GB2223031A - Electrochemical dehydration of nitric acid to dinitrogen pentoxide - Google Patents

Abstract

In an electrolytic process for the production of dinitrogen pentoxide in an electrochemical cell, dinitrogen pentoxide is anodically produced, hydrogen is evolved at the cathode and oxygen is evolved at the anode. Substantially no nitric oxide (N2O4) is produced. The process may be carried out in a cell comprising an anolyte chamber with the anolyte in contact with an ion exchange membrane, a catholyte chamber with the catholyte in contact with a cation exchange membrane, and further comprising a third chamber between the anolyte and catholyte chambers with the electrolyte therein in contact with the ion exchange membrane and the cation exchange membrane.

Description

Dehydration of Nitric Acid to Dinitrogen Pentoxide The present invention relates to the dehydration of nitric acid and in particular to the electrochemical dehydration of nitric acid to form dinitrogen pentoxide.

Dinitrogen pentoxide which exists in anhydrous acids in a dissolved state may be expressed as a salt of the form NO2 NO3- and is known to be a useful nitration agent, providing an alternative to nitric acid or nitric acid and sulphuric acid mixtures.

Furthermore the use of dinitrogen pentoxide enables nitration reactions to be undertaken which would not otherwise occur at an acceptable rate or yield, and further enables nitration reactions to be carried out in aprotic conditions when a suitable organic solvent is employed.

Dinitrogen pentoxide has conventionally been produced, on a laboratory scale only, by the dehydration of nitric acid with phosphorus pentoxide, the reaction of nitric oxide with ozone or the reaction of lithium nitrate with bromine pentafluoride. However none of these routes is viable commercially.

In US Patent Nos. 4,432,902 and 4,525,252 there is described a process in which dinitrogen pentoxide is produced by electrochemical dehydration of nitric acid.

In this process, in a cell divided by a membrane, nitric oxide (N204) is cathodically evolved, cycled to the compartment of the electrochemical cell containing the anode and anolyte, and anodically oxidised to dinitrogen pentoxide according to equation [1]: N204+ 2HNO3 = 2NO2++ NO3- + 2Htf 2e [1].

The product accumulates in the anolyte and water accumulates in the catholyte, which water must be continuously distilled off. For the product anolyte to be free from nitric oxide, it must be reacted to completion at a controlled potential. By this method concentrations of up to 23 wt% dinitrogen pentoxide in anhydrous nitric acid have been prepared.

However, this process is disadvantageous in that the cathodic evolution of nitric oxide also results in the formation of water according to equation [2]:- 2e-+4H+ + 2NO3- z N204 + 2H20 [2] The water thus formed will then undergo a parasitic reaction with the anodically formed nitronium ion component of the dinitrogen pentoxide, according to equation (3]:-

Thus the nitric oxide generated needs to be stripped from the catholyte, dried, and condensed prior to being recycled to the anolyte, in order to minimise the amount of water entering the anolyte.Furthermore, the electromigration and diffusion of nitronium ion towards the cathode and the diffusion of water across the membrane from catholyte to anolyte results in a current efficiency of only approximately 60%.

It is an object of the present invention to provide a process for the production of nitrogen pentoxide by the electrolytic dehydration of nitric acid which process produces substantially no nitric oxide.

It is another object of the present invention to provide a process in which dinitrogen pentoxide is anodically produced and in which oxygen is evolved at an anode and hydrogen is evolved at a cathode.

It is yet another object of the present invention to provide a process for the production of dinitrogen pentoxide in which the evolved hydrogen is substantially free from nitric oxide contamination.

It is a further object of the present invention to provide a process for the production of dinitrogen pentoxide which obviates the need to react the anolyte to completion, and which obviates the need for potential control.

Accordingly the present invention provides a process for the production of dinitrogen pentoxide by the electrolytic dehydration of nitric acid in an electrochemical cell wherein oxygen is evolved at an anode, hydrogen is evolved at a cathode and dinitrogen pentoxide is anodically produced, and by means of which process substantially no nitric oxide is produced.

In a first embodiment of the invention the electrochemical cell comprises (i) a first chamber bounded by the walls of the electrochemical cell and a first ion exchange membrane, which first chamber includes an anode and a first electrolyte, which first electrolyte includes nitric acid and is in contact with said first ion exchange membrane and said anode (ii) a second chamber bounded by the walls of the electrochemical cell and a second ion exchange membrane, which second ion exchange membrane is a cation exchange membrane, which second chamber includes a cathode and a second electrolyte different from said first electrolyte, which second electrolyte is in contact with said second ion exchange membrane and said cathode (iii) a third chamber bounded by the walls of the electrochemical cell and said first and second ion exchange membranes which third chamber includes a third electrolyte which third electrolyte is in contact with said first and second ion exchange membranes.

