Synthesis and characterization of magnetic bromochromate hybrid nanomaterials with triphenylphosphine surface-modified iron oxide nanoparticles and their catalytic application in multicomponent reactions

Article abstract of DOI:10.1039/c4ra04654d

In this work, the preparation of a new magnetic hybrid core-shell nanomaterial is reported in four steps. The magnetic Fe₃O₄ nanoparticles were synthesized via a simple hydrothermal system. Tetraethyl orthosilicate was used as a sour

Full text of DOI:10.1039/c4ra04654d

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Synthesis and characterization of magnetic
bromochromate hybrid nanomaterials with
triphenylphosphine surface-modified iron oxide
nanoparticles and their catalytic application in
multicomponent reactions
Cite this: RSC Adv., 2014, 4, 29765
Ali Maleki,* Rahmatollah Rahimi, Saied Maleki and Negar Hamidi
In this work, the preparation of a new magnetic hybrid core–shell nanomaterial is reported in four steps. The
magnetic Fe
was used as a source of SiO
functionalization of Fe
3
O
4
nanoparticles were synthesized via a simple hydrothermal system. Tetraethyl orthosilicate
2
for the creation of Fe @SiO core–shell nanoparticles. The subsequent
3
O
4
2
O
3 4
@SiO
2
was achieved by using (3-chloropropyl)trimethoxysilane and
ꢀ
triphenylphosphine. Finally, immobilization of the anionic part including the [CrO
3
Br] was carried out to
obtain Fe @SiO @PPh @[CrO
3
O
4
2
3
3
Br]. The structure and morphology of the prepared nanocatalyst was
characterized by using elemental analysis, scanning electron microscopy (SEM), transmission electron
microscopy (TEM), X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FT-IR)
and a vibrating sample magnetometer (VSM). The prepared catalyst showed efficient activity as an
heterogeneous catalyst in the synthesis of 1,4-dihydropyridine derivatives at room temperature.
Moreover, the catalyst can be readily recovered by simple magnetic decantation and can be recycled
several times with no significant loss of catalytic activity.
Received 17th May 2014
Accepted 11th June 2014
DOI: 10.1039/c4ra04654d
www.rsc.org/advances
purposes such as applications as an adsorbent, catalysis
support, and enzyme immobilization.
Introduction
6
Heterogeneous catalysts are especially interesting in academic
Multicomponent reactions (MCRs) are one of the most
and industrial research, mainly because of their simple sepa- effective and important tools in modern synthetic organic
1
ration from a mixture of products and due to their reusability.
chemistry. These reactions can make products by forming
However, the heterogeneous catalysts broadly need tedious multiple new bonds via a one-pot process without separating
preparation and/or separation processes, and there is a neces- the intermediates. Efficiency, atom economy and simplicity are
sity to obtain new materials with special properties such as the key features that make them a common area of research in
7
magnetism in order to overcome these weaknesses. Magnetic modern organic reactions.
nanoparticles are an important class of nanostructures mate-
1,4-Dihydropyridine (DHP) derivatives contain a large family
rials of current interest, mostly due to their advanced techno- of medicinally important compounds with diverse pharmaco-
logical and medical applications. Among the various magnetic logical and therapeutic properties, such as a vasodilator, hep-
nanoparticles, Fe O nanoparticles are arguably the most atoprotective, antiatherosclerotic, bronchodilator, antitumor,
3
4
8
extensively studied and have recently emerged as promising geroprotective and in antidiabetic activities. Although the
supports for the immobilization of core–shell metal nano- synthesis of DHPs has been endorsed by a variety of homoge-
particles. Fe
the reaction medium by an external permanent magnet. In suffer from limited scope, such as low yields, long reaction
3 4
O
-supported nanocatalysts can be separated from neous and heterogeneous acid catalysts, some of these methods
2
–5
9,10
general, in order to inhibit direct contact between magnetite times, expensive catalysts and difficult work-up procedures.
nanoparticles and also to prepare a suitable surface for the
In the present work, we aimed to prepare a magnetically
modication of magnetite nanoparticles, a coating of the recoverable nanocatalyst, in which bromochromate was
surface with silica shell is necessary. A silica surface can be immobilized on triphenylphosphine-functionalized silica-
easily functionalized with various organic groups for favourable coated iron oxide nanoparticles. One of the advantages of this
system is the relatively strong chemical interaction between the
bromochromate anion and the phosphonium groups func-
tionalized on the surface of the magnetite nanoparticles.
