CuO nanoclusters coated with mesoporous SiO2 as highly active and stable catalysts for olefin epoxidation

Article abstract of DOI:10.1039/c0jm04568c

A one-pot solvothermal synthetic method to prepare uniform CuO colloidal nanocrystal clusters (CNCs) with sizes of approximately 60 nm was developed. To enhance their stabilities, the CuO CNCs were coated with mesoporous SiO ₂ shells to form CuO CNCs@meso-SiO₂ nanocomposites. The CuO CNCs as well as the CuO CNCs@meso-SiO₂ composite catalyst were characterized by transmission electron microscopy, small-angle X-ray diffraction and N₂ adsorption-desorption methods. These results indicate that the SiO₂ shells have winding mesoporous channels with an average size of 3.7 nm, which are very favorable for mass transfer and catalytic reactions. This nanocomposite catalyst exhibited excellent activity and stability in olefin epoxidation reactions. The structure of the composite catalyst remained intact after eight consecutive runs, while pure CuO CNCs severely aggregated after only 1 run. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0jm04568c

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PAPER
CuO nanoclusters coated with mesoporous SiO2 as highly active and stable
catalysts for olefin epoxidation†
a
Chaoqiu Chen,ab Jin Qu,ab Changyan Cao,c Fang Niuab and Weiguo Song
*
Received 29th December 2010, Accepted 22nd February 2011
DOI: 10.1039/c0jm04568c
A one-pot solvothermal synthetic method to prepare uniform CuO colloidal nanocrystal clusters (CNCs)
with sizes of approximately 60 nm was developed. To enhance their stabilities, the CuO CNCs were
coated with mesoporous SiO2 shells to form CuO CNCs@meso-SiO2 nanocomposites. The CuO CNCs
as well as the CuO CNCs@meso-SiO2 composite catalyst were characterized by transmission electron
microscopy, small-angle X-ray diffraction and N2 adsorption-desorption methods. These results
indicate that the SiO2 shells have winding mesoporous channels with an average size of 3.7 nm, which are
very favorable for mass transfer and catalytic reactions. This nanocomposite catalyst exhibited excellent
activity and stability in olefin epoxidation reactions. The structure of the composite catalyst remained
intact after eight consecutive runs, while pure CuO CNCs severely aggregated after only 1 run.
trend, some strategies including alloying and encapsulation have
Introduction
been developed. Encapsulating nanoparticles in thermally stable
Morphology controlled transition metal oxide nanomaterials are
and chemically inert porous oxides shells can effectively prevent
promising materials in optical, electric and catalytic applica-
sintering of the nanoparticles by spatially separating the nano-
tions.1–4 In particular, cupric oxide (CuO) and cuprous oxide
particles. In addition, the porous structure of the porous coating
(Cu2O) have shown exciting properties as catalysts, solid-state
provides channels for the reaction species to reach the surface of
sensors and lithium-ion battery materials, owing to their low
the nanoparticles, thus allowing the catalytic reaction to occur.
band-gap energy, high catalytic activity, non-toxic nature and
The effectiveness of this strategy has been demonstrated by
abundant resources.5–13 Various CuO and Cu2O nano/micro
several papers for supported and unsupported metal nano-
materials have been synthesized to explore their properties and
particle catalysts.20–27 However, these researches are focused on
application possibilities.
noble metal nanoparticles. There are few studies on designing
Compared to bulk particles, nano-sized particles are much
aggregate-resistant systems with transition metal oxide nano-
more active and selective catalysts, due to their higher special
materials with a 3-dimensional structure.
surface areas and quantum-confinement effects.14–18 However,
In this study, a fast one-pot solvothermal process to prepare
a major obstacle for practical use of the nanoparticles as catalysts
uniform CuO colloidal nanocrystal clusters (CNCs) with a size of
is their tendency to aggregation under real reaction conditions.
approximate 60 nm was developed. Structural characterization
The nanoparticles are usually subject to sintering or coalescence
indicates that these CNCs consist of CuO nanocrystals. By
to minimize their surface energies, thus losing their catalytic
prolonged reaction time, the CuO CNCs were converted to Cu2O
activity during practical applications.19 In order to suppress this
hollow nanospheres. Encapsulating the CuO CNCs into meso-
porous SiO2 shells resulted in CuO CNCs@meso-SiO2 nano-
composites. The mesoporous SiO2 shell offers a physical shield to
aBeijing National Laboratory for Molecular Sciences (BNLMS), Institute
prevent the aggregation of the CuO CNCs. As a result, this
nanocomposite catalyst exhibited excellent activity and stability
in olefin epoxidation reactions. The composite catalyst remained
intact after eight runs. In sharp contrast, pure CuO CNCs lost
their morphology after only one run.
of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China.
