Solutions of TEMPO derivatives used as electrolytes in redox flow batteries

Abstract

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

Description

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

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

[0003] WO2018 / 028830 describes a method for producing 4-ammonium-2,2,6,6-tetraalkylpiperidinyl salts as typical TEMPO derivatives, which are redox-active compounds of the type commonly used in the cathode compartment of redox flow batteries. It is disclosed that 3 different production methods can be used to produce these compounds. The starting product is always a solution of 4-oxoalkylpiperidine, the corresponding imine or 4-aminoalkylpiperidine dissolved in different kinds of organic aprotic solvents such as alcohols, ethers, nitriles, halogenated hydrocarbons, aromatic hydrocarbons, aliphatic hydrocarbons or mixtures thereof. In both production methods described in WO 2018 / 028830, the intermediate compound is a solid, which must be separated from the solvent by filtration or centrifugation before being used in the next step. Additionally, the oxidation step described in WO 2018 / 028830 requires the addition of a base to keep the pH value in the optimal range during the oxidation process. By these described production methods, very pure compounds are obtained, but there are many intermediate stages in which solids must be separated and disposed, reaction steps in which solutions must be disposed of in a complex manner. Prior art methods use several different solvents or solvent mixtures, making solvent recycling particularly complex. In one method, anion exchange is required because bromide ions are not allowed in electrolyte solutions because they can be easily oxidized. In addition, there is no description of a production method in which water is mainly used as a solvent and anions can be selected at will without the use of ion exchange. For industrial-scale production, the described production method is not suitable because each filtering step, solid disposal step, solvent replacement or anion exchange step causes huge losses in terms of output and time, and increases the complexity of the method. In addition, if additional salts are to be removed from the electrolyte solution, the addition of additional alkali during the oxidation step will cause additional costs and efforts.

[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 cathode electrolyte material in a redox flow battery. Neither of these two publications describes the production of an aqueous solution containing compounds of formula (II) and (III) in addition to the compound of formula (I) and the preferred use of the aqueous solution as a cathode electrolyte in a redox flow battery.

[0005] T. Janoschka et al. described an aqueous organic redox flow battery comprising a compound of formula (I) in Chemical Communications, Vol. 54, No. 50, June 19, 2018, pp. 6871-6874, but did not describe the use of an aqueous solution comprising compounds of formula (II) and (III) in addition to the compound of formula (I) as a cathode electrolyte in a redox flow battery of the redox flow battery.

[0006] Therefore, one object of the present invention is to provide an aqueous solution comprising TEMPO derivatives having different chemical redox potentials comparable to those of the pure TEMPO derivatives described in the prior art, by using a simple and inexpensive method for producing the solution, wherein the anions in the resulting solution can be selected at will, no base addition is required during the oxidation process and it can be used on an industrial scale without extensive effort and large yield losses. Another object of the present invention is to provide a redox flow battery comprising an aqueous solution of a TEMPO derivative, which battery exhibits energy storage performance similar to or equal to that of a redox flow battery comprising an aqueous solution of 2,2,6,6-tetramethyl-1-piperidinyloxy-4-trimethylammonium chloride of the prior art.

[0007] This problem will be solved by a solution containing the following components:

[0008] a) water,

[0009] b) 20-55% by weight, based on the total weight of the solution, of a compound of formula (I) 2,2,6,6-tetramethyl-4-(trimethylammonio)-1-piperidinyloxy:

[0010]

[0011] c) less than 0.1% by weight of alkali metal cations, based on the total weight of the solution,

[0012] d) 0.1-12.5% ​​by weight, based on the total weight of the solution, of a compound of formula (II) N,N,N,1,2,2,6,6-octamethyl-4-piperidinammonium-1-oxide:

[0013]

[0014] e) 0.01-20% by weight, based on the total weight of the solution, of a compound of formula (III) 2,2,6,6-hexamethyl-4-(dimethylamino)-1-piperidinyloxy-N-oxide:

[0015]

[0016] The solution of the invention will be advantageous if the alkali metal cation is Na.

[0017] The solutions according to the invention are advantageous if the pH value is in the range of 2 to 7.

[0018] Solutions according to the invention are advantageous if the sum of the amount of compounds of the formula (I), (II), (III) and the amount of alkali metal cations in the solution is in the range from 20 to 50% by weight, relative to the total weight of the solution.

[0019] Another embodiment of the present invention is a method for producing the solution of the present invention, comprising the following steps: i) reacting a compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine

[0020]

[0021] With dimethyl carbonate in saturated C 1 -C 4 alcohol to obtain a saturated C 1 -C 4 A mixture of a compound of formula (V) N,N,N,2,2,6,6-heptamethyl-4-piperidinaminium, a compound of formula (VI) N,N,N,1,2,2,6,6-octamethyl-4-piperidinaminium and unreacted compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinaminium in alcohol:

[0022]

[0023] ii) Make C 1 -C 4 The alcohol solvent was exchanged for water.

[0024] iii) reacting the aqueous mixture obtained in step ii) with an aqueous hydrogen peroxide solution,

[0025] iv) adding an acid to the mixture obtained in step iii) until the pH value drops to a range of 3 to 5,

[0026] 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% by weight of the solution according to the invention.

[0027] If, in step i) of the process according to the invention, the compound of formula (IV) is reacted with dimethyl carbonate in a saturated C 1 -C 4 The process of the invention is advantageously carried out in the presence of an alcohol at a temperature in the range of 90 to 170°C.

