Efficient and convenient oxidation of cyclohexene to adipic acid with H2O2 catalyzed by H2WO4 in acidic ionic liquids

Article abstract of DOI:10.1007/s11696-017-0303-8

A simple, efficient, and eco-friendly catalytic system for the oxidation of cyclohexene to adipic acid with H₂O₂ catalyzed by H₂WO₄ in Br?nsted acidic ionic liquids under solvent-free conditions has been developed. Reaction conditions such as the catalysts, the types of anions and cations for Br?nsted acidic ionic liquids, reaction temperature, and the amount of hydrogen peroxide, were investigated. Moreover, the Hammett acidity functions (H₀) of Br?nsted acidic ionic liquids were determined using UV–visible spectrophotometry. The optimum reaction condition identified was n(H₂WO₄):n(Br?nsted acidic ionic liquids):n(cyclohexene):n(H₂O₂)?=?0.02:0.02:1:4.4, and the yield of adipic acid was 96% under the reaction scale of 10?mmol. The catalytic system can be easily recovered and reused for four reaction runs without significant loss of catalytic activity. Simple operation of the catalyst system and avoidance of the emission of nitrous oxide are the benefits of this work.

Full text of DOI:10.1007/s11696-017-0303-8

Chem. Pap.
DOI 10.1007/s11696-017-0303-8
ORIGINAL PAPER
Efficient and convenient oxidation of cyclohexene to adipic acid
with H2O2 catalyzed by H2WO4 in acidic ionic liquids
Bingtong Wang1,2 Zongbin Zhang1,2 Xinyu Zhang1,2 Shixin Sun1
•
•
•
•
Lin Wu1 Rong Xing1
•
Received: 26 July 2017 / Accepted: 21 September 2017
Ó Institute of Chemistry, Slovak Academy of Sciences 2017
Abstract A simple, efficient, and eco-friendly catalytic
system for the oxidation of cyclohexene to adipic acid with
H2O2 catalyzed by H2WO4 in Brønsted acidic ionic liquids
under solvent-free conditions has been developed. Reaction
conditions such as the catalysts, the types of anions and
cations for Brønsted acidic ionic liquids, reaction tempera-
ture, and the amount of hydrogen peroxide, were investigated.
Moreover, the Hammett acidity functions (H0) of Brønsted
acidic ionic liquids were determined using UV–visible spec-
trophotometry. The optimum reaction condition identified
was n(H2WO4):n(Brønsted acidic ionic liquids):n(cyclohex-
ene):n(H2O2) = 0.02:0.02:1:4.4, and the yield of adipic acid
was 96% under the reaction scale of 10 mmol. The catalytic
system can be easily recovered and reused for four reaction
runs without significant loss of catalytic activity. Simple
operation of the catalyst system and avoidance of the emis-
sion of nitrous oxide are the benefits of this work.
Introduction
Adipic acid, also referred to as hexanedioic acid, is a very
important raw material in the synthesis of chemically
industrial and commercial products such as nylon-6,6
fibers, polyurethanes, plasticizers, adiponitriles, low-tem-
perature synthetic lubricants, etc. (Ghosh et al. 2014; Shang
et al. 2016). The traditional industrial synthesis method of
adipic acid is the oxidation of a mixture of cyclohexanol
and cyclohexanone (so-called KA oil) using nitric acid as
´
oxidation(Van de Vyver and Roman-Leshkov 2013; Shang
et al. 2015; Usui and Sato 2003), which leads to a large
amount of nitrous oxide production during the oxidation
process. It is generally known that nitrous oxide is one of
the greenhouse gases with 120 year life-time in atmosphere
and high heat trapping effect (about 310 times more
powerful than CO2 per molecule). It also causes ozone
depletion and acid rain expected to be the dominant ozone-
depleting substance emitted in the 21st century (Vafaee-
zadeh and Mahmoodi Hashemi 2015; Wang et al. 2011).
Therefore, the development of energy-saving and eco-
friendly processes for the synthesis of adipic acid is
expected.
