Composite catalyst for alcoholysis of pet waste bottles and preparation method thereof

Abstract

The application belongs to the technical field of polymer material recycling and catalytic chemistry, and discloses a composite catalyst for PET waste bottle alcoholysis and a preparation method thereof. The composite catalyst is based on biomimetic enzyme engineering and dynamic covalent chemistry strategy, and comprises a binuclear metal cluster active center, a limited microenvironment shell layer and a dynamic covalent network skeleton. The binuclear metal cluster is constructed by zinc / cobalt / manganese ions and a tridentate ligand C9H8N2O4, the shell layer is a polydopamine derived functional polymer, and the skeleton is formed by cross-linking aromatic and aliphatic monomers through borate ester bonds. The application solves the performance degradation problem caused by the instability of the active center of the existing enzyme-like catalyst in industrial circulation application. The catalyst not only has the enzyme-like catalytic characteristics of high activity and high selectivity, but also realizes self-repairing after damage through the built-in multiple dynamic chemical mechanism, thereby providing a new catalytic material with performance and durability for efficient and sustainable chemical recycling of PET waste bottles.

Description

Technical Field

[0001] This invention belongs to the field of polymer material recycling and catalytic chemistry technology, and relates to a composite catalyst for alcoholysis of PET waste bottles and its preparation method. Background Technology

[0002] Polyethylene terephthalate (PET), as one of the world's largest-produced thermoplastic polyesters, has made efficient chemical recycling of its waste bottles a key link in realizing a circular economy for plastics. Among the many depolymerization pathways, alcoholysis has attracted much attention due to its relatively mild reaction conditions and the fact that the products can be directly used for repolymerization. The choice and performance of the catalyst directly determine the economic efficiency and sustainability of the process.

[0003] In recent years, to improve catalytic efficiency and selectivity, researchers have widely adopted the concept of biomimetic enzyme engineering, developing a series of composite catalytic systems with enzyme-like catalytic characteristics. These materials typically mimic the active pocket structure of natural enzymes, constructing confined microenvironments at the molecular scale to achieve targeted activation and cleavage of PET segments. These catalysts often combine metal active centers with organic ligands or functional polymer matrices, constructing reversible cross-linked networks using dynamic covalent chemistry strategies. This not only endows the materials with structural flexibility but also provides the possibility for dynamic regulation of catalytic sites.

[0004] Existing composite catalysts typically employ transition metal complexes such as zinc, cobalt, and manganese supported on porous supports (e.g., MOFs, mesoporous silica, or functionalized polymers), or embed catalytic units into self-assembled frameworks via dynamic covalent bonds such as Schiff bases and borate esters. In the initial reaction stage, such designs do significantly enhance catalytic activity and alcoholysis selectivity, with some systems even achieving complete depolymerization of PET waste bottles at relatively low temperatures.

[0005] Although dynamic covalent bonds endow materials with a macroscopically repairable appearance, their microscopic metal active centers are still highly susceptible to irreversible deactivation due to coordination environment disturbances. On the one hand, trace amounts of sulfur-, nitrogen-, or carboxylic acid impurities in the reaction system can be strongly adsorbed onto metal sites, leading to poisoning of active centers. On the other hand, driven by thermodynamics, highly dispersed nanoscale metal clusters or single-atom sites tend to undergo Ostwald ripening, inducing sintering and aggregation, thereby losing the catalytic advantages brought by high specific surface area. The dynamic network may undergo local dissociation during repeated swelling-contraction or acid-base fluctuations, causing some catalytic components to be lost to the liquid phase in the form of ions or complexes, resulting in permanent activity decay. Summary of the Invention

[0006] To achieve the aforementioned objectives, this invention provides a composite catalyst for the alcoholysis of PET waste bottles and its preparation method. Based on biomimetic enzyme engineering principles and a dynamic covalent chemistry strategy, the composite catalyst constructs a multi-level structural system with enzyme-like catalytic active pockets and built-in self-healing capabilities. After experiencing damage such as poisoning of metal active centers, sintering, or component loss, it can achieve in-situ regeneration and functional recovery of active sites through a dynamic reconstruction mechanism at the molecular level. This significantly improves its structural stability and service life under industrial cyclic conditions while maintaining high catalytic efficiency and selectivity.

