Process for producing monomers from polymers comprising monomers

Abstract

The present invention relates to a process for preparing a carboxylic acid from a hydrolysable polymer containing the carboxylic acid, comprising at least the following process steps: i) depolymerizing the polymer by hydrolyzing the polymer in an aqueous hydrolysis solution to form a carboxylic acid salt and optionally at least one further monomer component of the polymer; ii) optionally removing other monomer components and any other soluble and / or insoluble impurities from the hydrolysate solution produced in process step i); iii) transferring the hydrolysate solution produced in process step ii) to the anode chamber of the electrolysis device; iv) electrolyzing in the anode chamber using the hydrolysate solution, the electrolyzing device having, in addition to the anode chamber, a cathode chamber filled with a liquid, by connecting the electrolyzing device to a voltage source, a current flowing through the electrolyzing device and ion exchange in the liquid between the anode chamber and the cathode chamber, whereby the liquid in the cathode chamber becomes alkaline, and the liquid in the anode chamber becomes alkaline; protons are formed in the anode chamber, so that carboxylate is protonated, and carboxylic acid is precipitated; and v) removing the formed carboxylic acid from at least part of the hydrolysate solution, in which vi) the liquid generated in the cathode chamber of the electrolysis device in the process step iv) is used as a component of the hydrolysate solution in step i).

Description

Technical Field

[0001] The present invention relates to a method for producing a monomer from a polymer containing the monomer, and more particularly, to a method for regenerating a polymer by recovering the monomer from the polymer to achieve resource-saving recycling. Background Art

[0002] Sustainable and resource-saving methods of operation are becoming increasingly important in all industrial sectors. Therefore, it is of great importance to be able to process polymers in order to recover their components, especially the monomeric components of the polymers.

[0003] EP 3511451 discloses a process for recycling mixed fabrics, the process comprising: i) providing a mixed fabric, the fabric comprising cellulose fibers and synthetic fibers, wherein the synthetic fibers contain at least one synthetic plastic; ii) at least partially removing the synthetic plastic from the cellulose; and iii) further processing the mixed fabric from which the synthetic plastic has been removed after removal. The main objective of this document is to recover cellulose.

[0004] WO 2008 / 028429 A1 discloses a method for separating terephthalic acid from a terephthalate solution, comprising introducing an aqueous solution of a terephthalate into the cathode chamber of the cathode zone of an electrodialysis device, and introducing an electrolyte into the anode chamber, then subjecting the resulting salt solution and electrolyte solution to electrolysis, collecting terephthalic acid generated by the reaction of terephthalic acid anions with electrolyte cations from the anode chamber, and separating it from the electrolyte by filtration.

[0005] US 4,092,230 discloses a method for preparing terephthalic acid, comprising electrolyzing an aqueous solution of potassium terephthalate, preferably in the presence of a potassium salt of an acid stronger than terephthalic acid. In various embodiments of the present invention, terephthalonitrile is converted into terephthalic acid by hydrolyzing it in an aqueous medium containing dipotassium terephthalate, potassium bicarbonate and potassium hydroxide, stripping ammonia and carbon dioxide from the hydrolyzate, adding carbon dioxide to the stripped hydrolyzate to precipitate monopotassium terephthalate, electrolyzing the monopotassium terephthalate in the presence of an acid stronger than terephthalic acid to precipitate terephthalic acid, and isolating the terephthalic acid product.

[0006] US 6,312,582 discloses a process for recovering products from alkaline polyethylene terephthalate saponification products using soda, wherein both terephthalate ions in acidic form and sodium ions in soda form are present. In this method, a sodium terephthalate solution is formed when the saponification product is dissolved and electrochemically pre-acidified to a pH value of 4 to 7. Thereafter, an electrochemical acidification step is carried out by electrolysis, terephthalic acid is precipitated in the anode region, and sodium is obtained in the cathode region which can be reused.

[0007] EP 2736968 B1 discloses a process and an apparatus for recycling polymer materials (especially polyesters and polyamides), which uses a depolymerization process to depolymerize the treated material using a solvent decomposition mixture, and the reaction is carried out in at least one microwave depolymerization reactor (6) extending mainly along an axis (A), and the reactor comprises a system (7) for moving the reactants, which allows the reactor (6) to operate continuously; the moving system (7) is an Archimedean screw system, which moves the reactants through the reactor (6) mainly along the axis (A).

[0008] US 8,298,396 B2 discloses a process and apparatus for producing chemical compounds, especially fermentation products. The invention provides a method for producing one or more chemical substances, the method comprising a fermentation step to form these substances and a separation step to induce precipitation of the substances using at least one pair of electrodes, the pair of electrodes comprising at least one precipitation electrode and at least one counter electrode through which an electric current is passed to precipitate the one or more substances.

[0009] WO 2020 / 173961 A1 discloses a method for producing terephthalic acid (TPA) and / or ethylene glycol (EG) and / or other monomers constituting one or more plastic polymers by alkaline hydrolysis of one or more plastic polymers, the method comprising: a) contacting one or more plastic polymers with a metal oxide present in a solution in an alkaline environment to provide a reaction mixture; b) continuously stirring the reaction mixture under ultraviolet light for an appropriate period of time; c) obtaining terephthalic acid, ethylene glycol and / or other monomers from the reaction mixture.

[0010] However, the solutions known from the prior art may still have room for improvement, in particular with regard to the continued operation of efficient recycling of polyester, in particular polyethylene terephthalate. Summary of the invention

[0011] The object of the present invention is to provide a method by which at least one disadvantage of the prior art is at least partially overcome. In particular, the object of the present invention is to provide a solution that allows efficient production of said monomer from a polymer containing said monomer.

[0012] According to the invention, the object is achieved by a process having the features of claim 1. The object is further achieved by a carboxylic acid having the features of claim 11, a use having the features of claim 12, a polymer having the features of claim 13, a use having the features of claim 14 and a process having the features of claim 15. Preferred embodiments of the invention are disclosed in the dependent claims, the description and the drawings, wherein the further features shown or described in the dependent claims, the description or the drawings - alone or in any desired combination - constitute the subject matter of the invention, unless the context clearly indicates otherwise.

