Bimetallic Au/Pd catalyzed aerobic oxidation of alcohols in the poly(ethylene glycol)/CO2 system

Article abstract of DOI:10.1007/s11426-010-4014-z

Bimetallic Au/Pd nanoparticles were prepared and used to catalyze oxidation of alcohols in the poly(ethylene glycol) (PEG)/CO₂ biphasic system using O₂ as the oxidant without adding any base. The catalytic activity of Au/Pd bimetal with different mole ratios was studied using benzyl alcohol as the substrate. It was found that bimetallic Au/Pd nanoparticles with Au:Pd=1:3.5 had higher catalytic activity than monometallic Au, Pd and the bimetallic Au/Pd nanoparticles with other molar ratios. The effect of CO ₂ pressure on the oxidation of benzyl alcohol and 1-phenylethanol in PEG/CO₂ was investigated. It was demonstrated that CO₂ pressure could be used to tune the conversion and selectivity of the reactions effectively. α,β,-Unsaturated alcohols were also studied and found to be more reactive than benzyl alcohol and 1-phenylethanol. Recycling experiments showed that the Au/Pd/PEG/CO₂ catalytic system could be recycled at least four times without reducing the activity. In addition, the catalytic system is clean and the products can be separated easily.

Full text of DOI:10.1007/s11426-010-4014-z

SCIENCE CHINA
Chemistry
July 2010 Vol.53 No.7: 1592–1597
doi: 10.1007/s11426-010-4014-z
• ARTICLES •
Bimetallic Au/Pd catalyzed aerobic oxidation of alcohols
in the poly(ethylene glycol)/CO2 system
HE JinLing, WU TianBin, HU BaoJi, CHENG Yan, WU SuXiang,
JIANG Tao & HAN BuXing*
Beijing National Laboratory for Molecular Sciences; Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Received March 23, 2010; accepted April 30, 2010
Bimetallic Au/Pd nanoparticles were prepared and used to catalyze oxidation of alcohols in the poly(ethylene glycol)
(PEG)/CO2 biphasic system using O2 as the oxidant without adding any base. The catalytic activity of Au/Pd bimetal with dif-
ferent mole ratios was studied using benzyl alcohol as the substrate. It was found that bimetallic Au/Pd nanoparticles with
Au:Pd=1:3.5 had higher catalytic activity than monometallic Au, Pd and the bimetallic Au/Pd nanoparticles with other molar
ratios. The effect of CO2 pressure on the oxidation of benzyl alcohol and 1-phenylethanol in PEG/CO2 was investigated. It was
demonstrated that CO2 pressure could be used to tune the conversion and selectivity of the reactions effectively.
,-Unsaturated alcohols were also studied and found to be more reactive than benzyl alcohol and 1-phenylethanol. Recycling
experiments showed that the Au/Pd/PEG/CO2 catalytic system could be recycled at least four times without reducing the activ-
ity. In addition, the catalytic system is clean and the products can be separated easily.
oxidation, bimetallic Au/Pd nanoparticles, alcohols, poly(ethylene glycol)/CO2 system
compared with monometallic particles. Different groups
1 Introduction
[9–14] found that formation of Au/Pd bimetal could im-
prove the activity of the catalyst for different reactions. It
was demonstrated that supported Au/Pd nanoparticles could
catalyze the synthesis of H2O2 from H2 and O2 even under
low temperature [11, 12]. Furthermore, it was found that
Au/Pd/TiO2 could catalyze oxidation of alcohols without
the base [13]. Dimitratos et al. [14] studied oxidation of
alcohols with Au/Pd and Au/Pt supported on activated car-
bon as the catalysts and found that the Au/Pd catalysts had
positive synergistic effect, but Au/Pt showed negative syn-
ergistic effect. Hou et al. [15] reported oxidation of alcohols
with PVP-stabilized Au/Pd nanoparticles as catalysts in
basic aqueous solution.
Selective oxidation of alcohols to their respective aldehydes
or ketones is a key process in the production of fine chemi-
cals, and numerous approaches have been explored suc-
cessfully using different oxidants [1–5]. However, it is still
highly attractive to develop more satisfactory catalytic sys-
tems for the aerobic oxidation of alcohols.
Since the report of oxidation of CO and H2 catalyzed by
Au supported on metal oxides [6, 7], Au catalysts have been
studied extensively. Au nanoparticles are also active for
oxidation of alcohols, especially for diols and triols. How-
ever, Au catalyzed oxidation of alcohols usually occurred
under base conditions. Bimetallic catalysts are of great in-
terest and have been widely studied for many catalytic reac-
tions due to their synergistic and tunable properties [8]
In recent years, increasing attention has been paid to the
replacement of volatile organic solvents (VOS) in modern
organic synthesis and practical applications. Supercritical
(SC) CO2, low volatile ionic liquids and liquid polymers are
promising alternatives. CO2 is cheap, non-toxic, non-flam-
*Corresponding author (email: hanbx@iccas.ac.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2010
chem.scichina.com
www.springerlink.com