[0028] If, in step i) of the process of the invention, the compound of formula (IV) is reacted with 0.7 to 1.5 mol of dimethyl carbonate per mol of the compound of formula (IV) in a saturated C 1 -C 4 The reaction is carried out in the presence of an alcohol and the compound of formula (IV) in the feed mixture reacts with a saturated C 1 -C 4 The method of the present invention is advantageous if the mass ratio between the alcohols is in the range of 0.1 to 5.

[0029] The process according to the invention is advantageous if, in step iii) of the process according to the invention, the mixture obtained from step ii) is reacted with aqueous hydrogen peroxide solution at a temperature in the range from 20 to 80°C.

[0030] The process according to the invention is advantageous if, in step ii) of the process according to the invention, 1.5 to 5 mol of aqueous hydrogen peroxide solution in a concentration in the range from 25 to 70% by weight is used per mol of compound of the formula (IV) in the feed mixture.

[0031] The process according to the invention is advantageous if the addition of the acid in step iv) is started when the concentration of hydrogen peroxide in the mixture in step iii) has fallen to less than 0.5% by weight.

[0032] The process according to the invention is advantageous if the acid used in step iv) has a standard redox potential of greater than +1.35 V.

[0033] If, in step ii) of the process according to the invention, C 1 -C 4 The process of the present invention will be advantageous if the residual material is dissolved in water and the alcohol is removed.

[0034] If, in step ii) of the process according to the invention, water is added to the mixture obtained in step i) and thereafter or simultaneously C is distilled off 1 -C 4alcohol, the process of the present invention will be advantageous.

[0035] If saturated C 1 -C 4 The process according to the invention is advantageous if the alcohol is selected from the group of methanol and n-butanol.

[0036] Another embodiment is a method of making a redox flow battery, wherein the solution of the invention is used as electrolyte in one of the two compartments of the battery.

[0037] The method of the present invention for preparing a redox flow battery will be advantageous if it comprises the following steps:

[0038] a) providing two compartments for a catholyte solution and an anolyte solution, each compartment being connected to at least one storage tank for a catholyte solution and an anolyte solution, respectively,

[0039] b) the two compartments are separated by an ion-conducting membrane,

[0040] c) equipping each chamber with electrodes,

[0041] d) filling the cathode electrolyte compartment with a solution according to the invention as redox-active material,

[0042] e) filling the anolyte compartment with an anolyte solution comprising a further redox active material.

[0043] Another embodiment of the present invention is a redox flow battery obtained by the inventive method for producing a redox flow battery.

[0044] Another embodiment of the present invention is the use of the inventive redox flow battery for storing electrical energy.

[0045] The solution of the present invention comprises water, 20-55 wt % of the compound 2,2,6,6-tetramethyl-4-(trimethylammonium)-1-piperidinyloxy of formula (I), based on the total weight of the solution, less than 0.1 wt % of alkali metal cations, based on the total weight of the solution, 0.1-12.5 wt % of the compound N,N,N,1,2,2,6,6-octamethyl-4-piperidinammonium-1-oxide of formula (II), based on the total weight of the solution, and 0.01-20 wt % of the compound 2,2,6,6-hexamethyl-4-(dimethylamino)-1-piperidinyloxy-N-oxide of formula (III), based on the total weight of the solution.

[0046] According to all the compounds of formula (I), (II), (III) and alkali metal cations in the solution of the present invention, the compound of formula (I) is preferably the main compound. Preferably, the amount of the compound of formula (I) in the solution of the present invention is in the range of 25 to 50% by weight, in particular in the range of 35 to 50% by weight, more preferably in the range of 40 to 45% by weight, based on the total amount of the solution.

[0047] The solution of the present invention also contains alkali metal cations. Preferably, these alkali metal cations are selected from Na and K, more preferably Na as the alkali metal cation. The amount of alkali metal cations in the solution of the present invention is preferably less than 0.1% by weight, in particular in the range of 0.001 to 0.01% by weight, more preferably in the range of 0.001 to 0.008% by weight, based on the total amount of the solution.

[0048] The compound of formula (II) is a compound produced by oxidation of one of the two by-products formed by the methylation of the starting compound of formula (IV) in the method of the present invention. Preferably, the amount of the compound of formula (II) in the solution of the present invention is in the range of 0.1 to 12.5 wt %, particularly 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 amount of the solution.

[0049] The compound of formula (III) is another compound or a second compound produced by oxidation of one of the two by-products formed by the methylation of the starting compound of formula (IV) in the method of the present invention. Preferably, the amount of the compound of formula (III) in the solution of the present invention is in the range of 0.01 to 20% by weight, particularly in the range of 0.01 to 5.0% by weight, more preferably in the range of 0.02 to 1.0% by weight, based on the total amount of the solution.

[0050] The sum of all amounts of compounds of formula (I), (II), (III) and alkali metal cations is preferably in the range of 20 to 50 wt %, in particular in the range of 30 to 50 wt %, more preferably in the range of 40 to 50 wt %, based on the total amount of the solution.

[0051] The amount of water in the solution of the present invention is preferably in the range of 35 to 75% by weight, particularly in the range of 45 to 70% by weight, and more preferably in the range of 50 to 60% by weight, based on the total amount of the solution.

[0052] Preferably, the pH value of the solution of the present invention is in the range of 2-7, in particular in the range of 3-5, and more preferably in the range of 4-5.

