Three-phase electrode for redox reactions

Abstract

The invention relates to a method for carrying out redox reactions with a three-phase electrode, having two separate phases, comprising an organic phase containing an amphiphilic organic reactant and / or an electrochemically inert organic solvent and an aqueous phase containing a hydrophilic reactant and / or an aqueous solvent with a hydrophilic reactant, wherein the organic and the aqueous phase are separated from one another by a permeable membrane.

Description

[0001] THREE-PHASE ELECTRODE FOR REDOX REACTIONS

[0002] The invention relates to a method for carrying out redox reactions with a three-phase electrode having two separate phases, comprising an organic phase containing an amphiphilic organic reactant and / or an electrochemically inert organic solvent and an aqueous phase containing a hydrophilic reactant or an aqueous solvent with a hydrophilic reactant, wherein the organic and aqueous phases are separated from each other by a permeable membrane.

[0003] To date, many materials are made from petrochemicals, which are byproducts of refining crude oil into combustible fuels. As the transportation sector increasingly transitions to battery-powered electric vehicles, the need for conventional fuels will decrease, changing the business case for refineries. Indeed, it is not clear whether it is economically viable to operate a crude oil refinery solely for the purpose of producing petrochemicals. This has sparked interest in biomass, which can be refined into important commodity chemicals.

[0004] For example, it is possible to produce FDCA (2,5-furandicarboxylic acid) by oxidation of HMF (5-(hydroxymethyl)furfural) derived from fructose, which can replace terephthalic acid derived from petroleum in PET (polyethylene terephthalate).

[0005] Furthermore, 4-ethylguaiacol, a catechol derivative that is an important building block in a number of industries, can be obtained by hydrodeoxygenation of wood-derived acetovanillone. Biomass upgrading is therefore often achieved either by oxidation or hydrogenation of carbonyl compounds. These reactions are best carried out electrochemically, as this is a much milder process than thermal catalysis. At the high temperatures used in thermal catalysis, decomposition reactions of the biomass occur. The polymers formed ultimately deactivate the catalyst used and clog the reactor, hindering further processing. Scheme 1 below also shows that H +is a reactant in the electrochemical hydrogenation of carbonyl compounds, while OH' is a reactant in the electrochemical oxidation of aldehydes. Accordingly, the electrochemical reaction is only maintained if both the ion (H + or OH') and the organic reactant (carbonyl compound) are present at the electrode.

[0006] Aceto-4-ethyl-vanillone guajacole

[0007] Scheme 1

[0008] The electrochemical oxidation and reduction of carbonyl compounds proceed best in electrolytes with very high or very low pH values. However, H +or OH' ions also lead to the decomposition of the organic reactant by aldol condensation or other reactions, which is problematic because it wastes valuable starting materials and the formed by-products or decomposition products deactivate the electrode and / or require complex workup of the products.

[0009] Furthermore, efforts are also being made to catalyze other chemical reactions, such as those used in lignin-cellulose recycling. The following Scheme 2 shows a possible process sequence for the recycling of lignin-cellulose. In this process, lignin is separated from cellulose and hemicellulose using the established Kraft process. In order to be recycled, the resulting Kraft lignin must be desulfurized. In a second material stream, hemicellulose and cellulose can be converted into furfural or 5-(chloromethyl)furfural (CMF) by treatment with HCl. In Scheme 2, benzene and toluene can be produced by oxidation of the aldehyde function, reductive removal of the chlorine atom, the Diels-Alder reaction with ethylene, and decarboxylation.

[0010] Power

[0011] process

[0012] “crope residues" desulfurization raw material,

[0013] Power Lignocellulose lignin

[0014] chemical

[0015] Cellulose /

[0016] Hemicellulose Treatment with HCl

[0017] It is advantageous to perform hydrodesulfurization, hydrodechlorination, and oxidation of the aldehyde function electrochemically. It should be noted that the acidic reaction conditions required for electrochemical hydrodechlorination and electrochemical hydrodesulfurization can initiate decomposition reactions in the reactant, which are not necessarily limited to the functional groups to be electrochemically modified (thiol, halogen), but can also occur at other functional groups.

[0018] With the help of the three-phase electrode it is possible to prevent electrolyte-induced decomposition of aldehydes during electrochemical redox reactions.

[0019] There are other multiphase methods for the electrochemical conversion of organic compounds to prevent side or decomposition reactions, such as the emulsion process, but these have other disadvantages. In the emulsion process, the organic reactant is typically dissolved in a nonpolar organic solvent, and the solution is then dispersed in the aqueous electrolyte to form an emulsion.

[0020] However, due to the large surface area of ​​an emulsion, the organic and ionic reactants often collide at the liquid-liquid interface, which would not prevent the side reactions described above. Furthermore, it can be difficult to create and / or maintain a high degree of dispersion in the reaction mixture, necessitating more complex process control.

[0021] Other processes and devices are already known from the prior art, which comprise an electroconversion of carbonyl compounds in immiscible phases.

[0022] For example, US 2020385875 A1 describes porous electrodes that contact two immiscible liquid phases and are passable for reactants / products. Amphiphilic electrodes are disclosed that have a pore size of 0.1 pm to 500 pm. Desulfurization, hydrodesulfurization, or reduction of thiols / mercaptans are not explicitly mentioned, but only the oxidation of thiols / mercaptans is described. Hydrodechlorination is also not explicitly mentioned, but dichloromethane is only mentioned as the "first nonpolar solvent" and not as the "first organic material" that would be subjected to the electrochemical reaction as a reactant.

[0023] Furthermore, US2021 / 0062349 A1 describes membranes, electrodes, electrocatalytic systems, and methods for reducing CO2. In particular, the presented embodiments offer improved efficiencies and current densities for the production of useful hydrocarbons from CO2 reduction.

[0024] WO 2021209585 A1 discloses electrodes for the conversion of organic compounds, e.g., the oxidation of aldehydes. The electrode separates two immiscible phases and consists, for example, of PTFE with an additional hydrophobic layer.

[0025] US 4455208 A describes an electrochemical reduction in which organic product changes into the organic phase

[0026] US 2002014417 A1 discloses an electrochemical cell for the oxidation of organic compounds and an electrocatalytic oxidation process.

[0027] In WO 2007007343 A2, a gold-coated porous PTFE membrane is used as an electrode.

