Organophosphorous surfactant-assistant synthesis of SAPO-34 molecular sieve with special morphology and improved MTO performance

Article abstract of DOI:10.1039/c6ra06428k

With the aid of self-designed organophosphorous surfactant [2-(diethoxylphosphono)propyl]hexadecyldimethylammonium bromide (DPHAB), SAPO-34s with various special morphologies have been hydrothermally synthesized by using tetraethylammonium hydroxide (TEAOH), diethylamine (DEA) and triethylamine (TEA) as templates, respectively. The synthesized SAPO-34s were well characterized by XRD, XRF, SEM, N₂ adsorption-desorption, solid state NMR and NH₃-TPD measurements. The status of DPHAB was also investigated by using FT-IR, ¹³C NMR, TG-DTA and ³¹P NMR measurements as well as density functional theory calculations. It is found that the addition of DPHAB changes the crystal morphology dramatically and the aggregation degree of the crystals increases with the rising DPHAB/H₃PO₄ ratio. Moreover, microporous templates play an important role in the synthesis of mesoporous SAPO-34. TEAOH, which has the strongest interaction energy with the CHA framework among the investigated templates, shows the best cooperating ability with an organophosphorous surfactant to direct the formation of nanosized and mesoporous SAPO-34. The prepared SAPO-34s showed improved catalytic properties in the methanol-to-olefins (MTO) reaction.

Full text of DOI:10.1039/c6ra06428k

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Organophosphorous surfactant-assistant synthesis
of SAPO-34 molecular sieve with special
Cite this: RSC Adv., 2016, 6, 47864
morphology and improved MTO performance†
Chan Wang, Miao Yang, Wenna Zhang, Xiong Su, Shutao Xu, Peng Tian*ab
abc
ab
abc
abc
ab
ab
and Zhongmin Liu*
With the aid of self-designed organophosphorous surfactant [2-(diethoxylphosphono)propyl]
hexadecyldimethylammonium bromide (DPHAB), SAPO-34s with various special morphologies have
been hydrothermally synthesized by using tetraethylammonium hydroxide (TEAOH), diethylamine (DEA)
and triethylamine (TEA) as templates, respectively. The synthesized SAPO-34s were well characterized by
XRD, XRF, SEM, N
2
adsorption–desorption, solid state NMR and NH
3
-TPD measurements. The status of
31
13
DPHAB was also investigated by using FT-IR, C NMR, TG–DTA and P NMR measurements as well as
density functional theory calculations. It is found that the addition of DPHAB changes the crystal
morphology dramatically and the aggregation degree of the crystals increases with the rising DPHAB/
3 4
H PO ratio. Moreover, microporous templates play an important role in the synthesis of mesoporous
Received 10th March 2016
Accepted 6th May 2016
SAPO-34. TEAOH, which has the strongest interaction energy with the CHA framework among the
investigated templates, shows the best cooperating ability with an organophosphorous surfactant to
direct the formation of nanosized and mesoporous SAPO-34. The prepared SAPO-34s showed improved
catalytic properties in the methanol-to-olefins (MTO) reaction.
DOI: 10.1039/c6ra06428k
www.rsc.org/advances
Considerable efforts have been made to optimize the SAPO-
1
Introduction
7–11
3
4 catalyst,
and the product morphology control is consid-
Silicoaluminophosphate molecular sieves, as an important ered as an attractive solution to relieve the diffusion resistance.
class of crystalline microporous materials, have been widely Up to now, various strategies including modifying heating
1
2–15
16–20
8
used as heterogeneous acid catalysts due to their uniform modes,
adding additives,
changing raw materials and
11
microporous structure, medium/strong acidity and excellent synthetic routes have been tried to change the morphology of
thermal and hydrothermal stabilities. Especially, small-pore the SAPO-34 molecular sieve. Some SAPO-34s with smaller
1
SAPO-34 molecular sieves exhibit excellent performance in the crystal size and hierarchical porous structure have shown better
2–4
methanol-to-olens (MTO) reaction, based on which some catalytic performance for the MTO reaction. Recently, we have
commercial scale MTO plants have been successfully started also synthesized a spherical SAPO-34 nanosheet assembly by
5
up. In spite of this, the SAPO-34 catalyst suffers from rapid using [3-(trimethoxysilyl)propyl]octadecyldimethylammonium
deactivation due to the fast coke formation and mass transport chloride (TPOAC) as the mesoporogen and a part of silica
21
limitation caused by its small pore and large cavity structure. source, and diethylamine (DEA) as the microporous template.
So, the uidized-bed reaction technique with continuous Meanwhile, the hierarchical SAPO-34s have also been synthe-
regenerating and recycling has to be applied in the industrial sized by others with TPOAC mesoporogen together with other
6
17,22
MTO process. Improving the SAPO-34 catalyst would be of great microporous templates.
