Oxovanadium(iv) and iron(iii) salen complexes immobilized on amino-functionalized graphene oxide for the aerobic epoxidation of styrene

Article abstract of DOI:10.1039/c3nj00982c

Oxovanadium(iv) and iron(iii) salen complexes grafted onto amino-modified graphene oxide (NH₂-GO) have been synthesized via a stepwise procedure and the prepared materials were used as heterogeneous catalysts in the aerobic oxidation of styrene. The structures of these materials have been thoroughly investigated using different characterization techniques such as XRD, N₂ adsorption-desorption, TEM, FT-IR, diffuse reflectance UV-visible spectroscopy, ICP-AES, TEM, TG, Raman, and XPS. The results of XRD, N ₂ adsorption-desorption and TEM revealed that the structures of the prepared catalysts were well preserved. In addition, the results of FT-IR, diffuse reflectance UV-visible spectroscopy, ICP-AES, TG, Raman and XPS suggested that the oxovanadium(iv) and iron(iii) complexes had been incorporated onto amino-modified graphene oxide (GO). It was found that the heterogeneous oxovanadium(iv) catalyst was more active than its homogeneous analogue in the epoxidation of styrene, using acetonitrile as solvent and air as the oxidant in combination with the sacrificial co-reductant isobutyraldehyde, due to site-isolation. Furthermore, the tethered vanadium catalyst was quite stable and could be recycled many times.

Full text of DOI:10.1039/c3nj00982c

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Oxovanadium(IV) and iron(III) salen complexes
immobilized on amino-functionalized graphene oxide
for the aerobic epoxidation of styrene
Cite this: NewJ.Chem., 2013,
3
7, 4220
a
a
b
c
d
b
Zhifang Li, Shujie Wu, Hong Ding, Haiming Lu, Jiayin Liu, Qisheng Huo,
a
a
Jingqi Guan* and Qiubin Kan*
2
Oxovanadium(IV) and iron(III) salen complexes grafted onto amino-modified graphene oxide (NH –GO)
have been synthesized via a stepwise procedure and the prepared materials were used as heterogeneous
catalysts in the aerobic oxidation of styrene. The structures of these materials have been thoroughly investi-
gated using different characterization techniques such as XRD, N
2
adsorption–desorption, TEM, FT-IR, diffuse
adsorption–
reflectance UV-visible spectroscopy, ICP-AES, TEM, TG, Raman, and XPS. The results of XRD, N
2
desorption and TEM revealed that the structures of the prepared catalysts were well preserved. In addition,
the results of FT-IR, diffuse reflectance UV-visible spectroscopy, ICP-AES, TG, Raman and XPS suggested
that the oxovanadium(IV) and iron(III) complexes had been incorporated onto amino-modified graphene
oxide (GO). It was found that the heterogeneous oxovanadium(IV) catalyst was more active than its
homogeneous analogue in the epoxidation of styrene, using acetonitrile as solvent and air as the oxidant
in combination with the sacrificial co-reductant isobutyraldehyde, due to site-isolation. Furthermore, the
tethered vanadium catalyst was quite stable and could be recycled many times.
Received (in Montpellier, France)
2nd August 2013,
2
Accepted 11th September 2013
DOI: 10.1039/c3nj00982c
www.rsc.org/njc
catalysts by anchoring them onto insoluble supports such as
Introduction
6
7–9
10–12
13
14
SBA-16, SBA-15, montmorillonite,
zeolite-X and zeolite-Y
The oxidation of alkenes to the corresponding epoxides and etc. Covalent attachment is commonly used and is the most effective
carbonyl compounds is of significant importance for the synthesis method for anchoring homogeneous metal complexes. For example,
1–5
15
of fine chemicals and chemical intermediates.
