Low-temperature catalytic oxidation of aniline to azoxybenzene over an Ag/Fe2O3 nanoparticle catalyst using H2O2 as an oxidant

Article abstract of DOI:10.1039/c9nj01085h

An in situ modified hydrothermal synthesis of Ag/Fe₂O₃ nanoparticles (NPs) and studies of their catalytic activity as a simple, eco-friendly and recyclable catalyst for one-pot conversion of aniline to azoxybenzene were performed. The as-synthesized nanostructured material was characterised by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), SEM-mapping, temperature-programmed reduction (H₂-TPR), X-ray photoelectron spectroscopy (XPS), N₂ adsorption-desorption isotherms (BET), Fourier transform infrared spectroscopy (FT-IR), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), ultraviolet-visible spectroscopy (UV-vis) and vibrating sample magnetometer spectroscopy (VSM). The most active and recyclable catalyst with 2-5 nm diameters of the metallic Ag particles supported on 10-50 nm Fe₂O₃ nanoparticles was formed with a silver loading of 1.8 wt%. A high turnover number of ～592 was achieved with 92% conversion of aniline and 94% selectivity towards the target product azoxybenzene under atmospheric conditions. The effects of various reaction parameters including the reaction time, temperature and substrate to H₂O₂ molar ratio were screened and studied in detail. The results reveal the role of a synergistic effect between the surface Ag nanoparticles and Fe₂O₃ nanospheres for high catalytic activity.

Full text of DOI:10.1039/c9nj01085h

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Sharma, S. Adak, R. khatun, G. Singh, D. Das, V. Joshi, S. Bhandari, S. S. Dhar and R. Bal, New J. Chem.,
2019, DOI: 10.1039/C9NJ01085H.
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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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DOI: 10.1039/C9NJ01085H
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Low temperature catalytic oxidation of aniline to azoxybenzene
over Ag/Fe2O3 nanoparticle catalyst using H2O2 as an oxidant
Received 00th January 20xx,
Accepted 00th January 20xx
Bappi Paul,a Sachin K. Sharma,a Shubhadeep Adak,a Rubina Khatun,a Gurmeet Singh,a Dipak Das,a
Vedant Joshi,a Sahil Bhandari,a Siddhartha Sankar Dharb and Rajaram Bal*a
DOI: 10.1039/x0xx00000x
An in-situ modified hydrothermal synthesis of Ag/Fe2O3 nanoparticles (NPs) and studies of its catalytic activity as a simple,
eco-friendly and recyclable catalyst for one-pot conversion of aniline to azoxybenzene was performed. The as-synthesized
nanostructured material was characterised by powder X-ray diffraction (XRD), transmission electron microscopy (TEM),
scanning electron microscopy (SEM), SEM-mapping, temperature-programmed reduction (H2-TPR), X-ray photoelectron
spectroscopy (XPS), N2 adsorption-desorption isotherm (BET), Fourier transform infrared spectroscopy (FT-IR), inductively
coupled plasma−atomic emission spectroscopy (ICP-AES), ultraviolet-visible spectroscopy (UV-Vis) and vibrating sample
magnetometer spectroscopy(VSM). The most active and recyclable catalyst with 2–5 nm diameters of metallic Ag particles
supported on 10–50 nm Fe2O3 nanoparticles was formed with a silver loading of 1.8 wt%. High turnover number of ∼592
was achieved with 92% conversion of aniline and 94% selectivity towards the target product azoxybenzene under
atmospheric condition. The effect of various reaction parameters correspond to reaction time, temperature and
substrates to H2O2 molar ratio were screened and studied in detail. The results reveals that the role of synergistic effect
between the surface Ag nanoparticles and Fe2O3 nano-spheres for high catalytic activity.
