Recycling process of polyesters from mixed plastic waste

Abstract

The present invention relates to a process for depolymerizing plastic waste containing semi-aromatic polyesters, and in particular polyethylene terephthalate (PET), through methanolysis, for the production of dimethyl terephthalate (DMT). In particular, the invention relates to a process for depolymerizing plastic waste containing terephthalic polyesters, and in particular polyethylene terephthalate (PET), comprising the following steps: a) subjecting a plastic material containing terephthalic polyesters, in particular PET, to a methanolysis process in the presence of methanol and a catalyst based on a quaternary ammonium ionic compound and an iron(III), manganese(II), or cobalt(II) halide; b) separating the solid residue of step a) containing dimethyl terephthalate (DMT) from the liquid phase containing methanol, a glycol and the catalyst in solution; c) separating the DMT in substantially pure form from the solid residue of step b); d) optionally, subjecting the liquid phase of step b) to fractional distillation to recover the methanol, the glycol and the catalyst.

Description

[0001] "RECYCLING PROCESS OF POLYESTERS FROM MIXED PLASTIC WASTE" Description

[0002] Field of the invention

[0003] The present invention relates to a process for depolymerizing plastic waste containing semi-aromatic polyesters, and in particular polyethylene terephthalate (PET), through methanolysis, for the production of dimethyl terephthalate (DMT). State of the art

[0004] To date, the recycling of plastic material is an activity of extreme importance from both an environmental and economic perspective .

[0005] PET is currently recycled through mechanical processes for the production of bottles and fibres. These processes require high quality input material, which must be collected, separated and treated properly, for example, through successive washes, before it can undergo the recycling process.

[0006] The latter occurs by shredding the previously treated material followed by immersing it in tanks where the PET is separated by sedimentation from other materials such as HDPE or paper, ultimately obtaining flakes that can be used, after heating above the softening or melting temperature and extrusion and / or blow moulding, for the production of containers or fibres. However, the quality of the material thus obtained is often insufficient for the production of bottles, due to the partial depolymerization of the material.

[0007] To produce bottles from recycled PET it is therefore necessary to add virgin polymer or use cross-linkers during the extrusion step or perform a Solid State Polymerization (SSP), possibly combining these approaches. For this reason, still today, PET deriving from mechanical recycling is primarily used for low-added-value applications, such as the production of non-woven fabric (NWF) for textile fleece or for applications in the construction sector.

[0008] Further, the frequent presence of dyes in recycled bottles cause significant aesthetic issues and can lead to further economic devaluation of mechanically recycled PET. The so-called "floral" PET, i.e. deriving from coloured flakes, has a very low market price and finds few applications.

[0009] However, there are processes that involve the chemical recycling of PET by solvolysis. Such a process allows for the production of monomeric and oligomeric materials starting from recycled PET, which can then be used to produce virgin PET, thus of high added value, without the need of adding virgin PET to the recycled PET.

[0010] However, such processes must be economically viable, meaning they need to be simple to implement and capable of obtaining monomers with high yield and purity.

[0011] There is therefore a demand for solvolysis processes for PET or other terephthalate polyesters, such as polybutylene terephthalate, that meet said requirements. This demand is even more pressing when considering that at the end of 2022, the last European plant for the production of DMT (dimethyl terephthalate) was closed. Its use as a raw material for obtaining PET is thus dependent on imports from Asian countries and the USA, resulting in an increased unit cost of the material and greater difficulty in sourcing it on the market.

[0012] Summary of the invention

[0013] The underlying problem of the present invention is to provide a solvolysis process for a terephthalic polyester, in particular PET, that is economical, easy to implement, and that gives high purity monomers with high yields.

[0014] This problem is solved by a process as defined in the appended claims, the definitions of which are an integral part of the present description. The invention consists in a process for depolymerizing plastic waste containing semi-aromatic polyesters, and in particular polyethylene terephthalate (PET), through a solvolysis process in methanol (methanolysis), catalysed by ionic compounds based on transition metals, preferably iron, for the production of dimethyl terephthalate (DMT).

[0015] The process of the invention tolerates the presence, in the starting material, of other polymers and foreign substances, without for this reason encountering operational issues that compromise its efficiency.

[0016] The object of the present invention is a process for depolymerizing plastic waste containing terephthalic polyesters, and in particular polyethylene terephthalate (PET), comprising the following steps: a) subjecting a plastic material containing terephthalic polyesters, in particular PET, to a methanolysis process in the presence of methanol and a catalyst based on a ionic compound of a quaternary ammonium and an iron(III), manganese (II), or cobalt (IT) halide; b) separating the solid residue of step a) containing dimethyl terephthalate (DMT) from the liquid phase containing methanol, a glycol and the catalyst in solution; c) separating the DMT in substantially pure form from the solid residue of step b); d) optionally, subjecting the liquid phase of step b) to fractional distillation to recover the methanol, the glycol and the catalyst.

