Preparation and characterization of WO3 bonded imidazolium sulfonic acid chloride as a novel and green ionic liquid catalyst for the synthesis of adipic acid

Article abstract of DOI:10.1039/c6ra16098k

A novel nano WO₃-bonded imidazolium-sulfonic acid chloride (WO₃-IL) was prepared, and characterized by FT-IR, XRD, SEM, TEM, EDX and BET. The as synthesized WO₃ nanoparticle supported ionic liquid is studied as a heterogeneous catalyst for the liquid phase oxidation of cyclohexene to adipic acid. Apart from cyclohexene, a few other six-eight membered cyclic alcohols and ketones are converted into their dicarboxylic acids in high yields and in short reaction times employing the same reaction protocol. The synergy between the WO₃ and ionic liquid is assumed to play a significant role towards its very high catalytic activity. The recyclability of the catalyst is proved to be noteworthy as the catalyst exhibits no significant change in its catalytic activity even after five cycles of reuse.

Full text of DOI:10.1039/c6ra16098k

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Preparation and characterization of WO3 bonded
imidazolium sulfonic acid chloride as a novel and
green ionic liquid catalyst for the synthesis of adipic
acid†
Cite this: RSC Adv., 2016, 6, 99044
Received 21st June 2016
Accepted 11th October 2016
a
a
b
a
*
Bishal Bhuyan, Bappi Paul, Sethumathavan Vadivel and Siddhartha Sankar Dhar
DOI: 10.1039/c6ra16098k
www.rsc.org/advances
A novel nano WO3-bonded imidazolium-sulfonic acid chloride
During the last few decades, two signicant steps have been
(WO3-IL) was prepared, and characterized by FT-IR, XRD, SEM, TEM, taken by materials chemistry with the introduction and expanded
EDX and BET. The as synthesized WO3 nanoparticle supported ionic studies of nanoparticles on the one hand9,10 and ionic liquids
liquid is studied as a heterogeneous catalyst for the liquid phase (ILs) on the other hand.11 Nanoparticles have attracted consid-
oxidation of cyclohexene to adipic acid. Apart from cyclohexene, erable attention in recent times due to their numerous techno-
a few other six-eight membered cyclic alcohols and ketones are logical applications particularly in catalysis owing to the very high
converted into their dicarboxylic acids in high yields and in short surface to volume ratio and large number of active sites.12–14 The
reaction times employing the same reaction protocol. The synergy size, shape and also the composition of the nanoparticles plays
between the WO3 and ionic liquid is assumed to play a significant a very crucial role in the enhancement of their catalytic effi-
role towards its very high catalytic activity. The recyclability of the ciency.15 Oen the addition of a metal or support signicantly
catalyst is proved to be noteworthy as the catalyst exhibits no enhance the catalytic efficiency of the nanomaterial to a large
significant change in its catalytic activity even after five cycles of extent due to the synergistic effect.16 Noble metal nanoparticles
reuse.
such as silver and gold exhibit unique catalytic, electrical and
optical properties and are believed to be excellent supports on
various materials as well as transition metal oxide nano-
particles.17,18 Among the various transition metal oxides, WO3
offers an extensive array of potential applications in device such
as electrochromic display, semiconductor gas sensors and pho-
tocatalysis due to its outstanding electrochromic, gasochromic
and thermochromic properties.19–21
Ionic liquids have gained increasing popularity as innovative
and environmentally benign reaction media owing to its thermal
stability, very low vapor pressure and ability to dissolve various
compounds.22 Recently, a concept called supported ionic liquid
catalysis (SILC) has become popular for the ease of organic
transformations.23,24 The distinctive physicochemical properties
of ILs can thus be transferred to substrates. They not only retain
the advantages of ILs and supports but also derive a series of new
performances and novel properties due to their synergistic effect.
