Oxidation-reduction of maleic acid induced by argon-hydrogen plasma-jet

Article abstract of DOI:10.1246/bcsj.20100079

Plasma-jet in aqueous solutions is known to disproportionate water to give hydrogen and hydroxyl radicals. Maleic acid, which is a carbon-carbon double bond bearing dicarboxylic acid, underwent addition of hydrogen, hydrogen plus hydroxyl, hydroxyl species under an argon-hydrogen plasma-jet. Maleic acid (10mM) yielded 42% succinic acid (dihydrogenated product) under an argon-hydrogen (1.5 and 0.5 L min^-1) plasma-jet. This demonstrates an effective reduction method for maleic acid by plasma-jet in aqueous solutions.

Full text of DOI:10.1246/bcsj.20100079

1
264 Bull. Chem. Soc. Jpn. Vol. 83, No. 10, 1264–1268 (2010)
© 2010 The Chemical Society of Japan
Oxidation­Reduction of Maleic Acid Induced
by Argon­Hydrogen Plasma-Jet
1
1
2
1
Isao Yamakawa, Yoshimi Ito, Toratane Munegumi,* and Kaoru Harada
1Department of Chemistry, University of Tsukuba, Tennodai, Tsukuba 305-8571
2Department of Materials Chemistry and Bioengineering, Oyama National College of Technology,
Nakakuki, Oyama 323-0806
Received March 17, 2010; E-mail: munegumi@oyama-ct.ac.jp
Plasma-jet in aqueous solutions is known to disproportionate water to give hydrogen and hydroxyl radicals. Maleic
acid, which is a carbon­carbon double bond bearing dicarboxylic acid, underwent addition of hydrogen, hydrogen plus
hydroxyl, hydroxyl species under an argon­hydrogen plasma-jet. Maleic acid (10 mM) yielded 42% succinic acid
¹1
(
dihydrogenated product) under an argon­hydrogen (1.5 and 0.5 L min ) plasma-jet. This demonstrates an effective
reduction method for maleic acid by plasma-jet in aqueous solutions.
Plasma-induced reactions in gaseous, solid, and aqueous
would be useful for evaluating the effects of hydrogen and
hydroxyl radicals.
phases have been reported in the literature.1 Reactions
induced by plasma in aqueous solutions start with dispropor-
tionation of water molecules to hydrogen and hydroxyl radicals
as shown in Scheme 1. These radicals catalyze various
reactions: carbonium ion formation by hydrogen abstraction,
carbon­carbon bond formation between carbon radicals,
oxidation reactions by hydroxyl radicals, and so on. However,
compared to glow discharge7 into aqueous solutions, the
reactions by hydrogen radicals have not been well investigated.
­6
Experimental
Chemicals. Maleic acid for reactions, malonic acid, and
acetic acid were purchased from Wako Pure Chemical
Industries. Succinic acid, L-tartaric acid, meso-tartaric acid,
and dimethyl sulfate were purchased from Nacalai Tesque.
Malic acid was received courtesy of Fuso Chemical Co., Ltd.
Boron trifluoride methanol complex was purchased from Tokyo
Chemical Industry Co., Ltd.
,8
9
This study uses argon plasma-jet containing hydrogen gas in
the gas mixture for the redox reaction of carbon­carbon double
bonds. Maleic acid has one carbon­carbon double bond and
two carboxylic groups. Therefore, as shown in Scheme 2,
maleic acid can be transformed to succinic acid, malic acid, and
tartaric acid by hydrogen radicals, hydrogen plus hydroxyl
radicals, and hydroxyl radicals, respectively. The substrate
plasam-jet blowing
H
OH
H
+
OH
Scheme 1. Disproportionation of water to hydrogen and
hydroxyl radicals
H
H
2
H
H
C
C
H
HOOC
COOH
succinic acid
H
H
H
HO
H
OH
H
,
C
C
C
C
H
HOOC
COOH
HOOC
COOH
DL-malic acid
maleic acid
H
H
H
HO
H
2
HO
+
C
C
OH
COOH
HO
HOOC
C
C
OH
COOH
HOOC
meso-tartaric acid
DL-tartaric acid
Scheme 2. Oxidation­reduction of maleic acid induced by hydroxyl and hydrogen radicals.
Published on the web October 5, 2010; doi:10.1246/bcsj.20100079