[0007] The composite catalyst of the present invention comprises the following three core structural units: (1) a catalytic active center composed of a binuclear metal cluster; (2) a confined microenvironment shell constructed around the catalytic active center; and (3) a dynamic covalent network framework that penetrates the confined microenvironment shell and is covalently cross-linked with it. The binuclear metal cluster is formed by two adjacent transition metal ions connected by a bridging ligand. The transition metal ions are selected from any one or a combination of two of zinc, cobalt, and manganese. The bridging ligand is a tridentate organic ligand containing two carboxylic acid groups and an imidazole ring, with the molecular formula C9H8N2O4 and the structural formula HOOC—C4H2—Im—COOH, where Im is an imidazole ring. This ligand coordinates with two metal ions through two carboxylic acid oxygen atoms, while the imidazole nitrogen atom participates in the formation of a secondary coordination bond, thereby stabilizing the binuclear configuration and regulating the electron density distribution. The confined microenvironment shell is composed of a functionalized polymer containing polyphenolic structural units. This functionalized polymer, through coordination between catechol groups and metal ions and π-π stacking interactions, encapsulates the binuclear metal cluster, forming a dense shell with a thickness of 2-5 nm. This shell exhibits a molecular sieving effect, selectively allowing ethylene glycol molecules to enter while repelling large molecular impurities. Simultaneously, its abundant hydroxyl and phenolic hydroxyl groups can form hydrogen bonds with reaction intermediates, promoting the directional arrangement of PET segments near the active site. The dynamic covalent network framework is composed of alternating rigid aromatic monomers and flexible aliphatic monomers linked by borate ester bonds. The aromatic monomer is 1,3,5-tris(4-hydroxyphenyl)benzene, and the aliphatic monomer is 1,6-hexanediol. Under alkaline conditions, both undergo a condensation reaction with phenylboronic acid, forming a topologically reversible three-dimensional cross-linked network. This network not only provides mechanical support for the entire catalyst, but its borate ester bonds can also undergo reversible breakage and recombination in the presence of trace amounts of water or alcohol, thereby endowing the material with macroscopic deformation adaptability and microscopic component migration capabilities.

[0008] In the composite catalyst, the loading density of binuclear metal clusters is 0.8-1.2 clusters per square nanometer, the pore size distribution of the confined microenvironment shell is concentrated in the range of 0.6-0.9 nm, and the crosslinking density of the dynamic covalent network framework is 4.5 × 10⁻⁶ per cubic centimeter. 19 -6.0×10 19There are several crosslinking points. The overall specific surface area of ​​the catalyst is 180-250 m². 2 / g, pore volume 0.35-0.50m 3 / g, with an average pore size of 3.2-4.8nm.

[0009] In a preferred embodiment of the present invention, the preparation method of the composite catalyst includes the following steps:

[0010] Step 1, Synthesis of binuclear metal cluster precursor: A transition metal salt (such as zinc nitrate, cobalt chloride or manganese acetate) with a molar ratio of 1:1 and a tridentate organic ligand C9H8N2O4 were dissolved in anhydrous ethanol. The mixture was stirred at 70±5℃ for 6±2 hours under nitrogen protection. After cooling to room temperature, the mixture was filtered, washed three times with anhydrous ethanol, and dried under vacuum to obtain a dark red crystalline binuclear metal cluster precursor.

[0011] Step 2, constructing a confined microenvironment shell: The binuclear metal cluster precursor is dispersed in deionized water, and a dopamine hydrochloride solution with a mass fraction of 5±1% is added. The pH value is adjusted to 8.5±0.5, and the mixture is stirred at 25±5℃ for 12±2 hours to allow dopamine to self-polymerize on the surface of the metal cluster to form a polydopamine shell. The shell is then washed with deionized water by centrifugation until neutral to obtain a core-shell structure intermediate.

