An Industrial Process for Adipic Acid Production by the Liquid-Phase Oxidation of Cyclohexanone with Molecular Oxygen

Article abstract of DOI:10.1246/bcsj.76.1993

The liquid-phase O₂ oxidation (auto-oxidation) of cyclohexanone by the use of acetic acid as a solvent and Mn and Co salts as catalysts at 1 atm of pure O₂ at 70°C was presented. The selectivity of adipic acid was 77% and the combined selectivity of carboxylic acids, which consisted of adipic acid, glutaric acid, and succinic acid, was 93% for the 100% conversion of cyclohexanone. The combination of a Mn(OAc)₂ catalyst and a Co(OAc)₂ catalyst was effective for improving adipic acid selectivity in the liquid-phase oxidation of cyclohexanone with O₂. Adipic acid was probably generated via 2-hydrocyclohexanone and 6-oxohexanoic acid, and glutaric acid was probably generated via 5-oxopentanoic acid. Water concentration did not show significant effect on the reaction. Use of a acid, e.g., p-toluenesulfonic acid, accelerated the oxidation reaction. The oxidation reaction proceeded under high pressure, even if the O₂ concentration was < 10 vol %.

Full text of DOI:10.1246/bcsj.76.1993

ꢀ 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 1993–2001 (2003) 1993
An Industrial Process for Adipic Acid Production by the Liquid-Phase
Oxidation of Cyclohexanone with Molecular Oxygen
ꢀ
Yukito Nagamori, Hidefumi Hamachi, and Kazuyoshi Kimura
Atsushi Shimizu, Katsutoshi Tanaka, Hiroo Ogawa, Yuji Matsuoka, Masami Fujimori,
Nylon & Urethane Intermediates Division, Performance Chemicals Company, Asahi Kasei Corporation,
Yuraku-cho, 1-1-2, Chiyoda-ku, Tokyo 100-8440
Received April 9, 2003; E-mail: shimizu.ac@om.asahi-kasei.co.jp
Adipic acid is produced industrially by the oxidation of cyclohexanone and cyclohexanol with nitric acid. Dini-
trogen oxide (nitrous oxide, N2O), a non-CO2 greenhouse gas, is produced as a by-product in this process. We have
investigated an industrial process for the production of adipic acid without the generation of N2O. When acetic acid
was used as a solvent and Mn(OAc)2/Co(OAc)2 were used as catalysts under atmospheric pressure of pure O2 at 70
ꢁC, the selectivity of adipic acid was 77% and the combined selectivity of carboxylic acids, which consisted of adipic
acid, glutaric acid, and succinic acid, was 93% for the 100% conversion of cyclohexanone. We presume that adipic acid
is generated via 6-oxohexanoic acid. Glutaric acid was probably generated via 5-oxopentanoic acid in the reaction path
that branched at 6-oxohexanoic acid. When air containing less than 10 vol % of oxygen was used at 1.22 MPa in order to
avoid explosion hazards, the selectivity of adipic acid was 78% for a 99.3% conversion of cyclohexanone. We conclude
that the present reaction can be used as an industrial process for adipic acid production.
Adipic acid is used as an ingredient of nylon 66. It is also
used as an ingredient of aliphatic ꢀ,!-dihydroxy polyester (so-
called polyesterdiol), a soft segment in polyurethane. The total
production of adipic acid was 2300 kt in 1998 at plants all
around the world. A major industrial method to produce adipic
acid is the oxidation of KA oil (a mixture of cyclohexanol and
cyclohexanone) with nitric acid. This reaction gives a mixture
of adipic acid, glutaric acid, and succinic acid. The combined
yield of the three carboxylic acids is nearly 100%. Further-
more, the reaction is quite useful because the ratio of adipic
acid to the total is higher than 95%.1 Dinitrogen oxide (nitrous
oxide, N2O) is generated in almost stoichiometric quantities in
the nitric acid oxidation process (Scheme 1). Dinitrogen ox-
ide, which is used as an anesthetic, is known as a non-CO2
greenhouse gas.
Thiemence and Trogler pointed out the significance of N2O
generated from adipic acid plants to the atmospheric N2O
budget.2 The global generation of N2O, ascribed to both an-
thropogenic and natural sources, is estimated to be 26000 kt
per year.3 The concentration of N2O in the air is approximate-
ly 300 ppb at present. The rate of N2O generation now ex-
ceeds the global rate of N2O destruction, such that the concen-
tration of N2O is growing at the rate of 0.2% per year.4,5 The
amount of N2O generated during adipic acid production is
reported to be 0.25 kg per 1kg of adipic acid (survey by
Miyazaki Prefecture in 1994). Since the rate of adipic acid
production was 2300 kt/y in 1998, the total amount of N2O
produced in adipic acid plants in the world is estimated to
be ca. 580 kt/y. Because of the need to ensure the conserva-
tion of the global environment, major adipic acid manufactur-
ers have introduced N2O abatement facilities to their adipic
acid plants. As a result, 80% of the N2O generated at adipic
acid plants around the world is now being decomposed before
it is released to the environment.6
Considering the above background, we have investigated
new processes for the production of adipic acid without N2O
emission to the environment. The O2 oxidation process is
quite useful from the environmental viewpoint because it pro-
duces no N2O. Benzene derivatives such as cyclohexane and
cyclohexanone have been used as ingredients in many
papers. Originally, Tanaka reported the O2 oxidation of cyclo-
hexane in acetic acid using Co(OAc)2 as a catalyst for an in-
dustrial production process of adipic acid.7–9 Besides these pa-
pers, many scholars have reported the oxidation reaction of
cyclohexanone.10–14 More recently, Ishii et al.,15–18 Hirai,19
and Nakano and Ishii20,21 reported the O2 oxidation reactions
using N-hydroxyphthalimide radical (phthalimid-N-oxyl) as a
catalyst. A review on the cyclohexane oxidation was publish-
ed by Schuchardt et al.22 Meanwhile, many papers have dealt
with the liquid-phase oxidation of cyclohexanone with O2. For
example, cyclohexanone can be oxidized with O2 in acetic
acid by the use of a manganese salt or a cobalt salt as a
catalyst.23–32 A review concerning the catalytic oxidative car-
bon–carbon bond cleavage of ketones was published by
´
33
Bregeault et al. As for other preparation methods for adipic
Scheme 1. Nitric acid oxidation of cyclohexanone and cy-
clohexanol.
