Oxidation of Aniline to Nitrobenzene Catalysed by 1-Butyl-3-methyl imidazolium phosphotungstate Hybrid Material Using m-chloroperbenzoic Acid as an Oxidant

Article abstract of DOI:10.1007/s10562-017-2214-2

Abstract: Keggin ion based hybrid materials, [BmIm]₃[PW₁₂O₄₀], [TBA]₃PW₁₂O₄₀ and [BuPy]₃PW₁₂O₄₀, were prepared by proton exchange with organic cations, 1-butyl-3-methyl imidazolium ion, tetrabutylammonium ion and butylpyridinium ion, respectively. The formation of hybrid materials was confirmed by FTIR, PXRD, SEM, TG-DTA, DSC analysis. These hybrid compounds are active for oxidation of aniline using m-chloroperbenzoic acid as an oxidant. Among the three hybrid compounds, 1-butyl-3-methyl imidazolium phosphotungstate was found to be the best and efficient catalyst for selective aniline oxidation to nitrobenzene. It is a recoverable and reusable catalytic system. The redox property of the phosphotungstate cluster in the hybrid material is involved in the catalytic activity. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2214-2

Catal Lett
DOI 10.1007/s10562-017-2214-2
Oxidation of Aniline to Nitrobenzene Catalysed by 1-Butyl-3-
methyl imidazolium phosphotungstate Hybrid Material Using
m-chloroperbenzoic Acid as an Oxidant
2
1
2
1
R. Meenakshi · K. Shakeela · S. Kutti Rani · G. Ranga Rao
Received: 18 February 2017 / Accepted: 24 September 2017
©
Springer Science+Business Media, LLC 2017
Abstract Keggin ion based hybrid materials,
BmIm] [PW O ], [TBA] PW O and [BuPy] PW O ,
for oxidation of aniline using m-chloroperbenzoic acid as
an oxidant. Among the three hybrid compounds, 1-butyl-
3-methyl imidazolium phosphotungstate was found to be
the best and efficient catalyst for selective aniline oxidation
to nitrobenzene. It is a recoverable and reusable catalytic
system. The redox property of the phosphotungstate cluster
in the hybrid material is involved in the catalytic activity.
[
3
12 40
3
12 40
3
12 40
were prepared by proton exchange with organic cations,
1
-butyl-3-methyl imidazolium ion, tetrabutylammonium
ion and butylpyridinium ion, respectively. The formation
of hybrid materials was confirmed by FTIR, PXRD, SEM,
TG-DTA, DSC analysis. These hybrid compounds are active
Graphical Abstract
Keywords Phosphotungstic acid · Ionic liquid · Hybrid
*
*
S. Kutti Rani
material · Oxidation · Aniline
skrani@bsauniv.ac.in
G. Ranga Rao
grrao@iitm.ac.in
1
Introduction
1
2
Department of Chemistry, Indian Institute of Technology
Madras, Chennai, Tamil Nadu 600036, India
Ionic liquids (ILs) are the salts which can melt below
00 °C, exclusively contain ionic species. They have been
Department of Chemistry, B.S. Abdur Rahman University,
Vandalur, Chennai, Tamil Nadu 600048, India
1
Vol.:(0
1
123456789)
3

R. Meenakshi et al.
used as catalysts, electrolytes and reaction media since they
are thermally and electrochemically stable with no detect-
able vapour pressure, and high solvating power [1–5]. Keg-
gin anions are one type of polyoxometalates (POMs) with
general formula (M=W, Mo; X=P, Si) [6]. The structural
diversities and wide variety of applications of polyoxometa-
lates have been reviewed extensively [6–11]. They have been
widely used as catalysts for organic transformations due to
their controllable acidic and redox properties [11–17]. The
combination of ionic liquids and POMs gives rise to hybrid
materials in which the Keggin ions are fixed in the solid
state. These hybrid molecular materials retain the properties
of both the organic cations of ILs and inorganic anions of
POMs [17–20].
2 Experimental Section
2.1 Materials
1-bromobutane, 1-methyl imidazole and m-CPBA were pur-
chased from Sigma-Aldrich. Phosphotungstic acid (PWA),
tetrabutyl ammonium bromide (TBABr) were purchased
from Loba Chemie and used as received. Pyridine, aniline
and its substituted anilines were of analytical grade and used
as received without purification. Purified solvents were used
for all the catalytic reactions.
