Immobilization of molybdenum-based complexes on dendrimer-functionalized graphene oxide and their catalytic activity for the epoxidation of alkenes

Article abstract of DOI:10.1016/j.catcom.2021.106341

Two novel molybdenum-based heterogeneous catalysts immobilized on dendrimer-functionalized graphene oxide via electrostatic interactions (Mo-1) or covalent bonding (Mo-2) are reported. The catalysts show excellent catalysis in epoxidation of alkenes with high conversion, better selectivity and good recyclability. The characteristics were identified by SEM, TEM (EDS-mapping), FT-IR, XRD, and XPS. The cause of the difference between the two catalysts is supported by DFT calculations.

Full text of DOI:10.1016/j.catcom.2021.106341

Catalysis Communications 158 (2021) 106341
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Immobilization of molybdenum-based complexes on
dendrimer-functionalized graphene oxide and their catalytic activity for the
epoxidation of alkenes
Zhanfang Fan1, Bin Lin1, Yongqing Liu, Yang Liu*, Maosheng Cheng*
Key Laboratory of Structure-Based Drug Design & Discovery of Ministry of Education, School of Pharmaceutical Engineering, Shenyang Pharmaceutical University,
Shenyang 110016, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two novel molybdenum-based heterogeneous catalysts immobilized on dendrimer-functionalized graphene
oxide via electrostatic interactions (Mo-1) or covalent bonding (Mo-2) are reported. The catalysts show excellent
catalysis in epoxidation of alkenes with high conversion, better selectivity and good recyclability. The charac-
teristics were identified by SEM, TEM (EDS-mapping), FT-IR, XRD, and XPS. The cause of the difference between
the two catalysts is supported by DFT calculations.
Molybdenum
Graphene oxide
Dendrimer
Epoxidation
Alkenes
1. Introduction
areas, and hierarchical structures have been widely used. Poly-
oxometalate (POM), an anionic transition metal oxygen clusters, can be
Epoxides are important precursors for the production of fine chem-
icals, such as nanomaterials, surfactants, food additives, cosmetics and
epoxyresins. They are vital intermediates in organic synthesis and are
important to achieve structural and functional versatility. In recent
years, the epoxidation of alkenes has been the main route to produce
epoxides. In this process, metal-based catalysis has played an important
role because of the high selectivity and activities of the catalysts [1–4].
Molybdenum, an abundant and nonnoble metal, has naturally attracted
much attention from researchers [5].
used not only as an efficient catalyst in epoxidation directly [14], but
also as modificatory materials these have better stability and could be
easily reused without distinct loss in both mass and activity [15–17]. In
addition, Polyoxometalate-based catalysts immobilized on proper ma-
terials have also exhibited unexpected catalytic activity [18]. Graphene
oxide (GO), as an efficient support for heterogeneous catalytic reactions,
has been widely studied because of its abundant oxygen-containing
functional groups, superior mechanical strength and considerable spe-
cific surface area. Surface-functionalized GO, which was coordinated
with MoO2(acac)2 [19] or Sali-cylaldehydoethylenediamine (salen)-Mo-
complexes [20–22] has high conversion, good selectivity and excellent
reusability, and the major difference among them are the coupler and
the type of interaction. Furthermore, many other stable materials have
also been used as supports, such as dendrimer-functionalized magnetic
nanoparticles [23], porous silica materials [24] and photocatalyzed
epoxidation catalysts [25]. Overall, exploiting innovative molybdenum-
based heterogeneous catalysts for epoxidation of alkenes is inevitable to
overcome the problems with separation and recycling and improve their
properties and commercial value.
The historic progress of molybdenum-based catalysts for epoxidation
of alkenes can be traced back to the 1960s. Originally, molybdenum
complexes as homogeneous catalysts were widely used, including Mo
(CO)6, MoO2-phthalocyanine, MoO2(trans-cyclohexane-1,2-diol)2 [6,7]
and MoO2(acac)2 [8]. Moreover, molybdenum complexes with various
kinds of ligands, such as Schiff-base ligands [9–11], 1,2,4-triazole(trz)
ligands [12], salen ligands [13], etc., have also been certified as effi-
cient homogeneous catalysts. Unfortunately, although homogeneous
catalysts have high catalytic activity, their non-recyclability and diffi-
culty of separation have seriously limited their application. Therefore,
innovative effort has been made to design and prepare heterogeneous
catalysts to overcome the drawbacks of homogeneous catalysts. Support
materials with chemical and thermal stability, large specific surface
Herein, we report two novel molybdenum-based heterogeneous
catalysts for epoxidation of alkenes, that have been immobilized onto
dendrimer-functionalized GO via covalent bonding or electrostatic
* Corresponding authors.
