Highly robust magnetically recoverable Ag/Fe2O3 nanocatalyst for chemoselective hydrogenation of nitroarenes in water

Article abstract of DOI:10.1016/j.apcata.2017.03.007

This work reports on additive-free Ag nanoparticles (4–6?nm) deposition on magnetic α-Fe₂O₃ nanocrystals surface by the slow reduction of AgNO₃ with NaBH₄ in aqueous medium. The EDS analysis revealed that the new materials contain 3.93?wt% of Ag nanoparticle on the surface of α-Fe₂O₃ which are the active suite for hydrogenation reaction. The Ag/Fe₂O₃ nanocatalysts exhibited good catalytic ability toward chemoselective hydrogenation of nitroarenes in water. LC–MS study suggested that the catalytic reaction pathway is –NO₂, –NHOH, –NH₂ and certainly skips the nitrosoarene intermediate step. The nanocatalysts are very efficient in the hydrogenation of nitroarenes tolerating –H, –Br, –I, –OH, –OCH₃, –COOH and –CONH₂ functional groups. The nanocatalysts were separated by external magnet and recycled in aqueous medium which offer environmentally and safe approach to this hydrogenation reaction. The catalyst was tested up to 10 recycles and showed no significant loss of catalytic activity.

Full text of DOI:10.1016/j.apcata.2017.03.007

Accepted Manuscript
Title: Highly Robust Magnetically Recoverable Ag/Fe2O3
Nanocatalyst for Chemoselective Hydrogenation of
Nitroarenes in Water
Authors: Astam K. Patra, Nhat Tri Vo, Dukjoon Kim
PII:
S0926-860X(17)30096-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.03.007
APCATA 16166
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Applied Catalysis A: General
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Accepted date:
13-1-2017
20-2-2017
7-3-2017
Please cite this article as: Astam K.Patra, Nhat Tri Vo, Dukjoon Kim, Highly
Robust Magnetically Recoverable Ag/Fe2O3 Nanocatalyst for Chemoselective
Hydrogenation of Nitroarenes in Water, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2017.03.007
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Highly Robust Magnetically Recoverable Ag/Fe2O3 Nanocatalyst
for Chemoselective Hydrogenation of Nitroarenes in Water
Astam K. Patra, Nhat Tri Vo and Dukjoon Kim*
School of Chemical Engineering, Sungkyunkwan University, Suwon, Gyeonggi 16419,
Republic of Korea
Corresponding Author
*Tel.: +82-31-290-7250. Fax: +82-31-290-7270. E-mail: djkim@skku.edu (D.K.).
Graphical abstract
1

Highlight
 Highly efficient Ag nanoparticles incorporated magnetic α-Fe2O3 nanocatalyst were
developed by simple method.
 4-6 nm Ag nanoparticles on the surface of α-Fe2O3 are the active suite for the high
catalytic activity.
 LC-MS study suggested that the catalytic reaction pathway is –NO2, –NHOH, –
NH2 and certainly skips the nitrosoarene intermediate step.
 The nanocatalyst are very efficient in the hydrogenation of nitroarenes tolerating –
H, −Br, −I, −OH, −OCH3, −COOH, and −CONH2 functional groups in water.

High catalytic activity and excellent recyclability and can be separated by external
magnet and no significant loss of catalytic activity after 10 recycles.
Abstract
This work reports on additive-free Ag nanoparticles (4-6 nm) deposition on magnetic α-Fe2O3
nanocrystals surface by the slow reduction of AgNO3 with NaBH4 in aqueous medium. The
EDS analysis revealed that the new materials contain 3.93 weight % of Ag nanoparticle on the
surface of α-Fe2O3 which are the active suite for hydrogenation reaction. The Ag/Fe2O3
nanocatalysts exhibited good catalytic ability toward chemoselective hydrogenation of
nitroarenes in water. LC-MS study suggested that the catalytic reaction pathway is –NO2, –
NHOH, –NH2 and certainly skips the nitrosoarene intermediate step. The nanocatalysts are
very efficient in the hydrogenation of nitroarenes tolerating –H, −Br, −I, −OH, −OCH3,
−COOH and −CONH2 functional groups. The nanocatalysts were separated by external magnet
and recycled in aqueous medium which offer environmentally and safe approach to this
hydrogenation reaction. The catalyst was tested up to 10 recycles and showed no significant
loss of catalytic activity.
Keywords: Ag/Fe2O3 nanocatalyst; Heterogeneous catalysis; Hydrogenation; Nitroarene
reduction in water; Magnetically recoverable
2