[0053] In the solution of the present invention, the anion present as the counterion of the cationic species is selected from chloride, fluoride, perchlorate, sulfate, alkyl sulfonate, aryl sulfonate, phosphate, alkyl phosphonate, aryl phosphonate and nitrate or a mixture thereof. Preferably, the anion is selected from chloride, nitrate, sulfate and perchlorate, more preferably the anion is chloride.

[0054] The solution of the present invention is obtained by the method of the present invention. The method of the present invention comprises the following steps:

[0055] i) making a compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine

[0056]

[0057] With dimethyl carbonate in saturated C 1 -C 4 alcohol to obtain a saturated C 1 -C 4 A mixture of N,N,N,2,2,6,6-heptamethyl-4-piperidinium compound of formula (V), N,N,1,2,2,6,6-octamethyl-4-piperidinium compound of formula (VI) and unreacted N,N,2,2,6,6-hexamethyl-4-piperidinium compound of formula (IV) in alcohol:

[0058]

[0059] ii) Make C 1 -C 4 The alcohol solvent was exchanged for water.

[0060] iii) reacting the aqueous mixture obtained in step ii) with an aqueous hydrogen peroxide solution,

[0061] iv) adding an acid to the mixture obtained in step iii) until the pH value is in the range of 2 to 7,

[0062] v) partially removing water from the mixture obtained in step iv) until the concentration of the compound of the formula (I) is in the range from 20 to 55% by weight of the solution according to the invention.

[0063] For the first step i), the compound of formula (IV) is dissolved in saturated C 1 -C 4 Alcohol. Saturated C 1 -C 4 Alcohol refers to an alcohol selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, n-butanol, 2-butanol, isobutanol and tert-butanol. Methanol and n-butanol are preferably used, and methanol is more preferably used. Preferably, the compound of formula (IV) in the feed mixture is mixed with the saturated C1 -C 4 The mass ratio between the alcohols 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 "compound of formula (IV) in the feed mixture" shall mean the saturated C 1 -C 4 The amount of compound of formula (IV) in the alcohol is not the amount of compound of formula (IV) remaining in solution after the methylation in step i).

[0064] In the first step i) of the process according to the invention, the compound of formula (IV) is dissolved in saturated C 1 -C 4 Preferably, the alcoholic solution of the compound of formula (IV) is methylated with 0.7-1.5 mol, in particular 0.9-1.5 mol, more preferably 1.0-1.2 mol of dimethyl carbonate / mol of the compound of formula (IV) in the feed mixture used. In the methylation process, the reaction temperature is preferably in the range of 90 to 170° C., in particular in the range of 100 to 160° C., more preferably in the range of 120 to 140° C.

[0065] After the methylation in step i) of the method of the present invention is completed, the methylation reaction is carried out as described in step ii) of the method of the present invention from C 1 -C 4 The alcohol solvent is exchanged for water as the solvent. The solvent exchange can be performed in two different ways. One method is to replace C 1 -C 4 The alcohol is completely distilled off, so that a resulting solid material is obtained comprising the methylated compounds of formula (V), (VI) and unreacted compound of formula (IV). Thereafter, the resulting solid material is dissolved in water.

[0066] Another and more preferred solvent exchange method is to first add water to the alcohol solution obtained in step i) of the process of the present invention, and then remove the C in the form of a light boiler or a light boiling azeotrope with water. 1 -C 4 In a more preferred embodiment, water is added and C is removed. 1 -C 4 Alcohol can be done simultaneously. First add water or remove C 1 -C 4 This process step using methanol or n-butanol as the preferred C 1 -C 4If ethanol, n-propanol, isopropanol, isobutanol or tert-butyl alcohol are used in step i) of the present invention, then after adding water in step ii) of the present invention, they can be easily removed. Methanol can be removed as a low-boiling substance, while n-butanol can be removed as a heterogeneous azeotrope with water. If butanol is used, the aqueous phase obtained after condensation and phase separation can be returned to step i) of the present invention. Both methanol and n-butanol can be recycled to step i) of the present invention without any further cleaning or drying steps. If ethanol, n-propanol, isopropanol, isobutanol or tert-butyl alcohol are used in step i) of the present invention, these alcohols will be removed by distillation. These removed alcohols must be dried before being recycled to step i) of the present invention. It is particularly advantageous to use methanol as a solvent because it does not form an azeotrope with water and can be removed as a low-boiling substance, so that no separate drying step is required. Therefore, the recovered methanol can be used in step i) without further processing.

[0067] The aqueous solution obtained in step ii) of the process of the present invention comprises water, compounds of formula (V), (VI) and unreacted compound of formula (IV) and carbonate or methyl carbonate as counter ions. In step iii) of the process of the present invention, the aqueous mixture obtained in step ii) is reacted with an aqueous hydrogen peroxide solution.

[0068] Step iii) of the process according to the invention is an oxidation step, in which the compounds of formula (IV), (V) and (VI) are oxidized to compounds of formula (I), (II) and (III). Accordingly, the aqueous mixture obtained in 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 mole of compound of formula (IV) in the feed mixture used in step i) of the process according to the invention, the hydrogen peroxide being used as an aqueous solution.

[0069] Preferably, the concentration of the aqueous hydrogen peroxide solution used in step ii) of the process of the present invention is 25-70% by weight, in particular 30-70% by weight, more preferably 45-70% by weight of hydrogen peroxide.

[0070] During the oxidation, the temperature in step iii) 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.

