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F. Shahbazi, K. Amani / Catalysis Communications 55 (2014) 57–64
In this article, we report the synthesis and characterization of di-
was added dropwise into the dispersion and ultrasonicated for 30 min.
Subsequently, the mixture was refluxed at 110 °C under constant stir-
ring for 24 h. The products were magnetically separated and washed
two times with toluene and two times with ethanol and then dried at
60 °C.
amine modified silica coated magnetite-polyoxometalate nanoparticles
as a novel nanomagnetically-recoverable catalyst and evaluate its cata-
lytic activity in the synthesis of tetrahydrobenzo[b]pyran derivatives
and Knoevenagel condensation in aqueous media. To the best of our
knowledge, this is the first time that H3PW12O40 supported on
diamine-functionalized Fe3O4 magnetite nanoparticles is reported.
2.2.4. Synthesis of diamine-functionalized modified silica coated magnetite
nanoparticles (Fe3O4@SiO2@NH-NH2)
2. Materials and methods
0.7 g of Fe3O4@SiO2-Cl was dispersed in 70 mL of CH3CN and
ultrasonicated for 20 min. Then, KI (7 mmol, 1.162 g) and K2CO3
(7 mmol, 0.967 g) were added into the dispersion and ultrasonicated
for 15 min. Subsequently, 20 mmol ethylenediamine (1.34 mL) was
added to a solution and the mixture was stirred under reflux condition
for 24 h. The obtained solid was then magnetically collected from the
solution and washed abundantly with water/ethanol and dried at 60 °C.
2.1. Materials
All chemicals were purchased from Merck chemical company, with
no further purification applied to them. Fourier transform infrared spec-
tra were recorded using FT-IR Nicolet 6700 spectrometer using KBr
plates. Scanning electron micrographs (SEMs) of the samples were
taken with Hitachi S-4160. Transmission electron microscopy (TEM)
was performed with a Philips microscope (EM 208, Tokyo, Japan)
operating at 100 kV. The X-ray diffraction (XRD) was recorded
on a Philips X'Pert MPD diffractometer equipped with Cu Kα radiation
(λ = 1.54056 Å) operated at 40 kV and 30 mA. EDAX analysis was car-
ried out by NEW XL30 144-2.5 by an active area of 10 mm2 instrument.
Thermogravimetric analyses (TGA) were performed using a TGA Q50
V6.3 Build 189 instrument. The magnetic properties were investigated
by a home-made alternative gradient force magnetometer (AGFM) in
the magnetic field range of −5000 to 5000 Oe at room temperature.
Melting points were measured with an Electrothermal 9100 ap-
paratus.NMR spectra were recorded with a Bruker DRX-250 AVANCE in-
strument (250.1 MHz for 1H and 62.5 MHz for 13C) with DMSO as
solvent. Chemical shifts are given in ppm (δ) relative to internal TMS.
All yields refer to isolated products after purification. Products were
characterized by comparison to authentic samples and by spectroscopy
data (FT-IR, 1H NMR and 13C NMR spectra). The spectra and some of the
figures are given in Supporting Information.
2.2.5. Synthesis of diamine modified silica coated magnetite-
polyoxometalate nanoparticles (Fe3O4@SiO2@NH-NH2PW)
0.3 g of Fe3O4@SiO2@NH-NH2 was dispersed in 50 mL of de-
ionized water and ultrasonicated for 30 min. Then, a solution of
H3PW12O40.6H2O (0.42 mmol, 1.254 g) in 20 mL deionized water was
added dropwise into the solution and ultrasonicated for 30 min and
the mixture was stirred for 24 h at room temperature. Finally, the
formed Fe3O4@SiO2@NH-NH2-PW was magnetically separated and
washed twice with water and dried at 60 °C. H3PW12O40 supported on
diamine-functionalized Fe3O4 magnetite nanoparticles was synthesized
as a novel nanomagnetically-recoverable catalyst. SEM (Fig. 3c) and
TEM (Fig. 3d) analyses indicated that the average size of Fe3O4@SiO2@
NH-NH2-PW was approximately 60 nm.