[0013] The present invention relates to a method for preparing carboxylic acid from a hydrolyzable polymer containing carboxylic acid, the method comprising at least the following process steps:

[0014] i) depolymerizing the polymer by hydrolyzing the polymer in an aqueous hydrolysis solution to form a carboxylate salt and, optionally, at least one other monomer component of the polymer;

[0015] ii) optionally removing other monomer components and any other soluble and / or insoluble impurities from the hydrolyzate solution produced in process step i);

[0016] iii) transferring the hydrolyzate solution produced in process step ii) to the anode chamber of the electrolysis device;

[0017] iv) electrolysis using the hydrolysate solution in an anode chamber, wherein the electrolyser has a cathode chamber filled with liquid in addition to the anode chamber, by connecting the electrolyser to a voltage source, a current flows through the electrolyser and ion exchange occurs in the liquid between the anode chamber and the cathode chamber, whereby the liquid in the cathode chamber becomes alkaline and protons are formed in the anode chamber, protonating the carboxylate, e.g. causing precipitation of the carboxylic acid in the anode chamber; and

[0018] v) removing the formed carboxylic acid (from at least part of the hydrolysate solution), wherein

[0019] vi) In process step iv), the liquid produced in the cathode chamber of the electrolysis device is used as a component of the hydrolysis solution in step i).

[0020] This method is a particularly advantageous route to recover carboxylic acid as a monomer component of a polymer, making the acid easily processable, for example, repolymerizable, to create more value.

[0021] Therefore, the method is particularly useful for recovering carboxylic acid from a polymer comprising a carboxylic acid. It will be appreciated by those skilled in the art that carboxylic acid also includes polycarboxylic acids - for example, dicarboxylic acids or tricarboxylic acids. In addition, the polymer includes both homopolymers and copolymers.

[0022] According to process step i), the method first comprises depolymerizing the polymer by hydrolyzing the polymer in an aqueous hydrolysis solution to form a carboxylate salt and optionally at least one other monomer component of the polymer. Accordingly, the polymer can be present in an aqueous hydrolysis solution and hydrolytically depolymerize therein - i.e. can be broken down into its monomer components. Preferably, the monomer components are present in dissolved form in the solution. In principle, the meaning of the present invention includes both alkaline hydrolysis and enzymatic hydrolysis - alkaline hydrolysis may be more preferred here, as described below.

[0023] Depending on the hydrolysis process chosen, the hydrolysis solution contains the required ingredients, such as a base, an enzyme and / or any suitable initiator and / or catalyst.

[0024] However, for general hydrolysis, the solubility limits of the various products should not be exceeded, and heating the hydrolysis solution to 100°C and above, for example, with a lower limit of 130°C or 140°C and an upper limit of 200°C or 180°C, or at a higher temperature between 200°C and 300°C, can accelerate and more effectively convert the polymer part, especially the polyester part, into a carboxylate, i.e., a deprotonated carboxylic acid. The concept of the present invention also covers carboxylates with multiple functional groups, such as dicarboxylates, etc. Similar comments apply to other monomer components, such as alcohols, and therefore also polyols in polyesters. By analogy, this also applies to the forced mixing of the hydrolysis solution and the polymer, such as the polyester fraction present therein.

[0025] After at least partially completing the hydrolysis / depolymerization, the resulting solution is referred to as a hydrolyzate solution in the present invention. The hydrolyzate solution produced in process step i) is optionally treated by the method of step ii) to remove other monomer components, i.e., monomer components of non-target carboxylic acids, and any other soluble and / or insoluble impurities present in the hydrolyzate solution. In other words, at least one of any other monomer components, any soluble impurities, and any insoluble impurities present can be optionally removed. Therefore, in addition to the monomers to be recovered, all or at least most of the components present in the hydrolyzate solution can preferably be removed from the hydrolyzate solution.

[0026] To remove solids from the hydrolysate solution, it is possible in principle to employ solid separation techniques known to the person skilled in the art, such as filtration and / or centrifugation.

[0027] As one embodiment, impurities present may include dyes, which can be removed by adsorption or the like. For example, activated carbon is suitable for this step.

[0028] For example, other monomer components can be further removed by extraction using a suitable solvent. When polyester is used as the polymer, for example, polyols, such as ethylene glycol, can be removed from the hydrolyzate solution after the hydrolysis step by distillation (such as rectification) or extraction or a combination of the two. The solvent used for extraction can be regenerated and reused. Accordingly, for example, one or more other monomer components, such as ethylene glycol, can be separated simultaneously. The monomer components, such as ethylene glycol, can be processed again with the acid obtained in the subsequent electrolysis (such as terephthalic acid) to form a polymer, for example to form PET.

[0029] In order to extract hydrophilic components, such as ethylene glycol, for example, an organic solvent can be used, which forms a hydrophobic eutectic solvent with the ethylene glycol and forms a two-phase system with the aqueous hydrolyzate solution. The extraction process can be carried out in multiple stages. For example, menthol or thymol can be used as a solvent. Since the boiling point of ethylene glycol is lower than that of the eutectic solvent, the distillation of the eutectic solvent can allow the ethylene glycol to be removed as a top product. The solvent obtained in parallel as a bottom product can optionally be resupplied to the extraction process after further purification, and the other monomer components can also be treated accordingly.

[0030] However, it is worth noting that if the polymer contains only one monomer component, and / or any impurities have been removed in other steps, the removal steps of other components may be omitted. Thus, for example, the monomer component, dye or other impurities to be removed can be removed by electro(chemical) events in an electrolysis process, in particular oxidation reactions at electrodes, which will be described in detail below.

[0031] Non-hydrolyzable components, i.e. those not suitable for alkaline hydrolysis, originate from the starting materials, i.e. from products containing polymers, such as aliphatic plastics, cotton, etc., which essentially remain in solid form in the hydrolyzate solution, specifically in the form of dispersed particles, and can be removed from the system continuously or discontinuously by filtration or other mechanical means. Similarly, other particles insoluble in the hydrolyzate, such as adjuvants or impurities in the starting materials, can also be removed from the system in the same way.

[0032] As a result, a large amount of substantially solid-free hydrolyzate solution, in which polyvalent cations (especially metal ions), obtained carboxylates and other monomer components (such as polyols) are dissolved, is retained for further process steps. The liquid may also contain residues of unreacted hydrolyzed solution (usually small amounts), such as alkali, or organic residues such as dyes, etc. If necessary, these residues can be removed from the remaining hydrolyzed solution by suitable known techniques, such as adsorption on activated carbon.

