Superior catalytic properties in aerobic oxidation of alcohols over Au nanoparticles supported on layered double hydroxide

Article abstract of DOI:10.1016/j.cattod.2011.03.040

Extremely small-size Au nanoparticles mainly distributed at 1-5nm were successfully prepared on layered double hydroxide by ion-exchange and reduction procedures (Au/LDH). This catalyst showed superior catalytic properties in aerobic oxidation of a wide range of secondary and primary alcohols under very mild conditions (e.g. 1 atm pressure of oxygen and even at room temperature). In addition, this catalyst is stable and recyclable during oxidations. These advantages are reasonably attributed to the interaction between Au sites and basic LDH.

Full text of DOI:10.1016/j.cattod.2011.03.040

Catalysis Today 175 (2011) 404–410
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Superior catalytic properties in aerobic oxidation of alcohols over Au
nanoparticles supported on layered double hydroxide
b
b
a
b
a,∗
Liang Wang , Jian Zhang , Xiangju Meng , Dafang Zheng , Feng-Shou Xiao
a
Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University (Xixi Campus), Hangzhou 310028, China
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Extremely small-size Au nanoparticles mainly distributed at 1–5 nm were successfully prepared on
layered double hydroxide by ion-exchange and reduction procedures (Au/LDH). This catalyst showed
superior catalytic properties in aerobic oxidation of a wide range of secondary and primary alcohols
under very mild conditions (e.g. 1 atm pressure of oxygen and even at room temperature). In addition,
this catalyst is stable and recyclable during oxidations. These advantages are reasonably attributed to the
interaction between Au sites and basic LDH.
Received 29 September 2010
Received in revised form 10 February 2011
Accepted 16 March 2011
Available online 31 May 2011
Keywords:
Au nanoparticles
©
2011 Elsevier B.V. All rights reserved.
Layered double hydroxide (LDH)
Oxygen
Aerobic oxidation
Primary and secondary alcohols
1
. Introduction
dependent on Au nanosizes and support types [16,17]. For exam-
ple, compared with silica, CeO2 supported Au nanoparticles have
improved activity and selectivity for the desired products in the
aerobic oxidation of alcohols due to the synergetic electronic inter-
actions between Au sites and CeO2 [16a]. The use of Au/Ga Al O
The oxidation of alcohols to the corresponding aldehydes or
ketones is greatly important for the production of fine chemi-
cals [1–3]. Conventionally, these oxidations are carried out using
metallic salts as oxidants including permanganate, chromate and
bromate. These processes are typically environmentally unfriendly
because of the production of undesirable wastes and consequently
high costs [3b,4,5]. In recent years, environmentally benign oxi-
dants such as molecular oxygen with high atom efficiency have
been used [6]. However, heterogeneous oxidations with molecu-
lar oxygen under very mild conditions (1 atm O2 and even at room
temperature) without additives is still a great challenge [7].
It is well known that a lot of metal ions and metal nanoparti-
cles are catalytically active for alcohol oxidations using molecular
oxygen as oxidants. For example, ruthenium, copper, cobalt, and
palladium have been proved to be active for this kind of reactions
3
3
9
could effectively promote the conversion of alcohol oxidations,
since the Ga Al O support might substantially facilitate the crucial
3
3
9
alcohol-dehydrogenation step [16b].
More recently, we reported a communication that layered dou-
ble hydroxide supported Au nanoparticles (Au/LDH) were active
◦
for aerobic oxidation of a few alcohols at 80 C [17b]. Herein, we
showed a systematical study on catalytic oxidations of a wide range
of primary and secondary alcohols over Au/LDH catalyst under very
mild conditions such as room temperature.