E-mail addresses: y.liu@syphu.edu.cn (Y. Liu), mscheng@syphu.edu.cn (M. Cheng).
1
Zhanfang Fan and Bin Lin contributed equally to this work.
https://doi.org/10.1016/j.catcom.2021.106341
Received 2 June 2021; Received in revised form 8 July 2021; Accepted 16 July 2021
Available online 21 July 2021
1566-7367/© 2021 The Authors.
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Z. Fan et al.
Catalysis Communications 158 (2021) 106341
interactions [26,27] under mild conditions using tert-butyl hydroper-
oxide (TBHP) as an oxidant with good activity, high selectivity and
sufficient stability and reusability.
2.2. Catalysts synthesis
2.2.1. Synthesis of graphene oxide (GO)
In a typical experiment, graphite powder (6.0 g), H2SO4 (60 ml),
K2S2O8 (7.5 g), and P2O5 (7.5 g) were mixed in a 500 ml round-bottom
flask. The solution was heated to 80 ◦C and stirred for 12 h. Next, after
cooling to room temperature, the solution was diluted with 300 ml of
deionized (DI) water. After that, the mixture was filtered and washed
with 500 ml of deionized water. The product was dried at 60 ◦C under
vacuum for 12 h. The pretreated graphite powder was then stirred in
concentrated H2SO4 (150 ml) for 12 h. Afterwards, NaNO3 (3.0 g) was
added into the mixture, and then KMnO4 (30 g) was added slowly while
stirring at 0 ◦C. The mixture was then heated to 35 ◦C for 0.5 h, and later,
it was slowly diluted with DI water (1 L) to keep the temperature below
40 ◦C. Next, the mixture was heated to 90 ◦C and transferred to ambient
conditions after 15 min. After that, DI water (300 ml) was added quickly.
Afterward, 30% H2O2 (60 ml) was gradually added to the mixture. After
the oxidation process, the mixture was centrifuged and washed three
times with 5 wt% HCl to remove metal ions and then washed with large
amounts of deionized water and ethanol. The prepared dark-brown
product was dried in an oven at 40 ◦C for 24 h to afford graphene
oxide (black solid, 8.5 g).
2. Experimental
2.1. Materials
All reactants and solvents were directly obtained from commercial
sources (Energy Chemical, Shanghai, China) and used without further
purification, such as graphite powder (crystalline, ꢀ 325 mesh, 99%),
PAMAM-G1 (M.W = 516.38, 98%), H3PO4⋅12MoO3 (M.W = 1825.48 g/
mol, 98%), MoO2(acac)2 (98%), silica gel (100–200, 200–300 mesh).
