Rapid synthesis of hierarchical SSZ-13 zeolite microspheres: Via a fluoride-assisted in situ growth route using aluminum isopropoxide as aluminum source

Article abstract of DOI:10.1039/c9ra08895d

Hierarchical SSZ-13 zeolite microspheres were quickly synthesized by adding a small quantity of hydrofluoric acid in the starting gel using aluminum isopropoxide as aluminum source. The as-synthesized hierarchical SSZ-13 crystals with high crystallinity, pore volume and external surface area were obtained within 12 h, whereas a much longer time (144 h) was necessary to synthesize a fully crystalline zeolite in a fluorine free system. The obtained hierarchical SSZ-13 catalysts showed excellent MTO performance together with considerably prolonged catalytic lifetime and improved selectivity of light olefins in comparison with the conventional microporous SSZ-13.

Full text of DOI:10.1039/c9ra08895d

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Rapid synthesis of hierarchical SSZ-13 zeolite
microspheres via a fluoride-assisted in situ growth
route using aluminum isopropoxide as aluminum
source†
Cite this: RSC Adv., 2020, 10, 3566
Liancheng Bing,
and Guangjian Wang
Jiajia Liu, Kaifeng Yi, Fangni Li, Dezhi Han,
*
Fang Wang
Hierarchical SSZ-13 zeolite microspheres were quickly synthesized by adding a small quantity of
hydrofluoric acid in the starting gel using aluminum isopropoxide as aluminum source. The as-
synthesized hierarchical SSZ-13 crystals with high crystallinity, pore volume and external surface area
were obtained within 12 h, whereas a much longer time (144 h) was necessary to synthesize a fully
crystalline zeolite in a fluorine free system. The obtained hierarchical SSZ-13 catalysts showed excellent
MTO performance together with considerably prolonged catalytic lifetime and improved selectivity of
light olefins in comparison with the conventional microporous SSZ-13.
Received 29th October 2019
Accepted 12th January 2020
DOI: 10.1039/c9ra08895d
rsc.li/rsc-advances
obstacles in industrial applications.11 Recently, great efforts
have been paid in the synthesis of hierarchical SSZ-13 zeolites,
Introduction
offering the combined advantages of both microporous crystals
and mesoporous systems. This hierarchical structure can
provide efficient mass transfer and improved accessibility of
reactant molecules to the catalytically active sites on the
premise of preserving shape selectivity, high thermal/
hydrothermal stability and acid strength.12,13 Typical strategies
for the preparation of hierarchical zeolite materials, such as
alkaline treatment, does not enhance MTO catalytic perfor-
mance when applied to SSZ-13, because of the decrease of
surface area and acid sites.7,12 Hensen et al.13,14 synthesized
mesoporous SSZ-13 crystals by a dually structure-directing
Zeolites with unique microporous structure and acidity have
been widely used in the petrochemical industry as heteroge-
neous catalysts.1–3 As a successful non-petrochemical route to
produce light olens, the methanol-to-olen (MTO) process
over zeolite catalysts has received widespread attention and has
undergone considerable development over the past few
decades.4 To date, various zeolite catalysts have been used for
the conversion of methanol, among which SSZ-13 zeolite with
CHA framework topology exhibits high MTO catalytic perfor-
mance, mostly contributed to its large surface area, useful pore
structure in terms of shape selectivity, moderate acidity, and
high thermal/hydrothermal stability.5–7 However, the mass
transfer limitation and coke species formation in the cages
result in a low catalytic efficiency and the rapid deactivation of
SSZ-13.8
In order to prolong the catalytic lifetime and suppress the
deactivation of SSZ-13 catalyst in the MTO reaction, various
methods have been used to decrease the crystal size, which can
signicantly reduce the diffusion path length and enhance the
accessibility of reactants to the external active sites.9,10 However,
low crystallinity, low product yield, and difficulty in the sepa-
ration of nanosized particles from reaction mixture are main
synthetic
route
using
N,N,N-trimethyl-1-
adamantanammoniumhydroxide (TMAdOH) and diquaternary
ammonium type surfactant as templates. They further synthe-
sized hierarchical SSZ-13 zeolites by adding surfactant
C
22À4À4Br2 and sodium uoride to the synthesis gel.15 The ob-
tained SSZ-13 zeolites featured not only native micropores
characteristic of ordinary zeolites with CHA topological struc-
ture but also an additional micropore system with the average
pore size of ca. 0.5 nm as well as mesopores. The hierarchical
SSZ-13 catalysts exhibited remarkably prolonged catalyst life-
time in the MTO reaction compared to the conventional
micropore catalyst at similar ethylene and propylene yields.
