Shape-dependent catalytic activity of Fe3O4 nanostructures under the influence of an external magnetic field for multicomponent reactions in aqueous media

Article abstract of DOI:10.1039/c5ra14467a

High-quality and high-active one-dimensional (1-D) Fe₃O₄ nanostructures were synthesized via an external magnetic field (EMF) at the intensity range 0-526 μT in aqueous solution, without using any surfactant and organic solvent at room temperature. Characterization of the products was carried out using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectrophotometry (FT-IR), a laser particle size analyzer, surface area (Brunauer-Emmett-Teller, BET), and vibrating sample magnetometer (VSM). The results show that EMF at a critical adjusted intensity has a great influence on the structural features such as the morphology, particle size, surface pore size distribution, and magnetic properties of the Fe₃O₄ nanostructure. The morphology changed from peg-like network structures to needle-like cage structures by an exposure synthesis treatment with EMF at an intensity of 362 μT. The catalytic activity of two nanostructures prepared in the presence and absence of EMF were compared in a tricomponents reaction (TCR) for the synthesis of trisubstituted imidazoles under electromagnetic irradiation. The best results were obtained for the catalyst produced in the presence of EMF while the tricomponent reaction was carried out in the absence of EMF. This EMF condition was used for the preparation of a series of alkyl-, aryl-, and heteroaryl-substituted imidazoles from the corresponding benzoin and benzil in aqueous solution as green reaction conditions. It was found that the catalyst can be efficiently recycled and reused for several repeating cycles without significant loss of catalytic activity.

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Shape-dependent Catalytic Activity of Fe3O4 Nanostructures under
the Influence of External Magnetic Field for the Multicomponent
Reactions in Aqueous Media
Received 00th January 20xx,
Accepted 00th January 20xx
E. Rafiee,a,b† M. Joshaghania,b and P. Ghaderi-Shekhi Abadib
DOI: 10.1039/x0xx00000x
High-quality and high-active one-dimensional (1-D) Fe3O4 nanostructure was synthesized via the external magnetic field
(EMF) at intensity range 0–526 µT in aqueous solution, without using any surfactant and organic solvent at room
temperature. Characterization of the products were carried out using scanning electron microscopy (SEM), energy
dispersive X-ray spectroscopy (EDX), X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectrophotometry
(FT-IR), laser particle size analyzer, surface area (Brunauer-Emmett-Teller, BET), and vibrating sample magnetometer
(VSM). The results show that EMF at a critical adjusted intensity has a great influence on the structural features such as
the morphology, particle size, surface pores size distribution, and magnetic properties of the Fe3O4 nanostructure. The
morphology changed from peg-like network structures to needle-like cage structures by exposure synthesis treatment
with EMF at intensity 362 µT. The catalytic activity of two nanostructures prepared in the presence and absence of EMF
were compared in tricomponents reaction (TCR) for the synthesis of trisubstituted imidazoles under electromagnetic
irradiation. The best results were obtained for the catalyst produced in the presence of EMF while the tricomponent
reaction carried out in the absence of EMF. This EMF conditions was used for preparation of a series of alkyl–, aryl–, and
heteroaryl–substituted imidazoles from the corresponding benzoin and benzil in aqueous solution as green reaction
conditions. It was found that the catalyst can be efficiently recycled and reused for several repeating cycles without
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significant
loss
of
catalytic
activity.
and chemical properties,9 for example, 1-D magnetic materials have
technological applications by specific properties.10–16 It is known
that magnetic materials possess easy magnetic axes. For example,
Fe3O4 has easy magnetic axes along the (111) and (110) directions17
under external magnetic field to form 1-D nanostructures of
magnetic materials. Dimensional orientation can be tuned magnetic
properties of the materials. Many efficient techniques were
developed for preparing the 1-D Fe3O4 structures, mainly including
template-assisted strategy.18,19 However, the preparation and
removal of a template are difficult tasks in some cases.18 Template-
free strategies are always of great interest, since the post-
treatment of linear templates may have a bad effect on the
magnetic and electronic properties of the resulting structures.20
According to previous researches, the magnetic field can be
influence not only the synthesis of magnetic materials but also on
the organic reactions,21–27 but these researches mainly focused on
the synthesis of magnetic materials under EMF and the
nonmagnetic materials by this strategies. In the present work,
different Fe3O4 catalysts were prepared under various magnetic
field conditions as the magnetic catalysts. Reactivity of the
produced catalyst was also investigated in tricomponent reactions
(TCRs) of 1, 2– diketones, benzaldehyde, and ammonium acetate in
water as green reaction conditions.
