Simple and green oxidation of cyclohexene to adipic acid with an efficient and durable silica-functionalized ammonium tungstate catalyst

Article abstract of DOI:10.1016/j.catcom.2013.10.001

A novel silica-functionalized ammonium tungstate interphase catalyst has been reported as a non-nitric acid route for adipic acid production from one-pot oxidative cleavage of 30% hydrogen peroxide and catalytic amounts of p-toluenesulfonic acid (PTSA). The catalyst has been simply prepared by commercially available starting material. The structure of the catalyst has been investigated using FT-IR spectroscopy, atomic absorption, TEM, SEM and XRD analysis. The catalyst has shown good to high activity even up to 10 runs of reaction. Simple preparation of the catalyst, avoids using harmful phase transfer catalyst (PTC) and/or chlorinated additives are among the other benefits of this work.

Full text of DOI:10.1016/j.catcom.2013.10.001

Catalysis Communications 43 (2014) 169–172
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Simple and green oxidation of cyclohexene to adipic acid with an efficient
and durable silica-functionalized ammonium tungstate catalyst
Majid Vafaeezadeh, Mohammad Mahmoodi Hashemi ⁎
Department of Chemistry, Sharif University of Technology, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 August 2013
Received in revised form 28 September 2013
Accepted 2 October 2013
Available online 11 October 2013
A novel silica-functionalized ammonium tungstate interphase catalyst has been reported as a non-nitric acid
route for adipic acid production from one-pot oxidative cleavage of 30% hydrogen peroxide and catalytic
amounts of p-toluenesulfonic acid (PTSA). The catalyst has been simply prepared by commercially available
starting material. The structure of the catalyst has been investigated using FT-IR spectroscopy, atomic absorption,
TEM, SEM and XRD analysis. The catalyst has shown good to high activity even up to 10 runs of reaction. Simple
preparation of the catalyst, avoids using harmful phase transfer catalyst (PTC) and/or chlorinated additives are
among the other benefits of this work.
Keywords:
Adipic acid
Green oxidation
© 2013 Elsevier B.V. All rights reserved.
Hydrogen peroxide
Silica-functionalized ammonium tungstate
N O
2
1
. Introduction
2 2
catalyst (PTC) into the reaction mixture. Although H O is a cheap oxidant
with water as the sole by-product, the relatively expensive applied
quaternary ammonium compounds generally are not environmentally
benign reagents. Another limitation of this process is the cost of the
applied oxidant (H O ) which is relatively higher than O in the
2 2 2
traditional nitric acid oxidation route. However, applying recoverable
catalyst accompanied by environmentally benign protocols would
Adipic acid (1,6–hexanedioic acid) is a very important raw
material which is used in the chemical industries [1]. It is mainly
used in the manufacture of nylon-6,6 as a precursor for synthetic
fibers, tire reinforcements, plastics, urethane foams, elastomers,
and synthetic lubricants [1,2]. The current industrial method for
adipic acid production is oxidation of a cyclohexanone/cyclohexanol
2 2
significantly cover the cost of adipic acid production with H O as
(
so-called KA oil) mixture with nitric acid [1]. Emission of huge volume
of environmentally harmful gas, nitrous oxide (N O), is the major
drawback of this process. N O is a greenhouse gas with 120 years
life-time in atmosphere [3,4]. Owing to the strong IR absorption,
the ability of N O on warming the atmosphere is about 310 times
higher than that of carbon dioxide (CO ) [4]. The nitric oxide (NO)
which is produced from the N O decomposition can destroy ozone
oxidant.
2
In this regard, various methods have been developed to improve the
cyclohexene oxidation reaction such as using of surfactant-type
peroxotungstate and peroxomolybdates [6], H WO /H SO /H PO
2 4 2 4 3 4
mixture [7], ionic liquids [8,9], MIL-101 metal–organic framework
[10] complexes derived from heteropoly acid and glycine [11] and
Ti–AlSBA mesostructured catalysts [12].
2
2
2
2
molecules in a series of reactions. Additionally, N
gas which forms acid rains.
One-step oxidative cleavage of cyclohexene to adipic acid with
H O has been introduced as a green alternative to adipic acid [5].
2 2
In this method, in order to overcome organic starting material
and aqueous oxidant incompatibility and accelerate the reaction;
quaternary ammonium salt has been employed as a phase transfer
2
O is a harmful
However, uses of relatively expensive and corrosive reagents,
difficulties in catalyst recovery as well as catalysts degradation and/or
deactivation are among the drawbacks of these methodologies. Hence,
this oxidation reaction still requires further improvement in the point
of reducing hazardous reagents, improving catalyst recovery and
simplifying the overall process.
In the current study, we designed an interphase (organic–inorganic
hybrid) catalyst for oxidation of cyclohexene to adipic acid. The catalyst
comprises alkyl ammonium groups which covalently anchored on
the surface of the silica gel. The surface functionalities provide the
WO2
−
species which is necessary for oxidation. Additionally, the
4
functionalized porous material acts as quasi-homogenous system
with a large area of amphiphilic surface for better mass transfer of
⁎
Corresponding author. Fax: +98 21 66029165.
E-mail address: mhashemi@sharif.edu (M. Mahmoodi Hashemi).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.10.001