However, the use of expensive reagents and complicated
procedures as well as long reaction time ($6 days at 160 ꢀC) to
form a hierarchical network not only increases the cost but also
consumes much energy.14,16,17 Thus effectively shortening the
crystallization time using simple and cost-effective synthetic
procedure is of great practical signicance.
State Key Laboratory Base for Eco-Chemical Engineering in College of Chemical
Engineering, Qingdao University of Science and Technology, Qingdao 266042,
China. E-mail: wgjnet@126.com
† Electronic supplementary information (ESI) available: More details of
crystallization curves, XRD analyses, SEM, TEM, textural properties, pore size
distribution, and the detailed MTO results over SSZ-13 catalysts. See DOI:
10.1039/c9ra08895d
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Herein, we have developed a simple and fast synthetic 1760H2O under conventional hydrothermal conditions at
method to prepare hierarchically porous SSZ-13 via adding 160 ꢀC for 48–144 h. Aluminum isopropoxide and aluminum
a small quantity of hydrouoric acid in the starting gel using nitrate were used as aluminum sources, respectively. The
aluminum isopropoxide as aluminum source. The addition of synthesis process for these zeolite crystals was similar to that of
hydrouoric acid into the initial synthesis gel obviously shorten the system for synthesizing hierarchical SSZ-13 crystals without
the growth time of the crystal nucleus in the synthesis of hier- using HF. The synthesized SSZ-13 using aluminum nitrate as
archical SSZ-13 zeolites. The as-synthesized SSZ-13 zeolites aluminum source are denoted as SM-x, where x represents the
showed elliptical morphology comprising intracrystalline crystallization time; whereas SSZ-13 using aluminum isoprop-
parallel meso-channels. The obtained hierarchical SSZ-13 oxide as aluminum source is denoted as SAP.
catalysts showed outstanding MTO catalytic performance with
a prolonged catalytic lifetime and improved selectivity of light
Characterization
olens.
Powder X-ray diffraction (XRD) patterns were recorded on
a Rigaku D/max 2500 diffractometer in the 2 theta angle range
of 5 to 60ꢀ using Cu Ka radiation. The crystal morphology and
size of the obtained samples were identied on a JEOL JEM-
2100 electron microscope. Transmission electron microscopy
(TEM) images were obtained on a JEM-2100 instrument oper-
ating at 200 kV. Nitrogen adsorption/desorption experiments
were carried out on a Micromeritics ASAP 2010M system at
À196 ꢀC. The samples were degassed at 250 ꢀC for several hours
before measurement. The specic surface area (SBET) was
determined based on the BET equation. The t-plot methods
were used to determine the external surface area (Sexternal),
micropore surface area (Smicro) and micropore volume (Vmicro).
The mesoporous volume (Vmeso) was calculated by the BJH
method to the adsorption isotherm. The element analysis was
carried out by inductively coupled plasma atomic absorption
spectroscopy (ICP-OES) using PerkinElmer Optima 2100 DV
instrument. The acidity of the samples was measured by NH3–
TPD using a Micromeritics Autochem II 2920 chemisorption
analyser equipped with TCD as a detector under He ow. 50 mg
of the calcinated sample was loaded into a U-tube. The packed
Experimental
Chemical and materials
Fumed silica was obtained from Sigma Aldrich. N,N,N-
Trimethyl-1-adamantylammonium hydroxide (TMAdaOH,
25 wt%) was supplied by Hangzhou ICH Biofarm Co. Sodium
hydroxide, aluminum isopropoxide (AIP), aluminum nitrate,
sodium hydroxide, ammonium chloride, and hydrouoric acid
(HF, 40 wt%) were reagent grade from Sinopharm Chemical
Reagent Co. All chemical reagents were analytical grade and
were used without additional purication.