Although the synthesis method is
signicance from the scientic and practical points of view.
similar, the synthesized SAPO-34s displayed remarkably
different morphologies and MTO catalytic activity. It is thus
believed that microporous template plays an important role in
the crystallization of SAPO-34 which results in signicantly
a
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, different SAPO-34 morphologies and acidity.
Chinese Academy of Sciences, Dalian, P. R. China. E-mail: tianpeng@dicp.ac.cn;
Organosilane surfactant has been proved to be an effective
mesoporogen and silica source to synthesize hierarchical SAPO-
4. The existence of multifunctional groups in the designed
surfactant can effectively inhibit the phase separation of
microporous and mesoporous structures. At the same time, it
plays as a special silica source which would affect the acidity of
liuzm@dicp.ac.cn
b
National Engineering Laboratory for Methanol to Olens, Dalian Institute of
3
Chemical Physics, Chinese Academy of Sciences, Dalian, P. R. China
c
University of Chinese Academy of Sciences, Beijing, P. R. China
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra06428k
1
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1
the SAPO-34 product. To explore more effective method to tune product was obtained (yield: 86%). H NMR (CDCl
3
): d 1.14 (6H,
the SAPO crystal morphology, organophosphorous surfactant CH , m), 1.72 (2H, CH CH CH , m), 1.96 (2H, CH P, t, J 6.2 Hz),
3
2
2
2
2
1
3
seems to be an interesting choice. The incorporation of P from 3.30 (2H, BrCH , t, J 6.4 Hz), 3.91 (4H, OCH , m). C NMR
2
2
organophosphorous surfactant into the framework will not (CDCl ): d 16.15 (CH , d, J 4.6 Hz), 23.38 (CH CH CH , s), 25.69
3
3
2
2
2
change the acidity of SAPO framework, which allows an easier (CH
2
P, d, J 16 Hz), 33.27 (BrCH
2
, m), 61.30 (OCH
2
, d, J 20 Hz).
3
1
control of the product acidity. As we know, several organic
P NMR (CDCl
3
): d 31.8. Secondly, 53.28 g (0.21 mol)
phosphorus compounds have been used to synthesize molec- (C
2
5 2 3 6
H O) P(O)–C H Br and 56.75 g (0.21 mol) N,N-dimethylhex-
ular sieve materials. By using tetrabutylphosphonium as the adecylamine were stirred with 1.77 g (0.01 mol) KI catalyst in
ꢀ
only structure-directing agent (SDA), Tsapatsis et al. synthesized 55.38 g (1.2 mol) ethyl alcohol. The mixture was reuxed at 80 C
a self-pillared MFI zeolite composed of single unit cell lamella for 40 h under a nitrogen stream. Aer cooling to room
23
(
2 nm thick). The SDA creates an environment to encourage temperature and evaporating solvent, a light yellow thick liquid
the repetitive branching process which results in a “house-of- remained. The residual liquid was then puried by recrystalli-
cards” arrangement of the zeolite nanosheets with 2 to 7 nm zation to obtain the product [(C O) P(O)–C –N(CH
mesopores. These results suggest that organophosphorous 33]Br (denoted as DPHAB, yield: 83%). H NMR (CDCl
compound could play an important role in the synthesis of d 0.80 (3H, CH in C16 33, t), 1.17 (26H, CH in C16 33, m), 1.26
molecular sieve with special structure and morphology. (6H, CH in OC , t), 1.6–2.3 (6H, CH in PC N and C16
However, report on the synthesis of SAPO molecular sieves with m), 3.33 (6H, CH N, s), 3.42 (2H, CH in C16 33, t, 8 Hz), 3.71
(2H, CH in PC H N, m), 4.03 (4H, CH in OC H , m). C NMR
2
H
5
2
3
H
6
3 2
) –
3
):
1
C
16
H
3
H
2
H
3
2
H
5
2
3
H
6
33
H ,
3
2
H
24
13
the aid of organophosphorous compound is rare.
In the present work, a new organophosphorous surfactant (CDCl ): d 13.9 (CH in C H , s), 16.30 (CH in OC H , m),
2
3
6
2
2
5
3
3
16 33
3
2
5
containing a hydrolysable diethoxylphosphono part, a quater- 20.97 (CH in PC H N, s), 22.46 (CH in PC H N and C H , m),
2
3
6
2
3
6
16 33
nary ammonium group and a hydrophobic alkyl-chain tail was 26.08 (CH
designed, synthesized and used to synthesize SAPO-34 together (CH in C16
with different microporous templates, including tetraethy- 62.80 (CH in PC
lammonium hydroxide (TEAOH), diethylamine (DEA) and trie- (CDCl ): d 30.7. Little intermediate product remained in the
thylamine (TEA). The prepared SAPO-34s were well nal product. The purity of the intermediate and nal products
2
in C16
H
33, s), 29.0–29.5 (CH
N, s), 61.90 (CH
2 5
N, m), 64.11 (CH in OC H , s). P NMR
2
in C16
H
33, m), 31.70
2
H33, s), 51.05 (CH
3
2
in C16H33, s),
3
1
2
3
H
6
2
3
1
13
31
characterized, which showed different morphologies depend- was conrmed by H, C and P NMR analyses, and the spectra
ing on the different microporous templates used and the are shown in Fig. S1.†
DPHAB/H PO4 ratio. The status of the organophosphorous
3
molecule was investigated by various characterizations together 2.3 Synthesis of SAPO-34s
with the density functional theory calculation. The MTO cata-
lytic performances of the synthesized SAPO-34s were evaluated.
