Immobilized Cu(ii) and Co(ii) salen complexes on graphene oxide and their catalytic activity for aerobic epoxidation of styrene

Article abstract of DOI:10.1039/c3nj00099k

Novel graphene oxide (GO) tethered Cu(ii) and Co(ii) salen complexes [M-Salen-GO (M = Cu, Co)] were synthesized via a stepwise procedure and examined as catalysts in the epoxidation of styrene. Acetonitrile was used as a solvent and air as the oxidant in combination with a sacrificial co-reductant isobutyraldehyde. A comparison with homogeneous salen complexes of Co and Cu and graphene oxide (GO) tethered salen complexes was made. Heterogeneous copper(ii) and cobalt(ii) catalysts were confirmed by X-ray diffraction (XRD), FT-IR, diffusion reflection UV-visible spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICP-AES), TEM, TG, Raman and XPS. FT-IR, diffusion reflection UV-visible and inductively coupled plasma atomic emission spectroscopy (ICP-AES), along with TG, Raman and XPS results showed that Cu(ii) and Co(ii) salen complexes were successfully grafted on GO; X-ray diffraction (XRD) and TEM indicated that the sample structures were well preserved. It was found that heterogeneous catalysts were active and showed good recoverability without significant loss in activity and selectivity within successive runs.

Full text of DOI:10.1039/c3nj00099k

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Immobilized Cu(II) and Co(II) salen complexes on
graphene oxide and their catalytic activity for aerobic
Cite this: DOI: 10.1039/c3nj00099k
epoxidation of styrene†
Zhifang Li,a Shujie Wu,a Hong Ding,b Dafang Zheng,b Jing Hu,a Xu Wang,c
Qisheng Huo,b Jingqi Guan*a and Qiubin Kan*a
Novel graphene oxide (GO) tethered Cu(II) and Co(II) salen complexes [M–Salen–GO (M = Cu, Co)] were
synthesized via a stepwise procedure and examined as catalysts in the epoxidation of styrene. Acetonitrile was
used as a solvent and air as the oxidant in combination with a sacrificial co-reductant isobutyraldehyde. A
comparison with homogeneous salen complexes of Co and Cu and graphene oxide (GO) tethered salen
complexes was made. Heterogeneous copper(II) and cobalt(II) catalysts were confirmed by X-ray diffraction
(XRD), FT-IR, diffusion reflection UV-visible spectroscopy, inductively coupled plasma atomic emission
spectroscopy (ICP-AES), TEM, TG, Raman and XPS. FT-IR, diffusion reflection UV-visible and inductively coupled
plasma atomic emission spectroscopy (ICP-AES), along with TG, Raman and XPS results showed that Cu(II) and
Co(II) salen complexes were successfully grafted on GO; X-ray diffraction (XRD) and TEM indicated that the
sample structures were well preserved. It was found that heterogeneous catalysts were active and showed
good recoverability without significant loss in activity and selectivity within successive runs.
Received (in Montpellier, France)
24th January 2013,
Accepted 4th March 2013
DOI: 10.1039/c3nj00099k
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recycling of the catalyst. Therefore, the heterogenization of
homogeneous complexes on solids such as organic polymers,12,13
Introduction
SBA-15,8 MCM-41,14 zeolite Y,15 zeolite X,16 clays17 is fascinating.
Generally, to heterogenize homogenous complexes onto insoluble
solids, four methods have been developed: (i) encapsulated
method, (ii) covalent attachment method, (iii) coordination
method, and (iv) ion interaction method. However, covalent
attachment is the most commonly used and most effective
method for anchoring metal complexes. For example, Hu
et al.18 have recently reported that Schiff base molybdenum(VI)
complexes immobilized onto zirconium poly(styrene-phenylvinyl-
phosphonate)-phosphate showed comparable or even higher con-
version and chemical selectivity for epoxidation of olefins
compared with other heterogeneous catalysts known from
correlative literatures.19 Furthermore, these catalysts were easily
recovered by simple filtration and could be reused at least ten
times with little loss of activity. Xu et al.20 reported that composite
catalysts of Co-ZSM-5 coordinated with vanillic aldehyde salicyl-
hydrazone (L2) exhibited very good activity with 90.5% of styrene
conversion and 91.1% of epoxide selectivity in the epoxidation
of styrene with air as oxidant. Moreover, Co-ZSM-5 (L2) was
recyclable, and the leaching of cobalt from Co-ZSM-5 (L2) could
be controlled in the range of o9%.