[0028] DE 1596004 A1 describes a catalyst for the electrochemical oxidation of organic compounds and a process for its preparation, in particular aldehyde oxidations in aqueous solution.

[0029] JP 2007039737 A shows a gold-coated porous PTFE membrane as an electrode.

[0030] And from CN 110106514 A a porous electrode for aldehyde oxidation with two liquid phases is known.

[0031] Finally, WO 2020036675 A1 shows porous electrodes for aldehyde oxidation with two liquid phases in countercurrent, with gold and PTFE.

[0032] The object of the invention is therefore to develop new strategies which make it possible to achieve high current densities in the electroconversion of oxygenated and haloalkyl compounds and at the same time to avoid their electrolytically induced decomposition, wherein the reactants are present separately from one another in two immiscible phases before the reaction. The object is achieved by the features of independent claim 1.

[0033] A process is claimed for the conversion of oxygenated compounds using a three-phase electrode, in which an amphiphilic, organic reactant is provided alone or dissolved in an electrochemically inert solvent and which is separated by a hydrophobic and metal-catalytically coated permeable membrane from a hydrophilic, ionic other reactant, which is present alone or dissolved in an aqueous solvent.

[0034] According to the invention, an amphiphilic organic reactant is understood to mean an organic compound or an organic molecule that has both lipophilic and hydrophilic properties and dissolves, for example, in organic solvents, but can also pass into an aqueous phase.

[0035] According to the invention, a membrane is understood to be a sheet-like structure that can be rigid or flexible and is suitable for acting as a support for the electrocatalyst. A membrane according to the invention has porous structures with a pore diameter n of 800 nm or less, which enable direct contact between the organic phase and the aqueous phase within the pore structure and are intended to facilitate unhindered transfer of the respective reactant from the organic to the aqueous phase. At the same time, unhindered transfer of the respective reactant from the aqueous to the organic phase is prevented.Therefore, the special combination of the material properties of the membrane and the electrocatalyst ensures that a three-phase boundary is formed between the electrocatalyst, the organic phase and the aqueous phase, so that the phase transition of the organic reactant from the organic to the aqueous phase occurs in close proximity to the electrocatalyst.

[0036] According to the invention, a porous structure means that the membrane does not have to be completely covered with continuous pores, but at least in some areas has pores that ensure the transfer of a reactant from the organic into the aqueous phase. Both reactants are available for the electrochemical reaction if they are located at the phase boundary between the two liquids in the immediate vicinity of the catalyst. The membrane has a metal catalytic coating at one end, i.e. on one side of the membrane, which is advantageously present on at least one side of the membrane, wherein the porous structures can be at least partially coated with metal catalytic coating so that after contact of the respective reactants with the electrochemically active, metal catalytic coating, a redox reaction takes place and an unhindered transfer of the respective reactant from the organic phase into the aqueous phase can take place.

[0037] On the other hand, the membrane exhibits hydrophobicity or lipophilicity due to its structure or base, and its pore structures prevent the unhindered transfer of the respective reactant from the aqueous to the organic phase. Provided that one of the reactants crosses the phase boundary and both reactants are thus available at the electrode simultaneously, an (electro)chemical reaction is possible. At the same time, a chemical, electrolytically induced side or decomposition reaction is suppressed. This is achieved by electrochemically consuming the organic reactant immediately after the phase transition and thus making it unavailable for the chemical, electrolytically induced side or decomposition reaction.To achieve this, the phase boundary between the organic phase and the aqueous phase must be located directly at the electrocatalyst, eliminating the need for the organic reactant to reach the electrocatalyst through lengthy diffusion processes. This is achieved through the special design of the three-phase electrode (TPB).

[0038] The transition between the hydrophobic membrane and the hydrophilic electrocatalyst within the pores of the membrane means that the phase boundary between the organic phase and the aqueous phase forms directly at the electrocatalyst. Thus, the contact angle of the aqueous phase at the membrane is θ greater than 90°, preferably θ greater than 91°, particularly preferably θ greater than 107°. Due to the hydrophobicity of the membrane and the small pore size n < 800 nm, the aqueous phase is unable to penetrate the pores of the membrane. It exclusively wets the electrocatalytic film or penetrates the pores of the membrane only to the extent that the pore walls of the membrane are coated with the electrocatalyst. The membrane according to the invention consists in particular of PTFE or hydrophobized polycarbonate or hydrophobized aluminum oxide. However, this list is not exhaustive.Other hydrophobic compounds can also be used for membrane production if a different, more advanced membrane design appears necessary. The contact angle of the aqueous phase on the electrocatalyst used is 90° at most, preferably 86°. Furthermore, the contact angle of the organic phase on the electrocatalyst used is larger than that of the aqueous phase, and the contact angle of the organic phase on the membrane is smaller than that of the aqueous phase, and is 90° at most.

[0039] At the phase boundary, the side or decomposition reaction is suppressed if the reactant crossing the phase boundary is consumed by the electrochemical reaction immediately after crossing, thus reducing its concentration at the phase boundary to near zero and remaining permanently near zero throughout the duration of the electrochemical reaction. This requires that the rate of the electrochemical reaction is greater than the rate of the phase transition by selecting the process parameters (type of organic solvent, reactant concentration, applied potential, pore size, electrocatalyst, or electrolytically active metal layer of the membrane).

[0040] It was discovered according to the invention that in addition to the redox reactions of aldehydes and alcohols, reactions of thiols and haloalkanes with the TPB are also possible.

[0041] Thus, hydrodesulfurization of thiols, some of which are poorly soluble in water, can be carried out with current densities in the range of approximately 10 mA / cm 2 The electrochemical process used here allows the use of aqueous electrolytes in which oxygen evolution or aldehyde oxidation is possible as an anodic counter-reaction. Due to the high availability of protons during the phase transition, electrochemical desulfurization of thiols, such as kraft lignin, can be achieved.

[0042] This is important because the hydrodesulfurization of thiols is relevant for the desulfurization of kraft lignin. The kraft lignin produced during paper production contains large amounts of thiols. The sulfur in the thiols acts as a catalyst poison during the chemical processing of kraft lignin, and the unpleasant odor caused by the thiols prevents the use of kraft lignin as a building or insulation material. Therefore, kraft lignin is currently primarily utilized thermally. In thermal catalysis, several processes exist that allow the desulfurization of organic compounds. This process uses molecular hydrogen at elevated pressures and temperatures. The associated hazards (thermal breakdown of the reaction, risk of explosion) require expensive safety measures.

