US 3664936 A
A method of producing adiponitrile by electrolytic hydrodimerization of acrylonitrile which comprises conducting the electrolysis while purifying the catholyte by recovering the conductive supporting salt in the waste catholyte by means of a cation exchange resin.
Description (OCR text may contain errors)
United States Patent Seko et al.
[541 ELECTROLYTIC HYDRODIMERIZATION OF ACRYLONITRILE  Inventors: Maomi Seko, Tokyo; Akira Yomiyama, Nobeoka-shi; Yasunobu Takahashi,
Nobeoka-shi; Shigetoshi Seta, Nobeokashi; Koji Nakagawa, Nobeoka, all of Japan  Assignee: Asahi Kasei Kogyo Kabushiki Kaish Osaka, Japan  Filed: May 8, 1970 21 App]. No.: 35,727
[451 May 23, 1972 Primary Examiner-F. C. Edmundson AttorneyFlynn & Frishauf  ABSTRACT  Foreign Appli i n Priority Di l A method of producing adiponitrile by electrolytic hydrodimerization of acrylonitrile which comprises conduct- May 8, 1969 Japan ..44/34806 g the electrolysis while p y g the catholytc y recovering the conductive supporting salt in the waste catholyte by means  US. Cl. ..204/73 A, 204/237 of a cation exchange resin 51 C07b 29/06, C07c 121/26  Field of Search ..204/73, 73 A, 72 12 Claims, 1 Drawing Figure d I I l-2 l-3 |-I 2 J 3 A 12 H 15 Patented May 23, 1972 3 26. 4 7 v LE I c B ELECTROLYTIC I-IYDRODIMERIZATION OF ACRYLONITRILE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of electrolytically dimerizing acrylonitrile which comprises conducting the electrolysis while purifying the catholyte by recovering the conductive supporting salt in the waste catholyte by means of a cation exchange resin and reusing the same. More particularly, it relates to a process for conducting the electrolysis while successively purifying the catholyte by recovering the conductive supporting salt which is taken out of the system in association with the deposits due to the successive removal from the system of the deposits formed during prolonged electrolysis of acrylonitrile and accumulated in the catholyte. It is an object of this invention to provide a new method for producing adiponitrile by electrolytic hydrodimerization of acrylonitrile which is effective for preventing (1) formation of a by-product propionitrile, (2) formation of deposits on the surface of the cathode and (3) reduction of adiponitrile formation during prolonged electrolysis caused by the phenomena mentioned under (1) and (2) above. Other objects and efiects will be understood by the descriptions hereinbelow.
As used herein, conductive supporting salt means an electrolyte that will not discharge at the cathode when acrylonitrile is hydrodimerized at the cathode. Deposits accumulated in the catholyte means by-products formed during prolonged electrolysis of acrylonitrile or those generated by further deterioration of the same. In embodiment they are represented by amides and carboxylates formed by hydrolysis of acrylonitrile and adiponitrile, for example, amides such as acrylamide, w-cyanovaleramide and adipamide and carboxylates such as acrylic acid, w-carboxyvaleramide and adipic acid; and other by-products including lactones such as methylbutyrolactone, ethylbutyrolactone, or dimethylbutyrolactone, oxycarboxylates formed by hydrolysis of these lactones, and amides and carboxylates liberated from additives which, when used, will form or liberate the amides and carboxylates during electrolysis by deterioration, and also including polymers of acrylonitrile and deteriorated products thereof.
2, Description of the Prior Art Various methods are known of producing adiponitrile by electrolytic hydrodimerization of acrylonitrile, for example, as
disclosed in US. Pat. No. 3,193,481, Japanese Pat. Publication No. 4733/1965, US. Pat. Nos. 3,193,480 and 3,193,477, Belgian Pat. No. 664,436, Dutch Pat. Application No. 6,708,254 and Belgian Pat. Nos. 699,926 699,928.
