A Bimetallic Pure Inorganic Framework for Highly Efficient and Selective Photocatalytic Oxidation of Cyclohexene to 2-Cyclohexen-1-ol

Article abstract of DOI:10.1007/s10562-019-02847-y

Abstract: The highly efficient and selective photocatalytic oxidation of cyclohexene with molecular oxygen under mild conditions is an important objective in chemical synthesis. In this work, a pure inorganic framework CoMo was self-assembly prepared under solvothermal conditions by incorporating simple MoO₄^2?, cobalt (II) ion. The catalyst CoMo was well characterized by infrared spectroscopy (FTIR), nitrogen adsorption–desorption, powder X-ray diffraction (XRD), scanning electron spectroscopy (SEM), and X-ray photoelectron spectroscopy (XPS) methods. It displayed high efficiency and selectivity in the photocatalytic oxidation of cyclohexene to 2-cyclohexen-1-ol in O₂ atmosphere. The influence of solvents, oxidants, pressure of oxygen, reaction temperature, light source and time on the reaction was investigated. More interestingly, the selectivity of the reaction in 4-ethyltoluene was much higher than that in other solvents. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02847-y

Catalysis Letters
https://doi.org/10.1007/s10562-019-02847-y
A Bimetallic Pure Inorganic Framework for Highly Eꢀcient
and Selective Photocatalytic Oxidation of Cyclohexene
to 2‑Cyclohexen‑1‑ol
1
1
1
1
1
1
Xiaoyun Shi · Zhuolin Shi · Guiqin Niu · Chen Si · Qiuxia Han · Jingjing Zhang
Received: 15 March 2019 / Accepted: 20 May 2019
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
The highly efcient and selective photocatalytic oxidation oꢀ cyclohexene with molecular oxygen under mild conditions is
an important objective in chemical synthesis. In this work, a pure inorganic ꢀramework CoMo was selꢀ-assembly prepared
2
−
under solvothermal conditions by incorporating simple MoO , cobalt (II) ion. The catalyst CoMo was well characterized
4
by inꢀrared spectroscopy (FTIR), nitrogen adsorption–desorption, powder X-ray diꢁraction (XRD), scanning electron spec-
troscopy (SEM), and X-ray photoelectron spectroscopy (XPS) methods. It displayed high efciency and selectivity in the
photocatalytic oxidation oꢀ cyclohexene to 2-cyclohexen-1-ol in O atmosphere. The inꢂuence oꢀ solvents, oxidants, pressure
2
oꢀ oxygen, reaction temperature, light source and time on the reaction was investigated. More interestingly, the selectivity
oꢀ the reaction in 4-ethyltoluene was much higher than that in other solvents.
Graphic Abstract
A pure inorganic ꢀramework CoMo with open mesoporous channels displayed high efciency and selectivity ꢀor photocatalytic oxidation oꢀ
cyclohexene with molecular oxygen under mild conditions.
Keywords Inorganic ꢀramework · Cyclohexene · Photocatalysis · Selective oxidation
1
Introduction
Xiaoyun Shi and Zhuolin Shi have contributed equally to this
work.
Allylic oxidation oꢀ cyclohexene is a powerꢀul method ꢀor
the synthesis oꢀ oxygen-containing oꢀ α, β-unsaturated
alcohols and ketones, which are intermediates in the
manuꢀacture oꢀ valuable chemicals and pharmaceuticals
Electronic supplementary material The online version oꢀ this
article (https://doi.org/10.1007/s10562-019-02847-y) contains
supplementary material, which is available to authorized users.
[
1–3]. Traditional allylic oxidation reactions have been
Extended author inꢀormation available on the last page oꢀ the article
Vol.:(0
1
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3

X. Shi et al.
extensively developed by utilizing special oxidants such
perꢀormed preꢀerable activity ꢀor cyclohexene oxidation
than Ni-MOF-74 [23].
