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

• 3,5-cyclohexadiene-1,2-dioic acid may be obtained from o-phthalic acid by partial electrochemical reduction thereof using specially prepared, highly pure lead cathodes (Zeitschrift fiir Elektrochemie, vol. 35
• vol. ,33, pp. 2215 if. relating to the electrochemical refor overcoming the occurrence of poisoning at the c CC reduction reaction and allowing a current density of, say, 2 a./dm. to exist and then preparing them by a complicated process.
• 3,S-cyclohexadiene-l,2-dioic acid and 2,5-cyclohexadiene-1,4-dioic acid may be obtained at a very high degree of purity and without the occurrence of by-products by the electrochemical reduction of o-phthalic acid or terephthalic acid in electrolytic cells in which the anode and cathode chambers are separated by diaphragms, using catholytes containing organic solvents, provided that the flow of electrolysis current is periodically interrupted and the electrolytic cell is shortcircuited and, if desired, operated with poles reversed for relatively long periods.
• the advantage of the present process is that the formation of the said sparingly soluble by-products diphthalyl and a,u'-benzoin-dioic acid is completely suppressed.
• the period during which the flow of current is interrupted and short-circuiting is elfected is advantageouslyfrom 5 seconds to 15 minutes, especially from 1 to 5 minutes, short electrolysis times being associated with short no-current times.
• the no-current periods are from A to 7100 of the electrolysis time.
• one of a number of short-circuit periods may be replaced by a period of operation with poles reversed, for which purpose a current strength of from 1 to of the electrolysis current, preferably of from to A thereof, is used.
• the reversed-pole periods are usually equal to the short-circuit periods.
• one of them--selected at relatively large intervals as desired usually every th to 500th no-current periodmay be replaced by a period of operation with poles reversed.
• an electric time-switch is adjusted to switch off the rectifier at predetermined intervals and to short-circuit the electrolytic cell. Indeed, in a long-term experiment lasting for more than one year, the catholyte remained completely clear and no deposit of by-product could be found either on the diaphragms or on the cathodes. The current consumption, conversion rate and bath voltage remained constant throughout the entire period. t
• the process is eminently suitable for the partial electrochemical reduction of terephthalic acid and, preferably, of o-phthalic acid.
• the reduction is carried out in all other respects under known conditions and in conventional electrolysis apparatus in which the anode and cathode chambers are separated by diaphragms.
• the concentration of phthalic acid is usually in the range of from 5 to 25%, especially from 10 to 20% by weight.
• the catholyte used is usually a mixture of ethers, carboxamides and/ or nitriles which are liquid at room temperature and are miscible with water, for example dioxane, tetrahydrofuran, glycol monoethyl ether, dimethyl formamide or 'acetonitrile, together with water and sulfuric acid.
• the concentration of water and sulfuric acid in the catholyte is conveniently such that the catholyte shows good conductivity.
• the concentration of sulfuric acid is usually from 1 to 20%, especially from 2 to 10%,
• the concentration of water in the catholyte is usually between 5 and 40%, especially between and by weight.
• the concentration of organic solvent should be as high as possible consistent with maintaining good catholyte conductivity whilst achieving high phthalic acid solubility.
• the concentration of the organic solvent is usually in the range of from to 80%, in particular from 50 to 70%, by weight.
• a mixture of organic solvents may be used if desired.
• the anolyte used is generally dilute aqueous sulfuric acid having a concentration of from 2 to 10% by weight.
• the anode and cathode chambers are separated by a diaphragm.
• Ion exchanger diaphragms or plastics tissues may be used in place of the more commonly used ceramic materials if desired.
• the anode is normally made of lead, lead dioxide, graphite or platinum metals.
• the cathode may be made of the usual cathode materials with the necessary hydrogen overvoltage. No special alloys need be employed, and normal lead may be used for the cathode with good results without the cathode suffering from poisoning or fatigue.