In a second embodiment of the invention nitrate ion is substantially excluded from said second chamber.

In a third embodiment of the invention the first and third electrolytes are substantially the same.

In a fourth embodiment of the invention said first ion exchange membrane is an anion exchange membrane.

In a fifth embodiment of the invention said second electrolyte is acidic, substantially inert to oxidation at the anode, to reduction at the cathode and to nitration and is substantially non-oxidising, nonreducing and non-dehydrating and further is preferentially soluble with respect to nitric acid in said second ion exchange membrane and comprises large anions.

In a sixth embodiment of the invention said second electrolyte is a partially- or fully- halogenated alkane sulphonic or carboxylic acid.

In a seventh embodiment of the invention said second electrolyte is trifluoromethane sulphonic acid or trifluoroacetic acid.

In an eighth embodiment of the invention nitronium ion in said third chamber is titrated with water to form nitric acid and excess nitric acid in said third chamber is transferred to said first chamber.

In a ninth embodiment of the invention hydrogen evolved at the cathode is passed through a water trap to convert any contaminating nitric oxide to nitric and nitrous acids.

In a tenth embodiment of the invention, nitronium ion in said third chamber is titrated with water from said water trap.

In the present invention, NO2+ is not formed anodically via Reaction (1], but anodically by: 2HNO3 - NO2+ NO3-+2H+ + 1/2 2(g) + 2e [4] At the cathode, hydrogen is evolved, according to reaction [5] 2e~ + 2H+ = H2(g) [5] For hydrogen evolution to be made possible, the electrolyte in the vicinity of the cathode must be altered in such a way that the nitrate ion is substantially excluded.The net process of the invention is then: 2HNO3NO2+ NO3 + 1/2 O2(g)+H2(g) [6] (rather than 2HNO3 > NO2+ NO3 + H2O (distilled off) [7] as in the process of US 4,432,902 and US 4,525,252) In the process of the present invention, hydrogen ion is thus transported from the anode to the cathode across the cell and thus provides the required ionic conduction pathway.

Should any nitrate ion be present in the catholyte, it will reduce, as the standard potential for Reaction [2] is 0.94V, far higher than Reaction [5] at O.OV. Any nitric acid produced can then also dissociate in the catholyte (and elsewhere) via the equilibrium:-

Nitric oxide has a reputation for being difficult to contain in conventional plastic and elastomer electrolytic cell hardware. Thus contamination by nitric oxide of the hydrogen evolved at the cathode must be minimised.

For the process of the present invention, utilising Reactions [4] and [5], to be successful, a complete change of electrolyte from anolyte to catholyte is necessary, as is also the rigorous exclusion of nitrate ion from the catholyte.

It is desired that the catholyte has the following properties: - It should be an acid so that hydrogen ions are available for the cathodic process.

- It should be inert to reduction at the cathode.

- It should be inert to oxidation at the anode.

- It should be inert to nitration.

- It should be inert in nitrations, therefore nonoxidising, non-reducing, and non-dehydrating.

- The anion of the catholyte should be large in order to further restrict its diffusion and electromigration through ion exchange membranes.

- The catholyte should be preferentially soluble in the adjacent ion exchange membrane in order to increase the anion content of the membrane and further restrict the diffusion of nitrate ions.

- The catholyte should boil at a temperature sufficiently different from anhydrous nitric acid in order to separate them by distillation in either electrolyte reprocessing or in measures to minimise nitric acid in the catholyte.

- The catholyte should preferably be commercially available in an anhydrous form.

A preferred family of compounds having the above properties is the perfluorinated alkane sulfonic acids.

Trifluoromethanesulfonic acid is particularly preferred. Alkane sulfonic acids and other fully- or partially- halogenated sulfonic or carboxylic acids may also be used, of which trifluoroacetic acid is preferred.

Three principles (a,b and c, below) are utilised in the invention to achieve the change of electrolyte across the cell from nitric acid to one of the catholytes above and to enable the evolution of hydrogen as free as possible of nitric oxide contamination.

a) Ion exchange membranes are used. These may be cation exchange membranes which have a fixed anionic functionality. The fixed anionic functionality retards the ingress of similarly charged nitrate anions and thus restricts their diffusion to the catholyte.

