Supporting Information
Nb2O5·nH2O as a heterogeneous catalyst with water-tolerant Lewis acid sites
Kiyotaka Nakajima†, Yusuke Baba†, Ryouhei Noma†, Masaaki Kitano†, Junko N. Kondo‡, Shigenobu
Hayashi§ and Michikazu Hara †, П,*
†
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama
226-8503, Japan
‡
, Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama
226-8503, Japan
§
Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and
Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
П
Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, kawasaki 213-0012, Japan
Content
1. Method
2. Schematic structure of Nb2O5·nH2O.
3. Estimation of the amounts of Brønsted and Lewis acid sites for dehydrated Nb2O5·nH2O,
Na+/ Nb2O5·nH2O and H3PO4/Nb2O5·nH2O.
4. CO adsorption experiment and estimation of the amounts of Brønsted and Lewis acid sites
for hydrated Nb2O5·nH2O, Na+/ Nb2O5·nH2O and H3PO4/Nb2O5·nH2O.
5. FT-IR spectra for pyridine-adsorbed original and Na+-exchanged Nb2O5·nH2O.
6. Pyridine-adsorption experiment on Nb2O5·nH2O in saturated H2O vapor.
7. Catalyst reuse experiment with Nb2O5·nH2O for 5 runs.
8. MALDI-TOF-MASS for the reaction solution.
9. Catalyst reuse experiment with H3PO4-treated Nb2O5·nH2O for 5 runs.
10. 31P MAS NMR spectrum for H3PO4-treated Nb2O5·nH2O.
11. CO adsorption experiment for hydrated and dehydrated H3PO4/Nb2O5·nH2O.
S1
Figure legends in Supporting Information
Figure S1 Schematic structure of Nb2O5·nH2O
Figure S2 FT-IR spectra for Nb2O5·nH2O (a) after evacuation at room temperature for 24 h
and (b) after heating the sample at 423 K for 1 h under vacuum
Figure S3 FT-IR spectra for pyridine-adsorbed (a) original Nb2O5·nH2O and (b) its
Na+-exchanged form.
Figure S4 FT-IR spectra for Nb2O5·nH2O. (a) hydrated Nb2O5·nH2O in saturated H2O vapor,
(b) pyridine and H2O-adsorbed Nb2O5·nH2O in pyridine and H2O vapor, and (c) pyridine and
H2O-adsorbed Nb2O5·nH2O obtained by evacuation (room temperature) for 60 min after (b).
The marked square area in A is enlarged in B.
Figure S5 Catalytic activity of reused Nb2O5·nH2O for HMF production from D-glucose at
393 K. Catalyst: 0.2 g; water: 2.0 mL; D-glucose: 0.02 g.
Figure S6 MALDI-TOF-MASS for the reaction solution. Nb2O5·nH2O: 0.05 g; D-glucose:
0.05 g; water: 5.0 mL; reaction temperature: 393 K; reaction time: 6 h; ion detection: positive;
matrix: 2,5-dihydroxybenzoic acid-acetonitrile solution.
Figure S7 Catalytic activity of reused H3PO4/Nb2O5·nH2O for HMF production from
D-glucose at 393 K. Catalyst: 0.2 g; water: 2.0 mL; D-glucose: 0.02 g.
Figure S8 31P MAS NMR spectrum for H3PO4/Nb2O5·nH2O.
S2
Figure S9 Differential FT-IR spectra for a) dehydrated and b) hydrated H3PO4/Nb2O5·nH2O at
90 K.
(a) Prior to CO adsorption, the sample was heated at 423 K for 1 h under vacuum. CO
pressure: (a) 6.4×10-3, (b) 1.2×10-2, (c) 2.0×10-2, (d) 3.9×10-2, (e) 6.7×10-2, and (f) 1.3×10-1
kPa.
(b) Prior to CO adsorption, the sample was dehydrated at room temperature for 24 h under
vacuum. CO pressure: (a) 1.7×10-2, (b) 3.0×10-2, (c) 4.3×10-2, (d) 5.3×10-2, (e) 6.8×10-3, (f)
1.1×10-1, and (g) 1.4×10-1 kPa.
S3
1. Method
Preparation of Nb2O5·nH2O, Na+/ Nb2O5·nH2O and H3PO4/Nb2O5·nH2O
Nb2O5·nH2O was typically synthesized using a mixture of NbCl5 (5 g) and distilled water
(200 mL) stirred at room temperature for 3 h. The resulting white precipitate was repeatedly
washed with distilled water (ca. 500 mL) until the filtrate became neutral. Nb2O5·nH2O
powder was obtained by drying the precipitate overnight at 353 K.
