Acidic Ultrafine Tungsten Oxide Molecular Wires for Cellulosic Biomass Conversion

Article abstract of DOI:10.1002/anie.201602770

The application of nanocatalysis based on metal oxides for biomass conversion is of considerable interest in fundamental research and practical applications. New acidic transition-metal oxide molecular wires were synthesized for the conversion of cellulos

Full text of DOI:10.1002/anie.201602770

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201602770
German Edition: DOI: 10.1002/ange.201602770
Molecular Wires
Acidic Ultrafine Tungsten Oxide Molecular Wires for Cellulosic
Biomass Conversion
Zhenxin Zhang,* Masahiro Sadakane, Norihito Hiyoshi, Akihiro Yoshida, Michikazu Hara, and
Wataru Ueda*
Abstract: The application of nanocatalysis based on metal
oxides for biomass conversion is of considerable interest in
fundamental research and practical applications. New acidic
transition-metal oxide molecular wires were synthesized for the
conversion of cellulosic biomass. The ultrafine molecular wires
were constructed by repeating (NH4)2[XW6O21] (X = Te or Se)
along the length, exhibiting diameters of only 1.2 nm. The
nanowires dispersed in water and were observed using high-
angle annular dark-field scanning transmission electron mi-
croscopy. Acid sites were created by calcination without
collapse of the molecular wire structure. The acidic molecular
wire exhibited high activity and stability and promoted the
hydrolysis of the glycosidic bond. Various biomasses including
cellulose were able to be converted to hexoses as main
products.
Biomass utilization is important, owing to increasing
attention on energy resource consumption for a sustainable
society and development.[5] The condensation of glycosidic
bonds forms a large class of cellulosic biomass, typically
cellulose, and the conversion of cellulosic biomass into useful
chemicals is one of the most important biomass conversion
routes.[6] The first step is hydrolysis of a glycosidic bond to
form hexoses, which can be further transformed to other
chemicals. Brçnsted acids are active for glycosidic bond
hydrolysis. Various Brçnsted acids have been utilized for the
hydrolysis of cellulose, including traditional liquid acids and
heteropoly acids[7] as homogeneous catalysts as well as carbon
materials,[8,10] sulfonated resins,[9] zeolites,[10] solid super
acids,[10] and transition-metal oxides[11] as heterogeneous
catalysts. However, the current catalytic systems suffer from
many problems that need to be overcome. These limitations
include low catalytic activity, high catalyst loading, high
reaction temperature, long reaction time, and difficulty with
catalyst recycling.
A molecular wire based on metal oxide shows promise as
solid acids owing to several advantages, such as nanosize, high
stability, structure diversity, composition complexity, and
tunable properties. The design and development of a metal
oxide molecular wire with high acidity and stability are
expected to exhibit high activity for catalytic cellulosic
biomass conversion.
Herein, we report the successful design and synthesis of
acidic solid molecular wires based on tungstotellurate (W-Te
oxide) and tungstoselenate (W-Se oxide) and their applica-
tion for cellulosic biomass conversion. The structure of the
materials was confirmed by high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM)
combined with other characterization techniques. Single
molecular wires with a diameter of 1.2 nm were observed by
electron microscopy and atomic force microscopy (AFM). W-
Te oxide shows high thermal and hydrothermal stability and
acted as a robust acid catalyst for hydrolysis of cellulosic
biomass to produce hexoses.
T
he construction of molecular wires by polymerizing
a molecular fragment in length direction while maintaining
a molecular-level size in the other two dimensions has
attracted much attention. Organic molecular wires are
widely used.[1] However, inorganic molecular wires are rare
and difficult to obtain.[2] The assembly of metal–oxygen
octahedra is an ideal method for preparing well-crystallized
solids based on a one-dimensional (1D) molecular structure.[3]
Recent progress indicates that inorganic molecular wires can
be obtained using a “top-down” approach, resulting in a new
class of metal oxide molecular wires.[4] Therefore, this new
field, although full of challenges, is receiving intense inves-
tigation.
