Polyethyleneimine-modified superparamagnetic Fe3O4 nanoparticles: An efficient, reusable and water tolerance nanocatalyst

Article abstract of DOI:10.1016/j.jmmm.2014.09.044

A novel magnetically separable catalyst was prepared based on surface modification of Fe₃O₄ magnetic nanoparticle (MNPs) with polyethyleneimine (PEI) via covalent bonding. [3-(2,3-Epoxypropoxy)propyl]trimethoxysilane (EPO) was used a

Full text of DOI:10.1016/j.jmmm.2014.09.044

Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Polyethyleneimine-modified superparamagnetic Fe3O4 nanoparticles:
An efficient, reusable and water tolerance nanocatalyst
Mehdi Khoobi a, Tayebeh Modiri Delshad b, Mohsen Vosooghi a, Masoumeh Alipour a,
Hosein Hamadi c, Eskandar Alipour b, Majid Pirali Hamedani d,
n
Q1
Seyed esmaeil Sadat ebrahimi a, Zahra Safaei b, Alireza Foroumadi a, Abbas Shafiee a,
18
a Department of Medicinal Chemistry, Faculty of Pharmacy and Pharmaceutical Sciences Research Center, Tehran University of Medical Sciences, Tehran
14176, Iran
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b Department of Chemistry, Islamic Azad University, Tehran-North Branch, Zafar St., Tehran, Iran
c Department of Chemistry, College of Science, Shahid Chamran University, Ahvaz, Iran
d Iran Polymer & Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel magnetically separable catalyst was prepared based on surface modification of Fe3O4 magnetic
nanoparticle (MNPs) with polyethyleneimine (PEI) via covalent bonding. [3-(2,3-Epoxypropoxy)propyl]
trimethoxysilane (EPO) was used as cross linker to bond PEI on the surface of MNPs with permanent
stability in contrast to PEI coating via electrostatic interactions. The synthesized catalyst was character-
ized by Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), X-ray powder diffraction
(XRD), transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). The catalyst
show high efficiency for one-pot synthesis of 2-amino-3-cyano-4H-pyran derivatives via multi-compo-
nent reaction (MCR). This procedure offers the advantages of green reaction media, high yield, short
reaction time, easy purification of the products and simple recovery and reuse of the catalyst by simple
magnetic decantation without significant loss of catalytic activity.
Received 20 May 2014
Received in revised form
30 August 2014
Accepted 17 September 2014
Keywords:
Polyethyleneimine (PEI)
Magnetic Nanoparticle
Spiro-oxindole
Pyrano[3,2-c]chromene
Surface functionalization
& 2014 Published by Elsevier B.V.
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1. Introduction
increase the environmental concerns [2]. Catalyst plays an im-
portant role in green chemistry as it can provide the best yield of
The aim of green chemistry is to find alternative processes to
conserve resources and reduce costs. Application of environmen-
tally benign solvents instead of toxic and/or hazardous solvents
and also utilizing of mild conditions and inexpensive reagents are
the most fascinating strategies to develop a simple and green
synthesis of organic compounds [1,2]. Water as an available and
inexpensive solvent in large quantities can facilitate the rate of
organic reactions even for water-insoluble reactants as well as
product isolation by simple filtration. In addition, application of
catalytic rather than stoichiometric reagents and reducing con-
siderably the reaction times are important factors in order to
the reaction in the reduced temperatures. Nanocatalysts as bridge
between homogenous and heterogeneous catalysts can provide
both benefits of them including high activity and selectivity as
well as stability and ease of isolation or recovery. Considerable
properties of magnetic nanoparticles (MNPs) such as large surface
area to volume ratios, biocompatibility, non-toxicity and simple
modification made them attractive for several biomedical applica-
tions [3]. In addition to all the mentioned features, easy and
complete recovery of MNPs by means of an external magnet due to
their superparamagnetic property makes them the best catalyst
for green and sustainable chemistry. The surface of the MNPs has
to protect by a suitable coating materials in order to protect their
surface from oxidation, reduce the aggregating tendency and
subsequently improve their dispersibility and colloidal stability
[4–9].
