Process for dehalogenating chloro- and bromo-acetic acids

0001Chloro and bromoacetic acids are the mono-, di- and tri-haloacetic acids of the formulas<tables id="tabl0001" num="0001"><img file="EP0241685B1_D0001.tif" /></tables>For some purposes it is necessary to completely or partially dehalogenate the chlorine and bromoacetic acids that occur in certain processes. The partial dehalogenation of the triple and the double halogenated acetic acids is desirable or necessary, for example, if the intention is to obtain the monohalogenated acetic acids by chlorination or bromination of acetic acid in the highest possible yields. The chlorination and bromination of acetic acid in fact always produces more or less significant amounts of the di- and possibly also the tri-haloacetic acid, even if no more halogen than is necessary for the monohalogenation, which naturally impairs the yield of the desired monohalogen compound.

0002Various processes have therefore already been developed to dehalogenate the 2 and 3-fold halogenated acetic acids and to stop the dehalogenation even in the monohalogen stage. According to the process described, for example, in DE-B 848 807, this dehalogenation is carried out electrochemically by electrolysis of the corresponding mixtures or solutions in undivided electrolysis cells. Coal, Acheson graphite, lead and magnetite are mentioned as cathode materials, and coal and magnetite as anode materials. The presence of indifferent substances or inorganic impurities in the starting haloacetic acids should not be a problem here.

0003According to the examples, a current density of about 500 to 700 A / m² worked. The electrolysis temperature is below 100_°C.

0004The yields of the desired partially or completely dehalogenated products should be between 95 and 100% of theory lie.

0005According to example 2, the following mixture is electrolyzed:<tables id="tabl0002" num="0002"><img file="EP0241685B1_D0002.tif" /></tables>The electrolysis of the mixture was carried out according to the information in the example mentioned in the form of a 60% aqueous solution<ul id="ul0001" list-style="none"><li>using magnetite cathodes and carbon anodes at a voltage of 3.25 V on average and a current density of 500 to 600 A / m²</li><li>at 65_°C.</li><li>up to the dehalogenation of the di- and trichloroacetic acids up to the monohalogen stage. The yield of monochloroacetic acid is given as almost quantitative.</li></ul>

0006In Example 4, the electrolysis is continued until complete dehalogenation - that is to say halogen-free acetic acid.

0007The dehalogenation essential for this process is a reduction reaction taking place at the cathode. For the partial dehalogenation of dichloroacetic acid up to the monochloroacetic acid stage, the following reaction equation can be given:<ul id="ul0002" list-style="none"><li>CHCI₂COOH + 2 H + + 2 e → CH₂CICOOH + HCI</li></ul>The reaction of the aggressive haloacetic acids at the cathode has a considerable corrosive effect on the cathode material, as has also been shown by our own electrolysis experiments using magnetite and lead cathodes. Corrosion is hardly serious on carbon cathodes. A disadvantage of all the cathode materials mentioned here, however, is that increasing the current density leads to increasing hydrogen evolution at the cathode, and the electrodes are covered in a long-term test for 600 h with a coating which makes cleaning the cathode necessary, which of course is economical the procedure significantly affected.

0008At least partially the halogen ions formed on the cathode are discharged at the anode; in the case of chlorine ions:<ul id="ul0003" list-style="none"><li>2nd Cl- - Clz + 2 e</li></ul>In the undivided cells according to the aforementioned DE-B, the anodically formed halogen can easily come into contact with the product dehalogenated at the cathode and "react" back to the starting product; e.g.<ul id="ul0004" list-style="none"><li>CH₂CICOOH + Clz → CHCI₂COOH + HCI</li></ul>This "back reaction" can be prevented by performing the electrolysis in divided electrolysis cells. The diaphragm materials known at the time of registration of the aforementioned DE-B (in 1942) (for the division of the cells into the cathode and anode compartments) did not withstand the action of the aggressive halogen acetic acids and the at least as aggressive halogen, especially in the heat long standing. For this reason, divided electrolytic cells are also assessed as unsuitable for the electrolytic dehalogenation of haloacetic acids in the aforementioned DE-B.

0009With the development of chemically and thermally extremely stable membrane materials made of perfluorinated polymers in recent times, however, it has also become possible to carry out electrolysis with aggressive reagents in divided cells.

