Hierarchical PS/PANI nanostructure supported Cu(ii) complexes: Facile synthesis and study of catalytic applications in aerobic oxidation

Article abstract of DOI:10.1039/c4ra06944g

Hierarchical heterogeneous copper catalysts were prepared by immobilization of a homogeneous copper(ii) complex on the surface of polystyrene/polyaniline (PS/PANI) microspheres with oriented PANI nanofibers. EDX element maps and XPS spectra indicated that Cu²⁺ ions strongly coordinated with PANI imine. PS/PANI@Cu(OSO₂CF₃)₂ exhibited excellent catalytic activity for selective aerobic oxidation of alcohols and highly efficient aerobic epoxidation of alkenes under mild conditions. The supported copper(ii) catalyst maintained high levels of conversion and selectivity in these reactions after six cycles and showed good stability. This journal is

Full text of DOI:10.1039/c4ra06944g

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PAPER
Hierarchical PS/PANI nanostructure supported
Cu(II) complexes: facile synthesis and study of
catalytic applications in aerobic oxidation†
Cite this: RSC Adv., 2014, 4, 55028
a
a
a
a
b
a
a
*
Jie Yu, Yi Luan, Yue Qi, Junying Hou, Wenjun Dong, Mu Yang and Ge Wang
Hierarchical heterogeneous copper catalysts were prepared by immobilization of a homogeneous
copper(II) complex on the surface of polystyrene/polyaniline (PS/PANI) microspheres with oriented PANI
nanofibers. EDX element maps and XPS spectra indicated that Cu2+ ions strongly coordinated with PANI
imine. PS/PANI@Cu(OSO2CF3)2 exhibited excellent catalytic activity for selective aerobic oxidation of
alcohols and highly efficient aerobic epoxidation of alkenes under mild conditions. The supported
copper(II) catalyst maintained high levels of conversion and selectivity in these reactions after six cycles
and showed good stability.
Received 10th July 2014
Accepted 7th October 2014
DOI: 10.1039/c4ra06944g
www.rsc.org/advances
and non-toxic. A copper-based catalyst for aerobic oxidation
reaction is worth developing due to the excellent catalytic
1. Introduction
activity of copper and its ready availability.
Selective oxidations are among the most important organic
transformations in industry. During these processes, stoichio-
metric use of inorganic oxidants and large amounts of solvent
are generally employed.1,2 However, many of the commonly
used oxidants are expensive and/or toxic, for example chro-
mium compounds3,4 and permanganates.5,6 Catalytic oxidations
have attracted much attention recently because these processes
are more atom-efficient and less harmful to the environment.
The implementation of a catalytic oxidation process requires
stoichiometric amounts of oxidizing reagents, many of which
are also dangerous or toxic.7–9 As environmental concerns
increase, there has been an increase in the use of molecule
oxygen as the oxidant, which has the advantage of being inex-
pensive, abundant, safe, and environmentally friendly.10–13
It has been reported that noble metal nanoparticles,14–18
Ru19,20 and Pd21–24 can be used as catalyst to promote aerobic
oxidation of alcohols, alkanes, glucose and aryl carbonyl
derivatives. However, the use of noble metallic nanoparticles in
aerobic oxidation leads to high cost and limits its application
for use in industry. Recently, inexpensive transitional metals
including Co,25,26 Fe,27,28 V29,30 and Cu31,32 have been developed to
promote the oxidation reactions in the presence of molecule
oxygen. Among these transition metals, copper is one of the
most abundant elements in nature and relatively inexpensive
So far, several copper catalysts such as CuBr2(2,20-bipyr-
idine)/t-BuOK33, CuBr$Me2S,34 Cu(II)-diimine complexes,35
Cu(OAc)2$H2O/pytl-b-cyclodextrin,36 [Cu(m-Cl)Cl(phen)]237 and
Cu(C8F17(CH2)2–CO2)2(Rf-tacn)38 have been reported to show
excellent catalytic activity in aerobic oxidation of hydrocar-
bons, alkenes and alcohols. However, homogeneous cop-
per(II) catalysts are difficult to separate and recover from the
reaction system. Thus, studies of heterogeneous copper
catalysts have gained growing attention, due to the ease of
separation and recyclability. So far, there are only a few
heterogeneous copper catalysts that were studied for the
selective aerobic oxidation transformations.39,40 A variety of
conventional supports such as silica,41–44 graphene oxide45
and functional polymer46–48 have been utilized as the support
for copper salts. Amongst the available supports, polyaniline
would be an ideal support for immobilizing copper complex
because of its low cost, easy synthesis, good thermal and
chemical stability in most organic solvents. Furthermore,
facile control of morphology and a large number of imino
groups along polyaniline backbone would control copper
loading, which would determine the activity and reproduc-
ibility of the resulting catalysts.
