Preparation of nano-CuO-loaded halloysite nanotubes with high catalytic activity for selective oxidation of cyclohexene

Article abstract of DOI:10.1016/j.cclet.2015.08.002

A facile preparation method of nano-CuO catalysts, assembled in the hollow nanotube of halloysite nanotubes (HNTs), was developed. The characterizations of XRD, TEM, SEM, BET, XRF and FT-IR were used to analyze the structure and properties of the nano-CuO

Full text of DOI:10.1016/j.cclet.2015.08.002

G Model
CCLET-3405; No. of Pages 4
Chinese Chemical Letters xxx (2015) xxx–xxx
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Chinese Chemical Letters
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Original article
Preparation of nano-CuO-loaded halloysite nanotubes with high
catalytic activity for selective oxidation of cyclohexene
*
Zhi-Lin Cheng , Wei Sun
College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A facile preparation method of nano-CuO catalysts, assembled in the hollow nanotube of halloysite
nanotubes (HNTs), was developed. The characterizations of XRD, TEM, SEM, BET, XRF and FT-IR were
used to analyze the structure and properties of the nano-CuO/HNT loaded catalyst. The XRD patterns
indicated that the CuO nanoparticles on HNTs were monoclinic phase. The TEM-EDX and SEM images
confirmed that most of nano-CuO catalysts with the crystal size of ca. 20 nm were assembled into the
hollow nanotube of HNTs. The catalytic performance of the nano-CuO/HNT catalysts was evaluated by
using selective oxidation of cyclohexene. The reaction temperature and recycling times were
investigated. The results reveal that the nano-CuO/HNT catalysts exhibit an excellent catalytic oxidation
performance for selective oxidation of cyclohexene to 2-cyclohexene-1-one.
Received 10 June 2015
Received in revised form 2 July 2015
Accepted 8 July 2015
Available online xxx
Keywords:
Cyclohexene
CuO
Selective oxidation
HNTs
ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
catalysts offer excellent activity, which exhibited 21% cyclohexene
conversion and 84% selectivity toward epoxide [19]. The effect of
In the past, the
a
,
b
-unsaturated ketones found broad applica-
the size of gold nanoparticle-supported SiO2 catalyst on their
catalytic activity in the aerobic oxidation of cyclohexene revealed
that Au particles with a larger size than 2 nm were active in
cyclohexene oxidation [20]. CuO nanorods efficiently catalyzed
selective oxidation of cyclohexene to 2-cyclohexene-1-one (99%
conversion and 95% selectivity) using TBHP as oxidant [21]. A
trimetallic, hybrid, nano-mixed oxide (RuO2/Co3O4/CeO2) catalyst
was used to catalyze selective oxidation of cyclohexene to
2-cyclohexene-1-one (97.7% conversion and 95% selectivity)
[22]. More recently, the active nitrogen-doped CNTs (NCNTs), as
metal-free catalysts, have exhibited an excellent activity in the
allylic oxidation of cyclohexene in the liquid phase using oxygen as
oxidant [23]. Since the active metal, or metal oxide particles, are
prone to aggregate, resulting in difficultly being separated and
recovered from the solution after the reaction, therefore, the
supported technology was developed for the sake of blocking the
aggregation of the active metal or metal oxide particles. Based on
these reasons, the support of the loaded catalysts was mainly
focused on catalysts, such as TS-1 [11], MCM-41 [12], metal–
organic frameworks [11–18], SiO2 [19], CNTs [23] and so on.
tions in many fields, for example, as chemosensors for metal ions
[1], intermediates of many natural products and bioactive
compounds [2] and building blocks and products of pharmaceuti-
cal interest [3].
Cyclohexene is a promising starting material in the one-step
production of adipic acid with H2O2 as oxidant [4,5]. Cyclohexene
can be epoxidized to form epoxycyclohexane using hydroperoxide
as oxidant [6,7]. The allylic oxidation of cyclohexene is also
industrially relevant because it produces a,b-unsaturated alcohols
and ketones, which are important intermediates in the fragrance
industry and organic synthesis [8]. Numerous studies have
revealed that the epoxidation of cyclohexene is catalyzed by
homogeneous Fe, Mn, Cu, Cr catalysts, or their immobilized
versions [9,10]. Titanosilicate molecular sieves and metal–organic
frameworks are promising solid catalysts for the epoxidation [11].
Mesoporous silica-supported Fe, Co, and Au catalysts have been
reported as active for the aerobic oxidation of cyclohexene [12].
Metal–organic frameworks containing transition metals (e.g., Fe,
Cr, Cu, V, etc.) are regarded as active catalysts for the aerobic allylic
oxidation of cyclohexene [13–18]. The VO2-supported SiO2
Halloysite nanotubes (HNTs) 0.2–2
mm in length, 40–70 nm in
outer diameter and 10–30 nm in inner diameter, as two-layered
aluminosilicate clay, have exhibited promising results as a catalyst
support due to their inherent hollow nanotube structure and
different outside and inside chemistry [24,25]. A number of studies
*
Corresponding author.
E-mail address: zlcheng224@126.com (Z.-L. Cheng).
http://dx.doi.org/10.1016/j.cclet.2015.08.002
1001-8417/ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Z.-L. Cheng, W. Sun, Preparation of nano-CuO-loaded halloysite nanotubes with high catalytic activity
for selective oxidation of cyclohexene, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.08.002

