Size-Controlled Synthesis of Metallosilicates with MTW Structure and Catalytic Performance for Methanol-to-Propylene Reaction

Article abstract of DOI:10.1007/s10562-015-1683-4

The synthesis of metallosilicates with a MTW structure was achieved using tetraethylammonium bromide (TEABr) and methyltriethylammonium chloride (MTEACl) as organic structure-directing agents (OSDAs). MTW zeolites (Al-MTW, Si-MTW, Fe-MTW and (Al, Fe)-MTW) were obtained by hydrothermal synthesis. The crystal size of the resulting MTW zeolite strongly depended on the type of OSDA, in which the column-like and agglomerated nano-sized crystals could be produced by using MTEACl and TEABr, respectively as an OSDA. Isomorphous substitution of Fe for Si was demonstrated by UV-vis spectra and ac-NH₃-TPD profiles. These analyses indicated that Fe atoms were incorporated into the lattice framework and that almost all of the Fe species were well-dispersed without forming bulky iron oxide particles. The ac-NH₃-TPD profiles showed that the acid strength of the Fe-MTW zeolite was weaker than that of the Al-MTW zeolite. Al-MTW zeolites with different crystal sizes were applied to methanol-to-propylene reactions. The nano-sized Al-MTW zeolite exhibited a longer catalyst life time compared to the macro-sized Al-MTW zeolite. Since decreasing the acid strength of the MTW zeolite inhibited excessive reactions followed by the formation of aromatics, the (Al, Fe)-MTW zeolite exhibited higher yields of propylene and butenes and a lower yield of aromatics compared with the Al-MTW zeolite.

Full text of DOI:10.1007/s10562-015-1683-4

Catal Lett
DOI 10.1007/s10562-015-1683-4
Size-Controlled Synthesis of Metallosilicates with MTW Structure
and Catalytic Performance for Methanol-to-Propylene Reaction
1
1
1
2
•
Taichi Taniguchi
1
Takao Masuda
•
Yuta Nakasaka
•
Keitaro Yoneta
•
Teruoki Tago
Received: 19 November 2015 / Accepted: 13 December 2015
Ó Springer Science+Business Media New York 2016
Abstract The synthesis of metallosilicates with a MTW
structure was achieved using tetraethylammonium bromide
excessive reactions followed by the formation of aromatics,
the (Al, Fe)-MTW zeolite exhibited higher yields of
propylene and butenes and a lower yield of aromatics
compared with the Al-MTW zeolite.
(
(
TEABr)
and
methyltriethylammonium
chloride
MTEACl) as organic structure-directing agents (OSDAs).
MTW zeolites (Al-MTW, Si-MTW, Fe-MTW and (Al,
Fe)-MTW) were obtained by hydrothermal synthesis. The
crystal size of the resulting MTW zeolite strongly depen-
ded on the type of OSDA, in which the column-like and
agglomerated nano-sized crystals could be produced by
using MTEACl and TEABr, respectively as an OSDA.
Isomorphous substitution of Fe for Si was demonstrated by
Graphical Abstract
MTW zeolite
H C OH
3
etc.
Methanol
723 K
Propylene
Butenes
Aromaꢀcs
Crystal size-controlled
UV–vis spectra and ac-NH -TPD profiles. These analyses
3
1
µm
100 nm
indicated that Fe atoms were incorporated into the lattice
framework and that almost all of the Fe species were well-
dispersed without forming bulky iron oxide particles. The
Short catalyst life ꢀme
Long catalyst life ꢀme
Acidity-controlled
H δ+
H δ+
O
H δ+
O
ac-NH -TPD profiles showed that the acid strength of the
3
O
O
O
O
O
O
O
O
O
O
δ-
Si
Alδ- Si
Si
Si
Si
Fe δ-
Si
Fe-MTW zeolite was weaker than that of the Al-MTW
zeolite. Al-MTW zeolites with different crystal sizes were
applied to methanol-to-propylene reactions. The nano-
sized Al-MTW zeolite exhibited a longer catalyst life time
compared to the macro-sized Al-MTW zeolite. Since
decreasing the acid strength of the MTW zeolite inhibited
O
O
O
Selecꢀve producꢀon of propylene and butenes from methanol
Keywords Size-controlled synthesis Á Metallosilicate Á
MTW zeolite Á Methanol to propylene reaction
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-015-1683-4) contains supplementary
material, which is available to authorized users.
1
Introduction
&
Yuta Nakasaka
nakasaka@eng.hokudai.ac.jp
Zeolites, crystalline microporous aluminosilicates linked
through oxygen atoms, possess three-dimensionally con-
nected framework structures constructed from corner
sharing TO4 tetrahedra, where T is any tetrahedrally
coordinated cation such as Si or Al. These framework
structures are composed of n-rings, where n is the number
of T-atoms in the ring (e.g., 4-, 5-, or 6-rings), and large
1
Research Group of Chemical Engineering, Division of
Applied Chemistry, Faculty of Engineering, Hokkaido
University, N13 W8, Kita-ku, Sapporo 060-8628, Japan
2
Department of Chemical Engineering, Graduate School of
Science and Engineering, Tokyo Institute of Technology,
2
-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
123

T. Taniguchi et al.
pore openings of 8-, 10-, and 12- rings that are framed by
these small rings. Because zeolites have micropore of a
specific diameter almost equal to a diameter of lighter
hydrocarbons, zeolites exhibit a remarkable molecular
sieve effect for these hydrocarbons. Therefore, zeolites
have been widely used as catalysts and adsorbents. Cur-
rently, 229 types of zeolites and related structures are
recognized [1].
for cyclization reaction by which higher alkenes can be
converted into aromatics [14, 30].