E-mail: wsong@iccas.ac.cn; Fax: (+86)10-62557908; Tel: (+86)10-
62557908
bGraduate School of Chinese Academy of Sciences, Beijing, 100049, P. R.
China
cSchool of Materials Science and Engineering, Harbin Institute of
Technology, Harbin, 150001, P.R. China
† Electronic supplementary information (ESI) available: SEM images of
commercially available CuO and the sample obtained at the same
reaction conditions without the addition of surfactant PVP; TG profile
of CuO CNCs; X-ray diffraction patterns of samples obtained at
different reaction times; N2 adsorption and desorption isotherms of
CuO CNCs and Cu2O hollow nanospheres; TEM image of CuO
CNCs@meso-SiO2 nanocomposite used for 720 h under strong stirring.
See DOI: 10.1039/c0jm04568c
Experimental
Materials
Cupric acetate monohydrate ((CH3COO)2Cu$H2O), hexadecyl-
trimethylammonium bromide (CTAB), urea ((NH2)2CO),
ethanol (C2H6O), and commercial copper (II) oxide (CuO)
5774 | J. Mater. Chem., 2011, 21, 5774–5779
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powder were all purchased from Sinopharm Chemical Reagent
Co., Ltd. Polyvinylpyrrolidone (PVP) (Mw ¼ 55 000) was
purchased from Sigma-Aldrich. Norbornene, t-butylhydroper-
oxide (TBHP), cis-cyclooctene, styrene and trans-stilbene were
all purchased from Alfa Aesar. All materials were directly used as
received without further purification.
Olefin epoxidation
The epoxidation reactions were carried out in a 50 mL two-
necked round bottom flask fitted with a reflux condenser. 10 mL
of acetonitrile, 1 mmol alkene, 1 mmol diphenyl (as internal
standard for GC analysis) and 10 mg CuO CNCs or CuO
CNCs@meso-SiO2 nanocomposite catalyst was mixed within the
flask. Then 5 mmol TBHP was added drop-wise to the above
mixture under vigorous stirring. The reaction was maintained at
70 ꢀC for a desired time. The reaction mixture was sampled
periodically for analysis by gas chromatography (Agilent 6890N
GC) fitted with a HP-5 capillary column (30 m ꢂ 0.32 mm) and
flame ionization detector, and a gas chromatography-mass
spectroscopy (SHIMADZU, GCMS-QP2010S) method was
applied for the identification product mixtures.
Preparation of CuO CNCs and hollow Cu2O nanospheres
In a typical experiment, (CH3COO)2Cu$H2O (79 mg) and urea
(48 mg) were added into 32 mL ethanol and stirred until totally
dissolved, followed by the addition of PVP (145 mg) under
ultrasonic conditions for a few minutes. Then the navy-blue
solution was transferred into a 40 mL autoclave with a Teflon-
liner and was heated to 180 ꢀC for 1 or 12 h, for CuO CNCs and
hollow Cu2O nanospheres, respectively. Then the autoclave was
cooled naturally. The products were collected by centrifugation,
and washed with deionized water and ethanol several times.
Finally the samples were dried at 40 ꢀC in a vacuum oven
overnight.
