Method for producing a monomer from the polymer comprising the monomer

Abstract

The present invention relates to a method for producing a carboxylic acid from a hydrolyzable polymer containing the carboxylic acid, the method comprising at least the following method steps: i) depolymerizing the polymer by hydrolyzing the polymer in an aqueous hydrolysis solution, such that a carboxylate and possibly at least one additional monomeric component of the polymer are formed; ii) if necessary, separating additional monomeric components which are present in the hydrolysate solution produced in method step i) and possibly present additional soluble and / or insoluble impurities; iii) transferring the hydrolysate solution produced in method step ii) into an anode chamber of an electrolysis device; iv) performing an electrolysis with the hydrolysate solution in the anode chamber, the electrolysis device comprising, in addition to the anode chamber, a cathode chamber filled with a liquid, by connecting the electrolysis device to a voltage source, wherein current flows through the electrolysis device and ion exchange occurs between the liquids in the anode chamber and cathode chamber, such that the liquid in the cathode chamber becomes alkaline and protons which protonate the carboxylate are produced in the anode chamber, resulting in precipitation of the carboxylic acid; and v) separating the formed carboxylic acid from at least part of the hydrolysate solution, wherein vi) liquid arising in the cathode chamber of the electrolysis device in method step iv) is used as a component of the hydrolysis solution in step i).

Description

[0001] Process for producing a monomer from the polymer comprising the monomer

[0002] The present invention relates to a process for producing a monomer from the polymer comprising the monomer. In particular, the present invention relates to a process in which a polymer can be regenerated by recovering the monomer from the polymer, thus enabling a resource-saving cycle.

[0003] Sustainable and resource-efficient work is becoming increasingly important in all areas of technology. Accordingly, it is crucial to be able to process a polymer in such a way that its components, especially the monomeric components, can be recovered.

[0004] EP 3511451 describes a method for recycling a mixed textile, the method comprising: i) supplying the mixed textile, wherein the mixed textile comprises cellulosic fibers and synthetic fibers, wherein the synthetic fibers comprise at least one synthetic plastic, ii) at least partially removing the synthetic plastic from the cellulose, and iii) further processing the depleted mixed textile after depletion. The primary objective of this document is the recovery of cellulose.

[0005] WO 2008 / 028429 A1 describes a process for separating terephthalic acid from its salt solutions, in which an aqueous solution of a terephthalic acid salt is introduced into a cathode compartment of a cathode chamber of an electrodialysis device and electrolyte is introduced into an anode chamber, the resulting salt and electrolyte solutions are then subjected to electrolysis and terephthalic acid, which results from the reaction of terephthalic acid anions with electrolyte cations in the anode compartment, is withdrawn from the anode compartment and separated from the electrolyte by filtration.

[0006] US 4,092,230 describes a process for producing terephthalic acid, comprising the electrolysis of an aqueous solution of potassium terephthalate, preferably in the presence of a potassium salt of an acid stronger than terephthalic acid. In another embodiment of the invention, terephthalonitrile is converted to terephthalic acid by hydrolyzing terephthalonitrile in an aqueous medium containing dipotassium terephthalate, potassium bicarbonate, and potassium hydroxide, stripping ammonia and carbon dioxide from the hydrolyzed product, adding carbon dioxide to the stripped hydrolysis product to precipitate monopotassium terephthalate, and electrolyzing the monopotassium terephthalate in the presence of an acid stronger than terephthalic acid to precipitate terephthalic acid, and separating the terephthalic acid product.

[0007] US 6,312,582 describes a process for recovering saponification products of alkaline polyterephthalate with soda, in which both terephthalate ions are present in acidic form and the sodium ions are present in the form of soda. In this process, the sodium terephthalate solution resulting from the dissolution of the saponification products is subjected to electrochemical pre-acidification to bring the pH to 4 to 7. An electrochemical acidification step is then carried out by electrolysis to precipitate the terephthalic acid in the anode section and recover the sodium in the cathode section, which can be reused.EP 2 736 968 B1 describes that in a method and a device for recycling polymeric materials, in particular polyesters and polyamides, by means of a depolymerization process, the depolymerization reaction of the material to be treated is carried out with a solvolytic mixture in at least one microwave depolymerization reactor (6) which extends substantially along an axis (A) and is provided with a system (7) for moving the reactants which enables continuous operation of the reactor (6); the moving system (7) is an Archimedean screw system which moves the reactants substantially along the axis (A) through the reactor (6).

[0008] US 8,298,396 B2 describes methods and devices for producing chemical compounds, in particular fermentation products. According to the invention, a method for producing one or more chemical substances is provided, the method comprising a fermentation step in which the substances are formed and a separation step in which at least one pair of electrodes is used to induce precipitation of the substances. The pair comprises 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 describes a process for the alkaline hydrolysis of one or more plastic polymers to terephthalic acid (TPA) and / or ethylene glycol (EG) and / or other monomers forming the one or more plastic polymers, the process comprising a) contacting the one or more plastic polymers with a metal oxide in a solution in the presence of a base to provide a reaction mixture; b) stirring the reaction mixture for a suitable time under UV light; c) recovering terephthalic acid, ethylene glycol, and / or the other monomers from the reaction mixture. However, the solutions known from the prior art may still have potential for improvement, particularly with regard to the continuous operation of efficient recycling of polyesters, in particular polyethylene terephthalate.

[0010] It is therefore the object of the present invention to provide a measure by which at least one disadvantage of the prior art is at least partially overcome. In particular, it is an object of the present invention to provide a solution by means of which an efficient production of a monomer from a polymer containing the monomer is possible.

[0011] The object is achieved according to the invention by a process having the features of claim 1. The object is further achieved by a carboxylic acid having the features of claim 11, by a use having the features of claim 12, by a polymer having the features of claim 13, by a use having the features of claim 14 and by a process having the features of claim 15. Preferred embodiments of the invention are disclosed in the subclaims, in the description and in the figures, wherein further features described or shown in the subclaims or in the description or the figures can represent an object of the invention individually or in any combination, unless the context clearly indicates the opposite.

