Producing a stable activated nickel oxide hydroxide foam electrode

Abstract

The invention relates to a method by means of which metal electrodes and metal foam electrodes, in particular nickel electrodes and especially nickel foam electrodes, can be electrochemically activated. The invention also relates to metal electrodes and metal foam electrodes, in particular nickel electrodes and especially nickel foam electrodes, which have been obtained by means of the method for electrochemical activation. The invention additionally relates to the use, in electrochemical reactions, in the electrolysis of organic compounds, in the catalysis of organochemical reactions and in water electrolysis, of the metal electrodes and metal foam electrodes, in particular nickel electrodes and especially nickel foam electrodes, electrochemically activated by means of the method according to the invention.

Description

[0001] Preparation of a stable activated nickel oxide hydroxide foam electrode

[0002] The invention relates to a method by which metal electrodes and metal foam electrodes, in particular nickel electrodes and especially nickel foam electrodes, can be electrochemically activated. The invention further relates to metal electrodes and metal foam electrodes, in particular nickel electrodes and especially nickel foam electrodes, obtained by the electrochemical activation method. Furthermore, the invention relates to the use of the metal electrodes and metal foam electrodes electrochemically activated by the method according to the invention, in particular nickel electrodes and especially nickel foam electrodes, in electrochemical reactions, electrolysis of organic compounds, catalysis of organochemical reactions, and water electrolysis.

[0003] Metal electrodes and metal foam electrodes, especially nickel electrodes and especially nickel foam electrodes, are in great demand in a wide range of applications due to their high porosity, conductivity, mechanical stability and chemical resistance.

[0004] To date, only one electrocatalytic process for the activation of nickel foam electrodes is known in the state of the art. This process is based on alternating polarity between two nickel electrodes. This activation proceeds using solutions of 0.05 M - 0.1 M NiSO4(aq) and 0.1 M NaOAc(aq) or 0.1 M KOAc(aq), with the addition of base in concentrations of 10 -5M to 1 mM KOH(aq) or 5 mM NaOH(aq) (GWD Briggs, E. Jones, WFK Wynne-Jones, Trans. Faraday Soc. 1955, 51, 1433-1442; BV Lyalin, VA Petrosyan, Russ. Chem. Bull. 2004, 53, 688-692; W. Jud, CA Salazar, J. Imbrogno, J. Verghese, SM Guinness, J.-N. Desrosiers, CO Kappe, D. Canti Ho, Org. Process Res. Dev. 2022, 26, 1486-1495, H.-J. Schäfer, J. Kaulen, Tetrahedron 1982, 38, 3299-3308). In all known cases, activation requires a change of polarity between two nickel electrodes every 5 to 30 seconds, as specified.

[0005] A disadvantage of this known activation method is its low reproducibility, especially for cleaned nickel foam electrodes. The conventional activation method does not reproducibly produce stable nickel oxyhydroxides, especially for nickel foam electrodes cleaned after electrochemical oxidation.

[0006] Furthermore, the use of a polarity changer is required. This significantly increases the complexity of the electronic periphery of the conventional galvanostatic setup. Furthermore, two nickel electrodes are required, even though only one is used for the subsequent electrochemical reaction. Half of the deposited nickel species is thus deposited on an electrode that will not be used in the reaction, since only the last anodically polarized electrode bears an outer NiO(OH) layer.

[0007] It is an object of the present invention to provide a method for activating metal electrodes which does not have, or only to a reduced extent, the disadvantages of the method known from the prior art for activating metal electrodes and metal foam electrodes, in particular nickel electrodes and especially nickel foam electrodes.

[0008] This problem was solved by the subject matter of the patent claims and the description.

[0009] The present invention relates to a method for the electrochemical activation of metal electrodes comprising the steps

[0010] (a) Pretreatment of the metal electrode by bringing the metal electrode into contact with base and

[0011] (b) electrochemical activation of the metal electrode pretreated according to step (a) without

[0012] Reversing the polarity of the metal electrode to form an outer metal oxyhydroxide layer, wherein the metal electrode has 0 to 100 wt.% nickel and 100 to 0 wt.% cobalt, in each case based on the total content of nickel and cobalt in the electrode, or 10 to 100 wt.% nickel and 0 to 90 wt.% iron, in each case based on the total content of nickel and iron in the electrode, wherein the metal electrode has a content of other metals equal to or less than 10 wt.%, in each case based on the total metal content.

[0013] The method according to the invention enables the efficient activation of a single metal electrode, in particular a nickel electrode or nickel foam electrode, undivided and in direct current, without requiring polarity reversal of two metal electrodes, in particular two nickel electrodes. A polarity changer is eliminated when using the method according to the invention, thus simplifying the system design.