Results and discussion
Preparation of CuO CNCs and Cu2O hollow nanospheres
Black CuO CNCs were synthesized via a one pot solvothermal
reaction at 180 ꢀC for 1 h, using (CH3COO)2Cu$H2O and urea as
the starting materials, and ethanol as the solvent. PVP was
employed as the surfactant to control the morphology of the
product (Scheme 1). The morphology of the product was
examined by TEM. As shown in Fig. 1a, the product is composed
of a large quantity of well-dispersed colloidal clusters which
include approximately 120 nanoparticles on average. The
average diameter of these clusters was measured to be 60 ꢃ 5 nm
(Fig. 1a, 1b). The corresponding selected-area electron diffrac-
tion (SAED) shows typical ring patterns, indicating the poly-
crystalline nature of the CuO CNCs (Fig. 1c). HRTEM results
show two different set of atomic lattice fringes in the same
Preparation of CuO CNCs@meso-SiO2 nanocomposite
The mesoporous silica shells coated CuO CNCs were prepared
according to our previous method.28 In a typical synthesis
procedure, 24 mg of as-synthesized CuO CNCs were added to
a solution of 30 mL H2O, 20 mL ethanol and 0.124 g CTAB at
room temperature. After sonication for 1 h, 60 mL of 1 M NaOH
aqueous solution was added to the above solution under stirring.
Then 0.12 mL of TEOS was added slowly drop-wise and main-
tained at room temperature for 12 h with vigorous stirring. The
obtained samples were washed with deionized water and ethanol
several times. The resulting products were subjected to a Soxhlet
extraction with ethanol for 2 days to remove the CTAB.29,30
Finally the products were dried at 40 ꢀC in a vacuum oven
overnight.
̄
domain which correspond well with the (111) and (111) crystal-
lographic planes of the monoclinic CuO (Fig. 1d), further con-
firming the polycrystallinity of the CuO CNCs.
The XRD pattern of the CuO CNCs matches well those of
crystalline CuO (Fig. 2a, JCPDS 65-2309). The average size of
the CuO crystallites deduced from the Scherrer’s equation for the
strongest peak (111) is about 8 nm. The FT-IR spectrum
(Fig. 2b) of the CuO nanostructure indicates two signature peaks
at 524 cmꢁ1 and 586 cmꢁ1 from CuO.6,31 Three weak peaks at
2956 cmꢁ1, 1668 cmꢁ1, and 1426 cmꢁ1 were the characteristic
peaks of PVP,32 indicating the presence of residual PVP on the
CuO CNCs. Note that no calcination was carried out to prepare
the samples. Based on TG data, the PVP is approximately 1.8 wt
% in the sample (Fig. S1, ESI†). In a control experiment without
the addition of PVP, CuO cubes and CuO spheres were obtained
(Fig. S2, ESI†). These cubes and spheres have a wide size
Characterization
The size and morphology of the products were characterized by
transmission electron microscopy (TEM, JEOL JEM-1011,
operating at an accelerating voltage of 100 kV), scanning elec-
tron microscopy (SEM, JEOL-6701F) and high resolution TEM
(HRTEM, JEM 2100F, operating at an accelerating voltage of
200 kV). X-ray diffraction (XRD) patterns were collected on
a Rigaku D/max 2500 diffractometer with Cu Ka radiation (l ¼
0.1542 nm) at 40 kV and 100 mA. The small-angle XRD
measurements were carried out in Rigaku D/max-2400 diffrac-
tometer equipped with a secondary graphite monochromator
with Cu Ka radiation (l ¼ 0.154 nm). Data were collected in
a step-scan mode in the range of 0.6–6 degree with step-width of
0.02 and speed of 1 degree$minꢁ1. Fourier-transform IR (FTIR)
spectra were recorded on a Bruker Tensor 27 instrument. The
surface area of the products was measured by the Brunauer-
Emmet-Teller (BET) method using N2 adsorption and desorp-
tion isotherms on an Autosorb-1 analyzer. Pore size distribution
plot was obtained by the Barrett-Joyner-Halenda (BJH) method.
The weight content of the CuO in the CuO CNCs@meso-SiO2
nanocomposite was obtained from inductively coupled plasma
emission spectrometer (ICP-AES, Shimadzu) analysis.
Scheme 1 Overall synthetic scheme of CuO CNCs and the CuO
CNCs@meso-SiO2 nanocomposite.
