Vol. 25, No. 11 (2013)
Synthesis of Adipic Acid Oxidized by H
2
O
2
-Silicotungstic Acid Under Ultrasonication 6009
of 25, 40 and 59 kHz and a nominal power 250 W; Shanghai
Kudos Ultrasonic Instrument Co., Ltd.). The reaction flask
was located in the maximum energy area in the cleaner, where
the surface of reactants is slightly lower than the level of the
water. The reaction temperature was controlled by addition or
removal of water from ultrasonic bath.
TABLE-3
EFFECT OF ULTRASOUND FREQUENCY ON THE
OXIDATION OF CYCLOHEXENE TO ADIPIC ACID*
Ultrasound frequency (kHz)
Yield (%)
25
92
40
79
59
70
Stiring
33
*30 % H O : 10 mmol; silicotungstic acid: 0.04 mmol; reaction time: 4
h; reaction temperature: 30 ºC.
2 2
General procedure for the preparation of adipic acid:
A 50 mL round flask was charged with cyclohexene (2 mmol),
TABLE-4
2 2
silicotungstic acid (0.04 mmol) and 30 % H O (10 mmol) in
EFFECT OF REACTION TIME ON THE OXIDATION
OF CYCLOHEXENE TO ADIPIC ACID UNDER
ULTRASOUND IRRADIATION*
one portion. The reaction flask was located in the cleaner bath,
where the surface of reactants was slightly lower than the level
of the water. The mixture was irradiated 4 h (the reaction was
monitored by TLC). The reaction products were isolated by
filtration from the reaction mixture. Then the solvent was
poured into crushed ice. The precipitate was separated by
filtration, washed with water and crystallized from water to
obtain the adipic acid 265 mg, 92 %, m.p. 152-153 ºC.
Reaction time (h)
Yield (%)
3.0
84
4.0
92
5.0
83
6.0
78
*Ultrasound frequency: 25 kHz; 30 % H O : 10 mmol; silicotungstic
2
2
acid: 0.04 mmol; reaction temperature: 30 ºC.
As shown in Table-5, when the reaction was carried out
at 30 ºC, the adipic acid was obtain in 92 % yield, while when
the temperature was 25, 35 and 40 ºC, the yield was 87, 86
and 83 %, respectively. This may be that the lower reaction
temperature caused the lower reactivity, but the higher reaction
temperature would cause other complex oxidation by-products
and difficult workup procedures.
RESULTS AND DISCUSSION
To optimize the reaction conditions, a series of experi-
ments at varying amount of silicotungstic acid were carried
out. As shown in Table-1, the adipic acid obtained the higest
yield (92 %) at a molar ratio of cyclohexene: silicotungstic
acid of 1:0.02. The yields decreased when changing this
molar ratio. It indicates that the molar ratio had a significant
effect on this reaction.
TABLE-5
EFFECT OF REACTION TEMPERATURE ON THE
OXIDATION OF CYCLOHEXENE TO ADIPIC
ACID UNDER ULTRASOUND IRRADIATION*
TABLE-1
EFFECTS OF THE AMOUNT OF SILICOTUNGSTIC
ACID ON THE OXIDATION OF CYCLOHEXENE TO
ADIPIC ACID UNDER ULTRASOUND IRRADIATION*
Reaction temperature (ºC)
Yield (%)
25
87
30
92
35
86
40
83
*Ultrasound frequency: 25 kHz; 30 % H O : 10 mmol; silicotungstic
acid: 0.04 mmol; reaction time: 4 h.
2 2
Amount of silicotungstic
acid (mmol)
0.02
0.03
85
0.04
92
0.05
90
0.06
89
Yield (%)
66
4
-9
*
4
Ultrasound frequency: 25 kHz; 30 %, H O : 10 mmol; reaction time:
2 2
h; reaction temperature: 30 ºC.
As the data reported in literature , cyclohexene oxidized
by 30 % H to adipic acid in 55-94.5 % yield at 70-95 ºC
2 2
O
within 6-20 h. While under ultrasound and hydrogen peroxide-
silicotungstic acid-oxidation, the adipic acid was given in 92
The effect of amount of H
2
O
2
on reaction was investigated.
was 8, 9 and 10 mmol, the yield
was 65, 88 and 92 %, respectively. No improvement was
2 2
When the amount of H O
%
yields at 30 ºC within 4 h. The most important was the
useless of the phase transfer catalyst in ultrasonic system.
The reaction seems to occur via a classic multi-step
conversion including three kinds of oxidation pathway (olefin
epoxidation, oxidation of diol and Baeyer-Villiger oxidation)
2 2
observed while increasing the amount of H O (Table-2).
TABLE-2
2 2
EFFECTS OF THEAMOUNT OF H O ON THE OXIDATION
1
OF CYCLOHEXENE TO ADIPIC ACID UNDER
ULTRASOUND IRRADIATION*
and the hydrolysis process .
Amount of H
2
O
2
(mmol)
8
9
10
92
11
88
12
79
O
OH
Yield (%)
65
88
H O
H2O
H2O2
OH
2
2
O
*Ultrasound frequency: 25 kHz; silicotungstic acid: 0.04 mmol;
reaction time: 4 h; reaction temperature: 30 ºC.
OH
O
O
H2O
Under stirring condition, the adipic acid was obtained
in only 33 % yield within 4 h in the absence of ultrasound
H O
H2O2
COOH
COOH
2
2
O
O
OH
O
(
Table-3). While under ultrasound of 59, 40 and 25 kHz, the
Scheme-II: Synthesis of adipic acid via the oxidized by hydrogen peroxide
yield improved to70, 79 and 92 %, respectively in the same
time. It is apparent that the lower frequency of ultrasound can
accelerate the oxidation.
Conclusion
The effect of reaction time was shown in Table-4, no
improvement was found in a prolonged reaction time. The
reason may be that the prolonged reaction time would cause
many other by-products within the reaction system.
Ultrasound irradiation can accelerate the oxidation of
cyclohexene, high adipic acid yields, shorten reaction time,
optimize the reaction conditions and simplify operation
procedures.