Synthesis of core-shell ZSM-5@meso-SAPO-34 composite and its application in methanol to aromatics

Article abstract of DOI:10.1039/c5ra10296k

A core/shell-structured ZSM-5@meso-SAPO-34 composite catalyst was hydrothermally synthesized through overgrowing SAPO-34 molecular sieve on the external surface of ZSM-5. The catalyst was thoroughly characterized with regards to its crystallinity, morphology, elemental composition, surface area, pore volume, and acidity. The catalytic performance of the as-obtained ZSM-5@meso-SAPO-34 was tested by the conversion of methanol to aromatics reaction. ZSM-5@meso-SAPO-34 exhibited higher aromatics selectivity compared to that of pristine ZSM-5 and its physical mixture with SAPO-34. The unique catalytic synergistic behavior of ZSM-5@meso-SAPO-34 was ascribed to its effective enhancement of aromatization as a result of the acid sites on the external surface covered by the SAPO-34 shell.

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Journal Name
ARTICLE
Synthesis of coreꢀshell ZSMꢀ5@mesoꢀSAPOꢀ34
composite and its application in methanol to aromatics
Cite this: DOI: 10.1039/x0xx00000x
Ling Zhang,ab ZhongꢀXiang Jiang ,c Yue Yu, c ChongꢀShuai Sun,c YuꢀJia Wang,c
HaiꢀYan Wang*ac
Received 00th
Accepted 00th
A core/shellꢀstructured ZSMꢀ5@mesoꢀSAPOꢀ34 composite catalyst was hydrothermally synthesized
through overgrowing SAPOꢀ34 molecular sieve on the external surface of ZSMꢀ5. The catalyst was
thoroughly characterized with regards to their crystallinity, morphology, elemental composition, surface
area and pore volume, acidity. The catalytic performance of the asꢀobtained ZSMꢀ5@mesoꢀSAPOꢀ34
was tested by the conversion of methanol to aromatics reaction. ZSMꢀ5@mesoꢀSAPOꢀ34 exhibited
higher aromatics selectivity relative to that of pristine ZSMꢀ5 and its physical mixture with SAPOꢀ34.
ZSMꢀ5@mesoꢀSAPOꢀ34’s unique catalytic synergistic behaviors were ascribed to having an effective
enhancement of aromatization as a result of the acid sites on the external surface covered by SAPOꢀ34
shell.
DOI: 10.1039/x0xx00000x
www.rsc.org/
and species deposition in the pore structure, consequently
influencing zeolite molecular transport properties.
1. Introduction
Recent progress has demonstrated that fabrication of coreꢀshell
composite materials have paid particular attention to the fields of
catalysis due to their acidic properties of the external surface of
zeolites and diffusion efficiency can be altered in a desired way.18ꢀ21
Besides, it is noteworthy that coreꢀshell composite materials show
their synergetic effects and the multifunctional properties, e.g.,
nanoarchitectures, alterable compositions and sizes in catalytic
reactions. Generally, the synthesis of uniform coreꢀshell materials
contains various synthetic approaches such as polytypism and
epitaxy or overgrowth.19, 21ꢀ24 Particularly, overgrowth on different
materials can combine their different intrinsic properties based on
structure and acidity. For instance, ZSMꢀ5/SAPOꢀ11 composites
were synthesized by the in situ overgrowth, composed of an acidic
HꢀZSMꢀ5 zeolite core covered with a SAPOꢀ11 shell.25 It has been
reported previously that the coreꢀshell type composites have
desirable acidity, which is favourable to enhance the synergism
effect between Brönsted and Lewis acids. In addition, binary
structure ZSMꢀ5/SAPOꢀ5 composite zeolites synthesized by
overgrowing SAPOꢀ5 over the crystal surface of ZSMꢀ5 have shown
Currently, processes like the production of aromatics, including
benzene, toluene, and xylene (BTX) are of great significance in the
petrochemical industry. Classically, the production of aromatics
mainly relies on the catalytic reforming of naphtha. Other alternate
processes, such as methanolꢀtoꢀaromatics (MTA) as replacement for
the fossilꢀbased processes have grown to be a new focus for the
production of aromatics, owing to the rapid diminishing of fossil
resources.1 Recent studies have demonstrated that methanol can be
selectively converted to aromatics by using zeoliteꢀbased materials.2ꢀ
4
Aluminosilicate ZSMꢀ5 zeolite, as a shape selective catalyst,5, 6
has been extensively applied in the fields of petrochemical industry,
7
and the fine chemical industry involving the MTA process.