Herein, we have developed a novel system for aerobic
oxidation of alcohols and olens employing hierarchical
polystyrene/polyaniline microspheres supported copper(II)
salts. The catalytic activity of three heterogeneous copper(II)
catalysts were evaluated by the selective aerobic oxidation of
alcohols and olens. Great yields and recyclability were
demonstrated utilizing one of the as-synthesized heterogenous
copper catalysts, PS/PANI@Cu(OSO2CF3)2, under the optimized
reaction condition.
aSchool of Materials Science and Engineering, University of Science and Technology
Beijing, Beijing 100083, P. R. China. E-mail: gewang@mater.ustb.edu.cn; Fax: +86
10 62327878; Tel: +86 10 62333765
bCenter for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University,
Hangzhou 310018, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra06944g
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Na2CO3 (1 equiv.). The reaction mixture was stirred at
75 C under 1 atm of O2. Aer the reaction, the PS/PANI sup-
ported Cu(II) catalysts were ltered from the solution by
2. Experimental
2.1 Materials
ꢁ
centrifugation.
Styrene, aniline, iron(III)nitrate nonahydrate [Fe(NO3)3$9H2O],
ethanol, cupric chloride [CuCl2] and cupric nitrate [Cu(NO3)2]
were obtained from Beijing Chemical Reagent Co., Beijing,
China., potassium persulfate [K2S2O8, KPS], Copper(II) tri-
uoromethanesulfonate [Cu(OSO2CF3)2], 2,2,6,6-tetramethyl-
1-piperidinyloxy [TEMPO] were purchased from Alfa Aesar. All
the reagents above were used without any further purication
except the two monomers. Styrene was washed with 1 mol Lꢀ1
aqueous NaOH three times to remove the inhibitor, then
washed with water until neutral. Aniline was puried according
to literature procedure.49
2.6 Typical catalytic procedure for olen epoxidation
Olen (1 mmol) and PS/PANI@Cu(OSO2CF3)2 catalyst (0.2
mol%) were added to acetonitrile (5 mL) with trimethyl-
acetaldehyde (2 equiv.). The reaction mixture was stirred at
room temperature under 1 atm of O2. Aer the reaction, the
PS/PANI@Cu(OSO2CF3)2 catalyst were ltered from the solution
by centrifugation.
2.7 Catalyst recycling
The recyclability of the heterogenous catalyst were tested by
using the recovered catalyst according to the reaction condi-
tions described above. The catalyst was ltered from the reac-
tion solution aer the catalytic reaction ended, washed with
acetonitrile, dried and reused for the next run.
2.2 Synthesis of polystyrene (PS) microspheres
A soap-free emulsion polymerization was used to prepare the PS
seed nanoparticles according to literature.50 Deionized water (140
mL) and styrene (10 g) were added into a 250 mL four-necked
round-bottom ask with a mechanical stirrer, a N2 inlet and a
reux condenser. The oxygen in solution was removed by N2
bubbling for 10 min under stirring (300 rpm) before heated upꢀt1o
2.8 Characterization
Scanning electron microscope (SEM) images were taken by
ZEISS SUPRA 55 instrument at an acceleration voltage of 10 kV.
High resolution transmission electron microscope (HRTEM)
images and the EDX element maps were obtained by FEI Tecnai
F20 electron microscope with an acceleration voltage of 200 kV.