As described above, the suppression of the alkenes
cyclization followed by the formation of aromatics is a
desirable approach to increase propylene selectivity. MTW
zeolite, which was first reported by Rosinski and Rubin in
1974 [31], possesses one-dimensional 12-ring pores
˚
(5.6 9 6.0 A), so MTW zeolite provides pore space for the
Recently, methanol to olefins and propylene (MTO and
MTP, respectively) reaction using zeolite catalysts has
attracted much attention because of their potential as
alternative processes for the production of the lower olefins
from non-petroleum sources [2–5]. MFI zeolites have been
widely used for methanol conversion reactions [6–9]. In the
production of olefins from methanol, the initial step is
dehydration of the methanol to dimethyl ether, followed by
the production of light olefins including ethylene and
propylene. Moreover, butenes, higher alkenes (C5?
hydrocarbons), paraffin, and aromatics are usually obtained
reactive hydrocarbon-pool intermediates in the MTO
reaction. Although the pore size of MTW zeolite is larger
than that of MFI zeolite, it is smaller than the critical
dimension of channel intersection for MFI zeolite. The
non-intersecting one-dimensional structure of MTW zeolite
provides insufficient volume for cyclization reaction,
which is expected to suppress cyclization reactions, leading
to high propylene selectivity. In contrast, although iso-
morphous substitution of other hetero atoms in the MTW
zeolite framework [32] has been reported by Zhi et al.,
there are few reports concerning methanol-to-olefins reac-
tions over MTW zeolite [17, 33].
[
10]. In the detailed reaction mechanism, the hydrocarbon-
pool mechanism proposed initially by Dahl and Kolboe
11, 12], and Olsbye et al. suggested the dual-cycle concept
In this study, the substitution of Fe for Al in the MTW
structure was demonstrated using TEABr and MTEACl as
organic structure-directing agents (OSDAs). Metallosili-
cates with MTW structures (Al-, Si-, Fe-, and (Al, Fe)-
MTW zeolites) could be obtained using these OSDAs. We
found that the crystal size of the MTW metallosilicate
depended on the type of OSDA, and nano-crystalline MTW
zeolite could be obtained using TEABr. These zeolites
were applied to the methanol-to-propylene (MTP) reaction
as catalysts, and the effects of the crystal size and physico-
chemical properties of the prepared MTW zeolites on
catalytic stability and product selectivity during the MTP
reaction was examined.
[
consisting of a methyl aromatics route and an olefins
methylation/cracking route [13–15].
In this model, propylene and butenes are obtained from
the olefins methylation/cracking route, whereas ethylene
and aromatics are formed from the methyl aromatics route.
Moreover, the aromatics are formed through cyclization of
the higher alkenes formed in the olefins methylation/
cracking route [16]. Since propylene is mainly formed from
the olefins methylation/cracking route, suppression of
alkenes cyclization followed by the formation of aromatics
leads to high propylene selectivity.
Wang et al. reported that the MTO reaction is a space-
demanding process and the size of key reactive hydrocar-
bon-pool intermediates for the MTO process is between 5.7
2 Experimental
˚
and 6.0 A [17]. Because the channel intersection of MFI
zeolite provides sufficient volume for the intermediates,
MFI zeolite is a promising candidate for olefins production
from methanol. However, its strong acidity leads to
cyclization and intermolecular hydride transfer reactions to
form paraffin and aromatics [14, 18–20]. In order to
increase the yield of propylene from methanol, suppression
of cyclization of intermediates is necessary. It is well
known that the strength of Brønsted acid sites can be
modulated by isomorphic substitution of Si for trivalent
atoms other than Al [21, 22]. The substitution of Al for Fe
in the zeolite framework (e.g., LTA, MFI, and MOR zeo-
lites) has been reported [23–28] and highly selective pro-
duction of light olefins from methanol was achieved using
Fe containing MFI zeolite (Fe-MFI) [26–28]. On the other
hand, since the channel intersection for MFI have a critical
2.1 Synthesis of Metallosilicates with MTW
Structure
Raw materials used included the following: colloidal silica
(Sigma-Aldrich, Co., LLC.) as a Si source, and sodium
aluminate (Wako Pure Chemical Industries, Ltd.) and/or
iron (III) nitrate enneahydrate (Wako Pure Chemical
Industries, Ltd.) as T (T = Al and/or Fe, respectively)
sources. Sodium hydroxide (Wako Pure Chemical Indus-
tries, Ltd.) was used as an alkali source. A variety of
OSDAs, including alkylammonium ions, alkylamines, and
nitrogen-containing polymers, direct the crystallization of
the MTW zeolite, but the most commonly used OSDAs
include tetraethyl and methyltriethylammonium cations,
TEA and MTEA, respectively [34]. In this study,
tetraethylammonium bromide (TEABr, Wako Pure
˚
dimension of nearly 9.0 A [29], it provides enough volume
1
23

Size-Controlled Synthesis of Metallosilicates with MTW Structure and Catalytic Performance…
-
1
Chemical Industries, Ltd.) or triethylmethylammonium
chloride (MTEACl, Tokyo Chemical Industry Co., Ltd.)
was used as an OSDAs source.
catalyst [g] to the methanol feed rate [g h ] was in the
range from 0.1 to 0.2 h. The composition of the exit gas
was measured using on-line gas chromatography (GC-
2014, Shimadzu Co., Ltd.) using a Porapak-Q column with
a thermal conductivity detector (TCD), and a Gaskuropack
54 and SP-1700 column with a flame ionization detector
(FID). The conversion of each reaction was calculated
based on the amount of methanol transformed to reaction
products, excluding dimethyl ether. The amount of coke
deposited on each catalyst after the MTP reaction was
measured by thermogravimetry (TGA-50, Shimadzu Co.,
Ltd.).
