Hydrothermal synthesis of nanosized ZSM-22 and their use in the catalytic conversion of methanol

Article abstract of DOI:10.1016/S1872-2067(15)61099-3

ZSM-22 zeolite with different crystal lengths was prepared using a modified hydrothermal method. Rotation speed, Si/Al molar ratio and co-solvent have important effects on the crystal size of ZSM-22. The nanosized zeolite samples were characterized by X-ray diffraction, X-ray fluorescence, nitrogen adsorption, scanning electron microscopy, temperature-programmed desorption of ammonia and solid state nuclear magnetic resonance. The catalytic performance of nanosized ZSM-22 was tested using the conversion of methanol. Compared to conventional ZSM-22, the nanosized ZSM-22 zeolite exhibited superior selectivity to ethylene and aromatics and lower selectivity to propylene. Stability against deactivation was clearly shown by the nanosized ZSM-22 zeolite. A higher external surface area and smaller particle size make this nanosized ZSM-22 zeolite attractive for catalytic applications.

Full text of DOI:10.1016/S1872-2067(15)61099-3

Chinese Journal of Catalysis 37 (2016) 1381–1388
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Hydrothermal synthesis of nanosized ZSM‐22 and their use in the
catalytic conversion of methanol
Lei Chena,b, Peng Lua,b, Yangyang Yuana, Li Xua, Xiaomin Zhanga,*, Lei Xua,#
a Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
b University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Article history:
ZSM‐22 zeolite with different crystal lengths was prepared using a modified hydrothermal method.
Rotation speed, Si/Al molar ratio and co‐solvent have important effects on the crystal size of
ZSM‐22. The nanosized zeolite samples were characterized by X‐ray diffraction, X‐ray fluorescence,
nitrogen adsorption, scanning electron microscopy, temperature‐programmed desorption of am‐
monia and solid state nuclear magnetic resonance. The catalytic performance of nanosized ZSM‐22
was tested using the conversion of methanol. Compared to conventional ZSM‐22, the nanosized
ZSM‐22 zeolite exhibited superior selectivity to ethylene and aromatics and lower selectivity to
propylene. Stability against deactivation was clearly shown by the nanosized ZSM‐22 zeolite. A
higher external surface area and smaller particle size make this nanosized ZSM‐22 zeolite attractive
for catalytic applications.
Received 29 January 2016
Accepted 15 March 2016
Published 5 August 2016
Keywords:
Nanosized ZSM‐22 zeolite
Hydrothermal synthesis
Conversion of methanol
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
zeolites have a large external surface and high surface activity.
So it is preferred where the desired catalytic reactions take
Due to their diversity of structures and unique properties,
zeolites are widely used in a variety of applications including
catalysis, sorption‐separation and ion exchange processes. The
synthesis and application of zeolites have attracted a great deal
of interest [1–4]. The excellent performance in a variety of are‐
as by zeolites depends on their structural properties such as
crystal size and chemical composition [5–11]. In order to opti‐
mize zeolite performance, the synthesis of zeolite with a novel
structure has gained much attention in the past decades.
Lately, the preparation and application of nanosized zeolites
have attracted considerable attention. Compared to conven‐
tional molecular sieves of micron size, a zeolite with a crystal
size in the nano‐scale produced significant benefits in perfor‐
mance in separation and catalysis [5,12–16]. The nanosized
place on or near the external surface of the crystal like hydroi‐
somerization of long chain paraffins [17,18]. In addition, com‐
pared to the conventional micrometer‐sized zeolites, smaller
zeolite crystals have short diffusion path lengths. Therefore, it
is favorable to use a nanosized zeolite in some reactions be‐
cause mass transfer limitation is avoided. Furthermore, it has
been shown that deactivation can be slowed down on smaller
crystals due to their larger specific external area. Currently,
many strategies have been developed to prepare nanosized
zeolite crystals. The most common method uses a clear pre‐
cursor solution with an excess of the organic template [14].
With the development of the synthesis technology of nano‐
materials, some new strategies such as confined‐space synthe‐
sis [15,16], seed‐assisted approach [19], ionothermal synthesis,
* Corresponding author. Tel: +86‐411‐84379500; E‐mail: zhangxm@dicp.ac.cn
# Corresponding author. Tel/Fax: +86‐411‐82463032; E‐mail: leixu@dicp.ac.cn
This work was supported by the National Natural Science Foundation of China (21506202).
