Effect of silicon content on the catalytic behavior of chabazite type silicoaluminophosphate in the transformation of methanol to short chain olefins

Article abstract of DOI:10.1016/j.cattod.2013.04.031

AlPO-34 and SAPO-34 molecular sieves with different silicon content have been synthesized using a hydrothermal system in presence of tetraethylammonium hydroxide (TEAOH) and evaluated as catalysts for the transformation of methanol to short chain olefins. The results showed that SAPO-34 materials prepared with medium-high silicon concentration in the framework present quite similar silicon environment distributions and, when it is so, the key factor to obtain materials with improved catalytic performance is the crystallite size.

Full text of DOI:10.1016/j.cattod.2013.04.031

Catalysis Today 213 (2013) 219–225
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Effect of silicon content on the catalytic behavior of chabazite type
silicoaluminophosphate in the transformation of methanol to short
chain olefins
Teresa Álvaro-Mun˜oz, Carlos Márquez-Álvarez, Enrique Sastre∗
Instituto de Catálisis y Petroleoquímica, ICP-CSIC, C/ Marie Curie, 2, 28049 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
AlPO-34 and SAPO-34 molecular sieves with different silicon content have been synthesized using a
hydrothermal system in presence of tetraethylammonium hydroxide (TEAOH) and evaluated as cata-
lysts for the transformation of methanol to short chain olefins. The results showed that SAPO-34 materials
prepared with medium-high silicon concentration in the framework present quite similar silicon envi-
ronment distributions and, when it is so, the key factor to obtain materials with improved catalytic
performance is the crystallite size.
Received 20 December 2012
Received in revised form 26 March 2013
Accepted 5 April 2013
Available online 16 June 2013
Keywords:
AlPO-34
© 2013 Elsevier B.V. All rights reserved.
SAPO-34
Silicon content
Methanol-to-olefins
1. Introduction
influence of different synthesis parameters on the catalytic proper-
ties of these catalysts has been widely studied during the last few
The global energy crisis has renewed the interest in developing
technologies that allow diversifying the resources used to produce
energy and fuels, as well as chemicals currently obtained from
petroleum. Light olefins such as ethylene, propylene and butylenes
are important intermediates for the petrochemical industry. They
can be produced using several processes using mainly fossil, and
therefore non-renewable, raw materials. However, it is necessary
to seek alternative sources to satisfy the demand of these materials.
In these sense, the production of light olefins from methanol is an
attractive process because methanol can be obtained from a wide
variety of materials. Methanol can be efficiently produced from syn-
gas [1–3] obtained by natural gas reforming or carbon gasification,
and it might even provide an environmentally carbon neutral alter-
native to fossil carbon sources [4], if produced by chemical recycling
of carbon dioxide via hydrogenation [5] or from syngas obtained by
biomass gasification [6].
years. In this sense, the use of different structure directing agents
(SDA) has a notable influence on the physicochemical properties
and on the catalytic performance of SAPO-34 [9–15]. Moreover,
extensive studies indicate that crystallite size and morphology
affect catalyst properties and applications and, in this sense, it has
been demonstrated that the catalytic performance of SAPO-34 can
be strongly correlated with particle size due to the diffusion limita-
tions of the guest molecules in the micropores [14,16–20]. Another
parameter which could affect strongly the catalyst performance
is the content and distribution of silicon in the SAPO framework,
which determines the acid sites concentration and strength. In this
sense, some authors have mentioned the effect of low Si content of
SAPO-34 in the methanol transformation to olefins by reducing the
deactivation rate and the propane production [21]. Other authors
have estimated the stability and structure of silica species in SAPOs
based on theoretical calculations [22] or investigated the kinetics of
silicon substitution during SAPO-34 crystallization [23]. The acid-
ity of the different species of silicon presented in SAPO-34 has been
studied by different theoretical or experimental techniques, lead-
ing to differing conclusions. Several studies conclude that the acid
strength is independent of the content of Si atoms or particle size
[20,24] or that the number of strong acid sites is independent of the
silicon content [25]. However, other authors, based on FTIR, neutron
diffraction and NMR experiments, have described the presence of
three distinct Brönsted acid sites with different strength in SAPO-34
The transformation of methanol to olefins (MTO process) can
be effectively carried out by using small pore silicoaluminophos-
phates SAPO-34 (zeotype with chabazite structure) as catalyst
[7,8], obtaining exceptionally high selectivity to lower olefins. The
∗
Corresponding author. Tel.: +34 915854795; fax: +34 915854760.
