Rational construction of hierarchical SAPO-34 with enhanced MTO performance without an additional meso/macropore template

Article abstract of DOI:10.1039/d0ta08437a

A relatively facile strategy was rationally developed for the synthesis of hierarchical SAPO-34 using aluminum phosphate spheres as the nutrient and mesoporogen, without utilizing any additional meso/macropore-directing agents. The simple synthesis resulted in SAPO-34 (CHA) with classical rhombohedral morphology with dimensions between 2 and 3 μm and intracrystalline meso/macropores. It was demonstrated that the employment of spherical aluminum phosphate particles and the regulation of synthesis conditions to carry out the dissolution of aluminum phosphate spheres parallel to the crystallization process of the CHA phase were crucial factors leading to successful synthesis. The crystallization process of the hierarchical SAPO-34 was investigated and a possible crystallization model was proposed. The catalytic performances of the synthesized SAPO-34s in the MTO reaction were evaluated. The SAPO-34 with a hierarchical porous structure exhibited an improved catalytic lifespan and higher selectivity for light olefins. This work not only provides a new facile method to synthesize hierarchical SAPO-34 but also holds great promise for offering useful guidance for the synthesis of other aluminophosphate zeolites with hierarchical structures.

Full text of DOI:10.1039/d0ta08437a

Journal of
Materials Chemistry A
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Rational construction of hierarchical SAPO-34 with
enhanced MTO performance without an additional
meso/macropore template†
Cite this: J. Mater. Chem. A, 2021, 9,
859
1
a
a
a
a
b
Yafei Liang, Beibei Gao, * Lipeng Zhou,
Xiaomei Yang,
Tianliang Lu,
a
and Yunlai Su*a
Hongchang Yao
A relatively facile strategy was rationally developed for the synthesis of hierarchical SAPO-34 using
aluminum phosphate spheres as the nutrient and mesoporogen, without utilizing any additional meso/
macropore-directing agents. The simple synthesis resulted in SAPO-34 (CHA) with classical
rhombohedral morphology with dimensions between 2 and 3 mm and intracrystalline meso/macropores.
It was demonstrated that the employment of spherical aluminum phosphate particles and the regulation
of synthesis conditions to carry out the dissolution of aluminum phosphate spheres parallel to the
crystallization process of the CHA phase were crucial factors leading to successful synthesis. The
crystallization process of the hierarchical SAPO-34 was investigated and a possible crystallization model
was proposed. The catalytic performances of the synthesized SAPO-34s in the MTO reaction were
evaluated. The SAPO-34 with a hierarchical porous structure exhibited an improved catalytic lifespan and
higher selectivity for light olefins. This work not only provides a new facile method to synthesize
hierarchical SAPO-34 but also holds great promise for offering useful guidance for the synthesis of other
aluminophosphate zeolites with hierarchical structures.
Received 28th August 2020
Accepted 5th December 2020
DOI: 10.1039/d0ta08437a
rsc.li/materials-a
on different length scales including micro-, meso- and macro-
pores have been demonstrated to effectively decrease the intra-
Introduction
Zeolites are a family of microporous crystalline materials with crystalline diffusion path length and thus present enhanced
22
well-dened micropores of molecular dimensions that have been catalytic performance. So far, a series of synthesis strategies have
widely used as catalysts in a variety of industrial processes such as been developed to synthesize hierarchical SAPO-34 zeolites,
1–6
crude-oil rening and in bulk chemical industries. In recent including “bottom-up” and “top-down” approaches. Generally,
years, SAPO-34 with Brønsted acid sites of moderate acid strength the top-down approach involves post-synthetic treatment such as
23,24
and large chabazite cages connected via small 8-ring openings has uoride etching and demetallation using acids/alkalis.
The
become attractive due to its excellent catalytic performance in the mesopores are generated due to the extraction of framework
methanol-to-olen (MTO) reaction, which has been proved to be silicon and the pore structure can be tuned by varying the
the most successful nonpetrochemical route for the production of chemical etching temperature or time. Chen et al. developed
7
–13
light olens from natural gas or coal.
unit of the Shenhua Group has been constructed and entered into hierarchical SAPO-34.
The world's rst MTO a post-synthesis uoride etching route for the preparation of
25,26
The straight intersecting mesopores that
14
commercial operation successfully. However, the diffusion of enhance the accessibility of micropores in the SAPO-34 are
reactants and products is greatly restricted once a signicant introduced by the dissolution of the silicon-rich interfaces
fraction of the CHA cages are occupied by polycyclic aromatics between crystalline domains via treatment with HF–NH
F solu-
tion. However, SAPO-type zeolites are not as stable as alumino-
Considerable efforts have been made to overcome internal silicates and suffer from the loss of crystallinity and weight aer
4
15–18
(called coke), leading to the rapid deactivation of SAPO-34.