[0033] Chromatographic techniques or ion exchange techniques are likewise known and suitable for the same purpose.

[0034] It should be emphasized that impurities, especially organic impurities, are removed from the hydrolyzate solution in the presence of the acid in the ionic form of the carboxylate salt. This allows the selective removal of the generally non-polar organic impurities from the ionic terephthalate salt, thereby allowing the carboxylate salt to be selectively retained in the hydrolyzate solution at least to a large extent.

[0035] After the hydrolysate solution has been treated according to process step ii), the hydrolysate solution generated in process step ii) or, if process step ii) has been omitted, the hydrolysate solution correspondingly generated in process step i) in an obvious manner is transferred according to process step iii) to the anode chamber of the electrolyser. In principle, the electrolyser structure itself can be known, comprising an anode chamber with an anode, a cathode chamber with a cathode and a membrane which separates the anode chamber from the cathode chamber and is selectively permeable to ions. For example, the membrane can be a cation exchange membrane or a diaphragm. In addition, in a manner known per se, a voltage source can be connected to the electrolyser so that the electrolyser is operated in a manner known per se.

[0036] Accordingly, according to process step iv), electrolysis is performed in the anode chamber using a hydrolyzate solution. The cathode chamber is also filled with an electrolyte accordingly. For example, an aqueous solution containing an electrochemically inert conductive salt, such as sodium sulfate, potassium sulfate or phosphate, may initially be present in the cathode chamber. When the electrolysis device is connected to a voltage source, an electric current, in particular a direct current, flows through the electrolysis device. Therefore, ion exchange occurs between the liquids between the anode chamber and the cathode chamber, causing the liquid in the cathode chamber to become alkaline by forming hydroxide ions at the cathode, while forming protons at the anode chamber and / or the anode, which causes the pH value of the hydrolyzate solution in the anode chamber to decrease, and this electrolysis process can also be referred to as pH shift electrolysis. Therefore, the carboxylic acid located in the anode chamber and present in the form of a carboxylate can be protonated and precipitated. Specifically, the carboxylic acid precipitated in solid form can be easily removed from the anode chamber in the form of a solid suspended in a liquid. Therefore, preferably, the method of the present invention can produce such carboxylic acids on demand, and these carboxylic acids can be precipitated in an aqueous solution.

[0037] For example, the production of terephthalic acid takes advantage of the fact that the acid has a very low solubility in water in its protonated state (approximately 9.5 × 10 -5 mol / L), will accordingly crystallize in the hydrolyzate remaining in the anode chamber of the electrolyzer and / or can be obtained in solid form, such as a suspension or sediment, which solid can then be easily separated from the system. Of course, similar descriptions apply to other carboxylic acids with appropriate solubility.

[0038] In order to optimize the electrolysis step, it is preferably carried out at a pH value below 7, in particular in the buffer range of the corresponding acid (such as terephthalic acid). In the latter case, the advantageous pH value is from ≥2 to <7, for example from ≥2 to ≤6, for example from ≥5 to ≤6, for example 6. It is preferred to select a pH value at which the electrode still has electrochemical stability. This can be effectively achieved by the above-mentioned pH shift electrolysis method, so that the pH value of the hydrolyzate is reduced to a level at which the acid exists in a fully protonated form. Accordingly, this promotes the crystallization of the acid in the anode region and minimizes the transfer of protons to the cathode region of the electrolysis device. But in principle, the pH value can be reduced to 2.

[0039] In addition, for electrolysis, a suitable current may be between 0.1 and 1 A / cm 2 and the battery voltage is less than or equal to 12 volts.

[0040] Thus, the carboxylic acid and the monomers produced therefrom are present in the hydrolysate solution in the anode compartment in solid form and, according to process step v), can be removed from at least part of the hydrolysate solution. This can also be achieved by corresponding solid removal techniques known to the person skilled in the art, such as centrifugation or filtration. Furthermore, for this purpose, the hydrolysate solution can be removed from the anode compartment, or the carboxylic acid can be removed from the hydrolysate solution directly in the anode compartment. The carboxylic acid can then be processed in the desired manner.

[0041] The monomers, especially the carboxylic acids, produced in this way can easily have a purity of more than 80 mol%. Any remaining impurities can be removed by known methods, such as re-dissolving the acid in a suitable solvent and subsequently carrying out multiple crystallizations, adsorption and / or chromatography techniques to achieve the desired final purity. Further purification techniques are described in the Brown-Markering-Myerson paper: "Purification of terephthalic acid by crystal aging", published in Industrial Engineering Chemistry, Vol. 29, No. 10, 1990, pp. 2089-2093.

[0042] For example, the solid acid formed can be removed from the anode chamber as a suspension, for example by pumping, and continuously or discontinuously by at least one solids removal technology, for example along a bypass leading from the anode chamber, which is provided with an integrated sedimentation facility and a conduit for returning the residual liquid to the electrolysis device, for example to the cathode chamber.

[0043] Furthermore, according to process step vi), the liquid produced in the cathode chamber of the electrolysis device in process step iv) is used as a component of the hydrolysis solution in step i). In other words, the liquid produced in the cathode chamber is conveyed to the hydrolysis volume, where it can constitute at least a part of the hydrolysis solution.

[0044] Therefore, the base required for the hydrolysis can be prepared and used in situ during the electrolysis step. The catalyst used in this case, in particular the catalyst in the form of a soluble metal ion and / or a Lewis acid, especially a polyvalent metal ion, can be recovered after the hydrolysis step, for example in the following electrolysis step, the metal ions and the cations of the base migrate together to the cathode side by the action of the current and accumulate in the alkaline solution. At the same time, during the electrolysis process, the polyvalent cations enter the cathode chamber from the anode chamber through the ion permeable wall or membrane of the electrolytic cell and form a base with the hydroxide ions produced in the cathode chamber.

[0045] In addition, metal ions may also be attached to the electrode in metallic form due to electrochemical events (such as electrochemical deposition) and be released accordingly by brief potential reversals in the electrolysis device.

