
treatment for 1 h and the upper suspension was removed.
Finally, the drying and calcination of the pretreated seed at
550 °C for 4 h yielded seeds with TON nanorods firmly fixed
on the surface of the ZSM-5 zeolites. The secondary growth of
the preadsorbed seed was performed at 160 °C with a
solution having the molar composition SiO2 : 0.12K2O :
xAl2O3 : 0.2DAH : 40H2O, where x = 0–0.014. The resulting gel
Herein, the present work focuses on the formation of ZSM-
5@ZSM-22 core–shell composites comprising ZSM-5 zeolites
as the core and surface-inert ZSM-22 zeolite as the shell.
Several synthesis parameters were investigated to understand
the key factors affecting the crystallization process of the
composite. The formation mechanism of this material was
also systemically investigated. The core–shell material shows
significantly improved para-xylene selectivity in toluene
alkylation with methanol by suppressing the acid sites on the
external surface and improving the diffusion resistance of the
product molecules. This investigation may not only provide a
direct and efficient route for the crystallization of core–shell
zeolite composites but also prove the importance of acid site
distribution and diffusion behavior on improving the
para-xylene selectivity for alkylation.
2. Experimental
2.1 Chemicals and reagents
The detailed descriptions of the chemicals and reagents are
available in the ESI.†
Fig. 1 Schematic diagram of the preparation steps of the seed.
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was aged for 8 h at room temperature before performing the 3. Results and discussion
crystallization procedure in
a PTFE-lined stainless steel
3.1 Synthesis of core–shell materials
autoclave. After hydrothermal synthesis, the solid was washed
with distilled water to neutral and dried at 120 °C for 12 h.
The products were calcined at 550 °C for 8 h in air to remove
the organic template. With this method, several typical
Attempts to synthesize core–shell composites without using
TON seeds have been unsuccessful in cases where only
discrete MFI particles have been present. Therefore, TON
seeds are necessary in order to obtain the core–shell
structure. Meanwhile, the adsorption step of the TON
nanorods on MFI crystals is the key parameter for the growth
of TON nanorods and the full coverage of the MFI material.
Therefore, the present study focuses on the effect of chemical
compatibility and matching the crystallization field for the
core–shell MFI@TON composite.
3.1.1 Influence of Si/Al ratios of the crystallization gel.
Suppressing the growth of the core material and fabricating
the secondary crystallization of the shell zeolite were
considered to be important factors to prepare a core–shell
composite. After varying the different parameters of the
crystallization system, it was found that the chemical
compatibility and matching degree of the Si/Al ratios of the
initial gel with the seed were significant. Therefore, the
preadsorbed seeds containing TON and MFI zeolites with
different Si/Al ratios were prepared. Then, the prepared seed
was added into the gels at different initial Si/Al ratios for the
secondary synthesis process.
The obvious differences in the size and morphology of the
MFI crystals core and TON nanorods shell facilitate
distinguishing between these materials. Firstly, the seed
containing the TON zeolite (Si/Al ratio: 71.5) and MFI zeolite
(Si/Al ratio: 34) was used to prepare the composite. When the
Si/Al ratio of the initial gel composition ranged from 35 to
70, the composite material with the aggregated MFI zeolite
was obtained, as shown in Fig. 2a. Meanwhile, the diffraction
peaks assigned to the ZSM-22 zeolites were much weaker
than that for ZSM-5 zeolites, as shown in Fig. S1,† suggesting
that secondary crystallization in such synthesis conditions is
preferable for fabricating the MFI zeolite core. Closeup views
of the synthesized sample (Fig. 2b) provide solid evidence of
the crystallization of the MFI core sample. However, when
the Si/Al molar ratio of the initial gel is above 71.5, the
growth of the core is distinctly suppressed, leading to the
formation of spherical morphology with a large amount of
needle-like ZSM-22 zeolites, as shown in Fig. 2c.
samples
were
obtained
after
different
secondary
crystallization times to elucidate the growth process of the
core–shell structure. The final composite was finally
converted to its protonic form through the NH3-exchange
step, and it was denoted as MFI@TON.
The reference sample, termed as TON/MFI, was prepared
by using the same components as those used for preparing
the MFI@TON composite. The secondary crystallization of
the TON sample was the same as that used for composites
without the addition of MFI zeolites. After the crystallization
process, the post-treatment steps were the same as those for
the protonic zeolites. The resulting TON crystal (denoted as
TON-SG) was mechanically mixed with the MFI zeolites based
on the phase compositions of MFI and TON zeolites as
calculated from the XRD patterns of the composite.30
2.3 Preparation of catalysts
To obtain the catalysts for use in the catalytic tests for
toluene alkylation with methanol, the TON, MFI, TON/MFI,
TON-SG, and MFI@TON zeolites were extruded, crushed, and
sieved to a particle size of 10–20 mesh.
