A graphene oxide immobilized Cu(ii) complex of 1,2-bis(4-aminophenylthio)ethane: An efficient catalyst for epoxidation of olefins with tert-butyl hydroperoxide

Article abstract of DOI:10.1039/c5nj02309b

In this work, a new, recoverable and heterogeneous catalyst was prepared by covalent attachment of the Cu(ii) complex of 1,2-bis(4-aminophenylthio)ethane onto graphene oxide via the amide linkages. The structural and chemical nature of the catalyst was characterized by a variety of techniques such as Fourier transform infrared and diffuse reflectance UV-visible spectroscopies, X-ray diffraction, thermogravimetric analysis, transmission electron microscopy, field emission scanning electron microscopy and inductively coupled plasma atomic emission spectroscopy. The catalytic activity of this catalyst was investigated in the epoxidation of olefins with tert-butyl hydroperoxide. The catalyst has great reusability and stability in the epoxidation reactions.

Full text of DOI:10.1039/c5nj02309b

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A graphene oxide immobilized Cu(II) complex of
1,2-bis(4-aminophenylthio)ethane: an efficient
catalyst for epoxidation of olefins with tert-butyl
hydroperoxide
Cite this:DOI: 10.1039/c5nj02309b
Ali Zarnegaryan, Majid Moghadam,* Shahram Tangestaninejad,* Valiollah Mirkhani*
and Iraj Mohammdpoor-Baltork
In this work, a new, recoverable and heterogeneous catalyst was prepared by covalent attachment of
the Cu(II) complex of 1,2-bis(4-aminophenylthio)ethane onto graphene oxide via the amide linkages. The
structural and chemical nature of the catalyst was characterized by a variety of techniques such as
Fourier transform infrared and diffuse reflectance UV-visible spectroscopies, X-ray diffraction, thermogravimetric
analysis, transmission electron microscopy, field emission scanning electron microscopy and inductively
coupled plasma atomic emission spectroscopy. The catalytic activity of this catalyst was investigated in
the epoxidation of olefins with tert-butyl hydroperoxide. The catalyst has great reusability and stability in
the epoxidation reactions.
Received (in Montpellier, France)
28th August 2015,
Accepted 9th December 2015
DOI: 10.1039/c5nj02309b
www.rsc.org/njc
immobilized on various supports such as silica, organic polymers,
Introduction
carbon nanotubes, metal–organic frameworks, etc.5–19
Alkene epoxidation is a very useful reaction in industrial organic
Graphene, a monolayer of carbon atoms packed into a two
synthesis because the resultant epoxides are essential precursors in dimensional (2D) honeycomb lattice, is the basal building block
the synthesis of various important substances like plasticizers, in all graphitic materials.20–22 Graphene oxide (GO), obtained
cosmetics, perfumes and epoxy resins and pharmaceuticals.1 In by oxidation of graphene, is a layered material with oxygen-
the last few decades, soluble transition metal salts and transition containing groups, such as hydroxyl, epoxide, carboxyl and
metal complexes as conventional epoxidation catalysts have been carbonyl functional groups. This material is hydrophilic and
extensively used for homogeneous epoxidation of organic substrates readily disperses in water to form a stable colloidal suspension,23–26
such as cyclohexene, cyclooctene and styrene due to their high concurrently, GO has emerged as an important material due
catalytic performance.2,3 However, most of the existing methods to its unique nanostructure and a variety of fascinating char-
suffer from drawbacks such as difficult recovery and recycling of the acteristics such as very high specific surface area with intrinsic
catalyst, lower catalytic activity, tedious synthesis of catalysts, harsh low mass, ample oxygen carrying functionalities, and excellent
reaction conditions, and lower yields of the products.