Fourier-transformed infrared spectra (FT-IR) were recorded by a
NICOLET 6700 infrared spectrophotometer. X-Ray photoelec-
tron spectroscopy (XPS) spectra were tested by a Kratos AXIS
Ultra DLD photo-electron spectroscope. C 1s line at 284.8 eV
from adventitious carbon was used as the reference for binding
energy. Thermal gravimetric analysis (TGA) were operated on a
Netzsch model STA F409 instrument in nitrogen atmosphere
70 ꢁC in an oil bath. KPS aqueous solution (10 mL, 0.023 g mL
)
was injected into the solution to start the polymerization. Aer the
polymerization lasted for 24 h, the PS microspheres were collected
by centrifugation, followed by washing with water and ethanol
three times and dried under vacuum.
2.3 Synthesis of sea urchin-like PS/PANI microspheres
The PS/PANI microspheres with sea urchin-like morphology were
prepared by the method illustrated in previous literature51 with
Fe(NO3)3$9H2O as the oxidant and PS microspheres as the
template. First, PS microspheres template (0.3 g) were added in
deionized water (20 mL) to form emulsion, followed by addition of
aniline (3 mmol) under vigorous stirring at room temperature.
The solution was stirred for 12 h to allow the swelling of aniline
into the PS templates. Immediately the oxidant Fe(NO3)3$9H2O
(18 mmol) was added, the aniline was polymerized at the interface
of PS template and the solution to form the shell and bers. The
solution became dark green aer 12 h, the PS/PANI microspheres
were centrifuged, washed with water until the supernatant
colorless, and dried under vacuum.
with the heating rate of 10 K minꢀ1
.
3. Results and discussion
3.1 Synthesis of PS/PANI microspheres supported copper(II)
homogenous catalysts
The preparation of PS/PANI@Cu(II) heterogenous catalysts was
achieved by mixing homogenous copper(II) salts and PS/PANI
support in acetonitrile. The as-synthesized catalysts were char-
acterized with SEM, EDX, TEM, XPS and FTIR spectra. The sea
urchin like PS/PANI microsphere supports of PS/PANI@Cu(II)
catalysts were prepared by a chemical oxidation polymerization
using PS spheres as template utilizing soap-free emulsion
polymerization method. The scanning electron microscopy
(SEM) images of PS/PANI composite microspheres are shown in
Fig. 1a. The PS/PANI hybrid microspheres, with a uniform size
of 700 nm, exhibit unique hierarchical sea urchin-like archi-
tecture assembled from ultrathin PANI nanobers on the
surface of PS microspheres. Immobilizing the three copper(II)
homogeneous catalysts on the surface of the PS/PANI micro-
spheres had no effect on the morphology (Scheme 1).
2.4 Preparation of PS/PANI microspheres supported Cu(II)
homogenous catalysts
The PS/PANI microspheres-supported Cu(II) catalysts were
synthesized by mixing the sea urchin-like PS/PANI micro-
spheres (300 mg) and homogenous Cu(II) compounds (300 mg)
in acetonitrile (15 mL) with stirring at room temperature for 48
h. Aerwards, the solid was ltered, washed with acetonitrile,
and dried under vacuum to obtain a black powder.
2.5 Typical catalytic procedure for alcohol oxidation
Alcohol (1 mmol) and supported Cu(II) catalyst (1 mol%) were
The elemental maps showed additional evidence of the
added to acetonitrile (5 mL) with TEMPO (0.5 equiv.) and existence and distribution of Cu2+ and other elements in
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were examined by thermal gravimetric analysis (TGA). The PS/
PANI supported copper catalysts showed relatively good
ꢁ
thermal stability with the signicantly weight loss at ꢂ400 C.
The increase of the amount of homogenous copper species
loading on the supports led to the decrease of the weight loss of
the supported catalysts according to the TGA results (Fig. S5†).
