Surface modification of magnetite nanoparticles with molybdenum-dithiocarbamate complex: a new magnetically separable nanocatalyst

Article abstract of DOI:10.1007/s00706-017-1936-6

Surface modification of silica-coated magnetite nanoparticles (SCMNPs) by anchoring of molybdenum-dithiocarbamate complex resulted in the preparation of a new magnetically separable nanocatalyst for the epoxidation of olefins. The prepared nanocatalyst was characterized by various physicochemical techniques which indicated that the molybdenum complex is successfully supported on the SCMNPs support. This heterogeneous catalyst exhibited good catalytic activity and high selectivity in the epoxidation of olefins with tert-butyl hydroperoxide as oxidant under mild reaction conditions. It can be easily recovered by an external magnetic field and reused up to three times without noticeable deactivation. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s00706-017-1936-6

Monatsh Chem
DOI 10.1007/s00706-017-1936-6
ORIGINAL PAPER
Surface modification of magnetite nanoparticles with
molybdenum-dithiocarbamate complex: a new magnetically
separable nanocatalyst
1
1
Majid Masteri-Farahani
•
Maryam Modarres
Received: 1 July 2016 / Accepted: 13 February 2017
Ó Springer-Verlag Wien 2017
Abstract Surface modification of silica-coated magnetite
nanoparticles (SCMNPs) by anchoring of molybdenum-
dithiocarbamate complex resulted in the preparation of a
new magnetically separable nanocatalyst for the epoxida-
tion of olefins. The prepared nanocatalyst was
characterized by various physicochemical techniques
which indicated that the molybdenum complex is suc-
cessfully supported on the SCMNPs support. This
heterogeneous catalyst exhibited good catalytic activity
and high selectivity in the epoxidation of olefins with tert-
butyl hydroperoxide as oxidant under mild reaction con-
ditions. It can be easily recovered by an external magnetic
field and reused up to three times without noticeable
deactivation.
Introduction
Recently, easy separation and reuse of homogeneous
transition metal catalysts via their immobilization on the
surface of solid supports while maintaining their catalytic
performance has attracted great interest in catalytic
researches [1–4]. Various support materials such as poly-
mers [2, 3], zeolites [4, 5], and mesoporous materials [6–8]
have been used due to their high surface areas and well-
defined structures. However, most of this type of supported
catalysts suffers from reduced catalytic efficiency mainly
as a result of diffusion limitations. One of the most useful
methods for easy recovery and reuse of the catalysts is the
use of functionalized magnetic nanoparticles as catalyst
support. Separation by external magnetic field is a hopeful
method in comparison to conventional filtration or cen-
trifugation methods to attain improved reusability [9–11].
On the other hand, as the accessible surface area of the
magnetic nanoparticles is external, they have no the
internal diffusion limitations of porous materials. To pre-
serve the magnetite nanoparticles from probable chemical
reactions and agglomeration, their outer surface may be
coated with a silica layer which also provides an inert
barrier between the magnetite core and surface functional
groups.
Graphical abstract
Keywords Magnetite Á Molybdenum Á Dithiocarbamate Á
Immobilization Á Epoxidation
In this context, magnetite nanoparticles (MNPs) have
attracted much attention due to their unique properties such
as high stability, low toxicity, relatively high surface area,
and easy recovery. They have been used in many fields
such as catalysis of organic reactions [10–14], metal ion
separations [15–20], magnetic resonance imaging (MRI)
[
21–24], and biomedical applications [25–28].
&
Majid Masteri-Farahani
mfarahany@yahoo.com
Several types of transition metal homogeneous catalysts
have been immobilized on the surface of MNPs and have
exhibited excellent catalytic efficiencies in various
1
Faculty of Chemistry, Kharazmi University, Tehran, Iran
123

M. Masteri-Farahani, M. Modarres
reactions including hydrolysis [29], carbon–carbon cross-
coupling reactions [30–33], polymerization [34], hydro-
genation [35, 36], hydroformylation [37, 38], and olefin
epoxidation [39–42].
