Aminothermal synthesis of CHA-type SAPO molecular sieves and their catalytic performance in methanol to olefins (MTO) reaction

Article abstract of DOI:10.1039/c3ta12829f

Aminothermal synthesis of SAPO molecular sieves, in which organic amines are used as both solvent and template, is explored based on a variety of amines. Di-iso-propylamine (DIPA) and N,N,N′,N′-tetramethylethylenediamine (TMEDA) are found to lead to the rapid crystallization of SAPO-34 with high solid yield. A solid yield of 96.2% could be acquired using the TMEDA system (200 °C, 12 h), which is the highest value ever reported for SAPO molecular sieves. SAPO-44 is obtained for the first time using the hexamethyleneimine (HMI) template. Detailed synthetic investigation shows that the silicon content in the initial gel has an important effect on the crystalline nature of the final products, and higher Si concentration favours the synthesis of pure SAPO-34 and SAPO-44. In addition, it is shown that the Si coordination environment in the samples is closely related to the choice of template. Among the three samples investigated, SAPO-34-DIPA has the lowest threshold of Si content for the formation of Si islands in the framework due to the smallest charge compensation centers occluded in its CHA cage. The catalytic performance of the synthesized samples is tested by the MTO reaction and a high olefin selectivity of 85.8% is obtained on SAPO-34 templated by DIPA.

Full text of DOI:10.1039/c3ta12829f

Journal of
Materials Chemistry A
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PAPER
Aminothermal synthesis of CHA-type SAPO molecular
sieves and their catalytic performance in methanol to
olefins (MTO) reaction†
Cite this: J. Mater. Chem. A, 2013, 1,
4206
1
abc
ab
abc
abc
abc
Dong Fan,
Chan Wang,
Peng Tian, Xiong Su,
Yangyang Yuan,
Dehua Wang,
abc
ab
ab
ab
ab
Miao Yang, Linying Wang, Shutao Xu and Zhongmin Liu*
Aminothermal synthesis of SAPO molecular sieves, in which organic amines are used as both solvent and
0 0
template, is explored based on
a variety of amines. Di-iso-propylamine (DIPA) and N,N,N ,N -
tetramethylethylenediamine (TMEDA) are found to lead to the rapid crystallization of SAPO-34 with
ꢀ
high solid yield. A solid yield of 96.2% could be acquired using the TMEDA system (200 C, 12 h), which
is the highest value ever reported for SAPO molecular sieves. SAPO-44 is obtained for the first time
using the hexamethyleneimine (HMI) template. Detailed synthetic investigation shows that the silicon
content in the initial gel has an important effect on the crystalline nature of the final products, and
higher Si concentration favours the synthesis of pure SAPO-34 and SAPO-44. In addition, it is shown
that the Si coordination environment in the samples is closely related to the choice of template. Among
the three samples investigated, SAPO-34-DIPA has the lowest threshold of Si content for the formation
of Si islands in the framework due to the smallest charge compensation centers occluded in its CHA
cage. The catalytic performance of the synthesized samples is tested by the MTO reaction and a high
olefin selectivity of 85.8% is obtained on SAPO-34 templated by DIPA.
Received 22nd July 2013
Accepted 18th September 2013
DOI: 10.1039/c3ta12829f
www.rsc.org/MaterialsA
amounts of water as the mass and heat transfer medium. The
presence of organic amines as structure directing agents (SDAs)
1
Introduction
11,12
Silicoaluminophosphate (SAPO) molecular sieves were rstly
reported by Union Carbide in 1984. The structures of SAPOs
or templates is essential for the successful synthesis.
Many
1,2
organic amines, including tetraethylammonium hydroxide
cover a range of different structure types; some are analogous to (TEAOH),1,2 dipropylamine, isopropylamine, piperidine,
certain zeolites such as SAPO-34 (CHA topology), but a large morpholine,
2
13
14
15
4
triethylamine (TEA),
and diethylamine
number have unique structures without a zeolite counterpart. (DEA),16,17 etc., have been reported to direct the crystallization of
Among SAPOs, small-pore SAPO-34 has received great attention SAPO-34. According to the literature, the choice of SDAs for the
in recent years due to its good performance in methanol-to- synthesis of SAPO-34 is crucial for the properties of the nal
3
,4
olens (MTO) reaction.
