A new magnetically recoverable nanocatalyst for epoxidation of olefins

Article abstract of DOI:10.1016/j.molcata.2011.08.007

In this work, a new magnetically recoverable nanocatalyst was developed by covalent binding of a Schiff base ligand, N,N′-bis(3- salicylidenaminopropyl)amine (salpr), on the surface of silica coated magnetite nanoparticles (SCMNPs) and followed complexation with MoO₂(acac) ₂. Characterization of the prepared nanocatalyst was performed with different physicochemical methods such as FT-IR and atomic absorption spectroscopies, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). Finally, catalytic activity of the prepared MoO₂salpr/SCMNPs was examined in the epoxidation of olefins with tert-butyl hydroperoxide (TBHP) and cumene hydroperoxide (CHP).

Full text of DOI:10.1016/j.molcata.2011.08.007

Journal of Molecular Catalysis A: Chemical 348 (2011) 83–87
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A new magnetically recoverable nanocatalyst for epoxidation of olefins
∗
M. Masteri-Farahani , N. Tayyebi
Faculty of Chemistry, University of Tarbiat Moallem, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 April 2011
Received in revised form 25 June 2011
Accepted 15 August 2011
Available online 22 August 2011
In this work, a new magnetically recoverable nanocatalyst was developed by covalent binding of a Schiff
base ligand, N,N -bis(3-salicylidenaminopropyl)amine (salpr), on the surface of silica coated magnetite
ꢀ
nanoparticles (SCMNPs) and followed complexation with MoO2(acac)2. Characterization of the prepared
nanocatalyst was performed with different physicochemical methods such as FT-IR and atomic absorp-
tion spectroscopies, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron
microscopy (TEM) and vibrating sample magnetometry (VSM). Finally, catalytic activity of the prepared
MoO2salpr/SCMNPs was examined in the epoxidation of olefins with tert-butyl hydroperoxide (TBHP)
and cumene hydroperoxide (CHP).
Keywords:
Magnetite
Nanoparticle
Molybdenum
Immobilization
Epoxidation
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
activity due to limited diffusion of substrates through the narrow
pores of the support.
In recent years, nanosized materials have attracted great
Due to the high surface to volume ratio of the nanoparticles
surface modification of nanoparticles is an alternative rout for
preparation of highly active catalysts. Moreover, diffusion limita-
tions can be avoided, as all of the surface area of the nanoparticles
is external. However, the recovery of these modified nanoparticles
with filtration methods is very difficult. In this respect, immobiliza-
tion of homogeneous catalysts on modified magnetic nanoparticles
such as iron oxides gives heterogenized catalysts that can be quickly
and easily recovered in the presence of external magnetic fields
[11,27–33].
In this work, our objective is to functionalize magnetite
nanoparticles with chloropropyl groups and then incorporate a
molybdenum Schiff base complex with anchoring method to obtain
a heterogeneous molybdenum catalyst for the epoxidation of
olefins. To the best of our knowledge there is no any report about
preparation of such magnetically recoverable nanocatalyst for the
epoxidation of olefins. The advantage of this system is facile and
fast recovery of the solid catalyst at the end of reaction as well as
good activity and selectivity for epoxidation of olefins.
academic interests because of their special properties, such as rel-
atively large surface area and surface to volume ratio [1–7]. Among
them magnetic nanoparticles are widely used in the various fields
such as metal ion separations, enzyme immobilization, magnetic
resonance imaging (MRI), drug delivery and catalysis [8–15]. The
magnetite Fe O nanoparticles have superparamagnetic property
3
4
which enables them to attract to an external magnetic field, but
retain no residual magnetism after removing the magnetic field.
Thus, suspended magnetic nanoparticles can be adhered and then
removed very quickly from the sample solutions with utilization
of a magnetic field and do not agglomerate after removing the
external field. As the surface of Fe O4 nanoparticles are relatively
3
inert to chemical reactions, functionalization with reactive species
should be made to facilitate incorporation of other molecules on
the surface of the Fe O4 nanoparticles [16–18].