literature that various attempts have been made using
stoichiometric and catalytic routes for oxidative coupling of
aniline by using different oxidising agents. Aromatic amines
Introduction
Selective oxidation of anilines is an important step for
conversion of amines into a valuable industrial intermediates
in the field of pharmaceuticals, food additives, therapeutic
medicine, polymer stabiliser and liquid crystal display
devices.1-5 Azoxybenzene as one of the oxidised product of
get oxidized in presence of stoichiometric oxidants such as,
MnO2,8 Pb(OAc)4,9 peracetic acid,10 BaMnO4,11 Hg(OAc)212 and
also in presence of various types of molecular sieves.13 Corma
et al. has studied the oxidative coupling of aniline by molecular
oxygen to azobenzene with high selectivity over Au–TiO2
nanocatalyst.14 Recently, Ag/WO3 and CuCr2O4 were found to
be an excellent catalyst for selective oxidation of aniline to
azoxybenzene.15,16 Although Ag/WO3 catalyst has its own
advantages, however considering the loading of silver
percentage, conversion, selectivity and recovery of the
catalyst after reaction still provides us enough scope for
development of new catalyst. Li et al. has reported Ag
nanostructured catalyst for room temperature selective
oxidation of aniline to azobenzene at room temperature by
using strong potassium hydroxide (KOH) and dimethyl
sulfoxide (DMSO) as solvent.17 Lima et al. has reported that
niobium oxyhydroxide promotes the catalytic activity of δ-
FeOOH to convert aniline into azoxybenzene in the presence of
hydrogen peroxide.18 Das et al. has found that Titanium-
Loaded on MCM-48 act as an efficient catalyst for oxidation of
aniline to azoxybenzene in presence of hydrogen peroxide.19
Recently, Silva et al has reported sub-15nm CeO2 nanowires
with increased oxygen defects and Ce3+ Sites for Selective
oxidation of aniline at room temperature in presence of
H2O2.20 Among the numerous oxidising agents H2O2 and O2
anilines, is used as
a starting material for Wallach
rearrangement provide an easy synthetic route to synthesis
hydroxyazobenzene which is applicable in coloration of dye,
lacquer and resin.6,7 Azoxybenzene compounds falls in the
class of N-oxides of azo compounds which are produced as an
intermediate in the oxidation of aniline via dehydration of N-
phenylhydroxyl amine and nitrosobenzene. These reaction
pathway are quite harsh and lead to various intermediates.
Therefore, the challenge lies in manipulation of reaction
condition to increase the formation of desired product in the
center of all conceivable oxidized products with reasonable
conversion of the substrate. It is well documented in the
a. *Catalytic Conversion & Processes Division, CSIR-Indian Institute of Petroleum,
Dehradun- 248005, Uttrakhand, India.
b. Department of Chemistry, National Institute of Technology Silchar, Silchar-
788010, Assam, India.
*Corresponding authors. Fax: +91 135 2660202; Tel: +91 135 2525917
c. *E-mail: raja@iip.res.indress
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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have their own significance and regarded as a greener
oxidants. Although oxygen efficiency of molecular oxygen is
greater than hydrogen peroxide, but still it requires drastic
reaction condition to activate the oxygen at usual temperature
and pressure in presence of heterogeneous catalytic system.
Thus, from the safety and economical point of view, hydrogen
peroxide is a better oxidant over molecular oxygen. But the
reusability of the catalysts separation of the product from the
reaction mixtures become two major challenges in liquid-
phase organic transformation. Therefore, a recyclable catalyst
that works under mild reaction condition is in high demand.
Magnetic nanoparticles are highly desirable material as
catalysts or catalyst supported materials in comparison with
other transition metals oxides, iron oxide has been recognized
as a green catalyst or support because of its low toxicity, mild
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DOI: 10.1039/C9NJ01085H
O
NH2
Ag/Fe2O3, H2O2
Acetonitrile, 50°C
N N
Scheme 1. Oxidative of aniline to azoxybenzene
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Experimental
Synthesis of catalyst
Silver supported iron oxide nanoparticles was synthesized by
adapting our own surfactant assisted hydrothermal method.24
Adapting this synthesis procedure large quantity (up to 30 g) of
catalyst can be synthesized with highly reproducibility. An
reaction conditions, high thermal, mechanical stability and amount of 24.539
g
of ferric nitrate nonahydrate
high catalytic activity.21-23 Magnetic separation is a very
efficient technique for the separation of magnetic particles
from reaction mixture make them ideal catalyst for liquid
phase organic reaction.24 Among all the oxides of iron
hematite (α-Fe2O3) is one of the most desirable iron oxide
nanomaterials due to their versatile applications as a catalyst
in various reactions.25-26 Recently, nano-iron oxide catalyzed
oxidation of alcohols and olefins has been studied using
hydrogen peroxide as an oxidant.27 Noble silver metal
nanomaterials supported on iron oxide have shown
tremendous potentials in various fields including catalysis.
