Reduced Graphene Oxide Composite with Oxidizable Manganese/Cobalt Mixed Oxide for p-Cresol Oxidation by Using Molecular Oxygen

Article abstract of DOI:10.1002/cplu.201500053

A composite of graphene oxide (GO) with mixed oxide (MnCo) was prepared by using a solvothermal method. During the synthesis, both the reduction of GO and growth of metal oxides took place simultaneously. The as-prepared composite material was highly selective for the liquid-phase oxidation of p-cresol to form p-hydroxybenzaldehyde in 71% yield within 1 h. The composite material was characterised by SEM, X-ray photoelectron spectroscopy, high-resolution TEM and cyclic voltammetry (CV). A CV study revealed that the increase in the redox potential of the mixed oxide after being supported on GO, led to its higher activity of the catalyst for the oxidation reaction. The stability of the catalyst under the reaction conditions was studied by its successful reuse in three cycles.

Full text of DOI:10.1002/cplu.201500053

DOI: 10.1002/cplu.201500053
Full Papers
Reduced Graphene Oxide Composite with Oxidizable
Manganese/Cobalt Mixed Oxide for p-Cresol Oxidation by
Using Molecular Oxygen
Ajay Jha,[a] Sagar H. Patil,[b] Bhanu P. Solanki,[a] Ana P. C. Ribeiro,[c] Carlos A. N. Castro,[c]
Kashinath R. Patil,[b] Alberto Coronas,[d] and Chandrashekhar V. Rode*[a]
A composite of graphene oxide (GO) with mixed oxide (MnCo)
was prepared by using a solvothermal method. During the syn-
thesis, both the reduction of GO and growth of metal oxides
took place simultaneously. The as-prepared composite material
was highly selective for the liquid-phase oxidation of p-cresol
to form p-hydroxybenzaldehyde in 71% yield within 1 h. The
composite material was characterised by SEM, X-ray photoelec-
tron spectroscopy, high-resolution TEM and cyclic voltammetry
(CV). A CV study revealed that the increase in the redox poten-
tial of the mixed oxide after being supported on GO, led to its
higher activity of the catalyst for the oxidation reaction. The
stability of the catalyst under the reaction conditions was stud-
ied by its successful reuse in three cycles.
Introduction
Side-chain oxidation of p-cresol is a very challenging reaction
from both fundamental and application points of view because
the distribution of oxidation products, such as p-hydroxybenzyl
alcohol (PHBAlc), p-hydroxybenzaldehyde (PHBAld) and p-hy-
droxybenzoic acid (PHBAcid), is strongly influenced by the cat-
alyst and reaction conditions. All these products of p-cresol ox-
idation (Scheme 1) have versatile applications in industrial and
pharmaceutical sectors.[1,2] Industrially, PHBAld is synthesised
by side-chain chlorination of p-cresol and subsequent saponifi-
cation of the resulting dichloromethyl group.[3] Sharma and
Chandalia reported the oxidation of p-cresol in methanol by
using cobaltous chloride in the presence of NaOH to give 90%
conversion with 59% selectivity to PHBAld.[4] Major drawbacks
of this route are the employment and generation of chemicals
that are detrimental to health and the environment and longer
reaction times. These can be overcome by using a suitable het-
erogeneous catalyst, which clearly has advantages of ease of
Scheme 1. Reaction products of p-cresol oxidation.
separation from the reaction mixture and easy handling. Nev-
ertheless, disadvantages include limited activity and selectivity
because of the solid matrix, which limits mobility and accessi-
bility of the active sites.[5] In this respect, the dispersion of
metal oxide nanoparticles (NPs) on graphene sheets potentially
provides a novel way to develop more efficient catalysts for
oxidation reactions.[6] The use of graphene-based composite
materials, owing to their unique structure and extraordinary
properties, is known for various other applications, such as
electrocatalysts, electrodes and sensors; however, their explo-
ration in catalysis is limited.
