Electrolytic Condensation Of Carboxylic Acids

Abstract

Process for the electrochemical condensation of carboxylic acids in a solvent, in which the electrolyte is caused to flow at a rate of from 0.05 to 2 m/sec between pairs of electrodes which are impermeable to liquids and are spaced from 0.1 to 2 mm apart.

Description

This invention relates to a new and useful process for the electrochemical condensation of carboxylic acids.

In the field of organic electrochemistry the electrolytic condensation of carboxylic acids is known as Kolbe synthesis. For example, a method of producing dimethyl sebacate, which is not readily available via chemical routes, is to effect electrochemical condensation of monomethyl adipate in methanolic solution. To ensure that the electrolyte is sufficiently conductive, part of the carboxylic acid employed must be neutralized. The addition of foreign electrolytes is not possible, as all foreign anions strongly interfere with the electrochemical condensation process. Belgian Pat. No. 723,694, for example, describes the advantage of maintaining a low degree of neutralization. However, low degrees of neutralization make it difficult to maintain industrially satisfactory cell potentials without the current densities being too small. For example, higher current densities involve higher cell potentials, and it is then necessary to employ expensive equipment for the removal of the Joulian heat.

It is thus desirable to keep the distance between the electrodes as small as possible. However, the Kolbe synthesis involves electrolysis in which large quantities of gas are evolved at both electrodes, 0.5 moles of hydrogen per Faraday at the cathode and approximately 1 mole of carbon dioxide per Faraday at the anode. The result is that, when the electrode are close together, a substantial proportion of the space between the electrodes is filled with gas bubbles once a steady state has been reached (hereinafter referred to as gas-filling effect), and consequently a large portion of the cross-section between the electrodes is not available for the conduction of current through the electrolyte and the cell potential rises to high values.

The cited Belgian Patent also descloses a method of carrying out the Kolbe synthesis at low degrees of neutralization and at high current densities whilst maintaining low cell potentials. This method involves the use of a cell having vibrating electrode pairs which are permeable to liquids, but such cells suffer from the drawback that they are relatively complicated and do not lend themselves readily to use on an industrial scale.

On the other hand, no method of carrying out the Kolbe synthesis is known in which the difficulties caused by the gas-filling effect have been overcome using smooth, closely spaced electrodes impermeable to liquids.

In the electrolysis of water it is known that the gas-filling effect may be suppressed by carrying out the electrolysis under pressure. At a pressure as low as 10 atmospheres gage in the electrolytic cell the volume of the evolved gases is only 10 percent of that at atmospheric pressure and a normal cell potential is achieved. Surprisingly, however, attempts to apply this principle to the Kolbe synthesis fail. If, for example, a 40 percent solution of monomethyl adipate in methanol is electrolyzed in a pressure cell at a pressure of 15 atmospheres gage and at a current density of 25 A/dm.sup.2, a degree of neutralization .alpha. of 5 percent molar and an electrolyte temperature of 42.degree. C, the distance between the electrodes being 0.27 mm, very little sebacate is produced and the starting material is almost quantitatively re-found after electrolysis. At atmospheric pressure, however, the sebacate is produced under otherwise similar conditions at an 80 percent yield and 60 percent current efficiency. Table 1 shows how the current efficiency falls with increasing pressure.

Table 1 ______________________________________ Pressure (atm.) Yield (%) Current efficiency (%) ______________________________________ 1 83 63 2,5 80 56 6 79 45 16 79 28 ______________________________________

Research carried out in our laboratories has shown that the original current efficiency is retained if the electrolytically evolved gases are constantly flushed out of the system by means of nitrogen, argon or some other inert gas. Although it is possible to make a cell having means for flushing out the electrolytically evolved hydrogen and carbon dioxide under pressure, such a cell would be extremely complicated and unsatisfactory.