acid, the O2 oxidation of cyclohexanol,34 the nitric acid oxida-
Published on the web October 15, 2003; DOI 10.1246/bcsj.76.1993

1994 Bull. Chem. Soc. Jpn., 76, No. 10 (2003)
Production Process for Adipic Acid
of Mn(OAc)2 4H2O, 2.55 g (10.2 mmol) of Co(OAc)2 4H2O,
ꢂ
ꢂ
7.40 g of water, 0.85 g (4.47 mmol) of p-toluenesulfonic acid
(Practical Grade, Wako), and 121.7 g of acetic acid. Oxygen
3
and nitrogen were introduced at 4.9 N dm3 hꢃ1 and 48.1N dm
h
Scheme 2. O2 oxidation of cyclohexanone.
ꢃ1, respectively, and the pressure was controlled at 1.21 MPa
tion of 1,2-dihydroxycyclohexane without N2O generation,35
the H2O2 oxidation of cyclohexene,36 and the hydroformyla-
tion of butadiene have also been reported.37,38
(absolute pressure). All of the reagents were used without
purification. The temperature was controlled at 70 ꢁC. The oxida-
tion reaction proceeded for 5 h with 1200 rpm of agitation. Gas
chromatography and high-performance liquid chromatography un-
der the conditions described below were used to analyze the re-
maining cyclohexanone and the oxidation products.
HPLC Analysis of the Oxidation Products. Apparatus, Shi-
madzu LC10A. Column, ODS. Eluent, acetonitrile/H3PO4 (vol-
ume ratio, 0.15/0.85, H3PO4 conc., 0.15 wt %). Flow rate, 0.8
mL minꢃ1. Temperature, 15 ꢁC. Detector, RI and UV. Standards,
adipic acid (Asahi Kasei), glutaric acid (Special Grade, Wako),
succinic acid (Special Grade, Wako), 6-oxohexanoic acid pre-
pared using the method in the literature,39 5-oxopentanoic acid
prepared using the method described below, and 2-hydroxycyclo-
hexanone (Aldrich). Internal standard, pimelic acid (Special
Grade, Wako). Retention time for HPLC: succinic acid, 13.4
min; 5-oxopentanoic acid, 14.5 min; glutaric acid, 17.8 min; 2-hy-
droxycyclohexanone, 20.8 min; 6-oxohexanoic acid, 25.1min;
adipic acid, 30.3 min; pimelic acid, 68.8 min.
GC Analysis for Measuring Cyclohexanone Conversion.
Apparatus, Shimadzu GC-14B. Column, DB-1 (0.323 mmꢁ ꢄ
30 m; film thickness, 0.30 mm). Temperature, 100 ꢁC for 2
min; 100–180 ꢁC for 16 min; 180–300 ꢁC for 12 min; 300 ꢁꢁC
for 5 min. Carrier, He (60 kPa). Injection temperature, 300 C.
Detector temperature (FID), 350 ꢁC. Air pressure, 70 kPa. H2
pressure, 60 kPa. We assumed the oxidation reaction to be a pseu-
do first-order reaction with respect to cyclohexanone in order to
roughly estimate the reaction rates, k (hꢃ1). Reaction rates were
estimated from the relationship between cyclohexanone conver-
sion and time.
We have considered cyclohexanone to be a key intermediate
in developing a new O2 oxidation process for adipic acid
production. This is because cyclohexanone can be oxidized
by O2 easily, and is also an ingredient in the present nitric acid
oxidation process. Moreover, the use of cyclohexanone is de-
sirable since it is consumed in great quantities as an ingredient
of "-caprolactam. The oxidation of cyclohexanone (Scheme 2)
is a well-known reaction, but the possibility of any industrial
application is still ambiguous.
In this paper, we report the liquid-phase O2 oxidation (auto-
oxidation) of cyclohexanone by the use of acetic acid as a sol-
vent and Mn and Co salts as catalysts. One hundred percent
conversion of cyclohexanone should be a prerequisite condi-
tion in the new adipic acid plant, because recycling of cyclo-
hexanone complicates the process. Therefore, we investigated
the oxidation reaction for the high conversion of
cyclohexanone. We also report the possibility of the industrial
applicability of this reaction.
Experimental
Preparation of Adipic Acid under Atmospheric Pressure.