E-mail address: esastre@icp.csic.es (E. Sastre).
0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.04.031

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T. Álvaro-Mun˜oz et al. / Catalysis Today 213 (2013) 219–225
a capacity of 40 cm3, which were heated statically at the required
temperature under autogeneous pressure for the specified period
of time. The resulting solids were collected by centrifugation,
washed with water and ethanol and dried at room temperature
overnight. The organic template and the water trapped within
the micropores of the as-synthesized solids were removed by
calcination at 823 K prior to catalyst testing. Complete removal of
the organic molecule was assessed by thermogravimetric analysis.
materials [26–31], confirming the previous asseveration of Sastre
et al. [32] that the acid sites in the border of silicon islands are more
acidic than those next to isolated Si species. In a recent contribution,
we have demonstrated that samples of SAPO-34 obtained by adding
a surfactant to a two-liquid phase synthesis gel show a differ-
ent Si distribution than those synthesized in absence of surfactant
[33]. SAPO-34 synthesized by using this method, present greater
incorporation of silicon and a broader distribution of silicon envi-
ronments in the network and therefore, a substantial increase in
the material acidity. In conclusion, there are disagreements among
the different authors about the role of the silicon content on the
catalytic behaviour of SAPO-34 in the MTO process. In this sense,
the objective of this paper is intending to clarify some contradictory
results published up to now by different authors on the role of the
Si concentration in SAPO-34 materials on their catalytic behaviour
in the MTO reaction. So, in this contribution we report the synthesis
of SAPO-34 samples with different content of silicon in the frame-
work and describe their physicochemical properties and catalytic
behaviour in the MTO process trying to define clearly how the sil-
icon content influences the performance of the SAPO-34 catalysts
in the transformation of methanol to light olefins.
2.2. Characterization
Powder X-ray diffraction (XRD) patterns of as-synthesized and
calcined samples were recorded on a Philips X’PERT diffractome-
ter using Cu K␣ radiation with a nickel filter. The textural data
(pore volume and BET surface area) were determined by nitrogen
adsorption at 77 K using a Micrometrics ASAP 2010 volumet-
ric apparatus. Samples were degassed at 623 K under vacuum
for at least 20 h prior to measurement of the nitrogen adsorp-
tion/desorption isotherms. Due to the small size of pore windows of
chabazite, prolonged equilibration times (of at least 20 s) were used
in order to ensure that equilibrium was reached for every adsorp-
tive dose. The crystallite size and morphology were analyzed by
scanning electron microscopy (SEM) using a JEOL JSM 6400 or a
Philips XL30 microscopes, both operating at 20 kV.
The organic content of the samples was determined by ele-
mental analysis with a Perkin-Elmer 2400 CHN analyser and
by thermogravimetric analysis (TGA) using a Perkin-Elmer TGA7
instrument. TG analyses were carried out at a heating rate of
20 K/min under air flow. Elemental analysis for Al, P and Si was per-
formed for calcined samples by inductively coupled plasma optical
emission spectrometry (ICP-OES, Perkin-Elmer 3300DV instru-
ment) after sample dissolution by alkaline fusion.
2. Experimental
2.1. Synthesis of SAPO-34 molecular sieves
AlPO-34 and SAPO-34 catalysts were synthesized by
a
hydrothermal method. Aluminium hydroxide hydrate
(Sigma–Aldrich), 85% phosphoric acid (Riedel de Haën) and silica
sol (30 wt.% suspension in water, Aldrich) were used as sources
of the framework elements. Tetraethylammonium hydroxide
(TEAOH, 35 wt.% solution in water, Aldrich) has been used as
structure directing agent (SDA). Experimental conditions (tem-
perature and crystallization times) have been adjusted in order to
obtain pure phases of chabazite structure in all the cases. The gel
composition was: 1Al2O3:1P2O5:xSiO2:yTEAOH:40H2O (x = 0–0.8;
y = 1–1.5). The molar composition of the reaction mixtures and
the synthesis conditions for the different AlPO-34 and SAPO-34
materials obtained are given in Table 1. In all the cases, syntheses
were carried out at 423 K and crystallization time varied from 1 to
13 days. In a typical synthesis to get pure SAPO-34, the aluminium
source – aluminium hydroxide – was added slowly to a dilute
phosphoric acid solution, and the mixture was vigorously stirred
for 2 h to obtain a uniform gel. Silica solution was then added
dropwise to this mixture followed by addition of the template
(TEAOH). Finally, the mixture was stirred for about 4 h. The gel was
then transferred into Teflon-lined stainless steel autoclaves with
29Si CP/MAS NMR spectra were recorded at room temperature
using a Bruker AV-400-WB spectrometer operating at 79.5 MHz,
with a 4 mm probe spinning at 10 kHz. A ␲/2 pulse of 3 ␮s, contact
time of 6 ms and recycle delay of 5 s were used. The chemical shifts
were referenced to tetramethylsilane (TMS), taken as 0 ppm.
