Organosilane surfactant-directed synthesis of hierarchical porous SAPO-34 catalysts with excellent MTO performance

Article abstract of DOI:10.1039/c4cc02050b

Using an organosilane surfactant as the mesopore director, hierarchical porous silicoaluminophosphate SAPO-34 is obtained as an assembly of nanocrystallites intergrown into cubic micrometer-sized crystals, which show excellent performance in MTO reactions with a remarkably prolonged catalyst lifetime and enhanced selectivity of ethylene and propylene compared to the conventional microporous SAPO-34. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02050b

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Organosilane surfactant-directed synthesis of
hierarchical porous SAPO-34 catalysts with
Cite this:DOI: 10.1039/c4cc02050b
excellent MTO performance†
Received 19th March 2014,
Accepted 30th April 2014
a
a
a
b
a
Qiming Sun, Ning Wang, Dongyang Xi, Miao Yang and Jihong Yu*
DOI: 10.1039/c4cc02050b
www.rsc.org/chemcomm
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Using an organosilane surfactant as the mesopore director, catalyst acidity, and decreasing the crystal size. Another syn-
hierarchical porous silicoaluminophosphate SAPO-34 is obtained as thetic strategy is the introduction of meso- or/and macropores
an assembly of nanocrystallites intergrown into cubic micrometer- into the microporous zeolite crystals to obtain a hierarchical
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sized crystals, which show excellent performance in MTO reactions structure. In recent decades, some methods have been successfully
with a remarkably prolonged catalyst lifetime and enhanced selec- employed for the preparation of hierarchical porous alumino-
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tivity of ethylene and propylene compared to the conventional silicate zeolites, such as the post-treatment method, the hard-
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microporous SAPO-34.
templating method, and the soft-templating method. Notably,
Ryoo’s group demonstrated the soft-templating synthesis of zeolites
Zeolites are the most widely used solid catalysts in industry due by introducing organosilane surfactants as the mesopore director
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to their microporous structures and the resulting unique shape into conventional zeolite synthesis compositions. They further
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selectivity. Silicoaluminophosphate molecular sieve SAPO-34 extended this strategy to the synthesis of mesoporous alumino-
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with the topological structure of CHA has been intensively phosphate molecular sieves. Inspired by this design concept, a
studied as one of the most excellent catalysts for the few attempts have been made to synthesize hierarchical porous
methanol-to-olefin (MTO) conversion. Such an MTO process silicoaluminophosphate molecular sieves with the assistance of
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provides an alternative route to produce light olefins from organosilane surfactants. However, the synthesis of hierarchical
nonpetroleum sources, and has gained considerable attention porous SAPO-34 catalysts with excellent MTO catalytic performance
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in the last three decades. SAPO-34 with a large CHA cage (9.4 Å by using this method has seen less success.
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in diameter) and small 8-ring pore (0.38 Å Â 0.38 Å) opening,
In this work, the hierarchical porous SAPO-34 catalysts were
as well as mild acidity can induce a high selectivity of ethylene successfully synthesized using [3-(trimethoxysilyl)propyl]-octa-
and propylene in MTO reactions. However, the large CHA cage decyldimethyl-ammonium chloride (TPOAC) as the mesopore
and narrow pore opening also cause the problem of rapid director through a one-step hydrothermal crystallization. The
deactivation due to the limitation of mass transport and a large hierarchical porous SAPO-34 catalysts were prepared as cubic
amount of organic species accommodating in the large cavities agglomerates of small nanocrystals, which exhibit a remarkably
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during methanol conversion. Hence, overcoming the inherent prolonged catalytic lifetime and excellent light olefin selectivity
diffusion limitations and retarding coke deposition are of in the MTO conversions compared with the conventional micro-
significance to prolong the catalytic activity. So far, various porous SAPO-34 catalyst synthesized without adding organo-
approaches have been developed to reduce the coke deposition, silane surfactants.
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such as optimizing the synthesis conditions, modifying the
The hierarchical porous SAPO-34 catalysts (SH1 and SH2
were synthesized from the starting gels with the optimized
molar compositions of 1.0Al : 1.0P : 3.0Mor: 0.6SiO
0H O: x TPOAC (x = 0.1 and 0.15) under hydrothermal condi-
tions at 180 1C by using morpholine (Mor) as the template for
micropores and TPOAC for mesopores. As a comparison,
)
a
2
O
3
2
O
5
2
:
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
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2
College of Chemistry, Jilin University, Changchun 130012, P. R. China.
E-mail: jihong@jlu.edu.cn
b
National Engineering Laboratory for Methanol to Olefins, Dalian National
Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian 116023, P. R. China
the conventional microporous SAPO-34 (S ) was synthesized
under the same synthesis conditions without the addition
of TPOAC.