27
diffusion limitations associated with the micropores of zeolites acid/alkali treatment. Recently, Pan et al. prepared hierarchical
1
9–21
and minimize coke deposition.
possess a porous structure consisting of multiple levels of pores thylamine under hydrothermal conditions, during which the
Hierarchical zeolites which SAPO-34 with a relatively low weight loss by etching with trime-
ꢀ
28
recrystallization of the samples might occur at 180 C. The
bottom-up approach involves the use of sacricial mesoporogens,
including hard templates and so templates, together with the
a
Green Catalysis Center and College of Chemistry, Zhengzhou University, Zhengzhou
4
b
50001, China. E-mail: gaobeibei@zzu.edu.cn; yunlaisu@zzu.edu.cn
29
micropore template that induces the formation of zeolites. The
hard-template method is commonly performed by introducing
solid materials, which usually have a relatively rigid structure, into
School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, China
†
Electronic supplementary information (ESI) available: Additional XRD patterns
and SEM images of samples. See DOI: 10.1039/d0ta08437a
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the zeolite synthesis system. Currently, carbonaceous materials Tianjin Jindongtianzheng Chemical Reagent Co.), tetraethyl
and polymer spheres are the most widely investigated type of hard orthosilicate (TEOS, 98.0 wt%, Tianjin Kemiou Chemical
30–32
template.
Schmidt et al. introduced hierarchical pores in the Reagent Co.), ethanol (99.7 wt%, Sinopharm Chemical Reagent
SAPO-34 zeolites via templating of carbon nanoparticles and Co.), ethylene glycol (99.0 wt%, Sinopharm Chemical Reagent
30
carbon nanotubes. They found that the cave-like mesopores Co.), and n-propyl alcohol (99.0 wt%, Tianjin Yongda Chemical
created by carbon nanoparticles were inside the crystal and not Reagent Co.). The inorganic reagents used were aluminium
accessible from outside, while the use of carbon nanotubes led to nitrate nonahydrate (Al(NO ) $9H O, 99.0 wt%, Zhengzhou
3 3 2
mesopores that were accessible to the external surfaces of the Paini Chemical Co.), pseudoboehmite (67.5 wt%, Shandong
particles. In addition to hard templates, so templates are more Chemical Co.), phosphoric acid (85.0 wt%, Sinopharm Chem-
extensively used for synthesizing hierarchical zeolites due to their ical Reagent Co.), nitric acid (65.0–68.0 wt%, Yantai Shuang-
advantages such as exibility and diversity. To construct hierar- shuang Chemical Co.), fumed silica (99.8 wt%, Shenyang
3
3–38
chical SAPO-34, various so templates such as organosilane,
Chemical Co.), silica sol (30.6 wt%, Shenyang Chemical Co.),
cetyltrimethylammonium bromide (CTAB, 99.0 wt%, Tianjin
39–41
42,43
cetyltrimethylammonium bromide,
polyethylene glycol,
44
and polydiallyldimethylammonium chloride
have been Guangfu Chemical Co.) and ammonia water (25.0–28.0 wt%,
employed to direct the formation of meso/macroporous struc- Sinopharm Chemical Reagent Co.). All reagents were used
tures. However, most of the mesoporous templates involved in the without any further purication.
bottom-up approaches are expensive and the heat and water
generated from the combustion of the meso-template contribute
to the destabilization of the zeolite framework, which limits their
Synthesis of aluminum phosphate spheres (APSs)
Aluminum phosphate spheres (APSs) were prepared through
26
industrial application. Besides, the so-template-derived meso-
porous zeolites are oen comprised of nanosized zeolitic grains
the hydrothermal method. First, Al(NO
3
)
3
$9H
2
O
and
ꢁ1
(
NH ) HPO aqueous solutions (0.4 mol L ) were mixed in
4 2 4
35–41
with random orientations.
Therefore, more cost-effective and
a 500 mL beaker. Subsequently, the resultant solution was
titrated to pH ¼ 2.0 using nitric acid. Then certain amounts of
urea, ethylene glycol and n-propyl alcohol were introduced into
the solution. Aer stirring for 10 min, the solution was placed in
environmentally benign routes for the preparation of hierarchical
SAPO-34 zeolites with improved catalytic properties are still highly
required due to the lack of simple methods to introduce meso-
pores or macropores in zeolite crystals directly.
ꢀ
a Teon-lined stainless steel autoclave and heated at 100 C for
Previously, Schwieger et al. reported a very simple method for
1
2 h under static conditions. The obtained product was washed
45
the preparation of MFI-type zeolites. The interesting feature of
this technique is the formation of well-shaped macroporous
silicalite-1 crystals from mesoporous silica particles as the
sacricial template. However, this method is inapplicable to sil-
icoaluminophosphate molecular sieves which are formed by
ꢀ
with water and dried at 100 C. Finally, the powder was calcined
at 500 C for 2 h in air.