2.4 Characterizations
Detailed characterizations are available in the ESI.†
2.5 Catalytic test
The catalytic performance of toluene alkylation with
methanol over various catalysts was carried out in
a
continuous-flow fixed-bed reactor under atmospheric
pressure. The sieved catalyst (2.0 g) was packed in the
isothermal zone of the reactor tube. Before the catalytic test,
the catalyst was activated in situ under a N2 flow at 500 °C for
1 h. After decreasing the reaction temperature to 420 °C, a
mixture of toluene and methanol at a molar ratio of 4 : 1 as
the reactant was pumped into the reactor at a WHSV value of
2.0 h−1 via a double-plunger pump. All the products were
separated and the gas products were analyzed online by
means of a gas chromatograph (Agilent 6890 A) equipped
with a flame ionization detector and a capillary column
(Al2O3, 30 m × 0.32 mm × 0.25 μm). The liquid products were
collected with a cold trap and then kept in sealed vials for
posterior analysis via offline gas chromatography (Agilent
7890A equipped with a flame ionization detector and a
capillary column, DB-PONA, 50 m × 0.20 mm × 0.55 μm). In
all these experiments, the carbon balance was maintained
above 95%. Herein, toluene conversion was determined
based on the weight of the converted toluene. Selectivity
toward xylene was defined as the percentage of xylene in all
the products.
Furthermore,
a similar experiment was performed to
investigate the nature of this result. In this experiment, a
seed with TON zeolites (Si/Al = 71.5) and MFI zeolites (Si/Al =
121.5) was used. When the Si/Al ratio of the initial
composition was below 70, an amorphous material was
obtained. As the Si/Al ratio was improved to 70–120,
aggregated morphology and prominent diffraction peaks in
the ZSM-5 crystallites reappear, as shown in Fig. 2d and S1,†
respectively. As the Si/Al ratio increased above 120, the
desired composite with the secondary-crystallized TON shell
was obtained. Therefore, the growth of the MFI zeolite core
could be effectively inhibited when the initial Si/Al ratio of
the synthesis gel was higher than that of both the
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MFI@TON composite indicated no evidence of the formation
of amorphous silica, and the crystallinity reached 92% after
32 h. As the synthesis temperature further increased to 180
°C, the MFI@TON composite with high crystallinity (95%)
was obtained after 24 h. The formation of a zeolite with poor
crystallinity (Fig. S2† and 3a) and the amorphous phase at
lower temperatures indicate that the solution system is only
slightly favorable for the interactions between the organic
template and silica sol.31 Furthermore, the structure-
directing formation of the composite requires higher
temperatures. Nevertheless, as shown in the magnified SEM
images (Fig. S3†) of the composites with similar crystallinity,
the crystal of the TON shell prepared at 180 °C is much
thinner than the sample synthesized at 140 °C, suggesting
that an increase in crystallization time yields larger crystals.32
Fig. 2 Scanning electron microscopy (SEM) images of the composite
samples prepared under different conditions. Synthesis conditions: (a)
Si/Al ratios of TON and MFI zeolites in the preadsorbed seed are 71.5
and 34, respectively; Si/Al ratio of the initial gel is 70. (b) Closeup view
of Fig. 2a. (c) Si/Al ratios of TON and MFI zeolites in the preadsorbed
seed are 71.5 and 34, respectively; Si/Al ratio of the initial gel is 75. (d)
Si/Al ratios of TON and MFI zeolites in the preadsorbed seed are 71.5
and 121.5, respectively; Si/Al ratio of the initial gel is 120.
3.2 Crystallization procedure of core–shell materials
The seed comprising TON (Si/Al ratio: 71.5) and MFI (Si/Al
ratio: 34) zeolites was used to investigate the crystallization
procedure of core–shell MFI@TON materials. To obtain the
MFI@TON composite with a passivated surface, the molar
composition of the initial synthesis mixture was SiO2 :
0.12K2O : 0.2DAH : 40H2O.
3.2.1 XRD and SEM. Fig. 3b shows the XRD patterns of the
MFI@TON composite after different crystallization times at
160 °C. During the first 18 h, broad peaks between 10° and
40° and weak diffraction peaks attributable to MFI and TON
phases suggest that the product is still largely amorphous.
XRD intensities increase over the next few hours (24 h). The
clear signals at 8.10°, 20.28°, 24.14°, and 25.66° appear,
which are characteristic of the TON-type framework.33,34 After
crystallization for 32 h, the XRD patterns of the final
products reveal the intense diffraction peaks of the TON
phase together with the characteristic peaks of the MFI
phase. Moreover, Fig. S2† shows the dependence of
crystallinity on the crystallization time. This curve indicates
that the crystallization of the TON nanorods shell mainly
occurs within 18–32 h. The introduction period ends at 18 h
and the TON zeolite gets fully crystallized after 32 h.
components of the seed. Therefore, it can be concluded that
the Si/Al ratios compatibility of the seed and initial gel are
the key parameters for the formation of MFI@TON core–shell
materials.
