Conversion of methanol to propylene over nano-sized ZSM-5 zeolite aggregates synthesized by a modified seed-induced method with CTAB

Article abstract of DOI:10.1039/c6ra14753d

A facile and economical route to synthesize nano-sized ZSM-5 zeolite aggregates with mesopores was developed by a modified seed-induced method using silicate-1 (S-1) as seeds without additional templates in the presence of a trace amount of cetyltrimethylammonium bromide (CTAB). The effect of the amounts of CTAB and S-1 on the morphologies and textural properties were studied in detail, and the possible formation mechanism was proposed. The primary crystal sizes of the ZSM-5 zeolites could be adjusted and mesopores were generated in a certain range. The resulting ZSM-5 zeolites showed a uniformly conglobate morphology with particle sizes of 400-600 nm, aggregated with 20-50 nm crystallites, possessing a large external surface area and abundant inter-crystallite mesopores under the optimum synthesis conditions (CTAB to total SiO₂ molar ratio was 0.02 and SiO₂ in S-1 gel took up 8% to the total SiO₂). The catalytic performance of nano-sized ZSM-5 zeolites in methanol to propylene reaction showed a high stability and high propylene selectivity.

Full text of DOI:10.1039/c6ra14753d

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Conversion of methanol to propylene over nano-sized ZSM-5 zeolites aggregates synthesized
by modified seed-induced method with CTAB
Hengbao Chen, Yaquan Wang*, Fanjun Meng, Hongyao Li, Shougui Wang, Chao Sun, Shuhai
Wang, and Xiao Wang
Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical
Engineering & Technology, Tianjin University, Tianjin 300072, P. R. China
Abstract
A facile and economical route to synthesize nanoꢀsized ZSMꢀ5 zeolites aggregates with
mesopores was developed by a modified seedꢀinduced method using silicateꢀ1 (Sꢀ1) as seeds
without additional templates in the presence of trace amount of cetyltrimethylammonium bromide
(CTAB). The effect of the amounts of CTAB and Sꢀ1 on the morphologies and textural properties
were studied in detail, and the possible formation mechanism was proposed. The primary crystal
sizes of the ZSMꢀ5 zeolites could be adjusted and mesopores were generated in a certain range. The
resulting ZSMꢀ5 zeolites showed a uniformly conglobate morphology with particle sizes of 400ꢀ600
nm, aggregated with 20ꢀ50 nm crystallites, possessing large external surface area and abundant
interꢀcrystallite mesopores under the optimum synthesis conditions (CTAB to total SiO
2
molar ratio
was 0.02 and SiO in Sꢀ1 gel took up 8% to the total SiO ). The catalytic performance of nanoꢀsized
2
2
ZSMꢀ5 zeolites in methanol to propylene reaction exhibited high stability and high propylene
selectivity.
Keywords: Mesoporous ZSMꢀ5; CTAB; Seed; Nanoꢀsized; MTP
1
. Introduction
Propylene is one of important organic raw materials in the petrochemical industry, widely used
1
ꢀ3
for the production of propylene derivatives. In the past decade, Lurgi company has successfully
developed a methanol to propylene (MTP) technology based on ZSMꢀ5 zeolites catalysts,
demonstrating that it is a promising process for the production of propylene from nonꢀpetroleum
⃰
Corresponding author. Tel: +86 22 23507881; Fax: +86 22 23507881.
E-mail address: yqwang@tju.edu.cn (Y. Q. Wang).
1

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4,5
sources such as coal and natural gas. However, the microporous characteristic of ZSMꢀ5 often
leads to intraꢀcrystalline diffusion limitations and a rapid deactivation associated with coke
6
7,8
depositions, which severely hinders its practical applications.
To overcome these disadvantages of the pure microporosity in ZSMꢀ5, two main strategies have
9
been developed in recent years. One strategy is to reduce the crystal sizes of ZSMꢀ5. The decrease
in the crystal sizes from the micrometer to nanometer scale could increase the external surface area
and decrease the diffusion path length, which is conducive for the enhancement of catalytic
10
stability. Within the past few decades, nanoꢀsized ZSMꢀ5 zeolites have been successfully
11ꢀ13
synthesized by the hydrothermal crystallization.
However, zeolite crystals of particles no bigger
than 100 nm might be thermodynamically unstable due to high surface energy and vast amounts of
14
surface defects. Meanwhile, nanocrystals are difficult to handle and have low yields during
synthesis, in which the majority of the building units are left unused in the mother liquid. Another
strategy is to prepare ZSMꢀ5 with both micropores and mesopores, which are usually referred to as
15
mesoporous ZSMꢀ5 or hierarchical structured ZSMꢀ5. To this end, various attempts to synthesize
mesopores ZSMꢀ5 zeolites have been reported, including mainly post treatments and indirect
16
17ꢀ19
templating methods. Post treatments can be divided into steaming dealumination
, acid
2
0ꢀ22
23ꢀ26
dealumination
and base desilication
. Though mesopores can be developed, the etching of
zeolites is often accompanied by the loss of crystallinity, which leads to pernicious effects on the
catalytic performance. Indirect templating method is to add a lot of mesoꢀtemplate agents in the
27ꢀ30
synthetic gels with the micropore template of tetrapropylammonium hydroxide (TPAOH).
In
31
2
009, Choi et al. developed MFI zeolite nanosheets with a thickness of 2.5 nm along the bꢀaxis by
designing a diꢀquaternary ammoniumꢀtype surfactant, and the MFI zeolite nanosheets dramatically
suppressed the catalyst deactivation through coke deposition in methanolꢀtoꢀgasoline conversion.
