Selective conversion of cellobiose and cellulose into gluconic acid in water in the presence of oxygen, catalyzed by polyoxometalate-supported gold nanoparticles

Article abstract of DOI:10.1002/chem.201103262

Gold nanoparticles loaded onto Keggin-type insoluble polyoxometalates (Cs_xH_3-xPW₁₂O₄₀) showed superior catalytic performances for the direct conversion of cellobiose into gluconic acid in water in the presence of O₂. The selectivity of Au/Cs _xH_3-xPW₁₂O₄₀ for gluconic acid was significantly higher than those of Au catalysts loaded onto typical metal oxides (e.g., SiO₂, Al₂O₃, and TiO₂), carbon nanotubes, and zeolites (H-ZSM-5 and HY). The acidity of polyoxometalates and the mean-size of the Au nanoparticles were the key factors in the catalytic conversion of cellobiose into gluconic acid. The stronger acidity of polyoxometalates not only favored the conversion of cellobiose but also resulted in higher selectivity of gluconic acid by facilitating desorption and inhibiting its further degradation. On the other hand, the smaller Au nanoparticles accelerated the oxidation of glucose (an intermediate) into gluconic acid, thereby leading to increases both in the conversion of cellobiose and in the selectivity of gluconic acid. The Au/Cs_xH _3-xPW₁₂O₄₀ system also catalyzed the conversion of cellulose into gluconic acid with good efficiency, but it could not be used repeatedly owing to the leaching of a H⁺-rich hydrophilic moiety over long-term hydrothermal reactions. We have demonstrated that the combination of H₃PW₁₂O₄₀ and Au/Cs_3.0PW ₁₂O₄₀ afforded excellent yields of gluconic acid (about 85 %, 418 K, 11 h), and the deactivation of the recovered H₃PW ₁₂O₄₀-Au/Cs_3.0PW₁₂O₄₀ catalyst was not serious during repeated use. Crazy glu: Cellulose was oxidized into gluconic acid by combining polyoxometalate (POM)-supported Au nanoparticles (NPs) and acidic POMs. Au-NP size and POM acidity are the determining factors in the selective oxidation of glucose and cellobiose. Copyright

Full text of DOI:10.1002/chem.201103262

DOI: 10.1002/chem.201103262
Selective Conversion of Cellobiose and Cellulose into Gluconic Acid in Water
in the Presence of Oxygen, Catalyzed by Polyoxometalate-Supported Gold
Nanoparticles
Dongli An, Aihua Ye, Weiping Deng,* Qinghong Zhang, and Ye Wang*[a]
Abstract: Gold nanoparticles loaded
onto Keggin-type insoluble polyoxo-
metalates (CsxH3ÀxPW12O40) showed
superior catalytic performances for the
direct conversion of cellobiose into glu-
conic acid in water in the presence of
into gluconic acid. The stronger acidity
of polyoxometalates not only favored
the conversion of cellobiose but also
resulted in higher selectivity of glucon-
ic acid by facilitating desorption and
inhibiting its further degradation. On
the other hand, the smaller Au nano-
particles accelerated the oxidation of
glucose (an intermediate) into gluconic
acid, thereby leading to increases both
in the conversion of cellobiose and in
the selectivity of gluconic acid. The
Au/CsxH3ÀxPW12O40 system also cata-
lyzed the conversion of cellulose into
gluconic acid with good efficiency, but
it could not be used repeatedly owing
to the leaching of a H+-rich hydrophilic
moiety over long-term hydrothermal
reactions. We have demonstrated that
the combination of H3PW12O40 and Au/
Cs3.0PW12O40 afforded excellent yields
of gluconic acid (about 85%, 418 K,
11 h), and the deactivation of the re-
covered H3PW12O40–Au/Cs3.0PW12O40
catalyst was not serious during repeat-
ed use.
O2.
The
selectivity
of
Au/
CsxH3ÀxPW12O40 for gluconic acid was
significantly higher than those of Au
catalysts loaded onto typical metal
oxides (e.g., SiO2, Al2O3, and TiO2),
carbon nanotubes, and zeolites (H-
ZSM-5 and HY). The acidity of poly-
oxometalates and the mean-size of the
Au nanoparticles were the key factors
in the catalytic conversion of cellobiose
Keywords: cellulose
acid gold
catalysis · polyoxometalates
·
gluconic
·
·
heterogeneous
Introduction
lective conversion of cellulose under mild conditions still re-
mains a big challenge owing to its robust crystalline struc-
ture,[3] which results from the extensive hydrogen-bonding
networks formed through b-1,4-glycosidic linkages in cellu-
The transformation of renewable biomass resources into val-
uable chemicals has attracted much attention in recent
years.[1] The most-abundant source of biomass, cellulose, is
non-edible and can be used as a feedstock for the sustaina-
ble production of chemicals.[2] The pyrolysis or gasification
of cellulose at high temperatures, to produce bio-oils or
syngas, suffers from problems of high energy consumption
and low selectivity. From the viewpoint of sustainability, the
development of low-temperature, highly-selective catalytic
routes for the transformation of cellulose directly into valua-
ble chemicals or into a platform molecule (e.g., glucose),
which could then be facilely converted into chemicals in a
subsequent step, is of great significance.[2] However, the se-
À
À
lose. Moreover, there are various C O or C C bonds in cel-
lulose molecules, and the conversion of cellulose into a par-
ticular compound in high selectivities is also a challenging
task. Cellobiose, a d-glucose dimer that is connected by the
same b-1,4-glycosidic linkage as that in cellulose, is the sim-
plest model molecule of cellulose. Studies on the conversion
of cellobiose may provide important clues for the rational
design of efficient catalysts for cellulose transformations.
