Surfactant directed Ag1?xNix alloy nanoparticle catalysed synthesis of aromatic azo derivatives from aromatic amines

Article abstract of DOI:10.1016/j.apcata.2016.06.039

A bimetallic and multiply twinned Ag_1?xNi_x (x?=?0.2, 0.4, 0.6, 0.8) alloy nanoparticle (MTANP), for the first time has been explored as an efficient heterogeneous catalyst in the industrially important reaction conversion of aromatic amines to the symmetric or asymmetric azobenzenes by the activation of aerobic molecular oxygen. Here, we demonstrated that the as-synthesized and optimized Ag_0.6Ni_0.4 alloy NPs showed breakthrough in catalytic performance, enabling quantitative aromatic amine conversion and absolute selectivity in azobenzenes (100%) under aerobic green reaction conditions without using the environmentally unfriendly nitrites and any other hazardous materials. A high stability and recyclability of the catalyst are observed and investigated by several instrumental techniques. The key to the effective process is found to be size effect, electronic effect and structural defect, and the presence of surfactant at the surfaces of the AgNi MTANP catalyst.

Full text of DOI:10.1016/j.apcata.2016.06.039

Accepted Manuscript
Title: Surfactant directed Ag1 Ni alloy nanoparticle
−x
x
catalysed synthesis of aromatic azo derivatives from aromatic
amines
Author: Mukesh Kumar Kiran Soni Geeta Devi Yadav
Surendra Singh Sasanka Deka
PII:
S0926-860X(16)30353-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.06.039
APCATA 15935
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
21-3-2016
15-6-2016
30-6-2016
Please cite this article as: Mukesh Kumar, Kiran Soni, Geeta Devi Yadav, Surendra
Singh, Sasanka Deka, Surfactant directed Ag1−xNix alloy nanoparticle catalysed
synthesis of aromatic azo derivatives from aromatic amines, Applied Catalysis A,
General http://dx.doi.org/10.1016/j.apcata.2016.06.039
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Surfactant directed Ag1-xNi
x
alloy nanoparticle catalysed synthesis of
aromatic azo derivatives from aromatic amines
Mukesh Kumar, Kiran Soni, Geeta Devi Yadav, Surendra Singh and Sasanka Deka*
Department of Chemistry, University of Delhi, North campus, Delhi-110007, India
*
Corresponding Author Address: sdeka@chemistry.du.ac.in, ssdeka@gmail.com
1

Graphical Abstract
Highlights


Multiply twinned AgNi bimetallic alloy nanoparticles heterogeneous catalyst
Complete conversion of aromatic amines to the symmetric or asymmetric
azobenzenes


Breakthrough in catalytic performance, absolute selectivity (100%)
A high stability and recyclability of the catalyst without compromising morphology
Abstract
A bimetallic and multiply twinned Ag1-xNi (x = 0.2, 0.4, 0.6, 0.8) alloy nanoparticle
x
(MTANP), for the first time has been explored as an efficient heterogeneous catalyst in the
industrially important reaction conversion of aromatic amines to the symmetric or
asymmetric azobenzenes by the activation of aerobic molecular oxygen. Here, we
2

demonstrated that the as-synthesized and optimized Ag0.6Ni0.4 alloy NPs showed
breakthrough in catalytic performance, enabling quantitative aromatic amine conversion and
absolute selectivity in azobenzenes (100%) under aerobic green reaction conditions without
using the environmentally unfriendly nitrites and any other hazardous materials. A high
stability and recyclability of the catalyst is observed and investigated by several instrumental
techniques. The key to the effective process is found to be size effect, electronic effect and
structural defect, and the presence of surfactant at the surfaces of the AgNi MTANP catalyst.
Keywords: AgNi alloy nanoparticles; oxygen activation; aniline; azobenzene
1
. Introduction
Aromatic azo compounds and their derivatives are important class of commercial and
high-value chemicals used as organic dyes, indicators, food preservatives, additives radical
reaction initiators, polymers and therapeutic agents etc.[1-4]. Industrially azo compounds are
produced by different methods like diazotization [5],[6] and coupling (coupling of primary
arylamines with nitroso compounds) [7], condensation of nitro compounds with amines [8],
reduction of nitro compounds [9], oxidation of amino compounds (aromatic primary amines)
[
10], oxidation of hydrazo derivatives [11] and reduction of azoxybenzene [12] derivatives
3