A typical synthesis procedure was as follows. A certain amount
of DPHAB was rstly added into deionized water and stirred for
2
h. Then, desired amount of aluminium isopropoxide, organic
amine, tetraethyl orthosilicate and phosphoric acid were added
in sequence. Aer further stirring for 2–5 h, the obtained gel
2
Experimental section
2.1 Materials
was transferred into a stainless steel autoclave and heated
ꢀ
Orthophosphoric acid (H
3
PO
4
, 85 wt%), tetraethyl orthosilicate statically at 200 C for a certain time under autogenous pres-
(TEOS, 98 wt%), aluminium isopropoxide (Al(i-C
3
H
7
O)
3
,
sure. The products were recovered by ltration, washed with
99 wt%), tetraethylammoniumhydroxide (TEAOH, 25 wt%), deionized water and dried in air. The molar composition of the
diethylamine (DEA, 99 wt%) and triethylamine (TEA, 99 wt%) gel is (2.0–3.0)R : (1.0–1.2)P O : 1.0Al O : 0.5SiO : 150H O,
2
5
2
3
2
2
were purchased from Tianjin Kemiou Chemical Reagent Co. where R is the microporous template, and P O is the mixture of
2
5
Triethyl phosphite (98 wt%), 1,3-dibromopropane (99 wt%) and DPHAB and H PO . The molar ratio of DPHAB/H PO changes
3
4
3
4
N,N-dimethylhexadecylamine (99 wt%) were purchased from from 0 to N. The products are denoted as S-R-n, where R and n
Aladdin Chemical Inc. All chemicals were used as received refer to the microporous template and the molar ratio of
without further purication.
3 4
DPHAB/H PO in the gel, respectively. For comparison,
conventional SAPO-34s were prepared by using the corre-
sponding template but without any DPHAB. The detailed
synthesis and characterization information are provided in the
ESI.†
The yields of products were calculated by the following
equation:
2
.2 Synthesis of [2-(diethoxylphosphono)propyl]
hexadecyldimethylammonium bromide ([(C O)
–N(CH 33]Br)
2
H
5
2
P(O)–
C
3
H
6
3 2
) –C16H
The organophosphorous surfactant [2-(diethoxylphosphono)
propyl]hexadecyldimethylammonium bromide was prepared by
a
two-step reaction according to Scheme S1.† Firstly,
Yield ¼ (Msample ꢁ 75%)/M(Al
2
O
3
+ P
2
O
5
+ SiO
2
)
gel ꢁ 100%,
(C H O) P(O)–C H Br as an intermediate product was synthe-
2
5
2
3
6
sized by reuxing of 36.92 g (1.11 mol) triethyl phosphite and where Msample and M(Al O + P O + SiO ) stand for the weight
2
3
2
5
2 gel
ꢀ
2
24.33 g (5.55 mol) 1,3-dibromopropane at 165 C for 1 h under of the products and the dry mass of inorganic oxides in the
a nitrogen stream. Aer cooling to room temperature and starting mixture, respectively. 75% is the estimated average
evaporating the unreacted reactants, the colorless liquid value of framework compounds included in the samples.
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.4 Characterization
Paper
2
energies between the template and CHA structure were
described with the COMPASS force eld using the Forcite code.
The COMPASS force eld has been applied widely on the study
of interaction between zeolite framework and organic mole-
The unit cell and the positions of framework atoms
were xed in the optimization progress. The interaction energy
inter was calculated as: Einter ¼ E ꢂ Ezeolite ꢂ Etemplate, where E,
zeolite, Etemplate refer to calculation energies of the system, i.e.,
template inside CHA structure, CHA framework and organic
amine template, respectively. All the calculations were per-
formed in Material Studio soware.
The powder XRD pattern was recorded on a PANalytical X' 50
Pert PRO X-ray diffractometer with Cu-Ka radiation (l ¼ 1.54059
˚
A), operating at 40 kV and 40 mA. The chemical composition of
2
9,30
cule.
the samples was determined with Philips Magix-601 X-ray

uorescence (XRF) spectrometer. The crystal morphology was
E
E
observed by scanning electron microscopy (SEM) using
a TM3000 (Hitachi) and eld emission SEM (Hitachi SU8020).