2.3. Catalytic reactions
2.3.1. General procedure for the synthesis of tetrahydrobenzo[b]pyrans
A mixture containing an aromatic aldehyde (1 mmol), malononitrile
(1.2 mmol, 0.079 g), 5,5-dimethyl-1,3-cyclohexanedione (1 mmol,
0.140 g) and Fe3O4@SiO2@NH-NH2-PW (0.030 g) in water (4 mL) was
stirred at reflux for an adequate amount (Table 2). The progress of the
reaction was monitored by TLC (eluent: n-hexane/EtOAc, 10:7). After
completion of the reaction, 5 mL EtOAc was added and the catalyst
was separated by an external magnet. Then, the product was extracted
with EtOAc (3 × 5 mL) and dried with anhydrous Na2SO4. After evapo-
ration of EtOAc, the crude product was recrystallized from EtOH to af-
ford the solid product in excellent yield.
2.2. Methods
2.2.1. Synthesis of the magnetic Fe3O4 nanoparticles
Fe3O4 nanoparticles were prepared by chemical co-precipitation of
Fe3+ and Fe2+ions with a molar ratio of 2:1 [24]. FeCl3·6H2O
(21.6 mmol, 5.838 g) and FeCl2·4H2O (10.8 mmol, 2.147 g) were dis-
solved in 100 mL deionized water at 80–85 °C under N2 atmosphere.
Then, 10 mL of 25% NH4OH was immediately injected into the reaction
mixture. The reaction mixture was stirred for another 30 min and then
cooled to room temperature. The black precipitate was washed twice
with distilled water and twice with 0.02 M solution of NaCl. Magnetic
Fe3O4 nanoparticles were magnetically separated. The average diameter
of obtained magnetic Fe3O4 nanoparticles was estimated at approxi-
mately 20 nm by SEM (Fig. 3a).
2.3.2. General procedure for the Knoevenagel condensation
A mixture containing an aromatic aldehyde (1 mmol), malononitrile
or ethylcyanoacetate (1.2 mmol), and Fe3O4@SiO2@NH-NH2PW
(0.030 g) in water (4 mL) was stirred at reflux for an adequate
amount of time (Table 4). The progress of the reaction was monitored
by TLC (eluent: n-hexane/EtOAc, 2:1). After completion of the reaction,
5 mL EtOAc was added and catalyst was separated by an external mag-
net. Then, the product was extracted with EtOAc (3 × 5 mL) and dried
with anhydrous Na2SO4. After evaporation of EtOAc, the crude product
was recrystallized from EtOH to afford the solid product in excellent
yield.
2.2.2. Synthesis of silica-coated magnetite nanoparticles (Fe3O4@SiO2)
Silica-coated magnetite nanoparticles were prepared with Stöber
method, according to the reported method [25]. 0.7 g of magnetic
Fe3O4 was dispersed in a mixture of 32 mL ethanol, 8 mL deionized
water, and 0.6 mL concentrated ammonia aqueous solution by
ultrasonication for 30 min. Subsequently, 0.4 mL of tetraethyl
orthosilicate (TEOS) was added dropwise. After being stirred for 24 h
at room temperature, the resulted solid was magnetically separated,
washed twice with water and twice with ethanol, and then dried at
60 °C. The average diameter of obtained magnetic nanoparticles was es-
timated at approximately 30 nm by SEM (Fig. 3b).
3. Results and discussion
3.1. Catalyst preparation and characterization
First, the chemical co-precipitation of Fe2+ and Fe3+ ions in the
basic solution led to the formation of Fe3O4 magnetite nanoparticles
[24]. Then, in order to avoid possible oxidation or aggregation of the
iron oxide nanoparticles, a layer of SiO2 was coated on the nanoparticles.
This was achieved using the modified Stöber method [25]. Treatment of
2.2.3. Synthesis of chloropropyl-modified silica-coated magnetite nanopar-
ticles (Fe3O4@SiO2-Cl)
0.7 g of Fe3O4@SiO2 was dispersed in 100 mL of toluene and ul-
trasonicated for 30 min. Then, 1 mL of 3-chloropropyltrimethoxysilane