Abbreviations: ; DLS, dynamic light scattering; EG, ethylene glycol; EPO, [3-(2,3-
epoxypropoxy)propyl]trimethoxysilane; FT-IR, Fourier transform infrared; HD,
hydrodynamic diameter; MCR, multi-component reaction; MNPs, magnetic nano-
particle; PG, propylene glycol; PEI, polyethyleneimine; TLC, thin layer chromato-
graphy; TGA, thermogravimetric analysis; TEM, transmission electron microscopy;
VSM, vibrating sample magnetometry; XRD, X-ray powder diffraction; SEM,
scanning electron microscopy
PEI as a water soluble cationic polymer have been widely used
in drug/gene delivery [10–12], CO2 adsorption [13], nucleic acid
precipitation, protein coagulation/flocculation [14], nucleotide, cell
and enzyme immobilization [15–18]. It was found that PEI can
stabilize nanoparticles as a capping and reducing agent [19].
Although, there are several reports about preparation and
n
Corresponding author. Fax: þ98 21 66461178.
E-mail address: ashafiee@ams.ac.ir (A. Shafiee).
http://dx.doi.org/10.1016/j.jmmm.2014.09.044
0304-8853/& 2014 Published by Elsevier B.V.
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application of PEI-functionalized MNPs, must of them are based on
electrostatic adsorption of positively charged PEI on the negatively
charged surface of MNPs [18]. Only a few reports could be found
about application of PEI as a catalyst [23–28]. Amali et al. have
reported Pd entrapped by highly branched PEI on the surface of
Fe3O4 as successful catalyst in hydrogenation and ligand-free
Suzuki–Miyaura reactions [23]. Gold nanoparticles immobilized
on PEI modified Fe3O4 show good efficiency in reduction of
4-nitrophenol [24,27]. Layer-by-layer assembly via electrostatic
interaction have been used to prepare PEI coated Fe3O4 as catalyst
in all reports.
4H-pyrans are well known as privileged scaffolds of the
emerging drugs with numerous medicinal activities and show
diverse applications in laser dyes, pigments, cosmetics, optical
fluorescence markers and brighteners [28]. Bearing in mind the
efficiency of PEI as a water soluble catalyst and in connection with
our previous works [29], we decided to explore novel PEI cova-
lently bound on the surface of MNPs by [3-(2,3-epoxypropoxy)
propyl]trimethoxysilane (EPO) as cross linker (PEI@Si–MNPs) for
the preparation of chromene (4), pyrano[3]chromene (6), and
spiro-oxindole (9,11) derivatives (see Scheme 1).
size and morphology of the sample surfaces were analyzed by
scanning electron microscopy (VEGAII TESCAN) with an acceleration
voltage of 15 kV. The disk was pasted with copper tape, and the
sample was dispersed over the tape. The disk was coated with gold in
an ionization chamber. Transmission electron microscopy (TEM) was
carried out on EM208 Philips. Samples were prepared by placing a
droplet (1 mL) of nanocomposite dispersion latex, along with a droplet
of water, on a copper grid covered by Formvar foil (200 mesh). FT-IR
spectra were obtained using Nicolet FT-IR Magna 550 spectrographs
(KBr disks) spectrometer. Melting points were measured on a Kofler
hot stage apparatus and are uncorrected. NMR spectra were recorded
in CDCl3 on a Bruker 500 MHz spectrometer.
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2.2. Synthesis of PEI@Si–MNPs
Fe3O4 MNPs were synthesized using the chemical coprecipita-
tion method [30]. In order to graft PEI onto Fe3O4 MNPs, EPO
(1 mmol) was added to a stirred solution of 150 mL dry toluene
containing 1.5 g of PEI. The resultant mixture was allowed to react
at 80 °C for 24 h. To this solution, 2.5 g of Fe3O4 MNPs and 25 mL of
ethanol were added, and the solution was stirred at 80 °C for 24 h.
