EP0221790A1 - Process for the production of glyoxylic acid by the electrochemical reduction of oxalic acid - Google Patents

Abstract

This process carried out at a temperature between 0 and 30 ° C in an electrolyser comprising, at least one anode compartment containing an anode and an anolyte, consisting of an acidic aqueous solution, at least one cathode compartment and a catholyte consisting of an aqueous solution of oxalic acid and, between these two compartments, at least one separator, is characterized in that the anode is constituted by a solid conductor uniformly covered with lead dioxide.

Description

The present invention relates to a process for the manufacture of glyoxylic acid by electrochemical reduction of oxalic acid.

Glyoxylic acid is a synthetic industrial intermediate commonly used to access various raw materials such as p- _h ydroxymandelic acid, p-hydroxyphenylglycine. It is mainly obtained by gentle oxidation of glyoxal or by electrochemical reduction of oxalic acid.

The electrochemical reduction of oxalic acid to glyoxylic acid has been known for a very long time and it is generally carried out in an acid medium, at low temperature, with electrodes with a high hydrogen overvoltage, in the presence or not of a protonic mineral acid such as than sulfuric acid, an ion exchange membrane and it is usually carried out with a circulation of the electrolyte (German patents n ⁸ 163.842, 194.038. 204.787, 210.693, 292.866. 347.605, 458.436; French patents
no. 2.062.822, 2.151.150; Indian Patent No. 148,412; W. HOHRSCHULZ. Z. Elektrochem. 1926.82, 449; S. AVERY et al, Ber. 1899. 32. 2233-38; E. BAUR, Z. Elektrochem., 1919, 25. 104-5). In these long-term industrial operations, the methods described in the prior art are not entirely satisfactory. Indeed, we very often observe either a gradual drop in electrical efficiency (see German patent n ° 347.605, HDC RARSON et al, J. Appl. Chem. 1963, 13, 233-9), or an increasingly higher hydrogen at the cathode, a rapid consumption of the anode.

In order to remedy these drawbacks, it has been proposed to carry out the reduction either in the presence of tertiary amine or of quaternary ammonium (French patents No. 2,151,151, 2,208,876) or by carefully controlling the temperature of the catholyte (request Japanese Patent No. S5-58380). However, these solutions do not do not prevent progressive destruction of the anode.

However, the Applicant has surprisingly discovered a simple and economical process for the electrochemical reduction of oxalic acid to glyoxylic acid which overcomes this drawback. This process carried out at a temperature between 0 ° C and 30 ^° C in a having electrolyzer, at least one anode compartment containing an anode and an anolyte, at least one cathode compartment containing a cathode and a catholyte consisting of an aqueous solution of oxalic acid and, between these two compartments at least one separator, is characterized in that the anode is constituted by a solid conductor uniformly covered with lead dioxide.

According to the process of the invention, the coating of lead dioxide is uniform, compact, adherent to the substrate and it has a thickness of 0.2 to 5 mm. In order to facilitate its adhesion, it is sometimes advantageous to introduce between the substrate and the layer of lead dioxide, a thin metallic intermediate layer constituted by a metal chosen from copper, silver, gold.

The coating of lead dioxide is carried out by means known per se. For example, it can be carried out by electrolytic deposition on the solid conductive support chosen, previously cleaned, in an electrolyser equipped with a copper cathode and containing an electrolyte constituted by an acidic aqueous solution of lead (II) nitrate and nitrate. of copper (II) maintained at a pH = 2 by addition of copper (II) carbonate and lead (II) oxide under a current density of the order of 30 to 50 mA / cm ² and at a temperature of about 60 ^* C.

The uniformity of the deposit as well as its adhesion, its compactness and its thickness which is easily adjusted by the duration of the electrolytic deposit, are easily controlled by observation with a scanning electron microscope of the surface of the deposit and of the wafer after breaking of the electrode.

Analysis of the structure of the deposit using X-rays shows that it consists largely of lead dioxide crystallized in form B.

The solid conductive support is chosen from the materials commonly used in electrochemical processes, such as lead and its alloys, dense graphite, vitreous carbon, titanium, gold, platinum. Advantageously, the solid support is made of dense graphite or of titanium, and preferably, it is of titanium.

The cathode is made of lead or one of its alloys, advantageously with bismuth.

The anode and the cathode can take various forms, in particular they can be in the form of plate, disc, grid. They can also have a compact or porous structure permeable to gases. Advantageously, the anode and the cathode have a gas-permeable structure.

The method according to the invention is carried out at a temperature between 0 ° C and 30 ^° C, advantageously at a temperature of 20 ^° C, which very often involves a cooling of the cell, and / or anolyte and catholyte.

The anolyte consists of an acidic aqueous solution.

