Selective oxidation of alcohols to aldehydes/ketones over copper oxide-supported gold catalysts

Article abstract of DOI:10.1016/j.jcat.2012.11.018

Selective oxidation of alcohols to aldehydes/ketones with O₂ over a series of supported gold catalysts was studied. The catalytic performance depends strongly on the support. It is also strongly influenced by the preparation method and the pH value and stirring rate during the co-precipitation process as a result of their effect on the structure of the support and the particle size and electronic state of gold. The Au/CuO co-precipitated at pH 10 and stirring rate 100 rpm showed high activity, selectivity, and stability. The reaction might occur via oxidative dehydroxylation of alcohols to aldehydes/ketones by direct β-CH elimination, as indicated by the IR result. The oxidation of different cycloalcohols showed that the activity increased with an increase in their methyl groups. Both conversion and ketone selectivity higher than 99% were achieved for cyclooctanol and cyclododecanol.

Full text of DOI:10.1016/j.jcat.2012.11.018

Journal of Catalysis 299 (2013) 10–19
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selective oxidation of alcohols to aldehydes/ketones over copper
oxide-supported gold catalysts
a
a
b
b
Hui Wang a, Weibin Fan a, , Yue He , Jianguo Wang , Junko N. Kondo , Takashi Tatsumi
⇑
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, China
b Catalytic Chemistry Division, Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective oxidation of alcohols to aldehydes/ketones with O2 over a series of supported gold catalysts was
studied. The catalytic performance depends strongly on the support. It is also strongly influenced by the
preparation method and the pH value and stirring rate during the co-precipitation process as a result of
their effect on the structure of the support and the particle size and electronic state of gold. The Au/CuO
co-precipitated at pH 10 and stirring rate 100 rpm showed high activity, selectivity, and stability. The
reaction might occur via oxidative dehydroxylation of alcohols to aldehydes/ketones by direct b-CAH
elimination, as indicated by the IR result. The oxidation of different cycloalcohols showed that the activity
increased with an increase in their methyl groups. Both conversion and ketone selectivity higher than
99% were achieved for cyclooctanol and cyclododecanol.
Received 17 September 2012
Revised 2 November 2012
Accepted 7 November 2012
Available online 7 January 2013
Keywords:
Alcohol
Gold
Heterogeneous catalysis
Selective oxidation
Supported catalysts
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
of alcohols and alkenes, although gold is traditionally regarded as
an inert metal [10–13].
Selective oxidation of alcohols to aldehydes or ketones has at-
tracted a lot of attention because these products are important
intermediates for the synthesis of fine chemicals, and it opens up
the possibility of using renewable biomass-derived feedstocks
[1–3]. Usually, toxic and/or homogeneous oxidants, for example,
CrO3, KMnO4, MnO2, SeO2, and Br2, are used stoichiometrically
[1]. From the point of view of sustainable and green chemistry,
these oxidants should be replaced with clean oxidants such as
aqueous H2O2 and particularly molecular oxygen [2]. However, this
requires preparation of highly active, selective, and recyclable
O2-activating heterogeneous catalysts [3].
In this context, noble metals such as Pt and Pd have been used
to catalyze the oxidation of alcohols [4,5]. Although considerable
progress has been made in the past two decades, the low catalytic
stability or the quick catalyst deactivation arising from particle
aggregation or leaching of noble metals during the reaction and/
or regeneration process has still been a fatal problem limiting their
wide industrial utilization [5,6]. This particularly holds true for the
liquid-phase oxidation of alcohols with molecular oxygen over
supported catalysts. Nevertheless, supported gold catalysts have
recently been shown to be highly active and selective and more
resistant to deactivation than Pt and Pd catalysts for the oxidation
Galvagno et al. [7] reported that the catalytic properties of sup-
ported gold catalysts are strongly dependent on the types and
structures of supports. By comparatively studying Fe2O3-, ZnO-,
CaO-, and Al2O3-supported gold catalysts, they found that the basi-
city and lattice oxygen of supports have significant effects on the
catalytic performance. They also demonstrated that addition of
base to the reaction mixture is essential for achieving high activity
[14] as a result of favoring abstraction of hydrogen from alcohols
[9]. However, this is not true for the Au/CeO2 catalyst, which
showed high activity and selectivity in the oxidation of alcohols
to aldehydes or ketones in the presence of O2 without requirement
of additional base [8,9]. The high catalytic activity of the Au/CeO2
catalyst was ascribed to the stabilization of the positive oxidation
states of gold through its interaction with the nanometric ceria
surface, which creates Ce3+ and oxygen-deficient sites in the ceria,
and consequently stabilizes the reactive peroxy intermediate
formed from O2 [15,16]. A further increase in the activity could
be achieved by alloying Au and other noble metals such as Pd
and Ag [17–19]. Hutchings et al. reported that AuAPd alloy nano-
particles supported on TiO2 showed much higher activity and
selectivity than Au/TiO2 and Pd/TiO2 catalysts for the oxidation of
a series of alcohols [3,4].
Although the preparation methods, the nature of supports, and
the size of gold particles have been proved to affect the catalytic
performance considerably [4,14,15], these factors have not been
fully understood. This is because the catalytic reaction mechanism
⇑
Corresponding author. Fax: +86 351 4050350.
E-mail address: fanwb@sxicc.ac.cn (W. Fan).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.11.018

H. Wang et al. / Journal of Catalysis 299 (2013) 10–19
11
remains unclear, although considerable progress has been made in
the last decade. It is widely accepted that a synergetic effect might
exist between the gold nanoparticles and the support. It was also
suggested that hydroperoxy/peroxy intermediate species were
possibly formed during the reaction [4,16,10]. This is supported
by the finding that the addition of a peroxy initiator was necessary
to start the epoxidation of cyclooctene with molecular oxygen [4].