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Fig. 1 TEM images of the as-prepared CuO CNCs: (a) low-magnifica-
tion TEM image; (b) high-magnification TEM image and (c) the corre-
sponding SAED pattern of an individual CuO CNCs; (d) HRTEM image
taken from the individual CuO CNCs (insets: high-magnification
HRTEM image of area marked with (1) and (2), respectively; scale bar ¼
1 nm).
distribution from 200 nm to 1 mm in size. These results suggested
that the PVP helped to control the size of the nanocrystal clusters
and the formation of the spherical geometry. The as-synthesized
CuO CNC sample has a relatively high specific surface area of
80 m2$gꢁ1, which is favorable for the catalytic performance
(Fig. S3, ESI†).
Fig. 2 (a) X-ray diffraction patterns of as-prepared CuO CNCs and
Cu2O hollow spheres and (b) FT-IR spectra of PVP, as-prepared CuO
CNCs and as-prepared Cu2O hollow spheres.
Interestingly, when the reaction time was increased to 12 h,
Cu2O hollow nanospheres with a diameter of 110 ꢃ 15 nm were
produced (Fig. 3e). The HRTEM image shows the poly-
crystalline nature of these hollow nanospheres (Fig. 3f). As
shown in Fig. 2a, the XRD pattern of the hollow nanospheres
can be indexed to the cubic phase of Cu2O (JCPDS 65-3288). No
reflections of CuO were observed, confirming a complete trans-
formation from CuO to Cu2O. The FT-IR spectrum of the
hollow nanospheres (Fig. 2b) shows that the characteristic peaks
of CuO have disappeared. The new absorption band at 630 cmꢁ1
can be assigned to Cu2O.33 The specific surface area of the hollow
nanospheres is 62 m2$gꢁ1 (Fig. S3, ESI†).
Preparation of CuO CNCs@meso-SiO2 nanocomposite
For better mass transportation of the reaction species, porous
shells are usually preferred.34 In previous works, we developed
a method to produce silica nanotubes with mesoporous walls and
various internal morphologies. The hierarchical pore structure
shows faster mass transportation in catalysis.28 In this study, we
used a similar strategy to encapsulate CuO CNCs in mesoporous
SiO2 shells. As shown in Fig. 4a and 4b, the TEM images show
that CuO CNCs were evenly coated with a layer of mesoporous
shells about 40 nm thick, while the CuO CNCs remained in their
initial cluster morphologies (Fig. 1a). The inset of Fig. 4b shows
the HRTEM image of the CuO CNCs core in the CuO
CNCs@meso-SiO2 nanocomposite. Small-angle XRD and N2
adsorption-desorption analysis data are shown in Fig. 5. The
small-angle XRD pattern of a CuO CNCs@meso-SiO2 nano-
composite has a broad peak at around 2.5ꢀ (Fig. 5a), indicating
that the mesoporous SiO2 shells have relatively ordered meso-
pores. This is consistent with TEM results (Fig. 4b) and similar
mesoporous materials.28 The CuO CNCs@meso-SiO2 nano-
composite exhibits a type IV adsorption isotherm with a narrow
hysteresis loop (Fig. 5b). The surface area of the nanocomposite
To investigate the mechanism of the transformation from CuO
CNCs to Cu2O hollow nanospheres, products were sampled at
different reaction times and characterized by TEM (Fig. 3) and
XRD. As shown in Fig. 3b, solid nanospheres are obtained after
2 h at 180 ꢀC. After 4 h of reaction (Fig. 3c), small voids can be
seen in the center. With an increased reaction time, the voids in
the center became bigger. After 12 h, the conversion is
completed, leading to hollow nanospheres with well-centered
circular voids (Fig. 3e). XRD patterns show that the peaks from
CuO weakened sharply and new peaks from Cu2O appeared after
2 h (Fig. S4, ESI†). The exact conversion mechanism from CuO
to Cu2O will be the subject of another study. This work mainly
focused on the catalytic ability of CuO CNCs.
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Fig. 4 (a) Low-magnification TEM image of the CuO CNCs@meso-
SiO2 nanocomposite; (b) high-magnification TEM image of the CuO
CNCs@meso-SiO2 nanocomposite (inset: HRTEM image taken from the
CuO CNCs core in the CuO CNCs@meso-SiO2 nanocomposite); (c)
TEM image of the CuO CNCs@meso-SiO2 nanocomposite after eight
runs of olefin epoxidation; (d) TEM image of CuO CNCs used for 4 h of
olefin epoxidation.
whereas the commercial CuO only resulted in a 20% yield
after 1 h.
