Immobilization of [MoO2(acac)2] on surface of hydroxyapatite nanoparticles: A heterogeneous and reusable catalyst for olefin epoxidation reactions

Article abstract of DOI:10.1002/poc.3921

A new heterogeneous catalyst was prepared by grafting [MoO₂(acac)₂] on the hydroxyapatite (HAP) nanoparticle surfaces. The resulting catalyst, [MoO₂(acac)]@HAP, was characterized by the powder X-ray diffraction, FT-IR spectroscopy, inductively coupled plasma/atomic emission spectroscopy, field emission scanning electron microscopy, energy-dispersive X-ray analysis, thermogravimetery analysis, and transmission electron microscopy techniques. The results obtained confirmed that the catalyst could be successfully used for the efficient epoxidation of olefins with tert-butyl hydrogen peroxide in 1,2-dichloroethane as the solvent. This heterogeneous catalyst could be reused for several times without a significant loss in its catalytic activity and with no significant reduction in the reaction yield. This novel material showed a great catalytic performance, and the reactions carried out using this catalyst showed a good-to-excellent yield, high purity of the desired products, and a short reaction time.

Full text of DOI:10.1002/poc.3921

Received: 7 May 2018
DOI: 10.1002/poc.3921
Revised: 8 October 2018
Accepted: 31 October 2018
R E S E A R C H A R T I C L E
Immobilization of [MoO2(acac)2] on surface of
hydroxyapatite nanoparticles: A heterogeneous and
reusable catalyst for olefin epoxidation reactions
Farzane Ghanbari Resketi1 | Samie Salehi2,3 | Bahram Bahramian1
| Mahdi Mirzaee1
1 Faculty of Chemistry, Shahrood
Abstract
University of Technology, Shahrood, Iran
A new heterogeneous catalyst was prepared by grafting [MoO2(acac)2] on the
2 Department of Chemistry, Faculty of
Sciences, Ferdowsi University of
Mashhad, Mashhad, Iran
3 Department of Chemistry, University of
Mazandaran, Babolsar, Iran
hydroxyapatite (HAP) nanoparticle surfaces. The resulting catalyst,
[MoO2(acac)]@HAP, was characterized by the powder X‐ray diffraction, FT‐
IR spectroscopy, inductively coupled plasma/atomic emission spectroscopy,
field emission scanning electron microscopy, energy‐dispersive X‐ray analysis,
thermogravimetery analysis, and transmission electron microscopy techniques.
The results obtained confirmed that the catalyst could be successfully used for
the efficient epoxidation of olefins with tert‐butyl hydrogen peroxide in 1,2‐
dichloroethane as the solvent. This heterogeneous catalyst could be reused
for several times without a significant loss in its catalytic activity and with no
significant reduction in the reaction yield. This novel material showed a great
catalytic performance, and the reactions carried out using this catalyst showed
a good‐to‐excellent yield, high purity of the desired products, and a short reac-
tion time.
Correspondence
Bahram Bahramian, Faculty of Chemistry,
Shahrood University of Technology,
Shahrood, Iran.
Email: bahramian.b@shahroodut.ac.ir
KEYWORDS
alkene epoxidation, energy‐dispersive X‐ray analysis, hydroxyapatite, Mo heterogeneous catalysis,
supported molecular catalysis
1 | INTRODUCTION
to catalyze the oxidation of olefins, arenes, phenols,
alcohols, phosphines, and sulfides.[8–10]
In the recent decades, a huge number of efforts have been
made in the development of alkene epoxidation
catalysts.[1,2] Since these classes of organic compounds
are active intermediates either in laboratory research
works or in industry works, their production in a
large scale is of great interest.[3–5] Generally, epoxides
can be synthesized in a medium containing H2O2 or
alkylhydroperoxides in the presence of transition metal
complexes.[6,7] Oxido and peroxido transition metal com-
plexes have played, for some time, an important role in
the oxidation of organic substrates, and a variety of
peroxido complexes of various metals have been found
Oxoperoxo complexes of molybdenum (VI) have been
investigated extensively as either stoichiometric reagents
or catalysts for the oxidation of a variety organic com-
pounds such as olefins and alcohols.[11] A significant
advantage of many of these complexes is that they can be
prepared from inexpensive metal sources, eg, MoO3 or
(NH4)6Mo7O24, in combination with aqueous H2O2 and a
suitable organic ligand. Oxoperoxo‐molybdenum (VI)
complexes commonly contain MoO(O2)2, MoO(O2), and
MoO2(O2) fragments. These metal spices could be found
in mononuclear complexes such as [MoO(O2)2(L)n],[12–15]
[MoO(O2)Cl2(L)n], CpMoO(O2) R,[16] and [MoO2(O2)
J Phys Org Chem. 2018;e3921.
wileyonlinelibrary.com/journal/poc
© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/poc.3921

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GHANBARI RESKETI ET AL.