[0017] The object of the invention is also the use of a tetraalkylammonium Me-chloride, wherein Me is selected from Fe, Co and Mn, wherein preferably the tetraalkylammonium cation is tetramethylammonium, as a catalyst in a solvolysis reaction, in particular of terephthalic polyesters.

[0018] Further characteristics and advantages of the present invention become apparent from the description of some illustrative and non-limiting exemplary embodiments hereinafter.

[0019] Detailed description of the invention

[0020] The present invention relates to a process for depolymerizing plastic waste containing terephthalic polyesters, and in particular polyethylene terephthalate (PET), comprising the following steps: a) subjecting a plastic material containing terephthalic polyesters, in particular PET, to a methanolysis process in the presence of methanol and a catalyst based on a ionic compound of a quaternary ammonium and an iron(III), manganese (II), or cobalt (II) halide; b) separating the solid residue of step a) containing dimethyl terephthalate (DMT) from the liquid phase containing methanol, a glycol and the catalyst in solution; c) separating the DMT in substantially pure form from the solid residue of step b); d) optionally, subjecting the liquid phase of step b) to fractional distillation to recover the methanol, the glycol and the catalyst.

[0021] The plastic material containing terephthalic polyesters used in step a) can advantageously be selected from discarded bottles from mechanical recycling processes, with or without labels, material coming from the aspiration systems of mechanical recycling systems, non-food containers, with or without labels, white containers containing titanium dioxide, with or without labels, non-mechanically recyclable coloured bottles, with or without labels, multilayer food trays, with or without labels, biodegradable PET-based bags, PET-based fabrics and non-woven fabrics, also containing natural or synthetic fibres, special terephthalic polyesters for various applications. The plastic material is preferably ground before being subjected to the methanolysis process.

[0022] The reaction temperature is preferably between 110°C and 190°C, more preferably between 120°C and 185°C, even more preferably between 130°C and 180°C, or between 150°C and 170°C.

[0023] Methanolysis generally occurs at a pressure <20 bar.

[0024] The reaction time is preferably comprised between 0.5 and 6 hours, more preferably < 3 hours.

[0025] The methanol / terephthalic polyester weight ratio is preferably comprised between 2 and 10, more preferably between 2 and 6, even more preferably between 2.7 and 3.3.

[0026] The molar amount of catalyst, with respect to the moles of terephthalic polyester, is preferably greater than or equal to 0.1 moll and less than 6 moll, more preferably less than 2 moll, even more preferably less than 1.2 moll or less than 0.6 moll.

[0027] In certain embodiments, the catalyst is a tetraalkylammonium Me chloride, wherein Me is selected from Fe, Co and Mn. In preferred embodiments, the tetraalkylammonium cation is tetramethylammonium.

[0028] In particularly preferred embodiments, the catalyst is (CH3)4N+FeC14~. This catalyst is highly soluble and stable in methanol, allowing it to be pre-dissolved in methanol to the desired molar concentration and also stored for long periods of time for subsequent use.

[0029] This catalyst can be obtained by mixing tetramethylammonium chloride with ferric chloride.

[0030] Step a) can be carried out by conventional heating, for example in an autoclave, or by microwave heating. The latter heating mode allows the reaction to be carried out in short periods of time, even less than one hour, utilizing the ionic characteristic of the catalyst as a microwave absorber. In this case, a molar amount of catalyst within 3 mol% and 6 mol% may be required, which however is not a problem due to the very low cost of the catalyst used.

[0031] Step b) is carried out separating the solid residue by filtration or centrifugation, after cooling the mixture obtained in step a) to a temperature below 20°C, preferably below 15°C, even more preferably around 10°C.

[0032] In step c), by "DMT in substantially pure form" is intended DMT with a purity greater than 99%, preferably greater than 99.5%, more preferably greater than 99.8%, calculated with respect to the maximum moles obtainable if all the terephthalic polyester were 100% pure. In certain embodiments, step c) is carried out according to the following steps: i) dissolving the solid residue in hot ethyl acetate, preferably at the boiling temperature of ethyl acetate, ii) filtering the obtained DMT solution to remove suspended solids, iii) crystallizing the DMT from the ethyl acetate solution by cooling, iv) separating the crystallized DMT by filtration or centrifugation .

[0033] In other embodiments, step c) can be carried out by sublimation of DMT from the solid residue separated in step b), preferably at a temperature of around 170°C.

[0034] A further object of the invention is the use of a tetraalkylammonium Me-chloride, wherein Me is selected from Fe, Co and Mn, wherein preferably the tetraalkylammonium cation is tetramethylammonium, as a catalyst in a solvolysis reaction, in particular of terephthalic polyesters. It has been observed that the catalysts of the invention are active not only in the methanolysis reaction but also in glycolysis reactions, for example with ethylene glycol or propylene glycol. The reaction will now be further described by specific exemplary embodiments, provided for illustrative and nonlimiting purposes.

[0035] EXPERIMENTAL PART

[0036] Experimental conditions

[0037] All reactions were carried out in standard glassware as the reagents involved are air-stable, and the reaction conditions do not require an inert atmosphere.

