Starch as a green source for Fe3O4@carbon core-shell nanoparticles synthesis: A support for 12-tungstophosphoric acid, synthesis, characterization, and application as an efficient catalyst

Article abstract of DOI:10.1007/s11164-015-2229-5

12-Tungstophosphoric acid (PW) immobilized on carbon-coated Fe₃O₄ nanoparticles (Fe₃O₄@C-PW) was prepared through a combination of hydrothermal and chemical co-precipitation. The intermediate carbon layer, which was produced from starch as a green material, protects the magnetic core and also improves the dispersion and catalytic activity of the nanoparticles. Characterization of this catalyst was investigated by high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), vibrating sample magnetometry, and the acidic properties were studied by NH₃-temperature programmed desorption and potentiometric titration. The HRTEM image showed that the catalyst had a well-defined core-shell structure with an average particle size of 50 nm. The characterization data derived from FT-IR reveal that basic structure and geometry of the Keggin anion are preserved after synthesis of Fe₃O₄@C-PW. The as-prepared Fe₃O₄@C-PW was used as a nanocatalyst for the synthesis of various bis(indolyl)methanes and β-functionalized indoles in water. The catalyst can be recovered simply using an external magnetic field and reused several times without appreciable loss of its catalytic activity.

Full text of DOI:10.1007/s11164-015-2229-5

Res Chem Intermed
DOI 10.1007/s11164-015-2229-5
Starch as a green source for Fe3O4@carbon
core–shell nanoparticles synthesis: a support for
12-tungstophosphoric acid, synthesis,
characterization, and application as an efficient catalyst
Ezzat Rafiee1,2 Maryam Khodayari1
•
Received: 16 June 2015 / Accepted: 11 August 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract 12-Tungstophosphoric acid (PW) immobilized on carbon-coated Fe3O4
nanoparticles (Fe3O4@C-PW) was prepared through a combination of hydrothermal
and chemical co-precipitation. The intermediate carbon layer, which was produced
from starch as a green material, protects the magnetic core and also improves the
dispersion and catalytic activity of the nanoparticles. Characterization of this cat-
alyst was investigated by high resolution transmission electron microscopy
(HRTEM), Fourier transform infrared spectroscopy (FT-IR), vibrating sample
magnetometry, and the acidic properties were studied by NH3-temperature pro-
grammed desorption and potentiometric titration. The HRTEM image showed that
the catalyst had a well-defined core–shell structure with an average particle size of
50 nm. The characterization data derived from FT-IR reveal that basic structure and
geometry of the Keggin anion are preserved after synthesis of Fe3O4@C-PW. The
as-prepared Fe3O4@C-PW was used as a nanocatalyst for the synthesis of various
bis(indolyl)methanes and b-functionalized indoles in water. The catalyst can be
recovered simply using an external magnetic field and reused several times without
appreciable loss of its catalytic activity.
Keywords Core–shell particle Á Hydrothermal Á Co-precipitation method Á
Magnetically recoverable catalyst Á Heteropoly acid
&
Ezzat Rafiee
Ezzat_rafiee@yahoo.com; e.rafiei@razi.ac.ir
1
2
Department of Inorganic Chemistry, Faculty of Chemistry, Razi University,
Kermanshah 67149, Iran
Institute of Nano Science and Nano Technology, Razi University, Kermanshah 67149, Iran
123

E. Rafiee, M. Khodayari
Introduction
In the last few years, the literature has highlighted the importance of nanosized materials in
several scientific and technological areas, and many research councils have intensified
investments in nanotechnology for the coming years [1–7]. Mounting interest is recently
expressed for surface functionalization of nanocatalysts in which the nanoscale architecture
of active centers plays a dominant role in determining their efficiency and selectivity [8–12].
Surface functionalized iron oxide magnetic nanoparticles (MNPs) are a type of
novel functional materials, which have been widely used in biotechnology and
catalysis [13–26]. Magnetic particles can be coated with a protective layer of
different materials to improve their stability and to introduce new surface properties
and functionalities. Some of these coating materials are silica, alumina, and polymer
matrices such, as ion-exchanging resin [27–34].
