Electrochemical oxidation of fatty acids and fatty acid esters to form monocarboxylic acids and alpha-omega-dicarboxylic acids

Abstract

The invention relates to a process for preparing aliphatic monocarboxylic acids and α,ω-dicarboxylic acids or α,ω-dicarboxylic acid monoesters by electrochemical oxidation of unsubstituted or at least monosubstituted, monosaturated or polysaturated fatty acids or fatty acid esters in the presence of an inorganic or organic nitrate salt in an electrolytic cell in a reaction medium in the presence of oxygen.

Description

[0001] Electrochemical oxidation of fatty acids and fatty acid esters to monocarboxylic acids and a,u)-dicarboxylic acids

[0002] The invention relates to a process for the preparation of aliphatic monocarboxylic acids and a,w-dicarboxylic acids or a,w-dicarboxylic acid monoesters by electrochemical oxidation of unsubstituted or at least mono-substituted, mono- or polyunsaturated fatty acids or fatty acid esters in the presence of an inorganic or organic nitrate salt in an electrolysis cell in a reaction medium in the presence of oxygen.

[0003] Monocarboxylic acids, α,ω-dicarboxylic acids, and α,ω-dicarboxylic acid monoesters are important substrates for organic synthetic chemistry and monomer building blocks for polymer synthesis, respectively, and are therefore highly relevant for industrial applications. Conventional access to these substrates is primarily achieved by the oxidative cleavage of a C=C double bond in fatty acids and fatty acid esters using processes based on the use of transition metals, additional oxidants, and / or the principle of ozonolysis.

[0004] Known processes based on transition metals tend to pose toxic risks to humans and the environment due to their use. Added to this is the economic factor, as these methods are associated with ever-increasing costs due to the increasing scarcity of raw materials. Purification of the products and recovery of the catalysts entails additional labor. The necessary use of excess stoichiometric amounts of oxidants leads to additional waste reagents that must be disposed of. In most cases, the reaction necessarily takes place at elevated or reduced temperatures, which can also negatively impact the energy balance of a process. The intermediates formed during ozonolysis are potentially explosive, which poses a significant safety risk.In addition, ozone must be formed as a reactive species using special generators, which requires increased equipment costs.

[0005] It is an object of the invention to provide a sustainable and resource-saving process that enables the production of monocarboxylic acids, α,ω-dicarboxylic acids, and α,ω-dicarboxylic acid monoesters from fatty acids or fatty acid esters. This object is achieved by the subject matter of the patent claims and the description.

[0006] The present invention relates to a process for the preparation of aliphatic monocarboxylic acids and a,w-dicarboxylic acids or a,w-dicarboxylic acid monoesters by electrochemical oxidation of unsubstituted or at least mono-substituted, mono- or polyunsaturated fatty acids or fatty acid esters, comprising the process steps

[0007] (a) providing at least one unsubstituted or at least mono-substituted, mono- or polyunsaturated C6-C24 fatty acid or at least one unsubstituted or at least mono-substituted, mono- or polyunsaturated C6-C24 fatty acid ester;

[0008] (b) providing at least one inorganic or organic nitrate salt;

[0009] (c) electrochemical oxidation of the unsubstituted or at least mono-substituted, mono- or polyunsaturated fatty acid or the unsubstituted or at least mono-substituted, mono- or polyunsaturated fatty acid ester provided in step (a) in the presence of the inorganic or organic nitrate salt provided in step (b) in an electrolysis cell in a reaction medium in the presence of oxygen.

[0010] It has surprisingly been found that, with the aid of the inventive process for electrochemical oxidation, atmospheric oxygen can be used to introduce the oxygen function into fatty acids or fatty acid esters. The fatty acids used as starting materials can be obtained commercially by hydrolysis from their glycerol esters, which are widely present in vegetable fats and oils, among other things, and thus represent a renewable raw material. Methyl oleate is also obtained by transesterification of triglycerides with methanol and is used in biodiesel.

[0011] The process according to the invention for the production of aliphatic α,ω-dicarboxylic acids or α,ω-dicarboxylic acid monoesters, as well as monocarboxylic acids from these renewable raw materials, thus offers a direct, sustainable, and resource-saving alternative for the synthesis of important synthetic building blocks. α,ω-Dicarboxylic acids serve primarily as monomers for large-scale industrial polyamide synthesis. α,ω-Dicarboxylic acid monoesters can enable industrial access to the corresponding C2-shortened dimers via Kolbe electrolysis. To date, there has been little synthetic access to these resulting long-chain dicarboxylic acid diesters. Thus, both products are of great economic relevance.

[0012] The use of chemical oxidants, such as reactive peroxides, and expensive catalysts with complex ligand systems can be dispensed with in the process according to the invention. At the same time, the use of toxic and / or potentially carcinogenic reagents can be reduced or even completely avoided. The simple and safe process conditions allow for upscaling to an industrial scale, so that even larger quantities of the desired products can be produced. Thus, the present invention can significantly optimize previously cost- and time-intensive processes.

[0013] The process according to the invention is characterized in particular by high selectivity, low use of auxiliary chemicals, the use of electric current as an oxidizing agent and, associated with this, by a reduced generation of waste products.

[0014] It was also surprisingly found that the process according to the invention enables the use of electric current for the production of monocarboxylic acids, a,w-dicarboxylic acids and a,w-dicarboxylic acid monoesters using nitrate salts which act both as a conducting salt and as an electrochemical mediator.

[0015] It has further been surprisingly found that the process according to the invention can be carried out under ambient pressure and ambient temperature, which also has an advantageous effect on energy efficiency and thus also on environmental compatibility.

