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
  • C25B3/23—Oxidation

Definitions

• the present invention relates to an improved process for the electrochemical production of terephthalaldehyde tetraalkylacetals of the formula I. in which the radicals R are C1-C6-alkyl groups.
• Terephthalaldehyde tetraalkylacetals are valuable intermediates for organic syntheses and, after their hydrolysis, can be used in the form of terephthalaldehyde for the production of optical brighteners, fiber precursors and polymers.
• JP-A 85/39 183 it is known to produce terephthalaldehyde tetraalkylacetals by electrooxidation of p-tolylaldehyde dialkyl acetals in the presence of an aliphatic alcohol or an alcohol / acetic acid mixture and a neutral conductive salt. Furthermore, this document mentions without further explanation that p-xylene can be oxidized electrochemically to the corresponding terephthalaldehyde tetraalkylacetal in the presence of an aliphatic alcohol and acetic acid.
• EP-A 12 240 describes the electrochemical oxidation of p-xylene in alcoholic solution in the presence of a neutral conductive salt to give the corresponding p-tolylaldehyde dialkyl acetals. However, the direct oxidation to the respective tetraalkylacetals is not mentioned.
• DE-A 33 22 399 describes the anodic oxidation of p-xylene to p-tolylaldehyde dimethyl acetal, the electrochemical reaction taking place in methanol and sulfuric acid or a sulfonic acid.
• the direct oxidation to terephthalaldehyde tetramethyl acetal is also not observed in this process.
• the invention was therefore based on the object of making the terephthalaldehyde tetraalkylacetals (I), which are valuable for organic syntheses, electrochemically accessible in a simple and economical manner.
• a process for the electrochemical preparation of terephthalaldehyde tetraalkylacetals of the formula I in which the radicals R are C1-C6-alkyl groups which is characterized in that compounds (II) or mixtures thereof which are used in the electrochemical oxidation of p-xylene in addition to I and p-tolylaldehyde dialkyl acetal (III) in the presence of a neutral conductive salt or an acid in alkanolic solution, after extensive separation of I and, if desired, a part of the oxidation products II and III subject to further electrochemical oxidation.
• the electrochemical oxidation of p-xylene in alkanolic solution produces not only the valuable products terephthalaldehyde tetraalkylacetal (I) and p-tolylaldehyde dialkyl acetal (III) the following intermediate methylalkoxylated oxidation products of p-xylene, which are referred to as compounds (II):
• the compounds (II) are used either individually or as a mixture as starting compounds. They are usually about 1 to 50% by weight, preferably about 5 to 30% by weight, part of the alkanolic electrolysis solution.
• p-xylene and p-tolylaldehyde dialkyl acetal (III) in a mixture with II are oxidized electrochemically. They are contained in the electrolysis solution in an amount corresponding to the compounds (II).
• the amounts of the various starting compounds can vary.
• the mixture to be oxidized after recycling the components separated from I and optionally from III and adding fresh p-xylene, has the following composition: 50 to 90 wt% p-xylene 0.1 to 2% by weight of I 0.1 to 5% by weight IIa 0.1 to 5 wt% IIb 5 to 40 wt% IIc and 0 to 30% by weight III
• the following alkanols for example, are suitable as an alkanol component of the electrolysis solution: Methanol, ethanol, n-propanol, n-butanol, n-pentanol and n-hexanol, the isomers of the four last-mentioned compounds also being possible.
• Particularly preferred alkanols are methanol and ethanol.
• auxiliary electrolytes Another component of the electrolysis solution are neutral conductive salts or acids, which can also be referred to as auxiliary electrolytes and serve to improve conductivity.
• the neutral conductive salts are e.g. Halides, sulfonates, sulfates, phosphonates, phosphates, hexafluorophosphates, tetrafluoroborates and perchlorates.
• Its cationic part can be an alkali metal, e.g. lithium, sodium, potassium and rubidium, preferably sodium and potassium, as well as a quaternary ammonium residue, e.g. Tetramethylammonium, tetraethylammonium, tetra-n-propylammonium, dimethyldiethylammonium, dimethyldi-n-propylammonium and diethyldi-n-butylammonium.
• the relevant conductive salts are, for example, halides such as sodium and potassium fluoride, sulfonates such as sodium and potassium benzene sulfonate, sulfates such as tetramethylammonium methylsulfate, phosphate such as tetramethylammonium methylphosphate, hexafluorophosphates such as sodium and potassium hexate fluorate such as tetrafluorotoro fluorate such as tetrafluorotoro fluorate such as tetrafluorotoro fluorate such as tetrafluorotoro fluorate, such as tetra fluorine, such as tetra fluorine, such as tetra fluorine, such as tetra fluorine, such as tetra fluorine, as well as tetramethyl fluorate, such as tetra fluoride, such as tetrafluoride, and tetrafluoride
• mixtures for example of sodium and potassium benzene sulfonate, can also be used as conductive salts.
• the process according to the invention can also be carried out in the presence of an acidic auxiliary electrolyte.
• the following acids are suitable: sulfuric acid, half esters of sulfuric acid such as methyl sulfate, alkane sulfonic acids such as methyl and ethyl sulfonic acid and aromatic sulfonic acids such as benzene and p-toluenesulfonic acid, with sulfuric acid and benzenesulfonic acid being preferred.
• the auxiliary electrolytes are preferably used in the electrolysis solution in an amount of 0.1 to 5% by weight, in particular 0.2 to 3% by weight.
• terephthalaldehyde tetraalkylacetals (I) can be carried out in the electrolysis cells customary for electrochemical oxidation. As a rule, are suitable for this particularly undivided electrolysis cells.
