Solvent-free oxidation of cyclohexene over catalysts Au/OMS-2 and Au/La-OMS-2 with molecular oxygen

Article abstract of DOI:10.1016/j.catcom.2010.09.014

Supported gold catalysts Au/OMS-2 and Au/La-OMS-2 were prepared and used for liquid phase oxidation of cyclohexene with oxygen as an oxidant. These catalysts were characterized by XRD, SEM, TEM and EDX. The reactions were carried out in an autoclave at 80

Full text of DOI:10.1016/j.catcom.2010.09.014

Catalysis Communications 12 (2010) 197–201
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Solvent-free oxidation of cyclohexene over catalysts Au/OMS-2 and Au/La-OMS-2
with molecular oxygen
⁎
Zhen-Yu Cai, Ming-Qiao Zhu , Jun Chen, Yang-Yi Shen, Jing Zhao, Yue Tang, Xin-Zhi Chen
Department of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2010
Received in revised form 5 September 2010
Accepted 10 September 2010
Available online 22 September 2010
Supported gold catalysts Au/OMS-2 and Au/La-OMS-2 were prepared and used for liquid phase oxidation of
cyclohexene with oxygen as an oxidant. These catalysts were characterized by XRD, SEM, TEM and EDX. The
reactions were carried out in an autoclave at 80 °C without any solvent. Effects of Au content and La content
on catalytic performance were studied. Au/La-OMS-2 (0.24) was found to be an efficient catalyst for the
oxidation of cyclohexene with a high conversion (48.0%). More than 86% selectivity for ∑C6 (including
cyclohexene oxide, 2-cyclohexene-1-ol, 2-cyclohexene-1-one) was obtained.
Keywords:
Cyclohexene oxidation
Solvent-free
Au
© 2010 Elsevier B.V. All rights reserved.
OMS-2
Oxygen
1. Introduction
50.2%) along with many notable products [8]. The properties of
molecular sieves have made them highly desirable as support materials
The catalytic aerobic transformation of alkenes into value-added
oxygenated derivatives is still a challenge in the modern chemistry and
industry world [1]. Traditionally, these catalytic procedures produced a
great deal of environmentally undesirable wastes [2–4]. It is highly
desirable to replace the traditional process by a green procedure [5,6].
There are many easily available oxidants including molecular oxygen and
H2O2. Although the use of H2O2 is atom efficient and the only by-product
is water, compared with molecular oxygen, the relatively high cost of
H2O2 severely hinders its wide application in catalytic oxidation [7,8].
Therefore, many effective and recyclable heterogeneous catalytic systems
for olefin oxidation using oxygen in the liquid phase have been studied.
Unfortunately, most of these catalysts are prepared by immobilization of
biomimetic catalysts [9–11], preparation processes of which are
troublesome and expensive. Some of these catalytic systems are generally
performed in volatile and toxic organic solvents [12,13], which involve a
difficult process for product separation. In view of above, oxidation of
alkenes with oxygen under a solvent-free condition would be valuable.
Nowadays, the research of gold has made great development
[14–16]. It was discovered that gold has remarkable catalytic properties
for selectiveoxidation of cyclohexene[8]. Extensivestudies on oxidation
of cyclohexene have been carried out due to the potential uses of the
products, including 2-cyclohexene-1-ol, 2-cyclohexene-1-one and
many other chemical intermediates [17]. Cyclohexene oxidation over
the Au/C catalyst with oxygen gave a good conversion (approximately
for gold [18,19]. Some manganese compounds are good catalysts for
cyclohexene oxidation [20]. Manganese oxide octahedral molecular
sieve (K-OMS-2 or OMS-2) has been reported as an effective catalyst in
many oxidation reactions [21,22]. Ce-incorporated OMS-2 material
displays a good combination of high activity, stability patterns and
capacity for reactive phenol adsorption [23]. Therefore, we chose La (III),
which lies in the same group with Ce (III), to modify K-OMS-2. In this
work we report the application of Au/OMS-2 and Au/La-OMS-2 to
improve efficiency of the catalyst for cyclohexene oxidation using
oxygen as oxidant, and explore the effect of composition of catalysts on
oxidation of cyclohexene.