A series of Al-, Si-, Fe-, and (Al, Fe)-MTW zeolites
with different Si/T ratios (T = Al and/or Fe) was prepared
by hydrothermal synthesis under static conditions. An
aqueous solution containing Si, T sources and OSDA was
prepared. The synthetic gel composition was 10 Na O: x
2
OSDA: 100 SiO : y T O : 5555 H O, where (x,y) = (20,0)
2
2
2
3
to (60,1). The (x,y) value was adjusted depending on the
amount of OSDA and T source, respectively, added to the
synthetic gel. Scheme 1 shows the synthetic procedure of
MTW zeolites. The order of Si and T source addition is
different in synthetic procedure A from that of B.
For both procedures, the synthetic gel was transferred to
a Teflon-sealed stainless steel bottle, and hydrothermal
treatment was carried out at 423 K for a designated period
in an oven under static conditions with autogenous pres-
sure. After hydrothermal treatment, the product was sepa-
rated from the liquid by centrifugation, followed by
washing of the solid with distilled water and 2-propanol
and then drying overnight at 383 K. The OSDA was
removed by calcination in a muffle furnace at 823 K for
3 Results and Discussion
3.1 Synthesis of Al-MTW Zeolites and Effects of Gel
Compositions
Table 1 shows the compositions of the synthetic gels and
the characteristics of the products. Synthetic procedure A
was employed in preparation of Al-MTW zeolites. Fig-
ures 1 and 2 show X-ray diffraction patterns and SEM
photographs of the prepared samples at different Si/Al ratio
using TEABr (Al01, Al02, Al03) and MTEACl (Al04,
Al05, Al06) as OSDAs, respectively. The hydrothermal
time was 6 days. As shown in Fig. 1, all prepared samples
showed peaks corresponding to a MTW zeolite structure.
Though the BET areas of these samples showed almost the
1
2 h in air.
2
.2 Characterization of Metallosilicate with MTW
Structure
The crystallinity and phase purity of each zeolite crystal
was examined using X-ray diffract meter (XRD, JEOL
JDX). The morphology of the samples was observed using
a field-emission scanning electron microscope (FE-SEM,
JEOL JSM-6500F). BET area was measured using nitrogen
adsorption at 77 K (BEL Japan, Inc., Belsorp mini). The
composition of the samples (Si/T atomic ratio) was deter-
mined using X-ray fluorescence analysis (XRF, Rigaku
Supermini 200). Acidity was determined from ac-NH3-
TPD profiles [35]. UV–vis diffuse reflectance spectra were
measured using a JASCO V-670 unit, in the wavelength
range from 200 to 700 nm.
2
same values of approximately 330 m /g, the XRD pattern
of the prepared samples depended on the type of OSDA.
The MTW zeolite prepared using TEABr (Al01, Al02,
Al03) exhibited an XRD pattern with a larger half-width
compared to that of the samples prepared using MTEACl
(
Al04, Al05, Al06), indicating that the crystal morphology
is deferent.
The effect of the type of OSDA on the crystal size and
morphology of the prepared MTW zeolites is shown in
Fig. 2. The crystal size of the prepared MTW zeolites
depended strongly on the type of OSDA, regardless of the
Si/Al ratio. When using MTEACl, columnar grain growth
on a macro order was observed. In contrast, the use of
TEABr resulted in agglomeration of small crystals of ca.
2
.3 Methanol-to-Propylene Reaction
50 nm in diameter.
The MTW zeolite powders used for the MTP reaction were
pelletized, crushed and sieved to yield samples ca. 300 lm
in diameter. The Si/T ratio of samples used for the MTP
reaction was 100. The MTP reaction was carried out using
a fixed bed reactor under a N2 stream at 723 K and
atmospheric pressure. The methanol was introduced to the
reactor using syringe pump. The introduced methanol was
vaporized by passage of preheater at 423 K. The partial
pressure of methanol was 23.5 kPa. The weight ratio of the
The difference in crystal morphology was attributed to
different nucleation and growth rates of the zeolite crystal
during hydrothermal synthesis. To investigate this in detail,
changes in the crystallinity of MTW zeolites with
hydrothermal time were examined. The intensity of the
XRD pattern increased with hydrothermal time. After
6 days, well-crystallized MTW zeolites were obtained, and
no further improvement in the pattern was observed.
Accordingly, this sample was used as a standard to
123

T. Taniguchi et al.
NaOHaq + OSDA aq
T source
Al source
Si source
(
sodium aluminate)
(
colloidal silica)
Fe source
iron (III) nitrate enneahydrate)
(
T source
Al source
(
sodium aluminate)
Si source
colloidal silica)
Fe source
iron (III) nitrate enneahydrate)
(
(
Hydrothermal treatment: 423K 1~10 days
Procedure A
Procedure B
Scheme 1 Synthetic procedure of MTW zeolites
calculate the crystallinity of all synthesized samples. Fig-
ure 3 shows the changes in crystallinity of the MTW
zeolites with synthesis time. In MTEACl, the peaks cor-
responding to MTW zeolite appeared after 3 days, and the
crystallinity gradually increased with hydrothermal time. In
TEABr, a long induction period of approximately 4 days
was required to form zeolite nuclei, and the crystallinity
increased dramatically to reach almost 100 % for 1 day,
considering that the smaller crystals but higher in the
amount of nuclei is reason for longer induction period,
leading to formation of nanocrystal. When using MTEACl,
nucleation occurred at an initial stage, followed by crystal
growth. Because crystal growth occurred mainly during
hydrothermal synthesis, columnar macro-sized MTW
zeolites could be obtained. In contrast, when using TEABr,
a burst of nucleation occurred after a long induction period,
which caused rapid crystallization of MTW zeolites that
produced only nano-sized MTW zeolites.
