METHOD FOR PRODUCING AMMONIA NITROGEN

Description

The present invention relates to a method for producing ammoniacal nitrogen by feeding an electrochemical cell with dinitrogen (N₂), said electrochemical cell comprising at least one working electrode immersed in a composition comprising at least one compound (I) containing at least one element from group 13 of the periodic table, and at least one counter electrode; and an electrochemical cell for the reduction of dinitrogen into ammoniacal nitrogen.

Nitrogen (N) plays an essential role in the composition of living matter. In particular, it is a major constituent of amino acids, proteins including enzymes, and the nucleic acids that make up DNA and RNA. It is also an essential nutrient for growing crops. However, even though nitrogen is very abundant on the earth's surface (there is more nitrogen than carbon, hydrogen, and phosphorus combined in the biosphere, hydrosphere, and atmosphere), it is essentially present in the form of the extremely stable gas known as dinitrogen (N₂). Humans can therefore only marginally benefit from such abundance. Only microorganisms such as Rhizobia, involved in symbiotic nitrogen fixation by legumes, are capable of using this form of nitrogen and turning it into ammoniacal nitrogen and then organic nitrogen, which in turn can be used by other living beings and transformed into other forms of reactive nitrogen.

As a reactive nitrogen, ammonia is a key molecule with major applications in the agricultural sector, and also has the ability to store energy (in particular hydrogen). Ammonia in liquid form can also be used as a fuel to replace fossil-based Liquefied Petroleum Gas (LPG).

Until the 19^th century, ammonia was produced by distilling liquid manure or extracting domestic blackwater. After the second half of the 19^th century, it was obtained as a by-product of the manufactured gas industry (utility gas). It wasn't until the beginning of the 20^th century (1909-1913) that massive industrial production of ammonia began using the Haber-Bosch process. This process enables ammonia to be synthesized on an industrial scale from dinitrogen and dihydrogen (H₂) in the presence of a solid iron-based catalyst.

Ammonia production of around 200 million tons/year using this process has outstripped symbiotic fixation on a global scale since the end of the 20^th century, and is still used today. However, this Haber-Bosch process has the following disadvantages: it uses high pressures and temperatures, for example pressures of between 100 and 300 bar and temperatures of between 30° and 550° C., making this process extremely energy-intensive, requiring centralized and secure production, and implying high operating and transport costs. Furthermore, such a process generates very large quantities of carbon dioxide (around 1.5% of global CO₂ production), leading to environmental problems, and its yield remains low (20%).

In view of the ever-increasing demand for ammonia, and the environmental issues associated with its industrial production, other methods have been proposed, in particular using more efficient catalysts to reduce the temperatures and pressures involved. In particular, U.S. Pat. No. 6,037,459 describes a method comprising a first step of bringing dinitrogen into contact with a compound having the formula M(NR₁R₂)₃ wherein M is a transition metal (e.g. molybdenum), R₁ and R₂ are selected from tertiary alkyl groups, phenyl groups and substituted phenyl groups, to form a metal complex with nitride ligands; and a second step of reducing said complex in the presence of a hydrogen source to form ammonia. This method is carried out under ambient temperature and pressure conditions. In U.S. Pat. No. 6,037,459, only the yields of NΞN triple bond cleavage are described, and the formation of ammonia (second step) is not demonstrated.

Alternative methods based on biocatalysis, photocatalysis, or electrocatalysis have also been described. In particular, Li et al. [Adv. Mater., 2017, 29, 1700001, 1-6]propose the use of an electrode material comprising amorphous gold nanoparticles supported by graphite oxide and reduced cerium dioxide CeO_x-RGO. The Li et al. method uses an electrochemical cell fed continuously by a flow of dinitrogen, said cell comprising a saturated Ag/AgCl/KCl reference electrode, a platinum counter electrode, a working electrode consisting of said electrode material deposited on carbon paper, a pre-treated Nafion 211® membrane, and an electrolyte consisting of dilute hydrochloric acid. Said electrode material is used as a cathodic electrocatalyst. It therefore acts as a reducer of N₂, which then reacts with the H⁺ protons of the electrolyte on the surface of said material to form NH₃. Applying an electrical potential to said material is a means of reducing the activation barrier for the N₂ reduction reaction (NRR). However, the yields remain low, and the raw materials (noble metals such as gold or ruthenium) are rare and extremely expensive.

Moreover, the reduction of dinitrogen competes with the reduction of protons (H⁺) to dihydrogen H₂.

