A method for preparing aliphatic carboxylic acid ligand MOF by two-step hydrothermal degradation of waste nylon 66

Abstract

The present invention belongs to the technical field of waste polymer degradation and metal-organic framework (MOF) material preparation, and discloses a method for preparing aliphatic carboxylic acid ligand MOF by two-step hydrothermal degradation of waste nylon 66, comprising the following steps: S1. dispersing waste nylon 66 and a strong base compound in water to obtain a dispersion; S2. subjecting the dispersion to a hydrothermal reaction at 180-220°C for 12-48h to hydrolyze nylon 66 to obtain a nylon 66 degradation solution; S3. adding a metal salt solution thereto, and continuing the hydrothermal reaction at 60-160°C for 3-24h to coordinate the adipic acid salt and hexamethylenediamine in the degradation solution with metal ions to synthesize MOF, and the obtained product is an aliphatic carboxylic acid ligand MOF material. The present invention realizes the preparation of high-value-added MOF products from waste nylon 66 raw materials through a two-step hydrothermal method, the method is simple, realizes waste resource utilization, improves resource utilization, and reduces MOF production costs.

Description

Technical Field

[0001] This invention belongs to the technical field of waste polymer degradation and metal-organic framework (MOF) material preparation. More specifically, it relates to a method for preparing aliphatic carboxylic acid ligand MOFs through two-step hydrothermal degradation of waste nylon 66. This method provides a new, low-cost strategy for the synthesis of aliphatic carboxylic acid ligand MOF materials. Background Art

[0002] Nylon is a thermoplastic resin containing repeating amide groups -NHCO- in its molecular backbone. It exhibits excellent mechanical properties, heat resistance, wear resistance, chemical stability, and self-lubrication. It can replace metal to meet the requirements of lightweighting and cost reduction for downstream industrial products. With the increasing demand for engineering plastics in areas such as lightweight automobiles in recent years, the use of nylon products has evolved from chemical fibers to multi-purpose applications such as engineering plastics, films, and plastic alloys. It has become a typical alternative material for replacing wood, steel, and porcelain with plastic, and its market size has grown steadily year by year. In 2019, the global nylon market size was approximately US$28.15 billion, and it is expected to reach US$47 billion in 2027. However, the widespread use of nylon has also brought about a series of environmental problems. Waste is often discarded in landfills or natural environments such as oceans. Because it is difficult to biodegrade, the degradation process may take decades or even centuries. Currently, there are very few means for recycling waste nylon. Incineration of waste nylon will produce a large amount of toxic gases such as NO, NO2, CO, etc. Physical methods have high requirements for raw material purity, and the source of raw materials is limited to unused processing scraps. The quality of recycled products is poor and the added value is low. Chemical recycling degrades waste nylon into monomers for reuse in nylon production. It is a sustainable recycling model. Common recycling technologies include pyrolysis, hydrolysis, alcoholysis, and aminolysis. However, due to the large number of hydrogen bonds between nylon 66 polymer chains, the existing degradation process conditions are harsh, and the product recovery rate is difficult to improve, which seriously limits its recycling and conversion process. Eimontas et al. used copper-impregnated biochar-based catalysts to pyrolyze waste nylon to recover caprolactam and capronitrile (Justas Eimontas, Nerijus Striūgas, Zakarauskas,Ieva Inna Pitak. Metallised seaweed-derived bio-char catalyst preparation and its application in the pyrolysis process for the waste fishing nets and marine biomass utilisation. Fuel 2024, 357, 129922); however, the product is a mixture with a caprolactam content of only 54.3% and a capronitrile content of 31.7%, resulting in high energy consumption and difficulty in separating the mixed products. Zhao et al. degraded nylon 6,6 and converted it into a polysaccharide flocculant using Brucella (Haijuan Zhao, Su Sun, Yongming Cui, Muhammad Wajid Ullah, Khulood Fahad Alabbosh, Noureddine Elboughdiri, Jiangang Zhou. Sustainable production of bacterial flocculants by nylon-6,6 microplastics hydrolysate utilizing Brucella intermedia ZL-06. Journal of Hazardous Materials 2024, 465, 133435). However, the strain screening and cultivation cycle is long, and the degradation of nylon 6,6 requires the cumbersome and complex steps of neutralization with an organic solution, vacuum filtration, and recrystallization purification to extract adipic acid, which is not conducive to large-scale production. Currently, there is still a lack of simple methods that can recycle nylon waste on an industrial scale and convert it into high-value products.