Hierarchical SSZ-13 synthesis
Hierarchical SSZ-13 zeolites were prepared using in situ growth
route. The molar composition of the gel was Al2O3: 40 SiO2: 4
TMAdaOH: 4 Na2O: 1760H2O: 3.3 HF. First, NaOH was added
into a 3-neck ask containing deionized water and stirred until
dissolution. Then, TMAdaOH was added under vigorous stir-
ring, which was then magnetic stirred for another 30 min to
homogenize the organic and aqueous phases. Aer that, AIP as
alumina source was added and continuously stirred at 28 ꢀC for
1 h in order to obtain a clear resultant solution. Subsequently,
an optimum amount of fumed silica was added slowly into the
previously prepared solution, which was then magnetic stirred
at room temperature for another 4 h. Finally, a certain amount
of 40 wt% HF solution was added and then stirred for another
30 min. The resultant gel was sealed in a Teon-lined steel
autoclave and heated in a 160 ꢀC oven at under static conditions
for 6–48 h. Aer cooling down to room temperature, the ob-
tained solid-like products were separated by centrifugation,
washed with deionized water for several times, and dried over-
night at 80 ꢀC. Aer that, the powder samples were calcinated at
550 ꢀC for 10 h to remove the organic template. All samples were
exchanged with 2 mol LÀ1 NH4Cl solution under stirring at
ꢀ
column was purged at 350 C under the stream of helium for
activation. The sample was saturated with ammonia aer the
temperature decreased to 100 ꢀC. Subsequently, the sample was
ushed with a He ow for 30 min to remove the physically
adsorbed NH3. Finally, the measurement of the desorbed NH3
was performed from 100 to 700 ꢀC (10 ꢀC minÀ1) in the presence
of He ow. Solid state NMR experiments were carried out on
a Tesla Bruker DMX500 NMR spectrometer. 27Al MAS NMR
spectra were recorded at 156.25 MHz with a 90ꢀ pulse length of
3.6 ms. Other parameters included a spinning frequency of 12.0
kHz and a recycle time of 5 s. 29Si MAS NMR spectra were per-
formed at 99 MHz with a spinning rate of 20 kHz and a pulse
length of 2.2 ms with a 20 s repetition time.
Catalytic tests
ꢀ
80 C for three times to obtain NH4-type SSZ-13. Aer dryness,
Methanol conversion were performed at atmospheric pressure
in a xed-bed steel reactor with an inner diameter of 6 mm.
300 mg of calcinated catalyst (40–60 mesh) was loaded in the
center of the quartz reactor in each run. Prior to the MTO
reaction, the sample was activated at 500 ꢀC for 1 h under an N2
ow of 30 ml minÀ1, and then decreased the temperature to
the solids were calcinated at 500 ꢀC for 4 h. The obtained H-type
SSZ-13 catalysts are denoted as SH-x, where x represents crys-
tallization time.
Conventional SSZ-13 crystals synthesis
As reported in our previous work,18 conventional microporous 450 C to carry out the reaction. Subsequently, methanol was
SSZ-13 crystals were synthesized from the starting gel with introduced to the reactor by owing N2 (15 ml minÀ1) through
a molar composition of Al2O3: 40 SiO2: 4 TMAdaOH: 4 Na2O: a saturator at 40 ꢀC. The WHSV was kept at 3 hÀ1. All products
ꢀ
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from the reactors were analyzed on an online Agilent 6890 GC
instrument equipped with a FID detector and a HP-PLOT Q
capillary column.