The oxidation of olefins plays an important role in fine chemicals
and chemical intermediates.1–5 In recent years, a considerable
amount of research was dedicated to developing efficient catalysts
with easy product separation and inexpensive eco-friendly
oxidants,6–9 for example air, O2 and H2O2. However, compared
with molecular oxygen, the relatively high cost of H2O2 seriously
hinders its wide application in oxidation reactions.10 Therefore,
air and molecular oxygen are very attractive and desirable
oxidants for the epoxidation of alkenes with respect to environ-
mental and economic consideration.
Many transition metal (Cu and Co) complexes are well
known for active homogeneous catalysts for oxidation reactions
as well as reduction of organic substrates and for the synthesis
of fine chemicals.11 However, the homogenous catalysts suffer
from drawbacks such as deactivation, difficult recovery and
a College of Chemistry, Jilin University, Changchun, 130023, P.R. China.
E-mail: guanjq@jlu.edu.cn, qkan@mail.jlu.edu.cn; Tel: +86 431 88499140
b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
College of Chemistry, Jilin University, Changchun 130012, P.R. China
c State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, P.R. China
Salen is a bidentate ligand and possesses the ability to chelate
many metal ions and form stable complexes, which have been
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3nj00099k
c
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extensively studied due to their potential use as catalysts in oxidation
of alkenes. Jiang et al.21 reported that the catalytic activity of cobalt
salophen (N,N0-bis(salicylidene)o-phenylenediamine) complex
immobilized on montmorillonite in the epoxidation of cyclohexene
is very good. The catalyst was reused several times and its high
activity and epoxide selectivity remained almost unchanged.
Graphene oxide (GO), an oxidized form of graphene, is a layered
material consisting of hydrophilic oxygenated graphene sheets
(graphene oxide sheets) containing very high specific surface area
with intrinsic low mass, ample oxygen carrying functionalities, and
excellent mechanical strength.22,23 In 1958, William et al. prepared
graphite oxide.24 Consequently, in order to obtain graphene oxide
(GO) starting from graphite oxide several methods can be applied:
(a) the parental bulk graphite oxide has subsequently to be
dispersed in basic solutions to produce monomolecular sheets.25
(b) Graphite oxide is easily hydrated, bringing about a distinct
increase of the inter-planar distance.26 Instead of water as the polar
solvent (e.g. alcohols) can be exploited too.27 (c) Graphite oxide
exfoliates and decomposes when rapidly heated at moderately high
temperatures.28 It has been used as an important material due to
its unique nanostructure and the prospect of various applications
such as composite materials, sensors, solar cells, catalysis, and gas
storage.29–33 As we all know, graphene oxide has the abundance of
functional groups (such as carboxyl, carbonyl, hydroxyl and epox-
ide) on its basal planes and edges of GO and is inexpensive
compared with zeolites such as MCM-41 and SBA-15. Therefore,
GO performs as an excellent supporting material. Mungse et al.22
have reported that grafting of oxo-vanadium Schiff base on
graphene nanosheets was found to be highly efficient and showed
comparable catalytic reactivity as its homogenous analogue. More-
over, graphene-bound oxo-vanadium Schiff base catalyst was found
to be easily recoverable and recyclable without significant loss in
the catalytic activity. However, to the best of our knowledge, the
catalytic activity of the covalent tethering of Cu(II) and Co(II) Salen
complexes onto aminopropyl-functionalized GO in aerobic styrene
epoxidation have not been reported.
Fig. 1 High angle X-ray diffraction patterns of (a) GO, (b) SalenH2–GO, (c) Co–
Salen–GO, and (d) Cu–Salen–GO.
and (002) reflections of graphite oxide and graphite powder,
respectively, which clearly indicates that part of the graphite
powder was closer to graphite oxide and a small part of graphite
powder did not change (Fig. 1a).22,34 However, for the immobilized
complexes of cobalt and copper catalysts (Fig. 1b–d), the diffrac-
tion peaks become weak and broad at around 2y = 11.91 after the
introduction of bulky organometallic groups, indicating that the
developed catalysts have been successfully prepared.
TEM and HRTEM images of Co–Salen–GO and Cu–Salen–GO
showed that the immobilized complexes of cobalt and copper
catalysts have a layered structure (Fig. 2). It can be seen that
the crumpled nanosheets are in an agglomerated phase. In
addition, the supported catalysts with some layers can be seen
in Fig. 3.22 The N2 adsorption–desorption isotherms of
Co–Salen–GO and Cu–Salen–GO are depicted in Fig. 4. The sup-
ported materials maintain the characteristics of type IV isotherms.
The BET surface area of Co–Salen–GO and Cu–Salen–GO was 13
and 11 m2 gꢀ1 respectively.
