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Synthesis characterization and catalytic performance of SAPO-34 molecular sieves for methanol-to-olefin (MTO) reaction.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2011; 6: 596–605
Published online 20 May 2010 in Wiley Online Library
(wileyonlinelibrary.com) DOI:10.1002/apj.453
Research article
Synthesis, characterization and catalytic performance
of SAPO-34 molecular sieves for methanol-to-olefin (MTO)
reaction
Qian Wang,1 Lei Wang,2 Hui Wang,1 Zengxi Li,1 * Hui Wu,2 Guangming Li,2 Xiangping Zhang2 and Suojiang Zhang2 *
1
2
College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing 10049, China
State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
Received 16 December 2009; Revised 23 March 2010; Accepted 26 March 2010
ABSTRACT: The methanol-to-olefin (MTO) process is important because it offers an alternative to the conventional
oil route for the conversion of natural gas or coal to light olefins. The most promising catalysts for the MTO reaction
are solid acids with a zeolite structure, where reactants and products such as ethylene and propylene may freely
diffuse through active catalysts, but products with larger kinetic diameters are trapped within the cages. In this
work, SAPO-34 molecular sieves were synthesized by hydrothermal crystallization, using triethylamine (TEA) as the
template. These samples were characterized by X-ray diffraction, scanning electron microscopy, thermo-gravimetric
analysis–differential scanning calorimetry, energy dispersive spectroscopy, Fourier transform-infrared (FT-IR) and N2
adsorption–desorption. The effects of template concentration and crystallization time on the physicochemical properties
and catalytic performance were investigated. The crystallinity and morphology of SAPO-34 were influenced by the
concentration of the template. Pure SAPO-34 was obtained when the TEA/Al2 O3 molar ratio was higher than 2.0. With
increased crystallization time, the Si content in SAPO-34 crystals increased, which influenced the surface acidity. FT-IR
spectra indicated that all samples were dominated by Lewis acid sites and the sample crystallized for 11 h exhibited
the lowest number of Lewis acid sites. The catalytic performance of SAPO-34 was tested with the MTO reaction. High
selectivity to olefins (C2 H4 + C3 H6 ) was obtained over the catalyst synthesized with a TEA/Al2 O3 ratio of 3.0 and
crystallization time of 11 h. At the reaction temperature of 450 ◦ C, the methanol conversion approached 100% and the
yield of C2 –C4 olefins was more than 80%. Hence, SAPO-34 molecular sieves were shown to be excellent catalysts
for the MTO reaction.  2010 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: molecular sieve; SAPO-34; solid acid catalysts; methanol-to-olefin reaction; MTO process; TEA;
template concentration; crystallization time
INTRODUCTION
The methanol-to-olefin (MTO) process, regarded as
an alternative to the conventional oil route for the
conversion of natural gas or coal to light olefins,
has been attracting significant attention from academia
and industry.[1,2] The MTO technology developed by
Universal Oil Products Company (UOP) and Hydro
has been extensively demonstrated at a demo plant
in Norway.[3] The most promising catalysts for the
MTO reaction are solid acids with a zeolite structure.
Reactants and products such as ethylene and propylene
*Correspondence to: Zengxi Li, College of Chemistry and Chemical
Engineering, Graduate University of Chinese Academy of Sciences,
Beijing 10049, China. E-mail: zxli@home.ipe.ac.cn
Suojiang Zhang, State Key Laboratory of Multiphase Complex
System, Institute of Process Engineering, Chinese Academy of
Sciences, Beijing 100190, China. E-mail: sjzhang@home.ipe.ac.cn
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Curtin University is a trademark of Curtin University of Technology
may freely diffuse through active catalysts, but products with larger kinetic diameters are trapped within
the cages.[4,5] In the past decade, most of the literature has discussed the zeolite ZSM-5 as an appropriate
catalyst, which possesses a ten-member ring, interconnected channel system.[6] In these works, alkanes and
aromatics in the gasoline range are the main products.[7]
SAPO-34, a microporous silicoaluminophosphate with
chabazite structure (CHA) discovered by Lok et al .,[8] is
now demonstrated as being one of the best catalysts for
an MTO reaction. It gives a narrow range of product distribution with high selectivity to light olefins, which is
mainly attributed to its mild acidity and shape-selective
catalysis by an eight-ring pore opening.[9] Accordingly, many efforts have been focused on improving its catalytic performance by various physicochemical modifications.[10 – 12] The adjustment of acidity as
well as shape selectivity are two key properties to
Asia-Pacific Journal of Chemical Engineering SYNTHESIS, CHARACTERIZATION AND CATALYTIC PERFORMANCE OF SAPO-34
modify the reactive performance of such acidic crystalline material.[13] Although the catalytic process and
the role of the catalyst in the process are fairly well
studied, several parameters remain to be evaluated in
detail.