[0038] All chemicals and solvents employed are commercially available and were used exactly as received. For most of the depolymerization reactions carried out, R-PET flakes of different origins and purity were used as the PET source, as detailed in Table A.

[0039] Table A. Acronyms used to describe the different PET- based materials treated 1H-NMR analyses were performed with 300 MHz and 400 MHz spectrometers at room temperature. Coupling constants (J) are expressed in hertz (Hz), and chemical shifts (5) in ppm.13C-NMR analyses were performed with the same spectrometers operating at 75.5 MHz or 100 MHz, respectively.

[0040] Gas chromatographic analyses were performed using the GC-FAST technique with a Shimadzu GC-2010 equipped with a Supelco SLBTM-5ms capillary column.

[0041] Low-resolution MS spectra were acquired with instruments equipped with ESI / ion trap sources. High-resolution MS spectra were acquired on a Q-ToF SYNAPT G2-Si HDMS 8K (Waters) instrument equipped with a ZsprayTM ESI (Waters) source. The values are expressed as mass-to-charge ratio, with the relative intensities of the most significant peaks shown in parentheses. Elemental analyses were recorded at the analytical laboratories of the University of Milan.

[0042] Some of the glycolysis and methanolysis reactions, where indicated, were carried out in Personal Chemistry 300 W, 2,450 MHz monomodal microwave synthesizer "Emrys creator".

[0043] All high-temperature methanolysis reactions were carried out in stainless steel autoclaves.

[0044] Molecular weights of some depolymerization mixtures were analysed via SEC, using a system consisting of a Waters 1515 isocratic pump and four Waters Styrgel columns (HRS - HR4 - HR5 - HR2) with a Waters 2487 dual absorbance UV detector at 254 nm. Molecular weights are expressed in polystyrene (PS) equivalents. The internal standard is 1,2-dichlorobenzene.

[0045] The purity of samples was determined by DSC technique (DSC purity), using a Mettler Toledo DSC1 instrument. For the determination, the sample is weighed in an amount between 2 mg and 6 mg. The analysis involves the following temperature ramp and heating: 131°C to 144°C - l°C / min. Purity is determined through dedicated software, using the portion between 10% and 50% of the melting peak to avoid artifacts. SYNTHETIC PROCEDURE AND CHARACTERIZATION OF AMMONIUM

[0046] METALATE-BASED CATALYSTS

[0047] MANGANESE (II) METALATES 1) SYNTHESIS OF [TMA]2[MnCl4]

[0048] Procedure

[0049] A solution of MnC12*4H2O (1 mmol) in MeOH (5 ml) and a solution of TMAC1 (2 mmol) in MeOH (5 ml) are mixed under stirring; the reaction mixture is left under stirring at 40°C for 12 h, after which the solution is concentrated under reduced pressure, and the pink crystalline precipitate is filtered and dried under vacuum.

[0050] Alternative procedure: a solution of MnC12*4H2O (10 mmol) in DCM (50 ml) and a suspension of TMAC1 (20 mmol) in DCM (50 ml) are mixed under stirring; EtOH (20 ml) is added to the mixture to ensure complete dissolution. The reaction mixture is left under stirring for 3 h at room temperature at the end of which the (pink) precipitate formed is filtered and dried under vacuum.

[0051] Yield: 87%

[0052] Characterization

[0053] Elemental analysis (CsH24C14N2Mn):

[0054] Calculated: C: 27.85; H: 7.01; N: 8.12

[0055] Found: C: 27.90; H: 7.04; N: 8.10 Melting point: >300°C

[0056] 2) SYNTHESIS OF [TMA][MnCl3]

[0057] Procedure

[0058] TMAC5 |TMAHMnCls]

[0059] A solution of MnC12*4H2O (1 mmol) in MeOH (5 ml) and a solution of TMAC1 (1 mmol) in MeOH (5 ml) are mixed under stirring; the reaction mixture is left under stirring at 40°C for 12 h, after which the solution is concentrated under reduced pressure, and the pale pink precipitate is filtered and dried under vacuum.

[0060] Yield: 82%

[0061] Characterization

[0062] Elemental analysis (C4Hi2C13NMn):

[0063] Calculated: C: 20.41; H: 5.14; N: 5.95

[0064] Found: C: 20.73; H: 5.20; N: 6.07

[0065] IRON (III) METALATES

[0066] 3) SYNTHESIS OF [TMA][FeCl4]

[0067] Procedure

[0068] TMACS [TMA][F&CiJ A solution of FeCls*6H2O (20 mmol) in EtOH (75 ml) and a solution of TMAC1 (20 mmol) in EtOH (75 ml) are mixed under stirring; the reaction mixture is left under stirring at room temperature for 3 h, during which an immediate precipitation of a yellow-orange powder is observed. After 3 h the yellow-orange precipitate is filtered and dried under vacuum.