In the case of oxidation of alcohols with O2 over Au/TiO2, the
adsorption and activation of molecular oxygen could be enhanced
by the hydroxyl groups formed through the dissociation of H2 and
H2O molecules on the oxygen vacancies, possibly as a result of
facilitating O2 adsorption on TiO2, and hence activating O2 mole-
cules [20]. This is supported by a substantial increase in activity
in the presence of water. Nevertheless, to date, no unambiguous
evidence has been obtained. Detailed mechanistic studies aiming
at understanding why gold catalysts exhibit unique catalytic prop-
erties are required, since this would significantly promote the re-
search progress of gold catalysis. Unfortunately, the papers
concerning such mechanistic studies in recent years are few, prob-
ably because of extreme complexity.
For the physical mixture method, the pH value of aqueous
HAuCl4 solution was adjusted to 10. Then, CuO precipitated at
the same pH value was added and the mixture was stirred for
20 min. Finally, the solid was separated by centrifuging, washed
with distilled water, and dried at 100 °C to obtain the Au/CuOphy
catalyst.
For comparison, an Au/CuO catalyst was also prepared by the
incipient wetness impregnation method at ambient temperature
with aqueous HAuCl4 solution as Au precursor. The catalysts were
then dried at 100 °C, and the obtained sample was designated as
Au/CuOim
.
2.2. Catalyst characterization
XRD was performed on an advanced X-ray diffractometer (Bru-
ker AXS D8, Germany). The diffraction patterns of the samples
were recorded at room temperature with Cu K
a radiation in the
range of 2h between 10° and 80°. The HRTEM images of the sam-
ples were obtained on a JEM 2010 microscope operated at
200 kV and equipped with an energy-dispersive X-ray (EDX)
instrument. The X-ray photoelectron spectra (XPS) were measured
on a Shimadzu ESCA 3200 X-ray photoelectron spectrometer
In this work, a systematic study on the oxidation of alcohols
with molecular oxygen over the supported gold catalysts was con-
ducted. It shows that Au/CuO prepared by the co-precipitation
method is a highly active, selective, and stable heterogeneous cat-
alyst for the oxidation of alcohols to aldehydes or ketones, and that
the preparation method, the pH value of the precipitation mixture,
and the stirring rate in the co-precipitation process significantly
influence the catalytic performance.
equipped with an Mg K
compressed into a self-supported wafer for analysis. The survey
spectra were measured in binding energy (BE) range of
a target. About 70 mg of the sample was
a
0–1100 eV. The BE was calibrated with the C1s signal located at
284.6 eV. The data analysis involved spectral normalization,
Shirley background correction, and curve fitting by Gaussian–
Lorentzian functions. The Au4f regions were fitted by doublets with
fixed spectroscopic parameters, for example, of the 4f5/2 to 4f7/2
branch ratio of 3–4, but the full width at half maximum (FWHM),
positions, and intensities of the signals are independent and
variable, and optimized by the above-mentioned fitting program.
IR measurements were carried out on a Jasco FTIR 7300 spec-
trometer with an MCT detector at a resolution of 2 cmÀ1 by accu-
mulating 64 scans. Prior to the measurement, the sample was
treated at 100 °C under high-vacuum conditions (<1 Pa) to remove
adsorbed water and organics. After it was cooled in situ to room
temperature, the spectrum was collected. Then, the sample was al-
lowed to adsorb benzyl alcohol vapor for 10 min. This was followed
by evacuation to 1 Pa and collecting the spectrum again. Subse-
quently, the sample was treated with O2 (200 Pa) to oxidize ad-
sorbed benzyl alcohol, and the spectrum was recorded at a
different reaction time.
2. Experimental
2.1. Catalyst preparation
The supported gold catalysts were prepared by the co-precipita-
tion, HCHO-reduction, boiling, physical mixture, and impregnation
methods. The metal nitrates of Cu(NO3)2Á3H2O, Mn(NO3)2Á6H2O,
Ni(NO3)2Á6H2O, Co(NO3)2Á6H2O, Fe(NO3)3Á9H2O, Cr(NO3)3Á9H2O,
and Al(NO3)3Á9H2O were used to prepare the metal oxide supports
of CuO, MnO2, NiO, CoOx, Fe2O3, Cr2O3, and Al2O3, respectively,
while titanium tetra-butoxide monomer (TBOT) and tetraorthosili-
cate (TEOS) were used to prepare TiO2 and SiO2, respectively.
The preparation procedures for the co-precipitation method are
as follows: a certain amount of metal nitrates or TEOS was first dis-
solved in distilled water, followed by the addition of aqueous
HAuCl4 solution. The resultant solution was then slowly adjusted
to the designated pH value with 2.5 M aqueous NaOH solution un-
der stirring conditions. The precipitated solid was centrifuged and
washed several times with deionized water until no chlorine ions
were detected by AgNO3 solution. The resulting material was then
dried at 100 °C to obtain the Au/CuOco, Au/MnO2co, Au/NiOco, Au/
CoOxco, Au/Fe2O3co, Au/Cr2O3co, Au/Al2O3co, and Au/SiO2co catalysts.
The AuAAg/CuOco catalyst and the co-oxide-supported gold cata-
lysts were prepared by the co-precipitation method with AgNO3,
TBOT, and TEOS as the Ag, TiO2, and SiO2 sources, respectively.
The MCM-41 was formed through hydrolysis of TEOS in the pres-
ence of cetyltrimethylammonium bromide (CTAB).
2.3. Catalytic tests
The liquid-phase oxidation of alcohols with O2 in the presence
or absence of solvent was conducted at atmospheric pressure in
a round-bottom flask (50 mL) equipped with a condenser under
stirring conditions. The temperature was controlled with a water
bath and kept at 70 or 80 °C. The reaction time was 5–20 h. The de-
tailed reaction conditions for different batches are shown in the
footnotes of tables and the captions of figures. The obtained prod-
ucts were analyzed on two GC-14B gas chromatographs equipped
with flame ionization detectors and a 50-m OV-1 and a 30-m PEG
capillary column, respectively.
