WO1991019832A1

WO1991019832A1 - Method for producing glyoxylic acid by electroreduction with cathodic reactivation of oxalic acid

Info

Publication number: WO1991019832A1
Application number: PCT/ES1991/000035
Authority: WO
Inventors: José Ramón OCHOA GOMEZ; Asunción de Diego Zori; José María GARRIDO FRUTOS
Original assignee: Ercros S.A.
Priority date: 1990-06-12
Filing date: 1991-06-11
Publication date: 1991-12-26
Also published as: AU7965291A; ES2020475A6

Abstract

Method for producing glyoxylic acid by electroreduction, with cathodic reactivation, of oxalic acid in a divided eletrolytic cell comprised of an anodic compartment formed of an anode and an aqueous solution of a strong mineral acid as anolite and of a cathodic compartment formed of a lead cathod and an aqueous solution saturated with oxalic acid, the two compartments being separated by a cationic exchange membrane. The method comprises the addition of Pb(II) salts to the catholite or to the anolite in order to keep at high and constant levels throughout the operation both the current efficiency and the selectivity of the process. The effect of Pb (II) salts may be potentiated by washing the cathode with nitric acid. The glyoxylic acid is an intermediary in the industrial synthesis of p-hydroxyphenylglycine and vanillin, inter alia.

Description

PROCEDURE FOR OBTAINING GLIOXYLIC ACID BY ELECTRICAL EDUCATION, WITH CATHODICAL REACTIVATION, OF OXALIC ACID.

FIELD OF THE INVENTION

The invention relates, in general, to a process for synthesizing glyoxylic acid by electroreduction of oxalic acid by using a divided cell by means of a cation exchange membrane. In particular, the invention proposes the use of high purity Pb cathodes, high stability anodes in strongly acidic media and the addition of Pb (II) salts either to the anolyte or to the catholyte to easily prevent deactivation of the cathode. and keep the efficiency of the current constant and high over time, so it is not necessary to continuously disassemble the electrolytic cells to clean the cathode.

BACKGROUND OF THE INVENTION Glioxylic acid is an intermediate in the industrial synthesis of various products such as p-hydroxydandic acid, p-hydroxyphenylglycine and vanillin. The synthesis of glyoxylic acid by electroreduction of oxalic acid using high-voltage cathodes of hydrogen is known since 1903 (German patent 163,842) and can be carried out both in the presence of sulfuric acid and electrolyte (German patents 163,842 and 204,787) and in the absence thereof , that is, using oxalic acid itself as an electrolyte (French patents 2151150 and 2587039 and British patents 1411371 and 1319151).

However, from an industrial point of view all these procedures present a serious problem since as the electrolysis time increases, a Gradual decrease in current efficiency by deactivation of the cathode. An important percentage of the current is used to reduce the water, generating a large amount of hydrogen, which makes the process industrially economically unfeasible, unless the electrolytic cells are frequently disassembled in order to clean the cathode, a procedure that is highly expensive.

To avoid this problem, various solutions have been proposed. Thus, in the British Patent 1,446,179 nitrogen compounds (tertiary amines, quaternary ammonium salts) are added to the catholyte at concentrations between 0.00005% and 1% by weight. The authors of this invention have verified said process by adding tetrabutylammonium bromide to the catholyte, verifying that, during the first hours of electrolysis, the evolution of hydrogen is suppressed, but subsequently it is produced in an enormous amount. Another proposed solution to prevent deactivation of the cathode is mentioned in French Patent 2587039 in which it is claimed that the problem of deactivation of the cathode is eliminated by the use of a lead oxide anode supported on a suitable material. However, in a strongly acidic medium (the anolyte is an aqueous solution of a strong mineral acid) the stability of a lead oxide anode decreases with the operating time, according to our own experience and Pourbaix diagrams. On the other hand, both the efficiency of the current and the selectivity of the reaction are inferior to those obtained by the process of this invention. Another proposed solution to prevent deactivation of the cathode is mentioned in Japanese Patent 79 / 93,677, in which it is proposed to reactivate a lead cathode used in electroreduction of oxalic acid by washing with an alkaline solution, specifically NaOH. Therefore, an object of the present invention is to provide a process for obtaining glyoxylic acid by electroreduction of oxalic acid that overcomes the aforementioned drawbacks. With the process of the invention it is possible to keep the efficiency of the current constant and high throughout the operation time, the deactivation of the cathode is avoided and it is not necessary to disassemble the electrolytic cells to clean the cathode.

