Dioxomolybdenum(VI) complex covalently attached to amino-modified graphene oxide: Heterogeneous catalyst for the epoxidation of alkenes

Article abstract of DOI:10.1002/aoc.3127

Transition metal salen complex MoO₂-salen was successfully tethered onto amino-functionalized graphene oxide (designated as MoO ₂-salen-GO), which was tested in the epoxidation of various alkenes using tert-butylhydroperoxide or H₂O₂ as oxidant. Characterization results showed that dioxomolybdenum(VI) complex was successfully grafted onto the amino-functionalized graphene oxide and the structure of the graphene oxide was well preserved after several stepwise synthesis procedures. Catalytic tests showed that heterogeneous catalyst MoO₂-salen-GO was more active than its homogeneous analogue MoO ₂-salen in the epoxidation of cyclooctene due to site isolation. In addition, the MoO₂-salen-GO catalyst could be reused three times without significant loss of activity.

Full text of DOI:10.1002/aoc.3127

Full Paper
Received: 17 December 2013
Revised: 3 January 2014
Accepted: 4 January 2014
Published online in Wiley Online Library: 10 February 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3127
Dioxomolybdenum(VI) complex covalently
attached to amino-modified graphene oxide:
heterogeneous catalyst for the epoxidation
of alkenes
Zhifang Lia, Shujie Wua, Dafang Zhengb, Jiayin Liuc, Heng Liua, Haiming Lud,
Qisheng Huob, Jingqi Guana* and Qiubin Kana*
Transition metal salen complex MoO2–salen was successfully tethered onto amino-functionalized graphene oxide (designated
as MoO2–salen–GO), which was tested in the epoxidation of various alkenes using tert-butylhydroperoxide or H2O2 as oxidant.
Characterization results showed that dioxomolybdenum(VI) complex was successfully grafted onto the amino-functionalized
graphene oxide and the structure of the graphene oxide was well preserved after several stepwise synthesis procedures.
Catalytic tests showed that heterogeneous catalyst MoO2–salen–GO was more active than its homogeneous analogue
MoO2–salen in the epoxidation of cyclooctene due to site isolation. In addition, the MoO2–salen–GO catalyst could be reused
three times without significant loss of activity. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: epoxidation; dioxomolybdenum complex; alkene; graphene oxide
meantime, graphene oxide has been extensively applied to such
Introduction
fields as composite materials, catalysis, optoelectronics,
supercapacitors, memory devices and drug delivery.[13–18] It has
The epoxidation of alkenes has attained much attention recently
because the corresponding epoxides are significant raw materials
in the production of many fine chemicals and pharmaceuti-
cals.[1,2] Various transition metal compounds (Fe, V, Mn, Mo, Co,
etc.) were used in a variety of catalytic reactions and showed
excellent catalytic activity and selectivity for the homogeneous
oxidation of alkenes.[3] Many countermeasures have been taken
to heterogenize these homogeneous catalysts onto insoluble
supports such as SBA-15,[4] montmorillonite,[5] ZSM-5,[6] poly-
mer,[7] zeolite-X[8] and zeolite-Y[9] because heterogeneous cataly-
sis provides easy recovery of the catalyst and reaction products,
which is preferred in most industrial applications.[3] Therefore,
covalent attachment transition metal Schiff complexes onto
insoluble supports received much attention in the past decade.
For example, Masteri-Farahani et al.[10] reported that MCM-41
containing molybdenum catalyst (MoO2(salpr)/Si–MCM-41)
showed high selectivity, high turnover frequencies and good
stability for the oxidation of 1-hexene, 1-octene, cyclohexene
and cyclooctene using tert-butylhydroperoxide (TBHP) to pro-
duce the corresponding epoxides.
