Chitosan-stabilized gold, gold-palladium, and gold-platinum nanoclusters as efficient catalysts for aerobic oxidation of alcohols

Article abstract of DOI:10.1016/j.molcata.2011.03.019

Chitosan was used as a stabilizer for the synthesis of Au, AuPd and AuPt nanoclusters (NCs). The produced NCs had a narrow particle size distribution with sizes less than 2.3 nm in diameter and were characterized by various techniques such as UV-Visible s

Full text of DOI:10.1016/j.molcata.2011.03.019

Journal of Molecular Catalysis A: Chemical 341 (2011) 1–6
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Editor’s Choice paper
Chitosan-stabilized gold, gold–palladium, and gold–platinum nanoclusters as
efficient catalysts for aerobic oxidation of alcohols
Arumugam Murugadossa, Hidehiro Sakuraia,b,∗
a Research Center for Molecular Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444 8787, Japan
b PRESTO, Japan Science and Technology Agency, Tokyo 102-0075, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Chitosan was used as a stabilizer for the synthesis of Au, AuPd and AuPt nanoclusters (NCs). The produced
NCs had a narrow particle size distribution with sizes less than 2.3 nm in diameter and were charac-
terized by various techniques such as UV–Visible spectroscopy, X-ray diffraction, X-ray photoelectron
spectroscopy, transmission electron microscopy and scanning electron microscope energy dispersive
spectroscopy (STEM-EDS). These metal NCs exhibited high catalytic activity toward the aerobic oxida-
tion of various alcohols under ambient conditions comparable with the reported Au:PVP catalyst. Au NCs
protected by chitosan could easily be recovered for repeated use.
Received 5 February 2011
Received in revised form 22 March 2011
Accepted 25 March 2011
Available online 31 March 2011
Keywords:
Gold
Bimetallic cluster
Chitosan
© 2011 Elsevier B.V. All rights reserved.
Aerobic oxidation
Alcohol
1. Introduction
applied to biosensors [7] and other therapeutic materials [8] and
catalytic applications are scarce [9].
Polymer-stabilized gold nanoclusters (NCs) have recently
received a tremendous amount of attention due to their high
performance catalytic activity toward organic transformations,
including aerobic oxidation, as well as their potential to con-
trol chemical reactions by the design of their various protective
hydrophilic functional polymers [1]. Polyvinyl-2-pyrrolidone (PVP)
enhances the catalytic activity of gold for aerobic alcohol oxi-
dation better than other protective polymers, such as polyvinyl
alcohol or polyallylamine, owing to the electron donating effect
of the polymer matrices to the gold clusters [2,3]. Thermosensi-
tive star-shaped polymer matrices realize high reusability in the
direct aerobic oxidation of alcohols to acids under basic aque-
ous conditions [4]. These studies prompted us to develop other
hydrophilic polymers, in particular, biopolymer-stabilized gold NCs
and to investigate their catalytic activity.
Herein, we report a new preparative method of chitosan-
stabilized gold (Au:Chit), gold–palladium (AuPd:Chit) and
gold–platinum (AuPt:Chit) NCs. These NCs showed high cat-
alytic activity toward the aerobic oxidation of various alcohols into
corresponding carbonyl compounds and acids.
2. Experimental
2.1. Materials and methods
All chemicals and solvents were used as received with-
out further purification. Hydrogen tetrachloroaurate tetrahydrate
(HAuCl4·4H2O, Tanaka Kikinzoku), chloroplatinic acid (H2PtCl6,
Wako), palladium chloride (PdCl2, Wako), sodium tetraborohy-
dride (NaBH4, Wako), acetic acid (Wako), p-hydroxy benzyl alcohol,
benzylalcohol, p-methoxy benzylalcohol, p-nitrobenzylalochol, 1-
hydroxy indane, ascorbic acid and potassium hydroxide (KOH)
were obtained from Wako pure chemical industries. Ethyl acetate
and hexane were purchased from Wako. Chitosan, poly (d-
Chitosan (␤-1,4-linked poly d-glucosamine), derived from the
deacetylation of chitin, is the second most abundant biopolymer in
nature after cellulose [5]. The presence of primary amine, hydroxyl
and ester groups makes the polymer an excellent support for metal
NCs [6]. However, most chitosan-stabilized metal NCs have been
glucosamine), with
a medium molecular weight of 75–85%
deacetylated was from Sigma–Aldrich. Milli-Q grade water was
used in all experiments.