Scanning Electron Microscope (SEM) and Transmission Electron Mi-
croscope (TEM) were obtained by Q45-XFlash 6–30 (FEI-BRUKER,
Shanghai, China) and JEM2100F (JEOL, Tokyo, Japan). The Inductively
Coupled Plasma Optical Emission Spectrometer (ICP-OES) was
measured by Optima 8000 (PerkinElmer, Saint Paul, USA). Fourier
Transform Infrared Spectroscopy (FT-IR) were measured by a Nicolet
IS5 (Thermo Fisher, Waltham, USA). X-Ray Powder Diffraction (XRD)
and X-ray photoelectron spectroscopy (XPS) patterns were obtained by
X’Pert3PowderX (PANalytical, Almelo, Holland) and ESCALAB250Xi
(Thermo Fisher, Waltham, USA). The Density Functional Theory (DFT)
calculations were carried out at the b3lyp level, and the standard
LanL2DZ basis set was employed to represent the H, C, O, P, S and Mo
atoms to optimize these structural fragments, as implemented in the
Gaussian09 program. The conversion and selectivity of the substrates
and products were determined by Gas Chromatography-Mass Spec-
trometer (GC–MS) with internal standard curve method (Inert mass se-
lective datevtor: 5975, Network GC system: 6890 N, chromatographic
2.2.2. Synthesis of GO-PAMAM-G1
GO (3.0 g) was dispersed in 100 ml of deionized water by sonication
for 30 min. Then, 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hy-
drochloride (EDCI) (520 mg) and N-hydroxysuccinimide (NHS) (150
mg) were added to the solution and stirred for 15 min to activate the
carbonyl group on GO. Finally, Polyamide-amine dendritic polymer-G1
(PAMAM-G1) (3 g) in 20 ml DI water was added dropwise to the system,
◦
column: HP-5MS 19091S-433, 30.0 m*250
μ
m*0.25
μ
m, dodecane as an
and the reaction was allowed to proceed at 25 C for 24 h. The sus-
internal standard) 1H and 13C NMR spectra were recorded with Bruker
pension was filtered and washed with water and ethanol. The product
◦
Avance-III 600 spectrometers and referenced to DMSO‑d6.
was dried at 60 C under vacuum for 12 h. The resulting dark-brown
precipitates of the modified GO-PAMAM-G1 were ready to use (5.1 g).
2.2.3. Synthesis of Mo-1
H3PO4⋅12MoO3 (HPMo) (2.5 g) was added to a liquid suspension of
Fig. 1. The probable structures of catalysts.
2

Z. Fan et al.
Catalysis Communications 158 (2021) 106341
GO-PAMAM-G1 (2.5 g) in water. The mixture was stirred for 48 h at
room temperature. The black precipitate was collected and washed with
water and diethyl ether to yield a black solid (3.7 g).
2.2.4. Synthesis of GO-PAMAM-G1-salen
A solution of salicylaldehyde (0.52 ml, 5 mmol) in methanol (10 ml)
was added to GO-PAMAM-G1 (2.5 g), dispersed in 100 ml methanol in a
250 ml round bottom flask and refluxed for 12 h. The mixture was
cooled, filtered and dried at room temperature to obtain a black solid
(3.0 g).
2.2.5. Synthesis of Mo-2
A solution of MoO2(acac)2 (0.98 g, 3.0 mmol) in methanol was added
to a suspension of GO-PAMAM-G1-salen (3.0 g) in methanol. The
mixture was refluxed for 24 h, and the black precipitate was collected
and washed with methanol and diethyl ether to yield a black solid (3.5
g).
2.3. Evaluation of catalytic performance
Fig. 2. FT-IR spectra of the catalysts.
2.3.1. General procedure for epoxidation of alkenes catalyzed by Mo-1 and
Mo-2
In a typical procedure, the epoxidation reaction was carried out as
follows: 6 mmol TBHP (80% in di-tertiary butyl peroxide), as an oxidant,
was added to a mixture of catalyst (Mo-1, 50 mg or Mo-2, 50 mg) and
olefin (1 mmol) in toluene (20 ml), after the mixture had been stirred for
15 min. The mixture was then refluxed for an appropriate time, and the
final products were quantified by GC–MS with dodecane as an internal
standard, and the spectra of the products are given in the supporting
information.
2.3.2. Synthesis of synephrine
Acetic anhydride (2 mmol, 0.189 ml), triethylamine (2 mmol, 0.278
ml) and 4-dimethylaminopyridine (0.2 mmol, 24.4 mg) were added to a
mixture of 4-hydroxystyrene (in 10% butanediol solution, 1 mmol) in
20 ml dichloromethane (DCM). Then, the mixture was stirred for 1 h at
room temperature. The mixture was extracted by DCM, washed with
saturated sodium chloride aqueous solution, and dried with anhydrous
sodium sulfate. The organic layer was removed in vacuo. The residue
was purified by silica gel column chromatography (petroleum ether:
ethyl acetate =10:1) to afford the compound: 4-acetoxy-styrene. Then,
the epoxidation of 4-acetoxy-styrene was performed via the general
procedure to obtain the corresponding epoxide. Finally, the epoxide (1
mmol) was dissolved in 10 ml methylamine (40% in water), and
refluxed for 2 h. The solvent was removed in vacuo, and the remaining
product was purified by silica gel column chromatography (dichloro-
methane: methanol =10:1) to afford synephrine (yield: Mo-1, 91.2%;
Mo-2, 80.2%), the spectra of the synephrine are given in the supporting
information.