2.2 Materials Synthesis
The immobilized polyoxometalate hybrid materials have
attracted attention in the area of heterogeneous catalysis
The catalyst 1-butyl-3-methylimidazolium phosphotung-
state hybrid material [BmIm] [PW O ] was synthesized
3
12 40
[
21–27]. Leng et al. reported the synthesis of heteropoly-
as reported in literature [18]. 1-bromobutane was added to
1-methyl imidazole in a equimolar ratio of 0.24 mol and
stirred at 80 °C for 24 h. The yellowish ionic liquid 1-butyl
3-methyl imidazolium bromide (BMImBr) was obtained
in the lower phase. The unreacted starting material in the
upper phase was removed by washing with ethyl acetate.
N-butyl pyridinium bromide (BuPyBr) was prepared in a
similar manner according to reported procedure [37]. We
have chosen the ionic liquids which contain the cations,
anion based ionic liquids as reaction induced self- separation
catalysts for esterification of citric acid with n-butanol [21].
Series of Keggin type polyoxometalate-based ionic liquids
have been used as efficient catalysts for deep desulfuriza-
tion of fuels with H O as oxidant [23]. The esterification
2
2
of oleic acid for biodiesel production using SiW O -based
1
2
40
ionic liquid catalysts was reported by Zhen et al. [25]. In one
of our earlier reports, silicotungstic acid has been immo-
bilized onto the mesopores of amine-functionalized zirco-
nia by electrostatic interactions. This hybrid is found to be
an efficient heterogeneous catalyst for selective esterifica-
tion of maleic acid with 98% conversion [26]. In another
study, Yamaguchi et al. reported heterogeneous epoxidation
of wide range of olefins with H O using peroxotungstate
+
+
+
BMIm , TBA , and BuPy . Each of these cations is reacted
with H PW O nH O in 3:1 mol ratio to obtain one mole of
3
12 40
2
hybrid material, IL-POM. Typically, BMImBr (0.013 mol)
was added drop wise to the solution containing (0.0048 mol)
of phosphotungstic acid under constant stirring at room tem-
perature. The white precipitate obtained was washed with
distilled water and dried at 80 °C overnight. [TBA] PW O
12 40
2
2
immobilized on ionic liquid-modified SiO [27].
2
3
The oxidation of aniline with various oxidants results in
the formation of azoxybenzene, azobenzene, nitrosoben-
zene and nitrobenzene [28–32]. The product composition
depends upon the reaction conditions and the nature of the
catalyst employed, which need to be optimized to obtain
nitrobenzene as a desired product. The selective oxidation
of amines using polyoxometalates as catalysts has been
reported earlier [32–36]. However, IL-POM hybrid materi-
als are of current interest in organic transformations [12, 21].
We have prepared three IL-POM salts using ionic liquids
containing 1-butyl-3-methyl imidazolium cation, tetrabutyl
ammonium cation and butylpyridinium cation reacting with
phosphotungstate Keggin anion (PWA) [18]. These IL-PWA
hybrid materials are used as catalysts for selective oxidation
of aniline to nitrobenzene reaction. We report that 1-butyl-
and [BuPy] PW O were also synthesized in a similar
3 12 40
manner using TBABr and BuPyBr. The scheme of prepara-
tion of IL-POM hybrid catalysts is given Scheme 1.
2.3 Characterization
The FTIR spectra of the samples were recorded using
−
1
JASCO FT/IR-6300 in the range of 4000–400 cm . The
XRD pattern of the materials were obtained using Bruker
AXS D8 advanced powder XRD with CuK (λ=1.1548 Å)
α
radiation at settings of 40 kV and 30 mA with a divergence
of 0.982. The thermal stability was recorded on TGA (Per-
kin-Elmer) Q500 V20.10 high resolution Thermogravi-
metric Analyzer in nitrogen atmosphere (flow rate=20 ml/
min) in the temperature range of 30–1000 °C at a heating
rate of 5 °C/min. The phase transition of the materials was
tested on a DSC (Perkin-Elmer) Q200 MDSC V24.4 of build
116. The surface morphology and elemental composition of
3
-methyl imidazolium phosphotungstate is more efficient
catalyst for selective aniline oxidation using meta-chlorop-
erbenzoic acid(m-CPBA) as an oxidant.
