Electrolysis cell

Details Electrolysis cell Electrolysis cell

1999-10-05

2001-04-17

Valentine, Donald R. (Department: 1741)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S266000, C204S283000, C204S294000

Reexamination Certificate

active

06217728

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
This application is related to German Application DE 198 44 059.6, filed Sep. 25, 1998, which disclosure is incorporated herein by reference.
FIELD OF THE INVENTION
The invention relates to an electrolysis cell for carrying out chemical reactions. The electrolysis cell comprises an electrolyte chamber filled with electrically conductive filling material, a gas chamber and a gas diffusion electrode arranged between the electrolyte chamber and the gas chamber. The electrolysis cell may be used for the oxidation or reduction of components dissolved in the electrolyte, especially for converting cystine or cystine derivatives into cysteine or cysteine derivatives.
BACKGROUND OF THE INVENTION
Chemical reactions, especially oxidations and reductions, in an electrolysis cell using a gas diffusion electrode, are known. Thus, NL-A 9101022 teaches an electrolysis cell which comprises a gas chamber, an electrolyte chamber and a gas diffusion electrode arranged between them and having an electrolyte chamber filled with electrically conductive filling bodies acting as a fixed-bed electrode. The gas diffusion electrode to be used, which has a customary design, is in direct contact with the electrolyte chamber. Using an acidic electrolyte and a hydrogen-consuming anode, an Fe(III) salt dissolved in the electrolyte can be reduced to the corresponding Fe(II) salt. Inorganic and organic compounds can be oxidized in an analogous manner using an oxygen-consuming cathode, e.g., Fe(II) salts to Fe(III) salts or glucose to gluconic acid. The conversion is significantly increased by the fixed-bed electrode in the electrolyte chamber in comparison to using a customary a real electrode. Various influencing factors such as the particle size of the fixed bed and the electrolyte concentration, are described in J. Appl. Electrochemistry 28 (1998) 343-349. A disadvantage of this electrolysis cell resides in the fact that contact breaks occur on account of the rigidity of the gas diffusion electrode (GDE) resting on a rigid support grid and on account of the motion caused by the flow of the electrolyte, which contact breaks reduce the efficiency. A further disadvantage resides in the fact that the distribution of moisture between the gas chamber and the electrolyte chamber is problematic and flooding of the pores of the gas diffusion electrode on the side of the gas chamber hinders the passage of the gas to the catalytically active layer so that a higher operating voltage results and/or the electrolysis process comes to a halt. Finally, phenomena of poisoning readily occur as a result of the direct contact of the electrolyte, with compounds contained therein to be reduced or oxidized, with the catalytically active layer of the gas diffusion electrode, which brings about a drop in performance and a short service life of the GDE.
The problem of the passage of liquid through a gas diffusion electrode, which is caused by the hydraulic pressure of the electrolyte, is solved in the electrochemical cell according to EP-A 0,717,130, which comprises two electrolyte chambers separated by an ion exchange membrane but does not comprise a fixed-bed electrode, by forming the gas chamber in the form of gas pockets superimposed on each other in a cascade, which gas pockets are separated from each other and open downward toward the electrode. However, this design is technically expensive and also does not solve the problem of poisoning of the catalytic layer of the GDE.
The problem of the deactivation of the catalytic layer of the GDE in an electrolysis cell comprising two electrolyte chambers and a GDE along with a gas chamber can be avoided according to EP-A 0,522,382 by using a cation exchange membrane brought into direct contact with the catalytic layer of the H
2
-consuming GDE. In the cell described therein, the membrane is pressed on the GDE only by the pressure of the electrolyte and/or by a resilient element in the electrolyte chamber bordering on the GDE. Cells are also known having a gas diffusion electrode that comprises an ion exchange membrane laminated under pressure and temperature on the catalytic layer—see, e.g., U.S. Pat. No. 4,399,009. In both documents cited, no fixed-bed electrode is used. As the inventors of the present application determined using a cell comprising a fixed-bed electrode and an H
2
-consuming gas diffusion anode with an ion exchange membrane pressed on the electrolyte side, a delamination of the ion exchange membrane readily occurs given the customary design of the gas chamber behind the GDE, which leads to contact breaks and a reduction of the current gain or current efficiency.
EP-A 0,800 853 teaches a method and an electrolysis cell for cleaning gases, which cell comprises a fixed-bed electrode and a gas diffusion electrode (GDE) separated from it by a separator. The contacting of the GDE takes place via several contact strips of a noble metal or valve metal such as tantalum arranged on the electrode. Another contacting possibility suggested is to fill the gas chamber with graphite spheres in order that the electrode is contacted uniformly over the surface; the electric connection then takes place by means of an electrode end plate connected to the graphite spheres. It was determined that significant operational disturbances and a reduction of the current gain occur in continuous operation of the gas cleaning described in this document, during which the electrolyte and the gas are conducted in countercurrent manner through the catholyte chamber, as a consequence of the occurrence of delamination of an ion exchange membrane thermally pressed in a customary manner onto the catalytic layer of the GDE.
U.S. Pat. No. 4,876,115 teaches a fuel cell comprising two gas diffusion electrodes and a membrane of a solid polymeric electrolyte arranged between them. The production of the cell comprises, in order to form the membrane, the spraying of a solution of the membrane material such as NAFION® (E. I. Dupont and Company) onto the catalytic side of a gas diffusion electrode and the removal of the solvent. The cell does not comprise an electrolyte chamber provided with a fixed-bed electrode.
According to U.S. Pat. No. 5,106,433 the electroreduction of cystine and derivatives takes place in the presence of a nitrogen-containing base such as ammonia using a cell comprising two electrolyte chambers with a cathode having a high surface area, e.g., a carbon fleece. The expense for the recovery of the N-containing base is a disadvantage. A further disadvantage is the requirement for two electrolyte circuits. N-acetylcysteine is produced from L-cystine in the method of WO 97/42358 in which according to one embodiment a reaction mixture which can be obtained by the acetylization of cystine, contains N-acetylcystine that has been adjusted to a pH of 6 to 7 and that is electroreduced and, at the same time, desalinated electrodialytically. The electrolysis cell to be used in this instance can comprise a graphite cathode and a hydrogen-consuming gas diffusion anode and contains an ion exchange membrane between the electrolyte chambers. A Na-acetate solution is used as anolyte and the reaction mixture contains N-acetylcystine, Na-acetate and NaCl as catholyte. The economy of this method is inadequate for present requirements. According to the known method using an electrolysis cell comprising two electrolyte circuits and a cation exchange membrane arranged between them and using an alkaline anolyte, the catholyte is enriched with Na ions, which results in an increased expense for desalination of the solution during isolation of the product after the conclusion of the electrochemical conversion.
SUMMARY OF THE INVENTION
The invention therefore has the object of providing an electrolysis cell with a fixed-bed electrode in the electrolyte chamber, with a gas chamber and with a gas diffusion electrode arranged between them during the use of which said problems occur to a reduced extent or are preferably totally eliminated.
The electrolysis cell should be suitable in partic

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