Results and discussion
XRD patterns of the SSZ-13 crystals synthesized without using HF
are shown in Fig. 1. In the conventional synthesis, the XRD
pattern of the sample SM-48 crystallized for 48 h indicates that
the solid product was essentially amorphous, although several
very weak peaks were detected that can be associated with the
CHA-type zeolite framework topology.13,19 An increased crystalli-
zation time from 48 to 144 h results in an gradually increased
peak intensities, and crystallization completes aer crystalliza-
tion for 144 h, indicating the successful solid transformation of
SSZ-13 zeolite crystals from the amorphous phase, conrmed by
SEM (Fig. 2a and d). Fig. 3 shows XRD patterns of the hierarchical
SSZ-13 synthesized with HF/Al2O3 of 3.3 at different crystallized
time. The weak diffraction peaks associated with the CHA phase
appear aer 6 h of crystallization, which is interpreted by the
initial crystal growth in the amorphous phase. The correspond-
ing SEM image (Fig. 4d) indicates the formation of small
amounts of zeolite particles. Aer the crystallization time reaches
up to 12 h, the sample peak intensities increase greatly (Fig. 3),
while the SEM image shows that the amorphous phase in the
product disappears (Fig. 2b). Aer crystallization time over 12 h,
no large change in the peak intensity is observed, indicating
complete crystallization without any amorphous phase at 12 h,
conrmed also by SEM (Fig. 2c, 4b and c). Fig. S1† shows the
dependence of SSZ-13 crystallinity over the crystallization time,
the addition of small amount of HF results in the reduction of the
SSZ-13 synthesis time by 92% when compared with conventional
counterpart synthesized without using HF. Moreover, the addi-
tion of HF and the crystallization time also affect the crystal size
of the zeolitic products (Fig. 2 and 4). Some large cubic particles
(8–9 mm) can be seen in the SEM images of SM-144 (Fig. 2a and
d). When HF is added in the initial gel, the zeolitic product
exhibits typical elliptical morphology with the average particle
size of ca. 1–2 mm. The rough surfaces of the obtained particles
might indicate that they are the aggregates of zeolite nanocrystals
with abundant mesopores (Fig. 2e and f). The subsequent growth
Fig. 2 SEM images of SSZ-13 samples: (a and d) SM-144; (b and e) SH-
12; (c and f) SH-48. Scale bar: (a) 10 mm; (b–d) 1 mm; (e and f) 100 nm.
towards the elliptical morphology can be seen as the increase of
crystallization time, and aer crystallization for 48 h, the sample
SH-48 shows larger particle size of ca. 3–4 mm, indicating that
12 h is enough to obtain fully crystallized SSZ-13 microspheres
and probably the optimized value to completely transform to SSZ-
13 zeolite with smaller crystal size. Fig. S2† shows the TEM image
on the edge of a SH-12 microsphere. Bright areas can be seen in
the image because of differences in thickness, indicating the
existence of intracrystalline mesopores extending along the
crystal in the hierarchical SH-12 zeolite microsphere.20 In order to
investigate the formation of aggregates of zeolite nanocrystals,
SAP without HF was synthesized by using aluminum isoprop-
oxide as aluminum source. As shown in Fig. S3,† the SAP zeolite
shows a rough surface composed of 800 nm primary crystals,
similar to that of the SH-12 and SH-48 samples. Therefore, the
change of surface morphology of the SSZ-13 samples is attributed
to the difference of aluminum source.
Fig. 5 displays the N2 adsorption/desorption isotherms of the
hierarchical SSZ-13 zeolites and the conventional counterpart.
It can be seen that the sample SM-144 exhibits a standard type I
adsorption isotherm typical of microporous zeolite materials
without signicant mesoporosity.21,22 In contrast to the sample
SM-144, the addition of HF in the initial gel for samples SH-12
and SH-48 leads to an isotherm representing type IV behavior,23
the large hysteresis loops in the 0.45 < P/P0 < 0.95 partial pres-
sure region is observed, indicating the presence of mesoporous
structures formed by the aggregation of small crystallites. The
Fig. 1 XRD patterns of conventional SSZ-13 synthesized with different Fig. 3 XRD patterns of hierarchical SSZ-13 synthesized with different
crystallization times.
crystallization times.
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The Si/Al ratios of the hierarchical SSZ-13 samples measured
by ICP (Table S1†) are similar to that of the conventional one
synthesized without HF. The 27Al MAS NMR spectra in Fig. 6a
show that both SM-144 and SH-12 samples exhibit two peaks
centered at chemical shis of ca. 54 ppm and À2 ppm. The
signals at ca. 54 ppm are assigned to tetrahedrally coordinated
framework aluminum atoms.15,27 The relatively weak signals at
ca. À2 ppm indicate the existence of a limited number of
octahedrally coordinated extra framework aluminum
species.28,29 Their levels in the SM-144 and SH-12 zeolite are
however similar. Therefore, the introduction of HF does not
change the extra framework aluminum species appreciably. The
coordination environment of the Si atoms in SM-144 and SH-12
frameworks was characterized by 29Si MAS NMR spectrum
(Fig. 6b). Peaks at À113, À107, and À103 ppm are ascribed to
Si(0Al), Si(1Al), and Si(2Al), respectively.30 The distribution of
Si(nAl) does not obvious change with the addition of HF, indi-
cating that the sample SH-12 possesses almost similar coordi-
nation states of Si atoms compared to the conventional SSZ-13.