[0012] Step 3, in-situ growth of dynamic covalent network framework: The core-shell structure intermediate is dispersed in N,N-dimethylformamide, and 1,3,5-tris(4-hydroxyphenyl)benzene, 1,6-hexanediol and phenylboronic acid are added in sequence in a molar ratio of 1:1.2:3. Then, a catalytic amount of triethylamine is added, and the reaction is carried out at 80±5℃ under a nitrogen atmosphere for 24±2 hours, so that the borate ester bonds are cross-linked in situ on the surface of the polydopamine shell to form a three-dimensional network. After the reaction is completed, the mixture is centrifuged and washed in sequence with N,N-dimethylformamide, ethanol and deionized water, and then vacuum dried at 60±5℃ for 12±2 hours to obtain the target composite catalyst.

[0013] In another preferred embodiment of the present invention, the preparation method further includes a heat treatment step after step 3: the obtained composite catalyst is placed in a tube furnace and heated to 200±10℃ at a heating rate of 2±0.5℃ / min under a nitrogen atmosphere, held at that temperature for 2-3 hours, and then naturally cooled to room temperature. This heat treatment process promotes partial carbonization of the polydopamine shell, forming a conductive carbon layer rich in quinone structures. This carbon layer enhances the electronic coupling between the metal cluster and the shell, and its residual phenolic hydroxyl groups can still participate in hydrogen bonding. At the same time, the carbonization process does not destroy the integrity of the borate ester network.

[0014] The composite catalyst of this invention exhibits excellent catalytic performance and self-healing ability in the alcoholysis reaction of PET waste bottles. When the catalyst experiences any of the following damage modes, its initial activity can be restored through simple regeneration:

[0015] (1) For metal center poisoning: Immerse the deactivated catalyst in a 0.1 mol / L sodium citrate aqueous solution and stir at 60°C for 1 hour. The citrate ions adsorb sulfur- or nitrogen-containing impurities on the metal site through competitive coordination exchange. Then wash with deionized water to restore more than 90% of the original activity.

[0016] (2) For metal sintering aggregation: the deactivated catalyst is placed in an ethylene glycol vapor atmosphere and kept at 180°C for 30 min. The dynamic covalent network is locally relaxed due to swelling, releasing the encapsulated metal ions. At the same time, the phenolic hydroxyl groups in the confined shell are re-coordinated with the metal ions, which promotes the dissociation of the sintered metal particles and their redispersion into a binuclear cluster structure.

[0017] (3) For component loss: The used catalyst mother liquor and fresh catalyst are mixed at a volume ratio of 1:1 and left to stand at 150°C for 2 hours. The metal complexes or ligand fragments lost in the mother liquor are reintegrated into the dynamic network framework through borate ester exchange reaction and reconstructed in situ as active centers under the guidance of the confined shell.

[0018] The realization of the above self-healing mechanism relies on the unique structural design of the present invention: the binuclear metal cluster itself has high thermodynamic stability, and its bridging ligands inhibit the migration of single metal sites; the confined microenvironment shell not only provides spatial protection, but its polyphenol structure can release coordinating groups to participate in metal recoordination under thermal or chemical stimulation; the dynamic covalent network framework serves as a molecular highway, allowing catalytic components to migrate directionally between the damaged region and the reserve region, and to achieve structural recombination through reversible bonding.

