TEMPO derivative solutions for use as electrolytes in redox flow cells

Abstract

The present invention relates to a solution comprising water and various 2,2,6,6-tetramethyl-piperidinyl-oxyl (TEMPO) derivatives, a method for producing the solution, a method for making a redox flow cell comprising the solution as an electrolyte, a redox flow cell comprising the solution as an electrolyte in one chamber of the cell, and the use of a redox flow cell for storing electrical energy.

Description

[Technical Field]

[0001] The present invention relates to a solution comprising water and various 2,2,6,6-tetramethyl-piperidinyl-oxyl (TEMPO) derivatives, a method for producing the solution, a method for making a redox flow cell comprising the solution as an electrolyte, a redox flow cell comprising the solution as an electrolyte in one chamber of the cell, and the use of the redox flow cell for storing electrical energy. [Background technology]

[0002] There is a huge demand for storing electrical energy for a variety of applications. It has been found that a redox flow cell with new organic compounds as redox couples, including 2,2,6,6-tetramethylpiperidinyl-oxyl (TEMPO) derivatives with redox active potential, separated from each other by using a membrane that selects molecules by size, can be an easy and cheap way to provide a long-lasting redox flow cell without negative impact on the environment, as described in WO 2014 / 26728.

[0003] WO 2018 / 028830 describes a method for producing 4-ammonium-2,2,6,6-tetraalkylpiperidinyl salts as typical TEMPO derivatives, redox-active compounds commonly used in the cathode chamber of redox flow cells. Three different production methods are disclosed. The starting product is always a solution of either a 4-oxo-alkylpiperidine, the corresponding imine, or a 4-amino-alkylpiperidine dissolved in various 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 that must be separated from the solvent before use in the next step. While these described production methods yield very pure final compounds, there are many intermediate steps in which the solid must be processed, various solvents must be used, or anions must be exchanged. Furthermore, no production method is described in which only water is used as the solvent. On an industrial scale, the production methods described in the state of the art are unsuitable because each filtration step, solvent exchange, or anion exchange step causes significant losses in yield and time and produces waste solvent that requires further purification before it can be reused. Furthermore, because aqueous solutions are preferred as electrolytes in redox flow cells, the use of aprotic organic solvents in the production process may require at least one solvent exchange. Aqueous solutions have the advantage of being safer to handle than organic solvents, since water cannot be easily oxidized or reduced in redox flow cells and is not flammable, they are non-toxic, very cheap, and readily available. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] WO2014 / 26728 [Patent Document 2] WO2018 / 028830 Summary of the Invention [Problem to be solved by the invention]

[0005] It is therefore an object of the present invention to provide an easy and inexpensive method for producing aqueous solutions containing various TEMPO derivatives having chemical redox potentials comparable to those of the isolated pure TEMPO derivatives described in the state of the art, which can be used on an industrial scale without significant effort and loss of yield, and in which no solid intermediates need to be separated or processed, no solvent exchange is required, and no anion exchange is required. Another object of the present invention is to provide a redox flow cell containing an aqueous solution of a TEMPO derivative that exhibits similar or equivalent energy storage properties to the redox flow cell containing an aqueous solution of 2,2,6,6-tetramethyl-1-piperidinyloxy-4-trimethylammonium chloride described in the state of the art. [Means for solving the problem]

[0006] The challenge is, a) water, b) 20 to 55 wt.-% (% by weight) of the compound of formula (I) 2,2,6,6-tetramethyl-4-(trimethylammonio)-1-piperidinyloxy, relative to the total weight of the solution;

[0007] [ka] c) 0.1 to 6 wt.-% of alkali metal cations relative to the total weight of the solution; d) 0.5 to 12.5 wt.-% of the compound N,N,N,1,2,2,6,6-octamethyl-4-piperidineammonium-1-oxide of formula (II) relative to the total weight of the solution,

[0008] [ka] e) 0.1 to 20 wt.-% of the compound 2,2,6,6-hexamethyl-4-(dimethylamino)-1-piperidinyloxy-N-oxide of formula (III) relative to the total weight of the solution

[0009] [ka] This is solved by a solution containing

[0010] The solution of the present invention is advantageous when the alkali metal cation is Na.

[0011] The solution of the present invention is advantageous when the pH value is in the range of 2-7.

[0012] The solution of the present invention is advantageous when the sum of the amount of compounds of formulae (I), (II), (III) and the amount of alkali metal cations in the solution is in the range of 20 to 50 wt.-% relative to the total weight of the solution.

[0013] The solutions of the present invention are advantageous when at least 90 mol % of the counterions are chloride ions.

[0014] A further embodiment of the present invention is a method of producing a solution of the present invention, comprising the steps of: i) the compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine

[0015] [ka] with water and methyl chloride to produce an aqueous mixture containing the compound N,N,N,2,2,6,6-heptamethyl-4-piperidineaminium of formula (V), the compound N,N,N,1,2,2,6,6-octamethyl-4-piperidineaminium of formula (VI), and unreacted compound N,N,2,2,6,6-hexamethyl-4-piperidinamine of formula (IV).