Introduction
Tuning of magnetization is important technology in the synthesis of
magnetic material. The tuning of magnetization typically has been
achieved by external magnetic field (EMF- template-free) generated
from electromagnets. Recent developments have indicated that the
magnetic field, as an elegant way, could be applied to orient
magnetic material into nano- or microscale structures in which
dipole interactions between adjacent magnetic particles couple
them together and force the reversible formation of an anisotropic
structure.1–4 Anisotropic nanostructures have attracted wide spread
attention in the past decades owing to their unique electronic,
magnetic, and optical properties in various applications.5–8 In
particular, the controlled synthesis of multi-dimensional (M-D)
metallic nanostructures is one of the research hotspots, as
anisotropic morphologies can dramatically influence their physical
Scientists have made great achievements in M-D nanostructures of
magnetic elements.28–32 To the best of our knowledge, there are
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few reports on the effect of magnetic field in chemical reactivity of conditions (Table 2, entry 4). Possibly the agglomeration of
the process while competitive factors exist in the reaction magnetic catalysts which decrease the surface area while the
system.33–37 The effect of different reaction conditions including of reaction proceeds at EMF is the original reason for lower activity
an EMF on the TCR (Scheme 1) have been investigated. The TCR is when reaction carried out in magnetic field. Thus, in other
promoted by the synthesis of magnetic materials in the presence experiments, organic reactions were carried out in the absence of
and absence of EMF to derive a novel reaction condition that magnetic field but magnetic field was used for catalyst preparation.
possesses both adequate yield and reaction time. In this paper, four
different reaction types (Scheme 1) will be contrasted together.
In next step, various parameters were investigated, such as
temperature, solvent, catalyst loading, and type of the catalyst in
the model reaction (Table 3).
Results and discussion
Table 2 Investigation of the EMF effect on the model reaction.a
Catalytic behaviour
During our initial studies, to investigate the EMF effect on the
synthesis of magnetic catalysts and main organic reaction, the ZEMF
and AEMF1 (according to Table 1) were chosen as catalysts and the
model reaction (Scheme 1) was carried out in the presence and
absence of the EMF. Based on our recent success for the synthesis
of imidazoles,33 same experimental protocol for the synthesis of
trisubstituted imidazoles was employed. A solvent-free condition
was chosen as the reaction environment at 80 °C using 0.0025 g of
the magnetic catalyst. Therefore, four different reaction types were
obtained in the reaction of benzil, benzaldehyde, and NH4OAc as
model reaction (Scheme 1 and Table 2).
Entry Catalyst type
Magnetic field
Time (min)
10 (50)
50 (70)
5 (50)
50
50 (3 h)
Yield (%)b
36 (35)
60 (95)
35 (37)
95
1
2
3
4
5
ZEMF
ZEMF
AEMF1
AEMF1
AEMF1
Mc
MFd
Mc
MFd
MFe
20 (70)
aReaction conditions: benzil, (1.0 mmol); benzaldehyde, (1.0 mmol);
NH4OAc, (2.0 mmol); solvent-free; catalyst, (0.0025 g); at 80 °C.
bIsolated yield.
cEMF intensity = 432 µT.
dMagnetic field free, T = 80 °C.
eRoom temperature.
In the first set of experiments 0.0025 g of AEMF1 as catalyst was
used in solvent-free conditions, and four different temperatures
were evaluated (Table 3, entries 1–4). In all of these experiments
the desired product was obtained, with yields ranging from 90 to
94% but when the reaction was performed in 60 °C the expected
trisubstituted imidazole was afforded in lower reaction time
compare with 30 and 40 °C (Table 3, entry 3). Recently, the
development of new strategies for using water as solvent in organic
reaction has stimulated interest among the synthetic community
since it avoid environmental issues arising from the use of organic
solvents. Excellent yield was obtained at very short reaction time in
water as solvent (Table 3, entry 5). It should be noted that amount
of AEMF1 as catalyst has an influence on the product formation.