The majority of supports which immobilize ILs are metal/metal
oxide nanoparticles, mesoporous silica materials, zeolites and
clays (e.g., montmorillonite) and CNTs (carbon nanotubes).25,26 In
recent times metal/metal oxide nanoparticles have known to be
used as promising support because of their easy synthesis
process and functionalization, high surface area, facile separa-
tion, low toxicity, good stability and low cost.15,26 The covalent
interaction between the ILs and supports is accomplished by the
reaction between pre-functionalized solid surfaces and ILs with
1. Introduction
With an annual production of over 3.5 million tons globally,
adipic acid (AA) is one of the most important aliphatic dicar-
boxylic acids produced industrially. It is used chiey as a raw
material in the production of numerous valuable materials such
as polyurethanes, plasticizers, polyamides, intermediates for
pharmaceuticals and insecticides, etc.1–4 Most commonly,
adipic acid is produced industrially by the oxidation of a cyclo-
hexanol/cyclohexanone mixture or cyclohexane with nitric
acid.5,6 These industrial processes are associated with the co-
production of a considerable amount of environmentally
unfriendly and unwanted greenhouse gases like N2O, thus
contributing to some severe cases like global warming and
ozone depletion.7,8 Accordingly, to develop a clean technological
process, the use of environmentally benign reagents for the
production of adipic acid is highly desired.
aDepartment of Chemistry, National Institute of Technology, Silchar, Silchar-788010,
Assam, India. E-mail: ssd_iitg@hotmail.com; Fax: +91-03842-224797; Tel: +91-03842-
242915
bDepartment of Chemistry, PSG College of Technology, Coimbatore-641004, Tamil
Nadu, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra16098k
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special functional groups, such as trimethoxysilyl, ether-, cetyltrimethylammonium bromide (CTAB) (purity 99%), cyclo-
hydroxyl-group, etc. For metal/metal oxide nanoparticles, trime- hexene and other chemicals were purchased from Sigma
thoxysilyl group is realized to be the most stable linking group Aldrich Co. Hydrogen peroxide was purchased from Merck
between them and ILs. In some cases, cations of supported ionic India. Double distilled water was used for preparation of all
liquid acts themselves as catalysts. This kind of typical SILCs solutions. FT-IR spectrum was recorded on KBr pellets with
mainly involves supported sulfonic acid and metal-free func- Bruker 3000 Hyperion Microscope with Vertex 80 FT-IR system.
tionalized ILs. By and large, –SO3H or –SO2Cl functionalized ILs XRD measurements were carried out on a Phillips X'pert Pro
are immobilized onto silica materials, ZrO2, TiO2 or Al2O3. Multi purpose diffractometer (MPD) with Cu-Ka radiation (l ¼
Kotadia et al. synthesized a novel SILC which consisted of –SO3H 1.5418 A) with a scan speed of 2 minꢁ1. Transmission electron
functionalized benzimidazolium-based IL immobilized on microscopy images were obtained on a JEOL, JEM2100 equip-
a silica-based solid support. This IL based catalyst was used as an ment. The sample powders were dispersed in ethanol under
efficient heterogeneous catalyst, for solventless synthesis of 1- sonication and TEM grids were prepared using a few drops of
amidoalkyl naphthol. In comparison with other catalytic systems, the dispersion followed by drying in air. Field emission scan-
supported benzimidazolium-based IL –SO3H showed a much ning electron microscope (FESEM) images and EDX were
higher activity in terms of short reaction time, use of mild acquired with a JEOL JSM-7600F FEG-SEM eld emission
conditions and reusability. Recently, several reports on fabrica- scanning electron microscope.
ꢀ
˚
tion of hybrid magnetic NPs, including Fe3O4 anchored onto SiO2
have been reported and used as supports for Brønsted acidic 2.2 Synthesis of WO3 nanoparticles
ILs.27 Interestingly, this method offered excellent competence in
catalyst separation and recycling as well as avoiding large
amounts of acidic wastes.
An amount of 4.9 g of H2WO4 (20 mmol) solution was taken in
a 100 ml round bottomed ask and stirred at 100 ꢀC for 15 min.