Furthermore, it possesses MFIꢀtype topology connected by two types
of 10ꢀmembered ring channels, desirable catalytic properties, e.g.
high surface area, thermal stability and acidity, as well as adjustable
pore size comparable to the molecular dimension of BTX.8
Accordingly, ZSMꢀ5 zeolite exhibits both higher catalytic activity
and remarkable selectivity to aromatics in the process of
aromatization.9ꢀ11 However, the catalytic activity and selectivity for
aromatics formation are usually alleviated significantly by
deactivation and formation of coking associated with carbon
deposition, which may lead to the covering of the active sites on the
external surface.12, 13 Additionally, acid sites on the external surface
of ZSMꢀ5 can enhance effectively the BTXꢀselectivity as well as
side reactions, which inevitably influence the catalyst BTXꢀ
selectivity.14 Therefore, it is highly desirable to develop suitable
catalysts for the production of aromatics. In order to obtain higher
catalytic properties, various strategies have been attempted to reduce
contributions from external surface acid sites via introduction of
mesopores.15ꢀ17 Nevertheless, it may still result in partial destruction
a higher propylene yield and conversion of heavy oil, and a striking
26
decrease in the number of surface active sites.
More recently,
ZSMꢀ5/SAPOꢀ3427 composites with binary structures have been
successfully fabricated employing preꢀheated ZSMꢀ5 suspension
followed by secondary growth of SAPOꢀ34 layer, which represented
excellent catalytic performance and synergetic effect between ZSMꢀ
5 and SAPOꢀ34 in propane dehydrogenation reaction.28 Despite the
above described references, reports on the synthesis and application
of ZSMꢀ5/SAPOꢀ34 coreꢀshell structure composites are restricted.
To the best of our knowledge, the fabrication of this kind of
composite zeolites applying overgrowth strategy and its application
in
the
MTA
have
never
been
reported
before.
Silicoaluminophosphate SAPOꢀ34 exhibits higher performance in
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DOI: 10.1039/C5RA10296K
methanolꢀtoꢀolefin (MTO) reactions owing to its characteristic CHA SAPOꢀ34 in the absence of TPAOH and DEA. Next, TPAOH and
topology and small 8ꢀring pore (3.8 Å × 3.8 Å) opening, as well as DEA was subsequently added dropwise into the resultant solution
mild acidity.29, 30 However, the diffusion of bulky molecules such as under room temperature, then the mixture was dispersed in an
aromatics in the catalytic reaction severely is hindered by the aqueous solution of CTAB, continuous stirring until dispersed in a
microporous channels of the SAPOꢀ34 zeolites, probably a result of uniformed suspension. After aged at 313 K overnight, the gel was
32
the presence of the small pores.31,
This limitation could be crystallized at 453 K for 1ꢀ3 days. Finally, the product was filtrated,
overcome if the channels of the mesopore SAPOꢀ34 system is washed thoroughly with deionized water, and then dried overnight at
introduced in the configuration of the coreꢀshell ZSMꢀ5@SAPOꢀ34. 383 K. The organic template were eliminated by calcination at 823 K
In this work, we demonstrate a facile twoꢀstep hydrothermal for 4 h. The final samples were designated CZS (n), where n
synthesis strategy to prepare a series of core–shell heterostructures represents the weight ratio of ZSMꢀ5 precursor and SAPOꢀ34
ZSMꢀ5@mesoꢀSAPOꢀ34 formed by the growth of SAPOꢀ34 suspension gel.
crystalline overlayers on ZSMꢀ5 nanocrystals, and mesoporous
SAPOꢀ34 layer synthesized using diethylamine (DEA) and
2.1.3. Synthesis of mechanical mixture of HZSMꢀ5 and SAPOꢀ34.
tetrapropylammonium hydroxide (TPAOH) as coꢀtemplates. The For control experiment, a mechanical mixture of ZSMꢀ5 and SAPOꢀ
crystallization process of the ZSMꢀ5@mesoꢀSAPOꢀ34 coꢀcrystalline 34 was prepared by mixing ZSMꢀ5 asꢀsynthesized and amount of
zeolite was systematically investigated via the XRD, SEM, TEM, mesoporous SAPOꢀ34 asꢀsynthesized with a weight ratio of 2:1.The
and nitrogen adsorption characterizations. As
a hierarchicalꢀ composite zeolites were calcined at 873 K for 4 h and crushed into
structured composite catalyst, ZSMꢀ5@mesoꢀSAPOꢀ34 exhibits 20ꢀ40 meshes, and was denoted as HZS.
enhanced catalytic properties in the methanol to aromatics with high
selectivity for BTX opposed to the mechanical mixing.