We have now found, surprisingly, that the electrochemical condensation of carboxylic acids may be carried out in an industrially advantageous manner with good yields and no side reactions using a degree of neutralization of the carboxylic acid of less then 20 percent molar and preferably of less than 10 percent molar and a current density of more than 10 A/dm.sup.2 in a solvent and using electrodes which are impermeable to liquids, provided that the distance between the electrodes is from 0.1 to 2 mm and the electrolyte is caused to flow through the space between the paired electrodes at a rate of from 0.05 to 2 m/sec.

The rate of flow of the electrolyte between the electrodes, as required by the present invention for advantageous realization of the electrolytic condensation, is primarily governed by the current density, the distance between the electrodes and the length of the gap between the electrodes.

There is a definite lower limit to the rate of flow of the reaction mixture passing between the electrodes. If the rate of flow falls below said limit, the potential rises steeply and it is no longer possible to carry out the reaction under steady state conditions.

The rates of flow at which the process may be best carried out on an industrial scale range from 0.05 to 2 m/sec. The range 0.1 to 0.17 m/sec is preferred.

The distance between the plane-parallel electrodes is advantageously from 0.1 to 2 mm. A distance of from 0.3 to 0.8 mm is preferred. This distance is defined by spacers in the form of insulator strips, for example strips made from polypropylene or polyester sheeting. The length of the gap through which the reaction mixture flows is determined by the size of the electrodes and is advantageously from 5 to 100 cm.

Using the process conditions of the present invention, Kolbe syntheses may be carried out on an industrial scale at atmospheric pressure with excellent yields and at high current densities, low potentials and high conversion rates, no trouble being caused by the gas-filling effect, and there is no need for the use of a flushing gas such as nitrogen or argon to remove the electrolytically evolved hydrogen and carbon dioxide.

The process is applicable to all compounds susceptible to Kolbe synthesis. Of particular interest is the synthesis of difunctional compounds from substituted but not .alpha.-substituted alkanoic acids, particularly those containing from 2 to 20 carbon atoms in the acid radical.

The starting carboxylic acids may carry substituents in the .beta.-position or in a more remote position from the carboxyl group, such substituents being for example ester, acylamino, acyloxy, nitrilo, halo, aryl, alkyl or aralkyl groups or heterocyclic groups. Details of the range of application of the Kolbe synthesis are to be found, for example, in Russian Chemical Reviews, English translation, Vol. 29 (1960), pp. 161-180.

Examples of the use of the process of the invention are the synthesis of sebacic acid ester from adipic acid half-ester, of suberic acid ester from glutaric acid half-ester, of thapsic acid ester (C.sub.16) from azaleic acid half-ester, of 2,2'-, 5,5'-tetramethyladipic acid ester from 2,2'-dimethyl succinic acid half-ester, of N,N'-diacetyldicamethylene diamine from .epsilon.-acetylaminocaproic acid, of 1,8-octanediol diformate from 5-formyloxyvaleric acid, of decamethylene dicyanide from .epsilon.-cyanocaproic acid, of 1,20-dibromo-icosane from 11-bromoundecanoic acid and 1,10-dichlorodecane from .omega.-chlorocaproic acid.

The concentration of the starting materials in the solvent is usually from 10 to 20 percent by weight. The preferred solvent is methanol. Other useful solvents are water together with non-aqueous solvents such as lower alcohols, for example methanol, ethanol, isopropanol, or N,N-dialkylamides of lower alkanoic acids, in particular dimethyl formamide and dimethylacetamide, or mixtures of said solvents.

The degree of neutralization of the carboxylic acid used is less than 20 percent molar. We prefer to employ the carboxylic acid at a degree of neutralization of less than 10 percent molar, more preferably at from 2 to 5 percent molar. Convenient bases for adjusting the degree of neutralization are sodium methylate and anhydrous sodium carbonate. Other useful bases are potassium methylate, sodium or potassium ethylate, potassium carbonate or amines of sufficient basicity, such as triethylamine, or alkanol amines such as dimethyl aminoethanol or morpholine.