The pH of the solution was adjusted to ꢂ9 by dropwise addition
Catalytic methods by means of environmentally friendly
of NH4OH under continuous stirring conditions. Subsequently
molecular oxygen or H2O2 is highly demanding as it has a high
0.68 g of cetyltrimethylammonium bromide (CTAB) was added
content of active oxygen species (47 wt%). Moreover, H2O2 is
to the above suspension and stirred for another 24 h. Aer 24 h
of continuous stirring, the obtained yellowish colored suspen-
cheaper and safer to use than the other existing peroxides and
peracids.28–30 Metal supported WO3 nanoparticles have been
proven to be highly effective catalysts for conversion of cyclo-
sion was washed with ethanol for several times and oven dried
at 80 ꢀC for 8 hours. Finally the product was thermally heated at
hexene to AA. As for example, Ghosh et al. have reported on the
preparation of uniformly dispersed Ag nanoparticles (AgNPs)
supported on WO3 nanorods and found that the metal supported
nano-WO3 catalyst is highly active for selective oxidation of
cyclohexene to adipic acid.15 Although various methodologies for
oxidation of cyclohexene to adipic acid using tungsten oxide based
catalyst have been reported, most of these methods suffer from
limitations such as the use of expensive and hazardous reagents,
time consuming procedures, drastic reaction conditions with only
low or moderate yields and inefficiency of the catalyst in terms of
reusablility.31–33 Thus there exists sufficient scope for improve-
ment on WO3 NPs based catalysts for better catalytic performance.
Hence, knowing the utility of supported WO3 NPs in adipic
acid synthesis as well as combining the advantages of ILs, we
contemplated to develop a novel WO3 NPs supported IL catalyst
and study its efficacy in the oxidation of cyclohexene to adipic
acid. To the best of our knowledge, the use of nano WO3-bonded
ionic liquid has rarely been studied as catalyst in the oxidation
of cyclohexene to adipic acid. As a continuation of our ongoing
endeavor on synthesis and application of nanocatalysts in
synthetically important organic transformations,34–38 we report
herein nano WO3-bonded imidazolium sulfonic acid chloride
ꢀ
500 C for 6 hours.
2.3 Preparation of nano WO3-bonded imidazolium-sulfonic
acid chloride
A mixture of imidazole (0.34 g, 5 mmol) and (3-chloropropyl)trie-
thoxysilane (1.125 g, 5 mmol) were dissolved in 20 ml of dry
toluene and stirred for 18 h under reux conditions to obtain
a white solid compound. This was then ltered, and separated
from toluene. In the next step, 1.5 g of WO3 nanopowder was
separately dispersed in ethyl acetate (15 ml) under sonication for
15 min. To this dispersed suspension, the white solid obtained in
the previous step was slowly added and further stirred under reux
conditions for 8 h to give a yellowish suspension. The suspension
was then ltered off and washed thoroughly to remove the
unreacted reagents and excess of ethyl acetate. To this, a solution
of chlorosulfonic acid (5 mmol) in chloroform (10 ml) was added
dropwise with continuous stirring at 0 ꢀC (ice-water bath) for 2 h.
The solvent chloroform was removed by evaporation to obtain
nano-WO3-bonded imidazolium-sulfonic acid chloride (WO3-IL).
2.4 Oxidation of cyclohexene to adipic acid
(WO3-IL) as a novel catalyst in the oxidation of cyclohexene to An amount of 1.01 ml (10 mmol) of cyclohexene was dissolved
adipic acid by H2O2.
in 10 ml of acetonitrile. This was followed by addition of 50 mg
of the catalyst. Subsequently, 2.8 ml (120 mmol) of H2O2 (30%
aq. solution) was slowly added to the above reaction mixture
over a period of 30 minutes and stirred at 90 C in an oil bath
tted to a condenser till the complete conversion was achieved
2. Experimental
2.1 Materials and physical measurements
ꢀ
All the chemical reagents were of analytical grade and used as as conrmed by TLC. The catalyst was then recovered under hot
received without further purication. Tungstic acid (H2WO4), conditions from the reaction mixture through ltration. The
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product (adipic acid) thus formed in the reaction mixture was Further, to this functionalized compound, chlorosulfonic acid
then cooled to room temperature to obtain the white solid was added under ice-cooled conditions to produce nano WO3-
precipitate and recrystallized with methanol. The obtained bonded imidazolium-sulfonic acid chloride IV (Scheme 1). The
solid product was characterized by melting point, 1H NMR, 13
C
structure of WO3-IL was further characterized by FT-IR, XRD,
NMR, FTIR, and elemental analyses. Adipic acid: C6H10O4, MP SEM, EDX, TEM and BET analysis.
