IL32339A - Process for the production of adiponitrile - Google Patents

Description

Process for the production of adiponitrile UCB (tJHIOH CHIMIQUE-CHEF^ISOHE BEDRIJ VEN ) S . A - la - 32539/2 The present invention is concerned with a process for the production of adiponitrile from acrylonitrile.

It is well known to produce adiponitrile by the hydrodimerisation of acrylonitrile either by operating in the presence of a metal amalgam or by the direct electrolytic route.

For the direct electrolytic hydrodimerisation of acrylonitrile to give adiponitrile, which is what the present invention is concerned with, several methods arc already known. Thus, it has been proposed to carry out the cathodic hydrodimerisation of acrylonitrile in a concentrated aqueous solution of tetramethyl ammonium toluene sulfonate on a cathode having a hydrogen over-voltage greater than that of copper (British Patent Specifications Nos. 1,041,462 and 1,030,051). Another process consists in hydrodimerising acrylonitrile by the electrolysis of a mixture of acrylonitrile and of a small quantity of water saturated by an electrolyte, such as lithium bromide, on a platinum electrode (British Patent Specification Ho. 1,011,438).

These known processes present a number of disadvantages: high terminal voltages because of the low conductivity of the system, formation of polymers to the detriment of the selectivity of adiponitrile, consumption of the platinum electrode which greatly increases the cost of the product and difficulties of recovery caused by the dissolution of the electrolyte in the reaction product.

Subsequently, an improvement was achieved according - 2 - 32339/2 to which it was suggested to carry out the electrolytic hydrodimerisation of acrylonitrile emulsified in an aqueous alkali solution contained in an electrolysis device, without a diaphragm, furnished with a graphite cathode (British Patent Specification No. 1,014,428), a procedure which coped, to a large extent, with the difficulties mentioned above. Nevertheless, the yields of adiponitrile obtained by this process do not reach 75$ of the theoretical value and, to avoid the saponification of the nitrile groups by the alkali, the process must be carried out at a low temperature (about 0°C.) which results in the consumption of a large amount of energy for the refrigeration of the electrolytic solution.

However, the inconveniences of the above mentioned process have, to a large extent, been overcome by a recent process (British Patent Specification No. 1,089,707) in which the electrolysis is carried out in an - 3 - 32339/2 emulsion in an electrolyte containing alkali metal acid salts of a polybasic acid, as well as surface-active substances, ϊηβ electrolysis apparatus used is of the type without a diaphragm in w ich there is used an anode constituted by an iron oxide supported on metallic iron and possibly containing up to 10?» of oxides of silicon and up to 10?£ of oxides of titanium. By alkali metal acid salts of an inorganic or organic polybasic acid, there are intended the alkali metal acid sulfates, borates, perborates, phosphates, oxalates and the like.

The process according to British Patent Specification Eop 1,089,707 provides a remarkable technical advance in that it enables excellent yields to be obtained not only as referred to the acrylonitrile consumed but also as to the amount of electricity supplied to the system. However, a major disadvantage is that the iron oxide anode is subjected to a very considerable degree of corrosion during the course of the electrolysis. This corrosion necessitates the frequent replacement of the anodes, which represents a very heavy expense to tie point of compromising the industrial application of the invention. Consequently, there is a very great interest in preserving substantially all of the advan-tages provided by this process, while reducing the corrosion of the anode to a minimum. It is this which constitutes the object of the present invention.

According to the present invention, we have found that the electrolyte used according to the process of BritishPatent Specification Ho.1,08 , 0 is responsible for the corrosion of the iron oxide anode and that, if the salts - 4 - 32339/3 of the electrolyte are replaced wholly or partially by the alkali metal salts or pol condensed phosphoric acids, it is possible to maintain substantially all of the advantages of this process, while reducing the corrosion of the iron oxide anode to an amount which is technically acceptable. 3Jhe process according to the present invention for of the hydra$.merisation/acr lonitrile to adiponitrile by the direct electrolytic route, by passing a direct electrical current through an electrolytic, cell having the anode and cathode in contact with the electrolytic medium, comprises using an initial electrolysis medium consisting essentiall of (a) acrlonitrile, (b) water, (c) at least one alkali salt (as hereinafter defined) of condensed polyphosphoric acids of the formula: n H3?04 * (n-1) E20 (I) or in which n is from 2-100,/of polymetaphosphoric acids of the formula in which n is from 2-100 (d) a surface-active organic nitrogen base and (e) optionally at least one alkali metal acid sulfate, borate, perborate, phosphate and/or oxalate, the ratio by weight of (e) to (c) being from 99.9/0.1 to 0/100.