3.1.2 Influence of crystallization temperature. To
investigate the effect of synthesis temperature on the
crystallization of the core–shell materials, the synthesis was
carried out at different temperatures. The seed containing
the TON zeolite (Si/Al ratio: 71.5) and MFI zeolite (Si/Al ratio:
34) was used in this case. The secondary crystallization is
performed without the addition of Al. The XRD patterns are
shown in Fig. 3, and the crystallization curves at different
conditions are shown in Fig. S2 in the ESI.† At the lowest
crystallization temperature (140 °C), the mixture of
MFI@TON material and amorphous silica was obtained after
36 h, according to the relatively low intensity of the XRD
patterns shown in Fig. 3a. With a further increase in the
crystallization time to 60 h, the MFI@TON composite with
high crystallinity could be observed, as shown in Fig. S2.†
When the synthesis temperature improved to 160 °C, the
Fig.
4 and S4† show the SEM micrographs of the
MFI@TON composites after different crystallization stages at
various magnifications. As shown in Fig. 4a and b, a large
number of amorphous materials appear on the surface of the
seed. After crystallization for 24 h, the MFI crystals are fully
covered with elongated TON crystals, as shown in Fig. 4d and
S4a,† indicating that at this crystallization stage, the TON
nanorods shell grow along the needle axis, namely, the c-axis-
oriented growth is dominant during this crystallization
process.35 After synthesis for 32 h, the amorphous material is
totally consumed and all the MFI crystals are fully covered
with TON crystallites (width: 400–600 nm; length: 5–6 μm)
(Fig. 4e and f). These observations suggest that the TON
crystals bunched together perpendicular to the needle axis at
this stage. Combined with the closer inspection of the TEM
image (Fig. S5†), it is evident that the thickened crystals of
Fig. 3 XRD patterns of core–shell MFI@TON composites prepared at
140 (a), 160 (b), and 180 (c) °C.
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crystallization mixture is above 11, and the solubility of
amorphous silica is high. Moreover, the high Si content in
gel further increases the solubility. Therefore, it is not
surprising to observe the bands at 460 and 780 cm−1 in the
liquid phase, which can be assigned to the presence of
silicate species.37,38 The band at 620 cm−1 can be attributed
to the Al–O symmetric stretching mode of the AlIJOH)4 species
in the solution,38 which may originate from the partial
dissolution of the seed zeolites. The spectrum of the liquid
phase undergoes significant changes after crystallization for
24 h. The band at 620 cm−1 decreases, suggesting that a part
of the AlIJOH)4 species in the solution is incorporated into the
solid phase and then polymerized with the silicate species.
However, the band of silicate species remains unchanged
throughout the crystallization process, indicating that these
dissolved Si species are not incorporated into the solid phase.
No Raman bands assigned to the aluminosilicate species are
detected in the liquid phase.
Fig. 5b shows the Raman spectra of the solid phase of the
MFI@TON composite at various crystallization times. The
Raman spectrum of the initial precursor is characterized by a
broad band at 460 cm−1, while the bands assigned to the
seeds are not observed in the spectra due to low content. The
band at 460 cm−1 comes from amorphous or vitreous silica
and has been assigned to the symmetric vibration of the Si–
O–Si bond (νsIJSi–O–Si)) of the five-membered silicate rings.39
The band at 847 cm−1 observed in the Raman spectrum after
heating for 18 h can be attributed to DAH cations. These
cations cannot be extensively washed even after several
centrifugation treatments, indicating the presence of strong
interactions between the DAH cations with the framework.
With an increase in the crystallization time, the enhanced
intensity of the band at 460 cm−1 corresponds to an increased
amount of five-membered silicate rings in the amorphous
gel. Meanwhile, this broad band (at 460 cm−1) gradually
becomes sharper and shifts to 446 cm−1 after heating for 20
h. This result suggests that the siliceous species in the gel
mixture connect with each other and further form a TON-like
structure. The appearance of the band at 800 cm−1, which is
assigned to the symmetric stretching mode of T–O,40
indicates that the intermediate species containing five-
membered rings are obtained during the synthesis
procedure. By increasing the crystallization time from 20 to
24 h, the band at 446 cm−1 becomes much sharper, while the
band of the amorphous gel at 460 cm−1 rapidly decreases,
indicating the formation of a large amount of TON crystals,
which is in agreement with the XRD and SEM results. The
Raman band at 407 cm−1 associated with five-membered
rings41 appears after heating for 20 h, which becomes
narrower and stronger with an increase in the crystallization
time, suggesting the growth of crystals along the c-axis as well
as the formation of elongated crystals. The Raman band at
460 cm−1 becomes undetectable for crystallization beyond 24
h, suggesting that all the five-membered rings get almost
depleted and converted to form the TON-type structure.
