Preparation Of Glyoxylic Acid

Abstract

In the cathodic reduction of oxalic acid to glyoxylic acid production of hydrogen is reduced when the catholyte contains oxalic acid and 0.001 percent - 1 percent of an adjuvant which is a tertiary amine or quaternary ammonium derivative having more than 11 carbon atoms and, the nitrogen of which is not part of an unsaturated heterocyclic ring, or a heterocyclic tertiary amine or quaternary ammonium derivative thereof, the heterocyclic ring being unsaturated, containing a nitrogen atom and at least five carbon atoms.

Description

The present invention relates to a new process for the preparation of glyoxylic acid by the cathodic reduction of oxalic acid.

The preparation of glyoxylic acid by the cathodic reduction of oxalic acid is described in German Patent Specifications Nos. 163,842, 194,038 and 204,787 and Belgian Patent Specification No. 757,106; according to these processes, the oxalic acid, which is electrolysed, is in the form of an aqueous solution which may contain sulphuric acid. The majority of the processes described in the prior art do not state the results obtained during prolonged electrolyses; two main documents, however, mention this problem.

H. D. C. Rapson et al. (J. Appl. Chem., 13th June 1963, p 233) state that the current yield changes from 90 percent at the start of electrolysis to 30 percent at the end of electrolysis, but he makes no comment on the causes of this phenomenon.

German Patent Specification No. 347,605 also discloses that a decrease in yields, and especially in the current yield, is found after a few hours of electrolysis. In practice, the lowering of the current yield shows itself by an increase in the production of hydrogen at the cathode.

The reason for this increase in the production of hydrogen at the cathode is not completely clear; it probably originates, wholly or partially, from impurities present in the oxalic acid. Thus according to German Patent Specification No. 347,605, contamination of the cathode would occur, but the nature of the impurities responsible for this contamination is not stated, nor is the origin or the degree of purity of the oxalic acid employed.

We have now found that the production of hydrogen is greatly decreased if oxalic acid, which has been recrystallised several times is used, and that the production of hydrogen is particularly high if commercial oxalic acid is used, whether it is prepared from formates (Encyclopaedia of Chemical Technology, Kirk-Othmer, 2nd edition, 14, p 362-364) or whether it is prepared by the nitric acid oxidation of propylene (German Patent Specification No. 742,053 and French Patent Specifications Nos. 1,487,446, 1,501,725, 1,528,569 and 2,031,833). Though we do not intend to be bound to any particular theory, we consider that one of the factors to which the formation of hydrogen, and thereby the decrease in the electric current yields, can be attributed, could be the presence of transition metal ions (especially iron), it being possible for these ions to originate either from the starting reagents (especially oxalic acid), or, if these reagents do not contain any (i.e. the oxalic acid employed being pure acid),from the apparatus used for the electrolysis.

We have also found that with commercial acid, the current yields become insufficient after rather short periods of time, e.g. considerably less than 5 days (an Example of German Patent Specification No. 347,605). Even if the particular cathodes disclosed in German Patent Specification No. 347,605 are used, the advantages achieved with these cathodes (i.e. decrease in the production of hydrogen) disappear in experiments of long duration when commercial oxalic acid is reduced electrolytically. The current yields vary with the batches of commercial oxalic acid used.

The present invention provides a process for the preparation of glyoxylic acid by the cathodic reduction of oxalic acid which consists of carrying out an electrolysis in an electrolysis cell comprising a cathode, a cathode compartment, a separating diaphragm, an anode compartment, and an anode, the said cathode compartment containing a catholyte comprising an aqueous solution of oxalic acid, and 0.00005 to 1 percent by weight of an adjuvant which is:

a. a tertiary amine or quaternary ammonium compound which has a total of more than 11 carbon atoms and the nitrogen atom of which is not part of an unsaturated heterocyclic ring.

b. a heterocyclic tertiary amine or quaternary ammonium compound derived therefrom, the heterocyclic ring structure of which is unsaturated,contains nitrogen and possesses at least five carbon atoms.

The adjuvants defined above generally have less than 40 carbon atoms in all.

