Sulfonated-polyvinyl amine coated on Fe3O4 nanoparticles: a high-loaded and magnetically separable acid catalyst for multicomponent reactions

Article abstract of DOI:10.1007/s13738-019-01700-8

Abstract: In this study, a PVAm-bearing SO₃H groups/Fe₃O₄ nanocomposite was prepared via a simple and in situ polymerization of acrylamide onto the surface of magnetic nanoparticles without using organosilane precursors. In order to obtain amine-functionalized magnetic, Hofmann degradation was carried out. In the following, sulfonated-polyvinyl amine coated on Fe₃O₄ nanoparticles was prepared by covalent grafting of chlorosulfonic acid on amine groups. The introduction of polymer to the surface of magnetic nanoparticles increases the loading content with functional groups extremely. The resulting hybrid material was effectively employed as an unprecedented acid magnetic catalyst for the one-pot synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles under solvent-free conditions. High yields, mild reaction conditions, short reaction times, operational simplicity with reusability are the notable features of the catalyst. The structure of the newly developed catalyst was characterized using TGA, FT-IR, SEM, TEM, VSM, EDX, CHNS, and XRD analysis. Magnetic separation and recycling of the catalyst for at least 8 runs is possible without any significant deterioration in catalytic activity. This work is the first report of employing PVAm as a linker for immobilizing liquid acid on a support.

Full text of DOI:10.1007/s13738-019-01700-8

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-019-01700-8
ORIGINAL PAPER
Sulfonated‑polyvinyl amine coated on Fe3O4 nanoparticles:
a high‑loaded and magnetically separable acid catalyst
for multicomponent reactions
Maryam Arghan1 · Nadiya Koukabi1 · Eskandar Kolvari1
Received: 30 August 2018 / Accepted: 20 May 2019
© Iranian Chemical Society 2019
Abstract
In this study, a PVAm-bearing SO3H groups/Fe3O4 nanocomposite was prepared via a simple and in situ polymerization
of acrylamide onto the surface of magnetic nanoparticles without using organosilane precursors. In order to obtain amine-
functionalized magnetic, Hofmann degradation was carried out. In the following, sulfonated-polyvinyl amine coated on
Fe3O4 nanoparticles was prepared by covalent grafting of chlorosulfonic acid on amine groups. The introduction of polymer
to the surface of magnetic nanoparticles increases the loading content with functional groups extremely. The resulting hybrid
material was efectively employed as an unprecedented acid magnetic catalyst for the one-pot synthesis of 2,4,5-trisubstituted
and 1,2,4,5-tetrasubstituted imidazoles under solvent-free conditions. High yields, mild reaction conditions, short reaction
times, operational simplicity with reusability are the notable features of the catalyst. The structure of the newly developed
catalyst was characterized using TGA, FT-IR, SEM, TEM, VSM, EDX, CHNS, and XRD analysis. Magnetic separation and
recycling of the catalyst for at least 8 runs is possible without any signifcant deterioration in catalytic activity. This work is
the frst report of employing PVAm as a linker for immobilizing liquid acid on a support.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-019-01700-8) contains
supplementary material, which is available to authorized users.
Extended author information available on the last page of the article
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Journal of the Iranian Chemical Society
Graphical abstract
Keywords Nano-magnetic catalyst · Surface functionalization · High-loaded catalyst · Magnetic polymer · Multicomponent
reactions
the minimal work-up procedures required, low toxicity,
and price satisfy the requirements of the green chemistry
principles. Hence, since this catalyst has the advantages of
homogeneous catalysis (i.e., fast reaction rate) and heteroge-
neous catalysis (i.e., uncomplicated removal of the catalyst
after reaction), it can bridge the gap between homogeneous
and heterogeneous catalysis. An inherent disadvantage of
MNPs is the rapid aggregation to large cluster because of
the magnetic dipole–dipole attractions between particles
and consequent loss of inimitable features associated with
catalytic reactions. Therefore, to prevent irreversible aggre-
gation of magnetic nanoparticles, they are ordinarily coated
with specifc inorganic or organic materials to maintain their
nanoscale properties. On the other hand, low loading of
homogeneous catalyst onto a large amount of solid support is
a crucial problem with the usage of heterogeneous catalysts.
In the normal grafting, the organic compound can be coated
onto the surface of the solid substrate only by one layer.
