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 both production methods described in WO 2018 / 028830, the intermediate compound is a solid that must be separated from the solvent by filtration or centrifugation before use in the next step. Furthermore, the oxidation step described in WO 2018 / 028830 requires the addition of a base to maintain the pH within an optimal range during oxidation. While these described production methods yield very pure compounds, they involve many intermediate steps in which solids must be separated and processed, and reaction steps that require complex solution processing. Current methods use several different solvents or solvent mixtures, making solvent reuse particularly complicated. In one method, anion exchange is required because bromide is not tolerated in the electrolyte solution because it can be easily oxidized. Furthermore, no production methods have been described in which water is primarily used as the solvent, and the anion can be freely selected without the need for ion exchange. For industrial-scale production, the described production methods are unsuitable because each filtration step, solids treatment step, solvent exchange, or anion exchange step causes significant losses in yield and time and increases the complexity of the process. Furthermore, the addition of an extra base during the oxidation step results in additional cost and effort if additional salts must be removed from the electrolyte solution.

[0004] DE 102015010083 A1 and B. Ho Angewandte Chemie, Vol. 55, No. 46, October 18, 2016, pp. 14427-14430, disclose a method for synthesizing a compound of formula (I) and the use of a compound of formula (I) as a catholyte material in a redox flow cell. Neither disclosure describes the production of an aqueous solution containing compounds of formulas (II) and (III) in addition to the compound of formula (I), and the preferred use of such an aqueous solution as a catholyte in a redox flow cell.

[0005] T. Janoschka et al., in Chemical Communications, Vol. 54, No. 50, June 19, 2018, pp. 6871-6874, describe an aqueous organic redox flow battery containing the compound of formula (I), but do not describe the use of an aqueous solution containing compounds of formulas (II) and (III) in addition to the compound of formula (I) as a catholyte in the redox flow cell of the redox flow battery. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] WO2014 / 26728 [Patent Document 2] WO2018 / 028830 [Patent Document 3] DE 102015010083 A1 [Non-patent literature]

[0007] [Non-Patent Document 1] B. Ho Angewandte Chemie Volume 55, Issue 46 October 18, 2016, Pages 14427-14430 [Non-patent document 2] T. Janoschka et al., Chemical Communications, Vol. 54, No. 50, June 19, 2018, pp. 6871-6874 Summary of the Invention [Problem to be solved by the invention]

[0008] It is therefore an object of the present invention to provide aqueous solutions containing various TEMPO derivatives having chemical redox potentials comparable to those of the pure TEMPO derivatives described in the state of the art, which can be used on an industrial scale without significant effort and without significant loss of yield, by using a simple and inexpensive method for producing said solutions, the anions in the resulting solutions may be freely selected, and the addition of a base during oxidation is not required. Another object of the present invention is to provide a redox flow cell containing an aqueous solution of a TEMPO derivative that exhibits energy storage properties similar or equal to those of current redox flow cells containing an aqueous solution of 2,2,6,6-tetramethyl-1-piperidinyloxy-4-trimethylammonium chloride. [Means for solving the problem]

[0009] 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;

[0010] [ka] c) less than 0.1 wt.-% of alkali metal cations relative to the total weight of the solution; d) 0.1 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,

[0011] [ka] e) 0.01 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

[0012] [ka] This is solved by a solution containing

[0013] The solution of the present invention is advantageous when the alkali metal cation is Na.

[0014] The solution of the present invention is advantageous when the pH value is in the range of 2-7.

[0015] 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.

[0016] 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

[0017] [ka] with dimethyl carbonate in the presence of a saturated C1-C4 alcohol to produce a 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), dissolved in the saturated C1-C4 alcohol.

[0018] [ka] obtaining a step of ii) solvent exchange from C1-C4 alcohol to water; iii) reacting the aqueous mixture resulting from step ii) with aqueous hydrogen peroxide; iv) adding an acid to the mixture obtained in step iii) until the pH value falls to a range of 3 to 5; v) partially removing water from the mixture obtained in step iv) until the concentration of the compound of formula (I) is in the range of 20-55 wt.-% according to the solution of the invention. The method includes:

[0019] The process of the present invention is advantageous if, in step i) of the process of the present invention, the compound of formula (IV) is reacted with dimethyl carbonate in the presence of a saturated C1-C4 alcohol at a temperature in the range of 90-170°C.