Ammonia temperature programmed desorption (NH3-TPD) was
performed using a ChemBET-3000 TPR/TPD, Quantachrome TPD
equipment. Typically, 100 mg of sample pellets (20–40 mesh) were
pre-treated at 823 K for 1 h in helium flow (25 mL/min) and sub-
sequently cooled to the adsorption temperature (400 K). A gas
mixture of 5.0 vol.% NH3 in He was then allowed to flow over the
sample for 4 h at a rate of 15 mL/min followed by helium flow for
30 min to remove weakly adsorbed NH3. Finally, helium flow at
a rate of 25 mL/min was passed over the sample with increasing
temperature to 823 K at the rate of 10 K/min.
Table 1
Synthesis
2.3. Catalyst testing
of
SAPO-34
and
AlPO-34.
Molar
composition
of
gel:
Al2O3:P2O5:xSiO2:yTEAOH:40H2O.
Methanol conversion to olefins was tested at 673 and 723 K in
a continuous down flow packed bed reactor fully automated and
controlled from a personal computer (PID Eng&Tech Microactiv-
ity Reference), operating at atmospheric pressure. Previous to the
reaction, samples were pre-treated under nitrogen flow at 723 K
for 1 h. During the reaction, nitrogen was used as an inert dilu-
ent gas and co-fed with methanol into the reactor with a constant
methanol/nitrogen ratio of 1/1 mol. Nitrogen feed was regulated
using a mass flow controller. Methanol was fed as liquid using a
Gilson 307 HPLC pump, vaporized and mixed with the nitrogen
stream in a pre-heater set at 473 K. Catalyst weight (1.0 g; 20–30
mesh pellets size) and methanol flow rate (0.100 mL/min) were
adjusted in order to obtain a weight hourly space velocity (WHSV)
of 1.2 h−1. The reaction products were analyzed on-line by gas chro-
matography using a Varian CP3800 gas chromatograph equipped
Sample
x
y
Cryst. time (d)
Product phase
S-34(8)
S-34(6)
S-34(4)
S-34(4B)
0.8
0.6
0.4
0.4
1
1
1
1.5
5
5
5
5
CHA
CHA
CHA + AFI
CHA
A-34-A1
A-34-A3
A-34-A5
0
0
0
1
1
1
1
3
5
Amorphous
Amorphous
Amorphous
A-34-B1
A-34-B3
A-34-B5
A-34-B7
A-34-B10
A-34-B13
0
0
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1
3
5
7
10
13
CHA
CHA
CHA
CHA
CHA
CHA
Temperature of crystallization: 423 K.

T. Álvaro-Mun˜oz et al. / Catalysis Today 213 (2013) 219–225
221
A-34-B13
A-34-B10
S-34(8)
S-34(6)
A-34-B7
A-34-B5
A-34-B3
A-34-B1
S-34(4B)
S-34(4)
*
*
10
20
30
( )
40
50
10
20
30
40
50
º
º
Fig. 1. X-ray diffraction patterns of SAPO-34 samples synthesized with different
amount of silicon in the synthesis gel. The asterisks indicate reflections showing the
presence of and additional AFI phase.
Fig. 2. X-ray diffraction patterns of AlPO-34 samples synthesized at different crys-
tallization times.
100
0.00
with flame ionization (FID) and thermal conductivity (TCD) detec-
tors, with a Petrocol DH5 0.2 capillary column and a Porapack Q
80–100 mesh packed column for separation of hydrocarbons and
oxygenates, respectively.