Spectroscopic characterization
In our early work, SBA-15 supported Cu(II) and Co(II) Salen
complexes were prepared and examined as catalysts for styrene
oxidation. It was found that both heterogeneous copper(II) and
oxovanadium(IV) catalysts were more active than their homoge-
neous analogues.8 Furthermore, SBA-15 supported Cu(II), Co(II)
and Fe(III) tetrahydro-salen and salen complexes by covalent
tethering and their catalytic performances in aerobic epoxidation
of styrene were also investigated.7 Herein, we report the design
and characterization of GO supported Cu(II) and Co(II) Salen
complex catalysts synthesized via the covalent attachment
method. Moreover, the catalytic performances of both hetero-
geneous copper(II) and cobalt(II) catalysts in aerobic epoxidation
of styrene using air as oxidizing agent were also discussed.
Fig. 5 shows the FT-IR spectra of the pure support GO and
supported complexes of cobalt(II) and copper(II). The pure
support GO exhibited bands at 3409, 1725, 1621, 1220, and
1058 cmꢀ1 due to the stretching mode of O–H, CQO, CQC,
C–O and C–O–C bands, respectively.22 Compared with pure
support GO, A new band appeared at 1618 cmꢀ1 is due to imine
Results and discussion
Structural integrity studies
The high angle X-ray diffraction patterns of GO, SalenH2–GO,
Co–Salen–GO, and Cu–Salen–GO are shown in Fig. 1. For GO,
two peaks at ca. 2y = 11.81 and 2y = 26.41 correspond to (001)
Fig. 2 TEM images of (a) Co–Salen–GO and (b) Cu–Salen–GO.
c
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Fig. 6 UV-vis spectra of (a) GO, (b) NH2–GO, (c) Co–Salen–GO, and
(d) Cu–Salen–GO.
(CQN) in the Cu–Salen–GO material. In addition, in the region
of 1600–1300 cmꢀ1, the spectrum of Cu–Salen–GO material
shows weak peaks due to C–O, C–N and aromatic ring vibrations,
indicating that the salen ligand is successfully immobilized onto
GO.35 A similar result is obtained for Co–Salen–GO. Moreover,
ICP-AES analysis for Co–Salen–GO and Cu–Salen–GO catalysts
also indicates the successful introduction of metal ions.
The diffusion reflection UV-vis spectra of the solid GO, NH2–
GO, Co–Salen–GO and Cu–Salen–GO are depicted in Fig. 6.
Compared with GO and NH2–GO, the absorption bands
observed in the 260–430 nm region can be assigned to the
n-p*, p–p* ligand charge transfer and the ligand-to-metal
charge transfer (MLCT) in Co–Salen–GO and Cu–Salen–GO.7,36
In addition, the very weak bands at ca. 570 nm for Co–Salen–GO
and 630 nm for Cu–Salen–GO may be attributed to d–d transi-
tions of the centered Co and Cu ions, supporting the supposition
that the organometallic complexes have been successfully
anchored on GO.
Fig. 3 HRTEM images of (a) Co–Salen–GO and (b) Cu–Salen–GO.
Raman spectroscopy is the most direct and nondestructive
technique to distinguish ordered and disordered crystal struc-
tures of carbon materials. G band is generally assigned to the
E2g phonon of C sp2 atoms, whereas D band is a breathing
mode of k-point phonons of A1g symmetry.37–39 Fig. 7 displays
Raman spectra of GO, Co–Salen–GO and Cu–Salen–GO. The
Raman spectra of all the complexes exhibited two remarkable
peaks at around 1351, and 1596 cmꢀ1, which were associated
with the well-defined D band and G band, respectively. The
value of the D : G intensity ratio is B0.82 for GO while the value
of D : G intensity ratio is increased to B0.87 for Co–Salen–GO
and B0.84 for Cu–Salen–GO, indicating that a decrease in the
average size and an increase in the number of small crystalline
graphitic domains,37,38 which is consistent with the XRD
results.
Fig. 4 N2 adsorption–desorption isotherms profiles of (a) Co–Salen–GO and (b)
Cu–Salen–GO.
TG studies
TG analysis results of Co–Salen–GO and Cu–Salen–GO are
depicted in Fig. 8. The TG curve recorded under N2 atmosphere
shows a three step weight loss at temperatures ranging from 35 1C
to 800 1C. The first region at temperatures between 35 1C and
Fig. 5 FT-IR spectra of (a) GO, (b) Co–Salen–GO, and (c) Cu–Salen–GO.
c
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Fig. 9 XPS spectra of (a) GO, (b) Co–Salen–GO, and (c) Cu–Salen–GO.
Fig. 7 Raman spectra (G and D bands) of (a) GO, (b) Co–Salen–GO, and (c) Cu–
Salen–GO.
indicates that all the ligands were successfully covalently
anchored onto GO.
