Production of diesters of dicarboxylic acids by electrochemical condensation of monoesters of dicarboxylic acids
I United States Patent 1 [111 3,879,271  
 Nohe et a1. Apr. 22, 1975 [54] PRODUCTION OF DIESTERS OF 3,589,990 6/1971 Freidlin et a1. 204/72 DICARBOXYLIC ACIDS BY 3,652,430 3/1972 Beck et a1 204/72 3.787.299 1/1974 Beck et al. 204/59 R ELECTROCHEMICAL CONDENSATION OF MONOESTERS OF DICARBOXYLIC ACIDS Inventors: Heinz Nohe, Meckenheim; Franz Wenisch, Frankenthal; HubeFt Suter, Ludwigshafen, all of Germany Assignee: BASF Aktiengesellschaft,  
 Ludwigshafen, Rhine, Germany Foreign Application Priority Data Oct. 4, 1972 Germany 2248562 Filed: Oct. 4, 1973 Appl. No.: 403,614  
 US. Cl 204/72; 204/59 R; 204/79 Int. C1,. C07b 29/06; CO7c 69/34; CO7c 67/00 Field of Search 204/59 R, 72, 78, 79  
 References Cited UNITED STATES PATENTS 4/1967 Kuwata ct a1 204/79 FOREIGN PATENTS OR APPLICATIONS 11,116 7/1963 Japan 204/79 Primary E.\&#39;aminerF. C. Edmundson Attorney, Agent, or Firm-Johnston, Keil, Thompson &amp; Shurtleff [57] ABSTRACT The production of diesters of dicarboxylic acids having six to 18 carbon atoms and alcohols of four to 12 carbon atoms by electrochemical condensation of monoesters of dicarboxylic acids of four to 10 carbon atoms and alcohols of four to 12 carbon atoms in solution in methanol which contains from 0.01 to 5% by weight of water.  
 7 Claims, No Drawings PRODUCTION OF DIESTERS OF DICARBOXYLIC ACIDS BY ELECTROCHEMICAL CONDENSATION OF MONOESTERS OF DICARBOXYLIC ACIDS This invention relates to a new process for the production of diesters of dicarboxylic acids by electrochemical condensation of monoesters of dicarboxylic acids.  
  It is known that diesters of dicarboxylic acids can be prepared by electrochemical condensation (Kolbe synthesis) of monoesters of lower dicarboxylic acids. This is carried out in aqueous solution or preferably in solution in methanol. The monoester is partly neutralized. for example with sodium methylate. to increase conductivity. Platinum anodes are generally used in the electrolysis.  
  In continuous operation difficulties arise when starting from monoesters of higher alcohols such as the monobutyl, monoctyl. monopentyl. monohexyl. monocyclohexyl. mono-Z-methylhexy]. mono-2- cthylhexyl. monodecyl or monododecyl esters of dicarboxylic acids such as adipic acid or glutaric acid. Polymerized layers cover the anodes and these cause a marked rise in potential and hinder the passage of current. This may be counteracted of course by adding to the electrolyte for example a cosolvent such as tetrahydrofuran. methyl ethyl ketone. cyclohexanone or acetic acid or by periodically stopping the electrolysis and shortcircuiting the electrodes. but these measures result in a marked erosion of platinum from the anode and a consequent shortening of its life.  
  We have now found that in the production of diesters from dicarboxylic acids of six to 18 carbon atoms and alcohols of four to twelve carbon atoms by electrochemical condensation of monoesters of dicarboxylic acids of four to ten carbon atoms and alcohols of four to 12 carbon atoms in a 20 to 60 percent by weight solution in methanol the said disadvantages are obviated when the methanol solution contains from 0.0l to 5 percent by weight of water and the condensation is carried out in the presence of such an amount of a basic compound that the degree of neutralization is from I to 30 mole percent.  
  Suitable starting materials include the monoesters of dicarboxylic acids of four to ten carbon atoms such as glutaric. adipic. pimelic. suberic axelaic and sebacic acids with primary or secondary linear or branched alcohols of four to twelve carbon atoms and preferably from six to ten carbon atoms. Specific examples are the monobutyl. monoisobutyl. mono-Z-butyl. monopentyl. monohexyl. monocyclohexyl. monooctyl. mono-2- methylhexyl-(S). mono-Z-ethylhexyl. monodecyl or monododecyl esters of the said acids. Monoesters of adipic acid and particularly the Z-ethylhexanol monoester are of great industrial interest.  
  The monoesters of dicarboxylic acids are used according to this new process in from 20 to 60 percent and preferably in from 30 to 50 percent by weight solution in methanol. The acid solutions are partly neutralized to achieve adequate conductivity. Basic compounds such as sodium carbonate. sodium methylate. sodium hydroxide or amines are used for this purpose.  
