Facile template-free route to fabricate core–shell Fe3O4@PANI-SO3H urchin-like nanoparticles as reusable catalyst for Hantzsch reaction: change morphology upon sulfonation

Article abstract of DOI:10.1007/s11164-018-3514-x

Abstract: This paper describes a novel and effective approach to fabricate the core–shell Fe₃O₄@PANI-SO₃H urchin-like morphology in a self-assembly method as nano-magnetic solid acid catalyst. The Fe₃O₄@PANI microspheres were synthesized via an in situ surface polymerization of aniline monomer in an acidic condition, using ammonium persulfate as the oxidant. It was found that upon sulfonation of polyaniline (PANI), the morphology changes to morphology similar to urchin confirming the transmission electron microscopy and field emission scanning electron microscopy results. The as-prepared solid acid catalyst was also carefully analyzed by energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis and vibrating sample magnetometry. This new magnetic core–shell material was shown to have high catalytic performance for one–pot synthesis of 1,4-dihydropyridine derivatives. In addition, the catalyst can be easily separated using an external magnet. Recyclability and reusability for several runs, facile accessibility, and simple handling are the merits of this magnetic-heterogeneous acid catalyst. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11164-018-3514-x

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-018-3514-x
Facile template-free route to fabricate core–shell
Fe O @PANI-SO H urchin-like nanoparticles as reusable
3
4
3
catalyst for Hantzsch reaction: change morphology
upon sulfonation
Zahra Shahamat1 • Firouzeh Nemati1
Ali Elhampour
1
•
Received: 27 February 2018 / Accepted: 13 June 2018
Ó Springer Nature B.V. 2018
Abstract
This paper describes a novel and effective approach to fabricate the core–shell
Fe O @PANI-SO H urchin-like morphology in a self-assembly method as nano-
3
4
3
magnetic solid acid catalyst. The Fe O @PANI microspheres were synthesized via
3
4
an in situ surface polymerization of aniline monomer in an acidic condition, using
ammonium persulfate as the oxidant. It was found that upon sulfonation of
polyaniline (PANI), the morphology changes to morphology similar to urchin
confirming the transmission electron microscopy and field emission scanning
electron microscopy results. The as-prepared solid acid catalyst was also carefully
analyzed by energy-dispersive X-ray spectroscopy, Fourier transform infrared
spectroscopy, X-ray diffraction, thermogravimetric analysis and vibrating sample
magnetometry. This new magnetic core–shell material was shown to have high
catalytic performance for one–pot synthesis of 1,4-dihydropyridine derivatives. In
addition, the catalyst can be easily separated using an external magnet. Recycla-
bility and reusability for several runs, facile accessibility, and simple handling are
the merits of this magnetic-heterogeneous acid catalyst.
&
Firouzeh Nemati
fnemati@semnan.ac.ir
1
Department of Chemistry, Semnan University, Semnan, Iran
123

Z. Shahamat et al.
Graphical Abstract
Keywords Urchin-like morphology Á Self-assembly Á One-pot green
synthesis
Introduction
Polymer-supported catalysts are useful material [1], with considerable potentials in
a wide range of applications. Many attempts have been made to fabricate magnetic
nanoparticles Fe O -cross-linked synthetic polymers in order to enhance their
3
4
capabilities and fabricate best synergistic effects [2–7]. For instance, the Fe O
3
4
particles can be decorated on the surface of conjugated polymers (CPs) to increase
the conduction of electrons; otherwise the CPs can encapsulate the Fe O particles
3
4
to protect the latter from loss of activity [8]. Various organic polymers have been
applied as catalyst carriers, involving polyvinyl alcohol [9], polyvinyl pyrrolidone
[
10], polyvinyl pyridine [11], polyvinyl amine [12], and polyamides [13].
Researchers have found that among conjugated polymers, polypyrrole, polyaniline
and polyindole can be used as catalysts [14, 15]. Meanwhile, polyaniline (PANI) has
attracted extensive attention as a shell related to its unique electromagnetic features
[
16], desirable conductivity, convenience of preparation, suitable environmental
stability and extensive diversity of applications such as electrochromic instruments,
secondary batteries and catalysts [17–19]. Moreover, PANI is outstanding among
the family of conjugated polymers because its doping level can be easily controlled
via acid–base doping/dedoping steps [20–23].
Several basic factors affecting PANI morphology are the nature and quantity of
the dopant, oxidant reagent and monomer and molar proportion of the dopant and
oxidant to monomer, temperature, solvent components, presence of additives
including stabilizers and surfactants, and polymerization methods [24–28]. A
variety of different PANI micro/nanostructures can be synthesized by changing and
controlling the aforementioned factors. One-dimensional (1D) nanostructured
1
23