[a] Dr. A. Jha, B. P. Solanki, Dr. C. V. Rode
Chemical Engineering and Process Development
CSIR-National Chemical Laboratory
Pune 411008 (India)
E-mail: c.v.rode@ncl.res.in
[b] S. H. Patil, Dr. K. R. Patil
Centre for Materials Characterization
CSIR-National Chemical Laboratory
Pune 411008 (India)
[c] Dr. A. P. C. Ribeiro, Prof. C. A. N. Castro
Centro de Quꢀmica Estrutural
Faculdade de CiÞncias da Universidade de Lisboa
Campo Grande, 1749-016 Lisboa (Portugal)
Herein, a catalyst based on a graphene oxide (GO) compo-
site is reported for the first time for the liquid-phase oxidation
of p-cresol. The surface of GO hosts oxygen-containing func-
tional groups, such as hydroxyl (ꢀOH) and epoxide groups lo-
cated on the basal planes, and carbonyl (>C=O) and carboxyl
groups (ꢀCOOH) located at the edges of GO.[7] These function-
[d] Prof. A. Coronas
Department of Mechanical Engineering
Universitat Rovira I Virgili, 43007 Tarragona (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201500053.
ChemPlusChem 0000, 00, 0 – 0
1
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
al groups facilitate the anchoring of semiconductors, metal
NPs and so forth, which are useful for performing redox reac-
tions and open up new opportunities for designing novel cata-
lysts. GO is an excellent precursor for graphene synthesis,
through either chemical or thermal reduction processes. A
number of chemical reductants have been studied for the re-
duction of GO to form graphene/metal NPs composites, for ex-
ample, hydrazine and its derivatives.[8,9] However, these reduc-
tants are highly toxic and explosive, which limit their usage on
a larger scale. Therefore, we used ethylene glycol, which is safe
and environmentally friendly, as a reducing agent.[10] Although
cobalt is an excellent catalyst for activation of the relatively
inert CꢀH bond in oxidation reactions, the additional incorpo-
ration of manganese is known to improve the overall oxidation
activity and, more particularly, influence the product distribu-
tion owing to the formation of efficient redox couple between
cobalt and manganese.[11,12] The combination of these two
metal oxides (MnOx and CoOx) has been reported previously
for the oxidation of volatile organic compounds.[13] Recently,
our group has also reported manganese-doped cobalt oxide as
a highly active catalyst for the selective liquid-phase aerobic
oxidation of vanillyl alcohol under base-free conditions.[14] In-
spired by these results, and considering the catalytically useful
characteristics of reduced graphene oxide (r-GO), we attempt-
ed to develop a composite of r-GO with mixed metal oxides
(MnCo) as a novel heterogeneous catalyst. This is reported, for
the first time, for the selective oxidation of p-cresol to form
PHBAld. The activity of the catalyst was correlated with its
physicochemical characterisation data obtained from various
techniques.
attributed to the tetragonal phase of Mn3O4 (JCPDS no. 24-
0734), and those at 2q=19 (111), 31.1 (220), 36.6 (311), 44.7
(400), 59.1 (511) and 65.28 (440), were ascribed to the cubic
phase of Co3O4 (JCPDS no. 74-1657). The XRD pattern of the re-
duced graphene oxide composite with MnCo MO (r-GO–MnCo)
in Figure 1b shows the intact diffraction peaks of spinel oxides
of CoMn2O4, Co3O4 and Mn3O4, without any change in the
metal oxide phases after being supported on r-GO. However,
the peak intensities reduced owing to the decrease in crystal-
lite sizes. The crystallite size calculated by considering the
sharp peak of (311) was found to be 10 nm in the case of
MnCo MO, whereas it was 8 nm for r-GO–MnCo.
Figure 2a shows a SEM image of the GO powder, and indi-
cates that the surfaces of the GO sheets have a wrinkled
Results and Discussion
The powder XRD pattern of MnCo mixed oxide (MO) in Fig-
ure 1a shows diffraction peaks of three different spinel oxides.
