Fe3O4@Silica sulfuric acid core-shell composite as a novel nanomagnetic solid acid: Synthesis, characterization and application as an efficient and reusable catalyst for one-pot synthesis of 3,4-dihydropyrimidinones/thiones under sol

Article abstract of DOI:10.1007/s11164-013-1407-6

Abstract A simple and convenient method was used to prepare Fe₃O₄@Silica sulfuric acid core-shell composite using Fe₃O₄ spheres as the core and silica sulfuric acid nanoparticles as the shell. Magnetite nanopart

Full text of DOI:10.1007/s11164-013-1407-6

Res Chem Intermed
DOI 10.1007/s11164-013-1407-6
Fe3O4@Silica sulfuric acid core–shell composite
as a novel nanomagnetic solid acid: synthesis,
characterization and application as an efficient
and reusable catalyst for one-pot synthesis
of 3,4-dihydropyrimidinones/thiones under
solvent-free conditions
•
Ali Reza Kiasat Jamal Davarpanah
Received: 24 January 2013 / Accepted: 16 September 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A simple and convenient method was used to prepare Fe3O4@Silica
sulfuric acid core–shell composite using Fe3O4 spheres as the core and silica sul-
furic acid nanoparticles as the shell. Magnetite nanoparticles were synthesized by
the co-precipitation of FeCl2 and FeCl3 in ammonia solution. To improve the
chemical stability of magnetite nanoparticles, its surface engineering was success-
fully performed by the suitable deposition of silica onto nano-particles’ surface by
the ammonia-catalyzed hydrolysis of tetraethoxysilane. Next, the SiO2 spheres
served as a support for the immobilization of SO3H groups by simple mixing of the
core–shell composite and chlorosulfonic acid in CH2Cl2. The resulting solid acid
nanoparticles were characterized by infrared spectroscopy, scanning electron
microscope, thermogravimetric analysis, and vibrating sample magnetometer. The
catalytic activity of this solid acid nanocomposite was probed through one-pot
synthesis of 3,4-dihydropyrimidinones via three-component couplings of aldehydes,
b-diketone, and urea or thiourea under solvent-free conditions. In this reaction,
Fe3O4@Silica sulfuric acid shows a highly catalytic nature, easy to handle proce-
dure, short reaction time, recycle exploitation, and excellent isolated yields. The
nanomagnetic catalyst could be readily separated from the solution via application
of an external magnet, allowing straightforward recovery and reuse.
Keywords Fe3O4@Silica sulfuric acid Á Nanomagnetic solid acid Á
3,4-Dihydropyrimidinones Á Solvent-free
Introduction
Due to their high specific surface area, superparamagnetic properties and
biocompatibility, magnetic nanoparticles can effectively improve the loading and
A. R. Kiasat (&) Á J. Davarpanah
Chemistry Department, College of Science, Shahid Chamran University, 61357-4-3169 Ahvaz, Iran
e-mail: akiasat@scu.ac.ir
123