2

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32
Recently, Kim et al. also reported that MFI zeolite nanosheets synthesized with multiammonium
surfactants showed greater activity and selectivity to propylene and longer lifetimes than
commercial ZSMꢀ5 in the MTP reaction, this was attributed to the wellꢀdeveloped nanosheet
33
structure with a high specific surface area and a large mesopore volume. Zhang et al. developed a
hierarchical ZSMꢀ5 zeolite with bꢀoriented thinner dimension in fluoride medium, and the obtained
material displayed a greatly longer catalytic lifetime and higher propylene selectivity in MTP
reaction than conventional ZSMꢀ5 catalysts, due to the superior diffusivity of thinner bꢀaxis path
and intracrystal secondary mesopores. However, with these methods, the large usage of TPAOH
and mesoporogens makes the ZSMꢀ5 zeolites expensive and produces environmental pollution.
A seedꢀinduced method is an interesting alternative to synthesize the nanoꢀsized ZSMꢀ5 zeolites
34,35
since its low consumption of templates and rapid crystallization of ZSMꢀ5 zeolites.
Nevertheless, the intergrowth and secondary growth of crystals usually occur, leading to the
34
formation of compact aggregates with larger crystal sizes in respect to the seed particles. To
overcome the problem, the effort has been done to develop seedꢀinduced method in the presence of
various mesoporogens such as cationic surfactants and special organosilanes. Frunz and
3
6
coꢀworkers reported that the mesoporous ZSMꢀ5 crystals were synthesized by aging of ZSMꢀ5
synthesis solution to form the protozeolitic seeds, and subsequently adding a surfactant of
cetyltrimethylammonium chloride (CTACl). Nearly identical method was adopted to synthesize
hierarchical mesoporous ZSMꢀ5 using cetyltrimethylammonium bromide (CTAB) instead of
14
37
CTACl by Zhu et al. and Gonçalves et al. . However, in these methods, a large number of
38
TPAOH and CTAB were still used. Recently, Gao et al. prepared a series of ZSMꢀ5 nanozeolites
with hierarchical porosity by seedꢀassisted method and the addition of
a
phenylaminopropyltrimethoxysilane (PATMOS) into the medial synthesis system. The molar ratio
3

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of TPAOH introduced by the addition of seeds to total SiO
2
(TPAOH/SiO
2
) is 0.05ꢀ0.1, much lower
than above methods, but the use of expensive organosilanes restricts its wide range of applications
in industry.
Herein, nanoꢀsized ZSMꢀ5 zeolite aggregates were directly synthesized by a facile and
economical approach using silicaliteꢀ1 (Sꢀ1) as seeds in the presence of CTAB. The obtained
ZSMꢀ5 zeolites display high crystallinity, large specific surface area and external surface area,
abundant intercrystal mesoporosity due to selfꢀassembled nanocrystallites and a markedly
improvement in the stability and propylene selectivity in MTP reaction. Moreover, the synthesis
2
method has the advantage of low TPAOH and CTAB consumption (TAPOH/SiO =0.018,
CTAB/SiO = 0.02).
2
2
. Experimental
2
.1. Materials
2
The reagent grade chemicals including sodium aluminum (NaAlO , AR), CTAB (AR), Sodium
hydroxide (NaOH, AR), ethanol (EtOH, AR), TPAOH (24.8 wt.% in water) and methanol (AR)
were obtained from Tianjin Guangfu Fine Chemical Research Institute Co., Ltd. Tetraethyl
orthosilicate (TEOS, 98 wt.%) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd.
Silica sol (40 wt% suspension in water) was purchased from Qingdao Haiyang Chemical Co., Ltd.
Ammonium nitrate (NH
4
NO
3
, AR) was purchased from Guangdong Xilong Chemical Co., Ltd. A
/Al =160) was purchased from the Catalyst Plant of Nankai
commercial ZSMꢀ5 zeolites (SiO
2
2 3
O
University for comparison.
2
.2. Synthesis of the S-1 gel
Sꢀ1 gel used as seeds with a molar composition of 100 SiO
2
2
: 24 TPAOH: 800 EtOH: 2400 H O
3
9
was prepared according to our previous work. 10.98 g of TEOS, used as silica source, was added
4

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dropwise into 10.37 g of TPAOH solution mixed with 15.0 g deionized water and 9.71 g EtOH
while stirring. After homogenizing at room temperature for 2 h, the mixture was transferred to a
Teflonꢀlined autoclave, sealed, and hydrothermally treated at 100 °C for 96 h. After cooling, the
obtained Sꢀ1 gel was directly used for the synthesis of ZSMꢀ5 zeolites without any treatments.
2
.3. Preparation of nano-sized ZSM-5 aggregates
To synthesize the nanoꢀsized ZSMꢀ5 aggregates, silica sol was used as silica source and NaAlO
2
was used as aluminum source. NaOH was added to adjust the pH. In a typical run, 0.6782 g NaOH,
0
2 2
.0887 g NaAlO and 0.664 g CTAB were dissolved in 30 g H O. After vigorously stirring for 0.5 h,
6
.3755 g Sꢀ1 gel and 12.446 g silica sol were dropwise added into the clear solution in sequence
under stirring. Then, the mixture with a molar composition of 100 SiO : 0.6 Al O : 10 Na O: 2500
2
2
3
2
2 2 2
H O: 2 CTAB: 8 Sꢀ1(SiO molar content in respect to total SiO ) was stirred at ambient temperature
for 2 h, and transferred into Teflonꢀlined autoclave, aging at 120 °C for 12 h and subsequent
crystallization at 170 °C for 24 h were carried out. The products were centrifuged and washed with
distilled water until the pH reached about 7, dried at 120 °C for 12 h, and calcined at 550 °C for 6 h.
