Polyamide depolymerization method based on diester carbonate and haloid catalyst

Abstract

The invention discloses a polyamide depolymerization method based on diester carbonate and a haloid catalyst, and belongs to the technical field of waste plastic resource utilization. The method comprises the following step: in the presence of a haloid catalyst, reacting a polyamide material with diester carbonate at room temperature to 280 DEG C for 0.5-36 hours to realize chemical depolymerization of polyamide. The haloid catalyst is selected from halogen anion metal salt or ionic liquid. According to the method, the diester carbonate is used as a green reaction medium and reagent, the polyamide can be highly selectively depolymerized into corresponding carbamate, dibasic acid ester and other high-value monomers under mild conditions, and the depolymerization yield can reach 96% or above. The method provided by the invention has the advantages of mild reaction conditions, high efficiency, environmental friendliness, recoverable catalyst, high product value and the like, and provides a new way for chemical recovery of waste polyamide.

Description

Technical Field

[0001] This invention relates to a method for depolymerizing polyamide based on diester and halide salt catalysts, belonging to the field of waste plastic recycling technology. Background Technology

[0002] Polyamides, with their excellent properties, have been widely used in various fields of production and daily life. With the rapid development of human society, the demand for polyamides has increased dramatically, leading to a corresponding increase in waste. Polyamide waste has a serious impact on the ecological environment, thus necessitating a solution to this problem. Using chemical methods to convert it into useful substances is an important and effective way to address this issue. Currently, methods such as hydrolysis, alcoholysis, aminolysis, and acid hydrolysis can depolymerize polyamides into their monomers and chemicals, providing important means for their recycling. However, the depolymerization process of polyamides often involves problems such as high reaction temperatures and cross-contamination, resulting in high costs and technical difficulties in the chemical recycling of polyamides. Therefore, new methods for depolymerizing polyamides are urgently needed. Summary of the Invention

[0003] The purpose of this invention is to provide a method for depolymerizing polyamide materials. Using diester as the reaction medium and introducing halogen salt catalysts (including metal salts and ionic liquids), the method significantly reduces the temperature and pressure required for the reaction, achieving efficient depolymerization under near-room temperature to low-temperature (<220℃) conditions. This reduces energy consumption and equipment requirements, and selectively depolymerizes waste polyamide materials (such as nylon 6, nylon 66, etc.) into their corresponding monomers or high-value chemicals (such as ω-urethane, diesters, etc.). This provides a solution for the monomer-level closed-loop recycling of polyamides, realizing the recycling of resources.

[0004] The depolymerization method for polyamide materials provided by this invention includes the following steps: Polyamide materials are mixed with diesters and reacted under effective depolymerization conditions in the presence of halogen salt catalysts to depolymerize the polyamides into monomer products containing urethanes and / or diesters.

[0005] Preferably, the reaction conditions in the presence of the halide salt catalyst are as follows: The reaction was carried out at room temperature to 220°C for 0.5-24 hours, with the molar ratio of the halide salt catalyst to the polyamide structural unit being 1%-20%. Among them, the polyamide structural unit refers to the repeating unit in the polyamide chain, such as the structural unit of nylon 6 being -[NH-(CH2)5-CO]-, and the structural unit of nylon 66 being -[NH-(CH2)6-NH-CO-(CH2)4-CO]-. Preferably, the halide salt catalyst is selected from one of halide anionic metal salts, halide anionic ionic liquids, or composite systems thereof.

[0006] Preferably, the halogen in the halide anion metal salt is selected from Cl, Br, and I, and the metal is selected from Li, Na, K, Cs, Mg, Ca, Zn, Fe, Cu, Sn, and Zr, such as: LiCl, NaCl, KCl, RbCl, CsCl, MgCl2, CaCl2, SrCl2, BaCl2, ZnCl2, FeCl3, CuCl2, SnCl4, ZrCl4, CrCl3, MnCl2, CoCl2, NaBr, KBr, LiBr, CsBr, ZnBr2, FeBr3, CuBr2, SnBr4, ZrBr4, CrBr3, MgBr2, CaBr2, BaBr2, MnBr2, CoBr2, NaI, KI, ZnI2, FeI3, CuI2, ZrI4, CrI3, MgI2, CaI2, BaI2, MnI2, CoI2, etc.; Preferably, the halogen of the halogen anionic liquid is selected from Cl, Br and I, and the cation is selected from imidazole cation, pyrrole cation, tetraalkylammonium cation, tetraalkylphosphine cation, guanidine cation, and organic base cation.

