EP0198722B1 - Process for the preparation of amino-alcohols by electrochemical reduction of nitro-alcohols - Google Patents

Description

The present invention relates to the manufacture of amino alcohols by electrochemical reduction of nitro alcohols.

Nitro-alcohols are derivatives easily obtained by addition of formaldehyde on nitro-paraffins. Several processes have been described for transforming them into amino-alcohols (alkanol-amines) used in the manufacture of cosmetics, detergents or as intermediates for the synthesis of bactericides and pharmaceutical products.

The reduction of the -NO₂ group can be carried out by the Fe-Fe ⁺⁺ pair in sulfuric or acetic acid medium, but the weight of reagent used is approximately three times that of the nitro derivative to be reduced; this results in a large amount of solid residue to be removed and it is necessary to rectify the liquid phase containing the amine in order to obtain a pure product; the yield is around 80%

It is also possible to carry out a catalytic hydrogenation, for example on Raney nickel in methanol medium at 60 bars at 40 ° -45 ° C. In this case too, the yield does not exceed 80%; the side reactions are numerous leading to the formation of light amines and of heavy residue which must be separated from the desired amino alcohol, by several successive rectifications which necessitate significant investment and consumption of energy; on the other hand, one cannot avoid the formation of N-CH₃ derivative which is then difficult to separate from the desired amino derivative.

An electrochemical reduction process has already been described in US Pat. No. 2,485,982 according to which one operates in aqueous hydrochloric or sulfuric solution in an electrochemical cell provided with a porous porcelain diaphragm; an aqueous solution of amino alcohol hydrochloride or sulphate is obtained which must then be neutralized and / or precipitated in order to obtain the amine; in addition to the nitro derivative raw material, the acid and the neutralization or precipitation reagents are consumed which must then be discharged into the environment.

Another electrochemical reduction process has already been described in patent GB 1,166,363; it is the reduction of nitrobenzene to phenylhydroxylamine in a sulfuric bath which is neutralized before recovery of the para-aminophenol by distillation. In this process the acid and many neutralizing reagents are consumed, and it is moreover necessary to distill.

We sought an electrochemical reduction process in sulfuric medium to obtain on the one hand an aqueous solution concentrated in pure amino alcohol and on the other hand solutions of sulfuric acid reusable in the following operation.

According to the process of the invention, apart from water, the only raw material consumed stoichiometrically is nitro-alcohol; the consumption of sulfuric acid being reduced to a minimum and in certain cases possibly being zero. There is little or no release to the environment. And, the conversion of the nitro derivative into an amino derivative can reach 95-98% and in most cases remains above 90%.

In addition, the reaction conditions make it impossible to form derivatives (R) ₂-N-CH par, by Mannich reaction, often undesirable in the applications of amino alcohols, because of their physical properties and structure close to theirs, which increases the difficulty and the cost of the amino alcohol purification operations.

The reduction of the R-N0₂ group and the acid-amino derivative separation are carried out by electroreduction in sulfuric medium in three stages.
Thus, the invention relates to a process for the production of amino alcohols by electrochemical reduction of nitro-alcohols in sulfuric medium in three stages. During the first hydroxylamine reduction step, the nitro group is reduced on a cathode made of a material with a high hydrogen overvoltage by treating a sulfuric solution of the nitro derivative, the reaction being carried out on a cathode whose voltage is moderately electronegative. During the second amino alcohol reduction step, the electronegative voltage is higher in absolute value. In the third step, the sulfuric amino alcohol solution obtained is subjected to a purification operation by electro-electrodialysis and elimination of water.

In the first step there is a reduction to four electrons which transforms R-N0₂ into R-NOH. This reduction of the nitro group is carried out on a cathode produced from a material with a high hydrogen overvoltage by treating an aqueous sulfuric solution of the nitro derivative. This reaction is effective on a cathode whose voltage is moderately electronegative.

In the second step, the hydroxylamine amine is reduced to 2 electrons on a cathode whose electronegative voltage is higher in absolute value than previously.

In the third step, the sulfuric amino alcohol solution obtained is subjected to a purification operation by electro-electrodialysis, then to elimination of water.

The two stages of electrochemical reduction can be implemented in a diaphragm cell consisting of a cation exchanger (MEC) or anion exchanger (MEA) membrane; the purification phase can be carried out in the same device or in a specific device.