[0012] The present invention relates to a process for producing a carboxylic acid from a hydrolyzable polymer containing the carboxylic acid, the process comprising at least the following process steps: i) depolymerizing the polymer by hydrolyzing the polymer in an aqueous hydrolysis solution to form a carboxylate and optionally at least one further monomeric constituent of the polymer; ii) optionally separating further monomeric constituents present in the hydrolyzate solution produced in process step i) and any further soluble and / or insoluble impurities present; iii) transferring the hydrolyzate solution produced in process step ii) into an anode compartment of an electrolysis device;iv) carrying out an electrolysis with the hydrolysate solution in the anode compartment, wherein the electrolysis device has, in addition to the anode compartment, a cathode compartment filled with a liquid, by connecting the electrolysis device to a voltage source, wherein current flows through the electrolysis device and an ion exchange takes place between the liquids in the anode compartment and cathode compartment, so that the liquid in the cathode compartment becomes alkaline and protons protonating the carboxylate are formed in the anode compartment, whereby, for example, the carboxylic acid precipitates in the anode compartment; and v) separating the carboxylic acid formed, wherein vi) liquid obtained in a cathode compartment of the electrolysis device in process step iv) is used as a component of the hydrolysis solution in step i);

[0013] Such a process allows a carboxylic acid to be recovered as a monomer component in a polymer in a particularly advantageous manner, so that it can be repolymerized and thus further added value.

[0014] The process thus serves in particular to recover a carboxylic acid from the polymer comprising the carboxylic acid. In this case, a carboxylic acid also includes a polycarboxylic acid, for example a di- or tricarboxylic acid, in a manner understandable to the person skilled in the art. Furthermore, the polymers equally include homopolymers and copolymers. The process initially comprises, according to process step i), depolymerizing the polymer by hydrolysis of the polymer in an aqueous hydrolysis solution to form a carboxylate and optionally at least one further monomeric constituent of the polymer. Accordingly, the polymer can be present in the aqueous hydrolysis solution and can be depolymerized there by hydrolysis, i.e. broken down into its monomeric constituents. The monomeric constituents are preferably present in dissolved form in the solution.In principle, the invention encompasses both basic and enzymatic hydrolysis, whereby basic hydrolysis may be preferred, as described in more detail below.

[0015] Depending on the hydrolysis process chosen, the hydrolysis solution contains the required components, such as a base, an enzyme and / or suitable initiators and / or catalysts.

[0016] While the general rule for hydrolysis is that the solubility limit of the various products should not be exceeded, tempering the hydrolysis solution to 100°C and above, for example with lower limits at 130°C or 140°C and upper limits at 200°C or 180°C, but also at temperatures above 200°C to 300°C, leads to an accelerated and more effective conversion of the polymer component, in particular the polyester component, into carboxylates, i.e., deprotonated carboxylic acids. Carboxylates with a plurality of functional groups, such as dicarboxylates, are also encompassed within the meaning of the invention. The same applies to other monomeric constituents, such as alcohols, so that in the case of polyesters, for example, polyols are also encompassed. This applies analogously to forced mixing of the hydrolysis solution and the polymer, such as the polyester components contained therein.

[0017] After at least partial hydrolysis or

[0018] After depolymerization, the solution is referred to according to the invention as a hydrolyzate solution. This hydrolyzate solution produced in process step i) is optionally treated according to process step ii) by separating off further monomeric constituents present in the hydrolyzate solution, i.e. the monomeric constituents which are not the carboxylic acid to be recovered, and any further insoluble and / or soluble impurities that may be present. In other words, at least one from the group of any further monomer constituents that may be present, any soluble impurities that may be present, and any soluble impurities that may be present can optionally be separated off. This preferably allows all or at least a large part of the constituents present in the hydrolyzate solution in addition to the monomer to be recovered to be removed from the hydrolyzate solution.

[0019] With regard to the separation of solids from the hydrolysate solution, the separation methods for solids known to a person skilled in the art can in principle be used, such as filtration and / or centrifugation.

[0020] For example, dyes may be present as impurities, which can be removed by adsorption. Activated carbon, for example, is suitable for this step.

[0021] Further monomeric components can be removed, for example, by extraction with a suitable solvent. When using a polyester as the polymer, a polyol such as ethylene glycol can be separated from the hydrolyzate liquid after the hydrolysis step by distillation, e.g., rectification, or extraction, or a combination of both technologies. The solvent for the extraction can be regenerated and reused. Accordingly, one or more further monomeric components, such as ethylene glycol, can be separated in parallel. The monomeric components, such as ethylene glycol, can be processed back into a polymer, such as PET, using the acid obtained in the subsequent electrolysis, e.g., terephthalic acid.For the extraction of a hydrophilic component, such as ethylene glycol, an organic solvent can be used, for example, which forms a hydrophobic eutectic solvent with the ethylene glycol and forms a two-phase system with the aqueous hydrolyzate liquid. The extraction can be carried out in several stages. The solvent can be, for example, menthol or thymol. Since ethylene glycol has a lower boiling point than the eutectic solvent, ethylene glycol can be separated as the overhead product during rectification of the eutectic solvent. The solvent obtained in parallel as the bottom product can be returned to the extraction process after optional further purification; similar processes are possible for other monomeric components.

[0022] However, it should be noted that the separation of additional components may be omitted, for example, if only a single monomer component of the polymer is present and / or if impurities are removed elsewhere. For example, it is conceivable that monomer components, dyes, or other impurities to be separated are degraded by electrical processes, particularly oxidation, at the electrodes of the electrolysis process described in detail later.