BRIEF DESCRIPTION OF THE FIGURES

All the figures represent the variation of the efficiency of the current with respect to oxalic acid (Ees, in ordinates) against the ne of faradays (F, in abscissa), under different operating conditions. Thus, Figure 1 shows the variation of Ees against F using an anode consisting of a noble metal oxide supported on Ti (DSA-0 ₂ ), specifically iridium oxide on Ti; Figure 2 shows the variation of Ees against F using a Pt / Ti anode with polarity inversion (under different conditions) and without polarity inversion; Figure 3 shows the variation of Ees against F using a deteriorated anode of iridium oxide on Titanium and using demineralized water in one case and tap water in another; - Figure 4 shows the variation of Ees against F using a Pb / Ti anode, and addition of Pb (II) salts to the anolyte; Figure 5 shows the variation of Ees against F using an anode of Pb0 ₂ / Pb and addition of salts of Pb (II) to the anolyte; Figure 6 shows the variation of Ees against F using a deteriorated anode of iridium oxide on titanium and addition of salts of Pb (II) to the anolyte and the catholyte; - Figure 7 shows the variation of Ees against F using a Pt / Ti anode, addition of salts of Pb (II) to the catholyte and washing of the cathode with N0 ₃ H; and Figure 8 shows the variation of Ees against F using a deteriorated anode of iridium oxide on titanium with addition of salts of Pb (II) to the catholyte and without addition thereof.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a process for obtaining glyoxylic acid by electroreduction of oxalic acid in a divided electrolytic cell. The process of the invention further allows reactivation of the cathode in a simple and economical manner. The electroreduction of oxalic acid according to the process of this invention is carried out in a divided cell in which the anodic and cathodic compartments are separated by a cation exchange membrane from those usually available in the market. A cation exchange membrane should be understood as any selective membrane that allows the passage of cations but does not allow the passage of anions. By way of example, the membranes Neosepta CL-25 T, Naphion 117, Nafion 324, Nafion 417 and Sele ion CMV, all suitable for carrying out the process of the invention, can be cited.

The cathode to be used in the electrolytic cell must be of a high hydrogen overvoltage metal, preferably high purity lead (greater than 99.97% pure). As the anode, both platinum and a noble metal oxide supported on titanium (DSA) can be used. For practical reasons and due to its high stability in a strongly acidic medium, it is convenient to use a DSA (dimensionally stable anode) of oxygen, preferably iridium oxide on titanium. Once the electrolytic cell is prepared, the anolyte is circulated through the anodic compartment and the catholyte through the cathodic compartment. The anolyte is an aqueous solution of a strong mineral acid, preferably sulfuric acid of a concentration between 2% by weight and 25% by weight. The catholyte is a saturated aqueous solution of oxalic acid at the working temperature.

The current density to be applied can be between 100 A / m ² and 6000 A / m ² . However, for practical reasons and from an economic point of view it is preferable to work with current densities between 1000 and 3000 A / m ² .