The preparation procedures for the boiling method are simple.
Aqueous Cu(NO3)2Á3H2O and HAuCl4 solution were refluxed for
1 h under boiling conditions. Then, the solid was separated,
washed, and dried at 100 °C to obtain the catalyst of Au/CuOboil
.
3. Results and discussion
If the solution was first boiled for 0.5 h, and then aqueous HCHO
solution (37%) was added and further boiled for another 0.5 h, fol-
lowed by centrifuging, washing, and drying at 100 °C, we called
this the HCHO-reduction method in order to distinguish it from
the boiling method. The catalyst prepared by this method was des-
3.1. Effect of supports on the oxidation of benzyl alcohol with O2 over
the supported gold catalysts
Table 1 shows the catalytic results obtained over a series of sup-
ported gold catalysts prepared by the co-precipitation method in
ignated as Au/CuOHCHO
.

12
H. Wang et al. / Journal of Catalysis 299 (2013) 10–19
Table 1
Catalytic results for the oxidation of benzyl alcohol with O2 over various supported gold catalysts prepared by the co-precipitation method.
a
Catalysts
Au (wt.%)
Conversion (%)
Selectivity (%)
Benzaldehyde
TOF (hÀ1
)
Others
CuOco
–
1.6
58.5
12.2
40.7
28.7
35.0
3.3
>99
n.d.
1.8
–
56
8
25
19
15
4
Au/CuOco
Au/MnO2co
Au/NiOco
Au/CoOxco
Au/Fe2O3co
Au/Cr2O3co
Au/Al2O3co
Au/SiO2co
4.0
5.1
4.4
4.3
6.8
2.8
3.9
3.3
98.2
87.7
68.7
71.6
72.1
94.8
53.1
62.1
12.3
31.3
28.4
27.9
5.2
30.0
30.1
46.9
37.9
16
23
Note: Reaction conditions: 0.1 g catalyst, 10 mmol benzyl alcohol, 80 °C, 5 h, 5 mL/min O2.
a
TOF was calculated on the basis of total loading of gold.
Table 2
Effects of different co-supports or co-active component of Ag on the catalytic performance of Au/CuOco
.
Catalysts
Au (wt.%)
Conversion (%)
Selectivity (%)
Benzaldehyde
TOFa (hÀ1
)
Others
Au/3%CuO–1%TiO2
5.5
2.7
2.9
2.5
2.7
2.7
2.9
2.9
4.7
3.4
2.5
73.0
49.1
8.0
33.6
9.8
11.6
3.2
0.9
>99
94.3
97.4
>99
>99
>99
>99
>99
>99
92.2
n.d.
5.7
2.6
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
7.8
0.1
7.4
4.9
0.9
3.6
1.1
1.2
0.3
0.1
5.6
Au/1%CuO–3%MCM-41
Au/1%CuO–1%MCM-41
Au/3%CuO–1%MCM-41
Au/1%CuO–3%MCM-41b
Au/1%CuO–3%MCM-41c
Au/1%CuO–1%MCM-41b
Au/1%CuO–1%MCM-41c
Au(6%)–Ag(3%)/CuO
Au/MCM-41
71.1
Note: Reaction conditions: 0.1 g catalyst, 10 mL toluene, 3 mmol substrate, 5 mL/min O2, 80 °C, 20 h.
a
TOF was calculated on the basis of total loading of gold.
Extracted with anhydrous ethanol.
Extracted with 0.2 M HCl ethanol solution.
b
c
the oxidation of benzyl alcohol to benzaldehyde with O2 in the ab-
sence of solvent. The main by-products were benzoic acid and ben-
zoic acid benzyl ester for all the catalysts. The Au/MnO2co and Au/
Cr2O3co catalysts gave turnover frequencies (TOF) of 8 and 4 hÀ1
and benzaldehyde selectivities of 87.7% and 94.8%, respectively.
When the Au/NiOco, Au/Fe2O3co, Au/CoOxco, Au/Al2O3co, and Au/
When MCM-41 served as a support, a conversion of 71.1% was
obtained, although the benzaldehyde selectivity decreased to
92.2%. This suggests that the addition of MCM-41 to the Au/CuOco
catalyst would possibly increase the catalytic activity. Thus, the ra-
tio of CuO to MCM-41 was adjusted further to improve the cata-
lytic performance. It was found that the benzyl alcohol
conversion increased from 8.0% to 73.0% when the CuO/MCM-
41M ratio decreased from 3/1 to 1/3 (Table 2). Irrespective of this,
a further increase in MCM-41 had no positive effect. The as-synthe-
sized MCM-41-supported Au catalyst gave a conversion of benzyl
alcohol and a selectivity of benzaldehyde similar to those obtained
over the Au/CuOco–MCM-41 catalyst with a CuO/MCM-41M ratio
of 1/3. Attempt to remove CTAB surfactant by Soxhlet extraction
with anhydrous ethanol led to a decrease in the benzyl alcohol
conversion to 33.6%. A much more significant decrease in the ben-
zyl alcohol conversion was observed when the surfactant was re-
moved by extraction with 0.2 M HCl ethanol solution. This
results from severe leaching of gold nanoparticles during the
extraction process.
It was reported that the presence of base favored the selective
oxidation of alcohols over the supported gold catalysts [9,14].
Thus, a certain amount of Na2CO3, K2CO3, or K3PO4 was added to
the reaction mixture of oxidation of benzyl alcohol with molecular
oxygen over the Au/CuOco, and the obtained catalytic results are
shown in Table 3. Unexpectedly, addition of alkali metal salts to
the reaction mixture had a negligible or a negative effect on the
catalytic performance. When Na2CO3 was added, although the
activity increased slightly, the selectivity of benzaldehyde was re-
duced by 2.6%, which might be due to the promotion of the ester-
ification reaction. An increase in the basicity of added alkali metal
SiO2co catalysts were used, the TOF increased to 15–25 hÀ1
,
although the selectivities to benzaldehyde decreased to 53.1–
72.1%. As for Au/CuOco catalyst, a conversion of 58.5% was obtained
at the reaction time of 5 h, which corresponds to a TOF of 56 hÀ1
.
