Nb2O5?nH2O as a heterogeneous catalyst with water-tolerant lewis acid sites

Article abstract of DOI:10.1021/ja110482r

Niobic acid, Nb₂O₅?nH₂O, has been studied as a heterogeneous Lewis acid catalyst. NbO₄ tetrahedra, Lewis acid sites, on Nb₂O₅?nH₂O surface immediately form NbO₄-H₂O adducts in the presence of water. However, a part of the adducts can still function as effective Lewis acid sites, catalyzing the allylation of benzaldehyde with tetraallyl tin and the conversion of glucose into 5-(hydroxymethyl)furfural in water.

Full text of DOI:10.1021/ja110482r

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pubs.acs.org/JACS
Nb2O5 nH2O as a Heterogeneous Catalyst with Water-Tolerant Lewis
3
Acid Sites
Kiyotaka Nakajima,†,# Yusuke Baba,† Ryouhei Noma,† Masaaki Kitano,† Junko N. Kondo,‡
Shigenobu Hayashi,§ and Michikazu Hara*,†,#
†Materials and Structures Laboratory and ‡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
#Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan
S Supporting Information
b
they have been considered not to function in water.6 How-
ABSTRACT: Niobic acid, Nb2O5 nH2O, has been studied
as a heterogeneous Lewis acid catalyst. NbO4 tetrahedra,
Lewis acid sites, on Nb2O5 nH2O surface immediately form
NbO4-H2O adducts in the presence of water. However, a
part of the adducts can still function as effective Lewis acid
sites, catalyzing the allylation of benzaldehyde with tetra-
allyl tin and the conversion of glucose into 5-(hydroxy-
methyl)furfural in water.
ever, it has been reported that water molecules are coordi-
nated to Yb(OSO2CF3)3 (Yb(OTf)3), a rare earth metal
triflate highly polarized by OSO2CF3 groups, during reaction
in water, and this homogeneous catalyst functions as an
efficient Lewis acid in water, despite the coordination of
water.2 This suggests that Lewis acid sites in a highly
3
3
polarized field such as the NbO4 tetrahedra in Nb2O5 nH2O
3
can function even in water.
Fourier transform infrared (FT-IR) and Raman spectroscopies
were used to investigate the Lewis acid sites on Nb2O5 nH2O in
3
the presence of water. The Raman spectrum for Nb2O5 nH2O
3
ewis acids function as catalysts for the formation of C-C
powder dehydrated at 423 K in Figure 1 shows vibrational bands
at 550-750, 800-900, and 988 cm-1 that are attributable to
Nb-O-Nb, NbO6 octahedra, and NbO4 tetrahedra, respec-
L
bonds in organic compounds,1 and therefore the chemical
industry utilizes large amounts of Lewis acid catalysts, such as
AlCl3, BF3, and transition metal halides, for the production of
industrially important chemicals. However, these homogeneous
catalysts decompose or are ineffective in water, an environmen-
tally benign solvent, and have serious drawbacks such as the
production of waste, hazards of handling, separation from
products, and corrosion of equipment. In addition, the recovery
and reuse of homogeneous Lewis acid catalysts is an extremely
formidable obstacle. Any heterogeneous Lewis acid catalysts that
are insoluble, easily separable from products, and highly active in
water would be applicable for environmentally benign chemical
production. Rare earth metal triflates, which are homogeneous
Lewis acids, exhibit excellent catalytic activity and selectivity for
various organic reactions, even in the presence of water,2 and
polymer or inorganic oxide-supported rare earth metal triflates
have been developed as heterogeneous Lewis acids.3 While it is
well-known that many inorganic oxides including zeolites and
metal oxides have Lewis acid sites, these have generally been
regarded as inactive sites for reactions in water, due to the
formation of Lewis acid-base adducts by the coordination of
water to the Lewis acid sites. We have focused on niobic acid,
tively.6,7 Exposure of dehydrated Nb2O5 nH2O (BET surface
3
area: 179 m2 g-1, Brønsted acid amount: 0.17 mmol g-1, Lewis
acid amount: 0.15 mmol g-1, see the Supporting Information
[SI]) to water vapor diminishes the band at 988 cm-1, which
indicates that NbO4 tetrahedra are present only on the Nb2O5 n-
3
H2O surface and NbO4-H2O adducts are immediately formed
in the presence of water. When water-adsorbed Nb2O5 nH2O is
3
dehydrated at 423 K, the band at 988 cm-1 appears again. Such
reversibility between NbO4 tetrahedra and NbO4-H2O adducts
is not observed in isolated NbO4 on other metal oxide surfaces.7
Differential FT-IR spectra for CO-adsorbed dehydrated Nb2O5
3
nH2O are presented in Figure. 2a where CO is employed as a
basic probe molecule. The intensities of the three bands at 2145,
2168, and 2188 cm-1, assignable to physisorbed CO and CO
adsorbed on Brønsted and Lewis acid sites,8 respectively, in-
crease with the amount of introduced CO, and the band
intensities due to acid sites reach plateaus at Pco > 4.8 kPa.