[0071] After the addition of the aqueous hydrogen peroxide solution is completed, the resulting mixture of step iii) of the process of the present invention is stirred under the conditions of step iii), which means about 60° C., until the concentration of hydrogen peroxide in the solution is less than 0.5% by weight. The amount of remaining hydrogen peroxide is determined by cerium (IV) titration of the solution before and after catalytic decomposition of hydrogen peroxide with manganese dioxide. The difference between the two titration values ​​is used to calculate the residual amount of hydrogen peroxide.

[0072] After the addition of the aqueous hydrogen peroxide solution is complete and the concentration of hydrogen peroxide has dropped to less than 0.5% by weight, step iii) ends. In step iv), an acid is added to the solution obtained in step iii) to adjust the pH value. Preferably, the pH value is adjusted 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 by adding an acid. The acid used to adjust the pH value is an acid with a standard redox potential greater than +1.35V. Preferably, the acid is selected from hydrochloric acid, perchloric acid, sulfuric acid, phosphoric acid, nitric acid and methanesulfonic acid, in particular the acid is hydrochloric acid and perchloric acid, more preferably hydrochloric acid, which can be added as a gas or in the form of an aqueous solution. Preferably, an aqueous hydrochloric acid solution, commonly known as hydrochloric acid, is used.

[0073] In the last step of the process of the invention, the amount of water is reduced in step v) of the process of the invention. This will be done by distillation. The amount of water removed depends on the desired final concentration of the compound of formula (I) in the final solution. The water must be removed until the resulting solution shows a content of the compound of formula (I) in the range of 20 to 55% by weight, preferably in the range of 25 to 50% by weight, in particular in the range of 35 to 50% by weight, more preferably in the range of 40 to 45% by weight, based on the total weight of the solution. Preferably, the distillation will be carried out at a pressure in the range of 0.02 to 1.0 bar and at a temperature in the range of 20 to 100° C., in particular at a pressure in the range of 0.05 to 0.3 bar and at a temperature in the range of 30 to 70° C., more preferably at a pressure in the range of 0.08 to 0.2 bar and at a temperature in the range of 58 to 62° C.

[0074] After concentration, the solution of the present invention obtained can be used as an electrolyte in a redox flow battery without further treatment. Preferably, the solution of the present invention is used as a cathode electrolyte in the redox flow battery. Redox flow batteries are usually formed by using two compartments for a cathode electrolyte solution and an anode electrolyte solution, wherein each compartment is connected to a storage tank for the cathode electrolyte solution and the anode electrolyte solution, respectively, by a pump. The two compartments are separated by an ion-conducting membrane and are equipped with electrodes. In the cathode compartment and the connected cathode storage tank, the solution of the present invention is filled. In the anode compartment and the connected anode storage tank, the electrolyte of the anode is filled. The redox-active compounds in the redox flow battery change between their different redox levels during charging and discharging. For discharge, the electrolyte must be pumped from the storage tank to the electrode, while for charging, the reverse process is used. Therefore, a redox flow battery comprising a solution of the present invention as an electrolyte is a simple and versatile way of storing electrical energy for different applications. Example:

[0075] General:

[0076] pH value:

[0077] The pH values ​​were always measured using a calibrated glass electrode (EasyFerm Plus PHI S8 225, two-point calibration, using buffer pH=4.00 (citric acid, sodium hydroxide, sodium chloride from Fluka) and buffer pH=7.00 (monopotassium phosphate, disodium hydrogen phosphate from Fluka).

[0078] 1 H-NMR method:

[0079] The compound of formula (V) 1 H-NMR data:

[0080]

[0081] 1 H-NMR (500MHz, D 2 O): δ [ppm] = 3.68 (tt, J = 12.5Hz, 2.8Hz1H, H 1 ),3.07(s,9H,H 6 ),2.02–2.08(m,2H,H 3 ),1.32(t,J=12.5Hz,2H,H 2 ), 1.14(s,6H,H 5 ),1.12(s,6H,H 4 ).

[0082] The compound of formula (VI)1 H-NMR data:

[0083]

[0084] 1 H-NMR (500MHz, D 2 O): δ [ppm] = 3.62 (tt, J = 12.5Hz, 3.1Hz, 1H, H 7 ),3.00(s,9H,H 12 ),2.15(s,3H,H 13 ),2.08–2.02(m,2H,H 9 ),1.55(t,J= 12.5Hz,2H,H 8 ),1.15(s,6H,H 11 ),1.05(s,6H,H 10 ).

[0085] The compound of formula (IV) 1 H-NMR data:

[0086]

[0087] 1 H-NMR (500MHz, D 2 O): δ [ppm] = 2.83 (tt, J = 12.3Hz, 3.2Hz, 1H, H 14 ),2.27(s,6H,H 19 ),1.88(dd,J=12.7Hz,3.2Hz,2H,H 15 ),1.28(s,6H, H 17 ),1.24(s,6H,H 18 ),1.17(dd,J=12.7Hz,12.3Hz,6H,H 16 ).

[0088] The molar ratios of the compounds of formula (IV), (V) and (VI) can be determined by comparing 1 The integration of the H-NMR signals at δ = 3.68 ppm (from 1H of the compound of formula (V), 2.15 ppm (from 3H of the compound of formula (VI)) and 2.27 ppm (from 6H of the compound of formula (IV)) is most conveniently determined.

