2-Methyl-1,3-disulfoimidazolium polyoxometalate hybrid catalytic systems as equivalent safer alternatives to concentrated sulfuric acid in nitration of aromatic compounds

Article abstract of DOI:10.1002/aoc.5146

Ionic liquid-derived polyoxometalate salts [mdsim]₃[PM₁₂O₄₀] (where M?=?W and Mo) of two heteropolyacids H₃PW₁₂O₄₀.nH₂O and H₃PMo₁₂O₄₀.nH₂O were synthesized using 2-methyl-1,3-disulfoimidazolium chloride ([mdsim][Cl]) ionic liquid and the corresponding heteropolyacids. Three equivalents of [mdsim][Cl] were treated with the respective Keggin-structured heteropolyacids (one equivalent) in aqueous medium at room temperature to afford the water-stable ionic polyoxometalates as acidic solids. They were completely characterized using spectroscopic and other analytical techniques including thermal analysis and Hammett acidity studies. The inherent Br?nsted acidic properties of ─SO₃H group of these polyoxometalate salts were studied for the nitration of aromatic compounds with 69% HNO₃ at normal temperature and 80°C without use of any external concentrated sulfuric acid. These strongly acidic polyoxometalates display good recyclability and efficient reusability.

Full text of DOI:10.1002/aoc.5146

Received: 4 May 2019
DOI: 10.1002/aoc.5146
Revised: 1 July 2019
Accepted: 5 July 2019
F U L L P A P E R
2
‐Methyl‐1,3‐disulfoimidazolium polyoxometalate hybrid
catalytic systems as equivalent safer alternatives to
concentrated sulfuric acid in nitration of aromatic
compounds
Susmita Saikia
| Ruli Borah
Department of Chemical Sciences, Tezpur
University, Napaam, Tezpur 784028, India
Ionic liquid‐derived polyoxometalate salts [mdsim] [PM O ] (where M = W
3
12 40
and Mo) of two heteropolyacids H PW O .nH O and H PMo O .nH O
3
12 40
2
3
12 40
2
Correspondence
were synthesized using 2‐methyl‐1,3‐disulfoimidazolium chloride ([mdsim]
[Cl]) ionic liquid and the corresponding heteropolyacids. Three equivalents
of [mdsim][Cl] were treated with the respective Keggin‐structured
heteropolyacids (one equivalent) in aqueous medium at room temperature to
afford the water‐stable ionic polyoxometalates as acidic solids. They were
completely characterized using spectroscopic and other analytical techniques
including thermal analysis and Hammett acidity studies. The inherent
Ruli Borah, Department of Chemical
Sciences, Tezpur University, Napaam,
Tezpur 784028, India.
Email: ruli@tezu.ernet.in
Funding information
Science and Engineering Research Board,
Government of India, Grant/Award
Number: EMR/2016/002108
Brønsted acidic properties of ─SO H group of these polyoxometalate salts were
3
studied for the nitration of aromatic compounds with 69% HNO at normal
3
temperature and 80°C without use of any external concentrated sulfuric acid.
These strongly acidic polyoxometalates display good recyclability and efficient
reusability.
KEYWORDS
3
nitration, organic–inorganic hybrid, polyoxometalates, SO H functionalized
1
| INTRODUCTION
stability to undergo multi‐electron redox cycles in homo-
geneous or heterogeneous catalysis, as photocatalysts or
[
15–17]
As discrete anionic metal–oxygen clusters of intercon-
nected polyhedra of transition metal cations of group V
and VI elements in higher oxidation state (especially
electrocatalysts.
Industrial‐scale chemical reactions
involve utilization of POMs as heterogeneous catalysts
with efficient catalytic activity after immobilization on
suitable porous supports to eliminate non‐recyclability
in homogeneous phases. It has been observed that the
weak interactions between POMs and supports lead to
aggregation and leaching of POMs during catalysis as
well as during any work‐up step. In this situation, the
development of organic–inorganic hybrids of POMs has
emerged as powerful route for the creation of multifunc-
tional POMs in combination with various organic compo-
nents by covalent bonding or electrostatic or other weak
interactions that produce more efficient recyclable
4
+/5+
6+
1+/6+
V
, Mo
and W
), polyoxometalates (POMs)
have received considerable attention in various areas
such as catalysis, medicine, materials science, nanotech-
nology, molecular magnetism and photochemistry.
These materials possess several value‐added properties
including multi‐electron redox reactions, Brønsted acidic
protons, tunable rigid structure, large sizes, high negative
charges, nucleophilicity, good solubility in polar solvents,
etc. The catalytic behaviour of POMs is mainly concerned
with their super‐acidic nature and also unusual structural
[
1–14]
Appl Organometal Chem. 2019;e5146.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 15
https://doi.org/10.1002/aoc.5146

