FR2902098A1 - Monomers for production of ionic conductive membranes, e.g. for use in fuel cells, comprise imidazole compounds with a protecting group on nitrogen and an alkenyloxymethyl group at position 2 - Google Patents

Abstract

Heterocyclic monomers for use in the production of ionic conductive membranes, comprise imidazole compounds with a protective group such as benzyl or trialkylsilyl on the nitrogen at position 1 and an alkenyloxymethyl group at position 2. Monomers with an imidazole group for use in the production of ionic conductive membranes, having a structure with at least one unit of formula (1). R 1alkenyl; R 2a protective group, e.g. benzyl or trialkylsilyl. Independent claims are also included for (1) a method (M1) for the production of monomers (I) by making 1-benzyl-2-hydroxymethyl-imidazole (7) and using this to make the required monomer (6, 14) (2) protected polymers (12) containing the above monomer units (3) a method (M2) for the production of these protected polymers by polymerisation of monomers (6, 14) (4) deprotected polymers (12') based on the above protected polymers, with hydrogen atoms instead of the protecting group (R 2) in (12) (5) a method (M3) fo the production of (12') by replacing the protective groups (R 2) by hydrogen atoms (6) ionic conductive membranes made from these deprotected polymers (12') (7) electrochemical devices, fuel cells and electrolysers with these membranes as the electrolyte . [Image].

Description

Improved monomers and polymers bearing imidazole groups and

  The present invention mainly relates to monomers and polymers that can be used for the production of a proton conduction membrane in the absence of water. The invention also relates to a process for producing these monomers and polymers, wherein the membranes can be made from these monomers, and the use of these membranes as a solid polymer electrolyte or as a proton exchange membrane within a cell. fuel.

  In such a cell, an electrolyte is sandwiched between two active layers which are the seat of anodic and cathodic reactions ensuring the conversion of chemical energy into electrical energy. According to this configuration, the electrolyte must provide proton transfer from one active layer to the other. For this purpose, the fuel cells are solid polymer electrolyte (SPEFC for Solid Polymer Electrolyte Fuel Cell) or proton exchange membrane (Proton Exchange Membrane Fuel Cell) and are certainly the most elegant in terms of design and operation. To ensure the transport of protons, the known membranes contain 20 to 30% water, the water molecules ensuring the solvation of the protons which then become extremely mobile within the membrane.

  When the polymer constituting the membrane also comprises acid functions, the water molecules allow the dissociation of the acid groups and these functions, thus releasing the protons which they then transport.

  The performance of the membranes are therefore limited by the amount of water they contain, the possible dehydration of the membrane may lead to the interruption of proton transport.

  This dehydration can occur because of the mobility of the water molecules and / or because of the operating temperature of the cell. In fact, the water molecules, which are not bound to the polymer structure of the membrane, are mobile and can be driven in particular by large proton fluxes through the membrane, thereby reducing its conductive properties until the interruption. total proton transport. Only additional procedures would prevent dehydration due to the mobility of the water molecules, but they bring in part a surcharge and a complication of the battery system (auxiliaries dedicated to humidification). On the other hand, there is a limit operating temperature of the batteries naturally corresponding to the boiling temperature of water of 100 C at atmospheric pressure. This temperature is a physical limit, beyond which the amount of water decreases considerably until the membrane is completely dry. A solution indicated by the laws of thermodynamics would consist of increasing the operating pressure of the system, the boiling temperature of the water increasing accordingly. However, this solution is not satisfactory because for large operating pressures, the flow of protons through the membrane leads to even more water molecules, ultimately penalizing the performance of the battery. Most of the polymers constituting the ionomeric membranes used in fuel cells contain proton moieties SO3H, PO3H2. These functional groups are dissociated in the presence of water, or in the presence of molecules or basic functional groups. When these basic groups have different donor sites and proton acceptors, they can play a role identical to that of water molecules in proton transport.