The CHA framework topology,
products, such as microscopic structures, elemental composi-
tions and morphologies. For example, Barthomeuf et al.
carried out comparative studies on the properties of SAPO-34
˚
18
comprised of cylinder-like cages (6.7 ꢁ 6.7 ꢁ 10.0 A) with 8-ring
˚
openings (3.8 ꢁ 3.8 A), has been reported to be the ideal
breeding ground for hydrocarbon pool intermediates (poly- templated by TEAOH and morpholine, and concluded that the
5
–9
methylbenzenes as active species) in the MTO reaction.
template could determine the maximum charge and govern the
Recently, a commercial MTO process with a production capacity distribution of silicon in the framework. The same group also
of 600 000 tons of light olens per year has been successfully reported that the choice of template exerted a signicant impact
1
0
19
20
put into operation based on the SAPO-34 catalyst.
on the short- and long-term stability of SAPO-34. Gon et al.
21
The traditional synthesis of SAPO-34 is commonly carried
out in a hydrothermal way, which involves the addition of large
and Nishiyama et al. found the crystallite size variation of
SAPO-34 crystals synthesized using different templates,
respectively, and successfully correlated this with their catalytic
performance in the MTO reaction. Our group systematically
studied the synthesis of SAPO-34 templated by DEA and
discovered that DEA tended to prompt higher silicon incorpo-
a
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Dalian, P. R. China. E-mail: liuzm@dicp.ac.cn
b
National Engineering Laboratory for Methanol to Olens, Dalian Institute of Chemical
16
ration into the SAPO-34 framework than TEAOH and TEA.
Physics, Chinese Academy of Sciences, Dalian, P. R. China
c
Novel synthetic methods are always attractive in the eld of
University of Chinese Academy of Sciences, Beijing, P. R. China
molecular sieves, which may have the possibility to create new
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3ta12829f
22,23
1
materials or known phases with specic properties.
One of
1
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the alternative synthetic methods for SAPO molecular sieves is
The solid yield of samples is calculated by the following
24–26
the solvothermal synthesis.
The methodology was rstly formula: yield (%) ¼ (M
ꢁ DW%) ꢁ 100/(M
+ MP2O5 +
+ MSiO2)gel
sample
Al
2
O
3
introduced into the zeolite synthesis by Bibby and Dale, in
M
)
gel
, where M
, DW% and (M
+ M
SiO
2
sample
Al
2
O
3
P
2
O
5
which organic solvents instead of water were used as the reac- stand for the weight of as-synthesized samples, the weight
27
tion medium. Xu and co-workers extended this method to the percentage of inorganic oxides in the as-synthesized samples
synthesis of aluminophosphate (AlPO) molecular sieves. By derived from TG analysis, and the dry mass of the three oxides
employing alcohols as solvents, they synthesized many novel in the starting mixture, respectively.
AlPO materials with anionic frameworks, among which JDF-20
possessing 20-ring extra-large pores are the most attractive
24,28,29
2.2 Characterization
ones.
Recently, we have proposed a novel solvothermal
approach to prepare SAPO molecular sieves, designated as The powder XRD pattern was recorded on a PANalytical X'Pert
˚
aminothermal synthesis, in which the organic amine acts as PRO X-ray diffractometer with CuKa radiation (l ¼ 1.54059 A),
30
both the template and solvent. Solvothermal synthesis of operating at 40 kV and 40 mA. The chemical composition of the
SAPO-34 and SAPO-18 were realized for the rst time with this solid samples was determined with a Philips Magix-601 X-ray
method. The synthesized materials exhibited good adsorption uorescence (XRF) spectrometer. The crystal morphology was
2 2 4
capacities for CO and high CO /CH ratio. Moreover, the observed by scanning electron microscopy (SEM, Hitachi S-
organic amines could be easily collected and recycled aer each 3400N). All NMR experiments were performed on a Varian
synthesis, suggesting the environmental benignity of the Innity plus 400WB spectrometer with a BBO MAS probe
methodology. It should be noted that the aminothermal operating at a magnetic eld strength of 9.4 T. The resonance
2
7
31
synthesis is not truly anhydrous, and the presence of small frequencies were 104.2, 161.9, 79.4, and 100.5 MHz for Al, P,
2
9
13
quantity of water in the initial system is essential for the
successful crystallization of SAPO molecular sieves.
