An effective Mn-Co mixed oxide catalyst for the solvent-free selective oxidation of cyclohexane with molecular oxygen

Article abstract of DOI:10.1016/j.apcata.2016.06.001

Mn-Co mixed oxide (Mn_aCo_bO_x) catalysts were prepared by the precipitation method, and the effect of Mn/Co (atom) on their structural and catalytic performance for the solvent-free selective oxidation of cyclohexane with oxygen was investigated. The results showed that the atomic ratio of Mn/Co would affect the phase constitution, surface area, surface Mn valence states and the reducibility of the Mn_aCo_bO_x catalysts. When the atomic ratio of Mn/Co in the catalyst is 0.6, the Mn₂Co₃O_x catalyst behaved with excellent catalytic activity and stability, which should be attributed to its high specific surface area and the high atomic ratios of (Mn⁴⁺ + Mn³⁺)/Mn and Co³⁺/Co on its surface. The effects of the preparation conditions for the Mn₂Co₃O_x catalyst and the reaction conditions on the selective oxidation of cyclohexane over the Mn₂Co₃O_x catalyst was tested. Under the reaction conditions of 0.5 MPa O₂ and 140 °C for 4 h, 10.4% conversion of cyclohexane and 62.0% selectivity to a mixture of cyclohexanone and cyclohexanol (KA oil) were achieved. After this Mn₂Co₃O_x catalyst was repeatedly used 10 times, the cyclohexane conversion and the selectivity to KA oil had hardly changed.

Full text of DOI:10.1016/j.apcata.2016.06.001

Applied Catalysis A: General 523 (2016) 97–106
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
An effective Mn-Co mixed oxide catalyst for the solvent-free selective
oxidation of cyclohexane with molecular oxygen
Mingzhou Wu, Wangcheng Zhan∗, Yanglong Guo, Yun Guo, Yunsong Wang, Li Wang,
Guanzhong Lu∗
Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, PR
China
a r t i c l e i n f o
a b s t r a c t
Article history:
Mn-Co mixed oxide (MnaCobOx) catalysts were prepared by the precipitation method, and the effect
of Mn/Co (atom) on their structural and catalytic performance for the solvent-free selective oxidation
of cyclohexane with oxygen was investigated. The results showed that the atomic ratio of Mn/Co would
affect the phase constitution, surface area, surface Mn valence states and the reducibility of the MnaCobOx
catalysts. When the atomic ratio of Mn/Co in the catalyst is 0.6, the Mn2Co3Ox catalyst behaved with excel-
lent catalytic activity and stability, which should be attributed to its high specific surface area and the high
atomic ratios of (Mn4+ + Mn3+)/Mn and Co3+/Co on its surface. The effects of the preparation conditions
for the Mn2Co3Ox catalyst and the reaction conditions on the selective oxidation of cyclohexane over
the Mn2Co3Ox catalyst was tested. Under the reaction conditions of 0.5 MPa O2 and 140 ◦C for 4 h, 10.4%
conversion of cyclohexane and 62.0% selectivity to a mixture of cyclohexanone and cyclohexanol (KA oil)
were achieved. After this Mn2Co3Ox catalyst was repeatedly used 10 times, the cyclohexane conversion
and the selectivity to KA oil had hardly changed.
Received 16 January 2016
Received in revised form 31 May 2016
Accepted 1 June 2016
Available online 2 June 2016
Keywords:
Mn-Co mixed oxide
Selective oxidation of cyclohexane
Oxygen oxidant
Solvent-free reaction
Synthesis of cyclohexanone and
cyclohexanol
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
ane. Catalytic materials often play an important role for enhanced
catalytic reactions [4,5].
Terminally oxidized alkanes are extremely desirable poten-
tial feedstocks for the chemical and pharmaceutical industries,
which has prompted intensive research on new selective oxida-
tion reactions [1]. Among alkane oxidations, the selective oxidation
of cyclohexane is a very attractive reaction because its oxidation
products, cyclohexanone (K) and cyclohexanol (A), are the key
intermediates for producing nylon-6 and nylon-66 [2]. In the con-
ventional industrial processes, the conversion of cyclohexane is
usually lower than 5% in order to achieve high selectivity (approx-
imately 80%) to KA oil because cyclohexanone and cyclohexanol
are more active than the matrix. Moreover, in most cases, extreme
reaction conditions such as high pressure and high temperature are
required in current industrial processes. Hence, the reaction system
is potentially explosive, and the necessary care and safety precau-
tions should be taken [3]. Thus, it is necessary to design a new,
efficient catalytic system for the selective oxidation of cyclohex-
Until now, the heterogeneous catalysts for the selective oxi-
dation of cyclohexane include transition metal oxides (Co3O4 [6],
Ce1-xMnxO2 [7], Cu/Co/AC (active carbon) [8], and WO3/V2O5 [9]),
bimetallic nanocrystals [10], carbon nanotubes (CNTs) [11,12],
molecular sieves (AlPO-34 [13], HMS [14], MCM-48 [15], TUD-1
[16–18], and TS-1 [19]), etc., among which transition metal oxides
and mixed metal oxides have more comparative advantages and
have been studied widely. For example, the conversion of cyclohex-
ane over a mesoporous iron-titania catalyst reached 25.8% with the
selectivity of ∼90% to KA oil when acetic acid was used as the sol-
vent [20]. Using the Cu/Cr2O3 catalyst and the H2O2 oxidant, 86%
conversion of cyclohexane and 85% selectivity to cyclohexanone
were obtained after 3 h for the selective oxidation of cyclohexane
at room temperature [21]. In addition, the magnetic CoFe2O4 cat-
alyst was also used in the title reaction, and a 16.2% conversion of
cyclohexane was obtained under 1.6 MPa of initial oxygen pressure
at 145 ◦C [22]. Mn and Co oxides are the conventional oxidation
catalysts and are studied widely. Xu et al. reported that MnAlPO
molecular sieves were used as the catalyst for cyclohexane oxida-
tion with molecular oxygen, and a 3.7% conversion of cyclohexane
and 64.5% selectivity to KA oil were obtained [23]. We studied
∗
Corresponding authors.
E-mail addresses: zhanwc@ecust.edu.cn (W. Zhan), gzhlu@ecust.edu.cn (G. Lu).
http://dx.doi.org/10.1016/j.apcata.2016.06.001
0926-860X/© 2016 Elsevier B.V. All rights reserved.

98
M. Wu et al. / Applied Catalysis A: General 523 (2016) 97–106
the solvent-free selective oxidation of cyclohexane with oxygen
over AlPO-5 modified with rare earth elements [24,25], i.e., the
MCM-48 [26], A-HMS (A = Ce, Ti, Co, Al, Cr, V, Zr) [14] and MnOx
[27] catalysts. Using the MnOx catalyst, 8.0% conversion of cyclo-
hexane and 5.0% yield of KA oil was achieved, and this catalyst
behaved with good stability, that is, its catalytic activity had hardly
changed after 10 repeated uses [27]. The catalytic activity of Co3O4
nanocrystals for the cyclohexane oxidation was studied by Zhou
et al., and 7.6% conversion of cyclohexane and 89.1% selectivity to
KA oil were achieved [7]. Thus, manganese oxide or cobalt oxide
demonstrates catalytic activity for the selective oxidation cyclo-
hexane. The results reported by Todorova et al. demonstrated that
the catalytic activity of single cobalt or manganese oxide supported
on silica was similar for the complete n-hexane oxidation [28].
However, a combination between two elements can significantly
improve its catalytic activity for the oxidation of n-hexane, and its
temperature for the complete oxidation was decreased from 340 ◦C
to 270 ◦C [28].
