Catalyst for chemically recovering polyester as well as preparation method and application of catalyst

Abstract

The preparation method comprises the following steps: (1) dispersing a certain amount of ferric salt and ferrous salt in a solvent A under the protection of an inert atmosphere, dropwise adding ammonia water, and washing and drying by using magnetic separation after the reaction is finished, so as to obtain Fe3O4 magnetic nanoparticles; and (2) uniformly dispersing a certain amount of zinc salt and the Fe3O4 magnetic nanoparticles prepared in the step (1) into a solvent B, dropwise adding a solution B containing a certain amount of sodium hydroxide, and reacting at a certain temperature. And after the reaction is finished, washing by using magnetic separation, and drying, so as to obtain the Zn and Fe bimetallic synergistic catalyst Fe3O4 (at) ZnO with Lewis acid site for chemical recovery of polyester. The invention also comprises application of the catalyst for chemical recovery of polyester prepared by the method in catalysis of chemical recovery of polyester. The obtained catalyst has good polyester recovery performance and stability, waste plastics can be depolymerized to prepare regenerated plastics, and high-value resource utilization is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of thermal catalytic materials and polymer resource utilization, in particular to a preparation of a Zn, Fe bimetallic synergistic catalyst with Lewis acid sites for chemical recycling of polyesters and its application in catalytic depolymerization of high molecular polyesters for high-value utilization. BACKGROUND

[0002] Since polyesters have been mass-produced, they have been widely used in various industries due to their low cost and durability. According to the report released by the United Nations Environment Programme in 2023, the global annual production of polyesters exceeds 430 million tons. However, only 9% of plastics are currently recycled. Recycling waste polyethylene terephthalate (PET) polyesters not only produces high-value food-grade PET bottles, but also maximizes the reduction of environmental hazards and helps achieve carbon neutrality. Therefore, it is urgent to develop a better PET recycling method.

[0003] Currently, there are two main methods for recycling PET: mechanical recycling and chemical recycling. Mechanical recycling refers to collecting, sorting, crushing, cleaning, drying, and melting waste PET materials through mechanical methods, and then reshaping them into new products. Although the mechanical recycling process is relatively simple, due to the presence of various impurities in waste PET products, especially various dyes and additives, it is difficult to effectively remove these impurities through physical means, resulting in a decrease in the quality of recycled PET (r-PET).

[0004] Chemical recycling method is to utilize the reversibility of polycondensation reaction, by depolymerizing waste PET into monomers or polymer intermediates, and then polymerizing into high-quality r-PET after separation and purification. Compared with mechanical recycling, chemical recycling can usually better restore the original performance of the material. There are several methods for chemical recycling of PET, including methanolysis, hydrolysis, glycolysis, and ammonolysis. Among them, glycolysis is the most promising recycling method because it can be operated at atmospheric pressure and low temperature, and can be produced continuously; the product bis(2-hydroxyethyl) terephthalate (BHET) only needs to be separated and purified through hot water extraction, cooling crystallization, adsorption, etc. The purified monomer can be directly used for the synthesis of r-PET.

[0005] Although the glycolysis process has these outstanding advantages, the traditional PET glycolysis reaction is very slow without catalyst, and there is an urgent need to develop new catalysts that are cheap, highly active, and can be separated from the catalytic product to depolymerize PET waste plastics and produce recycled plastics for high-value resource utilization, which is of great significance to solve the problem of polyester pollution. SUMMARY

[0006] The present invention aims to overcome the shortcomings of the prior art by providing a Zn / Fe bimetallic synergistic catalyst with Lewis acidic sites for chemical recovery of polyester, as well as its preparation method and application. The resulting catalyst exhibits excellent catalytic activity in polyester chemical recovery applications and offers advantages such as simple and economical preparation, excellent stability, and recyclability.

[0007] A method for preparing a Zn and Fe bimetallic synergistic catalyst having Lewis acidic sites for chemical recovery of polyester comprises the following steps:

[0008] (1) A certain amount of iron salt and ferrous salt are dispersed in solvent A under inert atmosphere, and ammonia water is added dropwise. After the reaction is completed, magnetic separation is used for washing and drying to obtain Fe3O4 magnetic nanoparticles;

[0009] (2) A certain amount of zinc salt and the Fe3O4 magnetic nanoparticles prepared in step (1) are uniformly dispersed in solvent B, and a solution B containing a certain amount of sodium hydroxide is added dropwise, and the mixture is reacted at a certain temperature. After the reaction is completed, the mixture is washed by magnetic separation and dried to obtain the Zn and Fe bimetallic synergistic catalyst Fe3O4@ZnO having Lewis acidic sites for chemical recovery of polyester.