The XRD patterns of all samples show typical diffraction
peaks of the CHA structure (Fig. 1), proving the phase purity of
M
†
Electronic supplementary information (ESI) available: More details of experi-
mental and characterization processes, TEM, DFT pore distribution, FT-IR, TG-
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DTA, P, Al, H and Si MAS NMR, NH
results. See DOI: 10.1039/c4cc02050b
3
-TPD, and the MTO reaction catalytic
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the type-I N
2
adsorption–desorption isotherms, samples SH1
and SH2 exhibit the type-IV isotherms, and the large hysteresis
loops in the region 0.4 o P/P o 0.9 are observed due to the
o
1
3
capillary condensation in the mesopores (Fig. 1). Particularly,
sample SH2 exhibits a significantly increased external surface
2
À1
3
À1
area (123.6 m g ) and mesopore volume (0.26 cm g ) as
compared with the conventional microporous SAPO-34. Mean-
while, the pore size distribution also demonstrates the exis-
tence of hierarchical porosity in samples SH1 and SH2 (Fig. S2,
ESI†). The detailed surface area and pore volume data are
summarized in Table 1.
The FT-IR spectra further confirm the existence of the
TPOAC surfactant in the hierarchical porous SAPO-34 crystals
2
Fig. 1 XRD patterns (left) and N adsorption–desorption isotherms (right)
of conventional microporous SAPO-34 (S
SAPO-34 (SH1 and SH2).
M
) and hierarchical porous (Fig. S3, ESI†). Different from microporous SAPO-34 (S
), the
sharp IR peaks at 2922 cm and 2855 cm exist in samples
H1 and SH2, which are ascribed to the vibration of the C–H
bond derived from the hydrophobic alkyl chain of the TPOAC
M
À1
À1
S
1
6
surfactant. Elemental analyses show that the C/N ratios of
samples S , S and SH2 are increased due to the introduction
M
H1
of the TPOAC surfactant (Table 1). In addition, the TG-DTA
analyses also demonstrate that the TPOAC surfactants are
occluded into the hierarchical porous SAPO-34 crystals (Fig. S4,
ESI† and Table 1). The hierarchical porous SAPO-34 samples
have more weight loss and the exothermic peak at 340 1C is
distinct due to the thermal decomposition of the TPOAC
surfactant.
It is believed that the introduction of an organosilane
surfactant plays an important role in the formation of hierarch-
ical porous SAPO-34. The proposed assembly pathway of nano-
crystallites into micrometer-sized hierarchical porous SAPO-34
crystals is shown in Scheme 1. The silica portion of the TPOAC
M
Fig. 2 SEM images of conventional microporous SAPO-34 (S ) (a, b) and
hierarchical porous SAPO-34 (SH1) (c, d) and SH2 (e, f).
as-synthesized SAPO-34. Their SEM images show the character-
istic cubic morphology of the CHA crystals (Fig. 2). Notably,
different from the conventional microporous SAPO-34 (S ) with
M
a smooth crystal surface, samples SH1 and SH2 are observed to
be an aggregation of small cubic-like nanocrystals.
The size of the individual nanocrystals ranges from 100 to
500 nm. The TEM images of samples SH1 and SH2 clearly reveal
the existence of small mesopores (2–5 nm) and larger meso-
Scheme 1 Proposed assembly pathway of cubic hierarchical porous
pores (ca. 30 nm) in the nanocrystalline domains (Fig. S1, ESI†).
SAPO-34 crystals aggregated by small cubic nanocrystals assisted by the
In contrast to conventional microporous SAPO-34 (S
M
) showing organosilane surfactant.
M
Table 1 Compositions and textural properties of conventional microporous SAPO-34 (S ) and hierarchical porous SAPO-34 (SH1 and SH2) catalysts
a
b
c
d
2
À1
e
2
À1
e
2
À1
e
3
À1
f
3
À1
Sample no. Molar composition
TG (%) C/N
S
BET (m g
)
S
micro (m g
)
S
ext (m g
)
V
micro (cm g
)
V
meso (cm g )
S
S
S
M
Si0.18Al0.45
Si0.17Al0.45
Si0.16Al0.46
P
0.37
P
0.38
P
0.38
O
O
O
2
2
2
20.42
22.27
25.05
3.95
5.20
5.85
562.1
525.4
507.9
551.2
441.7
384.3
10.9
83.7
123.6
0.26
0.21
0.18
0.04
0.17
0.26
H1
H2
a
b
c
Measured by inductively coupled plasma (ICP). Weight loss of templates (%) measured by TG-DTA (Fig. S4, ESI). C/N of templates measured
d
using an element analyzer.
S
BET (total surface area) calculated by applying the BET equation using the linear part (0.05 o P/P
micro (micropore area), Sext (external surface area) and Vmicro (micropore volume) calculated using the t-plot method.
meso (mesopore volume) calculated using the BJH method (from desorption).
o
o 0.30) of the
e
adsorption isotherm.