HE JinLing, et al. Sci China Chem July (2010) Vol.53 No.7
1593
mable, and readily available. Liquid polymers such as PEGs
JEM-1011).
are superior to ionic liquids due to low price. PEG/CO2 bi-
phasic system is an excellent combinative reaction medium,
and reactions in the system can be conducted in the con-
tinuous mode and the products can be easily separated by
the extraction with SCCO2, which has been studied by sev-
eral groups [16–19]. Abundant and cheap molecular oxygen
is an attractive oxidant for oxidative reactions due to its
apparent economic and ecological advantages, and it has
been used in oxidation of alcohols with transition metals as
catalysts [20, 21].
In this work, bimetallic Au/Pd nanoparticles stabilized by
poly(vinylpyrrolidone) (PVP) were used as the catalyst for
oxidation of alcohols in PEG/CO2 system with O2 as the
oxidant in the absence of any base. The results indicated
that the greener catalytic system was very active, and the
activity and selectivity of the reactions could be tuned by
CO2 pressure. Furthermore, the separation of the product
and the catalyst was easy and the catalyst could be reused.
2.3 Oxidation of the alcohols
The oxidation reactions of the alcohols were carried out in a
stainless reactor of 20 mL with a magnetic stirrer. In a typi-
cal experiment, suitable amounts of Au/Pd/PEG and alcohol
were added into the reactor. After sealing, the air in the re-
actor was replaced by O2. The reactor was maintained at
80 °C and then more O2 was charged to 0.6 MPa. Then CO2
was charged to the desired pressure, and the stirrer was
started. After a suitable reaction time, the reactor was
cooled down and the gases were released slowly at 0 °C. The
reaction mixture was extracted by ethyl ether (3  10 mL).
The extracted liquid was analyzed by GC (Agilent 4890 D)
equipped with a flame-ionized detector using biphenyl as
the internal standard.
3 Results and discussion
3.1 Optimization of the Au/Pd molar ratio in the catalyst
2 Experimental
It was reported that the composition of bimetallic catalysts
has great influence on the activity [14, 15, 22, 24]. In this
work, the effect of the molar ratio of Au to Pd on the oxida-
tion of benzyl alcohol was investigated in PEG/CO2 bi-
phasic system using O2 as the oxidant. The results and reac-
tion conditions are shown in Table 1. It was demonstrated
that the catalytic activity for the alcohol oxidation was
closely related to the molar ratio of Au to Pd. Among the
catalysts investigated, the monometallic Au possessed the
lowest activity, and only 2.3% yield of benzaldehyde was
obtained (entry 8). Monometallic Pd gave 41.5% benzalde-
hyde yield under the same condition. At the Au:Pd molar
ratio of 1:3.5, the catalytic activity was the highest, and
61.0% benzaldehyde was obtained. However, the catalytic
activity decreased with further increasing the content of Au.
When the ratio of Au:Pd was 1:1, the yield of benzaldehyde
was decreased to 15.1%. Compared with monometallic Au
(entry 8), addition of Pd could enhance the catalyst activity
efficiently and avoid the use of bases. Compared with
monometallic Pd, bimetallic Au/Pd with appropriate com-
position (entry 5) had higher catalytic activity for the oxida-
tion reaction. The synergistic effect between Au and Pd has
been reported by other authors [14, 15, 22, 24].
tive electron effect of Au atom from Pd atom enhances the
interaction between Pd atom and the substrate. As reported
by other authors [29], Au and Pd may form an alloy at al-
most any ratio. Moreover, the monometallic Au was nearly
inactive for the oxidation of benzyl alcohol, suggesting that
the active sites of Au/Pd nanoparticles with Au:Pd=1:3.5
should exist mainly in Pd atoms, and Au atoms can improve
the activity of Pd. Au/Pd mole ratios in different Au/Pd
nanoparticles could be different, and therefore the mole
ratios are considered as an average value. The above results
2.1 Materials
Benzyl alcohol, 1-phenylethanol, allyl alcohol, cinnamic
alcohol, ethanol, HAuCl4·4H2O, and PdCl2 were of analyti-
cal grade and supplied by Beijing Chemical Reagent Com-
pany. PEG-600, PEG-800, PEG-1000 and PEG-1500 were
obtained from Beijing Yili Fine Chemical Company. NaBH4
and PVP (MW=30000) were provided by Beijing Chemical
Reagent Company. CO2 (> 99.99%) and O2 (> 99.9%) were
supplied by Beijing Analytical Instrument Factory.
2.2 Preparation and characterization of the catalysts
The pre-formed bimetallic Au/Pd nanoparticles with differ-
ent ratios were prepared using the reported method [22].
HAuCl4·4H2O aqueous solution (103 mol/L) and PdCl2
ethanol solution (103 mol/L) were mixed in a 100 mL flask,
and the total volume of the mixed solution was 50 mL.
0.278 g PVP (the ratio of PVP units to the metals was 50:1)
was added. Then the solution was refluxed for 2 h at 100 °C.
Au nanoparticles were prepared according to the method
reported by Tsukuda et al. [23]. In the experiment,
HAuCl4·4H2O aqueous solution (50 mL, 103 mol/L) and
PVP (0.555 g) were added to a flask (100 mL). The solution
was stirred for 30 min at 0 °C. Then NaBH4 aqueous solu-
tion (5 mL, 0.1 mol/L) was added under vigorous stirring.
9.72 g PEG was added to the above pre-formed nanopar-
ticle solutions. After mixing completely, the solvents were
removed by evaporation. The as-prepared catalytic system
was dried at 70 °C under vacuum overnight. The metal
concentration in the system was 5  103 mmol/g.
The attrac-
The original and used Au/Pd nanoparticles were ob-
served by transmission electron microscopy (TEM, JEOL