(C2O4)(H2O)]2−,[17] dinuclear complexes such as
of its interesting characteristics and catalytic properties.
It is known that HAP can form synergistic interactions
with a variety of transition metals because of its high
ion‐exchange capacity. The HAP supported Mo (IV)
nanoparticles were found to be highly active and reusable
catalyst in the olefin epoxidation reactions. In particular,
the effect of the Mo (IV) loading on the activity, selectiv-
ity, and stability of HAP‐supported Mo (IV) materials was
evaluated and discussed in relation to their catalystic
properties in olefin epoxidation reactions.
[{MoO(O2)2(H2O)n}2(μ‐O)]2−(n = 0, 1) and [{MoO(O2)(μ‐
OH)(C2O4)}2]2−,[18]
and
the
tetranuclearcomplex
[Mo4O16(3,5‐dimethylpyrazole)6].[19] Molybdenum (VI)
complexes have afforded some of the more promising cata-
lysts for olefin epoxidation reaction. Better catalytic results
have generally been obtained with alkylhydroper‐oxides
[usually tert‐butyl hydrogen peroxide (TBHP)] as oxidant
than with aq. H2O2, although a few systems have yielded
promising results with aq. H2O2.[20]
Supported molecular metal complexes in the field of
catalysis application have attracted significant atten-
tion.[21] These supported metal catalysts offer advantages
such as high selectivity and uniform accessibility (similar
to molecular catalysts in solution) as well as ease of sepa-
ration (characterization of solid catalysts).[22] The immo-
bilization of transition metal complexes on slid supports
is the object of intense research works in order to facili-
tate catalyst separation from the reaction mixture, simpli-
fication of the procedure for catalyst recycling, making
them useful for commercial applications, and maximizing
the site isolation of catalytic centers to minimize the pos-
sibility of inactive oxo‐bridged dimmer formation.[23]
Hydroxyapatites, [HAP‐x, Ca10 −x (HPO4)x (PO4)6 −x
(OH)2 −x, x = 0, 1], are an important class of biomaterials,
which are chemically similar to the mineral component
of mammalian bones.[24] HAPs, with a Ca/P molar ratio
ranging from 1.5 to 1.67, crystallize in a P63/m space
group with a hexagonal structure. Their structures can
2 | MATERIALS AND METHOD
Solvents and reagents were supplied from Merck or
Fluka, and used without any further purification. Ele-
mental analysis was obtained by inductively coupled
plasma/atomic emission spectroscopy (ICP/AES). Powder
X‐ray diffraction (PXRD) patterns were collected by a
Philips PW‐1800 or STOE diffractometer with Cu Kα
radiation (λ = 1.5418 Å). Transmission IR spectra were
obtained on a Shimadzu 470 IR spectrometer using KBr
disks. 1H NMR spectra were recorded on a BRUKER
1
AC80 spectrometer corresponding to a H Larmor fre-
quency of 80 MHz. Gas chromatography (GC) was carried
out by a Shimadzu GC‐16A instrument using a 2‐m col-
umn packed with silicon DC‐200. Transmission electron
microscopy (TEM) images were obtained on a LEO912AB
with 120 kV of acceleration voltage. Field emission‐
scanning electron microscopy (FE‐SEM) and energy‐
dispersive X‐ray (EDX) analysis were conducted using a
3−
be viewed as consisting of unconnected PO4 tetrahe-
drons with Ca2+ in the space between and a chain of
OH− ions along the c‐axis to balance the charge.[25] There
are different reported methods for producing HAP nano-
particles which among them sol‐gel synthesis of HAP has
attracted much attention. The most promising properties
of the sol‐gel processed HAPs are the formation of a crys-
talline single‐phase product with no organic residue, a
stoichiometric product, high‐phase purity, and using a
small number of reagents for the synthesized HAP. More-
over, HAP as support offers high stability, and various
substitutions are allowed by the apatite structure.[26,27]
The choice of HAP as support is also motivated because
of its unique advantages. HAP has nonporous structure
and does not cause significant mass transfer limitation;
furthermore, it has high ion exchange ability and adsorp-
tion capacity, and its low surface acidity prevents side
reactions arising from the support itself.[28] Recently,
HAP‐supported Pd,[29] Rh,[30] and Ru[31] made from
molecular metal precursors have been reported as effi-
cient catalysts for aerobic oxidation of alcohols.