Therefore, the low number of catalytic units compared to the
large surface area of magnetic nanoparticles (low loading),
clearly, leads to the application of more amounts of catalyst
[9, 10]. Therefore, it is necessary to develop and design a
simple method to obtain high-loading catalysts. One of the
good ways to overcome these limitations is the surface mod-
ifcation of magnetic iron oxide nanoparticles (NPs) with
diferent polymer grafting methods [11–13]. Improving the
solubility of magnetic nanoparticles in various solvents and
Introduction
Classical heterogeneous catalysts with constituents or parti-
cles typically on the micrometer dimensions are most widely
used in industries. In particular, with the advances in nano-
technology and nanoscience, an extensive variety of nano-
structure catalysts or solid-supported nanostructure catalysts
has been used to diverse catalytic reactions because they
have high selectivity and activity owing to their small size
(1–100 nm) and exposed active metallic or organometal-
lic center. Commonly, heterogenization is accomplished by
supporting homogeneous catalysts onto the surface of inor-
ganic solids and insoluble organic such as mesoporous silica
[1, 2], cross-linked polymers [3, 4], magnetic nanoparticles
[5, 6], and zeolites [7]. Magnetic nanoparticles (MNPs), a
type of catalyst-supported material or possible support, have
attracted extensive research interests. The most highlighted
feature of magnetic nanoparticles is their easy separation
from the reaction medium using an external magnet. This
property has made magnetic nanoparticles a desirable alter-
native to other heterogeneous catalysts, especially porous/
mesoporous systems that most of which need centrifugation
or fltration step or a monotonous work-up of the reaction
mixture for recovering the catalyst [8]. Magnetic nanopar-
ticles have also high mechanical and thermal stability and
are amenable to large-scale output. Furthermore, the negli-
gible amount of waste generated during their preparation,
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Journal of the Iranian Chemical Society
also enabling them to function for diferent applications are
some advantages of the surface-grafted polymers. Further-
more, using grafting polymer leads to higher catalyst loading
because more catalytic units can be attached to the surface
of magnetic nanoparticles. Using this polymer also ofers
the beneft of more catalytic units and thus a higher cata-
lyst loading. In continuing our attempts toward the expan-
sion of environmentally benign and efective heterogeneous
catalysts [14–20], in this research, a facile approach was
employed to prepare polyvinyl amine coated with Fe3O4
nanoparticles in which, the polyvinyl amine layer is directly
grafted onto the surface of Fe3O4 magnetic. In the following
step, the structurally stable spheres were subjected to the
sulfonation treatment to convert them into an efcient mag-
netically separable acid catalyst. Multicomponent reactions
(MCRs) continue to grow as outstanding status in medicinal
and organic chemistry because of their high degree of atom
economy and usage in the diversity-oriented synthesis [21,
22]. The optimal MCR is a mighty stratagem for the design
and the construction of diverse heterocyclic scafolds. Sub-
stituted imidazole derivatives are considered as one of the
most important group of fve-membered ring heteroaromatic
compounds because of their prevalence in natural products
and pharmacologically active compounds and wide utility. A
number of methods have been reported for the synthesis of
these classes of heterocyclic compounds [23–28]. However,
most of these methods sufer from some defciency such as
using equivalent or high weight percentage value of cata-
lyst, centrifuging or fltration methods for catalyst recovery,
long reaction times, nongreen solvents, and refux condition.
Thus, new routes for the synthesis of them have attracted
considerable attention in the search for new and rapid meth-
ods to access them. We were, therefore, interested in exam-
ining whether our as-prepared n-Fe3O4/PVAm-SO3H could
react well for the synthesis of these valuable compounds.
So, the one-pot synthesis of 2,4,5-trisubstituted imidazoles
and also 1,2,4,5-tetrasubstituted imidazoles under mild and
solvent-free conditions was carried out in the presence of
the introduced catalyst.
a Shimadzu 8400 s spectrometer using KBr pressed powder
disks. The particle size and morphology of the catalyst were
studied using scanning electron microscopy with a Philips
XL30 feld emission scanning electron microscope (Royal
Philips Electronics, Amsterdam, the Netherlands) instrument
operating at 25 kV and transmission electron microscopy
(TEM, CM30, Philips, the Netherland). The magnetic meas-
urement was carried out in a vibrating sample magnetometer
(VSM) (4 inches, Daghigh Meghnatis KashanCo., Kashan
Iran) at room temperature. Field emission scanning electron
microscope (SEM) images were acquired with a Philips XL30
feld emission scanning electron microscope (Royal Philips
Electronics, Amsterdam, the Netherlands) instrument operat-
ing at 10 kV. Energy-dispersive X-ray spectroscopic (EDX)
analysis (equipped with the SEM instrument) was done for the
elemental mapping of the nanocomposites. Thermogravimet-
ric analysis (TGA) was done using a DUPONT 951 thermal
analysis instruments heated from 25 °C to 1000 °C at ramp
5 °C/min under N2 atmosphere. CHNS analysis was carried
out on Elementar Vario-EL III series instrument. The purity
of products was checked by thin layer chromatography (TLC)
on commercial plates coated with silica gel 60 F254 using
n-hexane/ethyl acetate mixture as the mobile phase. The NMR
spectra were measured in pure deuterated chloroform and
dimethyl sulfoxide with a Bruker Avance 400 MHz instru-
ments (1H NMR 400 MHz and 13C NMR 125 MHz) and with
tetramethylsilane (TMS) as the internal standard. Melting
points were recorded on electrothermal 9100 apparatus.
Synthesis of Fe3O4 nanoparticles
The magnetic (Fe3O4) nanoparticles were synthesized based
on a reported method [18]. 3 mL FeCl3 (2 M dissolved in
2 M HCl) was added to 10.33 mL ultrapure water. In the next
step, 2 mL Na2SO3 (1 M) was added dropwise into the prior
solution in 1 min under magnetic stirring. Due to the forma-
tion of complex ions between the Fe3+ and SO32−, the color
of the solution changed from light yellow to red just after
mixing the solutions. Let the color of the solution turned
back again, after that it was added to 80 mL NH3·H2O solu-
tion (0.85 M) under intense stirring. A black precipitate was
quickly formed, which was allowed to crystallize completely
for another 30 min under magnetic stirring. The magnetic
nanoparticles were then purifed by a magnet, decantation,
and washed several times with deionized water (to pH<7.5).
The precipitates were dried in a vacuum oven at 60 °C for
12 h.
Experimental section
Materials and physicochemical characterization
techniques
The chemicals, reagents, and solvents were purchased from
Merck and Sigma-Aldrich chemical companies and employed
without further purifcation. The crystalline structures of the
samples were evaluated by X-ray difraction (XRD) analysis at
room temperature on a Siemens D5000 (Siemens AG, Munich,
Germany) using Cu-Ka radiation of wavelength 1.54°A. Fou-
rier-transform infrared spectroscopy (FT-IR) was recorded on
Preparation of polyacrylamide/Fe3O4
nanocomposite
Surface modifcation of the magnetic nanoparticles was car-
ried out using the method reported in the literature [29].