[0019] Furthermore, the preparation method of this invention has advantages such as controllable process, readily available raw materials, and no need for precious metals. The tridentate organic ligand used can be obtained by Strecker synthesis of commercially available phthalaldehyde and glycine. Dopamine, 1,3,5-tris(4-hydroxyphenyl)benzene, 1,6-hexanediol, and phenylboronic acid are all conventional chemical raw materials. The entire synthesis process is carried out under normal pressure, without the need for high-temperature and high-pressure equipment, making it suitable for large-scale production.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] This invention, through the rational design of a three-in-one composite structure of a binuclear metal cluster, a confined shell, and a dynamic network, successfully solves the performance degradation problem caused by the instability of active centers in existing enzyme-like catalysts during industrial recycling applications. This catalyst not only possesses highly active and selective enzyme-like catalytic characteristics, but also achieves self-repair after damage through built-in multiple dynamic chemical mechanisms, providing a novel catalytic material with both high performance and durability for the efficient and sustainable chemical recycling of PET waste bottles. Detailed Implementation

[0022] This invention provides a composite catalyst for the alcoholysis of PET waste bottles and its preparation method. Based on biomimetic enzyme engineering principles and a dynamic covalent chemistry strategy, this composite catalyst constructs a multi-level structural system with enzyme-like catalytic active pockets and built-in self-healing capabilities. After experiencing damage such as poisoning of metal active centers, sintering, or component loss, it can achieve in-situ regeneration and functional recovery of active sites through a dynamic reconstruction mechanism at the molecular level. This significantly improves its structural stability and service life under industrial cyclic conditions while maintaining high catalytic efficiency and selectivity.

[0023] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples, so as to ensure that those skilled in the art can fully understand and implement the present invention.

[0024] Example 1: The binuclear metal cluster consists of zinc ions and a tripentate ligand (molar ratio 1:1); the confined shell is polydopamine (thickness 3 nm); the dynamic network crosslinking density is 5.2 × 10⁻⁶. 19 pcs / cm 3 React at 190℃ for 2 hours;

[0025] Preparation process: Synthesis of binuclear metal clusters → Construction of confined shells → In-situ growth of dynamic covalent networks → Heat treatment → Drying.

[0026] Specifically, the following steps are included:

[0027] Step 1, Synthesis of binuclear metal cluster precursor: A transition metal salt (such as zinc nitrate, cobalt chloride or manganese acetate) with a molar ratio of 1:1 and a tridentate organic ligand C9H8N2O4 are dissolved in anhydrous ethanol. The mixture is stirred at 70°C for 6 hours under nitrogen protection. After cooling to room temperature, the mixture is filtered, washed three times with anhydrous ethanol, and dried under vacuum to obtain a dark red crystalline binuclear metal cluster precursor.

[0028] Step 2, constructing a confined microenvironment shell: The binuclear metal cluster precursor is dispersed in deionized water, and a 5% (w / w) dopamine hydrochloride solution is added to adjust the pH to 8.5. The mixture is stirred at 25°C for 12 hours to allow dopamine to self-polymerize on the surface of the metal cluster to form a polydopamine shell. The mixture is then centrifuged and washed with deionized water until neutral to obtain a core-shell structure intermediate.

[0029] Step 3, in-situ growth of dynamic covalent network framework: The core-shell structure intermediate is dispersed in N,N-dimethylformamide, and 1,3,5-tris(4-hydroxyphenyl)benzene, 1,6-hexanediol and phenylboronic acid are added in sequence in a molar ratio of 1:1.2:3. Then, a catalytic amount of triethylamine is added, and the reaction is carried out at 80°C under a nitrogen atmosphere for 24 hours to allow the borate ester bonds to crosslink in situ on the surface of the polydopamine shell to form a three-dimensional network. After the reaction is completed, the mixture is centrifuged and washed sequentially with N,N-dimethylformamide, ethanol and deionized water, and then vacuum dried at 60°C for 12 hours to obtain the target composite catalyst.

[0030] Step 4, heat treatment: The obtained composite catalyst is placed in a tube furnace and heated to 200°C at a heating rate of 2°C / min under a nitrogen atmosphere, held at that temperature for 2 hours, and then naturally cooled to room temperature.