In this paper, a Mn-Co mixed oxide (MnaCobOx) was developed
as an effective catalyst for the selective aerobic oxidation of cyclo-
hexane in a solvent-free system, with the purpose to study the
synergistic effect of cobalt and manganese oxide on the catalytic
performance. The effect of the Mn/Co atomic ratio on the structural,
physicochemical and catalytic properties of the MnaCobOx catalysts
was investigated. Furthermore, the effect of the preparation con-
ditions on the catalytic activity of the Mn2Co3Ox catalyst and the
effect of the reaction conditions on the selective oxidation of cyclo-
hexane over the Mn2Co3Ox catalyst were tested. After recycling the
catalysts 10 times, their catalytic activity hardly changed.
then collected on a copper grid. After the liquid phase was evapo-
rated, the grid was loaded into the microscope. The FT-IR spectra of
samples were recorded on a Nicolet Nexus 670 FT-IR spectrometer,
and the samples were ground with KBr and pressed into thin wafers.
X-ray photoelectron spectroscopy (XPS) spectra was recorded on a
Kratos Axis Ultra-DLD photoelectron spectrometer equipped with
a monochromatized AlK␣ X-ray source (1486.6 eV). C1s (binding
energy 284.8 eV) of adventitious carbon was used as the reference.
H2 temperature-programmed reduction (H2-TPR) was operated on
an apparatus PX200 (Tianjin Golden Eagle Technology Limited Cor-
poration) equipped with a TCD detector. 50 mg of the sample was
heated from room temperature to 800 ◦C at 10 ◦C/min in the reduc-
tion gas of 5 vol.% H2/N2 (45 mL/min).
2.3. Testing of catalytic activity
The activity of the catalyst for the selective oxidation of cyclo-
hexane was tested in a 100 mL stainless steel reactor equipped
with magnetic stirrer and explosion-proof pressure sensor. Molec-
ular oxygen was used as an oxidant, and solvent was not added
during the reaction process. In a typical reaction, 8 mL of cyclo-
hexane and 50 mg of catalyst (∼2000 mesh) were introduced into
the reactor. After O2 was charged to 0.5 MPa, the reactor was
heated to 140 ◦C under stirring at 800 rpm and maintained for 4 h.
In some cases, tert-butyl hydroperoxide and hydroquinone were
added into the reaction system as the free-radical initiator and
the free-radical scavenger, respectively. The effects of the rota-
tion speed and the particle size of catalyst on the conversion of
cyclohexane were evaluated under the conditions of 8 mL of cyclo-
hexane, 50 mg of catalyst, 140 ◦C and 0.5 MPa O2. Under the rotation
speed of 800 rpm and using a powder catalyst size (∼2000 mesh),
the effect of diffusion was excluded, that is, the oxidation reaction
was carried out in a regime of kinetics control.
2. Experimental
2.1. Synthesis of the catalysts
After the reaction was finished, the reaction mixture was diluted
with a certain amount of ethanol to dissolve the by-products, and
then the catalyst was separated by centrifugation. Because cyclo-
hexenyl hydroperoxide (CHHP) is easily decomposed in the GC
injector [29], excess triphenylphosphine (PPh3) was added to the
product mixture to quantitatively decompose CHHP to cyclohex-
anol. The concentrations of residual cyclohexane and the products
of cyclohexanol and cyclohexanone were analyzed by Perkin-Elmer
Clarus 500 gas chromatograph equipped with a PE-2 capillary
column (25 m × 0.32 mm × 1.0 ␮m) and an FID detector. Methyl-
benzene was used as the internal standard. The main by-products
are adipic acid, hexanoic acid, dicyclohexyl adipate and cyclohexyl
caproate. Adipic acid was analyzed by high performance liquid
chromatography.
The MnaCobOx catalysts were prepared by the precipitation
method. In a typical experiment, weighed Co(NO3)2·6H2O and
Mn(NO3)2 were completely dissolved in deionized water. The
molar ratio of Co/Mn was controlled from 2/0.5 to 2/4. The aque-
ous solution was added into a three-neck flask, and 1 mol/L NaOH
aqueous solution was then added dropwise to the synthesis solu-
tion at 60 ◦C under stirring until the pH was 8. After this synthesis,
the solution was aged at 60 ◦C for 4 h under stirring, in which the pH
of this solution was maintained at 8 by adding NaOH aqueous solu-
tion, if necessary. The formed solid was filtered, washed thoroughly
with deionized water, dried at 120 ◦C, and calcined in air at 400 ◦C
for 4 h to obtain the MnaCobOx samples. The samples were denoted
as Mn2Co0.5Ox, Mn2Co1Ox, Mn2Co2Ox, Mn2Co3Ox, and Mn2Co4Ox.
Pure MnOx and Co3O4 were also prepared by the same procedures
as the MnaCobOx samples.
Because there are numerous unstable products apart from K and
A in the reaction solution, the complete analysis for this reaction
mixture is not straightforward. Therefore, we calculated the con-
version of cyclohexane and the selectivity of the products by the
following formulas:
2.2. Characterization of catalysts
X-ray diffraction (XRD) analysis was performed on a Bruker
AXS D8 Focus X-ray diffractometer equipped with CuK␣ radi-
ation (␭ = 0.15406 nm) with a scanning rate of 6/min. Nitrogen
adsorption-desorption isotherms were measured at −196 ◦C on an
ASAP 2010 analyzer (Micromeritics Co. Ltd.). Before the measure-
ment, the sample was degassed at 300 ◦C for 6 h under vacuum to
remove moisture and impurities. Brumauer-Emmett-Teller (BET)
method was used to calculate the specific surface area of sample.
The chemical composition of sample was determined by an induc-
tively coupled-plasma atomic emission spectroscopy (ICP-AES) on
a TJA IRIS ADVANTAG 1000 instrument. Transmission electron
microscopy (TEM) images were taken on a JEOL JEM-2100 micro-
scope. Prior to testing, the sample was first dispersed in ethanol and
n
initial − n
residual
( )
−ane
initial
)
(
)
−ane
Con.−ane% =
× 100
n
(
−ane
n
−nol/−none
Sel.−nol/−none% =
× 100
residual
( )
−ane
n
initial − n
(
)
−ane
where n-nol and n-none denote the contents of cyclohexanol and
cyclohexanone in the reaction mixture, respectively, and n-ane (ini-
tial) and n-ane (residual) denote the content of cyclohexane in the
reaction solution before and after the reaction, respectively.

M. Wu et al. / Applied Catalysis A: General 523 (2016) 97–106
99
Fig. 3 shows the FT-IR spectra of the catalysts. All of the samples
mainly display peaks at 3420 and 1625 cm−1 as well as a group
of peaks below 750 cm−1. The peaks at 3420 and 1625 cm−1 are
associated with hydroxyl groups of water adsorbed on the sample
surface. The peaks at 631, 518 and 416 cm−1 in the spectra of the
Mn2Co0.5Ox and Mn2Co1Ox catalysts can be ascribed to the Mn-O
bending vibrations of MnOx [33,34]. However, the absorption peaks
in the spectra of the Mn2Co2Ox, Mn2Co3Ox and Mn2Co4Ox catalysts
shifted to 655 and 557 cm−1, which can be assigned to the vibration
of Co-O in the octahedral environment and the vibration of Mn-O in
the tetrahedral site of the spinel structure [35,36]. In other words,
the spinel structure was found in the Mn2Co2Ox, Mn2Co3Ox and
Mn2Co4Ox catalysts, which is consistent with the XRD results.