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have demonstrated that HNTs have been of great interest in
applications as nanomaterials over different fields, particularly on
catalysis. As proved, nano-CuO catalysts were highly active for
selective oxidation of cyclohexene to 2-cyclohexene-1-one [21].
The study of HNTs supported nano-CuO catalysts for selective
oxidation of cyclohexene has yet to been found.
Herein, this work presents the facile preparation method and a
series of characterizations of the nano-CuO-loaded HNT catalysts
using the impregnation method. The catalytic performance of the
nano-CuO/HNT catalysts was evaluated by the catalytic oxidation
of cyclohexene to 2-cyclohexene-1-one.
2. Experimental
Fig. 1. XRD patterns of HNTs, nano-CuO and nano-CuO/HNTs.
Catalyst preparation: The HNT mineral powder, as clay mineral,
was purchased from Tianjin Linruide Co., Ltd. in China. The original
HNT nanomaterials are approximately 0.5
mm in length and 27 nm
can be indexed to the characteristic peaks of the halloysite, as
shown in Fig. 1-HNTs [26]. But for nano-CuO/HNTs sample, there
appears five fresh peaks in the XRD pattern of nano-CuO/HNTs
besides the significantly reduced feature peaks of HNTs. Compared
to nano-CuO, these fresh peaks can be indexed to the main
characteristic peaks of the monoclinic phase of CuO, which is in
good accordance with JCPDS card No. 48-1548. The crystal size of
CuO particles on HNTs as evaluated by the Scherrer formula is
19.2 nm. Compared to HNTs, the characteristic peaks of HNTs in
the nano-CuO/HNT pattern are obviously reduced, which is
ascribed to the transformation of crystal-phase structure of the
tube wall of HNTs to the amorphous phase during calcining at
500 8C. Table 1 gives the compositions of HNTs and nano-CuO/
HNTs. The loaded amount of nano-CuO is about 8.4%.
The TEM-EDX and SEM photos of nano-CuO/HNTs are displayed
in Fig. 2. The EDX spectrum (inset) indicates that the nanoparticles
in hollow nanotube of HNTs contain a large concentration of the Cu
element, thereby confirming the existence of CuO. The CuO
particles are of the size of 20 nm, which is closer to the calculated
value achieved by XRD. As shown in the SEM photo, no
independent particles are found in outer area of HNTs. These
results indicate that the impregnation method developed in this
work is highly efficient on assembling nanoparticles in the hollow
nanotubes of HNTs. The BET data of HNTs and nano-CuO/HNTs are
listed in Table 2. Owing to nanoparticles occupying in nanotube of
HNTs, the specific surface area, pore volume and average pore
diameter of all nano-CuO/HNTs are decreased.
in inner diameter. Firstly, the HNT powder was dried in an oven at
120 8C before use. Next, the saturated solution of Cu(NO3)2 was
prepared at room temperature, and then an amount of urea was
added to prepare the transparently mixed solution. Then, the
mixed solution was added dropwise to the hot HNT powder taken
from oven until the surface of HNT powder was moist. The moist
powder was transferred to an oven and treated at 120 8C for 6 h.
Finally, the sample was calcined at 500 8C for 3 h. The obtained
sample was denoted as nano-CuO/HNTs. The nano-CuO particles
