Room temperature selective oxidation of aniline to azoxybenzene over a silver supported tungsten oxide nanostructured catalyst

Article abstract of DOI:10.1039/c4gc02123a

Heterogeneous catalysts comprising silver nanoparticles supported on nanostructured tungsten oxide were applied for room temperature oxidative coupling of aniline to azoxybenzene, an important chemical intermediate and a chemical of industrial interest. The catalytic protocol features high activity and selectivity to the target product azoxybenzene with a turnover number of ～368. The catalyst was characterized by XRD, XPS, ICP-AES, FT-IR, TGA, EXAFS, SEM and TEM. The silver-tungsten nanomaterial acts as an excellent catalyst for selective oxidation of aniline to azoxybenzene using H₂O₂ as an oxidant. An aniline conversion of 87% with 91% selectivity of azoxybenzene was achieved without the use of any external additives. Moreover, a high stability and recyclability of the catalyst is also observed under the investigated conditions. This journal is

Full text of DOI:10.1039/c4gc02123a

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Page 1 of 27
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Room Temperature Selective Oxidation of Aniline to Azoxybenzene over
Silver supported Tungsten Oxide Nanostructured Catalyst †
Shilpi Ghosh, Shankha S. Acharyya, Takehiko Sasaki and Rajaram Bal*
Abstract
Heterogeneous catalysts comprising of silver nanoparticles supported on nanostructured
tungsten oxide were applied for the room temperature oxidative coupling of aniline to
azoxybenzene, an important chemical intermediate and a chemical of industrial interest. The
catalytic protocol features high activity and selectivity to the target product azoxybenzene
with a turn over number of ~368. The catalyst was characterized by XRD, XPS, ICPꢀAES,
FTꢀIR, TGA, EXAFS, SEM and TEM. The silverꢀtungsten nanomaterial acts as an excellent
2 2
catalyst for selective oxidation of aniline to azoxybenzene using H O as an oxidant. An
aniline conversion of 87% with 91% selectivity of azoxybenzene was achieved without the
use of any external additives. Moreover, a high stability and recyclability of the catalyst is
also observed under the investigated conditions.
1. Introduction
Nowꢀaꢀdays growing attention is being directed towards the development of superior
catalytic systems with high performance from the view points of environmentally greener
process, economical efficiency and minimum consumption of resources. Nanostructured
materials are potential candidates because of their unique properties they exhibit, are different
1ꢀ4
compared to their bulk counterparts. Nanomaterials with controlled size and wellꢀdefined
shapes are promising candidates for catalysis as these materials could facilitate the control of

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tuning the catalytic properties. It is a well known phenomenon that both catalytic efficiency
and selectivity are highly dependent on the size and shape of the nanostructure materials and
5
the composition of the metals and support used. Therefore, the synthesis of nanomaterials
with specific morphology has sparked growing interest in the field of catalysis over the past
years. Simple preparation methods are required for easy manipulation of the nucleation and
growth kinetics to give a tunable shape and size of the nanoparticles. We report here the
simple preparation of very small Ag nanoparticles supported on nanocrytalline WO
procedure is highly reproducible and large quantity (~30 g) material can be prepared.
3
. This
Oxidation reactions are of fundamental importance in nature. Selective catalytic oxidation of
aniline is an important reaction for the synthesis of intermediates and a precursor to various
industrially valuable oxygenated products such as nitrobenzene, nitroso benzene, azo benzene
6
and azoxy benzene. Azoxy benzene is one of the most important and central building blocks
7
in naturally occurring compounds and functional materials. It is valuable both as
intermediates and high value compounds such as dyes, reducing agents, chemical stabilizers
8
and polymerisation inhibitors. Moreover, azoxybenzene is the precursor for Wallach
9
,10
rearrangement, which offers a simple way to prepare hydroxyazobenzene.
Therefore, the
challenge lies in controlling the selectivity to the target product in the midst of all possible
oxidation products at a reasonable substrate conversion in order to meet the market needs.
Various attempts were made for oxidative coupling of aniline using different oxidants like
11
12
13
14
peracetic acid,
Pb(OAc)4, Hg(OAc)2 and BaMnO4 but the employment of these
oxidants lacks environmental benignity. Molybdenum and vanadiumꢀcompounds catalyze
1
5
aniline oxidation in presence of tertꢀbutyl hydrogen peroxide (TBHP). The use of TBHP is
cost effective as compare to H and suffers from environmental problem. So, finding the
2
O
2
alternative, more benign methods of producing azoxybenzene with high conversion and
selectivity, with the aid of environmentally clean and cheaper oxidants like molecular oxygen