[
6],[8]. Notably, unsymmetrical azobenzenes were synthesized in two steps by proceeding
through aniline or nitrobenzene and stoichiometric amounts of nitrite salts or other oxidants
via diazonium salt or nitrosobenzene intermediates, with inorganic salts as waste [13],[14].
Even though numerous efforts have been made towards the synthesis of azo derivatives but
there is currently a lack of a generally applicable catalyst that is capable of oxidative coupling
of aniline under green conditions for getting high yields of product (symmetric as well as
unsymmetrical azobenzenes) and secondly, to prevent the use of stoichiometric and
environmentally unfriendly oxidants, such as lead salts, mercury salts etc [15].
In this context, the use of molecular oxygen or the aerial oxygen as primary oxidant in
the catalytic reactions will have great significance as the green chemistry point of view [16].
Oxidation of organic compounds by economical, readily available and environmental friendly
oxidants such as hydrogen peroxide, molecular oxygen etc. are attractive from the perspective
of industrial technology and synthetic purposes [17]. Because of the limited reports on the
efficient catalyst for the activation of the molecular oxygen, there is need to explore more in
this field [18]. In this process, nanoparticles (NPs) of various materials have been introduced
as new catalysts in many organic transformation reactions, such as in oxidation [19],[20], C-
C coupling [21],[22],[23], C-S coupling [24] etc. For the consideration of economical and
green reaction protocols, first time Grirrane et al. [1] designed a route for the synthesis of
o
aromatic azo compounds catalysed by gold NPs using O
2
(3-5 bar) as the oxidant at 100 C.
Yadav et. al.[8] have reported the silver substituted octahedral molecular sieve of
cryptomelane type (Ag-OMS-2 15 wt/wt%) catalyst for the conversion of nitroareane to
azobenzene with 80% selectivity. Bal group [25] have reported the use of Ag NP supported
on nanostructured tungsten oxide in the oxidative coupling of aniline to azoxybenzene using
H
2
2
O as an oxidant and found 87%91% conversion and selectivity. Furthermore, single-
crystalline octahedral Au−Ag nanoframes [26] and Ag/C support [27] has been used in the
formation of azobenzene in the presence of direct supply of oxygen. However, in most
reports, either high temperature or high pressure was used or the yields of the reactions were
very poor.
Bimetallic alloy nanoparticles are important class of materials because of their
outstanding and unique catalytic, electronic, optical, and magnetic properties arise from
synergistic effect and are used in organic synthesis, fuel cell, reduction reactions, magnetic
recording, antibacterial activity, etc. [28],[29],[30],[31]. However, to our knowledge, we do
not see the use of low cost bimetallic alloy NPs in catalysing the subjected organic
4

transformation reactions in literature till date. In this context to fill the gap we wish to report
the efficient catalysis of multiply twinned AgNi alloy nanoparticles in oxidative coupling of
aromatic amines under green condition. AgNi alloy NPs are one of the new additions to the
class of alloy particles that have recently drawn attention for their unique kinds of synthesis
(
complete immiscibility of metals, lower surface energy of Ag and 14% lattice mismatch) and
multifunctional properties [32]. Among the as-synthesized Ag1-xNi NPs (where x = 0, 0.2,
.4, 0.6, 0.8, 1.0), Ag0.6Ni0.4 alloy NP showed the best performance under mild conditions
x
0
and we demonstrated that our catalyst is highly selective, efficient, easily separable and
recyclable under the normal aerobic condition to give the required symmetric or asymmetric
azo compounds in very high yield. In addition, Ag0.6Ni0.4 MTANPs also catalyse oxidative
coupling of benzylamine with very high selectivity (99.9%) and activity (96.0%) towards N-
benzylidene-1-phenylmethanamine formation showing wide applicability of present alloy
nanocatalyst.
2
2
. Materials and Methods
.1. Materials
All the reagents were of analytical grade and were used without further purification.
Hexadecylamine (HDA, 98%), 1-octadecene (ODE, 90%) and oleylamine (OLA, 90%) were
from Sigma-Aldrich, USA. Nickel nitrate hexahydrate (Ni(NO
hydroxide (KOH) were from Merck, India and silver nitrate (AgNO
analytical reagent, India. Aniline and anisidine were obtained from SRL, India.
3
)
2
.6H
2
O) and potassium
3
, 99.9%) from Rankem
2
.2. Synthesis of Ag1-xNi Alloy and Ag Nanoparticles
x
The one pot wet chemical co-reduction method was used to synthesize alloy nanoparticle
) as reported in our earlier study [32]. In a typical synthesis, AgNO (0.8-0.2 mmol),
.6H O (0.2-0.8 mmol) and surfactant HDA (1.5 mmol) were added to 8 mL of ODE in
(Ag1-xNi
x
3
NiNO
3
2
a three necked round-bottom (RB) flask equipped with temperature controller, magnetic
stirring bar and connected to a condenser. The reaction mixture was stirred vigorously and
degassed at 110 °C for 30 min followed by increase of reaction temperature to 250 °C. At this
temperature the reaction mixture was kept for 40 min under a continuous flow of N . The
2
reaction mixture was cooled to room temperature naturally after the completion of the
reaction and the product was purified by precipitation with ethanol. The as obtained
precipitate was centrifuged and washed with ethanol for three times and finally dispersed in
toluene for further characterizations or dried for catalytic studies. Ag NPs were synthesized
5