Meanwhile, the selectivity to benzaldehyde reached 98.2%. These
results demonstrate that CuO is an effective support for the gold
catalyst in the selective oxidation of benzyl alcohol with molecular
oxygen. However, CuO itself is nearly inactive in this reaction. This
reveals that Au plays a major role in catalyzing this reaction, or
that Au and CuO collaboratively catalyze this reaction.
In order to further increase the activity of the Au/CuOco, the ef-
fects of the co-support or the co-active metal on the catalytic per-
formance of Au/CuO were investigated. Table 2 lists the catalytic
results obtained over various gold catalysts with co-supports or
co-active components. It is unexpected that the introduction of
the co-supports of TiO2 and MCM-41 or the co-active metal of Ag
led to a decrease in activity to the TOFs less than 10 hÀ1. Very
low conversion was obtained over the Au/(CuO + TiO2)co even if
the reaction time was increased to 20 h. This is probably due to
the reduction of CuO to Cu, as indicated by the bronze color of
the precipitation. For the AuAAg/CuO catalyst, it was also nearly
inactive for the oxidation of benzyl alcohol with O2, probably as
a result of the formation of AgCl when Au and Ag were simulta-
neously co-precipitated on the CuO support.

H. Wang et al. / Journal of Catalysis 299 (2013) 10–19
13
Table 3
3.2. Effect of preparation parameters on the oxidation of benzyl alcohol
with O2 over Au/CuO
Effect of the base added to the reaction mixture on the catalytic performance of Au/
CuOco for the oxidation of benzyl alcohol with O2.
a
Bases
Conversion (%)
Selectivity (%)
Benzaldehyde
TOF (hÀ1
)
3.2.1. Effect of preparation methods
Others
Table 4 summarizes the catalytic results obtained over the Au/
CuO samples prepared by the co-precipitation, boiling, HCHO-
reduction, physical mixture, and impregnation methods in the
oxidation of benzyl alcohol with molecular oxygen. Clearly, the
preparation method had a strong influence on the catalytic perfor-
mance of the Au/CuO. Although the selectivity of benzaldehyde
was higher than 99% for all the prepared samples, the activities
are markedly different. The Au/CuOco sample showed a high con-
version of 85.7%, whereas the Au/CuOHCHO and Au/CuOim gave a
No
45.3
47.8
38.8
26.3
98.7
96.1
94.4
95.6
1.3
3.9
5.6
4.4
87.6
90.0
71.8
49.3
Na2CO3
Na3PO4
K3PO4
Note: Reaction conditions: 0.1 g catalyst, 80 °C, 5 h, 20 mmol benzyl alcohol, 5 mL/
min O2, 0.6 g alkali metal salt.
a
TOF was calculated on the basis of total loading of gold.
conversion lower than 3%. In the case of Au/CuOboil and Au/CuOphy
,
Table 4
Catalytic results for the oxidation of benzyl alcohol with O2 over Au/CuO catalysts
prepared by different methods.
moderate conversions were obtained as a result of both having low
Au content. This is supported by the TOF obtained over these two
catalysts, which is similar to that of Au/CuOco. The low activity of
the Au/CuOim is probably due to the presence of large amounts
of chlorine ions, which was considered to poison gold catalysts
[11].
Preparation method
Au
(wt.%) (%)
Conversion Selectivity (%)
Benzaldehyde Others
TOF
(hÀ1
a
)
Co-precipitation
Boiling 0.5 h + HCHO
reduction + boiling 0.5 h
Boiling 1.0 h
Physically mixing
Impregnation
4.0
0.27
85.7
2.6
>99
>99
n.d.
n.d.
6.2
2.8
Fig. 1 shows the XPS spectra and their deconvoluted configura-
tions of the Au/CuO catalysts prepared by the different methods in
the BE range for Au4f signals. The binding energies of bulk metallic
Au, Au2O3, and Au(OH)3 are 83.9, 86.3, and 87.7 eV for 4f7/2 and
87.7, 89.6, and 91.4 eV for 4f5/2, respectively [22–25]. The very
small (nonmetallic) Au cluster or Au+ peaks locate at 84.9 and
88.6 eV [26–28]. Thus, it can be deduced that the Au/CuOim con-
tains mainly bulk metallic Au and Au2O3, whereas the Au/CuOco
contains mainly Au2O3, Au(OH)3, and Au+ and/or nonmetallic Au
clusters, but few metallic Au, since no signal was observed around
83.9 eV. For the Au/CuOboil, both bulk Au and nonmetallic Au clus-
ters are present. In the case of Au/CuOHCHO and Au/CuOphy, no obvi-
ous signals characteristic of Au species were observed because of
their very low Au content (<0.3%).
1.1
3.2
6.0
17.1
56.1
1.5
>99
>99
>99
n.d.
n.d.
n.d.
4.6
5.1
0.1
Note: Reaction conditions: 0.1 g catalyst, 80 °C, 20 h, 3 mmol benzyl alcohol, 10 mL
toluene, 5 mL/min O2.
a
TOF was calculated on the basis of total loading of gold.
salts conversely decreased the catalytic activity. Thus, the addition
of a base to the reaction mixture is not beneficial or even detrimen-
tal to the oxidation of benzyl alcohol over the Au/CuOco. Neverthe-
less, the leaching of CuO was completely inhibited by the addition
of base, as confirmed by the fact that the reaction liquid changed
from green under the solvent-free condition to colorless when
Na2CO3 was added.