[0046] This base and the metal ion acting as a Lewis acid can then be directed to the facility for carrying out the first process step (hydrolysis), thereby achieving complete recycling, i.e., in one cycle. Suitable metal ions include, but are not limited to, Zn, Cd, Hg, Cu, Ag, Ni, Pd, Co, Fe, Ti, Be, Mg, Ca, Sr, Al, Sb and Sn, with Sb, Ti, Zn and Ni being particularly preferred. Therefore, it may be advantageous if the liquid produced in process step iv) and used as a component of the hydrolysis solution in step i) contains a Lewis acid generated by an electrochemical event in the electrolysis or during the electrolysis process carried out by potential reversal.

[0047] Therefore, in the recycling process of the polymer, the recycling of the process base or Lewis acid as a hydrolysis catalyst is also included. This simultaneously achieves the effective use of resources and reduces energy consumption, emissions and transportation costs. Compared with chlor-alkali electrolysis, the use of pH shift electrolysis and the process described in the present invention significantly reduces or avoids the formation of salt waste and the management of gaseous chlorides during operation. From the perspective of technical process, this alkali can be particularly advantageously obtained in the following way: the solid acid formed in the electrolysis anode chamber, especially terephthalic acid, is removed, and the resulting liquid can enter the cathode chamber of the electrolyzer. Correspondingly, the liquid produced in process step v) can enter the cathode chamber of the electrolysis device. At the same time, cations, especially alkali metal cations or metal cations that migrate from the anode to the cathode chamber during the electrolysis process, form an alkaline solution with the hydroxide ions produced at the cathode during the electrochemical cracking of water. The alkaline solution contains an electrochemically generated base and also contains catalytically active alkali metal ions and metal ions. Therefore, the solution can be used for hydrolysis as described above.

[0048] In the hydrolysis solution, the concentration of Lewis acid is preferably in the range of ≤5% by weight, preferably from ≥0.5% to ≤2% by weight, preferably from ≥1% to ≤2% by weight, particularly preferably 1% by weight. If the amount of Lewis acid returned from the electrolysis device is insufficient, further Lewis acid can be added.

[0049] The process makes use of a particularly advantageous side effect: For example, in the polyester fibers in the fabric, such polyvalent cations based on zinc (Zn), iron (Fe) or titanium (Ti) are already present. They originate, for example, from catalyst residues in the polymerization reaction, auxiliaries in the composite material or impurities in the textile material. Such polyvalent cations can also be optionally released into the anolyte by oxidation reactions at the electrode surface or added separately as salts, for example, in order to establish the required catalyst concentration relative to the polyester content in the hydrolysis. By means of the pH-shift electrolysis in the process according to the invention, these catalytically active polyvalent metal ions are retained in a substantially closed operating cycle.

[0050] The advantage here is that the concentration and amount of base used are such that the carboxylic acid released during the hydrolysis is completely deprotonated.

[0051] Furthermore, the process described here is particularly advantageous since the reuse of the produced high-purity monomers in the production of plastics is becoming increasingly important in terms of sustainable operation. It is therefore preferred to remove the solvent from the plastic without residue. This has not been found to date in any relevant applications for this process, but according to the invention this is easily achievable, for example by drying it after the removal of the acid.

[0052] The invention is also based on the following considerations and findings. A large number of polymer-containing composite materials, such as PET-containing composite materials, have been mainly disposed of in this way - in particular by landfill or incineration. The recycling processes disclosed so far are complex and often still in the experimental stage. There is therefore an urgent need to utilize these composite materials as a raw material source for the monomers they contain for reprocessing. In particular, it is desirable to produce products recovered from the composite materials, also called "secondary" products, in particular terephthalic acid, in order to preserve the primary raw material sources. The secondary products thus obtained should have the key product properties of the corresponding products produced on a primary basis (or conventionally) in order to be able to provide a 1:1 substitute for these virgin products as far as possible.

[0053] It has been found that such secondary monomers, for example secondary terephthalic acid, can be obtained substantially from polymer-containing products by a combination of hydrolytic depolymerization and subsequent electrolysis of the specific hydrolysis products.

[0054] In a first process step, a polymer, such as a polyester, polyamide or other polymer containing carboxylic acids as monomers, is depolymerized by hydrolysis to form a hydrolyzate solution comprising at least one carboxylate, in particular a terephthalate, and possibly another monomer component.

[0055] Thus, the method of the present invention exhibits advantages that have not been previously achievable in a similar manner, as described below.

[0056] As a result, valuable materials that previously would have been landfilled can be further reused as new products, thus saving resources and reducing the environmental impact of the consumption of primary raw materials, especially fossil raw materials. Due to the free availability, the method can be independent of specific uses, for example, clothes can be transformed into completely different types of products. In addition, the energy consumption and emissions associated with the production of new products can be significantly reduced.

[0057] Preferably, the polymer comprises a polyester, in particular polyethylene terephthalate, wherein the carboxylic acid formed comprises in particular terephthalic acid. It should be noted in this regard that polyester-based fibers are currently mainly used for textiles. As of 2025, textiles in Europe must be collected and recycled [EU Strategy for Sustainable and Recyclable Textiles]. In Germany alone, more than 1,000,000 tons of waste or surplus textiles are collected each year. In 2025, the amount of such textiles in Europe is expected to exceed 5 million tons. Therefore, it is a significant advantage to reprocess these composite materials to obtain the individual pure components thereof. One outstanding example is polyethylene terephthalate, which can also be used particularly advantageously by the method according to the invention. For example, the solubility of terephthalic acid in water allows it to be effectively precipitated or crystallized in the anode chamber.

[0058] With regard to the use of polyesters, as an exemplary application, the polyester-containing starting material, i.e. the polymer-containing product, can, for example, comprise a blend of polyester and fiber, the blend consisting of polyester plastics such as PET, cotton, polyethylene (PE) and / or polypropylene (PP) and optionally other components, which can be components susceptible to alkaline hydrolysis (such as PET) or components that are not susceptible to alkaline degradation, such as cotton, PE or PP. Thus, the hydrolysis liquid contains monomers of the polyester, in particular at least one polyol and at least one dicarboxylic acid as the corresponding dicarboxylate salt, in each case in dissolved form.

[0059] By alkaline hydrolysis, these components can advantageously be separated, whereas components susceptible to alkaline hydrolysis, such as terephthalates, can be supplied to a subsequent electrolysis, such as the above-mentioned pH shift electrolysis, and then the corresponding monomers can be obtained in pure form in a further process step.