Organic or Gao et al. reported that a new immobilized dioxomolybdenum(VI)
inorganic oxidants such as H , TBHP and KMnO
2
O
2
4
are tradition- complex with bidentate Schiff base-type CPS-[MoO (ALGL) ] micro-
2
2
ally applied to the oxidation of alkenes, but the use of these spheres, exhibited excellent catalytic selectivity for the oxidation of
compounds can result in serious pollution and are economically benzyl alcohol to benzaldehyde.
3
wasteful. Therefore, from a sustainable and green chemistry
Graphene is a new member of the carbon material family
2
point of view, air is a very attractive and desirable oxidant for and is composed of two-dimensional sheets of sp -hybridized
the epoxidation of alkenes.
carbon with interesting physical properties such as excellent
Many Schiff transition metal complexes (Fe, V and Mn) thermal and mechanical stabilities compared to mesoporous
exhibit excellent catalytic activity and selectivity for the homo- zeolites such as MCM-41 and SBA-15. It is already well known
geneous oxidation of olefins. However, the vast majority of these that graphene can be synthesized from graphite, a low cost
catalysts have drawbacks such as difficult separation of the material. Since its discovery, graphene and its derivatives have
product, as well as deactivation, regeneration and recovery of received increasing attention due to their prospects for use in
the catalyst. In order to address these issues, a possible strategy various applications. Within the family of graphene derivatives,
to overcome these difficulties is to heterogenize homogeneous graphene oxide (GO) has a unique nanostructure (monolayer)
with ample oxygen carrying functionalities on its basal planes
1
6
1
7
a
b
c
and at its edges (hydroxyl, epoxide, and carboxylic groups),
excellent mechanical strength and hydrothermal stability. There-
fore, it has the potential for use in various applications such as
composite materials, catalysis, optoelectronics, supercapacitors,
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
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
College of Chemistry, Jilin University, Changchun 130012, P.R. China
State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, P.R. China
1
8–23
memory devices and as a drug delivery agent.
It has been
d
reported that GO can be synthesized by the exfoliation of
graphitic oxide. Graphite oxide was first synthesized through
Laboratory of Chemical Biology, Chinese Academy of Sciences,
Changchun Institute of Applied Chemistry, Changchun 130022, P.R. China
4
220 New J. Chem., 2013, 37, 4220--4229
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the oxidation of graphite by William et al. in 1958. Mungse
NJC
2
3
1
8
et al. reported a graphene-bound oxovanadium Schiff base that
was found to be highly efficient and showed comparable catalytic
reactivity to its homogenous analogue with the added benefits
of a facile recovery and recyclability of the heterogeneous
catalyst when used for the oxidation of various alcohols, diols,
and a-hydroxyketones to carbonyl compounds using TBHP as
an oxidant. Moreover, the prepared catalyst was successfully
reused for several runs without any significant loss of catalytic
activity. Furthermore, as far as we are aware, the catalytic activities
of oxovanadium(IV) or iron(III) salen complexes covalently tethered
onto aminopropyl-functionalized GO for the aerobic epoxidation
of styrene have not been reported.
In our earlier work, we found that transition metal
oxovanadium(IV) and iron(III) complexes tethered onto SBA-15
showed high catalytic activity and excellent selectivity towards
2
4,25
epoxides in the epoxidation of styrene.
Furthermore, the
heterogenized catalysts were quite stable and could be recycled
many times. Herein, we report the design and characterization of
functionalized VO(IV) and Fe(III) Schiff base complexes bearing
chloromethyl groups and their tethering onto amino-modified
GO materials using a covalent attachment method. In addition,
the catalytic properties of these complexes in the epoxidation of
styrene, using acetonitrile as solvent and air as the oxidizing
agent in combination with the sacrificial co-reductant isobutyr-
aldehyde, were also investigated.