Keywords Acidic ionic liquid Á H2WO4 Á Adipic acid Á
Solvent-free oxidation
Hydrogen peroxide (H2O2) is an economical, environ-
mentally friendly and atom-efficient oxidant owing to
production of only water during the oxidation processes
(Jones 1999; Zhang et al. 2011; Jiang et al. 2016). So far,
many routes have been developed for synthesis of adipic
acid with H2O2 coupled with ionic liquids (Vafaeezadeh
and Mahmoodi Hashemi 2014; Vafaeezadeh et al. 2012;
Vafaeezadeh and Hashemi 2013), H2WO4/H2SO4/H3PO4
Electronic supplementary material The online version of this
article (doi:10.1007/s11696-017-0303-8) contains supplementary
material, which is available to authorized users.
& Rong Xing
xingrr@126.com
1
College of Chemical and Environmental Engineering,
Yancheng Teachers University, Kaifang Dadao Rd. 50,
Yancheng 224002, Jiangsu, China
¨
mixture (Wen et al. 2012), mesoporous oxides (Bohstrom
et al. 2010), surfactant-type peroxotungstate and peroxo-
molybdates (Zhu et al. 2008), MIL-101 metal–organic
framework (Saedi et al. 2012), complexes derived from
2
College of Chemistry and Chemical Engineering, Nanjing
Tech University, Nanjing 210009, Jiangsu, China
123

Chem. Pap.
Preparation of Brønsted acidic ionic liquid
heteropoly acid and glycine (Ren et al. 2009), and oxo-
tungsten-containing SBA-15 (Cheng et al. 2007),
respectively.
The Brønsted acidic ionic liquid (BAIL) was synthesized
according to the procedures reported in the literature (Liu
et al. 2013, Bhandari et al. 2010).
Ionic liquids (ILs) have received increasing attentions for
their unique properties, such as negligible volatility, unique
permittivity, high thermal stability, catalytic ability, and good
solvents for both organic and inorganic reagents (Han et al.
2015; Li et al. 2014; Chen et al. 2015; Fang et al. 2011; Xun
et al. 2016; Karakulina et al. 2016). Based on these properties,
they usually serve as reaction media in a wide range of organic
reactions. From the viewpoints of practical and fundamental
research, functionalization of ILs with various groups has
been attempted with particular interest in catalysis. Brønsted
acidic ionic liquid (BAIL) is one of the functionalized ionic
liquids. The acidity of BAILs consists of two parts: (1) the
sulfonic group (–SO3H) which covalently bonded to the IL
and (2) a Brønsted acidic counter anion of the IL like bisulfate
(HSO4-) or dihydrogen phosphate (H2PO4-) (Vafaeezadeh
and Alinezhad 2016). According to the literature report, the
catalytic activity of H2WO4 was highly dependent on the
acidity of the reaction medium (Wen et al. 2012). Therefore,
BAILs was chosen asthe reactionmediumfor the oxidation of
cyclohexene to adipic acid.
First, 0.3 mol of N-methylimidazole (0.3 mol triethy-
lamine, 0.3 mol of pyridine, 0.15 mol of N,N,N0,N0-te-
tramethyl-1,3-propyldiamine or 0.15 mol of 4,40-
bipyridine) was dissolved in 100 mL of toluene, and
0.3 mol of 1,3-propyl sultone in 20 mL of toluene was
slowly added to the above mixture solution, which was
stirred in ice bath for 20 min. Then, the mixture solution
was heated to 50 °C for 24 h and was cooled down to room
temperature. The reaction mixture was filtered. The col-
lected precipitate was washed three times with ethyl acetate
(50 mL), followed by heating of the solution under vacuum
at 80 °C to remove the remaining solvent; the ionic liquid
(IL) was obtained. Second, a stoichiometric amount of
H2SO4 aqueous solution (the BAIL1, BAIL2 and BAIL3
with H2SO4/IL molar ration = 1:1, the BAIL4 and BAIL5
with H2SO4/IL molar ration = 2:1) was added into the
above IL and stirred at 90 °C for 5 h. The viscous colorless
liquid of BAILs was obtained by rotary distillation to
remove the water and vacuum drying. The mass yield of
BAIL was about 93%. The mass isolated of obtained
BAIL5 was 79.5 g. The structure and name of BAILs are
shown in Scheme 1.