The aim of the present invention is therefore to overcome the disadvantages of the prior art and, in particular, to provide a simple, economical, industrializable method for producing ammoniacal nitrogen, using abundant raw materials, which can preferably be recycled, which reduces carbon emissions and which implements relatively mild reaction conditions.

The first object of the invention is a method for producing ammoniacal nitrogen, characterized in that it comprises at least the following steps:

• • i) a step of feeding an electrochemical cell with dinitrogen (N₂), said electrochemical cell comprising at least one working electrode immersed in a composition maintained under stirring, and at least one counter electrode, said composition comprising at least one electrolyte solution and at least one compound responding to a following formula (I): R¹R²MY (I), wherein:
    • M is an element from group 13 of the periodic table, preferably selected from boron, aluminum, and mixtures thereof,
    • R¹ and R², identical or different, are chosen from an alkyl group, an aryl group, an aryl-alkyl group, an —OR group, and an —SR group, R being an alkyl group, an aryl group, or an aryl-alkyl group, and
    • Y is a group selected from a halogen −X, an −OR³ group, an −SR³ group, a triflate group, a mesylate group and a triflimidate group, R³ being an alkyl group, an aryl group or an aryl-alkyl group,
  • ii) a step of applying a potential difference between the working electrode and the counter electrode, or of applying a potential or a current to the working electrode, and
  • iii) a step of acid hydrolysis of the composition.

The method of the invention is simple, easy to implement, economical, and enables ammoniacal nitrogen to be obtained under relatively mild reaction conditions. In particular, the use of a compound of formula (I) as defined above in a reducing medium (that is, thanks to the supply of electrons from the working electrode) enables the activation of the triple bond of dinitrogen and the formation of intermediate species which then lead to ammoniacal nitrogen by hydrolysis. Last but not least, the method is industrializable, uses abundant raw materials that can be recycled, and makes it possible to reduce environmental impact.

Step i)

The compound of formula (I) R¹R²MY

According to the invention, boron is particularly preferred as the element M.

Groups R¹ and R²

The compound of formula (I) R¹R²MY is not a radical compound.

In the compound of formula (I), R¹ forms a single covalent bond with the element M and R² forms a single covalent bond with the element M.

R¹ and R², identical or different, are chosen from an alkyl group, an aryl group, an aryl-alkyl group, an —OR group, and an —SR group, R being an alkyl group, an aryl group, or an aryl-alkyl group.

An alkyl group as group R¹ and/or R² may be linear or branched, cyclic or non-cyclic. The alkyl group may comprise from 1 to 14 carbon atoms, and preferably from 2 to 10 carbon atoms. An alkyl group is preferably selected from ethyl, propyl, isopropyl, cyclohexyl, bicyclo[2.2.1]-2-heptyl and isopinocamphenyl. Among such groups, any one of cyclohexyl, bicyclo[2.2.1]-2-heptyl, or isopinocampheyl is particularly preferred.

The alkyl group as group R¹ and/or R² may comprise one or more heteroatoms, such as an oxygen atom, or a sulfur atom, with the proviso that one carbon atom of the alkyl group is directly bonded to the element M of formula (I) and none of the heteroatom(s) present in the alkyl group is directly covalently bonded to another heteroatom.

An aryl group as an R¹ and/or R² group may be substituted or unsubstituted. The aryl group may comprise from 6 to 30 carbon atoms, and preferably from 6 to 18 carbon atoms. An aryl group is preferably selected from a phenyl group, a —C₆F₅ group, a 2,4,6-(Me)₃-C₆H₂ group and a 2,4,6-(iPr)₃-C₆H₂ group. Among such groups, any of the 2,4,6-(Me)₃-C₆H₂ or 2,4,6-(iPr)₃-C₆H₂ groups is particularly preferred.

The aryl group as an R¹ and/or R² group may comprise one or more heteroatoms, particularly when the aryl group is substituted (that is, in the substituents of said aryl group), such as an oxygen atom or a nitrogen atom, it being understood that a carbon atom of the aryl group is directly bonded to the element M of formula (I).

An aryl-alkyl group as an R¹ and/or R² group is a group comprising at least one alkyl group and at least one aryl group which are linked directly by a covalent carbon (of the aryl group)-carbon (of the alkyl group) bond, or via an oxygen atom or a nitrogen atom, the aryl and alkyl groups being as defined above for the R¹ and R² groups. The alkyl-aryl group can be directly bonded to the element M of formula (I) via a carbon atom of the aryl group or via a carbon atom of the alkyl group.