[0003] MOFs are a class of crystalline compounds with periodic repeating network structures formed by the self-assembly of metal ions (or metal clusters) and organic ligands through coordination bonds. Their most notable characteristic is the abundance of pores and cavities within their crystal structure, resulting in a porosity exceeding 90%. This porous structure contributes to the large surface area typically found in MOFs. Furthermore, MOF properties such as porosity, pore size, and crystallinity can be customized through synthetic formulation adjustments and process control, yielding MOFs with targeted functionality. These properties have enabled MOFs to excel in applications such as gas storage, separation, and adsorption, as electrode materials and battery separators, catalysis, sensing, and drug delivery. Currently, the vast majority of MOFs utilize aromatic carboxylates, such as terephthalic acid and 1,3,5-benzenetricarboxylic acid, as ligands. MOFs with aliphatic carboxylic acids occupy a unique position, as their greater flexibility imparts a unique breathing effect. Moreover, their unique properties of high hydrophobicity, low optical absorption, gas adsorption selectivity, and thermal stability give them unique potential in the fields of photochemistry, photocatalysis, adsorption and diffusion, and environmental monitoring. However, compared with aromatic carboxylic acid ligand MOFs, there are very few types of aliphatic carboxylic acid ligand MOFs (for example, MOFs with adipic acid as ligands include Mn, Co, Tb, Zn, Al, UO, Nd, Ca, and Zr; there are also some MOFs with aliphatic ligands such as malonic acid, succinic acid, and glutaric acid, but the types are very limited). In addition, the synthesis process requires reactions in an organic solvent system, and the synthesis, purification, and separation processes all require the use of large amounts of organic solvents, which cause serious pollution to the environment. Currently, there is still a lack of industrial preparation methods.

[0004] In the past, MOF synthesis typically relied on high-purity, low-molecule monomers, which were expensive and unsuitable for large-scale production. In recent years, the upcycling of waste plastics to produce MOFs through chemical methods has attracted widespread attention. To date, reports have reported the preparation of MOFs using waste polymers such as polyethylene terephthalate and polylactic acid as precursors (ZL202211401078.5; ZL 202310475738.2; ZL 202310475998.X; ZL 202211401069.6). This represents a novel chemical recycling method that not only promotes waste recycling and addresses waste polymer pollution issues, but also produces high-value MOF products at low raw material costs, maximizing resource utilization and aligning with the principles of green, renewable, and sustainable production and a circular economy. However, the preparation of MOFs with aliphatic carboxylic acid ligands using waste nylon 6,6 as precursors has yet to be reported. Summary of the Invention

[0005] In response to the above-mentioned deficiencies or improvement needs of the prior art, the purpose of the present invention is to provide a method for preparing aliphatic carboxylic acid ligand MOFs by two-step hydrothermal degradation of waste nylon 66, wherein the overall process design of the method is improved, and a high-value-added MOF product is prepared from waste nylon 66 raw materials through a two-step hydrothermal method. The present invention utilizes a first-step hydrothermal reaction to degrade nylon 66 into adipate and hexamethylenediamine, and utilizes a second-step hydrothermal reaction to directly coordinate the degradation liquid with metal ions to prepare aliphatic carboxylic acid ligand MOF materials. The present invention upgrades the chemical recycling of waste nylon 66 and directly uses it as a precursor to synthesize aliphatic carboxylic acid ligand MOFs of various morphologies. The method is simple, realizes waste resource utilization, improves resource utilization, reduces MOF production costs, and simplifies the production process, making it suitable for industrial-scale production of MOFs.