Facile template-free route to fabricate core–shell…
PANIs such as nanotubes, nanorods, nanofibers, nanoribbons, nanobelts, nanowires
and nanoflowers are the best known examples that are synthesized in a controlled
manner and exhibit high surface area to volume ratio and homogeneous
morphology. In particular, many techniques are available to fabricate 1D
nanostructured PANI including a hard template [29, 30], a soft template [31, 32],
and template-free such as interfacial polymerization [33, 34] and rapid mixing
polymerization [35–38]. However, the morphology and the size controllability of
the self-assembled nanostructures often reported from other techniques are poor
compared with the hard-template method [39]. The immobilization of sulfonic acid
groups (–SO H) on polyaniline surface is a unique characteristic that has
3
encouraged remarkable research since the polymer materials can design a large
accessible surface area for capturing high density of acidic sites, thus used as
impressive solid Brønsted acids [40, 41]. meanwhile, acid catalysts are often
employed in organic transformations and industrial operations, replacement of
environmentally risky catalysts in existing processes, use of solid acid catalysts is an
innovative way to develop a clean and sustainable method for organic transforma-
tions [14].
In pursuit of development of durable and active acid heterogeneous catalysts
42–49], in this paper, an acidic magnetic catalyst consisting of urchin-like PANI
[
nanoparticles with highly oriented nanorods have been successfully synthesized in
three steps. These steps include oxidation polymerization of aniline in the surface of
Fe O microspheres, and addition of chlorosulfonic acid as sulfonation agents
3
4
during a self-assembly process. In this novel approach Fe O @PANI-SO H urchin-
3
4
3
like nanoparticles are synthesized with Fe O cores and PANI as shell, which results
3
4
in well-defined blackberry-like morphology. It is found that after sulfonation of
PANI, the morphology is changed to urchin-like. The Fe O @PANI-SO H urchin-
3
4
3
like nanoparticles show excellent catalytic performance in catalyzing one-pot
synthesis 1,4-dihydropiridine derivatives via Hantzsch condensation. The present
catalytic system has several remarkable advantages such as short reaction times
(
40 min), excellent yields (76–92%), environmentally friendly, and moderate
reaction conditions. The synthesized solid acid catalyst can be easily removed from
the reaction using a permanent magnet and recovered in excellent purity for direct
reuse.
Experimental works
Materials
All the chemicals utilized in this experiment were analytically pure and used
without further treatment. Ferric chloride hexahydrate (FeCl Á6H O), anhydrous
3
2
sodium acetate, ethylene glycol, aniline, ammonium peroxodisulfate (APS), poly-
(
vinylpyrrolidone) (PVP), 3-aminopropyltriethoxysilane (APTES) and cholorosul-
fonic acid, were purchased from the Fluka, Merck, and Aldrich chemical companies
and were used as received. Deionized water was using throughout the experiment.
123