In these prior methods, the yield of adiponitrile in terms of the consumed acrylonitrile is gradually reduced during a very long course of the electrolysis due to gradual increase in the by-production of propionitrile and biscyanoethyl ether, especially of the former, although the yield is fairly good at the early stage of the electrolysis. In such a prolonged electrolysis, moreover, there is a disadvantage that polyacrylonitrile or degenerated products of thesame tend to adhere to the surface of the cathode, thereby reducing the amount of adiponitrile formation with elapse of time, and it becomes impossible to produce adiponitrile in a high yield constantly throughout a sufficiently long period of time. These disadvantages give serious affects upon industrial application of the prior methods in two respects, i.e., yield and long term stability of the electrolysis, and attempts were made to find means for overcoming these disadvantages. For instance, in US. Pat. No. 3,335,162 there are described attempts to overcome the above-mentioned disadvantages by removing partly hydrolyzed polyacrylonitrile deposits formed in the interfacial phase on allowing the catholyte to stand. However, such attempts are not satisfactory in that the by-products are gradually accumulated in the catholyte during the course of the adiponitrile production in a long period of time to cause gradual decrease in the yield of adiponitrile and tendency for the polymer to adhere. As a matter of fact, these means can hardly permit maintenance of the yield of adiponitrile in terms of the consumed acrylonitrile over 90 percent during a long period of the electrolysis over 400 hours. According to the specification of said patent the electrolysis over 400 hours is conducted with a catholyte freshly prepared prior to the electrolysis from entirely freshly prepared acrylonitrile, conductive supporting salt and other additives and the results thereof are described. However, if the process is carried out on industrial scale the catholyte will not be freshly prepared following a batch of a long-term electrolysis but the adhered matter on the surface of the cathode will be removed followed by reassembling of the electrolytic cell and the next batch of electrolysis will be initiated. Under such conditions maintenance of the yield over percent will be surely diflicult throughout the several runs of more than 400 hours of the electrolysis, despite the yield of 80-90 percent achieved at an early stage of the electrolysis. The consideration is supported by Referential examples l and 2 described below.
The aforementioned facts suggest the cause for reducing the yield of adiponitrile to be a variety of deposits accumulated during the electrolysis in the catholyte.
SUMMARY OF THE INVENTION We have investigated into the cause for reducing the yield of adiponitrile to find that increase in the formation of a byproduct propionitrile and decrease in the yield of adiponitrile are ascribed to a variety of deposits as described above which are formed in association with a long-term electrolysis and accumulated in the system and further that among these deposits, carboxylates are especially causable. When the method according to the present invention is applied, it becomes possible to cause a remarkable increase in the yield of adiponitrile as well as to effect continued operation with a high yield constantly produced during a long period of time.
In carrying out the method of this invention, a portion of the catholyte is continuously or intermittently removed from a catholyte tank during electrolysis of a catholyte containing acrylonitrile and a conductive supporting salt. Said portion is then treated with a cation exchange resin and the cations of the supporting salt are adsorbed thereon. Then, the cation exchange resin on which the cations of the supporting salt are adsorbed is treated with an aqueous solution of an acid having the same anions as that of the supporting salt. Preferably, the passage of the solution of the acid is preceded by washing of the resin with water. Thus, the cations recovered from the cation exchange resin and the anions contained in the acid form the supporting salt which is used for the preparation of a fresh catholyte. Prior to the cation exchange resin treatment of the catholyte, if desired, acrylonitrile or adiponitrile or the two may be removed by such means as extraction, decantation or evaporation and recovered in a separate step. Recovery of acrylonitrile and/or adiponitrile may also be effected by applying the aforementioned means to the discharged solution following the resin treatment. It is also possible to recycle the acrylonitrile or adiponitrile thus removed and recovered directly into the mother catholyte. However, in such a case the possibility that the deposits such as the aforementioned carboxylates may be recycled together with acrylonitrile or adiponitrile should be given into consideration. In some cases, accordingly, the amount of the catholyte to be treated with a cation exchange resin should be increased or the treatment and circulation made more frequently.
As the catholyte of an aqueous electrolyte may be employed either a solution or an emulsion containing acrylonitrile and a conductive supporting salt. The method of this invention is effective independent of the type of conducting supporting salt and the concentration of acrylonitrile in the catholyte. The catholyte may contain, in addition to acrylonitrile and a conductive supporting salt, adiponitrile or other by-products formed during the electrolysis and additives necessary for effectively conducting the electrolysis such as an emulsifier or polymerization inhibitor.
