Solutions of TEMPO derivatives used as electrolytes in redox flow batteries

Abstract

The present invention relates to a solution comprising water and different 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) derivatives, a method for producing the solution, a method for preparing a redox flow battery comprising the solution as an electrolyte, a redox flow battery comprising the solution as an electrolyte in one compartment of the battery, and the use of the redox flow battery for storing electrical energy.

Description

[0001] The present invention relates to a solution comprising water and different 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) derivatives, a method for producing the solution, a method for preparing a redox flow battery comprising the solution as an electrolyte, a redox flow battery comprising the solution as an electrolyte in one compartment of the battery, and the use of the redox flow battery for storing electrical energy.

[0002] The demand for storing electrical energy in different types of applications is huge. It has been found that redox flow batteries with new organic compounds as redox pairs comprising 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) derivatives having redox potentials separated from each other by using a membrane selective for molecules by size may be a simple and inexpensive approach to providing long-life redox flow batteries that do not have a negative impact on the environment, as described in WO 2014 / 26728.

[0003] WO2018 / 028830 describes a method for producing 4-ammonium-2,2,6,6-tetraalkylpiperidinyl salts as typical TEMPO derivatives, which are redox-active compounds of this type that are commonly used in the cathode compartment of redox flow batteries. It is disclosed that 3 different production methods can be used to produce these compounds. The starting product is always a solution of 4-oxoalkylpiperidine, the corresponding imine or 4-aminoalkylpiperidine dissolved in different kinds of organic aprotic solvents such as alcohols, ethers, nitriles, halogenated hydrocarbons, aromatic hydrocarbons, aliphatic hydrocarbons or mixtures thereof. In the two production methods described in WO 2018 / 028830, the intermediate compound is a solid, which must be separated from the solvent before being used in the next step. By these described production methods, very pure final compounds are obtained, but there are many intermediate stages in which solids must be disposed of, different solvents must be used or anions must be exchanged. In addition, there is no description of a production method using only water as a solvent. For industrial scale, the methods described in the prior art are not suitable, since each filtration step, solvent exchange or anion exchange step causes huge losses in terms of yield and time and generates waste solvents, which require further purification before they can be reused. In addition, the use of aprotic organic solvents in the production process will require at least one solvent exchange, since aqueous solutions are preferred as electrolytes in redox flow batteries. Aqueous solutions are advantageous, since water is not easily oxidized or reduced in redox batteries, it is non-flammable and therefore safer to dispose of than organic solvents, it is non-toxic and very cheap and readily available.

[0004] Therefore, it is also an object of the present invention to provide an aqueous solution comprising TEMPO derivatives having different chemical redox potentials comparable to those of isolated pure TEMPO derivatives described in the prior art, which is provided in a simple and inexpensive process for producing the solution, in which no solid intermediates have to be separated or handled, no solvent exchange is required and no anion exchange is necessary and which can be used on an industrial scale without extensive effort and large yield losses. Another object of the present invention is to provide a redox flow battery comprising an aqueous solution of TEMPO derivatives, which battery shows similar or identical performance in storing energy as a redox flow battery comprising an aqueous solution of 2,2,6,6-tetramethyl-1-piperidinyloxy-4-trimethylammonium chloride as described in the prior art.

[0005] This problem will be solved by a solution containing the following components:

[0006] a) water,

[0007] b) 20-55% by weight, based on the total weight of the solution, of a compound of formula (I) 2,2,6,6-tetramethyl-4-(trimethylammonio)-1-piperidinyloxy:

[0008]

[0009] c) 0.1 to 6% by weight, based on the total weight of the solution, of an alkali metal cation,

[0010] d) 0.5-12.5% ​​by weight, based on the total weight of the solution, of a compound of formula (II) N,N,N,1,2,2,6,6-octamethyl-4-piperidinammonium-1-oxide:

[0011]

[0012] e) 0.1-20% by weight, based on the total weight of the solution, of a compound of formula (III) 2,2,6,6-hexamethyl-4-(dimethylamino)-1-piperidinyloxy-N-oxide:

[0013]

[0014] The solution of the invention will be advantageous if the alkali metal cation is Na.

[0015] The solutions according to the invention are advantageous if the pH value is in the range of 2 to 7.

[0016] Solutions according to the invention are advantageous if the sum of the amount of compounds of the formula (I), (II), (III) and the amount of alkali metal cations in the solution is in the range from 20 to 50% by weight, based on the total weight of the solution.

[0017] The solutions according to the invention are advantageous if at least 90 mol % of the counterions are chloride ions.

[0018] Another embodiment of the present invention is a method for producing the solution of the present invention, comprising the following steps: i) reacting a compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine

[0019]

[0020] Reaction with water and methyl chloride to obtain an aqueous mixture comprising a compound of formula (V), N,N,N,2,2,6,6-heptamethyl-4-piperidinaminium, a compound of formula (VI), N,N,N,1,2,2,6,6-octamethyl-4-piperidinaminium, and unreacted compound of formula (IV), N,N,2,2,6,6-hexamethyl-4-piperidinaminium:

[0021]

[0022] ii) reacting the aqueous mixture obtained in step i) with an aqueous hydrogen peroxide solution in the presence of a catalyst selected from alkali metal carbonates, alkali metal bicarbonates, carbon dioxide and mixtures thereof, iii) adding an acid to the mixture obtained in step ii) until the pH value is in the range of 2 to 7, iv) partially removing water until the concentration of the compound of formula (I) according to the solution of claim 1 is in the range of 20 to 55% by weight.

