A SOLUTION OF TEMPO-DERIVATIVES FOR USE AS ELECTROLYTE IN REDOX-FLOW CELLS

Abstract

The present invention relates to a solution comprising water and different 2,2,6,6-tetramethyl-piperidinyl-oxyl (TEMPO)-derivatives, a process for the production of this solution, a process for making a redox-flow cell comprising the solution as 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.

Claims

1.-18. (canceled)

19. A solution comprising a) water, b) 20 to 55 wt.-% according to the total weight amount of the solution of compound 2,2,6,6-tetramethyl-4-(trimethylammonio)-1-piperidinyloxy of formula (I), ##STR00014## c) less than 0.1 wt.-% according to the total weight amount of the solution alkali metal cation d) 0.1 to 12.5 wt.-% according to the total weight amount of the solution of compound N,N,N,1,2,2,6,6-octamethyl-4-piperidinammonium-1-oxide of formula (II) ##STR00015## e) 0.01 to 20 wt.-% according to the total weight amount of the solution of compound 2,2,6,6-hexamethyl-4-(dimethylamino)-1-piperidinyloxy-N-oxide of formula (III) ##STR00016##

20. The solution according to claim 19 wherein the alkali metal cation is Na.

21. The solution according to claim 19 wherein the solution has a pH-value in the range of 2 to 7.

22. The solution according to claim 19 wherein the sum of the amounts of the compounds of formula (I), (II), (III) plus the amounts of alkali metal cations in the solution is in the range of 20 to 50 wt.-% with respect to the total weight amount of the solution.

23. A process for the production of the solution according to claim 19 comprising the following steps: i) reacting the compound N,N,2,2,6,6-hexamethyl-4-piperidinamine of formula (IV) ##STR00017## with dimethyl carbonate in the presence of a saturated C.sub.1 to C.sub.4-alcohol to get a mixture comprising compound N,N,N,2,2,6,6-heptamethyl-4-piperidinaminium of formula (V), compound N,N,N,1,2,2,6,6-octamethyl-4-piperidinaminium of formula (VI) and non-reacted compound N,N,2,2,6,6-hexamethyl-4-piperidinamine of formula (IV) dissolved in the saturated C.sub.1 to C.sub.4 alcohol ##STR00018## ii) making a solvent change from the C.sub.1 to C.sub.4 alcohol to water iii) reacting the resulting aqueous mixture of step ii) with an aqueous hydrogen peroxide solution, iv) adding an acid to the resulting mixture of step iii) until the pH-value decreases to the range of 3 to 5, v) partially removing the water of the resulting mixture of step iv) until the concentration of compound of formula (I) is in the range of 20 to 55 wt.-% according to the solution of claim 19.

24. The process of claim 23 wherein in step i) of the process the compound of formula (IV) is reacted with dimethyl carbonate in the presence of a saturated C1 to C4 alcohol at a temperature in the range of 90 to 170° C.

25. The process according to claim 23 wherein in step i) of the process the compound of formula (IV) is reacted with 0.7 to 1.5 mol dimethyl carbonate in the present of a saturated C1 to C4 alcohol and the mass ratio between compound of formula (IV) in the feed mixture and the saturated C1 to C4 alcohol is in the range of 0.1 to 5.

26. The process according to claim 23 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.

27. The process according to claim 23 wherein in step iii) of the process 1.5 to 5 mol of aqueous hydrogen peroxide having a concentration in the range of 25 to 70 wt.-% are used per mol of compound of formula (IV) in the feed mixture.

28. The process according to claim 23 wherein the addition of the acid in step iv) will start when the concentration of hydrogen peroxide in the mixture of step iii) has decreased to less than 0.5 wt.-%.

29. The process according to claim 23 wherein the acid used in step iv) has a standard redox potential greater than +1.35 V.

30. The process according to claim 23 wherein in step ii) of the process the C1 to C4 alcohol is first distilled off and the remaining material is dissolved in water.