[0020] The process of the present invention is advantageous if, in step i) of the process of the present invention, the compound of formula (IV) is reacted with 0.7 to 1.5 mol of dimethyl carbonate per mol of compound of formula (IV) in the presence of a saturated C1-C4 alcohol, and the mass ratio of compound of formula (IV) to saturated C1-C4 alcohol in the feed mixture is in the range of 0.1 to 5.

[0021] The process of the invention is advantageous if, in step iii) of the process of the invention, the mixture resulting from step ii) is reacted with aqueous hydrogen peroxide at a temperature in the range of 20-80°C.

[0022] The process of the invention is advantageous when, in step iii) of the process of the invention, 1.5 to 5 mol of aqueous hydrogen peroxide having a concentration in the range of 25 to 70 wt.-% is used per mol of compound of formula (IV) in the feed mixture.

[0023] The process of the invention is advantageous if the addition of acid in step iv) is started when the concentration of hydrogen peroxide in the mixture of step iii) falls below 0.5 wt.-%.

[0024] The process of the invention is advantageous when the acid used in step iv) has a standard redox potential of more than +1.35V.

[0025] The process of the invention is advantageous if, in step ii) of the process of the invention, the C1-C4 alcohol is first removed by distillation and the remaining material is dissolved in water.

[0026] The process of the invention is advantageous if, in step ii) of the process of the invention, water is added to the mixture resulting from step i) and subsequently or simultaneously the C1-C4 alcohol is removed by distillation.

[0027] The process of the invention is advantageous when the saturated C1-C4 alcohol is selected from the group of methanol and n-butanol.

[0028] A further embodiment is a method of making a redox flow cell, wherein a solution of the present invention is used as the electrolyte in one of both chambers of the cell.

[0029] 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

[0030] A further embodiment of the present invention is a redox flow cell obtainable by the inventive method of making a redox flow cell.

[0031] 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]

[0032] [Figure 1] Cyclic voltammogram of the product of Example 4. [Figure 2] Cyclic voltammogram of the product of Example 5. [Figure 3] Cyclic voltammogram of the product of Example 6. DETAILED DESCRIPTION OF THE INVENTION

[0033] 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, less than 0.1 wt. % of an alkali metal cation, based on the total weight of the solution, 0.1 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 0.01 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.

[0034] 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 25 to 50 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.

[0035] The solution of the invention also contains 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 solution of the invention is preferably less than 0.1 wt.-% relative to the total amount of the solution, in particular in the range of 0.001 to 0.01 wt.-%, more preferably in the range of 0.001 to 0.008 wt.-%.

[0036] 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 0.1 to 12.5 wt. %, in particular in the range of 0.1 to 5.0 wt. %, more preferably in the range of 0.3 to 2.0 wt. %, based on the total weight of the solution.

[0037] 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 the compound of formula (III) in the solution of the present invention is in the range of 0.01 to 20 wt. %, in particular in the range of 0.01 to 5.0 wt. %, more preferably in the range of 0.02 to 1.0 wt. %, relative to the total amount of the solution.

[0038] 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.-%.

[0039] 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.-%.

[0040] 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.

[0041] In the solutions of the present invention, the anion present as a counterion to the cationic species is selected from the group of chloride, fluoride, perchlorate, sulfate, alkylsulfonate, arylsulfonate, phosphate, alkylphosphonate, arylphosphonate, and nitrate, or mixtures thereof. Preferably, the anion is selected from the group of chloride, nitrate, sulfate, and perchlorate, and more preferably, the anion is chloride.

[0042] 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

[0043] [ka] with dimethyl carbonate in the presence of a saturated C1-C4 alcohol to produce a 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), dissolved in the saturated C1-C4 alcohol.

[0044] [ka] obtaining a step of ii) solvent exchange from C1-C4 alcohol to water; iii) reacting the aqueous mixture resulting from step ii) with aqueous hydrogen peroxide; iv) adding an acid to the mixture obtained in step iii) until the pH value is in the range of 2 to 7; v) partially removing water from the mixture obtained in step iv) 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.

[0045] In the first step i), the compound of formula (IV) is dissolved in a saturated C1-C4 alcohol. Saturated C1-C4 alcohol refers to an alcohol selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, n-butanol, 2-butanol, isobutanol, and tert-butanol. The use of methanol and n-butanol is preferred, more preferably methanol. Preferably, in the first step i) of the process of the present invention, the mass ratio of the compound of formula (IV) in the feed mixture to the saturated C1-C4 alcohol used 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. The phrase "of the compound of formula (IV) in the feed mixture" refers to the amount of the compound of formula (IV) dissolved in the saturated C1-C4 alcohol 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).