2. Experimental
2.1. Catalyst preparation
K-OMS-2 was prepared according to procedures described in the
literature [24]. Potassium permanganate solution in deionized water
(0.4 mol L−1, 225 mL) was added to a mixture of manganese sulfate
hydrate solution (1.75 mol L−1, 67.5 mL) and concentrated nitric acid
(6.8 mL) in a 500 mL flask fitted with a reflux condenser. The slurry
was refluxed for 24 h, then filtered and washed with deionized water
several times. The catalyst was dried at 120 °C overnight before use.
K+ ions were then exchanged with La3+ ions by ion-exchanging K-
OMS-2 with La(NO3)3·6H2O to obtain La-OMS-2. 2.0 g K-OMS-2 was
added to 15 mL La(NO3)3·6H2O methanol solution. The slurry was
stirred vigorously at room temperature for 8 h. After filtration, the
solid was washed with deionized water several times. The product
⁎
Corresponding author. Tel.: +86 571 8795 2554; fax: +86 571 8795 1742.
E-mail address: zhumingqiao@zju.edu.cn (M.-Q. Zhu).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.09.014

198
Z.-Y. Cai et al. / Catalysis Communications 12 (2010) 197–201
was dried at 120 °C overnight and then calcined in air at 300 °C for 3 h.
The final material was named La-OMS-2 (0.12) with the number in
brackets indicating the molar concentration (mol L−1) of La
(NO3)3·6H2O methanol solution.
Au/OMS-2 and Au/La-OMS-2 catalysts with varied gold loadings
were prepared by the DP procedure. 2.0 g K-OMS-2 or La-OMS-2 was
stirred in 0.5 mmol L−1 HAuCl4 aqueous solution for 1 h at 60 °C . The pH
of the slurry was kept at 10 adjusted with 1.0 mol L−1 NaOH solution.
After filtration, the resulting solid was first washed with 15 mL
ammonia liquor (4.0 mol L−1) and then washed twice using 20 mL
deionized water to remove Cl− ions. Finally, the resulting solid was
dried at 120 °C overnight and calcined at 300 °C for 3 h, Au/OMS-2 or
Au/La-OMS-2 was obtained.
6 MPa). In a typical oxidation reaction, 20 mL cyclohexene and 0.20 g
catalyst were placed into the autoclave. The reactor was then heated
to the desired reaction temperature in oil bath under oxygen pressure
with a magnetic stirrer. After reaction, the reactor was cooled to room
temperature and the liquid phase was separated from the reaction
slurry. The solid catalyst was washed by acetone and dried at 120 °C
for 5 h. The liquid samples were analyzed by gas chromatography
(GC) with an SE-54 capillary column (30 m×0.32 mm×0.5 μm) and a
flame ionization detector (FID). N-heptane was used as the internal
standard for product analysis.
3. Results and discussion
3.1. Catalyst properties
2.2. Catalyst characterization
The scanning electron micrographs show a fibrous needle-like
morphology of K-OMS-2 and La-OMS-2 in Fig. 1. The morphology of K-
OMS-2 is similar to that of La-OMS-2. The result shows that the tunnel
cation substitution has no effect on the morphology. TEM images in
Fig. 2 show that the gold particle sizes of Au/OMS-2 and Au/La-OMS-2
are around 10–20 nm.
Scanning electron micrographs were taken on a Zeiss ULTRA 55
Gemini field emission scanning microscope with a Schottky Emitter at
an accelerating voltage of 3 kV. Transmission electron microscopy
(TEM) images were obtained on a JEM-200CX at 80 kV. The structure
of these materials was studied by X-ray diffraction experiments. A D/
max-RA instrument with CuKα radiation with a beam voltage of 40 kV
and a beam current of 40 mA was used to collect the X-ray data. The
chemical compositions of the synthesized catalysts were determined
by an energy dispersive X-ray analysis (EDX) on a Philips Oxford 7426
EDX spectrometer. This analysis provides a measure of the amount of
La3+ in the tunnel of K-OMS-2 which was exchanged with K+ cations.