Na+/Nb2O5·nH2O was obtained by Na+-exchange of Nb2O5·nH2O. 1 g of Nb2O5·nH2O was
stirred in 200 mL of 0.2 M NaCl solution maintained at pH = 5.5-5.8 by adding 0.05 M NaOH
solution. After 24 h, the collected sample was washed repeatedly with distilled water until
Na+ and Cl- ions were no longer detected and was then dried at 373 K for 12 h.
H3PO4/Nb2O5·nH2O was prepared by adsorbing H3PO4 on Nb2O5·nH2O. 1 g of
Nb2O5·nH2O was stirred in 200 mL of 1 M H3PO4 solution. After 48 h, the collected sample
was washed repeatedly with distilled water until phosphate ions were no longer detected and
was then dried at 373 K for 12 h.
Allylation of benzaldehyde with tetraallyl tin
The reaction was carried out in a Pyrex reaction vessel containing distilled water (15 mL),
benzaldehyde (0.4 mmol), tetraallyl tin (0.2 mmol), catalyst (0.1 g) and sodium dodecyl
sulfate (0.3 mmol).
After 1 h at 298 K, the reaction solution was analyzed by gas
chromatography-mass spectrometry (GC-MS).
HMF formation from glucose
Distilled water (2.0 mL) containing D-glucose (0.02 g) and catalyst (0.02 or 0.2 g) was
typically heated in a sealed Pyrex tube at 393 K. The solutions after reaction were analyzed
using HPLC and GC-MS.
S4
2. Schematic structure of Nb2O5·nH2O
Brønsted acid
H
O
O
O
Nb
O
O
Brønsted acid
H
O
Hδ+
Oδ −
O
O
δ−
δ+
OH
H
Nb
O
O
O
O
Lewis acid
Figure S1 Schematic structure of Nb2O5·nH2O
S5
3.
Estimation of the amounts of Brønsted and Lewis acid sites for dehydrated
Nb2O5·nH2O, Na+/ Nb2O5·nH2O and H3PO4/Nb2O5·nH2O.
The amounts of Lewis and Brønsted acid sites on Nb2O5·nH2O, Na+/ Nb2O5·nH2O and
H3PO4/Nb2O5·nH2O were estimated using FT-IR measurements for pyridine-adsorbed
samples at 298 K. Nb2O5·nH2O samples were pressed into self-supporting disks (20 mm
diameter, 25 mg) and placed in an IR cell attached to a closed glass-circulation system. Prior
to pyridine adsorption, the sample was dehydrated by heating at 423 K for 1 h under vacuum.
The intensities of the bands at 1450 cm-1 (pyridine coordinatively bonded to Lewis acid sites,
molecular absorption coefficient: 3.15 µmol·cm-1) and 1540 cm-1 (pyridinium ions formed by
Brønsted acid sites, molecular absorption coefficient: 1.20 µmol·cm-1) were plotted against
the amounts of pyridine adsorbed on the Lewis and Brønsted acid sites of the samples,
respectively. The intensities of both bands increased with the amount of chemisorbed pyridine,
reaching plateaus with the appearance of the band due to physisorbed pyridine (1440 cm-1).
While the band at 1440 cm-1 disappeared after evacuation at room temperature, there was no
significant difference in intensity of the bands at 1450 and 1540 cm-1 before and after
evacuation, which indicated that the maximum intensities of the bands at 1450 and 1540 cm-1
correspond to the amounts of Lewis and Brønsted acid sites available to chemisorb pyridine to
saturation, respectively. The amounts of Brønsted and Lewis acid sites on Nb2O5·nH2O, Na+/
Nb2O5·nH2O and H3PO4/Nb2O5·nH2O were estimated from the maximum band intensities and
molecular absorption coefficients at 1450 and 1540 cm-1.
S6
4. CO adsorption experiment and estimation of the amounts of Brønsted and Lewis acid
sites for hydrated Nb2O5·nH2O, Na+/ Nb2O5·nH2O and H3PO4/Nb2O5·nH2O.