2. Experimental
[
6d,8–11]. Recently, gold catalysts have been paid much atten-
tion because of their unprecedented catalytic properties, since the
works of Hutchings and Haruta [12–15]. One of the most important
findings is that Au nanoparticles are able to catalyze the oxida-
tion of alcohols [7,16–19]. Compared with Pd and Pt catalysts,
Au nanoparticles on supports show superior catalytic properties
under mild conditions. In these cases, the activities are strongly
2.1. Catalyst preparation
2.1.1. Preparation of LDH
As a typical run, 30.76 g of Mg(NO ) ·6H O (AR, Beijing Chem
3
2
2
Co.) and 15 g of Al(NO ) ·9H O (AR, Beijing Chem Co.) were dis-
3
3
2
solved in 400 ml of water, followed by the addition of 72 g of urea
AR, Beijing Chem Co.) under stirring at room temperature. After
(
heating to boiling point, a white precipitate was formed. After boil-
ing for 8 h and precipitating at room temperature for 12 h, the solid
LDH was collected by filtration and washing with water.
∗ Corresponding author. Tel.: +86 431 85168590; fax: +86 431 85168590.
E-mail address: fsxiao@mail.jlu.edu.cn (F.-S. Xiao).
0
920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.03.040

L. Wang et al. / Catalysis Today 175 (2011) 404–410
405
2.1.2. Preparation of Au/LDH
Au/LDH was prepared by ion-exchange and NaBH4 reduction.
b
For a typical run, 0.22 g of hydrochloroauric acid (HAuCl ·4H O,
4
2
AR, Shanghai Chem Co.) was dissolved in 80 ml of water, fol-
lowed by the addition of 6 g of LDH and stirring for overnight.
After filtrating, washing and drying, the sample was transferred
to 50 ml of toluene (AR, Beijing Chem Co., dried by P O5), fol-
2
lowed by the addition of NaBH4 (AR, Beijing Chem Co.). After
stirring for 10 min, 15 ml of ethanol was added and the mix-
ture was stirred for 6 h. Au/LDH with Au loading at 1.8% was
collected by filtration and washing with ethanol and water. The
Au loading was analyzed by inductively coupled plasma (ICP)
technique.
a
2
.1.3. Preparation of Pd/LDH
For a typical run, 0.38 g PdCl (AR, Shanghai Chem Co.) and NaCl
10
20
30
40
50
60
70
80
2
2
Theta/Degree
(
molar ratio of PdCl /NaCl is 1:2.3) were dissolved in 80 ml of water,
2
followed by the addition of 6 g of LDH and stirring for overnight. The
following ion-exchange and reduction procedures were the same
as the preparation of Au/LDH. The Pd loading analyzed by ICP was
Fig. 1. XRD patterns of (a) LDH support and (b) Au/LDH catalyst.
1
.8%.
3. Results and discussion
.1. Characterization of Au/LDH catalyst
Fig. 1 shows XRD patterns of LDH and Au/LDH samples. They
exhibited almost the same XRD peaks, indicating that the layered
structure of LDH is well remained after loading of Au nanoparticles.
Notably, the diffraction peaks associated with Au nanoparticles
were not observed, which might be related to high dispersion
of Au nanoparticles on LDH. TEM image of Au/LDH (Fig. 1(a))
confirms the presence of very small Au nanoparticles distributed
at 1–5 nm.
3
2.1.4. Preparation of Au/TiO2, Au/MgO, Au/Fe O3 and Au/SiO2
2
As typical run, solid supports such as TiO2 was added to
◦
the solution of hydrochloroauric acid. After stirring at 80 C for
overnight (pH = 9.0), filtrating and washing at room tempera-
◦
◦
ture, drying at 100 C for 12 h, and calcination at 400 C for
h, the sample was obtained. The Au loading analyzed by ICP
was 1.8%.