Several methods have been developed for the synthesis of
silver supported nanoparticles. However synthesis of
nanoparticles with desired shape, size and morphology is still
considered a challenging proposition. Jiang et al had reported
in situ synthesis of Ag nanoparticles on nanoporous Fe2O3
microboxes simply by annealing Prussian blue (PB) in the
presence of silver nitrate.28 Cui et al has successfully fabricated
(Fe(NO)3.9H2O) and 0.472 g of silver nitrate (AgNO3) were
dissolved distinctly in minimum volume of deionized water.
The resultant clear solutions of silver nitrate were then mixed
to the ferric nitrate solution and stirred magnetically. After
stirring for 15 min 16.822
g of hexamine and 1.8 g
cetyltrimethylammonium bromide (CTAB) was dissolved in
water and ethanol mixture (50 ml water and 2 ml ethanol) and
was added to the solution, followed by the addition of 0.4 g
hydrazine. After stirring for 4 h, the reddish suspension was
collected and the solution mixture was transferred into a
stainless steel autoclave and heated at 180 °C for 24. The
obtained solid material was centrifuged and washed with
excess of H2O and ethanol followed by drying for 12 h at 80 °C.
The as-obtained precursor powder was then calcined in a
muffle furnace for 4 h at 550°C to obtain powder of Ag/α-
Fe2O3. ICP-AES analysis was performed to confirm the loading
of the silver nanoparticles. The catalyst was also prepared by
co-precipitation and impregnation methods.
superhydrophobic Ag-Fe2O3/Fe surfaces on iron substrates Oxidative coupling of aniline
with a basic and simple replacement deposition method.29
Kulkarni et al has synthesized Ag@Fe2O3 nanoparticles by leaf
extract assisted process where leaf of Adathoda vasica plant
act as a capping agent as well as a reducing agent.30 Recently,
Paul et al has reported in situ synthesis of Ag/Fe2O3 decorated
on reduced graphene oxide nanocomposite via one-pot
hydrothermal synthesis procedure, where polyethylene glycol
act as surfactant to control the size and morphology of the
nanomaterials.31 We report herein the preparation of highly
dispersed ∼2–5 nm silver nanoparticles supported on
nanocrystalline iron oxide via cetyltrimethylammonium
bromide assisted hydrothermal method followed by
calcination for the selective oxidative coupling of aniline to
azoxybenzene under very mild reaction condition (Scheme 1).
Catalytic oxidation of aniline was performed in a 50 ml round
bottom flask with continuous stirring (800 rpm) in an oil bath.
In a typical procedure, 0.1 g nanostructured catalyst, was
dispersed in a 10 ml acetonitrile solution under sonication.
Subsequently, 1 g of substrate (aniline) and 2.15 g H2O2 (50%
aq. solution) was added drop wise under continuous stirring.
At regular intervals small portion of the reaction mixture were
withdrawn for analysis. The solid nanostructured catalyst was
separated from the reaction mixture by an external magnet.
The final products were examined using a gas chromatograph
(Agilent 7890B). The Carbon balance was performed for the
reaction and it was found to be between 98% ± 2.
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Results and discussion
Catalyst characterization
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The powder XRD patterns of the synthesized Ag/α-Fe2O3
nanoparticles was indexed with all the peaks at 2θ values
24.1°, 33.3°, 35.6°, 40.8° and 49.6° confirming the formation of
rhombohedrally structured α-Fe2O3 ((JCPDS # 87-1166, space
group: R3/c). (Fig. 1). The peaks patterns of α-Fe2O3 matches
well with the reported peaks, with the highest intensity peak
at a 2θ value of 33.3°, which is referred to the plane (104). In
addition to the diffraction peaks of α-Fe2O3 we could observed
two additional intense peak at 2θ values of 38.1 and 44.3°
corresponds to (111) and (200) of Ag(0) which also match well
with the literature reported values (JCPDS # 89-3722). No
change in phase of Ag was observed for the reused catalyst
indicting that the catalyst is active even after cycle of reuse.
The loading of silver present in the Ag/α-Fe2O3 nanocrystalline
material was confirmed by ICP-AES.
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Figure 2. SEM images of the (a, b) fresh and (c) spent catalyst and (d)
the SEM-EDS mapping of the fresh catalyst.
Figure 3. Elemental mapping of Ag/α-Fe2O3 (a) Fe, (b) O, and (c) Ag.
The particle size and distribution of the silver over Fe2O3
nanoparticles was carried out using high resolution
transmission electron microscopy (HRTEM) analysis (Fig. 4).
