Method for the electrochemical depolymerization of polyesters

Abstract

The invention relates to a method for the electrochemical depolymerization of a polyester, which method comprises the steps: a) providing a mixture comprising (i) a polyester selected from the group comprising polylactic acid, polyhydroxybutyrate, the copolymers thereof, mixtures thereof, and mixtures thereof with further plastics, and (ii) a solvent selected from the group comprising dioxanes, acetonitrile, alcohols, furan, dioxolanes, dimethyl sulfoxide, dimethyl formamide, sulfolane, N-methyl-2-pyrrolidone, the mixtures thereof and mixtures thereof with water; and b) electrolyzing the mixture from step a) in an electrochemical cell comprising an anode and a cathode which is electrically connected to the mixture, in order to form one or more decomposition products of the polyester.

Description

[0001] Process for the electrochemical depolymerization of polyesters

[0002] The invention relates to the technical field of plastics recycling and in particular to a process for the electrochemical depolymerization of polyesters.

[0003] Plastics have revolutionized everyday life due to their versatile uses. However, the rapidly growing production of plastics is leading to a significant increase in environmental pollution. Recycling is an attractive option for eliminating plastic waste, offering the opportunity to recover material value and conserve resources. Currently, plastics recovery is primarily achieved through mechanical recycling, in which plastics are ground and melted to create new products. However, this often requires single-variety polymer streams and results in the polymers degrading in quality with each recycling run. Furthermore, mechanically recycled plastics are only permitted for food packaging to a very limited extent.

[0004] In chemical recycling, plastics are broken down into their monomers or new chemical base materials. Chemical recycling processes include solvolytic, catalytic, and thermochemical processes (AL Merchan et al., Green Chem. 2022, 24, 9428-9449). MS Lehnertz et al., Green Chem. 2022, 24, 3957-3963 describe the catalytic depolymerization of bio-based polyhydroxybutyrate and polylactic acid over supported ruthenium catalysts. Polyhydroxybutyrate and polylactic acid are bio-based polyesters that can be produced from renewable raw materials and are used for applications such as cream and soap packaging, plastic bags, and disposable products. WO 2022 / 159040 A1 describes a process for recycling condensation polymers by catalytic degradation of the polymers into oligomers.US 10,059,650 B2 describes a process for producing a monomer by depolymerizing a biopolymer such as lignin within a system comprising a solvent and a porous metal oxide catalyst, wherein the system is heated to no more than about 220°C.

[0005] WO 96 / 10008 describes a process for recycling nylon 6 and nylon 6,6. Nylon 6 and nylon 6,6 are reacted with an aliphatic monocarboxylic acid to form an alkylamide monomer species. CN 106748649 B describes a process for the electrochemical decolorization and recycling of waste liquid from polyesters depolymerized with ethylene glycol. For this purpose, the waste liquid is decolorized in an electrolytic bath and polyol is subsequently recovered via rotary evaporation. CN 107556168 A discloses a process in which lignin is depolymerized under the catalytic action of a molecular sieve consisting of a base metal. US 2022 / 0170167 A1 describes a process for the electrochemical depolymerization of a polyester such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), or polyethylene naphthalate (PEN) in an alcoholic solvent. Electrochemical processes make it possible to use renewable energy for chemical reactions.This further increases the sustainability of the recycling process. Therefore, there is a need for processes for the electrochemical depolymerization of other plastics.

[0006] The object of the present invention was to provide a process for the electrochemical depolymerization of plastics that overcomes at least one of the aforementioned disadvantages of the prior art.

[0007] This object is achieved by a process for the electrochemical depolymerization of a polyester, wherein a) a mixture comprising (i) a polyester selected from the group comprising polylactic acid, polyhydroxybutyrate, their copolymers, mixtures, and mixtures with other plastics and (ii) a solvent selected from the group comprising dioxanes, in particular 1,3-dioxane or 1,4-dioxane, acetonitrile, alcohols, furan, dioxolanes, in particular 1,2-dioxolane or 1,3-dioxolane, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), sulfolane (tetrahydrothiophene-1,1-dioxide), N-methyl-2-pyrrolidone (NMP), mixtures thereof and mixtures with water are provided; and b) electrolyzing the mixture from step a) in an electrochemical cell comprising an anode and a cathode in electrical communication with the mixture to form one or more degradation products of the polyester.