Moreover, due to the presence of the magnetite core, the
Department of Chemistry, Iran University of Science and Technology, Tehran
1
7
6846-13114, Iran. E-mail: maleki@iust.ac.ir; Fax: +98-21-73021584; Tel: +98-21-
7240640-50
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Preparations of Fe
3 4
O nanoparticles (MNPs)
Fe O magnetite nanoparticles were provided by a known
3
4
11,12
procedure.
3 2
In general, FeCl $6H O (2.7 g) was dissolved in
ethylene glycol (70 mL) to form a clear solution, followed by the
addition of NaOAc (7.2 g). Then, the mixture was stirred vigor-
ously for 30 min and sealed in a 100 mL Teon-lined stainless-
steel autoclave. Next, the autoclave was heated and maintained
ꢁ
at 200 C for 8 h, and then cooled to room temperature. The
black Fe O particles were washed several times with ethanol
3
4
ꢁ
and dried at 60 C in a vacuum oven.
Preparation of silica-coated magnetite nanoparticles
3 4 2 3
Fig. 1 Preparation and catalytic application of Fe O @SiO @PPh @
(
Fe O @SiO ) (SCMNPs)
[
3
CrO Br].
3 4 2
The interlayers of SiO were prepared through a modied Stober
2
13–20
method.
In a typical process, as-prepared Fe O particles
3 4
prepared hybrid nanomaterials have superparamagnetic prop- (1.5 g) were dispersed in a mixture of ethanol (120 mL), deion-
erties, which make them easily separable in potential applica- ized water (30 mL) and concentrated ammonia aqueous solu-
tions in various elds, such as for oxidation of olens and tion (25 wt%, 3.8 mL) by ultrasonication for 15 min. Next, 5 mL
alcohols and for multicomponent reactions as magnetically of tetraethyl orthosilicate (TEOS) was added dropwise. Aer
recoverable catalysts. The catalytic activity of Fe O @ stirring for 12 h, the products were collected and washed with
3
4
ꢁ
SiO @PPh @[CrO Br] was investigated in a Hantzsch-DHPs deionized water and then dried under vacuum at 60 C for
2 3 3
synthesis via a four-component reaction strategy. Herein, DHP further use.
derivatives are prepared by a MCR of dimedone, a b-ketoester,
an aldehyde, and ammonium acetate (Fig. 1). In comparison
Functionalizing of silica-coated magnetite nanoparticles
Fe O @SiO (SCMNPs) with triphenylphosphine
with other catalysts, this nanocatalyst has both metal ions
and triphenylphosphine; therefore, it has phase transfer
catalyst properties. The phase transfer catalyst property of
the catalyst supports both a reduced time and increased yield
of reaction. Therefore, briey, these properties of the new
catalyst offer several advantages, such as high yields, short
reaction time, simple work-up procedure and reusability of
the catalyst.
3
4
2
(PCPTSCMNPs)
Initially, 1 g of silica-coated magnetite nanoparticles were
20
dispersed in dry toluene (80 mL) by ultrasonication. Then,
(
(
3-chloropropyl)trimethoxysilane (5 g) and triphenylphosphine
8 g) were added in dispersed nanoparticles, and the mixture
ꢁ
was reuxed at 110 C for 48 h. Aer that, the solid product was
cooled and isolated by a magnet, washed twice with deionized
water (100 mL) and ethanol (100 mL), and dried in vacuum
ꢁ
Experimental
at 60 C.