Figure 2b shows the results for CO-adsorption experiments on
hydrated Nb2O5 nH2O with 3.0 mmol of H2O adsorbed on 1 g
3
of sample. Assuming the adsorption cross section area of a H2O
molecule is 0.125 nm2, then 1.3 layers of H2O would be adsorbed
on the sample. Raman spectroscopy confirmed that most of the
NbO4 tetrahedra form NbO4-H2O adducts under the experi-
mental conditions; however, the band due to CO adsorbed on
Nb2O5 nH2O, as a potential solid Lewis acid candidate with
3
activity in water. Nb2O5 nH2O is readily prepared by the
3
hydrolysis of NbCl5 or Nb(OC2H5)5 and is an amorphous
metal oxide composed mainly of distorted NbO6 octahedra
and NbO4 tetrahedra, as shown in Figure S1.4 Nb-O bonds
are highly polarized in these distorted polyhedrons, so that a
part of the surface OH groups function as Brønsted acid
sites.4,5 While NbO4 tetrahedra function as Lewis acid sites,
Received: December 5, 2010
Published: March 03, 2011
r
2011 American Chemical Society
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|

Journal of the American Chemical Society
COMMUNICATION
Table 1. Catalytic Activity for the Allylation of Benzaldehyde
with Tetraallyl Tin in Watera
acid density/mmol g-1
catalystb
Sc(OTf)3
BAc
LAd
2.0
SDS/mmol yielde (%)
-
0.0
0.3
0.0
0.3
0.3
88
38
21
23
21
Figure 1. Raman spectra of Nb2O5 nH2O in a quartz cell. (a)
0.17f
0.14g
0.15f
0.03g
0.17f
0.03g
3
Nb2O5 nH2O
3
Nb2O5 nH2O evacuated at 423 K for 2 h to remove water. (b) Sample
3
(a) exposed to saturated water vapor at 300 K for 3 h. (c) Sample (b)
evacuated at 423 K for 2 h.
Naþ/Nb2O5 nH2O
-
3
-
a Reagents and conditions: distilled water, 15 mL; benzaldehyde, 0.4
mmol; tetraallyl tin, 0.2 mmol; sodium dodecyl sulfate, 0.3 mmol. b 0.1 g.
c Brønsted acid. d Lewis acid. e Yield of 4-phenyl-1-buten-4-ol (%) for 1 h.
f Dehydrated sample. g Water-adsorbed sample (see the SI).
NbO4 tetrahedra resulting in active NbO4-H2O adducts as
Lewis acid sites are inevitably formed during the formation of
Nb2O5 nH2O although the environment surrounding such
3
NbO4 tetrahedra has not been clarified yet.
The catalysis of Nb2O5 nH2O was evaluated through the
3
allylation of benzaldehyde with tetraallyl tin and the conversion
of glucose into 5-(hydroxymethyl)furfural (HMF) in water.
Allylation in water is catalyzed by water-tolerant homogeneous
Lewis acids such as rare earth metal triflates.2,3 Conversion of
glucose, a key component of cellulosic biomass, into HMF is an
attractive alternate route to sustainable chemical production;
HMF can be further converted into various polymers, synthetic
rubbers, plastics and pharmaceuticals.9 HMF formation from
glucose proceeds by isomerization of glucose to fructose, fol-
lowed by dehydration of fructose in the presence of catalysts.
