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.

AbstractA 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 30º
Keywordssynthesis, oxalic acid, contact glow discharge plasma.

Contact glow discharge is a kind of non-thermal plasma which creates a lot of energetic species such as ·OH, ·H, eaq, HO·, H, 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 H were detected in quantity during the contact glow discharge process in aqueous formic acid solutions, proposed the following reactions:
gasgas + eaq
gas →·OH + ·H
gas + H →·OH + H
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.

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 IIIelemental analyzer (Elementar, Germany), a DIGILΛB FTS 3000 FT-IR spectrometer (DIGILΛ,USA), and a WRS-1B digital melting point apparatus (Zhongshen, Shanghai, China). There is a post about organic chemistry news that you may like. You should check it out.
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 C·2HO (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 186.5-187.9º, 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

Roughly speaking, the yield of oxalic acid is proportional to the amount of formic acid in solution, as listing in Table 1. Note that this is an electrolysis process even though a non-Faraday electrolysis process, that is, a large of organic substances would cause the decrease of solution conductance to consume the electrical energy. At this moment for the excess existence of organic substances, the over-heat would make the electrodes molten. In this work, a ratio of formic acid-to-distilled water (v/v) was used to be 5 : 95.

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

Formic acid : distilled water
1:99 5:95 10:90 15:85 20:80
Formation of oxalic acid  (g) 0.0110 0.2080 0.2513 0.2756 0.2854

Perhaps, for the commonly chemical reaction, the yield of product would increase with increasing reaction time. However, for the reaction caused by plasma was not similar. Just as mentioned above, the attack coming from the energetic species to substances in aqueous solution is no selection. Thus, the formed oxalic acid could also be decomposed into CO and HO. From Table 2, it can be seen that the reaction time of 90 min was enough.

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

Reaction time  (min) 60 90 120 150 180
Formation of oxalic acid (g) 0.1676 0.2080 0.2125 0.2440 0.2675

The reaction temperature should be controlled due to being lots of Joule heating during glow discharge process. Otherwise, the electrodes would be destroyed. In fact, the higher temperature is not benefit to formation of oxalic acid, listing in Table 3.

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

Reaction temperature   (º 10-15 20-25 30-35 40-45 50-55 60-65
Formation of oxalic acid 0.2478 0.2140 0.2080 0.1980 0.1807 0.1601

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.

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.