It is well known that in the synthesis of molecular
sieves template plays important roles, such as structuredirecting, space-filling and charge-compensating.[9]
SAPO-34 can be synthesized with many templates,
in which tetraethylammonium hydroxide (TEAOH) is
the most commonly used. However, TEAOH is an
expensive reagent, and using this will result in an
increase in the cost for the production of the catalysts.
Instead of TEAOH, triethylamine (TEA) was successfully applied as a cheap template in the synthesis of
SAPO-34.[14] The template concentration has a strong
impact on the microscopic structure and morphology
of the obtained materials, and further influences their
catalytic performance.[15]
Owing to the fast deactivation of this catalyst,
acidity adjustment was considered to be an efficient
method for decreasing the rate of deactivation of
SAPO-34. The acidity of SAPO molecular sieves
could be varied through isomorphous substitution of
cations,[11,16] cation exchange[17] and adjustment of the
Si/Al ratio.[18,19] The acidity adjustment through the
Si/Al ratio of SAPO may be achieved in an easier and
cheaper way as it does not need any additional chemical materials.[20] The number and distribution of Si
in the framework are closely related to the synthesis
process, i.e. to the crystallization time.[21] He et al .[22]
analyzed the surface acidities of the SAPO-34 molecular sieve with IR and NH3 -TPD. The results showed
that the Brønsted acidic centers are the main sources
of the acidity of the molecular sieve. Up till now, the
effect of Lewis acidity on the catalytic performance of
SAPO-34 has been scarcely investigated.
In this work, SAPO-34 molecular sieves were synthesized by hydrothermal crystallization with different template concentrations and crystallization time,
using TEA as the template. Their physicochemical
properties were characterized using X-ray diffraction
(XRD), scanning electron microscopy (SEM), thermogravimetric analysis–differential scanning calorimetry
(TG–DSC), energy dispersive spectroscopy (EDS),
Fourier transform-infrared (FT-IR) and N2 adsorption–desorption. The catalytic performance of the catalysts in the MTO reaction was also tested.
EXPERIMENTAL
1.0P2 O5 :0.8SiO2 :(1.0–4.0)TEA:25H2 O. The gel was
prepared by addition of the silica sol (30 wt% SiO2 )
and TEA to the diluted phosphoric acid solution and
the mixture was stirred for 1 h at room temperature.
Pseudoboehmite (70.5 wt% Al2 O3 ) was added slowly
under continuous stirring for 10 min. The final mixture was stirred intensively until a homogeneous gel
was obtained. The gel was sealed in the stainless steel
autoclave lined with polytetrafluoroethylene. After precrystallizing at 100 ◦ C for 1 h, the initial gel was heated
and crystallized at 170 ◦ C for a certain time under autogenetic pressure. The solid product was obtained by
centrifugation, washed with deionized water and dried
at 110 ◦ C. Calcination was carried out in air at 550 ◦ C
for 6 h to remove the organic template.
Characterization
The XRD patterns of as-synthesized catalysts were
recorded on an X’ Pert PRO X-ray diffractometer
using Cu Kα radiation and an angle scanning speed of
4◦ /min between 5◦ and 50◦ . The crystal morphology was
observed using a JSM-6700F SEM. TG–DSC were performed using NETZSCH STA 449C under an air flow
rate of 60 ml/min at a heating rate of 10 ◦ C/min from
room temperature to 900 ◦ C. The Brunaucr-EmmettTeller (BET) surface areas and the pore volumes of
calcined samples were determined from isotherm data
of nitrogen adsorption–desorption obtained at −196 ◦ C
using a Micromeritics GemineV 2380 analyzer. Before
analysis, all samples were degassed at 105 ◦ C under
vacuum to remove the physically adsorbed water. The
chemical composition for Al, P and Si of the calcined
samples was analyzed by a scanning electron microscope equipped with an energy dispersive X-ray spectrometer JEOL-JSM-6301 SEM/EDS.