170
M. Vafaeezadeh, M. Mahmoodi Hashemi / Catalysis Communications 43 (2014) 169–172
organic starting material and aqueous oxidant. In other words, the
surface-bonded ammonium groups act as heterogenized PTC and
therefore there is no need to use harmful organic additive(s) to
overcome reagent phase incompatibility.
2
. Results and discussion
The catalyst was easily prepared by commercially available starting
materials (Scheme 1). The material A was prepared by grafting the 2-
2-(3-Trimethoxysilylpropylamino)ethylamino] ethylamine precursor
[
onto the high surface area silica gel (see electronic supplementary
information for details of the experimental procedure). Trifluor-
omethanesulfonic acid treatment of A produces surface-bonded
ammonium triflate B. The catalyst 1 is prepared by subsequence anion
exchange of the material B with an aqueous solution of Na
2 4 2
WO .2H O
at room temperature.
To investigate the structure of the functionalized materials, com-
parative FT-IR spectra of the pure silica, material A and the catalyst 1
are shown in Fig. 1. The top spectrum is for pure silica, while the next
two are for intermediate A and the final catalyst, which show important
differences relative to the silica. The characteristics bands of the A are at
−
1
2
922 and 2855 cm
which is attributed to C–H and N–H stretching
frequencies of the surface alkyl amine functionalities. In the case of
the catalyst 1, a very important characteristic band was observed at
−
1
8
75 cm
proves the presence of the WO
mentioning that the bands around 1470 and 1740 cm
which belongs to the W–O stretching frequency [8,9] and
2
−
Fig. 1. FT-IR spectroscopy of the pure silica, material A and catalyst 1.
4
species in the catalyst. It is worth
−
1
in both of A
and catalyst 1 are attributed to the N–H bending frequencies of the
surface-bonded amine and/or ammonium functionalities.
Atomic absorption analysis was performed to determine the accurate
step, the efficiency of catalyst 1 was investigated for oxidation of
cyclohexene to adipic acid in the presence of 200 mg of HCl and H SO .
2 4
The results indicated that in these cases the corresponding adipic acid
can be obtained in 29% and 47% (Table 1, entries 1 and 2).
The yield of the reaction significantly increases to 64% when p-
toluenesulfonic acid (PTSA) was used (Table 1, entry3). The increase
in the reaction yield may be attributed to better miscibility of the
organic acid into both starting material/intermediates and aqueous
oxidant.
2
−
loading of WO
4
in the catalyst. The result indicates that the loading of
WO2
−
in the catalyst 1 is 1.02 mmol.g
−1
.
4
XRD analysis of the pure silica, material A and catalyst 1 is shown in
Fig. 2. The diffraction peak which appeared at about 25° in all of the
structures is attributed to the amorphous silica gel [13]. This observation
proves that no essential changes in topological structure of the silica gel
occurred during the grafting process, chlorosulfonic acid treatment and
anion exchange of the B. In the case of catalyst 1, the decrease of the
intensity may be interpreted by the formation of hydrogen bond between
Oxidation of the cyclohexane to adipic acid is also a temperature-
dependent reaction with two distinct thermal steps [16]. To improve
the yield of the reaction we considered this issue and the reaction was
performed into two thermal steps. For this purpose, the mixture was
first heated at 73 °C (near the boiling point of the cyclohexene–water
mixture) to convert cyclohexene into 1,2-cyclohexanediol [16]. After
5 h, the flask immediately transfers into the second oil bath which its
temperature was adjusted at 87 °C and was heated for 15 h. In this
condition the pure adipic acid was obtained in 84% yield (Table 1,
entry 4) and 99% selectivity (determined by gas chromatography
(GC)) toward other dicarboxylic acids which might be produced from
WO2
–
and surface ammonium functionalities [14]. Our explanation about
4
the peaks which are observed at nearly 10 and 60° is that they may be
attributed to the silica-bonded ammonium tungstate species in catalyst
1. TEM and SEM of the catalyst are provided in the Supplementary data.
The activity of catalyst 1 was investigated in oxidation of cyclohexene
to adipic acid (Table 1). Typically, conversion of cyclohexene to adipic
acid theoretically needs 4 moles H for four step oxidation
Scheme 2). However, 4.4 moles of hydrogen peroxide was used to
ensure the complete conversion of cyclohexene [5]. Worthy of note is
2
O
2
(
2
−
that the activity of the WO
4
species in oxidation of cyclohexene to adipic
acid strongly depends on the acidity of the reaction media [15]. At the first
2 2
some rearrangement reactions [16]. Increasing the amount of H O to
Scheme 1. Schematic illustration of catalyst 1 preparation.

