Cs-substituted tungstophosphate-supported ruthenium nanoparticles as efficient and robust bifunctional catalysts for the conversion of inulin and cellulose into hexitols in water in the presence of H2

Article abstract of DOI:10.1039/c4ra05939e

Cellulose and inulin, two important biomasses, can be transformed to polyols using bifunctional catalysts combining acid sites for hydrolysis and metal nanoparticles for hydrogenation. Here, we report that Ru nanoparticles loaded on a Keggin-type polyoxometalate, i.e., Cs₃PW₁₂O₄₀, without intrinsic Bronsted acidity exhibit superior catalytic performances for the transformation of inulin and cellulose into hexitols in water in the presence of H₂. We demonstrated that new Bronsted acid sites were generated from H₂ on the Ru/Cs₃PW₁₂O₄₀ catalyst. The H₂-originated reversible Bronsted acid sites were robust during the transformation of biomass under hydrothermal conditions. We further found that the mean size of Ru nanoparticles determined the product selectivity in the conversion of inulin under H₂. The catalyst with larger Ru nanoparticles favoured the formation of fructose, the hydrolysis product, while the major products were hexitols over the catalyst with a smaller Ru particle size. We clarified that, as compared to that of inulin hydrolysis, the rate of fructose hydrogenation increased more rapidly upon decreasing the Ru particle size.

Full text of DOI:10.1039/c4ra05939e

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PAPER
Cs-substituted tungstophosphate-supported
ruthenium nanoparticles as efficient and robust
bifunctional catalysts for the conversion of inulin
and cellulose into hexitols in water in the presence
of H2†
Cite this: RSC Adv., 2014, 4, 43131
Weiping Deng, Enze Zhu, Mi Liu, Qinghong Zhang and Ye Wang*
Cellulose and inulin, two important biomasses, can be transformed to polyols using bifunctional catalysts
combining acid sites for hydrolysis and metal nanoparticles for hydrogenation. Here, we report that Ru
nanoparticles loaded on a Keggin-type polyoxometalate, i.e., Cs
acidity exhibit superior catalytic performances for the transformation of inulin and cellulose into hexitols
in water in the presence of H . We demonstrated that new Brønsted acid sites were generated from H
on the Ru/Cs PW12 40 catalyst. The H -originated reversible Brønsted acid sites were robust during the
transformation of biomass under hydrothermal conditions. We further found that the mean size of Ru
nanoparticles determined the product selectivity in the conversion of inulin under H . The catalyst with
3
PW12O40, without intrinsic Brønsted
2
2
3
O
2
2
Received 19th June 2014
Accepted 1st September 2014
larger Ru nanoparticles favoured the formation of fructose, the hydrolysis product, while the major
products were hexitols over the catalyst with a smaller Ru particle size. We clarified that, as compared to
that of inulin hydrolysis, the rate of fructose hydrogenation increased more rapidly upon decreasing the
Ru particle size.
DOI: 10.1039/c4ra05939e
www.rsc.org/advances
exist in cellulose macromolecules, making their crystalline
structure highly robust. The catalytic transformation of cellu-
1
. Introduction
Cellulose is a polymer of D-glucose linked by b-1,4-glycosidic lose under mild conditions is a difficult task.
bonds (Scheme 1) and is a major component of the abundant
On the other hand, inulin is also a non-digestible carbohy-
and inedible lignocellulosic biomass. The catalytic trans- drate composed of linear b-2,1-linked fructose chains with a
formation of cellulose into chemicals or fuels has attracted terminal glucopyranose unit (Scheme 1). Generally, inulin
much attention in recent years from the viewpoint of estab- amounts to 70–90 wt% of Jerusalem artichoke, which is a
1
lishing sustainable chemical and energy processes. Because perennial plant and can grow in wide range of ecological
2
of the huge numbers of hydroxyl groups and the linkage of conditions. To date, only a few studies have been devoted to the
b-1,4-glycosidic bonds, extensive hydrogen-bonding networks catalytic transformation of inulin into chemicals and fuels
State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation
Center of Chemistry for Energy Materials, National Engineering Laboratory for Green
Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical
Engineering, Xiamen University, China. E-mail: wangye@xmu.edu.cn; Fax: +86 592
2
183047; Tel: +86 592 2186156
†
Electronic supplementary information (ESI) available: XRD patterns of
Cs
of Cs
micrograph for colloidal Ru particles synthesized by the reduction of RuCl
with ascorbic acid; NH -TPD proles of several solid acids; TEM micrographs
for Ru nanoparticles loading on several solid acids; XRD patterns and XPS
spectra for Ru/Cs PW12 40 before and aer reaction; pyridine-adsorption IR for
40 and Ru/Al under different H pressures; Raman spectra of
40 under H and N ; TEM micrographs of Ru nanoparticles with
PW12 40 with
x
H3ꢀxPW12
O
40 and Ru/Cs
x
H3ꢀxPW12
O
40; N
2
adsorption–desorption isotherms
x
H3ꢀxPW12
O
40
;
XPS spectra for Ru/Cs
x
H3ꢀxPW12O40 catalysts; TEM
3
3
3
O
Cs
Cs
3
3
PW12
PW12
O
O
2
O
3
2
2
2
different mean sizes; hydrogenation of fructose over Ru/Cs
3
O
different mean sizes of Ru nanoparticles and the corresponding turnover
Scheme 1 Catalytic conversions of cellulose and inulin into hexitols
via hydrolysis and hydrogenation.
frequencies for these catalysts. See DOI: 10.1039/c4ra05939e
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3,4
under mild conditions. Moreover, there are various C–C and or systems capable of working under hydrothermal conditions
C–O bonds in both inulin and cellulose macromolecules. The remains highly challenging.
selective cleavage of specic chemical bonds in inulin or
Polyoxometalates, particularly the Keggin-type hetero-
cellulose to form a specic product is also a challenging polyacids, have recently attracted much attention for the cata-
research theme.
lytic hydrolysis of cellulose because of their strong Brønsted
11,12
The production of hexitols including sorbitol and mannitol acidity.
3 4
The combination of H PW12O40 or H SiW12O40 with
is one of the most attractive routes for the utilization of biomass Ru/C could catalyze the transformation of cellulose into hexitols
13
resources. Both sorbitol and mannitol nd many applications with good yields. However, the Keggin-type heteropolyacids
in food, pharmaceutical, and cosmetic industries. Sorbitol is are soluble in water and cannot be used as heterogeneous
also regarded as one of the top value-added chemicals from catalysts for the transformation of biomass in water. It was once
biomass for the production of isosorbide, 1,4-sorbitan, claimed that the H PW O incorporated inside the meso-
3
12 40
2
,5-anhydrosugars, lactic acid, glycerol, ethylene glycol, and porous cavities of metal–organic frameworks (MOFs) worked as
5
propylene glycol. For the production of hexitols from a biomass a heterogeneous catalyst for the conversion of cellulose in
such as cellulose or inulin, a bifunctional catalyst or catalytic water, but this catalyst could not be used recyclably. The salts
system that combines the hydrolysis and hydrogenation func- of heteropolyacids with larger monovalent cations such as Cs
14
+
tions is generally required. Liu and co-workers developed an are insoluble. Moreover, the H-form Cs salts of Keggin-type
intriguing bifunctional system, which combined the reversibly heteropolyacids with proper Cs contents may possess reason-
+
generated H O in hot water (518 K) with a hydrogenation ably large surface areas and mesoporosity due to the stacking of
3
15
catalyst, i.e., Ru/activated carbon (AC), for the conversion of primary particles. Thus, the Cs salts of Keggin-type hetero-
6
15
cellulose to hexitols with a yield approaching 40%. The polyacids may work as water-tolerant solid-acid catalysts.