Fig. 3. XRD patterns of the catalysts.
were slight differences in particle size between the two kinds of cata-
lysts, but both of them were granular and the sheet structure of GO was
not destroyed through the modification. TEM and element mapping
characterization are shown in Fig. S1c-S1d. There were marked differ-
ences between them. Mo was uniformly distributed in both of the cat-
alysts but with a difference in quantity. The catalysts were also
evaluated by ICP-OES to accurately measure the content of Mo, which
was calculated by mass ratio (Mo-1: 23.88%, Mo-2: 7.12%).
The optical properties were analysed by FTIR spectroscopy, as shown
in Fig. 2. In GO-PAMAMA-G1, the peaks at 3053 cmꢀ 1 could be ascribed
to bending vibrations of nitrogen hydrogen bond(ꢀ NH2); the peaks at
3448 cmꢀ 1 could be ascribed to bending vibrations of the amido bonds,
the corresponding signals appeared at 3405 cmꢀ 1 in GO [19] and 3279
cmꢀ 1 in PAMAM-G1 [29], which indicated that GO was functionalized
by dendrimers. But the peaks at 3053 cmꢀ 1 was disappeared in the other
solids, which meaned amino interacted with salicylaldehyde or Mo-
complexes. The peaks at 948 cmꢀ 1 and 793 cmꢀ 1 in catalyst Mo-1
3. Results and discussion
3.1. Preparation and characterization of catalysts
GO was prepared by following Hummers’ method [28]. Dendrimer-
functionalized GO was obtained following the procedure described by
Tarahomi et al. [27], whereby HPMo was immobilized as a molybdenum
complex to obtain Mo-1 (Fig. 1, and Scheme S1a, see Supporting In-
formation). Analogously, Mo-2 was prepared via condensation with
dendrimer-functionalized GO and salicylaldehyde to obtain a salen
ligand, followed by covalent bonding with MoO2(acac)2 (Fig. 1, and
Scheme S1b).
could be assigned to the interaction of Mo with the supports compared
ꢀ 1
with 1064 cmꢀ 1(P O) and 964 cm (Mo O) in HPMo. 900 cmꢀ 1 and
–
–
–
762 cmꢀ 1 in catalyst Mo-2 is characteristic of the presence of MoO2
group [22].
The characteristics of the catalysts were identified and characterized
by morphological and spectroscopic analysis.
Fig. 3 shows the XRD patterns of the catalysts. As we know, the XRD
pattern of GO shows the characteristic diffraction peak at 2θ ≈ 10◦, as
well as the HPMo at 2θ ≈ 26◦ [22].The samples show a distinct broad
The morphologies of the catalysts are shown in Fig. S1. Fig. S1a-S1b
displays the SEM images of the catalysts, which indicated that there
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Z. Fan et al.
Catalysis Communications 158 (2021) 106341
Fig. 4. XPS spectra (a) and N 1 s peak in the XPS spectra (b) of the catalysts.
diffraction peak between 20◦ and 30◦, which indicates that the GO was
complexed with Mo after functionalization by PAMAM-G1. And the
diffraction peak at 2θ ≈ 10◦ does not disappear, suggesting that the
structure of graphene oxide is not destroyed during sample preparation.
XPS analysis was conducted to further verify the formation of the
catalysts, and the elemental composition and valences of the various
elements, as shown in Fig. 4a. Fig. 4b shows the high-resolution XPS
spectrum of N 1 s. The bands at approximately 398, 400, and 401 eV can
be assigned to the bonds of
,
, and
The above results show that the type of interaction of the
catalysts are different(compared with the GO-PAMAM-G1 and GO-
Table 1
The epoxidation for alkene under optimum condition with catalysts of Mo-1 and Mo-2a.