[0089] Therefore, the molar ratio of the compounds of formula (IV):(V):(VI) is the same as the ratio of the following integrals:

[0090] (Integral signal of the compound of formula (V) at δ = 3.68 ppm): (Integral signal of the compound of formula (IV) at δ = 2.15 ppm) / 3: (Integral signal of the compound of formula (VI) at δ = 2.27 ppm) / 6 1 H-NMR measurement:

[0091] exist 1 Prior to H-NMR measurement, the solution of the present invention is reacted with an excess of phenylhydrazine (about 2 moles per mole of the compound of formula (I) and the compound of formula (III)) to convert the N-oxyl radical into the corresponding hydroxylamine. This procedure produces two isomeric forms of each reduced species (compounds of formula (Ia) and (Ib) / compounds of formula (IIa) and (IIIb)) and each isomer is present in 1 The H-NMR spectrum gives individual signals. 1 H-NMR measurement, the crude reaction mixture reduced with phenylhydrazine was treated with D 2 O dilution and reference to residual H at δ = 4.79 ppm 2 O proton signal.

[0092] As described in WO 2018 / 2883011 on page 28, the signal assignment of the compound of formula (I) was confirmed by synthesizing the compound of formula (I) as a pure crystalline material. As described herein, the signal assignment of the compound of formula (III) was confirmed by synthesizing the compound of formula (III) as a pure substance in aqueous solution.

[0093] The compound of formula (III) was synthesized in pure form in aqueous solution:

[0094] To a solution of the compound of formula (IV) (39.3 g) in water (40.1 g) was added 37 wt % hydrochloric acid (11.97 g), thereby reducing the pH of the solution to 9.0. Solid sodium bicarbonate (2.71 g) was then added and the mixture was heated to 60° C. When this temperature was reached, a 50 wt % aqueous solution of hydrogen peroxide (46.4 g) was added continuously over 4 hours. During the addition, the pH dropped and was maintained above 8.0 by adding a 50 wt % aqueous solution of sodium hydroxide (6.4 g) in approximately five equal portions. After the addition of hydrogen peroxide was complete, stirring was continued for 12 hours. The mixture was then cooled to room temperature and stirred. 1 HNMR spectroscopy and ESI MS mass spectrometry analysis. 1 According to H NMR, the mixture contained >99% by weight of the compound of formula (III) as organic substance.

[0095] The identification of compounds of formula (I) and (III) was also supported by HRMS (ESI in ACN:H 2O: HCOOH = 80:20:0.1, instrument: Q Extractive TM Hybrid-quadrupole-orbitrap mass spectrometer, ThermoFisher).

[0096] The reduced form of the compound of formula (I) 1 H-NMR:

[0097]

[0098] 1 H-NMR (500MHz, D 2 O): δ[ppm]=3.80–3.67(m,1H,H 1 +H 1’ ),3.12 (s,9H,H 6 or H 6’ , minor isomer), 3.09(s,9H,H 6 or H 6’ , major isomer), 2.24– 2.14 (m, 2H, H 2 +H 2’ ),1.99(t,J=12.1Hz,2H,H 3 or H 3’ , minor isomer), 1.75 (t, J=12.4 Hz, 2H, H 3 or H 3’ , major isomer), 1.34(s,6H,H 4 / 5 or H 4’ / 5’ , minor isomer), 1.26(s,6H,H 4 / 5 or H 4’ / 5’ , major isomer), 1.22(s,6H,H 4 / 5 or H 4’ / 5’ , major isomer), 1.12(s,6H,H 4 / 5 or H 4’ / 5’ , the minor isomer). The ratio of the two isomers is about 90:10.

[0099] HRMS:C 12 H 26 N 2 O + Theoretical value: 214.2040; Measured value: 214.2036 The reduced form of the compound of formula (III) 1 H-NMR:

[0100]

[0101] 1 H-NMR (500MHz, D 2 O): δ[ppm]=3.66–3.52(m,1H,H 7 +H 7’ ),3.17 (s,6H,H 12 or H 12’ , minor isomer), 3.14(s,6H,H 12 or H 12’ , major isomer), 2.27 –2.14 (m, 2H, H 8 +H 8’ ),1.93(t,J=12.5Hz,2H,H 9 or H 9’ , minor isomer), 1.72(t,J=12.5Hz,2H,H 9 or H 9’ , major isomer), 1.33(s,6H,H 10 / 11 or H 10’ / 11’ , minor isomer), 1.25(s,6H,H 10 / 11 or H 10’ / 11’ , major isomer), 1.21(s,6H,H 10 / 11 or H 10’ / 11’ , major isomer), 1.11(s,6H,H 10 / 11 or H 10’ / 11’ , minor isomer). The ratio of the two isomers is about 86:14.

[0102] HRMS:C 11 H 24 N 2 O 2 + Theoretical value: 216.1638; measured value: 216.1637

[0103] The compound of formula (II) remains unchanged during the reduction and gives signals that are well separated from the signals of the compounds of formula (Ia), (Ib), (IIIa) and (IIIb):

[0104]

[0105] 1 H-NMR (500MHz, D 2 O): δ [ppm] = 3.93 (tt, J = 13.3Hz, 3.2Hz, 1H, H 13 ),3.14(s,9H,H 19 ),3.03(s,3H,H 18 ),2.47(t,J=12.6Hz,2H,H14 ),2.07 (d,J=12.1Hz,2H,H 15 ,1.65(s,6H,H 16 ),1.56(s,6H,H 17 ).

[0106] HRMS: C 13 H 29 N 2 O + Theoretical value: 229.2274; measured value: 229.2271

[0107] The ratio of the compounds of formula (I), (II) and (III) can be determined by comparing 1 The integration of the H-NMR signals at δ == 3.80-3.67 ppm (from 1H of compounds of formula (I), 1.56 ppm (from 6H of compounds of formula (II)) and 3.66-3.52 ppm (from 1H of compounds of formula (III)) is most conveniently determined.