combination of a liquid mineral acid (e.g., HCl and H SO ) with few studies have reported the utilization of Cs
a supported Ru or Pt catalyst was further demonstrated to be Cs 40 for the transformation of cellulose. For
0.5
highly efficient for the selective conversion of cellulose into example, the combination of Cs2.5 0.5PW12O40 or Cs3.5H -
A
2
4
x
H3ꢀxPW12O40 or
x
H4ꢀxSiW12O
H
7
hexitols under mild conditions. Although excellent perfor- SiW12
O
40 with Ru/C was found to be highly efficient for the
atmo-
liquid acid, the problems of the recovery of the liquid acid and sphere. However, the leaching of tungsten species was
mances have been achieved by using these systems containing a conversion of cellulose to hexitols in water under H
2
16
16
the corrosion of the reactor would hinder the application of observed during the reaction.
these systems for large-scale production. In a short communication, we demonstrated that Ru nano-
Bifunctional catalysts based on solid acid-supported metal particles loaded on Cs salts of tungstophosphoric acid, i.e.,
nanoparticles have also attracted much attention for the Cs 40, were efficient bifunctional catalysts for the
transformation of cellulose in recent years. Fukuoka and Dhepe transformations of cellobiose, the model molecule of cellulose,
x
H3ꢀxPW12O
17
+
reported a pioneering work on the conversion of cellulose in into sorbitol. We observed the leaching of H -rich hydrophilic
water and obtained a hexitol yield of 31% over a Pt/g-Al moiety into H O under hydrothermal reaction conditions.
catalyst at 463 K under H atmosphere. We also demonstrated Unexpectedly, the Ru/Cs PW O without intrinsic Brønsted
2
O
3
2
8
2
3
12 40
that Ru nanoparticles supported on carbon nanotubes (CNTs) acidity could catalyze the conversion of cellobiose to sorbitol in
bearing oxygen-containing functional groups, which could work the presence of H . In the present paper, we report our recent
2
as acid sites, were efficient for the conversion of cellulose and comprehensive studies on the transformations of inulin and
cellobiose, a glucose dimer also connected by b-1,4-glycosidic cellulose in H
2
O under H
bond, into hexitols. Carbon-supported nickel or nickel phos- catalysts. We focus on the catalytic behaviours and functioning
phide catalysts could also catalyze the conversion of cellulose mechanisms of the Ru/Cs 40 catalyst and the origin of
the acidity for this catalyst. The size effect of Ru nanoparticles
2
over polyoxometalate-supported Ru
9
3
PW12O
10
into sorbitol under proper conditions.
On the other hand, the studies on the catalytic conversion of will be discussed. Our present work provides a novel strategy for
inulin to hexitols in water under H2 atmosphere are scarce. the design of stable bifunctional catalysts for the trans-
Heinen et al. demonstrated that a homogeneous Ru catalyst formation of biomass under H2 atmosphere under hydro-
(
Ru–P(m-C
production of mannitol from inulin. However, aer reaction
under H atmosphere at 363 K, the homogeneous Ru catalyst
6 4 3 3
H SO Na) ) in combination with HCl catalyzed the thermal conditions.
2
2
. Experimental section
3
a
was reduced and became precipitated. An acidied active
carbon supported-Ru catalyst was claimed to be also efficient
2.1 Catalyst preparation
and more stable for the transformation of inulin into hexitols at The Cs salts of tungstophosphates with different Cs contents
3
b
3
73 K. A Ni–W C/AC catalyst with C–C bond cleavage ability (Cs H PW O , x ¼ 1.0–3.0) were prepared by a reaction
2
x
3ꢀx
12 40
could catalyze the direct conversion of Jerusalem artichoke to between H PW O and Cs CO . The precipitates obtained
,2-propylene glycol with a yield of 35%.
3
12 40
2
3
4
1
2 3
aer the addition of the aqueous solution of Cs CO into that of
In short, although some bifunctional heterogeneous cata- H PW O were recovered by ltration, followed by washing
3
12 40
lysts have been reported for the conversion of cellulose or with deionized water, drying, and calcination to obtain the
inulin, the catalytic efficiency and catalyst stability are still Cs
x
H
3ꢀxPW12
O
40 samples. As an example, for the synthesis of
unsatisfactory. The development of robust solid acid materials Cs
3
PW12O
40
,
the aqueous solution of Cs CO with a
2
3
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ꢀ
1
concentration of 0.02 mol L (150 mL) was added into that of and placed in an in situ IR cell. Aer pretreatment in H
2
at 573 K,
ꢀ
1
H PW O with a concentration of 0.12 mol L (15 mL). Then, followed by evacuation at the same temperature, the sample was
3
12 40
the recovered precipitate was calcined in air at 673 K for 2 h cooled down to 423 K and was exposed to pyridine for 0.5 h. Aer
aer washing and drying. cooling down to 303 K followed by a brief evacuation to remove
The controlled syntheses of colloidal Ru nanoparticles with the gaseous pyridine, H
mean sizes ranging from 1.6 to 10.8 nm were performed by the were recorded under H
reduction of RuCl with ascorbic acid in H
2
or N
or N
2
was introduced. FT-IR spectra
with different pressures. The
2
2
3
2
O, ethylene glycol, or Raman spectra were collected on a Renishaw Raman System
butylene glycol at different temperatures and aging times. The 1000 spectrometer. Electron spin resonance (ESR) spectroscopic
detailed synthetic conditions will be described later. The Ru/ experiments were carried out on a Bruker EMX-10/12 operating
Cs H PW O catalysts were prepared by an adsorption of at X-band frequency of 9.43 GHz, modulation amplitude of 10.0
x
3ꢀx
12 40
the colloidal Ru nanoparticles onto the Cs H PW O
G, and modulation frequency of 100.0 kHz.
x
3ꢀx
12 40
samples. In brief, Cs H PW O white powders (1.0 g) were
x
3ꢀx
12 40
added into 20 mL aqueous solution containing the colloidal Ru
nanoparticles (Ru weight, 10 mg). The addition of the poly-
oxometalate powders decoloured the solution, while the colour
of the support was deepened, indicating the occurrence of the
adsorption of colloidal Ru nanoparticles onto the support. Aer
2
.3 Catalytic reaction
The conversion of inulin or cellulose was performed with a
batch-type high-pressure autoclave reactor. Typically, the cata-
lyst and the reactant (inulin or cellulose) were added into a
Teon-lined stainless-steel reactor pre-charged with H
the reaction was carried out at 363 or 433 K under 2 MPa H
h or 24 h. Aer the reaction, the solid catalyst was separated by
2
O. Then,

ltration, washing with deionized water, and drying at 373 K,
2
for
the Ru/Cs H PW O catalysts were obtained. The same
x
3ꢀx
12 40
4
adsorption procedure was also employed for the preparation of
Ru nanoparticles loaded on other solid acid supports. The
loading of Ru was kept at 1.0 wt% in each catalyst.
centrifugation, and the liquid products were analyzed by a
HPLC (Shimazu LC-20A) equipped with a RI detector and a
Shodex SUGARSC-1011 column (7 mm, 8 ꢁ 300 mm). The eluent
3
ꢀ1
was water with a ow rate of 0.5 cm min
.