  However, in this type of membrane, elution problems can occur during operation of the fuel cell. In this context, the invention aims primarily to overcome the aforementioned drawbacks by the development of a membrane providing proton transport at operating temperatures above 100 C and whose manufacturing process has a significant yield. For this purpose, the membrane according to the invention is made based on a heterocyclic polymer comprising heterocycles which transport protons within the membrane, including in the absence of water. Thus, proton migration is provided by means of heterocycles which offer an additional advantage over water molecules, since these heterocycles are anchored within the membrane, avoiding any elution problem. More specifically, the invention relates to a monomer carrying an imidazole heterocycle, which can be used for the manufacture of an ionic conductive membrane, and whose chemical structure comprises at least one unit of formula: in which R 1 comprises a grouping alkenyl, and wherein R2 comprises a protecting group of one of the nitrogen atoms of the heterocycle, such as a benzyl or trialkylsilyl group. According to an alternative embodiment, R 1 is of chemical formula CH = CH 2. According to another embodiment variant, R1 is of chemical formula (CH2) 2-O-CH = CH2. In addition, the process for producing the polymer constituting the membrane according to the invention consists in producing a monomer provided with an imidazole heterocycle and a protecting group of one of the nitrogen atoms of this heterocycle. in polymerizing this protected monomer to obtain a protected polymer, subjecting the protected polymer to a step of deprotecting the nitrogen atoms of the heterocycles, whereby a proton conducting polymer is obtained, and exhibits a high yield. The invention will be better understood and other objects, advantages and characteristics thereof will become clear from reading the description which follows, with reference to the appended figures in which: FIG. 1 represents examples of heterocycles which can be integrated into the polymer according to the invention; FIG. 2 illustrates the synthesis of a first monomer having an imidazole heterocycle according to the invention; FIG. 3 shows the synthesis of an intermediate molecule acting as reagent in the reaction of FIG. 2; FIG. 4 represents the synthesis of a polymer according to the invention from the first monomer of FIG. 2; FIG. 5 illustrates the 19F NMR characterization of the polymer of FIG. 4; Figure 6 shows the 1H NMR characterization of the polymer of Figure 4; FIG. 7 illustrates the formation of a second imidazole polymer; FIG. 8 shows the formation of a second heterocycle monomer of imidazole type; Figure 9 is an illustration of 1-benzimidazole 7 of Figure 3; FIG. 10 represents the intermediate molecule 9 of FIG. 3; FIG. 11 illustrates the 1 H NMR spectrum obtained for the intermediate molecule 7; FIG. 12 is an illustration of the first monomer according to the invention; FIG. 13 represents the 1 H NMR spectrum obtained for the monomer of FIG. 12. The present invention proposes the elaboration of particular polymers forming membranes making it possible to exhibit proton conduction properties even at temperatures greater than 100 C, such as than 200 C at atmospheric pressure. Proton migration is provided by means of heterocycles 2, immobilized and anchored to the polymer structure of the membrane. In this way, the heterocycles 2 do not accompany the proton in its migration, even for high proton fluxes, and therefore maintain the complete efficiency of the cell, including at high temperature. The approach is thus based on the use of heterocycles 2 such as imidazoles 3, pyrazoles 4, benzimidazoles 5, illustrated in FIG. 1, as proton solvents, replacing the water molecules of the known membranes. Indeed, just like water, these heterocycles 2 form networks of hydrogen bonds, and have proton transport properties similar to those of water. In such an environment of immobilized heterocycles 2, the proton transport is essentially based on a known Grotthuss diffusion mechanism, involving a transfer of the proton between heterocycles 2 and their reorganization, for example by reorientation. Numerous types of heterocycle-bearing polymers 2 can be synthesized and a family of polymers comprising imidazole heterocycles 3 as well as various processes for their manufacture are detailed below. A first process for producing this polymer consists in the cationic homopolymerization of imidazole 6 monomers, such as 1-benzyl-2- (vinyloxyethyloxymethyl) imidazoles 6 using initiators of the ZnI2, HgI2 or other iodinated complex type and even salts of Crivello. The synthesis of this monomer 6, represented in FIG. 2, is carried out by phase transfer catalysis by reaction of an intermediate molecule 7, such as 1-benzyl-2- (hydroxymethyl) imidazole 7, on 2-chloroethyl vinyl ether 8.