There was a slight decrease in the yield of the corresponding
product from 98 to 94% when the amount of AEMF1 catalyst was
changed from 0.0025 to 0.003 g but there was a high decrease in
the reaction time from 25 to 10 min. No significant alteration was
verified using 0.004 g of AEMF1 as catalyst (Table 3, entries 6–8).
According to the results presented in Table 1, since AEMF1 (which
was synthesized in the presence of EMF) shows excellent catalytic
activity in the model reaction, influence of different EMF intensity
was investigated in the catalyst synthesis process and catalytic
activity of these catalysts was studied in model reaction (Table 3,
entries 7 and 9–14). It seems that AEMF1 and AEMF6 as catalysts
showed the best performance (Table 3, entries 7 and 13). Because
the synthetic procedure of the AEMF6 is tedious and not technically
simple (as discussed in Experimental section) AEMF1 was chosen as
the best catalyst. In the other hand, magnetic-free (ZEMF) catalyst
exhibited similar activity with AEMF1 at very longer reaction time
which indicates predictable role of EMF as promoter and resister in
the activity of catalyst sites (Table 3, entry 14). Catalytic activity of
Table 1 Experimental conditions for preparation of different Fe3O4 catalysts.
Entry Symbol
EMF intensity (µT)
Morphology of the products
Peg-like network
Needle-like network
Aggregate structures
Stacked leafs
Rod-like particle
Purposeful nanorods
Cotton-like structure
1
2
3
4
5
6
7
ZEMFa
0
AEMF1
AEMF2
AEMF3
AEMF4b
AEMF5c
AEMF6d
362
432
485
362
362
362
aReaction time = 40 min.
bPreliminary treatment exposure the reaction solution by EMF.
cTerminally treatment exposure the reaction solution by EMF.
dPulsed magnetic field exposure the reaction solution.
Scheme 1 Different reaction conditions.
With the using of ZEMF as catalyst at the presence of 432 µT of the
EMF, only 36% product yield was obtained after 10 min but with the
using of ZEMF catalyst in 80 °C without using any EMF, the 95%
yield was obtained in longer reaction time (Table 2, entries 1 and 2).
According to the results of entries 3 and 4 of the Table 2, it seems
that using of EMF inhibits the progress of the organic reaction but
AEMF1 is more efficient than ZEMF as catalyst. Thus, using catalyst
which was prepared in magnetic field in the model reaction in the
absence of magnetic field was chosen as the best reaction
2 | J. Name., 2012, 00, 1-3
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bulk Fe3O4 was investigated for comparison (Table 3, entry 15). In structures displayed a well-packed layered structure through the
the absence of the catalyst, no product was formed after 10 h in cross section (Fig. 1g and h respectively).
optimized reaction conditions (Table 3, entry 16).
Fig. 2a-e shows typical low and high-magnification SEM images of
the Fe3O4 nanostructures under different EMF conditions
(preliminary, terminally, and pulsed exposure the reaction solution
by EMF). Morphology of the Fe3O4 nanostructures obtained at
preliminary exposure by EMF is that of the disarrangement rod-like
particles with average diameter and length about 60 nm and 1.7
µm, respectively (AEMF4 catalyst, Fig. 2a and b). If the magnetic
nuclei were synthesized first and then the EMF was used, the
density of nanorods would decrease (AEMF5 catalyst, Fig. 2c and d).
With pulsed exposure by EMF the cotton-like structures were
appeared with good monodispersity (AEMF6 catalyst, Fig. 2e).
According to SEM images pulsed exposure by EMF plays the force
role in promoting the formation of special morphology which seems
Table 3 Optimization of reaction conditions.a
O
CHO
Catalyst
N
+
NH4OAc
O
N
H
Entry Catalyst Tem.(°C) Solvent
Catalyst Time
Yield
(%)b
35
(90)
41
(g)
type
(min)
1
2
3
4
0.0025
0.0025
0.0025
0.0025
30
40
60
80
Solvent-
free
Solvent-
free
Solvent-
free
Solvent-
free
AEMF1
60 (7 h)
AEMF1
AEMF1
AEMF1
60 (7 h)
50
(92)
92
affect the catalytic activity (Table 3, entry 13)
.