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2 2
or H O is highly desirable. Applications of molecular oxygen and hydrogen peroxide as
oxidizing agents for selective oxidation of aniline in industry will have immense potential
interest. Recent years we have witnessed tremendous progress in selective oxidation of
aromatic amines. Hydrogen peroxide in presence of rhenium, tungsten or molybdenum
1
6ꢀ18
catalysts also gives nitrosoarenes with high yields.
Silica containing cobalt oxide catalyst
is reported to catalyse this reaction selectively at comparatively higher temperature (80 °C)
19
using H
2 2
O as oxidant. Titanium silicate molecular sieves, TSꢀ1 as catalyst affords selective
20
oxidation of aniline to nitrosobenzene using H
2
2 2 4
O as oxidant. Recently, spinel CuCr O
nanocatalyst was found to be suitable for oxidation of aniline to azoxybenzene at 70° C with
8
7
8% aniline conversion and 92% azoxybenzene seletivity. Corma et al. reported a Au/TiO
2
2
1
to catalyse aerobic oxidation aniline to azobenzene very selectively. Li et al. reported Ag
nanoparticles catalysed selective aerial oxidation of aniline to azobenzene in presence of
22
dimethyl sulfoxide as solvent in presence of KOH as a base at room temperature.
Heteropolyoxometalates (H PW O ) are also reported to catalyse aniline to nitrosobenzene
3
12 40
2
3
and nitrobenzene reaction at room temperature in presence of dichloromethane. Although
various catalysts have been efficiently used for the preparation of azoxybenzene, but a
successful catalytic system with very high selectivity and yield of azoxybenzene using
environmentally benign oxidant at room temperature has not been achieved. So, the
development of heterogeneous catalyst for selective oxidation of aniline to azoxybenzene
under mild condition is highly demanding.
We report here the preparation of ~2ꢀ5 nm Ag nanoparticles supported on nanocrystalline
3
WO for the selective oxidation of aniline to azoxybenzene at room temperature (25° C) (see
scheme I). An aniline conversion of 87% with 91% azoxybenzene selectivity was achieved at
room temperature with a turn over number of ~368.

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Scheme I
O-
N
N+
2
.5 wt% Ag/WO3
NH
2
O
CH CN, 25 C, H O
3
2
2
Aniline
Azoxybenzene
2
. Experimental
.1 Materials
2
Ammonium metatungstate hydrate, AgNO
3
, cetyltrimethylammonium bromide, HPLC grade
~99.9%) aniline, acetonitrile were purchased from SigmaꢀAldrich Co. Hydrogen Peroxide
50 wt % in water) was purchased from Merck KGaA, Darmstadt, Germany. All the
(
(
chemicals were used without further purification. Double distilled water was used during the
preparation of catalyst.
3
2.2 Preparation of Ag/WO nanostructured catalyst
24
The catalyst was prepared by modified our own preparation method. The advantage of this
method is that the catalyst was prepared at room temperature without the addition of
hydrazine. AgNO
CTAB) was mixed maintaining a molar ratio of Ag: CTAB: H
synthesis method, an aqueous solution of 0.32 g AgNO was added to 12.8 g ammonium
3
, ammonium metatungstate hydrate, cetyltrimethylammonium bromide
(
2
O = 1:0.70: 800. In a typical
3
metatungstate hydrate solution at room temperature with vigorous stirring for 1 h. Then 0.5 g
CTAB solution was added dropwise. The mixture was stirred for another 3 h. After that the
product was filtered, washed with ethanol and dried at 110 °C for 12 h and finally heated at
500°C for 6 h in presence of helium.

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Catalyst Characterization
A thorough characterization of the catalyst was done using different characterization
techniques like XRD, ICPꢀAES, SEM, TEM, BET surface area, EXAFS, XPS and TGA.
Powder Xꢀray diffraction patterns were collected on a Bruker D8 advance Xꢀray
diffractometer fitted with a Lynx eye highꢀspeed strip detector and a Cu K radiation source.
α
Diffraction patterns in the 2ºꢀ80º region were recorded at a rate of 0.5 degrees (2Ɵ) per
minute. Scanning electron microscopy (SEM) images were taken on a FEI Quanta 200 F,
6
using tungsten filament doped with lanthanumhexaboride (LaB ) as an Xꢀray source, fitted
with an ETD detector with high vacuum mode using secondary electrons and an acceleration
tension of 10 or 30 kV. Samples were analyzed by spreading them on a carbon tape. Energy
dispersive Xꢀray spectroscopy (EDX) was used in connection with SEM for the elemental
analysis. The elemental mapping was also collected with the same spectrophotometer.TEM
images were collected using a JEOL JEM 2100 microscope, and samples were prepared by
mounting an ethanolꢀdispersed sample on a lacey carbon formvar coated Cu grid. Xꢀray
photoelectron spectroscopy (XPS). XꢀRay photoelectron spectra were recorded on a Thermo
Scientific KꢀAlpha XꢀRay photoelectron spectrometer and binding energies (±0.1 eV) were
determined with respect to the position C 1s peak at 284.8 eV. Raman spectra was measured
at 298K by using a Laser Raman Spectrometer (JASCO, NRSꢀ3100) with the 532 nm line
from a diodeꢀpumped solidꢀstate laser for excitation. Measurements of extended Xꢀray
absorption fine structure (EXAFS) at AgꢀK edge were carried out in a transmission mode at
room temperature at the NW10A station of the Photon Factoryꢀ Advanced Ring for pulse Xꢀ
Rays at the Institute of Materials Structure Science, High Energy Accelerator Research
Organization in Japan (KEKꢀIMSSꢀPFꢀAR). The electron storage ring was operated at 6.5
GeV. Synchrotron radiation from the storage ring was monochromatized by a Si (311)
channel cut crystal at Ag Kꢀedge. Ionization chambers, which were used as detectors for

