A monodisperse nanocrystal catalyst for a method of recycling waste pet

Abstract

This invention discloses a method for recycling waste PET using a monodisperse nanocrystalline catalyst, comprising the following steps: S1, placing PET waste, reaction solvent, and monodisperse nanocrystalline catalyst into a depolymerization reactor; S2, after the depolymerization reaction is completed, filtering out unreacted insoluble matter while hot; S3, after cooling and precipitating the depolymerization product in the filtrate, fine filtration is performed to form a filter cake, and the monodisperse nanocrystalline catalyst in the filtrate is recycled along with the reaction solvent during depolymerization; S4, after washing, drying, and other treatments, the filter cake is sent to a polycondensation reactor with a polycondensation catalyst to synthesize new PET. The monodisperse nanocrystalline catalyst overcomes the problem of low activity caused by low mass transfer efficiency in depolymerization reactions of micron and nanoparticle catalysts. It can perform alcoholysis of waste polyester under mild conditions and in a short time, significantly improving the reaction rate and reducing reaction energy consumption. Its depolymerization product, after melt polycondensation, produces high molecular weight and high purity PET products.

Description

Technical Field

[0001] This invention relates to the field of chemical recycling technology for waste PET; more specifically, it relates to a method for recycling waste PET using a monodisperse nanocrystalline catalyst. Background Technology

[0002] Waste plastics severely pollute the environment, and their degradation is a global challenge. The International Union of Pure and Applied Chemistry (IUPAC) has included it in its list of "Ten Chemical Innovations That Will Change the World." Currently, less than 9% of waste plastics worldwide are downgraded to low-value plastics through mechanical recycling and other methods. However, emerging chemical recycling processes hold promise for providing a sustainable development path for the circular upgrading of plastics. Polyethylene terephthalate (PET) is a petroleum-based plastic polymerized from terephthalic acid. It is one of the most widely used plastics in film and sheet materials, packaging bottles, electronics, automotive parts, and machinery, with a lifespan far shorter than other petroleum-based plastics such as polyurethane and polyvinyl chloride. Compared to commonly used mechanical recycling methods, chemical depolymerization of PET can create an ideal circular polymer economy, depolymerizing waste plastics into hydrolyzed monomers such as terephthalic acid (TA), alcoholyzed monomers such as BHET and DMT. The obtained monomers can be converted into high-value chemicals or repolymerized into PET with the original physical and chemical properties.

[0003] Homogeneous catalysts commonly used in PET depolymerization reactions, including metal (Zn, Co, Mn, etc.) acetates and ionic liquids, are not easily separated from the products and reused. Chinese patent CN201911282127.6 describes a method for methanol hydrolysis of waste PET polyester using a homogeneous catalyst. The hydrolysis product is then subjected to DMT distillation, ethylene glycol transesterification, and BHET repolymerization to prepare recycled food-grade PET polyester. The new PET plastic polymerized from the purified BHET product exhibits a higher intrinsic viscosity during recycling than recycled PET obtained through direct melt processing, while still meeting food-grade hygiene requirements. However, its drawback lies in the highly complex process of separating the homogeneous catalyst from the reaction solvent.

[0004] To facilitate catalyst separation and recycling and improve reaction efficiency, nanoscale and highly dispersed metal particles, metal oxide particles, and graphitic carbon nitride have also been used to degrade PET. Among them, monodisperse nanocrystalline catalysts, compared with homogeneous catalysts, are separable and also have excellent activity, which can significantly promote mass transfer processes in heterogeneous reactions and increase the reaction rates of depolymerization and polycondensation reactions (Du JT, Sun Q, Zeng XF, et al. ZnO nanodispersion aspseudohomogeneous catalyst for alcoholysis of polyethylene terephthalate[J]. Chemical Engineering Science, 2020, 220: 115642.).

