Tri-templates synthesis of SAPO-34 and its performance in MTO reaction by statistical design of experiments

Article abstract of DOI:10.1016/j.apcata.2014.12.033

The effect of a tri-templating agent [i.e.; tetraethyl ammonium hydroxide (TEAOH), triethylamine (TEA) and morpholine (MOR)] on the catalytic performance of SAPO-34 catalyst was investigated in conversion of methanol to olefins (MTO). SAPO-34 catalysts were synthesized hydrothermally with nominal composition as 1 Al₂O₃:1 P₂O₅:0.4 SiO₂:2y TEAOH:2x TEA:2 (1 - (x + y)) MOR:70 H₂O. The products were characterized by XRD, SEM, BET, EDX, FT-IR, NH₃-TPD techniques. The relative molar ratios were obtained by response surface methodology applying central composite design. Tri-templating led to production of pure SAPO-34 phase with reduced particle sizes. Analysis of Variance (ANOVA) applied for investigating the significance of two independent variables indicated that TEA content was the most significant variable. The catalytic performance of the prepared catalysts was tested in MTO reaction at 410 °C and WHSV of 6.5 h^-1. Sample prepared with 0.5 MOR:0.5 TEA:1 TEAOH exhibited the highest yield of light olefins (88.7 wt%) owing to its highest crystallinity, smallest crystal size and highest surface area.

Full text of DOI:10.1016/j.apcata.2014.12.033

Applied Catalysis A: General 493 (2015) 103–111
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Tri-templates synthesis of SAPO-34 and its performance in MTO
reaction by statistical design of experiments
Shima Masoumia, Jafar Towfighia,∗, Ali Mohamadalizadehb,c, Zahra Kooshkia,
Kobra Rahimia
a Department of Chemical Engineering, Tarbiat Modares University, Chamran Highway Ale-Ahmad Cross, P.O. Box 14115-143, Tehran, Iran
b Gas Research Division, Research Institute of Petroleum Industry, Tehran 14665-1998, Iran
c Department of Chemical Engineering, Polytechnique Montreal, Succ. Centre-villem, Montréal, Québec H3C 3A7, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of a tri-templating agent [i.e., tetraethyl ammonium hydroxide (TEAOH), triethylamine (TEA)
and morpholine (MOR)] on the catalytic performance of SAPO-34 catalyst was investigated in conver-
sion of methanol to olefins (MTO). SAPO-34 catalysts were synthesized hydrothermally with nominal
composition as 1 Al2O3:1 P2O5:0.4 SiO2:2y TEAOH:2x TEA:2 (1 − (x + y)) MOR:70 H2O. The products were
characterized by XRD, SEM, BET, EDX, FT-IR, NH3-TPD techniques. The relative molar ratios were obtained
by response surface methodology applying central composite design. Tri-templating led to production of
pure SAPO-34 phase with reduced particle sizes. Analysis of Variance (ANOVA) applied for investigating
the significance of two independent variables indicated that TEA content was the most significant vari-
able. The catalytic performance of the prepared catalysts was tested in MTO reaction at 410 ◦C and WHSV
of 6.5 h−1. Sample prepared with 0.5 MOR:0.5 TEA:1 TEAOH exhibited the highest yield of light olefins
(88.7 wt%) owing to its highest crystallinity, smallest crystal size and highest surface area.
© 2015 Elsevier B.V. All rights reserved.
Received 3 September 2014
Received in revised form
16 December 2014
Accepted 18 December 2014
Available online 8 January 2015
Keywords:
MTO reaction
Light olefin
SAPO-34
Tri-templating
Central composite design
1. Introduction
imbalance in the framework’s charge then creation of Bronsted acid
sites [12–15].
Ethylene and propylene are the most important raw materials
in petrochemical industries used for production of major chemi-
cals [1,2]. The main source of these chemicals is steam cracking of
gas and liquid hydrocarbons at high temperature, but this method
demands an appreciable amount of energy with concomitant envi-
ronmental problems such as CO2 emissions and also has high
production cost due to high crude oil price [3,4].