∗
Corresponding author at: Research Center for Molecular Scale Nanoscience,
2.1.1. Preparation of Metal:Chit
Chitosan (0.15 g) was dissolved into 50 mL of 0.18% aqueous
acetic acid solution. To the solution was added 0.5 mM of HAuCl4
Institute for Molecular Science, 5-1 Higashiyama, Myodaiji, Okazaki 444 8787, Japan.
Tel.: +81 564 595525; fax: +81 564 595527.
E-mail address: hsakurai@ims.ac.jp (H. Sakurai).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.03.019

2
A. Murugadoss, H. Sakurai / Journal of Molecular Catalysis A: Chemical 341 (2011) 1–6
and the mixture was stirred for 30 min at room temperature
under 1600 rpm. Aqueous solution of 0.1 M NaBH4 (2.5 mL) was
rapidly added at 6 ◦C under vigorous stirring. The color of the reac-
tion mixture immediately turned from pale yellow to dark red in
color, indicating the formation of smaller Au NCs. The hydrosol of
Au:Chit was stored in refrigerator. The preparation of AuPd:Chit
and AuPt:Chit was based on the co-complexation of mixing of the
corresponding two metal ions. Chitosan (0.15 g) was dissolved into
50 mL 0.18% of aqueous acetic acid and followed by addition of
0.4 mM HAuCl4 and 0.1 mM PdCl2 aqueous solutions or 0.1 mM of
H2PtCl6 aqueous solutions. The molar ratio of metal ions are used
in these reaction is 4:1 (Au/Pd or Pt). Other procedures are followed
as same as Au:Chit for the preparation of bimetallic AuPd:Chit and
AuPt:Chit.
gelatinous Au:Chit were transferred to the reaction test tube to run
for the next cycle.
3. Results and discussion
For catalytic applications, it is very important to control the
size of the gold cluster during preparation because smaller clus-
ters exhibit superior catalytic activity for aerobic oxidations [1a,10].
Previous methods, such as a one-pot synthesis of chitosan with
metal salts at higher temperature, produced larger particle sizes
and a broad size distribution [11]. Generally, the higher temper-
ature treatment with weak stabilizing agents such as citric acid,
chitosan, PVP and starch produces the larger size gold NCs [7c,12].
Our preparative scheme for the synthesis of Au:Chit involves two
−
steps, the formation of ion pairs by AuCl4 with an aqueous acetic
2.1.2. Characterization studies
acid solution of chitosan, followed by rapid reduction with NaBH4
UV–vis spectra were measured by JASCO V-670 spectropho-
tometer at 24 ◦C. FTIR spectra of the chitosan and Au:Chit were
measured using a JASCO FTIR spectrometer with model of FTIR
4100 series. X-ray diffraction pattern of Au:Chit as in flake form,
placed on microscope glass slide was recorded using Rigaku Ultima
III, RINT-2000/PC (Cu K␣ radiation under operation at 40 kV and
40 mA), in thin film mode. XPS measurement of Au:Chit was carried
out using a Vacuum Generators ESCALAB 220iXL electron spec-
trometer. The base pressure in the analyzer was ca. 2 × 10−8 Torr.