[0043] An electrochemical process for the desulfurization of thiols is significantly safer because thermal runaway of the reaction is not possible and high pressures are not required. However, due to the low solubility of many thiols in aqueous electrolytes with low pH, desulfurization is only possible with low current densities. Even in organic electrolytes, the reaction only occurs with moderate current densities due to the limited availability of protons relevant for the reaction. Furthermore, hydrodesulfurization in organic electrolytes raises the question of the anodic counter-reaction. The oxidation of the thiols does not necessarily lead to their desulfurization. Partial or total oxidation of the electrolyte (solvent or supporting electrolyte) at the anode is generally undesirable. For this reason, expensive sacrificial anodes are often used in electrochemical processes in organic electrolytes.

[0044] The use of the TPB electrode has the advantage that desulfurization occurs at the interface between the organic electrolyte and the aqueous phase. This allows for oxygen evolution as an anodic counter-reaction.

[0045] Furthermore, the TPB electrode is intended for use in reactions with haloalkanes. Due to the limited availability of protons, the hydrodechlorination of alkyl halogens from organic electrolytes is currently only possible with limited current densities. Furthermore, reduction reactions in organic solvents pose the problem of the anodic counter-reaction, which should not result in the electrochemical degradation of the electrolyte. This means that it is generally undesirable for the anodic counter-reaction to partially or completely oxidize the solvent or the supporting electrolyte. This is often prevented by the use of expensive sacrificial anodes.

[0046] By using the TPB electrode, hydrodechlorination can occur at the interface between the organic and aqueous phases. This enables oxygen evolution as an anodic counter-reaction and allows the use of aqueous electrolytes in which oxygen evolution or aldehyde oxidation occurs as an anodic counter-reaction. Due to the high availability of protons, current densities of approximately 1 mA / cm 2 can be achieved.

[0047] In order to achieve the electrochemical conversion of the reactants at the three-phase boundary between the organic phase, the aqueous electrolyte and the solid electrode, a hydrophobic membrane is coated at one end, i.e. on one side, with an electrocatalyst. The membrane can, for example, be made entirely of PTFE or hydrophobic polycarbonate or hydrophobic aluminum oxide, or it can have a base body that is only coated with a PTFE or hydrophobic polycarbonate or hydrophobic aluminum oxide film and that, if appropriate, only consists of the compounds PTFE or polycarbonate or aluminum oxide in sections and not across the entire surface or on both sides, and thus only partially. To provide the TPB, an electrocatalyst is then applied as a layer to at least one side of the membrane as an electrode to provide the TPB. In particular, the electrocatalyst can be located in the area of ​​the porous structures.The membrane therefore has a hydrophobic structure as a whole, with the electrocatalytic coating being located at one end (i.e. on one side of the membrane) on the hydrophobic structure made of PTFE or polycarbonate or aluminum oxide, and at the other end (on the other side of the membrane) the membrane is hydrophobic and / or the electrocatalytic coating is missing.

[0048] The hydrophobic membrane according to the invention has porous structures or pores with a pore diameter n of 800 nm. This pore size, for example, in the case of PTFE, prevents the aqueous phase from penetrating the membrane pores beyond the area wetted by the electrocatalyst, provided the pressure differential is less than 1 bar. Thus, the pore size is also essential to ensure that the phase boundary between the organic and aqueous phases forms in the immediate vicinity of the electrocatalyst. Furthermore, a small pore size is crucial to minimize the residence time after the phase transition of the organic reactant into the aqueous phase and thus suppress the chemical, electrolytically induced side or decomposition reaction.This means that if the phase transition occurs in the center of the pore, the organic reactant must diffuse at least the length of the pore radius to reach the electrocatalyst and initiate a reaction. Therefore, the pore radius should be smaller than or at least within the range of the mean free path of the diffusing organic reactant. Therefore, the mean pore radius of the membrane is less than 800 nm, preferably less than 200 nm, and particularly preferably 20 nm.

[0049] The aqueous electrolyte is located on the side of the membrane where the electrocatalytic coating is located, while the organic phase, with a sufficiently low surface tension, is located on the other side of the membrane. Due to the low surface tension of the organic phase, it can penetrate the porous structures or pores of the membrane according to the invention. Conversely, the aqueous electrolyte only wets the electrode surface but cannot penetrate the porous structures or pores of the membrane and thus wet the membrane, whose pore diameters are smaller than 800 nm. This is due, on the one hand, to the high surface tension of water and, on the other hand, to the hydrophobicity of the membrane during phase crossover.The contact angle of the aqueous phase and the membrane is θ greater than 90°, preferably θ greater than 91°, particularly preferably θ greater than 107°, whereby the membrane can advantageously be constructed at least wholly or partially from PTFE or hydrophobicized polycarbonate or hydrophobicized aluminum oxide. PTFE forms a contact angle θ of 108° to 110° with water. This creates an interface between the aqueous electrolyte and the organic phase on the electrode surface or the membrane surface. In other words, due to the special design of the membrane according to the invention, an interface layer forms between the wetted electrode and the hydrophobic membrane, at which interface an electrochemical conversion takes place and which, through targeted control of the concentration of the reactants and the potential, enables an electrochemical conversion of the reactants free of decomposition and side reactions.

[0050] Various metals can be applied to the membrane surface as an electrocatalytic coating or electrode using established deposition techniques. This can be achieved by evaporation using CVD, PVD, spraying of nanoparticles, spin coating, or electroless deposition, which form a metal-catalytic layer as a sequential coating.

[0051] In this context, sequential coating refers to the coating of porous structures with meta II catalytically active metals, metal oxides, or alloys that macroscopically form a layer but microscopically fill only a portion of the porous structures or pores, thus not clogging the pores. The decisive factor is that the respective organic reactant can pass into the aqueous phase in the immediate vicinity of the metal catalytic layer without prior side or decomposition reactions. The metal catalytic layer thus forms a sequential coating, at least in partial areas of the membrane, that provides a permeable passage and enables an electrochemical reaction of the reactants after the phase transition has occurred.