Preferred adjuvants are those having the formulae: ##SPC1##

wherein each of R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6 and R.sub.7, which may be the same or different, represents a saturated or unsaturated, linear or branched, aliphatic hydrocarbon radical, or any pair from R.sub.1, R.sub.2,R.sub.3 and R.sub.4, or any pair from R.sub.5, R.sub.6 and R.sub.7 together forms a saturated alkylene or oxydialkylene radical, or a radical containing at least 2 oxyalkylene groups, for example, a radical of the formula --(CH.sub.2).sub.n --[--O--(CH.sub.2).sub.n --].sub.m --OH wherein n = 2 or 3 and m is an integer of 1 to 10,

.alpha. represents a hydrogen atom, or an alkyl radical of up to 20 carbon atoms, or a radical of the formula ##SPC2##

or two adjacent .alpha. symbols together form a radical of the formula: ##SPC3##

the number of unsaturated rings in the compound of the formula III or V being at most equal to 3; the preferred amines of the formula III or IV are pyridine, dipyridyl, quinoline, phenanthroline and their derivatives carrying alkyl substituents, especially picolines and lutidines.

R.sub.8 represents an alkyl radical of up to 20 carbon atoms.

y is equal to 1, 2 or 3.

A.sup.y .sup..theta. is the hydroxyl radical or an anion such that AH.sub.y represents an inorganic or organic acid.

The nature of A.sup.y .sup..theta. can be varied widely and any particular anion can be replaced by another according to the conventional techniques of ion exchange; examples of anions represented by A.sup.y .sup..theta. in addition to the hydroxyl radical, are nitrate, sulphate, phosphate, sulphonate, bicarbonate and oxalate.

The adjuvants used in the invention are preferably those which are soluble in water at the concentration considered, and in particular it is preferred to choose A.sup.y .sup..theta. so that the salts have this solubility.

Examples of preferred adjuvants are tetrabutyl-ammonium, tributyl-lauryl-ammonium, trimethyl-lauryl-ammonium, trimethyl-myristyl-ammonium, trimethyl-palmityl-ammonium, trimethyl-stearyl-ammonium, trimethyl-oleyl-ammonium, trimethyl-linoleyl-ammonium, trimethyl-linolenyl-ammonium, trimethyl-arachidyl-ammonium, trimethyl-behenyl-ammonium, trimethyl-erucyl-ammonium, triethyl-stearyl-ammonium and triethyl-hexyl-ammonium salts, especially the halides and bicarbonates and the hydroxides; pyridine, quinoline and 2,2'-dipyridyl.

The temperature of the catholyte is generally between 0.degree. and 70.degree.C., and preferably between 5.degree. and 35.degree.C.

Examples of metallic materials which are capable of forming the cathodes used in the process of the invention, are lead, cadmium, mercury and amalgams, as well as the alloys of these metals, particularly with silver, tin or antimony.

The anode of the electrolysis cells consists of an electrically conducting material which is electrochemically stable in the anolyte and under the operating conditions considered. Examples of such materials are metals and metalloids such as platinum, platinised titanium, graphite, lead and its alloys, particularly with silver, antimony or tin.

The separating diaphragm of the anode and cathode compartments is preferably a cation exchange membrane. Any known membrane can be used, but membranes of the homogeneous type and membranes of the heterogeneous type are preferred. These membranes can optionally be reinforced with a screen. For carrying out electrolysis operations over a long period, it is naturally preferred to use membranes, which do not swell and which are stable to the action of the various constituents of the catholyte and the anolyte. Examples of such membranes are those described in the following specifications. United States Patent No. 2,681,320 and French Patent Nos. 1,568,994, 1,575,782, 1,578,019, 1,583,089, 1,584,187 and 2,040,950.

The permeation selectivity of the membranes used (defined in and measured as in French Patent Specification No. 1,584,187) is preferably greater than 60%.

The catholyte used in the process according to the invention can comprise water, oxalic acid,glyoxylic acid, one or more adjuvants having one of the formulae I to V and, optionally, a strong inorganic acid such as sulphuric acid; however, the strong acid is preferably absent.