1. Introduction
Metallic nanoparticle incorporated magnetic iron oxide nanomaterials based catalysts have
concerned significant research interest in catalysis due to their easy separation, cost
effectiveness, nontoxic behaviour and environmentally safe [1-8]. Hematite (α-Fe2O3) have
better thermodynamic stability than other different iron oxides under ambient environments
[9]. On the other hand, only metal nanocatalysts show high catalytic activity but suffer from
several problems, such as separation, leaching and aggregation. So metal nanoparticle
incorporated magnetic α-Fe2O3 nanomaterials created new vision due to their ease magnetic
recovery and recyclability with high catalytic activity [1]. Recent research has demonstrated
that small size of Ag nanoparticle have high catalytic activity due to their new nanoscale
properties [10]. The Ag nanoparticles incorporated α-Fe2O3 nanomaterials have been
considered as efficient catalysts to replace conventional homogeneous catalysts, which are
normally hampered by separation and recyclability. Furthermore silver nanoparticle draw great
attention in catalysis due to plasmonic behavior of nanoscale silver [11, 12], high light
adsorption efficiency [13], oxygen reduction property [14], dissociation of H2 molecule [15]
and substrates for surface-enhanced Raman spectroscopy (SERS) [16]. The catalytic activity
of silver nanoparticles relies on upon the size, shape, atomic and electronic positioning in the
exposed facets of nanoparticles [10,17]. But the controlled synthesis of silver nanoparticles on
metal oxide surface by a simple method still remains a challenge to the researchers. In the past
decades, researchers have established many synthetic strategies, but most of the process are
energy exhaustive, need substantial template removal and formed large particles [18]. Here we
demonstrated a simple and eco-friendly method to grow monodisperse Ag nanoparticle on
magnetic bitruncated-octahedron shaped α-Fe2O3 nanocrystals surface. The size of the Ag
nanoparticle (4-6 nm) is governed by the controlled reduction of AgNO3 solution with NaBH4
in aqueous medium without any capping agent.
3

Nitroarenes compounds have different hazardous effect to the environment and heath. The
organic contaminant are mostly used in industry to prepare different pesticides, dyes, and
explosives [19, 20]. Some nitroarenes compounds (especially 4-nitrophenol) pollute water and
these compounds are a serious threat to the environment and public health due to their highly
toxic and carcinogenic in nature [21]. On the other hand aromatic amines are intermediates for
the productions of important chemicals in dyes, pharmaceuticals, and agricultural industries
[22-24]. Most of the hydrogenation of nitroarenes associated with hazardous solvent, required
higher temperature, dangerous hydrogen gas or use of expensive hydride source like silanes,
N2H2, LiAlH4 or LiBH4 [25-27]. Additionally in many methods, hydrogenation process often
ends at an intermediate step forming hydroxylamine, or hydrazine, azoarene etc like side
products [28]. Hence performing the hydrogenation of nitroarenes in sustainable method with
green solvent like water is challenging [29, 30]. Selective hydrogenation of different
nitroarenes and nitrobenzene has drawn much attention in the past decades, involving Ag, Au,
Pt, Ru, Ni/C and Co/C catalysts [31-36]. In 2012 Dey et al. used Fe(0) catalyst for selective
reduction of nitroarenes in water but after the reaction the catalyst transformed to Fe3O4 and
further cannot be used [37]. In 2014 Sharma et al. found that vasicine can be used for
hydrogenation of nitroarenes in water [38]. Dell’Anna et al. used polymer supported palladium
nanocrystals as efficient and recyclable catalyst for the reduction of nitroarenes to anilines
under mild conditions in water [39]. Kelly et al. selectively reduced the nitroaromatics in water
at room temperature using zinc dust, NH4Cl and commercially available designer surfactant
TPGS-750-M [40]. Recently Feng et al. used recyclable Fe/ppm Pd nanoparticles, PEG-
containing designer surfactants and NaBH4 for safe and selective nitrogroup reductions in
water at room temperature [41]. Here we synthesized Ag nanoparticle decorated magnetic
bitruncated-octahedron shaped α-Fe2O3 nanocrystals which show high catalytic hydrogenation
of nitroarene in water in presence of NaBH4 as hydrogen source. Furthermore, iron(III) is a
4