4
2.2. Sample characterization
Powder X-ray diffraction patterns (XRD) were obtained with
Rigaku D/MAX 2550 diffractometer with CuK␣ radiation
3.2. Oxidation of secondary alcohols
a
(
ꢀ = 0.1542 nm). Transmission electron microscopy (TEM) experi-
Table 1 presents catalytic activities and selectivities in oxidation
of a typical secondary alcohol of 1-phenylethanol to acetophenone
with molecular oxygen at an atmospheric pressure over various
ments were performed on a JEM-3010 electron microscope (JEOL,
Japan) with an acceleration voltage of 300 kV. The contents of
Au and Pd were determined by ICP with a Perkin-Elmer plasma
catalysts. Clearly, Au/LDH catalyst was very active. After reaction at
4
0 emission spectrometer. XPS spectra were performed a Thermo
◦
8
(
0 C for 2 h in toluene, 1-phenylethanol was completely converted
◦
ESCALAB 250 with Al K␣ radiation at ꢁ = 90 for the X-ray source,
the binding energies were calibrated using the C1s peak at 284.9 eV.
Temperature programmed surface reaction (TPSR) of adsorbed 2-
propanol was carried out as follows: the catalysts were treated
at 300 C for 3 h and cooled down to room temperature, then 2-
propanol vapor was introduced into the reaction system for 30 min.
After sweeping with Ar for 1 h, the temperature was increased
Table 1, entry 1). When the temperature was decreased down
◦
to 45 C, the complete conversion of 1-phenylethanol took for 4 h
(
Table 1, entry 2). Interestingly, when the reaction was performed
at room temperature for 12 h, the conversion still had 96% (Table 1,
entry 3). Compared with toluene, the use of water showed relatively
◦
◦
low reaction rate. When the reaction was performed at 45 C for 4 h,
the conversion was 40% (Table 1, entry 4). When the reaction time
was increased to 16 h, the conversion was 98% (Table 1, entry 5).
In contrast, Pd/LDH and the other Au-based catalysts showed very
low conversion for this reaction. For examples, Pd/LDH was almost
inactive (Table 1, entry 7). A series of Au-based catalysts with
similar Au content with Au/LDH (Au/TiO , Au/MgO, Au/Fe O and
◦
◦
(
10 C/min) from room temperature to 500 C, and the signals of
H2 (M/e = 2) were recorded by mass spectrometer with a thermal
conductivity detector (TCD).
2
.3. Catalytic tests
2
2
3
Au/SiO ) exhibited very low conversion (11–26%, Table 1, entries
2
The oxidation of alcohols was carried out in a 50-ml glass reac-
9–12). Even if the presence of additional Na CO , Au/SiO still gave
2
3
2
tor and stirred with a magnetic stirrer. The substrate, solvent and
catalyst were mixed in the reactor and heated to the reaction tem-
perature. Then molecular oxygen was introduced at air pressure.
After reaction, the product was taken out from the reaction system
and analyzed by gas chromatography (GC-14C, Shimadzu, using a
flame ionization detector) with a flexible quartz capillary column
coated with OV-17 and OV-1. The recyclability of Au/LDH catalyst
was carried out by separating the catalyst from the reaction sys-
tem by centrifugation, washing with a large amount of methanol
a low conversion (19%, Table 1, entry 13). These results indicate that
Au/LDH catalyst is very active, compared with Pd/LDH and the other
Au-based catalysts.
More importantly, when Au/LDH catalyst was recycled for 6
times, the conversion still have 97% (Table 1, entry 6), which is
still comparable with the fresh catalyst (loss of activity less than
3%, Table 1, entry 1). These results indicate that Au/LDH catalyst
was stable and reusable in aerobic oxidation of 1-phenylethanol.
Sample TEM images (Fig. 2) showed that Au nanoparticle sizes
of the recycled catalyst were similar to those of the fresh cata-
lyst, indicating that Au nanoparticles are basically stable during
◦
and drying at 100 C overnight, then the catalyst was reused in the
next reaction.

4
06
L. Wang et al. / Catalysis Today 175 (2011) 404–410
Table 1
Aerobic oxidation of 1-phenylethanol over various Au catalysts.
.