Notably, the band at 366 cm−1, which can be attributed to the
Fig.
4 SEM images of the core–shell MFI@TON composites at
different crystallization times of 12 (a), 18 (b), 21 (c), 24 (d), 32 (e), and
36 (f) h.
the TON shell comprised thinner nanorods along the a- and
b-axes. In summary, a growth model of the TON crystal shell,
where nanorods grow along the rod direction until the
medium is almost depleted and nanorods latterly fuse in an
aligned manner, can be speculated from the XRD and SEM
results supported by the TEM data.
3.2.2 UV Raman and magic-angle spinning nuclear
magnetic resonance (MAS NMR). During the crystallization
process, the liquid and solid phases coexist.36 These phases
are separated by centrifugation and independently analyzed.
The Raman spectra of the solution throughout the synthesis
process are shown in Fig. 5a. The bands at 460, 620, 780,
846, and 855 cm−1 are observed in the Raman spectrum of
the initial liquid phase, in which the bands at 846 and 855
cm−1 can be ascribed to the DAH cation. The pH value of the
Fig. 5 UV resonance Raman spectra of the liquid solution (a) and solid
phase (b) of MFI@TON composites after different crystallization times.
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six-membered rings,42 becomes intense after the
crystallization time exceeds 28 h, reflecting the growth and
fusion of TON-type crystals in an aligned manner. This band
becomes particularly prominent in the samples after the
crystallization time over 28 h, indicating the alignment and
fusion of the nanorods, which is in accordance with the SEM
and TEM results.
Fig. 7 Schematic representation of the hydrothermal crystallization
procedure of the core–shell MFI@TON composites.
27Al and 29Si MAS NMR measurements were performed to
determine the chemical states of silicon and aluminum
species during the crystallization process. As shown in
Fig. 6a, only one peak at around 54.6 ppm, which can be
assigned to the four-coordinated aluminum species, could be
detected throughout the synthesis procedure. Apparently,
when the solid was still largely amorphous (from 12 to 22 h),
the 27Al NMR signal was substantially broadened, suggesting
that the Al species from the MFI and TON of the seed were
partially dissolved. Furthermore, it is notable that the full
width at half maximum (FWHM) of the four-coordinated
aluminum species gradually decreased during the crystal
growth period (24–32 h) owing to the formation of a
tetrahedrally coordinated aluminosilicates structure.43
mainly comprised amorphous silicate species with five-
membered silicate rings, DAH, and partially dissolved Al in
the solution phase. The amount of five-membered silicate
rings increased with the crystallization time (∼0–12 h). Then,
the five-membered rings in the amorphous phase (some of
which may be incorporated by the Al species and interacted
with the structure-directing agent) link together to form the
nuclei of the TON-type structure with the assistance of TON
seeds. Simultaneously, TON-type nanorods grow along the
c-axis direction, leading to the generation of a Raman band
at 407 cm−1. When the crystallization time exceeds 24 h, all
the five-membered silicate and aluminosilicate rings in the
amorphous gel are transformed into the TON-type structure.
Then, these nanorods bunch together in an aligned manner
to form larger crystals accompanied by the appearance of a
Raman band at 336 cm−1 along with the enhanced intensity
of the XRD diffraction peaks as well as the sharp resonance
in the 29Si MAS NMR. Finally, the framework structures and
coordination environment of the TON shell become more
uniform after 32 h.
The 29Si MAS NMR spectra of the samples obtained after
various crystallization times are shown in Fig. 6b. Obviously,
in the initial stage of the crystallization process, the
resonance in the range from −107 to −100 ppm assigned to
silanol groups distinctly appears.44 As the crystallization
proceeds, the resonance of silanol groups gradually becomes
weaker and completely disappears at 24 h, suggesting the
transformation of the silica species into a highly ordered
framework.45 Then, with
a
further increase in the
crystallization time (28–36 h), the NMR signal sharpens and
the intensity gradually increases. Combined with the above
characterizations and the data from earlier reports,46,47 it can
be concluded that the growth of ZSM-22 zeolites involves two
steps. First, nanorods with the TON-type framework are
formed from the initial gel with the assistance of ZSM-22
seeds. In the second stage, the nanorods fuse together
perpendicular to the c-axis in an aligned manner, leading to
the formation of thick and long nanorods.
3.2.3 Proposed mechanism. Herein, based on the XRD,
SEM, TEM, UV Raman, and MAS NMR data, a growth
mechanism for the MFI@TON composite was proposed
(Fig. 7). During the initial stage, the precursor solution
3.3 Characterization of core–shell materials
The XRD patterns of different samples are shown in Fig. 8.
The sharper, better-defined Brägg diffraction peaks of MFI,
TON, and TON-SG zeolites reveal higher crystallinities.48 No
additional peaks can be observed in all the patterns,
suggesting that these materials are free of any impurities.