Fig. 4 shows the Mn 2p3/2 and Co 2p3/2 XPS spectra of the
Mn2Co0.5Ox, Mn2Co1Ox, Mn2Co3Ox and Mn2Co4Ox catalysts. There
is an asymmetrical Mn 2p3/2 peak for each sample, and this peak
can be fitted with a Shirley background and a convolution of Gaus-
sian and Lorentzian functions (80:20) to obtain three peaks, which
are assigned to the surface Mn2+, Mn3+ and Mn4+ species. The
binding energies of Mn2+, Mn3+ and Mn4+ in the Mn-Co oxides
are 640.9 0.1 eV, 642.0 0.1 eV and 643.7 0.1 eV, respectively
[37–39]. As shown in Fig. 4A, the binding energies of Mn 2p3/2
for the Mn2Co1Ox, Mn2Co3Ox and Mn2Co4Ox catalysts are higher
than those of the Mn2Co0.5Ox catalyst, due to the interaction of Mn
and Co with the increase in the Co amount. Similarly, as shown in
Fig. 4B, the Co 2p3/2 peak can be deconvoluted into two peaks at
780.4 0.2 eV and 781.8 0.2 eV, which are assigned to Co3+ and
Co2+, respectively. The peak at approximately 787.3 eV is the satel-
lite peak of Co 2p [28,40].
Table 1 shows the surface and bulk compositions of the
Mn2Co0.5Ox, Mn2Co1Ox, Mn2Co3Ox and Mn2Co4Ox catalysts, which
were calculated on the basis of the XPS data and ICP analysis. The
atomic ratios of Mn/Co (4.3, 1.7, 0.6 and 0.4) in the bulk of the four
catalysts are slightly deviated from the values (4.0, 2.0, 0.67 and
0.5) in their precursor solutions. When the Mn amount in the solu-
tion is much larger than the Co amount (as Mn2Co0.5Ox), the Mn/Co
atomic ratio in the bulk (4.3) is slightly higher than that in the solu-
tion (4.0), and with an increase in the Co amount in the solution, the
Mn/Co atomic ratio in the bulk decreased more and more compared
with that in the solution. For example, for the Mn2Co4Ox catalyst,
the Mn/Co atomic ratio in the bulk (0.4) is lower than that in the
solution (0.5). This should be ascribed to the difference between
the solubility of Co(OH)2 and Mn(OH)4 in the synthesis solution.
Because the pH in the synthesis solution is constant, and the pKsp
of Co(OH)2 (14.2-15.7 changed from blue to pink) is higher than that
of Mn(OH)4 (12.72), Mn(OH)4 could be quickly precipitated when
the Mn concentration in the solution is much larger than that of Co,
while when the Co concentration in the solution is larger than that
of Mn, Co(OH)2 could be more quickly and completely precipitated
than Mn(OH)4, which caused the Mn/Co ratio in the bulk to be less
than that in the solution.
Fig. 1. XRD patterns of (b-f) MnaCobOx and (c’) Mn2Co3Ox after being repeatedly
used 10 times.
2.4. Recycling usage of catalyst
The Mn2Co3Ox catalyst was filtrated from the reaction solution
after the reaction was ended and then washed with ethyl alcohol
three times, dried at 100 ◦C for 6 h and calcined in air at 400 ◦C
for 2 h, obtaining the recycled catalyst. Using the same procedure
as the activity testing for the fresh catalyst, the catalytic activity of
the recycled Mn2Co3Ox catalyst supplemented by the fresh catalyst
was tested because of the loss of the catalyst during the recycling
process. At the same time, the XRD pattern and the Mn and Co
contents of the Mn2Co3Ox catalyst after recycling 10 times were
analyzed to evaluate the stability of the catalyst.
3. Results and discussion
3.1. The structure and surface properties of Mn-Co mixed oxide
catalysts
The XRD patterns of the MnOx, Co3O4 and MnaCobOx catalysts
are shown in Fig. 1. The MnOx catalyst exhibits the diffraction
peaks of the hausmannite crystalline phase of Mn3O4 (JCPDS 24-
0734) and Mn2O3 (JCPDS 41-1442). After adding a small amount of
Co into MnOx, the Mn2Co0.5Ox and Mn2Co1Ox catalysts show the
diffraction peaks of (Co,Mn)(Co,Mn)2O4. With the further increas-
ing Co amount in MnOx, the diffraction peaks are transformed to
the tetragonal spinel structure of MnCo2O4 in the XRD spectra of
the Mn2Co3Ox and Mn2Co4Ox catalysts. As Co and Mn ions have
a similar ionic radius and the same valence state, it is generally
beneficial to form a MnaCobOx solid solution [30].
The TEM images of the Mn2Co0.5Ox, Mn2Co1Ox, Mn2Co3Ox and
Mn2Co4Ox catalysts are shown in Fig. 2. The Mn2Co0.5Ox cata-
lyst exhibits cubic morphology and good crystallinity and one set
of lattice fringes with a spacing of 0.49 nm (Fig. 2a2), which is
attributed to the (111) plane of (Co,Mn)(Co,Mn)2O4. As shown in
Fig. 2b2, the lattice fringes of the Mn2Co1Ox catalyst with a spacing
of 0.31 nm are ascribed to the (202) plane of (Co,Mn)(Co,Mn)2O4.
In comparison with the Mn2Co0.5Ox and Mn2Co1Ox catalysts, the
Mn2Co3Ox catalyst (Fig. 2c1) was agglomerated to form stacks with
unclear margins, but the cubic structure could be still observed
with fuzzy edges (Fig. 2c3). The interplanar spacings of 0.20 nm,
0.28 nm and 0.47 nm corresponding to the (400), (220) and (111)
planes of MnCo2O4 can be observed in Fig. 2c2 and c4 [31,32]. As the
Co content in the MnaCobOx catalysts was further increased, the
Mn2Co4Ox catalyst tended towards a spherical morphology with
crystallinity, as shown in Fig. 2d. The interplanar spacing of 0.25 nm
is assigned to the (311) plane of MnCo2O4.
However, the Mn/Co in the bulk is lower than the Mn/Co atomic
ratios on the surface of the corresponding catalysts. In addition,
the difference between the Mn/Co ratios in the bulk and on the
surface for a certain MnaCobOx catalyst would be increased as the
Co content in the MnaCobOx catalysts was increased because of the
formation of the tetragonal spinel structure MnCo2O4. Conversely,
the atomic ratios of (Mn4+ + Mn3+)/Mn and Co3+/Co on the surface of
the MnaCobOx catalysts initially increased and then decreased with
an increase in the Co amount, and the Mn2Co3Ox catalyst possessed
the highest atomic ratios of (Mn4+ + Mn3+)/Mn and Co3+/Co. It has
been reported that Mn species with the higher oxidation state are
preferable for the oxidation reactions over manganese-containing
catalysts [41]. It was also reported that the formation rates of
cyclohexanol and cyclohexanone synthesis are proportional to the
number of redox-active framework sites over heteroatom-doped

100
M. Wu et al. / Applied Catalysis A: General 523 (2016) 97–106
Fig. 2. TEM images of the (a) Mn2Co0.5Ox, (b) Mn2Co1Ox, (c) Mn2Co3Ox and (d) Mn2Co4Ox catalysts.
APO-5 and TUD-1 catalysts [42,16]. Therefore, the high atomic
ratios of (Mn4+ + Mn3+)/Mn and Co3+/Co for the Mn2Co3Ox catalyst
are beneficial to the improvement of its activity for the oxidation
of cyclohexane.
The H2-TPR profiles of the MnaCobOx, Co3O4 and MnOx cata-
lysts are shown in Fig. 5. Pure Co3O4 exhibited two overlapped
reduction peaks at ∼305 ◦C and ∼380 ◦C, which are assigned to the
reduction of Co3O4 to CoO and the reduction of CoO to metal Co,
respectively [43–45]. Pure MnOx also showed two strong reduction

M. Wu et al. / Applied Catalysis A: General 523 (2016) 97–106
101
Table 1
Compositions of the Mn2Co0.5Ox and Mn2Co3Ox catalysts determined by XPS and ICP.