0010A process for the electrochemical dehalogenation of dichloroacetic acid up to the stage of monochloroacetic acid in divided electrolysis cells is described in JP-A-54 (1979) -76521; In particular, cation exchange membranes made of perfluorinated polymers with COOH or SOgH groups on the polymer backbone are used as membrane materials.

0011In this process, lead or lead alloys are used as cathode materials; the catholyte is an aqueous solution of dichloroacetic acid + HCI and / or H₂S0₄ with a conductivity greater than 0.01 ohms^-1Cm-1.

0012Graphite, lead, lead alloys and titanium with a coating of oxides of platinum metals are mentioned as anode materials; An aqueous mineral acid solution serves as the anolyte, with oxygen acids being preferred as mineral acids because here there is no chlorine but only oxygen evolution:<ul id="ul0005" list-style="none"><li>H₂O → 1/2 0₂ + 2 H⁺ + 2 e</li></ul>For the membrane material, the required ion exchange capacity in grams of dry weight of the exchange resin is given, which is necessary to neutralize 1 gram equivalent of base. For membrane material with carboxyl groups, the exchange capacity should be 500 to 1500, preferably 500 to 1000, for membrane material with SO_aH groups are 500 to 1800, preferably 1000 to 1500.

0013The current densities are of the same order of magnitude as those of the process of the aforementioned DE-B 848 807. At a concentration of dichloroacetic acid below 25%, the current density should be below 10 A / dm² = 10₀₀ AJm², <ul id="ul0006" list-style="none"><li>at a dichloroacetic acid concentration below 15% below 800 A / m² and</li><li>at a dichloroacetic acid concentration below 10% below 400 A / m² lie.</li></ul>

0014Even the pure lead cathodes preferred here as cathodes are still subject to considerable corrosion. For electrolysis with a 99.99% lead cathode and an electrode area of ​​1 dm² and a current density of 4 A / dm² = 400 A / m² 59.6 mg of cathode weight loss should have occurred in 4 hours.

0015The following weight loss is stated for different lead alloys under the same conditions:<tables id="tabl0003" num="0003"><img file="EP0241685B1_D0003.tif" /></tables>According to the examples, the current yields are consistently around 95% and above.

0016Although the known electrochemical processes for the partial or complete dehalogenation of chloro and bromoacetic acids have various advantages, they still need to be improved, in particular with regard to the corrosion resistance of the cathode materials and the relatively low current densities; It was therefore the task of improving the known processes, particularly with regard to the cathode materials and the current densities, and thus making the processes even more economical.

0017According to the invention, this object could be achieved by using such aqueous solutions of chloro or bromoacetic acids as starting electrolysis solutions which still contain one or more salts of metals with a hydrogen overvoltage of at least 0.4 V (at a current density of 4000 A / m²) included dissolved.

0018The subject of the invention is therefore a process for the dehalogenation of chloroacids and bromoacetic acids by electrolysis of aqueous solutions of these acids using carbon cathodes and anodes also of carbon or of other conventional electrode materials in undivided or in divided (electrolysis) cells, which is characterized in that that the aqueous electrolysis solutions in the undivided cells and in the cathode compartment of the divided cells still contain one or more salts of metals with a hydrogen overvoltage of at least 0.4 V (at a current density of 4000 A / m²) included dissolved.

0019The salts of metals with a hydrogen overvoltage of at least 0.4 V (at a current density of 4000A / m2) are mainly the soluble salts of Cu, Ag, Au, Zn, Cd, Hg, Sn, Pb, Ti, Zr, Bi, V, Ta, Cr and / or Ni, preferably only the soluble Cu and Pb salts, in question. The most common anions of these salts are mainly CI-, Br, S0₄^2-, NO_s- and CH₃0CO-. However, these anions cannot be combined in the same way with all of the above-mentioned metals, because in some cases this results in salts which are difficult to dissolve (such as AgCI and AgBr; here, AgNOs are primarily suitable as soluble salts).

0020The salts can be added directly to the electrolysis solution or can also be generated in the solution, for example by adding oxides, carbonates etc. - in some cases also the metals themselves (if soluble).

0021The salt concentration in the electrolyte of the undivided cell and in the catholyte of the divided cell is expediently set to about 0.1 to 5000 ppm, preferably to about 10 to 1000 ppm.