50
94
5
6
7
8
0.0025
0.0015
0.003
0.004
0.003
0.003
0.003
0.003
0.003
0.003
0.003
60
60
60
60
60
60
60
60
60
60
60
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
AEMF1
AEMF1
AEMF1
AEMF1
AEMF2
AEMF3
AEMF4
AEMF5
AEMF6
ZEMF
25
35
10
8
23
35
20
30
7
98
97
94
98
85
51
94
87
89
93
5
9
10
11
12
13
14
15
H2O
H2O
H2O
70
30 (2 h)
Fe3O4
(bulk)
None
(38)
-
16
-
60
H2O
10 h
aReaction conditions: benzyl, (1.0 mmol); benzaldehyde, (1.0 mmol);
NH4OAc, (2.0 mmol); 1.0 mL H2O.
bIsolated yield.
Characterization of the catalysts
Fig. 1 SEM images of (a, b) AEMF1, (c, d) ZEMF, (e, f) AEMF2, and (g, h) AEMF3.
According to Table 3, the synthetized catalysts under difference
EMF conditions showed the different catalytic activities in the
model reaction. Therefore we are interested to investigate reasons
of the phenomena by the SEM analysis.
Under EMF (362 µT intensity) due to the dipolar interactions,
leading to 1-D configuration with the ripening interaction process,
alongside 1-D structures connected successively and the needle-like
network was formed (AEMF1 catalyst, Fig. 1a and b). We have
carried out a similar experiment without using an EMF, and the peg-
like network of Fe3O4 is obtained, as shown in Fig. 1c and d.
According to SEM images, AEMF1 exhibits the remarkably high
density compare with structures of the ZEMF, which can
dependence with catalytic activities of these two catalyst types
(Table 3, entries 7 and 14). The results indicated that an EMF is
helpful for higher aggregation of the product because the growth
rate of nuclei under EMF increases. It can be seen that the
connected 1-D structures increase with increasing EMF intensity to
432 µT and the aggregate structure formation (AEMF2 catalyst, Fig.
1e and f). Moreover, when the intensity of EMF is increased, the
locally induced magnetic field is enhanced too and the stacked leafs
were appeared (AEMF3 catalyst). The fracture edges of the Fe3O4
Fig. 2 SEM images of (a, b) AEMF4, (c, d) AEMF5, and (e) AEMF6.
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The phase and crystalline changes, and purity of the synthetic
catalysts under different EMF conditions were characterized by XRD
(Fig. 3a). The magnetic catalysts (except AEMF6) have crystalline
structures with peaks appearing at 29.90 °, 35.30 °, 42.50 °, 53.82 °,
and 62.44 °, correspond to the (220), (311), (400), (422), and (440)
planes. The peak positions are the same as reported peaks in the
standard file (JCPDS card: 00–65–3107) of Fe3O4. The reflection
peak in 48.01 °corresponded to the FeO phase. This peak observed
for all the synthesis catalysts under EMF. It seems that the AEMF6
was observed at the amorphous structure. The reflection peaks in
24.41 °, 33.43 °, 41.14 °, and 64.27 °, correspond to the (012), (104),
(113), and (018) planes of the ZEMF catalyst can be well indexed to
the α-Fe2O3 (JCPDS card: 00–73–0603). According to above results,
the magnetic process prevented from the phase change of the
Fe3O4 nanostructures (unlike ZEMF).39,40 The above results revealed
that growth rate of the crystal plates of the Fe3O4 nanostructures
could be induced under EMF. The average crystallite sizes of the
catalysts calculated using the Debye-Scherrer equation (Dc = [kλ ⁄
(√ꢀ2exp
– ꢀ2inst) cosθ]) from the peak width of 35.30 ° of ZEMF,
AEMF1, AEMF2, AEMF3, AEMF4, AEMF5, and AEMF6 are estimated
to 34.03, 23.10, 23.69, 23.57, 22.22, 20.98, and 17.20 nm,
respectively. According to above results, this found that the size of
the magnetic nanocatalysts is influenced by the EMF, but intensity
of the EMF shows no influence in the size of the particles.
It is considered that the synthetic catalysts under different EMF
conditions indicated the different morphology and crystalline
structure that these lead to the different catalyst activities.
Especially the AEMF1 catalyst indicated the high catalytic activity in
the model reaction. Therefore we have focused on the AEMF1 as
catalyst for further characterization. The hydrodynamic size of the
AEMF1 catalyst is about 25.76 nm with PDI (Polydispersity Index) of
0.225 measured by laser particle size analyzer, meaning that low
aggregation in colloidal suspensions was existed in catalyst. Fig. 3b
shows elemental mapping images obtained by EDX for the AEMF1
catalyst.