[*] Dr. Z. Zhang, Dr. A. Yoshida, Prof. Dr. W. Ueda
Faculty of Engineering, Kanagawa University
Rokkakubashi, Kanagawa-ku, Yokohama-shi, Kanagawa, 221-8686
(Japan)
E-mail: zhang.z.ag@m.titech.ac.jp
uedaw@kanagawa-u.ac.jp
Dr. Z. Zhang, Prof. Dr. M. Hara
Materials and Structures Laboratory, Tokyo Institute of Technology
Nagatsuta-cho 4259, Midori-ku, Yokohama-city, Kanagawa, 226-8503
(Japan)
The hydrothermal reaction of (NH4)6[H2W12O40] and
TeO2 or SeO2 under low pH conditions (ca. 1.5) resulted in
gel-like yellow products, indicating that the particle size in the
materials was small. The as-synthesized materials adsorbed
a large amount of water. After the materials were dried at
808C, the volume of the material substantially decreased
(Supporting Information, Figure S1). Powder X-ray diffrac-
tion (XRD) patterns exhibited broad peaks, indicating that
the crystallinity of the materials was low (Figure 1). The
precursor pH affected the resulting products. As shown in the
Supporting Information, Figure S2, the W-Te oxide synthe-
Dr. M. Sadakane
Department of Applied Chemistry, Hiroshima University
1-4-1 Kagamiyama, Higashi Hiroshima 739-8527 (Japan)
Dr. N. Hiyoshi
Research Institute for Chemical Process Technology, National
Institute of Advanced Industrial Science and Technology (AIST)
4-2-1 Nigatake, Miyagino, Sendai 983-8551 (Japan)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201602770.
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The structure of crystalline-W-Te oxide was determined
by the XRD pattern (Supporting Information, Tables S1–S3).
The initial structure was refined by the Rietveld method. The
simulated pattern was similar to the experimental pattern,
indicating that the proposed structure was correct (Support-
ing Information, Figure S7).[4]
The structure was confirmed by HAADF-STEM.[13] The
cross sectional image of the W-Te oxide (Figure 2a) shows
Figure 1. A) XRD patterns and B) FTIR spectra of a) W-Te oxide, b) W-
Se oxide, and c) crystalline-W-Te oxide.
sized at a high pH (ca. 5) was a crystalline sample, denoted as
crystalline-W-Te oxide. A decrease in the pH value of the
precursor solution by less than 1.0 produced a material that
had a different phase from that of W-Te oxide. For W-Se
oxide, a high pH precursor did not yield a solid compound but
a low pH yielded the same crystalline phase that was obtained
for W-Te oxide.
Figure 2. High-resolution HAADF-STEM images (left) and proposed
structures (right) of a) W-Te oxide in the a-b plane, b) in the (100)
plane, c) in the (2ꢁ10) plane, and d) W-Se oxide along the c-axis
(dashed line indicates the unit cell; W blue, Te/Se brown, O red).
The XRD pattern of the crystalline-W-Te oxide was
indexed with a hexagonal cell (a = 12.6798 ꢀ, c = 3.8192 ꢀ),
which was similar to that of Mo oxide molecular wire based
crystals (Supporting Information, Figure S3).[4] The main
XRD peaks of W-Te oxide and W-Se oxide can also be
indexed to nearly the same unit cell, indicating that these
materials have the same structure as that of crystalline-W-Te
oxide (Figure 1A). The broad diffraction peaks of the
materials may be due to the small size of the particles in the
material. The particle size of the materials was estimated by
Scherrerꢁs equation, showing ca. 15 nm in length along the c-
axis and ca. 9 nm in diameter across the a-b plane (Supporting
Information, Figure S4). The FTIR spectra of W-Te oxide, W-
Se oxide, and crystalline-W-Te oxide were identical (Fig-
ure 1B), and the FTIR spectra of the W-based oxide were
similar to those of the Mo-based oxide (Supporting Informa-
tion, Figure S3).[4] Therefore, the molecular structures of the
materials were basically the same.
