Process for depolymerizing polyethylene terephthalate by diolysis

Abstract

The present invention relates to a process for depolymerizing a material comprising polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG), the process comprising reacting a material comprising PET with EG in the presence of at least one catalyst to obtain a glycolysis product comprising bis(2-hydroxyethyl) terephthalate (BHET) and / or oligomers thereof, wherein the catalyst comprises an acetate salt of a metal selected from calcium, magnesium, aluminum, and mixtures thereof. The catalyst can be prepared in situ by a reaction between acetic acid and a compound containing the metal.

Description

[0001] This invention relates to a method for depolymerizing polyethylene terephthalate (PET) via glycolysis. In particular, the method uses a catalyst that has a reduced environmental impact and can be prepared from readily available raw materials.

[0002] Polyethylene terephthalate (PET) is a semi-crystalline thermoplastic polyester with high strength and transparency. PET is obtained through the esterification of terephthalic acid (TPA) with ethylene glycol (EG) or the transesterification of dimethyl terephthalate (DMT) with EG. Due to its physical and chemical properties, PET has many applications. In particular, PET is used to produce textile fibers and felts (fiber-grade PET), food containers (e.g., beverage bottles - bottle-grade PET), and food packaging films (film-grade PET).

[0003] Currently, global PET production exceeds 80 million tons annually. Given PET's high consumption and non-biodegradability, there is a strong technological need to recycle this material to reduce the environmental impact of end-of-life products and the consumption of fossil-derived raw materials for manufacturing new products.

[0004] The first method of recycling PET waste is through so-called "mechanical recycling." Mechanical recycling is based on the mechanical separation of polymers from contaminants and their subsequent regranulation. Specifically, mechanical recycling involves sorting and separating PET waste, removing contaminants, grinding, heat treatment, and extruding into pellets. Despite its widespread use, this recycling technology has limitations; it can only be used to recover PET from bottles in which the contaminant content is relatively low (less than about 10% w / w). Furthermore, the presence of dyes and other additives in PET waste, and the need to regranulate it to allow its reintroduction into the industrial production cycle, results in recycled polymers of lower quality than those obtained from the virgin feedstock. Mechanical recycling also carries the risk of the aforementioned contaminants accumulating in the polymer, leading to a decline in the quality of the polymer obtained with each recycling cycle.

[0005] The second method for recycling PET waste is so-called "chemical recycling." Chemical recycling is based on the chemical conversion of the polymer chains of PET into their starting monomers or oligomers through solvent decomposition (depolymerization). The PET depolymerization products can be easily purified from contaminants before being used in new polymerization processes, thus overcoming the drawbacks associated with mechanical recycling processes. Because chemical recycling allows for the acquisition of virgin feedstocks from the processing of PET waste, this technology has the added advantage of meeting the criteria that form the basis of the concepts of a circular economy and sustainable development.

[0006] PET depolymerization can occur in several ways:

[0007] - The monomers EG and TPA are produced via hydrolysis (acidic, alkaline, or neutral).

[0008] - DMT is produced via methanol decomposition, and then DMT is converted into PET via transesterification with EG;

[0009] -By diol hydrolysis, for example by breaking the polymer chain via EG, yields bis-2-hydroxyethyl terephthalate (BHET).

[0010] Currently, depolymerization via EG diol hydrolysis appears to be the most promising recycling technology, as it produces a product (compound BHET) that can be directly used as a monomer in the synthesis of PET or other widely used polymers (e.g., unsaturated polyester resins used to produce polyurethane).

[0011] Depolymerization of PET via EG diol hydrolysis typically occurs at temperatures and atmospheric pressures ranging from 180°C to 250°C, usually in the presence of a catalyst. During depolymerization, the polymer chains are transformed through solvent decomposition, theoretically resulting in complete depolymerization into monomers (BHET) or partial depolymerization, producing monomers and oligomers.

[0012] Down Figure 1 The diagram below illustrates the reactions involved in the depolymerization of PET using EG:

[0013] Figure 1

[0014]

[0015] BHET can be used as a monomer for producing various grades (bottle grade, fiber grade, and film grade) of PET with characteristics and properties completely equivalent to those of PET obtained from raw materials. Figure 2 The following diagram illustrates the polymerization reaction that enables the formation of PET from BHET:

[0016] Figure 2

[0017]

[0018] BHET polymerization releases one mole of EG, which can be recycled, for example, during depolymerization.

[0019] The depolymerization reaction can be carried out with or without a catalyst. However, given the lower conversion rate of PET to BHET and the significantly longer reaction time compared to catalytic diol hydrolysis, non-catalytic depolymerization is an economically unfeasible method.