[0069] Yield: 74%

[0070] Characterization

[0071] Elemental analysis (C4Hi2C14FeN):

[0072] Calculated: C = 17.68%; H = 4.45%; N = 5.15%

[0073] Found: C = 17.65%; H = 4.40%; N = 5.07%

[0074] Melting point: >3000C

[0075] 4) SYNTHESIS OF [TBA][FeCl4] Procedure

[0076] TBACI [TBA][FeClJ

[0077] A solution of FeC13*6H2O (0.36 mmol) in EtOH (10 ml) and a solution of TBACI (0.36 mmol) in EtOH (10 ml) are mixed under stirring; the reaction mixture is left under stirring at room temperature for 3 h, during which an immediate precipitation of a yellow powder is observed. The solution is then cooled to -20°C for 24 h and the yellow-orange precipitate is filtered and dried under vacuum (molecular pump, 10~2mBar).

[0078] Yield: 94%

[0079] Characterization

[0080] Elemental analysis (CieHseC^FeN):

[0081] Calculated: C, 43.66; H, 8.25; N, 3.18

[0082] Found: C, 43.82; H, 8.23; N, 3.16

[0083] MS (ESI +) m / z (%): calculated for CieHseN: 242.28, found: 242.32 (100%, M+).

[0084] HRMS (ESI -) m / z (%): calculated for FeC14: 193.8150, found: 193.8147 (100%, FeCl4~),158.8461 (40%, FeCl3).

[0085] Melting point: 113°C

[0086] 5) SYNTHESIS OF [TEBA][FeCl4]

[0087] A solution of FeC13*6H2O (0.36 mmol) in EtOH (10 ml) and a solution of TEBACI (0.36 mmol) in EtOH (10 ml) are mixed under stirring; the reaction mixture is left under stirring at room temperature for 3 h, during which an immediate precipitation of a yellow powder is observed. The solution is then cooled to -20°C for 24 h and the yellow-orange precipitate is filtered and dried under vacuum (molecular pump, 10~2mBar).

[0088] Yield: 84%

[0089] Characterization

[0090] Elemental analysis (Ci3H22C14FeN»H2O):

[0091] Calculated: C, 38.27; H, 5.93; N, 3.43

[0092] Found: C, 30.50; H, 5.70; N, 3.42

[0093] 6) SYNTHESIS OF [TEA][FeCl4]

[0094] Procedure

[0095] TEACI [TEA][F®Ctt]

[0096] A solution of FeC13*6H2O (0.36 mmol) in EtOH (10 ml) and a solution of TEACI (0.36 mmol) in EtOH (10 ml) are mixed under stirring; the reaction mixture is left under stirring at room temperature for 3 h, during which an immediate precipitation of a yellow powder is observed. The solution is then cooled to -20°C for 24 h and the yellow-orange precipitate is filtered and dried under vacuum (molecular pump, 10~2mBar).

[0097] Yield: 90%

[0098] Characterization Elemental analysis (C8H2oC14FeN»H20): Calculated: C, 27.78; H, 6.41; N, 4.05 Found: C, 27.31; H, 6.20; N, 3.95 COBALT (II) METALATES

[0099] 7) SYNTHESIS OF [TMA]2[C0CI4]

[0100] Procedure

[0101] TMACI [TMA]2|C$Gl4l

[0102] A solution of CoC12*6H20 (1 mmol) in MeOH (5 ml) and a solution of TMACI (2 mmol) in MeOH (5 ml) are mixed under stirring; the reaction mixture is left under stirring at 40°C for 12 h, after which the solution is concentrated under reduced pressure. Eventually, the blue precipitate is filtered and dried under vacuum.

[0103] Alternative procedure: the two reagents CoC12*6H20 (9.35 mmol) and TMACI (18.7 mmol) are mixed in the solid phase. A minimal amount of EtOH is then added under vigorous stirring at room temperature to promote the formation of a blue precipitate, which is then isolated and dried under vacuum.

[0104] Yield: 88%

[0105] Characterization

[0106] Elemental analysis (C8H24CI4N2C0): Calculated: C = 27.53%; H

[0107] Found: C = 27.25%; H =

[0108] Melting point: >300°C

[0109] COPPER (II) METALATES

[0110] 8) SYNTHESIS OF [TMA]2[CuCl4]

[0111] Procedure

[0112] A solution of CuC12*4H2O (1 mmol) in MeOH (5 ml) and a solution of TMAC1 (2 mmol) in MeOH (5 ml) are mixed together under stirring; the reaction mixture is left under stirring at 40°C for 12 h, after which the solution is concentrated under reduced pressure, and the yellow precipitate is filtered and dried under vacuum.