Although recent studies have demonstrated that the use
of reaction media such as ionic liquids,[4] methanol,[5] or su-
percritical methanol[6] may be beneficial for the catalytic
conversion of cellulose, the catalytic transformation of cellu-
lose in water would be the most desirable. The hydrolysis of
cellulose into glucose in water has been catalyzed by solid
Brønsted acids, such as carbon materials that contain SO3H
groups and layered transition-metal oxides (HNbMoO6),[7,8]
but the yields of glucose were low. Solid Lewis acids, such as
tungsten-containing Al2O3 and -ZrO2, were reported to cata-
lyze the conversion of cellulose into lactic acid in a yield of
27% (463 K, 24 h).[9] Some studies have succeeded in con-
verting cellulose directly into polyols by combining hydroly-
sis and hydrogenation (or hydrogenolysis) reactions.[10–14] Fu-
kuoka and Dhepe[10] reported a 31% yield of hexitols (sorbi-
tol and mannitol) by using a Pt/Al2O3 catalyst for the con-
[a] Dr. D. An, A. Ye, Dr. W. Deng, Prof. Dr. Q. Zhang,
Prof. Dr. Y. Wang
State Key Laboratory of Physical Chemistry of Solid Surfaces and
National Engineering Laboratory for Green Chemical Productions
of Alcohols, Ethers, and Esters
College of Chemistry and Chemical Engineering
Xiamen University, Xiamen 361005 (P. R. China)
Fax : (+86)592-2183047
E-mail: dengwp@xmu.edu.cn
wangye@xmu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103262.
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Chem. Eur. J. 2012, 18, 2938 – 2947

FULL PAPER
version of cellulose under a H2 atmosphere in water at
geneous-supported noble-metal catalysts could efficiently
catalyze the hydrolysis–hydrogenation of cellulose into hexi-
tols.[22] It is known that the Keggin-type polyoxometalates
with H+ ions that have been partially substituted with large
monovalent cations, such as Cs+ ions, are insoluble in water
and are expected to be water-tolerant solid acids.[19c] Howev-
er, to date, only scarce studies have been devoted to the
transformation of cellobiose or cellulose using catalysts
based on these insoluble substituted polyoxometala-
tes.[6,14,18b]
A combination of a polyoxometalate and Au nanoparti-
cles may be promising for the direct conversion of cellulose
into GA through a hydrolysis–oxidation route (Scheme 1).
Herein, we report the catalytic behavior of nanocomposites
of Au nanoparticles and Keggin-type polyoxometalates for
the conversion of cellobiose, the model molecule of cellu-
lose, into GA. Fundamental issues, such as the effects of the
acidity of the polyoxometalates and the size of Au nanopar-
ticles, on catalytic properties are studied in detail. Then,
these data were used for the development of catalysts or
catalytic systems that were capable of converting cellulose
into gluconic acid.
463 K. Kou and co-workers[11] showed that poly(N-vinyl-2-
pyrrolidone)ACHTUNGTRENNUNG(PVP)-stabilized Ru nanoclusters with a mean-
size of 2.4 nm dispersed in H2O could catalyze the hydroge-
nation of cellobiose into sorbitol at 393 K under acidic con-
ditions (pH 2). Liu and co-workers[12] used a combined Ru/C
catalyst with reversible protons generated in hot water
(518 K) to convert cellulose into hexitols with a yield of
almost 40% in the presence of H2. We found that Ru nano-
particles supported on carbon nanotubes (CNTs) that con-
tained acidic functional groups and a Keggin-type polyoxo-
metalate (Cs3PW12O40) with protons generated in the pres-
ence of gaseous H2 could catalyze the conversion of cellu-
[13,14]
lose and cellobiose into sorbitol in the presence of H2
Zhang et al.[15] demonstrated that Ni-modified tungsten–car-
bide catalysts catalyzed the hydrogenolysis of cellulose into
ethylene glycol with high yields under hydrothermal condi-
tions in a H2 atmosphere at 518 K. The transfer hydrogena-
tion of cellulose into hexitols also proceeded by using 2-
propanol as the hydrogen source with a Ru/C catalyst.[16]
In addition to glucose and polyols, organic acids are also
important platform molecules. Gluconic acid (GA) is widely
used in the pharmaceutical and food industries and is
mainly produced by the enzymatic oxidation of glucose.
Many studies have been reported concerning the production
of GA from the selective oxidation of glucose by O2 in the
presence of heterogeneous catalysts, particularly Au-based
catalysts.[17] To the best of our knowledge, there have been
no reports regarding the direct conversion of cellulose into
GA by combining hydrolysis and oxidation reactions
(Scheme 1). Only very few studies have investigated the cat-
alytic conversion of cellobiose, the model molecule of cellu-
lose, into gluconic acid.[18]
Results and Discussion
Structures
and
physicochemical
properties
of
CsxH3ÀxPW12O40-supported Au nanoparticles: XRD patterns
of samples of CsxH3ÀxPW12O40 with different Cs content (x=
1.2–3.0) could be attributed to the cubic-structured Keggin-
type polyoxometalate compounds (see the Supporting Infor-
mation, Figure S1).[23] The lattice structure did not undergo
significant changes after the loading of Au nanoparticles
onto the CsxH3ÀxPW12O40 (x=1.2–3.0) samples. N2 physi-
sorption measurements revealed that the surface areas of
the Au/CsxH3ÀxPW12O40 samples increased with increasing
the Cs content from x=1.2 to 3.0 (Table 1). The surface
areas of the CsxH3ÀxPW12O40 samples also increased accord-
ing to a similar trend. It has been proposed that the substitu-
tion of the smaller H+ ions with larger Cs+ ions in the poly-
oxometalate can cause the formation of micropores and
mesopores.[24]
Table 1. Selected physicochemical properties of the Au/CsxH3ÀxPW12O40
(x=1.2–3.0) catalysts.
Scheme 1. Direct conversion of cellulose into gluconic acid by using hy-
drolysis and oxidation reactions.