DOI: 10.1016/S1872‐2067(15)61099‐3 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 37, No. 8, August 2016

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Lei Chen et al. / Chinese Journal of Catalysis 37 (2016) 1381–1388
microwave and sonication synthesis approaches [20] as well as
microreactor synthesis [21] have appeared. Up to now, differ‐
ent zeolite structures on the nano‐scale have been prepared
and well studied including the FAU, MFI, MEL, SOD, GIS, LTA,
BEA, AEI and CHA framework types [11,13,22–25]. In short,
nanosized zeolites are ready to play an important role in a va‐
riety of application areas, and new fields will also open up due
to their exceptional properties. So broadening the kind of na‐
nosized zeolites and the development of mild synthesis meth‐
ods is of great importance.
ZSM‐22 with a TON topology structure was first synthesized
by Dwyer et al. [26]. It features 1‐dimensional 10‐membered
ring channels and tear‐drop‐shaped pores with a diameter of
0.45 nm × 0.55 nm. ZSM‐22 exhibits excellent catalytic activity
and shape selectivity for the hydroisomerization of long chain
n‐paraffins [27], skeletal isomerization of n‐butene [28] and
toluene alkylation [29]. Due to its potential application in in‐
dustry, more interests have been focused on the synthesis of
ZSM‐22. Hydrothermal synthesis is the most commonly meth‐
od to prepare ZSM‐22. The crystal sizes of ZSM‐22 are usually
in the range of 2–15 µm [30–34]. It has been reported that the
synthesis of ZSM‐22 is usually carried out under vigorous stir‐
ring conditions (usually up to 400 r/min is recommended) in
order to prevent co‐crystallization of ZSM‐5 and cristobalite.
Because of the metastable structure of ZSM‐22, one of the big‐
gest challenges in ZSM‐22 synthesis is the narrow synthesis
range. Babav et al. [31] have reported hydrothermal synthesis
of pure ZSM‐22 under mild conditions using an autoclave, in
which the synthesis vessel was rotated horizontally. However,
the particle size of the obtained ZSM‐22 was about 1 µm, which
was still too large for use as an effective catalyst. Muraza et al.
netic stirrer. Then DAH was dissolved in deionized water and
added into the Teflon beaker under stirring, followed by stir‐
ring for 1 h. Silica sol (30% SiO2) was added to the obtained
clear solution under stirring. The resulting gel, having a molar
composition of xSiO2:Al2O3:9KOH:27DAH:3600H2O was stirred
for another 2 h to get the final gel, where x was in the range of
30–90. Then the Teflon‐lined vessel was sealed in a stainless
container and aged for 6 h at 298 K. Finally, the gel was stirred
at varying rotation speed (0, 10, 20, 40 and 50 r/min) in a ro‐
tary furnace at 433 K for 38 h. After crystallization, the auto‐
clave was quenched in cold water and white crystals were re‐
covered by filtrating and washing with deionized distilled wa‐
ter. The obtained samples were dried at 353 K overnight. The
samples then were calcined at 823 K for 24 h to remove the
template. The obtained samples were denoted as N‐ZSM‐22.
Microsized ZSM‐22 was synthesized by a similar method
except that ethanol was added as co‐solvent when dissolving
KOH and Al2(SO4)3·18H2O. The obtained samples were denoted
as M‐ZSM‐22.
The calcined zeolites (K‐N‐ZSM‐22 and K‐M‐ZSM‐22) were
ion exchanged with 1 mol/L NH4Cl three times and calcined
again to obtain H‐N‐ZSM‐22 and H‐M‐ZSM‐22.
2.2. Characterization of ZSM‐22
X‐ray diffraction (XRD) patterns were recorded by a PANa‐
lytical X’Pert Pro X‐ray diffractometer using Cu Kα radiation (λ =
1.5418 Å) at 40 kV and 40 mA. The 2θ angles were scanned
from 5° to 65°. The particle size and morphology were meas‐
ured by field emission scanning electron microscopy (FE‐SEM,
Hitachi, SU8020). The chemical composition was determined
by a Philips Magix‐601 X‐ray fluorescence spectrometer (XRF).
N2 adsorption isotherms were measured on a Micromeritics
ASAP 2020 system at 77 K. The total surface area was calculat‐
ed based on the Brunauer‐Emmett‐Teller (BET) equation. The
micropore volume and micropore surface area were evaluated
by the t‐plot method. The mesopore volume was calculated by
the BJH method.