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HE JinLing, et al. Sci China Chem July (2010) Vol.53 No.7
Table 1 Effect of the Au/Pd mole ratio on benzaldehyde yield
in the maximum in the pressure versus conversion curve.
Our experiment showed that the selectivity was only
63% in the absence of CO2. CO2 pressure affected the selec-
tivity of benzaldehyde considerably, and a maximum oc-
curred at 7 MPa, as shown in Figure 1. The selectivity at
this pressure reached 93%. The reason for the occurrence of
the maximum in selectivity needs to be further studied. The
yield of benzaldehyde also reached the maximum at 7 MPa,
indicating the yield could be optimized by pressure.
Figure 2 shows the effect of pressure on the conversion,
selectivity, and yield for the oxidation of the secondary al-
cohol 1-phenylethanol. The pressure also considerably af-
fected the conversion for the reaction, and the maximum
also appeared at about 6 MPa. The selectivity of acetophe-
none was higher than 98% in the pressure range studied,
indicating that the catalytic system was highly selective for
the oxidation reaction.
Entry
Au/Pd (molor ratio)
Benzaldehyde yield (%)
1
2
3
4
5
6
7
8
1:1
15.1
25.9
29.2
50.9
61.0
37.0
41.5
2.3
1:2
1:2.5
1:3
1:3.5
1:4
0:1
1:0
Reaction conditions: 2.5 g Au/Pd/PEG-600, 2.5 mmol benzyl alcohol,
(Au+Pd):alcohol=1:200, 80 °C, 0.7 MPa O2, 10 MPa total pressure after
adding CO2, 8 h.
indicate that the catalytic system with Au:Pd ratio of 1:3.5
had the highest catalytic activity. Therefore, this catalyst
was used to study the effect of the reaction condition on the
oxidation reactions, and the results were discussed as fol-
lows.
3.3 Effect of reaction time on the reaction
The effect of reaction time on the oxidation of benzyl alco-
hol and 1-phenylethanol was investigated in PEG/CO2 using
the bimetallic Au/Pd nanoparticles as the catalyst. The reac-
tions were carried out at 80 °C with O2 pressure of 0.6 MPa
and a total pressure of 7 MPa. The products were benzalde-
hyde and acetophenone. Figure 3 shows that the yields of
the products increased with reaction time, and the apparent
acceleration of the reaction rate appeared at about 6 h. A
benzaldehyde yield of 90% was obtained in 9 h, and 92%
acetophenone yield was achieved in 12 h. Benzyl alcohol
was more reactive than 1-phenylethanol in the catalytic
system.
3.2 Influence of CO2 pressure on the reaction
The pressure usually influences the efficiency of the reac-
tions in CO2 considerably, which has been investigated by
many researchers. In this work, the effect of CO2 pressure
on conversion and selectivity was investigated with bi-
metallic Au/Pd nanoparticles in PEG-600 as the catalyst
using benzyl alcohol and 1-phenylethanol as the substrates.
In the investigation, the pressure of O2 was kept at 0.6 MPa
and the total pressure was changed. Figure 1 shows the ef-
fect of CO2 pressure on the conversion of benzyl alcohol.
The maximum conversion occurred at about 6 MPa with
changing of the pressure. This suggests that CO2 affects the
reaction rate in two opposite ways. First, the addition of
CO2 can decrease the viscosity of PEG [25], which is fa-
vorable to enhancement of the reaction rate. Second, CO2
can dilute the solution and may occupy some active sites of
the catalyst. The competition of the opposite factors resulted
3.4 Oxidation of ,-unsaturated alcohols
Besides benzyl alcohol and 1-phenylethanol, oxidations of
,-unsaturated alcohols were also carried out with the
Au/Pd catalyst in PEG/CO2 medium. The results at the op-
timized condition are listed in Table 2. The data indicated
Figure 2 Effect of total pressure on the conversion, selectivity, and yield
for the oxidation of 1-phenylethanol to acetophenone. Reaction conditions:
2.5 g Au/Pd/PEG-600, 2.5 mmol 1-phenylethanol, (Au+Pd):alcohol=1:200,
Au:Pd=1:3.5, 80 °C, 0.6 MPa O2, 10 h.
Figure 1 Effect of total pressure on the conversion, selectivity, and yield
for the oxidation of benzyl alcohol to benzaldehyde. Reaction conditions:
2.5 g Au/Pd/PEG-600, 2.5 mmol benzyl alcohol, (Au+Pd):alcohol=1:200,
Au:Pd=1:3.5, 80 °C, 0.6 MPa O2, 7 h.