◦
Entry
Catalyst
Solvent
Temperature ( C)
Time (h)
Conversion (%)
Selectivity (%)
1
2
3
4
5
Au/LDH
Au/LDH
Au/LDH
Au/LDH
Au/LDH
Au/LDH
Pd/LDH
Pd/LDH
Au/TiO2
Au/MgO
Au/Fe2O3
Au/SiO2
Au/SiO2
Toluene
Toluene
Toluene
H2O
80
45
R.T.
45
45
80
80
80
80
80
80
80
80
2
4
12
4
16
2.5
12
12
2
2
2
2
2
>99
>99
96
40
98
97
<1
7
24
11
26
4
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
H2O
a
6
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
7
8
b
c
77
9
>99.5
>99.5
>99.5
>99.5
>99.5
1
1
1
1
0
1
2
3
d
19
◦
Reaction conditions: 0.6 mmol of 1-phenylethanol, 80 mg of catalyst, 10 ml of solvent, O2 with rate 30 ml/min, room temperature at 26 C.
a
Recycled for 6 times.
Addition of t-butyl hydroperoxide (TBHP, 5 mol% based on substrate) as initiator.
Formation of benzaldehyde by-product.
Addition of 1 mmol of Na2CO3 base.
b
c
d
Fig. 2. TEM images and Au size distribution of (a) Au/LDH catalyst and (b) Au/LDH catalyst recycled for 6 times.

L. Wang et al. / Catalysis Today 175 (2011) 404–410
407
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
indicate that there is no leaching of active sites from Au/LDH
catalyst during heterogeneous oxidation of 1-phenylethanol to ace-
tophenone with molecular oxygen.
Table
2 presents aerobic oxidation of various secondary
alcohols over Au/LDH catalyst. The conversion of p-methyl-␣-
phenylethanol and p-methoxy-␣-phenylethanol was very high,
giving at 95% and 99%, respectively (Table 2, entries 1 and 2).
Diphenylmethanol also had good conversion (92%) (Table 2, entry
3), but 1-(2-aminophenyl)ethanol showed relatively low conver-
sion (81%, Table 2, entry 4), which might be related to the presence
of amine groups in 1-(2-aminophenyl)ethanol. Compared with
these aromatic secondary alcohols, aliphatic secondary alcohols
exhibit low activities (Table 2, entries 5–8). For example, cyclo-
hexanol had conversion at 60% at room temperature (Table 2,
entry 5), and 2-cyclohexan-1-ol gave conversion at 74%. Normally,
the oxidation of olefinic alcohols is considered as a model reac-
tion because side reactions such as hydrogenation, hydrogenolysis,
and decarbonylation exist during the oxidative process. Interest-
ingly, a typical olefinic alcohol of 2-cyclohexan-1-ol exhibited very
high selectivity for 2-cyclohexan-1-one (>99.5%, Table 2, entry 6),
suggesting that the side reactions are free in the oxidation. For
straight-chain alcohols, 2-hexanol and 2-butanol still had conver-
Removal of Au/LDH catalyst
0
.0
0.5
1.0
1.5
2.0
2.5
3.0
Time (h)
Fig. 3. Dependence of catalytic activity in oxidation of 1-phenylethanol on time for a
reaction system before and after separation of Au/LDH catalyst. Reaction conditions:
.6 mmol of 1-phenylethanol, 80 mg of Au/LDH catalyst, 10 ml of toluene, 80 C, O2
with rate 30 ml/min.
◦
0
◦
sion of 43 and 33% at 80 C (Table 2, entries 7 and 8). These results
indicate that Au/LDH catalyst is very active for aerobic oxidation of
a wide range of secondary alcohols.
catalyst recycles. The excellent stability of Au/LDH catalyst is poten-
tially important for its industrial applications for production of fine
chemicals.