[0016] [ka] obtaining a step of ii) reacting the aqueous mixture resulting from step i) with aqueous hydrogen peroxide in the presence of a catalyst selected from the group consisting of alkali metal carbonates, alkali metal bicarbonates, CO2 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) is in the range of 20-55 wt.-% according to the solution of claim 1. The method includes:

[0017] The process of the present invention is advantageous if, 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.

[0018] The process of the present 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 compound of formula (IV) to water in the feed mixture is in the range of 0.1 to 5.

[0019] The process of the present invention is advantageous if, in step ii) of the process, the mixture resulting from step i) is reacted with aqueous hydrogen peroxide in the presence of a catalyst at a temperature in the range of 20-80°C, wherein the pH value during the addition of the aqueous hydrogen peroxide solution is maintained between 7 and 10.

[0020] The process of the invention is advantageous if, in step ii) of the process, 1.5 to 5 mol of aqueous hydrogen peroxide are used, having a concentration in the range of 25 to 70 wt.-% 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.

[0021] The process of the invention is advantageous if the addition of acid in step iii) is started when the concentration of hydrogen peroxide in the mixture of step ii) falls below 0.5 wt.-%.

[0022] The process of the present invention is advantageous when the acid in step iii) is hydrogen chloride.

[0023] A further embodiment of the invention is a method of making a redox flow cell, wherein a solution of the invention is used as the electrolyte in one of the two chambers of the cell.

[0024] The method of the present invention for making a redox flow cell comprises the steps of: a) providing two chambers for catholyte and anolyte solutions, each connected to at least one storage tank for the catholyte and anolyte solutions, respectively; b) separating the two chambers with an ion-conducting membrane; c) equipping the chamber with electrodes; d) filling the solution of the present invention as a redox active material into the catholyte chamber; e) filling the anolyte chamber with an anolyte solution containing another redox active material; It is advantageous if the

[0025] A further embodiment of the present invention is a redox flow cell obtainable by the method of the present invention for making a redox flow cell.

[0026] A further embodiment of the present invention is the use of the redox flow cell of the present invention for storing electrical energy. [Brief explanation of the drawings]

[0027] [Figure 1] Cyclic voltammetry of the product of Example 3. [Figure 2] Example 5 Cyclic Bornometry. DETAILED DESCRIPTION OF THE INVENTION

[0028] The solution of the present invention comprises water, 20 to 55 wt. % of the compound of formula (I), 2,2,6,6-tetramethyl-4-(trimethylammonio)-1-piperidinyloxy, based on the total weight of the solution, 0.1 to 6 wt. % of an alkali metal cation, based on the total weight of the solution, 0.5 to 12.5 wt. % of the compound of formula (II), N,N,N,1,2,2,6,6-octamethyl-4-piperidineammonium-1-oxide, based on the total weight of the solution, and 0.1 to 20 wt. % of the compound of formula (III), 2,2,6,6-hexamethyl-4-(dimethylamino)-1-piperidinyloxy-N-oxide, based on the total weight of the solution.

[0029] The compound of formula (I) is preferably the major compound relative to all alkali metal cations and compounds of formulae (I), (II) and (III) in the solution of the invention. Preferably, the amount of compound of formula (I) in the solution of the invention is in the range of 20 to 55 wt.-%, in particular in the range of 35 to 50 wt.-%, more preferably in the range of 40 to 45 wt.-% relative to the total amount of the solution.

[0030] The solutions of the invention also contain alkali metal cations. Preferably, these alkali metal cations are selected from the group of Na and K, more preferably Na as the alkali metal cation. The amount of alkali metal cations in the solutions of the 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.-% relative to the total amount of the solution.

[0031] The compound of formula (II) is one of two by-products formed in the process of the present invention by methylation of the starting compound of formula (IV). 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. %, in particular 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 weight of the solution.

[0032] The compound of formula (III) is the second of two by-products formed in the process of the present invention by methylation of the starting compound of formula (IV), resulting from the oxidation of one of the two by-products. Preferably, the amount of compound of formula (III) in the solution of the present invention is in the range of 0.1 to 20 wt. %, in particular in the range of 0.1 to 5.0 wt. %, more preferably in the range of 0.1 to 1.5 wt. %, relative to the total amount of the solution.

[0033] The sum of all the amounts of compounds of formulae (I), (II), (III) and alkali metal cations is preferably in the range of 20 to 50 wt.-% relative to the total amount of the solution, in particular in the range of 30 to 50 wt.-% and more preferably in the range of 40 to 50 wt.-%.

[0034] The amount of water in the solution of the present invention is preferably in the range of 35 to 75 wt.-% relative to the total amount of the solution, in particular in the range of 45 to 70 wt.-%, more preferably in the range of 50 to 60 wt.-%.

[0035] Preferably, the pH value of the solution of the invention is in the range of 2-7, in particular in the range of 3-5, more preferably in the range of 4-5.