Elemental analysis indicated that the N:W:Te or Se ratio
was 2:6:1. X-ray photoelectron spectroscopy (XPS; Support-
ing Information, Figure S5) showed that the W ions in both
materials were WVI, and the Te and Se ions were TeIV and SeIV,
respectively, for the W-Te oxide and W-Se oxide, respectively.
The UV/Vis spectra of the materials also indicated that the W
ion in both materials was WVI, because no absorbance
between 500 and 800 nm was observed owing to the reduced
W species (Supporting Information, Figure S6).[12] The chem-
ical formulae for W-Te oxide and W-Se oxide were estimated
to be (NH4)2[TeIVWVI6O21] and (NH4)2[SeIVWVI6O21]. The
reactions for producing these materials can be expressed as
follows:
hexagonal units attributable to [TeW6O21]2ꢁ and their hex-
agonal array with a periodicity of ca. 1.2 nm. The side views of
W-Te oxide (Figure 2b,c) show that the hexagonal units are
stacked with a layer distance of ca. 0.4 nm to form a molecular
wire. The atomic positions observed in Figure 2a–c are in
good agreement with the proposed structures (Figure 2 right)
and that of crystalline-W-Te oxide. The side view of W-Se
oxide (Figure 2d) also shows a lattice fringe with a spacing of
ca. 0.4 nm owing to the stack of the hexagonal units
([SeW6O21]2ꢁ). A single molecular wire with a diameter of
ca. 1.2 nm was observed in the end of a bundle of molecular
wire.
The morphology of solid-state W-Te oxide and W-Se oxide
was observed by scanning electron microscopy (SEM;
Supporting Information, Figure S8). The crystalline-W-Te
oxide had a uniform diameter of ca. 200 nm. The nanowire
samples exhibited irregularly shaped particle aggregation. N2
adsorption–desorption measurement indicated the presence
of nanoparticles in the solid states (Supporting Information,
Figure S9). After dispersal, the surface areas of the materials
are expected to increase substantially.
W-Te oxide and W-Se oxide easily formed nanowires by
simply dispersing the samples in water by ultrasound or
heating. The SEM and transmission electron microscopy
(TEM) images show the ultrathin nanowires after dispersal in
water (Figure 3; Supporting Information, Figure S10). Single
molecular wires with diameters of about 1–2 nm can be easily
obtained, as shown in the typical high-resolution (HR)-TEM
images (Figure 3e,f).
½H2WVI12O40ꢀ6ꢁ þ 2 TeIVO2 Ð 2 ½TeIVWVI6O21ꢀ2ꢁ þ 2 OHꢁ
½H2WVI12O40ꢀ6ꢁ þ 2 SeIVO2 Ð 2 ½SeIVWVI6O21ꢀ2ꢁ þ 2 OHꢁ
The diameters of the materials after dispersal were further
analyzed by AFM. The AFM images of the materials after
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metric-differential thermal analysis (TG-DTA) indicated that
8.5% and 10.7% weight losses were observed for W-Te oxide
and W-Se oxide, respectively, during the heat treatment
(Supporting Information, Figure S11). The desorption of NH3
(m/z = 16) reached a peak maximum at 400 and 4308C for W-
Te oxide and W-Se oxide, respectively, in temperature-
programmed desorption-mass spectrometry (TPD-MS; Sup-
porting Information, Figure S12).
The thermal stability of W-Te oxide and W-Se oxide was
evaluated. The materials were calcined under air at different
temperatures for 2 h. W-Te oxide was stable at 3508C. The
XRD patterns and FTIR spectra (Supporting Information,
Figure S13a,b) of the materials calcined below 3508C did not
change. A further increase in the calcination temperature
generated material that exhibited different XRD peaks and
IR bands, which indicated damage to the original structure.
For W-Se oxide, the thermal stability was lower than that of
W-Te oxide. The material started to decompose at 3008C
(Supporting Information, Figure S13c,d).