The peaks at 2q=33.1 (103), 39.6 (004) and 54.68 (312) corre-
spond to the tetragonal phase of CoMn2O4 (JCPDS no.77-
0471). However, peaks at 2q=24.5 (101) and 41.78 (103) were
Figure 2. a) SEM image of the GO powder. b) and c) High-resolution (HR)
TEM images of the r-GO–MnCo catalyst. d) Lattice fringe pattern of r-GO–
MnCo.
paper-like morphology, which may be due to the oxidation
pre-treatment.[15] A TEM image of the r-GO sample exhibited
a thin, sheet-like structure with good separation and a fairly
large area (Figure S1). The few dark ripples that appeared on
the surface could arise from the scrolling and crumpling of
nanolayers during the formation of sheets. Figure 2b,c gives
HR-TEM images of the r-GO–MnCo composite, which clearly
show that the morphology of the MnCo NPs does not change
after the formation of a composite with r-GO because the bare
mixed metal oxide also has the same morphology (Figure S1),
leading to the uniform distribution of NPs on GO sheets with
high density. The r-GO–MnCo nanocomposite showed the
mixed morphology of spindle and spherical shapes. The spin-
dle-shaped particles were 6 nm in diameter with a length of
25 nm, whereas the spherical particles had a diameter of 8 nm.
The lattice fringe patterns of the r-GO–MnCo catalyst in Fig-
Figure 1. XRD patterns of a) MnCo MO and b) r-GO–MnCo.
&
ChemPlusChem 0000, 00, 0 – 0
www.chempluschem.org
2
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
ure 2d showed the dominant exposed planes as (111) in the r-
GO–MnCo nanocomposite with a lattice spacing of 0.46 nm,
which was related to spinel Co3O4. The other exposed planes
were (103) and (004) with lattice spacings of 0.26 and 0.22 nm,
respectively, which were related to the mixed spinel phase of
CoMn2O4.
Co2+ of CoMn2O4. The formation of CoMn2O4 was also con-
firmed by XRD and HR-TEM results. Another peak at 785.2 eV
was also present owing to Co2+, but this came from spinel
Co3O4. The appearance of two satellite peaks at 787.4 and
790.2 eV was indicative of Co2+ in spinel CoMn2O4 and Co3O4,
respectively. All peaks showed a positive shift towards higher
binding energy (BE) relative to the bare mixed metal oxides
(MnCo MO; Figure S2). The change in the electronic states of
the metals to higher valence
X-ray photoelectron spectroscopy (XPS) results for the C 1s
of GO and r-GO–MnCo are shown in Figures 3a,b. The C 1s
states confirmed the formation
of the r-GO–MnCo nanocompo-
site. This trend can be interpret-
ed as delocalisation of electrons
owing to dp–pp interactions be-
tween metal d orbitals and the
carbon p orbital of r-GO, as
a result of strong interactions
between r-GO and the metal
oxide.[17]
The XPS spectrum of the r-
GO–MnCo nanocomposite also
showed a broad peak of Mn 2p3/
2, which could be fitted satisfac-
torily to three major peaks and
a satellite peak after deconvolu-
tion, as shown in Figure 3d. The
peaks at 641.5, 642.7 and
643.9 eV corresponded to Mn2+
,
Mn3+ and Mn4+
, respectively,
whereas the small peak at
645.7 eV was assigned to the
satellite of Mn4+. The existence
of Mn3+, Mn2+ and Mn4+ species
was due to the formation of
CoMn2O4 and Mn3O4 spinel
oxides, the presence of which
was also confirmed by XRD dif-
fraction peaks at 2q=33.1 (103)
and 24.58 (101), respectively.
Figure 3. XPS results for a) C 1s of GO, b) C 1s of r-GO–MnCo, c) Co 2p3/2 and d) Mn 2p3/2 of the r-GO–MnCo
nanocomposite.
XPS region was deconvoluted into four peaks that correspond-
ed to four different bonding environments of carbon, including
sp2-carbon atoms in aromatic rings ((284.6ꢁ0.2) eV), namely,
carbon atoms bonded to hydroxy and epoxy (CꢀOH, (286.5ꢁ
0.1) eV), carbonyl (>C=O, (287ꢁ0.3) eV) and carboxyl groups
(COOH, (287.9ꢁ0.3) eV).[16] Compared with GO, the XPS results
for C 1s in r-GO–MnCo (Figure 3b) showed a reduction in the
intensity of oxygen-containing carbon species. At the same
time, we observed that there was also a decrease in the total
relative area of the peaks corresponding to these functional
groups. This result confirmed that the functional groups con-
taining oxygen were reduced during synthesis of the catalyst
in the presence of ethylene glycol.
