Facile synthesis of copper-based metal oxide nanoparticles with exceptional catalytic activity for the selective oxidation of styrenes into benzaldehydes

Article abstract of DOI:10.1002/cplu.201402319

The development of highly efficient catalysts for the selective oxidation of styrene to benzaldehyde has attracted great attention in recent years because of its significance in synthetic chemistry. In this study, two different kinds of copper-based metal oxide nanoparticles (NPs), namely, CuO and CuO/Co₃O₄ NPs, were synthesized by a simple and scalable method, and these nanoparticles demonstrated efficient catalytic abilities for the selective oxidation of styrene and its derivatives to the corresponding aldehydes in the presence of tert-butyl hydroperoxide (TBHP) under mild reaction conditions in excellent yields. Importantly, both of the heterogeneous catalysts can be recycled up to five runs while still maintaining their high catalytic activity. Talkin about our generation: Copper-based metal oxide (CuO, CuO/Co₃O₄) nanoparticles have been synthesized from coordination-driven self-assembling aggregates and calcination treatment. They exhibited exceptional catalytic activity and stability for the selective oxidation of styrene and its derivatives to generate the corresponding aldehydes (see figure; TBHP=tert-butyl hydroperoxide).

Full text of DOI:10.1002/cplu.201402319

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DOI: 10.1002/cplu.201402319
Facile Synthesis of Copper-Based Metal Oxide
Nanoparticles with Exceptional Catalytic Activity for the
Selective Oxidation of Styrenes into Benzaldehydes
Danhua Ge,[a] Jiaqing Wang,[a] Hongbo Geng,[a] Shuanglong Lu,[a] Dongtao Wang,[a]
Xinming Li,[a] Xianli Zhao,[b] Xueqin Cao,*[a] and Hongwei Gu*[a]
The development of highly efficient catalysts for the selective
oxidation of styrene to benzaldehyde has attracted great at-
tention in recent years because of its significance in synthetic
chemistry. In this study, two different kinds of copper-based
metal oxide nanoparticles (NPs), namely, CuO and CuO/Co3O4
NPs, were synthesized by a simple and scalable method, and
these nanoparticles demonstrated efficient catalytic abilities for
the selective oxidation of styrene and its derivatives to the cor-
responding aldehydes in the presence of tert-butyl hydroper-
oxide (TBHP) under mild reaction conditions in excellent yields.
Importantly, both of the heterogeneous catalysts can be recy-
cled up to five runs while still maintaining their high catalytic
activity.
Introduction
Styrene oxidation is of considerable importance in both aca-
demic and commercial sectors for the synthesis of benzalde-
hyde, which has wide applications in various fields as a crucial
intermediate, such as pharmaceuticals, dyestuffs, spices, and so
forth.[1,2] Traditionally, this transformation was performed using
organic peracid as the oxidant,[3] the wide application of which
was hindered because of it nocuous nature, high costs, and
the generation of undesirable by-products. However, taking
into account low catalytic activity, stability, and difficulty with
homogeneous catalyst separation, as well as the production of
many waste products during the reaction process, many efforts
have been devoted to develop valid and recyclable heteroge-
neous catalysts and safer oxidizing agents such as tert-butyl
hydroperoxide (TBHP), hydrogen peroxide, or molecular
oxygen to catalyze styrene selective oxidation.[4–6]
tion of styrene under mild conditions.[8] Although a large varie-
ty of supported catalysts have been well developed, catalysts
without supports have drawn more investigative attention on
account of their numerous advantages, such as making full use
of the effective reactive center of catalysts. Lambert et al.
adopted ultrasmall gold nanoclusters as catalyst, which exhibit-
ed relatively high catalytic activity and selectivity for styrene
oxidation under molecular oxygen.[9] Gu’s group reported that
ultrathin gold nanowires (GNWs) appeared much more active
for the oxidation of benzylic compounds under molecular
oxygen than the gold nanoclusters above.[10]
Copper oxide, an inexpensive and simple transition-metal
oxide, has also attracted much attention as novel nanocatalyst
in a variety of organic transformations such as dye degrada-
tion[11] and ring opening.[12] It demonstrated good catalytic per-
formance.