X-rays from the Mg K␣ line at 1253.6 eV (15 kV, 20 mA) were
used for excitation. Photoelectrons were corrected in the constant
analyzer energy mode with pass energy of 50 eV. The overall reso-
lution was 1 eV for the XPS measurements. The core level binding
energies (BE) were calibrated using graphitic carbon tap with the
adventitious carbon binding energy of 286.2 eV. Inductively cou-
pled plasmon-Atomic emission spectrometry (ICP-AES (LEEMAN
LABS INC, Profile plus)) was adopted to measure the Au, Pd and Pt
contents in chitosan. The Au:Chit, AuPd:Chit and AuPt:Chit were
added into aqua regia, respectively, and were allowed to react
for 3 h in order to dissolve the Au, Pd and Pt completely. The
resulting samples were analyzed by ICP-AES. The high-resolution
TEM images of chitosan stabilized metal NCs were recorded with
a JEOL JEM-3100FE at an accelerating voltage of 300 kV. Typical
magnification of the images was 100,000–120,000×. The compo-
sition of individual bimetallic NCs were evaluated using 0.25 nm
EDS (energy-dispersive spectrometry) probe in JEOL 3100FE TEM,
operating in the STEM mode. NMR spectra (400 MHz for 1H) were
recorded using JEOL JMN LAMBDA 400 spectrometer with CDCl3 as
solvent at 23 ◦C and (CH3)4Si as internal standard.
at 6 ◦C (Scheme 1). Strong electrostatic interactions between the
−
polycations of chitosan with AuCl4 might be the decisive factor
for controlling the size of gold NCs [13]. Furthermore, the reduc-
ing agent NaBH4 also played an important role in the formation of
smaller gold NCs than weak reducing agents (Fig. S1). This could
be due to the instaneously, formation of large number of nuclei,
whose growth can be further retarded with chitosan polymer by
anchoring them.
It is noteworthy to mention that Au:Chit, a 1:34 molar ratio
of gold and chitosan (monomer unit) is required as an optimized
condition. The amount of Au loading on chitosan was measured
to be 33 mg of Au per gram of chitosan on ICP analysis. At higher
pH, Au:Chit precipitates and the resulting filtrate showed neither
unreduced metal ions nor unbound metal NCs were present, which
was confirmed from the ICP analysis. This observation indicated
that Au NCs were efficiently entrapped on chitosan matrix. At high
molar ratio of Au:Chit (1:44), the reaction mixture was highly vis-
cous, which causes formation of bigger size gold clusters. Under the
condition of lower molar ratio of Au:Chit (1:22), the nanoclusters
were not well stable than 1:34 molar ratio of Au:Chit. Since a 1:100
molar ratio is necessary for the preparation of Au:PVP, the present
procedure allows significant reduction of the required monomer
unit per gold atom. In addition, the thus-prepared Au:Chit does not
require any special purification techniques and can be used as pre-
pared. Au:Chit is storable either as a solution or in solid form in the
refrigerator for months without any noticeable aggregation.
Fig. 1a shows the transmission electron microscopic analysis of
Au:Chit. The clusters are narrow in size distribution and the average
diameters are determined to be 2.3 0.2 nm from the histogram
plot of Fig. 1b. The X-ray diffraction studies (Fig. 1c) reveal a broad
(1 1 1) peak associated with a small face-centered cubic crystal. The
average crystallite sizes are also estimated to be 2.4 nm from the
diffraction peak of the Au (1 1 1) plane (Fig. S2), which is consistent
with diameters estimated from the TEM histogram plot. The UV–vis
spectrum (Fig. 1d) of Au:Chit exhibits a broad band at ∼500 nm,
which is blue shifted significantly from the well known surface
plasmon resonance (SPR) band of the Au clusters (cf. Au:PVP at
520 nm) [1a,2,14]. The blue shift of the SPR band might be attributed
to the chemical interaction between the gold core and stabilizing
molecule of chitosan [15].
2.1.3. General procedure for aerobic alcohol oxidation
In a test tube (ꢀ = 30 mm), benzyl alcohol (0.25 mmol), K2CO3
(103.7 mg, 0.75 mmol), and water (5 mL) were placed. The aqueous
solution of Metal (Au, AuPd, or AuPt):Chit (0.005 mmol, 10 ml, 2
atom%) was added and the reaction mixture was stirred vigorously
(1300 rpm) under air at a given temperature. If needed, the cata-
lysts were separated by centrifugation (KUBOTA 5200) at 4000 rpm.
