US3619388A - Process for electrolyzing nitriles - Google Patents

Abstract

A process of electrohydrodimerizing Alpha , Beta -mono-olefinic nitriles in a divided cell in which catholyte containing Alpha , Beta -mono-olefinic nitriles is separated from anolyte of an aqueous acid solution while reducing the concentration of hydrogen cyanide in the anolyte for preventing corrosion of the anode.

Description

United, States Patent Inventors Aklra Yomiyama Tokyo; Shinsaku Ogawa, Miyazaki-ken; Muneo Yoshida, Miyazaki-ken; Takamasa Sakai, Miyazaki-ken, all of Japan Appl. No. 796,912

Filed Feb. 5, 1969 Patented Nov. 9, 1971 Assignee Asahi Kasei Kogyo Kabushiki Kaisha Kita-ku, Osaka, Japan PROCESS FOR ELECTROLYZING NITRILES 3 Claims, No Drawings [56] References Cited UNITED STATES PATENTS 3,193,480 7/1965 Beizer et a1. 204/73 3,193,481 7/1965 Beizer 204/73 3,402,! 12 9/1968 Brubaker et a1. 204/74 Primary Examiner-F. C. Edmundson Attorney-Burgess, Dinklage and Sprung PROCESS FOR ELECTROLYZING NITRILES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electrolytic hydrodimerization of a,B-monoolefinic nitriles. More particularly, this invention relates to a process for electrolytically hydrodimerizing nitriles such as acrylonitrile while removing hydrogen cyanide produced by anodic oxidation of nitriles from an anolyte. In accordance with this invention, the inherently excellent resistance to corrosion of a lead peroxide anode comprising lead and silver can be preserved.

Recently, various electrolytic hydrodimerization reactions of nitriles are being practiced. For example, the electrolyses of afi-olefmic mononitriles of the formula wherein R R, and R represent hydrogen or an alkyl group having normally one-five carbon atoms, and aliphatic-01,13- olefinic dinitriles of the formula wherein R,, R and R have the same meaning as defined above, are known well. In addition, the electrolysis of lcyano-l,3-diene is also known. Typical examples of the nitriles known to be electrolyzed include acrylonitrile, methacrylonitrile, crotononitrile, Z-methylenebutyronitrile, 2- pentenenitrile, 2-methylenevaleronitrile, Z-methylenehexanenitrile, 2,3-dimethylcrotononitrile, tiglonitrile, senecinonitrile, 2-ethylidenehexanenitrile, fumaronitrile, itaconitrile, citraconitrile and butyrfumaronitrile.

in the electrolyses of these nitriles, the electrolytes contain other than these olefinic nitriles, derivatives thereof in which double-bonds are reduced, hydrodimers, hydrotrimers and hydrodimers-of mixtures of these olefinic nitriles and other olefms. In these electrolyses, nitriles such as acetonitrile byproduced may be used as solvents.

ln this invention, hereinafter, the term nitriles" is used to mean those nitriles referred to above.

The process for electrolyzing these nitriles has been known as disclosed in US. Pat. Nos. 3,193,476; No. 3,193,479; No. 3,193,480 and No. 3,193,481 and British Pat. No. 1,076,610.

Although the electrolyses of the prior arts referred to above employ catholytes in the form of a homogeneous solution, there has also been known an electrolysis process in which emulsified nitriles are used as a catholyte, as described in Japanese Pat. application Nos. 37988/ 1966 and No. 37989/1966.

in the electrolysis of nitriles as referred to above, if nitriles diffuse into anolyte and are oxidized at the anode, the resulting oxidized products cause extraordinary corrosion of the anode. For preventing this inconvenience, there has been proposed to carry out the electrolysis in a divided cell in which anolyte is separated from catholyte by a membrane as disclosed in U.S. Pat. No. 3,193,480.

However, although the extraordinary corrosion of anode may be prevented to some extent by the provision of the partition membrane, the effect is still unsatisfactory from the practical point of view, while the use of a membrane having porosity fine enough to completely prevent difi'usion of nitriles into an anode compartment is impracticable for economical reason, because such a membrane will have an exceedingly high electrical resistance.

