SAPO-34 templated by dipropylamine and diisopropylamine: Synthesis and catalytic performance in the methanol to olefin (MTO) reaction

Article abstract of DOI:10.1039/c5nj02351c

In this work, a comprehensive study of the hydrothermal synthesis and catalytic performance of SAPO-34 templated by the isomeric dipropylamine (DPA) and diisopropylamine (DIPA) was carried out. SAPO-34 with a faster crystallization rate and lower Si content could be obtained with DIPA as the template, suggesting the better templating efficacy of DIPA than DPA. Theoretical calculations reveal that DIPA possesses more favourable non-bonding interactions with the CHA framework and the electronic configuration is of vital importance in determining the template efficacy. SAPO-34-DIPA with low silicon contents exhibits excellent performance, over which a maximum selectivity of ethylene plus propylene (87.2%) is observed. This value should be among the top ever reported. The surface Si enrichment on the crystals, which is both template- and condition-dependent, is revealed to be of significant influence in the catalytic performance. The relatively homogenous Si distribution in the crystals, lower acid concentration and weaker acid strength corporately make SAPO-34-DIPA an excellent MTO catalyst.

Full text of DOI:10.1039/c5nj02351c

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RSCPublishing
ARTICLE
SAPO-34 Templated by Dipropylamine and
Diisopropylamine: Synthesis and Catalytic
Performance in the Methanol to Olefins (MTO)
Reaction
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Dong Fana, b, Peng Tian*, a, Shutao Xua, Dehua Wanga, b, Yue Yanga,
Jinzhe Lia, Quanyi Wanga, Miao Yanga and Zhongmin Liu*, a, c
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this work, a comprehensive study of the hydrothermal synthesis and catalytic performance of
SAPO-34 templated by the isomeric dipropylamine (DPA) and diisopropylamine (DIPA) was
carried out. SAPO-34 with faster crystallization rate and lower Si content could be obtained with
DIPA as the template, suggesting the better templating efficacy of DIPA than DPA. Theoretical
calculations reveal that DIPA possesses more favourable non-bonding interactions with the CHA
framework and the electronic configuration is of vital importance in determining the template
efficacy. SAPO-34-DIPA with low silicon contents exhibits excellent performance, over which a
maximum selectivity of ethylene plus propylene (87.2%) is observed. This value should be
among the top ever reported. The surface Si enrichment on the crystals, which is both template-
and condition-dependent, is revealed to be of significant influence in the catalytic performance.
The relatively homogenous Si distribution in the crystals, lower acid concentration and weaker
acid strength corporately make SAPO-34-DIPA an excellent MTO catalyst.
commercialized MTO utility with a production capacity of 600
000 tons light olefins per annum.20
1. Introduction
Silicoaluminophosphate (SAPO) molecular sieves are an
important class of open-framework crystalline materials, which
possess intrinsic cavities and channels and find widespread use
1-3
in gas separation and catalysis.
Since their first invention by
the Union Carbide scientists in the early 1980s,4, 5 a variety of
SAPO molecular sieves with unique structures have been
successfully synthesized.
The structures of SAPO molecular sieves cover a wide
range of different framework types; some are analogous to
certain aluminosilicate zeolites, while a lot have unique
topologies with no zeolitic counterparts.6 Among the various
SAPOs, SAPO-34, the SAPO isomorph of zeolite chabazite,
has been attracting special attention.7-12 The CHA (IZA code)
framework of SAPO-34 is fabricated by the ordered stacking of
14
the double 6-rings (D6R, Figure 1).13,
The resulting
framework, characterized by the barrel-like CHA cages with 8-
ring openings, is considered to be the ideal incubation cradle
for the hydrocarbon pool intermediates (active species) in the
methanol to olefins (MTO) reaction.15-19 In actual fact, SAPO-
34 based catalysts have been successfully applied in a
Figure 1 The barrel-like cages of SAPO-34 (Si omitted); two CHA cages are shown
in the structure
Till now, organic templates or structure directing agents
(SDAs) are indispensable in the synthesis of SAPO molecular
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sieves, in spite of the development of multiple synthetic under tumbling conditions. After the crystallization, the as-
21
strategies, including hydrothermal method,19,
method,22 dry-gel conversion,12, 23 and solvent-free synthesis.24 of centrifugation, washing and drying at 373 K overnight.
