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

• the present invention relates to a process for the production of glyoxylic acid by electrochemical reduction of oxalic acid.
• Glyoxylic acid is an important intermediate for the production of technically relevant compounds and can be produced either by a controlled oxidation of glyoxal or by an electrochemical reduction of oxalic acid.
• electrochemical reduction of oxalic acid to glyoxylic acid has long been known and is generally carried out in an aqueous, acidic medium, at low temperature, on electrodes with high hydrogen overvoltage, with or without the addition of mineral acids and in the presence of an ion exchange membrane (DE-AS 458 438) .
• the object of the present invention is to provide a process for the electrochemical reduction of oxalic acid to glyoxylic acid which avoids the disadvantages mentioned above, in particular has a high selectivity, reaches the lowest possible oxalic acid concentration at the end of the electrolysis and a cathode with a high one Long-term stability used.
• Selectivity is understood to mean the ratio of the amount of glyoxylic acid produced to the total amount of products formed during the electrolysis, namely glyoxylic acid plus by-products, for example glycolic acid, acetic acid and formic acid.
• the object is achieved in that the electrochemical reduction of oxalic acid is carried out on cathodes with a lead content of at least 50% and the aqueous electrolysis solution salts of metals with a hydrogen overvoltage of at least 0.25 V at a current density of 2500 A / m2 and optionally one Contains mineral acid.
• the present invention thus relates to a process for the preparation of glyoxylic acid by electrochemical reduction of oxalic acid in aqueous solution in divided or undivided electrolysis cells, characterized in that 50 to 99.999% by weight of the cathode consists of lead and the aqueous electrolysis solution in the undivided Cells or in the cathode compartment of the divided cells also contains at least one salt of metals with a hydrogen overvoltage of at least 0.25 V, preferably at least 0.40 V, based on a current density of 2500 A / m2 and a mineral acid or organic acid.
• cathodes which consist of 66 to 99.96% by weight, preferably 80 to 99.9% by weight, of lead and 34 to 0.04% by weight, preferably 20 to 0 , 1 wt .-%, consist of other metals.
• lead-containing materials can be used as cathodes.
• no high-purity lead is used.
• Preferred alloy components are V, Sb, Cu, Sn, Ag, Ni, As, Cd, Ca, in particular Sb, Sn, Cu and Ag.
• alloys which consist of 99.6% by weight of lead and 0.2% by weight of tin and silver are, for example, alloys which consist of 99.6% by weight of lead and 0.2% by weight of tin and silver.
• conventional lead alloys such as pipe lead (material no. 2.3201, 98.7 to 99.1% Pb; material no. 2.3202, 99.7 to 99.8% Pb), shot lead (material no.
• the method according to the invention is carried out in undivided or preferably in divided cells.
• the usual diaphragms made of polymers or other organic or inorganic materials, such as glass or ceramics, which are stable in the aqueous electrolysis solution, are used to divide the cells into anode and cathode compartments.
• Ion exchange membranes in particular cation exchange membranes made of polymers, preferably polymers having carboxyl and / or sulfonic acid groups, are preferably used.
• the use of stable anion exchange membranes is also possible.
• the electrolysis can be carried out in all customary electrolysis cells, such as, for example, in beaker or plate and frame cells or cells with fixed bed or fluidized bed electrodes. Both the monopolar and the bipolar circuit of the electrodes can be used.
• All materials on which the corresponding anode reactions take place can be used as anode material.
• lead, lead dioxide on lead or other carriers, platinum, metal oxides on titanium, for example titanium dioxide doped with noble metal oxides such as platinum oxide, are suitable for the development of oxygen from dilute sulfuric acid.
• carbon or titanium dioxide on titanium doped with noble metal oxides are used, for example, for the development of chlorine from aqueous alkali metal chloride solutions.
• Aqueous mineral acids or solutions of their salts such as, for example, dilute sulfuric or phosphoric acid, dilute or concentrated hydrochloric acid, sodium sulfate or sodium chloride solutions, can be used as anolyte liquids.
• the aqueous electrolysis solution in the undivided cell or in the cathode compartment in the divided cell contains the oxalic acid to be electrolyzed in a concentration expediently between about 0.1 mol of oxalic acid per liter of solution and the saturation concentration of oxalic acid in the aqueous electrolysis solution at the electrolysis temperature used.
• Salts of metals with a hydrogen overvoltage of at least 0.25 V are added to the aqueous electrolysis solution in the undivided cell or in the cathode compartment of the divided cell.
• Such salts are mainly the salts of Cu, Ag, Au, Zn, Cd, Hg, Sn, Pb, Tl, Ti, Zr, Bi, V, Ta, Cr, Ce, Co or Ni, preferably the salts of Pb , Sn, Bi, Zn, Cd and Cr.
• the preferred anions of these salts are chloride, sulfate, nitrate or acetate.
• the salts can be added directly or can also be generated in the solution, for example by adding oxides, carbonates, in some cases also the metals themselves.
• the salt concentration of the aqueous electrolysis solution in the undivided cell or in the cathode compartment of the divided cell is advantageously from about 10 auf to 10 wt .-%, preferably to about 10 ⁇ 5 to 0.1 wt .-%, each based on the total amount aqueous electrolysis solution.
• metal salts can also be used which form poorly soluble metal oxalates after addition to the aqueous electrolysis solution, for example the oxalates of Cu, Ag, Au, Zn, Cd, Sn, Pb, Ti, Zr, V, Ta, Ce and Co.
• the added metal ions from the product solution can be removed very easily by filtration after the electrolysis to the saturation concentration.