Experimental
Materials
Flaky graphite, H
hyde (99%), ZnCl
NaHCO , styrene (98%), isobutyraldehyde, 3-aminopropyl-
2
4 4 2 2
SO , KMnO , 30% H O , 5% HCl, salicylalde-
2
, diethyl ether, petroleum ether, MgSO ,
4
3
triethoxysilane 3-APTES (Aldrich), Fe(NO ) ꢀ9H O, V O , acetyl-
3
3
2
2 5
acetone and ethylenediamine. All organic solvents were
A.R. grade. Na/diphenylketone ketyl was used to dry toluene,
2
and the toluene was then distilled under a N atmosphere and
stored over 4A molecular sieves. All other reagents were used
without further purification.
Synthetic procedures (Scheme 1)
The stepwise procedure for the covalent attachment of the
oxovanadium(IV) and iron(III) salen complexes onto the amino-
modified graphene oxide to form VO-salen-GO and Fe-salen-GO,
respectively, is shown in Scheme 1 and described as follows.
Synthesis of graphite oxide (GO)
Scheme 1 Schematic outlining the syntheses of (a) graphene oxide, (b) CM–salenH
2
,
Graphite oxide was synthesized following the method described
2
6
and (c) the graphene oxide immobilized VO(IV) and Fe(III) complexes.
by Krishnamoorthy et al. In brief, 2 g flaky graphite and 45 ml
concentrated H SO were added to a 250 ml three-necked flask.
2
4
The mixture was stirred in an ice bath for 120 min. 6.0 g KMnO4 was heated to 95 1C for 15 min. Finally, the mixture was
was then added into the mixture gradually under vigorous transferred to a 1000 ml beaker then 280 ml deionized water
stirring and the reaction mixture was stirred at a temperature and 5 ml 30% H O solution were added. The desired brown
2 2
of less than 20 1C for 30 min. Afterwards, the mixture was product was obtained by filtration, washing with large amounts
stirred at 35 1C for 120 min and then 92 ml deionized water was of 5% HCl solution and deionized water and then drying at
slowly added under vigorous stirring. After this, the mixture 70 1C for two days. The dried sample is denoted as GO.
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Synthesis of amino-functionalized graphene oxide (NH
2
–GO)
isotherms were measured with a Micromeritics ASAP 2020
2
system at liquid N temperature. Before measurements, the
Amino-functionalized graphene oxide was synthesized according to
24,27
samples were outgassed at 130 1C for 6 h. The BET surface areas
were calculated using the Barrett–Emmett–Teller (BET) method.
The metal content of the catalysts was determined by inductively
coupled plasma atomic emission spectroscopy (ICP-AES) analysis
conducted with a Perkin-Elmer emission spectrometer. Thermo-
gravimetric analysis (TGA) was carried out using a Shimadzu
our earlier work with a slight modification.
In a typical proce-
dure, 10 ml dry THF and 20 ml dry toluene were added into a 50 ml
two-necked flask and then 1.0 g GO was dispersed using ultrasound.
Afterwards, 2.3 mmol 3-aminopropyltriethoxysilane (3-APTES) was
added to the stirred solution. The mixture was refluxed at 110 1C
under a N atmosphere for 24 h. The sample was filtered and
2
2
DTA-60 working in a N stream. The morphological features of
washed with a large amount of toluene and extracted with CH Cl .
2
2
the materials were characterized using a TECNAI F20 transmis-
sion electron microscope. Raman spectra were recorded with a
Renishaw Raman system model 1000 spectrometer using an
excitation wavelength of 514 nm. X-ray photoelectron spectra
were recorded with an ESCALAB 250 X-ray electron spectro-
meter using Al Ka radiation.
2
The obtained sample is denoted as NH –GO.
0
Preparation of N,N -bis(5-chloromethyl-
2
salicylidene)ethylenediamine (CM–salenH )
5-Chloromethyl-2-hydroxybenzaldehyde was also prepared and
24,25
characterized as described in our earlier work.