HE JinLing, et al. Sci China Chem July (2010) Vol.53 No.7
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3.5 Effect of the molecular weight of PEG
The properties of PEG depends on its molecular weight. In
this work, the effect of the molecular weight of PEG on the
oxidation of benzyl alcohol was investigated. Our experi-
ment demonstrated that PVP was not miscible with
PEG-1500. However, PEG-600, PEG-800 and PEG-1000
could dissolve PVP to stabilize Au/Pd nanoparticles. The
yields of benzaldehyde in PEGs of different molecular
weights are listed in Table 2. The results in the table show
that as the reaction time was long enough (9 h), a yield of
90% was reached. In order to study the effect of PEG mo-
lecular weight on the reaction, we carried out the reaction in
different PEGs in a shorter reaction time. The molecular
weight of PEG influenced the reaction considerably. The
benzaldehyde yield increased from 48% to 79% when the
molecular weight of PEG increased from 600 g/mol to 1000
g/mol with a reaction time of 6 h. Therefore, the molecular
weight of PEG is a significant factor for the reaction.
Figure 3 Dependence of product yields on the reaction time. Reaction
conditions: 2.5 g Au/Pd/PEG-600, 2.5 mmol alcohol, (Au+Pd):alcohol =
1:200, Au:Pd=1:3.5, 80 °C, 0.7 MPa O2, 7 MPa total pressure after adding CO2.
that ,-unsaturated alcohols were very reactive in the
catalytic system. Allyl alcohol and cinnamic alcohol could
be transformed completely at a lower temperature of 60 °C
and in short reaction time of 1 h, and the yields of corre-
sponding ,-unsaturated aldehydes were 98.7% and 99.3%,
respectively. The high reaction rate of ,-unsaturated al-
cohols was probably due to the activation of hydroxyl by
the unsaturated double bond.
3.6 Recyclability of the Au/Pd/PEG catalytic system
The recyclability of the Au/Pd/PEG catalytic system for
oxidation of alcohol was examined using benzyl alcohol as
the substrate. The oxidation reaction was carried out at 80 °C
Table 2 Oxidation of different substrates and effect of PEG molecular weight on the reaction
Substrate
Product
Solvent
T (°C)
Time (h)
1
Yield (%)
98.7
PEG-600
60
PEG-600
PEG-600
60
80
1
99.3
92.0
12
PEG-600
PEG-600
PEG-800
PEG-1000
80
80
80
80
9
6
6
6
90.0
48.0
57.8
79.0
Reaction conditions: 2.5 g Au/Pd/PEG, 2.5 mmol corresponding alcohol, (Au+Pd):alcohol =1:200, Au:Pd=1:3.5, 0.6 MPa O2, 7 MPa total pressure after
adding CO2.