[0046] In the first step i) of the process of the present invention, the compound of formula (IV) is dissolved in a saturated C1-C4 alcohol and methylated with dimethyl carbonate. Preferably, the alcohol solution of the compound of formula (IV) is methylated with 0.7 to 1.5 mol, in particular 0.9 to 1.5 mol, more preferably 1.0 to 1.2 mol of dimethyl carbonate per mol of the compound of formula (IV) in the feed mixture used. During this methylation, the reaction temperature is preferably in the range of 90 to 170°C, in particular 100 to 160°C, more preferably 120 to 140°C.

[0047] After the methylation in step i) of the method of the present invention is completed, a solvent exchange from the C1-C4 alcohol to water as the solvent is carried out as step ii) of the method of the present invention. The solvent exchange may be carried out in two different ways. One way is to completely remove the C1-C4 alcohol by distillation, resulting in a solid product material containing the methylated compound of formula (V) or (VI) and unreacted compound of formula (IV). The solid product material is then dissolved in water.

[0048] Another, more preferred, solvent exchange method is to first add water to the alcohol solution resulting from step i) of the present process, followed by removal of the C1-C4 alcohol as a light boiling product or in the form of a light boiling azeotrope with water. In a more preferred method, the addition of water and removal of the C1-C4 alcohol may occur simultaneously. This treatment step, in which water is added first or simultaneously with the removal of the C1-C4 alcohol, is preferred because no solid material needs to be treated. Furthermore, when methanol or n-butanol is used as the preferred C1-C4 alcohol in step i) of the present process, they can be easily removed after the addition of water in step ii) of the present process. Methanol can be removed by distillation as a light boiling product, and n-butanol can be removed by distillation as a heterophase azeotrope with water. When butanol is used, the resulting aqueous phase obtained after condensation and phase separation can be returned to step i) of the present process. Both alcohols—methanol and n-butanol—can be recycled to step i) of the present process without further washing or drying steps. When ethanol, n-propanol, iso-propanol, iso-butanol, or tert-butanol is used in step i) of the process of the present invention, these alcohols are removed by distillation. These removed alcohols must be dried before they can be reused in step i) of the process of the present invention. The use of methanol as a solvent is particularly advantageous because methanol does not form an azeotrope with water and can be removed as a light boiler, eliminating the need for a separate drying step. The recovered methanol can thus be used in step i) without further treatment.

[0049] The aqueous mixture produced in step ii) of the method of the present invention contains water, the compounds of formula (V) and (VI), unreacted compounds of formula (IV), and carbonate or methyl carbonate ions as counter ions. This aqueous mixture produced in step ii) is reacted with an aqueous hydrogen peroxide solution in step iii) of the method of the present invention.

[0050] Step iii) of the process of the present invention is an oxidation step in which compounds of formulae (IV), (V) and (VI) are oxidized to compounds of formulae (I), (II) and (III). Thus, the aqueous mixture resulting from step ii) is reacted with 1.5 to 5 mol, in particular 1.5 to 3 mol, more preferably 1.7 to 2.3 mol of hydrogen peroxide per mol of compound of formula (IV) in the feed mixture used in step i) of the process of the present invention, wherein the hydrogen peroxide is used as an aqueous solution.

[0051] Preferably, the concentration of the aqueous hydrogen peroxide solution used in step iii) 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.

[0052] During oxidation, the temperature in step iii) of the process of the invention preferably lies in the range of 20-80°C, in particular in the range of 40-60°C, more preferably in the range of 50-60°C.

[0053] After the addition of the aqueous hydrogen peroxide solution is complete, the mixture resulting from step iii) of the process of the present invention is stirred under the conditions of step iii), i.e., at approximately 60°C, 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.

[0054] Step iii) ends after the addition of the aqueous hydrogen peroxide solution is completed and the hydrogen peroxide concentration has dropped to less than 0.5 wt.-%. In step iv), an acid is added to the solution obtained in step iii) to adjust the pH. Preferably, the pH is adjusted to a range of 2 to 7, particularly a range of 3 to 5, more preferably a range of 4 to 5, by adding the acid. The acid used to adjust the pH has a standard redox potential of more than +1.35 V. Preferably, the acid is selected from the group consisting of hydrogen chloride, perchloric acid, sulfuric acid, phosphoric acid, nitric acid, and methanesulfonic acid, and in particular, the acid is hydrogen chloride and perchloric acid, more preferably hydrogen chloride. The 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.

[0055] In the final step of the process of the present invention, the amount of water is reduced in step v) of the process of the present invention. This is 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.-%, and 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.