been reported that GO can be prepared by the oxidation of
graphite into graphite oxide, followed by the exfoliation of this
graphitic oxide into GO.[12,19] Mungse et al.[13] reported that
grafting of oxo-vanadium Schiff base onto graphene nanosheets
exhibited high catalytic activity for the oxidation of various alco-
hols, diols and α-hydroxyketones to carbonyl compounds using
TBHP as an oxidant. Meanwhile, the immobilized catalyst could
be reused for several runs without significant loss in its catalytic
activity. Our group found that the tethered dioxomolybdenum
(VI) catalyst shows higher catalytic activity in the epoxidation of
styrene with TBHP as the oxidant and CHCl3 as the solvent than
its homogeneous analogue.[20]
Herein, we report the design and characterization of
dioxomolybdenum(VI) salen complex grafted onto the amino-
*
Correspondence to: Jingqi Guan and Qiubin Kan, College of Chemistry,
Jilin University, Changchun 130023, People’s Republic of China. E-mail:
guanjq@jlu.edu.cn; qkan@mail.jlu.edu.cn
Compared with conventional mesoporous silicates such as
MCM-41 and SBA-15, graphene is a fascinating two dimensional
carbon-based material with interesting physical properties such
as excellent thermal and mechanical stability.[11] As is well
known, the graphene can be prepared from natural graphite, a
low-cost material. Therefore, graphene and its derivatives have
aroused general attention. As a member of graphene derivatives,
graphene oxide (GO) possesses a unique nanostructure (a
monolayer or just a few stacked layers), ample oxygen-carrying
functionalities and excellent mechanical strength.[12,13] In the
a College of Chemistry, Jilin University, Changchun, 130023, People’s Republic
of China
b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College
of Chemistry, Jilin University, Changchun 130012, People’s Republic of China
c
Laboratory of Chemical Biology, Chinese Academy of Sciences, Changchun
Institute of Applied Chemistry, Changchun 130022, People’s Republic of China
d State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, People’s Republic of China
Appl. Organometal. Chem. 2014, 28, 317–323
Copyright © 2014 John Wiley & Sons, Ltd.

Z. Li et al.
modified graphene oxide (NH2–GO) via a stepwise procedure.
The catalytic properties in the epoxidation of various alkenes
using TBHP or H2O2 as oxidant and CH3CN or CHCl3 as solvent
were also investigated.
was added to the mixture gradually under vigorous stirring and
the reaction mixture was stirred at room temperature for
30 min. Afterwards, the mixture was stirred at 35°C for 120 min
and 92 ml deionized water was added under vigorous stirring.
The resulting mixture was heated to 95°C, stirred for 15 min and
transferred to a 1000 ml beaker; this was followed by addition of
30% H2O2 solution and distilled water to the mixture. Finally,
the resulting oxidized product was collected by filtering, washing
abundantly with 5% HCl solution and deionized water and drying.
The sample obtained was denoted as GO.
Experimental
Materials
Graphite powder, H2SO4, KMnO4, 30% H2O2, hydrochloric acid,
salicylaldehyde (99%), ZnCl2, diethyl ether, petroleum ether,
MgSO4, NaHCO3, 3-aminopropyltriethoxysilane (3-APTES), MoO3,
DMF and all organic solvents were AR grade. Na/diphenylketone
ketyl was used to dry toluene, and toluene was distilled under N2
atmosphere and stored over 4Å molecular sieves.
Synthesis of amino-functionalized graphene oxide (NH2–GO)
NH2–GO was synthesized according to our earlier work, with
slight modification.[21] Typically, 10 ml dry THF and 20 ml dry
toluene were added to a 50 ml two-necked flask and then 1.0 g
GO was dispersed by ultrasound. Then, 2.3 mmol 3-APTES was
added to the stirred solution and the mixture was allowed to re-
flux at 110°C under N2 atmosphere for 24 h. Finally, the resulting
sample was filtered, washed with absolute toluene, extracted
with CH2Cl2 and then dried. The obtained sample was denoted
as NH2–GO.
Synthetic Procedures (Scheme 1)
The procedure for the covalent attachment of salen complex of
molybdenum dioxide (VI) onto GO via a stepwise procedure is
depicted in Scheme 1 and is described as follows.