The X-ray diffraction patterns (Fig. 3) of K-OMS-2, Au/La-OMS-2
and regenerated Au/La-OMS-2 show that the catalysts are pure phases
and comparable to standard OMS-2 materials (JCPDS file #29-1020).
No typical signal of gold at 38.19° and 44.42° is observed in Au/La-
OMS-2 in Fig. 3. This is due to the low loadings (0.75 %) and high
dispersions of gold, which are out of detection limit for the
diffractioner. The XRD patterns show that the catalyst retains its
structure after ion-exchange with La3+, loading with Au, and
regeneration of Au/La-OMS-2.
2.3. Catalytic testing
The catalytic experiments for cyclohexene oxidation were carried
out in a PTFE-lined autoclave (Capacity=30 mL, pressure maximum
From the EDX analysis data the amount of K+ cation in OMS-2 and
Au/La-OMS-2 materials was obtained as shown in Table 1. The
Fig. 1. SEM micrograph of K-OMS-2 (a), (b) and La-OMS-2 (c), (d).

Z.-Y. Cai et al. / Catalysis Communications 12 (2010) 197–201
199
Fig. 2. Typical TEM images of Au/OMS-2 (0.75% Au) (a) and Au/La-OMS-2 (0.24) (b).
percentage of K+ exchanged with La3+ was calculated based on the
mass fraction % of K and La assuming that the mass fraction % of Mn
does not decrease in the ion-exchange process. The error of La content
and K content is b 0.5%. The content of gold was also obtained from
the EDX analysis; we could confirm that the DP method was effective
to prepare Au/OMS-2 and Au/La-OMS-2 catalysts. But the loadings of
gold were too low to get the accurate quantities, which resulted in
invalidity of the content of gold in quantitative analysis (Table 2).
the results show that such a small quantity of gold loading can
significantly enhance the catalytic performance. In the synthesis of the
Au/OMS-2 catalyst, loading gold on the support proceeds by a direct
deposition of [Au(OH)x]−(x−3) (x≥3) making gold disperse on a
molecular scale. Moreover, the subsequent activation of the catalyst
precursor at a low temperature might suppress the conglomeration of
gold nanoparticles. These lead to a high dispersion as well as a fine
size control of gold on the support, thus resulting in a high activity for
cyclohexene oxidation in spite of a relatively low gold loading of the
catalyst. Furthermore, a surprisingly high conversion up to 39.9% was
obtained over Au/OMS-2 (Au, 0.75%) as the reaction time was
extended to 24 h, and the selectivity of ∑C6 remains 89.9%.
3.2. Catalytic performance
In preliminary experiments, K-OMS-2 and various Au/OMS-2 were
tested for their catalytic activity for cyclohexene oxidation, the
products obtained were cyclohexene oxide, 2-cyclohexene-1-ol, and
2-cyclohexene-1-one, and they are collectively called ∑C6. Table 3
shows the conversion of cyclohexene and selectivity of products.
Compared with the uncatalyzed oxidation reaction (Table 3, Entry 1
and 7), both K-OMS-2 and Au/OMS-2 exhibit a good catalytic
performance. The conversion was only 19.5% with K-OMS-2 as the
catalyst while the reaction time was 12 h (Table 3, Entry 2), and the
Au/OMS-2 catalyst afforded much better conversion with higher
selectivity of ∑C6 at the same reaction condition (Table 3, Entry 3 to
Entry 6). Although the effect of the Au content is not obvious under
the reaction condition while the Au content varies from 0.5% to 1.25%,
To study the effect of La content of Au/La-OMS-2 on catalytic
performance in cyclohexene oxidation, reactions were done at 80 °C
and 0.4 MPa for 24 h (Table 4). The conversion of cyclohexene rises
with the increase of the La content of Au/La-OMS-2. Compared to K-
Table 1
Amount of La3+ exchange in the synthesized catalysts.
