A process for depolymerization of nylon for recycling into monomers

Abstract

This invention provides a process for depolymerizing nylon to recover monomers, belonging to the field of polymer depolymerization. The process specifically involves constructing a reaction system comprising nylon, a supported metal catalyst, and water, and carrying out a depolymerization reaction at 220℃~260℃ for 6-12 h to obtain caprolactam monomers. The supported metal catalyst includes a support and its supported active metal; the active metal is copper, the support is a metal oxide, and the mass ratio of active metal to support is 0.1:100-50:100. The process provided by this invention does not add any acids or alkalis, requires no precious metal catalysts, has a mild reaction temperature, and yields high-purity caprolactam monomers with a yield ≥90%. This fills the gap in existing technologies where catalytic methods are heavily polluting and high-temperature methods produce low-quality products, achieving green, low-carbon, and high-value recycling of waste nylon.

Description

Technical Field

[0001] This invention belongs to the field of polymer depolymerization, and specifically relates to a process for depolymerizing nylon and recovering it into monomers. Background Technology

[0002] Nylon 6 (PA6) has become a widely used polymer in the fields of fibers, engineering plastics and films due to its excellent mechanical properties, wear resistance and processability.

[0003] Current chemical recovery routes for nylon 6 can be categorized into pyrolysis, alcoholysis, ammonolysis, and hydrolysis. Among these, hydrolysis offers the highest atom economy due to its direct yield of caprolactam monomer. Pyrolysis is typically carried out at high temperatures (>400℃), resulting in complex products, monomer selectivity of less than 50%, and high separation costs. While alcoholysis and ammonolysis offer relatively milder reaction conditions, they require the introduction of chemical reagents such as methanol and ammonia, and face challenges related to solvent recovery and byproduct disposal. In contrast, hydrolysis uses water as a reaction medium, which theoretically allows for the breaking of amide bonds in the main chain of nylon 6 under milder conditions. For example, the invention patent with publication number CN114057621A discloses a method for efficient depolymerization of waste polyamide 6 and its application. The filtered waste polyamide 6 melt is fed into a depolymerization vessel containing subcritical water and mixed evenly. Depolymerization is carried out while maintaining the dynamic viscosity of the mixture at no higher than 10 Pa·s. The depolymerization time is 10-40 min. The depolymerization rate of the waste polyamide 6 melt is ≥98%. The depolymerized liquid is directly fed into a distillation column for purification to obtain caprolactam. The purified caprolactam is then fed into a polymerization vessel for ring-opening reaction. After polymerization, granulation, extraction and drying, recycled polyamide 6 chips are obtained. However, the practical bottleneck lies in the fact that the crystalline structure of Nylon 6 is dense, and water has extremely poor permeability below the melting point (~220℃), resulting in a very low reaction rate. Once the temperature rises to the supercritical or near-critical water state (>300℃), although depolymerization can be accelerated, it places extremely high demands on the pressure resistance and corrosion resistance of the equipment, significantly increasing industrial investment.

[0004] To overcome the kinetic barrier, current research generally employs catalytic enhancement strategies. Acid catalysis (such as sulfuric acid and p-toluenesulfonic acid) activates the carbonyl group by protonating the amide oxygen atom, while base catalysis (such as NaOH) promotes nucleophilic attack. Both can significantly increase the depolymerization rate in the 200℃-250℃ range, with monomer yields reaching over 85%. However, the introduction of strong acids and bases implies subsequent neutralization and desalination steps, which not only generate large amounts of inorganic salt waste liquid but also impose stringent requirements on the reactor materials.

[0005] Another mainstream approach is supported noble metal catalysis, which can cleave nylon 6 segments via hydrogenolysis under a hydrogen atmosphere, reducing the reaction temperature to 180℃~220℃. For example, Reference 1 (Upcycling of Polyamide Waste Tertiary Amines Using Mo Single Atoms and Rh Nanoparticles[J], 2025, 5,64) discloses a method for upgrading polyamide waste into tertiary amines using single molecular atoms and Rh nanoparticles. Specifically, it utilizes the catalyst Mo1Rh, which exhibits a synergistic effect between Mo single atoms and Rh nanoparticles. 1,N / TiO2, under mild conditions (e.g., 180°C), can catalyze the depolymerization of nylon with acetic acid to generate the corresponding carboxylic acid and diamide intermediates, and further catalyze the hydrogenation of the diamide to generate tertiary amines. However, this reaction still relies on acids, and the products are not nylon monomers, making it impossible to achieve a cyclical pathway of nylon depolymerization followed by repolymerization. Furthermore, the noble metal-catalyzed hydrogenation process is prone to generating large amounts of over-hydrogenation products, and the high cost of noble metals, catalyst deactivation due to carbon buildup, and safety risks associated with high-pressure hydrogen make its economic viability in large-scale waste plastic treatment scenarios questionable.