SCMNPs. Condensation of ethoxy groups of the silylating
agent with the silanol groups of SCMNPs leads to the
formation of stable siloxane (Si–O–Si) bonds through
which the aminopropyl groups have been attached cova-
lently to the surface of SCMNPs. Next addition of carbon
disulfide to the resulted nanomaterial gives the dithiocar-
bamate ligand supported on the surface of silica-coated
magnetite nanoparticles (dtc-SCMNPs). In this step,
nucleophilic attack of amine groups of Amp-SCMNPs to
In an extension of our previous researches on the
immobilization of homogeneous molybdenum catalysts on
various supports [40–44] we investigate, for the first time,
the immobilization of molybdenum-dithiocarbamate com-
plex on the surface of silica-coated magnetite nanoparticles
to achieve a heterogenized molybdenum catalyst for the
epoxidation of olefins. Furthermore, we discuss about full
characterization of the prepared catalyst such as its
microstructure, magnetic behavior, and surface area as well
as its nitrogen adsorption–desorption isotherms. Finally,
the catalytic activity of the prepared catalyst will be
illustrated in the epoxidation of olefins and allylic alcohols
with tert-butyl hydroperoxide as oxidant. The benefits of
this system are the easy recycling of the catalyst at the end
of reaction as well as high activity, selectivity, and
stability.
carbon disulfide as electrophile forms -NH-CS - groups
2
supported on the surface of silica-coated magnetite
nanoparticles. Finally, reaction of MoO (acac) with sup-
2
2
ported dithiocarbamate ligand produces the molybdenum-
dithiocarbamate complex immobilized on the surface of
magnetite nanoparticles, Mo-dtc-SCMNPs.
Characterization of the prepared Mo-dtc-SCMNPs
nanomaterials
Figure 2 shows the FT-IR spectra of SCMNPs, Amp-
SCMNPs, dtc-SCMNPs, and Mo-dtc-SCMNPs. The char-
acteristic peaks of the Fe–O bonds in the magnetite core of
the prepared nanomaterials are observed at around
Results and discussion
-
1
4
50–590 cm [45]. The strong and broad absorption band
-
1
Preparation of Mo-dtc-SCMNPs nanomaterial
at about 1000–1100 cm is due to the stretching vibra-
tions of Si–O–Si bonds which confirmed the presence of
silica layer on the outer surface of MNPs. The observed
The involved steps in the immobilization of molybdenum-
dithiocarbamate complex on the surface of magnetite
nanoparticles have been illustrated in Fig. 1. The first step
includes the preparation of magnetite nanoparticles
-
1
band at 2800–2900 cm is assigned to C–H stretching
vibrations and verifies the presence of aminopropyl chain
in Amp-SCMNPs (Fig. 2b). In the FT-IR spectrum of dtc-
-
1
(
MNPs). In the next step, the outer surface of magnetite
SCMNPs (Fig. 2c), the bands at 1465 and 678 cm are
assigned to N–CS2 and C–S vibrations, respectively
[46, 47]. Also, the appearance of the bands at 910 and
nanoparticles was coated with a silica layer to obtain
SCMNPs. Then, treatment of silanol groups of SCMNPs
with aminopropyltriethoxysilane (Amp) achieved Amp-
-
1
935 cm
in the FT-IR spectrum of Mo-dtc-SCMNPs
Fig. 1 Schematic representation of the preparation of Mo-dtc-SCMNPs
1
23

Surface modification of magnetite nanoparticles with molybdenum-dithiocarbamate complex…
Fig. 2 FT-IR spectra of
a SCMNPs, b Amp-SCMNPs,
c dtc-SCMNPs, and d Mo-dtc-
SCMNPs
(
Fig. 2d) is characteristic of cis-MoO group [48] which
2
attachment of molybdenum complex on the surface of
MNPs.
confirms the complexation of supported dithiocarbamate
ligand with molybdenum in the final nanomaterial.
The XRD pattern of the Mo-dtc-SCMNPs nanomaterial
has been shown in Fig. 3. Diffraction peaks in this pattern
can be indexed according to diffraction planes of inverse
cubic spinel structure (JCPDS No. 19-0629). This indicates
that the crystalline structure of the MNPs support has
maintained after the immobilization of molybdenum
complex. The broadness of the peaks is due to the
nanocrystalline nature of the prepared nanomaterial.