mechanical strength.27–29 GO has been used as an excellent
In recent years, to overcome these problems, heterogenization supporting material. This is due to the presence of plenty
of homogeneous catalysts has attracted much attention oxygenic functional groups on the surface which can be
because it combines the best properties of both homogeneous used for immobilization of various homogeneous materials
and heterogeneous counterparts.4 Among the various approaches as active sites. Scheuermann et al. reported the application
for immobilizing soluble catalysts, covalent attachment has been of GO supported Pd nanoparticles in the Suzuki–Miyaura
the most frequently used strategy, as the resulting heterogeneous coupling reaction with high activity and low palladium
catalysts have good stability during the course of catalytic leaching.30 Mungse et al. prepared oxo-vanadium Schiff
reactions. In this approach, catalyst leaching from the support covalently anchored onto graphene oxide sheets for the
is minimized and thus, provides a way to prepare site isolated oxidation of alcohols.31 Su et al. reported GO supported transi-
catalysts. In this manner, different soluble complexes have been tion metal Schiff base complexes as efficient and recyclable
catalysts for epoxidation of styrene.32 Nasseri et al. prepared a
GO supported Mn(II) Schiff base complex for epoxidation of
different alkenes.33
Department of Chemistry, Catalysis Division, University of Isfahan Institution,
In this work, the copper complex of 1,2-bis(4-aminophenyl-
thio)ethane was immobilized on graphene oxide nanosheets to
Isfahan, 81746-73441, Iran. E-mail: moghadamm@sci.ui.ac.ir, stanges@sci.ui.ac.ir,
mirkhai@sci.ui.ac.ir; Fax: +98 031-36689732
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Scheme 1 Alkene epoxidation with TBHP catalyzed by CuII(BAPTE)Cl2@GO.
obtain a novel heterogeneous Cu catalyst for epoxidation of
olefins with tert-butyl hydroperoxide (TBHP) (Scheme 1).
Results and discussion
Preparation and characterization of catalyst
Fig. 1 FT-IR spectra of (a) GO, (b) BAPTE, (c) BAPTE@GO and (d)
CuII(BAPTE)Cl2@GO.
In brief, graphene oxide (GO) was prepared from oxidation of
graphene powder by a modified Hummers method.34 Prior to
the immobilization of the copper catalyst, GO was treated with
SOCl2 to convert the –COOH groups into –COCl groups. GO–COCl
was reacted with 1,2-bis(4-aminophenlythio)ethane (BAPTE) in the
presence of triethylamine and the ligand was attached on GO–COCl
via amide linkages. The final catalyst, CuII(BAPTE)Cl2@GO, was
prepared by the reaction of BAPTE@GO with CuCl2 (Scheme 2).
Covalent attachment of CuII(BAPTE)Cl2 to the GO support
provided a catalyst which is not soluble in common organic
solvents.
The structural and chemical nature of the catalyst was
characterized by FT-IR and UV-vis spectroscopies, FE-SEM and
TEM microscopic techniques, XRD, TGA and ICP-AES analyses.
The FT-IR spectra of GO, BAPTE, BAPTE@GO and CuII(BAPTE)-
Cl2@GO are shown in Fig. 1. In pure GO (Fig. 1a), the strong band
at 1726 cmꢀ1 represents carboxylic acid groups and the bands at
1051, 1223 and 1618 cmꢀ1 are attributed to the C–O (epoxy), C–OH
and CQC bonds in graphene oxide, respectively. In addition, a
broad band at 3429 cmꢀ1, attributed to the stretching mode of
O–H bonds, reveals the presence of many hydroxyl groups.31 In
the FT-IR spectra of BAPTE@GO and CuII(BAPTE)Cl2@GO, the
band at 1566 cmꢀ1 is due to the N–H bending vibration. On the
other hand, a new band at 1711 cmꢀ1 is attributed to amide
(–NH–CO–) groups (Fig. 1c and d).35 The vibrational bands
corresponding to the –NH2 groups in BAPTE (Fig. 1b) disappeared
in the BAPTE@GO and CuII(BAPTE)Cl2@GO compounds. These
observations confirmed the covalent attachment of BAPTE to GO via
amide bond formation.
The UV-vis absorption spectra of GO, BAPTE@GO and
CuII(BAPTE)Cl2@GO are shown in Fig. 2. The UV-vis spectrum
of GO shows a strong absorption peak at 230 nm which can be
attributed to the p–p* transition of graphitic C–C bonds
(Fig. 2).36 The BAPTE@GO and CuII(BAPTE)Cl2@GO catalysts
exhibit a broad absorption at 274 nm which is completely
different from the parent GO.