[0113] Yield: 82%

[0114] Characterization

[0115] Elemental analysis (C8H24CI4N2CU):

[0116] Calculated: C, 27.17; H, 6.84; N, 7.92

[0117] Found: C, 27.15; H, 6.73; N, 7.65

[0118] Melting point: >3000C

[0119] ZINC (II) METALATES

[0120] 9) SYNTHESIS OF [TMA]2[ZnCl4] Procedure

[0121] TMAC1 [TMA]i2[ZnCl4J

[0122] A solution of ZnC12 (1 mmol) in MeOH (5 ml) and a solution of TMAC1 (2 mmol) in MeOH (5 ml) are mixed together under stirring; the reaction mixture is left under stirring at 40°C for 12 h, after which the solution is concentrated under reduced pressure, and the white precipitate is filtered and dried under vacuum.

[0123] Yield: 95%

[0124] Characterization

[0125] Elemental analysis (CgH24C14N2Zn):

[0126] Calculated: C, 27.03; H, 6.81; N, 7.88

[0127] Found: C, 26.88; H, 6.75; N, 7.31

[0128] MS (ESI +) m / z (%): calculated for CieHseN: 242.28, found: 242.28 (100%, M+).

[0129] HRMS (ESI -) m / z (%): calculated for ZnCl3170.83, found: 170.833 (100%, ZnCl3~).

[0130] Melting point: >300°C

[0131] METHANOLYSIS REACTIONS

[0132] Production of DMT (dimethyl terephthalate).

[0133] Example 1 - PRESSURE TUBE

[0134] Pressure tubes are generally glass reactors resistant to pressure buildup, capable of preventing solvent loss due to evaporation. They are equipped with rather thick edges and a screw cap. They have a maximum volume of 20 ml. Methanolysis tests were carried out in these reactors using 250 mg of R-PET flakes, 4 ml of MeOH and a catalytic loading of 5 moll with respect to the weight of the flakes (considered as pure PET). For heating, an aluminium block, in which the tubes can be inserted, was used.

[0135] The reaction mixture is heated to 130°C for 16 h under vigorous stirring. At the end of the reaction time, the mixture is allowed to cool down to room temperature; during cooling, the DMT precipitates and can then be isolated by filtration (the catalysts remain dissolved in MeOH).

[0136] The product is then purified by recrystallization from boiling AcOEt and eventually precipitated at -20°C. The recrystallization is carried out by dissolving the precipitate obtained directly from the reaction mixture in hot AcOEt, in this way allowing the product (DMT) to dissolve while any unreacted PET is isolated by filtration on paper and weighed to determine the process conversion (Table 1). The yields, here and subsequently, are determined with respect to the maximum amount in moles of DMT obtainable, considering the introduced flakes as being composed 100% of pure PET.

[0137] Table 1. Methanolysis reactions carried out in pressure tubes.a- a-Substrate: R-PET (250 mg), MeOH (4 ml), temperature = 130°C, reaction time = 16 h, catalytic loading = 5 mol% Example 2 - AUTOCLAVE 1

[0138] In a custom stainless steel 40 ml volume autoclave, methanolysis tests were carried out using 2.5 g or 5 g of R- PET, 16 - 20 ml of MeOH and a catalytic loading of 1 - 5%.

[0139] Heating was achieved using an oil bath and the reaction mixture was kept under constant stirring using a magnetic stirrer . The reaction mixture is maintained at 170°C for a period comprised between 0.5 and 3 h. After the reaction time, the autoclave is immediately cooled in an ice bath, causing the product to precipitate in crystalline form. The contents of the autoclave are recovered by filtration, and subsequently purified in different ways:

[0140] • Recrystallization in boiling ethyl acetate (AcOEt) with precipitation by cooling

[0141] • Direct sublimation of the solid mixture (product + impurities) by heating (170 - 200°C) under reduced pressure

[0142] • washing of the mixture with water, filtration, drying, and subsequent sublimation as previously described.

[0143] Table 2. Methanolysis reactions carried out in autoclave

[0144] 1.a

[0145] Table 2. Methanolysis reactions carried out in autoclave

[0146] 1.a

[0147] Tests were carried out on specific substrates, derived from post-consumer materials, that to date do not find employment in mechanical recycling, such as those listed in the previous Table A. Each test was carried out using 5 g of the substrate, 1% [TMA][FeC14] catalyst (70 mg), 20 ml of MeOH, at 170°C for 1 h (as reported for other examples).

[0148] At the end of the reaction, the autoclave is cooled, and the product is recovered by filtration. Subsequently, the residue is dissolved in boiling ethyl acetate and filtered to remove the unwanted residues. The filtrate is cooled, in this way resulting in the precipitation of DMT. The samples are then filtered, dried, and the purity is determined by DSC.

[0149] Table 3. Methanolysis reactions carried out in autoclave 1 using [TMA][FeCld as catalyst.a

[0150] 1 moll These materials were used without any washing and separation treatment from other polymers, which is instead routinely carried out in recycling plants through flotation. Hereinafter are reported the impurities identified by IR after depolymerization of the various samples:

[0151] - Trays: PVA, PR, paper

[0152] Flake_coloured_mix: PET oligomers, PVA, acrylates, PVC, paper

[0153] - White_bottles: PET oligomers, titanium dioxide

[0154] - NC_bottles: PP, PET oligomers

[0155] An alternative purification procedure tested consists in the sublimation of the solid product under vacuum at a temperature of 170°C. The obtained product is of very high purity and appears as a compact white crystalline solid.