All the solid state nuclear magnetic resonance (NMR) ex‐
periments were performed on a Bruker AvanceIII 600 spec‐
trometer equipped with a 14.1 T wide bore magnet. The reso‐
nance frequencies were 156.4 and 119.2 MHz for 27Al and 29Si,
respectively.
The acidity of H‐ZSM‐22 was determined by temperature‐
programmed desorption of ammonia (NH3‐TPD) on a chemical
adsorption instrument (Micromeritics AutoChem 2920). A 0.20
g sample was loaded in a U‐shaped reactor and pretreated at
923 K for 30 min under He atmosphere. After cooling to 373 K,
the sample was saturated with NH3, followed by purging with
He to remove physisorbed ammonia. Ammonia desorption was
carried out in a He flow (40 mL/min) by increasing the tem‐
perature from 373 to 873 K with a heating rate of 10 K/min
and monitored by a thermal conductivity detector (TCD).
[34] used
a microwave‐assisted hydrothermal synthesis
method to prepare submicrometer ZSM‐22 zeolite. The crystal
size of the obtained ZSM‐22 was in the range of 400–500 nm.
However, the crystal size was not adjustable and the synthesis
process needed a high power microwave reactor. Therefore, it
still remains a great challenge to synthesis pure nanosized
ZSM‐22 under mild hydrothermal synthesis conditions.
Here, we report the synthesis of pure nanosized ZSM‐22
crystals using a modified hydrothermal method. The influences
of the rotation speed, Si/Al molar ratio and ethanol as
co‐solvent on the crystal size were studied. The performance of
the nanosized and conventional ZSM‐22 was investigated in the
conversion of methanol.
2. Experimental
2.1. Preparation of ZSM‐22
Nanosized ZSM‐22 was synthesized by a modified tradition‐
al hydrothermal method. 1,6‐Diaminohexane (DAH) was used
as the structure directing agent, and silica sol and aluminum
sulfate were used as silicon source and aluminum source, re‐
spectively. A typical hydrothermal synthesis procedure of the
pure nanosized ZSM‐22 zeolite was carried out as follows. In a
Teflon beaker, a clear solution was prepared by mixing KOH,
Al2(SO4)3·18H2O and deionized water under stirring by a mag‐
2.3. Catalytic tests and product analysis
The ZSM‐22 sample was pressed and sieved to 20–40 mesh

Lei Chen et al. / Chinese Journal of Catalysis 37 (2016) 1381–1388
1383
and then loaded using 1 g of sample into a fixed‐bed reactor
with an inner diameter of 10 mm and a bed height of 40 mm.
The temperature of the fixed‐bed reactor was increased from
298 to 823 K with a heating rate of 15 K/min and then kept at
823 K for 60 min to activate the catalyst. After the catalyst was
activated, the temperature was then cooled to the reaction
temperature.
Methanol was fed into the reactor by a high performance
liquid chromatography infusion pump and vaporized. Then by
passing a carrier gas (He), methanol vapor was fed into the
catalyst bed and reacted under the catalysis of the catalyst. The
product was kept at 493 K and analyzed by gas chromatog‐
raphy (GC) online on an Agilent 7890A system with an FID
detector equipped with a CP WAX capillary column and a TCD
detector equipped with a PLOT Q capillary column. The con‐
version and selectivity were calculated on a CH2 basis. Dimethyl
ether (DME) was considered as reactant in the calculation.
Methanol conversion (X) and product selectivity (S) were cal‐
culated by the following equations:
(5)
(4)
(3)
(2)
(1)
10
20
30
40
50
2/(o )
Fig. 1. XRD patterns of N‐ZSM‐22 with Si/Al ratio of 60 synthesized
under varying rotation speeds (r/min): (1) 0; (2) 10; (3) 20; (4) 40; (5)
50.
zeolite includes two parts: the formation of a crystal nucleus
and growth of the crystal nucleus. In order to obtain a pure
zeolite, a certain rotation speed was needed to ensure the ho‐
mogenous mixing of the synthesis gel and to promote the for‐
mation of crystal nucleus. However, the growth space will be
restricted because of the aggregation of crystal nucleus, thus
limiting the size of the resulting crystal grain. When the rota‐
tion speed is slow during the growth process, the crystal nuclei
of N‐ZSM‐22 do not aggregate very well, so the resulting crystal
size of N‐ZSM‐22 is larger. When the rotation speed is faster,
the crystal nuclei of N‐ZSM‐22 aggregated more, limiting the
growth space of the crystal nucleus. However, the originally
formed crystal nucleus aggregation is broken due to the larger
external forces, and this resulted in the secondary growth of
the crystal particle during the process of crystal nucleus
growth; hence, the particle size is larger. That is to say, in order
to get a smaller particle size, a moderate rotation speed by the
present synthesis method was needed.