[0006] In summary, existing high-efficiency systems all rely on chemical additives or precious metals to increase reaction rates. How to achieve efficient depolymerization without relying on these factors is a key scientific problem that urgently needs to be solved in this field. Summary of the Invention

[0007] To address the problems existing in the prior art, the present invention aims to provide a process for the depolymerization and recycling of nylon into monomers. This process does not add any acid or alkali, requires no precious metal catalysts, has a mild reaction temperature, and produces caprolactam with a yield of ≥90%. It fills the gap in the prior art where catalytic methods are heavily polluting and high-temperature methods are of poor quality, and realizes the green, low-carbon, and high-value recycling of waste nylon.

[0008] The technical solution adopted by this application to solve the above problems is as follows: This invention provides a process for depolymerizing nylon to recover monomers, comprising: constructing a reaction system containing nylon, a supported metal catalyst and water, and carrying out a depolymerization reaction at a temperature of 220℃-260℃ for 6-12 h to obtain caprolactam monomers; Among them, the supported metal catalyst includes the support and the active metal supported thereon; the active metal is copper, the support is a metal oxide, and the mass ratio of active metal to support is 0.1:100-50:100.

[0009] To address the environmental burden and severe corrosion caused by existing technologies relying on acidic or basic catalysts, and the high reaction temperatures required for subcritical hydrothermal depolymerization routes leading to caprolactam thermal degradation and product quality deterioration, this invention develops a highly stable base metal catalyst. The active copper in this catalyst exhibits strong selectivity for the polar adsorption and activation of amide bonds in the nylon molecular chain, preferentially catalyzing the hydrolysis or alcoholysis of amide bonds rather than randomly breaking C-C or C-N bonds. The Lewis acidic sites on the surface of the metal oxide, acting as a support, adsorb carbonyl oxygen from the amide bonds, making the carbonyl carbon more susceptible to attack by nucleophiles (such as hydroxyl groups in water or solvents). Simultaneously, the copper sites activate the ammonia or amine intermediates generated during hydrolysis, promoting cyclization and ring closure to form caprolactam. This synergistic "acid-metal" dual-function accelerates the depolymerization rate and shifts the reaction towards monomer equilibrium, achieving high conversion rates, reduced energy consumption, and allowing the reaction to be carried out in a conventional high-pressure reactor, reducing the need for special corrosion-resistant materials. Furthermore, it avoids excessive degradation of nylon, preventing the generation of small molecule gases (such as CO, CO2, and NH3).

[0010] By further regulating the mild depolymerization reaction conditions, water can be brought to a subcritical state without raising it, and the thermal degradation of caprolactam can be avoided. This significantly reduces energy consumption and equipment requirements, and fundamentally avoids the equipment corrosion and post-processing problems caused by traditional acid / alkali processes. The resulting product is a high-purity caprolactam monomer that can be directly recycled in the polymerization process, realizing a closed-loop cycle of full value from waste nylon to new nylon.

[0011] Preferably, the supported metal catalyst is Cu / TiO2, Cu / CeO2, or Cu / Nb2O5.

[0012] Preferably, the mass ratio of nylon to supported metal catalyst is 2:1-10:1.

[0013] Preferably, the supported catalyst is prepared by impregnation using copper salt and metal oxide as raw materials, comprising: dissolving copper salt in water to obtain an impregnation solution; contacting the metal oxide with the impregnation solution, and then drying and calcining to obtain the supported catalyst; The mass of the copper salt is 0.1%-50% of the mass of the metal oxide.

[0014] More preferably, the impregnation method includes the over-impregnation method, the equal-volume impregnation method, the multiple impregnation method, or the impregnation-precipitation method.

[0015] More preferably, the metallic copper salt includes copper nitrate, copper chloride, copper acetate, copper hydroxide, or copper carbonate.

[0016] Further preferred, the calcination temperature is 200℃-800℃, and the calcination time is 0.5-12 h.