[0020] The most commonly used catalysts for PET depolymerization are primarily based on metal salts, particularly salts of Zn, Ti, Pb, Mn, and Na. These metal salts are typically acetates, carbonates, bicarbonates, and chlorides. In industrial processes, zinc acetate is by far the most widely used catalyst due to its high efficiency in terms of both rate and yield of the depolymerization reaction.

[0021] At the end of depolymerization, the metal introduced along with the catalyst must be removed from the monomer before it is used to produce another polymer, both to prevent interference with the quality of the final product and to avoid the accumulation of the metal in the polymer during subsequent recycling.

[0022] For example, catalytic depolymerization methods using metal salts as catalysts are described in EP 0723951 A1 and WO 2017 / 087752A1.

[0023] EP 0723951 A1 describes a method for preparing high-purity BHET by depolymerizing PET from recycled post-consumer or post-industrial waste. Preferably, the depolymerization is catalyzed by a catalyst based on Zn acetate, La acetate, and Ce acetate or titanium butoxide (Ti(OBu)4). A single embodiment of the method uses zinc acetate as a catalyst at an amount of approximately 2.2 mmol / kg (PET+EG). Among the catalysts tested, EP 0723951 A1 also mentions calcium acetate and other compounds, indicating that calcium acetate is less effective than zinc acetate.

[0024] WO 2017 / 087752 A1 describes a method for the chemical recovery of PET, in which microwave-assisted depolymerization is used. The reaction mixture comprises a catalytic system containing a catalyst and a microwave absorber. The catalyst is preferably a zinc salt, particularly zinc acetate. Alternative catalysts include magnesium acetate.

[0025] The metal catalysts currently used for PET depolymerization have several drawbacks. First, the most effective catalysts are based on transition metals and heavy metals (e.g., Zn, Ti, Pb), which have a high environmental impact and are elements that are quite difficult to remove from the BHET monomer at the end of depolymerization.

[0026] Second, due to the relatively large amount of catalyst used (the catalyst / PET ratio in the reaction mixture can be as high as 4% w / w to 6% w / w, depending on the type of catalyst and reaction conditions), it is crucial to ensure a continuous supply of catalyst in the plant to guarantee the continuity of the depolymerization process. This means that the plant is equipped with appropriate storage space for the catalyst, which must typically be stored under controlled humidity and temperature conditions to prevent deterioration (alkali metal and alkaline earth metal acetates are, for example, hygroscopic).

[0027] Third, since the catalyst is a synthetic compound, the cost of producing it is relatively high and significantly affects the final cost of the resulting BHET.

[0028] Given the aforementioned drawbacks, it is clear that the prior art explicitly demands catalytic depolymerization methods using alternative catalysts to those of known technologies. In particular, the catalyst must have a low environmental impact and be readily and inexpensively available.

[0029] Another drawback of PET recycling methods is the limited types of waste that can be depolymerized. Currently, almost all PET waste recovered through depolymerization comes from the sorting and collection of plastic containers (especially bottles). Other types of waste (especially PET fibers from the spinning process or post-consumer felt recycling) remain largely unused and end up in landfills due to the high levels of contaminants they contain.

[0030] PET-containing felts are, for example, composed of a multi-layered structure containing different types of materials (polymeric and non-polymeric) in which various inorganic materials are present as additives. Specifically, in the felt, polyester is used to manufacture the externally visible fibrous portion, the underlying support fabric or mesh to which the fibrous portion is attached (the so-called first backing), or the backing of the felt in contact with the support surface (the so-called second backing). An intermediate layer of adhesives or reinforcing materials, such as rubber latex, and filler materials (e.g., calcium and magnesium salts, aluminum compounds) may be placed between these layers. At the end of its life cycle, PET-containing felts typically undergo shredding and separation of contaminant fractions, producing a fibrous material with a cotton-like appearance (the so-called fluff). The fluff contains PET as well as a relatively large amount of contaminants (up to about 20% by weight), the most abundant of which are: calcium carbonate and / or magnesium carbonate, dolomite minerals, aluminosilicates, and aluminum trioxide.

[0031] The specific structure and chemical composition of felt blankets, as well as the contaminants (such as adhesives) absorbed during use, make the recycling of the PET contained within them quite complex and economically impractical. However, PET waste from felt blankets remains a very attractive source of material for recycling due to its large availability. Therefore, it is desirable to develop depolymerization methods that can also effectively utilize this type of material.

[0032] In view of the aforementioned prior art, the applicant therefore faces the following problem: providing a method for depolymerizing PET-containing materials that overcomes the aforementioned disadvantages of the known art. In particular, the applicant faces the following problem: providing a depolymerization method using a catalyst that has reduced environmental impact and an effect comparable to or even superior to that of known catalysts. Furthermore, the catalyst must be readily available or readily prepared from raw materials possessing these properties.