[0014] Furthermore, the method according to the invention enables, in particular, the reproducible activation of metal foam electrodes, especially nickel foam electrodes, after their use in electroorganic oxidations. Cleaning foam electrodes is more difficult than plate electrodes, which is why the previous method seemed less suitable for their activation. These cannot be easily polished, as is the case with plate electrodes.

[0015] Thus, the process claimed by the invention offers excellent access to electrochemically activated nickel surfaces, with increased reproducibility when using purified nickel foams or nickel foam electrodes. In particular, the process according to the invention enables the reproducible electrochemical activation of (purified) nickel foams and nickel plate electrodes. Depending on the extent of contamination, this can extend the service life of the electrodes.

[0016] Activated nickel foams are essential for the successful electrochemical oxidation of many substances, including (a)cyclic alcohols and ketones (H.-J. Schäfer, Electrochemistry I, Top. Curr. Chem. 1987, 142, 101-129). The present invention therefore allows a flexible and simplified method for activating nickel foams. A further advantage of the process according to the invention is that only very small volumes of a base are required to wet the metal electrode, in particular the nickel electrode, and the base used can be used continuously for this purpose. Further advantages include the flexibility in using various activation solutions, the suitability of the reaction conditions for industrially relevant scaling of the electrochemical activation, and the possibility of using the large activated nickel surface for various applications, such asin catalysis or electrolysis / battery technology.

[0017] Finally, the process according to the invention also enables the use of used nickel foams for electroorganic oxidation on activated nickel surfaces. Electrochemical activation in direct current enables controlled, rapid activation and avoids the deposited nickel residues on the second electrode.

[0018] Figure 1 shows on the left a scanning electron microscope (SEM) image of the activated nickel oxyhydroxide surface using the conventional activation method (taken from GWD Briggs, E. Jones, W.

[0019] F.K. Wynne-Jones, Trans. Faraday Soc. 1955, 51, 1433-1442) and on the right, an SEM image of the activated surface obtained using the method according to the invention. A finely porous fiber network is visible in both cases.

[0020] The conventional activation method therefore did not reproducibly produce stable nickel oxyhydroxides in nickel foams purified after electrochemical oxidation. This was evident even visually, as the lack of black coloration was not observed.

[0021] Figure 2 illustrates a flow electrolysis cell for carrying out a process using an electrochemically activated metal electrode according to the present invention.

[0022] Figure 3 shows a graphical evaluation of the parameter results with a plot corresponding to the qNMR yields of 3-ethyladipic acid after the electrochemical oxidation of 4-ethylcyclohexanol using an electrochemically activated metal electrode according to the present invention.

[0023] Figures 4 and 5 illustrate the qNMR yield change of 3-propyladipic acid without and with addition of 4-propylcyclohexanone over the course of the applied charge using an electrochemically activated metal electrode according to the present invention.

[0024] Bringing the metal electrode into contact with base has a dual function: on the one hand, it cleans the surface and, on the other hand, it accelerates the rapid formation of a metal oxyhydroxide layer, in particular a nickel oxyhydroxide layer, on the surface.

[0025] For further oxidation reactions on correspondingly electrochemically activated metal oxyhydroxide

[0026] For anodes, especially nickel oxyhydroxide anodes (Ni(O)OH), it may be advantageous to choose a basic environment, for example dilute sodium hydroxide solution as electrolyte, to keep the catalyst active during the reaction.

[0027] The content of other metals in the metal electrode is preferably equal to or less than 5 wt.%, very particularly preferably equal to or less than 2 wt.% and even more preferably equal to or less than 1 wt.%, in each case based on the total metal content.

[0028] The metal electrode preferably contains at most up to 1 wt.% each, particularly preferably at most up to 0.1 wt.% each and even more preferably at most 0.01 wt.% each of V, Wo and Mo. It should be noted that these metals are subject to corrosion, particularly in alkaline-aqueous emulsions.

[0029] Particularly preferred metal electrodes for use in the process according to the invention are those which contain at least 80% by weight, preferably at least 90% by weight, particularly preferably at least 95% by weight, of nickel, in each case based on the total weight of the metals in the electrode.

[0030] The metals in nickel and / or cobalt and optionally iron can be present in the metal electrode on a substrate, wherein the substrate is in particular at least one material selected from the group consisting of steel, copper, titanium and carbon, preferably graphite.

[0031] The metal electrode can preferably be doped with at least one element of the 5th and / or 6th main group, particularly preferably with one or more elements, preferably an element which is / are selected from the group consisting of phosphorus, arsenic, selenium and sulfur.

[0032] The doping content information refers to the elemental state of the doping relative to the mass of the metal of the electrode.