Carbonaceous materials, including carbon nanotubes, nanohorns, graphene,
activated carbon, amorphous carbon, and natural substrates, such as starch,
cellulose, sucrose, and glucose, are versatile coating material due to chemical
stability, biocompatibility, possibility of surface modification, and pore creation.
Here, we reported a novel way to prepare carbon-coated Fe3O4 nanoparticles
(NPs) by using starch as the carbon precursor. Starch is an ideal material to this
purpose because of the abundant availability of starch, low cost, renewability,
biocompatibility, biodegradability, and nontoxicity [35].
Synergetic combination of various heteropoly acids (HPAs) with inorganic supports as
nanocatalysts have been successfully developed and evaluated in a number of industrially
important reactions [36–45]. Immobilization of HPAs in the magnetically recoverable
nanoparticle support will provide easy treatment in the production process and open up an
opportunity for their applications in the chemical or pharmaceutical industry.
Several advantages to be highlighted using supported HPAs compared to homoge-
neous examples include easier recovery and recycling after carrying out reactions, and
easier product separation [46]. However, separation and recovery of the immobilized
HPAs are usually performed by filtration or centrifugation, which are not eco-friendly
processes in the case of nano-HPAs. The immobilization of 12-tungstophosphoric acid
(PW) on Fe3O4@C (Fe3O4@C-PW) can be employed to derive novel heterogeneous
catalytic systems that possess both high separation efficiency and a relatively high
surface area to maximize catalyst loading and activity. In this study Fe3O4@C-PW was
synthesized and characterized as a magnetically recoverable nanocatalyst, and the
reactivity of the produced catalyst was investigated in the synthesis of various
bis(indolyl)methanes (BIMs) and b-functionalized indoles in water.
Experimental
Materials and methods
FeCl3Á6H2O (99 %) and FeSO4Á7H2O (99 %), NaOH (98 %), HCl (37 %), starch,
PW ([99), other reagents, and solvents used in this work were obtained from
Merck, Aldrich, or Fluka without further purification.
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Starch as a green source for Fe3O4@carbon core–shell…
High resolution transmission electron microscopy (HRTEM) was obtained
using a TEM microscope (Jeol JEM-2100, with an accelerating voltage of
200 kV). Fourier transform infrared (FT-IR) spectra were recorded with KBr
pellets using a FT-IR spectrometer ALPHA. UV–Vis spectra were obtained with
an Agilent (8453) UV–Vis diode-array spectrometer using quartz cells of 1 cm
optical path. NMR spectra were recorded on a Bruker Avance 200 MHz NMR
spectrometer with CDCl3 as solvent and TMS as internal standard. Magnetic
properties of Fe3O4@C and Fe3O4@C-PW were measured using a BHV-55,
Riken, Japan, vibrating sample magnetometer (VSM). The potential variation was
measured with a Hanna 302 pH meter using a double junction electrode. The
NH3-temperature programmed desorption (NH3-TPD) measurements were carried
out with 200 mg samples on a Micromeritics 2900 TPD/TPR apparatus with a
thermal conductivity detector (TCD). The fitting performed by using the
Levenberg–Marquardt (L–M) algorithm.
Preparation of the catalyst
The Fe3O4 NPs were synthesized by a chemical co-precipitation technique of
ferric and ferrous ions in an alkaline solution under an atmosphere of dry nitrogen.
FeCl2Á4H2O (2.15 g) and FeCl3Á6H2O (5.8 g) were mixed with 175 mL of
deionized water. The resulting solution was heated to 80 °C while stirring
vigorously. Then, 10 mL of 25 % NH4OH was quickly added into the solution.
The resulting suspension was vigorously stirred for 5 min. The black Fe3O4
particles were separated using a magnet and were then washed three times
(sequentially) with distilled water and anhydrous ethanol. Then the aqueous
solution of Fe3O4 NPs (16 % w/w, 20 mL) that thoroughly dispersed by
sonication for 15 min at 40 °C were re-dispersed in 175 mL of distilled water
containing 2.26 g of soluble starch. After ultrasonic treatment for another 15 min,
the resulting suspension was transferred to the autoclave and maintained at 160 °C
for 12 h. After completion of the reaction, the autoclave was cooled to room
temperature, and the resulting Fe3O4@C NPs were collected using a magnet and
washed six times with distilled water. The final product was oven-dried at 60 °C
for 12 h.