A. R. Kiasat, J. Davarpanah
the catalytic efficiency of immobilized catalysts [1, 2]. In this field, iron oxides,
magnetite (Fe3O4), and maghemite (c-Fe2O3) are by far the most used magnetic
nanoparticles, because they are much less toxic than their metallic counterparts, and
they have high saturation magnetization and superparamagnetic behavior, among
which magnetite is a very promising choice due to its already proven biocompat-
ibility [3]. However, an unavoidable problem associated with the application of
‘‘naked’’ magnetite nanoparticles as catalyst support is its instability over longer
periods of time due to high initial chemical activity, easy oxidation in air, loss of
magnetism, dispersion, and agglomeration. In this regard, considerable attention has
recently been paid to the preparation of the Fe3O4-based nanoparticles with a core–
shell structure [3–6]. The silane agents are often considered as potential candidates
for directly modifying the surface of magnetite [7]. Such surface modification
enhances the biocompatibility and provides quite high density surface functional
end groups which allow for connecting to catalysts [8].
Sulfuric acid is one of the most popular acid catalysts for practical chemical
processes and is widely used in the production of industrially important chemicals.
Such homogeneous acids generally require special processing in the form of
neutralization, which involves energy-inefficient catalyst separation from products
and results in the formation of a large amount of sulfate wastes [9]. Consequently,
the development of solid acid catalysts, especially of the magnetic type, is expected
to have a major impact in industrial applications as well as for scientific aspects.
During the last few years, sulfonic acid-functionalized hydroxyapatite-encapsu-
lated Fe2O3 magnetic nanoparticles [10], sulfamic acid-functionalized magnetic
Fe3O4 nanoparticles [11], and nano-n-propylsulfonated Fe2O3 nanoparticles [12]
have been successfully considered as heterogeneous magnetic solid acids in organic
transformation.
On the other hand, 3,4-dihydropyrimidinones (DHPM) and their derivatives are
very well known for their wide range of biological activities [13] as calcium channel
blockers [14], antihypertensive agents, and neuropeptide Y antagonists [15]. By
considering all the above-mentioned points and in continuation of our interest in
using solid acid catalysts in organic synthesis [16–18], here, SO3H-functionalized
silica-coated magnetite nanoparticles with a core–shell structure, Fe3O4@Silica
sulfuric acid, were successfully prepared and their performance as a magnetic
powerful solid acid catalyst for the synthesis of DHPM/thiones under solvent-free
conditions was investigated.
Experimental
General
Chemical materials were purchased from Fluka and Merck and used without further
purification. MNPs were synthesized by co-precipitation of FeCl3.6H2O and
FeCl2.4H2O in ammonia solution, according to the reported procedure [19].
Products were characterized by comparison of their physical data, IR and 1H NMR,
and 13C NMR spectra with known samples.
123

Fe3O4@Silica sulfuric acid core–shell composite
The TGA curve of the Fe3O4@Silica sulfuric acid was recorded on a BAHR SPA
503 at heating rates of 10 °C min-1 under air atmosphere, over the temperature
range of 25–1,000 °C. The particle morphology was examined by SEM.
Preparation of silica-coated magnetic nanoparticles, Fe3O4@Silica
Silica-coating of the Fe3O4 nanoparticles was carried out in a water–ethanol
solution with ammonia-catalyzed reaction of tetraethylortosilane. Then, 2.0 g of the
synthesized Fe3O4 nanoparticles were diluted with 120 mL water and 450 mL
ethanol. This suspension was dispersed under ultrasonification for 25 min. In the
presence of a constant nitrogen flux and at room temperature, ammonium hydroxide
25 % solution (10 mL) was added to the suspension. Then, under continuous
mechanical stirring, 2.0 mL TEOS was slowly added to this dispersion, and, after
stirring for 12 h, silica was formed on the surface of the magnetite nanoparticles
through hydrolysis and condensation of TEOS. The coated particles were finally
separated from the liquid by a magnetic decantation and washed several times with
deionized water. They were then dried under vacuum at 60 °C overnight.
Preparation of SO3H functionalized silica-coated magnetite nanoparticles,
Fe3O4@Silica sulfuric acid
A 500-mL suction flask was equipped with a constant pressure dropping funnel. The
gas outlet was connected to a vacuum system through an adsorbing solution of alkali
trap. Fe3O4@Silica (2.5 g) was added to the flask and dispersed by ultrasonics for
10 min in CH2Cl2 (75 mL). Chlorosulfonic acid (1.75 g, ca. 1 mL, 15 mmol) in
CH2Cl2 (20 mL) was added dropwise over a period of 30 min at room temperature.
After completion of the addition, the mixture was shaken for 90 min, while the
residual HCl was eliminated by suction. Then, the Fe3O4@Silica sulfuric acid was
separated from the reaction mixture by a magnetic field and washed several times
with dried CH2Cl2. Finally, Fe3O4@Silica sulfuric acid was dried under vacuum at
60 °C.
pH-analysis of Fe3O4@Silica sulfuric acid
To an aqueous solution of NaCl (1 M, 25 mL) with an initial pH 5.9, the
Fe3O4@Silica sulfuric acid (100 mg) was added and the resulting mixture was
stirred for 24 h after which the pH of the solution decreased to 2.10. This is equal to
a loading of 1.98 mmol SO3H/g of acidic catalyst. Additionally, this result was
confirmed by back-titration analysis of the catalyst.
Typical procedure for the preparation of 3,4-dihydropyrimidin-2(1H)-ones/
thiones
A mixture of aromatic aldehyde (1.0 mmol), b-diketone (1.0 mmol), urea or
thiourea (2 mmol) and Fe3O4@Silica sulfuric acid (0.05 g) was heated at 80 °C for
the times shown in Table 1. After complete consumption of aromatic aldehyde as
123

