Catalytic properties of highly ordered crystalline nanoporous tungsten oxide in butanol dehydration

Article abstract of DOI:10.1166/jnn.2014.9952

Highly ordered mesoporous tungsten oxide (meso-WO₃) was successfully synthesized using mesoporous silica KIT-6 as a hard template via the nanoreplication method. The physicochemical properties of meso-WO₃ were characterized by X-ray diffraction, nitrogen adsorption-desorption, transmission electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption of ammonia, and infra-red spectroscopy of adsorbed pyridine. No oxidation state other than WO₃ was observed in the meso-WO₃ sample. Lewis acid sites were dominant in meso-WO₃, which could be confirmed by infra-red spectroscopy of pyridine and temperature-programmed desorption of ammonia. Its catalytic behavior in 2-butanol dehydration was investigated in a fixed bed reactor and compared with that of the WO₃/MCM-41 catalyst prepared by the atomic layer deposition method. The meso-WO₃ catalyst exhibited higher 2-butanol dehydration activity than that of the WO₃/MCM-41 catalyst, which is ascribed to the stronger acidity as well as higher amount of acid sites that are mainly composed of Lewis acid sites in the meso-WO₃ catalyst.

Full text of DOI:10.1166/jnn.2014.9952

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 8828–8833, 2014
Copyright © 2014 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Catalytic Properties of Highly Ordered Crystalline
Nanoporous Tungsten Oxide in Butanol Dehydration
1
2
2
3
Hyeonhee Choi , Eunok Lee , Mingshi Jin , Young-Kwon Park ,
2
ꢀ∗
1ꢀ∗
Ji Man Kim , and Jong-Ki Jeon
1
Department of Chemical Engineering, Kongju National University, Cheonan, 330-717, Korea
2
Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Korea
3
School of Environmental Engineering, University of Seoul, Seoul, 130-743, Korea
Highly ordered mesoporous tungsten oxide (meso-WO ) was successfully synthesized using meso-
3
porous silica KIT-6 as a hard template via the nanoreplication method. The physicochemical
properties of meso-WO3 were characterized by X-ray diffraction, nitrogen adsorption–desorption,
transmission electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed des-
orption of ammonia, and infra-red spectroscopy of adsorbed pyridine. No oxidation state other than
WO was observed in the meso-WO sample. Lewis acid sites were dominant in meso-WO , which
3
3
3
could be confirmed by infra-red spectroscopy of pyridine and temperature-programmed desorption
of ammonia. Its catalytic behavior in 2-butanol dehydration was investigated in a fixed bed reac-
tor and co mD p ea l ri ev de r we di thb tyh aP tuo bf l it sh he i nWg OT e/ Mc hC nM o- l4o 1g yc at toa :l y As td ep rl ae pi da er e Td h be yo lt oh eg i ca at ol mL ii cb rl a yr ye r deposition
3
method. The meso- WI PO : 1 c8 a2 t .a7 l y4 s. t8 2e x. 2h 1i b0i t eOd n h: i Wg h ee dr ,2 0- b9 u Dt a en co l2 d0e 1h 5y d 1r a4 t: i 5o 6n : 1a 4c tivity than that of the
3
WO /MCM-41 catalyst, wh Ci c oh pi ys r iag s hc tr :i bAe md et or i ct ha en sSt r co i ne gn et ri fi ac c Pi d ui t by l ias sh ew res ll as higher amount of acid
3
sites that are mainly composed of Lewis acid sites in the meso-WO catalyst.
3
Keywords: Mesoporous Tungsten Oxide, Nano-Replication, Butanol Dehydration, Butene.
1
. INTRODUCTION
Among the different synthetic approaches to meso-
porous metal oxide catalysts, atomic layer deposition
(ALD) processes offer a potential advantage–control of
the catalyst synthesis at the atomic level. Although ALD
refers mainly to gas-phase deposition processes, the same
principles of alternating the adsorption and reaction of
Metal oxides, such as tungsten oxide, zirconia, vana-
dia, and titania, are an important family of catalysts in
the petroleum and fine chemical industries. Many cat-
alytic applications such as dehydration, alkane oxida-
tive dehydrogenation, and isomerization are carried out
1
−5
10
over supported metal oxides.