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incident Xꢀray (I
0
2
) and transmitted Xꢀray (I), were filled with 50% Ar/N mixture gas and 100
%
Ar gas, respectively. The EXAFS raw data were analyzed with UWXAFS analysis
24
25
package, including background subtraction program AUTOBK, curve fitting program
2
6
2
0
FEFFIT. The amplitude reducing factor S for Ag was fixed at 0.95. The backscattering
27
28
amplitude and phase shift were calculated theoretically by FEFF 8.4 code. ATOMS was
used to obtain the FEFF input code for crystalline materials.Chemical analyses of the metallic
constituents were carried out by Inductively Coupled Plasma Atomic Emission Spectrometer;
model: PS 3000 uv, (DRE), Leeman Labs, Inc, (USA). Fourier Transformation Infraꢀred
Spectroscopy (FTIR) spectra were recorded on a Thermo Nicolet 8700 (USA) instrument
ꢀ1
with the operating conditions: resolution: 4cm , scan: 36, operating temperature: 23ꢀ25°C
ꢀ1
and the frequency range 4000ꢀ400 cm .Thermogravimetric Analyses (TGA) of the
uncalcined catalyst were carried out in a Pyris Diamond, Perkin Elmer instruments, and
technology by SII (Seiko Instruments Inc), USA] instrumentꢀbalance by heating 2.15 mg
º
ꢀ1
samples at 5 C min in flowing nitrogen.
Catalytic Oxidation of Aniline
Liquid phase oxidation of aniline was carried out at room temperature (25 °C) in a double
neck round bottom flask at room temperature in an oil bath using 0.10 g catalyst, 10 ml
2 2
acetonitrile solvent and 1 g of aniline to which 2.15 g H O (50% aq. solution) was added
dropwise. Small aliquots of the sample were withdrawn from the reaction mixture at regular
intervals for analysis. At the end of the reaction, the solid particles (catalyst) were separated
by filtering during the hot condition and products were analysed by gas chromatograph (GC,
Agilent 7890) connected with a HPꢀ5 capillary column (30 m length, 0.28 mm id and 0.25
µm film thickness) and a flame ionization detector (FID). Conversion of aniline was

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calculated based upon the GCꢀFID results, where aniline conversion = [moles of aniline
reacted]/[initial moles of aniline used] x 100 and selectivity of products calculated by [total
moles of product formed]/[total moles of aniline converted] x 100. The individual yields were
calculated and normalized with respect to the GC response factors. The product identification
was carried out by injecting authentic standard samples in GC and GCMS. For few cases, we
1
did the column chromatography to get the isolated yield and HꢀNMR was taken to identify
the product. The Cꢀbalance as well as material balance was carried out for most of the
experiments and it was found between 98ꢀ102%. For the reusability test, after completion of
the reaction, the catalyst was recovered from the reaction mixture by filtration and washed
thoroughly with acetone and reused as such for multiple cycles.
RESULTS AND DISCUSSIONS
Catalyst characterization
3
The amount of silver present in the Ag/WO nanostructured catalyst was estimated by ICPꢀ
AES. The related Xꢀray diffraction (XRD) pattern of the Ag/WO nanostructured catalyst
3
showed the peaks at 2θ values of 23.1º, 23.7º, 24.4º, 33.3º and 34.0º, confirm the formation of
3
monoclinic WO (JCPDS No. 43ꢀ1035, space group: P21/n) (Fig.1). However, we could not
observe any diffraction peaks assignable to metallic silver Ag (0) or oxides of silver,
indicating the very small silver particles size (2ꢀ5 nm). Xꢀray photoelectron spectroscopy
(XPS) analyses clearly the presence of metallic silver in the fresh sample from the
corresponding Ag 3d5/2 and Ag 3d3/2 binding energy values of 368.2 eV and 374.1 eV
29
respectively (Fig. 2). The W 4f5/2 and 4f7/2 spectra attributed to the binding energies 37.9
eV and 35.8 eV respectively suggesting that the tungsten in the tungsten oxide sample exists
+6 30
as W . (Fig. 3) The corresponding Ag 3d binding energy of the spent catalyst ~368.3 eV,
confirms that the oxidation state of metallic silver does not change during catalysis. (Fig.2b).