TEM image reveals almost spherical shape Fe2O3 nanoparticles
with size 20-50 nm. It was also observed from the TEM analysis
that small spherical size silver nanoparticles distributed
throughout the spherical shape Fe2O3 nanoparticles with an
average size of Ag particles within the range of 2–5 nm.
Moreover, from the HRTEM image we could observed the
lattice fringes separated by 0.25 nm and 0.23 nm, corresponds
to (110) and (111) plane for Fe2O3 and Ag(0) respectively.
Figure1. Powder XRD patterns of Ag/α-Fe2O3 nanoparticles.
The morphology and the structural properties of the
synthesized Ag/α-Fe2O3 nanoparticles was determined by SEM
analysis. The representative SEM images of the catalyst shows Moreover, the HRTEM image of the reused catalyst (Fig. S1 in
almost spherical shape of size 20-50 nm (Fig. 2(a,b,c). The EDS
spectra of the synthesized nanoparticles exhibited only the
presence of iron, silver and oxygen and no other peaks of
impurities could be identified in the spectra (Fig. 2d).
In order to confirm the dispersion of silver on the
nanocrystalline α-Fe2O3 elemental mapping of the as
synthesized material was performed. The results revels the
homogenous distribution of Ag on nanocrystalline α-Fe2O3
support (Fig. 3). It could be observed from the mapping that
silver nanoparticles are distributed throughout the support,
which confirms the uniform loading of silver.
ESI) shows almost same shape and size even after cycle of
recyclability test. The corresponding particle size distribution
histogram of Ag particles with sizes between 2 and 5 nm
indicates a very narrow particle size distribution which is in
good agreement with the PXRD (Fig. 4a, inset).
The metal-support interactions (Ag-Fe) and the reduction
behaviour within the catalysts were compared by H2-
temperature-programmed-reduction (TPR) (Fig. 5). The
synthesized nanomaterial exhibited two broad reduction peaks
at high temperature, the peak around 350 °C can be attributed
to the reduction of Fe2O3 to Fe3O4 and the other wide peaks at
higher temperatures around 500°C can be documented due to
transformation of Fe3O4→ FeO → Fe-metal.
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It was also observed from the TPR profile that with the loading
of silver on the support (Fe2O3), both the reduction peak
(around 400 and 800) shifted to lower temperatures compared
to the support. The study confirmed that loading of Ag
enhanced the reduction of Fe2O3 which further leads to strong
metal support interaction between the active metal (Ag) and
the support (α-Fe2O3). This perhaps takes place through the
spill over of hydrogen from the silver atoms to iron oxide.32,33
spectrums we could observed three different peaks at the
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binding energies of 533.2, 531.8 and 52D9O.8I:e10V.1c0o39rr/Ces9pNoJ0n1d0s85tHo
the adsorbed oxygen (Oads), hydroxyl oxygen (Ohyd) and lattice
oxygen (Olatt), respectively.33,34 Whereas the XPS analysis of
the spent catalyst confirms that the intact of oxidation state of
silver (Ag0) during the catalysis (Figure S2 in ESI).
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Figure 6. (a) Ag 3d3/2 spectrum (b) Fe 2p spectrum (c) O 1s spectrum of
the Ag/ α-Fe2O3 catalyst.
Figure 4. (a, b) TEM images (inset Ag particle size distribution), (c)
HRTEM image (lattice fringes) and (d) ED pattern of Ag/α-Fe2O3
nanoparticles
Figure 7. N2 adsorption-desorption isotherm and pore size distribution
curve (inset) of Ag/α-Fe2O3 nanoparticles.
To analysis the surface area of the synthesized nanoparticles
(Ag/α-Fe2O3) N2 physisorption measurements (BET) was
conducted. The N2 adsorption–desorption isotherms of the
synthesized nanostructured material exhibits high specific
surface area and pore volume. The amount of N2 adsorbed rise
gradually at lower relative pressures and then increased
sharply at higher relative pressures (Fig.7). The sample exhibits
type-IV adsorption–desorption isotherm with
a type H2
hysteresis loop in the P/P0 range from 0.80 to 0.95. The pores
are predominantly in the range from 3 to 15 nm as confirmed
by pore size distribution which is justified from the TEM
images (inset of Fig.7).
Figure 5. TPR profile of (a) Fe2O3 (b) Ag/Fe2O3 nanoparticles.