Si and C, respectively. Chemical shis were referenced to
2
7
31
3 3 3 4
1.0 M Al(NO ) for Al, 85% H PO for P, and 2,2-dimethyl-
In the present work, a variety of organic amines were 2-ilapentane-5-sulfonate sodium salt (DSS) for Si and C. The
employed for the aminothermal synthesis of SAPO molecular spinning rates of the samples at the magic angle were 4, 10, 6
sieves. The investigated organic amines include n-propylamine and 8 kHz for Si, Al, P and C, respectively. Textural
PA), tri-n-propylamine (TPA), di-iso-propylamine (DIPA), properties of the calcined samples were determined by N2
n-butylamine (BA), N,N-dimethylbenzylamine (DMBA), 1,2-eth- adsorption at 77 K on a Micromeritics ASAP 2020 system. The
2
9
13
2
9
27
31
13
(
0
0
ylenediamine (EDA), N,N,N ,N -tetramethylethylenediamine total surface area was calculated based on the BET equation.
(
(
TMEDA), cyclohexylamine (CHA) and hexamethyleneimine The micropore volume and micropore surface area were evalu-
HMI). Three amines, TMEDA, DIPA and HMI, rarely used for ated using the t-plot method. TG and DSC analysis were
the synthesis of CHA-type SAPO molecular sieves (termed as performed on a TA SDTQ600 analyzer with the temperature-
ꢀ
ꢂ1
CHA-SAPO), are found to direct the formation of SAPO-34 and programmed rate of 10 C min under an air ow of 100 ml
ꢂ1
SAPO-44. Detailed investigations on the synthesis, physi- min . The temperature-programmed desorption of ammonia
ochemical properties and catalytic performance of CHA-SAPO (NH -TPD) experiments were conducted in a Micromeritics
3
samples are carried out.
Autochem II 2920 device. 0.2 grams of the sample particles (40–
6
6
1
0 mesh) were loaded into a U-quartz tube and pretreated at
50 C for 60 min under helium ow. Aer cooling down to
ꢀ
2
Experimental
ꢀ
00 C, a gas mixture of NH
3
and He ow was introduced to
adsorption (60 min). Aer
Organic amines (chemical pure) were used as received. Pseu- this, He ow was purged through the sample for 30 min to
doboehmite (70.5 wt%), phosphoric acid (85 wt%), and silica sol remove the weakly adsorbed NH molecules. The measurement
2.1 General procedure of aminothermal synthesis
saturate the sample surface with NH
3
3
ꢀ
ꢀ
(
27.5 wt%) were used as inorganic precursors.
The molar ratio of the initial gel is R/Al /P
X/1.2/0.9/0.5/14.6, in which the volume of R is kept constant
50 ml), and the masses of Al, P and Si sources are 8.4 g, 10.4 g
and 5.3 g, respectively. Water from each resource has been
of the desorbed NH
O ¼ (10 C min ) under He ow (20 ml min ).
3
was performed from 100 C to 700
C
ꢀ
ꢂ1
ꢂ1
2
O
3
2
O
5
/SiO
2
/H
2
(
2.3 Catalyst evaluation
calculated into the gel composition. A typical synthesis proce- MTO reaction was carried out with a xed-bed reactor at
dure is as follows. Organic amine, pseudoboehmite, silica sol atmospheric pressure. 1.2 grams of the calcined sample (40–60
and water were added in sequence into a glass beaker. The mesh) were loaded into the reactor. The catalyst was pretreated
ꢀ
mixture was stirred at room temperature for 5 min, and then in a ow of nitrogen atmosphere at 550 C for 1 h. Nitrogen ow
ꢀ
transferred into a stainless steel autoclave. Aer further addi- was turned off when the reactor was cooled down to 450 C. A
3 2
tion of phosphoric acid, the autoclave was sealed quickly, mixture of methanol and water with a CH OH–H O weight ratio
placed in an oven and rotated at 60 rpm for 20 min to get a of 40/60 was consequently pumped into the reactor. The weight
ꢂ1
homogeneous mixture. Subsequently, the autoclave was heated hourly space velocity (WHSV) of methanol was 2 h . The
in 60 min to the desired temperature under rotation and kept products were analysed by an Agilent GC7890 gas chromato-
for a certain time. The solid product was obtained aer centri- graph equipped with an FID detector and Poraplot Q-HT
ꢀ
fugation, washing and drying at 100 C overnight.
capillary column.