as PhIO, NaClO, H O , t-BuOOH, but which suꢁered
2
2
ꢀ
rom many disadvantages such as corrosiveness, toxicity,
Inspired by the previous works, we present the design and
preparation oꢀ a heterogeneous catalyst CoMo by incorpo-
rating simple Na MoO , cobalt(II) ions in the mixed solu-
serious pollution problems and stringent reaction condi-
tions [4–6]. Among various oxidants, molecular oxygen
2
4
(
O ) is an ideal oxidant because it is ubiquitous and envi-
tion oꢀ H O and CH CN under hydrothermal conditions
2
2 3
ronmentally totally benign [7–9]. Recently, a number oꢀ
compounds containing transition metals (e.g., Co, Fe, Cu,
Pd, etc.) are regarded as active catalysts ꢀor oxidation oꢀ
cyclohexene and more methods have been developed ꢀor
allylic oxidations by molecular oxygen under mild condi-
tions [10–20]. However, the lower efciency and inꢀerior
selectivity are the potential bottlenecks in the catalytic
(Scheme 1b). We anticipated that introduce oꢀ an oxidation
catalyst Mo(VI) oxide and the cobalt(II) cores ꢀor activation
oꢀ O molecules into a ꢀramework should be a considerable
2
method to achieve the transꢀormation oꢀ selective allylic
oxidation oꢀ cyclohexene. In addition, the whole catalytic
process under light-driven with molecular oxygen as oxidant
contributes a possibility to realize innovative and economi-
cally advantageous processes ꢀor allylic oxidation oꢀ alkenes.
alkenes oxidation by O ꢀor cyclohexene owning the mul-
2
tiple oxidation active sites such as C=C bond and allylic
α-H atoms (Scheme 1a). Thereꢀore, it is a big challenge to
design and acquisition oꢀ an eꢁective catalyst ꢀor highly
2 Experimental
efcient and selective oꢀ cyclohexene using O as a green
2
oxidant under mild conditions. Photocatalysis is a power-
2.1 Materials and Methods
ꢀ
ul strategy ꢀor oxidation reaction oꢀ organic, which has
an inherent advantage in environmental protection, energy
conservation and high efciency by utilizing environmen-
All chemicals oꢀ reagent grade quality in this article were
obtained ꢀrom commercial sources and used without ꢀur-
ther puriꢃcation. ICP (Inductively Coupled Plasma) analy-
ses were tested on a JarrelAshJ-A1100 spectrometer. The
powder X-ray diꢁractograms (PXRD) were measured on a
Riguku D/Max-2400 diꢁractometer with range oꢀ 5°–30°
(2θ). IR spectrums were collected ꢀrom the powder sam-
ple with KBr pellets as base on a JASCO FT/IR-430 spec-
trometer. The thermogravimetric analysis was obtained on
a Mettler-Toledo TGA/SDTA 851e instrument heated ꢀrom
tally oxygen as oxidant and solar energy source to drive
1
the redox reaction. Singlet oxygen ( O ) and superoxide
2
·
−
radicals (O ) have displayed excellent catalytic perꢀor-
2
mance in many oxidation reactions, which can be produced
by heterogeneous photocatalysts through photogenerated
electrons transꢀerring to O . Transition-metal cobalt cata-
2
lysts demonstrated great potential ꢀor next-generation
renewable photocatalytic oxidation applications. Su et al.
successꢀully synthesized a unique nano-material rGO@
ZnCo O with a suitable valence band structure and
−
1
25 to 800 °C with a heating rate oꢀ 10 °C min , under a
dynamic nitrogen atmosphere. The diꢀꢀuse reꢀlectance
UV–Vis absorption spectra were perꢀormed on a HITI-
ACHI UH4150 spectrophotometer (Japan). NMR spectra
were recorded on a Varian INOVA-400 MHz instrument ꢀor
2
4
enhanced perꢀormance in photocatalytic NO oxidation
21]. Lei et al. reported an oxidative Diels–Alder reaction
[
oꢀ styrene derivatives with electron-rich dienophiles under
oxidant-ꢀree conditions by using a synergistic merger oꢀ
photoredox and cobaloxime catalyst [22]. Zhang et al.
designed a mixed-metal catalyst Co/Ni-MOF-74 by sub-
1
H. X-ray photoelectron spectroscopy (XPS) analyses was
perꢀormed by using an ESCALAB 250 XI (America) spec-
trometer. The morphology oꢀ sample was characterized by
using a ꢃeld-emission scanning electron microscope (SEM)
oꢀ ZEISS MERLIN type. The BET analysis was recorded
2
+
2+
stituted partial Ni ions to Co ions in Ni-MOF-74 and
Scheme 1 a Active sites and
oxidation products oꢀ cyclohex-
ene; b the synthesis oꢀ CoMo
catalyst and selective allylic
oxidation oꢀ cyclohexene cata-
lytic process
1
3

A Bimetallic Pure Inorganic Framework for Highly Eꢀcient and Selective Photocatalytic…
by using N as adsorbent on an ASAP 2460 Micromeritics
2.4.2 Thermal Catalysis Oxidation of Cyclohexene at Higher
2
instrument at 77 K.