By alkali salts of condensed polyphosphoric acids of formula (I), there are intended the sodium, potassium, lithium, ammonium and quaternary ammonium salts of acids, such as pyrophosphoric acid triphosphoric acid (H-j^O^) , tetraphosphoric acid (RgP^O^), polyphosphoric acids containing from 5 to 100 phosphorus atoms, and mixtures thereof B alkali sal s o olmetahoshric acids of - 5 - 32339/2 lithium, ammonium and quaternary ammonium salts of acids, such as dimetaphosphoric acid (Η2Ϊ>2^6 ^ ' trimetaphosphoric acid , meta- phosphoric acids containing from 5 to 100 phosphorus atoms and mixtures thereof.

The alkali salts of the condensed pol phosphoric acids of formula (I) and of the metaphosphoric acids of formula (II) may also be used in the form of mixtures with one another in any desired proportions; furthermore, there can be used the alkali salts of these acids such as are available commercially, for example, under the names of Graham's salt, ILurrol salt, sodium hexametaphosphate and SQ salt sold by Monsanto (Ua^P^O,^).

By alkali metal acid salts of polytasic acids, there are intended the acid salts of polybasic acids, such as sulfuric acid, boric acid, perboric acid, phosphoric acid or oxalic acid containing at least one hydrogen cation. di Thus, there may be mentioned mono- and/sodium orthophosphates> hydrogen mono- and dipotassium orthophosphates, sodium/sulfate and potassium hydrogen oxalate, as well as mixtures thereof.

As will be shown in tie following Examples* the salts used according to the present invention may wholly replace the alkali metal acid salts of polybasic acids, especially the alkali metal acid orthophosphates. However, as some of the salts according to the present invention are more expensive, especially with regard to the alkali orthophosphates, and as, on the other hand, their lower ionisation, increases the terminal voltage, it is intended, according to the present invention to use mixtures of alkali metal - 6 - 32339/2 acid salts of polybasic acids and of salts of acids according to formulae (I) and (II) in which the amount of polyphosphates according to the present invention is sufficien to maintain the corrosion of the anode at an acceptable level. Indeed, we have, surprisingly, found that the amount of polyphosphate may be relatively low, without prejudicing the anti-corrosive effect. Thus, the ratio by weight between the alkali metal acid salts of polybasic acids and the alkali salts of polyphosphates used according to the present invention may be 99.9/0ito 0/100, advantageously 99/1 to 80/20, and preferably 95/5 to 85/15·· The concentration by weight of the polyphosphates (or of the mixture of polyphosphates and of alkali metal acid salts of polybasic acids in the aqueous electrolytic solution may vary from 0.5>*> up to the concentration corresponding to saturation.

As surface-active substances, there may be used quaternary ammonium salts or pyridinium salts, such as bis-tetraehyl-ammonium acid phosphate, penta-tetraethyl-ammoniuai tripolyphosphate or bis-methyl-pyridinum acid phosphate. The concentration of these surface-active substances in the aqueous electrolytic solution may vary from 0.05 to by weight, preferably from 0.2 to 2# by weight.

Apart from the polyphosphates (and possibly the alkali metal acid salts of polybasic acids) and the surface-active substances, the initial electrolytic solution essentially contains water. However, there may also be added a small quantity of a base or of an acid in order to - 6a - 32339/2 In the course of the electrolysis, an emulsified mixture of acrylonitrile and of the initial aqueous electrolytic solution is circulated through the electrolysis apparatus, the volumetric ratio betweenthe aqueous phase and the acrylonitrile phase being preferably maintained within the limits of 1;1 and 6:1* The temperature during the electrolysis is preferably maintained within the limits of 0 to 40°C. and more, preferably within the region of room temperature.

The linear velocity of circulation of the emulsified mixture is preferably between 0.1 and 1 m./sec.

It is preferred to use an electrolysis apparatus without a diaphragm,having graphite cathodes and magnetite anodes, with or without a metallic support. The current density is preferably 1-20 amperes/dm and the voltage is preferably comprised between 4 and 10 volts and more, preferably between 4 and 7 volts.