250 1C corresponds to removal of physically adsorbed water. The
second region is due to the combustion of amine and undigested
oxygen carrying functionalities in the range of 250 to 450 1C.8,22 The
third region is mainly related to the decomposition of Schiff base
groups in the temperature range from 450 to 800 1C. Weight loss of
ca. 11.5% for Co–Salen–GO or ca. 13.5% for Cu–Salen–GO was
observed. The loadings of Schiff base ligands are 0.422 mmol gꢀ1
and 0.495 mmol gꢀ1 based on the weight losses for Co–Salen–GO
and Cu–Salen–GO. The contents of cobalt and copper are 0.210 and
0.195 mmol gꢀ1 estimated by ICP-AES, respectively. The molar
ratio of ligand to metal is greater than 1.0 : 1 in all immobilized
complexes catalysts, indicating that all metal ions have coordinated
to the ligand. Moreover, these results suggest that metal complexes
have been anchored onto NH2–GO.
Catalytic performance
The catalytic properties of various catalysts in the aerobic
epoxidation of styrene are shown in Table 1. For comparison,
the reaction results of the homogeneous complexes and Salen–
GO as catalysts are also given. When acetonitrile was used as
the solvent and air was used as the oxidant in combination with
a sacrificial co-reductant isobutyraldehyde, all catalysts are active
for the aerobic oxidation of styrene. The Co–Salen–GO catalyst
shows 70.1% conversion of styrene with a TOF of 83.5 hꢀ1 and
35.3% selectivity to the styrene oxide after 6 h. As for Cu–Salen–
GO catalyst, it exhibits high activity and good epoxidative
selectivity in MeCN, corresponding to 73.5% conversion of
styrene with a TOF of 94.2 hꢀ1 and 54.1% selectivity to the
styrene oxide. However, the homogeneous complexes and Salen–
GO show a lower activity than the corresponding tethered
analogues, Co–Salen–GO and Cu–Salen–GO under the same
reaction conditions. These results suggest that the site-isolation
and discrete active species promote the aerobic epoxidation
of styrene. At the same time, the higher activity of tethered
catalysts may also be ascribed to the active sites of the bulky
organometallic complexes and the synergistic beneficial catalytic
interaction between the metal complexes and GO, which is
in accordance with those reported in our early work and
literature.8,41
XPS studies
Results of XPS measurements are presented in Fig. 9. The
binding energies of C 1s and O 1s for all the complexes are
286.1 eV and 532.0 eV, respectively. The C element and the O
element derive from the APTES, Salen and the graphene oxide.
Compared with pure support GO, silicon element (Si) and
nitrogen element (N) are assigned to the presence of APTES
and Salen in Co–Salen–GO and Cu–Salen–GO.40 This result
The catalytic activity and product selectivity are also depen-
dent on the type of central metals and metal loadings. In our
experimental results, the conversion of styrene over two series
of catalysts decreases in the order Cu 4 Co (Table 1), which is
opposite to the results reported in our early work.7 This result
can be ascribed to the constraints and steric hindrance after
immobilization of cobalt complexes onto the GO.
The recoverability and stability of tethered catalysts are
studied in repeated epoxidation reactions. The catalysts
Co–Salen–GO and Cu–Salen–GO were recycled three times and
four times for the epoxidation of styrene, respectively and the
results are presented in Table 1. It is evident from the results that
the two recovered catalysts show good recoverability without
Fig. 8 TG curves of (a) Co–Salen–GO, and (b) Cu–Salen–GO.
c
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Table 1 Epoxidation of styrene over various catalysts
Product
selectivityc (mol%)
Styrene
Bza Others
Catalysts
Cycle conversiona (%) TOFb (hꢀ1
)
So
Salen–GO
1
1
2
3
1
2
3
4
1
1
46.5
70.1
69.0
67.6
73.5
73.2
64.5
63.9
64.2
63.1
—
83.5
—
80.9
94.2
—
—
142.6
52.2
52.0
35.3 64.7
0
Co–Salen–GO
Co–Salen–GO
Co–Salen–GO
Cu–Salen–GO
Cu–Salen–GO
Cu–Salen–GO
Cu–Salen–GO
Co–Salen
35.3 53.4 11.3
34.1 56.6 9.3
33.9 57.5 8.6
54.1 45.9
0
30.0 66.9 3.1
35.0 60.9 4.1
32.2 58.1 9.7
50.3 40.7 9.0
58.6 37.8 3.6
Cu–Salen
a
Reaction conditions: catalyst 50 mg (5 mg for the untethered catalyst),
styrene 1.14 ml (10 mmol), CH3CN 10 ml, flow of air 80 ml minꢀ1
,
isobutyraldehyde 2.28 ml (25 mmol), temperature 80 1C and time 6 h.