Two intense peaks at 934.9 eV and 954.7 eV were attributable
to Cu2+ 2p3/2 and Cu2+ 2p1/2, respectively. Shake-up satellite
peaks were also observed at 942.3–944.8 eV, which belongs to
the Cu2+ species.52 The Cu 2p XPS spectra also exhibits two
additional peaks centered at 932.4 and 952.2 eV, which were
attributable to the coordination between Cu2+ species and
amino groups.53
The peak-tted N 1s core-line spectrums of three PS/PANI@
Cu(II) composites were shown in Fig. 3. There are three different
N
species present in the spectra except that of PS/
PANI@Cu(NO3)2. The peaks attributed to neutral nitrogen in
imine (]N–) groups and amine (–NH–) groups along the chain
of PANI were at 398.4 and 399.7 eV, respectively. The binding
energy of the positively charged nitrogen (N+) atoms is at 400.8
eV. The existence of positively charged amine is the result of the
attachment of the Cu2+ ions to the PS/PANI particles surface. The
binding energies of the N species in PS/PANI@Cu(OSO2CF3)2 are
shied to higher regions as the result of the electronegativity of
the anions in copper(II) complex. Therefore, the electron with-
drawing triate was the strongest among three different copper
catalysts. The spectrum of PS/PANI@Cu(NO3)2 exhibited a
fourꢀth peak at 406.7 eV which is due to the existence of the N in
NO3 group.54
Fig.
1 SEM images of PS/PANI microspheres (a), PS/PANI@CuCl2
catalyst (b), PS/PANI@Cu(NO3)2 catalyst (c) and PS/PANI@Cu(OSO2CF3)2
catalyst (d).
corresponding anions (Fig.
2 and S3†). PS/PANI@CuCl2
anchored the most amount of copper(II) species among three
supported catalysts may affected by steric hindrance of anion.
In comparison to counterions NO3ꢀ and CF3SO3ꢀ, the smallest
anion, Clꢀ, has the smallest ionic radii and has very little bulk
with PANI-imine bound Cu2+. The small radii of Clꢀ results the
exceptionally high copper loadings in PS/PANI@CuCl2 catalyst.
The amount of Cu(II) was determined by ICP results, the
contents of Cu in PS/PANI@CuCl2, PS/PANI@Cu(NO3)2 and PS/
PANI@Cu(OSO2CF3)2 were 5.25 wt%, 1.79 wt% and 1.54 wt%,
respectively. The thermal stability of the heterogenous catalysts
Characteristic bands at 1570 cmꢀ1 and 1494 cmꢀ1 of FTIR
spectra (Fig. 4) were assigned to be the stretching deformation
of the C]C in quinoid and benzenoid rings, respectively. The
absorption bands at 1300 cmꢀ1 and 1141 cmꢀ1 were attributed
to the stretching of C–N of the secondary amines and the
bending of C–H.55 All four peaks above were observed in PANI
supports and three heterogeneous copper catalysts. The area
ratios of quinoid rings to benzenoid rings in three supported
Scheme 1 Schematic illustration of the synthesis process of the PS/PANI@Cu(II) catalysts.
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Fig.
3
Cu 2p XPS spectra of three supported catalysts ((1) PS/
PANI@CuCl2, (2) PS/PANI@Cu(NO3)2 and (3) PS/PANI@Cu(OSO2CF3)2)
(a) and N 1s XPS spectra of PS/PANI@CuCl2 (b), PS/PANI@Cu(NO3)2 (c)
and PS/PANI@Cu(OSO2CF3)2 (d).
Fig.
4 FTIR spectra of PS/PANI (a), PS/PANI@CuCl2 (b), PS/
Fig. 2 PS/PANI@CuCl2: (a) TEM image, (b) EDX elemental maps of Cu,
(c) EDX elemental maps of Cl; PS/PANI@Cu(NO3)2: (d) TEM image, (e)
EDX elemental maps of Cu, (f) EDX elemental maps of O and PS/
PANI@Cu(OSO2CF3)2: (g) TEM image, (h) EDX elemental maps of Cu, (i)
EDX elemental maps of S.
PANI@Cu(NO3)2 (c) and PS/PANI@Cu(OSO2CF3)2 (d).