Mesoporous transition
precursors can also be applied in solution. For the past
metal oxides have received enormous attention as cata-
lysts due to their excellent catalytic properties, which are
attributed to their highly ordered crystalline framework and
several years, an ALD method has been applied in the
liquid phase for the grafting of metal oxide species on
5
ꢀ10
the surface of mesoporous scaffolds.
By keeping the
6
high surface area. Various methods for the synthesis of
system under rigorous dry conditions using organic sol-
vents during grafting, the presence of oligomeric species,
which are normally formed in aqueous solutions, can
be avoided. Using this approach tungsten oxide can
be grafted on the surface of silica-based mesoporous
mesoporous transition metal oxides have been developed.
Among them, the nano-replication method has brought
forward incredible possibilities in the preparation of new
7
mesoporous materials. Generally, a precursor solution for
1
0
materials.
the targeted metal oxide is infiltrated into the mesopores of
silica templates. After drying the solvent, the composite is
heated under air conditions, and finally the silica template
is removed by a chemical etching technique using HF or
The 2nd generation bio-fuels based on butanol derived
from cellulose have attracted considerable attention as a
potential replacement for 1st generation bio-fuels based
on ethanol and edible oil. Butanol can be converted to
high value butenes through dehydration over metal oxide
catalysts. Butene itself is a highly-valued compound and
8
ꢀ9
NaOH, providing ordered mesoporous metal oxides.
∗
Authors to whom correspondence should be addressed.
8
828
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 11
1533-4880/2014/14/8828/006
doi:10.1166/jnn.2014.9952

Choi et al.
Catalytic Properties of Highly Ordered Crystalline Nanoporous Tungsten Oxide in Butanol Dehydration
can be used as a raw material for high quality fuel such
as gasoline.
was synthesized following previously reported methods.7
A triblock copolymer (Pluronic P123, EO PO EO ,
2
0
70
20
In the present study, we investigated the butanol dehy-
dration activity of highly ordered meso-tungsten (meso-
Aldirch) and tetraethylorthosilicate (Aldrich) were utilized
as the structure-directing agent and framework source,
respectively. 5.0 g of the mesoporous silica template, KIT-
WO ) catalyst, which was obtained from a mesoporous
3
ꢀ
silica template, KIT-6, with a 3-dimensional mesoporous
structure.7 We also compared the catalytic activity of
meso-WO material with that of the WO /MCM-41 cata-
6, was heated at 100 C for 1 h. 6.49 g of H [P(W O ꢁ ]
3
3
10 4
was dissolved in distilled water, and impregnated into the
silica template. Subsequently, the composites were placed
3
3
ꢀ
lyst prepared by the ALD method.
in an oven at 80 C overnight for the spontaneous infiltra-
tion of the precursor into the mesopores of the silica tem-
ꢀ
plate. The composites were heated at 550 C under static
2
2
. EXPERIMENTAL DETAILS
.1. Synthesis of Catalysts
MCM-41 was prepared following the procedures described
air conditions for 4 h. After the heat treatment, the silica
template was removed 2 times using HF solution. Finally,
the meso-WO material, thus obtained, was washed with
3
1
1
in the literature. After dissolving 12.2 g of cetyltrimethy-
lammonium bromide in 140 g of distilled water, the 50 g
distilled water several times and dried. Elemental analysis
of the meso-WO thus obtained indicated that the content
3
7
of sodium silicate solution (20 wt.% SiO ) was added. The
of SiO was less than 0.1 wt%.
2
2
ꢀ
mixture was heated to 100 C in an oven for 24 h, and
subsequently cooled to room temperature. After adjusting
the pH to 10 with acetic acid solution, the mixture was
reacted again at 100 C for 24 h. The cycle of cooling, pH
2
.2. Characterization of Catalysts
The specific surface area and pore volume of the catalyst
samples pretreated at 180 C under vacuum conditions
were examined by measuring the nitrogen adsorption–
desorption isotherms using a nitrogen adsorption ana-
ꢀ
ꢀ
adjusting and heating the mixture was carried out twice
more. The white precipitate was filtered, washed in dis-
ꢀ
tilled water and dried at 110 C overnight. The product
ꢀ
lyzer (BELSORP-mini II, BEL Japan) at −196 C.