Sample
Surface atom
Mn/Co (atom)
Ratio of Mn/Co (atom)
on surface to that in
the bulk
Mn4+/Mn Mn3+/Mn (Mn3+ + Mn4+)/Mn Co3+/Co
concentration (%)a
O
Mn
Co
In the solution In surf. by XPS In the bulk by ICP
Mn2Co0.5Ox 65.1 29.2 5.8
4
2
5.0
2.3
0.9
0.9
4.3
1.7
1.16
1.35
1.50
2.25
0.23
0.25
0.31
0.26
0.58
0.61
0.62
0.56
0.81
0.86
0.93
0.82
0.37
0.40
0.53
0.29
Mn2Co1Ox
Mn2Co3Ox
Mn2Co4Ox
61.0 27.1 11.9
64.2 17.4 18.4 2/3
60.6 18.9 20.6 0.5
0.6 (0.6)a
0.4
a
The atomic ratio of Mn/Co determined after being repeatedly used 10 times.
1625
416
655
557
Mn Co O
Mn
3420
B
Co3O4
A
Mn2Co4Ox
Mn2Co3Ox
222
C
Mn2Co2Ox
410
Mn2Co1Ox
Mn2Co0.5Ox
MnOx
425
b
a
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
100
200
300
400
500
600
700 800
Temperature (ºC)
Fig. 3. FT-IR spectra of the MnaCobOx catalysts.
Fig. 5. H2-TPR profiles of the Co3O4, MnOx and MnaCobOx catalysts.
peaks at ∼295 ◦C and ∼425 ◦C, which are ascribed to the reduction
of MnO2/Mn2O3 to Mn3O4 and the reduction of Mn3O4 to MnO,
respectively [45].
ascribed to the reduction of MnO2/Mn2O3 to Mn3O4 and Co3O4 to
CoO, and the broad reduction peak at >500 ◦C can be ascribed to
the reduction of Mn3O4 to MnO and CoO to Co [46,47]. However,
either there is a strong interaction between Co3O4 and MnOx, or
this is a Mn-Co composite oxide. Compared with the Mn2Co2Ox and
Mn2Co4Ox catalysts, the A reduction peak of the Mn2Co3Ox catalyst
is the lowest (222 ◦C), and the reduction peak at high temperature
is also the lowest, which shows that there is a very strong syner-
gistic effect between Co3O4 and MnOx in the Mn2Co3Ox catalyst,
resulting in the highest catalytic performance for the title reaction.
After adding the lower Co amount into MnOx (as Mn2Co0.5O2
and Mn2Co1O2), the a peak disappeared, and the b peak decreased
gradually, accompanied by the shift of the peaks towards a lower
temperature. For example, the top temperature and area of the
b peak for Mn2Co0.5O2 are 415 ◦C and 140 (a.u.), and those of
Mn2Co1O2 are 410 ◦C and 138 (a.u.), respectively. On the whole,
except for the appearance of the peak at ∼650 ◦C, the reduction
characteristics of Mn2Co0.5Ox and Mn2Co1Ox catalysts are similar
to those of the MnOx catalyst.
When the Co amount is equal or more than that of Mn in
MnaCobOx, the TPR curves of Mn2Co2Ox, Mn2Co3Ox and Mn2Co4Ox
exhibited the characteristics of the Co3O4 TPR curve, and the corre-
sponding A and B peaks decreased greatly, and the C peak appeared.
Because the three reduction peaks at <400 ◦C are overlapped, their
peak areas were calculated difficultly, but their total peak areas at
<400 ◦C are similar (160–164, a.u.) for these three samples. In the
TPR curves of MnaCobOx, the reduction peaks at <450 ◦C can be
3.2. The effect of the composition on the catalytic performance
Table 2 shows the catalytic performance of the Co3O4, MnOx and
MnaCobOx catalysts for the selective oxidation of cyclohexane with
O2 as the oxidant. In the blank testing, 1.2% conversion of cyclohex-
ane and 91.9% selectivity to KA oil were achieved. Using the MnOx
catalyst, 7.9% conversion of cyclohexane and 55.2% selectivity to
KA oil could be obtained. When the MnaCobOx samples were used
780.4
642.0
(A) Mn 2p3/2
(B) Co 2p3/2
781.8
Sat.
643.7
640.9
Mn2Co4O
x
Mn2Co4O
x
Mn2Co3O
x
Mn2Co3O
Mn2Co1O
x
Mn2Co1O
x
x
Mn2Co0.5
O
x
Mn2Co0.5
O
x
810
805
800
795
790
785
780
660
655
650
645
640
Binding energy (eV)
Binding energy (eV)
Fig. 4. Mn 2p3/2 and Co 2p3/2 XPS spectra of the MnaCobOx catalysts.

102
M. Wu et al. / Applied Catalysis A: General 523 (2016) 97–106
Table 2
Catalytic performance of the prepared catalysts for the oxidation of cyclohexane.a
Mass balance (%)b
SBET
Selectivity (%)
YKA
(%)
Catalyst
Time
(h)
Atmosphere/(MPa) Conversion
(%)
(m2/g)
−nol
−none
adipic acid
Blank test
MnOx
Mn2Co0.5Ox
Mn2Co1Ox
Mn2Co2Ox
Mn2Co3Ox
Mn2Co3Ox
Mn2Co4Ox
Co3O4
–
5
4
4
4
4
4
4
4
4
4
4
4
24
6
3
3
3
O2/0.5
O2/0.5
O2/0.5
O2/0.5
O2/0.5
O2/0.5
O2/0.5
O2/0.5
O2/0.5
O2/0.5
O2/0.5
O2/0.5
8% O2/N2/1.5
O2/1.0
O2/1.1
O2/1.1
O2/1.1
O2/1.1
O2/0.3
H2O2
1.2
7.9
6.8
8.0
9.5
10.4
10.4
8.8
7.7
2.4
13.2
–
3.8
2.8
3.1
2.8
5.0
2.9
5.0
81.2
43.0
25.5
30.5
32.9
30.0
31.7
31.6
29.5
44.2
–
30.1
–
56
51.1
26.2
24.4
38.1
25.0
50
48.9
29.7
24.4
19.8
27.1
30.3
30.3
26.3
22.4
–
28.2
–
31
33.3
46.5
45.2
44.6
43.8
29
1.7
5.4
4.8
6.2
5.5
6.3
6.3
5.7
7.2
–
95
94
94
95
94
94
13.3
16.3
22.2
37.9
40.8
40.8
36.0
18.3
40.8
40.8
40.8
–
4.4
3.7
4.2
5.4
6.5
6.5
4.9
5.1
–
c
94
95
d
Mn2Co3Ox
Mn2Co3Ox
e
6.6
–
7.7
–
f
Mn2Co3Ox
Co-TUD−1g [15]
3.3
2.4
2.3
1.9
4.1
2.0
4.0
h
Co3O4/Al2O3 [6]
MnAPO−5i [50]
MnAPSO−34i [50]
CoAPO−5i [50]
CoAPSO−34i [50]
Au/MgOj [51]
3
17
12
19
CoHMAk [52]
94.6
2.9
a
Reaction conditions: 8 mL cyclohexane, 50 mg catalyst, 0.5 MPa O2, at 140 ◦C.
b
c
d
e
f
Mass balance is equal to the ratio between the weight of the products and the reaction mixture without the catalysts.
Without adding triphenylphosphine to the reaction mixture before the determination of products.
The oxidation of cyclohexanone: 8 mL cyclohexanone, 50 mg catalyst, 0.5 MPa O2, at 140 ◦C.
Small amount (3 wt% cyclohexane) of tert-butyl hydroperoxide (free-radical initiator) was added.
Small amount (3 wt% cyclohexane) of hydroquinone (free-radical scavenger) was added.
g
h
i
Reaction conditions: 175 mmol cyclohexane, 0.05 mmol cyclohexyl hydroperoxide (radical initiator), 0.1 mmol active metal species, at 120 ◦C.
Reaction conditions: 15 g cyclohexane, 0.2 g catalyst, at 120 ◦C.