The process according to the present invention is equally applicable to anodes made of materials other than iron oxide, for example, anodes made of metallic iron.

In general, the electrolysis is carried out in a manner such that the conversion of the acrylonitrile is -70?*, pre erably 0-50 Below* 2Q , the economy o the process according to the present invention or the production rate is too poor for industrial use, while when the conversion is increased beyond 705¾, the selectivity of adiponitrile is less good.

The process according to the present invention may becarried out as well discontinuously as continuously. sr - 6b - 32339/2 Example 1 a) Comparative experiments for the corrosion of magnetite anodes.

The apparatus for measuring the rate of corrosion of the anodes comprises a 2 litre beaker provided with means for cooling and a mechanical stirrer.

The magnetite anode subjected to the experiments is a square plate of steel or of Armco iron, ("Armco" is a Registered Trade Mark) the edges of which are 10 cm. long and the thickness of which is 1 cm. , entirely covered with a coating of magnetite with a thickness of about 1 mm. , obtained by the superficial oxidation of the metal in water vapour at a temperature of 1000°C.

This anode is surrounded by two cathodes made of graphite and of the same dimensions, placed on both sides of the anodi at a distance of 1 cm.

The supply of current is made by means of steel threaded rods screwed to the upper part of each electrode. The assembly of the three electrodes is fixed vertically in the beaker.

The apparatus is provided with 1.8 litres of the electrolytic solution to be tested. A temperature of 20°C. is maintained and there is passed through a continuous current of 14 amperes. The current density is in the 2 region of 7 amperes/dm .

Every 24 hours, the electrolysis is interrupted for the time necessary to remove the anode, rinse it, dry and weigh it. The anode is then replaced and the electrolysis resumed under the same conditions.

In the following Table I, there are set out the results thus obtained by expressing the rate of corrosion of the anode in mm. loss of thickness per year.

TABLE I Experiment number JL 7 8 9 10 11 % composition of the electrolysis solution: H 2 O4 5.6 5.6 5.3 4.8 0 4.55 5.45 5.52 0 0.06 0.3 1.14 5.6 310 (Et N) HPO 1 1 1 1 1 1 1 1 1 1 1 Na .Jexametaphosphate - 5.55 - - 1 0.1 0.03 Na8P601cJ phosphate"oQ - 5.55 - - - 5.55 - Na4P2°7 rate of oorrosion mm./year first day 5.31 I2.3I 1.09 0.48 0.900.58 Ο.381.15 Ο.631.080.57 second day 3.I Ο.96 O.5I 0.16 0. 1 O.19 Ο.321.09 Ο.540.540.69 third day 2.59 0.960.48 0.16 Ο.38 Ο.320.420.800.130.030.09 fourth day 2.53 1.060.32^0.03<0.030.290.22 O.7O 0.25 O. I O.4I terminal voltage 5 5 5 5.6 5.6 6.6 6.6 5.6 5 5 5 The first experiment is the kind used according be seen that the corrosion of the electrode made of iron oxide brought about by this electrolyte is considerable. di hydrogen In experiments 2-5» in which the eeid±e/potassium/ortho-phosphate is replaced by an increasing amount of potassium tripolyphosphate, it is observed that the corrosion decreases with the increasing amount of tripolyphosphate and that on the fourth day it is not more than l/80th of the corrosion according to the first experiment. A comparison of experiments 1 and 4 also shows that it is possible to reduce eightyfold the rate of corrosion by replacing only 2(¾¾ of the orthophosphate by polyphosphate.

Experiments 6, 7 and 8 show that with other polyphosphates or polymeta- phosphates, there is also obtained, according to the present invention, a considerable diminution of the corrosion.

Experiments j 10 and 11 point out that at very low doses of sodium hexametaphosphate the decrease of corrosion is already considerable.

Comparative experiments for the corrosion of iron anodes.

When the magnetite anode of the experiment carried out as in Example 1 a) is replaced by an iron anode, the results represented in the following Table II are obtained: TABLE II Experiment number 1 2 3. 4 % composition of the electrolysis solution E2HP04 5.6 5.54 5.3 0 V 0 0.06 0.3 5.6 °10 (Et4N)2HP04 1 1 1 1 rate of corrosion mm./year -first day 1.1 0.5 0.5 0.03 second day 1.9 0.3 0.3 0.1 third day 2.0 0.3 0.25 0.2 fourth day 2.8 0.3 0.2 0.3 terminal voltage 5 5 5 5 The favorable effect of replacing a part of K^HPO^ by ^P^O^ can clearly been seen from this Table.