Either a strongly or a weakly acidic cation exchange resin may be used in the method of this invention and the fom'ler is especially effective in exchange and regeneration. Prior to the use of cation exchange resin the Na-form is converted to the H-form by conventional means and it is necessary prior to the practical use to make exchange with acid and washing with water sufficient to leave no Na ion remain.
As the conductive supporting salts are usually employed alkali metal salts and electrolyte salts represented by the general formula wherein R,, R R and R respectively are alkyl containing 14 carbon atoms, preferably alkyl consisting of 2-3 carbon atoms and X is sulfate radical or arylsulfonate radical of the general formula such as paratoluenesulfonate radical in which R, as usually used, is alkyl containing -4 carbon atoms. Anions forming conductive supporting salts known to be effective in electrolytic hydrodimerization of acrylonitrile other than the above such as halogens, nitrate radical and phosphate radical may be used except for the anions containing carboxylate radical. The tetraalkyl ammonium ion consisting of the Rs mentioned above is recovered from the catholyte and recycled.
In carrying out the method of this invention it is preferred to conduct recovery and purification of the catholyte by means of a cation exchange resin as frequently as possible or at a circulation rate as high as possible so far as economy of the process permits. The frequency should be determined depending upon the operation time rather than the amount of adiponitrile formed. In fact, continuous or intermittent treatment of the amount of catholyte corresponding to the entire catholyte in the system is conveniently made in l504,000 hours, preferably in ISO-1,000 hours.
In order to increase the recovery of conductive supporting salt, more exactly of the cation forming the same, as much as possible, the amounts of acid and water necessary for the generation of the resin are necessarily increased. These amounts are normally larger than the amounts of acid and water drawn out of the system. Therefore, it is necessary in the case of acid to balance between the amount of acid flowing through an exchange diaphragm to the anode side during electrolysis and the amount of acid added to adjust pH of the catholyte. If it is not balanced with the result that the acid, water or the both being accumulated, the excess amount thereof should be removed by means of an anion exchange resin or milk of lime in the case of acid or by evaporation or another means at a certain stage in the system in the case of water to maintain the balance. Recovery of the conductive supporting salt by means of a cation exchange resin can be achieved nearly in 100 percent yield by properly selecting the conditions of recovery. The recovery may be cut in a recovering rate below 100 percent, for example, in a yield of 4080 percent in view of the problem of balancing and supplementing with fresh conductive supporting salt.
DESCRIPTION OF PREFERRED EMBODIMENTS This invention will be illustratively described by a representative flowsheet as shown in the drawing. It is to be understood that the invention is not limited thereto.
In the flowsheet as shown in the drawing by 1 is indicated the electrolytic cell for electrolytic hydrodimerization of acrylonitrile to produce adiponitrile, which consists of two chambers, a cathode chamber 1-1 and an anode chamber 1-2 divided by a cation exchange diaphragm l-3. Circulation is made of an anolyte between the anode chamber l-2 and an anolyte tank 2, and of a catholyte between the cathode chamber l-l and a cathode tank 3.
The catholyte is a solution or an emulsion or a suspension containing acrylonitrile, adiponitrile and a conductive supporting salt and other additives. The catholyte in the catholyte tank 3 is consecutively in portions passed to an acrylonitrile stripper 4 and the acrylonitrile recovered is recycled to the catholyte tank 3. Water condensed and separated from acrylonitrile is discharged through a discharge end 6. The catholyte from which acrylonitrile and water have been removed in the acrylonitrile stripper 4 is allowed to stand in a decanter 7 for the separation. The oil layer mainly comprising adiponitrile is passed through a discharge end 8 to the step of purifying adiponitrile (not shown). On the other hand, the aqueous layer is recovered into the catholyte tank from the bottom of the decanter 7. Then, a portion of said aqueous layer is continuously or intermittently drawn out and, after cooled in a heat exchanger 9, is stored in a decanter 10. The oil layer separated on cooling the solution is passed to the catholyte tank 3, or a supplement pipe 5, or the discharge end 8. The aqueous layer is passed to a water addition tank 12, in which it is diluted with pure water to an appropriate dilution and then passed to a cation exchange resin column 13. In the cation exchange resin column 13 the cation in the solution is adsorbed. The effluent is taken out of the system through a discharge end 14. Pure water is then charged through an inlet 11 into the cation exchange resin column 13 thereby forcing out the catholyte in the column and then thoroughly washing the cation exchange resin inside the column. The waste liquor is discarded through the discharge end 14 outside the system.