2.2 Catalyst characterization
Powder X-ray diffraction (XRD) patterns were collected on a
Panalytical X'Pert Pro Super X-ray diffractometer equipped with
3
. Results and discussion
3.1 Structures and physicochemical properties of
X'Celerator detection. The Cu K
was used as the X-ray source. X-ray photoelectron spectra (XPS)
were recorded with a Quantum 2000 Scanning ESCA Micro- The crystalline structures of the synthesized Cs H PW O
12 40
a
radiation (40 kV and 30 mA)
Cs H PW O -supported Ru nanoparticles
x
3ꢀx
12 40
x
3ꢀx
probe instrument (Physical Electronics) using Al K radiation. samples with different Cs contents (x ¼ 1.0–3.0) were investi-
a
The binding energy was calibrated using C1s photoelectron peak gated by the XRD measurements (Fig. S1†). The XRD patterns
at 284.6 eV as a reference. Transmission electron microscopy for all the samples can be attributed to the cubic crystalline
18,19
(TEM) measurements were performed on a JEM-2100 electron structure of Keggin-type polyoxometalate compounds.
microscope operated at an acceleration voltage of 200 kV. The The loading of Ru nanoparticles onto the Cs H3ꢀxPW12O
x
40
mean size of Ru nanoparticles in the sample was estimated (x ¼ 1.0–3.0) samples did not signicantly change the XRD
from the TEM micrographs by counting around 150–200 patterns, suggesting that the crystalline structure was sustained
particles.
The CO chemisorption was measured with a Micromeritics areas of the Ru/Cs H PW O catalysts increased with
12 40
(Fig. S1b†). N physisorption studies revealed that the surface
2
x
3ꢀx
ASAP 2010C instrument. The acidity of the catalyst was exam- increasing the Cs content from x ¼ 1.0 to x ¼ 3.0 (Table 1). The
ined by ammonia temperature-programmed desorption (NH surface areas of the Cs 40 also increased in a similar
TPD) and pyridine-adsorbed Fourier-transform infrared (FT-IR) trend with increasing the Cs content. The N adsorption–
studies. NH -TPD was performed on a Micromeritics AutoChem desorption isotherms for the Cs 40 showed that the
920 II instrument. Typically, the sample loaded in a quartz samples with higher Cs contents (x $ 2) exhibited the type-IV
3
-
x
H3ꢀxPW12O
2
3
x
H3ꢀxPW12O
2
reactor was pretreated with high-purity He at 623 K for 1 h. Aer isotherm, in which the hysteresis of desorption branch was
the sample was cooled down to 373 K, the adsorption of NH3 clearly observed (Fig. S2†). This conrms that the substitution
+
+
was performed by switching the He ow to a NH –He (10 vol% of the smaller H with the larger Cs in the polyoxometalate
3
19
NH
.5 h. The gas phase and weakly adsorbed NH
purged by high-purity He at the same temperature. NH
was then performed in the He ow by raising the temperature to Fig. 1, a major desorption peak at 850–855 K was observed for
3
) gas mixture and then maintaining the sample at 373 K for causes the formation of micropores and mesopores.
molecules were NH -TPD measurements were performed to investigate the
-TPD acidities of the Ru/Cs 40 catalysts. As displayed in
0
3
3
3
x
H3ꢀxPW12O
ꢀ1
1
073 K at a rate of 10 K min . The desorbed NH
were detected by using a ThermoStar GSD 301 T2 mass spec- desorption of NH
trometer with the signal of m/z of 16. to be the Brønsted acid sites, over the Keggin-type poly-
Pyridine-adsorbed FT-IR was performed with a Nicolet Avatar oxometalates. As expected, no desorption of NH was observed
3
molecules the catalyst with x < 3.0. This peak could be ascribed to the
from the strong acid sites, which are believed
3
19
3
3
3
30 instrument equipped with an MCT detector with a resolution for the Ru/Cs PW12O40 catalyst. We have quantied the amount
ꢀ1
of 4 cm . The sample was pressed into a self-supported wafer, of NH
3
desorption using the peak at 850–855 K, and the result is
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x
Table 1 Some physicochemical properties of Ru/Cs H3ꢀxPW12O40 catalysts
2
ꢀ1
a
ꢀ1
b
c
Catalyst
Surf. area (m g
)
NH
3
desorption amount (mmol g
)
D(Ru)
d(Ru) (nm)
d
Ru/Cs1.0
Ru/Cs1.5
Ru/Cs2.0
Ru/Cs2.5
H
H
H
H
2.0PW12
1.5PW12
1.0PW12
0.5PW12
O
O
O
O
40
40
40
40
59
60
76
93
98
117
92
83
37
0
n.d.
—
0.47
0.45d
n.d.
0.49
1.9
2.0
—
Ru/Cs3.0PW12
O
40
1.8
a
b
c
Quantied from NH
3
-TPD. D(Ru) denotes the Ru dispersion obtained from CO chemisorption. d(Ru) denotes the size of Ru particles estimated
20
d
from D(Ru) by using d(Ru) ¼ 6V
m m
/[D(Ru) ꢁ a ] (nm).
Not detected.
displayed in Table 1. The amount of NH desorption decreased 3.2 Catalytic behaviours of polyoxometalates and
3
gradually with increasing the Cs content, becoming zero at x ¼ polyoxometalate-supported Ru nanoparticles for the
3
.0. No Brønsted acid sites exist on the Ru/Cs PW O catalyst. conversion of inulin
3 12 40
XPS studies were performed to gain information on the
chemical state of Ru over our catalysts. The result showed that
x 3ꢀx
We rst investigated the catalytic behaviours of the Cs H -
PW12
O40 samples for the hydrolysis of inulin in water. Over the
the binding energies of Ru 3d5/2 were at ꢂ280.4 eV for the Ru/
Cs
x
H
3ꢀxPW12
O
40 (x ¼ 1.0–2.5) samples with strong Brønsted
x
Cs H3ꢀxPW12O40 catalysts with different Cs contents (Fig. S3†).
acidity, inulin could be completely converted at 363 K, and
This suggests that the Ru species are all in metallic state over
these catalysts. We have performed TEM measurements for the
fructose was formed as a major product with a yield of 74–88%
(Fig. 2). Glucose and sucrose were the main by-products.
Ru/Cs H PW O catalysts. However, we failed to distinguish
x
3ꢀx
12 40
Glucose could be formed by the hydrolysis of the terminal
glucopyranose or by the isomerization of fructose over the Lewis
acid site on our catalyst, whereas sucrose may be produced via
partial hydrolysis of inulin without breaking the linkage
the Ru nanoparticles from the Cs H PW O supports in the
x
3ꢀx
12 40
TEM images possibly because of the weak contrast between Ru
and Cs. We measured the dispersion of Ru nanoparticles for
several typical catalysts by the CO chemisorption technique.