[0036] In the solution of the present invention, the anions present as counterions to the cationic species are selected from the group consisting of chloride, fluoride, perchlorate, sulfate, alkylsulfonate, arylsulfonate, phosphate, alkylphosphonate, arylphosphonate, and nitrate, or mixtures thereof. Preferably, the anions are selected from the group consisting of chloride, nitrate, sulfate, and perchlorate, and 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 solution of the present invention, preferably at least 95 mol% of all anions, and more preferably more than 99 mol% of all anions.

[0037] The solution of the present invention is obtained by the method of the present invention, which comprises the following steps: i) the compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine

[0038] [ka] with water and methyl chloride to form an aqueous solution containing the compound N,N,N,2,2,6,6-heptamethyl-4-piperidineaminium of formula (V), the compound N,N,N,1,2,2,6,6-octamethyl-4-piperidineaminium of formula (VI), and unreacted compound N,N,2,2,6,6-hexamethyl-4-piperidineamine of formula (IV).

[0039] [ka] obtaining a step of ii) reacting the aqueous mixture resulting from step i) with aqueous hydrogen peroxide in the presence of a catalyst selected from the group consisting of alkali metal carbonates, alkali metal bicarbonates, CO2 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) partial removal of water until the concentration of the compound of formula (I) is in the range of 20-55 wt.-% according to the solution of the invention. Includes:

[0040] In the first step i) of the process of the present 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 in the first step i) of the process of the present invention 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.

[0041] In the first step i) of the process of the present 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 to 1.2 mol, particularly 0.9 to 1.2 mol, more preferably 1.0 to 1.1 mol of methyl chloride per mol of the compound of formula (IV) in the feed mixture. The phrase "of the compound of formula (IV) in the feed mixture" refers to the amount of the compound of formula (IV) dissolved in water at the start of step i), and not the amount of the compound of formula (IV) remaining in solution after the methylation in step i). During this methylation, the reaction temperature is preferably in the range of 0 to 60°C, particularly 0 to 40°C, more preferably 15 to 25°C. The temperature may 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. Slow addition of methyl chloride and the use of external cooling are preferred so that the temperature does not rise above 60°C.

[0042] The mixture produced in step i) of the method of the present invention contains water, compounds of formulae (V) and (VI), and unreacted compounds of formula (IV). In step ii) of the method of the present invention, this aqueous mixture produced in step i) is reacted with aqueous hydrogen peroxide and an alkali metal catalyst selected from the group consisting of alkali metal carbonates, alkali metal bicarbonates, CO2, and mixtures thereof. Preferably, the catalyst is selected from the group consisting of Na2CO3, K2CO3, NaHCO3, KHCO3, CO2, and mixtures thereof. In particular, the catalyst is selected from the group consisting of Na2CO3 and NaHCO3. More preferably, NaHCO3 is used as the catalyst. The selected catalyst mixture is selected from the group consisting of 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. Preferred mixtures are selected from the group of NaHCO3 and Na2CO3.

[0043] The amount of catalyst used in step ii) of the process of the present 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 mol of compound of formula (IV) in the feed mixture used in step i) of the process of the present invention.

[0044] Step ii) of the process of the present invention is an oxidation step in which the compounds of formulae (IV), (V) and (VI) are oxidized to the compounds of formulae (I), (II) and (III). Thus, the aqueous mixture resulting from step i) is reacted in the presence of a catalyst with preferably 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 mol of compound of formula (IV) in the feed mixture used in step i) of the process of the present invention.

[0045] Preferably, the concentration of the aqueous hydrogen peroxide solution used in step ii) of the method of the invention falls in the range of 25 to 70 wt.-%, in particular 30 to 70 wt.-%, more preferably 45 to 70 wt.-% hydrogen peroxide.

[0046] During the oxidation, the temperature in 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. The adjustment of the pH value during the oxidation in step ii) of the process of the present invention is carried out by adding an acid or a base. Acids for adjusting the pH value in step ii) of the process of the present invention are selected from the group consisting of hydrogen chloride, perchloric acid, sulfuric acid, phosphoric acid, nitric acid, methanesulfonic acid, para-toluenesulfonic acid, methylphosphonic acid, and phenylphosphonic acid. Preferred acids are hydrogen chloride, sulfuric acid, nitric acid, and perchloric acid. More preferred is hydrogen chloride. Hydrogen chloride may 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 for adjusting the pH value in step ii) of the method of the present invention is selected from the group consisting of NaOH and KOH, more preferably NaOH. Usually, the addition of aqueous hydrogen peroxide solution leads to a decrease in the pH value of the solution during the oxidation in step ii) of the method of the present invention. Therefore, the initial pH value must be adjusted by adding a base. The use of a base, preferably NaOH, is preferred to maintain the pH value in the range of 7 to 10 during the oxidation.

[0047] After the addition of the aqueous hydrogen peroxide solution is complete, the mixture resulting from step ii) of the process of the present invention is stirred under the conditions of step ii), i.e., at approximately 60°C and a pH value in the range of 7-10, until the concentration of hydrogen peroxide in the solution is less than 0.5 wt.-%. The amount of remaining hydrogen peroxide is determined by cerium titration of the solution before and after catalytic decomposition of hydrogen peroxide by manganese dioxide. The difference between the two titrations is used to calculate the amount of remaining hydrogen peroxide.