With regard to the core–shell MFI@TON composite, the most
intense peaks of the TON-type material, in the ranges of 2θ =
7.0–10.0° and 20–22°, can be observed together with the
characteristic peaks of the MFI-type structure framework. The
signal-to-noise ratio of the diffraction lines in the patterns is
similar to that of the MFI and TON zeolites and is indicative
of a highly crystalline composite. In view of the diffraction
peaks belonging to the MFI and TON phases as well as the
physical mixtures using the pure TON and MFI phases as
references (as shown in Fig. S6†), the TON contents in the
MFI@TON composite can be estimated to be in the range of
∼75–85%. Therefore, the reference sample (TON/MFI)
exhibits similar XRD patterns as those of the MFI@TON
composite. The similar Si/Al ratios of the TON/MFI and
MFI@TON samples can also be observed, as listed in Table 1.
The surface Si/Al ratios of the different samples determined
by XPS are listed in Table 1, which confirm the uneven Al
distributions in the TON zeolites due to the fact that the Al
Fig. 6 27Al (a) and 29Si (b) MAS NMR spectra of MFI@TON composites
at different crystallization stages.
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Fig. 8 XRD patterns of TON, MFI, TON/MFI, TON-SG, and core–shell
MFI@TON materials.
Table 1 Characteristics of different samples
Surface areasd
Si/Al ratio
Initiala
(m2 g−1
)
Samples
Bulkb
Surfacec
SMicro
SEx
61.7
109.5
54.0
42.8
44.1
TON
MFI
TON/MFI
TON-SG
MFI@TON
75
35
—
—
∞
71.5
34.0
179.6
572.2
192.6
63.6
30.6
—
—
852.1
165.2
229.3
210.6
169.5
198.8
a
b
Fig. 9 SEM images of TON (a), MFI (b), seeds (c), TON-SG (d), and
core–shell MFI@TON (e) materials.
Initial composition during hydrothermal synthesis. Resulting Si/
c
Al ratio of the different samples determined by XRF. Surface Si/Al
ratio of the different samples determined by XPS. Obtained by N2
d
adsorption at −196 °C using a Micromeritics ASAP 2020 instrument.
up of nanorods. However, the completely covered structure
limits the observation for the combination of core and shell
zeolites. Fig. 10d and e show the composite crystals after
intensive post-treatment in order to destroy the core–shell
structure. Apparently, the nanorod crystals of ZSM-22 zeolites
as the shell are inserted in the ZSM-5 zeolite, forming bloom-
like morphology, as shown in Fig. 10e. The EDX analysis of
the Si and Al contents of different positions, as shown in
Fig. 10d, suggests that the Si/Al ratio of the nanorods is
874.2, which is close to the value of the TON shell as
species prefer to be located on the external surface.49 Because
of the extra-Al-free synthesis condition, the surface Si/Al ratio
of the composite is really high.
Fig. 9 shows the typical SEM images of MFI, TON,
adsorbed seed, TON-SG, and core–shell MFI@TON
composite. The TON and MFI zeolites exhibit needle- and
coffin-shaped crystals, as shown in Fig. 9a and b, which are
the typical morphologies for ZSM-22 and ZSM-5 zeolites,50,51
respectively. Evidently, as shown in Fig. 9e, the MFI@TON
composite displays chrysanthemum-like morphology with
dimensions of 20–30 μm, which is different from those of the
TON- or MFI-type zeolites used in our experiment. Almost no
unattached crystals with needle-shaped morphology could be
found, indicating that all the TON nanorods were
perpendicular to the core crystals. The bigger crystals of the
core–shell composite compared to the original zeolites could
be attributed to the growth of TON nanorods on the MFI
crystals. The isolated needles of the TON-SG sample are
shown in Fig. 9d, demonstrating the key role of the core
ZSM-5 crystal in maintaining the spherical morphology of the
MFI@TON composite.
Fig. 10 shows the representative high-resolution
transmission electron microscopy (HRTEM) images of the
core–shell materials before and after post-treatments. The
uniform growth of the TON nanorod shells on the external
surface of the MFI-type zeolites can be seen in
Fig. 10a and b. The corresponding HRTEM image (Fig. 10c)
for the TON shell reveals that the elongated crystals are made
Fig. 10 HRTEM and HAADF-STEM images of the core–shell composite
under different magnifications (a–c) and post-treated composites (d
and e).
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determined by the XPS analysis. Meanwhile, the Si/Al ratio of
MFI zeolites core is slightly higher than that of the parent
sample, indicating that the formation of MFI layers might
occur during the hydrothermal synthesis process. These
results reveal that the secondary growth of the TON materials
shell is dominant and the growth of the MFI material core is
suppressed.
core–shell material with a maximum at about 4.0 nm does
not suggest the “real” pores, which mainly results from the
tensile strength effect of the adsorbed phase,54 suggesting a
strong interaction between the TON shell and MFI zeolite
core.