an OSDA/Si ratio of 0.2. In contrast, well-crystallized Al-
MTW zeolites were obtained at an OSDA/Si ratio of 0.2 (see
Table 1; Fig. 1). The presence of aluminum in a synthetic
gel stabilizes the OSDAs during hydrothermal treatment [36,
37]. In the synthesis of metallosilicates with a MTW
structure, the OSDAs were not stable without aluminum,
leading to the formation of an amorphous phase at low
OSDA/Si ratios. Therefore, greater OSDA/Si ratios were
required to obtain pure Si-MTW zeolite compared to Al-
MTW zeolite. During the preparation of Fe-MTW zeolite, a
high OSDA/Si ratio was employed to achieve greater crys-
tallinity of the MTW phase.
In the preparation of Fe-MTW zeolites, the effect of the
synthetic procedure on crystal structure and the incorpo-
ration of Fe atoms was examined. Fe-MTW zeolites were
prepared at a Si/Fe ratio of 100 by using TEABr and
employing synthesis procedures A and B; the products are
denoted as samples Fe01 and Fe02, respectively. The XRD
patterns of Fe01 and Fe02 exhibited the typical MTW
structure. However, the XRD patterns did not provide
conclusive evidence of isomorphous substitution of Fe for
Si in the MTW framework.
3
.2 Synthesis of Metallosilicate with MTW
Structure
The synthesis of metallosilicates with a MTW structure
Si-MTW, Fe-MTW, and (Al, Fe)-MTW) was conducted
The color of the prepared sample acts as a simple
indicator of whether bulk iron oxide exists in the sample
[38]. The color of the calcined samples Fe01 and Fe02
were brown and completely white, respectively. The brown
color of sample Fe01 indicated that it contained aggregated
iron oxide clusters. For sample Fe02, the white color was
retained after calcination, indicating that no bulk iron oxide
apparently existed. On the basis of all the above evidence,
it was not concluded that isomorphous substitution of Fe
for Si in the crystalline lattice of MTW zeolite had been
(
hydrothermally at different Si/T ratio (Si/T = 50 to ?).
The hydrothermal time was 6 days. The synthetic gel
conditions and characteristics of the products are listed in
Table 2.
The synthesis of Si-MTW was conducted at different
OSDA/Si ratios using TEABr (Si01, Si02). Although the
prepared samples showed peaks corresponding to the MTW
zeolite structure, an amorphous phase was mainly formed at
1
23

Size-Controlled Synthesis of Metallosilicates with MTW Structure and Catalytic Performance…
Table 1 Composition of synthetic gel and product characteristics of Al-MTW zeolites
Run
Si/Al ratio
gel)
OH/Si
OSDA/Si
Synthesis
time (day)
OSDA
source
Phase
Si/Al ratio
(product)
BET area
2
(m /g)
(
AL01
50
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
6
TEABr
MTW
65
122
234
62
149
222
–
345
343
324
319
325
316
–
AL02
100
200
50
6
TEABr
MTW
AL03
6
TEABr
MTW
AL04
6
MTEACl
MTEACl
MTEACl
TEABr
MTW
AL05
100
200
50
6
MTW
AL06
6
MTW
AL07
1
Amorphous
Amorphous
Amorphous
Amorphous
MTW
AL08
50
2
TEABr
–
–
AL09
50
3
TEABr
–
–
AL010
AL011
AL012
AL013
AL014
AL015
AL016
AL017
AL018
50
4
TEABr
–
–
50
5
TEABr
–
344
338
–
50
10
1
TEABr
MTW
–
50
MTEACl
MTEACl
MTEACl
MTEACl
MTEACl
MTEACl
Amorphous
Amorphous
Amorphous ? MTW
MTW
–
50
2
–
–
50
3
–
82
50
4
–
210
301
280
50
5
MTW
–
50
10
MTW
–
Fig. 1 X-ray diffraction
patterns of Al-MTW zeolites
with different Si/Al ratios
obtained using a TEABr and
b MTEACl
(
a) Using TEABr
(b) Using MTEACl
Si/Al = 200
Si/Al = 200
Si/Al = 100
Si/Al = 50
Si/Al = 100
Si/Al = 50
Ref. MTW zeolite
Ref. MTW zeolite
10 15 20 25 30 35 40 45 50
5
10 15 20 25 30 35 40 45 50
angle 2θ [degree]
5
angle 2θ [degree]
demonstrated. These are necessary but not sufficient con-
ditions to prove the occurrence of isomorphous substitu-
tion, and consequently other characterization techniques
must be used to assess the incorporation of the Fe atoms
into framework positions.
Fe O and SiO is also shown. Bands between 200 and
2 3 2
3
300 nm are typically attributed to isolated Fe species,
?
either tetrahedrally coordinated in the zeolite framework or
3
with higher coordination. Octahedral Fe ions in small
?