• the current densities are in the range of from 1 to 30, preferably from 2 to 20 a./dm.
• the temperatures may be between 20 and 70 C.
• the process is usually carried out at temperatures ranging from 25 C. to C., this temperature range being maintained by cooling if necessary.
• Lower temperatures have the advantage that in the hydrogenation of o-phthalic acid the rearrangement of the 3,5-cyclohexadiene-1,2-dioic acid formed to 2,5- cyclohexadiene-1,2-dioic acid is substantially avoided.
• Isolation of the product is simple.
• the organic solvent may be removed by distillation, whereupon the cyclohexadiene-dioic acid precipitates, which may also be achieved by diluting the reaction solution with water.
• the precipitated product is then filtered off, washed and dried.
• bipolar, plate-shaped electrodes of lead which are connected in series and are assembled in the manner of a filter press.
• the electrodes may conveniently also be designed as cooling elements for water cooling.
• the five cathode and anode surfaces are separated by cation exchanger diaphragms, based on sulfonated polystyrene for example.
• the cathode chambers are interconnected by means of glass tubes and plastics tubing.
• the catholyte is pumped continuously through the five cathode chambers successively, in each case flowing upwardly through the chamber to be withdrawn at the top.
• the catholyte preferably consists of a mixture of from to of dioxane or tetrahydrofuran, from 10 to 25% of water, from 2 to 10% of sulfuric acid and from 10 to 20% of o-phthalic acid, by weight. We prefer to use dilute, 220% w./w. sulfuric acid as anolyte.
• the overall dimensions of the electrodes are 30 mm. x 730 mm., the 'internal dimensions of the effective area of the electrode being 250 mm. to 680 mm., equal to an area of 17 dm.
• EXAMPLE The electrolytic unit described above and comprising 6 lead electrodes connected in series and forming 5 cathode and anode chambers separated by cation exchanger diaphragms of sulfonated polystyrene is used.
• the anolyte used is 5% w./w. sulfuric acid.
• the catholyte consists of a mixture of 60% of dioxane, 20% of water, 5% of sulfuric acid and 15% of o-phthalic acid. Electrolysis is carried out under the following conditions:
• An electric time switch is set to switch ofi the rectifier automatically every 6 hours, the electrolytic cell then being short-circuited for 2 minutes. At intervals of four months one of the short-circuit periods is replaced by a period of operation with poles reversed using a current at a strength of that of the working current, giving a current density of 0.5 a./dm.
• the discharged catholyte remains completely clear and the electrodes and diaphragms remain free of deposits. No diphthalyl or a,abenzoin-dioic acid could be detected. Theconditions stated remained constant for a period of operation lasting longer than one year.
• the total amount of by-productscomprising approximately equal of diphthalyl and a,a-benzoin-dioic acid is 0.2% by weight of the phthalic acid introduced or 0.03% by weight of the catholyte solution.
• I v V I 2 A process as claimed in claim 1 wherein the ratio of the length of the electrolysis periods to that of the no-current periods is from 1,000:1 to :5.
• a process as claimed in claim 1 wherein. at relatively long intervals a no-current period is replaced by a period in which the electrolytic cell is operated with its poles reversed instead of being short-circuited.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

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Cited By (2)

• 0
  • BE BE757871D patent/BE757871A/fr unknown
• 1969
  • 1969-10-23 DE DE19691953259 patent/DE1953259B2/de active Granted
• 1970
  • 1970-10-19 US US82206A patent/US3684668A/en not_active Expired - Lifetime
  • 1970-10-19 NL NL7015304A patent/NL7015304A/xx unknown
  • 1970-10-20 CA CA096091A patent/CA930327A/en not_active Expired
  • 1970-10-21 FR FR7037963A patent/FR2066298A5/fr not_active Expired
  • 1970-10-22 GB GB5011770A patent/GB1318795A/en not_active Expired
  • 1970-10-22 AT AT951570A patent/AT302265B/de not_active IP Right Cessation
  • 1970-10-23 JP JP45092912A patent/JPS507596B1/ja active Pending

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