[0010] In the above preparation route, the specific process conditions of each step are as follows:

[0011] (1) In step (1):

[0012] The iron salt can be at least one of ferric nitrate, ferric chloride, and ferric sulfate, the ferrous salt can be at least one of ferrous chloride and ferrous sulfate, and the solvent A can be water, ethanol, or an ethanol aqueous solution in any proportion. Preferably, the iron salt is ferric chloride, the ferrous salt is ferrous chloride, and the solvent A is water. More preferably, 32g of ferric chloride and 15g of ferrous chloride are dissolved in 100mL of water, and 40mL of aqueous ammonia is added dropwise.

[0013] (2) In step (2):

[0014] The zinc salt can be at least one of ferric nitrate, ferric chloride, ferric sulfate, and zinc acetate, and the solvent B can be water, ethanol, or an ethanol-water solution in any proportion. In the sodium hydroxide-containing solution B, the mass of the sodium hydroxide is 0.5 to 2 g. The molar ratio of zinc to iron is 1 to 5, the reaction time is 2 to 6 hours, and the reaction temperature is 60 to 80°C. Preferably, the zinc salt is zinc acetate and the solvent B is ethanol. More preferably, 3.4 g of zinc acetate and 0.48 g of Fe3O4 are dissolved in 200 mL of ethanol and stirred to uniformly disperse them. 20 mL of an ethanol solution containing 1.25 g of sodium hydroxide is added dropwise and the mixture is reacted at 70°C for 4 hours.

[0015] The application also comprises a method for efficiently depolymerizing PET polyester by using the prepared Zn, Fe bimetallic synergistic catalyst with Lewis acid sites Fe3O4@ZnO, and then effectively obtaining polyester monomer BHET with high added value. Specifically comprising the following steps: adding Fe3O4@ZnO as a catalyst in an alcohol solution containing polyester, heating the reactor and continuously stirring, magnetically separating the catalyst after the reaction is completed, and filtering and recrystallizing the reaction liquid to obtain polyester monomer BHET. As a preferred, the alcohol solution is at least one of methanol, ethanol, ethylene glycol, and propylene glycol, the reaction temperature is 150-210 DEG C, and the reaction time is 10-120 min. Further preferably, the alcohol solution is ethylene glycol, the reaction temperature is 190-200 DEG C, and the reaction time is 20-40 min.

[0016] Compared with the prior art, the application has the following beneficial effects:

[0017] (1) The application prepares a composite catalyst Fe3O4@ZnO for efficiently depolymerizing polyester, which utilizes the synergistic effect of Zn and Fe two transition metals and the porous structure of ZnO nanoparticles with Lewis acid sites to quickly strip the lone electron of carbonyl oxygen in polyester and activate the free lone pair of electrons on ethylene glycol to attack it nucleophilically, and efficiently and selectively depolymerize PET polyester into BHET polyester monomer with high added value.

[0018] (2) The preparation process of the Fe3O4@ZnO catalyst is simple and has high stability. By adjusting parameters such as the zinc-iron ratio and the reaction time, Fe3O4@ZnO catalysts with different molar contents are formed.

[0019] (3) The raw materials used in the application are cheap, and no pollutants are generated in the preparation process, which is conducive to further realizing large-scale production. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 are the morphology pictures of (a) Fe3O4, (b) Fe3O4@ZnO-1, (c) Fe3O4@ZnO-2, and (d) Fe3O4@ZnO-3 under a scanning electron microscope;

[0021] Figure 2 are the magnetic hysteresis loops of Fe3O4 and Fe3O4@ZnO-2;

[0022] Figure 3 are the catalytic PET polyester chemical recycling performance comparison charts of Fe3O4, Fe3O4@ZnO-1, Fe3O4@ZnO-2, and Fe3O4@ZnO-3;

[0023] Figure 4High performance liquid chromatogram of bis(2-hydroxyethyl) terephthalate obtained by Fe3O4@ZnO-2 catalyzed PET chemical recycling;

[0024] Figure 5 Thermogravimetric curve of (a) Fe3O4@ZnO-2, (b) cycle test diagram of Fe3O4@ZnO-2, (c) PET depolymerization rate of Fe3O4@ZnO-2 after high-temperature calcination, (d) bis(2-hydroxyethyl) terephthalate yield of Fe3O4@ZnO-2 after high-temperature calcination. DETAILED DESCRIPTION

[0025] The application will be further described in detail below with reference to the accompanying drawings and specific examples.