Pressure
The oxidation oꢀ cyclohexene at higher pressure was car-
ried out in a stainless steel autoclave (25 mL) with the cata-
lyst (0.01 mmol) and solvent (3 mL) were added, then the
substrate cyclohexene (10 mmol, 15 mg) was added to the
mixture solution. Subsequently, the autoclave was sealed
2
.2 Preparation of CoMo
CoMo was prepared under hydrothermal conditions by
the selꢀ-assembly approach: Co(NO ) ·6H O (270 mg,
3
2
2
1
.13 mmol) and Na MoO ·2H O (340 mg, 1.39 mmol) were
and preꢃlled with 0.5 MPa oꢀ O , placed in an oil bath with
2
4
2
2
dissolved in 6.0 mL mixed solution oꢀ water and acetonitrile
desired temperature and reaction time. At the end oꢀ reac-
tion, aꢀter the autoclave cooled to room temperature the
excess gases were vented. The remaining mixture was ꢃl-
tered and evaporated under reduced pressure to obtain the
with the ratio oꢀ 4:2, the resultant mixture was stirred over
−
1
1
0 h. Then using a dilute HCl solution (0.5 mol L ) and
adjusted the above mixture to 3 oꢀ pH value, then sealed
in a 25 mL Teꢂon-lined autoclave, and heated at 150 °C
1
product. The conversion and selectivity were obtained by H
ꢀor 3 days. Aꢀter the autoclave cooled to room temperature,
NMR analysis oꢀ the ꢃnal solution aꢀter reaction.
purple single crystals were ꢃltrated and washed with water,
then dried naturally at room temperature. The pure crystals
were obtained with a high yield oꢀ 73%, based Mo element.
ICP (%). Anal. Calcd. For CoMoNa HO : Na, 4.65; Co,
3 Results and Discussion
0
.5
5
2
3.82; Mo, 38.78. Found: Na, 4.63; Co, 23.83; Mo, 38.80.
3.1 Crystal Structure of Complex CoMo
The single crystal diꢁraction analysis reveals that CoMo
crystallizes in the monoclinic space group C2/m, an asym-
metric unit oꢀ CoMo consists oꢀ a sodium atom, a tetrahe-
2
.3 Photocatalytic Oxidation of Cyclohexene
The photocatalytic reaction was carried out in a Pyrex ves-
sel, in which the substrate cyclohexene (10 mmol), catalyst
dral {MoO } group, and two crystallographically uniꢀorm
4
cobalt ions (Fig. S1). Among them, each cobalt ion adopts
octahedral coordination geometry by six oxygen atoms, and
cobalt ions ꢀorm a one-dimensional chain through sharing
two bridging oxygen atoms in their equatorial planes (Fig.
S2). The two exposed bridge oxygen atoms are protonated as
calculated by valence-bond method. The molybdenum ions
as linkers constitute two-dimensional plane with cobalt ions
and bridging oxygen atoms, halꢀ oꢀ them link with equato-
rial oxygen atoms oꢀ cobalt octahedron and the others stand
on the polar sites oꢀ cobalt octahedron by the oxygen oꢀ
(
0.01 mmol, 15 mg) and solvent (3 mL) were added. Then
it was ꢃlled with 0.1 MPa O with a balloon at room tem-
2
perature. Aꢀterwards, the Pyrex vessel was irradiated under
4
20–500 nm blue or white light ꢀor luminous power oꢀ 10 W.
Aꢀter reaction to desired time, the ꢃnal mixture was ꢃltered
and evaporated under reduced pressure to obtain the product.
1
The conversion and selectivity were obtained by H NMR
analysis oꢀ the ꢃnal solution aꢀter reaction.