ꢀ
Synthesis of mesoporous silica spheres (MSSs)
incorporation of silicon atoms in aluminophosphate crystalline In a typical procedure for the synthesis of mesoporous silica
frameworks. Inspired by this design concept, a relatively facile spheres (MSSs), TEOS was added into an ethanol–water solution
and cost-effective synthetic method for the preparation of hier- including certain amounts of CTAB and ammonium hydroxide.
archical SAPO-34 zeolites was rationally developed in the present Aer stirring for 2 h at room temperature, the white precipitate
ꢀ
work, using predesigned aluminum phosphate spheres as was collected, washed with deionized water and dried at 100 C
nutrients as well as temporary templates. The employment of overnight. Finally, the as-synthesized MSSs were calcined at
ꢀ
spherical aluminum phosphate particles as the aluminium and 550 C for 6 h in air to remove CTAB.
phosphorus source was demonstrated to play a vital role in
macropore formation. Moreover, the SAPO-34 particle obtained
was demonstrated to be a zeolite single crystal with intracrystal-
line meso/macropores rather than random aggregates of nano-
crystals. The signicance of regulating the synthesis conditions
to carry out the dissolution of aluminum phosphate spheres
parallel to the crystallization process of the CHA phase was also
proved. Meanwhile, the MTO catalytic performance of SAPO-34
was investigated in detail. This work offers not only a facile
approach to synthesize hierarchical SAPO-34 with enhanced
catalytic performance but also useful guidance for the synthesis
of other aluminophosphate zeolites with hierarchical structures.
Synthesis of hierarchical SAPO-34 samples
Hierarchical SAPO-34 was prepared via a vapor-phase transport
(VPT) method. Firstly, 1.22 g of aluminum phosphate spheres
were ground with 0.15 g fumed silica and 0.03 g SAPO-34 seeds
for 20 min. The average particle size of the SAPO-34 seed is 1 mm
ꢀ
and was calcined at 600 C before used. Then the mixture was
transferred to a Teon plate and placed in a Teon-lined
stainless steel autoclave with 1.50 g of distilled water and
ꢀ
0.76 g TEA at the bottom. Aer crystallization at 200 C for 6 d,
the solid product was recovered, washed with water and dried at
ꢀ
1
00 C.
Experimental
Synthesis of the conventional SAPO-34 sample
Chemical reagents
The conventional SAPO-34 sample was synthesized with
The organic reagents used in this work were triethylamine (TEA, a mixture of 3TEA : 1.0Al
2 3 2 5 2 2
O : 1.0P O : 0.5SiO : 50H O. Typi-
99.0 wt%, Sinopharm Chemical Reagent Co.), urea (99.0 wt%, cally, pseudoboehmite was rst mixed with deionized water at
1860 | J. Mater. Chem. A, 2021, 9, 1859–1867
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room temperature under stirring for 30 min. Then phosphoric
acid was added to the resultant solution, followed by contin-
uous stirring for 2 h. Then TEOS was added and stirred for
30 min. Aer further addition of TEA, the reaction mixture was
further stirred for 4 h and then transferred to a 100 mL Teon-
lined stainless steel autoclave. Crystallization was conducted at
ꢀ
2
00 C for 2 d under autogenous pressure. The solid product
was centrifuged, washed with water several times, and then
ꢀ
dried at 100 C in an oven overnight.
Characterization
X-ray diffraction (XRD) patterns were collected to determine the
crystalline phase on a Panalytical X'pert PRO X-ray diffractom-
˚
eter (40 kV, 40 mA) using Cu Ka (l ¼ 1.5406 A) radiation.
Fig. 1 XRD patterns of hierarchical SAPO-34 (SAPO-34-H1 and SAPO-
Scanning electron microscopy (SEM) images were observed
using Zeiss-sigma 500 and SU8020 electron microscopes.
Chemical compositions were determined using a Philips Magix-
34-H2) and conventional microporous SAPO-34 (SAPO-34-C).
6
01 X-ray uorescence (XRF) spectrometer. Nitrogen
ꢀ
1
.00Al
2
O
3
: 1.00P
2
O
5
: 0.50SiO
2
: 1.50TEA at 200 C. This was
adsorption/desorption measurements were measured on
achieved through three steps: preparation of aluminum phos-
phate spheres, grinding these particles with a silica source, and
crystallization through a vapor-phase transport (VPT) method.
