Gold- and gold-palladium/poly(1-vinylpyrrolidin-2-one) nanoclusters as quasi-homogeneous catalyst for aerobic oxidation of glycerol

Article abstract of DOI:10.1016/j.tetlet.2011.03.027

Glycerol was oxidized catalytically under aerobic conditions in the presence of monometallic nanoclusters of gold on poly(1-vinylpyrrolidin-2-one) (PVP) to give hydroxymalonic acid (tartronic acid) as the major product, together with 2,3-dihydroxypropanoic acid (glyceric acid) and hydroxyacetic acid (glycolic acid) as minor products. In contrast, oxalic acid was selectively obtained when bimetallic Au-Pd:PVP nanoclusters were used as the catalyst.

Full text of DOI:10.1016/j.tetlet.2011.03.027

Tetrahedron Letters 52 (2011) 2633–2637
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Tetrahedron Letters
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Gold– and gold–palladium/poly(1-vinylpyrrolidin-2-one) nanoclusters
as quasi-homogeneous catalyst for aerobic oxidation of glycerol
Salprima Yudha S a, Raghu Nath Dhital b, Hidehiro Sakurai b,c,d,
⇑
a Department of Chemistry, Faculty of Mathematic and Natural Science, University of Bengkulu, Jalan Raya Kandang Limun, Bengkulu 38371A, Indonesia
b Department of Functional Molecular Science, The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8787, Japan
c Research Center for Molecular Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan
d 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:
Glycerol was oxidized catalytically under aerobic conditions in the presence of monometallic nanoclus-
ters of gold on poly(1-vinylpyrrolidin-2-one) (PVP) to give hydroxymalonic acid (tartronic acid) as the
major product, together with 2,3-dihydroxypropanoic acid (glyceric acid) and hydroxyacetic acid
(glycolic acid) as minor products. In contrast, oxalic acid was selectively obtained when bimetallic
Au–Pd:PVP nanoclusters were used as the catalyst.
Received 4 January 2011
Revised 28 February 2011
Accepted 7 March 2011
Available online 21 March 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Aerobic oxidation
Glycerol
Gold
Palladium
1. Introduction
environmentally sustainable systems. We have previously re-
ported that Au nanoclusters stabilized by hydrophilic polymers,
The use of nanosized gold as a catalyst for aerobic oxidation
reactions has been of considerable interest to chemists for the last
two decades.1,2 A characteristic feature of gold nanocluster cata-
lysts is that they show superior activity in water or water-contain-
ing solvents, which should permit the development of
such as poly(1-vinylpyrrolidin-2-one) (PVP), behave as a quasi-
homogeneous catalyst in aqueous media and that they exhibit an
excellent activity toward the aerobic oxidation reactions, including
the oxidation of alcohols.3,4 Au:PVP-catalyzed oxidation reactions
of alcohols have been carried out under relatively mild conditions
hydroxyacetic acid
Dihydroxy Acetone
Hydroxypyruvic Acid
(Glycolic Acid)
Oxalic Acid
O
O
OH
OH
O
OH
HO
OH
HO
O
OH OH
O
O
OH
OH OH
OH
OH
OH
O
HO
OH
H
OH
HO
OH
O
O
OH
2,3-dihydroxypropanal 2,3-dihydroxypropanoic acid hydroxymalonic acid oxomalonic acid
(Glyceraldehyde) (Glyceric Acid) (Tatronic Acid) (Meso Oxalic Acid)
O
OH
O
O
O
Scheme 1. Reaction pathways for oxidation of glycerol under basic conditions.
⇑
Corresponding author. Tel.: +81 564 59 5525; fax: +81 564 59 5527.
E-mail address: hsakurai@ims.ac.jp (H. Sakurai).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.03.027