Entry
Substrate
Product
Catalyst
Conversionb (%)
Selectivityb (%)
1
2
Mo-1
Mo-2
43.8
21.8
55.6
73.4
3
4
5
6
Mo-1
Mo-2
Mo-1
Mo-2
60.1
30.4
99.7
99.5
72.3
78.6
93.7
76.3
7
Mo-1
Mo-2
Mo-1
Mo-2
89.0
90.1
87.5
94.6
12.2
13.5
89.2
66.1
8
9
10
11
12
13
14
Mo-1
Mo-2
Mo-1
Mo-2
50.6
45.3
99.9
99.9
34.1
28.4
50.5
66.4
15
16
Mo-1
Mo-2
97.2
92.1
76.4
54.6
17
18
Mo-1
Mo-2
81.1
79.9
71.2
56.1
19
20
Mo-1
Mo-2
99.4
99.0
98.8
86.1
a
b
Reaction conditions: alkene (1 mmol), toluene (20 ml), TBHP (6 mmol), catalyst (Mo-1, 50 mg, Mo-2, 50 mg), 24 h.
Determined by GC–MS.
4

Z. Fan et al.
Catalysis Communications 158 (2021) 106341
model substrate, TBHP as oxidant and reaction 24 h similarly. The cat-
alysts were easily recovered by filtration and vacuum drying at 60 ◦C.
After recycling for 5 runs, the catalytic activity (conversion) and selec-
tivity of the catalysts were maintained with minor loss, as shown in
Fig. 5. In addition, there were no obvious differences between the fresh
and reused catalysts (named R-Mo-1 and R-Mo-2) both in morphology
and spectroscopy (Fig. S3-S5).
3.3. DFT calculations
All the morphologic and spectroscopic analysis revealed that the new
catalysts had tiny differences in morphology, molybdenum content and
distribution, but there were some differences between them in catalytic
activity. To further study the cause of the different catalytic properties, a
DFT study was conducted. In the reaction, the molybdenum complex
first reacts with the oxidant to form the peroxymolybdenum complex,
and then the alkene interacts with the oxygen atom of the peroxygen
complex by nucleophilic attack to obtain the epoxidized products, as
shown in Fig. S6 [30]. Therefore, the natural population analysis (NPA)
charge of each of the optimized structures was calculated using the same
basis set to illustrate the charge value, explain the activity of the Mo-1
and Mo-2 by comparing the NPA charge of oxygen atoms.
Fig. 5. Study of the catalysts’ recyclability for the epoxidation of cyclohexene.
PAMAM-G1-salen, as shown in Fig. S2).
3.2. Evaluation of catalytic performance
To simplify the calculation, representative key structural fragments
of the two peroxygen complexes were selected because of the relatively
complex structures of the two catalysts. Due to the steric hindrance of
the surrounding atoms, the O2 atoms of the catalysts do not easily
combine with alkenes. Therefore, the charge of the O1 atom was used for
comparison, as shown in Fig. S7. In a comparison of the charges of the
oxygen atoms in the two catalyst peroxygen complexes, the values in
decreasing order was Mo-1 (ꢀ 0.290e) < Mo-2 (ꢀ 0.301e). Because this
reaction is a nucleophilic reaction, the order of catalyst activity was Mo-
1 > Mo-2, which was consistent with the experimental results (Mo-1 >
Mo-2). In conclusion, the reaction activity of the catalysts may be related
to the charge of the peroxygen atom of the catalystic peroxygen com-
plex, which increased with the decrease in the charge of the oxygen
atom.
We chose cyclohexene as a model compound and TBHP as an oxidant
to investigate the performance of the catalysts in toluene at 80 ◦C, and
the results are shown in Table S1. Under the same conditions, Mo-1 and
Mo-2 exhibited better catalytic activity in the order of Mo-1 > Mo-2 in
comparison to the intermediates and a previously reported catalyst,
which named PAMAMA-G1-Mo, a dendritic phosphomolybdate hybrid
by composite based on PAMAM-G1 and HPMo.