Table 4 shows that the Au/CuOboil showed a TOF of 4.6 hÀ1, low-
er than that (6.2 hÀ1) of the Au/CuOco. This indicates that besides
Au-Au(bulk)
(a)
(b)
Au(OH)3
Au-Au(bulk)
Au2O3
Au cluster/Au+
Au2O3
Au2O3
Au2O3
Au cluster
Au(OH)3
/Au+
82
84
86
88
90
92
94
84
86
88
90
92
94
Binding Energy/eV
Binding Energy/eV
Au-Au(bulk)
Au cluster/Au+
Au-Au(bulk)
(c)
Au cluster/Au+
84
86
88
90
Binding Energy/eV
Fig. 1. XPS spectra and the deconvoluted configurations of Au4f signals of the Au/CuO prepared by the (a) impregnation, (b) co-precipitation, and (c) boiling methods.

14
H. Wang et al. / Journal of Catalysis 299 (2013) 10–19
Table 5
molecular oxygen. TOF increased with increasing pH value of the
co-precipitated mixture up to 10. The selectivity of benzaldehyde
was 27.7% when the pH value of the co-precipitated mixture was
8, while it reached more than 99% over the sample co-precipitated
at pH 10. Nevertheless, a further increase in the pH value of the
co-precipitated mixture decreased the benzyl alcohol conversion
despite the fact that the benzaldehyde selectivity was still higher
than 99%.
Effects of the pH value of the co-precipitated gel on the catalytic performance of
prepared Au/CuOco for the oxidation of benzyl alcohol with O2.
pH
value
Au
(wt.%)
Conversion
(%)
Selectivity (%)
TOF
(hÀ1
a
)
Benzaldehyde Others
8
9
10
11
4.7
5.0
4.0
4.0
39.7
16.4
85.7
76.7
27.7
>99
>99
>99
72.3
n.d.
n.d.
n.d.
0.7
1.0
6.3
5.6
Fig. 2 shows the HRTEM images of the Au/CuOco samples
co-precipitated at pH 8, 9, 10, and 11. The Au particles were homo-
geneously distributed on the CuO for all the four samples. Their
sizes centered at 1–3, 3–6, 3–6, and 6–9 nm for the samples
co-precipitated at pH 8, 9, 10, and 11, respectively. The size of
the gold particles is one of the key factors that determine the activ-
ity of supported gold catalysts. The Au–oxygen interaction is highly
dependent on the gold particle size at the nanoscale, which results
in an interesting size selectivity in heterogeneously catalyzed reac-
tions [15]. Generally, small gold particles enhance oxidation ability
by facilitating oxygen activation [16]. Thus, the Au/CuOco catalyst
precipitated at pH 8 showed low selectivity for benzaldehyde be-
cause its gold particles were in the range 1–3 nm, which could fur-
ther oxidize the aldehyde to benzyl acid and subsequently form
esters. As for the Au/CuOco catalysts precipitated at pH 9, 10, and
11, they had moderately large particles and consequently were
highly selective for the oxidation of benzyl alcohol to benzalde-
hyde due to the inhibition of deep oxidation of benzaldehyde. This
shows that gold particles larger than 3 nm seem not to be active for
the further oxidation of benzaldehyde to benzoic acid.
Note: Reaction conditions: 0.1 g catalyst, 80 °C, 20 h, 3 mmol benzyl alcohol, 10 mL
toluene, 5 mL/min O2.
a
TOF was calculated on the basis of total loading of gold.
the Au content and the chlorine ions, the gold electronic state may
strongly influence the reactivity of Au/CuO, too. The higher activity
of the Au/CuOco than of the Au/Cuboil might be due to the presence
of abundant cationic (nonmetallic) Au species originated from the
metal–support interaction [12]. The gold atoms interacted with the
oxide support to form nonmetallic Au clusters or bond to surface
hydroxyl or lattice/surface oxygen to form Au+ and Au3+ species
[15,16]. The interaction between the gold and the oxide support
depends strongly on how the gold atoms arranged in the support
[26], which is closely related to the preparation methods.
3.2.2. Effect of the pH value of the co-precipitated mixture
For the gold catalyst prepared by the co-precipitation method,
the pH value of the co-precipitated mixture has a significant
influence on the electronic state and particle size of gold and
the structure of support [13,14]. Table 5 displays the catalytic
results obtained over the Au/CuOco catalysts co-precipitated at
pH values of 8, 9, 10, and 11 in the oxidation of benzyl alcohol with
The XRD patterns of the Au/CuOco co-precipitated at pH 8, 9, 10,
and 11 are shown in Fig. 3. The Au/CuOco co-precipitated at pH 8
showed sharp diffraction lines at 2h of 13.3°, 25.7°, 33.6°, and
58.1°. All these lines could be attributed to crystalline Cu(OH)2ÁH2O
(a)
(b)
5 nm
10 nm
(c)
(d)
5 nm
20 nm
Fig. 2. HRTEM images of Au/CuOco catalysts co-precipitated at pH (a) 8, (b) 9, (c) 10, and (d) 11.

H. Wang et al. / Journal of Catalysis 299 (2013) 10–19
15
♦
♦
•
0.05
d
♦
♦
♦
♦
♦
♦
♦
20 min
15 min
c
•
♦
♦
b
a
1 min
0 min
•
•
•
1200
1400
1600
1800
2000
10
20
30
40
2θ/°
50
60
70
80
Wavenumber/cm-1
Fig. 3. XRD patterns of Au/CuOco catalysts co-precipitated at pH (a) 8, (b) 9, (c) 10,
and (d) 11. Crystalline phases detected: Ç CuO; d Cu(OH)2ÁH2O.