following the similar method as described above, however 1 mmol AgNO was used without
3
the addition of any Ni metal precursor while other parameters (surfactant, solvent, time,
temperature, etc) were kept constant.
2
.3. Synthesis of Ni Nanoparticles
Nickel NPs were separately synthesized following a similar approach as followed in our
earlier report [32]. In a typical synthesis, Ni(OCOCH
3
)
2
.4H
2
O (2 mmol) was added to OLA
°
(
16 mmol, 5.26 mL) in a three necked RB flask and degassed at 110 C for 30 minutes. At
this temperature, 1.6 mmol of TOP was injected to the reaction mixture under continuous N
2
°
flow and reaction mixture was brought to 220 C and stirred at this temperature for next 2h.
After completion of the reaction, product was precipitated and washed with ethanol and
finally dispersed in toluene for further characterizations or dried for catalytic studies.
2
.4. Characterization
The morphology and composition of the as-prepared products were characterized by
transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) imaging
2
obtained from a Philips Technai G 30 transmission electron microscope operating at an
accelerating voltage of 200 kV equipped with an energy dispersive X-ray spectroscopy
(EDAX) attachment and elemental mapping feature. The phase and the crystallographic
structure of the products were identified by X-ray diffraction (XRD, Bruker D8 Advance, Cu
Kα, λ = 0.154 056 nm). Elemental compositions in the as-synthesized alloy samples were
1
studied using an atomic absorption spectrometer (AAS, Zeenit 700p Analytik Jena). H and
13C NMR spectra were recorded in DMSO on a Bruker Avance 400 MHz spectrometer. FTIR
and UV-visible spectra were recorded in Perkin Elmer (Spectrum-RXI-59333, Bruker-Tensor
2
7) and Perkin Elmer Lambda-35 UV-Vis spectrophotometer, respectively. GC analysis was
performed using a Shimadzu GC-2010 Plus instrument using N as a carrier gas on FID-
2
detector using capillary column, Rtx-5 column (30 m x 0.25 mm x 0.25 mm, Restek, USA).
GC conditions were as follows: initial oven temperature (80 °C for 3 min); ramp (15 °C/min);
final temperature (180 °C); final time (10 min); injector temperature (230 °C); detector
temperature (250 °C); injection volume (1 μL). An automatic micropore physisorption
analyzer (Micromeritics ASAP 2020, U.S.A.) was used to determine surface areas of samples
following the Brunauer−Emmett−Teller (BET) principle, and the pore parameters of the
samples were determined with the Barrett−Joyner−Halenda (BJH) method, derived from N
2
6

adsorption−desorption measurements carried out at 77 K. Prior to analysis, samples were
degassed in situ at 200 °C for 8 h.
2
.5. Catalytic Experiments
All catalytic reactions were carried out in a RB flask. In a typical reaction for the synthesis of
azobenzene, a mixture of 1 mmol aniline, 1 mmol KOH, 2.6 mg Ag1-xNi nanocatalyst and 2
x
°
mL of DMSO solvent were treated at 60 C in open air. All the reactions were performed in
aerial condition. The reaction mixture was vigorously stirred at the required temperature for
the required time. The progression of the oxidation reactions were monitored by TLC in 5%
ethyl acetate and hexane mixture and finally by GC analysis. For optimization, various
catalysts (Ag, Ni and Ag Ni1-x) were applied under similar reaction condition as followed for
x
azobenzene synthesis. The solvent variation study and catalyst loading and effect of reaction
temperature study was done using Ag0.6Ni0.4 alloy NPS, since this composition showed the
best catalytic activity. The substrate scope study for oxidative coupling reaction was done
using Ag0.6Ni0.4 alloy NP under similar reaction conditions as followed for azobenzene
synthesis and percentage conversion was again calculated by GC. For a symmetric and
asymmetric coupling final product (azobenzene and 1-(4-methoxyphenyl)-2-phenyldiazene )
1
13
was completely studied using various techniques like H NMR, C NMR, IR and UV-Vis
spectroscopy as shown below.
Oxidative coupling of aromatic amines to symmetric azobenzene: Ag Ni1-x, Ag or Ni
x
NPs (2.6 mg) was added to a reaction mixture of aniline or derivatives of aniline (1 mmol,
methoxyaniline, chloroaniline, thioaniline, nitroaniline), KOH (1 mmol) in DMSO (2 mL)
o
and was vigorously stirred at 60 C for 24 h in air. After cooling down to room temperature,
the mixture was extracted with diethyl ether and purified by column chromatography to
1
afford 84 mg (94.40%) of (E)-1,2-Diphenyldiazene. Orange solid; H NMR (400 MHz,
1
3
CDCl
CDCl
3
3
): δ (ppm) = 7.93 - 7.91 (d, J = 7.6 Hz, 4H), 7.54 - 7.47 (m, 6H); C NMR (101 MHz,
): δ (ppm) = 152.7, 131.08, 129.18, 122.93; IR (KBr): ν = 3062, 1613, 1483, 1453,
-1
1
299, 1221, 1250, 1071, 1020, 776, 689 cm ; UV-Vis (in ethyl acetate): λmax = 317.5 nm.
Synthesis of N-benzylidine-1-phenylmethanamine: Ag0.6Ni0.4 alloy NP (2.6 mg) was added
to a reaction mixture of benzylamine (1 mmol), KOH (1 mmol) in Toluene (2 mL) and was
o
vigorously stirred at 60 C for 24 h in air. After cooling down to room temperature, the
mixture was extracted with diethyl ether and purified by column chromatography to obtain
N-benzylidine-1-phenylmethanamine. In this case % conversion was calculated from NMR.
7