The obtained hierarchical SAPO-34 samples using fumed silica
and MSSs as silicon sources were denoted as SAPO-34-H1 and
SAPO-34-H2, respectively. For comparison, conventional SAPO-
ꢀ
a Quantachrome autosorb-1 analyzer at ꢁ196 C. The samples
ꢀ
were degassed at 350 C under vacuum before the measure-
ments. The temperature-programmed desorption of ammonia
3
(NH -TPD) experiments were performed using a TP-5080 auto-
mated chemisorption analysis unit. The samples (0.1 g, 40–60
mesh) were pretreated under a He ow at 600 C for 120 min.
Then the temperature was brought to 100 C, followed by the
adsorption of NH
sample for 30 min to remove the physically adsorbed NH
molecules. The desorption of NH was carried out from 100 to
ꢀ
3
4 (SAPO-34-C) was also synthesized with a similar gel compo-
ꢀ
sition under hydrothermal conditions. The SEM image in
Fig. S1† shows that the aluminum phosphate precursor aer
calcination is spherical with a uniform size of about 100–
00 nm and the XRD pattern indicates that the precursor is
amorphous. The XRD patterns of all zeolite samples show
3
for 30 min. Then He was purged through the
3
3
ꢀ
ꢀ
ꢁ1
29
2
6
00 C at a heating rate of 10 C min . The solid-state Si
magic-angle spinning (MAS) NMR spectra were recorded on an
Agilent 600M spectrometer with a 4 mm MAS probe with
a spinning rate of 8 kHz using high-power proton decoupling.
Chemical shis were referenced to tetramethylsilane (TMS).
characteristic diffraction peaks of the CHA-type structure
Catalytic tests
The methanol-to-olen (MTO) reaction was tested with a quartz
tubular xed-bed reactor at atmospheric pressure. 0.4 g
calcined catalyst (40–60 mesh) was pretreated in owing
ꢀ
nitrogen at 550 C for 1 h. Aer the temperature was brought to
ꢀ
the reaction temperature of 450 C, a mixture of methanol and
water with a weight ratio of 40 : 60 was pumped into the reactor
ꢁ
1
with a weight hourly space velocity (WHSV) of 2 h . The reac-
tion products were analyzed using an on-line gas chromato-
graph (GC-7900) equipped with a ame ionization detector
(FID) and Plot-Q column. The methanol conversion and selec-
tivity of the products were calculated on a CH basis. Dimethyl
2
ether in the products was regarded as a reactant in the
calculations.
Results and discussion
Synthesis and characterization of hierarchical SAPO-34
zeolites
Hierarchical porous SAPO-34 zeolites were synthesized from the
Fig. 2 SEM images of hierarchical SAPO-34-H1 (a, b) and SAPO-34-
starting gels with
a
typical molar composition of H2 (c, d) and conventional microporous SAPO-34-C (e, f).
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Fig. 1), suggesting the formation of SAPO-34. The SEM images
Paper
(
of SAPO-34 samples in Fig. 2 all display typical rhombohedral
morphology. Specically, the conventional microporous SAPO-
34 (SAPO-34-C) shows a smooth crystal surface, while intra-
crystalline meso/macropores with wide pore size distribution
can be observed to be distributed over the external surface of the
crystals for SAPO-34-H1 and SAPO-34-H2 (Fig. 2b and d).
Besides, SAPO-34-H2 synthesized with silica spheres contains
regular circular holes with diameters of about 300 nm in the
crystal surface as is evident from the SEM image in Fig. 2c, the
sizes of which are greatly consistent with that of silica spheres.
To further investigate the internal structures of the samples,
the TEM images and selected area electron diffraction (SAED)
images of SAPO-34-H1 were obtained and are shown in Fig. 3. As
is seen in Fig. 3a, the crystal dimensions are consistent with that
in the SEM images. In particular, intracrystalline meso/
macropores are observed in the whole crystal and seem to be
well interconnected. However, the meso/macropores in the
crystals do not present a regular circular shape and show
a relatively wide pore size distribution (Fig. 3b). This is possibly
because the APSs used in the synthesis group into agglomerates
to some extent. Besides, successive crystal growth might occur
aer the APSs dissolve completely, resulting in the decrease of
meso/macropores. The SAED pattern containing discrete
diffraction spots is shown in Fig. 3d, indicating that each SAPO-
Fig. 4
N
2
adsorption/desorption isotherms of hierarchical SAPO-34
(SAPO-34-H1 and SAPO-34-H2) and conventional microporous
SAPO-34 (SAPO-34-C).
Table 1 Textural properties of the samples
3
Pore volume (cm
2
ꢁ1
ꢁ1
Surface area (m g
)
g
)
a
total
b
b
b
c
Sample
S
S
micro
S
ext
V
micro
V
meso
SAPO-34-C
758
712
733
743
679
702
15
33
31
0.27
0.25
0.26
0.01
0.12
0.11
34-H1 particle is a single crystal despite being highly meso- SAPO-34-H1
SAPO-34-H2
porous and tends to show better hydrothermal stability than
hierarchical structures constructed by aggregation of
nanocrystals.