[0023] In principle, non-hydrolyzable components (e.g., components not hydrolyzable by alkaline hydrolysis) from the starting material, i.e., the polymer-containing product, such as aliphatic plastics, cotton, etc., remain in the hydrolyzate solution as a solid fraction, particularly as dispersed particles, and can be removed from the system continuously or discontinuously, for example, by filtration or another type of mechanical separation. Other particles that are insoluble in the hydrolysis liquid, such as additives or impurities in the starting material, can be removed from the system in the same way. As a result, a largely solids-free hydrolyzate liquid containing dissolved polyvalent cations (especially metal ions), recovered carboxylates, and other monomeric components, such as polyols, remains for the subsequent process steps.Likewise, residues, usually in small amounts, of unreacted hydrolysis solution, such as a base, and organic residues such as dyes, etc., may be present in the liquid. These can be removed from the remaining hydrolyzate liquid, if necessary, using suitable known methods, such as adsorption on activated carbon. Chromatographic methods or ion exchange processes are also known and suitable for the same purpose.

[0024] It is important to note that the separation of the organic contaminants, in particular, from the hydrolyzate solution occurs in a state in which the acid is present in ionic form as a carboxylate. This enables the selective separation of the often non-polar organic contaminants from the ionic terephthalates, allowing at least a large portion of the carboxylates to remain selectively in the hydrolyzate solution.

[0025] After treatment of the hydrolyzate solution according to process step ii), the hydrolyzate solution produced in process step ii) or, if process step ii) is omitted, in a directly visible manner correspondingly in process step i) according to process step iii) is transferred to the anode compartment of an electrolysis device. The electrolysis device can have a basically known structure and comprise an anode compartment with an anode, a cathode compartment with a cathode, and a membrane separating the anode compartment and the cathode compartment and which is particularly permeable to ions. This membrane can be, for example, a cation exchange membrane or a diaphragm. Furthermore, a voltage source can be connected to the electrolysis device in a manner known per se, so that the electrolysis device can be operated in a manner known per se.Accordingly, according to process step iv), electrolysis is carried out with the hydrolysate solution in the anode compartment. The cathode compartment is also filled with electrolysis liquid. Initially, for example, an aqueous solution with electrochemically inert conducting salt, such as sodium sulfate, potassium sulfate, or phosphates, can be present in the cathode compartment. If the electrolysis device is connected to a voltage source, current, in particular direct current, flows through the electrolysis device. This results in an ion exchange between the liquids in the anode compartment and cathode compartment, so that the liquid in the cathode compartment becomes alkaline through the formation of hydroxide ions at the cathode, and protons are formed in the anode compartment or at the anode. This lowers the pH of the hydrolysate solution in the anode compartment, and the electrolysis can also be referred to as pH-swing electrolysis.This allows the carboxylic acid present in the anode compartment, which is present as a carboxylate, to be protonated and precipitate. In particular, the carboxylic acid precipitates as a solid, which can be easily removed from the anode compartment as a solid suspended in the liquid. Accordingly, the process can be preferred for producing such carboxylic acids that precipitate in an aqueous solution.

[0026] In the production of terephthalic acid, for example, the discovery is exploited that in its protonated form, it is only poorly soluble in water (approximately 9.5*10-5 mol / liter). Consequently, it crystallizes in the remaining hydrolyzate liquid of the anode compartment or forms a solid, such as a suspension or sediment, which can then be easily removed from the system. The same applies, of course, to other carboxylic acids with suitable solubilities.

[0027] To optimize the electrolysis step, it can preferably be carried out at a pH value of less than 7, in particular in the buffer range of the corresponding acid, such as terephthalic acid. In the latter case, a pH value of > 2 to < 7, for example > 2 to < 6, approximately > 5 to < 6, approximately 6, is advantageous. A pH value is preferably selected at which the electrode is still electrochemically stable. This can be easily achieved with the described pH alternating electrolysis, whereby the pH value in the hydrolysate can be reduced to a value at which the acid is present in fully protonated form. Accordingly, the crystallization of the acid in the anode region is increased and the transfer of protons to the cathode region of the device is minimized. In principle, however, the pH value can be reduced to values ​​of 2.

[0028] During electrolysis, a current flow of between 0.1 and 1 A / cm 2be suitable for a cell voltage less than or equal to 12 volts.

[0029] Thus, the carboxylic acid, and hence the monomer, is present as a solid in the hydrolyzate solution in the anode compartment and can be separated from at least a portion of the hydrolyzate solution according to process step v). This can be achieved by appropriate solid separation processes known to those skilled in the art, such as centrifugation or filtration. Furthermore, the hydrolyzate solution can be removed from the anode compartment for this purpose, or the carboxylic acid can be separated from the hydrolyzate solution directly in the anode compartment. The carboxylic acid can then be processed as desired.

[0030] The purity of the monomers produced in this way, especially carboxylic acids, is easily above 80 mol%. Any remaining impurities can be brought to the desired final purity by redissolving the acid using known methods, for example, in a suitable solvent and subsequent multi-stage crystallization, adsorption, and / or chromatographic processes. Further purification methods are disclosed in the article by Brown-Marquering-Myerson: "Purification of terephthalic acid by crystal aging" in Ind. Eng. Chem. Res. 1990, 29, 10, 2089-2093.For example, the solid acid formed can be removed as a suspension from the anode compartment, for example pumped out, and separated continuously or discontinuously by means of at least one solid separation process, for example along a bypass leading away from the anode compartment with integrated separation device and return line of the residual liquid into the electrolysis device, such as into the cathode compartment.

[0031] Furthermore, according to process step vi), liquid accumulating in a cathode chamber of the electrolysis device during process step iv) is used as a component of the hydrolysis solution in step i). In other words, liquid accumulating in the cathode chamber is transferred to a hydrolysis volume in which it can form at least part of the hydrolysis solution.

[0032] This allows the base required for hydrolysis to be produced and used in-situ in conjunction with the described electrolysis step. A catalyst used in this process, particularly in the form of soluble metal ions or a Lewis acid, especially multivalent metal ions, can be recovered after the hydrolysis step, for example, in the subsequent electrolysis step. Due to the flow of electric current, the metal ions migrate to the cathode side together with the cations of the base, where they accumulate in the basic solution. At the same time, during electrolysis, multivalent cations pass from the anode compartment into the cathode compartment via the ion-permeable wall or membrane of the electrolysis cell and form a base with the hydroxide ions produced in the cathode compartment.