It is worth noting that aerobic oxidation of 1-phenylethanol
could occur in the absence of solvent. When a reaction system
3.3. Oxidation of primary alcohols
(
1
160 mmol and 10 mg of Au/LDH) was performed for 0.5 h at
Generally, compared with secondary alcohols, the oxidation
of primary alcohols shows relatively low activities [16]. Table 3
presents catalytic activities and selectivities in aerobic oxidation
of benzyl alcohol over Au/LDH catalyst, and the choice of benzyl
alcohol as a model reaction is due to the presence of side reactions
such as over-oxidation of benzaldehyde and esterification of benzyl
alcohol with benzoic acid. Notably, Au/LDH catalyst was very active
and selective for the formation of benzaldehyde in the presence of
toluene solvent (Table 3, entries 1–3). When the temperature was
◦
60 C, Au/LDH gives a extremely high turnover frequency (TOF)
−1
of 21,040 h , which is comparable with the most active Au species
reported in literature [16b,23].
Fig. 3 shows dependence of catalytic conversion on time for a
reaction system before and after separation of Au/LDH catalyst.
Before the removal of Au/LDH catalyst, the conversion increased
with time. However, after separation of Au/LDH catalyst from the
reaction system, the conversion became a constant. These results
Table 2
Aerobic oxidation of secondary alcohols over Au/LDH catalyst.
◦
Entry
1
Substrate
Product
Temperature ( C)
Solvent
Toluene
Time (h)
20
Conversion (%)
95
Selectivity (%)
>99.5
R.T.
2
3
R.T.
45
Toluene
Toluene
20
8
99
92
>99.5
>99.5
4
5
6
80
Toluene
Toluene
Toluene
8
24
12
81
60
74
92
R.T.
45
>99.5
>99.5
7
8
80
80
Toluene
Toluene
24
40
43
33
>99.5
>99.5
◦
Reaction conditions: 0.6 mmol of alcohol, 80 mg of catalyst, 10 ml of solvent, O2 with rate 30 ml/min, room temperature at 26 C.

4
08
L. Wang et al. / Catalysis Today 175 (2011) 404–410
Table 3
Aerobic oxidation of benzyl alcohol over various Au catalysts.
.
◦
Selectivitya (%)
Entry
Catalyst
Solvent
Temperature ( C)
Time (h)
Conversion (%)
89
1
2
3
4
5
6
7
Au/LDH
Au/LDH
Au/LDH
Au/LDH
Au/LDH
Au/LDH
Pd/LDH
Pd/LDH
Au/TiO2
Au/MgO
Au/Fe2O3
Au/SiO2
Au/SiO2
Toluene
Toluene
Toluene
H2O
H2O
H2O
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
80
45
R.T.
80
45
R.T.
45
45
80
80
80
80
80
20
20
20
20
20
40
24
24
20
20
20
20
20
93
96
99
71
71
82
–
98
99
97
99
–
77
49
96
90
69
<1
3
14
26
6
b
8
9
10
11
12
13
<1
6
c
91
Reaction conditions: 0.6 mmol of benzyl alcohol, 80 mg of catalyst, 10 ml of solvent, O2 with rate 30 ml/min.
a
Selectivity for benzaldehyde.
Addition of t-butyl hydroperoxide (TBHP, 5 mol% based on substrate) as initiator.
Addition of 1 mmol of Na2CO3 base.
b
c
◦
8
(
0 C, the catalyst gave the conversion at 89% and selectivity at 93%
Table 4 presents catalytic activities and selectivities in oxida-
tion of 2-phenylethanol and 1-hexanol in the presence of water.
2-phenylethanol had good conversion (87%) and selectivity for
phenylacetaldehyde (81%), while 1-hexanol showed low activity
(14%) and selectivity for n-hexaldehyde (22%).
◦
Table 3, entry 1); when the temperature was reduced to 45 C,
the catalyst showed lower conversion (77%) but higher selectiv-
ity (96%) (Table 3, entry 2); when the reaction was performed at
room temperature, the conversion was much lower (49%) and the
benzaldehyde was selectively formed (>99%) (Table 3, entry 3). In
contrast, Pd/LDH and the other Au-based catalysts exhibited very
low conversion (1–25%, Table 3, entries 7–13).