3.2 Catalytic aerobic oxidation of alcohol
The aerobic oxidation activity of the as-synthesized supported
copper(II) catalysts was evaluated employing alcohol substrates
employing TEMPO as the radical initiator.58 Benzaldehyde
cannot be produced through aerobic oxidation of benzyl alcohol
in the absence of catalyst or in the presence of PS/PANI support
(Table 1, entries 1 and 2). Decent yield was achieved using PS/
PANI@CuCl2, which suggested the crucial role of copper cata-
lyst (Table 1, entry 3). The amounts of PS/PANI@CuCl2, PS/
PANI@Cu(NO3)2 and PS/PANI@Cu(OSO2CF3)2 added in this
reaction were 12.1 mg, 35.4 mg and 41.1 mg based on the same
Cu loading. The evaluation of all three supported copper cata-
lysts suggested that PS/PANI@Cu(OSO2CF3)2 was the best
catalysts were all about 0.6 which showed good agreement with
the results calculated by XPS spectra. Due to the difference on
the anion of copper salts, the characteristic peaks vary signi-
cantly. For PS/PANI@CuCl2, the characteristic peak at 1600
cmꢀ1 is caused by the vibration of chloride anion. For PS/
PANI@Cu(NO3)2, the peak at 1380 cmꢀ1 was attributed to
asymmetric stretching of NO3. For PS/PANI@Cu(OSO2CF3)2,
peaks at 1248 and 1033 cmꢀ1 are assigned to asymmetric and
symmetric vibrations of SO3, respectively, which are the indic-
ative of the coordination behavior of the triate ligand.56,57
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Table 1 The optimization of aerobic oxidation of benzyl alcohola
Table
2
Benzylic alcohol aerobic oxidation using PS/
PANI@Cu(OSO2CF3)2 catalysta
Conversionb Selectivityb
Entry Catalyst
Solvent
Yieldb Selectivityb
Entry Substrate
Product
1
2
3
4
5
6
7
8
—
PS/PANI
CH3CN
CH3CN
CH3CN
CH3CN
0%
0%
53%
45%
99%
—
—
1
2
99%
95%
99%
99%
PS/PANI@CuCl2
PS/PANI@Cu(NO3)2
PS/PANI@Cu(OSO2CF3)2 CH3CN
PS/PANI@Cu(OSO2CF3)2 ClCH2CH2Cl 85%
PS/PANI@Cu(OSO2CF3)2 PhCH3
PS/PANI@Cu(OSO2CF3)2 EtOH
99%
99%
99%
99%
99%
99%
72%
22%
3
4
5
99%
92%
65%
99%
99%
99%
a
Reaction conditions: alcohol (1 mmol), 1 mol% catalyst based on
copper, 5 mL solvent, TEMPO (0.5 mmol), Na2CO3 (1 mmol) 75 ꢁC for
24 h, 1 atm O2. Determined by GC-MS using nitrobenzene as the
internal standard.
b
catalyst (Table 1, entries 3–5). The superb catalytic performance
of PS/PANI@Cu(OSO2CF3)2 is attributed to the coordination
ability of the anions to the Cu2+. The interactions between the
copper(II) center and the chloride and nitrate ions are stronger
than triate because triate anion is known to be weakly coor-
dinated to various metal ions.59 Therefore, the alcoholate
intermediate can coordinate with the Cu2+ in copper triate
complex much easier than the other supported copper salts.
Further solvent screens showed that CH3CN is the most suitable
solvent for the benzyl alcohol oxidation (Table 1, entry 6),
because of its polarity and ability to stabilize the Cu(I) species
formed during the catalytic cycle.59 Chlorinated solvents, such
as 1,2-dichloroethane, gave slightly compromised yields
(Table 1, entry 7). Low polarity solvents, such as toluene, gave
only low yield at the standard reaction condition (Table 1, entry
8). Furthermore, alcoholic solvents, such as ethanol, were not
suitable due to coordination and competing oxidation
reactions.