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S. Y. S et al. / Tetrahedron Letters 52 (2011) 2633–2637
Table 1
Au:PVP-catalyzed oxidation of glycerol under open air15,16
OH
OH
OH
OH
Au:PVP (x atom%)
Base (x mol%)
temp.(oC), time (h)
O
O
O
O
+
+
OH OH
OH OH
OH
OH OH
1
2
3
Entry
Au:PVP (atom %)
Base
Temp (°C)
Time (h)
Conversion (%)
Selectivity
Type
Amount (mol %)
1
2
3
1
2
3
4
5
6
7
8
9
2.5
2.5
2.5
5.0
5.0
5.0
5.0
5.0
5.0
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
—
NaOH
Cs2CO3
K2CO3
300
300
300
300
300
—
300
300
600
30
50
80
80
80
80
80
80
80
24
24
24
24
48
48
24
24
24
0
0
53
89
100
0
94
96
100
—
—
—
25
30
29
—
32
40
45
—
—
25
37
21
—
25
24
29
—
21
12
9
—
11
9
9
in basic aqueous solution at atmospheric pressure and room tem-
perature, in the case of benzylic alcohols, or at temperatures of up
to 80 °C, in the case of aliphatic alcohols. Under these conditions,
carboxylic acids are obtained directly by oxidation of primary
alcohols.4c
a mixture of 2,3-dihydroxypropanoic acid (1), hydroxymalonic acid
(2), and hydroxyacetic acid (3) was obtained with a low glycerol
conversion (53%) after 24 h (entry 3). Because the products differed
from those obtained under previous conditions for heterogeneous
catalysis, attempts were made to achieve 100% conversion by
changing the amounts of catalyst and the reaction times.10 When
the amount of catalyst was increased to 5.0 atom % (entry 4), the
conversion increased to 89% and when the reaction time was
Biorenewable products are attractive as raw materials because
they can be produced in a sustainable fashion by agriculture or for-
estry. Triglycerides are typical biorenewable materials that are
present as major components in vegetable oils and animal fats. Re-
cent interest in triglycerides has focused on their reaction with
methanol to produce biodiesel fuels. Each molecule of the triglyc-
eride reacts with three molecules of methanol to produce three
molecules of biodiesel and one of glycerol, so that about 10% of
the product mixture consists of glycerol,5 which is a versatile syn-
thon and an important biorenewable compound and is, therefore,
widely used in syntheses of many useful organic derivatives. The
development of environmentally benign and selective procedures
for chemical transformations of glycerol into useful compounds
is now a rapidly emerging field of chemistry.6 Here, we report
our studies on the aerobic oxidation of glycerol catalyzed by
Au:PVP or bimetallic Au–Pd:PVP nanoclusters.
2. Results and discussion
Several reports have appeared on oxidation reactions of glycerol
catalyzed by gold nanoclusters.5–9 The oxidation of glycerol under
basic condition follows the path shown in Scheme 1.5,6 In the pres-
ence of Au/C catalysts, glycerol is oxidized to 2,3-dihydroxypropa-
noic acid (glyceric acid) with 100% selectivity under an oxygen
atmosphere in the presence of a base.7
The efficient conversion of glycerol into lactic acid by aerobic
oxidation on a Au–Pt/TiO2 catalyst has recently been reported.8
However, there are only a few reports on the use of acidic or
base-free conditions for gold-catalyzed oxidation reactions of glyc-
erol.9 Typical conditions for these aqueous-phase reactions gener-
ally involve temperatures in the range of 30–80 °C and oxygen
pressures in the range of 1–10 atm.
The results of our studies on the aerobic oxidation of glycerol in
the presence of Au:PVP (mean size / = 1.3 0.3 nm)4 catalyst are
listed in Table 1. Usually, Au:PVP-catalyzed aerobic oxidations of
alcohols are carried out in the presence of 300 mol % of K2CO3 in
water under air at room temperature to 60 °C.4b However, when
2.5 atom % of Au:PVP was used, no reaction was observed at
30 °C (entry 1) or at 50 °C (entry 2) after 24 h. Oxidation did, how-
ever, occur when the reaction temperature was raised to 80 °C, and
Figure 1. Typical TEM images and particle size distributions of (a) Au–
Pd(80:20):PVP, (b) Au–Pd(50:50):PVP, and (c) Au–Pd(20:80):PVP.