ꢀ
was washed with HCl/ethanol mixture, dried at 110 C and
finally calcined at 550 C for 4 h.
Specific surface area, pore volume, and pore size were
calculated by the Brunauer–Emmett–Teller (BET), t-plot,
and Barret–Joyner–Halenda (BJH) methods, respectively.
The structure of the catalyst was examined with transmis-
ꢀ
Tungsten oxide was added on MCM-41 using the ALD
method.12 5 g of MCM-41 was suspended in 150 ml of
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anhydrous toluene and was refluxed for 3 h under bub-
sion electron microscopy (TEM). The JEOL JEM-2011F
(22 kV) model was utilized for TEM.
Copyright: American Scientific Publishers
bled nitrogen. 8.21 g of WCl6 was also suspended in
3
6
00 ml of anhydrous toluene and was also refluxed for
h under bubbled nitrogen. Then a support solution was
X-ray diffraction (XRD) patterns were obtained in the
reflection mode using a Rigaku D/MAX-2200 Ultima
equipped with Cu Kꢂ radiation at 30 kV and 40 mA. X-ray
photoelectron spectroscopy (XPS) was used to analyze the
oxidation state of the tungsten oxide on the samples. The
XPS analyses were conducted on a MultiLab ESCA 2000
X-ray photoelectron spectrometer with Mg Kꢂ radiation at
300 W. The effects of sample charging were eliminated by
correcting the observed spectra for a Si 2p binding energy
value of 103.5 eV. The powdered catalysts were mounted
onto the sample holder and were degassed overnight at
mixed with the precursor solution and was refluxed for
2
4 h under bubbled nitrogen. After washing with toluene,
ꢀ
the mixed solution was filtered, dried at 100 C in an oven
for 30 min, and finally calcined at 500 C. WO /MCM-41
ꢀ
3
samples with tungsten oxide loadings of 27.0 wt% were
prepared. Even if the concentration of the WCl solution
6
that was used in the modified ALD method was excessive,
a WO /MCM-41 sample with more than 27.0 wt% tung-
3
sten oxide loading could not be obtained. Herrera et al.
reported that the maximum loading for a single deposition
is based on the assumption that three silanol groups on
−
7
room temperature and 10 Torr pressure.
The acid characteristics of the catalyst were analyzed
1
0
the mesoporous silica can hydrolyze one WCl molecule.
with ammonia temperature programmed desorption (NH -
6
3
1
6
With this assumption, one monolayer of WO (100% cov-
TPD) utilizing BEL-CAT-B (BEL Japan). The charac-
teristics of the acid sites were investigated by infra-red
spectroscopy using pyridine as the probe molecule (Py-
x
erage) corresponds to 30 wt% of WO , which agrees well
3
with the maximum tungsten oxide loading of this study.
Meso-WO3 material was synthesized by the nano-
1
3ꢀ14
IR).
Pyridine vapor was admitted in doses until the
1
5
replication method. In the nano-replication method,
ordered mesoporous silicas are used as templates. First,
the metal precursor solution is filled inside the pores
of mesoporous silicas and solidified. Subsequently, the
silica templates are removed by NaOH or HF solu-
tion, and after washing, drying, a material replicat-
ing the mesostructure of the hard template is obtained.
In this research, a mesoporous silica template, KIT-6
surface of the catalyst wafer was saturated. Infra-red spec-
tra over a wafer that contained chemisorbed pyridine were
recorded using a Spectrum GX (Perkinelmer) with an
ꢀ
MCT detector at a temperature range of 100–350 C.
2.3. Butanol Dehydration Activity
2-Butanol dehydration reaction was performed using a
fixed bed reactor containing 0.01 g of the catalyst.