[0005] The use of monodisperse nanocrystalline catalysts to alcoholyze waste PET into BHET or DMT significantly reduces reaction energy consumption compared to the direct esterification of purified terephthalic acid (PTA) to synthesize BHET. This redirects the demand for high-purity terephthalic acid raw materials to the recycling, washing, and decolorization of waste PET plastics. Therefore, providing a highly efficient chemical conversion method based on monodisperse nanocrystalline catalysts for the alcoholysis of waste PET into new PET via polycondensation is of great significance in reducing energy consumption in PET production and improving the quality of recycled PET. By reducing the cost of BHET, an intermediate product in polyester synthesis, PET production's dependence on petrochemical products is lessened, while simultaneously reducing PET pollution in the natural environment and promoting a circular economy for waste plastics. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide a method for recycling waste PET using a monodisperse nanocrystalline catalyst. The monodisperse nanocrystalline catalyst overcomes the problem of low activity caused by low mass transfer efficiency in depolymerization and polycondensation reactions of micron and nanoparticle catalysts. It enables the alcoholysis and melt polycondensation of waste polyester under mild conditions and in a short time, significantly improving the reaction rate, reducing reaction energy consumption, and producing PET products with high intrinsic viscosity and high purity.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A method for recycling waste PET using a monodisperse nanocrystalline catalyst includes the following steps:

[0009] S1. Place the PET waste, reaction solvent, and monodisperse nanocrystalline catalyst into the depolymerization reactor;

[0010] S2. After the depolymerization reaction is complete, filter out the unreacted insoluble matter while it is still hot.

[0011] S3. After the depolymerization products in the filtrate are cooled and precipitated, they are finely filtered to form a filter cake. The monodisperse nanocrystals in the filtrate are recycled along with the reaction solvent during the depolymerization process.

[0012] S4, after being washed, dried and processed, the filter cake is taken to the polycondensation reactor with the polycondensation catalyst to synthesize new PET.

[0013] As a further improvement to the technical solution, in step S1, the monodisperse nanocrystals are selected from one or more of TiO2, CeO2, MnO2, CuO, Cu2O, ZnO, MgO, NiO, ZrO2, Al2O3, ZIF-67, ZIF-8, and MAF-6.

[0014] Preferably, the average particle size of the monodisperse nanocrystals is 1-10 nm; more preferably, the average particle size of the monodisperse nanocrystals is between 1-5 nm.

[0015] As a further improvement to the technical solution, in step S1, the monodisperse nanocrystals are nanocrystals that exist in a stable monodisperse form in a liquid medium, the liquid medium being selected from the reaction solvent, and the content of nanocrystals in the dispersion system being 1-30 wt%; more preferably, the content being 5-20 wt%.

[0016] As a further improvement to the technical solution, in step S1, the amount of the monodisperse nanocrystalline catalyst used in the depolymerization process is 0.1-1 wt% of the PET waste mass, more preferably 0.3-0.8 wt%.

[0017] Preferably, in step S1, the waste PET is decolorized to reduce the color value b to less than 2.5.

[0018] Preferably, in step S1, the reaction solvent is selected from one or more of methanol, ethanol, ethylene glycol, butanediol, and hexanediol.

[0019] Preferably, in step S1, the mass ratio of the reaction solvent to PET waste is 3-10:1; more preferably, the mass ratio is 4-8:1.

[0020] Preferably, in step S1, the reaction temperature of the depolymerization reactor is 150-197℃, the reaction time is 0.5-3h, and the stirring rate of the reactor is 50-500rpm; more preferably, the reaction temperature is 180-197℃, and the reaction time is 0.75-1.5h.

[0021] As a further improvement to the technical solution, in step S2, the temperature for hot filtration after the depolymerization reaction is 120-180℃; more preferably, the hot filtration temperature is 140-160℃.

[0022] Preferably, in step S2, the insoluble matter includes some oligomers and unreacted PET, which can be directly sent to step S1 for depolymerization until completely converted into alcoholysis monomers after drying.