Z. Shahamat et al.
Apparatus
FT-IR spectra were achieved with potassium bromide pellets in the range of
-
1
4
00–2000 cm
using a Shimadzu 8400 s spectrometer. Scanning electron
microscopy, FESEM–EDX, analysis was performed using Tescanvega II XMU
digital scanning microscope. Samples were coated with gold at 10 mA for 2 min
prior to analysis. The transmission electron microscopy (TEM) experiments were
taken with a CM30300Kv field emission transmission electron microscope. XRD
˚
patterns were measured by Philips using Cu-Ka radiation of wavelength 1.54 A. The
magnetic properties were investigated using a vibrating sample magnetometer
(
were carried out with a LINSEIS model STS PT 16,000 thermal analyzer under
VSM, Lakeshore7407) at room temperature. Thermogravimetric analyses (TGA)
-
1
atmospheric condition at a heating rate of 5 °C min . Melting points were obtained
in open capillaries using an Electrothermal 9100 without further corrections. The
progress of the reactions was monitored by TLC on commercial aluminum-backed
plates of silica gel 60 F254, visualized using ultraviolet light.
General experimental and synthetic details
Synthesis and chemical modification of Fe O microspheres
3
4
The magnetic Fe O particles were synthesized through a solvothermal reaction.
3
4
FeCl Á6H O (1.35 g, 5 mmol) and sodium acetate (3.6 g, 26.6 mmol) were
3
2
dissolved in ethylene glycol (40 mL) under magnetic stirring. In the next step
prepared homogeneous yellow solution was sealed to heat at 200 °C by transferring
to a Teflon-lined stainless-steel autoclave. The reaction proceeded for 8 h and then
the autoclave was cooled to room temperature. The obtained black magnetite
particles were washed with ethanol three times and finally dried in an oven at 60 °C
overnight. Subsequently, chemically modification of Fe O particles was performed
3
4
by using (3-aminopropyl) triethoxysilane (APTES) to serve as a linker between the
polyaniline nanonoparticles and the Fe O surface. Typically, a suspension (50 mL)
was prepared using Fe O microspheres (0.2 g) and APTES (4 mL) dissolved in
3
4
3
4
anhydrous ethanol. The suspension was refluxed for 15 h under dry nitrogen. The
resulting modified Fe O microspheres were collected with the help of a magnet and
then washed with ethanol. Ultimately, the product was dried in a vacuum oven at
3
4
6
NH ) [50].
0 °C overnight to obtain the amine functionalized Fe O microspheres (Fe O –
2
3
4
3 4
Synthesis of Fe O @PANI core–shell microspheres
3
4
In situ surface polymerization of Fe O microspheres were carried out in the
3
4
presence of PVP. In a typical procedure, PVP (0.3 g) was dissolved in deionized
water (600 mL) then the NH –Fe O particles (0.09 g) were added. After sonication
2
3 4
the mixture was added into a mixture of aniline (0.12 mL) and HCl (0.6 mL, 2 M)
with vigorous stirring and stirred for 30 min. In the next step, an aqueous solution of
APS (1.2 g in 120 mL) was added into the mixture instantly to start the oxidative
1
23

Facile template-free route to fabricate core–shell…
polymerization. The reaction was allowed to continue under stirring for 3 h. The
resulting precipitate was washed with deionized water and ethanol several times and
collected with a magnet. Finally, the product was dried in an oven at 60 °C
overnight to obtain the desired Fe O @PANI microspheres [50, 51].
3
4
Loading of –SO H on Fe O @PANI core–shell microspheres (Fe O @PANI-SO H)
3
3
4
3
4
3
Fe O @PANI powder (0.1 g) was dispersed in 10 mL of 1,2-dichloroethane (DCE)
3 4
for 15 min. The chlorosulfonic acid (0.24 g, 0.138 mL) diluted with 0.7 mL of DCE
and was added dropwise for 30 min into the dispersion liquid, and the reaction
mixture was stirred at 80 °C and then kept for 5 h. Next, chlorosulfonated
polyanilines were collected with a magnet, immersed in 200 mL water and heated
for 4 h at 100 °C to promote Fe O @PANI-SO H hydrolysis. In this way, the
3
4
3
PANI-SO Cl was transformed into PANI-SO H. Subsequently, the solvent was
2
3
carefully removed and washed with acetone and dried at 70 °C in an oven [52].
1
Loading of H
?
The loading of H for acidic magnetic nanoparticles was investigated using ion-
change pH analysis. For this purpose, 0.1 g of the catalyst was added to 10 mL of
distillated water and dispersed continuously for 15 min at room temperature. The
aqueous suspension was titrated with NaOH (0.1 M) to neutrality, and subsequently
two endpoints were detected by a potentiometer. In the first endpoint, which is equal
?
?
to free protons from –SO H group, loading H was obtained 1.2 mmol H per g
3
catalyst. In the second endpoint, the neutralization of protons from imine nitrogen
?
take place, that is, for Fe O @PANI-SO H was 2 mmol H /g [52]. According to
3
4
3
?
the obtained results, total loading of acid sites was about 3.2 mmol H per g catalyst
and S/N ratio is 0.65.
Synthesis of 1,4-dihydropyridine derivatives
In a 10-mL round-bottom flask, a mixture of aldehyde (1 mmol), ethyl acetoacetate
2 mmol), ammonium acetate (NH OAc) (2.5 mmol) and Fe O @PANI-SO H as
4 3 4 3
(
?
catalyst (0.01 g, equal to 0.03 mmol H ) was stirred under solvent-free conditions
for the specified time and the appropriate temperature according to Table 1. The
progress of the reaction was monitoring by TLC. Upon completion, and after adding
hot EtOH to the reaction mixture and stirring for 3–5 min, the catalyst was
separated from the mixture by means of an outer magnet. The catalyst was washed
with ethanol and acetone and dried under vacuum. The recovered Fe O @PANI-
3
4
SO H could be used for the next run without any other treatment. The pure 1,4-
3
dihydropyridine derivatives were crystallized from ethanol.
Results and discussion
The schematic diagram for steps of synthesis Fe O @PANI-SO H urchin-like
3
4
3
nanoparticles is depicted in Scheme 1.
123