95
-0.02
90
-0.04
85
S-34(6)
-0.06
80
3. Results and discussion
S-34(8)
75
70
65
60
-0.08
-0.10
-0.12
A-34-B7
S-34(4B)
According to the experimental procedure a series of samples
of SAPO-34 and AlPO-34 has been synthesized. The experimental
conditions and the gel compositions used for the different syn-
theses are presented in Table 1. In the case of SAPO materials, it
was observed that when the silicon content in the synthesis gel
was decreased to a SiO2/Al2O3 ratio of 0.4 it was necessary to
increase the concentration of the structure directing agent (TEAOH)
to obtain materials with pure chabazite phase (sample S-34(4B)
compared to sample S-34(4), which showed a mixture of CHA and
AFI phases). Under the experimental conditions used in this work
it has not been possible to synthesize pure SAPO-34 with higher
silicon content than sample S-34(8) (with x = 0.8, Table 1). The
X-ray powder diffraction patterns of the as-synthesized samples
confirm the structure type SAPO-34 (CHA structure) in these mate-
rials (Fig. 1). The peaks position and the intensities are identical to
those reported for SAPO-34 [34]. As shown in Table 1, the synthe-
ses carried out without silicon in the gel only rendered amorphous
solids when the hydrothermal treatment was prolonged up to 5
days using the lowest amount of SDA (y = 1). However, when the
amount of TEAOH in the synthesis gel was increased up to y = 1.5,
materials having pure chabazite phase were obtained for crystal-
lization times from 1 up to 13 days [35]. XRD patterns of these
samples are presented in Fig. 2. It can be observed that the intensity
of the XRD peaks of these samples increases with the crystallization
time up to 7 days and then remain unchanged for longer times. For
that reason, sample A-34-B7 has been selected to be compared with
SAPO samples along this work. It is important to notice that AlPO
samples present narrower diffraction peaks than the correspond-
ing samples synthesized with silicon (Fig. 1), probably pointing to
a smaller crystallite size for the later samples.
400
600
800
1000
Temperature (K)
Fig. 3. Thermogravimetric analyses (solid lines) and derivatives (dotted lines) of
AlPO-34 and SAPO-34 samples.
Several weight loss steps can be observed in the TG profiles,
depending on the sample. Samples S-34(6) and S-34(8) present
a first weight loss, at temperatures of 323–343 K, which can be
attributed to desorption of adsorbed water. The main weight loss,
between 573 and 823 K, corresponds to the decomposition of the
template (TEAOH) located inside the channels of the SAPO structure
compensating the charge associated to the silicon atoms. Finally,
the third weight loss at temperatures higher than 823 K has been
associated with the further removal of organic residues occluded
in the channels and cages of the SAPO-34 caused by combustion.
Sample S-34(4B), in addition to these peaks, presents a notable
weight loss at temperatures below 573 K which can be attributed
to SDA physisorbed on the SAPO crystallites. It has been calculated
that the amount of TEA+ cations required to fill up the void vol-
ume of SAPO-34 is around 1 molecule of template per chabazite
cage, so the organic species in excess present in this sample should
then be located probably on the external surface of the crystals.
Finally, the AlPO sample A-34-B7 presents a different TG profile
with the peaks corresponding to the desorption of water (tem-
perature below 373 K), an important weight loss centred at 473 K
attributed also to physisorbed SDA and an additional weight loss at
750–800 K which could be assigned to the removal of the organic
residues by combustion.
The chemical and CHN elemental analyses of selected samples
are detailed in Table 2. As it could be expected the amount of sil-
icon incorporated to the solids increases linearly with the silicon
concentration in the synthesis gel.
Theorganiccontentof theas-synthesizedsamplescanberelated
with the incorporation of the SDA to the solids, which has been
studied by thermogravimetric and elemental (CHN) analyses. Ther-
mogravimetric analyses of these samples are plotted in Fig. 3.
The quantification of the different weight losses from the
thermogravimetric analyses is presented in Table 3 and can be
compared with the CHN elemental analyses (Table 2). It can be
observed that values for the total amount of organic compounds in
the solids determined from both techniques are in good agreement,

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T. Álvaro-Mun˜oz et al. / Catalysis Today 213 (2013) 219–225
Table 2
Chemical and CHN elemental analyses of selected samples of SAPO-34 and AlPO-34.
Sample
Si/(Al + P)gel
Si/(Al + P)solid
Organic content (wt%)
C/N at. ratio
Mol SDA/u.c.