Experimental procedure
FT-IR spectra were obtained at a resolution of 4 cm-1 using a spectrometer (FT/IR 6100,
Jasco) equipped with an extended KBr beam splitting device and a mercury cadmium telluride
(MCT) detector. A total of 64 scans were averaged for each spectrum. The samples were
pressed into self-supporting disks (20 mm diameter, 20-30 mg) and placed in an IR cell
attached to a closed glass-circulation system. The disk was dehydrated by heating at 423 K for
1 h under vacuum in order to remove physisorbed water. In the experiments for the hydrated
samples, the self-supported disk was evacuated at room temperature for 24 h under vacuum in
order to remove weakly adsorbed water. CO, as a basic probe molecule, was adsorbed on the
dehydrated and hydrated sample disks at 90 K in the IR cell using liquid N2. Each IR
spectrum was measured after adsorbed CO and CO in the gas phase reached an equilibrium.
The IR spectra of the sample at 90 K before CO adsorption were used as the backgrounds for
the differential spectra obtained by subtracting the backgrounds from the spectra for
CO-adsorbed samples.
Hydrated Nb2O5·nH2O sample for FT-IR experiments
Figures S2a and S2b show FT-IR spectra for Nb2O5·nH2O after evacuation at room
temperature for 24 h and 423 K for 1 h, respectively. Figure S2a has two distinctive signals at
3800-2600 and 1600 cm-1 that are assignable to the OH stretching and bending modes of
physisorbed H2O, respectively, which indicates that there is a considerable amount of
physisorbed H2O molecules on the sample evacuated at room temperature for 24 h. In contrast,
the signals due to physisorbed H2O disappeared after evacuation at 423 K for 1 h under
S7
vacuum. The amount of physisorbed H2O on the hydrated Nb2O5·nH2O sample was estimated
to be 3.0 mmol·g-1 by measuring the total amount of desorbed H2O during heating the sample
at 423 K under vacuum. Assuming that the adsorption cross section area of a H2O molecule is
0.125 nm2, then 1.3 layers of H2O would be adsorbed on the hydrated Nb2O5·nH2O sample.
absorbance
Nb-O str.
OH str.
0.5
OH bend.
a
b
Nb-OH str.
4000
3000
2000
1000
-1
Wavenumber / cm
Figure S2 FT-IR spectra for Nb2O5·nH2O (a) after evacuation at room temperature for 24 h
and (b) after heating the sample at 423 K for 1 h under vacuum
Estimation of the amounts of Brønsted and Lewis acid sites for hydrated Nb2O5·nH2O,
Na+/ Nb2O5·nH2O and H3PO4/Nb2O5·nH2O.
The amounts of effective Brønsted and Lewis acid sites on hydrated Nb2O5·nH2O, Na+/
Nb2O5·nH2O and H3PO4/Nb2O5·nH2O were estimated from FT-IR measurements of CO
adsorbed samples. The correlation between the amount and band intensity of each CO species
adsorbed on the Brønsted and Lewis acid sites was obtained by FT-IR for CO-adsorbed
dehydrated Nb2O5·nH2O (Lewis acid sites: 0.15 mmol g-1, Brønsted acid sites 0.14 mmol g-1).
First, CO was adsorbed on the dehydrated sample at 90 K. The intensities of three bands at
S8
2145, 2168 and 2188 cm-1, assignable to physisorbed CO, CO adsorbed on Brønsted and
Lewis acid sites, respectively, increased with the amount of introduced CO, and the band
intensities due to acid sites reaches plateaus at Pco > 4.8 kPa. The correlation of the band
intensity and amount of each CO species adsorbed on the Brønsted and Lewis acid sites was
estimated from each acid density obtained by pyridine-adsorption experiment and maximum
band intensity. FT-IR spectra for CO-adsorbed hydrated Nb2O5·nH2O, Na+/ Nb2O5·nH2O and
H3PO4/Nb2O5·nH2O were then measured at 90 K. The intensities of the bands due to CO
adsorbed on Brønsted and Lewis acid sites increased with the amount of introduced CO,
reaching plateaus at Pco > 4.8 kPa, as was the case for dehydrated Nb2O5·nH2O. Each
effective acid density on the hydrated samples, hydrated Nb2O5·nH2O (Brønsted acid; 0.14
mmol g-1, Lewis acid; 0.032 mmol g-1), Na+/ Nb2O5·nH2O (Brønsted acid; below limitation of
detection, Lewis acid; 0.034 mmol g-1) and hydrated H3PO4/Nb2O5·nH2O (Brønsted acid;
0.043 mmol g-1, Lewis acid; 0.019 mmol g-1) was calculated from the band intensity-amount
correlation.
S9
Abs.