Nowadays, research interest has been focused on methanol to
olefins (MTO) process because of the availability of huge quantities
of natural gas and an increasing public awareness for providing raw
materials from natural gas [5,6]. There have been many molecular
sieve catalysts applied for the MTO reaction [7–9]. SAPO-34, has
received great attention due to relatively mild acidity, chabazite
structure, good thermal/hydrothermal stability and high selectiv-
ity to olefins in MTO reaction [10,11]. SAPO-34 is formed by the
introduction of silicon atoms into the neutral framework of alu-
minophosphate (ALPO4) molecular sieve, which causes a negative
SAPO-34 is normally synthesized hydrothermally from a gel
containing sources of Al, Si and P. Also, one or a mixture of
components is added to reacting mixture as structure directing
agents (SDA) [16–18]. It is well known that template can play
structure-directing, space-filling, and charge-compensating roles
in the synthesis of SAPO molecular sieves [19–21].
Alvaro-Munoz et al. [22] used different templates such as
tetraethyl ammonium hydroxide (TEAOH), triethylamine (TEA) and
morpholine (MOR) in SAPO-34 synthesis. The sample synthesized
with TEAOH as template rendered the best catalytic performance
owing to its enhanced external surface area and stronger acidity,
but its high price limits its application in an industrial scale. How-
ever, TEA and MOR cost considerably lower than TEAOH, but MOR
had little effect on the nucleation so that accelerated the crystal
growth of SAPO-34 resulting in large particle size and lower cat-
alytic performance in MTO reaction [23]. On the other hand, TEA
was reported to cause to the presence of impurity phase and small
size particles [24,25].
Recently, mixed templates such as TEAOH/MOR [26,27] and
TEAOH/TEA [28] have been employed in the synthesis of SAPO-
34 in order to improve the properties of SAPO-34 and increase
its catalytic performance. Although three kinds of amines, namely
∗
Corresponding author. Tel.: +98 21 82883311; fax: +98 21 82883311.
E-mail address: towfighi@modares.ac.ir (J. Towfighi).
http://dx.doi.org/10.1016/j.apcata.2014.12.033
0926-860X/© 2015 Elsevier B.V. All rights reserved.

104
S. Masoumi et al. / Applied Catalysis A: General 493 (2015) 103–111
TEAOH, TEA, MOR and a mixture of two these templates have
been used as most common SDA in the synthesis gel, but the
effect of different combinations of a new tri-templating agent
TEAOH/TEA/MOR has not been investigated.
The BET specific surface areas of calcined samples were acquired
from isotherm data of nitrogen adsorption–desorption at −196 ◦C
using Micromeritics ASAP-2010 analyzer. The chemical composi-
tion of the catalysts was determined by TESCAN system (VEGA
model) scanning electron microscope equipped with an energy
dispersive X-ray (EDX) spectrometer. The catalyst acidic proper-
ties were measured by temperature programmed desorption of
ammonia (NH3-TPD) using Micromeritics 2000. About 0.06 g of the
catalyst was pretreated to remove adsorbed water at 300 ◦C for
3 h and was subsequently cooled to the adsorption temperature
of 100 ◦C. After purging with helium for 20 min, the analysis was
carried out at a heating rate of 10 mL/min from 100 to 600 ◦C.
In this work, central composite design (CCD) was applied to
investigate how the ratios of TEA/TEAOH/MOR influence the char-
acteristics of SAPO-34 as well as its catalytic performance in MTO
reaction. Different samples were synthesized using two or three
templates with different molar ratios of TEA/TEAOH/MOR in the
starting gel. Analysis of Variance (ANOVA) was employed to study
the effect of the main factors and the associated interaction on rel-
ative crystallinity and maximum yield of light olefins. A quadratic
model was proposed for maximum yield of light olefins and relative
crystallinity as a function of TEA and TEAOH content in synthesis
gel. The crystal size, crystallinity, specific surface area, chemical
composition and acidic properties of the samples were character-
ized by XRD, SEM, BET, EDX, FTIR and NH3-TPD techniques. The
reactor tests were performed over the prepared catalysts in order
to obtain the best ratios of TEA/TEAOH/MOR in MTO reaction to find
the proper catalyst for this reaction.
2.3. Catalyst performance test
Methanol conversion to olefins was tested under atmospheric
pressure at 410 ◦C. The SAPO-34 catalyst weighing 1 and 2.5 g sili-
con carbide (as an inert) [30] were packed in the center of stainless
steel reactor (internal diameter: 6 mm, length: 8 cm) and heated by
a tubular furnace. The catalysts were pretreated with 150 mL/min
flow of N2 at 550 ◦C for 1 h and then the temperature was reduced
to reaction temperature (410 ◦C).