151–152 ꢀC; 1H NMR (500 MHz, CD3OD): d 1.72 (m, 4H), 2.42 (m,
4H), 5.30 (s, 2H); 13C NMR (500 MHz, CD3OD): d 175.9, 33.1,
3.1 Catalyst characterization
24.1; FTIR 1696, 2959 cmꢁ1; elemental analysis (%) calculated
The chemical composition of the as-synthesized nanoparticles
for C6H10O4: C, 49.31, H, 6.90; found: C, 49.38; H, 6.79.
were examined by FT-IR spectroscopy. Fig. 1(a) showed the
FTIR spectrum of synthesized WO3 nanoparticles. Two prom-
inent peaks at around 818 and 757 cmꢁ1 correspond to the
symmetric stretching modes of (O–W–O), which conrms the
formation of WO3 nanoparticles. Likewise, Fig. 1(b) showed
the FT-IR spectral peaks of the synthesized nano WO3-bonded
imidazolium-sulfonic acid chloride. Four new characteristic
2.5 Oxidation of C6–C8 cyclic alcohols and ketones
An amount of 10 mmol of substrate was dissolved in 10 ml of
acetonitrile. This was followed by addition of 50 mg of the
catalyst. Subsequently, 2.8 ml (120 mmol) of H2O2 (30% aq.
solution) was slowly added to the above reaction mixture over
a period of 30 minutes and stirred at 90 ꢀC in an oil bath tted to
a condenser till the complete conversion was achieved as
conrmed by TLC. The catalyst was then recovered under hot
conditions from the reaction mixture through ltration. The
product thus formed in the reaction mixture was then cooled to
room temperature to obtain the white solid precipitate and
recrystallized with methanol. The resulted product was char-
acterized by melting point, 1H NMR, 13C NMR, FTIR, and
elemental analyses.
peaks appeared around 3148, 2840, 1580 and 1448 cmꢁ1
,
which corresponds to sp2 C–H stretching vibration of imid-
azole moiety, alkyl chain stretching and vibration of C]N and
C]C of the imidazole ring respectively.39 A characteristic broad
peak at the region 2500–3600 cmꢁ1 can be related to the O–H
stretching of the SO3H group. On the other hand, two prominent
peaks were observed at around 1058 and 1168 cmꢁ1 may corre-
spond to the vibrational modes of N–SO2 and O–SO2 bonds.40 All
these characteristics peaks unambiguously indicate successful
functionalization of the WO3 nanoparticles with imidazolium
sulfonic acid chloride.
3. Results and discussions
To conrm the formation of WO3 nanoparticles, X-ray
Synthesis of WO3 nanoparticles has been successfully achieved powder diffraction (XRD) measurement was done on WO3 NPs
by precipitation method followed by calcination. Tungstic acid (Fig. 2(a)). Fig. 2(a) shows XRD pattern of WO3 NPs. The powder
(H2WO4) was used as a tungsten source whereas NH4OH was XRD pattern showed several intense and sharp diffraction peaks
used as the precipitating agent. CTAB was used as surfactant to at 2q values of 22.8ꢀ, 23.5ꢀ, 24.06ꢀ, 33.2ꢀ and 34.4ꢀ (2q), which
prevent agglomeration and in the formation of uniformly sized corresponds to the primitive orthorhombic phase of WO3
nanoparticles. The calcined WO3 NPs were then characterized (JCPDS no. 89-4480). The XRD analysis was also carried out for
by FT-IR, XRD, SEM and EDX analysis which conrmed the WO3-bonded imidazolium sulfonic acid (Fig. 2(b)). The XRD
formation of WO3 nanoparticles. For the preparation of nano patterns are consistent with standard data pattern of WO3 NPs
WO3 supported IL, at rst imidazole and (3-chloropropyl)trie- conrming retention of the orthorhombic phase during the
thoxysilane were reacted in toluene under reux conditions to functionalization process. An additional broad peak was
produce I. With the release of a molecule of HCl, white observed in the range of 2q equals to 18–27ꢀ which can be
precipitate I transformed to II. Subsequently with the addition attributed to the presence of amorphous silica shell formed
of WO3 NPs under reux conditions, product III was formed. around the nanoparticles.41
Scheme 1 Schematic representation of the formation of nano-WO3-bonded imidazolium-sulfonic acid chloride (WO3-IL).
