Effect of additives doping on catalytic properties of Mg 3(VO4)2 catalysts in oxidative dehydrogenation of cyclohexane

Article abstract of DOI:10.1016/j.cattod.2012.09.019

Effects of additives comprising alkali and alkaline earth metals (Li, Na, K and Ca) introduced to Mg₃(VO₄)₂ on their structures, physicochemical properties and the catalytic behaviors in the oxidative dehydrogenation of cyclohexane were investigated. The characterization and the experimental results showed that the additive does not affect markedly the structure of the catalyst, but blocks the active sites and hinders the reducibility of the active species thus decreasing the catalytic activity. Moreover, the additive could enhance the selectivity to cyclohexene by enhancing the nucleophilicity, the redox property as well as the type and number of the oxygen species on the catalyst surface. The doped catalysts could give the catalytic activity and selectivity to cyclohexene in the orders of K- < Na- ≈ Ca- < Li- < non-doped and non-doped < Li- < Na- < K- < Ca-, respectively. Among the doped catalysts, Ca-Mg₃(VO ₄)₂ catalyst demonstrated a yield of cyclohexene of 8.2%, which was a promotion compared with 6.4% of Mg₃(VO₄) ₂.

Full text of DOI:10.1016/j.cattod.2012.09.019

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Effect of additives doping on catalytic properties of Mg3(VO4)2 catalysts in
oxidative dehydrogenation of cyclohexane
M. Jina, P. Lua, G.X. Yua,∗, Z.M. Chengb, L.F. Chenc, J.A. Wangc
a School of Chemistry and Environmental Engineering, Jianghan University, Wuhan, Hubei 430056, PR China
b State Key Laboratory of Chemical Engineering, UNILAB Research Centre of Chemical Reaction Engineering, East China University of Science and Technology, Shanghai 200237, PR
China
c Escuela Superior de Ingeniería Química e Industrias Extractivas (ESIQIE), Instituto Politécnico Nacional, Av. Politécnico S/N, Col. Zacatenco, 07738 México, D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Effects of additives comprising alkali and alkaline earth metals (Li, Na, K and Ca) introduced to Mg3(VO4)2
on their structures, physicochemical properties and the catalytic behaviors in the oxidative dehydrogena-
tion of cyclohexane were investigated. The characterization and the experimental results showed that
the additive does not affect markedly the structure of the catalyst, but blocks the active sites and hinders
the reducibility of the active species thus decreasing the catalytic activity. Moreover, the additive could
enhance the selectivity to cyclohexene by enhancing the nucleophilicity, the redox property as well as the
type and number of the oxygen species on the catalyst surface. The doped catalysts could give the catalytic
activity and selectivity to cyclohexene in the orders of K- < Na- ≈ Ca- < Li- < non-doped and non-doped < Li-
< Na- < K- < Ca-, respectively. Among the doped catalysts, Ca–Mg3(VO4)2 catalyst demonstrated a yield of
cyclohexene of 8.2%, which was a promotion compared with 6.4% of Mg3(VO4)2.
Received 7 February 2012
Received in revised form 21 August 2012
Accepted 1 September 2012
Available online xxx
Keywords:
Cyclohexane
Oxidative dehydrogenation
Alkali/alkaline earth metal
Cyclohexene
© 2012 Published by Elsevier B.V.
Modified catalyst
1. Introduction
the selectivity to the objective alkene which could be explained
by modification of acid–base properties and the reducibility of the
The oxidative dehydrogenation (ODH) of cyclohexane to cyclo-
hexene is a challenging pursuit in both economic and scientific
terms. Previous investigations of the catalysts used in the ODH reac-
tion of cyclohexane demonstrated Mg3(VO4)2 has been considered
as a favorite catalyst for producing a promising yield of cyclohexene
[1–3]. However, a major challenge in the commercial development
of cyclohexane ODH is further improvement in cyclohexene yield
since the main cause of the selectivity limitation arises from the
conversion–selectivity relationship [4,5].
One of the solutions for the catalyst is to ensure easy desorp-
tion of intermediate product cyclohexene from the catalyst surface.
