Synthesis, characterization, and catalytic activity of supported molybdenum Schiff base complex as a magnetically recoverable nanocatalyst in epoxidation reaction

Article abstract of DOI:10.1080/00958972.2015.1137290

A heterogeneous nanocatalyst was prepared via covalent anchoring of dioxomolybdenum(VI) Schiff base complex on core-shell structured Fe₃O₄@SiO₂. The properties and the nature of the surface-fixed complex have been identifi

Full text of DOI:10.1080/00958972.2015.1137290

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, characterization and catalytic activity
of supported molybdenum Schiff base complex
as a magnetically–recoverable nanocatalyst in
epoxidation reaction
Zeinab Moradi–Shoeili, Maryam Zare, Mojtaba Bagherzadeh, Saim Özkar &
Serdar Akbayrak
To cite this article: Zeinab Moradi–Shoeili, Maryam Zare, Mojtaba Bagherzadeh, Saim Özkar
& Serdar Akbayrak (2016): Synthesis, characterization and catalytic activity of supported
molybdenum Schiff base complex as a magnetically–recoverable nanocatalyst in epoxidation
reaction, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2015.1137290
To link to this article: http://dx.doi.org/10.1080/00958972.2015.1137290
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Jan 2016.
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Date: 12 January 2016, At: 22:54

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2015.1137290
Synthesis, characterization and catalytic activity of supported molybdenum
Schiff base complex as a magnetically–recoverable nanocatalyst in
epoxidation reaction
ZEINAB MORADI–SHOEILI*1, MARYAM ZARE2, MOJTABA BAGHERZADEH3, SAIM ÖZKAR4 and
SERDAR AKBAYRAK4
1Department of Chemistry, Faculty of Sciences, University of Guilan, P.O. Box 41335–1914, Rasht, Iran
2Materials Science & Engineering Department, Golpayegan University of Technology,
P.O. Box 8771765651, Golpayegan, Iran
3Department of Chemistry, Sharif University of Technology, Azadi Ave., P.O. Box 11155–3516, Tehran, Iran
4Chemistry Department, Middle East Technical University, 06800 Ankara, Turkey
A heterogeneous nanocatalyst was prepared via covalent anchoring of dioxomolybdenum(VI)
Schiff base complex on core–shell structured Fe3O4@SiO2. The properties and the nature of the
surface–fixed complex have been identified by a series of characterization techniques such as
SEM, EDX, XRD, TGA, FT–IR and VSM. The synthesized hybrid material was an efficient
nanocatalyst for selective oxidation of olefins to corresponding epoxides with t–BuOOH in high
yields and selectivity. The catalyst could be conveniently recovered by applying an external
magnetic field and reused several times without significant loss of efficiency.
Keywords: Molybdenum complex; Magnetic nanoparticles; Heterogeneous catalyst; Schiff base;
Epoxidation reaction
1. Introduction
Epoxidation of alkenes and arenes is important in organic synthesis and industrial production due
to its applications in the manufacturing of a variety of valuable materials [1, 2]. Design and
fabrication of a catalyst with high selectivity, stability and recyclability is challenging. Among
*Corresponding author. Email: zmoradi@guilan.ac.ir
1