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Fig. 1 FT-IR spectra of (a) nano WO3 particles, (b) nano-WO3-bonded imidazolium-sulfonic acid chloride.
Fig. 2 Powder XRD patterns of (a) nano WO3 particles, (b) nano-WO3-bonded imidazolium-sulfonic acid chloride.
Fig. 3 shows the SEM images of as synthesized pure WO3 nanoparticles was retained during the functionalization
NPs, where uniform distribution of the nanoparticles was process. BET analysis of as synthesized catalyst showed high
observed. The micrographs of the catalyst showed that the WO3 surface area and pore volume of 47.25 m2 gꢁ1 and 0.133 cm3
particles are of hexagonal shape. Fig. 4 shows SEM images of gꢁ1 respectively.
WO3 supported imidazolium sulfonic acid. It can be seen from
the images that there is a thick layer of the IL coated over the
3.2 Catalytic performance of nano WO3-bonded
imidazolium-sulfonic acid chloride in the oxidation of
cyclohexene to adipic acid
surface of the WO3 nanoparticles, conrming its successful
functionalization. HR-SEM images (Fig. 4(c) and (d)) clearly
exhibit thick layers of the IL coated over the surface of WO3
nanoparticles.
EDX spectra of the WO3 NPs and WO3-IL catalyst shows the
elemental composition (Fig. 5). Fig. 5(a) indicates the presence
of elements W and O in as-synthesized WO3 NPs. However, EDX
spectrum of the WO3-IL catalyst indicates the presence of W, O,
Si, C, S, N and Cl (Fig. 5(b)). These observations further
conrmed the successful functionalization of the WO3 nano-
particles with imidazolium sulfonic acid chloride.
Aer successful synthesis and full characterization of nano
WO3-bonded imidazolium-sulfonic acid chloride (WO3-IL), its
catalytic activity was examined in oxidation of cyclohexene to
adipic acid and a variety of other cyclic C6–C8 alcohols and
ketones to their corresponding dicarboxylic acids using H2O2 as
a green oxidant. In order to get best results, several reaction
parameters such as amount of catalyst, reaction time and
temperature were varied in which cyclohexene was chosen as
the model substrate. It was observed from the optimization
studies that best results were obtained when 50 mg of catalyst
and 1 : 12 equiv. of substrate to oxidant ratio were used at
Fig.
6 displays TEM images of nano WO3-bonded
imidazolium-sulfonic acid chloride. From the TEM image
(Fig. 6(b)), it is seen that the catalyst is hexagonal like particles
with IL coated to its surface in accordance with the XRD and
SEM ndings. The spacing of the lattice fringes was found to
be separated by 0.240 nm, possibly due to the (200) plane of
WO3, while ED patterns conrmed the polycrystalline nature
of the nanocatalyst. From these observations, it can be
concluded that the crystalline orthorhombic phase of the
ꢀ
a reaction temperature of 90 C (Table 1). No further improve-
ment in yield was observed on changing the concentration of
oxidant or catalyst. The catalyst did not show any activity at
room temperature but with increase in temperature, the yield
gradually increased. At 90 ꢀC, we found that the formation of the
product was maximum, while further rise in temperature (up to
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Fig. 3 SEM images of synthesized WO3 NPs (a and b) low magnification, (c and d) high magnification.