The reaction mixture was quenched with 10 mL of 0.1 M HCl and
then extracted with ethyl acetate (3× 10 mL). The organic layer
was dried over anhydrous Na2SO4 and evaporated in vacuo. The
crude product was purified either by column chromatography or
by preparative thin layer chromatography (50% ethyl acetate in
hexane) to provide pure product(s) as listed. The products were
confirmed by 1H NMR spectra.
The interaction between gold core and chitosan was also
depicted by XPS analysis (Fig. 2). The smaller BE in Au 4f7/2 (82.9 eV)
is ascribed that core Au has negative charge in state due to the inter-
action of chitosan with surface of the Au clusters [2]. The higher BEs
of both N1s (399.5 eV) and O1s (532.7) in Au:Chit than those of N1s
(398.1 eV) and O1s (531.7 eV) in chitosan (Fig. S3) are observed.
Miyama and coworkers reported that core level N1s (400.5 eV)
spectrum of amine group of chitosan coordinated with gold exhibits
higher BEs than free chitosan molecules [16]. Kumar et al., also
2.1.4. Procedure for the reuse of Au:Chit catalyst
After completion of the reaction, the catalysts were separated by
centrifugation (KUBOTA 5200) at 4000 rpm, and then washed with
water several times (preferably more than 4 times). Thus-recovered

A. Murugadoss, H. Sakurai / Journal of Molecular Catalysis A: Chemical 341 (2011) 1–6
3
−
Scheme 1. The schematic representation of formation of Au:Chit. (a) Polycationic form of chitosan, (b) formation of ion pairs with AuCl4 and (c) Au:Chit formation.
Fig. 1. (a) TEM image of chitosan-stabilized Au NCs. (b) Particle size histogram plot obtained from (a). (c) X-ray diffraction pattern of Au:Chit. (d) UV–vis spectrum of Au:Chit.
Fig. 2. (a) XPS spectra of core Au 4f, (b) N1s and (c) O1s. (d) FTIR spectra of pure chitosan (i) and Au:Chit (ii).

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A. Murugadoss, H. Sakurai / Journal of Molecular Catalysis A: Chemical 341 (2011) 1–6
Table 1
reaction rate was slow, with completion in 30 h, giving the acid
3d in 98% yield (Entry 7). The reaction of 1-indanol (1e) afforded
the corresponding ketone (2e) in 96% yield after 1.5 h (Entry 9)
and in quantitative yield for 30 min by Au:PVP (Entry 10), respec-
tively. Thus, Au:Chit has comparable catalytic activity to Au:PVP
toward the oxidation of various aromatic primary/secondary alco-
hols [20]. Such similar catalytic activity of Au:Chit and Au:PVP is
unexpected because of the following two reasons. First, Au:PVP
NCs are smaller in diameter (1.3 nm) than Au:Chit (2.3–2.4 nm).
The catalytic activity of Au:PVP depends highly on its cluster size
and the activity drastically decreases above 2 nm [10]. The activity
of Au:Chit is significantly higher than that of Au:PVP of comparable
mean size. Second, in basic media Au:PVP and Au:Chit behave as
quasi-homogeneous and heterogeneous catalysts, respectively, and
even though the former is believed to be more active, the Au:Chit
catalyst works as efficiently as Au:PVP. This unexpectedly high cat-
alytic activity of Au:Chit might arise from the matrix dependence
interaction between chitosan and Au NCs as depicted by XPS and
FTIR analyses. The detailed mechanistic studies for the aerobic oxi-
dation of alcohol catalyzed by quasi-homogeneous Au nanocatalysts
have been described well in elsewhere [2]. In addition, the use of
excess capping agents or protective agents of the polymers may
completely cover the active sites of the cluster surface, which act
as poison to the catalysts [21]. In comparison to Au:Chit (1:34 molar
ratio), more PVP (1:100 molar ratio) is used to stabilize the Au NCs
for the preparation of Au:PVP. Since Au:Chit works as a heteroge-
neous catalyst, it is expected to be easily recoverable and reusable
by simple filtration or centrifugation.
Aerobic oxidation of aromatic primary alcohols and comparative studies with
Au:Chit and Au:PVP catalysts.