A. R. Kiasat, J. Davarpanah
judged by TLC (using n-hexane-ethylacetate as eluent), the mixture was washed
with distilled water to separate the excess of urea or thiourea. The crude product was
dissolved in hot ethanol and the magnetic catalyst was concentrated on the sidewall
of the reaction vessel using an external magnet. The hot solution was separated by
decantation and was allowed to reach room temperature.
Results and discussion
Investigation around the approach of Fe3O4@Silica sulfuric acid preparation
and its structural and morphological analysis
SO3H-functionalized silica-coated magnetite nanoparticles have the advantages of
both magnetic separation techniques and nano-sized materials, which can be easily
recovered or manipulated with an external magnetic field. As the acidic groups are
usually immobilized on the surface of the magnetic nanoparticles, easy access of
reactants to this active site can also be achieved. In the present study, Fe3O4
magnetic nanoparticles were prepared by a coprecipitation method from ferrous and
ferric ion solutions in basic media. The magnetic nanoparticles prepared by this
method had significant numbers of hydroxyl groups on the surface from contact
with the aqueous phase. The MNPs are easily coated with amorphous silica using
the conventional sol–gel method. The sol–gel process is based on the hydrolysis of
TEOS, and subsequent condensation of hydrolyzed TEOS on surface iron
hydroxyls. Next, the SiO2 spheres served as a support for the immobilization of
SO3H groups by simple mixing of the core–shell composite and chlorosulfonic acid
in CH2Cl2. The schematic diagram for the synthetic procedure is shown in
Scheme 1.
Table 1 Solvent-free reaction of benzaldehyde, ethylacetoacetate, and urea under different conditions
Entry
Solvent
T (oC)
Catayst (g)
Time (min)
Yield (%)a
1
Ethanol
Reflux
Reflux
Reflux
Reflux
130
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.1
180
180
300
300
30
80
2
Acetonitrile
Water
78
3
35
4
Toluene
35
5
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Mixb
Mix
Mix
78
6
120
30
7
100
30
8
90
35
9
80
35
81
10
11
12
a
80
0.05
0.02
–
35
92
80
45
62
80
180
20
Isolated yield
b
Mixture of products
123

Fe3O4@Silica sulfuric acid core–shell composite
Due to the paramagnetic nature of the nanoparticle core, the nuclear magnetic
resonance technique could not be used to confirm Fe3O4@Silica sulfuric acid
synthesis. Instead, titration and FT-IR were used to characterize the core–shell
composite structure. Sulfonic acid loading was calculated based on titration of the
proton-exchanged brine solutions and showed the loading of 1.98 mmol SO3H/g of
catalyst.
The FT-IR spectra of Fe3O4 nanoparticles, Fe3O4@SiO2, and Fe3O4@Silica
sulfuric acid are shown in Fig. 1. The FT-IR analysis of the Fe3O4@SiO2 and
Fe3O4@Silica sulfuric acid exhibits two basic characteristic peaks at *3,300 cm-1
(O–H stretching) and 580 cm-1 (Fe–O vibration) [20]. The bands at 1,092 cm-1
,
800 cm-1, and 447 cm-1 come from the Si–O–Si group. The peak at *900 cm-1
in Fe3O4@SiO2 is attributed to t (:Si–OH) of free silanol groups [21]. The
presence of sa ulfonyl group in Fe3O4@Silica sulfuric acid is confirmed by 1,217
and 1,124 cm-1 bands, which were covered by a stronger absorption of Si–O bonds
at 1,092 cm-1, and the peak of the Si–OH group at 900 cm-1 disappears for
Fe3O4@Silica sulfuric acid [11]. A wide peak at 3,393 cm-1 is due to the stretching
of OH groups in the SO3H.
Figure 2 shows the TGA curves of Fe3O4@SiO2 and Fe3O4@Silica sulfuric acid
powders. For two curves, the weight loss below 200 °C might be due to the
evaporation of the H2O molecules adsorbed onto the surface and the release of the
structural water resulted from the bonded hydroxyl groups [22]. Curve (a) indicates
that the silica-coated MNPs are thermally stable, and no obvious weight loss was
observed below 1,000 °C, while in curve (b), the mass loss of Fe3O4@Silica sulfuric
acid at 650–710 °C is attributed to the SO3H groups.
Scanning electron microscopy is a useful tool for determining the size
distribution, particle shape, and porosity. It has been a primary tool for
FeCl3.6H2O
NH4OH, N2
80 oC, 30 min
Fe3O4
MNP
+
FeCl2.4H2O
TEOS, H2O\ EtOH, N2
NH4OH, rt, 12h
SiO2
SiO2
OSO3H
OSO3H
OSO3H
OH
OH
OH
ClSO3H, CH2Cl2
rt, 30 min
MNP
MNP
2
2
O i S
O i S
Fe3O4 @ Silica sulfuric acid
Scheme 1 Schematic diagram for the synthesis of Fe3O4@Silica sulfuric acid
Fe3O4 @ Silica
123