2. Experimental
The liquid mixture of methanol in water (30 wt%) with a weight
hourly space velocity (WHSV) of 6.5 h−1, was fed into the reactor.
The gas product was analyzed by a Hewlett-Packard 5890 flame
ionization detector (FID) gas chromatograph (GC) equipped with
Agilent J&W GS-alumina and plot columns. The oven was operated
at 50 ◦C, then ramped at 5 ◦C/min up to 180 ◦C and held for 5 min
at 180 ◦C. Finally the temperature ramped to 50 ◦C at the same rate
for the next test. The yield of products is defined by Eq. (2):
2.1. Catalyst preparation
The SAPO-34 samples were synthesized hydrothermally using
the mixture of three different organic templates namely, MOR
(Merck), tetraethyl ammonium hydroxide (20 wt% aqueous solu-
tion of TEAOH, Merck) and TEA (Merck). Aluminum isopropoxide,
silica gel and phosphoric acid (85 wt%, H3PO4, Merck) were used
as the source of Al, Si and P, respectively. The molar composition
of synthesis gel was 1 Al2O3:1 P2O5:0.4 SiO2:2y TEAOH:2x TEA:2
(1 − (x + y)) MOR:70 H2O.
mg(out) × xp
mMaoH(in)
Yp
=
× 100
(2)
The synthesis gel was prepared by slowly adding aluminum iso-
propoxide powder to a solution containing phosphoric acid and
deionized water with continuous stirring, then silica gel was added
drop wise to the above solution, followed by addition of tem-
plates. The obtained gel was aged at room temperature for 8 h
with agitation. After the aging period, the gel was transferred into
a Teflon-lined stainless steel autoclave, which was heated at 190 ◦C
for 24 h. The synthesized material was recovered by centrifugation,
washed several times with distilled water, and then oven-dried at
110 ◦C for 10 h. The final product was calcined at 550 ◦C for 5 h in
order to remove the organic templates that resided in the pores of
the samples.
where in Eq. (2), Yp is yield of product (wt%), mg(out) is mass flow rate
of outlet gas product, and xp is mass fraction of product that were
analyzed by GC, and mMeoH(in) is mass flow rate of inlet methanol.
The conversion of methanol was determined by Eq. (3), using frac-
tional distillation of outlet liquid.
(mMeoH(in) − mMeoH(out)) or (consumed method)
XMeoH
=
× 100
mMeoH(in)
(3)
where in Eq. (3), XMeoH is conversion of methanol (%), mMeoH(in)
and mMeoH(out) are the inlet and outlet mass flow rate of methanol,
respectively.
2.2. Catalyst characterization
The X-ray diffraction (XRD) patterns of catalysts were obtained
by powder X-ray diffractometer (Bruker D8) using CuK␣ radi-
˚
ation (ꢀ = 1.54 A). For zeolite SAPO-34, the relative crystallinity
2.4. Design of experiments
was determined from the main peak intensities at 2ꢁ ≈ 9.6, 13.0,
and 20.6 [29] on the base of SAPO-34 catalyst prepared with
0.5 MOR:0.5 TEA:1 TEAOH molar ratio possessing the highest
XRD intensities among other samples. Therefore, the relative crys-
tallinity of the samples was calculated by Eq. (1):
Central Composite Design (CCD) was applied to investigate the
effect of two independent variables namely, TEA content (A) and
TEAOH content (B) on relative crystallinity and maximum yield of
light olefins. CCD method of experimental design and molar com-
position as 2x TEA:2y TEAOH:2 (1 − (x + y)) MOR was used to find
molar ratios of these templates. These two independent factors
were coded as +1 for high level, 0 for center value, −1 for low levels
and ˛ for axis point. The distance from the center of the design
space to axial point is called ˛ [31]. The five examined levels for
each independent variable are shown in Table 1. The applied molar
ratios of three templates are given in Table 2.
ꢀ
I
ꢀ
% relative crystallinity =
× 100
(1)
I2
where I is the line intensity of the sample and I2 is the line intensity
of the S2 sample. The crystal size and morphology was analyzed
using Philips XL30 scanning electron microscope (SEM). Diffuse
reflectance FTIR was conducted using a Bruker Tensor-27 spec-
trophotometer. IR spectra of the samples in the region of the
framework stretching vibrations (450–4000 cm−1) were measured.