.
Entry
Substrate
Catalysts
Time (h)
Yield (%)
2
3
1
2
3
4
5
6
7
8
9
1a
1a
1b
1b
1c
1c
1d
1d
1e
1e
Au:Chit
Au:PVP
Au:Chit
Au:PVP
Au:Chit
Au:PVP
Au:Chit
Au:PVP
Au:Chit
Au:PVP
8
6
8
8
8
0
0
98
91
71
0
0
0
96
99
96
85
0
0
25
91
98
99
0
3
30
24
1.5
0.5
10
0
claimed that N1s spectrum of primary amine bound gold particles
shifted to higher BEs than that of unbound primary amine due to
strong electrostatic interaction between amine and gold clusters
[17]. Therefore, it can be concluded that the interaction between
chitosan and Au NCs exists through the multidendate coordination
of amine and hydroxyl group. These observations were further sup-
ported from the FTIR analysis of chitosan and Au:Chit as shown in
Fig. 2d. Significant differences are observed in the 1800–1300 cm−1
region of the spectra between chitosan and Au:Chit. The intense
band of bending vibration of –NH is observed at 1595 cm−1 in chi-
tosan and at 1560 cm−1 in Au:Chit. This lower frequency shift might
be due to the interaction with gold clusters [9a,18]. The shifting of
the –OH bending vibration from 1426 and 1313 cm−1 (I) to 1416
and 1305 cm−1 (II) may also account for the interaction of gold
clusters with –OH group of chitosan. Furthermore, Au:Chit exhibits
more intense set of bands in the region of 650–500 cm−1 com-
pared to chitosan and it has been proposed that these new bands
are ascribed to the stretching vibration of Au–N and Au–O bonds
[9a,19].
We found that the catalyst could easily be recovered by sim-
ple centrifugation after the reaction and no special treatment
was required to reuse the catalyst in the oxidation reaction. The
reusability of Au:Chit was tested by the oxidation of 1b under the
same reaction conditions as listed in Entry 3 of Table 1, and the yield
of 2b in three cycles were 95, 93, and 97%, respectively. The size
of the Au in Au:Chit remained the same even after the third cycle,
which was confirmed by TEM analysis of recovered Au:Chit (Fig. 3).
Histogram plot of recovered Au:Chit (Fig. 3c) indicates that Au clus-
ters are uniform in size and particles are not aggregated during
the catalytic reactions. The X-ray diffraction pattern of recovered
Au:Chit (Fig. 3d) shows the crystallite size is 2.4 nm as fresh Au:Chit.
The leaching of Au was also checked by inductively coupled Plas-
mon (ICP) analysis of the filtrate, which showed that no Au was
present (detection limit: 0.02 ppm).
As demonstrated above, chitosan possesses great potential as
an excellent matrix for metal cluster catalysts. These results fur-
ther motivated us to apply chitosan matrices to stabilize bimetallic
cluster systems because the doping of other metals into gold NCs
may enhance the catalytic activity and selectivity of monometallic
Au nanocatalysts [22]. AuPd:Chit and AuPt:Chit bimetallic clus-
ters were efficiently prepared based on the procedure as shown in
Scheme 1 by co-complexation via mixing of the corresponding two
metal ions (HAuCl4/PdCl2 or HAuCl4/H2PtCl6). The average diame-
ters of AuPd:Chit and AuPt:Chit were determined to be 2.4 0.5 nm
and 2.4 0.4 nm, respectively, from HRTEM analysis by measuring
more than 350 particles (Fig. 4a and b).
The distribution of each bimetallic NCs were analyzed by
0.25 nm scanning transmission electron microscope-energy
dispersive spectroscopy (STEM-EDS) shown in Supporting
information (Figs. S4 and S5). These spectra indicate that each
bimetallic NCs were made by composition of two metal and
the ratio of the metals (4:1 of Au and Pd or Pt) were almost
consistent with the initial concentration of metals used dur-
ing the preparation of bimetallic NCs. The amount of metal
loading in AuPd:Chit and AuPt:Chit was found to be 26 mg
(Au) and 3.5 mg (Pd), 26 mg (Au) and 6.5 mg (Pt) per gram
of chitosan, respectively, on ICP elemental analysis. As same
The catalytic activity of Au:Chit for the aerobic oxidation of alco-
hols was then tested and compared with that of previously reported
Au:PVP (mean size 1.3 nm) as shown in Table 1.