b
TOF, hꢀ1: (turnover frequency) moles of substrate converted per mole
c
metal ion per hour. So: Styrene oxide, Bza: benzaldehyde and Others:
including benzoic acid, phenylacetaldehyde.
significant loss of activity or selectivity in the same manner as that
of the fresh catalysts. We filtered the reaction mixture after the
recycled reaction and analyzed the solution with inductively
coupled plasma analysis techniques. The result indicates that there
is no cobalt in the solution. But the copper content for the used
catalysts just decreased slightly (after 4 cycles) compared with the
fresh one. We also did a leaching test to verify the heterogeneity of
the catalytic process. A leaching result of a representative catalyst,
Cu–Salen–GO, is listed in Fig. 10. The catalyst was filtered at the
reaction temperature (80 1C). Half the volume was filtered (2 h) and
the resulting clear solution was allowed to react for 2 h. It seemed
clear from this finding that styrene could just be converted at a very
low rate. The filtrate neither shows any activity towards oxidation of
styrene nor shows the presence of copper by the ICP-AES analysis.
These results suggest that the large majority of the catalysis is
carried out by a copper catalyst truly heterogeneous in nature.
In order to characterize the recycled catalyst Cu–Salen–GO (after
4 cycles), FT-IR spectra (Fig. 1S, ESI†) and XRD (Fig. 2S, ESI†) were
Scheme 1 Proposed mechanism for aerobic epoxidation of styrene with metal
complexes in the presence of isobutyraldehyde.
done for the recovered complex after four catalytic run. Fig. 1S and
2S are given in the ESI†. IR spectra and XRD of recycled catalysts
were found be nearly identical to that of fresh catalysts, which
suggests the stability and recyclability of the catalysts. These
observations are in consonance with the result of a leaching test.
According to our catalytic experimental results, our early
work and comparison with the proposed mechanism reported
in the literature for related systems,42,43 the epoxidation of
styrene performed on the Schiff base metal center was oxidized
by O2, possibly based on the mechanism depicted in Scheme 1.
It is proposed that the aerobic epoxidation of styrene with
iron(III) complexes in the presence of isobutyraldehyde proceeds
via free radical intermediates.42 Furthermore, Bakhvalov et al.43
have reported the mechanism and kinetics for the epoxidation of
alkenes in the presence of Ni(acac)2, using air as oxidant in
combination with a sacrificial co-reductant isobutyraldehyde. In
our case, isopropylacyl radical produced through the electron
transfer to the resultant metal complexes forms the corres-
ponding oxo-metal-complex radical 1 in reaction with O2, which
leads to peroxide-metal-complexes radical 2. A radical chain
reaction leads to styrene oxide benzaldehyde and other products.
Experimental
Materials
Graphite powder, H2SO4, KMnO4, 30% H2O2, 5% HCl, salicyl-
aldehyde (99%), ZnCl2, diethyl ether, petroleum ether, MgSO4,
NaHCO3, styrene (98%), isobutylaldehyde, 3-aminopropyl-
triethoxysilane 3-APTES (Aldrich), Co(NO3)2ꢁ6H2O, Cu(CH3COO)2,
ethylenediamine. All organic solvents are A.R. grade. Na/diphenyl-
ketone ketyl was used to dry toluene, and then the toluene is
Fig. 10 Kinetic profile of aerobic epoxidation of styrene in combination with
isobutyraldehyde as co-reductant (J) and leaching experiment of Cu–Salen–GO
catalyst (n) (continuing the reaction after removing the catalyst after 2 h).
Reaction conditions are analogous to Table 1.
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distilled under a N2 atmosphere. The other reagents were used
without further treatment.
Synthetic procedures (Scheme 2)
The procedure for the covalent attachment copper(II) and
cobalt(II) salen complexes to amino-modified graphene oxide
by a stepwise procedure is depicted in Scheme 2 and described
as follows.
Preparation of graphene oxide (GO)
Graphene oxide was synthesized according to the litera-
ture.22,24,40,44 As a typical run, 2.5 g graphite powder was added
to a 250 ml flask containing concentrated H2SO4. The mixture
was stirred in an ice bath for 30 min. 7.5 g KMnO4 was then
added into the mixture gradually with stirring and cooling. The
reaction mixture was stirred at 35 ꢂ 3 1C for 30 min. Afterwards,
115 ml deionized water was slowly added under vigorous
stirring. After that, the mixture was heated to 95 1C, stirred
for 15 min and transferred to a 1000 ml beaker. Finally, 350 ml
deionized water and 25 ml 30% H2O2 solution were added to
the beaker. The resulting solid was filtered, washed with a large
amount of 5% HCl solution and deionized water and dried at
70 1C for one week. The dried sample was denoted as GO.