The FT-IR spectra of the AEMF1 catalyst is shown in Fig. 4a. The
AEMF1 catalyst shows the strong absorption in the region 574 cm–1
corresponds to the metal-oxygen (Fe–O) stretching of Fe3O4.41
Moreover, the relatively high intensity of this peak indicates the
purity level and the prevalence of very small amount of defects.42
From the results of XRD and FT-IR analysis, it is suggested that
synthesis under EMF allow forming very stable Fe3O4 particles
without other metal oxide impurities.
Fig. 3 (a) XRD patterns of ZEMF, AEMF1-AEMF6 and (b) EDX spectrum of the AEMF1
catalyst.
Furthermore BET test was also performed. The result made it clear
that an applied magnetic field for the synthetic catalyst produced a
pronounced increase in the BET specific surface area. The AEMF1
exhibits the remarkably high BET value of 101.52 m2/g compare to
BET specific surface area of ZEMF (23.02 m2/g). May be this is the
main reason for higher catalytic activity of AEMF1 (Table 3, entry 7
and 14).
The M-H loop of the as-prepared Fe3O4 AEMF1 was measured at
room temperature (Fig. 5). The specific saturation magnetization
(Ms) of the nanocatalyst is 7.32 emu/g. Reducing of Ms under EMF is
same as offer results of the literature43 (M-H loop of ZEMF was
presented for comparison in Fig. 5). Furthermore, the high shape
anisotropy of 1-D structures in AEMF1 would prevent needle
magnetizing in directions other than along their easy magnetic axes.
Despite reduced Ms, the retrieving of magnet material is still alive.
This phenomena activated by high magnetic field may open up a
new way to endow nanomaterials with special magnetic properties.
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of hydrogen bond between the hydroxide and carbonyl present on
the salicylaldehyde does not effect on the yield of corresponding
product (Table 4, compare entries 1 and 12 as an example).44,45 In
the case of sterically demanding groups of the aryl aldehydes yields
of the corresponding products were acceptable (Table 4, entries 9,
17, 19, and 21). Benzo[d]imidazole was functionalized with 2, 3, 4,
5, 6– pentafluoro phenyl (Table 4, entry 19) was also synthesized
but in reflux conditions. 2– (2, 3, 4, 5, 6–Pentafluoro phenyl)-1H-
benzo[d]-imidazole is an attractive building block for the synthesis
of new organic fluorine semiconductors.46 It was observed that
there is a slight decrease in the product yield as the carbon length
chain was increased in the case of the alkyl aldehydes (Table 4,
entries 25–27).
Table 4 Preparation of trisubstituted imidazoles from 1, 2– diketones and
aldehydes catalyzed by AEMF1 as catalyst.a
Ph
O
Ph
Ph
O
N
AEMF1
R
+
O
O
H
R
Ph
N
H
NH4OAc, H2O, 60 C
Entry
Product
Time
(min)
10
Yield
(%)b
95
Ref.
[47]
Fig. 4 FT-IR spectra of AEMF1 (a) fresh catalyst, (b) after first run, and (c) seventh run of
model reaction.
1
2c
3
30
20
5
91
89
85
92
[47]
[48]
[48]
[49]
4
5
15
Fig. 5 Magnetization measurements of AEMF1 and ZEMF catalysts.
6
7
15
30
93
88
[50]
[51]
Synthesis of various trisubstituted imidazoles
The above results demonstrated that the AEMF1 catalyst under
optimized conditions can achieve excellent yield of the
trisubstituted imidazoles. A broader range of aldehydes and 1, 2-
diketones was investigated in order to evaluate the scope and
limitations of the method as presented in Table 4. In general, the
reactions are very clean with high yielding and indicating this
reaction is sensitive to electronic and steric properties of the alkyl–,
aryl–, or heteroaryl–substituents.
8
9
20
90
90
72
[51]
When benzoin was used instead of benzil comparable yield was
obtained with longer reaction time (Table 4, entries 1 and 2).