Fig. 3 TEM characterization of the samples obtained with different
reaction times (a) 1 h, (b) 2 h, (c) 4 h, (d) 8 h, and (e) 12 h (inset: SEM
image of a broken Cu2O hollow sphere); (f) HRTEM image taken from
the Cu2O hollow sphere (insets: high-magnification HRTEM image of
area marked with (1) and (2), respectively).
Besides TBHP, other cheaper oxidants such as H2O2, O2 and
air were also tested for norbornene epoxidation, but unfortu-
nately these oxidants didn’t work under these reaction condi-
tions.
Based on the catalytic results within 1 h of reaction in Fig. 6a,
the turnover frequency (TOF) of norbornene epoxide formation
in CuO CNCs@meso-SiO2 nanocomposite and commercial CuO
were 5.77 ꢂ 10ꢁ3 sꢁ1 and 0.44 ꢂ 10ꢁ3 sꢁ1, respectively, indicating
that CuO CNCs@meso-SiO2 nanocomposite is apparently much
more active than commercial CuO powder. However, the surface
area of CuO CNCs is 80 m2/g, and the surface area of commercial
is 860.6 m2$gꢁ1. The inset of Fig. 5b shows the BJH pore size
distribution with a sharp peak at 3.7 nm, which was likely due to
the average diameter of the pores in the mesoporous SiO2 shells.
All these results suggested that the silica coating on the
composite is mesoporous silica, which provide efficient channels
for mass transfer and catalytic reactions. The weight content of
the CuO in the CuO CNCs@meso-SiO2 nanocomposite was
27.2% according to ICP analysis results.
Catalytic performance in olefin epoxidation
The CuO CNCs, CuO CNCs@meso-SiO2 nanocomposite and
Cu2O hollow nanospheres were used as catalysts for norbornene
epoxidation using t-butylhydroperoxide (TBHP) as the oxidant
and acetonitrile as the solvent. For comparison, commercial
available CuO powder (SEM image is shown in Fig. S5, ESI†)
was also tested. The norbornene conversion as a function of
reaction time is shown in Fig. 6a. Both CuO CNCs and CuO
CNCs@meso-SiO2 nanocomposite had decent initial activities,
resulting in 68% and 71% yield of norbornene epoxide in 1 h,
respectively, and 100% selectivity for the epoxidation product;
Fig. 5 (a) Small-angle XRD pattern of the CuO CNCs@meso-SiO2
nanocomposite; (b) N2 adsorption-desorption isotherms of the CuO
CNCs@meso-SiO2 nanocomposite (inset: the BJH pore diameter distri-
bution curve at desorption branch).
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Catalytic epoxidations of styrene and trans-stilbene as a func-
tion of the reaction time and reaction temperature were carried
out to compare the selectivities of these two reactions at different
conversions. We found interesting patterns of selectivity changes
at different reaction times as shown in Fig. S7, S8 and Table S1,
ESI†. At lower alkene conversion (at the beginning of the reac-
tion, or with lower reaction temperature, the selectivities were
actually much lower, i.e. the undesired side reaction dominated).
However, with higher alkene conversions (70 ꢀC), the alkene
epoxide selectivities were enhanced with increased reaction time.
We propose that the side reactions (oxidative cleavage of
alkene10) were faster than the epoxidation reactions, but the
active sites for these side reactions were deactivated quickly, thus
the epoxide products’ selectivities were enhanced with longer
reaction times. At higher reaction temperature, the alkene
conversions increased but the selectivities of epoxides decreased,
probably due to ‘‘over reaction’’.
More importantly, the CuO CNCs@meso-SiO2 nano-
composite showed remarkable stability. The cycling performance
of the CuO CNCs and CuO CNCs@meso-SiO2 nanocomposite
are shown in Fig. 6b, the CuO CNCs@meso-SiO2 nano-
composite had no obvious loss of activity for eight consecutive
runs; the norbornene epoxide yields were around 90% in all eight
runs. In contrast, the norbornene epoxide yields decreased
steadily in four runs using CuO CNCs as catalyst.
