Method for the electrochemical hydrogenation of organic compounds

Abstract

The present invention relates to a process for the electrochemical hydrogenation of an organic compound selected from the group consisting of ketones, enones, aromatics and nitriles in a divided cell with H2O as hydrogen source and in the presence of H2SO4 in anolyte and catholyte, in which a pure material electrode made of a material selected from graphite, nickel and steel is used as the cathode and an electrode selected from solid-body and supported electrodes with an active material selected from platinum, graphite, boron-doped diamond, ruthenium oxide, platinum oxide and / or iridium oxide is used as the anode.

Description

[0001] The present invention relates to a process for the electrochemical hydrogenation of organic compounds.

[0002] The hydrogenation of organic compounds is of great importance to the chemical industry, as it enables the conversion of unsaturated compounds into saturated ones. Also very important is the production of alcohols from ketones and enones, as well as the production of amines from nitriles.

[0003] Previously used processes for the catalytic hydrogenation of organic compounds required the use of high temperatures, high pressures and expensive catalysts, as disclosed, for example, in EP 1 318 130 A1 for the hydrogenation of isophorone to 3,3,5-trimethylcyclohexanol.

[0004] Due to the high technical complexity of catalytic hydrogenation, recent attempts have been made to achieve the hydrogenation of organic compounds electrochemically. The protons required for the hydrogenation / reduction of organic compounds on the cathode side are preferably generated from water on the anode side, meaning the reaction preferably takes place in aqueous solution.

[0005] Navarro et al. (Tetrahedron Letters 44 (2003), 4725–4727) describe the electrocatalytic hydrogenation of organic compounds in an undivided cell using water-methanol mixtures and ammonium acetate or ammonium chloride as the electrolyte. These electrolytes have the disadvantage that the preparation of the materials and the resulting purification steps for the electrolyte use are very complex. Furthermore, the use of chloride-containing electrolytes releases chlorine as a byproduct of the electrochemical process, which must be removed and treated separately.

[0006] CN 116497375 A discloses a process for the electrochemical hydrogenation of benzaldehyde to benzyl alcohol in a ternary electrolyte / water / solvent mixture, which is carried out in an undivided cell. Various salts, particularly imidazolium salts and tetraalkylammonium salts, can be used as electrolytes. These have the disadvantage that the preparation of the materials and the purification steps resulting from the use of the electrolyte are very complex.

[0007] Furthermore, the use of undivided cells has the disadvantage that anodic reactions lead to the formation of oxidative by-products and thus reduce the selectivity of the process.

[0008] For this reason, electrochemical hydrogenation processes have been developed that use split cells. In these, the two half-cells are separated by the use of a separator.

[0009] RU 2 198 158 C2 discloses the electrochemical hydrogenation of aldehydes and ketones in a split cell with a platinum anode and a copper cathode activated with a nickel skeleton catalyst. Aqueous solutions of non-oxidizing salts or aqueous solutions of alkali metal hydroxides are used as the electrolyte. Benzene is also used as a solvent. The use of organic solvents such as benzene increases the necessary downstream processing effort and reduces economic viability. Furthermore, regular renewal of the nickel skeleton catalyst is necessary, and the products become contaminated with nickel-containing material. This leads to higher production costs and increased purification effort.

[0010] Pintauro et al. (J. Appl. Electrochem. 21 (1991), 799-804) disclose the electrocatalytic hydrogenation of benzene, aniline, and nitrobenzene in a split cell in the presence of a solvent mixture consisting of water and t-butanol containing a hydrotrophic salt (a toluenesulfonate or tetraalkylammonium salt) as the electrolyte. These electrolytes have the disadvantage that the preparation of the materials and the purification steps resulting from the use of the electrolyte are complex.

[0011] Lessard et al. (Can. J. Chem 73, 846–852 (1995)) discloses a process for the electrocatalytic hydrogenation of conjugated enones in a divided cell with a carbon anode and a pressed powder cathode made of materials selected from nickel boride and nickel (fractal or Raney) and in the presence of aqueous solutions of sodium chloride and boric acid. The use of sodium chloride leads to the formation of chlorine as a byproduct of the electrochemical hydrogenation. Boric acid is carcinogenic and must be laboriously removed from the product and byproduct streams of the electrochemical process. Both Raney nickel and fractal nickel electrodes have the disadvantage that the electrodes must be laboriously catalytically activated by pressing on nanoparticulate nickel (referred to as "activation" or "modification") before use. These electrodes are disadvantageous because they are sensitive to abrasion and are oxidation-labile.

