Magnetically separable core-shell iron oxide@nickel nanoparticles as high-performance recyclable catalysts for chemoselective reduction of nitroaromatics

Article abstract of DOI:10.1039/c4cy00673a

A magnetically separable core-shell iron oxide@nickel (IO@Ni) nanocatalyst was synthesized by reduction of Ni²⁺ ions in the presence of iron oxide (Fe²⁺, Fe³⁺) by a simple one-pot synthetic route using NaBH₄ as a reducing agent and starch as a capping agent. The synthesized nanoparticles (NPs) were characterized by several techniques such as X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS). The core-shell iron oxide@nickel nanoparticles (IO@NiNPs) were found to have excellent activity for the hydrogenation reactions of aromatic nitro compounds under mild conditions using water as a green solvent. Excellent chemoselectivity and recyclability up to 30 cycles for the nitro group reduction was demonstrated.

Full text of DOI:10.1039/c4cy00673a

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Magnetically separable core–shell iron
oxide@nickel nanoparticles as high-performance
recyclable catalysts for chemoselective reduction
of nitroaromatics†
Cite this: DOI: 10.1039/c4cy00673a
a
b
c
a
Puran Singh Rathore, Rajesh Patidar, T. Shripathi and Sonal Thakore*
A magnetically separable core–shell iron oxide@nickel (IO@Ni) nanocatalyst was synthesized by reduction
2
+
2+
3+
4
of Ni ions in the presence of iron oxide (Fe , Fe ) by a simple one-pot synthetic route using NaBH as
a reducing agent and starch as a capping agent. The synthesized nanoparticles (NPs) were characterized by
several techniques such as X-ray diffraction (XRD), high resolution transmission electron microscopy
Received 24th May 2014,
Accepted 1st August 2014
(HR-TEM), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS). The
core–shell iron oxide@nickel nanoparticles (IO@NiNPs) were found to have excellent activity for the hydro-
genation reactions of aromatic nitro compounds under mild conditions using water as a green solvent.
Excellent chemoselectivity and recyclability up to 30 cycles for the nitro group reduction was demonstrated.
DOI: 10.1039/c4cy00673a
www.rsc.org/catalysis
developing materials from waste such as iron tailings. Tail-
ings refer to the waste material of beneficiation processes.
Introduction
The concept of “Green Chemistry” has become one of the
guiding principles of environmentally benign synthesis.
Among several iron-based solid waste materials, iron ore
tailings (IOTs) are one of the fast accumulating wastes
throughout the world due to the rapid expansion of iron and
steel based industries and is becoming one of the serious
environmental threats for the future generation. Presently,
IOTs are mainly used with very little intervention of high
technology and moreover, value addition is low. As a result,
there is an increasing interest in the search for possible alter-
native uses of waste IOTs such as adsorption and catalysis or
as precursors to value added products before their final
1
–3
Nanomaterial-catalyzed transformations in an aqueous reac-
tion medium are one of the ideal solutions for the develop-
ment of green and sustainable protocols. Nanocatalysis is a
rapidly growing field which involves the use of metal nano-
4
particles as catalysts for a variety of organic reactions. The
preparation and use of magnetic nanoparticles (MNPs) offers
advantages in clean and sustainable chemistry as they are
5
non-toxic, readily accessible, and retrievable. Additionally,
1
4–19
the activity and selectivity of magnetic nano-catalysts can be
manipulated by their surface modification.
discharge.
Here we assess the potential of synthesizing
magnetic iron oxide from IOTs and its use as a template for
the synthesis of magnetically separable nickel nanoparticles
(NiNPs).
The development and design of core–shell nanomaterials
has emerged in recent years because of their multifaceted
6
,7
applications especially as catalysts for organic reactions.
The reduction of nitroarenes is an important process as
the products, aromatic amines, are versatile intermediates
and precursors in the preparation of dyes, pharmaceuticals,
Magnetically separable nanocatalysts are an important class
of core–shell nanocatalyst, which are robust, air stable, avoid
8
20
traditional filtration processes and are recyclable. Various
pigments, agrochemicals and polymers. The process is
methods have been developed to synthesize specific
complicated as the reduction of a nitro group proceeds in
stages and it often stops at an intermediate stage producing
9
–13
application-based MNPs.