[0008] Surprisingly, it was found that the process according to the invention allows for the electrochemical depolymerization of polylactic acid (PLA) and polyhydroxybutyrate (PHB) in good yield. Thus, by electrochemical depolymerization of a commercially available PLA cup (M n = 48300 g mol' 1 ) a yield of lactic acid of 87% after passing through 1000 Colomb during electrolysis on platinum electrodes in a dioxane / water mixture (vol: 9:1) using 0.25 M lithium perchlorate as electrolyte at 60 °C and a current density of 50 mA / cm 2 In blind tests conducted under these conditions without electricity, no lactic acid was detected. A particular advantage is that the process can be applied to real waste materials such as cutlery and packaging made of PLA. Similar yields were achieved regardless of the presence of additives such as color pigments, plasticizers, or stabilizers.

[0009] Thus, electrochemical depolymerization of a sustainable polymer can be made available for chemical recycling. This allows for expanding the range of recycling options and promoting the use of waste materials as a resource in a circular economy for polymers.

[0010] For the purposes of this application, the term "polyester" refers to a polymeric compound whose monomer units are linked together via an ester group. The term "polyester" includes, in particular, polylactic acid (PLA), polyhydroxybutyrate (PHB), and their copolymers.

[0011] Mixtures can be depolymerized which comprise a polyester selected from the group comprising polylactic acid, polyhydroxybutyrate, their copolymers, mixtures, and mixtures with other plastics. Preferred copolymers of polyhydroxybutyrate, also referred to as polyhydroxybutyric acid, are selected from the group comprising poly(3-hydroxybutyrate-co-3-hydroxyvalerate), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) and poly(3-hydroxybutyrate-co-3-hydroxyoctanoate), in particular poly(3-hydroxybutyrate-co-3-hydroxyvalerate). Preferred copolymers of polylactic acid are selected from the group comprising polylactic acid-polyhydroxybutyrate, polylactic acid-polyhydroxyalkanoates, and polylactic acid-polyhydroxybutyrate-co-polyhydroxyvalerate.

[0012] Polylactic acid and polyhydroxybutyrate can also be depolymerized from mixtures containing one or more other plastics, for example, mixtures containing fossil plastics such as polyethylene terephthalate (PET), polyethylene (PE), polystyrene (PS), polyethylene furanoate (PEF), or polyethylene naphthalate (PEN). Polylactic acid and polyhydroxybutyrate can also be depolymerized in or from such mixtures, and their degradation products can be recovered. Mixtures arise, for example, in the form of mixtures of substances in post-consumer waste (PCW). Depolymerization of such mixtures using the described process can provide the advantage of breaking down the biopolymers while the "industrial" or fossil polymers retain their substance completely or largely. This enables material separation, which is an essential and important step in recycling.

[0013] Preferably, a mixture comprising a polyester selected from polylactic acid or polyhydroxybutyrate is depolymerized. Commercially used materials containing polylactic acid or polyhydroxybutyrate usually contain additives such as color pigments, plasticizers, or stabilizers. Accordingly, the mixture can contain additives selected from the group comprising color pigments, plasticizers, lubricants, light stabilizers, nucleation nuclei, flame retardants, and other organic or inorganic components of industrially used plastics. Other organic or inorganic components can, in particular, be nanocomposites, including the group of non-functionalized and functionalized cellulose nanocrystals and chitin nanocrystals.

[0014] According to the described process, the mixture is electrolyzed in an electrochemical cell comprising an anode and a cathode in electrical communication with the mixture to form one or more degradation products of the polyester.

[0015] For the purposes of this application, the term "electrochemical cell" refers to devices used in electrochemistry or based on electrochemical processes. Electrolysis cells are used to obtain various substances by applying a voltage. Electrolytic reduction takes place at the cathode, while electrolytic oxidation takes place at the anode. Electrolysis in a divided cell demonstrated the cathodic nature of the depolymerization. Without being bound to a specific theory, it is assumed that water splitting occurs as a competing reaction with the formation of hydrogen and hydroxyl ions, which act as nucleophiles and lead to alkaline hydrolysis of the polyester.