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Fe3O4 (0.25 g) and acrylamide monomer (0.125 g) in THF
(7 mL) were placed in a round-bottom fask and sonicated
for 10 min. 0.0125 g ammonium persulfate as initiator was
then added to the reaction mixture at 75 °C under vigorous
magnetic stirring. After 5 h of stirring, the obtained n-Fe3O4/
PAM nanocomposite was separated by a magnet and dried
at 60 °C overnight under vacuum.
rotary vacuum evaporation, the obtained solid product was
washed with water and recrystallized from ethanol to ofer
the pure product. The products were known and character-
ized by melting points, 1H NMR, and FT-IR spectroscopy
techniques.
General procedure for the synthesis
of 1,2,4,5‑tetrasubstituted imidazoles
Preparation of polyvinyl amine/Fe3O4
nanocomposite
The reaction was conducted in a round-bottom fask with
a magnetic stirrer and placed in an oil bath which could
control the reaction temperature. In a general reaction, a
mixture of n-Fe3O4/PVAm-SO3H nanocomposite (0.015 g),
aldehyde (1.0 mmol), benzil (1.0 mmol), ammonium acetate
(1.0 mmol), and primary aromatic amine (1.0 mmol) was
placed in a round-bottom fask. The mixture was stirred at
100 °C for an appropriate time as mentioned in Table 3.
After completion of the reaction as monitored by TLC, 5 mL
of hot ethanol was added and the magnetic catalyst was sepa-
rated by an external magnet. Finally, the solvent was evapo-
rated and the crude product was recrystallized from EtOH
to aford the pure product. The products were identifed with
1H NMR, 13C NMR, FT-IR spectroscopy techniques, and
melting points.
In order to convert polyacrylamide to polyvinyl amine, Hof-
mann degradation was carried out with Ca(OCl)2 [29]. A
mixture of Fe3O4/PAM nanocomposite (0.1 g), deionized
water (15 mL), and Ca(OCl)2 (0.08 g) was dispersed for
10 min. The suspension was then refuxed for 6 h. After-
ward, it was removed using a magnet, washed with deionized
water, and fnally dried at 60 °C overnight under vacuum
to yield polyvinyl amine-coated Fe3O4(n-Fe3O4/PVAm
nanocomposite).
Preparation of sulfonated‑polyvinyl amine/Fe3O4
nanocomposite
The n-Fe3O4/PVAm nanocomposite was sulfonated using
two-neck flask glass reactor equipped with a constant
pressure dropping funnel, containing chlorosulfonic acid
(0.6059 g, 0.0091 mmol), and a gas inlet tube for conducting
HCl gas into water as an adsorbing solution. The n-Fe3O4/
PVAm nanocomposite (1 g) was dispersed in 20 mL of dry
CH2Cl2 in the glass reactor. Chlorosulfonic acid was added
dropwise into the mixture at room temperature. During stir-
ring the mixture, HCl gas was immediately removed from
the reaction vessel. Upon completion of the addition, the
mixture was stirred for 2 h, while the remaining HCl was
snatched by suction. The resulted magnetic nanocomposite
was separated from the suspension by an external magnet,
washed several times with anhydrous ethanol and dried at
100 °C to give n-Fe3O4/PVAm-SO3H nanocomposite.
Results and discussion
Reaction processes used in order to prepare
sulfonated‑polyvinyl amine/Fe3O4 nanocomposite
We conducted this study to synthesize the polymer-grafted
magnetic nanoparticle. The strategy for preparing the
n-Fe3O4/PVAm-SO3H nanocomposite was planned accord-
ing to a simple four-step procedure as shown in Scheme 1.
The n-Fe3O4/PAM nanocomposite was obtained by the
polymerization of acrylamide in the presence of Fe3O4 using
ammonium persulfate as an initiator. To ensure unreacted
monomer, and make ungrafted polymers not remain on the
surface of magnetic nanoparticles, the obtained nanocom-
posite was magnetically separated and washed several times
with deionized water. Hofmann degradation on n-Fe3O4/
PAM nanocomposite was done by Ca(OCl)2 in order to pre-
pare an n-Fe3O4/PVAm nanocomposite. After separation
of the catalyst by an external magnet, to obtain sulfonated-
polyvinyl amine-coated Fe3O4 nanocomposite, the n-Fe3O4/
NH2 nanoparticle was transformed to n-Fe3O4/NH-SO3H
via a condensation reaction with chlorosulfonic acid. Fur-
thermore, uncoated magnetic nanoparticles were dissolved
and their uncoated magnetic nanoparticles were removed.
The catalyst was washed again several times with water and
ethanol. Multilayer and polymeric identity of the material
coated on the surface of magnetic nanoparticle provides
General procedure for the synthesis
of 2,4,5‑trisubstituted imidazoles
In a general reaction, a mixture of aromatic aldehyde
(1.0 mmol), ammonium acetate (2.0 mmol), benzil
(1.0 mmol), and n-Fe3O4/PVAm-SO3H nanocomposite
(0.015 g) as catalyst was taken to a round-bottom fask with
a magnetic stirrer and placed in an oil bath to keep the tem-
perature at 100 °C for the period of time (the reaction was
monitored by TLC) as indicated in Table 2. After comple-
tion of the reaction, the reaction mixture was dissolved in
ethanol (5 mL) and the catalyst was separated by an external
magnet. After the reaction mixture was concentrated under
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Journal of the Iranian Chemical Society
Scheme 1 Schematic of the procedure for the preparation of n-Fe3O4/PVAm-SO3H nanocomposite
FT‑IR spectra
The FT-IR spectra of n-Fe3O4, n-Fe3O4/PAM, n-Fe3O4/
PVAm, and n-Fe3O4/PVAm-SO3H nanocomposites were
recorded (Fig. 1a–d). The interpretation of FT-IR analy-
sis of n-Fe3O4/PAM and n-Fe3O4/PVAm was discussed in
detail in the preceding paper [29]. Figure 1d presents char-
acteristic absorption peaks including 1199, 1345, 1635, and
3155 cm−1 that correspond to O=S=O symmetric, O=S=O
asymmetric stretching, bending vibration of N–H, and
stretching vibration of the O–H groups in the structure of
n-Fe3O4/PVAm-SO3H nanocomposite, respectively. These
results show the successful conjugation of sulfamic acid
onto the surface of the nano-Fe3O4.