[0026] Example 1

[0027] The preparation method of the Zn, Fe bimetallic synergistic catalyst with Lewis acid sites in this example includes the following steps:

[0028] (1) 32 g of ferric chloride and 15 g of ferrous chloride were dispersed in 100 mL of solvent water under nitrogen protection, 40 mL of ammonia water (25 wt%) was added dropwise, and after the reaction was completed, magnetic separation was used for washing and drying, and Fe3O4 magnetic nanoparticles were obtained;

[0029] (2) 3.4 g of zinc acetate and 0.48 g of Fe3O4 prepared in step (1) were dissolved in 200 mL of ethanol and stirred, 20 mL of an ethanol solution containing 1.25 g of sodium hydroxide was added dropwise, and then the reaction was carried out at 70°C for 4 hours. After the reaction was completed, magnetic separation was used for washing and drying, and a Zn, Fe bimetallic synergistic catalyst with Lewis acid sites was obtained, which was named Fe3O4@ZnO-2.

[0030] Comparative Example 1

[0031] In order to facilitate performance comparison, the operation steps of Example 1 were repeated, except that only step (1) was performed, and step (2) was not performed, and then a Fe3O4 catalyst was prepared.

[0032] Comparative Example 2

[0033] In order to facilitate performance comparison, the operation steps of Example 1 were repeated, except that the addition amount of Fe3O4 in step (2) was 1.2 g, and other test conditions were the same, and then a Fe3O4@ZnO-1 catalyst was prepared.

[0034] Comparative Example 3

[0035] For the purpose of performance comparison, the operation steps of Example 1 were repeated, except that the amount of Fe3O4 added in step (2) was 0.24 g, and other test conditions were the same, and then the Fe3O4@ZnO-3 catalyst was prepared.

[0036] Figure 1 The morphology pictures of (a) Fe3O4, (b) Fe3O4@ZnO-1, (c) Fe3O4@ZnO-2, and (d) Fe3O4@ZnO-3 under a scanning electron microscope. Figure 1 (a) is the micro-morphology diagram of Fe3O4, and it can be seen that the surface of Fe3O4 without loading ZnO is very smooth. From Figure 1 In (b, c, d), it can be observed that ZnO is well attached to the surface of Fe3O4, and with the increase of the amount of zinc acetate added, a porous structure of nanoparticle accumulation is formed. Such surface morphology is conducive to the adsorption of PET polyester and the desorption of bis(2-hydroxyethyl) terephthalate, and can provide a large number of active sites to improve the performance of polyester chemical recycling.

[0037] Figure 2 The hysteresis loop of Fe3O4@ZnO-2 obtained in Example 1 and Fe3O4 obtained in Comparative Example 1. Compared with Fe3O4, the saturation magnetization of Fe3O4@ZnO-2 is reduced due to the surface loading of weakly magnetic ZnO. Even so, the magnetic sensitivity of Fe3O4@ZnO-2 is high enough, as shown in the inset of Figure 2 As shown in the inset of FIG. 6, Fe3O4@ZnO-2 can be concentrated and separated in the solvent within a few seconds by an external magnet.

[0038] Application Example 1

[0039] 0.05 g of Fe3O4@ZnO-2 obtained in Example 1, Fe3O4 obtained in Comparative Examples 1, 2 and 3, Fe3O4@ZnO-1 and Fe3O4@ZnO-3 catalysts were weighed, respectively, and added to a reactor equipped with a temperature sensor and a stirring device together with 5 g of PET and 30 mL of ethylene glycol. Then the reactor was transferred to a metal bath, and after the system was heated to 197℃, the reaction was carried out for 30 min. After the reaction, the catalyst was separated by magnetic separation, and the unreacted PET polyester particles were separated by filtration and dried. The filtrate was refrigerated at -10℃ for 8 hours, and white bis(2-hydroxyethyl) terephthalate crystals were precipitated. Then, after filtration, washing and drying, pure bis(2-hydroxyethyl) terephthalate was obtained.