Reaction conditions: catalyst (0.195 mmol metal), 18 g cyclohexane, 0.04 g TBHP, at 130 ◦C.
j
Reaction conditions: 10 mL cyclohexane, 0.05 mmol tert-butylhydroperoxy (radical initiator), 6 mg catalyst amount, T = 140 ◦C.
k
Reaction conditions: substrate: oxidant (H2O2) = 1:1, 50 mg catalyst, 10 mL acetic acid (solvent), 5 mmol methyl ethyl ketone (initiator), at 100 ◦C.
−1 molcatalyst−1) is
−1 molmetal−1),
) and CoAPSO-34
as the catalysts, the conversion of cyclohexane showed a volcano-
shaped curve with the increase in the amount of Co in the catalyst,
among which the highest yield of KA oil (6.45%), 10.4% conver-
sion of cyclohexane and 62% selectivity to KA oil, could be obtained
over the Mn2Co3Ox catalyst. The TOF on the Mn2Co3Ox catalyst is
on the Mn2Co3Ox catalyst (14.3 molsubstrate
h
higher than that on MnAPO-5 (11.4 molsu−b1strate
h
MnAPSO-34 (10.3 molsubstrate
h
−1 molmetal
(10.6 molsubstrate h
−1 molmetal−1) [50]. On the contrary, the con-
version of cyclohexane over the Mn2Co3Ox catalyst is lower than
that in the liquid-phase oxidation of cyclohexane, especially the
reaction system containing both solvent and initiator. For example,
the conversion of cyclohexane could be sharply improved to 81.2%
over the Co-HMA catalyst with H2O2 as the oxidizer in acetonitrile
as the solvent [52]. The results show that the Co-Mn mixed oxide
is an effective catalyst for cyclohexane oxidation with O2 as the
oxidizer in the solvent-free system.
For the selective oxidation of cyclohexane over the solid catalyst,
it was reported that the addition of TBHP (tert-butyl hydroperox-
ide) as free-radical initiator can accelerate the initiation step of
the auto-oxidation process with molecular oxygen as an oxidant
to improve the activity of the catalyst [7,53]. Therefore, the effect
of adding TBHP on the activity of the Mn2Co3Ox catalyst for the
selective oxidation of cyclohexane was also examined. As shown in
Table 2, after adding TBHP, the conversion of cyclohexane increased
to 13.2% over the Mn2Co3Ox catalyst, but the total selectivity to KA
oil slightly decreased to 58.3%. On the contrary, after hydroquinone
(a free-radical scavenger) was added, the oxidation of cyclohexane
was almost completely inhibited, proving that the reaction follows
the free radical mechanism [54].
The characterization results of the catalysts showed that the
presence of Co in the MnOx catalyst promoted the formation of the
MnaCobOx solid solution and the MnCo2O4 spinel structure. In addi-
tion, the crystallinities of the Mn2Co2Ox and Mn2Co3Ox catalysts
were lower than those of the Mn2Co0.5Ox and Mn2Co1Ox catalysts.
Consequently, when the Co content in MnOx was increased, the
special surface areas of the MnaCobOx catalysts first increased and
then decreased slightly. The XPS results in Table 1 revealed that
Mn is inclined to enrich on the surface of the Mn-Co mixed oxide,
14.3 molsubstrate h
−1 molcatalyst−1. With a further increase in the Co
amount of the catalyst, the conversion of cyclohexane decreased;
for example, 8.8% conversion of cyclohexane was obtained over the
Mn2Co4Ox catalyst. Although the Co3O4 catalyst exhibited a high
selectivity to KA oil (66.6%), the yield of KA oil (5.13%) was much
lower than that (6.45%) of the Mn2Co3Ox catalyst.
In addition to the products of cyclohexanol and cyclohexanone,
another important product, adipic acid (AA), could also be detected
in the reaction products. However, the selectivity of adipic acid in
this reaction system over the Mn2Co3Ox catalyst is very low, which
is much different from that on the Mn-doped titanosilicate and
Co/Mn cluster complex catalysts [48,49]. Zou et al. reported that
the presence of Mn3+ sites in Mn-HTS can promote the formation
of CyOO·, which facilitated the formation of cyclohexyl radicals and
thus enhanced the AA formation [48]. In addition, the keto-form
intermediate (1,2-cyclohexanedione) was also revealed to be able
to promote the production of AA on the Co/Mn cluster [49]. On the
contrary, the prepared MnaCobOx catalyst did not exhibit a high
activity for oxidizing cyclohexanone to AA, as shown in Table 2,
resulting in a low selectivity to adipic acid for the oxidation of
cyclohexane.
Compared with the reported catalysts including Co-TUD-1
[16], Co3O4/Al2O3 [7], MnAPO-5, MnAPSO-34, CoAPO-5, CoAPSO-
34 [50] and Au/MgO [51] (Table 2), the Mn2Co3Ox catalyst
exhibits superior catalytic properties under similar reaction
conditions and using O2 as the oxidant. For example, only 5.0%
and 3.1% conversion of cyclohexane was achieved after 3 h of
reaction over the CoAPO-5 and MnAPO-5 catalysts, respec-
tively, with the addition of a small amount of TBHP. The TOF

M. Wu et al. / Applied Catalysis A: General 523 (2016) 97–106
103
60
40
20
0
60
Conversion %
Conversion %
Sel. of -none %
Sel. of -nol %
Sel. of acid %
a
b
Sel. of -none %
Sel. of -nol %
Sel. of acid %
40
20
At 60 oC & pH = 8
pH = 8 & aging time 4 h
0
20
40
60
80
100
0
4
8
12
Aging time (h)
Precipitation and aging temperature (°C)
80
60
40
20
0
Conversion %
Sel. of -none %
Sel. of -nol %
Sel. of acid %
c
d
Conversion %
Sel. of -none %
Sel. of -nol %
Sel. of acid %
60
40
20
0
pH=8, aging time 4 h & at 60 °C
At 60 oC & aging time 4 h
7
8
9
10
11
300
400
500
600
pH value
Calcination temperature (°C)
Fig. 6. Effect of the preparation conditions on the catalytic activity of the Mn2Co3Ox catalyst for cyclohexane oxidation, and (c inset) XRD patterns of Mn2Co3Ox catalysts
prepared at different pH values. (Reaction condition: 8 mL cyclohexane, 50 mg catalyst, 140 ◦C, 0.5 MPa O2, 4 h).
and an increase in the Co content can aggravate the enrichment
of Mn. In particular, the atomic ratios of (Mn4+ + Mn3+)/Mn and
Co3+/Co on the surface of the Mn2Co3Ox catalyst are 0.93 and 0.53,
respectively, which are higher than those on the surface of other
MnaCobOx catalysts. The H2-TPR results show that the Mn2Co3Ox
catalyst possesses a higher redox ability at the low temperature.
The results shown in Table 2 revealed that single Co or Mn can act
as the active sites for the oxidation of cyclohexane, and even the
formation rates of cyclohexanol and cyclohexanone synthesis are
proportional to the number of redox-active framework metal sites
in MnAPO-5 catalysts [42]. When we related the catalytic activity
of the MnaCobOx catalyst to its physicochemical and surface prop-
erties, it could be found that the excellent catalytic activity of the
Mn2Co3Ox catalyst should be attributed to its high specific surface
area and the high atomic ratios of (Mn4+ + Mn3+)/Mn and Co3+/Co
on its surface, accompanied by its high redox ability. This view-
point will be further elaborated in discussing the following reaction
mechanism.
Fig. 6b presents the effect of the aging time on the catalytic activ-
ity of the Mn2Co3Ox catalyst for the oxidation of cyclohexane. No
obvious changes in the cyclohexane conversion were observed with
the increase in the aging time of the Mn2Co3Ox catalyst. When
the catalyst aged at 60 ◦C for 4 h was used, the selectivity to KA
oil reached the maximum, which consisted of 30.3% selectivity to
cyclohexanone and 31.7% selectivity to cyclohexanol.