The effect of TEA and TEAOH content in synthesis gel on max-
imum yield of light olefins and relative crystallinty is explained

S. Masoumi et al. / Applied Catalysis A: General 493 (2015) 103–111
105
Table 1
Variables and their examined levels used in experimental design.
Factor
Level
Axis
−˛
−1
0
+1
0.43
+˛
0.5
A – x (TEA content)
B – y (TEAOH content)
0.07
0.07
0.25
0.25
0
0
0.43
0.5
by the following quadratic polynomial equation as a function of
independent variables is given in Eq. (4):
k
k
k
k
ꢁ
ꢁ
ꢁꢁ
Y = ˇ0
+
ˇiXi +
ˇiiXi2
+
ˇijXiXj
(4)
i=1
i=1
i=1 i<j
where Y is the calculated response, k is the number of variables,
ˇ0 is the constant coefficient, ˇj is the linear coefficient, ˇjj is the
squared coefficient, ˇij is the interaction coefficient, Xi and Xj are
uncoded independent variables.
Analysis of variance (ANOVA) utilized to investigate the sig-
nificance of the main factors and their interactions is given in
Tables 3 and 5. The significance of each factor is investigated using
F and p values. Values less than 0.05 for p indicate that a variable
is significant. The F test, is defined as F = MSF/MSE, where MSF and
MSE are the mean squares of factors or interactions, and errors,
respectively. The statistical significance of a factor is determined
by comparison of the value in the F table at the desirable probabil-
ity level (e.g., F0.05 (9,7) = 2.53) [31,32]. The greater value of F for
each factor indicates that its effect is statistically significant.
Fig. 1. XRD patterns of as-synthesized SAPO-34 with different molar ratios of tem-
plates.
3. Results and discussion
templates. The intensities of peaks associated with S2 sample pre-
pared from a mixture of 0.5 MOR:0.5 TEA:1 TEAOH are the highest.
It can be regarded as the base sample for determining the relative
crystallinity of the other samples. The relative crystallinity of each
catalyst are presented in Table 2.
According to ANOVA results for relative crystallinity, the fac-
tor A and interactions AB and A2 were proved to have statistically
significant effects on the relative crystallinity (Table 3).
After the ANOVA test, the quadratic polynomial equation for
relative crystallinity as a function of actual variables is given in Eq.
(5):
3.1. Crystallinity and crystal size (XRD, SEM)
The X-ray diffraction patterns of the synthesized catalysts pre-
pared by different molar ratios of templates are shown in Fig. 1.
The as-synthesized SAPO-34 exhibits a mixed trigonal and tri-
clinic phase, whereas after calcination an exclusive trigonal phase
is detectable [2]. As Fig. 1 shows diffraction peaks of trigonal phase
appeared for all the samples.
All the synthesized samples possessed typical powder diffrac-
tion patterns corresponding to chabazite structure of SAPO-34 as
Relative crystallinty(%) = +62.05685 + 202.82944XA + 85.14315XB − 176.00000XAXB
(5)
−391.60000XA2 − 63.60000XB2
In which XA and XB denote actual variables of TEA and TEAOH
content, respectively. Determination coefficient (R2) of Eq. (5) is
0.92, indicating that this model can describe the experimental data
of relative crystallinity.
Main effects of each factor on relative crystallinity are presented
in Fig. 2. Increasing TEA content in synthesis gel led to increase
in relative crystallinity initially, reaching to a maximum and then
falling (Fig. 2(a)). The intensity of main peaks in the XRD pattern
is seen in Fig. 1. The intensity and position of each peak match well
with that of reported for SAPO-34 material without the presence of
any impurity phase [29]. Furthermore, there is no additional peak
of impurity phase in any of the samples implying that SAPO-34
was successfully crystallized. According to the XRD patterns, reflec-
tion intensities of each peak varied with changing molar ratios of
Table 2
Molar ratios, relative crystallinity (%) and mean crystal size of SAPO-34 samples.