Textural properties of the calcined samples were determined by
N adsorption–desorption at 77 K on a Micromeritics ASAP 2020
2
system. The total surface area was calculated based on the BET
equation. The micropore volume, micropore surface area and
external surface area were evaluated using the t-plot method.
Mesopore volume was evaluated from the adsorption isotherm
by Barrett–Joyner–Halenda (BJH) method. The temperature-
programmed desorption of ammonia (NH -TPD) was carried
out with an Autochem II 2920 equipment (Micromeritics). 0.2
grams of the sample particles (40–60 mesh) were loaded into
a U-quartz tube and pretreated at 650 C for 60 min under He
Density functional theory (DFT) calculation is a useful tool to
understand molecular properties and describe the atoms
31–33
behavior in the molecule.
It was used herein to investigate
the reactivity and the stability of the DPHAB molecule. Geom-
etry optimization of organophosphorous surfactant was per-
formed at the DFT B3LYP level theory using a 6-31G* basis set
with Gaussian 09 program. Molecular electrostatic potential
(MEP) was determined at the same level of theory B3LYP/6-31G*
with the optimized structure.
3
ꢀ
ꢀ

ow. Aer cooling down to 100 C, a gas mixture of NH
3
and He

ow was introduced to saturate the sample surface with NH
3
adsorption (60 min). Aer this, He ow was purged through the 2.6 Catalytic tests
sample for 30 min to remove the weakly adsorbed NH
3
mole-
Methanol to olens (MTO) reaction was performed in a quartz
tubular xed-bed reactor at atmospheric pressure. 0.3 g
calcined catalyst (40–60 mesh) was loaded in the quartz reactor
cules. The measurement of the desorbed NH was performed
3
ꢀ
ꢀ
ꢂ1
ꢂ1
from 100 to 650 C (10 C min ) under He ow (20 ml min ).
The liquid nuclear magnetic resonance (NMR) spectra were
recorded on a Bruker Avance III 400 MHz NMR spectrometer.
ꢀ
ꢂ1
and activated at 550 C in a He ow of 30 ml min for 1 h
before starting each reaction run, and then the temperature was
CDCl
3
and D
2
O were used as the solvent in the measurement of
ꢀ
ꢂ1
adjusted to a reaction temperature of 450 C. The methanol was
1
13
31
31
H, C and P NMR of DPHAB, and P NMR of mother liquid
aer crystallization, respectively. Tetramethylsilane (d ¼ 0 ppm)
fed by passing the carrier gas (42 ml min ) through a saturator
ꢀ
containing methanol at 30 C, which gave a weight hourly space
1
or CDCl
3
(d ¼ 7.27 ppm) serve as the internal standard for H
ꢂ1
velocity (WHSV) of 3 h . The reaction products were analyzed
1
3
NMR, and CDCl (77.16 ppm) for C NMR. All the solid state
3
using an online gas chromatograph (Agilent GC 7890N),
equipped with a ame ionization detector (FID) and Plot-Q
column.
NMR experiments were performed on a Bruker Avance III 600
spectrometer equipped with a 14.1 T wide-bore magnet. The
resonance frequencies were 150.9, 242.9, 156.4 and 119.2 MHz
1
3
31
27
29
13
31
27
for C, P, Al and Si, respectively. C, P and Al MAS
NMR experiments were performed on a 4 mm MAS probe with 3 Results and discussion
2
9
a spinning rate of 12 kHz. Si MAS NMR spectrum was recor-
ded with a 7 mm MAS probe with a spinning rate of 6 kHz.
Chemical shis were referenced to adamantane for C, 85%
3
.1 Synthesis and characterization of SAPO-34 molecular
sieve
1
3
3
1
27
SAPO-34s were synthesized by using DPHAB together with
different microporous templates including TEAOH, DEA and
TEA. The results are shown in Table 1 and Fig. S2–S4.† When
TEAOH is used, pure SAPO-34 was readily synthesized in the
DPHAB/H PO ratio range of 0 to 1/2. Dense phase is the nal
3 4
product when DPHAB is used as the only phosphor source.
When DEA or TEA is used instead, pure SAPO-34 can only be
H PO for P, (NH )Al(SO ) $2H O for Al and 4,4-dimethyl-4-
3
4
4
4 2
2
2
9
silapentane sulfonate sodium salt (DSS) for Si. Thermogravi-
metric and differential thermal analysis (TG and DTA) were
performed on a TA SDTQ600 analyzer with the temperature-
programmed rate of 10 C min in air. The Fourier trans-
formed infrared (FT-IR) spectra were carried out on a Bruker
ꢀ
ꢂ1
ꢂ1
Tensor 27 instrument with a resolution of 4 cm . The pallets
3 4
obtained with the DPHAB/H PO smaller than 1/5. Larger
were made by mixing the KBr and samples quantitatively.
amount of DPHAB leads to impurity or amorphous phase. It is
also noted that the addition of DPHAB slows down the crystal-
lization rate. Conventional SAPO-34 (S-TEAOH-0) can be well
2.5 Calculations
Templating ability to the formation of SAPO-34 was evaluated by crystallized within 3 days. When DPHAB is introduced, the
the interaction energy between template–zeolite in CHA struc- reaction takes 6, 5 and 10 days for TEAOH, DEA and TEA system,
25–27
ture.
