Efficient dimerization of formic acid in aqueous solution induced by Ar-Arc plasma

Article abstract of DOI:10.1246/bcsj.74.2175

Introduction of argon-arc plasma into an aqueous solution of formic acid resulted in the formation of oxalic acid. The reaction conditions for the dimerization were studied. The maximum yield of oxalic acid reached 47-50% when the reaction was carried out using sodium formate or calcium formate. Dimerization of acetic acid was also examined.

Full text of DOI:10.1246/bcsj.74.2175

c. Jpn.
© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 2175–2179 (2001) 2175
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Efficient Dimerization of Formic Acid in Aqueous Solution Induced by
Ar-Arc Plasma
Dimerization of Formic Acid by Ar-Arc Plasma
K. Harada et al.
Kaoru Harada,# Yoshimi Ito, Toshinori Wada, and Toratane Munegumi*,†
01
75
79
SJA8
09-2673
150
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8677
†Department of Materials Chemistry and Bioengineering, Oyama National College of Technology, Nakakuki,
Oyama, Tochigi 323-0806
(Received May 1, 2001)
01
01
Introduction of argon-arc plasma into an aqueous solution of formic acid resulted in the formation of oxalic acid.
The reaction conditions for the dimerization were studied. The maximum yield of oxalic acid reached 47–50% when the
reaction was carried out using sodium formate or calcium formate. Dimerization of acetic acid was also examined.
.2
3.1
Various chemical reactions induced by plasma have been
Results and Discussion
studied1 in laboratory and also in industry. However, most of
the reactions have been carried out in gaseous or solid phase,
and few studies on plasma induced reactions in aqueous solu-
tion have been reported.2 We have studied various types of
plasma-induced chemical reactions3–5 in aqueous solutions:
these include various types of oxidation,6–9 amination,10–12 car-
boxylation,13–15 hydrogenation,16–17 and fixation of nitro-
gen.18-22
Formic acid has been used for carboxylation reactions in
aqueous solution by the use of glow discharge,13–15 argon-arc
plasma,23 or burning flame.24 In these reactions, the primary
products are hydroxyl and hydrogen radicals25,26 which are
formed by dissociation of water molecules. These hydroxyl
and hydrogen radicals substract hydrogen from formic acid to
form carboxyl radical (OCOOH). The latter radical is relative-
ly stable in aqueous solution compared with other radicals and
can be used for various carboxylation reactions of organic
compounds in aqueous solution. In this paper, studies on the
coupling reactions of formic acid and acetic acid are presented
as shown in Scheme 1.
When argon-arc plasma was introduced into an aqueous so-
lution of formic acid, a small amount of oxalic acid was detect-
ed. Oxalic acid was formed by dimerization of formic acid
radicals. The main factors influencing the formation of oxalic
acid could be the stability of monomer radical and the stability
of the dimerized product. Therefore, the stability of oxalic ac-
id, sodium oxalate, calcium oxalate, and oxamide in aqueous
solution was examined by the introduction of argon arc plas-
ma. Figure 1 shows the time course of the degradation of oxal-
ic acid and its derivatives. Free oxalic acid was found to be
very unstable. Therefore, it could be expected that oxalic acid
formed by the dimerization of formic acid radical was rapidly
decomposed under the reaction conditions. By employing the
new information on the instability of various forms of oxalic
Fig. 1. Degradation of oxalic acid, oxamide, calcium ox-
alate, and sodium oxalate induced by Ar-arc plasma (10 V,
10 A).
ꢀ: Calcium oxalate (1.01 mmol/50 mL H2O), ꢁ: Oxamide
(1.18 mmol/50 mL H2O), ꢂ: Sodium oxalate (1.09 mmol/
50 mL H2O), ꢃ: Oxalic acid (1.06 mmol/50 mL H2O).
Scheme 1. Dimerization reactions of formic acid and acetic
acid.
# Present address: Kobe Shoin Women’s University, Nada-ku,
Kobe, Hyogo 657-0015