[0023] As a further improvement to the technical solution, in step S3, the temperature of the cooling precipitation is 0-50℃, the content of monodisperse nanocrystalline catalyst in the filtrate is 80-100% of the original added amount, and the amount of reaction solvent and catalyst dispersed in the filtrate is replenished to a sufficient amount before being recycled to the depolymerization reactor.

[0024] As a further improvement to the technical solution, in step S4, the washing solution for washing the filter cake is selected from one or more of water, methanol, ethanol, ethylene glycol, cyclohexane, tetrahydrofuran, toluene, acetone, N,N'-dimethylformamide, and N,N'-dimethylacetamide.

[0025] Preferably, when ethylene glycol is used as the reaction solvent in step S1, the filter cake is diethyl terephthalate (BHET) and its oligomers, which are directly sent to the polycondensation unit; when methanol, ethanol, 1,2-propanediol, and 1,4-butanediol are used as the reaction solvent in step S1, the filter cakes are dimethyl terephthalate (DMT), diethyl terephthalate (DET), propylene glycol terephthalate, and butylene glycol terephthalate, respectively, which are exchanged with ethylene glycol to obtain BHET and its oligomers, and then sent to the polycondensation unit.

[0026] As a further improvement to the technical solution, in step S4, the polycondensation catalyst is selected from one or more of TiO2, Sb2O3, Sb2S3, V2O5, and GeO2;

[0027] Preferably, the amount of the polycondensation catalyst used in the catalytic polycondensation is 0.1-2 wt% of the mass of the depolymerization product; more preferably, the amount is 0.3-0.8 wt%.

[0028] Preferably, the polycondensation reactor includes a pre-polymerization section and a final polymerization section; the reaction temperature of the pre-polymerization section is 240-300℃, the vacuum degree is 20-100Pa, the stirring rate is 50-1000rpm, and the vacuuming time is 0.5-1.5h; the reaction temperature of the final polymerization section is 250-320℃, the stirring rate is 50-1000rpm, and the reaction time is 0.5-4h.

[0029] Any range described in this invention includes the endpoint, any value between the endpoints, and any subrange consisting of the endpoint or any value between the endpoints.

[0030] Unless otherwise specified, all raw materials used in this invention can be obtained commercially, and the equipment used in this invention can be conventional equipment in the relevant field or refer to existing technology in the relevant field.

[0031] Compared with the prior art, the present invention has the following beneficial effects. :

[0032] 1) This invention reduces the production cost of petroleum-based plastic PET by utilizing the high activity of monodisperse nanocrystals for depolymerization of waste PET and BHET polycondensation. Compared with common waste PET melt recycling processes, the PET obtained by chemically recovering alcoholysis monomers BHET or DMT and then polycondensing can achieve the effect of not degrading physical and chemical properties.

[0033] 2) Monodisperse nanocrystalline catalysts can completely degrade waste PET in a short time under relatively mild conditions, significantly improving depolymerization efficiency and reducing the need for high-temperature and high-pressure equipment for PET depolymerization reaction. Waste PET can be completely depolymerized under reaction temperature of 170-197℃ and reaction time of 30-80min, and the product purity can reach more than 90%.

[0034] 3) The monodisperse nanocrystalline catalyst has a very low dosage and good cyclic depolymerization effect. Attached Figure Description

[0035] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0036] Figure 1 The images shown are transmission electron microscope (TEM) images, particle size distribution diagrams, and photographs of the monodisperse nanocrystals in Example 1.

[0037] Figure 2 Here is a scanning electron microscope image of the melt-recycled PET fiber from Example 1;

[0038] Figure 3 Thermogravimetric curve of the melt-recycled PET fiber in Example 1;

[0039] Figure 4 This is a photograph of the product obtained from the degradation and melting of recycled PET fibers in Example 1.