[0033] If the metal electrode is doped with phosphorus, it can preferably contain 2 to 10 wt.% phosphorus, particularly preferably 3 to 9 wt.% and very particularly preferably 4 to 9 wt.%, wherein phosphorus is considered as an element in each case and is related to the metal mass of the electrode.

[0034] The determination of the phosphorus doping content can preferably be carried out according to DIN EN ISO 5427, Annex D.1

[0035] Preferably, the metal electrode has a thickness of several millimeters, more preferably more than 3 mm, further more preferably more than 5 mm and particularly preferably equal to or thicker than 6 mm.

[0036] Preferably, in the process according to the invention, a Ni(O)OH foam electrode is used as the metal electrode, which preferably contains at least 80 wt.%, particularly preferably at least 85, 90, 95, 98 or 99 wt.%, very particularly preferably at least 99.9, and even more preferably at least 99.99 wt.% nickel, in each case based on the metal content of the Ni(O)OH foam electrode.

[0037] Preferably, the metal electrode is pretreated by bringing it into contact with base, preferably at least one inorganic base and / or at least one organic base.

[0038] Suitable for use in the contacting step are customary inorganic and organic bases known to those skilled in the art. Preferably, one or more bases, preferably one base, are independently selected from the group consisting of N(R A )4OH, N(R A )4acetate, with R Aeach independently of one another H or Ci-4-alkyl I, linear or branched, NaOH, sodium acetate, KOH, potassium acetate, LiOH, lithium acetate, Na2CO3, K2CO3 and U2CO3 are used, particularly preferably selected from the group consisting of NaOH and KOH.

[0039] The base is preferably used in aqueous solution in the contacting step, the concentration of the base preferably being 10 mM to 10 M, particularly preferably 0.2 to 5 M, very particularly preferably 0.5 to 1.5 M and even more preferably 1 ± 0.1 M.

[0040] Bringing the metal electrode into contact with the base can be done by spraying, immersing, in a jet or in a flow.

[0041] The metal electrode and the base can be brought into contact with each other over a wide temperature range. Contacting the metal electrode with the base preferably takes place at a temperature of 10 to 40°C, particularly preferably at 18 to 27°C.

[0042] According to the method according to the invention, the electrochemical activation of the metal electrode can advantageously be carried out, one or more salts of nickel and / or cobalt and optionally iron being particularly suitable for this step.

[0043] If nickel or cobalt salts are used, the salts selected from the group consisting of Ni2SO4 and CoSO4 are particularly suitable.

[0044] For electrochemical activation of the metal electrode, the salt or salts may preferably be present in aqueous basic solution, preferably in a concentration of 0.05 to 0.15 M.

[0045] In this case, customary bases known to the person skilled in the art are suitable. Preferably, one or more bases, in particular one base, are used which are independently selected from the group consisting of N(R A )4OH, N(R A )4acetate, with R Aeach independently of one another is H or C 1-4 -alkyl, linear or branched, NaOH, sodium acetate, KOH, potassium acetate, LiOH, lithium acetate, Na2CO3, K2CO3 and U2CO3, particularly preferably selected from the group consisting of NaOH and KOH. The concentration of the base during the electrochemical activation of the metal electrode is preferably 10 mM to 10 M, preferably 0.2 to 5 M, particularly preferably 0.5 to 1.5 M, most preferably 1 ± 0.1 M.

[0046] The activation of the metal electrode preferably takes place at a charge of 2 to 10 coulombs, preferably 5 to 7 coulombs.

[0047] The activation of the electrochemical metal electrode can occur over a wide temperature range. Preferably, the activation of the metal electrode occurs at a temperature of 10 to 40°C, particularly preferably at 18 to 27°C.

[0048] A further subject of the present invention is an electrochemically activated metal electrode, in particular obtainable by the process according to the invention.

[0049] A further object of the present invention is an electrochemically activated metal electrode, in particular obtainable or obtained by the process according to the invention, for carrying out oxidation reactions of organic components, for water electrolysis, in batteries or in catalysis.

[0050] The metal electrode electrochemically activated according to the process according to the invention can preferably be used in a batch electrolysis cell or in a continuously flowing electrolysis cell, preferably in a continuously flowing electrolysis cell.

[0051] The metal electrode electrochemically activated by the process according to the invention can preferably be used in an undivided electrolysis cell.

[0052] The following examples further illustrate the present invention, but are not to be construed as limiting the scope of the invention.

[0053] Examples:

[0054] General information and methods

[0055] Analytical-grade chemicals were purchased and used from common suppliers (such as TCI, Aldrich, Rotisolv, Fisher Chemical, VWR, Thermos Scientific, and Acros). Solvents were purified by standard methods such as distillation or demineralization.