1H NMR (400 MHz, CDCl
) δ (ppm) = 8.34 (s, 1H), 7.71-7.70 (m, 2H), 7.40-7.25 (m, 8H),
3
1
3
4
1
.76 (s, 2H). C NMR (100 MHz, CDCl
27.9, 127.6, 126.7, 126.4, 64.5.
3
) δ (ppm) = 161.6, 138.9, 135.6, 130.4, 128.1,
Oxidative coupling of aromatic amines to asymmetric azobenzene. Ag0.6Ni0.4 alloy NP
(2.6 mg) was added to a reaction mixture of aniline (0.5 mmol), KOH (1 mmol), 4-
o
methoxybenzenamine (0.5 mmol) in DMSO (2 mL) and was vigorously stirred at 60 C for
2
4 h in air. After cooling down to room temperature, the mixture was extracted with diethyl
ether and later on the mixture was purified by column chromatography on silica gel to afford
1
6
4
7
1
1
3.1 mg (60 %) of 1-(4-methoxyphenyl)-2-phenyldiazene; yellow solid; H NMR (CDCl
3
,
00 MHz): δ (ppm) = 7.95 - 7.93 (m, 2 H), 7.90 - 7.88 (m, 2 H), 7.52 - 7.44 (m, 3 H), 7.03 -
1
3
.01 (m, 2 H), 3.88 (s, 3 H); C NMR (CDCl
3
, 101 MHz): δ (ppm) = 162.15, 152.85, 147.1,
30.48, 129.15, 124.87, 122.67, 114.31, 55.69; IR (KBr): ν = 2962, 2923, 1601, 1501, 1252,
-1
140, 1029, 839, 799, 769, 688 cm ; UV-Vis (in Ethyl acetate): λmax = 345.0 nm.
3
. Results and Discussion
The synthesis protocol used for the synthesis of Ag1-xNi MTANPs was followed from
x
our earlier report [32]. Since Ag0.6Ni0.4 composition showed the best catalytic activity as
corroborated in the experimental section in the present study, characterizations of Ag0.6Ni0.4
alloy NPs are discussed in this section as a representative case. The size, morphology and
crystal structure of MTANPs was investigated by TEM analysis. Fig. 1. displays the low
magnification TEM, high resolution TEM and SAED pattern for as synthesized Ag0.6Ni0.4
alloy NPs. The low resolution TEM images of alloy NPs shows the multiple twinned nature
that appears spherical at low resolution (Fig. 1a-b). The average particle diameter was found
to be 20±1 nm which is consistent with our previous report. Furthermore, an image of a
single isolated particle with higher magnification is shown in inset of panel b that clearly
represent twinned structure in which primarily five crystalline domains oriented in different
directions separated with twinned boundary are seen clearly. Therefore the particles are
termed as multiply twinned alloy nanoparticles (MTANPs). The interplaner distance
calculated from lattice fringes was 0.24 nm which is consistent with (111) plane of AgNi
alloy NPs (Fig. 1c). SAED pattern confirms crystalline nature of alloy NPs where the (111),
(200), (220) and (311) lattice planes can be ascribed to fcc structure of Ag0.6Ni0.4 MTANPs
(Fig. 1d).
8