However, the presence of Mn4+ indicates the possibility of the
presence of MnO2, which we did not observe in the XRD re-
sults owing to peak broadening. Similar to cobalt, a shift in the
BE of manganese was also observed; this could be due to ex-
tremely effective delocalised donor–acceptor interactions with
the p system of r-GO (Figure S3).
The manner in which oxygen is bound to metals in the r-
GO–MnCo nanocomposite can be determined by the XPS of
oxygen. Deconvolution of the O 1s spectrum of pure GO pro-
duced three peaks at approximately 530.1, 531.8 and 532.8 eV.
These peaks have been assigned to >C=O, ꢀCOOH and ꢀOH
residual oxygen-containing groups, respectively, bound with
carbon atoms in GO (Figure S4).[18] However, the deconvolution
of O 1s spectra in the case of metal oxide composites with r-
GO yielded four peaks. The peaks at lower BE values of 529.5–
530.5 eV could be assigned to oxygen species involved in
metal oxide bonds in the composite and must therefore corre-
spond to CoꢀO and MnꢀO, respectively.[19] However, peaks at
In the r-GO–MnCo nanocomposite, the peak owing to Co
2p3/2 was unsymmetrical and broad; this could be fitted satis-
factorily to three principal and two satellite peaks after decon-
volution (Figure 3c). The peak at 780.5 eV was assigned to
Co3+ of Co2O3, whereas the peak at 782.8 eV was assigned to
ChemPlusChem 0000, 00, 0 – 0
www.chempluschem.org
3
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
higher binding energies in the case of metal composites were
assigned to >C=O, ꢀCOOH and ꢀOH residual oxygen-contain-
ing groups, which helped to bind metals on the surface of par-
tially reduced GO.
Catalyst activity
The performance of different catalysts for p-cresol oxidation
was studied under the same reaction conditions (Table 1). A
blank run without catalyst gave only 6% yield of product with
higher selectivity to PHBAcid (60%). However, in the absence
The active sites present in the r-GO–MnCo catalyst could be
approximately estimated by cyclic voltammetry (CV). CV meas-
urements were performed at a scan rate of 100 mVsꢀ1 over
the potential range of 0–0.8 V in a 0.5m solution of H2SO4 by
using a glassy carbon electrode. Figure 4 shows the plots of
Table 1. Catalyst screening for p-cresol oxidation.
Catalysts
Yield [%]
Selectivity [%]
PHBAld PHBAcid
PHBAlc
PHBEth
WO[a]
r-GO–Co
r-GO–Mn
MnCo (1:1)
r-GO–MnCo (1:1)
r-GO–MnCo (1:2)
r-GO–MnCo (2:1)
r-GO–MnCo (1:1)
MnCo (1:1)/SiO2
MnCo (1:1)/Al2O3
6
20
28
42
71
57
60
NR[b]
9
2
–
100
1
–
–
1
–
–
–
38
89
–
95
96
97
95
–
60
10
–
3
3
2
3
–
6
–
1
–
1
1
1
1
–
1
1
93
93
31
6
[a] WO=without catalyst. [b] NR=no reaction (reaction performed in the
absence of base).
of NaOH, no reaction took place. It is known that NaOH forms
the phenolate ion, which is the active species for the oxidation
reaction.[14] A systematic study on the effect of NaOH concen-
tration was also reported, and showed that a minimum ratio of
NaOH/p-cresol was required, below which the selectivity to
PHBAld could be as low as 11% owing to the oxidative dimeri-
sation of p-cresol to give the dihydroxy diphenyl compound.[24]
Therefore, it was concluded that NaOH was essential for p-
cresol oxidation. Table 1 also shows that monometallic oxide
catalysts exhibited lower activity than the bimetallic catalysts.
Interestingly, the r-GO–Co catalyst was highly selective for
PHBAld formation, whereas the r-GO–Mn catalyst was highly
selective for PHBAlc formation.