[0023] The process according to the invention is advantageous if, in step i) of the process, the compound of the formula (IV) is reacted with methyl chloride and water at a temperature in the range from 0 to 60°C.

[0024] The process according to the invention is advantageous if, in step i) of the process, the compound of formula (IV) is reacted with 0.7 to 1.2 equivalents of methyl chloride in the presence of water and the mass ratio of the compound of formula (IV) to water in the feed mixture is in the range from 0.1 to 5.

[0025] The process according to the invention is advantageous if, in step ii) of the process, the mixture obtained from step i) is reacted with an aqueous hydrogen peroxide solution in the presence of a catalyst at a temperature in the range from 20 to 80° C., wherein during the addition of the aqueous hydrogen peroxide solution the pH value is maintained between 7 and 10.

[0026] The process according to the invention is advantageous if, in step ii) of the process, 1.5 to 5 mol of aqueous hydrogen peroxide in a concentration in the range from 25 to 70% by weight is used per mol of compound of formula (IV) in the feed mixture and the reaction is carried out in the presence of 0.005 to 0.4 mol of catalyst per mol of compound of formula (IV) in the feed mixture.

[0027] The process according to the invention is advantageous if the addition of the acid in step iii) is started when the concentration of hydrogen peroxide in the mixture from step ii) has fallen to less than 0.5% by weight.

[0028] The process according to the invention is advantageous if the acid in step iii) is hydrochloric acid.

[0029] Another embodiment of the invention is a method for producing a redox flow battery, wherein the solution of the invention is used as electrolyte in one of the two compartments of the battery.

[0030] The method of the present invention for preparing a redox flow battery will be advantageous if it comprises the following steps:

[0031] a) providing two compartments for a catholyte solution and an anolyte solution, each compartment being connected to at least one storage tank for a catholyte solution and an anolyte solution, respectively,

[0032] b) the two compartments are separated by an ion-conducting membrane,

[0033] c) equipping each chamber with electrodes,

[0034] d) filling the cathode electrolyte compartment with a solution according to the invention as redox-active material,

[0035] e) filling the anolyte compartment with an anolyte solution comprising a further redox active material.

[0036] Another embodiment of the present invention is a redox flow battery obtained by the inventive method for producing a redox flow battery.

[0037] Another embodiment of the present invention is the use of the inventive redox flow battery for storing electrical energy.

[0038] The solution of the present invention comprises water, 20-55 wt % of the compound 2,2,6,6-tetramethyl-4-(trimethylammonium)-1-piperidinyloxy of formula (I), based on the total weight of the solution, 0.1-6 wt % of alkali metal cations, based on the total weight of the solution, 0.5-12.5 wt % of the compound N,N,N,1,2,2,6,6-octamethyl-4-piperidinammonium-1-oxide of formula (II), based on the total weight of the solution, and 0.1-20 wt % of the compound 2,2,6,6-hexamethyl-4-(dimethylamino)-1-piperidinyloxy-N-oxide of formula (III), based on the total weight of the solution.

[0039] According to all the compounds of formula (I), (II), (III) and alkali metal cations in the solution of the present invention, the compound of formula (I) is preferably the main compound. Preferably, the amount of the compound of formula (I) in the solution of the present invention is in the range of 20 to 55% by weight, in particular in the range of 35 to 50% by weight, more preferably in the range of 40 to 45% by weight, based on the total amount of the solution.

[0040] The solution of the present invention also contains alkali metal cations. Preferably, these alkali metal cations are selected from Na and K, more preferably Na is an alkali metal cation. The amount of alkali metal cations in the solution of the present invention is preferably in the range of 0.1 to 6 wt. %, in particular in the range of 0.3 to 3.0 wt. %, more preferably in the range of 0.5 to 1.7 wt. %, based on the total amount of solution.

[0041] The compound of formula (II) is a compound produced by oxidation of one of the two by-products formed by the methylation of the starting compound of formula (IV) in the method of the present invention. Preferably, the amount of the compound of formula (II) in the solution of the present invention is in the range of 0.5 to 12.5 wt %, particularly in the range of 0.5 to 5.0 wt %, more preferably in the range of 2.0 to 3.0 wt %, based on the total amount of the solution.

[0042] The compound of formula (III) is another compound or a second compound produced by oxidation of one of the two by-products formed by the methylation of the starting compound of formula (IV) in the method of the present invention. Preferably, the amount of the compound of formula (III) in the solution of the present invention is in the range of 0.1 to 20% by weight, particularly in the range of 0.1 to 5.0% by weight, more preferably in the range of 0.1 to 1.5% by weight, based on the total amount of the solution.

[0043] The sum of all amounts of compounds of formula (I), (II), (III) and alkali metal cations is preferably in the range of 20 to 50 wt %, in particular in the range of 30 to 50 wt %, more preferably in the range of 40 to 50 wt %, based on the total amount of the solution.

[0044] The amount of water in the solution of the present invention is preferably in the range of 35 to 75% by weight, particularly in the range of 45 to 70% by weight, and more preferably in the range of 50 to 60% by weight, based on the total amount of the solution.

[0045] Preferably, the pH value of the solution of the present invention is in the range of 2-7, in particular in the range of 3-5, and more preferably in the range of 4-5.