2
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SAIKIA AND BORAH
heterogeneous catalysts. The presence of organic ligands
increases the porosity and hydrophobicity of hybrid struc-
[
18]
tures
and thus modifies the polarity of POM frame-
The Brønsted and Lewis acidic sites of hybrid
[19]
works.
materials are associated with POMs and the metal ions
of their frameworks. Thus combining all their structural
changes and newly added unique physical/chemical
[20–26]
properties of ionic liquids
makes POMs water‐toler-
ant and recyclable heterogeneous catalysts. This concept
of organic–inorganic hybrids has been employed to syn-
thesize a new class of task‐specific ionic liquid‐based
POM salts by pairing of anionic POMs with or without
SCHEME
functionalized hybrid POM salts
2
3
Nitration reaction catalysed by N─SO H‐
[27]
functional group‐tethered organic cations.
The pres-
ence of Brønsted acidic functional groups (e.g. ─COOH,
─
SO H) with the organic cations of hybrid POMs may
provide more Brønsted acidic character to these ionic
salts. A literature search reveals the synthesis of a few
oxidative side products and safety problems of common
nitrating agents involving mixtures of concentrated or
3
[
42–44]
fuming nitric acid with sulfuric acid.
The modified
N‐alkylsulfonic‐functionalized
organic
cation‐based
nitration methods have been reported to utilize ionic liq-
[
42,43,45,46]
POM hybrids and their uses as strong Brønsted acidic cat-
uids,
or metal nitrates,
In these reports, ionic liquids acted as better reaction
superacids, alkyl nitrates, nitronium salts
[28–31]
[35,47–51]
[52,53]
alysts in organic reactions.
No reports are found of
supported catalysts,
etc.
the preparation of direct N─SO H‐functionalized POM
ionic salts of imidazolium cations. In this context, we
3
[54–59]
media in the presence of concentrated nitric acid.
planned to synthesize direct N─SO H‐functionalized 2‐
Moosavi‐Zare et al. first employed [pyridine–SO H]
3
3
methyl‐1,3‐disulfoimidazolium POM salts of phospho-
tungstic acid and phosphomolybdic acid (Scheme 1) and
examined their catalytic activity as reusable alternatives
to concentrated H SO for the nitration of aromatic com-
[NO ] ionic liquid as safe and reusable nitrating agent
3
[
60]
for the nitration of arenes.
Yang et al. in 2011 designed
novel Keggin heteropolyacid anion‐based Brønsted acidic
ionic salts, i.e. [SO H(CH ) Mim] n[H PMo O ]
2
4
3
2 4
3
3n
12 40
[
28]
pounds using 69% HNO at room temperature and also at
(n = 1, 2, 3), for aromatic nitration.
Thus, the replace-
3
8
0°C within short reaction times (Scheme 2).
ment of corrosive and hazardous concentrated H SO by
2
4
As a unit process, nitration is widely employed for the
N─SO H‐functionalized heterogeneous POM hybrids in
3
preparation of chemical feedstocks of large numbers of
essential chemical products such as dyes, perfumes, phar-
concentrated H SO –HNO mixtures will lead to the
2
4
3
development of environmentally benign routes of aro-
matic nitration which are free from the already men-
tioned limitations of traditional nitration methods.
[
32,33]
maceuticals, fertilizers, plastics and explosives.
Nitro‐aromatics are reaction intermediates used in func-
[
34–36]
tional group transformation reactions.
ber of modified
A large num-
[37–41]
nitrating reagents and catalysts
have been developed to avoid release of excess acid waste
streams, use of excessive reagents, over‐nitration,
2 | EXPERIMENTAL
2.1 | Materials and instrumentation
Hydrated phosphotungstic acid (H PW O .nH O),
3
12 40
2
phosphomolybdic acid (H PMo O .nH O) and other
3
12 40
2
aromatic hydrocarbons were used directly as received
from Tokyo Chemicals Industry Pvt Limited (India). A
PerkinElmer Frontier MIR‐FIRFTIR spectrophotometer
was utilized to record Fourier transform infrared (FT‐
IR) spectra. Thermogravimetric analysis (TGA) was per-
formed with a Shimadzu TGA‐50. Hammett acidities of
the POM hybrid salts were measured with a UV 2550
spectrophotometer using 4‐nitroaniline as basic indicator.
Scanning electron microscopy (SEM) images were
obtained using a JEOL JSM‐6390LVSEM, along with
energy‐dispersive X‐ray analysis (EDX). Raman analyses
SCHEME
disulfoimidazolium POM salts [mdsim]
mdsim] [PMo12
1
Synthesis of N─SO
3
H‐functionalized 1, 3‐
40] and
3
[PW12
O
[
3
40
O ]