ThemagneticpropertiesofMNPs, Amp-SCMNPs, and Mo-
dtc-SCMNPs were investigated by using vibrating sample
magnetometry (Fig. 4). As can be seen in Fig. 4, the magne-
tization curves of all nanomaterials showed no hysteresis loop
and they exhibited superparamagnetic properties. The satura-
tion magnetization of the nanomaterials decreases successively
The amounts of supported aminopropyl group and
dithiocarbamate ligand were determined from CHNS
analyses of the materials and are given in Table 1.
Molybdenum content of the soxhlet extracted Mo-dtc-
-
1
SCMNPs was found to be 0.19 mmol g
inductively coupled plasma-optical emission spectroscopy
ICP-OES) chemical analysis and further confirmed the
from the
(
Table 1 Elemental analysis of the prepared nanomaterials
Sample
N content/ S content/ Mo content/
-1
BET
S /
-1
-1
2
m g
-1
mmol g
mmol g
mmol g
MNPs
0.02
0.41
0.37
–
–
6
(
from 60 emu/g in MNPs to 30 emu/g in Mo-dtc-SCMNPs)
Amp-SCMNPs
dtc-SCMNPs
–
–
nd
nd
73
nd
with surface modification of the magnetite core.
0.58
0.45
nd
–
The superparamagnetic property of the catalyst is
important for its employment in catalytic applications.
Despite the decrease in the saturation magnetization in
comparison with parent MNPs, the Mo-dtc-SCMNPs can
be recovered in the presence of external magnetic field and
Mo-dtc-SCMNPs 0.30
a
Mo-dtc-SCMNPs nd
0.19
0.18
nd not determined
a
First recovered catalyst
123

M. Masteri-Farahani, M. Modarres
Fig. 3 XRD pattern of the
prepared Mo-dtc-SCMNPs
Fig. 4 Magnetization curves of
a MNPs, b Amp-SCMNPs, and
c Mo-dtc-SCMNPs
redispersed rapidly without aggregation when the magnetic
field is removed.
Emmet–Teller (BET) method. Interestingly, it was found that
the surface area of Mo-dtc-SCMNPs is greater than that of
MNPs. This can be explained by considering the aggregation in
these nanomaterials. Aggregation of the nanoparticles decrea-
ses the available external surface area of the nanoparticles. So,
in the case of Mo-dtc-SCMNPs, the decrease in the aggregation
of the nanoparticles due to the presence of functional groups
results in greater surface area in this nanomaterial. The results
reveal that the Mo-dtc-SCMNPs material possesses adequate
surface area after the immobilization of molybdenum-dithio-
carbamate complex on the surface of MNPs.
As the surface area and textural properties of the hetero-
geneous catalysts are important parameters in determining
their catalytic performance, the nitrogen adsorption–des-
orption isotherms of MNPs and Mo-dtc-SCMNPs were
studied (Fig. 5). As can be seen in Fig. 5, adsorption–des-
orption isotherms for both materials exhibited the type II
isotherm [49]. The small hysteresis loops encountered in the
case of Mo-dtc-SCMNPs indicates the presence of some
mesoporosity in this material. The observed mesoporosity
may be caused by some aggregation of individual nanopar-
ticles due to van der Waals forces.
Figure 6 shows the SEM and TEM images of the prepared
Mo-dtc-SCMNPs nanomaterial. The SEM image illustrates
that the prepared Mo-dtc-SCMNPs nanomaterial is com-
posed of spherical nanoparticles. Aggregation results in
Total surface areas (SBET) of MNPs and Mo-dtc-SCMNPs
(given in the Fig. 5) were obtained based on the Brunauer–
1
23

Surface modification of magnetite nanoparticles with molybdenum-dithiocarbamate complex…
increasing the size of the nanoparticles as seen in the SEM
image. Also, the TEM image of the prepared nanomaterial
shows that most of the nanoparticles are aggregated. As can
be seen nm in the edges of the aggregated nanoparticles, their
particle size can be estimated about 15 nm.
Catalytic epoxidation of olefins and allylic alcohols
in the presence of Mo-dtc-SCMNPs
Catalytic efficiency of the prepared Mo-dtc-SCMNPs was
evaluated in the epoxidation of some olefins and allylic
alcohols with tert-butyl hydroperoxide (TBHP) as oxidant.