[0056] Unless otherwise stated, ambient pressure and temperature were used as reaction conditions. The electrodes applied for the reactions were commercially available and contained small channels on the inlet and outlet sides: stainless steel (1.4571) and Recemat™ nickel foam. They match the setup of the IKA ElectraSyn flow 6-18. For activation, a separate sheet electrode (6 x 18 cm²) made of stainless steel (1.4571) was cut by the university workshop. A TDK Lambda Genesys 750 W / 1500 W power supply was used as the galvanostat. The reaction mixture was pumped through Ismapren tubing (size 16) using a Regio Digital Masterflex™ peristaltic pump (model no. 78018-42) from Ismatec. A heating circuit was installed and connected to an IKA WICO CBC 5 C refrigeration and circulation thermostat (model RN41).

[0057] Melting point determination: Melting points were measured using an M-565 device (Büchi, Essen, Germany). The heating rate was 1 °C per minute.

[0058] NMR spectroscopy: A Bruker Avance III 600 (Bruker, probe head: 5 mm TCI-CryoProbe head with z-gradient and ATM) NMR spectrometer was used to measure 1 H (600 MHz) and 13 C (151 MHz) inverse gated spectra at 25°C. Chemical shifts (δ) are given in parts per million (ppm). Quantification was performed by integrating the respective signals in the 13 C inverse-gated spectrum against 1,3,5-trimethoxybenzene as internal standard (δ = (55.4±0.02) ppm). The DMSO-d6 signal was chosen as reference (δ = 39.52 ppm).

[0059] Liquid chromatography: Photodiode array analysis was performed using a Shimadzu DUGA-20A3 instrument equipped with a Knauer C18 column (Eurospher II, 100-5 C18, 150 x 4 mm). The column was conditioned to 25°C, and the flow rate was set to 1 mL / min. The aqueous eluent was buffered with formic acid (0.8 mL / 2.5 L) and stabilized with acetone (5 vol%). To prepare samples, the carboxylic acid product mixture was extracted from the reaction mixture by acid-base extraction with ethyl acetate, dried over MgSO4, and the solvent evaporated. The product was filtered (VWR 13 mm syringe filter with 0.45 pm PTFE membrane), dissolved, and diluted in acetonitrile (MS grade). This organic solution was analyzed by liquid chromatography.

[0060] High-resolution mass spectra were obtained using a G6545A Q-ToF (Agilent GmbH, Waldbronn, Germany) with a dual AJS electrospray ion source (Dual AJS ESI). The MS parameters were as follows: mass range: 80 to 3200 m / z, scan rate: 1 spectrum s 1, Nebulizer pressure: 25 psig, Capillary voltage: 3500 V, Fragmentor: 50 V, Skimmer: 45 V, Dry gas temperature: 275°C, Dry gas flow: 10 L min-1, Sheath gas temperature: 350°C, Sheath gas flow: 10 L min-1. Mass calibration was performed on the day of measurement using an external standard. The mass accuracy of the measurement results is better than 5 ppm. The chromatographic separation was performed on a 1260 Infinity II HPLC system (Agilent GmbH, Waldbronn, Germany) equipped with a G7111B 1260 Quaternary Pump, G7129A 1260 Vialsampler, and G7116A 1260 Multicolumn Thermostat, equipped with an Agilent EclipsePlus C18 RRHP (2.1 x 50 mm, 2.1 pm particle size) analytical column. Eluents were 98% H2O with 2% ACN and 0.05% formic acid (eluent A) and 2% H2O with 98% ACN and 0.05% formic acid (eluent B).The following gradient elution was used for separation at a flow rate of 200 pl min-1: starting at 10% B for 1 min, followed by a linear increase to 95% B at 10 min, maintaining 95% B until 30 min before proceeding back to 10% B at 33 min. The column was equilibrated at 10% B for 15 min before the next measurement. The injection volume was 2 pl. Data were acquired using Agilent MassHunter Workstation LC / MS software version 11.0, and data analysis was performed using Agilent MassHunter Workstation Qualitative Analysis software version 10.0.

[0061] MALDI-TOF MS: The Autoflex MALDI-TOF mass spectrometer from Bruker Daltonik GmbH (Fahrenheitstraße 4, 28359 Bremen, Germany) equipped with a smartbeam™ll Nd:YAG laser (355 nm) was used. The image was measured in linear mode. 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) was used as the matrix.

[0062] An electrolysis cell with an electrochemically activated nickel anode was prepared according to the following description and used in the following reactions.