Materials and instrumentation
Synthesis of bromochromate (VI), [CrO
3
Br]
All chemicals were purchased from Merck, Fluka and Aldrich.
Melting points were measured on an Electrothermal 9200 To a solution of CrO
3
(0.01 mol/100 mL) in H O, a solution of
2
apparatus and are uncorrected. Fourier transform infrared (FT- (0.01 mol) in HBr was added with stirring at room temperature
IR) spectra were registered using a Perkin-Elmer Spectrum RXI until an orange solution was formed.
FT-IR spectrometer using pellets of the nanomaterials diluted
with KBr. Chemical analyses were performed with Philips-
PW1480 X-ray uorescence (XRF) spectrometer. The crystalline
Synthesis of Fe
PCPTSCMNPs)
3 4 2 3 3
O @SiO @PPh @[CrO Br] (BCr-
phase of the nanoparticles were recognized by X-ray diffraction
Fe
3
O
4
@SiO
2
@PPh
3
(1 g) was dispersed into 100 mL of distilled
˚
ꢁ
(
XRD) measurements using Cu K radiation (l ¼ 1.54 A) on a
a
ꢀ
water and [CrO
mixture was stirred at room temperature for 24 h. Aer that, the
resulting Fe O @SiO @PPh @[CrO Br] was washed with water
and diethyl ether thoroughly and dried in a vacuum oven at
3
Br] (0.1 mmol) was added. The resultant
Philips-PW1800 diffractometer in the 2q range of 4–90 . Scan-
ning electron microscopy (SEM) images of the samples were
taken with a Zeiss-DSM 960A microscope with an attached
camera. Transmission electron microscopy (TEM) images were
obtained through a Zeiss EM 900 electron microscope operating
at 80 kV. Magnetic susceptibility measurements were carried
out using a vibrating sample magnetometer (BHV-55, Riken,
3
4
2
3
3
ꢁ
6
60 C.
General procedure for the preparation of 1,4-dihydropyridines
Japan) in the magnetic eld range of ꢀ10 000 Oe to +10 000 Oe To a mixture of dimedone (1 mmol), ethylacetoacetate (1 mmol)
at room temperature. The Cr leaching content of the catalyst and BCr-PCPTSCMNPs (0.01 g) in EtOH (5 mL), aldehyde
was measured by inductively coupled plasma (ICP) by Shimadzu (1 mmol) and ammonium acetate (1 mmol) were added at
S 7000.
room temperature. The reaction mixture was stirred at room
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temperature until the reaction was completed (monitored by SCMNPs, PCPTSCMNPs, BCr-PCPTSCMNPs are shown in Fig. 2.
TLC). Then, the mixture was cooled to room temperature and As shown in Fig. 2a, Fe O usually presents bands at ꢂ430 and
3
4
ꢀ
1
BCr-PCPTSCMNPs were separated by an external magnet. Aer 576 cm due to Fe–O vibrations in octahedral and tetrahedral
22,23
ꢀ1
evaporation of the solvent, the solid product was ltered and sites, respectively.
The strong absorption band at 1095 cm
recrystallized from EtOH to obtain the pure products. The pure was ascribed to Si–O–Si vibrations, demonstrating the presence
2
4,25
ꢀ1
products weight and yields were obtained via calculation.
of SiO
Fe. These outcomes indicate that SiO
surfaces of Fe @SiO core–shells (Fig. 2b).
of propyl and triphenylphosphine were veried by the stretch-
2
.
The weak bond at 810 cm is characteristic of Si–O–
2
is immobilized on the
16–19
3
O
4
2
The presence
Results and discussion
Preparation of magnetite bromochromate nanomaterials
ꢀ1
ing vibrations appearing at about 2830, 2900 cm in the FT-IR
6,26
spectrum of PCPTSCMNPs (Fig. 2c). In Fig. 2d, the presence
The order of steps in the immobilization of the magnetite core–
shell nanoparticles is shown in Fig. 1. First, Fe O magnetic
3 4
of the absorption bands at 770, 805, 895, 905 (nasymmetric Cr]O
(A )), 940 (nsymmetric Cr]O (E)) demonstrate the immobilization
1
nanoparticles were synthesized by a hydrothermal procedure.