harder Lewis acid, compared to late-transition-metal cations, allowing better interaction with
nitrogroup of the nitroarene to facilitie the hydrogenation reaction. It is worth pointing out that
NaBH4 is sustainable reducing agent which is mild, efficient and very effective for the
reduction of nitroaromatic compounds and after reduction it converted water soluble boric acid.
In the absence of catalysts, NaBH4 cannot reduce nitrogroups.
2. Experimental Section
2.1 Synthesis of bitruncated-octahedron shaped α-Fe2O3 nanocrystals
Bitruncated-octahedron shaped α-Fe2O3 nanocrystals were first synthesized according to the
procedure reported in our previous work [9]. In this synthesis process, sodium salicylate (0.8
g) and NaOH (0.2 g) were added in 10 mL of water. Iron(III) nitrate nonahydrate (2.02 g) with
3.0 g of water was gradually added to the above solution. 2M NaOH solution was used to adjust
the pH = 8. The following mixture solution was stirred for 3 hrs. Then, this solution was
hydrothermally treated at 200 °C for 72 hrs in a Teflon-lined stainless steel autoclave. The
resulting material was separated and washed several times with water and ethanol. The material
was dried at 25 °C under vacuum. Furthermore, to remove the salicylate molecule, the material
was extracted with acid-ethanol solution. The material is denoted as S200-BTO.
2.2 Preparation of Ag/Fe2O3 nanomaterials
Ag nanoparticles were deposited on the surface of bitruncated-octahedron shaped α-Fe2O3
nanocrystals without any capping agent. In this synthesis, 0.2 g S200-BTO (α-Fe2O3)
nanocrystal was dispersed in 150 mL water by sonication. Then the solution was putted into
ice bath with constant stirring. 20 mL 0.01 M AgNO3 solution was added to the above solution
and stirred for 30 min. Then 50 mL 0.002 M NaBH4 solution was added slowly to the above
mixture. The resultant mixture was stirred for 3 hrs. Then the reaction mixture removes from
ice bath and keeps in room temperature for 12 hrs. The sample was collected by centrifugation
5

and wash with water and methanol several times. The sample was dry in vacuum in room
temperature. The material is denoted as Ag/Fe2O3 nanocatalyst.
2.3 Catalysis procedure
In the hydrogenation of nitroarene, the kinetics, effect of solvent, amount of catalyst and
amount of NaBH4 were carried out on 4-nitrophenol as model reagent in room temperature. In
the typical experiment, 2 ml of 0.1 mmol/L 4-nitrophenol aqueous solution was taken into a 3
mL cuvettes and 500 µL of 1 mg/mL catalyst aqueous solution was added to it. Then 200 µL
of 10 mmol/L of NaBH4 aqueous solution was added. The reaction was monitoring by UV-Vis
spectrometer at regular intervals of time. The solvent effect, amount of catalyst and amount of
NaBH4 was studies in similar reaction condition. The intermediate products of 4-nitrophenol
reduction were identified by using a liquid chromatograph-mass spectrometer (Agilent Model
1100 LC-MS ion trap with C18 Column and negative ion mode). Here to decrease the rate of
reaction, 20 mL of 0.5 mmol/L 4-nitrophenol aqueous solutions was taken and 1 mL of 1
mg/mL catalyst aqueous solution was added to it. Here we increase the ratio between 4-
nitrophenol to catalyst to decrease the rate of reaction. Then 1 mL of 0.1 mmol/mL of NaBH4
aqueous solution was added and the solution was stirred. 10 μL of the aliquot was injected for
analysis and acetonitrile–water mixture (30: 70) was used as an eluent with the column
temperature at 298 K and 1 mL/ min flow rate. For different nitroarene reduction, 0.2 mmol of
nitroarene compound was dispersed in 10 mL water and 5 mg of catalyst was added into a 25
mL sealed tube. The solution was sonicated for 5 min and was placed in an oil bath at 100 °C
under vigorous stirring. 2 mmol of NaBH4 was added to the solution and closed the cap and
continues reaction. After 30 min of reaction, the magnetic catalyst was recovered by an external
magnet and the solution was transferred to 100 mL beaker. Then 20 mL of ethyl acetate was
added to it. Then take all the solution in separating funnel and shake it. Then the organic layer
was collect and dried over Na2SO4 and evaporated to dryness to give the colorless crude
6