the many transition metal complexes used as catalysts for selective epoxidation reactions [3-6],
molybdenum(VI) compounds with Schiff base ligands have been intensively investigated as
efficient homogeneous catalysts [7-10]. In most cases, the recovery of these kinds of catalysts
from the reaction medium is extremely difficult and associated with environmental pollution
[6, 11, 12]. As a result, heterogenizing of the catalytically active species to the surface of
traditional solid supports has attracted attention [13-15]. However, immobilization of the
homogeneous systems usually decreases the catalytic site accessibility for the substrate and
consequently reduces the catalytic activity [16]. Hence, providing new types of heterogeneous
catalysts that have an elevated rate of reaction and can be easily recovered from the reaction
medium is ongoing [17-20].
Magnetite Fe3O4 nanoparticles have attracted interest as support materials due to their
low toxicity, high saturation magnetization and biocompatibility [21, 22]. Unprotected magnetic
nanoparticles are found to be unstable and easily aggregate during catalytic transformations,
resulting in deduction of magnetic properties, catalytic capability and dispersibility [23]. An
approach which could effectively solve these problems is protecting the magnetic nanoparticles
with a non–magnetic and chemically stable shell such as silica [24-26]. In addition, the outer
shell of silica could be easily functionalized with various ligands which can also provide efficient
binding sites to numerous catalytic species including transition metal complexes [13, 27].
Therefore, silica–coated magnetic nanoparticles (Fe3O4@SiO2), as supports, have attracted
interest to provide magnetically reusable heterogeneous catalysts [16, 28-30].
In this study, the synthesis and structural characterization of a new core–shell structured
nanomaterial Fe3O4@SiO2–NHAlMo are described. The catalytic properties of the nanocatalyst
were investigated in the epoxidation of olefins using t–BuOOH as oxygen source, which
exhibited excellent catalytic performance with high yields and selectivity, mild reaction
conditions and short reaction times. The catalyst could be conveniently separated using an
external magnet and efficiently recovered several times without a significant loss in catalytic
activity.
2

2. Experimental
2.1. Materials and methods
Air–sensitive operations were carried out using standard techniques under a nitrogen atmosphere.
All reagents were obtained from commercial sources and used as received. Bis(acetylacetonato)
dioxomolybdenum(VI), [MoO2(acac)2] [31] was prepared as described in the literature. Fe3O4
magnetite nanoparticles [32] and silica–coated Fe3O4 (Fe3O4@SiO2) [33] were prepared
following the previously published procedure. Powder X-ray diffraction (PXRD) data were
collected with a Philips X’Pert MPD diffractometer (Co-Kα [au: please check] X-radiation, λ =
1.79 Å). The metal loading of the host materials and leaching of molybdenum into the solution
were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a
Perkin-Elmer AA-300 spectrophotometer. FT-IR spectra of samples as KBr pellets were
recorded using an ABB Bomem MB-100 FT-IR spectrophotometer; FT-IR spectra from catalytic
studies were taken using a Unicam Matson 1000 FT-IR spectrophotometer. Scanning electron
microscopy (SEM) images were taken on a KYKY-EM3200 scanning electron microscope. TGA
and DTA analyses were carried out with a Thermo Gravimetric System Mettler Toledo
TGA/DSC1 at a heating rate of 5 °C min−1 under air. Elemental analysis was recorded with an
energy dispersive X-ray (EDX) analyzer (KEVEX Delta series) mounted on the Hitachi S-800. A
vibration sample magnetometer (VSM) was used to measure the magnetic properties of the
samples at room temperature. Ni was used as a standard. Gas chromatographic (GC) analyses
were performed on an Agilent Technologies 6890N, 19019 J-413 HP-5, 5% phenyl methyl
siloxane, capillary column 60.0m×250 μm×1.00 μm.
2.2. Synthesis of the Fe3O4@SiO2–NH2
The core–shell Fe3O4@SiO2 nanospheres were modified with 3–aminopropyltriethoxysilane
(APTES) according to the procedure described previously [34]. 1 g of prepared Fe3O4@SiO2
nanoparticles was sonicated in 100 mL dry toluene for 30 min to achieve a uniform dispersion.
Subsequently, 1 mL of APTES was added under vigorous stirring at room temperature for 24 h.
The solid material was washed by toluene and ethanol several times and dried in an oven at
70 °C for 8 h.
3