Subsequently, we chose Mo-1 as the catalyst to optimize the condi-
tions for the epoxidation of cyclohexene as compiled in Table S2. First,
the effects of the oxidant in the catalyzed reaction system were inves-
tigated. The conversions were very low, less than 5%, as was the
selectivity, when we chose different oxidants, such as air, 30% H2O2,
NaClO or PhIO; all the reactions were performed under the same con-
ditions except that of TBHP (Table S2, entries 1–5). After that, we
screened several solvents, viz., DCM, EtOH, 95% EtOH, THF, and
acetonitrile (Table S2, entries 6–10), as reaction media, but all the above
solvents showed poor results both in conversion and selectivity, except
for acetonitrile, which showed a conversion of 55.4%. Homogeneously,
catalyzing the epoxidation of cyclohexene in different solvents with 30%
H2O2 and NaClO was also investigated (Table S2, entries 11–20), but we
observed that all the results were unsatisfactory. Ultimately, the epox-
idation of alkenes was carried out in 20 ml toluene at 110 ◦C using 6 eq.
TBHP as the oxidant and 50 mg catalyst.
4. Conclusion
In summary, we synthesized two new catalysts by immobilizing Mo-
based complex onto dendrimer-functionalized graphene oxide, and the
structures were confirmed and characterized by SEM, TEM(ESD-
mapping), FT-IR, XRD and XPS. The catalysts can be applied to the
epoxidation of alkenes under mild conditions using TBHP as an oxidant
with favourable conversion and selectivity, and the catalysts can be
easily reused at least five runs with no obvious loss in catalytic activity.
The activity differences of the catalysts were calculated via DFT, which
indicated that the decrease in the charge of the peroxygen atom of the
catalystic peroxygen complex was beneficial for improving the reaction
activity of the catalysts. The catalysts could also be employed to syn-
thetize medicines containing epoxide intermediates (such as synephrine,
which had yields of up to 91.2%). These catalysts will have broad
prospects in the field of catalytic oxidation.
By keeping these facts in mind and based on our past knowledge and
experience, a diverse range of alkenes was chosen to confirm the
application scope of this methodology, as shown in Table 1. All of the
alkenes were converted to the corresponding epoxide both in better
conversion and selectivity under the optimum conditions; likewise, the
order of active activity was Mo-1 > Mo-2. The alkene structure played an
important role in the catalytic activity. The cyclic olefins were more
reactive than the linear olefins (Table 1, entries 1–12), the aromatic
alkene showed the best activity, and the substituted groups and steric
hindrance had little effect on the results (Table 1, entries 13–20). The
types of byproducts mainly include aldehyde or ketone compound,
alcohol compound and diol compounds.
Credit author statement
Dr. Maosheng Cheng and Dr. Yang Liu supervised the whole exper-
iment and provided technical guidance. Zhanfang Fan designed and
synthesized all of novel catalysts and evaluated their catalytic activity.
Dr. Bin Lin supervised and carried out calculation via DFT. Yongqing Liu
assisted in the synthetic research.
Encouraged by the overall results of the epoxidation of alkenes in
terms of conversion and selectivity, for the first time, we evaluated the
synthetic potential of this protocol for the preparation of synephrine, an
α
1-receptor agonist, that can be synthesized via an epoxidation inter-
mediate (Scheme S2). Synephrine was obtained in high yields, all
greater than 80%, under mild reaction conditions and with convenient
post-processing. Mo-1 had a higher yield at 91.2%. The results provide a
new method for the synthesis of amino alcohol compounds.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
To study the recyclability of the catalysts, we chose cyclohexene as a
5

Z. Fan et al.
Catalysis Communications 158 (2021) 106341
´
[13] G. Chahboun, J.A. Brito, B. Royo, M.A.E. Amrani, E. Gomez-Bengoa, M.E.
the work reported in this paper.
G. Mosquera, T. Cuenca, E. Royo, Olefin epoxidation catalyzed by cis-
dioxidomolybdenum(VI) complexes containing chiral alkoxo-imino ligands derived
Acknowledgements
from (+)-α-Pinene, Eur. J. Inorg. Chem. 17 (2012) 2940–2949.
[14] A. Rezaeifard, M. Jafarpour, R. Haddad, F. Feizpour, {Mo72Cr30} nanocluster as a
novel self-separating catalyst for hydrogen peroxide olefin epoxidation, Catal.
Commun. 95 (2017) 88–91.