To confirm the valence state of Ag nanoparticles in Ag/α-Fe2O3
nanostructured catalyst X-ray photoelectron spectroscopy
(XPS) was performed where the presence of metallic silver
could be clearly identified which corresponds to 3d5/2 and
3d3/2 with binding energy around 368.3 eV and 374.2 eV
respectively (Fig. 6a). The Fe 2p3/2 and 2p1/2 spectra attributed
to the binding energies 710.6 eV and 724.2 eV respectively
suggest that the binding energies of Fe3+ in Fe2O3. (Fig. 6(b)). In
addition to that a satellite peak at 717.6 eV is characteristic of
Fe3+ ions in α-Fe2O3.33 After deconvolution of oxygen (O 1s)
The surface-coordinating organic template (CTAB) on the
uncalcined catalyst (Ag/Fe(OH)3 precursor) was analysed by
FTIR spectroscopy (Fig. S3 in ESI). To study the nature of
interaction and occurrence of the template molecules with the
Fe-O surface, FTIR-spectrum of clean CTAB with that of
uncalcined Ag/Fe(OH)3 precursor were studied. For pure CTAB,
the stretching vibrations of the ammonium group appeared at
1487 and 1472 cm-1 (νasym(CH3–N+)) and 1439 cm-1 (νsym(CH3–
N+)). The characteristic absorption bands for CTAB in the Ag–Fe
sample appeared at 1640 cm-1 (νasym(CH3–N+)) and 1410 cm-1
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(νsym(CH3–N+)). It can be incidental that mutual interactions a ferromagnetic behaviour with a saturation magnetization of
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DOI: 10.1039/C9NJ01085H
between template and the surface of Ag–Fe have taken place. around 33.4, coercivity (Hc) of 310.06 G and remnant
The peaks at 809 and 1062 cm-1 corresponds to the C–N+ magnetization (Mr) of 8.42 emu g-1.
stretching vibration of CTAB molecules. The frequencies
beyond 1600 cm-1 to 3000 cm-1 can be attributed to the CH2
symmetric and antisymmetric stretching vibrations
respectively. The band at 609 and 471 cm−1 are attributed to
Fe–O bond in tetrahedral and octahedral site in Fe2O3. After
the calcination of nanostructured material we could observe
the absence of all the typical frequencies of the surfactant
(CTAB) which confirm that the removal of surfactant moieties
from the materials. However, we could observe the absorption
bands at 608 and 517 cm−1 even after calcination which refers
to the typical peak of Fe-O (metal oxide). Furthermore, no
peaks of C, N or Br were detected from the SEM-EDS diagram
of catalyst which confirms the removal of the template (CTAB)
after calcination. The surface-coordinating organic surfactant
CTAB molecules on the pre-calcined catalyst surface were
further confirmed from the TGA. Two step decomposition
pathways were observed from the TGA curve (Fig.S4 in ESI). In
the first step (30-100°C) the weight loss of 8.04% was observed
due to the decomposition of physically adsorbed moisture. In
the second step the weight loss of 73.33 % around 280-520°C
was observed, indicating the removal of CTAB from the
nanoparticles surface and conversion of Ag/Fe(OH)3 to Ag/α-
Fe2O3 nanoparticles. TGA curve of the catalysts after
calcination does not show the presence of any organic
impurities which conforms the complete removal of the
surfactant CTAB (Fig. S5 in ESI). The UV-Vis DRS absorption
spectrum of Ag/α-Fe2O3 (after calcination) is shown in Fig. S6
in ESI. The absorption peak shows bands around 475 nm and
550 nm which are due to the presence metallic silver
nanoparticles and hematite nanostructures, which confirm the
presence of metallic silver nanoparticles supported over iron
oxide nanostructures. The physicochemical properties of the
fresh and spent catalyst is summarised in Table 1.
Catalytic activity
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After the fruitful synthesis and characterization of
nanostructured catalyst (Ag/α-Fe2O3), we extend our studies to
evaluate its catalytic efficiency for oxidative coupling reaction
of various aromatic substituted amines where aniline was
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selected as
a standard substrate and various reaction
parameter was evaluated such as temperature, time, catalyst
amount and aniline: H2O2 molar ratio in order to get finest
selectivity and yield of azoxybenzene. When the catalytic
performance were evaluated by using H2O2 as oxidant and
acetonitrile (CH3CN) as a solvent with aniline: H2O2 mole ratio
(1 : 3) for 8 h at 50 °C, the catalyst exhibited 94% selectivity
towards azoxybenzene with maximum aniline conversion of
92% were nitrobenzene, nitrosobenzene and azobenzene were
spotted as the by-products of the reaction. When the molar
ration of aniline and H2O2 was 1 : 1, we observed 40%
conversion of aniline with 60% azoxybenzene selectivity (very
low), which is believed due to the lesser interaction between
active oxidising species and the reactant. When the aniline to
H2O2 mole ratio is increased up to five times (1:5), the
conversion of aniline is increased up to 97% and the selectivity
of azoxybenzene decreased to 72% with the increase in
selectivity for nitrobenzene, which may be due to over
oxidation in the presence of excessive active oxygenated
species (Fig. 8).