Synthesis of graphene oxide
Synthesis of MoO2Cl2(dmf)2
GO was synthesized as follows. Graphite powder and concen-
trated H2SO4 were added to a 250 ml three-necked flask and the
mixture was stirred in an ice bath for 120 min. Then, 6.0 g KMnO4
MoO2Cl2(dmf)2 was synthesized and characterized as described
in our earlier work.[22] A mixture of 1.0 g MoO3 and 10 ml (6 M)
HCl was heated to boiling until most of the MoO3 dissolved,
and then cooled immediately to room temperature. Finally, the
clear solution was collected by filtration. 2.5 ml DMF was added
to the clear solution and the mixture was stirred for 2 h. The white
solid was collected by filtration, washed and dried.
Synthesis of GO-tethered N,N′-bis(5-chloromethyl-salicylidene)ethylenediamine
(CM–salenH2) metal complexes
5-Chloromethyl-2-hydroxybenzaldehyde was synthesized and
characterized as described in our earlier work.[21] Typically, a
mixture of 80 mmol salicylaldehyde, 12 ml 38% aqueous formal-
dehyde and 0.6 g ZnCl2 was introduced into a three-necked flask.
Then, 50 ml concentrated HCl was slowly added under N2
atmosphere. Finally, the mixture was stirred moderately at room
temperature under N2 atmosphere for 24 h. The solution was
filtered and 5-chloromethyl-2-hydroxybenzaldehyde was recov-
ered by crystallization.
N,N′-Bis(5-chloromethyl-salicylidene)ethylenediamine was also
synthesized and characterized as described in our earlier work.[21]
In brief, a solution of 5-chloromethyl-2-hydroxybenzaldehyde
(10 mmol, 10 ml) was added to a solution of ethylenediamine
(5 mmol, 10 ml) in CH2Cl2. Afterwards, the resulting mixture was
stirred at room temperature under N2 atmosphere for 24 h. The
solvent was removed under vacuum. The sample was designated
as CM–salenH2.
GO-tethered CM–salenH2 was synthesized by nucleophilic
reaction between the chloromethyl modified salen and the
aminopropyl-functionalized GO. The general procedure was as
follows. 1.15 mmol CM–salenH2 was added to the above pre-
pared NH2–GO in 20 ml toluene and then the mixture was
refluxed under N2 atmosphere for 24 h. The solution was filtered
and the resultant solid was washed with toluene and dried.
Uncomplexed ligands were removed by full Soxhlet extraction
with CH2Cl2. The solid material was denoted as salenH2–GO.
Mo–salen complexes M–salen–GO (M = Mo) were prepared by
adding a certain amount of MoO2Cl2(dmf)2 to a CHCl3 solution
containing SalenH2–GO, which was stirred at room temperature
Scheme 1. Schematic outlines of syntheses of (a) GO, (b) CM–salen
and (c) dioxomolybdenum(VI) complex covalently attached on amino-
modified GO.
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Appl. Organometal. Chem. 2014, 28, 317–323

Epoxidation of alkenes over MoO2–salen–GO
for 24 h. The resulting material was filtered, washed with large
quantities of CHCl3, Soxhlet-extracted with CH2Cl2 for 24 h and
then dried.
Synthesis of pure dioxomolybdenum(VI) salen complex
Dioxomolybdenum(VI) salen complex was synthesized and char-
acterized as described in our earlier work.[20] In brief, 2.0 mmol
salen ligand and 2.0 mmol MoO2Cl2(dmf)2 were added to a
round-bottomed flask containing 20 ml CH2Cl2 and the mixture
was stirred at room temperature for 24 h. The mixture was then
filtered and the obtained sample was denoted as MoO2–salen.