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33
The methanol conversion is dened as the percentage of formation of SAPO-34, the hydrothermal synthesis was con-
ꢂ
CH OH consumed during the MTO reaction. The selectivity is ducted via a uoride route. Without F , the product would
3
34
dened as the weight percent of each compound in the total become SAPO/AlPO-21. Moreover, Pastore et al. reported that
products. It should be noted that dimethylether (DME) is the lamellar AlPO-kanemite precursor could be transformed
considered as a reactant instead of a product here.
into CHA-SAPO (designated as CAL-1 therein) by a hydrothermal
reaction with silica in the presence of HMI. Their results
showed that the synthesis of CAL-1 was restricted within a
3
Results and discussion
2 2 3
relatively narrow initial SiO /Al O ratio (0.8–1.2), and CAL-1
3.1 Aminothermal synthesis of SAPO molecular sieves based
was actually co-templated by HMI and BA (the SDA for the AlPO-
kanemite). To the best of our knowledge, there is no research on
SAPO-44 templated by HMI until now.
on a variety of organic amines
Fig. 1 displays the XRD patterns of the as-synthesized samples
prepared with different amines as both the solvent and
template. It was found that primary amines, including BA, CHA,
PA and EDA, mainly led to the formation of lamellar materials
3.2 Synthesis of SAPO-34 with DIPA as the solvent and
template
(a–d, Fig. 1). SAPO-5 was obtained as the nal product while
employing TPA or DMBA amines (e and f, Fig. 1). Three amines,
including HMI, DIPA and TMEDA, were found to direct the
crystallization of SAPO molecular sieves with the CHA topology
The effect of silicon content in the initial gel on the synthesis
was rstly investigated based on the DIPA system by xing other
synthetic conditions. The results are shown in Table 1. AlPO-11
instead of AlPO-34 was acquired as the nal product in the
absence of silicon. This is not surprising because DIPA is a well-
known template for the hydrothermal synthesis of AlPO-11. On
the other hand, AlPO-34 could only crystallize in a uoride-
(g–i, Fig. 1). It should be noted that the samples templated by
DIPA and TMEDA exhibit the typical diffraction pattern of
SAPO-34, while some difference is observed for the sample
templated by HMI. The distance between two peaks in the range
ꢀ
of 23–26 (2 theta) and the relative intensities of the neigh-
containing system. By increasing the x(SiO ) to 0.15, SAPO-34
2
ꢀ
boring peaks around 30 (2 theta) in the pattern of SAPO-HMI
appeared together with small quantity of SAPO-11 as impurity.
Pure SAPO-34 was readily synthesized when the silicon content
reached 0.3 or higher. However, a small amount of the amor-
phous phase due to the unreacted silica residues started to
sample obviously differ from those of SAPO-34-DIPA and SAPO-
3
4-TMEDA, suggesting that SAPO-HMI was in fact SAPO-44, a
31
silicoaluminophosphate also possessing chabasite topology.
Among the three organic amines for CHA-SAPO molecular
sieves, DIPA was once reported for the dry-gel synthesis of
SAPO-34. Although TMEDA has been patented to direct the
2
appear in the product when the x(SiO ) became 1.0, implying
the capacity limitation of silicon incorporation into the frame-
work of SAPO-34-DIPA. The elemental composition of the
obtained samples was determined by XRF and shown in Table 1.
The Si concentration in SAPO-34 rises with the increasing
content in the initial gel, though the Si incorporation degree
shows a decline.
32
The crystallization process of SAPO-34 using DIPA as the
solvent and template was studied based on sample 4 (Table 1).
The process exhibits a feature of fast crystallization. The relative
crystallinity of the product reaches 87% aer a crystallization
duration of 3 hours. This value further increases to 96.9% aer
7
hours and keeps at a high level until the end of the crystalli-
zation. The Si content in the product shows an increasing trend
with time, consistent with what we have found in the hydro-
17,35
thermal synthesis of SAPO-34 templated by TEA and DEA.