It is found that one template may produce molecular sieves with The reference sample R1 was synthesized using
solvothermal synthesized samples were obtained through combined processes
different structures by varying the synthetic conditions; and one triethylamine (TEA) as the template according to the same
type of molecular sieve could also be synthesized using procedure mentioned above.
different templates.25 The elemental composition, local
The solid yield of the samples was calculated by the
microscopic structure and morphology of one specific following formula: Yield (%) = (Msample * 83%) * 100 /
molecular sieve may change with the templates.26-28 Therefore, (MAl2O3+ MP2O5+ MSiO2)gel, where Msample and (MAl2O3+ MP2O5
the catalytic performance of the products may be different. MSiO2 gel stand for the weight of as-synthesized solid samples
+
)
Research on the synthesis of SAPO-34 and its and the dry mass of three inorganic oxides in the starting
physicochemical/catalytic property is one of the most mixtures, respectively. The weight percentage of dry mass in
interesting issues in the area of molecular sieves. Many amines the as-synthesized samples (83%, template and water removed)
have been claimed to be used as the template for the synthesis was derived from the TG analysis.
of SAPO-34.29-32 The structure-directing ability varies among
2.2 Characterization. The powder XRD patterns were
these organic amines, as indicated by the distinct crystallization recorded on a PANalytical X'Pert PRO X-ray diffractometer
rate, compositions range etc. Among the amines, two isomeric with CuKα radiation (= 1.54059 Å), operating at 40 kV and 40
amines, DPA and DIPA arouse our great interests. Both amines mA. The chemical composition of the solid samples was
have been extensively studied as the templates for SAPO-11, determined with a Philips Magix-601 X-ray fluorescence (XRF)
SAPO-31, and SAPO-41, but less attention has been paid to spectrometer. The crystal morphology was observed by
their structure-directing ability to the formation of SAPO-34.33, scanning electron microscopy (SEM, KYKY-AMRAY-1000B).
34 Our recent work shows that DIPA could be an efficient SDA All the solid state NMR experiments were performed on a
to direct the aminothermal synthesis of SAPO-34 with high Bruker Avance III 600 spectrometer equipped with a 14.1 T
solid yield and good MTO catalytic performance.35 It would be wide-bore magnet. The resonance frequencies were 156.4,
therefore interesting to study the hydrothermal synthesis of 242.9, 119.2, 150.9 and 600.1 MHz for 27Al, 31P, 29Si, 13C and
1
SAPO-34 templated by DIPA and DPA and their 1H, respectively. 27Al, 31P, 13C and H MAS NMR experiments
physicochemical properties (catalytic behaviors). In addition, were performed on a 4 mm MAS probe with a spinning rate of
considering the closely related molecular structures of both 12 kHz. 27Al MAS NMR spectra were recorded using one pulse
amines, a theoretical study on their templating ability might sequence. 200 scans were accumulated with a π/8 pulse width
also help us better understand the host-guest relationship and of 0.75 μs and a 2 s recycle delay. Chemical shifts were
the role of the organic amines in the synthesis of molecular referenced to (NH4)Al(SO4)2.12H2O at -0.4 ppm. 31P MAS
sieves, which is of fundamental importance towards a priori or NMR spectra were recorded using high-power proton
de novo synthesis.25, 36-40
decoupling. 32 scans were accumulated with a π/4 pulse width
In this contribution,
a
comparative study of the of 2.25 μs and a 10 s recycle delay. Chemical shifts were
hydrothermal synthesis of SAPO-34 using DPA and DIPA as referenced to 85% H3PO4 at 0 ppm. 13C MAS NMR spectra
the template is carried out. Theoretical calculations are were recorded using high-power proton decoupling. 244 scans
performed to elucidate the different templating efficacy of two were accumulated with a π/2 pulse width of 5 μs and a 4 s
amines. The obtained SAPO-34 samples are characterized in recycle delay. Chemical shifts were referenced to admantane
1
detail and tested as catalysts for the MTO reaction. Distinct with the upfield methine peak at 29.5 ppm. H MAS NMR
catalytic performance over samples is observed. Factors that spectra were recorded using one pulse program. 4 scans were
might influence the catalytic properties are discussed. accumulated with a π/2 pulse width of 4.4 μs and a 60 s recycle
Fortunately, some enlightening results are discovered.