While, the difficulty or the ease of cyclohexene desorption depends
on the properties of the catalyst surface. Therefore, an effective
way for improving the selectivity is to modify the catalyst sur-
face by the introduction of additive, especially the alkali/alkaline
earth metal [6–11]. The introduction of alkali/alkaline earth metal
to V2O5/Al2O3 [6,7], V2O5/TiO2 [8,9], V2O5/SiO2, V2O5/MgO [10,11]
and V Mg O catalysts [12] has been carried out in the ODH of
lower alkanes to produce the corresponding alkenes. Most of the
additives, such as Ca, Li, Na and Cs, played a promoting effect on
active phases by alkalis. While the other additive, such as K, was
found to decrease the selectivity to the objective product since K
was found to inhibit the formation of Mg3(VO4)2 from a mixture
of MgO and NH4VO3 and/or trace K was segregated onto the sur-
face of magnesium vanadates [13–16]. Compared with the ODH of
lower alkanes, Patcas et al. [17,18] only investigated the effect of
alkali metal addition on the catalytic activity in the ODH of cyclo-
hexane. They first modified NiO/Al2O3 catalyst by alkali (Na, K and
Cs) and found no promotion effect owing to a lower active species
content and a larger amount of bulk nickel aluminate leaded by
the modification. Then they modified the egg-shell NiO/Al2O3 cat-
alyst by Li and proposed Li could enhance the catalytic activity
and the selectivity to cyclohexene by decreasing the formation of
bulk nickel aluminates. Besides, it is a pity that few or no stud-
ies have been reported on the influence of alkali/alkaline earth
metal doping on catalytic property of Mg3(VO4)2 in the ODH of
cyclohexane.
In this work, the catalytic performance of the alkali/alkaline
metal (Li, Na, K and Ca) doping on Mg3(VO4)2 was studied in the
ODH of cyclohexane. N2-adsorption, XRD, H2-TPR and XPS tech-
niques were applied to give further information on the structural,
the reducibility of active species as well as the type and distribution
of the surface oxygen species of the catalyst. Furthermore, the rela-
tionship among the reducibility of the active species, the type and
∗
Corresponding author. Tel.: +86 027 84226806.
E-mail address: guoxianyu828@yahoo.com.cn (G.X. Yu).
0920-5861/$ – see front matter © 2012 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.cattod.2012.09.019
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distribution of the surface oxygen species and the catalytic behavior
of the catalysts were investigated in detail.
Ca-Mg3(VO4)2
2. Experimental
K-Mg3(VO4)2
2.1. Catalyst preparation
Mg3(VO4)2 was synthesized via the citrate complexation
method, which has been described in detail in the literature [2,3].
First, a transparent solution of Mg(NO3)2·6H2O and NH4VO3 with
Mg/V atomic ratio of 3/2 was prepared. Then citric acid was added
in the mixing solution with a 20% excess over the number of ionic
equivalents of cations. Subsequently, ammonia was added to adjust
the pH value of 4.8. The solution obtained was evaporated at 343 K
to form a gel, and the gel was dried subsequently in a vacuum
oven overnight at 343 K. Finally, the resulting solid was ground and
heated up to 673 K for 18 h at a constant rising rate of 1 K/min to
decompose the gel precursor, then calcined in the air at 823 K for
6 h. The final sample was crushed and sieved to obtain a particle
size of 20–40 mesh.
Na-Mg3(VO4)2
Li-Mg
Mg
3
(VO
4
)
2
3(VO4)
2
The doped samples were denoted as A-, where A were Li, Na,
K and Ca, respectively. In this work, the atomic ratio A/V of 0.1
was adopted, which was based on the following aspects [19]: (1)
the K/V ratio of 0.1 was shown in previous studies as optimal for
V-based catalysts; (2) the A/V ratio of 0.1 can avoid formation of
10
20
30
40
50
60
70
80
2
mixed A
V O bulk compounds and/or trace A onto the catalyst
Fig. 1. XRD patterns of the catalysts.
surface; (3) the evident modification of the catalytic properties can
be observed at the A/V ratio of 0.1. The doped samples were pre-
pared by incipient impregnation. The additive was introduced by
adding the appropriate amount of the corresponding nitrate solu-
tion onto Mg3(VO4)2 particles. After impregnation, the obtaining
sample was dried at 393 K overnight and calcined in the air at 823 K
for 6 h.
by a HP 6890 gas chromatography equipped with a PEG 20,000
column.