[0060] In addition, the anode is preferably at least partially designed to have effective non-stick properties for the carboxylic acid produced in step iv). This embodiment can achieve significant advantages in terms of the process. In fact, preventing the solids (especially the formed carboxylic acid) from adhering to the electrode surface increases the amount of acid (e.g. terephthalic acid) suspended in the anode compartment. In addition, there is no need to interrupt the process to remove deposits on the electrode. Therefore, the process has long-term stability and can be achieved with little maintenance work. There are many ways to produce non-stick properties.

[0061] For example, in the presence of a corresponding anolyte, the anode is provided so that at least the adhesion of solid terephthalic acid to the electrode can be suppressed to a great extent, mainly due to the electrochemical reaction between the electrode and the liquid. Specific examples include, for example, that the anode is composed of at least one metal or metal alloy at least on its surface, the metal or metal alloy comprising at least one selected from vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), zirconium (Zr), niobium (Nb) and molybdenum (Mo). The alloy may include at least one of the above metals, for example at least two.

[0062] Furthermore, the electrodes are advantageously less expensive than the platinum electrodes commonly used. Furthermore, for example, nickel electrodes or electrodes based on alloys with a nickel content (eg nickel content> 50 mol %) can simultaneously be electrochemically stable in this process step.

[0063] In addition, it is preferred that the hydrolysis carried out in process step i) is an alkaline hydrolysis. One advantage of alkaline hydrolysis over enzymatic hydrolysis is the high temperature stability of the hydrolysis. Despite its advantages, enzymatic hydrolysates can usually only be used below the glass transition temperature of PET (about 80° C.). Thus, in the case of alkaline hydrolysis, for example, the crystalline fraction of PET can be obtained in an improved manner, since the alkaline hydrolysis can also be carried out above the glass transition temperature of the polyester.

[0064] Another advantage that has emerged is that no organic auxiliary is required, as is often the case with other hydrolysis processes. For example, the polyester fraction can be converted more quickly into carboxylates and polyols, so that the process in this embodiment is more efficient. In the case of alkaline hydrolysis, in particular an aqueous hydrolysis solution, the carboxylic acid formed can be irreversibly withdrawn from the reaction equilibrium by neutralizing it in the presence of a base, thereby further enhancing the effect.

[0065] In addition, it is preferred that the process is carried out continuously. For example, it can be seen that a particular advantage of carrying out the process continuously is that, in addition to supplying the product for recovery and removing the acid formed as a secondary product (e.g. terephthalic acid), a continuous cycle of alkali can be returned from the electrolysis unit to the hydrolysis unit and back to the electrolysis unit again. The latter can therefore be advantageous, in particular when carrying out alkaline hydrolysis.

[0066] It is further preferred that the content of polymer in the hydrolysis solution is ≥ 0.5 mol / L based on the hydrolysis solution. For example, the upper limit may be limited by the maximum solubility of the generated carboxylate. For example, exemplary fractions may be in the range of ≤ 5 mol / L, for example from ≥ 1 mol / L to ≤ 2 mol / L.

[0067] Particularly in this embodiment, the polymer can be hydrolyzed efficiently under the existing conditions, so that the process can be carried out in a particularly advantageous manner, since the solubility limit is generally not exceeded and the polymer can be present in dissolved form in the hydrolysis solution.

[0068] As mentioned above, the polymer depolymerized in process step i) has particular advantages as a component of a product selected from the group consisting of textiles (including clothing), plastic packaging, plastic films and plastic bottles. Due to the complexity of recycling and / or obtaining raw materials, such products that are considered valuable substances would have been abandoned in the past. However, according to the present invention, the advantages are obvious, even these types of products as well as monomer units contain other substances, making subsequent processing more difficult, can be easily processed by the method of the present invention. In other words, the above-mentioned products can be characterized as mixed products, which can be easily processed as starting materials in the present invention even if they only partially consist of hydrolyzable polymers.

[0069] According to the electrochemical standard potential, electrolysis may lead to the reduction of catalytically active ions and their deposition at the cathode. Non-shaped parts of the electrode can be removed from the electrode by temporarily reversing the potential and reoxidizing it in combination with a higher flow rate.

[0070] The hydrolysis can be carried out in a particularly efficient and time-saving manner if the product is introduced into the hydrolysis liquid in comminuted form. The reaction conditions can thereby be significantly improved.

[0071] The above advantages also apply to the carboxylic acid produced by the above process. In detail, the carboxylic acid can be produced in an extremely resource-efficient and sustainable manner, since it is recovered from used materials or waste. Furthermore, the carboxylic acid itself and the materials used in its preparation can be recycled, which further emphasizes the above advantages. Such a carboxylic acid can be distinguished from a carboxylic acid formed in a different way, since it may still contain at least traces of components from the hydrolysis. For example, the carboxylic acid may still include the metal or metal ions of the Lewis acid used in the hydrolysis, which can act as a catalyst in a possible polymerization reaction.

[0072] As mentioned above, it may be particularly advantageous if the carboxylic acid is terephthalic acid, since this allows polyethylene terephthalate to be used and constructed in one cycle. This polymer has great potential for reprocessing.

[0073] Accordingly, further described is the use of the carboxylic acids produced by the process according to the invention, in particular terephthalic acid, as monomers for the production of polymers, in particular polyethylene terephthalate.

[0074] Therefore, another subject of the present invention is a polymer, in particular polyethylene terephthalate, produced from the above-mentioned carboxylic acids, in particular terephthalic acid. This polymer can be distinguished from a polymer formed in a different way, because it may still contain at least traces of components from the hydrolysis. For example, the polymer may still include metals or metal ions of the Lewis acid used in the hydrolysis, which metals or metal ions may have been used as catalysts in the previously carried out polymerization.

[0075] Therefore, the present invention also relates to the use of a polymer for producing a product, wherein the product is selected from the group consisting of textiles (including clothing), plastic packaging, plastic films and plastic bottles, characterized in that the polymer is a polymer as described above.

[0076] This can be particularly advantageous if the monomers produced from the polymer by hydrolysis or depolymerization and electrolysis are converted back into the polymer and then converted into the monomers according to the above-described process.