2176 Bull. Chem. Soc. Jpn., 74, No. 11 (2001)
Dimerization of Formic Acid by Ar-Arc Plasma
acid shown in Fig. 1, the dimerization reaction was carried out
by using sodium formate, calicium formate or formamide in
aqueous solution by using Ar-arc plasma.
Figure 2 shows the time course of the dimerization reaction
of 0.5 M sodium formate (1 M = 1 mol dm−3). The maximum
yield of oxalic acid reached 47%. In a similar way, Figs. 3 and
4 show the time course of the dimerization reaction of 0.05 and
0.25 M calcium formate. The maximum yield of oxalic acid
reached 48% and 38%, respectively.
The great enhancement in the yield of oxalic acid from for-
mic acid salts compared with the yield (1.3% and 0.44%
shown in Table 1) of oxalic acid from formic acid could be due
to the following two reasons. One is the enhancement of the
stability of the resulting anion radical (OCOO−) compared with
formic acid radicals (OCOOH) under Ar-arc plasma blowing.
The other reason is the stability of sodium and calcium salts of
oxalic acid shown in Fig. 1. Under basic conditions, both the
stability of the intermediate monomer radical and that of the
resulting products contribute in the same direction to the in-
crease in the yield of the dimerized products. In addition, in the
case of the dimerization of calcium formate, the solubility of
the resulting calcium oxalate is very low, and the product is
precipitated out of the reaction mixture. This enables the prod-
uct to further resist decomposition to carbon dioxide under the
harsh high energy reaction conditions.
Figure 5 shows the time course of dimerization of a 0.5 M
formamide solution. The maximum yield of oxamide was
14%. As shown in Fig. 1, the stability of oxamide under the
conditions used is rather high. Therefore, the reason for the
relatively low yield may be the instability of the carbamoyl
radical (OCONH2). However, the yield is much higher than
that of the dimerization product of free formic acid. Thus, by
the use of sodium or calcium formate and formamide, the
dimerization reactions were carried out successfully. Summa-
rized results of the dimerization reaction of formic acid and
formamide are shown in Table 1.
Although the free formic acid gave much lower maximum
Fig. 2. Formation of sodium oxalate from 0.5M sodium for-
mate solution induced by Ar-arc plasma (10 V, 10 A).
ꢂ: Sodium formate, ꢄ: Sodium oxalate.
Fig. 4. Formation of calcium oxalate from 0.25 M calcium
formate solution induced by Ar-arc plasma (10 V, 10 A).
ꢃ: Calcium formate, ꢀ: Calcium oxalate.
Fig. 5. Formation of oxamide from 0.5 M formamide solu-
tion induced by Ar-arc plasma (10 V, 10 A).
ꢅ: Formamide, ꢁ: Oxamide.
Fig. 3. Formation of calcium oxalate from 0.05 M calcium
formate solution induced by Ar-arc plasma (10 V, 10 A).
ꢃ: Calcium formate, ꢀ: Calcium oxalate.

K. Harada et al.
Bull. Chem. Soc. Jpn., 74, No. 11 (2001) 2177
Table 1. Dimerization Reaction of Derivatives of Formic Acid Induced by Ar-Arc Plasma
Substrate
Concentration
Product
Maximum yield
Reaction time
M
0.1
0.5
%
1.3
0.44
min
10
140
Formic acid
(HCOOH)
Oxalic acid
(HOOC-COOH)
Formamide
(HCONH2)
0.005
0.01
0.05
0.10
0.50
Oxamide
(H2NOC-CONH2)
4.3
9.8
10
14
14
10
20
30
45
120
Sodium formate
(HCOONa)
0.1
0.5
Sodium oxalate
(NaOOC-COONa)
33
47
15
60
Calcium formate
((HCOO)2Ca)
0.05
0.25
Calcium oxalate
((COO)2Ca)
50
38
90
105
yields (1.3%, 0.44%) of the dimerized product (oxalic acid),
formamide and formic acid salts gave higher maximum yields
of their dimerized products (oxamides: 4.3–14% and oxalic
acid salts: 33–50%). The reasons were already described
above. The maximum yields of oxamide were similar inde-
pendently of the initial concentration of formamide. However,
it took a longer time with the higher initial concentration of
formamide until oxamide reached the maximum. Sodium for-
mate and calcium formate gave almost the same yields of oxal-
ic acid salts. The results showed clear effects of sodium and
calcium salts of formate on the great enhancement of dimeriza-
tion.
Similar reaction conditions were applied for the dimeriza-
tion of acetic acid. However, the yields of the resulting succin-
ic acid using free acetic acid, sodium acetate, calcium acetate,
and acetamide at concentrations of 0.1–0.5 M were all in the
range of 1–5%. In these reactions, the formation of malonic
acid (2–6%) was observed. Malonic acid would be formed by
the oxidative degradation of succinic acid, because the maxi-
mum yield of malonic acid was obtained after the decreasing
of succinic acid. Figure 6 shows the time course of the dimer-
ization of calcium acetate induced by Ar-arc plasma in aque-
ous solution. Degradation of succinic acid and malonic acid
were studied under similar conditions, as shown in Figs. 7 and
8. The results indicate that succinic acid and malonic acid
were unstable in the free and also in the salt form under the
conditions used, compared with the salts of oxalic acid as
shown in Fig. 1. The other reason for the low yield of succinic
acid could be that the carboxymethyl radical (OCH2COOH) is
unstable and is rapidly oxidized to glycolic acid, glyoxylic ac-
id, and oxalic acid. These oxidized products were confirmed
in the reaction mixture of aqueous acetic acid applied to elec-
tric discharge.5 Thus, it was found that the effective dimeriza-
tion of acetic acid by the use of Ar-arc plasma is difficult.
Fig. 6. Formation of calcium succinate and calcium ma-
lonate from 0.25 M calcium acetate solution induced by
Ar-arc plasma (10 V, 10 A).
ꢃ: Calcium acetate, ꢀ: Calcium succinate, ꢁ: Calcium
malonate.
Fig. 7. Degradation of succinic acid, sodium succinate, cal-
cium succinate and succinamide induced by Ar-arc plasma
(10 V, 10 A).
ꢆ: Succinamide (1.02 mmol/50 mL H2O), ꢀ: Succinic
acid (1.04 mmol/50 mL H2O), ꢁ: Sodium succinate (1.02
mmol/50 mL H2O), ꢄ: Calcium succinate (1.03 mmol/50
mL H2O).
Experimental
Reagents: Formic acid, sodium formate, calcium formate,
oxalic acid, calcium oxalate, formamide, oxamide, acetic acid,
malonic acid, acetamide, succinamide, sodium hydroxide, and cal-
cium hydroxide used in this study were all special grade and were
used without purification. Succinic acid was recrystallized from