Next, the external surface of the MNPs was coated with silica
shell to create SCMNPs. Sturdy materials such as silica as
coupling layers are used in the preparation of magnetically
recyclable nanocatalysts (MRNCs) frequently. The hard
coupling layers can act as anchoring sites for the nanocatalysts
or the precursors of the nanocatalysts to support them, and thus
combine both superparamagnetic components and nano-
catalysts. Magnetic cores are completely protected by the solid
silica outer layer against the external environment; thus,
MRNCs can be applied in relatively difficult reaction conditions
of the bromochromate.
3 4 2 3 3
Chemical analysis of the Fe O @SiO @PPh @[CrO Br] by X-
ray uorescence (XRF) is shown in Table 1. As can be seen in
this table, the quantity of chromium is nearly 0.39%, which
conrms the immobilization amount of [CrO Br] was 0.0075
mol/100 g. Loss on ignition (L.O.I.) calculations indicate the
quantity of organic matter in Fe O @SiO @PPh @[CrO Br].
The crystalline structure of the magnetite bromochromate
nanoparticles is specied by XRD as demonstrated in Fig. 3. The
results are in accord with standard patterns of an inverse cubic
ꢀ
3
3
4
2
3
3
spinel magnetite (Fe
3 4
O ) crystal structure, showing six diffrac-
(strong acidic and strong oxidizing).
ꢁ
ꢁ
ꢁ
tion peaks at 2q about 29.70 (2 2 0), 35.21 (3 1 1), 42.96 (4 0 0),
Moreover, the silica surface can be functionalized using a
ꢁ ꢁ ꢁ
5
1
3.53 (4 2 2), 56.97 (5 1 1) and 62.52 (4 4 0) (JCPDS Card no.
multiplicity of known surface modiers. Compared with the
hard coupling layer materials indicated above, so organic
molecules with several functional groups, such as –SH, –COOH,
27,28
9-0629).
The strong and sharp peaks indicated that Fe O4
3
crystals are highly crystalline. As can be seen, the resultant
nanomaterials show only the typical diffraction peaks of
magnetite nanoparticles, and there are no specic peaks for
bromochromate. This illustrates that the bromochromate
species are well dispersed on the surface of PCPTSCMNPs, and
there is no longer any crystalline phase of bromochromate in
the resultant nanomaterials. The average crystallite size of
nanoparticles was also determined from X-ray line broadening
using the Debye–Scherrer formula (D ¼ 0.9l/b cos q, where D is
–
2
OH, –NH and pyrrole rings, can be more simply attached to
the surface of magnetic components, as well as to catalytic
active species, through covalent bonds, coordination bonds,
and electrostatic interaction. Most of the time, hard and so
coupling layers were used together for the convenient fabrica-
tion of MRNCs. The hard coupling layers mainly serve as a
protective shell for the magnetic components, while the so
coupling layers with multiple functional groups act as
anchoring sites for the catalytic active species. The most
frequently used combination of hard and so buffer layers is
silica and silane coupling agents or polymers. The action of the
silanol groups of SCMNPs with triphenylphosphonium (3-
chloropropyl)trimethoxysilane produced PCPTSCMNPs.
The next step involved the reaction of the PCPTSCMNPs
groups with bromochromate to yield the magnetite bromo-
chromate nanomaterials BCr-PCPTSCMNPs. In this step,
phosphonium groups give positively charged cations, which
bonds electrostatically to the bromochromate. As a result, the
excess of bromochromate was removed by washing the resulting
materials with water. The attached bromochromate anions are
established on the surface of modied magnetite and could not
6,21
be leached to aqueous solution.