J. Nanosci. Nanotechnol. 14, 8828–8833, 2014
8829

Catalytic Properties of Highly Ordered Crystalline Nanoporous Tungsten Oxide in Butanol Dehydration
Choi et al.
After maintaining the reactor temperature at a fixed level
under a nitrogen flow of 200 ml/min, 2-butanol was sup-
plied in a flow of 1 ml/hr. In this case, the WHSV was
−
1
8
0 h (for butanol). A syringe pump was used to inject
-butanol into the reactor at a fixed rate. Nitrogen flow
2
was regulated with a mass flow controller, and the reac-
tor temperature was controlled with a tubular furnace
equipped with a programmable temperature controller. The
gas phase products were analyzed using an online gas
chromatograph (YL 6100 GC) equipped with FID and an
alumina capillary column.
Figure 2. TEM image of meso-WO and WO 27.0 wt%/MCM-41.
3
3
3
. RESULTS AND DISCUSSION
which is characteristic of mesoporous materials. The BET
3
.1. Catalyst Characterization
2
3
surface area and pore volume are 65 m /g and 0.10 cm /g,
respectively (Table I). In previous studies, the meso-
MOx materials showed values of BET surface area in the
Figure 1 compares the XRD patterns of meso-WO3
and WO /MCM-41. In previous studies, we reported
that meso-MOx materials, such as CeO , Co O , Cr O ,
Mn O , NiO, RuO , SnO , and TiO , showed an XRD
peak at the low angle region, which corresponded to the
position of the [110] reflection for Ia3d symmetry.
The presence of a [110] reflection indicates that the
cubic Ia3d mesostructure was transformed to the tetrag-
onal I4 /a (or lower) mesostructure after the removal of
the silica template. The low-angle XRD pattern of meso-
WO was similar to those of the above-mentioned meso-
MOx materials, indicating that the mesopore structure
was successfully established. The high-angle XRD pat-
tern of meso-WO exhibited the high crystallinity of WO .
The low-angle XRD pattern of the WO /MCM-41 cata-
lyst exhibited diffraction peaks indexed to (100), (110),
3
2
2
3
4
2
3
range of 108–241 m /g and pore volume in the range
3
7
2
3
2
2
2
of 0.21–0.67 cm /g. Although the present meso-WO3
material has smaller BET surface area and pore volume
than those of meso-MOx materials such as CeO , Co O ,
7
ꢀ15
2
3
4
Cr O , Mn O , NiO, RuO , SnO , and TiO , the meso-
2
3
2
3
2
2
2
WO exhibits well-defined mesostructure with BJH pore
3
1
diameter of 3.3 nm. Not only the meso-WO but also the
3
WO /MCM-41 shows a typical type-IV isotherm with hys-
3
3
teresis, which is characteristic of mesoporous materials.
In case of WO /MCM-41, the BET surface area and pore
3
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2
3
volume are 772 m /g and 0.57 cm /g, respectively, which
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3
3
Copyright: American S ci si e mn tui fci ch Pl a ur gb el ir s thh ea rns those of meso-WO .
3
3
The composition and chemical bonding state of catalyst
surface were inferred from XPS analysis. Figures 4 and 5
show the XPS spectra of meso-WO and WO /MCM-41.
(200), and (211), originating from the characteristics of the
3
3
hexagonal structure of mesopores in MCM-41, indicating
that the impregnation of tungsten did not alter the hexag-
onal mesopore structure of MCM-41. The highly ordered
mesostructures of meso-WO as well as WO /MCM-41
The W 4f spectra had two peaks (4f7/2, 4f5/2), which were
separated by 2.2 eV (Fig. 4). No oxidation state other
than WO (e.g., WO or W) was observed in the two
3
2
3
3
can also be confirmed by TEM images, as shown in
Figure 2.
Adsorption
Desorption
Figure 3 shows the nitrogen adsorption–desorption
isotherms for meso-WO and WO /MCM-41. The nitro-
3
3
gen adsorption–desorption isotherm of meso-WO mate-
3
WO /MCM-41
3
rial is a typical type IV isotherm with a hysteresis loop,
Meso-WO3
0
.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/Po)
Figure 1. XRD patterns of the samples (low angle XRD: left, high
angle XRD: right).
