Aerobic oxidation of ethane to acetic acid catalyzed by N,N′-dihydroxypyromellitimide combined with Co species

Article abstract of DOI:10.1016/S0040-4039(02)02208-6

Aerobic oxidation of ethane to acetic acid was successfully performed through a catalytic radical process using N,N′-dihydroxypyromellitimide (NDHPI) combined with a Co(II) salt in acetonitrile or propionic acid. The turnover number of NDHPI reached 9.5 in acetonitrile and 15.3 in propionic acid.

Full text of DOI:10.1016/S0040-4039(02)02208-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 8859–8861
Aerobic oxidation of ethane to acetic acid catalyzed by
N,N%-dihydroxypyromellitimide combined with Co species
Akihiro Shibamoto, Satoshi Sakaguchi and Yasutaka Ishii*
Department of Applied Chemistry, Faculty of Engineering and High Technology Research Center, Kansai University, Suita,
Osaka 564-8680, Japan
Received 19 August 2002; revised 2 October 2002; accepted 4 October 2002
Abstract—Aerobic oxidation of ethane to acetic acid was successfully performed through a catalytic radical process using
N,N%-dihydroxypyromellitimide (NDHPI) combined with a Co(II) salt in acetonitrile or propionic acid. The turnover number of
NDHPI reached 9.5 in acetonitrile and 15.3 in propionic acid. © 2002 Elsevier Science Ltd. All rights reserved.
Methane and ethane are the main components of natu-
ral gas which has been long used worldwide as a clean
fuel. Since ethane comprises 5–10% of the natural gas,
a vast amount of ethane is formed with production of
methane. Like methane, ethane is also a most unreac-
tive class of hydrocarbons which are very difficult to be
converted into useful organic chemicals. If a new
methodology for converting ethane efficiently to useful
oxygen-containing compounds like ethanol and acetic
acid is developed, it would vastly contribute to the
efficient use of feedstock. There have been several
works for the conversion of ethane to acetic acid under
heterogeneous catalysts using mixed metal oxides,1 but
the transformation of ethane under homogeneous con-
ditions is rarely studied.2 Therefore, the direct catalytic
conversion of ethane to oxygenates is an enormous
challenge in oxidation chemistry as well as industrial
chemistry. Previously, Sen et al. reported liquid-phase
oxidation of ethane to acetic acid and formic acid by
Pd/C with H2O2 generated in situ from H2, arising from
a metal-catalyzed water gas shift reaction with CO and
water, and O2.3 Fujiwara et al. showed that ethane can
be converted into N,N-dimethylpropylamine by the
reaction with N,N-trimethylamine N-oxide in using
Cu(OAc)2 as the catalyst.4 Ethane oxidation with H2O2
catalyzed homogeneously by V-containing polyphos-
phomolybdates has been carried out by Shul’pin et al.5
In previous papers, we have reported that alkanes like
cyclohexane and toluene can be oxidized with molecu-
lar oxygen by the use of N-hydroxyphthalimide (NHPI)
which serves as a carbon radical-producing catalyst
(CRPC) from alkanes.6 In this paper, we would like to
report the oxidation of ethane to acetic acid with air
using CRPC like NHPI and its analogues.
In order to confirm suitable catalysts in this transfor-
mation, ethane was reacted under various conditions
using NHPI and its analogues as catalysts (Table 1).
A typical reaction was examined as follows: The reac-
tion was carried out in a 50-mL teflon-coated auto-
clave. To acetonitrile (5 mL) was added NHPI (100
mmol) and Co(OAc)2·4H2O (30 mmol), and then ethane
(20 atm, ca. 70 mmol) and air (20 atm) were charged.
The mixture was stirred at 150°C for 15 h. After the
reaction, products were subjected to GC and GC–MS
analyses.
Among the catalysts examined, N,N-dihydroxypy-
romellitimide (NDHPI) was found to be the best cata-
lyst. For instance, when a mixture of ethane (20 atm)
and air (20 atm) in acetonitrile (5 mL) was allowed to
react in the presence of NDHPI (100 mmol) and
Co(OAc)2 (30 mmol) at 150°C for 15 h (standard condi-
tions), 830 mmol of acetic acid was obtained and the
turnover number (TON) of NDHPI reached 8.3 (Run
5). It is noted that other products such as ethanol or
acetaldehyde were not detected at all under these reac-
Keywords: oxidation; ethane; N-hydroxyphthalimide; catalysis.
* Corresponding author. Tel.: +81-6-6368-0793; fax: +81-6339-4026;
e-mail: ishii@ipcku.kansai-u.ac.jp
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02208-6