PEI@Si–MNPs was magnetically isolated by an external magnet
and repeatedly washed with ethanol (Scheme 2). Subsequently, it
was washed repeatedly with water and ethanol to remove un-
reacted substrates and byproducts, and dried at 40 °C for several
days [24].
2. Experimental
2.1. General
[3-(2,3-Epoxypropoxy)propyl]trimethoxysilane (EPO, 98% purity),
aqueous ammonia solution (28 wt%), ferric chloride hexahydrate
(FeCl3 ꢀ 6H2O), and ferrous chloride tetrahydrate (FeCl2 ꢀ 4H2O) were
purchased from Merck (Darmstadt, Germany). Hyperbranched PEI
(Mw¼60,000) were purchased from Sigma-Aldrich (St. Louis, MO,
USA). X-ray diffraction (XRD) patterns were recorded with a XPert
MPD advanced diffractometer. The magnetic properties of the samples
were detected at room temperature using a vibrating sample magnet-
ometer (VSM, Meghnatis Kavir Kashan Co., Kashan, Iran). The particle
2.3. General procedure for catalytic synthesis of compounds 4, 6.9,11
A stirring mixture of an active carbonyl compound (aldehyde 1
or isatin 7, 1 mmol), malononitrile 2 (1.2 mmol), magnetic catalytic
system ([PEI@Si–MNPs], 5 mg) and ethylene glycol/water (EG/H2O
20/80, 2 mL) were sonicated for one minute. To this stirred
mixture, an enolizable compounds (3-(dimethylamino)phenol 3,
4-hydroxycoumarin 5 or dimedone 10, 1 mmol) was added. The
OH
NH2
CN
O
Ar
N
OH
CN
O
O
O
CN
CN
Ar
3
5
+
Ar
H
Cat.
H2O , 40 oC
O
O
N
O
NH2
Cat.
H2O , Reflux
1
2
6
4
H2N
NH
O
O
OH
H
N
N
Si
N
N
H
O
n
Cat:
O
HPEI
NH
NH2
H2N
n
O
OH
O
O
O
NH2
O
O
O
R
O
CN
CN
O
O
NH2
8
10
Cat.
+
CN
O
O
N
H
CN
N
Cat.
H2O , Reflux
R
N
H
O
H
H2O , 40 oC
11
7
2
Scheme 1. PEI@Si–MNPs catalyzed one-pot synthesis of various 4 H-pyrans.
Please cite this article as: M. Khoobi, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j.
jmmm.2014.09.044i

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H2N
1
2
3
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NH
O
Fe (II)
+
Fe (III)
+
NH4OH
OH
O
H
N
N
N
Si
N
H
Fe3O4
O
4
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n
O
5
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NH
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7
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9
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H2N
NH
H2N
H2N
NH
O
OH
NH2
O
O
O
H
N
n
N
N
Si
N
N
O
N
H
+
Si
O
N
H
O
n
n
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O
O
PEI@Si-MNPs
NH
NH
EPO-PEI
H2N
Scheme 2. Schematic representation of the formation of PEI@Si–MNPs.
HPEI
EPO
NH2
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Table 1
Optimization of reaction conditions for three-component reaction of benzaldehyde, malononitrile and 4-hydroxycoumarin or 3-(dimethylamino)phenol.