[0031] Example 2: The binuclear metal cluster is cobalt ions + tripenteric ligand (molar ratio 1:1), and the rest of the formulation and process are the same as in Example 1;

[0032] Preparation process: Same as in Example 1.

[0033] Example 3: The binuclear metal cluster is manganese ion + tripenteric ligand (molar ratio 1:1), and the rest of the formulation and process are the same as in Example 1;

[0034] Preparation process: Same as in Example 1.

[0035] Example 4: The binuclear metal cluster is composed of zinc ions:cobalt ions = 1:1 + tripentate ligand (total molar ratio 1:1), and the rest of the formulation and process are the same as in Example 1;

[0036] Preparation process: Same as in Example 1.

[0037] Example 5: The polydopamine shell thickness is 2nm, and the rest of the formulation and process are the same as in Example 1;

[0038] Preparation process: Same as in Example 1.

[0039] Example 6: The polydopamine shell thickness is 5 nm, and the rest of the formulation and process are the same as in Example 1;

[0040] Preparation process: Same as in Example 1.

[0041] Example 7: The 200℃ heat treatment step is omitted; the rest of the formula and process are the same as in Example 1.

[0042] Preparation process: Same as in Example 1 (without heat treatment step).

[0043] Example 8: Dynamic covalent network crosslinking density 6.0 × 10 19 pcs / cm 3 The remaining formulas and processes are the same as in Example 1;

[0044] Preparation process: Same as in Example 1.

[0045] Comparative Example 1: No binuclear metal clusters, confined shells, or dynamic networks were used; only zinc nitrate was used as a catalyst; the remaining reaction conditions were the same as in Example 1.

[0046] Preparation process: Zinc nitrate is dissolved → dried → directly used in alcoholysis reaction.

[0047] Comparative Example 2: No dynamic covalent network, only a dual-core metal cluster + confined shell; the rest of the formulation and process are the same as in Example 1;

[0048] Preparation process: synthesis of binuclear metal clusters → construction of confined shells → drying.

[0049] Test method:

[0050] Catalytic performance testing: PET conversion rate and BHET selectivity were determined by high performance liquid chromatography; the reaction was controlled at 190℃ for 2 hours, and the reaction kinetics were recorded.

[0051] Stability test: After 10 consecutive cycles of use, the conversion rate was monitored; after simulating poisoning and sintering damage, the regeneration activity recovery rate was tested.

[0052] The test data comparisons are shown in Table 1 and Table 2.

[0053] Table 1. Comparison of PET conversion rate, BHET selectivity, and conversion rate after 10 cycles:

[0054]

[0055] Table 2. Comparison of Poisoning Regeneration Recovery Rate and Conversion Rate after 3 Cycles:

[0056]

[0057] Examples 1-8 showed a PET conversion rate ≥98% and a conversion rate ≥91% after 10 cycles, which was far superior to the comparative examples. Comparative example 1 had extremely poor activity and stability due to the lack of a composite structure, and comparative example 2 lacked a dynamic network and self-healing ability, confirming that the composite system is the key to high efficiency and stability.

[0058] The zinc-cobalt mixed metal cluster (Example 4) exhibits the best catalytic performance; the stability is optimal when the confined shell thickness is 3-5 nm; and the cycle life is extended when the dynamic network crosslinking density is increased (Example 8).

[0059] The catalyst in this example is easy to regenerate and can quickly recover its activity after poisoning and sintering; the conversion rate after three cycles is still ≥92%, which is much higher than that of traditional catalysts and is suitable for continuous production.

[0060] Compared to traditional single-metal catalysts (Comparative Example 1), Example 1 showed a 16% increase in conversion rate and a 20% increase in selectivity; compared to catalysts without dynamic network (Comparative Example 2), it showed improved cycle stability, solving the industry problem of easy deactivation and difficult cycling of traditional catalysts.

[0061] The composite catalyst described in this invention achieves efficient alcoholysis of PET through the synergistic effect of multi-core metal clusters, confined shells, and dynamic networks, and can be applied to industrial plastic recycling scenarios with different parameter combinations.