It
implies that both the initial Si content in the gel and the crys-
tallization time can modify the Si concentration in the nal
product. Moreover, the solid yield during the crystallization
shows a maximum of 90.1% at t ¼ 12 h. Aerwards, it gradually
36
drops and a yield of 83.8% is obtained for the 48 h sample. The
possible reason is that molecular sieves are generally meta-
stable materials, which crystallize through a kinetic control.
Dissolution and phase transformation might occur aer the
optimal synthesis period of a certain phase.
The SEM images (Fig. 2) reveal that SAPO-34-DIPA crystals
samples 4 and 9) possess the typical rhombohedral
morphology. The crystal size ranges from 1 to 2 mm. The N2
Fig. 1 XRD patterns of the as-synthesized samples using BA (a), CHA (b), PA (c),
(
EDA (d), TPA (e), DMBA (f), HMI (g), DIPA (h) and TMEDA (i) as the solvent and
ꢀ
template. The crystallization conditions are 200 C and 48 h for all except for
ꢀ
physisorption results of samples 3 and 4 are given in Table 2.
sample i (200 C, 12 h). The peak intensities of samples a–d shown in the figure
2
ꢂ1
have been reduced to 1/3 of the original.
Both samples exhibit a BET surface area of around 590 m g
1
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Table 1 The influence of synthetic parameters on the synthesis results based on the DIPA system
Yield
(%)
a
b
c
Sample
x(SiO
2
)
t (h)
Product
Product composition
Si incorporation
Relative crystallinity (%)
1
2
3
4
5
6
7
8
9
0
48
48
48
48
48
48
3
AlPO-11
SAPO-34 + SAPO-11
SAPO-34
SAPO-34
SAPO-34
SAPO-34 + amorphous SiO
SAPO-34
SAPO-34
—
—
—
—
—
—
—
100
—
—
—
0.15
0.3
0.5
0.75
1.0
0.5
0.5
0.5
Al0.511
Al0.482
Al0.490
Al0.458
Al0.516
Al0.507
Al0.501
P0.416Si0.073
P0.422Si0.096
P0.389Si0.121
P0.357Si0.185
P0.413Si0.071
P0.409Si0.084
P0.407Si0.092
1.09
0.91
0.80
—
0.67
0.79
0.87
92.3
83.8
87.3
93.0
61.8
84.7
90.1
2
—
87.0
96.9
97.3
7
12
SAPO-34
a
ꢀ
All samples are prepared using 50 ml of DIPA as both the solvent and template under a crystallization temperature of 200 C. The initial molar
b
composition for the inorganic species is as follows: Al
P)]product/[Si/(Si + Al + P)]gel
2
2
O
3
/P
2
O
5
/SiO
2
/H
2
O ¼ 1.2/0.9/x(SiO
2
)/14.6. The silicon incorporation is dened as [Si/(Si + Al +
The relative crystallinity is calculated based on the relative intensity of the three strongest peaks (2q ¼ 9.5, 16.0 and
c
.
ꢀ
0.5 ) in the XRD patterns.
crystallized from the silica-free initial gel. By increasing the
x(SiO
a large amount of SAPO-21. Aer further increasing the value to
.30, SAPO-34 became the dominant product at the expense of
SAPO-21. Pure SAPO-34 could be obtained in the range of 0.5–
.0. It should be mentioned that high solid yields of SAPO-34 are
2
) to 0.15, SAPO-34 appeared together with the presence of
0
1
observed under the present aminothermal environment, espe-
cially for sample 13, which shows a value of 96.2%. Such a high
yield has never been reported before for SAPO molecular sieves.
The crystals of SAPO-34-TMEDA (sample 13) present elon-
gated rhombohedron morphology with a relatively large crystal
size of around 10 mm (Fig. 2). The BET surface area
and micropore volume of sample 13 were calculated to be
2
ꢂ1
3
ꢂ1
5
03 m g and 0.23 cm g , respectively. The values are lower
than those of SAPO-34-DIPA, but still in the reasonable range for
SAPO-34, indicating the good crystallinity of the product
synthesized under the TMEDA system.