Typical reaction conditions (Run 7) are as follows. A 200 mL
four-necked flask with a condenser, an agitator, a thermocouple,
and a nozzle for O2 in-feed was placed in a water bath. Into this
reactor we placed 9.00 g (0.0918 mol) of cyclohexanone (1st
Grade by Wako Pure Chemicals Industries, Ltd.), 1.60 g (6.55
mmol) of Mn(OAc)2 4H2O (Special Grade, Wako), 1.63 g (6.55
mmol) of Co(OAc)2 4H2O (Special Grade, Wako), 4.50 g of wa-
GC/MS Analysis for Oxidation Products Processed with a
Methyl Esterification Reagent. GC apparatus, HP-5890A. Col-
umn, CPB10 (0.25 mmꢁ ꢄ 20 m; film thickness, 0.25 mm). Tem-
ꢂ
ꢂ
ter, 1.04 g (5.47 mmol) of p-toluenesulfonic acid monohydrate,
and 72.23 g of acetic acid (Special Grade, Wako). All of the re-
agents were used without purification. O2 from a cylinder was in-
troduced at 4 dm3 hꢃ1 and the temperature was controlled at 70
ꢁC. The oxidation reaction was allowed to proceed for 5 h with
1500 rpm agitation. The color of the reaction solution was dark
brown, which was the color of the Mn(c) and Co(c) during the
reaction, and the color gradually became light after we stopped
feeding with O2. Gas chromatography and high-performance liq-
uid chromatography under the conditions described below were
used to analyze the remaining cyclohexanone and the oxidation
products.
Preparation of Adipic Acid under High Pressure. A 0.660
dm3 autoclave (i.d. = 60 mmꢁ) made of Hastelloy-C incorporat-
ing an agitator, two baffle plates, a thermocouple, a nozzle for O2
in-feed, and a condenser was placed in a hot water bath. The flow
rates of O2 from a cylinder and N2 from a cylinder were controlled
automatically to adjust the partial pressure of oxygen at a constant
value for the oxidation reaction. Fine bubbles of the mixture gas
were injected into the reactor through a fritted metal filter set at
the tip of the O2 feed nozzle. Waste gas was cooled by the con-
denser fitted to the top of the reactor in order to return the conden-
sate to the reactor and then released to the outside through a back-
pressure valve. Typical reaction conditions (Run 79) are as fol-
lows: 15.0 g (0.153 mol) of cyclohexanone, 2.71 g (11.1 mmol)
ꢁ
ꢁ
perature, 70–250 C (5 C minꢃ1). Carrier, He of 15 mL minꢃ1
.
MS apparatus, JEOL AX-500 (EI, CI); methyl pentanoate acid,
CI-MS m=z, 117 (M + 1, 100), 59 (5); 2-hydroxycyclohexanone,
CI-MS m=z, 115 (M + 1, 100), 99 (10); dimethyl succinate, CI-
MS m=z, 147 (M + 1, 100), 115 (20); dimethyl glutarate, CI-
MS m=z, 161 (M + 1, 100), 129 (30); methyl 6-oxohexanoate,
CI-MS m=z, 145 (M + 1, 100), 113 (20); dimethyl adipate, CI-
MS m=z, 175 (M + 1, 100), 143 (25); dimethyl azelate, CI-MS
m=z, 217 (M + 1, 100), 185 (10); dimethyl sebacate, CI-MS
m=z, 231 (M + 1, 100), 199 (10).
Preparation of 5-Oxopentanoic Acid. 5-Oxopentanoic acid
was prepared as an HPLC standard by the following procedure.
Using a method reported in the literature,40 cyclopentanone was
oxidized in water by O2 with FeCl3 and VOSO4 as catalysts. A
200 mL four-necked flask fitted with a condenser, an agitator, a
thermometer, and an O2 bag was placed in a water bath. Into this
reactor we added 5.21g (0.0620 mol) of cyclopentanone, 1.04 g
(6.41mmol) of FeCl , 1.34 g (8.22 mmol) of VOSO4, and 58.5
3
ꢁ
g of water. The temperature was controlled at 70 C. The oxida-
tion reaction proceeded for 5 h with 550 rpm of agitation. GC/
MS analysis indicated that the main products were 5-oxopentanoic
acid and glutaric acid. The reaction solution was used as an LC
standard for 5-oxopentanoic acid. The results of the GC/MS anal-
ysis are shown below. GC apparatus, HP-5890A. Column, Dura-

A. Shimizu et al.
Bull. Chem. Soc. Jpn., 76, No. 10 (2003) 1995
bond DB-1(0.25 mm ꢁ ꢄ 30 m; film thickness, 0.25 mm). Tem-
sion in our study. The combination of Mn(OAc)2 and
Co(OAc)2, for which the selectivity of the adipic acid was
77%, was the most effective in improving the selectivity with
respect to adipic acid. Kamiya and Kotake have already
reported that a combination of an Mn salt and a Co salt accel-
erated the rate of oxidation at low catalyst concentration
(2 ꢄ 10ꢃ4 M).23 However, they did not mention the effective-
ness in terms of adipic acid selectivity. The combination of a
Mn salt and a Co salt was effective in increasing selectivity
with respect to adipic acid under the high concentration (ca.
10ꢃ1 M) of catalyst employed in this study.
Reaction Mechanism. Scheme 3 illustrates a possible
mechanism for the oxidation reaction. When a Mn catalyst
is used in the liquid-phase oxidation (auto-oxidation) of cyclo-
hexanone, the first oxidation product, 2-hydroperoxycyclohex-
anone 6, is probably generated via the enol tautomer of cyclo-
hexanone 2.23 Pritzkow47 and Ogawa et al.25 took the inter-
mediates to be 2-hydroperoxycyclohexanone 6 and 6-oxohex-
anoic acid 12 via compound 11 using a Mn salt catalyst.