[0046] In the solutions of the present invention, the anions present as counterions to the cationic species are selected from chloride, fluoride, perchlorate, sulfate, alkyl sulfonate, aryl sulfonate, phosphate, alkyl phosphonate, aryl phosphonate and nitrate or mixtures thereof. Preferably, the anions are selected from chloride, nitrate, sulfate and perchlorate, more preferably the anions are chloride. Since chloride anions are the most preferred anions, they should account for at least 90 mol% of all anions in the solutions of the present invention, preferably at least 95 mol% of all anions, more preferably more than 99 mol% of all anions.

[0047] The solution of the present invention is obtained by the method of the present invention. The method of the present invention comprises the following steps:

[0048] i) making a compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine

[0049]

[0050] Reaction with water and methyl chloride to obtain an aqueous mixture comprising the compound N,N,N,2,2,6,6-heptamethyl-4-piperidinium of formula (V), the compound N,N,N,1,2,2,6,6-octamethyl-4-piperidinium of formula (VI) and unreacted compound N,N,2,2,6,6-hexamethyl-4-piperidinium of formula (IV):

[0051]

[0052] ii) reacting the aqueous mixture obtained in step i) with an aqueous hydrogen peroxide solution in the presence of a catalyst selected from alkali metal carbonates, alkali metal hydrogen carbonates, carbon dioxide and mixtures thereof, iii) adding an acid to the mixture obtained in step ii) until the pH value is in the range of 2 to 7, iv) partially removing water until the concentration of the compound of formula (I) according to the invention is in the range of 20 to 55% by weight.

[0053] For the first step of the process of the invention, the compound of formula (IV) is dissolved in water. Preferably, the mass ratio of the compound of formula (IV) to water in the feed mixture is in the range of 0.1 to 5, in particular in the range of 0.3 to 2, more preferably in the range of 0.7 to 1.2.

[0054] In the first step i) of the process according to the invention, the starting compound of formula (IV) dissolved in water is methylated with methyl chloride. Preferably, the aqueous solution of the compound of formula (IV) is methylated with 0.7-1.2 mol, in particular 0.9-1.2 mol, more preferably 1.0-1.1 mol of methyl chloride per mol of the compound of formula (IV) in the feed mixture. The phrase "compound of formula (IV) in the feed mixture" shall refer to the amount of compound of formula (IV) dissolved in water in step i) at the beginning of step i), not the amount of compound of formula (IV) remaining in solution after methylation in step i). During the methylation, the temperature of the reaction is preferably in the range of 0 to 60° C., in particular in the range of 0 to 40° C., more preferably between 15 and 25° C. The temperature can be controlled by external heating, cooling or by slowly adding methyl chloride to the aqueous solution of the compound of formula (IV) so that the temperature does not rise above 60° C. Preferably, the methyl chloride is added slowly and external cooling is used so that the temperature does not rise above 60° C.

[0055] The resulting mixture of step i) of the process of the present invention comprises water, compounds of formula (V), (VI) and unreacted compound of formula (IV). In step ii) of the process of the present invention, the resulting aqueous mixture of step i) is reacted with an aqueous hydrogen peroxide solution and an alkali metal catalyst selected from alkali metal carbonates, alkali metal bicarbonates, carbon dioxide and mixtures of these. Preferably, the catalyst is selected from Na2CO3, K2CO3, NaHCO3, KHCO3, CO2 and mixtures of these, in particular the catalyst is selected from Na2CO3 and NaHCO3, more preferably NaHCO3 is used as a catalyst. The mixture of selected catalysts is selected from Na2CO3 and NaHCO3, K2CO3 and KHCO3, Na2CO3 and KHCO3, K2CO3 and NaHCO3, Na2CO3 and CO2, K2CO3 and CO2, NaHCO3 and CO2 and KHCO3 and CO2. A preferred mixture is selected from NaHCO3 and Na2CO3.

[0056] The amount of catalyst used in step ii) of the process according to the invention is preferably in the range of 0.005 to 0.4 mol, in particular in the range of 0.01 to 0.2 mol, more preferably in the range of 0.02 to 0.1 mol, per mole of the compound of formula (IV) in the feed mixture used in step i) of the process according to the invention.

[0057] Step ii) of the process according to the invention is an oxidation step, in which the compounds of formula (IV), (V) and (VI) are oxidized to compounds of formula (I), (II) and (III). Therefore, the aqueous mixture obtained in step i) is preferably reacted with 1.5 to 5 mol, in particular 1.5 to 3 mol, more preferably 1.7 to 2.3 mol of aqueous hydrogen peroxide per mole of compound of formula (IV) in the feed mixture used in step i) of the process according to the invention in the presence of a catalyst.

[0058] Preferably, the concentration of the aqueous hydrogen peroxide solution used in step ii) of the process of the present invention is 25-70% by weight, in particular 30-70% by weight, more preferably 45-70% by weight of hydrogen peroxide.