2.3. Synthesis of Fe3O4@SiO2–NHAl
Supported Schiff base was prepared by addition of 5–bromo salicylaldehyde (0.4 g, mmol) to a
mixture of Fe3O4@SiO2–NH2 (1 g) in ethanol (100 mL). The resultant mixture was refluxed for
24 h. The product was magnetically separated and washed with hot ethanol several times to
remove unreacted species and dried at 70 °C for 6 h. This product will be referred as
Fe3O4@SiO2–NHAl.
2.4. Synthesis of Fe3O4@SiO2–NHAlMo nanoparticles
260 mg of MoO2(acac)2 was dissolved in 50 mL ethanol and Fe3O4@SiO2–NHAl (0.4 g) was
then added to this solution. The resultant mixture was refluxed for 24 h. The solid was
magnetically separated and then washed several times with hot ethanol. The final product was
dried in an oven at 70 °C for 8 h to afford supported molybdenum catalyst, Fe3O4@SiO2–
NHAlMo.
2.5. Catalytic reactions
The liquid–phase olefin epoxidation reactions were carried out in air and autogenous pressure in
a 5 mL batch micro–reactor equipped with a stir bar and a sampling valve, and immersed in a
thermostated oil bath. Typically, a mixture of the substrate (1 mmol), Fe3O4@SiO2–NHAlMo
(10 mg), chlorobenzene (1 mmol) as internal standard and C2H4Cl2 (1 mL) was stirred in a 5 mL
tube for 10 min at 84 °C. Then 182 μL of 5.5 M solution of t–BuOOH in decane (1 mmol) was
added and the system was stirred for 20-60 min under reflux. The progress of the reaction was
monitored by GC. All the reactions were performed at least two times and the products were
compared with standard samples.
3. Results and discussion
3.1. Catalyst preparation and characterization
The synthetic procedure for the Fe3O4@SiO2–NHAlMo nanocomposite is illustrated in
scheme 1. The magnetic Fe3O4 nanoparticles of approximately 40 nm diameter were synthesized
by co–precipitation method. In order to prevent decomposition and aggregation, magnetic
nanoparticles were subsequently coated with a silica shell using tetraethoxysilane (TEOS) as the
silica source. The silica layered system, Fe3O4@SiO2 was functionalized in two steps using
4

3-aminopropyltriethoxysilane (APTES) and 5-bromo salycilaldehyde (scheme 1) which affords
suitable chelating Schiff base ligand for grafting of the molybdenum metal on the surface of the
nanoparticles. Then, the prepared nanospheres were reacted with MoO2(acac)2 in ethanol for
24 h to yield the corresponding Fe3O4@SiO2–NHAlMo nanoparticles.
NH2
Si
O
O
O
FeCl3. 6H2O
+
FeCl2. 4H2O
(EtO)3Si
NH2
TEOS
NH4OH
N2
dry toluene, 24h
Fe3O4
nanoparticles
Fe3O4@SiO2
Fe3O4@ SiO2-NH2
core shell
Br
EtOH, reflux
O
HO
Br
Br
N
N
O
O
Si
O
O
Si
O
O
O
HO
Mo
O
MoO2(acac)2
EtOH, reflux
O
O
O
Fe3O4@ SiO2-NHAl
Fe3O4@ SiO2-NHAlMo
nanoparticles
Scheme 1. Synthetic pathways for the synthesis of Fe3O4@SiO2–NHAlMo nanocomposite.
Figure 1 represents the FT–IR spectra of synthesized materials which are in agreement
with the proposed structure shown in scheme 1. The FT–IR spectrum for Fe3O4 nanoparticles
(figure 1a) shows strong bands at 3433 and 1623 cm–1, indicating water molecules adsorbed to
the surface. Significant absorption at 581 cm–1 (evident in all spectra) and 1061 cm–1 (figures 1b,
1c and 1d) were attributed to stretching frequency of Fe–O and Si–O, respectively [35]. These
peaks in FT–IR spectra reveal the formation of Fe3O4 nanoparticles which were successfully
coated with SiO2 layer. The sharp band at 1635 cm–1 (figure 1c) which was assigned to ν(C=N)
has shifted to the 1615 cm–1 range (figure 1d) in the spectra of the complex suggesting
coordination of nitrogen to molybdenum, in agreement with literature values [36, 37]. The band
5