Druliner and Wasserman concluded that cyclohexane-1,2-di-
one 5 generated by the disproportionation of compound 4
was the main intermediate from their study of the oscillatory
ꢁ
ꢁ
ꢁ
perature, 100 C for 2 min; 100–300 C (20 C minꢃ1). Carrier,
He 100 dm3 minꢃ1. MS apparatus, JEOL AX-500 (EI, CI); CI-
MS m=z 117 (M + 1, 100), 99 (70).
Results and Discussion
Effect of Metal Salt Catalyst Used with Mn(OAc)2. The
selectivity of adipic acid in the liquid-phase oxidation reaction
of cyclohexanone with a Mn(OAc)2 catalyst is low. Table 1
summarizes the selectivity of adipic acid quoted in the litera-
ture and in Run 1. These results show that the selectivity of
adipic acid is 72% or less. It is necessary for the selectivity
of adipic acid to be improved to create a viable industrial
process. Various attempts have been made to improve the se-
lectivity of adipic acid. Kulrestha and Roch reported a combi-
nation of Mn, Cu, and Ba salts,41 Nishino reported a combina-
tion of Mn and Ce salts,42 and Kusunoki and Ogawa reported a
combination of Mn and Pb salts.43 Shinohara et al. reported
the combination of a Co salt and a Mn salt as catalysts in
the oxidation of KA oil (a mixture of cyclohexanone and
cyclohexanol).44–46 Figure 1summarizes the results of screen-
ing for different catalysts at over 90% of cyclohexane conver-
Table 1. Adipic Acid Selectivity by O2 Oxidation 1 (Cyclohexanone) with Mn(OAc)2 from the Literatures
1
Mn(OAc)2 Solvent Temp Conv. of 1 Selectivity of 17 Time
ꢁC
M
mM
%
%
h
Kamath and Chandalia, 197327
Constantini and Krumenacker, 198331
Kamiya and Kotake, 197323
Ogawa et al., 196725
Rao and Raghunathan, 198430
Ohashi and Mizutani, 194128
1.2
5.4
7.4
84
1AcOH
12
AcO05H
AcOH
AcOH
80
65
82
70
95
44.2
85
61.5
53
5
5
59
2.8
15
5
AcOH
1 97.6
AcOH
>90
70.5
72
33.6
73
AcOH Run7011
5
99.3
68.1
Other conditions of Run 1, H2O, 5.0 wt %; O2, 4 dm3 hꢃ1
.
Fig. 1. Effect of metal salt combined with Mn(OAc)2. Reaction conditions: 1 (cyclohexanone), 1.02 M. Mn(b), 72.8 mM. H2O, 5
wt %. Temp, 70 ꢁC. O2, 4 dm3 hꢃ1. Solvent, Acetic acid. All metal salts were metal acetate. Molar ratio of Metal to Mn, Ag/Mn,
0.53; Sn/Mn, 0.25; Cu/Mn, 0.43; Pb/Mn, 0.27; Ni/Mn, 0.49; Nd/Mn, 0.38; Pd/Mn, 0.52; Ba/Mn, 0.40; Bi/Mn, 0.27; Others/
Mn, 1.00. Molar ratio of p-toluenesulfonic acid (p-TS) to Mn, Runs 18, 22, 23, 24, 25, 27, 29, p-TS/Mn = 0.84; Others, p-TS/
Mn = 0.42. Reaction time, Run 24, 2.5 h; Runs 7, 9, 18, 22, 23, 25, 27, 29, 30, 3 h; Runs 16, 20, 4 h; Others, 5 h. Conversion of 1
(cyclohexanone), Run 3, 92.1%; Run 10, 94.9%; Run 11, 92.6%; Run 17, 92.5%; Run 18, 93.9%; Run 19, 96.1%; Run 20, 95.6%;
Run 28, 94.2%; Others, >97%.

1996 Bull. Chem. Soc. Jpn., 76, No. 10 (2003)
Production Process for Adipic Acid
Scheme 3. Reaction mechanism for the oxidation of cyclohexanone.
Fig. 2. Changes of intermediates and products with time. , Conv. of 1; , Total selectivity; , 12; , 17; , 8; ꢀ, 24; , 28; ꢄ,
29. Reaction conditions: Temp, 70 ꢁC. O2, 6 dm3 hꢃ1. (a) Run 31, 1 (cyclohexanone), 9.76 g (1.27 M); AcOH, 50.9 g; H2O, 15.1
g; Mn(OAc)2 4H2O, 2.9 g (151 mM). (b) Run 32, 1 (cyclohexanone), 18.62 g (1.89 M); AcOH, 74.0 g; H2O, 4.90 g;
Mn(OAc)2 4H2O, 1.60 g (64.7 mM); Co(OAc)2 4H2O, 1.67 g (66.6 mM); p-toluenesulfonic acid, 0.56 g (29.1mM).
ꢂ
ꢂ
ꢂ
oxidation of cyclohexanone with 20 mM of Co(OAc)2.48 The
conversion of cyclohexanone and the selectivity of the oxida-
tion products with time when using the Mn(OAc)2 catalyst and
the Mn(OAc)2/Co(OAc)2 catalyst are shown in Fig. 2. The
main products besides adipic acid 17 were 2-hydroxycyclohex-
anone 8, 6-oxohexanoic acid 12, 5-oxopentanoic acid 24, glu-
taric acid 28, and succinic acid 29. Figure 2 indicates that 2-
hydroxycyclohexanone 8 is generated first, followed by 6-oxo-
hexanoic acid 12. As shown in Table 2, compound 17, which
was accompanied by 28, 29, 12, and 24, was the main product

A. Shimizu et al.
Bull. Chem. Soc. Jpn., 76, No. 10 (2003) 1997
Table 2. Oxidation of 12 (6-Oxohexanoic Acid) and 8 (2-Hydroxycyclohexanone)
Raw material
Conc./M
Cat.