The obtained samples were denoted NꢀZSMꢀ5.
To better investigate the effect of CTAB added in the precursor solution on the properties of the
final ZSMꢀ5 products, five samples with different CTAB/SiO
2
ratios (0.005, 0.015, 0.02, 0.03 and
0
.05, respectively) were synthesized with the same synthesis method of NꢀZSMꢀ5, and they were
denoted as ZꢀxCꢀ0.08S (x represents the CTAB/SiO
2
molar ratio). Here, NꢀZSMꢀ5
(
CTAB/SiO =0.02) was reꢀsynthesized in order to prove the reliability of this synthesis method, and
2
denoted as Zꢀ0.02Cꢀ0.08S. For comparison, conventional ZSMꢀ5 (denoted as ConꢀZSMꢀ5) was
synthesized under the same condition of NꢀZSMꢀ5, but in the absence of CTAB. Both
5

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representative NꢀZSMꢀ5 and ConꢀZSMꢀ5 zeolites were ion exchanged three times in 1 M NH
4 3
NO
solution at 80 ºC for 2 h, and then calcined at 550 °C to obtain the Hꢀtype ZSMꢀ5.
Similarly, to investigate the effect of seeds on the properties of ZSMꢀ5 zeolites, four samples
with different Sꢀ1/SiO ratios (0.005, 0.02, 0.04 and 0.06, respectively) were synthesized with the
2
same synthesis method of NꢀZSMꢀ5, and they were denoted as Zꢀ2CꢀyS (y represents the Sꢀ1/SiO2
ratio).
2
.4. Catalyst characterization
Scanning electron microscopy (SEM) and transmission electron microscope (TEM) were used
to determine the size of the crystals and particles and the morphology of the ZSMꢀ5 samples. SEM
images were recorded on an Sꢀ4800 field emission scanning electron microscopy with an
accelerating voltage of 3 kV. TEM was carried on a JEMꢀ2100F instrument operating at 200 kV.
For the TEM image, the specimens were dispersed in ethanol and placed on holey copper grids.
Xꢀray diffraction (XRD) patterns were obtained at room temperature on a Rigaku D/max2500
diffractometer employing the graphite filtered Cu Kα radiation (λ = 0.1542 nm) with a scanning rate
ꢀ1
of 8° min in the 2θ ranges from 5° to 55°.
Nitrogen adsorption and desorption isotherms of the samples were measured at liquid N
2
temperature (77 K) on a Micromeritics TriStar 3000 automated physisorption instrument. Prior to
the measurements, all the samples were degassed at 300 °C for 4 h. The total specific surface area
(SBET) was derived from the BrunauerꢀEmmettꢀTeller (BET) equation. The micropore surface area
(
Smicro) was derived from the tꢀplot method, and external surface area (S ) was calculated by
ext
0
subtracting Smicro from SBET. The total pore volume (Vtotal) was calculated at p/p = 0.99. The
micropore volume (Vmicro) was also derived from the tꢀplot method, and mesopore volume (Vmeso
)
6

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was calculated by subtracting Vmicro from Vtotal. The mesopore size distributions were estimated by
using the BarrettꢀJoynerꢀHalenda (BJH) method from the desorption branch of the isotherms.
Fourier transform infrared (FTꢀIR) spectra of the samples were recorded in a Bruker Vertex 7.0
spectrometer, using the KBr wafer technique. The infrared absorbance spectra were recorded from
ꢀ1
ꢀ1
2
000 to 400 cm with a resolution of 4 cm .
3
Temperatureꢀprogrammed desorption of ammonia (NH ꢀTPD) measurements were recorded
using a Micromeritics 2910 chemical adsorption instrument. The samples were pretreated in a
nitrogen flow at 400 °C for 1 h, and then cooled down to 100 °C, and ammonia was introduced with
nitrogen as the carrier gas. After 60 min, the flow was switched to nitrogen, and the sample was
ꢀ1
heated to 700 °C at a rate of 10 °C min . The desorbed ammonia was monitored by a thermal
conductivity detector.
Solidꢀstate magic angle spinning nuclear magnetic resonance (MAS NMR) was used to
investigate the coordination of Al and Si atoms in the ZSMꢀ5 framework. The measurements were
performed on a Varian infinityplus 300MHz NMR spectrometer equipped with two radio frequency
2
7
channels at a field strength of 7.0 T. Al MAS NMR spectra were recorded at 104.2 MHz at a
2
9
spinning frequency of 5 kHz and 2.05 s intervals between successive accumulations. Si MAS
NMR spectra were recorded at 79.4 MHz at a spinning frequency of 5 kHz and 6.05 s intervals
between successive accumulations.
Elemental analyses were carried out on an inductively coupled plasma optical emission
spectroscope (ICPꢀOES) using a Varian VistaꢀMPX emission spectrometer.
Thermogravimetric (TG) analysis was evaluated by Shimadzu TGAꢀ50 apparatus using a
ꢀ1
temperature ramp from 30 to 800 °C with a heating rate of 10 °C min in oxygen atmosphere.