3
x
?
oligonuclear Fe O complexes give rise to broad bands
y
Characterization by UV–vis spectra and ac-NH -TPD
3
between 300 and 450 nm, and bands above 450 nm are
3
?
profiles was applied to determine the nature and distribu-
tion of iron species in the prepared samples. Figure 4
shows the UV–vis spectra of Fe01 and Fe02. For com-
parison, the spectrum of a sample with mechanically mixed
characteristic of Fe ions in large iron oxide aggregates
[39]. The UV–vis spectrum of Fe01 showed a charge
transfer band between 200 and 300 nm, as well as broad
absorption bands of comparatively low intensity at
123

T. Taniguchi et al.
Fig. 2 FE-SEM photographs of
Al-MTW zeolites prepared by
using a TEABr and b MTEACl
(a1) Nano-Al-MTW (Si/Al=50)
(a2) Nano-Al-MTW (Si/Al=100)
(a3) Nano-Al-MTW (Si/Al=200)
1
00 nm
100 nm
100 nm
(b1) Macro-Al-MTW (Si/Al=50)
(b2) Macro-Al-MTW (Si/Al=100)
(b3) Macro-Al-MTW (Si/Al=200)
1
µm
1 µm
1 µm
wavelengths higher than 450 nm, indicating the presence
3
of octahedral Fe
1
00
?
3?
ions in small oligonuclear Fex Oy
TEABr
complexes (possibly iron oxide). In contrast, the UV–vis
spectrum of Fe02 displayed a band at 200–300 nm, with no
contribution above 450 nm, indicating that the majority of
8
6
4
2
0
0
0
0
0
MTEACl
3
?
Fe species in the samples were well isolated. This result
demonstrates that no iron oxide particles were formed in
Fe02.
Figure 5 shows the ac-NH -TPD profiles of Fe01 and
3
Fe02. The peaks at desorption temperatures above 550 K
were associated with desorption of NH adsorbed on strong
3
acid sites, whereas peaks below 550 K were assigned to
0
1
2
3
4
5
6
NH weakly held or physically adsorbed on zeolite crys-
3
Synthesis time [day]
tals. Since these strong acid sites could have been gener-
ated only by protons associated with trivalent atoms in
lattice positions, the presence of strong acid sites was
Fig. 3 Crystallization kinetics of Al-MTW zeolites during synthesis
with TEABr (circle) and MTEACl (triangle)
Table 2 Composition of synthetic gel and product characteristics of MTW metallosilicates
Synthetic gel
Product
Run
Procedure
Si/Al Si/Fe OH/Si OSDA/ Synthesis
Si
OSDA source Phase
Si/Al Si/Fe BET area
ratio
ratio
ratio
time (day)
ratio
[m2/g]
Si01
–
–
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
6
6
6
6
6
6
6
6
6
6
6
6
TEABr
Amorphous ? MTW
–
–
–
Si02
–
–
TEABr
MTW
–
–
–
Si03
–
–
MTEACl
TEABr
MTW
–
–
–
Fe01
Fe02
Fe03
Fe04
Fe05
Fe06
Fe07
AlFe01
AlFe02
A
B
B
B
B
B
B
B
B
–
100
100
50
MTW
–
88
88
–
302
352
–
–
TEABr
MTW
–
–
TEABr
Amorphous
MTW
–
–
200
100
50
TEABr
–
165
68
–
–
–
MTEACl
MTEACl
MTEACl
TEABr
MTW
–
248
–
–
Amorphous
MTW
–
–
200
200
200
–
161
183
163
–
200
200
MTW
235
233
304
–
MTEACl
MTW
1
23

Size-Controlled Synthesis of Metallosilicates with MTW Structure and Catalytic Performance…
state. Because the Fe species were added to the synthetic
(
(
b)
a)
gels in which Si species was well dispersed, the formation
of ferric hydroxide was suppressed by interaction of Fe and
Si species, leading to incorporation of Fe atoms into the
MTW framework.
(
(
(
a) Sample Fe01
b) Sample Fe02
c) Mechanically mixed Fe O and SiO
2
2
3
MTW zeolites containing Fe atoms were prepared at
different Si/Fe ratios and with different OSDAs employing
synthetic procedure B. Figure 6 shows the XRD patterns of
samples prepared at different Si/T ratios using TEABr
(Fe02, Fe03, Fe04, AlFe01, Si02) and MTEACl (Fe05, Fe
(
c)
0
6, Fe07, AlFe02, Si03) as OSDAs. No peaks corre-
sponding to the MTW zeolite structure were observed in
samples prepared at Si/Fe ratios below 50, indicating that
the addition of an excess amount of the Fe source (iron (III)
nitrate enneahydrate) suppressed the formation of MTW
zeolite nuclei. In contrast, peaks corresponding to the
MTW zeolite structure were observed in samples prepared
at Si/Fe ratios above 100, regardless of the type of OSDA.
The crystal morphologies of the prepared metallosili-
cates with a MTW structure were obtained using FE-SEM.
As shown in Fig. 7, the crystal size of metallosilicates with
a MTW structure depended strongly on the type of OSDA
and nano-sized zeolites could be synthesized using TEABr.
The dependence of the crystal morphology of Si-, Fe-, and
2
00
300
400
500
600
700
Wavelength [nm]
Fig. 4 UV-vis spectra of Fe-MTW (Si/Fe = 100) prepared using
TEABr by synthetic procedure A and B, corresponding to a Fe01 and
2 3 2
b Fe02, and c mechanically mixed Fe O and SiO
7
6
5
4
3
2
1
0
Sample Fe01
Sample Fe02
(Al, Fe)-MTW zeolites on the type of OSDA was similar to
that of Al-MTW, indicating that the type of OSDA was the
most important factor affecting the crystal morphology and
size of the MTW zeolite. The samples prepared by TEABr
and MTEACl were denoted as nano-MTW and macro-
MTW, respectively.