X = (n(fed of MeOH) – n(unreacted MeOH))/n(fed of MeOH)  100%
S = n(carbon atom in a product)/(n(fed of MeOH) – n(unreacted MeOH))  100%
3. Results and discussion
3.1. Synthesis of nanosized ZSM‐22 zeolite
3.1.1. Effect of rotation speed
It is important to synthesize a pure zeolite for many applica‐
tions. ZSM‐22 has a metastable crystal structure, and the syn‐
thesis conditions are harsh. It has been reported that high
speed (usually up to 400 r/min) mixing of the synthesis gel
during the hydrothermal synthesis process is required in order
to prevent generating impurities of ZSM‐5 and cristobalite [31].
It still remains a great challenge to synthesize pure ZSM‐22, not
to mention nanosized ZSM‐22.
First of all, N‐ZSM‐22 with a Si/Al molar ratio of 60 was
synthesized under different rotation speeds. The XRD patterns
of the samples are shown in Fig. 1. It was found that an amor‐
phous phase was formed under static synthesis conditions sim‐
ilar to the findings of others. During the synthesis of N‐ZSM‐22
in the rotating oven under varying rotation speed, the XRD
patterns of the product showed diffraction peaks characteristic
of ZSM‐22 zeolite with the TON topology. No other peak was
observed suggesting pure N‐ZSM‐22. It has been reported that
the synthesis of pure ZSM‐22 depends mostly on the homoge‐
neous mixing of the synthesis gel during the hydrothermal
synthesis process. Thus, this result suggested that the present
preparation method in a rotating oven at a relatively slow rota‐
tion speed could effectively mix the synthesis gel homogene‐
ously and give phase‐pure ZSM‐22.
3.1.2. Effect of Si/Al ratio
A series of nanosized zeolite with varying Si/Al ratios were
also prepared under the rotation speed of 20 r/min. The effect
(a)
(c)
(b)
200 nm
200 nm
200 nm
200 nm
(d)
In order to investigate the morphology and crystal size of
the obtained ZSM‐22, SEM was used to characterize the zeolite.
As displayed in Fig. 2, N‐ZSM‐22 possessed a uniform nee‐
dle‐like morphology, typical of ZSM‐22, with a particle length
ranging from 150 to 250 nm. These N‐ZSM‐22 particles were
much shorter than those previously reported, which were syn‐
thesized under more severe conditions. The crystallization of a
Fig. 2. SEM images of N‐ZSM‐22 with Si/Al ratio of 60 synthesized un‐
der varying rotation speeds (r/min): (a) 10; (b) 20; (c) 40; (d) 50.

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Lei Chen et al. / Chinese Journal of Catalysis 37 (2016) 1381–1388
(6)
(6)
(5)
(4)
(5)
(4)
(3)
(2)
(1)
(3)
(2)
(1)
0.0
0.2
0.4
0.6
0.8
1.0
10
20
30
2/(o )
40
50
Relative pressure (p/p0)
Fig. 5. N2 adsorption isotherms of N‐ZSM‐22 with varying Si/Al ratios:
Fig. 3. XRD patterns of N‐ZSM‐22 with varying Si/Al ratios: (1) 30; (2)
(1) 30; (2) 45; (3) 60; (4) 75; (5) 90. (6) M‐ZSM‐22 with Si/Al ratio of
90.
45; (3) 60; (4) 75; (5) 90. (6) M‐ZSM‐22 with Si/Al ratio of 90.
of Si/Al ratio on the crystal size of nanosized ZSM‐22 was in‐
vestigated. N‐ZSM‐22 was synthesized at a molar ratio of xSiO2:
Al2O3:9KOH:27DAH:3600H2O at 433 K for 38 h, where x was
varied from 30 to 90. The resulting XRD patterns are shown in
Fig. 3. As shown, no other peak was observed for the N‐ZSM‐22
samples with the Si/Al ratio from 30 to 90, indicating a high
purity of the products.