Scheme 2 Schematic representation of PVAm interaction with nano-
Fe3O4
many catalytic units resulting in a high loading level and
excellent stability of the catalyst [30]. Hence, the catalyst
can be used in low weight percentages compared to the sub-
strates. This is certainly useful for large-scale applications
[31]. Scheme 2 was developed to clarify the possible adsorp-
tion mode for n-Fe3O4/PVAm, which is a lone pair interac-
tion between the nitrogen atom and the hydroxyl protons on
the Fe3O4 surface [32].
X‑ray difraction spectra
X-ray difraction (XRD) characterization analysis was per-
formed to identify crystalline phases of the nano-Fe3O4
before and after modifcations (Fig. 2). As shown in Fig. 2,
all the samples have diffraction peaks at around 30.5°,
35.9°, 43.5°, 53.9°, 57.2°, and 63° 2θ ascribed to the inverse
cubic spinel structure of Fe3O4 [33]. The crystal size of the
n-Fe3O4/PVAm-SO3H nanocomposite was determined using
the Debye–Scherrer equation. The results show that cubic
spinel structure of n-Fe3O4/PVAm-SO3H nanocomposite
was synthesized at the size of 22 nm.
The characterization of sulfonated‑polyvinyl amine/
Fe3O4 nanocomposite
To elude any errors during the preparation procedure and
investigate stability of the catalyst before and after the cata-
lytic reaction, we performed a comprehensive characteriza-
tion of magnetic samples at diferent stages. The synthesized
magnetic nanoparticles, n-Fe3O4/PAM, n-Fe3O4/PVAm,
n-Fe3O4/PVAm-SO3H nanocomposite, and the recovered
catalyst after the eighth cycle (n-Fe3O4/PVAm-SO3H-8),
were all characterized by means of Fourier-transform infra-
red spectroscopy (FT-IR), scanning electron microscopy
(SEM), transmission electron microscopy (TEM), X-ray
difraction (XRD), thermogravimetric analysis (TGA), and
vibrating sample magnetometer analysis (VSM).
Vibrating sample magnetometer
The magnetic properties of bare n-Fe3O4 and sulfonated-
polyvinyl amine coated on Fe3O4 were measured at room
temperature through a vibrating sample magnetometer
(VSM) (Fig. 3a, b). It can be seen that the Fe3O4 nano-
particles are superparamagnetic with magnetic saturation
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Journal of the Iranian Chemical Society
Fig. 1 FT-IR spectra of a
n-Fe3O4, b n-Fe3O4/PAM, c
n-Fe3O4/PVAm, d n-Fe3O4/
PVAm-SO3H nanocomposite
(d)
1635
3155
1345
1199
(c)
(b)
1633
1452
1614
3440
3172 3423
1652
(a)
3168
3442
570
557
3850
3350
2850
2350
1850
1350
850
350
Wavenumber (cm-1)
Fig. 2 XRD pattern of a
n-Fe3O4, b n-Fe3O4/PAM, c
n-Fe3O4/PVAm, d n-Fe3O4/
PVAm-SO3H nanocomposite
(d)
(c)
(b)
(a)
10
20
30
40
50
60
70
80
90
2θ/degrees
(MS) value of about 70. The magnetic saturation value
dramatically decreased to 13 emu/g for n-Fe3O4/PVAm-
SO3H nanocomposites. However, the level of catalyst
magnetizations is still adequate to strongly respond to an
external magnet thorough catalyst recovering.
Field emission scanning electron microscopy
transmission electron microscopy
The morphology and particle shape of the n-Fe3O4,
n-Fe3O4/PVAm, and n-Fe3O4/PVAm-SO3H nanocomposite
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Fig. 3 Room temperature mag-
netization curves of a n-Fe3O4,
b n-Fe3O4/PVAm-SO3H nano-
composite
were also investigated by TEM and SEM (Fig. 4). As can
be seen in Fig. 4a and b, after conversion of polyvinyl
amine into sulfonated-polyvinyl amine, the aggregation of
the nanoparticles is clearly seen, which occurs during the
sulfonation process [34, 35]. The TEM image of n-Fe3O4/
PVAm-SO3H nanocomposite confrms the nanometer size
of the catalyst. Very similar morphology observation has
been previously reported [13, 30, 36–38].
Thermogravimetric analysis
In order to investigate the thermal stability of the n-Fe3O4/
PVAm-SO3H, the TGA curve of the nanocomposite under
N2 atmosphere was represented in Fig. 5. It can be observed
that the catalyst narrates three stages of decomposition. The
frst region, which is ascribed to the loss of trapped water
from the catalyst, is below 100 °C. The second weight loss
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Fig. 4 a SEM images of
n-Fe3O4/PVAm nanocomposite,
b SEM images of n-Fe3O4/
PVAm-SO3H nanocomposite,
c TEM image of n-Fe3O4 and
d–f TEM images of n-Fe3O4/
PVAm-SO3H nanocomposite
(10%) in the region about 200–400 °C is attributed to slow
decomposition of the grafted SO3H groups. The third weight
loss (about 550–630 °C) is related to the sudden mass loss
of SO3H groups (about 32%) [16, 17, 35, 39–41] and the
degradation of the polymer [29, 42]. In comparison with the
TGA curve of PVAm polymer [42], the thermodegradation
of the polymer begins at 150 °C and ends at 550 °C, so, as
is clear, the thermal stability of the polymer is intensifed
markedly after hybridization. According to the TGA, the
amount of sulfamic acid functionalized on nano-Fe3O4 is
evaluated to be 5.2 mmol/g.