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The representative SEM images of the catalyst are shown in Fig. 4. The images showed that
the Ag/WO nanostructured catalyst is comprised of very small particles. The SEM elemental
mapping exhibits a uniform dispersion of silver nanoparticles on WO support (Fig. S1). The
3
3
electron diffraction (ED) pattern showed only the presence of silver, oxygen and tungsten and
no impurities could be observed in the spectra (Fig.4c). Transmission electron microscopy
(TEM) measurements were carried out to check the particle size and distribution of the silver
nanoparticles (Fig 5). High resolution transmission electron microscopy (HRTEM) revealed
that the catalyst is composed of highly dispersed very small silver nanoparticles of ~2ꢀ5 nm
on WO support. The corresponding TEM histogram of Ag nanoparticles showed a very
3
narrow particle size distribution with sizes between 2ꢀ5 nm (Fig 4b, inset). The lattice fringe
3
distance of 0.38 nm indicates the [020] lattice spacing of WO and the distance of 0.20 nm
corresponds to [200] plane of Ag (Fig. 3d). SAED pattern (Fig. 5c) exhibited the inter planar
spacing distances of 0.23 nm, 0.20 nm and 0.14 nm, these are indexed to Ag (111), (200) and
(
220) respectively, which refer to the unique characteristic of the fcc structure of Ag. The
lattice fringes with a dꢀspacing of 0.38 nm, 0.37 nm and 0.26 nm were also observed in the
SAED pattern which corresponds to (002), (020) and (022) lattice planes of WO with
3
monoclinic structure. The TEM elemental mapping indicates the presence of Ag, W and O
species and further ascertains the uniformly dispersion of AgNPs on tungsten oxide support
(Fig 6). Furthermore, the TEM image of the spent catalyst showed that the topology and the
silver particle size of the catalyst were hardly changed after four reuses (Fig 5e, f). TEM
EDX pattern also showed the presence of Ag and W in the sample (Fig. S2). The surface area
2
2
3 3
of the fresh Ag/WO and spent Ag/WO were 50 m /g and 47 m /g respectively, which also
indicates that the available active sites remained constant during the catalysis. AgꢀK edge
extended Xꢀray absorption fine structure (EXAFS) analysis of the Ag/WO nanostructure
3
catalyst further revealed that no significant changes occurred in oxidation state of the silver

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3
species of the fresh and the spent Ag/WO nanostructure catalyst during the oxidative
coupling of aniline to azoxybenzene (Table 1, Fig. S3). Table 1 shows the structural
parameters of the catalyst by curveꢀfitting of the Ag Kꢀedge EXAFS data. It was observed
theta AgꢀAg contribution at 0.2872 nm with a coordination number (C.N.) of 6.3 and the
absence of AgꢀO contribution indicates the formation of metallic Ag in the fresh catalyst. For
the spent catalyst, AgꢀAg bond at 0.2873 nm with C.N. 7.7 was observed. The observation
indicated that the oxidation state of Ag (0) remained unchanged after the catalysis which was
supported by XPS also. The embedment of the cetyltrimethyl ammonium bromide molecules
over the precalcined catalyst was confirmed by the FTIR analysis (see Figure 7). The peaks of
ꢀ1
+
the sample at 811 and 1099 cm can be assigned to the CꢀN stretching modes of CTA
ꢀ1
molecules. The peaks at 1361 cm are assigned to the symmetric mode of vibration of the
+
31
head groups of the methylene moiety (N ꢀCH
3
). The frequencies above 1600 cm to 3000
ꢀ1
cm are due to CH symmetric and antisymmetric vibrations, respectively. The characteristic
ꢀ1
ꢀ1
peaks at 628, 890 cm represent the stretching vibration of WꢀOꢀW and the peak at 948 cm
3
2
associated with the stretching vibration of W=O. The embedment of CTAB molecules on
the preꢀcalcined catalyst surface was further confirmed from TGA analysis. TGA analysis
was operated to understand the various decomposition regimes as shown in Fig. 8. The TGA
diagram showed that the weight loss has two regimes, first being the ~ 3% mass loss of water
at 200 to 300 °C and finally the major mass loss of 12% around 450 °C is assigned to the
decomposition of CTA moiety. Further weight loss was not observed when the temperature
was further increased from 500°ꢀ700° C, indicating the stability of the catalyst upto 700° C.
Total mass reduction of ~15% confirmed the complete the removal of the surfactant CTAB.
UVꢀvis DRS of the Ag/WO
3
catalyst in Fig. 9 showed an absorption maxima at 401 nm.
+6
According to literature, the oxygen ligand to metal charge transfer (LMCT) bands for W ,
33
appeared ~300ꢀ400 nm. The UVꢀvis absorption spectra of silver nanoparticles occurs ~410