In the diagrams below, the global reactions used and the transfers of matter are indicated (solid lines: migration caused by the electric field - dotted lines: transfers by diffusion).

1st stage

Avec ce dispositif, le passage du courant s'effectue grâce à la migration sous champ électrique du cation H₃O⁺ entraînant une dilution du catholyte. Dans la première étape, l'efficacité du courant étant totale, les quatre protons générés à l'anode par oxydation de l'eau sont consommés pour la réduction cathodique ; il n'y a pas de dégagement d'hydrogène. Dans la deuxième étape, l'efficacité du courant n'est pas complète et une partie de celui-ci va être utilisée à la réduction de protons en H₂. Cette consommation de protons sera compensée par une production plus élevée à l'anode et un flux d'H₃O⁺ plus élevé.

With this device, the current flows through the migration of the H₃O⁺ cation under an electric field, resulting in a dilution of the catholyte. In the first step, the current efficiency being total, the four protons generated at the anode by oxidation of water are consumed for cathodic reduction; there is no release of hydrogen. In the second step, the efficiency of the current is not complete and part of it will be used to reduce protons to H₂. This consumption of protons will be compensated by a higher production at the anode and a higher H₃O⁺ flux.

In any case, the optimum H⁺ / RX acidity ratio (RX = RNO₂ + RNHOH + RNH₂) will be kept constant on the cathode side.

One can, in fact, operate on anodic and cathodic solutions of concentration, such that the transfer by diffusion of mole of H₂SO₄ is negligible. These concentrations will be chosen according to the properties of the membrane used. One cannot however completely avoid an ion exchange leading to a transfer of the catholyte to the anolyte of the cations R-NH₂OH⁺, R-NH₃⁺.

The diagram of the 3rd stage represents an electro-electrodialysis device provided with an MEA diaphragm and electrodes with low hydrogen and oxygen overvoltage in which thanks to the transfer of ions SO₄ ⁼ under the effect of the electric field and when the cations are reduced on the cathode, ions of the R-NH₃⁺ type are destroyed and a pure aqueous solution of R-NH₂ is collected on the cathode side and a dilute and recyclable solution of H₂SO₄ solution on the anode side afterwards in the cathode compartment addition of RNO₂.

Cette utilisation de MEA met à profit les propriétés spécifiques de cette membrane en utilisant, d'une part, ses sites d'échange pour provoquer le passage des ions SO₄ ⁼ du compartiment cathodique vers l'anodique et assurer ainsi le passage du courant électrique dans la cellule et d'autre part ses propriétés de dialyse qui permettent une diffusion sélective de l'acide sulfurique.The case of an MEA diaphragm cell can be diagrammed in a similar manner.

This use of MEA takes advantage of the specific properties of this membrane by using, on the one hand, its exchange sites to cause the passage of SO₄ ⁼ ions from the cathode compartment to the anode and thus ensure the passage of electric current in the cell and on the other hand its dialysis properties which allow a selective diffusion of sulfuric acid.

Thus, during the first two stages, an aqueous solution of sulfuric acid appreciably more concentrated than the catholyte is used on the anode side; the diffusion rate through the membrane from the anolyte to the catholyte thus balances the transport of SO₄ ⁼ ions in the opposite direction. It is thus possible to maintain, on the cathode side, a constant acid concentration and always operate the electrochemical reduction on a solution in which the H⁺ / RX molar ratio remains constant and at its optimum value in particular for the reduction of the hydroxylamine group into an amino group. (RX = R-NO₂ + R-NHOH + R-NH₂).

The final purification by electro-electrodialysis can be carried out in a special apparatus as shown in Figure 1 (3rd step) which differs from the MEA cell only in the nature of the electrode materials.

It is also possible to carry out the fine ale purification in the cell used for carrying out the electrochemical reduction by replacing the concentrated anodic sulfuric solution with pure water and by sending into the cell the quantity of electricity necessary for the transfer of the SO₄ ions ⁼ ; the modification of the difference in sulfuric concentration on either side of the membrane results in the inversion of the diffusion current. As in the electro-electrodialysis machine (fig. 1, 3rd step), the transfer of solvation water takes place in the catholyte-anolyte direction and the titer of the catholyte is increased. However, since the cathode material for electrochemical reduction is chosen because of its high hydrogen overvoltage, the use of the reaction cell for purification unnecessarily leads to additional energy consumption.

