EP0476115A1

EP0476115A1 - Process to separate dioxygen from a gas mixture

Info

Publication number: EP0476115A1
Application number: EP91907588A
Authority: EP
Inventors: Christian Amatore; Said Aziz; Anny Jutand; François Draskovic; Kenneth Yamaguchi; Panayotis Cocolios
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 1990-03-27
Filing date: 1991-03-19
Publication date: 1992-03-25
Also published as: ZA912276B; CA2056410A1; US5296105A; FR2660210A1; AU7665391A; FR2660210B1; PT97183A; CS80591A2; BR9105169A; PL293018A1; AU632232B2; JPH04506627A; WO1991014649A1; HU913557D0

Abstract

Le procédé de séparation de dioxygène d'un mélange de gaz le renffermant comprend: l'absorption du dioxygène par des complexes des métaux de transition à bas degré d'oxydation correspondant à la formule générale (A): {Ln(M+p)m(X-z)x}mp-xz dans laquelle L représente un site de coordination appartenant à un ou plusieurs ligand(s) capable(s) de stabiliser les faibles valences de M, M est un métal de transition capable de fixer O2 en formant des composés dioxygénés peroxydiques dihapto, X est un anion coordinant organique, n est compris entre 2 et 12, p représente le degré d'oxydation de M dans le complexe de formule (A), m est égal à 1 ou 2, z est compris entre 1 et 3, x est compris entre 0 et 4, la désorption du dioxygène par oxydation électrochimique du produit peroxo dihapto (B) obtenu par réaction du complexe de formule (A) avec O2, la récupération du dioxygène libéré, et, le cas échéant pour un procédé en continu, la réduction électrochimique du complexe obtenu à l'issue de l'étape d'électrooxydation du composé (B), pour régénérer le complexe (A).The process for separating dioxygen from a mixture of gases containing it comprises: absorption of dioxygen by complexes of the transition metals with a low degree of oxidation corresponding to the general formula (A): {Ln (M + p) m (Xz) x} mp-xz in which L represents a coordination site belonging to one or more ligand (s) capable (s) of stabilizing the low valences of M, M is a transition metal capable of fixing O2 by forming dihapto peroxidic dioxygenated compounds, X is an organic coordinating anion, n is between 2 and 12, p represents the degree of oxidation of M in the complex of formula (A), m is equal to 1 or 2, z is included between 1 and 3, x is between 0 and 4, the desorption of dioxygen by electrochemical oxidation of the peroxo dihapto product (B) obtained by reaction of the complex of formula (A) with O2, the recovery of the dioxygen released, and, if appropriate for a continuous process, the electrochemical reduction of the complex obtained at the end of step d 'electrooxidation of the compound (B), to regenerate the complex (A).

Description

PROCESS FOR THE SEPARATION OF DIOXYGEN FROM A GAS MIXTURE.

The subject of the invention is the use of transition metal complexes for the separation of dioxygen from a mixture of gases containing it. Numerous transition metal complexes of type C _y M, in which the transition metal M is linked to y molecules of organic complexing agents C, are capable of fixing molecular oxygen.

The complexes with the resulting dioxygen are either of the superoxo type: C _y MOO, μ-peroxo: C _y MOO-MC _y , peroxo dihapto: C _y H l hydroperoxo: C _y MO-OH, oxo: C _y M = O or y-oxo: C _y MO-MC _y . Among these structures, only the first three consist of a non-destructive fixation of the O ₂ molecule on the metal and are therefore likely to intervene in the processes of separation of the oxygen molecule from a gaseous mixture. The most complex families of complexing agents used in this regard include cobalt linked to a Schiff base, an amino acid, a porphyrin, a polyalkyamine or a polyalkylamino acid, or also iron linked to a cryptand or to a porphyrin. Other families include, as transition metal, manganese or copper linked respectively to one or more phosphines or to a protein.

The fixation of dioxygen on these complexing agents leads to species of the superoxo or μ-peroxo type and is thermodynamically reversible, the desorption phase being carried out by raising the temperature and decreasing the partial pressure of the gas. However, to improve the efficiency of the separation of dioxygen from a gas mixture using such complexing agents, it is advisable to work continuously, using metal complexes which have a very high affinity for oxygen. Unfortunately, this can only be achieved if the strength of the metal-oxygen bond is high, which requires a very high energy expenditure for the desorption step. US Patent No. 4,475,994 in the name of Maxdem Inc. makes it possible to remedy at least party to this drawback by proposing the use of a solution of a cobalt complex capable of fixing the oxygen in contact with the gas mixture and the use of the electrochemical route to desorb the oxygen. The metal complex used in this process fixes the oxygen in the superoxo CyCo-OO or y-peroxo CyCo-OO-CoCy form when the cobalt at the start is in the +11 oxidation state. With a suitably chosen ligand, it is possible to electrochemically oxidize the complex without altering the organic ligand and thus to release oxygen in the gaseous state at the anode of an electrochemical cell.

This oxidation reaction leads to the formation of a complex of Co (III) inactive towards O ₂ , which is reduced to the cathode compartment of the same cell to regenerate a species of Co (II) capable of fixing O ₂ . By way of example, in the case where the oxygen is fixed in superoxo form, this cycle is illustrated by the following diagram I:

w Λ

The cobalt complexes of the superoxo type (usually soluble in organic medium) or γ-peroxo (usually soluble in aqueous medium) known to be able to coordinate, transport, then decomplex molecular oxygen according to the electrochemical cycle presented in diagram I have however limited lifetimes. Understanding the functioning and analysis of the degradation of complex molecules in the presence of oxygen and under electrochemical stress are very delicate, especially in an aqueous medium where there is a very high number of equilibria to take into consideration.

