Advances in Chemical Engineering and Science, 2011, 1, 15-19
doi:10.4236/aces.2011.11003 Published Online January 2011 (
Copyright © 2011 SciRes. ACES
Halloysite Nanotubes Supported Gold Catalyst for
Cyclohexene Oxidation with Molecular Oxygen
Zhen-Yu Cai1, Ming-Qiao Zhu1*, Huan Dai1, Yi Liu1, Jian-Xin Mao2, Xin-Zhi Chen1, Chao-Hong He1
1Department of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou, China
2Department of Chemistry, Zhejiang University, Hangzhou, China
Received January 6, 2011; revised January 18, 2011; accepted January 20, 2011
The selective oxidation of cyclohexene to 2-cyclohexene-1-ol and 2-cyclohexene-1-one has been investi-
gated over Au/HNTs (HNTs: halloysite nanotubes) catalysts with molecular oxygen in a solvent-free system.
The catalysts were prepared by deposition precipitation method and characterized by ICP-AES, TEM and
XRD. The results show that the catalytic performance of Au/HNTs is quite well and the catalytic activity
over recycled catalyst remains highly. Moreover, the nano-size effect of gold is also reported for the reaction.
Keywords: Gold Catalyst, HNTs, Cyclohexene Oxidation, Oxygen
1. Introduction
The catalytic oxidation of hydrocarbons into value-added
oxygenated derivatives is still a challenge in modern
chemistry and industry world [1-3]. In particular, the
oxidation of cyclohexene is often inefficient as there is a
C = C bound and four α-H atoms in the cyclohexene
molecule. Oxidation of cyclohexene is an important
method for the synthesis of chemical intermediates like
2-cyclohexene-1-ol and 2-cyclohexene-1-one in the
manufacture of high-value pharmaceuticals [4]. A greater
demand for these oxidation products and increased envi-
romental concerns warrant the introduction of catalytic
systems using heterogeneous catalyst and the environ-
mentally friendly oxidants such as molecular oxygen or
hydrogen peroxide [5]. The use of H2O2 is atom efficient
and the only by-product is water, but the relatively high
cost of H2O2 severely hinders its wide application in
catalytic oxidation [6]. On the other hand, catalytic sys-
tems using oxygen as the oxidant instead resulted in
three important advantages: the facility to separate the
catalyst after the reaction, lower energy costs and a
higher stability of the irreversible reaction of over-
oxidantion products [7,8]. Therefore, oxidation of
cyclohexene with oxygen under solvent-free condition
would be valuable.
In recent years, an increasing interest has been di-
rected to the catalytic potential of gold catalysts [9-12].
Supported gold catalysts have been extensively studied
for a wide range of oxidation reactions including CO
oxidation [13], propylene epoxidation [14], the direct
synthesis of hydrogen peroxide from oxygen and hydro-
gen [15,16], oxidation of cyclohexane to KA oil [17-19],
etc.. Particularly, the partial liquid-phase cyclohexene
oxidation using gold catalysts including Au/C and
Au/CNTs makes gold even more attractive [20,21]. Un-
fortunately, as for the Au/C catalyst, it has good catalytic
performance only with the addition of special organic
solvent [20]. The performance of Au/CNTs catalyst is
influenced by the amount of TBHP [21]. Compared
Au/C and Au/CNTs, we can see that the structure of the
carrier has great effect on the catalytic performance of
supported gold. As silica and Al2O3 are quite common
industrial materials as catalysts support material because
of their relative stability, high surface area and low price.
Au/Al2O3 and Au/SiO2 are quite effective in cyclohexane
oxidation [8,22], but there is little report about alumi-
nosilicate-supported Au catalyst. HNTs (halloysite
nanotubes) is a special kind of aluminosilicate. The ob-
jective of this work is to report catalyst Au/HNTs of very
low metal loadings and the effect of nano-size of gold on
catalytic performance for the selective oxidation of
cyclohexene using molecular oxygen in a solvent-free
2. Experimental
Au/HNTs catalysts with varied gold loadings were pre-
Copyright © 2011 SciRes. ACES
pared by the deposition-precipitation procedure. 2.0 g
HNTs support 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 4.0 mol·L-1 ammonia solu-
tion. After filtration, the resulting solid was washed
twice using 20 mL of deionized water for each wash to
remove Cl- ion. Finally, the resulting solid was dried at
80˚C overnight and calcined at 300˚C for 3 h, Au/HNTs
was obtained.