2.2. Catalyst characterizations
Xꢀray diffraction (XRD) patterns were performed to determine the
phase structures of the samples using D/max –RB diffractometer
with CuꢀKα radiation (40 k V, 50 m A). The morphology and size of
the crystal materials were measured by scanning electron
microscopy (SEM) on a HITACHI SUꢀ8010 microscope equipped
with an energyꢀdispersive Xꢀray spectrometer (EDS), and operated
at 10ꢀ15 kV. Transmission electron microscopy (TEM) and high
resolution transmission electron microscopy (HRTEM) of the
samples were performed on a JEOL JEMꢀ2100F with an acceleration
voltage of 200 kV.
Infrared absorption spectra were recorded on a WQFꢀ510 Fourier
transform infrared (FTꢀIR) with the resolution of 4 cmꢀ1. The sample
powder and KBr were extruded into the selfꢀsupported wafer with
the mass ratio of 1:300, and the spectra were recorded in the range of
4000~400 cmꢀ1.N2 adsorptionꢀdesorption isotherms were obtained on
a Micromeritics ASAP 2405 analyzer at ꢀ196 ℃ . Prior to the
measurement, each sample (100mg) was degassed at 300℃ for 10 h.
The specific surface areas were calculated according to the BET
method. The surface area, pore volume of the samples was
calculated by BET, tꢀplot and BJH methods, respectively.
2. Experimental
2.1．Material preparation
2.1.1. Synthesis of ZSMꢀ5 and mesoporous SAPOꢀ34.
Zeolite ZSMꢀ5: ZSMꢀ5 crystals with a Si/Al ratio of 25 were
synthesized by hydrothermal method. For the typical procedure,
colloidal silica sol (30%) was mixed with sodium aluminate and
deionized water, and the mixture was stirred for 2 h before
tetrapropylammonium hydroxide (TPAOH, >25%) was added. The
molar ratio of the reactants in this synthesis was set as
1.0SiO2:0.035Al2O3:0.07Na2O:0.2TPAOH:30H2O. The mixture was
vigorously stirred for 4 h before it was transferred into a 100 ml
Teflonꢀline autoclave, which was preheated to 40 °C for 12 h.
Zeolite ZSMꢀ5 was then obtained by hydrothermal treatment at 453
K for 48 h under autogenous pressure, after which the products were
washed with distilled water and dried at 383 K. The final products
were obtained after calcination at 823 K for 5 h to remove any
organic substances.
The acidic properties of the samples was determined by NH3ꢀTPD
with a Micromeritics AutoChem II 2920 in the range of 150 ~ 600℃
at a ramp rate of 20 ℃ min−1, and the desorbed ammonia was
Mesoporous SAPOꢀ34：
Mesoporous SAPOꢀ34 was prepared hydrothermally using DEA and
TPAOH as the coꢀtemplates with the molar composition of
1.0Al2O3:1.0P2O5:0.6SiO2:2.0DEA:0.2TEAOH:0.03CTAB:60H2O.
TPAOH and DEA was employed as structure directing agent and
coꢀtemplate. Typically, 2.9 g pseudoboehmite (70% Al2O3) mixed
with the required amount of deionized water was added to 2.7 ml
tetraethylorthosilicate (TEOS) under room temperature. After
successive stirring for 2 h, 2.7 ml phosphoric acid (85%) was
dispersed drop into the above solution with continuous stirring for a
further 2 h. subsequently, 3.2 ml TPAOH and 4.2 ml diethylamine
(DEA) was added to the mixture, and the mixture was stirred for 5 h.
Next, the solution was mixed with an aqueous (80 g of water)
solution of CTAB (0.21 g) and stirred for 4 h. Final mixture was
transferred into autoclave and aged at 313 K overnight, and then the
resulting gel was heated at 453 K for 48 h. All the resulting solid
were thoroughly washed with deionized water, filtrated, and dried 12
h at 383 K. The organic template were eliminated by calcination at
823 K for 6 h.
monitored by a gas chromatograph with a TCD detector.