We gratefully acknowledge the National Natural Science Foundation
of China (No. 81473087), the Program for Liaoning Innovative Talents
in University (LR2017043), and the Career Development Support Plan
for Young and Middle-aged Teachers in Shenyang Pharmaceutical Uni-
versity (ZQN2016005) for the financial support.
[15] M. Ghorbanloo, R. Bikas, G. Małecki, New molybdenum(VI) complexes with
thiazole-hydrazone ligand: preparation, structural characterization, and catalytic
applications in olefin epoxidation, Inorg. Chim. Acta 445 (2016) 8–16.
[16] H.C. Gao, Y. Yan, X.H. Xu, J.H. Yu, H.L. Niu, W.X. Gao, W.X. Zhang, M.J. Jia,
Catalytic epoxidation of olefin over supramolecular compounds of molybdenum
oxide clusters and a copper complex, Chin. J. Catal. 36 (2015) 1811–1817.
[17] H. Taghiyar, B. Yadollahi, New perspective to catalytic epoxidation of olefins by
Keplerate containing Keggin polyoxometalates, Polyhedron 156 (2018) 98–104.
[18] U. Jameel, M.Q. Zhu, X.Z. Chen, Z.F. Tong, Polyoxometalate-nano gold hybrid
mesostructured catalyst for green cyclohexene epoxidation, Curr. Org. Chem. 21
(2017) 2585–2596.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.catcom.2021.106341.
[19] M. Masteri-Farahani, S. Mirshekar, Covalent functionalization of graphene oxide
with molybdenum-carboxylate complexes: new reusable catalysts for the
epoxidation of olefins, Colloid. Surf. A 538 (2018) 387–392.
References
[20] Z.F. Li, S.J. Wu, D.F. Zheng, J.Y. Liu, H. Liu, H.M. Lu, Q.S. Huo, J.Q. Guan, Q.
B. Kan, Dioxomolybdenum(VI) complex covalently attached to amino-modified
graphene oxide: heterogeneous catalyst for the epoxidation of alkenes, Appl.
Organometal. Chem. 28 (2014) 317–323.
[1] E.Z. Ayla, D.S. Potts, D.T. Bregante, D.W. Flaherty, Alkene epoxidations with H2O2
over groups 4ꢀ 6 metal-substituted BEA zeolites: reactive intermediates, reaction
pathways, and linear free-energy relationships, ACS Catal. 11 (2020) 139–154.
[2] D.Q. Xu, Q.S. Sun, J. Lin, W. Sun, Ligand regulation for manganese-catalyzed
enantioselective epoxidation of olefins without acid, Chem. Commun. 56 (2020)
13101–13104.
[21] Z.F. Li, C.L. Yang, Y.Y. Ma, P. Li, J.Q. Guan, Q.B. Kan, Facile fabrication of
magnetic MoO2–Salen-modified graphene-based catalyst for epoxidation of
alkenes, Appl. Organometal. Chem. 31 (2017), e3742.
[22] B. Rezazadeha, A.R. Pouralia, A. Banaeib, H. Behniafar, Schiff base complexes of
Mo(VI) immobilized on functionalized graphene oxide nano-sheets for the catalytic
epoxidation of alkenes, J. Coord. Chem. 72 (2019) 3401–3416.
[3] J.-K. Li, H.J. Liao, Y.J. Tang, J.L. Huang, L.D. Cha, T.S. Lin, J.L. Lee, I.V. Kurnikov,
M.G. Kurnikova, W.C. Chang, N.L. Chan, Y.S. Guo, Epoxidation catalyzed by the
nonheme Iron(II)- and 2-oxoglutarate-dependent oxygenase, AsqJ: mechanistic
elucidation of oxygen atom transfer by a ferryl intermediate, J. Am. Chem. Soc.
142 (2020) 6268–6284.
[23] M. Niakan, Z. Asadi, M. Masteri-Farahani, Immobilization of salen molybdenum
complex on dendrimer functionalized magnetic nanoparticles and its catalytic
activity for the epoxidation of olefins, Appl. Surf. Sci. 481 (2019) 394–403.
[24] T. Luts, R. Frank, W. Suprun, S. Fritzsche, E. Hey-Hawkins, H. Papp, Epoxidation of
olefins catalyzed by novel Mn(III) and Mo(IV)-Salen complexes immobilized on
mesoporous silica gel part II: study of the catalytic epoxidation of olefins, J. Mol.