Catalyst
La content
(wt.%)
K content
(wt.%)
La3+/K+
(mol:mol)
a
(mol L−1 La3+
)
K-OMS-2
0
4.6
4.9
4.4
4.5
5.5
4.6
0
Au/La-OMS-2 (0.06)
Au/La-OMS-2 (0.12)
Au/La-OMS-2 (0.18)
Au/La-OMS-2 (0.24)
Au/La-OMS-2 (0.24)b
4.5
5.1
6.6
9.2
6.2
0.26
0.33
0.41
0.47
0.38
OMS-2
(c)
a
The number in brackets indicate the molar concentration (mol L−1
) of La
(NO3)3·6H2O methanol solution.
b
The catalyst was regenerated after used for 4 times.
(b)
(a)
Table 2
Content of gold in the synthesized catalysts.
Catalyst (wt.% Au)
Au content(wt.%)
Theoretical
Error ( %)
Actual
Au/OMS-2 (0.5)
Au/OMS-2 (0.75)
Au/OMS-2 (1.0)
Au/OMS-2 (1.25)
0.50
0.75
1.00
1.25
0.11
0.93
0.96
1.26
0.69
0.54
0.47
0.68
20
30
40
50
60
Fig. 3. XRD patterns of K-OMS-2 (a), Au/La-OMS-2 (b) and regenerated Au/La-OMS-2 (c).

200
Z.-Y. Cai et al. / Catalysis Communications 12 (2010) 197–201
Table 3
Table 5
Effects of Au contents and preparation methods of catalysts on catalytic performance in
Catalytic performance of fresh and regenerated Au/La-OMS-2 catalysta.
cyclohexene oxidationa.
Cycles Catalystb
Time Conversion Selectivity (%)
Entry Catalyst
(Au (wt.)%)
Time Conversion Selectivity (%)
(h)
(%)
Cy-oxide Cy-nol Cy-one ∑C6
(h)
(%)
Cy-oxide Cy-nol Cy-one ∑C6
1
2
3
4
1[6]
4[6]
Au/La-OMS-2 24
Au/La-OMS-2 24
Au/La-OMS-2 24
Au/La-OMS-2 24
48.0
47.1
46.0
44.0
52.2
41.5
2.5
2.8
2.3
2.5
3.1
3.7
40.3
40.3
36.7
34.6
14.0
11.2
44.0
42.0
47.7
49.0
71.2
74.2
86.2
85.1
86.7
86.1
88.3
89.1
1
2
3
4
5
6
7
8
9
No
K-OMS-2
Au-OMS-2 (0.5)
Au-OMS-2 (0.75) 12
Au-OMS-2 (1.0) 12
Au-OMS-2 (1.25) 12
No
K-OMS-2
12
12
12
12.1
19.5
27.1
28.6
26.2
25.8
21.9
28.9
39.9
8.1
4.1
4.1
4.4
4.4
4.9
7.1
3.2
2.5
15.4
41.4
40.0
37.7
36.5
37.9
26.8
42.7
35.5
21.7
42.3
46.1
48.1
49.1
46.7
35.0
42.6
51.9
45.2
87.7
90.4
90.2
89.9
89.4
68.9
88.5
89.9
CrMCM-41c
CrMCM-41c
24
24
a
Reactions were done with 0.20 g of catalyst, 20 mL cyclohexene, at 80 °C, and the
pressure of oxygen 0.4 MPa.
24
24
b
Au content of the catalyst is 0.75 wt. % theoretically, and the molar concentration
Au-OMS-2 (0.75) 24
(mol L−1) of La(NO3)3·6H2O methanol solution is 0.24.
a
c
All reactions were done with 0.20 g of catalyst, 20 mL cyclohexene, at 80 °C, and the
Reaction conditions: cyclohexene, 1 g; catalyst, 20 mg; O2 pressure, 1 atm; time,
pressure of oxygen 0.4 MPa.
24 h; temperature, 343 K.