[0012] US 2021 / 0348283 A1 discloses a process for the hydrogenation of nitriles, using a basic buffer solution containing a chelating agent and a tetraalkylamine. A split cell is used in the examples. Anodes disclosed include those made of (Raney) nickel, carbon steel, a Pt-Ir dimensionally stable anode material, and palladium. Cathode materials include metals, carbon, and combinations thereof.

[0013] The addition of basic buffer solutions, chelating agents and tetraalkylammonium salts has the disadvantage that the provision of the materials and the resulting purification steps are complex.

[0014] RU 2 218 325 C2 discloses a process for the electrocatalytic hydrogenation of, among other things, nitriles, in which the reaction takes place in a divided cell at a cathode activated with a nickel catalyst and a magnetite or platinum anode. Basic aqueous solutions containing non-oxidizing metal salts are optionally used as catholytes. A disadvantage is that the nickel catalyst-activated electrode requires regular replacement, and the products are contaminated by nickel-containing material. This leads to increased production and purification costs.

[0015] DE 10 2021 119 761 A1 discloses a process for the electrocatalytic hydrogenation of organic compounds in a divided cell, in which the cathode comprises or consists of a transition metal chalcogenide selected from sulfides, selenides, and tellurides. However, corresponding cathodes must first be manufactured in a complex process.

[0016] US 2015 / 0008139 A1 also discloses a process for the electrochemical hydrogenation of organic compounds in divided cells using aqueous electrolyte solutions. An electrode comprises porous activated carbon on which metal catalyst particles are immobilized. However, these electrodes also require complex manufacturing.

[0017] WO 03 / 054286 A1 discloses a process for the electrolytic hydrogenation of vat and sulfur dyes from aqueous solution, carried out in a divided cell, using alkali metal hydroxide as the catholyte and sulfuric acid or alkali metal hydroxide as the anolyte. All materials stable in the alkaline range, electrically conductive, large-area, catalytically active materials with a low hydrogen overvoltage, can be used as cathodes, in particular Raney nickel and graphite granules as fluidized-bed or fixed-bed electrodes. The process is carried out by applying a defined cell voltage. However, the alkaline electrolyte has the disadvantage that, in the presence of ketones / enones with enolizable protons, e.g., isophorone, byproducts from base-catalyzed reactions, including aldol reactions, can be formed. This reduces the selectivity of the process.In addition, the reaction solution must be neutralized during processing, which leads to increased effort and the formation of salt-containing waste streams.

[0018] Raney nickel and similar catalytically active electrodes require a complex process, such as pressing or coating them onto suitable support materials. This increases the manufacturing effort and results in lower physical and chemical stability of the electrodes. The use of graphite granules as fixed / fluidized bed electrodes requires greater technical effort to stabilize the bed around the main conductor, e.g., platinum wire, and thus has limited economic viability.

[0019] Finally, electrolysis requires the use of a reference electrode (Ag / AgCl) by applying a defined voltage (also known as "potentiostatic"). This increases the technical complexity of the process.

[0020] Miller et al., J. Org. Chem., Vol. 43, No. 10, 1978, 2059-2061, discloses a process for the electrochemical hydrogenation of phenols in which the hydrogenation takes place in a divided cell in sulfuric acid solution. The anode consists of platinum, and the cathode of carbon electroplated with catalyst metal or platinized platinum. The cathodes used have the disadvantage of not being stable over the long term, especially under industrial-scale conditions. They therefore must be re-electroplated. They also require a complex activation process before use.

