Process for the electrocatalytic hydrogenation of alkynes, and electrochemical cell for said process

Abstract

The invention relates to a process for the electrocatalytic hydrogenation of alkynes in an electrochemical cell and to an electrochemical cell that can be used for this purpose. The alkyne is provided in gaseous, liquid or at least partially dissolved form and hydrogenated on the cathode; the cathode comprises elementary silver as the catalyst and / or a silver compound that is reduced to elementary silver during the electrocatalytic reaction. In particular the selective production of (E)-alkenes is possible using the process.

Description

[0001] Patent application:

[0002] Process for the electrocatalytic hydrogenation of alkynes and electrochemical cell for this process

[0003] The application relates to a process for the electrocatalytic hydrogenation of alkynes to alkenes, in particular for the production of alkenes with an increased proportion of the trans-isomer ((E)-alkene), as well as to an electrochemical cell with which this process can be carried out.

[0004] According to the state of the art, there are three different preparation options for the stereoselective hydrogenation of alkynes: Preparation using stoichiometric hydrogenation reagents, thermally catalyzed processes using homogeneous or heterogeneous catalysts, and electrocatalytic hydrogenation reactions.

[0005] In principle, the hydrogenation of alkynes to alkenes can be achieved using any of the three methods, and further hydrogenation to alkanes can usually be suppressed. In general, the formation of (Z)-alkenes is preferred. Without special measures, (E)-alkenes are not formed at all or only on a small scale using the state-of-the-art processes.

[0006] The use of stoichiometric reagents has the inherent disadvantage of generating large amounts of waste. With stoichiometric reagents, selective hydrogenation to (E)-alkenes can be achieved, particularly with reducing agents such as Na in NH3(I). However, this process has the disadvantage that it takes place under conditions unsuitable for industrial application. Other stoichiometric reagents are also used primarily on a laboratory scale; however, these are often highly toxic or contain CMR substances such as chromium salts or boranes. Thermally catalyzed processes (catalytic reactions with gaseous hydrogen) have the inherent disadvantage of requiring upstream hydrogen production. Current technology involves producing hydrogen from fossil sources, for example, by electrolysis or steam reforming.In addition, many hydrogenation reactions must be carried out at elevated temperatures and pressures to ensure sufficient conversion. This not only represents an additional energy input but also places increased demands on process reliability and the stability of the reacted substances.

[0007] Catalysts based on expensive transition metal complexes with Pt, Pd, Ru, Re, Ir, or Rh are often used as catalysts for thermally catalyzed processes. These highly specialized catalysts are capable of catalytic hydrogenation of alkynes selectively not only to (Z)-alkenes but also to (E)-alkenes. Due to their limited stability, complex preparation, and difficulty in separating the catalysts from the desired product, homogeneous catalysts are often unsuitable for large-scale industrial applications.

[0008] In heterogeneously catalyzed processes, noble metals such as Pt, Ru, or Rh are also frequently used. To avoid complete hydrogenation of the desired alkenes to alkanes, the catalysts are sometimes treated with toxic catalyst poisons such as lead (salts). Lindlar catalysts have been described as heterogeneous catalysts with which high (Z) selectivity can be achieved. For the production of (E)-alkenes, only the use of mixtures of a Pd-containing hydrogenation catalyst and a Rh-containing catalyst is known, with activated alkynes containing at least one phenyl ring and / or one carbonyl group being used as starting materials.

[0009] State-of-the-art electrocatalytic processes have so far been used primarily on a laboratory scale. Nickel, palladium, palladium / platinum alloys, and copper have been used as catalysts. However, only reaction products with high (Z) selectivities have been described. Continuous systems are also largely unknown.

[0010] DE 10 2016 218 230 A1 describes the hydrogenation of alkynes using copper as a catalyst. However, this document does not disclose the formation of (E)-alkenes. (E)-selectivity is generally unknown in the electrochemical processes of the state of the art. Furthermore, no industrially relevant current densities (from 100 mA cm) are available for the known electrochemical processes. -2 ) are known; these are often also measured with concentrations in the range of a few mmol I' 1 carried out.

[0011] The present invention is therefore based on the object of overcoming the disadvantages of the prior art and providing an improved process and an electrochemical cell for the electrocatalytic hydrogenation of alkynes, with which at least one of the disadvantages described above can be overcome. In particular, the process should also be applicable on an industrial scale and preferably also allow a certain selectivity with regard to the isomers formed. Finally, the process should also avoid the use of expensive precious metals as catalysts.

[0012] At least one of these objects is achieved by the method and the electrochemical cell according to the independent claims. Subclaims and the description teach advantageous further developments.