• Mineral acids such as phosphoric acid, hydrochloric acid, sulfuric acid or nitric acid, or organic acids, for example trifluoroacetic acid, formic acid or acetic acid, are added to the aqueous electrolysis solution in the undivided cell or in the cathode space in the divided cell. It is preferred to add mineral acids, particularly preferably nitric acid.
• the concentration of the abovementioned acids is between 0 and 10% by weight, preferably between 10 ⁇ 6 and 0.1% by weight. If acids are added to the catholyte or to the electrolyte of an undivided cell in the concentrations given above, the current yield surprisingly remains above 70% even after several batchwise tests, while the current yield in the absence of the acid is clearly below 70%. At the beginning of the electrolysis, the addition of acid can initially be dispensed with if there are simultaneously salts of the above-mentioned metals in the aqueous electrolysis solution.
• the addition of the above-mentioned metal salts can be dispensed with if one or more of the above-mentioned mineral acids are present in the aqueous electrolyte solution.
• the current density of the method according to the invention is advantageously between 10 and 5000 A / m2, preferably 100 to 4000 A / m2.
• the cell voltage of the method according to the invention is dependent on the current density and is expediently between 1 V and 20 V, preferably between 1 V and 10 V, based on an electrode spacing of 3 mm.
• the electrolysis temperature can range from - 20 ° C to + 40 ° C. Surprisingly, it was found that at electrolysis temperatures below + 18 ° C, even at oxalic acid concentrations less than 1.5% by weight, the formation of glycolic acid as a by-product can be less than 1.5 mol% compared to the glyoxylic acid formed. The proportion of glycolic acid increases at higher temperatures.
• the electrolysis temperature is therefore preferably between + 10 ° C and + 30 ° C, in particular between + 10 ° C and + 18 ° C.
• the catholyte flow rate of the process according to the invention is between 1 and 10,000, preferably 50 and 2000, in particular 100 and 1000, liters per hour.
• the product solution is worked up using customary methods.
• the electrochemical reduction is stopped when a certain turnover has been reached.
• the resulting glyoxylic acid is separated from any oxalic acid still present in accordance with the prior art mentioned above.
• the oxalic acid can be selectively fixed to ion exchange resins and the aqueous solution free of oxalic acid can be concentrated in order to obtain a commercial 50% by weight glyoxylic acid.
• the glyoxylic acid is continuously extracted from the reaction mixture by customary methods and the corresponding equivalent proportion of fresh oxalic acid is added simultaneously.
• the reaction by-products in particular glycolic acid, acetic acid and formic acid, are not or not completely separated from the glyoxylic acid by these methods. It is therefore important to have a high selectivity in the To achieve procedures to avoid complex cleaning processes.
• the process according to the invention is characterized in that the proportion of the sum of by-products can be kept very low. It is between 0 and 5 mol%, preferably below 3 mol%, in particular below 2 mol%, relative to the glyoxylic acid.
• the selectivity of the process according to the invention is all the more remarkable in that, even at a low final oxalic acid concentration, that is to say in the range of 0.2 mol of oxalic acid per liter of electrolysis solution, the proportion of by-products is preferably below 3 mol%, based on glyoxylic acid.
• the particular advantage of the cathode used according to the invention is that a high-purity, expensive lead cathode can be dispensed with and conventional, technically available lead-containing materials can be used. Periodic rinsing with nitric acid can also be dispensed with, so that the lead wear is kept very low and a long service life of the cathode can be achieved in the technical process.
• a divided circulation cell which is constructed as follows:
• Example 3 (comparative example) Rinsing with 10% nitric acid
• the electrochemical cell was rinsed with 5 l of 10% HNO3 for 20 minutes at about 20 ° C.
• the content of lead (II) ions after the rinsing process was 0.88 g / l, which corresponds to a lead wear of 4.4 g.
• the example confirms the strong corrosion of the lead cathode when rinsing with nitric acid.
• Example 4 with the addition of lead salts and nitric acid
• the weight of the cathode increased slightly during the electrolysis from 1958.3 g before experiment a) to 1958.9 g after experiment e).
• Example 5 with the addition of lead salts and nitric acid
• Example 6 With the addition of nitric acid, without the addition of lead salts
• the cathode was rinsed with 2 liters of 10% nitric acid for about 10 minutes at about 25 ° C.
• the electrolysis was carried out analogously to Example 4, but a lead-antimony alloy, material no. 2.3202 with a lead content between 99.7 and 99.8%.

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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)
• Electrolytic Production Of Metals (AREA)

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• 1992
  • 1992-05-26 DE DE4217338A patent/DE4217338C2/de not_active Expired - Fee Related
• 1993
  • 1993-05-18 EP EP93108108A patent/EP0578946B1/fr not_active Expired - Lifetime
  • 1993-05-18 AT AT93108108T patent/ATE134224T1/de not_active IP Right Cessation
  • 1993-05-18 DE DE59301621T patent/DE59301621D1/de not_active Expired - Fee Related
  • 1993-05-24 BR BR9302036A patent/BR9302036A/pt not_active Application Discontinuation
  • 1993-05-24 US US08/066,533 patent/US5395488A/en not_active Expired - Fee Related
  • 1993-05-25 JP JP5122838A patent/JPH0657471A/ja not_active Withdrawn
  • 1993-05-25 CA CA002096901A patent/CA2096901A1/fr not_active Abandoned

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