[0021] The object of the present invention is to provide a process for the electrochemical hydrogenation of organic compounds that does not have the disadvantages of the prior art. In particular, the object of the present invention is to provide a process for the electrochemical hydrogenation of organic compounds that is highly selective and produces few undesired by-products, and that uses electrolytes and electrodes that can be manufactured, operated, and purified or reused with little effort. With regard to the electrodes, it is further desirable that they do not require (re)activation or (re)plating. With regard to the electrolytes, it is further desirable that they be as harmless to health as possible, in particular, they are not carcinogenic.This sum of objects posed here is achieved by the process according to the invention for the electrochemical hydrogenation of an organic compound selected from the group consisting of ketones, enones, aromatics and nitriles. in a divided cell with H 2 O as hydrogen source and in the presence of sulfuric acid (H 2 SO 4 ) in anolyte and catholyte, in which a pure material electrode made of a material selected from graphite, nickel and steel is used as the cathode and an electrode selected from solid-body and supported electrodes with an active material selected from platinum, graphite, boron-doped diamond, ruthenium oxide, platinum oxide and / or iridium oxide is used as the anode.

[0022] The present process is a process for the electrochemical hydrogenation of an organic compound selected from the group consisting of ketones, enones, aromatics, and nitriles. If a ketone is used as the starting material, the corresponding hydroxyl-containing compound is formed by hydrogenation. If an enone is used as the starting material, the corresponding unsaturated hydroxyl-containing compound, the corresponding saturated carbonyl-containing compound (i.e. ketone or aldehyde), or the corresponding saturated hydroxyl-containing compound can be produced by hydrogenation. The process according to the invention is particularly suitable for the hydrogenation of enones to the corresponding saturated hydroxyl-containing compounds. If an aromatic is used as the starting material, the corresponding, optionally partially unsaturated, cyclic hydrocarbon compound is formed by hydrogenation.The process according to the invention is particularly suitable for the hydrogenation of aromatics to the corresponding saturated cyclic hydrocarbon compounds. The organic compounds used, selected from the group consisting of ketones, enones, aromatics, and nitriles, cannot exclusively be organic compounds having the functionalities mentioned. The term "organic compound selected from the group consisting of ketones, enones, aromatics, and nitriles" thus includes here and below organic compounds which, in addition to the functionalities mentioned, also have other, non-electrochemically hydrogenatable functionalities or substituents. The organic compounds mentioned can also be organic compounds which, in addition to the functionalities listed in the optionallyThe oxygen and nitrogen atoms present in the keto, enone and nitrile functionalities contain further oxygen and / or nitrogen and / or other hetero atoms.

[0023] The process according to the invention is particularly suitable for the electrochemical hydrogenation of organic compounds which contain exclusively carbon and hydrogen and optionally also nitrogen and / or oxygen.

[0024] The process according to the invention is particularly suitable for the hydrogenation of isophorone, isophoronenitrile, a mixture of 2,2,4- and 2,4,4-trimethylhexanedinitrile, 4,4'-methylenedianiline and naphthalene.

[0025] The process according to the invention is suitable for hydrogenating the preferred reactants mentioned to the products mentioned: Isophorone to 3,3,5-trimethylcyclohexanol or 3,3,5-trimethylcyclohexanone, isophoronenitrile to isophoroneaminoalcohol or isophoronediamine, the mixture of 2,2,4- and 2,4,4-trimethylhexanedinitrile to 2,2,4- and 2,4,4-trimethylhexane-1,6-diamine, 4,4'-methylenedianiline to 4,4'-diaminodicyclohexylmethane and naphthalene to tetrahydronaphthalene or decahydronaphthalene.

[0026] It is clear to the person skilled in the art how to choose the reaction conditions in order to achieve the respective product mentioned.

[0027] The process according to the invention is also particularly suitable for the electrochemical hydrogenation of ketones. Even more preferably, the process according to the invention is a process for the electrochemical hydrogenation of ketones containing exclusively carbon, hydrogen, and oxygen.

[0028] The process according to the invention is particularly suitable for the hydrogenation of isophorone to 3,3,5-trimethylcyclohexanol.

[0029] The concentration of the organic compound is preferably 0.01 - 1 mol / l, preferably 0.048 - 0.1 mol / l, since the organic compound can then be dissolved particularly well.