The textural properties derived from the N2 adsorption–
desorption isotherms are listed in Table 1. Expectedly, the
external surface area of the core–shell material is much lower
than that of the reference sample (TON/MFI) due to the
totally covered MFI zeolites. To further confirm this, several
samples after different crystallization times were obtained, as
shown in Fig. S9,† and the textural properties are listed in
Table S1.† When the crystallization time exceeds 24 h, fusion
and aggregation led to a gradual decrease in the interspace
between the individual TON nanorods. Therefore, the
hysteresis loop and corresponding external surface area of
the core–shell materials gradually decreased with an increase
in the synthesis time. This result revealed that the growth of
the TON shell could be performed through the aggregation of
the formed nanorods.
Fig. S10 and S11† show the temperature-programmed
desorption of ammonia (NH3-TPD) and pyridine-adsorbed
infrared (Py-IR) data for the different samples, and the acidity
results are listed in Table 2. Among all the samples, the
highest Al concentration in the MFI sample results in the
largest amount of acid sites. TON zeolites exhibit a slightly
higher amount of acid sites than that in the TON/MFI sample
due to the comprehensive effect of the addition of MFI
zeolites and high Si/Al ratio of the latter. Meanwhile, the
MFI@TON composite exhibits a lesser amount of acid sites
as compared to the physically mixed TON/MFI sample. This
result suggests that the acid sites of the MFI zeolite core may
not be detected by NH3 molecules due to the complete
coverage by TON zeolites. As compared to the original TON
zeolite, the TON-SG sample with a higher Si/Al ratio exhibits
a lower amount of acid sites.
The N2 adsorption–desorption isotherms of the protonic
TON, MFI, TON/MFI, TON-SG, and the MFI@TON composites
are shown in Fig. 11. All these isotherms are typical of
microporous materials due to the steep rise at low relative
pressures. The subsequent upswing can be attributed to the
multilayer adsorption on the surface of the crystals.52 With
regard to the isotherm of TON zeolites, the hysteresis loop
mainly results from polycrystalline accumulation.53 However,
the composite exhibits a smaller hysteresis loop at higher
relative pressures as compared to that observed in the TON/
MFI sample, which is probably due to the densely stacked
TON nanorods. A similar conclusion can be drawn by
comparing the isotherms of the TON and TON-SG samples.
Nevertheless, in order to further verify the core–shell
structure, the composites with a calcined core and un-
calcined shell obtained after different crystallization times
are investigated by N2 adsorption measurements. The
isotherms for the calcined and un-calcined composites (36 h)
are shown in Fig. S7.† Obviously, the un-calcined composite
shows a much smaller hysteresis loop at higher relative
pressure, and no steep uptake is observed. As listed in Table
S2,† with an increase in the crystallization time, the
microporous areas and volumes of the different samples
gradually decrease due to which the organic template in the
channels of the shell ZSM-22 zeolite inhibits N2 adsorption.
Therefore, the coverage of the shell correspondingly improves
with an increase in the crystallization time. These results
demonstrate that the MFI zeolite core with open channels is
completely covered by the un-calcined TON shell. This result
also further provides solid evidence of the formation of a
core–shell structure with higher coverage.26
Py-IR results provide more information about the acidity
of the various samples. The concentrations of Brønsted (B)
and Lewis (L) acid sites for different samples increase with
the Al contents, which is similar to the NH3-TPD results.
Apparently, the coverage of the TON shell with a high Si/Al
ratio on the MFI zeolite results in a decreased amount of B
acid sites over the MFI@TON composite as compared to that
in the TON/MFI sample with exposed MFI zeolites. However,
The micropore diameters (as shown in Fig. S8a†) of all the
samples are narrowly centered at about 0.5 nm, indicating
the formation of a 10-membered ring. Furthermore, Fig. S8b†
shows that the mesopore distributions are not observed in
the TON, TON-SG, and MFI zeolites, as well as the TON/MFI
sample. However, the mesopore distribution curve of the
Table 2 Acidity properties of the different samples determined by NH3-
TPD and Py-IR
Acid types (μmol g−1
)
Acid amount
(μmol g−1
)
B acid sites
L acid sites
Samples
Weak
Strong
200 °C
350 °C
200 °C
350 °C
TON
MFI
TON/MFI
TON-SG
MFI@TON
182
385
136
73
153
327
128
42
75
187
62
39
42
62
159
69
33
35
69
115
58
59
78
59
96
48
51
69
Fig. 11 N2 adsorption–desorption isotherms of different materials.
78
45
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Table 3 Catalytic performances of toluene alkylation with methanol over
different catalysts (reaction conditions: 420 °C; nToluene/nMethanol = 4/1;
WHSV = 2.0 h−1; atmosphere)
the core–shell composite exhibits a higher amount of L acid
sites as compared to the TON/MFI sample because of which
the interaction between the MFI core and TON zeolite shell
generates crystal lattice defects in the form of L acid sites.55
Notably, the similar densities of B acid sites in the TON-SG
and MFI@TON samples further indicate that the pyridine
molecules do not reach the MFI zeolites core.