[0048] After the addition of aqueous hydrogen peroxide is complete and the hydrogen peroxide concentration has fallen to less than 0.5 wt.-% in the solution obtained in step ii), the pH value of this solution is adjusted by adding an acid in step iii) of the method of the present invention. Preferably, the pH value 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 lower the pH value is selected from the group consisting of hydrogen chloride, perchloric acid, sulfuric acid, phosphoric acid, nitric acid, and methanesulfonic acid. Preferred acids are hydrogen chloride, sulfuric acid, nitric acid, and perchloric acid. More preferred is hydrogen chloride, used as a gas or as an aqueous solution known as hydrochloric acid.

[0049] In the final step of the process of the present invention, in step iv) of the process, the amount of water is reduced. This is preferentially carried out by distillation. The amount of water removed by distillation depends on the desired final concentration of the compound of formula (I) in the final solution. Water should be removed by distillation until the resulting solution exhibits a content of the compound of formula (I) in the range of 20 to 55 wt.-%, preferably in the range of 25 to 50 wt.-%, in particular in the range of 35 to 50 wt.-%, more preferably in the range of 40 to 45 wt.-%, based on the total weight of the solution. Preferably, the distillation is carried out at a pressure in the range of 0.02 to 1.0 bar and 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 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 a temperature in the range of 58 to 62°C.

[0050] After concentration, the resulting solution of the present invention can be used as an electrolyte in a redox flow cell without further processing. Preferably, the solution of the present invention is used as the catholyte in such a redox flow cell. Redox flow cells are typically constructed by using two chambers, one for the catholyte solution and one for the anolyte solution, each connected via a pump to a storage tank for the catholyte and anolyte solution, respectively. Both chambers are separated by an ion-conducting membrane and equipped with electrodes. The cathode chamber and the associated cathode storage tank are filled with the solution of the present invention. The anode chamber and the associated anode storage tank are filled with the electrolyte for the anode. The redox-active compounds in the redox flow cell change between their various redox levels during charge and discharge. During discharge, the electrolyte must be pumped from the storage tank to the electrodes, and during charge, the reverse process is used. Thus, a redox flow cell containing the solution of the present invention as an electrolyte is a facile and versatile way to store electrical energy for a variety of applications. [Example]

[0051] [Example] General rules: pH value: The pH value is always measured using a calibrated glass electrode (EasyFerm Plus PHI S8 225, two-point calibration with buffer pH=4.00 (citric acid, sodium hydroxide, sodium chloride from Fluka) and buffer pH=7.00 (potassium dihydrogen phosphate, disodium hydrogen phosphate from Fluka)).

[0052] 1 H-NMR method: Compounds of formula (V) 1 H-NMR data:

[0053] [ka] 1H-NMR (500 MHz, D2O): δ [ppm] = 3.68 (tt, J = 12.5 Hz, 2.8 Hz 1 H, H1), 3.07 (s, 9 H, H6), 2.02 - 2.08 (m, 2 H, H3), 1.32 (t, J = 12.5 Hz, 2 H, H2), 1.14 (s, 6 H, H5), 1.12 (s, 6 H, H4).

[0054] For the compound of formula (VI) 1 H-NMR data:

[0055]

Chem.

[0056] For the compound of formula (IV) 1 H-NMR data:

[0057]

Chem.

[0058] The molar ratios of the compounds of formula (IV), (V) and (VI) are at δ = 3.68 ppm (1H from the compound of formula (V)), 2.15 ppm (3H from the compound of formula (VI)) and 2.27 ppm (6H from the compound of formula (IV)). 1 It can be most conveniently determined by comparing the integrals of the 1 H-NMR signals.

[0059] Thus, the molar ratio of compounds of formula (IV):(V):(VI) is the same as the ratio of the integrals: (integral value of the signal at δ=3.68 ppm from the compound of formula (V)):(integral value of the signal at δ=2.15 ppm from the compound of formula (VI)) / 3:(integral value of the signal at δ=2.27 ppm from the compound of formula (IV)) / 6.

[0060] The solution of the present invention 1 H-NMR measurement: 1 Prior to H-NMR measurement, the solution of the present invention is reacted with an excess of phenylhydrazine (approximately 2 mol per mol of the compound of formula (I) and the compound of formula (III)) to convert the N-oxyl radical to the corresponding hydroxylamine. This procedure results in two isomeric forms of the reduced species (compounds of formula (Ia) and (Ib) / compounds of formula (IIIa) and (IIIb)), each of which is 1 This results in individual signals in the H-NMR spectrum. 1 For all H-NMR measurements, the crude reaction mixture from the reduction with phenylhydrazine was diluted with DO and the signal of the remaining HO proton at δ=4.79 ppm was used as the reference.

[0061] The signal assignments for the compound of formula (I) were confirmed by synthesizing the compound of formula (I) as a pure crystalline material as described in WO2018 / 2883011, page 28. The signal assignments for the compound of formula (III) were confirmed by synthesizing the compound of formula (III) as a pure material in aqueous solution as described herein.