2.3. Catalytic activity measurement
MTA reaction was carried out in a continuousꢀflow fixedꢀbed
reactor with inner diameter of 10 mm. The catalyst（5 ml）was
loaded into the reactor. Methanol was pumped into the reactor after
the catalyst was pretreated in nitrogen flow at 748 k for 3 h. The
reaction was conducted under pressure (0.5 MPa), weight hourly
space velocity (WHSV =1.2 hꢀ1). Liquid products were analyzed on
a gas chromatograph (Agilent 7890A) equipped with a flame
ionization detector and a capillary column.
3. Results and discussion
3.1. Preparation and characterization of coreꢀshell ZSMꢀ5@mesoꢀ
SAPOꢀ34
3.1.1 XRD
2.1.2. Synthesis of coreꢀshell ZSMꢀ5@mesoꢀSAPOꢀ34. Coreꢀshell
ZSMꢀ5@mesoꢀSAPOꢀ34 materials were synthesized by adding
amount of premade ZSMꢀ5 precursor into the gel mentioned in
2 | J. Name., 2012, 00, 1-3
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ZSMꢀ5@mesoꢀSAPOꢀ34 is prepared with a hydrothermal method Fig.2. XRD patterns of ZSMꢀ5@mesoꢀSAPOꢀ34 (CZSꢀ2) synthesized at different
by adding an amount of premade ZSMꢀ5 precursor into the gel of crystallization time: 24 h (a); 36 h (b); 48 h (c); 60 h (d); 72h (e).
SAPOꢀ34. As displayed in Fig.1, the Xꢀray diffraction patterns of a
Fig.2 shows XRD patterns of ZSMꢀ5@mesoꢀSAPOꢀ34
composite comprising ZSMꢀ5 and SAPOꢀ34 treated for 48 h with
(CZSꢀ2) synthesized at different crystallization time, all of
the weight ratio of ZSMꢀ5 precursor and SAPOꢀ34 suspension gel
which exhibit wellꢀresolved peaks in the range of 5–50°, and
varying from 1 to 2. The samples both show wellꢀresolved
match well with those of Fig.1c. With the crystallization time
diffraction peaks at 2θ=7.9°, 8.8°, 22ꢀ25°, and 9.5°, which are
continuously increased, the crystallinity and intensity of CHA
assigned to the MFI and CHA phases, respectively.33 The XRD
in the ZSMꢀ5@mesoꢀSAPOꢀ34 was relatively weaker than that
results indicate that the presence of both ZSMꢀ5 and SAPOꢀ34 in the
intergrowth zeolites. In addition, it appears that the crystallinity and
intensity of CZSꢀ2 are higher than CZSꢀ1. In contrast, the reflections
of the ZSMꢀ5@mesoꢀSAPOꢀ34 coreꢀshell are very comparable to
those of a mechanical mixture except for weaker relative intensity
and crystallinity of the diffraction peak. When compared with those
of the pristine zeolites (ZSMꢀ5 and SAPOꢀ34), the characteristic
peaks of coreꢀshell ZSMꢀ5@mesoꢀSAPOꢀ34 shift slightly to higher
of pure of SAPOꢀ34 crystal, also the intensity of MFI gradually
increased. It is observed that the intensity of MFI is higher than
that of CHA in Figꢀ2e. This can be explained as follows: On the
one hand, the growth rate of the ZSMꢀ5 phase is fast enough to
surpass that of the SAPOꢀ34 phase when crystallization time
extended, the former will protrude out of the latter, as shown in
Fig. 3e. Therefore, the intensity of MFI is observed higher than
CHA. On the other hand, because the weight ratio of ZSMꢀ5
precursor and SAPOꢀ34 suspension gel is 2, which maybe leads
to peaks of ZSMꢀ5 are higher than that of SAPOꢀ34 phase
during the secondary crystallization process.
angles due to the distortion of the crystal lattice, in agreement with
34
the XRD analysis the published
.
In the case of the coreꢀshell
zeolite, the shrinkage resulting from intergrowth shows less
noticeable due to the ionic radius of silicon is similar to Al and P 3.1.2 SEM and TEM
ions when forming composite zeolites. The observed phenomena
To further demonstrate the targeted coreꢀshell structure in
evidenced that SAPOꢀ34 has successfully grown as an outer layer of
ZSMꢀ5 in CZS simultaneously.