Example 2.

Electrolytic hydrodimerisation of acrylonitrile in preBaace of polyphosphates.

In a semi-industrial electrolytic cell made of polypropylene of the filter-press type comprising 6 compartments, each of which is delimited 2 by a flat graphite cathode with an effective area of 3·4 dm and by a flat steel anode covered with a coating of megnetite, which also has an 2 effective surface area of 3.4 dm , having a distance between the cathodes and anodes of 5 mm. , there is carried out the electrolysis of an emulsion containing acrylonitrile and the aqueous solution (electrolyte). The composition of this emulsion and the conditions of electrolysis are given in the following Table III.

In experiment no. 1, the composition of the electrolyte is of the British 1,089,707 type used in Bolgian Patent Specification no. -6βνΦ36; it contains, in hydrogen particular, aei ie dipotassium/orthophosphate.

In the second experiment, which is according to the present iiydrogen invention, a part of the -ee¾4ie-di otassium^rthophosphate of the first experiment is replaced by potassium tripolyphosphate.

In the third experiment, according to the present invention, a Hydrogen part of the eeietie dipotassium/orthophosphate of the first experiment is replaced by sodium hexametaphosphate.

In the fourth experiment, according to the present invention, the hydrogen whole of the dipotassium/orthophosphate is replaced by potassium tripolyphosphate.

In the four experiments, the electrolysis device functions in a continuous manner, with a constant supply of acrylonitrile and of water (to compensate for the electrolytic decomposition of this latter) and a continuous removal, by decantation, of an organic phase containing unchanged acrylonitrile, adiponitrile; propionitrile and the products of hydro-oligomerisation. The pH value is permanently controled and maintained at 8.4.

In the following Table: AN = acrylonitrile ADIT = adiponitrile P = propionitrile Eff. ADN/AN = % of the number of 1/2 moles of AM formed referred to the number of moles of W supplied, Eff. PN/AN = % of the number of moles of PN formed, referred to the number of moles of AN supplied.

Eff.Hydr. = fraction in % of AN supplied which is transformed into hydrooligomers.

Yield ADN/AN = quotient in % of the efficiency of A by the conversion of AN.

Yield ADN/Elec. = ratio in % between the number of l 2 moles of ADN formed and the number of faradays provided to the cell in the course of the electrolysis.

TABLE III Experiment number 1 2 3 4_ duration of electrolysis (hrs.) 227 215 II7. 83 emulsion: % composition of the -Bri.-fe.eai- initial (/aqueous phase)e»wleAe»i ¾0 93.46 93.30 3.3Ο 93.5 2HP04 5.57 5.36 5.36 0 Na hexametaphosphate 0 0 0.53 0 initial organic phase AN AN AN AN ratio by vol.aq.ph./org. h. 2 2 2 2 current intensity (amps) 157 157 158 158 current density (amp/dm ) 7.9 7» 9 7.9 7 rate of circulation of emulsion supply of AN (g./hr) 634 634 634 634 supply of water (g./hr) 72 78. 78.5 78.5 unchanged AN (%) 51.2 51.7 51.9 52.9 efficiency ADN/AN (%) 39.0 36.1 37.4 35.7 efficiency PN/AN {%) 3.6 44 4.35 3.2 efficiency Hydr./AN {%) 4.8 7,8 - 8.2 yield ADN/AN {%) 80.0 74.7 77.8 75.7 yield ADN/elec. current ( ) 77.8 70. 73.Ο 73.6 rate of corrosion of the magnetite anodes (mm./year) 2.2 0.6 0.3 1.2 terminal voltage 5.8 6.3 6.3 6.1 specific conductivity of the 18.10"3 14-io"3 12,10"5 — emulsion (SL~ cm" ) The Table shows that the yield of adiponitrile referred to the supplied acrylonitrile is appreciably maintained and that the yield of adiponitrile with reference to the electric current is subjected to a slight diminution. However, this small loss in yield is largely compensated by the diminution of the speed of corrosion of the anode which, compared with that found in experiment no.l, is only about one half in experiment ηο·4, is less than one third in experiment no.2 and is less than one seventh in experiment no.J.