The in situ FT-IR measurements were used to determine the acidity of the catalyst with pyridine as the
probe molecule. The spectra were recorded using a
Thermo Nicolet380 spectrometer equipped with a heatable and evacuable IR cell with CaF2 windows, connected to a gas-evacuating system. The powdered samples were pressed into self-supporting wafers with a
diameter of 13 mm and weight of ∼10 mg. Prior to
pyridine adsorption, the samples were pretreated at
300 ◦ C in a vacuum (10−3 Pa) followed by cooling to
room temperature. Then, pyridine was adsorbed on the
catalysts for 30 min and the desorption experiments
were conducted at 20, 100, 200 and 300 ◦ C for 10 min,
respectively.
Synthesis
Catalyst testing
The SAPO-34 molecular sieve was synthesized by
hydrothermal crystallization, using TEA as the template.
The initial gel composition was kept as (1.0–2.0)Al2 O3 :
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
The MTO reactions were carried out in a fixed-bed
microreactor in the temperature range of 390–510 ◦ C
Asia-Pac. J. Chem. Eng. 2011; 6: 596–605
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under atmospheric pressure. A total of 0.5 g of catalyst
mixed with an equivalent volume of quartz (30–40
mesh) was loaded into the center of the stainless steel
(i.d., 1 cm; length, 30 cm) reactor. The sample was
activated in a flow of nitrogen at 550 ◦ C for 1 h and then
cooled to the reaction temperature. Methanol was fed by
a tranquil flow pump and mixed with N2 at a flow rate of
60 ml/min. The weight hourly space velocity (WHSV)
was 3 h−1 . The analysis of the reaction products was
performed using an on-line gas chromatograph Agilent
GC 7890A equipped with a flame ionization detector
(FID) and a thermal conductivity detector (TCD) for
hydrocarbons and oxygenates, respectively. Methanol
conversion and light olefin selectivity are defined by
Eqns (1) and (2):
(a)
(b)
MeOH conversion (%) =
[(moles of MeOH fed)
−(moles of MeOH unreacted)]
× 100%
(moles of MeOH fed)
Olefin carbon selectivity (%) =
(moles of carbon atom
in specific olefin produced)
× 100%
[(moles of MeOH fed)
−(moles of MeOH unreacted)]
(1)
(2)
RESULTS AND DISCUSSION
Effect of template concentration
The effect of template concentration on the structure of
SAPO-34 samples was investigated with the gel composition of 1.0Al2 O3 :1.0P2 O5 :0.8SiO2 :25H2 O:x TEA. The
XRD patterns of the fresh and used samples are shown
in Fig. 1A and B. For the fresh samples, when the
TEA/Al2 O3 ratio is 1.0, the crystalline phase is found to
be quite different from others, in which there are multiple components. SAPO-5 with characteristic diffraction
peaks at 2θ = 7.2◦ , 20.9◦ , 22.4◦ as well as AlPO4 , a
dense phase of alumino-phosphate structure, with peaks
appearing around 2θ = 12.5◦ and 17.9◦ cocrystallize
with SAPO-34 (2θ = 9.5◦ ). The diffraction peaks are
weak and broad, indicating that the crystallinity is quite
low. When the TEA/Al2 O3 ratio was higher than 2.0,
typical powder diffraction patterns corresponding to the
CHA structure of SAPO-34 could be observed, where
intensity and peak position of each pattern matched
well with those reported for SAPO-34 material with no
impurity phases.[23] The diffraction peaks of the used
samples are similar to those of the fresh samples, which
reveals that the structure of the used catalysts keeps
invariant. With the increase in the TEA/Al2 O3 ratio
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 1. XRD patterns of (A) the fresh samples
and (B) the used samples synthesized with different
TEA/Al2 O3 ratios (a) 1.0, (b) 2.0, (c) 3.0 and (d) 4.0.
Figure 2. Intensity of reflection at 2θ ≈ 9.5◦ , 16◦ , 20.5◦
in XRD patterns for the fresh samples synthesized with
different TEA/Al2 O3 ratios.