Table 1. Physicochemical Properties of the Ag/ α-Fe2O3 Nanoparticles
Catalyst
Catalyst
Ag
loadinga
(%)
BET
Surface
area
Morphology
Average particles size
From
XRDb
Fe2O3
From TEM
(m2/g)
Fe2O3
32
Ag
Ag/Fe2O3
1.8
1.6
35.4
35.9
Uniform
dispersed
Spherical
Uniform
dispersed
Spherical
33.7
33.1
3.8
3.6
Ag/Fe2O3
(Spent,
after 5
cycle
32
Figure 8. Effect of the aniline: H2O2 molar ratio on catalytic oxidation
of aniline. [ ] aniline conversion, [ ] azoxybenzene selectivity, [
]
nitrobenzene selectivity, [ ] azobenzene selectivity, [ ] nitrosoaniline
selectivity. Reaction Condition solvent = CH3CN; catalyst weight = 0.10
g; substrate= aniline (1 g); time = 8 h; temperature 50ºC.
When the temperature is low, the selectivity of azoxybenzene
was found to be 90% with 42 % conversion of aniline, with the
increase of temperature up to 70 °C, the conversion increases
aEstimated by ICP-AES. bMeasured using Scherrer equation.
The room temperature magnetic hysteresis studies of Ag/α-
Fe2O3, with the magnetic field applied extensive from -20000 G
to +20000 G (Fig.S7 in ESI). The VSM hysteresis loops exhibited
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results of the optimization studied of the reaction parameter
progressively but the selectivity decreases as the increase in
formation of nitrobenzene as the side product which may be
due to the over oxidation of aniline or decomposition of H2O2
takes place at high temperature (Fig. 9). The effect of reaction
time was also studied and it was observed that with the
increase in time both the azoxybenzene selectivity and aniline
conversion increases (Fig. 10). A sharp increase in the
selectivity of azoxybenzene and conversion of aniline and with
the decrease in the selectivity of nitroso benzene was
observed after 4 h of reaction. Figure 10 shows the maximum
selectivity towards azoxybenzene (∼94%) was achieved at 8 h
of reaction, but after 10 h the selectivity of azoxybenzene
decreases to ∼78% due to the formation of nitrobenzene. The
individual role of Ag and Fe2O3 was explored for the catalytic
activity, where we have conducted a series of experiments.
View Article Online
DOI: 10.1039/C9NJ01085H
a standard catalyst, various other
and Ag/α-Fe2O3 as
substituted aniline reactions were performed. Table
3
summarized the results of various other substituted aniline.
Various substituted anilines bearing electron withdrawing and
electron donating groups were examined and was found to be
selectively transformed to their corresponding substituted
azoxy products with high to excellent yield.
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Figure 10. Effect of time on catalytic oxidation of aniline. [ ] aniline
conversion of [ ] azoxybenzene selectivity, [
] nitrobenzene
selectivity, [ ] azobenzene selectivity, [ ] nitrosoaniline selectivity.
Reaction condition solvent = CH3CN; catalyst weight = 0.10 g;
substrate= aniline (1 g); substrate: H2O2 mole ratio = 1 : 3; temperature
50ºC.
Figure 9. Effect of temperature (°C) on catalytic oxidation of aniline.
[ ] aniline Conversion, [ ] azoxybenzene selectivity, [ ] nitrobenzene
selectivity, [ ] azobenzene selectivity, [ ] nitrosoaniline selectivity.
Reaction Condition solvent = CH3CN; catalyst weight = 0.10 g;
substrate= aniline(1 g); substrate :H2O2 mole ratio = 1: 3; time = 8 h.