Q2
Entry
Catalyst
Solvent
Cat. (mg)
T (°C)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
9
10
11
12
13b
14b
15
16
–
H2O
H2O
H2O
H2O
H2O
–
–
5
5
5
5
5
5
5
5
5
5
–
5
5
5
5
100
100
100
rt
100
100
100
100
100
100
100
100
rt
10
2
2
5
1
1
1
1.5
1
1
1
2
1
1.5
1
21
45
62
52
82
72
85
89
88
85
98
72
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70
45
Fe3O4 MNPs
PEI
PEI@Si–MNPs
PEI@Si–MNPs
PEI@Si–MNPs
PEI@Si–MNPs
PEI@Si–MNPs
PEI@Si–MNPs
PEI@Si–MNPs
PEI@Si–MNPs
–
Propylene glycol (PG)
Ethylene glycol (EG)
EG/H2O (50/50)
EG/H2O (80/20)
EG/H2O (20/80)
EG/H2O (20/80)
EG/H2O (20/80)
EG/H2O (20/80)
EtOH
97
98
99
PEI@Si–MNPs
PEI@Si–MNPs
PEI@Si–MNPs
PEI@Si–MNPs
40
78
60
100
101
102
103
104
105
106
107
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132
CH3CN
1
Reaction conditions: 4-hydroxycoumarin (1 mmol), malononitrile (1.2 equiv.), benzaldehyde (1.2 equiv), solvent (2 mL) and the required amount of catalyst.
a
The yields refer to the isolated product.
b
Reaction of 3-(dimethylamino) phenol (1 mmol), malononitrile (1.2 equiv.), 4-nitro benzaldehyde (1.2 equiv.), solvent (2 mL) and the required amount of catalyst.
Table 2
Results of three-component reaction of 4-hydroxycoumarin, malononitrile and different aldehydes catalyzed by PEI@Si–MNP in EG/H2O.
a
Entry
1
Product
Yielda
94
Time (min)
60
m.p.b (°C)
4250
Entry
4
Product
Yield
91
Time (min)
60
m.p. (°C)
221–223
2
3
20
96
10 hc
4250
4250
5
6
89
81
55
70
4250
4250
50
a
Isolated yield.
Similar to the literatures m.p. [30–35].
No catalyst, H2O, reflux, 10 h.
b
c
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Table 3
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Results of three-component reaction of 3-(dimethylamino)phenol, malononitrile and different aldehydes catalyzed by PEI@Si–MNPs in EG/H2O.
3
4
5
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Entry
1
Product
Yielda
94
Time (min)
90
m.p.b
Entry
7
Product
Yielda
81
Time (min)
100
m.p.b(°C)
204–206
224–226
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2
91
90
194–196
8
85
90
220–22
3
88
100
163–166
9
87
80
174–178
212–214
4
92
80
214–218
10
89
80
5
86
100
158–160
11
83
100
4250
6
93
89
90
80
207–209
12
17
82
81
100
182–184
192–194
13
155–157
90
14
15
90
85
90
80
161–163
209–211
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19
86
81
100
120
142–144
162–164
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Table 3 (continued )
Entry
Product
Yielda
Time (min)
m.p.b
Entry
Product
Yielda
Time (min)
m.p.b(°C)
4
70
5
71
6
72
7
73
8
74
9
75
10
11
12
13
14
15
16
17
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129
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16
81
100
201–203
20
78
100
195–197
a
Isolated yield.
Similar to the literatures m.p. [30–35].
b
reaction mixture was allowed to cool at room temperature and
diluted with ethyl acetate and the catalyst was easily separated
from the reaction mixture with an external magnet and washed
twice with ethyl acetate. The combined organic layers were
concentrated in vacuum and the residue was purified by crystal-
lization from ethanol. All compounds gave satisfactory spectral
data and they were identical with those reported in the literature
[29,31–37].
Table 4
Results of three-component synthesis of spirooxindole derivatives 9 using isatin,
malononitrile and dimedone and 11 using isatin, malononitrile and 4-hydroxycou-
marin by PEI@Si–MNP in EG/H2O
Entry
1
Product
Yielda
97
Time (min)
30
m.p.b
4250
3. Results and discussion
As illustrated in Scheme 2, the catalyst was prepared easily by
two step process. Initially, the amine groups of PEI reacted with
epoxy group of EPO, and then the latter was covalently anchored
on the surface of MNPs through reaction of trimethoxysilane
groups of EPO-PEI with hydroxyl groups of MNPs via a silanization
reaction.