Characterization
Powder X-ray diffraction (XRD) patterns of the samples were
recorded with a Shimadzu XRD-6000 diffractometer using Ni-
filtered Cu-Kα radiation scanning from 5 to 40° (2θ). FT-IR spec-
tra were recorded as KBr pellets in the range 400–4000 cmÀ1
using the NICOLET impact 410 spectrometer. Diffuse reflec-
tance UV–visible spectra of the solid samples were performed
from 200 to 800 nm on a Shimadzu UV-3600 spectrophotome-
ter with an integrating sphere using BaSO4 as standard. N2
Figure 1. XRD patterns of (a) GO, (b) salen–GO and (c) MoO2–salen–GO.
very weak and broad after the introduction of the organometallic
group (Fig. 1b), indicating that the salen complex was success-
fully tethered onto the GO. Meanwhile, a very weak and broad
diffraction peak centered at around 25.0° emerged for salen–
GO, indicating that a part of the graphite oxide was changed into
graphene[23] (Fig. 1b). A similar pattern was found for MoO2–
salen–GO (Fig. 1c).
TEM and high-resolution (HR)-TEM images of MoO2–salen–GO
are shown in Fig. 2. It is evident from Fig. 2(a) that the
immobilized catalyst has a layer structure (monolayers or just a
few layers). Additionally, the crumpled nanosheets can also be
seen due to an agglomerated phase (Fig. 2a). As is shown in Fig. 2
(b), the d-spacing value of 0.485 nm is calculated according to the
spacing of the (002) plane, which is larger than that of graphite
(d002 = 0.34 nm).[24]
adsorption–desorption isotherms were measured with
a
Micromeritics ASAP 2020 system at liquid N2 temperature.
Before measurements, the sample was outgassed at 130°C for
6 h. The surface area was calculated using the Barrett–
Emmett–Teller (BET) method. The metal contents of the cata-
lysts were determined by inductively coupled plasma atomic
emission spectroscopy (ICP-AES) analysis conducted on a
PerkinElmer emission spectrometer. TGA was carried out on a
Shimadzu DTA-60 instrument working in an N2 stream with
a heating rate of 10°C minÀ1. Transmission electron microscopy
(TEM) was performed using a TECNAI F20 transmission electron
microscope with an acceleration voltage of 200 kV. Raman
spectra were recorded on a Renishaw Raman system model
1000 spectrometer with an excitation wavelength of 514 nm.
X-ray photoelectron spectra were obtained on an ESCALAB
250 X-ray electron spectrometer using Al-Kα radiation.
The N2 adsorption–desorption isotherm of MoO2–salen–GO is
shown in Fig. 3. It exhibits typical type IV sorption isotherms with
a hysteresis loop at a relative pressure of 0.4 < p/p0 < 1.0,
suggesting that MoO2–salen–GO is a kind of mesoporous mate-
rial.[25] In addition, the BET specific surface area is 18.0 m2 gÀ1
for MoO2–salen–GO.
Catalytic Tests
The epoxidation of various alkenes (cyclooctene, styrene, gera-
niol or 1-octene), with TBHP or H2O2 as the oxidant plus CH3CN
or CHCl3 as the solvent, was carried out in a round-bottomed
flask at 70°C under magnetic stirring, using substrate (5 mmol),
solvent (5 ml), dodecane (2.5 mmol, internal standard), catalyst
(50 mg) and oxidant (5 mmol). The filter liquor was analyzed by
GC with a Shimadzu GC-8A (Japan) instrument equipped using
an HJ-5 capillary column and flame ionization detector. At the
end of the reaction the catalyst was filtered off, washed with
copious solvent and dried. The catalyst was then reused in the
next reaction without further purification.
Spectroscopic Characterization
GO and MoO2–salen–GO were characterized by FT-IR. As shown
in Fig. 4(a), the FT-IR spectrum of GO exhibited characteristic IR
bands at ~3400, 1729, 1621, 1376, 1220, and 1048 cmÀ1, which
are associated with the stretching mode of O―H, C O, C―C,
C―OH, C―O―C and C―O, respectively.[12] Comparing the FT-
IR spectrum of MoO2–salen–GO with that of GO, some new weak
peaks in the range 1600–1200 cm-1 were observed, which can be
ascribed to C―O, C―N and aromatic ring vibrations.[20] In addi-
tion, there were new bands at ~942 and 906 cmÀ1, which could
be due to the characteristic bands of Mo O. The FT-IR results sug-
gest that dioxomolybdenum(VI) salen has been successfully
immobilized onto the GO. Moreover, ICP-AES analysis of MoO2–
salen–GO also suggests the successful introduction of metal ion.