Therefore according to these results, the benzil was selected as 1,
2– diketone source for more investigations in the synthesis of
trisubstituted imidazoles. Various electron donating and
[52,50]
withdrawing groups at the orto, meta, and para-position of the aryl
aldehydes were used and afforded high yields (Table 4). Formation
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10
35
25
10
83
86
98
[48]
26
27
55
76
67
[55]
[56]
11
12
[49,50]
[48]
120
aReaction conditions: the AEMF1, (0.003 g); benzil, (1.0 mmol); aldehyde,
(1.0 mmol); NH4OAc, (2.0 mmol); 1.0 mL H2O; T = 60 °C.
bIsolated yield.
cThe AEMF1, (0.003 g); benzoin, (1.0 mmol); aldehyde, (1.0 mmol); NH4OAc,
(2.0 mmol); 1.0 mL H2O; T = 60 °C.
dThe reaction performed at reflux conditions.
13
20
92
[53]
In comparison with other homogeneous and heterogeneous
catalysts used in the synthesis of trisubstituted imidazoles, the
AEMF1 catalyst shows remarked higher activity in shorter reaction
times (Table 5).
14
15
10
20
90
84
[48]
[48]
Table 5 Comparison of the activity of the AEMF1 with other
homogeneous and heterogeneous catalysts used in model reaction.
No. Catalyst
Reaction
conditions/T (°C)
Time
(min)/Yield
(%)
Ref.
[52]
[57]
[58]
16
17
30
40
95
93
[48]
[48]
1
2
3
Bioglycerol-
based carbon
catalyst
CH3CN/55
10/84
HOAc
Flow microreactor
system under
pressure/180
2/93
Silica-bonded S- Solvent-free/130
sulfonic acid
80/95
18
20
98
93
[44]
-
4
5
Wang resin
HOAc
AcOH/100
Microwave
20 h/56
5/98
[59]
[60]
irradiation/160
Ultrasound
irradiation/40
Solvent-free/60
Neat/120
19d
9 h
F
F
F
F
6
Sulfamic acid-
Fe3O4
InF3
HBF4-SiO2
InCl3.3H2O
Silica sulfuric
acid
25/98
[61]
Ph
N
F
7
8
9
10
26/91
15/92
8.3 h/82
50/83
[62]
[63]
[64]
[65]
N
H
Ph
MeOH/30
Solvent-free/130
20
21
35
50
81
84
[47]
[48]
11
12
13
L- Proline
Y(OTF)3
Nano SBA-15
MeOH/60
C10F18/80
2, 2, 2-
9 h/90
6 h/80
3 h/92
[48]
[55]
[47]
Trifluoroethanol/90
H2O/60
14
Fe3O4
10/95
Present
work
22
23
24
25
25
5
90
96
86
90
[54]
[50]
[50]
[48]
Reusability of the magnetic catalysts
Finally, the AEMF1 catalyst was recyclable very easy with slight loss
of activity. In order to facilitate the catalyst recovery the runs were
made in tenfold scale. We consider the loss of catalyst during the
recovery process was 3 wt.% after seventh run. After completion of
the reaction, the catalyst was recovered from the reaction mixture
by external magnet and reused for the fresh reaction. It was found
that the AEMF1 catalyst could be reused at least seven times with a
slight decrease in activity (Fig. 6). According to the FT-IR spectra in
Fig. 4b and c, it seems that magnetic catalyst is stable in the
reaction and recovery process at least after seven reusing.
15
30
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ALPHA spectrophotometer at a spectral resolution of 4 cm-1
using KBr pellets in the range of 400–4000 cm-1 with a delicate
beam condenser and a liquid nitrogen cooled MCT-detector.
XRD patterns were recorded by an Inel French, EQUINOX 3000
model X-ray diffractometer using Cu Kα radiation. Scanning
electron microscopy (SEM) has been performed using an
AIS2300C microscope with scanning range from 0 to 20 keV.
Energy dispersive X-ray (EDX) measurements were made with
an IXRF model 550i attached to SEM. SEM/EDX samples were
prepared by coating of solid particles into a conductive layer.
The size distribution of the samples was obtained using a laser
particle size analyzer (HPPS5001, Malvern, UK). The magnetic
properties of Fe3O4 nanostructure were measured using a
BHV-55, Riken, Japan vibrating sample magnetometer (VSM).
The specific surface area (using BET and BJH methods), the
total pore volume, and the mean pore diameter were
measured using a N2 adsorption-desorption isotherm at liquid
Fig. 6 Recycling test of the AEMF1 catalyst and weight losing in model react ion
after 10 min.