The catholyte can contain oxalic acid without glyoxylic acid only at the start of electrolysis; in the same way, the catholyte can contain glyoxylic acid without oxalic acid only at the end of electrolysis. The concentrations of the oxalic and glyoxylic acid can be either constant when the reaction is carried out continuously, or variable when the reaction is carried out discontinuously or at the start of a continuous operation. In all cases, the concentration of oxalic acid is less then the saturation value at the temperature of electrolysis; generally, this concentration is greater than 2 percent by weight, this value relating particularly to the constant concentration when the reaction is carried out continuously and to the final concentration when the reaction is carried out discontinuously. The concentration of glyoxylic acid is usually between 3 and 25 percent by weight, and preferably between 5 and 15 percent by weight, these values relating particularly to the constant concentration of glyoxylic acid when the reaction is carried out continuously and to the final concentration of this acid when the reaction is carried out discontinuously.

The oxalic acid can be the commercially available material, or if preferred the acid recrystallised from this can be used. Oxalic acid produced by any known process can be used with apparatus in which no special precautions are necessary to remove harmful ions.

As has been stated above, the concentration of adjuvant in the catholyte is usually 0.00005 to 1 percent by weight. This concentration is preferably between 0.0001 and 0.5 percent; the use of these small amounts has the value of avoiding the need to remove the adjuvant from the glyoxylic acid produced, as the adjuvant in these amounts hardly exerts any harmful effect on the properties of the acid.

The adjuvant helps to reduce the amount of hydrogen produced at the cathode and improves the electrical yield.

The catholyte can also contain reaction byproducts in small amounts, e.g. generally less than 1 percent.

An aqueous acid solution is preferably used as the anolyte, though any other anolyte capable of providing electrical conductivity between the two electrodes can be used. Aqueous solutions of sulphuric or phosphoric acids are usually employed in a concentration generally of 0.1 to 5 mols/litre, and preferably 0.5 to 2 mols/litre.

The current density at the cathode is preferably 3 to 50 A/dm.sup.2, and especially 10 to 35 A/dm.sup.2.

In order to carry out the invention, electrolysis cells of any known type can be used, for example, those disclosed in the patent specifications mentioned above and especially Belgian Patent Specification No. 757,106.

However, it is preferred to use electrolysis cells with solid electrodes, which makes it possible to produce a compact apparatus, especially of the filter press type. The electrodes and the separating diaphragm are advantageously located in parallel planes.

Also advantageously, either or both of the catholyte and the anolyte can be circulated in their respective compartments, which makes it possible to achieve better results.

Finally, spacers, for example, woven fabrics or grids, can be located between the electrodes and the separating diaphragm.

The following Examples illustrate the invention.

The concentrations of solutions expressed as a percentage denote, unless otherwise stated, the number of grams of solute per 100 cm.sup.3 of solution; however, these concentrations in g/100 cm.sup.3 differ only slightly from concentrations in % (weight/weight) because the solutions employed in the Examples generally have a density of about 1.

The "commercial" oxalic acid used in the Examples is an acid prepared according to the techniques described in French Patent Specification No. 331,498 and British Patent Specification No. 11,487/1915, the various reactions carried out leading to an oxalic acid solution which is dried in vacuo and then crystals separated from mother liquor. The product is an oxalic acid dihydrate with a degree of purity of about 99.2 percent. The recrystallizations mentioned in the Examples were carried out from water.

EXAMPLE 1

The reduction of commercial oxalic acid is carried out in a cell possessing the following characteristics:

Both the electrodes are rectangular plates of lead with a usable surface area of these electrodes being 2.5 dm.sup.2.

The cation exchange membrane is of the heterogeneous type consisting of a crosslinked sulphonated styrene/divinylbenzene copolymer dispersed in a polyvinyl chloride matrix, and is reinforced with a screen in the form of a woven fabric.

Permeation selectivity measured in 0.6 M KCl solution: 77.5 percent

Substitution resistance measured in 0.6 M KCl solution: 7 .OMEGA..cm.sup.2.

Electrodes-membranes distance: 3mm.