Conc./mM
Conv. of
Selectivity/%
29 12
9.4 4.5
Time/h
raw material/% 17 28
24
ꢀ
Run 33
Run 34
Run 35
8
8
8
1.08
Mn(OAc)2
Co(acac)3
Mn(OAc)3
Mn(OAc)3
138
438
438
111
116
9
97
100
100
94
54 2.4
24 15
7.4
2
11
4
trace
0.438
0.438
0.356
4.5
4.5
9
4
10
3
3
38
3
Run 36 12
Run 37 12
58 17
76 12
2
trace
—
—
0.331Co(OAc)
3
100
2
ꢀ The color of solution was light brown. Temp, 70 ꢁC. O2, 6 dm3 hꢃ1. H2O, Run 33, 18 wt %; Run 34, 0 wt %; Run
35, 2 wt %; Run 36, 7.7 wt %; Run 37, 7.3 wt %. Solv., AcOH.
Fig. 3. The influence of the molar ratio of Mn to Co. , 17; , 28 + 29; , Conv. of 1; , Consumption rate of 1. Reaction
conditions: Total conc. of Metals, Run 38, 136 mM; Run 39, 138 mM; Run 40, 140 mM; Run 41, 143 mM; Run 42, 146 mM.
1 (cyclohexanone), 1.02 M; H2O, 6 wt %; p-TS, 32.0 mM; O2, 4 dm3 hꢃ1. Solv., AcOH. Temp, 70 ꢁC. Reaction time, Run
38, 4 h; Run 39, 4.5 h; Run 40, 6 h; Run 41, 5.5 h; Run 42, 12 h (containing 2 h of induction period). Conversion of 1, Run
38, 100%; Run 39, 99%; Run 40, 100%; Run 41, 99%; Run 42, 97%.
generated in the oxidation reaction of 2-hydroxycyclohexa-
none under a Mn catalyst and a Co catalyst (Runs 33–35).
This result probably indicates that 2-hydroxycyclohexanone
8 is one of intermediates. Compound 12, 6-oxohexanoic acid,
which was prepared using the method in the literature,39 was
also converted to compounds 17, 28, 29, 12, 24 under a Mn
catalyst and a Co catalyst (Runs 36, 37). These fact shows that
6-oxohexanoic acid 12 is also one of the intermediates. The
oxidation of 6-oxohexanoic acid 12 under a Co(b) catalyst pro-
ceeded with a respectable selectivity of 76% in Run 37. This
result shows that the Co(b) ion is probably the main catalyst in
the oxidation of 6-oxohexanoic acid 12. In general, an alde-
hyde compound such as acetaldehyde can be easily converted
to carboxylic compounds via peroxyacid.48,49 Therefore, 6-oxo-
hexanoic acid 12 is probably converted to adipic acid 17 via
monoperoxyadipic acid 15. Measurement of the reaction mix-
ture (processed with a methyl esterification agent) using GC/
MS indicated that valeric acid 19, azelaic acid 20, and sebacic
acid 21 were generated. Valeric acid 19 is generated in the re-
action between the pentanoic acid radical (4-carboxybutyl rad-
ical) 18, which is generated from the decarboxylation reaction
of monoperoxyadipic acid 15, and the hydrogen radical. Aze-
laic acid 20 is probably generated in the reaction between rad-
ical 18 and the butanoic acid radical (3-carboxypropyl radical)
27 generated from the decarboxylation reaction of monoper-
oxyglutaric acid 25. A possible mechanism for the generation
of sebacic acid 21 is the dimerization of radical 18. The reac-
tion between O2 and radical 18 would give 5-oxopentanoic acid
24, which would then be converted to glutaric acid 28 via
monoperoxyglutaric acid 25. Succinic acid 29 is probably gen-
erated from radical 27 branched at the radical compound 26.
Influence of the Ratio of Mn to Co and Metal
Concentration. Figure 3 shows the relationship between
the molar ratio of Mn to Co and the selectivity of dibasic acids
at 136–146 mM of Mn and Co catalysts. The peak of the adip-
ic acid selectivity, which was shown to be 77%, occurred at
around 1/1 of Mn/Co (molar ratio). The selectivity of glutaric
acid and succinic acid increased with increasing Co. The re-
action-rate for cyclohexanone decreased with increasing Co
concentration. From the viewpoint of adipic acid selectivity,
a suitable molar ratio of Mn to Co is 1/1. The metal ion con-
centration dependence of the adipic acid selectivity and reac-
tion-rates is indicated in Fig. 4. The acceleration effect on
the reaction-rate caused by Co was observed at a low concen-
tration of the catalyst (ca. 10ꢃ3 M), as reported by Kamiya and
Kotake.23 The selectivity and the reaction-rates increased with
metal ion concentration and become constant at a metal ion
concentration of more than 150 mM. From the viewpoint of
productivity of adipic acid, the concentration of metal ion
should be settled at ca. 150 mM.