Fig. 4 SEM images of nano-WO3-bonded imidazolium-sulfonic acid chloride (a and b) low magnification, (c and d) high magnification.
ꢀ
120 C), led to decrease in the yield. This may be due to rapid cyclohexene. But CH3CN has provided best results in terms
decomposition of H2O2 above 90 ꢀC takes place.42 We have tried of yield, selectivity and reaction temperature. Moreover,
a few solvents like DMF, CH2Cl2 and CH3CN for the oxidation of combination of CH3CN and H2O2 is already reported to be green
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Table 1 Effect of reagent and catalyst amounts on oxidation of
cyclohexenea to AA
Substrate
(equiv.)
Catalyst
(mg)
Oxidant
(equiv.)
Time
(h)
Temp
(ꢀC)
Yieldb
(%)
Entrya
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
20
50
60
10
20
50
50
—
12
12
14
10
10
12
12
12
8
8
8
8
8
8
8
24
90
90
90
90
90
rt
90
97
93
91
90
—
79
65
120
rt
a
Reaction conditions: cyclohexene (1 equiv.), catalyst (50 mg), oxidant
(12 equiv.), acetonitrile (10 ml), temp. (90 ꢀC), time (8 h), b Isolated yield.
To ascertain the role of catalyst, as prepared WO3 NPs was
used as catalyst in the oxidation of cyclohexene to adipic acid. It
was notable that WO3 NPs showed minimal conversion of
adipic acid. Further imidazolium sulfonic acid chloride was
also prepared separately and used as catalyst where no
conversion was observed at all. From these observations, it is
evident that WO3 NPs and imidazolium sulfonic acid chloride
independently failed to carry out catalytic oxidation of cyclo-
hexene to AA, however when IL support on WO3 NPs was
synthesized and evaluated as catalyst, its efficiency for the
Fig. 5 EDX spectrum of (a) synthesized WO3 NPs, (b) nano-WO3-
bonded imidazolium-sulfonic acid chloride (WO3-IL).
oxidizing system because interaction of H2O2 with CH3CN is
believed to generate peroxycarboxyimidic acid which a very
good oxygen donor species.43
Fig. 6 (a and b) TEM images of nano-WO3-bonded imidazolium-sulfonic acid chloride (WO3-IL) (c) HR-TEM, (d) ED pattern of the synthesized
nanocatalyst.
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oxidation increased signicantly. Hence, it can be concluded results are summarized in Table 2. It was observed that oxida-
that there exists a synergy between IL and WO3 NPs which tion of ketone is comparatively faster than alcohol. This might
enhances the performance of the catalyst.
be explained by the fact that in the rst step alcohol forms the
It is worth noting that the nano WO3-bonded imidazolium- intermediate ketone, which is then further oxidized to acid. In
sulfonic acid chloride had a better dispersion due to the pres- order to provide conrmation about the stability of the
ence of the supported IL compared with those without the IL synthesized catalyst aer its treatment with 30% H2O2 at 90 ꢀC,
support, which made it more suitable for the reaction condi- we have conducted TGA of the catalyst. It is clearly seen from the
tions.39 To generalize the scope of this catalytic protocol, a few TGA curve that the catalyst is stable with about 2–3% weight loss
other cyclic C6–C8 alcohols and ketones were subjected to observed up to 100 ꢀC (Fig. S4 in ESI†). This very little decrease
oxidation under the optimized reaction conditions and the in the weight may be due to loss of loosely attached H2O
Table 2 Oxidation of cycloalkenes, cycloalkanols and cycloalkanones by nano-WO3-bonded imidazolium-sulfonic acid chloride using 30%
H2O2
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Table 3 Comparison of the present catalyst with other reported
catalyst in literature
Time Temp Yield/conversion
Entry Catalyst
Oxidant (h)
(ꢀC) (%)
Ref.