2
3
95
95
30
40
4250
4250
3.1. Chemical–physical characterization of PEI@Si–MNPs
3.1.1. FT-IR
The as-synthesized PEI@Si–MNPs catalyst was characterized by
various techniques. The FT-IR spectrum of PEI@Si–MNPs, shows
bands at 1581 and 1457 cmꢁ1 assigned to the stretching vibration
of C–N and bending vibration of C–H bonds of PEI macromolecular
chains respectively, which are chemically linked on the surfaces of
MNPs and the bands at around 2924 and 2831 cmꢁ1 are attributed
to stretching vibration of the aliphatic C–H bands. In addition, the
characteristic peaks of Fe–O at 584 cmꢁ1 and strong adsorption
band at 1029–1023 cmꢁ1 of Si–O–Si and Si–OH were also observed
(Fig. 1).
4
97
92
45
30
4250
4250
5
3.1.2. XRD
The X-ray diffraction patterns of MNPs and PEI@Si–MNPs are
shown in Fig. 2. The position and relative intensity of all the
diffraction peaks suitably matched those of standard Fe3O4 [38].
The MNPs are in the form of inverse spinel Fe3O4 with a face-
centered cubic structure, suggesting that the sample has a cubic
crystal system. Moreover, no characteristic peaks of impurities
were observed in the XRD spectrum. The comparison of the
diffraction patterns of MNPs and PEI@Si–MNPs shows that the
broadening of these diffraction peaks is reflective of polycrystal-
line as-prepared MNPs. The Bragg diffraction angles are nearly
a
Isolated yield.
Similar to the literatures m.p. [33].
b
reaction mixture was heated at appropriate temperature as men-
tioned in Table 1. The progress of the reaction was monitored by
TLC. After completion of the reaction (as shown in Tables 2–4), the
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Fig.1. FT-IR spectrum of Fe3O4 MNPs and PEI@Si–MNPs.
Fig. 2. XRD patterns of Fe3O4 MNPs and PEI@Si–MNPs.
identical, indicating that the structures of the MNPs are preserved
after PEI coating. The average crystallite sizes of MNPs were
estimated 17.2 nm using Scherrer's equation.
from the lattice. Final weight loss of 0.8% is appeared at the range
300–800 °C which is due to the phase transition of Fe3O4 to FeO
which is thermodynamically stable above 570 °C in phase diagram
of the Fe–O system [39]. Rahman et al. [39] were also exhibited
two steps of weight loss including 0.4%, 2% and 2.1% at tempera-
ture ranges of 25–150 °C, 150–300 °C and 650–800 °C, respec-
tively. Thermogram of MNPs@PEI was also show three steps of
weight loss attributed to the 5% weight loss in the temperature
range of 50–150 °C relating to the vaporization of water, followed
by 20% weight loss at the temperature range of 150–500 °C, which
could be ascribed to oxidative decomposition of organic moiety. In
the final step, the weight loss (5% at the temperature range of
3.1.3. TGA
TGA curves of Fe3O4 MNPs and PEI@Si–MNPs are shown in
Fig. 3a. Three steps of weight loss are found in the thermogram of
MNPs. The first weight loss of about 1% at the range of 25–150 °C
could be attributed to the desorption of hydrogen bonded water
molecule, and the second weight loss of approximately 0.5% at
150–300 °C may be due to the removal of trapped water molecules
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Fig. 3. (a) TGA of Fe3O4 MNPs and PEI@Si–MNPs; (b) hysteresis loops of MNPs and PEI@Si–MNPs; (c–e) visible color change from red to gray during spiro-oxindole
preparation and catalyst ability to effective recovery at the end of reactions. (For interpretation of the references to color in this figure legend, the reader is referred to the
web version of this article.)