[0062] Synthesis of the compound of formula (III) in pure form in aqueous solution: To a solution of the compound of formula (IV) (39.3 g) in water (40.1 g) is added 37 wt.-% hydrochloric acid (11.97 g), which reduces the pH of the solution to 9.0. Solid sodium bicarbonate (2.71 g) is then added, and the mixture is heated to 60°C. Once this temperature is reached, a 50 wt.-% aqueous solution of hydrogen peroxide (46.4 g) is added continuously over a period of 4 hours. During the addition, the pH is reduced and maintained above 8.0 by adding 5 approximately equal portions of a 50 wt.-% aqueous solution of sodium hydroxide (6.4 g). After the addition of hydrogen peroxide is complete, stirring is continued for 12 hours. The mixture is then cooled to room temperature. 1 The mixture is analyzed by H NMR spectroscopy and ESI MS mass spectrometry. 1 It contains more than 99 wt.-% of the compound of formula (III) as organic material as determined by H NMR.

[0063] The identity of the compounds of formula (I) and (III) is also confirmed by HRMS (ESI with ACN:H2O:HCOOH = 80:20:0.1, instrument: Q Extractive™ Hybrid Quadrupole Orbitrap Mass Spectrometer, ThermoFisher).

[0064] The reduced form of the compound of formula (I) 1 H-NMR:

[0065] [ka] 1H-NMR (500 MHz, D2O): δ [ppm] = 3.80 - 3.67 (m, 1 H, H1+H 1’ ), 3.12 (s, 9 H, H6 or H 6’ , minor isomer), 3.09 (s, 9 H, H6 or H 6’ , main isomer), 2.24 - 2.14 (m, 2 H, H2+H 2’ ), 1.99 (t, J = 12.1 Hz, 2 H, H3 or H 3’ , minor isomer), 1.75 (t, J = 12.4 Hz, 2 H, H3 or H 3’ , major isomer), 1.34 (s, 6 H, H 4 / 5 or H 4’ / 5’ , minor isomer), 1.26 (s, 6 H, H 4 / 5 or H 4’ / 5’ , main isomer), 1.22 (s, 6 H, H 4 / 5 or H 4’ / 5’ , main isomer), 1.12 (s, 6 H, H 4 / 5 or H 4’ / 5’ , minor isomer).The ratio of the two isomers is approximately 90:10. HRMS:C 12 H 26 N2O + Theoretical value: 214.2040, measured value: 214.2036

[0066] The reduced form of the compound of formula (III) 1 H-NMR:

[0067] [ka] 1 H-NMR (500 MHz, D2O): δ [ppm] = 3.66 - 3.52 (m, 1 H, H7+H 7’ ), 3.17 (s, 6 H, H 12 or H 12’ , minor isomer), 3.14 (s, 6 H, H 12 or H 12’, main isomer), 2.27 - 2.14 (m, 2 H, H8+H 8’ ), 1.93 (t, J = 12.5 Hz, 2 H, H9 or H 9’ , minor isomer), 1.72 (t, J = 12.5 Hz, 2 H, H9 or H 9’ , main isomer), 1.33 (s, 6 H, H 10 / 11 or H 10’ / 11’ , minor isomer), 1.25 (s, 6 H, H 10 / 11 or H 10’ / 11’ , main isomer), 1.21 (s, 6 H, H 10 / 11 or H 10’ / 11’ , main isomer), 1.11 (s, 6 H, H 10 / 11 or H 10’ / 11’ , minor isomer).The ratio of the two isomers is approximately 86:14. HRMS:C 11 H 24 N2O2 + Theoretical value: 216.1638, Measured value: 216.1637

[0068] Compounds of formula (II) remain unchanged upon reduction and give signals well separated from those from compounds of formulae (Ia), (Ib), (IIIa) and (IIIb):

[0069] [ka] 1 H-NMR (500 MHz, D2O): δ [ppm] = 3.93 (tt, J = 13.3 Hz, 3.2 Hz, 1 H, H 13 ), 3.14 (s, 9 H, H 19 ), 3.03 (s, 3 H, H 18 ), 2.47 (t, J = 12.6 Hz, 2 H, H 14 ), 2.07 (d, J = 12.1 Hz, 2 H, H 15 , 1.65 (s, 6 H, H 16 ), 1.56 (s, 6 H, H 17). HRMS:C 13 H 29 N2O + Theoretical value: 229.2274, Measured value: 229.2271

[0070] The ratios of compounds of formula (I), (II) and (III) are as follows: δ = 3.80-3.67 ppm (1H from compound of formula (I)), 1.56 ppm (6H from compound of formula (II)) and 3.66-3.52 ppm (1H from compound of formula (III)). 1 It can be most conveniently determined by comparing the integrals of the 1 H-NMR signals.

[0071] Thus, the molar ratio of compounds of formula (I):(II):(III) is equivalent to the ratio of the integrals: (integral value of the signal at δ=3.80 to 3.67 ppm derived from the compound of formula (I)): (integral value of the signal at δ=1.56 ppm derived from the compound of formula (II)) / 6: (integral value of the signal at δ=3.66 to 3.52 ppm derived from the compound of formula (III)).