Among these equilibria, those skilled in the art will cite those between the protonated and deprotonated ligands (function of pH), between the ligand and the metal (function of the basicity of the ligand), between species of different coordination schemes (eg : taco-coordinated complex pentacoordinated complex hexacoordinated complex), the equilibrium of

spin, for example: Co ^II low spin - Co ^II high spin (function of the field strength of the ligand used), of complexation of oxygen, and of dimerization of the oxygen complex.

Poorly controlled chemical and / or electrochemical reactions can alter either the organic ligand or the metal itself, causing irreversible degradation of the complex.

In order to have dioxygen complexing agents which are more satisfactory with regard to the requirements of the technique for the separation of dioxygen from a gas mixture, the inventors have used a new family of transition metal complexes having the advantage of a coordination scheme. simplified, of a limited number of reactions in solution, comprising simple organic ligands, generally available commercially, and operating according to an electrochemical cycle whose principle is similar to that of scheme I.

This process for the separation of dioxygen from a mixture of gases containing it is characterized in that it comprises:

- the absorption of dioxygen by transition metal complexes with a low degree of oxidation, corresponding to the general formula (A):

[L _n (M + P) _a (Xz) _x ] mp-xz (A)

in which :

* L represents a coordination site belonging to one or more inorganic or organic ligand (s), mono or polydentate (s) identical or different, said ligands being capable of stabilizing the low valences of M, chosen in particular among the following ligands: . carbon monoxide

. phosphines

. phosphine oxides

. phosphites

. aliphatic or aromatic amines

. amides

. carboxylic acids

. aliphatic or aromatic thiols

. sulfoxides

. nitriles

. isocyanates

. arsines

. alkyl (aryl) silanes or alkyl (aryl) siloxanes

nitrogen, phosphorus, oxygenated, saturated or unsaturated heterocycles

* M is a transition metal capable of fixing O ₂ by forming dihapto dioxygenated peroxide compounds,

M

chosen in particular from Pd, Pt, Rh, Ni, Ir and Mo.

* X is an organic coordinating anion, for example a carboxylate ion, or an inorganic anion, for example a halide and in particular a chloride ion,

* n, between 2 and 12, represents the number of coordination sites L,

* p represents the degree of oxidation of M in the complex of formula (A),

* m, equal to 1 or 2, represents the number of metal centers of the complex,

* z, between 1 and 3, represents the charge of the anion X ^-z ,

* x, between 0 and 4, represents the number of anions X, identical or different, coordinated at the metal center (s) M

- the said absorption leading to a dioxygenated complex of the peroxo dihapto type corresponding to the following formula (B): [L _N , (M ^{P + 2} O ₂ ) _m , (X ^-z ) _x ,] ^m'p-x'z (B)

in which :

* L, M, X, p, z, have the same meanings as before,

* n ', m', x 'have the same meanings as n, m, x respectively with: 2≤n'≤n, 1≤m'≤m, 0≤x'≤x.

the desorption of dioxygen by electrochemical oxidation of the product peroxo dihapto (B) obtained by reaction of the complex of formula (A) with O ₂ ,

- recovery of the released oxygen, and where appropriate for a continuous process:

the electrochemical reduction of the complex obtained at the end of the electrooxidation step of the compound (B), to regenerate the complex (A),

A group of metal complexes usable in the invention is illustrated by the formula (C), [L _n M ^{+ p} ] ^{+ p} , in which the symbols have the meanings given above.

In another group of complexes of the invention, the metal M is coordinated, in addition to the ligands comprising L coordinating sites, to anions so as to at least partially block the unoccupied coordination sites of M.

This group is illustrated by the formula (D):

[L _n (M ^{+ p} ) (X ^-x ) _x ] ^p-xz (D)

in which L, M, X, n, p, x and z, are as defined above.

The invention also relates to dinuclear complexes corresponding to the formula:

[L _n (M ^{+ p} ) ₂ (X ^-2 ) _x ] ^2p-xz (E)

in which L, M, X, n, p, z, and x are as defined above, where the dinuclear structure is provided either by bridging X ^{- z} anions, or by polydentate chelating ligands carrying at least two coordinating sites L, either by a metal-metal bond, or by a combination of these bonds.

In the above formulas, (A) to (E), the metal complexes which can be used in the invention can be in the state of anions or cations. They are then associated with one or more against mineral or organic non-coordinating ion (s) which counterbalances their charge. These ions come from the support electrolyte and can in particular be quaternary ammonium salts, quaternary phosphonium salts, alkali or alkaline-earth complexed or not or the like, for cations, and halide ions, tetrafluoroborates, hexafluorophosphates, sulfates, carbonates, phosphates or the like, for anions. The dioxygenated complexes of formula (B) result from the fixation of dioxygen by the compounds of formula (A) and more particularly by the compounds of formula (C), (D) or (E). Compounds (A), (C), (D) and (E) can be synthesized beforehand or they can be formed in situ from commercial products or not.