Then, an aqueous solution of an acid containing the same anion as of the conductive supporting salt, prepared in an aqueous acid solution tank 15, is introduced into the cation exchange resin column 13 to regenerate the cation while forcing out the pure water remaining in the column. The conductive supporting salt thus regenerated is stored in a tank 16 and thereafter continuously introduced into the catholyte tank. The pipe 5, described in more detail, is a supplement pipe for supplementing acrylonitrile being decreased in the system by the transformation into adiponitrile.
In the drawing is shown a batch-system ion exchange apparatus as the cation exchange resin apparatus used in this invention. Of course embodiment with a continuous ion exchange resin apparatus is involved within the scope of this invention.
The invention will be described in greater detail in conjunction with the following specific examples.
EXAMPLE l An experimental apparatus similar to that shown by the flowsheet on the drawing was used, which includes an electrolytic cell in which a cathode of lead alloy containing 6% antimony 10 cm. by 10 cm. in conductive area and an anode of lead alloy containing 0.7% silver and 6% antimony with the same conductuve area were divided by a cation exchange diaphragm 1 mm. in thickness, thus forming the cathode and anode chambers each being 10 cm. by 10 cm. by 1 mm. in size. 2N aqueous sulfuric acid was used as the anolyte and flowed at a rate of 200 cm./sec. As the catholyte was used an emulsion consisting of an aqueous phase containing 2% acrylonitrile, 9% adiponitrile, 79.95% water, 8.0% tetraethylammonium sulfate and 0.05% emulsifier polyvinyl alcohol and an oil phase containing 17% acrylonitrile, 75% adiponitrile, 5% water and 3% electrolytic organic by-products mainly comprising propionitrile, and having a pH of 7. It was passed and circulated through the electrolytic cell at a flow rate of 200 cm./sec. Electrolysis was conducted at a temperature of the liquor of 50 C. with an electric current of 10 A. As the cation exchange resin was employed the sulfonate-type one. The conductive supporting salt was recovered by passing 200 cc. of the aqueous layer from the decanter 7, which amount corresponds to percent of the total amount of the catholyte which was 2,000 cc., per 24 hours to the cation exchange column 13 containing 300 cc. of the cation exchange resin. The conductive supporting salt in the aqueous layer from the decanter 7, which had been concentrated to 12 percent following passage through the acrylonitrile stripper 4, was continuously taken out of the system over 24 hours and stored in the decanter 10, which was regenerated in one exchange cycle. The exchange capacity of the resin was 0.4 equivalents/l. and the sulfuric acid for regeneration fed from the aqueous acid solution tank 15 was 130 cc. of 3N sulfuric acid. 100 cc. of the washing water left, in the cation exchange resin column 13 was forced out with the same volume of aqueous sulfuric acid and was wasted off, while 200 cc. of the solution forced out with cc. of the remaining aqueous sulfuric acid and another water additionally introduced was drawn into the tank 16 and recycled to the catholyte tank 3. The yield of recovery of the conductive supporting salt was approximately 50 percent and the balance due was supplemented by fresh conductive supporting salt.
From the aqueous solution after the cation exchange resin treatment, which was taken out of the discharge end 14, was recovered adiponitrile by solvent extraction, which was passed to the step succeeding to the discharge end 8.
Not only according to the procedures described above but also in general, polyvinyl alcohol, the emulsifying effect of which is gradually lost during the progress of electrolysis, should be supplementarily and successively added in order to maintain the emulsion. In carrying out the method according to this invention more polyvinyl alcohol should be supplementarily added because the catholyte was successively drawn out of the system. Experiment in this example was made while adding to the tank 3 polyvinyl alcohol at a rate of 3.4 mg./hour.