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A. Shibamoto et al. / Tetrahedron Letters 43 (2002) 8859–8861
Table 1. Aerobic oxidation of ethane (1) to acetic acid (2) by NHPI analoguesa
Run
Catalyst
Solv. (mL)
Co(OAc)2 (mmol)
Yieldb (mmol)
TON
1
2
3
4
5
6
7
NHPI
NHPI
NHPI
NHNI
NDHPI
NDHPI
–
CH3CN (5)
CH3CN (10)
PhCN (5)
CH3CN (5)
CH3CN (5)
CH3CN (7)
CH3CN (5)
20
20
10
40
30
25
30
530
612
150
200
830
570
B5
2.7
3.1
0.75
1.0
8.3
5.7
–
a The reaction was carried out in the presence of catalyst (200 mmol for NHPI and NHNI and 100 mmol for NDHPI) at 150°C for 15 h.
b GC yield.
It is known that acetonitrile and benzonitrile are
hydrolyzed in basic or acidic medium to yield the
corresponding carboxylic acids. Since the present eth-
ane oxidation is carried out in acetonitrile which is
liable to lead to acetic acid in the cource of the reac-
tion, the ethane oxidation was examined in deuterated
acetonitrile, CD3CN, under the influence of NDHPI
and CoCl2 (Eq. (1))
tion conditions. In the oxidation of ethane using NHPI
as a catalyst under these conditions, the amount of
NHPI used was twice that of DHPI, but the yield of
acetic acid and the TON of the catalyst were 530 and
2.7 mmol, respectively (Run 1). Similarly, N-hydroxy-
naphthalimide (NHNI) was less active to form 200
mmol of acetic acid. Although the oxidation was exam-
ined in various solvents like benzonitrile, nitromethane,
benzene, and trifluorotoluene, the solvents other than
acetonitrile gave in unsatisfactory results. No oxidation
was induced by Co(OAc)2 alone without a radical-pro-
ducing catalyst like NHPI (Run 7).
(1)
On the basis of these results, the oxidation of ethane
catalyzed by NDHPI was carried out by varying the
conditions (Table 2).
The GC–MS spectrum of the reactant indicated the
formation of CH3COOH having m/e=60 formed by
the oxidation of ethane and no evidence of the forma-
tion of CD3COOH arising from hydrolysis of CD3CN.
This observation suggests that the acetic acid is derived
from ethane.
The yield of acetic acid was found to slightly increase
with rise of the reaction temperature over 150°C (Runs
1 and 2). The oxidation took place to some extent even
at 120°C, but the yield of acetic acid became very low
(Run 5). Although the reaction under lower ethane
pressure than 20 atm resulted in decrease of acetic acid,
no large effect of air pressures was observed (Runs
6–8). We next examined the effect of a Co species in the
oxidation of ethane under the standard conditions by
using the NDHPI catalyst. Divalent Co species were
found to be better additives than a trivalent Co one
(Runs 9 to 11). Among the divalent cobalt species
examined, CoCl2 and Co(NO3)2 were found to be good
additives to the NDHPI catalyst leading to 946 or 892
mmol of acetic acid, respectively (Runs 9 and 10).
In a previous paper, it was reported that acetic acid is
a good solvent in the NHPI-catalyzed oxidation of
cyclohexane and toluene with O2. Since the present
ethane oxidation produced acetic acid as the sole
product, it is difficult to estimate the amount of the
resulting acetic acid by the oxidation of ethane using
acetic acid as the solvent. Thus, the oxidation was
examined in a homologue of the acetic acid, propionic
acid, under standard conditions (Eq. (2)). It was found
Table 2. Aerobic oxidation of ethane (1) to acetic acid (2) by NDHPI under various conditionsa
Run
Temp. (°C)
Co species
Ethane (atm)
Air (atm)
Yield (mmol)
TON
1
2
3
4
5
6
7
8
170
160
150
140
120
150
150
150
150
150
150
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
CoCl2
20
20
20
20
20
30
10
10
20
20
20
20
20
20
20
20
10
10
30
20
20
20
888
845
829
470
135
709
380
309
946
892
338
8.9
8.5
8.3
4.7
1.4
7.1
3.8
3.1
9.5
8.9
3.4
9
10
11
Co(NO3)2
Co(acac)3
a A mixture of ethane (1) and air was reacted using NDHPI (100 mmol) and Co species (30 mmol) at 120–170°C for 15 h.

A. Shibamoto et al. / Tetrahedron Letters 43 (2002) 8859–8861
8861
that the oxidation of ethane in the propionic acid
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Acknowledgements
This work was partially supported by a Grant-in-Aid for
Scientific Research (KAKENHI) (S) (No. 13853008)
from Japan Society for the Promotion of Science (JSPS).
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