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anchored onto graphene oxide sheets for the oxidation of
alcohols, which showed good catalytic behavior.27
In this paper, we report design and characterization of
transition metal Schiff base complexes supported onto gra-
phene oxide. Moreover, the catalytic performance of different
transition metal (Fe2+, Co2+, VO2+ or Cu2+) Schiff base complexes
graed onto graphene oxide in the epoxidation of styrene using
acetonitrile as solvent and tert-butyl hydroperoxide as oxidant
was investigated. The obtained hybrid catalyst Cu–NH2–GO
leads to a heterogeneous epoxidation of styrene, showing high
conversion, high selectivity, easy recovery, and steady reuse.
Experimental
Materials and methods
Graphite power (320 mesh, 99.5%), potassium permanganate
(KMnO4), concentrated sulfuric acid (H2SO4), hydrochloric acid
(HCl 5%), hydrogen peroxide (H2O2 30%) 3-amino-
propyltriethoxysilane (3-APTES) (Aldrich), acetylacetone, styrene
(98%), Cu(CH3COO)2, Co(NO3)2$6H2O, Fe(NO3)2$9H2O, and
tert-butyl hydroperoxide (TBHP 70%) were used without further
treatment. Na/diphenylketone ketyl was used to dry toluene,
and then the toluene was distilled under an N2 atmosphere.
XRD patterns were collected on a Shimadzu XRD-6000
diffract meter equipped with Ni-ltered Cu-Ka radiation (oper-
ating at 40 kV, 30 mA). Diffractions were carried out in the 2q
ranges of 5–40ꢁ at a scanning speed of 6ꢁ/minꢂ1. The infrared
spectra were carried out on a NICOLET impact 410 spectrometer
in the range of 400–4000 cmꢂ1 aer the samples were mixed
with KBr and pressed into tablets. Thermo gravimetric (TG)
curves were obtained on a NETZSCH STA 449C analyzer in an N2
Scheme 1 Synthetic schematic outline of NH2–GO and Cu–NH2–GO.
added into the mixture at 0 ꢁC. Aer oxidizing for 2 h, the
reaction mixture was subsequently transferred to a pre-heated
ꢁ
water bath at 35 C and stirred for further 2 h. Then, 96 ml of
deionized water was slowly added, and the temperature was
increased to 95 ꢁC. The mixture was maintained at that
temperature for 15 minutes and treated by adding 30% H2O2.
The resulting solid was ltered, washed by 5% HCl and deion-
ized water and dried at 70 ꢁC for one week. The resulting solid
powder was labelled as GO.
ꢁ
ꢁ
Synthesis of amino-functionalized graphene oxide (NH2–GO)
stream in the range of 100–800 C with a heating rate of 10 C
minꢂ1. Scanning electron microscopy (SEM) images were
obtained using a Hitachi S-4800 eld emission scanning elec-
tron microscope (FE-SEM) and iridium (IXRF Systems) soware.
Transmission electron microscopy (TEM) photographs were
taken on a Tecnai F20 (FEI Company) eld emission trans-
mission electron microscope system at an acceleration voltage
of 120 kV. The specic surface area was determined using the
Brunauer–Emmett–Teller (BET) equation. Raman spectra were
collected from a Lab-RAM HR 800 confocal microscope Raman
system using an excitation wavelength laser of 532 nm. Metal
content was estimated by inductively coupled plasma atomic
emission spectroscopy (ICP-AES) analysis conducted on a Per-
kinElmer emission spectrometer.
The obtained GO (1.0 g, 30 ml) was suspended in anhydrous
toluene under N2 atmosphere. 3-aminopropyltrimethoxysilane
(3-APTES) was graed on graphene oxide by targeting their
hydroxyl and carboxyl groups.31,32 Aer adding 2.3 mmol 3-
APTES, the mixture was reuxed at 110 ꢁC under nitrogen
protection for 24 h. Then, the resultant solid was washed with
toluene at least 3 times to remove the residual APTES. The
resulting coated grapheme oxide was dried in a vacuum oven at
ꢁ
40 C for 24 h, which was labeled as NH2–GO.
Synthesis of copper(II) Schiff base graed graphene oxide
0.5 g NH2–GO was dispersed in 30 ml ethanol and then 100 mg
Cu(CH3COO)2 and 6 mmol acetylacetone were added. The
mixture was reuxed at 70 ꢁC for 3 h. Aer cooling to room
temperature, the mixture was ltrated and washed with ethanol
to remove the undigested Cu(CH3COO)2, followed by drying.
The resulting solid powder was labeled as Cu–NH2–GO.
Synthetic procedures
The synthesis of heterogeneous catalysts was shown in Scheme
1 and described as follows.
For comparison, Fe–NH2–GO, Co–NH2–GO, and VO–NH2–
GO were synthesized under the similar synthetic conditions.
Synthesis of graphene oxide (GO)
Graphene oxide was prepared and puried by modied
Hummers method.28–30 In a typical procedure, 46 ml concen-
Catalytic tests
trated H2SO4 was added to the graphite power (2.0 g) in a 250 ml The epoxidation of styrene reactions was carried out in a 10 ml
ꢁ
ask, and the resulting mixture was cooled down to 0 C in an round bottom ask equipped with a magnetic stirrer and reux
ice bath under stirring for 2 h. Then, KMnO4 (6.0 g) was slowly condenser. Typically, styrene (5 mmol), acetonitrile (5 ml) and
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 9990–9996 | 9991