The amount of NH3 removed from W-Te oxide and W-Se
oxide by calcination at 3508C and 3008C, respectively, was
estimated by TPD-MS (Supporting Information, Figure S12),
and the calcined samples are referred to W-Te-AC350 and W-
Se-AC300, respectively. After calcination, about 50% of the
NH3 was removed. We assume that desorption of NH3 from
the material produced proton in the materials as acid sites.
The chemical formulae of the calcined materials were
estimated to be (NH4)[HTeW6O21] and (NH4)[HSeW6O21]
for W-Te-AC350 and W-Se-AC300, respectively.
Hydrothermal stabilities of W-Te-AC350 and W-Se-
AC300, were tested at 1758C for 2 h. After the treatment,
the dispersed materials were recovered by evaporating all of
the H2O (Supporting Information, Figure S14). W-Te-AC350
was stable, and the structure did not change after the
hydrothermal treatment. By contrast, the structure of W-Se-
AC300 changed under the same conditions.
Figure 3. Electron microscopy images of the dispersed materials:
a) SEM and b) TEM images of W-Te oxide; c) SEM and d) TEM images
of W-Se oxide; HR-TEM images of e) W-Te oxide and f) W-Se oxide.
The dispersed samples were prepared in water followed by sonication
for 1 h, and then the solution was dropped on a TEM grid and dried.
dispersal indicated the presence of nanowires (Figure 4). In
most cases, the nanowire consisted of an aggregation of
several single molecular wires with a diameter less than 5 nm.
A single molecular wire with the thickness of ca. 1.2 nm were
also observed. All of the microscopy observations were
conducted after the solution with the molecular wire was
dried, and therefore particle aggregation could not be
completely avoided.
FTIR spectra of the materials contained bands at ca. 1620
W-Te-AC350 was characterized and used as an acid
catalyst for biomass hydrolysis in an aqueous solution. TEM
(Supporting Information, Figure S10e,f) and AFM (Fig-
ure S15) images indicated that W-Te-AC350 formed well-
dispersed nanowires after hydrothermal treatment. Size
distribution based on TEM indicated that the distribution
maximum for diameter and length was 2–3 nm and 40–60 nm,
respectively (Figure S16). Oxidation states of W and Te were
WVI and TeIV (Figure S5). Elemental analysis and TG-DTA
(Figure S11c) showed the ratio N:W:Te was 1:6:1, which was
identical to the formula of (NH4)[HTeIVWVI6O21]·2H2O.
Using NH3 as a probe molecule combined with TPD
showed that the acid amount of the material was
0.41 mmolgꢁ1 in solid state (Figure S17). Titration showed
that the acid concentration of the dispersed material was
0.80 mmolgꢁ1.
and 1400 cmꢁ1, which correspond to H2O and NH4 , respec-
+
+
tively (Figure 1). H2O and NH4 were near the molecular
wires and could be removed by calcination. Thermogravi-
As shown in Table 1, cellobiose was hydrolyzed by W-Te-
AC350 at 1308C for 4 h under hydrothermal conditions, and
glucose was obtained as the main product with a high yield
(90.5%). Other soluble polysaccharides, such as sucrose and
starch, were converted into glucose under the same conditions
(Table 1, entries 2,3). Compared with the reported catalyst,
W-Te-AC350 exhibited high activity for hydrolysis of poly-
Figure 4. AFM images of a) W-Te oxide and b) W-Se oxide, top:
thickness analysis indicated by yellow dashed lines in the AFM images.
The sample was prepared using the same method as used for the TEM
images.
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Table 1: Biomass hydrolysis by W-Te-AC350 under different conditions.[a]
Entry Biomass t [h] T [8C] Conversion [%]
Yield based on carbon [%]
glucose mannose formic acid levulinic acid HMF
Total yield of organics [%]
1
2
3
4
5
6
7
cellobiose
sucrose
starch
4
4
4
2
2
2
2
130
130
130
175
175
175
175
93.1
99
–
–
–
90.5
43.9
78.4
8.8
19.0
25.8
0
0
0.1
3.3
0.5
0.5
0.7
3.0
0
0.1
15.1
1.0
1.1
2.5
8.5
0
0.7
7.8
1.2
0.7
1.2
2.0
0
91.4
72.0
81.3
14.0
30.6
47.1
0
1.9
0.2
2.9
7.2
7.8
0
microcrystalline cellulose[b]
ball-milled cellulose
ball-milled cellulose[c]
microcrystalline cellulose[d]
–
–
[a] Reaction conditions: biomass: 0.308 mmol based on glucose unit, W-Te-AC350: 0.05 g, water 0.5 mL. [b] Carbon balance was 86.7% (see details in
the Supporting Information). [c] cellulose: 0.185 mmol. [d] Without catalyst.
saccharides (Supporting Information, Tables S4 and S5).
