Direct hydroxylation of benzene to phenol over metal oxide supported graphene oxide catalysts

Article abstract of DOI:10.1016/j.catcom.2015.04.014

A series of metal oxide (V, Fe, Cu) supported graphene oxide (M/GO) catalysts have been prepared by the impregnation method. The catalysts were characterized by TEM, XRD, FT-IR, and Raman spectroscopies and the results showed that the metal oxide nanopart

Full text of DOI:10.1016/j.catcom.2015.04.014

Catalysis Communications 68 (2015) 1–5
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Direct hydroxylation of benzene to phenol over metal oxide supported
graphene oxide catalysts
Cheng Wang, Liya Hu, Yichen Hu, Yuanhang Ren, Xueying Chen, Bin Yue ⁎, Heyong He ⁎
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai
200433, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of metal oxide (V, Fe, Cu) supported graphene oxide (M/GO) catalysts have been prepared by the
impregnation method. The catalysts were characterized by TEM, XRD, FT-IR, and Raman spectroscopies and
the results showed that the metal oxide nanoparticles were highly dispersed and successfully incorporated on
graphene oxide sheets. Among the catalysts, V/GO exhibited the highest efficiency in direct hydroxylation of
Received 23 January 2015
Received in revised form 3 April 2015
Accepted 9 April 2015
Available online 16 April 2015
2 2
benzene using H O as an oxidant in acetic acid solvent with a phenol yield of 22.7% and a selectivity of 98.4%
under the optimized condition along with good recycle performance.
Keywords:
©
2015 Published by Elsevier B.V.
Graphene oxide
Nanocomposites
Benzene hydroxylation
Phenol
1
. Introduction
since its discovery, graphene has attracted a remarkable attention in
the field of catalysis because of its unique thermal, mechanical and
chemical properties [7]. Frank et al. reported that pristine graphene
could be a promising catalyst for selective oxidation of acrolein to acrylic
acid [8]. Nevertheless, the graphene with unfunctionalized carbons
limits its application in catalysis. The functionalization of graphene is
expected to introduce sufficient O-containing groups on the graphene
sheet, which plays an important role in further chemical modification.
Graphene oxide (GO), which has a graphene-like structure, containing
plentiful functional groups, including hydroxyl, carboxyl, carbonyl and
epoxy groups, shows great potential as an efficient catalyst support
[9]. Dreyer et al. reported high activity of GO in the selective catalytic
oxidation of various alcohols, alkenes and alkynes to aldehydes, acids
and ketones, respectively, under relatively moderate conditions,
where GO not only acted as the catalyst, but also as the assisted oxidant
in addition to ambient oxygen [10]. Furthermore, the open π-electron
system of GO may especially promote the interaction between GO and
reactants possessing a π-electron system [11]. Our previous work indi-
cated that the increase of hydrophobicity of supports facilitated the ad-
sorption of benzene to enhance the catalytic activity of catalysts in the
titled reaction [12]. Therefore, it is anticipated that the hydrophobicity
of GO and the π–π interaction between GO and benzene will further en-
hance the adsorption and activation of benzene to improve the catalytic
performance of direct hydroxylation of benzene to phenol. Many kinds
of transition metal species have been used in the benzene hydroxylation
reaction, such as Fe, Cu, V, Co, and Ti [13,14]. Among them, Fe-, Cu-, and
V-containing catalysts have shown good catalytic activity.
Phenol is an important organic intermediate used as a precursor to
synthesize many useful chemical materials, such as resins, pesticides,
medicine and dyestuffs. Currently, phenol is mainly produced with a
relatively low yield through the three-step cumene process, accompa-
nying the production of by-product acetone, high pollution and high en-
ergy consumption [1]. Therefore, it is desirable to explore efficient,
economic and environment-friendly reaction routes. The direct one-
step hydroxylation of benzene has become a research hotspot and
gained important progress. Many attempts have been made to employ
molecular oxygen, nitrous oxide, hydrogen peroxide and a mixture of
oxygen and hydrogen as oxidants in the oxidation of benzene to phenol
[
2–5]. However, using molecular oxygen as an oxidant requires the
existence of reducing agents while nitrous oxide is of high cost and dif-
ficult to store, which limits their application in the benzene hydroxyl-
ation. By contrast, hydrogen peroxide is a clean and mild oxidant with
water as the only by-product and widely used in the study of liquid
phase hydroxylation of benzene [4].
Carbon nanomaterial is one of the most commonly used materials in
catalysis mainly due to their distinct structure and inherent nature in-
cluding easy surface modification, remarkable chemical inertness and
high specific surface areas [6]. Graphene, a monolayer carbon material,
2
consists of sp hybridized carbon atoms which are arranged in hexago-
nal structure with a two-dimensional (2D) honeycomb lattice. Ever
In this work, we report a facile approach to prepare metal oxide (V,
Fe, Cu) supported GO nanocomposites. Their catalytic performance and
⁎
Corresponding authors.
E-mail addresses: yuebin@fudan.edu.cn (B. Yue), heyonghe@fudan.edu.cn (H. He).
http://dx.doi.org/10.1016/j.catcom.2015.04.014
566-7367/© 2015 Published by Elsevier B.V.
1