Metal salts, at varying degrees of oxidation, can serve as precursors to the active species, such as the halogen, acetate, nitrates, sulfates, fluoroborates, perchlorates or the like salts. Compounds which are particularly suitable for the process in the case where M represents Pd include PdCl ₂ and Na ₂ PdCl ₄ .

Ligands which are particularly suitable for the preparation of the complexes of the invention are chosen from phosphines. Mention will in particular be made of triphenylphosphines, alkyl-, aryl- or alkylarylphosphines, mono- or bidentates and in particular triphenylphosphine and tri-n-butylphosphine. In order to have water-soluble ligands, use will more particularly be made of compounds containing a hydrophilic functional group, on one of the phosphorus substituents, for example sulfonated phosphines such as the trisulfonated triphenylphosphine P (C ₆ H ₄ SO ₃ H) ₃ or disulfonated P (C ₆ H ₅ ) (C ₆ H ₄ SO ₃ H) ₂ or monosulfonated P (C ₆ H ₅ ) ₂ (C ₆ H ₄ SO ₃ H). Phosphine oxides and phosphites are also suitable when operating in an aqueous medium.

Metal complexes particularly preferred for the implementation of the invention contain as transition metal M palladium, platinum, nickel or rhodium.

The electrochemical oxidation step is carried out in the compar anode cell of an electrolysis cell. Suitable electrolytes are of the perfluoroborate or tetraphenylborate or perfluorophosphate or halide or sulphate or carbonate or carbonate or phosphate of alkali or alkaline earth, complexed or not, of quaternary ammonium salt, of phosphonium and include, for example, tetrafluoroborate of tetra-n- butyl ammonium or triethylbenzylammonium hexafluorophosphate.

The operation is carried out at a potential chosen as a function of the oxygenated metal complex to be oxidized and the composition of the medium.

The oxidation stage is followed by the desorption of the oxygen which is also carried out in the anode compartment of the electrolysis cell, the separation of the oxygen from the solution being carried out by means of a gas-liquid separation tower placed in downstream of the anode compartment.

To carry out the continuous oxygen separation process, the solution obtained is transferred at the outlet of the previous tower, into the cathode compartment of the electrolysis cell, where the electrochemical reduction is carried out at a more negative potential than that used in the oxidation step, which leads to a lower valence complex, capable of fixing the oxygen again in an absorption tower placed between the exit of the cathode compartment and the entry of the anode compartment, the electrochemical oxidation-reduction cycle then being resumed. The electrochemical oxidation-reduction cycle to which reference is made in the foregoing is illustrated by one of the following schemes IIa, IIb, Ilc, IId, or L, M, X, p, z, n, m, x , n ', m', x 'have the same meanings as before and n ", m", x "have the same meanings as n', m 'and x' respectively:

The intermediate species (F) and (H) being of very short life to be properly characterized, the four schemes lIa, Ilb, Ile, Ild are possible. Similarly (G) can represent several intermediate species resulting from the exchange of ligands during the cycle. According to one embodiment of the invention, the oxygen absorption step is carried out by contacting the gas mixture with the metal complex in reduced form in solution.

According to another variant of the invention, the active species (A) is formed in situ. For this, a mixture of metal halide, for example PdCl ₂ or Na ₂ PdCl ₄ , and of ligand, for example triphenylphosphine or tri-n-butylphosphine, which is rapidly introduced into a species (G), is introduced into the cathode compartment. 'example above (R ₃ P) ₂ PdCl ₂ , with R = phenyl or n-butyl). The active species (A) is obtained by reduction of the species (G) to an appropriate potential value and the cycle is then continued normally.

When the species (G) is reducible to a potential close to or more negative than the potential for reducing dioxygen to superoxide ion, an advantageous variant of the process consists in reacting this superoxide ion generated in the cathode compartment on the species (G ) to lead directly to the dioxygenated complex (B). For example, in the case where (G) is the complex (R ₃ P) ₂ PdCl ₂ this reaction is written

0 ₂ + e- 0 ₂ ^"

(R ₃ P) ₂ PdCl ₂ +20 ₂ ^; - *> (R ₃ P) ₂ Pd0 ₂ + 2C1- + 0 ₂

I

The materialization of this transformation can advantageously be carried out by the use of an electrochemical cell under pressure, or the use of a porous cathode with gas diffusion. The porous cathode also has the following two advantages:

* compared to the conventional process, it allows the elimination of the absorption tower, * compared to the use of pressure, it avoids saturation of the solution with the other constituents of the mixture containing the oxygen.

In an alternative embodiment, a solution of the complex is used in an organic solvent. A solvent preferably having a low ohmic drop is preferably chosen. Suitable solvents of this type include dimethylformamide, dimethylsulfoxide, acetonitrile, or even benzonitrile, dichloromethane, tetrahydrofuran.

According to another variant, the operation is carried out in an aqueous medium, the ligands used being then chosen from water-soluble ligands.

According to the invention, the separation of dioxygen is carried out at atmospheric pressure.

Alternatively, one can operate under high pressure of the order of 1 to 100 bar and preferably from 1 to 20 bar. According to one embodiment, the complex solution is then brought into contact with compressed air.

In another embodiment, the absorption of oxygen is carried out at atmospheric pressure and the liquid is compressed using a pump to the desired pressure.

This process makes it possible to separate the oxygen from the air, in particular in a continuous process.