[0059] During the oxidation process, the temperature of step ii) of the process of the present invention is preferably in the range of 20 to 80° C., in particular in the range of 40 to 60° C., more preferably in the range of 50 to 60° C. During the addition of the aqueous hydrogen peroxide solution, the pH value of the reaction solution in step ii) of the process of the present invention is preferably in the range of 7 to 10, in particular in the range of 8 to 10, more preferably in the range of 8 to 9. Adjustment of the pH value during the oxidation process in step ii) of the process of the present invention will be carried out by adding an acid or a base. The acid used for adjusting the pH value in step ii) of the process of the present invention is selected from hydrochloric acid, perchloric acid, sulfuric acid, phosphoric acid, nitric acid, methanesulfonic acid, p-toluenesulfonic acid, methylphosphonic acid or phenylphosphonic acid. Preferred acids are hydrochloric acid, sulfuric acid, nitric acid and perchloric acid. More preferred is hydrochloric acid. Hydrochloric acid can be added as a gas or in the form of an aqueous solution. Preferably, an aqueous solution of hydrogen chloride, commonly known as hydrochloric acid, is used. The base used for adjusting the pH value in step ii) of the process of the present invention is selected from NaOH and KOH, more preferably NaOH. During the oxidation process in step ii) of the process of the present invention, the addition of aqueous hydrogen peroxide solution usually leads to a decrease in the pH value of the solution. Therefore, the initial pH must be adjusted by adding a base. Preferably a base, preferably NaOH, is used to keep the pH during the oxidation process in the range of 7 to 10.

[0060] After the addition of the aqueous hydrogen peroxide solution is complete, the resulting mixture of step ii) of the process of the present invention is stirred under the conditions of step ii), which means about 60° C. and a pH value in the range of 7 to 10, until the concentration of hydrogen peroxide in the solution is less than 0.5% by weight. The amount of remaining hydrogen peroxide is determined by cerium (IV) titration of the solution before and after catalytic decomposition of hydrogen peroxide with manganese dioxide. The difference between the two titration values ​​is used to calculate the residual amount of hydrogen peroxide.

[0061] After the addition of the aqueous hydrogen peroxide solution has been completed and the concentration of hydrogen peroxide in the solution obtained in step ii) has dropped to less than 0.5% by weight, the pH of the solution will be adjusted in step iii) of the process of the invention by adding an acid. Preferably, the pH after the addition of the acid is in the range of 2 to 7, in particular in the range of 3 to 5, more preferably in the range of 4 to 5. The acid used to reduce the pH is selected from hydrochloric acid, perchloric acid, sulfuric acid, phosphoric acid, nitric acid and methanesulfonic acid. Preferred acids are hydrochloric acid, sulfuric acid, nitric acid and perchloric acid. More preferably hydrochloric acid is used as a gas or in an aqueous solution called hydrochloric acid.

[0062] In the last step of the process of the invention, the amount of water is reduced in step iv) of the process of the invention. This will preferably be carried out by distillation. The amount of water distilled off depends on the final concentration of the compound of formula (I) desired in the final solution. The water must be distilled off until the resulting solution shows a content of the compound of formula (I) in the range of 20 to 55% by weight, preferably in the range of 25 to 50% by weight, in particular in the range of 35 to 50% by weight, more preferably in the range of 40 to 45% by weight, based on the total weight of the solution. Preferably, the distillation will be carried out at a pressure in the range of 0.02 to 1.0 bar and at a temperature in the range of 20 to 100° C., in particular at a pressure in the range of 0.05 to 0.3 bar and at a temperature in the range of 30 to 70° C., more preferably at a pressure in the range of 0.08 to 0.2 bar and at a temperature in the range of 58 to 62° C.

[0063] After concentration, the solution of the present invention obtained can be used as an electrolyte in a redox flow battery without further treatment. Preferably, the solution of the present invention is used as a cathode electrolyte in the redox flow battery. Redox flow batteries are usually formed by using two compartments for a cathode electrolyte solution and an anode electrolyte solution, wherein each compartment is connected to a storage tank for the cathode electrolyte solution and the anode electrolyte solution, respectively, by a pump. The two compartments are separated by an ion-conducting membrane and are equipped with electrodes. In the cathode compartment and the connected cathode storage tank, the solution of the present invention is filled. In the anode compartment and the connected anode storage tank, the electrolyte of the anode is filled. The redox-active compounds in the redox flow battery change between their different redox levels during charging and discharging. For discharge, the electrolyte must be pumped from the storage tank to the electrode, while for charging, the reverse process is used. Therefore, a redox flow battery comprising a solution of the present invention as an electrolyte is a simple and versatile way of storing electrical energy for different applications. Example:

[0064] General:

[0065] pH value:

[0066] The pH values ​​were always measured using a calibrated glass electrode (EasyFerm Plus PHI S8 225, two-point calibration, using buffer pH=4.00 (citric acid, sodium hydroxide, sodium chloride from Fluka) and buffer pH=7.00 (monopotassium phosphate, disodium hydrogen phosphate from Fluka).

[0067] 1 H-NMR method:

[0068] The compound of formula (V) 1 H-NMR data:

[0069]

[0070] 1 H-NMR (500MHz, D2O): δ [ppm] = 3.68 (tt, J = 12.5Hz, 2.8Hz 1H,H1),3.07(s,9H,H6),2.02–2.08(m,2H,H3),1.32(t,J=12.5Hz,2H,H2),1.14(s,6H,H5),1.12(s,6H,H4).

[0071] The compound of formula (VI) 1 H-NMR data:

[0072]

[0073] 1 H-NMR (500MHz, D2O): δ [ppm] = 3.62 (tt, J = 12.5Hz, 3.1Hz, 1H, H7), 3.00 (s, 9H, H 12 ),2.15(s,3H,H 13 ),2.08–2.02(m,2H,H9),1.55(t,J=12.5Hz,2H,H8),1.15(s,6H,H 11 ),1.05(s,6H,H 10 ).