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34
nm. Therefore, the absorption maxima ~401 nm could be assigned to the presence of both
silver and WO nanoparticles.
3
Catalytic Activity
We have investigated the catalytic activity of Ag/WO nanostructured catalyst towards the
3
oxidative coupling of aniline to azoxybenzene at room temperature and at atmospheric
pressure using acetonitrile as solvent and H
the Ag/WO nanostructured catalyst afforded an azoxybenzene selectivity of 91% with an
aniline conversion of ~87% under catalyst/aniline weight ratio of 1:10 and aniline: H
molar ratio of 1:3. Nitroso benzene, nitrobenzene and azo benzene were detected as the side
products of this reaction. A series of catalytic experiment was carried out using Ag/WO
nanostructured catalyst in order to understand the variation of aniline to azoxybenzene
conversion as a function of time, temperature and aniline: H mole ratio. The catalyst
exhibits an aniline conversion of 87% at room temperature with azoxybenzene selectivity of
1%. We observed that with the increase of temperature although the conversion increases
2 2
O as an oxidizing agent. The results showed that
3
2
O
2
3
2
O
2
9
steadily but the selectivity drops down due to the formation of nitrobenzene as the major side
product (Fig. 10). The results also showed an increasing trend in both the aniline conversion
as well as azoxybenzene selectivity with increase of time (Fig. 11). After 6h of reaction, the
catalyst exhibited an aniline conversion of 38% with 63% azoxybenzene selectivity; nitroso
benzene was found to be the major side product. The results indicated that after 24 h of
reaction, maximum selectivity of azoxybenzene (~91%) was achieved, but after 30 h, the
selectivity of azoxybenzene drops down to ~78% due to formation of nitrobenzene. The mole
ratio of aniline: H
adjusted to 1:1, the conversion of aniline was very low (25%) with azoxybenzene selectivity
63% and nitrosobenzene selectivity ~35%. But when aniline : H O mole ratio was
2 2 2 2
O was used in three fold excess (Fig. 12). In fact when aniline: H O was
~
2
2

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increased to 1:3, aniline conversion increases to 87 % with 91% azoxybenzene selectivity,
further increase in the aniline : H mole ratio to 1:5, we noticed that although the aniline
2
O
2
conversion increased to 93% but the azoxybenzene selectivity decreased significantly to 70%,
owing to the formation of nitrobenzene as the major side product. Table 2 showed the activity
of the different Ag/WO catalysts in the selective oxidation of aniline to azoxybenzene in
3
2 2 3
liquid phase using H O as oxidant. The Ag/WO nanostructured catalyst showed 87%
aniline conversion and ~91% azoxybenzene selectivity with turn over number (TON) value
of ~ 367.8 in the absence of any external additives at room temperature (Table 2, entry 6).
Blank run in the absence of the catalyst gave almost no reactivity under the optimized
reaction condition (Table 2, entry 8). Ag/WO nanostructured catalyst prepared by surfactant
3
promoted method provided excellent yield of azoxybenzene (Table 2, entry 6). When
3
Ag/WO catalyst synthesized by the conventional impregnation method, it showed very poor
catalytic activity due to the irregular shape and larger particle sizes and excessive leaching of
the metals (Table 1, entry 5). So, particles in the nanodomain definitely have a role towards
the higher catalytic efficiency. Furthermore, Ag/WO nanostructured catalyst showed greater
3
TON of 367.8 for azoxybenzene production and greater H
compared to alone Ag or WO catalyst prepared through surfactant promoted method (Table
, entries 3, 4). Results showed that when Ag, prepared by surfactant assisted method, was
2 2
O efficiency of ~26.4%,
3
2
exclusively employed as catalyst, selective formation of azobenzene as the principal product
was observed under the identical reaction conditions, with a lower aniline conversion of 37%.
Notably WO
3
catalyst, prepared by surfactant promoted method, was also capable of
selectively catalyzing aniline to azoxybenzene with 30.7% yield. Although Ag and WO
3
catalysts exhibited different catalytic profiles, these results further indicated a clear
participation of both Ag and WO species in the Ag/WO nanostructured catalyst during the
3
3
oxidative coupling of aniline under the investigated conditions. We believe that a synergistic