The effect of the pH value of the synthesis solution on the
catalytic activity of the Mn2Co3Ox catalyst for the oxidation of
cyclohexane is shown in Fig. 6c. With the increase in the pH value
of synthesis solution, the conversion of cyclohexane was hardly
changed, but the selectivities to cyclohexanone and cyclohexanol
obviously varied. The selectivity to KA oil reached a maximum
(62.0%) over the catalyst prepared in the synthesis solution with
pH = 8. The pH value of the synthesis solution could affect the bulk
structure (see their XRD patterns shown in Fig. 6c inset) and sur-
face composition. The surface composition would affect the surface
acidity/basicity and the nature of the active sites, resulting in the
change in the selectivities to products K and A.
3.3. The effect of the preparation conditions on the catalytic
performance
The effect of the calcination temperature on the catalytic activ-
ity of the Mn2Co3Ox catalyst for the oxidation of cyclohexane was
also evaluated (Fig. 6d). When the calcination temperature was
increased, the conversion of the cyclohexane decreased slightly,
while the selectivities to cyclohexanol and cyclohexanone exhib-
ited a volcano-shaped curve. The selectivity to cyclohexanol and
cyclohexanone reached the maximum value (31.7% and 30.3%,
respectively) over the Mn2Co3Ox catalyst calcined at 400 ◦C for 4 h.
These results indicated that the preparation conditions of the
Mn2Co3Ox catalyst have a small influence on its catalytic activity
for the oxidation of cyclohexane. When the Mn2Co3Ox catalyst was
prepared in the synthesis solution (pH = 8) at 60 ◦C, aged at 60 ◦C for
The effect of the precipitation and aging temperatures on the
catalytic activity of the Mn2Co3Ox catalyst for the oxidation of
cyclohexane is shown in Fig. 6a. The results show that the catalytic
activity of the Mn2Co3Ox catalyst was less affected by the pre-
cipitation temperature. Using the Mn2Co3Ox catalyst prepared at
60 ◦C, 10.4% conversion of cyclohexane and 62.0% selectivity to KA
oil (including 30% selectivity to cyclohexanone) could be obtained.
With the variation in the preparation temperature of catalyst, the
selectivity to cyclohexanol was hardly changed.

104
M. Wu et al. / Applied Catalysis A: General 523 (2016) 97–106
60
40
20
0
60
Conversion %
Conversion %
Sel. of -none %
Sel. of -nol %
Sel. of acid %
a
b
Sel. of -none %
Sel. of -nol %
Sel. of acid %
40
20
0
120
130
140
150
160
0.2
0.4
0.6
0.8
1.0
Oxygen pressure (MPa)
Reaction temperature (°C)
60
40
20
0
60
40
20
0
Conversion %
Sel. of -none %
Sel. of nol %
Sel. of acid %
d
c
Conversion %
Sel. of -none %
Sel. of -nol %
Sel. of acid %
20
40
60
80
100
2
4
6
8
Catalyst amount (mg)
Reaction time (h)
Fig. 7. Effect of the initial oxygen pressure (a), reaction temperature (b), reaction time (c) and amount of catalyst (d) on the cyclohexane oxidation over the Mn2Co3Ox
catalyst. (Standard reaction condition except the investigated parameter: 8 mL cyclohexane, 50 mg catalyst, 140 ◦C, 0.5 MPa O2, 4 h).
4 h, calcined at 400 ◦C for 4 h and used as the catalyst, 10.4% conver-
sion of cyclohexane and 62.0% selectivity to KA oil were achieved.
tion. On the contrary, when the reaction temperature was increased
from 120 ◦C to 160 ◦C, the selectivity to acid increased from 4% to
10.8%.
3.4. Effect of the reaction conditions on the catalytic oxidation of
cyclohexane
3.4.3. Reaction time
3.4.1. Initial oxygen pressure
Fig. 7c shows the effect of the reaction time on the cat-
alytic performance of the Mn2Co3Ox catalyst for the oxidation of
cyclohexane. The conversion of cyclohexane increased during the
reaction time of 0–4 h and then remained stable when the reaction
time was further prolonged from 4 h to 8 h. At the same time, the
selectivity to cyclohexanol and cyclohexanone decreased, and the
selectivity to acid did not obviously change.
Fig. 7a shows the effect of initial oxygen pressure on the cat-
alytic performance of the Mn2Co3Ox catalyst for the oxidation
of cyclohexane. With an increase in the initial oxygen pressure
from 0.3 to 1.0 MPa, the conversion of cyclohexane first increased
and then remained stable at 0.5–1.0 MPa. At the same time, the
selectivity to KA oil was decreased from 84.1% to 55.1% with
the increase in the initial oxygen pressure because of the deep
oxidation of KA oil under higher oxygen pressure. However, the
selectivity to cyclohexanol and cyclohexanone exhibited different
trends: the selectivity to cyclohexanol gradually decreased from
31.7% to 20.5%, but the selectivity to cyclohexanone only slightly
changed. The reason is that cyclohexanol is easier to further oxidize
to byproducts (such as acids) at high oxygen pressure.
3.4.4. Amount of catalyst
Fig. 7d shows the effect of the catalyst amount on the catalytic
performance of the Mn2Co3Ox catalyst for the oxidation of cyclo-
hexane. With the increase in the amount of catalyst, the conversion
of cyclohexane increased, while the selectivity to cyclohexanol
and cyclohexanone decreased. Meanwhile, the selectivity to acid
decreased first and then remained unchanged when the catalyst
amount was increased from 70 to 90 mg.
The results above show that the appropriate reaction conditions
for the solvent-free cyclohexane oxidation are 0.5 MPa of initial O2
pressure, 140 ◦C of reaction temperature, 4 h of reaction time and
50 mg of catalyst, in which 10.4% conversion of cyclohexane and
62.0% selectivity to KA oil can be obtained.
3.4.2. Reaction temperature
Fig. 7b presents the effect of the reaction temperature on the
catalytic performance of the Mn2Co3Ox catalyst for the oxidation
of cyclohexane. With the increase in the reaction temperature, the
conversion of cyclohexane increased as the temperature increased
up to 140 ◦C and then remained stable when the reaction temper-
ature was further increased to 160 ◦C. Meanwhile, the selectivity
to cyclohexanol and cyclohexanone gradually decreased with the
increasing reaction temperature due to the enhanced deep oxida-

M. Wu et al. / Applied Catalysis A: General 523 (2016) 97–106
105
ROO*H can desorb from the active sites as ROOH, accompanied by
the return of the active sites.
Conversion
70
60
50
Sel. of K+A oil
∗ + RH → R ∗ H
(1)
(2)
(3)
R ∗ H + O2 → ROO ∗ H
ROO ∗ H → ∗ + ROOH
9
6
3
0
As far as metal-based homogeneous catalysts are concerned,
ROOH decomposition has been proposed to involve Me-ROOH com-
plexes and one-electron redox cycles [60]. Therefore, the sequence
of reactions related to ROOH decomposition on MnaCobOx catalysts
can be proposed as shown in Eqs. (4) and (5) [16,62,63].
1
2
3
4
5
6
7
8
9
10
Recycling (times)
ROOH + Men+ → ROO• + H+ + Me(n−1)+
ROOH + Me(n−1)+ → RO• + OH− + Men+
(4)
(5)
Fig. 8. The effect of recycling times on the catalytic activity of the Mn2Co3Ox catalyst
for the cyclohexane oxidation.
The propagation sequence involves H abstraction from RH by
ROO• and RO• to form ROOH and ROH accompanied by the for-
mation of cyclohexyl radicals (R•), which can cause O2 to rapidly
regenerate ROO•. The related sequence of reactions is shown in
Eqs. (6)–(8). The chain-termination involves two self-reactions of
ROO•radicals to form oxidation products of ROH and R = O, as shown
in Eq. (9).