0022As a result of this modification of the known methods, the electrodes are extraordinarily resistant to corrosion, combined with the possibility of working at current densities which are about 10 times higher (up to about 8000 A / m²) ensures that deposits do not form on the electrodes even during long continuous operation; the process is therefore extremely economical and progressive.

0023According to the prior art, it was in no way to be expected that the combination of carbon cathodes and the presence of certain metal salts in the electrolyte or. Such an increase in the cost-effectiveness of the process is achieved in particular by the possibility of working with higher current densities without the formation of deposits on the electrodes.

0024Trichloric and dichloroacetic acid and tribromoic and dibromoacetic acid, in particular only trichloric and / or dichloroacetic acid, are preferably used as starting compounds for the process; the electrolysis here is preferably carried out only up to the monohalogen stage (monochloro or monobromoacetic acid).

0025The continuation of the electrolysis up to (completely dehalogenated) acetic acid is of course possible, but not preferred.

0026In principle, aqueous solutions of the starting haloacetic acids of all possible concentrations (approx. 1 to 95%) can be used as the electrolyte (in the undivided cell) or catholyte (in the divided cell). The solutions can also contain mineral acids (e.g. HCI, H₂S0₄ etc.) and must contain the content of certain metal salts according to the invention.

0027The anolyte (in the divided cell) is preferably an aqueous mineral acid, in particular aqueous hydrochloric acid and sulfuric acid.

0028In principle, all possible carbon electrode materials are possible as carbon cathodes, such as electrode graphites, impregnated graphite materials and also glassy carbon.

0029During the electrolysis, the metal on which the metal salt added according to the invention is based is deposited on the cathode, which leads to a change in the properties of the cathode. As a result, the cathodic current density can reach values ​​of up to approximately 8000 A / m², preferably up to about 6000 A / m², are increased without excessive hydrogen evolution and a progress of the dehalogenation reaction beyond the desired stage occurring as side reactions. The metal deposited on the cathode is repeatedly partially dissolved by the acidic solution surrounding the cathode and then deposited again, etc. There is no annoying deposit formation on the cathode.

0030The same material as for the cathode can be used as the anode material. In addition, it is also possible to use other conventional electrode materials which, however, must be inert under the electrolysis conditions. A preferred such other common electrode material is titanium coated with Ti0₂ and doped with a noble metal oxide such as platinum oxide.

0031Preferred anolyte liquids are aqueous mineral acids such as, for example, aqueous hydrochloric acid or aqueous sulfuric acid. The use of aqueous hydrochloric acid is preferred here when working in divided cells and other uses exist for the anodically formed chlorine; otherwise the use of aqueous sulfuric acid is cheaper.

0032Of the two possibilities of the electrolysis cells in which the method according to the invention can be carried out - undivided and divided cells - the implementation in the divided cells is preferred. The same ion exchange membranes as those described in the aforementioned JP-A-54 (1979) -76521 are suitable for dividing the cells into the anode and cathode compartments; ds thus those made of perfluorinated polymers with carboxyl and / or sulfonic acid groups, preferably also with the ion exchange capacities specified in JP-A. In principle, the use of diaphragms made of other perfluorinated polymers or inorganic materials which are stable in the electrolyte is also possible.

0033The electrolysis temperature should be below 100_°C lie; preferably it is between about 5 and 95_°C, especially between about 40 and 80_°C.

0034It is possible to carry out the electrolysis both continuously and batchwise. A method of operation in divided electrolysis cells with a discontinuous execution of the cathode reaction and continuous operation of the anode reaction is particularly expedient. If the anolyte contains HCI, the anodic chlorine evolution constantly consumes CI, which can be compensated for by the continuous addition of gaseous HCI or aqueous hydrochloric acid.

0035The electrolysis product is worked up in a known manner, for example by distillation. The metal salts remain behind here and can be returned to the process.