Catalyst
Surface area NH3 desorption[a]
[m2 gÀ1 [mmolgÀ1
Mean-size of
U
]
A
]
Au NPs[b] [nm]
Au/Cs1.2H1.8PW12O40 35
Au/Cs1.7H1.3PW12O40 47
Au/Cs2.2H0.8PW12O40 56
Au/Cs2.6H0.4PW12O40 73
121
90
76
31
trace
2.7
2.6
2.5
2.5
2.7
Polyoxometalates have attracted much attention as water-
tolerant Brønsted acid catalysts.[19] It has been reported that
the acidity of Keggin-type H3PW12O40 is stronger than that
of H-ZSM-5, and even stronger than that of H2SO4.[20] Sev-
eral recent studies have demonstrated that H3PW12O40 and
H4SiW12O40 are active for the hydrolysis or alcoholysis of
cellobiose and cellulose into glucose or alkyl glucosides.[5,21]
The combination of homogeneous H3PW12O40 with hetero-
Au/Cs3.0PW12O40
83
[a] Calculated from NH3-TPD analysis. [b] Evaluated by TEM.
The acidities of the Au/CsxH3ÀxPW12O40 catalysts were in-
vestigated by NH3-TPD. A major peak at 850–855 K was ob-
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2939

W. Deng, Y. Wang et al.
served, which could be ascribed to desorption of NH3 from
the strong Brønsted acid sites on the Keggin-type polyoxo-
metalates (Figure 1).[24] Quantification revealed that the
Figure 1. NH3-TPD profiles for the Au/CsxH3ÀxPW12O40 (x=1.2–3.0) cata-
lysts.
amount of NH3 desorbed gradually decreased with increas-
ing Cs content (Table 1). Only a trace amount of NH3 de-
sorption was observed when x increased to 3.0. Recently,
Han and co-workers[18b] argued that the Au species in Au/
Cs2HPW12O40, which was prepared by an impregnation
method, might be cationic. Our XPS studies showed that the
binding energies of Au 4f7/2 were about 83.8 eV for all sam-
ples of Au/CsxH3ÀxPW12O40 with different Cs content pre-
pared by the deposition–precipitation (DP) method (see the
Supporting Information, Figure S2), thus indicating that the
Au species were in the metallic state in these catalysts. This
difference may arise from the different preparation meth-
ods. TEM micrographs of some typical catalysts are shown
in Figure 2. The CsxH3ÀxPW12O40 supports were almost
spherical in morphology with diameters of 20–100 nm. Au
nanoparticles with sizes smaller than 5–6 nm (mostly 1.5–
4 nm) were located over the CsxH3-xPW12O40 nanospheres.
The particle-size distributions for these catalysts were also
plotted. The mean-sizes of the Au particles in Au/
CsxH3ÀxPW12O40 (x=1.2–3.0) were almost all the same, in
the range 2.5–2.7 nm (Table 1).
Figure 2. Typical TEM micrographs and particle-size distributions for the
Au/CsxH3ÀxPW12O40 (x=1.2–3.0) catalysts: a) x=1.2, b) x=1.7, c) x=2.2,
and d) x=3.0.
cellobiose conversions for the catalysts with different Cs
contents, the performances of Au/CsxH3ÀxPW12O40 catalysts
after 1 h at 418 K are also shown (Table 2, entries 6–10).
The cellobiose conversion with catalysts with higher Cs con-
tent was lower than those with smaller Cs content. Correla-
tion of these observations with the characterization results
(Table 1) suggested that stronger acidity favored both the
activity and the selectivity for the oxidative conversion of
cellobiose into gluconic acid.
The selectivities of gluconic acid obtained with metal-
oxide-supported Au catalysts were significantly lower than
those with polyoxometalate-supported catalysts; although
some of them (Au/Al2O3 and Au/TiO2) also provided higher
cellobiose conversions. Au/H-ZSM-5 showed moderate GA
selectivity but lower activity, possibly because the cellobiose
molecules could not enter the micropores of H-ZSM-5,
where the acid sites were located. Compared to Au/H-ZSM-
5, Au/HY, which had larger micropores, exhibited relatively
Catalytic properties of CsxH3ÀxPW12O40-supported Au nano-
particles for the oxidative conversion of cellobiose: Table 2
shows the catalytic behaviors of the Au/CsxH3ÀxPW12O40 cat-
alysts for the oxidative conversion of cellobiose in the pres-
ence of O2 in water. For comparison, the performances of
Au nanoparticles loaded onto several metal oxides, H-ZSM-
5, HY, and a carbon nanotube (CNT) containing acidic func-
tional groups[18a] are also shown. The Au/CsxH3ÀxPW12O40
catalysts with relatively lower Cs contents (x=1.2, 1.7, and
2.2) exhibited cellobiose conversion and gluconic acid selec-
tivityꢀ95% after 3 h at 418 K. A further increase in Cs con-
tent to xꢀ2.6 decreased the selectivity. With the Au/
Cs3.0PW12O40 catalyst, the GA selectivity was 85%, which
was similar to that with Au/CNTs. To distinguish between
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Conversion of Cellobiose and Cellulose into Gluconic Acid
FULL PAPER
Table 2. Oxidative conversion of cellobiose by O2 in H2O with Au/
CsxH3ÀxPW12O40 (x=1.2–3.0) catalysts as well as Au catalysts loaded onto
several metal oxides, zeolites, and CNTs.[a]
Catalyst
Conv. [%]
Selectivity[b] [%]
Yield
GA
Glucose
GA[%]
1
2
3
4
5
Au/Cs1.2H1.8PW12O40
Au/Cs1.7H1.3PW12O40
Au/Cs2.2H0.8PW12O40
Au/Cs2.6H0.4PW12O40
Au/Cs3.0PW12O40
Au/Cs1.2H1.8PW12O40
Au/Cs1.7H1.3PW12O40
Au/Cs2.2H0.8PW12O40
Au/Cs2.6H0.4PW12O40
Au/Cs3.0PW12O40
Au/SiO2
97
98
96
97
95
73
70
66
60
58
67
95
96
45
64
84
>99
96
95
90
85
96
95
92
88
82
61
31
63
76
62
86
0
1
1
0
0
2
2
3
5
5
0
0
0
0
0
0
97
94
91
87
81
70
67
61
53
48
41
29
60
34
40
72
6[c]
7[c]
8[c]
9[c]
10[c]
11
12
13
14
15
16
Figure 3. Conversions of gluconic acid with Au catalysts loaded onto vari-
ous supports. Reaction conditions: catalyst (0.050 g, Au loading:
1.0 wt%), gluconic acid (10 mmolLÀ1), H2O (25 mL), P(O2)=0.5 MPa,
T=418 K, t=1 h.