[0033] The applicant has also addressed the problem of providing a depolymerization method that can supply PET-containing materials from various types of waste and processing waste to the depolymerization method, including PET-containing materials from waste containing a large amount of contaminants (e.g., waste from post-consumer felt recycling).

[0034] It has been found that the above and other objectives, which will be further described below, can be achieved by a method for depolymerizing PET-containing materials in which calcium acetate, magnesium acetate, or aluminum acetate is used as a catalyst. It has been found that, since the metal components are not composed of heavy metals or transition metals, but rather of alkaline earth metals (Ca and Mg) and amphoteric metals (Al), these salts have efficiencies comparable to, and sometimes even exceeding, those of the most efficient catalysts known in the art, such as zinc acetate, while also having a reduced environmental impact. Furthermore, these readily available and relatively inexpensive salts can be easily removed from the BHET obtained at the end of depolymerization, even by simple washing with water.

[0035] In some cases (e.g., calcium acetate), the catalyst according to the invention exhibits a higher reaction rate than known catalysts, even at reduced weight concentrations, allowing depolymerization to be completed in a shorter time compared to known methods. This provides the advantage that a smaller reactor volume can be used for the same production capacity compared to that required in prior art methods.

[0036] The applicant has also surprisingly discovered that the depolymerization catalyst can also be formed in situ in a reaction mixture from acetic acid and a compound containing a metal counterion (e.g., an oxide, hydroxide, carbonate, or bicarbonate of the aforementioned metal). By appropriately adding acetic acid and the metal-containing compound to the reaction mixture, an acetate of the metal can be produced that catalyzes the depolymerization reaction with an efficiency completely comparable in both reaction rate and PET to BHET conversion to that of the same compound prepared in situ and added to the reaction mixture.

[0037] The in-situ formation of catalysts from readily available and low-cost reactants with low environmental impact overcomes the drawbacks associated with the supply of catalysts in the aforementioned known technologies.

[0038] It was also observed that, advantageously, the catalyst can be prepared in situ using compounds that are present as contaminants in PET-containing materials and are supplied to the depolymerized material as metallic compounds (e.g., calcium carbonate, mixed calcium carbonate and magnesium carbonate, or aluminum hydroxide, commonly found in materials derived from post-consumer blanket waste). This allows for the production of BHET using a wider range of PET waste (particularly waste currently primarily disposed of via landfill, such as post-consumer blanket waste).

[0039] Furthermore, the in-situ catalyst preparation method described herein is not limited to acetates of Ca, Mg, and Al. In fact, it can be advantageously used to depolymerize PET-containing materials via diol hydrolysis in the presence of any metal acetate capable of being effectively used as a depolymerization catalyst (e.g., catalysts of known techniques such as zinc acetate, lead acetate, iron acetate, manganese acetate, and antimony acetate).

[0040] Therefore, according to a first aspect, the present invention relates to a method for depolymerizing a material comprising polyethylene terephthalate (PET) by diol hydrolysis with monoethylene glycol (EG), the method comprising reacting the PET-containing material with EG in the presence of at least one catalyst to obtain a diol hydrolysis product comprising bis(2-hydroxyethyl) terephthalate (BHET) and / or oligomers thereof, wherein the catalyst comprises an acetate of a metal selected from calcium, magnesium, aluminum, and mixtures thereof.

[0041] According to a second aspect, the present invention relates to a method for depolymerizing a material comprising polyethylene terephthalate (PET) by diol hydrolysis of ethylene glycol (EG), the method comprising reacting PET with EG in the presence of at least one catalyst comprising a metal acetate to obtain a diol hydrolysis product comprising bis(2-hydroxyethyl) terephthalate (BHET) and / or oligomers thereof, wherein the catalyst is obtained in situ by a reaction between acetic acid and a compound containing said metal.

[0042] Typically, the PET-containing material supplied to the depolymerization method according to the invention can be any material containing PET, preferably containing PET in an amount greater than or equal to 60% by weight, more preferably greater than or equal to 70% by weight, and even more preferably greater than or equal to 80% by weight. The material may contain contaminants (i.e., compounds other than PET) in an amount up to 40% by weight, preferably up to 30% by weight, and even more preferably up to 20% by weight.

[0043] Examples of contaminants commonly found in PET-containing materials from recycled waste include: dyes, polyolefins (e.g., polyethylene or polypropylene), rubber latex, polyamides, polyvinyl alcohol (EVOH), polyvinyl acetate (EVA), UV absorbers; fillers (e.g., calcium carbonate, magnesium carbonate, aluminum hydroxide, titanium dioxide, carbon black, silica), metal particles (e.g., aluminum), and other contaminants derived from the use of the product.