[0007] In this invention, based on the Lewis acid activation mechanism, halogen salts catalyze the depolymerization of polyamides. The metal cation in the halogen salt acts as a Lewis acid, interacting with the amide bonds on the polyamide chain. This significantly activates the carbonyl group in the amide bond, making it more susceptible to nucleophilic attack from the alkoxy group in the diester, thereby significantly lowering the reaction energy barrier and enabling the depolymerization reaction to proceed efficiently under mild conditions. The catalytic principle is explained in detail below: Metal cations in halide salts (such as ZnCl2, FeCl3) (such as Zn 2+ Fe 3+ It has empty electron orbitals and can accept lone pairs of electrons, therefore it is a strong Lewis acid. The amide bond (-NH-CO-) on the polyamide molecular chain is its most critical chemical bond, in which the carbonyl (C=O) oxygen atom carries a partially negative charge and possesses a lone pair of electrons. A Lewis acid metal cation (M... +The amide interacts with the carbonyl oxygen atom to form a transient complex. This coordination strongly attracts the electron cloud of the carbonyl oxygen atom, leading to a significant decrease in the electron cloud density of the carbonyl carbon atom (C) and a significant increase in its positive charge. The alkoxy group (RO-) in the diester is a nucleophile with a lone pair of electrons on its oxygen atom. The activated, highly positively charged carbonyl carbon atom is now highly susceptible to nucleophilic attack by the alkoxy group. The tetrahedral intermediate formed after the nucleophilic attack is very unstable, and bond breaking occurs. The CN bond in the amide bond breaks, ultimately forming urethane and carboxylic acid esters. Finally, the polyamide chain is completely cleaved, generating the target product (such as methyl 6-methoxycarbonylaminohexanoate).

[0008] In the method of this invention, although the core of catalysis is the metal cation, the anion also plays an important role: Coordination ability: The coordination ability of the anion affects the Lewis acidity of the cation. Typically, Cl... - >Br - >I - Ion pair effect: Smaller anions (such as Cl-) - It binds more tightly to cations, making the cations more "exposed", resulting in stronger acidity and better catalytic activity.

[0009] In the method of this invention, the catalytic principle of haloionic liquids (such as [BMIm]Cl) is similar to that of halosal salts: Anion interaction: its Cl - Anions themselves have a certain nucleophilicity and coordination ability, which can help activate amide bonds.

[0010] Cationic effect: Organic cations (such as imidazolium) can bind to polyamide chains through π-π interactions or hydrogen bonds, helping to shorten the distance between reactants and improve reaction efficiency.

[0011] Synergistic effect: Sometimes the cations and anions of ionic liquids can act as a whole and synergistically catalyze reactions through the "ion pair" mechanism.

[0012] Preferably, the chemical formula of the diester is R1O(C=O)OR2, wherein R1 and R2 are independently selected from C1-C12 alkyl, C1-C12 cycloalkyl or C6-C12 aryl, more preferably from C1-C6 alkyl, C1-C6 cycloalkyl or C6-C10 aryl, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, dibutyl carbonate, diphenyl carbonate, etc.

[0013] The molar ratio of the diester to the polyamide structural unit is 1:1 to 25:1.

[0014] Preferably, the polyamide material is selected from at least one of nylon 6 (PA6, polycaprolactam), nylon 66 (PA66, polyhexamethylene adipamide), nylon 11 (PA11, polyω-aminoundecanoyl), nylon 12 (PA12, polydodecanoyl lactam), nylon 610 (PA610, polydecanoyl hexamethylene diamine), nylon 612 (PA612, polydodecane-1,12-dihexamethylene adipamide), nylon 46 (PA46, polybutadiene adipamide), or blends, composites, and waste materials thereof.

[0015] Preferably, the depolymerization method further includes a post-processing step: after the reaction is completed, the unreacted solid material is separated and recovered, and the excess diester, monomer product and halide catalyst are recovered by distillation.