The oxidative coupling of aniline to azoxybenzene over Ag/α-
Fe2O3 nanostructured catalyst is assumed to follow the radical
pathway as illustrated in Scheme S1 in ESI. According to the
previous reported literature, in the first step H2O2 decompose
over Ag NPs to form superoxide (O2•−) conferring to the
reaction mechanism shown in eqn (i). In the second step the
generated superoxide (O2•−) from Ag(0) yields peroxide species
•OOH, which acts as the active species for this catalytic
reaction eqn (ii)).35-38 Subsequently, the adsorption of aniline
onto the surface of iron oxide take place through electrostatic
interactions. This active peroxide species (•OOH) then
combine with the N–H bond of aniline, which is slightly
electropositive in nature by donating its lone pair of electron
to iron. Nitrosobenzene and phenyl hydroxyl amine were
found to be a main intermediates for this reaction, which
consensuses well with the literature report for the oxidation of
aniline in presence of H2O2.15,16
Negligible amount of aniline conversion was observed in
absence of the catalyst (Table 2, entry 10). When the catalytic
activity of Ag/α-Fe2O3 synthesized by our present preparation
method was studied it was observed that the catalyst showed
excellent conversion of aniline (Table 2, entry 6). When the Ag
loaded on α-Fe2O3 by impregnation method (imp), it exhibit
poor activity behaviour due to increase in sizes of particle and
the leaching of active metals (Table 2, entry 5). Furthermore,
the Ag/α-Fe2O3 nanostructured catalyst synthesized by our
preparation method showed high TON of 592.0 for the
formation of azoxybenzene with greater H2O2 efficiency of
∼27.1%, compared to simple Ag or Fe2O3 catalyst (Table 2,
entries 3 and 4). It is believed that excellent catalytic activity
shown by the present catalyst (Ag/α-Fe2O3) synthesized by our
present method is due to high specific surface area (35.4 m2/g)
and excellent dispersion of small sized Ag nanoparticles
supported on Fe2O3 which leads to easy accessibility active
catalytic sites of the catalyst and the reactants. Thus, from the
It is assumed that the reaction take place step by step, where
at the preliminary aniline get oxidizes to phenyl hydroxyl
amine, which further oxidizes to azobenzene. Finally
azobenzene oxidizes to form azoxybenzene.
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Table 2. Oxidative coupling of aniline to azoxybenzenea
View Article Online
DOI: 10.1039/C9NJ01085H
Entry
Catalyst
Selectivity SPc (%)
H2O2
efficiency
b
CT
Yield
YA
(%)
Turnover
number
(TON)
(%)
Nitro
benzen
Nitroso
benzene
Azo
benzen
Azoxy
benzene
d
(%)
e
11
8
13
17
38
4
e
48
6
52
2
1
2
3
4
5
6
Agcom
Fe2O3
Agus
Fe2O3
Ag/Fe2O3
3.6
11
32
27
18
92
41
47
31
36
3
-
22
4
45
52
94
-
-
-
-
-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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27
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29
30
31
32
33
34
35
36
37
38
39
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45
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com
2.4
1.3
12.2
9.4
86.5
205.9
173.7
115.8
592
0.4
3.8
2.9
27.1
us
imp
7
Ag/Fe2O3 nano
1
1
catalyste
7
8
Ag/Fe2O3 nano
catalystf
91
99
99
1
7
-
1
-
1
4
-
92
96
99
-
83.72
95.0
98.0
585.6
543.7
325.6
26.2
35
Ag/Fe2O3 nano
catalystg
9
Ag/Fe2O3 nano
catalysth
No Catalyst
-
-
60.3
10
-
-
-
a Reaction conditions: CH3CN = 10 ml, aniline = 1 g, catalyst = 0.10 g with silver loading = 1.8 wt%, temperature = 50°C; time = 8 h; aniline
: H2O2 mole ratio = 1 : 3; b CT: aniline conversion analysed by GC = [aniline reacted in mols/aniline used in mols] × 100. c SP: moles of the
d
e
f
product / moles of aniline converted; YA Yield = conversion × selectivity/100; ∼Silver nanoparticles supported on Fe2O3. Catalyst
after 5 cycle of reuse; g When reactant phenyl hydroxyl amine was used, time = 6 h; h When reactant azobenzene was used, time = 4 h;
imp = impregnation method; com = commercial; us = bare Ag and Fe2O3 synthesized by our method. H2O2 efficiency = [moles of
azoxybenzene formed/total moles of H2O2 added] × 100.
