Process for the Manufacture of Phenylhydrazine

This invention relates to a process for the manufacture of phenylhydrazine.

Our copending Application Ser. No. 307776 now U.S. Pat. 3,836,440 issued Sept. 17, 1974, provides a process for the manufacture of phenylhydrazine from diazoaminobenzene by cathodic reduction with separation of cathode and anode spaces by a diaphragm, wherein diazoaminobenzene, dissolved in an electrolyte, is reduced on solid electrode materials. In a special variant of the process a graphite disc is used as cathode.

Since the thermally sensitive diazoaminobenzene which is unstable in acid solution cannot withstand an acid working up of the electrolysis solution -- which is by far the most expedient process for the isolation of the phenylhydrazine, in order to avoid a large decrease in yield it is necessary to reduce cathodically the diazoaminobenzene dissolved in the catholyte completely, i.e. the solution must be subjected to electrolysis until it no longer contains any diazoaminobenzene.

It is known, however, that concentration polarization occurs increasingly during this electrolysis of the solution, i.e. above all towards the end of the electrolysis, so that the cathode potential in the final electrolysis reaches values under constant cell current, which are in the range of those that cause phenylhydrazine scission. This cathodic cleavability of the phenylhydrazine is especially disturbing in the electrosynthesis of phenylhydrazine from diazoaminobenzene, and the problem arises of completely electrolysing diazoaminobenzene solutions at technically interesting current densities of approximately 1000 A/m.sup.2 and above (at a cell voltage of approximately 4 V) without a part of the phenylhydrazine already formed being split into aniline and ammonia at the same time.

It has now been found that phenylhydrazine can be obtained by cathodic reduction in especially good yields by electrolysing diazoaminobenzene solutions on a graphite cathode with a three dimensional surface in a flow type diaphragm cell.

As graphite cathodes with a three dimensional surface according to the invention preferably masses of loose particles, nets, fabrics or structured plates are used, through the cavities and recesses of which the electrolysis solution flows optionally in a cycle.

On electrodes of this type even towards the end of the reduction, i.e. with low concentration of diazoaminobenzene, no noticeable scission of the phenylhydrazine occurs, which leads to good yields of phenylhydrazine. Since, furthermore, surprisingly with the use of a graphite electrode with a three dimensional surface according to the invention for the cathodic electrolysis of diazoaminobenzene solutions with technically interesting current densities a noticeable hydrogen development begins only toward the end of the electrolysis, not only good yields but also good current efficiencies are obtained.

For the manufacture of graphite electrodes with a three dimensional surface according to the invention, for example, for bulk electrodes, balls, grains, lumps, grit, lamellae, granules, coils of hollow cylinders of from 0.5 mm to 20 mm, preferably from 1.5 mm to 10 mm mean diameter are used, which fill the total cathode space up to the diaphragm. The particles of such a mass or filling consist either completely of graphite or of a non-conducting carrier material which is coated with graphite. The mass is contacted with a current carrying graphite or other current-conducting plate which may be smooth, or contured plate. Suitable graphite nets or grids are the known nets and loose fabrics of graphite fibres or of mixtures of graphite fibres with fibres of inert non-conducting materials or grids of graphite bars of 8 .mu. to 4 mm fibre or bar diameter and with mesh widths between 0.2 mm and 10 mm, preferably 0.5 mm to 4 mm mesh width, 10 .mu. to 3 mm fibre or bar diameter. As structured graphite plates there are used those which are made by suitable processing, for example by grooving plane plates, or using those which already have the desired surface structures when manufactured. The structured graphite plates of the invention obtained in this way may be irregular or regular, for example, they may have round, oval, rhombic, rectangular or square elevations with any arbitrary means diameters, intervals and heights of these elevations. Mean diameters of from 0.3 mm to 20 mm, intervals of between 0.2 mm and 25 mm and heights of between 0.2 mm and 15 mm, may be used. Especially preferred are mean diameters of from 1.5 mm to 8 mm, intervals of between 1 mm and 6 mm and heights of from 0.5 mm to 8 mm. Those structured plates are preferred, the structures of which have equal heights.

According to the invention the graphite-electrodes with three-dimensional electrode surface are used as cathode in a flow electrolysis cell, the cathode and anode spaces of which are separated from one another by a diaphragm and the electrolysis solution containing the diazoaminobenzene is circulated during the electrolysis through the cathode space and the cavities and recesses of the cathode body. The flow speed is variable within wide limits, preferably, however, in the range of from 0.5 m to 4 m per second.

For a continuous operation of the electro-synthesis of the invention a cascade connection of an unlimited number of flow cells with electrodes of the invention is also possible.

To obtain a cell voltage as low as possible and to protect the diaphragm from mechanical damage the diaphragm is separated from the structured plates, nets or grids or the particles of the bulk electrodes by a fine net of inert, non-conducting material, such as, for example, glass fibres or organic polymer material, such as polyethylene, polyacrylamide, polyacrylonitrile or polyamides (nylon), in such a way that a direct contact of graphite and diaphragm is avoided.