The elemental composition of Ag and Ni was investigated using EDAX spectroscopy.
The representative EDAX analysis of various Ag Ni1-x (X = 0.2-0.8) alloy samples is shown
x
in Fig. 2a-d and obtained stoichiometric Ag/Ni ratios are entered in Table 1. EDAX results
clearly represent expected Ag/Ni elemental ratios which indicates controlled synthesis of
alloy NPs. To check the distribution of Ag and Ni elements in alloy NPs, elemental mapping
in the STEM mode was carried out on Ag0.6Ni0.4 MTANPs sample which display
homogeneous distributions of Ag and Ni in alloy NPs. (Fig. 1e−g). Further, we have carried
out AAS measurements to determine the bulk elemental ratio of Ag and Ni in alloy samples
and obtained results are entered in Table 1. In all alloy samples, compositional ratios of
Ag:Ni were found to be consistent with the corresponding stoichiometric elemental ratio and
the ratios obtained from EDAX measurements.
In the present synthesis of MATNPs, ODE acts as solvent and HDA acts both as
reducing agent and surfactant. AgNi alloy NP catalysts were conveniently prepared by
the colloidal reduction method. The as-synthesized particles are stable in organic
solvents and there is no secondary impurity phase present, as corroborated by powder
XRD patterns as shown in Fig. 3a. These results are consistent with our previous report
on the synthesis and characterization of the as-synthesized material. Powder XRD
pattern for Ag
200), (220), (311), and (222) planes of Ag fcc nanostructure but with slight deviation
from the actual 2θ values. With increase in Ni content, the XRD peak corresponding to
200) plane of Ag is shifted toward higher 2θ values with decrease in intensity. The XRD
x
Ni1-x MTANPs shows five characteristic peaks corresponding to (111),
(
(
peak corresponding to (200) plane AgNi alloy NPs can be clearly seen in between pure
Ag and Ni confirming solid solution of Ag and Ni according to Vegard’s law. Similar
shifting towards higher 2θ values and decrease in the intensity of the XRD peaks with
increasing amounts of Ni content was observed for the Ag (111), (220) and (311) peaks
also. However, an XRD peak centered at 52.15°, corresponding to the (200) plane of fcc
Ni was not observed in MTANPs, ruling out any heterogeneous growth of Ni NPs. This
observation is consistent with earlier reports of AgNi alloy particles synthesized using
o
different methods Hence, the absence of characteristic peak of Ni (at 52.13 ) and
resemblance with Ag fcc structure of MTANPs confirmed the formation of solid
solution in which Ni atoms are incorporated in Ag lattice.
To further justify the high surface area of our Ag0.6Ni0.4 MTANPs and application in
catalysis, BET adsorption-desorption isotherm was performed at 77 K on the as-synthesized
sample. By using the multipoint BET equation, the specific surface area of AgNi sample was
2
-1
found to be 76.0 m g . This high surface area is mainly attributed to the small sized (20±1
nm) alloy NPs. Further, the pore size distribution was calculated using the Barrett–Joyner–
Halenda (BJH) method, and the maximum pore volume distribution was observed for a 1-2
9

nm pore radius (inset of Fig. 3b). This observation of a high number of pores could be
indicative of the presence of essential dense defects on the twin boundaries.
Catalytic Study
To test the composition controlled catalytic activity, Ag, Ni and Ag1-xNi MTANPs
x
were applied in coupling of aniline to azobenzene. The progress of reaction was monitored by
GC analysis and obtained results were entered in Table 2 (also see Table S1 in SI). For GC
analysis, final reaction solution was centrifuged at 3000 rpm to separate out the catalyst, and
aliquots were then transferred to an eppendrof and 10 μL of the aliquot was diluted with the
1
.5 mL of ethylacetate. The diluted solution was then run through the GC column. The
oxidation products were identified by their retention times in comparison with standard
samples. Each peak of the GC chromatogram was integrated and the actual concentration of
each component was obtained from the pre-calibrated plot of peak area against concentration.
The yields of azo compounds were calculated with reference to products obtained.
Quantitative data from the obtained results viz., substrate conversion, product selectivity and
product yield were calculated from the GC data as follows:
m o les o f co n verted su b stra te  1 0 0
S u b stra te C o n versio n

%


(1)
(2)
m o les o f rea cta n t in feed
m o les o f p ro d u ct fo rm ed  1 0 0
P ro d u ct selectivity

%


m o les o f rea cta n t co n verted
S u b stra te co n versio n

%

 P ro d u ct selectivity

%

P ro d u ct yield

%


(3)
1
0 0
From obtained results it was found Ag metal have higher activity over Ni for
oxidative coupling reaction which can be due to high electron density of Ag metal which help
in faster oxygen activation over metallic surface. From initial study, it has been investigated
that catalytic property of Ag can be enhanced by doping of Ni atoms in Ag lattice and hence
synergistic effect between more electronegative noble Ag and less electronegative transition
metal Ni. This metal-metal synergistic effect of Ag and Ni metals may be resulted into
optimum adsorption-desorption behaviour of substrate and product over catalyst surface as
per Sabatier principle [33]. As per the volcano plot drawn according to the Sabatier principle,
Ag is on the left side of the volcano maximum and Ni is on the right side. Thus Ag is limited
by desorption of product and Ni is limited by activation of reactants. Therefore, an optimum
adsorption-desorption characteristics could be expected from a proper combination or
1
0