Figure 4. CV curves of MnCo MO and the r-GO–MnCo nanocomposite.
current density versus potential for the MnCo MO and r-GO–
MnCo nanocomposite. It is clear that the r-GO–MnCo nano-
composite presents the largest integrated area, compared with
bare MnCo MO, which indicates that the redox potential of the
nanocomposite has been significantly increased owing to the
addition of GO.[20] The enhancement in redox potential of the
r-GO–MnCo nanocomposite material can be ascribed to 1) the
larger specific surface area (154 m2 gꢀ1) than that of bare MnCo
MO (85 m2 gꢀ1), 2) the smaller particle size (8 nm) and 3) the
synergistic effect between the r-GO and MnCo NPs. Therefore,
a decrease in particle size increased the active surface that
took part in the redox reactions.[17] Both samples (MnCo MO
and r-GO–MnCo nanocomposite) showed clearly the presence
of oxidation and reduction peaks. However, the r-GO–MnCo
nanocomposite showed two very sharp reduction peaks: one
at higher potential value (0.2 V), which could be assigned to
the reduction of Mn3+ to Mn2+, and the second one at a lower
potential value (ꢀ0.55 V), which could be assigned for the fur-
ther reduction of Mn2+ and Co2+ to metallic Mn and Co, re-
spectively.[21] Compared with the bare MnCo MO, a substantial
negative shift (from ꢀ0.28 to ꢀ0.55 V) of the cathodic peak
was observed for r-GO–MnCo, which indicated increased elec-
tron density of the Co2+ and Mn2+ species.[22] The reduction
peak indicates the reduction ability of oxygen; hence, as the
potential value shifted to more and more negative values, the
oxygen reduction ability increased, which led to the higher oxi-
dation ability of r-GO–MnCo for p-cresol to form PHBAld. A
similar study was reported for Mn3O4 composite with multiwal-
led carbon nanotubes (MWCNTs).[23]
The addition of manganese to the cobalt oxide led to a com-
plete change in both the selectivity pattern and catalyst activi-
ty. The mixed metal oxides (MnCo MO) exhibited higher cata-
lytic activity than that of the monometallic catalyst, regardless
of whether cobalt or manganese alone was used as the active
phase. The higher activity of the mixed metal oxides was due
to the cooperative effect between the metals towards an incre-
ment in the mobility of oxygen as well as stabilising the more
active species. The redox cycles, which also permit reactivation
of the catalyst, are also favoured. Manganese, in synergism
with cobalt oxide, improves the superficial oxygen availability
at the catalyst surface, which is attributed to the high oxygen
storage capacity of manganese.[25] The r-GO–MnCo catalyst
showed a higher activity (71% product yield) than those of
bare mixed metal oxide (42% product yield) without affecting
the selectivity pattern. This observation suggests that the GO-
doped catalyst was efficient for sustainable redox couple for-
mation (Mn+$M(n+1)+). The change in the oxidising ability of
the catalyst by increasing the electron density on to the metal
oxide through delocalisation of electrons through p–p interac-
&
ChemPlusChem 0000, 00, 0 – 0
www.chempluschem.org
4
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
tions of r-GO was supported by results from CV anal-
ysis. The GO-based catalysts also provide anchoring
sites for aromatic substrates through p–p interac-
tions; this increases the probability of adsorption of
a
substrate on the catalyst surface, leading to
a higher reaction rate. Furthermore, to reveal the im-
portance of r-GO as a support, we performed the oxi-
dation experiments with MnCo (1:1) catalysts sup-
ported on traditional supports, such as silica and alu-
mina, and the results are also shown in Table 1. Both
catalysts gave much lower activities than those for
r-GO-supported MnCo MO catalysts.
In this study, all catalysts were prepared by varying
the ratios of cobalt and manganese with a constant
amount of GO (1%). The Mn/Co ratio in the catalysts
was determined by energy-dispersive X-ray spectros-
copy (EDS), the results of which were close to the cal-
culated values (Figure S5). Among the screened cata-
Figure 6. Recycling studies for the r-GO–MnCo catalyst. Reaction conditions: p-cresol
(55.5 mmol), NaOH (155 mmol), r-GO–MnCo catalyst (0.1 g), 1008C, methanol (50 mL),
oxygen (2.7 bar), 1 h.
