Process for the electrochemical hydrogenation of isophorone

Abstract

The subject of the present invention is a process for the electrochemical hydrogenation of isophorone in a divided electrolytic cell using H2O as hydrogen source and in the presence of H2SO4 in the anolyte and catholyte, wherein the cathode used is a pure material electrode made of a material selected from graphite, nickel and steel, the anode used is an electrode selected from a solid body electrode and a supported electrode, these electrodes having an active material selected from platinum, graphite, boron-doped diamond, ruthenium oxide, platinum oxide and / or iridium oxide.

Description

[0001] This invention relates to a method for the electrochemical hydrogenation of isophorone.

[0002] Hydrogenation of organic compounds is of great significance to the chemical industry because it can convert unsaturated compounds into saturated compounds. Equally important, it enables the production of alcohols from ketones and enones, and amines from nitriles.

[0003] The methods used to date for the catalytic hydrogenation of organic compounds require high temperatures, high pressures, and expensive catalysts, such as those disclosed 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 focused on achieving hydrogenation of organic compounds through electrochemical methods. The protons required for hydrogenation / reduction of organic compounds on the cathode side are preferably generated from water on the anode side; that is, the reaction is preferably carried out in an aqueous solution.

[0005] Navarro et al. (Tetrahedron Letters 44 (2003), 4725-4727) described the electrocatalytic hydrogenation of organic compounds in an unseparated electrolytic cell using a water-methanol mixture and ammonium acetate or ammonium chloride as electrolytes. A drawback of these electrolytes is the complexity of material provision and the purification steps resulting from their use. Furthermore, the use of chloride-containing electrolytes releases chlorine gas as a byproduct of the electrochemical process, requiring separate emission and treatment.

[0006] CN 116497375 A discloses a method for the electrochemical hydrogenation of benzaldehyde to produce benzyl alcohol in a ternary electrolyte / water / solvent mixture, which is carried out in an unseparated electrolytic cell. Available electrolytes include various salts, particularly imidazolium salts and tetraalkylammonium salts. Their disadvantage lies in the complexity of material provision and the purification steps resulting from the use of the electrolyte.

[0007] In addition, the use of undivided electrolytic cells has the disadvantage that the anodic reaction can lead to the formation of oxidative byproducts, thereby reducing the selectivity of the process.

[0008] Therefore, an electrochemical hydrogenation method using a partitioned electrolytic cell was developed. In these electrolytic cells, the two half-cells are separated by a diaphragm.

[0009] RU 2 198 158 C2 discloses the electrochemical hydrogenation of aldehydes and ketones in a partitioned electrolytic cell with a platinum anode and a copper cathode activated with a nickel framework catalyst. The electrolyte used is an aqueous solution of a non-oxidizing salt or an aqueous solution of an alkali metal hydroxide. Furthermore, benzene is used as a solvent. Using organic solvents such as benzene increases the complexity of the required downstream processes and reduces economic efficiency. In addition, the nickel framework catalyst needs to be replaced periodically, and the product is contaminated with nickel-containing materials. This leads to higher production complexity and higher purification complexity.

[0010] Pintauro et al. (J. Appl. Electrochem. 21 (1991), 799-804) disclosed the electrocatalytic hydrogenation of benzene, aniline, and nitrobenzene in a separated electrolytic cell in the presence of a solvent mixture consisting of water and tert-butanol, containing a water-soluble solubilizing salt (toluenesulfonate or tetraalkylammonium salt) as the electrolyte. A drawback of these electrolytes is the complexity of material preparation and the purification steps required by their use.