Electrolysis was initiated with an entirely freshly prepared catholyte under the conditions described above and continuously operated for 300 hours. The surface of the electrode was then cleaned and thereafter continuous operation was conducted with the same catholyte for additional 300 hours. Totally five runs of the electrolysis was made over 1,500 hours with the results shown in Table 1 below. A certain amount of polymers was adhered on the surface of electrode after each run, but the amount was far smaller than that in the case where the method according to this invention was not applied to indicate very satisfactory results.
COMPARATIVE EXAMPLE 1 For comparison sake, an electrolysis was carried out without purification of the catholyte conducted in the absence of the members from the heat exchanger to the tank 16 on the flowsheet of the drawing. Experimental conditions were the same as in Example 1 except for the above and the amount of polyvinyl alcohol added being 2.7 mg./hour to counterbalance the decrease in the emulsifying effect. in fact, the emulsifying effect of polyvinyl alcohol was to be lost approximately at such a rate. The results are shown in Table 2, indicating that continuous operation was not feasible in and after the third run.
TABLE 2 The results of electrolysis (2) Yield 11 on consumed acrylonitrile On com- Operation Adiponitrile Propionitrile pletion of hour (Total) lst run 300 92.5 6.4 2nd 600 86 7.6 3rd 850 8.4 4th 1,140 78 10.8
Adhesion of polymers was too much on completion of the third and fourth runs, especially on completion of the fourth run, to continue the electrolytic experiment.
EXAMPLE 2 An electrolysis was carried out under entirely the same conditions as in Example 1 except that on the flowsheet the outlet pipe A of the catholyte tank 3 was connected directly with the pipe B of the heat exchanger thereby the catholyte being Only a little polymer was adhered on the surface of the electrode on completion of every run in a similar manner to in Example 1.
EXAMPLE 3 An electrolysis was carried out under the same conditions as in Example 1 except that a weakly acidic cation exchange resin of carboxylate type was used in place of the strongly acidic cation exchange resin of sulfonate type. The results were similar to those in Examples 1 and 2, as shown in Table 4. Only a little polymer was adhered on the surface of the electrode on completion of every run in a similar manner to in Examples l and 2.
TABLE 4 The results of electrolysis (4) Yield on consumed acrylonitrile On com- Operation Adiponitrile Propionitrile pletion of 7 hour (Total) lst run 300 92 5.7 3rd 900 90 4.9 5th 1,500 91 6.1
EXAMPLE 4 An electrolysis was made using the same electrolytic cell as in Example 1 where a homogeneous solution containing 35% tetraethylammonium paratoluenesulfonate, 12% acrylonitrile and 10% adiponitrile, and having a pH of 7, was employed as the catholyte and an electric current of 40 A was applied at 50 C. During progress of the electrolysis was drawn out portionwise the solution in the catholyte tank, to which was added acrylonitrile and water. There were separated two layers, adiponitrile being thereby separated, and the aqueous phase was treated with a cation exchange resin in the same way as in Example 1. The tetraethylammonium ion then adsorbed was regenerated with two equivalents of a 2N aqueous solution of paratoluenesulfonic acid, followed by concentration and recycling into the catholyte for reuse.
Conditions under which the catholyte was treated by means of a cation exchange resin and experimental conditions such as composition and flow rate in the electrolytic cell of the anolyte and flow rate of the catholyte in the electrolytic cell were similar to those in Example 1.
The results of an experiment under the aforementioned conditions are shown in Table 5. The polymer on the surface of the electrode was satisfactory, being slightly more than in Example 1.
TABLE The results of electrolysis (5) An electrolytic experiment was carried out under the same conditions as in Example 2 except for omission of the cation exchange resin treatment. The results are shown in Table 6. Too much polymer adhered on the surface of the electrode in the third and fourth runs to conduct continuous operation for more than 300 hours.