NH3–TPD curves of the hierarchical SSZ-13 zeolites and the
conventional one are shown in Fig. 7. The conventional SSZ-13
Fig. 4 SEM images of hierarchical SSZ-13 samples: (a and d) SH-6; (b
and e) SH-24; (c and f) SH-36. Scale bar: (a) 10 mm; (b–d) 1 mm; (e and f)
100 nm.
ꢀ
exhibits two ammonia desorption peaks at ca. 140 and 490 C,
corresponding to the weak and strong acid sites, respectively.31,32
ꢀ
Ammonia molecules desorbs before 300 C from the weak acid
sites. Thus, methanol as a less basic molecule cannot be able to
be converted into light olens at 450 ꢀC on these sites. This
means that weak acid sites have no evident effect on the MTO
catalytic performance of SSZ-13 catalysts.33 The high temperature
desorption peaks corresponding to the interaction of NH3
Fig. 5 N2 adsorption/desorption isotherms of conventional micro-
porous SSZ-13 and hierarchical SSZ-13 samples.
steep uptake at low partial pressure (0 < P/P0 < 0.01) is still
associate with the adsorption in the microstructure of the
hierarchical SSZ-13 zeolites, which indicates that the formation
of secondary pores does not decrease the micropore volume of
the hierarchical SSZ-13 samples. Notably, compared with SH-48,
SH-12 takes up N2 in the low pressure range and the micropore
adsorption capacity is also to be higher at high partial pressure.
The textural properties of the crystals are summarized in Table
S1.† The hierarchical SSZ-13 samples exhibit larger micropo-
rous volumes (0.27–0.28 cm3 gÀ1) than the conventional coun-
terpart (0.24 cm3 gÀ1), which mainly resulted from the fact that
a small amount of amorphous phase and/or defect zones in the
conventional SSZ-13 is easy to be attacked.24 This is in good
agreement with the increase of crystallinity based on the anal-
ysis of XRD spectra as shown in Fig. S4.† The specic surface
areas and mesoporous volumes of the hierarchical SSZ-13
samples (559–591 m2 gÀ1) are also increased, because of the
increase of external surface area. The pore-size distribution also
demonstrates the existence of mesopores centered at around
4 nm in samples SH-12 and SH-48 (Fig. S5†). The large meso-
porous volume of hierarchical SSZ-13 samples would be bene-
cial for decreasing the diffusion limitation of reactants and
products, with the corresponding effect on the MTO
performance.25,26
Fig. 6 (a) 27Al MAS NMR and (b) 29Si MAS NMR spectra of SSZ-13
samples.
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the main reaction products. Methanol conversion of the
conventional microporous SSZ-13 synthesized without adding
HF eventually decreases with the increase of TOS. The hierar-
chical SSZ-13 catalysts exhibit a remarkably longer lifetime than
conventional counterpart with only microporous structure. The
most stable catalyst is SH-12, which displays a lifetime of
485 min before the methanol conversion decreased to less than
99%. Notably, the catalytic activity of SH-48, hierarchical SSZ-13
synthesized with longer crystallization time, is better than that
of SM-144 yet worse than that of SH-12. This proves that the use
of the uoride in the synthesis of SSZ-13 zeolite catalyst with
shortened crystallization time is consistent with respect to MTO
catalytic performance.
Fig. 7 NH3–TPD of conventional microporous SSZ-13 (SM-144) and
hierarchical porous SSZ-13 (SH-12 and SH-48) catalysts.