S. Y. S et al. / Tetrahedron Letters 52 (2011) 2633–2637
2635
prolonged to 48 h (entry 5), the conversion reached 100% (entry 5),
but the selectivity of the reaction remained unsatisfactory. In at-
tempts to improve the selectivity of the reaction, we examined
the effect of the base. As shown in entry 6, the reaction did not pro-
ceed at all in the absence of the base, showing that the presence of
the base in this catalytic system is required because it initiates the
reaction by abstracting a hydrogen atom from a hydroxyl group.
On the basis of our results, we examined other combinations of
catalysts and bases, and we found that the selectivity toward
hydroxymalonic acid (2) increased to 32% or 40% when the base
was changed to NaOH or Cs2CO3, respectively (entries 7 and 8).
On the other hand, the amount of base also affected the selectivity.
Quantitative conversion was observed after 24 h and the selectivity
to 2 was improved to 45% when the amount of K2CO3 was in-
creased to 600 mol % (entry 9). However, under these reaction con-
ditions, it appeared to be very difficult to suppress the formation of
2,3-dihydroxypropanoic acid (1) and hydroxyacetic acid (3) as
byproducts.
As a result of synergistic effects, bimetallic clusters often show
superior reactivity and/or selectivity compared with the corre-
sponding monometallic clusters.11 Recently, Liu reported that
Au–Pt alloy nanoclusters on TiO2 showed a high selectivity in the
conversion of glycerol into lactic acid.8 Among the various combi-
nations of bimetallic systems, Au–Pd alloy catalysts has been
extensively studied.12 We were therefore encouraged to study
the catalytic activity of Au–Pd:PVP in the oxidation of glycerol.
Au–Pd bimetallic catalysts containing various ratios of the two
metals were prepared by rapid reduction of mixed aqueous solu-
tions of HAuCl4 and H2PdCl4 with NaBH4.13 Figure 1 shows typical
TEM images of a series of Au–Pd:PVPs. The average diameters of
Au–Pd(80:20):PVP, Au–Pd(50:50):PVP, and Au–Pd(20:80):PVP are
determined to be 3.2 1.2, 3.2 0.8, and 3.9 1.2 nm, respectively.
Figure 2. Typical STEM image of Au–Pd(50:50):PVP and EDS analysis.