31. The process according to claim 23 wherein in step ii) of the process water is added to the resulting mixture of step i) and afterwards or simultaneously the C1 to C4 alcohol is distilled off.

32. The process according to claim 31 wherein the saturated C1 to C4 alcohol is selected from the group of methanol and n-butanol.

33. A process for making a redox-flow cell wherein a solution according to claim 19 is used as electrolyte in one of both chambers of the cell.

34. The process according to claim 33 comprising the following steps: a) providing two chambers for catholyte and anolyte solutions, each connected to at least one storage tank for catholyte and anolyte solutions respectively b) separating the two chambers with an ion-conducting membrane c) equipping the chambers with electrodes d) filling the solution comprising a 2,2,6,6-tetramethyl-4-(trimethylammonio)-1-piperidinyloxy salt as redox active material in the catholyte chamber e) filling an anolyte solution comprising another redox active material in the anolyte chamber.

35. A redox-flow cell obtained by a process according to claim 33.

36. Use of the redox flow cell according to claim 35 for storing electrical energy.

Description

EXAMPLES

[0067] General:

[0068] pH-Values:

[0069] pH values are 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).

[0070] .sup.1H-NMR Method:

[0071] .sup.1H-NMR data of compound of formula (V):

##STR00008##

[0072] .sup.1H-NMR (500 MHz, D.sub.2O): δ [ppm]=3.68 (tt, J=12.5 Hz, 2.8 Hz 1H, H.sub.1), 3.07 (s, 9H, H.sub.6), 2.02-2.08 (m, 2H, H.sub.3), 1.32 (t, J=12.5 Hz, 2H, H.sub.2), 1.14 (s, 6H, H.sub.5), 1.12 (s, 6H, H.sub.4).

[0073] .sup.1H-NMR data of compound of formula (VI):

##STR00009##

[0074] .sup.1H-NMR (500 MHz, D.sub.2O): δ [ppm]=3.62 (tt, J=12.5 Hz, 3.1 Hz, 1H, H.sub.7), 3.00 (s, 9H, H.sub.12), 2.15 (s, 3H, H.sub.13), 2.08-2.02 (m, 2H, H.sub.9), 1.55 (t, J=12.5 Hz, 2H, H.sub.8), 1.15 (s, 6H, H.sub.11), 1.05 (s, 6H, H.sub.10).

[0075] .sup.1H-NMR data of compound of formula (IV):

##STR00010##

[0076] .sup.1H-NMR (500 MHz, D.sub.2O): δ [ppm]=2.83 (tt, J=12.3 Hz, 3.2 Hz, 1H, H1.sub.4), 2.27 (s, 6H, H.sub.19), 1.88 (dd, J=12.7 Hz, 3.2 Hz, 2H, H.sub.15), 1.28 (s, 6H, H.sub.17), 1.24 (s, 6H, H.sub.18), 1.17 (dd, J=12.7 Hz, 12.3 Hz, 6H, H.sub.16).

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

[0078] Thus the molar ratio of compound of formula (IV):(V):(VI) is the same as the ratio of the following integrals:

[0079] (integral of signal at δ=3.68 ppm from compound of formula (V)):(integral of signal at δ=2.15 ppm from compound of formula (IV))/3:(integral of signal at δ=2.27 ppm from compound of formula (VI))/6.

[0080] .sup.1H-NMR Measurements of the Inventive Solution:

[0081] Prior to .sup.1H-NMR measurements, the inventive solution is reacted with excess phenyl hydrazine (approx. 2 mol per mol of compound of formula (I) plus (III)) to convert the N-oxyl radicals to the corresponding hydroxylamines. This procedure yields two isomeric forms of each reduced species (compound of formula (Ia) and (Ib)/compound of formula (IIa) and (IIIb)) and each isomer gives individual signals in the .sup.1H-NMR spectrum. For all .sup.1H-NMR measurements the crude reaction mixture from reduction with phenyl hydrazine was diluted with D.sub.2O and referenced to the signal of residual H.sub.2O protons at δ=4.79 ppm.