3
00
400
500
600
700
800
Figure 8 shows the ac-NH -TPD profiles of the nano-Al,
3
Temperature [K]
Fe-, and (Al, Fe)-MTW zeolites (Al02, Fe02, AlFe01). The
Fig. 5 ac-NH3-TPD profiles of Fe-MTW prepared using procedure
A and procedure B, corresponding to Fe01and Fe02, respectively
Si/T ratio of these samples was 100. The NH desorption
3
peak was shifted to lower temperatures in the order: Al-
MTW [ (Al, Fe)-MTW [ Fe-MTW, indicating that the
acid strength depended on the type of heteroatom. Since
the acid amount of these samples is almost the same, these
samples were applied to MTP reaction as catalysts, and
catalytic performance was measured.
strong evidence of isomorphous substitution of Fe for Si in
the MTW framework. Fe01 exhibited only one NH des-
3
orption peak, this result clearly shows that Fe01 possessed
no strong acid sites, indicating that Fe atoms were not
incorporated into the MTW framework. In contrast, Fe02
exhibited two NH desorption peaks, and the presence of
3
3.3 Methanol-to-Propylene Reaction
strong acid sites demonstrated the incorporation of Fe
atoms into the MTW framework. These results indicated
that the synthetic procedure was an important factor
affecting efficiency in the incorporation of Fe atoms into
the MTW framework.
3.3.1 Effect of Crystal Size on Catalyst Lifetime
The effects of the crystal size and acidity of the MTW
zeolite on the MTP reaction were examined. Before the
reaction, the zeolite catalysts underwent ion-exchange in
an NH NO aqueous solution to prepare the H-type coun-
In accordance with procedure A, the synthetic gel
became a red-brown color after addition of the Fe source,
indicating that agglomerated ferric hydroxide had been
formed during synthetic gel preparation. In procedure B,
the synthetic gel became a yellow–brown color after
addition of the Fe source, which indicated that the Fe
species were adsorbed on the zeolite synthetic gel as ions
4
3
terpart of each zeolite. The MTP reaction was conducted
over Al-MTW (Si/Al = 100) with different crystal sizes at
723 K with a W/F ratio of 0.1 h to clarify the effect of
crystal size on catalytic performance. Figure 9 shows the
changes in conversion with time on stream over the nano-
123

T. Taniguchi et al.
Fig. 6 X-ray diffraction
patterns of zeolites containing
Fe atoms obtained at different
Si/Fe ratios using a TEABr and
b MTEACl
(a) Using TEABr
(b) Using MTEACl
Si/T = ∞
Si/T = ∞
Si/Fe = 200, Si/Al = 200
Si/Fe = 200
Si/Fe = 200, Si/Al = 200
Si/Fe = 200
Si/Fe = 100
Si/Fe = 50
Si/Fe = 100
Si/Fe = 50
Ref. MTW zeolite
Ref. MTW zeolite
5
10 15 20 25 30 35 40 45 50
angle 2θ [degree]
5
10 15 20 25 30 35 40 45 50
angle 2θ [degree]
(
a1)Nano-Fe-MTW (Si/Fe=100)
(a2)Nano-(Al,Fe)-MTW
Si/Fe=200, Si/Al = 200)
(a3)Nano-Si-MTW (Si/T=∞)
(
2
00 nm
200 nm
200 nm
(
b1)Macro-Fe-MTW (Si/Fe=100) (b2)Macro-(Al,Fe)-MTW
(b3)Macro-Si-MTW (Si/T=∞)
(
Si/Fe=200, Si/Al = 200)
2
μm
2 μm
5 μm
Fig. 7 SEM photographs of MTW zeolites obtained using a TEABr and b MTEACl
Al-MTW and macro-Al-MTW. As shown in Fig. 9, con-
version dramatically decreased with time on stream in the
macro-Al-MTW. In contrast, the nano-Al-MTW main-
tained the initial conversion after 260 min. The amounts of
coke formed on the macro- and nano-Al-MTW during the
MTP reaction were 5.9 and 0.9 wt%, respectively.
Although the nano-Al-MTW possessed physical/chemical
properties (i.e., BET area, acid amount, acid strength)
similar to those of the macro-Al-MTW, the changes in
conversion and the amount of coke deposition were dif-
ferent, indicating that the crystal size of the catalyst
strongly affected the catalyst lifetime and catalytic
performance.
4
Nano-Fe-MTW (Si/Fe = 100)
Nano-(Al,Fe)-MTW
(Si/Fe = 200, Si/Al = 200)
3
Nano-Al-MTW (Si/Al = 100)
2
1
0
450
500
550
600
650
700
750
800
Temperature [K]
Because the diffusion length of the reactant and products
was assignable to the crystal size, the residence time of the
reactant and products inside the zeolite channels becomes
3
Fig. 8 ac-NH -TPD profiles of nano-Fe-MTW, nano-(Al, Fe)-MTW,
nano-Al-MTW. Si/T ratio of samples is 100
1
23

Size-Controlled Synthesis of Metallosilicates with MTW Structure and Catalytic Performance…
longer in the macro crystal, so that the excessive reactions
to form aromatics and coke tended to proceed in the macro-
Al-MTW. In the nano-Al-MTW, larger external surface
area and smaller diffusion resistance of reactant/product
within the micropore resulted in stable activity.
nano-Al-MTW as catalysts. The MTP reaction was con-
ducted at 723 K with a W/F ratio of 0.1 h. The Si/T ratio of
these catalysts was 100. For comparison, Al-MFI zeolite
with crystal size of approximately 100 nm (nano-Al-MFI,
Si/Al = 100) was applied to MTP reaction at 723 K with a
W/F ratio of 0.1 h. It was confirmed that the ac-NH -TPD
3
3
.3.2 Effect of Zeolite Acidity on Product Yield
profiles of the nano-Al-MFI is similar to that of the Nano-
Al-MTW, indicating that the two zeolites possessed almost
same acid amount and acid strength. The detailed prepa-
ration method of nano-Al-MFI was explained in our pre-
vious study [40].