The SEM images of N‐ZSM‐22 with different Si/Al ratios are
shown in Fig. 4. It has been reported for ZSM‐22 that the typical
morphology of TON‐type zeolite was needle‐like crystallites
with a length of about 2–15 µm [26,27,30,31,33,34]. According
to the SEM images, all of the N‐ZSM‐22 crystals exhibited a
needle‐like morphology, and the crystal length of individual
needles varied from 150 to 800 nm. The crystal length of
N‐ZSM‐22 decreased from 800 to 150 nm with the increase of
the Si/Al ratio from 30 to 60. Conversely, the crystal length
increased from 150 to 650 nm when the Si/Al ratio continued
to increase to 90. It has been reported that the Si/Al ratio af‐
fected the crystal size and morphology of ZSM‐5 and ZSM‐23
[35,36]. Shirazi et al. [35] found that by increasing the alumi‐
num content of ZSM‐5, the surface areas increased and the
crystal size decreased. Liu et al. [36] found that the Si/Al ratio
affected the ZSM‐23 morphology. In this study, it was clear that
the Si/Al ratio had a large effect on the crystal size of ZSM‐22.
N2 adsorption isotherms were used to estimate the porosity
of the samples. Typical isotherms with a sharp uptake at a rela‐
tive pressure p/p0 above 0.01 are shown in Fig. 5. This indicat‐
ed a characteristic micropore framework of N‐ZSM‐22. The
detailed textural properties of the samples are listed in Table 1.
Combined with the results above, some trends were observed
for the data in Table 1. The external surface areas increased
gradually with decreasing crystal size, which is in accordance
with the reported results. The Si/Al ratio obtained from the
XRF method was lower than in the original precursor gel, sug‐
gesting that part of the Si atoms were not incorporated into the
(b)
(a)
(e)
200 nm
500 nm
500 nm
(d)
(c)
Figure 4
5 µm
500 nm
Fig. 4. SEM images of N‐ZSM‐22 with varying Si/Al ratios: (a) 30; (b) 45; (c) 75; (d) 90. (e) M‐ZSM‐22 with starting Si/Al ratio of 90.

Lei Chen et al. / Chinese Journal of Catalysis 37 (2016) 1381–1388
1385
Table 1
Surface area, pore volume and XRF results of ZSM‐22 with different Si/Al ratios.
Original Si/Al
ratio
Surface area (m2/g)
Microporeb External
Pore volume (cm3/g)
Si/Al ratio
by XRF
Crystal size
(μm)
Sample
BETa
150.6
183.4
235.1
206.3
209.7
184.6
Totalc
0.17
0.33
0.25
0.26
0.21
0.14
Microporeb
Mesopore
0.12
0.29
N‐ZSM‐22
N‐ZSM‐22
N‐ZSM‐22
N‐ZSM‐22
N‐ZSM‐22
M‐ZSM‐22
30
45
60
75
90
90
104.4
94.9
46.3
88.5
70.1
60.4
51.3
39.3
0.80
0.20
0.15
0.20
0.65
7.20
0.05
0.04
30.8
39.7
51.3
55.9
66.9
74.2
165.0
145.9
158.4
145.3
0.08
0.07
0.07
0.07
0.17
0.19
0.14
0.07
a BET method. b t‐plot method. c Volume adsorbed at p/p0 = 0.99.
framework of the zeolite. To confirm the coordination envi‐
ronment of the Al and Si atoms on a molecular level, 27Al MAS
NMR and 29Si MAS NMR were performed for all the N‐ZSM‐22
samples. The spectra are shown in Fig. 6. In the 27Al MAS NMR
spectra of N‐ZSM‐22, two signals at δ = 55 and 0 were ob‐
served, which were assigned to framework Al species in the
tetrahedral coordination state and extra‐framework octahedral
Al species, respectively. The intensity of the signal at δ = 0 de‐
creased gradually with increasing Si/Al ratios. This result sug‐
gested that the amount of extra‐framework octahedral Al spe‐
cies decreased with increasing Si/Al ratio. The signals with
chemical shifts at δ = –112, –109, –106, –102 and –96 in the 29Si
MAS NMR spectra were ascribed to Si(0Al), Si(1Al), Si(OH) and
Si(OH)2 species, respectively.