Some of them are aimed at
2
1
hydroxylamine and hydrazine as side products. A variety of
procedures and reducing agents are available for this
a
Department of Chemistry, Faculty of Science, The M. S. University of Baroda,
Vadodara, 390002, India. E-mail: chemistry2797@yahoo.com;
Fax: +91 0265 2429814; Tel: +91 0265 2795552
2
2
purpose. In the literature, a few procedures involving noble
metal NPs such as Pd, Pt, Ag, Au, as well as Fe, Fe O @Ni
3
4
b
Analytical Discipline and CIF, Central Salt and Marine Chemicals Research
and Ni@Ag NPs have been demonstrated for the reduction of
Institute (CSIR-CSMCRI), G. B. Marg, Bhavnagar, 364002, Gujarat, India
7
,22–24
c
nitro groups.
The main limitations of earlier reported
UGC-DAE Consortium for Scientific Research, Indore 452 001, India
work were the necessity of high H2 pressures, prolonged
reaction times, lower turnover numbers, organic solvents,
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00673a
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reduced recyclability and high temperatures. Further, the
selection of metal and its support, the hydrogen source and
operational simplicity, which are the important parameters
for effective conversions, are restricted. However, an alterna-
tive procedure which is efficient, simple, chemoselective,
green and cost-effective would be highly beneficial.
carried out based on the Crystallography Open Database
(COD). All of the crystallite lines identified with their respec-
tive phase(s) are shown in Fig. 1. Details of composition
along with its peak positions are shown in ESI.† It is also
observed that the pattern is dominated by maghemite
2 3 3 4
(γ-Fe O ) and magnetite (Fe O ) phases. It is to be noted here
In the past, we have investigated the use of NiNPs as cata-
that the peak positions of both the phases are very close
to each other, hence exact identification is very difficult.
However, based on the intensity distribution, it can be con-
cluded that the majority of the sample consists of maghemite
or magnetite phases along with hematite as a secondary
phase and goethite as an impurity phase. The XRD pattern is
2
5
lysts for transesterification as well as reduction. In this
paper, we report the first protocol for the synthesis of a
starch capped iron oxide@Ni (IO@Ni) core–shell mag-
netically separable nanocatalyst for the hydrogenation of
aromatic nitro compounds. Initially p-nitroaniline (PNA) was
used as the model system to simplify the analysis and to
accelerate the screening speed. All reactions were carried out
at room temperature (rt) in aqueous medium (green solvent).
Consequently, the optimized conditions were used for the
synthesis of various aromatic amines.
2
6,27
similar to reported data.
presence of Fe metal (ESI†).
SEM-EDX of IO also showed the
Synthesis and characterizations of IO@NiNPs
A template synthesis method was used for synthesis of
magnetic separable core–shell nanoparticles. This method is
mainly used to generate specific nanostructures of bimetallic
nanoalloys which are otherwise very difficult to prepare. This
approach utilizes pre-formed iron oxide as a base metal and
subsequently directs the deposition of the Ni metal onto
its surface site. In the present study, nanosized nickel was
deposited on iron oxide under sonication and starch was
used as a capping agent. This method is commonly used
to prepare core–shell or multi-shell morphologies. Starch
capping was proven by FT-IR (ESI†).
Results and discussions
Synthesis and characterizations of iron oxide (IO)
The IOTs selected for the present study primarily consist of
Fe, Si and Al with magnetite, hematite and goethite as the
main iron bearing phases (ESI†). Si and Al are present in the
form of kaolinite which is evident from X-ray diffraction
(
XRD) patterns (ESI†). Fig. 1A shows the XRD patterns of the
as synthesized IO. The lines in Fig. 1A correspond to crystal-
line phases. It is deduced from this pattern that the sample
is composed of mixed phases. Detailed analysis has been
Fig. 1B shows the XRD pattern of IO@NiNPs. The signals
of Ni were not detected in the XRD pattern of IO@NiNPs
although the position and relative intensity of the diffraction
2
4b,28
peaks are reasonably close to the reported pattern.