[0056] 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]

[0057] [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)).

[0058] 1 H-NMR method: Compounds of formula (V) 1 H-NMR data:

[0059] [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).

[0060] For the compound of formula (VI) 1 H-NMR data:

[0061] [Chemical formula] 1 H-NMR (500 MHz, D2O): δ [ppm] = 3.62 (tt, J = 12.5 Hz, 3.1 Hz, 1 H, H7), 3.00 (s, 9 H, H 12 ), 2.15 (s, 3 H, H 13 ), 2.08 - 2.02 (m, 2 H, H9), 1.55 (t, J = 12.5 Hz, 2 H, H8), 1.15 (s, 6 H, H 11 ), 1.05 (s, 6 H, H 10 ).

[0062] For the compound of formula (IV) 1 H-NMR data: [[ID=三十二]]

[0063] [Chemical formula] 1 H-NMR (500 MHz, D2O): δ [ppm] = 2.83 (tt, J = 12.3 Hz, 3.2 Hz, 1 H, H 14 ), 2.27 (s, 6 H, H 19 ), 1.88 (dd, J = 12.7 Hz, 3.2 Hz, 2 H, H 15 ), 1.28 (s, 6 H, H 17), 1.24 (s, 6 H, H 18 ), 1.17 (dd, J = 12.7 Hz, 12.3 Hz, 6 H, H 16 ).

[0064] 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.

[0065] 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 (IV)) / 3:(integral value of the signal at δ=2.27 ppm from the compound of formula (VI)) / 6.

[0066] 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 moles per mole of the compounds of formulae (I) and (III)) to convert the N-oxyl radicals to the corresponding hydroxylamines. This procedure results in two isomeric forms of the reduced species (compounds of formulae (Ia) and (Ib) / compounds of formulae (IIa) 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.

[0067] 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.

[0068] 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.

[0069] 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).

[0070] The reduced form of the compound of formula (I) 1 H-NMR:

[0071] [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’ , main 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

[0072] The reduced form of the compound of formula (III) 1 H-NMR:

[0073] [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

[0074] Compounds of formula (II) remain unchanged upon reduction and give signals well separated from those from compounds of formulae (Ia), (Ib), (IIIa) and (IIIb):

[0075] [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

[0076] 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.

[0077] 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)).

[0078] Cerium redox titration: The total content of hydrogen peroxide and N-oxyl species (compounds of formula (I) + (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:

[0079]

number

[0080] 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 the cerium(IV) sulfate solution (ΔV) is detected. Use the following equation to calculate the difference in consumption of the cerium(IV) sulfate solution (ΔV): C =V C2 -V C1 ) the concentration of hydrogen peroxide can be calculated:

[0081]

number

[0082] 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).

[0083] Example 1: In a stainless steel autoclave, 92 g (116.5 ml) of methanol, 40.0 g of the compound of formula (IV), and 23.5 g of dimethyl carbonate are mixed and heated to 120°C. The mixture is stirred at 120°C for 24 h. The autoclave is then cooled to room temperature and depressurized. The volatiles are removed by distillation to give a solid residue (49.6 g), which is dissolved in 50 g of water to give a 50 wt.-% aqueous solution of the compounds of formulae (IV), (V), and (VI) as a clear, yellowish solution. 1 The ratio of compounds of formula (IV):(V):(VI) determined by H-NMR was 1.0:98.1:0.9, which corresponds to 0.4 wt.-% of compound of formula (IV), 49.1 wt.-% of carbonate of compound of formula (V) and 0.5 wt.-% of carbonate of compound of formula (VI).

[0084] Example 2: In a stainless steel autoclave, 92 g (116.5 ml) of methanol, 40.0 g of the compound of formula (IV), and 23.5 g of dimethyl carbonate are mixed and heated to 120°C. The mixture is stirred at 120°C for 24 h. The autoclave is then cooled to room temperature and depressurized. 40 mL of water is then added, and the mixture is heated to 90°C, collecting 46.1 g of distillate. An additional 40 g of water is added to the sump, and the mixture is heated to 107°C, collecting 47.9 g of distillate. The sump (124.6 g) contains a 40 wt.-% solution of the compounds of formulae (IV), (V), and (VI) in water. 1 The ratio of compounds of formula (IV):(V):(VI) determined by H-NMR was 1.0:98.1:0.9, which corresponds to 0.3 wt.-% of compound of formula (IV), 39.3 wt.-% of carbonate of compound of formula (V), and 0.4 wt.-% of carbonate of compound of formula (VI). No methanol was detected by NMR.