[0052] The electrolytically active, catalytic layer of the membrane can contain at least one element and / or at least one oxide from main group 5 or 6 or transition group 8 or 1, in particular silver and gold. These metals and / or metal oxides, as well as combinations of these metals, metal oxides, or alloys of these metals, can form the metal-catalytic layer. Examples of metals that can be used, but are not limited to, include cobalt, nickel, copper, gold, silver, and platinum, as well as alloys such as gold-platinum alloys, platinum-indium alloys, platinum-palladium alloys, or nickel-iron alloys, as well as their metal oxides. Transition metal oxides, chalcogenides, and pnictides can also be used as electrocatalysts.

[0053] In a preferred embodiment, the metal catalytic layer can be deposited on the membrane using electroless deposition. The advantage of such electroless deposition is that the process is more scalable than vapor deposition or sputtering and represents an alternative to vapor deposition or sputtering of the metal catalytic layer on the membrane.

[0054] It is also possible to deposit metals, metal oxide alloys or combinations thereof onto the electrode by electrochemical processes, for example by (reactive) co-sputtering, vapor deposition techniques or by surface immobilization of pre-synthesized catalyst nanomaterials by spinning, spraying or dipping, by drop casting, wet impregnation or other processes common to the person skilled in the art.

[0055] The organic phase used in the TPB electrode consists of the organic reactant alone or of an organic reactant and an electrochemically inert organic solvent. Electrochemically inert means that the organic solvent is electrochemically inactive within the potential window in which the reactant undergoes a reaction. Furthermore, the organic phase must be immiscible with the aqueous electrolyte. This latter condition is generally met by solvents from the substance classes (i) aromatics, (ii) ethers, (iii) alkanes, and (iv) haloalkanes. In particular, toluene, diethyl ether, n-heptane, and dichloromethane are suitable for preparing the organic phase as organic solvents for the TPB electrode that are electrochemically inert to many catalysts.

[0056] For the purposes of the invention, organic reactants are understood to mean oxygenated compounds which preferably comprise the substance classes of aldehydes, ketones, thiols, haloalkanes and alcohols, alone or in combination. In the present case, oxygenated compounds are understood to mean compounds which comprise a reactive carbonyl or ketone function or an oxidizable alcohol including a thiol and haloalkanes and which are available alone, i.e. in the pure phase or mixed, as reactant or reactants for the electrochemical conversion. The advantage of using the organic reactant in the pure phase is that quantitative conversion of the reactant can be achieved at a constant reaction rate / current density. A further advantage of using a pure organic phase is that the current remains almost constant during the electrolysis and only drops rapidly towards the end of the electrolysis, once the organic reactant has been consumed.

[0057] A reaction control with a constant current transient during complete conversion of the reactant is not possible in conventional arrangements in which the reactant is present in the electrolyte. This means that in conventional arrangements, the concentration of the reactant decreases continuously over the duration of the electrolysis, so that the reaction rate decreases over time. If the reaction rate is determined, for example, by the diffusion of the reactant to the electrode, the mass transport of the reactant slows with decreasing concentration, and the current decreases exponentially over the duration of the electrolysis in conventional arrangements. If the reaction rate is determined, for example, by the kinetics of the reaction, the current in conventional arrangements decreases according to the reaction order in the reactant.Therefore, quantitative conversion of a reactant is theoretically never achieved in conventional arrangements, and quantitative conversion by practically relevant standards is only achieved after very long electrolysis times due to decreasing reaction rates. This problem can be remedied by using pure organic reactants in the present TPB.

[0058] Furthermore, the transition rate can be controlled by selecting the concentration and identity of the conducting salt in the aqueous phase. In addition to water as a solvent, alkali solutions, especially sodium hydroxide, aqueous solutions of perchlorates, and aqueous solutions of mineral acids, especially sulfuric acid, can be used as aqueous electrolytes.

[0059] Thus, almost decomposition-free reactions of the reactants can be controlled by at least one of the following steps a) to f) in TPB a.) by choosing a suitable concentration of the reactant in the organic or aqueous phase, b.) by choosing a suitable organic or aqueous solvent for the organic or aqueous phase, c.) by choosing the concentration and identity of a conducting salt in the aqueous phase d.) by choosing a metal-catalytically active coating and the pore size of the membrane e.) by choosing a suitable potential and f.) by choosing a suitable reaction temperature.

[0060] In addition, selectivity control for mixtures of several organic compounds can be achieved by concerted selection of at least one of the process steps a) to f).

[0061] Thus, by increasing the reaction temperatures, an exponential increase in current densities is achieved and, for example, current densities of 250 mA / cm 2 in the oxidation of furfural at 50 °Cm, which means that high reaction rates are achieved by choosing the higher temperature range.

[0062] Secondly, the choice of the concentration and identity of the conducting salt in the aqueous phase can allow for control of the reaction by increasing the transfer rate (extraction rate). This allows the extraction rate to be adapted to the kinetics of the electrochemical reaction. This avoids extracting a larger amount of reactants than can be converted. The corresponding excess would otherwise undergo a decomposition reaction.

[0063] The choice of experimental parameters can also enable selectivity control in mixtures of several organic compounds. This allows reactants from mixtures (e.g., chemicals from biomass) to be specifically converted. Chemicals derived from biomass, in particular, are often found in complex mixtures of many components and tend to undergo decomposition reactions if used uncontrolled.

[0064] Controlling the transfer rate (extraction rate), for example, of furfural into the aqueous electrolyte solution, enables selectivity control over the reactants to be extracted and then converted in an electrochemical reaction. The extraction rate depends not only on the nature of the aqueous and organic phases, but also on the type of dissolved substance to be extracted. For other organic molecules, different distribution coefficients and thus different extraction rates result than those of furfural. With a suitable composition of the aqueous electrolyte solution, a significantly different extraction rate of the individual organic components into the electrolyte solution can be achieved. This makes it possible to control which of the various starting materials from a mixture are converted in a subsequent reaction.

[0065] Numerous experiments have shown that the suppression of side or decomposition reactions depends, among other things, on the rate of conversion of the reactants at the electrode and on the concentration of the organic reactant at the electrode. This finding is important because the limiting factor for side- or decomposition-product-free oxidative production, for example, of carboxylic acids from alcohols or aldehydes, depends solely on the rate of distribution of the organic reactant at the electrode, which is slow compared to the rate of the electrochemical reaction at the electrode.