1
3

Oxidation of Aniline to Nitrobenzene Catalysed by 1-Butyl-3-methyl imidazolium…
the materials were recorded using an FEI QUANTA 200F
m-chloroperbenzoic acid is a stronger oxidizing agent with
3
1
microscope. P NMR spectra of fresh and reused cata-
outstanding reactivity and greater selectivity when compared
to hydrogen peroxide and other per acids [38]. The addition
of 2.67 mmol of m-CPBA to a stirred solution containing
0.053 mmol of the catalyst and 0.53 mmol of aniline in 3 ml
of acetonitrile resulted in green coloured solution which
indicates the formation of nitrosobenzene intermediate. The
reaction mixture was stirred at 85 °C for 2 h. Diethyl ether
was added to the reaction mixture to re-precipitate the cata-
lyst and recover it by filtration. The ether layer was washed
with saturated bicarbonate solution to remove the m-chlo-
robenzoic acid byproduct. The final product was recovered
by evaporating ether and analyzed by gas chromatograph
and FID detector. The formation of the products were further
confirmed by gas chromatography-mass spectrometry. The
conversion of aniline and product selectivity were calculated
by using the expressions,
lysts in DMSO-d solvent were recorded on a Bruker-FT-
6
NMR-400 MHz spectrometer. The products formed in the
aniline conversion reaction were analysed by gas chromato-
graph (Shimadzu 2014) employing capillary column (Rtx-5,
3
0 m×0.32 mm, ID 1 µm) and flame ionisation detector and
further confirmed by gas chromatography-mass spectrom-
etry (Perkin Elmer). The Clarus 680 chromatograph with
capillary column (Elite-5MS, 30 m x 0.25 mm ID x 250 µm
df) was connected with Clarus 600 (EI) mass spectrometer.
2
.4 Catalytic Oxidation of Aniline
The oxidation of aniline using 1-butyl-3-methyl imida-
zolium phosphotungstate and m-CPBA was investigated.
Initial mol % of aniline − Final mol % of aniline
Conversion (mol %) =
Initial mol % of aniline
× 100
GC peak area of nitrobenzene
Selectivity of nitrobenzene (%) =
× 100
Sum of GC peak areas of azoxybenzene and nitrobenzene
Scheme 1 Synthesis of differ-
ent cation substituted phos-
photungstates. a BMImBr, b
[
[
BMIm] PW O , c TBABr, d
3 12 40
TBA] PW O e BuPyBr and
3
12 40,
f [BuPy] PW O
3
12 40
1
3

R. Meenakshi et al.
3
Results and Discussion
of bulky tetra butyl ammonium ions around phosphotung-
state anions [18, 40].
3
.1 Physicochemical Characterization
The thermal behaviour of all the catalysts under nitrogen
atmosphere has been studied by TGA and DSC as shown
in the Figs. 3 and 4, respectively. The results are tabulated
inTable 1. From Fig. 3, it is observed that all the catalysts
showed dual weight losses corresponding to the organic
cation followed by inorganic Keggin anion. The pure phos-
photungstic acid generally decomposes at 550 °C leading
to WO and P O residues. From Table 1, it could be seen
The FTIR spectra of the prepared catalysts and their pre-
cursors are shown in Fig. 1. The inorganic moiety and
phosphotungstate anion show four noticeable stretching
−
1
vibrations in the region of 700–1100 cm [39]. The P–O
−
1
stretching of all the hybrids observed at 1079 cm is
analogous to that of the parent phosphotungstate anion.
3
2
5
−
1
W=O
stretching is shifted from 985 to 974 cm
that the thermal stability (T ) and maximum degradation
terminal
onset
and the W–O –W vibration also marginally blue shifted.