[0016] For the purposes of this application, the term "depolymerization" refers to a reaction that is the opposite of polymerization, i.e., the splitting off of a monomer from a polymer. The polyester can be partially, predominantly, or completely split into degradation products of the polyester, preferably its monomers. Thus, depolymerization represents the opposite reaction to polymerization and can be used as a method for recycling plastics (chemical recycling). In preferred embodiments, the polyester is polylactic acid, and the degradation product of electrochemical depolymerization is lactic acid. Thus, after depolymerization of PLA in a mixture of dioxane and water, lactic acid was found to be the sole degradation product. Measurements showed that decomposition of dioxane was the only side reaction.Thus, an advantageously good selectivity of the electrochemical depolymerization can be provided compared to other catalytic approaches in which by-products such as 1,2-propanediol or propionic acid can be formed.

[0017] In also preferred embodiments, the polyester is polyhydroxybutyrate and the degradation product is selected from the group comprising 3-hydroxybutanoic acid, 1,2-propanediol, butanoic acid, also referred to as butyric acid, and / or 1,3-butanediol, in particular 3-hydroxybutanoic acid.

[0018] The electrolyzable mixture can be a suspension or a solution of the polyester in a solvent. The solvent is selected from the group comprising dioxanes, acetonitrile, alcohols, furan, dioxolanes, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), sulfolane (tetrahydrothiophene-1,1-dioxide), 3-sulfolene, N-methyl-2-pyrrolidone (NMP), mixtures thereof, and mixtures with water. The term "dioxanes" includes 1,3-dioxane and 1,4-dioxane. The term "dioxolanes" includes 1,2-dioxolane and 1,3-dioxolane. The term "alcohols" includes monohydric and polyhydric, in particular dihydric, alcohols. The alcohol is preferably selected from the group comprising methanol, ethanol, propanol, isopropanol, butanol, and / or ethylene glycol. Preferably, the alcohol is selected from the group comprising methanol, ethanol and / or ethylene glycol.Organic solvents with a dipole moment that completely or partially dissolve polyester, for example selected from 1,3-dioxane, 1,4-dioxane, acetonitrile, furan, dioxolane, dimethylformamide (DMF), and / or sulfolane, are preferred. In embodiments, the solvent is selected from 1,4-dioxane and acetonitrile. Further preferred are mixtures of solvents selected from the group comprising dioxanes, acetonitrile, alcohols, furan, dioxolanes, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), sulfolane (tetrahydrothiophene-1,1-dioxide), N-methyl-2-pyrrolidone (NMP), and mixtures thereof with water. Particular preference is given to mixtures of dioxane and alcohol, for example dioxane and methanol, ethanol, and / or ethylene glycol, mixtures of dioxane and water, and mixtures of dioxane, alcohol, and water.

[0019] In embodiments, the solvent is a mixture of dioxane selected from 1,3-dioxane and 1,4-dioxane and water, preferably with a volume ratio of dioxane to water in the range of 9.5:1 to 7:1. The mixture of dioxane and water may further contain an alcohol. In preferred embodiments, the solvent is a mixture of dioxane, alcohol, and water. Preference is given to mixtures of dioxane, methanol, and water; dioxane, ethanol, and water; and dioxane, ethylene glycol, and water.

[0020] In embodiments, the solvent is selected from acetonitrile, dimethyl sulfoxide (DMSO), or a mixture of water and alcohol, in particular selected from methanol, ethanol, or ethylene glycol. Preference is given to mixtures of alcohol and water, for example, mixtures of water and ethanol, water and methanol, and water and ethylene glycol. The proportion of water in a mixture of alcohol and water is preferably in the range from >1 wt% to <20 wt%, preferably in the range from >10 wt% to <15 wt%, based on the total weight of the solvent.

[0021] With regard to the yield of polyester degradation products from electrolytic depolymerization, an influence of the polymer's molecular weight has been observed. Commercial materials are made from polylactic acid and polyhydroxybutyrate with a molecular weight of up to 2,000,000 g / mol and are generally depolymerizable. In embodiments, the molecular weight of the polyester is in the range of >7700 g / mol to <100,000 g / mol, preferably in the range of >10,000 g / mol to <78,000 g / mol. Good monomer yields have been achieved with polyester molecular weights in these ranges.