In the present work, we report an efficient and envi-
ronmentally friendly route for synthesis of adipic acid
catalyzed by H2WO4 with H2O2 in BAILs, which can
not only possess the significant ability to solubilize
organic and inorganic reagents and form the homoge-
neous reaction systems, but also provide large number of
acid sites during the catalytic processes. The present
method has several advantages, such as simple and safe
manipulation, recycled catalytic system, organic solvent-
free and halide-free system, and no N2O produced
compared to the traditional method of nitric acid
oxidation.
General procedure for oxidation of a cyclohexene
to adipic acid
A mixture of 0.2 mmol of H2WO4, 44 mmol of 30% H2O2
and 0.2 mmol of BAIL was added into a 50 mL round-
bottomed flask with a reflux condenser and stirred at room
temperature for 30 min to prepare the peroxytungstate
catalyst. Then, 10 mmol of cyclohexene was added and the
mixture was heated with vigorous stirring at 73 °C for 5 h,
and then at 87 °C for 7 h. After the reaction completion,
the reaction solution was cooled to room temperature and
then placed in the refrigerator at 4 °C for 12 h. The adipic
acid product was collected by filtration from the aqueous
solution as a colorless crystalline solid. The solid was
washed with three small portions of cold water and ethyl
acetate. Drying the solid under vacuum at 60 °C gave
1.397 g of adipic acid. The melting point of the isolated
adipic acid was 151.6 °C.
Experimental
General
All chemicals and solvents were purchased from the
Sinopharm Chemical Reagent Co., Ltd and Aladdin, and
1
used without further purification. H NMR spectra were
recorded on a Bruker 400 MHz spectrometer using D2O or
DMSO as the solvent. All the products were identified by
Agilent 7890B-7000C gas chromatography–mass spec-
trometry with HP-5-MS capillary column. Melting points
were determined on a WRS-2 microscope melting appa-
ratus and reported uncorrected.
Reusability of the catalyst
The aqueous solution containing the remaining catalyst and
BAIL obtained from the previous reaction was concen-
trated with rotary evaporator to obtain an oily mixture, and
then washed with diethyl ether (2 9 5 mL) and charged
123

Chem. Pap.
N
SO3H
SO3H
N
N
N
SO3H
[X-]
-
-
[HSO4 ]
[HSO4 ]
BAIL2
BAIL3
BAIL1a (X=HSO4)
BAIL1b (X=Cl)
BAIL1c (X=NO3)
BAIL1d (X=H2PO4)
BAIL1e (X=HOOCCOO)
SO3H
N
N
SO3H
N
N
SO3H
SO3H
-
2[HSO4 ]
BAIL5
-
2[HSO4 ]
BAIL4
Scheme 1 Structure of synthesized Brønsted acidic ionic liquid
with cyclohexene and 30% H2O2. The next run was carried
out under optimized reaction conditions.
provide more protons than Na2WO4. The protons were
beneficial to the reaction of the substrates. On the basis of
the above results of the experiment, H2WO4 was the best
catalyst among the four catalysts in terms of yield which
varied with catalytic materials, and chosen as the model
catalyst for further investigation.
Results and discussion
We first investigated the catalytic activities of various
catalysts in the oxidation cleavage of cyclohexene to adipic
acid using H2O2 as the oxidant in BAIL 1 (Table 1, entries
1–4). H4[Si(W3O10)4]ÁxH2O and H3PW12O40Á5H2O used as
a catalyst in the oxidation reaction, respectively, which did
not show any appreciably catalytic performance in com-
parison with H2WO4, which gave a yield of 85%. It is
speculated that the high catalytic performance of the
H2WO4 is due to significant ability to form easily mono-
protonated peroxotungstate species (see the following
mechanism), which was the active catalytic species formed
in the acidic media (Noyori et al. 2003). H2WO4 also could
In the reaction system, the BAILs act as acidic pro-
moters and phase transfer agents. Therefore, the perfor-
mance of BAILs displays an important influence on the
catalytic activity. First, the effect of anions in BAILs was
investigated for oxidation of cyclohexene to adipic acid.