[0016] The C6-C24 fatty acids and C6-C24 fatty acid esters provided according to the invention in step (a) are mono- or polyunsaturated, i.e. they have one or more C=C double bonds, for example 1, 2, 3 or 4 0=0 double bonds. The fatty acids and fatty acid esters can be present in either the c / s configuration or the trans configuration. If a fatty acid or fatty acid ester has more than one C=C double bond, both configurations can be present in one molecule. The fatty acids and fatty acid esters can be linear or branched, with linear chains being preferred. The fatty acids and fatty acid esters can be unsubstituted or at least mono-substituted. If they are mono- or polysubstituted, they are preferably substituted by 1, 2, 3, 4 or 5 substituents, independently of one another, each selected from the group consisting of methyl, phenyl or benzyl. The phenyl orBenzyl substituents may themselves be unsubstituted or mono- or polysubstituted with 1, 2 or 3 substituents, independently of one another, each selected from the group consisting of F, Cl, Br, and NO2.

[0017] In a preferred embodiment of the process according to the invention, step (a) comprises providing at least one unsubstituted, mono- or polyunsaturated C6-C24 fatty acid or at least one unsubstituted, mono- or polyunsaturated C6-C24 fatty acid ester.

[0018] In a further preferred embodiment of the process according to the invention, at least one unsubstituted, monounsaturated C6-C24 fatty acid or at least one unsubstituted, monounsaturated C6-C24 fatty acid ester is provided in step (a).

[0019] In a particularly preferred embodiment of the process according to the invention, in step (a) at least one mono- or polyunsaturated fatty acid is provided, selected from the group consisting of hex-3-enoic acid, undecylenic acid, myristoleic acid, palmitoleic acid, margaroleic acid, petroselinic acid, oleic acid, elaidic acid, vaccenic acid, gadoleic acid, gondoic acid, cetoleic acid, erucic acid, nervonic acid, linoleic acid, alpha-linolenic acid, gamma-linolenic acid, calendulic acid, punicic acid, alpha-eleostearic acid, beta-eleostearic acid, arachidonic acid, eicosapentaenoic acid, docosadienoic acid, docosatetraenoic acid, docosahexaenoic acid and tetracosahexaenoic acid, optionally in the form of an ester, in particular selected from the group consisting of hex-3-enoic acid, myristoleic acid, palmitoleic acid, petroselinic acid, oleic acid, elaidic acid, vaccenic acid, Gadoleic acid, gonadoic acid, cetoleic acid, erucic acid, nervonic acid, linoleic acid, docosadienoic acid, linolenic acid, arachidonic acid, if applicable.in the form of an ester.

[0020] A particularly preferred fatty acid or fatty acid ester according to step (a) of the process according to the invention is an oleic acid selected from the group consisting of oleic acid, elaidic acid, erucic acid, and linoleic acid, optionally in the form of an ester. If a fatty acid ester is provided according to step (a) of the process according to the invention, methyl esters or ethyl esters of the fatty acid are preferred.

[0021] According to step (b) of the process according to the invention, at least one inorganic or organic nitrate salt is provided. This nitrate salt acts both as a conductive salt and as a mediator in the electrochemical oxidation process according to the invention. Preferably, an inorganic or organic nitrate of the general formula

[0022] [Cation + ][NO3'] is used, wherein the [cation*] is selected from the group consisting of Na +, K + , ammonium ions with the general structure [R 1 R 2 R 3 R 4 N + ] with R 1 , R 2 , R 3 , R 4 , independently of one another, each selected from the group consisting of Ci- to Cw-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched, imidazolium cations having the general structure (I) with R 1 and R 2 , independently of one another, each selected from the group consisting of Ci- to Cw-alkyl, straight-chain or branched, in particular Ci- to Cs-alkyl, straight-chain or branched, and R 3 selected from the group consisting of H and Ci- to C-alkyl, straight-chain or branched, in particular from the group consisting of H and Ci- to Cs-alkyl, straight-chain or branched,

[0023] Pyridinium cations with the general structure (II) with R 1selected from the group consisting of Ci- to Cw-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched and R 2 , R 3 and R 4 , independently of one another, each selected from the group consisting of H and Ci- to Cw-alkyl, straight-chain or branched, in particular from the group consisting of H and Ci- to Cs-alkyl, straight-chain or branched, and

[0024] Phosphonium ions with the general structure [R 1a R 2a R 3a R 4a P + ] with R 1a , R 2a , R 3a , R 4a , independently of one another, each selected from the group consisting of Ci- to Cw-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched.

[0025] If an organic nitrate based on the imidazolium cations is used in the process according to the invention, preference is given to cations of the general formula (I) in which R 1 and R 2, independently of one another, are each selected from the group consisting of Ci- to Cw-alkyl, straight-chain or branched, in particular Ci- to Cs-alkyl, straight-chain or branched and R 3 represents hydrogen. Particularly preferred imidazolium cations of the general formula (I) are those in which R 1 for methyl and R 2 represent ethyl or R 1 for methyl and R 2 represents methyl and R 1 for methyl and R 2 stands for butyl, and R 3 each stands for hydrogen.

[0026] If a nitrate based on the pyridinium cations is used in the process according to the invention, preference is given to cations of the general formula (II) in which R 1 represents Ci- to C-alkyl, straight-chain or branched, in particular represents Ci- to Cs-alkyl, straight-chain or branched. Particularly preferred are pyridinium cations of the general formula (II) in which R 1represents Ci- to Cw-alkyl, straight-chain or branched, in particular Ci- to Cs-alkyl, straight-chain or branched and the radicals R 2 , R 3 and R 4 , independently of one another, are each selected from the group consisting of Ci- to Cs-alkyl, straight-chain or branched, with a single substitution in the 2-, 3- or 4-position, a double substitution in the 2,4-, 2,5- or 2,6-position or a triple substitution in the 2,4,6-position being preferred.