[0006] To achieve the above object, the present invention provides a method for preparing aliphatic carboxylic acid ligand MOF by two-step hydrothermal degradation of waste nylon 66, characterized in that it comprises the following steps:

[0007] S1. Dispersing waste nylon 66 and a strong base compound in water to obtain a dispersion; wherein the mass ratio of the strong base compound to the waste nylon 66 is 0.17 to 0.4:1;

[0008] S2. The dispersion obtained in step S1 is subjected to a hydrothermal reaction at 180 to 220°C for 12 to 48 hours to hydrolyze nylon 66 to obtain a nylon 66 degradation solution;

[0009] S3. Add a metal salt solution to the nylon 66 degradation solution obtained in step S2, and continue the hydrothermal reaction at 60-160° C. for 3-24 hours to coordinate the adipate and hexamethylenediamine in the degradation solution with the metal ions to synthesize MOF. The obtained product is an aliphatic carboxylic acid ligand MOF material; wherein the metal element in the MOF material is the same as the metal element in the metal salt solution.

[0010] As a further preferred embodiment of the present invention, in step S1, the strong base compound is at least one of sodium hydroxide and potassium hydroxide.

[0011] As a further preferred embodiment of the present invention, in step S3, the metal salt in the metal salt solution is at least one of cobalt nitrate, bismuth nitrate, terbium nitrate, europium chloride, lanthanum nitrate, and cadmium nitrate.

[0012] As a further preferred embodiment of the present invention, the ratio of the metal salt contained in the metal salt solution in step S3 to the waste nylon 66 used in step S1 satisfies a mass ratio of 0.6 to 2.5:1.

[0013] The above technical solution conceived by the present invention is different from the prior art. In the method of the present invention, the first hydrothermal reaction is to hydrolyze and degrade nylon 66 to obtain a nylon 66 degradation liquid, which is a degradation reaction; the second hydrothermal reaction is to directly coordinate the nylon 66 degradation liquid with metal ions to prepare an aliphatic carboxylic acid ligand MOF material, which is a coordination reaction. In the first hydrothermal reaction, the nucleophilic attack of the hydroxide ion of the strong base compound is used to break the amide bond to achieve nylon 66 degradation. The resulting nylon 66 degradation liquid is weakly alkaline, which not only helps to ensure the purity and stability of the degradation product and reduce the occurrence of side reactions, but also effectively reduces equipment losses and further ensures the safety of the reaction. The nylon 66 degradation liquid obtained by the first hydrothermal reaction of the present invention is directly used as a precursor for MOF synthesis without further purification or acid addition to adjust the pH, simplifying the tedious steps of polyamide recycling and maximizing raw material utilization. The weakly alkaline degradation solution provides a mild reaction environment, promoting the coordination reaction between adipate and hexamethylenediamine with metal ions and minimizing side reactions. Adipate's high solubility allows for uniform distribution in aqueous solution, improving reaction efficiency and product uniformity. Furthermore, the amino groups of hexamethylenediamine remain unprotonated. Its long-chain structure and flexibility allow the molecule to adapt to varying coordination requirements in three-dimensional space. Together with adipate, it synthesizes MOF materials with varying pore sizes, morphologies, and functionalities. Metal ions and organic ligands self-assemble through coordination bonds, gradually forming highly ordered MOF crystals.

[0014] Specifically, the present invention can achieve the following beneficial effects:

[0015] (1) Compared to traditional MOF synthesis methods, which typically require de novo synthesis or the purchase of purified small molecule organic ligands, the present invention uses waste nylon as raw material, and the adipate in its degradation solution can serve as the organic ligand for MOF synthesis. Utilizing waste plastics as raw materials for the production of high-value products can reduce MOF production costs, promote waste plastic recycling, address waste plastic pollution issues, and achieve waste resource utilization.

[0016] (2) Existing chemical recovery methods, such as thermally decomposing nylon with a catalyst to produce a mixture of caprolactam and capronitrile, or converting it into a flocculant, require complex separation and purification steps for the nylon degradation products before product recovery can be achieved. The present invention eliminates the need to separate and purify the hexamethylenediamine and adipic acid products from the degradation solution after nylon degradation, thus reducing the number of steps and related equipment required in chemical production, simplifying the production process, thereby reducing production costs and improving production efficiency.