To investigate the textural properties of the obtained
2
samples, N adsorption–desorption measurements were con-
a
b
BET surface area.
Smicro (micropore area), Sext (external surface area)
and Vmicro (micropore volume) determined from the t-plot method.
c
V
meso (mesopore volume) ¼ Vtotal ꢁ Vmicro, Vtotal is determined from
the adsorbed volume at P/P
0
¼ 0.98.
ducted and the results are displayed in Fig. 4. The SAPO-34
samples all exhibit the characteristic type-I isotherm with
a steep increase at low relative pressure, indicating the micro-
porosity of the samples. Besides, in accordance with the results
in the SEM images, the typical type-IV isotherm with a hyster-
esis loop at higher relative pressures can be seen for the hier-
archical samples SAPO-34-H1 and SAPO-34-H2, demonstrating
the existence of meso/macropores in the hierarchically struc-
tured SAPO-34. The detailed results are summarized in Table 1.
In particular, in comparison with the conventional SAPO-34-C,
SAPO-34-H1 and SAPO-34-H2 samples possess larger external
surface areas and higher mesopore volumes, without severely
sacricing the intrinsic micropore volume.
3
The acidic properties of samples were investigated by NH -
TPD measurements and the corresponding proles are shown
in Fig. 5. All the SAPO-34 samples exhibit two desorption peaks
ꢀ
ꢀ
at about 210 C and 430–450 C, which could be ascribed to the
weak and strong acid sites, respectively. Given that the acidic
properties of SAPO-34 are closely related to the Si incorporation
in the framework, XRF was also used to characterize the bulk
chemical composition of the obtained SAPO-34 samples.
Accordingly, the actual Si contents of samples and the number
3
of acid sites were determined from the XRF results and NH -
TPD proles, and the results are listed in Table 2. It is observed
that although the Si contents of the samples are similar, the
Fig. 3 TEM images of a SAPO-34-H1 particle (a–c) and the corre-
sponding SAED pattern of the part in the circle in c (d).
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2 3
Fig. 6 SEM images of SAPO-34s synthesized with a TEA/Al O molar
ratio of 2.0 (a) and water/dry gel molar ratio of 22.22 (b).
Al O decreases to 1.0, as is oen the case in the conventional
2
3
hydrothermal synthesis of SAPO-34. However, as can be seen in
Fig. 6a, the meso/macropores distributed in the SAPO-34 crys-
Fig. 5 NH
3
-TPD profiles of hierarchical SAPO-34 (SAPO-34-H1 and tals almost disappear for the sample synthesized with a rela-
SAPO-34-H2) and conventional microporous SAPO-34 (SAPO-34-C).
2 3
tively high TEA/Al O ratio of 2.0. This can be ascribed to the
increase of the TEA concentration in the system, which would
lead to the fast dissolution of APSs. Consequently, the sacricial
template effect of aluminum phosphate spheres for meso/
macropore formation would become poor, resulting in fewer
meso/macropores in the crystal. Therefore, adopting
a moderate amount of TEA in the system to regulate the
dissolution rate of APSs is pivotal during the crystallization to
obtain hierarchical SAPO-34 with abundant intracrystalline
meso/macropores.
hierarchical SAPO-34 samples have lower acidic concentration
compared with the conventional SAPO-34.
The chemical environments of the Si atoms of the hierar-
chical SAPO-34-H1 and the conventional SAPO-34 samples were
2
9
investigated by solid-state Si MAS NMR spectroscopy. As
shown in Fig. S2,† both the samples exhibit similar spectra, and
the dominant peak at about ꢁ91 ppm is attributed to the iso-
lated Si(4Al) species. The overlapping peaks around ꢁ95 ppm
and ꢁ100 ppm are ascribed to the Si(3Al) and Si(2Al),
respectively.
Effect of the water/dry gel molar ratio
The water placed at the bottom of the autoclave that contained
dry gel has been found to play a vital role in zeolite crystalliza-
2 3
Effect of the TEA/Al O molar ratio
To elucidate the parameters that were critical for the formation tion using dry-gel conversion. Therefore, syntheses using
of the meso/macropores of hierarchical SAPO-34, controlled different water/dry gel ratios were carried out to investigate the
experiments with varied gel compositions were performed. effect of the water content on the crystalline phase and
During the crystallization process, APSs which might gradually morphology of the products.