Energy‑dispersive X‑ray (EDX) analysis
The EDX analysis was performed to analyze the chemical
composition of n-Fe3O4/PVAm-SO3H and verify the suc-
cessful incorporation of polymeric segment on Fe3O4 sur-
face and SO3H group attachment to them. The EDX analysis
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Journal of the Iranian Chemical Society
Fig. 5 TGA curve of the
n-Fe3O4/PVAm-SO3H nano-
composite
104
94
84
74
64
54
44
0
100
200
300
400
500
600
700
800
900
1000
Temperature (°C)
(Fig. 6) reveals the appearance of Fe, C, N, O, and S in the
body of the catalyst. The existence of S peak confrms the
successful covalent bond of sulfonic acid group on the sur-
face of the nanoparticles.
[39, 40]. 10 mL of NaCl solution (1 M) was added to 0.1 g
of this composite and stirred for 24 h at room tempera-
ture. During this time, ion exchange occurred between
protons and sodium ions. The catalyst was separated, and
the obtained fltrate was titrated with a standard NaOH
solution (0.01 M) in the presence of phenolphthalein as
indicator. 54.5 mL of NaOH was required to attain equiva-
lence point. According to the obtained result, the acid sites
content was determined to be 5.45 mmol/g of the catalyst
which is in good agreement with the result obtained from
Measurement of the acid site content of the n‑Fe3O4/
PVAm‑SO3H nanocomposite
The content of acid sites of the n-Fe3O4/PVAm-SO3H
nanocomposite was estimated by ion-exchange pH analysis
Fig. 6 EDX spectra of n-Fe3O4/
PVAm-SO3H nanocomposite
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Table 1 CHNS elemental analyses for n-Fe3O4/PVAm-SO3H nano-
was comparably measured with the fresh catalyst (Table 1,
entry 2).
composite
Entry
Product
Elemental analysis (%)
Assessment of catalytic activity of n‑Fe3O4/
PVAm‑SO3H nanocomposite for the synthesis
of 2,4,5‑trisubstituted and 1,2,4,5‑tetrasubstituted
imidazoles
C (wt%)
N (wt%)
12.57
S (wt%)
16.48
1
2
n-Fe3O4/PVAm- 23.26
SO3H (fresh
catalyst)
n-Fe3O4/PVAm- 22.64
SO3H (after the
8th run)
11.77
15.88
In order to investigate the catalytic performance of
n-Fe3O4/PVAm-SO3H nanocomposite, valuable com-
pounds including 2,4,5-trisubstituted and 1,2,4,5-tet-
rasubstituted imidazoles were synthesized over the
catalyst with the finest content of acid sites (reported in
section 3.2.6, 5.45 mmol/g). Therefore, the as-prepared
catalyst was applied in a mixture of aromatic aldehyde,
benzil, and ammonium acetate (Scheme 3) as well as in
the reaction mixture of benzil, ammonium acetate, alde-
hyde, and primary aromatic amine (Scheme 4) to provide
2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imida-
zoles, respectively.
TGA. As can be seen, because of multilayer and poly-
meric identity of this nanocomposite, it could have a large
amount of sulfonic acid-functionalized PVAm onto the
surface.
CHNS analysis
Table 1 illustrates the percentage of C, N, and S content
which was calculated from the CHNS analysis. The load-
ing amount of SO3H groups on the surface of n-Fe3O4/
PVAm was calculated about 5.15 mmol/g based on S atom
(Table 1, entry 1) which is in good agreement with the
loading amount calculating from titration and TGA data.
The result shows around 58% of success on sulfonation
treatment. Also, the CHNS analysis for recovered catalyst
Optimization of the reaction parameters
for the synthesis of 2,4,5‑trisubstituted imidazoles
Benzaldehydes, benzil, and NH4OAc are taken as model
substrates to optimize reaction conditions. To determine
the effect of catalyst, initially, the model reaction was car-
ried out in the absence of the catalyst that led to low yield
of the product in the prolonged reaction time (Table 2,
Scheme 3 Synthesis of 2,4,5-trisubstituted imidazoles using n-Fe3O4/PVAm-SO3H as nanocatalyst
Scheme 4 Synthesis of 1,2,4,5-tetrasubstituted imidazoles using n-Fe3O4/PVAm-SO3H as nanocatalyst
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Journal of the Iranian Chemical Society
Table 2 Optimization of the
reaction conditions for the
synthesis of 2,4,5-trisubstituted
imidazoles
Entry
Amount of
catalyst (g)
Medium
Temperature (°C)
Time (min)
Yielda (%)
1
2
3
4
5
6
7
8
–
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Water
Ethanol
Methanol
Acetonitrile
Dichloromethane
THF
90
90
90
90
90
90
60
100
120
20
20
15
10
9
50
10
9
20
20
20
20
20
20
55
90
94
97
96
96
78
98
98
71
80
85
78
56
45
0.008
0.009
0.01
0.015
0.02
0.015
0.015
0.015
0.015
0.015
0.015
0.015
0.015
0.015
9
120
10
11
12
13
14
15
Refux
Refux
Refux
Refux
Refux
Refux
Reaction conditions: benzaldehyde (1.0 mmol), benzil (1.0 mmol), and NH4OAc (2.0 mmol)
aIsolated yields
entry 1). In the following, different amounts of n-Fe3O4/
PVAm-SO3H nanocomposite were used to determine
their effects on the reaction under solvent-free condition
at 90 °C. The results in Table 2 indicated that 0.015 g
of the catalyst was sufficient for the best yield (Table 2,
entries 2–6). Because of the importance of temperature
for the synthesis of imidazole compounds, after the opti-
mization of the amount of catalyst, the model reaction
was investigated under solvent-free condition in different
temperatures (Table 2, entries 5 and 7–9). It is clear that
at lower temperatures, even if the time was prolonged
to 50 min, only low yields were observed. As a result,
the best choice was 100 °C as the optimal temperature
for our reaction. Investigation of the effect of reaction
medium for the process was carried out as follows. The
model reaction was carried out in conventional solvents
and under solvent-free condition. According to Table 2,
the results showed that solvent-free condition is the best
medium for the reaction, which is in line with the green
chemistry protocols (Table 2, entries 8 and 10–15).