Depending on the structure of the prepared products, the use of MEA membrane may have the advantage of a more strict suppression of the transfer by ion exchange of the cations R-NH₃⁺ and R-NH₂OH⁺ to the anolyte; one can also more easily use the anode compartment for carrying out the oxidation reaction. However, in most cases, it is simpler and more convenient to carry out the reduction operations in electrochemical cells provided with an MEC diaphragm and purification in an electro-electrodialysis machine provided with an MEA diaphragm.

dans laquelle R₁ et R₂ ensemble ou séparément sont l'hydrogène, le groupement hydroxyalkyle, tel hydroxyméthyle, ou un groupement alcoyle linéaire ou ramifié, en particulier, méthyle, éthyle, propyle ou contenant un nombre d'atomes de carbone supérieur à trois.The process is applicable to the nitro-alcohols represented by the formula

in which R₁ and R₂ together or separately are hydrogen, the hydroxyalkyl group, such as hydroxymethyl, or a linear or branched alkyl group, in particular, methyl, ethyl, propyl or containing a number of carbon atoms greater than three.

Among these products, there are nitro products leading to industrially important alkanol-amines such as, nitro-2-methyl-2-propanol-1, nitro-2-methyl-2-propanediol 1-3, nitro- 2-ethyl-2-propanediol 1-3, nitro-2-butanol-1, tris (hydroxymethyl) nitromethane.

The cathode is constructed from a material with a high hydrogen overvoltage such as pure or alloyed lead, mercury in the form of an amalgam (with copper, lead, Zn, etc.), zinc, zirconium. etc ...

The anode is made of a material chemically inert in the anodic solution and preferably with low oxygen overvoltage such as for example Pb, ruthenium titanium, platinum Pt, etc.

The diaphragm is made with a cation exchange membrane or commercial anion exchanger such as, for example, those sold under the brands "Nafion" (Du Pont) "IONAC" (Ionac), "ARP" and "CRP" (Rhône Poulenc) or those marketed by ASAHI Chem Ind or ASAHI CLASS CO etc ...

The cathode current density has the maximum value compatible with the usable electrode voltages and the properties of the membrane; with lead or mercury and an "IONAC 3470" membrane, we can operate under 50A / dm² and more.

The temperature of the cathode solution can be between 20 ° C and 100 ° C; it will preferably be carried out between 60 ° C. and 90 ° C. for the second step, in the case where Pb cathodes are used, and at 30 ° C. on amalgamated copper.

The catholyte is an aqueous sulfuric solution which can be saturated with nitro derivative; for nitro-2-methyl-2-propanediol, it is possible, for example, to operate at 333 g / l (or 286 g / kg).

soit compris entre 1 et 1,5 de préférence entre 1,05 et 1,18.The H₂SO₄ content of the catholyte will be such that the molar ratio

is between 1 and 1.5, preferably between 1.05 and 1.18.

The anolyte is an aqueous sulfuric solution; its composition will depend on the type and properties of the membrane used and in particular on its permeability to sulfuric acid.

In the case of an MEC cation exchanger diaphragm, (H₂SO₄) in the anolyte will have a value such that the migration flow by diffusion of H₂SO₄ is minimized as well as the transfer of organic cations by ion exchange.

In the case of an anion exchanger diaphragm MEA, (H₂SO₄) _A in the anolyte has a value such that the concentration difference (H₂SO₄) _A - (H₂SO₄) _C results in a diffusion flow of H₂SO₄ balanced with the migration of SO₄ ⁼ ions under the effect of the electric field; this composition will therefore depend on the diffusion characteristics of the membrane, the density of the current on the diaphragm, and the concentration of H⁺ (therefore R-NO₂) of the catholyte.

The sulfuric solution of nitro-alcohols used as a catholyte can be prepared from solid products obtained by crystallization and purified by recrystallization.

One can also use the aqueous solution obtained by reaction of nitro-paraffin and formaldehyde; in this case, we can proceed (for example) as follows:

An aqueous formaldehyde solution titrating from 35 to 40% is placed in a stirred reactor; it is brought to 40 ° C; we adjust the pH at 9 and the nitro paraffin is added dropwise while maintaining the temperature between 40 and 50 ° C and the pH at 9-10 by addition of a 15N aqueous NaOH solution; after one hour, the addition of nitro paraffin is complete; the mixture is stirred again for 1 hour at the same temperature while maintaining the pH above 9; the amount of nitro derivative is exactly stoichiometric or in slight excess (1 mol%) over the amount of formaldehyde.