[0044] Preferably, the PET-containing material includes post-consumer PET waste and / or post-industrial PET waste, which can originate from, for example, a wide variety of products, such as:

[0045] - Transparent and / or colored PET bottles for food products (such as water, soft drinks, etc.);

[0046] - Multilayer PET products, typically used in the food industry, are in which the PET layer is combined with layers of other polymer or metal materials.

[0047] - Products containing PET fibers, such as textiles, blankets, etc.

[0048] In a particularly preferred embodiment, the depolymerization material comprises post-consumer waste and / or post-industrial waste containing PET, preferably post-consumer felt waste containing PET.

[0049] Materials containing PET can be supplied to the depolymerization process in the form of granules, flakes, fibers, or fluff.

[0050] The depolymerization catalyst is present in the reaction mixture in an amount effective for depolymerizing PET present in the reaction mixture. Preferably, the catalyst is present in the reaction mixture in an amount ranging from 0.1% to 5% by weight, more preferably from 0.4% to 2.0% by weight, relative to the weight of PET contained in the material undergoing depolymerization.

[0051] Preferably, the catalyst is present in the reaction mixture at a ratio of greater than 5, more preferably greater than 15, and even more preferably greater than 20, for example in the range of 20 to 50 mmol catalyst / kg (PET+EG), wherein “PET+EG” represents the total weight of EG and PET present in the material containing PET.

[0052] As described above, the catalyst can be prepared in situ and added to the reaction mixture as is, or the catalyst can be prepared in situ by reacting acetic acid with a compound containing at least one metal that is capable of forming a metal salt with acetic acid.

[0053] In this specification, the term "metal acetate" also includes mixed salts, i.e., salts of metals containing one or more counterions other than the acetate anion (hereinafter also referred to as "Ac"). An example of a mixed salt is calcium bicarbonate acetate (hereinafter also referred to as Ca(HCO3)Ac), which can be obtained by reacting acetic acid and calcium carbonate in appropriate stoichiometric ratios.

[0054] Preferably, the metal-containing compound is selected from oxides, hydroxides, carbonates, bicarbonates, chlorides, and mixtures thereof.

[0055] Preferably, the metal-containing compound is selected from: calcium carbonate, calcium bicarbonate, calcium hydroxide, magnesium carbonate, magnesium bicarbonate, magnesium hydroxide, aluminum hydroxide, and aluminum trichloride.

[0056] As described above, when preparing the catalyst in situ, the catalyst may include acetates other than Ca, Mg, and Al acetates. In such cases, the metal-containing compound comprises at least one metal selected from zinc, iron, manganese, antimony, and mixtures thereof; preferably, the aforementioned compound is in the form of an oxide, hydroxide, carbonate, bicarbonate, or chloride.

[0057] The reactants required to form an in-situ catalyst can be added together or separately to EG, or to a mixture containing PET-containing materials and EG.

[0058] In a preferred embodiment, the PET-containing material further comprises a metal-containing compound in a catalytically effective amount (i.e., an amount sufficient to provide an acetate capable of catalyzing the depolymerization reaction by reacting with acetic acid). If the concentration of the metal-containing compound is insufficient to obtain a catalytically effective amount of metal acetate, the metal compound may be intentionally added to the reaction mixture.

[0059] Preferably, acetic acid is added to the reaction mixture in an amount ranging from 0.01% to 5% by weight, more preferably from 0.03% to 1.5% by weight, and even more preferably from 0.1% to 1% by weight, relative to the PET-containing material.

[0060] Preferably, the metal-containing compound is added to the reaction mixture in an amount ranging from 0.01% to 5% by weight, more preferably from 0.02% to 1.8% by weight, and even more preferably from 0.06% to 1.2% by weight, relative to the PET-containing material.

[0061] Preferably, the molar ratio between acetic acid and the metal compound is in the range of 1:1 to 1:10.

[0062] The applicant notes that the metal acetate formation reaction may compete with the reaction between acetic acid and EG to form the corresponding ethylene glycol monoacetate polyester (EGMA) or ethylene glycol diacetate polyester (EGDA), thus affecting the yield of the acetate catalyst formation reaction. However, it has been found that this can be overcome by adding acetic acid and a metal compound to EG or a mixture of EG and PET under appropriate conditions. In particular, preferably, this is done by first mixing acetic acid and a metal compound with EG and possibly PET-containing materials at a relatively low temperature, preferably in the range of 20°C to 70°C, more preferably 20°C to 50°C, and even more preferably 20°C to 40°C, and then raising the temperature of the mixture to a temperature selected for the depolymerization reaction. Without referring to any particular theory, it is considered that at low temperatures, the catalyst formation reaction is more favorable than the formation of EGMA and EGDA esters; once the metal acetate is formed, the temperature of the reaction mixture can be raised to the depolymerization temperature, which will not induce the formation of EGMA and EGDA esters.