  The phase transfer catalyst used in this reaction is triethyl benzyl ammonium chloride and the yields obtained vary between 85 and 90%. The formation step of the intermediate molecule 7, known to those skilled in the art, is represented in FIG. 3, and consists of a reaction between 1-benzylimidazole 9 and formaldehyde or paraformaldehyde 10. above-mentioned cationic homopolymerization of monomer 6, a polymer on which are anchored imidazole heterocycles 3, the nitrogen atoms of which are protected by a benzyl group and can be deprotected by reaction of the polymer with Pd (OH) 2. A second process for producing an imidazole polymer 3 according to the invention consists in a radical copolymerization represented in FIG. 4 of the monomer 6 described above with fluorinated monomers 11, such as tetrafluoroethylene (TFE), hexafluoropropene ( HFP, X = CF3) and / or chlorotrifluoroethylene (CTFE, X = C1), or bromorotrifluoroethylene (XTFE, X = Br), trifluoroethylene, pentafluoropropene, 3,3,3-trifluoropropene, perfluoro methyl vinyl ether (PMVE), perfluoroethyl vinyl ether (PEVE), perfluoro propyl vinyl ether (PPVE), which results in a first fluorinated polymer 12 carrying imidazole functions 3.

  The radical copolymerization is carried out with radical initiators of the peroxide, azo, peroxycarbonate or peroxypivalate type in an autoclave at a pressure of between 5 and 50 bar, at a temperature set according to the nature of the initiator used, so that at this temperature the initiator has a half-life of about 1 hour. This polymer 12 thus formed was characterized, in particular, by Nuclear Magnetic Resonance spectroscopy of 19F and 1H by analysis of the product of the copolymerization reaction, after evaporation of the solvent and purification by precipitation in pentane.

  The 19F NMR spectrum obtained (FIG. 5) confirms the obtaining of the fluorinated copolymer 12. Indeed, the complex signals visible on this spectrum, located between -70 and -85 ppm and between -108 and -125 ppm, are characteristic of the chemical shift of fluorine atoms of chlorotrifluoroethylene (CTFE) units in the CTFE-vinyl ether copolymer. On the other hand, the 1 H NMR spectrum confirms the incorporation of the monomer 6 into the polymer 12 (FIG. 6) and the absence of transfer to the monomer, solvent, initiator and polymer.

  The polymer 12 is thus constituted by a succession of imidazole monomers 6 which each comprise a nitrogen-protecting group, namely a benzyl group. The protected polymer obtained 12 is subjected to a deprotection step shown in FIG. 7 during which the benzyl protective groups are replaced by hydrogen atoms, so as to form the imidazole groups which will provide the proton conduction. This deprotection step consists of a reaction of the first polymer 12 in the presence of dihydrogen and palladium dihydroxyl Pd (OH) 2. The deprotected polymer 12 'thus obtained is similar to the first polymer 12, but comprises a hydrogen atom. replacing the benzyl group of the first polymer 12, and accordingly has proton conductive properties. As an indication, the following is given in detail the experiments carried out for the formation of the deprotected polymer 12 ':

  Synthesis of 1-benzylimidazole 9 illustrated by step (a) of Figure 3.

  According to the experimental protocol adopted, 44 g of imidazole (0.65 mol), 100 g of benzyl chloride (0.78 mol) and 60 g of KOH (1.07 mol) in 700 mL of THF are placed in a 1L flask. The reaction mixture is refluxed for 120 h and then cooled to room temperature. The organic phase is filtered and the solvent is evaporated under reduced pressure. The solid product obtained is dissolved in 500 mL of CH 2 Cl 2 and then washed with water (3 × 100 mL). The organic phase is dried over Na 2 SO 4, filtered and then evaporated under reduced pressure. The collected solid is recrystallized from toluene to give 67 g of desired product.