dissolve from the outer shell to the core acted as the sacricial
template for macropore formation as well as the raw material. be seen that strong peaks at 20.3 , 21.5 , and 23.0 belonging to
The XRD patterns of the samples are shown in Fig. S4.† It can
ꢀ
ꢀ
ꢀ
Therefore, conducting the dissolution of APSs parallel to the dense phase AlPO -tridymite arise when the water/dry gel molar
4
ꢀ
crystallization process of the SAPO-34 is crucial for the forma- ratio is 11.11 and a peak at 21.8 associated with dense phase
4
tion of meso/macropores. Considering that TEA serves as the AlPO -cristobalite also exists. This suggests that the existence of
chemical etching agent which could promote the dissolution of sufficient water during the crystallization is a prerequisite for
the aluminum phosphate precursor, the amount of TEA might the successful synthesis of pure SAPO-34. When the water
play a key role during the construction of the hierarchical content is low, the aluminum phosphate precursor is more
structure.
likely to transform into dense phase AlPO4. By increasing the
Fig. S3† displays the XRD patterns of the samples synthe- water/dry gel molar ratio from 11.11 to 16.67, SAPO-34 without
sized with different amounts of TEA using fumed silica as the other impurities was obtained. Upon further increasing the
silicon source. It can be seen that the product turns into water content, the crystallinity of the product improved,
a mixture of SAPO-5 and SAPO-34 when the molar ratio of TEA/ whereas the AEI phase which tends to coexist with the CHA
Table 2 Composition and the acid amount of the samples
Medium and strong acid
a
b
ꢁ1
b
ꢁ1
Sample
Molar composition
Weak acid amount (mmol g
)
amount (mmol g
)
SAPO-34-C
SAPO-34-H1
SAPO-34-H2
Al0.496
Al0.485
Al0.487
P0.385Si0.119
P0.394Si0.121
P0.388Si0.125
0.51
0.41
0.42
0.70
0.53
0.57
a
b
3
Determined by X-ray uorescence (XRF) analysis. Calculated from the peak area of NH -TPD proles.
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phase appears. In agreement with the XRD results, the SEM
image in Fig. 6b shows that the sample acquired when a larger
amount of H O was introduced exhibits nanosheet crossed
2
morphology with pronounced meso/macropores. In conclusion,
a sufficient amount of external bulk water which is brought to
the surface of the dry gel via vapor is necessary for successful
crystallization. Increasing the water content would enhance the
adsorption and condensation of water molecules on or inside
the dry gel, making the crystal structure more regular.
2 2 3
Effect of the SiO /Al O molar ratio
During the synthesis of SAPO-34, the silica source has a signif-
icant effect on silicon incorporation into the alumino-
phosphate building blocks in the crystallization process and
thus may also affect the morphology of the nal product in this
strategy. Therefore, control experiments using different Si
contents were carried out with the other experimental condi-
tions unchanged.
Fig. 7 XRD patterns of the products with different crystallization
times.
with fumed silica as the silicon source was performed by XRD and
The XRD patterns of the samples are shown in Fig. S5.† It can SEM. Firstly, the SEM image of fumed silica is shown in Fig. S7,†
be seen that the characteristic peaks match well with the which indicates that fumed silica is composed of monodisperse
simulated XRD pattern of the AFI-type molecular sieve, sug- spheres with uniform dimensions of about 20 nm. As shown in
gesting the formation of AlPO
4
-5 crystals when no silica source Fig. 7, the XRD pattern of the gel without heating shows a broad,
ꢀ
was added. Besides, a peak at 22.3 belonging to the dense amorphous hump, indicating the sample is amorphous in
phase also exists. Therefore, the existence of a silica source is nature. The corresponding SEM images (Fig. 8a) reveal the
crucial to the formation of the CHA phase. The corresponding uniform mixing of the spherical APSs and fumed silica nano-
SEM image is shown in Fig. S6a.† It can be seen that the AlPO -5 particles. With further hydrothermal reaction for 24 h, charac-
4
crystals obtained in the absence of a silica source have a typical teristic diffraction peaks indexed to SAPO-34 could be observed.
rod-like shape without the formation of mesopores. In consid- The corresponding SEM image (Fig. 8b) reveals that the SAPO-34
eration of this, adopting a moderate amount of TEA in the crystals form in the voids between the APSs, along with the
system to regulate the dissolution rate of APSs is pivotal to consumption of the dissolved APSs. Upon further heating the gel
obtain hierarchical zeolites; the TEA/Al O ratio was changed to for 72 h, the XRD peaks of SAPO-34 become higher and the broad
2
3
1.0. As can be seen in Fig. S6b,† for the sample fabricated with peaks arising from amorphous materials disappear completely
a relatively low TEA/Al O ratio of 1.0, spherical crystals with
2
3
dimensions of about 200 nm were produced, which are analo-
gous to the morphology of APSs. This indicates that a different
dissolution behaviour of APS precursors might occur when no
silica source is added. Besides, syntheses of hierarchical SAPO-
2 2 3
34 with SiO /Al O ratios of 0.3 and 0.7 were also investigated.