2
.5. Catalytic tests
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The MTP reaction was tested in a continuous flow fixed bed reactor at 470 °C under
ꢀ1
atmospheric pressure and the weight hourly space velocity (WHSV) was kept constant at 8 h . The
catalyst loading was 0.5 g (mesh size: 20 ꢀ 40), and it was loaded into the middle section of a quartz
tubular reactor with a 20 mm internal diameter and a total length of 400 mm. A thermocouple was
positioned in the center of the catalyst bed in order to monitor the temperature. The catalyst was
ꢀ1
2
first activated at 470 °C for 1 h under high purity N at a flow rate of 60 ml min , prior to the start
of the MTP reaction. HPLC infusion pump fed a liquid solution of methanol in water (50 mol.%) at
a flow rate of 0.12 ml/min. After catalyst activation, high purity nitrogen at a flow rate of 60 ml
ꢀ1
min controlled using a mass flow controller carried the vaporized methanol into the reactor. To
avoid possible condensation of hydrocarbons, the temperature of the effluent line was constantly
maintained at 180 °C. An onꢀline gas chromatograph (GCꢀSPꢀ3420) equipped with a flame
ionization detector (FID) and a 50 m capillary column (HPꢀPLOTꢀQ) analyzed the products. The
reaction performance results, including methanol conversion and product selectivity, were
subsequently calculated by Eqs. (1) and (2).
i
o
o
N
ꢀ(N
+ 2N
)
MeOH
MeOH
DME
Methanol conversion (%) =
×100
(1)
i
N
MeOH
i
x× N
C H
x
y
Selectivity (%)
=
×100
(2)
i
o
o
N
ꢀ(N
+ 2N
)
DME
MeOH
MeOH
i
o
are to the components at the inlet and
Where, N is the number of moles. Superscript
and
outlet of reactor, respectively, x is the number of carbon atoms. Dimethyl ether (DME) is
4
0
considered as reactant.
3
. Results and discussion
3
.1. Characterization of nano-sized ZSM-5 zeolites aggregates
8

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SEM images (Fig. 1aꢀc) clearly shows a uniformly globular morphology of NꢀZSMꢀ5 with
particle sizes of 400ꢀ600 nm homogeneously aggregated with 20ꢀ50 nm nanoꢀcrystallites. The
aggregation due to the in situ selfꢀassembly of these crystallites during the hydrothermal synthesis
2
may give rise to abundant interꢀcrystallite mesopores. The N adsorptionꢀdesorption isotherm (Fig.
1
d) shows that there is a large hysteresis loop at p/p of 0.45ꢀ0.95 in NꢀZSMꢀ5, which indicates the
0
presence of mesoporous structure, formed in aggregates, and the hysteresis loop is a typeꢀH4
isotherm, suggesting that both the micropores and mesopores exist. BJH pore size distribution of
NꢀZSMꢀ5 sample shown in Fig. 1d inset exhibits a narrow peak at 3.6 nm and a relatively broad
peak at 52.9 nm, revealing that NꢀZSMꢀ5 contains abundant mesopores. The texture properties of
2
ꢀ1
NꢀZSMꢀ5 are listed in Table 1. NꢀZSMꢀ5 exhibits a high specific surface area (414 m g ) and a
3
ꢀ1
large mesoporous volume (0.23 cm g ). TEM image (Fig. 1e) shows that NꢀZSMꢀ5 particles are
loosened texture, containing abundant mesopores open at the external surface, which is agreement
2
with the results of SEM and N adsorptionꢀdesorption isotherm. Under high magnification (Fig. 1f),
clear lattice fringes are observed, confirming that nanoꢀcrystallites are highly crystalline.
XRD patterns (Fig. 2a) show that NꢀZSMꢀ5 has a typical MFI structure (peaks at 2
θ
of 7.9°,
3
9
8
.7°, 23.1°, 23.9° and 24.4°). FTꢀIR spectra (Fig. 2b) of NꢀZSMꢀ5 show intensive absorption
ꢀ1
ꢀ1
ꢀ1
bands at 1150ꢀ1050 cm , 795 cm and 455 cm , associated with internal asymmetric stretch,
7
external symmetric stretch and TꢀO bend respectively. The optical density ratios of the absorption
ꢀ1
ꢀ1
bands at 550 cm and 455 cm for NꢀZSMꢀ5 is 0.71, which is near to 0.7 for pure pentasil samples
7
with high crystallinity of ZSMꢀ5 , indicating that NꢀZSMꢀ5 has a good MFI frame structure.
2
7
29
Al and Si MAS NMR was used to further reveal the coordination of Al and Si atoms in
27
NꢀZSMꢀ5. One distinctive resonance peak at a chemical shift of 53 ppm in the Al MAS NMR
spectrum (Fig. 2c) which corresponds to tetrahedrally coordinated Al in the zeolite framework is
9

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observed. Moreover, no resonance at 0 ppm, typically associated with the octahedrally coordinated
41
Al in the extraꢀframework positions, confirms that the Al atoms are totally incorporated into the
2
9
crystal frameworks of the NꢀZSMꢀ5. Fig. 2d shows the Si MAS NMR spectra of NꢀZSMꢀ5. The
intensive resonances at 114.5 ppm and 111.0 ppm are associated to the Si (4Si, 0Al) sites, and the
4
2
peak at 104.2 ppm corresponds to Si (3Si, 1Al) sites. Resonances representing Si (2Si, 2Al) with
chemical shift below 100 ppm sites are not observed, indicating that [AlꢀOꢀSiꢀOꢀAl] sequences are
4
3
not present in the highly siliceous NꢀZSMꢀ5. The resonance at 100.7 ppm of NꢀZSMꢀ5 belongs to
the Si (3Si, 1OH) sites, which can be attributed to the hydroxyl groups on the large external surface
28
of NꢀZSMꢀ5.