[0156] As proof of the high purity achievable by this process, the previously described product resulting from the depolymerization of food trays, and obtained by crystallization with a purity of 99.863%, was subjected to sublimation, obtaining a product with a purity of 99.962%.

[0157] A direct sublimation test of the product recovered at the end of the reaction was also performed, using milk bottles as substrate which was subjected to a water washing for the separation of catalyst and excess ethylene glycol. The sublimated product was recovered with a total yield of 62% (greater than the crystallization method) and with a purity of 98.6%. This lower purity is likely attributable to the presence of residual water and / or glycol which may have been carried along with the sublimated product.

[0158] In this regard, a "simulated" reaction test was also performed, where 10 g of previously crystallised DMT was combined with 3.2 g of ethylene glycol and 140 mg of [TMA][FeC14], and subsequently subjected to water washing and sublimation. The product was again in this case recovered with a mean purity of 98.4%.

[0159] Two tests were also performed on mixed materials created ad hoc in the laboratory, by adding 1 g of another polymer, specifically HDPE and PS, to 10 g of R-PET yielding the following results.

[0160] 10 g R-PET + 1 g HDPE: 100% R-PET conversion - DMT yield 76.4%

[0161] 10 g R-PET + 1 g PS: 100% R-PET conversion - DMT yield 73.2%

[0162] Two additional tests were carried out under the same conditions previously described (5 g of R-PET in 20 ml of MeOH, T = 170°C, t = 1 h), using catalysts wherein the cation is modified. The following catalysts were used:

[0163] - Tetraethylammonium tetrachloroferrate (TEAFeC14)

[0164] - triethylbenzylammonium tetrachloroferrate (TEBAFeC14)

[0165] Both catalysts were prepared following the methods previously described and dried at 50°C before use.

[0166] Both reactions resulted in a complete depolymerization of the starting material, but a significant presence of oligomers is observed, with a consequently lower yield of DMT: 2.46 g for TEAFeC14, corresponding to a 48.7% yield, and 2.60 g for TEBAFeC14, corresponding to a 51.5% yield, respectively. Additionally, poorer separation of the catalyst from the product, that consequently leads to a lower purity of the isolated DMT, is observed.

[0167] Example 3 - AUTOCLAVE 2

[0168] This second autoclave has a total volume of 100 ml, is made entirely of stainless steel, and is equipped with a manometer, to monitor the pressure that develops during the tests (about 20-22 bar). Typically, in this reactor methanolysis tests were carried out on 12.5 g or 25 g of plastic material (depending on the desired concentration), in 80 ml of MeOH and with a [TMA][FeC14] catalytic loading of 5% or 2.5%. The latter is the metallated (ferrate) catalyst that, during previous screenings, proved to be the best in terms of activity and efficiency. Further, being based on iron, an abundant and non-toxic metal, it is also a good choice in terms of cost-effectiveness and environmental sustainability. For all these reasons, many tests were carried out with this catalyst.

[0169] Heating was achieved using a block of aluminium in which the autoclave can be inserted. The reaction mixture is heated to 170°C for 3 h, under very vigorous stirring. After the reaction time, the autoclave is cooled in an ice bath. The product is observed to precipitate in crystalline form (DMT), which is readily isolated by filtration and then purified through recrystallization from boiling AcOEt and subsequent cold precipitation.

[0170] In any case, only few tests were carried out utilizing the "AUTOCLAVE 2" reactor, as "AUTOCLAVE 3" (example 4), being equipped with mechanical stirring, resulted being more efficient .

[0171] Table 4. Methanolysis reactions carried out in autoclave 2 using [TMA][FeCld as catalyst.a- Table 4. Methanolysis reactions carried out in autoclave 2 using [TMA][FeCl4] as catalyst.a- a- Substrate: F -PET or coloured R-PET (25 g), MeOH (80 ml), temperature = 170°C, reaction time= 3 h, [TMA][FeCl4] catalytic loading == 2.5 or 5 moll

[0172] As can be seen, Test 3 was performed on coloured R-PET. In this case, no issues were observed during any of the process steps due to the fact that the dyes are very soluble both in MeOH during the actual depolymerization and then in AcOEt (the solvent used for the crystallization), resulting in pure DMT in the form of white crystals (as occurs with R- PET) as the product.

[0173] Example 4 - AUTOCLAVE 3

[0174] This third autoclave, as already mentioned, proved to be the most effective, most likely due to the mechanical agitation: the reaction mixture being in fact heterogeneous, due to the presence of plastic material, very efficient agitation significantly facilitates the depolymerization process. Specifically, due to this characteristic not only was it possible to reduce reaction times but also to reduce the catalytic loading and, most importantly, the MeOH / PET ratio. In fact, it is noteworthy that under these conditions, in the optimized reactions, a MeOH / PET ratio = 3.2, very convenient compared to the standard processes reported, was used.