An R alkyl group of the —OR or —SR group can be linear or branched, cyclic or non-cyclic. The R alkyl group may comprise from 1 to 10 carbon atoms, and preferably from 1 to 4 carbon atoms.

An R aryl group of the —OR or —SR group may be substituted or unsubstituted. The R aryl group may comprise from 6 to 30 carbon atoms, and preferably from 6 to 18 carbon atoms. An R aryl group is preferably selected from a phenyl group, a naphthyl group, an anthracenyl group or a pyrenyl group.

An R aryl-alkyl group of the —OR or —SR group is a group comprising at least one alkyl group and at least one aryl group which are linked directly by a covalent carbon (of the aryl group)-carbon (of the alkyl group) bond or via an oxygen atom, or a sulfur atom, the aryl and alkyl groups being as defined above for the R group.

The R¹ and R² groups may be covalently linked, in particular via a carbon-carbon bond, to form a divalent group, said R¹ and R² groups being as defined above. In this embodiment, the divalent group does not form a planar ring with the element M.

For example, the divalent group may be an alkyl group (that is, R¹ and R² are alkyl groups), preferably a 9-bicyclo[3.3.1]nonane group.

According to one embodiment of the invention, R¹ and R², which may be identical or different, are selected from an alkyl group, an aryl group and an aryl-alkyl group.

According to a preferred embodiment of the invention, at least one of the R¹ and R² groups is an alkyl group, and more particularly preferred, both the R¹ and R² groups are alkyl groups.

According to a particularly preferred embodiment of the invention, R¹ and R₂ are identical.

The R¹ and R² groups of compound (I) are non-stabilizing groups. In other words, their function is to not stabilize the radical R¹R²M^o generated during the method, and consequently to make it more reactive towards dinitrogen N₂.

The group Y

Y is a group selected from a halogen —X, an —OR³ group, an —SR³ group, a triflate group (—OSO₂CF₃), a mesylate group (—OSO₂CH₃), and a triflimidate group (NTf₂ or N(SO₂CF₃)₂), R³ being an alkyl, aryl or aryl-alkyl group.

X is preferably a chlorine or bromine atom, and particularly preferably a chlorine atom.

An R³ alkyl group may be linear or branched, cyclic or non-cyclic. The R³ alkyl group may comprise from 1 to 10 carbon atoms, and preferably from 1 to 4 carbon atoms.

An R³ aryl group may be substituted or unsubstituted. The R³ aryl group may comprise from 6 to 30 carbon atoms, and preferably from 6 to 18 carbon atoms. An R³ aryl group is preferably selected from phenyl, 2,4,6-(Me)₃-C₆H₂, 2,4,6-(iPr)₃-C₆H₂, and naphthyl.

An R³ aryl-alkyl group is a group comprising at least one alkyl group and at least one aryl group which are linked directly by a covalent carbon (of the aryl group)-carbon (of the alkyl group) bond or via an oxygen atom, or a sulfur atom, the aryl and alkyl groups being as defined above for the R³ group.

Y is preferably a halogen X.

Group Y of compound (I) is a group with nucleofugal properties. In other words, its function is to facilitate the formation of the radical R¹R²M^o.

According to a particularly preferred embodiment of the invention, the compound of formula (I) is selected from dialkylchloroboranes, dialkylbromoboranes, dialkylchloroaluminium compounds, and dialkylbromoaluminium compounds, such as diisopinocamphenylborane halides, dicyclohexylborane or bis(bicyclo[2.2.1]-2-heptyl)borane, or haloboranes based on 9-borabicyclo[3.3.1]nonane.

The compound of formula (I) has the advantages of being readily available commercially or of being readily synthesizable.

The compound of formula (I) has the characteristics of a Lewis acid, that is, a chemical entity in which one of its constituent atoms has an electron gap.

The Working Electrode

The working electrode preferably comprises (or preferably consists of) at least one inert, electrically conductive material.

The electrically conductive inert material can be selected from carbon, platinum, stainless steel, and metal oxides.

The carbon can be vitreous carbon, pyrolytic carbon, or diamond doped, for example with boron or sulfur.

The electrically conductive material is preferably in the form of a porous material such as a foam (e.g. carbon foam), felt, mesh or fabric.

When the electrically conductive material is a metal oxide, it can be transparent.

Metal oxides that can be used as electrically conductive materials include indium-tin oxide.

In the invention, the expression “inert electrically conductive material” means that the electrically conductive material does not react chemically with the various elements present in the electrochemical cell.