alloying of both metals. Thus following this assumption, we found that Ag0.6Ni0.4
composition demonstrated the best catalytic activity towards the oxidative coupling of aniline
o
(
1 mmol) in DMSO (2 mL) at 60 C in aerobic condition with excellent conversion (94.4%)
and azobenzene selectively (99.9%) (See Fig. S2). The decrease in activity for Ag0.2Ni0.8 may
be attributed to the excess content of Ni (more than alloying limit for AgNi alloys) which gets
oxidized and passivate the surface for catalysis. To further gain insight, physical mixture of
Ag and Ni (60/40 %) was used in similar catalytic reaction conditions which showed 67 %
conversion confirms the synergistic effect of Ag and Ni in alloy NPs.
This is a first report where such kind of low cost alloy nanoparticle catalyst is used
under very mild reaction condition and obtained excellent yield and absolute selectivity. To
confirm this optimum condition, several controlled reactions have been carried out using
Ag0.6Ni0.4 MTANPs and the results are summarized in Fig. 4 and Table 3. The effect of
solvent was first investigated, keeping the catalyst loading (2.6 mg) and time (24 h) intact.
Fig. 2a shows that only DMSO solvent demonstrated the best yield of azobenzene, whereas
no progression of reaction in other mentioned solvents. Other than DMSO, DMF is another
possible solvent matching properties with DMSO. However here too, the activity of the
catalyst and conversion of aniline depend on the interaction of solvent-catalyst, solubility of
air and interaction of solvent-aniline in the solution phase. We assume that, in the strong
basic condition amidate formed from the DMF binds to the AgNi alloy and inhibits the
catalytic process. On the other hand since the polarity of DMSO is higher than isopropanol
and at the same time it is aprotic solvent than water, it is assumed that DMSO is showing the
best result. Further reaction time was varied keeping the catalyst loading (2.6 mg) in DMSO
intact (see Fig. 2b, Table S3). Maximum product was obtained after 24 h of reaction time.
Notably, AgNi alloy NPs catalysts have been tested in various conditions such as with and
without base, at different temperature and different amount of catalyst loading. However, no
product has been detected without the use of KOH and also maximum conversion of substrate
and maximum yield obtained when the catalyst loading was 2.6 mg and the reaction
o
temperature was 60 C as seen in Table 3. However, in particular, effect of catalyst loading
was studied by varying catalyst amount from 1.3-7 mg and obtained results were entered in
Table 3. It seems that 2.6 mg catalyst loading is the optimum condition, as increase of
catalyst amount is not enhancing the conversion efficiency. However, no other product
formed in these higher catalysts loading other than azobenzene. On the other hand, higher
1
1

reaction temperature leads to over oxidation of substrates/intermediate and yields multiple
by-products (GC data is not shown).
In the UV-Vis spectrum (Fig. 5a), the optical absorption peak at 317 nm suggested the
formation of trans-azobenzene as the only product. The FTIR data of azobenzene (I) in Fig.
−
1
1
5
b show a band at 1613 cm confirming the synthesis of the azo (N=N) group. In the H-
NMR spectrum (Fig. 5c), the protons of the aromatic ring appeared at 7.00–8.00 ppm,
however, there is no peak for the NH group in the product; proving the formation of
1
3
azobenzene. Further, this is confirmed from C-NMR analysis, where four signals for the
aromatic ring carbons in the range of 165-114 ppm shown in Fig. 5(d). The reason behind the
excellent conversion of aniline to azobenzene with complete selectivity in the present study is
the multiply twinned nature of AgNi alloy NPs catalyst and the controlled aerial oxidation
facilitate by it. In addition the observed efficiency comes from size effect (high surface area),
structural effect (defects generated from twinned structure) and electronic effect (difference
in oxidation potential of Ag/Ni) offered by AgNi MTANPs [32]. In most of the previous
reports [1], [34] over oxidation of the hydrazine intermediate occurred to form azoxy benzene
and nitroso benzene as by-products. However in the present case over oxidation is not
observed, hence absolute selectivity in product formation.
In a measure to check the reaction process, catalytic reactions were carried out
in three more conditions. Interestingly, in these cases no product was detected in the absence
of catalyst, under a nitrogen atmosphere and surfactant removed AgNi MTANPs (entry 1-3 in
Table 3). Thus based on all the above experimental facts we found the key to the effective
process can be divided to four factors: (a) the activation of molecular oxygen in air in the
presence on the surface of AgNi NPs (no reaction under inert atmosphere, entry 2 Table 3),
(b) size effect, availability of high effective surface area for catalysis due to small sized alloy
NPs (BET measurements showed high surface area, Fig. 3b), (c) electronic effect, the
electronegativity difference between Ni (1.91) and Ag (1.93) metals would cause electron
transfer to Ag from Ni resulting in the creation of electron rich and electron poor regions on
the bimetallic surface and last but not the least (d) presence of surfactant and structural defect
at the surfaces due to twinned morphology (corroborated from the Table 2 and 3). The
importance of twinning effect can be ascertain from entry 7 Table 2, where % of conversion
was very less for physical mixture as compared to twinned alloy NPs catalyst. In addition, it
is found in entry 3 Table 3 that, when the HDA surfactants on the surface of AgNi NPs were
removed by annealing no reaction occurs under comparable reaction conditions
demonstrating the function of long chain hydrocarbon. The removal of HDA surfactant from
the surface of AgNi NPs is ascertain from Fig. 6, where C-N, C-H and N-H stretching and
bending frequencies related to the amine molecules are absent on the annealed sample. Since
the polar amine group is bounded to inorganic metal alloy surface, the hydrophobic chains
1
2