[0040] Figure 5 The nuclear magnetic resonance spectrum of the product obtained from the degradation and melt recycling of PET fibers in Example 1;

[0041] Figure 6 The liquid chromatogram of the product obtained from the degradation and melt recycling of PET fibers in Example 1;

[0042] Figure 7 This is a transmission electron microscope image of the monodisperse nanocrystals in Example 1 after recycling;

[0043] Figure 8 This is a transmission electron microscope (TEM) image of the nanoparticles in Comparative Example 1.

[0044] Figure 9 This is a transmission electron microscope (TEM) image of the nanoparticles in Comparative Example 2. Detailed Implementation

[0045] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments, further clarifies the invention. Those skilled in the art should understand that the specific descriptions below are illustrative rather than restrictive, and should not be construed as limiting the scope of protection of the present invention.

[0046] As one aspect of the present invention, a method for recycling waste PET using a monodisperse nanocrystalline catalyst includes the following steps:

[0047] S1. Place the PET waste, reaction solvent, and monodisperse nanocrystalline catalyst into the depolymerization reactor;

[0048] S2. After the depolymerization reaction is complete, filter out the unreacted insoluble matter while it is still hot.

[0049] S3. After the depolymerization products in the filtrate are cooled and precipitated, they are finely filtered to form a filter cake. The monodisperse nanocrystals in the filtrate are recycled along with the reaction solvent during the depolymerization process.

[0050] S4, after being washed, dried and processed, the filter cake is taken to the polycondensation reactor with the polycondensation catalyst to synthesize new PET.

[0051] In some embodiments, in step S1, the monodisperse nanocrystalline catalyst comprises monodisperse nanocrystalline crystals and a liquid-phase dispersion medium; the monodisperse nanocrystalline crystals are selected from one or more of TiO2, CeO2, MnO2, CuO, Cu2O, ZnO, MgO, NiO, ZrO2, Al2O3, ZIF-67, ZIF-8, and MAF-6.

[0052] In some embodiments, the average particle size of the monodisperse nanocrystals is 1-10 nm; more preferably, the average particle size of the monodisperse nanocrystals is between 1-5 nm. The nanocrystals exist in a stable monodisperse form in a liquid medium selected from a reaction solvent, and the content of the nanocrystals in the dispersion system is 1-30 wt%; more preferably, the content is 5-20 wt%. The monodisperse nanocrystal catalyst used as a catalyst in this invention is prior art, and its preparation can refer to the methods disclosed in Chinese Patent CN201710303695.4 or Chinese Patent CN201710794562.1, in which suitable surface coating agents such as polyvinylpyrrolidone, polyethylene glycol, silane coupling agents, etc., are grafted onto the surface of the nanocrystals through existing in-situ addition or synthesis, so that they can be stably monodispersed in the liquid medium.

[0053] In some embodiments, in step S1, the amount of the monodisperse nanocrystalline catalyst used in the depolymerization process is 0.1-1 wt% of the PET waste mass, more preferably 0.3-0.8 wt%.

[0054] In some embodiments, in step S1, the waste PET is decolorized to reduce the color value b to less than 2.5.

[0055] In some embodiments, in step S1, the reaction solvent is selected from one or more of methanol, ethanol, ethylene glycol, butanediol, and hexanediol.

[0056] In some embodiments, in step S1, the mass ratio of the reaction solvent to PET waste is 3-10:1; more preferably, the mass ratio is 4-8:1.

[0057] In some embodiments, in step S1, the reaction temperature of the depolymerization reactor is 150-197°C, the reaction time is 0.5-3h, and the stirring rate of the reactor is 50-500rpm; more preferably, the reaction temperature is 180-197°C, and the reaction time is 0.75-1.5h.

[0058] In some embodiments, in step S2, the temperature for hot filtration after the depolymerization reaction is completed is 120-180°C; more preferably, the hot filtration temperature is 140-160°C.

[0059] In some embodiments, in step S2, the insoluble material includes some oligomers and unreacted PET, which can be directly sent to step S1 for depolymerization until completely converted into alcoholysis monomers after drying.