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effect between the support WO
3
and Ag nanoparticles may exist at the interface, which
further confirms that WO
catalytic process.
3
does not simply act as a support, but also intervenes in the
To illustrate the general applicability of Ag/WO nanostructured catalyst, the scope of the
3
reaction was extended to oxidative coupling of a range of aromatic amines (Table 3). Under
the optimized conditions, we found that a variety of structurally different aromatic amines
were selectively transformed to their corresponding azoxy products with high yield.
The mechanism of the reaction was supposed to follow the path as shown in scheme S1. We
believe that the initial step is the decomposition of H O over AgNPs to form superoxide
2
2
•ꢀ
35ꢀ37
(O
2
), according to the reaction shown in eq (i) in the supporting information.
2 2
H O
dissociation is believed to occur over the Ag surface and probably follows the mechanism,
3
8
suggested by Kazarnovsky (Supporting Information). We can rationally assume that since
tungsten oxide is acidic in nature; the basic aniline can be easily adsorbed onto the surface of
3
acidic WO through electrostatic interactions. During the oxidation process, dissociation of
•ꢀ
H
2 2 2
O over Ag generates superoxide (O ), which ultimately produces peroxide species
•
OOH, which is supposed to be the active species for this reaction (Supporting Information,
•
eq (ii). This OOH then attacks the NꢀH bond of aniline, which is partially electropositive in
nature after donating its electron pair to tungsten. We found phenyl hydroxyl amine and
nitrosobenzene to be the intermediates for this reaction, which accords well with the reported
8
literatures for the oxidative coupling of aniline with H
2
O
2
. The oxidation reaction occurs
step by step, where initially aniline oxidizes to phenyl hydroxyl amine, which further
oxidizes to azobenzene. Finally azobenzene oxidizes to form azoxybenzene. To obtain insight
into the reaction pathway, the reaction intermediates were separately subjected to oxidative
coupling reaction using 2.5 wt% Ag/WO nanostructured catalyst. When phenyl hydroxyl
3
amine was subjected to oxidation under the optimized condition, it produced azoxybenzene

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within 16 h (Table 2, entry 9) and when azobenzene was subjected to oxidation, it produced
azoxybenzene within 14 h (Table 2, entry 10). These experiments indicate that phenyl
hydroxyl amine and azobenzene are the intermediates for this reaction for the formation of
azoxybenzene. Interestingly when we added pꢀbenzoquinone at optimized condition initially,
we observed aniline conversion of 80% with 97 % azoxy benzene selectivity within just 16 h
of reaction. Thereafter, in a separate reaction, equimolar of 2,6ꢀdiꢀtertꢀbutylꢀ4ꢀ(4‘ꢀ
methoxyphenyl)phenoxyl (DBMP) was added as a radical scavenger in the reaction medium
to understand the possible formation of radical species. The reaction was almost suppressed
under this condition and gave aniline conversion of ~17% after 24 h with major formation of
azoxybenzene. All these results further suggest that addition of pꢀbenzoquinone accelerates
the reaction rate, whereas, DBMP retards it. This result further demonstrates the involvement
of radical species in our system.
Reusability Test
We tested the reusability of the Ag/WO nanostructured catalyst without further regeneration.
3
The oxidative coupling of aniline was carried out with the reused catalyst under the same
reaction condition. Activity of the recovered catalyst after four consecutive runs did not lead
to any significant decline in its catalytic activity in terms of conversion and selectivity
(Figure S4). After completion of the reaction, the solid catalyst was removed from the
reaction mixture by filtration during hot condition and the reaction was allowed to proceed
with the filtrate under the same conditions. The reaction was completely stopped after the
removal of the catalyst. Another important finding is that the oxidation pathway does not
involve homogeneous catalysis by dissolving tungsten or silver species. TEM image does not
show any change compared to the initial morphology and particle size distribution (Fig. 5f).
We also observed that the amount of Ag and W present in the spent catalyst after 4 reuses is

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almost same with that of the fresh catalyst as estimated by ICPꢀAES, which further confirms
the true heterogeneity of the catalyst. XPS also confirmed that almost no change was
occurred in the metallic state of the silver before and after four reuses.
Conclusions
In summary, we have demonstrated a simple preparation method for the one pot synthesis of
2
ꢀ5 nm Ag nanoparticles supported on nanocrystalline WO
by XRD, XPS, BETꢀ surface area, ICPꢀAES, FTꢀIR, TGA, EXAFS, SEM and TEM. In the
light of the findings, we can say that the Ag/WO nanostructured catalyst serves as a highly
selective heterogeneous catalyst for the one step selective oxidation of saturated primary NꢀH
bonds of aniline to azoxybenzene with H as oxidant at room temperature. The oxidative
3
. The catalyst was characterized
3
2
O
2
coupling proceeds in a highly selective manner without the use of any external additives
yielding an aniline conversion of 87% with 91% azoxybenzene selectivity. The significantly
enhanced activity of our catalyst stems from the combination of the factors like: a) nanosize
effect, the uniformly dispersed very small silver particles (~2 nm) generates more active sites
for the reaction and b) the synergistic interaction between the ~2ꢀ5 nm AgNPs and
3
nanocrystalline WO .
Acknowledgements
S.G. thanks UGC, S.S.A. thanks CSIR, India for the fellowship. R.B thanks CSIR, New Delhi
for the financial support in the form of 12 FYP Project (CSCꢀ0125, CSC 0117). Director,
CSIRꢀIIP, is acknowledged for his support and encouragement. The authors thank Analytical