Hence, when using conventional membranes in a divided electrolysis cell, small amounts of nitriles inevitably diffuse into an anode compartment and leads to anodie oxidation thereof at the anode. Thus, it is necessary that either an anode I which is insusceptible to such extraordinary corrosion due to anodic oxidation of nitriles be used, or, alternatively, the

LII

anolyte be subjected to a special treatment as described hereinafter.

Normally, platinum, nickel, carbon, magnetic iron oxide, lead and lead peroxide are known as materials for anodes. Among these materials enumerated above, most preferable are lead and lead alloy on which a coating layer of lead peroxide is formed, considering its low cost, small amount of corrosion and workability and in view of the fact that it has resistance to corrosion even in an acidic solution and has no adverse influence on hydrodimerization reaction.

It is also known from various literature that the corrosion resistance may be enhanced by alloying lead with silver or further with antimony, or by applying heat treatments to the resulting alloys, or by incorporating thereinto small amounts of tellurium, tin, cadmium, calcium, copper and cobalt.

Among these suggested method of improving the corrosion resistance of anodes, it has been believed that the alloying of lead with silver was most effective.

However, it has been found that when an electrolysis is carried out for 200-500 hours in a divided cell partitioned by a membrane employing the lead-silver alloy as an anode and using a solution or emulsion of nitriles as a catholyte and sulfuric acid, which is most preferable when using the lead-silver alloy anode, as an anolyte, because of anodic oxidation of nitriles which are diffused through the membrane, corrosion occurs to create on the surface of anode quite unusual blisters of more than 1 mm. heights which subsequently come off and the anode becomes no longer usable. The higher the silver content becomes, the more drastic the detrimental phenomenon prevails. This is contradictory to the accepted theory that, in general, up until silver content exceeds 5%, the higher the silver content becomes, the more the corrosion resistance in sulfuric acid solution is increased.

In producing adiponitrile by the electrolytic hydrodimerization of acrylonitrile in a divided cell partitioned by a membrane using a lead-silver alloy anode and a 0.5N aqueous sulfuric acid as an anolyte, there are accumulated in the anolyte nitric acid ions and the amount of corrosion is increased. To eliminate this inconvenience, French Pat. No. 1,487,571 proposes a process for removing the nitric acid ion by using a liquid ion exchange resin.

The present inventors have conducted an extensive study in an attempt to clarify the mechanism of the extraordinary corrosion of lead-silver alloy anode. As a result, it has been found that when nitriles are oxidized at anode there is produced hydrogen cyanide which is attributable to the extraordinary corrosion of lead-silver alloy anode and that the extraordinary corrosion of anode and the formation of nitric acid may be prevented by removing hydrogen cyanide from anolyte to reduce the concentration thereof. The present invention has its basis on this novel finding.

The aeration process comprises recycling an anolyte, feeding thus recycled anolyte wholly or partly from the top of a packed tower and blowing air into the packed tower from the tower bottom.

Considering the use of lead-silver alloy anode, easiness of removing hydrogen cyanide and cost, sulfuric acid is the most preferred anolyte.

When using a 10% sulfuric acid as an anolyte and an anode in which 0.1% of silver is alloyed, the permissible concentrations of hydrogen cyanide and nitric acid in the anolyte are about 200 p.p.m. and about 1000 ppm. respectively. The higher the silver content and the lower the sulfuric acid concentration become, the lower the permissible concentrations of hydrogen cyanide and nitric acid become.

Although the formation of nitric acid can be suppressed by removing hydrogen cyanide from sulfuric acid, there may be used other methods such as substitution of a fresh sulfuric acid for a part of anolyte or removal by using an ion exchange resin, may be conveniently adopted in combination with the operation mentioned above.

With regard to the composition of lead-silver alloy used for anodes, the content of silver in this alloy is from 0.05 to 5%,

and preferably from 0.05 to l.5%. The overall amount of corrosion of lead peroxide is reduced as the silver content is increased. But, if the silver content is excessively high, the layer of lead peroxide is blistered by about I mm. due to the extraordinary corro'sion by hydrogen cyanide and thus blistered layer tends to come off. From this point of view, the higher the silver content in the alloy becomes, the lower the permissible concentration of hydrogen cyanide in the anolyte becomes.