1
product. The isolated crude product was characterized by H (shown in the Supporting
Information), respectively.
2.4 Leaching test
To know the true heterogeneous nature of the catalyst, we performed the leaching test. 1 mmol
of 4-nitrophenol was dissolve in 10 ml water and 5 mg of catalyst was added to it. The solution
was sonicated for 2 min and was placed in an oil bath at 100 °C under vigorous stirring. 2 mmol
of NaBH4 was added to the solution and closed the cap and continues reaction. After 8 min,
the reaction was stopped and removed all the catalyst from the solution by centrifuging at
10,000 rpm. Then collect the aliquot and continue the reaction for another 10 min.
2.5 Recyclability of the Ag/Fe2O3 nanocatalyst
The reusability of the Ag/Fe2O3 nanocatalyst was examined for 4-nitrophenol reduction
reaction in water. For this reaction, 0.2 mmol of 4-nitrophenol was dissolved in 10 ml water
into a 25 mL sealed tube and 5 mg catalyst was added. The solution was sonicated for 2 min
and was placed in an oil bath at 100 °C under vigorous stirring. 2 mmol of NaBH4 was added
to the solution and closed the cap and continues reaction. After completion of the reaction, the
magnetic catalyst was recovered by an external magnet and the solution was decanted. The
catalyst was washed thoroughly with ethanol several times. For the next reaction, the catalyst
was activated through drying at 100 °C for 3 hrs and used for subsequent recycling
experiments. The recycling was performed for ten repetitive reaction cycles.
2.6 Characterization
The Ag/Fe2O3 nanocatalyst were examined by different characterization techniques. Purity and
crystallinity of the samples were studied by a Bruker D-8 Advance diffractometer operated at
40 kV voltage and 40 mA current using Cu Kα (λ = 0.15406 nm) radiation. HRTEM, EDS and
lattice indices images were recorded in a JEOL JEM-2100F TEM operated at 200 kV. FE SEM
7

(JEOL JEM-7600F) was used for the morphology analysis. Nitrogen sorption analysis were
carried out by a Micromeritics Instrument ASAP 2000 surface area analyzer at 77 K. XPS was
done on a Thermo Scientific (Model No ESCALAB 250Xi) X-ray Photoelectron Spectrometer
operated at 15 kV and 20 mA with a monochromatic Al K X-ray source. UV-visible spectra
were recorded on a UV/Vis Scanning Spectrophotometer (Beckman Coulter DU 730 Model).
The intermediates of 4-nitrophenol reduction were supervised by liquid chromatograph-mass
spectrometer (LC-MS, Agilent Model 1100). The hydrogenation products were recognised by
Fourier transform-nuclear magnetic resonance spectroscopy (FT-NMR) using a UniyInovq 500
1
(Varian, U.S.A.) NMR spectrometer. The products were dissolved in DMSO-d6 before H
NMR measurement.
3. Results and Discussion
3.1 Ag/Fe2O3 Nanocatalyst Synthesis and Structural Study
The synthesis process for Ag nanoparticle decorated bitruncated-octahedron-shaped α-Fe2O3
nanocrystal (denoted as Ag/Fe2O3) via simple method is shown in Scheme 1. Here we have
developed a facile and simple method to prepare Ag/Fe2O3 nanocatalyst. Bitruncated-
octahedron shaped α-Fe2O3 nanocrystals were first synthesized by hydrothermal method
according to our previous work [9]. The Ag nanoparticle was deposited on bitruncated-
octahedron-shaped α-Fe2O3 nanocrystal without any capping agent. Here α-Fe2O3 nanocrystal
was dispersed in water along with AgNO3 solution and stirred in ice bath. The hydrophilic
hydroxyl groups on α-Fe2O3 surface can easily coordinate with Ag+ ions.
The Ag+ cation was then reduced to Ag(0) slowly by addition of NaBH4 solution following the
chemical equation.
2NaBH4 + 2AgNO3 + 6H2O = 2Ag + 7H2 + 2H3BO3 + 2NaNO3
8

But in the presence of Ag nanoparticles in the reaction medium, the reduction rate of the
reaction was increased rapidly which increased the crystal growth of Ag. So we used ice bath
to control reaction rate and decrease the growth of Ag nanoparticles. In literature several
research groups used template molecule to control the size of Ag nanoparticles. But template
molecule were cover the surface of Ag nanoparticle and subsequent removal are very dificult
and show low catalytic reactivity. Here small size Ag nanoparticle were deposited on the
surface of α-Fe2O3 with out any template molecule which make it a promissing catalyst. The
structural features, crystallinity, and purity of the prepared Ag/Fe2O3 nanomaterials were
measured, and the achieved XRD pattern is shown in Fig. 1. The X-ray diffraction pattern
clearly specified the presence of two phases, i.e. Ag and α-Fe2O3; because the obtained XRD
pattern matches the JCPDS 04-0783 for cubic Ag metal and JCPDS 01-084-0308 for α-Fe2O3
[42,43]. The XRD pattern are indexed according to the 2θ values for silver and iron oxide as
shown in Fig. 1. The peaks at 2θ values 24.2, 33.3, 35.7, 41.0, 43.7, 49.7, 54.3, 57.7, 62.7, 64.3
and 64.3° are assigned to (012), (104), (110), (113), (024), (116), (122), (214), and (300) planes
of α-Fe2O3, respectively [43,44]. The peaks at 2θ values 38.1, 44.3 and 64.5° are assigned to
(111), (200) and (220) planes of silver, respectively [42]. This wide angle XRD analysis
evidently specifies that the Ag and α-Fe2O3 are both highly crystalline in nature.
3.2 Nanostructure and Composition Analysis
Fig. 2 displays the high resolution field-emission scanning electron microscope (FE-SEM)
images of S200-BTO and Ag/Fe2O3 nanomaterials. FE-SEM images were used to study the
size and morphological features of nanomaterials. Fig. 2 clearly indicates that α-Fe2O3
nanocrystals are identical in particle size (length: 310 ± 50, width: 220 ± 20, height: 150 ± 10)
and Ag nanoparticles were deposited on the surfaces. The images also exhibit that the
nanomaterials are well dispersed and moreover, we could clearly see the faces of all the
bitruncated-octahedron shaped nanocrystals. High resolution transmission electron microscopy
9