The TGA curve of pure GO and the heterogeneous catalyst
are shown in Fig. 3. Graphene oxide shows a low weight loss
below 100 1C, evidently owing to evaporation of water molecules
which are held in the material. The second significant weight
loss is observed in the range 190–220 1C, due to thermal
decomposition of oxygen carrying functionalities. The TGA
curve of the heterogeneous catalyst shows a low weight loss at
75–300 1C due to the decomposition of the residual oxygen
containing groups. Compared with the thermogram of GO, the
weight loss of CuII(BAPTE)Cl2@GO below 300 1C is lower,
indicating that the main oxygen-containing functional groups
of GO have been converted after reacting with BAPTE.
The XRD patterns are used to study the changes in the
structure (Fig. 4). For GO, two peaks at 2y = 11.31 and 24.261
correspond to (001) and (002) reflections of GO and graphite
powder, respectively (Fig. 4a).31 The presence of various oxygen
carrying functionalities on the basal planes of graphene oxide
Scheme 2 Schematic preparation of CuII(BAPTE)Cl2@GO.
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Fig. 4 XRD patterns of (a) graphene oxide, (b) BAPTE@GO, and (c)
CuII(BAPTE)Cl2@GO catalyst.
Fig. 2 UV-vis absorption spectra of (a) graphene oxide, (b) BAPTE@GO
and (c) CuII(BAPTE)Cl2@GO catalyst.
Fig. 5 FE-SEM images of (a) GO and (b) CuII(BAPTE)Cl2@GO.
Energy dispersive (X-ray spectroscopy) analyses of GO and
CuII(BAPTE)Cl2@GO are shown in (Fig. 6). As can be seen, the
in the EDX spectrum of GO, the major elements are carbon and
oxygen (Fig. 6a). In the EDX spectrum of CuII(BAPTE)Cl2@GO
(Fig. 6b), the presence of Cu, N and S peaks originates from
CuII(BAPTE)Cl2 is a good indication for attachment of CuII-
(BAPTE)Cl2 on GO sheets.
The TEM image of CuII(BAPTE)Cl2@GO shows twisted
nanosheets (Fig. 7). In the TEM image of CuII(BAPTE)Cl2@GO.
the appearance of dark spots is due to the attachment of
CuII(BAPTE) molecules on the GO surface.39
Fig. 3 Thermograms of (a) graphene oxide and (b) CuII(BAPTE)Cl2@GO
catalyst.
Investigation of catalytic activity of CuII(BAPTE)Cl2@GO in the
epoxidation of olefins with TBHP
layers and the trapped water molecules between these layers
increased the d-spacing in graphene oxide.27,37,38 By immobilization Initially, the catalytic activity of the prepared catalyst was investigated
of BAPTE, the intense (001) peak almost disappears, whereas the in the epoxidation of cyclooctene using TBHP as a model reaction
appearance of diffraction peaks of pure BAPTE in the supported under different conditions. To find optimum reaction conditions,
catalyst at around 2y = 14–281 indicates the immobilization of the influence of different factors that may affect the conversion and
1,2-bis(4-aminophenylthio)ethane (BAPTE) on GO (Fig. 4b and c). selectivity of the cyclooctene epoxidation was investigated.
The surface morphologies of GO and CuII(BAPTE)Cl2@GO
For choosing the reaction media, the model reaction was carried
were studied using field emission scanning electron micro- out in different solvents such as acetonitrile, 1,2-dichloroethane
scopy (FE-SEM). As shown in Fig. 5a, there are large flakes of (DCE), methanol, ethanol, tetrahydrofuran (THF) ethanol–water and
GO with macroscopic wrinkling. Compared to GO sheets, the water using 0.7 mol% of the heterogeneous Cu(II) catalyst, (Table 1,
FE-SEM image of CuII(BAPTE)Cl2@GO (Fig. 5b) exhibits an entries 1–8). The best yield was obtained in acetonitrile. The effect of
agglomerated layered structure incorporating copper complex reaction temperature was also investigated in the model reaction.
moieties between the sheets.