3
.2. Effect of CTAB/SiO ratio on the properties of ZSM-5
2
2
The synthetic conditions of five samples with different CTAB/SiO ratios (0.005, 0.015, 0.02,
0
.03 and 0.05, respectively) and ConꢀZSMꢀ5 are summarized in Table 1. Here, NꢀZSMꢀ5
=0.02) was reꢀsynthesized in order to prove the reliability of this synthesis method.
molar ratios and texture properties of all samples are also summarized in Table 1.
(CTAB/SiO
2
2 2 3
The SiO /Al O
All the ZSMꢀ5 samples have similar SiO /Al O ratio, however, it is obvious that the CTAB/SiO
2
2
2
3
ratio has a significant impact on the textural properties of the ZSMꢀ5 zeolites. At the relatively low
CTAB/SiO ratio, increasing the addition amounts of CTAB gives rise to the increase of total
specific surface area (SBET), external surface area (Sext), pore volume (Vtotal) and mesoporous
volume (Vmeso). As the CTAB/SiO is increased to 0.02, the sample (NꢀZSMꢀ5) exhibits the most
excellent textural properties, It is noteworthy that SBET, S , V and Vmeso are 416 m g , 271 m
2
2
2
ꢀ1
2
2
ext
total
ꢀ1
3
ꢀ1
3
ꢀ1
2
ꢀ1
g , 0.30 cm g and 0.23 cm g , significantly higher than those of ConꢀZSMꢀ5 (342 m g ,161 m
ꢀ1
3
ꢀ1
3
ꢀ1
g , 0.21 cm g and 0.09 cm g , respectively). Further increasing the CTAB/SiO
2
ratio from 0.02
to 0.05, SBET, Sext, Vtotal and Vmeso rapidly decrease.
1
0

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The XRD patterns (Fig. 3) of all the synthesized samples show typical diffraction characteristics
for ZSMꢀ5 structure, and no amorphous phase is detected, suggesting that they are successfully
2
synthesized. The corresponding SEM images of the samples with different CTAB/SiO ratios are
shown in Fig. 4. Though all the samples have similar particle sizes, their morphologies are
distinctly different. ConꢀZSMꢀ5 (Fig. 4a) has irregular blockꢀlike morphology aggregated with
nanocrystals above 100 nm sizes. After the addition of CTAB, the morphologies of the samples (Fig.
4
bꢀf) show significant change compared to that of ConꢀZSMꢀ5. At the relatively low CTAB/SiO
2
ratio from 0.005 to 0.02, the particles of ZSMꢀ5 zeolites comprise the globular agglomerates of
nanoꢀcrystallites, and the average sizes of the nanoꢀcrystallites gradually decrease to 30 nm, which
is much smaller than that of ConꢀZSMꢀ5. The results indicate that CTAB may interact with the
crystal nucleus and inhibit the crystal growth. The SEM image (Fig. 4d) of the sample with
CTAB/SiO
repeatability of this method to synthesize nanoꢀsized ZSMꢀ5 aggregates with mesopores. While the
samples with CTAB/SiO ratio of 0.03 and 0.05 (Fig.4e and f) show less rough surface, indicating
2
ratio of 0.02 is in good accordance with that of the NꢀZSMꢀ5 (Fig. 1b), confirming the
2
that excessive amounts of CTAB results in the growth of crystal again.
+
+
To clarify the behaviour of CTA in the synthesis, it is useful to evaluate the amount of CTA
incorporated in the asꢀsynthesized samples by TG analysis. Fig. 5a presents the TG curves of
+
ConꢀZSMꢀ5 and NꢀZSMꢀ5 synthesized with different CTA /SiO
2
ratio before calcination. All
samples show two regions of weight loss. The first region below 300 ºC is assigned to desorption of
water. Both ConꢀZSMꢀ5 and the sample with CTAB/SiO ratio of 0.005 have a weight loss of 1.3%.
2
2
Increasing the CTAB/SiO ratio to 0.5, the weight loss increases greatly to 6.8%. This may be
ascribed to hydrophilicity of the surface with more CTAB, which adsorbs more water. The second
4
4
region of 300 ºCꢀ500 ºC is ascribed to the combustion of the organic template. For ConꢀZSMꢀ5,
1
1

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+
the organic template of TPAOH is introduced in trace amount (TPA /SiO
2
=0.018) with the Sꢀ1
+
seeds, but the weight loss is 7.4%, which is much larger than the mass percentage of TPA in the
total mass (2.2%). Therefore, it is believed that the more weight loss is attributed to the
condensation water from the elimination of the surface hydroxyl groups or removal of chemically
45
bound water. For NꢀZSMꢀ5 with CTAB/SiO of 0.005, the weight loss increases by 1.8% in
2
+
comparison with that of ConꢀZSMꢀ5, proving that most of CTA is occluded into the asꢀsynthesized
2
sample. However, when more CTAB is added, i.e., with CTAB/SiO of 0.02 and 0.05, the weight
loss only increases slightly, indicating that the excessive amounts of CTAB cannot be occluded into
+
the asꢀsynthesis samples. Fig. 5b further shows the amount of CTA incorporated into the samples
+
and the variation of mesopore volumes as a function of CTA /SiO ratios. With the addition of
2
+
CTAB at low level, the mesopore volume increases due to the incorporation of CTA into the
+
samples and/or the interaction of CTA with the crystal nucleus to restrain the growth of crystals
and form nanoꢀsized aggregates. However, the addition of excessive amounts of CTAB is not
needed and has a negative effect with the mesopore volume being decreased.