[0040] Figure 3A comparison chart of the PET polyester chemical recycling performance of the four catalysts in Application Example 1 is shown. The results show that the PET degradation rate of Fe3O4@ZnO-2 is 100%, and the monomer bis(2-hydroxyethyl) terephthalate yield is 90.76%; the PET degradation rate of Fe3O4 catalyst is 7.8%, and the monomer bis(2-hydroxyethyl) terephthalate yield is 6.45%; the PET degradation rate of Fe3O4@ZnO-1 catalyst is 70.24%, and the monomer bis(2-hydroxyethyl) terephthalate yield is 50.8%; the PET degradation rate of Fe3O4@ZnO-3 catalyst is 98.2%, and the monomer bis(2-hydroxyethyl) terephthalate yield is 86.62%. It can be seen that Fe3O4@ZnO-2 has the best PET polyester chemical recycling performance. In addition, the catalytic performance of the four catalysts shows a trend of first increasing and then decreasing with the loading amount of ZnO, and reaches the optimum at a molar ratio of Zn:Fe = 2.5:1. This shows that the porous structure of the Fe3O4@ZnO-2 with Lewis acid ZnO nanoparticles stacked significantly enhances the chemical recycling ability of PET polyester.

[0041] Figure 4 A high-performance liquid chromatogram of bis(2-hydroxyethyl) terephthalate obtained by Fe3O4@ZnO-2 catalytic PET chemical recycling. Only one sharp single peak appears, indicating that the product bis(2-hydroxyethyl) terephthalate has high purity.

[0042] Figure 5 (a) is the thermogravimetric analysis curve of Fe3O4@ZnO-2 obtained in Example 1, to investigate the stability of the prepared Fe3O4@ZnO-2. Figure 5 As can be seen from (a), the weight loss of Fe3O4@ZnO-2 catalyst at 197°C is only 0.75%, which can be ignored, indicating that the catalyst has good stability at this reaction temperature. When the temperature rises to 250°C, the catalyst mass decreases slightly, but still retains more than 96% of the mass at 800°C.

[0043] Application Example 2

[0044] The Fe3O4@ZnO-2 obtained in Example 1 was studied for its PET polyester chemical recycling performance. The operation steps of Application Example 1 were repeated, except that the reaction time was 40 min after the system was heated to 197°C. Under this condition, the PET degradation rate was 100%, and the monomer bis(2-hydroxyethyl) terephthalate yield was 92.99%.

[0045] Application Example 3

[0046] The performance of the Fe₃O₄@ZnO-2 catalyst obtained in Example 1 in chemical recovery of PET polyester was investigated. The steps of Example 1 were repeated, except that the reaction time was extended to 20 minutes after the system temperature was raised to 197°C. Under these conditions, the PET degradation rate was 100%, and the yield of bis(2-hydroxyethyl) terephthalate monomer was 85.61%.

[0047] Application Example 4

[0048] The chemical recovery performance of PET polyester catalyzed by the Fe₃O₄@ZnO-2 obtained in Example 1 was investigated. The steps of Example 1 were repeated, except that the reaction time was extended to 10 minutes after the system temperature was raised to 197°C. Under these conditions, the PET degradation rate was 64.12%, and the yield of bis(2-hydroxyethyl) terephthalate monomer was 62.2%.

[0049] Application Example 5

[0050] The chemical recovery performance of PET polyester catalyzed by the Fe₃O₄@ZnO-2 obtained in Example 1 was investigated. The steps of Example 1 were repeated, except that the reaction time was extended to 5 minutes after the system temperature was raised to 197°C. Under these conditions, the PET degradation rate was 37.42%, and the yield of bis(2-hydroxyethyl) terephthalate monomer was 29.96%.

[0051] Application Example 6

[0052] The Fe3O4@ZnO-2 catalyst from Application Example 1 was used to catalyze the chemical recovery of PET polyester for reusability testing. The steps of Application Example 1 were repeated, except that the catalyst, after magnetic separation, was washed and dried, and the steps of Application Example 1 were repeated five times. Figure 5 (b) Comparison of five consecutive chemical recycling of PET polyester using Fe3O4@ZnO-2 at 197°C. The results show that the catalyst performance remains good after five cycles of testing, and the yield of monomer bis(2-hydroxyethyl) terephthalate remains over 90%.