[0082] Signal assignments for compound of formula (I) were confirmed by synthesizing compound of formula (I) as a pure crystalline material as described in WO 2018/2883011 on page 28. Signal assignments for compound of formula (III) were confirmed by synthesizing compound of formula (III) as a pure material in aqueous solution as described here.

[0083] Synthesis of Compound of Formula (III) in Pure Form in Aqueous Solution:

[0084] To a solution of compound of formula (IV) (39.3 g) in water (40.1 g), 37 wt.-% hydrochloric acid (11.97 g) is added, whereby the pH value of the solution decreases to 9.0. Then, solid sodium bicarbonate (2.71 g) is added and the mixture is heated to 60° C. When this temperature is reached a 50 wt.-% aqueous solution of hydrogen peroxide (46.4 g) is continuously added over a period of 4 hours. During addition the pH value decreases and is kept above 8.0 by the addition of a 50 wt.-% aqueous solution of sodium hydroxide (6.4 g) in five approximately equal portions. After the addition of the hydrogen peroxide is completed, stirring is continued for 12 hours. Then, the mixture is allowed to cool down to room temperature and analyzed by .sup.1H NMR spectroscopy and ESI MS mass spectrometry. The mixture contains >99 wt.-% of compound of formula (III) as organic material as determined by .sup.1H NMR.

[0085] The identity of compound of formula (I) and (III) is also supported by HRMS (ESI in ACN:H.sub.2O:HCOOH=80:20:0.1, instrument: Q Extractive™ hybrid-quadrupole-orbitrap mass spectrometer, ThermoFisher).

[0086] .sup.1H-NMR of the reduced form of compound of formula (I):

##STR00011##

[0087] .sup.1H-NMR (500 MHz, D.sub.2O): δ [ppm]=3.80-3.67 (m, 1H, H.sub.1+H.sub.1′), 3.12 (s, 9H, H.sub.6 or H.sub.6′, minor isomer), 3.09 (s, 9H, H.sub.6 or H.sub.6′, major isomer), 2.24-2.14 (m, 2H, H.sub.2+H.sub.2′), 1.99 (t, J=12.1 Hz, 2H, H.sub.3 or H.sub.3, minor isomer), 1.75 (t, J=12.4 Hz, 2H, H.sub.3 or H.sub.3, major isomer), 1.34 (s, 6H, H.sub.4/5 or H.sub.4′/5′, minor isomer), 1.26 (s, 6H, H.sub.4/5 or H.sub.4′/5′, major isomer), 1.22 (s, 6H, H.sub.4/5 or H.sub.4′/5′, major isomer), 1.12 (s, 6H, H.sub.4/5 or H.sub.4′/5′, minor isomer). The ratio of the two isomers is approximately 90:10.

[0088] HRMS: theory for C.sub.12H.sub.26N.sub.2O.sup.+: 214.2040; found: 214.2036

[0089] .sup.1H-NMR of the reduced form of compound of formula (III):

##STR00012##

[0090] .sup.1H-NMR (500 MHz, D.sub.2O): δ [ppm]=3.66-3.52 (m, 1H, H.sub.7+H.sub.7′), 3.17 (s, 6H, H.sub.12 or H.sub.12′, minor isomer), 3.14 (s, 6H, H.sub.12 or H.sub.12′, major isomer), 2.27-2.14 (m, 2H, H.sub.8+H.sub.8′), 1.93 (t, J=12.5 Hz, 2H, H.sub.9 or H.sub.9′, minor isomer), 1.72 (t, J=12.5 Hz, 2H, H.sub.9 or H.sub.9′, major isomer), 1.33 (s, 6H, H.sub.10/11 or H.sub.10′/11′, minor isomer), 1.25 (s, 6H, H.sub.10/11 or H.sub.10′/11′, major isomer), 1.21 (s, 6H, H.sub.10/11 or H.sub.10′/11′, major isomer), 1.11 (s, 6H, H.sub.10/11 or H.sub.10′/11′, minor isomer). The ratio of the two isomers is approximately 86:14.