[0063] In cases where the PET-containing material to be depolymerized contains a metal compound, the PET-containing material is also added to EG and acetic acid, preferably at low temperature.

[0064] Specifically, in one embodiment, the depolymerization method involving in-situ catalyst formation includes:

[0065] a. Acetic acid, a metal-containing compound, and EG are mixed at a temperature ranging from 20°C to 70°C, preferably from 20°C to 50°C;

[0066] b. Heat the mixture obtained in step a to a temperature in the range of 170°C to 230°C;

[0067] c. Feed the PET-containing material into the mixture obtained in step b to obtain the diol hydrolysis product.

[0068] In-situ catalyst preparation is particularly useful when the material containing PET also contains one or more metal compounds that serve as precursors for in-situ catalyst formation via reaction with acetic acid. In practice, the metal content of the material can be determined, and acetic acid can thus be added to the reaction mixture. If the metal compound is present in a relatively large amount in the PET-containing material, sufficient acetic acid can always be added to convert only a portion of the metal compound and produce the required amount of catalyst. The remaining metal compound, present as a contaminant, is then removed in a purification step of the diollysis product. This is especially true for recycled PET (fluff) from recycled felt, which typically contains at least 3% to 5% by weight of calcium carbonate and / or aluminum hydroxide.

[0069] Specifically, in one embodiment, the depolymerization method involving in-situ catalyst formation includes:

[0070] a. Mixing EG, acetic acid, and materials containing PET and compounds containing the metal at a temperature ranging from 20°C to 70°C, preferably from 20°C to 50°C;

[0071] b. Heat the mixture obtained in step a to a temperature in the range of 170°C to 230°C and allow the reaction to occur to obtain the diol hydrolysis product.

[0072] Preferably, the diol hydrolysis reaction is carried out at a temperature in the range of 170°C to 230°C, more preferably in the range of 190°C to 210°C. The heating of the reaction mixture is not performed by microwave radiation.

[0073] In one embodiment, when the catalyst comprises magnesium acetate, the heating of the reaction mixture is not performed by microwave radiation.

[0074] In the diol hydrolysis reaction, preferably, EG is used in a weight ratio of EG:PETm in the range of 1:1 to 8:1, more preferably in the range of 1:1 to 4:1, and even more preferably in the range of 1:1 to 2:1, where PETm represents a material containing PET.

[0075] The duration of the diollysis reaction can vary widely depending on the reaction conditions, such as temperature, stirring, and reactor type. Typically, the reaction time ranges from 1 hour to 6 hours, preferably from 1.5 hours to 4 hours. The reaction can be carried out as a batch or continuous process. The reaction pressure is typically atmospheric pressure, but reduced or increased pressures can be used.

[0076] Diololysis results in the formation of diololysis products containing BHET and / or its oligomers. At the end of depolymerization, the diololysis products typically also contain residual fractions of material containing unreacted PET and other depolymerization products (e.g., water, contaminants insoluble in EG, etc.).

[0077] BHET and / or its oligomers can be separated from the diol hydrolysis products and purified using appropriate techniques known to the public, such as distillation, filtration, crystallization, etc.

[0078] BHET and / or its oligomers obtained by the methods described herein can be used as feedstocks in new production processes for polyester polymers and copolymers. For example, they can be polymerized to produce PET entirely from recycled BHET or in combination with BHET made from virgin feedstocks.

[0079] The methods described herein can be performed using apparatus and equipment known in the art for the production of polymers (particularly polyesters).

[0080] Advantageously, the depolymerization reaction can be carried out in an acid-resistant stainless steel reactor equipped with a suitable inlet for dispensing liquid acetic acid and a solid compound for in-situ catalyst formation. The reactor may be equipped with heating and mixing devices to heat the reaction mixture to the desired temperature and maintain stirring.

[0081] The following embodiments are provided for illustrative purposes only and should not be construed as limiting the scope of protection defined by the appended claims.

[0082] In the examples, the time-related trends of the overall yields of the diollysis reactions in Examples 1 to 14 are illustrated with reference to the appendix. Figures 1 to 7 . Example

[0083] 1. De-aggregation test

[0084] The effectiveness of the method according to this specification was verified through a series of catalytic depolymerization tests on different types of PET-containing waste. The depolymerization tests were performed as follows.

[0085] The diolization reaction was carried out in a 1-liter round-bottom Pyrex glass reaction flask. The flask was heated via an isomantle. The reaction flask had four necks with the following functions:

[0086] - Insert a thermocouple into the first neck to detect the temperature of the liquid mixture during the reaction: the temperature setpoint can be digitally set on the heating jacket;

[0087] - Insert a reflux condenser into the second neck to condense the vapors generated during the reaction by a continuously supplied flow of cooling water;

[0088] The remaining two necks are used to supply ethylene glycol (EG) and PET-containing materials; during the depolymerization reaction, these two necks are kept closed by two ground glass stoppers.