[0013] According to the application, it was recognized that the electrocatalytic hydrogenation of alkynes to alkenes is achieved by using silver or a silver compound as the catalyst for the electrocatalytic reaction. A process according to the application for the electrocatalytic hydrogenation of alkynes is accordingly carried out in an electrochemical cell comprising a cathode (which can also be designed as a gas diffusion electrode), an anode, and an electrolyte, with the hydrogenation taking place at the cathode. The alkyne to be hydrogenated is used in gaseous, liquid, and / or at least partially dissolved form (where the state of aggregation is based on the boiling point or melting point at room temperature, i.e., 20°C); it can therefore be used in its pure form or in a solvent, although it need not be completely dissolved.If the alkyne is used in solution, the solution can in particular be obtained from a solid, but in principle a solution of a liquid alkyne or a gaseous alkyne is also conceivable. The cathode contains the silver catalyst; this can either comprise or consist of elemental silver (where elemental silver is primarily understood to mean the pure element, but in individual cases can also be understood to be a silver alloy with another metal) or alternatively comprise or consist of a silver compound. Since the silver catalyst is present in particular in or on the cathode, any silver compound is usually completely reduced to elemental silver during operation of the electrochemical cell.

[0014] Electrochemical cells can contain both solid and liquid electrolytes as electrolytes. In the case of liquid electrolytes, the alkyne (sometimes referred to simply as the "substrate") can also be dissolved or at least partially dissolved in the electrolyte. An aqueous electrolyte can be used as the liquid electrolyte—for cost reasons alone. The pFI of this aqueous electrolyte will typically be alkaline, neutral, or slightly acidic. These aqueous electrolytes can contain salts (which do not change the pFI), particularly in the case of neutral electrolytes, thus ensuring sufficient conductivity of the electrolyte. In some cases, slightly higher selectivities with respect to the trans isomer of the alkyne formed are achieved with alkaline electrolytes.In addition to aqueous electrolytes, other solvents or mixtures of water with other solvents can also be used as electrolytes. Alcohols such as methanol or ethanol are particularly suitable. The only important factors regarding the solvent are that it does not cause unwanted decomposition under the electrolysis conditions and that sufficient conductivity or a change in the pFI value can be achieved by adding salts.

[0015] For all electrolytes, especially solid electrolytes, a separate reaction phase with the substrate can also be used, with this reaction phase bypassing the cathode. The reaction phase can contain the liquid or gaseous alkyne in dissolved or at least partially dissolved form; however, it can also consist of the liquid or gaseous alkyne. In this case, the substrate is typically not dissolved in the electrolyte, or only dissolved to a very small extent.

[0016] If a solvent is used for the alkyne in addition to the electrolyte, any common solvent can be used, provided it is stable under the conditions of the electrocatalytic process. Such a solvent can also be used as a proton source for the formation of hydrogen at the cathode surface. Finally, the reaction phase can also contain salts to control the activity.

[0017] Finally, the electrochemical cell can be either a batch cell or a cell that allows continuous flow operation. Equally high Faradaic efficiencies and similar cis / trans ratios of the formed alkenes have been observed for both cell types.

[0018] Hydrogenation in the electrochemical cell proceeds in principle in a manner known to those skilled in the art from the literature on electrocatalytic reactions. Atomic hydrogen is formed at the cathode by a suitable hydrogen source (in particular, a liquid electrolyte or an optionally used solvent for the alkyne). This means that the definition of "cathode" according to the application also includes a catalyst layer on a solid electrolyte that may be present (for clarification: the electrolyte itself cannot, by definition, be the cathode; however, the cathode or part of the cathode can be applied directly to it in the form of a catalyst layer). Atomic hydrogen can therefore also be formed on its surface (provided the electrocatalyst required by the application is arranged thereon), so that reduction of the substrate can occur.In addition to a hydrogen source in the cathode compartment, protons generated by the anode reaction and diffused into the cathode compartment can also serve as a hydrogen source. The atomic hydrogen then reacts (typically before H2 is formed from the hydrogen atoms) with the alkyne to form an alkyne. The process according to the application not only eliminates the need for stoichiometric reducing reagents, but also eliminates the use of gaseous hydrogen. Accordingly, the production process selected according to the application can be carried out at ambient pressure with increased process reliability and without complex upstream hydrogen production. This represents a clear advantage with regard to the sustainability of the process according to the application. The complete conversion of the substrate, or the conversion in general, can be adjusted by the applied current on the one hand and the residence time on the other.According to the invention, it was found that the alkenes formed are not further reduced to alkanes (at least with the reaction parameters reasonably chosen to ensure complete conversion).

[0019] With the process according to the application, it is not only possible to carry out electrocatalytic hydrogenation at high current densities and on an industrial scale due to the use of silver as a catalyst; it has also been found that the specific form of silver used in the cathode also offers the possibility of controlling the alkene isomer formed. Depending on the reaction procedure, either a preferential formation of Z isomers or the preferential formation of E isomers (which cannot be produced using state-of-the-art electrocatalytic processes) can be achieved.