lysts, we observed that a 1:1 ratio of Co/Mn showed a higher
catalytic activity (71% product yield) than those of GO–MnCo
(1:2) and GO–MnCo (2:1), which gave product yields of 57 and
60%, respectively. Because the r-GO–MnCo (1:1) catalyst gave
a better performance, further study on the optimisation of the
reaction parameters was performed with this catalyst. Figure 5
shows the time course of the catalytic performances in
MnCo nanocomposite, the catalyst was filtered and washed
several times with methanol, dried at 1008C for 6 h, and
reused for subsequent runs by adding fresh reactants. Figure 6
shows that the catalyst retained its activity, even after third
cycle; this indicates the high efficiency of the r-GO–MnCo cata-
lyst for the oxidation of p-cresol. To check the stability of the r-
GO–MnCo catalyst, and verify that the reaction was truly heter-
ogeneously catalysed, a leaching experiment was also per-
formed. After 30 min of reaction, the catalyst was filtered and
the reaction continued for another 30 min. The product yields
measured before and after catalyst filtration were 68 and
66.5%, respectively, which confirmed that the reaction was
heterogeneous and also that there was no metal leaching
under these reaction conditions.
Conclusion
In the liquid-phase oxidation of p-cresol to form PHBAld, r-GO
proved to be a highly efficient support for the mixed metal
oxide (MnCo MO) catalyst. The structural characteristics of r-GO
provided active sites for adsorbing p-cresol through p–p stack-
ing interactions. In addition, the r-GO support caused charge
transfer, which led to the enhancement in the redox potential
of the MnCo MO. Both these factors led to the increased rate
of oxidation to give a product yield of 71% with >96% selec-
tivity to PHBAld in 1 h from p-cresol oxidation under mild reac-
tion conditions.
Figure 5. Effect of time on p-cresol oxidation. Reaction conditions: p-cresol
(55.5 mmol), NaOH (155 mmol), r-GO–MnCo catalyst (0.2 g), 1008C, methanol
(50 mL), oxygen (2.7 bar).
p-cresol oxidation with r-GO–MnCo (1:1) catalyst under 2.7 bar
of oxygen. The product yield increased from 68 to 71% by ex-
tending the reaction time from 30 to 60 min. Thereafter, a de-
crease in the product yield occurred from 71 to 60% in 2 h;
this might be due to the formation of a tarry product, which
was detected only after the reaction was continued for
a longer reaction time (2 h). However, selectivity to PHBAlc,
PHBAcid and PHBEth throughout the reaction remained only
<4% and the selectivity for PHBAld was >96%.
Experimental Section
Preparation of GO and r-GO–MnCo nanocomposite
A modified Hummers’ method was used to synthesise GO, in
which graphite powder (1.0 g) was poured into a solution of
NaNO3 (0.5 g) in concentrated H2SO4 (30 mL) and cooled to 08C.
KMnO4 (3.0 g) was then added, during which the temperature of
the mixture was maintained below 208C. Successively, the mixture
was stirred at 358C for 1 h, and then diluted with deionised water
(46 mL) by keeping the temperature at 858C and then increasing it
to 1008C for 30 min. Warm deionised water (140 mL) was then
The stability of the r-GO–MnCo catalyst for p-cresol oxida-
tion was established by performing experiments to reuse the
catalyst. Briefly, after the first oxidation run with fresh r-GO–
ChemPlusChem 0000, 00, 0 – 0
www.chempluschem.org
5
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
added to the mixture, followed by the dropwise addition of a 50%
aqueous solution of H2O2 (15 mL), and the solution was stirred for
30 min. The mixture was then centrifuged and washed with a 5%
aqueous solution of HCl (1 L) to remove metal ions, followed by
deionised water (1 L) to remove the acid. The resulting solid was
dried in air for 12 h at 608C.
again filled with oxygen. This was continued for up to 2 h. The
progress of the reaction was monitored by observing the pressure
drop in the reservoir vessel as a function of time. After 2 h, the re-
actor was cooled to room temperature and excess oxygen gas was
safely vented. The contents of the reactor were discharged and the
final volume was recorded and the liquid samples were analysed
by HPLC.