It also makes it possible to separate the dioxygen from a mixture of gases, which is of particular interest for the purpose of purifying gases such as N ₂ , Ar or CO ₂ .

The invention will be illustrated below by examples relating to the study of the oxidation-reduction of complexes used according to the invention.

More particularly, Figures 1 to 4 represent voltamograms referring to selected examples:

- Figure 1 relates to the electrochemistry of the L ₄ Pd ° complex in which L represents triphenylphosphine. The curve LA is the voltamogram of the complex (PPh ₃ ) ₄ Pd ° under an argon atmosphere; curve 1B that of the same complex after introduction of oxygen; curve 1C represents the voltamogram of (PPh ₃ ) ₄ Pd ° in the presence of an excess of O ₂ and the curve 1D results from the previous one after addition of two equivalents of nBu ₄ N ⁺ Cl- with respect to the starting complex.

- Figures 2 and 3 show the voltamograms of the complex (PPh ₃ ) ₂ PdO ₂ obtained chemically, then dissolved in dimethylformamide. More particularly, Figure 2 reproduces the voltammograms obtained with an initial cathode scan for the complex (PPh ₃ ) ₂ PdO ₂ alone (curve 2A) and added with two equivalents of nBu ₄ N ⁺ Cl- (curve 2B).

- Figure 3 reproduces the voltammograms obtained with an anodic initial scan for (PPh ₃ ) ₂ PdO ₂ alone (curve 3A) and added with two equivalents of nBu ₄ N ⁺ Cl- (curve 3B).

- Figure 4 shows the voltamograms recorded for the complex (PPh ₃ ) ₂ PdO ₂ prepared in situ in dimethylformamide from PdCl ₂ and PPh ₃ . More specifically, curves 4A to 4D represent:

* the complex (PPh ₃ ) ₂ PdCl ₂ alone (curve 4A)

* the complex (PPh ₃ ) ₂ PdCl ₂ under low partial pressure of O ₂ (curve 4B)

* the complex (PPh ₃ ) ₂ PdCl ₂ in air (curve 4C)

* the complex (PPh ₃ ) ₂ PdCl ₂ under an atmosphere of pure O ₂ (curve 4D).

EXAMPLE 1: EISCTROCHIHICAL STUDY OF THE METAL COMPLEX L ₄ Pdº IN WHICH THE METAL IS COORDINATED BY FOUR LIGANDS HONODENTATES TRIPHENYLPHOSPHINE PPh ₃ .

The oxidation-reduction potential studies were carried out in dimethylformamide (DMF) or dichloromethane (CH ₂ Cl ₂ ) in the presence of 0.3 M tetra-n-butylammonium tetrafluoroborate (nBu ₄ NBF ₄ ) as the electrolyte Support . The L ₄ Pd ° concentration is 2mM.

The potentials were measured with respect to the saturated calomel electrode (DHW), and determined by cyclic voltammetry on the gold (Au) or carbon (C) electrode, at the scanning speeds of 200 and 100 mV.s ^{- 1} respectively.

A - ELECTROCHEMISTRY OF L ₄ Pd ° IN THE ABSENCE OF DIOXYGEN Oxidation: L ₄ Pd ° ---> [L ₂ Pd ^II ] ⁺² + 2e- + 2L

E _ox = + 0.13V (DMF, Au)

= + 0.07V (DMF, C)

E _1/2 = + 0.02V (CH ₂ Cl ₂ , C, reversible)

B - ELECTROCHEMISTRY OF L ₄ Pd ° IN THE PRESENCE OF DIOXYGEN

In the presence of a mixture of gases containing dioxygen (eg air), the L ₄ Pd ° complex, leads to the dioxygen compound L ₂ Pd ^II O ₂ , according to:

L ₄ Pd ° + O ₂ ---> L ₂ Pd ^II O ₂ + 2L

Oxidation:

L ₂ Pd ^II O ₂ ---> [L ₂ Pd ^II ] ^{2 +} + O ₂ + 2e-

E _ox = + 0.52V (DMF, Au)

= + 0.48V (DMF, C)

= + 0.64V (CH ₂ Cl ₂ , Au)

= + 0.59V (CH ₂ Cl ₂ , C)

The oxidation of the L ₂ Pd ^II O ₂ complex is irreversible at potentials close to + 0.50V (vs ECS). The chemical reaction is followed by a gas evolution observable with the naked eye in the case of electrolysis with controlled potential (+ 0.60V).

Reduction:

L ₂ Pd ^II O ₂ + 2e- → [L ₂ Pd ^q ] ^{+ q} + O ₂ ^{- (2-q)} where q = 0 or 1

E _red = -1.68V (DMF, Au)

= -1.62V (DMF, C)

= -1.73V (CH ₂ Cl ₂ , Au)

= -1.74V (CH ₂ Cl ₂ , C)

The reduced species is able to fix dioxygen again and gives rise to the same oxidation reaction as above with 2 electrons around + 0.50V. This species is much more hungry for dioxygen than its precursor L ₄ Pdº.