[0074] The compound of formula (IV) 1 H-NMR data:

[0075]

[0076] 1 H-NMR (500MHz, D2O): δ [ppm] = 2.83 (tt, J = 12.3Hz, 3.2Hz, 1H, H 14 ),2.27(s,6H,H 19),1.88(dd,J=12.7Hz,3.2Hz,2H,H 15 ),1.28(s,6H,H 17 ),1.24(s,6H,H 18 ),1.17(dd,J=12.7Hz,12.3Hz,6H,H 16 ).

[0077] The molar ratios of the compounds of formula (IV), (V) and (VI) can be determined by comparing 1 The integration of the H-NMR signals at δ = 3.68 ppm (from 1H of the compound of formula (V), 2.15 ppm (from 3H of the compound of formula (VI)) and 2.27 ppm (from 6H of the compound of formula (IV)) is most conveniently determined.

[0078] Therefore, the molar ratio of the compounds of formula (IV):(V):(VI) is the same as the ratio of the following integrals:

[0079] (Integral signal of the compound of formula (V) at δ=3.68 ppm): (Integral signal of the compound of formula (VI) at δ=2.15 ppm) / 3: (Integral signal of the compound of formula (IV) at δ=2.27 ppm) / 6 1 H-NMR measurement:

[0080] exist 1 Prior to H-NMR measurement, the solution of the present invention is reacted with an excess of phenylhydrazine (about 2 moles per mole of the compound of formula (I) and the compound of formula (III)) to convert the N-oxyl radical into the corresponding hydroxylamine. This procedure produces two isomeric forms of each reduced species (compounds of formula (Ia) and (Ib) / compounds of formula (IIIa) and (IIIb)) and each isomer is present in 1 The H-NMR spectrum gives individual signals. 1 For H-NMR measurement, the crude reaction mixture reduced with phenylhydrazine was diluted with D2O and referenced to the signal of residual H2O protons at δ = 4.79 ppm.

[0081] As described in WO 2018 / 2883011 on page 28, the signal assignment of the compound of formula (I) was confirmed by synthesizing the compound of formula (I) as a pure crystalline material. As described herein, the signal assignment of the compound of formula (III) was confirmed by synthesizing the compound of formula (III) as a pure substance in aqueous solution.

[0082] The compound of formula (III) was synthesized in pure form in aqueous solution:

[0083] To a solution of the compound of formula (IV) (39.3 g) in water (40.1 g) was added 37 wt % hydrochloric acid (11.97 g), thereby reducing the pH of the solution to 9.0. Solid sodium bicarbonate (2.71 g) was then added and the mixture was heated to 60° C. When this temperature was reached, a 50 wt % aqueous solution of hydrogen peroxide (46.4 g) was added continuously over 4 hours. During the addition, the pH dropped and was maintained above 8.0 by adding a 50 wt % aqueous solution of sodium hydroxide (6.4 g) in approximately five equal portions. After the addition of hydrogen peroxide was complete, stirring was continued for 12 hours. The mixture was then cooled to room temperature and stirred. 1 H NMR spectroscopy and ESI MS mass spectrometry analysis. 1 According to H NMR, the mixture contained >99 wt % of the compound of formula (III) as organic substance.

[0084] The confirmation of the compounds of formula (I) and (III) was also supported by HRMS (ESI in ACN:H2O:HCOOH=80:20:0.1, instrument: Q Extractive TM Hybrid-quadrupole-orbitrap mass spectrometer, ThermoFisher).

[0085] The reduced form of the compound of formula (I) 1 H-NMR:

[0086]

[0087] 1 H-NMR (500MHz, D2O): δ[ppm]=3.80–3.67(m,1H,H1+H 1’ ),3.12(s,9H,H6 or H 6’ , minor isomer), 3.09 (s, 9H, H6 or H 6’ , major isomer), 2.24–2.14 (m, 2H, H2+H 2’ ),1.99(t,J=12.1Hz,2H,H3orH 3’ , minor isomer), 1.75 (t, J = 12.4 Hz, 2H, H3 or H 3’ , major isomer), 1.34(s,6H,H 4 / 5 or H 4’ / 5’ , minor isomer), 1.26(s,6H,H 4 / 5 or H 4’ / 5’ , major isomer), 1.22(s,6H,H 4 / 5 or H 4’ / 5’, major isomer), 1.12(s,6H,H 4 / 5 or H 4’ / 5’ , the minor isomer). The ratio of the two isomers is about 90:10.

[0088] HRMS:C 12 H 26 N2O + Theoretical value: 214.2040; Measured value: 214.2036 The reduced form of the compound of formula (III) 1 H-NMR:

[0089]

[0090] 1 H-NMR (500MHz, D2O): δ[ppm]=3.66–3.52(m,1H,H7+H 7’ ),3.17(s,6H,H 12 or H 12’ , minor isomer), 3.14(s,6H,H 12 or H 12’ , major isomer), 2.27–2.14 (m, 2H, H8+H 8’ ),1.93(t,J=12.5Hz,2H,H9orH 9’ , minor isomer), 1.72 (t, J = 12.5 Hz, 2H, H9 or H 9’ , major isomer), 1.33(s,6H,H 10 / 11 or H 10’ / 11’ , minor isomer), 1.25(s,6H,H 10 / 11 or H 10’ / 11’ , major isomer), 1.21(s,6H,H 10 / 11 or H 10’ / 11’ , major isomer), 1.11(s,6H,H 10 / 11 or H 10’ / 11’ , minor isomer). The ratio of the two isomers is about 86:14.