(HR TEM) was used to investigate the shape, size, and different surfaces of exposed facets of
the nanomaterials. The Ag and α-Fe2O3 nanocrystals both are clearly seen in Fig. 3a, having
the shape of spherical and bitruncated-octahedron respectively.
We found that through the specimen, the Ag nanoparticles had a uniform size of 4-6 nm along
with some 25 nm particle. Fig. 3b displays an individual bitruncated-octahedron particle along
with Ag nanoparticles. A closer view of the image in Fig. 3b is acquired from the lower side
and different Ag nanoparticles are seen more clearly in Fig. 3c. The Fig. 3d is acquired from a
closer view of Fig. 3c, and different classes of lattice fringes are resolved. The lattice spacing
of the bitruncated-octahedron is 0.25 nm, which agrees to (110) lattice planes in α-Fe2O3 crystal
[9].The lattice spacing in the Ag nanoparticles is 0.23 nm, matching to the (111) and equivalent
lattice planes of the cubic Ag crystal phase [21]. The TEM images indicated that the Ag
nanoparticles with particle sizes less than 10 nm were grew in the surfaces of the bitruncated-
octahedron shaped α-Fe2O3 nanocrystals with well define nanostructure.
The elemental composition of the prepared Ag/Fe2O3 nanomaterials was investigated by
elemental mapping and EDS analysis. Fig. 4 shows the elemental mapping and EDS spectrum
of Ag/Fe2O3 nanomaterials. We recorded a dark-field STEM image (Fig. 4a) and performed
elemental mapping for Ag/Fe2O3 nanomaterials. Elemental mapping for Fe, O and Ag is shown
in Fig. 4b, c, and d respectively, and all the elements are spread over the whole image. Peaks
for the elements Fe, O and Ag are clearly seen in the EDX spectrum which revealed that the
Ag/Fe2O3 nanomaterials is composed of Fe, O and Ag and the elements are present 60.23,
35.84 and 3.93 wt.% respectively.
3.3 XPS and Surface Composition Analysis
XPS analysis was carried out before and after Ag nanoparticles loading on α-Fe2O3
nanocrystals surface to examine the oxidation states, surface composition of the elements and
10

amount of Ag metal present in the Ag/Fe2O3 nanomaterials. Fig. 5 displays the XPS results
corresponding to the Ag/Fe2O3 materials. The binding energy of C 1s line to 284.6 eV is used
as reference to correct the binding energies of elements obtained in the XPS analysis [45,46].
The survey XPS profile (Fig. 5a) revealed that the S200-BTO and Ag/Fe2O3 nanomaterials are
composed of Fe, O and Fe, O, Ag respectively. The elemental composition of Ag/Fe2O3 is
calculated as (atomic %): Fe 55.62, O 43.26, Ag 1.12. In the high-resolution Fe spectra (Fig.
5b), both materials show two peaks at binding energies of 710.68 eV for Fe 2p3/2 and 724.36
eV for Fe 2p1/2. The satellite peaks appear at 718.75 eV binding energy which is characteristic
of Fe3+ in α-Fe2O3 [44]. The deposition of Ag on surface of the particle does not change the
oxidation state of Fe significantly. The high-resolution O 1s spectra of S200-BTO (Fig. 5c)
show two peaks which are located at 529.3, and 530.9 eV. But O 1s spectra of Ag/Fe2O3 show
three peaks which are located at 529.3, 530.9 and 531.9 eV. The peaks at 529.3 eV are assigned
to the lattice oxygen atoms binding with Fe (Fe–O) and the peaks at 530.9 are assigned to the
surface hydroxyl groups (Fe–OH) [47]. The new peak at 531.9 is assigned to Fe-O-Ag (surface
complexation) [42]. The high-resolution Ag 3d spectrum (Fig. 5d) show two peaks and these
two peaks are located at 367.75 eV for Ag 3d5/2 and 373.79 eV for Ag 3d3/2 with a spin–orbit
splitting of 6 eV for the Ag 3d5/2 and Ag 3d3/2 states which is characteristic of silver metal [48].
The results indicate silver nanocrystals were successfully deposited on the surface of the α-
Fe2O3 nanocrystals.
3.4 Catalytic Hydrogenation Reaction
The catalytic reactivity of the Ag/Fe2O3 nanocatalyst is examined with different nitroarene
compounds. Reactivity of the nanocatalyst depends on many factor like surface area, electronic,
and atomic structure etc. The BET N2 sorption method is used to measure the surface area [49-
11