[0060] In some embodiments, in step S3, the temperature of the cooling precipitation is 0-50°C, the content of monodisperse nanocrystalline catalyst in the filtrate is 80-100% of the original added amount, and the amount of reaction solvent and catalyst dispersed in the filtrate is replenished to a sufficient amount before being recycled to the depolymerization reactor.

[0061] In some embodiments, in step S4, the washing solution for washing the filter cake is selected from one or more of water, methanol, ethanol, ethylene glycol, cyclohexane, tetrahydrofuran, toluene, acetone, N,N'-dimethylformamide, and N,N'-dimethylacetamide.

[0062] Preferably, when ethylene glycol is used as the reaction solvent in step S1, the filter cake is diethyl terephthalate (BHET) and its oligomers, which are directly sent to the polycondensation unit; when methanol, ethanol, 1,2-propanediol, and 1,4-butanediol are used as the reaction solvent in step S1, the filter cakes are dimethyl terephthalate (DMT), diethyl terephthalate (DET), propylene glycol terephthalate, and butylene glycol terephthalate, respectively, which are exchanged with ethylene glycol to obtain BHET and its oligomers, and then sent to the polycondensation unit.

[0063] In some embodiments, in step S4, the polycondensation catalyst is selected from one or more of TiO2, Sb2O3, Sb2S3, V2O5, and GeO2;

[0064] Preferably, the amount of the polycondensation catalyst used in the catalytic polycondensation is 0.1-2 wt% of the mass of the depolymerization product; more preferably, the amount is 0.3-0.8 wt%.

[0065] Preferably, the polycondensation reactor includes a pre-polymerization section and a final polymerization section; the reaction temperature of the pre-polymerization section is 240-300℃, the vacuum degree is 20-100Pa, the stirring rate is 50-1000rpm, and the vacuuming time is 0.5-1.5h; the reaction temperature of the final polymerization section is 250-320℃, the stirring rate is 50-1000rpm, and the reaction time is 0.5-4h.

[0066] Example 1

[0067] A method for recycling waste PET using a monodisperse nanocrystalline catalyst includes the following steps:

[0068] The ethylene glycol-phase ZnO monodisperse nanocrystalline catalyst with an average particle size of 4 nm was used to degrade melt-recycled PET fibers. The process included the following steps: The monodisperse nanocrystalline catalyst, 10 g of PET waste, and 60 g of ethylene glycol were placed in a depolymerization reactor. The catalyst addition was 0.35 wt% of the PET mass. The reaction temperature was 180 °C, and the reaction time was 1 h. After the depolymerization reaction, unreacted insoluble matter was filtered off while hot. The monodisperse nanocrystalline catalyst and molten BHET were added to the filtrate. The depolymerization product in the filtrate was cooled and precipitated, then finely filtered to form a filter cake. The filter cake was washed and dried to obtain the depolymerized product. The product yield was determined by weighing the product, and the BHET content was obtained by high-performance liquid chromatography (HPLC). The PET degradation rate was 98.3%, the product yield was 75.7%, and the BHET purity was 93.0%.

[0069] Figure 1 The images shown are transmission electron microscope (TEM) images, particle size distribution diagrams, and photographs of the monodisperse nanocrystals in Example 1. The monodisperse nanocrystals are stably dispersed in ethylene glycol, with an average particle size of 4 nm. They are highly transparent at a solid content of 8.35% and no precipitation has occurred for more than a month.

[0070] Figure 2 The image shows a scanning electron microscope (SEM) image of the melt-recycled PET fiber from Example 1, with an average diameter of 100 μm.

[0071] Figure 3The thermogravimetric curve of the melt-recycled PET fiber in Example 1 is shown. The air atmosphere thermogravimetric curve shows that the inorganic impurity content is 1.56%. Combined with the XPS test in Table 1, the C / O content ratio is 2.25. The waste PET contains trace amounts of P, N, S, and Cl elements, as well as trace amounts of small molecule additives.