Fig. 2 FT-IR spectra of: (a) Fe
O
3 4
3 4 2 3 4
, (b) Fe O @SiO , (c) Fe O @
SiO @PPh and (d) Fe @SiO @PPh
2
3
3
O
4
2
3
@[CrO Br].
3
Characterization of the prepared magnetite–bromochromate
nanomaterials
Table 1 XRF results of Fe
O
3 4
2 3 3
@SiO @PPh @[CrO Br]
In order to exhibit the surface modication of the magnetite
nanoparticles and preparation of the magnetite bromochro- Element
mate nanomaterials, the FT-IR spectra of the prepared MNPS, Percentage
Fe
23.28
Si
26.85
O
32.80
Cr
0.39
P
1.92
L.O.I.
14.76
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3 4 2 3 3
Fig. 3 XRD pattern of Fe O @SiO @PPh @[CrO Br].
3 4 2
Fig. 5 TEM images of Fe O @SiO .
the average crystalline size, l is the X-ray wavelength used, b is
the angular line width at half maximum intensity and q is the
Bragg's angle). For the 3 1 1 reection, the average size of the
The average particle size of Fe
3
O
4
@SiO
2
was about 15 nm.
During the seeded sol–gel method, the thickness of the silica
shells of the Fe @SiO particles can be appropriately adapted
7,29
Fe O NPs was calculated to be around 24 nm.
3
4
3
O
4
2
The morphology and size details of the nanocatalyst were
investigated by SEM measurement, which is illustrated in
by controlling the addition amount of silica source (TEOS).
With the growing of the particle sizes, the magnetic properties
of the particles reduces, which was inadequate for the separa-
tion of the catalyst from the reaction mixture. Usually,
compared with the large-size support solid, nanoparticles with
small particle sizes have a larger specic surface area and better
dispersion in the solvent, which can have useful effects for the
Fig. 4a. The SEM image of pure Fe
3 4
O indicated that the mean
diameter of the Fe particles is around 15 nm. Aggregation
3 4
O
enhances the size of the observed nanoparticles as observed in
the SEM image. Fig. 4b and c show that the iron oxide nano-
particles have a core–shell mode and spherical morphology.
Fig. 4d–f show images of the obtained Fe O @SiO @PPh @
3
4
2
3
30
catalysis.
3
[CrO Br].
Magnetic measurements were carried out using a vibrating
sample magnetometer (VSM) at 300 K. The magnetization
The TEM images shown in Fig. 5 indicated that the silica–
magnetite composites made as supports for the catalyst had
good spherical morphologies and regular core–shell structures.
curves measured for the Fe
3 4 3 4
O nanoparticles and for Fe O @
SiO @PPh @[CrO Br] are compared in Fig. 6. There was no
2
3
3
hysteresis in the magnetization for the two examined nano-
particles. As can be seen in Fig. 6, the values of the saturation
ꢀ
ꢀ1
1
magnetization were 53 emu g for Fe
3
O
4
nanoparticles, 37 emu
, 31 emu g for Fe @SiO @PPh and 28
3
@SiO @PPh @[CrO Br]. The reduction of
ꢀ1
g
for Fe
3
O
4
@SiO
2
4
3
O
4
2
3
ꢀ
1
emu g for Fe
3
ꢀ
O
2
3
1
nearly 25 emu g in the saturation magnetization suggests the
presence of some SiO @PPh @[CrO Br] on the surface of the
2
3
3
magnetic supports. Even with this reduction in the saturation
magnetization, the catalyst still can be competently and easily
separated from the solution with the use of an external
magnetic force.
Fig. 4 SEM images of: (a) Fe
Fe @SiO @PPh @[CrO Br].
O
3 4
, (b and c) Fe
3
O
4
@SiO
2
, (d–f) Fig.
Fe
6
Magnetization curves of (a) Fe (b) Fe
3
O
4
,
3 4 2
O @SiO , (c)
3
O
4
2
3
3
3
O
4
@SiO @PPh (d) Fe @SiO @PPh @[CrO Br].
