A new approach to synthesize oxalic acid from formic acid by contact glow discharge plasma

Gao Jinzhang, Fu Yan, Wang Aixiang, Wu Jianlin, Li Yan, Yang Wu
(College of Chemistry & Chemical Engineering,Northwest Normal University, Lanzhou 730070, China)

Received June 6, 2008.

Abstract A new approach to synthesize oxalic acid from formic acid by using contact glow discharge plasma was made successfully. The product obtained was characterized by using high-performance liquid chromatography, FT-IR spectra, elemental analysis and melting point apparatus. The effects of the ratio of reactant, reaction time and temperature were examined in detail. The yield of oxalic acid (after recrystallizing) can reached 10.6% under the following conditions: the ratio of 5:95 (v/v) for formic acid to water, the reaction time of 90 min, and the reaction temperature of 30ºC.
Keywords synthesis, oxalic acid, contact glow discharge plasma.

1. INTRODUCTION
Contact glow discharge is a kind of non-thermal plasma which creates a lot of energetic species such as ·OH, ·H, e^-_aq, HO₂·, H₂O₂, and so on; and these energetic species can be used to induce some unusual chemical changes in the solution ^[1,2]. So far, this technique has been studied widely in the field of wastewater treatment and gotten many potential applications^[3]. However, relatively few papers are devoted to synthetic chemistry, that is, how to use these energetic species to initiate a formation reaction. As far back as the 1970s, Harada et al ^[4,5] found firstly the formation of some amino acids during the contact glow discharge process in aqueous solution containing aliphatic carboxylic acid or elemental carbon. Then, Sengupta and co-workers ^[6] reported that the oxalic acid and H₂O₂ were detected in quantity during the contact glow discharge process in aqueous formic acid solutions, proposed the following reactions:
H₂O_gas →H₂O⁺_gas + e^-_aq
H₂O_gas →·OH + ·H
H₂O⁺_gas + H₂O →·OH + H₃O⁺
·OH + HCOOH →·COOH + H₂O
·COOH +·COOH →HOOC-COOH
    The above results imply that a potential application in the synthetic chemistry would appear in future.
    The primary aim of this study is to obtain the solid oxalic acid. It is not a simple way to do so, because the attack coming from the energetic species to substances in aqueous solution is no selection. In other words, the oxalic acid formed could also be destroyed by energetic species. Thus, variable parameters should be optimized firstly in this study.

2. EXPERIMENTAL
The experimental apparatus used consists of a high voltage power supply and a reactor just as shown in previous works ^[2,3]. The power supply was a Model LW100J1 DC power supply (Liyou, Shanghai, China) providing voltages of 0-1000 V and a current range of 0-1 A. The reactor contains a platinum wire anode and a graphite stick as cathode. A pointed platinum anode with a diameter of 0.5 mm was sealed into a Pyrex glass tube to generate glow discharge in aqueous solution. The cathode was a graphite stick (diameter: 10 mm) being parallel to the anode with a distance of about 20 mm. The discharge reactor was coated with an outer water jacket to keep the solution at a constant temperature. A magnetic stirring bar was used at the bottom of reactor to keep the solution mixed well. The reaction process was monitored by means of a high-performance liquid chromatography (Shimadzu HPLC-LC-6A) equipped with a spectrophotometric UV/V detector (Shimadzu-SPD-6AV) and a Kromasil 100-5 C18 column (250×4.6 mm). The detection wavelength was at 210 nm with a sulfuric acid solution (pH=2.10) as mobile phase. The pure product was examined by a VARIO EL III elemental analyzer (Elementar, Germany), a DIGILΛB FTS 3000 FT-IR spectrometer (DIGILΛB，USA), and a WRS-1B digital melting point apparatus (Zhongshen, Shanghai, China).
    General procedure for synthesizing oxalic acid was given below: a desired ratio of formic acid to distilled water was added into a 100 mL flask with stirring, the reaction with glow discharge was carried out at a certain temperature and time. After finishing the reaction, the resulting solution was evaporated to near dryness and then, brought to room temperature, finally, filtered to obtain the white crystal. Recrystallization was done with distilled water. Elemental analysis data were calculated for a formula of C₂H₂O₄·2H₂O (Found): C,19.05 (19.23); H,4.76 (5.52); O,76.19 ( 75.25). After dehydrating, the melting point of product was detected by WRS-1B digital melting (without correction) to be 186.5-187.9ºC, implying that a good quality oxalic acid was prepared. To assess further the quality of product, the FT-IR spectra of two oxalic acid samples, in which one is analytical reagent grade, were given in Figure 1.
    It is clear that two profiles are the same. The broad absorption near 3430 cm^-1 was devoted by the stretching vibration of -OH group; the peak at 1690 cm^-1 was attributed to the C=O group; the absorption at 1257 cm^-1 was corresponding to the C-O stretching vibration; and the absorption at 722 cm^-1 should be the structure vibration of C-C group.
    　
Figure 1. FT-IR spectra of oxalic acid
a: synthesized product;   b: analytical reagent

Figure 2. Chromatograms of formic acid under glow discharge plasma
   a: formic acid;    b: oxalic acid

Table 1 Influence of formic acid dosage on the formation of oxalic acid

Table 2 Influence of reaction time on the formation of oxalic acid

Table 3 Effect of reaction temperature on the formation of oxalic acid

    In conclusion, it may be said that it is possible to synthesize oxalic acid in solid from formic acid by using the glow discharge plasma technique. In appropriate conditions the solid oxalic acid yield of 10.6% was obtained. Based on the data of HPLC, about 48% of formic acid was converted (maybe including degradation itself). The contact glow discharge plasma is, truly, a green energy source, even though it has not been used in practice. A strongly potential ability for application in synthetic chemistry will appear in future.

Acknowledgements
This work was supported in part by the Project of Key Science and Technology of Education Ministry (00250), the Nature Science Foundation of Gansu Province (3ZS041-A25- 028), the Invention Project of Science & Technology of NWNU (KJCXGC-01), and Gansu Key Lab of Polymer Materials, China.

REFERENCES
[1] A.R. Denaro, A. Hicking, J. Electrochem. Soc., 1958, 105: 265.
[2] J.Z. Gao, A.X. Wang, Y. Fu et al., Plasma Sci. Technol., 2008, 10: 30.
[3] J.Z. Gao, Pakistan J. Biol. Sci., 2006, 9: 323.
[4] K. Harada, T. Iwasaki, Nature, 1974, 250: 426.
[5] K. Harada, S. Suzuki, Nature, 1977, 266: 275.
[6] S.K. Sengupta, R. Singh, A.K. Srivastava, Indian J. Chem., 1995, 34A: 459.

利用接触辉光放电电解等离子体在甲酸溶液中合成草酸的尝试
高锦章，付燕，王爱香，吴建林，李岩，杨武
(西北师范大学 化学化工学院， 730070 兰州)
摘要 接触辉光放电电解等离子体技术在水处理中的研究与应用已有许多报道，相对而言，在合成化学中的应用尚不多见，常见有报道在溶液中检测到的产物，很难分离。由于羟基自由基缺乏选择性，产物的生成与降解两个过程并存。另外，必须考虑到反应物在水中的溶解度，若难溶于水，则很难利用该技术，因为这毕竟属于电解过程，等离子体只能在含电解质的水溶液中产生。由于这是一项绿色能源技术，有待深入研究。本文在不损坏电极与设备的前提下，在稀甲酸溶液中合成并提纯得到了白色结晶草酸，产率达到10.6%，当属一种新的尝试。
关键词 合成 草酸 接触辉光放电电解等离子