1
2
Na2WO4
WO3
H2O2
H2O2
8
9
90
90
93 yield
79 yield
4
21
nanorods
Ti–AlSBA
H2WO4/TS-1 H2O2
3
4
TBHP
24
—
rt
90
84 yield
GC 31
26
32
conversion
GC 87 yield
GC 94 yield
GC 85 yield
GC 92 yield
GC 90 yield
81 yield
5
6
7
8
9
10
11
H2WO4
Ag/WO3 NPs H2O2
Ag2WO4-IL
InCl3
RuCl3
H2O2
20
18
18
90
90
75
90
rt
33
42
46
47
48
49
Present
work
H2O2
t-BuOOH 9
NaIO4
KHSO5
0.5
24
8
RuPcS
Nano-WO3-IL H2O2
20
90
97 yield
Fig.
7
Catalytic activity and reusability of nanostructured WO3-
bonded imidazolium-sulfonic acid chloride catalyst against number of
cycles.
molecules. Moreover, there are works already reported where
nanostructured WO3 has been shown to be quite stable to H2O2
at higher temperature (or upto 100 ꢀC).42 Likewise numerous
reports on the thermal stability of imidazolium IL catalyst
(above 100 ꢀC) are reported.44,45 In addition to that, SEM results
of the spent catalyst showed an almost similar morphology to
that of the fresh catalyst, indicating that the catalyst sustained
the reaction conditions (Fig. S5 in ESI†). All these analytical
studies of the catalyst and relevant information obtained from
the literature for similar compounds unambiguously suggest
that the catalyst is quite stable towards treatment of 30% H2O2
at the reaction temperature for different reaction times.
A comparison between our present work and some of the
previously reported works have been incorporated in Table 3.
The comparison clearly reveals that the catalytic reaction
described herein is better than some of the most efficient
catalytic route for the synthesis of adipic acid in terms of yield
and reaction time. From the comparison, it is also observed that
Ru based catalyst seems to be most efficient catalyst in per-
forming the oxidation. However, the major drawback of the
catalyst is its high cost and non-recoverability. It is pertinent to
say that easy recovery, synthesis and recyclability of our present
catalyst adds to its advantages over other contemporary
catalysts.
of the fresh catalyst whereas EDX pattern conrmed the pres-
ence of elements W, O, Si, C, S, N and Cl (Fig. S5 in ESI†). These
ndings truly contribute to the heterogeneity and easy reus-
ability of our synthesized catalyst.
4. Conclusions
In summary, WO3 nanoparticles were successfully synthesized
by precipitation method. The synthesized WO3 NPs were then
covalently graed with imidazolium sulfonic acid chloride to
produce the novel nano WO3 bonded IL catalyst. The catalyst
was characterized by XRD, FT-IR, SEM, EDX, TEM and BET
analysis. All the characterization results conclusively indicated
the formation of nano WO3 supported imidazolium sulfonic
acid chloride. This nano WO3 bonded IL compound was used as
a recyclable catalyst for oxidation of cyclohexene to adipic acid.
Synergy between WO3 NPs and ionic liquid has also been
established in its catalytic studies. Cost-effectiveness, enhanced
activity and stability of as-synthesized catalyst are some of the
salient advantages. The catalyst did not show any signicant
loss in its activity even aer 5 cycles of reuse, conrming the
true heterogeneity of the catalyst.
3.3 Reusability of catalyst
In recent times, the extensive use of new recyclable catalyst has
been the subject of immense interest in the eld of heteroge-
neous catalysis. Aer completion of the reaction, the solid
catalyst was ltered off from the reaction mixture followed by
washing with ethanol and centrifugation for several times. The
recovered catalyst was then reused under the optimized condi-
tions for multiple cycles to check its catalytic efficiency. From
Fig. 7, it can be observed that the selectivity remains almost
constant, whereas there is a slight decline in the activity of the
recovered catalyst with each catalytic run. The SEM image of the
spent catalyst showed an almost similar size and shape to that
Acknowledgements
The authors acknowledge SAIF IIT Bombay, SAIF NEHU, Shil-
long and SAIF, Gauhati University for providing analytical
results of the samples. The Director, NIT Silchar is acknowl-
edged for nancial support (through STIS project sanction No.
PA/254/23093-96).
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