2
3
3
O
4
2
3
3
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Preparation of 1,4-DHPs in the presence of
Fe O @SiO @PPh @[CrO Br]
3 4 2 3 3
methyl, nitro and halides under the reaction conditions. Both
the electron-rich and electron-decient aldehydes worked well
and led to the target products in high yields. The structures of
all the products were recognized from their melting points and
FT-IR spectral data. Aer the h cycle, the Cr leaching of the
catalyst was also determined. ICP analysis of the clear ltrate
showed that the Cr content was less than 0.11 ppm, which
To investigate the catalytic activity of the prepared nanocatalyst,
a one-pot four-component synthesis of 1,4-DHP derivatives
through the Hantzsch reaction of dimedone, b-ketoesters,
aldehydes, and ammonium acetate was carried out in EtOH at
room temperature (Table 2). This protocol had the ability to
endure a variety of functional groups such as hydroxyl, methoxy,
a
Table 2 Synthesis of 1,4-dihydropyridines in the presence of Fe
O
3 4
2 3 3
@SiO @PPh @[CrO Br] nanocatalyst
b
ꢀ1
ꢁ
ꢁ
Lit. mp ( C)
Entry Aldehyde Product Time (min) Run, yield (%)
IR, KBr (cm
)
Mp ( C)
1, 91
2, 90
3, 89
4, 88
5, 85
3
275, 3200, 2957, 1705, 1647,
2
58–260
1
45
1605, 1497, 1382,
255
c
(ref. 33)
1217, 1032, 849, 536 (ref. 31)
1
2
3
4
5
, 96
, 94
, 94
, 93
, 90
3
276, 3207, 3078, 2960, 1701,
258–260
(ref. 34)
2
3
4
5
50
60
48
60
1647, 1604, 1492, 1380,
258–260
1280, 1217, 1072, 848, 554 (ref. 31)
1, 93
2, 92
3, 91
4, 89
5, 88
3278, 3199, 3076, 2960, 1701, 1643,
240–242
(ref. 35)
1608, 1517, 1488, 1344,
240
1218, 1072, 836, 696 (ref. 32)
1, 94
2, 93
3, 91
4, 90
5, 89
3390, 3082, 2958, 1700, 1645,
232–234
(ref. 35)
1600, 1483, 1379, 1220,
235–236
240–242
1074, 782, 684, 540 (ref. 31)
1, 89
2, 87
3, 86
4, 85
5, 84
3274, 3203, 3076, 2960, 1704, 1647,
241–243
(ref. 36)
1604, 1490, 1382, 1278,
1215, 1070, 840, 530 (ref. 32)
1, 87
2, 86
3, 84
4, 82
5, 81
3
292, 3209, 2960, 1720, 1637,
202–205
(ref. 37)
6
60
1608, 1483, 1380, 1215,
072, 754, 588 (ref. 31)
202–203
1
a
Reaction conditions: aldehyde (1 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol), ammonium acetate (1 mmol), catalyst (0.01 g), EtOH (5
b
c
mL), r.t. Isolated yield. IR active modes: 3275 (N–H, s), 3200 (C–H, Ar, s), 2975 (C–H, CH
and 1497 (C]C, Ar, s), 1217 and 1032 (C–O, s), 849 cm (]C–H, b) [s: strong; b: broad].
3
, s), 1705 (C]O, ester, s), 1647 (C]O, ketone, s), 1605
ꢀ1
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a
Table 3 Synthesis of 1,4-dihydropyridines in the presence of various catalysts
ꢁ
b
Entry
Catalyst
Solvent
Temp. ( C)
Time (min)
Yield (%)
Lit.
1
2
3
4
5
6
7
Cellulose sulfuric acid
H
2
O
100
Reux
r.t.
r.t.
r.t.