Preparation of amino-functionalized graphene oxide (NH2–GO)
Amino-functionalized graphene oxide was synthesized according
to our early work.8,45 The general producing procedure is as
follows: 2.3 mmol 3-aminopropyltriethoxysilane (3-APTES) was
added to a stirred solution of GO (1.0 g, 30 ml) in dry toluene.
After that, the mixture was refluxed at 110 1C under N2 atmo-
sphere for 24 h. The sample was filtered, washed repeatedly with
toluene and dried. The dried sample was denoted as NH2–GO.
Synthesis of N,N0-bis(5-chloromethyl-
salicylidene)ethylenediamine (CM–SalenH2)
5-Chloromethyl-2-hydroxybenzaldehyde was prepared and
characterized as described in our early work.8 A mixture of
80 mmol salicylaldehyde, 12 ml 38% aqueous formaldehyde
and 0.6 g ZnCl2 was introduced into a three-necked flask.
Afterwards, 50 ml conc. HCl was added and the mixture was
stirred at room temperature under N2 atmosphere for 24 h. The
white solid was filtered and dissolved with diethyl ether. The
combined organic phases were collected by filtration, washed and
then dried. Furnished 5-chloromethyl-2-hydroxybenzaldehyde was
recovered by distillation and crystallization. N,N0-bis(5-chloro-
methyl-salicylidene)ethylenediamine was also prepared and
characterized as described in our early work.8 In a typical synth-
esis, 5-chloromethyl-2-hydroxybenzaldehyde (10 mmol, 10 ml) was
added to a solution of ethylene diamine (5 mmol, 10 ml) in CH2Cl2
and the resulting mixture was allowed to stir at room temperature
under N2 atmosphere for 24 h. The solvent was removed by rotary
evaporation.
Scheme 2 Schematic outlines of syntheses of (a) graphene oxide, (b) CM–Salen,
and (c) graphene immobilized copper(II) and cobalt(II) complexes.
aminopropyl-functionalized GO.7,8 The above prepared NH2–GO
was refluxed in toluene with 1.15 mmol CM–SalenH2 under N2
atmosphere for 24 h. The solid was then filtered and washed
Synthesis of GO tethered CM–SalenH2 metal complexes
GO tethered CM–SalenH2 were synthesized by a nucleophilic abundantly with toluene, Soxhlet-extracted with CH2Cl2 and then
reaction between the chloromethyl-modified salen and the dried in vacuo. The obtained samples were denoted as SalenH2–GO.
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Cu(II), Co(II) SalenH2 tethered complexes M–Salen–GO (M = CH3CN, and drying at 100 1C for 24 h. The recovered catalyst
Cu, Co) were prepared by addition of SalenH2–GO into the was used in the recycling experiments.
corresponding metal salt [Co(NO3)2ꢁ6H2O and Cu(CH3COO)2]
solution, stirring at room temperature for 12 h, filtering,
Conclusions
washing with abundant water and extracting with CH2Cl2.
We have successfully synthesized new cobalt and copper com-
Synthesis of pure metal salen complexes
plexes with salen ligands tethered onto amino-functionalized
SalenH2 and metal salen complexes M–Salen (M = Cu, Co) were
prepared and characterized as described in our early work.7 As
a typical run, to a solution of ethylenediamine (0.1 mmol,
25 ml) was added to a solution of salicyladehyde (0.2 mmol,
150 ml) in ethanol and the resulting mixture was refluxed for
1 h. The sample was filtered and recrystallized from ethanol.
The dried sample was denoted as SalenH2. 0.01 mol metal salt
[Co(NO3)2ꢁ6H2O and Cu(CH3COO)2] in 125 ml ethanol and
10 ml NaOH (2 M) aqueous solution were added to a solution
of 0.01 mol SalenH2 in hot ethanol and the resulting mixture
was refluxed for 1 h. The solid was filtered recrystallized from
CHCl3–petroleum ether.
GO via a stepwise procedure, as evidenced by XRD, TEM, FT-IR,
UV-vis spectroscopy, Raman spectra, ICP-AES, XPS and TG
techniques. The heterogenized cobalt and copper complexes
were examined as catalyst for the epoxidation of styrene, using
acetonitrile as solvent and air as oxidant in combination with a
sacrificial co-reductant isobutyraldehyde. The results showed
that heterogeneous catalysts were active for the epoxidation of
styrene. Furthermore, heterogeneous catalysts show good reco-
verability and relatively high stability against leaching of active
species and can be recycled many times without significant loss
of catalytic activity and selectivity.