[0072] Cerium redox titration: The total content of hydrogen peroxide and N-oxyl species (compounds of formula (I) and (III)) is determined using cerium redox titration according to the following method: N-oxyl species content: 100 mg of manganese dioxide is added to approximately 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 stir bar and diluted with 45 mL of purified water and 5 mL of dilute sulfuric acid (10 wt.-% aqueous solution). The resulting solution is placed in an automatic titrator (905 Titrando, Metrohm) equipped with a Pt-Titrode (Metrohm) and stirred at 20-25°C. The redox potential jump (V C1) by adding cerium(IV) sulfate solution (0.10 mol / L) via a titrator. The total concentration in weight-% of compounds of formula (I) + (III), w, can then be calculated from the consumption of cerium(IV) sulfate solution using the following equation: I+III can be calculated:

[0073]

number

[0074] Sum of hydrogen peroxide and N-oxyl species: 250 ± 2 mg of the analyte is placed in a beaker equipped with a magnetic stir bar and diluted with 45 mL of purified water and 5 mL of dilute sulfuric acid (10 wt.-% aqueous solution). The resulting solution is placed in an automatic titrator (905 Titrando, Metrohm) equipped with a Pt-Titrode (Metrohm) and stirred at 20-25 °C. The redox potential jump (V C2 Add cerium(IV) sulfate solution (0.10 mol / L) via the titrator until a difference in consumption of cerium(IV) sulfate solution (ΔV) is detected. Use the following equation to calculate the difference in consumption of cerium(IV) sulfate solution (ΔV) C =V C2 -V C1 ) the concentration of hydrogen peroxide can be calculated:

[0075]

number

[0076] Cyclic voltammetry method: The solution obtained from each example was diluted with 0.1 mol / L aqueous sodium chloride until the concentration of the N-oxyl compound reached 1.0 wt.%. The solution was placed in an electrochemical cell equipped with a standard three-electrode set (working electrode: glassy carbon (φ=2 mm), counter electrode: platinum wire, reference electrode: Ag / AgCl, 3 mol / L aqueous KCl). Using a PGU20V-2A-E potentiostat (IPS), the potential was increased to 1200 mV and then cycled between 1200 mV and -700 mV at a scan rate of ±20 mV / s (a total of three cycles).

[0077] Example 1: In a stainless steel autoclave, 800 ml of water and 645 g of the compound of formula (IV), N,N,2,2,6,6-hexamethylpiperidin-4-amine, are mixed and the temperature is adjusted to 20°C. Then, 185.5 g of methyl chloride are added to the autoclave at approximately 3.2 bar within 70 minutes. The mixture is then stirred at 20-25°C for 6 hours. The autoclave is then depressurized and purged with nitrogen for approximately 10 minutes. 1630 g of an aqueous solution of the compounds of formulae (IV), (V), and (VI) (50 wt.-% organics in water) is obtained. 1 The molar ratio of compounds of formula (IV):(V):(VI) determined by H-NMR was 2.5:92.8:4.7, which corresponds to 1.0 wt.-% of compound of formula (IV), 46.5 wt.-% of the chloride salt of compound of formula (V) and 2.5 wt.-% of the chloride salt of compound of formula (VI).

[0078] Example 2 (comparison): Methylation was carried out as described in WO2018 / 28830, page 27, line 20 to page 28, line 15 (corresponding to DE102016009904A1, paragraph

[0112] and thereafter). 1 In H-NMR, only the signals of the compound of formula (V) are visible.

[0079] Example 3: 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), 37 wt.-% hydrochloric acid (1.67 g) is added, which reduces the pH value of the solution to 9.0. Solid sodium bicarbonate (2.79 g) is then added, and the mixture is heated to 60°C. Once the temperature is reached, 50 wt.-% aqueous hydrogen peroxide solution (32.7 g) is added continuously over a period of 4 hours. During the addition, the pH value is reduced and maintained above 8.0 by adding 5 approximately equal portions of 50 wt.-% aqueous sodium hydroxide solution (1.73 g). After the addition of hydrogen peroxide is complete, stirring is continued for 12 hours. The mixture is then cooled to about 30° C., and 37 wt.-% hydrochloric acid (approximately 2 g) is added to lower the pH value of the solution to 4.5. Water is then distilled off under reduced pressure (70 mbar abs) until the concentration of the N-oxyl species of the compounds of formulae (I) and (III) is 46 wt.-% (determined by cerium redox titration).

[0080] 1 The molar ratio of compounds of formula (I):(II):(III) determined by H-NMR was 93.0:4.7:2.3, which corresponds to 45.0 wt.-% of the chloride salt of compound of formula (I), 2.4 wt.-% of the chloride salt of compound of formula (II), and 1.0 wt.-% of compound of formula (III).