The dispersions of Ru, denoted as D(Ru), were quite similar
0.45–0.49) for these catalysts (Table 1). It is known that the size
of metallic Ru particles, denoted as d(Ru), can be evaluated
3 40
between terminal glucopyranose and fructose. The Cs PW12O ,
which had no Brønsted acidity, showed very low inulin
conversion and fructose yield (<5%). Such a low activity was
probably due to the presence of weak Lewis acid sites on the
(
from the Ru dispersion. We have estimated the size of Ru
Cs
several other typical solid acids are also displayed in Fig. 2.
Al exhibited a low activity for the hydrolysis of inulin prob-
3
PW12O40. For comparison, the catalytic performances of
particles by using the following relationship, d(Ru) ¼ 6V /
m
[
D(Ru) ꢁ a
m m m
] (nm), where D(Ru), V , and a are the Ru
2 3
O
dispersion, the volume occupied by a metal atom in the bulk,
and the surface area occupied by an exposed surface metal
ably because of its relatively weaker acidity (Fig. S5 and Table
S1†). In addition to fructose, glucose, sucrose and some oligo-
mers were also formed over Al O . Zeolites HZSM-5 and MCM-
20
ꢀ3
3
atom, respectively. By using the V
m
(13.65 ꢁ 10 nm ) and a
m
2
3
ꢀ
2
2
20
(
9.09 ꢁ 10 nm ) values reported in literature, we obtained
22 bearing stronger acidities and larger densities of acid sites
the sizes of Ru nanoparticles of 1.8–2.0 nm for the catalysts with
Ru dispersions of 0.49–0.45 (Table 1). These size values are close
to that of Ru nanoparticles (1.6 nm) in the colloids used for the
preparation of the supported catalysts by an adsorption method
(Fig. S5 and Table S1†) showed higher fructose yields. Although
HZSM-5 possessed stronger acidity than MCM-22, its activity for
inulin hydrolysis was lower. This is probably due to the smaller
(Fig. S4†).
Fig. 2 Catalytic behaviours of the Cs
40 catalysts with typical solid acids for the hydrolysis of inulin. Reaction conditions:
inulin 0.20 g; catalyst, 0.050 g; H
O, 10 mL; T ¼ 363 K; t ¼ 4 h.
x
H3ꢀxPW12O40 as well as some
Fig. 1 NH
different Cs contents.
3 x
-TPD profiles for the Ru/Cs H3ꢀxPW12O
2
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pore size of HZSM-5 (Table S1†). The results in Fig. 2 demon- products. Under our reaction conditions, no hexitols were
strate that the Cs H PW O (x < 3) samples with strong formed without a catalyst. The yields of hexitols over the Ru/
x
3ꢀx
12 40
Brønsted acidity catalyze the hydrolysis of inulin very efficiently, Cs H PW O catalysts were higher than those over the Ru
x
3ꢀx
12 40
and the Cs PW O is almost inactive for the hydrolysis reac- nanoparticles loaded on Al O , H-ZSM-5, or H-MCM-22 (Fig. 3).
3
12 40
2 3
tion because of the lacking of Brønsted acidity.
Then, we investigated the catalytic performances of Ru was also efficient for the conversion of cellulose, providing a
nanoparticles loaded on the Cs 40 catalysts
40 (x ¼ 1–3) series of hexitol yield of 45% at 433 K. The Ru/Cs
samples for the conversion of inulin in water under H
3
The Ru/Cs PW12O40 catalyst without intrinsic Brønsted acidity
x
H
3ꢀxPW12
O
x
H3ꢀxPW12O
2
atmo- with lower Cs contents (x ¼ 1.0–2.0), which had strong Brønsted
sphere. For comparison, Ru catalysts supported on other typical acidity, showed comparable or slightly higher hexitol yields
solid acids, i.e., Al O , MCM-22, and HZSM-5, were also inves- than the Ru/Cs PW O . In other words, the presence of
2
3
3
12 40
tigated (Table 2). All these supported Ru catalysts were prepared intrinsic Brønsted acid sites only contributed a little to the
by the adsorption of colloidal Ru nanoparticles with a mean size catalytic conversion of cellulose to hexitols.
of 1.6 nm onto the supports. The mean sizes of Ru particles over
different supports were quite similar, being 1.6–2.3 nm Ru/Cs
Fig. S6†). Catalytic studies showed that, in the absence of a conversion of ball-milled cellulose in water under H
catalyst (blank reaction), no inulin conversion was observed. As The yields of hexitols decreased to some extent in the initial two
compared to the catalyst with Al , MCM-22, or HZSM-5 as the recycles over both catalysts, and then remained stable in the
We have examined the stability of the Ru/Cs
HPW12 40 catalysts during the repeated uses for the
at 433 K.
3
PW12O40 and
2
O
(
2
2 3
O
support, polyoxometalate-supported Ru nanoparticles exhibited next recycles (Fig. 4). However, the hexitol yield decreased more
higher inulin conversions and higher hexitol (sorbitol and quickly over the Ru/Cs HPW O catalyst than that over the Ru/
2
12 40
mannitol) yields. Hexitol yields of 79–84% were achieved over Cs PW O catalyst. V ´e drine and co-workers once reported that
3
12 40
3ꢀxPW12
the Ru/Cs
63 K for 4 h under a H
was roughly equal to that of mannitol over these catalysts. This solved in hot water. Okuhara et al. found that the treatment of
x
H3ꢀxPW12
O
40 catalysts for the conversion of inulin at the Cs
x
H
O
40 might be composed of Cs
3
PW12O40 with
3
2
pressure of 2 MPa. The yield of sorbitol
H PW12O40 dispersed in its pores, and the latter might be dis-
3
22
+
is likely because fructose, the reaction intermediate, can be Cs2.5
H0.5PW12O40 in water at 393 K caused the leaching of H -
transformed equally into a- and b-form furans, which are then rich hydrophilic moiety into water although they claimed that
2
1
hydrogenated to form sorbitol and mannitol, respectively. It is the protons were homogeneously dispersed in the Cs H
-
3ꢀx
x
23
+
quite unexpected that the Ru/Cs PW O catalyst, which does PW O . We also observed the leaching of H -rich hydrophilic
3
12 40
12 40
not possess intrinsic Brønsted acidity, shows the highest yield moiety from the Au/Cs H PW O catalysts for the oxidative
x
3ꢀx
12 40
2
4
of hexitols (84%).
transformation of cellulose under hydrothermal conditions.
+
Thus, we speculate that the leaching of H -rich hydrophilic
moiety occurs for the Ru/Cs HPW12 40 catalyst. Aer four
recycles, the hexitol yield over the Ru/Cs PW O catalyst (41%)
2
O
3.3 Catalytic behaviours of polyoxometalate-supported Ru
3
12 40
nanoparticles for the conversion of cellulose
2 12 40
was slightly higher than that over the Ru/Cs HPW O catalyst.