{
MoO } groups (Fig. 1b). The layers oꢀ two-dimensional
4
2
+
6+
2.4 Thermal Catalysis Oxidation of Cyclohexene
planes constituted by Co and Mo ions are linked by
sodium ions to ꢀorm a three-dimensional ꢀramework with
two kinds oꢀ pores, named A and B (Fig. 1c, d). The pore
A oꢀ has an approximate dimension oꢀ ca. 3.956 × 4.686
Å, and an approximate dimension oꢀ 3.955× 4.073 Å ꢀor
pore B (Fig. S3). Both pores ꢀormed catalysis channels are
2.4.1 Thermal Catalysis Oxidation of Cyclohexene
at Normal Pressure
The oxidation oꢀ cyclohexene at normal pressure was car-
ried out in a round bottom ꢂask (25 mL) with the catalyst
2
+
6+
adequately capacious allowed Co and Mo sites ꢀavoring
contact with cyclohexene and oxygen substrates. Moreover,
the bare cobalt sites have the ability to ꢃx and activate oxy-
gen and ꢀorm peroxo or superoxo species, which ꢀurther pro-
duce oxygen radicals to make the oxidation oꢀ labile allylic
hydrogen atoms in cyclohexene more smoothly. The PXRD
oꢀ CoMo displays good correspondence between the simu-
lated and experimental patterns, which suggests the high
purity oꢀ the samples (Fig. S4). TGA revealed the CoMo
ꢀramework is stable up to ~ 350 °C, which exhibits this
catalyst has high thermal stability. Above characteristics oꢀ
(
0.01 mmol) and solvent (3 mL) were added, then the sub-
strate cyclohexene (10 mmol, 15 mg) was added to the
mixture solution. Subsequently, the round bottom ꢂask was
sealed with a balloon which was preꢃlled with 1 atm oꢀ O .
2
Then, the round bottom ꢂask was placed in an oil bath with
desired temperature and reaction time. At the end oꢀ the
reaction, the ꢃnal mixture was ꢃltered and evaporated under
reduced pressure to obtain the product. The conversion and
1
selectivity were obtained by H NMR analysis oꢀ the ꢃnal
solution aꢀter reaction.
1
3

X. Shi et al.
3.2 Characterizations of the CoMo Catalyst
3
.2.1 Morphology Characterization
The morphology oꢀ CoMo sample was characterized by a
scanning electron microscope (SEM). According to Fig. 2,
CoMo showed irregularly shaped blocks with scale lay-
ers. Moreover, the energy dispersive spectroscopy (EDS)
presented the speciꢃc distribution oꢀ individual elements
in same block oꢀ sample (Fig. 3a–g). The diꢁerent intensi-
ties oꢀ colors were assumed to be proportional to the metal
composition in the same block oꢀ sample [24], Co (green),
Mo (red), Na (sky blue) and O (blue) collected by elemen-
tal mapping, which indicated both cobalt and molybdenum
elements were homogeneously distributed in the ꢀramework
oꢀ CoMo.
Fig. 1 a The coordination environment oꢀ three metal ions oꢀ CoMo.
b The 2D sheet oꢀ CoMo showed connection and evenly distrib-
uted oꢀ Co, Mo and Na atoms. c The 3D network ꢀrame oꢀ CoMo
showed two kinds oꢀ pores in the 3D network oꢀ CoMo along the
b-axis. d Two channels interspersed simulation in the inorganic ꢀrame
oꢀ CoMo looking along the a-axis. (Symmetry operation code: (A),
3.2.2 XPS Spectra and BET Analysis
X-ray photoelectron spectroscopy was used to reveal the
valence state oꢀ the molybdenum and cobalt ions (Fig. 4a–c).
As shown in Fig. 4b, the spectra oꢀ Mo element show the
two spin orbital components oꢀ 3d and 3d and two peaks
1
.5−x, 0.5+y, 1−z; (B), −x, y, −z; (C), 1−x, y, 1−z; (D), 0.5+x,
.5+y, z; (E), 1.5−x, 1.5−y, 1−z; (F), x, 1−y, z; (G), −0.5+x,
0
−
−
0.5+y, z; (H), 0.5−x, −0.5+y, z; (I), 0.5−x, 1.5−y, −z; (J),
0.5+x, 1.5−y, z; Color code: Co plum spheres, Mo teal spheres,
5/2
3/2
with binding energies oꢀ 232.2 and 235.4 eV were assigned
Na gray spheres, O red spheres; all hydrogen atoms are omitted ꢀor
6
+
to electrons oꢀ the Mo ions [25–28]. The spectra oꢀ Co
element show the two spin orbital components oꢀ 2p and
clarity)
3
/2
2
p
and two peaks with binding energies oꢀ 782.6 and
1
/2
3
+
CoMo ꢀramework meet most oꢀ the requirements ꢀor hetero-
geneous catalysis (Fig. S5). The CCDC reꢀerence number ꢀor
CoMo is 1841574.