Mechanism of the reaction
The plausible mechanism (Scheme 2) for the synthesis of
corresponding imidazole in model reaction in the presence of
AEMF1 involves the initial interaction of AEMF1 with aldehyde
leading to the formation of intermediate (I), which attacked by the
amine source gives another intermediate (II). This intermediate
reacts with amine source generating intermediate (III).
Intermediate (III) at the presence of AEMF1 reacted with benzil and
leads to the desired product.66–68
nitrogen temperature (−196 °C), using
a NOVA 2200
instrument (Quantachrome, USA). Prior to the adsorption-
desorption measurements, all the samples were degassed at
110 °C in a N2 flow for 3 h to remove the moisture and other
adsorbates. Thin layer chromatography on precoated silica gel
Fluorescent 254 nm (0.2 mm) on aluminum plates was used for
monitoring the reactions.
Conclusions
AEMF1
O
O
Fe3O4 nanostructure has been synthesized in the presence of
EMF as an efficient and stable catalyst. EMF provide a good
opportunity to design 1-D Fe3O4 with high surface area. This
magnetic catalyst showed excellent activity in the synthesis of
trisubstituted imidazole with easy separation from the
reaction mixture using EMF along with organic process do not
increased the catalytic activity of magnetic catalyst. The
catalyst could be recovered and reused at least seventh times
without significant decreasing in catalytic activity. This study is
an initial attempt toward synthesis of the 1-D Fe3O4
nanostructures. Modification of synthetic approach and
preparation, properties, and application in a wide range of
other chemical reactions are in progress and will be reported
in due course.
NH4OAc
-OAcH
NH3
H
H
AEMF1
+
(I)
NH
NH4OAc
-OAcH
H
+
NH3
(II)
-H2O
O
O
AEMF1
NH2
NH2
N
H
NH2
O
AEMF1
III
Experimental
(
)
Material and methods
N
Analytical grades of FeCl3.6H2O, FeCl2.4H2O, NaOH, 1, 2–
diketones, aldehydes, and NH4OAc were purchased from
Merck and were used without further purifications. The
N
H
magnetic field was generated by
a Helmholtz cylinder
Scheme 2 Possible mechanism for the synthesis of trisubstituted imidazoles.
permanent magnet and fuel source. The bore diameter was 3
cm, and a magnetic flux density of range of 362 to 526 µT
could be imposed. 1 H and 13 C NMR spectra were recorded on
a Bruker Avance 200 MHz NMR spectrometer with CDCl3 as
solvent and TMS as internal standard. The FT-IR spectra of the
samples were recorded at room temperature using a Bruker,
Preparation of the 1-D Fe3O4 nanostructure catalysts
In a typical procedure, FeCl3.6H2O (0.06 mol) and FeCl2.4H2O
(0.03 mol) were dissolved into the 5.0 ml of deoxygenated
water (by nitrogen gas bubbling). The resulting solution was
added drop wise into 25 ml NaOH (1.5 M) under vigorous
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mechanical stirring at room temperature. The overall
experimental process was performed in a Helmholtz cylinder
permanent magnet with a value of 362–526 µT. A black
precipitate formed after 20 min. After synthesis treatment, the
resulting black precipitate was collected, filtered, and washed
with deoxygenated distilled water several times to remove any
possible impurities, and finally dried in a desiccator at room
temperature before characterization. The as-obtained
crystalline Fe3O4 sample was denoted as the AEMF catalyst.
Preparation conditions for all catalysts and their abbreviations
were summarized in Table 1. For synthesis of AEMF6, a
mixture of iron salts in deoxygenated water and NaOH solution
was exposured by EMF (362 µT) for 5 min then the EMF was
removed (the mixture of solution was stirred). This condition
was repeated for four times.
2
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General procedure for the one-pot synthesis of trisubstituted
imidazoles
In a typical reaction, a suspension of benzil (1.0 mmol),
aldehyde (1.0 mmol), and ammonium acetate (2.0 mmol) were
added to mixture of catalyst (0.003 g) and 5 mL distilled water.
The resulting mixture was heated to 60 °C for an appropriate
time. In the case of exposure by EMF, the resulting mixture
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and exposed with 432 µT intensity for an appropriate time.
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room temperature and the solid (containing catalyst and
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imidazole. Analytical data for this compound is presented
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