As a consequence, the design of novel and more effective
catalysts combined with clean oxidants to replace traditional
strategies is extremely urgent from an environmental point of
view. For example, Dapurkar et al. found that supported gold
nanoparticle catalysts could efficiently catalyze the oxidation of
benzylic compounds at 1 atm O2.[7] Luque’s group reported
that iron oxide nanoparticles supported on the mesoporous
aluminosilicates exhibited high catalytic activity for the oxida-
Herein, we describe the use of copper oxide (CuO) nanopar-
ticles and bimetal oxide (CuO/Co3O4) nanoparticles as nonsup-
ported catalysts for the selective oxidation of styrene to ben-
zaldehyde under mild conditions. In this study, metal oxide
nanocatalysts were synthesized by a calcination treatment of
metal–ligand complexes, which were achieved by means of
a coordination-driven aggregation process. The reaction was
performed in acetonitrile and using tert-butyl hydroperoxide
(TBHP) as the oxidant. The results revealed that our metal
oxide nanoparticles showed high catalytic activities and the
overall yield of benzaldehyde was as high as 80%. These inex-
pensive and unsupported catalysts were found to be more
active than our previously reported Pt@Fe2O3 NWs catalysts.[13]
Furthermore, compared with a Cu-Ni-Co/g-Al2O3 system, our
CuO and CuO/Co3O4 compounds provided higher conversion
and selectivity.[14] It was noted that CuO NPs displayed com-
plete conversion of styrene and its derivatives, whereas CuO/
Co3O4 NPs exhibited higher selectivity for the corresponding al-
[a] D. Ge, J. Wang, H. Geng, S. Lu, D. Wang, Prof. X. Li, Prof. X. Cao, Prof. H. Gu
Key Laboratory of Organic Synthesis of Jiangsu Province
College of Chemistry, Chemical Engineering and Materials Science
& Collaborative Innovation Center of Suzhou Nano Science and
Technology, Soochow University, Suzhou 215123 (P. R. China)
E-mail: hongwei@suda.edu.cn
xqcao@suda.edu.cn
[b] X. Zhao
Jiangsu Sunshine Co., Ltd.
Jiangyin 214426 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402319.
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dehydes. Importantly, these metal oxide NPs could be recycled
multiple times without any notable decrease in activity.
formation). These particles were calcinated at 5008C, and the
color of the products changed from yellowish-brown into
brownish-black, which resulted in the formation of the CuO
nanoparticles. The typical SEM of the CuO NPs sample (Fig-
ure 1b) showed that the shape and size were very different
from the Cu–LCs. The particles almost can be separated from
each other, which could be further confirmed by the corre-
sponding TEM image (Figure S2). It was clear that the CuO NPs
were very uniform solid nanospheres with perfectly smooth
surfaces and diminishing sizes from about 1.5 mm to 150 nm in
diameter. This could be attributed to the successive release
and loss of CO2 and H2O during
Results and Discussion
The metal oxide spheres were synthesized according to our
previous reported method.[15] As illustrated in Scheme 1, organ-
ic ligands ((Z)-4-amino-5-(hydroxyimino)-2-(4-n-octyloxyphen-
yl)-2,5-dihydro-1H-imidazole-3-oxide (AHODIO)) were mixed
with metal acetate (copper(II) acetate and cobalt(II) acetate) in
a 2:1 molar ratio to form metal–ligand complexes (M–LCs) by
the thermal decomposition. The
corresponding
EDS
further
proved the removal of released
organic ligands (Figure S3). In
addition, the crystalline structure
and phase purity of the as-pre-
pared CuO were analyzed by
XRD in Figure 1c. All the diffrac-
tion peaks were satisfactorily in-
dexed to the standard XRD data
card of CuO (JCPDS card no. 48-
1548).[16] Clearly, no additional
peaks were detected, thus indi-
cating the absence of any other
Scheme 1. Preparation of copper-based metal oxide nanoparticles.
impurities such as Cu2O and Cu
in the XRD patterns.
means of a coordination-driven aggregation process (CDAP).