Then acidified with H₂SO₄ to pH 5.

The catholyte can then be prepared by adding H₂SO₄, and optionally H₂O, in proportions such that the composition of the final solution is at the H⁺ / R-NO₂ ratio corresponding to the optimum of the cathodic reduction.

The method can be implemented in an apparatus allowing continuous or discontinuous manufacture.

The following examples illustrate, without limitation, the present invention.

Example 1

A multicell electrolyser is used comprising 3 cathode compartments alternating with 4 anode compartments; the cathodes are lead plates whose useful surface immersed in the electrolyte is 72 cm2 (2 x 36 cm2); the anodes are identical Pb plates.

The electrodes have undergone a preliminary degreasing treatment with detergent and then electronic pickling.
Electrochemical oxidation: 10 min in H₂SO₄ 4%, 2 A / dm2
Electrochemical reduction: 15 min in H₂SO₄

The compartments are separated by 6 diaphragms of 37.5 cm2 useful cut from a membrane sold under the brand "IONAC 3475" consisting of a polypropylene support and anion exchange sites of the quaternary ammonium type.

The 7 compartments are polypropylene frames 20 mm thick, joined by threaded rods; sealing is obtained by polyvinyl chloride PVC seals; each compartment has a useful volume of 77 ml.

The cathode liquor is distributed in the three compartments from a thermal conditioning circuit consisting of a pump and a heat exchanger; this recirculation has the effect of causing agitation of the reaction medium; the compartments are not fitted with turbulence promoters. The total volume of cathode liquor thus brought into play is 340 ml. The anode liquor is not stirred.

The catholyte contains 500 mmol (67.6 g; 179.1 g / kg of nitro-2-methyl-2-propanediol-1-3 and 29 g H₂SO₄ (7.7% by weight). The molar ratio H⁺ / R-NO₂ is therefore 1.186. The anolyte is a 39% aqueous solution of sulfuric acid.

The catholyte is brought to 50 ° C. and a cathode current density of 10 A / dm2 is established. The voltage measured on the central cathode compared to a saturated calomel ECS electrode, thanks to an assembly consisting of a capillary tube and a sintered glass in contact with the cathode is close to -0.6V / ECS.

At the end of the first stage, the variation of the cathode voltage is rapid; the cathode liquor is then brought to 80 ° C. and the operation is continued with the same current density; the cathode voltage takes a value close to - 1.5 V / DHW.

The progress of the reaction is checked in parallel by potentiometric analysis of the cathode liquor which measures the contents of free acidity, R-NHOH and R-NH₂; a semi-quantitative phmetric test indicates the disappearance of R-NO₂; the volume variations of catholyte and anolyte are also measured in which water is optionally added.

At the end of the second step, checked by chemical analysis (disappearance of the inflection point corresponding to R-NHOH on the potentiometric graph), the anode compartments are emptied and immediately filled with pure water, leaving the electrodes energized ; the interpolar voltage takes a high value, then decreases because of the progressive increase in acidity of the anolyte, goes through a minimum and increases again because of the decrease in conductivity of the catholyte caused by its progressive depletion in ions.

Finally, a cathode solution containing 1.57 mole / kg R-NH₂ (165 g / kg), 0.2 g H₂SO₄ / kg is collected; the content of nitro derivative and hydroxylamine is zero.

By evaporation to dryness under reduced pressure, a white precipitate containing 99% amino-2-methyl-2-propanediol-1-3 is finally obtained (determination by vapor phase chromatography); its melting point is 108 ° C.

The overall yield compared to the initial nitro derivative is greater than 95%; the current efficiency is 67% for electrochemical reduction. Total energy expenditure (including electrodialysis) is 11 kWh / kg.

The anode solution collected is an aqueous sulfuric solution titrating 39% H₂SO₄ and it can be recycled. The aqueous sulfuric solution collected after electro-electrodialysis can be used in part on the cathode side after delivery to the title in H₂SO₄ and addition of a new charge of nitro derivative.

Example 2

The same electrolyser is used as in Example 1, but a sulfuric solution of tris (hydroxymethyl) nitromethane is treated at a concentration of 1.068 moles / kg (161 g / kg); the H₂SO₄ concentration is such that the H⁺ / R-NO₂ ratio = 1.19; the title of the anolyte is 35.5% H₂SO₄.