delay. Chemical shifts were referenced to admantane at 1.74
ppm. 29Si MAS NMR spectra were recorded with a 7 mm MAS
probe with a spinning rate of 6 kHz using high-power proton
decoupling. 5000-6000 scans were accumulated with a π/4
pulse width of 2.5 μs and a 10 s recycle delay. Chemical shifts
were referenced to 4, 4-dimethyl-4-silapentane sulfonate
sodium salt (DSS).Textural properties of the calcined samples
were determined by N2 adsorption at 77 K on a Micromeritics
ASAP 2020 system. The total surface area was calculated using
the BET equation. The micropore volume and micropore
surface area were evaluated using the t-plot method. TG and
DSC analysis was performed on a Perkin-Elmer Pyris-1 TGA
and DTA-7 analyzer with the temperature-programmed rate of
2 Experimental
2.1 Synthesis procedures.
A
typical hydrothermal
synthesis procedure using DPA or DIPA as the template was as
follows. Pseudoboehmite (70.0 wt%), water, phosphoric acid
(85 wt%), and silica sol (30 wt%) were sequentially added into
a plastic beaker. The resulting mixture was stirred at room
temperature for 30 min to form a homogenous gel. After further
addition of DPA or DIPA as the template, the mixture was
transferred into a stainless steel autoclave and sealed quickly.
Subsequently, the autoclave was transferred into an oven,
heated to 200 oC and kept for certain crystallization period
2 | J. Name., 2012, 00, 1-3
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Table 1. Summary of the synthesis of SAPO molecular sieves using DPA or DIPA as the template.
Sample [a]
R
Al2O3
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.0
1.2
1.2
1.2
1.0
P2O5
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
1.0
SiO2
0.5
0.0
0.3
0.5
0.75
0.5
0.5
0.0
0.2
0.3
0.5
0.75
0.3
H2O
40
40
40
40
40
40
40
40
40
40
40
40
50
Product
SAPO-31
AlPO-11
Yield
-
-
55.6%
74.0%
77.6%
80.4%
-
1
2
3
4
5
6
7
8
9
10
11
12
R1
1.0 DPA
3.0 DPA
3.0 DPA
3.0 DPA
3.0 DPA
5.0 DPA
1.0 DIPA
3.0 DIPA
3.0 DIPA
3.0 DIPA
3.0 DIPA
3.0 DIPA
3.5 TEA
SAPO-34+minor SAPO-11
SAPO-34
SAPO-34
SAPO-34
SAPO-11
AlPO-11
-
SAPO-34+minor SAPO-11
SAPO-34
90.6%
83.1%
84.5%
85.2%
70%
SAPO-34
SAPO-34
SAPO-34
[a] All the synthesis experiments are carried out at 200 oC for 48 h under tumbling conditions.