The results are given in Tables 2 and 3. Also, the epoxidation
of cyclooctene in the presence of parent SCMNPs as well as
blank (no catalyst) test was investigated to clarify the cat-
alytic effect of the prepared Mo-dtc-SCMNPs (Table 4). As
can be seen in this table, the epoxidation reactions do not
proceed significantly in the absence of the Mo-dtc-SCMNPs.
Moreover, we found that the catalytic efficiency of the
prepared Mo-dtc SCMNPs nanomaterial is comparable to
Fig. 5 Nitrogen adsorption–desorption isotherms of MNPs and Mo-
dtc-SCMNPs
Table 2 Results of catalytic epoxidation of some olefins with TBHP
in presence of Mo-dtc-SCMNPs
Run no. Olefins
Time/h Conversion/% Selectivity/%
1
2
3
4
5
1-Hexene
8
4
8
4
8
4
8
4
8
4
42
89
29
59
22
52
99
99
83
92
[99
[99
[99
[99
[99
[99
[99
[99
[99
[99
2
2
2
2
2
1-Octene
1- Decene
Cyclooctene
Cyclohexene
3
Reaction conditions: 50 mg catalyst, 4 mmol olefin, 0.75 cm TBHP,
3
cm refluxing chloroform
5
Table 3 Results of catalytic epoxidation of some allylic alcohols
with TBHP in presence of Mo-dtc-SCMNPs
Run Olefins
no.
Time/h Conversion/% Selectivity/%
1
2
3
3-Methyl-2-buten-1-ol
8
4
8
4
8
4
89
99
81
96
69
82
[99
[99
[99
[99
[99
[99
2
2
2
Trans-2-hexen-1-ol
1-Octen-3-ol
Reaction conditions: 50 mg catalyst, 4 mmol allylic alcohol,
0
Fig. 6 The SEM (top) and TEM (bottom) images of prepared Mo-
dtc-SCMNPs
3
.75 cm TBHP, 5 cm refluxing chloroform
3
123

M. Masteri-Farahani, M. Modarres
Table 4 Results of the epoxidation of cyclooctene with TBHP in
different conditions
molybdenum catalysts [54–57]. In the first step, the TBHP
is coordinated to the molybdenum center to give molyb-
denum-alkyl peroxide species. Then, the peroxidic oxygen
would have electrophilic character and the catalytic epox-
idation proceed through electrophilic oxygen transfer from
the resulted molybdenum-alkyl peroxide to the olefin as
nucleophilic reagent (Fig. 7). Hence, olefins with higher
electron density have more nucleophilic character and
show higher reactivities in comparison with terminal ones.
The Mo-dtc-SCMNPs catalyst was reused for the
epoxidation of cyclooctene to investigate the reusability
and stability of the catalyst. The results are presented in
Fig. 8 and show that the activity and selectivity do not
decrease significantly for at least three recycles.
a
Selectivity /%
Run no.
Time/h
Conversion/%
b
1
12
12
2
14
16
84
86
41
36
c
2
d
3
[99
[99
3
2
4
Reaction conditions: 50 mg catalyst, 4 mmol cyclooctene, 0.75 cm
3
TBHP, 5 cm refluxing chloroform
a
Selectivity toward epoxycyclooctane
b
Reaction in the absence of catalyst
c
Reaction in the presence of SCMNPs
d
Results after filtering the catalyst
To examine the stability of the surface-attached molyb-
denum complex, filtration test was conducted in the
epoxidation of cyclooctene which after 2 h the Mo-dtc-
SCMNPs catalyst was separated magnetically at the reaction
temperature. The solution was transferred into another flask
and further refluxed for 22 h to investigate whether con-
version resulted from solubilized molybdenum complex
leached from the MNPs or supported molybdenum catalyst.