3
On the other hand, it has been found that dioxomolybdenum (VI)
complexes of multidentate Schiff base ligands are selective homo-
geneous catalysts for the epoxidation of olefins. Many attempts
have been done for heterogenization of molybdenum catalysts
within different supports such as silicates, zeolites and other molec-
ular sieves [19–26] to facilitate the recovery and reuse of the
catalysts. But, the resulted catalysts suffer from decreased catalytic
2. Experimental
2.1. Materials and instrumentation
Ferrous chloride tetrahydrate (FeCl ·4H O), ferric chloride
2
2
(
FeCl ·6H O), chloropropyl trimethoxysilane, tetraethyl orthosil-
3
2
∗ Corresponding author. Tel.: +98 261 4551023; fax: +98 261 4551023.
E-mail address: mfarahany@yahoo.com (M. Masteri-Farahani).
icate (TEOS), ammonia (25 wt.%) were purchased from Merck
chemical company. The salpr ligand was prepared according to
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.08.007

8
4
M. Masteri-Farahani, N. Tayyebi / Journal of Molecular Catalysis A: Chemical 348 (2011) 83–87
the reported method [34]. MoO (acac)2 was prepared according
2
to literature method [35].
Fourier transform infrared spectra were recorded using Perkin-
Elmer Spectrum RXI FT-IR spectrometer, using pellets of the
materials diluted with KBr. The transmission electron micrographs
(
TEM) of the nanoparticles were recorded using a Philips EM 208
S instrument with an accelerating voltage of 100 kV. Samples were
prepared for TEM by placing droplets of a suspension of the sam-
ple in acetone on a polymer microgrid supported on a Cu grid.
Scanning electron micrographs (SEM) of the samples were taken
with ZEISS-DSM 960A microscope with attached camera. Magnetic
susceptibility measurements were carried out using a vibrating
sample magnetometer (VSM) (BHV-55, Riken, Japan) in the mag-
netic field range of −8000 Oe to 8000 Oe at room temperature. The
crystalline phase of the nanoparticles were identified by means of
X-ray diffraction measurements using Cu k␣ radiation (ꢀ = 1.54 A˚ )
◦
on a SIEFERT XRD 3003 PTS diffractometer in the 2Â range of 10–80 .
Chemical analyses of samples were carried out with VARIAN VISTA-
MPX ICP-AES atomic absorption spectrometer.
Fig. 1. FT-IR spectra of (a) salpr/SCMNPs and (b) MoO2salpr/SCMNPs materials.
magnetic stirrer. Tert-butylhydroperoxide (TBHP) (obtained from
Merck as 80% in di-tertiary butyl peroxide) or cumene hydroper-
oxide (80% in cumene, Merck) were used as oxidants. In a typical
procedure, to a mixture of catalyst (100 mg) and olefin (8 mmol)
in chloroform (20 ml) was added oxidant (14.4 mmol) under nitro-
gen atmosphere and the mixture was refluxed for appropriate time.
Samples were withdrawn periodically and after dilution with chlo-
roform and cooling were analyzed using a gas chromatograph (HP,
Agilent 6890N) equipped with a capillary column (HP-5) and a FID
detector. Products were quantified using isooctane (1 g, 8.75 mmol,
Merck) as internal standard. GC–MS of products were recorded
using a Shimadzu-14A fitted with a capillary column (CBP5-M25).
The molybdenum content of recycled catalyst was measured with
above mentioned atomic absorption spectrometer after dissolution
of the solid in hydrogen fluoride solution.