Cellulose is more difficult to convert and was hydrolyzed at
1758C for 2 h. Hexoses (glucose and mannose) were the
primary products. Some other products were detected, such as
5-hydroxymethylfurfural (HMF), levulinic acid, and formic
acid. Ball-milling can decrease the crystallinity of cellulose
(Supporting Information, Figure S18) and activate the cellu-
lose.[14] The activity of the reaction increased using the ball-
milled cellulose (Table 1, entries 5,6). In the absence of the
catalyst, no products were detected (Table 1, entry 7).
The catalytic activities of different materials were inves-
tigated for comparison with W-Te-AC350. As shown in the
Supporting Information, Table S6, under the same conditions,
W-Te-AC350 was more active for microcrystalline cellulose
conversion than other solid acids based on the amount of acid.
Moreover, the activity of W-Te-AC350 was comparable to
homogeneous catalysts (Supporting Information, Table S6).
The calcination temperature affected the catalytic activity
of W-Te-AC350. As shown in the Supporting Information,
Figure S19, the as-synthesized material exhibited low activity,
and an increase in the calcination temperature enhanced the
activity. When the calcination was conducted at 3508C, the
highest activity was obtained. This result demonstrated that
acid sites were generated by calcination. However, a further
increase in the calcination temperature (4008C) deactivated
the catalyst, which was most likely due to structural decom-
position of the material at a high temperature. Therefore,
maintaining the nanowire structure is a key factor for high
catalytic activity.
The catalyst concentration was investigated. As the
catalyst concentration increased, the total detected products
increased, and hexoses tended to be further converted into
other products, such as levulinic acid and formic acid. A
decrease in the catalyst concentration effectively increased
the selectivity to hydrolysis products (Supporting Informa-
tion, Figure S20). Prolonging the reaction time yielded more
organic acids and HMF, although the total yield of detected
products increased (Supporting Information, Figure S21). A
decrease in the reaction temperature decreased the activity
and effectively suppressed the conversion of the generated
hexoses. The selectivity to hexose increased at a low reaction
temperature (Supporting Information, Figure S22).
low (Supporting Information, Figure S23). TEM (Supporting
Information, Figure S10i,j) and the size distribution (Sup-
porting Information, Figure S16) indicated that some nano-
wires were still in the solution. When a high-speed centrifu-
gation condition (48000 G, 24 h) was used, a higher activity of
the recovered catalyst was achieved.
The high catalytic activity of W-Te-AC350 was primarily
due to two factors: ultrafine nanosized catalyst particles after
dispersal and the strong interactions between the catalyst and
the biomass. W-Te-AC350 could be well dispersed in water
under hydrothermal conditions (1758C, 2 h). After the
hydrothermal treatment, the photographic images indicated
that the solution containing the material became transparent
and exhibited laser scattering, which indicated that the
material dispersed and formed ultrafine nanowires in water
(Supporting Information, Figure S24a,b). For a sample with
a low concentration, the laser scattering phenomenon still
existed after the hydrothermal treatment (Figure S24c,d),
indicating that the material was not dissolved in water but
dispersed. Before and after the hydrothermal treatment,
dynamic light scattering (DLS) was conducted for W-Te-
AC350, and the results revealed that after the hydrothermal
treatment, the particle size of the material decreased (Sup-
porting Information, Figure S25). The SEM and TEM images
(Figure S10e,f) revealed that W-Te-AC350 remained a nano-
wire. After the hydrothermal treatment, W-Te-AC350 was
dried at ꢁ308C under high vacuum. The surface area of the
dispersed W-Te-AC350 (15 m2 gꢁ1) was higher than that of the
as-synthesized W-Te oxide (Supporting Information, Fig-
ure S9). The adsorption under low pressure demonstrated the
existence of micropores that may be due to the gap formed by
isolated molecular wires. The calcined crystalline-W-Te oxide
(Supporting Information, Figure S26) exhibited a lower activ-
ity than that of W-Te-AC350, which indicated that the
nanosized material exhibited improved activity for the
reaction (Supporting Information, Table S6, entry 2). All of
the results demonstrated that the hydrothermal treatment
promoted dispersal of the material in water and increase in
the surface area.