M. Vafaeezadeh, M. Mahmoodi Hashemi / Catalysis Communications 43 (2014) 169–172
171
monoprotonated peroxotungstate species and thus reduced the
conversion of cyclohexene to adipic acid.
Increasing the amount of the catalyst to 500mg did not significantly
increase the yield of reaction (Table 1, entry 6). Thus the condition
which has been stated in entry 4 of Table 1 which has higher turnover
number (T.O.N) was selected as the optimum reaction condition.
Notably, no noticeable adipic acid was obtained by using either solely
catalyst 1 (Table 1, entry 7) or PTSA (Table 1, entry 8). Furthermore,
neither material A nor B can catalyze the reaction (Table 1, entries 9
and 10).
A proposed mechanism of the reaction is shown in Scheme 2
considering four steps oxidation. In this mechanism, the reaction can
be catalyzed via in situ formation of peroxotungstate species from
WO2
−
in the acidic media [15,17]. At the first oxidation step, cyclohexene
4
is converted to 1,2-epoxycyclohexane (step 1) following formation of
,2-cyclohexanediol (confirmed by GC) in the acidic media. Oxidation
1
of the resulted diol to 2-hydroxy cyclohexanone (step 2) and 7-
hydroxyoxepan-2-one (step 3) are the second and the third oxidation
steps, respectively. The fourth oxidation step leads to produce adipic
anhydride (step 4). The final intermediate undergoes ring opening to
produce adipic acid. Table 1 provides some useful data about the by-
products and intermediates.
The reusability of catalyst 1 was also investigated in direct oxidation of
cyclohexene to adipic acid. In this regard after the reaction termination,
the catalyst was vacuum filtrated and the material is dried in an oven.
Nearly 97% of the initial weighted catalyst could be recovered. The
recovered catalyst was then subjected into another reaction run with
identical conditions which stated at the entry 4 of Table 1. The results
indicated that the catalyst can be recovered at least for five reaction
sequences with an average 82% yield of adipic acid (Fig. 3).
Fig. 2. XRD patterns of pure silica gel, material A and catalyst 1.
According to the FT-IR spectrum, the recovered catalyst after five
runs had no noticeable changes in structure in comparison of initial
prepared catalyst (See Supplementary material).
6
mmol per mol of cyclohexene had negative effect on the reaction yield
(
Table 1, entry 5). This observation can be explained via the effect on the
reaction pH in the higher ratio of hydrogen peroxide/cyclohexene
/CyH). To investigate this issue, the initial pH of the reaction
The loading of tungsten of the five-time recovered catalyst was
2 2
(H O
–1
determined by atomic absorption analysis to be 0.93mmolg . The result
mixture in the optimum condition (Table 1, entry 4) was determined
to be 0.89. According to the literature report [15], the suitable pH for
formation of monoprotonated peroxotungstate species in the reaction
mixture is between pH 0.4 and 3. By increasing the molar ratio (not