[0020] According to an embodiment frequently realized due to the good controllability of the size of the catalyst regions, an electrode can be used as the cathode, which consists of a support material and a catalyst layer arranged thereon, or which comprises, in addition to the support material and catalyst layer, further layers (in particular arranged between the support material and catalyst layer). A "further layer" is particularly an ionomer layer, with which the pH value or the ion availability at the cathode for the formation of atomic hydrogen can be controlled (if an ionomer layer is included, the cathode can consist of the support material, the catalyst layer, and the ionomer layer, or can in turn contain other "further" layers). In the case of solid-state electrolyte cells, according to the application, the support material also comprises or consists of the solid electrolyte.The support materials used for electrodes, which are typically known to those skilled in the art, can be used as the support material - particularly if the solid electrolyte is not already serving as the support material. The support material can in particular comprise or consist of a metal and / or a non-metal (but theoretically also a non-conductive material such as a polymer) and can in particular be a carbon-containing material such as carbon paper, carbon fabric, carbon fleece or carbon foam or a metal such as nickel or titanium or a metal alloy such as stainless steel, in each case in grid form or as a metal foam. Like an electrode consisting of the catalyst, the catalyst layer can also consist of the catalyst (i.e. the silver or the silver compound) or contain it.The catalyst layer can also completely cover the support material (for example, a carbon fleece as a support material can be completely covered by the catalyst layer), but it can also be present only on partial areas of the support material (for example, a catalyst layer that is only present on partial areas of a porous carbon support). Often, the support material will not contain an expensive precious metal, but rather cheaper metals such as titanium or carbon-containing support materials. Copper is also typically not used as a support material. The catalyst layer can also be formed from a prefabricated layered material that is applied to the support material (so that a material-to-material bond is formed between the support material and the layered material); however, the catalyst layer is often deposited on the support material using a suitable process.One possibility for depositing silver is, for example, the reductive electrochemical deposition of dissolved silver salts.

[0021] An embodiment in which the catalyst layer or the catalyst is present only on partial areas of the surface can be advantageously used to adjust the selectivity between the E-isomer and the Z-isomer. According to the invention, it was observed that with particularly small-scale catalyst regions (i.e., small silver particles or small-area "silver regions" obtained, for example, by deposition), control can be achieved such that the E-isomer of the aiken is increasingly formed during hydrogenation. Accordingly, the catalyst layer can be applied in a deposition process in such a way that no continuous silver layer is formed, but only "silver islands." For this purpose, for example, appropriate impregnation of the substrate onto which silver or a silver compound is subsequently deposited can be carried out beforehand.Alternatively, a small-scale catalyst region can be achieved by selecting the particle size of the silver particles used, or alternatively, when using a silver precursor, the size of the precursor particles applied to the electrode surface can be selected accordingly. Alternatively or additionally, a suitable starting material for the catalyst layer, in particular using a suitable binder, can be used to ensure the formation of spaced "silver islands" from the silver precursor (for example, by applying a voltage to the electrode used as the cathode). In particular, the silver catalysts according to the application can generally be formed from a precursor (in situ) or applied to the electrode by reductive electrochemical deposition of dissolved silver salts.

[0022] It should be noted here that for the formation of an increased rate of the E isomer, it is advantageous to provide an electrode material or electrode coating, or to use an appropriately selected starting material, that prevents the formation of large silver regions and / or ensures that the silver present in the electrode is present only in nanoscale dimensions in contiguous regions of elemental silver. Nanoscale dimensions are understood in particular to mean dimensions that are a maximum of 200 nm (as defined below) or smaller (these can be determined, for example, using scanning tunneling microscopy images).

[0023] Without wishing to be limited to this, the above finding is currently attributed to the fact that the Z-isomer - as is usual in hydrogenation reactions - is formed as a kinetic reaction product under the conditions of electrocatalysis, but that smaller silver areas or small particles ensure better coordination of the alkenes formed by the hydrogenation and therefore - through the brief formation of a transition state involving another atom of atomic hydrogen - support the isomerization to the thermodynamic product, the E-isomer, whereas larger, connected flat or planar silver areas do not.

[0024] According to one embodiment, the process according to the application is usually carried out at room temperature or in the temperature range between 20°C and 80°C, and typically also without cooling. Low temperatures are preferred, particularly for economic reasons. Furthermore, excessively high temperatures carry the risk of triggering agglomeration of nanoscale silver particles or silver regions. Typically, the reaction according to the application is carried out at atmospheric pressure, regardless of the selected temperature. Depending on the starting materials and products or their physical state, a pressure higher than atmospheric pressure or a pressure lower than atmospheric pressure may also be appropriate (for example, a reduced pressure when gaseous alkenes are formed as a product).

[0025] According to a further embodiment, the silver contained in the cathode or in the catalyst layer has particle sizes of less than 1 pm, in particular less than 0.5 pm, and frequently also less than 200 nm. Particularly high trans-selectivities are achieved in particular with particle sizes of less than 150 nm, in particular less than 100 nm, and frequently also less than 50 nm. The particle size refers to the D determined by laser diffraction. 50 -value; in a preferred embodiment, however, the specified particle size is a D 90-value (since a larger proportion of smaller particles is usually advantageous, particularly with regard to increased trans formation). These definitions apply - unless stated otherwise - to all particle size data stated in the application. If the silver particles are not introduced as such into the electrode material or the catalyst layer but are formed from a precursor, the particle size of the silver islands or silver regions formed can only be determined retrospectively. For this purpose, the particle sizes can be determined by evaluating electron micrographs. If the cathode has regions of deposited silver, the average diameters found here are often at least partly less than 1 pm, in particular less than 0.5 pm and usually less than 200 nm or even less than 100 nm.Typically, at least 50% of the deposited silver surface has such average diameters, and frequently even 90% of the deposited silver surface. The evaluation can also be carried out based on electron microscopic images.