S
f
V
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surfactant is firstly incorporated into the resultant silicoalumino- sample S
M
(71.9%) (Table S1, ESI†). The prominent difference
phosphate framework, and the organic tails of the TPOAC surfac- in the catalytic performance of hierarchical porous SAPO-34
tant forming the organic layers direct the mesoporous structure. and conventional microporous SAPO-34 catalysts can be explained
Meanwhile, the organic tails of the TPOAC surfactant attached on by the difference in the level of porosity, acidity, as well as
1
2b
the surface of nanocrystals provide a capping effect,
limiting the further growth of the nanocrystals, forming the SAPO-34 can greatly enhance the transfer of the reaction products
cubic-like agglomerates with some intervoid space. from the narrow pore to outside space and reduce the coke
thus crystallite size. The generated mesopore in the hierarchical porous
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1
29
The solid-state P, Al, H, and Si MAS NMR spectra of formation and thus prolong the lifetime. Furthermore, the
calcined SAPO-34 samples are presented in Fig. S5 (ESI†). Com- decreased acidic strength and acidic concentration of hierarchical
pared with the conventional microporous SAPO-34, the hierarch- porous SAPO-34 catalysts can also retard the coke formation
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ical porous SAPO-34 crystals show more pentacoordinated and and thus prolong the catalyst lifetime. Hydrogen transfer index
octahedral aluminum atoms attributed to the extra-framework (HTI, C H /C H ) indicates that the hierarchical porous SAPO-34
3
8
3 6
17
aluminum species in the formation of hierarchical structure.
catalysts can decrease the hydrogen transfer level of secondary
Inductively coupled plasma (ICP) analyses show that all transformation of olefin products and lower the coking rate
samples have similar molar compositions (Table 1). It is (Fig. S10 and Table S2, ESI†), especially, sample SH2 exhibits the
À1
noticed that with the increase of the TPOAC surfactant, the lowest coking rate of 0.132 mg min , which is only about 40% of
silicon contents of samples decrease slightly from S
M M
, SH1 to sample S . In addition, the decreased nanocrystallite size in the
S
H2. This may be because the increased steric hindrance caused agglomerates of the hierarchical porous SAPO-34 catalysts is also
7
by the mutual repulsion of surfactant decreases the incorpora- an important reason for enhancing the lifetime. The smaller
tion of silicon into the frameworks. Meanwhile, NH -TPD crystals can shorten the diffusion length reactant and generated
3
analyses reveal that the introduction of the TPOAC surfactant products and greatly enhance the mass transfer during the
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can also affect the acidity of catalysts (Fig. S6, ESI†). The methanol conversion, and thus prolong the lifetime.
hierarchical porous SAPO-34 samples have lower acidic In summary, hierarchical porous silicoaluminophosphate
strength and acidic concentration in strong acid sites com- SAPO-34 catalysts have been successfully synthesized using
pared with the conventional microporous SAPO-34. Further- the organosilane surfactant as the mesopore director by direct
more,
SH2 has the lowest acidic strength and acidic hydrothermal crystallization. The hierarchical porous SAPO-34
concentration in strong acid sites that is connected with the crystals are obtained as cubic aggregates of nanocrystallites.
increased amount of TPOAC and the decreased silicon content Thanks to the hierarchical porosity, decreased acidity as well as
in the framework.
Catalytic tests of methanol conversion were carried out at catalysts exhibit four-times prolonged catalytic lifetime and
3 6
73 K in a fixed-bed reactor over the catalysts. Significantly, the more than 10% improvement of light olefin (C + C H )
reduced nanocrystallite size, the hierarchical porous SAPO-34
6
2
H
4
hierarchical porous SAPO-34 catalysts exhibit a remarkably selectivity in MTO conversion compared with the conventional
prolonged catalyst lifetime and enhanced light olefin selectivity microporous SAPO-34 catalyst. This work demonstrates an
compared with the conventional microporous SAPO-34 catalyst effective organosilane surfactant-directed approach to prepare
(
Fig. 3, Table S1, ESI†). The yields of products of samples SM, hierarchical porous silicoaluminophosphate molecular sieves
SH1 and SH2 are shown in Fig. S7–S9 (ESI†). Particularly, the with improved catalytic properties.
lifetime of sample SH2 (466 min) with hierarchical porous We thank the State Basic Research Project of China (Grant
structure is more than four-times higher than that of S No. 2011CB808703 and 2014CB931802), National Natural
106 min) with only microporous structure. Meanwhile, the Science Foundation of China (Major Program, Grant No.
M
(
selectivity of ethylene and propylene of sample SH2 reaches up 91122029) and the Major International Joint Research Project
to 82.7%, which has improved more than 10% compared with of China for financial support (Grant No. 21320102001).
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