Z. Shahamat et al.
Scheme 1 Schematic diagram for synthesis of Fe
Nanocatalyst characterizations
FT-IR analysis
3 4 3
O @PANI-SO H urchin-like nanoparticles
-
1
FT-IR spectrum analysis in the region of 400–2000 cm
characterize the molecular structure of the resulting Fe O , Fe O @PANI and
was performed to
3
4
3 4
Fe O @PANI-SO H urchin-like nanoparticles. The infrared spectrum (a) exhibits a
3
4
3
-
1
band at 570 cm that belongs to the Fe–O stretching of Fe O in core–shell
3
4
microspheres. The characteristic and sharp peaks of PANI are completely revealed
-
1
-1
at 1392–1593 cm after polymerization. The peaks including 1593 and 1506 cm
are attributed to the C=N and C=C stretching of the quinoid and benzenoid rings.
-
Also, the peak at around 1392 cm is assigned to C–N stretching of the benzenoid
1
-
1
ring (Fig. 1b) [50]. The presence of absorption peaks in 1178 and 1105 cm can be
assigned to asymmetric and symmetric O=S=O stretching vibration which confirm
-
1
sulfonation of PANI (Fig. 1c). Moreover, peaks close to 973 and 806 cm are due
to the S–O and C–S stretching vibrations, respectively [52]. The greater intensity of
area peaks in curve b compared to sulfonated poyaniline in curve c indicate that the
–SO H groups are successfully attached to the aromatic rings of the PANI
3
backbone.
1
23

Facile template-free route to fabricate core–shell…
3 4 3 4 3 4 3
Fig. 1 FT-IR spectra of a Fe O , b Fe O @PANI and c Fe O @PANI-SO H urchin-like nanoparticles
XRD analysis
The X-ray diffraction pattern (XRD) of (a) Fe O4 (b) Fe O @PANI and
3
3 4
(
c) Fe O @PANI-SO H urchin-like nanoparticles prepared under the present
3 4 3
experimental condition is presented in Fig. 2. Scan range is ranging from 20.00° to
9
0.00°. Figure 2a shows diffraction peaks at 2h = 29.955°, 35.22°, 42.89°, 56.67°,
2.13°. These data are in good agreement with that of Fe O . In Fig. 2b, obviously
6
the sharp peaks and their relative intensities observed for Fe O @PANI micro-
3
4
3
4
spheres are nearly corresponding to those of Fe O . This indicates that there is no
3
4
obvious chemical interaction between Fe O and PANI in the composite, and the
4
3
crystalline structure of Fe O is preserved. In Fig. 2c, the same peaks of
3
4
Fe O @PANI are observed, where the intensity of diffraction peaks from PANI-
3
4
SO H/magnetite due to incorporation of SO H groups. These are rather different
3
3
from those of the pure magnetite particles. In addition, there is no considerable shift
in the positions of the peaks, indicating the structural stability of Fe O @PANI-
SO H during sulfonation process. The crystallite size (D) from X-ray patterns was
3
4
3
also measured using the Scherrer equation given as D = K k/b Cos h where K is a
˚
coefficient and is equal to 0.9, k is the wavelength (k = 1.5406 A), D is the diameter
of the crystallites, b is the full width at a half maximum (FWHM) and h is the
diffraction angle of (101) peak. As a result, crystallite size of particles was assessed
around 31.168 nm.
123