The
XRD pattern shows that the MNPs are well-crystalline and
exhibit diffraction peaks corresponding to (111), (220), (311),
(
400), (511) and (440) planes of a cubic crystal system. The
XRD pattern indicates mixed phases of iron oxides with
nickel.
The weight percentage of Ni was determined to be 1.98%
by AAS analysis and Energy Dispersive X-ray analysis (EDX)
also confirmed the ratio 1.89 : 98.11 (Ni : iron oxide) (Fig. 2A).
EDX analysis indicates that the clean final product is mostly
composed of O, Fe and Ni, with no other signals (Fig. 2A).
The magnetic measurements on the as-prepared iron
oxide and IO@NiNPs have been carried out at room tempera-
ture, and the data are shown in Fig. 2B. From the recorded
M–H loop, it is clear that both the as-prepared samples are
ferromagnetic at RT. The magnetization of the as-prepared
iron oxide was 1.6269 emu, and it decreased to a value of
1
.5259 emu after coating of NiNPs. The coercivity values of
the IO and IO@NiNPs were 73.65 and 61.76 G, respectively.
The low magnetization value exhibited by the IO@NiNPs
compared to the iron oxide could be due to the presence
of impurities/defects in the IO@NiNPs. Since the size of
the nanoparticles synthesized by us lies in the range of
20–50 nm, they are still ferromagnetic.
In order to give further evidence of the formation of a
Fig. 1 XRD pattern of (A) iron oxide and (B) IO@NiNPs.
NiNP shell over the iron oxide core and to determine the
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Fig. 2 (A) EDX pattern and (B) magnetic hysteresis curve of as-prepared IO and IO@NiNPs.
oxidation state of Ni, XPS resolution Ni 2p spectroscopy
results of iron oxide and IO@NiNPs were compared (ESI†).
The signals of Ni were very faint in the XPS spectrum of
IO@NiNPs (Fig. 3) because of low loading of nickel metal.
Nevertheless, for a better understanding of the oxidation
state of Ni metal, the binding energy obtained from the cata-
lyst was compared with the Ni metal 2p3/2 peak position. The
peak value obtained at 852.9 eV is within the range of the
Fig. 4B and C are HR-TEM images of typical IO@NiNPs at
different magnifications. The high resolution images in
Fig. 4D and E shows well developed lattice fringes and the
fringes extend throughout the particle confirming the mono-
crystalline nature of the individual particles. The distance
between adjacent lattice fringes measured as 0.221 nm in
Fig. 4D corresponds to the 311 reflection. The selected area
electron diffraction (SAED) pattern shown in Fig. 4F corre-
2
9
30
literature value (852.7 eV ± 0.4 eV). In agreement with the
above analyses, it can be concluded that the oxidation state
of nickel in the catalyst is zero.
sponds to the higher order reflections of IO@NiNPs. The
HR-TEM images (Fig. 4) clearly show that nickel is deposited
on the iron oxide surface. In accordance with the above
analyses, it can be concluded that the nanoparticles prepared
by this method are a mixture of iron oxide and nickel metal
nanoparticles.
The Brunauer–Emmet–Teller (BET) surface area of a mag-
2
−1
netic IO@NiNPs sample was determined to be 17.6546 m g ,
demonstrating that these MNPs have a high surface area
(
ESI†). The pore size of the MNPs was determined to be
5.4629 nm by the Barrett–Joyner–Halenda (BJH) analysis
of the isotherms (ESI†). The total pore volumes were
2
Catalytic reduction of p-nitroaniline and other aromatic
nitro compounds
3
−1
0
.051547 cm g (ESI†). The High-Resolution Transmission
Electron Microscopy (HR-TEM) images of IO@NiNPs
show a somewhat spherical morphology with some cubic
particles and an average size range of 20–50 nm (Fig. 4A).
The IO@Ni core–shell nanoparticles were used as a catalyst
for the hydrogenation of aromatic nitro compounds using
NaBH4 as the hydrogen donor and water as a solvent as
shown in Scheme 1. The initial screening of the reduction of
p-nitroaniline (PNA) to yield p-phenylenediamine (PPDA) was
investigated under various reaction conditions (Table 1). The
conversion was monitored by UV-vis spectroscopy, where the
product and reactant show different absorption bands at
307 nm and 381 nm, respectively as shown in Fig. 5.