2
C. Wang et al. / Catalysis Communications 68 (2015) 1–5
stability were tested in the hydroxylation of benzene with hydrogen
peroxide as an oxidant.
of 80 wt.% acetic acid were added into a 25 mL three-necked flask con-
nected with a reflux setup. After heating to 65 °C, 3.5 mL of 30 wt.% H
2 2
O
was added dropwise for 30 min with vigorous stirring, the reaction was
continued for another 3 h. After reaction, the mixture was separated by
centrifugation. The content of liquid products was analyzed by gas chro-
matography (GC) using toluene as the internal standard. The conver-
sion of benzene, the selectivity of phenol and the yield of phenol were
calculated as n (converted benzene) / n (initial benzene), n (phenol) /
n (converted benzene) and n (phenol) / n (initial benzene),
respectively.
2
. Experimental
4 3 3 3 2 3 2 2
Graphite powder, NH VO , Fe(NO ) ·9H O and Cu(NO ) ·3H O
were purchased from Aladdin Reagent Inc. Graphite oxide was prepared
by the Hummers method [15]. To prepare V/GO, 0.20 g of dried graphite
oxide was dispersed in 50 mL of deionized water by sonication for 2 h to
4 3
form a stable graphene oxide suspension. Then 0.05 g of NH VO was
added, followed by vigorous stirring for 3 h, and drying at 60 °C
for 16 h. The resulted solid sample was transferred into a porcelain
3. Results and discussion
boat and calcined in N
2
at 550 °C for 3 h with a heating rate of
−
1
−1
1
.5 °C·min . The product was washed with 0.10 mol·L
HCl and
The morphology of GO, V/GO, Fe/GO and Cu/GO nanocomposites
was characterized by TEM. As shown in Fig. 1a, the large lamellar nano-
sheets with low contrast grade can be identified to be multi-layer GO
sheets. TEM images exhibit that vanadium oxide (Fig. 1b), iron oxide
(Fig. 1c), and copper oxide (Fig. 1d) nanoparticles (the dark dots in
the white circles) with different particle sizes are dispersed on the GO
sheets. Also, it can be seen that most of the metal oxide nanoparticles
are dispersed on the edge or rough area of GO sheets, leading to the in-
homogeneous distribution of active species on the GO surface and the
nonuniform decoration of GO sheets with metal oxide nanoparticles.
The corresponding EDX images of V/GO, Fe/GO and Cu/GO were also
listed in Fig. 1 (SI).
The XRD patterns of V/GO, Fe/GO, Cu/GO and GO are presented in
Fig. 2. All the samples show the characteristic peak at 2θ of ca. 13°,
corresponding to the (001) crystalline planes of GO [16]. After introduc-
ing metal oxide species on GO sheets, the intensity of the peak at 13° de-
creases whereas a new sharp diffraction peak at 26.5° appears; the latter
deionized water to remove the residue, and then dried at 60 °C in air
overnight. To prepare Fe/GO and Cu/GO, the same procedure of prepar-
ing V/GO was followed except using the equimolar of Fe(NO
3 3 2
) ·9H O
and Cu(NO ·3H O instead, respectively.
)
3 2
2
XRD patterns were recorded on a Bruker D8 Advance X-ray diffrac-
tometer using Cu-Kα radiation with a voltage of 40 kV and a current
of 40 mA. FT-IR spectra were recorded on a Nicolet iS10 infrared instru-
ment using KBr discs. TEM images were obtained from a JEOL JEM2011
microscope operated at 200 kV. Thermogravimetric (TG) measure-
ments were carried out on a Perkin-Elmer TGA 7 thermal analyzer at a
−
1
heating rate of 10 °C·min under a 40 mL/min air flow. Raman spectra
were recorded on a Dilor LABRAM-1B spectrometer with a 514 nm laser
excitation. Elemental analysis was performed on a Thermo Elemental
IRIS Intrepid ICP atomic emission spectrometer.
The experiments of benzene hydroxylation to phenol were carried
out as follows. Typically, 40 mg of catalyst, 1 mL of benzene and 10 mL
Fig. 1. TEM images of (a)GO, (b) V/GO, (c) Fe/GO, (d) Cu/GO. Some metal oxide particles are exemplified in circle.