From XRD, XPS and CO chemisorption measurements, we
x
We have investigated the catalytic behaviours of the Ru/Cs -
conrmed that the structure of the Ru/Cs PW O catalyst did
H
3ꢀxPW12
O40 catalysts for the conversion of ball-milled cellu-
3
12 40
not undergo signicant changes except for a slight increase in
the size of Ru nanoparticles (Fig. S7 and Table S2†). The slight
lose in water under H atmosphere. The catalytic conversion of
2
cellulose provided hexitols, particularly sorbitol, as the major
decrease in the activity of the Ru/Cs
initial 2 recycles (Fig. 4) may be due to the increase in the size of
Ru nanoparticles (Table S2†). Therefore, the Ru/Cs 40 is a
3
PW12O40 catalyst during the
PW12O
Table 2 Catalytic performances of Ru nanoparticles loaded on poly-
3
oxometalates and other supports for the conversion of inulin in water
a
2
under H
b
Yield (%)
Catalyst
Blank
Conv. (%) Sor. Man. Fru. Glu. Suc.
0
0
0
0
0
0
Ru/Al
Ru/HZSM-5
Ru/MCM-22
Ru/Cs1.0
Ru/Cs1.5
Ru/Cs2.0
Ru/Cs2.5
2
O
3
6.1
85
0.9
30
0.8
27
0.7 0.4
1.1
8.2
8.4
4.9
5.5
5.0
4.9
6.0
18
2.1
94
100
100
100
100
100
36
41
39
43
40
43
37
38
40
40
39
41
2.6 0.8
H
H
H
H
2.0PW12
1.5PW12
1.0PW12
0.5PW12
O
O
O
O
40
40
40
40
11
2.3
8.5 2.0
10
2.0
6.6 1.8
4.9 2.1
Ru/Cs3.0PW12O
40
a
Fig. 3 Catalytic behaviours of Ru nanoparticles loaded on Cs H
3ꢀx
-
Reaction conditions: catalyst (Ru loading, 1.0 wt%), 0.05 g; inulin 0.2
x
b
PW12O
40 and several typical solid acids for the conversion of cellulose
. Reaction conditions: cellulose (ball-milled), 0.10 g;
catalyst, 0.10 g; H O, 10 mL; T ¼ 433 K; t ¼ 24 h.
g; H
2
O, 10 mL; H
2
, 2 MPa; T ¼ 363 K; t ¼ 4 h. Sor., man., fru., glu., and
suc. denote sorbitol, mannitol, fructose, glucose, and sucrose, in water under H
2
respectively.
2
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MPa, the yield of hexitols increased from 52% to 59%, while
that of fructose decreased from 17% to 13%. Beside fructose, a
part of oligomers, which were produced by the partial hydrolysis
of inulin, may also contribute to the formation of hexitols under
higher H pressures.
2
The loading of Ru is another key factor for the efficient
conversion of inulin. As displayed in Fig. 6, without loading Ru,
the Cs
3
PW12O
40 was less active for the conversion of inulin even
under H
2
atmosphere (inulin conversion < 10%). The conver-
sion of inulin increased to ꢂ100% aer loading a small amount
of Ru (0.2 wt%) onto the Cs PW O . Meanwhile, the intro-
3
12 40
Fig. 4 Repeated uses of the Ru/Cs
3
PW12
O
40 and Ru/Cs
2
HPW12
. Reaction
O, 15 mL;
O
40
duction of a small amount of Ru signicantly enhanced the
yield of fructose, the hydrolysis product. At higher loadings of
Ru, the yields of sorbitol and mannitol increased at the expense
of that of fructose due to the hydrogenation catalyzed by Ru
nanoparticles.
catalysts for the conversion of cellulose in water under H
conditions: cellulose (ball-milled), 0.10 g; catalyst, 0.10 g; H
, 2 MPa; T ¼ 433 K; t ¼ 24 h.
2
2
H
2
In short, the results described above suggest that new acid
sites effective for the hydrolysis of inulin to fructose are
generated over the Ru/Cs PW O catalyst. The presence of H2
and the loading of Ru play crucial roles in the generation of
such acid sites. These generated acid sites are believed to be
highly efficient and relatively robust heterogeneous catalyst for
the transformation of biomass (inulin and cellulose) under
hydrothermal conditions.
3
12 40
3 40
responsible for the superior activity of the Ru/Cs PW12O
catalyst, which does not possess intrinsic Brønsted acidity, for
the conversion of inulin or cellulose into hexitols. Under a lower
3
.4 Key factors determining the catalytic behaviour of Ru/
Cs 40 catalyst for the conversion of inulin
As described above, it is signicant that the Ru/Cs PW12O
40
3
PW12O
3
2
H pressure or over the catalyst with a lower Ru loading, the
catalyst without intrinsic Brønsted acidity shows superior
catalytic performances for the production of hexitols from both
inulin and cellulose. Taking inulin as an example, we have
formation of fructose can be signicantly promoted because of
the accelerated hydrolysis ability and the relatively lower
hydrogenation ability under such circumstances.
claried that the H pressure is one key factor determining the
2
catalytic behaviour of this catalyst. Fig. 5 shows the effect of H2
3
.5 Characterizations of Brønsted acid sites generated in the
presence of H over the Ru/Cs 40 catalyst
signicantly To gain direct evidence for the generation of the Brønsted acid
increased the inulin conversion; the inulin conversion sites in the presence of H , we have performed FT-IR spectro-
pressure on the conversion of inulin at 363 K. In the absence of
2
3
PW12O
H
2
(under 2 MPa N
2
), the inulin conversion was only 20%, and
the major product was fructose. The presence of H
2
2
increased from 20% to 97% by feeding H
2
with a low pressure scopic studies of adsorbed pyridine. In these studies, the Ru/
(
0.2 MPa) (Fig. 5). It is of interest that the yield of fructose, Cs PW O catalyst was pretreated in an in situ IR cell under H2
3
12 40
which is formed by the acid-catalyzed hydrolysis of inulin at 573 K for 1 h, followed by evacuation at the same tempera-
Scheme 1), increases signicantly by feeding H2 with a low ture. Then, the Ru/Cs PW O catalyst was exposed to pyridine
(
3
12 40
pressure (0.2 MPa). A further increase in the H2 pressure at 423 K for 0.5 h. Aer cooling down to 303 K followed by a brief
increased the yields of sorbitol and mannitol owing to the evacuation, H was introduced into the in situ IR cell. In the
2
ꢀ
1
occurrence of hydrogenation of fructose or other intermediates absence of H , an IR band at 1425–1450 cm was observed.
2
2 2
at higher H pressures. When H pressure increased from 2 to 4 This band could be ascribed to the pyridine adsorbed on Lewis
Fig. 5 Effect of H
Cs PW12 40 catalyst for the conversion of inulin in water under H
Reaction conditions: inulin, 0.2 g; catalyst (Ru loading, 1.0 wt%), 0.020 Reaction conditions: inulin, 0.20 g; catalyst, 0.050 g; H
g; H MPa; T ¼ 363 K; t ¼ 4 h.
O, 10 mL; T ¼ 363 K; t ¼ 4 h.