[0030] The process is carried out in a divided cell. In divided cells, the two half-cells are separated by a separator. Divided cells have the advantage of preventing the formation of oxidized by-products due to anodic side reactions, thus improving the selectivity of the overall process. In principle, the separator material can be any material that can withstand the sulfuric acid used. Separators made of microporous plastics and nonwovens made of polyethylene or glass fiber have proven particularly suitable. Separators made of microporous plastics can also be reinforced to achieve better strength and tear resistance. Cation exchange membranes can also be used with preference. These are also more preferably reinforced to achieve better strength and tear resistance. Most preferably, the separator comprises a perfluorosulfonic acid-polytetrafluoroethylene copolymer.Preferably, this material is also reinforced to achieve better strength and tear resistance. Suitable separators can be selected from the NAFION® membranes from The Chemours Company FC, LLC. Particularly preferred are membranes with the NAFION® 424 type or similar types.

[0031] In the electrochemical hydrogenation process according to the invention, water, i.e. H 2 O, is used as the hydrogen source. This means that water must be present during the reaction. In addition to the electrolyte used and the organic compound used selected from ketones, enones, aromatics and nitriles, water is therefore present as the hydrogen source and solvent. In addition to water, one or more further solvents can preferably also be present. Particularly preferably, a solvent selected from the group consisting of methanol, isopropanol, tetrahydrofuran, acetone and acetonitrile can be used as the further solvent. If a further solvent is present, this is preferably present in mass proportions of 0.01:1 to 10:1 and particularly preferably in mass proportions of 0.5:1 to 2:1 based on the mass of the water present. Further preference is given to using a mixture of water and methanol.Very particular preference is given to using a mixture of methanol and water in which the mass fraction of methanol used is 0.8:1 to 1.2:1, even more preferably 1:1, based on the mass of water present.

[0032] The electrochemical hydrogenation process is carried out in the presence of sulfuric acid (H 2 SO 4 ) in the anolyte and catholyte. Sulfuric acid is preferably the only electrolyte present. In principle, however, one or more additional electrolytes can also be used. If additional electrolytes are added, they can preferably be selected from the group consisting of boric acid, sodium sulfate, and alkylammonium salts. If additional electrolytes are added, they are preferably used in proportions of 0.01-30 mol%, more preferably 0.01-10 mol%, and most preferably 0.01-5 mol%, based on the total amount of electrolytes.

[0033] The sulfuric acid (H 2 SO 4 ) is preferably used in proportions of 0.01 - 10 wt.%, more preferably 0.1 - 5 wt.%, even more preferably 0.5 - 2.5 wt.%, based on the mass of the water present.

[0034] The pH of the electrolyte solution used in the process according to the invention, i.e., the aqueous catholyte solution containing the organic compound and optionally the anolyte solution present during the reaction, more preferably the aqueous catholyte solution containing the organic compound and the anolyte solution present during the reaction, is preferably between 0.01 and 5, more preferably between 0.1 and 2, even more preferably between 0.8 and 1.5. The pH is determined at room temperature (20°C) using pH paper or pH probes, preferably with pH paper.

[0035] The cathode used in the process according to the invention is a pure-material electrode made of a material selected from graphite, nickel, and steel. Pure-material electrodes are unsupported and untreated solid-body electrodes made of a defined alloy or a defined material composition. A solid body has a uniform structure on both the front and back sides (no carrier material). A solid body is homogeneous (a defined structure both within the body and on the surface). A solid body can be divided as desired, resulting in another solid body. This is due to the same composition in the body and on the surface.

[0036] Preferably, the cathode is selected from the group consisting of Steel, graphite and nickel plate electrodes and nickel foam electrodes.

[0037] The cathode is particularly preferably a nickel foam electrode, as this leads to particularly high yields.

[0038] The anode used in the process according to the invention is an electrode selected from solid and supported electrodes with an active material selected from platinum, graphite, boron-doped diamond, ruthenium oxide, platinum oxide and / or iridium oxide.

[0039] The corresponding anodes are solid-body electrodes or supported electrodes, in which the respective active material is the aforementioned electrode material. Thus, the aforementioned anodes are solid-body or supported electrodes with an active material selected from platinum, graphite, boron-doped diamond, ruthenium oxide, platinum oxide, and / or iridium oxide. If a support is used, this is preferably a material selected from graphite, tantalum, titanium, iron, and silicon. Electrodes with a support selected from titanium, iron, and silicon with a coating selected from ruthenium oxide, platinum oxide, and / or iridium oxide are also commercially available as DSA electrodes (dimensionally stable anodes).