Preferably, the electrically conductive inert material of the working electrode has a specific surface area, measured by the BET method, of at least 0.1 m²/g, and particularly preferably of at least 100 m²/g. This optimizes the contact surface between the working electrode and the composition, and in particular between the working electrode and the compound of formula (I).

During step i), the working electrode is fully or partially immersed in the composition. This helps to promote contact between the working electrode and the compound of formula (I), thus obtaining a large exchange surface.

The Counter Electrode

The counter electrode preferably comprises (or preferably consists of) at least one inert, electrically conductive material.

The electrically conductive inert material can be selected from carbon, platinum, stainless steel, and metal oxides.

The carbon can be graphite, pyrolytic carbon, or diamond doped, for example with boron or sulfur.

The electrically conductive material may be in the form of a porous material such as a foam (e.g. carbon foam), felt, mesh or fabric.

When the electrically conductive material is a metal oxide, it can be transparent.

Metal oxides that can be used as electrically conductive materials include indium-tin oxide.

Preferably, the electrically conductive inert material of the counter electrode has a specific surface area, measured by the BET method, of at least 0.1 m²/g, and particularly preferably of at least 100 m²/g. This makes it possible to optimize the exchange surface between the counter electrode and the electrolyte solution (multiple compartments) or the composition (1 single compartment).

The counter electrode is preferably fully or partially immersed in an electrolyte solution.

The electrolyte solution can be that of the composition. In this embodiment, in step i), the counter electrode is fully or partially immersed in the composition.

This promotes contact between the counter electrode and the composition, resulting in a large exchange surface.

The Electrolyte Solution of the Composition

The electrolyte solution carries the current through the electrochemical cell.

The electrolyte solution may comprise (or consist of) a combination of an organic solvent and a salt; or an ionic liquid.

An ionic liquid is well known to the person skilled in the art and can be considered as a molten salt at room temperature (e.g. 18-25° C.). The ionic liquid has an organic cationic component and acts as a solvent in the present invention, in the same way as a conventional organic solvent.

In the invention, organic solvent is taken to mean a conventional organic solvent, that is, one which is salt-free or not in the form of a salt.

The organic solvent is preferably selected from aprotic organic solvents, and in particular selected from ethers, carbonates, nitriles, amides and phosphoramides, such as THE (tetrahydrofuran), methyl-THF, N,N′-dimethylformamide, acetonitrile, benzonitrile, hexamethylphosphoramide, or propylene carbonate.

The salt is soluble in the organic solvent, preferably at concentrations of at least about 0.1 mol/L, for example at concentrations ranging from about 0.1 to 0.5 mol/L.

The salt is preferably an inert salt.

In the invention, the term “inert salt” means that the salt does not react chemically with the various elements present in the electrochemical cell, and in particular is not reduced at the working electrode within the operating potential range of the electrochemical cell.

The salt can be selected from alkali metal salts and quaternary ammonium salts, and preferably from quaternary ammonium salts.

Examples of alkali metal salts include lithium salts such as LiTFSI, LiPF₆, or LiClO₄.

Examples of quaternary ammonium salts include tetraalkylammonium salts such as tetra-n-butylammonium bis(trifluoromethanesulfonyl)imidate (TBATFSI), tetra-n-ethylammonium perchlorate, tetra-n-butylammonium hexafluorophosphate, or tetra-n-propylammonium tetrafluoroborate.

The ionic liquid can be selected from ammonium salts, imidazolium salts, phosphonium salts, pyrrolidinium salts, and piperidinium salts, and preferably from alkylammonium salts, alkylimidazolium salts, alkylphosphonium salts, alkylpyrrolidinium salts, and alkylpiperidinium salts.

The ionic liquid preferably comprises an anionic part of the bis(trifluoromethanesulfonyl)imidate type.

Examples of ionic liquids include triethylbutylammonium bis(trifluoromethanesulfonyl)imidate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imidate, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imidate, trimethylbutylammonium bis(trifluoromethanesulfonyl)imidate, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imidate, N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imidate, or 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imidate.

The ionic liquid is preferably water-immiscible. This facilitates subsequent purification.

The Reference Electrode

The electrochemical cell may also comprise a reference electrode. This can be used to control the potential of the working electrode and limit ohmic drops.

The reference electrode can be selected from a saturated silver nitrate electrode Ag/AgNO₃, a saturated calomel electrode (SCE), or a silver chloride electrode Ag/AgCl.

The reference electrode is preferably fully or partially immersed in an electrolyte solution.

The electrolyte solution can be that of the composition. In this embodiment, in step i), the counter electrode is fully or partially immersed in the composition.

