Method for catalyzing waste nylon 6 to be depolymerized into caprolactam

Abstract

The invention discloses a method for catalyzing waste nylon 6 to be depolymerized into caprolactam, and belongs to the technical field of polyamide waste material depolymerization. The problems that an existing nylon 6 depolymerization method lacks a cheap, easily available and efficient catalytic system, and the purity of caprolactam monomers obtained by catalyzing nylon 6 depolymerization is low are solved. Sodium caprolactam or potassium caprolactam which is cheap, easy to obtain and low in cost and can be prepared from caprolactam, sodium hydroxide / potassium hydroxide, sodium alkoxide / potassium alkoxide and the like through simple process steps is adopted as a depolymerization catalyst, participation of additional solvents such as water is not needed, and the caprolactam is efficiently depolymerized through nylon 6 under the reduced pressure heating condition; the catalyst is prevented from remaining in the product, the purity of the recovered monomer is high, the monomer product can be directly recycled, and the recycling efficiency of the nylon 6 is improved.

Description

Technical Field

[0001] This invention relates to a method for catalytically depolymerizing waste nylon 6 into caprolactam, belonging to the field of polyamide waste material depolymerization technology. Background Technology

[0002] Nylon 6 (PA6) is an important polyamide engineering plastic, widely used in textiles, automotive, electronics, and packaging due to its excellent mechanical strength, abrasion resistance, chemical resistance, and good processing properties. Its global annual production capacity is enormous; however, large-scale production also generates a large amount of waste, such as discarded fishing nets, carpets, and industrial waste fibers. These wastes are difficult to degrade in the natural environment, posing a threat to the ecosystem. Therefore, developing efficient recycling technologies is crucial for the sustainable development of the nylon 6 industry.

[0003] Currently, there are three main methods for recycling plastics: physical recycling, energy recovery, and chemical recycling. When Nylon 6 is physically recycled through melt-and-remolde, the high-temperature melting causes partial degradation, typically leading to a decline in performance, and the number of times Nylon 6 can be physically recycled is limited. Energy recovery refers to recovering heat through combustion; however, the combustion of Nylon 6 releases toxic HCN, CO, CO2, and NH3, causing air pollution problems, and the resulting small molecules are difficult to convert back into high-value products. In contrast, chemical recycling to obtain monomers or high-value chemicals is more valuable.

[0004] Currently, hydrolysis is the main method for the chemical recovery of nylon 6. Under high temperature and pressure, water is used as the reaction medium to break the amide bonds in the nylon 6 polymer chain, producing caprolactam monomer. However, this method typically requires high-temperature and high-pressure water or steam, and the product often contains byproducts such as aminocaproic acid, requiring complex purification steps. Depressurized directional pyrolysis to monomer can more efficiently obtain the depolymerized product, caprolactam. Although some directional pyrolysis systems have been disclosed, such as lanthanum metal complex catalysis (Angew. Chem. Int. Ed. 2023, 135, e202212543; Chem 2024, 10, 172-189), they suffer from cumbersome catalytic system preparation, high cost, and the possibility of catalyst entering the product and affecting monomer purity. Patent CN 202410248101.4 reports catalytic depressurized directional pyrolysis under the action of a metal amide catalyst; this catalytic system also suffers from high cost and the possibility of catalyst entering the caprolactam monomer. Developing inexpensive, readily available, and efficient catalytic systems to catalyze the depolymerization of nylon 6 to obtain high-purity caprolactam monomers remains a need in this field. Summary of the Invention

[0005] In order to solve the problems of the lack of inexpensive, readily available and efficient catalytic systems in existing methods for depolymerizing nylon 6, and the low purity of caprolactam monomers obtained by catalytic depolymerization of nylon 6, this invention provides a method for catalytic depolymerization of waste nylon 6 into caprolactam.

[0006] The technical solution of this invention: One of the objectives of this invention is to provide a method for catalytically depolymerizing waste nylon 6 into caprolactam. The method involves depolymerizing nylon 6 under solvent-free, heated, and reduced pressure conditions using caprolactam salt as a catalyst to obtain the depolymerization product caprolactam.

[0007] Further specifying, the caprolactam salt is sodium caprolactam or potassium caprolactam, with the following structural formula: .

[0008] Further, the depolymerization temperature is specified as 250-320℃.

[0009] Furthermore, the depolymerization temperature is specified as 280℃.

[0010] Furthermore, the depolymerization temperature is specified as 300℃.

[0011] Further, the depolymerization pressure is limited to 0.01-100 mbar.

[0012] Furthermore, the depolymerization pressure is set at 0.1 mbar.

[0013] Further specified, the amount of caprolactam salt added is 2-20 wt% of the mass of nylon 6.

[0014] Further defined, waste nylon 6 refers to substandard materials from the production of nylon 6, processing scraps, and recycled products.

[0015] Furthermore, the number-average molecular weight of waste nylon 6 is 10. 2 -10 7 g / mol.