Au/Al2O3
Au/TiO2
Au/H-ZSM-5
Au/HY
Au/CNT
Effect of mean-sizes of Au nanoparticles on the oxidation of
cellobiose and glucose into gluconic acid: It is well-known
that the size of Au nanoparticles significantly affects their
catalytic behavior in many reactions.[25] We found that the
mean-size of Au nanoparticles with Cs2.2H0.8PW12O40 could
be easily regulated in the range 2.5–20 nm by changing the
concentration of the Au precursor (HAuCl4) or the aging
time used in the preparation of the catalyst by the DP
method. Table 3 shows the conditions used and the corre-
sponding mean-sizes of Au nanoparticles, derived from
[a] Reaction conditions: catalyst (0.050 g, Au loading: 1.0 wt%), cello-
biose (12 mmolLÀ1), H2O (25 mL), P(O2)=0.5 MPa, T=418 K, t=3 h.
[b] GA=gluconic acid. The remaining products included acetylpropionic
acid, glucuronic acid, acetic acid, glyceric acid, and malic acid. [c] t=1 h.
higher cellobiose conversion. Our TEM studies clarified that
the mean-sizes of Au nanoparticles with metal oxides and
H-ZSM-5 were in the range 2.5–2.8 nm (see the Supporting
Information, Figure S3), which were quite similar to those
with CsxH3ÀxPW12O40. These observations allowed us to con-
clude that the nature of the support played a key role in de-
termining the catalytic behavior, particularly in determining
the selectivity for GA.
Table 3. Effect of preparation conditions on the mean-size and dispersion
of Au nanoparticles and on the acidity of the Au/Cs2.2H0.8PW12O40 cata-
lysts.
A
Aging t
[h]
Mean-size of
Au disper-
sion[b] [%]
NH3 desorp-
The consecutive conversion of GA was expected to affect
its selectivity in the oxidative conversion of cellobiose
(Table 2). Therefore, we investigated the conversion of GA
with Au catalysts loaded onto different supports under reac-
tion conditions similar to those used for cellobiose oxida-
tion. For the series of Au/CsxH3ÀxPW12O40 catalysts, the con-
version of gluconic acid increased gradually with increasing
Cs content from x=1.2 to 3.0 (Figure 3). This increasing se-
quence corresponded well with a decrease in the selectivity
for GA in cellobiose oxidation with increasing Cs content
(Table 2). We speculated that the higher acidity of the cata-
lyst at lower Cs content may hinder the adsorption of GA,
thereby resulting in the lower conversion. In other words,
the more-facile desorption of gluconic acid from catalysts
with stronger acidities may contribute to their higher selec-
tivities for gluconic acid (Table 2). Figure 3 shows that the
Au/Al2O3 was the most-active catalyst for the conversion of
gluconic acid. This observation may be the main reason for
it affording the lowest selectivity for gluconic acid in the ox-
idative conversion of cellobiose. Compared to the metal-
oxide-supported catalysts, the CsxH3ÀxPW12O40-supported
Au catalysts were relatively inert for the conversion of glu-
conic acid. This feature made these catalysts more suitable
for the selective formation of gluconic acid.
[mm]
Au NPs[a] [nm]
tion [mmolgÀ1
]
0.12
0.8
0.24
0.8
1.2
0.8
5
5
16
7
4
10
16
2.5
3.3
4.2
5.0
6.3
12
49 (47)
38 (35)
29 (28)
26 (23)
21 (19)
12 (9.6)
9.6 (5.9)
n.d.
76
n.d.
78
n.d.
77
80
0.8
20
[a] Obtained from TEM. [b] Measured by H2–O2 titration, and estimated
from the mean-size of Au NPs (in parenthesis).
TEM measurements for the final catalysts after H2 reduction
(see the Supporting Information, Figure S4). An increase in
the concentration of HAuCl4 increased the mean-size of the
Au nanoparticles. We expected that a larger number of Au
precursors per unit volume would increase the probability
of collision among the particles during the particle-growth
process, and would lead to larger Au nanoparticles.[26] Pro-
longing the aging time also significantly increased the size of
the Au nanoparticles, probably owing to Ostwald ripening,
which was caused by the dissolved smaller particles precipi-
tating onto the larger particles.[26]
We also measured the dispersion of Au particles over
these catalysts by H2–O2 titration.[27] By assuming the spheri-
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W. Deng, Y. Wang et al.
cal model of Au nanoparticles, the Au dispersion (D) was
roughly estimated by using the following relationship: D=
1.17/dꢁ100%, in which d is particle diameter in nm.[28] The
dispersions of Au nanoparticles thus estimated are also
listed in Table 3. The Au dispersions obtained from the two
techniques were consistent with each other, thereby validat-
ing the mean-size of Au nanoparticles for each catalyst.
Because the acidity may also affect the catalytic perfor-
mance, we performed NH3-TPD measurements for some
typical Au/Cs2.2H0.8PW12O40 catalysts with different mean-
sizes of Au nanoparticles. These results clarified that both
the NH3-desorption temperature and the NH3-desorption
amount (Table 3) were not significantly different for these
Au/Cs2.2H0.8PW12O40 catalysts with different mean-sizes of
Au nanoparticles, although Cs2.2H0.8PW12O40 alone, without
the loading of Au particles, showed a larger amount of NH3
desorption (see the Supporting Information, Figure S5).
Thus, the loading of Au slightly decreased the number of
acid sites, but the acidity was independent of the mean-size
of Au nanoparticles in the Au/Cs2.2H0.8PW12O40 catalysts.