It is seen from the Table 1 (entries 1–3) that the presence
of a catalyst along with NaBH is required for hydrogenation
4
of PNA. In our previous study we have observed that nickel
nanoparticle-catalysed hydrogenation of PNA takes more than
2
5b
one hour.
IO, the reaction yields were very low (~50 to 55%) even after
4 h at rt (Table 1, entry 4). When the reaction conditions
When the reaction was carried out using only
2
mentioned in the Table (entry 6) were used, the reaction
proceeded faster, within 12 min, and the λmax shifted from
381 nm to 307 nm. Whereas, in the absence of NPs, the peak
Fig. 3 X-ray photoelectron spectrum of IO@NiNPs.
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Fig. 4 HR-TEM images of magnetic IO@NiNPs at different magnifications (A) 20 nm; (B) 10 nm and (C) 5 nm showing particle size distribution; the
resolved lattice fringes and SAED pattern of IO@NiNPs are (D), (E) and (F), respectively.
at 381 nm corresponding to PNA did not disappear even after
4 h of reflux. The optimum reaction conditions for PNA
12 min (Table 1, entry 6). However, further lowering the
2
amount of NaBH caused an increase in the reaction time
4
reactions are given in Table 1 (entry 6), which yields 96 ± 2%
of product at rt after 12 min and selectivity was 100%.
Nanocatalysts can catalyze the reduction of nitro com-
(entries 6 to 10) and hence an adequate quantity of NaBH
was used for the optimum reaction conditions.
4
Variation in the quantity of NPs (Table 1, entries 11 to 16)
under optimum reaction conditions showed that even 0.3 mg
was sufficient for catalyzing the reaction at rt. But the reac-
tion time increased to 180 min for maximum conversion
(Table 1, entry 16). On the other hand, with 9 mg the reaction
time decreased to 23 min (Table 1, entry 9). Beyond 12 mg
the reaction time and yield did not change (Table 1,
entries 11 and 12).
In order to assess the efficiency of the NPs further, the
quantity of water and substrate were also optimized (ESI†).
After optimization of the reaction conditions (Table 1 and
ESI†) the catalytic activity of magnetic IO@Ni core–shell NPs
with other nitro substrates was further explored (Scheme 2).
The catalytic performance of the nanocatalyst was not signifi-
cantly influenced by the nature and position of the substitu-
ents on the aromatic nitro compounds. Electron-donating as
pounds by acting as an electronic relay system to overcome
−
the kinetic barrier, in which the electrons donated by BH
4
3
1
can be transferred to the acceptor nitro groups. The
−
p-nitroaniline and BH
4
were adsorbed by the electronic
hole in the surface of the NiNPs via chemical adsorption.
Immediately, p-phenylenediamine was produced on the
surface of NPs. In the second step, the PPDA was desorbed
from the surface of the NPs. A detailed mechanism has been
3
2
proposed by Xie et al.
Furthermore, experiments were performed using a differ-
ent mole ratio of NaBH in the standard reaction. When we
4
decreased the molar quantity of NaBH from 1.85 mmol to
4
1
.44 mmol the yield and reaction time did not change
(Table 1, entries 5 and 6), whereas using 1.44 mmol of NaBH4
resulted in a remarkable yield (96 ± 2%) at 25–30 °C within
3
well as -withdrawing groups such as –Cl, –Br, CH and –OH
did not have a significant influence on the reaction (Table 2).
Notably, electron transfer over the IO@Ni core–shell nano-
catalyst exhibits excellent activity and selectivity for the
hydrogenation of a series of halogenated nitrobenzenes
to the corresponding halogenated amines, without any
dehalogenation (Table 2, entries 10, 11, 16–20 and 23). All
the nitro arenes were reduced in excellent yields affording a
single product, which minimizes the efforts to separate
unreacted starting compounds. In all cases, the turnover
number (TON) was as high as ~102 to 115 and turnover
frequency (TOF) ~2 to 12 per site per min. (Table 2).
Scheme 1 Schematic representation of IO@NiNPs catalysed reduction
of PNA to PPDA by NaBH .