(T ) followed the trend [TBA] <[BuPy] <[BmIm] . The
e
max
3
3
3
Hence, the primary Keggin structure has been well
high stability of [BmIm] PW O indicated the stronger
3 12 40
−
1
retained in all the catalysts. The peak at 620 cm corre-
sponds to C –N –C bending related to [BmIm] PW O
12 40
interaction between imidazolium cation and phosphotung-
state anion, primarily due to better delocalisation of positive
charge than pyridinium and ammonium cations [40, 42]. The
TGA curve of [TBA] PW O in Fig. 3b shows a steep fall
2
1
5
3
is analogous to that observed in the parent ionic liquid
+
BmImBr [18]. The Im ring stretching vibration observed
3
12 40
−
1
−1
at 1569 cm for the ionic liquid shifted to 1564 cm
(19% weight loss) in the region 330–460°C, resulting from
the decomposition of organic moiety possessing greater
number of butyl groups around Keggin anion, when com-
pared to [BmIm] PW O (9% weight loss, Fig. 3a) and
+
in the hybrid material. The Im ring C–H stretch of the
−
1
hybrid material is shifted to 2947 and 3128 cm [18].
All the hybrids, [BmIm] PW O , [TBA] PW O ,
3
12 40
3
12 40
3
12 40
[
BuPy] PW O revealed peak shifts in the region
[BuPy] PW O (7% weight loss, Fig. 3c).
3
12 40
3 12 40
−
1
2
800–3000 cm which are attributed to butyl C–H
The endothermic peak of DSC curves (Fig. 4; Table 1)
at 189, 206 and 238 °C demonstrate the solid to liquid
transition of [BmIm] PW O , [TBA] PW O and
stretching vibrations. The two hybrid materials, except
[
TBA] PW O , show vibrational bands characteristic of
3 12 40
3
12 40
3
12 40
aromatic C=C bonds. The heterocyclic C-H stretching and
[BuPy] PW O respectively. T followed the trend
3 12 40 m
−
1
bending of BuPyBr are observed at 2957 and 1175 cm .
For [BuPy] PW O hybrid material, these frequencies
[BmIm] <[TBA] <[BuPy] . Generally, melting point will
3 3 3
be high when packing of molecules is even and ordered.
[BuPy] PW O is planar and symmetric. Hence its melting
3
12 40
−
1
are shifted to 2953 and 1167 cm respectively. The bend-
3
12 40
−
1
ing vibration of Ar-H observed at 682 cm is analogous
to that of BuPyBr.
point is higher than [BmIm] PW O and [TBA] PW O .
3 12 40 3 12 40
The peak observed at 169 °C for [BmIm] PW O corre-
3
12 40
Figure 2 shows the powder XRD pattern of the three
hybrid materials. Phosphotungstic acid shows the charac-
teristic high intensity peak at 10.29° (JCPDS No: 752125).
The hybrid material, however, shows new intense peaks in
the low angle region while the intensity of the peaks in the
high angle region decreased [40]. The three acidic protons
sponds to the presence of an eutectic impurity in the sample.
Elemental analysis revealed the presence of two water mol-
ecules in the [BmIm] PW O hybrid (Calculated C-8.5,
3
12 40
H-1.48, N-1.52, and Experimental C-8.73, H-1.50, N-2.35).
Hence, the eutectic impurity is expected to be due to weak
hydrogen bonding in [BmIm] PW O hybrid.
3
12 40
and most of the H O molecules of phosphotungstic acid are
The surface morphologies of the three powder mate-
rials are shown in Fig. 5. Capsule like particles with
smooth surfaces are observed for [BmIm] PW O and
2
+
replaced by three molecules of organic cations (BmIm ,
+
+
TBA , BuPy ). The intense peaks in the low angle region
in Fig. 2 indicate the formation of well-defined layered-type
structures in all the three compounds. For [BmIm] PW O ,
3
12 40
[BuPy] PW O due to the regular arrangement of three
3
12 40
butyl groups around the phosphotungstate ions. However,
3
12 40
the diffraction pattern shows four distinct peaks with an
intense peak at 2θ = 8.7 corresponding to the d-spacing
of 1.0 nm. The d-spacing value agreed with the theoreti-
cal size of the IL-POM hybrids [41]. Similar pattern has
been observed for [BuPy] PW O with little shift in the 2θ
granular like particles are observed for [TBA] PW O
3
12 40
which might be due to the random arrangement of large
number of butyl groups around the phosphotungstate ion.
This can be correlated with the presence of merged diffrac-
tion peaks observed in PXRD of [TBA] PW O .