3.5. The stability of the Mn2Co3Ox catalyst
The stability of the Mn2Co3Ox catalyst was tested by the hot fil-
tration testing [55]. The Mn2Co3Ox catalyst was separated from the
reaction solution after 1.5 h, and the residual solution was continu-
ally reacted at 140 ◦C for another 2.5 h under the same conditions. It
was found that the conversion of cyclohexane increased from 5.6%
to 6.2%, which is lower than that (10.4%) over the catalyst after 4 h.
Subsequently, the catalytic activity of the recycled Mn2Co3Ox
catalyst was tested under the same reaction conditions as the first
run, and the results of 10 repeated usages are shown in Fig. 8. There
is no obvious change in the cyclohexane conversion or the selec-
tivity to KA oil after the recycled catalyst was repeatedly used 10
times. These results indicate that the Mn2Co3Ox catalyst is stable for
cyclohexane oxidation by O2 in a solvent-free system. In addition,
the contents of Mn and Co in the product mixture after the first run
were found by ICP-AES to be 0.48 ppm and 0.85 ppm, respectively,
which shows that Mn and Co are barely leaching from the catalyst
during the reaction. Furthermore, the mole ratio of Mn/Co in the
Mn2Co3Ox catalyst after being repeatedly used 10 times was also
detected by ICP-AES, and could be still maintained at the same ratio
of 0.6 as the fresh catalyst (Table 1). The XRD pattern (Fig. 1c’) of the
recycled Mn2Co3Ox catalyst confirmed that after being repeatedly
used for 10 times, the Mn2Co3Ox catalyst could maintain well the
crystal structure of the fresh catalyst. The above results indicate
that this is a heterogeneous catalytic process, and the Mn2Co3Ox
catalyst possesses high stability for cyclohexane oxidation by O2 in
a solvent-free system.
ROO• + RH → ROH + R•
RO• + RH → ROH + R•
R• + O2 → ROO•
(6)
(7)
(8)
(9)
ROO• + ROO• → ROH + R = O + O2
4. Conclusions
In summary, the highly efficient MnaCobOx catalysts for the oxi-
dation of cyclohexane with molecular oxygen was prepared by
a precipitation method, and their structures and phase composi-
tions, surface areas, valence states of surface Mn and Co species
and reducibility were affected by the molar ratio of Mn/Co. The
Mn2Co3Ox catalyst showed the highest activity for the oxidation
of cyclohexane with molecular oxygen. After the Mn2Co3Ox cat-
alyst was used repeatedly 10 times, its activity hardly changed,
which shows that the Mn2Co3Ox catalyst possesses a high stabil-
ity for the oxidation of cyclohexane with O2. Relating the catalytic
activity of the MnaCobOx catalyst with its physicochemical and sur-
face properties, it can be found that the excellent activity of the
Mn2Co3Ox catalysts should be attributed to its high specific surface
area and the high atomic ratios of (Mn4+ + Mn3+)/Mn and Co3+/Co
on its surface, which can activate C H bonds at early stages of the
reaction, resulting in shorter initial induction periods required for
ROOH mediated pathways. Conversely, the effects of the prepara-
tion conditions and reaction conditions on the catalytic activity of
the Mn2Co3Ox catalyst were also studied. Under the proper reaction
conditions, 10.4% conversion of cyclohexane and 62.0% selectivity
to KA oil can be obtained at 140 ◦C for 4 h.
3.6. The reaction mechanism of cyclohexane oxidation over the
Mn2Co3Ox catalyst
It is well known that the oxidation of cyclohexane (RH) occurs
through a radical chain mechanism, and it generally comprises
three steps: initiation, propagation and termination [56–58], in
which cyclohexyl hydroperoxide (ROOH) is an intermediate in the
formation of cyclohexanol (ROH) and cyclohexanone (R = O) [59].
By inference from other low-temperature catalytic oxidations
with solvated Co and Mn complexes and O2 [60], the reaction of
cyclohexane oxidation is assumed to occur on MnaCobOx catalysts
via initial formation of ROOH, which splits into alkoxy (RO) and
alkylperoxy (ROO) free radicals. It should be noted that ROOH can
also be formed by autoxidation of cyclohexane, but the active Me
(Me = Co and Mn) species (*) can activate R-H bonds, which leads to
shorter initial induction periods [42,51,61]. The related sequence of
reactions can be shown in Eqs. (1)–(3). Initially, RH can react with
active Me sites (predominantly Mn4+, Mn3+ and Co3+) to activate the
Acknowledgement
This project was supported financially by the National Natural
Science Foundation of China (21103048, 91545103).
References
[1] L. Gómez-Hortigüela, F. Corà, C.R.A. Catlow, ACS Catal. 1 (2010) 18–28.
[2] A.K. Suresh, M.M. Sharma, T. Sridhar, Ind. Eng. Chem. Res. 39 (2000)
3958–3997.
[3] D.D. Davis, D.R. Kemp, in: J.I. Kroschwitz, M. Howe-Grant (Eds.), Kirk-Othmer
Encyclopedia of Chemical Technology, Wiley, New York, 1991.
C
H bond of RH, followed by O2 insertion to form ROO*H. Finally,

106
M. Wu et al. / Applied Catalysis A: General 523 (2016) 97–106
[4] C.C. Guo, M.F. Chu, Q. Liu, Y. Liu, D.C. Guo, X.Q. Liu, Appl. Catal. A: Gen. 246
[35] G.N. Kustova, E.B. Burgina, G.G. Volkova, T.M. Yurieva, L.M. Plyasova, J. Mol.
Catal. A: Chem. 158 (2000) 293–296.
(2003) 303–309.
[5] C.C. Guo, G. Huang, X.B. Zhang, D.C. Guo, Appl. Catal. A: Gen. 247 (2003)
261–267.
[36] G.N. Pirogova, N.M. Panich, R.I. Korosteleva, Yu V. Voronin, Russ. Chem. Bull.
45 (1996) 2525–2528.
[6] P.R. Makgwane, S.S. Ray, Catal. Commun. 54 (2014) 118–123.
[7] L.P. Zhou, J. Xu, H. Miao, F. Wang, X.Q. Li, Appl. Catal. A: Gen. 292 (2005)
223–228.
[37] X.Y. Wang, K. Qian, D. Li, Appl. Catal. B: Environ. 86 (2009) 166–175.
[38] H.F. Li, G.Z. Lu, Q.G. Dai, Y.Q. Wang, Y. Guo, Y.L. Guo, Appl. Catal. B: Environ.
102 (2011) 475–483.
[8] A. Selvamani, M. Selvaraj, M. Gurulakshmi, R. Ramya, K. Shanthi, J. Nanosci.
Nanotechnol. 14 (2014) 2864–2870.
[9] M. Zabihi, F. Khorasheh, J. Shayegan, RSC Adv. 5 (2015) 5107–5122.
[10] L. Wang, S. Zhao, C. Liu, C. Li, X. Li, H. Li, Y. Wang, C. Ma, Z. Li, J. Zeng, Nano
Lett. 15 (2015) 2875–2880.
[39] Y.G. Zhang, Z.F. Qin, G.F. Wang, H.Q. Zhu, M. Dong, S.N. Li, Z.W. Wu, Z.K. Li, Z.H.
Wu, J. Zhang, T.D. Hu, W.B. Fan, J.G. Wang, Appl. Catal. B: Environ. 129 (2013)
172–181.
[40] D. Schanke, S. Vada, E.A. Blekkan, A.M. Hilmen, A. Hoff, A. Holmen, J. Catal. 156
(1995) 85–95.
[11] H. Yu, F. Peng, J. Tan, X.W. Hu, H.J. Wang, J.A. Yang, W.X. Zheng, Angew. Chem.
Int. Ed. 50 (2011) 3978–3982.
[41] Q. Ye, J.S. Zhao, F.F. Huo, D. Wang, S.Y. Cheng, T.F. Kang, H.X. Dai, Microporous
Mesoporous Mater. 172 (2013) 20–29.