The high selectivity of light olens is another important
benet of the use of MTO catalyst. The introduction of
secondary porosity has been proven to improve the catalytic
selectivity because of the greatly enhanced diffusion effi-
ciency.15,26,35 As shown in Fig. S6–S8,† the selectivity data with
>95% methanol conversion shows that ethylene plus propylene
selectivity of hierarchical SH-12 (85.1%) is improved around 6%
compared to the conventional microporous SM-144 (80.5%),
and slightly higher than the hierarchical SH-48 (82.2%). The
generated mesopore in the hierarchical SSZ-13 catalysts not
only enhances the mass transfer of the reagents and products
but also exposes more active sites for MTO reaction as well as
reduce the coke deposition.13,36 In addition, the decrease of
strength and concentration in strong acid sites of hierarchically
structured SH-12 can also reduce the coke deposition.
Furthermore, the decreased crystal size of the hierarchical SSZ-
13 catalysts is another important reason for prolonging the
lifetime. The smaller crystal size of sample SH-12 can shorten
the diffusion pathway of reactants and products and suppresses
the coke formation rate during the methanol conversion, and
thus prolong the catalyst lifetime and enhance the light olen
selectivity.9,37 Above results demonstrate that the improved
catalytic lifetime and light olens selectivity of the hierarchical
SSZ-13 catalysts can be attributed to the high crystallinity, the
enhanced diffusion efficiency, the increased microporous
volume and the appropriate acidity in strong acid sites of the
hierarchically structured zeolite materials.
molecules with strong acid sites are ascribed to the Brønsted acid
sites that is connected with the MTO catalytic performance.34 The
NH3–TPD proles of the hierarchical SSZ-13 samples are similar
to that of the conventional microporous SSZ-13, but the position
and intensity of the high temperature peak are slightly lower,
indicating lower strong acidity for the hierarchical SSZ-13 zeolites
compared to the conventional counterpart.
Catalytic reaction tests were carried out in a xed-bed steel
À1
ꢀ
reactor at 450 C with a WHSV of 3 h over the conventional
and hierarchical SSZ-13 catalysts. The catalytic conversions of
methanol and selectivities of the products as a function of time-
on-stream (TOS) are shown in Fig. 8 and S6–S8,† and the
detailed catalytic results are summarized in Table S2.† Obvi-
ously, the initial methanol conversion for all catalysts is
complete, and ethylene and propylene as the combined yield is
Conclusions
In summary, a simple, rapid and low-cost approach has been
developed to synthesize hierarchical SSZ-13 by one-step hydro-
thermal synthesis method using aluminum isopropoxide as
aluminum source with the assistance of HF. The as-synthesized
hierarchical SSZ-13 crystals were highly crystalline, and the
crystal growth process was completed within 12 h, whereas
144 h was necessary to synthesize a fully crystalline zeolite in
uorine free system. The hierarchical SSZ-13 catalysts showed
higher micropore volume than the conventional microporous
SSZ-13 synthesized without HF, and the assistance of HF could
lead to the increase of specic surface areas and mesoporous
volumes because of the increase of external surface area caused
by the formation of mesopores. Taking advantage of the large
micropore volume and accessible active sites, enhanced mass
Fig.
8 Methanol conversion (a) and selectivity to ethylene plus
propylene (b) with time-on-stream over SSZ-13 samples.
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transfer and reduced coke formation rate, and appropriate 15 X. Zhu, J. P. Hofmann, B. Mezari, N. Kosinov, L. Wu, Q. Qian,
´
strong acidic strength and concentration, the hierarchical SSZ-
B. M. Weckhuysen, S. Asahina, J. Ruiz-Martınez and
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´
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Conflicts of interest
There are no conicts to declare.
19 Z. Liu, T. Wakihara, K. Oshima, D. Nishioka, Y. Hotta,
S. P. Elangovan, Y. Yanaba, T. Yoshikawa, W. Chaikittisilp,
T. Matsuo, T. Takewaki and T. Okubo, Angew. Chem., Int.
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Acknowledgements
The authors gratefully acknowledge nancial support of this
work by the Foundation of State Key Laboratory of High-
efficiency Utilization of Coal and Green Chemical Engineering
(2017-K17 and 2017-K23), the Shandong Provincial Natural
Science Foundation, China (ZR2018027, ZR2019BB052), and the
opening Project of Shandong Eco-chemical Engineering
Collaborative Innovation Center (XTCXQN15).
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