Antimony may be incorporated into the lead-silver alloy and the content of antimony in the alloy is preferably from I to 10%. By the incorporation of antimony, the permissible concentrations of hydrogen cyanide and nitric acid for preventing the extraordinary corrosion by hydrogen cyanide are increased.

lf antimony is incorporated, the lead-silver alloy may be hardened by quenching the alloy from a temperature of 230-250 C. to normal temperature. By this quenching, the mechanical strength of the anode prepared therefrom, such as hardness, flexal strength, tensile strength, etc, can be greatly enhanced. Antimony content of 2-6% affords most satisfactory hardening effect. Moreover, the amount of corrosion of lead peroxide in a sulfuric acid solution containing hydrogen cyanide can be reduced by about 50%, maximum.

in view of these advantages described above, lead-silver alloy into which is incroporated antimony is a preferable material for electrodes used in the electrolysis of nitriles.

The corrosion resistance of lead-silver alloy anode may also be improved by incorporating less than about 1% of tellurium, tin, calcium, copper, cadmium and cobalt.

As described above, sulfuric acid is most preferable as an anolyte and the concentration of about 530% is particularly preferred. if the concentration of sulfuric acid is too low, not only are lowered the permissible concentrations of hydrogen cyanide and nitric acid but also the electric conductivity of anolyte is decreased.

Besides sulfuric acid, solutions of phosphoric acid or aryl sulfonic acid may also be used as an anolyte.

In the process of this invention, membranes capable of preventing diffusion of nitriles from a cathode compartment into an anode compartment as thoroughly as possible and those having a high electric conductivity are preferred. Generally, a cation exchange membrane is preferably used for this application.

In practising the electrolysis in accordance with this invention, an anode electric current density of about 5 to amperes per dm. particularly 5 to amperes per dm. is most preferably. The amount of corrosion of lead peroxide is increased if the anode current density is either lower or higher than the range specified above.

Conventional catholyte of an aqueous concentrated solution of a supporting electrolyte represented by tetraethylammonium p-toluenesulfonate containing more than 5% of nitriles homogeneously dissolved therein may be conveniently used. However, the process disclosed in Japanese Pat. application Nos. 37988/66 and No. 37989/66 in which emulsified nitriles are used as catholytes has advantages in that supporting electrolytes having high electric conductivities such tetraalkylammonium sulfate or halides can be used and that the separation and purification of the product are simplified.

DESCRIPTION OF PREFERRED EMBODIMENTS The following examples will illustrate this invention more fully. However, it should not be construed that these examples restrict this invention in any manner.

Example l An alloy consisting of 0.1% silver, 6% antimony and the balance of lead, which was hardened by quenching from 245 C. to normal temperature was used as an anode. The same material was used as a cathode. A cation exchange membrane of 1 mm. thickness obtained by sulfonating a divinylbenzenestyrene-butadiene copolymer was used as a partition membrane. An electrolysis cell partitioned into a cathode compartment and an anode compartment by said membrane was employed. An anolyte in the anode compartment and a catholyte in the cathode compartment were recycled, respectively, between an anolyte tank and a catholyte tank by recycling pumps.

A 2N aqueous sulfuric acid was used as anolyte which was circulated in the anode compartment at a rate of 20 cm./sec. to discharge fine particles of lead peroxide having a particle size of about 50 removed from the anode.

An emulsion having a ratio of an oil phase to an aqueous phase of 1:3 was used as catholyte. The oil phase was consisted of 20% acrylonitrile, 6% adiponitrile, 10% propionitrile, 2% 2-cyanoethyl adiponitrile, 1% biscyanoethyl ether and 6% water. The aqueous phase comprised of 20% tetraethylammonium sulfate, 8% of said nitriles and the balance of water.

The electrolysis was carried out by maintaining the conditions set forth above. Other conditions of electrolysis included an electrolysis temperature of 50 C., an electric current density of 10 amperes per dm. for both anode and cathode and an amount of anolyte of 500 cc./amperes.

One-fifth of the recycling flow to the anode compartment was supplied to the top of a packed tower filled with one inch Raschig rings in a height of 3 m. and an aeration was carried out by blowing into the packed tower from the bottom 10 volumes of air per a volume of gaseous oxygen produced at the anode by electrolysis.