[0062] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. 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 composite catalyst for the alcoholysis of waste PET bottles, characterized in that, The composite catalyst comprises: Catalytic active centers composed of binuclear metal clusters: A confined microenvironment shell constructed around the catalytic active center: And a dynamic covalent network framework that penetrates and covalently cross-links with the confined microenvironment shell; The binuclear metal cluster is formed by two adjacent transition metal ions connected by a bridging ligand. The transition metal ions are selected from any one or a combination of two of zinc, cobalt, and manganese. The bridging ligand is a tridentate organic ligand with the molecular formula C9H8N2O4, which coordinates with the two metal ions through two carboxylic acid oxygen atoms and forms a secondary coordination bond through imidazole nitrogen atoms. The confined microenvironment shell is a polydopamine-derived shell, whose surface is rich in phenolic hydroxyl groups and quinone structures. It can form hydrogen bonds with reaction intermediates to promote the directional arrangement of PET segments near the catalytic active center. It wraps around the binuclear metal cluster through the coordination of catechol groups with metal ions and π-π stacking, forming a dense shell with a thickness of 2-5 nm. The dynamic covalent network framework is composed of alternating 1,3,5-tris(4-hydroxyphenyl)benzene and 1,6-hexanediol linked by borate ester bonds, and forms a three-dimensional network through phenylboronic acid crosslinking.

2. The composite catalyst for alcoholysis of PET waste bottles according to claim 1, characterized in that, The loading density of the binuclear metal clusters is 0.8-1.2 clusters per square nanometer, the pore size distribution of the confined microenvironment shell is concentrated in the range of 0.6-0.9 nm, and the crosslinking density of the dynamic covalent network framework is 4.5 × 10⁻⁶ per cubic centimeter. 19 -6.0×10 19 One cross-linking point.

3. The composite catalyst for alcoholysis of PET waste bottles according to claim 1, characterized in that, The borate ester bonds in the dynamic covalent network framework can undergo reversible breakage and recombination in the presence of trace amounts of water or alcohol, endowing the catalyst with the ability to migrate microscopic components and adapt to macroscopic deformation.

4. The composite catalyst for alcoholysis of PET waste bottles according to claim 1, characterized in that, The confined microenvironment shell is partially carbonized after heat treatment to form a conductive carbon layer. The conductive carbon layer retains residual phenolic hydroxyl groups and enhances the electronic coupling between the conductive carbon layer and the catalytic active center.

5. A method for preparing a composite catalyst for the alcoholysis of PET waste bottles as described in any one of claims 1-4, characterized in that, Includes the following steps: Step 1, Synthesis of binuclear metal cluster precursor: A transition metal salt with a molar ratio of 1:1 and a tridentate organic ligand C9H8N2O4 are dissolved in anhydrous ethanol. The mixture is stirred under nitrogen protection, cooled, filtered, washed and vacuum dried. Step 2, constructing a confined microenvironment shell: disperse the binuclear metal cluster precursor in water, add dopamine hydrochloride solution, adjust the pH to 8.5±0.5, stir the reaction, and then centrifuge and wash until neutral; Step 3, in-situ growth of dynamic covalent network framework: The obtained core-shell structure intermediate was dispersed in N,N-dimethylformamide, and 1,3,5-tris(4-hydroxyphenyl)benzene, 1,6-hexanediol and phenylboronic acid were added in sequence at a molar ratio of 1:1.2:

3. Triethylamine was added as a catalyst, and the reaction was carried out under a nitrogen atmosphere. After centrifugation, washing and drying were performed.

6. The method for preparing the composite catalyst for alcoholysis of PET waste bottles according to claim 5, characterized in that, The third step is followed by a heat treatment step: the obtained composite catalyst is placed in a nitrogen atmosphere and heated to 200±10℃ at a rate of 2±0.5℃ / min, held at that temperature for 2-3 hours, and then naturally cooled.

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