In step i), the composition comprising the compound of formula (I) and the electrolyte solution is brought into contact with dinitrogen (N₂).

During step i), dinitrogen is fed to the electrochemical cell, preferably in continuous flow. The electrochemical cell is preferably supplied with dinitrogen by bubbling dinitrogen into the electrochemical cell composition.

Step i) is preferably carried out in a dry or anhydrous medium. In other words, step i) is preferably carried out in a glovebox or in a suitable device to avoid contact of the composition with air and/or moisture.

Contact with moisture and/or air leads to the formation of by-products such as H₂ and/or R¹R²MOMR¹R².

The dinitrogen fed to the electrochemical cell is preferably dry dinitrogen.

To protect the reaction medium from moisture and/or air, the electrochemical cell is preferably hermetically sealed.

Step i) can last from about 0.5 h to 20 h, and preferably from about 2 h to 12 h.

Step i) is preferably carried out at a temperature ranging from −80° C. to about 60° C., and particularly preferably from 0° C. to about 30° C.

In step i), the composition is kept under agitation, for example by means of a mechanical or magnetic stirrer; or by means of a flow or circulation electrochemical cell; or by means of the bubbling of dinitrogen alone into the composition in the electrochemical cell. Agitation promotes contact between the composition and the dinitrogen and the electrodes, and thus encourages the reaction in step ii).

Step i) can be carried out at a pressure ranging from around 1 bar to 200 bar, and preferably from 1 bar to 100 bar. A pressure of 1 bar is industrially advantageous.

Step i) is preferably carried out at atmospheric pressure.

The compound of formula (I) can have a molar concentration in the composition ranging from about 10⁻⁵ mol/L to 10⁻¹ mol/L, and preferably from about 10⁻³ mol/L to 10⁻² mol/L.

Step ii) Step ii) involves applying a potential or current to the working electrode, or a potential difference between the working electrode and the counter electrode.

The potential, the potential difference, or the current applied must be sufficient to enable reduction of compound (I) to the radical R¹R²M^o. The range of potential or current values may vary according to the type of working electrode used, the temperature, the nature of R¹ and/or R², the electrolyte solution, etc.

The potential difference applied between the working electrode and the counter electrode can range from about 2V to 50V, and preferably from about 3V to 20V.

This method of applying a potential difference between the working electrode and the counter electrode is particularly suitable when the electrochemical cell does not comprise a reference electrode.

The potential applied to the working electrode can range from about −4V to −1V, and preferably from about −3V to −2V, preferably relative to a saturated silver nitrate Ag/AgNO₃ reference electrode.

This method of applying a potential to the working electrode is particularly suitable when the electrochemical cell comprises a reference electrode.

The current applied to the working electrode can range from about 0.01 A to 10 A, and preferably from about 0.5 A to 1 A.

The working electrode and counter electrode are electrically connected to a source of voltage or current.

In step ii), the working electrode acts as an electron supplier. The electrons go directly into the solution upon contact with compound (I). The working electrode is then used to reduce the compound (I), which then reacts with dinitrogen to form one or more species based on nitrogen and on the element M, in particular of the following formula (II): N(MR¹R²)_3-xH_x, x being an integer from 0 to 3. The working electrode is therefore not involved in the chemical process of dinitrogen reduction as such.

The formation of one or more species based on nitrogen and the element M as defined above can be explained in particular by carrying out a radical chain reaction involving one or more radicals based on the element M which are sufficiently unstable to react with the nitrogen in the dinitrogen.

Compound (I), by virtue of its formula (I) and thus the definition of Y, M, R¹, and R², has the ability to activate the triple bond of dinitrogen in a reducing medium, and to minimize or even avoid dimerization of the radical R¹R²M^o.

Consequently, step ii) implements the electrochemical reduction of compound (I), which is totally different from the electrochemical methods of the prior art, which implement surface processes in which the working electrode acts as both electron supplier and catalyst.

Step ii) can be carried out under potentiostatic or galvanostatic conditions.

According to a particularly preferred embodiment of the invention, steps i) and

ii) are concomitant. In other words, a potential, a potential difference, or a current is applied while the electrochemical cell is being fed with dinitrogen.

The electrochemical cell used in step i) may comprise a single compartment containing the composition as defined in the invention, the electrodes and the dinitrogen; it may comprise two compartments C₁ and C₂, in particular so as to isolate the counter electrode from the working electrode. This avoids diffusion phenomena between the two electrodes; or may comprise three compartments C₁, C₂ and C₃, to ensure electrical contact between compartments C₁ and C₂, while insulating the counter electrode from the working electrode.