These redox reactions are illustrated in FIG. 1 reproducing the cyclic voltammograms noted in the DMF of the complexes:

- (PPh ₃ ) ₄ Pd ° under an argon atmosphere (Figure 1A)

- (PPh ₃ ) ₄ Pd ° under an oxygen atmosphere (Figures 1B and 1C)

In addition, the attribution of the various redox reactions is confirmed by the study of the redox behavior, under the same conditions, of the (PPh ₃ ) ₂ PdO ₂ complex illustrated in Figure 2 (curves 2A and 2B). - All of these observations lead to establishing for the L ₄ Pd ° complex the following cycle (Diagram III):

DIAGRAM III EXAMPLE 2: ELECTROCHEMISTRY OF L ₄ Pd ° IN THE PRESENCE OF DIOXYGEN AND CHLORIDE IONS. REDOX POTENTIALS OF CHLORIDE SOLUTIONS OF L ₄ Pdº

The presence of chloride ions leads to the formation of new, more stable complexes:

L ₂ Pd ^II O ₂ + xCl- ---> [L ₂ Pd ^II O ₂ Cl _x ] ^-x where x = 1 or 2

Oxidation:

[L ₂ Pd ^II O ₂ Cl _x ] ^-x ---> [L ₂ Pd ^II O ₂ Cl _x ] ^(2-x) + 2e-

E _ox = + 0.48V (DMF, Au)

= + 0.48V (DMF, C)

= + 0.57V (CH ₂ Cl ₂ , Au)

= + 0.62V (CH ₂ Cl ₂ , C)

Reduction:

[L ₂ Pd ^II O ₂ Cl _x ] ^-x + 2e- ---> [L ₂ Pd ^q Cl _x ] ^(qx) + O ₂ ^{- (2-q)} where q = 0 or 1

E _red = -1.68V (DMF, Au)

= -1.56V (DMF, C)

= -1.73V (CH ₂ Cl ₂ , Au)

= -1.69V (CH ₂ Cl ₂ , C)

These redox reactions are illustrated in Figure 1D (voltamograms) of (PPh ₃ ) ₄ Pd under an oxygen atmosphere in the presence of 2 equivalents of gilding ions) and Figures 2B and 3B (voltamograms of (PPh ₃ ) ₂ PdO ₂ in an atmosphere of argon in the presence of 2 equivalents chloride ions). From these observations we conclude that the interaction of Cl- ions with the species present in solution gives rise to the following cycle where x = 1 or 2:

EXAMPLE 3: ELECTROLYSIS UNDER POTENTIAL CONTROL OF A SOLUTION OF

L ₂ PdO ₂ (L = PPh ₃ ) IN THE PRESENCE OF Cl- IONS

A solution of L ₂ PdO _{2 is} oxidized under an argon atmosphere in the

CH ₂ Cl ₂ in the presence of two equivalents of nBu ₄ NCl in a cell with separate compartments. After passing 1.54 Faraday / mole, the formation of a yellow solid which precipitates in the cell is observed, accompanied by gas evolution.

The solid obtained after filtration (Yield = 892 relative to the amount of electricity consumed) has the same physicochemical characteristics as the L ₂ PdCl ₂ complex prepared by chemical means ( ³¹ P NMR: signal at 23.70 ppm relative to H ₃ PO ₄ , taken as an external reference; IR: v _pd-cl = 350 cm ^-1 ).

Therefore :

The oxidation of L ₂ Pd ₂ O ₂ involves two electrons per mole. It releases molecular oxygen and leads to the L ₂ PdCl ₂ complex, according to:

L ₂ PdO ₂ + 2Cl- ---> O ₂ + L ₂ PdCl ₂ + 2e-

Apart from the gas evolution at the anode, the dioxygen is highlighted in the following way: a cyclic voltammetry carried out on the solution after electrolysis under argon in DMF, where L ₂ PdCl ₂ is soluble, shows the presence of a reduction wave corresponding to L ₂ PdO ₂ around -1.68V while the oxidation wave of L ₂ PdO ₂ is absent if the scanning is carried out anodically in a first time. The reduction of L ₂ PdCl ₂ does not lead to the formation of L ₂ Pd °, but to that of L ₂ Pd ^II O ₂ , which proves the presence of dioxygen in solution.

EXAMPLE 4: ELECTROLYSIS UNDER POTENTIAL CONTROL OF A L ₂ PdO ₂ SOLUTION (L = PPh ₃ ) IN THE PRESENCE OF Cl- IONS. QUANTITATIVE MEASUREMENT OF DESORBED DIOXYGEN.

The electrolysis of a solution of L ₂ PdO ₂ is carried out under the following conditions:

Installation: cell with double jacket for circulation of heat transfer fluid with compartments separated by porous glass; carbon anode (RVC 1000, Carbone Lorraine); platinum grid cathode; reference Saturated Calomel Electrode (ECS) SERVOHEX oxygen analyzer, fitted in series with a BR00KS volumeter for respectively analyzing the O ₂ content and quantifying the gas produced.

Composition of the medium: solvent, dimethylformamide, 70 ml; L ₂ PdO ₂ 5mM; benzyltri-n-butyl ammonium chloride, 11mM; support electrolyte, tetra-n-butylammonium hexafluorophosphate, 0.3M, medium saturated with oxygen.

Operating conditions: temperature of the solution maintained at 20 ° C. by circulation of water in the double jacket, potential applied + 0.85 V vs. E.C.S.

Results: 7.1 Nml of gas, with an average content of 98.4% of oxygen, are collected corresponding to a production of 0.31 mmol of O ₂ (86% of the theoretical quantity expected). The faradic yield, relative to the oxygen produced by assuming an exchange of 2 electrons per molecule of O ₂ released, is 80.8%.