make the NPs dispersible in DMSO. Further these hydrocarbon ends of HDA plays an
important role for aniline substrates to get oxidized on the AgNi NP catalyst surface with
adsorbed oxygen (transformed to superoxide radical) due to easy access through
hexadecyleamine as proved earlier in different reactions [32][35][36]. These factors give the
evidence about the mechanism of reaction which cannot be possible without oxygen and a
mechanistic model can be proposed as shown in Scheme 1, based on the earlier work by
Dutta et al. and Cai et al. [37][38]
Recyclability and stability of catalyst
Stability and reusability of the alloy NP catalyst were measured in subsequent batches of
catalytic cycles using the recovered catalyst, in identical reaction condition set up by adding
fresh aniline (1 mmol), KOH and DMSO (2 mL). At the end of the reaction, the reaction
mixture was collected, tested by TLC in 5% ethylacetate and hexane mixture and finally
analyzed by GC. The results (as shown in Fig. 7, Table S4) suggest that the catalyst can be
reused up to minimum 5 cycles with negligible decrease in conversion of aniline to
azobenzene.
These results concluded recyclability of alloy NPs with the minor decrease in product
yield after five successive cycles. This trifling decrease in efficiency may be attributed to
catalyst lost and removal of capping surfactant lost during successive cycles. After each test,
the catalyst was recovered by centrifugation followed by washing with diethyl ether, distilled
water and drying. For a heterogeneous catalyst it is important to retain original properties.
Therefore, to check the structural changes in crystal structure, morphology and surface
capping agents of the recovered AgNi catalyst after five catalytic runs, we have carried out
post-characterization of recovered catalyst using FT-IR, XRD and TEM analysis and the
results are shown in Fig. 6 (FT-IR) and 8 (XRD and TEM). FT-IR results shows that
th
surfactant was not removed from NPs surface even after 5 catalytic cycle indicating
consistent catalytic activity, strong attachment of surfactant molecules to NPs surface and
stability of alloy NPs towards azobenzene synthesis. It can be seen from XRD pattern of
reused Ag0.6Ni0.4 MTANPs (Fig. 8a) that peak positions remain unchanged and matching well
with the fresh AgNi MTANPs, which presents the structural stability of present sample.
However, the peak intensity is found to be reduced in the case of reused catalyst. This
observation can be understood from the TEM micrograph as shown in Fig. 8b, where slight
reduction in particle size of the reused AgNi alloy NP is manifested. This might be due to
removal (leaching) of some AgNi surface atoms during repeated cycles.
Substrate scope of the Ag0.6Ni0.4 MTANPs catalyst
1
3

We have in addition evaluated the generality of AgNi alloy NPs as catalyst for
oxidative coupling of amines. Aniline afforded product symmetric azobenezene in 94.4%
yield with 99.9% selectivity (Table 4, entry 1). In case of electron donating groups on aniline
also afforded corresponding aromatic azo compounds in good to excellent yield with 99.9%
selectivity (Table 4, entries 2-4). The catalyst was unable to catalyse the reaction of 4-
nitroaniline (electron withdrawing group on aniline) (Table 4, entry 5). We have also carried
out the oxidative coupling reaction of benzylamine in toluene and azomethine compound was
obtained in 96% yield with 99.9% selectivity (Table 4, entry 6). See Fig. S2-S8 for detail
characterizations.
Later, Ag0.6Ni0.4 alloy NPs was also tested for oxidative coupling of asymmetric
aniline. The oxidative coupling of anilines and p-anisidine was studied and major product 1-
(4-methoxyphenyl)-2-phenyldiazene was obtained in 60% yield (Scheme 2) and the
diazobenzene was a minor product. The major product was characterized using FTIR, UV-
Vis and NMR measurements and the spectra are shown in Fig. S9-S12. This result further
corroborates the usefulness of Ag0.6Ni0.4 MTANPs catalyst in the efficient production of
symmetric and asymmetric azo compounds with considerable recyclability and stability.
4
. Conclusions
In summary, we demonstrated that optimized Ag0.6Ni0.4 MTANPs efficiently catalyse
oxidative coupling of aromatic amines to form azo compounds under mild reaction
conditions. The synthesis reaction was catalysed by using the alloy NPs (2.6 mg loading) at
6
0 °C in open air, which results in symmetrical and asymmetrical azobenzene with 99.9%
selectivity. Excellent recyclability of the catalyst, with only 5% activity loss after five cycles,
makes the process environmentally and economically sustainable. This advanced catalytic
activity of the present catalyst is due to its smaller size, presence of twinned defect, activation
of molecular oxygen effectively at the surface and electronic effect from two different metals.
We expect that such alloy nanoparticle will open the way for a variety of catalysis reactions
for the synthesis of important organic compounds.
Acknowledgments
K.S. and M.K. thanks CSIR-India for research fellowship. S.D. gratefully acknowledges the
financial support received from CSIR-India (01(2773)/14/EMR-II) and University of Delhi
(RC/2015/9677/882). We thank USIC, DU for instrumentation facility.
1
4