2
pressure on catalytic behaviours of the Ru/ Fig. 6 Effect of Ru loading on catalytic behaviours of the Ru/
Cs PW12 40 catalyst for the conversion of inulin in water under H
O, 10 mL; H , 2
3
O
2
.
3
O
2
.
2
2
2
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2
5
acid sites. The introduction of H
2
decreased the IR band at
ꢀ
1
1
450 cm and resulted in a signicant increase in the IR band
ꢀ
1
at 1540 cm (Fig. 7A). This IR band could be assigned to the
pyridine adsorbed on the Brønsted acid sites. The intensity of
25
ꢀ1
the IR band at 1540 cm increased with the H
gesting the generation of the Brønsted acid sites under H
When N was introduced into the IR cell instead of H , the IR
band at 1540 cm was not enhanced (Fig. 7B), conrming that
the Brønsted acid sites were originated from H . We also per-
formed the pyridine-adsorbed FT-IR studies for the Cs PW O
2
pressure, sug-
2
.
2
2
ꢀ1
2
3
12 40
ꢀ
1
sample without Ru. The IR band at 1540 cm
was not
enhanced in the presence of H2 (Fig. S8†). This provides Fig. 8 Raman spectra for the Ru/Cs
3
PW12
O
40 under N
2
and H
2
ꢀ
1
ꢀ1
atmospheres at 423 K. (A) 200–1040 cm , (B) 940–1040 cm . (a)
evidence that Ru nanoparticles are essential for the generation
of Brønsted acid sites over the Ru/Cs 40 under H
2
After the introduction of N
switching to H , (c) 10 min after switching again to N
switching again to H , (e) 10 min after switching again to N
after switching again to H2.
2
into the pretreated sample, (b) 10 min after
, (d) 10 min after
, (f) 10 min
3
PW12
O
2
2
atmosphere. Actually, many recent studies have demonstrated
2
2
26
that Ru nanoparticles can activate H
2
under mild conditions.
Furthermore, we carried out the same study for the Ru/Al
2
O
3
ꢀ1
catalyst. The IR band at 1540 cm ascribed to the Brønsted acid
sites could not be observed over the Ru/Al O in the presence of
2
3
bridging W–O–W bonds of the intact Keggin structure
ꢀ1
27
H (Fig. S9†). It is noteworthy that the formation of protons
2
(ꢂ240 cm ). When the gas atmosphere was switched from N2
from H
over effect in an early study. However, we could not observe
the generation of the Brønsted acid sites from H over our Ru/
Al . Therefore, the Ru/Cs 40 is a quite unique system
for the generation of Brønsted acid sites in the presence of H
2 2 3
was once proposed over a Pt/Al O catalyst via a spill-
to H , all these major bands attributable to Keggin unit still
2
8
a
remained (Fig. 8A). However, a shi of W]O stretching vibra-
tion bands to higher wavenumbers was observed (Fig. 8B),
suggesting a perturbation of W]O bonds in the primary Keggin
2
2
O
3
3
PW12O
2
.
structure in the presence of H
stretching vibration bands shi back aer H
. In other words, the shis of Raman bands are reversible. On
. It is noteworthy that the W]O
2
It is expected that one electron will be released simulta-
neously with the generation of one proton, the Brønsted acid
2
was replaced by
N
2
site, from the hydrogen species over the Ru/Cs PW O cata-
3
12 40
the other hand, no such shis of Raman bands were observed
lyst. To gain further information on this process, we have
monitored the structure change of the catalyst by Raman
spectroscopic measurements. Fig. 8 shows the Raman spectra
by switching the atmosphere from N2 to H2 for Cs PW O
3
12 40
alone (Fig. S10†). These results suggest that the shi of the
Raman bands belonging to the W]O stretching vibrations is
related to the generation of Brønsted acidity. We speculate that
the electrons released during the formation of protons from
hydrogen species might migrate to Cs PW12O40, and the
3
reduction of tungsten species may cause the shi of Raman
bands belonging to W]O stretching vibrations.
of the Ru/Cs
3
PW12
O40 catalyst under N
2 2
and H atmospheres at
4
5
23 K. Under N
48, and ꢂ240 cm were observed. These bands could be
2
atmosphere, Raman bands at 1002, 990, 897,
ꢀ1
attributed to the strong W]O symmetric and asymmetric
ꢀ
1
stretching modes (1002 and 990 cm ), the asymmetric
stretching vibration of bridging W–O –W (O denotes a corner
sharing bridging oxygen atom) (897 cm ), the symmetric
stretching of bridging W–O –W (O denotes an edge-sharing
bridging oxygen atom) (548 cm ), and the bending mode of the
b
b
1
We further performed ESR spectroscopic studies for the Ru/
Cs PW O catalyst in the presence of H . An ESR signal
ꢀ
3
12 40
2
c
c
appeared at ꢂ3700 G aer contact of the sample with H
at 423
K (Fig. 9). This signal could be attributed to W . The ESR
signal disappeared when H was purged and replaced by N
The appearance of W in the presence of H provides further
2
ꢀ1
5+
28
2
2
.
5
+
2
evidence that the released electrons are transferred to
Cs PW O and are stored in the tungsten species.
3
12 40
Actually, the concept of generation of new Brønsted acid
sites from molecular H has been proposed by Hattori and co-
2
workers for a few metal oxide-supported transition metal cata-
2
ꢀ
29
lysts such as Pt/SO
can only appear under H
acidity has found applications in the hydroisomerization of
4
–ZrO
2
. These acid sites are reversible and
2
atmosphere. Such a kind of Brønsted
29
alkanes. Based on our present studies described above and the
model suggested by Hattori and co-workers, we have proposed a
mechanism for the generation of Brønsted acid sites on our Ru/
Cs PW O catalyst (Scheme 2). In brief, molecular H is rst
Fig. 7 Pyridine-adsorbed FT-IR spectra for the Ru/Cs
(A) and N (B) atmospheres with different pressures. (a) At 323 K
after pretreatment in H at 573 K followed by evacuation, (b) under 13
kPa H or N , (c) under 19 kPa H or N , (d) under 26 kPa H or N , (e) H species spill over to the Cs
under 33 kPa H or N , (f) under 45 kPa H or N
3
PW12O40 under
3
12 40
2
H
2
2
dissociated on Ru nanoparticles, and then, the formed atomic
40 surface. The H atom may
2
2
2
2
2
2
2
3
PW12O
2
2
2
2
.
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larger Ru nanoparticles. A longer aging time also resulted in a
larger mean size of colloidal Ru nanoparticles due to the Ost-
wald ripening, which was caused by the dissolved smaller
31
particles precipitating onto the larger particles.
Then, the colloidal Ru nanoparticles were loaded onto the
40. We measured the dispersion of Ru particles over
3
Cs PW12O
each supported catalyst by CO chemisorption, and estimated
the size of Ru particles using the dispersion as mentioned
above. The sizes of Ru nanoparticles estimated from Ru
dispersions over these catalysts are close to those in the
colloidal precursors estimated by TEM except for the catalyst
with the largest mean size of Ru particles (Table 3).
Fig. 9 ESR spectra of the Ru/Cs
3
PW12
O
40 catalyst. (a) After contact
with H
with N
2
at 423 K. (b) After contact with H at 423 K, followed by purging
.