The energy was obtained by molecular modeling. The respectively. The yields of the three systems are similar and
locator and maximum loading of templates were performed decrease as the increasing DPHAB/H PO ratio. It is deduced
3
4
using Monte Carlo-simulated annealing in adsorption locator that DPHAB is not as active as H
3
PO
a
4
, therefore, the increased
deciency of effective
28,29
code.
The organic templates structure and the interaction DPHAB/H
3
PO
4
ratio results in
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Table 1 Reaction conditions and results of SAPO molecular sieve synthesized with DPHAB
RSC Advances
a
Sample
DPHAB/H
3
PO
4
Product
Crystallization time/days
Product yield
Product composition
b
S-TEAOH-0
0
SAPO-34
SAPO-34
SAPO-34
SAPO-34
Dense phase
SAPO-34
SAPO-34
SAPO-34 + lamellar phase
Amorphous
SAPO-34
3
6
6
6
6
5
5
5
5
47.3%
74.6%
65.3%
49.3%
—
52.3%
49.6%
39.6%
—
Al0.498
Al0.488
Al0.468
Al0.490
Al0.411
Al0.487
Al0.487
—
P
P
P
P
P
P
P
0.390Si0.112
0.394Si0.118
0.419Si0.113
0.393Si0.117
0.506Si0.083
0.392Si0.117
0.406Si0.107
O
O
O
O
O
O
O
2
2
2
2
2
2
2
S-TEAOH-1/9
S-TEAOH-1/6
S-TEAOH-1/2
S-TEAOH-N
S-DEA-1/6
S-DEA-1/5
S-DEA-1/3
S-DEA-1/2
S-TEA-1/6
1/9
1/6
1/2
N
1/6
1/5
1/3
1/2
1/6
1/5
1/3
—
10
10
10
62.5%
60.3%
—
Al0.487
Al0.489
—
P0.404Si0.109
O
O
2
2
S-TEA-1/5
S-TEA-1/3
SAPO-34
Amorphous
P0.410Si0.101
a
The initial gel molar composition are Al
2
O
3
/Pb
2
O
5
/SiO
2
/TEAOH/H
2
O ¼ 1.0/1.2/0.5/2.0/150, Al
2
O
3
/P
2
O
5
/SiO
2
/DEA/H
2
O ¼ 1.0/1.0/0.5/2.0/150 and
Al
2
O
3
/P /SiO /TEA/H
2
O
5
2
2
O ¼ 1.0/1.0/0.5/3.0/150. The estimated value of framework compounds is 85% used for the yield calculation.
phosphorus source and a reduced yield. The Si contents of the SAPO-34. It indicates that both the type of template and DPHAB
obtained products are comparable although they were synthe- have signicant impact on the crystallization of SAPO-34. The
sized with different template and DPHAB/H PO ratio. combined use of TEAOH and DPHAB leads to the smallest
3
4
Comparing the three synthetic systems, TEAOH leads to the primary (50–500 nm) and secondary (5–10 mm) particles, which
widest crystallization region and the relatively short crystalli- suggests that the TEAOH template may favor more SAPO-34
zation time. It is speculated that TEAOH has better templating nucleation. The crystal morphology change and aggregation
ability. The templating abilities of the three templates for the should be related to the addition of DPHAB. Surfactants have
formation of SAPO-34 were evaluated by comparing the inter-
action energy (Einter) between template and the CHA framework,
and the Einter were obtained by the molecular modeling and
calculation. The modeled positions of the three templates in the
CHA structure are shown in Fig. S5.† The calculated Einter for
TEAOH, DEA and TEA are ꢂ79.70, ꢂ41.88 and ꢂ24.30 kcal
ꢂ1
mol , respectively, which give the order of ETEAOH < EDEA
<
E
TEA. The result indicates that TEAOH has the best templating
ability within the three structure directing agents.