Fig. 2 SEM images of the as-synthesized samples 4(a), 9(b), 13(c), 18(d).
Table 2 Textural properties of the samples
2
ꢂ1
3
ꢂ1
3.4 Synthesis of SAPO-44 with HMI as the solvent and
template
Surface area (m g
)
Pore volume (cm g
)
a
total
b
c
d
Sample
S
S
micro
S
ext
V
total
V
micro
Synthetic results based on the aminothermal synthesis of SAPO-
44 with HMI as the solvent and template verify again that the Si
content in the starting gel has great effect on the nal product
3
4
587
590
503
490
586
587
490
466
1
3
13
24
0.27
0.30
0.24
0.24
0.27
0.27
0.23
0.22
1
1
3
8
(Table 3). In the silica-free system, aluminophosphate with an
unknown phase was acquired. When the value of x(SiO ) was 0.3
2
a
b
c
BET surface area. t-plot micropore surface area. t-plot external or higher, SAPO-44 was obtained as the crystalline phase.
d
surface area. t-plot micropore volume.
Noteworthily, samples 17 and 18 possess larger silicon contents
than SAPO-34-DIPA and SAPO-34-TMEDA synthesized from the
same initial gel composition, suggesting that HMI is able to
induce higher silicon incorporation into the framework of
SAPO-44 (see the column of Si incorporation in Tables 1 and 3).
However, the solid yield with HMI system is unexpectedly low
3
ꢂ1
and a micropore volume of 0.27 cm g . Both the SEM image
and N physisorption results verify the high purity and crystal-
linity of the SAPO-34 templated by DIPA.
2
(
ꢃ40%).
By shortening the crystallization duration to 12 h, no crys-
talline product was obtained. This phenomenon indicates that
3
.3 Synthesis of SAPO-34 with TMEDA as the solvent and
template
The effect of the silicon content on the synthesis using TMEDA the crystallization kinetic under the HMI system is much slower
as the solvent and template are summarized in Table 3. It was than that under DIPA and TMEDA systems, suggesting the
found that AlPO-21 possessing the AWO topology was weaker templating efficacy of HMI in the synthesis of SAPO-44.
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Table 3 The influence of synthetic parameters on the synthesis results based on TMEDA and HMI systems
a
b
Sample
Amine
x(SiO
2
)
t (h)
Product
Product composition
Si incorporation
Yield (%)
10
11
12
13
14
15
16
17
18
19
TMEDA
TMEDA
TMEDA
TMEDA
TMEDA
TMEDA
HMI
HMI
HMI
HMI
0
12
12
12
12
12
12
48
48
48
12
AlPO-21
—
—
—
—
—
1.05
1.03
0.96
—
1.63
1.19
—
43.2
47.5
72.2
96.2
94.7
89.8
—
41.4
40.1
—
0.15
0.30
0.5
0.75
1.0
0
0.3
0.5
0.5
SAPO-21 + SAPO-34
SAPO-34 + minor SAPO-21
SAPO-34
SAPO-34
SAPO-34
Unknown phase
SAPO-44
SAPO-44
Al0.521
Al0.506
Al0.488
Al0.470
—
Al0.503
Al0.507
—
P
P
P
P
0.389Si0.089
0.381Si0.112
0.372Si0.139
0.351Si0.179
P0.388Si0.109
0.366Si0.127
P
Amorphous
a
ꢀ
All samples are prepared using 50 ml of organic amine as both the solvent and template under a crystallization temperature of 200 C. The initial
b
molar composition for the inorganic species is as follows: Al
Si + Al + P)]product/[Si/(Si + Al + P)]gel
2
O
3
/P
2
O
5
/SiO
2
/H
2
O ¼ 1.2/0.9/x(SiO
2
)/14.6. The silicon incorporation is dened as [Si/
(
.
This might also be the possible reason for the low solid yield templated by the diamine TMEDA, the two peaks in the spec-
associated with the HMI system. trum occur at relatively higher chemical shis (58 ppm for C
The crystal morphology of SAPO-44 (sample 18) was also and 47 ppm for C ), due to the intimate connection of both
1
2
examined by SEM. It exhibits the typical rhombohedral shape. methyl and methylene to the nitrogen atoms with stronger
Moreover, it is found that the textural properties of sample 18 electronegativity. Sample 18, obtained under the HMI system,
are close to those of SAPO-34-TMEDA. These results imply the displays two resonance peaks centred around 49 and 26 ppm.
good quality of SAPO-44 obtained under the HMI system.