2
2
Fig. 10 shows the catalytic performances of the Ru/
Cs
3
PW12
O
40 catalysts with different mean sizes of Ru particles
atmosphere.
for the conversion of inulin in water under H
2
release one electron, resulting in the formation of Brønsted acid
site (proton) on the Cs PW O surface. Meanwhile, a small
fraction of tungsten species on Cs PW12O40 is reduced to W .
3
Fructose and hexitols (sorbitol and mannitol) were major
products. Small amounts of sucrose and glucose were also
formed. Over the catalyst with smaller Ru nanoparticles, inulin
was mainly transformed into hexitols. A hexitol yield of 84% was
attained aer a reaction at 363 K for 4 h by using the catalyst
with a mean Ru particle size of 1.9 nm. Upon increasing the
mean size of Ru particles from 1.9 to 27 nm, the yield of hexitols
gradually decreased, while that of fructose increased gradually.
The fructose yield increased from 4.9% to 75% with an increase
in the mean size of Ru particles from 1.9 to 13 nm. It should be
noted that the conversion of inulin over the Cs PW O
3
12 40
5
+
Our experimental results indicate that these processes are
reversible. Thus, the proton may recombine one electron from
5
+
W , forming atomic H species. The reverse spill-over of the H
species to Ru nanoparticles occurs subsequently, leading to the
formation of molecular H . In the presence of a reactant, the
2
reversibly generated protons can catalyze the hydrolysis reac-
tion. The co-existence of protons (Brønsted acid sites) and H
atoms on catalyst surfaces leads to synergistic effects for the
conversion of inulin to hexitols.
3
12 40
without Ru provided fructose with a yield of <5%. It is quite
interesting that the size of Ru nanoparticles determines the
product selectivities. By regulating the size of Ru nanoparticles,
the product distributions can be tuned.
3.6 Size effects of Ru nanoparticles on catalytic conversion
of inulin
To gain further insights into the Ru particle size effect on the
reaction pathways, we have investigated the time courses for the
conversions of inulin over the Ru/Cs PW12O40 catalysts with
3
It is known that the size of metal nanoparticles plays crucial
30
roles in their catalytic behaviours in many reactions. To
investigate the size effect of Ru nanoparticles in our system, we
have synthesized colloidal Ru nanoparticles with four different
mean sizes (1.6, 2.7, 5.0, and 10.8 nm) by changing the reduc-
tion temperature and the aging time during the reduction of
different mean Ru particle sizes. Fig. 11 shows that the catalysts
with smaller Ru particles provide higher inulin conversions at a
short reaction time. Aer reaction for 1 h, inulin could be
completely converted over the catalyst with a mean Ru particle
size of 1.9 nm, whereas only ꢂ60% of inulin conversion was
attained on the catalyst with a mean Ru particle size of 27 nm.
Thus, the catalyst with a smaller mean size of Ru nanoparticles
was more active for the transformation of inulin. Fructose was
formed as the major product at the initial reaction stage irre-
spective of the mean size of Ru particles in the catalyst. For the
catalysts with mean Ru particle sizes of 1.9, 2.3 and 6.0 nm, the
prolonging of the reaction time decreased the yield of fructose
and increased those of sorbitol and mannitol (Fig. 11A–C). This
observation conrms that sorbitol and mannitol are formed by
the hydrogenation of fructose, which is the reaction interme-
diate, over these catalysts. On the other hand, for the Ru/
Cs PW O catalyst with a mean Ru particle size of 27 nm
RuCl with ascorbic acid. Table 3 shows the conditions used for
3
the synthesis of colloids and the obtained mean sizes of Ru
nanoparticles derived from TEM measurements (Fig. S11†). The
increase in the reduction temperature increased the mean size
of Ru nanoparticles. This is possibly because a higher reduction
temperature may increase the probability of collision among
the particles during the particle growing process, leading to
3
12 40
(Fig. 11D), the yields of hexitols (sorbitol and mannitol) were
very low (<8%) even aer 6 h of reaction. The yield of fructose
increased to ꢂ80% aer 6 h of reaction over this catalyst. Thus,
the hydrogenation of fructose proceeded very slowly over the
catalyst with a larger mean Ru particle size.
We further studied the effect of Ru particle size on catalytic
Scheme 2 Proposed mechanism for the generation of Brønsted acid
3
sites over the Ru/Cs PW12O
40 catalyst in the presence of H
2
.
3
behaviours of the Ru/Cs PW12O40 catalysts for the
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Table 3 Mean sizes and dispersion of Ru particles in colloidal precursors or over the Cs
3
PW12O
40-supported catalysts
a
b
c
Conditions used for colloid synthesis
Ru size in colloids (nm) Ru dispersion
Ru size in catalyst (nm)
Reduction temp., 353 K; aging time, 1 h; solvent, water
Reduction temp., 353 K; aging time, 12 h; solvent, water
Reduction temp., 383 K; aging time, 0.5 h; solvent, 60% ethylene glycol
Reduction temp., 383 K; aging time, 12 h; solvent, 60% butylene glycol
1.6
2.7
5.0
0.49
0.40
0.15
0.033
1.9
2.3
6.0
27
10.8
a
b
c
Evaluated from TEM micrographs (Fig. S11). Measured from CO chemisorption for Ru/Cs
3
PW12O
40
.
Calculated from Ru dispersion.
sharply with decreasing the mean size of Ru particles from 6 nm
to 1.9 nm (Fig. S13†). This result suggests that the hydrogena-
tion of fructose over the Ru/Cs PW O catalyst is a structure-
3
12 40
sensitive reaction.
For comparison, the initial rates of inulin conversions over
the Ru/Cs 40 catalysts with different Ru particle sizes
3
PW12O
were also evaluated using the conversions at a shorter reaction
time (0.17 h) in Fig. 11. The hydrolysis product, i.e., fructose,
was mainly formed at such a shorter reaction time. It should be
noted that the hydrolysis of inulin proceeded very slow over the
3
Fig. 10 Catalytic behaviours of the Ru/Cs PW12O40 catalysts with
different mean sizes of Ru nanoparticles for the conversion of inulin in
water under H . Reaction conditions: inulin, 0.20 g; Ru/Cs PW12
.050 g; H O, 10 mL; H
, 2 MPa; T ¼ 363 K; t ¼ 4 h.
Cs PW O alone without loading Ru particles. Thus, the
2
3
O
40
,
3
12 40
0
2
2
presence of Ru nanoparticles plays a pivotal role in the hydro-
lysis of inulin in our system. As displayed in Table 4, the rate of
inulin hydrolysis also depended on the mean size of Ru parti-
cles; the smaller mean Ru particle size afforded a higher rate of
inulin hydrolysis. This can be interpreted by our proposed
model that the Brønsted acid sites for hydrolysis of inulin are
generated from H on Ru nanoparticles (Scheme 2). We spec-
2
ulate that the smaller Ru particles, which contain higher frac-
tions of coordinatively unsaturated Ru sites, favour the
dissociation of H to atomic hydrogen species. Thus, the cata-
2
lyst with smaller Ru particles can generate higher concentra-
tions of Brønsted acid sites over catalyst surfaces for the
hydrolysis reaction.