[0077] Therefore, according to the above, the solid acid produced according to the method, in particular terephthalic acid, can be used as a monomer to produce polymers (such as polyethylene terephthalate (PET)) or to produce products composed of these polymers. Any additional purification steps are described in the embodiments described above. The polymer or polymer-containing product produced can be used as a substitute for polymers produced from fossil raw materials. For example, the above-disclosed features of the new method for producing terephthalic acid and its use can be similarly applied to the production and use of monomers with similar chemical properties to terephthalic acid. This includes the process of secondary production of 2,5-furandicarboxylic acid (FDCA) from the corresponding product (polyethylene 2,5-furandicarboxylate=PEF). Accordingly, PEF can be secondary produced from ethylene glycol and recovered FDCA. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 : A schematic diagram showing an embodiment of the method of the present invention;

[0079] Figure 2 :The general reaction mechanism of Lewis acid-catalyzed alkaline polyester hydrolysis as the preferred polymer hydrolysis is demonstrated;

[0080] Figure 3 :Taking the alkaline hydrolysis of polyethylene terephthalate (PET) as an example, the process of its hydrolysis to produce ethylene glycol and sodium terephthalate is demonstrated;

[0081] Figure 4 : shows an electrolytic cell in which pH shift electrolysis is being performed;

[0082] Figure 5 : Demonstrates the regeneration of a Lewis acid catalyst as part of an embodiment of the process of the present invention;

[0083] Figure 6 : An exemplary operating range of pH values ​​in pH shift electrolysis is shown;

[0084] Figure 7 : The concentration distribution of terephthalic acid during the depolymerization of bottle flakes (1) and fibers (2) is shown;

[0085] Figure 8 :The current and voltage distribution of Ir anode electrolysis is shown;

[0086] Fig. 9 :The current and voltage distribution of Ir anode electrolysis is shown;

[0087] Fig.10 : Current and voltage distributions for nickel anode electrolysis are shown. DETAILED DESCRIPTION

[0088] The present invention will be described hereinafter by way of example with reference to the accompanying drawings and embodiments, wherein the features listed below can not only represent one aspect of the present invention independently, but also can represent one aspect of the present invention in combination, and the present invention is not limited to the following drawings, embodiments, descriptions and working examples.

[0089] Figure 1 A schematic process flow diagram of one embodiment of the method according to the present invention is shown. The method is used to generate carboxylic acids from polymers comprising carboxylic acids as monomer components, and the polymers may be selected from, for example, polyesters and polyamides.

[0090] Here, according to reference numeral 10, firstly a polymer-containing product is provided, the product being selected from the group consisting of textiles (including clothing), plastic packaging, plastic films and plastic bottles.

[0091] According to process step 1, the polymer is initially depolymerized by hydrolyzing the polymer in an aqueous hydrolysis solution to form a carboxylate salt and, optionally, at least one further monomer component of the polymer.

[0092] This hydrolysis Figure 2 and Figure 3 More specifically, Figure 2 The general reaction mechanism of Lewis acid-catalyzed alkaline polyester hydrolysis as a preferred polymer hydrolysis is demonstrated. Figure 3Taking the alkaline hydrolysis of polyethylene terephthalate (PET) as an example, the process of its hydrolysis to ethylene glycol and sodium terephthalate is demonstrated. Both hydrolysis here are catalyzed by Lewis acids.

[0093] Back to Figure 1 Then, in process step 2, solid removal is advantageously performed to remove the solids present in the hydrolyzate formed during the hydrolysis process, so that these solid-containing impurities (reference number 11) are removed. In addition, according to process step 3, other components, such as additives, can be further removed, and these removed components are represented by reference number 12.

[0094] The process step according to reference number 4 also shows the removal of another monomer, such as a polyol. This can be achieved in particular by extraction with a water-immiscible solvent. According to reference number 13, the monomer, such as a polyol, is regenerated together with the solvent; reference number 5 is intended to represent this regeneration process. Subsequently, the regenerated monomer can be collected according to reference number 9, while the solvent is reused according to reference number 14.

[0095] The carboxylate salts formed during the hydrolysis to generate carboxylic acids can subsequently enter an electrolysis device, wherein reference numeral 6a indicates the cathode side and reference numeral 6b indicates the anode side.

[0096] Figure 4 demonstrated this pH shift electrolysis method, Figure 4 The electrolytic process for obtaining crystalline terephthalic acid by crystallization in the anolyte and recovery of the base and Lewis acid catalyst on the cathode side is presented more precisely.

[0097] The liquid in the cathode chamber or cation side 6a of the electrolyzer 15 of the electrolyzer is preferably alkaline, and the solution may be a hydrolyzate solution. When the voltage source of the electrolyzer applies a DC voltage to the electrolyzer 15, current flows through the electrolyzer 15, and ions are able to flow through the cation permeable membrane 18, the following effects occur. Preferably, monovalent cations (Li + 、Na + , K + ) and polyvalent metal ions Me 2+ (Zn 2+ , Cu 2+ , Fe 3+ The monovalent cations form a base on the cathode side of the electrolytic cell 15, and the polyvalent metal ions have catalytic activity in the ester cleavage of alkaline hydrolysis.

[0098] Between two conductively connected electrodes (each in a water environment), water is electrolyzed according to the following reaction:

[0099] In the anode chamber: 2H2O→4H ++O2+4e -

[0100] In the cathode chamber: 2H2O+4e - →4OH - +2H2

[0101] If the circuit in the electrolytic cell 15 is neither proton- + ) nor by hydroxide ions (OH - ) is not closed, but mainly closed by different ions (such as alkali metal ions or metal ions), then the electrochemically generated protons form acids in the anode region (anode chamber) and bases in the cathode region (cathode chamber). Therefore, an electrochemically induced pH shift occurs. The formation of acid in the anode chamber causes the dissociation equilibrium to shift to the protonated form. Taking terephthalic acid as an example, protonated terephthalic acid has a lower solubility than hydrogen terephthalate (HTPA-) or terephthalate (TPA2-), so terephthalic acid can be precipitated as a solid during the electrochemical pH shift. At the same time, an alkaline solution is generated on the cathode side because the (alkali) metal ions contained in the terephthalate close the circuit in the electrolytic cell 15 as charge carriers and migrate to the cathode chamber through the electrodialysis effect. This electrolysis is also called pH swing electrolysis or pH shift electrolysis.