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HE JinLing, et al. Sci China Chem July (2010) Vol.53 No.7
with constant pressure and reaction time. In each cycle, af-
ter extraction by ethyl ether, the catalytic system
Au/Pd/PEG was dried and used for the next run. In order to
show the effect of run time on the reaction, we carried out
the reaction in a shorter time. The yield and selectivity of
benzaldehyde for the four repeated runs are shown in Figure
4. The virgin and the recycled catalytic systems showed
very high selectivity at the experimental condition. The
yield increased noticeably with the reusing time. This phe-
nomenon needs to be further studied. In addition, the Au/Pd
nanoparticles in the virgin Au/Pd/PEG and those in the
catalytic system after used for four times were observed by
transmission electron microscopy (TEM), and the images
and size distributions of the Au/Pd nanoparticles are shown
in Figure 5. Figures 5(b) and (d) show that the size distribu-
tion of the Au/Pd nanoparticles changed slightly after using
for four times.
the bimetallic Au/Pd nanoparticles with other molar ratios.
Among the studied substrates, ,-unsaturated alcohols are
more reactive than benzyl alcohol and 1-phenylethanol. CO2
pressure can be used to tune the conversion and selectivity
for oxidation of the alcohols effectively, and at optimized
conditions the yields of the desired products are higher than
90% for all the reactions. The Au/Pd/PEG can be easily
separated from the products and is reusable. In addition, the
Au/Pd/PEG/CO2 system is greener. These characteristics
make it a promising catalytic system.
4 Conclusions
In this work, bimetallic Au/Pd nanoparticles stabilized by
PVP have been used to catalyze oxidation of different kinds
of alcohols in PEG/CO2 biphasic system using O2 as the
oxidant without adding any base. The ratio of Au/Pd has
great influence on the catalytic activity for oxidation of al-
cohols, and bimetallic nanoparticles with the Au/Pd molar
ratio of 1:3.5 are more active than monometallic Au, Pd and
Figure 4 The yield and selectivity of benzaldehyde in the four runs of
recycling experiments. Reaction conditions: 2.5 g Au/Pd/PEG-600, 2.5
mmol alcohol, (Au+Pd):alcohol=1:200, Au:Pd=1:3.5, 80 °C, 0.6 MPa O2, 7
MPa total pressure after adding CO2, 6 h.
Figure 5 TEM image (a) and size distribution (b) of the Au/Pd nanoparticles in the virgin Au/Pd/PEG system, and TEM image (c) and size distribution (d)
of the Au/Pd nanoparticles in the Au/Pd/PEG catalytic system after being used for four times.

HE JinLing, et al. Sci China Chem July (2010) Vol.53 No.7
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The authors are grateful to the National Natural Science Foundation of
China (20973177 & 20932002), Ministry of Science and Technology of
China (2006CB202504), and Chinese Academy of Sciences (KJCX2.
YW.H16) for financial supports.
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