A. R. Kiasat, J. Davarpanah
Fig. 1 FT-IR spectra of a Fe3O4, b Fe3O4@SiO2, and c Fe3O4@Silica sulfuric acid
Fig. 2 TGA curves of a Fe3O4@SiO2 and b Fe3O4@Silica sulfuric acid powders
characterizing the surface morphology and fundamental physical properties of the
surface [21]. According to Fig. 3, both kinds of nanoparticles are apparently similar
in size and shape and have nano-dimensions ranging from 90 to 170 nm. The
particles are rather uniform in their morphology.
The magnetic properties of the uncoated magnetic iron oxide (Fe3O4),
Fe3O4@SiO2, and Fe3O4@Silica sulfuric acid were measured by vibrating sample
123

Fe3O4@Silica sulfuric acid core–shell composite
magnetometer, VSM, at room temperature (Fig. 4). In Fig. 4, the hysteresis loops
that are characteristic of superparamagnetic behavior can be clearly observed for all
the nanoparticles. Superparamagnetism is the responsiveness to an applied magnetic
field without retaining any magnetism after removal of the applied magnetic field.
From M versus H curves, the saturation magnetization value (Ms) of uncoated
MNPs was found to be 66 emu g-1. For Fe3O4@SiO2 and Fe3O4@Silica sulfuric
acid, the magnetization obtained at the same field were 38 and 17 emu g-1
,
respectively, lower than that of uncoated Fe3O4. This is mainly attributed to the
existence of nonmagnetic materials on the surface of the nanoparticles.
Application of Fe3O4@Silica sulfuric acid as nanomagnetic solid acid catalyst
for one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones
To evaluate the catalytic activity of Fe3O4@Silica sulfuric acid as a nanomagnetic
heterogeneous solid acid catalyst in the synthesis of 3,4-dihydropyrimidin-2(1H)-
ones/thiones (DHPMs) through multicomponent reaction (Scheme 2), a model
three-component coupling reaction of benzaldehyde (1 mmol), ethylacetoacetate
(1 mmol), and urea (2 mmol) was examined to determine whether the use of
Fig. 3 SEM images of a Fe3O4@SiO2 and b Fe3O4@Silica sulfuric acid
123

A. R. Kiasat, J. Davarpanah
Fe3O4@Silica sulfuric was efficient and to investigate the optimized conditions
(Table 1).
Subsequently, with optimal conditions in hand, 1:1:2 molar ratios of aldehyde,
ethylacetoacetate, and urea and 0.05 g of Fe3O4@Silica sulfuric acid at 80 °C under
solvent-free conditions, the generality and synthetic scope of this coupling protocol
were demonstrated by synthesizing a series of 3,4-dihydropyrimidin-2(1H)-ones
(Table 2).
As shown in Table 2, aromatic aldehydes, having electron-releasing as well as
electron-withdrawing groups, were uniformly transformed into the corresponding
3,4-dihydropyrimidin-2(1H)-ones in high yields within 25–55 min. Substituents on
the aromatic ring had no obvious effect on yield or reaction time under the above
optimal conditions.
The reusability of the catalyst in the reaction of benzaldehyde, ethylacetoacetate,
and urea under solvent-free conditions at 80 °C was evaluated. In this procedure,
Fig. 4 VSM magnetization curves of the a Fe3O4, b Fe3O4@SiO2, and c Fe3O4@Silica sulfuric acid
nanoparticles
R
O
O
O
X
O
Fe3O4@silica sulfuric acid
Solvent free, 80 oC
R1
NH
+
+
R1
R2
H2N
NH2
H
R2
N
X
H
R2= OEt, Me
R1= Me
X= O, S
R
Scheme 2 Synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones through multicomponent reaction
promoted by Fe3O4@Silica sulfuric acid
123