[0053] Application Example 7

[0054] The stability of the Fe3O4@ZnO-2 catalyst obtained in Example 1 was investigated. After calcining the Fe3O4@ZnO-2 at 300°C, 400°C, 500°C, and 600°C under a nitrogen atmosphere, its catalytic performance in chemical recovery of PET polyester was determined according to the procedures of Application Example 1. Figure 5 (c) and (d) are the PET depolymerization rate and the yield of bis(2-hydroxyethyl) terephthalate after high temperature calcination of Fe3O4@ZnO-2. Figure 5(c, d) It can be seen that the catalytic performance of the catalysts slightly decreases with the increase of calcination temperature, but still exhibits the performance close to that of the original Fe3O4@ZnO-2 after the reaction time is prolonged, indicating that the catalysts still retain most of the active sites after high-temperature calcination and have good stability.

[0055] The above detailed description of the specific embodiments has described the technical solutions and beneficial effects of the present application. It should be understood that the above description is only the most preferred embodiment of the present application and is not intended to limit the present application. Any modifications, supplements, and equivalent replacements made within the principle range of the present application should be included in the protection scope of the present application.

Claims

1. A method for preparing a catalyst for chemical recovery of polyester, characterized in that: The steps include: (1) A certain amount of iron salt and ferrous salt are dispersed in solvent A under inert atmosphere, and ammonia water is added dropwise. After the reaction is completed, magnetic separation is used for washing and drying to obtain Fe3O4 magnetic nanoparticles; (2) A certain amount of zinc salt and the Fe3O4 magnetic nanoparticles prepared in step (1) are uniformly dispersed in solvent B, and a solution B containing a certain amount of sodium hydroxide is added dropwise, and the mixture is reacted at a certain temperature. After the reaction is completed, the mixture is washed by magnetic separation and dried to obtain the Zn and Fe bimetallic synergistic catalyst Fe3O4@ZnO having Lewis acidic sites for chemical recovery of polyester.

2. The method for preparing a catalyst for chemically recovering polyester according to claim 1, characterized in that: In step (1), the iron salt may be at least one of ferric nitrate, ferric chloride, and ferric sulfate, the ferrous salt may be at least one of ferrous chloride and ferrous sulfate, and the solvent A may be water, ethanol, or an ethanol-water solution in any proportion.

3. The method for preparing a catalyst for chemical recovery of polyester according to claim 1, characterized in that: In step (2), the zinc salt can be at least one of ferric nitrate, ferric chloride, ferric sulfate, and zinc acetate, and the solvent B can be water, ethanol, or an ethanol-water solution in any proportion.

4. The method for preparing a catalyst for chemical recovery of polyester according to claim 1, characterized in that: In step (2), the molar ratio of zinc to iron is 1 to 5.

5. The method for preparing a catalyst for chemical recovery of polyester according to claim 1, characterized in that: In step (2), the mass of sodium hydroxide in the solution B containing sodium hydroxide is 0.5 to 2 g.

6. The method for preparing a catalyst for chemical recovery of polyester according to claim 1, characterized in that: In step (2), the reaction time is 2 to 6 hours, and the reaction temperature is 60 to 80°C.

7. A catalyst for chemical recovery of polyester prepared according to the method according to any one of claims 1 to 6, characterized in that: The catalyst has the ability to catalyze chemical recovery of polyester.

8. Use of the catalyst for chemical recovery of polyester according to claim 7 in chemical recovery of polyester, characterized in that: The following steps are involved: A catalyst for chemically recovering polyester is added to an alcohol solution containing polyester, the reactor is heated and continuously stirred, and after the reaction is completed, the catalyst is magnetically separated and the reaction solution is filtered and recrystallized to obtain polyester monomer.

9. Use of the catalyst for chemical recovery of polyester according to claim 8 in chemical recovery of polyester, characterized in that: The alcohol solution is at least one of methanol, ethanol, ethylene glycol, and propylene glycol. The reaction temperature is 150 to 210° C., and the reaction time is 10 to 120 minutes.

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