[0085] Example 3 (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.

[0086] Example 4: To the solution taken from Example 1 (100 g, containing 0.4 wt.-% of the compound of formula (IV), 49.1 wt.-% of the carbonate salt of the compound of formula (V), and 0.5 wt.-% of the carbonate salt of the compound of formula (VI) in water), 37 wt.-% hydrochloric acid (0.82 g) is added, thereby lowering the pH value of the solution to 10.0. The mixture is then heated to 60°C. Once the temperature is reached, a 50 wt.-% aqueous solution of hydrogen peroxide (32.7 g) is added continuously over a period of 4 hours. 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 19 g) is added to lower the pH value of the solution to 4.3. Therefore, 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 50 wt.-% (determined by cerium redox titration).

[0087] 1 The molar ratio of compounds of formula (I):(II):(III) determined by H-NMR was 98.1:0.8:1.1, which corresponds to 49.5 wt.-% of the chloride salt of compound of formula (I), 0.4 wt.-% of the chloride salt of compound of formula (II), and 0.5 wt.-% of compound of formula (III).

[0088] Example 5: To the solution taken from Example 1 (100 g, containing 0.4 wt.-% of the compound of formula (IV), 49.1 wt.-% of the carbonate salt of the compound of formula (V), and 0.5 wt.-% of the carbonate salt of the compound of formula (VI) in water), 10 wt.-% nitric acid (21.1 g) is added, thereby lowering the pH value of the solution to 9.5. The mixture is then heated to 60°C. Once the temperature is reached, a 50 wt.-% aqueous solution of hydrogen peroxide (32.7 g) is added continuously over a period of 2 hours. After the addition of hydrogen peroxide is complete, stirring is continued for 12 hours. The mixture is then cooled to about 30°C, and 10 wt.-% nitric acid (approximately 19 g) is added to lower the pH value of the solution to 4.5. Therefore, 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 49 wt.-% (determined by cerium redox titration).

[0089] 1 The molar ratio of compounds of formula (I):(II):(III) determined by H-NMR was 98.1:0.8:1.1, which corresponds to 48.6 wt.-% of the nitrate of compound of formula (I), 0.4 wt.-% of the nitrate of compound of formula (II), and 0.5 wt.-% of compound of formula (III).

[0090] Example 6 (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.

[0091] Example 7: Cyclic voltammogram of the product of Example 4 (see Figure I).

[0092] Example 8: Cyclic voltammogram of the product of Example 5 (see Figure II).

[0093] Example 9: Cyclic voltammogram of the product of Example 6 (see Figure III).