The results are summarized in Table 1. The anions of the
BAILs including NO3-, Cl-, H2PO4 and HOOCCOO-
-
used in the oxidation reaction showed relatively low cat-
alytic performance in comparison with HSO4-. It is spec-
-
ulated that HSO4 could provide stronger acidity, which
was beneficial to the oxidation of cyclohexene to adipic
acid. The experimental results suggested that the optimum
anion was HSO4-. In the second step, the effect of cations
in BAILs was studied for oxidation of cyclohexene to
adipic acid. The results are summarized in Table 2. To our
delight, all of the BAILs showed good catalytic activity for
oxidation of cyclohexene to adipic acid, in which the
BAIL5 had the highest catalytic activity and gave a yield
of 96%. The above results suggested that BAIL5 was
optimal for the oxidation reaction.
Table 1 Effect of different catalysts and ILs
Entry
IL
Catalyst
Yielda/%
1
2
3
4
5
6
7
8
BAIL 1a
BAIL 1a
BAIL 1a
BAIL 1a
BAIL 1b
BAIL 1c
BAIL 1d
BAIL 1e
H2WO4
85
32
53
18
70
35
69
36
H4[Si(W3O10)4]ÁxH2O
H3PW12O40Á5H2O
Na2WO4
To confirm the important role of BAILs, the oxidation
reaction of cyclohexene was carried out using H2WO4 as a
catalyst under ionic liquids free and 1,3-dibutylimida-
zolium bromide ([BBIM]Br), respectively (Table 2, entries
6 and 7), which both gave low yield of adipic acid. The
high catalytic performance of BAILs was probably due to a
large amount of Brønsted acidic sites which was necessary
for the oxidation reaction of cyclohexene (Vafaeezadeh
and Mahmoodi Hashemi 2015). Based on this point, the
Brønsted acidity was evaluated from the determination of
H2WO4
H2WO4
H2WO4
H2WO4
Reaction conditions: H2WO4, BAIL, cyclohexene, and H2O2 in a
molar ratio of 0.02:0.02:1:4.4
All of the reaction mixtures were carried out at 73 °C for 5 h and then
87 °C for 7 h
a
Isolated yield of adipic acid
123

Chem. Pap.
Table 2 Effect of different ILs
Yielda/%
4 \ BAIL 1a \ BAIL 2 \ BAIL 3. Moreover, the Ham-
mett function (H0) could be calculated, and the results are
listed in Table 3. Based on H0 values, Brønsted acidic
character of BAILs increased in the order of BAIL 5
(0.68) [ BAIL 4 (0.74) [ BAIL 1a (0.79) [ BAIL 2
(0.82) [ BAIL 3 (0.84), suggesting that the highest cat-
alytic performance of BAIL5 was due to the relatively
stronger Brønsted acidity in comparison with the other four
BAILs used in oxidation reaction, which was in agreement
with the literature reported (Vafaeezadeh and Mahmoodi
Hashemi 2015). They implied that the oxidative cleavage
of cyclohexene was not only an oxidation reaction, but also
an acid-catalyzed reaction (Jin et al. 2011).
Entry BAILs
1
2
3
4
5
6
7
BAIL 1a
BAIL 2
BAIL 3
BAIL 4
BAIL 5
–
85
81
81
90
96
18
[BBIM]Br 47
The reaction mixtures were
carried out at 73°C for 5 h and
then 87°C for 7 h
Reaction conditions: H2WO4,
BAIL, cyclohexene, and H2O2
It is well known that the reaction temperature influences
greatly the reaction rate and the catalytic properties of the
catalyst. As shown in Table 4, the oxidation reaction did
not occur at room temperature, indicating the catalytic
system was catalytically inactive (Table 4, entry 1). When
the oxidation reaction was carried out at the 73 °C for 5 h,
and then at 87 °C for 7 h, the yield of adipic acid was 96%.