Nafionr, a product of E.I. Du Pont de Nemours and Company (Inc.) is a preferred example. Nation is a highly stable perfluorinated polymeric material having appended sulfonic acid functionality up to equivalent concentrations of 1 to 2 moles per litre. The ion exchange membrane in contact with the anolyte of the first chamber, and with the electrolyte of the third chamber is preferably an anion exchange membrane of fixed cationic functionality. Tosflex, a product of Tosoh Corporation, Japan, is particularly preferred.

b) The electrolyte selected for the catholyte is chosen on the basis of stability to nitronium ion and hydrogen and oxygen evolution and for similar structural properties to the ion exchange membrane, such that it would be expected to achieve a higher solubility in the ion exchange membrane than would nitric acid. The preferential presence of catholyte anions in the membrane further restricts the diffusion of nitrate anions. In the case of Nafion, above, a perfluorinated alkane sulfonic acid is a preferred choice of catholyte. The higher the molecular weight of the perfluorinated alkane, the greater the preferential solubility would be expected to be.

c) Measures [a] and [b] alone are insufficient for the adequate exclusion of the nitronium ion.

The third electrolyte chamber is therefore formed by the inclusion of two ion exchange membranes in the electrolytic cell. The chamber between the two ion exchange membranes is filled with a third electrolyte, which is preferably identical to the anolyte.

Nitronium ion formed at the anode will electromigrate toward the cathode, and in the case of use of cationic exchange membranes, will readily penetrate the membranes and react as would the nitrate ion, to produce nitric oxide according to: 2e + 2N02+v- N204 [9] This would result in contamination of the hydrogen evolved. With a central chamber added, controlled additions of water may be made to this compartment in order to convert the N02+ passing through the anolyteside ion exchange membrane to nitric acid via Reaction [3] before it may penetrate the second ion exchange membrane protecting the catholyte. In this case the central compartment would accumulate nitric acid on a continuous basis which may be returned to the anolyte.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example only, to the following Figure 1 which represents a process flow diagram of the process of the present invention, In Figure 1 there is shown an electrochemical cell 19 bounded by walls 8 which electrochemical cell 19 includes a first chamber 20, which chamber 20 contains an anode 1. The anode 1 is in contact with anolyte 5 and the anolyte 5 is in contact with an ion exchange membrane 7. The ion exchange membrane 7 is preferably an anion exchange membrane. The electrochemical cell 19 further includes a second chamber 21, which chamber 21 contains a cathode 3. The cathode 3 is in contact with catholyte 6, and the catholyte 6 is in contact with a cation exchange membrane 22. The electrochemical cell further includes a third chamber 23, which chamber 23 contains a third electrolyte 10.

The third electrolyte 10 is in contact with the ion exchange membrane 7 and the cation exchange membrane 22.

The anode and cathode preferably comprise a material selected from glassy carbon, graphite, other forms of carbon, platinum, so called dimensionally stable electrode materials (mixed noble metal oxides and valve metal oxides on titanium, tantalum or niobium substrates), Ebonex (Ti407), or platinum on titanium, tantalum or niobium. The anode may further preferably comprise lead dioxide or tin dioxide, and the cathode may further preferably comprise nickel or stainless steel.

The anolyte 5 which includes nitric acid is fed from a storage tank 17 to the first chamber 20 via a heat exchanger 16a by means of which the anolyte 5 is cooled. The anolyte 5 and the product dinitrogen pentoxide exit the first chamber 20 to a storage tank 18 which tank 18 is cooled by a heat exchanger 16b.

The heat exchangers 16a and 16b are each connected to a cooling means 15. The electrochemical cell 19 may also be cooled. It is preferred that the storage tanks 17,18 and the electrochemical cell 19 are maintained at a temperature in the range -10 to +100C. Oxygen formed at the anode 1 is vented via a line 2 and an alkaline scrubber 14b.

Hydrogen, which may contain trace nitric oxide, is evolved at the cathode 3 and is passed via a line 4 to a water trap 13. In the water trap 13 the nitric oxide, if present, dissolves to form nitric and nitrous acids. The hydrogen is vented via an alkaline scrubber 14b. Any remaining contaminating nitric oxide is reacted to nitrate and nitrite in the alkaline scrubber 1 4b.

Water containing nitric and nitrous acids from the water trap 13 is fed via a line 11 and a valve 24 to the third chamber 23. Nitronium ion which is electromigrating through the third chamber 23 from the anolyte 5 towards the cathode 3 is thus titrated with water from the water trap 13 and forms nitric acid.

The excess nitric acid thus formed, together with the trace nitrous acid from the water of the water trap 13 is fed via a line 12 to the anolyte storage tank 17.

The trace nitrous acid is oxidised to nitric acid at the anode 1.

The product dinitrogen pentoxide which is dissolved in nitric acid may be equilibrated with an organic solvent in order to transfer the dinitrogen pentoxide to the organic phase.