Consequently, the optimum reaction conditions were
observed using 15 mg of the catalyst (n-Fe3O4/PVAm-
SO3H) at 100 °C under solvent-free condition in the reac-
tion of benzaldehyde (1.0 mmol), benzil (1.0 mmol), and
NH4OAc (2.0 mmol).
electron-donating or electron-withdrawing substituents
on an aromatic ring of araldehyde, we tested divergent
aryl aldehydes for the synthesis of 2,4,5-trisubstituted
imidazoles (Table 3). As can be seen in Table 3, alde-
hydes including electron-withdrawing (activated) and
electron-rich (deactivated) groups performed equally well
in the reaction, and the corresponding 2,4,5-trisubstituted
imidazoles were provided in good yields (Table 3). In
a scale-up experiment, we probed the reaction among
benzaldehyde, benzil, and NH4OAc as a model reaction
under optimized conditions in 10 mmol scales. The yield
of scale-up reaction did not change. This illustrated that
catalyst performance in a scale-up process is as good as
that in a 1 mmol scale reaction.
Synthesis of tetrasubstituted imidazole derivatives
In order to broaden the scope of this new protocol, for
the synthesis of tetrasubstituted imidazoles via the one-
pot four-component condensation of benzil (1.0 mmol),
aromatic aldehyde (1.0 mmol), ammonium acetate
(1.0 mmol), primary aromatic amine (1.0 mmol), and
n-Fe3O4/PVAm-SO3H (15 mg) as catalyst, we used sev-
eral substituted aromatic aldehydes and amines under the
optimized condition (i.e., 100 ̊ C, 15 mg of the catalyst,
and solvent-free condition). The results are summarized
in Table 4. The reaction proceeded efficiently with dif-
ferent kinds of aromatic aldehydes in good to excellent
yields and all of the aldehydes bearing either electron-
donating or electron-withdrawing groups could be sub-
jected successfully to this protocol.
Performance of the catalyst on several substrates
for the synthesis of 2,4,5‑trisubstituted imidazoles
To further extend the scope of n-Fe3O4/PVAm-
SO3H-catalyzed reaction and summarize the effect of
1 3

Journal of the Iranian Chemical Society
Table 3 Synthesis of
Entry
Aldehyde
Time (min)
Yielda (%)
Mp (°C)
Mpref (°C)
2,4,5-trisubstituted imidazoles
in the presence of n-Fe3O4/
PVAm-SO3H nanocomposite
1
2
3
4
5
6
7
8
PhCHO
10
20
15
15
12
20
15
12
12
15
25
30
15
20
98
82
90
95
97
95
89
92
88
97
95
89
92
94
271–273
257–258
231–234
211–212
228–230
267–268
209–211
259–261
199–200
261–263
301–302
230–232
270–272
232–233
270–272 [43]
256–257 [18]
230–233 [43]
210–211 [18]
228–231 [43]
267–268 [18]
211–212 [18]
260–261 [43]
200–201 [18]
262–263 [18]
301–303 [43]
230 [44]
4-NMe2C6H4CHO
4-MeC6H4CHO
2-OMeC6H4CHO
4-OMeC6H4CHO
3-OEt-4-OH C6H3
2-OH-C6H4CHO
3-OH-C6H4CHO
2-Cl-C6H4CHO
4-Cl-C6H4CHO
3-Br-C6H4CHO
4-Br-C6H4CHO
3-NO2C6H4CHO
4-NO2C6H4CHO
9
10
11
12
13
14
269–271 [43]
231–233 [18]
Reaction condition: aldehyde (1.0 mmol), benzil (1.0 mmol), NH4OAc (2.0 mmol), and nano-Fe3O4-
NHSO3H (15 mg), solvent-free, at 100 °C
aIsolated yields
Table 4 Synthesis of
1,2,4,5-tetrasubstituted
imidazoles in the presence
of n-Fe3O4/PVAm-SO3H
nanocomposite
Entry
Aldehyde
Amine
Time (min)
Yield (%)
Mp (°C)
Mpref (°C)
1
2
3
4
5
6
7
8
PhCHO
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
4-Cl-C6H4 NH2
4-Cl-C6H4 NH2
4-MeC6H4 NH2
4-MeC6H4 NH2
4-MeC6H4 NH2
4-MeC6H4 NH2
20
25
35
25
22
25
35
20
30
15
40
30
20
95
95
91
90
90
92
93
90
88
98
93
92
96
217–219
189–190
183–184
231–233
280–281
163–164
185–188
166–168
187–189
187–190
279–280
151–153
218–220
217–218 [18]
189–191 [18]
181–183 [18]
230–232 [18]
281–282 [18]
163–164 [18]
185–187 [18]
165–167 [18]
188–190 [18]
188–190 [18]
278–280 [18]
150–152 [18]
220 [45]
4-MeC6H4CHO
4-OMeC6H4CHO
3-OH-C6H4CHO
4-OH-C6H4CHO
4-Cl-C6H4CHO
4-NO2C6H4CHO
4-MeC6H4CHO
4-Cl-C6H4CHO
4-MeC6H4CHO
4-OH-C6H4CHO
3-NO2C6H4CHO
4-NO2C6H4CHO
9
10
11
12
13
Reaction condition: benzil (1.0 mmol), aromatic aldehyde (1.0 mmol), aromatic amine (1.0 mmol),
NH4OAc (1.0 mmol), and nano-Fe3O4-NHSO3H (15 mg), solvent-free, at 100 °C
Therefore, it can be seen that n-Fe3O4/PVAm-SO3H nano-
composite is a very benefcial and advantageous catalyst
in the synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasu-
bstituted imidazoles.