MIRA3
TESCAN‐XMU
SEM
instrument.
Thermogravimetry experiments were performed using a
Stanton Redcroft, STA‐780 series with an aluminum cru-
cible applying at heating rate of 10°C min‐1 in a tempera-
ture range of 50°C to 600°C under N2 atmosphere.
3 | EXPERIMENTAL
3.1 | Preparation of hydroxyapatite
Ca10(PO4)6(OH)2 (HAP‐0)
Stoichiometric hydroxyapatite Ca10(PO4)6(OH)2 (HAP‐0)
was prepared following a modified literature method.[32]
In a 1‐L three‐necked flask fitted with a mechanical stir-
rer was dissolved 15.75 g of Ca (NO3)2.4H2O (66.7 mmol)
in 120 mL de‐ionized water, and the pH was adjusted
to 11 using aqueous ammonia. In a separate flask,
5.28 g of (NH4)2HPO4 (40 mmol) was dissolved in 150‐
mL de‐ionized water, and the pH was adjusted to 11
using aqueous ammonia. Then, the (NH4)2HPO4 solution
was added dropwise to the Ca (NO3)2 solution at room
temperature via an addition funnel in the course of
In the present study, the textural, structural, and
chemical properties of a hydroxyapatite‐supported Mo
(IV) were investigated. We chose HAP for study because

GHANBARI RESKETI ET AL.
3 of 11
30 minutes with vigorous stirring. The milky mixture was
aged in an oven at 105°C for 30 minutes, centrifuged, and
washed four times with a copious amount of water. The
white solid obtained was dried in an oven at 110°C
overnight. Yield: 6.10 g; its XRD pattern matched that of
a standard HAP.[33]
by GC. Since different alkenes have different reactivities
toward oxidation, these reactions were continued until
no further progress was observed. All reactions were
run at least in duplicate. The details of the reaction condi-
tions are summarized in Tables 2 and 3.
3.2 | Synthesis of complex
di‐oxo‐acetylacetonato molybdenum
[MoO2(acac)2]
3.5 | Catalyst recovery and reuse
The reusability of a supported catalyst is one of the most
important benefits of heterogeneous catalysts because
transition metal complexes are often expensive to be pur-
chased or prepared. Therefore, the recycling experiments
were also studied in this work. The catalyst recycling tests
were also conducted using [MoO2(acac)]@HAP. For reus-
ability of the catalyst, it was filtered after the reaction
from the reaction mixture, washed with methanol, and
dried before reuse. The recovered catalyst was then used
for the next run by adding cyclooctene under similar
reaction conditions as used for the fresh catalyst. The
detailed procedures of the recycling experiment are sum-
marized in Table 5.
The complex was prepared by literature methods.[34]
(NH2)6Mo7O24·4H2O (30.0 g) was dissolved in H2O
(100 mL), and acetylacetone (40 mL) was added. The
pH of the solution was adjusted to 3.5 using 10% HNO3,
and a solid began to precipitate. Yellow MoO2(acac)2
was isolated by filtration, washed with H2O, ethanol,
and ether, and dried in vacuo.
3.3 | Preparation of supported Mo catalyst
on hydroxyapatite using [MoO2(acac)]
@HAP
An amount of 0.063 g (1.8 mmol) of [MoO2(acac)2] was
dissolved in 30‐mL methanol to give a light yellow solu-
tion, toward which 1 g of the HAP support was added.
The mixture was refluxed for 24 hours, and the mixture
turned to dark brown. The mixture was filtered, and the
resulting solid was washed with a copious amount of
methanol and dried under vacuum to give a dark brown
solid. ICP/AES revealed that the Mo level was
0.04 mmol/g (0.38%) on the HAP support (Scheme 1).