Z. Shahamat et al.
3 4 3 4 3 4 3
Fig. 2 X-ray scattering patterns of a Fe O , b Fe O @PANI and c Fe O @PANI-SO H urchin-like
nanoparticles
TGA analysis
The stability of the prepared Fe O @PANI-SO H catalyst and the percent of
3
4
3
organic functional groups on the surface of magnetic particle were determined by
TGA analysis. Figure 3 demonstrates the results of the thermogravimetric analysis
of Fe O @PANI and Fe O @PANI-SO H urchin-like nanoparticles under atmo-
3
4
3
4
3
-
1
spheric flow at a heating rate of 10 °C min from 0 to 800 °C. Three main
inflection regions can be observed for Fe O @PANI. The first region at lower
3
4
temperatures (\ 200 °C) originates from loss of absorbing water and solvent. The
second region at higher temperature (200–525 °C) is associated with the gradual
decomposition of PANI, and finally for the complete burning of organic parts. As
Fig. 3 TG curves of the a Fe
atmosphere flow at a heating rate of 10 °C min from 0 to 800 °C
3 4 3 4 3
O @PANI, b Fe O @PANI-SO H urchin-like nanoparticles under
-1
1
23

Facile template-free route to fabricate core–shell…
Fig. 4 DSC curves Fe
rate of 10 °C min from 25 to 800 °C
3
O
4
@PANI-SO
3
H urchin-like nanoparticles under atmosphere flow at a heating
-1
seen from the TGA curve b, the rate of weight loss of PANI-SO H nanorods is
3
different from the rate of weight loss of PANI, which could be due to incorporation
of SO H group in PANI nanorods [53]. In the third region of weight loss of
3
Fe O @PANI-SO H (525–640 °C) a sharp loss in mass is observed, possibly
3
4
3
implies a large scale thermal degradation of the PANI chains and anion dopant [54].
From the TG analysis, the mass ratio of the Fe O in the Fe O @PANI is about 82%
3
4
3 4
and the mass ration of the –SO H group in Fe O @PANI-SO H is about 5%.
3
3
3
4
DSC analysis
DSC measurement was accomplished in order to provide further evidence for the
presence of the various species and evaluate their thermal behavior. As shown in
Fig. 4, correspondingly, two endotherms were observed in the DSC thermograms.
The first endotherm in the temperature region 50–200 °C corresponds to loss of
absorbing water and solvent. Second, observed in the temperature range of
5
50–660 °C is attributed to the breakdown of the PANI chains and anionic dopant.
Correspondingly, the DSC thermogram shows two exothermic peaks with two
maximums appearing at 281 and 653 °C, confirming decomposition of PANI and
finally for the complete burning of organic parts.
VSM analysis
A vibrating sample magnetometer (VSM) was used to investigate the magnetic
property of the synthesized Fe O @PANI-SO H urchin-like nanoparticles, and the
corresponding result is shown in Fig. 5. Obviously, good magnetization saturation
3
4
3
-
1
(
property of the Fe O @PANI-SO H. As a result, it can be easily recovered using an
M ) (38.16 emu g ) and no hysteresis can be implied to super-paramagnetic
s
3
4
3
external magnet.
123

Z. Shahamat et al.
3 4 3
Fig. 5 VSM curve of Fe O @PANI-SO H urchin-like nanoparticles
EDX analysis
The EDX analysis was used as a powerful technique to investigate the existence of
elements well-structured shell on the surface of the synthesized core–shell
nanoparticles (CSNPs). Thus, the existence of PANI-SO H shell on the surface of
3
Fe O microspheres was confirmed by the EDX detector coupled to the SEM which
3
4
clearly illustrates the presence of Fe, C, N and O in Fe O @PANI and Fe, C, N, O
3
4
and S in Fe O @PANI-SO H urchin-like nanoparticles (Fig. 6).
3
4
3
FESEM and TEM analysis
The size and morphology of the Fe O , Fe O @PANI and Fe O @PANI-SO H
3
4
3
4
3
4
3
urchin-like nanoparticles was studied by scanning electron microscopy. The FE-
SEM images of (a) Fe O (b) Fe O @PANI and (c, d) Fe O @PANI-SO H are
3
4
3
4
3
4
3
shown in Fig. 7. Impressively, it is found that aniline polymerization first occurs on
the surface of Fe O microspheres, which leads to generation of PANI coating shell
3
4
3 4 3 4 3
Fig. 6 EDX spectrum and elemental analysis of Fe O @PANI (a) and Fe O @PANI-SO H urchin-like
nanoparticles (b)
1
23