[0040] The anode used is particularly preferably a platinum electrode. With this electrode material, the platinum is preferably pressed, welded, or coated onto a carrier material. Many different carrier materials that are stable to the electrolyte are used, preferably tantalum.

[0041] The aforementioned cathodes and anodes have the advantage of particularly long service lives under the reaction conditions. This makes them particularly suitable for large-scale use.

[0042] The hydrogenation is preferably carried out between 10-30 °C and 950-1050 mbar. The reaction is particularly preferably carried out under SATP conditions (25 °C, 1013 mbar).

[0043] The process according to the invention enables hydrogenation in batch and continuous processes.

[0044] In principle, the process according to the invention can be carried out both potentiostatically, i.e., at a constant voltage and varying current, and galvanostatically, i.e., at a varying voltage and constant current. The process according to the invention is particularly preferably carried out galvanostatically. The galvanostatic reaction procedure eliminates the need for an additional third electrode as a reference electrode. This offers an economic advantage and allows for a simpler design.

[0045] Furthermore, the process according to the invention is particularly suitable for galvanostatic use at high current densities. The present invention thus provides a hydrogenation process that can be carried out at current densities of 15–250 mA / cm² and has the advantage of leading to particularly high yields. The comparatively high current intensity for organic electrochemical reactions enables the use of smaller electrode surfaces to achieve a sufficient charge. This enables a reduction in investment costs for electrochemical processes on an industrial scale. The process according to the invention can most preferably be carried out at current densities of 25–35 mA / cm². Examples Example 1 - Hydrogenation of isophorone to 3,3,5-trimethylcyclohexanol

[0046] The reaction setup consisted of a 5 ml half-cell equipped with a tantalum-supported Pt electrode as the anode half-cell and a 5 ml half-cell equipped with a nickel foam electrode as the cathode half-cell, separated from each other by a Nafion 424 membrane (CEM). Both half-cells were filled with 5 ml of electrolyte solution (1:1 water:methanol; 1 wt% H 2 SO 4 , pH = 1). Both electrodes were immersed 1 cm into the electrolyte solution. 0.24 mmol of isophorone was added to the cathode side. Galvanostatic electrolysis was carried out at room temperature and ambient pressure with a constant current density of 35 mA / cm 2 for a period of 2.1 h (185 °C, 8 °F). GC analysis against an internal standard showed 91% conversion of isophorone to 3,3,5-trimethylcyclohexanol.

Claims

1. A process for the electrochemical hydrogenation of an organic compound selected from the group consisting of ketones, enones, aromatics and nitriles - in a divided cell - with H2O as hydrogen source and - in the presence of H2SO4 in anolyte and catholyte, characterized in that - as cathode a pure material electrode made of a material selected from graphite, nickel and steel and - as anode an electrode selected from solid body and supported electrodes with an active material selected from platinum, graphite, boron-doped diamond, ruthenium oxide, platinum oxide and / or iridium oxide is used.

2. Method according to claim 1, characterized in that it is a process for the hydrogenation of ketones.

3. Method according to claim 2, characterized in that it is a process for the hydrogenation of isophorone to 3,3,5-trimethylcyclohexanol.

4. Method according to one of the preceding claims, characterized in thatthe separator used in the divided cell comprises a perfluorosulfonic acid-polytetrafluoroethylene copolymer.

5. Method according to one of the preceding claims characterized in that the electrolysis is carried out in a solvent mixture consisting of water and a solvent selected from the group consisting of methanol, isopropanol, tetrahydrofuran, acetone and acetonitrile.

6. Method according to one of the preceding claims, characterized in that the pH value of the electrolyte solution is 0.01 to 5.

7. Method according to one of the preceding claims, characterized in that the cathode is selected from the group consisting of - steel, graphite and nickel plate electrodes and - nickel foam electrodes.

8. Method according to one of the preceding claims, characterized in that the anode is a platinum electrode.

9. Method according to one of the preceding claims, characterized in thatit is carried out galvanostatically.

10. Method according to claim 9, characterized in that hydrogenation at current densities of 15 - 250 mA / cm 2 is carried out.

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