Figure 1 shows the SEM image of as-prepared Cu-based MLCs,
which clearly indicated that the precipitate consisted of
spheres with a diameter of about 1.5 mm and exhibited exten-
sive corrugation with deep cavities at the surface. The chemi-
cal composition of Cu–LCs was determined by energy-disper-
sive X-ray spectroscopy (EDS; Figure S1 in the Supporting In-
According to the above procedure, CuO/Co3O4 nanoparticles
were synthesized by calcination treatment of Cu/Co–ligand
complexes under the same temperature. The size of Cu/Co–
LCs (Figure 2a) was approximately 100 nm, whereas the size of
CuO/Co3O4 (Figure 2b) was less than 50 nm. This size reduction
came from the release of the organic ligands. The XRD pat-
terns (Figure 2c) illustrate the pure mixed-crystalline structure,
Figure 1. SEM images of as-prepared (a) Cu–LCs and (b) CuO NPs. (c) XRD
Figure 2. SEM image of as-prepared (a) Cu-Co–LCs and (b) CuO/Co3O4 NPs.
patterns of the CuO NPs.
(c) XRD patterns of the CuO/Co3O4 NPs.
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which consisted of CuO and Co3O4. Moreover, the chemical
composition of complexes was also determined by EDS (Fig-
ures S4 and S5). The measured Brunauer–Emmett–Teller (BET)
pore volume and specific surface area of the CuO NPs and
CuO/Co3O4 NPs are described in Table S1.
Table 2. Time-conversion data for benzaldehyde formation from the oxi-
dation of styrene using CuO NPs as catalyst.[a]
Time [h]
Conversion [%][b]
Selectivity [%][b]
A/B/C
Yield [%][b]
1
3
5
6
7
8
10
12
24
80
87
96
96
95
96
97
98
100
87:13:0
82:14:4
74:18:6
76:16:6
78:15:6
77:14:7
85:13:3
82:16:3
81:16:0
70
71
70
73
74
74
82
80
81
The test of the catalytic activity of the CuO NPs was per-
formed by using styrene as the model substrate under air in
the presence of TBHP (Scheme 2). Table 1 shows the results of
the selective oxidation of styrene under different reaction con-
ditions. The reactions in N,N-dimethylformamide (DMF) and
tetrahydrofuran (THF) exhibited complete conversions with rel-
[a] Reaction conditions: styrene (1 mmol), TBHP (2.5 mmol), CuO NPs
(5 mg), CH3CN (2 mL), 708C. [b] Determined by GC analysis.
Scheme 2. Styrene oxidation with CuO NPs for the generation of the benzal-
dehyde and other by-products.
over the entire reaction, the values were between 74 and
87.1%, which suggested that the benzaldehyde was formed by
the direct oxidative cleavage of C=C of styrene, and one of the
by-products, styrene oxide, can be further oxidized to form
benzaldehyde.[17]
Table 1. The optimization of the different solvents for styrene oxidation
to benzaldehyde.[a]
To further evaluate the catalytic activity of CuO nanoparti-
cles, we conducted further studies to screen other factors in-
cluding different temperatures, oxidants, and the amount of
oxidants and catalysts. The reactions were carried out in the
presence of different oxidizing agents, such as TBHP, air,
oxygen, hydrogen peroxide, or benzoyl peroxide (BPO), respec-
tively (entries 1–5, Table S2). Among these, no benzaldehyde
was achieved in air, O2, or hydrogen peroxide. BPO gave rela-
tively high conversion but quite low selectivity for benzalde-
hyde. When the amount of TBHP was increased, the conver-
sion for styrene was higher. The styrene could be fully convert-
ed and the selectivity was as high as 84.4% when using
4 mmol of TBHP (Table 3, entry 7). Furthermore, it was found
that the conversion for styrene was 100% and the selectivity
for benzaldehyde was 86.4% when adding 7 mg CuO NPs as
the catalyst for the oxidation of styrene (Table S3).