For the immobilization of PW on Fe3O4@C particles, 0.7 g of PW was dissolved
in 5 mL of dry methanol. This solution was added dropwise to a suspension of 1.0 g
Fe3O4@C-PW in methanol (50 mL) while being dispersed by sonication. The
mixture was stirred to obtain Fe3O4@C-PW NPs. The catalyst was collected by a
permanent magnet and dried in a vacuum overnight.
Acidity measurement
For the potentiometric titration, 0.05 g of solid was suspended in acetonitrile
(90 mL) and stirred for 3 h. The suspension was titrated with a 0.05 mol/L solution
of n-butylamine in acetonitrile.
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E. Rafiee, M. Khodayari
Catalytic experiments
General procedure for the synthesis of products 5a–5h
A mixture of aldehyde (1 mmol), indole (2 mmol), catalyst (0.15 g), and H2O (5 mL)
was stirred at room temperature. At the end of the reaction, water was removed from
the mixture by centrifugation followed by decantation. The product was dissolved in
acetonitrile, and the catalyst was separated from the product by an external magnet.
The solvent was evaporated in vacuum to give the product, which was purified by
column chromatography on silica gel using EtOAc/petroleum ether as eluent.
General procedure for the synthesis of products 6a–6d
A mixture of indole (1 mmol), aldehyde (1 mmol), b-dicarbonyl compound (1 mmol),
and catalyst (0.15 g) in H2O (5 mL) was stirred at room temperature. Purification of
the products and recovery of the catalyst are similar to the above procedure.
General procedure for the synthesis of products 7a–7c
A mixture of indole (1 mmol), amine (1.2 mmol), formaldehyde (1.2 mmol), and
catalyst (0.15 g) in H2O (5 mL) was stirred at room temperature. At the end of the
reaction, CH2Cl2 (2 9 5 mL) was added and the reaction mixture was stirred. The
catalyst was separated from the product by an external magnet onto the reaction
vessel, followed by decantation of the reaction mixture. The organic layer was dried
over anhydrous sodium sulfate, filtered, and concentrated. The residue was purified
by column chromatography on silica gel to afford the pure product.
Large scale synthesis
Reaction of indole and benzealdehyde was selected for large-scale synthesis. The
reaction of indole (1a, 20.0 mmol) with benzaldehyde (2a, 10.0 mmol) in the presence
of Fe3O4@C-PW (1.5 g) was carried out at room temperature. Purification of the
products and recovery of the catalyst are similar to the ‘‘General procedure for the
synthesis of products’’ section.
Results and discussion
Scheme 1 presents the synthetic strategy for Fe3O4@C-PW catalyst. Magnetic
Fe3O4 NPs were prepared according to the literature using ferric chloride
hexahydrate and ferrous chloride tetrahydrate upon addition of NH4OH through
the chemical co-precipitation method. A layer of carbon was coated Fe3O4 NPs by
using starch as the carbon precursor to protect the Fe3O4 magnetic core from
oxidizing. The outer shell of carbon not only improves the dispersibility, but also
provides suitable sites (C–OH groups) for surface functionalization with PW.
Immobilization of Fe3O4@C NPs with a methanol solution of PW occurs via
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Starch as a green source for Fe3O4@carbon core–shell…
Scheme 1 Schematic illustration for preparation of Fe3O4@C-PW catalyst
Fig. 1 HRTEM images of a a fresh and b recovered Fe3O4@C-PW with typical core–shell structure
formation of hydrogen bonding between hydroxyl group on the surface of carbon
and PW molecule.
HRTEM image indicated that Fe3O4@C-PW had a well-defined core–shell
structure, and the mean diameter of the particle is about 50 nm (Fig. 1a). The dark
core of the Fe3O4 NP and the grey carbon shell are clearly observable. The typical
carbon shell thickness was estimated to be around 5 nm. Because of the strong mass
difference between carbon and PW, it could be assumed that the outside darker
contrast indicates the shell of PW particles immobilized onto the Fe3O4@C NPs.