4 | RESULT AND DISCUSSION
The HAP was prepared by a sol‐gel process and was
used as an inorganic support for the [MoO2(acac)2] com-
plex. This supported catalyst was used in the epoxidation
of cis‐cyclooctene. The catalytic procedures for
[MoO2(acac)]@HAP were optimized for different param-
eters such as the catalyst, solvent, and oxidant. Then,
the catalyst was used for the catalytic epoxidation of dif-
ferent olefins under the optimum conditions. In addition,
recycling experiments were done to reveal the reusability
of the catalyst.
3.4 | General procedure for catalytic
epoxidation of alkenes
In a typical experiment, to a solution of alkene
(0.63 mmol) and [MoO2(acac)]@HAP in C2H4Cl2 (3 mL)
was added TBHP as an oxidant. The mixture was vigor-
ously stirred at 80°C under an N2 atmosphere for the
required time, and the reaction progress was monitored
SCHEME 1 Proposed formation pathway and structure for
FIGURE 1 Powder XRD pattern for the hydroxyapatite (HAP)
[MoO2(acac)]@HAP heterogeneous catalysis
and [MoO2(acac)]@HAP

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GHANBARI RESKETI ET AL.
important impurities that appear in the HAP composition
is CO32−, which appears in the range of 1400 to 1500 cm
−1. In the corresponding spectrum, the intensity of the
mentioned peak was very weak and verified the highly
pure synthesized HAP. The peak at 1032 cm−1 was
assigned to the triply degenerated asymmetric stretching
mode of the P―O bond of the phosphate group. The band
at 936 cm−1 is characteristic of the hydrogen phosphate
group. The absorption bands at 743 and 564 cm−1 were
assigned to the triply degenerated bending mode of the
O―P―O bonds of the phosphate group. The absence of
absorption at 631 cm−1 for OH of HA together with the
broad absorption characteristic of phosphates of HAP
also indicate the low crystallinity of the products.[36] It
should be noticed that the addition of molybdenum to
the HAP framework does not cause any significant
change in the IR spectra, confirming the conclusion pre-
viously obtained from X‐ray diffraction patterns. None
of the bands due to the free [MoO2(acac)2] could be found
in the [MoO2(acac)]@HAP spectrum.[37] Maybe in the IR
spectrum, the low complex content in [MoO2 (acac)]
@HAP (ICP‐AES data) has not been enough to display
some noticeable change (Figure 2).
Figure 3A represents the FE‐SEM images for the
[MoO2 (acac)]@HAP nanoparticles. From the SEM anal-
ysis, the morphology of the catalyst is composed of
agglomerated particles with different sizes. There is a
good agreement between this result and the XRD data.
The size of particles was estimated to be 30 to 40 nm for
[MoO2 (acac)]@HAP.
From the EDX analysis, the chemical composition
and the weight percentage of each element in the
[MoO2 (acac)]@HAP compound were determined. The
peaks for P, Ca, O, C, and Mo were observed (Figure 3
B). The amount of Mo doped on the surfaces of HAP is
in accordance with the results obtained from the ICP
analysis. These data confirm that [MoO2(acac)]@HAP
4.1 | Characterization of [MoO2(acac)]
@HAP
Inductively coupled plasma (ICP) and elemental analysis
confirmed that the Mo (IV) level was 0.04 mmol/g
(0.38%) on the HAP surfaces. The powder XRD pattern
for the HAP and [MoO2(acac)]@HAP is shown in
Figure 1. The PXRD pattern for the synthesized HAP
and supported metal complex was compared with stan-
dard HAP (Table 1).
The XRD pattern shows the characteristic peaks of
HAP as (002), (211), and (202) plan which can be indexed
as a pure hexagonal phase with a P63/m space group
according to the international center for diffraction data
database, ICDD‐PDF 9‐0432. The particle size of the pre-
pared HAP was computed using the X‐ray line broaden-
ing method with the Debye‐Scherrer equation[35]
:
0:94λ
D ¼
βDCOS θ
where D is the particle size in nanometres, λ is the wave‐
length of the radiation (1.54056 Å for Cu Kα radiation), θ
is the peak position, and βD is the peak width at half‐
maximum intensity. The calculation of particle size from
the PXRD pattern according to the Scherer equation
shows 36‐nm particles for the HAP.