[0066] For example, if both an organic reactant and OH- are present at the interface, side or decomposition reactions can occur. However, if the distribution rate of the organic reactant is slow compared to the rate of the electrochemical reaction, a situation can arise in which the concentration of the organic reactant on the aqueous side of the interface is very low. In other words, if the rate of reaction between the reactants is not determined by the electrochemical conversion but by the rate of phase transition from the organic to the aqueous phase, then the organic reactant is always removed from the aqueous phase faster than it is replenished.This dynamic equilibrium only reduces the concentration of the organic reactant in the aqueous phase to near zero if the organic reactant does not have to undergo lengthy diffusion processes within the aqueous phase to reach the electrocatalyst after the phase transition. This requires that the phase boundary between the organic phase and the aqueous phase is located directly at the electrocatalyst, so that the organic reactant is in close proximity to the electrocatalyst after the phase transition.

[0067] In the electrolyte-induced decomposition of aldehydes, such as aldol condensation, the reaction is second-order for small aldehyde concentrations. Thus, the reaction rate essentially drops to almost zero when the aldehyde concentration at the interface becomes negligible. This occurs when the same amount of reactant is consumed by the organic phase at the electrode surface as is supplied. In this case, no decomposition reaction (side reaction) occurs when the reactant passes into the aqueous phase. This was demonstrated by coupled differential electrochemical mass spectrometry (DEMS) measurements, where, by selecting the appropriate potential and varying the concentration, it was determined that above a certain limit, the current density did not increase further, and at the same time, no significant organic reactant concentration could be measured beyond the electrode surface.Accordingly, it can be concluded that by appropriately selecting the type and concentration of the respective reactant, the choice of the appropriate organic solvent, the selection of the metal-catalytic coating of the electrode, the choice of the pore size, and the choice of the applied potential, a conversion of organic or aqueous reactants free of side reactions or decomposition products can be achieved. It should be noted that this finding primarily applies to the decomposition- and side-reaction-influenced conversion of aldehydes in aqueous solvents. It can also be stated that the electrochemical conversion of organic molecules using TPB is not limited to the oxidation of aldehydes.The variable design of the porous structures on the hydrophobic membrane, the variation of the metal catalytic layer, and the selection and concentration of reactants make it possible to convert other classes of substances with the TPB. For example, it is possible to oxidize alcohols and thiols or to hydrogenate ketones and haloalkanes. In these reactions, the rate of the electrochemical conversion is lower than that of the phase transition. However, in the case of alcohols and ketones, this is less significant than in the oxidation of aldehydes, since the chemical, electrolytically induced side or decomposition reaction of alcohols in alkaline conditions (eliminations) and ketones in acidic conditions (aldol condensation) is slower than that of aldehydes.

[0068] Such a wide range of possible applications of TPB represents a significant economic advantage over previously known processes, particularly for biomass utilization or in the production of platform chemicals or basic or commodity chemicals.

[0069] The invention is explained in more detail using the following exemplary embodiments:

[0070] First, the electroless deposition of silver as a metal catalytic layer on a membrane according to the invention is described:

[0071] The PTFE membrane is exposed to a hydrogen plasma on one side for 100 seconds to obtain a hydrophilic PTFE surface. After this pretreatment, the surface is exposed to an aqueous solution of 0.11 mol / L SnCl2 and 0.48 mol / L HCl for 6 minutes. After washing with water, the membrane is exposed to an aqueous solution of 1.4 mmol / L PdCl2 and 30 mmol / L HCl for 2 minutes.

[0072] To deposit the catalyst layer, the prepared membrane is immersed in a suitable deposition solution. For example, for the deposition of silver, two aqueous solutions, A and B, can be prepared (A: 90 mmol / L glucose, 80 mmol / L sodium tartrate, 10 vol% ethanol; B: 50 mmol / L AgNO3, 278 mmol / L NaOH, 11 vol% NH3*H2O). Immediately before deposition, solutions A and B are mixed in a ratio of 5:2. The treated surface of the PTFE membrane is exposed to this mixture for 45 minutes to obtain a silver-coated PTFE membrane.

[0073] There are a number of deposition solutions known from the literature that allow the deposition of various metal layers (Au, Cu, Ag, Ru, etc.).

[0074] Figures 1 and 2 show the cyclic voltammograms (CVs) of the compounds toluene, diethyl ether, n-heptane and dichloromethane as examples of organic solvents such as aromatics, ethers, alkanes and haloalkanes, respectively, and demonstrate that they are electrochemically inactive in acidic and alkaline electrolytes.

[0075] Figure 1 shows the CVs with v=50 mV / s measured on a gold-TPB electrode (100 nm Au sputtered onto a PTFE membrane with a 20 nm pore diameter) exposed to an aqueous electrolyte of 0.5 M H2SO4, while the backside of the PTFE membrane was exposed to argon or the organic solvent. Figure 2 shows the same measurements as in Figure 1 for an aqueous phase containing 10 mM NaOH at v=50 mV / s.

[0076] The comparison of the CVs in the presence and absence of an organic phase (Figures 1 and 2) shows that the organic solvents used are electrochemically inactive in the potential window between -0.4 V and 1.1 V (for 0.5 M H2SO4) and between -1.0 V and 0.3 V (for 10 mM NaOH). This means that in the presence of the organic phase, there is no additional current that would indicate the degradation of the organic solvent. Accordingly, toluene, diethyl ether, n-heptane, and dichloromethane, and (in a broader sense) the solvent classes they represent (aromatics, ethers, alkanes, and haloalkanes), have been shown to be suitable for the preparation of the organic phase of the TPB electrode, ensuring the oxidation of butanal as a model compound for biomass-derived compounds.