[0081] Example 4: 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), 37 wt.-% hydrochloric acid (5.83 g) is added, thereby lowering the pH value of the solution to 8.5. Solid sodium carbonate (1.17 g) is then added, and the mixture is heated to 40°C. Once the temperature is reached, 50 wt.-% aqueous hydrogen peroxide solution (32.7 g) is added continuously over a period of 6 hours. During the addition, the pH value is lowered and kept above 8.0 by adding 5 approximately equal portions of 50 wt.-% aqueous sodium hydroxide solution (6.39 g). After the addition of hydrogen peroxide is complete, stirring is continued for 12 hours. The mixture is then cooled to about 30° C., and 37 wt.-% hydrochloric acid (approximately 2.5 g) is added to adjust the pH value of the solution to 4.0. Subsequently, water is removed by distillation under reduced pressure (70 mbar abs) until the concentration of the N-oxyl species of the compounds of formulae (I) and (III) is 48 wt.-% (determined by cerium redox titration).

[0082] 1 The molar ratio of compounds of formula (I):(II):(III) determined by H-NMR was 93.1:4.8:2.1, which corresponds to 47.1 wt.-% of the chloride salt of compound of formula (I), 2.6 wt.-% of the chloride salt of compound of formula (II) and 0.9 wt.-% of compound of formula (III).

[0083] Example 5 (comparison): The oxidation was carried out as described in WO2018 / 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 In 1 H-NMR, only the signals of the compounds of formula (Ia / Ib) are visible.

[0084] Example 6: The cyclic voltammetry of the product of Example 3 is measured (see Figure I).

[0085] Example 7 (comparison): The cyclic bornimetry of Example 5 is measured (see Figure II).

[0086] The cyclic voltammogram in Figure I is nearly identical to that of Comparative Example 5 in Figure II. Thus, the inventive solution of Example 3 exhibits nearly the same redox potential as the solution obtained in Example 5, which represents the state of the art. Thus, the inventive solution can be used in a redox flow cell as described in the state of the art. Some embodiments are given below. Item 1 a) water, b) 20 to 55 wt.-% of the compound of formula (I) 2,2,6,6-tetramethyl-4-(trimethylammonio)-1-piperidinyloxy, relative to the total weight of the solution; [ka] c) 0.1 to 6 wt.-% of alkali metal cations relative to the total weight of the solution; d) 0.5 to 12.5 wt.-% of the compound N,N,N,1,2,2,6,6-octamethyl-4-piperidineammonium-1-oxide of formula (II) relative to the total weight of the solution, [ka] e) 0.1 to 20 wt.-% of the compound 2,2,6,6-hexamethyl-4-(dimethylamino)-1-piperidinyloxy-N-oxide of formula (III) relative to the total weight of the solution [ka] A solution containing Section 2 Item 1. The solution according to item 1, wherein the alkali metal cation is Na. Section 3 Item 3. The solution according to any one of items 1 and 2, having a pH value in the range of 2 to 7. Section 4 Item 4. The solution according to any one of items 1 to 3, wherein the sum of the amount of the compound of formula (I), (II), or (III) and the amount of the alkali metal cation in the solution is in the range of 20 to 50 wt. % based on the total weight of the solution. Section 5 Item 5. The solution according to any one of items 1 to 4, wherein at least 90 mol % of the counter ions are chloride ions. Section 6 Item 1. A method for producing the solution according to item 1, comprising the steps of: i) the compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine [ka] with water and methyl chloride to produce an aqueous mixture containing the compound N,N,N,2,2,6,6-heptamethyl-4-piperidineaminium of formula (V), the compound N,N,N,1,2,2,6,6-octamethyl-4-piperidineaminium of formula (VI), and unreacted compound N,N,2,2,6,6-hexamethyl-4-piperidinamine of formula (IV). [ka] obtaining a step of ii) reacting the aqueous mixture from step i) with aqueous hydrogen peroxide solution to form a solution of alkali metal carbonate, alkali metal bicarbonate, CO 2 and mixtures thereof; iii) adding an acid to the mixture obtained in step ii) until the pH value falls to a range of 2 to 7; iv) Partially removing water until the concentration of the compound of formula (I) is in the range of 20-55 wt.-% in the solution described in item 1. A method comprising: Section 7 Item 7. The method according to item 6, wherein in step i) of the method, the compound of formula (IV) is reacted with methyl chloride and water at a temperature in the range of 0 to 60°C. Section 8 8. The method according to any one of items 6 to 7, wherein in step i) of the method, the compound of formula (IV) is reacted with 0.7 to 1.2 mol 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 of 0.1 to 5. Section 9 Item 9. The method according to any one of items 6 to 8, wherein in step ii) of the method, the mixture obtained in 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, and the pH value during the addition of the aqueous hydrogen peroxide solution is maintained between 7 and 10. Section 10 10. The method according to any one of paragraphs 6 to 9, wherein in step ii) of the method, 1.5 to 5 moles of aqueous hydrogen peroxide having a concentration in the range of 25 to 70 wt.-% are reacted per mole of compound of formula (IV) in the feed mixture in the presence of 0.005 to 0.4 moles of catalyst per mole of compound of formula (IV) in the feed mixture. Section 11 11. The method according to any one of paragraphs 6 to 10, wherein the addition of acid in step iii) is initiated when the concentration of hydrogen peroxide in the mixture of step ii) falls below 0.5 wt.-%. Section 12 12. The method of any one of paragraphs 6 to 11, wherein the acid in step iii) is hydrochloric acid. Section 13 A method of making a redox flow cell, wherein the solution of any one of paragraphs 1 to 5 is used as an electrolyte in one of the two chambers of the cell. Section 14 Follow these steps: a) providing two chambers for catholyte and anolyte solutions, each connected to at least one storage tank for the catholyte and anolyte solutions, respectively; b) separating the two chambers with an ion-conducting membrane; c) equipping the chamber with electrodes; d) filling a catholyte chamber with the solution according to any one of items 1 to 5, which contains 2,2,6,6-tetramethyl-4-(trimethylammonio)-1-piperidinyloxy salt as a redox active material; e) filling the anolyte chamber with an anolyte solution containing another redox active material Item 14. The method according to Item 13, comprising: Section 15 Item 15. A redox flow cell obtained by the method according to any one of items 13 to 14. Section 16 16. Use of a redox flow cell according to paragraph 15 for storing electrical energy.