  Synthesis of 1-benzyl-2-hydroxymethylimidazole 7 represented by step (b) of FIG. 3 50 g of 1-benzylimidazole of the crude product of the preceding reaction and 15 g of paraformaldehyde are dissolved in 150 ml of dioxane. The mixture is brought to 135 ° C. under an autoclave for 20 hours. The solvent is evaporated off under reduced pressure, the product obtained in the form of a black oil is dissolved in 400 mL of CH 2 Cl 2 and then washed with water (2 × 100 mL). The organic phase is dried over Na 2 SO 4, filtered and then evaporated under reduced pressure. The product is drawn under vacuum for 24 h and then recrystallized from 100 mL of ethyl acetate to give 30 g of pure product (yield = 50% from imidazole). The R.MN 1H spectrum obtained for the final product, visible in FIG. 11, confirms the production of 1-benzyl-2-hydroxymethyl-imidazole 7. Synthesis of 1-benzyl-2- (vinyloxyethyloxymethyl) imidazole 6 represented by Step (c) of Figure 3 To a solution of 1-benzyl-2-hydroxymethylimidazole (4.7 g, 0.024 mol) in toluene (15 mL) is added 0.54 g of TEBAC (2.4 × 10 3). mol) and 5.6 g of CEVE (0.05 mol). A 20 N basic solution in NaOH (100 ml) is then added to the medium and the heterogeneous mixture is heated at 90 ° C. for 24 hours under vigorous stirring. After cooling, the solution is diluted with 250 ml of decanted ether and the organic phase is then washed with water to pH = 7 (10 × 100 ml). The organic phase is dried over Na2SO4, filtered and then evaporated under reduced pressure to give 5.3 g (85%) of desired product. Given the satisfactory purity of the product obtained, no further purification is required.

  The 1H NMR spectrum of the product obtained, represented in FIG. 13, confirms the production of 1-benzyl-2- (vinyloxyethyloxymethyl) imidazole 6.

  Synthesis of the Poly (E.V.-alt-CTFE) or Protected Polymer Copolymer 12 Illustrated by Step (d) of Figure 4

  The copolymerization of 1-benzyl-2 (vinyloxyethyloxymethyl) imidazole 6 with CTFE is carried out in an autoclave of 100 cm3 in hastelloy equipped with a magnetic bar, two valves (gas inlet and outlet), a disc of safety, a precision manometer and a thermometer. The autoclave is first purged of the air it contains thanks to a high vacuum. 10 g of the liquid monomer (vinyl ether having the protected imidazole group), 0.210 g of tert-butylperoxypivalate initiator and 60 ml of 1,1,1,3,3-pentafluorobutane are introduced into the system.

  The whole is then cooled in an acetone bath at -80 ° C. and 10.1 g of CTFE are introduced into the system. The autoclave is finally placed in an oil bath at 75 ° C. for 14 hours, the pressure reaches 15 bar at the beginning of handling and ends at 12 bar after 14 hours. The system is then allowed to warm to room temperature, the unreacted CTFE is purged and the reaction crude is evaporated to give a black viscous oil. The brown product is solubilized in dichloromethane and then precipitated in pentane to obtain an orange powder.

  Deprotection of imidazoles of the poly (EV-alt-CTFE) copolymer by visible hydrogenation in FIG. 7 (step (e)) In the same autoclave as that previously used, 100 ml of acetic acid, 4 g of poly-copolymer, are introduced. (EV-alt-CTFE) and 1 g of palladium dihydroxide catalyst. The autoclave is then closed and 32 bars of hydrogen are introduced. The reaction mixture is heated at 70 ° C. for 48 hours. The reaction crude is filtered on celite and the solvent is evaporated. The product obtained is refluxed in 50 ml of a solution of Na2CO3 at 5%. The product is filtered, washed with three times 50 ml of distilled water and then dried. It is finally taken up in methanol and precipitated in ether. 2 g of orange solid is obtained with a deprotection rate of 90%. The analyzes carried out on the final product show that it has a glass transition temperature of -30 ° C. and a thermal stability of up to 200 ° C. for an average molar mass of 8500 g / mol (PMMA equivalent).