[0091] HRMS: theory for C.sub.11H.sub.24N.sub.2O.sub.2.sup.+: 216.1638; found: 216.1637

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

##STR00013##

[0093] .sup.1H-NMR (500 MHz, D.sub.2O): δ [ppm]=3.93 (tt, J=13.3 Hz, 3.2 Hz, 1H, H.sub.13), 3.14 (s, 9H, H.sub.19), 3.03 (s, 3H, H.sub.18), 2.47 (t, J=12.6 Hz, 2H, H.sub.14), 2.07 (d, J=12.1 Hz, 2H, H.sub.15, 1.65 (s, 6H, H.sub.16), 1.56 (s, 6H, H.sub.17).

[0094] HRMS: theory for C.sub.13H.sub.29N.sub.2O.sup.+: 229.2274; found: 229.2271

[0095] The ratio of compound of formula (I), (II) and (III) can be determined most conveniently by comparing the integrals of the .sup.1H-NMR signals at 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)).

[0096] Thus the molar ratio of compound of formula (I):(II):(III) is the same as the ratio of the following integrals:

[0097] (integral of signal at δ=3.80-3.67 ppm from compound of formula (I)):(integral of signal at δ=1.56 ppm from compound of formula (II))/6:(integral of signal at δ=3.66-3.52 ppm from compound of formula (III)).

[0098] Cerimetric Redox Titration:

[0099] Cerimetric redox titration is used to determine the total content of hydrogen peroxide and N-oxyl species (compound of formula (I)+(III)) according to the following method:

[0100] Content of N-Oxyl Species:

[0101] 100 mg of manganese dioxide is added to approx. 1 g of analyte. The mixture is stirred at 20 to 25° C. for 5 minutes or until 5 minutes after the end of gas evolution. Then the analyte is filtered. 250±2 mg of filtered analyte is placed in a beaker equipped with a magnetic stirring bar and is diluted with 45 mL purified water and 5 mL dilute sulfuric acid (10 wt.-% in water). The obtained solution is placed on an automated titration device (905 Titrando, Metrohm) equipped with a Pt-Titrode (Metrohm) and is stirred at 20-25° C. Cerium (IV) sulfate solution (0.10 mol/L) is added via the titration device until a redox potential jump is detected (V.sub.C1). The concentration of the sum of compound of formula (I)+(III) in weight-%, w.sub.I+III, can then be calculated from the consumption of cerium (IV) sulfate solution using the following equation:

[00001]$w_{I+\mathrm{III}}=100*\frac{V_{c1}{C}_c}{\text{?}}*\left[\left(x_I*M_{I-\mathrm{cl}}\right)+\left(x_{\mathrm{III}}*M_{\mathrm{III}}\right)\right]$$\text{?}\text{indicates text missing or illegible when filed}$

[0102] Where the symbols have the following meaning:

[0103] V.sub.c is the volume of the cerium sulfate solution used given in liter

[0104] C.sub.c1 is the concentration of the cerium sulfate solution used given in mol/liter

[0105] m.sub.s is the mass of the analyte given in grams

[0106] M.sub.I-CI is 249.8 g/mol, the molar mass of compound of formula (I) as the chloride salt

[0107] M.sub.III is 215.3 g/mol, the molar mass of compound of formula (III)

[0108] x.sub.I is the molar fraction of compound of formula (I) calculated as the ratio of (integral at δ=3.80-3.67 ppm in .sup.1H-NMR): [(integral of signal at δ=3.66-3.52 ppm in .sup.1H-NMR)+(integral at δ=3.80-3.67 ppm in .sup.1H-NMR)]