The precise nature of this anolyte is not characteristic of the invention because it essentially aims to ensure the electrical conductivity between the two electrodes. Usually aqueous solutions of sulfuric, phosphoric acid are used. The concentration of these solutions is generally understood between 0.1 and 5 moles / liter, preferably between 0.5 and 2 moles / liter.

The catholyte, at the start of electrolysis, is an aqueous solution of oxalic acid with a concentration of between 0.1 M and its saturation at the temperature considered. The concentration of sulfuric acid in the anolyte is advantageously 1 M. During the electrolysis, the concentrations of oxalic acid and of glyoxylic acid formed can be either constant, when operating continuously, or variable when we operate discontinuously. Advantageously, the concentration of oxalic acid in the catholyte is 0.7 + 0.1M. When operating discontinuously, it is advantageous to regularly introduce oxalic acid into the catholyte so as to keep its concentration as close as possible to this value. We also know that glyoxylic acid is easily reduced. The parasitic current for reduction of glyoxylic acid is proportional to its concentration in the reaction medium. In order to avoid a possible electrochemical reduction of the glyoxylic acid formed, it is advantageous to limit the conversion rate of the oxalic acid used to around 60% in molar proportions.

When operating in discontinuous operation, it is therefore advantageous to stop the reduction when this conversion rate is reached, then to separate the glyoxylic acid formed from the residual oxalic acid by means known per se, such as in particular the selective fixation of oxalic acid on an ion exchange resin, then recovery of an aqueous solution of glyoxylic acid free from other mineral and / or organic acid which can, if necessary, be concentrated so as to obtain a solution commercial aqueous glyoxylic acid 50% by weight. When operating in continuous mode, the acid is continuously extracted glyoxylic formed by means known per se, and simultaneously introducing an equivalent amount of fresh oxalic acid.

The method according to the invention is implemented in an electrolyser equipped with at least one separation membrane delimiting at least one anode compartment and at least one cathode compartment. This membrane is an ion exchange membrane, advantageously a cation exchange membrane. The nature of this is not characteristic of the invention; this is how any known membrane can be used, in particular membranes of the homogeneous type and membranes of the heterogeneous type. The permselectivity of the membranes used is preferably greater than 60% (determination made according to French patent No. ⁸ 1,584,187). Advantageously, the anode and the cathode, having a structure permeable to gases, are plated on either side of the separation membrane. The current density at the cathode is generally between 3 and 50 A / dm ² .

The evacuation of the gases formed both at the cathode and at the anode is favored by an upward circulation of the anolyte and of the catholyte along the respective electrodes. Advantageously, the anolyte can be circulated faster than the catholyte. Also advantageously, the electrolysis cell can be equipped with a total anode surface greater than the total cathode surface, advantageously this difference is about 20 Z.

The following examples are given by way of explanation and in no way limit the invention.

EXAMPLE 1: A - Preparation of the anode -

A 2239 quality graphite disc from LE CARBONE-LORRAINE, with a geometric surface of 12.5 cm ² is carefully etched by polarization anodic for 30 minutes in sodium hydroxide at 10% by weight under a current density of the order of _{4 mA /} cm ² , then after washing with distilled water it is placed in an aqueous solution of nitric acid at 10% by weight for 10 minutes and finally it is washed with distilled water.

This disc is then used as a central anode in an electrolyser equipped with two lateral copper cathodes and containing as an electrolyte an acidic aqueous solution (pH = 4 to 4.5) containing 325 to 350 g / 1 of lead (II) nitrate and 25 to 30 g / 1 of copper (II) nitrate, maintained in circulation at a temperature of 60 ^° C. the deposit lead dioxide on the anode is carried out under a current density of 30-50 mA / cm ² , at 60 ± 5 ° C, and maintaining the pH of the electrolyte at around 2 by additions of lead (II) oxide and copper (II) carbonate. The electrolysis is stopped when the thickness of lead dioxide deposited on the anode is close to 0.4 mm. An examination of this deposit under a scanning electron microscope shows that it is uniform, compact and adherent to the substrate and that it consists of pyramidal grains with projecting faces.

B - Electrochemical reduction of oxalic acid -

• - 1 litre d'une solution aqueuse d'acide oxalique 0,71 M comme catholyte ;
• - 1 litre d'une solution aqueuse d'acide sulfurique 1 M comme anolyte.

In a laboratory electrolyser equipped with a flat 99.99% purity lead cathode disc, with a surface area of 12.5 cm ² , the graphite anode covered with lead dioxide prepared previously, a membrane in "Nafion" (Kirk-Othmer, Encyclopedia of Chemical Technology, Volume 15, page 120, 3rd edition, 1981, J. WILEY and Sons, New-York) delimiting a cathode compartment and an anode compartment, of an annex system of pumps allowing continuous upward flow, at a regulated temperature, of the catholyte and anolyte along the electrodes and an electric current supply continuous, we place:

• - 1 liter of an aqueous solution of 0.71 M oxalic acid as catholyte;
• - 1 liter of a 1 M aqueous sulfuric acid solution as an anolyte.