EXAMPLE 5: ELECTROCHEMISTRY OF THE L ₂ PdO ₂ COMPLEX PREPARED IN SITU FROM PdCl ₂ AND TRIPHENYLPHOSPHINE, PPh ₃ .

The addition of 2 equivalents of triphenylphosphine, PPh ₃ , to a suspension of PdCl ₂ in DHF rapidly leads to a yellow solution characteristic of the (PPh ₃ ) ₂ PdCl ₂ complex. The cyclic voltammetry (scanning speed 200mV / sec, gold electrode) carried out on this complex generated in situ appears to be identical to that of the complex (PPh ₃ ) ₂ PdCl ₂ prepared independently isolated in the solid state, then put back in solution.

A - ELECTROCHEMISTICS IN THE ABSENCE OF DIOXYGEN

- Reduction:

L ₂ Pd ^II Cl ₂ + 2e- ---> [L ₂ Pd ° Cl _x ] ^x- + (2-x) Cl- E _red = - 0.97V (DMF, Au)

= - 0.92V (DMF, C)

- Oxidation of [L ₂ Pd ° Cl _x ] ^-x :

[L ₂ Pd ° Cl _x ] ^-x + (2-x) Cl- ---> L ₂ PdCl ₂ + 2e-

E _ox = + 0.015V (DMF, Au)

= + 0.05V (DMF, C)

B - ELECTROCHEMISTRY IN THE PRESENCE OF DIOXYGEN

In the presence of dioxygen, the wave of oxidation of the zerovalent electrogenerated complex disappears [(PPh ₃ ) ₂ Pd ° Cl _x ] ^-x at + 0.015V and the appearance of a new oxidizing complex at more positive potentials. characteristic of [(PPh ₃ ) ₂ Pd ^II O ₂ Cl _x ] ^-x .

Reduction of the starting complex:

(PPh ₃ ) ₂ Pd ^II Cl ₂ + 2e- ---> [(PPh ₃ ) ₂ Pd ° Cl _x ] ^-x + (2-x) Cl-

E _red = - 0.99 (DMF, Au) (confused with the O ₂ reduction wave)

Formation of the dioxygenated complex:

[(PPh ₃ ) ₂ Pd ° Cl _x ] ^-x + O ₂ > [(PPh ₃ ) ₂ Pd ^II O ₂ Cl _x ] ^-x

Reduction of the oxygenated complex:

[(PPh ₃ ) ₂ Pd ^II O ₂ Cl _x ] ^-x + 2e ----> [(PPh ₃ ) ₂ Pd ^q Cl _x ] ^(qx) + O ₂ ^{- (2-q)} where q = 0 or 1.

E _red = - 1.67V (DMF, Au)

= - 1.60V (DMF, C)

Oxidation of the dioxygenated complex:

[(PPh ₃ ) ₂ Pd ^II O ₂ Cl _x ] ^-x ---> [(PPh ₃ ) ₂ PdCl _x ] ^2-x + O ₂ + 2e- E _ox = + 0.43 (DMF, Au)

= + 0.43V (DMF, C)

The above results are illustrated in Figure 4 representing the voltamograms of the complex (PPh ₃ ) ₂ PdCl ₂ recorded under argon and under O ₂ atmosphere.

EXAMPLE 6 ELECTROCHEMICAL STUDY OF THE METALLIC COMPLEX L ₂ PdCl ₂ IN WHICH THE METAL IS COORDINATED BY TWO MONODENTATE LIGANDS

tri-n-BU TYLPHOSPHINE AND TWO CHLORIDE IONS

This study was carried out on 2πM solutions of complex by cyclic voltammetry on a gold electrode at a speed of 200mV / sec in DMF in the presence of 0, 3M of (nBu) ₄ NBF ₄ as support electrolyte.

The voltamogram recorded under argon shows,

- Reduction:

. L ₂ Pd ^II Cl ₂ + 2e- ---> [L ₂ PdºCl _x ] ^{- x} + (2-x) Cl- E _{r é d} = - 1, 39V

. a forecast at - 0.97V

- in Oxidation (return sweep):

. [L ₂ Pd ° Cl _x ] ^-x + (2-x) Cl- ---> L ₂ Pd ^II Cl ₂ + 2e- E _ox = -0.40V The voltamogram recorded under an oxygen atmosphere presents three waves of reduction at - 0.92, - 1.39 and - 2.17 V respectively attributable to the mono-electronic reduction of dissolved oxygen in solution in superoxide ion O ₂ - to the centrometallized reduction of the L ₂ Pd ^II Cl ₂ complex and to the reduction of the dioxygenated complex [L ₂ Pd ^II O ₂ Cl _x ] ^x- .

In oxidation we observe the disappearance of the wave of [L ₂ Pd ° Cl _x ] ^-x at -0.40V and the appearance of three waves at - 0.70, + 0.22 and (+0.60 to + 0.80) V respectively attributable to the oxidation of the superoxide ion O ₂ - to dioxygen, to the oxidation of the complex

dioxygenated [L ₂ PdO ₂ Cl _x ] ^-x and to the oxidation of Cl- ions.