Samples
Molar composition
Relative crystallinity (%)
Mean crystal size (␮m)
S1
S2
S3
S4
S5
S6
S7
S8
S9
Al2O3:0.4 SiO2:1 P2O5:1.5 MOR:0.5 TEA
Al2O3:0.4 SiO2:1 P2O5:0.5 MOR:0.5 TEA:1 TEAOH
Al2O3:0.4 SiO2:1 P2O5:1.5 MOR:0.5 TEAOH
Al2O3:0.4 SiO2:1 P2O5:1 MOR:0.5 TEA:0.5 TEAOH
Al2O3:0.4 SiO2:1 P2O5:0.5 MOR:1 TEA:0.5 TEAOH
Al2O3:0.4 SiO2:1 P2O5:1 MOR:0.86 TEA:0.14TEAOH
Al2O3:0.4 SiO2:1 P2O5:0.28 MOR:0.86 TEA:0.86 TEAOH
Al2O3:0.4 SiO2:1 P2O5:1 MOR:0.14 TEA:0.86 TEAOH
Al2O3:0.4 SiO2:1 P2O5:1.72 MOR:0.14 TEA:0.14 TEAOH
85
100
83
95
62
81
65
88
80
3.25
0.64
2.92
1.2
1.45
1.5
1.32
1.25
1.63

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S. Masoumi et al. / Applied Catalysis A: General 493 (2015) 103–111
Table 3
Analysis of variance (ANOVA) for relative crystallinity (%).
Factor
Sum of squares
df
Mean square
F
p
Model
A – x (TEA content)
B – y (TEAOH content)
1527.40
341.71
21.82
121.00
1041.78
27.48
5
1
1
1
1
305.48
341.71
21.82
121.00
1041.78
27.48
15.46
17.30
1.10
6.12
52.73
1.39
0.0012
0.0042
0.3282
0.0425
0.0002
0.2768
AB
A2
B2
1
Error
Core total
1.20
1665.69
4
12
0.30
for S5 sample was weak. This sample exhibited the lowest relative
crystallinity due to the highest amount of TEA content in synthesis
gel. TEA content has the largest effect on the relative crystallinity.
According to Table 3, the F values for A and A2 that related to TEA
content in synthesis gel are 17.30 and 52.73, respectively, which
are much higher than other terms. These results are well reflected
by Fig. 2(a) and (b). For instance, Fig. 2(a) shows that the increase of
TEA changes the relative crystallinity in the range of 75–95% while
Fig. 2(b) shows that the increase of TEAOH results in increasing of
relative crystallinity in the range of 91–95%. These values confirm
that TEAOH content does not have any significant effect on relative
crystallinity in comparison with TEA.
Contour plots as the graphical representations that were
obtained from Eq. (2), can be used to study the effects of process
variables on the relative cryatallinity. It is shown in Fig. 3, high
level of TEAOH and low level of TEA content in the synthesis gel
correlated with high relative crystallinity.
Wang et al. [28] found that, when TEA template individually or
a mixture of TEA/TEAOH is used to prepare SAPO-34, the SAPO-
5 impurity phase could be also observed. XRD patterns confirmed
that mixture of TEAOH/TEA/MOR can improve the purity of SAPO-
34. So using MOR as a third template may help to obtain pure phase
of SAPO-34.
The SEM images of the synthesized samples are presented in
Fig. 4. All the samples exhibited the cubic shape of typical SAPO-
34, distinct in crystal size. The SEM study confirmed that crystal
size of samples depended on the type and number of templates in
the initial gel. According to Refs. [23,24,26,33,34] average crystal
size of SAPO-34 samples is 5–20 ␮m when MOR is used as a tem-
plate agent. From Table 2, the mean crystal size of S1 and S3 were
3.15 and 2.7 ␮m, respectively. Therefore, our results showed that
using TEA or TEAOH as a template in synthesis gel can have a pos-
itive effect on crystal size reduction. The results are in agreement
with other studies [23,24]. S3 sample with larger width and lower
intensity of diffraction peaks illustrated smaller particle size com-
pared to S1. It is concluded that the presence of TEAOH compared
to TEA in synthesis gel caused more particle size reduction. The tri-
templating utilizing both TEA and TEAOH as factors for decreasing
the crystal size, resulting in decreasing the crystal size.
Nucleation improvement may be responsible for the crystal size
reduction [35]. It could be due to the role of templates as space
filling and interaction between the three different types of tem-
plates. The greater number of templates causes small space filling
around template molecules results in increasing the number of
small nuclei.