Asia-Pac. J. Chem. Eng. 2011; 6: 596–605
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering SYNTHESIS, CHARACTERIZATION AND CATALYTIC PERFORMANCE OF SAPO-34
Figure 3. SEM images of as-synthesized samples with different TEA/Al2 O3
ratios: (a) 1.0, (b) 2.0, (c) 3.0 and (d) 4.0.
Figure 4. N2 adsorption–desorption isotherms of samples synthesized with different
TEA/Al2 O3 ratios: (a) 1.0, (b) 2.0, (c) 3.0 and (d) 4.0.
from 1.0 to 4.0, the intensity of the peak at 2θ = 9.5◦
is obviously enhanced (Fig. 2).
SEM results (Fig. 3) display that the sample synthesized with the TEA/Al2 O3 ratio of 1.0 is constituted of irregular crystal particles with some hexagonal
crystals (SAPO-5) and rarely cubic-like morphology
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
(SAPO-34). By increasing the template concentration, crystals of exclusively cubic shape are observed
and the crystal size becomes larger, with average
crystal diameters of 2.0, 3.0 and 5.0 µm for the
samples synthesized at the TEA/Al2 O3 ratio of 2.0–4.0,
respectively.
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Table 1. BET surface areas, pore volumes and pore
diameters of the catalysts synthesized with different
TEA/Al2 O3 ratios.
TEA/Al2 O3
molar
ratio
1.0
2.0
3.0
4.0
BET
surface area
(m2 /g)
Pore
volume
(cm3 /g)
Pore
diameter (nm)
(BJH method)
63
304
388
435
0.11
0.23
0.25
0.25
7.2
3.0
2.5
2.3
The N2 adsorption–desorption isotherms of samples
are given in Fig. 4, and the BET surface areas, total pore
volumes and pore diameters are tabulated in Table 1.
From Fig. 4, it is seen that increasing the TEA/Al2 O3
ratio leads to a rise in the amount of N2 adsorbed
at the low-relative pressure region. This result reveals
the increase in the amount of micropores presented in
the samples with the enhancement of the TEA/Al2 O3
ratio. Accordingly, when the TEA/Al2 O3 ratio increased
from 1.0 to 4.0, the surface area increased from 63
to 435 m2 /g and the pore diameter decreased from 7.2
to 2.3 nm. A steep enhancement in the surface area is
observed when the TEA/Al2 O3 ratio increases from 1.0
to 2.0. This may be attributed to the change in crystal
phase from SAPO-5 (AFI-type structure) to SAPO-34.
Generally, the CHA-type molecular sieve has a larger
BET surface area than that with AFI.[24] The surface
area increases gradually in the range of the TEA/Al2 O3
ratio from 2.0 to 4.0. This phenomenon strongly suggests that the crystallinity of SAPO-34 enhances, and
the impurity phase disappears. This result accords well
with the XRD data, which show that the crystallinity
increases with the TEA concentration. The presence of
the hysteresis loop at the high-relative pressure region
in Fig. 4 generally indicates the presence of mesopores
or macropores, which is usually attributed to the voids
between the primary particles constituting the main bulk
phase.
The TG–DSC profiles of the synthesized samples
are described in Fig. 5. Weight losses are summarized
in Table 2. The TG curves of the samples with the
TEA/Al2 O3 ratio of 2.0–4.0 show similar patterns with
weight losses in the temperature ranges of 25–120,
300–450 and 450–900 ◦ C (Fig. 5b–d). The first weight
loss of about 6% in the low temperature, accompanied
with a small endothermic peak observed in the DSC
curve, corresponds to the loss of physically adsorbed
water. The second weight loss between 300 and 450 ◦ C,
associated with the strong exothermic DSC peak, is
attributed to the oxidative decomposition of organic
templates. The third weight loss at a temperature higher
than 450 ◦ C may be related to the further removal of
organic residue occluded in the channels and cages
of SAPO-34. The water loss in the sample with a
TEA/Al2 O3 ratio of 1.0 is much larger (12.23%), as
is seen in Fig. 5a; this is attributed to a higher fraction
of noncondensed hydroxyl groups, which is expected
for the amorphous samples.[25]
Effect of crystallization time
The powder XRD patterns of samples synthesized for
different crystallization time (the gel molar composition is 2.0Al2 O3 :1.0P2 O5 :0.8SiO2 :3.0TEA:25H2 O) are
shown in Fig. 6. The XRD pattern of the sample crystallized for 2 h demonstrates that the sample mainly
consists of the amorphous phase. However, very weak
peaks emerge at 2θ = 9.5◦ and 20.5◦ , suggesting the
appearance of a very small amount of SAPO-34 crystals. The diffraction peaks of SAPO-34 become evident
when the samples are crystallized for more than 5 h.