Facile template-free route to fabricate core–shell…
Fig. 7 SEM images of a Fe
synthesized in three steps by oxidation polymerization of aniline on the surface of Fe
b) and adding chlorosulfonic acid as the sulfonation agent (c, d)
3
O
4
, b Fe
3
O
4
@PANI, c, d Fe
3
O
4
@PANI-SO
3
H urchin-like nanoparticles
3 4
O
microspheres
(
and subsequent Fe O –PANI core–shell structures. The Fe O @PANI is firstly
3
4
3 4
formed in near-spherical morphology (blackberry-like morphology) [54] (Fig. 7b).
Afterwards, when the Fe O @PANI particles are reacted with chlorosulfonic acid in
3
4
DCE as solvent and subsequent hydrolysis in H O at 100 °C for 4 h, the shape of
2
the particles looks like nanorods due to the sulfonation of emeraldine salts, made in
a uniform urchin-like morphology. This is confirmed with FE-SEM images (Fig. 7c,
d). In high magnification, the particles consist of PANI nanorods, which are
interconnected and entered into the microspheres. Two factors can be attributed to
this morphological change: reduction of the extent of p-conjugation in the PANI
123

Z. Shahamat et al.
Fig. 8 Typical TEM images of Fe
3
O
4
@PANI-SO
3
H urchin-like nanoparticles and changing morphology
during the process of sulfonation
backbone due to electron-withdrawing effect of –SO H groups and steric repulsion
3
between large –SO H groups.
3
More information about morphology of Fe O @PANI-SO H can be provided by
3
4
3
TEM images (Fig. 8). The representative TEM images confirm that the Fe O @
4
3
PANI-SO H nanoparticles have urchin-like morphology.
3
As is well known, when a conjugated polymer is doped with chlorosulfonic acid,
the polymeric chains become protonated and positively charged. The negatively
charged dopant anions reside near the positively charged polymer backbone due to
ionic attractions. The size and charge density of the doping acids thus influence the
interchain packing distance and ultimately the supramolecular morphology.
Catalytic evaluation
The obtained results show that the Hantzsch reaction can be design in the presence
of one reusable acid catalytic reagent as a heterogeneous catalyst; this is
significantly attractive from a sustainability and environmental point of view. As
a result, the synthesis of 1, 4-dihydropyridine derivatives was accomplished by
reacting ethyl acetoacetate with benzaldehydes in the presence of ammonium
acetate (NH OAc) as a model reaction. In order to evaluate the influence of the
4
substituent and solvent on the reaction yield, reaction was investigated in different
conditions and results summarized in Table 1. Initially, different amounts of
heterogeneous acid catalyst of Fe O @PANI-SO H were scrutinized at 110 °C. The
3
4
3
best results were gained when the reaction was performed in presence of 0.01 g of
?
catalyst that is equal to 0.032 mmol H . The yield of the corresponding product was
not changed even by increasing the quantity of the catalyst. As illustrated in
Table 1, in the absence of the catalyst, the corresponding product was attained in
trace. In the next stage, the effect of various temperatures on model reaction using
Fe O @PANI-SO H (0.01 g) was evaluated. The results suggest that a temperature
3
4
3
1
23

