Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B3/00—Electrolytic production of organic compounds
  • C25B3/20—Processes
  • C25B3/25—Reduction

Definitions

• Monochloroacetic acid and its derivatives are important intermediates in industrial organic synthesis. They are used for the production of adhesives, pesticides or pharmaceutical products.
• the production of monochloroacetic acid by chlorinating acetic acid is always associated with the formation of di- and trichloroacetic acid.
• electrochemical dehalogenation is also available for removing di- and trichloroacetic acid from the product mixture (EP-B 0 241 685).
• the latter dehalogenation is carried out using graphite cathodes in the presence of small amounts of metal salts with a hydrogen overvoltage of at least 0.4 V (at a current density of 4000 A / m2), preferably in water-containing, acidic electrolytes.
• This process has a high selectivity, since the thermodynamically favored reduction of protons to hydrogen takes place at the cathode at low concentrations of the partially dehalogenated di- and trichloroacetic acid. In this way, undesired dehalogenation of the monochloroacetic acid is avoided, but the di- and trichloroacetic acid are also dehalogenated only with poor current efficiency.
• This method is not suitable for dehalogenation down to a very low concentration level of di- and trichloroacetic acid, since an increasing proportion of the electrical charge for the Reduction of protons to hydrogen is consumed.
• An economical implementation of dehalogenation to monochloroacetic acid at a low concentration of di- and trichloroacetic acid has therefore only been insufficient to date (comparative example).
• the task was therefore to selectively dehalogenate di- and trichloroacetic acid with very extensive conversion to monochloroacetic acid - that is, not completely.
• Nekrasov et al. Investigated the dehalogenation of trichloroacetic acid and monochloroacetic acid in the presence of tetramethylammonium or tetraethylammonium salt in a non-protic electrolyte (Nekrasov et al., Elektrokhimiya 1988, 24, 560-563). However, the effects they observed in no way suggest that ammonium salts in an aqueous electrolyte could inhibit the above-mentioned undesirable reduction of protons to hydrogen.
• di- and trichloroacetic acid can be dehalogenated continuously or discontinuously to give monochloroacetic acid with very extensive conversion in divided electrolysis cells if electrolysis is carried out in aqueous solutions in which, in addition to metal salts, a hydrogen overvoltage of at least 0.4 V (at a Current density of 4000 A / m2) quaternary ammonium and / or phosphonium salts are dissolved.
• Another object is an electrolysis solution for the partial dehalogenation of tri- and / or dichloroacetic acid, which contains at least one of these two acids, one or more metal salts with a hydrogen overvoltage of at least 0.4 V (at a current density of at least 4000 A / m2) contains at least one compound selected from the group of compounds of the formula I to V.
• At least one compound of the formula I or II or III or IV or V or any mixtures of compounds of the formulas I, II, III, IV and V are used in the electrolysis.
• the compounds of the formulas I to V are used in concentrations of 1 to 5000 ppm, preferably 10 to 1000 ppm, but in particular 50 to 500 ppm.
• metal salts with a hydrogen overvoltage of at least 0.4 V at a current density of 4000 A / m2
• the soluble salts of Cu, Zn, Cd, Hg, Sn, Pb, Ti, Bi, V, Ta and / or Ni preferably the soluble salts of Cu, Zn, Cd, Sn, Hg and Pb are used.
• Cl ⁇ , Br ⁇ , SO42 ⁇ , NO3 ⁇ or CH3COO ⁇ are preferably used as anions, the anion being chosen so that a soluble metal salt is formed (for example PbNO3).
• the salts can be added directly to the electrolysis solution or, e.g. by adding oxides or carbonates - or by adding the metals themselves, such as Zn, Cd, Sn, Pb, Ni - in the solution.
• the salt concentration in the catholyte is expediently set to about 0.1 to 5000 ppm, preferably to about 10 to 1000 ppm.
• the starting material for the process is di- and / or trichloroacetic acid or mixtures thereof with monochloroacetic acid which are inevitably formed in the acetic acid chlorination.
• aqueous solutions of chlorinated acetic acids in all possible concentrations can be used.
• the proportion by weight of di- and trichloroacetic acid in the total amount of chlorinated acetic acids is less than 10% by weight. This weight fraction can easily be less than 5% by weight, or even less than 2% by weight, which was particularly surprising.
• the catholyte can also contain mineral acids (eg HCl, H2SO4 etc.).
• the anolyte is preferably an aqueous mineral acid, especially an aqueous hydrochloric acid or sulfuric acid.
• the same material as the cathode can generally be used as the anode material.
• other customary electrode materials which, however, must be inert under the electrolysis conditions, for example titanium, coated with titanium dioxide and doped with a noble metal oxide, such as e.g. Ruthenium dioxide.
• Cation exchange membranes made of perfluorinated polymers with carboxyl and / or sulfonic acid groups are generally used to divide the cells into the anode and cathode compartments. It is also generally possible to use anion exchange membranes which are stable in the electrolyte, diaphragms made of polymers or inorganic materials.
• the electrolysis temperature should generally be below 100 ° C, preferably between 10 and 90 ° C.
• the electrolysis can be carried out either continuously or batchwise.
• a continuous process is preferred, especially at a low concentration of di- and trichloroacetic acid.
• chloride is constantly consumed due to the anodic chlorine evolution. In general, the chloride consumption is then compensated for by continuously introducing gaseous HCl or aqueous hydrochloric acid.
• the electrolysis product is worked up in a known manner, e.g. by distillation.
• the metal salts and the quaternary ammonium and phosphonium compounds remain in the residue and can be returned to the process.
• Examples 1-9 are followed by a comparative example.
• the comparative example shows that under the electrolysis conditions of EP-B 0 241 685, when a dichloroacetic acid concentration of 31% is reached (based on the total amount of the dissolved acetic acids), the majority of the electrical charge is used for the reduction of protons to hydrogen .
• Electrode area 0.02 m2
• Cation exchange membrane ®Nafion 423 (DuPont, 1-layer membrane made from copolymers of perfluorosulfonylethoxy vinyl ether and tetrafluoroethylene with an equivalent weight of 1200)
• Flow 400 l / h
• Catholyte 2400 g water, 1050 g monochloroacetic acid, 60 g dichloroacetic acid.
• concentrations of the metal salt and the compound of formula I can be seen from the table.

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• Chemical & Material Sciences (AREA)
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• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)

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• 1990
  • 1990-05-18 DE DE4016063A patent/DE4016063A1/de not_active Withdrawn
• 1991
  • 1991-05-16 JP JP3111938A patent/JPH0593290A/ja not_active Withdrawn
  • 1991-05-16 EP EP91107944A patent/EP0457320B1/fr not_active Expired - Lifetime
  • 1991-05-16 FI FI912381A patent/FI912381A7/fi unknown
  • 1991-05-16 DE DE59103750T patent/DE59103750D1/de not_active Expired - Fee Related
  • 1991-05-17 CA CA002042862A patent/CA2042862A1/fr not_active Abandoned
  • 1991-05-17 BR BR919102050A patent/BR9102050A/pt not_active Application Discontinuation
• 1993
  • 1993-10-19 US US08/139,337 patent/US5362367A/en not_active Expired - Fee Related

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