The FT-IR spectra of Fe3O4@C, PW, and Fe3O4@C-PW after drying are
presented in Fig. 2a–c. The primary structure of PW could be identified by four
characteristic IR bands appearing within the range 700–1200 cm-1. Characteristic
FT-IR bands of the bulk PW appeared at 1080 (P–O), 985 (W–O), 890, and
814 cm-1 (W–O–W) [47]. The strong absorption peak at around 569 cm-1 is
characteristic of the Fe–O stretching vibration [48]. The peaks at 1617 and
1695 cm-1 can be attributed to the C=C and C=O stretching vibrations,
respectively, which reflect the carbonization of the soluble starch [21]. Additionally,
the peaks at 1000–1400 cm-1 correspond to the C–OH stretching and O–H bending
vibrations [49], also the strong peak at 3396 cm-1 represents the vibration mode of -
OH, providing further evidence of these hydroxyl groups.
The magnetic property of Fe3O4@C, Fe3O4@C-PW, and recovered Fe3O4@C-PW
were characterized using VSM (Fig. 3). The magnetic measurement (Ms) of
Fe3O4@C is about 45 emu/g (Fig. 3a). However, the saturation magnetization of
Fe3O4@C-PW is about 17 emu/g (Fig. 3b), which is significantly smaller than that
of Fe3O4@C. There is no hysteresis, suggesting that Fe3O4@C-PW is
123

E. Rafiee, M. Khodayari
Fig. 2 FT-IR spectra of the a Fe3O4@C, b PW, and c Fe3O4@C-PW
Fig. 3 Magnetization measurements for
Fe3O4@C-PW
a Fe3O4@C, b fresh Fe3O4@C-PW, and c recovered
superparamagnetic. The saturation magnetization of recovered Fe3O4@C-PW is
about the same as fresh Fe3O4@C-PW, which is about 14 emu/g (Fig. 3c).
To investigate the acidic property of the catalyst, NH3-TPD was evaluated
(Fig. 4). Four types of desorption peaks in the range of 300–800 °C are present for
this catalyst. The peaks observed at low temperature, about 130–300 °C, are related
to weak acidic sites [50]. The peak observed at lower temperature (T = 307 °C)
corresponds to the weak acidic sites on the support. The moderate to strong acidic
123

Starch as a green source for Fe3O4@carbon core–shell…
Fig. 4 TPD analysis
sites in the range 400–600 °C correspond to strong acidic sites generated due to the
presence of PW. The remaining peak around 800 °C is probably due to dehydration
rearrangements of the Keggin unit [51].
The acidity of the catalyst was also measured by potentiometric titration with
n-butylamine. n-Butylamine is considered a strong base, so its adsorption could be
expected on sites of different acid strength. The total solid acidity without
distinguishing the type of acidity is titrated. The potentiometric titration curves
obtained for the supported catalyst during the n-butylamine titration are shown in
Fig. 5. This technique evaluated the total number of acid sites (meq. of amine = g
of solid) and their strength (Ei) on the catalyst surface. The strength of the acid sites
600
500
400
300
200
Fe3O4@C-PW
100
Fe3O4@C
0
-100
-200
-300
0
1
2
3
4
meq amine/g catalyst
Fig. 5 Potentiometric titration curves of Fe3O4@C and Fe3O4@C-PW
123

E. Rafiee, M. Khodayari
can be classified according to the following scale: Ei [ 100 mV (very strong sites),
0\Ei \100 mV (strong sites), -100 \Ei \0 mV (weak sites), and Ei \-100 mV
(very weak sites) [52, 53]. Fe3O4@C-PW have stronger acid sites (Ei = 520 mV) in
comparison with Fe3O4@C (Ei = 130 mV), and according to the mentioned scale,
Fe3O4@C-PW presents very strong acid sites. Also, the total number of acid sites of
Fe3O4@C-PW increased compared to the Fe3O4@C.
With these results, it seems that this nanocatalyst is active in organic reactions.
Among the different protocols for the synthesis of useful building blocks, we
selected the synthesis of 3-substituted indoles, particularly 3-alkylindoles and BIMs,
because these compounds are known to possess a wide range of biological activities
including antioxidant, antibacterial, and insecticidal activity [54].