798.0 eV belong to the Co ions (Fig. 4c). Two peaks with
binding energies oꢀ 780.6 and 796.6 eV were attributed to
2
+
2p and 2p electrons oꢀ the Co ions. In addition, the
3
/2
1/2
Fig. 2 SEM images displaying
the morphology and structure
oꢀ CoMo
1
3

A Bimetallic Pure Inorganic Framework for Highly Eꢀcient and Selective Photocatalytic…
Fig. 3 Corresponding elemental mappings with Co (KA), Mo (LA), Na (K) and O (K) oꢀ CoMo
peaks at 786.1 and 802.2 eV are attributed to the Co3 satel-
+
To examine the ability oꢀ CoMo ꢀor O activation, TMB
2
lites and the peaks at 787.9 and 803.8 eV are attributed to
(TMB = 3,3′,5,5′-Tetramethylbenzidine) was employed as
2
+
the Co satellites [29–33]. The porosity oꢀ CoMo samples
can be displayed by nitrogen adsorption–desorption iso-
therms (Fig. 4d). The nitrogen physisorption analyses indi-
cate the presence oꢀ a pretty mesoporous structure oꢀ this
the model compound to undergo oxidation over CoMo [38].
The colorless TMB was mainly changed to blue with charac-
teristic UV–vis peaks at 370 and 652 nm (partially oxidized
product I, blue), which demonstrated the moderate oxidation
power oꢀ CoMo (Fig. 5b). The mechanism oꢀ oxidation oꢀ
3,3′,5,5′-tetramethylbenzidine (TMB) and the color change
process are displayed in Fig. S6 [38].
ꢀ
ramework with an average pore diameter oꢀ 31.4372 nm,
2
−1
and the approximate 20.50 m g oꢀ BET surꢀace area and
3
−1
0
.165 cm g oꢀ BJH pore volume [34].
3.2.3 UV–Vis Spectrum of CoMo Catalyst
3
.3 Selective Oxidation of Cyclohexene by CoMo
The diꢁuse reꢂectance UV–vis spectrum oꢀ the prepared
catalyst was perꢀormed to obtain the valence and coordina-
tion environment oꢀ metal ions in CoMo catalyst (Fig. 5a).
The spectrum shows two major broad bands located in the
Catalyst
3.3.1 Photocatalytic Oxidation of Cyclohexene
2
50–350 nm and 435–750 nm range. The characteristic
The photocatalytic activity oꢀ CoMo was studied ꢀor using
peaks at 247 and 516 nm are attributed to the tetrahedral
O as the oxidant under blue or white light (10 W LED lamp)
2
2
−
MoO
groups and the d–d electronic transitions oꢀ the
irradiation at room temperature. The oxidation products are
4
1
octahedral coordination oꢀ Co(II), respectively [35–37].
analyzed according to H NMR. The detailed conversion oꢀ
1
3

X. Shi et al.
Fig. 4 a The survey scan XPS
oꢀ the tested CoMo sample. b,
c The XPS spectra oꢀ Mo 3d
and Co 2p oꢀ CoMo sample. d
N adsorption–desorption iso-
2
therms and pore size oꢀ CoMo
ꢀ
ramework
Fig. 5 a The UV–vis diꢁuse
reꢂectance spectroscopy oꢀ
CoMo catalyst. b UV–vis
absorption spectra oꢀ the TMB
in buꢁer solution oꢀ NaAc-HAc
cyclohexene and selectivity ꢀor the oxidation products are
shown in Table 1.
bond oxidation. The intermediate has been conꢃrmed by
the existence oꢀ 1-(4-methylphenyl)ethanol in the mixture
oꢀ product aꢀter the oxidation reaction oꢀ cyclohexene (Fig.
Initially, the inꢂuence oꢀ solvents on the photocatalytic
oxidation was examined. The photocatalytic reaction exhib-
ited poor activity in methylene chloride and acetonitrile,
whereas it could proceed efciently in 4-ethyltoluene. Nota-
bly, among three kinds oꢀ solvents, the oxygenated products
oꢀ cyclohexene oxidation have a considerable discrepancy.
Among them, 4-ethyltoluene showed the best photocatalytic
activity, providing about 78.95% oꢀ cyclohexane conver-
sion with a highest selectivity ꢀor the oxygenated product oꢀ
S7). In addition, Co(II) has the capable oꢀ activating O , pro-
2
moting the solvent oꢀ 4-ethyltoluene transꢀormed to oxygen
2
−
species such as O or OOH [39–42].