r-GO–MnCo nanocomposite was synthesised by a solvothermal
method. In a typical synthesis, GO was dispersed into ethylene
glycol (60 mL) and sonicated for 1 h, followed by the subsequent
addition of aqueous NH3 (1.6 mL), and stirred again for 1 h. Manga-
nese and cobalt acetates were added in a 1:1 molar ratio and the
solution was gradually heated to 1608C. As the temperature
reached 1608C, a 0.3m aqueous solution of Na2CO3 (200 mL) was
added dropwise and the slurry was further aged for 2 h under a ni-
trogen atmosphere. The resulting product was filtered, thoroughly
washed with distilled water until neutralisation, and dried over-
night at 1008C, then calcined at 3508C for 4 h.
Acknowledgements
A.J. gratefully acknowledges the Council of Scientific and Industri-
al Research (CSIR, New Delhi) for the award of a senior research
fellowship.
Keywords: cyclic voltammetry
· graphene · oxidation ·
supported catalysts · transition metals
MnCo (1:1) oxide supported on SiO2 and Al2O3 catalysts were ob-
tained by a wet impregnation method. The synthesis involved dis-
solution of Mn and Co precursors into methanol (50 mL). After
complete dissolution of the metal precursors, SiO2/Al2O3 was
added to the solution. The suspension was stirred for 6 h and then
the solvent was removed in vacuo. The obtained solid product was
dried at 1008C for 12 h and then calcined at 3508C for 4 h.
[1] H. V. Borgaonkar, B. C. Sampatraj, J. Chem. Technol. Biotechnol. 1984, 34,
107–112.
[2] V. S. Kshirsagar, A. C. Garade, K. R. Patil, R. K. Jha, C. V. Rode, Ind. Eng.
Chem. Res. 2009, 48, 9423–9427.
[3] A. Thoer, G. Denis, M. Delmas, A. Gaset, Synth. Commun. 1988, 18,
2095–2101.
[4] S. N. Sharma, S. B. Chandalia, J. Chem. Technol. Biotechnol. 1990, 49,
141–153.
Catalyst characterisation
[5] M. Gruttadauria, F. Giacalone, R. Noto, Green Chem. 2013, 15, 2608–
2618.
XRD patterns of the catalysts were recorded in the 2q range of 10–
808 (scan rate of 5.38 minꢀ1) on a PANalytical PXRD Model X-Pert
PRO-1712 instrument by using Ni-filtered CuKa radiation (l=
0.154 nm) as a source (current intensity, 30 mA and 40 kV) and an
Xcelerator detector. The microscopic features of the GO were char-
acterised by means of an Environ scanning electron microscope (E-
SEM, Quanta 200 3D). The morphology and size of the as-prepared
products were observed by HR-TEM (JEOL 2010F) at an accelera-
tion voltage of 300 kV. The surface chemistry was analysed by
using an X-ray photoelectron spectrometer (XPS, VG Micro Tech
ESCA-3000). The chamber pressure was about 1ꢁ10ꢀ9 torr under
the testing conditions. The surface area of the samples was ob-
tained by the BET method.
[6] B. F. Machadoab, P. Serp, Catal. Sci. Technol. 2012, 2, 54–75.
[7] V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C.
Kemp, P. Hobza, R. Zboril, K. S. Kim, Chem. Rev. 2012, 112, 6156–6214.
[8] S. Wang, X. Wang, S. P. Jiang, Phys. Chem. Chem. Phys. 2011, 13, 6883–
6891.
[9] Y. Hu, H. Zhang, P. Wu, H. Zhang, B. Zhou, C. Cai, Phys. Chem. Chem.
Phys. 2011, 13, 4083–4094.
[10] C. Xu, X. Wang, J. Zhu, J. Phys. Chem. C 2008, 112, 19841–19845.
[11] F. Wang, G. Yang, W. Zhang, W. Wu, J. Xu, Chem. Commun. 2003, 1172–
1173.
[12] D. A. Aguilera, A. Perez, R. Molina, S. Moreno, Appl. Catal. B 2011, 104,
144–150.
[13] Z. Y. Tian, P. H. Tchoua Ngamou, V. Vannier, K. K. Hçinghaus, N. Bahla-
wane, Appl. Catal. B 2012, 117 – 118, 125–134.