[0109] x.sub.III is the molar fraction of compound of formula (III) calculated as the ratio of (integral of signal at δ=3.66-3.52 ppm in .sup.1H-NMR): [(integral of signal at δ=3.66-3.52 ppm in .sup.1H-NMR)+(integral at δ=3.80-3.67 ppm in .sup.1H-NMR)]

[0110] Sum of Hydrogen Peroxide and N-Oxyl Species:

[0111] 250±2 mg of analyte is placed in a beaker equipped with a magnetic stirring bar and is diluted with 45 mL purified water and 5 mL dilute sulfuric acid (10 wt.-% in water). The obtained solution is placed on an automated titration device (905 Titrando, Metrohm) equipped with a Pt-Titrode (Metrohm) and is stirred at 20-25° C. Cerium (IV) sulfate solution (0.10 mol/L) is added via the titration device until a redox potential jump (V.sub.C2) is detected. The concentration of hydrogen peroxide can be calculated from the difference of the consumptions of cerium (IV) sulfate solution (ΔV.sub.C=V.sub.C2−V.sub.C1) using the following equation:

[00002]$w_{H_2{O}_2}=100*\frac{\Delta V_c*C_c}{\text{?}}*M\left(H_2{O}_2\right)$$\text{?}\text{indicates text missing or illegible when filed}$

[0112] Where the symbols have the same meanings as defined above and M.sub.H2O2 is 34.0 g/mol, the molar mass of hydrogen peroxide.

[0113] Cyclic Voltammetry Method:

[0114] The solution obtained from the respective example is diluted with 0.1 mol/L aqueous sodium chloride solution until the concentration of the N-oxyl compounds is 1.0 wt.-%. Said solution is placed in an electrochemical cell equipped with a standard 3 electrode setup (working electrode: glassy carbon (ø=2 mm), counter electrode: platinum wire, reference electrode: Ag/AgCl, 3 mol/L KCl in water). The potential is ramped to 1200 mV and then cycled between 1200 mV and −700 mV at a scan rate of ±20 mV/s (in total 3 cycles) using PGU 20V-2A-E potentiostat (IPS).

Example 1

[0115] In a stainless-steel autoclave 92 g (116.5 ml) of methanol, 40.0 g of compound of formula (IV) and 23.5 g dimethyl carbonate are mixed and heated to 120° C. The mixture is stirred for 24 h at 120° C. Then the autoclave is allowed to cool down to room temperature and depressurized. Volatiles are distilled off and a solid residue (49.6 g) is obtained and dissolved in 50 g water to obtain a 50 wt.-% aqueous solution of compound of formula (IV), (V) and (VI) as a clear yellowish solution. The ratio of compound of formula (IV):(V):(VI) as determined by .sup.1H-NMR is 1.0:98.1:0.9, which corresponds to 0.4 wt.-% of compound of formula (IV), 49.1 wt.-% of compound of formula (V) carbonate salt and 0.5 wt.-% of compound of formula (VI) carbonate salt.

Example 2

[0116] In a stainless-steel autoclave 92 g (116.5 ml) of methanol, 40.0 g of compound of formula (IV) and 23.5 g dimethyl carbonate are mixed and heated to 120° C. The mixture is stirred for 24 h at 120° C. Then the autoclave is allowed to cool down to room temperature and depressurized. Then, 40 mL of water are added, the mixture is heated to 90° C. and 46.1 g distillate are collected. Another 40 g of water are added to the sump, the mixture is heated to 107° C. and 47.9 g of distillate are collected. The sump (124.6 g) is a 40 wt.-% solution of compound of formula (IV), (V) and (VI) in water. The ratio of compound of formula (IV):(V):(VI) as determined by .sup.1H-NMR is 1.0:98.1:0.9, which corresponds to 0.3 wt.-% of compound of formula (IV), 39.3 wt.-% of compound of formula (V) carbonate salt and 0.4 wt.-% of compound of formula (VI) carbonate salt. No methanol is found in the NMR.