The temperature showed a remarkable effect on the epoxide yield.
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Table
1
Optimization of reaction conditions in the epoxidation of
cyclooctene catalyzed by CuII(BAPTE)Cl2@GOa
Catalyst amount
Entry (mol%)
T (1C) Solvent
Oxidant Yieldb (%)
1
2
3
4
5
6
7
8
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0
0.25
0.5
1.0
0.7
0.7
0.7
80
80
80
80
80
80
80
80
25
40
60
70
90
80
80
80
80
80
80
80
CH3CN
EtOAc
C2H4Cl2
MeOH
EtOH
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
97
80
62
35
26
45
22
14
18
33
58
75
97
0
31
63
97
70
42
0
THF
EtOH–H2O TBHP
H2O
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
NaIO4
—
9
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
10
11
12
13
14
15
16
17
18
19
20
a
Reaction conditions: cyclooctene (1 mmol), oxidant (3 mmol), solvent
b
(5 mL) and catalyst after 8 h. GC yields based on the initial cyclooctene.
such as H2O2, TBHP and NaIO4. The highest epoxide yield was
obtained using TBHP, while in the absence of any oxidant; no
conversion was observed (Table 1, entries 1 and 18–20).
Fig. 6 EDX spectra of (a) GO and (b) CuII(BAPTE)Cl2@GO.
Under the optimized reaction conditions, the scope of this
protocol was further investigated in the epoxidation of different
olefins with TBHP. The results are summarized in Table 2.
Cyclooctene was converted selectively to its corresponding
epoxide in 97% yield. Cyclohexene was oxidized in 95% yield
with 92% epoxide selectivity and a trace amount of cyclohex-2-
ene-1-one was produced. In the epoxidation of styrene and
a-methylstyrene, benzaldehyde and acetophenone were produced
as minor products. 1-Octene and 1-decene as linear alkenes were
epoxidized efficiently by this catalyst in high yields and selectivity.
The results obtained by using CuII(BAPTE)Cl2@GO in the
epoxidation of cyclohexene were compared with the results
reported for some other copper(II) catalysts. As can be seen,
our reported method is superior in terms of reaction times,
yields or epoxide selectivity (Table 3). Also, in most cases,
higher TOFs have been obtained by our reported method.
A plausible mechanism for this reaction is shown in Scheme 3.
According to this mechanism, homolytic cleavage of the O–O
Fig. 7 TEM image of the CuII(BAPTE)Cl2@GO catalyst.
Amongst the tested temperatures, 80 1C was the optimum peroxy bond produces the t-BuOꢁ and OH radicals. Addition of
ꢁ
temperature for obtaining the highest conversion (Table 1, 2,6-di-tert-butylphenol as a radical scavenger stopped the reaction
entries 1 and 9–13).
and no product was produced. Upon reaction of these radicals
For optimization of the catalyst amount, different amounts with the Cu catalyst, a metal oxo was formed which can transfer its
of catalyst were used in the model reaction and the best results oxygen to alkene to produce the epoxide.
were obtained in the presence of 25 mg (0.7 mol%) of hetero-
The reusability of the catalyst was investigated in the sequential
geneous CuII(BAPTE)Cl2@GO catalyst after 8 h. Increasing the epoxidation of cyclooctene with TBHP. In this manner, after each
catalyst amount did not affect the reaction rate. Note that in the run, the catalyst was separated from the reaction mixture by
absence of the catalyst, no product was detected in the reaction simple filtration and washed with acetonitrile and ethanol, dried
mixture (Table 1, entries 1 and 14–17).
at 40 1C and used with fresh cyclooctene and TBHP. The results,
To evaluate the oxidizing potential of other common oxidants, which are summarized in Fig. 8, show that the catalyst could be
cyclooctene was subjected to the oxidation using different oxidants reused four consecutive times without loss of its catalytic activity.