In particular, it is observed that the species oxidized to + 0.22V does not come from that reduced to - 2.17V, but from the species which results from the reaction of dioxygen with [L ₂ Pd ° Cl _x ] ^-x ( reversal of the return sweep at -1.5 V or of the reaction of the superoxide ion O ₂ - with the L ₂ Pd ^II Cl ₂ complex, (in the case of a sweep back to

- 1.0V) according to:

2O ₂ - + L ₂ Pd ^II Cl ₂ ---> [L ₂ Pd ^II O ₂ Cl _x ] ^-x + (2-x) Cl- + O ₂ In the presence of an excess of chloride ions (addition of ( n-Bu) ₄ N ⁺ Cl-) the same general behavior is observed with, however, a slight shift in the oxidation potentials of the species [L ₂ PdºCl _x ] ^-x and [L ₂ PdO ₂ Cl _x ] ^-x respectively from -0.40 to -0.43V and from +0.22 to

+ 0.19V. This displacement, an indicator of more facilitated oxidation for these two species, is attributable to a stronger interaction of the Cl- ions with L ₂ Pd ° and L ₂ Pd ^II O ₂ .

EXAMPLE 7 ELECTROCHEMISTRY OF THE L ₂ PdO ₂ COMPLEX PREPARED IN SITU A

STARTING FROM NaPdCl ₄ AND TRI-n-BUTYLPHOSPHINE, P (n-Bu) ₃ .

The operating conditions are the same as those used in Example 6.

The starting metal salt, NaPdCl ₄ , being soluble in DMF, the addition of two equivalents of tri-n-butyl-phosphine very quickly leads to the formation of the L ₂ PdCl ₂ complex (color change of the solution of the brown to yellow).

The cyclic voltammetry performed on this complex generated in situ in the absence of oxygen shows on the side of the reduction the presence of a prevague at -0.97V and a wave at -1.39V identical to that of the complex (n-Bu ) ₂ PdCl ₂ .

On the oxidation side, there is a wave at -0.47V attributed to the oxidation of [L ₂ Pd ° Cl _x ] ^-x . This potential is 0.07V more negative than that observed in the case of L ₂ PdCl ₂ in the absence of chloride ions (-0.40V), but only 0.04V more negative than that observed for the same complex added with (n -Bu) ₄ N + Cl- (cf. Example 6). Consequently, this variation in potential is attributable to the concentration of Cl- ions in solution and accounts for the interactions between Cl- ions and intermediate species formed.

In the presence of dioxygen, the recorded voltammogram shows on the side of the reduction a wave at -0.94V attributed to the reduction in dioxygen and a second wave at -1.39V attributed to the reduction in L ₂ Pd ^II Cl ₂ . On the oxidation side, the disappearance of the wave of [L ₂ PdºCl _x ] ^-x and the appearance of the wave of oxidation of [L ₂ PdO ₂ Cl _x ] ^-x at + 0.22V are observed. An oxidation wave at + 0.79V is attributed to the oxidation of chloride ions.

Still in the presence of dioxygen, but by performing the return sweep just after the reduction of the oxygen (-0.94V), we notice on the oxidation side a wave at + 0.22V corresponding to the oxidation of [L ₂ PdO ₂ Cl _x ] ^-x and the absence of the oxidation wave of the zerovalent complex [L ₂ Pd ° Cl _x ] ^-x at -0.47V. These observations indicate the formation of the dioxygenated complex according to:

20 ₂ - + L ₂ Pd ^II Cl ₂ ---> [L ₂ Pd ^II O ₂ Cl _x ] ^-x + (2-x) Cl ^{- +} O ₂

Claims

1. Process for the separation of dioxygen from a mixture of gases containing it, characterized in that it comprises:

[L _n (M ^{+ p} ) _m (X ^-z ) _x ] ^mp-xz (A)

in which :

* L represents a coordination site belonging to one or more inorganic or organic ligand (s), mono or polydentate (s), identical (s) or different (s) said ligand (s) being capable (s) of stabilizing the low valences of H, chosen in particular from:

. carbon monoxides

. phosphines

. phosphine oxides

. phosphites

. aliphatic or aromatic amines

. amides

. carboxylic acids

. aliphatic or aromatic thiols

. sulfoxides

. nitriles

. isocyanates

. arsines

. alkyl (aryl) silanes or alkyl (aryl) siloxanes. nitrogen, phosphorus, oxygenated, saturated or unsaturated sulfur heterocycles,

* M is a trans-ition metal capable of fixing O ₂ by forming dioxygenated peroxidic dihapto compounds,

0

chosen in particular from Pd, Pt, Rh, Ni, Ir and Mo,

* X is an organic coordinating anion, for example a carboxy ion late, or inorganic, for example a halide and in particular a chloride ion,

* n, between 2 and 12, represents the number of coordination sites L,

* p represents the degree of oxidation of M in the complex of formula (A),

* m, equal to 1 or 2, represents the number of metal centers of the complex,

* z, between 1 and 3, represents the charge of the anion X ^-z , * x, between 0 and 4, represents the number of anions X, identical or different, coordinated at (x) center (s) metallic (s) M;

- the said absorption leading to a dioxygenated complex of the peroxo dihapto type of formula (B):

[L _n , (M ^{p + 2} O ₂ ) _m '(X ^-z ) _x ,] ^m'p-x'z (B)

in which :

* L, M, X, p and z have the same meanings as before,

* n ', m' and x 'have the same meanings as n, m and x respectively with: 2≤n'≤n, 1≤m'≤m, 0≤x'≤x;

- the electrochemical reduction of the complex obtained at the end of the step of electrooxidation of the compound (B), to regenerate the complex (A);

2. Method according to claim 1, characterized in that the metal complex used corresponds to the formula (C) [L _n M ^{+ p} ] ^{+ p} in which the symbols have the meanings given in claim 1.