The reactions were carried out in water in the presence of 2
atom% of Au:Chit and 300 mol% of K2CO3 at 300 K under aerobic
conditions. Under basic conditions Au:Chit immediately precipi-
tates out, functioning as a heterogeneous catalyst. This is in sharp
contrast with Au:PVP, which behaves as a quasi-homogeneous cata-
lyst. The oxidation of benzyl alcohol (Entry 1: 1a) was tested, giving
the corresponding acid (3a) in 96% yield after 8 h (Entry 1). In the
case of Au:PVP, the reaction was completed within 6 h although the
yield was somewhat lower (85%) (Entry 2). p-Hydroxybenzyl alco-
hol (1b) is oxidized into p-hydroxy benzaldehyde under the same
conditions as in the case of Au:PVP (Entry 4). Indeed, the aldehyde
2b was selectively obtained in 98% yield in the reaction catalyzed
by Au:Chit (Entry 3). The electronic effect was monitored by the
reactions of p-methoxybenzyl alcohol (1c) and p-nitrobenzyl alco-
hol (1d). The former reaction gave the aldehyde 2c in 71% yield and
the acid 3c in 25% yield after 8 h (Entry 5). In contrast, the Au:PVP
reaction affords 3c in 91% yield (Entry 6). In the latter case, the

A. Murugadoss, H. Sakurai / Journal of Molecular Catalysis A: Chemical 341 (2011) 1–6
5
Fig. 3. (a) TEM picture of recovered Au:Chit catalysts, after third cycle, (b) high resolution TEM image of (a), (c) histogram plots of (b) and (d) X-ray diffraction pattern of (a).
as Au:Chit, metal NCs are efficiently entrapped on chitosan
matrix.
The thus-prepared bimetallic clusters were tested for aero-
bic alcohol oxidation, which showed that both AuPd:Chit and
AuPt:Chit have comparable catalytic activity to Au:Chit (Table 2).
All the reactions were carried out in water in the presence of 2
atom% of M:Chit and 300 mol% of K2CO3 at 300 K under aerobic con-
ditions. As shown in Entries 1–3, in the oxidation of benzyl alcohol
(1a) to benzoic acid, AuPt:Chit took a longer time (10 h, 95% yield)
than either AuPd:Chit (8 h, 97% yield) or Au:Chit (8 h, 96% yield).
In contrast, oxidations of 1b, 1c and 1d were completed within 4,
or 26, or 0.5 h and 8, or 29, or 1.5 h by AuPt:Chit and AuPd:Chit,
UV–vis spectra of AuPd:Chit and AuPt:Chit are shown in Fig. 4c
and d. An exponantially increasing absorbance toward higher
energy due to the interband transition of newly formed bimetal-
lic alloy structure of AuPd NCs is observed [23]. The absence of SPR
band of Au also indicates the alloy form rather than the formation of
segregation. On the contrary, bimetalic AuPt:Chit shows an broad
band at ∼502 nm, which is ascribable as the segregation of gold in
the AuPt NCs.
Fig. 4. (a) TEM images of AuPd:Chit and (b) AuPt:Chit. (c) and (d) are UV–vis spectra of AuPd:Chit and AuPt:Chit, respectively.

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Table 2
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Time (h)
Yield (%)
2
3
1
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4
5
6
7
8
9
1a
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98
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96
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97
95
0
0
0
98
97
98
0
0
0
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Chit–AuPt NCs
Chit–Au NCs
Chit–AuPd NCs
Chit–AuPt NCs
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Acknowledgments
This work was supported by MEXT and NEDO. AM thanks JSPS
(Japan Society for the Promotion of Science) for fellowship support.
We also thank Ms. Noriko Kai for technical support.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2011.03.019.