  These steps make it possible to form a proton conducting polymer having as a pattern the monomer 6 of FIG. 12.

  It is possible to prepare a polymer with imidazole groups 3 according to a third manufacturing process consisting of a polymerization of a second imidazole monomer 14, 1-benzyl-2- (vinyloxymethyl) imidazole. As can be seen in FIG. 8, the synthesis of this monomer 14 consists of a transetherification reaction of the intermediate molecule 7 of FIG. 3 with ethyl vinyl ether 15, in the presence of transetherification catalysts such as palladium acetate Pd ( OAc) 2 or mercury Hg (OAc) 2 16. Thereafter, this monomer 14 is polymerized, either by homopolymerization previously described with reference to the first process, or by copolymerization with fluorinated monomers 11 as described with reference to the second process, this results in the formation of a protected fluorinated imidazole polymer or not.

  As described for the first and second processes described above, the polymer obtained by the third process can react with Pd (OH) 2 to obtain a deprotected polymer, that is to say without benzyl groups and which will be conductive proton.

  A fourth method of forming the polymer according to the invention may consist of a terpolymerization of one or other of the monomers 6 and 14 described above and respectively illustrated in FIGS. 2 and 8, with two fluorinated alkenes chosen from tetrafluoroethylene (TFE), hexafluoropropene (HFP, X = CF3) and / or chlorotrifluoroethylene (CTFE, X = C1), or bromorotrifluoroethylene (XTFE, X = Br), trifluoroethylene, pentafluoropropene, 3,3 , 3-trifluoropropene, perfluoroalkyl methyl vinyl ether (PMVE), perfluoroalkyl ethyl vinyl ether (PEVE), perfluoroalkyl propyl vinyl ether (PPVE) or any other analogous molecule having a longer fluorinated chain. As for the polymers made by the above processes, the polymer obtained by the fourth method can be deprotected by reaction with Pd (OH) 2 and form a proton conductive polymer.

  The different deprotected polymers obtained from the four processes described above can constitute the base of proton conducting membranes.

  It should be noted that different monomers bearing imidazole function can be manufactured according to a manufacturing method similar to the first process for the manufacture of monomer 6, but in which the chloroethylvinylether 8 reagent would be replaced for example by a longer chain compound, comprising a such chloroethylvinylether 8.

  Likewise, other monomers according to the invention can be produced by means of a process similar to the second process for producing monomer 14, in which the reagent ethylvinyl ether would be replaced in particular by a longer chain compound and comprising this ethylvinylether 15.

  Thus, other polymers than those described above can be made from these monomers and constitute different ionic conducting membranes, which can serve as electrolytes in different electrochemical devices such as an electrolyzer or a fuel cell used for example to ensure the traction of a motor vehicle. The membrane according to the invention used within a fuel cell makes it possible to exceed the operating limit temperature of known polymer solid electrolyte fuel cells, to reduce the complexity of the battery system (battery and its auxiliaries), and to reduce by consequently its cost (the auxiliaries dedicated to humidification are no longer necessary). The heterocycles of the deprotected polymers formed according to the abovementioned manufacturing processes, provide proton transport within the membrane, whereas the fluorinated groups present within the polymer for certain manufacturing processes (copolymerization with fluorinated alkenes, or monomers carrying fluorinated heterocycle) endow the polymer with a certain chemical inertness in an oxidizing or reducing medium and a thermostability which participate in the stabilization of the membrane during operation, in particular for use within a fuel cell operating at higher temperatures. at 100 C.