[0108] Therefore, the molar ratio of the compounds of formula (I): (II): (III) is the same as the ratio of the following integrals:

[0109] (Integrated signal of the compound of formula (I) at δ=3.80-3.67 ppm): (Integrated signal of the compound of formula (II) at δ=1.56 ppm) / 6: (Integrated signal of the compound of formula (III) at δ=3.66-3.52 ppm).

[0110] Cerium(IV) Redox Titration:

[0111] Cerium (IV) redox titration was used to determine the total content of hydrogen peroxide and N-oxyl species (compounds of formula (I) + (III)) according to the following method:

[0112] Content of N-oxyl species:

[0113] 100 mg of manganese dioxide is added to about 1 g of analyte. The mixture is stirred at 20-25° C. for 5 minutes or until 5 minutes after the end of gas evolution. The analyte is then filtered. 250±2 mg of the filtered analyte is placed in a beaker equipped with a magnetic stirring bar and diluted with 45 ml of pure water and 5 ml of dilute sulfuric acid (10 wt % in water). The obtained solution is placed on an automatic titration device (905 Titrando, Metrohm) equipped with a platinum Titrode (Metrohm) and stirred at 20-25° C. Cerium (IV) sulfate solution (0.10 mol / L) is added through the titration device until a redox potential jump (V C1). Then the concentration (weight %) of the sum of the compounds of formula (I) + (III), w I+III , can be calculated from the consumption of cerium (IV) sulfate solution using the following formula:

[0114]

[0115] The symbols have the following meanings:

[0116] V c is the volume of cerium sulfate solution used given in litres,

[0117] C c1 is the concentration of the cerium sulfate solution used given in mol / l,

[0118] m s is the mass of the analyte given in grams,

[0119] M I-Cl The molar mass of the compound of formula (I) as a chloride salt is 249.8 g / mol.

[0120] M III The molar mass of the compound of formula (III) is 215.3 g / mol.

[0121] x I is the molar fraction of the compound of formula (I), expressed as ( 1 Integral at δ=3.80-3.67 ppm in H-NMR): [( 1 Signal integration at δ = 3.66-3.52 ppm in H-NMR) + ( 1 The ratio of the integral at δ = 3.80-3.67 ppm in H-NMR)] was calculated

[0122] x III is the molar fraction of the compound of formula (III), 1 Signal integration at δ=3.66-3.52 ppm in H-NMR): [( 1 Signal integration at δ = 3.66-3.52 ppm in H-NMR) + ( 1 The ratio of the integral at δ = 3.80-3.67 ppm in H-NMR)] was calculated

[0123] Sum of hydrogen peroxide and N-oxyl species:

[0124] 250 ± 2 mg of the analyte was placed in a beaker equipped with a magnetic stirring bar and diluted with 45 ml of pure water and 5 ml of dilute sulfuric acid (10 wt % in water). The obtained solution was placed on an automatic titration device (905 Titrando, Metrohm) equipped with a platinum Titrode (Metrohm) and stirred at 20-25° C. Cerium (IV) sulfate solution (0.10 mol / L) was added via the titration device until a redox potential jump (V C2 The concentration of hydrogen peroxide can be calculated from the difference in the consumption of cerium (IV) sulfate solution (ΔV C =V C2 -V C1 ) is calculated:

[0125]

[0126] where the symbols have the same meanings as defined above and M H2O2 It is 34.0 g / mol, which is the molar mass of hydrogen peroxide.

[0127] Cyclic voltammetry method:

[0128] The solution obtained from the corresponding example was diluted with 0.1 mol / L sodium chloride aqueous solution until the concentration of the N-oxyl compound was 1.0 wt %. The solution was placed in an electrochemical cell equipped with a standard three-electrode setup (working electrode: glassy carbon black Counter electrode: platinum wire, reference electrode: Ag / AgCl, 3 mol / L KCl in water). Using a PGU 20V-2A-E potentiostat (IPS), the potential was raised to 1200 mV and then cycled between 1200 mV and -700 mV with a scan rate of ±20 mV / s (3 cycles in total).

[0129] Embodiment 1:

[0130] 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 were mixed and heated to 120° C. The mixture was stirred at 120° C. for 24 hours. The autoclave was then cooled to room temperature and depressurized. The volatiles were evaporated and a solid residue (49.6 g) was obtained and dissolved in 50 g of water to obtain a 50% by weight aqueous solution of the compounds of formula (IV), (V) and (VI) in the form of a clear yellow solution. By 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).

[0131] Embodiment 2:

[0132] 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 hours. The autoclave is then cooled to room temperature and depressurized. Then, 40 ml of water is added, the mixture is heated to 90°C and 46.1 g of distillate is collected. Another 40 g of water is added to the tank, the mixture is heated to 107°C and 47.9 g of distillate is collected. The tank (124.6 g) is a 40 wt% aqueous solution of the compounds of formula (IV), (V) and (VI). By 1 The ratio of compounds of formula (IV): (V): (VI) determined by H-NMR was 1.0:98.1:0.9, corresponding to 0.3 wt% of compound (IV), 39.3 wt% of carbonate of compound (V) and 0.4 wt% of carbonate of compound (VI). No methanol was found in the NMR.