Page 15 of 27
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DOI: 10.1039/C4GC02123A
Section Division, CSIRꢀIIP. The XAFS measurements were performed at KEKꢀIMSSꢀPF
with the approval of the Photon Factory Advisory Committee (project 2013G210).
Notes and References:
a
Catalytic Conversion & Processes Division, CSIR-Indian Institute of Petroleum, Dehradun
248005, India
b
Department of Complexity Science and Engineering, Graduate School of Frontier Sciences,
The University of Tokyo, Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan
Corresponding author. Tel.: +91 135 2525797; Fax: +91 135 2660202
Eꢀmail addresses: raja@iip.res.in
†
Electronic supplementary information (ESI) available.
1
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e
d
c
)
(
b
a
2
0
40
Tw o Th et a/ de g
60
80
Fig. 1 XRD patterns of a) W (VI) Oxide, b) Ag (I) oxide, c), Ag (0), d) fresh Ag/WO
spent Ag/WO
3
and e)
3
.

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3
68.3
3
74.4
b)
3
68.2
3
74.1
a)
3
65
370
Binding energy (eV)
375
3
Figure 2. Ag 3d core level spectra a) fresh Ag/WO catalyst and b) spent catalyst.
30000
25000
20000
15000
10000
35.8 eV
37.9 eV
5000
0
35
40
Binding energy (eV)
45
Figure 3. W4f 7/2, W4f 5/2 core level spectra of the fresh Ag/ WO catalyst.
3

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a
c
b
1µm
500 nm
Figure 4. (a), (b) SEM and (c) SEMꢀEDX of Ag/WO
3
nanostructured catalyst
a
c
b
50
40
30
20
10
0
2
3 4 5 6 7
Size (nm)
200 nm
1 nm
e
d
f
d
0
(
.2 nm
50
200) Ag
4
0
0
.38 nm
30
2
10
0
0
(
020) WO3
2
3 4 5 6 7
Size (nm)
2
nm
200 nm
3
Figure 5. (a), (b) TEM image of fresh Ag/WO nanostructure catalyst (inset Ag particle size
distribution), (c) selected area electron diffraction image (SAED, d) lattice fringes and e), f)
TEM image of used catalyst (inset Ag particle size distribution).

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a
b
d
c
Figure 6. TEM elemental mapping of Ag/WO
3
catalyst a) TEM image, b) position of silver,
c) position of oxygen and d) position of tungsten.

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b)
-1
1099 cm
-1
1633 cm
-1
948 cm
-1
a)
-1
1361 cm
8
90 cm
-1
8
11 cm
-1
-1
6
28 cm
2935 cm
500 1000 1500 2000 2500 3000 3500
-1
Wavenumber (cm )
Figure 7. Infraꢀred spectra of a) uncalcined Ag/WO
3
catalyst and b) calcined catalyst.
0
.2
.0
100
0
-0.2
-0.4
-0.6
-0.8
-1.0
95
90
85
80
0
4
50 C
100
200
300
400 0500
600
700
Temperature ( C)
Figure. 8. TG and DTG analyses of uncalcined Ag/ WO
3
catalyst.

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0
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
.1
4
01 nm
3
00
400
500
600
700
Wavenumber (nm)
Figure. 9. UVꢀvis DRS spectrum Ag/ WO
3
nanostructured catalyst.
100
80
60
40
20
0
30
40
50
600
70
80
Temperature/ C
Figure. 10 Effect of temperature on aniline oxidation.
[
[
■ ] Conversion of aniline; [■] Selectivity to nitrobenzene; [■] Selectivity to nitrosoaniline,
■] selectivity to azobenzene, [■] Selectivity to azoxybenzene. Reaction Condition: solvent=
2 2
acetonitrile; aniline =1g; weight of catalyst = 0.10 g; aniline: H O mole ratio =1:3; time = 24
h.

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90
80
70
60
50
40
30
20
10
0
5
10
15
20
Time/h
25
30
Figure. 11 Effect of time on aniline oxidation.
[
[
■ ] Conversion of aniline; [■] Selectivity to nitrobenzene; [■] Selectivity to nitrosoaniline,
■] selectivity to azobenzene, [■] Selectivity to azoxybenzene. Reaction Condition: solvent=
acetonitrile; aniline =1g; weight of catalyst = 0.10 g; substrate: H O mole ratio =1:3; time =
2
2
24 h.
100
90
80
70
60
50
40
30
20
10
0
1:1
1:3
1:5
Aniline : H O mole ratio
2
2
2 2
Figure. 12 Effect of aniline:H O mole ratio on aniline oxidation.
[
[
■ ] Conversion of aniline; [■] Selectivity to nitrobenzene; [■] Selectivity to nitrosoaniline,
■] selectivity to azobenzene, [■] Selectivity to azoxybenzene. Reaction Condition: solvent=
2 2
acetonitrile; aniline =1g; weight of catalyst = 0.10 g; : H O mole ratio =1:3; time = 24 h.