ZSMꢀ5@mesoꢀSAPOꢀ34, the scanning electron microscope
(SEM) was used to characterize the morphology of the asꢀ
synthesized. As displayed in Fig.3a, the sample of ZSMꢀ5 asꢀ
synthesized possesses very uniform sphericalꢀlike morphology
with particle sizes ranging from 1 to 2 ꢁm, and consisting of
hexagonal columnar monocrystals. Similar morphologies are
observed in ZSMꢀ5@mesoꢀSAPOꢀ34 coreꢀshell (Fig.3cꢀf), and
the size of the crystal particle is in agreement with the above. It
is attributed to the aggregation of ZSMꢀ5 crystals under the
action of Gibbs free energy. In addition, the morphology did
not suffer any influence from the gel system of secondary
crystallization.
e
d
c
b
Additionally, Fig.3 mainly displays the morphology of ZSMꢀ
5@mesoporous SAPOꢀ34 synthesized during the different
crystallization time. Initially, only uniform cubicꢀlike
morphology SAPOꢀ34 phase with smooth crystalꢀface and
particle sizes of ca.10ꢁm in diameter was observed in the
composite structure after 24 h crystallization (Fig.3b), which is
typical morphology of CHA crystalline zeolite. Furthermore, no
trace of ZSMꢀ5 crystal was detected on the surface of ZSMꢀ5@
mesoꢀSAPOꢀ34 other than SAPOꢀ34 phase. It was assumed that
a layer densely shell of the SAPOꢀ34 formation begins to occur
on the surface of ZSMꢀ5 and ZSMꢀ5 further grew into core
phase until entirely covered by SAPOꢀ34 shell. However, it did
not ensure the coreꢀshell structure only relied on the results.
When crystallization time was prolonged to 48 h (Fig.3cꢀd),
spherical ZSMꢀ5 particles began to appear on the surface of
ZSMꢀ5@ mesoꢀSAPOꢀ34 and part of particle embedded into
the composite structure, which changed the surface of skeletons
from smoothness to coarseness. With the crystallization time
increased to 72 h (Fig.3eꢀf), the amount of spherical ZSMꢀ5
particles increased continuously and scattered on the surface of
SAPOꢀ34 shell. It suggests that ZSMꢀ5 precursors gradually
form in the original silicoaluminophosphate gel of SAPOꢀ34
and further fabricates coreꢀshell composite structure with an
increase of crystallization time. Thus, appropriate synthesis
time is a critical factor in the synthetic procedure for the
forming of selfꢀassembled coreꢀshell structure.
a
10
20
30
40
50
2θ (degree)
Fig.1. XRD patterns of asꢀsynthesized mesoporous SAPOꢀ34 (a), CZSꢀ1
(b), CZSꢀ2 (c), HZS (d), ZSMꢀ5 (e).
e
d
c
b
a
10
20
30
40
50
2θ (degree)
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and few spherical ZSMꢀ5 particles
with SEM results
，
which in good agreement
（
Fig.3d). Moreover, the lattice fringes
between core and shell can be better observed in the enlarged
image (Fig.5B). The results indicate that SAPOꢀ34 was grown
on the surface of ZSMꢀ5 crystallites successfully and fabricated
a coreꢀshell structure composite.
Fig.5. TEM images of coreꢀshell ZSMꢀ5@mesoꢀSAPOꢀ34 (CZSꢀ2)
synthesized from crystallization time at 48 h: Lowꢀmagnification (A), Highꢀ
magnification (B), and 72 h Lowꢀmagnification (C), Highꢀmagnification
(D).
Fig.3. SEM images of ZSMꢀ5 asꢀsynthesized (a), hierarchically mesoporous
SAPOꢀ34 (b), coreꢀshell ZSMꢀ5@mesoꢀSAPOꢀ34 (CZSꢀ2) synthesized at
different crystallization time: 48 h, highꢀmagnification (c) and corresponding lowꢀ
magnification (d), 60 h (e), 72 h (f).