[0033] Furthermore, the metal ions can also adhere to the electrode in the form of metals through electrochemical processes, such as electrochemical deposition, and thus be released by a brief potential reversal of the electrolysis device. This base and the metal ions acting as Lewis acids can then be fed into a device for carrying out the first process step (hydrolysis) and thus be fully recycled, i.e., circulated. Suitable metal ions include, for example, Zn, Cd, Hg, Cu, Ag, Ni, Pd, Co, Fe, Ti, Be, Mg, Ca, Sr, Al, Sb, and Sn, of which Sb, Ti, Zn, and Ni may be particularly preferred. Accordingly, it can be advantageous if the liquid obtained in process step iv) and used as a component of the hydrolysis solution in step i) comprises a Lewis acid generated by electrical processes during the electrolysis or by a process carried out with potential reversal for the electrolysis.

[0034] Within polymer recycling, the process base, or rather the Lewis acid that acts as a catalyst for hydrolysis, is also recycled. This simultaneously conserves resources and reduces energy consumption, emissions, and transport costs. The use of pH-swing electrolysis and thus the process according to the invention avoids or significantly reduces the generation of salt waste and—compared to chlor-alkali electrolysis—the handling of gaseous chlorine compounds in the process. From a process engineering perspective, this base can be obtained particularly advantageously as follows: Separation of the solid acid, particularly terephthalic acid, formed in the anode compartment of the electrolysis produces a liquid that can be fed into the cathode compartment of the electrolysis cell. Accordingly, the liquid obtained in process step v) can be fed into the cathode compartment of the electrolysis device.At the same time, cations, in particular alkali metal or metal cations, which migrate from the anode to the cathode compartment during electrolysis, form an alkaline solution with hydroxide ions which are formed during the electrochemical water splitting at the cathode. This alkaline solution contains the electrochemically produced base as well as other catalytically active alkali metal and metal ions. This solution can thus be used for hydrolysis as described above. The concentration of the Lewis acid during hydrolysis can preferably be in a range from < 5 wt.%, preferably > 0.5 wt.% to < 2 wt.%, approximately > 1 wt.% to < 2 wt.%, in particular 1 wt.%, based on the hydrolysis solution. If the amount of Lewis acid returned from the electrolysis device is insufficient, further Lewis acid can be added.

[0035] The process takes advantage of a particularly advantageous side effect: In polyester fibers in textiles, for example, such multivalent cations, for example based on zinc (Zn), iron (Fe) or titanium (Ti), are already present. They originate, among other things, from residues of the catalyst from the polymerization, from additives in the composite materials or from impurities in the textile material. Optionally, such multivalent cations can also be released into the anolyte through oxidation reactions on the electrode surface or added separately, for example as salts, in order to adjust a desired concentration of the catalyst, based on the polyester content in the hydrolysis. The pH-changing electrolysis of the process described here keeps these catalytically active multivalent metal ions in a largely closed process cycle.

[0036] It is advantageous to use the base in a concentration and amount to achieve complete deprotonation of the carboxylic acid, which is cleaved during hydrolysis.

[0037] Furthermore, the process described here is particularly advantageous because, in the interests of sustainable processes, it is becoming increasingly important to ensure high purity of produced monomers for reuse in the manufacture of a plastic. Accordingly, residue-free removal of the solvents from the plastic is preferred. Such processes have not yet found any relevant application, but are easily feasible according to the invention, for example by drying the acid after its separation. The invention is also based on the following considerations and findings. Large quantities of polymer-containing composite materials, such as PET-containing ones, are available, which have so far predominantly been disposed of, i.e. in particular landfilled or incinerated. Recycling processes known to date are complicated and are often still in the experimental stage.Accordingly, there is an urgent need to use these composite materials or composite materials as a raw material source for the recycling of the monomers they contain. Specifically, it would be desirable to produce recycled products, also referred to as "secondary" products, from the composite materials, especially terephthalic acid, and thus conserve primary raw material sources. The secondary products thus obtained should possess the essential product properties of a corresponding primary, i.e., conventionally produced product, in order to be able to replace it as closely as possible.

[0038] It has been found that such secondary monomers, for example secondary terephthalic acid, can be obtained from polymer-containing products essentially by a combination of depolymerization by hydrolysis and subsequent specific electrolysis of the hydrolysate.

[0039] In a first process step, the depolymerization of the polymer, such as the polyester, polyamide or another polymer containing a carboxylic acid as a monomer, by means of hydrolysis serves to form a hydrolyzate liquid which, in addition to optionally a further monomer component, comprises at least one carboxylate, in particular terephthalate.

[0040] The process according to the invention thus offers previously unattainable advantages, as outlined below. For example, recyclable materials that were previously mostly landfilled can be reused as new products, adding value. This conserves resources and reduces the burden on the environment while reducing the consumption of primary raw materials, especially fossil fuels. Due to their free availability, this can be done without any specific purpose, allowing, for example, clothing materials to be converted into entirely different products. Furthermore, energy consumption and emissions during the production of new products can be significantly reduced.

[0041] It may be preferred that the polymer comprises a polyester, in particular polyethylene terephthalate, especially where the carboxylic acid formed comprises terephthalic acid. In this regard, it should be mentioned that polyester-based fibers are currently predominantly used in textiles. From 2025, textiles in Europe must be collected and recycled [EU Strategy for Sustainable and Recyclable Textiles]. In Germany alone, >1,000,0001 used or surplus textiles are currently collected per year. In Europe, over 5 million tons of these textiles are expected for 2025. Reprocessing to obtain individual and pure components of these composite materials is therefore particularly advantageous. Among these, polyethylene terephthalate is a prominent example, which can also be used particularly advantageously by the process according to the invention.For example, with regard to the solubility of terephthalic acid in water, which can enable effective precipitation or crystallization in the anode compartment.