concentration) of hydrogen peroxide to H O /CyH = 6, this value
2 2
increased to 1.08. The increase of pH led to decrease of the amounts of
indicates that after 5th reaction run, more than 91% of the WO2
−
species
4
has been retained by the surface ammonium functionalities and a little
but not negligible) catalyst loss of ~9% occurred after 5th reaction run.
(
It is worth mentioning that continuing reusability experiment of the
catalyst up to 10 runs gave moderated yield (63%) of adipic acid at 10th
run of reaction.
3. Conclusion
In summary, an efficient and reusable silica-functionalized ammo-
Table 1
The data of the catalytic performance in direct oxidation of cyclohexene to adipic acid.
nium tungstate catalyst was reported as a non-nitric acid route for
oxidation of cyclohexene to adipic acid using commercially available
reagents. In this method the adipic acid was synthesized using H O as
2 2
Product (%)
Entry Catalyst
Acid
Catalyst 1 HCl
Catalyst 1 SO
CyH conv. (%) T.O.Na Adipic acidb,c,d Diol Other
an oxidant which produces only water as by-product. We showed that
the surface ammonium functionalities strongly retain WO2
−
species.
1
2
3
4
5
6
7
8
9
1
1
95
98
100
100
100
100
10
0
8
29
47
64
24
14
–
42
33
31
11
10
6
4
H
2
4
13
18
24
21
17
–
Additionally, it provided suitable polarity to the surface of the catalyst
for efficient diffusions of both organic starting materials and aqueous
hydrogen peroxide. The catalyst was simply separated and reused for
five reaction runs with an average 82% yield. Because of elimination of
nitrous oxide emission, the presented method is green and significantly
less hazardous than nitric acid oxidation pathway.
Catalyst 1 PTSA
Catalyst 1 PTSA
Catalyst 1 PTSA
Catalyst 1 PTSA
Catalyst 1
–
e
84 (89)
–
75f
–
g
88
–
–
Trace
N.R
N.R
–
–
PTSA
–
–
–
h
Material A PTSA
Material B PTSA
Pure silica PTSA
0
8
–
–
–
h
0
1
–
Trace
–
–
i
Acknowledgment
0
–
N.R
–
–
a
b
c
d
e
f
2−
4
T.O.N (turnover number) refers to moles of adipic acid produced per mol of WO
Yield refers to isolated pure product and gave satisfactory 1H NMR spectra.
.
The authors gratefully acknowledge Sharif University of Technology
research council for financial support of this work.
350 mg of catalyst 1 was used in all experiments, unless otherwise stated.
Reaction time (h)/temperature: entries 1–3: 15/73; entries 4–11: 5/73 then 15/87.
GC yield.
Appendix A. Supplementary data
2 2
Molar ratio of H O /CyH = 6.
g
h
i
5
3
00 mg of catalyst 1 was used for the reaction.
00 mg of material A and B was used as catalyst.
Picture of the catalyst; FT-IR spectra of the catalyst; TEM of catalyst 1,
1
200 mg of un-functionalized silica gel was used as catalyst.
SEM of catalyst 1 and H NMR spectra of the adipic acid associated with

172
M. Vafaeezadeh, M. Mahmoodi Hashemi / Catalysis Communications 43 (2014) 169–172
Scheme 2. Proposed mechanism of cyclohexene oxidation to adipic acid.
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