3.1.3 FTꢀIR
FTꢀIR spectroscopy assists in identifying surface species of
shell formed during the crystallization. Fig.6 displays FTꢀIR
spectra of asꢀsynthesized samples of ZSMꢀ5, SAPOꢀ34, ZSMꢀ
5@mesoꢀSAPOꢀ34 coreꢀshell (CZSꢀ2) and mechanical mixture
(HZS). Framework vibrations at 455, 793 and 1094 cmꢀ1 are
observed in CZSꢀ2, which corresponds to the SiꢀO bending
mode vibration of tetrahedron inner links of TꢀOꢀT similar to
those of ZSMꢀ5 zeolites. In addition, two bands at 546 cmꢀ1 and
640 cmꢀ1 for CZSꢀ2 and HZS are assigned to the asymmetric
vibration of the fiveꢀmembered rings of T–O–T (T=Si or Al) in
the MFI frameworks and the bend of double 6ꢀring,
respectively. These results indicate that the coreꢀshell
composite sample contains the primary units of CHA zeolites
besides MFIꢀtype zeolites, which further proved the forming of
shell phase around core phase during the second crystallization.
It's worth noting that distinct absorptions appear at about
1221 cmꢀ1. The peak for CZSꢀ2 slightly shifts to 1218 cmꢀ1 and
the intensity is lower than those in HZS and pure ZSMꢀ5. This
phenomenon can be ascribed to a particular conjunction form of
tetrahedrons and a special skeleton structure at the interface
between core and shell zeolites. Because the 1221 cmꢀ1 IR band
is sensitive to change in the zeolite structure, the decrease in
intensity proves it maybe includes the presence of CHA
structure besides MFI phase. In case of CZSꢀ2 and HZS, all
bands exhibit a lower intensity corresponding to pure ZSMꢀ5
Fig.4. SEMꢀEDS line scanning of ZSMꢀ5@mesoꢀSAPOꢀ34 coreꢀshell (CZSꢀ2).
The formation of the synthesized coreꢀshell ZSMꢀ5@mesoꢀ
SAPOꢀ34 was also confirmed by SEMꢀEDX analysis results
presented in Fig.4. Fig.4 reveals that the results of SEMꢀEDS
scanning along the edge of external surface of ZSMꢀ5@mesoꢀ
SAPOꢀ34 coreꢀshell (CZSꢀ2). According to the data listed in
the Fig.4, obviously, a larger number of P element but less Si
element were measured on the crystallite surface except for Al
element where the P is the chief ingredient of SAPOꢀ34 phase
whereas it does not exist in ZSMꢀ5. Thus, the EDS
investigation further verified that ZSMꢀ5 phase in the
composite may be covered by the SAPOꢀ34 layer generated in
the secondary synthesis.
Fig.5 shows the TEM micrographs of the coreꢀshell ZSMꢀ
5@ mesoꢀSAPOꢀ34 synthesized, which clearly exhibits the
ZSMꢀ5 covered tightly by a about 20ꢀ30 nmꢀthick SAPOꢀ34
shell in a coreꢀshell structure, and no other crystalline phase
was detected except the characteristic cubic SAPOꢀ34 phase
and SAPOꢀ34. It is inferred that the introduction of SAPOꢀ34
shell decreases the crystallinity of synthesized coreꢀshell
zeolite, which is in accordance with the result of XRD.
（
）
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isotherm curves of ZSMꢀ5@mesoꢀSAPOꢀ34 coreꢀshell and
mechanical mixture are more or less similar to those of pristine
ZSMꢀ5 zeolite combined micropores and mesopores. Whereas,
typical hysteresis loops of them obviously show differently.
Compared with the latter, the isotherm curve of the coreꢀshell
composite sample appears a relatively flat curve with a type H4
hysteresis loop. The phenomenon is associated with narrow slitꢀ
like pores in accordance with the IUPAC,36 which is attributed
to the presence of a large number of mesopores in the
structured composite. In contrast, the sharp uptake in the
adsorptionꢀdesorption isotherms of the HZS sample can be
observed at higher relative pressure P/P0 > 0.9. Perhaps it arises
from the formation of an agglomerate of particles from ZSMꢀ5
and SAPOꢀ34.