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3
Table 1. EXAFS curveꢀfitting parameters at Ag–Kꢀedge Fourierꢀfiltered k ꢀweighted EXAFS
functions
-
1
Catalyst
Path
R (10 nm)
CN
DW
∆k
∆R
∆E (eV)
f
R (%)
0
-
5
2
-1
-1
(
10 nm )
(10 nm )
(10 nm)
Fresh
Spent
AgꢀAg 2.872±0.032 6.3±0.5
AgꢀAg 2.873±0.031 7.7±0.6
8.8±0.4
9.4±0.4
3 – 14
1.5–3.0
ꢀ1.1±0.7
1.8±0.6
0.79
1.04
3 – 14
1.0– 3.0
EXAFS = extended Xꢀray absorption fine structure; CN= coꢀordination number; R = bond
length; DW: DebyeꢀWaller factor; △K: the range of wavenumbers used in the fitting; △r: the
2
2
range of bond distances used in the fitting; So =0.95, S : amplitude reducing factor; △E0:
0
shift of the edgeꢀposition; R : Reliability factor.
f

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a
Table 2. Oxidative coupling of aniline to azoxybenzene
N
O-
N+
Ag/WO catalyst
3
NH
2
RT, H O2
2
b
Entry
Catalyst
C
(
T
Selectivity
S (%)
P
Yield Turn over
2 2
H O
efficiency
(%)
c
d
%)
Y
A
Number
(TON)
(%)
Nitro
Nitroso
Azo
Azoxy
benzene benzene benzene benzene
com
1
2
3
4
5
6
Ag
3.2
4.8
27
21
15
87
11
9
40.5
53
27
34
ꢀ
48.5
3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
com
WO
3
35
ꢀ
us
Ag
14
9
59
ꢀ
ꢀ
us
WO
3
57
54
91
12
8.1
ꢀ
4
imp
Ag/WO
3
39
3
5
37.6
367.8
2.7
26.4
Ag/WO
nano
catalyst
Ag/WO
nano
3
e
3
f
2
4
79.1
7
86
4
2
4
90
77.4
359.5
25.8
catalyst
8
9
No
Catalyst
0.3
99
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Ag/WO
nano
catalyst
3
5
95
94.0
436.9
31.3
g
1
0
Ag/WO
nano
catalyst
3
99
ꢀ
ꢀ
ꢀ
99
98.0
455.3
32.7
h
a
Reaction conditions: solvent (acetonitrile) = 10ml, substrate (aniline) = 1g, weight of the
catalyst = 0.10g, silver loading= 2.5 wt%, reaction temperature = Room Temperature; time =
b
2
4h; aniline: H O mole ratio = 1: 3; C : conversion of aniline based upon the FIDꢀGC
2 2 T
c
results = [moles of aniline reacted/initial moles of aniline used] x 100; S : selectivity of
product calculated by total moles of product formed/total moles of aniliene converted; Y
P
d
A
e
(Yield of azoxybenzene) = conversion × selectivity/100; TON calculation ~Ag nanoparticles
f
g
supported on WO
amine used as reactant, time= 16 h; Ag/WO
3
; catalyst after 4 reuse; Ag/WO
3
nano catalyst = when phenyl hydroxyl
h
3
nano catalyst = when azobenzene used as
reacant, time = 14 h; com = Commercial; us = Bare Ag and WO
imp = impregnation method.
3
prepared by our method;

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Table 3. Activities of Ag/WO
3
nanostructure catalyst for the formation of azoxybenzenes via
selective oxidation of different substituted anilines.
Entries
Substrate
C
T
Product
Time Selecti
vity
h
Yield
(%)
(
%)
N
O-
1
65
24
24
88
95
57.2
N+
NH
2
NH
2
O
2
78
Et
Et
88.35
N
N
N
O
N
3
4
87
81
Cl
Cl
24
24
92
89
84.6
72.0
Cl
NH
2
Cl
H
2
N
Cl
Cl
N+
-O
N
-O
O
N
5
6
82
24
24
92
87
71.7
74.8
N+
NH
2
O
2
N
N
NO
2
O
O-
86
O
NH
2
N
N+
O
O
a
Reaction conditions: Solvent (acetonitrile) = 10ml, substrate = 1g, weight of the Ag/WO
mole ratio = 1: 3.
3
2 2
catalyst = 0.10g, silver loading= 2.5 wt%, time = 24 h; Aniline: H O