The main byproducts of oxidation of cyclohexene were
1,2-cyclohexandiol and pentanedioic acid, which detected
by GC–MS. The yield of 1,2-cyclohexandiol and pen-
tanedioic acid were about 0.9 and 2.2%, respectively. The
thermal gravity analysis showed that the BAILs had high
thermal stability (Figs. S3–7).
in
a
0.02:0.02:1:4.4
molar
ratio
of
a
Isolated yield of adipic acid
the Hammett acidity. The Hammett acidity functions (H0)
were determined using UV–visible spectrophotometer with
a basic indicator (denoted as I) according to the literature
reported previously (Wang et al. 2008; Thomazeau et al.
2003; Cui et al. 2010). The indicator and protons can
interact rapidly to form protonated indicator (denoted as
IH?) when BAILs were added into indicator solution.
Then, the UV absorbance of the protonated indicator will
decrease. Therefore, the [I]/[IH?] ration could be deter-
mined from the measure of UV absorbance, and then, the
Hammett function was calculated using Eq. (1) according
to the measurable ratio [I]/[IH?]:
Based on the previous literature (Sato 1998), a possible
mechanism for the oxidation of cyclohexene to adipic acid
is proposed (Scheme 2). In this oxidation reaction, the
catalyst H2WO4 was oxidized by H2O2 and then formed the
peroxotungstate species in the acidic media. The process of
cyclohexene oxidation has undergone three kinds of oxi-
dation reactions of olefin epoxidation, 2-diol oxidations,
and Baeyer–Villiger oxidation, which included four oxi-
dation steps. The oxidation reaction pathway of cyclo-
hexene to adipic acid is shown in Scheme 2. The amount of
H2O2 applied in the oxidation reaction was four times the
amount of cyclohexene, and a vital factor influencing the
progress of cyclohexene oxidation. However, the molar
H0 ¼ pKðIÞaqþ logð½IŠ=½IHþŠÞ:
ð1Þ
As shown in Fig. 1, concerning the effect of Brønsted
acidity of BAILs, the absorbance of the protonated form of
the indicator decreased in the order of BAIL 5 \ BAIL
1.5
1.2
Blank
BAIL 3
BAIL 2
Table 3 Calculation and comparison of H0 values of different ILs in
H2O (20 °C)
0.9
BAIL 1a
BAIL 4
BAIL 5
Entry
ILs
Amax
[I] (%)
[IH] (%)
H0
0.6
1
2
3
4
5
6
–
1.486
0.583
0.605
0.619
0.538
0.495
100
0
–
BAIL 1a
BAIL 2
BAIL 3
BAIL 4
BAIL 5
39.2
40.7
41.6
36.2
33.3
60.8
59.3
58.4
63.8
66.7
0.79
0.82
0.84
0.74
0.68
0.3
0.0
300
400
500
600
Wavelength (nm)
4-nitroanline (0.145 mmol/L, pKa = 0.99), ILs (32 mmol/L) in H2O
Fig. 1 Absorption spectra of 0.145 mmol/L of 4-nitroaniline for
various ionic liquids in H2O
123

Chem. Pap.
Table 4 Optimization of
oxidation conditions
Yielde/%
Table 5 Effect of the molar ratio of BAIL 5/cyclohexene on the
oxidation reaction of cyclohexene to adipic acid
Entry
BAILs
1
2
3
a
BAIL 5a,b
BAIL 5a,c
BAIL 5c,d
0
96
86
Entry
n(BAIL 6)/n(Cyclohexene)
Yield (%)
1
2
3
4
5
0
17
64
96
89
88
0.01
0.02
0.04
0.06
Reaction conditions: H2WO4,
BAIL, cyclohexene, and H2O2
in
0.02:0.02:1:4.4
a
molar
ratio
of
b
The reaction mixtures were
carried out at room temperature
for 24 h
Reaction conditions: H2WO4, cyclohexene, and H2O2 in a molar ratio
of 0.02:1:4.4
c
The reaction mixtures were
carried out at 73 °C for 5 h and
then 87 °C for 7 h
Reaction conditions: H2WO4,
BAIL, cyclohexene, and H2O2
in a molar ratio of 0.02:0.02:1:6
d
as the molar ratio of BAIL 5/cyclohexene was increased
from 0 to 0.02, the yield of the adipic acid increased sig-
nificantly and reached the highest (Table 5, entries 1–3).