[0094] The cyclic voltammograms in Figures I and II are nearly identical to that of Comparative Example 6 in Figure III. Thus, the inventive solutions of Examples 4 and 5 exhibit nearly the same redox potential as the solution obtained in Example 5, which represents the state of the art. Thus, the inventive solutions can be used in redox flow cells 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) less than 0.1 wt.-% of alkali metal cations relative to the total weight of the solution; d) 0.1 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) 0.01 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. A method for producing the solution according to any one of items 1 to 4, comprising the steps of: i) reacting a compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine with [ka] with dimethyl carbonate in the presence of a saturated C1-C4 alcohol to produce a 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), dissolved in the saturated C1-C4 alcohol. [ka] obtaining a step of ii) solvent exchange from C1-C4 alcohol to water; iii) reacting the aqueous mixture resulting from step ii) with aqueous hydrogen peroxide; iv) adding an acid to the mixture obtained in step iii) until the pH value falls to a range of 3 to 5; v) partially removing water from the mixture obtained in step iv) until the concentration of the compound of formula (I) is in the range of 20 to 55 wt.-% in the solution described in item 1. A method comprising: Section 6 Item 6. The method according to item 5, wherein in step i) of the method, the compound of formula (IV) is reacted with dimethyl carbonate in the presence of a saturated C1-C4 alcohol at a temperature in the range of 90-170°C. Section 7 7. The method according to any one of items 5 to 6, wherein in step i) of the method, the compound of formula (IV) is reacted with 0.7 to 1.5 mol of dimethyl carbonate in the presence of a saturated C1-C4 alcohol, and the mass ratio of the compound of formula (IV) to the saturated C1-C4 alcohol in the feed mixture is in the range of 0.1 to 5. Section 8 Item 8. The method according to any one of items 5 to 7, wherein in step iii) of the method, the mixture obtained in step ii) is reacted with an aqueous hydrogen peroxide solution at a temperature in the range of 20 to 80°C. Section 9 9. The method according to any one of paragraphs 5 to 8, wherein in step iii) of the method, 1.5 to 5 mol of aqueous hydrogen peroxide having a concentration in the range of 25 to 70 wt.-% is used per mol of compound of formula (IV) in the feed mixture. Item 10 10. The method according to any one of paragraphs 5 to 9, wherein the addition of acid in step iv) is initiated when the concentration of hydrogen peroxide in the mixture of step iii) falls below 0.5 wt.-%. Section 11 11. The method according to any one of paragraphs 5 to 10, wherein the acid used in step iv) has a standard redox potential of greater than +1.35 V. Section 12 12. The method according to any one of paragraphs 5 to 11, wherein in step ii) of the method, first the C1-C4 alcohol is removed by distillation and the remaining material is dissolved in water. Item 13 Item 12. The method according to any one of items 5 to 11, wherein in step ii) of the method, water is added to the mixture obtained in step i), and then or simultaneously, the C1-C4 alcohol is removed by distillation. Section 14 Item 14. The method according to item 13, wherein the saturated C1-C4 alcohol is selected from the group consisting of methanol and n-butanol. Section 15 A method of making a redox flow cell, wherein the solution described in any one of paragraphs 1 to 4 is used as an electrolyte in one of both chambers of the cell. Item 16 Steps below: 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 4, 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 16. The method according to Item 15, comprising: Item 17 17. A redox flow cell obtained by the method according to any one of items 15 to 16. Section 18 18. Use of a redox flow cell according to paragraph 17 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) less than 0.1 wt.-% of alkali metal cations relative to the total weight of the solution; d) 0.1 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) 0.01 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. A method for producing a solution according to any one of claims 1 to 4, comprising the steps of: i) the compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine 【Chemistry 4】 with dimethyl carbonate in the presence of a saturated C1-C4 alcohol to produce a 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), dissolved in the saturated C1-C4 alcohol. 【Transformation 5】 obtaining a step of ii) solvent exchange from C1-C4 alcohol to water after methylation in step i); iii) reacting the aqueous mixture resulting from step ii) with aqueous hydrogen peroxide; iv) adding acid to the mixture resulting from step iii) until the pH value falls to the range of 3 to 5; v) in the final step, partially removing the water from the mixture obtained in step iv) 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:

6. 6. The process according to claim 5, wherein in step i) of the process, the compound of formula (IV) is reacted with dimethyl carbonate in the presence of a saturated C1-C4 alcohol at a temperature in the range of 90-170°C.

7. 7. The process according to any one of claims 5 to 6, wherein in process step i) the compound of formula (IV) is reacted with 0.7 to 1.5 mol of dimethyl carbonate in the presence of a saturated C1-C4 alcohol, and the mass ratio of compound of formula (IV) to saturated C1-C4 alcohol in the feed mixture is in the range of 0.1 to 5.

8. 8. The method according to any one of claims 5 to 7, wherein in process step iii) the mixture resulting from step ii) is reacted with aqueous hydrogen peroxide at a temperature in the range of 20 to 80°C.

9. 9. The process according to any one of claims 5 to 8, wherein in process step iii) 1.5 to 5 mol of aqueous hydrogen peroxide having a concentration in the range of 25 to 70 wt.-% is used per mol of compound of formula (IV) in the feed mixture.

10. 10. The method according to any one of claims 5 to 9, wherein the addition of acid in step iv) is initiated when the concentration of hydrogen peroxide in the mixture of step iii) falls below 0.5 wt.-%.

11. 11. The method according to any one of claims 5 to 10, wherein the acid used in step iv) is selected from the group of hydrogen chloride, perchloric acid, sulfuric acid, nitric acid and methanesulfonic acid.

12. 12. The method according to any one of claims 5 to 11, wherein in step ii) of the method, first the C1-C4 alcohol is removed by distillation and the remaining material is dissolved in water.

13. 12. The method according to any one of claims 5 to 11, wherein in step ii) of the method, water is added to the mixture resulting from step i) and subsequently or simultaneously the C1-C4 alcohol is removed by distillation.

14. 14. The method of claim 13, wherein the saturated C1-C4 alcohol is selected from the group of methanol and n-butanol.

15. 10. A method for making a redox flow cell, wherein a solution according to any one of claims 1 to 4 is used as an electrolyte in one of both chambers of the cell.

16. Steps below: 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 4, 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 16. The method of claim 15, comprising:

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