The conversions and selectivities were determined after 2
and 24 h (Table 4). It was found that the conversion
increased slightly (2%) and then remains constant. So, it can
be concluded that the epoxidation reaction proceeds mostly
by the heterogeneous Mo-dtc-SCMNPs catalyst. Moreover,
after separating the Mo-dtc-SCMNPs catalyst, no consider-
able leaching of molybdenum was detected as indicated by
ICP-OES (Table 1). Thus, from these results it can be
concluded that the molybdenum-dithiocarbamate complex is
tightly anchored to the surface of SCMNPs and the prepared
heterogeneous catalyst is stable in the reaction conditions.
that of our previous catalysts based on large surface area
mesoporous materials [50–53]. This observation may be
explained by taking to account the fact that compared to
earlier catalysts with greater surface area but small pore
size, the Mo-dtc-SCMNPs has not diffusion limitations as
all of the surface area of the Mo-dtc-SCMNPs nanomate-
rial is external. Also, it seems that the presence of S-donor
dithiocarbamate ligand in the catalyst increases its catalytic
activity in comparison with similar catalysts immobilized
on the surface of SCMNPs [40–42].
The Mo-dtc-SCMNPs catalyst can be recycled easily by
using a permanent magnet compared to other catalysts
supported on mesoporous networks which needs a time
consuming filtration of reaction mixture.
Results of Tables 2 show that reactivities of the olefins
with internal double bond are higher than that of terminal
ones. This can be interpreted by considering the proposed
mechanism of the olefin epoxidation in the presence of
Fig. 7 The proposed
mechanism for the epoxidation
of olefins in the presence of
prepared Mo-dtc-SCMNPs
catalyst
1
23

Surface modification of magnetite nanoparticles with molybdenum-dithiocarbamate complex…
mixture by an external magnetic field and reused for three
times without considerable loss of its catalytic efficiency.
Experimental
FT-IR spectra of the samples were provided on Perkin–
Elmer Spectrum RXI FT-IR spectrometer in the wavenum-
-
1
ber range of 400–4000 cm , using pellets of the
nanomaterials diluted with potassium bromide. Molybde-
num content of the catalyst was measured by Varian Vista-
MPX ICP-OES spectrometer. CHNS analysis of the samples
was performed on Thermo Finnigan (Flash 1112 Series EA)
Analyzer. X-ray diffraction (XRD) patterns of the prepared
nanomaterials were recorded with a SIEFERT XRD 3003
PTS diffractometer using Cu Ka radiation (k = 0.154 nm).
Measurement of magnetic susceptibilities were done using a
vibrating sample magnetometer (BHV-55, Riken, Japan) in
the magnetic field range of -8000 to 8000 Oe at room
temperature. Scanning electron microscopy (SEM) analyses
of the samples were performed with ZEISS-DSM 960A
microscope. Nitrogen adsorption–desorption analyses were
performed using Quanta chrome Nova 2200, Version 7.11
Analyzer at liquid nitrogen temperature (77 K). Before the
adsorption analysis the samples were outgassed at 383 K.
The analysis of epoxidation products were conducted by a
gas chromatograph (HP, Agilent 6890N) equipped with
capillary column (HP-5) using helium as carrier gas. GC–MS
of the products was provided using a Shimadzu-14A fitted
with a capillary column (CBP5-M25).
Fig. 8 Results of the epoxidation of cyclooctene with TBHP in the
presence of recycled Mo-dtc-SCMNPs. Reaction conditions: 50 mg
3
catalyst, 4 mmol olefin, 0.75 cm TBHP, 5 cm refluxing chloroform
3
Table 5 Comparison of the results obtained for cycooctene epoxi-
dation catalyzed by various molybdenum-based catalysts
a
Time Conversion Selectivity Reference
Run Catalyst
No.
/h
/%
/%
1
2
Mo-dtc-SCMNPs
MoO pyprMCM-
8
7
99
98
[99
[99
This work
[51]
2
4
1
3
4
Mo-POM
8
8
99
99
[99
[99
[58]
[59]
Mo–AMP–
CuBTC
5
MoO (acac)
2
2
4
99
[99
This work
a
Selectivity toward epoxycyclooctane
Furthermore, Table 5 compares the efficiency of the
prepared Mo-dtc-SCMNPs catalyst with some of the pre-
viously reported heterogeneous catalysts as well as
MoO (acac) as homogeneous one. It can be seen that the
Preparation of modified SCMNPs
with dithiocarbamate ligand (dtc-SCMNPs)
2
2
catalytic activity of the Mo-dtc-SCMNPs catalyst is com-
parable to that of other molybdenum catalysts.