We operate with a cathodic current density of 9 A / dm2; the catholyte temperature is maintained at 50 ° C during the first phase and then raised to 80 ° C for the second phase.

After electro-electrodialysis carried out as in Example 1, a pure amine solution is obtained which no longer contains hydroxylamine or sulfuric acid; its titer is 1.171 moles / kg (142 g / kg).

The chemical yield compared to the initial nitro derivative is 91 mol%: the efficiency of the current is 55%; the energy consumption is 8.6 kwh / kg for electrolysis and 12.3 kwh / kg for the electrolysis-electro-electrodialysis unit.

By dry evaporation of the catholyte, a white solid product titrating 96% is collected (assay by vapor phase chromatography).

Example 3

A cell similar to the previous one is used, but having only one cathode compartment between two anode compartments; the cathode is made of Pb, the anodes of ruthenian titanium; the diaphragm is an anion exchange membrane, sold under the brand "IONAC" 3475.

The reduction of nitro-2-methyl-2-propanediol is carried out by operating with a catholyte containing 1 mole / kg of nitro derivative; one operates at 20 A / dm2 at 80 ° C; the H⁺ / RX ratio varies from 1.5 to 1.1 during operation.

A solution containing 0.880 mole / kg of amino-2-methyl-2-propanediol-1-3 is obtained before electrodialysis.

The chemical yield compared to the nitro derivative is 94.6% The efficiency of the current is 74.7%. The energy consumption is 7.8 kwh / kg. The solution obtained contains only the amino alcohol and sulfuric acid and it can be very easily purified and concentrated by electro-electrodialysis.

Example 4

The operation is carried out in the electrolysis cell used in Example 3 in which the Pb cathode has been replaced by a cathode consisting of a Cu-Hg amalgam prepared by immersion for 10 minutes of a Cu plate of thickness 1 mm in a solution of mercuric sulfate (3%) and H₂SO₄ (10%).

A cathode current density of 10 A / dm2 is used; the treated solution contains 0.737 mole / kg of methyl-2-nitro-2-propanediol-1-3; it is maintained at 30 ° C.

Before the final purification treatment, an aqueous solution of methyl-2-amino-2-propanediol-1-3 and sulfuric acid is obtained. The efficiency of the current is 80%. The chemical yield compared to the nitro derivative is 97%.

Example 5

The operation is carried out on a cell with three compartments, identical to that of Example 3, except that it is provided with a cation exchange membrane diaphragm, sold under the brand "IONAC" MC 3470.

The 2-nitro-2-methyl-propanediol 1-3 obtained in solution is reduced by adding nitroethane to a formaldehyde solution at 50 ° C., the pH being maintained at 9.5 by adding a 15 N sodium hydroxide solution. The concentration of the solution is then adjusted to 0.95 moles / kg of nitro derivative and 0.97 H₂SO₄ equivalent / kg.

One operates on a mercury cathode (amalgamated copper) at 30 ° C; the current density is 10 A / dm2 on the cathode and 9.6 A / dm2 on the diaphragm.

After electrolysis, a solution is obtained containing 0.831 mole / kg of amino alcohol and 0.867 H₂SO₄ equivalent per kg, from which the pure amino alcohol is very easily extracted by EED and evaporation to dryness. The conversion of the nitro derivative into the amino derivative is quantitative and the efficiency of the cumulative current over the two stages of the electrochemical reduction is 80%; the energy consumption for electrolysis is 9.8 kwh / kg of finished product.

Example 6

We operate with the same cell equipped with an MEC diaphragm on a mixture rich in 2-nitro-1-butanol, having the following composition:

The reduction is carried out on the amalgamated copper cathode at 10 A / dm2 on the cathode and 9.6 A / dm2 on the diaphragm. The temperature of the catholyte is 30 ° C; it is an aqueous solution containing 1.075 mole / kg of nitro derivatives and 1.21 equ / kg H₂SO₄.

After electrolysis, an aqueous sulfuric solution is obtained containing 0.845 mole / kg of amino derivatives and 1.03 equ / kg H₂SO₄; a residue of hydroxylamine function is measured in the solution representing only 2.5 mol% of the organic products. By electro-electrodialysis EED and extraction of water, one will easily obtain an almost pure solution of amino alcohols.

The overall efficiency of the current is 75% and the energy expenditure for electrolysis is 9 kWh / kg of amino derivatives; the chemical yield, compared to the initial nitro derivatives, is 90.9%.