SAIKIA AND BORAH
3 of 15
were conducted using a Horiba LabRAM HR spectropho-
tometer equipped with a He–Ne laser with an excitation
wavelength of 514.5 nm. Powder X‐ray diffraction
heteropolyoxometalate salts were dried in a vacuum oven
at 80°C overnight to afford 98–99% yields of the products.
(
XRD) patterns were recorded with a Rigaku Multiflex
2
.3 | Typical procedure for nitration of
instrument using a nickel‐filtered Cu K (0.15418 nm)
radiation source and scintillation counter detector. A
Buchi‐560 melting point apparatus was used to determine
the melting points of organic nitro compounds. UV–visi-
ble diffusion reflectance spectra were obtained with a
aromatic compounds
Nitration of various aromatic compounds was done by
direct treatment of aromatic substrate (1 mmol) with
69% concentrated HNO (0.12 ml, 3 mmol) in the pres-
1
3
Shimadzu UV 2450 spectrophotometer. The H NMR,
ence of 0.3 mol% of [mdsim] [PW O ] or 0.5 mol% of
1
3
31
3
12 40
C NMR and P NMR spectra of hybrid POM salts and
aromatic nitro compounds were recorded using a JEOL
[mdsim] [PMo O ] catalyst in a 50 ml round‐bottom
3
12 40
flask fitted with an air condenser at room temperature
or other temperature to afford the desired nitro‐aromatic
compounds (Scheme 2). Ice‐cold water (5 ml) was added
to stop the reaction upon completion (monitored by
TLC) and the nitro product was extracted with dichloro-
methane (3 × 5 ml) from the aqueous solution of the reac-
tion mixture. The organic extract was dried over
anhydrous sodium sulfate and evaporated to afford solid
4
00 MHz spectrophotometer in DMSO‐d and CDCl .
6
3
2
.2 | Preparation of 2‐methyl‐1,3‐
disulfoimidazolium salts of Keggin anions
(
Mo)
[mdsim] [PM O ], where M = W and
3 12 40
(
or liquid) crude nitro products. The catalyst dispersed
in aqueous extract was recovered as a solid residue after
evaporation of water under reduced pressure and it was
then dried in a vacuum oven at 80°C for reactivation.
Further purification of the nitro products was done using
preparative TLC technique or recrystallization from suit-
able solvent based on the single‐product selectivity or
physical state of crude nitro products.
The
preparation
of
hybrid
2‐methyl‐1,3‐
disulfoimidazolium heteropolyanions of Keggin units
3
−
[
PM O ] (where M = W and Mo) was completed in
12 40
a two‐step process (Scheme 1). The parent 2‐methyl‐1,3‐
disulfoimidazolium chloride ([mdsim][Cl]) ionic liquid
was prepared through dropwise addition of
chlorosulfonic acid (20 mmol) to a stirred solution of 2‐
methylimidazole (10 mmol) in dry CH Cl (30 ml) in a
2
2
1
00 ml round‐bottom flask at 0°C for 10 min under nitro-
2.4 | Procedure for Hammett acidity
gen atmosphere. Then the mixture was stirred at room
temperature for 1 h to complete the reaction. The brown
viscous ionic liquid formed a separate layer with dichlo-
romethane which was later removed through a decanta-
tion process. Repeated addition and decantation of dry
CH Cl (3 × 10 ml) were done for three more times for
calculation
The procedure for Hammett acidity calculation involved
mixing of equal concentration of 4‐nitroaniline (5 mg l ,
−
1
−
1
pK = 0.99) and the solid acid (5 mmol l ) in acetonitrile
a
solvent. The absorbance of 4‐nitroaniline decreased as
2
2
the acidity of POMs increased due to protonation of the
purification of the ionic liquid phase by dissolving dichlo-
romethane‐soluble impurities. Drying the residue in vac-
uum oven at 50°C yielded 95% of the product as brown
viscous liquid.
+
+
indicator [HI] in acetonitrile. The [I]/[IH ] ratio can be
obtained from the absorption differences. Equation (1)
gives us the H° values of the respective hybrid POM salts,
where pK(I)aq represents the pK value of the indicator:
The second step involved the treatment of H PW O .
a
3
12 40
nH O or H PMo O .nH O with the parent [mdsim][Cl]
2
3
12 40
2
H° ¼ pKðIÞaq þ log½Iꢀ=½IHþꢀ
(1)
ionic liquid in a 1:3 ratio to achieve the desired
mdsim] [PM O ] heteropolyoxometalate salts as strong
[
3
12 40
acidic solid materials (Scheme 1). To a stirred aqueous
solution (20 ml of water) of the respective heteropolyacid
3 | RESULTS AND DISCUSSION
(2 mmol), viscous ionic liquid (6 mmol) was added
dropwise at room temperature within 5 min. Formation
of white and yellow precipitates was observed for the
The
formation
of
─SO H‐functionalized
2‐
3
methylimidazolium POMs of phosphotungstate and
[
mdsim] [PW O ] and [mdsim] [PMo O ] com-
3 12 40 3 12 40
phosphomolybdate, namely [mdsim] [PM O ], was
3
12 40
pounds, respectively, after 12 h of stirring at room tem-
perature. Then the precipitate was centrifuged and
washed three to four times with distilled water. The pure
achieved following a two‐step method (Scheme 1) as
described in Section 2.2. The solid POM salts were sub-
jected to various analytical techniques to complete their