640
d
c
457
1097
793
546
1221
b
a
The textural properties of samples are listed in Table 1, in
which the specific BET surface area and total pore volume of
pristine ZSMꢀ5 asꢀsynthesized are 398 m2gꢀ1 and 0.21 cm3gꢀ1
respectively, with lower contribution to mesopores from
intrinsic ZSMꢀ5, only 81 m2gꢀ1and 0.13 cm3gꢀ1, respectively. In
contrast, the ZSMꢀ5@mesoꢀSAPOꢀ34 composites possess a
lower micropore volumes but more mesopores. The micropore
volumes and total pore volumes of ZSMꢀ5@mesoꢀSAPOꢀ34
coreꢀshell are higher than the mechanical mixture, with about
0.16 cm3gꢀ1 and 0.23 cm3gꢀ1, respectively. It is in accordance
with the result of Fig.7.
1600
1400
1200
1000
800
600
400
Wavenumber/cm-1
Fig.6. IR spectra of samples：(a) ZSMꢀ5, (b) CZSꢀ2, (c) HZS, (d) mesoporous
SAPOꢀ34.
3.1.4 N2 adsorptionꢀdesorption characterization
N2 adsorptionꢀdesorption isotherms and pore size
distributions were used to estimate the porosity features of
samples shown in Fig.7. The initial part of all samples exhibit
the type I isotherm curves with a sharp uptake at relative
pressure P/P0 below 0.1, indicating a characteristic micropore
framework. The ZSMꢀ5@mesoꢀSAPOꢀ34 sample displays type
IV isotherm with a hysteresis loop at a relative pressure P/P0 of
0.45ꢀ0.95, as a result of the presence of mesopores, which is
associated with capillary condensation.35 Comparatively, the
Table 1 Textual properties of the zeolites samples
Sample
Smesoa (m2.gꢀ1) Smicrob (m2.gꢀ1) SBETc (m2.gꢀ1) Vmesod (cm3.gꢀ1) Vmicroe (cm3.gꢀ1) Vtotalf (cm3.gꢀ1)
ZSMꢀ5
CZSꢀ2
81
68
57
52
317
309
299
270
398
377
356
322
0.08
0.16
0.14
0.09
0.13
0.07
0.08
0.16
0.21
0.23
0.22
0.25
HZS
SAPOꢀ34
a, b surface area covered by mesopores and micropores calculated from tꢀplot, respectively. c total surface area calculated using BET equation in a range of
relative pressure from 0.05 to 0.3. d,e pore volume covered by mesopores and micropores calculated from tꢀplot; f Single point pore volume at P/P0
＝0.99.
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Table 2 Product distributions of methanol transformation over the investigated catalystsa
Product distribution (wt. %)
YBTX (%)
SBTX (%)
Catalyst
b
+h
C1ꢀC5
64.5
62.3
97
B c
2.1
2.9
0.2
3.4
4.1
4.2
4.3
Td
oꢀX e
5.6
6.0
0.4
8.1
7.9
7.9
7.5
mꢀX f
9.5
pꢀX g
3.1
5.9
0.4
3.3
4.1
4.6
4.4
C9
ZSMꢀ5
HZS
SAPOꢀ34
CZSꢀ1
CZSꢀ1.5
CZSꢀ2
CZSꢀ2.5
9.8
12.1
0.2
5.4
5.3
0.5
9.7
8.8
8.5
8.7
30.1
37.7
2.5
40.2
45.4
48.6
47.5
84.7
85.5
87.1
81.5
83.8
85.1
84.5
10.8
1.3
50.1
45.8
42.9
43.8
10.5
11.2
13.4
13.1
14.9
18.1
18.5
18.2
a Reaction conditions: 0.5 MPa, 460 °C, WHSV=1.2 hꢀ1, timeꢀonꢀstream = 4 h. b C1−5 alkenes. c Benzene. d Toluene. e orthoꢀXylene. f metaꢀXylene. g paraꢀXylene. h C9 and
C9+ aromatics
3.2 The acidity of ZSMꢀ5@mesoꢀSAPOꢀ34 core–shell catalysts
3.3 Catalytic performance
Methanolꢀtoꢀaromatics (MTA) conversion as a typical shapeꢀ
selective reaction was used to evaluate the catalytic performance of
the ZSMꢀ5@mesoporous SAPOꢀ34 catalysts. Its catalytic properties
were compared with those of ZSMꢀ5 and physical mixing ZSMꢀ
5/SAPOꢀ34 catalysts. As shown in Table 2, SAPOꢀ34 showed an
extremely low yield of aromatics (2.5 %) on account of weak acidity
as characterized by NH3ꢀTPD. The yields to aromatics are around
30.1 wt % on parent ZSMꢀ5 zeolites with the Si/Al ratio = 25.