4
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Table 1 Optimization of reaction conditions for hydrogenation of p-nitroanilinea
c
Sr. no.
Catalyst (mg) (IO@NiNPs)
NaBH
4
(mmol)
Temperature (°C)
Time
% Yield (±2%)
1
2
3
4
5
None
12
None
None
1.85
1.85
1.85
1.44
1.03
0.61
0.20
0.02
1.44
1.44
1.44
1.44
1.44
1.44
25–30 to reflux
25–30 to reflux
25–30 to reflux
24 h
24 h
24 h
24 h
12 min
12 min
20 min
30 min
24 h
—
<5
<10
55
96
96
96
96
90
40
96
96
96
96
96
96
None
d
IO
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
12
12
12
12
12
12
18
15
9
b
6
7
8
9
1
1
1
1
1
1
1
0
1
2
3
4
5
6
24 h
12 min
12 min
23 min
32 min
39 min
180 min
6
3
0.3
a
b
c
d
Reaction conditions: p-nitroaniline-0.362 mmol, solvent 2 mL (water). Optimized reaction conditions. Isolated yield. Iron oxide only
(20 mg).
using magnesium hydride (MgH
2
) or magnesium silicide
(
(
Mg Si) by annealing (350–750 °C) under high H pressure
0.1–7 MPa) for 2–4 h. This opens up new possibilities for
2
2
3
3
the eco-friendly synthesis of aromatic amines.
Recycling of IO@NiNPs
The IO@Ni core–shell NPs were recovered by external
magnetic separation (Fig. 6A; a and b) with negligible loss.
Recycling experiments carried out under optimized condi-
tions showed interesting results. It was observed that the NPs
could be reused directly without further purification for
3
0 consecutive runs, as can be seen in Table 3, with a very
high total turnover number (TTN) of 3456. Up to 16 cycles
there was no loss in activity. After that, however, the reaction
time increased with each successive recycling experiment
from 12 min up to 140 min. This may be due to gradual loss
of the catalytic activity of the nanocatalyst with the number
of runs, which may be due to various reasons. One of the rea-
sons may be surface modification due to the deposition of
Fig. 5 UV-vis spectra indicating the conversion of p-nitroaniline to
p-phenylenediamine in the presence of IO@NiNPs.
3
4
matter during the reaction. The HR-TEM images of the
th
nanocatalyst recorded after the 30 run display an agglomer-
ation of NPs due to deposited matter (Fig. 6B and C). The
image in Fig. 6D shows that lattice fringes were damaged by
deposited matter throughout the particle. The SAED pattern
shown in Fig. 6E corresponds to the reflections of IO@NiNPs,
confirming that the catalyst is still crystalline in nature;
however, the catalytic activity reduces due to deposited
matter. Nevertheless, the reaction does proceed to comple-
tion at the same temperature at the end of 30 cycles giving
the same yield. Further, the reused IO@NiNPs were also char-
acterized by EDX and atomic absorption spectrophotometry
(AAS) to confirm the leaching of nickel during the reaction.
EDX analysis reveals that the Ni content in IO@NiNPs, which
was 1.89% before the reaction, showed a marginal decrease
to 1.53% (Fig. 6F) after the reusability study of 30 cycles. This
Scheme
nitroaromatics.
2 General scheme for the reduction of various
Further, this catalytic system works efficiently in aqueous
systems and therefore, no hazardous solvent was required.