[0026] All the above statements regarding small particle sizes and silver regions can advantageously contribute to supporting the formation of the E isomer.

[0027] As already stated several times above, the increased formation of E isomers can be promoted by specific measures. Accordingly, the solution according to the application can be used particularly advantageously for the reduction of alkynes containing internal alkynes of the formula R 1 -C=CR 2 are (i.e. where there is a substituent R on both sides of the triple bond 1 or R 2 which is not hydrogen).

[0028] In principle, the alkynes used as substrate can be all substituted or unsubstituted alkynes R'-C^CR 2 The substituents R 1 or R 2 can independently contain heteroatoms or be alkyl, aryl, aralkyl, or olefin groups, or hydrogen. The substituents can independently be substituted or unsubstituted, with particular examples being alcohol groups, amine groups, and carboxylic acid groups. Ketones are also of secondary importance. Regardless of the substitution of the alkynes, alkynes with a maximum of 14 carbon atoms are often used for economic reasons alone.

[0029] Also advantageous for the process are liquid alkynes or alkynes that are at least partially soluble in aqueous electrolytes when used. The use of alkynes with one or more hydrophilic groups, such as alcohol groups, can therefore be advantageous. Alkynes (or the e-alkenes produced from them) of particular commercial interest are 1,4-butynediol, 2-butyne-1-ol, and acetylenedicarboxylic acid.

[0030] As already mentioned, the alkynes can be used either in bulk or as a solution. In particular, very highly concentrated systems can be used in solutions without significant changes in the isomer ratio formed and the Faradaic efficiency being observed. Therefore, when using solutions or suspensions in which the alkyne is only partially dissolved, the alkyne is typically used in an amount of at least 100 mmol, typically at least 200 mmol, and often at least 1 mol per liter of solvent (which can be either the electrolyte and / or an additional solvent).

[0031] According to a further embodiment, the catalyst (or cathode) comprises, in addition to the silver or the silver compound, a binder or consists of the silver and / or the silver compound and the binder. In particular, the catalyst layer comprises, in addition to the silver or the silver compound, a binder or consists of the silver and / or the silver compound and the binder.

[0032] If a binder is also used, several aspects can be addressed. Firstly, the hydrophilicity of the electrode can be adjusted. The binder can therefore influence the mass transport to the electrode, particularly proton transport (which is essential for the reduction to atomic hydrogen). Secondly, the binder controls the extent to which hydrogen atoms are available on the surface of the electrode as a reducing agent. Finally, it is assumed (without this being experimentally verified) that the binder can also significantly influence the silver structure formed on the electrode surface and thus, to a certain extent, control the proportion of Z-isomers formed or the isomerization from the Z-isomer to the E-isomer.

[0033] Ionomers, resins (with or without ion-exchange groups), or polymeric binders have proven particularly suitable in the above sense. Among polymeric binders, binders selected from the group consisting of polyolefins, fluorinated polyolefins, polyaromatic polymers, copolymers with polyolefins and / or fluorinated polyolefins, and polymer blends containing polyolefins and / or fluorinated polyolefins are particularly suitable. Examples of polyolefins and fluorinated polyolefins include, in particular, polyethylene, polypropylene, polyvinylidene fluoride (PVDF), fluoroethylene-propylene (FEP), and polytetrafluoroethylene.Ionomers include polymers and copolymers containing sulfonic acid groups, particularly polymers and copolymers based on perfluorosulfonic acid and hydrocarbon-based polymers and copolymers (e.g., with aromatic hydrocarbons) containing sulfonic acid groups, commercially available, for example, as Nation® from DuPont. Other ionomers include polymers and copolymers containing ammonium groups or polymers with quaternized nitrogen heterocycles, commercially available, for example, as Sustainion® and Fumion® FAA3 from Fumatec.

[0034] According to the application, it was recognized that binders that contain at least partially fluorinated polymers and / or ionomers, which in particular typically contain at least 50 wt.% fluorinated polymers and / or ionomers and which often consist of fluorinated polymers and / or ionomers, are particularly favorable for the electrocatalytic reduction of alkynes, as they achieve high Faradaic efficiencies and high selectivities with respect to the trans product. PVDF, PP, and PTFE, as well as Nafion® and fluorine-free Fumion® as ionomers, have proven particularly suitable.

[0035] According to a further embodiment, the proportion of binder in the electrode or in the catalytically active electrode coating (based on the total weight of the electrode or electrode coating—where the silver weight refers to the silver species during operation of the electrochemical cell) is 0.1 to 40 wt.%, for example 1 to 33 wt.%, frequently 3 to 30 wt.%, and often 5 to 25 wt.%. With such silver / binder ratios, the small silver regions advantageous according to the application can be achieved particularly well. Furthermore, it has been shown that in these weight ranges, the formation of the E isomer can be enhanced with a larger binder proportion.