In brief, 80 mmol
salicylaldehyde, 12 ml 38% aqueous formaldehyde and 0.6 g ZnCl
2
Catalytic tests
were added to a three-necked flask. Afterwards, 50 ml conc. HCl
was introduced and then the mixture was stirred at room tempera-
The epoxidation of styrene was used as a test reaction for the
catalysts and was carried out as follows. A mixture of styrene
ture under a N atmosphere for 24 h. A white solid was obtained
2
(
5
10 mmol), MeCN (10 ml), and catalyst (50 mg) was added to a
0 ml two-necked flask equipped with a condenser and an air
pump in an oil bath. The temperature was increased to 80 1C
by filtration. The furnished 5-chloromethyl-2-hydroxybenzalde-
0
hyde was recovered by crystallization. N,N -Bis(5-chloromethyl-
salicylidene)ethylenediamine was also prepared and characterized
ꢁ
1
and then air was introduced at a flow rate of ca. 80 ml min . After
filtering the solid, the filtrate was analyzed using a gas chromato-
graph (GC-8A, Shimadzu) equipped with a FID-type detector
and a HJ-5 column, in order to determine the conversion of
styrene and the product yield. At the end of the reaction, the
catalyst was recovered by centrifugation, then washed with
copious MeCN, and dried. Afterwards, the catalyst was reused
in the next reaction without any further purification.
2
4,25
as described in our earlier work.
The general procedure is
as follows: a solution of 5-chloromethyl-2-hydroxybenzaldehyde
(
(
10 mmol, 10 ml) was added to a solution of ethylene diamine
5 mmol, 10 ml) in CH Cl . Soon afterwards, the resulting
2
2
mixture was stirred at room temperature under a N atmosphere
2
for 24 h. A solid was obtained using rotary evaporation. The
collected sample is denoted as CM–salenH2.
Preparation of GO tethered CM–salenH
2
metal complexes
Results and discussion
GO tethered CM–salenH was prepared using a nucleophilic reac-
2
tion between the chloromethyl modified salen and aminopropyl- XRD characterization
functionalized GO. 1.15 mmol CM–salenH
suspension of the above prepared NH –GO in toluene and the
mixture was then refluxed under a N atmosphere for 24 h. A solid
2
was introduced to a
Fig. 1 shows the XRD patterns of the pure GO support and the
covalently attached salen complex catalysts. For GO (Fig. 1a),
2
2
was obtained after filtration, washing with toluene, and drying. Any
uncomplexed ligands were removed using a full Soxhlet extraction
with CH Cl . The obtained solid is denoted as salenH –GO.
2
2
2
The VO(IV) and Fe(III) salenH supported complexes, M-salen-GO
2
(
M = VO, Fe), were synthesized by the addition of a specific amount
of each metal, in the form of VO(acac) or Fe(NO O, into a
ꢀ9H
methanol solution of salenH –GO, which was stirred at room
temperature for 24 h, then filtered, washed with a large quantity
of methanol and water and finally extracted with CH Cl
2
3
)
3
2
2
2
2
.
Characterization
Powder X-ray diffraction (XRD) patterns of the solid products
were collected using a Shimadzu XRD-6000 diffractometer with
Ni-filtered Cu Ka radiation and scanning from 5 to 401 (2y).
FT-IR spectra were obtained using KBr pellets in the range
ꢁ
1
4
00–4000 cm with a NICOLET impact 410 spectrometer. Diffuse
reflectance UV-vis spectra of the solid samples were recorded
using a Shimadzu UV-3600 spectrophotometer with an integrating
Fig. 1 High angle X-ray diffraction patterns of (a) GO, (b) VO-salen-GO and
4 2
sphere using BaSO as a standard. N
adsorption–desorption (c) Fe-salen-GO.