[0027] In principle, two or more of the above-mentioned nitrate salts can also be used in the process according to the invention. Preferably, one nitrate salt according to the invention is used, in particular an organic ammonium nitrate salt of the composition [R 1 R 2 R 3 R 4 N + ][NOs'] or an organic phosphonium salt of the composition [R 1a R 2a R 3a R 4a P +][NO3'], wherein an organic ammonium nitrate salt of the composition [R 1 R 2 R 3 R 4 N + ][NOs'] is particularly preferred. The organic ammonium nitrate salt is very particularly preferably tetra-n-butylammonium nitrate or methyltri-n-octylammonium nitrate. The organic phosphonium nitrate salt is very particularly preferably tetra-n-butylphosphonium nitrate or methyltri-n-octylphosphonium nitrate. The organic imidazolium nitrate salt is preferably 1-butyl-3-methylimidazolium nitrate.

[0028] Tetra-n-butylammonium nitrate or methyltri-n-octylammonium nitrate is most preferably used as the organic nitrate salt in the process according to the invention.

[0029] The order in which the components used in the process according to the invention are provided may vary, as may the order in which the individual components are brought into contact with one another or with the respective reaction medium.

[0030] In one embodiment of the process according to the invention, the fatty acid or fatty acid ester or inorganic or organic nitrate salt is initially introduced and combined with the reaction medium, preferably at least partially or completely dissolved in the reaction medium or mixed therewith, and then the other of these two components is added. In a further embodiment of the process according to the invention, the fatty acid or fatty acid ester and the inorganic or organic nitrate salt are initially introduced and then combined with the reaction medium, preferably at least partially or completely dissolved in the reaction medium or mixed therewith.Furthermore, it is also possible that in the process according to the invention the fatty acid or the fatty acid ester and the inorganic or organic nitrate salt are added to the reaction medium simultaneously or in chronological sequence to one another, preferably at least partially or completely dissolved in the reaction medium or mixed therewith.

[0031] The reaction medium used in the process according to the invention is liquid under the conditions under which the process is carried out and is suitable for partially or completely dissolving the components used, i.e. in particular the fatty acid or fatty acid ester used and the inorganic or organic nitrate salt. If at least one of these components is used in liquid form, the reaction medium is preferably readily miscible with this component(s). A polar aprotic reaction medium is preferably used in the process according to the invention for electrochemical oxidation. This can be used in anhydrous form, in dried form, or in combination with water.

[0032] If an inorganic nitrate salt, in particular potassium nitrate or sodium nitrate, is used in the process according to the invention, the reaction medium advantageously contains water, with an aprotic reaction medium in combination with water being preferred. The water content in the reaction medium can vary. Preferably, the water content is up to 20 vol.%, more preferably up to 15 vol.%, most preferably up to 10 vol.%, and even more preferably up to 5 vol.%, in each case based on the total amount of reaction medium.

[0033] Preferably, the polar aprotic reaction medium is selected from the group consisting of aliphatic nitriles, aliphatic ketones, cycloaliphatic ketones, dialkyl carbonates, cyclic carbonates, lactones, aliphatic nitroalkanes, and dimethyl sulfoxide, esters and ethers or a combination of at least two of these components.

[0034] Particularly preferably, the reaction medium is selected from the group consisting of acetonitrile, isobutyronitrile, adiponitrile, acetone, dimethyl carbonate, methyl ethyl ketone, 3-pentanone, cyclohexanone, nitromethane, nitropropane, tert-butyl methyl ether, dimethyl sulfoxide, gamma-butyrolactone and epsilon-caprolactone or a combination of at least two of these components.

[0035] Most preferably, the reaction medium is selected from the group consisting of acetonitrile, isobutyronitrile, adiponitrile, dimethyl carbonate and acetone or a combination of at least two of these components.

[0036] The reaction medium is most preferably acetonitrile, isobutyronitrile or adiponitrile in dried or anhydrous form.

[0037] Also particularly preferred is the reaction medium acetonitrile, isobutyronitrile or adiponitrile, optionally in combination with water.

[0038] If one or more of the above-mentioned components are used in the reaction medium in combination with water, the water content is preferably up to 20 vol.%, particularly preferably up to 15 vol.%, very particularly preferably up to 10 vol.%, even more preferably up to 5 vol.%, in each case based on the total amount of reaction medium.

[0039] To carry out the process according to the invention, it may be advantageous to add additional solubilizing components to the reaction medium. Suitable solubilizing components can be determined by simple preliminary tests on the solubility behavior.

[0040] Suitable solubilizing components include, for example, primary alcohols, secondary alcohols, monoketones, or dialkyl carbonates, or mixtures of at least two of these components, optionally in combination with water. Aliphatic C 1-6 alcohols can preferably be used in the process according to the invention, with particularly preferred solubilizing components being selected from the group consisting of methanol, ethanol, isopropanol, 2-methyl-2-butanol, or mixtures of at least two of these components, optionally in combination with water.

[0041] The use of dimethyl carbonate as reaction medium, optionally in combination with at least one Ci-e alcohol, in particular selected from the group consisting of methanol, ethanol, isopropanol, 2-methyl-2-butanol, optionally in combination with water, can be particularly advantageous.

[0042] If one or more of these solubilizing components are used in combination with water, the water content is preferably up to 20 vol.%, particularly preferably up to 15 vol.%, very particularly preferably up to 10 vol.%, even more preferably up to 5 vol.%, in each case based on the total amount of solubilizing component and water.