[0089] The reaction mixture is kept stirred using a magnetic stirrer with a PTFE magnetic stirring rod. The rotation speed of the PTFE magnetic stirring rod can be digitally set on the Isomantle device. The reaction temperature and rotation speed are set to 195°C and 1400 rpm, respectively.

[0090] First, EG and the catalyst (if not prepared in situ) are loaded into a reaction flask. The mixture, while being stirred, is then heated to the reaction temperature, and the resulting vapors are condensed using a bubble condenser. Once the mixture of glycol and catalyst has reached the reaction temperature, PET-containing material (hereinafter referred to as "PET only"), preheated to 110°C, is introduced into the flask.

[0091] The moment when PET is first loaded into the reaction flask is considered the reaction start "time 0" (t0). Starting at t0, samples of the diol hydrolysate are taken out at fixed intervals using a glass Pasteur pipette; the liquid is then transferred to a vial cooled with cold water to stop the reaction process, and then analyzed by HPLC to determine the content of BHET and its oligomers.

[0092] The time it took to observe PET completely dissolving in the diol was also recorded.

[0093] At the end of the diol hydrolysis, the contents of the flask are drained and the liquid is subjected to HPLC analysis at the end of the process.

[0094] 2. Examples 1 to 3 (comparative) - Catalyst: Zinc acetate (ZnAc2)

[0095] Following the steps described in section 1, the catalyst was added at different concentrations to depolymerize granular PET in the presence of zinc acetate. The PET used was in substantially pure form (approximately 100% purity). Tables 1 to 3 provide the composition of the reaction mixture and the analytical results of the reaction liquid taken out at different reaction times.

[0096] The weight ratios EG / PET and catalyst / PET refer to the weight ratios of EG and catalyst to the weight of materials containing PET, respectively.

[0097] The parameters “BHET yield”, “dimer yield” (of BHET) and “total yield” refer to the ratio of the amount of BHET, BHT dimer and “BHET+dimer” to the weight of the material containing PET, respectively, expressed as mole / mole percentage.

[0098] Table 1

[0099]

[0100] Table 2

[0101]

[0102] Table 3

[0103]

[0104] Figure 1 The time-related trend of the overall yield of the polymerization reaction is shown.

[0105] 3. Examples 4 and 5 - Catalyst: Calcium acetate (CaAc2)

[0106] Following the process described in 1, granular PET (100% purity) is depolymerized in the presence of calcium acetate, which is added at 1% by weight and 3% by weight of the PET being loaded.

[0107] Tables 4 and 5 provide the composition of the reaction mixture and the analytical results of the reaction liquid taken out at different reaction times.

[0108] Table 4

[0109]

[0110] Table 5

[0111]

[0112] Figure 2 The time-related trend of the overall yield of the polymerization reaction is shown.

[0113] A comparison of the results of Examples 1 to 3 and 4 to 5 shows that, for the same catalyst weight, the CaAc2 catalyst has an efficiency comparable to that of ZnAc2 and a faster depolymerization kinetics than ZnAc2.

[0114] 4. Examples 6 to 7

[0115] Since the physical form of the PET-containing material may also affect the degree of depolymerization, the steps described in step 1 are carried out by feeding a PET-containing waste material (with a purity of about 97.5%) consisting of spinning waste and using ZnAc2 or CaAc2 as a catalyst.

[0116] Tables 6 and 7 provide the composition of the reaction mixture and the analytical results of the reaction liquid taken out at different reaction times.

[0117] Table 6

[0118]

[0119] Table 7

[0120]

[0121] Figure 3 The time-related trends of the total yield of the depolymerization reactions in Examples 6 and 7 are shown.

[0122] As can be seen, these examples compare two different weight doses, but they correspond to the same molar dose (approximately 5.5 mmol of catalyst).

[0123] Figure 3The graphical comparisons show that CaAc2 is a depolymerization catalyst that achieves results very comparable to those of conventional ZnAc2, even at lower weight concentrations. The reaction is completed faster in the presence of CaAc2 than with ZnAc2.

[0124] 5. Examples 8 to 10 - Catalyst: Calcium acetate (CaAc2)

[0125] The effectiveness of the CaAc2 catalyst was tested in the depolymerization reaction of PET waste (fluff) from felt recycling. The fluff used had a PET content of 80% by weight.

[0126] The process described in section 1 is carried out by feeding fluff into a reaction mixture containing CaAc2 as a catalyst.