[0017] On the other hand, the present invention also provides the application of the monomer obtained by the process for depolymerizing and recovering nylon into monomer in the preparation of high molecular weight nylon.

[0018] Preferably, the caprolactam monomer obtained by depolymerization according to the present invention can be repolymerized to a number-average molecular weight greater than 1.5 × 10⁻⁶. 4 Regenerated Nylon 6.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. An acid-free catalytic system was constructed, which fundamentally avoids the equipment corrosion and post-treatment problems caused by traditional acid / alkali processes.

[0020] 2. This invention develops a highly stable base metal catalyst, which effectively reduces catalytic costs. In the process of depolymerizing nylon to recover monomers, only mild reaction conditions are required, without raising water to a subcritical state, effectively avoiding the thermal degradation of caprolactam. The resulting product is a high-purity caprolactam monomer that can be directly reused in the polymerization process, realizing a closed-loop cycle of full value from waste nylon to new nylon.

[0021] 3. The process provided by this invention has a low environmental burden, reduces energy consumption, and the reaction can be carried out in a conventional high-pressure reactor, which significantly reduces energy consumption and equipment requirements, and can be widely used in large-scale waste plastic treatment scenarios. Attached Figure Description

[0022] Figure 1 Transmission electron microscopy (TEM) image and energy dispersive spectroscopy (EDS) image of the supported catalyst Cu / TiO2 prepared in Example 1.

[0023] Figure 2 The yield of the supported catalyst Cu / TiO2 prepared in Example 1 for depolymerizing nylon and recovering caprolactam monomer at different temperatures.

[0024] Figure 3 Transmission electron microscopy (TEM) image and energy dispersive spectroscopy (EDS) image of the supported catalyst Cu / CeO2 prepared in Example 2.

[0025] Figure 4 The yield of the supported catalyst Cu / CeO2 prepared in Example 2 for depolymerizing nylon and recovering caprolactam monomer at different temperatures.

[0026] Figure 5 Transmission electron microscopy (TEM) image and energy dispersive spectroscopy (EDS) image of the supported catalyst Cu / Nb2O5 prepared in Example 3.

[0027] Figure 6 The yield of the supported catalyst Cu / Nb2O5 prepared in Example 3 for depolymerizing nylon and recovering caprolactam monomer at different temperatures.

[0028] Figure 7The molecular weight result is for the repolymerization into high molecular weight nylon. Detailed Implementation

[0029] The specific embodiments of this application will be further described in detail below with reference to the accompanying drawings and examples. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available products. The specific embodiments of this application will be further described in detail below with reference to the accompanying drawings and examples.

[0030] Example 1 Take 2 g of TiO2 and use the excess impregnation method to support a copper acetate aqueous solution containing 5% of TiO2 by mass. Dry overnight, then heat at a uniform rate of 5 °C / min and hold at 300 °C for 6 h to obtain the supported metal catalyst Cu / TiO2.

[0031] The morphology of the supported metal catalyst Cu / TiO2 analyzed by transmission electron microscopy is shown in the appendix. Figure 1 Meanwhile, the elemental composition of the catalyst was analyzed using energy dispersive spectroscopy, confirming that Cu was successfully loaded onto the TiO2 support.

[0032] Weigh 1 g of the supported metal catalyst Cu / TiO2, 5 g of nylon 6 powder, and 100 mL of water, and add them to a reaction vessel. After reacting at 220℃-260℃ for 6 h, filter the resulting mixture and analyze it by high-performance liquid chromatography (HPLC) to obtain caprolactam and its corresponding yield, as shown below. Figure 2 As shown, caprolactam content exceeded 90%.

[0033] Example 2 Take 2 g of CeO2 and use the equal-volume impregnation method to support a gold copper nitrate solution containing 20% ​​of the CeO2 mass. Dry overnight, then heat at a uniform rate of 5 °C / min and hold at 500 °C for 12 h to obtain the supported metal catalyst Cu / CeO2.

[0034] The morphology of the supported metal catalyst Cu / CeO2 analyzed by transmission electron microscopy is shown in the appendix. Figure 3 Meanwhile, the elemental composition of the catalyst was analyzed using energy dispersive spectroscopy, confirming that Cu was successfully loaded onto the CeO2 support.