(6)
(5)
(4)
(3)
(2)
(1)
473
673
Temperature (K)
873
973
573
773
NH3‐TPD was used to evaluate the acidity of the nanosized
ZSM‐22 samples with different crystal sizes. As is shown in Fig.
7, all the samples exhibited two desorption peaks: a low‐tem‐
perature peak at 480 K and a high‐temperature peak at 713 K,
corresponding to weak acid sites and strong acid sites, respec‐
tively. With increasing Si/Al ratio, there were slight shifts to‐
ward lower temperature, suggesting a decreased strength of
the strong and weak acid sites caused by the reduction of Al
atoms in the framework of ZSM‐22. The results discussed
above showed that the Si/Al ratio of the initial gel mixture has a
significant effect on the physical and chemical properties of the
N‐ZSM‐22 samples.
Fig. 7. NH3‐TPD profiles of N‐ZSM‐22 with varying Si/Al ratios: (1) 30;
(2) 45; (3) 60; (4) 75; (5) 90. (6) M‐ZSM‐22 with Si/Al ratio of 90.
such as rotation speed and composition of starting gel, organic
solvents like alcohols and diols have also been reported as a
promising alternative to control the morphology and crystal
size of zeolites [37]. Here, ethanol was used in the synthesis
mixture of the zeolite for the purpose of tuning the crystal
morphology and crystal size of ZSM‐22.
The XRD pattern of the obtained M‐ZSM‐22 sample is shown
in Fig. 3(6). A phase‐pure ZSM‐22 was still obtained, suggesting
that adding ethanol to the starting gel did not change the for‐
mation of the zeolite crystal phase. The morphologies and
crystal sizes of the sample were characterized by SEM. The SEM
image is shown in Fig. 4(e). Clearly, the sample consisted of
3.1.3. Effect of ethanol
Several techniques have been used to control the morphol‐
ogy and crystal size of zeolites. Beside the synthesis conditions
55
(b)
(a)
-109
-106
-112
0
(5)
(4)
-96
-102
(5)
(4)
(3)
(2)
(3)
(2)
(1)
(1)
120 100 80 60 40 20
0
-20 -40
-80
-90 -100 -110 -120 -130 -140 -150


Fig. 6. 27Al MAS NMR (a) and 29Si MAS NMR (b) spectra of N‐ZSM‐22 samples with different Si/Al ratios: (1) 30; (2) 45; (3) 60; (4) 75; (5) 90.

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Lei Chen et al. / Chinese Journal of Catalysis 37 (2016) 1381–1388
needle‐shaped crystals with crystal size of 7.2 μm. Compared to
nanosized ZSM‐22, the addition of ethanol effectively induced
the elongation of ZSM‐22 crystals to form bigger particles. The
N2 adsorption isotherm of M‐ZSM‐22 is shown in Fig. 5(6) and
the textural properties of the sample are listed in Table 1.
Compared to M‐ZSM‐22, the larger external surface area of
N‐ZSM‐22 benefits a high surface activity. The acid property of
M‐ZSM‐22 was also characterized by NH3‐TPD. The high‐tem‐
perature and low‐temperature desorption peaks were shifted
down to lower temperatures for M‐ZSM‐22, indicating a de‐
creased strength of the strong acid sites and weak acid sites
with increasing Si/Al ratio.
M‐ZSM‐22. The selectivity to ethylene and aromatics over
N‐ZSM‐22 was higher than that over M‐ZSM‐22. It was found
that the initial low but detectable production of olefins over
both zeolites was catalyzed by external and/or pore mouth acid
sites by the hydrocarbon pool mechanism [40]. Therefore, the
larger external surface area of N‐ZSM‐22 benefited a higher
selectivity of ethylene and aromatics.