Catalysts
MFI TON TON/MFI TON-SG MFI@TON
Methanol
conversion (%)
Toluene
conversion (%)
Selectivity
99.96 99.85 99.68
26.67 24.25 21.15
83.52 87.80 85.62
99.84
18.02
88.86
99.92
17.69
90.69
to xylene (%)
para-Selectivity (%)
p-Xylene yield (%)
Aromatics
3.4 Catalytic performance
22.13 42.43 31.95
5.90 10.29 6.76
52.24
9.41
75.85
13.42
The catalytic performance of the composite catalyst is
investigated in the alkylation of toluene with methanol. Since
the alkylation reaction is typically an acid-catalyzed process,
the activity and product selectivity are influenced by multiple
factors, such as acidity, porosity, crystal size, and reaction
conditions. Therefore, the MFI, TON, TON/MFI, and TON-SG
catalysts are also evaluated under the same conditions.
The catalytic performance and detailed product selectivity
are shown in Fig. 12 and listed in Table 3. All the catalysts
exhibit high methanol conversion (above 99%) under these
reaction conditions. The toluene conversion decreases in the
order of MFI (26.67%) > TON (24.25%) > TON/MFI (21.15%)
> TON-SG (18.02%) ≈ TON/MFI (17.69%), directly validating
the important role of the amounts of acid sites determined
by NH3-TPD and Py-IR techniques. When compared with the
core–shell MFI@TON catalyst, the mechanically mixed TON/
MFI catalyst exhibits higher toluene conversion because of
which the isolated MFI crystals can act as the active sites for
the alkylation reaction, while in the core–shell structure, the
MFI zeolites are completely covered by the TON shell. The
similar toluene conversions of MFI@TON and TON-SG
catalyst further indicate that the MFI zeolites core are inert
in this reaction.
Benzene (%)
p-Xylene (%)
m-Xylene (%)
o-Xylene (%)
4.89 0.10 3.56
22.13 42.43 31.95
32.53 25.28 28.87
18.71 16.31 16.82
0.10
52.24
19.62
13.45
3.45
0.10
75.85
8.02
4.14
2.58
C9+ hydrocarbons (%) 5.25 3.68 4.42
improved effect of TON shell on suppressing the strong acid
sites of the MFI zeolites. This result also indicates that the
MFI zeolites core does not directly participate in toluene
alkylation. Apparently, the selectivity toward xylene, namely,
o-, m-, and p-xylene, over the different catalysts under
consideration follows the order of MFI@TON (90.69%) >
TON-SG (88.86%) > TON (87.80%) > TON/MFI (85.62%) >
MFI (83.52%), indicating the essential effect of the TON
zeolite shell on the increase in xylene selectivity. Since the
large crystals with enhanced diffusion resistance can
suppress the further transformation of the as-generated
xylenes to highly methylated aromatics, the MFI@TON and
+
TON-SG catalysts exhibit lower C9 aromatics selectivity as
compared to that by the original TON catalyst.
Within the distribution of xylene isomers, the para-xylene
selectivity over different catalysts decrease in the order of
MFI@TON (75.85%) > TON-SG (52.24%) > TON (42.43%) >
TON/MFI (31.95%) > MFI (22.13%). In general, the increase
for zeolite crystals is correlated with the increased diffusion
length and resistance of the products, and the transport of
m-xylene and o-xylene in the micropores is considerably
inhibited, thereby resulting in the further isomerization of
these isomers into p-xylene with a higher diffusion rate.57
Therefore, the TON-SG catalyst with big crystals exhibits
higher para-selectivity than the parent TON catalyst.
Meanwhile, the acid sites also have an important role in
affecting the product distribution. As determined by the XPS
and TEM-EDX data, the composition of the preparation
process results in almost no Al sites and correspondingly
marginal amount of acid sites on the external surface of the
shell component. Therefore, as compared to the parent TON
catalyst, the enhanced selectivity toward p-xylene over TON-
SG and core–shell MFI@TON catalysts can also be attributed
to the almost inert external surface, which cannot isomerize
the p-xylene diffused from the micropores into its isomers,
consequently guaranteeing the high para-selectivity.
Moreover, much higher para-selectivity can be achieved by
the MFI@TON catalyst than that by the TON-SG catalyst,
Significant differences in product selectivity and
distribution are observed between these catalysts. With
regard to the distribution of the aromatic products (Table 3),
a certain amount of benzene is generated over the MFI and
TON/MFI catalysts from toluene disproportionation. It is
widely accepted that toluene disproportionation needs a
stronger acid strength than toluene alkylation.56 Therefore,
no benzene is observed in the products over the TON, TON-
SG, and MFI@TON catalysts, not only suggesting the much
weaker acidity of the TON zeolites but also showing the
Fig. 12 Toluene conversion and aromatics selectivity over MFI, TON,
TON/MFI, TON-SG, and MFI@TON catalysts (reaction conditions: 420
°C; nToluene/nMethanol = 4/1; WHSV = 2.0 h−1; atmosphere).