Since acid strength affects the progress of the methanol
conversion reaction, the effect of acidity on catalytic per-
formance was investigated using the nano-Fe-MTW and
Figure 10 shows the product yield after 20 min for the
MTP reaction at 723 K. As shown in Fig. 10, the nano-Al-
MFI exhibited higher aromatics and ethylene yield and
lower propylene yield compared to the nano-Al-MTW.
Because channel intersection of the MFI zeolite provides
enough volume for cyclization reactions, higher alkenes
was converted into aromatics [14, 30], leading to high yield
of aromatics due to greater contribution of the methyl
aromatics route. Aromatics and ethylene were formed to a
slight extent in the nano-Fe-MTW and nano-Al-MTW,
indicating that the formation of aromatics was effectively
suppressed due to the non-intersecting one-dimensional
structure of the MTW zeolite.
1
00
Nano-Al-MTW
Macro-Al-MTW
8
0
0
0
0
6
4
2
However, the initial conversion using the nano-Fe-
MTW was as low as 72 C-mol% and yield of C5?
hydrocarbons was higher than that of the reaction using the
nano-Al-MTW, indicating that the weak acidity of the
nano-Fe-MTW was not enough to crack the C5? hydro-
carbons. In order to increase the yields of propylene and
0
0
60
120
180
240
300
Time on stream [min.]
Fig. 9 Changes in conversion with time on stream for MTP reaction
over Al-MTW with different crystal sizes
Fig. 10 Product yield of exit
gas for MTP reaction after
Conversion
[C-mol%]
72
98
98
99
2
0 min over nano-Fe-MTW,
100
nano-Al-MTW, nano-(Al, Fe)-
MTW and nano-Al-MFI
C5+
hydrocarbons
8
6
4
2
0
0
0
0
0
methane
others
paraffin
aromatics
ethylene
butenes
propylene
Nano-Fe-MTW
W/F = 0.1 h
Nano-Al-MTW
W/F = 0.1 h
Nano-(Al,Fe)-MTW
W/F = 0.2 h
Nano-Al-MFI
W/F = 0.1 h
123

T. Taniguchi et al.
Nano-(Al,Fe)-MTW, W/F = 0.2 h
Therefore, although MTW zeolite possesses one-dimen-
sional pore, it was concluded that the pore size was suffi-
ciently large for the olefins methylation/cracking route to
produce propylene and that the co-existence of Al and Fe in
the MTW zeolite framework was effective in increasing the
yields of propylene and butenes.
1
00
Conversion
8
6
4
2
0
0
0
0
Light olefins
4
Conclusion
C5+ hydrocarbons
A series of Al-, Si-, Fe-, and (Al, Fe)-MTW zeolites were
synthesized using TEABr and MTEACl as OSDAs. The
crystal size and morphology were founded to depend on the
type of OSDA, nano-sized MTW zeolites obtained using
TEABr, regardless of variations in the T site atom. The
synthetic procedure is an important factor affecting effi-
ciency in the incorporation of Fe atoms into the framework.
The isomorphous substitution of Fe for Si was demon-
Aromatics: (
)
Paraffins: ( )
0
0
8
6
4
2
Propylene
0
0
0
strated using UV–vis spectra and ac-NH -TPD profiles.
3
These analyses showed that Fe atoms were incorporated
3
into the lattice framework and that almost all of the Fe
Butenes
?
species was well dispersed without forming bulky iron
Ethylene
oxide particles. In addition, the ac-NH -TPD profiles
3
demonstrated that the acid strength of the Fe-MTW zeolite
was weaker than that of the Al-MTW zeolite. In the
methanol-to-propylene reaction, the nano-Al-MTW zeolite
exhibited a longer catalyst lifetime than the macro-Al-
MTW zeolite. Accordingly, the nano-sized zeolite is con-
sidered a promising material for increasing the external
surface area as well as decreasing the diffusion resistance
of the reactant within the micropore, which improves the
catalytic activity and catalyst lifetime. Moreover,
employing the nano-(Al, Fe)-MTW zeolite during the MTP
reaction gave higher yields of propylene and butenes in
conjunction with lower yields of aromatics and ethylene,
compared with the nano-Al-MTW zeolite, indicating that
the cyclization of C5? hydrocarbons was suppressed while
retaining cracking ability. The nano-(Al, Fe)-MTW zeolite,
0
0
60
120
180
240
300
Time on stream [min.]
Fig. 11 Changes in methanol conversion, product selectivity and
olefin composition with time on stream over nano-(Al, Fe)-MTW
butenes from the cracking of C5? hydrocarbons, the nano-
(
Al, Fe)-MTW, which possesses strong and weak acid sites
ascribable to Al and Fe, respectively, was applied to the
MTP reaction. In order to obtain the same amount of Fe
and Al within the reactor, a W/F ratio of 0.2 h was
employed.