[0063] A flow electrolysis cell with a geometric anodic surface of 108 cm 2 used (Figure 2). The modular design of the cell further includes a self-supporting frame and a cage for the foam electrode - all with an active geometric surface area of ​​108 cm 2 - make this electrolyzer particularly interesting for scaled electrolysis in flow mode. Furthermore, the high heat dissipation is enabled by channels behind the electrodes, which allow for better temperature control than smaller commercial flow electrolyzers, where only the reservoir can be set to a specific temperature.

[0064] Figure 2 shows the reaction setup in an undivided flow electrolysis cell with a stainless steel cathode and a nickel foam anode contacted by a nickel plate electrode. The distance between the electrodes was 3 mm. The cooling circuit used water to maintain the temperature between 25 and 50 °C. The reaction mixture was emulsified using an IKA Magie lab mixing unit with a DR mixing element (IKA-Werke GmbH & Co. KG, Staufen, Germany).

[0065] Activation of the nickel anode:

[0066] First, an activation solution was prepared consisting of 0.1 M NiSO4(aq), 0.1 M NaOAc(aq), and 5 mM NaOH(aq). This activation solution (420 mL) was filled into a cup-shaped cell made of polymethyl methacrylate (PMMA). A stainless steel electrode (6 x 18 cm) was immersed and connected. Before fully immersing and connecting the nickel foam anode at a distance of 10 mm, it was immersed in 1 M sodium hydroxide solution or thoroughly rinsed with it and then drained. A current density of 5.0 mA was achieved. 2 was applied for 30 minutes. This corresponds to a charge of 9 C cm 2The activation solution was stirred at 300 rpm. The activated dark nickel foam was then rinsed with demineralized water. The stainless steel cathode was cleaned by briefly immersing it in dilute sulfuric acid (4M), then washed with water and treated with sandpaper (pore size 300) until it achieved a homogeneous optical shine. The cell was then rearranged. Although SEM images showed that the cleaning process did not fully restore the original surface morphology, the activation procedure described above could also be successfully applied to the cleaned nickel foams.

[0067] Implementation of flow cell electrolysis:

[0068] The commercially available modular flow electrolyzer IKA ESF 6-18 was used, as described above. This was equipped with a nickel foam anode (Recemat™ RCM-Ni4753.05), connected by a nickel sheet, and a stainless steel cathode (1 .4571), each measuring 6 x 18 cm. 2 A polytetrafluoroethylene (PTFE) spacer (0.4 mm) was used. The nickel foam anode was activated as described above.

[0069] The commercially available IKA magic LAB® mixing device was connected to the IKA magic PLANT® stirred tank. The electrolyte, consisting of NaOH(aq) (1 M) and the respective cyclic reactant 4-alkylcyclohexanol (1), was emulsified in it. This mixing unit features a powerful DR mixing element and cyclically pumps the liquid into the storage tank and back again. The flow rate is 12,000 rpm (corresponding to approximately 70 L per hour). From there, a peristaltic pump feeds the upright flow cell from below. Outlet pipes are connected to the storage tank, allowing the hydrogen formed to be safely removed. A specified charge quantity and current density were applied and monitored using Python software control and the TDK Lambda Genesys galvanostat. After electrolysis, the reaction mixture was pumped out of the electrolyzer in the reverse flow direction.Then, methyl tert-butyl ether (MTBE, 80 ml) and 1 M sodium hydroxide solution (80 ml) were used to purify the cell sequentially. All fractions were combined and processed by liquid-liquid extraction in a 2L perforator.

[0070] First, non-acidic components were extracted from the basic aqueous reaction mixture using MTBE. After acidification to pH 1-2, the acids were extracted with ethyl acetate. The organic fractions were dried over MgSO4 and the solvents removed. The remaining acid fraction was weighed, and the products were quantitatively determined by 13 C Inverse gated NMR spectroscopy. 1,3,5-trimethoxybenzene (0.1 mmol) was used as an internal standard.

[0071] Example 1 :

[0072] Anodic oxidation of 4-methylcyclohexanol (1') to 3-methyladipic acid (2') was carried out in the flow cell described above.

[0073] 4-Methylcyclohexanol Oxidation: 4-Methylcyclohexanol (1', 10 mmol, c = 0.1 M, isomer mixture) was mixed with sodium hydroxide solution (1 M) in a beaker (V = 100 mL). The cell was heated to 50 °C using an external IKA-HBC-5 thermostat. The reaction mixture was pumped through the electrolyzer at a flow rate of 61 mL min-1. The applied current density was set to 5.0 mA cm-2, and a total charge of 8.0 F was used. After the reaction, the product mixture was collected, tert-butyl methyl ether was pumped through the cell, and an acid-base extraction was performed. For this purpose, the basic reaction medium was extracted three times with tert-butyl methyl ether (100 mL each), acidified to pH 1-2 with 4.5 M sulfuric acid and extracted four times with ethyl acetate (100 mL each).After drying over MgSO4 and subsequent solvent removal, the product mixture was analyzed by 1 H and 13C inverse-gated NMR against 1,3,5-trimethoxybenzene as internal standard.