2
.1.1. Preparation of silica coated magnetite nanoparticles
(
SCMNPs) and chloropropyl modified SCMNPs (ClpSCMNPs)
Magnetite nanoparticles (MNPs) were prepared according to
reported method [36]. Modification of the prepared MNPs was
performed as followed: The magnetite nanoparticles (1 g) were dis-
persed in deionized water in a 250 ml round-bottom flask with
sonication and then an aqueous solution of TEOS (10% (v/v), 80 ml)
was added, followed by glycerol (60 ml). The pH of the suspen-
sion was adjusted to 4.6 using glacial acetic acid, and the mixture
◦
was then stirred and heated at 90 C for 2 h under a nitrogen
atmosphere. After cooling to room temperature, the silica coated
magnetite nanoparticles was separated from the reaction mixture
using a permanent magnet and washed several times with distilled
water and methanol. The obtained SCMNPs (2 g) were suspended
in ethanol (100 ml) and then chloropropyl trimethoxysilane (2 ml,
Merck) was added under dry nitrogen atmosphere. The mixture
was refluxed for 12 h and the resulted solid was magnetically sep-
arated, washed with ethanol to remove the unreacted residue of
silylating reagent and then vacuum dried at 353 K.
3. Results and discussion
3
.1. Preparation of MoO salpr/SCMNPs
2
Scheme 1 shows the sequence of events in the functionaliza-
2.1.2. Preparation of salpr/SCMNPs
tion of MNPs with molybdenum Schiff base complex. In the first
step, the external surface of MNPs was coated with a silica shell to
obtain SCMNPs. Then, treatment of silanol groups of SCMNPs with
chloropropyl trimethoxysilane affords chloropropylated magnetite
nanoparticles (ClpSCMNPs). The second step involves substitution
reaction of ClpSCMNPs chloro groups with secondary amine group
of salpr Schiff base to yield the salpr/SCMNPs, a tetradentate Schiff
base ligand supported on SCMNPs. Addition of triethylamine in this
step facilitates the substitution reaction. Based on the literature
method [20], complexation of the salpr/SCMNPs was performed
ClpSCMNPs (2 g) was suspended in 100 ml of acetonitrile with
sonication. To this mixture was added excess of salpr ligand (1 g,
.2 mmol) and triethylamine (0.5 ml) and the resulted mixture was
refluxed for 12 h. The resultant solid was separated magnetically
and then washed with ethanol several times to remove the unre-
acted residue of the salpr ligand and dried under vacuum at 353 K.
3
2
(
.1.3. Preparation of MoO salpr modified SCMNPs
2
MoO salpr/SCMNPs)
2
For preparation of MoO salpr/SCMNPs, excess of MoO (acac)
2
2
2
with excess of MoO (acac)2 to afford MoO salpr/SCMNPs in the
(
500 mg, 1.5 mmol) was dissolved in ethanol (50 ml). Salpr /SCM-
2
2
third step. Soxhlet extraction of the product was carried out sub-
NPs (1 g, dried in vacuum oven at 353 K) was then added to this
solution and after sonication the mixture was refluxed for 12 h.
After separation with an external magnet, the product was washed
sequently in order to remove the unreacted MoO (acac)2 from the
2
resulted material.
with methanol to remove unreacted MoO (acac) . The resulted
2
2
MoO salpr/SCMNPs material was then dried under vacuum at
3.2. Characterization of the prepared nanomaterial
2
3
53 K and characterized with FT-IR spectroscopy, X-ray diffraction,
scanning and transmission electron microscopies and vibrating
sample magnetometry.
In order to confirm the modification of the magnetite sur-
face the FT-IR spectra of the prepared salpr/SCMNPs and
MoO salpr/SCMNPs materials were obtained and have been
2
2
.2. Catalytic epoxidation of olefins in the presence of
shown in Fig. 1. The observation of two broad bands at
−
1
MoO salpr/SCMNPs
around 430–584 cm
indicates the presence of magnetite core
2
in the prepared nanoparticles. The silica coating of magnetite
nanoparticles was confirmed by observation of a broad band
about 1000–1100 cm assigned to Si–O–Si and Si–OH stretching
Epoxidation of olefins (purchased from Merck) was carried out
in a 25 ml round bottomed flask equipped with a condenser and a
−
1

M. Masteri-Farahani, N. Tayyebi / Journal of Molecular Catalysis A: Chemical 348 (2011) 83–87
85
Scheme 1. The sequence of events in the preparation of MoO2salpr/SCMNPs.
vibrations. The presence of anchored propyl groups were confirmed
broad peaks indicate the nanocrystalline nature of the prepared
−
1
by stretching vibrations appeared at about 2923 cm in the FT-IR
spectrum of ClpSCMNPs (not shown here). In the FT-IR spectrum
MoO salpr/SCMNPs.