[0022] The electrolyzable mixture can be a suspension or a solution of the polyester in a solvent. Thus, variations in the concentration of the polyester have shown that electrolytic depolymerization can be carried out using suspensions. In embodiments, the concentration of the polyester in the mixture is in the range from > 1 mg / mL to < 100 mg / mL, preferably in the range from > 5 mg / mL to < 20 mg / mL, preferably in the range from > 5 mg / mL to < 15 mg / mL. For example, at the start of the reaction, the mixture can be a suspension with a polyester concentration, for example PLA, of 100 mg / mL, wherein the polymer can be redissolved in a suspension once the already dissolved polymer has reacted.

[0023] The mixture comprising a polyester and solvent preferably contains one or more salts that ensure charge transport in the electrolyte. In principle, all salts soluble in the solvents can be used. Suitable conducting salts are, for example, selected from the group comprising tetraalkylammonium salts, perchlorates, hydroxides, and halides. In embodiments, the mixture contains a salt selected from perchlorates, in particular LiClO4, KClO4, or NaClO4; tetraalkylammonium salts, in particular tetrabutylammonium chloride, tetrabutylammonium bromide, tetrabutylammonium iodide, or tetrabutylammonium hexafluorophosphate; alkali metal halides, in particular NaCl, KBr, NaBr, or KCl; alkali metal nitrates, in particular NaNO3 or KNO3; alkali metal hydroxides, in particular NaOH, KOH, or LiOH; or mixtures thereof.

[0024] The cation of the tetraalkylammonium salt can be a tetraalkylammonium cation, where the four radicals of the tetraalkylammonium cation can, independently of one another, preferably be an alkyl radical having 1, 2, 3, 4, 5, or 6 carbon atoms. Tetramethyl, tetraethyl, tetrapropyl, and tetrabutyl radicals are preferred. The tetraalkylammonium cation is particularly preferably a tetramethylammonium cation, a tetraethylammonium cation, or a tetrabutylammonium cation. The tetraalkylammonium salt can be a halide, in particular chloride, bromide, or iodide, or a hexafluorophosphate.Preferred are tetrabutylammonium salts selected from tetrabutylammonium chloride, tetrabutylammonium bromide, tetrabutylammonium iodide, and tetrabutylammonium hexafluorophosphate, or tetramethylammonium salts selected from tetramethylammonium chloride, tetramethylammonium bromide, tetramethylammonium iodide, and tetramethylammonium hexafluorophosphate, or tetraethylammonium salts selected from tetraethylammonium chloride, tetraethylammonium bromide, tetraethylammonium iodide, and tetraethylammonium hexafluorophosphate, as well as tetrapropylammonium salts selected from tetrapropylammonium chloride, tetrapropylammonium bromide, tetrapropylammonium iodide, and tetrapropylammonium hexafluorophosphate. Tetrabutylammonium chloride is particularly preferred. These were found to successfully increase conductivity and show no interaction with the polymer or negative influence on the reaction.

[0025] Further preferred are perchlorates selected from LiClCe, KClO4, and NaClCe. LiClCu is particularly preferred. Also preferred are hydroxides and halides, in particular alkali metal halides selected from NaCl, KBr, NaBr, and KCl, and alkali metal hydroxides selected from NaOH, KOH, and LiOH, or mixtures thereof. Perchlorates, hydroxides, and halides are soluble in alcohol / water mixtures, increase conductivity, and have shown no interaction with polymer or negative influence on the reaction.

[0026] The concentration of the salt or hydroxide can range from > 0.1 M to < 1 M, preferably from > 0.25 M to < 0.5 M. Other parameters such as electrode material, current intensity, temperature, or pressure can also have an influence, albeit to a lesser extent, on depolymerization.

[0027] In principle, electrode materials selected from the iron, cobalt, nickel, copper, and zinc groups, as well as titanium and graphite, graphite flux, boron-doped diamond, glassy carbon, and carbon-containing electrodes, can be used. In embodiments, the cathode material is selected from the group comprising platinum, titanium, graphite, steel, silver, copper, iron, cobalt, nickel, tin, zinc, lead, gold, and boron-doped diamond electrodes. In particular, the cathode material is selected from platinum, titanium, graphite, and steel. These showed similar yields in the depolymerization of PLA. Titanium, graphite, and steel can thus replace the more expensive platinum. In particular, titanium produced comparable yields of lactic acid to platinum in the depolymerization of PLA and can thus form a cost-effective alternative cathode material for the depolymerization reaction.