Raman spectroscopy is a non-destructive way to characterize
graphitic materials (e.g. graphite, graphene and GO), in particular
to decide ordered and disordered crystal structures of these
Results and Discussion
Structural Integrity Studies
Figure 1 exhibits the wide-angle X-ray diffraction patterns of GO,
salen–GO and MoO2–salen–GO. GO shows an obvious peak at
2θ = 10.9° (Fig. 1a), which can be ascribed to the (001) reflection
of graphite oxide, suggesting that graphite powder was oxidized
to graphite oxide.[13] Nevertheless, the diffraction peak becomes
Appl. Organometal. Chem. 2014, 28, 317–323
Copyright © 2014 John Wiley & Sons, Ltd.
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Z. Li et al.
Figure 4. FT-IR spectra of (a) GO and (b) MoO2–salen–GO.
carbon materials.[24] The Raman spectra of GO and MoO2–salen–
GO are shown in Fig. 5. Both of the catalysts exhibit peaks at
~1581 and 1331 cmÀ1, which are generally attributed to the G
band (the graphitic lattice) and D band (defects in the graphitic
domain), respectively.[26] However, ID/IG (the Raman intensity
ratio of D and G bands) is a little different. Compared with GO
(1.10), MoO2–salen–GO gives a slightly high ratio of ID/IG (1.13),
indicating that the size of the in-plane sp2 domains decreases
and the covalent attachment of dioxomolybdenum(VI) salen to GO
by a stepwise procedure causes some defects or deoxygenation.[27,28]
TG Studies
The TG curves of GO and MoO2–salen–GO are shown in Fig. 6.
The TG curve of GO shows two obvious weight losses in the
temperature range 25–700°C (Fig. 6a), which are assigned to
the loss of the physically adsorbed water and the decomposition
of oxygen-carrying functionalities.[13] In contrast, the TG curve of
MoO2–salen–GO shows three-step weight loss in the range
25–700°C. The first loss below 250°C was attributed to the phys-
ically adsorbed water and the decomposition of undigested
oxygen-carrying functionalities, which do not interact with
3-aminopropyl-trimethoxysilane.[13] The second loss at tempera-
tures between 250 and 450°C corresponds to the combustion of
amine. The third loss of ~9.0% in the range 450–700°C is probably
attributed to the decomposition of salen ligand. Therefore, the
loading of salen ligand is ~0.33 mmol gÀ1 based on the weight
loss for the MoO2–salen–GO catalyst. In the meantime, the content
of molybdenum is 0.16 mmol gÀ1, as estimated by ICP-AES.
Therefore, the molar ratio of ligand/metal is close to 2/1, which
indicates that all metal ions have coordinated to ligand.
Figure 2. TEM and HR-TEM images of MoO2–salen–GO.
X-ray Photoelectron Spectroscopy (XPS) Studies
Figure 7 shows the X-ray photoelectron spectra of GO and MoO2–
salen–GO. Compared with GO, XPS of MoO2–salen–GO gives
signals mainly associated with N 1s, Cl 2p, Si 2p and Mo 3d,
confirming the presence of dioxomolybdenum(VI) salen complex
in MoO2–salen–GO, in good agreement with the above charac-
terization results. The high-resolution C 1s X-ray photoelectron
Figure 3. N2 adsorption–desorption isotherms profile of MoO2–salen–GO.
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Epoxidation of alkenes over MoO2–salen–GO
Figure 7. X-ray photoelectron spectra of (a) GO and (b) MoO2–salen–GO.
spectrum (Fig. 8a,b) shows peaks at ~284.2, 285.3, 287.4 and
289.3 eV, which are attributed to C C, C―OH, C―O―C and
O―C O groups, respectively, indicating that GO has been suc-
cessfully synthesized.[29] Obviously, the relative intensity of
C―OH signal for MoO2―salen―GO decreases compared with
the pure support GO, suggesting that dioxomolybdenum(VI)
salen complex has been successfully introduced onto GO. The
Mo 3d spectrum of the MoO2–salen–GO catalyst (Fig. 8) shows
two peaks centered at ~232.9 eV and 236.1 eV in the 3d orbits,
which could be due to Mo 3d5/2 and Mo 3d3/2, respectively,
suggesting that the molybdenum species with such a binding
energy could be assigned to Mo(VI).[30]
Catalytic Tests
Figure 5. Raman spectra of (a) GO and (b) MoO2–salen–GO.