Electrolysis is started at 20 ± 1 ° C, at an electrical voltage of 8 volts, a current density of 100 mA / cm ² and a circulation of the catholyte and the anolyte, maintained at 20 + 1 ° C, d '' about 400 cm / min.

After 6 hours of operation, 84 mmol of oxalic acid are dissolved in the catholyte, then 119 mmol after 12 hours, 367 mmol after 24 hours and finally 125 mmol after 30 hours.

After 36 hours of operation, consumption of 162,000 coulombs and 0.79 mole of oxalic acid, a catholyte is obtained containing 0.615 mole of oxalic acid and 0.685 mole of glyoxylic acid. The chemical yield is 86.7% of the theory calculated relative to the oxalic acid consumed and the electrical yield is 81.6% of the theory calculated relative to the glyoxylic acid formed. The anode and the cathode show no point of attack or weight loss.

EXAMPLE 2: A - Preparation of the anode -

A titanium plate of 99.6% purity and a thickness of 0.25 mm is deployed in the form of a grid having identical meshes 3-34-25. Then, this grid, carefully sanded and rinsed successively with acetone, alcohol and water, is uniformly covered with lead dioxide according to a process identical to that described in Example 1-A-. Its active surface determined after deposition by electrochemical control is 9cm ² .

B - Electrochemical reduction of oxalic acid -

• - 1 litre d'une solution aqueuse d'acide oxalique 0,653 M comme catholyte,
• - 1 litre d'une solution aqueuse d'acide sulfurique 1 M comme anolyte.

In an equipped laboratory chlorinator a 99.99% purity lead cathode in the form of a grid with an active surface of 9 cm ² , the titanium anode covered with lead dioxide prepared previously, a "Nafion" membrane on which are placed on both sides the two electrodes, an annex system of pumps making it possible to ensure continuously and at a regulated temperature, the upward circulation of the catholyte and the anolyte along the electrodes and of a supply in direct current, we place:

• - 1 liter of an aqueous solution of oxalic acid 0.653 M as catholyte,
• - 1 liter of a 1 M aqueous sulfuric acid solution as an anolyte.

The electrolysis is started at 20 ± 1 ° C under an electrical voltage of 3.2 volts, a current density of 100 mA / cm2 and a circulation of the catholyte and the anolyte maintained at 20 ± 1 ° C 400 cm ³ / min.

After 21 hours of operation, consumption of 68,040 coulombs, a catholyte is obtained containing 334 mmol of oxalic acid and 227 mmol of glyoxylic acid, ie a yield of 86.8 X of the theory calculated relative to oxalic acid consumed and an electrical efficiency of 78.6% of the theory calculated with respect to the glyoxylic acid formed. The anode and the cathode show neither point of attack nor appreciable weight loss. In the anolyte, only 1 ppm of lead is detected, which corresponds to a consumption of 0.004 mmol of lead dioxide by Faraday.

COMPARISON EXAMPLES 3-5 Example 3

Example 1-B is reproduced by replacing the dense graphite anode covered with lead dioxide with a lead anode of identical shape. During the electrolysis, there is a consumption of this anode of 1.64 mmol of lead by Faraday.

Example 4

Example 1-8 is repeated, replacing the dense graphite anode covered with lead dioxide with a dense graphite anode of the same quality and identical shape. During electrolysis, a consumption of this anode of 341.3 mmol of carbon is observed by Faraday.

Example 5

When reproducing Example 2-8 by replacing the titanium anode covered with lead dioxide with a titanium anode of the same quality and identical shape, a rapid stopping of the electrolysis is observed due to the formation on the anode of an insulating layer of titanium oxide.

It goes without saying that the present invention has been described for purely explanatory and in no way limitative and that any useful modification may be made without departing from its scope.

Claims (7)

1. A method for producing glyoxylic acid by electrochemical reduction of oxalic acid carried out at a temperature between 0 ° C and 30 ^° C in an electrolyser comprising at least one anode compartment containing an anode and an anolyte consisting of a solution aqueous acid, at least one cathode compartment containing a cathode and a catholyte constituted by an aqueous solution of oxalic acid and, between these two compartments, at least one separator, said method being characterized in that the anode consists of solid conductor evenly coated with lead dioxide. 2.- Method according to claim 1 characterized in that said separator is constituted by an ion exchange membrane. 3. Method according to claim 2 characterized in that the ion exchange membrane is a cation exchange membrane. 4. Method according to any one of claims 1 to 3 characterized in that the anode consists of titanium covered with lead dioxide. 5. Method according to any one of claims 1 to 3 characterized in that the anode consists of dense graphite covered with lead dioxide. 6. Method according to any one of claims 1 to 5 characterized in that the conductive solid has a structure permeable to gases. 7. Method according to claim 6 characterized in that the two electrodes are plated on either side of the ion exchange membrane.