Optimized experiments were carried out by using different amounts of
Fe3O4@C-PW catalyst in the model reaction (Scheme 2), and results are
summarized in Table 1. A blank experiment in the absence of the catalyst showed
that the presence of catalyst is essential in this reaction (Table 1, entry 1). When
using the Fe3O4@C-PW as catalyst, a significant improvement was observed
(Table 1, entry 2). Increasing the catalyst quantity from 0.10 to 0.15 g led to a
decrease in reaction time. Further increase from 0.15 to 0.20 g of the catalyst
quantity showed no improvement in the yield or reaction time (Table 1, entries 3
and 4). Therefore, 0.15 g of the catalyst was selected as the best catalyst loading in
further investigations. The catalytic activity of 0.15 g of Fe3O4@C-PW catalyst was
compared with support only (Fe3O4@C) and bulk PW on unit weight basis (Table 1,
entries 5 and 6). The results showed that Fe3O4@C was not efficient for the
completion of the reaction, and the yield was much lower than that obtained using
the Fe3O4@C-PW catalyst. However, when PW was used as the catalyst, a moderate
Scheme 2 Reaction of indole and benzaldehyde as a model reaction
Table 1 Optimization of the
reaction conditions
Entry
Catalyst
Time (min)
Yield (%)a
1
2
3
4
5
6
–
60
20
10
10
60
20
–
Fe3O4@C-PW (0.10 g)
Fe3O4@C-PW (0.15 g)
Fe3O4@C-PW (0.20 g)
Fe3O4@C (0.15 g)
PW (0.15 g)
98
96
98
10
60
a
Reaction conditions: indole
(2 mmol), benzaldehyde
(1 mmol), H2O (5 mL), room
temperature
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Starch as a green source for Fe3O4@carbon core–shell…
yield of the product was obtained. Nevertheless, the reaction proceeded in a
homogeneous system, which makes catalyst recovery very difficult.
In order to evaluate the scope and generality of this protocol, various aldehydes,
ketones, and b-dicarbonyl compounds were tested as substrate (Scheme 3). Some
aldehydes and ketones with electron-donating and electron-withdrawing groups
underwent electrophilic substitution reaction with indole and 2-methyl indole and
gave the corresponding BIMs in excellent yields and low reaction times (Table 2,
entries 1–6 and 8–10). Moreover, when heteroaromatic aldehydes are employed, the
reaction proceeds smoothly with longer reaction times to afford the corresponding
product in excellent yields. It’s remarkable to note that when formaldehyde as an
aliphatic aldehyde was reacted, the corresponding BIM was obtained in low yield
(entry 7). In addition, the reactivity of some b-diketone and b-ketoesters were also
screened to carry out the electrophilic substitution reaction with indole and
benzaldehyde. As expected, three molecular condensation reaction of indole with
benzaldehyde and b-dicarbonyl compounds afforded the desired products in
superior yields (entries 10–14).
The reusability of the catalyst was studied by employing the recovered catalyst in
the model reaction. At the end of the reaction, BIMs were obtained as pinkish solid
products. The reaction mixture was centrifuged and water was removed from the
mixture to leave residue. The product was dissolved in acetonitrile, and the catalyst
was easily separated from the product by attaching an external magnet onto the
reaction vessel, followed by decantation of the solution including product (Fig. 6).