4
222 New J. Chem., 2013, 37, 4220--4229
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the peak at around 10.91 can be attributed to the (001) reflec- distributed on the GO. In addition, very weak and broad
tion of graphite oxide, indicating that the flaky graphite was diffraction peaks centered at 22.71 appeared in the patterns
successfully changed into graphite oxide and oxygen species of the VO-salen-GO and Fe-salen-GO catalysts, indicating that
were successfully inserted into the graphitic layers. At the same part of the graphite oxide was changed into graphene oxide
1
8
time, the diffraction peak at ca. 2y = 10.91 did not disappear in (Fig. 1b and c).
the patterns of the supported catalysts, which clearly suggests Fig. 2 shows a transmission electron microscopy (TEM) image of
that the structure of graphene oxide was not destroyed during the pure GO support and HR-TEM images of the catalyst with
the sample preparation procedure. Nevertheless, for the catalysts immobilized complexes of oxovanadium(IV) and iron(III). It is
containing the immobilized complexes of VO(IV) and Fe(III) apparent that the pure GO support has a layered structure (either
(Fig. 1b and c), the diffraction peaks are weak and broad at a monolayer or just a few layers). A similar result was also observed
around 2y = 10.91 after the introduction of the bulky organo- for the catalysts with immobilized complexes of oxovanadium(IV)
metallic groups, suggesting that the salen complexes are well and iron(III). However, the crumpled nanosheets are in an
Fig. 2 TEM images of (a) GO, (b) VO-salen-GO, (c) Fe-salen-GO and HR-TEM images of (d) VO-salen-GO, (e) Fe-salen-GO.
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Fig. 5 UV-vis absorption spectra of (a) VO-CM-salen, (b) Fe-salen, (c) GO,
Fig. 3
2
N adsorption–desorption isotherms profiles of (a) VO-salen-GO and
(
d) VO-salen-GO and (e) Fe-salen-GO.
(
b) Fe-salen-GO.
agglomerated phase for Fe-salen-GO (Fig. 2c). From Fig. 2d and e,
d-spacing values of 0.480 and 0.492 nm were measured, respec-
tively, corresponding to the spacing of (002) planes, which are
corresponding to the stretching modes of O–H, CQO, CQC,
QC–H, C–O and C–O–C, respectively. Comparing the FTIR
spectra of the covalently attached oxovanadium(IV) and iron(III)
salen complexes on the amino-modified graphene oxide with
26
2
8
larger than that of graphite (d002 = 0.34 nm).
The N adsorption–desorption isotherms of VO-salen-GO and
2
that of the graphite oxide, some new weak peaks in the range of
Fe-salen-GO are shown in Fig. 3. In Fig. 3a and b, both of the
tethered catalysts exhibit typical IV shapes with hysteresis loops
in the relative pressure range of 0.4–1.0, indicating that their
graphene oxide sheets are mesoporous materials. The Brunauer–
Emmett–Teller (BET) specific surface areas were 26.5 and
ꢁ1
1
600–1200 cm were observed due to C–O, C–N and aromatic
ring vibrations, indicating that the salen ligands were success-
29
fully immobilized on GO. Furthermore, ICP-AES analysis of
the covalent attachment of the oxovanadium(IV) and iron(III)
salen complexes onto the amino-modified graphene oxide also
indicates the successful introduction of the metal ions.
2
ꢁ1
2
6.3 m g for VO-salen-GO and Fe-salen-GO, respectively.
Fig. 5 shows the UV-vis absorption spectra of pure support,
tethered catalysts (VO-salen-GO and Fe-salen-GO) and their
homogeneous analogues. For the homogeneous analogues,
absorption bands at ca. 217, 267, 332 and 430 nm emerged,
which can be attributed to n–p* and p–p* ligand charge
transfer. Similar to the homogeneous analogues and pure
support, the absorption bands in the range of 210–420 nm
also appeared in both of the tethered catalysts, indicating
that the salen ligand had been anchored onto GO. The expected
d–d transitions at ca. 580 nm for Fe-salen-GO indicate that the
Fe(III) salen complex has been successfully immobilized
onto GO. Meanwhile, a weak band frequently emerges in the
range of 500–600 nm for the supported oxovanadium(IV)
catalyst verifying that the oxovanadium salen complex has been
Spectroscopic characterization
Fig. 4 shows the FT-IR spectra of the pure GO support and the
covalently attached salen complex catalysts (VO-salen-GO and
Cu-salen-GO). Characteristic bands of the pure GO support
ꢁ
1
(Fig. 4a) appeared at ca. 3387, 1728, 1620, 1369, 1220, 1055 cm ,
3
0
anchored onto GO.