This reaction has been investigated in the presence of other
protic solvents such as methanol, ethanol, and ethylene
glycol; however, none of them gave satisfactory results (data
not given). In an aqueous system, NaBH gives sodium meta-
4
borate (NaBO2) which is the only waste generated during
the reaction. However, sodium metaborate can be recycled
3
3
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Table 2 Chemoselective reduction of nitroaromatics to aromatic amines at rt by IO@NiNPsa
b
c
−1
S. no
Substrate
Product
Time (min ± 2)
Yield (% ± 3)
TON (±2)/TOF (min
)
1
2
3
4
5
6
7
8
9
o-Nitrotoluene
p-Nitrotoluene
m-Nitrotoluene
o-Nitrophenol
p-Nitrophenol
m-Nitrophenol
p-Nitrobenzyl alcohol
m-Nitrobenzyl alcohol
o-Nitrobenzyl alcohol
4-Chloro-3-nitroaniline
4-Chloro-2-nitroaniline
Nitrobenzene
o-Nitroaniline
m-Nitroaniline
p-Nitroaniline
o-Chloronitrobenzene
m-Chloronitrobenzene
p-Bromonitrobenzene
o-Bromonitrobenzene
m-Bromonitrobenzene
6-Nitroquinoline
1-Nitronaphthalene
4-Chloro-2-methyl-1-nitrobenzene
3-Nitroquinoline
o-Toulidine
p-Toulidine
m-Toulidine
o-Aminophenol
p-Aminophenol
m-Aminophenol
p-Aminobenzyl alcohol
m-Aminobenzyl alcohol
o-Aminobenzyl alcohol
4-Chlorobenzene-1,3-diamine
4-Chlorobenzene-1,2-diamine
Aniline
o-Phenylenediamine
m-Phenylenediamine
p-Phenylenediamine
p-Chloroaniline
m-Chloroaniline
p-Bromoaniline
o-Bromoaniline
m-Bromoaniline
Quinolin-6-amine
1-Naphthylamine
2-Amino-5-chlorotoluene
Quinolin-3-amine
m-Phenylenediamine
o-Phenylenediamine
p-Phenylenediamine
24
13
16
09
10
17
13
13
15
30
40
25
16
14
12
19
22
17
21
29
35
38
50
40
30
28
28
89
92
94
92
90
90
89
94
95
91
90
95
94
94
96
93
94
95
95
94
94
93
88
92
89
85
92
106/4.45
110/8.49
112/7.05
110/12.26
108/10.8
108/6.3
106/8.15
112/8.61
114/7.6
109.2/3.64
108/2.7
114/4.56
112.8/7.05
112.8/8.05
115/9.6
111.6/5.87
112.8/5.12
114/6.7
114/5.4
112/3.86
112/3.2
111.6/2.93
105.6/2.11
110.4/2.76
106.8/3.56
102/3.64
110.4/3.94
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
1,3-Dinitrobenzene
1,2-Dinitrobenzene
1,4-Dinitrobenzene
a
Reaction conditions: all reactions were carried out at rt, substrate 0.362 mmol, catalyst 12 mg (0.45 wt% or 1.06 mol% NiNPs of substrate),
b
c
4
NaBH 1.44 mmol and solvent 2 mL (water). Isolated yield after column chromatography. TON and TOF was calculated on the basis of
NiNPs.
confirmed that only negligible leaching of nickel occurred,
candidate for other important organic conversions and indus-
trial applications. Further investigation, modification, and
applications are under progress in our laboratory.
which was further supported by AAS results (ESI†). Ferrous
and ferric ion leaching were also confirmed by AAS (ESI†).
The high recyclability and stability of IO@NiNPs can be
attributed to the presence of a starch coating which makes
the IO@NiNPs stable to water and air and maintains the
Experimental
Materials
3
5
metallic state of nickel in the catalyst. This eventually
increases their utility and enhances recycling efficiency.
p-Nitroaniline (PNA), other aromatic nitro compounds,
NaBH , Ni(CH COO) ·4H O, liquid ammonia, and starch
4
Thus, the metal surface, capping agent and NaBH simulta-
4
3
2
2
neously make the IO@NiNPs system an effective catalyst.
were purchased from Merck Mumbai, India. All of the solu-
tions were prepared using double-distilled and demineralized
water. Iron ore tailings, collected from iron industries (Silverline
Exporters Private Limited, Gujarat, India), were screened for
their iron content and one sample with relatively high iron
content (~36.04%) was used as one of the starting materials.