Entry
Solvent
Conversion [%][b]
Yield [%][b]
1
2
3
4
5
6
7
8
9
CHCl3
CH3CN
toluene
isopropanol
ethanol
DMF
96
96
55
92
64
100
100
81
60
76
77
74
34
53
56
48
63
38
43
57
THF
1,4-dioxane
methanol
H2O
10
[a] Reaction conditions: styrene (1 mmol), TBHP (2.5 mmol), CuO NPs
(5 mg), solvent (2 mL), 708C, 8 h. [b] Determined by GC analysis.
atively lower selectivity towards benzaldehyde. Compared to
other solvents, CHCl3 offered excellent yield for the oxidation
of styrene, but its real application is hindered by its high toxici-
ty. When using water as solvent, the yield of benzaldehyde
was 57.3% (Table 1, entry 10). It was found that acetonitrile
was one of the best solvents for the reactions, and the yield of
benzaldehyde was 74.9% over eight hours (Table 1, entry 2).
The time-dependent conversion of styrene to benzaldehyde
was based on the GC analysis, which is illustrated in Table 2.
Styrene (1 mmol) was oxidized at 708C under an air atmos-
phere by using CuO nanoparticles (5 mg) as catalyst, tert-butyl
hydroperoxide (2.5 mmol) as oxidizing agent, and acetonitrile
(2 mL) as solvent. As shown in Table 2, after one hour, the reac-
tion afforded benzaldehyde in a yield of 70.4%, and the selec-
tivity towards acetophenone was 12.9%. When the reaction
time increased, the yields of benzaldehyde remained un-
changed. In addition, styrene oxide started to appear and was
gradually reduced as time went on. The highest yield of ben-
zaldehyde was achieved after ten hours and reached as high
as 82.1%. Compared with the selectivities of benzaldehyde
The general applicability of the catalytic oxidation was
tested with styrene derivates under the optimized conditions,
and the results are summarized in Table 4. Clearly, all the sub-
strates can be completely converted with an overall yield be-
Table 3. The different ratios of TBHP and styrene for styrene oxidation to
benzaldehyde.[a]
Entry
TBHP [mmol]
Conversion [%][b]
Yield [%][b]
1
2
3
4
5
6
7
8
0.5
1.0
1.5
2.0
2.5
3.0
4.0
5.0
39
66
83
91
97
100
100
100
32
46
72
73
82
70
84
74
[a] Reaction conditions: styrene (1 mmol), CuO NPs (5 mg), CH3CN (2 mL),
708C, 10 h. [b] Determined by GC analysis.
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whereby the oxygen was desor-
bed from the oxidant. As
a result, the reactive sites would
be favorable for the formation of
benzaldehyde in view of the cat-
alytic reaction that probably
adopts an approach similar to
the Eley–Rideal mechanism.[14]
Our CuO and CuO/Co3O4 cata-
lysts could be easily recycled
and reused over five times and
Table 4. Selective oxidation of styrene and its derivatives to the corresponding aldehydes.
[b]
Entry
Substrate
Product
CuO[a]
Selectivity [%][c]
CuO/Co3O4
Conv. [%][c]
Conv. [%][c]
Selectivity [%][c]
1
2
3
4
5
100
100
100
100
100
86
37
54
73
62
100
99
84
84
51
56
91
100
96
100
exhibited
excellent
stability
(Figure 3). The yields were able
to be maintained (over 65%)
and the conversion and selectivi-
ty could basically remain un-
changed after five cycles.
6
7
8
100
100
100
77
56
34
97
94
75
73
66
100
9
100
46
96
71
Conclusion
[a] Reaction conditions: styrene (1 mmol), TBHP (4 mmol), CuO NPs (7 mg), CH3CN (2 mL), 708C, 10 h. [b] Reac-
tion conditions: styrene (1 mmol), TBHP (3 mmol), CuO/Co3O4 NPs (5 mg), CH3CN (2 mL), 708C, 5 h. [c] Conver-
sion and selectivity determined by GC analysis.
In summary, we have synthe-
sized M–LCs by means of a coor-
dination-driven aggregation pro-
cess, and we obtained two kinds
tween 34.8 and 86.4% (Table 4, entries 1–9). Furthermore, we
examined the possibility of the nanoparticles to catalyze 2-
vinyl naphthalene to the corresponding 2-naphthaldehyde,
which was generated with a yield of 46.1% (Table 4, entry 9).
