Chemistry: electrical and wave energy – Processes and products
Patent
1994-10-24
1995-12-12
Gorgos, Kathryn
Chemistry: electrical and wave energy
Processes and products
C25B 304
Patent
active
054746582
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for preparing glyoxylic acid by electrochemical reduction of oxalic acid.
Glyoxylic acid is an important intermediate in the preparation of industrially relevant compounds and can be prepared either by controlled oxidation of glyoxal or by electrochemical reduction of oxalic acid.
The electrochemical reduction of oxalic acid to give glyoxylic acid has been known for a long time and is generally carried out in an aqueous, acidic medium, at low temperature, on electrodes having a high hydrogen overpotential, for example on electrodes made of lead, cadmium or mercury, with or without the addition of mineral acids and in the presence of an ion exchanger membrane (German Published Application 163 842, 292 866, 458 438).
The conventional electrolytic processes used hitherto involving oxalic acid on an industrial scale, or experiments with prolonged electrolysis did not give satisfactory results, since the current yield fell off significantly as the electrolysis progressed (German Published Application 347 605) and the generation of hydrogen increased.
To overcome these drawbacks, the reduction of oxalic acid was carried out on lead cathodes in the presence of additives, for example tertiary amines or quaternary ammonium salts (German Laid Open Applications 22 40 759, 23 59 863). The concentration of the additive in these cases is between 10.sup.-5 % and 1%. This additive is then contained in the glyoxylic acid product and must be removed by a separation process. The documents mentioned do not provide any detailed information on the selectivity of the process.
In Goodridge etal., J. Appl. Electrochem., 10, 1 (1980), pp. 55-60, various electrode materials are studied with regard to their current yield in the electrochemical reduction of oxalic acid. It was found in this study that a hyperpure lead cathode (99.999%) is most suitable for this purpose, while a graphite cathode results in a distinctly lower current yield.
International Patent Application WO-91/19832 likewise describes an electrochemical process for preparing glyoxylic acid from oxalic acid, in which process, however, hyperpure lead cathodes having a purity of more than 99.97% are used in the presence of small amounts of lead salts dissolved in the electrolysis solution. In this process, the lead cathodes are periodically rinsed with nitric acid, as a result of which the service life of the cathodes is reduced. A further drawback of this process consists in the oxalic acid concentration having to be constantly maintained in the saturation concentration range during the electrolysis. The selectivity in this case is only 95%.
Hitherto, only the use of graphite cathodes and cathodes having a high hydrogen overvoltage, such as lead, mercury or cadmium and alloys of these metals has been described. With respect to industrial application of the said process, these materials have grave drawbacks regarding toxicity and use and workability in an electrochemical cell.
The object of the present invention is to provide a process for the electrochemical reduction of oxalic acid to give glyoxylic acid, which avoids the drawbacks mentioned above, which, in particular, has a high selectivity, achieves as low as possible an oxalic acid concentration at the end of the electrolysis and uses a cathode having good long-term stability. At the same time, the cathode is to be composed of an industrially readily available or easily worked material. Selectivity is understood as the ratio of the amount of glyoxylic acid produced to the amount of all the products formed during the electrolysis, namely glyoxylic acid plus by-products, for example glycolic acid, acetic acid and formic acid.
The object is achieved in that the electrochemical reduction of oxalic acid is carried out on cathodes which comprise carbon or at least 50% by weight of at least one of the metals Cu, Ti, Zr, V, Nb, Ta, Fe, Co, Ni, Zn, Al, Sn and Cr, and the electrolyte is composed of, or contains, salts of metals having a hydrogen overpotential of at least 0.25 V at a
REFERENCES:
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patent: 4707226 (1987-11-01), Dapperheld
patent: 4800012 (1989-01-01), Dapperheld et al.
Scott, A Preliminary Investigation of the Simultaneous Anodic and Cathodic Production of Glyoxylic Acid, Electrochimica Acta, 36(9), 1991 (no month), pp. 1447-1452.
Goodridge, F. et al, J. of Appl. Electrochemistry 10:55-60 (1980) (no month).
Scott, K., Electrochimica Acta 36:1477-1452 (1991) (no month).
Babusiaux Pierre
Dapperheld Steffen
Scharbert Bernd
Gorgos Kathryn
Hoechst AG
Wong Edna
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