[0091] HRMS:C 11 H 24 N2O2 + Theoretical value: 216.1638; measured value: 216.1637

[0092] The compound of formula (II) remains unchanged during the reduction and gives signals that are well separated from the signals of the compounds of formula (Ia), (Ib), (IIIa) and (IIIb):

[0093]

[0094] 1 H-NMR (500MHz, D2O): δ [ppm] = 3.93 (tt, J = 13.3Hz, 3.2Hz, 1H, H 13 ),3.14(s,9H,H 19 ),3.03(s,3H,H 18 ),2.47(t,J=12.6Hz,2H,H 14 ),2.07(d,J=12.1Hz,2H,H 15 ,1.65(s,6H,H 16 ),1.56(s,6H,H 17 ).

[0095] HRMS: C 13 H 29 N2O + Theoretical value: 229.2274; measured value: 229.2271

[0096] The ratio of the compounds of formula (I), (II) and (III) can be determined by comparing 1 The integration of the H-NMR signals at δ==3.80-3.67 ppm (from 1H of the compound of formula (I), 1.56 ppm (from 6H of the compound of formula (II)) and 3.66-3.52 ppm (from 1H of the compound of formula (III)) is most conveniently determined.

[0097] Therefore, the molar ratio of the compounds of formula (I): (II): (III) is the same as the ratio of the following integrals:

[0098] (Integrated signal of the compound of formula (I) at δ=3.80-3.67 ppm): (Integrated signal of the compound of formula (II) at δ=1.56 ppm) / 6: (Integrated signal of the compound of formula (III) at δ=3.66-3.52 ppm).

[0099] Cerium(IV) Redox Titration:

[0100] Cerium (IV) redox titration was used to determine the total content of hydrogen peroxide and N-oxyl species (compounds of formula (I) and (III)) according to the following method:

[0101] Content of N-oxyl species:

[0102] 100 mg of manganese dioxide is added to about 1 g of analyte. The mixture is stirred at 20-25° C. for 5 minutes or until 5 minutes after the end of gas evolution. The analyte is then filtered. 250±2 mg of the filtered analyte is placed in a beaker equipped with a magnetic stirring bar and diluted with 45 ml of pure water and 5 ml of dilute sulfuric acid (10% by weight in water). The obtained solution is placed on an automatic titration device (905 Titrando, Metrohm) equipped with a platinum Titrode (Metrohm) and stirred at 20-25° C. Cerium (IV) sulfate solution (0.10 mol / L) is added through the titration device until a redox potential jump (V C1 ). Then the concentration (weight %) of the sum of the compounds of formula (I) + (III), w I+III , can be calculated from the consumption of cerium (IV) sulfate solution using the following formula:

[0103]

[0104] The symbols have the following meanings:

[0105] V c is the volume of cerium sulfate solution used given in litres,

[0106] C c1 is the concentration of the cerium sulfate solution used given in mol / l,

[0107] m s is the mass of the analyte given in grams,

[0108] M I -Cl is 249.8 g / mol, the molar mass of the compound of formula (I) as a chloride salt

[0109] M III The molar mass of the compound of formula (III) is 215.3 g / mol.

[0110] x I is the molar fraction of the compound of formula (I), expressed as ( 1 Integral at δ=3.80-3.67 ppm in H-NMR): [( 1 Signal integration at δ = 3.66-3.52 ppm in H-NMR) + ( 1 The ratio calculation of the integral at δ = 3.80-3.67 ppm in H-NMR

[0111] x III is the molar fraction of the compound of formula (III), 1 Signal integration at δ=3.66-3.52 ppm in H-NMR): [( 1Signal integration at δ = 3.66-3.52 ppm in H-NMR) + ( 1 The ratio calculation of the integral at δ = 3.80-3.67 ppm in H-NMR

[0112] Sum of hydrogen peroxide and N-oxyl species:

[0113] 250 ± 2 mg of the analyte was placed in a beaker equipped with a magnetic stirring bar and diluted with 45 ml of pure water and 5 ml of dilute sulfuric acid (10 wt % in water). The obtained solution was placed on an automatic titration device (905 Titrando, Metrohm) equipped with a platinum Titrode (Metrohm) and stirred at 20-25° C. Cerium (IV) sulfate solution (0.10 mol / L) was added via the titration device until a redox potential jump (V C2 The concentration of hydrogen peroxide can be calculated from the difference in the consumption of cerium (IV) sulfate solution (ΔV C =V C2 -V C1 ) is calculated:

[0114]

[0115] where the symbols have the same meanings as defined above and M H2O2 It is 34.0 g / mol, which is the molar mass of hydrogen peroxide.

[0116] Cyclic voltammetry method:

[0117] The solution obtained from the corresponding example was diluted with 0.1 mol / L sodium chloride aqueous solution until the concentration of the N-oxyl compound was 1.0 wt %. The solution was placed in an electrochemical cell equipped with a standard three-electrode setup (working electrode: glassy carbon black Counter electrode: platinum wire, reference electrode: Ag / AgCl, 3 mol / L KCl in water). Using a PGU 20V-2A-E potentiostat (IPS), the potential was raised to 1200 mV and then cycled between 1200 mV and -700 mV with a scan rate of ±20 mV / s (3 cycles in total).