C. Wang et al. / Catalysis Communications 68 (2015) 1–5
3
Fig. 2. The XRD patterns of (a) V/GO, (b) Fe/GO, (c) Cu/GO and (d) GO.
Fig. 4. Raman spectra of (a) V/GO, (b) Fe/GO, (c) Cu/GO and (d) GO.
is ascribed to the (002) reflection of graphite-like stacking of partially
reduced GO sheets because of their large conjugated domains [17].
The appearance of crystalline phase of metal oxides illustrates that the
metal oxide species are agglomerated and crystallized during the calci-
nation treatment, confirming that the metal oxide nanoparticles were
successfully introduced onto GO sheets.
D G
I /I values of V/GO, Fe/GO and Cu/GO are lower than that of GO, indi-
2
cating an increase of both the ordering degree and the size of sp do-
mains, which is due to the decrease of defects with the decline of
oxygenic groups of V/GO, Fe/GO and Cu/GO. In addition, the splitting
−
1
of the G band of graphene oxide around 1590 cm into two peaks at
−
1
−1
1580 cm and 1615 cm after introduction of metal oxides can be as-
cribed to the interaction between graphene oxide and metal oxides [24].
The TG–DTG curves of GO, V/GO, Fe/GO and Cu/GO conducted under
air atmosphere are shown in Fig. 5. For GO (Fig. 5a), the weight loss
below 100 °C is due to the evaporation of the adsorbed water. The
decomposition of oxygen-containing functional groups to form carbon
dioxide and carbon monoxide occurs in the range of 200–300 °C. By
contrast, the curves of V/GO, Fe/GO and Cu/GO (Fig. 5b, c, d) show no
weight change from 200 °C to 400 °C, indicating that most of oxygen-
contained functional groups have been removed after the calcination
treatment during the preparation of M/GO. The main exothermic peak
of V/GO, Fe/GO and Cu/GO illustrates that the loaded metal oxides can
effectively improve the stability of GO, which further implies the strong
interaction between metal oxides and GO sheets.
The FT-IR spectra of V/GO, Fe/GO, Cu/GO and GO are shown in Fig. 3.
−
1
The peaks at 3440, 1630, 1400 and 1100 cm can be assigned to the
stretching vibration of O–H groups, the skeletal vibration of unoxidized
2
sp C_C bonds, and the bending vibration of O–H groups and C–O
stretching vibrations, respectively [18,19]. The new bands at 502 and
−
1
−1
−1
6
05 cm of V/GO, 470 cm of Fe/GO and 530 cm of Cu/GO illustrate
the stretching modes of M–O [20,21]. In addition, the new band at
−
1
1
573 cm corresponds to the formation of monodentate or bidentate
complex between the carboxyl group and metal atoms [22]. These re-
sults imply that metal oxide nanoparticles have strong interactions
with GO sheets.
Raman spectroscopy is an effective tool for characterizing the struc-
−
1
ture of carbon nanomaterials. Since the G peak at around 1590 cm
2
represents the sp bonding structure of the carbon atom and the D
The activity of the catalysts was tested in the hydroxylation of ben-
zene with hydrogen peroxide as an oxidant, and the results are listed
in Table 1. The benzene conversion of V/GO (23.1%) is much higher
than that of Fe/GO (15.9%) and Cu/GO (10.2%) (Table 1, entries 3–5).
The turnover number (TON) based on the metal content also implies
that V/GO is more active than Fe/GO and Cu/GO. It is known that the
capacity of transition metals to activate oxidant species like hydrogen
peroxide is dependent on the outer d electron density [25]. The lower
the electron density, the higher the activity of the metal. The order of
metal electron density is V b Fe b Cu, which is in the same order with
the corresponding catalytic activity. The corresponding controlled ex-
−
1
peak at around 1360 cm
structure of graphene, the intensity ratio of I
uate the graphitization and structural ordering of carbon nanomaterials
23]. The lower the I /I value, the higher the integrity of the graphene.
Fig. 4 shows the Raman spectra of V/GO, Fe/GO, Cu/GO and GO, and their
ratios of I /I are 0.99, 1.29, 0.98 and 1.64, respectively. Obviously, the
traces the edge defects and amorphous
D
/I has been used to eval-
G
[
D G
D
G
periments were conducted with bare GO and NH
4 3
VO (Table 1, entries
1–2). The GO exhibits a benzene conversion of 0.6% and a phenol selec-
tivity of 64.3%, attributing to the strong π–π interaction between GO and
2
the benzene ring and the intrinsic activity of the dominant sp carbon
atoms of GO in the selective oxidation [26]. A benzene conversion of
4 3
13.4% and a phenol selectivity of 91.3% are obtained over NH VO . How-
ever, it is difficult to recycle the catalyst in homogeneous reaction.
Therefore, the V/GO catalyst is more efficient than the individual com-
ponent in this reaction. Since the V/GO catalyst showed the best catalyt-
ic activity in benzene hydroxylation reaction, another three cycle
experiments have been performed to further investigate its reusability
(Table 1, entries 6–8). After each reaction, the V/GO catalyst was sepa-
rated by centrifugation and washed three times with ethanol and
water and dried at 60 °C for the next fresh reaction. The phenol yield de-
creases from 22.7% to 18.5% after four recycles. Moreover, the vanadium
content of V/GO before the reaction is 10.9% while it turns out to be
Fig. 3. FT-IR spectra of (a) V/GO, (b) Fe/GO, (c) Cu/GO and (d) GO.