[12] X. Yang, H. Yu, F. Peng, H. Wang, ChemSusChem 5 (2012) 1213–1217.
[13] L. Gomez-Hortigueela, F. Cora, G. Sankar, C.M. Zicovich-Wilson, C.R.A. Catlow,
Chem.-Eur. J. 16 (2010) 13638–13645.
[14] J. Li, Y. Shi, L. Xu, G.Z. Lu, Ind. Eng. Chem. Res. 49 (2010) 5392–5399.
[15] Y. Fu, W.C. Zhan, Y. Guo, Y.Q. Wang, X.H. Liu, Y. Guo, Y.S. Wang, G.Z. Lu,
Microporous Mesoporous Mater. 214 (2015) 101–107.
[16] A. Ramanathan, S.M. Hamdy, R. Parton, T. Maschmeyer, C.J. Jansen, U.
Hanefeld, Appl. Catal. A: Gen. 355 (2009) 78–82.
[17] M.S. Hamdy, A. Ramanathan, T. Maschmeyer, U. Hanefeld, J.C. Jansen,
Chem.-Eur. J. 12 (2006) 1782–1789.
[18] R. Anand, M.S. Hamdy, P. Gkourgkoulas, Th. Maschmeyer, J.C. Jansen, U.
Hanefeld, Catal. Today 117 (2006) 279–283.
[19] W.Z. Zhong, T. Qiao, J. Dai, L.Q. Mao, Q. Xu, G.Q. Zou, X.X. Liu, D.L. Yin, F.P.
Zhao, J. Catal. 330 (2015) 208–221.
[20] D.N. Srivastava, N. Perkas, A. Gedanken, I. Felner, J. Phys. Chem. B 106 (2002)
1878–1883.
[21] B. Sarkar, P. Prajapati, R. Tiwari, R. Tiwari, S. Ghosh, S.S. Acharyya, C. Pendem,
R.K. Singha, L.N.S. Konathala, J. Kumar, T. Sasaki, R. Bal, Green Chem. 14 (2012)
2600–2606.
[22] J.H. Tong, L.L. Bo, Z. Li, Z.Q. Lei, C.G. Xia, J. Mol. Catal. A: Chem. 307 (2009)
58–63.
[42] B. Modén, B.Z. Zhan, J. Dakka, J.G. Santiesteban, E. Iglesia, J. Catal. 239 (2006)
390–401.
[43] T. Garcia, D. Sellick, F. Varela, I. Vázquez, A. Dejkoz, S. Agouram, S.H. Taylor, B.
Solsona, Appl. Catal. A: Gen. 450 (2013) 169–177.
[44] S.C. Kim, W.G. Shim, Appl. Catal. B: Environ. 98 (2010) 180–185.
[45] J. Li, G.Z. Lu, G.S. Wu, D.S. Mao, Y.L. Guo, Y.Q. Wang, Y. Guo, RSC Adv. 3 (2013)
12409–12416.
[46] Y. Tanaka, T. Utaka, R. Kikuchi, T. Takeguchi, K. Sasaki, K. Eguchi, J. Catal. 215
(2003) 271–278.
[47] T. Nissinen, M. Leskelä, M. Gasik, J. Lamminen, Thermochim. Acta 427 (2005)
155–161.
[48] G.Q. Zou, W.Z. Zhong, Q. Xu, J.F. Xiao, C. Liu, Y.Q. Li, L.Q. Mao, S. Kirk, D.L. Yin,
Catal. Commun. 58 (2015) 46–52.
[49] S.A. Chavan, D. Srinivas, P. Ratnasamy, J. Catal. 212 (2002) 39–45.
[50] P. Tian, Z.M. Liu, Z.B. Wu, L. Xu, Y.L. He, Catal. Today 93–95 (2004) 735–742.
[51] M. Conte, X. Liu, D.M. Murphy, K. Whiston, G.J. Hutchings, Phys. Chem. Chem.
Phys. 14 (2012) 16279–16285.
[52] P. Selvam, S.K. Mohapatra, J. Catal. 233 (2005) 276–287.
[53] H.X. Yuan, Q.H. Xia, H.J. Zhan, X.H. Lu, K.X. Su, Appl. Catal. A: Gen. 304 (2006)
178–184.
[54] C.A. Tolman, J.D. Druliner, M.J. Nappa, N. Herron, in: C.L. Hill (Ed.), Activation
and Functionalisation of Alkanes, Wiley, New York, 1989.
[55] R.A. Sheldon, M. Walau, I.W.C.E. Arends, U. Schuchardt, Acc. Chem. Res. 31
(1998) 485–493.
[23] L.P. Zhou, J. Xu, H. Miao, X.Q. Li, F. Wang, Catal. Lett. 99 (2005) 231–234.
[24] R. Zhao, Y.Q. Wang, Y.L. Guo, Y. Guo, X.H. Liu, Z.G. Zhang, Y.S. Wang, W.C.
Zhan, G.Z. Lu, Green Chem. 8 (2006) 459–466.
[25] J. Li, X. Li, Y. Shi, D.S. Mao, G.Z. Lu, Catal. Lett. 137 (2010) 180–189.
[26] W.C. Zhan, G.Z. Lu, Y.L. Guo, Y. Guo, Y.Q. Wang, Y.S. Wang, Z.G. Zhang, X.H. Liu,
J. Rare Earth. 26 (2008) 515–522.
[56] Y. Wen, O.E. Potter, T. Sridhar, Chem. Eng. Sci. 52 (1997) 4593–4605.
[57] U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, R.S. da Cruz, M.C. Guerreiro,
D. Mandelli, E.V. Spinacé, E.L. Pires, Appl. Catal. A: Gen. 211 (2001) 1–17.
[58] I.W.C.E. Arends, R.A. Sheldon, Appl. Catal. A: Gen. 212 (2001) 175–187.
[59] I. Hermans, T.L. Nguyen, P.A. Jacobs, J. Peeters, ChemPhysChem 6 (2005)
637–645.
[60] J.F. Black, J. Am. Chem. Soc. 100 (1978) 527–535.
[61] B. Modén, L. Oliviero, J. Dakka, J.G. Santiesteban, E. Iglesia, J. Phys. Chem. B 108
(2004) 5552–5563.
[27] M.Z. Wu, W.C. Zhan, Y. Guo, Y.S. Wang, Y.L. Guo, X.Q. Gong, L. Wang, G.Z. Lu,
Chin. J. Catal. 37 (2016) 184–192.
[28] S. Todorova, H. Kolev, J.P. Holgado, G. Kadinov, C. Bonev, R. Peren˜íguez, A.
Caballero, Appl. Catal. B: Environ. 94 (2010) 46–54.
[29] G.B. Shul’pin, J. Mol. Catal. A: Chem. 189 (2002) 39–66.
[30] G.C. Bond, Appl. Catal. A: Gen. 149 (1997) 3–25.
[31] J.F. Li, S.L. Xiong, X.W. Li, Y.T. Qian, Nanoscale 5 (2013) 2045–2054.
[32] J.K. Zhu, Q.M. Gao, Microporous Mesoporous Mater. 124 (2009) 144–152.
[33] H.E. Wang, D. Qian, Mater. Chem. Phys. 109 (2008) 399–403.
[34] C.M. Julien, M. Massot, C. Poinsignon, Spectrochim. Acta 60 (2004) 689–700.
[62] X. Liu, M. Conte, M. Sankar, Q. He, D.M. Murphy, D. Morgan, R.L. Jenkins, D.
Knight, K. Whiston, C.J. Kiely, G.J. Hutchings, Appl. Catal. A: Gen. 504 (2015)
373–380.
[63] M. Conte, X. Liu, D.M. Murphy, S.H. Taylor, K. Whiston, G.J. Hutchings, Catal.
Lett. 146 (2016) 126–135.