51]. The BET surface areas for the S200-BTO and Ag/Fe2O3 nanocatalysts are determined as
16.3 and 19.58 m2g-1, respectively. Further the EDS and XPS analysis revealed that the
materials contain 3.93 weight % of Ag metal nanoparticles on the surface of α-Fe2O3
nanocrystals which are the active suite for hydrogenation reaction using NaBH4. Here we
explored the kinetics, effect of solvent, amount of catalyst and amount of NaBH4 in the
hydrogenation reaction and 4-nitrophenol was chosen as model reagent in room temperature
(25 °C). As seen from Table 1, Ag/Fe2O3 nanocatalyst showed high catalytic activity in
hydrogenation of 4-nitrophenol to aminophenol by NaBH4 in the water solvent. But in the
absence of catalyst, the hydrogenation rate of nitrophenol is very slow.
The hydrogenation reaction also carried out with α-Fe2O3 (S200-BTO) but without Ag, α-Fe2O3
does not show any activity. From Table 1, we observed that conversions of 99%, 94%, and 8%
take place in water, water-ethanol (50/50 in v/v) and ethanol solvent respectively. It was found
that organic solvents such as DMF, DCM, THF and acetronitrile almost failed to initiate the
reaction, whereas the reduction proceeded efficiently in water. Interestingly the reaction
proceeds very slowly in ethanol too. Thus, water has a critical role in this reaction. The
hydrogenation of 4-nitrophenol is shown in Fig. 6a. The progress of the hydrogenation reaction
was monitored through their UV-vis absorption spectra. 4-nitrophenol in aqueous solution
shows absorption maxima around 317 nm. With the addition of sodium borohydride solution,
the 4-nitrophenolate ion was formed immediately in the basic medium and the peak has been
shifted to around 400 nm. The corresponding final product, 4-aminephenol shows an
absorption band around 296 nm. From the UV-Vis spectra, it is clearly seen that a gradual
decrease in absorption band around 400 nm is accompanied by simultaneous increase in
absorption band around 296 nm which indicates the gradual conversion of 4-nitrophenolate
into 4-aminophenol [52]. The two isosbestic points were seen at 277 and 312 nm. It specifies
the conversion of 4-nitrophenolate into 4-aminophenol occurred without any side reactions.
12

Fig. 6b and c show that the conversion was increase with increase the amount of NaBH4 and
catalyst. Fig. 6d shows that the catalyst can be separated by external magnet. The evolution of
the reaction was monitored by liquid chromatograph-mass spectroscopy. The hydrogenation
over Ag/Fe2O3 nanocatalyst, major intermediates were recognized by LC-MS and are
summarized in Table S1. The intermediates were confirmed by ESI-MS (Fig. S1 and S2). The
intermediates were recognized by liquid chromatography (retention time 6.06 min, m/z 138
denoted compound P1; retention time 1.71 min, m/z 214 denoted compound P2; and retention
time 1.25 min, m/z 109 denoted compound P3). We observed the formation of –NHOH
transient species in the LC-MS spectra of reaction mixture at an intermediate step during
monitoring the progress of the reaction. The –NHOH transient species vanishes with time as
the reaction proceed.
According to the observations described above, we propose a general mechanistic pathway for
the Ag/Fe2O3 catalyzed reduction of nitroarenes and shown in Fig. 7. It is suggested that the
reaction pathway is –NO2, –NHOH, –NH2. Other product like azoxy-, azo-, or
hydrazobenzenes was not detected. Here the B−H bond cleavage occurs on the surface of
Ag/Fe2O3 nanocatalyst to give the [Ag]-H species (Fig. 7). Such high reactive species are
responsible for the rapid reduction of nitroarenes into the corresponding hydroxylamines with
very fast reaction kinetics and possibly skips the nitrosoarene intermediate. Fountoulaki et al.
also found similar type of mechanistic pathways in reduction of nitroarenes in the presence of
supported gold nanoparticle[26]. So the performance of Ag/Fe2O3 nanocatalyst depends on so
many parameters especially amount of catalyst, solvent, amount of NaBH4 and the substrate
concentration. Further we started our study for hydrogenation of different nitrocontaining
aromatics at 100 °C because many nitroaromatics does not dissolve in water in room
temperature. But solubility of these nitro compounds was increased in water at 100 °C. We
13