The crystallinity further increases with crystallization
Figure 5. TG–DSC profiles of as-synthesized samples with different TEA/Al2 O3 ratios:
(a) 1.0, (b) 2.0, (c) 3.0 and (d) 4.0.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2011; 6: 596–605
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering SYNTHESIS, CHARACTERIZATION AND CATALYTIC PERFORMANCE OF SAPO-34
Table 2. Thermal analysis results of SAPO-34 samples
synthesized with different TEA/Al2 O3 ratios.
Weight loss (%)
TEA/Al2 O3
◦
molar ratio 25–120 C 300–450 ◦ C 450–900 ◦ C Total
1.0
2.0
3.0
4.0
a
12.2
6.3
6.6
5.8
–
8.8
10.0
8.5
a
5.1
2.9
3.7
4.0
17.3
18.0
20.3
18.3
At 120–900 ◦ C.
Figure 6. XRD patterns of as-synthesized samples with
different crystallization time.
time. And the crystallinity reaches almost 90% at 11 h
compared to that of the sample crystallized for 48 h.
SEM images of as-synthesized samples are given in
Fig. 7. At a crystallization time of 2 h, there exists
a large amount of amorphous phase with thimbleful
cubic-like crystals of SAPO-34. With an increase in
the crystallization time, a fast transformation of the
crystalline phase is observed. Upon crystallizing for
11 h or longer, the amorphous phase almost disappears.
Generally, SAPO-34 crystals grow larger and larger as
the process of crystallization proceeds.
The chemical compositions of samples obtained by
EDS are presented in Table 3. Only Al, P and Si elements were detected. Before 11 h, the contents of P and
Si in the samples increased but those of Al decreased
rapidly. These results indicate that the amorphous materials observed by SEM at the early crystallization period
were the unreacted alumina, which dissolved gradually
into the liquid phase with time. With the extension of
the crystallization time, the ratio of Si/(Al + P + Si)
ranges from 0.017 to 0.135 for the products. Compared
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Table 3.
Elemental compositions of calcined
samples synthesized with different crystallization time
obtained by SEM–EDS analysis.
Crystallization
time (h)
2
5
11
48
Elemental compositions
Al
Si
P
Relative
yield (%)
0.760
0.655
0.552
0.706
0.017
0.033
0.065
0.135
0.223
0.312
0.383
0.159
57
73
90
100
to Si/(Al + P + Si) that equals 0.118 for all initial
gels, the ratio for the product crystallized for 48 h was
found to be higher than that for the initial gel, which
means that more amount of Si was incorporated into the
framework of SAPO molecular sieves and remained as
an amorphous silica phase on the extra framework.
The relative solid yield as a function of the crystallization time is also listed in Table 3. Before 2 h,
the relative solid yield was low. A fast increase in
the yield was observed with the crystallization time
from 2 to 11 h. The solid yields were influenced by
the counteraction parameters between the dissolution of
amorphous alumina into the solution and the generation
of crystals.[26] After 11 h, the yield still increased with
a much flatter slope.
The adsorption of pyridine on the catalysts has been
monitored with the FT-IR spectrum in order to evaluate the relative amount of Lewis and Brønsted acid
sites. The spectra obtained from pyridine adsorbed at
room temperature and evacuated at different temperatures on the sample crystallized for 11 h are depicted
in Fig. 8. The strong absorption bands around 1450 and
1490 cm−1 indicate the presence of coordinated pyridine at the Lewis acid sites of the sample. There was
no detectable pyridine adsorbed on Brønsted acid sites
in the sample, which should appear in the adsorption
region at around 1540 cm−1 . A decreasing tendency
was seen in the intensity of various bands with increasing desorption temperature.