C
H
N
Total
A-34-B7
S-34(4B)
S-34(6)
S-34(8)
0
0
4.47
2.99
2.54
2.60
4.96
0.70
2.82
1.55
1.60
8.16
25.62
13.92
13.73
7.5
7.4
7.4
7.0
0.49
2.03
1.00
1.04
0.10
0.15
0.20
0.15
0.20
0.28
17.84
9.77
9.59
Table 3
Thermogravimetric analyses of selected samples of SAPO-34 and AlPO-34.
(substitution of one P atom by a Si atom in the AlPO framework) but
also by simultaneous substitution of a pair of adjacent Al–P atoms
(SM3 mechanism), thus giving rise to aluminosilicate domains
(commonly referred to as Si islands) in the SAPO network which is in
good agreement with observations published previously for SAPO-
34 catalysts with similar silicon framework content [23,31,35]. The
size and concentration of these Si islands will be different depend-
ing on the extension of SM3 to SM2 substitutions [37,38] and this
can be correlated with the strength of the acid sites generated at
the border of the Si islands which is higher than that of the acid
sites created by the isolated Si atoms [39]. Thus, a higher number
of acid sites are generated through the SM2 mechanism, while sub-
stitution via SM2 + SM3 yields less but stronger acid sites. In all the
spectra shown in Fig. 5, this broad band is centred at −100 ppm and
only slight differences can be appreciated among the different sam-
ples. The broad spectra obtained for these samples, with maxima
at around −100 ppm, indicate that there are both isolated Si atoms
and very small silica domains, but no large silicon islands, in these
catalysts.
Sample
Weight loss (%)
T < 573 K
573 K < T < 823 K
T > 823 K
A-34-B7
S-34(4B)
S-34(6)
S-34(8)
16.6
19.7
3.1
8.1
15.8
14.2
14.4
1.0
1.6
1.8
1.7
3.6
Table 4
Textural properties of selected calcined samples of SAPO-34.
Sample
Specific surface area (m2/g)
SBET
Smicro
Sext
S-34(4B)
S-34(6)
S-34(8)
424
652
545
384
608
483
40
44
62
The acidity of the calcined samples has been evaluated by NH3-
TPD (Fig. 6). It can be observed that the AlPO material possess some
acidity, although the total amount of adsorbed ammonia in that
sample is significantly smaller than that obtained for the SAPO-
34 samples. Besides, the TPD profile of the AlPO sample shows a
maximum at around 525 K, indicating that these acid sites are rel-
atively weak. This acidity could be related to weak Brönsted sites
corresponding to terminal P OH and/or Al OH groups. However,
it is well known that this technique does not allow distinguish-
ing between Brönsted and Lewis acid sites. Therefore, it cannot be
excluded that some Lewis acid centres able to adsorb ammonia at
moderate temperature are present in the AlPO material, probably
formed during the calcination treatment. SAPO-34 samples with
different Si content show slightly different TPD profiles, although
the total amount of ammonia adsorbed is similar for the three sam-
ples in spite of the different Si framework content. All three samples
show a desorption peak with maximum at around 525 K, as in the
case of the AlPO sample, that would correspond to weak acid sites,
and an additional desorption peak at higher temperature (with
maximum at around 625 K), which reveal the presence of stronger
acid sites in these samples. The relative proportion of the high-
and low-temperature desorption peaks changes with the total sil-
icon content, indicating different distribution of acid strength. The
sample with intermediate silicon content, i.e. S34(6), exhibits the
highest proportion of stronger acid sites, while the sample with the
higher silicon content, i.e. S34(8), possesses the lowest proportion
of stronger acid sites, the weak acid sites being predominant.
Samples of AlPO-34 and SAPO-34 with different content of sil-
icon in the framework have been tested in the transformation
of methanol to short chain olefins at 673 and 723 K, under the
experimental conditions described previously. Except the AlPO-34
catalyst, all the samples showed very high initial activity in the
MTO reaction at both temperatures (Fig. 7), and full conversion of
oxygenates (both methanol and intermediate dimethyl ether) was
obtained at short reaction time. In all the cases, the catalyst life-
time decreased when the temperature was increased from 673 to
723 K (Fig. 7). The process is characterized by fast deactivation of the
considering the weight losses attributed to the SDA. The C/N ratios
calculated from the elemental analyses are close to that corre-
sponding to tetraethylammonium hydroxide (C/N = 8), indicating
that the organic molecules occluded or adsorbed in the different
samples should be mainly SDA molecules which are stable during
all the synthesis.
