Structure and catalysis of layered Nb-W oxide constructed by the self-assembly of nanofibers

Article abstract of DOI:10.1016/j.cattod.2012.06.034

Treatment of crystalline Nb₂O₅-WO_x fibers with an aqueous solution of oxalic or tartaric acid resulted in the formation of nanosheets with a layered structure in which fibers were self-assembled. The oxalic and tartaric acid-treated oxides had dense layered structures containing mesopores between the layers with mean diameters of 4.6 and 1.9 nm, respectively. These oxides exhibited much higher recyclability as catalysts for the Friedel-Crafts alkylation and acylation of anisole compared to untreated ones. Furthermore, no deactivation was observed in a continuous flow process for the reaction of anisole and benzyl alcohol at least over 87 days' period in which turnover number = 33,000 was obtained. The improved catalytic performance was ascribed to the formation of layered structure, which was preserved during reactions.

Full text of DOI:10.1016/j.cattod.2012.06.034

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Catalysis Today 204 (2013) 197–203
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Structure and catalysis of layered Nb–W oxide constructed by the self-assembly
of nanofibers
Kazu Okumura∗, Soichiro Ishida, Ryota Takahata, Naonobu Katada
Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101 Koyama-cho Minami, Tottori-city, Tottori 680-8552, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 May 2012
Received in revised form 19 June 2012
Accepted 29 June 2012
Available online 19 August 2012
Treatment of crystalline Nb2O5–WOx fibers with an aqueous solution of oxalic or tartaric acid resulted in
the formation of nanosheets with a layered structure in which fibers were self-assembled. The oxalic and
tartaric acid-treated oxides had dense layered structures containing mesopores between the layers with
mean diameters of 4.6 and 1.9 nm, respectively. These oxides exhibited much higher recyclability as cat-
alysts for the Friedel-Crafts alkylation and acylation of anisole compared to untreated ones. Furthermore,
no deactivation was observed in a continuous flow process for the reaction of anisole and benzyl alcohol
at least over 87 days’ period in which turnover number = 33,000 was obtained. The improved catalytic
performance was ascribed to the formation of layered structure, which was preserved during reactions.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Niobium
Tungsten
Nano fibers
Layered structure
Friedel-Crafts reaction
1. Introduction
the evolution of catalytic function in the Friedel-Crafts reactions.
The activity of Nb2O5–WOx was greater than 10 times that of ␤-
Fabrication of solid acid oxides that show active and durable
catalytic performance is important in the field of heterogeneous
catalysis. Such metal oxides are useful in practice, especially in
organic synthesis and the petroleum industry. Tungsten-based
acidic oxides, represented by WO3/ZrO2 and heteropoly acid, are
promising because of their strongly acidic characteristics. These
catalysts have been attracting attention not only as acidic catalysts,
but also as fundamental materials such as electrochromic devices
and gas sensors [1–3]. We reported that Nb2O5–WO3 prepared by a
co-precipitation method was active for the Friedel-Crafts alkylation
[4]. Further, we have recently attempted to synthesize Nb2O5–WOx
using hydrothermal methods [5]. The hydrothermally synthesized
Nb2O5–WOx was a long nanocrystalline with several nanometers
in length, tetragonal tungsten bronze structure as observed by elec-
tron microscopy. A similar nanowire structure of tungsten bronze
and tungsten oxide has been synthesized [6–12], and the wires
of oxides have been studied as the photocatalyst and the sensors
[13–16]. The N2-calcined Nb2O5–WOx exhibited a much higher
catalytic activity for Friedel-Crafts alkylations and acylations than
Nb2O5–WOx prepared by the co-precipitation. The W L3-edge X-ray
absorption near edge structure (XANES) and electron microscopy
suggested that the formation of partially reduced tungsten atoms
(probably W4+) in the crystalline Nb2O5–WOx was responsible for
zeolite, which has been known as one of the best catalysts reported
to date [17]. However, hydrothermally synthesized Nb2O5–WOx
needs centrifugation in the separation from the reaction product
due to its needle-like morphology. Moreover, the hydrothermally
synthesized Nb2O5–WOx was readily destroyed during reaction or
by crushing it with a mortar to form inactive WO3.
In the present work, the hydrothermally synthesized fibrous
Nb2O5–WO3 was treated with an aqueous solution of oxalic or tar-
taric acid. We found that the fibrous structure changed to a layered
one in which fragmented fibers assembled to produce the layers.
Preliminary data were recently reported elsewhere as a commu-
nication [18]. Although it has been reported that the assembly of
tungsten–bronze nanowires occurred spontaneously on a silicon
wafer after evaporation of the solvent [7] and films were prepared
by assembling the W18O49 nanobundle suspension onto tin-doped
indium oxide coated glass [19], formation of a regularly stacked
structure through treatment with acid has not been reported. Our
aim was to synthesize a solid acid catalyst that exhibited superior
catalytic performance through the treatment of Nb2O5–WO3 with
oxalic or tartaric acids. The untreated Nb2O5–WOx was compared
with the catalysts that had been treated with oxalic acid, tartaric
acid, or plain water. It has been reported that the particle size and
crystallinity of the thin film of WO3·H2O could be easily controlled
by adjusting the concentration of oxalic acid in a colloid solution
of tungsten oxide hydrate [20]. Flower-like assemblies of tungsten
oxide hydrate were prepared in the presence of oxalic acid [21].
These studies suggest that the oxalic acid had a profound effect
∗
Corresponding author. Tel.: +81 857 31 5257; fax: +81 857 31 5684.
E-mail address: okmr@chem.tottori-u.ac.jp (K. Okumura).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.06.034