[0077] Figure 3A shows how the current density observed at the TPB electrode (with respect to the geometric surface area of ​​the electrode) depends on the applied potential and the concentration of butanal in the organic phase. When butanal is added to toluene, an additional current appears that is not present in Figure 3A, thus indicating the electrochemical oxidation of butanal at the TPB electrode. The current caused by the oxidation of butanal scales linearly with the concentration of butanal in the organic phase, as shown in Figure 4B. Linear regression shows that the current density due to butanal oxidation increases with the butanal concentration by 72 mA cm / s. 2 / (mol / l). This means that the current density at the TPB electrode increases by 72 mAcrrr 2 (0.72kArrr 2) when the concentration of butanal in the organic phase increases by 1 mol / l. Accordingly, technically relevant current densities, which, for example, in the case of chlor-alkali electrolysis, are 4 kArrr 2 can be achieved in electrochemical aldehyde oxidation if the organic phase has sufficiently high butanal concentrations.

[0078] Since the current increases linearly with the butanal concentration in the organic phase, it can be concluded that the reaction is limited by the mass transport of the reactant to the electrode surface.

[0079] However, the behavior observed in Figure 3 is limited to a specific concentration range. This means that in Figure 3, the current caused by butanal oxidation no longer reaches a limit. Although the peak current density of 68 mAcrrr 2 at 0.17 V only slightly from the expected current density of 72 mAcrrr 2deviates, the reaction is limited by electrode kinetics, and the mass transport limitation can no longer be reached. This limits the reaction rates to lower than expected values.

[0080] Figure 4 shows DEMS (Differential Electrochemical Mass Spectrometry) results obtained on a TPB gold electrode with a solution of 10 mM butanal in toluene as the organic phase. The experiment in Figure 4 was performed using a double thin-film cell, in which the aqueous electrolyte (1.0 M NaOH) is constantly pumped through the cell at a flow rate of 10 pl / s. In this experimental setup, the electrolyte first passes through the interface with the organic phase and is then pumped to a vacuum-electrolyte interface, where volatile compounds present in the electrolyte evaporate and are detected by a mass spectrometer. In the case of the TPB electrode, this allows one to determine how much of the reactant, in the case of the butanal that enters the aqueous electrolyte, undergoes an electrochemical reaction.

[0081] The CV in Figure 4A shows that no positive current flows in the potential range between -1.0 V and -0.7 V, indicating that no butanal oxidation is taking place. In this potential range, the mass spectrometric signal for mass 72 (butanal) has a positive value. This means that butanal is transferring from the organic phase into the aqueous electrolyte but does not react and can therefore be detected by mass spectrometry.

[0082] In the potential range between -0.7 V and 0.3 V, the current in the CV of Figure 4A increases, reaching a value of approximately 0.4 mA, indicating that electrochemical aldehyde oxidation is taking place. As in Figure 3, the current becomes independent of potential, suggesting that the reaction in butanal is mass-transport limited.

[0083] In parallel with the increase in current in the CV, the mass spectrometric response for mass 72 decreases to the initial level in Figure 4B. This means that in this potential window, no butanal is transported from the TPB electrode to the electrolyte-vacuum interface, which means that all butanal transferring from the organic phase to the aqueous phase undergoes an electrochemical reaction.

[0084] Furthermore, this suggests that the rate of aldehyde oxidation is not limited by the kinetics of the electrochemical reaction, but rather by the rate at which the aldehyde transfers from the organic phase to the aqueous electrolyte (i.e., the partition rate). This correlates with the current curves from Figure 3A, which exhibit the same limiting current: Since the aldehyde oxidation rate exceeds the butanal partition rate, the current can no longer increase with the potential. However, when the butanal concentration in the organic phase reaches 1.0 M, the kinetics of the electrochemical reaction cannot, or only just, reach the butanal partition rate.

[0085] Since the aldehyde oxidation rate in Figure 3B increases linearly with the butanal concentration in the organic phase, it is reasonable to assume that the partition rate of butanal remains rate-limiting even at higher concentrations than those used for the experiment in Figure 4. Accordingly, the slope of the linear fit in Figure 3B would be defined by the rate at which butanal passes from the organic phase to the aqueous phase. Furthermore, it is expected that the electrolyte-induced decomposition of butanal may be avoided under the experimental conditions of Figure 3 when the butanal concentration in the organic phase is between 0 and 100 mM.Figure 5 shows the results of high-performance liquid chromatography (HPLC) experiments in this regard, where the amount of butanal formed is expressed as a percentage of the amount expected if all the butanal originally added to the organic phase is converted to butanal. The same applies to the transferred charge and the amount of butanal extracted. The transferred charge is plotted as a percentage of the charge required to convert all the added butanal to butanal, and the detected amount of butanal is plotted as a percentage of the butanal originally added to the organic phase.

[0086] The very good agreement between the transferred charge and the amount of butyric acid formed shows that the Faradaic efficiency of the reaction is about 100% when a gold-sputtered TPB electrode is used.

[0087] The results presented in Figure 5 are insightful because they support the interpretation of the present DEMS results: In the experiment where the butanal concentration in the organic phase is 0.1 M (Figure 5A), little or no butanal is extracted into the aqueous electrolyte. These results support the interpretation of the results in Figure 3A that the oxidation rate of butanal is limited by the mass transport of butanal. This means that only a very small fraction of the butanal extracted into the aqueous phase fails to undergo an electrochemical reaction, suggesting that the kinetics of electrochemical butanal oxidation are fast compared to the partitioning rate of butanal.

[0088] This changes when the butanal concentration is increased to 1.0 M. Figure 5B shows that a significant concentration of butanal can be detected in the aqueous electrolyte. This indicates that significantly more butanal is extracted into the aqueous phase than the TPB electrode can oxidize. Accordingly, at sufficiently high concentrations, the distribution rate of butanal becomes rapid compared to the kinetics of butanal oxidation.

[0089] Figure 6A shows how the current density observed at the TPB electrode (relative to the geometric surface area of ​​the electrode) depends on the applied potential and the concentration of butanol in the organic phase. As in the case of butanal oxidation in Figure 3A, the current density in Figure 6A increases with increasing butanol concentration in the organic phase, which can be attributed to the electrochemical oxidation of butanol. The use of the TPB electrode is therefore not limited to the oxidation of aldehydes, but can also be used for the oxidation of other classes of compounds, such as alcohols. However, unlike aldehyde oxidation, no current limitation occurs in the oxidation of butanol, suggesting that the reaction rate is not limited by the partitioning rate of the organic reactant, but rather by the kinetics of the electrochemical reaction.Furthermore, the plot in Figure 6B shows that the current caused by the oxidation of butanol does not increase linearly with the butanol concentration and that the achievable current density remains far below the value achieved by the oxidation of butanal at the same concentration in the organic phase.