As can be seen in Fig. S5,† the XRD patterns of the as-
synthesized samples both show the pure structure of the CHA
crystal. Consistent with the XRD results, the SEM images in
Fig. S6† show that the samples acquired exhibit typical rhom-
bohedral morphology with pronounced meso/macropores.
Besides, the bulk elemental composition of the product
synthesized with a SiO
analysis is Al0.455 0.376Si0.169, showing that the Si content
increases as the SiO /Al O ratio of the synthetic gels increases.
2 2 3
/Al O ratio of 0.7 derived from XRF
P
2
2 3
In conclusion, the existence of a silica source is necessary for
mesopore formation during the crystallization of SAPO-34 and
hierarchical SAPO-34 molecular sieves could be obtained with
2 2 3
different SiO /Al O ratios.
Crystallization process and proposed formation mechanism
of hierarchical SAPO-34 zeolites
In order to gure out the formation mechanism of the hierar-
Fig. 8 SEM images of the products with different crystallization times:
chical SAPO-34, an investigation of the crystallization process (a) 0 h; (b) 24 h; (c and d) 72 h; (e and f) 144 h.
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(Fig. 7). The ongoing crystallization continues to consume the Catalytic performance in the MTO reaction
dissolved APSs and the crystal morphology becomes relatively
regular and uniform, while the APSs are dissolved to leave
a network of interconnected meso/macropores distributed in the
crystal with a classical CHA morphology (Fig. 8c and d). Further
prolonging the crystallization to 144 h, the XRD peak intensities
continue to increase and this process will continue until well-
shaped CHA crystals with dimensions of about 3 mm are
formed (Fig. 8e and f).
To conrm the template effect of APSs for the formation of
the meso/macropores, controlled experiments employing
aluminium phosphate powder without specic morphology
were performed and the corresponding morphology of the
product is presented in Fig. S8.† It can be seen that SAPO-34
with typical rhombohedral morphology of the CHA crystals
was obtained without the existence of meso/macropores, con-
The MTO catalytic performance of the SAPO-34 catalysts was
investigated in a xed-bed reactor and the results are displayed
in Fig. 9. Signicantly, the hierarchical SAPO-34-H1 and SAPO-
34-H2 catalysts exhibit a considerably prolonged catalyst life-
time compared with the conventional microporous SAPO-34-C.
Specically, the catalytic lifetime, dened as the reaction
duration during which >99% methanol conversion can be
achieved, is only 160 min for SAPO-34-C (Table 3). Remarkably,
the lifetime of sample SAPO-34-H1 with a hierarchical porous
structure is 310 min, almost twice the lifetime of SAPO-34-C
with only micropores. The corresponding product distribu-
tions of the MTO reaction are shown in Fig. 10. It can be
observed that light olens (ethylene and propylene) are the
main products for all the catalysts and show an increasing trend
with prolonging the reaction time. Specically, the selectivity of
ethylene increases time on stream, while that of propylene
decreases slightly for the SAPO-34 catalysts. It could be attrib-
uted to the increasing coke content which result in pronounced

rming the crucial role of APSs in meso/macropore formation.
Meanwhile, a small number of amorphous particles appeared
around the rhombohedral crystals, which are supposed to be
unreacted aluminium phosphate.
15
shape selectivity effects in the CHA cages of the SAPO-34.
Based on the aforementioned observation of the crystalli-
zation process, the formation mechanism proposed is depic-
ted in Scheme 1. Initially, spherical APSs are mixed with fumed
silica nanoparticles uniformly. Then when the APSs and
fumed silica mixtures are heated under VPT conditions, the
APSs and the silicon source start to dissolve gradually and
thereby give rise to the nucleation and formation of the SAPO-
Notably, the average selectivity of ethylene and propylene of
sample SAPO-34-H2 is higher than that of the conventional
catalyst, which is improved from 75.6% for the conventional
SAPO-34-C catalyst up to 81.6%.
Generally, the catalytic properties of SAPO-34 catalysts in the
MTO reaction are correlated with the acidic properties and the
crystal sizes. On one hand, the nanosized and hierarchical
SAPO-34 zeolites could effectively overcome the diffusion limi-
tation and subsequently inhibit the secondary reaction, thus
3
4 nanocrystals around the APSs. During the process, the
dissolved APSs will be consumed as the nutrient and trans-
ported to the growing CHA crystals. Meanwhile, the APSs serve
as the sacricial templates which are dissolved to leave
intracrystalline meso/macropores distributed in the crystal. As
the crystallization goes on, the crystallinity of the SAPO-34
crystals increased and the crystallization will continue until
crystals become well-shaped. Different from the conventional
hydrothermal process which contains continuous aqueous
medium, the limited added water amount in the precursors
during this VPT process causes limited mass transfer and
decreased mobility of precursor species, leading to the good
conservation of the intracrystalline meso/macropores formed
during the crystallization.