started our study with hydrogenation of 4-nitrophenol (0.2 mmol) in water (10 mL) with the
Ag/Fe2O3 nanocatalyst (5 mg) in a sealed tube at 100 °C. No catalytic conversion was taking
place in the absence of NaBH4. When NaBH4 was added as hydrogen source, the reaction was
taking place and conversion reached 99.5% and complete the reaction within 30 min with high
selectivity. The turnover frequency for Ag/Fe2O3 nanocatalysts was reached 218.5 h-1 when the
NaBH4/4-nitrophenol molar ratio was 10:1 with 1.821 x 10-3 mmol Ag catalyst. Interestingly,
it is prominent that Ag/Fe2O3 nanocatalysts shows a superior catalytic activity in the reduction
of 4-nitrophenol to that of Ag reported in literature (see Table S2). Remarkably, NaBH4 is mild,
efficient and very active for the hydrogenation of nitroarenes compounds in water. Because in
the presence of Ag metal in the reaction medium, it produces hydrogen gas and water soluble
sodium borate following the reaction:
NaBH4 + 2H2O → NaBO2 + 4H2
NaBO2 + H2O = H3BO3 + NaOH
The hydrogen gas reduced the nitro compound to amine and NaBO2 produced water soluble
boric acid which can be separated easily. We employed this method several nitroarenes. The
results are summarized in Table 2. Interestingly, 4-nitrobenzoic acid was successfully reduced
to 4-aminobenzoic, because it is a highly challenging example of such a reduction in the
presence of a free carboxylic acid. The hydrogenation of nitroarenes is also proceeds further
with other valuable substituents (CONH2, OCH3, OH, Br, I etc).
The catalyst shows poor catalytic activity for 2-nitrophenyl phenyl ether due solubility problem
in water and steric hindrance effect of O-phenoxy group. The α-Fe2O3 (S200-BTO) materials
does not show any activity in the hydrogenation reaction of nitrophenol in hot water also. To
know the true heterogeneous nature of the catalyst we performed the leaching test (see
experimental section) [45]. After removal of catalyst no further reaction was taking place. The
14

reusability of the Ag/Fe2O3 nanocatalyst in the hydrogenation reaction was examined using 4-
nitrophenol as a reference, and the recycling of the catalyst was conducted for ten repetitive
cycles. The results are shown in Fig. 8, suggesting good catalytic efficiency and nanocatalysts
did not show significant deactivation during the ten consecutive catalytic runs. In order to check
the stability and possibility of leaching, we performed high resolution TEM and powder XRD
analyses of the reused catalyst Ag/Fe2O3 nanocatalysts. Fig. S3 and S4 suggested that the
catalyst structure remain same after the catalytic hydrogenation of nitroarene to corresponding
amine compound. From these results, we can conclude that our Ag/Fe2O3 nanomaterial is
highly efficient in catalysing hydrogenation of nitroarene under mild and eco-friendly reaction
conditions.
4. Conclusions
In conclusion, a robust and green protocol has been developed for the hydrogenation of
functionalized nitroarenes to the corresponding primary amines. Here Ag/Fe2O3 nanocatalyst
has been developed by a very simple and an environmentally friendly method. Small particle
sizes (4-6 nm) of Ag nanoparticles were successfully deposited on the surface of bitruncated-
octahedron shaped α-Fe2O3 nanocrystals without any capping agent. Ag/Fe2O3 nanocatalyst
showed groups in the presence of NaBH4 in the aqueous medium. This new Ag/Fe2O3
nanocatalyst show good heterogeneous nature and presence of magnetic α-Fe2O3 particle, the
recovery of the catalyst can be easily done by external magnet and the catalyst was reused very
efficiently. The catalyst shows high TOF value in the reaction. The unique catalytic role played
by the Ag/Fe2O3 nanocatalyst may contribute significantly in heterogeneous catalyst in green
water solvent.
15