Optimum Reaction Temperature. Figure 5 shows the
temperature dependence of the reaction. The selectivity de-
creased with temperature above 70 ꢁC. Unknown peaks on
an LC chart increased and the selectivity of glutaric acid and
succinic acid remained almost constant at the elevated temper-
atures (Run 57, glutaric acid, 5%, succinic acid, trace; Run 58,

1998 Bull. Chem. Soc. Jpn., 76, No. 10 (2003)
Production Process for Adipic Acid
Fig. 4. Selectivity of adipic acid vs concentration of metal ions. , Selectivity of 17; , Consumption rate constant of 1 k (hꢃ1).
Reaction conditions: 1 (cyclohexanone), 1.02 M. Temp, 70 ꢁC. O2, 6 dm3 hꢃ1. Reaction time, Run 43, 5 h; Run 44, 9 h; Run 45, 5
h; Run 46, 9 h; Run 47, 11 h, Run 48, 9 h; Run, 49, 5 h; Run 50, 5 h; Run 51, 7 h. H2O, Runs 43–45, 0 wt %; Runs 46–48, 6 wt %;
Run 49, 0 wt %; Run 50, 11 wt %; Run 51, 7 wt %. Molar ratio of Mn/Co (Runs 49–51), 1/1. Solv., AcOH. Conv. of 1, Run 43,
14%; Run 44, 99.4%; Run 45, 85%; Run 46, 99.3%; Run 47, 99.9%; Run 48, 99.3%; Run 49, 91%; Run 50, 95.4%; Run 51,
98.9%.
Fig. 5. Temperature dependence of adipic acid selectivity. Reaction conditions: 1 (cyclohexanone), 1.02 M. Mn, 146 mM. H2O, 3.3
M (6 wt %). O2, 4 dm3 hꢃ1. p-Toluenesulfonic acid, Runs 53–56, 9.9 mM; Others, 0 mM. Solv., AcOH. Reaction time, Run 52, 5
h; Run 53, 3 h; Run 54, 3 h; Run 55, 3 h; Run 56, 3 h; Run 57, 5 h; Run 58, 5 h. Conversion of 1, Run 52, 86.0%; Run 53, 93.5%;
Run 54, 93.6%; Run 55, 94.7%; Run 56, 96.1%; Run 57, 100%; Run 58, 100%. (a) 1 (cyclohexanone), 1.02 M. Mn, 73 mM. Co,
68 mM. H2O, 3.3 M (6 wt %). O2, 4 dm3 hꢃ1. p-Toluenesulfonic acid, 33.7 mM. Solv., AcOH. Reaction time, Run 59, 7 h; Run
60, 5 h; Run 61, 3 h; Run 62, 3 h. Conversion of 1, Run 59, 99.4%; Run 60, 99.5%; Run 61, 99.4%; Run 62, 99.4%.
glutaric acid, 7%, succinic acid, trace; Run 62, glutaric acid,
7%, succinic acid, 17%). This phenomenon probably indicates
the existence of side reactions having large activation energies.
From this result, the optimum temperature for the reaction is
70 ꢁC. Ogawa et al. reported that the peak selectivity occurred
around 70 ꢁC at low concentrations of Mn (2.8 mM).25
Kamath and Chandalia also reported peak selectivity around
27
ꢁ
80 C at low concentrations of Co (7 mM).
The Effect of the Addition of Acid and Water. Costantini
and Krumenacker reported that acids accelerated the oxidation
reaction using a Mn salt in acetic acid.31 Figure 6 shows the
relationship between the reaction-rate and the concentration
of p-toluenesulfonic acid. The addition of acid accelerated
Fig. 6. The effect of p-toluenesulfonic acid.
, Mn (72.9
the oxidation reaction, both in the reactions with a Mn/Co
catalyst as well as a Mn catalyst. The acceleration effect of acid
on the oxidation reaction using the Mn catalyst was larger than
the effect of acid on the reaction using the Mn/Co catalyst. The
acceleration effect can be explained as the catalytic effect of
acid on the tautomerism between cyclohexanone 1 and com-
pound 2. From the viewpoint of productivity, the addition of
acid is desirable for an industrial process.
Figure 7 shows the relationship between the concentration
of water and the selectivity of adipic acid and the relationship
between the concentration of water and the reaction-rate at 52/
48 (mol%/mol%) of Mn/Co. Although the effect of water on
the selectivity of adipic acid is not prominent below 20 wt %,
mM) + Co (73.0 mM); , Mn (72.9 mM); , Mn (146
mM). Reaction conditions: 1 (cyclohexanone), 1.02 M;
H2O, 6 wt %; Temp, 70 ꢁC; O2, 4 dm3 hꢃ1. Solv., AcOH.
Water, 6 wt %. Reaction time, 3–4 h.
it has a small peak at around 10% of water. The reaction-rate
decreased slightly with increasing water content. Ogawa
reported that the selectivity of adipic acid in the oxidation re-
action of a cyclohexanone/cyclohexanol mixture increased
with the amount of water, and it was stable at more than 1%
water under 8 mM of Mn(OAc)2.26 Kamiya reported that
the reaction-rate had a peak at around 1% water at 20 mM
of Co(OAc)2.24 From the result of this study, the effect of wa-

A. Shimizu et al.
Bull. Chem. Soc. Jpn., 76, No. 10 (2003) 1999
Fig. 8. Conversion and selectivity vs O2 partial pressure.