3
.3. Effect of S-1/SiO ratio on the properties of ZSM-5
2
Seed also plays an important role in the synthesis of zeolites and tunes the properties of the final
1
2
products. Four samples with different Sꢀ1/SiO
were synthesized with the same synthesis method of NꢀZSMꢀ5. Their SiO
properties are also listed in Table 1. The four samples all have similar SiO
2
ratios (0.005, 0.02, 0.04 and 0.06, respectively)
/Al ratios and texture
/Al ratios. With the
2
2 3
O
2
2 3
O
2
ꢀ1
2
ꢀ1
increase of Sꢀ1/SiO ratio from 0.005 to 0.06, the SBET increases from 323 m g to 381 m g , and
2
2
ꢀ1
3
ꢀ1
the corresponding Sext, Vtotal and Vmeso of the samples increases to 213 m g , 0.26 cm g and 0.19
3
ꢀ1
cm g , respectively. This means the textural property especially external surface area and
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mesoporosity can be tuned, to a certain extent, by changing the using amount of the Sꢀ1 seeds in the
preparation of nanoꢀsized ZSMꢀ5 zeolites.
XRD patterns in Fig. 6 show that all the synthesized samples with high crystallinity are obtained.
SEM images of ZSMꢀ5 zeolites synthesized with different Sꢀ1/SiO
the ZSMꢀ5 zeolites are the aggregated spheres of nanoꢀcrystallites. Although the average particle
sizes of ZSMꢀ5 zeolites have a reduction from 800 nm to 500 nm with increasing the Sꢀ1/SiO ratio
of 0.005 to 0.02, all the ZSMꢀ5 zeolites show almost same average particle sizes around 500 nm
further increasing the Sꢀ1/SiO ratio up to 0.06. It should be pointed out that the average sizes of
primary nanoꢀcrystallites gradually decrease from 100 nm to 40 nm with the increase of the
Sꢀ1/SiO ratio, which conduces to the increase of S , S , V and Vmeso, as shown in Table 1.
2
ratios are shown in Fig. 7. All
2
2
2
BET
ext
total
Similar results are also reported by Teng et al., who believed that more nuclei and surface were
3
5
provided with the increase of seeds, and accordingly the primary particle became smaller. The
result of SEM images reveals that the employed amount of seeds has an important effect not only
on the size of the ZSMꢀ5 zeolites particles but also on the primary nanoꢀcrystallites.
3
.4. Form mechanism of nano-sized ZSM-5 with mespores
According to seed surface crystallization mechanism suggested by Tang and coꢀworkers, the
46
crystallization of ZSMꢀ5 zeolites only occurs on the surface of seeds and accordingly the primary
particle becomes larger than the seed crystals, but the primary crystals of the NꢀZSMꢀ5 aggregates
are obviously smaller than the Sꢀ1 seeds with the sizes of 180 nm (Fig. 8a) in our syntheses.
Therefore, a different process of the zeolite growth may be followed. Based on the above research
by varying the CTAB/SiO
2 2
and Sꢀ1/SiO ratios of synthesis precursor, a possible formation
mechanism of NꢀZSMꢀ5 is proposed in this paper. Firstly, the Sꢀ1 seeds are dissolved into small
47
fragmentations to provide more nuclei and surface of crystal growth, as reported in literatures.
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Then these fragmentations induce the precursor aluminosilicates into ZSMꢀ5 phase. Nevertheless, it
is difficult for the growth of single isolated crystals because of abundant intergrowth and secondary
growth, such as the ConꢀZSMꢀ5 (Fig. 4a). When the amphiphilic surfactants CTAB added into the
precursor, each crystal nucleus interacts with CTAB and is dispersed, accordingly the intergrowth
and secondary growth are inhibited. Meanwhile, large amounts of nanocrystals are formed, and
further selfꢀassemble into aggregates. Increasing the added amounts of seeds, the more crystal
nuclei are provided, and the sizes of primary crystals of ZSMꢀ5 aggregates are smaller. Based on
the study above, the possible mechanism of the nanoꢀsized ZSMꢀ5 aggregates synthesized by
modified seedꢀinduced method with CTAB is proposed as Fig. 8b.
3
.5. Catalytic performance on MTP reaction
To investigate the catalytic properties of nanoꢀsized ZSMꢀ5 aggregates synthesized in this paper,
both representative NꢀZSMꢀ5 and ConꢀZSMꢀ5 catalysts were characterized by
temperatureꢀprogrammed desorption of ammonia (NH ꢀTPD). Fig. 9 displays that the curves of
both catalysts with two peaks attributed to the desorption of NH on the weak acidic sites and strong
3
3
acidic sites are similarꢀshaped, respectively. Distinctly, the amounts of weak acid and strong acid of
NꢀZSMꢀ5 are higher than those of ConꢀZSMꢀ5 (Table 2). Besides, the peak of weak acid displays a
slight shift to lower temperature. It is probably due to that there are more accessible acid sites in
NꢀZSMꢀ5 because of abundant mesopores and larger external surface area. Considering the
excellent textural properties and modified acidity, NꢀZSMꢀ5 catalyst is expected to show better
catalytic performances in the MTP reaction, compared with ConꢀZSMꢀ5.