[0016] The polyamide depolymerization products obtained by the method of this invention also fall within the scope of protection of this invention: ω-alkoxycarbonylaminoalkyl esters obtained by depolymerization of polylactams; and / or, Dicarboxylic acid diesters and diamine dicarboxylic acid esters obtained by depolymerization of diamine-diacarboxylic acid type polyamides; like Figure 1 and Figure 2 As shown.

[0017] Preferably, nylon 6 is depolymerized to obtain 6-alkoxycarbonylaminohexanoate, such as nylon 6 being depolymerized in DMC to obtain methyl 6-methoxycarbonylaminohexanoate, and nylon 6 being depolymerized in DEC to obtain ethyl 6-ethoxycarbonylaminohexanoate. Nylon 11 is depolymerized to yield 11-alkoxycarbonylaminoundecanoate, such as methyl 11-methoxycarbonylaminoundecanoate in DMC. Nylon 12 is depolymerized to obtain 12-alkoxycarbonylaminododecanoate, such as methyl 12-methoxycarbonylaminododecanoate ester when Nylon 12 is depolymerized in DMC. Nylon 66 is depolymerized to yield dimethyl adipate and dimethyl hexane-1,6-dimethyldicarbamate, such as dimethyl adipate (MDA) and dimethyl hexane-1,6-dimethyldicarbamate (HDC) in DMC. Nylon 610 is depolymerized to yield dimethyl sebacate and dimethyl hexane-1,6-dimethyldicarbamate, such as when nylon 610 is depolymerized in DMC to yield dimethyl sebacate and dimethyl hexane-1,6-dimethyldicarbamate (HDC). Nylon 612 is depolymerized in DMC to yield dimethyl dodecanoate and dimethyl hexane-1,6-dimethyldicarbamate (HDC).

[0018] The application of halogen salt catalysts in the depolymerization reaction of polyamide in diester is also within the scope of protection of this invention.

[0019] The present invention has the following beneficial technical effects: Mild conditions: Under the action of halogen salt catalysts, efficient depolymerization can be achieved at lower temperatures (room temperature - 220℃) and in shorter times (0.5-24h), significantly reducing energy consumption and equipment requirements.

[0020] Highly efficient depolymerization: It can achieve highly selective and high-yield (up to 96%) depolymerization of various polyamides (such as PA6, PA66, etc.), converting them into valuable monomers.

[0021] Green and environmentally friendly: It uses low-toxicity, biodegradable diesters as the reaction medium, and the catalyst can be recycled and reused. The whole process is environmentally friendly and in line with the principles of green chemistry.

[0022] High product value: The depolymerization products are monomers or their derivatives that can be used for repolymerization, realizing the high-value closed-loop recycling of polyamide, with great economic potential. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the depolymerization of polyamide in dimethyl carbonate.

[0024] Figure 2 This is a schematic diagram of the depolymerization of polyamide in diethyl carbonate. Detailed Implementation

[0025] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0026] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0027] This invention provides a green, efficient, and mild method to solve the problems of high reaction temperature, high energy consumption, and serious process pollution in existing polyamide (nylon) chemical recycling technologies, and to achieve the upgraded recycling of waste polyamide plastics into its high-value monomers.

[0028] The method for catalytic depolymerization of polyamide provided by this invention mainly includes: Reaction system: Dicarbonates (such as dimethyl carbonate (DMC) and diethyl carbonate (DEC)) are used as both the solvent and reactants.

[0029] Catalytic system: The reaction is carried out under the conditions of halide salt catalysts (including metal halides and halide ionic liquids).

[0030] Process conditions: The polyamide can be completely depolymerized by reacting for 0.5 to 36 hours within a temperature range of room temperature to 280°C.

[0031] Post-processing: After the reaction, excess solvent, high-purity monomer products and catalyst are separated and recovered through simple operations such as filtration and distillation to achieve closed-loop recycling.

[0032] The polyamide depolymerization reaction involved in this invention is a catalytic alcoholysis reaction that uses diester carbonate as a reaction medium and reagent to break the amide bonds (-NH-CO-) in the polyamide molecular chain under specific conditions, converting it into more valuable urethane and carboxylic acid ester monomers.