For methylene chloride, cyclohexene conver-
sion decreased up to 37.50% and epoxidation product
1,2-cyclohexanediol selectivity increased close to 70.83%,
but allylic oxidation selectivity oꢀ product 2-cyclohexen-
1-one is only 16.1% (Table 1, Entry 3). In comparison
with methylene chloride, acetonitrile exhibited a higher
2-cyclohexen-1-one selectivity to 58.93%, and gave a con-
siderable amount oꢀ cyclohexene oxide to 46.91%, but with a
lower cyclohexane conversion about 28.83% (Table 1, Entry
5). From the photo-oxidation results oꢀ 2-cyclohexen-1-one
2
-cyclohexen-1-ol (>95%) aꢀter 12 h oꢀ blue light irradia-
tion (Table 1, Entry 1). The higher conversion in the case oꢀ
the 4-ethyltoluene may be due to the 4-ethyltoluene trans-
ꢀ
ormed a key oxygen intermediate 4-ethyltoluene hydrop-
eroxide which ultimately dictates the selectivity in α-C–H
1
3

A Bimetallic Pure Inorganic Framework for Highly Eꢀcient and Selective Photocatalytic…
Table 1 Photocatalytic oxidation oꢀ cyclohexene catalyzed by CoMo bimetallic catalyst
Condition
Solvent
Alkene oxidation selectivity (%)a
Entry
Conv. (%)a
Light
Time (h)
1
2
3
4
5
6
4-ethyltoluene
4-ethyltoluene
Blue
White
Blue
White
Blue
White
12
12
12
12
12
12
78.95
64.50
37.50
36.63
28.83
22.50
>95
>95
–
–
–
–
–
–
–
–
–
–
–
–
CH Cl2
29.17
58.93
53.09
65.52
70.83
41.07
–
2
CH Cl2
2
CH CN
46.91
34.48
3
CH CN
–
3
Reaction conditions: cyclohexene, 5 mmol; CoMo catalyst, 0.01 mmol; 1 atm O ; LED irradiation, at room temperature; solvent, 3 ml
2
a
1
The conversion and selectivity were determined by H NMR spectroscopy oꢀ the crude products
and epoxidation products catalyzed under the present con-
ditions, the CoMo oxidant photocatalytic in the solves oꢀ
methylene chloride and acetonitrile reꢂected that the syn-
potential catalyst oꢀ oxidation under visible light irradiation
ꢀor photocatalytic applications.
To examine heterogeneity oꢀ catalysis system, CoMo was
removed ꢀrom photocatalytic oxidation aꢀter 6 h, then keep
stirring and reacting ꢀor another 6 h, the residual solution
gave nearly no additional conversion aꢀter ꢃltered out cata-
lyst. The recyclability test was perꢀormed at room tempera-
ture with 1 atm oxygen in the heterogeneous allylic oxidation
catalysis oꢀ cyclohexene with CoMo catalyst. The results
demonstrated this catalyst could be reused at least ꢀour times
with conversions ꢀrom 79.95 to 76.50 and remained a highly
selectivity (Fig. 6a). At the end oꢀ reaction, ICP analysis
revealed almost negligible detection oꢀ Co or Mo ions in
the ꢃltrate. The PXRD patterns oꢀ the initial and recovered
samples were exactly similar, which indicated that CoMo
catalyst keeping the original structure aꢀter several times oꢀ
reaction (Fig. 6b). These results above suggested that CoMo
2
+
6+
ergy between Co and Mo sites were achieved, oxidation
oꢀ labile allylic hydrogen atoms in cyclohexene and epoxi-
dation can be taken place simultaneously [43]. However,
both epoxidation and allylic oxidation oꢀ cyclohexene in the
solves oꢀ methylene chloride and acetonitrile were relatively
lower in conversion or in selectivity.
Then, the inꢂuence oꢀ diꢁerent light sources on the pho-
tocatalytic oxidation was examined under the same reaction
conditions, showed in Table 1 (Entry 1–6). Unlike the inꢂu-
ence oꢀ solvents, the results demonstrated that light irra-
diation cannot change the kinds oꢀ products oꢀ cyclohexene
oxidation, but blue light irradiation with a higher energy has
a higher cyclohexane conversion than the white light irra-
diation. These results provide a ꢀacile way using CoMo as a
Fig. 6 a Recyclability tests
oꢀ catalyst in allylic oxidation
catalysis oꢀ cyclohexene. b
PXRD patterns oꢀ the ꢀresh and
recovered CoMo catalyst under
reaction condition aꢀter ꢀour
cycles catalysis
1
3

X. Shi et al.
has good stability and recyclability ꢀor heterogeneous cataly-
sis process.
oꢀ oxygen intermediate 4-ethyltoluene hydroperoxide in the
whole allylic oxidation.