To quickly evaluate the catalytic activities, MTP reactions over NꢀZSMꢀ5, ConꢀZSMꢀ5 and
ꢀ1
commercial ZSMꢀ5 were carried out with WHSV of 8 h . The conversion of methanol is presented
as a function of time on stream (TOS) for the three catalysts in Fig. 10a. Methanol conversions over
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NꢀZSMꢀ5, ConꢀZSMꢀ5 and commercial ZSMꢀ5 are approximately 100% during the initial 37 h, 11
h and 20 h TOS, respectively, indicating the high initial stability of all catalysts. If the catalyst
deactivation is defined as methanol conversions decreasing to around 95%, the catalytic lifetime of
NꢀZSMꢀ5 (40 h) is much longer than that of ConꢀZSMꢀ5 (15 h) and commercial ZSMꢀ5 (25 h).
Therefore, compared with ConꢀZSMꢀ5 and commercial ZSMꢀ5, NꢀZSMꢀ5 exhibits a pronounced
improvement of the stability.
Fig. 10b displays the selectivity of propylene over NꢀZSMꢀ5, ConꢀZSMꢀ5 and commercial
ZSMꢀ5 catalysts as a function of TOS. At the initial stage (0.3 h), the selectivity towards propylene
is amount to 38.6% for NꢀZSMꢀ5, remarkably higher than that of ConꢀZSMꢀ5 (32.2%) and
commercial ZSMꢀ5 (37.4%). The steady state with the stable propylene selectivity of the reaction is
observed after about 2 h TOS for NꢀZSMꢀ5, 5 h for ConꢀZSMꢀ5 and 8 h for commercial ZSMꢀ5.
The steady period of reaction for NꢀZSMꢀ5 reaches more than 30 h, much longer than ConꢀZSMꢀ5
=
=
(
13 h) and commercial ZSMꢀ5 (18 h). The selectivity of ethylene and light olefin (C
2 4
ꢀC ) for
NꢀZSMꢀ5, ConꢀZSMꢀ5 and commercial ZSMꢀ5 are shown in Fig. 10c and d. The average values of
=
2
=
selectivity towards propylene, ethylene and C ꢀC for NꢀZSMꢀ5 at steady state are 39.5%, 12.2%
4
and 71.7%, respectively, higher than ConꢀZSMꢀ5 (35.7%, 10.6% and 62.3%, respectively) and
commercial ZSMꢀ5 (37.9%, 11.5% and 67.9%, respectively), leading to a similar
propylene/ethylene ratio (3.23, 3.36 and 3.29 respectively). With prolonged TOS, the deactivation
stage occurs, and the propylene selectivity for all catalysts decreases slowly.
Fig. 11 further shows the coke deposition on the deactivated NꢀZSMꢀ5 and ConꢀZSMꢀ5
catalysts measured by TG analysis. NꢀZSMꢀ5 contains 2.43 wt.% coke deposit after 41 h TOS,
while for ConꢀZSMꢀ5 the coke deposit is 2.11 wt.% after 16 h. The average coking rate on
NꢀZSMꢀ5 is 0.06% per hour, much lower than that of 0.13% on ConꢀZSMꢀ5 catalyst. NꢀZSMꢀ5
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exhibits a strong capability for coke tolerance, and retardation of catalyst deactivation. The
excellently catalytic performance of NꢀZSMꢀ5 catalyst owes to that the nano sizes, large external
surface area, and abundant mesopores of NꢀZSMꢀ5 can improve the diffusivity of methanol and
48
products, and restrain the coke deposition, thereby leading to the pronounced improvement of
catalytic stability and propylene selectivity in MTP reaction.
4
. Conclusions
Highly crystalline nanoꢀsized ZSMꢀ5 zeolite aggregates (NꢀZSMꢀ5) with abundant interꢀcrystal
mesopores are directly synthesized by a modified seedꢀinduced method in the presence of CTAB.
The addition of a small amount of CTAB can restrain the intergrowth and secondary growth of the
primary nanoꢀcrystallites, therefore the nanoꢀsized ZSMꢀ5 aggregates are facilely obtained by the
selfꢀassembly of the nanoꢀcrystallites. Increasing the amounts of Sꢀ1 seeds gives rise to the gradual
decrease of the primary crystallites sizes. NꢀZSMꢀ5 has a remarkable improvement on the stability
and propylene selectivity in the methanol to propylene reaction compared with the conventional
ZSMꢀ5 and the commercial ZSMꢀ5. This difference is due to the large external surface area and
mesopore volume, which improve the diffusion rate and enhance the catalytic efficiency. In
particular, the synthetic strategy provides a facile and economical method for the synthesis of
NꢀZSMꢀ5 because of the low consumption of seeds and CTAB. The synthetic zeolites are expected
to be widely usable in chemical industry as catalysts or adsorbents.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (21276183).
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Table captions
Table 1 List of synthesis conditions and derived parameters obtained from the N2
adsorptionꢀdesorption isotherms of all ZSMꢀ5 zeolites.
Table 2 Acid properties of the NꢀZSMꢀ5 and ConꢀZSMꢀ5 catalysts.