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K. Okumura et al. / Catalysis Today 204 (2013) 197–203
on the morphology of WO3 crystals. In this study, hydrothermally
synthesized Nb2O5–WOx was treated with an aqueous solution of
oxalic or tartaric acid, and the resultant structurally layered cat-
alysts were applied to the Friedel-Crafts alkylation of the anisole
with benzyl alcohol or to the acylation of the anisole using octanoic
acid as the acylating agent. The obtained catalyst was applied to a
continuous flow-type Friedel-Crafts alkylation as well. While the
flow-type reaction has been previously carried out over zeolite cat-
alysts, those catalysts suffered deactivation within several hours
[22,23]. Therefore, improvement in the sustainability of the cata-
lyst has remained a problem to be solved. The catalytic performance
of the layered Nb–W oxide in the flow-type reaction was compared
to that of the untreated catalyst.
77 K by liquid N2. As-prepared sample was utilized for the collec-
tion of IR data. The sample diluted with potassium bromide was
pelletized prior to the measurements.
2.3. Catalytic reaction
Benzylation of anisole (Friedel-Crafts alkylation) was carried out
over 0.02 g of catalyst. The reaction was performed using anisole
(10 g) and benzyl alcohol (0.675 g, 6.25 mmol) in an oil bath at
353 K while purging with Ar (30 mL min−1). Acylation of anisole
with octanoic acid (Friedel-Crafts acylation) was carried out in
a manner analogous to the benzylation of anisole, using anisole
(10 g) and octanoic acid (0.288 g, 2 mmol) in an oil bath at 413 K
(catalyst weight: 0.1 g). The products were analyzed using a gas
chromatograph (GC-2010, Shimadzu) equipped with a capillary
column (InertCap 1) and a flame ionization detector (FID). Tride-
cane was used as an internal standard. Continuous Friedel-Crafts
alkylation of anisole and benzyl alcohol was performed using a
flow-type reactor: a reactant solution was fed into the catalyst
(0.1 g) loaded in a stainless steel column. A filter with 1 ␮m pore
size was placed at the end of column. Reaction was carried out at
393 K. Flowing rate was 0.6 g h−1. The effluent solution collected
periodically was analyzed by GC.
2. Experimental
2.1. Sample preparation
Fibrous crystal of Nb2O5–WOx was synthesized using
a
hydrothermal method: a solution of ammonium paratungstate,
(NH4)10W12O41·5H2O (Wako Chemical Co., 2.03 g) dissolved in
water (60 mL) was mixed in a flask with ammonium niobium
oxalate, NH4[NbO(C2O4)2(H2O)]·xH2O (CBMM Co., 0.263 g) dis-
solved in water (10 mL) and then the liquid phase was purged
with N2. The mixed solution was placed in a Teflon-sealed auto-
clave in a glove box under N2. Hydrothermal synthesis was carried
out at 443 K for 48 h while the bottle was continuously rotated
at a speed of 15 rpm. The precipitate (1.2 g) was treated with an
aqueous solution of oxalic acid or tartaric acid (40 mL) at 353 K
for 10 h with vigorous stirring. The typical acid concentration was
1.9 mol L−1, unless otherwise stated. The solid was separated from
the solution by suction filtration using a filter (Advantec, 5C). Fil-
tration was slow (up to 4 h) due to the fine particulate nature of
the treated solid. The treated pale yellow sample was calcined in
an N2 flow (50 mL min−1) at 723 K for 2 h prior to characteriza-
tion and use. The oxalic- and tartaric-acid-treated catalysts, o-NbW
and t-NbW, respectively, comprised shiny grains with a dark blue
color (Fig. S1, Supplementary information). The as-synthesized
Nb2O5–WOx nanofibers were treated in water without the addition
of acids as a reference (designated as w-NbW). The as-synthesized
material was also calcined in an N2 flow at 773 K without any treat-
ment (denoted as NbW).
3. Results
3.1. Structural and acidic properties
Fig. 1 shows FE-SEM images of NbW and w-, o-, and t-NbW. The
formation of fiber-like shapes was clearly visible in NbW (Fig. 1(a)).
The maximum length of the fibers was several micrometers and the
fiber thickness was ca. 25 nm. The fiber bundles in NbW were ran-
domly oriented. In the case of w-NbW (Fig. 1(b)), a partially layered
sheet was seen in which fragmented fibers assembled. However,
the fiber packing was sparse, and the fiber orientation was inho-
mogeneous. In the o-NbW and t-NbW images (Fig. 1(c) and (e)),
characteristic sheet-like structures were seen, in a marked dif-
ference from NbW. The sheets showed a layer-by-layer stacking
configuration. This feature did not change after calcination in N2,
meaning the formation of the stacked sheets was realized when
as-synthesized NbW was treated with oxalic or tartaric acid rather
than through calcination in N2. The stacked configuration of o-
NbW was insensitive to oxalic acid concentration in the range of
0.2–4.6 mol L−1. A similar structure was observed in the o-NbW
separated by centrifugation (Fig. 2). However, formation of the lay-
ered sheet structure of the NbW fibers was not observed in a sample
treated with an aqueous solution of oxalic acid without stirring.
Fig. 1(d) and (f) shows the FE-SEM images of o-NbW and t-NbW sur-
faces measured at higher magnification, respectively. The images
show that the nanofibers were assembled to form sheets. The thick-
ness of a sheet in o-, t-NbW was estimated to be 25 nm, which
agreed with the diameter of one nanofiber (25 nm). For compar-
ison, the NbW oxide fibers were treated with aqueous solutions
of other acids, including hydrochloric, phosphoric, acetic, and cit-
ric acids (1.9 mol L−1). The results for these samples were identical
with that of w-NbW: partial formation of the stacked layers of fibers
was observed in these oxides. Table 1 lists the distribution of the
lengths of the fibers in these oxides estimated from the FE-SEM
images. The fiber lengths varied widely, ranging from 0.3 to 2.1 ␮m
in NbW with a mean value of 1.1 ␮m. The distribution of the fiber
lengths in w-NbW was slightly lower than that of NbW, with a mean
length of 0.6 ␮m. The lengths of the fibers composing the sheets of
o-NbW and t-NbW were approximately 0.27 and 0.33 ␮m, respec-
tively, which were much shorter than that of NbW. This indicates
that the fibers fragmented during the treatments with oxalic and
2.2. Characterization
Field emission scanning electron microscopy (FE-SEM) images
were taken with a JEOL JSM-6701F microscope with an accelera-
tion voltage of 5 kV. The crystalline structure was analyzed by X-ray
diffraction (XRD) under ambient conditions using a Rigaku Ultima
IV X-ray diffractometer with Cu K␣ radiation. Data of N2 adsorp-
tion isotherms were collected with BELSORP-max. Samples were
dehydrated at 573 K under vacuum prior to the measurements.
Acid properties of synthesized Nb2O5–WOx were measured by
means of the temperature programmed desorption of ammonia
(NH3 TPD) on a BEL Japan TPD-1-AT(NH3) system. The sample was
evacuated at 673 K prior to the measurement. Ammonia (13.3 kPa)
was equilibrated with the pretreated sample at 373 K. The TPD data
was collected with a temperature ramping rate of 10 K min−1. A
mass spectrometer was used to measure the desorbed NH3. For
the measurement, m/e = 16 was monitored to analyze the desorbed
NH3. This mass number was used instead of m/e = 17 in order to
avoid interference caused by water.
Fourier transform infrared spectra (FT-IR) were recorded by
using a Spectrum one (Perkin Elmer, Japan) spectrometer equipped
with a with a mercury cadmium telluride (MCT) detector kept at