However, the dependence of the rate of inulin hydrolysis on
the Ru particle size is less strong than that of fructose hydro-
genation. A comparison of the date in Table 4 reveals that, for
the catalyst with a larger mean size of Ru nanoparticles
(particularly, 27 nm), the rate of inulin hydrolysis is higher than
3 40
Fig. 11 Time courses for conversion of inulin over the Ru/Cs PW12O
catalysts with different mean Ru particle sizes (A) 1.9 nm, (B) 2.3 nm, (C) that of fructose hydrogenation. This can explain the observation
6
0
.0 nm, (D) 27 nm. Reaction conditions: inulin, 0.20 g; Ru/Cs
.050 g; H O, 10 mL; H
, 2 MPa; T ¼ 363 K.
3 40
PW12O ,
that fructose is formed as the dominant product over the cata-
lyst with a mean Ru particle size of 27 nm even aer a long-time
2
2
hydrogenation of fructose. The major products in the hydro-
genation of fructose were hexitols (sorbitol and mannitol) and
the selectivity to hexitols was >98% over the Ru/Cs PW O
Table 4 Effect of mean size of Ru nanoparticles on the rates of inulin
hydrolysis and fructose hydrogenation over the Ru/Cs
catalysts
3
PW12O
40
3
12 40
catalyst irrespective of the mean Ru particle size. However, the
conversion of fructose was strongly dependent on the size of Ru
particles (Fig. S12†). We evaluated the rates of fructose
conversions over the catalysts with different mean Ru particle
sizes by using the conversions at the initial reaction stage. The
results are summarized in Table 4. We also calculated the
turnover frequency (TOF), i.e., the moles of fructose converted
per unit time at the initial stage per mole of surface Ru atoms.
The TOF for the hydrogenation of fructose to hexitols increased
catꢀ
1
ꢀ1
Initial reaction rate (mmol g
h )
Mean Ru size
nm)
a
b
(
Fructose hydrogenation
Inulin hydrolysis
1
2
6
2
.9
.3
.0
7
117
72
13
1
30
23
20
10
a
b
Catalyst, 0.050 g; fructose, 0.20 g; H
2
O, 10 mL; H
2
, 2 MPa; T ¼ 363 K.
Catalyst, 0.050 g; inulin, 0.20 g; H O, 10 mL; H
2
2
, 2 MPa; T ¼ 363 K.
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reaction (Fig. 11D). Upon decreasing the mean size of Ru Education (no. 20130121130001), and the Program for Chang-
nanoparticles, the rate of fructose hydrogenation increased jiang Scholars and Innovative Research Team in Chinese
more rapidly. As a result, the rate of fructose hydrogenation Universities (no. IRT1036).
exceeded that of inulin hydrolysis over the catalysts with smaller
Ru nanoparticles. In other words, the rate-determining step
changed from the fructose hydrogenation to inulin hydrolysis
Notes and references
with decreasing the mean size of Ru nanoparticles over the Ru/
Cs 40 catalysts. Thus, it becomes understandable that
hexitols are produced as the main products over the catalyst
with a smaller mean size of Ru particles (in particular 1.9 or 2.3
nm), while fructose is formed dominantly over the one with
bigger Ru particles.
1 (a) D. Klemm, B. Heublein, H. P. Fink and A. Bohn, Angew.
Chem., Int. Ed., 2005, 44, 3358; (b) P. L. Dhepe and
A. Fukuoka, ChemSusChem, 2008, 1, 969; (c) S. Van de
Vyver, J. Geboers, P. A. Jacobs and B. F. Sels,
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PW12O
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4
. Conclusion
Ru nanoparticles loaded on the Keggin-type polyoxometalates,
i.e., Ru/Cs 40, were highly efficient for the conver-
sions of inulin and cellulose into hexitols in water in the pres-
ence of The Ru/Cs 40 catalysts exhibited
signicantly higher yields of hexitols than the Ru catalysts
loaded on other solid acids such as Al O , HZSM-5 and MCM-
x
H3ꢀxPW12O
H
2
.
x
H3ꢀxPW12
O
2
3
2
2. It is unexpected that the Ru/Cs
3
PW12
O
40 catalyst, which does
3 (a) A. W. Heinen, G. Papadogianakis, R. A. Sheldon,
J. A. Peters and H. van Bekkum, J. Mol. Catal., 1999, 142,
17; (b) A. W. Heinen, J. A. Peters and H. van Bekkum,
Carbohydr. Res., 2001, 330, 381.
not possess strong intrinsic Brønsted acidity, exhibits superior
performances for the conversions of both inulin and cellulose
into hexitols. Hexitol yields of 84% and 45% were achieved for
the conversions of inulin and cellulose over the Ru/Cs
3
PW12
O
40
4 L. Zhou, A. Wang, C. Li, M. Zheng and T. Zhang,
ChemSusChem, 2013, 5, 932.
catalyst at 363 K for 4 h and 433 K for 24 h, respectively. This
catalyst was robust during the recycling uses. We have
demonstrated that new Brønsted acid sites can be generated
5 T. Werpy and G. Petersen, Top Value Added Chemicals from
Biomass, 2004, vol. 1, pp. 58–60, http://www.pnl.gov/main/
publications/external/technical_reports/PNNL-14808.pdf.
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7636.
7 (a) R. Palkovits, K. Tajvidi, J. Procelewska, R. Rinaldi and
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over the Ru/Cs
characterizations by pyridine-adsorbed FT-IR, Raman, and ESR
spectroscopic measurements, we propose that molecular H is
rst dissociated on Ru particles, and then, the H atoms spillover
to the Cs surface, which are subsequently converted to
protons by releasing electrons to the support. The H -originated
3 2
PW12O40 catalyst from molecular H . Through
2

3
PW12O
4
2
Brønsted acid sites are reversible and responsible for the
superior catalytic performance of the Ru/Cs PW O catalyst.
3
12 4
We further found that the mean size of Ru nanoparticles over
the Ru/Cs 40 catalyst exerted signicant effects on the
products distribution in the conversion of inulin in water under
atmosphere. For the catalyst with a larger mean size of Ru
3
PW12
O
H
2
nanoparticles (27 nm), fructose, a hydrolysis product, was
attained as the major product, whereas the catalysts with
smaller Ru nanoparticles afforded hexitols with higher yields.
The rates of both inulin hydrolysis and fructose hydrogenation 10 (a) S. Van de Vyver, J. Geboers, W. Schutyser, M. Dusselier,
were dependent on the mean size of Ru particles, and smaller
Ru particles favoured the reaction rates for both steps. The rate
of fructose hydrogenation increased more signicantly upon
decreasing the mean Ru particle size, and the rate determining
step changed from fructose hydrogenation to inulin hydrolysis
at the same time.
P. Eloy, E. Dornez, J. W. Seo, C. M. Courtin,
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11 W. Deng, Q. Zhang and Y. Wang, Dalton Trans., 2012, 41,
9817.
Acknowledgements
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This work was supported by the National Natural Science
Foundation of China (no. 21173172, 21103143, and 21033006),
the Research Fund for the Doctorial Program of Higher
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