[0011] Lessard et al. (Can. J. Chem 73, 846-852 (1995)) disclosed a method for the electrocatalytic hydrogenation of conjugated enones in a partitioned electrolytic cell in the presence of an aqueous solution of sodium chloride and boric acid, the electrolytic cell comprising a carbon anode and a pressed powder cathode made of a material selected from nickel boride and nickel (fractal nickel or Raney nickel). The use of sodium chloride results in the production of chlorine gas as a byproduct during electrochemical hydrogenation. Boric acid is carcinogenic and requires complex removal from the product and byproduct streams of the electrochemical process. A drawback of both Raney nickel and fractal nickel electrodes is the need for complex catalytic activation (referred to as “activation” or “modification”) of the electrodes by pressing nickel nanoparticles prior to use. The corresponding electrodes are also disadvantageous because they are sensitive to wear and prone to oxidation.

[0012] US 2021 / 0348283 A1 discloses a method for hydrogenating nitriles using an alkaline buffer solution comprising a chelating agent and a tetraalkylamine. Separate electrolytic cells are employed in the examples. Those made of (Raney) nickel, carbon steel, Pt-Ir size-stabilized anode materials, and palladium are disclosed as anodes. Metals, carbon, and combinations thereof are disclosed as cathode materials.

[0013] The disadvantage of adding alkaline buffer solutions, chelating agents, and tetraalkylammonium salts is that the provision of these materials and the purification steps resulting from their use are complex.

[0014] RU 2 218 325 C2 discloses an electrocatalytic hydrogenation method, particularly for nitriles, wherein the reaction takes place in a separated electrolytic cell at a cathode activated with a nickel catalyst and a magnetite or platinum anode. The cathode electrolyte used is an optional alkaline aqueous solution containing a non-oxidizing metal salt. A disadvantage is the need for periodic replacement of the nickel catalyst-activated electrode and the potential for product contamination by nickel-containing materials. This leads to increased complexity in the production and purification processes.

[0015] DE 10 2021 119 761 A1 discloses a method for electrocatalytic hydrogenation of organic compounds in a partitioned electrolytic cell, wherein the cathode comprises or is composed of a transition metal chalcogenide selected from sulfides, selenides, and tellurides. However, the corresponding cathode first requires complex manufacturing.

[0016] US 2015 / 0008139 A1 also discloses a method for electrochemical hydrogenation of organic compounds using an aqueous electrolyte solution in a partitioned electrolyzer. The electrode comprises porous activated carbon with metal catalyst particles immobilized on it. However, these electrodes also require complex manufacturing processes.

[0017] WO 03 / 054286 A1 discloses a method for the electrolytic hydrogenation of vat dyes and sulfur dyes from aqueous solutions in a separated electrolytic cell, wherein an alkali metal hydroxide is used as the cathode electrolyte and sulfuric acid or an alkali metal hydroxide is used as the anode electrolyte. Available cathodes include catalytically active materials with a wide range of alkali resistance, high conductivity, large surface area, and low hydrogen overpotential, particularly Raney nickel and graphite particles as fluidized or fixed-bed electrodes. The process is accomplished by applying a specific electrolytic cell voltage. However, a disadvantage of the presence of an alkaline electrolyte is that byproducts of alkaline-catalyzed reactions (especially aldol reactions) may form in the presence of ketones / enones with enolizable protons (such as isophorones). This reduces the selectivity of the method. Furthermore, the reaction solution must be neutralized during post-treatment, which increases complexity and creates a saline waste stream.

[0018] Raney nickel and similar catalytically active electrodes require complex manufacturing processes involving pressing or coating onto suitable support materials. This increases complexity and leads to lower physical and chemical stability of the electrodes. Using graphite particles as fixed-bed / fluidized-bed electrodes requires even greater technical complexity for bed stabilization around a host material such as platinum wire, thus limiting their economic practicality.

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

[0020] Miller et al., J. Org. Chem., Vol. 43, No. 10, 1978, 2059-2061, disclosed an electrochemical hydrogenation method for phenols, wherein the hydrogenation is carried out in a separate electrolytic cell in a sulfuric acid solution. The anode is made of platinum, and the cathode is made of carbon electroplated with a catalyst metal or platinum plated on platinum. A drawback of the cathodes used is their lack of long-term stability, particularly under large-scale industrial conditions. Therefore, they need to be re-electroplated. They also require complex activation before being put into use.