Two pumps cause the catholyte and the anolyte to flow in the corresponding compartments of the cell.

The circuits where the anolyte and the catholyte flow each contain an expansion vessel equipped with supply and removal pipelines.

The circuit of the catholyte also contains a heat exchanger.

The electrolysis conditions are as follows:

current density: -- 14 A/dm.sup.2

voltage: -- 4.45 V

temperature -- 20.degree.C.

speed of flow of the electrolytes over the electrodes: -- 1 m/second.

The catholyte is degassed with a stream of nitrogen of 300 1/hour and the hydrogen formed is measured by gas phase chromatography, in the gas coming from the expansion vessel of the catholyte circuit.

Catholyte introduced initially: 7 l of a 3.28% oxalic acid solution.

This solution is electrolysed for 7 hours 15 minutes, supplying 0.510 l/hour of a 15.6 percent strength aqueous solution of oxalic acid and removing the catholyte so as to keep its volume constant; in a second stage, electrolysis is carried out continuously at constant concentration for 21 hours, supplying the catholyte with 0.825 l/hour of a solution containing 10.5 percent of oxalic acid. During this entire period, the volume of the catholyte is kept constant at 7 l.

At the end of this period, which has lasted for a total of 28 hours 15 minutes, the rate of production of hydrogen is measured; from this rate, an instantaneous current yield of 18.25 percent (yield for the production of hydrogen) is deduced.

The catholyte was found to contain 4.4 percent glyoxylic acid and 4.2 percent oxalic acid.

50 cm.sup.3 of a 40 percent (weight/weight) solution of tetra-(n-butyl)-ammonium hydroxide are then added, as an adjuvant, and electrolysis is continued, supplying 0.815 l/hour of a 10.5 percent solution of oxalic acid until the end of the experiment (with corresponding removal of catholyte to constant volume).

Because of the continuous removal of the catholyte solution, the content of adjuvant decreases at the end of the experiment.

Table I shows the development of the instantaneous current yield (yield for the production of hydrogen) during the various time intervals studied. ##SPC4##

Yields for the production of glyoxylic acid:

The average yields during the period of the experiment which extends from 39 hours 15 minutes to 79 hours 15 minutes, namely 40 hours, were evaluated The catholyte removed during the 40 hours represents 33.990 l. The material balance for the 40 hour period considered is as follows:

oxalic acid employed -- 3,423 g

oxalic acid consumed -- 2,220 g

glyoxylic acid produced -- 1,674 g

current yield -- 86.6%

yield of glyoxylic acid relative to the oxalic acid consumed -- 91.7%

EXAMPLE 2

In this experiment, an apparatus similar to that described in Example 1 is used, but the usable surface area of the electrode is 0.8 dm.sup.2.

Electrolysis is carried out under the following conditions:

current density -- 25 A/dm.sup.2

temperature -- 23.degree.C

electrolysis voltage -- 5.3 V

speed of flow of the electrolytes over the electrodes -- 1 m/second

the catholyte is degassed with a stream of nitrogen of -- 100 l/hr.

Catholyte introduced initially: 1.630 l of a 5.8% aqueous solution of oxalic acid (commercial oxalic acid, recrystallised once from water). This solution is electrolysed for 1 hour and recrystallised oxalic acid containing 17.2 g of pure oxalic acid is then introduced again into the catholyte. This addition is repeated every 30 minutes during the first 9 hours of electrolysis. The volume of catholyte is kept constant at 1.600 l. After a total electrolysis time of 9 hours, the hydrogen, which is liberated from the catholyte, represents an instantaneous current yield of 10.7 percent.

The catholyte is found to contain 8.5 percent glyoxylic acid and 4.35 percent oxalic acid.

3 cm.sup.3 of a 40 percent (weight/weight) solution of tetra-(n-butyl)-ammonium hydroxide, (i.e. to provide a concentration of 2.9 .times. 10.sup.-.sup.3 mol/l) are then added to the catholyte. From this time onwards, 0.220 l/hour of a 17.2 percent aqueous solution of oxalic acid, to which tetra(n-butyl)-ammonium hydroxide has been added so that the concentration of tetra-butyl-ammonium ions is 2.9 .times. 10.sup..sup.-3 mol/l, is run in. Electrolysis is carried out under these conditions for 14 hours. The evolution of hydrogen remains constant during this entire period and represents an instantaneous current yield of 3.5 percent.