The process of the present invention is substantially closed, except for anodic oxygen and cathodic hydrogen which are vented via the alkaline scrubbers 14a and 14b respectively. The rate of nitrate diffusion from the third chamber 23 is equal to the rate of nitric oxide formation at the cathode 3.

Only if the rate of nitric oxide formation exceeds the amount which can be absorbed by the water trap 13 (as determined by solubility and rate of water use) will any excess need to be reacted to nitrate and nitrite in the alkaline scrubber 14b.

An important consideration in the present invention is to ensure that- sufficient nitrate ion is excluded from the catholyte so that the management of the nitric oxide content of the evolved hydrogen does not become a problem. It is also desirable that the loss of catholyte into the central compartment (and anolyte) is minimised.

Factors influencing the nitrate ion concentration in the catholyte include: (a) forces leading to accumulation of nitrate ion (i) osmosis of higher concentration to lower concentrations (ii) diffusion (b) forces retarding the rate of accumulation of nitrate ion (i) electromigration of nitrate toward the anolyte (ii) the presence of the cation exchange membrane (iii) the equivalent weight of the cation exchange membrane (iv) the preferential solubility of the catholyte in the membrane leading to an increase of equivalent weight (c) forces leading to depletion of nitrate ion (i) electromigration of nitrate toward the anolyte (ii) low mobility of large catholyte anions forces nitrate to preferentially electromigrate (iii) reaction of nitrate to nitric oxide Factors influencing the concentration of catholyte in the anolyte include: (a) forces leading to loss from the catholyte (i) diffusion (ii) electromigration towards the anolyte (b) forces retarding the rate of loss from the catholyte (i) the presence of a cation exchange membrane (ii) the equivalent weight of the cation exchange membrane (iii) the preferential solubility of the catholyte in the cation exchange membrane (iv) osmosis (v) the low mobility of the large catholyte anion leading to preferential nitrate ion electromigration (vi) a high anticipated viscosity of the catholyte (which may be enhanced by use of addition agents) (c) forces leading to depletion (i) preferential solubility The energy requirements of the process are not expected to be economically constraining due to the high-value-added nature of dinitrogen pentoxide.The current efficiency is improved with respect to the process of US 4,432,902 and US 4,525,252 in that no water is produced within the cell, and thus this is not a source of nitronium ion loss. Certainly, greater than 80% is achievable, dependent on the anolyte flow rate and concentration of product desired. Current efficiency losses arise only from the diffusion and electromigration of nitronium ion to the third chamber.

At a 1 molar target nitronium ion concentration, and 20 molar for hydrogen ion in anhydrous nitric acid, if one can assume equal diffusion and electromigration, a 95% current efficiency results. The current efficiency of the process of US 4,432,902 and US 4,525,252 is 55-65%.

The cell voltage at 200 mA/cm2 will be composed of 2.5V vs. the Standard Hydrogen Electrode (S.H.E.) at the anode and -0.5V vs. S.H.E. at the cathode, with resistive losses of 0.6V in the two membranes and 0.1V in each of the three electrolyte compartments. (The resistivity of anhydrous nitric acid is 30 ohm-cm.) Total cell voltage at 200 mA/cm2 will thus be in the vicinity of 4.OV. At 95% current efficiency, 4.0V shows dinitrogen pentoxide to be produced at 2.0 kwh/kg.

Reagent grade nitric acid contains 10 wt.% water, which will ultimately electrolyse away to hydrogen and oxygen. However, until this takes place and the limiting value of water content is achieved (determined by the equilibrium: 2HNO3 H2Oo NO3- t NO2+ [10]) power consumption will be greater than 2 kwh/kg through start-up.

Energy requirements for cooling the electrochemical cell and the product storage vessel must also be considered.

Thus it can be seen that by the process of the present invention, in contrast to the process of US 4,432.902 and US 4,525,252, nitric oxide is produced only at levels which may be considered as effluent and there is accordingly no requirement for afterprocessing of nitric oxide, such as stripping, drying, compression and condensation. Moreover, the electrochemical cell in the process of the present invention need not be optimised in order completely to react nitric oxide from the anolyte so that the product stream contains no unreacted material. Furthermore, since in the present invention no nitric oxide is present in the anolyte, the anolyte may be stripped of pure dinitrogen pentoxide by means of organic solvents.

The use of organic solvents for the stripping of the dinitrogen pentoxide further enables the inventory of nitric acid to be greatly reduced. Since in the present invention water is not produced by the reactions employed, the need to remove water from the catholyte is obviated. In comparison with the process of US 4,432,902 and US 4,525,252, both a higher current efficiency and a higher current density are achieved.