In order to obtain good results it is convenient to keep the concentration of oxalic constant throughout the entire electrolysis, to maintain a high linear velocity of the catholyte in the cell to facilitate the transfer of matter at the dissolution-cathode interface, a working temperature included between 10 ° C and 152C and pass a maximum amount of electricity corresponding to 86.5% of the theoretical, since in this way it is possible to obtain a selectivity equal to or greater than 95% and a current efficiency greater than 85%, together with an acceptably high concentration of glyoxylic acid (approximately 100 g / 1) which simplifies its purification. Under these conditions the conversion of oxalic is at least 82%. The electrosynthesized glyoxylic acid according to the process of the invention can be purified simply by evaporation of water and crystallization of excess oxalic acid to obtain an aqueous solution. 50% by weight, directly marketable. The previously described way of operating leads to excellent results during the first electrolysis. Subsequently, as the number of electrolysis performed accumulates, there is a decrease in current efficiency and selectivity as a consequence of the deactivation of the cathode. In parallel, there is a sharp increase in electrogenerated hydrogen. The deactivation of the cathode is generally associated with the electrodeposition on its surface of low hydrogen overvoltage metals (F. Goodridge et al. Journal of Applied Electrochemistry JL0_, 55-60, 1980) originating well from a gradual corrosion of the anode (especially in the case of using a platinum anode, for example) either of impurities contained in the oxalic acid or in the anolyte, or by the presence of said metals dissolved in the water.

Therefore, in principle, the cathode can be kept active using high purity oxalic acid, demineralized water and a very stable anode such as the oxygen DSA mentioned above (iridium oxide on titanium). Indeed, this aspect is demonstrated in example 1 that accompanies the present description. However, the activity of the cathode is highly dependent on the anode used and its condition. Thus, in the case of using a platinum anode supported on titanium, it produces a ^'rapid decrease of both the current efficiency and the selectivity of the process (see Example 2). In the case of using an iridium oxide supported on deteriorated titanium as an anode (for example, by operating time) there is also a decrease in current efficiency and selectivity (see example 3).

A method frequently used in electroorganic processes to regenerate the activity of a electrode consists of reversing its polarity for a certain time. In our case, this would only be possible with a platinum anode, since the reversal of polarity when a metal oxide is used as an anode supposes its destruction (see example 3). Additionally, in Example 2 that accompanies this description, it is demonstrated that the polarity inversion using a platinum anode on titanium does not regenerate the activity of the cathode. However, surprising manner it has been found and constitutes one of the novel aspects of this _invention. that the addition of a lead salt, especially a lead salt (II) either to the anolyte or to the catholyte allows both the efficiency of the current and the selectivity of the process to be high and constant over time (see examples 4 to 8 of this description). In principle, any salt of Pb (II) seems adequate, although it has been found that the salts of lead sulfate (II), lead carbonate (II) and lead oxalate (II) are salts of Pb (II) especially suitable for carrying out the process of the invention, for its ability to reactivate the cathode.

The concentration of these salts of Pb (II) either in the anolyte or in the catholyte is minimal since according to its solubility product, the concentration of Pb (II) in a 1 M aqueous solution of S0 „H ₂ , Typical concentration of the anolyte is 5'56.10 ^'β g / 1 at room temperature; and in a 0.5 M aqueous solution in oxalic acid, typical concentration of the catholyte, it is of the order of 1.3'10 ^'8 g / 1, whereby said Pb (II) salt is added to the catholyte, the concentration of Pb (II) present in the commercial glyoxylic acid solution obtained is depreciable. Consequently, it has been proven that the addition of traces of Pb (II) salts are sufficient to maintain the cathode activity. The effect of these salts is really spectacular as can be seen in examples 4 and following of this description, since not only is the process performance maintained over time, but also the activity of the process can be regenerated. a cathode completely deactivated, increasing the efficiency of the current with respect to the substrate in a drastic way, from 54% to 94% (see example 4). Based on the results obtained, it seems reasonable to think that the Pb ²⁺ ions present in the catholyte solution (either because the Pb (II) salt has been added to the catholyte or because it has been added to the anolyte and Pb ² ions ⁺ have crossed the cation exchange membrane in the course of electrolysis) are electrodeposited in the lead cathode, and therefore, continuously regenerate its surface, so that the efficiency of the current is ensured for long periods of operating time without affect at all the purity of the commercial glyoxylic acid solutions obtained.