[0017] (3) The method of the present invention prepares aliphatic carboxylic acid ligand MOF by two-step hydrothermal degradation of waste nylon 66, wherein the first hydrothermal reaction is to degrade nylon 66, and the hydrothermal reaction needs to be carried out at 180-220°C for 12-48 hours to ensure sufficient degradation of nylon 66; the second hydrothermal reaction is to directly coordinate the nylon 66 degradation liquid with metal ions to prepare aliphatic carboxylic acid ligand MOF material, and the hydrothermal reaction needs to be continued at 60-160°C for 3-24 hours to avoid excessive temperature affecting the synthesis of the target MOF material.

[0018] In summary, the present invention realizes the high-value-added conversion of waste plastics through a two-step hydrothermal degradation method for preparing aliphatic carboxylic acid ligand MOF from waste nylon 66. The proposed waste nylon recycling method can solve the pollution problem of waste polymers. High-value MOF products can be produced through a simple process flow at low raw material costs, achieving the goal of maximizing resource utilization, realizing cost reduction and efficiency improvement in large-scale production, and also conforming to the green, renewable and sustainable green production and circular economy concepts. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 These are the X-ray powder diffraction spectrum and scanning electron microscopy image of the Co-MOF material prepared in Example 1.

[0020] in, Figure 1 a in the equation corresponds to the X-ray powder diffraction spectrum. Figure 1 b in FIG corresponds to a scanning electron microscope image.

[0021] Figure 2 The X-ray powder diffraction spectrum and scanning electron microscope image of the Bi-MOF material prepared in Example 2.

[0022] in, Figure 2 a in the equation corresponds to the X-ray powder diffraction spectrum. Figure 2 b in FIG corresponds to a scanning electron microscope image.

[0023] Figure 3 These are the X-ray powder diffraction spectrum and scanning electron microscope image of the Tb-MOF material prepared in Example 3.

[0024] in, Figure 3 a in the equation corresponds to the X-ray powder diffraction spectrum. Figure 3 b in FIG corresponds to a scanning electron microscope image.

[0025] Figure 4 These are the X-ray powder diffraction spectrum and scanning electron microscope image of the Eu-MOF material prepared in Example 4.

[0026] in, Figure 4 a in the equation corresponds to the X-ray powder diffraction spectrum. Figure 4 b in FIG corresponds to a scanning electron microscope image.

[0027] Figure 5 These are the X-ray powder diffraction spectrum and scanning electron microscope image of the La-MOF material prepared in Example 5.

[0028] in, Figure 5 a in the equation corresponds to the X-ray powder diffraction spectrum. Figure 5 b in FIG corresponds to a scanning electron microscope image.

[0029] Figure 6 These are the X-ray powder diffraction spectrum and scanning electron microscope image of the Cd-MOF material prepared in Example 6.

[0030] in, Figure 6 a in the equation corresponds to the X-ray powder diffraction spectrum. Figure 6 b in FIG corresponds to a scanning electron microscope image.

[0031] Figure 7 This is the X-ray powder diffraction spectrum of the material prepared in Comparative Example 1.

[0032] Figure 8 This is the X-ray powder diffraction spectrum of the material prepared in Comparative Example 2. DETAILED DESCRIPTION

[0033] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0034] In general, the method of the present invention is to add nylon 66 and a strong base compound to a reactor, add water to dissolve the strong base compound, then seal the solution in the reactor, react at 180-220°C, then add a metal salt solution, and react at 60-160°C to obtain an aliphatic carboxylic acid ligand MOF material.

[0035] The following are specific embodiments:

[0036] Example 1

[0037] 3.0 g of waste nylon 66 and 0.7 g of potassium hydroxide were weighed and added to a reactor, 30 mL of deionized water was added, the reactor was sealed and placed in an oven, and heated at 160 ° C for 18 h. Then 4.27 g of cobalt nitrate hexahydrate was dissolved in 30 mL of deionized water, added to the reactor and sealed again, and the reaction was continued at 60 ° C for 20 h. The product was washed and dried to obtain Co-MOF material with a yield of 85%.