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1
6

Fig. 1. (a-b) Low resolution TEM images (Inset: single isolated alloy nanoparticle showing
five-fold twinning), (c) HRTEM image and (d) SAED pattern for of Ag0.6Ni0.4 MTANPs.
1
7

Fig. 2. EDAX analysis of (a) Ag0.8Ni0.2, (b) Ag0.6Ni0.4, (c) Ag0.4Ni0.6, (d)Ag0.2Ni0.8 samples. (e)
Dark-field TEM image and (f-g) elemental maps of Ag and Ni of Ag0.6Ni0.4 MTANPs.
1
8

Fig. 3. (a) Powder XRD pattern of pure Ag, Ni and synthesized alloy compositions. (b) BET
1
9

Fig. 4. (a) Amount of azobenzene yield vs. the variation of reaction solvent. (b) Progress of
reaction at different time interval.
2
0

Fig. 5. (a) UV-Vis spectrum and (b) FTIR spectrum of as-obtained azobenzene dissolved in
1
13
ethyl acetate. (c) H-NMR and (d) C NMR spectrum of as-obtained azobenzene dissolved in
CDCl
3
.
2
1

Fig. 6. FTIR spectra of as-synthesized (black curve), reused (blue) and annealed (red curve)
Ag0.6Ni0.5 MTANPs showing the presence, retainment and removal of hexadecyle amine
surfactant at three different conditions.
2
2

Fig. 7. Recyclability study of the Ag0.6Ni0.4 MTANPs catalyst for aniline oxidation in DMSO
o
at 60 C.
2
3

th
Fig. 8. Characterization result of Ag0.6Ni0.4 nanocatalyst after 5 cycle (a) XRD pattern, (b)
low magnification TEM image.
2
4

Scheme 1. Proposed mechanistic model of the oxidation of aniline into azobenzene over
AgNi alloy NP catalyst.
2
5

Scheme 2. Synthesis of asymmetric 1-(4-methoxyphenyl)-2-phenyldiazene in the identical
reaction condition using AgNi alloy NPs as catalyst.
2
6

Table 1. Comparison of stoichiometric elemental ratios with EDAX and AAS results.
Sr.
Ag/Ni
(Precursors
composition)
Ag0.8Ni0.2
Ag/Ni
(EDS atomic
percentage)
81.5/18.5
Ag/Ni
(AAS atomic
percentage)
83.6/16.4
No.
1
2
3
4
Ag0.6Ni0.4
60.3/39.7
61.2/38.8
Ag0.4Ni0.6
41.9/58.1
40.8/59.2
Ag0.2Ni0.8
18.1/81.9
17.2/82.8
2
7

Table 2. Studies on the variation of catalyst and catalyst composition in the oxidative
coupling of aniline to azobenzene.a
Entry
Nanocatalysts
composition
Conversion
(%)
1
2
3
4
5
6
7
Ag
64.4
94.0
94.4
93.0
71.3
15.3
67
Ag0.8Ni0.2
Ag0.6Ni0.4
Ag0.4Ni0.6
Ag0.2Ni0.8
Ni
Physical mixture of
Ag and Ni NPs
a
o
Aniline (1 mmol), KOH (1 mmol), DMSO (2 mL) were taken in a reaction flask (at 60 C) and composition of
(x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) catalyst (2.6 mg) was added and stirred for 24 h.
Ag1-xNi
x
2
8

Table 3. Controlled reactions at indicated conditionsa.
o
Entry
Catalyst
loading
T ( C)
Variation in
solvents
Conversion
(%)b
1
No catalyst 60
DMSO
0
2c
3d
4
2.6 mg
2.6 mg
2.6 mg
2.6 mg
2.6 mg
2.6 mg
2.6 mg
2.6 mg
2.6 mg
1.3 mg
4.3 mg
7.0 mg
2.6 mg
2.6 mg
60
60
60
60
60
60
60
60
60
60
60
60
30
DMSO
DMSO
O-Xylene
Toluene
Acetonitrile
Water
trace
trace
0
5
0
6
0
7
0
8
DMF
0
9
Isopropanol
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
20.0
94.4
68
1
1
1
1
1
1
0
1
2
3
4
5
90.5
85.0
40.0
20.2
100
aAniline (1 mmol), Ag0.6Ni0.4 (indicated amount) and KOH (1 mmol) were taken in solvent (2 mL) and stirred at
b
specified temperature. The conversion was determined by GC and product was characterized by NMR.
c
d
2
Reaction under nitrogen environment. Surfactant stripped (annealed under N atmosphere) catalyst.
2
9