Claims (24)

  1. Imidazole-type heterocycle-bearing monomer, which can be used for the manufacture of an ionic conductive membrane, characterized in that its chemical structure comprises at least one unit of formula: in which R 1 comprises an alkenyl group, and in which R2 comprises a protecting group of one of the nitrogen atoms of the heterocycle, such as a benzyl or trialkylsilyl group.   2. The monomer according to claim 1, wherein R1 is of chemical formula CH = CH2.   3. The monomer according to claim 1, wherein R1 is of chemical formula (CH2) 2-O-CH = CH2.   4. Process for the manufacture of the monomer of any one of claims 1 to 3, characterized in that it comprises at least one step of forming (a) 1-benzyl-2- (hydroxymethyl) imidazole (7) and at least one step of synthesizing said monomer (6, 14) from this 1-benzyl-2- (hydroxymethyl) imidazole (7).   The process according to claim 4, wherein the step of synthesizing the monomer (14) is a transetherification of 1-benzyl-2- (hydroxymethyl) imidazole (7) with ethyl vinyl ether (15), which 1-Benzyl-2- (vinyloxymethyl) imidazole (14) results. R1   The process according to claim 5, wherein the transetherification is carried out in the presence of mercury acetate (16).   The process according to claim 4, wherein the step of synthesizing the monomer (6) consists of a reaction between 1-benzyl-2- (hydroxymethyl) imidazole (7) and chloroethyl vinyl ether (8) in the presence of a phase transfer catalyst, which results in 1-benzyl-2- (vinyloxyethyloxymethyl) imidazole (6).   The process of claim 7, wherein the phase transfer catalyst is triethyl benzyl ammonium chloride.   A protected polymer comprising the monomer of any one of claims 1 to 3, or comprising the monomer made by any one of the processes of claims 4 to 7.   The protected polymer of claim 9, further comprising acidic functions.   11. The method of manufacturing the protected polymer of claim 9 or 10, from the monomer of any one of claims 1 to 3, comprising at least one polymerization step of this monomer (6, 14).   The process according to claim 11, wherein the polymerization step consists of cationic homopolymerization of the monomer (6, 14) in the presence of a cationic initiator, such as ZnI 2, HgI 2 or Crivello salts.   13. The method of claim 11, wherein the polymerization step consists of a radical copolymerization of this monomer with a second monomer.   14. The method of claim 11, wherein the polymerization step is a terpolymerization of the monomer (6, 14) with two second monomers.   The method of claim 13 or 14, wherein the second monomer is an electron-accepting vinyl monomer, such as a fluorinated or perfluorinated monomer (11).   16. The method of claim 15, characterized in that the fluorinated monomer (11) is selected from the family of fluorinated alkenes, such as chlorotrifluoroethylene, hexafluoropropene, trifluoroethylene, tetrafluoroethylene, 3,3,3-trifluoropropene , pentafluoropropene, perfluorinated olefins, and / or vinylidene fluorides, perfluoroalkyl vinyl ether, perfluoroalkoxyalkyl vinyl ether, perfluoroalkylvinylether.   17. A deprotected polymer (12 ') made from the protected polymer (12) of claims 9 or 10 or based on the polymer made from any of the processes of claims 11 to 16, comprising hydrogen atoms in the form of replacing the protective groups (R2) of the protected polymer (12).   The process for producing the protected polymer of claim 17, using the protected polymer (12) of claim 9 or 10 or the polymer made from any of the processes of claims 11 to 16, characterized in that it comprises a step of deprotecting the nitrogen atoms of the heterocycles of the protected polymer, 12 by which the protective groups (R2) of the polymer (12) are replaced by hydrogen atoms, forming a deprotected polymer (12 ') provided heterocycles of imidazole type.   19. The method of claim 17, characterized in that the deprotection step consists of a reaction of the protected polymer (12) obtained by the polymerization step with Pd (OH) 2.   20. An ionic conductive membrane, characterized in that it is made from the deprotected polymer (12 ') of claim 17 or the polymer made according to any one of the processes of claims 18 to 19.   An electrochemical device involving as an electrolyte the ionic conductive membrane of claim 20.   22. Fuel cell involving as electrolyte the ionic conductive membrane of claim 20.   23. Use of the fuel cell of claim 22 for traction of a motor vehicle.   Electrolyser involving as electrolyte the ionic conductive membrane of claim 20.