[0021] Kunugi et al. (J. Electroanal. Chem. 313 (1991) 215-225) disclosed a method for the hydrogenation of ketones (acetone, acetophenone) using a nickel cathode and a platinum anode. However, the disclosed method only yielded low yields.

[0022] The object of this invention is to provide a method for the electrochemical hydrogenation of isophorone that avoids the disadvantages of prior art. Specifically, this invention provides a method for the electrochemical hydrogenation of isophorone that offers high yield, high selectivity, and minimal unwanted byproducts, and utilizes electrolytes and electrodes that can be produced, handled, purified, or reused with low complexity. Regarding the electrodes, it is desirable that they do not require (re)activation or (re)plating. Regarding the electrolytes, it is desirable that they pose as little health hazard as possible, particularly low carcinogenicity. These objectives are achieved by the method for the electrochemical hydrogenation of isophorone according to the present invention. -In a partitioned electrolytic cell -Use H2O as the hydrogen source, and - The process is carried out in the presence of sulfuric acid (H₂SO₄) in both the anolyte and catholyte. in The cathode used is a pure material electrode made from materials selected from graphite, nickel, and steel, and - The anode used is an electrode selected from solid electrodes and loaded electrodes, which have an active material selected from platinum, graphite, boron-doped diamond, ruthenium oxide, platinum oxide and / or iridium oxide.

[0023] The method of this invention is a method for the electrochemical hydrogenation of isophorones (i.e., a type of enone). If the reactant used is an enone, hydrogenation may produce corresponding unsaturated hydroxyl-containing compounds, corresponding saturated carbonyl-containing compounds (i.e., saturated ketones), or corresponding saturated hydroxyl-containing compounds due to the presence of different functional groups. The method of this invention is particularly suitable for hydrogenating enones to obtain corresponding saturated hydroxyl-containing compounds.

[0024] The method of the present invention is particularly suitable for hydrogenating isophorone to obtain 3,3,5-trimethylcyclohexanol or 3,3,5-trimethylcyclohexanone. The method of the present invention is even more preferred for hydrogenating isophorone to obtain 3,3,5-trimethylcyclohexanol, because this achieves particularly good yields.

[0025] It will be clear to those skilled in the art how to select reaction conditions to obtain the respective products.

[0026] Isophorones readily undergo hydrogenation on plate electrodes, particularly nickel or steel electrodes, to obtain 3,3,5-trimethylcyclohexanone. Therefore, the subject of this invention is still a method for hydrogenating isophorone to obtain 3,3,5-trimethylcyclohexanone, wherein the cathode used is a nickel or steel plate electrode. When hydrogenating isophorone to obtain 3,3,5-trimethylcyclohexanone, a stainless steel plate cathode and an anode selected from solid electrodes and supported electrodes, having an active material selected from graphite and boron-doped diamond, are particularly preferred.

[0027] Isophorones readily undergo hydrogenation on porous electrodes, particularly nickel foam electrodes, and on graphite electrodes to obtain 3,3,5-trimethylcyclohexanol. Therefore, the subject of this invention is a method for hydrogenating isophorones to obtain 3,3,5-trimethylcyclohexanol, wherein the cathode used is a pure material electrode made of a material selected from graphite and nickel foam. When hydrogenating isophorones to obtain 3,3,5-trimethylcyclohexanol, a cathode selected from nickel foam or graphite and a tantalum-supported platinum anode are particularly preferred.

[0028] The concentration of the organic compound is preferably 0.01-1 mol / L, more preferably 0.048-0.1 mol / L, because the organic compound is particularly easy to dissolve at this concentration.