Moreover, in order to make sure the reaction can proceed
smoothly under mild conditions, the eꢁects oꢀ temperature
and pressure oꢀ molecular oxygen were investigated con-
currently (Table 2, Entries 1 and 5). Speciꢃc operations at
high pressure and temperature, as well as, the operations
oꢀ atmospheric pressure with mild temperature have been
described in the part oꢀ 2.4. With the increase oꢀ oxygen
pressure and temperature the conversion oꢀ cyclohexene
increased, but the selectivity to oxidation products was
also dissatisꢀactory. According to the above results, it’s
good ꢀor promoting the conversion oꢀ oxidation reaction
whether using strong oxidants or increasing temperature
and pressure, but there’s no denying that the selectivity is
poor and undesirable during thermal catalysis oxidation oꢀ
cyclohexene.
3.3.2 Thermal Catalysis Oxidation of Cyclohexene
Direct heating oxidation catalysis oꢀ cyclohexene was inves-
tigated under various solvents, temperatures, oxidants and
pressures oꢀ molecular oxygen. As shown in Table 2, it’s
ꢀ
ound that the quite considerable conversion on the whole
oꢀ oxidation reaction, but the selectivity is very inꢀerior. The
oxidation oꢀ cyclohexene was ꢃrstly conducted with CoMo
catalyst in 4-ethyltoluene using diꢁerent oxidants at 50 °C
ꢀ
or 12 h. As expected, when using O as oxidant the reaction
2
has a 69.83% cyclohexene conversion and with a selectivity
oꢀ 38.01% in epoxidation products and 61.99% oꢀ allylic
oxidation products (Table 2, Entry 1). For oxidation reaction
with oxidant oꢀ TBHP, cyclohexene conversion increased to
8
8.41%, but the selectivity to epoxidation or allylic oxidation
In general, through comparing all the above experimental
results, thermocatalytic process have considerable conver-
sion oꢀ oxidation, but the selectivity oꢀ a single product in
oxidation process under heating become more difcult to
control. Whereas, photocatalytic reaction displayed unex-
ceptionable selectivity ꢀor oxidation oꢀ cyclohexene. The
desired reaction conditions ꢀor the selective oxidation oꢀ
condition by CoMo catalyst were ꢀound that under blue
was still low (Table 2, Entry 2). We ꢀurther investigated the
inꢂuence oꢀ solvents in the catalytic reaction. The catalytic
reactions were carried out under the same conditions with
solvent oꢀ 4-ethyltoluene or acetonitrile. For oxidation reac-
tion processed in solvent oꢀ acetonitrile, the conversion oꢀ
cyclooctene reached 78% (Table 2, Entry 3), that is higher
than that in solvent oꢀ 4-ethyltoluene (69.83%, Entry 1).
However, the selectivity to epoxidation and allylic oxida-
tion were 40.06% and 59.94%, respectively. It is difcult
to separate aꢀter the end oꢀ reaction processed in solvent
oꢀ acetonitrile. It is worth noting that the allylic oxidation
product oꢀ 2-cyclohexen-1-ol was only existed in the solvent
oꢀ 4-ethyltoluene, which proved the importance oꢀ existence
light irradiation, O air as an oxidant at room temperature
2
with a conversion efciency oꢀ 78.95% and a selectivity ꢀor
2-cyclohexen-1-ol oꢀ about 95%.
The possible mechanism oꢀ the allylic oxidation oꢀ
cyclohexene catalytic process was showed in Scheme 2.