With the optimal reaction condition on hand, several alcohol
substrates were chose to study the aerobic oxidation catalytic
performance of PS/PANI@Cu(OSO2CF3)2 under the optimal
reaction condition. Benzyl alcohol was transformed to the cor-
responding benzaldehyde in 99% yield aer 24 h (Table 2,
entry 1). For aerobic oxidation of benzyl alcohol, the turnover
frequency (TOF) was calculated to be 4.125 hꢀ1 using PS/
PANI@Cu(OSO2CF3)2 as catalyst. In the meanwhile, M. F.
Semmelhack reported homogeneous TEMPO/CuCl system with
TOF of 4.7 hꢀ1 for aerobic oxidation of benzyl alcohol.60 Roger A.
Sheldon reported PIPO/CuCl catalyzed aerobic oxidation of
benzyl alcohol with a TOF of 2.3 hꢀ1 in dimethylformamide
(DMF) solvent.61 MCM-41-ether-TEMPO/CuCl synthesized by D.
Brunel gave an average TOF of 0.5 hꢀ1 in aerobic oxidation of
benzyl alcohol reaction.62 The TOF of benzyl alcohol catalyzed
by Cu–Mn oxide/C oxidation was calculated to be 5.8 hꢀ1 at 80–
100 ꢁC in CH2Cl2.63 The efficiency of our heterogenous catalytic
6
7
8
33%
57%
92%
99%
99%
99%
9
5%
99%
a
Reaction conditions: alcohol (1 mmol), 1 mol% PS/PANI@Cu(OSO2CF3)2
catalyst, 5 mL acetonitrile, TEMPO (0.5 mmol), Na2CO3 (1 mmol), 75 ꢁC
for 24 h, 1 atm O2. b Determined by GC-MS.
system is comparable to other efficient aerobic oxidation
system. p-Methyl benzyl alcohol also reacted smoothly under
the same reaction conditions (Table 2, entry 2). Electron-rich
benzyl alcohols, such as p-methoxy benzyl alcohol and
m-methoxy benzyl alcohol were suitable reactive under the
optimal reaction conditions, which gave 99% and 92% yield,
respectively (entries 3 and 4). Electron-withdrawing functional
groups were tolerated utilizing PS/PANI@Cu(OSO2CF3)2 cata-
lyst, however, compromised yields were observed for p-F and o-F
benzyl alcohols due to lower reactivity (entries 5 and 6). Heter-
oaromatic alcohol, 2-thiophenemethanol, reacted well in the
presence of the as-synthesized catalyst (entry 7). In order to test
the transformation from secondary alcohols to ketone, 1-phenyl-
ethanol was catalyzed to acetophenone as the model reaction.
Cinnamyl alcohol was also evaluated as the allylic alcohol and
cinnamaldehyde was formed as the only product in 92% yield
(Table 3, entry 8). Unfortunately, our catalytic system is not
compatible with secondary alcohols, such as 1-phenylethanol,
and low conversions were observed. Prolonged reaction time
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Table 3 Epoxidation of olefins using PS/PANI@Cu(OSO2CF3)2 catalyst with oxygena
Entry
1
Substrate
Product
Time (h)
18
Conversionb (%)
>99
Selectivityb (%)
>99
2
3
30
48
65
50
>99
>99
4
9
>99
>99
a
Reaction conditions: olen (1 mmol), 0.2 mol% PS/PANI@Cu(OSO2CF3)2 catalyst, 5 mL acetonitrile, trimethylacetaldehyde (2 mmol), room
temperature, 1 atm O2. b Determined by GC-MS.
and increased reaction temperature did not improve the yields aerobic epoxidation of cyclic alkenes were evaluated by PS/
of the desired acetophenone product (Table 2, entry 9).