Example 3 (Comparative)

[0117] The methylation was made as described in the WO 2018/28830, page 27, line 20 to page 28 line 15 (entspricht DE102016009904A1, paragraph [0112] and following). In the .sup.1H-NMR of the product obtained, only signals for compound of formula (V) are visible.

Example 4

[0118] To a solution taken from example 1 (100 g, contains 0.4 wt.-% compound of formula (IV), 49.1 wt.-% of compound of formula (V) carbonate salt and 0.5 wt.-% of compound of formula (VI) carbonate salt in water) 37 wt.-% hydrochloric acid (0.82 g) is added, whereby the pH value of the solution decreases to 10.0. Then the mixture is heated to 60° C. When the temperature is reached a 50 wt.-% aqueous solution of hydrogen peroxide (32.7 g) is continuously added over a period of 4 hours. After the addition of the hydrogen peroxide is completed, stirring is continued for 12 hours. The mixture is then allowed to cool down to about 30° C. and 37 wt.-% hydrochloric acid (ca. 19 g) is added to decrease the pH value of the solution to 4.3. Water is consequently distilled off at reduced pressure (70 mbar abs) until the concentration of N-oxyl species of compound of formula (I) and (III) is 50 wt.-% (as determined by cerimetric redox titration). The molar ratio of compound of formula (I):(II):(III) as determined by .sup.1H-NMR is 98.1:0.8:1.1 which corresponds to 49.5 wt.-% of compound of formula (I) chloride salt, 0.4 wt.-% of compound of formula (II) chloride salt and 0.5 wt.-% of compound of formula (III).

Example 5

[0119] To a solution taken from example 1 (100 g, contains 0.4 wt.-% of compound of formula (IV), 49.1 wt.-% of compound of formula (V) carbonate salt and 0.5 wt.-% of compound of formula (VI) carbonate salt in water) 10 wt.-% nitric acid (21.1 g) is added, whereby the pH value of the solution decreases to 9.5. Then the mixture is heated to 60° C. When the temperature is reached a 50 wt.-% aqueous solution of hydrogen peroxide (32.7 g) is continuously added over a period of 2 hours. After the addition of the hydrogen peroxide is completed, stirring is continued for 12 hours. The mixture is then allowed to cool down to about 30° C. and 10 wt.-% nitric acid (ca. 19 g) is added to decrease the pH value of the solution to 4.5. Water is consequently distilled off at reduced pressure (70 mbar abs) until the concentration of N-oxyl species of compound of formula (I) and (III) is 49 wt.-% (as determined by cerimetric redox titration).

[0120] The molar ratio o compound of formula (I):(II):(III) as determined by .sup.1H-NMR is 98.1:0.8:1.1, which corresponds to 48.6 wt.-% of compound of formula (I) nitrate salt, 0.4 wt.-% of compound of formula (II) nitrate salt and 0.5 wt.-% of compound of formula (III).

Example 6 (Comparative)

[0121] The oxidation was performed as described in the WO 2018/028830, page 28, line 20 to page 29 line 22. In HRMS of the product obtained no signals for compound of formula (II) and (IIIa)/(IIIb) are visible; in .sup.1H-NMR of the reduced samples only signals for compound of formula (Ia/Ib) are visible.

Example 7

[0122] Cyclic voltammogram of the product from example 4 (see Figure I).

Example 8

[0123] Cyclic voltammogram of the product from example 5 (see Figure II).

Example 9

[0124] Cyclic voltammogram of the product from example 6 (see Figure III).

[0125] The cyclic voltammogram in Figure I and II are nearly identical to that of the comparative example 6 in Figure III. Therefore, the inventive solution of example 4 and 5 show nearly the same redox potential as the solution obtained in example 5 which represents the state of the art. The inventive solution can thus be used in a redox flow cell as it is described in the state of the art.