Conversion rates based on the free carboxylic acid may be pushed up to very high values, for example to over 90 percent and, at very low degrees of neutralization, to over 95 percent.

Suitable electrodes are those having smooth surfaces. Examples of suitable materials for the anode are platinum, platinium-rhodium, platinum-irridium, gold or gold-platinum. Advantageously, these precious metals are applied as a thin layer, for example as a layer having a thickness of from 2 to 70.mu., to a conducting substrate, for example a substrate of aluminium, refined steel, titanium or graphite, by electroplating or ceramic processes or by rolling, soldering, welding or bonding by means of a conducting cement. The material to be selected for the cathode is not critical and stainless steel or nickel may be advantageously used.

In the process of the invention, the current density may be kept high at a moderate cell potential despite the low degree of neutralization. The process is generally carried out at current densities ranging from 10 to 60 amps/dm.sup.2 and preferably from 15 to 40 amps/dm.sup.2. To obtain such current densities, cell potentials of from 5 to 25 volts are generally necessary.

Usually the temperature of the electrolyte during electrolysis is maintained in the range from 20.degree. to 60.degree. C, preferably 40.degree. to 55.degree. C.

Working up of the material discharged from the electrolytic cell is particularly simple due to the low electrolyte concentration. Normally the solvent will be distilled off. The starting material may then be readily separated from the residue, which mainly consists of a mixture of reaction product and unreacted carboxylic acid, for example by extraction with water or aqueous sodium carbonate solution using normal methods. The reaction product may be obtained in a pure state for example by distillation or freezing out. Alternatively, the product may be separated with the aid of organic solvents or by steam distillation.

The process may be carried out batchwise or continuously. For continuous operation use may be made, for example, of an electrolytic cell having a plurality of plane, plate-shaped electrodes in a bipolar series arrangement. The electrodes have a slightly trapezoid shape such that they rest against the side walls of the trough to form a liquid-tight seal therewith. Alternatively, they may be provided with a frame containing inlets and outlets for the solution and assembled in the manner of a filter press. The current connections to the cell are to the end plates. In order to maintain constant spacing between the plates, narrow insulating strips produced, for example, from plastics film such as polyester film, may be interposed between the plates in a direction parallel to the direction of flow. The thickness of the strips is governed by the desired spacing of the electrodes and may be from 0.05 to 2 mm. A gas escape is provided in the cover of the cell.

The reaction mixtures passes through an inlet, flows between the paired electrodes and leaves the cell through an outlet to be recirculated by a centrifugal pump, the circulating material being caused to pass through a heat exchanger and a flowmeter. For continuous operation, the cell is also equipped with an inlet for fresh reaction solution and an outlet for the partially converted mixture. There is no difficulty in observing the pH and temperature.

Alternatively, use may be made of a cell containing round plate-shaped electrodes forming a stack and provided with a central inlet for the electrolyte such that the latter flows between the electrodes in radial directions. This arrangement is contained in a cell of glass. The end electrodes are suitable connected to a source of direct current. Narrow strips of from 0.2 to 0.7 mm thick polyester film are provided between the paired electrodes. A gas escape is provided in the cover. The reaction solution is pumped by a centrifugal pump through inlets to the central cavity of the electrode system, flows radially through the gap between the electrodes and is recycled via a cooler and a rotameter. The pH and temperature of the circulated reaction mixture may be observed by means of a glass electrode and a thermometer.

However, electrodes of some other configuration may be used, such as pairs of electrodes consisting of concentric cylinders.

The compounds produced by the process of the invention are well suited, for example, for use as intermadiates in the manufacture of polyamides or polyesters and in the manufacture of special plastizizers or ester oils.