For the anode space a simple common construction approximately in the form of a narrow gap between anode and diaphragm with lateral electrolyte-inlet and outlet is sufficient.

The diaphragm intervals of cathode and anode are not critical and can be designed randomly. They are chosen expediently for economical reasons but are as small as possible.

As diaphragms there are suitable all materials resistant to alkalies, acids and organic solvents, such as, for example, porous ceramic material, felts, porous sheets or permselective membranes, preferably cation exchange membranes.

In contradistinction to the cathodic reduction of diazotates, the electrolysis temperature used in the present process can be varied over a wide range and its upper limit is determined only by the thermal decomposition of the diazoaminobenzene. A temperature in the range of from -20.degree. to +90.degree.C, especially +15.degree. to +50.degree.C is preferred.

The electrolysis can be carried out with any electrolytes, preferably with protic, especially aqueous electrolytes. Preferably organic cosolvents are added especially to the catholyte, whereby the solubility of the diazoaminobenzene is considerably improved. As organic cosolvents there are suitable above all watersoluble compounds such as, for example, lower alcohols, such as methanol, ethanol, i- or n-propanol, butanol, ethers, such as for example, tetrahydrofurane dioxane, glycol monomethyl ether, glycol dimethyl ether, carboxylic acid amides, such as, for example, dimethyl formamide, diethylacetamide, diethyl formamide and/or nitriles, such as, for example, acetonitrile, propionitrile, or mixtures of the solvents named.

A sufficient conductivity in the catholyte is obtained expediently by adding an auxiliary electrolyte in the form of suitable "conducting salts" in known manner. The pH-value of the catholyte has to be chosen in such a manner that the diazoaminobenzene does not decompose to give benzenediazonium salt and aniline, a pH-value above 5, especially approximately 6 to 14 being preferred. Higher pH-values are also possible but they do not offer any advantage.

Suitable conducting salts are especially alkali metal and ammonium hydroxides as well as organic bases, for example the various tetraalkyl ammonium hydroxides. The catholyte can contain up to approximately 35% by weight of diazoaminobenzene, preferably the process should be carried out in a range of from approximately from 15 to 25%. The diazoaminobenzene solutions can be electrolysed with up to approximately 3500 A/m.sup.2, the cell voltage being, for example, between approximately 3.0 and 10 V, depending on the current density used. The electrolysis should be carried out preferably at current densities from approximately 1500 to approximately 2500 A/m.sup.2 and cell voltages of approximately 4.0 to approximately 6.0 V.

As anolyte conducting salt solutions, acids or alkaline solutions of any concentrations are suitable, preferably however about 0.1 to 10 normal aqueous alkaline solutions. The anode material is not critical. Any material may be used, which does not corrode under the anode charge such as, for example, platinum metals, lead coated with PbO.sub.2, graphite coated with PbO.sub.2, a titanium electrode coated with precious metal in sulphuric acid or phosphoric acid, or nickel or graphite or steel coated with nickel in aqueous alkaline solution. As anodes there may be used each of the usual structural types such as, for example plane, smooth plates, nets or expanded metals or also one of the bulked or structured plates according to the invention.

The flow speeds of the catholyte are from 0.1 to 7 m/sec, preferably from 0.5 to 4 m/sec, especially from 1 to 2.5 m/sec. The flow speed of the anolyte can be lower. However, there should be a difference in pressure between anode and cathode spaces. Preferably the process is carried out at a low excess pressure of from approximately 0.01 to 0.5 atmosphere gauge in the anolyte, so that the diaphragm is not subjected to too great a mechanical burden.

The invention will now be described by way of example only and in further detail with reference to the accompanying drawing of which

FIG. 1 and FIG. 2 show structured graphite plates with elevations 1 and grooves 2 through which the catholyte flows.

FIG. 3 illustrates an electrolysis cell with a mass of graphite balls.

The anode side with the anode 3 and the anolyte inlet 8 and outlet 8a is separated from the cathode by the diaphragm 6. The cathode current is supplied through the contact plate 4 to the particles 5 of the graphite mass. The catholyte flows through the free spaces via the inlet 7 at a flow speed of from 0.1 to 7 m/sec, preferably from 0.5 to 4 m/sec, especially from 1 to 2.5 m/sec, to the outlet 7a. The spacing between the cathode contact plate 4 and the diaphragm 6 is not critical and is variable within wide limits. Preferably a spacing of from 1 to 50 mm, especially from 3 to 20 mm. is used. The net 9 protects the diaphragm 6 from direct contact with the cathode material.

The following examples illustrate the invention. The material yields given are calculated on the quantity of starting compound used.