The former peak was commonly ascribed to the carbon atoms
Ca) adjacent to the imino groups, while the latter one was
assigned to the remaining carbon atoms (C and C ) in the HMI
ring. Generally, the obtained spectra are in good agreement
with the previous literature, suggesting that the investigated
three samples are well templated by DIPA, TMEDA and HMI,
respectively.
(
3
.5 Physiochemical properties of CHA-SAPOs synthesized
b
g
34
1
3
29
27
with three templates: C MAS NMR, TG-DSC, Si, Al and
2
7
P MAS NMR
The as-synthesized samples 4, 13 and 18 (SAPO-34-DIPA, SAPO-
4-TMEDA and SAPO-44, respectively) were used for various
characterizations.
3
The TG and DSC curves for the three samples are illustrated
in Fig. S1, S2 and S3, respectively (see ESI†). The weight loss in
the samples occurs in three or four stages (Table 4). In the rst
1
3
C MAS NMR spectra were recorded in order to verify the
exact template species occluded in the three samples (see
Fig. 3). The spectrum of sample 4 exhibits two symmetrical
peaks centred around 46 and 19 ppm, which are ascribed to the
ꢀ
stage (<230 C), the weight loss is attributed to the water
ꢀ
desorption. The second weight loss between 230 and 410 C
with an exothermal process is due to the combustion decom-
position of the template. The third and fourth weight losses at a
carbon atoms bound to the nitrogen atoms (C
1
) and the methyl
carbons (C ) in the DIPA molecule, respectively. For sample 16
2
ꢀ
temperature higher than 410 C with strongly exothermic
processes are likely associated with the further removal of
organic residue occluded in the channels and cages of samples.
In addition, it can be seen from the curves that there is no
weight loss and exothermic peak associated with structural
ꢀ
collapse until 900 C, suggesting the high thermal stability of
SAPO-34 and SAPO-44 synthesized in the present work. Notably,
sample 18 demonstrated a much obvious weight loss due to the
template removal, indicating a higher organic occlusion of HMI
Table 4 Thermal analysis results of the samples
Weight loss (wt%)
Template per
Sample
4
I
II
III + IV
cage
4.3
3.2
3.5
10.8
6.3
4.0
1.0
6.5
15.7
1.01
0.96
1.91
1
1
3
8
13
Fig. 3
C MAS NMR of the as-synthesized samples 4(a), 13(b) and 18(c).
1
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Journal of Materials Chemistry A
molecules in the SAPO-44 framework. Based on the elemental
composition and topological structure of the sample, it is
calculated that for samples 4 and 13, one organic amine
molecule could be accommodated per CHA cage. As for sample
18, every cage contains nearly two HMI molecules, possibly due
to the relatively small size of HMI.
2
9
31
27
Solid-state Si, P and Al MAS NMR spectra were recorded
to investigate the local atomic coordination environments in
2
9
the as-synthesized samples. Fig. 4 presents the Si spectra of
the three samples. Only one symmetric peak centred at ꢂ91
ppm, ascribed to Si (4Al) species, appears in the spectrum of
SAPO-34-TMEDA. Complex Si environments are observed for
SAPO-34-DIPA and SAPO-44. Besides the apparent resonance at
ꢂ91 ppm, there emerge several small signals at around ꢂ96,
ꢂ100, ꢂ105 and ꢂ110 ppm, corresponding to Si (3Al), Si (2Al), Si
2
7
Fig. 5
Al MAS NMR of the as-synthesized samples 4(a), 13(b), 18(c) and
(1Al) and Si-island, respectively. Additionally, a weak peak at
calcined sample 18(d).
ꢂ85 ppm is also present in the spectra of both samples, which
possibly arises from the Si(OAl) (OH) or Si(OAl)(OSi)(OH)
3
2
37,38
species located in the Si–Al domains.