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K. Okumura et al. / Catalysis Today 204 (2013) 197–203
199
Fig. 1. FE-SEM images: (a) NbW, (b) w-NbW, (c and d) o-NbW, and (e and f) t-NbW.
tartaric acids. FE-SEM images of NbW and o-NbW taken after cal-
cination in air at 773 K are given in Fig. 3. In the case of NbW, the
fibers completely disappeared after calcination in air. In turn, round
particles were observed. Unlike the NbW sample, the layered struc-
ture of o-NbW remained intact even after calcination in air and, the
preservation of the fibrous structure is clearly visible (Fig. 3(b)). This
indicated that the formation of the layered structure enhanced the
thermal strength of the fibers composed of layers.
XRD patterns of these samples are displayed in Fig. 4. Peaks
assignable to the (0 0 1) and (0 0 2) planes of tungsten bronze can be
Table 1
Distribution (%) of fiber lengths in NbW, w-NbW, o-NbW and t-NbW.
Sample
0.00–0.25
0.26–0.50
0.51–0.75
0.76–1.00
1.01–1.25
1.26–1.50
1.51–1.75
1.76–2.00
2.01–2.25
(␮m)
(␮m)
(␮m)
(␮m)
(␮m)
(␮m)
(␮m)
(␮m)
(␮m)
NbW
0
8.8
54.4
26.0
5.1
29.4
39.8
70.0
16.2
38.7
4.8
22.7
18.0
1.0
0
17.2
4.1
0
19.7
1.0
0
11.1
7.0
0
0
1.0
0
0
w-NbW
o-NbW
t-NbW
0
0
0
4.0
0
0
0
0