Fig. 5. IR spectra collected following the oxidation of benzyl alcohol with O2 over
the Au/CuOco catalyst co-precipitated at pH 10: adsorption of benzyl alcohol (0 min)
and further reaction with O2 for 1 min, 15 min, and 20 min.
surrounding OAH on the catalyst. In addition, another two broad
bands were observed in the regions 1200–1600 cmÀ1 and 3000–
3600 cmÀ1. These two bands are related to phenyl groups and
OAH species [19,20]. When benzyl alcohol adsorbed on the surface
of the Au/CuOco was obtained at pH 9, these bands decreased
greatly in intensity. This indicates that the adsorption of benzyl
alcohol on the sample co-precipitated at pH 9 is significantly weak-
er than that on the sample co-precipitated at pH 8. Although the
bands between 1200 and 1600 cmÀ1 were more resolved and the
broad band attributed to the hydrogen-bonded OH species was
lower in intensity, similar IR spectra were obtained for the cata-
lysts co-precipitated at pH 10 and 11. However, after penetration
of O2 for 10 min, the band attributed to C@O-related species
(ACHO) appeared at 1690 cmÀ1 [17] in the IR spectra of the Au/
CuOco co-precipitated at pH 10 (Fig. 5), while this band cannot be
differentiated from the spectrum of the sample obtained at pH 9.
It should be noted that no evidence was found for the occurrence
of deep oxidation for the three samples. Thus, it is not difficult to
understand that the sample co-precipitated at pH 8 showed a
low benzaldehyde selectivity of 27.7%, while those co-precipitated
at pH 9, 10, and 11 gave benzyl aldehyde selectivity higher than
99% and that co-precipitated at pH 9 showed a low conversion of
16.4%.
1
a
b
c
d
1600
2400
3200
-1
4000
Wavenumber/cm
Fig. 4. IR spectra of the adsorption of benzyl alcohol over Au/CuOco catalysts co-
precipitated at the pH of (a) 8, (b) 9, (c) 10, and (d) 11.
(PDF number 42-0746). However, when the pH value of the
co-precipitated mixture was increased to 9, these diffraction lines
decreased in intensity, while new lines (at 2h of 35.5°, 38.7°, and
48.8°) characteristic of CuO (PDF number 45-0937) appeared. This
indicated that part of Cu(OH)2ÁH2O transformed into CuO. A further
increase in the pH of the co-precipitation mixture to 10 led to the
complete transformation of Cu(OH)2ÁH2O into CuO. This is coinci-
dent with the color change of the precipitation mixture from light
green to black. XRD measurements demonstrated that the support
co-precipitated at pH 11 was also pure CuO. On the other hand, no
evident diffraction lines characteristic of gold species were de-
tected for the four samples, showing that gold species were highly
dispersed on the supports for all of these samples. Nevertheless,
the HRTEM analysis indicated the presence of a larger number of
6–9 nm gold particles on the sample co-precipitated at pH 11. This
part of the gold particles exhibited lower activity. The above results
show that the pH value of the co-precipitated mixture influences
not only the gold particle size but also the support structure and
consequently the catalytic properties of the Au/CuOco in the oxida-
tion of alcohols with molecular oxygen.
3.2.3. Effect of the stirring rate during the co-precipitation process
Like the pH value of the co-precipitated mixture, the stirring
rate during the co-precipitation process also strongly affects the
size of gold particles, although this point has not been received
much attention. Fig. 6 shows the HRTEM imagines of the samples
co-precipitated at different stirring rates. The samples co-precipi-
tated at the stirring rates of 50 and 100 rpm had a narrow size dis-
tribution of gold nanoparticles, being approximately in Gauss
distribution with average values of 4.3 and 4.5 nm, respectively.
However, when the stirring rate was 50 rpm, CuO particle aggrega-
tion was observed to some extent. An increase in the stirring rate
resulted in not only an increase in the gold particle size but also
a broadening of the size distribution. The sample co-precipitated
at a stirring rate of 300 rpm had a gold particle size mainly in
the ranges of 3–6 and 7–19 nm. When the stirring rate was in-
creased to 500 rpm, most of the gold particles increased to 7–13
and 14–17 nm, although the average size (ꢀ12 nm) was not signif-
icantly increased.
Fig. 4 depicts the IR spectra of benzyl alcohol adsorbed on these
four catalysts. The Au/CuOco co-precipitated at pH 8 showed four
bands at 1763, 2345, 2466, and 2740 cmÀ1, which may be due to
the formation of carboxylic acid and phenyl ester [17,18]. This
shows that benzyl alcohol could be oxidized to acid and further
to ester although no O2 was introduced. This could be attributed
to the strong oxidation ability of the small gold particles, which
can catalyze the reaction between benzyl alcohol and the
Table 6 shows the catalytic results of the Au/CuOco catalysts co-
precipitated at stirring rates of 50, 100, 300, and 500 rpm. The
function for benzyl alcohol conversion vs. stirring rate showed a
volcanic shape despite the fact that the benzaldehyde selectivity

16
H. Wang et al. / Journal of Catalysis 299 (2013) 10–19
30
20
10
0
30
20
10
0
0
1 2 3 4 5 6 7 8 9 10
1
2
3
4
5
6
7
8
Particle diameter (nm)
Particle diameter (nm)
(b)
(a)
5 nm
5 nm
15
12
9
20
15
10
5
6
3
0
0
6
8
10 12 14 16
4
6
8 10 12 14 16 18
Particle diameter (nm)
Particle diameter (nm)
(c)
(d)
5 nm
5 nm
Fig. 6. HRTEM images and gold particle size distribution of the Au/CuOco catalysts co-precipitated at the stirring rate of (a) 50 rpm, (b) 100 rpm, (c) 300 rpm, and (d) 500 rpm.
Table 6
was higher than 99% for all the samples, as the sizes of most of
Effects of the stirring rate during the co-precipitation process on the catalytic
their gold particles were larger than 3 nm. The sample precipitated
performance of Au/CuOco in the oxidation of benzyl alcohol with O2.
at 100 rpm gave the highest conversion, 85.7%. A decrease or an in-
crease in the stirring rate led to a reduction in benzyl alcohol
conversion.