Facile template-free route to fabricate core–shell…
of 110 °C is the foremost condition for the present reaction. In addition, in order to
study of the effect of the solvent, the model reaction was investigated inside
different solvents such as water, THF, DMF, and toluene. The best results were
achieved without any additional solvents. Finally, after obtaining the best conditions
of temperature, solvent, and catalyst, the quantity of ammonium acetate was
evaluated. The best yield was achieved in the presence of 2.5 mmol ammonium
acetate. All of these observations have been shown in entry 3 of Table 1 describing
the optimized reaction condition. So, the optimal conditions are reached when the
model reaction was performed by reacting of benzaldehyde (1 mmol), ammonium
acetate (NH OAc) (2.5 mmol), ethyl acetoacetate (2 mmol) and catalyst (0.01 g)
4
stirred at 110 °C in the absence of any solvent. After optimizing the reaction
condition, different types of aldehydes were applied to investigate the scope and
application of this method. The results are demonstrated in Table 2. As shown, for
aldehydes bearing electron-donating groups, such as the methoxy and methyl
groups, the reaction accomplished a satisfactory yield in comparison with the
electron-withdrawing groups, those including the Br, Cl, F and nitro groups. A
notable decrease in yield was observed when the nitro group was attached to the
aromatic ring. The reason behind this decrease can be attributed to the nitro group in
comparison to chlorine and bromine groups with much stronger electron-withdrawal
ability. Furthermore, heterocyclic aldehyde (Table 2, entry 14), as well as a,b-
unsaturated aldehyde (Table 2, entry 13) were converted into corresponding 1,4-
DHPs in good yields.
Table 1 Optimization of reaction condition for catalytic synthesis of 1,4-DHP
O
O
O
O
O
H
NH
4
OAc
2
+
C H
2 5
O
2 5
OC H
2 5
OC H
Reaction condition
N
H
a
Entry
Catalyst (g)
Time (min)
Reaction condition
Yield (%)
1
2
3
4
5
6
7
8
–
40
40
40
40
40
40
40
40
40
40
40
Solvent free/110
Solvent free/110
Solvent free/110
Solvent free/110
Toluene/110
Trace
50
0.005
0.01
0.02
0.01
0.01
0.01
0.01
0.01
0.01
0.01
92
90
60
THF/110
50
DMF/110
Trace
Trace
65
H2O/110
1
1
1
a
0
1
2
Solvent free/85
Solvent free/r.t.
Solvent free/130
Trace
92
Reaction condition: benzaldehyde (1 mmol), ammonium acetate (NH4OAc) (2.5 mmol), ethyl ace-
toacetate (2 mmol) and catalyst (0.01 g) stirred at 110 °C during 40 min
123

Z. Shahamat et al.
Table 2 Catalytic synthesis of 1,4-DHPs 3a–3k via Hantzsch reaction using Fe
3
O
4
@PANI-SO
3
H catalyst
O
R
O
O
O
O
NH4OAc
+
C2H5O
OC2H5
2
R
H
0.01 g Catalyst
OC2H5
N
H
1
2a-2n
3
a-3n
Yield: 76-92%
Time: 40min
Solvent-free, 110 C
o
a
Entry Aldehyde
R
C
Product Yield (%) M.P. (°C) (Obs) M.P. (°C) (L)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
2a
2b
2c
2d
2e
2f
6
H
5
3a
3b
3c
3d
3e
3f
92
85
83
80
85
76
51
54
62
88
87
87
89
90
158
149
126
142
167
148
162
160
130
158
226
135
140
170
157–158 [55]
149 [56]
4-Cl C
2-Cl C
3-Cl C
6
6
6
H
5
5
5
H
H
126–128 [57]
141–143 [58]
165–167 [55]
147–148 [56]
162–164 [48]
160–161 [59]
129–130 [48]
157–158 [56]
227–228 [60]
135–137 [61]
139–140 [56]
169–170 [59]
4-Br C
6 5
H
4-F C H
6 5
2g
2h
2i
2-NO
3-NO
4-NO
2
C
6
H
H
H
5
3g
3h
3i
2
C
6
6
5
5
2
C
0
1
2
3
4
2j
4-MeO C
4-HO C
6 5
4-CH C H
6
H
5
3j
2k
2l
6
H
5
3k
3l
3
2m
2n
Cinnamaldehyde
3m
3n
Thiophen-2-
carbaldehyde
a
Reaction condition: Arylaldehyde (1 mmol), ammonium acetate (NH4OAc) (2.5 mmol), ethyl ace-
toacetate (2 mmol); isolated yield
Reusability of the catalyst
In order to clarify the possible catalytic power, after completion of the optimized
reaction including benzaldehyde (1 mmol), ethylacetoacetate (2 mmol), ammonium
acetate (NH OAc) (2.5 mmol) and catalyst (0.01 g) stirred at 110 °C during 40 min
4
under solvent-free condition in the first run, the catalyst was removed by an outer
magnetic field and washed with hot ethanol. It was dried using an oven and then
added to another vessel containing the starting reactants. The sequential reaction
was performed under the same conditions as discussed before. At the end of
reaction, the recycling was conveniently repeated for four cycles without significant
decline in the catalytic activity, which can reflect persistent activity during the reuse
process (Fig. 9). Furthermore, comparison of FT-IR and XRD spectra, intensity and
peak positions between the used and reused catalyst after the reaction indicates that
structure of the catalyst had remained approximately intact after four recoveries
(
Fig. 10).
1
23