PANI@Cu(OSO2CF3)2 in acetonitrile at room temperature. The
Hot ltration test was conducted to conrm the heteroge- results of the catalytic epoxidation are listed in Table 3. Bulkier
neous nature of the aerobic oxidation catalysis with PS/ cycloalkenes such as cyclooctene, cyclododecene and norbor-
PANI@Cu(OSO2CF3)2. The PS/PANI@Cu(OSO2CF3)2 catalyst was nene were transformed to corresponding epoxides with excel-
centrifuged and isolated at 9 h and the mixture was stirred for lent selectivity. The aerobic oxidation of cyclooctene reacted
ꢁ
another 15 h at 75 C. The increase in conversion was stopped smoothly exhibiting remarkable conversion over 99% of cyclo-
aer the ltration step, and the yield of aldehyde remained 65% octene oxide as the sole product. Cyclododecene resulted in
(Fig. 5). This result indicated Cu(OSO2CF3)2 was strongly bound moderate yields of cyclododecene oxide (conversion 65%,
to the surface of the PS/PANI support and only very trace selectivity 99%). Poor yield of exo-epoxynorbornane was
amounts of Cu2+ species exist in solution.
obtained even aer a reaction time of 48 h. The tri-substituted
alkene such as a-pinene resulted in a-pinene oxide with high
selectivity and conversion of over 99% (Table 3, entry 4). For
aerobic olen epoxidation reaction, PS/PANI@Cu(OSO2CF3)2
catalyzed cyclooctene epoxidation and a-pinene with TOF of
27.5 hꢀ1 and 55 hꢀ1, respectively. This result is signicantly
higher than literature reported TOF for epoxidation reaction.
Shun-Ichi Murahashi synthesized copper-crown ether catalyzed
3.3 Catalytic aerobic epoxidation of olens
The catalytic activity was further extended to the epoxidation of
a variety of olens in the presence of molecule oxygen. The
olens epoxidation using oxygen as oxidant with TOF of 5.82
hꢀ1
.
Shun-Ichi Murahashi reported Cu(OH)2 catalyzed olen
64
epoxidation with TOF of 5.88 hꢀ1
.
65
The recyclability of aerobic oxidation of benzyl alcohol and
cyclooctene were studied at 75 ꢁC and room temperature,
respectively, utilizing the PS/PANI@Cu(OSO2CF3)2 catalyst in
acetonitrile. Six recycling tests of PS/PANI@Cu(OSO2CF3)2
catalyst were carried out and the selectivity of the product was
retained at 99%. Conversion of benzyl alcohol and cyclooctene
maintained 90% and 94%, respectively, with slight loss
compared with the rst run. Yields of benzaldehyde decreased a
little more than that of cyclooctene oxide during the each
successive reaction because of the higher reaction temperature
(Fig. 6). The high conversion and selectivity aer six cycles
indicated the high stability of supported copper heterogeneous
catalyst and low leakage of copper ion. FTIR spectrum showed
Fig. 5 Leaching test for benzyl alcohol oxidation catalyzed by PS/
PANI@Cu(OSO2CF3)2.
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Fig. 6 Recycling tests of PS/PANI@Cu(OSO2CF3)2 for benzyl alcohol
(red) and cyclooctene (gray) oxidation.
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no obvious difference between fresh PS/PANI@Cu(OSO2CF3)2
and the one aer six uses (Fig. S4†).
4. Conclusions
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In summary, three novel supported copper(II) catalysts have
been prepared by anchoring homogeneous copper(II) salts onto
the hierarchical PS/PANI microspheres. Selective aerobic
oxidation of the alcohols were studied utilizing supported cop-
per(II) catalysts in different solvents. PS/PANI@Cu(OSO2CF3)2
catalysts showed the best catalytic activity towards the aerobic
oxidation of the alcohols and a wide variety of alcohol
substrates were tolerated. The great activity of as-synthesized
PS/PANI@Cu(OSO2CF3)2 was further extend to catalyze the
aerobic epoxidation reaction. The catalytic activity of heteroge-
neous catalysts was maintained without signicant loss of yield
aer six recycles, which indicated the high stability of the as-
synthesized PS/PANI@Cu(OSO2CF3)2 catalyst during the reac-
tion. These results indicate the advantages and great immobi-
lization ability of the PANI support.
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We thank the National High Technology Research and the Devel-
opment Program of China (863 Program) (no. 2013AA031702) and
Beijing Natural Science Foundation (Grant no. 2144052) for nan-
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