[14] A. Jha, K. R. Patil, C. V. Rode, ChemPlusChem 2013, 78, 1384–1392.
[15] L. Zhang, L. Jiang, H. Yan, W. D. Wang, W. Wang, W. Song, Y. Guo, J.
Wan, J. Mater. Chem. 2010, 20, 5462–5467.
Catalytic reactions
All catalytic oxidation reactions were performed in a 300 mL high-
pressure Hastelloy reactor supplied by Parr Instruments Co. USA.
The reactor was connected to an oxygen cylinder held at a pressure
higher than that of the reactor. A Thermo Scientific HPLC model
AS3000 liquid chromatograph equipped with a UV detector was
used for analysis. HPLC analysis was performed on a 25 cm RP-18
column. The products and reactant were detected by using a UV
detector at lmax =254 nm. Aqueous methanol (35%) was used as
a mobile phase at a column temperature of 358C and a flow rate
[16] X. Huo, J. Liu, B. Wang, H. Zhang, Z. Yang, X. She, P. Xi, J. Mater. Chem. A
2013, 1, 651–656.
[17] J. Zhu, A. Holmen, D. Chen, ChemCatChem 2013, 5, 378–401.
[18] H. Karami, B. Kafi, S. N. Mortazavi, Int. J. Electrochem. Sci. 2009, 4, 414–
424.
[19] Z. S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li, H. M.
Cheng, ACS Nano 2010, 4, 3187–3194.
[20] C. Du, Z. Yao, Y. Chen, H. Bai, L. Li, RSC Adv. 2014, 4, 9133–9138.
[21] L. Wang, B. Liu, S. Ran, L. Wang, L. Gao, F. Qu, D. Chen, G. Shen, J.
Mater. Chem. A 2013, 1, 2139–2143.
[22] J. Xu, P. Gao, T. S. Zhao, Energy Environ. Sci. 2012, 5, 5333–5339.
[23] M. Biswal, V. V. Dhas, V. R. Mate, A. Banerjee, P. Pachfule, K. L. Agrawal,
S. B. Ogale, C. V. Rode, J. Phys. Chem. C 2011, 115, 15440–15448.
[24] C. V. Rode, M. V. Sonar, J. M. Nadgeri, R. V. Chaudhari, Org. Process Res.
Dev. 2004, 8, 873–878.
of 1 mLminꢀ1
.
In a typical experiment, p-cresol (6.0 g), NaOH (6.2 g) and methanol
(50 mL) were heated in a round-bottomed flask fitted with a reflux
condenser until all NaOH dissolved completely. This reaction mix-
ture was charged into a 300 mL Parr autoclave. Then, the catalyst
(0.2 g) was added and the reaction mixture was heated to 373 K.
After the desired temperature was attained, the reactor was pres-
surised with oxygen (2.7 bar). Then, the reaction was started by ag-
itating at 1000 rpm. When the pressure decreased, the reactor was
[25] Q. Zhang, X. Liu, W. Fan, Y. Wang, Appl. Catal. B 2011, 102, 207–214.
Received: February 5, 2015
Revised: March 18, 2015
Published online on && &&, 0000
&
ChemPlusChem 0000, 00, 0 – 0
www.chempluschem.org
6
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

FULL PAPERS
Composite doubles: A reduced gra-
phene oxide (r-GO) composite with
MnCo oxide nanoparticles increases the
selective oxidation of p-cresol to form
p-hydroxybenzaldehyde (PHBAld; see
scheme, PHBAlc=p-hydroxybenzyl alco-
hol). The high catalytic activity of the r-
GO–MnCo catalyst is related to the in-
crease in the redox potential of the
mixed oxide after being supporting on
r-GO.
A. Jha, S. H. Patil, B. P. Solanki,
A. P. C. Ribeiro, C. A. N. Castro, K. R. Patil,
A. Coronas, C. V. Rode*
&& – &&
Reduced Graphene Oxide Composite
with Oxidizable Manganese/Cobalt
Mixed Oxide for p-Cresol Oxidation by
Using Molecular Oxygen
ChemPlusChem 0000, 00, 0 – 0
www.chempluschem.org
7
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