However, ZSMꢀ5 exhibits a slightly higher yield of 84.7 wt % to
BTX. Compared with the purely acidic ZSMꢀ5, ZSMꢀ5/SAPOꢀ34
(HZS) prepared by physical mixing improves the aromatics yield to
37.7 wt % and gives obviously higher selectivity for BTX (85.5 wt
%). In contrast, ZSMꢀ5@mesoꢀSAPOꢀ34 catalyst exhibited high
catalytically activity. It is worth noting that the yield to aromatics is
Fig.8. NH3–TPD patterns of different samples ：ZSMꢀ5, CZSꢀ2, HZS and SAPOꢀ
obviously enhanced with the increase in ZSMꢀ5/SAPOꢀ34 weight
34.
ratio. The selectivity to BTX and yields to aromatics at reaction
temperature 460℃ and WHSV=1.2 hꢀ1 reach 81.5%, 85.1% and
The surface acidities of the asꢀsynthesized samples were
determined by the temperature programmed desorption of
ammonia (NH3ꢀTPD). As illustrated in Fig.8, the spectrums of
the ZSMꢀ5@mesoꢀSAPOꢀ34 display two distinct desorption
40.2%, 48.6%, respectively. This result should be related to the
increased acidity and acid amount in ZSMꢀ5, which caused the
enhancement of aromatics formation. Moreover, the addition of
suitable amounts of SAPOꢀ34 in the coreꢀshell structure can be
peaks, the highꢀtemperature peak at ca. 440
℃ and lowꢀ beneficial to aromatics. The direct connection between cores and
shells lead to the fabrication of hierarchical porous structure and
enhance the accessibility to active sites.
temperature peak centered ca. 183 ℃，which mainly arise
from the strong and weak acid sites, respectively. It is can be
seen that the area of the highꢀtemperature peak shows an
obvious decrease, and simultaneously slightly shifts towards
lower temperature relative to that of pristine ZSMꢀ5. It can be
deduced that a portion of the strong acid sites of ZSMꢀ5 or the
surface of ZSMꢀ5 core may be covered and replaced by the
weaker acidity of SAPOꢀ34 shell after the incorporation of
SAPOꢀ34, which is in agreement with our previous results. In
addition, the mechanical mixture shows lower temperature and
smaller acidity relative to that of coreꢀshell ZSMꢀ5@mesoꢀ
SAPOꢀ34.
Fig. 9 shows the methanol conversion and selectivity of benzene,
toluene, and xylene with time on over ZSMꢀ5@mesoꢀSAPOꢀ34. As
presented in the figure, the selectivity of BTX rises with time and
can reach the maximum of 85.1% under approaching 100%
methanol conversion. Whereas, selectivity levels decrease with time.
The change of BTX selectivity could be related to the coking effect,
which reduced the pore size of SAPOꢀ34 shell in ZSMꢀ5@mesoꢀ
SAPOꢀ34.
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DOI: 10.1039/C5RA10296K
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The hierarchical pore system with coreꢀshell structured
ZSMꢀ5@mesoꢀSAPOꢀ34 is successfully prepared by the
hydrothermal method via introducing mesoporous SAPOꢀ34
system. The characterization of SEM, BET, XRD, FTIR, TEM,
EDS and NH3ꢀTPD indicated that the cetyltrimethylammonium
bromide (CTAB) and TPAOH aided prepared ZSMꢀ
5@mesoporous SAPOꢀ34 coreꢀshell composites had large
surface area, and high catalytic activity for the MTA. These
excellent physicochemical properties for ZSMꢀ5@mesoporous
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mesoporous SAPOꢀ34, which shorten diffusion path length
within the composites zeolites. Under optimal conditions, the
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achieved a power conversion efficiency of approximately 100%
and higher yield of aromatics, which is comparable with the
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This project was financially supported by scientific and
technological department of Liaoning Province (No. 201202126).
We appreciate anonymous reviewers for their helpful suggestions on
the quality improvement of our present paper.
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a
College of Chemical Engineering, China University of Petroleum,
Qingdao 266555, P. R. China. Eꢀmail: fswhy@126.com; Tel: +86ꢀ24ꢀ
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College of Shun Hua , Liaoning Shihua University, Fushun 113001;
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College of Chemistry, Chemical Engineering and Environmental
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