3
12 40
3
12 40
values. The d-spacing value analogous to the high intense
peak (2θ=8.8) is 0.99 nm. In contrast to other two catalysts,
the XRD pattern of [TBA] PW O exhibits peaks merged
3
12 40
with high intensity peak at 2θ= 8.0 with d-spacing value
of 1.1 nm. The combined peaks indicated the presence of
closely packed crystal structures relating to the large number
1
3

Oxidation of Aniline to Nitrobenzene Catalysed by 1-Butyl-3-methyl imidazolium…
Fig. 1 FTIR spectrum of a (i) H PW O , (ii) BMImBr, (iii) [BMIm] PW O , b (i) H PW O , (ii) [TBA]Br, (iii) [TBA] PW O , c (i)
3
12 40
3
12 40
3
12 40
3
12 40
H PW O , (ii) [BuPy]Br, (iii) [BuPy] PW O
12 40
3
12 40
3
3
.2 Aniline Oxidation Reaction
.2.1 Influence of Reaction Conditions
conversion is 100% in acetonitrile with greater selectivity
(
93%) to nitrobenzene. Under homogeneous conditions, the
3
nitroso intermediate would be better stabilised by the imi-
dazolium groups in the catalyst which on further oxidation
gives nitro product [43].
The oxidation of aniline gives variety of products as depicted
in Scheme 2. The reaction parameters like solvents, tempera-
ture, concentration of aniline and meta-chloro-perbenzoic
acid (m-CPBA) are optimized for [BmIm] PW O cata-
Under homogeneous condition, variation of reaction
parameters like amount of oxidant, substrate, catalyst and
the temperature on aniline oxidation was studied as shown in
Fig. 6. The equimolar concentration of substrate and oxidant
(0.53 mmol) at 85 °C resulted in moderate conversion of
aniline (67%). As seen in Fig. 6a, the formation of insignifi-
cant amount of azobenzene (not shown) and azoxybenzene
is favoured initially due to the insufficient amount of oxi-
dant. The oxidative coupling of nitrosobenzene intermediate
3
12 40
lyst. The effect of solvents on the oxidation of aniline over
[
BMIm] PW O catalyst at 85 °C is given in Table 2. The
3
12 40
compound [BMIm] PW O dissolves in acetonitrile and
3
12 40
acts as a homogeneous catalyst while in other solvents it
acts as solid–liquid heterogeneous catalyst. High conversion
(
>90%) of aniline in all the solvents was observed, but the
1
3

R. Meenakshi et al.
Fig. 2 XRD pattern of (a) H PW O , (b) [BMIm] PW O , (c)
3
12 40
3
12 40
[
TBA] PW O , (d) [BuPy] PW O
3 12 40 3 12 40
Fig. 4 DSC pattern of (a) [BMIm] PW O , (b) [TBA] PW O ,
3
12 40
3
12 40
(
c) [BuPy] PW O
3 12 40
Table 1 Thermal properties of various cation substituted phospho-
tungstates
Catalyst
Tonset (°C) Tmax (°C) Residue (%) Tm (°C)
[
(
(
BmIm] [PW O ] 378
493
432
446
85
78
88
189
206
238
3
12 40
TBA) PW O
336
355
3
12 40
BuPy) PW O
3
12 40
Also, the oxidation of aniline in acetonitrile using m-CPBA
as oxidant, in the absence of catalyst, for 10 h resulted in
lower yield of nitrobenzene, as reported by Liu et al. [44].