With further increase of the molar ratio of BAIL 5/cyclo-
hexene from 0.02 to 0.06, the yield of adipic acid decreased
slightly (Table 5, entries 4 and 5). This result is probably
due to a large amount of BAIL 5 in the oxidation reaction,
which diluted the H2O2 and further limited to form active
catalytic species of monoprotonated peroxotungstate spe-
cies. The results showed that the optimum molar ratio of
BAIL 5/cyclohexene was 0.02.
e
Isolated yield of adipic acid
ratio of H2O2/cyclohexene increased from 4.4 to 6, and the
yield of adipic acid decreased slightly (Table 4, entries 2
and 3). This indicated that the excess amount of H2O2 and
catalyst could cause side effects.
We further investigated effect of the molar ratio of
BAIL 5/cyclohexene, which can significantly affect the
yield of adipic acid in the oxidation reaction of cyclohex-
ene. The results are summarized in Table 5. As expected,
As shown in Table 6, influence of the H2WO4/cyclo-
hexene molar ratio was also studied. The oxidation of
cyclohexene to adipic acid reaction did not occur without
Scheme 2 Reaction pathway
for oxidation of cyclohexene to
adipic acid
H2WO4, H2O2
COOH
COOH
BAIL
OH
O
O
COOH
COOH
OH
O
H2O,H+
OH
H2O,H+
OH
O
O
O
O
N
N
SO3H
HO3S
-
2[HSO4 ]
H2O2
O
O
H2O
O
O
O
O
O
O
W
W
O
OH
H2O
HO
123

Chem. Pap.
Table 6 Effect of the molar ratio of H2WO4/cyclohexene on the
oxidation reaction of cyclohexene to adipic acid
Conclusion
In summary, a durable, recyclable, and highly efficient
catalytic oxidation system with H2WO4, BAILs, and H2O2
has been developed, which could effectively oxidize
cyclohexene to adipic acid under mild reaction conditions,
such as atmospheric pressure, as well as solvent-free and
halide-free conditions. The present method is green and can
eliminate emission of nitrous oxide without danger in
comparison with the nitric acid oxidation pathway. More-
over, the catalytic system may offer many advantages, such
as simplicity of operation, high yields, stability, and easy
isolation of products.
Entry
n(H2WO4)/n(Cyclohexene)
Yield (%)
1
2
3
4
5
0
0
59
96
77
38
0.01
0.02
0.03
0.04
Reaction conditions: BAIL 5, cyclohexene, and H2O2 in a molar ratio
of 0.02:1:4.4
catalyst (Table 6, entry 1). The yield of the adipic acid was
the highest when the molar ratio of H2WO4/cyclohexene
was 0.02 (Table 6, entry 3). When the molar ratio further
increased, the yield of adipic acid decreased obviously
(Table 6, entries 4 and 5). The result indicated that an
excess amount of the catalyst led to side effects in the
oxidation reaction. The optimum molar ratio of H2WO4/-
cyclohexene was 0.02.
Acknowledgements This work was supported by the Qian-Lan
Project of Jiangsu Province, the Natural Science Foundation of
Jiangsu Province (No. BK20140460), the University Science
Research Project of Jiangsu Province (No. 17KJD150008) and the
Excellent Specialties Program and Development of Jiangsu High
Education Institutions (PPZY2015B113).
From the viewpoint of practical application, recycla-
bility of the catalyst is very important. The recycling
experiments of the oxidation reaction of cyclohexene were
performed under the optimized reaction conditions. As
shown in Fig. 2, the H2WO4 and BAIL 5 catalytic system
could conveniently recycled at least four runs without
appreciable loss in catalytic activities. The catalytic system
still gave a yield of 89% after the fourth recycles. These
results suggested that the catalytic system was recyclable
and stable for oxidation reactions of cyclohexene.
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