The electrolysis was carried out under these conditions for more than 50 hours and the concentrations of hydrogen cyanide and nitric acid were about 50 ppm. and about 400 ppm. respectively. The electrolysis was further continued for the total of 2000 hrs., but the conditions and the concentrations of hydrogen cyanide and nitric acid remained unchanged.

During the period, amounts of lead peroxide discharged from the cell were measured. As a result, it was found that 5-7 mglamperehour of lead was corroded throughout the period.

After the electrolysis of 2000 hours, the anode was disassembled and the surface was closely examined with a finding that the whole surface was uniformly coated with a layer of lead peroxide of about 0.2 mm. thickness and no extraordinary corrosion was observed.

Comparative Example 1 Example I was repeated under the same electrolysis conditions using the same cell, electrode material, membrane, anolyte and catholyte as used therein except that no removal of hydrogen cyanide by the aeration was effected.

As a result, after the operation of 200 hours of electrolysis, the concentrations of hydrogen cyanide and nitric acid in the anolyte reached 500 ppm. and 2000 p.p.m., respectively. The amount of lead peroxide discharged from the cell during the 200 hours period was about 5-7 mg./ampere-hour of operation. However, there were observed choking in the anode compartment and decrease in the anolyte recycling amount at 200th hour of operation so that the anode was disassembled to be examined. it was found that the layer of lead peroxide on the anode surface was peeled off in a depth of about 1 mm. and the whole surface was extraordinarily corroded.

Example 2 An alloy consisting of 0.3% silver, 4% antimony, 0.15% tellurium and the balance of lead which was hardened by quenching from 245 C. to normal temperature was used as an anode. The same material was used for a cathode. Other conditions were remained same as described in Example I The aeration conditions were the same as described in example except that a part of the anolyte was replaced by a fresh sulfuric acid at a rate of 2 ccJampere'hQur.

The electrolysis was carried out under these conditions and after the operation of about hours, the concentrations of replacement of the anolyte was effected.

At the 200th hour of operation, there were observed chok- As a result, it was found that the layer of lead peroxide on the anode surface was peeled off in a depth of about 1 mm. and the whole surface thereof was extraordinarily corroded.

Example 3 ment by said partition membrane was employed.

A 3N aqueous sulfuric acid was used as an anolyte which was circulated in the anode compartment at a rate of 2 m./sec. A 40% aqueous solution of tetraethylammonium p-toluenesulfonate containing 10% each of l-cyano-l,3-diene, methacrylonitrile and acetonitrile was used as a catholyte.

The electrolysis conditions included an electrolysis temperature of 50 C., an electric current density of 30 amperes per dm. and an amount of anolyte of 500 cc./ampere.

flow to the anode compartment was supplied to the top of a packed tower filled with one inch Raschig rings in a height of 3 m., and an aeration operation was carried out by blowing into the packed tower from the bottom 50 volumes of air per a volume of gaseous oxygen produced at the anode by electrolysis.

The electrolysis was carried out under these conditions described above and the concentration of hydrogen cyanide operation, no accumulation of observed and the concentration was invariably below 200 p.p.m.

During the electrolysis, amounts of lead peroxide discharged from the cell were measured and it was found that 1-2 mg./ampere-hour of lead was corroded throughout the period.

ry corrosion was observed.

We claim:

1. A process for electrolyzing nitriles using a lead-silver alloy anode characterized in that said electrolysis being car- 2. A process according to claim I wherein said electrolysis being carried out by maintaining hydrogen cyanide concentration lower than 200 p.p.m.

3. A process according to claim 1 wherein said electrolysis being carried out in a divided cell in which a catholyte containing nitriles is separated from an anolyte of an aqueous sulfuric acid by a partition membrane.

Claims (2)

1. 2. A process according to claim 1 wherein said electrolysis being carried out by maintaining hydrogen cyanide concentration lower than 200 p.p.m.
2. 3. A process according to claim 1 wherein said electrolysis being carried out in a divided cell in which a catholyte containing nitriles is separated from an anolyte of an aqueous sulfuric acid by a partition membrane.