This emphasizes the importance of the hybrid catalyst in
the oxidation of aniline. Increasing catalyst concentration
up to 0.053 mmol resulted in high selectivity to nitroben-
zene (Fig. 6b). The imidazolium groups in the catalyst would
stabilise the nitroso intermediate under homogeneous condi-
tions, which on further oxidation resulted in the nitro prod-
uct [43]. At higher catalyst concentrations, there is a pos-
sibility of parallel oxidative coupling between nitroso with
phenyl hydroxylamine, thereby leading to slight decrease
in nitro selectivity (78%). Low substrate concentrations
result in nitrobenzene as a major product (Fig. 6c). The sub-
strate concentration above 1 mmol favours the formation of
nitrosobenzene intermediate. The nitrosobenzene intermedi-
ate might have reacted with phenylhydroxylamine to form
azoxybenzene which would have been converted to azoben-
zene for further increase in substrate concentration. Fig-
ure 6d shows the enhanced selectivity towards nitrobenzene
with increase in temperature [33]. Nitrosobenzene inter-
mediate was not observed at the temperatures studied. The
optimized reaction conditions for obtaining nitrobenzene
Fig. 3 TGA pattern of (a) [BMIm] PW O , (b) [TBA] PW O ,
3
12 40
3
12 40
(
c) [BuPy] PW O
3 12 40
formed might have reacted primarily with unreacted ani-
line or phenyl hydroxylamine resulting in the formation of
azoxybenzene, which can convert to azobenzene as shown in
Scheme 2. At this point, the doubling the catalyst concentra-
tion resulted in 86% conversion with 54 and 44% selectivity
for azoxy and azobenzene, respectively. Further increase in
the concentration of oxidant with 0.053 mmol of catalyst
resulted in complete conversion of aniline with high selectiv-
ity towards nitrobenzene (Fig. 6a). The use of 2.67 mmol of
oxidant predominantly favoured the formation of the selec-
tive product nitrobenzene (96% GC yield). With 2.67 mmol
of oxidant, in the absence of catalyst, oxidation after two
hours at 85 °C resulted in a mixture of products with 48, 35,
and 10% of azoxy, nitroso and nitrobenzene, respectively.
1
3

Oxidation of Aniline to Nitrobenzene Catalysed by 1-Butyl-3-methyl imidazolium…
Fig. 5 SEM images of a [BmIm] PW O , b [TBA] PW O , and c [BuPy] PW O
3
12 40
3
12 40
3
12 40
OH
O
O-
N+
[
O]
[O]
Step 2
[O]
Step 3
NH2
NH
N
Step 1
O
Aniline
phenyl hydroxylamine
Nitrosobenzene
Nitrobenzene
Step 4
O-
N+
N
N
Step 5
N
azoxybenzene
azobenzene
Scheme 2 Products of aniline oxidation
Table 2 Influence of various solvents on aniline oxidation using
as oxidant are summarized in Table 3. The selectivity for
nitrobenzene is only 10% in the absence of the catalyst. The
a
[
BMIm] PW O catalyst
3 12 40
homogeneous Keggin structure H PW O resulted in 100%
Sl. No.
Solvent
Solubility
Conver-
Selec-
3
12 40
b
c
sion (%)
tivity
conversion with 83% selectivity to nitrobenzene. Though
this catalyst gives better selectivity towards nitrobenzene, the
complete recovery of the catalyst is difficult. The oxidation
(
%)
1
2
3
4
Ethyl acetate
Toluene
Ethanol
Insoluble
Insoluble
Insoluble
Soluble
100
92
92
45
75
70
93
of aniline catalyzed by Na [WZnZn (H O) (ZnW O ) ]
12
2
2
2
9
34 2
in aqueous biphasic reaction medium resulted in azoxyben-
zene as the major product [33]. Tetranuclear peroxotungstate
TPA [H{W O (O ) (μ − O)} ] resulted in nitrosobenzene
Acetonitrile
100
3
2
2
2 4
2
a
Aniline (0.53 mmol), m-CPBA (2.67 mmol), catalyst (0.053 mmol),
as 85% selective product in the presence of H O oxidant
2
2
8
5 °C, 2 h
[
34]. Various other polyoxometalate catalysts have shown
b
c
Conversion of aniline
Selectivity for nitrobenzene
much less selectivities compared to [BmIm] PW O and
3
12 40
[
TBA] PW O IL-POM hybrid catalysts. Even though the
3 12 40
aniline oxidation by [BmIm] PW O catalyst is homoge-
3
12 40
selectively are 0.53 mmol of substrate, 2.67 mmol of
neous, the recovery of the catalyst can be done by precipi-
tation method using low polar solvent like diethyl ether.
However, recovery of [TBA] PW O catalyst is not pos-
m-CPBA, 0.053 mmol of [BmIm] PW O catalyst and
3
12 40
8
5 °C in 2 h.
3
12 40
The product distribution of aniline oxidation using cat-
ion substituted phosphotungstates as catalysts and m-CPBA
sible because the tetra butyl groups seem to enhance partial
solubility of [TBA] PW O in ether. The hybrid catalyst
3
12 40
1
3

R. Meenakshi et al.
Fig. 6 Effect of reaction parameters on the oxidation of aniline using m-CPBA oxidant over [BmIm] PW O catalyst: a oxidant concentration,
3
12 40
b catalyst concentration, c substrate concentration and d reaction temperature
[
BuPy] PW O shows good conversion under heterogene-
as catalysts for aniline oxidation with TBHP and m-CPBA
as oxidants. About 77% of unreacted aniline was observed
when TBHP was used as an oxidant (Entry 3, Table 4).