10 oC/min under an air flow of 100 ml/min. X-ray cooled down to 723 K. A mixture of methanol and water with a
photoelectron spectroscopy (XPS) analysis was conducted on CH3OH/H2O weight ratio of 40/60 was consequently pumped
VG ESCALAB MK2 XPS system. AlKα X-ray (1486.6 eV) into the reactor. The weight hourly space velocity (CH3OH
radiation was used for exciting the photo electron spectra. A WHSV) was 4 h-1. The products were analyzed by an Agilent
hemispherical analyzer was employed for electron detection, GC7890 gas chromatograph equipped with a FID detector and
and the pass energy of the analyzer was set at 50 eV. HP-PLOT Q capillary column.
Quantification analysis of the surface elemental compositions
The methanol conversion was defined as the percentage of
was determined by the areas of the core-level peaks of Si 2p, Al CH3OH consumed during the MTO reaction. The selectivity
2p and P 2p. The temperature-programmed desorption of was defined as the weight percent of each compound in the
ammonia (NH3-TPD) experiments were conducted in
a
total products. It should be noted that dimethylether (DME) was
Micromeritics Autochem II 2920 device. 0.1 grams of the considered as a reactant instead of product here.
sample particles (40–60 mesh) were loaded into a U-quartz tube
and pretreated at 650 oC for 60 min under a gas mixture of NH3
and He flow was introduced to saturate the sample surface with
3 Results and Discussion
3.1 The influence of the synthetic parameters. Table 1
NH3 adsorption (60 min). After this, He flow was purged
through the sample for 30 min to remove the weakly adsorbed
NH3 molecules. The measurement of the desorbed NH3 was
lists the overall results of the hydrothermal syntheses using
both DPA and DIPA as the template. The powder XRD patterns
of the selected samples are presented in Figure S1. As can be
seen, the crystallization of SAPO-34 is significantly affected by
the silicon ratio in the initial gel for both amine systems. With
DPA as the template, AlPO-11 having the AEL (IZA code)
structure is obtained in the absence of silicon. Increasing the
molar ratio of silicon to 0.3, SAPO-34 crystallizes with minor
SAPO-11 as impurities. Pure SAPO-34 could only be obtained
when the value is increased to as high as 0.5. A similar trend is
observed for the samples synthesized using the DIPA template.
With the increasing silicon content, a gradual phase transition
from AEL to CHA is observed.
Another notable parameter that is found to exert great
influence on the phase selectivity of the syntheses is the
template dosage. In the DPA system, SAPO-31 is synthesized
when the molar ratio of DPA in the initial gel equals to 1.
Increasing the value to 3 or 5, SAPO-34 is readily formed in
pure phase, suggesting that higher DPA loading is favourable
for the CHA phase selectivity. Similarly, larger amount of
DIPA template is also found to be the prerequisite for the
synthesis of pure SAPO-34. This is in agreement with the
previous reports that increasing the gel alkalinity is better for
the formation of pure phase of SAPO-34.
o
o
o
performed from 100 C to 700 C (10 C min-1) under He flow
(20 ml min-1).
2.3 Calculation details. The host-guest interactions
between the SAPO-34 framework and template molecules are
theoretically studied based on molecular mechanics. All the
calculations are carried out using the Material Studio 6.0
software provided by the Accelrys Inc. The conformations of
the investigated templates and the interaction energies are
described with the COMPASS forcefield. A super cell
comprised of 4 rhombohedra unit cells of the CHA structure is
built to model the host framework of SAPO-34. Periodic
boundary conditions are applied. The framework atoms are kept
fixed during the calculations. The optimal docking positions of
the organic molecules inside the host framework are determined
via a simulated annealing process. Ewald summation method is
chosen for the calculation of the Coulombic electrostatic energy.
For the van de Walls interaction energy, atom-based summation
method with a cutoff value set at 12.5 Å is applied.
2.4 Catalyst Evaluation. MTO reaction was carried out
with a fixed-bed reactor at atmospheric pressure. 1.2 grams of
calcined SAPO-34 crystals was loaded into the reactor. The
catalyst was pretreated in a flow of nitrogen atmosphere at 823
K for 1 h. Nitrogen flow was turned off when the reactor was
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