[0072] Figure 4 This is a photograph of the product obtained from the degradation and melting of recycled PET fibers in Example 1.

[0073] Figure 5 The nuclear magnetic resonance spectrum of the product obtained by degrading and melting recycled PET fiber in Example 1 is shown. Compared with commercial BHET with 85% purity, it can be seen that the main component of the degradation product is BHET, with a small amount of dimer, diethylene glycol and oxalic acid impurities.

[0074] Figure 6 The liquid chromatogram of the product obtained from the degradation and melt recycling of PET fibers in Example 1 shows that the retention time is 3.7 min for BHET monomer and 4.9 min for BHET dimer, with a BHET purity of 93.0%.

[0075] Figure 7 The image shown is a transmission electron microscope (TEM) image of the monodisperse nanocrystals from Example 1 after recycling. The monodisperse nanocrystals remain monodisperse in ethylene glycol, with no significant change in morphology.

[0076] Example 2

[0077] Example 1 was repeated, except that the reaction solvent was 60g methanol, the catalyst was methanol-phase CuO monodisperse nanocrystals with an average particle size of 3nm, the product was DMT, and the PET degradation rate was 96.4%, the yield was 80.6%, and the purity of DMT was 91.0%.

[0078] Example 3

[0079] Example 1 was repeated, except that the reaction solvent was 60g methanol, the catalyst was a methanol-phase CeO2 monodisperse nanocrystal catalyst with an average particle size of 3nm, the product was DMT, and the PET degradation rate was 99.6%, the product yield was 82.1%, and the DMT purity was 95.2%.

[0080] Example 4

[0081] Example 1 was repeated, except that: ethylene glycol-phase NiO monodisperse nanocrystalline catalyst with an average particle size of 3 nm was used. After the reaction, the PET degradation rate was measured to be 82.6%, the product yield was 74.9%, and the BHET purity was 90.2%.

[0082] Example 5

[0083] Example 1 was repeated, except that different PET wastes were used as PET sources. The PET degradation rate and product yield were measured after the reaction, as shown in Table 1.

[0084] Table 1

[0085] PET raw materials Particle size (μm) PET degradation rate (%) Product yield (%) Virgin PET fibers 100 99.1 79.3 Melt-recycled PET fiber 150 98.7 77.2 Waste PET fabrics 50 99.3 75.7 Waste PET powder 100 100 85.3 Virgin PET Sheets 3000 12.6 11.3 Melt-recycled PET sheets 3000 15.8 13.6

[0086] Example 6

[0087] Example 1 was repeated, except that the amount of catalyst in step (3) was changed to 0.165%-0.7%, and the PET degradation rate and product yield were measured after the reaction as shown in Table 2.

[0088] Table 2

[0089] Catalyst dosage (%) PET degradation rate (%) Product yield (%) 0.17 18.4 65.3 0.26 91.8 73.5 0.35 98.3 75.7 0.7 100.0 78.2 1.0 100.0 80.6

[0090] Example 7

[0091] Example 1 was repeated, except that after the product precipitated in ethylene glycol, the monodisperse nanocrystals in the filtrate obtained by fine filtration were recycled along with the reaction solvent in the next round of depolymerization. The PET degradation rate and product yield measured after the reaction are shown in Table 3.

[0092] Table 3

[0093] Reaction cycles (number of times) PET degradation rate (%) Product yield (%) 1 98.3 75.7 2 96.2 75.2 3 94.9 72.1 4 90.6 73.9 5 87.2 70.3