4
of 15
SAIKIA AND BORAH
structural, thermal and Brønsted acidic characterization
process. The acidic properties of the hybrid salts were
examined with a view to the salts being equivalent safer
replacements for concentrated sulfuric acid in the nitra-
tion of aromatic compounds.
13.79 ppm for the ─SO H groups of imidazolium cation
3
13
(Figure 1). Their C NMR spectra also evidenced the
existence of imidazolium ring carbons within the struc-
31
tures of the hybrid materials (Figure 2). The P NMR
spectra of the hybrid POMs are shown in Figure 3. The
phosphorus peak of phosphotungstic acid (Figure S1) at
−14.98 ppm is changed to −15.25 ppm for
3
.1 | NMR analysis
[mdsim] 40]. The same peak of phosphomolybdic
acid (Figure S2) at −3.47 ppm is shifted to −3.94 ppm for
[PW12O
3
1
The H NMR spectra of both the hybrid compounds in
[mdsim] [PMo O ]. The characteristic sharp and sym-
3
12 40
DMSO‐d displayed one singlet of two protons at 13.77–
metric resonance peaks of phosphorus in tetrahedral
6
1
FIGURE 1 H NMR spectra of (a) [mdsim]
3
[PW12
O40] and (b) [mdsim]
3
[PMo12
O
40
]

SAIKIA AND BORAH
5 of 15
1
3
FIGURE 2
C NMR spectra of (a)
[
mdsim]
mdsim]
3
[PW12O40] and (b)
[
3
40
[PMo12O ]
position of the Keggin‐structured anions in each of the
heteropolyacids are slightly moved towards higher fre-
quencies in the case of the hybrid compounds and thus
indicate strong ionic interaction between the
imidazolium cation and Keggin anion in the hybrid com-
þ
þ
þ
H5O2 ↔H3O þ H2O↔H þ 2H2O
(2)
3.2 | FT‐IR analysis
31
pounds. The up‐field resonance of P signals signifies the
To verify the presence of Keggin structure of
retention of heteropolyanions with a small hydration
heteropolyanions within the POM compounds, their FT‐
sphere observed from dynamic dehydration equilibrium
IR spectra were analysed and compared in two regions:
+
−1
involving dioxonium group H O2 (equation (2)) as pro-
500–2000 and 2000–4000 cm
(Figure 4). The
5
3−
posed to explain the behaviour of hydration environment
as a function of temperature close to room temperature
[PM O ] Keggin units of the two salts exhibited typi-
12 40
−
1
cal absorptions at 1061–1083 cm (asymmetric P─O
[61,62]
−1
using NMR study:
stretch), 961–983 cm (asymmetric M═O stretch), 874–
t