The TEM image of CuO CNCs sampled after refluxing for 4 h
(Fig. 4d) shows that the CuO CNCs severely aggregated during
the reaction process, resulting in a lower yield for norbornene
epoxidation in the following runs. However, the TEM image of
the CuO CNCs@meso-SiO2 nanocomposite remained
unchanged after 8 runs (Fig. 4c). In an extended stability test, the
CuO CNCs@meso-SiO2 nanocomposite was continuously stir-
red in the same reaction mixture for 720 h, most of the nano-
composites still maintained their morphologies (Fig. S9, ESI†).
These results demonstrate that the mesoporous SiO2 shell could
serve as an effective protection against the destruction of CuO
CNCs.
Fig. 6 (a) Norbornene conversion as a function of the reaction time over
the CuO CNCs@meso-SiO2 nanocomposite, CuO CNCs, Cu2O hollow
nanospheres and commercial CuO. Conditions: 1 mmol norbornene,
5 mmol TBHP, 10 mg catalyst, stirred in 10 mL of acetonitrile under
reflux (70 ꢀC). (b) Recycling results for epoxidation of norbonene,
conditions: 1 mmol norbornene, 5 mmol TBHP, 10 mg catalyst, stirred in
10 mL of acetonitrile under reflux (70 ꢀC) for 6 h.
CuO is 1.2 m2/g. Thus we also compared the TOF per surface Cu
atom (by calculating the TOF per unit of surface area, as the
number of surface Cu atoms is proportional to the surface area
of the CuO), we found that the per surface area based TOF ratio
between CuO CNCs@meso-SiO2 nanocomposite and commer-
cial CuO is about 1 : 5, indicating that the active sites on
commercial CuO were actually more active than those on CuO
CNCs. The apparently higher activity of CuO CNCs catalyst was
due to its much higher surface area and much more active sites
than commercial CuO.
Conclusions
In summary, we prepared uniform CuO CNCs via a fast one-pot
solvothermal synthetic route. CuO CNCs were coated with
mesoporous SiO2 shells to build an aggregate-resistant catalyst
system. The CuO CNCs@meso-SiO2 nanocomposite exhibited
excellent catalytic activity and stability for olefin epoxidation
reactions. The nanocomposite remained intact after eight runs.
This strategy can be extended to design other catalytic systems
with different shapes and compositions.
The Cu2O hollow nanospheres also can catalyze the epoxida-
tion, giving a 66.3% conversion and 100% selectivity for nor-
bornene epoxide within 1 h. XRD patterns of the used catalysts
(Fig. S6, ESI†) show that the oxidation states of Cu in the
catalysts remain after the reaction, indicating that all CuO and
Cu2O can catalyze the epoxidation.
Acknowledgements
Epoxidation reactions of cis-cyclooctene, styrene and trans-
stilbene were also carried out and the results are summarized in
Table 1. After refluxing in acetonitrile for 8 h, cis-cyclooctene
gave a 66.5% conversion and 100% selectivity to the corre-
sponding epoxide. However, other olefins, such as styrene and
trans-stilbene have lower epoxide selectivities.
We thank the financial support from the National Natural
Science Foundation of China (NSFC 50725207, 20821003),
Ministry of Science and Technology (MOST 2007CB936400,
2009CB930400) and the Chinese Academy of Sciences (KJCX2-
YW-N40).
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Table 1 Epoxidation of Alkenes with CuOa
Catalyst
Substrate
Product
Time, h
6
Conversion, %
90
Selectivity, %
100
CuO CNCs@meso-SiO2
nanocomposite
CuO CNCs
6
6
6
8
86
100
100
100
100
Cu2O hollow nanospheres
Commercial CuO
82.3
64.2
66.5
CuO CNCs@meso-SiO2
nanocomposite
CuO CNCs@meso-SiO2
nanocomposite
2
4
88.6
87.7
61.2
80.9
CuO CNCs@meso-SiO2
nanocomposite
a
1 mmol alkene, 5 mmol TBHP, and 10 mg catalyst, stirred in 10 mL of acetonitrile under reflux (70 ꢀC).
19 A. Cao, R. Lu and G. Veser, Phys. Chem. Chem. Phys., 12, pp. 13499–
13510.
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