120–300
300
390
300–480
240
180
78–92
72
32–75
75–90
60
10
38
39
40
PPh
MgAl
NaHSO
3
C
2
H
5
OH
2
@hydrotalcites
CH
CH
3
CN
3
CN
4
@SiO
2
Fe
3
Fe
3
Fe
3
O
O
O
4
4
4
@SiO
@SiO
@SiO
2
2
2
C
2
C
2
C
2
H
5
H
5
H
5
OH
OH
OH
Present work
Present work
Present work
@PPh
@PPh
3
r.t.
r.t.
70
96
3
@[CrO
3
Br]
50
a
Reaction conditions: 4-methylbenzaldehyde (1 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol), ammonium acetate (1 mmol), catalyst
b
(0.01 g), and solvent (5 mL). Isolated yield.
shows that the BCr-PCPTSCM NPs catalyst was very stable and from the Iran University of Science and Technology, and the
could tolerate this reaction.
reviewers for valuable comments and suggestions.
As indicated in Table 3, to compare the efficiency of this new
nanocatalyst, we carried out a model reaction in the presence of
various catalysts. Fe O @SiO and Fe O @SiO @PPh needed Notes and references
3
4
2
3
4
2
3
longer reaction times than Fe O @SiO @PPh @[CrO Br] at
3
4
2
3
3
1
For a review, see: M. J. Climent, A. Corma and S. Iborra, RSC
Adv., 2012, 2, 16–58.
S. Prasad and B. Satyanarayana, J. Mol. Catal. A: Chem., 2013,
room temperature. Previously, PPh
3
was reported as a catalyst
for this reaction, in which the reaction time was between 120
2
38
and 300 min and reux conditions were required. In addition,
the present catalyst is capable of catalyzing the four-component
reaction but the other ones were applied in three-component
reactions.
370, 205–209.
3
4
J. K. Lim and S. A. Majetich, Nano Today, 2013, 8, 98–113.
N. C. Bigall, W. J. Parak and D. Dorfs, Nano Today, 2012, 7,
282–296.
5
6
A. Maleki and M. Kamalzare, Catal. Commun., 2014, 53, 67.
M. M. Farahani, J. Movassagh, F. Taghavi, P. Eghbali and
F. Salimi, Chem. Eng. J., 2012, 184, 342–346.
Reusability of the catalyst
The reusability of the catalyst is very important in large scale
industrial synthetic processes. The recyclability of the catalyst
was screened in the synthesis of 1,4-DHPs. Separating the catalyst
from the reaction mixture was simple. When the reaction was
complete, the catalyst was recovered nearly quantitatively from
the reaction ask by an external magnet and was subsequently
reused in several runs. As illustrated in Table 2, it demonstrated
nearly no loss of activity aer ve successive runs. The activities
7 M. V. Marques, M. M. Ruthner, L. A. M. Fontoura and
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ꢀ
active composition [CrO Br] . The reaction for regenerating the
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3
ꢀ
catalysts was carried out in water with [CrO Br] .
3
1
2 M. Xie, F. Zhang, Y. Long and J. Ma, RSC Adv., 2013, 3,
Conclusions
10329–10334.
This work has explained the preparation of functionalized 13 R. B. N. Baig and R. S. Varma, RSC Adv., 2014, 4, 6568–
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6572.
modied silica-coated magnetite nanoparticles with bromo- 14 J. Safari and Z. Zarnegar, J. Mol. Catal. A: Chem., 2013, 379,
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269–276.
ꢀ
spectra of the magnetite catalysts show that the [CrO
immobilized onto the surface of the Fe
3
Br] was 15 J. Davarpanah and A. R. Kiasat, Catal. Commun., 2013, 42,
3
O
4
magnetite nano-
98–103.
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Acknowledgements
The authors gratefully acknowledge the nancial support from 21 D. Zhang, C. Zhou, Z. Sun, L. Z. Wu, C. H. Tung and
the Iran National Science Foundation (INSF), partial support T. Zhang, Nanoscale, 2012, 4, 6244–6255.
29770 | RSC Adv., 2014, 4, 29765–29771
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