An electrochemical cell with two compartments C₁ and C₂ preferably comprises:

• • a compartment C₁ containing the composition as defined in the invention, the working electrode immersed (in whole or in part) in said composition, and the reference electrode immersed (in whole or in part) in said composition if the reference electrode exists,
  • a compartment C₂ containing the counter electrode immersed (in whole or in part) in an electrolyte solution, and
  • a membrane separating the two compartments C₁ and C₂.

The membrane can be a sintered glass, an ion exchange membrane such as a cationic membrane (e.g. Nafion®), or a polymer material such as a fluorinated organic polymer material.

The electrolyte solution of the compartment C₂ may comprise (or consist of) a combination of an organic solvent and a salt; or an ionic liquid.

The organic solvent is preferably selected from aprotic organic solvents, in particular selected from ethers, carbonates, nitriles, amides and phosphoramides, such as THE (tetrahydrofuran), methyl-THF, N,N′-dimethylformamide, acetonitrile, benzonitrile, hexamethylphosphoramide, or propylene carbonate.

The electrolyte solution in compartment C₂ may additionally comprise an oxidizable species which, in particular, prevents oxidation of the organic solvent in compartment C₁.

The oxidizable species can advantageously be selected from ferrocene, tetrathiafulvalene or one of the tetrathiafulvalene derivatives.

The salt is soluble in the organic solvent, preferably at concentrations of at least about 0.1 mol/L, for example at concentrations ranging from about 0.1 to 0.5 mol/L.

The salt is preferably an inert salt.

The salt can be selected from alkali metal salts and quaternary ammonium salts, and preferably from quaternary ammonium salts.

Examples of alkali metal salts include lithium salts such as LiTFSI, LiPF₆, or LiClO₄.

Examples of quaternary ammonium salts include tetraalkylammonium salts such as TBATFSI, tetra-n-ethylammonium perchlorate, tetra-n-butylammonium hexafluorophosphate or tetra-n-propylammonium tetrafluoroborate.

The ionic liquid can be selected from ammonium salts, imidazolium salts, phosphonium salts, pyrrolidinium salts, and piperidinium salts, and preferably from alkylammonium salts, alkylimidazolium salts, alkylphosphonium salts, alkylpyrrolidinium salts, and alkylpiperidinium salts.

The ionic liquid preferably comprises an anionic part of the bis(trifluoromethanesulfonyl)imidate type.

Examples of ionic liquids include triethylbutylammonium bis(trifluoromethanesulfonyl)imidate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imidate, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imidate, trimethylbutylammonium bis(trifluoromethanesulfonyl)imidate, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imidate, N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imidate, or 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imidate.

The ionic liquid is preferably water-immiscible. This facilitates subsequent purification.

The compartment C₂ contains no compound (I).

The electrolyte solution (respectively salt) in the compartment C₂ can be identical to the electrolyte solution (respectively salt) in the compartment C₁.

Step iii) At the end of step ii), species based on nitrogen and the element M are present in the composition, and are then hydrolyzed in an acid medium in step iii), to form ammoniacal nitrogen.

In the invention, ammoniacal nitrogen refers to the two most reduced forms of nitrogen: ammonium (NH₄+) and ammonia (NH₃). Ammoniacal nitrogen is therefore selected from ammonium (NH₄+), ammonia (NH₃), and mixtures thereof. Generally speaking, depending on the conditions of step iii), and in particular the quantity of acid, ammonium (excess acid), or ammonia (stoichiometric quantities with respect to N(MR¹R²)_3-xH_x) will be obtained.

Step iii) hydrolysis in an acid medium can be carried out by bringing a reaction crude obtained in preceding step ii) into contact with an acid solution or with a gaseous acid (in the form of a gas).

The acidic solution may comprise an aqueous solvent (e.g. water) and at least one acid such as hydrochloric acid, hydrobromic acid, sulfuric acid or nitric acid, or an aprotic organic solvent and at least one acid such as hydrochloric acid, hydrobromic acid, sulfuric acid or nitric acid.

The aprotic organic solvent can be selected from ethers such as diethyl ether or dioxane, and alkanes such as hexane or heptane.

The acid gas may be hydrochloric acid gas.

Step iii) advantageously leads to ammonium, particularly when the acid is used in excess relative to the compound of formula (I).

The aqueous solvent is preferably water.

The acid solution can have a pH ranging from 0 to 6.