The yield of glyoxylic acid during the last 14 hours of the experiment was evaluated. In order to make this evaluation, the catholyte, which was removed, and the contents remaining in the cell are combined, giving 4.930 l.

This solution was found to contain 435 g glyoxylic acid and 207 g. oxalic acid.

Electric current yield -- 77.5 percent

Yield of glyoxylic acid relative to the oxalic acid used up: 92.5 percent.

EXAMPLE 3

The apparatus described in Example 2 is used and electrolysis is carried out under the following conditions:

current density -- 25 A/dm.sup.2

voltage -- 5.55 V

temperature -- 10.degree.-12.degree.C

speed of flow of the electrolytes over the electrodes -- 1 m/second

degassing of the catholyte with a stream of nitrogen of about 150 l/hour.

Catholyte introduced initially: 1 litre of a 3.57 percent aqueous solution of oxalic acid (commercial acid recrystallised once from water).

Electrolysis is carried out for 1 hour 45 minutes, supplying the catholyte with 0.110 l/hour of a 35.7 percent hot (80.degree.C) solution of oxalic acid and removal as in previous Examples to keep the volume constant. The hyrodgen produced at this instant represents an instantaneous electrical yield of 3.9 percent.

Five g of a 40 percent by weight aqueous solution of tetra-(n-butyl)-ammonium hydroxide are then added. Electrolysis is continued for 4 hours 15 minutes with the same rate of feed catholyte as above; 2 g of the tetra-butyl-ammonium hydroxide solution mentioned above are then added. Electrolysis is continued under the same conditions for 1 hour 30 minutes. The volume of the catholyte is brought to 1.600 l and electrolysis is continued for 6 hours, keeping the volume of the catholyte at 1.6 l by suitable removal.

From this time onwards and until the end of the experiment, the catholyte is supplied with a 19.1 percent aqueous solution of oxalic acid, at the rate of 0.230 l/hour. The evolution of hydrogen represents an electrical yield which is constant and substantially equal to 1 percent.

At the end of the experiment, the apparatus is emptied and the liquid obtained is combined with the liquid removed.

Glyoxylic acid was obtained with an average current yield of 91% and a yield of 93.2 percent relative to the oxalic acid used up.

EXAMPLE 4

The apparatus described in Example 2 is used.

The electrolysis conditions are as follows:

current density -- 25 A/dm.sup.2

voltage -- 5.3 V

temperature -- 23.degree.C

speed of flow of the electrolytes over the electrodes -- 1 m/second

The catholyte is degassed with a stream of nitrogen of 100 l/hour and the hydrogen formed is measured in the gas which comes off.

Catholyte introduced initially: 2 l of a 5.85 percent aqueous solution of oxalic acid (commercial acid which has not been recrystallised. This solution is electrolysed for 1 hour 15 minutes and 17.9 g of oxalic acid are then added to the catholyte every 30 minutes until the end of the experiment. After a total electrolysis time of 4 hours, the hydrogen produced represents a current yield of 10.5 percent. 4 cm.sup.3 of an aqueous solution containing 0.5 mol of tributyl-lauryl-ammonium hydroxide/litre are then added. Electrolysis is continued for 2 hours and the hydrogen evolved, which then represents a current yield of 0.1 percent, is measured.

EXAMPLE 5

The reduction of commercial oxalic acid is carried out in a cell similar to that of Example 1.

The electrolysis conditions are as follows:

current density -- 25 A/dm.sup.2

voltage -- 6.3 V

temperature -- 20.degree.C

speed of flow of the electrolytes over the electrodes -- 1 m/second

The catholyte is degassed with a stream of nitrogen of 300 l/hour.

Catholyte introduced initially: 10 l of a 5 percent aqueous solution of oxalic acid (commercial acid recrystallised once from water).