6
of 15
SAIKIA AND BORAH
3
1
FIGURE 3
3 3 40
P NMR spectra of (a) [mdsim] [PW12O40] and (b) [mdsim] [PMo12O ]
−
1
8
93 cm (asymmetric stretch of M─O2c2─M bonds of
S─O symmetric stretch of the ─SO H group. The S─O
3
−
1
corner‐sharing MO octahedra), 770–804 cm (asymmet-
ric stretch of M─O2c1─M bonds involving edge‐sharing
MO octahedra) and 586–590 cm (P─O bending). The
antisymmetric stretch may be observed in the same
6
−
1
region as the P─O stretch at 1061–1083 cm . It is
expected that vibration frequency of N─S stretch will
exist within the absorption peak of Keggin anions at
−
1
6
−
1
peak at 1623 cm with medium intensity can be assigned
to overlapped O─H bending with C═C stretching of
imidazole ring. In addition, the small shoulder at 1427–
−
1
874–893 cm . The aliphatic C─H stretch of ─Me group
−
1
is distinctly observed at 2719, 2919 and 3037 cm for
−
1
1
435 cm can be assigned to the imidazole ring vibra-
the hybrid salt of phosphotungstic acid. There are also
−
1
−1
tion. The peak at 1213–1284 cm could be attributed to
two strong absorptions at 3327 and 3207 cm which

SAIKIA AND BORAH
7 of 15
minimum amount of strongly hydrogen‐bonded water
3
−
molecules in the rigid framework of the [PMo O ]
1
2 40
Keggin anion. TGA (Figure 5) also evidenced the absence
of loosely bound physisorbed water in the structures of
the hybrid POM salts and thus supports the hydrophobic
nature of the hybrid materials.
3.3 | TGA results
The TGA profiles of the two POM salts simply indicate
their hydrophobic nature and marked increase in thermal
stability as compared to the parent [mdsim][Cl] ionic liq-
uid
(Figure
5).
Thermal
stability
of
the
[
mdsim] [PW O ] salt is extended up to 440°C which
3
12 40
[66]
is observed at 320°C for the [mdsim] [PMo O ] salt.
3
12 40
The initial 15% weight loss of the parent ionic liquid
around 100°C can be observed for physisorbed water
followed by another slow decomposition between 220
and 320°C due to stepwise removal of the two ─SO H
3
groups and imidazole moiety.
3.4 | Powder XRD analysis
3
−
3−
The Keggin structures of [PW O ] and [PMo O ]
12
40
12 40
anions are retained in both hybrid POM salts as assessed
from their characteristic powder XRD peaks (Figure 6).
For [mdsim] [PW O ], strong reflections at 2θ = 10.6°,
3
12 40
1
4.4°, 20.1°, 24.5°, 29.0° and 35.8° can be correlated with
matched reflection planes (1 1 0), (2 0 0), (2 2 0), (2 2 2),
4 0 0) and (3 3 2) of powder XRD database (JCPDS card
number 75‐2125). Similarly, the intense peaks of
mdsim] [PMo O ] at 2θ = 11.0°, 19.2°, 25.9°, 28.6°
(
[
3
12 40
FIGURE
mdsim]
phosphomolybdic acid (PMA) and [mdsim][Cl]
4
FT‐IR
spectra
of
3
[mdsim] [PW12O40],
[
3
[PMo12O40], phosphotungstic acid (PTA),
can be assigned to overlapped C─H stretch of
imidazolium cation and O─H stretch of ─SO H groups
3
involving strong intramolecular hydrogen bonds with
[63–65]
the phosphotungstate anion.
The weak O─H stretch
band of the phosphotungstate hybrid compound at
−
1
3
545 cm can be observed owing to a very small amount
of hydrogen‐bonded water molecules around the Keggin
anion after elimination of most of its water content
through the dynamic dehydration equilibrium in the
strong Brønsted acidic environment of the material
(equation (2)). The medium strength broad shoulder of
the phosphomolybdate hybrid compound near 3413 cm
−
1
can be expected as
a merged absorption of
imidazolium C─H stretch, intramolecular hydrogen‐
FIGURE 5 TGA curves of POMs with respect to the parent
bonded O─H stretch of ─SO H groups in addition to
[mdsim][Cl]
3