at 1539 cm−1 for Fe3O4@SiO2–NHAlMo nanoparticles (figure 1d, inset) is not observed for
Fe3O4@SiO2–NHAl. This band could correspond to the vibration of the C=O group of acac,
suggesting involvement of acac in coordination of molybdenum [38]. Two strong absorptions at
903 and 939 cm–1 (figure 1d) are indicative of symmetrical and anti–symmetrical stretch of two
cis doubly–bonded oxygens at the molybdenum core [39].
The structures of the Fe3O4, Fe3O4@SiO2–NHAl and Fe3O4@SiO2–NHAlMo
nanoparticles were determined by powder X–ray diffraction (XRD). As shown in figure 2, the
diffraction patterns and relative intensities of all the peaks are in agreement with those of
magnetite which can be indexed to the face–centered cubic phase of Fe3O4 [23, 40, 41]. The
broad peak between 20–30° in figures 2b and c indicates the amorphous structure of silica layer.
In the XRD pattern of Fe3O4@SiO2–NHAl and Fe3O4@SiO2–NHAlMo, the characteristic peaks
of Fe3O4 did not change, revealing that the structure of Fe3O4 is well maintained, after being
coated with silica. No obvious reflections for SiO2 and Mo complex are observed, which
indicates that they are relatively thin amorphous layers on the surface of Fe3O4 nanospheres.
The morphology and sizes of Fe3O4 and Fe3O4@SiO2–NHAlMo were observed by
scanning electron microscope (SEM). Fe3O4 nanoparticles are relatively monodisperse particles
with an average diameter of about 40 nm (figure 3a). As shown in figure 3b, the nanocomposite
particles were relatively uniform with a thickness of approximately 50 nm, indicating the
successful coating of silica around each magnetic Fe3O4 core particle. Furthermore, SEM images
showed that the nanoparticles still maintain their spherical shape after the coating procedures
with silica and molybdenum complex. The EDX spectrum of Fe3O4@SiO2–NHAlMo presented
in figure 3c clearly demonstrates the presence of molybdenum in the composite. Using elemental
microanalysis results (wt%) of the sample measured by EDX, the typical loading of the
molybdenum moieties on the surface is 0.79 mmol/g.
Thermogravimetric analysis of the Fe3O4@SiO2–NHAlMo nanoparticle was carried out
from 25–800 °C at a heating rate of 10 °C min–1 (figure 4). The complex showed weight losses at
temperature below 150 °C that could be due to the removal of physically adsorbed water
molecule and/or organic solvents which were adsorbed during the synthesis of nanoparticles.
With further increase in temperature, the TG curves show two steps of weight loss. The first of
1.52% observed from 200 to 260 °C may be ascribed to desorption of the chemisorbed water on
6