The broad XRD peaks clearly indicate the nanocrys-
talline nature of the material. The XRD pattern for the
catalyst demonstrates that due to the low loading of
molybdenum species on the surface of HAP nanoparti-
cles, no obvious changes occur in the PXRD pattern of
[MoO2(acac)]@HAP. The HAP peaks for [MoO2(acac)]
@HAP become slightly sharper than those for neat HAP
due to the additional calcination step (at 500°C) after
the impregnation of HAP by molybdenum.
In order to characterize the nanocomposite and to
confirm the immobilization of the active components on
the HAP surface, FT‐IR spectroscopy was initially utilized
for the HAP and [MoO2(acac)]@HAP. In the IR spectra of
the prepared HAP, the OH− frequency observed in the of
3421 cm−1 (OH− bending) and the peak at 1632 cm−1
were assigned to the associated water. One of the most
TABLE 1 Texture parameters for standard HAP, HAP prepared
sample, and [MoO2(acac)]@HAP obtained from XRD
ICDD(9‐432)
HAP Sample
[MoO2(acac)]@HAP
hkl
D
002
211
202
3.44
2.81
2.63
3.23
2.48
2.67
3.84
2.69
2.56
FIGURE 2 FT‐IR spectra for HAP and [MoO2(acac)]@HAP

GHANBARI RESKETI ET AL.
5 of 11
FIGURE 3 FE‐SEM image A, and EDX analysis B, of [MoO2(acac)]@HAP nanoparticles
has been a successful supported on the HAP nanoparticle
surfaces.
nanoparticles was prominent in both samples, attributed
to the Van der Waals forces of attraction.[38]
In order to provide a direct evidence of any preferen-
tial morphology and facet orientation of supported
molecular catalysis, the HAP and [MoO2(acac)]@HAP
particles were studied directly by TEM. Figure 4A and B
presents the TEM images for the purified HAP and the
[MoO2(acac)]@HAP catalyst, respectively. Observations
from the TEM micrographs confirmed that HAP and
[MoO2(acac)]@HAP showed similar morphological
To determine the stability of the pure HAP and its
supported Mo (IV) catalyst, the thermogravimetric analy-
sis was conducted in an atmosphere of N2 from 25°C to
600°C with a temperature ramp of 10°C min−1, and the
resultant thermograms are shown in Figure 5. The total
weight loss for the pure HAP is due to the evaporation
of physically and chemically adsorbed water. Grafting of
Mo complex onto HAP surfaces correspondingly
increased the weight loss. The first weight loss occurred
rocky‐like
structures.
Agglomeration
of
these
FIGURE 4 TEM images for A, HAP and B, [MoO2(acac)]@HAP

6 of 11
GHANBARI RESKETI ET AL.
temperature due to the crystallinity of HAP nanoparti-
cles. No such major weight loss was observed at the
higher temperature. The thermogravimetric study,
heating under controlled atmosphere, explains the ther-
mal stability of these nanoparticles. All these results con-
firm that the material was synthesized, and the Mo (VI)
sites are directly anchored on the surface of HAPs.[39]
4.2 | Alkene epoxidation with
tert‐butylhydroperoxide catalyzed by
[MoO2(acac)]@HAP
FIGURE
[MoO2(acac)]@HAP
5
Thermogravimetric analysis of HAP and
In the catalytic epoxidation of alkenes, the choice of oxy-
gen donor and solvent is of crucial importance. To this
content, the influence of solvent, temperature, amount
of oxidant, amount of catalyst, and calcination time on
at 80°C to 170°C due to trapped physical water, and the
second weight loss is recorded at 180°C to 300°C