were prepared by using sol-gel method in previous work [26].
Catalyst characterization: The morphology and element com-
position of samples was observed by a transmission electron
microscope (TEM) and energy-dispersive X-ray spectrometry
analysis (EDX) (Tecnai 12, Philips Co.). The surface morphology
was examined by a scanning electron microscopy (SEM) (S4800II,
Hitachi) at an acceleration voltage of 15 kV. The FT-IR spectrum
was analyzed by a Varian IFS66/S spectrometer. The BET properties
of the HNTs and catalysts were characterized by using
a
Quantachrome Autosorp 2000. The composition of HNTs and
catalysts was analyzed by LAB CENTER XRF-1800 X ray fluores-
cence spectrometer (Shimadzu Corp.).
Cyclohexene oxidation: The catalytic oxidation of cyclohexene
with tertiobutylhydroperoxyde TBHP (AR grade), as an oxidant,
was carried out in a two neck glass round-bottom flask equipped
with a magnetic stirrer and a reflux condenser. Typically, 10 mmol
of cyclohexene, 100 mL of acetonitrile, and 50 mg of the nano-CuO/
HNT catalysts were placed in the above flask and then stirred at a
stirring speed of 1000 rpm for several minutes. Then, a certain
amount of TBHP was rapidly added with stirring at the reaction
temperature for 6 h. The reaction products were identified by
comparison with authentic products and the course of reactions
was followed by gas chromatography–mass spectroscopy (GC–MS:
Trace DSQ II, Thermo Co. USA). After reaction completion, the
catalysts were calcined for regeneration at 500 8C for 3 h. The
oxidation reaction of cyclohexene employed CuO/HNTs catalyst is
presented in Scheme 1.
Fig. 3 shows the FT-IR spectra of HNTs, nano-CuO and nano-
CuO/HNTs catalysts. The nano-CuO/HNTs possess some typical
signals of HNTs, such as the deformations of Al–O–Si at 538 cmÀ1
,
Si–O–Si at 462 cmÀ1, and Si–O broad stretching band at about
1010 cmÀ1. However, the O–H groups of the inner hydroxyl groups
in HNTs at 909 cmÀ1 have disappeared, which may be assumed to
be due to the transformation of the crystal-phase structure of the
tube wall. This verifies the concluded results of XRD above. As to
the feature peaks of CuO at the stretching band of 584 cmÀ1 and the
lattice vibration of 530 cmÀ1, respectively, and the nano-CuO/
HNTs are only detected at the characteristic signal of 586 cmÀ1
.
3. Results and discussion
The catalytic activities of HNTs, nano-CuO and nano-CuO/HNT
catalysts as a function of reaction temperature are shown in Fig. 4.
As to the HNT catalysts for selective oxidation of cyclohexene to
2-cyclohexene-1-one, almost no catalytic activity is found.
Fig. 1 shows the XRD patterns of HNTs, nano-CuO and nano-
CuO/HNTs. For the HNTs sample, all of the observed peaks mainly
O
Table 1
HNTs/CuO
XRF analysis of chemical composition of HNTs and nano-CuO/HNTs.
+
O
TBHP
Composition (m/%)
SiO2
Al2O3
CuO
Other oxide
1
2
HNTs
50.95
46.35
40.76
38.35
0
8.29
6.90
Nano-CuO/HNTs
8.40
Scheme 1. Oxidation of cyclohexene by TBHP in the presence of CuO/HNTs catalyst.
Please cite this article in press as: Z.-L. Cheng, W. Sun, Preparation of nano-CuO-loaded halloysite nanotubes with high catalytic activity
for selective oxidation of cyclohexene, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.08.002