[0127] In tests involving the supply of fluff, a fraction of suspended solids with a yellowish-brown appearance was observed in the reaction mixture at 195°C at the end of the diollysis reaction. This fraction was removed by hot filtration of the reaction mixture (to approximately 190°C). Approximately 10% to 15% by weight of the fluff, separated by filtration, was analyzed by DSC, TGA, and FT-IR. The analysis revealed the presence of polypropylene, gum, and a mixture of other inorganic substances, including calcium carbonate.

[0128] Tables 8 to 10 provide the composition of the reaction mixture and the analytical results of the reaction liquid taken out at different reaction times.

[0129] Table 8

[0130]

[0131] Table 9

[0132]

[0133] Table 10

[0134]

[0135] Figure 4 The time-related trend of the total yield of the depolymerization reaction in Example 8 is shown, while Figure 5 The time-related trends of the total yield of the reactions in Examples 9 and 10 are shown.

[0136] Examples 9 and 10 demonstrate that even when operating with relatively low CaAc2 catalyst concentrations and EG / PET ratios, PET to BHET and oligomer conversion rates comparable to those achieved with catalysts (e.g., ZnAc2) used in known diol hydrolysis methods at higher EG / PET ratios were obtained.

[0137] 6. Examples 11-12 - Catalyst: Acetic acid and in-situ produced calcium bicarbonate acetate (Ca(HCO3)Ac)

[0138] The following depolymerization test was performed following the steps described in section 1. In Example 11 (comparative), the depolymerization test was performed using PET (approximately 97.5% purity) in the form of substantially uncontaminated fibers. Acetic acid was added to the reaction mixture only to test its potential effect as a depolymerization catalyst. Acetic acid was added to EG at 30°C, and the mixture was then heated to 195°C and maintained under reflux. At this temperature, the PET fibers were added. After 3 hours of reaction, no dissolution of the fibers was observed, and HPLC analysis of the reaction mixture liquid did not detect the presence of BHET.

[0139] Example 12 was carried out under the same conditions as Example 11, except that equimolar amounts of CaCO3 and acetic acid were added to EG at 30°C. The mixture was then heated to 195°C, maintained under reflux, and PET fibers were added. The reaction was continued until the fibers were completely dissolved. The data shown in Table 11 thus demonstrate that an active stoichiometric catalyst, Ca(HCO3)Ac, was produced in the reaction mixture at a concentration of 2.66 wt% relative to the weight of the PET fibers.

[0140] Table 11

[0141]

[0142] Figure 6 The time-related trend of the total yield of the depolymerization reaction in Example 12 is shown compared to the time-related trend of the total yield of the depolymerization reaction in Example 13.

[0143] 7. Example 13 - Catalyst: Acetic acid and in-situ produced calcium bicarbonate acetate (Ca(HCO3)Ac)

[0144] Example 13 was carried out under the same conditions as Example 12, except that acetic acid and PET fluff were added to EG at 30°C, and CaCO3 contained in the PET fluff (80% purity) was used. The CaCO3 content in the fluff was approximately 5% by weight relative to the weight of the fluff.

[0145] Table 12 provides the composition of the reaction mixture and the analytical results of the reaction liquid taken out at different reaction times.

[0146] Table 12

[0147]

[0148] Figure 6 The time-related trend of the total yield of the depolymerization reaction in Example 13 is shown. The results indicate that the PET diolization process in Example 13 occurred to a similar extent to that in Example 12, confirming that an effective depolymerization catalyst can be prepared in situ using metal compounds present as contaminants in materials containing recycled PET.

[0149] 8. Example 14 - Catalyst: Magnesium acetate (MgAc2)

[0150] Following the process described in 1, magnesium acetate, prepared at 0.45% by weight of MgAc2 relative to the weight of the PET loaded, is used to depolymerize the granular PET (100% purity).

[0151] Table 13 provides the composition of the reaction mixture and the analytical results of the reaction liquid taken out at different reaction times.

[0152] Table 13

[0153]

[0154] Figure 7 The time-dependent trend of the overall yield of the depolymerization reaction in Example 14 is shown. The results indicate that magnesium acetate is also an effective catalyst in the depolymerization of PET.

[0155] 9. Examples 15 to 18 - Other catalysts produced in situ

[0156] Examples 15 to 18 were carried out under the same conditions as Example 12, with the following added to EG at 30°C:

[0157] - In Example 15, acetic acid and zinc carbonate are present in a 2:1 molar ratio;

[0158] - In Example 16, acetic acid and calcium oxide were present in a 2:1 molar ratio;

[0159] - In Example 17, acetic acid and magnesium hydroxide are present in a 2:1 molar ratio;

[0160] - In Example 18, acetic acid and aluminum hydroxide were present in a 3:1 molar ratio.

[0161] In all cases, the mixture is then heated to 195°C, kept under reflux, and PET pellets are added.

[0162] Tables 14 to 17 provide the composition of the reaction mixture and the analytical results of the reaction liquid taken out at different reaction times.