Step iii) can last from about 1 min to 30 min, and preferably from about 2 min to 10 min. Step iii) is a very fast, almost instantaneous step.

Step iii) is preferably carried out at a temperature ranging from −20° C. to about 40° C., and particularly preferably from 0° C. to about 20° C.

Step iii) is preferably carried out with stirring.

Step iii) is preferably carried out at atmospheric pressure.

Step iii) produces ammonia NH₃ and/or ammonium NH₄+.

In step iii), a reaction crude obtained in the previous step ii) can be brought into contact with an acid solution by adding the acid solution to the composition.

Other Steps of the Method

The process may further comprise, after step ii) and before step iii), a step ii-1) in which the current or potential applied to the working electrode or the potential difference applied between the working electrode and the counter electrode is stopped.

After step ii), or ii-1) and before step iii), the method may further comprise a purification step ii-2). Step ii-2) removes at least some of any by-products (e.g. salts) formed during step ii). In other words, step ii-2) separates the nitrogen- and element M-based species formed in step ii) from the salts.

Step ii-2) can be carried out by extraction of a reaction mixture formed in step ii), or ii-1), in particular using an apolar organic solvent. The nitrogen- and element M-based species formed in step ii) or ii-1) is/are soluble in said apolar organic solvent and can be separated from the salts by filtration.

The apolar organic solvent can be chosen from alkanes such as hexane, pentane or heptane.

The preferred apolar organic solvent is pentane.

The method may further comprise, prior to step i), a step i-0) for preparing the compound of formula (I).

The compound of formula (I) can be prepared according to a double hydroboration or hydroalumination protocol as described in the following articles: H. C. Brown, N. Ravindran, J. Am. Chem. Soc. 1976, 98, 1798-1806 and H. C. Brown, N. Ravindran, J. Am. Chem. Soc. 1976, 98, 1785-1798; or according to the reaction of two equivalents of alkene with one equivalent of a mono-halogenoborane (e.g. having the formula YBH₂ wherein Y is as defined in the invention) in THE or diethyl ether, at room temperature.

In general, one equivalent of the compound MH₂Y reacts with two equivalents of an alkene R′CH═CH₂ to form the compound (R′CH₂CH₂)₂MY.

The method of the invention preferably uses no gaseous species other than dinitrogen (N₂) as the starting reagent.

The method can also comprise a step iv) for recycling the compound (I).

In this embodiment, step iii) preceding step iv) is preferably carried out by contacting under an inert atmosphere the reaction crude obtained in the preceding step ii), ii-1), or ii-2) with an acid solution comprising an aprotic organic solvent and at least one acid, or a gaseous acid, said acid solution and said gaseous acid being as defined above, so as to preferably form ammonia NH₃, and then step iv) can be carried out by distillation of the ammonia NH₃.

The remaining liquid composition includes compound (I) in the electrolyte solution and can be reused in another reaction.

A second object of the invention is an electrochemical cell for the reduction of dinitrogen into ammoniacal nitrogen, characterized in that it comprises:

• • a composition comprising at least one electrolyte solution and at least one compound responding to a following formula (I): R¹R²MY (I), wherein:
  • M is an element from group 13 of the periodic table, preferably selected from boron, aluminum, and mixtures thereof,
  • R¹ and R², identical or different, are chosen from an alkyl group, an aryl group, an aryl-alkyl group, an —OR group, and an —SR group, R being an alkyl group, an aryl group, or an aryl-alkyl group, and
  • Y is a group selected from a halogen —X, an —OR³ group, an —SR³ group, a triflate group, a mesylate group and a triflimidate group, R³ being an alkyl group, an aryl group or an aryl-alkyl group,
  • a working electrode immersed (in whole or in part) in said composition, and
  • a counter electrode immersed (in whole or in part) in said composition or in an electrolyte solution.

The electrochemical cell may also comprise a reference electrode immersed (in whole or in part) in said composition or in an electrolyte solution. The electrochemical cell, the electrolyte solution, the composition, the compound responding to formula (I), the working electrode, the counter electrode, and the reference electrode are as defined in the first object of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings show the invention.

FIG. 1 shows an example of an electrochemical cell according to the invention or one implemented according to the method of the invention.

FIG. 2 shows another example of an electrochemical cell according to the invention or one implemented according to the method of the invention.

Further features and advantages of the present invention will become apparent from the description of non-limiting examples of the method and electrochemical cell according to the invention.