TABLE 2 Epoxidation of cis‐cylooctene under reflux conditions in the presence of [MoO2(acac)]@HAPa
Oxidant/Substrate
Entry Catalyst, mg
Solventb
Oxidant
Mo Loadingc, mol% Molar Ratio
Epoxide Yield, %d
1
[MoO2(acac)]@HAP (40) CH3CN
[MoO2(acac)]@HAP (40) CH3OH
TBHP
TBHP
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
0.4
0.9
1.3
1.8
2
2
No reaction
3
[MoO2(acac)]@HAP (40) (CH3)2CO TBHP
No reaction
4
[MoO2(acac)]@HAP (40) C6H5CH3
[MoO2(acac)]@HAP (40) CH2Cl2
[MoO2(acac)]@HAP (40) CHCl3
[MoO2(acac)]@HAP (40) C2H4Cl2
[MoO2(acac)]@HAP (40) C2H4Cl2
[MoO2(acac)]@HAP (40) CH3CN
[MoO2(acac)]@HAP (40) C2H4Cl2
[MoO2(acac)]@HAP (40) CH3CN
[MoO2(acac)]@HAP (40) C2H4Cl2
[MoO2(acac)]@HAP (40) CH3CN
[MoO2(acac)]@HAP (10) C2H4Cl2
[MoO2(acac)]@HAP (15) C2H4Cl2
[MoO2(acac)]@HAP (20) C2H4Cl2
[MoO2(acac)]@HAP (30) C2H4Cl2
[MoO2(acac)]@HAP (50) C2H4Cl2
[MoO2(acac)]@HAP (20) C2H4Cl2
[MoO2(acac)]@HAP (20) C2H4Cl2
[MoO2(acac)]@HAP (20) C2H4Cl2
[MoO2(acac)]@HAP (20) C2H4Cl2
TBHP
TBHP
TBHP
TBHP
H2O2
H2O2
55
24
62
95
2
5
6
7
8
9
5
10
11
12
13
14
15
16
17
18
19
20
21
22
H2O2.CON2H4 0.25
2
H2O2.CON2H4 0.25
5
e
NaIO4
NaIO4
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
0.25
0.25
0.064
0.09
0.12
0.19
0.31
0.12
0.12
0.12
0.12
2
3
75
77
95
95
95
46
66
95
95
aTime of reactions is 150 minutes.
bAmount of solvent is 3 mL.
cCalculated for 0.63 mmol cyclooctene.
dGC yields are based on the starting cyclooctene.
eTetra‐butyl phosphonium bromide was used as the phase transfer reagent.

GHANBARI RESKETI ET AL.
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TABLE 3 Epoxidation of some olefins with TBHP catalyzed by [MoO2(acac)]@HAP in refluxing conditiona
[MoO2(acac)]@HAP Catalystb
Entry
Alkene
Conversion (%)
Epoxide (%)
Selectivity (%)
Time (h)
TONg
TOFh
1
Cyclooctene
Cyclohexene
1‐Octene
95
95
95
100
2
2
6
4
4
2
2
2
5
2
3
771
748
567
716
630
669
574
732
771
708
748
385
374
94
2
95
100
3
72
72
100
4
1‐Hexene
91
91
100
179
157
334
287
366
154
354
249
5
Styrene
86c
95d
73
80
93
6
α‐Methyl styrene
α‐Pinene
85
89
7
73
100
8
1‐Hepten
93
93
100
9
Trans‐stilbene
Cis‐stilbene
Indene
98e
90e
95
98 (trans)f
100
10
11
90
95
82 (cis), 8 (trans)i
100
aReaction conditions: alkene (0.63 mmol), TBHP (0.84 mmol), catalyst (20 mg), 1,2‐dichloroethane (3 mL).
bGC yield based on the starting alkenes.
cThe by‐product is 7% for reaction.
dThe by‐product is 11% for catalyst.
eBoth the 1H NMR and GC data approved the reported yields.
fTrans‐isomer is the only product according to the 1H NMR data.
gTurn‐over number (TON) = (mol product/mol catalyst).
hTurn‐over frequency (TOF) = [mol of epoxide/(mol of catalyst)] per reaction time.
iCis and trans isomers are produced according to1H NMR data.
the epoxidation reaction was evaluated in order to find
the optimized situation for the epoxidation reaction. The
catalytic activity of the supported Mo (IV) complex on
the HAP surfaces was first monitored in the epoxidation
of cyclooctene as a model substrate in the presence of dif-
ferent oxidants.
TBHP to bind to the molybdenum center. However, the
solvent containing chlorine atom(s) has a positive effect
on the catalytic activity. Clearly, the best result was
obtained in the presence of 1,2‐dichloroethane with the
highest conversion (95%) at 2 hours.