The electrodeposition of the Pb ²⁺ ions on the lead cathode was evident when the cells were dismantled, since a lead precipitate was visible on the cathode. The adhesion of electrodeposited lead is not good, so after several electrolysis a deposit of lead begins to accumulate on the cathode which causes a slow decrease in the efficiency of the current over time. To solve this problem, simply wash the cathode, without disassembling the cell, with nitric acid at room temperature, so that the efficiency of the current is regenerated (see examples 7 and

8). Consequently, washing the cathode with nitric acid enhances the effect of the addition of salts of Pb (II) in the course of electrolysis. Nitric acid The cathode wash may have a concentration between 2% by weight and 15% by weight, preferably 10% by weight.

Therefore, according to the process of this invention, it is possible to maintain the activity of the cathode during several electrolysis by adding traces of salts of Pb (II) either to the catholyte or to the anolyte, and optionally, if this effect is desired it can be enhanced by washing the cathode with nitric acid, so it is not necessary to disassemble the electrolytic cell only for routine maintenance checks once or twice a year.

The following examples serve to illustrate this invention without being considered limiting thereof.

EXAMPLES

* General mode of operation

All examples were performed in a press filter cell equipped with a lead cathode (of purity> 99 '97%) of 20 cm ² of geometric area and an anode of the same area and whose material is specified in each example. The membranes used were indiscriminately Selemión CMV, Nafión 417 and Nafión 324. The interelectrodic distance was 0.9 cm. The anolyte was formed by an aqueous solution of sulfuric acid of 10% by weight, and the catholyte by a saturated aqueous solution of oxalic acid, the concentration of oxalic acid being maintained throughout the entire electrolysis by periodic addition thereof. For each kilogram of oxalic acid (of a purity greater than 99.5% by weight), 3.31 kg of water were used.

The temperature of the anolyte and the catholyte was kept constant at 132C, recirculating a mixture between the electrolyte tanks containing a mixture ethylene glycol / HgO from a cryostat. By means of two magnetic displacement pumps I AKI MD-20R the electrolytes were recirculated through the corresponding compartments of the cell, so that the linear velocity of the catholyte was 0.31 / sg. Using a PROMAX FAC-365 direct current power supply, a current of 4 amps was applied, stopping electrolysis when 86.5% of the theoretical amount of electricity needed to reduce all the oxalic acid present was passed. The level of the anolyte was kept constant by adding the necessary amount of water during each experiment (approximately 45 ml H ₂ 0 / faraday is transferred from the anolyte to the catholyte). After the electrolysis, the catholyte was collected, the tank and the cathodic compartment were washed with water, adding said water to the catholyte, and the whole was weighed once homogenized. The oxalic acid concentration was determined by visible spectroscopy at 440 nm by measuring the absorbance produced by the complex formed between oxalic acid and Fe (III) in acidic medium (E. Szetey et al. Acta Chimica Acad. Sci. Hungaricae, .98. (4), 367-373, 1978).

The glyoxylic acid was determined by differential pulse polarography using a mercury drip electrode in 0.25 N NaOH (peak potential -131 volts vs Ag / AgCl electrode (sat)), using a Polaroprocesseur Tacussel.

In all cases the cell voltage was between 5'8 and 8 volts, and the specific energy consumption between 5 and 6'5 K h / g. These values can be lowered by decreasing the inter-electrode distance.

The results obtained in each example are reflected in the figures that accompany this description. In the The figures represent the Ees or efficiency of the current with respect to oxalic acid (ordinate) against F or ne of faradays (abscissa). Each of the points that appear in the graphs of the figures corresponds to an electrolysis. Each Faraday corresponds to 6'7 hours of operation. Also, the following notation is used in the following examples:

Ees: Efficiency of the current with respect to oxalic acid;

Ec: Efficacy of the current with respect to glyoxylic acid; S: Selectivity; F: ne de Faradays. Example 1

Following the general mode of operation described above, a series of electrolysis were carried out with demineralized water and an oxygen DSA as an anode (iridium oxide deposited on titanium). As can be seen in Figure 1, the Ees remained constant at around 89%. The average selectivity of all experiments was 98%. The efficiency of the current with respect to Ec glyoxylic acid remained constant at around 87%. This example demonstrates how working with pure substances, demineralized water and an oxidation stable anode it is possible to maintain the efficiency of the current and the selectivity constant over time.