Catal. A 273 (2007) 250–258.
[4] Y.R. Shen, P.P. Jiang, P.T. Wai, Q. Gu, W.J. Zhang, Recent progress in application
of molybdenum-based catalysts for epoxidation of alkenes, Catalysts 9 (2019) 31.
[5] S. Morales-delaRosa, J.M. Campos-Martin, P. Terreros, J.L.G. Fierro, Catalytic
epoxidation of cyclohexene with tertbutylhydroperoxide using an immobilized
molybdenum catalyst, Top. Catal. 58 (2015) 325–333.
[25] G. Bian, P.-P. Jiang, F. Wang, Y.-R. Shen, K.-L. Jiang, L. Liu, W.-J. Zhang, Light
driven epoxidation of olefins using a graphene oxide/g-C3N4 supported Mo (salen)
complex, New J. Chem. 42 (2018) 85–90.
[6] R.A. Sheldon, Molybdenum-catalysed epoxidation of olefins with alkyl
hydroperoxides, Recl. Trav. Chim. Pays Bas. 92 (1973) 253–266.
[7] A.O. Chong, K.B. Sharpless, On the mechanism of the molybdenum and vanadium
catalyzed epoxidation of olefins by alkyl hydroperoxides, J. Org. Chem. 42 (2002)
1587–1590.
[26] Q.-L. Tong, Y. Liu, X.-Z. Gao, Z.-F. Fan, T.-F. Liu, B. Li, D.-S. Su, Q.-H. Wang, M.-
S. Cheng, A deoximation method for deprotection of ketones and aldhydes using a
graphene-oxide-based co-catalysts system, Adv. Synth. Catal. 361 (2019)
3137–3145.
[8] A. Fusi, R. Ugo, G.M. Zanderighi, Homogeneous catalysis by Transition Metal
Complexes IV. The use of mixed catalysts in the oxidation of cyclohexene, J. Catal.
34 (1974) 175–190.
[27] M. Tarahomi, H. Alinezhad, B. Maleki, Immobilizing Pd nanoparticles on the
ternary hybrid system of graphene oxide, Fe3O4 nanoparticles, and PAMAM
dendrimer as an efficient support for catalyzing sonogashira coupling reaction,
Appl. Organometal. Chem. 33 (2019), e5203.
[9] J.W. Kuck, R.M. Reich, F.E. Kuhn, Molecular epoxidation reactions catalyzed by
Rhenium, Molybdenum, and Iron complexes, Chem. Rec. 16 (2016) 349–364.
[10] M.E. Judmaier, C. Holzer, M. Volpe, N.C. Mosch-Zanetti, Molybdenum(VI) dioxo
complexes employing Schiff Base ligands with an intramolecular donor for highly
selective olefin epoxidation, Inorg. Chem. 51 (2012) 9956–9966.
[11] M. Bagherzadeh, S. Ataie, H. Mahmoudi, J. Janczak, Synthesis, structure
characterization and study of a new molybdenum Schiff base complex as an
epoxidation catalyst with very high turnover numbers, Inorg. Chem. Commun. 84
(2017) 63–67.
[28] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80
(2002) 1339.
[29] Y.-J. Chen, R. Tan, W.-G. Zheng, Y.-Y. Zhang, G.-W. Zhao, D.-H. Yin, Dendritic
phosphotungstate hybrids efficiently catalyze the selective oxidation of alcohols
with H2O2, Catal. Sci. Technol. 4 (2014) 4084–4092.
[30] Y.G. Abashkin, S.K. Burt, (Salen)Mn-catalyzed epoxidation of alkenes: a two-zone
process with different spin-state channels as suggested by DFT study, Org. Lett. 6
(2004) 59–62.
[12] M.M. Antunes, T.R. Amarante, A.A. Valente, F.A.A. Paz, I.S. Gonçalves,
M. Pillinger, A linear trinuclear oxidodiperoxido-molybdenum(VI) complex with
single triazole bridges: catalytic activity in epoxidation, alcoholysis, and
acetalization reactions, ChemCatChem 10 (2018) 2782–2791.
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