TABLE 6 The results of electrolysis (6) Yield on consumed acrylonitrile On com- Operation Adiponitrile Propionitrile pletion of hour (Total) lst run 300 89 6.8 2nd 600 88 7.3 3rd 850 83 8.6 4th 1,110 76 12.1 5th 1,260 70 13.8
EXAMPLE 5 An electrolysis was carried out using the same electrolytic cell under the same conditions as in Example 4 except that the aqueous phase from the separation of adiponitrile after the addition of acrylonitrile and water was passed prior to the treatment with a cation exchange resin to the acrylonitrile stripper to remove the acrylonitrile. The results are shown in Table 7. Adhesion of the polymer on the surface of the electrode was similar to that in Example 4.
TABLE 7 The results of electrolysis (7) Yield on consumed acrylonitrile On com- Operation Adiponitrile Propionitrile pletion of hour (Total) 1st run 300 88 7.9 2nd 600 88 7.6 3rd 900 9.1 4th 1,200 86 8.8 5th 1,500 86 8.5
EXAMPLE 6 An electrolysis was carried out using the same electrolytic cell as in Example 1 where an emulsion having a pH of 7 and consisting of an aqueous phase containing 20% sodium hydrophosphate, 1% tetraethylammonium hydrophosphate, 1% acrylonitrile, 2% adiponitrile, 17% polyvinyl alcohol and the balance of water and an oil phase containing 17% acrylonitrile, 75% adiponitrile, 5% water and 3% by-products of the electrolysis such as propionitrile'was employed as the catholyte, which was passed and circulated through the electrolytic cell at a flow rate of 200 cm./sec. at a liquid temperature of 40 C. under application of an electric current of 10 A. Ion exchange was conducted in the same way as in Example 1 using a cation exchange resin of sulfonate type.
The electrolysis thus made while purifying the catholyte gave the following results:
TABLE 8 The results of electrolysis (8) Yield on consumed acrylonitrile On com- Operation Adiponitrile Propionitrile pletion of hour (Total) lst run 300 86 10.1 2nd 600 85 11.2 3rd 900 86 10.3 4th 1,200 85 10.9 5th 1,500 86 9.8
Without the purification of the catholyte by means of a cation exchange resin the results were as follows:
TABLE 9 The results of electrolysis (9) 1. A method of continuously producing adiponitrile by electrolytic hydrodimerization of acrylonitrile in an aqueous catholyte comprising acrylonitrile and a conductive supporting salt, which comprises, while conducting electrolysis, intermittently or continuously removing a portion of said catholyte, treating said portion of said catholyte with a cation exchange resin to adsorb cations of the supporting salt contained in said catholyte on the cation exchange resin, treating the resulting resin with an aqueous solution of an acid having the same anion as that of the supporting salt to form the supporting salt, and recirculating said recovered supporting salt for reuse in the preparation of a fresh catholyte.
2. A method according to claim 1, wherein said conductive supporting salt is a quarternary ammonium salt.
3. A method according to claim 1, wherein said catholyte is an emulsion containing an aqueous phase as continuous phase.
4. A method according to claim 3 wherein said catholyte is separated into an aqueous phase and an oil phase and the separated aqueous phase is treated with said cation exchange resin.
5. A method according to claim 1, wherein said catholyte is a homogeneous aqueous solution.
6. A method according to claim 5 wherein said catholyte is separated into an aqueous phase and an oil phase by addition thereto of acrylonitrile and/or water and the aqueous phase is treated with a cation exchange resin.
7. A method according to claim 1 wherein said cation exchange resin is in acid form.
8. A method according to claim 1 wherein said recovery of adiponitrile is made by solvent extraction of the catholyte prior to treatment with the ion exchange resin.
9. A method according to claim 1, wherein said cation exchange resin treatment is made in such a way that the liquid volume corresponding to the volume of entire catholyte is treated once per a to 1,000 hour operation of the hydrodimerization reaction of acrylonitrile.
10. Method according to claim 1, wherein acrylonitrile is removed from said portions of said catholyte prior to the treatment of said portions of said catholyte with the cation exchange resin. 7
11. Method according to claim 1, wherein acrylonitrile is removed from said portions of said catholyte prior to the treatment of said portions of said catholyte with the cation exchange resin; and adiponitrile formed during the electrolysis is recovered from the effluent obtained from the cation exchange resin treatment.
12. Method according to claim 10, wherein said adiponitrile is also recovered from the efiluent obtained from the cation exchange resin treatment.