The crystallite size and shape of the different samples has been
determined by SEM (Fig. 4). Both parameters seem to be influenced
by the amount of silicon employed in the synthesis. The AlPO sam-
ple A-34-B7 presents a mixture of large rhombohedral crystallites
of approximately 1 ␮m and very small nanocrystals with irregular
shape, while SAPO samples present a more uniform morphology
consisting in plate-like crystallites. The dimensions of crystallites
are around 0.8 ␮m × 0.8 ␮m × 0.3 ␮m for sample S-34(4B). Crys-
tallite size decreases when the amount of silicon in the material
increases. In this sense, SEM micrographs of samples S-34(6) S-
34(8) show that these samples crystallize also with a plate-like
morphology and crystallites of around 0.5 ␮m × 0.5 ␮m × 0.2 ␮m
in size.
Textural properties of calcined materials were analyzed by
nitrogen adsorption–desorption at 77 K. All the SAPO-34 samples
present type I isotherms (according to the IUPAC classification [36]),
corresponding to microporous materials (Fig. I in Supplementary
information). Data of surface area calculated from the isotherms
are collected in Table 4. It can be observed that the samples have
surface area in the range 420–650 m2/g. The sample with larger
crystallites (S-34(4B)) possesses the lower surface area and all the
samples show very low non-microporous (external) surface, values
characteristic of this type of materials.
29Si CP/MAS NMR spectra of the SAPO samples are presented
in Fig. 5. It can be observed that the spectra obtained for the three
samples are quite similar and consist of a broad envelope in the
−80 to −120 ppm range, corresponding to several resonances due
to different Si(nAl) environments (n = 0–4), in which the Si atoms
are tetrahedrally coordinated to oxygen atoms and surrounded by n
Al and 4-n Si atoms in the second coordination sphere. Therefore, in
these samples Si incorporation occurs not only by SM2 mechanism

T. Álvaro-Mun˜oz et al. / Catalysis Today 213 (2013) 219–225
223
Fig. 4. Scanning electron micrographs of AlPO-34 and SAPO-34 samples.
Si (4Al)
Si (3Al)
Si (2Al)
Si (1Al)
Si (0Al)
S-34(4B)
S-34(6)
S-34(8)
S-34(4B)
S-34(6)
S-34(8)
A-34-B7
-50
-60
-70
-80
-90
-100
-110
-120
-130
450
500
550
600
650
700
750
800
Chemical shift (ppm)
Temperature (K)
Fig. 5. 29Si CP/MAS NMR spectra of calcined SAPO-34 catalysts synthesized with
Fig. 6. NH3-TPD plots of calcined SAPO-34 catalysts synthesized with different
different amount of silicon in the synthesis gel.
amount of silicon in the synthesis gel.
100
80
60
40
20
0
673 K
723 K
catalyst as a result of the formation of pore-blocking products in the
course of the reaction, making the active centres of the catalyst less
accessible for reactant molecules, and this mechanism is enhanced
when the reaction temperature increases. All catalysts tested con-
verted methanol with high selectivity to short chain (C2–C4) olefins,
reaching values as high as 90–95%, which only decayed slightly
when the conversion levels started to decrease (Fig. 8). For all the
catalysts, the ethylene/propylene ratio increases with the reaction
S-34(4B)
S-34(8)
S-34(6)
S-34(8)
=
temperature (Supplementary Information, Fig. II). C2=/C3 ratios
AlPO-34-7B
around 1 are obtained at 673 K while they increase up to 1.3–1.4
when the temperature rises at 723 K. This fact has been previously
attributed to the secondary reactions of oligomerization and crack-
ing which are favoured when the reaction temperature increases
[40,41].
It is important to remark that data in Figs. 7 and 8 are referred to
conversion of methanol into hydrocarbon products, that is, in prod-
ucts other than dimethyl ether. Therefore, conversion levels are
S-34(6)
S-34(4B)
AlPO-34-7B
0
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Reaction time (h)
Fig. 7. Conversion of oxygenates (MeOH and DME) vs. reaction time obtained with
the different catalysts. Test conditions: T = 673–723 K, WHSV = 1.2 h−1, 1 g of catalyst.