[0028] In embodiments, the current density of the electrolysis is in the range of > 1 mA / cm2 up to < 100 mA / cm 2 , preferably in the range of > 10 mA / cm 2 up to < 50 mA / cm 2 , preferably in the range of > 20 mA / cm 2 up to < 40 mA / cm 2 It was found that increasing the current density from 5 to 10 mA / cm 2 led to an increase in the lactic acid yield during the depolymerization of PLA. It is assumed that at such low concentrations, the reaction at the cathode is not limited by mass transport and a linear increase in the reaction can occur. A further increase in the current density to 20 mA / cm 2In contrast, this only led to a slight further increase in the lactic acid yield. Overall, a low correlation was found between yield and current density. The potential can be in the range from > 0.01 V to < 230 V, preferably in the range from > 0.01 V to < 50 V, more preferably in the range from > 0.01 V to < 30 V. In embodiments, the electrolysis is carried out at a temperature in the range from > 5 °C to < 95 °C, preferably in the range from > 20 °C to < 60 °C. For example, an electrochemical reactor operating under pressure can enable a working temperature above the actual boiling point of the solvents. The electrolysis can be carried out at a pressure in the range from > 0 to < 100 bar, preferably at 1 bar. Advantageously, the electrochemical depolymerization only requires atmospheric pressure and, if necessary,increased temperatures, which positively influences the energy balance of the electrochemical degradation.

[0029] The electrochemical depolymerization of polylactic acid and polyhydroxybutyrate, their copolymers, blends, and blends with other plastics can provide a promising approach for commercial application due to the moderate reaction conditions. Furthermore, the applicability to real-world waste materials such as PLA cutlery and packaging has been demonstrated by achieving similar yields regardless of the presence of additives such as color pigments, plasticizers, or stabilizers. Overall, the electrochemical depolymerization of renewable polylactic acid and polyhydroxybutyrate provides a promising method that expands the possibilities of chemical recycling and can promote the use of waste streams as valuable raw materials in a closed or open circular economy for polymers.

[0030] Unless otherwise specified, the technical and scientific terms used have the meaning commonly understood by one of ordinary skill in the art to which this invention belongs.

[0031] Examples and figures illustrating the present invention are given below.

[0032] Chemicals used: Chemical Manufacturer Purity

[0033] 1,4-Dioxane Sigma Aldrich 99.8%

[0034] Acetonitrile-d3 Sigma Aldrich 99.8%

[0035] DL-lactic acid Sigma Aldrich 90%

[0036] Ethanol absolutely Supelco. Merck > 99.9%

[0037] Graphite foil 0.4 mm Alfa Aesar 99.8%

[0038] Lithium perchlorate Sigma Aldrich > 95%

[0039] L-(+)-Lactic Acid Sigma Aldrich > 99.9%

[0040] Mesitylene Alfa Aesar > 98%

[0041] High performance liquid chromatography (HPLC):

[0042] The liquid phase products were analyzed by HPLC using a Shimadzu system (Prominence LC-20 system, Cs Organic Acid Resin 8 x 250 mm, 5 mmol L' 1 H2SO4). 100 μl of the reaction solution was taken and diluted with 100 μl of water to precipitate the polymer. The mixture was filtered through a Chromafil Xtra polyamide filter (25 mm, 0.45 μm) before analysis.

[0043] Example 1

[0044] Depolymerization of PLA to lactic acid

[0045] Electrochemical depolymerization of PLA to lactic acid on platinum electrodes was performed in an electrochemical cell composed of two electrodes in a 20 mL glass vessel. A 3D-printed cap held the electrodes in place. Electrodes measuring 5 x 1 cm were used, with an active surface area of ​​1.2 cm. 2 .

[0046] A commercially available PLA cup (ISAP Packaging SpA, Verona, Italy, M n = 48300 g mol' 1 ) was cut into pieces measuring 3-5 mm x 3-5 mm with scissors. 35 mg of the pieces were then weighed and added to the glass vessel containing a mixture of 6.3 mL dioxane, 0.7 mL H2O (vol. 9:1), and 185 mg LiClO4 (0.25 M). The reaction mixture was stirred with a magnetic stirrer. A water bath controlled the reaction temperature at 60 °C. At a current density of 40 mA / cm 2 was electrolyzed on a platinum cathode using a platinum anode. After a transfer of 1000°C, a yield of approximately 87% lactic acid was obtained; no byproducts were observed.