[0118] Embodiment 1:

[0119] In a stainless steel autoclave, 800 ml of water and 645 g of the N,N,2,2,6,6-hexamethylpiperidin-4-amine compound of formula (IV) are mixed and the temperature is adjusted to 20°C. Then, 185.5 g of methyl chloride are pressed into the autoclave at about 3.2 bar over 70 minutes. The mixture is then stirred at 20-25°C for 6 hours. Thereafter, the autoclave is depressurized and purged with nitrogen for about 10 minutes. 1630 g of an aqueous solution of compounds of formula (IV), (V) and (VI) (50% by weight of organic matter in water) is obtained. By 1 The molar ratio of the compounds of formula (IV): (V): (VI) determined by H-NMR was 2.5:92.8:4.7, which corresponds to 1.0 wt% of the compound of formula (IV), 46.5 wt% of the chloride salt of the compound of formula (V) and 2.5 wt% of the chloride salt of the compound of formula (VI).

[0120] Embodiment 2 (comparison):

[0121] The methylation is carried out as described in WO 2018 / 028830, page 27, line 20 to page 28, line 15 (corresponding to DE102016009904A1, paragraph

[0112] and subsequent contents). 1 In H-NMR, only the signal of the compound of formula (V) was observed.

[0122] Embodiment 3:

[0123] To the solution taken from Example 1 (100 g, containing 1.0 wt % of the compound of formula (IV), 46.5 wt % of the chloride salt of the compound of formula (V) and 2.5 wt % of the chloride salt of the compound of formula (VI) in water) was added 37 wt % hydrochloric acid (1.67 g), thereby reducing the pH of the solution to 9.0. Solid sodium bicarbonate (2.79 g) was then added and the mixture was heated to 60° C. When this temperature was reached, 50 wt % aqueous hydrogen peroxide solution (32.7 g) was added continuously over 4 hours. During the addition, the pH dropped and was maintained above 8.0 by adding 50 wt % aqueous sodium hydroxide solution (1.73 g) in about five equal portions. After the addition of hydrogen peroxide was complete, stirring was continued for 12 hours. The mixture was then cooled to about 30° C. and 37 wt % hydrochloric acid (about 2 g) was added to reduce the pH of the solution to 4.5. The water was subsequently distilled off under reduced pressure (70 mbar abs.) until the concentration of the N-oxyl species of the compounds of the formulae (I) and (III) was 46% by weight (determined by cerium(IV) redox titration).

[0124] pass 1The molar ratio of the compounds of formula (I): (II): (III) determined by H-NMR is 93.0:4.7:2.3, which corresponds to 45.0 wt % of the chloride salt of the compound of formula (I), 2.4 wt % of the chloride salt of the compound of formula (II) and 1.0 wt % of the compound of formula (III).

[0125] Embodiment 4:

[0126] To the solution taken from Example 1 (100 g, containing 1.0 wt % of the compound of formula (IV), 46.5 wt % of the chloride salt of the compound of formula (V) and 2.5 wt % of the chloride salt of the compound of formula (VI) in water) was added 37 wt % hydrochloric acid (5.83 g), thereby reducing the pH of the solution to 8.5. Solid sodium carbonate (1.17 g) was then added and the mixture was heated to 40° C. When this temperature was reached, 50 wt % aqueous hydrogen peroxide solution (32.7 g) was added continuously over 6 hours. During the addition, the pH dropped and was maintained above 8.0 by adding 50 wt % aqueous sodium hydroxide solution (6.39 g) in approximately five equal portions. After the addition of hydrogen peroxide was complete, stirring was continued for 12 hours. The mixture was then cooled to approximately 30° C. and 37 wt % hydrochloric acid (approximately 2.5 g) was added to adjust the pH of the solution to 4.0. The water was subsequently distilled off under reduced pressure (70 mbar abs.) until the concentration of the N-oxyl species of the compounds of the formulae (I) and (III) was 48% by weight (determined by cerium(IV) redox titration).

[0127] pass 1 The molar ratio of the compounds of formula (I): (II): (III) determined by H-NMR is 93.1:4.8:2.1, which corresponds to 47.1% by weight of the chloride salt of the compound of formula (I), 2.6% by weight of the chloride salt of the compound of formula (II) and 0.9% by weight of the compound of formula (III).

[0128] Embodiment 5 (comparison):

[0129] The oxidation was carried out as described in WO 2018 / 028830, page 28, line 20 to page 29, line 22. In the HRMS of the obtained product, no signals of the compounds of formula (II) and (IIIa) / (IIIb) were visible; 1 Only the signal of the compound of formula (Ia / Ib) was visible in H-NMR.

[0130] Example 6

[0131] Measure the cyclic voltammetry of the product of Example 3 (see Figure I ).

[0132] Example 7 (comparison):

[0133] Measure the cyclic voltammetry of Example 5 (see Figure II ).

[0134] Figure I The cyclic voltammogram and Figure II The cyclic voltammogram of Comparative Example 5 in Example 3 is almost the same. Therefore, the solution of the present invention of Example 3 shows almost the same redox potential as the solution obtained in Example 5 representing the prior art. Therefore, the solution of the present invention can be used in the redox flow battery described in the prior art.

Claims

1. A solution comprising: a) water, b) 20-55% by weight, based on the total weight of the solution, of a compound of formula (I) 2,2,6,6-tetramethyl-4-(trimethylammonium)-1-piperidinyloxy: c) 0.1 to 6% by weight, based on the total weight of the solution, of an alkali metal cation, d) 0.5-12.5% ​​by weight, based on the total weight of the solution, of a compound of formula (II) N,N,N,1,2,2,6,6-octamethyl-4-piperidinium-1-oxide: e) 0.1-20% by weight, based on the total weight of the solution, of a compound of formula (III) 2,2,6,6-hexamethyl-4-(dimethylamino)-1-piperidinyloxy-N-oxide:

2. The solution of claim 1, wherein the alkali metal cation is a Na ion.

3. The solution according to claim 1, wherein the pH value is in the range of 2 to 7. The solution according to claim 2 , wherein the pH value is in the range of 2 to 7.