[0036] A catalyst layer with a binder can be applied in two ways. Either the finished layer can be applied to the support material, or, in rarer cases, only the binder can be deposited first on the support material, followed by the actual catalyst, i.e., the silver or silver compound. Various methods are known to those skilled in the art for applying a binder layer, or optionally also a layer of binder and catalyst, to the support material; in particular, these include doctor blade coating, pressing, spray coating, magnetron sputtering, and electrospinning. Electrospinning has the advantage of being a simple process that enables the creation of highly porous structures from smooth, nonwoven fibers with interconnected, uniform pores.

[0037] According to a further embodiment, either a porous support material is used on which the catalyst coating or the formed silver islands are at least partially present on the inner surface; alternatively, a porous catalyst can be used in which the silver islands crucial for the reduction of the alkyne are also present on the inner surface. In the simplest case, a microporous carbon support is considered as the porous support material, which comprises, for example, carbon black as a carbon-containing material mixed with a (second) binder. The (second) binder is typically a hydrophobic polymer such as polytetrafluoroethylene (PTFE) to prevent penetration of an aqueous electrolyte, which is particularly relevant when the alkyne is not dissolved in this aqueous phase.Furthermore, layered systems are also suitable as porous support materials, for example, layered systems consisting of porous metal layers with gas-permeable polymer layers arranged thereon, the surface of which is formed by a porous metal substrate (e.g., a metal grid) and a membrane-like polymer layer of a hydrophobic polymer. The catalyst layer can be arranged either on the metal substrate or on the polymer layer.

[0038] According to a further embodiment, the method according to the application is carried out in such a way that a current density of at least 10 mA*cm' 2 Typically, at least 50 mA*cm' 2 applied and often at least 100 mA*cm' 2 It has been found that even with higher current densities sufficient Faraday efficiencies can be achieved (they are often greater than 90% at more than 50 mA cm -2and often greater than 85% at more than 100 mA cm -2 ). Accordingly, the process according to the application is suitable not only for laboratory operation, but can also be easily applied to industrial processes. In addition, it should be noted that a certain increase in selectivity with respect to the formed trans isomer can also be achieved by adjusting the current density.

[0039] In summary, the presented electrochemical process, due to its intrinsic properties, offers the possibility of performing the reduction from alkyne to alkene continuously and on an industrial scale. Furthermore, the process can replace conventional hydrogenation reactions as a sustainable synthesis method when using renewable energy.

[0040] To achieve particularly high proportions of the E isomer in the crude product, it is often advisable to use particle sizes of up to 200 nm with binder contents of at least 5 wt.%, especially at least 10 wt.%, and simultaneously employ a polyolefin or fluorinated polyolefin as the binder. Separation of the E and Z isomers can be achieved, as is known to those skilled in the art, for example, by chromatography, distillation, and the like.

[0041] The object of the application is also achieved by an electrochemical cell for the electrocatalytic hydrogenation of alkynes. This cell comprises a reaction chamber, a cathode in the reaction chamber, an electrolyte, an anode, and an ion exchange membrane arranged between the anode and cathode. The reaction chamber has an inlet for a substrate and an outlet for the reaction products formed. The cathode comprises or consists of elemental silver as a catalyst and / or comprises a silver compound that is reducible to elemental silver during operation of the electrochemical cell.

[0042] The electrolyte can be liquid or solid, and the electrochemical cell can be a flow cell or a batch cell. Further optional equipment features are described in the process description above.

[0043] Without limiting its generality, the method according to the application and the electrochemical cell are described in more detail below using examples and figures.

[0044] Figure 1 shows an electrochemical flow cell for electrochemical reduction. A (bi-)polar plate with a flow guide 203 is arranged on each of the two end plates 201, each with an insulator layer 202 arranged thereon. The cathode 204 is arranged on one of the bipolar plates 203 (the electrocatalyst layer is not shown for clarity). The cathode 204 is followed by an electrolyte compartment 206, an anion exchange membrane 207, another electrolyte compartment 206, and an anode 208. Sealing is also achieved here by means of several seals 206. Also not shown are a reservoir for the substrate, as well as the connections for supplying the reactants and the outlets for the formed products.

[0045] Figure 7 shows a zero-gap solid electrolyte cell for electrochemical reduction. A (Bi-)polar plate with flow guide 103 is arranged on each of the two end plates 101, each with an insulator layer 102 arranged thereon. The cathode 104 (which comprises the cathode support material, the electrocatalyst layer, and the solid electrolyte) is arranged on one of the bipolar plates 103. The cathode 104 is followed by an anion exchange membrane 106 and an anode 107. The spaces between the cathode 104 and the anion exchange membrane 106 on the one hand, and the anion exchange membrane 106 and the anode 107 on the other, are sealed to the outside with a seal 106. Not shown are a reservoir for the substrate, as well as the connections for supplying the reactants and the outlets for the formed products. of butynediol at an electrode with Si icles on