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Table 2 Epoxidation of alkenes with tert-BuOOH catalyzed by the heterogeneous Cu(II) complex under reflux conditionsa
Entry
1
Olefin
Product
Epoxide selectivity (%)
100
Yieldb (%)
TOF (hꢀ1
17.3
)
97
2
3
92c
88d
95
93
16.9
16.6
4
90e
96
17.1
5
6
a
92
90
94
91
16.8
16.3
b
Reaction conditions: alkene (1 mmol), TBHP (3 mmol) and catalyst (0.7 mol% based on Cu) in acetonitrile at 80 1C after 8 h. Determined by GC
c
d
e
based on the starting alkene. The by-product is cyclohex-2-ene-1-one. The by-product is benzaldehyde. The by-product is acetophenone.
Table 3 Comparison of the catalytic activity of CuII(BAPTE)Cl2@GO with other reported copper(II) catalysts in the epoxidation of cyclohexene
Epoxide
selectivity (%)
Entry
Catalyst
Time (h)
Conversion (%)
TOF (hꢀ1
)
Ref.
1
2
3
4
5
6
7
8
CuII(BAPTE)Cl2@GO
PS–Cu–Sal
Cu–S–AMSiO2
Cu–S–AM(PS)
Cu(salen)
8
7
95
74
32.4
57.4
30
78
85
88
90
46
76
92
30
1.3
2.9
1.4
90
87
92
87
74
84
79
66
73
6
16.9
17.6
2.2
4.0
2.1
12.0
13.3
17.0
12.0
88.0
12.1
49.8
99.3
79.0
13.0
This study
40
41
41
41
42
42
42
42
43
44
45
46
47
48
12
12
12
24
24
24
24
24
24
6
[Cu4(O)(L1)2(CH3COO)4]
[Cu4(O)(L2)2(CH3COO)4]CH3CN
[Cu4(O)(L3)2(CH3COO)4]
[Cu4(O)(L4)2(CH3COO)4]
[Cu2Ni(L1)(2,20-bpy)2(NO3)2][ClO4]2
[Cu2L(m1,1,-N3)(m1,3-N3)(m1,1,1-N3)]
PS–Cu–amp-Cl
9
10
11
12
13
14
15
61
75
65
20
Cu(salen)–f-GO
[Cu(H2btec)(bipy)]
[Cu2(bipy)2(btec)]N MOF
12
24
6
Fig. 8 Investigation of catalyst reusability in the epoxidation of cyclooctene
with TBHP.
Scheme 3 The proposed mechanism for epoxidation of alkenes with
TBHP.
Conclusion
The surface of graphene oxide is rich in different functional
The amount of Cu leached was determined by ICP which showed a groups which can be used for supporting of various catalysts.
value of about 1% in the first run and 0.4% in the second run. No The Cu(II) complex of 1,2-bis(4-aminophenylthio)ethane, CuII-
Cu was detected in the filtrates of third and fourth run.
(BAPTE)Cl2, supported on graphene nanosheets was prepared
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by covalent interactions between amino groups of 1,2-bis(4- cooled and poured into H2O (300 ml). The solid was filtered off,
aminophenylthio)ethane and acyl groups on graphene oxide washed with H2O and dried. The product was recrystallized
followed by a reaction with CuCl2. The prepared heterogeneous from EtOH, and a yellowish residue was obtained. Yield: 68%,
1
catalyst was used as a highly efficient catalyst in the epoxidation m.p. 79 1C. H NMR (400 MHz, CDCl3): d (ppm) = 7.19 (2H, d),
of various olefins with TBHP. Moreover, the catalyst was recovered 7.21 (2H, m), 6.60 (2H, m), 6.62 (2H, d), 3.73 (4H, m), 2.89 (4H, m).
easily and reused without significant loss in its catalytic activity.
FT-IR (KBr) (cmꢀ1): 3416 (s), 2932 (w), 2280 (w), 1620 (w), 1592 (m),
1493 (s), 1283 (s), 1176 (m), 1117 (m), 819 (s).