  From 0.0] to 5 percent and preferably 0.2 to 2 percent by weight of water is added to the methanol electrolyte which consists for example of a solution in methanol of the monoester which has been partly neutralized with sodium methylate. The required concentration of water may however be set up particularly advantageously by carrying out the neutralization with the calculated amount of aqueous caustic soda solution or aqueous sodium carbonate solution. Finally it is possible to carry out the neutralization with sodium or potassium hydroxide or with sodium carbonate. The neutralization and control of the water content in a single stage by adding caustic soda solution is particularly suitable for continuous operation.  
  The degree of neutralization, i.e. the molar percentage of ester which is neutralized. is from 1 to 30 and preferably 2 to 15 mole7c. The electrolyte may also contain other organic solvents such as ethers, ketones. acetic acid or formic acid. In this case the water content of from 0.01 to 5 percent by weight is based on the electrolyte.  
  Electrochemical dimerization is carried out for example at a current density of from 5 to 50 amps/dm and preferably of from 10 to 30 amps/dm&#34;. Depending on the conditions the potential thus set up is from 5 to 30 volts but usually from 6 to volts.  
  The reaction temperature is generally kept at from to 65C and preferably from to C.  
  Smooth platinum anodes are particularly suitable. but other anode materials which contain platinum as an alloy component such as Pt-Rh, Pt-Ru. Pt-lr. Pt-Au or platinized metals such as Ti or Ta may also be used. The cathodes may consist of the materials conventionally used in the Kolbe synthesis such as alloy steel. nickel. titanium or platinum.  
  Cells of various types are suitable as the electrolytic cells. Thus the cells described in Belgian Patent Specification No. 723.694 and having vibrating pairs of electrodes which are permeable to liquid may be used and also those cells which vibrate and have an electrode which is permeable to liquid or cells which have two liquid-permeable electrodes and do not vibrate. Capillary-slit cells such as described in Chem. lng. Techn. 41 (1969 page 945. are also suitable. It is particularly advantageous to carry out the reaction in an electrolytic cell such as is described in German Printed Application DOS 2.039.590. The electrolyte in this case is circulated from a buffer vessel through an undivided electrolytic cell with bipolar electrode plates of similar construction to a filter press and through a cooler and gas separator. The reacted electrolyte is discharged through an overflow and fresh electrolyte is fed in at the rate of reaction which is determined by titration or by measurement of the pH.  
  The life of the anodes used in the electrochemical condensation according to the new process is prolonged by a factor of up to four as compared with conventional methods without any decline in current yield or substance yield. This advantageous result is surprising because investigations in the Kolbe synthesis of dimethyl sebacate in methanol and in glycol with an addition of water have shown that loss of platinum is least in the absence of water and increases with increasing content of water (see .lourn. Appl. Chem. (USSR) 37 I964). 224) and that current yield and substance yield are markedly decreased by the presence of water (see Journal. Appl. Chem. (USSR) 38 (1965). I776).  
  It is unnecessary to add a cosolvent. acetic acid or formic acid in the advantageous novel process: periodic shortcircuiting of the electrodes is also not necessary. Platinum erosion is however clearly less even in the presence of these additives and when periodically shortcircuiting the electrodes. The ratio of working periods to rest periods may conveniently be from one to thrity minutes to from two to sixty seconds. preferably from two to twenty minutes to from live to fifteen seconds. I  
 The following Examples illustrate the invention. The  
 &#39; parts and percentages indicated are by weight.  
 EXAMPLE I A solution of 40 parts of mono-Z-ethylhexyl adipate in 60 parts of methanol is adjusted to a degree of neutralization of with sodium hydroxide. Then 0.5 part of water is added to the solution. The solution obtained is pumped at a flow rateof 50 cm/second at 50 to 55C through a cell having bipolar electrodes arranged in the manner of a filter press. The anodes are of platinum and the anode and cathode surfaces each amount to 1.9 dm&#34;: the distance between electrodes is 0.5 mm. The electrolysis proceeds at a cell potential of from 1 l to 12 volts and a current density of amps/dm&#39;-&#39;. After twenty minutes the current is interrupted by a relay for fifteen seconds. Fresh electrolyte is fed in at such a rate that the degree of reaction is 80 percent. The reacted electrolyte is discharged continuously.  
  The discharge is worked up by evaporating the solvent and washing the residue first with 2.5 percent caustic soda solution and then three times with water. The crude di-Z-ethylhexyl sebacate is purified from byproducts by steam distillation at 150C and mm Hg. The purity of the ester which remains is determined by gas chromatography.  