Also, the effect of the nature of peroxide on the cata-
lytic activity of [MoO2(acac)]@HAP was investigated
(Table 2, entries 8‐13). TBHP with three other peroxides
using cyclooctene as a model substrate under the opti-
mized reaction conditions were used. In the presence of
The solvent has
a
significant effect on the
[MoO2(acac)]@HAP activity (Table 2, entries 1‐7). Appar-
ently, highly coordinating solvents cause a significant
decrease in the catalytic activity since they compete with
FIGURE 7 Epoxidation of 0.63 mmol of cis‐cyclooctene with
0.84 mmol of TBHP catalyzed by 20 mg of [MoO2(acac)]@HAP in
3‐mL C2H4Cl2 under reflux conditions
FIGURE 6 Effect of calcination temperature on epoxidation of
cyclooctene catalyzed by [MoO2(acac)]@HAP for 2 h

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GHANBARI RESKETI ET AL.
TABLE 5 Reusability of [MoO2(acac)]@HAP in epoxidation of
cyclooctene under reflux conditionsa
Mo
Epoxide, Time, Leached,
TOF,
TONd h−1e
b
c
Entry
%
h
%
First recycle
95
2
2
2
2
0.8
0.4
0
748
724
700
669
374
362
350
334
Second recycle 92
Third recycle 89
Fourth recycle 85
0
aReaction conditions: cyclooctene (0.63 mmol), TBHP (0.84 mmol), catalyst
(20 mg, 0.0008 mmol), C2H4Cl2 (3 mL).
bGC yield based on the starting alkene.
cDetermined by ICP.
dTON = (mol product/mol catalyst).
eTOF) = [mol of epoxide/(mol of catalyst)] per reaction time.
SCHEME 2 Proposed mechanism for epoxidation of cyclohexene
with TBHP catalyzed by [MoO2(acac)]@HAP
The presence of a metallic center causes to increase the
electrophilicity of the oxidant to transfer the oxygen atom
in the C═C bond to the corresponding olefin.[41]
H2O2, urea hydrogen peroxide, and sodium periodate
(NaIO4), no significant product formation was observed
either in acetonitrile or in C2H4Cl2; only a small amount
of cyclooctene oxide was produced in the reaction
medium, but TBHP cyclooctene oxide was found as the
only product in 94% yield. The high efficiency of TBHP
rather than H2O2 could be attributed to TBHP weaker
O―O bonds with respect to H2O2. Furthermore, the by‐
product of the reaction (t‐butyl alcohol) is easily sepa-
rated by distillation, and it can be recycled or used for
other industrial processes.[40] In addition, under the mild
conditions, the epoxidation of cyclooctene using TBHP
did not proceed in the absence of the catalyst.
Alkene conversion to epoxide was influenced by
changing the amount of catalyst loaded. Interestingly,
an epoxide production increase was observed with an
increase in the amount of catalyst loading to 20 mg
(0.0008 mmol), but no change was observed with a fur-
ther increase in the amount of catalyst up to 50 mg
(0.002 mmol) (Table 2, entries 14‐18). The ratio of oxidant
to substrate is one of the most crucial parameters in the
catalytic epoxidation processes (Table 2, entries 19‐22).
The new catalyst [MoO2(acac)]@HAP can also be
used for the epoxidation of a wide range of substituted
alkenes (Table 3). Blank experiments performed without
the catalyst or HAP show a very low conversion even
after 6‐hour reflux. However, in the presence of the cata-
lyst, a wide range of both cyclic and linear alkenes can
efficiently and selectively be converted to epoxides.
Sobczak and co‐worker have suggested that a higher
electron‐donating ability of the olefin double bond is
expected to show a greater epoxidation reactivity.[39]
Therefore, cyclooctene and cyclohexene with inner dou-
ble bonds should exhibit a greater activity in comparison
to 1‐hexene and 1‐octene, which contain terminal double
bonds. Moreover, the present catalytic system is
completely selective to the epoxide for all substrates
except for α‐methyl styrene and methylstyrene. Interest-
ingly, epoxidation of trans‐stilbene produced the trans‐
stilbene epoxide in 100% yield. This reflects a high selec-
tivity of this catalyst in the epoxidation of alkenes.