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S. Y. S et al. / Tetrahedron Letters 52 (2011) 2633–2637
Table 2
Glycerol oxidation on monometallic Au:PVP or bimetallic Au–Pd:PVP nanoclusters15,16
OH
OH
O
O
OH
OH OH
Au-Pd:PVP (%ratio) (5 atom%)
OH
1
3
O
K2CO3 (600 mol%)
80 oC, 24h
OH OH
H
OH
OH
OH
5
O
O
O
O
OH OH
OH
2
4
Entry
Au/Pd ratioa
Conversion (%)
Selectivity
1
2
3
4
5
1
2b
3b
4b
5b
6c
100:0
80:20
50:50
20:80
0:100
2.5:2.5
100
100
100
100
82.4
100
29
5
—
—
28
10
45
47
9
8
27
27
9
4
5
4
22
4
—
—
—
—
—
17
—
10
57
28
5
10
a
b
c
Total Au + Pd = 0.5 mmol.
Precipitation occurred during the reaction and the solution became colorless.
Au/PVP cluster + Pd/PVP cluster.
The size of these bimetallic clusters is significantly larger than that
of monometallic Au:PVP and Pd:PVP probably due to the difference
of the preparative conditions. The bimetallic composition is further
confirmed by STEM/EDS measurement as shown in Figure 2 as an
example of the analysis of Au–Pd(50:50):PVP. EDS analysis
revealed that the ratio of Au and Pd within the metal clusters is
close to 1:1. The Au:Pd ratio was further confirmed by the ICP
analysis.14
try 6); this gave a complex mixture of products. The results, there-
fore, showed that the Au–Pd bimetallic structure may play an
important role in relation to the high selectivity to oxalic acid in
entry 3. In the case of monometallic Au:PVP, C3 products were pro-
duced mainly, and hydroxyacetic acid (3), obtained in a minor
amount, was the sole C2 product. In contrast, Au–Pd bimetallic cat-
alyst gave mainly C2 products [oxalic acid (4) and hydroxyacetic
acid (3)], together with a C1 product (formic acid). The oxidative
C–C bond scission reaction may involve H2O2;5 therefore, the
bimetallic catalyst might be efficient in forming of H2O2 by reduc-
tion of O2 and/or it may be efficient in catalyzing C–C bond-cleav-
age reactions involving H2O2.13f
Representative results obtained using the resulting catalysts are
shown in Table 2. All the reactions were carried out by using
5.0 atom % of metal (Au + Pd):PVP in the presence of 600 mol % of
K2CO3 in H2O at 80 °C for 24 h. In the case of Au–Pd(80:20):PVP,
glycerol was completely consumed and several products, including
unidentifiable materials, were obtained. Of the major products, the
yield of 2,3-dihydroxypropanoic acid (1) was drastically decreased
to 5% and, instead, oxalic acid (4) was obtained in 10% yield; how-
ever, hydroxymalonic acid (2) was still obtained as the major prod-
uct in 47% yield (entry 2). A comparison of entries 1 and 2 suggests
that doping with Pd could enhance the activity of the catalyst in
aerobic oxidation. When the ratio of Pd was increased to 50%, the
yield and the selectivity to oxalic acid (4) were significantly in-
creased, as shown in entry 3. In contrast, when the ratio of Au/Pd
was increased to 20:80, the reaction became more complicated,
and a complex mixture of products was obtained (entry 4). It is
noteworthy in this case that the Pd or Pd-rich catalyst immediately
precipitated out at the beginning of the reaction, leading to heter-
ogeneous conditions probably due to the poor stabilizing ability of
PVP toward the Pd as well as relatively larger mean size of the clus-
ters. In contrast, in the cases of entries 1–3, the reaction proceeded
mostly under quasi-homogeneous conditions and no significant
amount of precipitates was observed at the beginning. In the case
of the reaction in the presence of monometallic Pd:PVP (mean size
/ = 1.5 0.3 nm),4a the rate was slower than that in the presence of
Au:PVP or of Au–Pd:PVP and the conversion was 82% after 24 h
(entry 5). Although the product distribution was complex, formic
acid (5) was obtained in 17% yield. These results showed that Pd
could play some role in accelerating the oxidative C–C bond scis-
sion process in comparison with the Au catalyst. To confirm the ef-
fect of alloying the metals, a physical mixture of 2.5 atom % of
Au:PVP and 2.5 atom % of Pd:PVP was examined as a catalyst (en-
3. Conclusion
Au:PVP acted as a quasi-homogeneous catalyst for the aerobic
oxidation of glycerol under relatively mild conditions and exhib-
ited a unique selectivity for the formation of hydroxymalonic acid
(2). A 1:1 alloy of Au and Pd showed a different selectivity, giving
oxalic acid as the major product. These results suggest that the
product distribution could be controlled by changing the catalyst
design through an appropriate choice of the protecting polymer3
and the nature of the alloyed structure of the metals.
Acknowledgments
This work was supported by MEXT, NEDO, and PRESTO-JST.
S.Y.S. thanks the Japan Society for the Promotion of Sciences (JSPS)
for an Invitation Fellowship for Research in Japan. We also thank
Ms. Noriko Kai for her technical support.
References and notes
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unit) of PVP (K-30) was dissolved in 44 mL of MilliQ water and to the solution
was added quantitative amount of HAuCl4 (25 mM) and PdCl2 (12.5 mM) and
final concentration of metals solution was made 0.5 mM, that is, 1:100 metals
to polymer ratios. The resulting solution was stirred for 30 min at 27 °C. The
solution was maintained 15 °C before reduction, and an aqueous solution
(5 mL) of NaBH4 (20 mg, 0.52 mmol) was added rapidly under vigorous
stirring. The color of the mixture was turned from pale yellow to brown,
indicating the formation of Au–Pd bimetallic clusters. Thus obtained Au–
Pd:PVP clusters were subsequently dialyzed through the membrane filter to
remove the inorganic impurities such Na+ and ClÀ, which is a crucial treatment
to enhance the stability of the bimetallic clusters against coalescence. The
dialyzed hydrosol of Au–Pd:PVP was diluted to 25 mL and stored in refrigerator
for catalytic reactions and characterizations.
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14. Detailed characterization of these Au–Pd:PVP clusters will be discussed
elsewhere.
15. General procedure for aerobic oxidation: Glycerol (0.1 mmol) was placed in a test
tube and dissolved in H2O (10 mL). A 0.5 mM aqueous solution (0.5 mM) of the
metal-containing PVP catalyst (Au or Au–Pd) was added with stirring at
1300 rpm, followed by K2CO3 (600 mol %). The mixture was heated at 80 °C for
24 h then cooled to room temperature. The solidified materials, including the
catalyst, were separated by ultracentrifugation filtration (10,000 MWCO), and
the residual solution was analyzed by HPLC.
16. Unfortunately, these PVP-stabilized clusters are not suitable enough for
multiple reuses due to the significant aggregation during the reaction.
Further studies for the pursuit of the best combination of the metals and the
polymers are under investigation.