[0043] The solubilizing components can preferably be added in amounts of < 50 vol.%, particularly preferably < 30 vol.% and very particularly preferably < 10 vol.%, in each case based on the total amount of reaction medium.

[0044] Preferably, the inorganic or organic nitrate salt is used in the process according to the invention in an amount of 0.1 to 2.0, preferably 0.2 to 1.0, particularly preferably 0.3 to 0.8 and very particularly preferably 0.4 to 0.8 equivalents, in each case based on the amount of fatty acid or fatty acid ester.

[0045] According to the invention, the electrochemical oxidation of the fatty acid or fatty acid ester is carried out in the presence of the inorganic or organic nitrate salt in an electrolysis cell in a reaction medium in the presence of oxygen, wherein the electrochemical oxidation is preferably carried out in an electrolysis cell.

[0046] For this purpose, a gas atmosphere containing oxygen is advantageously provided in spatial connection with the reaction medium.

[0047] For this purpose, a gas atmosphere containing oxygen is advantageously provided in spatial connection with the reaction medium.

[0048] The proportion of oxygen in the gas atmosphere can vary. The proportion of oxygen in the gas atmosphere is preferably 10 to 100 vol.%, particularly preferably 15 to 30 vol.%, particularly preferably 15 to 25 vol.%, and most preferably 18 to 22 vol.%.

[0049] In one embodiment, the proportion of oxygen in the gas atmosphere can be 10 to 100 vol.%, particularly preferably 15 to 100 vol.%, particularly preferably 20 to 100 vol.%.

[0050] The gas atmosphere air is particularly preferred.

[0051] Advantageously, a gas exchange is enforced between the gas atmosphere and the reaction medium, preferably by introducing a gas atmosphere into the reaction medium or by stirring the liquid phase in the presence of the gas atmosphere.

[0052] Gas exchange between the gas atmosphere and the reaction medium, in particular stirring, for example via the geometry of the stirrer or the stirring speed, can be used to control the electrochemical oxidation. The amount of oxygen dissolved in the reaction medium is preferably at least 1 mmol / L of the reaction medium, particularly preferably at least 5 mmol / L of the reaction medium.

[0053] Also preferably, the amount of oxygen dissolved in the reaction medium is at least 10 mmol / L reaction medium.

[0054] The process according to the invention can be carried out in both a divided and an undivided electrolysis cell, with the process being carried out in an undivided electrolysis cell being preferred.

[0055] In order to avoid undesirable chemical reactions, it is sometimes advantageous to separate the cathode compartment and the anode compartment and to allow the charge exchange between the anode and cathode compartments to take place only through a porous diaphragm - often an ion exchange resin.

[0056] The undivided electrolysis cell preferably used according to the invention has at least two electrodes. Anodes and cathodes made of conventional materials can be used, for example, glassy carbon, boron-doped diamond (BDD), or graphite. The use of glassy carbon electrodes is preferred.

[0057] The undivided electrolysis cell preferably has at least one glassy carbon anode or at least one glassy carbon cathode. Preferably, both the anode and the cathode are glassy carbon electrodes.

[0058] The distance between the electrodes can vary within a certain range. The distance is preferably 0.1 mm to 2.0 cm, more preferably 0.1 mm to 1.0 cm, and most preferably 0.1 mm to 0.5 cm.

[0059] Furthermore, the process according to the invention can be carried out batchwise or continuously, preferably in an undivided flow-through electrolysis cell.

[0060] The process according to the invention is preferably carried out with a charge quantity of at least 190 C (2 F) to 970 C (10 F), preferably 290 C (3 F) to 870 C (9 F), particularly preferably 330 C (3.5 F) to 820 C (8.5 F), very particularly preferably 380 C (4 F) to 775 C (8 F), most preferably 380 C (4 F) to 580 C (6 F), in each case for 1 mmol of fatty acid or fatty acid ester used, in each case for one double bond in the fatty acid or fatty acid ester used.

[0061] Preferably, the electrochemical oxidation in the process according to the invention takes place at a constant current intensity.

[0062] The current density at which the process according to the invention is carried out is preferably at least 5 mA / cm 2 or at least 10 mA / cm 2 or at least 15 mA / cm 2 or at least 20 mA / cm 2 or 20 mA / cm 2 up to 50 mA / cm 2, where the area refers to the geometric area of ​​the electrodes.

[0063] A significant advantage of the process according to the invention is that electric current is used as the oxidizing agent, which is a particularly environmentally friendly agent when it comes from renewable sources, in particular from biomass, solar thermal energy, geothermal energy, hydropower, wind power or photovoltaics.

[0064] The process according to the invention can be carried out over a wide temperature range, for example at a temperature in the range from 0 to 60 °C, preferably from 5 to 50 °C, particularly preferably from 10 to 40 °C, most preferably from 15 to 30 °C.

[0065] The process according to the invention can be carried out at elevated or reduced pressure. If the process according to the invention is carried out at elevated pressure, a pressure of up to 16 bar is preferred, particularly preferably up to 6 bar.

[0066] The process according to the invention can also preferably be carried out under atmospheric pressure.

[0067] The products prepared by the process according to the invention can be isolated or purified by conventional methods known to those skilled in the art, in particular by extraction, crystallization, centrifugation, precipitation, distillation, evaporation, or chromatography. The following examples further illustrate the present invention but are not to be construed as limiting the scope of the invention.

[0068] General information and methods

[0069] Analytical-grade chemicals were purchased and used from common suppliers (such as TCI, Aldrich, and Acros). Oxygen was purchased in 2.5 grade from NIPPON GASES Deutschland GmbH, Düsseldorf, Germany, and used directly.