[0046] 2-Butyne-1,4-diol is electrocatalytically hydrogenated to the corresponding (E / Z)-alkenes in 0.3 M aqueous KOH solution. A carbon fabric (W1 S1010 - CeTech) serves as the cathode, onto which an ink consisting of silver nanoparticles (Alfa Aesar, Ag Nanoparticles APS 20-40 nm, 99% - measured by laser diffraction: D 10 : 32nm, D 50 : 38 nm, D 90 : 49 nm) and a binder according to Table 1 (given as mass percent based on the total mass of binder and silver) at 80°C, so that a loading of 2 mg cm -2 The fluid or solvent used for the binder is an isopropanol / water mixture or (in the case of PVDF) methanol. Electrolysis takes place in a flow cell as shown in Figure 1 at 100 mA cm -2The alkenes are obtained with the Faradaic efficiencies (FE) and selectivities shown in Figure 2. The total Faradaic efficiency (for the formation of E and Z isomers) is indicated by the triangles on the left, the Faradaic efficiencies for the formation of the (Z)-alkene are shown by the bars on the left, and that of the (E)-alkene by the bars on the right; the (E)-selectivity is indicated by the asterisks in the right column (the triangles and asterisks are also used accordingly in all subsequent figures). It can be seen that the binder plays an important role in selectivity and Faradaic efficiencies. Different binders and binder mixtures are shown. Here, FAA3 stands for the fluorine-free, anion-conducting ionomer Fumion® FAA-3 from Fumatec, PTFE for polytetrafluoroethylene, PVDF for polyvinylidene fluoride and Naf (or Naf) for 2-[1-[difluoro[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2-tetrafluoroethanesulfonic acid.The (E)-selectivity is consistently very high, and the Faradaic efficiencies are above 75% in most cases. Table 1 shows the absolute values ​​for the binders and binder mixtures mentioned. The results are the mean values ​​from measurements at two electrodes, with the standard deviation given. According to the application, the Faradaic efficiencies were always determined after completion of the electrolysis from the quantification of the resulting alkenes. 1 H NMR determined using potassium hydrogen phthalate as internal standard in D2O.

[0047] Table 1

[0048] Binder i Binder 2 FE A | ken [%] FE cis [%] FE^ns [%] Selectivity trans [%]

[0049] 10% Fumion 12% PVDF 82.0 25.0±2.1 57.0±12.0 69.5

[0050] 10% Fumion - 63.6 16.9±6.1 46.7±5.9 73.5

[0051] 12% Fumion 12% PTFE 52.5 8.3±0.2 44.2±0.9 84.2

[0052] 10% Nation - 75.2 22.3±8.8 52.8±4.6 70.3

[0053] 12% Nation 12% PTFE 79.2 17.4±0.8 61 .8±4.2 78.0

[0054] 8.2% Nation 10% PVDF 83.7 29.3 54.4 65.0

[0055] 10% PTFE 10% PVDF 83.2 17.9 65.3 78.5

[0056] 5% PTFE 5% PVDF 87.2 21.7±3.4 65.5±7.4 75.2

[0057] 10% PTFE 1% PVDF 87.2 18.6±0.7 68.5±2.4 78.6

[0058] 10% PVDF - 87.0 14.5±3.4 72.5±3.8 83.3

[0059] Furthermore, the influence of current density on the (E / Z) selectivity is shown. For the electrodes according to rows 3 and 4 of Table 1, Figure 3 and Table 2 show that, depending on the binder, there is also a change in the current density (given in mA*cm' 2) influences the selectivity. Thus, with higher current densities, the trans selectivity can be increased with NaF, whereas with FAA3 / PTFE, the high trans selectivity of over 80% is maintained while simultaneously significantly increasing the Faradaic efficiency. The higher Faradaic efficiencies at lower current densities are due to mass transport limitations. Table 2

[0060] Binder current density FE^ [%] FE cis [%] FE tens [%] Selectivity trans [%]

[0061] 12% PTFE / 10% Fumion -50 71 1 1 60 85

[0062] 12% PTFE / 10% Fumion -100 52.5 8.3±0.2 44.2±0.9 84.2

[0063] 12% PTFE / 10% Fumion -140 50 7 43 86

[0064] 10% Nation -50 92 41 51 55

[0065] 10% Nation -100 75.2 22.3±8.8 52.8±4.6 70.3

[0066] 10% Nation -140 75 21 54 72

[0067] Example 2 - Isomerization experiments of butenediol

[0068] To investigate possible isomerizations, the alkene 2-butene-1,4-diol (cis:trans 97:3) was used as the starting material. Otherwise, the same experimental setup as in Example 1 was chosen, using 10% NaCl / 12% PTFE as the binder. Without applying a potential, no isomerization of the cis-alkene to the trans-alkene was observed even after 20 hours. Even with successive increases in the potential, only an increasing isomerization from the Z-isomer to the E-isomer was observed. The results are presented in Table 3. Therefore, it can be concluded that the significant influence on the formation of Z-isomers or E-isomers occurs during the electrocatalytic production.