According to XRD patterns and SEM images, the phase purity
and particle size of the samples significantly depend on the nature
and number of templates. The S2 sample showed the highest crys-
tallinity (100%) and smallest mean crystal size (0.64 ␮m).
Fig. 3. Contours describing the response surface for relative crystallinity (%) as a
function of y(TEAOH) vs. x(TEA).
Fig. 2. Main effects of each factor on relative crystallinity (%).

S. Masoumi et al. / Applied Catalysis A: General 493 (2015) 103–111
107
Fig. 4. SEM images of as-synthesized samples.
3.2. Chemical composition and surface area (EDX, BET)
amount of Si was incorporated into the framework of SAPO-34
samples and/or remained as amorphous silica phase on extra-
framework [27,33]. The amount of the chosen templating agents
plays a significant role in silicon incorporation, as well as in the
recovery of pure structure with high crystallinity. Therefore, for
these samples with high crystallinity, more Si content was incor-
porated into the framework in the crystallization process. S2 and
S4 samples with molar ratios of 0.5 MOR:0.5 TEA:1.0 TEAOH and 1
MOR:0.5 TEA:0.5 TEAOH possessed about 1.1 and 1.2 times increase
of the final Si content.
Samples with the best crystallinity and smallest crystal size
were selected for EDX, BET and NH3-TPD analyses. The surface area
of samples is given in Table 4. The surface area of pure SAPO-34
can reach up to 650 m2/g [36]. The surface area of prepared sam-
ples was very close to pure SAPO-34. Also based on XRD patterns,
these samples exhibited high crystallinity without any detectable
impurity phase. It can be seen that the surface area of samples did
not vary considerably.
The final SAPO-34 compositions with respect to Al, Si and P
based on (AlXPYSiZ) O2 formula were obtained by the results of
EDX analysis (Table 4). Compared to Si/(Al + P + Si) = 0.091 (%Si con-
tent = 9.09) belonging to all starting gels, this ratio in products
was higher than 0.091 after crystallization which means the more
3.3. Acidity (NH3-TPD, FTIR)
The TPD spectra of NH3 adsorbed on SAPO-34 samples were
used to characterize the acidic properties of samples and the results
Table 4
Physico-chemical properties of synthesized SAPO-34 samples.
Sample
Product composition
Surface
Acid strength
distribution (%)
area (m2/g)
Composition in solid
% Si content
Si incorporationa
Strong
Weak
S2
S3
S4
S6
S8
S9
Al0.5521Si0.1043P0.4110O2
Al0.5301Si0.0964P0.3976O2
Al0.5741Si0.1126P0.4127O2
Al0.5093Si0.094P0.3997O2
Al0.5323Si0.0949P0.3728O2
Al0.5153Si0.0936P0.3911O2
10.05
9.41
10.24
9.37
9.49
9.36
1.073
1.034
1.125
1.030
1.042
1.028
625.8
589.0
610.3
591.6
598.9
578.2
70
48
74
42
63
40
30
52
26
58
37
60
a
The level of silicon incorporation is defined as the molar ratio of [Si/(Si + Al + P)solid]/[Si/(Si + Al + P)gel].

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S. Masoumi et al. / Applied Catalysis A: General 493 (2015) 103–111
Fig. 5. NH3-TPD profiles of synthesized SAPO-34 samples.
are shown in Table 4 and Fig. 5. Desorption temperature signifies
the strength of acid sites, the stronger acid sites require a higher
desorption temperature. The area under the NH3 desorption curve
indicates the amount of ammonia desorbed and the acid strength
distribution.
Two desorption peaks, at the range of 150–210 and 370–420 ◦C
as low and high temperature desorption sites were recognized. The
first desorption peak of TPD curves, corresponding to weak acid
sites, was attributed to the hydroxyl groups (–OH) bounded to the
defect sites, i.e. P-OH hydroxyl groups not fully linked to AlO4 tetra-
hedral, SiOH and AlOH [37,38]. The samples with lower crystallinity
have a higher amount of such kind of these sites.
The weak acid sites, desorb ammonia molecules before reaching
to 300 ◦C [39]. Therefore, these type of acid sites would not be able
to adsorb and convert methanol as a less basic molecule at reaction
temperature of 410 ◦C into olefins.