Raman spectroscopy is a non-destructive technique for
characterizing graphitic materials (graphite, graphene, GO
and so on), in particular it can be used to determine the
2
8
ordered and disordered crystal structures of carbon materials.
2
The G band refers to the graphitic lattice (sp carbon structure)
3
1
and the D band refers to defects in the graphitic domain.
The Raman spectra of GO, VO-salen-GO and Fe-salen-GO are shown
in Fig. 6. The spectra of all of the catalysts exhibit two strong bands
ꢁ1
at ca. 1334 and 1596 cm , which correspond to a well-defined
D band and G band, respectively. I /I (the Raman intensity ratio of
Fig. 4 FT-IR spectra of (a) GO, (b) VO-salen-GO and (c) Fe-salen-GO.
D G
4
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1
8
and the decomposition of oxygen carrying functionalities. In
contrast, the TG curves of the tethered catalysts show a three-
step weight loss at temperatures ranging from 30 to 800 1C
(Fig. 7b and c). The weight loss below 250 1C is due to the loss
of physically adsorbed water and the decomposition of unused
oxygen carrying functionalities, which have not interacted with
1
8
APTMS. The weight loss within the temperature range of 250
to 450 1C can mainly be attributed to the combustion of amines.
Weight loss between 450–670 1C can probably be attributed to
3
3
the decomposition of the Schiff ligand. Weight losses of
ca. 10.5% and 9.1% were noted for VO-salen-GO and Fe-salen-GO,
respectively. The loadings of the Schiff base ligands, calculated
based on the weight losses for VO-salen-GO and Fe-salen-GO, are
ꢁ
1
ꢁ1
0
.385 mmol g and 0.334 mmol g , respectively. Meanwhile,
the vanadium and iron content, estimated from the ICP-AES
ꢁ
1
data, is 0.118 and 0.109 mmol g , respectively. Hence, the
molar ratios of ligand/metal for VO-salen-GO and Fe-salen-GO
are 3.26/1 and 3.06/1, respectively, indicating that all of the metal
ions are coordinated to ligands. Moreover, these results demon-
Fig. 6 The Raman spectra of (a) GO, (b) VO-salen-GO and (c) Fe-salen-GO.
the D and G bands) is regarded as a unique characteristic strate that the metal salen complexes have been successfully
feature that can be used to investigate the degree of graphitiza- anchored onto GO.
3
1
tion in graphitic materials. The value of the D/G intensity
ratio is B1.11 for GO, while the value of the D/G intensity ratio XPS studies
increases to B1.39 for VO-salen-GO and B1.18 for Fe-salen-
Fig. 8 shows the X-ray photoelectron spectra (XPS) of GO,
2
GO, suggesting that the size of the in-plane sp domains
VO-salen-GO and Fe-salen-GO. They mainly exhibit signals
associated with C1s and M2p, confirming the presence of metal
salen complexes in GO, which is in good agreement with the above
results. The high resolution C1s XPS spectrum of GO (Fig. 8a)
decreases and the covalent attachment of salen complexes
to GO by a stepwise procedure causes many defects or
3
2
de-oxygenation.