The effects of Si/Al ratio of the ZSM‐22 zeolite on methanol
conversion was also investigated (Fig. 8(a)). Considering the
more extra‐framework octahedral Al species in the N‐ZSM‐22
samples with Si/Al ratios of 30 and 45 (Fig. 6), the catalytic
performance of N‐ZSM‐22 with Si/Al ratios of 60, 75 and 90
was investigated. As shown in Fig. 8(a), methanol was com‐
pletely converted to hydrocarbons at the beginning of the reac‐
tion for all the nanosized ZSM‐22 zeolite. It has been reported
that with the decrease of Si/Al, the acid strength of the zeolite
increased and the zeolite deactivated faster. However,
N‐ZSM‐22 with Si/Al ratios of 90 deactivated faster than
N‐ZSM‐22 with Si/Al ratios of 75 and 60. The probably reason
was the larger crystal size of N‐ZSM‐22 with Si/Al ratio of 90,
which is in accordance with the above results. The product
distribution of N‐ZSM‐22 with different Si/Al ratios in the con‐
version of methanol are shown in Fig. 8(b). As shown, the se‐
lectivity of light olefins including ethylene and propylene in‐
creased with increasing of Si/Al ratio, while the selectivity of
aromatics decreased. The results discussed above suggested
that the higher Si/Al ratio facilitated the formation of light ole‐
fins and prevented the formation of aromatics.
3.2. Catalytic performance
Due to the unique one‐dimensional pore system and small
pore opening size, ZSM‐22 has been reported to be useful in the
conversion of methanol to olefins (MTO) [38,39]. The crystal
size was reported to have important effects on the conversion
of methanol for SAPO‐34 and ZSM‐5. However, there are few
reports yet regarding the effect of ZSM‐22 crystal size on the
conversion of methanol. Here, the prepared nanosized and
microsized ZSM‐22 samples were tested in the conversion of
methanol.
Fig. 8 presents the effects of zeolite crystal size on methanol
conversion and product distribution with time on stream
(TOS). As shown in Fig. 8(a), M‐ZSM‐22 with the larger crystal
size deactivated faster than N‐ZSM‐22. The conversion of
methanol declined with TOS due to coke formation on the acid
sites located on the pore mouths and external surface [40]. In
addition, the enhancement of reactant and product diffusion in
the nanosized ZSM‐22 also benefited a longer catalyst lifetime.
Thus, the large external surface area and smaller crystal size
effectively slowed down the deactivation of the catalyst as pre‐
viously reported.
4. Conclusions
Nanosized ZSM‐22 with various Si/Al ratios was synthe‐
sized by a modified hydrothermal method using a relatively
low rotation speed. The nano‐crystal structure of ZSM‐22 was
confirmed by XRD and SEM. Through tuning the rotation speed
and Si/Al ratio, nanosized ZSM‐22 with crystal size in the range
of 150–800 nm was synthesized. Ethanol as co‐solvent used in
the precursor gel induced the elongation of the ZSM‐22 crystals
to form bigger particles. The catalytic performance of the
ZSM‐22 zeolite samples was tested with the conversion of
At the beginning of the reaction, methanol was completely
converted to hydrocarbons over N‐ZSM‐22 and M‐ZSM‐22.
Light olefins, which included ethylene, propylene and butylene,
were the major products. The total selectivity to ethylene and
propylene was almost the same for both N‐ZSM‐22 and
(b)
(a)
100
95
90
85
80
75
70
25
(2)
(1)
(3)
20
(3)
(2)
(1)
15
10
5
(4)
(4)
0
C2H4 C3H6
C4
C5+ Aromatics Alkanes
0
30
60
90
120
150
180
210
TOS (min)
Fig. 8. (a) Methanol conversion as a function of time on stream (TOS) at 753 K with WHSV of 1.60 h–1 and (b) the product distribution at 10 min on
N‐ZSM‐22 with Si/Al ratios of (1) 60, (2) 75, and (3) 90 and (4) M‐ZSM‐22 with Si/Al ratio of 90.

Lei Chen et al. / Chinese Journal of Catalysis 37 (2016) 1381–1388
1387
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alytic applications.
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Graphical Abstract
Chin. J. Catal., 2016, 37: 1381–1388 doi: 10.1016/S1872‐2067(15)61099‐3
Hydrothermal synthesis of nanosized ZSM‐22 and their use in the catalytic conversion of methanol
Lei Chen, Peng Lu, Yangyang Yuan, Li Xu, Xiaomin Zhang*, Lei Xu*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences; University of Chinese Academy of Sciences
100
95
90
Nanosized
ZSM-22
MTH
0.2 µm
2 µm
85
80
75
70
Conventional
ZSM-22
0
30
60
90
120
150
180
210
Time on stream (min)
Different crystal length ZSM‐22 zeolite samples were prepared using a modified hydrothermal synthesis method. Nanosized ZSM‐22
showed better catalytic stability in the conversion of methanol.

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