[0029] This method is carried out in a partitioned electrolytic cell. In a partitioned electrolytic cell, two half-electrolytic cells are separated by a diaphragm. The advantage of a partitioned electrolytic cell is that it prevents the formation of oxidative byproducts due to anodic side reactions, thereby improving the selectivity of the entire method. In principle, the diaphragm material can be any material capable of withstanding the sulfuric acid used. Diaphragms made of microporous plastics and nonwoven fabrics made of polyethylene or glass fiber have proven particularly suitable. Diaphragms made of microporous plastics can be further reinforced to obtain better strength and tear strength. Also preferably, a cation exchange membrane can be used. More preferably, the diaphragm is also reinforced to obtain better strength and tear strength. Very particularly preferably, the diaphragm comprises a perfluorosulfonic acid-polytetrafluoroethylene copolymer. More preferably, it is also reinforced to obtain better strength and tear strength. The corresponding diaphragm can be selected from membranes of the model name NAFION® manufactured by The Chemours Company FC, LLC. Very particularly preferably, membranes of the model name NAFION® 424 or similar are used.

[0030] In the electrochemical hydrogenation method of the present invention, water, i.e., H2O, is used as the hydrogen source. This means that water must be present in the reaction. Therefore, water is present as both the hydrogen source and the solvent, in addition to the electrolyte used and the organic compound selected from ketones, enones, aromatics, and nitriles. One or more other solvents are preferably present besides water. Particularly preferred other solvents are those selected from methanol, isopropanol, tetrahydrofuran, acetone, and acetonitrile. If other solvents are present, their mass ratio is preferably from 0.01:1 to 10:1, particularly preferably from 0.5:1 to 2:1, based on the mass of water present. More preferably, a mixture of water and methanol is used. Very particularly preferred is a mixture of methanol and water, wherein the mass ratio of methanol used is from 0.8:1 to 1.2:1, and even more preferably 1:1, based on the mass of water present.

[0031] The electrochemical hydrogenation method is carried out in the presence of sulfuric acid (H₂SO₄) in both the anolyte and the catholyte. Sulfuric acid is preferably the only electrolyte present. However, in principle, one or more other electrolytes may be added to the anolyte and / or the catholyte. If other electrolytes are added, these electrolytes are preferably selected from boric acid, sodium sulfate, and alkylammonium salts. Sodium sulfate is particularly preferred. If other electrolytes are added, their usage is preferably 0.01-30 mol%, more preferably 0.01-10 mol%, and very particularly preferably 0.01-5 mol%, based on the total amount of all electrolytes.

[0032] The preferred proportion of sulfuric acid (H2SO4) used is 0.01-10% by weight, more preferably 0.1-5% by weight, and even more preferably 0.5-2.5% by weight, based on the mass of water present.

[0033] The pH value of the electrolyte solution used in the method of the present invention, i.e., the aqueous solution of the cathode electrolyte containing organic compounds and the optional anolyte solution present in the reaction, is preferably 0.01 to 5, more preferably 0.1 to 2, and even more preferably 0.8 to 1.5. The pH value is determined using pH test paper or a pH probe at room temperature (20°C), preferably using pH test paper.

[0034] The cathode used in the method of this invention is a pure material electrode made of a material selected from graphite, nickel, and steel. The pure material electrode is a non-loaded and untreated solid electrode composed of a specific alloy or a specific material composition. The front and back sides of the solid electrode have the same structure (no carrier material). The solid electrode is homogeneous (the specific structure is present both within the body and on the surface of the body). The solid electrode can be arbitrarily divided to obtain further solid electrodes. This is because the composition is identical within the body and on the surface.

[0035] The cathode is preferably selected from: - Steel, graphite, and nickel plate electrodes, and - Nickel foam electrode.

[0036] The cathode is particularly preferably a nickel foam electrode because it yields exceptionally high yields.

[0037] The anode used in the method of the present invention is an electrode selected from solid electrodes and loaded electrodes, which have an active material selected from platinum, graphite, boron-doped diamond, ruthenium oxide, platinum oxide and / or iridium oxide.