OMS-2 (Table 3, Entry 8) and La-OMS-2 (Table 4, Entry 1), Au/La-OMS-2
presented a much better catalytic activity (Table 4, Entry 2 to 5). When
the reaction was done with K-OMS-2 (Table 3, Entry 8), 28.9%
conversion was obtained, which increased to 38.7% with La-OMS-2
(0.24) (Table 4, Entry 1) as the catalyst, and the conversion was up to
48.0% on Au/La-OMS-2 (0.24) (Table 4, Entry 5).
oxygen initially formed 2-cyclohexene-1-hydroperoxide as shown in
Scheme 1. 2-Cyclohexene-1-hydroperoxide was unstable and easily
formed other products. There were three main paths. As the
selectivity of cyclohexene oxide was 2.0%–4.0%, we could infer that
step II played a minor role in the reaction. Comparing the selectivity of
2-cyclohexene-1-one with that of 2-cyclohexene-1-ol, it is inferred
that there was 5%–15% of 2-cyclohexene-1-hydroperoxide dehydra-
tion through step III. Most of 2-cyclohexene-1-one and 2-cyclohex-
ene-1-ol were obtained from step I. The radical-chain sequence
mechanism of step I was proposed as follows (Scheme 2).
The stability of a catalyst is an important consideration for the
industrial application. As Au/La-OMS-2 (0.24) possesses the highest
La doping among the doped K-OMS-2 supported catalysts and has a
good catalytic performance, Au/La-OMS-2 (0.24) was used for the
recycling test to evaluate the stability of the catalyst. The catalyst was
recovered by filtration after the reaction and regenerated by a simple
regeneration procedure. By washing with acetone and drying at
120 °C for 5 h, Au/La-OMS-2 (0.24) restored the original activity. As
shown in Table 5, there is only a slight decrease of conversion as well
as selectivity of 2-cyclohexene-1-ol and 2-cyclohexene-1-one after
the first run. Moreover, the activity and the selectivity of the catalyst
were well retained after subsequent three runs. This is because
leaching was minimal in the oxygen and solvent-free system
comparing to the reaction carried out in an organic solvent with
other oxidant. A similar trend has been observed using CrMCM-41 in a
solvent-free/dioxygen condition [6]. The result of La3+/K+ (Table 1)
shows that the composition of Au/La-OMS-2 changed only a little after
four runs of the reaction. This suggests that the supported gold
catalyst Au/La-OMS-2 is quite stable under the reaction conditions.
In order to explain how the oxidation occurred, and how the major
products were formed, we also speculated the oxidation mechanism.
According to the literature [20,25,26] and the analysis of the relevant
factors of the oxidation, the oxidation of cyclohexene with molecular
OH
O
+
cat I
OOH
OH
+
+ O2
+
O
II
III
O
H2O
+
Scheme 1. Mechanism of cyclohexene oxidation.
Table 4
Effect of La contents of catalysts on catalytic performance in cyclohexene oxidationa.
Entry Catalyst
(mol L°1 La3+
Time Conversion Selectivity (%)
OO
b
OOH
OO
)
(h)
(%)
Cy-oxide Cy-nol Cy-one ∑C6
cat
1
2
La-OMS-2 (0.24) 24
Au/La-OMS-2
(0.06)
38.7
41.2
2.8
2.7
40.0
40.9
47.0
47.4
89.7
91.0
+
H
O
24
24
24
24
3
4
5
Au/La-OMS-2
(0.12)
Au/La-OMS-2
(0.18)
Au/La-OMS-2
(0.24)
43.8
46.7
48.0
3.2
2.1
2.5
42.7
40.0
40.3
41.9
45.0
44.0
87.8
87.1
86.2
O
O O
OH
O
2
+
a
All reactions were done with 0.20 g of catalyst, 20 mL cyclohexene, at 80 °C, and the
pressure of oxygen is 0.4 MPa.
b
Au content of the catalysts was kept constant, Au (wt.%)=0.75.
Scheme 2. Radical-chain sequence mechanism of step I.

Z.-Y. Cai et al. / Catalysis Communications 12 (2010) 197–201
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In summary, the OMS-2 supported gold catalyst with low gold
loading and La doping has shown high efficiency for cyclohexene
oxidation to 2-cyclocyhexene-1-ol and 2-cyclocyhexene-1-one using
molecular oxygen in the solvent-free system under mild conditions.
The good activity and stability of the catalyst Au/La-OMS-2 are
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of the La-OMS-2 molecular sieve. Au/La-OMS-2 may be useful in the
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Acknowledgments
This material is based upon work funded by the financial support
by Zhejiang Provincial Natural Science Foundation of China under
Grant No. Y4080247 and No. R4090358.
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