Under the optimised reaction conditions, no selective prod-
uct was observed using H O and TBHP as oxidants. Hence,
3
12 40
ous conditions but the selectivity towards nitrobenzene is
very less due to poor stabilization of nitroso intermediate
by the catalyst.
The efficiency of m-CPBA is compared with per-
oxo oxidants like H O and tetrabutyl hydrogen perox-
2
2
only m-CPBA is the best oxidant to obtain nitrobenzene with
2
2
ide (TBHP) under optimised reaction conditions using
highest selectivity (Entry 1, Table 4).
[
BmIm] [PW O ] catalyst (Table 4). The conversion of
The plausible reaction mechanism of the oxidation of
aniline by m-chloroperbenzoic acid as oxidant is given
in Scheme 3. The phosphotungstate moiety in the hybrid
[BmIm] [PW O ] reacts with m-CPBA to generate per-
3
12 40
aniline is 66% with H O as an oxidant resulting in nitroben-
2
2
zene as a minor product. The use of H O as oxidant gener-
2
2
ally results in product nitrosobenzene [32, 34], azoxyben-
zene [33, 35] and azobenzene [45]. Aniline reacting with
H O in CHCl , in the presence of peroxotungstophosphate
3
12 40
3
−
oxo- species {PO [WO(O ) ] } , which can act as active
4
2 2 4
oxygen transfer agent [46]. This further reacts with aniline
2
2
3
as heterogeneous catalyst, does not produce nitrobenzene at
room temperature [32]. There have been no reports in the
literature regarding the use of POM and IL-POM hybrids
to produce nitrobenzene, as depicted in Scheme 3.
In order to evaluate the ease of the oxidation process,
the reaction is carried out using different substituted aniline
1
3

Oxidation of Aniline to Nitrobenzene Catalysed by 1-Butyl-3-methyl imidazolium…
Table 3 Oxidation of aniline catalysed by various polyoxometalates with m-CPBAa
Sl. No.
Catalyst
Oxidant
Reaction medium
Product selectivity
Selectivity %
References
1
2
3
4
5
6
7
8
9
No catalyst
H PW O
m-CPBA
m-CPBA
H O
Homo
Homo
Homo
Homo
Homo
Homo
Homo
Homo
Homo
Hetero
Nitro
Nitro
Azoxy
Nitroso
Nitro
Nitro
Nitro
Nitro
Nitro
Nitro
10
83
95
85
13
71
62
93
90
4
Present work
Present work
[33]
[34]
[32]
3
12 40
b
Na [WZnZn (H O) (ZnW O ) ]
12
2
2
2
9
34 2
c
2
2
2
2
2
TPA [H{W O (O ) (μ−O)} ]
H O
3
2
2
2 4
2
2
+
3−d
3−e
[PyN (CH ) CH ] (PO [W(O)(O ) ] )
H O
2
15
3 3
4
2 2 4
2
+
[PyN (CH ) CH ] (PO [W(O)(O ) ] )
H O
[32]
2
15
3 3
f
4
2 2 4
2
[BmIm] PW O
m-CPBA
m-CPBA
m-CPBA
m-CPBA
Present Work
Present work
Present work
Present work
3
12 40
[BmIm] PW O
3
12 40
[TBA] PW O
3
12 40
1
0
[BuPy] PW O
3
12 40
Reaction conditions:
a
Aniline (0.53 mmol), m-CPBA (2.67 mmol), acetonitrile (3 ml), 85 °C, 2 h
b
c
d
e
f
Aniline (1 mmol), H O (5 mmol), water (1 ml), 75 °C, 7 h
2
2
Aniline (1 mmol), H O (2 mmol) catalyst (5 μmol)
2
2
Aniline (3 mmol), H O (9 mmol), chloroform (7.5 ml), RT, 2 h
2
2
Aniline (3 mmol), H O (9 mmol), chloroform (7.5 ml), reflux, 4 h
2
2
Aniline (0.53 mmol), m-CPBA (2.67 mmol), acetonitrile (3 ml), RT, 2 h
substrates under the same optimized reaction conditions
the corresponding nitro compounds with sodium perborate
in the presence of tungstophosphoric acid and surfactants
[35]. Though nitrobenzene is obtained as the selective
product, catalyst recovery is not possible. The use of excess
(
Table 5). Firouzabadi et al. reported the conversion of ani-
lines substituted with electron-donating groups selectively to
sodium perborate (NaBO ) in acetic acid for the oxidation of
3
Table 4 Effect of various oxidants on aniline oxidation using
a
anilines containing electron-withdrawing groups to the cor-
[
BmIm] [PW O ] catalyst
3 12 40
responding nitro compounds has been reported by McKillop
b
Entry
Oxidant
Conversion (%)
%
[
47]. But anilines with electron-donating groups are oxidized
c
Nitrobenzene
to the nitro compounds in very low yields [48]. Hence this
method appears inappropriate for this type of oxidation reac-
tions. Under optimized conditions, we have observed that the
substituents with both electron withdrawing and donating
groups irrespective of their positions (ortho, meta and para)
resulted in corresponding nitro derivatives with high selec-
tivity, which is in contrast to the reported procedures [35,
47, 48]. The products obtained by the oxidation of aniline
derivatives have been confirmed using gas chromatography-
mass spectrometry.