[0016] Beneficial effects: (1) The catalytic system used in this invention is inexpensive, readily available, and low in cost. It can be prepared from caprolactam and sodium / potassium hydroxide, sodium / potassium alkoxide, etc. through a simple process, which makes the depolymerization process economically efficient.

[0017] (2) The catalytic system used in this invention can efficiently depolymerize nylon 6 and selectively obtain caprolactam monomer, with fewer byproducts such as aminocaproic acid and oligomers. Moreover, the catalyst is derived from the monomer caprolactam, which will not introduce other impurities into the reaction system. The purity of the recovered monomer is high, and the monomer product can be directly reused, thus improving the efficiency of nylon 6 recycling.

[0018] (3) The present invention has good catalytic depolymerization effect on nylon 6 waste from different sources and has good universality. Detailed Implementation

[0019] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0020] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art may make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0021] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0022] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0023] Example 1: In this embodiment, sodium caprolactam is used to catalyze the depolymerization of nylon 6. The reaction process is as follows:

[0024] The specific depolymerization process is as follows: (1) Add 5 g of nylon 6 (molecular weight 30.0 kg / mol) particles to a 25 mL reaction flask, add 0.5 g of sodium caprolactam, connect a vacuum distillation apparatus, and stir the reaction at 0.1 mbar and 300 °C.

[0025] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle, and 4.1 g of the depolymerization product caprolactam was obtained, with a yield of 82%.

[0026] Example 2: In this embodiment, potassium caprolactam is used to catalyze the depolymerization of nylon 6. The specific depolymerization process is as follows: (1) Add 5 g of nylon 6 (molecular weight 30.0 kg / mol) particles to a 25 mL reaction flask, add 0.5 g of potassium caprolactam, connect a vacuum distillation apparatus, and stir the reaction at 0.1 mbar and 300 °C.

[0027] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle to obtain 4.0 g of the depolymerization product caprolactam, with a yield of 80%.

[0028] Example 3: The difference between this embodiment and Embodiment 1 is that the depolymerization temperature is different, and the specific depolymerization process is as follows: (1) Add 5 g of nylon 6 (molecular weight 30.0 kg / mol) particles to a 25 mL reaction flask, add 0.5 g of sodium caprolactam, connect a vacuum distillation apparatus, and stir the reaction at 0.1 mbar and 320 °C.

[0029] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle, and 3.8 g of the depolymerization product caprolactam was obtained, with a yield of 76%.

[0030] Example 4: The difference between this embodiment and Embodiment 1 is that the depolymerization temperature is different, and the specific depolymerization process is as follows: (1) Add 5 g of nylon 6 (molecular weight 30.0 kg / mol) particles to a 25 mL reaction flask, add 0.5 g of sodium caprolactam, connect a vacuum distillation apparatus, and stir the reaction at 0.1 mbar and 280 °C.

[0031] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle, and 3.9 g of the depolymerization product caprolactam was obtained, with a yield of 78%.

[0032] Example 5: The difference between this embodiment and Embodiment 1 is that the depolymerization temperature is different, and the specific depolymerization process is as follows: (1) Add 5 g of nylon 6 (molecular weight 30.0 kg / mol) particles to a 25 mL reaction flask, add 0.5 g of sodium caprolactam, connect a vacuum distillation apparatus, and stir the reaction at 0.1 mbar and 260 °C.

[0033] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle to obtain 3.5 g of the depolymerization product caprolactam, with a yield of 70%.

[0034] Comparing Example 1 with Examples 1-4, it can be seen that increasing the temperature can promote the depolymerization reaction and increase the depolymerization yield. However, the temperature should not be increased excessively, as excessively high temperatures can easily lead to localized carbonization of the polymer, affecting the yield of the reaction product.

[0035] Example 6: The difference between this embodiment and Embodiment 1 is that the depolymerization pressure is different, and the specific depolymerization process is as follows: (1) Add 5 g of nylon 6 (molecular weight 30.0 kg / mol) particles to a 25 mL reaction flask, add 0.5 g of sodium caprolactam, connect a vacuum distillation apparatus, and stir the reaction at 1 mbar and 300 °C.

[0036] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle to obtain 3.5 g of the depolymerization product caprolactam, with a yield of 70%.

[0037] Example 7: The difference between this embodiment and Embodiment 1 is that the depolymerization pressure is different, and the specific depolymerization process is as follows: (1) Add 5 g of nylon 6 (molecular weight 30.0 kg / mol) particles to a 25 mL reaction flask, add 0.5 g of sodium caprolactam, connect a vacuum distillation apparatus, and stir the reaction at 10 mbar and 300 °C.

[0038] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle to obtain 3.0 g of the depolymerization product caprolactam, with a yield of 60%.

[0039] Comparing Examples 1 and 6-7, it can be seen that the lower the pressure, the more it promotes the depolymerization reaction.