[0033] The depolymerization reaction involved in this invention is essentially a nucleophilic substitution reaction. Its essence is that the alkoxy group (RO-) in the carbonate diester acts as a nucleophile, attacking the carbonyl carbon atom in the polyamide chain, thereby breaking the amide bond (CN bond).

[0034] For polyamides prepared from lactams (such as nylon 6, nylon 11, and nylon 12): their depolymerization reaction produces a single monomeric derivative.

[0035] For polyamides (such as nylon 66 and nylon 610) prepared from diamines and diacids, their molecular chains contain both "amine ends" and "acid ends". The depolymerization reaction will generate two different monomer derivatives: dicarboxylic acid esters and dicarboxylic acid esters.

[0036] In the method of this invention, the halide salt catalyst plays a crucial role, and its mechanism is mainly Lewis acid activation: Lewis acid action: metal cation (such as Zn) 2+ As a strong Lewis acid, it coordinates with the carbonyl oxygen atom on the amide bond of polyamide. This coordination greatly enhances the positive charge of the carbonyl carbon atom, making it more susceptible to attack by the alkoxy group (nucleophile) in the carbonate diester. By activating the reaction site, the catalyst significantly lowers the activation energy of the reaction. This allows the reaction to proceed efficiently at lower temperatures (e.g., 180°C) and for shorter time periods.

[0037] The system after the reaction of this invention is a mixture containing: unreacted excess diester solvent; the generated monomer product (liquid); a solid catalyst (such as an insoluble metal salt); and possibly a small amount of undepolymerized solids or impurities. After cooling-filtration (separating the solid catalyst, which can be directly recycled)-filtrate-vacuum distillation (first distilling off and recovering diester)-the residue is the high-purity monomer product.

[0038] This invention transforms a traditional, energy-intensive process into a green chemical process that is mild (low temperature, normal pressure), highly efficient (high depolymerization yield), environmentally friendly (recyclable green solvents and catalysts), and economically valuable (producing high-value monomers), providing technical support for the circular economy of nylon.

[0039] Examples 1-12: Metal salt catalysis of halide anions in the depolymerization of dimethyl carbonate (PA6) 1.13 g (approximately 10 mmol of polyamide structural units) of PA6 powder, 0.5 mmol of metal carboxylate, and approximately 9 g (100 mmol) of dimethyl carbonate were added to a 100 mL pressure-resistant reaction tube. The reaction tube was then placed in a heating device at a set temperature and reacted for a certain time. After cooling in an ice-water bath, the solid catalyst was recovered by filtration and reused. The filtrate was rotary evaporated to remove unreacted dimethyl carbonate, yielding methyl 6-methoxycarbonylaminohexanoate (MAME) liquid. In this example, the molar ratio of dimethyl carbonate to PA6 structural units was 10:1, and the molar ratio of metal salt to PA6 structural units was 5%.

[0040] Table 1. Reaction conditions and results of Examples 1-12

[0041] The above examples show that ZnCl2 has the best catalytic effect, and the catalytic activity order of halide anions is Cl... - >Br - >I - .

[0042] Examples 13-21: Catalysis of dimethyl carbonate depolymerization (PA6) by ionic liquids with different halogen anions 1.13 g (approximately 10 mmol of polyamide structural units) of PA6 powder, 0.5 mmol of ionic liquid, and approximately 9 g (100 mmol) of dimethyl carbonate were added to a 100 mL pressure-resistant reaction tube. The reaction tube was then placed in a heating device at a set temperature and reacted for a certain time. The mixture was then cooled in an ice-water bath. Water was added to extract the ionic liquid, which could be recycled. The dimethyl carbonate phase was then rotary evaporated to remove unreacted dimethyl carbonate, yielding methyl 6-methoxycarbonylaminohexanoate (MAME) liquid. In this example, the molar ratio of the ionic liquid to the PA6 structural units was 5%.

[0043] Table 2 Reaction conditions and results of Examples 13-21

[0044] The above examples demonstrate that ionic liquids can serve as highly efficient catalysts, with a reaction temperature (160°C) lower than that of most metal salt systems (180°C), and their catalytic activity order is Cl. - >Br - >I - .