Fe3O4@Silica sulfuric acid core–shell composite
Table 2 A solvent-free
Entry
Rb
Xb
R2b
Time
(min)
Yield
(%)a
reaction for synthesis of 3,4 -
dihydropyrimidin-2(1H)-ones/
thiones in presence of
1
H
O
S
OEt
OEt
OEt
OEt
Me
35
50
25
25
25
25
30
35
30
35
50
45
25
25
45
45
45
55
55
35
35
92
86
92
87
94
92
88
84
90
86
86
88
90
92
83
81
85
82
80
92
90
Fe3O4@Silica sulfuric acid
2
H
3
4-O2N
4-O2N
4-O2N
4-O2N
4-Cl
O
S
4
5
O
S
6
Me
7
O
S
OEt
OEt
Me
8
4-Cl
9
4-Cl
O
S
10
11
12
13
14
15
16
17
18
19
20
21
4-Cl
Me
4-CH3O
4-CH3O
4-CN
4-CN
2-HO
2-HO
2-HO
4-H3C
4-H3C
H
O
O
O
O
O
S
OEt
Me
OEt
Me
OEt
OEt
Me
O
O
S
OEt
OEt
Me
O
S
a
Isolated yield
H
Me
b
According to scheme 2
100
80
60
92
90
86
81
40
20
1
2
3
4
No of runs
Fig. 5 Recyclabilitry of Fe3O4@Silica sulfuric acid
after completion of each reaction, hot ethanol was added and the catalyst was
separated by an external magnet. The recovered catalyst was washed with ethanol,
dried, and reused four times (Fig. 5). The pH change during the reaction process
was also considered for fresh and recovered catalyst, and the pH decrease was not
observed after the reaction process. Based on titration of the proton-exchanged brine
solutions, fresh and recovered catalysts showed the loading of 1.98 mmol SO3H/g.
123

A. R. Kiasat, J. Davarpanah
Table 3 Comparison of the efficacy of Fe3O4@Silica sulfuric acid catalyst with some of those reported
in the literature
Entry
Catalyst
Catalyst loading
(g or wt%)
Time
(h)
Yield
(%)
Ref
1
2
3
4
5
6
7
8
Fe3O4@Silica sulfuric acid
Montmorillonite KSF
Sulfated zirconia
0.05 g
15 %
0.6
48
4
92
77
94
80
56
91
80
80
This work
[23]
15.15 %
0.3 g
[24]
PS-PEG-SO3H
10
2
[15]
ZrOCl2.8H2O
10 mol%
35 %
[25]
Silica-sulfuric acid
H3PMo12O40
6
[13]
11 %
5
[26]
Zeolite [HY(Si/Al = 2.43,
H
15 %
12
[27]
56(SiO2)136(AlO2)56
]
9
Bentonite/PS-SO3H
0.1 g
0.5
89
[28]
To evaluate the generality and synthetic scope of this coupling protocol, this
three-component condensation reaction also proceeded with acetyl acetone and
thiourea (Table 2).
In order to show the merits of the presented protocol in DHPMs synthesis, we
have compared the results of the Fe3O4@silica sulfuric acid catalyst with some of
those reported in the literature. The model reaction of benzaldehyde, ethyl
acetoacetate, and urea was considered as a representative example (Table 3). As can
be seen, in addition to having the general advantages attributed to the inherent
magnetic property of nanocatalysts, Fe3O4@silica sulfuric acid is an equal or more
efficient catalyst for this three-component reaction.
Conclusion
Fe3O4@Silica sulfuric acid core–shell composite was successfully achieved by a
multiple synthetic procedure which was confirmed with FT-IR, TGA, SEM, and VSM.
The most interesting features of the present work include durability as well as efficient
catalytic activity for one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones
via three-component coupling reactions, under solvent-free conditions at 80 °C. This
method offers several advantages including high yield, short reaction time, simple
work-up procedure, ease of separation, and recyclability of the magnetic catalyst, as
well as the ability to tolerate a wide variety of substitutions in the reagents.
Acknowledgment We are grateful to the Research Council of Shahid Chamran University for financial
support.
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