EXAMPLE 1

The electrolytic cell used is made up of three round discs of graphite having a diameter of 117 mm and a thickness of 10 mm. The discs have a central bore of 30 mm diameter for the electrolyte feed. The effective area is exactly twice 1 dm.sup.2. The anode side is provided with a 40.mu. thick foil of platinum bonded thereto by a conducting epoxy cement, and the cathode side is provided by a sheet of refined steel 1 mm thick. The arrangement consists of two series-connected cells. Four strips of 0.5 mm thick polypropylene are disposed radially between the electrodes to act as spacers.

At the commencement of electrolysis a mixture of 400 g of monomethyl adipate and 500 g of methanol adjusted to a degree of neutralization of 5 percent molar by the dropwise addition of a mixture of 22.5 g of 30 percent methanolic sodium methylate solution in 77.5 grams of methanol at 0.degree.C, is fed to the cell. The mixture is circulated through the cell and a heat exchanger and electrolysis is carried out using a current of 25 amps giving a current density of 25 A/dm.sup.2. The temperature is maintained at 42.degree. C by water cooling applied to the circulating mixture outside the cell. The electrolytically generated hydrogen and carbon dioxide leave the cell via a brine-cooled reflux condenser. The throughput through the two bipolar series-connected cells is maintained at 150 liters per hour, equivalent to a rate of flow in the cell of from 41 to 11 cm/sec (inlet and outlet rates of radial flow of material through the gap). The potential across each cell is 15.4 volts at the commencement of electrolysis, 12.8 volts after 1 hour and 11.7 volts at the end of electrolysis. The pH rises from an initial value of 5.4 to a final value of 9. After the passage of 132 percent of the theoretical amount of current (84 Ah) required for complete conversion of the monoester the reaction mixture, which is clear and colorless, is worked up by distillation. The solvent is driven off in a rotary evaporator and the residue is diluted with hexane and washed with water and then freed from hexane in a rotary evaporator. The residue, weighing 232 g, is shaken with a little 8 percent sodium bicarbonate solution. The final residue consists of 227 g of crude dimethyl sebacate. 0.4 g of half-ester are found in the aqueous phase titrimetrically. In all, therefore, 5.4 g of unreacted half-ester are found. Analysis by gas chromatography (with reference to dimethyl phthalate as internal standard) shows a concentration of 95 percent of sebacate in the crude ester. This corresponds to a yield of 81.5 percent and a current efficiency of 61.2 percent. Fractional distillation of the crude ester in vacuo produces a 99.9 percent pure dimethyl sebacate having a melting point of 35.degree. C.

EXAMPLE 2

Using the cell described in Example 1, 1,000 g of a mixture of 40 percent of mono(2-ethylhexyl) adipate in methanol neutralized with sodium methylate to a degree of neutralization of 10 percent molar are reacted at 40.degree. C and a current density of 20 amps/dm.sup.2, the theoretical current conversion being 140 percent. In this Example the throughput is 200 liters per hour giving a rate of flow of the reaction mixture in the cell of 55 to 15 cm/sec (inlet and outlet rates of radial flow through the gap between the electrodes). To avoid the rise of potential, which is characteristic of the electrolysis of this half-ester, the flow of current is stopped for 10 second periods at intervals of 5 minutes. By this means the cell potential across each pair of electrodes is held virtually constant at 16.5 volts. If the current is not switched off periodically in this manner, the cell potential rises to about 30 volts due to the formation of a polymeric deposit on the anode.

Working up is effected by neutralizing the unreacted half-ester with aqueous 10 percent sodium hydroxide and distilling off the methanol, elutriating the sodium salt of the half-ester from the residue with water and subjecting the resiude to steam distillation at 20 mm of Hg and 130.degree. C to remove further by-products. The resulting product, bis-2-ethylhexyl sebacate, is confirmed by gas chromatography and the ester value. The yield is 70.3 percent and the current efficiency 48.0 percent.