[0090] Figure 7 shows the butanol oxidation current densities for electrodeposited nickel on a 100 nm gold-sputtered PTFE membrane, recorded on a TPB electrode with a pore diameter of 20 nm and continuous rotation at 540 rpm. The aqueous electrolyte is 0.1 M NaOH, and the organic phase comprises toluene with 1000 mM butanol as the reactant. The scan rate is 20 mV / s.

[0091] In this study, nickel was investigated as an active electrocatalyst for the selective oxidation of alcohols. An attempt was made to improve the performance of the TPB electrode for the oxidation of butanol by modifying the gold-sputtered TPB electrode with nickel. For this purpose, nickel was electrodeposited from an electrolyte consisting of 30 mM NiSO4, 200 mM K2SO4, and 15 mM H3BO3. The pH of this solution was adjusted to 4.4 by adding the appropriate amount of sulfuric acid. A potential of -0.95 V versus a Ag / AgCl 3M KCl reference electrode was applied for ten minutes during the deposition.

[0092] Furthermore, a TPB electrode was fabricated with a platinum electrocatalyst, which exhibits higher activity for the hydrogenation of carbonyl compounds than gold. To achieve sufficient conductivity, a 100 nm gold film was first sputtered onto the PTFE membrane (pore size of 20 nm) and then a 100 nm platinum film was sputtered onto the gold film. CVs were recorded with this electrode in an aqueous electrolyte of 0.5 M H2SO4, while the organic phase contained the indicated concentrations of acetophenone. Due to its structural similarity to acetovanillone (shown in Scheme 1) and vanillin, the hydrogenation of acetophenone can be considered a model reaction for the electrochemical upgrading of lignin compounds.

[0093] Finally, Figure 8 shows that the current at -0.22 V increases with the concentration of acetophenone in the organic phase. This demonstrates that the TPB electrode can hydrogenate acetophenone. Furthermore, the results once again demonstrate that it is possible to modify the TPB electrode with a variety of electrocatalysts (here platinum). The scaling of the reduction current at -0.22 V with the acetophenone concentration shows that the TPB electrode can be used to perform the selective hydrogenation of the ketone function. Figure 9 shows cyclic voltammograms recorded on a silver TPB electrode, which was manufactured and coated electrolessly, for example, analogously to the exemplary embodiment of a TPB electrode used for the desulfurization of 6-mercaptohexanol, with an aqueous electrolyte as 1 M H2SO4 and a scan rate of 50 mV / s and loading as indicated.Figure 9 compares cyclic voltammograms recorded at the same TPB electrode under argon (solid) and 6-mercaptohexanol (dotted) loading. Under 6-mercaptohexanol loading, an additional current is observed in the hydrogen evolution region, indicating a reduction of 6-mercaptohexanol.

[0094] Figure 10 shows a DEMS measurement for the hydrodesulfurization of 6-mercaptohexanol on a silver-TPB electrode loaded with 6-mercaptohexanol. Figure 10, top, shows a cyclic voltammogram. Figure 10, bottom, shows a mass spectrometric signal for mass 34 (H2S). Aqueous phase: 1 M H2SO4; scan rate: 50 mV / s; flow rate: 5 pL / s. It can be seen from the DEMS measurement in Figure 10 that, parallel to the reduction in Figure 9, a mass spectrometric signal for mass 34 (H2S) is detected, indicating that this reduction is the hydrodesulfurization of 6-mercaptohexanol.

[0095] The TPB electrode can also be used for the hydrodechlorination of alkyl halogens. This is illustrated by Figure 11, which compares cyclic voltammograms recorded on the same silver TPB electrode with argon (solid), with an aqueous electrolyte such as 1 M H2SO4, and a scan rate of 50 mV / s. It was recorded with 1,2-dichloroethane (dotted). An additional, reductive current prior to hydrogen evolution, which is only observed in the presence of 1,2-dichloroethane, indicates the hydrodechlorination of 1,2-dichloroethane.

[0096] Figure 12 shows the measurement on a gold-TPB electrode with a pure phase of furfural and then in an aqueous electrolyte solution of 5 mol / kg NaOH and 1 mol / kg NaClO4, and illustrates how this potentiodynamically cycles. By increasing the potential, currents of up to 67 mA / cm 2Likewise, a potential range is evident in which the obtained currents are plateau-like, which suggests a limitation of the currents, and thus of the reaction, due to a limitation in the furfural extraction.

[0097] In a further measurement, a pure furfural phase was again potentiodynamically cycled in the same electrolyte solution (Figure 13A). A cyclic voltammogram was recorded at a gold TPB electrode. The aqueous electrolyte was 5 mol / kg NaOH and 1 mol / kg NaClO4 at various temperatures as indicated. The loading of the TPB electrode was furfural, and the scan rate was 20 mV / s. After each cycle, the temperature of the electrolyte solution was increased to the indicated value. The recorded current densities increase exponentially with temperature, which can be verified by graphically plotting a linear curve in an Arrhenius graph (Figure 13B with the logarithmic representation of the plateau regions in the cyclic voltammograms against the reciprocal temperature of the aqueous phase).

[0098] In addition, an electrolysis experiment (Figure 14) was carried out with an applied potential of 0.2 V (vs. Ag|AgCl) on the gold-TPB electrode under the conditions as in Figure 13 in an electrolyte solution that was constantly heated to 50 °C. High and only slowly decreasing current densities were recorded over a period of just over one hour. The product analysis of the electrolyte solution, determined by HPLC at the indicated time points, also shows that 2-furo acid is formed from furfural with a Faradaic efficiency of 100%. At the end of the electrolysis, over 93% of the originally used amount of furfural is now in solution as 2-furo acid. Only less than 1% of furfural has been extracted into the aqueous phase, i.e., has passed over without being oxidized on the electrode surface. HPLC product determination reveals 2-furoic acid (diamonds) and furfural (circles) in the aqueous phase.Faraday efficiency of the oxidation of furfural to 2-furoic acid (triangles).