Example 7

An aqueous solution of tris (methyl hydroxy) nitro methane in sulfuric acid and containing 1.94 mole / kg of nitro derivative and 2.32 equ / kg is treated at 30 ° C. on the same cell as in the previous example. H₂SO₄; the current density is 13.9 A / dm2 on the cathode and 13.3 A / dm2 on the diaphragm.

An aqueous sulfuric solution is obtained containing 1.134 mol / kg of tris (methylhydroxy) amino methane, ie 137 g / kg and 1.391 eq / kg H₂SO₄; it is very easy to extract the pure amino alcohol by an EED treatment followed by dry evaporation; the overall efficiency of the electrolysis current is 65%.

Claims (13)

1. Process for manufacture of amino-alcohols. by electro-chemical reduction of nitro-alcohols in sulphuric medium in three stages, in which during the first stage of reduction into hydroxylamine, the reduction of the nitrated group is performed on a cathode constructed in a material having a high hydrogen overpressure by treating a sulphuric solution of the nitrated derivative, the reaction being conducted on a cathode the voltage of which is moderately electro-negative; during the second stage of reduction into amino-alcohol, the electronegative voltage is higher as an absolute value; and during the third stage the sulphuric solution of amino-alcohol obtained is subjected to an operation of purification by electro-electrodialysis and removal of water.
2. Process for manufacture of amino-alcohols, as claimed in claim 1, characterised in that the nitro-alcohol subjected to the electrochemical reduction is represented by the formula

   

   in which R₁ and R₂ together or separately are hydrogen, a hydroxyalkyl group such as hydroxymethyl, a linear or branched aykyl group, in particular methyl, ethyl, propyl or containing a number of carbon atoms greater than 3.
3. Process for manufacture of amino-alcohols as claimed in claim 2, characterised in that the nitro-alcohol is 2-nitro-2-methyl-1-propanol, 2-nitro-2-methyl-1,3-propanediol, 2-nitro-1-butanol, tris (hydroxymethyl) nitromethane.
4. Process for manufacture of amino-alcohols as claimed in any of claims 1 to 3, characterised in that the nitrated derivative raw material used is in the form of the raw solution obtained by reaction of formaldehyde and the corresponding nitroparaffin.
5. Process for manufacture of amino-alcohols, as claimed in any of claims 1 to 4, characterised in that the cathodic solution subjected to the electrolysis has a composition such that the molar ratio H⁺/R-NO₂ is between 1 and 1.5, its temperature being fixed at a value of between 20 and 100°C.
6. Process for manufacture of amino-alcohols as claimed in any of claims 1 to 5, characterised in that the electrochemical reduction of the nitro-alcohols is implemented in an apparatus in which the anodic and cathodic compartments are separated by a cation exchange membrane.
7. Process for manufacture of amino-alcohols as claimed in any of claims 1 to 5, characterised in that the electrochemical reduction of the nitro-alcohols is implemented in an apparatus in which the anodic and cathodic compartments are separated by an anion exchange membrane.
8. process for manufacture of amino-alcohols as claimed in any of claims 1 to 5 and 7 characterised in that the final purification by electro-electrodialysis is performed in an apparatus the anodes and cathodes of which are constituted by materials having respectively a slight overpressure of oxygen and of hydrogen and in which the anodic and cathodic compartments are separated by a diaphragm constituted by an anion exchange membrane.
9. Process for manufacture of amino-alcohols as claimed in any of claims 1 to 8, characterised in that the electro-electrodialysis apparatus is fed on the cathode side with the sulphuric amino-alcohol solution and on the anode side with pure water.
10. Process for manufacture of amino-alcohols as claimed in any of claims 1 to 5 and 7 characterised in that the final purification of the catholyte is performed in the electrochemical reactor by replacing the concentrated solution of sulphuric acid contained in the anodic compartment by pure water.
11. Process for manufacture of amino-alcohols as claimed in claim 1, characterised in that the cathodes are constructed in a material having a high overpressure of hydrogen, such as mercury, in amalgam form, lead, zirconium.
12. Process for manufacture of amino-alcohols as claimed in claim 1, characterised in that the aqueous concentrated sulphuric solution constituting the anolyte is collected and reused in a subsequent operation.
13. Process for manufacture of amino-alcohols as claimed in claim 1, characterised in that the diluted aqueous sulphuric solution collected on the anode side is reused on the cathode side after recharging with nitro-alcohol.