[0045] Examples 22-27: Catalytic depolymerization of dimethyl carbonate (PA6) in composite systems Add 1.13 g (approximately 10 mmol of polyamide structural units) of PA6 powder, 0.5 mmol of metal salt, 0.5 mmol of additive, and approximately 9 g (100 mmol) of dimethyl carbonate to a 100 mL pressure-resistant reaction tube. Then, place the reaction tube in a heating device at a set temperature and react for a certain time. Cool to room temperature, filter to recover the catalyst and additive, and rotary evaporate the liquid to remove unreacted dimethyl carbonate, thus obtaining methyl 6-methoxycarbonylaminohexanoate (MAME) liquid.

[0046] Table 3 Reaction conditions and results of Examples 22-27

[0047] The above examples demonstrate that a composite system consisting of metal salts and other additives can further improve catalytic efficiency or maintain a high yield at lower temperatures.

[0048] Examples 28-34: Catalysis of PA6 depolymerization in DMC using a composite catalytic system Add 1.13 g (approximately 10 mmol of polyamide structural units) of PA6 powder, 0.5 mmol of metal salt, 0.5 mmol of ionic liquid, and approximately 9 g (100 mmol) of dimethyl carbonate to a 100 mL pressure-resistant reaction tube. Then, place the reaction tube in a heating device at a set temperature and react for a certain time. Cool to room temperature, filter to recover the metal salt, and then add saturated sodium chloride solution to separate the ionic liquid. Separate the supernatant to recover the catalyst, and rotary evaporate the lower liquid to remove unreacted dimethyl carbonate, thus obtaining methyl 6-methoxycarbonylaminohexanoate (MAME) liquid.

[0049] Table 4. Reaction conditions and results of Examples 28-34

[0050] The above examples demonstrate that the composite system of metal salt and ionic liquid can further improve catalytic efficiency or maintain high yield at lower temperatures.

[0051] Examples 35-42: Depolymerization of PA6 by DMC in other solvent systems The experimental procedure involves adding 1.13 g (approximately 10 mmol of polyamide structural units) of PA6 powder, 0.5 mmol of metal salt, 0.5 mmol of additive, approximately 0.9 g (10 mmol) of dimethyl carbonate, and 5 mL of solvent to a 50 mL pressure-resistant reaction tube. The reaction tube is then placed in a heating device at a set temperature and allowed to react for a certain period of time. After cooling to room temperature, the metal salt is recovered by filtration. A saturated sodium chloride solution is then added to separate the ionic liquid. The supernatant is separated to recover the catalyst, and the lower liquid is rotary evaporated to remove unreacted dimethyl carbonate and solvent, thus obtaining methyl 6-methoxycarbonylaminohexanoate (MAME) liquid.

[0052] Table 5. Reaction conditions and results of Examples 35-42

[0053] Example 43: ZnCl2-catalyzed depolymerization of PA6 in diethyl carbonate (DEC) 1.13 g (approximately 10 mmol of substance containing polyamide structural units) of 100% PA6 textile (nylon), 0.5 mmol of zinc chloride, and approximately 12 g (100 mmol) of diethyl carbonate were added to a 50 mL pressure-resistant reaction tube. The reaction tube was then placed in a heating device at 180 °C and reacted for 16 hours. After cooling in an ice-water bath, the metal salt was recovered by filtration, and the liquid was rotary evaporated to remove unreacted diethyl carbonate, yielding ethyl 6-ethoxycarbonylaminohexanoate (78%).

[0054] This embodiment demonstrates that the method of the present invention is applicable to actual waste (textiles) and other dicarbonates (such as diethyl carbonate, DEC).

[0055] Examples 44-48: Depolymerization of other polyamides in DMC catalyzed by ZnCl2 Approximately 1 mmol of polyamide containing polyamide structural units, 0.05 mmol of zinc chloride, and 900 mg (approximately 10 mmol) of DMC were added to a 15 mL pressure-resistant reaction tube. The reaction tube was then placed in a heating device at 180 °C and reacted for 2 hours. After cooling in an ice-water bath, 60 mg (0.5 mmol) of mesitylene was added as an internal standard. The yield was then determined by NMR analysis using the internal standard method.