Zhang et al. reported that CrCl2, a homogeneous Lewis acid in
ionic liquids, functions as an efficient catalyst for one-step HMF
formation under mild reaction conditions (ca. 373 K).9c The
hydrolysis of cellulosic biomass generally proceeds in water,
resulting in an aqueous solution of glucose. Therefore, a catalytic
system that could convert glucose into HMF in water, for
example, a water-tolerant heterogeneous Lewis acid, would be
favorable as a large-scale environmentally benign process. Table 1
summarizes the results for allylation in the presence of the tested
catalysts. Sodium dodecyl sulfate (SDS) was used in this experi-
ment as a surfactant, which solubilizes hydrophobic organic
reactants and/or forms emulsion in water.2a Therefore, the
addition of SDS to the reaction system results in a remarkable
increase in the catalytic performance of Sc(OTf)3. Naþ-ex-
changed Nb2O5 nH2O (Naþ/Nb2O5 nH2O) (see the SI),
Figure 2. Differential FT-IR spectra for (A) dehydrated and (B)
hydrated Nb2O5 nH2O at 90 K. (A) Prior to CO adsorption, the
3
sample was heated at 423 K for 1 h under vacuum. Gas phase CO
pressure: (a) 7.1 ꢀ 10-3, (b) 1.2 ꢀ 10-2, (c) 2.2 ꢀ 10-2, (d) 4.0 ꢀ 10-
2, (e) 6.6 ꢀ 10-2, and (f) 1.4 ꢀ 10-1 kPa. () Prior to CO adsorption, the
sample was dehydrated at room temperature for 24 h under vacuum.
Gas-phase CO pressure: (a) 6.1 ꢀ 10-3, (b) 9.3 ꢀ 10-3, (c) 1.4 ꢀ 10-2
,
(d) 2.6 ꢀ 10-2, (e) 4.3 ꢀ 10-2, (f) 6.8 ꢀ 10-2, and (g) 1.4 ꢀ 10-1 kPa.
Lewis acid sites is observed even on hydrated Nb2O5 nH2O. In
3
addition, it was confirmed that the Lewis acid sites interact with
pyridine molecules even on Nb2O5 nH2O in saturated H2O
3
vapor, multilayer adsorption of water (see the SI). A part of the
Lewis acid sites on Nb2O5 nH2O are therefore expected to function
3
as water-tolerant active sites. The amount of NbO4-CO on
hydrated Nb2O5 nH2O was approximately one-fifth of that on
3
dehydrated Nb2O5 nH2O (see the SI). The vibration (2178-
3
2185 cm-1) of CO bonded to NbO4 on the hydrated sample is
weaker than that of the dehydrated sample (2186-2190 cm-1).
This can be attributed to CO lateral interaction with H2O or
dipole-dipole coupling interactions, rather than strong/weak
Lewis NbO4-H2O adducts.
3
3
where the Brønsted acid sites are blocked with Naþ, also
catalyzes the reaction as well as Nb2O5 nH2O. Neither dehy-
3
Such active and inactive NbO4-H2O adducts may be attrib-
uted to the difference in environment surrounding NbO4 tetra-
hedra. There was no significant difference in effective Lewis acid
density (on dehydrated and hydrated samples) and catalytic activity
drated nor hydrated Naþ/Nb2O5 nH2O samples have Brønsted
3
acid sites, and the number of the Lewis acid sites on dehydrated
and hydrated Naþ/Nb2O5 nH2O is the same on dehydrated and
3
hydrated Nb2O5 nH2O, respectively, as shown in Table 1. This
means that the formation of NbO4-H2O adducts does not result
in the formation of Brønsted acid sites on Nb2O5 nH2O.
3
(see below) among commercial Nb2O5 nH2O (supplied by CB-
3
MM) and Nb2O5 nH2O samples prepared by the hydrolysis of
3
3
NbCl5 or Nb(OC2H5)5 at room temperature, suggesting that the
FT-IR measurements confirmed that all the Brønsted acid sites
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dx.doi.org/10.1021/ja110482r |J. Am. Chem. Soc. 2011, 133, 4224–4227

Journal of the American Chemical Society
COMMUNICATION
Table 2. Catalytic Activity for the Conversion of Glucose into
HMF in Watera
formation of polymerized species as shown in Figure S6 (SI).
Complex side reactions, including aldol condensation among
reducing saccharides with formyl groups (-CHO) in the pre-
sence of acid catalysts, result in the formation of complex
polymers as unknown species that cannot be detected by HPLC
and GC-MS.10 Table 2 also shows that H3PO4-treated
Nb2O5 nH2O (H3PO4/Nb2O5 nH2O) results in a large de-
3
3
crease in the undetectable products and an increased HMF yield.