: Conversion of 1, : Selectivity of 17, : Selectivity
of 28, : Selectivity of 29. Reaction conditions: 1 (cyclo-
hexanone), 1.02 ꢁM. Mn(OAc)2, 72.9 mM. Co(OAc)2, 67.9
mM. Temp, 70 C. Press. (absolute), Run 77, 0.81MPa;
Run 78, 1.01 MPa; Run 79, 1.21 MPa; Run 80, 4.65
MPa. O2 partial press., Run 77, 0.071MPa; Run 78,
0.091 MPa; Run 79, 0.111 MPa; Run 80, 0.384 MPa.
Flow rate of N2 and O2, Runs 77–79, 53 Ndm3 hꢃ1; Run
80, 70 Ndm3 hꢃ1. Solv., AcOH. p-Toluenesulfonic acid,
29.7 mM. Water, 6 wt %. Reaction time, Run 77, 5 h;
Run 78, 5 h; Run 79, 5 h; Run 80, 3.7 h.
Fig. 7. The effect of water. , Selectivity of 17; , Reac-
tion rate. Reference data, , Selectivity of 17 in Run 74;
, Reaction rate of 17 in Run 74. Reaction conditions: 1
(cyclohexanone), 1.02 M. Mn(OAc)2, 72.9 mM. Co-
ꢁ
(OAc)2, 67.9 mM. Temp, 70 C. O2, 4 dm3 hꢃ1. p-Tolu-
enesulfonic acid, Run 72, 28.5 mM; Run 73, 30.8 mM;
Run 74, 24.4 mM; Runs 75, 76, 29.1mM. Solv., AcOH.
Reaction time, 3–6 h. Conversion, Run 72, 99.7%; Run
73, 99.6%; Run 74, 99.6%; Run 75, 99.6%; Run 76, 99.8%.
ter in the presence of a high concentration of catalyst is small.
When water wasn’t used in the reaction, adipic acid precipitat-
ed easily. This would cause trouble in an actual industrial
plant. Therefore, the existence of water is necessary for the
operation of a practical process.
for an industrial process. When the concentration of cyclohex-
anone was 2 M (Run 82), the selectivity was the same as in the
case of 1M cyclohexanone (Run 7). Therefore, the concentra-
tion of cyclohexanone should be 2 M in a practical plant.
When the reaction is roughly assumed to be a pseudo first-or-
der reaction with respect to cyclohexane concentration, the
rate-constant k (hꢃ1) of the reaction using Mn(OAc)2/
Co(OAc)2 with p-toluenesulfonic acid was calculated to be
1.04 hꢃ1 (Fig. 9). Figure 10 shows the relationship between
the adipic acid yield and the cyclohexanone conversion.
The relationship was almost linear and the slope of the line
was 0.745. Therefore, the rate of adipic acid generation can
be represented by 0:745 ꢄ 1:04 (hꢃ1)ꢄ cyclohexanone conc.
(mol dmꢃ3). Figure 11 explains the block diagram for manu-
facturing adipic acid from cyclohexanone. The plant consists
of 3 high-pressure CSTRs combined in series with each other.
The O2 concentration of the gas phase of those reactors should
be controlled at around 5% to avoid explosion as well as tere-
phthalic acid production. The first reactor and the second re-
actor are operated at 70 ꢁC and the third reactor is operated at
over 70 ꢁC. The concentration of cyclohexanone in the first re-
actor decides the productive capacity of adipic acid. When the
concentration of cyclohexanone at the exit of the first reactor is
2 M, adipic acid productivity is 150 kg mꢃ3 hꢃ1. This produc-
The Effect of O2 Pressure. Pure oxygen is hard to handle
in an organic chemical reaction in an industrial process be-
cause of explosion hazards. The concentration of oxygen in
the gas emitted during terephthalic acid production, which is
commercialized in an acetic acid solvent with a Co(OAc)2 cat-
alyst, is limited to 2–5 vol % to avoid explosion.50 Figure 8
shows the result of the liquid-phase oxidation reaction under
high pressure and low O2 concentration in the autoclave
described in the experimental section. The selectivity of adip-
ic acid has a peak (78%) at around 0.101 MPa of O2 (partial
pressure). We found that the oxidation reaction proceeded un-
der high pressure, even if the O2 concentration was less than
10 vol %. We think that the industrial process should employ
a high-pressure reactor with a low O2 concentration. From this
result, 0.101 MPa of O2 (partial pressure) should be used in an
industrial process.
Industrial Process for Oxidation of Cyclohexanone.
Table 3 shows the results of the liquid-phase oxidation using
Mn(OAc)2/Co(OAc)2 under preferable reaction conditions
Table 3. Results of the Oxidation Reaction under Preferable Reaction Conditions for an Industrial Process
1
Mn(OAc)2 Co(OAc)2 H2O p-Toluenesulfonic acid O2
N2 Press. Temp. Time Conv. of 1
Selectivity/%
M
mM
mM
wt %
mM
dm3/h dm3/h MPa ꢁC
h
%
17 28 29 8 12 24 Total
Run 7 1.02
Run 40 1.02
Run 82 2.00
Run 79 1.02
72.6
72.5
73.2
73.4
72.8
67.5
72.9
67.9
5.0
6.0
5.0
6.0
60.6
32.0
30.4
29.7
4.0
4.0
4.0
—
—
—
0.10 70.0 3.0
0.10 70.0 6.0
0.10 70.0 4.0
97.5 77 12 2 0 1
100.0 77 12 2 0 1
99.9 77 11 2 1 0
99.3 78 12 2 0 0
100.0 95 3 2
1
1
2
0
93
93
92
92
ꢀ
ꢀ
4.9 48.1 1.21 70.0 5.0
Nitric acid oxidation (SRI, 1996)1
100
ꢀ Ndm3 hꢃ1. Solvent in Runs 7, 82, 79, AcOH.

