Method for Producing Ammonia Nitrogen

The present invention relates to a method for producing ammoniacal nitrogen by feeding an electrochemical cell with dinitrogen (N), 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 (N). Humans can therefore only marginally benefit from such abundance. Only microorganisms such as, 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 19century, ammonia was produced by distilling liquid manure or extracting domestic blackwater. After the second half of the 19century, it was obtained as a by-product of the manufactured gas industry (utility gas). It wasn't until the beginning of the 20century (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 (H) 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 20century, 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 COproduction), 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(NRR)wherein M is a transition metal (e.g. molybdenum), Rand Rare 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-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 N, which then reacts with the Hprotons of the electrolyte on the surface of said material to form NH. Applying an electrical potential to said material is a means of reducing the activation barrier for the Nreduction 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 H.

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:

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.

The compound of formula (I) RRMY

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

Groups Rand R

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

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

Rand R, 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 Rand/or Rmay 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 Rand/or Rmay 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 Rand/or Rgroup 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 —CFgroup, a 2,4,6-(Me)-CHgroup and a 2,4,6-(iPr)-CHgroup. Among such groups, any of the 2,4,6-(Me)-CHor 2,4,6-(iPr)-CHgroups is particularly preferred.

The aryl group as an Rand/or Rgroup 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 Rand/or Rgroup 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 Rand Rgroups. 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 Rand Rgroups may be covalently linked, in particular via a carbon-carbon bond, to form a divalent group, said Rand Rgroups 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, Rand Rare alkyl groups), preferably a 9-bicyclo[3.3.1]nonane group.

According to one embodiment of the invention, Rand R, 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 Rand Rgroups is an alkyl group, and more particularly preferred, both the Rand Rgroups are alkyl groups.

According to a particularly preferred embodiment of the invention, Rand Rare identical.

The Rand Rgroups of compound (I) are non-stabilizing groups. In other words, their function is to not stabilize the radical RRMgenerated during the method, and consequently to make it more reactive towards dinitrogen N.

The group Y

Y is a group selected from a halogen —X, an —ORgroup, an —SRgroup, a triflate group (—OSOCF), a mesylate group (—OSOCH), and a triflimidate group (NTfor N(SOCF)), Rbeing an alkyl, aryl or aryl-alkyl group.

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

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

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

An Raryl-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 Rgroup.

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 RRM.

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 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 m/g, and particularly preferably of at least 100 m/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).