[0094] Example 8

[0095] Example 1 was repeated, except that the waste PET raw material used for depolymerization was first decolorized with N,N'-dimethylformamide solvent, and the condensation raw material BHET obtained from depolymerization was washed, dried, and then sent to the condensation reactor for further condensation reaction. According to the test results of inductively coupled plasma atomic emission spectrometry (ICP), the residual amount of monodisperse nanocrystalline catalyst in the depolymerization product was less than 20 ppm after simple washing. The condensation catalyst was Sb2O3, with a dosage of 0.3 wt%, the reaction temperature was 270°C, the vacuum was slowly evacuated to a vacuum degree of 80 Pa, the stirring rate was 400 rpm, the reaction time was 2.5 h, and the product PET was dried under vacuum at 90°C. The molecular weight of the melt-regenerated PET fiber from the raw material was 16.3 kDa, the molecular weight of the product PET was 17.5 kDa, the terminal carboxyl group content was 0.035 mol / kg, the yellowness (b value) was 4.9, and the diethylene glycol impurity content was 4.8%.

[0096] Comparative Example 1

[0097] Example 1 was repeated, except that 50nm commercially available nano-ZnO powder was used as the catalyst. After the reaction, the PET degradation rate was measured to be 12.3%, the product yield was 2.1%, and the BHET purity was 70.2%.

[0098] Figure 8 The image shows a transmission electron microscope (TEM) image of nano-ZnO powder in Comparative Example 1. Compared to monodisperse nanocrystals, the nanoparticles exhibit severe agglomeration. The deagglomeration reaction results catalyzed by both methods indicate that the monodisperse nanocrystal catalyst is significantly more efficient than the nanoparticle catalyst.

[0099] Comparative Example 2

[0100] Example 1 was repeated, except that 30nm commercially available nano CeO2 powder was used as the catalyst. After the reaction, the PET degradation rate was measured to be 10.2%, the product yield was 4.1%, and the BHET purity was 59.2%.

[0101] Figure 9 The image shows a transmission electron microscope (TEM) image of the nano-CeO2 powder in Comparative Example 2. Compared to monodisperse nanocrystals, the nanoparticles exhibit severe agglomeration. The deagglomeration reaction results catalyzed by both methods indicate that the monodisperse nanocrystal catalyst is significantly more efficient than the nanoparticle catalyst.

[0102] Comparative Example 3

[0103] Example 1 was repeated, except that the reaction temperature in the depolymerization reactor was 150°C, and the PET degradation rate was only 20.7% after the reaction. This shows that the reaction efficiency of PET depolymerization is significantly reduced below 170°C.

[0104] Comparative Example 4

[0105] Example 1 was repeated, except that after the depolymerization reaction was completed and cooled to 80°C, the ethylene glycol alcoholysis product BHET was filtered. It was observed that most of BHET had precipitated in the filter residue and could not be separated from the unreacted PET. The product yield recovered in the filtrate was as low as 50.1%.

[0106] Therefore, it can be seen that the thermal filtration process can maximize the collection of BHET products while separating unreacted PET residue.

[0107] Comparative Example 5

[0108] Example 8 was repeated, except that the reaction time of the polycondensation stage was shortened to 1.5 h, the molecular weight of the PET product was 10.5 kDa, the terminal carboxyl group content was 0.052 mol / kg, the yellowness (b value) was 3.5, and the diethylene glycol impurity content was 2.8%.

[0109] This indicates that a shorter reaction time results in a higher carboxyl group content at the end of the reaction, suggesting incomplete polycondensation.

[0110] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is impossible to exhaustively list all embodiments here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A method for recycling waste PET using a monodisperse nanocrystalline catalyst, characterized in that, Includes the following steps: S1. Place the PET waste, reaction solvent, and monodisperse nanocrystalline catalyst into the depolymerization reactor; S2. After the depolymerization reaction is complete, filter out the unreacted insoluble matter while it is still hot. S3. After the depolymerization products in the filtrate are cooled and precipitated, they are finely filtered to form a filter cake. The monodisperse nanocrystals in the filtrate are recycled along with the reaction solvent during the depolymerization process. S4. After washing and drying the filter cake, it is sent to the polycondensation reactor with the polycondensation catalyst to synthesize new PET. In step S1, the monodisperse nanocrystals are selected from one or more of CeO2, CuO, ZnO, and NiO; In step S1, the average particle size of the monodisperse nanocrystals is 1-10 nm; In step S1, the reaction temperature of the depolymerization reactor is 180-197℃, and the reaction time is 0.5-3 h; In step S2, the temperature for hot filtration after the depolymerization reaction is completed is 120-180℃.