[0035] Weigh 0.5 g of the supported metal catalyst Cu / CeO2, 5 g of nylon 6 powder, and 100 mL of water, and add them to a reaction vessel. After reacting at 220℃-260℃ for 12 h, filter the resulting mixture and analyze it by high-performance liquid chromatography to obtain caprolactam and its corresponding yield, as shown below. Figure 4 As shown, caprolactam content exceeded 90%.

[0036] Example 3 Take 2 g of Nb₂O₅ and use a multiple impregnation method to support an aqueous solution of copper chloride containing 50% of the mass of Nb₂O₅. Dry overnight, then heat at a uniform rate of 5 °C / min and hold at 800 °C for 6 h to obtain the supported metal catalyst Cu / Nb₂O₅.

[0037] The morphology of the supported metal catalyst Cu / Nb₂O₅ analyzed by transmission electron microscopy is shown in the appendix. Figure 5 Meanwhile, the elemental composition of the catalyst was analyzed using energy dispersive spectroscopy, confirming that Cu was successfully loaded onto the Nb2O5 support.

[0038] Weigh 2 g of the supported metal catalyst Cu / Nb₂O₅, 5 g of nylon 6 powder, and 100 mL of water, and add them to a reaction vessel. After reacting at 220℃-260℃ for 8 h, filter the resulting mixture and analyze it by high-performance liquid chromatography (HPLC) to obtain caprolactam and its corresponding yield, as shown below. Figure 6 As shown, caprolactam content exceeded 90%.

[0039] Application Example 1 Take 40 g of caprolactam obtained from the depolymerization in Example 1, and add caprolactam and deionized water at a mass ratio of 100:5 to a polymerization reactor. After purging with nitrogen, seal the reactor. Heat to 250℃-260℃, control the pressure inside the reactor at 1.0-1.5 MPa, and maintain the pressure for 2 hours. Then slowly release the pressure to atmospheric pressure, and continue heating at atmospheric pressure to 260℃-270℃, maintaining the temperature for condensation polymerization for 3 hours. After the reaction is complete, cool and discharge the material; high molecular weight nylon 6 has been successfully polymerized. The number average molecular weight, determined by gel permeation chromatography, is approximately 1.5 × 10⁻⁶. 4 (See attached) Figure 7 The above results demonstrate that the depolymerization process provided by this invention can effectively recover and repolymerize nylon 6, and the regenerated product has good polymerization activity.

[0040] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A process for depolymerizing and recovering nylon into monomers, characterized in that, A reaction system containing nylon, a supported metal catalyst, and water was constructed, and a depolymerization reaction was carried out at 220℃-260℃ for 6-12 h to obtain caprolactam monomer; Among them, the supported metal catalyst includes the support and the active metal supported thereon; the active metal is copper, the support is a metal oxide, and the mass ratio of active metal to support is 0.1:100-50:

100.

2. The process for depolymerizing and recycling nylon into monomers according to claim 1, characterized in that, The supported metal catalysts are Cu / TiO2, Cu / CeO2, or Cu / Nb2O5.

3. The process for depolymerizing and recycling nylon into monomers according to claim 1, characterized in that, The mass ratio of nylon to supported metal catalyst is 2:1-10:

1.

4. The process for depolymerizing and recycling nylon into monomers according to claim 1, characterized in that, The supported catalyst is prepared by impregnation using copper salt and metal oxide as raw materials, comprising: dissolving copper salt in water to obtain an impregnation solution; contacting the metal oxide with the impregnation solution, and then drying and calcining to obtain the supported catalyst; The mass of the copper salt is 0.1%-50% of the mass of the metal oxide.

5. The process for depolymerizing and recovering nylon into monomers according to claim 4, characterized in that, Impregnation methods include over-impregnation, equal-volume impregnation, multiple impregnation, or impregnation-precipitation.

6. The process for depolymerizing and recovering nylon into monomers according to claim 4, characterized in that, Metallic copper salts include copper nitrate, copper chloride, copper acetate, copper hydroxide, or copper carbonate.

7. The process for depolymerizing and recovering nylon into monomers according to claim 4, characterized in that, The roasting temperature is 200℃-800℃, and the roasting time is 0.5-12 h.

8. The use of the monomer obtained from the process for depolymerizing and recovering nylon into monomer according to any one of claims 1-7 in the preparation of high molecular weight nylon.

9. The application according to claim 8, characterized in that, The prepared high molecular weight nylon has a number average molecular weight of at least 1.5 × 10⁻⁶. 4 .

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