  After operation for 480 hours di-Z-ethylhexyl sebacate is obtained in a material yield of 80% and a current yield of 60%. The energy consumption is 2.6 kwh/kg of ester. Unreacted monoester can be precipitated from the caustic soda solution by acidification with sulfuric acid and-reused after it has been washed with water. A loss of platinum of 15 &#39;y/amperehour (3.3 mg/kg of ester) is determined by weighing the anodes before and after the experiment. Without the addition of water the loss of platinum is 56 &#39;y/amperehour 12.1 mg/kg of ester).  
 EXAMPLE 2 A solution of 30 parts of mono-2-ethylhexyl adipate in 30 parts of methanol is adjusted with sodium methylate to a degree of neutralization of 10 percent and 1 part of water is added. Electrolysis is carried out in the cell described in Example 1 at a degree of conversion of 90 percent. The electrolytic current is interrupted for ten seconds every ten minutes.  
  The discharge is processed as in Example 1. Di-2- ethylhexyl sebacate is obtained in a material yield of 78 percent and a current yield of57 percent. After the experiment has been carried on for 240 hours the loss of platinum is 18 y/amperehour (3.9 mg/kg of di-2- ethylhexyl sebacate). Under the same conditions but without the addition of water the platinum loss at the anodes is 86 y/amperehour.  
 EXAMPLE 3 A soluution of 40 parts of mono-2-cthylhexyl glutarate in 60 parts&#39;of methanol is neutralized with 0.66  
 part of sodium hydroxide to a neutralization degree of 10 percent and 0.5 part of water is added. Electrolysis is carried out batchwise in a bipolar cell at to C. 14 to 15 volts and a current density of 25 amps/dm&#34; to a degree of conversion of 97 percent. The current is interrupted for 15 seconds every 15 minutes. After the electrolyte has been reacted for two hours it is processed as described in Example 1. Di-Z-ethylhexyl suberate is obtained in a material yield of percent and a current yield of 49 percent. Loss of platinum after the experiment has proceeded for two hours is 38 &#39;y/amperehour (8.4 mg/kg of di-Z-ethylhexyl suberate). Without adding water and with shortcircuiting of the electrodes the loss of platinum is 66 y/amperehour 19.9 mg/kg of di-Z-ethylhexyl suberate).  
 EXAMPLE &#34;4 A solution of 40 parts of mono-2-ethylhexyl glutarate in 60 parts of methanol is neutralized to the extent of 10 percent with the calculated amount of caustic soda solution. Electrolysis of the solution which contains 1 part of water is carried out continuously ina bipolar plate cell at 50 to 55C. a current density of 20 ampercs/dm and at 12 volts. The degree of conversion is 80%. Current is interrupted for 15 seconds every 15 minutes. After the experiment has been carried on for 60 hours di-2-ethylhexyl suberate is obtained in a material yield of 64 percent and a current yield of 51 percent. Loss of platinum is 27.2 y/amperehour (7.9 mg of platinum per kg of di-Z-ethylhexyl suberate).  
  under the same conditions without the addition of water the anoidic loss of platinum is 71 &#39;y/amperehour.  
 We claim:  
 1. A process for the production of dicsters of dicarboxylic acids of six to 18 carbon atoms and alcohols of four to twelve carbon atoms wherein a monoester of a dicarboxylic acidof four to ten carbon atoms and an alcohol of four to twelve carbon atoms is electrochemically condensed at a current density of from 5 to 50 amps/dm and a temperature of from 20 to 65C in a 20 to 60% by weight solution in methanol which contains from 0.2 to 2 percent by weight of water in the presence of such an amount ofa basic compound that the degree of neutralization is from 1 to 30mole percent.  
  2. A process as claimed in claim I wherein the electrochemical condensation is carried out with a monoester of a dicarboxylic acid of four to 10 carbon atoms and a primary or secondary. linear or branched alcohol of six to 10 carbon atoms.  
  3. A process as claimed in claim I wherein a monoester of adipic acid is used.  
  4. A process as claimed in claim 1 wherein mono-2- ethylhexyl adipate is used.  
  5. A process claimed in claim 1 wherein a 30 to 50 percent by weight methanol solution is used.  
  6. A process as claimed in claim 1 wherein the addition of water and the neutralization are carried out by adding a solution of sodium hydroxide or sodium carbonate.  
 7. A process as claimed in claim 1 wherein the degree of neutralization is from 2 to 15 mole percent.  
 UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,879,271 DATED April 22, 1975 tNVENTOR(S) I NOHE et a] It rs certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 In Column 1, Line 45, delete &#34;axelaic&#34; and insert--azelaic-- In Column 1, Line 18, delete &#34;monoctyl&#34; and insert-mono0ctyl- Signed and Scaled this eleventh Day of May 1976 {st-:AL]  
 Arrest:  
 RUTH C. MASON C. MARSHALL DANN Allcsiing Officer (ummissium&#39;r nflau&#39;nls and Trmlemurkx