2
indicates the oxidation of four types of sp -hybridized C1s with
TG studies
four peaks centered at ca. 284.3, 285.4, 287.4, and 289.1 eV, which
Fig. 7 shows the TG/DTA curves of GO, VO-salen-GO and correspond to CQC, C–O–H, C–O–C, and O–CQO groups, respec-
34
Fe-salen-GO. The TG curve of graphene oxide exhibits two tively, suggesting that GO has been successfully synthesized. The
obvious weight losses in the range of 25 to 800 1C (Fig. 7a), C1s XPS spectra of the supported catalysts (Fig. 8b and c) also
which can be assigned to the loss of physically adsorbed water exhibit the same oxygen functionalities. Moreover, the relative
Fig. 7 TG/DTA curves of (a) GO, (b) VO-salen-GO and (c) Fe-salen-GO.
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Fig. 9 XPS analysis of M2p for (a) VO-salen-GO and (b) Fe-salen-GO.
metal complexes were anchored onto GO. The M2p XPS spectrum
of VO-salen-GO (Fig. 9a) shows peaks at ca. 517.1 eV and
524.1 eV, which can be attributed to the vanadium(IV) atom in
the tethered catalyst. The two peaks appearing at ca. 711.8 eV
and 725.3 eV in the M2p spectrum in Fig. 9b are due to Fe(III) in
the Fe-salen-GO catalyst.
Catalytic properties
The catalytic activity of the derivative GO materials and the
complexes (VO-salen-GO and Fe-salen-GO) for the epoxidation
of olefins was investigated using styrene as a substrate and air
as an oxidant in combination with a sacrificial co-reductant
3
(isobutyraldehyde) and CH CN as a solvent (Table 1). Notably,
all of the catalysts are active for the aerobic oxidation of styrene.
The VO-salen-GO catalyst shows an 88.0% conversion of styrene
ꢁ
1
after 8 h with a TOF of 186.4 h
and 20.7% selectivity
Fig. 8 XPS analysis of C1s for (a) GO, (b) VO-salen-GO and (c) Fe-salen-GO.
for styrene oxide. Meanwhile, Fe-salen-GO has a lower activity,
but better epoxide selectivity, yielding a 76.5% conversion of
ꢁ1
intensity of the C–O–H signal was decreased for these catalysts styrene with a TOF of 175.5 h
and 49.8% selectivity for
compared with the pure GO support, indicating that the salen the corresponding epoxide. However, the tethered Fe-salen-GO
4
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Table 1 Epoxidation catalyzed by various catalysts
c
Product selectivity (mol%)
Metal content
Styrene
conversion (%)
ꢁ
1
a
b
ꢁ1
Catalyst
Cycle
(mmol g
)
TOF (h
)
So
Bza
Others
Ref.
Salen-GO
VO-CM-salen
Fe-salen
1
1
1
1
1
1
2
3
1
2
3
—
—
46.5
69.0
83.1
78.6
80.9
88.0
79.0
89.0
76.5
80.4
77.5
—
35.3
58.2
51.2
71.2
59.7
20.7
25.8
17.2
49.8
22.2
9.0
64.7
41.8
44.1
28.3
32.8
74.5
74.2
81.3
46.4
73.7
90.2
—
—
27
24
25
24
63.4
67.0
63.0
96.0
186.4
—
196.9
175.5
—
3.104
0.312
0.211
0.118
—
0.113
0.109
—
4.7
0.5
7.5
4.8
—
1.5
3.8
4.1
0.8
VO-salen-SBA
Fe-salen-SBA
VO-salen-GO
VO-salen-GO
VO-salen-GO
Fe-salen-GO
Fe-salen-GO
Fe-salen-GO
25
Herein
Herein
Herein
Herein
Herein
Herein
0.107
181.1
a
ꢁ1
Reaction conditions: catalyst: 50 mg (5 mg for the untethered catalyst), styrene: 1.14 ml (10 mmol), CH
3
CN: 10 ml, air flow rate: 80 ml min
,
b
ꢁ1
isobutyraldehyde: 2.28 ml (25 mmol), temperature: 80 1C and time: 8 h. TOF, h : (turnover frequency) moles of substrate converted per mole
metal ion per hour. So: styrene oxide; Bza: benzaldehyde; Others: including benzoic acid and phenylacetaldehyde.