[0074] Example 2:

[0075] First, 4-ethylcyclohexanol was synthesized according to the following procedure. 4-Ethylphenol (100 g, 0.82 mol) was dissolved in methanol (500 mL), ruthenium / carbon (10 g, 5% Ru on C) was added to the 1-liter autoclave, and a hydrogen atmosphere (10 bar) was applied. The autoclave was heated to 120 °C. To monitor the yield of 4-ethylcyclohexanol (1"), samples (0.1 mL of the reaction medium) were taken once daily. Product formation was monitored by gas chromatography, and the reaction was stopped when no further product formation was observed. 4-Ethylphenol was purchased from Sigma Aldrich, and ruthenium was purchased from Heraeus Chemicals. 4-Ethylcyclohexanol (1") was synthesized on a 200 g scale with isolated yields of up to 95% after vacuum distillation.

[0076] Subsequently, the electrochemical oxidation of 4-ethylcyclohexanol to 3-ethyladipic acid was carried out according to the following Scheme 1 The oxidation was subjected to a design-of-experiment screening procedure with regard to the parameters reaction temperature, flow rate, concentration of the cyclic reactant, and current density. A nickel foam anode activated as described above and a V5A stainless steel plate cathode were used. The reaction takes place in a semi-stable emulsion of the cyclic reactant with 1 M NaOH(aq), as the two liquids are immiscible under standard conditions.

[0077] To conduct the screening procedure, a Masterflex™ PharMed™ BPT, L / S 16, tube setup for long-term use was installed. A Masterflex™ Ismatec™ 78018-42 peristaltic pump was used for pumping. An IKA Magie Lab / IKA Magie Plant system with a DR mixing element (IKA-Werke GmbH & Co. KG, Staufen, Germany) was used for continuous mixing. In all experiments, the mixing system operated at 12,000 rpm (= 70 L h-1 circulation rate) to achieve an optically semi-stable emulsion for the time it was pumped through the flow-through electrolyzer and back into the mixer. A charge of 9 F was used for all reactions.

[0078] The reaction mixture was processed as follows: After each experiment, the reaction mixture was collected in a vial, and tert-butyl methyl ether (50 mL) was pumped through the electrolyzer in cycle mode for 1 minute to minimize losses. The same procedure was performed with 1 M sodium hydroxide solution (50 mL). Subsequently, an acid-base extraction was performed using a Ludwig perforator. High stirring speeds can be more easily achieved by using a small stir bar. Additionally, the solvent should be added to the main compartment. The yellow basic reaction mixture was extracted overnight with MTBE. The two phases were then separated, and the aqueous layer was acidified to pH 1-2 with 4.5 M H2SO4(aq). The neutralized crude product was then extracted in a perforator with ethyl acetate.The two organic fractions were dried over MgSO4, the solvent was removed under low pressure and the product was analyzed by 13C inverse gated NMR with 1,3,5-trimethoxybenzene as internal standard.

[0079] For isolation, the resulting carboxylic acid mixture was passed through a Vigreux column in vacuo at 10°- 10 -2mbar. The main product was distilled at 180-220°C. The remaining yellow color could be removed by recrystallization from heptane. If ethyl esters of 3-ethyladipic acid were present, the product mixture was dissolved in sodium hydroxide solution (3M), heated to 80°C, and stirred vigorously for 3 hours. The mixture was then extracted with tert-butyl methyl ether (3 x one-third of the aqueous volume) in a sufficiently large separatory funnel, acidified to pH 2-3 with H2SO4(aq) (4.5M), and extracted with ethyl acetate. The organic fraction was dried over MgSO4, and after removal of the solvent, the crystals were collected. A second recrystallization resulted in optically higher purity.

[0080] For the parameter DoE screening, four parameters were varied: the concentration of the cyclic reactant c, the reaction temperature T, the flow rate of the reaction medium <D sowie die Stromdichte j, wobei die entsprechenden Parametergrenzen in der nachstehenden Tabelle 1 wiedergegeben sind. Neben den gesetzten Parametergrenzen wurden auch Zentralpunktversuche durchgeführt. Alle Versuche wurden einmal wiederholt, um die Reproduzierbarkeit der Umsetzungen zu bestimmen, und die Reaktionsreihenfolge dieser 18 chemischen Versuche wurde randomisiert. Die Reaktion dauerte zwischen 0,9 und 2,3 Tagen. Quantitative NMR-Ausbeuten von nahezu 40 % wurden für das Zielprodukt (2") erzielt. Die maximale isolierte Ausbeute betrug 39 %. Die Nickelschaumanode wurde für alle hier berichteten Reaktionen wiederverwendet, wodurch sich der Abfallstrom verringerte.