Stirring rate (rpm)
Conversion (%)
Selectivity (%)
Benzaldehyde
Others
The precipitation is a complex process, including the primary
process of nucleation, growth, and dissolution of a metastable
phase and the secondary process of nucleation and growth of a sta-
ble phase [21]. The kinetics of nucleation and growth of each phase
determines which phase appears first and how long the transfor-
mation from the metastable form to the stable phase takes. The
overall kinetics results from a complex interplay between the
nucleation, the growth, and the dissolution of the involved solids.
Cornel et al. studied the effect of the stirring rate on the transfor-
mation process using a population balance model [22]. It was
found that the stirring rate integration controlled the growth of
the metastable phase. Thus, an integration mechanism in which
the growth of the stable phase would be limited at low supersatu-
ration in a stirring vessel was assumed, where the nucleation time
50
100
300
500
47.4
85.7
62.5
51.4
>99
>99
>99
99.3
n.d.
n.d.
n.d.
0.7
Note: Reaction conditions: 0.1 g catalyst, 10 mL toluene, 3 mmol benzyl alcohol,
5 mL/min O2, 80 °C, 20 h.
of the stable phase was strongly influenced by the stirring rate
[22]. Such a mechanism might dominate the co-precipitation pro-
cess of the Au/CuOco catalyst, since this process occurred in the
stirring system with low supersaturation. The low stirring rate fa-
vors the nucleation of the metastable phase but limits the growth
of the stable form, giving small gold particles. An increase in the

H. Wang et al. / Journal of Catalysis 299 (2013) 10–19
17
50
100
80
60
40
20
0
♦
♦
40
30
20
10
0
♦
♦
♦
♦
♦
♦
♦
c
•
•
b
a
10
20
30
40
50
60
70
80
1
2
3
4
5
6
2θ/°
Runs
Fig. 8. XRD patterns of the fresh (a), spent (b), and regenerated (c) Au/CuOco
catalyst: Ç CuO; d Cu2O.
Fig. 7. Catalytic results for the oxidation of benzyl alcohol with O2 over the Au/
CuOco catalyst within six repeated runs with regeneration (reaction conditions:
catalyst/Na2CO3/substrate = 0.1 g/0.3 g/20 mmol, 80 °C, 5 h, 5 mL/min O2).
3.4. Catalytic mechanism of oxidation of benzyl alcohol by O2 over Au/
CuOco
stirring rate not only led to a remarkable decrease in the nucleation
time of the stable phase but also promoted its crystal growth.
When the stirring rate was increased to a certain value, a popula-
tion balance was established between the dissolution of the meta-
stable phase and the nucleation and growth of the stable form. As a
result, no obvious increase in the average size was observed for
gold particles.
Fig. 5 shows the IR spectra collected at 1, 15, and 20 min for the
oxidation of benzyl alcohol by molecular oxygen over the Au/CuOco
co-precipitated at pH 10. When benzyl alcohol adsorbed onto the
surface of the sample, four bands at 1597, 1540, 1405, and
1365 cmÀ1 were observed in the IR spectrum. The band at
1597 cmÀ1 was due to the C@C vibration of the phenyl ring [23],
whereas those at 1540 and 1405 cmÀ1 were attributed to the
asymmetric and the symmetric stretching vibrations of the ad-
sorbed OACAO groups in bridge mode [17]. After O2 was intro-
duced, the band at 1365 cmÀ1, ascribed to the scissor vibration of
CH2, disappeared, while a new band at 1690 cmÀ1 was observed,
and increased in intensity with increasing reaction time. This dem-
onstrates that benzyl alcohol was oxidized to benzaldehyde. How-
ever, this does not hold true for the pure CuO support as catalyst,
on which benzaldehyde was not formed after O2 treatment,
although benzyl alcohol could adsorb weakly onto its surface. This
is confirmed by the absence of the 1690 cmÀ1 band in the IR spec-
trum of the O2-purged sample (Fig. 1S). This might be because ben-
zyl alcohol adsorbed onto the surface of CuO so weakly that it
could not be activated, and/or the pure CuO support could not acti-
vate the O2 molecule. As a result, very low conversion (1.6%) was
obtained on CuO (Table 1).
XRD measurements show that part of the CuO in the fresh Au/
CuOco catalyst was reduced to Cu2O after reaction and mostly
recovered after regeneration (Fig. 8), indicating that the CuO sup-
port participated in the reaction through supply of active oxygen.
The interaction between Au and CuO enhances the oxygen mobility
and facilitates the formation of oxygen vacancy through the redox
of Au0 M Aud+ and CuO M Cu2O. In combination with the FTIR and
XPS spectral and catalytic results, this suggests that the gold spe-
cies and the CuO support collaboratively promoted the activation
of benzyl alcohol and O2. Thus, a possible mechanism, shown in
Fig. 9, is proposed for the oxidation of benzyl alcohol over the
Au/CuOco by modifying the conventional dehydrogenation mecha-
nism [24,25]. Au acts as the sole catalytic center where reaction
takes place, and the oxygen transfer is completed through the
redox cycle of Au0 M Aud+ and CuO M Cu2O. Benzyl alcohol mole-
cules adsorbed onto the Au nanoparticles, forming metal–alkoxide
intermediates and adsorbed hydrogens via OAH bond cleavage.
The adsorbed hydrogen bonded to the surface lattice oxygen to
form surface OAH, which is coordinated to the carbon atom of
the methyl group in the benzyl alcohol molecule and accompanied
by the transformation of CuO to Cu2O. Hence, the OACAO group in
3.3. Catalytic recyclability of the Au/CuOco in the oxidation of benzyl
alcohol with O2
Fig. 7 shows the catalytic results for Au/CuOco in the oxidation
of benzyl alcohol with O2 under the same reaction conditions in
the presence of Na2CO3 in six repeated runs with regeneration.