[0042] With regard to the use of a polyester as an exemplary application, the polyester-containing starting material, i.e., the polymer-containing product, can comprise, for example, polyester and fiber blends of polyester plastics such as PET, cotton, polyethylene (PE), and / or polypropylene (PP), and optionally other components. These components can be alkaline-hydrolyzable components (such as PET) or non-alkaline-hydrolyzable components (such as cotton, PE, or PP). The resulting hydrolysis liquid contains the polyester monomers, specifically in the form of at least one polyol and at least one dicarboxylic acid as the corresponding dicarboxylate, each in dissolved form.

[0043] With alkaline hydrolysis, these components can be advantageously separated and the alkaline-hydrolyzable components, for example terephthalate, can be subjected to a subsequent electrolysis, for example a pH-change electrolysis as described above, in order to then obtain the corresponding monomers in pure form in further process steps.

[0044] It may further be preferred that the anode be configured, at least in part, with anti-adhesion properties effective with respect to the carboxylic acid formed in step iv). This configuration can allow for significant process-related advantages. This is because preventing the adhesion of solids, such as in particular the carboxylic acid formed, to the electrode surface increases the amount of suspended acid, for example, terephthalic acid, in the anode compartment. Furthermore, there is no need to interrupt the process to remove deposits from the electrodes. The process can thus be carried out with minimal maintenance and long-term stability. Anti-adhesion properties can be achieved in a variety of ways.

[0045] For example, it can be provided that the anode, in the presence of the respective anolyte, at least largely suppresses the adhesion of solid terephthalic acid to the electrode, in particular due to electrochemical reactions between the electrode and the liquid. Concrete examples include, for example, that the anode is formed, at least on its surface, from at least one metal or a metal alloy comprising at least one metal from the group consisting of vanadium (v), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), zirconium (Zr), niobium (Nb), and molybdenum (Mo). The alloy can comprise at least one, for example at least two, of the aforementioned metals.

[0046] Another advantage is that these electrodes are less expensive than conventional platinum electrodes. Furthermore, nickel electrodes or electrodes based on an alloy with a nickel content, for example, >50 mol% nickel, are simultaneously electrochemically stabilized during this process step.

[0047] It may further be preferred that the hydrolysis carried out in process step i) be a basic hydrolysis. One advantage of basic hydrolysis, in contrast to enzymatic hydrolysis, for example, is that the hydrolysis is very temperature-stable. Despite their advantages, enzymatic hydrolyses can usually only be used below the glass transition temperature of PET (approximately 80°C). The crystalline components of PET, for example, are therefore more easily accessible with basic hydrolysis, since alkaline hydrolysis can also be carried out above the glass transition temperature of the polyester.

[0048] A further advantage has been shown to be that no organic additives are required, as is often the case with other hydrolysis processes. The polyester component, for example, can be converted more quickly into carboxylate and polyols, making the process more effective in this configuration. This can be further enhanced by the fact that, when using alkaline hydrolysis, particularly with an aqueous hydrolysis solution, the resulting carboxylic acid can be neutralized in the presence of a base and thus irreversibly removed from the reaction equilibrium.

[0049] Furthermore, it may be preferred that the process be carried out continuously. A particular advantage of the process when carried out continuously can be seen, for example, in the fact that, in addition to the supply of the product to be recycled and the separation of the secondarily formed acid, for example, terephthalic acid, the recirculation of the aforementioned base from the electrolysis device to the hydrolysis device and from there back to the electrolysis device can also be carried out continuously. The latter can therefore be particularly advantageous when conducting basic hydrolysis.

[0050] It may further be preferred that the polymer be present in the hydrolysis solution in a proportion of >0.5 mol / L, based on the hydrolysis solution. The upper limit may be determined, for example, by the maximum solubility of the carboxylate formed. Exemplary proportions may, for example, be in a range from <5 mol / L, for example >1 mol / L to <2 mol / L.

[0051] In particular, in this embodiment, the polymer can be effectively hydrolyzed under the given conditions, so that the process can be carried out particularly advantageously, since the solubility limit is usually not exceeded and the polymer is thus dissolved in the hydrolysis solution.

[0052] As already indicated above, it can be particularly advantageous if the polymer depolymerized in process step i) is a component of a product selected from the group consisting of textiles, including clothing, plastic packaging, plastic films, and plastic bottles. In particular, such products, which are considered valuable materials, were previously disposed of because recycling or extracting the raw materials was complex. However, the invention provides the advantage that even such products, which, in addition to the monomer units, contain other substances that complicate further processing, can be easily treated using the process according to the invention. In other words, the aforementioned products are to be characterized as mixed products, which can be easily treated as starting materials according to the invention, even if they consist only partially of hydrolyzable polymers.

[0053] Depending on the electrochemical standard potential, electrolysis can lead to a reduction of the catalytically active ions and their deposition at the cathode. The non-forming parts of the electrode can be reoxidized and thus removed from the electrode by a temporary potential reversal combined with a higher flow rate.

[0054] Hydrolysis can be particularly efficient and time-saving if the product is introduced into the hydrolysis liquid as a comminuted fraction. The resulting fraction can significantly improve the reaction conditions.

[0055] The above advantages are also realizable for a carboxylic acid produced using the process described above. Specifically, the carboxylic acid can be produced in a highly resource-efficient and sustainable manner, as it is recovered from old material or waste. Furthermore, the carboxylic acid itself, as well as the materials used in its production, can be recycled, which further emphasizes the advantages described above. Such a carboxylic acid can be distinguished from other types of carboxylic acids because it can still contain, at least in traces, components of the hydrolysis. For example, the carboxylic acid can still contain metals or metal ions from the Lewis acids used in the hydrolysis, which can serve as catalysts in a potentially feasible polymerization.

[0056] As stated above, it can be particularly advantageous if the carboxylic acid is terephthalic acid, since in this way, polyethylene terephthalate, in particular, can be used and recycled. This polymer has great potential, particularly with regard to further processing. Thus, the use of the carboxylic acid produced by the process according to the invention, in particular terephthalic acid, as a monomer for producing a polymer, in particular polyethylene terephthalate, is also described.