3
.4. XPS spectra of Au/LDH, Au/TiO , and Au/SiO catalysts
2 2
Interestingly, when water was used as solvent in the oxida-
tion, Au/LDH catalyst showed high conversion and low selectivity
To understand the highly active Au nanoparticles on the sur-
face of LDH support, XPS spectra of Au/LDH, Au/TiO , and Au/SiO2
2
(
Table 3, entries 4–6), compared with toluene solvent. For example,
catalysts have been compared, as shown in Fig. 4. Au/LDH cata-
lyst showed Au 4f7/2 binding energy at 83.2 eV (Fig. 4(a)), which
is obviously lower than that of Au/TiO2 (83.5 eV, Fig. 4(b)) and of
Au/SiO2 catalyst (84.0 eV, Fig. 4(c)). Compared with Au/TiO2 and
◦
the conversion of benzyl alcohol at 45 C in water can reach 90%,
which is much higher than that in toluene solvent (77%). On the con-
trary, the selectivity in water was only 71%, which is less than that in
toluene solvent (96%). This phenomenon is possibly assigned to the
difference in solubility of alcohols [6a,20] and in reactivity of prod-
ucts [19b]. For example, primary alcohols have better solubility in
water than secondary alcohols, resulting in higher conversion of
primary alcohols than secondary alcohols [6a,20]. However, water
could react with aldehyde oxidized by primary alcohols to form
aldehyde hydrate, which could be easily oxidized into carboxylic
acid, a major by-product of primary alcohol oxidations [19b]. In
contrast, the acetones oxidized by secondary alcohols in water are
basically stable.
Au/SiO , the downshift of Au 4f
over Au/LDH indicates that Au
2
7/2
nanoparticles are negative charged, which might be resulted from
the strong interaction between Au nanoparticles with LDH support
17a,21,22]. The negatively charged Au nanoparticles are favorable
for activation of molecular oxygen in aerobic oxidation of alcohols
23]. It was reported that a hydrogenation step is key step for the
[
[
oxidation of alcohols [19b,24,25]. In addition, the support alkalin-
ity is important to facilitate the dehydrogenation step [19b,24]. This
conclusion is also confirmed by the fact that the addition of solid
base (Na CO ) could effectively enhance catalytic activities in this
2
3
Table 4
Aerobic oxidation of primary alcohols over Au/LDH catalyst.
◦
Entry
Substrate
Product
Temperature ( C)
Solvent
Time (h)
Conversion (%)
Selectivity (%)
1
2a
45
80
H2O
H2O
24
24
87
14
81
22
Reaction conditions: 0.6 mmol of alcohol, 80 mg of catalyst, 10 ml of solvent, O2 with rate 30 ml/min.
a
O2 pressure at 0.5 MP.

L. Wang et al. / Catalysis Today 175 (2011) 404–410
409
a
b
c
(
d)
8
3.2 eV
(c)
(
b)
a)
(
8
0
85
Binding Energy (eV)
90
95
1
00
200
300
400
o
Temperature
C
Fig. 5. TPSR spectra of 2-propanol adsorbed on (a) Au/TiO2, (b) Au/SiO2, (c) LDH and
(d) Au/LDH for H2 signals.
8
3.5 eV
can greatly facilitate this process, which could be the reason of high
activities of Au/LDH for the alcohol oxidations.
4
. Conclusions
Extremely small Au nanoparticles distributed at 1–5 nm on LDH
support (Au/LDH) is successfully prepared by ion-exchange and
reduction procedures. Compared with the other Au-based cat-
alysts, Au/LDH catalyst shows good activities, high selectivities,
and recyclability in aerobic oxidation of a wide range of pri-
mary and secondary alcohols under very mild conditions, which
is of great importance for potentially industrial production of fine
chemicals.
80
85
Binding Energy (eV)
90
95
8
4.0 eV
Acknowledgements
This work is supported by the National Natural Science Founda-
tion of China (20973079) and State Basic Research Project of China
(2009CB623501).
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