Example 5» The experiment was carried out in a tubular electrolytic cell constituted by a cylindrical anode made of molten magnetite, surrounded by a graphite tube which functions both as oathode and as container. The anode has a diameter of 6 cm. and an effective length of 61 cm., i.e. a 2 surface of 11* 3 dm . The cathode has an interior diameter of 1 cm., i.e. 2 an effective surface of 13.2 dm · The distance between the electrodes is 0.5 o » The cell proper is completed by a tubing system with pumps, hydrocyclone and filter allowing the circulation of the reaction medium between the electrodes and the collection of the solid products resulting from the corrosion of the anode (phosphate + iron hydroxide). Cooling is effected by circulating brine in a double jacket surrounding the graphite tube.

The working conditions and the obtained results are given in the following Table IV: TABLE IV Experiment number 1 2 % composition of the emulsion initial aqueous phase ^O 93- 3 93.5 Na hexametaphosphate zero 0.3 initial organic phase AN AN ratio by vol.aq.ph./org.ph. 2 2 current intensity (amps) 90 90 anodic current density (amp/dm ) 7.95 7.95 cathodic ourrent density (amp/dm ) 6.80 6.80 rate of circulation (dm sec.) 3 3 Experiment number 1 2 supply of AN (g./hr) 372 393 supply of water (g./hr) 47 47 unchanged AN (%) 53.4 58.3 efficiency ADN/AN { ) 38.8 34.6 efficienc PN/AN ( ) 3.7 3.0 efficiency Hydr./AN ( ) 4.1 4.1 yield ADN/AN {%) 83.2 82.9 yield ADN/elec.ourrent (?ό) 80.2 75.5 rate of corrosion of the magnetite anodes (mm./year) 15.4 0.3 terminal voltage 5.6 5.6 specific conductivity of the emulsion (A"^cm»"^) /2.1θ "' 1

Claims (1)

1. CLAIMS Process for the of acrylonitrile to by the direct electrolytic b passing a direct electrical through an electrolytic cell having the anode and cathode thereof in with the electrolytic which comprises using an initial electrolysis medium consisting essentially of at least one alkali salt herein of condensed polyphosphoric acids of the general in which n is from 2 to or of acids of the general P n n in which n is from 2 to a organic nitrogen base and optionally at least one alkali metal acid phosphate the ratio by weight of to being from to Process according to Claim wherein the ratio by weight of to is to preferably from to Process according to any of the preceding wherein the concentration of the in the aqueous electrolytic solution is from by weight to the concentration corresponding to 14 Process to any of the preceding wherein the salts of general are or quaternary salts of pyro tetraphosphoric or polyphosphoric acids containing from 5 to 100 phosphorus and Process according to any of the preceding wherein the salts of general formula are ammonium or quaternary ammonium salts of tri tetrametaphosphoric acids or metaphosphoric acids having 5 to 100 phosphorus and mixtures Process according to any of the preceding wherein the salts of general formula are used in mixture with salts of general formula Process according to any of the the nitrogen base is a quaternary ammonium or pyridinium th acid onium tripolyphosphate or acid Process according to any of the preceding wherein the concentration of the nitrogen base in the aqueous electrolytic solution is from to to 2if by Process according to any of the preceding wherein the alkali metal acid salt is a sodium or potassium acid or sodium hydrogen sulphate or potassium hydrogen 15 Process according to any of the preceding wherein an amount of or acid is added to the water so as to maintain a of between and between 8 and Process according any of the preceding in the course of the the ratio of the aqueous electrolytic solution to acrylonitrile is maintained between and Process according to any of the preceding in the course of the the temperature is maintained within the limits of 0 to at or near the ambient Process according to any of the preceding wherein the linear velocity of circulation of the emulsified mixture of the aqueous electrolytic solution and nitrile in the electrolysis apparatus is between and 1 Process according to any of the preceding wherein the electrolysis is carried out in an electrolysis apparatus without a with the aid of graphite cathodes and inagnetite or iron Process according to sny of the preceding where in the current density is from 1 to 20 and the voltage is from 4 to 10 from 4 to 7 roeess according to any of the preceding wherein the electrolysis is carried out in a manner such 16 5 that the conversion of acrylonitrile is 20 to Process according to any of the preceding wherein the electrolysis is carried out Process according to any of 1 to wherein the electrolysis is carried out Process according to Claim 1 for the sation of acrylonitrile to substantially as hereinbefore described and For the Applicants insufficientOCRQuality