Conclusion
The present study demonstrated the feasibility of using a
waste from iron ore beneficiation plants (IOTs) as one of the
starting materials for the synthesis of IO@Ni core–shell
MNPs and their possible application in catalysis. A highly
efficient, viable heterogeneous IO@Ni core–shell MNP cata-
lyst was developed for the hydrogenation reaction using the
environmentally friendly solvent water. Due to strong interac-
tions between NiNPs and iron oxide, the catalyst could be
reused by recycling 30 times without any significant loss in
catalytic activity and selectivity. The reaction was broadly
applicable as diverse aromatic nitro compounds were succes-
sively converted to the corresponding amines in excellent
yield (85–96%), high TON and TOF. Being robust and
magnetically separable, this core–shell catalyst is a promising
Synthesis of iron oxide
A mixture of iron oxide was synthesized from the waste iron
ore tailings (IOTs) via acid leaching and precipitation
through a sequential precipitation method, as previously
3
5
reported. For almost complete recovery of iron from IOTs it
was first neutralized by adding Ca(OH)2 followed by the
settling of aluminium hydroxide and calcium sulfate; subse-
quently, it was neutralized by adding NaOH followed by
precipitation of Fe(OH) . The Fe(OH) was gradually oxidized
2
2
to iron oxide. The thermal treatment of iron oxide was
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Fig. 6 (A) Reaction mixture of PPDA (a) before and (b) after magnetic separation by a simple magnet; HR-TEM of reused magnetic IO@NiNPs at
different magnifications (B) 20 nm, (C) 0.2 μm, (D) 1 nm (lattice fringes) and (E) SAED pattern and (F) EDX pattern of IO@NiNPs.
Table 3 Reduction of PNA to PPDA at rt (25–30 °C) using IO@NiNPs under optimum conditions (recycling experiments)
a
b
a
b
Cycle
Time (min ±2)
Yield (±1%)
IY (±3%)
Cycle
Time (min)
Yield (±1%)
IY (±3%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
12
12
14
13
13
16
15
16
21
25
24
21
25
21
24
98
99
98
98
98
98
97
98
98
98
99
99
98
98
98
96
96
96
96
96
96
96
96
96
96
96
96
96
96
96
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
22
30
36
35
42
45
46
45
45
47
45
44
59
70
140
99
98
98
99
99
99
99
99
98
99
98
99
98
98
98
96
96
96
95
95
96
96
96
95
96
96
96
96
95
96
0
1
2
3
4
5
a
b
GC yield. Isolated yield.
performed in an oven, after drying at 100 °C, at 450 °C. Fur-
Synthesis of iron oxide@nickel nanoparticles (IO@NiNPs)
ther details on the synthesis of iron oxide are discussed
3
6–38
+
elsewhere.
These results indicate that IOTs could be used
IO@Ni core–shell NPs were synthesized by reduction of Ni
as a source material for the production of iron oxide, with
comparable quality to those produced using analytical-grade
reagents.
ions in the presence of iron oxide. To deposit NiNPs, 100 mg
of iron oxide was dispersed in 10 mL of deionized water and
sonicated for 30 min. Nickel nanoparticles were synthesized
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on magnetic core through a wet chemical reduction process
Kα radiation (λ = 0.15406 nm) on an advance X-ray powder
diffractometer. Samples were prepared by pressing dried
powder and patterns were collected with a scanning rate of
2
5b
by earlier reported methods with minor modifications.
Ni(CH COO) ·4H O and soluble starch were used as a metal
salt precursor and stabilizing agent, respectively. NaBH4
10%, W/V) was used as a reducing agent and liquid ammo-
3
2
2
−
1
2° min and 2θ ranging from 0 to 80°. Surface area and
porosity of the nanocatalyst were measured by a volumetric
adsorption system (Micromeritics Instrument corporation,
USA, model ASAP 2010) using N2 adsorption/desorption
isotherms at 77 K up to 1 bar. Prior to the measurements,
the samples were activated (degassed) by heating at the rate
(
nia as a complexing agent. Briefly, 10 mL of 0.02 M nickel
acetate solution was added to 10 mL of starch solution
(
1%, W/V) and stirred on a magnetic stirrer at RT. The pH of
the solution was adjusted to 10 with liquid ammonia
600 μL) and reduction was carried out with 200 μL of NaBH4
−
1
(
of 1 K min up to 383 K under vacuum. The temperature as
well as vacuum were maintained for 7 hours prior to the
measurements. Surface area was calculated by the Brunauer–
Emmet–Teller (BET) method and the porosity by the Barrett–
Joyner–Halenda (BJH) method. High-Resolution Transmis-
sion Electron Microscopy (HR-TEM) was carried out using a
Jeol (Jem-2100) electron microscope operated at an accelera-
tion voltage of 200 kV. For this purpose, the dry powered
sample was dispersed in methanol and ultrasonication treat-
ment was carried out for 30 min. Subsequently, that sample
was deposited onto a carbon-coated grid at room temperature
and it was allowed to air dry (about 6 hours). Selected area
electron diffraction patterns (SAED) and Energy-dispersive
X-ray spectroscopy (EDX/EDS) were also investigated from the
electron micrographs. FT-IR spectra were recorded with KBr
pellets on a Perkin Elmer RX1 model in the range of
solution. On further stirring for 10 min, the IO were gradually
coated by NiNPs when the colour of the solution changed
from red to reddish black. The resultant IO@NiNPs were
collected by an external magnet and washed with deionized
water several times to remove unreacted materials. Finally,
NPs were washed with acetone, and dried at 60 °C under a
vacuum.