Aminopropyl-modified silica-coated magnetite nanoparti-
cles (Amp-SCMNPs) were synthesized according to our
previous report [40]. The prepared Amp-SCMNPs (1.0 g)
3
were suspended in 50 cm of 0.1 M sodium hydroxide with
3
3
Conclusion
sonication and then 5 cm carbon disulfide in 10 cm iso-
propanol were added. The mixture was stirred for 4 h at
room temperature. The solid was collected with a perma-
nent magnet and repeatedly washed with ethanol. The
resultant solid (named as dtc-SCMNPs) was dried under
vacuum at 353 K.
Molybdenum-dithiocarbamate complex was immobilized
on the surface of silica-coated magnetite nanoparticles. The
external surface of SCMNPs was functionalized with
aminopropyl group and then dithiocarbamate ligand. Then,
complexation of the supported dithiocarbamate ligands
with molybdenum complex gave a magnetically recover-
Immobilization of molybdenum complex
on the surface of magnetite nanoparticles (Mo-dtc-
SCMNPs)
able
molybdenum
catalyst.
Physicochemical
characterizations showed superparamagnetic properties as
well as relatively high surface area of the prepared catalyst.
This recoverable catalyst was active in the epoxidation of
olefins with high activity and excellent selectivity. In
addition, it could be readily removed from the reaction
First, 2 mmol of MoO (acac) (prepared according to lit-
2 2
3
erature method [60]) was dissolved in 50 cm ethanol. The
prepared dtc-SCMNPs (1.0 g, dried in vacuum oven at
123

M. Masteri-Farahani, M. Modarres
3
53 K) was then added to this solution with sonication and
22. Kim J, Park S, Lee JE, Jin SM, Lee JH, Lee IS, Yang I, Kim JS,
Kim SK, Cho MH, Hyeon T (2006) Angew Chem Int Ed
the mixture was refluxed for 24 h under nitrogen atmo-
sphere. The product was separated with a permanent
magnet and washed with ethanol. The obtained Mo-dtc-
SCMNPs nanomaterial was soxhlet extracted with metha-
nol to remove unreacted reagents and was dried under
vacuum at 353 K.
1
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(
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Catalytic epoxidation of olefins and allylic alcohols
in the presence of Mo-dtc-SCMNPs
28. Barrera C, Herrera A, Zayas Y, Rinaldi C (2009) J Magn Magn
Mater 321:1397
2
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Commun 4435
The catalytic tests for the epoxidation reactions were per-
formed in the presence of Mo-dtc-SCMNPs as catalyst and
tert-butyl hydroperoxide (TBHP, 80% in di-tert-butyl
peroxide) as oxidant. The general procedure was as fol-
lows: to a mixture of 50 mg catalyst and 4 mmol olefin or
31. Zheng Y, Stevens PD, Gao Y (2006) J Org Chem 71:537
3
2. Duanmu C, Saha I, Zheng Y, Goodson BM, Gao Y (2006) Chem
Mater 18:5973
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3
34. Ding S, Xing Y, Radosz M, Shen Y (2006) Macromolecules
39:6399
3
3
allyl alcohol in 5 cm chloroform was added 0.75 cm
3
3
3
5. Hu AG, Yee GT, Lin WB (2005) J Am Chem Soc 127:12486
6. Guin D, Baruwati B, Manorama SV (2007) Org Lett 9:1419
7. Abu-Reziq R, Alper H, Wang DS, Post ML (2006) J Am Chem
Soc 128:5279
TBHP and the mixture was refluxed under nitrogen atmo-
sphere for appropriate time. Samples were withdrawn in
given times and were analyzed using GC analysis.
3
8. Yoon TJ, Lee W, Oh YS, Lee JK (2003) New J Chem 27:227
Acknowledgements The authors gratefully acknowledge financial
support from the Iran National Science Foundation (INSF) (Grant No.
39. Shylesh S, Schweizer J, Demeshko S, Schunmann V, Ernst S,
Thiel WR (2009) Adv Synth Catal 351:1789
40. Masteri-Farahani M, Kashef Z (2012) J Magn Magn Mater
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