[0057] Accordingly, the present invention further provides a polymer, in particular polyethylene terephthalate, produced from the above-described carboxylic acid, in particular terephthalic acid. Such a polymer can be distinguished from polymers formed in other ways because it may still contain, at least in traces, components of the hydrolysis. For example, the polymer may still contain metals or metal ions of the Lewis acids used in the hydrolysis, which can serve as catalysts in the previously conducted polymerization.

[0058] The present invention thus 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, plastic bottles, characterized in that the polymer is one as described above.

[0059] It can be particularly advantageous if the monomer produced from a polymer by hydrolysis or depolymerization and electrolysis is converted back into a polymer and this is then converted into a monomer according to the process described above.

[0060] Following the above, a solid acid produced by the process, in particular terephthalic acid, can thus be used as a monomer for the production of polymers, such as polyethylene terephthalate (PET), or for the production of products from these polymers. Any additional purification steps are mentioned above by way of example. The polymer produced or the polymer-containing product can serve as a substitute for a polymer produced from fossil raw materials. The features of the novel process for producing, for example, terephthalic acid and its uses disclosed above can be applied analogously to the production and use of monomers that are chemically comparable to those of terephthalic acid. This includes a process for the secondary production of 2,5-furandicarboxylic acid (FDCA) from a corresponding product (polyethylene furanoate = PEF).Accordingly, secondary PEF can be produced from recycled FDCA and ethylene glycol.

[0061] The invention is explained below by way of example with reference to the attached drawings and examples, wherein the features presented below can represent an aspect of the invention both individually and in combination, and wherein the invention is not limited to the following drawings, examples, the following description and the following embodiments.

[0062] They show:

[0063] Fig. 1 is a schematic view of an embodiment of a method according to the present invention;

[0064] Fig. 2 shows the general reaction mechanism of Lewis acid catalyzed alkaline polyester hydrolysis as hydrolysis of a preferred polymer;

[0065] Fig. 3 shows the hydrolysis using the concrete example of basic hydrolysis of polyethylene terephthalate to ethylene glycol and sodium terephthalate.

[0066] Fig. 4 an electrolysis cell with a pH-shift electrolysis taking place in it;

[0067] Fig. 5 shows a regeneration of a Lewis acid catalyst in an embodiment of the process according to the invention;

[0068] Fig. 6 shows an exemplary operating range of the pH value of pH-swing electrolysis; Fig. 7 shows the concentration curve of terephthalic acid during the depolymerization of bottle flakes (1) and fibers (2);

[0069] Fig. 8 shows the current and voltage curve of an electrolysis with an IR anode;

[0070] Fig. 9 shows the current and voltage curves of an electrolysis with an IR anode; and Fig. 10 shows the current and voltage curves of an electrolysis with a Ni anode.

[0071] Figure 1 shows a schematic representation of one embodiment of a process according to the present invention. Such a process serves to produce a carboxylic acid from the polymer comprising the carboxylic acid as a monomer component, wherein the polymer can be selected, for example, from a polyester and a polyamide.

[0072] According to reference numeral 10, a polymer-containing product is first provided, which is selected, for example, from the group consisting of textiles, including clothing, plastic packaging, plastic films, plastic bottles.

[0073] According to process step 1, the polymer is first depolymerized by hydrolysis of the polymer in an aqueous hydrolysis solution to form a carboxylate and optionally at least one further monomeric constituent of the polymer.

[0074] Such a hydrolysis is shown as basic hydrolysis in the scheme of Figures 2 and 3. More specifically, Figure 2 shows the general reaction mechanism of Lewis acid-catalyzed alkaline polyester hydrolysis as the hydrolysis of a preferred polymer, and Figure 3 shows the hydrolysis using the specific example of the basic hydrolysis of polyethylene terephthalate to ethylene glycol and sodium terephthalate. Both hydrolyses are Lewis acid-catalyzed. Returning to Figure 1, in process step 2, a solids separation advantageously takes place to separate the solids present in the hydrolyzate solution formed during the hydrolysis, so that the solid-containing impurities can be removed according to reference numeral 11. Furthermore, other components, such as additives, can be separated according to process step 3 as separated components according to reference numeral 12.

[0075] The process step according to reference numeral 4 further shows the separation of another monomer, such as a polyol. This can be achieved, in particular, by extraction with a water-insoluble solvent. According to reference numeral 13, the monomer, such as the polyol, is regenerated together with the solvent, with the regeneration being indicated by reference numeral 5. Subsequently, the regenerated monomer can be collected according to reference numeral 9, and the solvent can be reused according to reference numeral 14.

[0076] The carboxylate of the carboxylic acid to be produced resulting from the hydrolysis can then be fed into an electrolysis device, of which reference numeral 6a indicates the cathode side and reference numeral 6b indicates the anode side.

[0077] Figure 4 shows such a pH-swing electrolysis, with Figure 4 showing in more detail the electrolysis for obtaining crystalline terephthalic acid by crystallization in the anolyte and cathode-side recovery of base and Lewis acid catalyst.

[0078] The liquid in the cathode compartment or the cation side 6a of the electrolysis cell 15 of an electrolysis device is preferably alkaline, and this solution can be the hydrolyzate solution. This results in the following effects: as soon as a direct voltage is applied to the electrolysis cell 15 by a voltage source of the electrolysis device, a current flows through the electrolysis cell 15, and ions can flow through the cation-permeable membrane 18. The circuit is preferably formed by monovalent cations (Li+, Na+, K+), which form a base on the cathode side of the electrolysis cell 15, and by multivalent metal ions Me 2+(Zn2+, Cu2+, Fe3+, etc.), which have catalytic activity in the ester cleavage of alkaline hydrolysis.