[0038] Figure 1 a in the figure is the X-ray powder diffraction spectrum of Co-MOF material. Figure 1 Figure b is a scanning electron micrograph of the Co-MOF material. The X-ray powder diffraction spectrum shows that the Co-MOF material exhibits significant characteristic diffraction peaks, indicating successful synthesis and good crystallinity. The scanning electron micrograph reveals that the Co-MOF material morphology is stacked flake crystals measuring 5 to 10 μm.

[0039] Example 2

[0040] 1.5 g of waste nylon 66 and 0.45 g of sodium hydroxide were weighed and added to a reactor, 30 mL of deionized water was added, the reactor was sealed and placed in an oven, and heated at 180 ° C for 24 h. Then 2.78 g of bismuth nitrate pentahydrate was dissolved in 30 mL of deionized water, added to the reactor and sealed again, and the reaction was continued at 120 ° C for 12 h. The product was washed and dried to obtain Bi-MOF material with a yield of 88%.

[0041] Figure 2 a in the figure is the X-ray powder diffraction spectrum of Bi-MOF material. Figure 2 Figure b is a scanning electron micrograph of the Bi-MOF material. The X-ray powder diffraction spectrum shows that the Bi-MOF material exhibits significant characteristic diffraction peaks, indicating successful synthesis and good crystallinity. The scanning electron micrograph reveals that the Bi-MOF material morphology consists of stacked nanosheets measuring 150 to 300 nm.

[0042] Example 3

[0043] 1.0 g of waste nylon 66 and 0.25 g of sodium hydroxide were weighed and added to a reactor, 30 mL of deionized water was added, the reactor was sealed and placed in an oven, and heated at 220°C for 12 h. 2.47 g of terbium nitrate hexahydrate was dissolved in 30 mL of deionized water, added to the reactor and sealed again. The reaction was continued at 90°C for 5 h. The product was washed and dried to obtain Tb-MOF material with a yield of 93%.

[0044] Figure 3 a in the figure is the X-ray powder diffraction spectrum of Tb-MOF material. Figure 3Figure b is a scanning electron micrograph of the Tb-MOF material. The X-ray powder diffraction spectrum shows that the Tb-MOF material exhibits significant characteristic diffraction peaks, indicating successful synthesis and good crystallinity. The scanning electron micrograph reveals that the Tb-MOF material morphology is stacked flake-like crystals measuring 15 to 30 μm.

[0045] Example 4

[0046] 1.5 g of waste nylon 66 and 0.5 g of potassium hydroxide were weighed and added to a reactor, 30 mL of deionized water was added, the reactor was sealed and placed in an oven, heated at 200 ° C for 24 h, 3.12 g of europium chloride hexahydrate was dissolved in 30 mL of deionized water, added to the reactor and sealed again, and the reaction was continued at 160 ° C for 3 h. The product was washed and dried to obtain Eu-MOF material with a yield of 98%.

[0047] Figure 4 a in the figure is the X-ray powder diffraction spectrum of Eu-MOF material. Figure 4 Figure b is a scanning electron micrograph of the Eu-MOF material. The X-ray powder diffraction spectrum shows that the Eu-MOF material exhibits significant characteristic diffraction peaks, indicating successful synthesis and good crystallinity. The scanning electron micrograph reveals that the Eu-MOF material morphology consists of stacked plate-like crystals measuring 2 to 2.5 μm.

[0048] Example 5

[0049] 2.0 g of waste nylon 66 and 0.8 g of sodium hydroxide were weighed and added to a reactor, 30 mL of deionized water was added, the reactor was sealed and placed in an oven, and heated at 160 ° C for 36 h. Then 1.33 g of lanthanum nitrate hexahydrate was dissolved in 30 mL of deionized water, added to the reactor and sealed again, and the reaction was continued at 90 ° C for 10 h. The product was washed and dried to obtain La-MOF material with a yield of 97%.