Figure 3. N adsorption–desorption isotherms of the samples.
2
8
830
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Choi et al.
Catalytic Properties of Highly Ordered Crystalline Nanoporous Tungsten Oxide in Butanol Dehydration
Table I. BET surface area and pore volume.
O 1s
2
3
Catalyst
SBET (m /g)
VP (cm /g)
DP (nm)
WO /MCM-41
3
WO /MCM-41
Meso-WO3
772
65
0.57
0.10
2.4
3.3
3
Meso-WO3
samples, implying that meso-WO as well as WO /MCM-
3
3
4
1 can produce the WO species. Figure 5 shows the O 1s
3
binding energies of the two catalysts. It is noticeable that
two different states of surface oxygen are shown in the
spectra. The binding energy peak of around 530.0 eV of
meso-WO represents the lattice oxygen of tungsten oxide.
3
5
34
532
530
528
526
According to our previous results on meso-Mn O , a dom-
2
3
Binding Energy (eV)
inant peak appeared at 529.9 eV, which indicated that the
main surface oxygen state was lattice oxygen. Meanwhile
in the case of Mn/SBA-15, the major peak appeared at
Figure 5. O 1s photoelectron spectra of the samples.
5
31.4 eV, which could be attributed to oxygen existing
ꢀ
WO /MCM-41 presents a broad peak up to 450 C, which
3
as surface oxygen and surface hydroxyl group. In this
study, the high binding energy peak of around 532.8 eV
of WO /MCM-41 seems to be due to surface oxygen and
indicates a distribution of medium and strong acid sites.
In addition, meso-WO has a much higher amount of acid
3
3
sites than WO /MCM-41.
3
surface hydroxyl group.
Figure 7 shows the FT-IR spectra of the pyridine
adsorbed on the meso-WO3 sample, followed by des-
orption at elevated temperatures from 25 to 300 C.
An increase in desorption temperature seems to reduce
the intensity of the band at 1446 cm more drastically
than that of the band at 1438 cm . It is clear that the
band at 1438 cm show the presence of a higher strength
acid site, which could be assigned to a Lewis acid site.
The acid properties of the catalysts were analyzed with
NH -TPD and pyridine-IR. Figure 6 shows the result of
NH -TPD analysis. The WO /MCM-41 sample showed
weak acid sites only at around 180 C, which means
that only weak acid sites were produced. Meanwhile,
ꢀ
3
3
3
ꢀ
−1
−1
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−
1
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1
6
Copyright: American Scientific Publishers
WO /MCM-41
3
In addition, the meso-WO3 sample did not show any
band corresponding to a Brönsted acid site. In a previ-
ous study, it was suggested that not only the Lewis acid
sites but also very weak acid sites generated by hydrogen-
bonded pyridine on a silanol group were produced on the
W
WO3
WO2
WO /MCM-41 samples, which are attributed to the cre-
3
ation of tungsten oxide-induced acid sites. Both the NH3-
TPD and Py-IR indicate that meso-WO has much stronger
3
Meso-WO3
WO /MCM-41
3
Meso-WO3
1
00
200
300
400
500
4
0
38
36
34
32
30
Temperature (ºC)
Binding Energy (eV)
Figure 4. W 4f photoelectron spectra of the samples.
J. Nanosci. Nanotechnol. 14, 8828–8833, 2014
Figure 6. NH -TPD of the meso-WO3 and the WO /MCM-41
sample.
3
3
8831

Catalytic Properties of Highly Ordered Crystalline Nanoporous Tungsten Oxide in Butanol Dehydration
Choi et al.