This solution is electrolysed for 29 hours, adding 114 g of oxalic acid to the catholyte every hour. After operating for 18 hours, 0.500 l of water is added every hour at the same time as the oxalic acid, and the volume of the catholyte is kept at 10 l.

At the 29th hour, the catholyte contains 9.55 percent glyoxylic acid and 4.55 percent oxalic acid.

At this instant, the hydrogen produced represents an instantaneous current yield of 9.55 percent.

1 cm.sup.3 of a solution containing 0.36 mol of triethyl-(n-stearyl)-ammonium hydroxide/litre is then added to the catholyte. During the 13 hours of electrolysis which follow, the hydrogen evolved approximately represents a current yield of about 1.7 percent (constant instantaneous yield). Since the additive is gradually removed by the withdrawal, the hydrogen evolved then increases and represents 6.7 percent of the current yield 2 hours later.

0.2 cm.sup.3 of the solution containing 0.36 mol of triethyl-(n-stearyl)-ammonium hydroxide/litre is then added to the catholyte. During the 14 hours which follow, the hydrogen evolved represents an instantaneous current yield which remains at about 1.2 percent.

EXAMPLE 6

The procedure of Example 4 is followed, using, as the quaternary ammonium derivative, 5 cm.sup.3 of a solution containing 1 mol of triethyl-(n-hexyl)-ammonium bicarbonate/litre.

After 6 hours 40 minutes of electrolysis without an adjuvant, the hydrogen produced represents a current yield of 3.7 percent. The solution of triethyl-(n-hexyl)-ammonium bicarbonate is added to the catholyte, the volume of which is 2 l. After the addition, the current yield corresponding to the hydrogen produced becomes 0.9 percent.

EXAMPLE 7

The procedure of Example 4 is followed, using pyridine as the adjuvant.

After 10 hours 15 minutes of electrolysis without an adjuvant, the hydrogen produced represents a current yield of 9.6 percent.

One g of pyridine is then added to the catholyte; at the end of 15 minutes the current yield for the hydrogen is no more than 4.6 percent; 1 cm.sup.3 of pyridine is again added to the catholyte and electrolysis is continued for a further 3 hours 30 minutes under the same conditions. During all this time, the instantaneous current yield corresponding to the hydrogen produced remains constant and equal to 4 percent.

EXAMPLE 8

The procedure of Example 4 is followed, using quinoline as the adjuvant. After 5 hours of electrolysis without an adjuvant, the hydrogen produced represents a current yield of 6.5 percent. 0.25 cm.sup.3 of quinoline is added to the catholyte, the volume of which is 2 l. The current yield corresponding to the hydrogen produced after the addition of the adjuvant is no more than 4.1 percent.

EXAMPLE 9

The procedure of Example 4 is followed, using 2,2'-dipyridyl as the adjuvant.

After 1 hour of electrolysis without an adjuvant, the hydrogen produced represents a current yield of 21 percent. 0.25 g of 2,2'-dipyridyl is then added to the catholyte, the volume of which is 2 l. The current yield corresponding to the hydrogen produced after the addition of the adjuvant is no more than 2.2 percent.

EXAMPLE 10

Electrolysis is carried out in an apparatus similar to that of Example 1, under the following conditions.

Anolyte: 10 percent by weight aqueous solution of sulphuric acid.

Catholyte: during the first 10 hours, the apparatus is supplied at the rate of 0.27 l/hour with an aqueous solution, at 80.degree.C, containing 57.2 percent by weight of oxalic acid and 0.007 g/l of trimethyl-stearyl-ammonium chloride.

After the 10th hour and up to the 105th hour, the apparatus is supplied at the rate of 0.68 l/hour with a solution, at 60.degree.C, containing 25.3 percent by weight of oxalic acid and 0.014 g/l of trimethyl-stearyl-ammonium chloride.

The catholyte is removed at the rate of 0.74 1/hour.

Current density 24 A/dm.sup.2

Electrolysis temperature : about 11.degree.C (maintained by cooling)

Electrolysis voltage 5.9 V

The anolyte and the catholyte are degassed with a stream of nitrogen (100 l/hour)

Speed of flow of the electrolytes : l m/second.