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further suggesting the improved effects of spherical
morphology and core–shell structure.
microporous channels of the TON zeolites shell without
reaching the MFI zeolite core. Then, the methanol molecules
are converted into methoxonium ions on the interior acid
sites. Subsequently, the formed methoxonium ions attack the
aromatic ring of the toluene molecules with the assistance of
acid sites, mainly forming para-xylene due to the steric
hindrance of the channels and certain amounts of m-xylene
and o-xylene. The much-improved diffusion time and
distance lead to the further transformation of side products
(o- and m-xylene) into p-xylene and a significantly increased
para-selectivity. In addition, the p-xylene products diffusing
from the microporous channels can hardly isomerize on the
almost inert surface, and the high para-xylene selectivity can
be largely maintained.
Furthermore, a significant difference between the TON/
MFI and MFI@TON catalysts can also be observed. Since
these samples contain similar Si/Al ratios and pore channels,
the key difference in these catalysts is the combination of the
two zeolites components. The components of the TON/MFI
catalyst prepared by mechanical mixing are randomly
distributed, while the zeolite components of the MFI@TON
catalyst yield spherical morphology with
a core–shell
structure. The uncovered MFI zeolite in the TON/MFI catalyst
causes further conversion of the desired products and lower
selectivity toward para-xylene in comparison to the
MFI@TON catalyst. When combined with the catalytic
performance of the TON-SG catalyst, it can be inferred that
the reactant molecules have double diffusion resistance and 4. Conclusion
longer diffusion times in the shell of the ZSM-22 crystals (of
Core–shell MFI@TON composites with uniform morphology
and well-ordered pore channel were successfully synthesized
from precisely designed seeds using hydrothermal
the MFI@TON catalyst) when they penetrate into the ZSM-22
channels and get converted at the interior acid sites. It
should be noted that the spherical morphology of the core–
shell MFI@TON composite also limits the further
isomerization of the para-xylene products by exposing more
pore mouths and lesser portion of the external surface.
Therefore, the considerably enhanced para-xylene selectivity
of the MFI@TON catalyst can not only be attributed to the
passivation of the acid sites on the external surface of the
TON shell by forming spherical morphology and inert surface
but also resulting from the special structure with extended
diffusion pathways of the TON crystals shell.
Fig. S11 and Table S3† list the catalytic performances of
the MFI@TON composite and various zeolite catalysts found
in the literature. The ZSM-5 zeolite catalyst in the present
work exhibits a close performance value with the reported
values, indicating the reliability of these experimental data.
As compared to the 10- (MFI, SAPO-11) and 12-membered
ring (SAPO-31, SAPO-5) zeolites, the considerably enhanced
para-selectivity can be mainly attributed to the structural
characteristics of the MFI@TON composite.
a
crystallization method. Investigations about the synthesis
conditions revealed the significance of the Si/Al ratios on
determining the formation of a core–shell structure by
suppressing the growth of the MFI zeolite core and ensuring
the crystallization of the TON nanorods shell. A broad
combination of characterizations assisted in formulating the
crystallization mechanism of the MFI@TON composite,
including the oriented crystal growth and selective fusion.
TEM and N2-adsorption measurements provided solid
evidence for the total coverage of the uniform TON shell on
the MFI crystal. Both the pure silica composition and
spherical morphology ensured the passivation of the surface
acid sites as determined by the acidity measurements and
XPS. As a result, the core–shell composite exhibited much
higher para-xylene selectivity in the alkylation of toluene with
methanol as compared to the original, unattached, and
mechanically mixed catalysts. The enhanced catalytic
performance can be attributed to the optimized acidity of the
TON zeolites shell and the spherical morphology by
suppressing the further isomerization of para-xylene as well
as the considerably prolonged diffusion for the intermediates
inside the channels due to the spherical structure with
largely exposed pore mouths. The synthesis of core–shell
MFI@TON composites offers a pathway to optimize the
diffusion properties of a zeolite material, providing further
insights into the growth of ZSM-22 zeolites.
Based on the above discussions, the reaction pathway of
the toluene alkylation with methanol over the core–shell
MFI@TON composite is proposed, as shown in Fig. 13.
Firstly, both toluene and methanol penetrate into the
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are grateful to Synfuels China Co., Ltd. and
Beijing Key Laboratory of Coal to Cleaning Liquid Fuels for
financial support. This project is also supported by the
National Natural Science Foundation of China (No. 21902171).
Fig. 13 Simplified scheme of the diffusion pathways for the reactant
(toluene and methanol) and products (p-xylene) over the core–shell
MFI@TON composites.
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