2
To study the magnetic properties of magnetite nanoparticles
before and after silica coating we investigated the hysteresis loops
of magnetite and functionalized magnetite nanoparticles at room
temperature using vibrating sample magnetometry (VSM).
Magnetization curves of prepared materials were shown in
Fig. 3. The magnetization curve of MNPs exhibited no remanence
effect (superparamagnetic property) with saturation magneti-
zation of about 60 emu/g. Also, the silica coated magnetite
nanoparticles showed superparamagnetic behavior with decreased
−
1
of salpr /SCMNPs (Fig. 1a) a band at 1631 cm , assigned to C
N
stretching vibration of salpr ligand [20,35], and some weak bands
−1
at 1400–1500 cm assigned to stretching vibrations of aromatic
rings of salpr ligand were observed that were not present in par-
−
1
ent SCMNPs. The band at 1631 cm upon reaction of slapr ligand
−
1
with MoO (acac) shifted to lower frequency (1617 cm ) (Fig. 1b)
2
2
indicating complexation of C N group of supported ligand with
molybdenum. Also, appearance of two adjacent bands at 911 and
−1
9
38 cm
in the FT-IR spectrum of complexed material is char-
saturation magnetization about 42 emu/g. The MoO salpr/SCMNPs
2
acteristic of presence of cis-MoO2 group [37] and formation of
nanomaterial exhibited superparamagnetic properties with sat-
uration magnetization about 37 emu/g. The superparamagnetic
MoO salpr/SCMNPs nanomaterial.
2
Molybdenum content of the prepared nanomaterial was found
to be 0.71 wt.% on the basis of ICP-AES chemical analysis.
properties of the prepared MoO salpr/SCMNPs are critical for their
application, which prevents aggregation and enables them to redis-
perse rapidly when the magnetic field is removed.
2
Fig.
2 depicts the XRD pattern of prepared nanomate-
rial. The diffraction peaks can be assigned to the planes of
Transmission electron micrographs of the magnetic nanomate-
rial are shown in Fig. 4a. As can be seen, most of the nanoparticles
inverse cubic spinel structured Fe O4 (JCPDS no. 19-0629). The
3
Fig. 2. XRD pattern of prepared MoO2salpr/SCMNPs.
Fig. 3. Magnetization curves of (a) MNPs, (b) SCMNPs, (c) MoO2salpr/SCMNPs.

8
6
M. Masteri-Farahani, N. Tayyebi / Journal of Molecular Catalysis A: Chemical 348 (2011) 83–87
Table 1
Results of catalytic epoxidation of some olefins with TBHP in presence of
MoO2salpr/SCMNPs.
TOF (h 1)b
−
Olefin
Time (h)
Conversion
%)
Selectivity
(%)
a
(
Cyclooctene
Cyclooctenec
Cyclohexene
Cyclohexenec
6
6
6
6
6
6
3
99.9
80
61
72
56
22
85
89
99
98
14
9
99
99
98
99
99
99
99
95
98
99
41
45
180
59
110
53
101
16
–
1
1
-hexene
-hexenec
d
Cyclooctene
2
0
6
6
–
Cyclooctene (1st)e
Cyclooctene (2nd)
178
177
–
e
f
Cyclooctene
12
12
g
Cyclooctene
–
Reaction conditions: catalyst (100 mg), olefin (8 mmol), TBHP (1.6 ml, 14.4 mmol),
refluxing chloroform (20 ml).
a
Selectivity toward the corresponding epoxide.
Calculated as mmol of product formed per mmol of molybdenum in the catalyst
b
per hour.
c
Values from Ref. [20].
Catalytic test after filtration of the catalyst.
Catalytic test with first and second recovered catalyst.