The morphologies of the SAPO-34s synthesized with TEAOH
are presented in Fig. 1. Without DPHAB, the S-TEAOH-0 pres-
ents typical rhombohedral crystals with the particle size of ca. 2
mm (Fig. 1a and b). Upon addition of DPHAB, the primary
particle size of SAPO-34 product decreases to ca. 50–500 nm,
which aggregate in different degree (Fig. 1c–h). Interestingly,
the aggregation degree of nanocrystalline raises with the
3 4
increased DPHAB. When the DPHAB/H PO ratio reaches to 1/2,
the SAPO-34 product becomes regular 5–10 mm sphere which is
consisted of SAPO-34 slices, and the thickness of the SAPO-34
slice is 50–100 nm (inset of Fig. 1h). The crystal morphologies
of SAPO-34s synthesized with DEA and TEA templates were also
examined by SEM. As seen in Fig. 2, S-DEA-1/6 presents a worm-
like morphology with a crystal size of around 2 mm (Fig. 2a);
while S-DEA-1/5 presents a spherical morphology with a rough
surface and relatively large crystal size of around 30 mm
(Fig. 2b), which are quite different from the conventional
rhombohedral morphology (Fig. S6†). Similarly, S-TEA-1/6 and
S-TEA-1/5 exhibit the dispersed pyramidal morphology (ca.
5
mm) and a spherical aggregation (ca. 25 mm), respectively
Fig. 1 SEM images of as-synthesized samples prepared with TEAOH
and different DPHAB/H PO : S-TEAOH-0 (a and b), S-TEAOH-1/9 (c
and d), S-TEAOH-1/6 (e and f) and S-TEAOH-1/2 (g and h).
(Fig. 2c and d). The type of template and minor variation of
3
4
DPHAB can trigger obvious changes for the morphology of
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Table 2 Textural properties of the SAPO-34 samples
a
b
c
d
e
S
BET
S
mic
S
Ext
V
mic
V
meso
2
ꢂ1
2
ꢂ1
2
ꢂ1
3
ꢂ1
3
ꢂ1
Sample
(m g
)
(m g
)
(m g
)
(cm g
)
(cm g )
S-TEAOH-0
S-TEAOH-1/9 613
S-TEAOH-1/2 673
S-DEA-1/6
S-DEA-1/5
S-TEA-1/6
S-TEA-1/5
602
593
583
577
608
597
573
575
9
30
96
23
25
18
12
0.28
0.27
0.27
0.28
0.28
0.27
0.27
0.02
0.09
0.18
0.10
0.02
0.07
0.02
631
622
591
587
a
b
c
BET surface area. t-Plot micropore surface area. t-Plot external
d
e
surface area. t-Plot micropore volume. BJH adsorption volume.
Fig. 2 SEM images of as-synthesized samples prepared with DEA/TEA
and different DPHAB/H
c) and S-TEA-1/5 (d).
3
PO
4
: S-DEA-1/6 (a), S-DEA-1/5 (b), S-TEA-1/6
2
ꢂ1
2
ꢂ1
(
673 m g ) and external surface area (96 m g ) owing to the
(
high crystallinity and the presence of intercrystal mesopores.
The other samples also exhibit high BET surface area and large
micropore volume indicating their good crystallinity.
Solid-state P, Al and Si NMR characterization were
measured for the as-synthesized samples S-TEAOH-1/2, S-DEA-
been widely used to control the morphology of various materials
due to their efficient self-assembly properties.
previous work, a spherical self-assembly of SAPO-34 nanosheets
was achieved by using TPOAC as a mesogorogen. The surfac-
tant is a very potent surface stabilizer, which can lower the
energy cost for creating new surfaces, thus affecting crystal
splitting and changing the morphologies of the crystal. In our
current work, DPHAB plays a similar role as TPOAC which leads
to the self-assembly of SAPO-34 primary crystals to reduce the
product surface energy.
3
1
27
29
34–37
In our
3
1
1
/6 and S-TEA-1/6. The P NMR spectra are shown in Fig. 4,
and the corresponding Al and Si NMR spectra are given in
21
2
7
29
3
1
Fig. S8.† All the P NMR spectra give only strong resonance
peak at around ꢂ29 ppm suggesting the predominant tetrahe-
38
dral PO environment in the framework for the three samples.
4
The missing of signal at around 30.7 ppm indicates that the
samples do not include any organic phosphorous species. The
Al NMR spectra (Fig. S8A†) give a strong resonance at 39 ppm
arising from tetrahedral Al species, and another weak signal at
0 ppm attributed to penta-coordinated Al formed by additional
interactions of water or template molecule with the framework
2
7
The
N
2
adsorption–desorption isotherms of SAPO-34
samples are given in Fig. 3 and S7.† All the samples have type
I isotherms typical for pure microporous material except for
S-TEAOH-1/2. S-TEAOH-1/2 shows combined features of type I
1
3
9
29
aluminum. Regarding to the Si NMR spectra (Fig. S8B†), all
the three samples have a apparent resonance at ꢂ91 ppm
and IV isotherms with two steep steps in the P/P < 0.01 and 0.40
0
<
0
P/P < 0.90 regions, corresponding to lling of the micropore
40
ascribed to Si(4Al) species. The presence of signal at around
volumes and capillary condensation in the mesopores, respec-
tively. Correspondingly, the mesopore size distribution of
S-TEAOH-1/2 is centered at 5.6 nm, as shown in the inset of
Fig. 3. The textural properties of the samples are given in
Table 2. S-TEAOH-1/2 exhibits the largest BET surface area
40
ꢂ96 ppm is ascribed to the environment of Si(3Al). For sample
S-TEAOH-1/2, more remarkable peaks emerge at around ꢂ101,
ꢂ105 and ꢂ110 ppm corresponding to Si(2Al), Si(1Al) and
40
Si-island, respectively. The presence of Si(nAl) (n ¼ 0–2)
Fig.