EXAMPLES

FIG. 1 shows an electrochemical cell 1 for the reduction of dinitrogen into ammoniacal nitrogen, comprising a composition 2 comprising at least one electrolyte solution and at least one compound responding to formula (I), a working electrode 3 immersed in said composition, and a counter electrode 4 immersed in said composition. The electrochemical cell 1 may also comprise a reference electrode 5, as well as a system 6 (e.g. tube) for introducing dinitrogen into the electrochemical cell via an inlet 7. The system 6 is positioned so that dinitrogen 8 can be incorporated into or released from the composition 2 in the vicinity of the working electrode 3.

The electrochemical cell 1 may further comprise inlets and outlets (not shown in FIG. 1) for causing the composition to flow through the electrochemical cell 1, for example with a pumping system.

The electrochemical cell 1 may further comprise an outlet (not shown in FIG. 1) to allow dinitrogen to exit, and thereby to flow through the electrochemical cell 1.

FIG. 2 shows an electrochemical cell 10 for the reduction of dinitrogen into ammoniacal nitrogen comprising:

• • a first compartment C₁ 11 containing a composition 21 comprising at least one electrolyte solution and at least one compound responding to formula (I), a working electrode 30 immersed in said composition and, optionally, a reference electrode 50; and
  • a second compartment C₂ 12 containing a counter electrode 40 immersed in an electrolyte solution 22.

The two compartments are separated by a membrane 90.

Compartment C₁ also includes a system 60 (e.g. tube) for introducing dinitrogen into the electrochemical cell via an inlet 70. The system 60 is positioned so that dinitrogen 80 can be incorporated into or released from the composition 21 in the vicinity of the working electrode 30.

Example 1: Method for Producing Ammoniacal Nitrogen from Dinitrogen Using

an electrochemical cell comprising a composition containing dicyclohexylchloroborane as the compound of formula (I) An electrochemical cell comprising three compartments C₁, C₂, and C₃ has been implemented in the method of the invention and comprises:

• • a compartment C₁ containing:
  • a composition maintained under stirring comprising 3 mL of anhydrous THF, 6×10⁻⁴ moles of tetra-n-butylammonium bis-trifluoromethanesulfonimidate (TBATFSI) [TBATFSI concentration 0.2 mol/L], and 9×10⁻⁵ moles of dicyclohexylchloroborane [compound (I) concentration 0.03 mol/L];
  • a working electrode consisting of a vitreous carbon rod planted in a vitreous carbon foam (10 mm×5 mm×7 mm), said working electrode being immersed in said composition; and
  • a reference electrode consisting of a silver wire immersed in an acetonitrile solution comprising 10⁻² mol/L silver nitrate and 0.2 mol/LTBATFSI,
  • a C₂ compartment containing a counter electrode consisting of a vitreous carbon (or graphite) rod planted in a piece of carbon felt (10 mm×10 mm×100 mm), said counter electrode being immersed in an electrolyte solution comprising DMF and TBATFSI [TBATFSI concentration 0.2 mol/L],
  • a C₃ compartment containing an electrolyte solution comprising anhydrous tetrahydrofuran (THF) and TBATFSI [TBATFSI concentration 0.2 mol/L],
  • a membrane separating compartments C₁ and C₃, consisting of a sintered glass disk, and
  • a membrane separating compartments C₃ and C₂, consisting of a sintered glass disk.

The compartment C₃ ensures electrical contact between compartments C₁ and C₂, while limiting the diffusion of constituents between the working electrode and the counter electrode.

The body of the reference electrode is brought into contact with the composition of compartment C₁ via a glass extension containing an electrolyte solution comprising anhydrous THE and TBATFSI [TBATFSI concentration 0.2 mol/L]. Said reference electrode is thus immersed in said composition.

The electrochemical cell as described above is supplied with dinitrogen (N₂), the electrochemical cell being placed in a glovebox under a nitrogen atmosphere (step i)). A potential of −2.7 V is then applied to the working electrode for 4 hours (step ii). The progress of the reaction is monitored by coulometry. After electrolysis has stopped, the amount of ammonium produced is estimated from a 0.5 mL sample of the electrolyzed solution. This sample is treated by adding excess HCl (in ether solution) (step iii)), followed by evaporation of all volatile species under reduced pressure. The amount of ammonium formed is estimated via analysis of the residue by 1H NMR spectroscopy recorded in DMSO-d₆ in the presence of trimethoxybenzene, used as an internal reference.

Example 2: Control Methods (not Part of the Invention)

The control method consists in reproducing the experiment described above in the absence of the compound of formula (I) or in the absence of the application of a potential. No ammonium formation was observed.