4
C. Wang et al. / Catalysis Communications 68 (2015) 1–5
Fig. 5. TG-DTG curves of (a) GO, (b) V/GO, (c) Fe/GO and (d) Cu/GO.
Table 1
The hydroxylation of benzene to phenol over various catalysts.
Entry
Catalyst
GO
Metal content (wt.%)a
Benzene conversion (%)
Phenol selectivity (%)
Phenol yield (%)
TONb
1
2
3
4
5
6
7
8
–
0.6
13.4
10.2
15.9
23.1
21.3
20.8
19.2
64.3
91.3
79.7
94.1
98.4
96.0
94.5
96.4
0.4
12.2
8.1
15.0
22.7
20.5
19.7
18.5
–
4
NH VO
3
–
–
Cu/GO
Fe/GO
V/GO (1st run)
V/GO (2nd run)
V/GO (3rd run)
V/GO (4th run)
11.3
11.1
10.9
–
16.3
22.6
30.5
–
–
–
10.2
–
Reaction condition: 40 mg of catalyst, 1 mL of benzene, 10 mL of 80 wt.% acetic acid, 3.5 mL of 30 wt.% H
2
O
2
, and 65 °C for 3 h.
a
Analyzed by ICP.
b
Turnover number (TON) was calculated as the molecular numbers of converted benzene per metal atom.
1
0.2% after four recycles according to the ICP-AES results (Table 1). The
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