[0070] Glassy carbon (SIGRADUR® G, from HTW Hochtemperatur Werkstoffe GmbH, Thierhaupten, Germany) was used as the electrode material.

[0071] Gas chromatographic investigations were performed on a Shimadzu GC-2010 (Shimadzu, Japan) equipped with a ZB-FFAP capillary GC column (Zebron, USA; length: 30 m, inner diameter: 0.25 mm, film thickness: 0.25 pm, carrier gas: argon).

[0072] NMR spectrometry of 1 H-NMR and 13 C-NMR spectra were recorded at 25 °C using a Bruker Avance II 400 (400 MHz, 5 mm BBFO head with z-gradient and ATM, SampleXPress 60 sample changer, Analytische Messtechnik, Karlsruhe, Germany).

[0073] Column chromatography was performed using silica gel 60 M (0.040–0.063 mm, Macherey-Nagel GmbH & Co., Düren, Germany) and a mobile phase mixture of cyclohexane and ethyl acetate (9:1 to 7:3) with 1.0 vol% acetic acid as an additive. Thin-layer chromatography was performed on silica gel 60 plates coated on aluminum (F254, Merck KGaA, Darmstadt, Germany). KMnO4 solution (potassium permanganate reagent: 3 g KMnO4, 20 g K2CO3, 5 mL NaOH (5%), in 300 mL water) was used to stain the TLC plates.

[0074] The undivided Teflon cells used for electrolysis are described in the literature (a) C. Gütz, B. Klöckner, SR Waldvogel, Org. Process Res. Dev. 2016, 20, 26-32; b) A. Kirste, G. Schnakenburg, F. Stecker, A. Fischer, SR Waldvogel, Angew. Chem. Int. Ed. 2010, 49, 971-975; Angew. Chem. 2010, 122, 983-987. (see Sl). The complete range of these cells is commercially available from IKA Screening System (IKA-Werke GmbH & Co. KG, Staufen, Germany). The dimensions of the electrodes were 7 cm x 1 cm x 0.3 cm.

[0075] The gas introduction was controlled by two Model 5850S ​​mass flow controllers (MFCs) from Brooks Instrument BV, Veenendaal, Netherlands. One controller each was used for the oxygen and nitrogen lines. The controllers were controlled using the Smart DDE software and Matlab R2017b. The volume flow was additionally controlled by a DK800 variable area flow meter from KRÖHNE Messtechnik GmbH, Duisburg. For all experiments, the total volume flow was constant at 20 mL / min, which, limited by the MFCs used, also represents the maximum achievable volume flow. The percentage volume flows of the two gases were adjusted using the MFCs and their software. The gas cylinders used were from the following suppliers: Oxygen 2.5 from NIPPON GASES Deutschland GmbH, Düsseldorf, and Nitrogen 5.0 from NIPPON GASES Deutschland GmbH, Düsseldorf.The gas distributor and the gas inlet covers of the electrolysis cells are described in the literature (M. Dörr, D. Waldmann, SR Waldvogel, GIT Labor-Fachz. 2021 , 7-8, 26-28) and were purchased from IKA (IKA-Werke GmbH & Co. KG, Staufen, Germany).

[0076] General Working Instruction AAV1

[0077] The fatty acid or fatty acid ester (0.5 mmol) and tetrabutylammonium nitrate (0.2–1.0 eq.) were placed in an undivided 5 mL Teflon pot cell and dissolved in the solvent (5 mL). The cell was equipped with glassy carbon electrodes spaced 0.5 cm apart. The immersion area of ​​the electrodes was 1.8 cm. 2 An oxygen atmosphere (20-100 vol%) is established in the gas space of the electrolysis cell. After the cell has been fixed in a stainless steel block, galvanostatic electrolysis is carried out at a current density of 5-20 mA / cm 2at 5-50 °C. The stirring speed is 100-500 rpm. After applying a charge of 8-20 °C (386-965 °C based on 0.5 mmol substrate) and termination of the reaction, 50.5 pL of propionic acid are added as an internal standard, and the yields are determined by gas chromatography or, in the case of azelaic acid 3b, by isolation by column chromatography (eluent: cyclohexane / ethyl acetate = 9:1 to 7:3, with 1.0 vol.% acetic acid as additive).

[0078] According to AAV1, the following fatty acids and fatty acid esters 1a-1c were converted to the corresponding carboxylic acids:

[0079] Scheme 1.

[0080] The process according to the invention was carried out under varying reaction parameters. Examples 1-17 are summarized in Table 1, indicating the respective conditions.

[0081] Table 1 :

[0082] Example oleic acid methyl ester 1a Pelargonic acid Azelaic acid mono- Nonanal No. Tech, (ca 70%) 2 [bl methyl ester 3a [bl 4 [bl

[0083] 1 Standard condition [al 33% 37% 15%

[0084] 2 MeCN as solvent 18% 19% 15%

[0085] 3 Load quantity 8 F 31% 33% 13%

[0086] 4 Load quantity 12 F 23% 27% 9%

[0087] 5 Current density 5 mA / cm 2 21% 23% 12%

[0088] 6 Current density 20 mA / cm 2 10% 16%

[0089] 7 Temperature = 50 °C 22% 27%

[0090] 8 Temperature = 5 °C 30% 35% 15%

[0091] 9 Conductive salt 0.02 M (0.2 eq.) 24% 26% 14%

[0092] 10 Conductive salt 0.1 M (1.0 eq.) 30% 35% 11%

[0093] 11 Stirring speed 31% 33% 14%

[0094] 500 rpm

[0095] 12 Stirring speed 14% 23%

[0096] 100 rpm

[0097] 13 O2 / N2= 20 / 80 8% 13%

[0098] 14 O2 / N2= 0 / 100

[0099] Oleic acid methyl ester 1a Pelargonic acid Azelaic acid mono- Nonanal

[0100] Pure (99.0%) 2 [bl methyl ester 3a [bl 4 [bl

[0101] 15 Standard condition 46% 46% 21%

[0102] Oleic acid 1b Pelargonic acid A , . .. M Nonanal

[0103] 2[b] Azelaic acid 3b [cl [b]