Starting from the tenth hour, the various parameters defining the electrolysis conditions remained practically constant.

The material balance of the operation is as follows:

oxalic acid (dihydrate) employed -- 18.025 kg

oxalic acid (dihydrate) recovered -- 3.855 kg

glyoxylic acid produced -- 7.747 kg ##SPC5##

current yield for the production of glyoxylic

acid 89.2%

current yield for the production of hydrogen: 2 percent

Claims

I claim:

1. A process for the preparation of glyoxylic acid by the cathodic reduction of oxalic acid which consists of carrying out an electrolysis in an electrolysis cell comprising a cathode, a cathode compartment, a separating diaphragm, an anode compartment and an anode, the said cathode compartment containing a catholyte comprising an aqueous solution of oxalic acid, and 0.00005 to 1 percent by weight of an adjuvant which is:

a. a tertiary amine or quaternary ammonium compound which has a total of more than 11 carbon atoms and the nitrogen atom of which is not part of an unsaturated heterocyclic ring

b. a heterocyclic tertiary amine or quaternary ammonium compound derived therefrom; the heterocyclic ring structure of which is unsaturated, contains nitrogen and possesses at least 5 carbon atoms.

2. A process according to claim 1, wherein the adjuvant is of the formula ##SPC6##

wherein each of R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6 and R.sub.7 independently represents a saturated or unsaturated, linear or branched, aliphatic hydrocarbon radical, or any pair from R.sub.1, R.sub.2, R.sub.3 and R.sub.4, or any pair from R.sub.5, R.sub.6 and R.sub.7 together forms a saturated alkylene or an oxydialkylene radical, or a radical containing at least two oxyalkylene groups

.alpha. represents a hydrogen atom, or an alkyl radical of up to 20 carbon atoms, or a radical of the formula ##SPC7##

or two adjacent .alpha. symbols together form a radical of the formula ##SPC8##

the number of unsaturated rings in the compound of the formula III or V being at most equal to 3;

R.sub.8 represents an alkyl radical of up to 20 carbon atoms.

y is equal to 1, 2 or 3

A.sup.y.sup..theta. is the hydroxyl radical or an anion such that AH.sub.y represents an inorganic or organic acid.

3. A process according to claim 1, wherein the adjuvant is present in the catholyte in solution.

4. A process according to claim 2 wherein the adjuvant is selected from the group consisting of tetrabutyl-ammonium, tributyl-lauryl-ammonium, trimethyl-lauryl-ammonium, trimethyl-myristyl-ammonium, trimethyl-palmityl-ammonium, trimethyl-stearyl-ammonium, trimethyl-oleyl-ammonium, trimethyl-linoleyl-ammonium, trimethyl-linolenyl-ammonium, trimethyl-arachidyl-ammonium, trimethyl-behenyl-ammonium, trimethyl-erucyl-ammonium, triethyl-stearyl-ammonium and triethyl-hexyl-ammonium salt and hydroxide, pyridine, quinoline and 2,2'-dipyridyl.

5. A process according to claim 1, wherein at least one of the catholyte and anolyte are circulated outside the cathode and anode compartments respectively.

6. A process according to claim 5, wherein the catholyte is circulated, oxalic acid is continuously added to the catholyte and catholyte is withdrawn to extract the glyoxylic acid produced therein.

7. A process according to claim 1 wherein the cathode and anode are made of lead, the diaphragm is a crosslinked sulphonated styrene-divinylbenzene copolymer dispersed in a polyvinyl chloride matrix, the catholyte and anolyte are separately circulated outside the cathode and anode compartments respectively, the catholyte is degassed with nitrogen and contains 2.9 .times.10.sup.-.sup.3 to 0.01 mol/l of an adjuvant selected from tetra-n-butylammonium, tributyl lauryl ammonium, triethyl-n-stearyl ammonium hydroxides, triethyl-n-hexylammonium bicarbonate, trimethyl-n-stearyl ammonium chloride, pyridine, quinoline and 2,2'-dipyridyl.