• the reaction can also be carried out in divided electrolysis cells.
• noble metals such as platinum, metal oxides such as lead dioxide, ruthenium dioxide, chromium-III-oxide and metals such as titanium, which are coated with a metal oxide layer such as ruthenium dioxide, and graphite can be used as the anode material.
• Suitable cathode materials are e.g. Metals such as lead, iron, copper, nickel and zinc, metal alloys such as steel, precious metals such as platinum and graphite.
• Graphite is preferably used for both electrodes.
• the electrochemical oxidation can be carried out at 0.5 to 20 and preferably at 1 to 10 bar.
• electrolysis can be carried out at a current density of preferably 0.1 to 30 A / dm2, particularly 1 to 10 A / dm2.
• the theoretically necessary amount of charge is 8 F. In practice, 8.5 to 12 F per mole of starting compounds used is expediently used.
• the electrolysis can be carried out batchwise or continuously.
• the electrolyzed solution can be worked up using customary methods such as distillation, extraction, filtration and crystallization.
• the process products are preferably distilled off.
• the compounds (II) or their mixtures are subjected to further electrochemical oxidation.
• the complete or partial removal of the likewise desired p-tolylaldehyde dialkyl acetal (III) is recommended when the ratio of I to III is in a range from about 0.03 to 1 to 0.33 to 1, in particular 0.2 to 1 to 0.3 to 1 falls.
• dialkylacetal (III) can be subjected to the electrochemical oxidation either individually or together with II.
• the process according to the invention is of particular importance if the compounds (II) or their mixtures are returned to the stage of the electrochemical oxidation of p-xylene.
• the reaction solution obtained in the electrochemical oxidation in the electrolysis cell is expanded to a pressure which is 10 mbar to 10 bar lower than the pressure in the electrolysis cell.
• the electrolysis cell preferably has an overpressure, based on normal pressure, of 0.1 to 6 bar. This pressure can preferably be built up in the electrolysis cell by means of a pump, but can also be generated by an inert gas such as nitrogen. After the oxidation step, the reaction solution is preferably let down to normal pressure.
• the first of these preferred embodiments also provides the following continuous mode of operation: After the reaction solution has been let down, a partial stream of the reaction solution is separated off and worked up. This partial stream is generally less than 5% by weight, preferably 0.01 to 1% by weight, of the total stream. Part of the gas dissolved in the reaction solution is discharged from the electrolysis circuit through this partial flow. A separate degassing of the entire reaction solution is not necessary, but can be advantageous in the case of small partial flows and relatively large amounts of gas.
• the partial flow is worked up as described above. Solvents, auxiliary electrolyte, starting compounds and, if appropriate, incompletely oxidized intermediates can be added to the reaction solution which is returned to the electrolytic cell. The recycled reaction solution is further replaced by the amount of starting compounds which corresponds to the amount of the separated product. After recycling and oxidation, the cycle described is repeated as often as required.
• the gas released during the expansion of the reaction solution after the electrolysis is also separated off, which is predominantly hydrogen discharged from the electrolysis cell.
• the reaction solution is then returned to the electrolysis cell, electrolyzed and then expanded.
• This sequence of process steps is referred to below as cycles. It has proven to be beneficial exposing the reaction solution to a large number of cycles, as a result of which a higher yield can be achieved economically than in just two cycles. 50 to 5000 cycles are preferred, particularly preferably 200 to 3000 cycles.
• the oxidation of p-xylene in one cycle is generally not carried out until conversion is complete.
• the turnover is generally 0.01 to 5% of the theoretical turnover.
• reaction solution is worked up on the product. This is done in a manner known per se, predominantly by distillation. If a solvent is present in the reaction solution, it is distilled off. If neutral salts are used as auxiliary electrolyte, these can then be filtered off before the acetal I is distilled. Solvents, electrolyte and unreacted starting compound can be used again in further process approaches.
• preferably 60 to 95, in particular 70 to 90,% by weight of the p-xylene to be reacted are initially introduced into the electrolysis cell in a batchwise procedure, and the remaining amount is added, preferably continuously, during the electrolysis.
• the process according to the invention has the advantage over the prior art that terephthalaldehyde tetraalkylacetals (I) are selectively accessible directly from p-xylene by electrooxidation. It is also advantageous that the corresponding p-tolylaldehyde dialkyl acetals (III) can also be obtained here as valuable products.
• the electrolysis apparatus used in the following examples consisted of an undivided flow cell with 11 bipolar graphite electrode plates.
• the electrode plates each had an area of 150 cm2.
• the distance between the electrodes was 1 mm in each case.
• the individual electrolyte solutions were each passed through the cell at a rate of 200 l / h.

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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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Citations (2)

• 1993
  • 1993-03-19 DE DE4308846A patent/DE4308846A1/de not_active Withdrawn
• 1994
  • 1994-03-09 EP EP19940103543 patent/EP0621352A3/fr not_active Withdrawn

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