(
selectivity)
1
2
3
m-CPBA
100
66
23
93
10
<1
H O
2
2
TBHP
a
Aniline (0.53 mmol), catalyst (0.053 mmol), acetonitrile (3 ml),
8
5 °C, 2 h
b
Conversion of aniline
Selectivity for nitrobenzene
c
Scheme 3 Mechanism of
O
O
O
H
aniline oxidation catalysed by
Diethyl ether
[
BmIm] [PW O ]
N
3
12 40
O
[
BmIm] PW O
3
12 40
N
NO2
Cl
+
O
OH
NH2
[
BmIm] {P [W ( ) ] }
Cl
3
O4
O O
2 2 4
1
3

R. Meenakshi et al.
Table 5 Oxidation
of
derivatives
of
aniline
using
3.2.2 Reusability Test
Though [BmIm] PW O is soluble in acetonitrile, it is re-
a
[
BmIm] [PW O ] as catalyst
3 12 40
Sl. No.
Substrate
Product
%Nitroben-
3
12 40
zene (selec-
tivity)
precipitated with diethyl ether and then recovered by filtra-
tion for reuse. Nearly 100% conversion of aniline has been
observed in all the four runs indicating the steady reusability
of the catalyst. But the selectivity to nitrobenzene decreased
from 92% in the first run to 83% in the fourth run. Hence, it
could be established that the catalyst could be reused thrice
without much loss of catalytic activity, as shown in Fig. 7.
The IR spectrum of the reused catalyst is similar to that of
fresh catalyst with reduced intensity (Fig. 8). Though the
1
2
98
100
3
4
96
92
5
6
98
98
Fig. 7 Reusability of [BmIm] PW O , reaction conditions:
3
12 40
m-CPBA: 2.67 mmol, Catalyst: 0.053 mmol, acetonitrile: 3 ml,
8
5 °C, 2 h
7
8
100
98
9
87
a
m-CPBA : 2.67 mmol, Catalyst: 0.053 mmol, acetonitrile: 3 ml,
5 °C, 2 h
8
Fig. 8 FTIR spectrum of 1-butyl-3-methyl imidazolium phospho-
tungstate catalyst (a) fresh and (b) four times reused
1
3

Oxidation of Aniline to Nitrobenzene Catalysed by 1-Butyl-3-methyl imidazolium…
catalyst. The use of m-CPBA oxidant produced nitrobenzene
with high selectivity when compared to hydrogen perox-
ide and TBHP. The substituted anilines, irrespective of the
electron donating or withdrawing groups present and their
positions (ortho, meta and para), also produced the corre-
sponding nitrobenzenes with high selectivities. The catalyst
can be recovered easily and reused thrice without loss of
catalytic activity.
Acknowledgements The DST-FIST facilities in IIT Madras have
been very helpful to carry out this work. We thank VIT-SIF lab, SAS,
Chemistry Division, Vellore, for GC-MS analysis.
Compliance with Ethical Standards
Conflict of interest The authors declare that there is no conflict of
interest.
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