I. Yamakawa et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010) 1265
Plasma torch
Power supply
Gas mixer
Gas flow
Cooling
water
Reaction
mixutre
Cooling
water
Retention time / min
Stirrer
Hydrogen
Argon
Figure 2. A typical gas chromatogram of methyl esters
derived from organic acids contained in a reaction mixture.
Column: FFAP glass capillary column (25 m © 0.25 mm
i.d.); Temperature program: 90 to 180 °C (3 °C min );
Carrier gas: helium.
Figure 1. Apparatus for argon­hydrogen plasma-jet reac-
tions into aqueous solutions. Volume of the reaction
solution: 300 mL.
¹1
Reaction Apparatus and Reaction Conditions.
The
carried out by a flame ionization detector. The injection port
temperature was 180 °C. The column temperature was pro-
apparatus for the plasma-jet reaction is shown in Figure 1. The
plasma torch was immersed into the reaction solution inside of
the reaction vessel. The reaction vessel and the solution were
cooled by circulating water in the outside of the vessel. The
reaction apparatus has another vent leading outside of the
vessel. A Liebig condenser was attached to the vent for cooling
the water vapor of the reaction solution. The plasma torch was
made by Nippon Welding Co., Ltd. The current and voltage
were maintained at 30 A and 10 V. The total flow rate of the gas
¹
1
grammed from 90 to 180 °C at a rate of 3 °C min
.
Analysis of Carbon Dioxide. The gas flowing out from
the reaction vessel was passed through a barium hydroxide
(Ba(OH) ) solution. Carbon dioxide was quantified by back
2
titration.
Results and Discussion
Ratio of Hydrogen to Argon in the Plasma-Jet. Figure 2
shows a gas chromatogram of standard methyl esters of maleic
acid, succinic acid, oxalic acid, and malonic acid. These
compounds were separated completely on the gas chromato-
gram.
¹
1
mixture introduced into the plasma torch was 2.0 L min to
mix the reaction solution.
Analysis of Maleic Acid, Malic Acid, DL-Tartaric Acid,
Malonic Acid, and Acetic Acid. The solution taken from
the reaction mixture was analyzed by means of reversed-
phase high-performance liquid chromatography. A Jasco TRI
ROTAR-V flow pump, a Jasco UVDEC-100-IV detector, and
a TSK gel ODS-80TM (4.6 mm i.d. © 250 mm) column were
used for analyses. The eluant (50 mM ammonium phosphate
The analytical method was applied to determine the
concentration of maleic and succinic acids under the plasma-
jet with different ratios of hydrogen to argon (hydrogen­argon:
¹
1
0, 2.0 (0%); 0.1, 1.9 (5%); 0.3, 1.7 (15%); 0.5, 1.5 L min
(25%)). The hydrogen ratio of 25% was the maximum ratio to
generate a stable plasma-jet. When pure argon in the plasma-jet
was used for the reaction, hydrogenated product, succinic acid
reached a maximum yield (8%) in 20 min as shown in
Figure 3a. On the other hand, when a mixture (25% hydrogen)
of argon­hydrogen was used for the reaction, the yield of
succinic acid reached 42% in 80 min as shown in Figure 3b.
The substrate maleic acid disappeared by a reaction time of
60 min (argon plasma) and 80 min (argon­hydrogen mixture
(25% hydrogen) plasma), respectively. The decrease of the
substrate was faster under argon than argon­hydrogen plasma-
jet and the increase of product was faster under argon­
hydrogen than argon plasma. What caused the difference
seemed to be the additional hydrogen radical provided from
argon­hydrogen plasma.
¹
1
pH 2.4) flowed at a constant rate of 0.3 mL min . The eluted
solution was detected at 210 nm absorbance.
Method for Analysis of meso-Tartaric Acid.
Sample
solution (1 mL) taken from the reaction solution was lyophi-
lized to dryness. The resulting residue was dried in a desiccator
under vacuum overnight in the presence of diphosphorus
pentaoxide and was followed by esterification using boron
trifluoride methanol complex (0.5 mL) at 70 °C for 30 min. The
reaction mixture was diluted to 5 mL using distilled water. The
solution was analyzed by means of the HPLC system described
above using 10% acetonitrile in water.
Analysis of Succinic Acid and Oxalic Acid. A 2 mL
portion of the diluted solution described in the analysis of
meso-tartaric acid was extracted with benzene (1 mL © 4), and
the extracted benzene solution was analyzed with a Hitachi 163
gas chromatograph equipped with a 5% FFAP glass capillary
column (3 mm i.d. © 3 m, GL Sciences, Inc.). Detection was
Figure 4 shows the dependence of yield of succinic acid on
the ratio of hydrogen to argon in the plasma-jet. The reaction
time at which the yield of succinic acid achieved the maximum