Figure captions
Figure 1 The morphology and textural property of NꢀZSMꢀ5: (aꢀc) SEM images under different
magnification, (d) N adsorption/desorption isotherm and BJH pore size distribution, (e) low
2
magnification of TEM image and (f) high magnification of TEM image.
27
29
Figure 2 XRD patterns (a), FTꢀIR spectra (b), Al MAS NMR spectra (c) and Si MAS NMR
spectra (d) of NꢀZSMꢀ5.
Figure 3 XRD patterns of ConꢀZSMꢀ5 and ZSMꢀ5 zeolites synthesized with different
CTAB/SiO ratios: 0.005, 0.015, 0.02, 0.03 and 0.05.
2
Figure 4 SEM images of ConꢀZSMꢀ5 (a), and ZSMꢀ5 zeolites synthesized with different
CTAB/SiO ratios: (b) 0.005, (c) 0.015, (d) 0.02, (e) 0.03 and (f) 0.05. All scale bars are 1 ꢁm.
2
+
Figure 5 (a) TG curves of ConꢀZSMꢀ5 and NꢀZSMꢀ5 synthesized with different CTA /SiO2
+
ratios before calcination; (b) The amount of CTA incorporated in the samples and variation of
+
mesopore volumes as a function of increasing CTA /SiO molar ratios.
2
Figure 6 XRD patterns of ZSMꢀ5 zeolites synthesized with different Sꢀ1/SiO ratios: 0.005, 0.02,
2
0.04 and 0.06.
Figure 7 SEM images of ZSMꢀ5 zeolites synthesized with different Sꢀ1/SiO ratios: (a) 0.005, (b)
2
0.02, (c) 0.04 and (d) 0.06. All scale bars are 1 ꢁm.
Figure 8 SEM image of Sꢀ1 seed (a) and proposed mechanism for the synthesis of nanoꢀsized
ZSMꢀ5 aggregates by modified seedꢀinduced method with CTAB (b).
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Figure 9 NH ꢀTPD profiles of NꢀZSMꢀ5 and ConꢀZSMꢀ5 catalysts.
3
Figure 10 Methanol conversion (a), propylene selectivity (b), ethylene selectivity (c) and light
olefin selectivity (d) of MTP reaction with time on stream over NꢀZSMꢀ5, ConꢀZSMꢀ5 and
commercial ZSMꢀ5. Reaction conditions: 470 ºC; 0.1 MPa; zeolite 0.5g; methanol WHSV = 8
ꢀ1
ꢀ1
h ; molar ratio of methanol and water = 1:1; carrier gas: high purity N , 60 ml min . Testing was
2
stopped at ~93% conversion.
Figure 11 TG curves of the NꢀZSMꢀ5 and ConꢀZSMꢀ5 catalysts after deactivation.
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Table 1
b
c
c
d
e
e
CTAB/SiO2 Sꢀ1/SiO2 SiO /Al O SBET
Smicro
Sext
2
Vtotal
3
Vmicro
Vmeso
2
2
3
Samples
a
2
ꢀ1
2
ꢀ1
ꢀ1
ꢀ1
3
ꢀ1
3
ꢀ1
ratio
0.02
0
ratio
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.005
0.02
0.04
0.06
ratio
(m g ) (m g ) (m g ) (cm g ) (cm g ) (cm g )
NꢀZSMꢀ5
ConꢀZSMꢀ5
157
155
146
152
158
151
157
152
147
155
159
414
342
366
388
416
358
318
323
369
373
381
141
181
174
169
145
154
169
171
192
173
168
273
161
192
219
271
204
149
152
177
200
213
0.31
0.21
0.22
0.24
0.30
0.22
0.17
0.19
0.23
0.26
0.26
0.08
0.12
0.10
0.08
0.07
0.07
0.10
0.07
0.09
0.09
0.07
0.23
0.09
0.12
0.16
0.23
0.15
0.07
0.12
0.14
0.17
0.19
Zꢀ0.005Cꢀ0.08S
Zꢀ0.015Cꢀ0.08S
Zꢀ0.02Cꢀ0.08S
Zꢀ0.03Cꢀ0.08S
Zꢀ0.05Cꢀ0.08S
Zꢀ0.02Cꢀ0.005S
Zꢀ0.02Cꢀ0.02S
Zꢀ0.02Cꢀ0.04S
Zꢀ0.02Cꢀ0.06S
0.005
0.015
0.02
0.03
0.05
0.02
0.02
0.02
0.02
a
b
c
Determined by ICPꢀOES. Derived from the BET equation. Micropore surface area (Smicro) was derived from the
d
e
tꢀplot method, Sext was calculated by subtracting Smicro from SBET. Calculated at p/p = 0.99. Micropore volume
0
(Vmicro) was evaluated by the tꢀplot method, Vmeso was calculated by subtracting Vmicro from Vtotal.
Table 2
Total acidity
Strong acidity
Weak acidity
Catalysts
ꢀ1
ꢀ1
ꢀ1
/
(mmol g )
0.314
/(mmol g )
/(mmol g )
NꢀZSMꢀ5
0.149
0.140
0.165
0.150
ConꢀZSMꢀ5
0.290
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Figure 1
Figure 2
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RSC Advances
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DOI: 10.1039/C6RA14753D
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RSC Advances
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DOI: 10.1039/C6RA14753D
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RSC Advances
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DOI: 10.1039/C6RA14753D
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RSC Advances
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DOI: 10.1039/C6RA14753D
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RSC Advances
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DOI: 10.1039/C6RA14753D
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