Synthesis of GO–COCl. The obtained GO (3.0 g) was
suspended in thionyl chloride (40 ml) and the mixture was at
stirred at 60 1C under nitrogen atmosphere for 6 h. Then, the
resultant solid was filtered, washed with anhydrous toluene at
Experimental
General remarks
Materials and methods. All chemicals were purchased from least 5 times to remove the residual thionyl chloride and dried
Merck and Sigma-Aldrich chemical companies and used with- at 40 1C for 24 h in a vacuum oven.
out purification. FT-IR spectra were obtained as potassium
Preparation of the CuII(BAPTE)Cl2@GO catalyst. GO–COCl
bromide pellets in the range 400–4000 cmꢀ1 using a JASCO (1 g) was dispersed in anhydrous toluene (80 ml) using an
6300D instrument. Diffuse reflectance UV-vis (DR UV-vis) spectra ultrasonic bath. Then, 1,2-bis(4-aminophenylthio)ethane (100 mg)
were obtained on a JASCO, V-670 (190–2700 nm) spectrophoto- and triethylamine (4 ml) were added to this mixture and the
meter. The ICP analyses were performed on an ICP-Spectro resulting mixture was refluxed for 48 h under nitrogen atmosphere.
Ciros CCD instrument. The XRD measurements were carried After the reaction was completed, the mixture was filtered and
out using a Bruker D8-advance X-ray diffractometer with Cu Ka washed with anhydrous toluene (4 ꢂ 10 mL) to remove the non-
radiation (l = 1.5406 Å). The TEM images were obtained using a reacted BAPTE. In the final step, the prepared powder was
Philips CM10 instrument. TGA analysis was performed by dispersed in ethanol (60 ml), CuCl2.2H2O (250 mg) was added
heating at a constant rate of 10 1C minꢀ1 from 50 1C to 600 1C and the mixture was stirred under reflux conditions for 24 h. At
under argon atmosphere. The Cu content of the catalysts was the end of the reaction, CuII(BAPTE)Cl2@GO was filtered and
determined by Shimadzu AA-6300 atomic absorption spectroscopy washed with ethanol several times to remove the unreacted
(AAS). The yields were determined using a Shimadzu GC-16A gas CuCl2ꢃ2H2O.
chromatograph (GC) equipped with a flame ionization detector.
General procedure for catalytic epoxidation of olefins
Scanning electron microscopy (SEM) images were obtained using
a Hitachi S-4800 field emission scanning electron microscope
(FE-SEM) and iridium (IXRF Systems) software.
In a round bottom flask, a mixture of alkene (1 mmol) and
CuII(BAPTE)Cl2@GO (25 mg, 0.007 mmol) in acetonitrile (5 ml)
was prepared. TBHP (3 mmol) was added drop-wise to the
reaction vessel and the mixture was stirred under reflux conditions.
The progress of the reaction was monitored by GC. After completion
of the reaction, the catalyst was separated by centrifugation, washed
with acetonitrile and ethanol, and dried at 50 1C. The recovered
catalyst was used for investigation of the catalyst reusability.
The copper loading of the catalyst was determined using an
inductively coupled plasma atomic emission spectrometer
(ICP-AES). Loading of copper in the prepared catalyst was found
to be 0.28 mmol gꢀ1
.
Syntheses of supported catalyst, CuII(BAPTE)Cl2@GO Synthesis
of graphene oxide. Graphene oxide (GO) was oxidized from graphite
powder by a modified Hummer method.34 3 g of graphite power
(Sigma-Aldrich) and 12 mL of H2SO4 were added to 2.5 g of K2S2O8
and P2O5 at 80 1C and mixed for 4.5 h. Then, the mixture was cooled
to room temperature and washed with deionized water (500 mL).
After then, the mixture was stirred overnight at room temperature
and then filtered and washed with deionized water and ethanol,
subsequently and dried. The collected powder was added to H2SO4
(120 mL) with KMnO4 (15 g) in a two-neck flask placed in an ice
bath and stirred slowly until the contents were completely dissolved.
Then, deionized water (250 mL) was added into the mixture with
stirring for 2 h at 35 1C. Finally, deionized water (250 mL) was added
along with 20 mL of H2O2 and the reaction was terminated via
stirring for 30 min in an ice bath. The resulting mixture was washed
until reaching pH = 7, and then a brown powder was collected after
drying in a vacuum oven.
Acknowledgements
The authors are grateful to the Research Council of the University
of Isfahan for financial support of this work.
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