Hydrogen-bonded pyridine are in a similar range as
those of pyridine adsorbed at Lewis and Brønsted acid
sites, and at 200 ◦ C the entire hydrogen-bonded pyridine
is desorbed.[27] Therefore, Fig. 9 shows the spectra
after pyridine desorption at 200 ◦ C on the samples
synthesized with different crystallization time. The
results indicate that surface acid species on all samples
were dominated by Lewis acid sites. An estimation
of the amount of Lewis acid sites is obtained by the
determination of the integral intensity of the band at
around 1450 cm−1 . The results are listed in Table 4.
The sample with a crystallization time of 2 h exhibits
the highest Lewis acidity, whereas that of 11 h exhibits
the lowest one. The reduced amount of Lewis acid sites
is perhaps due to the increase in Si content, which is in
a good agreement with the EDS analysis. There is no
Asia-Pac. J. Chem. Eng. 2011; 6: 596–605
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Asia-Pacific Journal of Chemical Engineering
Figure 7. SEM images of as-synthesized samples with different
crystallization time.
Figure 8. FT-IR spectra of pyridine adsorbed at room
temperature and evacuated at 20, 100, 200, 300 ◦ C on
the sample crystallized for 11 h.
Figure 9. FT-IR spectra of pyridine adsorbed at room
temperature and evacuated at 200 ◦ C on the samples with
different crystallization time.
obvious shift of Lewis bands of all samples, indicating
that the Lewis acid strength of the surface sites is almost
the same.[28]
Table 4. Integral intensities of the bands around
1450 cm−1 for the catalysts with different crystallization time.
Catalytic performance
The catalytic performance of SAPO-34 samples synthesized with different template concentrations and crystallization time in the MTO reaction was tested at 450 ◦ C.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Crystallization time (h)
2
5
11
48
Integral intensity (a.u.)
114.5
81.4
35.5
40.2
Figure 10 gives the methanol conversion as well as
selectivity of C2 H4 and C3 H6 on the catalysts synAsia-Pac. J. Chem. Eng. 2011; 6: 596–605
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering SYNTHESIS, CHARACTERIZATION AND CATALYTIC PERFORMANCE OF SAPO-34
Figure 10. Effects of TEA/Al2 O3 ratios of as-synthesized
samples on methanol conversion and selectivity of C2 H4 and
C3 H6 .
Figure 11. Effects of crystallization time of as-synthesized
samples on methanol conversion and selectivity of C2 H4 and
C3 H6 .
thesized with different template concentration. As presented in the figure, when the TEA/Al2 O3 ratio is 1.0,
the catalyst has almost no selectivity to light olefins.
On the basis of the XRD and SEM data, the material is a mixture of three crystals with large amount
of amorphous phase. This can explain both the lower
conversion and an unusual selectivity toward the reaction products. As the template concentration increases,
the methanol conversion and olefin selectivity improves
correspondingly. When the TEA/Al2 O3 ratio rises to
3.0, the selectivity of light olefins (C2 H4 + C3 H6 )
achieves a maximum under 100% methanol conversion. This result confirms that the activity of SAPO-34
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
samples improves with the rise in the crystallinity. Nevertheless, the catalytic activity of the sample synthesized
with the TEA/Al2 O3 ratio of 4.0 decreases, indicating
that the highest degree of crystallinity may not be the
best for catalytic activity of SAPO-34 catalysts in an
MTO reaction.
The effect of crystallization time on the catalytic
performance is presented in Fig. 11. The catalyst crystallized for 2 h shows the lowest olefin selectivity on
account of the absence of the crystal phase. With the
extension of crystallization time from 2 to 11 h, the
methanol conversion as well as the olefin selectivity
increased sharply, and the sample crystallized for 11 h
showed the optimum olefin selectivity. This result is
associated with the decreased amount of Lewis acidity
in SAPO-34, shown previously by FT-IR, which caused
a reduction in coke formation and hydride transfer reaction. Kang and Inui[29] proved that SAPO-34 has the
analogical structure of CHA zeolite that contains large
cavities inside the crystal channel. Thus, it involves a
risk of rapid coke formation, when the concentration of
acid sites inside the cavity is very high. The catalytic
activities of the samples crystallized for 25 h or more
are unexpectedly lower than those of the sample crystallized for 11 h. The acid amount in these samples is
nearly identical and the crystal size with longer crystallization time is found to be larger. These results suggest
that the decrease in the olefin selectivity was not caused
by the change in the acid amount, but by the crystal size
of the catalysts. This phenomenon was also observed by
Nishiyama et al .[12] and he found that the effectiveness
of the catalyst decreased with increasing crystal size.