As shown in Fig. 10, the nano-(Al, Fe)-MTW gave
higher yields of propylene and butenes compared to the
nano-Al-MTW, and the total yield reaching more than 70
C-mol%. In addition, the aromatics and ethylene yields of
the nano-(Al, Fe)-MTW were lower than those of the nano-
Al-MTW and the C5? hydrocarbons yield of the nano-(Al,
Fe)-MTW was lower than that of the nano-Fe-MTW,
indicating that the introduction of weak acid sites in the Al-
MTW led to suppression of the formation of aromatics
while retaining cracking ability. Figure 11 shows the
changes in conversion, product selectivity and olefin
composition with time on stream over the nano-(Al, Fe)-
MTW. The conversion remained at approximately 98
C-mol% during the reaction. Moreover, the product yields
and olefin selectivity were also stable, indicating that the
nano-(Al, Fe)-MTW possesses high catalytic stability.
which possesses
a non-intersecting one-dimensional
structure and contains strong and weak acid sites derived
from Al and Fe atoms, respectively, is responsible for the
greater contribution of the olefins methylation/cracking
route, leading to the high yields (above 70 C-mol%) of
propylene and butenes.
References
1
. Baerlocher Ch, McCusker LB. Database of zeolite structure.
http://www.iza-structure.org/database/
2. Stocker M (1999) Microporous Mesoporous Mater 29:3
3. Zhao T, Takemoto T, Tsubaki N (2006) Catal Commun 7:647
4
. Firoozi M, Baghalha M, Asadi M (2009) Catal Commun 10:1582
1
23

Size-Controlled Synthesis of Metallosilicates with MTW Structure and Catalytic Performance…
5
6
7
8
9
. Mei C, Wen P, Liu Z, Liu H, Wang Y, Yang W, Xie Z, Hua W,
Gao Z (2008) J Catal 258:243
. Sahoo SK, Viswanaoham N, Ray N, Gupta JK, Smgh LD (2001)
Appl Catal A 205:1
. Heemsoth J, Tegeler E, Roessner F, Hagen A (2001) Microporous
Mesoporous Mater 46:185
. Tessonnier JP, Louis B, Rigolet S, Ledoux MJ, Huu CP (2008)
Appl Catal A 336:79
21. Cejka J, Vondrova A, Wichterlova B (1994) Zeolites 14:147
22. Chu Cynthia T-W, Chang C (1985) J Phys Chem 89:1569
23. Fejes P, Kiricsi I, Lazar K, Marsi I, Rockenbauer A, Korecz L
(2003) Appl Catal A 242:63
24. Yu Y, Xiong G, Feng CL, Xiao S (2000) J Catal 194:487
25. Kumar R, Thangaraj A, Bhat RN, Ratnasamy P (1990) Zeolites 10:85
26. Inui T, Matsuda T, Yamase O, Nagata H, Fukuda K, Ukawa T,
Miyamoto A (1986) J Catal 98:491
27. Ahmed S (2012) J Porous Mater. 19:111
28. Jin Y, Asaoka S, Zhang S, Lin P, Zhao S (2013) Fuel Process
Technol 115:34
29. Derouane EG, Gabelica Z (1980) J Catal 65:486
30. Saxena SK, Viswanadham N, Al-Muhtaseb AH (2014) J Ind Eng
Chem 20:2876
31. Rosinski EJ, Rubin MK (1974) US Patent 3, 832, 449
32. Zhi YX, Tuel A, Taarit YB, Naccache C (1992) Zeolites 12:138
33. Salehirad F, Anderson MW (1998) J Chem Soc Faraday Trans
94:1911
34. Chiche BH, Dutartre R, Renzo FD, Fajula F (1995) Catal Lett 31:359
35. Masuda T, Fujikata Y, Mukai SR, Hashimoto K (1997) Appl
Catal A 165:57
. Liu C, Deng Y, Pan Y, Gu Y, Qiao B, Gao X (2004) J Mol Catal
A 215:195
1
0. Olah GA, Goeppert A, Prakash GKS (2006) Beyond oil and gas:
the methanol economy. Wiley, Weinheim
1. Dahl IM, Kolboe S (1994) J Catal 149:458
2. Dahl IM, Kolboe S (1996) J Catal 161:304
3. Svelle S, Joensen F, Nerlov J, Olsbye U, Lillerud KP, Kolboe S,
Bjørgen M (2006) J Am Chem Soc 128:14770
4. Bjørgen M, Svelle S, Joensen F, Nerlov J, Kolboe S, Bonino F,
Palumbo L, Bordiga S, Olsbye U (2007) J Catal 249:195
5. Bjørgen M, Lillerud KP, Olsbye U, Svelle S (2007) Stud Surf Sci
Catal 167:463
6. Bjørgen M, Joensen F, Lillerud KP, Olsbye U, Svelle S (2009)
Catal Today 142:90
7. Wang Q, Cui ZM, Cao CY, Song WG (2011) J Phys Chem C
1
1
1
1
1
1
1
1
1
2
36. Chaikittisilp W, Yokoi T, Okubo T (2008) Microporous Meso-
porous Mater 116:188
1
15:24987
37. Matsukata M, Ogura M, Osaki T, Rao PRHP, Nomura M,
Kikuchi E (1999) Top Catal 9:77
38. Ratnasamy P, Kumar R (1991) Catal Today 9:329
39. Ram ´ı rez JP (2004) J Catal 227:512
40. Tago T, Konno H, Nakasaka Y, Masuda T (2012) Catal Surv Asia
16:148
8. Zhang GQ, Bai T, Chen TF, Fan WT, Zhang X (2014) Ind Eng
Chem Res 53:14932
9. Choudhary TV, Kinage A, Banerjee S, Choudhary VR (2006)
Catal Commun 7:166
0. Song Y, Zhu X, Xie S, Wang Q, Xu L (2004) Catal Lett 97:31
123