1
266 Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010)
00
Oxidation­Reduction of Maleic Acid by Plasma
1
100
90
80
70
60
50
40
(
a)
(
b)
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
30
20
10
0
0
20
40
60
80
0
20
40
60
80
100
Time / min
Time / min
Figure 3. Yield of succinic acid ( ) and recovery of maleic acid ( ) in the reactions induced by pure argon plasma (a) and 75%
argon­25% hydrogen plasma (b) into 10 mM maleic acid.
Figure 4. Succinic acid formation in 10 mM maleic acid
induced by argon­hydrogen plasma at different ratios as
follows: ( ), pure argon; ( ), 95% argon­5% hydrogen;
Figure 5. Succinic acid formation induced by 75% argon­
25% hydrogen plasma in different concentrations of maleic
acid as follows: ( ), 1; ( ), 5; ( ), 10; ( ), 50; and ( ),
100 mM.
(
), 85% argon­15% hydrogen; ( ), 75% argon­25%
hydrogen.
Table 1. Maximum Yield of Succinic Acid and the Initial
Concentration of Maleic Acid
became later with a higher rate of hydrogen to argon in the
plasma-jet. In the case of 25% hydrogen plasma, the time
proceeded and the maximum yields of succinic acid at the
following times: 80 min, 42%; 40 min, 30%; 30 min, 15%;
Initial concentration
of maleic
Maximum yield
of succinic
acid/%
Time maximum
yield was
acid/mM
achieved/min
2
0 min, 8%.
1
5
10
50
100
36
40
42
40
40
40
60
80
80
120
Concentration of Substrates. Figure 5 shows succinic
acid formation induced by plasma-jet with mixtures of 75%
argon­25% hydrogen in different concentrations of maleic
acid. In all cases the yield of succinic acid increased with time,
achieved maximum, and then decreased. The reaction time at
which the yield of succinic acid achieved maximum became
later with higher concentration of maleic acid in the plasma-jet.
The maximum yield of succinic acid and the initial concen-
tration of maleic acid are shown in Table 1 with the time
maximum yield was achieved. The maximum yield of succinic
acid did not depend on the initial concentration of maleic acid.
Oxidative Products. Instead of using GC for the analysis
of reduction product succinic acid, oxidative products (acetic
acid, tartaric acid, malic acid, and malonic acid) were analyzed
by HPLC. Figure 6 shows a HPLC of standard methyl esters of
maleic acid, succinic acid, oxalic acid, and malonic acid. These
compounds were separated completely by HPLC. The oxida-
tive products obtained from the reactions using argon plasma
and argon­hydrogen plasma are shown in Figures 7a and 7b,
respectively.
The plasma-jet using pure argon gave 13% malonic acid at
50 min as shown in Figure 7a. Malonic acid has three carbons

I. Yamakawa et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010) 1267
and seems to be formed by the oxidation of an alfa carbon of
maleic acid followed by decarboxylation. Maximum yields of
malic acid, DL-tartaric acid, and meso-tartaric acid were obtain-
ed in 5.5%, 3.0%, and 1.5% at 20 min, respectively. However,
maximum yield of oxalic acid was obtained in 7.0% at 40 min.
The time delay could mean that oxalic acid was formed by the
oxidative degradation of malic acid, DL-tartaric acid, acetic
1
0,11
acid, and/or meso-tartaric acid. Although other research
on
argon plasma jet reactions has shown the pathway of oxalic
acid formation through acetic acid from malonic, a detailed
reaction mechanism cannot be proposed at this stage.
The plasma-jet using 25% hydrogen in argon gave little
oxidative product compared with those in argon only as shown
in Figure 7b. Even malic acid, the yield of which was the
highest among the oxidative products, gave a maximum yield of
only 5.5%. Other products were obtained in less than 2% yield.
In particular, meso-tartaric acid was not detected under these
reaction conditions. A plausible mechanism is shown in
Scheme 3. The series of reactions would start with the addition
of hydroxyl radical to the C­C double bond. The resulting first
radical intermediate (intermediate 1) may be in equilibrium
with the second radical intermediate (intermediate 2). The
intermediate 1 yields DL-tartaric acid by coupling with a
hydroxyl radical and also yields DL-malic acid by coupling
with a hydrogen radical, respectively. Although intermediate 2
yields meso-tartaric acid, it can also be produced by hydrogen
abstraction from DL-malic acid. In this case, there are two
pathways for the formation of intermediate 2 that leads to meso-
tartaric acid. However, meso-tartaric acid was not detected in the
reaction using 75% argon­25% hydrogen plasma. This suggests
that 75% argon­25% hydrogen plasma is not effective for the
abstraction of hydrogen radical from malic acid to afford
intermediate 2. Malonic acid would be formed by the hydrox-
ylation of the intermediate 1, and the acetic acid from the
decarboxylation of malonic acid. Oxalic acid was detected in
lower yield than that in the reaction using pure argon plasma.
Degradation of Succinic Acid, DL-Malic Acid, and DL-
Tartaric Acid. Although maleic acid can be converted into
succinic acid, malic acid, and tartaric acid, these products
undergo oxidative degradation to give several by-products.
Degradation of succinic acid must proceed during the forma-
tion of succinic acid induced by the plasma-jet. Degradation of
Retention time / min
Figure 6. A reversed-phase high performance liquid chro-
matogram of organic acids contained in the reaction solu-
tion. Column: TSK gel ODS-80TM (250 mm © 4.6 mm
¹1
i.d.); Buffer: 50 mM NH4H2PO4; Flow rate: 0.3 mL min
Detection: UV at 210 nm.
;
1
1
1
4
2
0
14
(
b)
(
a)
12
10
8
8
6
4
2
0
6
4
2
0
0
20
40
60
80
0
20
40
60
80
Time / min
Time / min
Figure 7. Oxidative products in the reaction solution of 10 mM maleic acid induced by pure argon (a) or 75% argon­25% hydrogen
b) plasma. Oxidative products are as follows: ( ), malonic acid; ( ), malic acid; ( ), oxalic acid; ( ), DL-tartaric acid; ( ), acetic
acid; and ( ), meso-tartaric acid.
(