3. Results and discussion
3.1. Textural and structural properties
2.2. Catalyst characterization
Examination of the surface areas of the samples reveals that
the doping treatment produces a modification during the prepa-
ration procedure. The doped catalysts show a lower BET surface
area with respect to the non-doped one (Table 1); meanwhile, the
results show that the surface area decreases along with the ion size
increasing, which could be explained by the formation of O−A+
on the catalyst surface [20,21].
As shown in Fig. 1, no features of crystalline additive vanadates
or additive metal involving material can be observed in the doped
catalysts, which is different from the research of Kung and Kung
[13,14]. The XRD characterization result suggests that the addi-
tive coordinates with the surface active species in modifying its
structure instead of forming bulk compounds.
BET surface area of the as-prepared sample was measured by
N2 physisorption at 77.3 K (ASAP 2010, Micromeritics). The XRD
pattern was measured using Cu K␣ radiation (D/MAX 2550 X-ray
diffractometer) at 40 kV and 40 mA. Actual composition of the sam-
ple was determined on an ICP-AES equipment (Varian 710 ES). XPS
measurements were carried out on a PHI 5300/ESCA system with
Al K␣ as a radiation source (Perkin-Elmer), the correction of the
surface charge electricity effect was using C1s (284.60 eV) and the
range of the spectrum of scanning energy was from 0 to 1200 eV.
XPSPEAK software was used for the deconvolution and analysis the
XPS characterization results.
The temperature-programmed reduction (H2-TPR) was mea-
sured on a Micromeritics AutoChem II 2920 apparatus. Before
reduction, about 0.20 g sample was pre-treated in air at 423 K for
40 min to remove the adsorbed water, followed by cooling to 313 K
under Ar flow (30 ml/min). Subsequently, the sample was reduced
with a 10 vol.% H2/Ar mixture (30 ml/min) by temperature pro-
gramming from 313 to 1273 K at a rate of 10 K/min.
3.2. Characterization results of TPR
As shown in Fig. 2, the temperature of maximum hydrogen con-
sumption (Tmax) is 970 K for Mg3(VO4)2, while it shifts to 979 (Li-),
1002 (Na-), 1054 (K-), and 1023 K (Ca-) for the doped catalysts,
respectively. The shift to higher temperature could be due to the
stronger interaction between the active species and the additive,
which is in agreement with the views of Valenzuela et al. [12], and
Balderas-Tapia et al. [22]. It is obviously that the reducibility of the
active species decreases in the order of Mg3(VO4)2 > Li- > Na- > Ca-
> K-. In addition, based on the previous researches [24,25], Tmax
ranged from 873 to 973 K is responsible for the adsorption of the
incompletion reduction oxygen Oı− (0 < ı < 2), and the value ranged
from 973 to 1173 K is involved in the adsorption of lattice oxygen.
Therefore, the oxygen species on the doped-catalysts surface are
the mixture of the lattice oxygen and Oı− (0 < ı < 2) species.
2.3. Catalytic test
The catalyst activity test was carried out isothermally at atmo-
spheric pressure in a fixed-bed microreactor made of stainless steel
with an inner diameter of 9 mm. About 0.5 g of as-prepared catalyst
diluted with inert quartz sands at a mass ratio of 1:4 was loaded into
the reactor so as to ensure a uniform catalyst distribution. In each
run, the catalyst was first pretreated in an air stream of 50 ml/min
at 773 K for 1 h, and then adjusted to the reaction conditions. The
system was allowed to stabilize for 1.5 h before the first product
sample was taken for analysis. The liquid products were analyzed
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Table 1
Catalytic performance of cyclohexane ODH over the catalysts.
Catalysts
SBET (m2/g)
ICP (mol) Mg/V/A
Temperature (K)
Conversion (%)
Cyclohexane
Selectivity (%)
Cyclohexene
Yield (%)
COx
Cyclohexane
673
723
773
7.8
15.5
21.1
65.7
41.5
27.2
16.2
20.7
28.2
5.12
6.43
5.73
Mg3(VO4)2
Li-
1.5/1/–
17.5
15.5
14.8
14.3
15.2
673
723
773
5.8
11.5
18.0
77.6
61.4
42.1
11.2
17.3
25.3
4.50
7.06
7.57
1.5/1/0.1
1.5/1/0.1
1.5/1/0.1
1.5/1/0.1
673
723
773
4.9
10.8
17.1
79.9
65.9
43.5
10.3
16.9
24.1
3.92
7.12
7.43
Na-
K-
673
723
773
4.2
10.0
15.4
81.8
69.6
49.5
9.9
16.5
23.6
3.43
6.96
7.62
673
723
773
4.7
10.6
16.5
82.1
69.5
49.9
10.2
16.8
23.5
3.86
7.37
8.23
Ca-
Experimental condition: cyclohexane flow rate of 0.107 mol/h, air flow rate of 100 ml/min, atmospheric pressure.