[0133] Embodiment 3 (comparison):

[0134] The methylation is carried out as described in WO 2018 / 028830, page 27, line 20 to page 28, line 15 (corresponding to DE102016009904A1, paragraph

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

[0135] Embodiment 4:

[0136] 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 of the compound of formula (V) and 0.5 wt % of the carbonate of the compound of formula (VI), in water) was added 37 wt % hydrochloric acid (0.82 g), thereby reducing the pH value of the solution to 10.0. The mixture was then heated to 60° C. When this temperature was reached, a 50 wt % aqueous solution of hydrogen peroxide (32.7 g) was added continuously over 4 hours. After the addition of hydrogen peroxide was complete, stirring was continued for 12 hours. The mixture was then cooled to about 30° C. and 37 % hydrochloric acid (about 19 g) was added to reduce the pH value of the solution to 4.3. Water was subsequently distilled off under reduced pressure (70 absolute mbar) until the concentration of the N-oxyl species of the compounds of formula (I) and (III) was 50 wt % (determined by cerium (IV) redox titration).

[0137] pass 1The molar ratio of the compounds of formula (I): (II): (III) determined by H-NMR was 98.1:0.8:1.1, corresponding to 49.5 wt% chloride salt of the compound of formula (I), 0.4 wt% chloride salt of the compound of formula (II) and 0.5 wt% of the compound of formula (III).

[0138] Embodiment 5:

[0139] 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 of the compound of formula (V) and 0.5 wt % of the carbonate of the compound of formula (VI), in water) was added 10 wt % nitric acid (21.1 g), thereby reducing the pH value of the solution to 9.5. The mixture was then heated to 60° C. When this temperature was reached, a 50 wt % aqueous solution of hydrogen peroxide (32.7 g) was added continuously over 2 hours. After the addition of hydrogen peroxide was complete, stirring was continued for 12 hours. The mixture was then cooled to about 30° C. and 10 % nitric acid (about 19 g) was added to reduce the pH value of the solution to 4.5. Water was subsequently distilled off under reduced pressure (70 absolute mbar) until the concentration of the N-oxyl species of the compounds of formula (I) and (III) was 49 wt % (determined by cerium (IV) redox titration).

[0140] pass 1 The molar ratio of the compounds of formula (I): (II): (III) determined by H-NMR was 98.1:0.8:1.1, corresponding to 48.6 wt % nitrate of the compound of formula (I), 0.4 wt % nitrate of the compound of formula (II) and 0.5 wt % of the compound of formula (III).

[0141] Embodiment 6 (comparison):

[0142] The oxidation was carried out as described in WO 2018 / 028830, page 28, line 20 to page 29, line 22. In the HRMS of the obtained product, no signals of the compounds of formula (II) and (IIIa) / (IIIb) were visible; 1 Only the signal of the compound of formula (Ia / Ib) was observed in H-NMR.

[0143] Example 7

[0144] The cyclic voltammogram of the product of Example 4 (see Figure 1 ).

[0145] Example 8

[0146] The cyclic voltammogram of the product of Example 5 (see Figure 2 ).

[0147] Example 9

[0148] Cyclic voltammogram of the product of Example 6 (see Figure 3 ).

[0149] Figure 1 and 2 The cyclic voltammogram and Figure 3 The cyclic voltammogram of Comparative Example 6 in is almost the same. Therefore, the solutions of the present invention of Examples 4 and 5 show almost the same redox potential as the solution obtained in Example 5 representing the prior art. Therefore, the solutions of the present invention can be used in the redox flow battery described in the prior art.

Claims

1. A solution comprising: a) water, b) 20-55% by weight, based on the total weight of the solution, of a compound of formula (I) 2,2,6,6-tetramethyl-4-(trimethylammonium)-1-piperidinyloxy: c) less than 0.1% by weight of alkali metal cations, based on the total weight of the solution, d) 0.1-12.5% ​​by weight, based on the total weight of the solution, of a compound of formula (II) N,N,N,1,2,2,6,6-octamethyl-4-piperidinium-1-oxide: e) 0.01-20% by weight, based on the total weight of the solution, of a compound of formula (III) 2,2,6,6-hexamethyl-4-(dimethylamino)-1-piperidinyloxy-N-oxide:

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

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

5. The solution according to claim 1, wherein the sum of the amount of the compound of formula (I), (II), (III) and the amount of the alkali metal cation in the solution is in the range of 30 to 50 wt % relative to the total weight of the solution.

6. The solution according to claim 2, wherein the sum of the amount of the compound of formula (I), (II), (III) and the amount of the alkali metal cation in the solution is in the range of 30 to 50 wt % relative to the total weight of the solution.

7. The solution according to claim 3, wherein the sum of the amount of the compound of formula (I), (II), (III) and the amount of the alkali metal cation in the solution is in the range of 30 to 50 wt % relative to the total weight of the solution.

8. The solution according to claim 4, wherein the sum of the amount of the compound of formula (I), (II), (III) and the amount of the alkali metal cation in the solution is in the range of 30 to 50 wt % relative to the total weight of the solution.

9. A method for producing a solution according to any one of claims 1 to 8, comprising the following steps: i) making a compound of formula (IV) N,N,2,2,6,6-hexamethyl-4-piperidinamine with dimethyl carbonate in the presence of a saturated C1-C4 alcohol to obtain a mixture comprising N,N,N,2,2,6,6-heptamethyl-4-piperidinium compound of formula (V), N,N,N,1,2,2,6,6-octamethyl-4-piperidinium compound of formula (VI) and unreacted N,N,2,2,6,6-hexamethyl-4-piperidinium compound of formula (IV) dissolved in a saturated C1-C4 alcohol: ii) exchanging the solvent from a C1-C4 alcohol to water, iii) reacting the aqueous mixture obtained in step ii) with an aqueous hydrogen peroxide solution, iv) adding an acid to the mixture obtained in step iii) until the pH value decreases 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 % according to the solution of claim 1.