1
00
8
6
4
2
0
0
0
0
0
0
3
00 ºC
2
50 ºC
2
00 ºC
1
50 ºC
trans-2-butene
-butene
1
00 ºC
1
6
4
2
cis-2-butene
1
456
1
438
0
0
0
RT
1
490
1
446
1
600
1550
1500
1450
1400
trans-2-butene
Wavenumber (cm–1
40
)
1
-butene
cis-2-butene
Figure 7. FTIR of adsorbed pyridine over meso-WO sample. (The
3
−3
ꢀ
sample was desorbed under 1D0 el it vo rer r aet d1 0 b0 ,y 1 P5 0 u, b2 0l i0s, h2 i5 n0 ga n Td e3 c0 0h n Co . )l ogy to: Ad30elaide Theological Library
IP: 182.74.82.210 On: Wed, 09 Dec 2015 14:56:14
Copyright: American Scientific Publishers
acidity as well as a higher amount of acid sites than
2
0
0
0
WO /MCM-41, which could be ascribed to the Lewis acid
3
sites.
1
3
.2. Butanol Dehydration
Figure 8 compares the catalytic performance in 2-butanol
dehydration over the catalysts and was performed at
Without
catalyst
MCM-41
WO3
/MCM-41
Meso-WO3
ꢀ −1
2
50 C, 1 atm, and 80 h WHSV for 2 h of time-on-
Catalyst
stream. In the case of reaction without a catalyst, 2-butanol
hardly reacted under the above-mentioned reaction condi-
tion. While the pure MCM-41 recorded lower than 5.0%
Figure 8. Conversion, selectivity and yield of 2-butanol dehydration
over various catalysts (Reaction condition: 250 C, 1 atm, time-on-stream
2 hr, WHSV 80 h ).
ꢀ
−1
2
-butanol conversion, the 2-butanol conversion signifi-
cantly increased over the WO /MCM-41 catalyst. It was
3
very noticeable that the meso-WO3 catalyst exhibited
4. CONCLUSIONS
Highly ordered meso-WO was successfully synthesized
higher 2-butanol conversion than that over the WO /MCM-
3
3
4
1 catalyst, while product distribution did not significantly
by the nano-replication method using a mesoporous silica
template, KIT-6. Lewis acid sites were dominant in meso-
WO , which could be confirmed by Py-IR and NH -TPD.
depend on the catalyst. It was apparent that the Lewis acid
sites were dominant in the meso-WO , which could be
3
3
3
confirmed by Py-IR and NH -TPD. According to our pre-
The meso-WO catalyst exhibited higher 2-butanol dehy-
3
3
vious results on 2-butanol dehydration, it was suggested
that the activity of dehydration of 2-butanol to butenes
is proportional to the increase of the Lewis acid sites
that could be mainly active for 2-butanol dehydration.5
dration activity than that of the WO /MCM-41 catalyst,
which is ascribed to the stronger acidity as well as higher
amount of acid sites that are mainly composed of Lewis
3
acid sites in the meso-WO catalyst.
3
Consequently, the higher activity of the meso-WO cat-
3
alyst can be ascribed to its stronger acidity as well
as the higher amount of acid sites of the meso-WO3
catalyst.
Acknowledgments: This research was supported by the
Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the
8
832
J. Nanosci. Nanotechnol. 14, 8828–8833, 2014

Choi et al.
Catalytic Properties of Highly Ordered Crystalline Nanoporous Tungsten Oxide in Butanol Dehydration
Ministry of Education, Science and Technology (2010-
023600). J. M. Kim also thanks the cooperative
R&D program (B551179-12-06-00) funded by the Korea
Research Council for Industrial Science and Technology.
7. M. Jin, J. N. Park, J. K. Shon, Z. Li, E. Lee, and J. M. Kim, J. Porous
Mat. 20, 989 (2013).
0
8. J. K. Shon, S. S. Kong, Y. S. Kim, J. H. Lee W. K Park, S. C. Park
and J. M. Kim, Micropor. Mesopor. Mat. 120, 441 (2009).
9. T. Valdés-Solís and A. B. Fuertes, Mater. Res. Bull. 41, 2187 (2006).
10. J. E. Herrera, J. H. Kwak, J. Z. Hu, Y. Wang, and C. H. F. Peden,
Top. Catal. 39, 245 (2006).
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Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 14, 8828–8833, 2014
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