The flow type electrolysis cell consisted of two polyethylene halves in the form of flat cases measuring 300 .times. 300 .times. 50 mm with side inlet and outlet, between which was placed a cation exchanger membrane (type Nafion XR 475 DuPont). The cathode space was completely filled with graphite fragments (mean diameter approximately 4 mm, type P 127 from Sigri), which were contacted from behind by a plate of the same material. The distance of the contacting plate from the diaphragm was 6 mm. The free space in the graphite mass was approximately 40% of the total cathode space. A direct contact of the graphite fragments with the diaphragm was prevented by a fine polyethylene net placed in between. The anode consisted of a nickel plate which was kept approximately 1.5 mm away from the diaphragm by polyethylene nets. The anolyte was aqueous 6N NaOH.

The catholyte contained 151 g of diazoaminobenzene in a mixture of 150 ml of tetrahydrofurane, 650 ml of methanol and 50 ml of 50% aqueous NaOH. The electrolytes were circulated at approximately 25.degree. to 30.degree.C at a speed of approximately 1 m/sec through the electrolysis cell. 59 AH were passed through the cell at 15 A/dm.sup.2 (cell voltage approximately 4.0 V) and subsequently 31 AH at 10 A/dm.sup.2 (cell voltage approximately 3.6 v), whereupon the initially deep blackish red solution had turned pink. It was acidified with concentrated HC1 and evaporated to dryness. The residue was rendered alkaline with aqueous 50% NaOH, the separated organic phase was separated and the aqueous phase repeatedly extracted with ether. The combined organic phases were concentrated by evaporation after drying over MgSO.sub.4 and a gas chromatogram was taken from the remaining residue (147 g of oil). At the same time the content of the oil of phenylhydrazine was determined iodometrically. Both determinations gave a content of 50.6% by weight phenylhydrazine, corresponding to 89.6% material yield and 81.7% current efficiency. 47.5% by weight of the oil was aniline the rest non-evaporated solvent. The oil was separated in a simple distillation apparatus with a short column and with weak reflux into the main constituents. After removal of 3 g of solvent 69 g of aniline was distilled at 60.degree. to 65.degree.C/4 mm Hg and thereafter at 100.degree. to 101.degree.CC/4 mm Hg 71.4 g of phenylhydrazine was distilled.

Under similar conditions, using only a smooth graphite plate as cathode against the diaphragm, a material yield of only phenylhydrazine of approximately 60% was obtained.

In the cell described in Example 1 the cathode space was filled with approximately 4 mm fragments of electrographite (Sirri). A direct contact of the fragments with the diaphragm was prevented by a fine polyethylene net placed between. The cation exchange membrane was of the type C 61 AZL 183 (Ionics), the anode of lead was coated with PbO.sub.2 and as anolyte aqueous 2N H.sub.2 SO.sub.4 was used. The catholyte contained 90 ml of tetrahydrofurane, 800 ml of methanol, 35 ml of 50% aqueous NaOH and 75.6 g of diazoaminobenzene. The electrolytes were recycled at approximately 25.degree. to 30.degree.C through the electrolysis cell at a speed of approximately 1 m/sec. At 14 A/dm.sup.2 (cell voltage approximately 6 V) 45 AH were passed through. After working up (as in Example 1) 74.5 g of oil containing 47.7% by weight of phenylhydrazine and 50.5% by weight of aniline were obtained. The content of phenylhydrazine corresponded to 85.6% material yield and 77.8% current efficiency.

In the cell described in Example 1 as cathode a structured graphiteplate (evs-material from Sigri) with diamond shaped elevations (approximately 3.5 mm edge length, approximately 1.5 mm groove depth, approximately 1.5 mm groove width, approximately 30.degree. angle between the grooves and the current direction of the electrolyte) was fitted in such a way that the upper edge of the elevations was still approximately 0.5 mm away from the diaphragm. This distance was maintained by a polyethylene net placed between. The cation exchange membrane was of the type Nafion XR 475, the anode was of nickel and the anolyte was aqueous 6N NaOH. The electrolytes were circulated at 1 m/sec at approximately 25.degree. to 30.degree.C through the electrolysis cell.

Through the solution of 600 ml of tetrahydrofurane, 2000 ml of methanol, 400 ml of 30% aqueous NaOH and 462 g of diazoaminobenzene there were passed at 20 A/dm.sup.2 111 AH (cell voltage approximately 4.6 V), at 15 A/dm.sup.2 128 AH (cell voltage approximately 4.0 V) and at 10 A/dm.sup.2 60 AH (cell voltage approximately 3.6 V). After the conclusion of the electrolysis the solvent was distilled off from the catholyte under reduced pressure, whereupon the residue formed two layers. The upper organic phase was separated. In 458 g of oil it contained 49.3% by weight phenylhydrazine, corresponding to 89% material yield and 81% current efficiency and 47.9% by weight of aniline. The raw base mixture (as in Example 1) was separated by distillation in a short column. 215 g of aniline and 203 g of phenylhydrazine were obtained.