Furthermore, the effect of the calcination temperature
of HAP on the catalytic activity of [MoO2(acac)]@HAP
TABLE 4 Comparison between this work and those previously reported in the literature for epoxidation of alkenes
Catalyst
Time, h
Conversion, %
Selectivity, %
Reference
[46]
MCM‐41‐SB‐MoO2(acac)
MoO2(acac)2 supported on polybenzimidazole
Mo‐IFBNPs
2
94
90
90
93
97
87
95
100
97
[47]
[48]
[49]
[49]
[50]
2
2
100
95
Mo‐Im‐BNPs
1
Mo‐A‐BNPs
3.5
9
97
MoO2alaacacAmpMCM‐41
[MoO2(acac)]@HAP
99
2
100
This work

GHANBARI RESKETI ET AL.
9 of 11
was investigated. The calcination temperature‐dependent
curves in Figure 6 confirm a low epoxidation yield at a low
temperature (100°C); a significant growth in the product
yield was observed at higher temperatures (400°C‐900°C).
Figure 7 shows the effect of reaction time on the epox-
idation of cyclooctene with TBHP. Clearly, the epoxide pro-
duction increased during the time, and after 2 hours, the
conversion reached a maximum. However, after 2 hours,
no significant increase was observed in the epoxide yield.
On the basis of the Sobczak's ideas and other experimen-
tal and theoretical reports,[42] probable mechanisms can be
proposed for the epoxidation of cyclooctene with TBHP cat-
alyzed by [MoO2(acac)]@HAP. The proposed mechanism
for alkene epoxidations is illustrated in scheme 2. In the first
step, pending molybdenum complex is oxidized, and an
oxo‐molybdenum complex is formed. The next step for both
catalysts involves transferring the TBHP proton to a termi-
nal oxygen atom of these oxo complexes, which results in
the coordination of tert‐butylperoxide anion to Lewis acidic
metal centers.[43] Then, the olefin substrate coordinated to
the metal center and, as a nucleophile, is inserted to the
metal‐oxygen bond of coordinated peroxide electrophile
anion.[44] This mechanism easily explains the faster
reaction of electron‐rich olefins in comparison to electron‐
poor olefins. In the next step, epoxide product is formed
and tert‐butylperoxide anion is converted into tert‐butoxide
anion. Then, the peroxide product is released, and the cata-
lytic cycle continued by substitution of a new tert‐
butylperoxide instead of tert‐butoxide anion.[45]
The results obtained using the [MoO2(acac)]@HAP cat-
alyst in the epoxidation of alkenes were compared with
those reported for some other supported Mo catalysts
(Table 4). As it can be seen, in most cases, our reported
method is superior in terms of reaction time, reaction yield,
and epoxide selectivity. Also, in most cases, higher TOFs
were obtained by our reported method.[47–50]
As discussed earlier, the reusability of solid supported
catalysts is one of their most important benefits. There-
fore, the reusability of the [MoO2(acac)]@HAP catalyst
was monitored by means of multiple sequential epoxida-
tions of cyclooctene with TBHP (Table 5, Figure 8). It is
clear that the supported catalysts could be successfully
recycled for four times without a significant decrease in
activity or selectivity under the test conditions.
The solution and recovered catalysts were analyzed by
the ICP analysis technique. The results obtained
FIGURE 8 A, Basic XRD pattern and B, recovered XRD pattern of [MoO2(acac)]@HAP nanocatalysts; C, FESEM image of [MoO2(acac)]
@HAP nanocatalysts after fourth cycles and D, the recycling experiment of [MoO2(acac)]@HAP catalyst for oxidation of cis‐cyclooctene at
optimum conditions

10 of 11
GHANBARI RESKETI ET AL.
ACKNOWLEDGEMENT
The authors wish to thank the Vice‐President's Office for
Research Affairs in the University of Shahrood for
supporting this work.
ORCID
Bahram Bahramian
https://orcid.org/0000-0002-0665-
2697
Mahdi Mirzaee https://orcid.org/0000-0003-4093-8944
FIGURE
homogeneous reaction pathway
9
Sheldon's test for study of heterogeneous or
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How to cite this article: Ghanbari Resketi F,
Salehi S, Bahramian B, Mirzaee M. Immobilization
of [MoO2(acac)2] on surface of hydroxyapatite
nanoparticles: A heterogeneous and reusable
catalyst for olefin epoxidation reactions. J Phys Org
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