We studied the catalytic behavior of Au/Cs2.2H0.8PW12O40
catalysts with different mean-sizes of Au nanoparticles for
the conversion of cellobiose. Table 4 shows the results ob-
tained at 418 K after 0.67–6 h. Cs2.2H0.8PW12O40 without Au
NPs only catalyzed the hydrolysis of cellobiose into glucose,
and the conversion of cellobiose was quite low under such a
mild temperature (Table 4, entries 1 and 2). After loading
the Au particles, the conversion of cellobiose increased sig-
nificantly, and GA was formed as a major product, thus sug-
gesting that the Au nanoparticles accelerated the conversion
of cellobiose and were responsible for the oxidation of glu-
cose, a possible intermediate during the conversion of cello-
biose into gluconic acid. For each supported Au catalyst,
prolonging the reaction time increased the selectivity for
gluconic acid and decreased that for glucose (Table 4),
thereby clearly suggesting that the oxidation of cellobiose
proceeded through a two-step route, that is, the hydrolysis
of cellobiose into glucose followed by the oxidation of glu-
cose into gluconic acid. Both the cellobiose conversion and
the gluconic acid selectivity with the Au/Cs2.2H0.8PW12O40
catalysts were dependent on the mean-size of the Au nano-
particles: the smaller the mean-size, the higher the cello-
biose conversion and the higher the gluconic acid selectivity.
We expected that Au nanoparticles mainly participated in
the oxidation of glucose. To gain an insight into the effect of
Au particle size, we performed the oxidation of glucose with
Au/Cs2.2H0.8PW12O40 catalysts with different mean-sizes of
Au nanoparticles under reaction conditions similar to those
used for the oxidation of cellobiose. We measured glucose
conversions at the initial reaction stage for these catalysts
and evaluated the initial rate of glucose conversion
(Table 5) by using the slopes of the straight lines obtained in
Table 5. Effect of mean-size of Au nanoparticles on the oxidative conver-
sion of cellobiose and glucose with Au/Cs2.2H0.8PW12O40.
Mean Au
size [nm]
(cat)À1 hÀ1
Initial conversion rate [molgACHNUTGRTNEGNU ]
Glucose
Cellobiose
conversion[a]
conversion[b]
2.5
3.3
4.2
5.0
6.3
12
234
168
114
84
60
12
17.1
14.4
12.2
11.2
9.0
6.8
20
6.0
5.8
[a] Reaction conditions: Catalyst (0.0020 g), glucose (24 mmolLÀ1), H2O
(25 mL), P(O2)=0.5 MPa, T=418 K. [b] Reaction conditions: Catalyst
(0.050 g), cellobiose (12 mmolLÀ1), H2O (25 mL), P(O2)=0.5 MPa, T=
418 K.
the plots of the conversion versus reaction time (see the
Supporting Information, Figure S6). The selectivity for GA
was>99% with each catalyst under these conditions. We
also calculated the turnover frequency (TOF), that is, the
moles of glucose converted per unit time at the initial stage
per mole of surface Au atoms. The catalysts with smaller Au
nanoparticles exhibited higher TOFs for the oxidation of
glucose into gluconic acid (Figure 4). A study of unsupport-
ed Au nanoparticles dispersed in water suggested that the
TOF based on surface Au atoms was independent of the
size of Au NPs in glucose oxidation,[17a] whereas Haruta and
co-workers[17g] observed an increase in the TOF for the
same reaction with a decrease in the mean diameter of Au
particles loaded onto the metal oxides. Our results suggested
that the oxidation of glucose with Au/Cs2.2H0.8PW12O40 was a
structure-sensitive reaction. The facile activation of O2 or
glucose may be the reason for the higher TOFs with the
smaller Au nanoparticles,[29] which contained higher propor-
tions of coordinatively unsaturated Au sites.[26]
Table 4. Effect of the mean-size of Au nanoparticles on the oxidative
conversion of cellobiose with Au/Cs2.2H0.8PW12O40.[a]
Mean-size of
Au NPs [nm]
t
Conv.
[%]
Selectivity[b] [%]
Yield
GA [%]
[h]
GA
Glucose
1[c]
2[c]
3
4
5
6
7
8
9
–
–
0.67
3
0.67
3
0.67
3
0.67
3
0.67
3
0.67
3
0.67
3
0.67
3
6
25
38
96
32
89
27
80
25
78
20
73
15
65
13
49
94
0
0
100
92
12
1.0
13
1.0
20
9.0
29
13
36
18
49
23
48
26
10
0
0
2.5
2.5
3.3
3.3
4.2
4.2
5.0
5.0
6.3
6.3
12
88
95
87
97
80
83
71
75
64
71
51
62
52
59
86
33
91
28
86
22
66
18
59
13
52
8
10
11
12
13
14
15
16
17
12
20
20
20
40
7
29
81
6
[a] Reaction conditions: catalyst (0.050 g, Au loading: 1.0 wt%), cello-
biose (12 mmolLÀ1), H2O (25 mL), P(O2)=0.5 MPa, T=418 K. [b] GA=
gluconic acid. [c] Cs2.2H0.8PW12O40 alone.
For comparison, the rates of cellobiose conversion with
these catalysts were also evaluated using the data for shorter
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Chem. Eur. J. 2012, 18, 2938 – 2947

Conversion of Cellobiose and Cellulose into Gluconic Acid
FULL PAPER
Table 6. Effect of the addition of gluconic acid on the conversion of cel-
lobiose with Cs2.2H0.8PW12O40 and Au/Cs2.2H0.8PW12O40.[a]
Catalyst
GA
Conv.
[%]
Selectivity [%]
addition[b]
GA
Glucose
1
2
3
4
5
6
–
–
no
yes
no
2
7
6
12
13
24
0
0
0
0
52
44
100
100
100
100
48
Cs2.2H0.8PW12O40
Cs2.2H0.8PW12O40
Au (20 nm)/Cs2.2H0.8PW12O40 no
Au (20 nm)/Cs2.2H0.8PW12O40 yes
yes
56
[a] Reaction conditions: catalyst (0.050 g, Au loading: 1.0 wt%), cello-
biose (12 mmolLÀ1), H2O (25 mL), P(O2)=0.5 MPa, T=418 K, t=
40 min. [b] GA=gluconic acid, amount of GA added=15 mmol.