For the large crystals, successive polymerization easily
occurs and the formation of coke is promoted as a result
of the resistance to diffusion.
The influence of the reaction temperature on the product distribution for methanol conversion reaction on
the sample synthesized with TEA/Al2 O3 ratio of 3.0
and crystallized for 11 h is summarized in Table 5.
The yield of reaction products corresponds to a timeon-stream (TOS) of 40 min at each temperature. As
the reaction temperature increased from 390 to 510 ◦ C,
the methanol conversion was almost complete and the
yield of C2 –C4 olefins was more than 80% at 450 ◦ C;
the yield of saturated hydrocarbon was maintained at
less than 3% in the whole reaction temperature. C4
olefins gave a maximum value of about 16% at 390 ◦ C,
and it gradually decreased with increasing temperature. In the case of propylene, the yield increased with
increasing temperature up to 450 ◦ C and then decreased,
whereas the yield of ethylene steadily increased in the
whole reaction temperature. The reason for this phenomenon is that propylene and butylenes can oligomerize to oligomers, which cracks to form ethylene at a
higher temperature. Therefore, yields of propylene and
butylenes have maximum values at low or medium temperature, while the yield of ethylene steadily increases
Asia-Pac. J. Chem. Eng. 2011; 6: 596–605
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Table 5. Influence of reaction temperature on the product distribution over the catalyst with a TEA/Al2 O3 ratio of
3.0 and crystallized for 11 h.
Reaction temperature (◦ C)
Methanol conversion (%)
Product yields (%)
CH3 OCH3
390
420
450
480
510
99.3
99.4
99.5
99.6
99.3
with the reaction temperature. The molar ratio of ethylene to propylene increased from 0.75 at 390 ◦ C to 1.89
at 510 ◦ C and that of ethylene to butylenes increased
from 0.9 to 5.0.
1.9
1.5
1.1
0.6
2.0
=
C3 =
C4 =
C1 –C4
C5 +
14.6
18.5
29.6
32.1
36.0
29.5
32.2
40.3
31.7
28.6
15.8
14.0
14.1
8.9
7.1
2.1
1.8
1.8
1.8
2.8
36.1
32.1
13.2
24.9
23.6
C2
2008AA06Z324) and by the Innovation Project of
Institute of Process Engineering, Chinese Academy of
Sciences (No. 082702).
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CONCLUSIONS
In this work, SAPO-34 molecular sieves were synthesized with different template concentration and crystallization time, which have evident effects on their
physicochemical properties and catalytic performance.
At low-TEA concentration, the product was cocrystallized with AlPO4 and SAPO-5, and the morphology of
the crystal was irregular. Pure SAPO-34 was obtained
at the TEA/Al2 O3 molar ratio of more than 2.0. The
relative crystallinity, crystal size and surface area were
greatly enhanced with TEA concentration.
The SAPO-34 crystals appeared when crystallization
time was over 5 h and the crystallinity increased
rapidly. Afterward, the crystallinity leveled off and the
crystal size became larger. The crystallization process
had a crucial effect on the incorporation of Si atoms,
and the Si content in SAPO-34 crystals increased with
the crystallization time. The FT-IR spectra showed that
only the Lewis acid sites were present on the catalyst
surface and the number of Lewis acid sites reduced with
crystallization time.
SAPO-34 molecular sieves have proved to be excellent catalysts for the MTO reaction. The template concentrations and crystallization time play important roles
in methanol conversion as well as in selectivity to
olefins. The catalyst with a TEA/Al2 O3 ratio of 3.0 had
the highest light olefin selectivity and the optimum crystallization time was 11 h. When the temperature was
450 ◦ C, yield of C2 –C4 olefins was more than 80% and
the methanol conversion approached 100%.
Acknowledgements
This research was supported financially by the National
High Technology Research and Development Program
of China (863 Program) (Nos. 2006AA06Z371 and
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
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synthesis, molecular, reaction, mto, catalytic, performance, olefin, characterization, sapo, sieve, methanol
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