5. FT-IR spectra for pyridine-adsorbed original and Na+-exchanged Nb2O5·nH2O
1445 cm-1
0.5
1540 cm-1
a
b
1600
1400
Wavenumber / cm-1
Figure S3 FT-IR spectra for pyridine-adsorbed (a) original Nb2O5·nH2O and (b) its
Na+-exchanged form.
The Brønsted acid sites of Na+/ Nb2O5·nH2O was examined by FT-IR measurement. We
did not adopt CO but pyridine as a basic molecular probe on this purpose because the signal
for bridged Na+···CO···Na+ species (2158 cm-1) formed on Na+ exchanged solid acid obscures
the band for adsorbed CO on Brønsted acid site (2165 cm-1).1 Figure S3 shows FT-IR spectra
for pyridine-adsorbed Nb2O5·nH2O and Na+/ Nb2O5·nH2O. The band at 1445 cm-1 assignable
to adsorbed pyridine on Lewis acid site is observed in both spectra. On the other hand, the
band for pyridinium ion formed on Brønsted acid sites (1540 cm-1) is not observed in Na+/
Nb2O5·nH2O (Figure S3 (b)). This indicates that the Brønsted acid sites on Na+/ Nb2O5·nH2O
are blocked with Na+.
Ref. 1) Otero Areán, C.; Nachtigallová, D.; Nachtigall, P.; Garrone, E.; Rodriguez Delgado, M.
Phys. Chem. Chem. Phys., 2007, 9, 1421.
S10
6. Pyridine-adsorption experiment on Nb2O5·nH2O in saturated H2O vapor
B
0.5
Abs.
Abs.
A
OH str.
0.5
OH bend.
(a)
(a)
4000
3000
2000
(b)
(b)
(c)
(c)
1000
Wavenymber / cm-1
1800
1600
1400
Wavenymber /
1200
cm-1
Figure S4 FT-IR spectra for Nb2O5·nH2O. (a) hydrated Nb2O5·nH2O in saturated H2O vapor,
(b) pyridine and H2O-adsorbed Nb2O5·nH2O in pyridine and H2O vapor, and (c) pyridine and
H2O-adsorbed Nb2O5·nH2O obtained by evacuation (room temperature) for 60 min after (b).
The marked square area in A is enlarged in B.
Pyridine adsorption on Nb2O5·nH2O in saturated H2O vapor was examined by FT-IR to
study the Lewis acid sites on Nb2O5·nH2O. Figure S4 shows FT-IR spectra for (a) hydrated
Nb2O5·nH2O in saturated H2O vapor, (b) pyridine and H2O-adsorbed Nb2O5·nH2O in pyridine
and H2O vapor, and (c) pyridine and H2O-adsorbed Nb2O5·nH2O obtained by room
temperature evacuation for 60 min after (b). Nb2O5·nH2O was pressed into a self-supporting
disk (20 mm diameter, 20-30 mg) and placed in an IR cell attached to a closed
glass-circulation system. After the disk was exposed to saturated H2O vapor (20~25 Torr) at
room temperature for 60 min (Figure S4(a)), small amount of pyridine vapor (ca. 0.08 mmol)
was further added to the reaction system (391.6 cm3). Figure S4(b) was measured after 60 min
in the presence of H2O and pyridine vapor. The sample was evacuated at room temperature
S11
for 60 min to remove the physisorbed H2O and pyridine molecules on the surface (Figure S4
(c)).
Figure S4(a) clearly exhibits two signals at 3700~2500 cm-1 and 1700~1500 cm-1
assignable to OH stretching and bending mode of H2O molecule, respectively. There are
several peaks in Figure S4(b) after introduction of small amount of pyridine. A broad and
weak peak at 1540 cm-1 is due to pyridine on Brønsted acid sites (cationic pyridinium ion).
Two sharp peaks at 1487 and 1445 cm-1 are assignable to both of pyridine on Brønsted and
Lewis acid sites and pyridine on Lewis acid sites (coordinated pyridine), respectively. These
peaks due to pyridine adsorbed on acid sites remain even after evacuation (Figure S4(c)), thus
indicating that the Lewis acid sites can interact with pyridine molecules even on Nb2O5·nH2O
in saturated H2O vapor, multilayer adsorption of water.
S12
7.
Catalyst reuse experiment with Nb2O5·nH2O for 5 runs
: Glucose conversion
: HMF yield
Conversion, yield (%)
100
80
60
40
20
0
1st
2nd
3rd
4th
5th
Figure S5 Catalytic activity of reused Nb2O5·nH2O for HMF production from D-glucose at
393 K.
Catalyst: 0.2 g; water: 2.0 mL; D-glucose: 0.02 g.