Reaction was carried out without catalyst.
d
e
f
g
Reaction was carried out in the presence of SCMNPs.
Table 2
Results of catalytic epoxidation of olefins with CHP in presence of
MoO2salpr/SCMNPs.
TOF (h 1
−
)
Olefin
Time (h)
Conversion
%)
Selectivity
(%)
(
Cyclooctene
Cyclohexene
6
6
6
95
62
45
99
99
98
171
112
81
1-Hexene
Reaction conditions: catalyst (100 mg), olefin (8 mmol), CHP (14.4 mmol), refluxing
chloroform (20 ml).
As seen in Tables 1 and 2, catalytic activities and TOFs increase
in the order of cyclooctene > cyclohexene > 1-hexene as internal
double bond olefins with higher electron density show higher reac-
tivities than terminal olefins.
Comparison of the obtained results with our earlier work on
the similar catalyst derived from mesoporous material (MCM-41),
i.e. MoO salpr@MCM-41 [20], is interesting and instructive. Com-
2
pared to MoO salpr@MCM-41 with 1.75 wt% molybdenum, the
2
MoO salpr/SCMNPs with 0.71 wt% molybdenum shows more cat-
2
Fig. 4. (a) SEM and (b) TEM images of prepared MoO2salpr/SCMNPs.
alytic activity, e.g. TOF = 180 for the last vs. TOF = 59 in the former
in cyclooctene epoxidation or TOF = 110 vs. TOF = 53 in cyclohex-
ene epoxidation in the same reaction conditions (results showed
in Table 1). This can be interpreted by considering the fact that
are almost spherical in shape. The particle size is about 10 nm and
relatively monodispersed.
Fig. 4b shows the SEM image of the prepared nanomaterial.
The SEM image indicates that the obtained product is composed
of spherical nanoparticles. Aggregation gives rise to increasing the
size of observed nanoparticles as seen in the SEM image.
compared to the MoO salpr@MCM-41 catalyst with higher surface
2
area but narrow pore size, the MoO salpr/SCMNPs has not diffu-
2
sion limitations, as all of the surface area of the nanoparticles is
external. On the other hand, the MoO salpr/SCMNPs nanocatalyst
2
can be recovered easily with application of a permanent magnet
3.3. Catalytic epoxidation of olefins in the presence of
Table 3
MoO salpr/SCMNPs
Results of catalytic epoxidation of allyl alcohols with CHP in presence of
MoO2salpr/SCMNPs.
2
Catalytic activity of the prepared nanomaterial was examined
in the epoxidation of some olefins with TBHP and CHP. The oxi-
dation results as well as turnover frequencies (TOF’s), i.e. mmol
of product formed per mmol of molybdenum in the catalyst are
given in Tables 1–3. We have included the results of epoxidation
of cyclooctene in the presence of parent SCMNPs and blank (no
catalyst) in Table 1 in order to clear up the catalytic effect of the
−1
Olefin
Time (h) Conversion
(%)
Selectivity
(%)
TOF (h
)
3
1
-Methyl-2-butene-1-ol
-Octene-3-ol
6
6
6
6
83
15
73
54
99
99
98
98
150
27
132
97
3-Butene-2-ol
Trans-2-hexene-1-ol
Reaction conditions: catalyst (100 mg), olefin (8 mmol), CHP (14.4 mmol), refluxing
chloroform (20 ml).
prepared MoO salpr/SCMNPs.
2

M. Masteri-Farahani, N. Tayyebi / Journal of Molecular Catalysis A: Chemical 348 (2011) 83–87
87
property. Thus, the magnetite core is stable during the course of
epoxidation reaction as a result of protective function of the silica
shell against TBHP.
4. Conclusion
In this work, we have shown that functionalization of silica
coated magnetite nanoparticles with a tetradentate salpr Schiff
base ligand and subsequent complexation with molybdenum
affords an easily recoverable truly heterogenized molybdenum
nanocatalyst which is active and selective in catalytic epoxidation
of olefins and allylic alcohols.
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