[0104] 16 Standard condition 27% 35% 15%

[0105] Elaidic acid 1c Pelargonic acid A , . .. M Nonanal a [b] Azelaic acid 3b [cl [b]

[0106] 17 Standard condition 23% 32% 14% a) Standard condition: reactant = 0.5 mmol, NBU4NO3 = 0.05 M (0.5 eq.), isobutyronitrile = 5.0 mL, electrodes = GC||GC, O2 / N2= 100 / 0 (20 mL / min), charge = 10 F, current density = 10 mA / cm 2, stirring speed = 350 rpm, T = 23 °C. b) Yields determined via GC using propionic acid as the internal standard (ISTD). c) Isolated yields. Example No. 15:

[0107] According to AAV1, oleic acid methyl ester 1a (pure, 99%, 0.149 g, 0.5 mmol, 1.0 eq.) was galvanostatically electrolyzed under an oxygen atmosphere (100 vol%) and application of 10 F. The yield of the products was determined via gas chromatography using propionic acid as the ISTD: pelargonic acid 2 (0.23 mmol),

[0108] Azelaic acid monomethyl ester 3a (0.23 mmol), nonanal 4 (0.11 mmol).

[0109] Example No. 16:

[0110] According to AAV1, oleic acid 1b (0.141 g, 0.5 mmol, 1.0 eq.) was galvanostatically electrolyzed under an oxygen atmosphere (100 vol%) and application of 10 F. The yield of the products was determined via gas chromatography with propionic acid as the ISTD: pelargonic acid 2 (0.14 mmol), nonanal 4 (0.08 mmol).

[0111] Azelaic acid 3b was isolated via column chromatography: 44 mg, 0.18 mmol and the structure was 1 H-NMR spectroscopy confirms:

[0112] 1 H-NMR (300 MHz, DMSO-d6) δ [ppm] = 11.97 (s, 2H); 2.18 (t, J = 7.3 Hz, 4H); 1.50–1.43 (m, 4H); 1.26–1.24 (m, 6H). These analytical data are consistent with the literature values.

[0113] Example No. 17:

[0114] According to AAV1, elaidic acid 1c (0.141 g, 0.5 mmol, 1.0 eq.) was galvanostatically electrolyzed under an oxygen atmosphere (100 vol%) and application of 10 F. The product yield was determined via gas chromatography using propionic acid as the ISTD: pelargonic acid 2 (0.12 mmol), nonanal 4 (0.07 mmol).

[0115] Azelaic acid 3b was isolated via column chromatography: 40 mg, 0.16 mmol and the structure was 1 H-NMR spectroscopy confirms:

[0116] 1H-NMR (300 MHz, DMSO-d6) δ [ppm] = 11.97 (s, 2H); 2.18 (t, J = 7.3 Hz, 4H); 1.50–1.43 (m, 4H); 1.26–1.24 (m, 6H). The analytical data are consistent with the literature values. Example No. 18:

[0117] According to AAV1, polyunsaturated fatty acids can also be oxidized, as shown for the example of linoleic acid 5 (see Scheme 2, Table 2).

[0118] Scheme 2.

[0119] Table 2:

[0120] Example linoleic acid 5 _ “ rhl ,, , _ hl k l Caprylic acid Malonic acid

[0121] No. Tech. (approx. 60-70%) K

[0122] 18a Standard condition [al 20% 28%

[0123] 18b Conductive salt 0.1 M (1.0 eq.) 21% 25% a) Standard condition: reactant = 0.5 mmol, NBU4NO3 = 0.05 M (0.5 eq.), isobutyronitrile = 5.0 mL, electrodes = GC||GC, O2 / N2 = 100 / 0 (20 mL / min), charge = 20 F, current density = 10 mA / cm 2, stirring speed = 350 rpm, T = 23 °C. b) Yields determined via GC using propionic acid as the internal standard (ISTD). Azelaic acid 3b was qualitatively detected via HPLC-MS.