2178 Bull. Chem. Soc. Jpn., 74, No. 11 (2001)
Dimerization of Formic Acid by Ar-Arc Plasma
were maintained at 10 V and 10 A.
Analytical Method: In the analysis of the dimerization of
formic acid, formic was analyzed by a Dionex 2000i ion chro-
matograph equipped with a Dionex HPICE-AS1 column (250 mm
× 9 mm I.D.). Eluent: 0.125 mM octanesulfonic acid. Flow rate:
1.0 mL/min. Oxalic acid was analyzed using a capillary isotacho-
electrophoresis apparatus (Shimadzu IP-2A). Leading electrolyte
solution: 0.01 M hydrochloric acid containing 0.5% triton X-100
and β-alanine. Terminal electrolyte solution: 0.01 M hexanoic ac-
id. Capillary column: 100 mm × 0.5 mm I.D. Detector: electropo-
tential gradient detector at 20 °C.
The analyses of formic acid and oxalic acid formed in the
dimerization of sodium formate were carried out on an ion chro-
matograph equipped with a Dionex HPICE-AS1 (250 mm × 9
mm I.D.) column and a TSK gel IC-Anion-PW column (50 mm ×
4.6 mm I.D.), respectively. Eluent: 0.125 mM octanesulfonic acid
(for formic acid) and boric acid buffer solution (pH 8.5) (for oxal-
ic acid) which was composed of boric acid (360 mg), sodium tet-
raborate (500 mg), glycerol (5.0 g), potassium gluconate (300
mg), acetonitrile (120 mL), and 1-butanol (30 mL) in one liter of
solution. Flow rate: 1.0 mL/min in both analysis.
Fig. 8. Degradation of malonic acid, sodium malonate and
calcium malonate induced by Ar-arc plasma (10 V, 10 A).
ꢃ: Malonic acid (1.03 mmol/50 mL H2O), ꢅ: Sodium ma-
lonate (1.11 mmol/50mL H2O), ꢂ: Calcium malonate
(1.06 mmol/50 mL H2O).
Analyses of the reaction products obtained by dimerization of
calcium formate were carried out using a capillary column isota-
choelectrophoresis instrument. Leading electrolyte solution was
similar to the one mentioned above, except that the pH was 3.7.
In the dimerization reaction of formamide, the resulting oxam-
ide was analyzed by HPLC (JASCO Trirotar-V with a UV-Spec-
trophotometer UVIDEC-100-IV) at 215 nm, column ODS (250
mm × 4.6 mm I.D.), eluent liquid: H2O, flow rate 0.5 mL/min.
Formamide was analyzed by a gas chromatograph (Hitachi 163)
under the following conditions: column, (UNISOLE F-200 (2 m
× 3 mm I.D.) at 150°C; injection temp., 250°C; carrier gas, N2;
detector, FID (flame ionization detector).
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Fig. 9. Apparatus for Ar-arc plasma induced reaction in
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