[0069] Table 3

[0070] Time [h] Potential vs. RH E [V] Cis [%] Trans (%)

[0071] 0 - 97 3

[0072] 20 - 97 3

[0073] 2 -0.500 98 2

[0074] 2 -1,000 97 3

[0075] 2 -1,500 96 4 Electrode with different

[0076] Silver areas on a

[0077] To investigate how smaller and larger silver surfaces affect the selectivity for the formed isomers, different particle sizes were used, and agglomeration was induced by thermal treatment. Instead of the nanoparticles from Example 1, silver particles with an average diameter of 2 pm from Alfa were used. Alternatively, the electrodes from Example 1 can be thermally treated at 240 °C, resulting in agglomeration of the Ag nanoparticles. Here, too, a loading of 2 mg cm -2Figure 4 shows the result of an electrocatalytic fluorination of 2-butyne-1,4-diol, carried out according to Example 1, except for the different silver particles. It can be seen that both the microparticles ("pm") and the agglomerated nanoparticles ("NPs") - each with PTFE as a binder - show an increased formation of the Z-isomer due to the larger silver areas; compared to the results from Example 1 (e.g., silver nanoparticles with PVDF as a binder / row 8 in Table 1), the cis / trans ratio is almost reversed. of butynediol on an electrode with Si icles on slightly acidic environment

[0078] In contrast to the binder and the silver particle size, the pH of the electrolyte has no significant influence on the cis / trans ratio. When using an electrode according to Example 1 with 10% PVDF as a binder, a trans selectivity of 61% (FE cis= 23%; FE trans = 37%) was also obtained when using a 1 M phosphate buffer (pH = 6) instead of 0.3 M aqueous KOH.

[0079] Under the same conditions as in Example 1, 2-butyn-1-ol is electrocatalytically hydrogenated instead of butynediol. Slightly lower efficiencies (42% and 54%, respectively) for the alkene and a slightly increased H2 formation are obtained. However, the (E) selectivity is also 72% (with 10% FAA3 / 12% PVDF as binder) and 66% (with 10% PVDF as binder).

[0080] Figure 5 shows the total Faradaic efficiency (for the formation of E and Z isomers) as squares on the left. The Faradaic efficiencies for the formation of the (Z)-alkene are shown by the bars on the left and those for the (E)-alkene by the bars on the right. The (E)-selectivity is indicated by the asterisks in the right column. Compared to the values ​​from Example 1 shown in the upper half of Figure 5, slightly lower efficiencies are found for butynol (lower half); however, the selectivity remains unchanged (since the binder, current density, and silver particle size have not changed).

[0081] 6 - Flow of butynediol in a zero-gap cell at an electrode with

[0082] The electrolysis takes place in a zero-gap solid electrolyte cell (marked ZG) according to Figure 7 at 80 mA cm -2In contrast to the flow cell (marked with F) according to Example 1, only the cells are swapped here. 10% PVDF was used as a binder for the cathode (cf. Table 1, line 8). Figure 6 shows the differences between (top) a flow cell according to Example 1 and (bottom) the solid electrolyte cell according to this example. The alkenes are obtained in the Faraday efficiencies (FE) and selectivities according to Figure 2. Very high E-selectivities are also obtained in the zero-gap cell. However, the Faraday efficiencies are slightly lower and, in addition, there is a proportionally higher alkene content in the anolyte (A) compared to the catholyte (C) than in the flow cell.

[0083] The results show that the type of electrocatalytic cell does not have a significant influence on the formation of the different isomers. of butynediol at an electrode with

[0084] In contrast to example 6, a zero-gap cell was used with a cathode containing copper nanoparticles as a catalyst (measured by laser diffraction: D 10 : 52nm, D 50 : 71 nm, D 90 : 1 17 nm; 2 mg cm -2 , 10% PVDF as a binder on graphite fleece from SGL Carbon). The other conditions correspond to those of Example 6. In addition to the anion exchange membrane / AEM (Fumatech FM-FAA3-PK-130) used there, alternative experiments were also carried out with a bipolar membrane (reverse bias) / BPM (Fumatech FM-FBM-PK). In contrast to the embodiment according to the invention, no E-isomers were obtained with copper nanoparticles. In addition, the Faraday efficiencies for the Z-isomers are very low. This does not change even when experimenting with a lower current density (20 mA cm -2 Figure 8 shows the results obtained by 1 H-NMR spectroscopy of the product solution in D2O.

[0085] 8 - Hydrogenation of butynediol in a zero-gap cell at an electrode with different

[0086] In contrast to Example 6, in the zero-gap cell, electrodes with silver particles of different sizes are used as catalyst (2 mg cm -2 , 10% PVDF as a binder on graphite fleece from SGL Carbon). The tests were carried out with the nanoparticles already used in Example 6 and with silver particles with a particle size of 150 nm (Sigma Aldrich - measured by laser diffraction: D 10 : 229nm, D 50 : 312 nm, D 90: 513 nm). Figure 9 shows that the nanoparticles, on the one hand, result in better Faradaic efficiencies and, on the other hand, that a significant excess of the E isomer can be achieved only with the nanoparticles. This applies equally when using an anion exchange membrane / AEM (Fumatech FM-FAA3-PK-130) or a bipolar membrane (reverse bias) / BPM (Fumatech FM-FBM-PK) and a current density of 80 mA cm -2 However, in comparison to Example 3, it is also evident that the formation of the E isomer is preferred for particle sizes of 150 nm and smaller. Example 9 - Hydrogenation of butynediol in a zero-gap cell at different current densities and binder contents

[0087] Under the same conditions as in Example 9, using the silver nanoparticles from Example 1, the proportions of the PVDF binder (expressed in wt. %) and the current intensities are varied. Figure 10 shows that with increasing binder content, the selectivity towards the E isomer increases, and that a slightly increased formation of the E isomer is observed even at higher current densities. This applies equally to an anion exchange membrane (AEM) and the bipolar membrane (BPM).