Comparison of results using n‑Fe3O4/PVAm
nanocomposite with results obtained by other
workers for the synthesis of 2,4,5‑trisubstituted
and 1,2,4,5‑tetrasubstituted imidazoles
Plausible mechanism for the synthesis
of 2,4,5‑trisubstituted and 1,2,4,5‑tetrasubstituted
imidazoles by n‑Fe3O4/PVAm‑SO3H nanocomposite
The efciency of n-Fe3O4/PVAm-SO3H nanocomposite
performance for the synthesis of 2,4,5-triphenyl-1H-im-
idazole (Table 5) and 1,2,4,5-tetraphenyl-1H-imidazole
(Table 6) is compared with some other catalysts reported.
The present catalyst has the advantages of magnetic recy-
clability, using a low weight percent of the catalyst, high
loading level of the active catalyst site, shorter reaction
time, and a volatile solvent-free and high product yield.
A probable mechanism for the condensation of ammo-
nium acetate, benzaldehyde, and benzil for the synthesis
of trisubstituted imidazoles using n-Fe3O4/PVAm-SO3H
nanocomposite is proposed as shown in Scheme 5. As
1 3

Journal of the Iranian Chemical Society
Table 5 Comparison of results
using the n-Fe3O4/PVAm-SO3H
nanocomposite with those
obtained by other catalysts for
the synthesis of 2,4,5-triphenyl-
1H-imidazole
Entry Catalyst
Conditions
Time (h(min)) Yield (%) References
1
2
SA-MNPs
(NH4)6Mo7O24·4H2O
C2H5OH/u.s./refux
2
90
94
[43]
[46]
100 °C/neatl/micro- (10)
wave irradiation
3
4
5
6
7
8
9
Polymer-supported zinc chloride C2H5OH/refux
Yb(OTf)3 70 °C/HOAc
nano-crystalline sulfated zirconia C2H5OH/refux
Fe3O4@chitosan
MoO3/SiO2
InCl3.3H2O
n-Fe3O4/PVAm-SO3H
1 (30)
2
(45)
2
2 (12)
8
(10)
96
95
87
95
95
82
98
[47]
[48]
[49]
[50]
[51]
[52]
Present work
C2H5OH/refux
80 °C/CH3CN
RT/CH3OH
100 °C/neat
Table 6 Comparison of
results using n-Fe3O4/PVAm-
SO3H nanocomposite with
those obtained by other
catalysts for the synthesis
of 1,2,4,5-tetraphenyl-1H-
imidazole
Entry
Catalyst
Conditions
Time (h(min))
Yield (%)
References
1
2
3
4
5
6
7
8
nano-TiCl4.SiO2
InCl3.3H2O
[Dsim]HSO4
110 °C/neat
RT/CH3OH
90 °C/neat
(30)
6 (24)
(15)
1
(45)
1 (30)
1
2
2
2
(20)
82
89
84
80
87
75
98
80
90
90
95
[53]
[52]
[54]
[49]
[49]
[49]
[55]
[56]
Zeolite
refux/C2H5OH
refux/C2H5OH
refux/C2H5OH
refux/C2H5OH
140 °C/neat
140 °C/neat
140 °C/neat
100 °C/neat
Nano-crystalline SZ
Montmorillonite K10
FeCl3.6H2O
BF3-SiO2
K5CoW12O40.3H2O
SbCl5/SiO2
9
10
11
[57]
[58]
Present work
n-Fe3O4/PVAm -SO3H
shown in the scheme, the polarization of carbonyl group
is induced by the interaction between carbonyl oxygen in
aldehyde as well as in benzil and n-Fe3O4/PVAm-SO3H
(H+). Then, diamine intermediate 1 is formed, through
nucleophilic attack of the lone pair of ammonia, on car-
bonyl group of the aldehyde. Afterward, the intramolecular
interaction between intermediate (I) and benzil-activated
by H+ leads to the cyclization and formation of interme-
diate (II), which, in turn, rearranges to the trisubstituted
imidazole. Similarly, the plausible mechanism for the syn-
thesis of the tetrasubstituted imidazole involves the reac-
tion of an aldehyde, primary amine, and ammonium acetate
in the presence of the catalyst to form intermediate (III).
Intermediate (IV) is formed as a result of condensation of
intermediate (III) with benzil and then tautomerizes to the
tetrasubstituted imidazole (Scheme 6).
assayed for the synthesis of 2,4,5-triphenyl-1H-imidazole
(Fig. 7). After the completion of each cycle, the n-Fe3O4/
PVAm-SO3H nanocomposite was simply taken out with
a permanent magnet, washed several times with deion-
ized water and ethanol and dried in an oven at 60 °C.