3. Method according to claim 1, characterized in that in the metal complex used, the metal M is coordinated in addition to the ligands comprising coordinating sites L, to anions so as to at least partially block the coordination sites unoccupied of M, the metal complex responding more in particular to formula (D):

[L _n (M ^{+ p} ) (X ^-z ) _x ] ^p-xz (D)

wherein L, M, X, n, p, x and z, are as defined in claim 1.

4. Method according to claim 1, characterized in that the metal complex used is a dinuclear complex of formula [L _n (M ^{+ p} ) ₂ (X ^-z ) _x ] ^2p-xz in which the different symbols are such as defined in claim 1, in which the dinuclear structure is provided either by bridging X ^-z anions, and / or by polydentate chelating ligands carrying at least two coordinating sites L and / or by a metal / metal bond, and / or by a combination of these bonds.

5. Method according to any one of claims 1 to 4, characterized in that the metal complexes used are in the state of anions or cations, and are associated with one or more non-ion (s) mineral or organic coordinator (s) which counterbalances their charge, said counterions being either cations chosen in particular from quaternary ammonium salts, quaternary phosphonium salts, alkalis or alkalines -terrous complexed or not, are anions chosen in particular from halide ions, tetrafluoroborates, hexafluorophosphates, sulfates, carbonates, phosphates.

6. Method according to any one of claims 1 to 5, characterized in that the ligands are chosen from phosphines, phosphine oxides, phosphites, the phosphine being chosen especially from triphenylphosphines, alkyl-, aryl- or alkylaryl- phosphines, mono- or bidentates and in particular, triphenylphosphine and tri-n-butylphosphine, the phosphine used comprising, where appropriate, a hydrophilic functional fragment on one of the phosphorus substituents.

7. Method according to any one of claims 1 to 6, characterized in that the transition metal M is chosen from palladium, platinum, nickel, rhodium, iridium or molybdenum.

8. Method according to any one of claims 1 to 7, characterized in that the compounds (A), (B), (C), (D) and (E) defined in claims 1 to 4 respectively are synthesized beforehand or are formed in situ.

9. Method according to any one of claims 1 to 8, characterized in that the electrochemical oxidation step is carried out in the anode compartment of an electrolysis cell, suitable electrolytes being in particular of the perfluoroborate or tetraphenylborate type or perfluorophosphate or halide or sulphate or carbonate or phosphate of alkali or alkaline earth complexed or not, of quaternary ammonium salt, of phosphonium and include for example tetrafluoroborate of tetra-n-butylammonium or hexafluorophosphate of triethylbenzylammonium.

10. Method according to any one of claims 1 to 9, characterized in that the desorption of dioxygen is carried out in the anode compartment of the electrolysis cell, the separation of dioxygen from the solution being carried out by means of a tower of gas-liquid separation placed downstream of the anode compartment, and that in order to carry out the continuous oxygen separation process, the solution obtained is transferred at the outlet of the previous tower, into the cathode compartment of the electrolysis cell, where the electrochemical reduction is carried out at a more negative potential than that used in the oxidation step, which leads to a lower valence complex, capable of fixing the oxygen again in an absorption tower placed between leaving the cathode compartment and entering the anode compartment, the electrochemical oxidation-reduction cycle then being resumed.

11. Method according to any one of claims 1 to 10, characterized in that the oxygen absorption step is carried out by contact of the gas mixture with the metal complex in reduced form in solution, or as a variant , the active species (A) is formed in situ by adding the metal salts and the appropriate ligand to the cathode compartment, which generates an intermediate species (G) of formula:

[L _{n "} M ^{+ p} _m" X ^-z _{x "} ] ^{m" (p + 2) -x "z} (G) in which n ", m" and x "have the same meanings as n ', m' and x 'respectively, the other symbols being as defined in claim 1, the reduction of the species (G) to a value of appropriate potential to obtain (A), the cycle then being continued.

12. Method according to claim 11, characterized in that when the species (G) is reducible to a potential close to or more negative than the reduction potential of dioxygen into superoxide ion, this superoxide ion generated in the cathode compartment reacts on the 'species (G) to lead directly to the dioxygen complex (B) defined in claim 1.

13. Method according to claim 12, characterized in that an electrochemical cell under pressure or a porous cathode with gas diffusion is used.

14. Method according to any one of claims 1 to 13, characterized in that it operates in solution in an organic solvent, preferably in a solvent having a low ohmic drop such as dimethylformamide, dimethylsulfoxide, acetonitrile, benzonitrile, dichloromethane or tetrahydrofuran.

15. Method according to any one of claims 1 to 13, characterized in that the aqueous ligands used are then chosen from water-soluble ligands.

16. Method according to any one of claims 1 to 15, characterized in that the dioxygen separation step is carried out at atmospheric pressure.

17. Method according to any one of claims 1 to 15, characterized in that the step of separation of the oxygen is carried out under high pressure, in particular from 1 to 100 bar, preferably from 1 to 20 bar approximately.

18. Application of the method according to any one of claims 1 to 17, to the separation and continuous recovery of oxygen from the air.

19. Application of the method according to any one of claims l to 17, for the purification of gas mixtures containing dioxin.