The result for reuse experiment on Nb2O5·nH2O is shown in Figure S5. The glucose
conversion and HMF yield were measured after 3 h of reaction. The collected catalyst was
repeatedly rinsed with distilled water and was then reused for the subsequent reaction. No
significant decrease in glucose conversion and HMF yield was observed even after 5 reuses of
the catalyst, thus indicating that Nb2O5·nH2O can function as a stable acid catalyst for the
reaction.
S13
8.
MALDI-TOF-MASS for the reaction solution.
Glucose+Na
60
203.13
*: Matrix-derived signal
Mass intensity / a.u.
50
427.28
40
30
20
467.29
149.18
575.36
617.38
277.19
629.41
10
0
100
743.46
280
460
640
820
893.54
1000
m/z
Figure S6 MALDI-TOF-MASS for the reaction solution.
Nb2O5·nH2O: 0.05 g; D-glucose: 0.05 g; water: 5. 0 mL; reaction temperature: 393 K; reaction
time: 6 h; ion detection: positive; matrix: 2,5-dihydroxybenzoic acid-acetonitrile solution.
S14
9.
Catalyst reuse experiment with H3PO4-treated Nb2O5·nH2O for 5 runs.
: Glucose conversion
: HMF yield
Conversion, yield (%)
100
80
60
40
20
0
1st
2nd
3rd
4th
5th
Figure S7 Catalytic activity of reused H3PO4/Nb2O5·nH2O for HMF production from
D-glucose at 393 K.
Catalyst: 0.2 g; water: 2.0 mL; D-glucose: 0.02 g.
The result for reuse experiment on H3PO4/Nb2O5·nH2O is shown in Figure S7. The
glucose conversion and HMF yield were measured after 3 h of reaction. The collected catalyst
was repeatedly rinsed with distilled water and was then reused for the subsequent reaction.
The glucose conversion and HMF yield of the catalyst retained unchanged even after 5 reuses
for the reaction. Therefore, H3PO4/Nb2O5·nH2O could be used repeatedly as solid acid catalyst
without loss of original activity.
S15
31
10.
P MAS NMR spectrum for H3PO4-treated Nb2O5·nH2O
100
50
0
-50
-100
Chemical shift / ppm
Figure S8 31P MAS NMR spectrum for H3PO4/Nb2O5·nH2O
The 31P MAS NMR spectrum for the sample was measured at room temperature and at a
Larmor frequency of 162.0 MHz using a single-pulse sequence with high-power proton
decoupling. A Bruker MAS probehead was used with a 4 mm zirconia rotor. The spinning rate
of the sample was 8 kHz. The
31
P chemical shift was referenced to 85% H3PO4 at 0.0 ppm.
(NH4)2HPO4 was used as a second experimental reference material with the signal set at 1.33
ppm.
The amount of phosphorous species on H3PO4/Nb2O5·nH2O was estimated to be ca. 1.0
mmol g-1 by inductively coupled plasma-atomic emission spectroscopy (ICP-AES).
S16
CO adsorption experiment for hydrated and dehydrated H3PO4/Nb2O5·nH2O
O
C≡
δ−
O
C≡
O
O
0.05
Nb
O
O
absorbance
ph
ys
is
CO orb
ed
absorbance
B
b)
O
A
a)
O
H
-O δ−
H δ+
C≡
O
O
Nb
O
H
11.
O
0.03
g
f
e
f
e
d
c
b
a
2200
2150
2100
d
c
b
a
2200
2150
2100
Wavenumber / cm-1
Wavenumber / cm-1
Figure S9 Differential FT-IR spectra for a) dehydrated and b) hydrated H3PO4/Nb2O5·nH2O at
90 K.
a) Prior to CO adsorption, the sample was heated at 423 K for 1 h under vacuum. CO
pressure: (a) 6.4×10-3, (b) 1.2×10-2, (c) 2.0×10-2, (d) 3.9×10-2, (e) 6.7×10-2, and (f) 1.3×10-1
kPa.
b) Prior to CO adsorption, the sample was dehydrated at room temperature for 24 h under
vacuum. CO pressure: (a) 1.7×10-2, (b) 3.0×10-2, (c) 4.3×10-2, (d) 5.3×10-2, (e) 6.8×10-3, (f)
1.1×10-1, and (g) 1.4×10-1 kPa.
S17