Q3
Fig. 4. SEM image of (a) Fe3O4 MNPs, (b) PEI@Si–MNPs and (c, d) TEM image of PEI@Si–MNPs. (e) HDs sizes of Fe3O4 MNPs and (f) PEI@Si–MNPs.
400–800 °C) could be ascribed to the phase transition of Fe3O4 to
FeO. From the percentage of weight loss in the TGA curve, the
weight loss of MNPs@PEI nanoparticles was 30%. Comparison
between the TGA results of Fe3O4 MNPs and PEI@Si–MNPs show
the percentage of EPO and PEI grafting density is about 20%.
3.2. Magnetic measurements
3.2.1. VSM
Magnetic measurements for Fe3O4 MNPs and PEI@Si–MNPs
were performed using a vibrating sample magnetometer (VSM)
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with a peak field of 8 kOe and their hysteresis curves are presented
in Fig. 3b. It could be seen from the loops in Fig. 3b that the specific
saturation magnetization (ss) and the coercive field (Hc), which
represents the force required to rotate the magnetization vector of
a magnet out of its equilibrium direction, for the Fe3O4 MNPs are
46.4 emu gꢁ1 and 32 Oe, respectively and those for PEI@Si–MNPs
are 30.2 Oe and 33.8 emu gꢁ1, respectively. The decrease in mass
saturation magnetization can be ascribed to the contribution of
the non-magnetic silica and PEI shell. Although the ss values of
the PEI@Si–MNPs have decreased; they still could be efficiently
separated from solution with a permanent magnet. Fig. 3c–e
shows the ability of the catalyst to effective recovery at the end
of reactions.
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3.3. Size and shape
Fig. 5. Catalyst recycling experiments.
3.3.1. SEM and TEM
of the nanocatalyst which cause to increase the local concentration
of reactants around the active sites of the catalyst.
The SEM and TEM images of MNPs and PEI@Si–MNPs are
presented in Fig. 4a–d. They show that most of the particles have
quasi-spherical shape with the average size between 15 and 25 nm
and they present uniform particles. These results are in good
agreement with the XRD analysis (Fig. 2).
With this result in hand, same procedure was used for the
preparation of chromene derivatives 4 in excellent yields by means
of three-component reactions of 3-(dimethylamino) phenol, mal-
ononitrile, and aldehyde catalyzed by mentioned magnetic cata-
lytic system in EG/water at 40 °C and very short reaction times
(Table 1, entry14).
In order to explore the scope and the limitations of this
catalytic method, we investigated various aldehydes containing
either electron withdrawing or electron donating functional
groups under the optimized reaction conditions. The results given
in Tables 2 and 3 show that this one pot, three component
condensation completed within 50–120 min, with good isolated
yields. The results clearly indicate that reactions can tolerate a
wide range of differently substituted aldehydes. In addition, it was
observed that the reaction of 4-hydroxycoumarin was completed
under reflux condition while the 3-(dimethylamino) phenol react
at 40 °C.
In continuation to this research, a simple and an efficient one
pot synthetic approach was used for the preparation of biologically
interesting spirooxindole derivatives by means of three-compo-
nent reactions of isatin, malononitrile, and dimedone or 4-hydro-
xycoumarin catalyzed by mentioned magnetic catalytic system in
water (Table 4). Both dimedone and 4-hydroxycoumarin could
give the product in excellent yields (92–97%). We noticed that
using of 4-hydroxycoumarin need reflux condition to complete in
relative short time while the dimedone react at 40 °C.
3.4. Colloidal
3.4.1. DLS measurements
The hydrodynamic diameter (HD) sizes of Fe3O4 MNPs and
PEI@Si–MNPs were obtained in the range of 30–65 and
40–100 nm, respectively (shown in Fig. 4e and f). This increase
in HD size of aqueous dispersion of MNPs@PEI could be attributed
to coated silica and polymer layers.