The amount of weak acid sites represented by first peak has no
significant effect on MTO performance. For illustrating that weak
acid sites is not responsible for methanol conversion to light olefins,
Ye et al. [40] performed NH3-TPD test on deactivated samples after
reaction compared with before; the results showed that the second
peak of deactivated samples was absolutely missing and the first
peak maintained intact after the reaction.
Therefore, the second desorption peak as strong Bronsted acid
sites should be activated for MTO reaction. This peak was attributed
to the bridging hydroxyl groups, i.e. –SiOHAl–, formed by replace-
ment of phosphorous by silicon [12,41–43]. As is seen in Table 2
the type and ratio of templates employed in the synthesis affect
the distribution of acid sites strength.
There is a consensus that in the case of SAPO-34 molecular
sieves, acid sites generation is a consequence of Si incorporation
into ALPO framework [44,45]. As shown in Table 4, S2, S4 and S8
samples that were prepared by mixing three types of different tem-
plates exhibited high distribution of strong acid sites due to higher
amount of silicon incorporation.
TPD results agreed with the results from in situ FTIR spectra
of SAPO-34 samples at the OH-stretching vibration region (Fig. 6).
The framework vibrations were characteristically similar to those
of CHA-structure [14,41,46,47].
There were two adsorption bands at about 3600 and 3625 cm−1
,
which were assigned to –SiOHAl– groups interacting with the
oxygen atoms of the framework and located inside the double-six-
rings. These hydroxyl groups are suggested to be the active sites for
Fig. 6. In situ Fourier transform IR spectra of synthesized samples.

S. Masoumi et al. / Applied Catalysis A: General 493 (2015) 103–111
109
Fig. 7. Products distribution with TOS over synthesized SAPO-34 catalysts (S1–S9) at 410 ◦C, WHSV of 6.5 h−1
.
acid catalyzed reactions [41,46]. S2, S4 and S8 samples indicated
higher concentration of the strong acidic hydroxyl groups which
also showed higher amount of strong acid sites.
time on stream (TOS) are shown in Fig. 7. All the prepared sam-
ples have catalytic performance at the given condition, but the
distribution of products and maximum yield of light olefins varies
significantly. Obviously, type of templates and changing the ratio
of MOR/TEA/TEAOH affect the products distribution and maximum
yield of light olefins.
Maximum yield of ethylene and propylene in methanol con-
version reaction over SAPO-34 catalysts are given in Table 5. The
maximum yield of ethylene is more than propylene for all cata-
lysts. It is proposed that at reaction temperature 410 ◦C (>400 ◦C),
propylene and butenes may oligomerize to bigger oligomers, and
subsequently formed oligomers could be cracked to ethylene [33].
SAPO-34 catalyst is deactivated to form olefins by two routes.
First, deactivation is caused by the formation of coke from the
hydrocarbons that entrapped within the cages of SAPO-34 catalyst
during the process. Second, it is caused by losing acidity after many
cycles of MTO [48]. In methanol to olefin process over SAPO-34
The band at 1635 cm−1 is assigned to the bonding vibrational
mode of water weakly adsorbed in the SAPO-34 cages. Bending
around 1100 and 730 cm−1 correspond to asymmetric and sym-
metricstretchof O–P–O. Furthermore, absorptionpeaksat640, 580,
530 and 480 cm−1, indicated the bend of double 6-ring, PO4, AlO4
and SiO4, respectively [14,47].
3.4. MTO reaction performance
Catalytic performances of SAPO-34 catalysts synthesized by a
mixture of three templates for methanol to olefins (MTO) reac-
tion were tested at 410 ◦C and WHSV of 6.5 cm−1. For all SAPO-34
catalysts, over 98% conversion of methanol was obtained in MTO
reaction. The products distribution of the catalysts with reaction
Table 5
Products yield in methanol conversion reaction over SAPO-34 catalysts.