[0081] Table 1: DoE screening parameters. The geometric current density refers to the current relative to the geometric anodic surface area of ​​the anodic foam - 108 cm². Reaction conditions: 1 M NaOH(aq), water, activation = 7.5 mA cm² 2 (geometric current density), activation = 10 C cm 2 , 9 F. The flow rate of the reaction <D ist eine kumulierte Durchflussrate beider Röhren. Jeder Parameter hat drei Datenpunkte.

[0082] It was found that the flow rate and the concentration of the reactant have the greatest influence on the yield of the desired product. Higher flow rates, especially from 30 to 50 mL 1 and lower concentrations of the reactant, especially from 0.2 to 0.35 M, have a positive effect on the yield of 3-ethyladipic acids.

[0083] The analyzed product spectrum of the investigated oxidation of 4-ethylcyclohexanol is shown below:

[0084] Structures of 3-ethyladipic acid (2"), 3-ethylglutaric acid (3), 2-ethylglutaric acid (4) and ethylsuccinic acid (5). A graphical evaluation of the parameter results with a plot corresponding to the qNMR yields of 3-ethyladipic acid (2) is shown in Figure 3. The tested parameters are reaction temperature T (25 °C, 37.5 °C and 50 °C), flow rate of the reaction mixture <D (10 mL min-1 , 30 mL min-1 und 50 mL min-1), Konzentration von 4 Ethylcyclohexanol c(1) (0,2 m, 0,35 m und 0,5 m) und applizierte Reaktions-Stromdichte j (3,0 mA cm-2, 4,0 mA cm-2 und 5,0 mA cm-2).

[0085] The screening showed that a particular combination of the four tested parameters leads to the highest yields, namely a flow rate of 50 mL mim 1 , a reaction temperature of 25 °C, a current density j of 5.0 mA cm 2and a reactant concentration of 0.2 mol / l. This enabled yields of the desired product (2") of well over 30% to be achieved.

[0086] With these parameters, the best quantitative NMR yields of the products were as follows:

[0087] YqNMR (3-ethyladipic acid) = 38%

[0088] YqNMR (3-ethylglutaric acid) = 4%

[0089] YqNMR (2-ethylglutaric acid) = 4%, YqNMR (ethylsuccinic acid) = 1%.

[0090] After the workup procedure, 2" was isolated with a yield of 38%. This is the most efficient isolation of 2" by an electrochemical synthesis reported to date.

[0091] Characterization of 3-ethyladipic acid (2")

[0092] 2" was synthesized electrochemically from 4-ethylcyclohexanol (6.41 g, 50 mmol, 0.2 M) according to the procedure described above. The product 2" was obtained by acid-base extraction, vacuum distillation, and recrystallization from n-heptane as a light yellow crystalline solid (3.31 g, 19 mmol, 38%).

[0093] 1 H NMR (600 MHz, DMSO-d6): ö [ppm] = 12.04 (s, 2H), 2.18 (t, J = 7.9 Hz, 2H), 2.12 (d, J = 6.9 Hz, 2H), 1 .68 (hept, J = 6.5 Hz, 1 H), 1 .57 - 1 .42 (m, 2H), 1 .33 - 1 .21 (m, 2H), 0.81 (t, J = 7.5 Hz, 3H).

[0094] 13 C NMR (151 MHz, DMSO-d6): δ [ppm] = 174.75, 174.29, 38.07, 35.41, 31.20, 28.17, 25.63, 10.69. Melting point: (47.4 - 49.8) °C.

[0095] HRMS (ESI+) m / z: calculated for C8H14O4 + H+ 175.0965 [M+H+]+, found: 175.0965. Example 3:

[0096] Anodic oxidation of 4-propylcyclohexanol to 3-propyladipic acid was carried out in the flow cell described above and the development of the yield of 3-propyladipic acid during the reaction was analyzed.

[0097] Figure 4 shows the qNMR yield change of 3-propyladipic acid with the applied charge. The qNMR yield was analyzed against the standard 1,3,5-trimethoxybenzene.

[0098] The formation of 3-propyladipic acid was influenced by the formation of 4-propylcyclohexanone. 1 F after the maximum available concentration of 4-propylcyclohexanone, the production rate of 3-propyladipic acid was highest. The final result was a yield of over 30%.