Catalytic reduction of p-nitroaniline and other aromatic
nitro compounds
In a typical reaction PNA 50 mg (0.362 mmol) was used as a
starting material, NaBH 54.6 mg (1.44 mmol) as a source of
4
hydrogen, water (2 mL) as a solvent and IO@Ni-MNPs 12 mg
(
0.45 wt% NiNPs of PNA) as a nanocatalyst. All the compo-
nents were mixed together in a 10 mL round-bottom flask
and the reaction was carried out at room temperature (rt,
−
1
4
000–400 cm . Magnetic measurements were done by a
vibrating sample magnetometer (EG&G Model 155 VSM) at
room temperature in the range +20 000 to −20 000 G. The
surface composition was investigated using X-ray photoelectron
spectroscopy (XPS) on a VSW X-ray photoelectron spectrometer
2
5–30 °C) under stirring for 12 min. Reaction monitoring was
done by thin-layer chromatography (TLC) and gas chromatog-
raphy. After completion of the reaction, the catalyst was sepa-
rated magnetically. The product was isolated by extraction in
dichloromethane, and evaporation of solvent followed by
column chromatography (10 : 90, ethyl acetate in hexane v/v)
over basic alumina furnished p-phenylenediamine (PPDA).
The spectroscopic data of this compound are in good agree-
ment with those reported. The reaction was also carried out
with IO, NiNPs and without catalyst for comparison.
(
UK) using a Mg and Al twin anode X-ray gun with
multichannel detector and hemispherical analyser having
resolution of 1.0 eV. The binding energies obtained in the XPS
analysis were calibrated against the C1s peak at 284.6 eV.
IO@NiNPs-catalyzed hydrogenation reactions were moni-
tored on a PerkinElmer Lambda 35 UV-vis spectrophotometer
by corresponding λmax. Hydrogenation reaction monitoring
was also done by thin-layer chromatography (TLC, using
ninhydrin as staining reagent) and gas chromatography (GC).
All products of the reduction of nitroarenes are commercially
available and were identified by comparing their physical
and spectral data (m.p., TLC (silica gel 60 F254, Merck,
Mumbai, India), GC (Perkin Elmer clarus 500 GC) and
Reduction of other nitro aromatics was carried out in a
similar manner. The products purified by short-path basic
alumina chromatography (0–40% ethyl acetate in hexane v/v)
1
were analyzed by H NMR (ESI†).
All experiments have been repeated three times and the
reproducibility confirmed. The recyclability of the NPs was
also surveyed. The NPs were recovered with a magnet and
washed with water followed by methanol and again with
water. They were dried at 60 °C under vacuum and used for
the next cycle.
1
H NMR (BRUKER 400 MHz)) with those of authentic samples
or reported data.
Acknowledgements
Characterization methods
The authors are grateful to the Solvay group, Research, Devel-
opment and Technology Centre at Savli, Gujarat, India, for
financial assistance.
The chemical analyses for the major constituents of IOT, acid
insoluble residue and aqueous suspension of IO@NiNPs were
carried out by conventional wet chemical analysis while AAS
(
AA 6300: Shimadzu, Japan) or ICP-OES (Perkin Elmer, 4300)
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