The catalytic activity and epoxide yield over the catalysts were
explored for the epoxidation of various substrates (cyclooctene,
geraniol, styrene and 1-octene) using TBHP or H2O2 as oxidant
plus (CHCl3 or CH3CN) as the solvent system. The results are
summarized in Table 1. Notably, the listed catalysts are active
and remarkably selective for the epoxidation of the vast majority
of substrates. When CHCl3 was used as solvent and TBHP as oxi-
dant, the freshly prepared MoO2–salen–GO catalyst shows 91.7%
conversion of cyclooctene, with a turnover frequency (TOF) of
69.5 hÀ1 and 91.7% yield of the cyclooctene epoxide after 8 h.
However, it only exhibits 12.5% conversion of cyclooctene with
a TOF of 9.5 hÀ1 and 12.5% yield of the corresponding epoxide
after 8 h when TBHP was used as the oxidant plus CH3CN as the
solvent. Furthermore, it shows 44.3% conversion of cyclooctene
with a TOF of 33.6 hÀ1 and 44.3% yield of the corresponding
epoxide after 8 h when H2O2 was used as the oxidant plus CH3CN
as the solvent. In addition, it only shows 5.6% conversion of
cyclooctene with a TOF of 4.2 hÀ1 and 5.6% yield of the corre-
sponding epoxide after 8 h when H2O2 was used as the oxidant
plus CHCl3 as the solvent. It seems obvious that the MoO2–
salen–GO catalyst performs well with TBHP as the oxidant in
CHCl3, which is in accordance with the findings reported in the
literature and our earlier work.[20,31]
Figure 6. TG curves of (a) GO and (b) MoO2–salen–GO.
Appl. Organometal. Chem. 2014, 28, 317–323
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Z. Li et al.
Figure 8. High-resolution C1s spectrum of (a) GO and (b) MoO2–salen–
GO, and Mo3d spectrum of MoO2–salen–GO.
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Appl. Organometal. Chem. 2014, 28, 317–323

Epoxidation of alkenes over MoO2–salen–GO
For comparison, the catalytic properties of neat complex were
also investigated and the results are listed in Table 1. The MoO2–
salen catalyst shows 54.3% conversion of cyclooctene with a TOF
of 26.7 hÀ1 after 8 h when using TBHP as the solvent in CHCl3,
which is much lower than the corresponding heterogeneous cat-
alyst MoO2–salen–GO. A similar result is observed when using
H2O2 as the solvent in CH3CN. A blank experiment performed
over SalenH2–GO showed only 2.8% conversion of cyclooctene.
These results suggest that the active sites are the transition metal
ions and the supported catalyst could effectively isolate the
active species and prevent formation of μ-oxo and μ-peroxo
dimeric or other polymeric species.
Furthermore, the results of the epoxidation of various sub-
strates (cyclooctene, geraniol, styrene and 1-octene) using differ-
ent oxidants over the MoO2–salen–GO catalyst were also
investigated (Table 1). It is obvious that the heterogeneous cata-
lyst showed comparable or even higher conversions in oxidation
of cyclooctene, geraniol, styrene and 1-octene using TBHP as
oxidant in CHCl3 than those achieved using H2O2 as oxidant in
CH3CN, which might be attributed to the decomposition of
H2O2 into molecular oxygen and water.[32] Moreover, the conver-
sion of cyclooctene is highest, followed by geraniol and then 1-
octene using TBHP as oxidant in CHCl3 (Table 1), which may be
due to electron density.[31]
Specialized Research Fund for the Doctoral Program of Higher
Education (20100061120083).
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This work was supported by the National Natural Science Foun-
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