224
T. Álvaro-Mun˜oz et al. / Catalysis Today 213 (2013) 219–225
100
90
80
70
60
50
40
From the results presented here it can be concluded that, under
the experimental synthesis conditions used in this work, with
relatively high concentration of silicon in the framework, the dis-
tribution of the silicon environments is quite similar for all SAPO
catalysts, despite the different samples incorporate quite differ-
ent amounts of silicon into their frameworks and, as different
authors have proposed previously, it appears that this parameter
has a minor effect in the catalytic performance of these materi-
als [20,24,25]. Indeed, the catalysts with different silicon content
showed similar total concentration of acid sites, although some
differences in acid strength distribution were apparent, which
nevertheless seemed not to have a significant effect on catalysts sta-
bility. On the contrary, the crystallite of the SAPO-34 catalysts has
been shown as the key parameter to control the catalytic behaviour
in order to obtain more stable materials with longer lifetime. The
results demonstrate that samples synthesized with smaller crys-
tallite rendered the best catalytic performance independent of the
silicon content.
AlPO-34-7B
S-34(4B)
S-34(6)
S-34(8)
673 K
723 K
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Reaction time (h)
Fig. 8. Selectivity to short chain olefins (C2–C4) in function of the time on stream
for the different samples in the MTO reaction. Test conditions: T = 673–723 K,
WHSV = 1.2 h−1, 1 g of catalyst.
4. Conclusions
Different samples of SAPO-34 with different amounts of silicon
in the framework have been synthesized by following a conven-
tional hydrothermal method. The results showed that SAPO-34
materials prepared with medium-high silicon concentration in the
framework present quite similar silicon environment distributions
and, when it is so, the key factor to obtain materials with improved
catalytic performance is the crystallite size of the catalyst.
determined as conversion of oxygenates (transformation of both
methanol and dimethyl ether) and selectivity values indicate selec-
tivity to the different hydrocarbons and are calculated excluding
the intermediate product dimethyl ether. Thus, the low conver-
sion values calculated for AlPO-34 are due to the fact that this
catalyst produces mainly dimethyl ether. Also, the decay of con-
version levels of the SAPO-34 catalysts is due to the production
of dimethyl ether that is observed once the catalysts deactivation
starts. The lower activity of the AlPO-34 sample can be attributed
both to its low concentration of acid sites and their weak acid
strength (as shown by the NH3-TPD analyses), associated with
defects in the framework, since the absence of silicon in this cat-
alyst precludes the presence of Brønsted acid centers like those
characteristic of SAPO materials (Si OH Al). At both tempera-
tures, as the reaction time increases, differences in the catalytic
behaviour began to appear among the different samples. In the
case of sample S-34(4B) conversion at 673 K goes down to less
than 50% after 4 h of time on stream (TOS), while samples S-
34(6) and S-34(8) maintain conversion levels above 90% during
6 h. As it is well known, the main problem of this kind of cata-
lysts in the MTO process is the rapid deactivation attributed to
the deposition of high molecular weight hydrocarbons on the pore
entrances [7,17,42,43], but it has been demonstrated that an ade-
quate optimization of the synthesis parameters can lead to catalysts
with improved lifetime [14,15,21,44]. Among the different param-
eters that influence the stability of these catalysts, especially the
crystallite size and the acid strength of the active centres have
been considered the most important factors affecting the deac-
tivation during the reaction. The NH3-TPD results showed that
the three SAPO-34 catalysts possess similar total concentration of
acid sites, but different acid strength distribution. The TPD pro-
files indicate that the proportion of stronger acid sites increases
in the order S34(8) < S34(4B) < S34(6). These results indicate that
catalytic behaviour of this series of SAPO-34 samples is not deter-
mined mainly by acidity. The faster deactivation of sample S34(4B)
should be attributed to the larger crystallite size of this catalyst. The
smaller crystallites of samples S-34(6) and S-34(8) would facilitate
the accessibility of the reactant molecules to the acid sites and the
diffusion of the reaction products and could justify their higher
stability in the MTO process under the experimental conditions
tested here. The fact that both samples showed comparable cat-
alytic stability despite their different distribution of acid strength,
also supports the hypothesis that crystallite size has a more impor-
tant effect on catalytic behaviour than acidity.
Acknowledgements
We are thankful for the financial support of the Spanish Min-
istry of Science and Innovation, projects MAT2009-13569 and
MAT2012-31127. TAM acknowledges CSIC for a PhD grant.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.cattod.
2013.04.031.
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