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268 Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010)
Oxidation­Reduction of Maleic Acid by Plasma
HO
H
COOH
C
H
HOOC
C
oxalic acid
OH
DL-tartaric acid
OH
H
H
OH
OH
OH
C
OH
C
OH
C
H
C
C
COOH
OH
H
C
H
H
H
H
HOOC
COOH
COOH
HOOC
HOOC
COOH
HOOC
maleic acid
intermediate 1
intermediate 2
meso-tartaric acid
H
H
-H
H
HO
HO
H
O
O
COOH
OH
H2O
COOH
C
C
C
C
C
C
H
HOOC
H
-
HO
OH
H
HOOC
oxalic acid
DL-malic acid
OH
H
C
OH
C
H
H
OH
C
H
H
HO
COOH
- CO2
- CO2
C
C
C
HOOC
O
COOH
O
H
H
HO
malonic acid
acetic acid
Scheme 3. Plausible mechanism for the formation of oxidative products.
1
00
containing 75% argon and 25% hydrogen. This shows that
argon­hydrogen plasma-jets can be used to reduce carbon­
carbon double bonds in good yields. This method can also
cause an oxidation­reduction of carbon­carbon double bonds to
give DL-malic acids by hydroxyl and hydrogen radicals as well
as tartaric acid by hydroxyl radicals. This research also suggests
a mechanism of oxidation­reduction of other products.
9
0
0
0
0
0
0
0
0
0
8
7
6
5
4
3
2
1
References
1
J. R. Hollaham, A. T. Bell, Techniques and Applications of
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2
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0
0
10
20
30
40
50
60
70
80
Time / min
3
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Figure 8. Degradation of succinic acid ( , ), DL-malic
acid ( , ), and DL-tartaric acid ( , ) induced by pure
4
K. Harada, M. Nomoto, J. Synth. Org. Chem., Jpn. 1982,
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,
) or 75% argon­25% hydrogen
4
plasma ( , ). Concentration of substrates: 10 mM.
,
5
by K. Matsuno, K. Dose, K. Harada, D. L. Rohlfing, Plenum Press,
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6
7
K. Harada, J. Synth. Org. Chem., Jpn. 1990, 48, 522.
E. Kokufuta, T. Sodeyama, K. Fujimori, K. Harada, I.
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succinic acid can be consumed slower than both malic acid and
DL-tartaric acid under the plasma-jet conditions.
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Lett. 1985, 1569.
8
Conclusion
9
K. Harada, Y. Ito, T. Wada, T. Munegumi, Bull. Chem. Soc.
This research demonstrated that an argon­hydrogen plasma-
jet in aqueous solutions of maleic acid (10 mM) gave succinic
acid up to 42% as a reduction product using a plasma-jet
Jpn. 2001, 74, 2175.
10 K. Harada, M. Takasaki, Tetrahedron Lett. 1983, 24, 4839.
11 M. Takasaki, K. Harada, Tetrahedron 1985, 41, 4463.