[0050] Figure 5 a in the figure is the X-ray powder diffraction spectrum of La-MOF material. Figure 5 Figure b shows a scanning electron micrograph of the La-MOF material. The X-ray powder diffraction spectrum shows that the La-MOF material exhibits significant characteristic diffraction peaks, indicating successful synthesis and good crystallinity. The scanning electron micrograph reveals that the La-MOF material exhibits elongated, rod-like crystals measuring 0.5 to 1 μm.

[0051] Example 6

[0052] 2.5 g of waste nylon 66 and 0.4 g of sodium hydroxide were weighed and added to a reactor, 30 mL of deionized water was added, the reactor was sealed and placed in an oven, and heated at 200 ° C for 18 h. Then 4.31 g of cadmium nitrate tetrahydrate was dissolved in 30 mL of deionized water, added to the reactor and sealed again, and the reaction was continued at 100 ° C for 24 h. The product was washed and dried to obtain Cd-MOF material with a yield of 99%.

[0053] Figure 6 a in the figure is the X-ray powder diffraction spectrum of Cd-MOF material. Figure 6 Figure b is a scanning electron micrograph of the Cd-MOF material. The X-ray powder diffraction spectrum shows that the Cd-MOF material exhibits significant characteristic diffraction peaks, indicating successful synthesis and good crystallinity. The scanning electron micrograph reveals that the Cd-MOF material is composed of stacked plate-like crystals ranging in size from 400 to 700 nm.

[0054] Comparative Example 1

[0055] Weigh 3.0g of waste nylon 66 and 0.7g of potassium hydroxide into a reactor, add 30mL of deionized water, seal the reactor and place it in an oven, heat it at 120°C for 12h, then dissolve 4.27g of cobalt nitrate hexahydrate in 30mL of deionized water, add it to the reactor and seal it again, continue the reaction at 60°C for 20h, wash and dry the product, and its X-ray powder diffraction spectrum is as follows Figure 7 As shown in the figure, the analysis showed that it was a mixture of nylon and Co(OH)2. It can be seen that the first step hydrothermal reaction failed to degrade nylon 66.

[0056] Comparative Example 2

[0057] Weigh 1.0g of waste nylon 66 and 0.25g of sodium hydroxide into a reactor, add 30mL of deionized water, seal the reactor and place it in an oven, heat it at 220°C for 12h, then dissolve 2.47g of terbium nitrate hexahydrate in 30mL of deionized water, add it to the reactor and seal it again, continue the reaction at 200°C for 24h, wash and dry the product, and its X-ray powder diffraction spectrum is as follows Figure 8 As shown, no obvious characteristic diffraction peak appears, indicating that the high reaction temperature during the second step coordination reaction (i.e., the second step hydrothermal reaction) is not conducive to the synthesis of MOF.

[0058] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing aliphatic carboxylic acid ligand MOF by two-step hydrothermal degradation of waste nylon 66, characterized in that: The following steps are involved: S1. Dispersing waste nylon 66 and a strong base compound in water to obtain a dispersion; wherein the mass ratio of the strong base compound to the waste nylon 66 is 0.17 to 0.4:1; S2. The dispersion obtained in step S1 is subjected to a hydrothermal reaction at 180 to 220 ° C for 12 to 48 h to hydrolyze nylon 66 to obtain a nylon 66 degradation solution; S3. Adding a metal salt solution to the nylon 66 degradation solution obtained in step S2, and continuing the hydrothermal reaction at 60-160°C for 3-24 hours to coordinate the adipate and hexamethylenediamine in the degradation solution with the metal ions to form a MOF. The resulting product is an aliphatic carboxylic acid ligand MOF material; wherein the metal element in the MOF material is the same as the metal element in the metal salt solution; In step S1, the strong base compound is at least one of sodium hydroxide and potassium hydroxide.

2. The method according to claim 1, wherein: In step S3, the metal salt in the metal salt solution is at least one of cobalt nitrate, bismuth nitrate, terbium nitrate, europium chloride, lanthanum nitrate, and cadmium nitrate.

3. The method according to claim 1, wherein: The ratio of the metal salt contained in the metal salt solution in step S3 to the waste nylon 66 used in step S1 satisfies a mass ratio of 0.6 to 2.5:1.

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