In order to get more idea about the reaction pathway and the catalytic cycles to check the adeptness and efficiency of the
formation of intermediate,
a reaction was carried out catalyst. The nanostructured catalyst remain active even after
separately with all the intermediates under the same reaction 5 consecutive cycle without decline in the catalytic activity
condition in presence of Ag/α-Fe2O3 as catalyst. Where phenyl which confirms true heterogeneity of the catalyst (Fig.11(a)).
hydroxyl amine and azobenzene produced azoxybenzene
within 6 h and 4 h of reaction time respectively (Table 2, entry
9) which confirm the presence of phenyl hydroxyl amine as
well as azobenzene as intermediates in the reaction for the
formation of azoxybenzene.
In order to confirm the radical formation in the reaction
mixture, p-benzoquinone was added in the reaction mixture
where we observed aniline conversion up to 84% with 95%
selectivity towards azoxy benzene within 6 h of reaction time.
On the other hand, equimolar quantity of 2,6-di-tert-butyl-4-
(4-methoxyphenyl)phenoxyl (DBMP) was added to the
reaction mixture as a radical scavenger to confirm the radical
formation. The reaction was virtually suppressed under this
reaction condition with aniline conversion of ∼14% even after
24 h of continuous reaction. From all these above observation
it propose, the increasing in the reaction rate with the addition
Table 3. Oxidative coupling of of different substituted anilinesa
Entries
Substrate
CT (%)
Product
Selectivity (%) Yield (%) TON
O
H3C
N
N
84
94
62.1
84.6
413.8
422.8
74
1
H3C
NH2
CH3
Cl
Cl
O
Cl
90
90
2
3
NH2
N
N
O
Cl
F
NH2
NH2
Cl
N
N
Cl
82.8
80.1
92
90
422.8
480.0
O
F
N
N
F
89
90
72
4
5
O
H3CO
O2N
N
N
OCH3
H3CO
O2N
NH2
NH2
86
68
77.4
49.0
437.9
312.6
O
of
p-benzoquinone,
whereas
2,6-di-tert-butyl-4-(4-
6
N
N
NO2
methoxyphenyl)phenoxyl retards it. Which confirm a radical
pathway for our present catalytic system.
After the reaction was completed, the nanostructured solid
catalyst was magnetically recovered from the reaction mixture
followed by washing with ethanol and reused for numerous
a Reaction conditions: solvent (CH3CN) = 10 ml, substrate = 1 g,, catalyst = 0.10 g, time = 8 h; substarte: H2O2 molar
ratio = 1 : 3; CT: conversion analysed by GC = [substarte reacted in mols/substarte used in mols] × 100; Selectivity total
moles of the product formed/total moles of substarte converted; Yield = conversion × selectivity/100
The TEM image of the reused catalyst exhibited an almost
similar shape and size to that of the fresh synthesized catalyst
(Fig S1 in ESI). The heterogeneity of the catalyst was further
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confirmed from the ICP-AES analysis where the amount of Ag
and Fe remain nearly same even after five consecutive cycle
with negligible amount of silver leaching (concentration of
both metals were <2 ppb). Conventional hot filtration test was
performed to confirm no leaching of silver particles from the
support during the reaction, where the catalyst was removed
after 4 hour of the catalytic reaction.39 After 4 hour of catalytic
reaction the yield of the product was noted. The catalyst was
separated and the reaction mixture and was further continued
for another 6 hours, no noticeable increase in the yield of the
product was observed. Which confirms the absence of
94% azoxybenzene selectivity at 50 °C with H2O2 as oxidant. In
this present studies the synthesized nanostructured catalyst
showed true heterogeneity as its effectiveness remains
unchanged even after five consecutive cycles.
View Article Online
DOI: 10.1039/C9NJ01085H
Conflicts of interest
There are no conflicts to interest.
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Acknowledgements
leaching of Ag particles from the support (Fe2O3) in the course B.P thanks SERB-DST, New Delhi, for financial support in the
of reaction (Fig.11(b)).
form of National Post-Doctoral fellowship (PDF/2016/001948).
Director IIP, is acknowledged for his encouragement and
support.
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We have described herein a facile and simple surfactant
assisted hydrothermal preparation of Ag NPs supported Fe2O3
nanoparticles catalyst. The synthesized material were
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interacted with Fe2O3 support. The catalyst with 1.8% Ag
loading was found to be the optimum loading for the selective
oxidative coupling of aniline with 92% aniline conversion and
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Graphical Abstract
Highly dispersed Ag supported α-Fe2O3 found to be recyclable and efficient catalyst for one-pot conversion of aniline to
azoxybenzene
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