[0038] The corresponding anode is either a solid electrode or a supported electrode, wherein the active material of each is the electrode material. Therefore, the anode is an electrode selected from solid electrodes and supported electrodes, which have an active material selected from platinum, graphite, boron-doped diamond, ruthenium oxide, platinum oxide, and / or iridium oxide. If a support is used, it is preferably a material selected from graphite, tantalum, titanium, iron, and silicon. Electrodes with supports selected from titanium, iron, and silicon and coatings selected from ruthenium oxide, platinum oxide, and / or iridium oxide are also commercially available as DSA electrodes (size-stabilized anodes).

[0039] The anode used is particularly preferably a platinum electrode. For this electrode material, platinum is preferably pressed, welded, or coated onto a carrier material. Many different carrier materials that are stable to the electrolyte are used here, with tantalum being preferred.

[0040] The advantage of the cathode and anode is their exceptionally long service life under reaction conditions. Therefore, they are particularly suitable for large-scale industrial applications.

[0041] The hydrogenation is preferably carried out at a temperature of 10-30°C and a pressure of 950-1050 mbar. The reaction is particularly preferably carried out under SATP conditions (25°C, 1013 mbar).

[0042] The method of the present invention enables hydrogenation in both batch and continuous processes.

[0043] The method of the present invention can, in principle, be carried out under constant potential conditions (i.e., constant voltage and varying current intensity) or constant current conditions (i.e., varying voltage and constant current intensity). It is particularly preferred to carry out the method of the present invention under constant current conditions. The constant current reaction mode eliminates the need for an additional third electrode as a reference electrode. This has economic advantages and achieves a simpler construction.

[0044] Furthermore, the method of the present invention is particularly suitable for use at high current densities under constant current conditions. Therefore, the subject of the present invention is a hydrogenation method that can operate at 15-250 mA / cm². 2 The method is carried out at a current density of 25-35 mA / cm², and has the advantage of producing particularly high yields. The relatively high current intensity used for organic electrochemical reactions allows for the use of a smaller electrode surface area to obtain sufficient charge. This reduces the investment cost of electrochemical processes on an industrial scale. The method of the present invention is particularly preferably carried out at 25-35 mA / cm². 2 The experiment was conducted at a current density of [specific value]. Example

[0045] Example 1 - Hydrogenation of isophorone to obtain 3,3,5-trimethylcyclohexanol The reaction structure consists of a 5 ml semi-electrolytic cell equipped with a tantalum-supported platinum electrode as the anode and a 5 ml semi-electrolytic cell equipped with a nickel foam electrode as the cathode, separated by a Nafion 424 membrane (CEM). Each semi-electrolytic cell contains 5 ml of electrolyte solution (1:1 water:methanol; 1 wt% H₂SO₄, pH=1). Both electrodes are immersed 1 cm in the electrolyte solution. 0.24 mmol of isophorone is added to the cathode side. Constant current electrolysis is performed at room temperature and atmospheric pressure, with a constant current density of 35 mA / cm². 2 The duration was 2.1 h (185°C, 8°F). GC analysis with reference internal standard showed that the conversion of isophorone to 3,3,5-trimethylcyclohexanol was 91%.

[0046] Example 2 - Hydrogenation of isophorone to obtain 3,3,5-trimethylcyclohexanone The reaction structure consists of a 5 ml semi-electrolytic cell equipped with a BDD electrode as the anode semi-electrolytic cell and a 5 ml semi-electrolytic cell equipped with a stainless steel electrode as the cathode semi-electrolytic cell, separated from each other by a Nafion 424 membrane (CEM). Each semi-electrolytic cell contains 5 ml of electrolyte solution (1:1 water:methanol; 0.05 M H₂SO₄, 0.1 M sodium sulfate). Both electrodes are immersed 1 cm in the electrolyte solution. 0.50 mmol of isophorone is added to the cathode side. Constant current electrolysis is performed at room temperature and atmospheric pressure, with a constant current density of 45 mA / cm². 2 The duration was 2.8 h. GC analysis using the internal standard showed that the conversion of isophorone to 3,3,5-trimethylcyclohexanone was 72%.