[0040] Example 8: The difference between this embodiment and Example 1 is that the amount of catalyst added is different, and the specific depolymerization process is as follows: (1) Add 5 g of nylon 6 (molecular weight 30.0 kg / mol) particles to a 25 mL reaction flask, add 0.25 g of sodium caprolactam, connect a vacuum distillation apparatus, and stir the reaction at 0.1 mbar and 300 °C.

[0041] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle, and 3.8 g of the depolymerization product caprolactam was obtained, with a yield of 76%.

[0042] Example 9: The difference between this embodiment and Example 1 is that the amount of catalyst added is different, and the specific depolymerization process is as follows: (1) Add 5 g of nylon 6 (molecular weight 30.0 kg / mol) particles to a 25 mL reaction flask, add 1 g of sodium caprolactam, connect a vacuum distillation apparatus, and stir the reaction at 0.1 mbar and 300 °C.

[0043] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle, and 4.2 g of the depolymerization product caprolactam was obtained, with a yield of 84%.

[0044] Comparing Examples 1 and 8-9, it can be seen that increasing the amount of catalyst added can promote the depolymerization reaction.

[0045] Example 10: The difference between this embodiment and Embodiment 1 is that the source of nylon 6 is different, and the specific depolymerization process is as follows: (1) Add 5 g of nylon 6 fishing net recycled green material (source: Lianyungang Yongtai Plastics Industry) granules to a 25 mL reaction flask, add 0.5 g of sodium caprolactam, connect a vacuum distillation apparatus, and stir the reaction at 0.1 mbar and 300℃.

[0046] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle, and 3.9 g of the depolymerization product caprolactam was obtained, with a yield of 78%.

[0047] Example 11: The difference between this embodiment and Embodiment 1 is that the source of nylon 6 is different, and the specific depolymerization process is as follows: (1) Add 5 g of Nylon 6 fishing net recycled yellow material (source: Lianyungang Yongtai Plastics Industry) granules to a 25 mL reaction flask, add 0.5 g of sodium caprolactam, connect a vacuum distillation apparatus, and stir the reaction at 0.1 mbar and 300℃.

[0048] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle to obtain 4.0 g of the depolymerization product caprolactam, with a yield of 80%.

[0049] Comparing Examples 1 and 10-11, it can be seen that the catalytic system also has a good depolymerization effect on the actual recycled waste.

[0050] Comparative Example 1: The difference between this comparative example and Example 1 is that NaHMDS is used instead of sodium caprolactam. The specific depolymerization process is as follows: (1) Add 5 g of Nylon 6 (molecular weight 30.0 kg / mol) particles to a 25 mL reaction flask, add 0.5 g of NaHMDS, connect a vacuum distillation apparatus, and stir the reaction at 0.1 mbar and 300 °C.

[0051] (2) After 4 hours of reaction, the depolymerization product was collected in the collection bottle, and 4.3 g of the depolymerization product caprolactam was obtained, with a yield of 86%.

[0052] In large-scale reactions, the catalytic effect of NaHMDS was comparable to that of sodium caprolactam. Analysis of the monomers collected in Example 1 and Comparative Example 1 revealed that the monomer obtained in Comparative Example 1 contained 2 wt% NaHMDS and HMDS. This indicates that the NaHMDS catalyst readily volatilizes into the product under heated and reduced pressure conditions, and that the HMDS generated after its reaction with the polymer substrate also readily enters the product. In contrast, the substance generated after the reaction of sodium caprolactam with the polymer substrate is caprolactam, which does not affect the monomer purity upon entering the product.

[0053] The above description is only a preferred embodiment of the present invention. Given that those skilled in the art can make appropriate changes and modifications to the above embodiments, the present invention is not limited to the specific embodiments described above, and some modifications and changes to the present invention should also fall within the protection scope of the claims of the present invention.

Claims

1. A method for catalytically depolymerizing waste nylon 6 into caprolactam, characterized in that, Nylon 6 was depolymerized under solvent-free, heated, and reduced pressure conditions using caprolactam salt as a catalyst to obtain the depolymerization product caprolactam.

2. The method according to claim 1, characterized in that, Caprolactam salts are sodium caprolactam or potassium caprolactam, with the following structural formulas: 。 3. The method according to claim 1, characterized in that, The depolymerization temperature is 250-320℃.

4. The method according to claim 3, characterized in that, The depolymerization temperature is 280℃.

5. The method according to claim 3, characterized in that, The depolymerization temperature is 300℃.

6. The method according to claim 1, characterized in that, The depolymerization pressure is 0.01-100 mbar.

7. The method according to claim 6, characterized in that, The depolymerization pressure is 0.1 mbar.

8. The method according to claim 1, characterized in that, The amount of caprolactam salt added is 2-20 wt% of the mass of nylon 6.

9. The method according to claim 1, characterized in that, Waste Nylon 6 is derived from substandard materials produced from Nylon 6, processing scraps, and recycled products.

10. Caprolactam obtained by the method according to any one of claims 1 to 9.

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