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The catalytic activity of PEI@Si–MNPs was investigated for the
synthesis of different 2-amino-3-cyano-4H-pyran derivatives
using a MCR approach. For a model reaction, the reaction between
benzaldehyde, malononitrile, and 4-hydroxycoumarin was chosen
in the presence of innocuous solvents (Table 1). The catalytic
performances of Fe3O4 MNPs, PEI and PEI@Si–MNPs were screened
in water using 5 mg of catalyst (Table 1, entries 2–5). Without the
catalyst, the product was detected in 21% yields after 10 h (Table 1,
entry 1). Fe3O4 MNPs and PEI in bare form produced the product
with 45 and 62% yields, respectively (Table 1, entries 2 and 3). Yet,
their analogous combination in the form of PEI@Si–MNPs in-
creased the yield up to 82% in a shorter reaction time (Table 1,
entry 5). We then tried to screen the efficiency of the catalyst in
different solvents in order to optimize the reaction conditions. A
good result was obtained under neat conditions (Table 1, entry 6).
But, the mixture was viscous without solvent and well dispersion
and magnetically separation of the catalyst was difficult. Slightly
higher yields were obtained when propylene glycol (PG) or
ethylene glycol (EG) were used as the solvent instead of water
(Table 1, entries 7 and 8). We noticed also that reducing in the
quantity of the catalyst (5 mg) decreased the efficiency of the
reaction and no meaningful difference was obtained when the
catalyst increased slightly (data not shown). Interestingly, the
reaction takes place in high yield without catalyst in the presence
of EG/water (20/80) as solvent (72%, Table 1, entry 12). But the best
result was obtained in EG/water (20/80) by 5 mg of the catalyst in
a shorter reaction time (98%, Table 1, entry 11). The corresponding
product was also obtained in 70% and 45% yields for ethanol and
acetonitrile as the solvent, respectively (Table 1, entries 15 and 16).
These results confirmed the crucial role of solvent in the disper-
sion of the nanocatalyst and adsorption of reactants on the surface
3.6. Catalyst recycling
To evaluate the stability and level of reusability of the catalyst,
we decanted the vessel by use of an external magnet and the left
used catalyst was washed with ethanol several times to remove
residual product, dried under vacuum and reused in a subsequent
reaction. A new reaction was then conducted with fresh reactants
under similar conditions. It was found that the developed catalyst
could be used at least five times and the conversion stayed with no
detectable loss, more than 90% (see Fig. 5). Moreover, the FT-IR
spectrum of the recovered catalyst showed no change after using it
for five times. This indicates that no leaching of the PEI species
occurred from support on using and reusing the catalyst.
4. Conclusions
In summary, we showed that PEI grafted on silica coated Fe3O4
nanoparticles fabricated by covalently anchoring, was a novel
and effective heterogeneous catalyst for the one-pot synthesis
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of pyrano[3,2-c]chromene, chromenes and spiro-oxindoles deri-
vatives from commercially available starting materials. The pre-
sent method requires remarkably small amounts of non-toxic and
environmentally friendly PEI@Si–NMP as catalyst. In addition the
aqueous conditions, excellent yields, operational simplicity, prac-
ticability, product purity, cost efficiency and environmentally
benefits are the worthy advantages of this protocol. The intro-
duced magnetically nanocatalyst was highly stable and could be
reused in 5 successive runs with no significant structural change
and loss of activity.
Based on these observations, it could be concluded that this
green and cost-effective catalyst, with simple experimental and
work-up procedure, which avoids the use of large volumes of
hazardous organic solvents, makes it a useful alternative for the
scale-up of these three component reactions.
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Acknowledgment
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This work was supported by grants from the research council of
Tehran University of Medical Sciences and Health Services and
Q7 from the Iran National Science Foundation (INSF).
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Please cite this article as: M. Khoobi, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j.
jmmm.2014.09.044i