Example 2 In this case a platinum anode deposited on titanium was used in addition to demineralized water. As can be seen in Figure 2, the Ees decreases rapidly. Successive reversals of polarity did not restore cathode activity. These investments were made in the final moments of the experiment The precedent in such a way that in case of redisolving the metallic impurities deposited in the cathode would be eliminated when removing the already electrolyzed catholyte.

Example 3

In this case an iridium oxide anode deposited on titanium was used. The same one used in Example 1 but which was previously operated as a cathode for 1 hour, after which the cell was disassembled, said electrode was washed with abundant water to remove the non-adhered oxide on the titanium and again the cell performing the corresponding experiments using this deteriorated DSA as an anode. Figure 3 shows the variation of Ees as a function of the faradayε passed both for the case of using demineralized water and tap water (31 ppm as CaC0 ₃ ). The results clearly show the pernicious effect of a deteriorated DSA on the effectiveness of the process. Likewise, the positive effect of demineralized water is demonstrated.

Example 4

In this case a platinum anode deposited on titanium was used. In Figure 4 it can be seen how the Ees decreases dramatically from 88% to 54% after the passage of 4 faradays. At that time, lead (II) sulfate was added to the anolyte producing a rapid increase in current efficiency up to 94% and then slowly decreasing and stabilizing around 88%, the original value. The selectivity remained constant at around 95%.

The results are clearly significant and clearly demonstrate the positive effect of the addition of lead (II) sulfate to the anolyte on the activity of the lead cathode. Example 5

In this case a Pb0 ₂ anode deposited on lead was used. The anode was formed in situ, using H ₂ S0 ₄ 10% as an anolyte, by applying a current of 0.2 A (100 A / m ² ) for 30 minutes at room temperature. Then after adjusting the temperature to 13 ^e C the experiments were started. Figure 5 clearly shows how the addition of lead (II) sulfate to the anolyte rapidly regenerates the activity of the cathode. The selectivity remained between 93% and 97%.

Example 6

In this case, the damaged DSA used in example 3 was used. Since the beginning of the experiments, lead (II) sulfate was added to the anolyte but in this case this way of acting was completely ineffective. In addition there was a 2 volt increase in cell voltage. For the reason that the addition of lead (II) sulfate to the anolyte using an iridium oxide anode is ineffective. However, adding 100 g of lead carbonate to the catholyte produced, as can be seen in Figure 6, a dramatic increase in the efficiency of the current with respect to the substrate, going from 65% to 91%, to maintain around this value. The example unequivocally demonstrates the positive effect of the addition of lead (II) carbonate to the catholyte on the activity of the cathode. The selectivity remained constant at around 95%.

Example 7

In this case a platinum anode deposited on titanium was used, the same as in examples 2 and 4, with 38 mg of lead (II) sulfate being added to the faraday catholyte. The results are represented in the Figure 7. As can be seen, the Ees remained well above the values obtained without adding lead sulfate (II) (see figures 2 and 4 before the addition of lead sulfate (II)). However, the decrease in Ees is much more pronounced than when this salt is added to the anolyte (Figure 2). Consequently, the addition of lead (II) sulfate to the catholyte improves the activity of the cathode but to a lesser extent than adding lead (II) carbonate and lead (II) oxalate (example 8) also to the catholyte or lead sulfate ( II) to the anolyte (example 4). After 52 hours of operation the Ees had decreased to 74%.

At that time the cathode was washed, without disassembling the cell, three times with 10% HN0 ₃ for 15 minutes at room temperature. After washing the cathode with abundant demineralized water to remove the remains of nitric acid, a new electrolysis was carried out, increasing the efficiency of the current with respect to the substrate to 90%. The selectivity was 96%. When dismantling the cell, no deposit was seen on the cathode, unlike in the previous examples.