The catalyst was then dried and used in the next run. The reusability and catalytic
activity of the catalyst without calcinations was found unfavorable after the first run
(40 % yield after 10 min in second run). Calcinations of reused catalyst can help to
activate the catalyst and lead to increase reactivity and reusability. Thus, after each
run, the catalyst was calcinated at 150 °C for 2 h. At the fourth reaction run,
Scheme 3 Various substrates
123

E. Rafiee, M. Khodayari
Table 2 Synthesis of various indole derivatives in the presence of Fe3O4@C-PW
Entry
Substrates
Producta
Time (min)
Yield (%)b
1
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1a
1a
1a
1a
2a
2b
2c
2d
2e
2f
5a
5b
5c
5d
5e
5f
10 (30)
15
98 (93)
94
2
3
15
95
4
20
91
5
15
62
6
25
93
7
2g
2h
2a
2d
2a
2a
2a
2f
5g
5h
5i
30
42
8
20
91
9
10
98
10
11
12
13
14c
5j
20
92
3a
3b
3c
3b
6a
6b
6c
6d
25
97
30
96
30
95
30
94
aAll products were identified by comparing their spectral data with those of the authentic samples [31–35]
b
Isolated yields, results in parentheses, refer to large scale syntheses
c
0.3 g of the catalyst was used in this reaction. Corresponding product in the presence of 0.15 g of the
catalyst was obtained in 40 % after 60 min
87.5 wt% of the catalyst (compared with the amount of fresh one used in the first
run) was recovered and yield of the product decreased from 95 to 83 %. Stability of
core–shell structure of the catalyst after reusing was checked by HRTEM. Figure 1b
shows the image of the recovered catalyst indicated that the core–shell structure of
the catalyst was not changed. A decrease in weight of the recycled catalyst mainly is
due to the recovering processes including collecting technique and washing after
each run (Fig. 7).
A plausible mechanism of the reaction in the presence of Fe3O4@C-PW as a
¨
Bronsted acid catalyst is proposed (Scheme 4). The condensation of heteroaromatic
aldehydes to afford the corresponding BIMs is explained by pathway 1. The
carbonyl group of the aldehydes was activated by the catalyst to give intermediate
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Starch as a green source for Fe3O4@carbon core–shell…
Fig. 6 Separation of the Fe3O4@C-PW catalyst and product from the reaction mixture using an external
magnet a start of the reaction, b completion of the reaction, c centrifugation of the mixture, and
d recovery of the catalyst by adding solvent
Fig. 7 Reusability of the Fe3O4@C-PW in the model reaction (after 10 min)
I and then an electrophilic substitution was carried out at C-3 of an indole to afford
intermediate II. Afterward, in pathway 1 the intermediate II served as an
electrophile to attack a second molecule of indole, and BIMs were formed after loss
of water. Production of the b-functionalized indoles is explained by pathway 2.
Similar to other routes, by action of the catalyst, the intermediate II was produced
from initial aldehyde and indole. Then this intermediate could be quickly combined
with the employed 1,3-dicarbonyl compound to produce, after the release of H2O,
the final b-functionalized indoles.
The applicability of this catalyst for the large-scale synthesis was investigated in
the reaction of 1a (20.0 mmol) with 2a (10.0 mmol) under the optimized conditions
123

E. Rafiee, M. Khodayari
Scheme 4 Proposed mechanism for the synthesis of BIMs and b-functionalized indoles in the presence
of Fe3O4@C-PW
(Table 2, entry 1). The result showed that Fe3O4@C-PW is a good candidate for
large-scale synthesis, and yield of the reaction was 93 % in a larger reaction time
(30 min) compared to small scale.
Conclusion
In this study, we synthesized Fe3O4@C-PW magnetic nanocatalyst by combining
hydrothermal and chemical co-precipitation methods. The commercial availability
and low cost of starch as the carbon precursor allow carrying out synthesis of
Fe3O4@C-PW on a macro-scale. The Fe3O4@C-PW nanocatalyst that we obtained
has a mean diameter of 50 nm, with a carbon shell of about 5 nm characterized by
HRTEM. The carbon layer protects the magnetic core against oxidation, making it
possible to control precisely the positions of the nanoparticles. The carbon layer also
adsorbs the PW nanoparticles on its surface, improving their dispersion and activity.
Superparamagnetism of this nanocatalyst is confirmed from VSM studies. NH3-TPD
shows strong acidic sites. Advantages of the present approach rely not only on the
simplicity, rapidity, and efficiency of the procedure, but also on the formation of a
controlled core–shell structure. More importantly, Fe3O4@C-PW is intrinsically
heterogeneous and recovered simply using an external magnetic field. In addition,
this catalyst was used in a novel methodology for the syntheses of BIMs and b-
functionalized indoles, which are biologically interesting compounds.
Acknowledgments The authors thank the Razi University Research Council for support of this work.
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