Facile template-free route to fabricate core–shell…
Fig. 9 Recycling results of the Fe
Conditions: benzaldehyde (1 mmol), ammonium acetate (NH
2 mmol) and catalyst (0.01 g) stirred at 110 °C during 40 min
3
O
4
@PANI-SO
3
H urchin-like nanoparticles on model reaction.
4
OAc) (2.5 mmol), ethyl acetoacetate
(
Fig. 10 FT-IR spectra (a) and XRD spectra (b) fresh and reused nanocatalyst
In addition, the slight decrease in the yield of the corresponding product could
probably take place due to the gradual loss of the catalyst during removal or
washing. Another test was done to determine whether the catalyst operates in a
heterogeneous or homogeneous way. As such, the model reaction was accomplished
in the presence of Fe O @PANI-SO H in the reaction condition. Before completion
3
4
3
of the reaction, hot toluene was added into the reaction vessel and the catalyst was
removed using an external magnet. It was found that 40% yield of the corresponding
product was attained at this stage. The residue of toluene was then removed under
reduced pressure from the filtrate and the reaction was permitted to proceed further
at 110 °C for another 25 min. In this regard, no appreciable reaction progress was
observed, so we can conclude that no leaching of –SO H groups occurred and the
3
catalyst worked in a heterogeneous way.
123

Z. Shahamat et al.
Table 3 The comparison of the performance of various catalysts on the model reaction
Entry Catalyst
Condition
Product Time
min)
Yield
(%)
References
[56]
(
1
Dimethyl phosphate ionic
liquids
Solvent free/
80 °C
EtOH/reflux
3a
180
61
2
3
TiO
2
NPs
3a
3a
135
20
92
90
[55]
[59]
Chitosan
Solvent free/
8
0 °C
Solvent free/
0 °C
EtOH/80 °C
4
Silica sulphuric acid (SSA)
3a
45
96
[62]
8
a
5
6
Fe-TDU-1
3a
3a
300
15
76
98
[63]
[64]
Phosphor sulfonic acid (PSA) Solvent free/
0 °C
Solvent free/
10 °C
Solvent free/
10 °C
8
7
8
a
3
H-Fe O
4
3
@DA-SO H
3a
3a
40
40
85
92
[48]
1
Fe
3
O
4
@PANI-SO
3
H
Present
work
1
Lewis acidic mesoporous material (TUD-1) containing Fe
Comparison of the catalytic activity
Table 3 compares the efficiency of the present method for the synthesis of various
of 1,4-DHPs with results recently reported by other catalysts in the related literature.
Each of these methods has its own merits, but some of them often suffer from some
problems such as the use of organic solvents (entries 2 and 5), long reaction times
(entries 1, 2 and 5) and use of non-recyclable or expensive catalyst (entries 1).
Fe O @PANI–SO H acts as a highly effective and economical catalyst and
performs better in terms of time and yield produced by the reaction.
3
4
3
Conclusions
In this paper, we report successful synthesis of urchin-like PANI nanoparticles by
using ClSO H as a dopant through a template-free method in a self-assembly
3
process and its usability also as a catalyst. The morphology of PANI is changed
upon sulfonation due to reduced p–p stacking and steric repulsion between –SO H
groups and converted to urchin-like morphology. The prepared nano-material shows
3
?
-1
high acidity (3.2 H mmol g ), and because of such acidity, it can be used as a
solid acid catalyst. The potential application is for one-pot synthesis of 1,4-DHPs.
The reaction is performed in high yields and short reaction times without using any
hazardous solvents. The recyclability test shows that the Fe O @PANI–SO H
3
4
3
catalyst could be recycled and reused without significant decrease in catalytic
activity even after four cycles. Therefore, this catalyst provides an effective and
easy way to control rout for the acid catalyzed Hantzsch reaction. As a result, this
1
23

Facile template-free route to fabricate core–shell…
certainly can be envisaged as a solid acid for the catalysis of other transformations.
Moreover the high stability, magnetic ease-to-recover of the catalyst and the use of
the low-cost starting materials make this protocol economically attractive.
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