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K. Okumura et al. / Catalysis Today 204 (2013) 197–203
Fig. 3. FE-SEM images of (a) NbW and (b) o-NbW calcined in air at 773 K.
Fig. 2. FE-SEM images of o-NbW prepared by centrifugation.
number of OH groups on the surface of the nanofibers through
dehydration or the hydrogen-bonding of the OH groups.
The acidic properties of o-NbW were characterized by the NH3
TPD. A broad desorption of ammonia was observed in the tem-
perature range between 400 and 700 K (Fig. 7(b)). The amounts
seen in the data for NbW. The intensity of the diffraction decreased
in the o- and t-NbW samples. This change is most likely due to the
cleavage of the fibers to give fragments.
Fig. 5 shows the N2 adsorption isotherms of the untreated
and treated samples. The isotherm of NbW was similar to that
of w-NbW, which was categorized as type II. Similar isotherms
were obtained for NbW oxides treated with other kinds of acids
(1.9 mol L−1) including hydrochloric, phosphoric, acetic, and citric
acids (Fig. S2). The isotherms of o-NbW and t-NbW were different
from those of NbW and w-NbW, and could be categorized as type
IV, indicating the presence of mesopores. The mesopore volumes of
o- and t-NbW were in the range 0.10–0.15 cm3 g−1. The specific sur-
face areas calculated on the basis of the isotherms of NbW, w-NbW,
o-NbW, and t-NbW were 62, 60, 64, and 63 m2 g−1, respectively; the
surface area was independent of the samples. Distribution of pore
sizes in these oxides determined by the Barrett–Joyner–Halenda
(BJH) method showed the presence of mesopore with ca. 4.6 and
1.9 nm in o- and t-NbW, respectively (Fig. 6). The pore struc-
ture probably arises from the interspaces of the nanofibers or the
boundaries of sheets. We measured the N2 adsorption isotherms
of o-NbW prepared using different concentrations of oxalic acid
(0.2–4.6 mol L−1). The pore sizes of o-NbW did not depend on the
oxalic acid concentration (Fig. S3).
100000cps
Nb2O5
WO3
NbW, after reaction
(001)
(002)
NbW
w-NbW
o-NbW, after reaction
o-NbW
t-NbW
FT-IR spectra of the as-prepared oxides are shown in Fig. 7(a).
In the spectrum of NbW, two peaks appeared with maxima at 3448
and 3151 cm−1 in the range of 2900–3700 cm−1, which may be
assigned to the OH stretching band. The intensities of the OH
bands in the o- and t-NbW samples were smaller compared to
those of NbW and w-NbW. Treatment of the as-synthesized NbW
with oxalic or tartaric acid most likely resulted in a decrease in the
20
40
60
80
2θ / degrees
Fig. 4. XRD patterns of untreated and treated NbW and reference samples. XRD
of NbW and o-NbW measured after Friedel-Crafts alkylation (single batch) are
included.