c
catalyst exhibits a somewhat lower activity than the corre-
sponding homogeneous complex under the same reaction condi-
tions. The lower activity observed can be attributed to the
presence of the bulky organometallic complex and linker moieties
in GO, resulting in some diffusion limitation. The catalytic results
of VO-salen-GO and its corresponding homogeneous complex
show the reverse behavior due to the active sites of the bulky
organometallic complexes and the catalytically beneficial syner-
gistic interaction between the metal complexes and GO. This
result also suggests that the supported catalyst probably isolates
the active species and prevents the formation of m-oxo, m-peroxo
dimeric or other polymeric species. These results are consistent
24,25
8
with our earlier work.
SBA-Fe(acac) demonstrates a higher activity than Fe(acac) for the
oxidation of styrene using H as an oxidant. It has also been
For example, Anand et al. reported that
2 2
O
reported in our earlier work that heterogeneous catalysts show
higher activities than the corresponding homogeneous catalysts
24
in the epoxidation of styrene.
The recoverability and stability of the supported catalysts
were investigated in repeated epoxidations of styrene. The catalysts,
VO-salen-GO and Fe-salen-GO, were recycled three times for the
epoxidation of styrene and the results are shown in Table 1. It seems
apparent from the results that the recovered VO-salen-GO catalyst
exhibits a good recoverability without a significant loss of activity
or selectivity. Nevertheless, for the recovered Fe-salen-GO catalyst,
the selectivity for styrene oxide is evidently decreased in the
epoxidation of styrene compared with the fresh catalyst, probably
due to the variation of coordination environment around Fe
during the reaction. We centrifuged the reaction mixture after
the recycling reactions and analyzed the solution using an
inductively coupled plasma technique. The results indicate that
there is almost no metal in the filter liquor. However, the metal
content of the used catalysts decreased slightly (after 3 cycles)
compared with the fresh ones. A leaching test was carried out
in order to verify the heterogeneity of the catalytic processes of
VO-salen-GO and Fe-salen-GO. The leaching results are shown in
Fig. 10 Kinetic profiles for the aerobic epoxidation of styrene in combination
with isobutyraldehyde as a co-reductant (J) and the leaching experiment
for VO-salen-GO and Fe-salen-GO catalysts (n) (in which the reaction is con-
tinued after removing the catalyst after 2–3 h). The reaction conditions are
Fig. 10. We filtered each catalyst at the reaction temperature (80 1C). analogous to Table 1.
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Conclusions
The covalent attachment of oxovanadium(IV) and iron(III) salen
complexes onto amino-modified graphene oxide [M-salen-GO
(M = VO, Fe)] was achieved using a stepwise procedure and the
resulting materials were examined as catalysts in the epoxidation of
styrene using acetonitrile as a solvent and air as an oxidant in
combination with isobutyraldehyde as a sacrificial co-reductant.
The prepared catalysts were characterized by a comprehensive
range of techniques such as XRD, N adsorption–desorption,
2
TEM, FT-IR, diffuse reflectance UV-visible spectroscopy,
ICP-AES, TEM, TG, Raman, and XPS. XRD, N adsorption–
2
desorption and TEM results demonstrated that the structure
of GO remained intact throughout the grafting procedure.
FT-IR, UV-vis spectroscopy, XPS, ICP-AES, and TG data etc,
indicated that the oxovanadium(IV) and iron(III) salen complexes
had been successfully anchored onto amino-modified GO. It was
found that the heterogeneous catalysts were active for the
epoxidation of styrene. Furthermore, the catalysts showed good
recoverability without significant loss of activity and selectivity in
successive runs.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21303069), Jilin province (201105006),
and Specialized Research Fund for the Doctoral Program of
Higher Education (20100061120083).
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Scheme 2 A reaction scheme showing the aerobic epoxidation of styrene with
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Half of the volume was filtered (at 2 h) and the resulting clear
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