The chemical compositions of the samples were de-
termined by ICP-AES (IRIS Intrepid XPS). 0.1 g of
the solid samples was leached by 4 mL of aqua regia for
4 h and the leaching liquid was collected for determining
gold. The specific surface areas were obtained by the
Brunauer-Emmett-Teller (BET) method using an Auto-
sorb-1-C instrument. 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. Transmission electron microscopy (TEM) images
were obtained on a JEM-1230 at 80 kV. The samples
were dispersed in ethanol and then dropped on cop-
per-coated grid. The gold particle size distribution was
obtained by measuring the diameter of metal particles.
The catalytic experiments for cyclohexene oxidation
were carried out in a PTFE-lined autoclave (Capacity =
30mL, pressure maximum 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 con-
stant stirring with a magnetic stirrer. After the reaction
was over, 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 80˚C for 3 h. Reactants and products were iden-
tified by gas chromatography-mass spectroscopy
(GC-MS) as well as by comparing retention time to re-
spective standards in GC traces. GC analyses were done
using a GC 1690 instrument with a flame ionization de-
tector (FID). The column used was an SE-54 capillary
column (30 m × 0.32 mm × 0.5 μm). N-Heptane was used
as an internal standard for product analysis.
3. Results and Discussion
3.1. Catalyst Characterization
The actual gold contents and specific surface areas of
samples were shown in Table 1. With the increase of
gold loadings, the specific surface area of Au/HNTs
catalysts differs only slightly, which means loading gold
has little effect on that.
Figure 1 gives the XRD patterns of the HNTs,
Au(0.37%)/HNTs, Au(0.80%)/HNTs and Au(1.35%)
20 4060 80
2 θ (°)
Figure 1 XRD patterns of HNTs (a), Au(0.37%)/HNTs (b),
Au(0.80%)/HNTs (c), Au(1.35%)/HNTs (d).
Table 1. Au content, specific surface area of HNTs and
supported gold catalysts.
Au content (wt. %)
Samples Theoretical Actual
HNTs 60
Au/HNTs 0.5 0.37 57
Au/HNTs 1.0 0.80 58
Au/HNTs 1.5 1.35 58
/HNTs, respectively. The typical signals of gold at 38.19°,
44.42°and 64.57° were observed and became more obvi-
ous in steps from Figure 1 (b-d), which indicated that the
particle size of gold increased according to the increasing
of gold content of the catalyst.
Figure 2 shows the typical TEM images of supported
Au/HNTs catalysts and the black round dots in the im-
ages are gold particles. Although gold is unevenly dis-
tributed and the particle size is discrepancy in the same
sample, it is easy to see that as the gold loading changes
from 0.37% to 1.35%, the gold particles become bigger,
agreeing well with the results of Figure 1, which means
the gold loading greatly affects the particle size.
3.2. Catalytic Oxidation of Cyclohexene
In preliminary experiments, an uncatalyzed oxidation
reaction was carried out under the typical reaction condi-
tions as shown in Table 2 (Entry 1). There were four
main products could be obtained: cyclohexene oxide,
2-cyclohexene-1-ol, 2-cyclohexene-1-one and cyclohex-
ane-1, 2-diol, and their selectivity was 8.1%, 15.4%,
21.7% and 23.1% respectively. Moreover, the activity of
pure HNTs support was also studied under the same
conditions (Table 2, Entry 2). Compared with uncata-
Copyright © 2011 SciRes. ACES
Figure 2. TEM images of Au/HNTs catalysts of (a) 0.37%,
(b) 0.80%, (c) 1.35% Au.
Table 2. Effect of Au loadings on catalytic performance in
cyclohexene oxidationa.