Figure 4. TOFs for the oxidation of glucose into gluconic acid by using
Au/Cs2.2H0.8PW12O40 catalysts with different mean-sizes of Au nanoparti-
cles. Reaction conditions: glucose (24 mmolLÀ1), H2O (25 mL), P(O2)=
0.5 MPa, T=418 K.
of cellobiose conversions could be explained by the fact that
the smaller Au nanoparticles exhibited higher activity for
the conversion of glucose into gluconic acid.
reaction times (0.67 h; Table 4). As shown in Table 5, for a
sample with a larger mean-size of Au nanoparticles, the rate
of cellobiose conversion was similar to that of glucose con-
version. This result suggested that the oxidation of glucose
was a rate-determining step during the oxidative conversion
of cellobiose with catalysts with larger Au nanoparticles.
This result explains the observation that a larger proportion
of glucose remained for these catalysts (Table 4). Decreasing
the mean-size of Au nanoparticles caused the rate of glucose
conversion to rise more steeply, although the rates of both
glucose and cellobiose conversions increased. As a result,
the rate of glucose oxidation was remarkably higher than
that of cellobiose conversion with catalysts with smaller Au
nanoparticles. Thus, the selectivity for glucose decreased
and that for gluconic acid increased significantly with de-
creasing mean-size of Au nanoparticles (Table 4). Clearly,
with the catalysts with smaller Au nanoparticles, the hydrol-
ysis of cellobiose into glucose became the rate-determining
step.
The rate of cellobiose conversion increased gradually with
decreasing mean-size of Au nanoparticles, even for the cata-
lysts that exhibited much-higher rates of glucose oxidation
(Table 5). These catalysts possessed almost the same acidity
(Table 3; also see the Supporting Information, Figure S5),
which should not influence the rate of cellobiose hydrolysis.
We speculated that the gluconic acid formed in the presence
of Au nanoparticles may function as a liquid acid and may
accelerate the hydrolysis of cellobiose. This acceleration
may be one of the reasons why the presence of Au NPs sig-
nificantly accelerated the conversion of cellobiose, whereas
the catalyst acidity was slightly decreased. We found that
the addition of gluconic acid into the reaction medium
caused enhancement in the hydrolysis of cellobiose in the
case of a blank reaction, with Cs2.2H0.8PW12O40, and with Au/
Cs2.2H0.8PW12O40 with a larger mean-size of Au nanoparti-
cles, where glucose was the main product (Table 6). This
result confirmed the enhancing effect of gluconic acid
formed with the bifunctional catalyst on the overall rate of
conversion of cellobiose. The observation that the catalysts
containing smaller Au nanoparticles exhibited higher rates
Recycling the polyoxometalate-supported Au catalyst for
the oxidative conversion of cellobiose: The results described
above suggested that acidity and Au-particle size are two
crucial factors in determining the catalytic behavior of the
polyoxometalate-supported Au nanoparticles for the oxida-
tion of cellobiose. The Au/Cs1.2H1.8PW12O40 catalysts with
larger concentrations of acid sites and a mean-size of Au
particles of 2.7 nm afforded a gluconic acid yield of 97% at
418 K after 3 h. Besides activity and selectivity, the stability
of a catalyst is also a key factor for its practical application.
Thus, we examined the reusability of this catalyst for the ox-
idative conversion of cellobiose. After each run, the catalyst
was recovered by filtration, washed, and dried, and then re-
used in the next run. Figure 5 shows that both the conver-
sion of cellobiose and the selectivity for gluconic acid de-
creased only slightly over repeated use. After two cycles,
glucose was also formed with low selectivity, thereby indi-
cating that the ability to oxidize glucose into gluconic acid
declined, presumably because of the aggregation of Au
nanoparticles. Despite this slight decrease, after five cycles,
Figure 5. Recycling of Au/Cs1.2H1.8PW12O40 for the oxidative conversion
of cellobiose. Reaction conditions: catalyst (0.050 g, Au loading:
1.0 wt%), cellobiose (12 mmolLÀ1), H2O (25 mL), P(O2)=0.5 MPa, T=
418 K, t=3 h.
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2943

W. Deng, Y. Wang et al.
the cellobiose conversion and the gluconic acid selectivity
remained >90%.
Catalytic conversion of cellulose into gluconic acid in water
in the presence of O2: To date, there has been no report re-
garding the direct conversion of cellulose into gluconic acid
by O2 with reasonable selectivity. We investigated the cata-
lytic behavior of Au/CsxH3ÀxPW12O40 for the conversion of
ball-milled cellulose (crystallinity: 33%) in water in the
presence of O2 at a relatively mild temperature (418 K).
Figure 6 shows that catalysts with x=1.2–2.6 provided excel-
Figure 7. Recycling of the Au/Cs1.2H1.8PW12O40 catalyst for the oxidative
conversion of cellulose. Reaction conditions: catalyst (0.20 g, Au loading:
1.0 wt%), ball-milled cellulose (0.20 g), H2O (10 mL), P(O2)=1.0 MPa,
T=418 K, t=11 h.
unfortunately, despite the high effectiveness, the H+-contain-
ing polyoxometalates were not suitable for the conversion
of cellulose under hydrothermal conditions over long reac-
tion times. Catalyst deactivation during the recycling for cel-
lulose conversion was more significant than that for cello-
biose conversion. We speculated that this result was because
the conversion of cellulose was more difficult than that of
cellobiose. A similar conclusion was reached for the trans-
formation of cellulose into sorbitol in water in the presence
of H2 with Ru/CsxH3.0ÀxPW12O40 catalysts.[14]
Figure 6. Oxidative conversion of cellulose in the presence of O2 in H2O
with Au/CsxH3.0ÀxPW12O40 (x=1.2–3.0), Au/Al2O3, and Au/TiO2. Reaction
conditions: catalyst (0.20 g, Au loading: 1.0 wt%), ball-milled cellulose
(0.20 g), H2O (10 mL); P(O2)=1.0 MPa, T=418 K, t=11 h.