[0099] Subsequently, the reaction procedure was modified so that 4-propylcyclohexanone was present at the most stable concentration possible, thus keeping the production rate of 3-propyladipic acid constant. To achieve this, the reactant was added at a constant rate, adjusted to the production rate of 3-propyladipic acid.

[0100] Surprisingly, a high yield of the desired product was observed here as well. This procedure thus has a positive impact on the yield and also enables continuous reaction.

[0101] The qNMR yield change of 3-propyladipic acid upon addition of 4-propylcyclohexanone over the course of the applied charge is shown in Figure 5.

[0102] List of reference symbols:

[0103] 1 flow electrolysis cell

[0104] 2 self-standing frame and cage for the electrode 3 mixer

[0105] 4 Emulsion

[0106] 5 stainless steel electrode

[0107] 6 nickel foam electrode

[0108] 7 Nickel plate electrode 8 Cooling circuit

[0109] 9 Cooling circuit

[0110] 10 Cooling circuit

[0111] 11 Pump

Claims

1 . A method for the electrochemical activation of metal electrodes comprising the steps (a) Pretreatment of the metal electrode by bringing the metal electrode into contact with base and (b) electrochemical activation of the metal electrode pretreated according to step (a) without reversing the polarity of the metal electrode to form an outer metal oxyhydroxide layer, wherein the metal electrode comprises 0 to 100 wt.% nickel and 100 to 0 wt.% cobalt, in each case based on the total content of nickel and cobalt in the electrode, or 10 to 100 wt.% nickel and 0 to 90 wt.% iron, in each case based on the total content of nickel and iron in the electrode, wherein the metal electrode has a content of other metals equal to or less than 10 wt.%, in each case based on the total metal content.

2. The method according to claim 1, wherein the metal electrode has a content of other metals equal to or less than 5 wt.%, more preferably equal to or less than 2 wt.% and particularly preferably equal to or less than 1 wt.%, in each case based on the total metal content, in each case based on the total metal content.

3. The method according to claim 1 or 2, wherein the metal electrode is a Ni(O)OH foam electrode, wherein the Ni(O)OH foam electrode contains as metal in particular at least 80 wt.%, preferably at least 85, 90, 95, 98 or 99 wt.%, particularly preferably at least 99.9, especially preferably at least 99.99 wt.% nickel, in each case based on the metal content of the Ni(O)OH foam electrode.

4. The method according to at least one of the preceding claims, wherein the metal electrode is pretreated by bringing it into contact with base, at least one inorganic base and / or at least one organic base, preferably using one or more bases which are independently selected from the group consisting of N(R A )4OH, N(R A )4acetate, with R A each independently of one another H or Ci-4-alkyl, linear or branched, NaOH, sodium acetate, KOH, potassium acetate, LiOH, lithium acetate, Na2CO3, K2CO3 and U2CO3, particularly preferably selected from the group consisting of NaOH and KOH.

5. The method according to claim 4, wherein the base is used in aqueous solution, in particular in a concentration of the base of 10 mM to 10 M, preferably 0.2 to 5 M, particularly preferably 0.5 to 1.5 M, most particularly preferably 1 ± 0.1 M.

6. The process according to at least one of the preceding claims, wherein the contacting of the metal electrode with the base takes place at a temperature of 10 to 40°C, preferably at 18 to 27°C.

7. The method according to at least one of the preceding claims, wherein in the electrochemical activation according to step (a) pretreated metal electrode one or more salts of nickel and / or cobalt and optionally iron, particularly preferably salts selected from the group consisting of Ni2SO4 and / or CoSO4 are used.

8. The process according to claim 7, wherein the salt or salts are present in aqueous basic solution, wherein one or more bases, preferably one base, are independently selected from the group consisting of N(R A )4OH, N(R A )4acetate, with R Aeach independently of one another H or Ci-4-alkyl, linear or branched, NaOH, sodium acetate, KOH, potassium acetate, LiOH, lithium acetate, Na2CO3, K2CO3 and U2CO3, particularly preferably selected from the group consisting of NaOH and KOH.

9. The method according to any one of claims 7 or 8, wherein the electrochemical activation of the metal electrode takes place at a temperature of 10 to 40°C, preferably at 18 to 27°C.

10. The method according to any one of claims 7 to 9, wherein the electrochemical activation of the metal electrode takes place at a charge amount of 2 to 10 coulombs, preferably 5 to 7 coulombs.

11. Electrochemically activated metal electrode obtainable by a process according to one or more of the preceding claims.

12. Use of a metal electrode according to claim 11 for carrying out oxidation reactions of organic components.

13. Use of a metal electrode according to claim 11 for carrying out water electrolysis.

14. Use of a metal electrode according to claim 11 in batteries.

15. Use of a metal electrode according to claim 11 in catalysis.

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