The catalytic performance remained unchanged even after
several catalyst reuses (Figure S7, SI). 31P MAS NMR spectros-
copy (Figure S8, SI) and ICP-AES analysis revealed that 1.0
mmol of phosphate ions are tightly fixed on 1 g of H3PO4/
selectivity %
catalystb
HCl
BAc
LAd conv.e Fruf HMF FAg LAh unknown
Nb2O5 nH2O and ca. 70% (0.10 mmol g-1) of the Brønsted acid
3
9.9
22.4
4.8
0.9
1.1
-
-
-
-
100
100
89
-
-
-
-
5.7 27.1
8.4 56.4
42.3 42.3
9.8 35.4
65.5
35.2
15.4
54.8
64.8
sites are covered with phosphate ions. Considering that the Lewis
acid sites are not covered with phosphate ions, as shown in
Table 2, and the Brønsted acid sites do not play an important role
for the reaction, it is expected that most of the phosphate ions
H2SO4
Amberlyst-15
NafionNR50
H-mordenite
(Si/Al = 90)
H-ZSM-5
65
0.26
12 35.2
-
-
-
-
-
(0.90 mmol g-1) are fixed on neutral OH groups on Nb2O5 n-
3
0.15 0.05
34
-
-
3.8
96.2
84.6
84.6
43.3
H2O, which may reduce undesirable side reactions.
(Si/Al = 90)
In summary, niobic acid Nb2O5 nH2O functions as a hetero-
3
Nb2O5 nH2O
0.17i 0.15i 100
12.1
3.2
2.5
3
geneous catalyst with water-tolerant Lewis acid sites for the
allylation of benzaldehyde with tetraallyl tin and the conversion
of glucose into 5-(hydroxymethyl)furfural (HMF) in water. The
Lewis acid sites workable in water are due to NbO4 tetrahedra
that still have effective positive charges as Lewis acid sites even
after the formation of NbO4-H2O adducts.
0.14j 0.03j
i
Naþ/
-
-
0.17i 100
0.03j
0.5 12.4
0.8 52.1
j
Nb2O5 nH2O
3
H3PO4/
0.04i 0.11i
0.04j 0.02j
92
2.6 1.2
Nb2O5 nH2O
3
a Reagents and conditions: distilled water, 2.0 mL; D-glucose, 0.02 g
(0.11 mmol); temperature, 393 K. b 0.2 g. c Brønsted acid amount (mmol
g-1), d Lewis acid amount (mmol g-1), e Yield conversion (%) for 3 h,
’ ASSOCIATED CONTENT
g
i
j
f Fructose, formic acid, h Levulinic acid Dehydrated sample, Water-
adsorbed sample (see the SI).
S
Supporting Information. Experimental section and Fig-
b
ures S1-S9. This material is available free of charge via the
Internet at http://pubs.acs.org.
on Nb2O5 nH2O were blocked with Naþ (Figure S3, SI), even
3
after the reaction, and they could not function as effective active
sites for the reaction. Therefore, the Lewis acid sites appear to be
active for the reaction, even in water. The addition of SDS to the
reaction system is not effective for increase in catalytic activity of
’ AUTHOR INFORMATION
Corresponding Author
mhara@msl.titech.ac.jp
Nb2O5 nH2O. This may be attributed to the adsorption cap-
3
ability of Nb2O5 nH2O for the reactant molecules without SDS.
3
Table 2 summarizes the results for the conversion of glucose
into HMF in water (393 K). The homogeneous and hetero-
geneous Brønsted acids (HCl, H2SO4, NafionNR50 and Amber-
lyst-15) produce no HMF but instead yield levulinic and formic
acids under reaction conditions. This is due to hydration ability
of Brønsted acids in water:9c the evolved HMF is rapidly
converted into levulinic and formic acids.9d-g In contrast,
’ ACKNOWLEDGMENT
This work was supported by the Research and Development in
a New Interdisciplinary Field Based on Nanotechnology and
Materials Science programs of the Ministry of Education, Cul-
ture, Sports, Science and Technology (MEXT) of Japan.
Nb2O5 nH2O produces HMF yield, and no decrease in activity
3
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3
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