16
retarding coke formation and improving the catalytic stability.
On the other hand, acidic strength and acid density are also
revealed to exert signicant effects on the catalytic performance
10,46
of SAPO-34 catalysts in the MTO reaction.
Higher acidic
strength and higher concentration of Brønsted acid sites could
give rise to further condensation of hydrocarbon pool
compounds, accelerate the coke formation and cause a short-
ened catalyst lifetime. Therefore, the enhanced catalytic
Fig. 9 Methanol conversion with time-on-stream over hierarchical
porous SAPO-34 (SAPO-34-H1 and SAPO-34-H2) and conventional
ꢀ
Scheme
1
Schematic illustration of the proposed crystallization microporous SAPO-34 (SAPO-34-C). Reaction conditions: 450 C,
ꢁ
1
process of hierarchical SAPO-34.
WHSV ¼ 2 h , catalyst weight ¼ 400 mg.
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a
Table 3 Catalytic performance over SAPO-34 samples in the MTO reaction
b
]c
2
]c
3
]
2
]c
3
]
2
]d
3
Sample
Lifetime (min)
C
(%)
C
(%)
C
+ C
(%)
C
+ C
(%)
SAPO-34-C
SAPO-34-H1
SAPO-34-H2
160
310
285
43.66
43.47
43.80
38.61
40.81
40.26
82.27
84.28
84.06
75.60
81.84
81.56
a
ꢀ
ꢁ1
b
Reaction conditions: 450 C, WHSV ¼ 2 h , catalyst weight ¼ 400 mg. The lifetime is dened as the reaction duration with >99% methanol
c
]
2
]
d
conversion. The selectivity to C
selectivity to C
and C
3
refers to the highest selectivity of ethylene and propene under >99% methanol conversion. The
]
]
2 3
and C refers to the average selectivity of ethylene and propene under >99% methanol conversion.
intracrystalline meso/macropores in the crystals has been
rationally developed without the aid of additional surfactants.
The adoption of aluminum phosphate spheres as both the
aluminium and phosphorus source was revealed to be crucial to
the meso/macropore formation. SAPO-34 with typical rhombo-
hedral morphology without meso/macropores was obtained
when employing the aluminium phosphate powder without
specic morphology. Adopting a moderate amount of TEA in
the system to regulate the dissolution rate of APSs was also
pivotal during the crystallization. Besides, the water placed at
the bottom of the autoclave was found to play a vital role in the
zeolite phase formation. Compared with the conventional
microporous SAPO-34, the hierarchical SAPO-34 catalysts
exhibited a prolonged catalytic lifetime and increased selectivity
to ethylene and propylene in the MTO reactions, which could be
Fig. 10 Selectivity to ethylene plus propylene (a), ethylene (b), ascribed to the hierarchical structures and decreased acidities.
propylene (c) and hydrogen transfer index (C
3
H
8
/C
3
H
6
) (d) over SAPO-
This work demonstrates a feasible strategy for constructing
hierarchical SAPO-34 catalysts with improved MTO perfor-
mance and holds great promise for fabricating more highly
effective hierarchical SAPO zeolites.
ꢀ
3
4 catalysts with time-on-stream. Reaction conditions: 450 C, WHSV
ꢁ1
¼
2 h , catalyst weight ¼ 400 mg.
performance of hierarchical SAPO-34-H1 and SAPO-34-H2
catalysts can be explained by their lower acid concentrations
and hierarchical pore structure, which enhance the mass
transfer inside the crystals and thus inhibit the catalyst deac-
tivation. The increased light olen selectivity should also result
from the decreased acidities of hierarchical SAPO-34-H1 and
Conflicts of interest
There are no conicts to declare.
SAPO-34-H2 catalysts, which could inhibit the occurrence of Acknowledgements
side reactions such as olen oligomerization, cyclization and
This work was supported by the China Postdoctoral Science
Foundation (2018M642769).
hydrogen transfer. Correspondingly, the selectivity of alkanes
such as ethane and propane) decreases, while the selectivity of
light olens increases and a longer catalyst lifetime is achieved.
The hydrogen transfer index (HTI, C /C ratio) is oen
employed to evaluate the hydrogen transfer level of secondary
(
3
H
8
3 6
H
Notes and references
47
transformation of olens products. The corresponding HTI
values of MTO reactions over the selected samples are shown in
Fig. 10d. This shows that the HTI values of the SAPO-34-H1 and
SAPO-34-H2 samples are lower than that of SAPO-34-C, well
consistent with their lower acid densities and hierarchical
structures. That is, hierarchical porous SAPO-34 catalysts can
decrease the hydrogen transfer level and thus improve the
selectivity for ethylene and propylene.
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4
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