TEAOH-0, S-TEAOH-1/9 and S-TEAOH-1/2, and BJH pore size Fig. 4
distribution of calcined S-TEAOH-1/2 (inset). 6 (b) and S-TEA-1/6 (c).
3 Nitrogen adsorption/desorption isotherms of calcined S-
31
P NMR spectra of as-synthesized S-TEAOH-1/2 (a), S-DEA-1/
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species may be caused by the larger external surface of
S-TEAOH-1/2.
3.2 Status of DPHAB
The above characterizations clearly demonstrate that the
introduction of DPHAB has a signicant impact on the crys-
tallization and morphology of SAPO-34, but the organic phos-
phorous species may be not involved in the SAPO-34 products.
So it is necessary to investigate the status of DPHAB in the
reaction. FT-IR was used to check the organic species in the
product. As seen in Fig. 5, both S-TEAOH-0 and S-TEAOH-1/2
ꢂ1
exhibit bands at 2927 and 2850 cm assigned to the charac-
24
teristic stretching vibration of C–H groups. Obviously, the Fig. 7 TG–DTA curves of as-synthesized S-TEAOH-0 and S-TEAOH-
1
/2.
bands for S-TEAOH-1/2 are much stronger than that of
S-TEAOH-0, which suggests that more organic species are
included in S-TEAOH-1/2. In contrast, the FT-IR spectra of
S-DEA-1/6 and S-TEA-1/6 are similar to their corresponding the strong signal at 29.5 ppm is assigned to methylene moieties
conventional samples (Fig. S9†). It indicates that there is not which should come from the long-alkyl chain of DPHAB. The
any additional organic species involved in the two samples result conrms the existence of the long alkyl chains in
except the microporous template.
In order to reveal the involved organic species in sample the amount of organics in S-TEAOH-1/2. The weight loss below
S-TEAOH-1/2. TG and DTA analyses were performed to examine
1
3
ꢀ
S-TEAOH-1/2, C MAS NMR was further measured, and the 200 C corresponds to the removal of physically adsorbed water,
ꢀ
spectrum is given in Fig. 6. Two signals at 51.8 and 6.4 ppm can while the weight loss between 200 and 800 C mainly corre-
be attributed to –CH and –CH groups of TEAOH template, and sponds to the decomposed organics in the sample. As shown in
2
3
ꢀ
Fig. 7, S-TEAOH-1/2 has a larger weight loss below 200 C than
S-TEAOH-0. It may come from physically adsorbed water on the
ꢀ
external surface. The weight loss at 200–800 C is ca. 16.5 wt%
for S-TEAOH-0, and 18.7 wt% for S-TEAOH-1/2. The additional
weight loss is ascribed to the removal of the organic groups
coming from the DPHAB.
1
3
The FT-IR, C NMR and TG–DTA characterizations clearly
show that some long-alkyl chains belonging to DPHAB are
introduced in S-TEAOH-1/2. However, the (C
2 5 2
H O) P(O)–C^
groups are not involved in the nal product according to the
3
1
solid-state P NMR. So, to check what happened to DPHAB
during the reaction, the mother liquid of S-TEAOH-1/2 aer
3
1
crystallization was collected, and measured the P NMR spec-
trum (Fig. 8). It shows a strong signal at 24 ppm due to the
Fig. 5 FT-IR spectra of as-synthesized S-TEAOH-0 and S-TEAOH-
/2.
presence of (HO) P(O)–C^. Another weak signal at 27 ppm is
2
1
31
Fig. 8
P NMR spectrum of mother liquid after crystallization based
13
Fig. 6
C MAS NMR spectrum of as-synthesized S-TEAOH-1/2.
on sample S-TEAOH-1/2.
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4
1
a
b
ascribed to the small amount of (C
2
H
5
O)(HO)P(O)–C^.
Table 3 Lifetime and product selectivity of SAPO-34s in the MTO
ꢂ1
ꢀ
reaction (WHSV ¼ 3 h , T ¼ 450 C)
(
HO) P(O)–C^ and (C H O)(HO)P(O)–C^ are the hydrolysis
2 2 5
products of (C H O) P(O)–C^. Bases on the above analyses, it is
2
5
2
Selectivity (%)
believed that DPHAB decomposed during the reaction, and the
generated (C O)
P(O)–C^ groups or its hydrolyzed groups Sample
H
5
2
Lifetime (min)
C
2
H
4
C
3
H
6
2 4 3 6
C H + C H
2
were inactive under the current reaction condition which stayed
in the mother liquid.