Selectivity (%)
(wt.% Au)
sion (%) Cy-oxide Cy-ol Cy-one Cy-ol + Cy-one
Nob 12.1 8.1 15.4 21.7 37.1
HNTs 16.9 6.5 29.8 43.4 73.2
(0.37% Au) 25.9 4.0 32.5 46.9 79.3
(0.80% Au) 29.5 3.5 35.5 49.0 84.5
(1.35% Au) 21.2 4.1 34.3 48.0 82.3
a All reactions were done with 0.20 g of catalyst, 20 mL cyclohexene,
at 80˚C, 12 h, and the pressure of oxygen is 0.4 MPa. b The selectivity
of cyclohexane-1,2-diol is 23.1%.
lyzed reaction, conversion of cyclohexene increased
4.8% over HNTs while the distribution of product
changed a lot. No cyclohexane-1,2-diol was detected in
the product as the selectivity of 2-cyclohexene-1-ol and
2-cyclohexene-1-one increased to 29.8% and 43.4% re-
The catalytic performance of Au/HNTs catalysts with
different gold loadings was investigated for cyclohexene
oxidation using molecular oxygen as an oxidant in a sol-
vent-free system (Table 2). In this work, the best cata-
lytic performance is Au(0.80%)/HNTs, which presents a
conversion of 29.5%, a little better than Au(0.37%)/
HNTs. As gold content increased from 0.80% to 1.35%,
a sharp decrease of cyclohexene conversion is evident.
This phenomenon may result from the different quanti-
ties of active sites of the catalysts. There existed an ap-
parent nano-size effect of gold in cyclohexene oxidation.
From the TEM images, we can see that the gold particles
grow bigger as the gold content increasing. The particle
sizes of Au(0.37%)/HNTs are 10nm, Au(0.80%)/
HNTs are 20nm, Au(1.35%)/HNTs are 20~40 nm.
There are much more gold particles smaller than 10 nm
of Au(0.80%)/HNTs comparing with Au(0.37%)/HNTs
and Au(1.35%)/HNTs, and the Au(0.80%)/HNTs has
better catalytic performance. Therefore, we infer that the
gold particles 10nm could be more active.
As Au/HNTs (0.80% Au) shows the best results with
respect to the conversion and the selectivity to the two
desired oxygenates, it is employed to investigate the
progress of the reaction with time under the typical con-
ditions. As shown in Figure 3, the oxidation of cyclo-
hexene make a large progress from 6 h to 12 h, affording
a conversion of 29.5% and 84.5% selectivity to the two
desired oxygenates at 12 h. It is also evident that the
oxidation reaction gradually slowdowns with time, sug-
gesting a gradual loss of catalytic activity of the catalyst.
We believe this could be ascribed to a strong affinity of
HNTs with the products formed increasing in the reac-
tion, through which the adsorption of apolar cyclohexene
on the catalyst can be suppressed.
Recycling tests were performed using Au/HNTs
(0.80% Au) under the typical reaction conditions for 12h,
and the results are given in Figure 4. Both the conver-
sion and the overall selectivity to two oxygenates are
well retained with a slightly enhanced selectivity to
2-cyclohexene-1-one and a little decreased selectivity to
2-cyclohexene-ol, suggesting a high stability of the cata-
In order to explain how the oxidation was occurred,
and how the major products were formed, we also
speculated the oxidation mechanism. The oxidation of
cyclohexene with molecular oxygen initially formed
2-cyclohexene-1-hydroperoxide [23,24]. 2-Cyclohexene-
Copyright © 2011 SciRes. ACES
6 121824
Re actio n time (h)
Conversion (%)
Selectivity (%)
Figure 3. Cyclohexene oxidation over Au/HNTs (0.80% Au)
with different reaction time. (Cyclohexene, ●∑C6, 2-
cyclohexene-1-ol, 2-cyclohexene-1-one).
Conversion or selectivity (%)
Figure 4. Results of recycling test over Au/HNTs (0.80%
Au). (Cyclohexene, ●∑C6, 2-cyclohexene-1-ol, 2-
1-hydroperoxide was unstable and easily formed other
products as shown in Scheme 1.
4. Conclusions
In summary, halloysite nanotubes supported gold cata-
lysts have been prepared successfully by the deposi-
tion-precipitation procedure. The nano-size effect of gold
was found in the reaction. The results show that
Au/HNTs catalysts with the gold particles smaller than
10 nm are highly active for the selective cyclohexene
oxidation to 2-cyclohexene-ol and 2-cyclohexene-one
under relatively mild conditions.
5. Acknowledgments
This material is based upon work funded by financial
support by Zhejiang Provincial Natural Science Founda-
Scheme 1. Radical-chain sequence mechanism of 2-Cyclohexene-
1-hydroperoxide to form other products.
tion of China under Grant No. Y4080247 and No.
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