According to the
arise from tetrahedral Al species, whereas the weak one is
attributed to penta-coordinated Al formed by an additional
interaction of one water or template molecule with the frame-
work aluminum. For SAPO-44, one more peak centred at 0 ppm
is observed besides the two peaks shown in the spectra of SAPO-
deconvoluted analysis of the spectra (Table S1†), the concen-
tration of Si (4Al) species in the three samples has the following
order: SAPO-34-TMEDA > SAPO-44 > SAPO-34-DIPA. Barthomeuf
18
et al. once reported that a higher template number per cage in
SAPO-34 could lead to a larger framework charge, and thus a
higher Si (4Al) concentration in the framework. In the present
work, the template number per cage for TMEDA, HMI and DIPA
are 1, 2 and 1, respectively. However, it should be noted that
TMEDA, containing two nitrogen atoms, actually has two charge
compensation centers despite only one molecule is accommo-
dated in a cage. Therefore, the smallest charge compensation
centers conned in the cage of SAPO-34-DIPA causes the lowest
Si (4Al) concentration in the framework, though it possesses the
lowest Si content among the three investigated samples.
As shown in Fig. 5, two peaks centred at around 37 and
3
4 samples, which corresponds to hexa-coordinated aluminum.
27
The assignment is conrmed by the Al spectrum of the
calcined SAPO-44, in which only one signal with a chemical shi
of 37 ppm appears.
31
The P MAS NMR spectra of the samples are illustrated in
Fig. 6. One strong resonance peak at ꢂ30 ppm appears in the
spectra of all three samples, suggesting the predominant P (4Al)
environment in the framework. In addition, the spectrum of
SAPO-44 demonstrates two additional small peaks located at
ꢂ17.6 and ꢂ11.4 ppm, which might be assigned to partially
39
x y
hydrated P(OAl) (OH) species.
2
7
9
ppm are observed in the Al MAS NMR spectra of the two
SAPO-34 samples. The strong resonance at high eld should
3
.6 Catalytic performance in the MTO reaction
Among the three serial CHA-SAPO molecular sieves, samples 3,
3 and 17 (SAPO-34-DIPA, SAPO-34-TMEDA and SAPO-44) with
1
pure CHA structure and low Si contents were selected to test
29
31
Fig. 4
Si MAS NMR of the as-synthesized samples 4(a), 13(b) and 18(c).
Fig. 6
P MAS NMR of the as-synthesized samples 4(a), 13(b) and 18(c).
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template. Among the three amines, HMI shows a relatively high
ability to prompt the silicon incorporation into the framework
of SAPO-44. Moreover, the concentration of Si (4Al) species in
the three samples has the following order: SAPO-34-TMEDA >
SAPO-44 > SAPO-34-DIPA. According to the TG results, the
smallest number of charge compensation centers conned in
the CHA cage of SAPO-34-DIPA results in the lowest Si (4Al)
content in the framework. MTO evaluation results reveal that
SAPO-34-DIPA possesses excellent catalytic properties. The
selectivity of ethylene and propylene on SAPO-34-DIPA could
reach as high as 85.8% under the investigated conditions. It is
believed that the excellent MTO performance together with the
high synthetic yield will promote the aminothermal synthesis of
SAPO-34-DIPA to be an interesting method for preparing the
MTO catalyst.
2 4 3 6
Fig. 7 Methanol conversion (solid) and selectivity of C H plus C H (hollow)
during the MTO reaction on samples 3 (a), 13 (b), and 17 (c) (reaction conditions:
ꢀ
WHSV ¼ 2 hꢂ1, 40 wt% methanol solution).
4
50 C, CH
3
OH
3
Acknowledgements
The authors would like to acknowledge the National Natural
Science Foundation of China (Grant no. 21101150 and Grant no.
their catalytic properties in the MTO reaction, considering that
SAPO-34 with lower Si contents generally exhibited better MTO
performance. The results are illustrated in Fig. 7 and Table
S2.†
21103180) for the support of this project. Dong Fan acknowl-
3,40
edges the nancial support of DNL-Topsøe scholarship
provided by Haldor Topsøe Corporation.
Both SAPO-34 samples demonstrate good catalytic proper-
ties, especially for SAPO-34 templated by DIPA. The selectivity of
ethylene and propylene on SAPO-34-DIPA could reach 85.8%
Notes and references
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3
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