Acknowledgements
Characterization
XRD measurements were carried out using Cu Ka radiation on a
Shimadzu XRD-6000 diffractometer equipped with Ni-filtered
Cu Ka radiation (operating at 40 kV, 30 mA, wavelength k =
0.15418 nm). Diffractions were carried out in the ranges (2y) of
51–401 at the scanning speed of 61 minꢀ1. The infrared spectra
were recorded in KBr disks using a NICOLET impact 410
spectrometer in the range 400–4000 cmꢀ1. Diffuse reflectance
UV-vis spectra were recorded from 200 to 800 nm on a Shimadzu
UV-2550 spectrophotometer using BaSO4 as a reference. Thermo-
gravimetric (TG) analysis was carried out on a Shimadzu DTA-60
working in a N2 stream with a heating rate of 10 1C minꢀ1. in the
range of 25–800 1C. Metal content was estimated by inductively
coupled plasma atomic emission spectroscopy (ICP-AES) analysis
conducted on a PerkinElmer emission spectrometer. Before mea-
surements, the sample was dissolved by concentrated nitric acid,
diluted with to volume, and mixed. X-Ray photoelectron spectro-
scopy (XPS) was measured on a ESCALAB 250 X-ray electron
spectrometer using AlKa radiation. Transmission electron micro-
scopy photographs were taken on a TECNAI F20 electron micro-
scope with an acceleration voltage of 200 kV. Raman spectra were
recorded using a Renishaw Raman system model 1000 spectro-
meter with an excitation wavelength of 514 nm.
This work was supported by Jilin province (20090591 and
201105006), Jilin University (450060445017) and Specialized
Research Fund for the Doctoral Program of Higher Education
(20100061120083).
Notes and references
1 G. A. Barf and R. A. Sheldon, J. Mol. Catal. A: Chem., 1995,
102, 23.
2 B. Notari, Stud. Surf. Sci. Catal., 1991, 60, 343.
3 J. R. Monnier, Appl. Catal., A, 2001, 221, 73.
4 M. D. Hughes, Y. J. Xu, P. Jenkins, P. McMorn, P. Landon,
D. I. Enache, A. F. Carley, G. A. Attard, G. J. Hutchings,
F. King, E. H. Stitt, P. Johnston, K. Griffin and C. J. Kiely,
Nature, 2005, 437, 1132.
5 S. M. Islam, A. S. Roy, P. Mondal, M. Mubarak, S. Mondal,
D. Hossain, S. Banerjee and S. C. Santra, J. Mol. Catal. A:
Chem., 2011, 336, 106.
6 V. Mirkhani, M. Moghadam, S. Tangestaninejad,
I. Mohammadpoor-Baltork and N. Rasouli, Inorg. Chem.
Commun., 2007, 10, 1537.
7 Y. Yang, Y. Zhang, S. J. Hao and Q. B. Kan, Chem. Eng. J.,
2011, 171, 1356.
Catalytic tests
8 Y. Yang, Y. Zhang, S. J. Hao, J. Q. Guan, H. Ding, F. P. Shang,
P. P. Qiu and Q. B. Kan, Appl. Catal., A, 2010, 381, 274.
9 Q. H. Tang, Y. Wang, J. Liang, P. Wang, Q. H. Zhang and
H. L. Wan, Chem. Commun., 2004, 440.
The epoxidation of styrene with air was carried out with a 50 ml
two-necked flask equipped with condenser and an air pump.
Typically, styrene (10 mmol), MeCN (10 ml), 25 mmol isobutyr-
aldehyde and catalyst (50 mg) were added to the reactor. At the 10 X. J. Meng, K. F. Lin, X. Y. Yang, Z. H. Sun, D. Z. Jiang and
desired temperature (80 1C), air was introduced with a flowing F.-S. Xiao, J. Catal., 2003, 218, 460.
rate of ca. 80 ml minꢀ1. After reaction, the catalyst was filtered 11 M. R. Maurya, A. K. Chandrakar and S. Chand, J. Mol. Catal.
off, and the products were analyzed by a gas chromatograph A: Chem., 2007, 274, 192.
(Shimadzu GC-8A) equipped with a capillary column (HJ-5, 12 C. E. Song, E. J. Roh, B. M. Yu, D. Y. Chi, S. C. Kim and
30 M ꢃ 0.25 mm ꢃ 0.25 mm) and a FID detector. In order to K. J. Lee, Chem. Commun., 2000, 615.
investigate the stability and recyclability of catalysts, the solid 13 L. Canali, E. Cowan, H. D. Deleuze, C. L. Gibson and
catalyst was recovered by centrifugation, washing with copious
D. C. Sherrington, J. Chem. Soc., Perkin Trans. 1, 2000, 2055.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem.

View Article Online
Paper
NJC
14 G.-J. Kim and J.-H. Shin, Tetrahedron Lett., 1999, 40, 6827.
15 Y. Yang, H. Ding, S. J. Hao, Y. Zhang and Q. B. Kan, Appl.
Organomet. Chem., 2011, 25, 262.