[0099] Figure 14 further shows that the TPB electrode allows quantitative conversion of the reactant at a constant current transient when the TPB electrode is loaded with the reactant as a pure phase. This is because the transition of the organic reactant from the organic phase to the aqueous phase represents the rate-determining step. The rate of this step is determined by the concentration gradient of the reactant across the phase boundary. As long as the organic reactant is present as a pure phase, the concentration gradient and therefore the reaction rate remain constant.

[0100] Figure 15 shows another major advantage of the TPB electrode. The reaction rate depends on the extraction process of the reactants into the electrolyte solution, i.e., on their partition coefficient. The partition coefficient and thus the extraction rate can be changed by selecting the composition of the electrolyte solution. Therefore, it is possible to adapt the extraction rate to the rate of the electrochemical reaction. This prevents a larger amount of reactants from passing into the aqueous phase than can be electrochemically reacted. The remaining excess would otherwise undergo decomposition reactions in the aqueous phase. Figure 15 illustrates this using the example of the oxidation of furfural. The three electrolyte solutions of 1 mol / kg NaOH and 1 mol / kg NaCIO4, 5 mol / kg NaOH and 1 mol / kg NaCIO4 as well as 1 mol / kg NaOH and 5 mol / kg NaCIO4 all show the same extraction-limited current densities at 67 mA / cm 2at a feed rate of 20 mV / s. However, if an electrolyte solution of 5 mol / kg NaOH and 5 mol / kg NaCIO4 is used under otherwise identical conditions, the extraction-limited currents are only 1 mA / cm 2 This demonstrates the excellent control over the extraction rate achievable with the TBP electrode by adjusting the electrolyte concentration.

[0101] The inventors would like to thank the European Union and the European Research Council (ERC) for financial support through an ERC Starting Grant, project acronym MITICAT, GA number 949724, funded within the Horizon 2020 initiative. The inventors also thank the German Research Foundation (DFG) for funding under the German Excellence Strategy (EXC 2033-390677874-RESOLV) and through the Collaborative Research Center / Transregio (TRR 247-388390466) (Project A09). Christoph Bondü also acknowledges financial support from the European Union's Horizon 2020 Science and Innovation Program under the Marie Sklodowska-Curie grant agreement number 801459-FP-RESOMUS.

Claims

Patent claims 1. A method for carrying out redox reactions by means of a three-phase electrode, comprising two separate phases, comprising an organic phase containing an amphiphilic, organic reactant, an electrochemically inert organic solvent and an aqueous phase containing a hydrophilic reactant with or without an aqueous solvent, wherein the organic and the aqueous phase are separated from one another by a permeable membrane, characterized in that the membrane of the three-phase electrode has porous structures with a pore diameter in a range of n « 800nm ​​and carries a metal catalytic coating at one end and comprises a hydrophobic structure or base body at the other end, wherein a transfer of the reactant from the organic to the aqueous phase represents the rate-determining step of the redox reaction.

2. Process according to claim 1, characterized in that the concentration of the organic reactant in the aqueous phase or of the aqueous reactant in the organic phase before the reaction is almost zero, whereby their side and / or decomposition reaction is suppressed.

3. Process according to claim 1 or 2, characterized in that a decomposition-free electrochemical redox reaction is carried out with the substance classes comprising the group of aldehydes, ketones, thiols, haloalkanes or alcohols.

4. Process according to one of the preceding claims, characterized in that hydrodesulfurization of thiols takes place.

5. Process according to one of the preceding claims, characterized in that hydrodechlorination of haloalkanes takes place.

6. Method according to one of the preceding claims, characterized in that the decomposition-free reaction of the reactants is carried out by at least one of the following steps a) to f) a.) by selecting a suitable concentration of the reactant in the organic or aqueous phase, b.) by selecting a suitable organic or aqueous solvent for the organic or aqueous phase, c.) by selecting the concentration and identity of a conducting salt in the aqueous phase d.) by selecting a metal-catalytically active coating and the Pore ​​size of the membrane e.) is controlled by selecting a suitable potential and f.) by choosing a suitable reaction temperature.

7. Process according to claim 6, characterized in that selectivity control in mixtures of several organic compounds is carried out by concerted selection of at least one of the process steps a) to f).

8. Process according to one of the preceding claims, characterized in that the organic reactants are used in pure phase.

9. Three-phase electrode for carrying out redox reactions according to one of claims 1 to 8, characterized in that the membrane of the three-phase electrode is meta II catalytically coated at one end and is hydrophobic at the other end and the membrane has porous structures which prevent unhindered transfer of the reactant from the aqueous phase into the organic phase and enable unhindered transfer of the reactant from the organic phase into the aqueous phase, wherein the porous structures have a pore diameter n which is in a range of n « 800nm.

10. Three-phase electrode for carrying out redox reactions according to one of claims 1 to 9, characterized in that the membrane has at one end a metal-catalytic layer made of at least one element of metals or metal oxides of the 5th or 6th main group or 8th or 1st subgroup, in particular silver or gold, as well as combinations of these metals and / or transition metals and / or transition metal oxides or alloys of these metals and / or transition metals and at the other end comprises a hydrophobic plastic, in particular PTFE or hydrophobized polycarbonate or hydrophobized aluminum oxide.

11. Three-phase electrode for carrying out redox reactions according to one of the preceding claims, characterized in that the membrane side coated with a metal catalyst at one end and hydrophobic at the other end ensures the formation of a phase boundary directly at the catalyst.

12. Three-phase electrode for carrying out redox reactions according to one of the preceding claims, characterized in that the contact angle of the aqueous phase on the membrane is 0 greater than 90°, preferably 0 greater than 91°, particularly preferably 0 greater than 107°.

13. Three-phase electrode for carrying out redox reactions according to one of the preceding claims, characterized in that the organic reactants preferably comprise oxygenated compounds, particularly preferably compounds of the substance classes of aldehydes, ketones, haloalkanes, thiols, alcohols alone or in combination, and the organic solvent comprises the substance class of aromatics, ethers, alkanes and haloalkanes, in particular toluene, diethyl ether, n-heptane and dichloromethane.

14. Three-phase electrode for carrying out redox reactions according to one of the preceding claims, characterized in that an electroless deposition of the metal catalytic layer takes place on the membrane.

15. Use of a three-phase electrode for carrying out redox reactions according to one of claims 1 to 14, for use in biotechnology, fuel production or for the production of basic or basic chemicals.

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