Claims

Process for the production of aliphatic monocarboxylic acids and α,γ-dicarboxylic acids or α,γ-dicarboxylic acid monoesters by electrochemical oxidation of unsubstituted or at least singly substituted, mono- or polyunsaturated fatty acids or fatty acid esters, comprising the process steps (a) Provision of at least one unsubstituted or at least singly substituted, mono- or polyunsaturated C6-C24 fatty acid or at least one unsubstituted or at least singly substituted, mono- or polyunsaturated C6-C24 fatty acid ester, wherein the substituents of the C6-C24 fatty acid are selected from the group consisting of methyl, phenyl or benzyl, wherein the phenyl or benzyl substituents themselves are each unsubstituted or singly or polysubstituted, with 1, 2 or 3 substituents, independently of each other, each selected from the group consisting of F, CI, Br, and NO2; (b) Provision of at least one inorganic or organic nitrate salt, wherein the nitrate salt of process step (b) is a nitrate of the general formula [cation + ][NOs'] is present, with [cation*] selected from the group consisting of Na + , K + , Ammonium ions with the general structure [R 1 R 2 R 3 R 4 N + ] with R 1 , R 2 , R 3 , R 4 , independently of each other, each selected from the group consisting of Ci- to Cw-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched, Imidazolium cations with the general structure (I) with R 1 and R 2 , independently of each other, each selected from the group consisting of Ci to Cw alkyl, straight-chain or branched, in particular Ci to Cs alkyl, straight-chain or branched, and R 3selected from the group consisting of H and Ci- to C-alkyl, straight-chain or branched, especially from the group consisting of H and Ci to Cs alkyl, straight-chain or branched, Pyridinium cations with the general structure (II) with R 1 selected from the group consisting of Ci- to C-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched and R 2 , R 3 and R 4 , independently of each other, each selected from the group consisting of H and Ci- to C-alkyl, straight-chain or branched, in particular from the group consisting of H and Ci- to Cs-alkyl, straight-chain or branched, and phosphonium ions with the general structure [R 1a R 2a R 3a R 4a P + ] with R 1a , R 2a , R 3a , R 4a, independently of each other, each selected from the group consisting of Ci- to C-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched; (c) Electrochemical oxidation of the unsubstituted or at least singly substituted, mono- or polyunsaturated fatty acid or the unsubstituted or at least singly substituted, mono- or polyunsaturated fatty acid ester provided in step (a) in the presence of the inorganic or organic nitrate salt provided in step (b) in an electrolysis cell in a reaction medium in the presence of oxygen. The method according to claim 1, wherein in step (a) at least one unsubstituted, mono- or polyunsaturated C6-C24 fatty acid or at least one unsubstituted, mono- or polyunsaturated C6-C24 fatty acid ester is provided.The method of claim 2, wherein in step (a) the provision of at least one mono- or polyunsaturated fatty acid selected from the group consisting of hexa-3-enoic acid, undecylenic acid, myristoleic acid, palmitoleic acid, margaroleic acid, petroselinic acid, oleic acid, elaidic acid, vaccenic acid, gadoleic acid, gondoic acid, cetoleic acid, erucic acid. Nervonic acid, linoleic acid, alpha-linolenic acid, gamma-linolenic acid, calendic acid, punicic acid, alpha-eleostearic acid, beta-eleostearic acid, arachidonic acid, eicosapentaenoic acid, docosadienoic acid, docosatetraenoic acid, docosahexaenoic acid and tetracosahexaenoic acid, possibly in the form of an ester.

4. Method according to one of claims 2 or 3, wherein in step (a) the provision of oleic acid, erucic acid, linoleic acid and / or elaidic acid, optionally in the form of an ester, takes place.

5. A method according to any of the preceding claims, wherein a methyl ester or an ethyl ester is provided as the ester.

6. A method according to any of the preceding claims, wherein in the imidazolium cations of general formula (I) the residues R1 and R2 are each selected independently from the group consisting of C1 to C18 alkyl, straight-chain or branched, in particular C1 to C8 alkyl, straight-chain or branched, and R3 represents hydrogen, preferably R1 represents methyl and R2 represents ethyl, or R1 represents methyl and R2 represents methyl, and R1 represents methyl and R2 represents butyl, and R3 represents hydrogen.

7. A method according to any of the preceding claims, wherein in the pyridinium cations of general formula (II) the residue R1 represents C1 to C18 alkyl, straight-chain or branched, in particular C1 to C8 alkyl, straight-chain or branched, and the residues R2, R3 and R4 are each selected independently from the group consisting of C1 to C8 alkyl, straight-chain or branched, wherein a simple substitution in the 2-, 3- or 4-position, a double substitution in the 2,4-, 2,5- or 2,6-position or a triple substitution in the 2,4,6-position is preferred.

8. Method according to any one of the preceding claims, wherein the organic The nitrate salt selected is from the group consisting of tetra-n-butylammonium nitrate, methyl-tri-n-octylammonium nitrate, and tetra-n-butylphosphonium nitrate. Methyl tri-n-octyl phosphonium nitrate and 1-butyl-3-methylimidazolium nitrate.

9. A method according to any of the preceding claims, wherein the fatty acid or fatty acid ester or the inorganic or organic nitrate salt is presented and combined with the reaction medium, preferably in the A process according to any one of the preceding claims, wherein the fatty acid or fatty acid ester and the inorganic or organic nitrate salt are placed and then combined with the reaction medium, preferably being at least partially or completely dissolved or mixed in the reaction medium. A process according to any one of the preceding claims, wherein the fatty acid or fatty acid ester and the inorganic or organic nitrate salt are added to the reaction medium simultaneously or sequentially, preferably being at least partially or completely dissolved or mixed in the reaction medium. A process according to any one of the preceding claims, wherein the reaction medium is a polar aprotic reaction medium, optionally...in combination with water, wherein a polar aprotic reaction medium is selected from the group consisting of aliphatic nitriles, aliphatic ketones, cycloaliphatic ketones, dialkyl carbonates, cyclic carbonates, lactones, aliphatic nitroalkanes, dimethyl sulfoxide, esters and ethers, or a combination of at least two of these components. A process according to claim 12, wherein the water content is up to 20 vol%, preferably up to 15 vol%, particularly preferably up to 10 vol%, and most preferably up to 5 vol%, in each case based on the total amount of reaction medium. A process according to claim 12 or 13, wherein the polar aprotic reaction medium is selected from the group consisting of acetonitrile, isobutyronitrile, adiponitrile, acetone, dimethyl carbonate, methyl ethyl ketone, 3-pentanone, cyclohexanone, nitromethane, nitropropane, tert-1, ...-Butyl methyl ether, dimethyl sulfoxide, gamma-butyrolactone and epsilon-caprolactone, preferably from the group consisting of acetonitrile, isobutyronitrile, adiponitrile, dimethyl carbonate and acetone or a combination of at least two of these components, each optionally in combination with water.