29 S. Stankovich, D. A. Dikin, G. H. B. Dommett,
K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner,
S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282.
16 I. L. Viana Rosa, C. M. C. P. Manso, O. A. Serra and 30 C. H. Lu, H. H. Yang, C. L. Zhu, X. Chen and G. N. Chen,
Y. Iamamoto, J. Mol. Catal. A: Chem., 2000, 160, 199. Angew. Chem., Int. Ed., 2009, 48, 4785.
17 R. I. Kureshy, N. H. Khan, S. H. R. Abdi, I. Ahmad, S. Singh 31 Y. H. Hu, H. Wang and B. Hu, ChemSusChem, 2010, 3, 782.
and R. V. Jasra, J. Catal., 2004, 221, 234. 32 J. Pyun, Angew. Chem., Int. Ed., 2011, 50, 46.
18 Z. K. Hu, X. K. Fu and Y. D. Li, Inorg. Chem. Commun., 2011, 33 L. Wang, K. Lee, Y. Y. Sun, M. Lucking, Z. Chen, J. J. Zhao
14, 497. and S. B. Zhang, ACS Nano, 2009, 3, 2995.
19 K. Yu, L.-L. Lou, F. Ding, S. J. Wang, Z. L. Wang and 34 Y. H. Zu, J. Y. Tang, W. C. Zhu, M. Zhang, G. Liu, Y. Liu,
S. X. Liu, Catal. Commun., 2006, 7, 170.
20 G. Xu, Q.-H. Xia, X.-H. Lu, Q. Zhang and H.-J. Zhan, J. Mol.
Catal. A: Chem., 2007, 266, 180.
21 J. Jiang, K. Ma, Y. F. Zheng, S. L. Cai, R. Li and J. T. Ma, Appl.
Clay Sci., 2009, 45, 117.
22 H. P. Mungse, S. Verma, N. Kumar, B. Sain and O. P. Khatri,
J. Mater. Chem., 2012, 22, 5427.
W. X. Zhang and M. J. Jia, Bioresour. Technol., 2011,
102, 8939.
35 Y. Yang, S. J. Hao, Y. Zhang and Q. B. Kan, Solid State Sci.,
2011, 13, 1938.
36 J. Huang, X. K. Fu, G. Wang, Y. Q. Ge and Q. Miao, J. Mol.
Catal. A: Chem., 2012, 357, 162.
37 Z.-G. Le, Z. R. Liu, Y. Qian and C. Y. Wang, Appl. Surf. Sci.,
2012, 258, 5348.
23 D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner, G. H.
B. Dommett, G. Evmenenko, S. T. Nguyen and R. S. Ruoff, 38 J. J. Wu, D. Zhang, Y. Wang, Y. Wan and B. R. Hou, J. Power
Nature, 2007, 448, 457. Sources, 2012, 198, 122.
24 W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 39 F. Tuinstra and J. L. Koenig, J. Chem. Phys., 1970, 53, 1126.
80, 1339. 40 Y. Lin, J. Jin and M. Song, J. Mater. Chem., 2011, 21, 3455.
25 D. R. Dreyer, S. Park, C. W. Bielawski and R. S. Ruoff, Chem. 41 G. Gonzalez-Arellano, A. Corma, M. Iglesias and F. Sanchez,
Soc. Rev., 2010, 39, 228. Inorg. Chim. Acta, 2004, 357, 3071.
26 A. V. Talyzin, V. L. Solozhenko, O. O. Kurakevych, T. Szabo, 42 Y. Yang, J. Q. Guan, P. P. Qiu and Q. B. Kan, Transition Met.
´
´
I. Dekany, A. Kurnosov and V. Dmitriev, Angew. Chem., Int.
Ed., 2008, 47, 8268.
27 A. V. Talyzin, B. Sundqvist, T. Szabo, I. Dekany and
V. Dmitriev, J. Am. Chem. Soc., 2009, 131, 18445.
28 A. V. Talyzin, T. Szabo, I. Dekany, F. Langenhorst,
Chem., 2010, 35, 263.
43 O. V. Bakhvalov, V. V. Fomenko and N. F. Salakhutdinov,
Chem. Sustainable Dev., 2008, 16, 633.
44 N. R. Khalid, E. Ahmed, Z. L. Hong, Y. W. Zhang and
M. Ahmad, Curr. Appl. Phys., 2012, 12, 1485.
P. S. Sokolov and V. L. Solozhenko, J. Phys. Chem. C, 2009, 45 H. S. Wang, J. H. Huang, S. J. Wu, C. Xu, L. H. Xing, L. Xu
113, 11279. and Q. B. Kan, Mater. Lett., 2006, 60, 2662.
c
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