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Z.-L. Cheng, W. Sun / Chinese Chemical Letters xxx (2015) xxx–xxx
3
Fig. 2. TEM-EDX (inset) and SEM photos of nano-CuO/HNT catalysts (a) TEM, (b) SEM.
However, the nano-CuO catalysts exhibit a higher selective
oxidation performance, achieving an 84% maximum conversion
and 82% maximum selectivity for 2-cyclohexene-1-one at 70 8C.
But for the nano-CuO/HNT catalysts, as the reaction temperature
elevated to 70 8C, the conversion of cyclohexene and selectivity of
the target product of 2-cyclohexene-1-one both are remarkably
increased up to the maximum values of 99% and 98%, respectively,
exhibiting 15% higher than those of the nano-CuO catalysts. This
result can be further confirmed through GC diagram, as shown in
Fig. 4b. With the further increasing of reaction temperature, the
selectivity of the target product of 2-cyclohexene-1-one descends
to 65% at 80 8C, whereas the conversion of cyclohexene exhibits
little change. This is attributed to the increases of the by-products
at that reaction temperature. In addition, the nano-CuO/HNT
catalysts have a higher selectivity of the target product of
2-cyclohexene-1-one than that of CuO nano-rods reported by
Zhu et al. [21].
of nano-CuO catalysts, provides
a
native nano-space, thus
improving the dispersion of nano-CuO particles and preventing
their aggregation in the reaction. Furthermore, the structure and
composition properties of HNTs also produce an influence for
catalytic activity.
100
a
80
60
nano-CuO/HNTs conversion
nano-CuO/HNTs selectivity
HNTs conversion
HNTs selectivity
nano-CuO conversion
nano-CuO selectivity
40
20
0
Fig. 5 shows the recyclability of the nano-CuO/HNT catalysts. It
can be interestingly found that the nano-CuO/HNT catalysts via the
three-times-recycling still maintain the stabilizing conversion and
excellent selectivity. This suggests the nano-CuO/HNT catalysts are
desirable and promising catalysts applied in the selective oxidation
of cyclohexene to 2-cyclohexene-1-one.
50
60
70
80
Reaction temperature/oC
(1) 5.71
b
The reason the nano-CuO/HNT catalysts possess good catalytic
performance is that the nanotube structure of HNTs, in the forming
Table 2
(2)
4.45
BET data of HNTs and nano-CuO/HNTs.
5.13
Sample name
SBET (m2/g)
Pore volume
(cm3/g)
Average pore
4
6
8
10
diameter (nm)
time/min
HNTs
45
28
0.26
0.13
27
15
Nano-CuO/HNTs
Fig. 4. Catalytic activities of HNTs, nano-CuO and nano-CuO/HNT catalysts as a
function of reaction temperature (a) and GC diagram of the oxidation system of
cyclohexene after reaction at 70 8C (b).
nano-CuO/HNTs
100
% (conversion)
% (selectivity)
HNTs
80
60
40
20
0
586
536
909
538
462
nano-CuO
1010
584
536
4000
3500
3000
2500
2000
1500
1000
500
0
1
2
3
4
Wavenumber (cm-1)
Recycling times
Fig. 5. The recyclability of nano-CuO/HNT catalysts.
Fig. 3. FT-IR spectra of HNTs, nano-CuO and nano-CuO/HNT catalysts.
Please cite this article in press as: Z.-L. Cheng, W. Sun, Preparation of nano-CuO-loaded halloysite nanotubes with high catalytic activity
for selective oxidation of cyclohexene, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.08.002

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Acknowledgments
This work was supported by the Talent Introduction Fund of
Yangzhou University, Jiangsu Social Development Project (No.
BE2014613) and Six Talent Peaks of Jiangsu Province (No. 2014-
XCL-013). The authors also acknowledge the Project Funded by the
Priority Academic Program Development of Jiangsu Higher
Education Institutions. The data of this paper originated from
the Test Center of Yangzhou University.
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