[0079] Between the two electrically connected electrodes (each in an aqueous environment), the water is electrolytically split according to the following reactions:

[0080] In the anode compartment: 2 H2O -> 4H + + O2+ 4e~

[0081] In the cathode compartment: 2 H2O + 4e~ -> 4 OH~ + 2 H2

[0082] If the circuit within the electrolysis cell 15 is closed neither by protons (H+) nor by hydroxide ions (OH-), but predominantly by other ions such as alkali metal ions or metal ions, the electrochemically produced protons form an acid in the anode compartment and a base in the cathode compartment. This results in an electrochemically induced pH change. The formation of an acid in the anode compartment leads to a shift in the dissociation equilibria towards a protonated form. In the case of terephthalic acid, the protonated form of terephthalic acid has a lower solubility than hydrogen terephthalate (HTPA) or terephthalate (TPA). 2), so that terephthalic acid can precipitate as a solid during the electrochemical pH change. At the same time, a basic solution is generated on the cathode side, since the (alkali) metal ions contained in the terephthalate act as charge carriers, closing the circuit in the electrolysis cell 15 and experiencing a shift into the cathode compartment due to the electrodialytic effect. This electrolysis is also called pH swing electrolysis or pH-change electrolysis. In other words: On the anode side (in the anode compartment) of the electrolysis device, terephthalic acid can form through crystallization in the anolyte, such as precipitating or sedimenting. The terephthalate obtained in the remaining hydrolyzate liquid is converted into largely fully protonated terephthalic acid by electrochemically produced protons.The protonated terephthalic acid crystallizes in the anode compartment and can be withdrawn from the electrolysis device in suspended form. At the same time, a base is formed on the cathode side (in the cathode compartment) of the electrolysis device, which may contain polyvalent cations, as explained in more detail below.

[0083] In a subsequent process step, the solid terephthalic acid thus formed can be withdrawn from the anode compartment of the electrolysis device and treated for subsequent use. The residual liquid resulting from the separation of the solid terephthalic acid can also be reused by being fed into the cathode compartment of the electrolysis cell. The cations migrating through the membrane in the electrolysis device then combine with the hydroxide ions produced during water splitting at the cathode to form the desired alkaline solution in the cathode compartment, which can then be fed back into the hydrolysis stage.

[0084] In Figure 5, the left-hand image shows how a Lewis acid catalyst can form into a metal during electrolysis. The right-hand image shows that the Lewis acid catalyst can be regenerated by a potential reversal and thus reintroduced into the process, particularly for hydrolysis.

[0085] Returning to Figure 1, reference numeral 7 denotes the separation of the desired monomer or carboxylic acid, as shown in a preferred example of terephthalic acid, which can be removed according to reference numeral 8. Figure 6 also shows an advantageous operating pH range for the pH-swing electrolysis for the example of terephthalic acid. Terephthalic acid has two dissociation stages. Above pH 5, hydrogen terephthalate and terephthalate are predominantly present. By protonating the hydrogen terephthalate near the anode, terephthalic acid crystals can be formed as low as pH 6. The pH-swing electrolysis is operated within the buffer range of terephthalic acid.In Figure 6, the operating area 2 corresponds to the process status b shown in Figure 1, the operating area 1 corresponds to the process status a shown in Figure 1 and the operating area 3 corresponds to the process status c shown in Figure 1.

[0086] The above process provides an alternative, environmentally friendly method for the recovery 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 recourse to fossil raw materials or primary raw materials. At the same time, it can demonstrate a way to avoid, in particular, the landfilling or incineration of polyester-containing products and the resulting further environmental pollution.

[0087] Examples

[0088] Determination of the concentration of the monomers TPA and EG

[0089] The concentration of dissolved terephthalic acid (TPA) and its salts can be determined using HPLC. An Agilent 1200 HPLC equipped with a C18ec column (CS chromatography) and a DAD set to a signal wavelength of 250 nm with a 20 nm bandwidth is used. The eluent is an isochratic mixture of 50v% methanol and 50v% of an aqueous eluent consisting 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 µL / min. Samples are diluted 1:500 in the eluent before measurement and filtered using a Chromafil Xtra H-PTFE-20 / 13 syringe filter.

[0090] The concentration 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 chromatography) and a DAD tuned to a signal wavelength of 254 nm, 210 nm, and 250 nm with a 4 nm bandwidth is used. In addition, a refractive index detector (RI) at 35°C is used to quantify non-UV active components. An aqueous solution containing 2.5 mM H2SO4 is used as the eluent. The measurement is carried out at a column temperature of 30°C and an eluent flow rate of µmL / min. The samples are diluted 1:5 in 0.1 M H2SO4 before measurement and filtered with a Chromafil Xtra H-PTFE-20 / 13 syringe filter.

[0091] Example 1

[0092] Hydrolysis of PET bottle flakes

[0093] In preparation, clear 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 diameter of < 2 mm for hydrolysis. 0.5 L of an alkaline hydrolysis solution was poured into a 1 L Erlenmeyer flask. 20.83 g of ground PET was added to achieve a final concentration of 0.3 M based on the aqueous solution. In addition, 0.43 g of ZnSC>4*7H2O was added as a catalyst to achieve a ratio of 1:100 catalyst:PET in the hydrolyte. The reaction solution was boiled under reflux and ambient pressure on a magnetic stir plate at 300 rpm for the desired time. After cooling to room temperature, solids were removed by filtration through a quantitative filter paper with a particle retention of 2.7 micrometers to obtain a clear hydrolysate.With a base concentration of 0.75 M, a conversion of 52.2% ±1.2% is achieved after 8 hours, and with 2 M, 88.6% ±0.7%, based on the initial PET amount. Example 2.

[0094] Hydrolysis of solid-containing production waste

[0095] PET granules with a TiCh content of 0.3 wt% are milled as described in Example 1. A 0.75 M NaOH solution is used for the hydrolysis. The other reaction conditions are selected analogously to Example 1. After 8 hours of reaction time, a conversion of 48.05% ±0.15% based on the initial PET amount is achieved. After filtration, an apparently clear hydrolyzate is obtained, while a white solid is retained in the filter.