the silica surface [42]. At the last stage, the mass losses of 7.51% at 300–440 °C can be
attributed to complete removal of the fragment parts of the ligand on the surface.
The magnetic properties of Fe3O4 and the Fe3O4@SiO2–NHAlMo nanocomposite were
investigated using vibrating sample magnetometer (VSM) at room temperature in an applied
magnetic field sweeping from −10 to 10 kOe. The magnetization curves of the samples are
shown in figure 5. The absence of hysteresis phenomenon indicates that the magnetic particles
have superparamagnetic behavior at room temperature. The results show that Fe3O4 and
Fe3O4@SiO2–NHAlMo nanoparticles have saturated magnetization values of 61.0 and
45.6 emu/g, respectively. Compared with Fe3O4 nanoparticles, a decrease of about 15.4 emu/g in
saturation magnetization was observed for Fe3O4@SiO2–NHAlMo nanocomposite which can be
attributed to increase in mass and size after the coating of magnetite nanoparticles with silica and
anchoring the Mo(VI) catalyst. Nevertheless, the final synthesized nanocatalyst can still be
efficiently separated from the solution using an external magnet.
3.2. Catalytic epoxidation of olefins
Catalytic studies were initiated by testing the activity of Fe3O4@SiO2–NHAlMo nanocatalyst in
oxidation of cyclooctene using t–BuOOH as an oxidant. This reaction was best carried out using
10 mg catalyst and 1 mmol of t–BuOOH for 20 min under reflux in dichloroethane. The catalytic
ability of the magnetite nanoparticle–supported catalyst was evaluated in catalyzing a series of
olefins in the optimized conditions. As summarized in table 1, different alkenes are generally
excellent substrates for these catalysts. The catalytic oxygenation of cyclohexene with t–BuOOH
was carried out in the presence of material for 30 min under reflux. The selective conversion of
cyclohexene to the corresponding epoxide indicates that the reaction proceeds via a non–radical
mechanism [43, 44]. The least reactive aliphatic terminal alkenes such as n–octene and n–
heptene were catalytically oxygenated in high yields and excellent selectivities. The olefins
conjugated with a phenyl group such as α–methylstyrene and 4–methylstyrene are converted into
these epoxides in moderate yield. A proposed mechanism of the reaction is shown in scheme 2,
postulated based on the result of the study and previously reported investigations by Thiel [16].
The catalytic reaction involves transfer of one t–BuOOH proton to one oxo group of dioxo
complex and coordination of t–BuOO anion to the Lewis acidic metal center. Nucleophilic attack
of an olefin to one oxygen of the peroxo ligand, as an electrophile, yields epoxide and t-butanol
7

as a by-product. The six-coordinate species is subsequently activated for oxygen transfer to the
olefin.
Br
N
OH
O
,
O
O
Si
O
O
O
O
O
OH
Mo
O
O
O
O
Si
Si
O
O
Br
N
O
N
O
Br
O
O
O
O
O
O
O
O
O
O
O
O
H
OBu
H
OBu
Scheme 2. Proposed mechanism for oxidation of alkenes catalyzed by Fe3O4@SiO2–NHAlMo in
the presence of t–BuOOH.
The recyclability of the modified catalyst was examined by reusing the material isolated
from a previous run in the next run of epoxidation of cyclooctene under the same conditions. For
the reusability test, the catalyst was isolated from the reaction solution after complete
epoxidation of cyclooctene by external magnet, washed with C2H4Cl2, and dried at room
temperature. The conversions at 20 min were similar for all six runs and selectivity to the
epoxide remained very high (table 2). The leaching test was carried out after the catalyst was
reused six times, the reaction mixture was filtered, and ICP measurement of the filtrate showed
that no detectable molybdenum can be found, indicating that almost no leaching of the catalyst
can be observed. In order to verify further whether the observed catalysis was truly
heterogeneous, the solid was filtered off from the reaction mixture and the fresh alkene and
oxidant was added to filtrate and no reaction was observed. Increase in the conversion in run 2
with respect to run 1 (table 2) could be attributed to trace amount of Mo(VI) leached from the
catalyst into the solution.
The IR spectrum of Fe3O4 supported catalyst before and after six reuses shows no major
changes (figure 6). The spectrum contained medium intensity bands at 2856 and 2976 cm–1,
attributed to t-butanol formed during the epoxidation process. The used catalyst may therefore
8

possess some active metal centers coordinated with t–butanol which could account for slight
decrease in the olefin conversion rate.
Table 3 summarizes the comparison of catalytic activity of Fe3O4@SiO2–NHAlMo in
conversion of olefins to epoxides using t-BuOOH, with some of the reported molybdenum(VI)
nanocatalysts. From these results, it is clear that Fe3O4@SiO2–NHAlMo is more efficient and
effective than other heterogeneous catalytic systems.
4. Conclusion
Functionalized iron oxide nanoparticle was prepared and used as a support for immobilizing the
dioxomolybdenum Schiff base complex. The catalytic activity of the prepared nanocatalyst was
studied for selective oxidation of olefins to corresponding epoxides using t–BuOOH. The
catalyst in olefin epoxidation reactions give high yield and selectivity of the products, in most
cases. After completion of the reaction, the catalysts could be easily recovered by simple
magnetic decantation. The heterogeneous nanocatalyst was chemically stable and can be reused
at least six times with nearly constant catalytic activity; no metal leaching was detected after the
consecutive catalytic reactions. Therefore, chemical stability, high activity, short reaction time,
convenient recoverability and reusability make Fe3O4@SiO2–NHAlMo nanocomposite a
valuable catalyst in industrial applications.
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11