[0175] This reactor (the largest used) has a total volume of about 300 ml, is made entirely of stainless steel, and is equipped with a manometer to monitor the pressure that develops during the tests (about 20-22 bar), and an internal temperature probe.

[0176] Methanolysis tests carried out with this reactor used 24 g or, more frequently, 48 g of plastic material (allowing for an initial scale-up), in 195 ml of MeOH and with a catalytic loading varying between 5%, 2.5%, 1%, 0.5% and 0.1%.

[0177] Heating was achieved by placing the autoclave in a thermal blanket directly connected to a thermostat, which, thanks to the coupling with the internal probe, ensured very precise temperature control of the reaction. The reaction mixture, already under mechanical stirring, is then gradually heated until reaching the desired temperature of 170°C (starting from room temperature, in about 30 min 170°C are reached). The studied reaction times were those comprised between 1 h and 4 h, always considered from the moment the reaction mixture reached 170°C.

[0178] As in the previously described cases, at the end of the reaction time the autoclave is cooled in an ice bath and consequently the product (DMT) precipitation in crystalline form is observed, which is readily isolated by filtration and then purified via recrystallization from boiling AcOEt and subsequent cold precipitation.

[0179] Effect of varying the catalytic loading:

[0180] Table 5. Methanolysis reactions carried out in autoclave

[0181] 3 using [TMA][FeCld as catalyst.a- a- Substrate: R-PET (48 g), MeOH (195 ml), temperature =

[0182] 170°C, reaction time = 4 h, catalytic loading = variable

[0183] [TMA][FeCl4] Effect of varying the reaction time with constant catalytic loading (comparison between 1 moll and 0.5 moll catalytic loading):

[0184] Table 6. Methanolysis reactions carried out in autoclave a- Substrate: R-PET (48 g), MeOH (195 ml), temperature =

[0185] 170°C, variable reaction time, [TMA][FeC14] catalytic loading = 1 moll

[0186] Table 7. Methanolysis reactions carried out in autoclave

[0187] 3, [TMA][FeC14] catalyst loading = 0.5 moll.a a- Substrate: R-PET (48 g), MeOH (195 ml), temperature =

[0188] 170°C, variable reaction time, [TMA][FeC14] catalytic loading = 0.5 moll

[0189] Catalyst screening (catalytic loading 1 moll): Table 8. Methanolysis reactions carried out in autoclave

[0190] 3.a- a- Substrate: R-PET (48 g), MeOH (195 ml), temperature =

[0191] 170°C, variable reaction time (1 h or 2 h), catalytic loading

[0192] = 1 mo1%

[0193] Subsequent tests with recovered catalyst (initial catalytic loading 1 moll):

[0194] To study the catalyst's activity and assess the possibility of reusing it multiple times, tests were carried out in which, at the end of the reaction, the catalyst was recovered by distilling part of the reaction mixture. The distilled mixture contains MeOH, EG, a small amount of dissolved DMT, and the catalyst itself. During operation, it was observed that part of the DMT does not distil over, thereby causing the weight of the recovered catalyst to progressively increase.

[0195] Table 9. Methanolysis reactions carried out in autoclave

[0196] 170°C, reaction time = 2 h, initial catalytic loading = 1 moll

[0197] Tests for studying the effect of the presence of other polymers (catalytic loading 1 moll): The ratio used between PET and the other polymer is 10:1 by weight. In particular, to compose the reaction mixture, 48 g PET and 4.8 g of the other plastic material are combined.

[0198] Table 10. Methanolysis reactions carried out in autoclave

[0199] 3 using [TMA][FeCld as catalyst.a- a- Substrate: clear R-PET (48 g) + PP or LDPE or PA (4.8 g), MeOH (195 ml), temperature = 170°C, reaction time = 3 h, catalytic loading [TMA][FeC14] = 1 moll

[0200] All plastics other than PET are introduced into the reaction mixture in the form of granules. In the case of PP, LDPE, HDPE and PS, a fused agglomerate of these polymers is observed at the end of the reaction, but they are not chemically altered by the process and do not significantly affect its efficiency. However, with PA (specifically nylon 6,6), it does not remain recognizable at the end of the reaction; however, further analysis showed that the PA remains in the polymer matrix although it changes form, in that from initially being pellets it appears as a white powder (a change likely caused by heating).

[0201] Tests on actual material provided by Montello spa (catalytic loading 1 moll):

[0202] Table 11. Methanolysis reactions carried out in autoclave a- Substrate: "Aspirated" or "Flake coloured mix" or "Trays" (24 or 48 g), MeOH (195 ml), temperature = 170°C, reaction time = 1 h, [TMA][FeC14] catalytic loading = 1 moll

[0203] • "Aspirated": PET from aspiration from mechanic recycling plant. This material, being very fine, is very reactive.

[0204] • "Flake coloured mix": mixed material containing PET,

[0205] PVA, acrylates, PVC and paper. • "Trays": multilayer food trays. This material, in addition to PET, contains PVA, PP and paper.