10. The process according to claim 9, 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 to 170°C.

11. The process according to claim 9, wherein in step i) of the process, the compound of formula (IV) is reacted with 0.7-1.5 mol of dimethyl carbonate in the presence of a saturated C1-C4 alcohol and the mass ratio between the compound of formula (IV) and the saturated C1-C4 alcohol in the feed mixture is in the range of 0.1 to 5.

12. The process according to claim 10, wherein in step i) of the process, the compound of formula (IV) is reacted with 0.7-1.5 mol of dimethyl carbonate in the presence of a saturated C1-C4 alcohol and the mass ratio between the compound of formula (IV) and the saturated C1-C4 alcohol in the feed mixture is in the range of 0.1 to 5.

13. The process according to claim 9, wherein in step iii) of the process, the resulting mixture of step ii) is reacted with an aqueous hydrogen peroxide solution at a temperature in the range of 20 to 80°C.

14. The process according to claim 10, wherein in step iii) of the process, the resulting mixture of step ii) is reacted with an aqueous hydrogen peroxide solution at a temperature in the range of 20 to 80°C.

15. The process according to claim 11, wherein in step iii) of the process, the resulting mixture of step ii) is reacted with an aqueous hydrogen peroxide solution at a temperature in the range of 20 to 80°C.

16. The process according to claim 12, wherein in step iii) of the process, the resulting mixture of step ii) is reacted with an aqueous hydrogen peroxide solution at a temperature in the range of 20 to 80°C.

17. The process according to claim 9, wherein 1.5 to 5 molar aqueous hydrogen peroxide solution in a concentration ranging from 25 to 70% by weight is used per mole of the compound of formula (IV) in the feed mixture.

18. The process according to any one of claims 10 to 16, wherein 1.5 to 5 molar aqueous hydrogen peroxide solution in a concentration in the range of 25 to 70% by weight is used per mole of the compound of formula (IV) in the feed mixture.

19. The process according to claim 9, wherein the addition of the acid in step iv) is started when the concentration of hydrogen peroxide in the mixture in step iii) drops to less than 0.5 wt%.

20. The process according to any one of claims 10 to 17, wherein the addition of the acid in step iv) is started when the concentration of hydrogen peroxide in the mixture in step iii) has dropped to less than 0.5% by weight.

21. The process of claim 18, wherein the addition of the acid in step iv) is initiated when the concentration of hydrogen peroxide in the mixture in step iii) drops to less than 0.5 wt%.

22. The method of claim 9, wherein the acid used in step iv) has a standard redox potential greater than +1.35 V.

23. The process according to any one of claims 10 to 17, wherein the acid used in step iv) has a standard redox potential greater than +1.35 V.

24. The method of claim 18, wherein the acid used in step iv) has a standard redox potential greater than +1.35 V.

25. The method of claim 19, wherein the acid used in step iv) has a standard redox potential greater than +1.35 V.

26. The method of claim 20, wherein the acid used in step iv) has a standard redox potential greater than +1.35 V.

27. The method of claim 21, wherein the acid used in step iv) has a standard redox potential greater than +1.35 V.

28. The process of claim 9, wherein in step ii) of the process, the C1-C4 alcohol is first distilled off and the remaining material is dissolved in water.

29. The process according to any one of claims 10 to 17, wherein in step ii) of the process, the C1-C4 alcohol is first distilled off and the remaining material is dissolved in water.

30. A process according to any one of claims 24 to 27, wherein in step ii) of the process, the C1-C4 alcohol is first distilled off and the remaining material is dissolved in water.

31. The process according to claim 9, wherein in step ii) of the process, water is added to the mixture obtained in step i) and thereafter or simultaneously the C1-C4 alcohol is distilled off.

32. The process according to any one of claims 10 to 17, wherein in step ii) of the process, water is added to the mixture obtained in step i) and thereafter or simultaneously the C1-C4 alcohol is distilled off.

33. The process according to any one of claims 24 to 27, wherein in step ii) of the process, water is added to the mixture obtained in step i) and thereafter or simultaneously the C1-C4 alcohol is distilled off.

34. The process of claim 31, wherein the saturated C1-C4 alcohol is selected from the group consisting of methanol and n-butanol.

35. The process of claim 32, wherein the saturated C1-C4 alcohol is selected from the group consisting of methanol and n-butanol.

36. The process of claim 33, wherein the saturated C1-C4 alcohol is selected from the group consisting of methanol and n-butanol.

37. Method for producing a redox flow battery, wherein a solution according to any one of claims 1 to 8 is used as electrolyte in one of the two compartments of the battery.

38. The method according to claim 37, comprising the steps of: a) providing two compartments for a catholyte solution and an anolyte solution, each compartment being connected to at least one storage tank for a catholyte solution and an anolyte solution, respectively, b) the two compartments are separated by an ion-conducting membrane, c) equipping each compartment with electrodes, d) filling the cathode electrolyte compartment with a solution comprising 2,2,6,6-tetramethyl-4-(trimethylammonium)-1-piperidinyloxy salt as redox-active material according to any one of claims 1 to 8, e) filling the anolyte compartment with an anolyte solution comprising a further redox-active material.

39. A redox flow battery obtained by the method according to any one of claims 37-38.

40. Use of a redox flow battery according to claim 39 for storing electrical energy.

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