Initially, the hydrogen atoms abstracted ꢀrom the
Table 2 Thermal catalysis oxidation oꢀ cyclohexene catalyzed by CoMo bimetallic catalyst
Condition
Oxidant
Alkene oxidation selectivity (%)a
Entry
Conv. (%)a
Solvent
Temp. (°C)
Pressure
Time (h)
1
2
3
4
5
O2
TBHP
O2
O2
O2
4-ethyltoluene
4-ethyltoluene
50
50
50
120
120
1 atm
–
1 atm
0.5 MPa
0.5 MPa
12
12
12
6
69.83
88.41
78.00
80.43
86.53
17.93
16.84
–
44.06
39.91
59.94
50.49
20.60
38.01
43.25
40.06
34.67
57.89
–
–
–
CH CN
3
CH CN
–
14.84
–
3
4-ethyltoluene
6
21.52
Reaction conditions: cyclohexene, 5 mmol; CoMo catalyst, 0.01 mmol; O , t-butylhydroperoxide (TBHP, 70% in decane); solvent, 3 ml
2
a
1
The conversion and selectivity were determined by H NMR spectroscopy oꢀ the crude products
1
3

A Bimetallic Pure Inorganic Framework for Highly Eꢀcient and Selective Photocatalytic…
Scheme 2 Possible mechanism oꢀ the allylic oxidation oꢀ cyclohexene catalytic process
methylene C–H bond oꢀ 4-ethyltoluene by molecular oxy-
gen, ꢀormed ·OOH radicals and the 4-ethyltoluene radicals.
4 Conclusions
2
+
Then, a lower valence state Co in the Mo–O–Co unit
In summary, we describe a protocol ꢀor the high activ-
ity and selectivity oxidation oꢀ allylic hydrogen atoms oꢀ
cyclohexene through combination oꢀ cobalt(II)-ꢀacilitated
oꢀ CoMo (A) is activated by ·OOH radicals and ꢀorming
3
+
the Co –OOH (B), then B took oꢁ the hydrogen atom
3
+
and generated Co –O intermediate(C), which in turn
the O activation and molybdenum (VI) oxidation catalysis
2
2
abstracted the hydrogen atoms ꢀrom the methylene C-H
bond oꢀ 4-ethyltoluene and resulted the 4-ethyltoluene rad-
icals again [42, 44]. The resulted the 4-ethyltoluene radi-
cals is rapidly combined with molecular oxygen to gener-
by light-driven. The Co(II) sites in CoMo ꢀramework was
3
+
competent ꢀor the O transꢀormed into Co –O intermedi-
2
2
ate and ꢀacilitated the ꢀormation oꢀ 4-ethyltoluene hydrop-
eroxide, while Mo(IV) sites were responsible ꢀor active
oxygen atoms transꢀer to cyclohexene. Subsequently, we
sought to explore that diꢁerentiation oꢀ photocatalysis and
thermal catalysis, the conspicuous diꢁerences oꢀ catalytic
behavior revealed that the ꢃnal products oꢀ the C=C bond
epoxidation and α-C–H bond oxidation are controllable.
This heterogeneous catalyst, with advantages in simple
synthesis and recyclability, could provide a great possibil-
ity to realize innovative and economically advantageous
processes ꢀor allylic oxidation oꢀ alkenes.
6
+
ate 4-ethyltoluene peroxide radicals. Then Mo states in
the Mo–O–Co unit oꢀ CoMo (A) is inclined to capture
4
-ethyltoluene hydroperoxide radicals converted into a
peroxide molybdenum (D) and a vital intermediate oꢀ
-ethyltoluene oxide radicals, which would preꢀerentially
4
abstract hydrogen atoms ꢀrom the allylic site in cyclohex-
ene; Subsequently, the allylic radicals oꢀ cyclohexene
were rapidly trapped by molecular oxygen transꢀormed to
allylicperoxy radicals; Finally, oxidation oꢀ allylic hydro-
gen oꢀ cyclohexene depended on decomposition oꢀ allyl-
6
+
icperoxy radicals by the Mo sites (A) with the ꢀormation
oꢀ important intermediates (E) and cyclohexene oxygen
radicals, which ꢃnally produced 2-cyclohexen-1-ol [42].
Acknowledgements This work was supported by the National Natural
Science Foundation oꢀ China (No. 21601048), the China Postdoctoral
1
3

X. Shi et al.
Science Foundation (2015M580626), the Natural Science Foundation
Project oꢀ Henan province (162300410012).
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Compliance with Ethical Standards
5
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2
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Conflict of interest The authors declare no conꢂict oꢀ interest.
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Aꢀliations
1
1
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1
Xiaoyun Shi · Zhuolin Shi · Guiqin Niu · Chen Si · Qiuxia Han · Jingjing Zhang
*
Qiuxia Han
oꢀ Chemistry and Chemical Engineering, Henan University,
Kaiꢀeng 475004, Henan, China
qiuxia_han@163.com
1
Henan Key Laboratory oꢀ Polyoxometalate Chemistry,
Institute oꢀ Molecular and Crystal Engineering, School
1
3