2000 Bull. Chem. Soc. Jpn., 76, No. 10 (2003)
Production Process for Adipic Acid
Fig. 9. Time vs ꢃ lnð1 ꢃ Conv.Þ. Conv., conversion of 1
(cyclohexanone).
Fig. 10. Yield of adipic acid vs conversion of cyclohexa-
none in Run 32.
Fig. 11. Block diagram for manufacturing adipic acid from cyclohexanone.
tivity means that a ca. 42 m3 reactor is necessary for 50000 t/y
(50000 t per 8000 h) of adipic acid production. More than
90% of the cyclohexanone is consumed in the second
reactor. The remaining cyclohexanone and the intermediates
are consumed completely in the third reactor at elevated
temperature. The reaction solution is fed into the adipic acid
recovery section, where adipic acid is purified by re-
crystallization. Three-stage re-crystallization may be neces-
sary, because the selectivity of adipic acid is low when com-
pared to the present adipic acid production process that uses
2-stage re-crystallization.1 The product, (the adipic acid), is
obtained from the adipic acid recovery section. The solution,
after removing the adipic acid, is moved to the solvent recov-
ery section where AcOH is recovered by distillation. Water is
added in the adipic acid recovery section to wash the adipic
acid crystals and to dissolve the adipic acid crystals for re-
crystallization. This water is removed in the solvent recovery
section. Part of the concentrated solution in the solvent recov-
ery section is moved to the DBA (dibasic acid) recovery sec-
tion, where DBA is purified. The by-products are removed in
the DBA recovery section. The other portion of the concen-
trated solution in the solvent recovery section is moved to
the catalyst recovery section. By-products are removed in
the catalyst recovery section as well as in the DBA recovery
section. Acetic acid recovered in the solvent recovery section
and the catalysts recovered in the catalyst recovery section are
recycled to the first reactor. The liquid-phase oxidation proc-
ess with acetic acid and metal acetate has been commercial-
ized as a production process of terephthalic acid.50 We think
that technologies used in terephthalic acid production, such as
solvent recovery, catalyst recovery, and explosion avoidance50
are applicable to the liquid-phase oxidation of cyclohexanone.
The selectivity of adipic acid and the total selectivity of dicar-
boxylic acids are low in the oxidation reaction of this study
when compared to the present adipic acid process (Table 3).
The difference in the combined selectivity of dicarboxylic
acids between both processes is 7%. Compared to the nitric
acid cost in the conventional process, this value is small.
(See Appendix.) Furthermore, the fact that N2O isn’t generated
and that the HNO3 plant isn’t necessary are significant merits
for the liquid-phase oxidation of cyclohexanone.
Conclusion
We summarize the results as follows: (1) The combination
of a Mn(OAc)2 catalyst and a Co(OAc)2 catalyst is effective
for improving adipic acid selectivity in the liquid-phase oxida-
tion of cyclohexanone with O2. (2) Adipic acid is probably
generated via 2-hydroxycyclohexanone and 6-oxohexanoic
acid, and glutaric acid is probably generated via 5-oxopentano-
ic acid. (3) The optimum molar ratio of Mn to Co is 1/1 from
the viewpoint of adipic acid selectivity. (4) The concentration
of the metal salt should be set at ca. 150 mM from the view-
point of productivity. (5) The selectivity of adipic acid shows
ꢁ
a maximum at 70 C. (6) Water concentration, which is less
than 20 wt %, doesn’t have a significant effect on the
reaction. (7) Use of an acid, such as p-toluenesulfonic acid,
accelerates the oxidation reaction. (8) The oxidation reaction
proceeds under high pressure, even if the O2 concentration is
less than 10 vol %. An O2 partial pressure of 0.101 MPa
should be used in the industrial process.

A. Shimizu et al.
Bull. Chem. Soc. Jpn., 76, No. 10 (2003) 2001
21Y. Ishii and T. Nakano, JP 9327626 A2 9(917);
Abstr., 127, 206051(1997).
22 U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, R. S. da
Chem.
The feasibility of technology required for an industrial adip-
ic acid process depends on not only technical aspects but also
capital investment due to the equipment and conditions at the
location. Although we should continue further evaluations of
the technology from various viewpoints, we think that the O2
oxidation of cyclohexanone is one of possibility for an indus-
trial adipic acid process.
´
Cruz, M. C. Guerreiro, D. Mandelli, E. V. Spinace, and E. L.
Pires, Appl. Catal. A, 211, 1(2001).
23 Y. Kamiya and M. Kotake, Bull. Chem. Soc. Jpn., 46, 2780
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24 Y. Kamiya, Kogyo Kagaku Zasshi, 74, 91 (1971).
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Appendix
The valuable cost of KA oil and the valuable cost of nitric acid
are explained as follows:
KA oil valuable cost=Nitric acid valuable cost ¼ approx. 3=2:
The proportion of nitric acid cost in the total production cost of
adipic acid is relatively large. Therefore, the selectivity of carbox-
ylic acids in the present process doesn’t have a large influence on
the production cost of the carboxylic acids compared with the ni-
tric acid cost in the conventional process.
29 H. Shen and H. Weng, Ind. Eng. Chem. Res., 27, 2246
(1988).
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(1983); Chem. Abstr., 102, 25201(1985).
32 K. Tanaka, Y. Matsuoka, and A. Shimizu, JP 2001213841
A2 (2001); Chem. Abstr., 135, 154342 (2001).
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