[0088] Example 10 - Hydrogenation of butynediol in a zero-gap cell using a silver membrane as electrode

[0089] In contrast to Example 6, electrodes made of pure silver, rather than catalyst layers made of silver / binder mixtures, are used. Various Ag membranes (Sterlitech), often used as filters due to their porosity (pore sizes of 0.45, 0.8, 3.0, and 5.0 μm), are used for the electrochemical hydrogenation of 2-butyne-1,4-diol in polymer electrolyte cells.

[0090] In addition, different carbon materials are used as carrier materials under the silver membranes.

[0091] Figure 11 shows that with the two support materials (H23 stands for carbon paper from Freudenberg; CF for carbon fleece from SGL Carbon Sigracell GFD 2.5) and without support material (neat), significantly larger proportions of cis-alkenes (shaded areas of the bars) are formed, although a certain proportion of trans-alkenes is also formed. Ag membranes with larger pores tend to show somewhat better Faraday efficiencies (FE); with the use of carbon fleece, a tendency toward trans-alkenes is still most observed, especially at pore sizes of 0.8 pm.

[0092] The results show that flow effects have a certain impact on cis / trans selectivity. However, this effect is far from comparable to the effects of binder, current intensity, and silver particle size.

Claims

Patent claims 1 . A process for the electrocatalytic hydrogenation of alkynes in an electrochemical cell comprising a cathode, an anode and an electrolyte, wherein the alkyne is in gaseous, liquid or at least partially dissolved form and is hydrogenated at the cathode, wherein the cathode comprises or consists of elemental silver as catalyst and / or comprises a silver compound which is reduced to elemental silver during the electrocatalytic reaction.

2. Process according to the preceding claim, wherein the electrode used as cathode comprises a carrier material and a catalyst layer arranged thereon, wherein the catalyst layer comprises or consists of the elemental silver and / or the silver compound.

3. Method according to one of the preceding claims, wherein the catalyst or the catalyst layer is present at least in partial areas of the surface of the cathode.

4. A method according to any one of the preceding claims, wherein the silver is present as silver powder, in the form of a porous silver membrane or in the form of silver deposited on the cathode surface, and / or the silver compound is a silver salt.

5. Method according to the preceding claim, wherein the silver powder at least partially has particle sizes of less than 1 pm, in particular less than 0.5 pm, for example less than 100 nm, or the areas of deposited silver at least partially have an average diameter of less than 1 pm, in particular less than 0.5 pm, for example less than 100 nm.

6. Process according to one of the preceding claims, wherein an internal alkyne is used as the alkyne.

7. The method according to any one of the preceding claims, wherein the catalyst or the catalyst layer additionally comprises a binder or consists of a binder and the silver and / or the silver compound.

8. The method according to the preceding claim, wherein the binder is an ionomer, a resin, an ion exchange resin or a polymeric binder, wherein the polymeric binder is in particular selected from polyolefins, fluorinated polyolefins, copolymers with polyolefins and / or fluorinated polyolefins and polymer blends containing polyolefins and / or fluorinated polyolefins, wherein the polyolefins and fluorinated polyolefins are in particular selected from the group consisting of PE, PP, PVDF, FEP and PTFE.

9. Process according to the preceding claim, wherein the binder comprises or consists of PVDF, PTFE, PP and / or a cation-conducting or anion-conducting ionomer.

10. Process according to one of the three preceding claims, wherein the catalyst layer or the catalyst has a binder content of 0.1 to 40 wt.%, in particular 3 to 33 wt.%, for example 10 to 25 wt.%. 1 1. A method according to any one of the preceding claims, wherein the catalyst layer is porous or the support material is at least partially formed from a porous material and the catalyst layer is arranged as a coating at least partially also on the inner surface of the porous support material 12. Process according to one of the preceding claims, wherein the electrocatalytic hydrogenation is carried out in batch mode or in continuous flow mode.

13. Method according to one of the preceding claims, wherein the applied current density is greater than 10 mA cm -2 , for example greater than 50 mA cm -2 and especially greater than 100 mA cm -2 amounts. Use of an electrode comprising silver or a silver compound for the electrocatalytic hydrogenation of alkynes. An electrochemical cell for the electrocatalytic hydrogenation of alkynes, comprising a reaction chamber, a cathode in the reaction chamber, an electrolyte, an anode, and an ion exchange membrane arranged between the anode and cathode. The reaction chamber has an inlet for a substrate and an outlet for the reaction products formed. The cathode comprises or consists of elemental silver as a catalyst and / or comprises a silver compound that is reducible to elemental silver during operation of the electrochemical cell.

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