15. Method according to any one of claims 12 to 14, wherein the reaction medium is acetonitrile, isobutyronitrile or adiponitrile in dried or anhydrous form.

16. Method according to any of the preceding claims, wherein the reaction medium comprises one or more solvent-mediating components.

17. The method of claim 16, wherein the solubilizing components are primary alcohols, secondary alcohols, monoketones or dialkyl carbonates or mixtures of at least two of these components, optionally in combination with water.

18. Method according to one of claims 16 or 17, wherein aliphatic Ci-6 alcohols are present as one or more solubilizing components, preferably one or more alcohols selected from the group consisting of methanol, ethanol, isopropanol, 2-methyl-2-butanol and mixtures of at least two of these components, optionally in combination with water.

19. Method according to one of the preceding claims, wherein the reaction medium is dimethyl carbonate, optionally in combination with at least one C1-6 alcohol, preferably selected from the group consisting of methanol, ethanol, isopropanol, 2-methyl-2-butanol.

20. A method according to any of the preceding claims, wherein the reaction medium comprises water.

21. Method according to one of claims 16 to 18, wherein one or more solubilizing components are added in an amount of < 50 vol.%, particularly preferably < 30 vol.% and most preferably < 10 vol.%, in each case based on the total amount of reaction medium.

22. Method according to any of the preceding claims, wherein the inorganic or organic nitrate salt is present in an amount of 0.1 to 2.0, preferably 0.2 to 1.0, particularly preferably 0.3 to 0.8 and most preferably 0.4 to 0.8 equivalents, in each case based on the amount of fatty acid or fatty acid ester used. A method according to any one of the preceding claims, wherein a gas atmosphere containing oxygen is provided in spatial connection with the reaction medium, wherein the proportion of oxygen in the gas atmosphere is preferably 10 to 100 vol.%, particularly preferably 15 to 30 vol.%, most preferably 15 to 25 vol.%, and most preferably 18 to 22 vol.%. A method according to any one of the preceding claims, wherein the gas atmosphere is air. A method according to claim 24, wherein gas exchange is forced between the gas atmosphere and the reaction medium, preferably by introducing the gas atmosphere into the reaction medium or by stirring the reaction medium in the presence of the gas atmosphere. A method according to claim 25, wherein the stirring is used to control the electrochemical oxidation.A method according to any one of the preceding claims, wherein the amount of oxygen dissolved in the reaction medium is at least 1 mmol / L, preferably 5 mmol / L, and more preferably 10 mmol / L. A method according to any one of the preceding embodiments, wherein the electrolysis cell is a single electrolysis cell. A method according to any one of the preceding claims, wherein the single electrolysis cell comprises a glassy carbon anode, a graphite anode, or a BDD anode, preferably a glassy carbon anode. A method according to any one of the preceding claims, wherein the single electrolysis cell comprises a glassy carbon cathode, a graphite cathode, or a BDD cathode, preferably a glassy carbon cathode. A method according to any one of the preceding claims, wherein the distance between the electrodes in the electrolysis cell is 0.1 mm to 2.0 cm, preferably 0.1 mm to 1.0 cm, and particularly preferably 0.1 mm to 0.5 cm.

32. Method according to any of the preceding claims, wherein the charge quantity is at least 190°C (2°F) to 970°C (10°F), preferably 290°C (3°F) to 870°C (9°F), particularly preferably 330°C (3.5°F) to 820°C (8.5°F), most preferably 380°C (4°F) to 775°C (8°F), most preferably 380°C (4°F) to 580°C (6°F), in each case for 1 mmol of fatty acid or fatty acid ester used, in each case for one double bond in the fatty acid or fatty acid ester used.

33. Method according to one of the preceding claims, wherein the electrochemical oxidation is carried out at constant current.

34. Method according to any of the preceding claims, wherein the current density is at least 5 mA / cm² 2 , preferably 10 mA / cm 2 , preferably 15 mA / cm 2 , preferably 20 mA / cm 2 , where the area specification refers to the geometric area of ​​the electrodes.

35. Method according to any of the preceding claims, wherein the current density is at least 20 mA / cm² 2 up to 50 mA / cm 2 is, where the area specification refers to the geometric area of ​​the electrodes.

36. Method according to any of the preceding claims, wherein the current used for electrochemical oxidation is derived from renewable sources, in particular from biomass, solar thermal energy, geothermal energy, hydropower, wind power or photovoltaics.

37. Method according to one of the preceding claims, wherein the electrochemical oxidation takes place at a temperature in the range of 0 to 60 °C, preferably from 5 to 50 °C, particularly preferably from 10 to 40 °C, most preferably from 15 to 30 °C.

38. A method according to any of the preceding claims, wherein it is carried out under atmospheric pressure.

39. Method according to any of the preceding claims, wherein it is carried out under suppression.

40. Method according to one of the preceding claims, wherein it is carried out under overpressure, preferably up to 16 bar, particularly preferably up to 6 bar.

41. Method according to any of the preceding claims, wherein it is carried out in an undivided cell.

42. A method according to any of the preceding claims, wherein it is carried out in batches.

43. Method according to one of the preceding claims, wherein it is carried out continuously in an undivided flow-through electrolysis cell.

44. Method according to any of the preceding claims, wherein it is carried out continuously, preferably in an undivided flow-through electrolysis cell.

45. A method according to any of the preceding claims, wherein it is carried out without the addition of catalysts, in particular without the addition of transition metal catalysts.

46. ​​A method according to any of the preceding claims, wherein no further oxidizing agents are added besides oxygen or atmospheric oxygen.

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