Sample
MOR:TEA:TEAOH
Light olefins yield (wt%)
C2H4
C3H6
C2= C3=
S1
S2
S3
S4
S5
S6
S7
S8
S9
1.5:0.5:0.0
0.5:0.5:1.0
1.5:0.0:0.5
1.0:0.5:0.5
50.50
58.41
48.78
57.38
35.46
46.54
37.13
56.35
45.60
20.48
30.29
20.44
29.04
14.68
19.11
15.09
28.23
18.47
70.98
88.70
69.22
86.42
50.14
65.65
52.22
84.58
64.07
0.5:1.0:0.5
1.0:0.86:0.14
0.28:0.86:0.86
1.0:0.14:0.86
1.72:0.14:0.14

110
S. Masoumi et al. / Applied Catalysis A: General 493 (2015) 103–111
Table 6
Analysis of variance (ANOVA) for the yield of light olefins.
Factor
Sum of squares
df
Mean square
F
p
Model
A – x (TEA content)
B – y (TEAOH content)
2268.15
417.07
128.78
287.98
1398.30
117.23
0.14
5
1
1
1
1
1
4
12
453.63
417.07
128.78
287.98
1398.30
117.23
0.035
51.50
47.35
14.62
32.69
158.74
13.31
<0.0001
0.0002
0.0065
0.0007
<0.0001
0.0082
AB
A2
B2
Error
Core total
2329.81
Fig. 9. Contours describing the response surface for light olefins yield (wt%) as a
function of y(TEAOH) vs. x(TEA).
Product distribution in MTO reaction is affected by diffusion lim-
itation of propene within pores of SAPO-34 with particle size of
>0.5 ␮m [28]. This observation can be related to the coking effect,
which was suggested to be responsible for reduction in pore size
of SAPO-34. Coke formation hindered the scape of heavier prod-
uct molecules from the cage of SAPO-34, so that the production of
these molecules suppressed, resulting in increasing yield of lighter
alkenes such as ethylene. By the formation of larger hydrocarbons,
activity loss of the catalysts became more significant, so the deacti-
vation of the catalysts is caused by diffusional barriers being created
rather than loss of active sites [28,50].
Compared with synthesized samples, S2 sample possessed the
smallest crystal size about 0.6 ␮m. According to Ref. [48,50] the
particle size range of 0.5–1 ␮m is suitable for the process because
of minimizing the possibility of inhomogeneous coke distribution.
As is seen in Fig. 7, this sample presented the best results in terms of
light olefins yield and stability with TOS. So, S2 sample possessed
the highest yield of light olefins (88.7 wt%) and the total yield of
C2= C3= maintained upper 60% after 250 min. The S4 sample pos-
sessed highest yield of light olefins at the first of reaction (about
50%). The highest strong acid sites associated with high surface area
of this catalyst that could promote the hydrogen transfer reaction.
From the ANOVA results for light olefins yield (Table 6), the
factor A and B and their interactions have statistically significant
effects on yield of light olefins. After the ANOVA test, the quadratic
polynomial equation for maximum yield of light olefins as a func-
tion of actual variables is given in Eq. (6):
Fig. 8. Main effects of each factor on maximum yield of light olefins (wt%).
catalysts, it is widely accepted that methanol is first dehydrated to
DME, then DME can easily be converted to light olefins over strong
acid sites of catalysts until the cages inside the SAPO-34 catalysts
are occupied by coke. Therefore, after deactivation of the catalysts,
conversion of methanol to DME can still take place at the weak
acidic sites [10,26,49].
According to Fig. 7, the rate of formation of ethylene and propyl-
ene increased with time on stream and reached a maximum. At the
first propylene yield reached a maximum then leveled off, whereas
Maximum light olefins yield = +37.47561 + 253.87725XA + 156.25832XB − 271.52000XAXB
(6)
−453.68400XA2 − 131.36400XB2
production of ethylene monotonically increased. After activation of
the catalyst with a propene pulse, ethylene is able to form stable.
In which XA and XB denote actual variables of TEA and TEAOH,
respectively. The R2 value of Eq. (6) is 0.973, indicating a good fitting

S. Masoumi et al. / Applied Catalysis A: General 493 (2015) 103–111
111
between predicted values and experimental data of light olefins
yield.
Graphical representations of experimental design are shown
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SAPO-34 samples were synthesized hydrothermally using
different combinations of MOR/TEA/TEAOH as templates that
exhibited different physicochemical properties. The morphology
of the samples was similar to cubic shape of typical SAPO-34. It
was revealed that using a mixture of three templates in synthesis
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affected on light olefins yield of the different catalysts. The cat-
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the highest yield of light olefins (88.7 wt%) due to its small-
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