[0102] In other words, terephthalic acid can, for example, precipitate or deposit as a result of anolyte crystallization on the anode side of the electrolyzer (in the anode chamber). Here, the terephthalic esters obtained in the remaining hydrolyzate are converted into almost completely protonated terephthalic acid by electrochemically generated protons. The protonated terephthalic acid crystallizes in the anode chamber and can be discharged from the electrolyzer in a suspended state. At the same time, on the cathode side of the electrolyzer (cathode chamber), a base is formed which may contain polyvalent cations, as will be described in detail below.

[0103] In a subsequent process step, the solid terephthalic acid thus formed can be discharged from the anode chamber of the electrolyzer and processed for further use. The residual liquid produced during the solid terephthalic acid separation process can also be reused by introducing it into the cathode chamber of the electrolyzer. The cations that migrate through the membrane in the electrolyzer together with the hydroxide ions produced in the cathode chamber during the hydrolysis process form the required alkaline solution in the cathode chamber, which can ultimately be fed back to the hydrolysis stage.

[0104] Figure 5 The left figure shows how a Lewis acid catalyst forms a metal during electrolysis. The right figure shows that by reversing the potential, the Lewis acid catalyst can be regenerated and can be reintroduced into the process, in particular for hydrolysis.

[0105] Back to Figure 1, reference number 7 shows the removal of a desired monomer or carboxylic acid, such as terephthalic acid in a preferred embodiment, which can be removed according to reference number 8.

[0106] also, Figure 6 The favorable operating range of pH values ​​in the pH-shift electrolysis is shown using terephthalic acid as an example. Terephthalic acid has two dissociated states. At pH values ​​above 5, terephthalic acid hydrogen and terephthalic acid esters are mainly present. By protonation of the terephthalic acid hydrogen near the anode, terephthalic acid crystals can be formed at a pH of only 6. Here, the pH-shift electrolysis is operated in the buffer range of terephthalic acid. Figure 6 In the operating range 2, Figure 1 The process state b shown, operating range 1 corresponds to Figure 1 The process state a shown in the figure corresponds to the operating range 3 Figure 1 The process status c.

[0107] The above method provides an environmentally friendly alternative method for recycling or secondary production of carboxylic acids such as terephthalic acid, which can provide the main monomer for the production of polymers (such as PET) without the use of fossil raw materials and / or primary raw materials. At the same time, the present invention also indicates a method to avoid landfilling or incineration of polyester-containing products and thus further burdening the environment.

[0108] Example

[0109] Determination of monomeric TPA and EG concentrations

[0110] The concentration of dissolved terephthalic acid (TPA) and its salt can be determined by HPLC. Using an Agilent 1200 high performance liquid chromatograph equipped with a C18ec column (CS chromatograph), the DAD is set to a signal wavelength of 250nm and a bandwidth of 20nm. The eluent used is an isocratic mixture of 50v% methanol and 50v% aqueous eluent, which is composed of 5v% trifluoroacetic acid, 10v% methanol and 85v% water. The measurement is carried out at a column temperature of 30°C and an eluent flow rate of 1mL / min. Before measurement, the sample is diluted in the eluent in a ratio of 1:500 and filtered with a Chromafil Xtra H-PTFE-20 / 13 syringe filter.

[0111] The concentrations of ethylene glycol (EG) and short-chain carboxylic acids can be determined by HPLC. An Agilent 1260 HPLC equipped with an organic acid resin column (CS chromatograph) was used, and the DAD was set to signal wavelengths of 254 nm, 210 nm and 250 nm with a bandwidth of 4 nm. In addition, a refractive index detector (RI) was used at 35°C to detect non-UV active ingredients. The eluent used was an aqueous solution containing 2.5 mM H2SO4. The measurement was carried out at a column temperature of 30°C and an eluent flow rate of 1 mL / min. Before measurement, the sample was diluted in 0.1 M H2SO4 in a ratio of 1:5 and filtered with a Chromafil Xtra H-PTFE-20 / 13 syringe filter.

[0112] Example 1

[0113] Hydrolysis of PET flakes

[0114] During the preparation process, transparent PET bottle flakes were cryogenically ground in a Retsch ZM 200 with a 1 mm impact screen to obtain a starting material with a particle size of <2 mm for hydrolysis. 0.5 L of the alkaline hydrolysis solution was injected into a 1 L conical flask. 20.83 g of the ground PET was added to reach a final concentration of 0.3 M based on the aqueous solution. In addition, 0.43 g of ZnSO4·7H2O was added as a catalyst to give a catalyst to PET ratio of 1:100 in the hydrolyzate. The reaction solution was placed on a magnetic stirring plate and boiled under reflux at 300 rpm for the specified time at ambient pressure. After cooling to room temperature, the solids were removed by filtration using a quantitative filter paper with a particle cutoff size of 2.7 μm to obtain a clear hydrolyzate. Based on the amount of PET initially used, the conversion after 8 hours was 52.2% ± 1.2% when the base concentration was 0.75 M and 88.6% ± 0.7% when the base concentration was 2 M.

[0115] Example 2

[0116] Hydrolysis of solid production waste

[0117] PET particles containing 0.3w% TiO2 were ground as described in Example 1. Hydrolysis was carried out using 0.75M NaOH solution. Other reaction conditions were similar to those in Example 1. After 8 hours of reaction, the conversion was 48.05% ± 0.15% based on the amount of PET initially used. Filtration produced a significantly clear hydrolyzate with a white solid remaining on the filter.

[0118] Example 3

[0119] Hydrolysis of PET fibers

[0120] 20.83 g of PET-containing fibers from old textiles were weighed out. The fibers were placed in a reaction vessel without further pretreatment. A 0.75 M NaOH solution was used for hydrolysis. The other reaction conditions were selected similarly to Example 1. The reaction vessel was initially filled with fibers only to the extent that it was still possible to mix well. As the experiment progressed, the remaining fibers were gradually added as the volume of fibers in the reaction vessel decreased.

[0121] Regarding this, Figure 7 The concentration profiles of terephthalic acid during the depolymerization of bottle flakes (1) and fibers (2) are shown. The dots are measured values, the line is the calculated concentration profile, and the black and white dots / boxes represent duplicate experiments in each case.