Then, the recovered catalyst was conducted squarely to
the subsequent cycle without any extra treatment. The
results show that this polymer magnetic catalyst can be
reused eight times with no significant loss of activity and
products were obtained in high yield. Figure 8 shows the
FT-IR spectra of n-Fe3O4/PVAm-SO3H nanocompos-
ite before and after the catalyst was used eight times in
the reaction medium. As seen in Fig. 8, no significant
change in FT-IR spectra of the catalyst was observed after
eight cycles. Moreover, according to scanning electron
microscope (SEM) in Fig. 9, the identity of the recovered
catalyst was checked which suggested that the nature of
the catalyst remains intact and there was no change in
the morphology during the reaction and recycling stages
as compared to the pristine catalyst. The titration of the
catalyst by NaOH after cycles shows that the loading of
proton ion was 5.4 mmol/g. In addition, the elemental
composition (Fig. 10) shows the presence of C, N, O, S,
and Fe in the nanocomposite structure. Also, the VSM
Reusability and leaching of the catalyst
As an aspect of green chemistry and industrial applicabil-
ity, the reusability of the catalyst was tested. As a trait of
the supported catalysis system, the effortless separation
from the reaction medium, the recyclability, and the sta-
bility of n-Fe3O4/PVAm-SO3H nanocomposite was also
1 3

Journal of the Iranian Chemical Society
Scheme 5 Plausible mechanism for the synthesis of 2,4,5-trisubstituted imidazoles through acid site of catalyst
Fig. 7 Recyclability of n-Fe3O4/
PVAm-SO3H nanocomposite
for the synthesis of 2,4,5-tri-
phenyl-1H-imidazole; reaction
conditions: benzaldehyde
98
98
98
97
97
97
95
95
100
90
80
70
60
50
40
30
20
10
0
(1,0 mmol), benzil (1.0 mmol),
and NH4OAc (2.0 mmol), and
catalyst (15 mg), reaction time:
10 min at 100
free condition
̊C under solvent-
1
2
3
4
5
6
7
8
Cycle
analysis was performed to control the magnetic property
of the recovered nanocatalyst. As observed in Fig. S1, the
recovered catalyst is superparamagnetic with magnetic
saturation (MS) value of about 10 emu/g, and so there is
no significant change in its magnetic property. Therefore,
the magnetite nanoparticles are stable during the course
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Journal of the Iranian Chemical Society
Fig. 8 FT-IR spectra of
n-Fe3O4/PVAm-SO3H nano-
composite before use (a) and
after reused eight times (b)
(b)
(a)
3900
3400
2900
2400
1900
1400
900
400
Wavenumber (cm-1)
Fig. 9 SEM images n-Fe3O4/PVAm-SO3H nanocomposite before use (a) and after reused eight times (b)
of reaction due to protective function of the polymer. In
general, when using a supported catalyst, a crucial point
is the feasibility by which some active sites are lost from
the solid support to the liquid phase, and these leached
species are accountable for a remarkable loss of the
catalytic activity. Therefore, the proof of heterogeneity
of the catalyst is an essential task. For this purpose, a
heterogeneity test was performed, in which the reaction
between benzaldehyde (1.0 mmol), benzil (1.0 mmol),
and NH4OAc (2.0 mmol) was performed under optimized
condition, and the n-Fe3O4/PVAm-SO3H catalyst was
entirely removed from the reaction mixture by an external
magnet at approximately 50% conversion of the starting
material [34]. Then, the reaction was allowed to stir for
3 h (Fig. 11) and it was found that no further reaction
and increase in the product yield occurred (monitored by
TLC). These results convinced us that the leaching of the
catalyst was negligible in the catalytic process and con-
firmed that the observed catalyst was completely hetero-
geneous. Moreover, a certain stability of this catalyst was
also observed, because during the period of our study,
n-Fe3O4/PVAm-SO3H nanocomposite kept on shelves
remained equally active after at least up to three months
(Scheme 6).
1 3

Journal of the Iranian Chemical Society
Fig. 10 EDX spectra of recov-
ered n-Fe3O4/PVAm-SO3H
nanocomposite
Fig. 11 Heterogeneity test
for condensation of benzil,
NH4OAc, and benzaldehyde
were performed in order to synthesize a new polymer-
grafted magnetic nanoparticle (n-Fe3O4/PVAm-SO3H
nanocomposite). The surface functionalization of the
magnetic nanoparticles by sulfonated-polyvinyl amine was
confrmed by several analytical techniques. The resulted
catalyst proved to be an efective magnetic catalyst for
green synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasu-
bstituted imidazoles in excellent yields. This new acidic
magnetic catalyst is better than soluble acids because it
has the following advantages: (a) simple catalyst prepara-
tion; (b) high loading level; (c) high thermal stability; (d)
self-stability without any particular protection; (e) easy
Conclusion
In conclusion, a novel procedure was reported that the
polyvinyl amine layer is directly grafted onto the surface
of Fe3O4 magnetic without using any bridged organosi-
lanes compound, and in the following, it was efectively
employed as a scafold for immobilizing liquid acid. The
magnetic nanoparticles were initially prepared by a simple
method, then, after polymerizing the acrylamide, the mon-
omer is turned into a polymer. As a result, the n-Fe3O4/
PAM is synthesized via radical polymerization. In con-
tinuation, Hofmann degradation and sulfonation treatment
1 3

Journal of the Iranian Chemical Society
Scheme 6 Plausible mechanism for the synthesis of 1,2,4,5-tetrasubstituted imidazoles through acid site of catalyst
7. J. Liang, Z. Liang, R. Zou, Y. Zhao, Adv. Mater. 29, 1701139
magnetic recyclability and reusability; (f) reusable several
times without loss of activity; (g) short reaction time; (h)
excellent yields; and (i) easy separation of product. The
advantages of this catalyst make it a good candidate for
laboratory and industrial utilization.
(2017)
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Acknowledgements The authors acknowledge Semnan University
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Afliations
Maryam Arghan1 · Nadiya Koukabi1 · Eskandar Kolvari1
1
* Nadiya Koukabi
Department of Chemistry, Semnan University, P.O.
n.koukabi@semnan.ac.ir
Box 35195-363, Semnan, Iran
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