[0056] Example 44 depolymerized PA11 to obtain methyl 11-methoxycarbonylaminoundecanoate, with a yield of 78%.

[0057] Example 45 depolymerized PA12 to obtain methyl 12-methoxycarbonylaminododecanoate, with a yield of 79%.

[0058] Example 46 depolymerized PA66 to obtain dimethyl adipate (MDA) in 80% yield and dimethyl hexane-1,6-dimethyldicarbamate (HDC) in 72% yield.

[0059] Example 47 depolymerized PA610 to obtain dimethyl sebacate in 77% yield and dimethyl hexane-1,6-dimethyldicarbamate (HDC) in 72% yield.

[0060] Example 48 depolymerized PA612 to obtain dimethyl dodecanoate in 80% yield and dimethyl hexane-1,6-dimethyldicarbamate (HDC) in 75% yield.

[0061] The above embodiments demonstrate that the method of the present invention is applicable to a variety of polyamides and can efficiently recover their monomers or derivatives.

[0062] The foregoing has described in detail the implementation of the present invention. However, the present invention is not limited to the above-described embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for depolymerizing a polyamide material, comprising the following steps: Polyamide materials are mixed with diesters and reacted under effective depolymerization conditions in the presence of halogen salt catalysts to depolymerize the polyamide into monomer products containing urethane and / or diesters.

2. The depolymerization method according to claim 1, characterized in that: The reaction conditions in the presence of the halide salt catalyst are as follows: The reaction is carried out at room temperature to 220°C for 0.5-24 hours, and the molar ratio of the halogen salt catalyst to the polyamide structural unit is 1% to 20%.

3. The depolymerization method according to claim 1 or 2, characterized in that: The halogen salt catalyst is selected from one of the following: halogen anionic metal salts, halogen anionic ionic liquids, or their composite systems.

4. The depolymerization method according to claim 3, characterized in that: The halogens in the halide anion metal salts are selected from Cl, Br and I, and the metals are selected from Li, Na, K, Cs, Mg, Ca, Zn, Fe, Cu, Sn and Zr.

5. The depolymerization method according to claim 3, characterized in that: The halogen of the halogen anionic liquid is selected from Cl, Br and I, and the cation is selected from imidazole cation, pyrrole cation, tetraalkylammonium cation, tetraalkylphosphine cation, guanidine cation, and organic base cation.

6. The depolymerization method according to any one of claims 1-5, characterized in that: The chemical formula of the carbonate diester is R1O(C=O)OR2, wherein R1 and R2 are independently selected from C1-C12 alkyl, C1-C12 cycloalkyl or C6-C12 aryl. The molar ratio of the diester to the polyamide structural unit is 1:1 to 25:

1.

7. The depolymerization method according to any one of claims 1-6, characterized in that: The polyamide material is selected from at least one of nylon 6, nylon 66, nylon 11, nylon 12, nylon 610, nylon 612, nylon 46, or blends, composites, and waste materials thereof.

8. The depolymerization method according to any one of claims 1-7, characterized in that: The depolymerization method also includes a post-processing step: after the reaction is completed, unreacted solid materials are separated and recovered, and excess diester, monomer products and halogen salt catalyst are recovered by distillation.

9. A polyamide depolymerization product obtained by the method of any one of claims 1-8, the product comprising: ω-alkoxycarbonylaminoalkyl esters obtained by depolymerization of polylactams; and / or, Dicarboxylic acid diester and diamine dicarboxylic acid ester obtained by depolymerization of diamine-diacid type polyamide.

10. Application of halide salt catalysts in the depolymerization of polyamides in diesters; The halogen salt catalyst is selected from one of the following: halogen anionic metal salts, halogen anionic ionic liquids, or their composite systems. The halogens in the halide anion metal salts are selected from Cl, Br and I, and the metals are selected from Li, Na, K, Cs, Mg, Ca, Zn, Fe, Cu, Sn and Zr; The halogen of the halogen anionic liquid is selected from Cl, Br and I, and the cation is selected from imidazole cation, pyrrole cation, tetraalkylammonium cation, tetraalkylphosphine cation, guanidine cation, and organic base cation.

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