2. The method for recycling waste PET using the monodisperse nanocrystalline catalyst according to claim 1, characterized in that: The average particle size of the monodisperse nanocrystals is between 1 and 5 nm.

3. The method for recycling waste PET using the monodisperse nanocrystalline catalyst according to claim 1, characterized in that: The monodisperse nanocrystals are nanocrystals that exist in a stable monodisperse form in a liquid medium. The liquid medium is selected from the reaction solvent in step S1, and the content of nanocrystals in the dispersion system is 1-30 wt.

4. The method for recycling waste PET using the monodisperse nanocrystalline catalyst according to claim 1, characterized in that: In step S1, the amount of the monodisperse nanocrystalline catalyst used in the depolymerization process is 0.1-1 wt% of the PET waste mass.

5. The method for recycling waste PET using the monodisperse nanocrystalline catalyst according to claim 1, characterized in that: In step S1, the PET waste is decolorized to reduce the color value b to less than 2.5; In step S1, the reaction solvent is selected from one or more of methanol, ethanol, 1,2-propanediol, 1,4-butanediol, and ethylene glycol; In step S1, the mass ratio of the reaction solvent to PET waste is 3-10:1; In step S1, the stirring rate of the reactor is 50-500 rpm.

6. The method for recycling waste PET using the monodisperse nanocrystalline catalyst according to claim 1, characterized in that: In step S2, the insoluble matter includes some oligomers and unreacted PET, which are dried and then sent directly to step S1 for depolymerization until they are completely converted into alcoholysis monomers.

7. The method for recycling waste PET using the monodisperse nanocrystalline catalyst according to claim 1, characterized in that: In step S3, the temperature of the cooling precipitation is 0-50℃, the content of monodisperse nanocrystalline catalyst in the filtrate is 80-100% of the original added amount, and the amount of reaction solvent and catalyst dispersed in the filtrate is replenished to a sufficient amount before being sent to the depolymerization reactor for recycling.

8. The method for recycling waste PET using the monodisperse nanocrystalline catalyst according to claim 1, characterized in that: In step S4, the washing solution for washing the filter cake is selected from one or more of water, methanol, ethanol, ethylene glycol, cyclohexane, tetrahydrofuran, toluene, acetone, N,N'-dimethylformamide, and N,N'-dimethylacetamide. When ethylene glycol is used as the reaction solvent in step S1, the filter cake is diethyl terephthalate and its oligomers, which go directly to the polycondensation unit. When methanol, ethanol, 1,2-propanediol, and 1,4-butanediol are used as the reaction solvent in step S1, the filter cakes are dimethyl terephthalate, diethyl terephthalate, propylene glycol terephthalate, and butylene glycol terephthalate, respectively. After transesterification with ethylene glycol, diethyl terephthalate and its oligomers are obtained and sent to the polycondensation unit.

9. The method for recycling waste PET using the monodisperse nanocrystalline catalyst according to claim 1, characterized in that: In step S4, the polycondensation catalyst is selected from one or more of TiO2, Sb2O3, Sb2S3, V2O5, and GeO2; The amount of the polycondensation catalyst used in the catalytic polycondensation is 0.1-2 wt% of the mass of the depolymerization product; The polycondensation reactor includes a pre-polymerization section and a final polymerization section; the reaction temperature of the pre-polymerization section is 240-300℃, the vacuum degree is 20-100 Pa, the stirring rate is 50-1000 rpm, and the vacuuming time is 0.5-1.5 h; the reaction temperature of the final polymerization section is 250-320℃, the stirring rate is 50-1000 rpm, and the reaction time is 0.5-4 h.

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