Example 8

In this case, an impaired oxygen DSA (iridium oxide on titanium) was used as the anode. The same as in Examples 3 and 6, adding lead (II) oxalate in varying amounts throughout the different electrolysis and washing the cathode once for 15 minutes with HN0 ₃ 10% ^' when the Ees decreased to a value close to 90% The results can be seen in Figure 8, where they are compared with those obtained under the same conditions but without the addition of lead (II) oxalate, the beneficial effect on the cathode activity of the addition of oxalate addition being clearly demonstrated. lead (II) to the catholyte. You can also see how to measure which decreases the amount added per faraday and it is possible to maintain the activity of the cathode for a longer time, so that the washings with 10% HN0 ₃ are spaced more in time. It is also seen in Figure 8 how after each wash with 10% HN0 ₃ the Ees is increased. Thus, by the joint addition of lead (II) oxalate to the catholyte and periodic washing with 10% nitric acid of the cathode without disassembling the cell, it is possible to keep the Ees at an average value of (94-95)%. In all cases the electrolysis selectivity was around 96% so that the efficiency of the current with respect to Ec glyoxylic acid was (90-91)%.

TRANSLATION OF THE LEGENDS OF THE FIGURES - Figures 1 to 8

(a) Efficiency of the current with respect to oxalic acid in% (Ees%).

(b) n2 Faradays (F).

- Figure 1 (c) DSA anode - 0 ₂ .

- Figure 2

(c) Pt / Ti anode.

(d) Without polarity inversion.

(e) Polarity inversion: 30 sec. at 2000 A / m ² . (f) Polarity inversion: 10 min. at 100 A / m ² 2

- Figure 3

(c) damaged DSA-0 ₂ node.

(d) Demineralized water.

(e) Tap water (31 pp as Ca C0 ₃ ). - Figure 4

(c) Pt / Ti anode.

(d) Addition PbS0 ₄ to the anolyte.

- Figure 5 (c) PbO _a / Pb node. (d) Addition PbS0 ₄ to the anolyte. - Figure 6

(c) Damaged DSA-0 ₂ anode.

(d) Addition PbS0 ₄ to the anolyte.

(e) Addition PbC0 ₃ to the catholyte (100 mg) - Figure 7

(c) Washing the cathode with 10% HN0 ₃

- Figure 8

(c) Damaged DSA-0 ₂ anode.

(d) Addition of lead oxalate (II) to the catholyte in the amounts indicated in (g), (h) and (i).

(e) No addition of lead oxalate.

(f) Washing the cathode with 10% HN0 ₃ .

(g) 33.5 mg / F. ^' (h) 1.67 mg / F. (i) 0.75 mg / F.

Claims

1. Procedure for obtaining glyoxylic acid by electroreduction, with cathodic reactivation, of oxalic acid at a temperature between IO ^ C and 15sc in an electrolytic cell divided by a cation exchange membrane, comprising an anode compartment formed by an anode and an anolyte constituted by an aqueous solution of a strong mineral acid, and a cathodic compartment formed by a lead cathode and a catholyte constituted by a saturated aqueous solution of oxalic acid, said process being characterized in that the efficiency of the current is kept constant and the selectivity of the process during long periods of operation, by adding lead salts either to the anolyte or to the catholyte.

2. Method according to claim 1, characterized in that said lead salts are lead (II) salts which are selected from lead sulfate (II), lead carbonate (II) and lead oxalate (II).

3. Method according to claim 1, characterized in that the strong mineral acid used as an anolyte is sulfuric acid of a concentration between 2% by weight and 25% by weight, preferably 10% by weight.

4. Method according to the preceding claims, characterized in that the action of the lead salts is enhanced by periodic washing of the cathode with nitric acid.

5. Method according to claim 1, characterized in that the anode is constituted by platinum supported on titanium, or by iridium oxide supported on titanium.

Method according to claim 1, characterized in that the cathode is constituted by lead of purity greater than 99.97%.

7. Method according to claim 1, characterized in that for a charge conversion of at least 82% oxalic acid, a selectivity equal to or greater than 95% is obtained.