Clearly, Au/CuOco showed similar benzyl alcohol conversion and
benzaldehyde selectivity for all the runs, except for the second re-
cycle. This shows that the prepared Au/CuOco catalyst is highly sta-
ble during the reaction process. This is also supported by the facts
that the reaction liquid was kept colorless after filtration of the cat-
alyst and that no leaching of gold particles was detected by ICP.
The decrease in benzyl alcohol conversion from 27.9% to 17.2% in
the second run is due to the regeneration method employed. If
the Au/CuOco was regenerated by washing with only acetone for
three times, as done in the case of run 2, its catalytic activity signif-
icantly decreased. However, when the spent catalyst was washed
with water and subsequently with acetone for three times each,
as done in the runs 3–6, its catalytic reactivity was recovered. This
shows that accumulation of organic and inorganic species ad-
sorbed on its surface caused its deactivation, suggesting that the
decrease in activity with the alkalinity of the added base may be
due to the increased adsorption of organic and inorganic species
onto the surface of the catalyst (Table 3). The effective regeneration
method for the spent Au/CuOco is to clean its surface by thoroughly
washing it with both the inorganic solvent water and the organic
solvent acetone.
It is interesting that although part of the CuO was transformed
into Cu2O after each run, as revealed by XRD measurements, it was
mostly reoxidized to CuO again after regeneration and drying
(Fig. 8). This demonstrates that the CuO support participated in
the oxidation reaction. Also, it is worth noting that no diffraction
peaks characteristic of gold species are detected in all the spent
catalysts, indicating that the high dispersion of Au on the support
was maintained during the regeneration and reaction processes.

18
H. Wang et al. / Journal of Catalysis 299 (2013) 10–19
the ring structure to be more unstable but more reactive. As a con-
sequence, the conversions of cycolalcohols decreased in the order
cyclododecanol > cyclooctanol > cyclohexanol. It is worth noting
that the Au/CuOco was also highly active and selective for the oxi-
dation of unsaturated alcohols. The conversion of cinnamyl alcohol
and the selectivity of cinnamyl aldehyde reached 83.8% and 99.4%,
respectively (entry 2). These results show that the Au/CuOco is
effective at least for benzylic, cyclic, and unsaturated alcohols.
H2O
H
H
O
R
H
O
H
H
H
C
H
O
HO
O
H
O2
HO
Au
Au
H Au
R
O
O
O
O
O
O
O
[Cu+]
[O2-]
4. Conclusions
[Cu2+]
CuO
Au/CuOco is highly active and selective for the oxidation of ben-
zylic, cyclic, and unsaturated alcohols to the corresponding alde-
hydes or ketones by molecular oxygen at atmospheric pressure.
When bulky cyclooctanol and cyclododecanol are oxidized, both
the conversion and the ketone selectivity are higher than 99%.
The preparation method and the pH value and stirring rate during
the co-precipitation process have significant effects on the catalytic
performance as a result of influencing the structure of the support
and/or the electronic state and particle size of gold. The sample
prepared by the co-precipitation method showed higher activity
and selectivity than the samples prepared by the HCHO-reduction
and impregnation methods. The optimum pH value and stirring
rate during the co-precipitation process are 10 and 100 rpm,
respectively. The Au/CuOco is a stable heterogeneous catalyst and
can be recycled to use with regeneration. Addition of alkali metal
salts to the reaction mixture is not positive for the increase in
the activity but in the catalytic stability. After being activated by
Au nanoparticles, the benzyl alcohol molecule interacts with sur-
face lattice oxygen of CuO to form bridged OACAO group as inter-
mediate, which is followed by the b-H elimination to form
benzaldehyde and water.
Fig. 9. Proposed reaction mechanism for the oxidation of benzyl alcohol by O2 over
the Au/CuOco catalyst.
Table 7
Catalytic results for the oxidation of a variety of alcohols with O2 over Au/CuOco
.
Entry Substrate
Conversion
(%)
Selectivity (%)
Aldehyde/
ketone
Others
1
2
3
4
5
6
Benzyl alcohola
85.7
83.8
66.0
99.0
>99.0
85.3
>99
99.4
>99.0
>99.0
>99.0
>99.0
n.d.
0.6
n.d.
n.d.
n.d.
n.d.
Cinnamyl alcohola
Cyclohexanolb
Cyclooctanolb
Cyclododecanolb
2-Methyl benzyl
alcohola,c
7
4-Methyl benzyl
87.7
>99.0
n.d.
alcohola,c
8
9
4-Methyl cyclohexanolb
3,5-Aimethyl
71.4
92.3
>99.0
>99.0
n.d.
n.d.
cyclohexanolb
b
Notes: Reaction conditions: 0.1 g catalyst, 5 mL/min O2, 10 mL toluene, a 3 or
2 mmol substrate, 80 or c 70 °C, 20 h.
-
Acknowledgments
The authors are grateful for financial support by the
National Basic Research Program (Nos. 2011CB201403 and
2009CB226101), the National Science Foundation of China (Nos.
21273264, 21203231, and 20876163), and the Natural Science
Foundation of Shanxi Province (Nos. 2009011009-2 and
2011011006-3).
bridge mode was observed by IR. Finally, the CAH bond in the b po-
sition was activated, leading to the formation of benzaldehyde and
water [26,27]. This is consistent with the finding of Mullins et al.
that the b-H elimination mechanism dominates the dehydroxyla-
tion of alcohols to aldehydes or ketones over bulk gold catalyst
[28]. The oxygen adsorbed onto the support surface disassociates
into chemisorbed oxygen species and diffuses into the support lat-
tice vicinity. This is followed by activation at the oxygen vacancies
and conversion into lattice oxygen O2À. The lattice oxygen further
transferred to the surface of CuO through oxygen-deficient sites,
restoring the interface between the support and Au active sites,
and reacted with benzyl alcohol, as shown in Fig. 9.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2012.11.018.
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