5. A solution according to any one of claims 1 to 4, wherein the sum of the amount of the compound of formula (I), (II), (III) and the amount of the alkali metal cation in the solution is in the range of 30 to 50 wt % based on the total weight of the solution.

6. The solution of any one of claims 1 to 4, wherein at least 90 mol % of the counterions are chloride ions.

7. The solution of claim 5, wherein at least 90 mole % of the counter ions are chloride ions.

8. A method for producing a solution according to claim 1, comprising the steps of: i) making a compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine: Reaction with water and methyl chloride to obtain an aqueous mixture comprising the compound N,N,N,2,2,6,6-heptamethyl-4-piperidinium of formula (V), the compound N,N,N,1,2,2,6,6-octamethyl-4-piperidinium of formula (VI) and unreacted compound N,N,2,2,6,6-hexamethyl-4-piperidinium of formula (IV): ii) reacting the aqueous mixture obtained in step i) with an aqueous hydrogen peroxide solution in the presence of a catalyst selected from the group consisting of alkali metal carbonates, alkali metal bicarbonates, carbon dioxide and mixtures thereof, iii) adding an acid to the mixture obtained in step ii) until the pH value drops to within the range of 2 to 7, iv) partially removing water until the concentration of the compound of formula (I) is within the range of 20 to 55% by weight of the solution according to claim 1.

9. The process according to claim 8, wherein in step i) of the process, the compound of formula (IV) is reacted with methyl chloride and water at a temperature in the range of 0 to 60°C.

10. The process according to claim 8, wherein in step i) of the process, the compound of formula (IV) is reacted with 0.7-1.2 mol of methyl chloride in the presence of water and the mass ratio between the compound of formula (IV) and water in the feed mixture is in the range of 0.1 to 5.

11. The process according to claim 9, wherein in step i) of the process, the compound of formula (IV) is reacted with 0.7-1.2 mol of methyl chloride in the presence of water and the mass ratio between the compound of formula (IV) and water in the feed mixture is in the range of 0.1 to 5.

12. The process according to claim 8, wherein in step ii) of the process, the resulting mixture of step i) is reacted with an aqueous hydrogen peroxide solution in the presence of a catalyst at a temperature in the range of 20 to 80° C., wherein during the addition of the aqueous hydrogen peroxide solution, the pH value is maintained between 7 and 10.

13. The process according to claim 9, wherein in step ii) of the process, the resulting mixture of step i) is reacted with an aqueous hydrogen peroxide solution in the presence of a catalyst at a temperature in the range of 20 to 80° C., wherein during the addition of the aqueous hydrogen peroxide solution, the pH value is maintained between 7 and 10.

14. The process according to claim 10, wherein in step ii) of the process, the resulting mixture of step i) is reacted with an aqueous hydrogen peroxide solution in the presence of a catalyst at a temperature in the range of 20 to 80°C, wherein during the addition of the aqueous hydrogen peroxide solution, the pH value is maintained between 7 and 10.

15. The process according to claim 11, wherein in step ii) of the process, the resulting mixture of step i) is reacted with an aqueous hydrogen peroxide solution in the presence of a catalyst at a temperature in the range of 20 to 80°C, wherein during the addition of the aqueous hydrogen peroxide solution, the pH value is maintained between 7 and 10.

16. The process according to any one of claims 8 to 15, wherein in step ii) of the process, 1.5 to 5 moles of an aqueous hydrogen peroxide solution having a concentration in the range of 25 to 70% by weight per mole of the compound of formula (IV) in the feed mixture is reacted in the presence of 0.005 to 0.4 moles of a catalyst per mole of the compound of formula (IV) in the feed mixture.

17. The process according to any one of claims 8 to 15, wherein the addition of the acid in step iii) is started when the concentration of hydrogen peroxide in the mixture in step ii) has dropped to less than 0.5% by weight.

18. The process according to claim 16, wherein the addition of the acid in step iii) is started when the concentration of hydrogen peroxide in the mixture in step ii) drops to less than 0.5% by weight.

19. The process according to any one of claims 8 to 15, wherein the acid in step iii) is hydrochloric acid.

20. The process of claim 18, wherein the acid in step iii) is hydrochloric acid.

21. Method for producing a redox flow battery, wherein a solution according to any one of claims 1 to 7 is used as electrolyte in one of the two compartments of the battery.

22. The method according to claim 21, comprising the steps of: a) providing two compartments for a catholyte solution and an anolyte solution, each compartment being connected to at least one storage tank for a catholyte solution and an anolyte solution, respectively, b) the two compartments are separated by an ion-conducting membrane, c) equipping each compartment with electrodes, d) filling the cathode electrolyte compartment with a solution comprising 2,2,6,6-tetramethyl-4-(trimethylammonium)-1-piperidinyloxy salt as redox-active material according to any one of claims 1 to 7, e) filling the anolyte compartment with an anolyte solution comprising a further redox-active material.

23. A redox flow battery obtained by the method according to any one of claims 21-22.

24. Use of a redox flow battery according to claim 23 for storing electrical energy.

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