8
of 15
SAIKIA AND BORAH
FIGURE 6 Powder XRD analysis of two polyoxometalate salts
FIGURE 7 Raman spectra of polyoxometalates
−
1
band at 543 cm is attributed to combined stretching
and bending vibrations of W─O2c1─W, W─O2c2─W
and O─P─O bonds. Furthermore, the medium intensity
peak at 242 cm can be assigned to W─O4c stretching
and W─O2c2─W bending vibrations.
and 36.1° can be correlated with the reflection planes (1 1
0
), (2 1 2), (0 0 6), (3 0 5) and (4 2 2) of JCPDS card num-
ber 46‐0482. Other sharp peaks at 2θ = 17.4°, 17.9°, 21.5°
and 26.05° can be attributed to the 2‐methylimidazolium
−
1
[67,68]
unit.
Similarly, in the case of the [mdsim] [PMo O ] salt,
3
12 40
−
1
the strong band at 1006 cm can be assigned to sym-
metric stretch of terminal Mo─O bond followed by
3.5 | Raman analysis
t
−
1
another shoulder at 979 cm corresponding to asym-
metric stretch of the same bond. The medium strength
‘inter ligand vibration’ band defined by Bridgeman at
Raman spectra for the POM salts are displayed in
Figure 7. The presence of Keggin‐structured
3
−
3−
−1
heteropolyanions [PW O ]
and [PMo O ]
was
906 cm is observed for asymmetric Mo─O2c2─Mo
12
40
12 40
[
70]
−1
evidenced from typical Raman bands as observed
stretch.
from combined stretching and bending vibration of
Mo─O2c1─Mo bonds of the Mo 13 groups. The sharp
Another broad peak at 620 cm may result
[69,70]
according to Bridgeman's assignments.
In the spec-
trum of [mdsim] [PW O ], the sharp intense peak at
3
O
3
12 40
−
1
−1
1
023 cm is observed for symmetric stretching mode
band at 266 cm can be assigned to stretching vibra-
of W─O bond along with another weak band at
tion of Mo─O4c bonds and bending of the intra‐ligand
Mo─O2c2─Mo bonds.
t
1
−
[70]
9
26 cm for asymmetric W─O2c2─W stretch. The weak

SAIKIA AND BORAH
9 of 15
3
.6 | UV–visible diffuse reflectance
spectra
3
−
The existence of Keggin structure of [PM O ] can be
1
2 40
evidenced from a comparison of the UV–visible diffuse
reflectance absorption of the hybrid POMs and the corre-
sponding heteropolyacids (Figure 8).
3.7 | SEM analysis
The surface morphology of [mdsim] [PW O ] indicates
3
12 40
a heterogeneous surface with smaller size crystalline par-
ticles and clusters (Figure 9a). The crystallinity of
[
mdsim] [PMo O ] particles is higher as compared to
3 12 40
the phosphotungstate POM salt (Figure 9b). The sizes of
FIGURE 9 SEM images of (a) [mdsim]
mdsim] [PMo12
3
[PW12
O40] and (b)
[
3
40
O ]
crystalline particles are larger with minimum clustering
on its surface. The agglomeration of each solid material
can be expected as an outcome of secondary interactions
via intermolecular hydrogen bonds of ─SO H functional-
3
ity of the imidazolium cation.
3.8 | EDX analysis
Figure 10 exhibits the presence of key elements of the
POM salts in their respective EDX patterns, except the
detection of
P
atom in the spectrum of the
[
mdsim] [PW O ] salt. The absence of the P atom peak
3
12 40
can be assumed as a low abundance of this element
within the complex and large cage of Keggin anion as
compared to oxygen (O) and tungsten (W) atoms. How-
31
ever, the P NMR spectrum of this salt confirmed the
presence of P atoms in the Keggin‐structured anion
(
Figure 3a). The EDX mapping of the hybrid materials
FIGURE
8
UV–visible diffuse reflectance spectra of (a)
40] and H 40 (PTA)and (b)
40] and H PMo12 40 (PMA)
[
mdsim]
mdsim]
3
[PW12
O
3
PW12
O
revealed a uniform dispersion of the component elements
for the two salts (Figure 11).
[
3
[PMo12
O
3
O