Table 1. Fe3O4@SiO2–NHAlMo catalyzed epoxidation of different olefins by
t−BuOOHa.
Time
(min)
Entry
Substrate
Conversionb%
Selectivityb%
1c
2
3
4
5
6
7
8
cyclooctene
cyclooctene
cyclohexene
n-heptene
n-octene
26
98
98
97
94
99
100
98
60
20
30
30
60
60
60
60
100
100
100
45
styrene
4-methoxystyrene 50
72
α–methylstyrene 62
60
aReaction condition: 10 mg catalyst: 1 mmol alkene: 1 mmol t–BuOOH. The
reactions were run under reflux in 1 mL dichloroethane. bGC yield was determined
by GC with respect to an internal standard (chlorobenzene). cWithout the catalyst.
12

Table 2. Reuses of Fe3O4@SiO2–NHAlMo in the selective oxidation of cyclooctenea
Run
Conversionb%
Selectivityb%
1
2
3
4
5
6
98
98
98
98
98
98
98
99.6
99.6
95
94
94
aReaction condition: 10 mg catalyst: 1 mmol cyclooctene: 1 mmol
t−BuOOH. The reactions were run under reflux for 20 min. bThe
conversion and selectivity were determined by GC with respect to an
internal standard (chlorobenzene).
13

Table 3. Comparison of the Fe3O4@SiO2–NHAlMo catalytic activity with those of reported
molybdenum nanocatalysts.
Temp.
C)
Molar ratio
cat:sub:ox
Conversion
%
Entry
Catalyst
Time
Ref.
(
°
1
2
3
4
5a
6
7
Fe3O4@SiO2–NHAlMo
Mo-MCM-41
0.008.:1:1
1:68:205
Reflux
40
20 min 98
This work
3 h
6 h
6 h
12 h
2 h
2 h
97
11
16
16
40
45
45
1Mo@SMNP
0.01:1:1.2
0.01:1:1.2
Reflux
Reflux
96
2Mo@SMNP
58
Mo-Fe3O4@SiO2@P4VP
MCM-41-MoO2Cl(THF)2
MCM-41-L/MoO2Cl2
0.059:3.5:22 60
0.082:7.3:11 55
0.082:7.3:11 55
85.6
31
29
a H2O2 has been used as oxidant.
14

Figure 1. FT–IR spectra of (a) Fe3O4, (b) Fe3O4@SiO2, (c) Fe3O4@SiO2–NHAl and
(d) Fe3O4@SiO2–NHAlMo (inset: high resolution spectrum of 1d).
15

Figure 2. The XRD patterns of (a) Fe3O4, (b) Fe3O4@SiO2–NHAl and (c) Fe3O4@SiO2–
NHAlMo.
16

Figure 3. SEM images of (a) Fe3O4, (b) Fe3O4@SiO2–NHAlMo and (c) the EDX pattern of
Fe3O4@SiO2–NHAlMo.
17

Figure 4. Thermogravimetric curves of Fe3O4@SiO2–NHAlMo nanocomposite.
18

Figure 5. VSM magnetization curves of (a) Fe3O4 microspheres and (b) Fe3O4@SiO2–NHAlMo
nanocomposite (inset: photos for magnetic separation of catalyst by external magnet).
19

Figure 6. IR spectra of (a) Fe3O4@SiO2–NHAlMo and (b) used catalyst after the 6th cycle.
20

Graphical abstract
21