[0047] Table 1 shows the course of the lactic acid yield over time / transferred charge amount.

[0048] Table 1: Overview of yield versus charge transfer. Reaction conditions: Pt cathode, Pt anode, 40 mA / cm 2 , dioxane / water (vol. 9:1), 0.25 M LiCICU, 60°C, 5 mg / mL PLA (48300 g mol' 1 ).

[0049] As can be seen from Table 1, the yield increased with the amount of charge. Using 1000°C, a yield of approximately 87% based on the amount of material used was achieved. For a sample weight of 10 mmol of polylactic acid, where the amount of material is based on the repeating unit, 8.7 mmol of lactic acid were generated. One molecule of lactic acid can be formed from one PLA subunit (condensed monomer). The control without applied voltage showed no formation of lactic acid.

[0050] Examples 2-14

[0051] Depolymerization of PLA to lactic acid under variation of cathode material, current density, concentration and molecular weight The electrochemical depolymerization of PLA to lactic acid was carried out as described in Example 1 in an electrochemical cell consisting of two electrodes in a 20 mL glass vessel and electrodes measuring 5 x 1 cm, the active surface being 1.2 cm 2 The reaction mixture consisted of 35 mg PLA in 6.3 mL dioxane and 0.7 mL H2O (vol. 9:1) and 185 mg LiCl04 (0.25 M) and was stirred with a magnetic stirrer. A water bath controlled the reaction temperature at 60 °C. In each reaction, the cathode material, current density, concentration, or molecular weight were modified. The transferred charge of 150 °C was kept constant or adjusted to the concentration. The yields of lactic acid achieved in each case are listed in Table 2.

[0052] Table 2: Overview of the yield under different reaction conditions, under

[0053] Use of: Pt anode, dioxane / water (vol. 9:1), 0.25 M LiClO4, 60°C. As can be seen from Table 2, comparable yields to platinum were achieved using titanium, graphite, and steel as cathode materials. Titanium, in particular, produced comparable yields of lactic acid to platinum in the depolymerization of PLA. Increasing the current density from 5 to 10 mA / cm 2 also led to a near doubling of the yield, while a further increase to 20 mA / cm 2no further increase was observed. Overall, a low correlation was found between yield and current density. PLA concentrations ranging from 1.25 mg / mL up to a suspension of 15 mg / mL resulted in comparable lactic acid yields. Increasing the molecular weight of the polylactic acid used from 40,900 g / mol to 78,800 g / mol resulted in a decrease in yield.

[0054] Examples 15-17

[0055] Depolymerization of PLA material containing additives

[0056] The electrochemical depolymerization of PLA material containing additives such as dyes, plasticizers, and reinforcing agents was investigated. For this purpose, a commercially available PLA cup (ISAP Packaging SpA, Verona, Italy, M n = 48300 g mol' 1), PLA packaging film (bio-mi Ltd., Matulji, Croatia), and a PLA knife (Weita AG, Aesch BL, Switzerland) were used. The reactions were carried out as described in Example 1 in an electrochemical cell comprising two electrodes in a 20 mL glass vessel. The other conditions were maintained, and the transferred charge was kept constant at 150 °C. The yields of lactic acid obtained using the various materials are listed in Table 3.

[0057] Table 3: Overview of the lactic acid yield for different materials.

[0058] Reaction conditions: Pt cathode, Pt anode, 40 mA / cm 2 , dioxane / water (vol. 9:1), 0.25 M LiClO4, 60°C, 5 mg / mL PLA material, 150 C. As can be seen from Table 3, PLA materials containing additives could also be successfully depolymerized electrochemically.

[0059] Example 18

[0060] Depolymerization of polyhydroxybutyrate (PHB)

[0061] The electrochemical depolymerization of PHB was carried out in the electrochemical cell described in Example 1, containing two electrodes in a 20 mL glass vessel. A mixture of DMSO and water (9:1 by volume) was used as the solvent, while the other conditions remained the same. Commercially available PHB powder (Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany) was used, of which 35 mg was weighed and added to a glass vessel containing 0.25 M LiClO4 in DMSO / water (9:1 by volume).