Even though quantification of H2-TPR profiles does not give
precise information about V oxidation states, it can provide some
useful indications. According to the total hydrogen consumption,
the average valences of vanadium species in the samples are prob-
ably +4.69 (K-), +4.71 (Na-), +4.72 (Li-), +4.70 (Ca-) and +4.74
(Mg3(VO4)2), respectively. It is indicated that the type of the addi-
tive has an effect on the V oxidation states, which is in same with
the view proposed by Sloczynski [23].
variations in the vanadium oxidation state, changes of the elec-
tronic density of V O and the type and the distribution of the oxy-
gen species, XPS were recorded shown in Figs. 3 and 4 and Table 2.
Fig. 3 demonstrated XPS spectra of V2p on the catalysts sur-
face. The assignment of bands is a complex problem, as can be
seen from the different results of V4+ and V5+ reported in litera-
ture [27,28]. For V2O4, binding energy values of 515.5, 515.7 and
516.3–516.6 eV have been assigned, and for V2O5, values between
517.1 and 518.0 eV have been assigned. It can be suggested that
bands with a high binding energy value can be assigned to V5+ while
those with a low binding energy value may be attributed to V4+. The
results listed in Table 2 of the peak resolution of V species show that
In addition, no feature of the reduction peaks of crystalline addi-
tive vanadates or additive metal involving material, which confirms
the result of the XRD characterization.
there is a small shift to lower binding energies in V2p 3/2 (Vꢀ2p 3/2
)
3.3. Characterization results of XPS
and V2p 1/2 (Vꢀ2p 1/2) of the doped catalysts with respect to the non-
doped one, which indicates that there should be an increase of the
electron density around vanadium atoms. Considering the limita-
tion in the assignment of the bands, an approximate value of the
atomic ratios of V5+/V4+ can be obtained.
When the additive is an ion with a larger ionic radius than that
of V5+/V4+/Mg2+, it might enter into the interstitial site of the cata-
lyst lattice rather than replace the cations in the bulk crystal [26]. In
order to investigate the composition of the catalyst surface, possible
XPS characterization also gives some information of the oxygen
species on the catalyst surface shown in Fig. 4. Based on the peak
resolution of O1s, oxygen species on the catalyst surface involves
the lattice (O2−) and adsorbed oxygen species (O− and O2−), which
is in a good agreement with the H2-TPR characterization. Mean-
while, the amount of three different oxygen species O2−, O− and
O2− (denoted as OI, OII, and OIII) can be obtained in Table 2 [29,30],
indicating that the lattice oxygen species increases is in the order
of Mg3(VO4)2 < Li- < Na- < K- < Ca-.
Ca-Mg3(VO4)2
K-Mg3(VO4)2
Na-Mg3(VO4)2
3.4. Catalytic performances
It is well known that the reaction is accepted to proceed by the
Mars–van Krevelen reaction mechanism [31], in which adsorbed
cyclohexane reacts with lattice oxygen and the reduced metal oxide
reacts with adsorbed, dissociated oxygen. Besides the main prod-
uct cyclohexene, benzene, a trace of 1,3-cyclohexadiene and the
nonselective oxidation production COx are also observed.
Table 1 presents the catalytic conversions and the selectivity to
cyclohexene over all the samples at various temperatures. Given
a similar surface area of the doped samples, the comparison is
made in terms of the conversion of cyclohexane and the selectiv-
ity to cyclohexene. The doped catalysts shows a lower conversion
of cyclohexane and a higher selectivity to cyclohexene; mean-
while, the conversion of cyclohexane increases and the selectivity
to cyclohexene decreases with elevation of reaction temperature,
Li-Mg3(VO4)2
Mg3(VO4)2
2
7
3
4
7
3
6
7
3
8
7
3
107
3
1273
Temperature/K
Fig. 2. H2-TPR profiles of the catalysts.