[0122] Example 4

[0123] pH shift electrolysis with conventional electrodes

[0124] The anolyte used was the hydrolyzate described in Example 1. The catholyte introduced at the beginning was a 0.1 M Na2SO4 solution. The active electrode area used was 100 cm 2 The double-chamber electrolyser was a 1.5-kcal / 1.2-kilometer-long double-chamber electrolyser. The anode used was an iridium-coated titanium electrode supplied by Electrocell and the cathode used was a nickel electrode. To separate the electrolyte compartments, a fumasep F-14100 cation exchange membrane supplied by Fumatech BWT was used. A voltage was applied to establish a constant current of 5A. The electrolysis process continued until a solid suspension formed in the anolyte and the cell voltage reached the limit value of 12V.

[0125] Figure 8 The case of pH shift electrolysis of PET bottle flake hydrolysate using Ir anode is described. After 3600 seconds, the voltage rises significantly and the current drops significantly. This is due to the formation of a solid layer of terephthalic acid at the anode.

[0126] Example 5

[0127] pH shift electrolysis with conventional electrodes

[0128] The anolyte used was the hydrolyzate described in Example 2. The catholyte introduced at the beginning was a 0.1 M Na2SO4 solution. The active electrode area used was 100 cm 2 The double-chamber electrolyser was a 1.5-kcal / 1.2-kilometer-long double-chamber electrolyser. The anode used was an iridium-coated titanium electrode supplied by Electrocell and the cathode used was a nickel electrode. To separate the electrolyte compartments, a fumasep F-14100 cation exchange membrane supplied by Fumatech BWT was used. A voltage was applied to establish a constant current of 5A. The electrolysis process continued until a solid suspension formed in the anolyte and the cell voltage reached the limit value of 12V.

[0129] Fig. 9 The pH shift electrolysis of fiber hydrolysate using an Ir anode is described. After 3300 seconds, the voltage rises significantly and the current drops significantly. This is due to the formation of a solid layer of terephthalic acid at the anode.

[0130] Example 6

[0131] pH shift electrolysis with conventional electrodes

[0132] The anolyte used was the hydrolyzate described in Example 1. The catholyte introduced at the beginning was a 0.1 M Na2SO4 solution. The active electrode area used was 100 cm 2 A double-chamber electrolyser. Nickel electrodes were used for the anode and cathode. To separate the electrolyte chambers, a fumasep F-14100 cation exchange membrane from Fumatech BWT was used. A voltage was applied to establish a constant current of 5 A. The electrolysis process continued until a solid suspension formed in the anolyte and the cell voltage reached the limit value of 12 V. In contrast to the above example, the current could be maintained without decreasing for a longer operating time.

[0133] Fig.10 pH-shift electrolysis of fiber hydrolysate using a nickel anode is demonstrated. The current does not drop due to the suppression of coating formation and / or the prevention of terephthalic acid accumulation at the anode.

Claims

1. A method for producing carboxylic acid from a hydrolyzable polymer containing carboxylic acid, the method comprising at least the following process steps: i) depolymerizing the polymer by hydrolyzing the polymer in an aqueous hydrolysis solution to form a carboxylate salt and optionally at least one other monomer component of the polymer; ii) optionally removing other monomer components and any other soluble and / or insoluble impurities from the hydrolysate solution produced in process step i); iii) transferring the hydrolyzate solution produced in process step ii) to the anode chamber of the electrolysis device; iv) electrolysis using the hydrolyzate solution in the anode chamber, wherein the electrolysis device has a cathode chamber filled with liquid in addition to the anode chamber, by connecting the electrolysis device to a voltage source, current flows through the electrolysis device, and ion exchange occurs in the liquid between the anode chamber and the cathode chamber, so that the liquid in the cathode chamber becomes alkaline, and protons are formed in the anode chamber, protonating the carboxylate, resulting in precipitation of carboxylic acid; and v) removing the formed carboxylic acid from at least part of the hydrolysate solution, in, vi) The liquid produced in the cathode chamber of the electrolysis device in process step iv) is used as a component of the hydrolysis solution in step i).

2. The method according to claim 1, characterized in that The polymers include polyesters, more particularly polyethylene terephthalate, particularly where the carboxylic acid formed includes terephthalic acid.

3. The method according to any one of claims 1 and 2, characterized in that The anode has at least partially effective non-stick properties for the carboxylic acid formed in step iv).

4. The method according to any one of claims 1 to 3, characterized in that The anode is composed of at least one metal or metal alloy at least on its surface, wherein the metal or metal alloy contains at least one metal selected from the group consisting of vanadium, chromium, manganese, iron, cobalt, nickel, zirconium, niobium, molybdenum, zinc and antimony.

5. The method according to any one of claims 1 to 4, characterized in that The liquid produced in process step v) is fed to the cathode chamber of the electrolysis device.

6. The method according to any one of claims 1 to 5, characterized in that The hydrolysis carried out in process step i) is an alkaline hydrolysis.

7. The method according to claim 6, characterized in that The liquid produced in process step iv) and the liquid used as a component of the hydrolysis solution in step i) comprises a Lewis acid which is generated by electrical events during the electrolysis or during the electrolysis performed by potential reversal.

8. The method according to any one of claims 1 to 7, characterized in that The polymer is present in the hydrolysis solution at a concentration of ≥0.5 mol / L based on the hydrolysis solution.

9. The method according to any one of claims 1 to 8, characterized in that In process step v), the pH in the anode space is ≥2 to <7.

10. The method according to any one of claims 1 to 9, characterized in that The polymer depolymerized in process step i) is part of a product selected from the group consisting of textiles (including clothing), plastic packaging, plastic films and plastic bottles.

11. A carboxylic acid prepared according to the process of any one of claims 1 to 10.

12. Use of the carboxylic acids according to claim 11 as monomers for the preparation of polymers.

13. A polymer prepared from the carboxylic acid according to claim 11.

14. Use of a polymer for producing a product, in particular wherein the product is selected from the group consisting of textiles (including clothing), plastic packaging, plastic films and plastic bottles, characterised in that The polymer is the polymer according to claim 13.

15. The method according to any one of claims 1 to 10, characterized in that The polymer is the polymer according to claim 13.

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