[0062] After electrolysis, the reaction mixture was analyzed by NMR. 1H NMR spectra were recorded at 300 MHz using a Bruker AV300 spectrometer. Measurements were performed at room temperature. Acetonitrile-d3 (D3CCN) was used as the NMR solvent, with mesitylene as the internal standard. The chemical shifts were referenced to the aromatic proton signal of mesitylene at 6.28 ppm ('H NMR (300 MHz, D3CCN) 5 = 2.26(s), 6.28(s) ppm). The formation of the monomer hydroxybutyric acid was confirmed by NMR spectroscopy.

[0063] Overall, the results show that electrochemical depolymerization of polylactic acid to yield lactic acid and of polyhydroxybutyrate to yield the monomer hydroxybutyric acid can be provided.

Claims

Patent claims 1. A process for the electrochemical depolymerization of a polyester, characterized in that a) a mixture comprising (i) a polyester selected from the group comprising polylactic acid, polyhydroxybutyrate, their copolymers, mixtures, and mixtures with other plastics and (ii) a solvent selected from the group comprising dioxanes, acetonitrile, alcohols, furan, dioxolanes, dimethyl sulfoxide, dimethylformamide, sulfolane, 3-sulfolene, N-methyl-2-pyrrolidone, mixtures thereof, and mixtures with water; and b) the mixture from step a) is electrolyzed in an electrochemical cell comprising an anode and a cathode which is in electrical connection with the mixture, in order to form one or more degradation products of the polyester.

2. Process according to claim 1, characterized in that the polyester is polylactic acid and the degradation product is lactic acid or the polyester is polyhydroxybutyrate and the degradation product is selected from 3-hydroxybutanoic acid, 1,2-propanediol, butanoic acid and / or 1,3-butanediol.

3. Process according to claim 1 or 2, characterized in that the solvent is a mixture of dioxane, in particular selected from 1,3-dioxane and 1,4-dioxane, and water, preferably with a volume ratio of dioxane to water in the range from 9.5:1 to 7:1, wherein the mixture of dioxane and water may further contain an alcohol.

4. Process according to one of the preceding claims, characterized in that the solvent is selected from acetonitrile, dimethyl sulfoxide or a mixture of water and alcohol, in particular selected from methanol, ethanol or ethylene glycol.

5. Process according to one of the preceding claims, characterized in that the molecular weight of the polyester is in the range from > 7700 g / mol to < 100,000 g / mol, preferably in the range from > 10,000 g / mol to < 78,000 g / mol.

6. Method according to one of the preceding claims, characterized in that the concentration of the polyester in the mixture is in the range from > 1 mg / mL to < 100 mg / mL, preferably in the range from > 5 mg / mL to < 15 mg / mL.

7. The method according to any one of the preceding claims, characterized in that the mixture contains a salt selected from perchlorates, in particular LiClO4, KCIO4 or NaClO4, tetraalkylammonium salts, in particular tetrabutylammonium chloride, tetrabutylammonium bromide, tetrabutylammonium iodide or tetrabutylammonium hexafluorophosphate, alkali metal halides, in particular NaCl, KBr, NaBr, or KCl, alkali metal nitrates, in particular NaNOa or KNO3, and alkali metal hydroxides, in particular NaOH, KOH or LiOH, or mixtures thereof.

8. Method according to one of the preceding claims, characterized in that the cathode material is selected from the group comprising platinum, titanium, graphite, steel, silver, copper, iron, cobalt, nickel, tin, zinc, lead, gold, and boron-doped diamond electrodes, in particular selected from platinum, titanium, graphite, and steel.

9. Method according to one of the preceding claims, characterized in that the current density of the electrolysis is in the range of > 1 mA / cm 2 up to < 100 mA / cm 2 , preferably in the range of > 10 mA / cm 2 up to < 50 mA / cm 2 , preferably in the range of > 20 mA / cm 2 up to < 40 mA / cm 2 , lies.

10. Process according to one of the preceding claims, characterized in that the electrolysis is carried out at a temperature in the range from > 5 °C to < 95 °C, preferably in the range from > 20 °C to < 60 °C.

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