[0096] Example 3

[0097] Hydrolysis of PET-containing fibers

[0098] 20.83 g of PET-containing fibers from used textiles are weighed. The fibers are added to the reaction vessel without further pretreatment. A 0.75 M NaOH solution is used for hydrolysis. The other reaction conditions are selected analogously to Example 1. The reaction vessel is initially filled with only as much fiber as is necessary to ensure thorough mixing. The remaining fibers are then added gradually over the course of the experiment as the fiber volume in the reaction vessel decreases.

[0099] In this regard, Figure 7 shows the concentration curve of terephthalic acid during the depolymerization of bottle flakes (1) and fibers (2). The points are measured values ​​and the lines are calculated concentration curves, with black and white dots and boxes indicating duplicate experiments, respectively.

[0100] Example 4 pH change electrolysis with conventional electrodes

[0101] A hydrolysate as described in Example 1 is used as the anolyte. A 0.1 M NaSO4 solution is used as the catholyte. A two-chamber electrolysis cell with 100 cm 2 The active electrode area is used. An iridium-coated titanium electrode from Electrocell serves as the anode, and a nickel electrode serves as the cathode. A fumasep F-14100 cation exchange membrane from Fumatech BWT separates the electrolyte compartments. A constant current of 5 A is established by applying a voltage. Electrolysis is carried out until a solid suspension forms in the anolyte and the cell voltage limit of 12 V is reached.

[0102] Figure 8 describes the pH-alternating electrolysis of the hydrolyzate from PET bottle flakes using an IR anode. After 3600 s, a significant increase in voltage and a decrease in current are observed. This is due to the formation of a solid layer of terephthalic acid on the anode.

[0103] Example 5 pH change electrolysis with conventional electrodes

[0104] A hydrolysate as described in Example 2 is used as the anolyte. A 0.1 M NaSO4 solution is used as the catholyte. A two-chamber electrolysis cell with 100 cm 2The active electrode area is used. An iridium-coated titanium electrode from Electrocell serves as the anode, and a nickel electrode serves as the cathode. A fumasep F-14100 cation exchange membrane from Fumatech BWT separates the electrolyte compartments. A constant current of 5 A is established by applying a voltage. Electrolysis is carried out until a solid suspension forms in the anolyte and the cell voltage limit of 12 V is reached.

[0105] Figure 9 describes the pH-cycling electrolysis of the fiber hydrolysate with an Ir anode. After 3300 s, a significant increase in voltage and a decrease in current are observed. This is due to the formation of a solid layer of terephthalic acid on the anode. Example 6: pH-cycling electrolysis with conventional electrodes

[0106] A hydrolysate as described in Example 1 is used as the anolyte. A 0.1 M Na2SO4 solution is used as the catholyte. A two-chamber electrolysis cell with 100 cm 2 active electrode area is used. A nickel electrode is used as the anode and cathode. A fumasep F-14100 cation exchange membrane from Fumatech BWT is used to separate the electrolyte compartments. By applying a voltage, a constant current flow of 5 A is established. Electrolysis is carried out until a solid suspension forms in the anolyte and the cell voltage limit of 12 V is reached. It is clearly evident that, in contrast to the previously mentioned examples, a longer operating time is possible without a drop in current.

[0107] Figure 10 shows the pH cycling electrolysis of the fiber hydrolysate with a Ni anode.

[0108] The drop in current intensity does not occur because the formation of the covering layer is suppressed or because terephthalic acid is prevented from accumulating on the anode.

Claims

Patent claims 1. A process for producing a carboxylic acid from a hydrolyzable polymer containing the carboxylic acid, the process comprising at least the following process steps: i) depolymerizing the polymer by hydrolyzing the polymer in an aqueous hydrolysis solution to form a carboxylate and optionally at least one further monomeric constituent of the polymer; ii) optionally separating further monomeric constituents present in the hydrolyzate solution produced in process step i) and any further soluble and / or insoluble impurities present; iii) transferring the hydrolyzate solution produced in process step ii) into an anode compartment of an electrolysis device;iv) carrying out an electrolysis with the hydrolysate solution in the anode compartment, wherein the electrolysis device has, in addition to the anode compartment, a cathode compartment filled with a liquid, by connecting the electrolysis device to a voltage source, wherein current flows through the electrolysis device and an ion exchange takes place between the liquids in the anode compartment and cathode compartment, so that the liquid in the cathode compartment becomes alkaline and protons protonating the carboxylate are formed in the anode compartment, whereby the carboxylic acid precipitates; and v) separating the carboxylic acid formed from at least a portion of the hydrolysate solution, wherein vi) liquid obtained in the cathode compartment 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 polymer comprises a polyester, in particular polyethylene terephthalate, in particular wherein the carboxylic acid formed comprises terephthalic acid.

3. Process according to one of claims 1 or 2, characterized in that the anode is at least partially designed with anti-adhesion properties effective with respect to the carboxylic acid formed in step iv).

4. Method according to one of claims 1 to 3, characterized in that the anode is formed, at least on its surface, from at least one metal or a metal alloy comprising at least one metal from the group consisting of vanadium, chromium, manganese, iron, cobalt, nickel, zirconium, niobium, molybdenum, zinc and antimony.

5. Process according to one of claims 1 to 4, characterized in that liquid obtained in process step v) is passed into the cathode chamber of the electrolysis device.

6. Process according to one of claims 1 to 5, characterized in that the hydrolysis carried out in process step i) is a basic hydrolysis.

7. The process according to claim 6, characterized in that the liquid obtained in process step iv) and used as a component of the hydrolysis solution in step i) comprises a Lewis acid which was generated by electrical processes during the electrolysis or in a process carried out with potential reversal for the electrolysis.

8. Process according to one of claims 1 to 7, characterized in that the polymer is present in the hydrolysis solution in a proportion of > 0.5 mol / L, based on the hydrolysis solution.

9. Process according to one of claims 1 to 8, characterized in that in process step v) a pH value of > 2 to < 7 is present in the anode compartment.

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

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

12. Use of the carboxylic acid according to claim 11 as a monomer for producing a polymer.

13. A polymer prepared from a 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, plastic bottles, characterized in that the polymer is one according to claim 13.

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

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