10 of 15
SAIKIA AND BORAH
FIGURE 10 EDX patterns of (a)
[
mdsim]
mdsim]
3
[PW12
O40]and (b)
[
3
[PMo12
40
O ]
3 3 40
FIGURE 11 EDX mapping of (a) N, S, O and W for [mdsim] [PW12O40] and (b) N, S, O, P and Mo for [mdsim] [PMo12O ]
3
.9 | Hammett acidity determination
spectrophotometric basic indicator and CH
CN as polar
3
using UV–visible spectrometry
aprotic solvent according to the standard procedure
[
71]
described in Section 2.4.
Characteristic absorbance of
Determination of Brønsted acidity of the POM salts using
the Hammett acidity method utilized 4‐nitroaniline as
the basic indicator decreases with increased acidity of
the acidic POM ionic salts as the degree of protonation

SAIKIA AND BORAH
11 of 15
3.10 | Catalytic activity
The catalytic activity of the hybrid compounds was investi-
gated for the nitration of various aromatic hydrocarbons
containing electron‐withdrawing or donating groups in
the benzene ring. Various catalytic amounts of
[
mdsim] [PW O ], 0.16, 0.32 and 0.64 mol%, were ini-
3 12 40
tially studied for optimization of reaction of naphthalene
1 mmol) with 69% HNO (3 mmol) at room temperature
(
3
with stirring (Table 2, entry 1) in the absence of any addi-
tional solvent. The reaction showed best catalytic activity
using 0.32 mol% the catalyst by taking 5 min to produce
98% yield of the product (Table 2, entry 1). This optimized
amount of catalyst was also used for conducting the same
reaction with the other [mdsim] [PMo O ] catalyst
3
12 40
under identical conditions with a lower catalytic activity
Table 2, entry 2). Then the amount of this catalyst was
increased to 0.5 and 1 mol% to get a better outcome of the
model reaction, which was found for a catalyst amount of
FIGURE 12 Hammett plots of 2‐methyldisulfoimidazolium POM
salts with respect to the parent acids
(
0.5 mol% (Table 2, entry 2). Also, varying the reactant
TABLE 1 Calculation of Hammett acidity function (H°)
and HNO (69%) ratio from 1:1 to 1:2 gave different results
3
+
(Table 2, entries 3 and 4). A 1:3 ratio of substrate to HNO3
led to a satisfactory result in the case of the optimization
reaction in the presence of 0.32 mol% of the
[mdsim] [PW O ] salt (Table 2, entry 1) and 0.50 mol%
of the [mdsim] [PMo O ] salt (Table 2, entry 2). The sub-
3 12 40
strate scope was extended with these optimized amounts of
Entry
A
max
[I] (%)
[IH ] (%)
H°
p‐Niroaniline
2.60
1.50
1.29
1.02
0.86
100
0
—
H
H
3
PMo12
PW12
O
40
57.6
49.6
39.2
33.2
42.4
50.4
60.8
66.8
1.02
3
12 40
3
O
40
0.98
0.79
0.68
[
mdsim]
3
3
[PMo12
[PW12
O
40
]
catalysts for electron‐rich and electron‐deficient aromatic
[
mdsim]
O
40
]
compounds with varied amount of 69% HNO as nitrating
3
agent (Table 3). It was observed that electron‐deficient sys-
tems required 80°C and more acid to undergo nitration
(Table 3, entries 3, 4 and 6), whereas hydrocarbons and
phenolic compounds (Table 3, entries 1, 2, 5, 7 and 8) easily
nitrated at room temperature in the presence of minimal
nitric acid. Nitration of phenol was not possible with these
catalysts because of oxidative degradation. Aniline also did
enhances with their acidic strength (Figure 12). The order
of acidity was found as [mdsim] [PW O ]
>
>
3
12 40
[
mdsim] [PMo O ]
>
phosphotungstic acid
3
12 40
phosphomolybdic acid from the calculated Hammett
acidity function (Table 1).
TABLE 2 Optimization of amount of POM catalyst for nitration of naphthalene
Entry
Catalyst
Amount (mol%)
Time (min)
Yield of 1a (%)
a
1
2
3
4
[mdsim]
[mdsim]
[mdsim]
[mdsim]
3
[PW12
O
40
]
0.16/0.32/0.64
0.32/0.50/1
0.30/0.50
‐do‐
15/5/5
20/10/10
20/30
92/98/98
90/96/98
25/20
a
3
3
3
[PMo12
O
40
]
b
[PW12
[PW12
O
40]/[mdsim]
3
[PMo12
40
O ]
c
O
40
]
3
/[mdsim]
3
[PMo12
O
40
]
20/25
68/65
a
3
Nitration carried out using 1:3 ratio of naphthalene to 69% HNO acid at room temperature.
b
3
Using 1:1 ratio of naphthalene to 69% HNO .
c
3
Using 1:2 ratio of naphthalene to 69% HNO with optimum amount of the two catalysts.