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Li-Mg3(VO4)2
Mg3(VO4)2
512
514
516
518
520
522
524
526
528
512
514
516
518
520
522
524
526
528
Binding energy/eV
Binding energy/eV
K-Mg3(VO4)2
Na-Mg3(VO4)2
512
514
516
518
520
522
524
526
528
512
514
516
518
520
522
524
526
528
Binding energy/eV
Binding energy/eV
Ca-Mg3(VO4)2
512
514
516
518
520
522
524
526
528
Binding energy/eV
Fig. 3. XPS spectra supplementary information of V2p on the catalysts.
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Li-Mg3(VO )
Mg (VO )
4 2
3
4 2
526
528
530
532
534
536
526
528
530
532
534
536
Binding energy/eV
Binding energy/eV
Na-Mg3(VO )
K-Mg (VO )
4 2
3
4 2
526
528
530
532
534
536
526
528
530
532
534
536
Binding energy/eV
Binding energy/eV
Ca-Mg3(VO )
4 2
526
528
530
532
534
536
Binding energy/eV
Fig. 4. XPS spectra supplementary information of O1s on the catalysts.
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Table 2
XPS results for the catalysts.
Catalysts
V5+
V4+
V5+/V4+
(mol%)
BE O1s
OI
OI/OII/OIII
(mol%)
BEL V2p 3/2
BEL V2p 1/2
BEL Vꢀ2p 3/2
BEL Vꢀ2p 1/2
OII
OIII
Mg3(VO4)2
Li-
Na-
K-
Ca-
517.9
517.7
517.6
517.6
517.5
525.8
525.6
525.5
525.6
525.3
515.8
515.6
515.8
515.7
515.4
524.9
524.8
524.7
524.7
524.5
2.75
2.58
2.47
2.30
2.39
530.4
530.4
530.4
530.5
530.3
531.8
531.8
531.9
532.4
531.9
532.9
532.8
532.7
533.6
532.8
85.4/8.4/6.2
86.3/7.7/6.0
88.1/9.5/2.4
89.1/8.9/2.0
90.4/7.1/2.5
as a typical behavior for the ODH over oxide catalysts. Among the
samples, K-catalyst shows the lowest conversion of cyclohexane
and Ca-catalyst shows the highest selectivity to cyclohexene and
the highest yield of cyclohexene, as shown in Table 1.
Fig. 5 demonstrates the model of cyclohexane ODH reaction-
showing of the catalyst surface. The introduction of additive would
geometrical block a certain amount of active sites, which leads to
a lower conversion of cyclohexane. Moreover, it is interesting to
find that the effect becomes more pronounced as the ionic radius
of the additives increase from Li to Na, Ca and K, explaining the
differentiation among the catalytic activities in the same order of
K- < Ca- ≈ Na- < Li- < non-doped.
Furthermore, the lower reducibility of the active species and the
lower V oxidation states are essential to a better catalytic perfor-
mance [34,35]. According to the results of H2-TPR characterization,
the additive plays a beneficial effect on cyclohexene selectivity.
However, the selectivity to cyclohexene showed in Table 1 follows
the order of Mg3(VO4)2 < Li- < Na- < K- < Ca-, which is different from
that of the reducibility of the active species. It can be suggested
that the selectivity to cyclohexene does not depend clearly on the
reducibility of the active species.
According to the research of Valenzuela et al. [36] and Sugiyama
et al. [35], when the additive was doped into Mg3(VO4)2, unbal-
anced charges and lattice distortion occur on its surface. From XPS
characterization of the samples, a smallꢀshift to lower binding ener-
gies in V2p 3/2 (Vꢀ2p 3/2) and V2p 1/2 (V 2p 1/2) implies an increase
of the electron density around vanadium atoms, which result in
a easy desorption of cyclohexene from the catalyst surface [37].
Besides, alkali/alkaline earth metal is known to facilitate oxygen
dissociation in metal-based catalyst by transferring an electron to
Due to the high electron density at ␲ bond, cyclohexene is
considered nucleophilicity. If the catalyst surface is of almost the
same property, the selectivity to cyclohexene can be improved.