The higher current density results in a better utilisation of the cell hardware, since, because the process is solvent decomposition, no limiting current considerations arise. Further, the process of the present invention does not require potential control, the provision of which is expensive.

Example It is desired to produce 1 molar dinitrogen pentoxide at a rate of 10,000 litres/day. At 2:1 stoichiometry, such a rate of production would be suitable, for example, for the nitration of 1770 kg/day of the intermediate DADN to 1184 kg/day of the explosive HMX, using the known 80% yield.

At 95% current efficiency, a current of 23,500A is required. At a current density of 200mAcm'2 an area of the anode and of the cathode of 11,750cm2 is required.

Using third chamber and catholyte gaps of 5mm each, and an additional 10% volume for associated plumbing, 12.9 litres of catholyte are required.

At 95% current efficiency, 526 moles/day of nitronium ion are lost to the third chambers of the cells, and must be reacted with water to nitric acid.

Thus, 9.58 litres/day of water must be added to the third chambers. This results in the formation of 22.2 litres/day of nitric acid in the third chambers, which must be returned to the anolyte.

Assuming, in the absence of depleting processes a 1 molar nitrate ion concentration could diffuse into the catholyte per day gives (26.8 Ah/mole) (1M/l) (12.9 moles) = 346Ah/day.

The current efficiency at the cathodes is thus given by: 100(1-346 Ah/day) (23,500A) (24 hr/day) = 99.9% 9.5 litres of feed water should dissolve 12.9 moles of N204 in the water scrubber, to give a closed system.

For 12.9 litres of catholyte and 10,000 litres of nitric acid anolyte 20 (for example 10 day) replenishment cycles could be achieved before a 2.6% content of catholyte in nitric acid would be reached.

At this point, reprocessing to the pure constituents through distillation could be justified. The catholyte, anolyte and the electrolyte of the third chamber are preferably distilled or vacuum distilled in order to separate them into pure components for the restart of the process.

Claims (15)

1. A process for the production of dinitrogen pentoxide by the electrolytic dehydration of nitric acid in an electrochemical cell wherein oxygen is evolved at an anode, hydrogen is evolved at a cathode and dinitrogen pentoxide is anodically produced, and by means of which process substantially no nitric oxide is produced.

2. A process according to claim 1 wherein the electrochemical cell comprises (i) a first chamber bounded by the walls of the electrochemical cell and a first ion exchange membrane, which first chamber includes an anode and a first electrolyte, which first electrolyte includes nitric acid and is in contact with said first ion exchange membrane and said anode (ii) a second chamber bounded by the walls of the electrochemical cell and a second ion exchange membrane, which second ion exchange membrane is a cation exchange membrane, which second chamber includes a cathode and a second electrolyte different from said first electrolyte, which second electrolyte is in contact with said second ion exchange membrane and said cathode (iii) a third chamber bounded by the walls of the electrochemical cell and said first and second ion exchange membranes which third chamber includes a third electrolyte which third electrolyte is in contact with said first and second ion exchange membranes.

3. A process according to claim 2, wherein nitrate ion is substantially excluded from said second chamber.

4. A process according to claim 2 or 3 wherein said first and third electrolytes are substantially the same.

5. A process according to any of claims 2 to 4 wherein said first ion exchange membrane is an anion exchange membrane.

6. A process according to any of claims 2 to 5 wherein said second electrolyte is acidic, substantially inert to oxidation at the anode, to reduction at the cathode and to nitration and is substantially non-oxidising, non-reducing and nondehydrating.

7. A process according to any of claims 2 to 6 wherein said second electrolyte is preferentially soluble with respect to nitric acid in said second ion exchange membrane.

8. A process according to any of claims 2 to 7 wherein said second electrolyte comprises large anions.

9. A process according to any of claims 2 to 8 wherein said second electrolyte is a partially- or fully- halogenated alkane sulphonic or carboxylic acid.

10. A process according to claim 9 wherein said second electrolyte is trifluoromethane sulphonic acid or trifluorocetic acid.

11. A process according to any of claims 2 to 10 wherein nitronium ion in said third chamber is titrated with water to form nitric acid.

12. A process according to claim 11 wherein excess nitric acid in said third chamber is transferred to said first chamber.

13. A process according to any of claims 2 to 12 wherein hydrogen evolved at the cathode is passed through a water trap to convert contaminating nitric oxide to nitric and nitrous acids.

14. A process according to claims 11 and 13 wherein nitronium ion in said third chamber is titrated with water from said water trap.

15. A process substantially as hereinbefore described with reference to and as illustrated in Figure 1.