EXAMPLE 3

The cell consists of a rectangular plate of refined steel measuring 80 .times. 160 .times. 20 mm and having a groove cut near each of the short sides, which grooves serve as inlet and outlet for the reaction mixture to be circulated as described in Example 1. The anode is an aluminium plate of the same size, to which a 40.mu. thick foil of platnium has been bonded by means of a conducting cement. A frame having a thickness of 0.5 mm is interposed between the two plates to leave an effective electrode area of 0.5 dm.sup.2. The assembly is held together by means of 12 screws.

At the commencement of electrolysis, the reactor is charged with a reaction mixture consisting of 220 g of methanol and 146 g of 5-formyloxyvaleric acid which has been neutralized to 5 percent molar with sodium methylate. Electrolysis is carried out at a current of 12.5 g giving a current denisty of 25 amps/dm.sup.2, at a temperature of 40.degree. C and a theoretical current conversion of 119 percent. The throughput is 60 liters per hour giving a rate of flow of reaction mixture between the electrodes of 67 cm/sec. At the commencement of electrolysis the cell potential adjusts itself to 12.3 volts and falls to 10.6 volts toward the end of electrolysis. During the reaction the pH rises from 6.0 to 8.6. After working up in a manner similar to that described in Example 1, there are obtained 69,8 g of 1,8-octanediol diformate, equivalent to a yield of 76.7 percent and a current efficiency of 64.6 percent.

EXAMPLE 4

Using the arrangement described in Example 3, 300 g of a mixture of 40 percent by weight of 6-acetylaminocaproic acid in methanol neutralized to 5 percent molar with sodium methylate is reacted at 40.degree. C, a current density of 25 amps/dm.sup.2 and a theoretical current conversion of 130 percent. The pumping rate is 36 1/hr giving a rate of flow of mixture between the electrodes of 40 cm/sec. During the electrolysis the cell potential rises from 16.3 to 22.0 volts and the pH from 6.6 to 7.1.

Working up is effected by distilling off the methanol, extracting the residue with hot water and, after cooling, filtering off undissolved N,N'-diacetyldecamethylene diamine. After drying, there are obtained 14.9 g of product, m.p. 128.degree.-130.degree. C. The unreacted 6-acetylaminocaproic acid in the filtrate is determined by titration. The yield is 26.1 percent and the current efficiency 13.5 percent.

EXAMPLE 5

A solution of monomethyl glutarate having a degree of neutralization of 5 percent molar and produced by stirring 296 g of glutaric anhydride (2,60 moles) in a solution of 0.7 g (0.13 moles) of sodium methylate in 650 g of methanol for 1 hour at the boil, is electrolyzed for 400 minutes in the system described in Example 3 using a current of 12.5 amps giving a current density of 25 amps/dm.sup.2, at a temperature of 45.degree. C. During this period the initial potential of 14.1 volts falls to 10.5 volts. The distance between the electrodes being 0.5 mm and the throughput 60 liters per hour, the rate of flow of the solution between the electrodes is 67 cm/sec.

After working up as described in Example 1 there are obtained 9.1 g of unreacted monomethyl glutarate and 194.1 g of crude dimethyl suberate, which is found to contain 97.3 percent of dimethyl suberate when analyzed by gas chromatography. This is equivalent to a conversion rate of 97.6 percent, a yield of 77.7 percent and a current efficiency of 60.6 percent.

Claims

1. A process for the electrochemical condensation of carboxylic acids at a degree of neutralization of the carboxylic acid of less than 20 percent molar and a current density of more than 10 A/dm.sup.2 in a solvent using electrodes which are impermeable to liquids, wherein the distance between the electrodes is from 0.1 to 2 mm and the electrolyte is caused to flow through the space between the paired electrodes at a rate of from 0.05 to

2. A process as claimed in claim 1, wherein methanol is used as solvent.

3. A process as claimed in claim 1 wherein the distance between the paired

4. A process as claimed in claim 1 wherein the electrolyte is caused to

5. A process as claimed in claim 1 wherein more than 90 percent of the

7. A process as claimed in claim 1 wherein the degree of neutralization of the carboxylic acid is less than 10 percent molar.