0036The invention is now explained in more detail by the following examples. After the (invention) examples A there follow some comparative examples B, from which it can be seen that magnetite cathodes (instead of carbon cathodes), in the presence of, for example, a lead salt in the electrolyte solution, undergo not inconsiderable corrosion and, at higher current densities, considerable hydrogen evolution. Another comparative example with a carbon cathode, but without the addition of a metal salt to the electrolytic solution according to the invention, shows that hydrogen is formed to a considerable extent even when current densities are not too high; if, on the other hand, a lead salt is added to the electrolyte solution, the evolution of hydrogen is suppressed and the current density can be increased.

0037The electrolytic cell used in all (invention and comparative) examples was a split (plate and frame) circulation cell.<tables id="tabl0004" num="0004"><img file="EP0241685B1_D0004.tif" /></tables>

A) Examples of Invention

Examples 1 to 8

Electrolysis conditions

0038Circulation cell with 0.02 m² Electrode area, electrode spacing 4 mm<ul id="ul0007" list-style="none"><li>Electrodes: Electrode graphite EH (from Sigri, Meitingen)</li><li>Cation exchange membrane: (R) Nafion 315 (from DuPont); it is a 2-layer membrane made from copolymers of perfluorosulfonylethoxy vinyl ether + tetrafluoroethylene. There is a layer with the equivalent weight 1300 on the cathode side and one with the equivalent weight 1100 on the anode side.<img file="EP0241685B1_D0005.tif" /></li></ul>

0039The composition of the catholyte and the electrolysis result are shown in the following table:

Example 9

Electrolysis conditions

0040Circulation cell with 0.25 m² Electrode area, electrode spacing 4 mm<ul id="ul0008" list-style="none"><li>Electrodes: Electrode graphite EH (from Sigri, Meitingen)</li><li>Cation exchange membrane: (^R) Nafion 324 (from DuPont) it is a 2-layer membrane with the same composition as Nafion 315, only with slightly thinner layers.<img file="EP0241685B1_D0006.tif" /><img file="EP0241685B1_D0007.tif" /><img file="EP0241685B1_D0008.tif" /><img file="EP0241685B1_D0009.tif" /></li></ul>

B) Comparative Example 1

Electrolysis conditions:

0041Circulation cell with 0.02m² Electrode area, electrode spacing 6 mm<ul id="ul0009" list-style="none"><li>Anode: Electrode graphite EH (from Sigri, Meitingen)</li><li>Cathode: completely and tightly coated stainless steel with magnetite</li><li>Cation exchange membrane: (^R) Nafion 324 (from DuPont)</li><li>Spacers: polyethylene nets</li><li>Flow: 500 l / h</li><li>Temp .: 39_°C.</li><li>Anolyte: conc. HCI, continuously supplemented by gaseous HCI</li><li>It became a catholyte with the composition<img file="EP0241685B1_D0010.tif" />at a current density of 2000 A / m² electrolyzed. The terminal voltage was 3.2 V. The proportion of the current that was used for the development of hydrogen was 14.3%.</li><li>After adding 0.75 g Pb (OAc)₂· 2 h₂0 (100 ppm Pb²+) The hydrogen development decreased briefly, but then rose again.</li><li>After 270 Ah, 28% of the electricity was used for hydrogen development, after 350 Ah the value was 45% and then rose further to approx. 80%.</li><li>After a charge consumption of 752 Ah, an electrolyte with the composition:<img file="EP0241685B1_D0011.tif" /></li><li>The current yield for this slight depletion of dichloroacetic acid was only 44%. Serious corrosion damage was found on the magnetite layer of the cathode. The corrosion rate was 14 mgFe / Ah.</li></ul>

Veraleichsbeisoiel2 2

0042Under the conditions described in Inventive Examples (A) 1-8, but without the addition of a metal salt, a catholyte with the composition<tables id="tabl0005" num="0005"><img file="EP0241685B1_D0012.tif" /></tables>at a current density of 1250 A / m² electrolyzed. The terminal voltage was 3.9 V. After a current consumption of 1104 Ah, the proportion of the current that was used for the development of hydrogen rose to 49%.

0043After adding 10 g Pb (N0₃)₂ (* 400 ppm Pb²⁺) there was no more hydrogen evolution to the catholyte. The current density could reach 4000 A / m² be increased (terminal voltage 4.1 V; temperature 52_°C). The side reaction of hydrogen evolution started again at a dichloroacetic acid concentration of 3%. The current efficiency for reducing the dichloroacetic acid content to 0.15 kg was 97.2%.