Claims

1. a) water, b) 20 to 55 wt.-% of the compound of formula (I) 2,2,6,6-tetramethyl-4-(trimethylammonio)-1-piperidinyloxy, relative to the total weight of the solution; 【Chemistry 1】 c) 0.1 to 6 wt.-% of alkali metal cations relative to the total weight of the solution; d) 0.5 to 12.5 wt.-% of the compound N,N,N,1,2,2,6,6-octamethyl-4-piperidineammonium-1-oxide of formula (II), relative to the total weight of the solution; 【Chemistry 2】 e) 0.1 to 20 wt.-% of the compound 2,2,6,6-hexamethyl-4-(dimethylamino)-1-piperidinyloxy-N-oxide of formula (III), relative to the total weight of the solution 【Transformation 3】 A solution containing

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

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

4. 4. The solution according to claim 1, wherein the sum of the amount of compounds of formula (I), (II), (III) and the amount of alkali metal cations in the solution is in the range of 20 to 50 wt.-% relative to the total weight of the solution.

5. 5. The solution of claim 1, wherein at least 90 mol % of the counterions are chloride ions.

6. 10. A method of producing the solution of claim 1, comprising the steps of: i) the compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine 【Chemistry 4】 with water and methyl chloride to produce an aqueous mixture containing the compound N,N,N,2,2,6,6-heptamethyl-4-piperidineaminium of formula (V), the compound N,N,N,1,2,2,6,6-octamethyl-4-piperidineaminium of formula (VI), and unreacted compound N,N,2,2,6,6-hexamethyl-4-piperidinamine of formula (IV). 【Transformation 5】 obtaining a step of ii) reacting the aqueous mixture from step i) with aqueous hydrogen peroxide solution to form a solution of alkali metal carbonate, alkali metal bicarbonate, CO 2 and mixtures thereof; iii) adding an acid to the mixture resulting from step ii) until the pH value falls to a range of 2 to 7; iv) in the last step, partial removal of water until the concentration of the compound of formula (I) is in the range of 20 to 55 wt.-% relative to the total weight of the solution of claim 1. A method comprising:

7. 7. The process according to claim 6, wherein in process step i) the compound of formula (IV) is reacted with methyl chloride and water at a temperature in the range of 0 to 60°C.

8. 8. The process according to any one of claims 6 to 7, wherein in process step i) the compound of formula (IV) is reacted with 0.7 to 1.2 mol of methyl chloride in the presence of water, and the mass ratio of compound of formula (IV) to water in the feed mixture is in the range of 0.1 to 5.

9. 9. The method according to any one of claims 6 to 8, wherein in step ii) of the method, the mixture resulting from step i) is reacted with aqueous hydrogen peroxide in the presence of a catalyst at a temperature in the range of 20 to 80°C, and the pH value during the addition of the aqueous hydrogen peroxide is maintained between 7 and 10.

10. 10. The process according to any one of claims 6 to 9, wherein in process step ii) 1.5 to 5 mol of aqueous hydrogen peroxide having a concentration in the range of 25 to 70 wt.-% are reacted per mol of compound of formula (IV) in the feed mixture in the presence of 0.005 to 0.4 mol of catalyst per mol of compound of formula (IV) in the feed mixture.

11. 11. The method according to any one of claims 6 to 10, wherein the addition of acid in step iii) is initiated when the concentration of hydrogen peroxide in the mixture of step ii) falls below 0.5 wt.-%.

12. 12. The method according to any one of claims 6 to 11, wherein the acid in step iii) is hydrochloric acid.

13. 10. A method of making a redox flow cell, wherein a solution according to any one of claims 1 to 5 is used as an electrolyte in one of the two chambers of the cell.

14. Follow these steps: a) providing two chambers for catholyte and anolyte solutions, each connected to at least one storage tank for the catholyte and anolyte solutions, respectively; b) separating the two chambers with an ion-conducting membrane; c) equipping the chamber with electrodes; d) filling the catholyte chamber with a solution according to any one of claims 1 to 5, comprising 2,2,6,6-tetramethyl-4-(trimethylammonio)-1-piperidinyloxy salt as redox active material; e) filling the anolyte chamber with an anolyte solution containing another redox active material 14. The method of claim 13, comprising:

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