On the other hand, Cs3.0PW12O40 has been demonstrated
to be stable under hydrothermal conditions.[14] We also
found that the Au/Cs3.0PW12O40 catalyst showed superior
performance to Au catalysts loaded onto metal oxides for
the oxidative conversion of cellobiose into gluconic acid
(Table 2). This catalyst was relatively inert toward the con-
secutive degradation of gluconic acid as compared to other
metal-oxide-supported Au catalysts (Figure 3), and was ad-
vantageous for the selective formation of gluconic acid. We
also confirmed that this catalyst was quite efficient and
stable for the oxidation of glucose into gluconic acid (see
the Supporting Information, Table S1). However, Au/
Cs3.0PW12O40 does not possess strong-enough Brønsted acidi-
ty required for the hydrolysis of cellulose. Our strategy was
to combine this catalyst with H3PW12O40, a strong Brønsted
acid. Although H3PW12O40 is dissolved in water during the
reaction, it can be recovered by recrystallization by extrac-
tion with diethyl ether after the reaction. Moreover, the
problem of corrosion of H3PW12O40 is not as serious as that
of other liquid acids, such as H2SO4. The combination of
H3PW12O40 with supported noble-metal catalysts (e.g., Ru/
C) has been demonstrated to be effective for the conversion
of cellulose into polyols.[22] Herein, we examined the possi-
bility of the combination of H3PW12O40 and Au/Cs3.0PW12O40
for the oxidative conversion of cellulose into gluconic acid
in H2O.
lent yields of gluconic acid. GA yields of 47–60% were ach-
ieved with selectivities of 79–87%, depending on the Cs
content after 11 h at 418 K. These performances were com-
parable to those obtained for the conversion of cellulose
into polyols (e.g., sorbitol) by hydrolysis–hydrogenation,
where relatively higher reaction temperatures (>433 K)
were required and H2 was consumed.
We examined the stability of Au/Cs1.2H1.8PW12O40 for the
oxidative conversion of cellulose in water at 418 K. The con-
version of cellulose decreased remarkably in the second and
the third cycles, although the selectivity for gluconic acid
only changed slightly (Figure 7). After three cycles, the cel-
lulose conversion and gluconic acid yield decreased from
about 70% and 60% to 41% and 32%, respectively. We
speculated that the relatively long reaction time (11 h)
under hydrothermal conditions may cause changes in the
structure of Cs1.2H1.8PW12O40. Vꢂdrine and co-workers[30]
suggested that CsxH3.0ÀxPW12O40 might be composed of
Cs3PW12O40 with H3PW12O40 dispersed in its pores, and that
the latter might be dissolved in hot water. Okuhara and co-
workers[31] demonstrated that protons were homogeneously
dispersed in CsxH3.0ÀxPW12O40, but the treatment of
Cs2.5H0.5PW12O40 in water at 393 K could cause leaching of
the H+-rich hydrophilic moiety into water. In fact, we ob-
tained H3PW12O40 from the reaction solution by using dieth-
yl ether as an extracting agent after the filtration of the
solid Au/Cs1.2H1.8PW12O40 catalyst. This result indicated that,
Table 7 shows the effect of O2 pressure on the conversion
of ball-milled cellulose (crystallinity: 33%) by using the
H3PW12O40–Au/Cs3.0PW12O40 catalyst at 418 K. The cellulose
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Conversion of Cellobiose and Cellulose into Gluconic Acid
FULL PAPER
Table 7. Oxidative conversions of cellulose in water catalyzed by
H3PW12O40–Au/Cs3.0PW12O40.[a]
mation, Figures S7 and S8). We also measured the Au con-
tent in solution after the reaction by ICP-MS, and almost no
Au was detected in the liquid, thereby indicating that the
leaching of Au was negligible. On the other hand, we found
that the Au nanoparticles in Au/Cs3.0PW12O40 grew slightly
from 2.7 to 3.9 nm after the sixth cycle (see the Supporting
Information, Figure S9). This result might explain the slight
decrease in the catalytic performances during the initial four
reaction cycles.
O2 pressure
[MPa]
Conv.
[%]
Selectivity
Yield of
G
of GA [%][b]
GA [%][b]
1
2
0.5
1.0
1.5
1.0
72
87
97
49
90
89
88
80
65
77
85
39
3
4[c]
[a] Reaction conditions: catalyst H3PW12O40 (0.10 g)-Au/Cs3.0PW12O40
(0.080 g, Au loading: 1.0 wt%), ball-milled cellulose (0.20 g), H2O
(10 mL), T=418 K, t=11 h. [b] GA=gluconic acid. [c] Microcrystalline
cellulose (0.20 g).
Conclusion
Keggin-type polyoxometalate-supported Au nanoparticles,
that is, Au/CsxH3ÀxPW12O40, were highly efficient for the oxi-
dative conversion of cellobiose into gluconic acid in the
presence of O2 in water without pH control. These catalysts
exhibited significantly higher selectivity for gluconic acid
than Au catalysts supported on typical metal oxides (SiO2,
Al2O3, and TiO2), H-ZSM-5, HY, and CNTs. The consecu-
tive conversion of gluconic acid by Au/CsxH3ÀxPW12O40 was
slower. We investigated the effects of the acidity of the cata-
lysts and of the mean-size of the Au nanoparticles by study-
ing two series of Au/CsxH3.0ÀxPW12O40 catalysts: 1) with the
same mean-size of Au but different acidities, and 2) with the
same acidity but different Au mean-sizes. Both the acidity
and mean-size of the Au nanoparticles played crucial roles
in the conversion of cellobiose into gluconic acid. The acidic
sites were proposed to catalyze the hydrolysis of cellobiose,
with the formation of glucose as an intermediate, whilst the
Au nanoparticles accounted for the oxidation of glucose
into gluconic acid. The acidity also contributed to the selec-
tive formation of gluconic acid by suppressing its consecu-
tive conversion. On the other hand, the smaller Au nanopar-
ticles accelerated the oxidation of glucose, thereby resulting
in higher selectivity for gluconic acid. For the catalysts con-
taining smaller Au nanoparticles, the rate of cellobiose con-
version was determined by the acid-catalyzed hydrolysis
step. Catalysts with smaller Au nanoparticles showed higher
conversions of cellobiose, presumably because the gluconic
acid formed in the reaction could promote the hydrolysis re-
action. A gluconic acid yield of 97% was attained by using
Au/Cs1.2H1.8PW12O40 with a mean Au particle size of 2.7 nm,
at 418 K for 3 h. We demonstrated that Au/CsxH3.0ÀxPW12O40
could efficiently catalyze the oxidative conversion of cellu-
lose into gluconic acid with considerable selectivity (about
80%) and yield (about 60%) in water at 418 K. However,
the CsxH3.0ÀxPW12O40 system was unstable over long-term
hydrothermal conditions, and Au/CsxH3.0ÀxPW12O40 became
deactivated for the conversion of cellulose during repeated
use. Serious deactivation could be overcome by using Au/
Cs3.0PW12O40 in combination with H3PW12O40. This combina-
tion afforded a gluconic acid yield of 85% at 418 K for the
conversion of the ball-milled cellulose, and a gluconic acid
yield of 39% for the conversion of microcrystalline cellu-
lose. H3PW12O40–Au/Cs3.0PW12O40 could be recovered from
the liquid phase and used repeatedly.