In order to further understand the properties and status of
organophosphorous surfactant, the molecular electrostatic
potential (MEP) related to the electronic density was calculated
S-TEAOH-0
S-TEAOH-1/9
S-TEAOH-1/2
208
292
242
46.39
45.28
45.64
32.60
34.72
35.20
78.99
80.00
80.84
a
Catalyst lifetime dened as the reaction duration with >99% methanol
b
conversion. The highest selectivity of ethylene and propylene under
at the optimized geometry. For comparison, the MEP of orga- >99% methanol conversion.
nosilane TPOAC molecule was also calculated. Fig. 9 shows the
MEP maps of the two molecules. The positive (blue) regions of
the maps are related to nucleophilic reactivity and the negative
(
red) regions to electrophilic reactivity. For both DPHAB and is used instead of DPHAB together with TEAOH to conduct the
TPOAC, the positive (blue) regions are mainly focused on the synthesis, pure SAPO-34 cannot be obtained (Fig. S10†).
quaternary ammonium group, and the adjacent diethox-
ylphosphono and methoxysilyl groups are in green color. These
groups are relatively susceptible sites to interact with the
4
Catalytic performances in the MTO
anionic silicoaluminophosphate species through nucleophilic reaction
reactions. For TPOAC, methoxysilyl groups could hydrolyze and
Because of the optimized morphology of SAPO-34s synthesized
react with the silicoaluminophosphate species forming covalent
bonds. The synergy between the ammonium groups and the
hydrolyzed methoxysilyl groups enables the organosilane to be
integrated into the SAPO product efficiently and intactly. Unlike
TPOAC, there is a negative (red) region nearby the diethox-
ylphosphono groups due to the existence of P]O for DPHAB.
This part may disturb the interaction between DPHAB and
silicoaluminophosphate species. Moreover, the relatively inac-
tivity of the hydrolyzed diethoxylphosphono groups prevents
DPHAB to participate in the formation of SAPO framework. The
conicting interaction between quaternary ammonium and
diethoxylphosphono groups with silicoaluminophosphate
species may break up the integrity of the DPHAB molecule. As
a result, the quaternary ammonium groups together with
long-alkyl chains of DPHAB are involved in the SAPO product,
with TEAOH/DPHAB, the MTO catalytic performances of
S-TEAOH-1/9 and S-TEAOH-1/2 were evaluated. The results are
listed in Table 3. Both samples exhibit better catalytic perfor-
mance than S-TEAOH-0. The methanol conversion over 99%
could be maintained for 292 and 242 min for S-TEAOH-1/9 and
S-TEAOH-1/2, respectively, which are longer than that of
S-TEAOH-0 (208 min). The light olen (ethylene plus propylene)
selectivity for S-TEAOH-1/9 and S-TEAOH-1/2 reach to 80.00%
and 80.84%, respectively, which are higher than that of
S-TEAOH-0 (78.99%). The large secondary particle size of
S-TEAOH-1/2 may be responsible for the less extension of MTO
catalytic lifetime. Moreover, it is noted that the selectivity for
ethylene reduces a little bit, and the propylene selectivity
increases comparing with that of S-TEAOH-0. The improved
mass transfer ability is associated with the enhanced propylene
while (C
stay in the mother liquid. Nevertheless, the role of DPHAB is
unique and cannot be replaced. When N(CH 33Br (CTAB)
2 5 2
H O) P(O)–C^ groups and its hydrolyzed products
42–44
and reduced ethylene selectivity.
In addition, the improved
catalytic property should also thank to the well kept acidity.
Several negative results of the mesoporous SAPO-34 synthesized
by organosilane surfactant and TEAOH template have been re-
3 3 16
) C H
22,45
ported.
tivation rate is still fast due to the dramatically decreased acidity
aer mesopore integration. In our case, NH -TPD measurement
Although the mesoporosity is introduced, the deac-
3
shows that the acidity of obtained samples change little aer
implanting mesopores, as seen in Fig. S11.† All the three
samples have two similar desorption peaks at about 190 and
ꢀ
4
40 C in the NH -TPD proles, which indicates that the use of
3
organophosphorous surfactant has little effect on the acidity of
SAPO products.
5
Conclusions
By using the self-designed organophosphorous surfactant
DPHAB and microporous template TEAOH, DEA and TEA,
highly crystallized SAPO-34 molecular sieve with special
morphologies have been hydrothermally synthesized and
Fig. 9 Molecular electrostatic potential maps of DPHAB and TPOAC
molecules.
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characterized. It is found that the crystal morphology changes 16 Y. Cui, Q. Zhang, J. He, Y. Wang and F. Wei, Particuology,
obviously with the addition of DPHAB and the crystals aggregate 2013, 11, 468–474.
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3
4
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We are thankful for the nancial support from the National
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