12 of 15
SAIKIA AND BORAH
TABLE 3 Substrate scope for nitration of aromatic compounds using hybrid catalysts
a
b,f
Entry
Substrate
Product
Time (min) A/B
Temp. (°C)
Yield (%) A/B
c
1
5/10
r.t.
98/96
c
e
2
15/20
r.t.
64:30 (A)
62:27 (B)
d
3
4
5
6
15/20
80
80
r.t.
80
95/92
d
10/12
90/88
90/90
85/80
c
10/15
d
20/28
c
7
8
10/15
r.t.
80
90/85
75/72
g
6 h/6.5h
a
b
c
d
A and B represent [mdsim]
3
[PW12
O
40] and [mdsim]
3
[PMo12
O
40] catalysts, respectively. Isolatedyields. Reactant: conc. HNO
3
= 1:3. Reactant: conc.
e
f
3
HNO = 1:10. Selectivity of p‐ and o‐products calculated based on isolated yields of products. All nitro compounds were known compounds and also charac-
terized by their melting points and H NMR, C NMR and FT‐IR analyses. Reactant: conc. HNO = 1:10.
3
1
13
g
SCHEME 3 Mechanism for nitration of
naphthalene using POM salt as catalyst
not form any nitration product and led to the formation of
took part in the reaction followed by hydrolysis of amide
bonds in strong acidic solution. Aromatic aldehydes with-
out electron‐donating groups were inactive for the
some complex mixture of polar products involving ─SO H‐
3
functionalized acidic hybrid material. Acetanilide also

SAIKIA AND BORAH
13 of 15
nitration reaction and produced oxidative side products.
Interestingly, nitration of 4‐hydroxybenzaldehyde yielded
4‐hydroxy‐3‐nitrobenzaldehyde at 80°C with excess
amount of nitric acid within a short time without oxidation
of aldehyde group (Table 3, entry 6).
3.11 | Plausible reaction mechanism
The proposed mechanism (Scheme 3) followed the usual
path for formation of nitronium ion from the interac-
tion of ─SO H group of the catalyst with nitric acid
3
which later acts as an electrophile to give the nitro
product.
3.12 | Recyclability study
The catalytic recyclability was studied for 5 mmol of
naphthalene with 15 mmol of 69% HNO at room temper-
3
ature with stirring using the optimized amount of
[
mdsim] [PW O ] catalyst for the specified reaction
3 12 40
time (Table 3, entry 1). The catalyst was recovered as a
solid residue from acidic solution of water after comple-
tion of the reaction according to the standard work‐up
steps as described for thw general method of nitration
reaction (Section 2.3). Figure 13 exhibits the reusability
profile of spent catalyst with decreasing catalytic activity
for four consecutive runs that may be due to the loss of
a small amount of the used catalyst during repeated isola-
tion in every cycle. After the fourth cycle of the reaction,
the reused catalyst almost retains the original powder
XRD pattern and FT‐IR spectrum (Figure 14) similar to
the fresh POM salt.
FIGURE 14 (a) Powder XRD pattern and (b) FT‐IR spectra of
fresh and recycled [mdsim] [PW12O40] catalyst
3
4
| CONCLUSIONS
This work developed thermally stable and water‐stable
hydrophobic ─SO H‐functionalized imidazolium‐based
3
POM hybrid salts of corresponding heteropolyacids
(
namely phosphotungstic acid and phosphomolybdic
acid) as solid acidic materials. The internal ─SO H
3
groups of these salts function as equivalent to concen-
trated sulfuric acid during nitration of aromatic com-
pounds in combination with stoichiometric amount of
6
9% HNO at room temperature or 80°C depending on
3
the nature of electron‐rich and electron‐deficient aro-
matic substrates. Thus, the catalytic use of these hydro-
phobic POM salts introduced several advantages to
aromatic nitration as compared to traditional nitration
3
FIGURE 13 Reusability profile of [mdsim] [PW12O40] catalyst

14 of 15
SAIKIA AND BORAH
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[
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How to cite this article: Saikia S, Borah R. 2‐
Methyl‐1,3‐disulfoimidazolium polyoxometalate
hybrid catalytic systems as equivalent safer
alternatives to concentrated sulfuric acid in
nitration of aromatic compounds. Appl
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