Encouraged by the above results, we tested the catalytic ac-
tivities of bimetal oxide (CuO/Co3O4). When CuO/Co3O4 was
used as the catalyst for the selective oxidation of styrene,
these two kinds of catalysts provided different catalytic activity.
In acetonitrile, the conversion of styrene by the CuO/Co3O4
over five hours was 100% and the yield of benzaldehyde was
84.4%, a little lower than that of CuO (86.4%).
The time-conversion profile for the oxidation of styrene is
shown in Table S4. After one hour, 66.2% of styrene conversion
Figure 3. Catalytic stability of CuO NPs (black) and CuO/Co3O4 NPs (diagonal
was observed with 88.2% selectivity of benzaldehyde at 708C.
The highest yield (73.9%) was obtained after five hours with
94.8% styrene conversion. The conversion was able to reach
100% after 24 hours, but the selectivity gradually decreased.
Temperature-dependent analysis revealed that the selectivity
of benzaldehyde was decreased as the conversion of styrene
increased while the reaction temperature was raised from 40
to 1008C.
A series of styrene derivates could also converted to their
corresponding aldehyde under the optimized reaction condi-
tions (Table 4). Compared to the catalytic performance of CuO
NPs, not all substrate could be completely converted except
for 4-methoxystyrene (Table 4, entry 3), 4-methylstyrene
(Table 4, entry 5), and 3-bromostyrene (Table 4, entry 8). Simi-
larly, the yields of the selective oxidation for the electron-rich
and electron-withdrawing substrates to the corresponding al-
dehydes ranged from 51.4 to 91.2% and were relatively higher
than that of CuO. It was reported that CuO shows greater
Brønsted acidity than Co3O4, whereby the oxidant was ad-
sorbed on the surface. The Co3O4 displayed high Lewis basicity,
stripes) in the oxidation of styrene to benzaldehyde.
of inexpensive copper-based metal oxide (CuO, CuO/Co3O4)
NPs after calcination treatment. As heterogeneous catalysts,
the as-prepared copper-based metal oxide NPs exhibited ex-
ceptional catalytic performance for the selective oxidation of
styrene and its derivatives to the corresponding aldehydes
with TBHP as oxidant under mild reaction conditions, thereby
resulting in yields as high as 86.4% for benzaldehyde from sty-
rene. In comparison, CuO/Co3O4 NPs displayed relatively higher
selectivity than that of CuO NPs for the generation of the cor-
responding aldehydes, whereas CuO NPs exhibited higher cat-
alytic capability for the complete conversion of styrene and its
derivates. Beyond these, the copper-based metal oxide NPs
showed excellent cyclic stability over five runs. The oxidation
process was environmentally friendly and demonstrated their
potential for industrial application.
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Experimental Section
An explosion stack (50 mL) was enclosed with styrene (1 mmol),
CuO NPs (7 mg), TBHP (4 mmol), and CH3CN (2 mL). Then the reac-
tion was stirred for 10 h at 708C. When the reaction was stopped,
the reaction mixtures were analyzed by GC using chlorobenzene as
the internal standard. The catalysts were gained by the centrifuga-
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An explosion stack (50 mL) was enclosed with styrene (1 mmol),
CuO/Co3O4 NPs (5 mg), TBHP (3 mmol), and CH3CN (2 mL). Then
the reaction was stirred for 5 h at 708C. The following procedures
were the same as the above.
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Keywords: copper · nanoparticles · oxidation · self-assembly ·
styrene
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Received: September 15, 2014
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D. Ge, J. Wang, H. Geng, S. Lu, D. Wang,
X. Li, X. Zhao, X. Cao,* H. Gu*
Talkin’ about our generation: Copper-
based metal oxide (CuO, CuO/Co3O4)
nanoparticles have been synthesized
from coordination-driven self-assem-
bling aggregates and calcination treat-
ment. They exhibited exceptional cata-
lytic activity and stability for the selec-
tive oxidation of styrene and its deriva-
tives to generate the corresponding al-
dehydes (see figure; TBHP=tert-butyl
hydroperoxide).
&& – &&
Facile Synthesis of Copper-Based
Metal Oxide Nanoparticles with
Exceptional Catalytic Activity for the
Selective Oxidation of Styrenes into
Benzaldehydes
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