In the previous studies [32,33], the introduction of the addi-
tive should effect the distribution of electronic cloud densities
around vanadium and oxygen atoms; meanwhile, if the addi-
tive is an ion of lower electronegativity than that of vanadium,
oxygen atoms in the active species become more nucleophilicity
and vanadium less electrophilicity. Accordingly, the alkali/alkaline
metal increase the nucleophilicity of the catalyst surface through
its coordination with surface vanadium species. As a result, com-
pared with the non-doped catalyst, the doped one could lead to
a lower cyclohexane conversion and higher cyclohexene selectiv-
ity, which is in accordance with the experimental results listed in
Table 1.
metal center, which is then back-donated to the 1 ␲ antibond-
g
ing orbital of the oxygen molecule [38]. A similar effect may be
present in metal oxide system, where dissociated oxygen species
may be easily incorporated into the lattice as O2− [7]. Among the
three types of oxygen species [39–41], the lattice oxygen species
O2− are mainly responsible for selective oxidation, whereas the
adsorbed oxygen species O− and O2 are responsible for the acti-
−
vation of cyclohexane and deep oxidation to the formation of COx.
As a result, the less adsorbed oxygen species (O− + O2−) the catalyst
has, the higher selectivity to cyclohexene can be obtained. Accord-
ingly, the selectivity to cyclohexene should be increased as the
amount of lattice oxygen species O2− in the order of Mg3(VO4)2 < Li-
< Na- < K- < Ca-. As a matter of fact, the order of the selectivity to
cyclohexene listed in Table 1 shows a good agreement with that
of the amount of lattice oxygen species of all catalysts. Then, it is
suggested the selectivity to cyclohexene appear to be mostly influ-
enced by the type and number of the oxygen species on the catalyst
surface.
According to the Mars–van Krevelen mechanism, gaseous oxy-
gen participates in the reaction only after adsorption in other parts
of the catalyst and then migrates through the lattice to the active
sites by the reoxidation between V4+ and V5+. Therefore, the coex-
istence of V4+ and V5+ on the surface is necessary for the ODH, as
shown in Table 2. Among the vanadium species, V5+ O in an octa-
hedral structure is responsible for providing the center of oxidation
for the adsorbed alkyl species, which contributes to a high conver-
sion of cyclohexane and a high selectivity to COx, whereas V4+
O
in a tetrahedral structure is responsible for activating cyclohexane
and adsorbing oxygen, which attributes a high selectivity to cyclo-
hexene. As a result, a lower atomic ratio of V5+/V4+ on the catalyst
surface suggests a lower conversion of cyclohexane and a higher
selectivity to cyclohexene, as shown in Tables 1 and 2.
In addition, the results also reveal that the contributions of the
lattice oxygen in the catalysts to the reaction should be strongly
influenced by the introduction of the additive, especially Ca. Due to
the different valence with the alkali metal, the incorporation of Ca2+
into Mg3(VO4)2 not only affects the abstraction/the incorporation
Fig. 5. Model of the ODH of cyclohexane reaction showing of the catalysts.
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of oxygen species from and into the lattice vacancy but also influ-
ences the stabilization of the catalyst structure during the redox
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The addition of alkali/alkaline earth metal to Mg3(VO4)2 affects
the physical, chemical nature and the catalytic behavior in the
ODH of cyclohexane to cyclohexene. During the coordination
of the additive with the surface active species, the additive
blocked the active sites and hindered the reducibility of the
active species thus decreasing the catalytic activity. Moreover, the
additive could improve the selectivity to cyclohexene by enhanc-
ing the nucleophilicity, the redox property as well as the type
and number of the oxygen species as investigated by H2-TPR
and XPS characterization. However, the selectivity to cyclohexene
did not depend clearly on the reducibility of the active species,
but appears to be strongly influenced by the acid-basic prop-
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Among the doped catalysts, Ca-catalyst could give a relatively
higher yield of cyclohexene of 8.2% compared with the non-
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Acknowledgements
The present work was conducted under support from the Doc-
toral Scientific Research Fund of Jianghan University (No. 2010018)
and the Project of Education Department of Hubei Province
(Q20123402).
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Please cite this article in press as: M. Jin, et al., Effect of additives doping on catalytic properties of Mg3(VO4)2 catalysts in oxidative dehydro-
genation of cyclohexane, Catal. Today (2012), http://dx.doi.org/10.1016/j.cattod.2012.09.019