Acknowledgements
DK received grant from the National Research Foundation of Korea, which is funded by the
Korean Government (MEST) (NRF-2010-0027955).
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18

Fig. 1. Wide angle XRD pattern of the highly crystalline Ag/Fe2O3 nanocrystals and the peaks
are indexed to the α-Fe2O3 and cubic Ag phase.
19

Fig. 2. High resolution FE-SEM images of (a) S200-BTO and (b) Ag/Fe2O3 nanomaterials.
20

Fig. 3. (a) TEM image of Ag/Fe2O3 nanomaterials, (b) HRTEM image of a single particle and
Ag nanoparticles size histogram, (c) HRTEM image of particle, showing the Ag nanoparticles
on the surface of α-Fe2O3, (d) HRTEM image of Ag/Fe2O3 nanomaterial having a lattice plane
with (110) indices of α-Fe2O3 and (111) and equivalent indices of Ag nanoparticle.
21

Fig. 4. (a) Dark-field STEM image of Ag/Fe2O3 nanomaterial and elemental mapping of (b)
Fe, (c) O, and (d) Ag. The EDX spectrum of Ag/Fe2O3 nanomaterial is shown in (e).
22

Fig. 5. XPS analysis of S200-BTO and Ag/Fe2O3 nanomaterials (a) survey XPS and High
resolution XPS of (b) Fe 2p (c) O 1s (d) Ag 3d.
23

Fig. 6. (a) Kinetics study of hydrogenation reaction of 4-nitrophenol [Reaction condition: 2 ml
of 0.1 mmol/L 4-nitrophenol, 500 µL of 1 mg/mL Ag/Fe2O3 nanocatalyst, and 200 µL of 10
mmol/L of NaBH4], (b) Effect of NaBH4 amount [Reaction condition: 2 ml of 0.1 mmol/L 4-
nitrophenol, 500 µL of 1 mg/mL Ag/Fe2O3 nanocatalyst, and different amount of 10 mmol/L
of NaBH4 solution, and reaction carried out for 10 min], (c) Effect of catalyst amount [Reaction
condition: 2 ml of 0.1 mmol/L 4-nitrophenol, different amount of 1 mg/mL Ag/Fe2O3
nanocatalyst, and 200 µL of 10 mmol/L of NaBH4 solution, and reaction carried out for 5 min],
and (d) Catalyst separate by external magnet.
24

Fig. 7. Proposed reaction mechanism for Ag/Fe2O3catalyzed hydrogenation of nitroarene.
25

Fig. 8. Reusability of magnetic Ag/Fe2O3 nanocatalyst for hydrogenation of nitroarene in
water.
26

Scheme 1: Synthetic route of Ag nanocrystal decorated bitruncated-octahedron-shaped α-
Fe2O3 nanocrystal via simple method.
27

Table 1. Optimization of Reaction Conditions on hydrogenation of 4-nitrophenol in the
presence of Ag/Fe2O3 nanocatalyst in room temperature
Hydrogen
Source
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
Conversion
Entry
Solvent
Time (min)
(%)a
99
6
1
2b
3c
4
5
6
7
8
9
H2O
H2O
H2O
10
10
30
30
30
30
30
30
30
0
94
8
0
0
H2O-Ethanol
Ethanol
THF
DMF
DCM
Acetonitrile
0
0
aReaction condition: 2 ml of 0.1 mmol/L 4-nitrophenol, 500 µL of 1 mg/mL
Ag/Fe2O3 nanocatalyst, and 200 µL of 10 mmol/L of NaBH4; bReaction carried
out in absence of catalyst, cReaction carried out with α-Fe2O3 (S200-BTO).
28

Table 2. Magnetic Ag/Fe2O3 nanocatalyst for hydrogenation of nitroarene in water in the
presence of NaBH4.
Time
(min)
Yield (%)/
TOF
Product
Entry
Reactant
Selectivity (%) (h-1)
1
30
30
30
30
30
30
30
99/100
99.5/100
99.4/100
99/100
217.4
218.5
218.3
217.4
214.3
217.4
210.8
2
3
4
5
6
7
97.6/100
98/100
96/100
8
30
20.1/96
44.2
9a
30
0
-
Reaction condition: nitroarenes substrate (0.2 mmol), Ag/Fe2O3 catalyst (5 mg, 1.821x10-6
mole Ag), NaBH4 (2 mmol) and 10 mL of water. TOF (turnover number) = number of moles
of substrate converted/number of moles of active site of the catalyst per hour, aReaction carried
out with α-Fe2O3 (S200-BTO).
29