Four oxygen sites are present in W-Te-AC350, including
corner-sharing oxygen (O1), edge-sharing oxygen (O2), layer-
sharing oxygen (O3), and terminal oxygen (O4), for proto-
nation without considering the interactions between protons
The catalyst can be reused. Elemental analysis indicated
that 47% of the catalyst was recovered by a low-speed
centrifugation (9800 G, 1 h). The activity of the catalyst was
+
with NH4 (Supporting Information, Figure S27). When O2
was protonated, the system energy was the lowest among all
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a model for the catalyst. A Monte Carlo simulation (Support-
ing Information, Figure S28) showed that cellobiose inter-
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molecular wire with the distance between O and H shorter
than 3 ꢀ, indicating that strong or medium hydrogen bonds
existed between the catalyst and cellobiose. The interaction
between W-Te-AC350 and cellobiose was investigated using
1H NMR (Supporting Information, Figure S29). When only
cellobiose was present, peaks corresponding to the hydroxyl
groups of cellobiose were observed by 1H NMR.[15] After the
addition of W-Te-AC350 to the cellobiose solution, the peaks
corresponding to hydroxyl group of cellobiose was broad-
ened, indicating fast proton exchange between the molecular
ꢁ
ꢁ
wire (W OH) and cellobiose ( OH) as well as the existence
of an interaction between the catalyst and the biomass.
In summary, an acidic nanocatalyst based on a W-oxide
molecular wire was synthesized and characterized. The
structure of the molecular wire was confirmed using an
atomic-resolution HAADF-STEM image combined with
XRD, FTIR, XPS, TPD, TG, and elemental analysis, and
the result indicated that the growth of a hexagonal unit,
(NH4)2[TeW6O21] or (NH4)2[SeW6O21], resulted in the for-
mation of an inorganic 1D polymer. W-Te oxide exhibited
high thermal and hydrothermal stability and was acidified by
calcination. The acidic molecular wire was dispersed in water
and acted as an efficient catalyst for biomass hydrolysis to
produce hexoses.
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Acknowledgements
This work was supported in part by the Novel Cheap and
Abundant Materials for Catalytic Biomass Conversion
(NOVACAM, FP7-NMP-2013-EU-Japan-604319) program
of the Japan Science and Technology Agency (JST). The
authors also thank Professor Atsushi Kameyama for the DLS
analysis and the Material Analysis Suzukake-dai Center,
Technical Department, Tokyo Institute of Technology, for the
elemental analysis and NMR measurement.
[15] B. Bernet, R. Bꢂrli, J. Xu, A. Vasella, Helv. Chim. Acta 2002, 85,
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Keywords: cellulosic biomass · hexose · hydrolysis ·
molecular wires · tungsten oxide
Received: March 19, 2016
Revised: June 20, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Molecular Wires
A molecular wire based on tungsten oxide
with a diameter of 1.2 nm was synthe-
sized. Acid sites were created in aqueous
solution by calcination without collapse
of the molecular wire structure. This
material was applied for the hydrolysis of
cellulosic biomass to produce hexose.
Z. Zhang,* M. Sadakane, N. Hiyoshi,
A. Yoshida, M. Hara,
W. Ueda*
&&&& — &&&&
Acidic Ultrafine Tungsten Oxide
Molecular Wires for Cellulosic Biomass
Conversion
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!