conversion increased with O2 pressure, whilst the selectivity
for gluconic acid decreased slightly. A glucose yield of 85%
was obtained with a selectivity of 88% as the O2 pressure
rose to 1.5 MPa. The combination of H3PW12O40–Au/
Cs3.0PW12O40 was also employed to transform the microcrys-
talline cellulose (crystallinity: 85%) in water at 418 K. Mi-
crocrystalline cellulose without pretreatment was also con-
verted into GA in 39% yield and a selectivity of 80% after
11 h.
We considered the repeated use of H3PW12O40–Au/
Cs3.0PW12O40 for the conversion of ball-milled cellulose.
After each run, the solid Au/Cs3.0PW12O40 was first separat-
ed from the reaction mixture by simple centrifugation.
Then, H3PW12O40 was recovered through recrystallization by
extraction with diethyl ether. Figure 8 shows the results ob-
tained with the recovered catalyst. The conversion of cellu-
lose and the selectivity of GA only decreased slightly during
the initial four cycles and then remained almost unchanged
during the fifth and the sixth cycles. After six cycles, the cel-
lulose conversion, GA selectivity, and GA yield were 81%,
81%, and 66%, respectively. We characterized the catalyst
after the sixth recycle. The Keggin-type structures of both
H3PW12O40 and Au/Cs3.0PW12O40 did not undergo significant
changes after repeated reactions (see the Supporting Infor-
Figure 8. Recycling of the H3PW12O40–Au/Cs3.0PW12O40 catalyst for the
oxidative conversion of cellulose into gluconic acid in presence of O2 in
water. Reaction conditions: H3PW12O40 (0.10 g), Au/Cs3.0PW12O40
(0.080 g, Au loading: 1.0 wt%), ball-milled cellulose (0.20 g), H2O
(10 mL), P(O2)=1.0 MPa, T=418 K, t=11 h.
Chem. Eur. J. 2012, 18, 2938 – 2947
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2945

W. Deng, Y. Wang et al.
Acknowledgements
Experimental Section
This work was supported by the National Natural Science Foundation of
China (Nos. 21173172, 21103143, 21033006, and 20873110), the National
Basic Research Program of China (No. 2010CB732303), and the Program
for Changjiang Scholars and Innovative Research Team in University
(No. IRT1036).
Catalyst preparation: The Cs salts of Keggin-type tungstophosphoric
acids with different Cs content (CsxH3ÀxPW12O40, x=1.2–3.0) were pre-
pared by the reaction between H3PW12O40 and Cs2CO3 in aqueous solu-
[24]
tion with different molar ratios of Cs /
(PW12O40)3À
.
The precipitates
obtained were recovered by filtration, washed with deionized water, and
dried at 373 K for 2 h to obtain CsxH3ÀxPW12O40. The supported Au cata-
lysts were prepared by a deposition–precipitation (DP) method, by using
urea as a homogeneous precipitant. Typically, first, urea was added into
an aqueous solution of HAuCl4 (0.12 mmolLÀ1) with a molar ratio of
urea/Au of 100:1. Then, the support was added into the mixed aqueous
solution, and the suspension was vigorously stirred at 353 K. After aging
for a certain time (typically 5 h), the solid product was recovered by cen-
trifugation followed by thorough washing with deionized water to
remove the remaining chloride anions. The resulting powdery solid was
dried in air at 353 K for 1 h, and finally reduced in H2 at 523 K for 2 h.
The concentration of HAuCl4 and the aging time were regulated to con-
trol the size of the Au nanoparticles, as reported previously.[26] The target
Au loading in each catalyst was 1.0 wt%.
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with a Micromeritics Tristar 3000 Surface Area and Porosimetry analyzer
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Catalytic reaction: The conversions of cellobiose and cellulose were per-
formed in a batch-type Teflon-lined stainless-steel autoclave. Typically,
for the conversion of cellobiose, cellobiose (Alfa Aesar; 0.30 mmol) and
the catalyst (typically, 0.050 g) were added into a reactor that had been
precharged with H2O (25 mL). No pH regulation was carried out for the
accumulation of gluconic acid. The oil bath for heating the reaction
system was preheated to the reaction temperature. After the introduction
of pressurized O2 (typically 0.5 MPa), the reactor was placed in an oil
bath, and the temperature of the reaction system increased at a rate of>
10 KminÀ1. After the system reached the reaction temperature (typically
418 K), the reaction was started by vigorous stirring. We confirmed that
almost no reaction occurred during the heating process without stirring.
After a fixed reaction time, the reaction was quickly stopped by cooling
the reactor to RT in cold water. The liquid products were analyzed by
HPLC (Shimazu LC-20 A) equipped with an RI detector and a Shodex
SUGARSH-1011 column (8ꢁ300 mm) by using dilute H2SO4 aqueous so-
lution as a mobile phase. The conversion of cellulose was performed ac-
cording to the same procedure, except that cellulose was used instead of
cellobiose.
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Received: October 17, 2011
Published online: February 1, 2012
Chem. Eur. J. 2012, 18, 2938 – 2947
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www.chemeurj.org
2947