[0047] Example 3 - Hydrogenation of acetophenone The reaction structure consists of a 5 ml semi-electrolytic cell equipped with a tantalum-supported platinum electrode as the anode and a 5 ml semi-electrolytic cell equipped with a nickel foam electrode as the cathode, separated by a Nafion 424 membrane (CEM). Each semi-electrolytic cell contains 5 ml of electrolyte solution (1:1 water:methanol; 0.05 M H₂SO₄, 0.1 M sodium sulfate). Both electrodes are immersed 1 cm in the electrolyte solution. 0.50 mmol of acetophenone is added to the cathode side. Constant current electrolysis is performed at room temperature and atmospheric pressure, with a constant current density of 35 mA / cm². 2 The duration was 2.1 h. GC analysis using the internal standard showed that the conversion of acetophenone to 1-phenylethanol was 15%.

[0048] Example 4 - Hydrogenation of acetone The reaction structure consists of a 5 ml semi-electrolytic cell equipped with a tantalum-supported platinum electrode as the anode and a 5 ml semi-electrolytic cell equipped with a nickel foam electrode as the cathode, separated by a Nafion 424 membrane (CEM). Each semi-electrolytic cell contains 5 ml of electrolyte solution (1:1 water:methanol; 0.05 M H₂SO₄, 0.1 M sodium sulfate). Both electrodes are immersed 1 cm in the electrolyte solution. 0.50 mmol of acetone is added to the cathode side. Constant current electrolysis is performed at room temperature and atmospheric pressure, with a constant current density of 35 mA / cm². 2 The duration was 2.1 h. GC analysis using the internal standard showed that acetone was not converted to 2-propanol.

Claims

1. A method for the electrochemical hydrogenation of isophorone, wherein the method -In a partitioned electrolytic cell -Use H2O as the hydrogen source, and - The process is carried out in the presence of H2SO4 in both the anolyte and the catholyte. Its features - The cathode used is a pure material electrode made of materials selected from graphite, nickel, and steel, and - The anode used is an electrode selected from solid electrodes and loaded electrodes, which have an active material selected from platinum, graphite, boron-doped diamond, ruthenium oxide, platinum oxide and / or iridium oxide.

2. The method according to claim 1, characterized in that, It is a method for hydrogenating isophorone to obtain 3,3,5-trimethylcyclohexanol or 3,3,5-trimethylcyclohexanone.

3. The method according to claim 2, characterized in that, It is a method for hydrogenating isophorone to obtain 3,3,5-trimethylcyclohexanol.

4. The method according to claim 3, characterized in that, The cathode is a pure material electrode made of a material selected from graphite and nickel foam.

5. The method according to any one of the preceding claims, characterized in that, The diaphragm used in the partitioned electrolytic cell contains a perfluorosulfonic acid-polytetrafluoroethylene copolymer.

6. The method according to any 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 methanol, isopropanol, tetrahydrofuran, acetone and acetonitrile.

7. The method according to any one of the preceding claims, characterized in that, The pH value of the electrolyte solution is between 0.01 and 5.

8. The method according to any one of the preceding claims, characterized in that, The cathode is selected from: - Steel, graphite, and nickel plate electrodes, and - Nickel foam electrode.

9. The method according to any one of the preceding claims, characterized in that, The anode is a platinum electrode.

10. The method according to any one of the preceding claims, characterized in that, It is carried out under constant current conditions.

11. The method according to claim 10, characterized in that, The hydrogenation is carried out at 15-250 mA / cm. 2 The experiment was conducted at a current density of [specific value].

12. The method according to any one of the preceding claims, characterized in that, At least one electrolyte selected from boric acid, sodium sulfate and alkylammonium salts is added to the anode electrolyte and / or cathode electrolyte.