[0163] Table 14

[0164]

[0165] Table 15

[0166]

[0167] Table 16

[0168]

[0169] Table 17

[0170]

[0171] Figure 8 The time-related trend of the total yield of the depolymerization reaction in Example 15 is shown compared to that in Example 3, where the same catalyst was prepared in situ and then added to the reaction mixture. Figure 9 The time-related trend of the total yield of the depolymerization reaction in Example 16 is shown compared to that in Example 5, where the same catalyst was prepared ex-situ and then added to the reaction mixture. The results indicate that the in-situ generated catalyst is just as effective as the ex-situ generated catalyst.

[0172] 10. Examples 19-20 - Recovery of BHET by Evaporation and Crystallization of EG

[0173] Using the same process as in Examples 12 and 13, the focus in Examples 19 and 20 is to separate BHET and the dimer to obtain a material that can be polymerized into PET.

[0174] In this first step, the glycololysis residue is filtered to separate unconverted PET and other impurities from the glycololysis material. A portion of the diol in the permeate is evaporated by flash evaporation under vacuum.

[0175] At this point, a large excess of water (4 times the weight of the diol) is added to precipitate the product. The product is then filtered again, washed with water, and finally dried.

[0176] The obtained material was then analyzed by HPLC, FT-IR, DSC and NMR to determine that the obtained material was BHET and its dimer.

[0177] As shown in Table 18, the characteristics of the separated products (content of COOH groups and diethylene glycol (DEG), melting point) are consistent with the characteristics typically required for the materials to be polymerized.

[0178] Table 18

[0179] Starting PET PET fiber PET fleece Process parameters As in Example 12 As in Example 13 COOH of BHET [mEq / kg] 36.7 35.4 DEG content in BHET [%) 0.12 0.11 BHET's melting point [°C] 108.9 107.5

Claims

1. A method for depolymerizing a material comprising polyethylene terephthalate (PET) by diol hydrolysis with ethylene glycol (EG), the method comprising reacting the PET-containing material with EG in the presence of at least one catalyst comprising a metal acetate to obtain a diol hydrolysis product comprising bis(2-hydroxyethyl) terephthalate (BHET) and / or oligomers thereof, wherein the method comprises the step of preparing the catalyst by an in-situ reaction between acetic acid and a compound containing the metal.

2. The method according to claim 1, wherein the step of preparing the catalyst by in-situ reaction between acetic acid and a compound containing the metal comprises: a. Mixing acetic acid, a compound containing the metal, and EG at a temperature ranging from 20°C to 70°C; b. Heat the mixture obtained in step a to a temperature in the range of 170°C to 230°C; c. The material containing PET is fed into the mixture obtained in step b to obtain the diol hydrolysis product.

3. The method according to claim 1, wherein the step of preparing the catalyst by in-situ reaction between acetic acid and a compound containing the metal comprises: a. Mixing EG, acetic acid, and materials containing PET and compounds containing the metal at a temperature ranging from 20°C to 70°C; b. Heat the mixture obtained in step a to a temperature in the range of 170°C to 230°C and allow the reaction to occur to obtain the diol hydrolysis product.

4. The method according to any one of claims 1 to 3, wherein the metal acetate is selected from: calcium acetate, magnesium acetate, aluminum acetate, zinc acetate, lead acetate, iron acetate, manganese acetate, antimony acetate, and mixtures thereof.

5. The method according to any one of claims 1 to 3, wherein the compound containing the metal is selected from: oxides, hydroxides, carbonates, bicarbonates, chlorides, and mixtures thereof.

6. The method according to any one of claims 1 to 3, wherein the compound containing the metal is selected from: calcium carbonate, calcium bicarbonate, calcium hydroxide, magnesium carbonate, magnesium bicarbonate, magnesium hydroxide, aluminum hydroxide, and aluminum trichloride.

7. The method according to any one of claims 1 to 3, wherein the material comprising PET contains a compound containing the metal.

8. The method according to any one of claims 1 to 3, wherein the material comprising PET includes post-consumer waste and / or post-industrial waste comprising PET.

9. The method according to any one of claims 1 to 3, wherein the material comprising PET includes post-consumer felt waste comprising PET.

10. The method according to any one of claims 1 to 3, wherein the material comprising PET is in the form of granules, flakes, fibers or fluff.

11. The method according to any one of claims 1 to 3, wherein the weight ratio EG:PETm is 1:1 to 8:1, wherein PETm represents the material comprising PET.

12. The method according to any one of claims 1 to 3, wherein the weight ratio EG:PETm is 1:1 to 4:1, wherein PETm represents the material comprising PET.

13. The method according to any one of claims 1 to 3, wherein the weight ratio EG:PETm is 1:1 to 2:1, wherein PETm represents the material comprising PET.

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