[0206] Even when processing these materials high purity DMT is obtained.

[0207] Example 5 - AUTOCLAVE 4 (microwave)

[0208] This final autoclave has a maximum volume of 100 ml and is made entirely of Teflon, a material that is transparent to microwaves, which therefore allows tests to be carried out using this type of heating. A methanolysis test was carried out using 2 g of R-PET, in 32 ml of MeOH and with a [TMA] [FeC14] catalytic loading of 5%. The reaction mixture was heated to 170°C for 1 h, resulting in complete conversion and a DMT yield of 74.4%.

[0209] Although this is not an excellent result it is reasonable to believe that microwaves could bring several benefits to the process, in particular a drastic reduction in reaction times.

[0210] Some advantages of the process of the invention are: use of environmentally sustainable and non-toxic catalysts that are inexpensive and, anyway, can be used in small quantities - high solubility of the catalyst in methanol, thereby allowing for almost complete recovery thereof (and thus subsequent reuse) through a simple distillation

[0211] - DMT is not contaminated by the catalyst

[0212] - high activity of the catalyst

[0213] - production of DMT in nearly quantitative yields and ease of recovery thereof by filtration

[0214] - high purity of DMT that allows its reuse as monomer for the production of terephthalic polyesters

[0215] - low toxicity and recyclability of ethyl acetate used for the crystallization of the DMT short reaction times, especially in the case of methanolysis using microwaves

[0216] - ability to use the catalyst even in the presence of other polymers or materials such as paper, metal and wood, which can be readily separated by filtration

[0217] - no influence of residual moisture on the outcome of the process.

[0218] It is clear that only certain specific embodiments of the present invention have been described, to which the skilled in the art will be able to apply all those modifications necessary for their adaptation to particular applications, without however departing from the scope of protection of the present invention as defined in the appended claims.

Claims

CLAIMS1. A process for depolymerizing plastic waste containing terephthalic polyesters, and in particular polyethylene terephthalate (PET), comprising the following steps: a) subjecting a plastic material containing terephthalic polyesters, in particular PET, to a methanolysis process in the presence of methanol and a catalyst based on a ionic compound of a quaternary ammonium and an iron(III), manganese (II), or cobalt (II) halide; b) separating the solid residue of step a) containing dimethyl terephthalate (DMT) from the liquid phase containing methanol, a glycol and the catalyst in solution; c) separating the DMT in substantially pure form from the solid residue of step b); d) optionally, subjecting the liquid phase of step b) to fractional distillation to recover the methanol, the glycol and the catalyst.

2. The process according to claim 1, wherein in step a) the reaction temperature is between 110°C and 190°C, or between 120°C and 185°C, or between 130°C and 180°C, or between 150°C and 170°C, and the pressure is ≤ 20 bar.

3. The process according to claim 1 or 2, wherein the reaction times are comprised between 0.5 and 6 hours, or are < 3 hours.

4. The process according to any one of claims 1 to 3, wherein the methanol / terephthalic polyester weight ratio is comprised between 2 and 10, or between 2 and 6, or between 2.7 and 3.3.

5. The process according to any one of claims 1 to 4, wherein the molar amount of catalyst, with respect to the moles of terephthalic polyester, is greater than or equal to 0.1 moll and less than 6 moll, or less than 2 moll, or less than 1.2 moll, or less than 0.6 moll.

6. The process according to any one of claims 1 to 5, wherein the catalyst is a tetraalkylammonium Me-chloride, wherein Me is selected from Fe, Co and Mn, wherein preferably the tetraalkylammonium cation is tetramethylammonium.

7. The process according to claim 6, wherein the catalyst is (CH3)4N+FeClr.

8. The process according to any one of claims 1 to 7, wherein step a) is carried out by microwave heating.

9. The process according to any one of claims 1 to 8, wherein step b) is carried out by separating the solid residue by filtration or centrifugation, after cooling themixture obtained in step a) to a temperature below 20°C, or below 15°C, or about 10°C.

10. The process according to any one of claims 1 to 9, wherein step c) is carried out according to the following steps: i) dissolving the solid residue in hot ethyl acetate, preferably at the boiling temperature of ethyl acetate, ii) filtering the obtained DMT solution to remove suspended solids, iii) crystallizing the DMT from the ethyl acetate solution by cooling, iv) separating the crystallized DMT by filtration or centrifugation .

11. The process according to any one of claims 1 to 9, wherein step c) is carried out by sublimation of DMT from the solid residue separated in step b).

12. A use of a tetraalkylammonium Me-chloride, wherein Me is selected from Fe, Co and Mn, wherein preferably the tetraalkylammonium cation is tetramethylammonium, as a catalyst in a solvolysis reaction.

13. The use according to claim 12, wherein the catalyst is (CH3)4N+FeCl4~.

14. The use according to claim 12 or 13, wherein the solvolysis reaction is a methanolysis or glycolysis reaction of a terephthalic polyester.

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