Upcycling waste polyamide into sustainable plastics by catalyst-free and solvent-free melt polycondensation

Abstract

Polyamide 66 (PA66), a nearly non-biodegradable polymer, has caused significant environmental damage, especially through marine debris accumulation, threatening marine ecosystems and biodiversity. Current recycling methodologies universally necessitate the use of metal catalysts and involve complex recycling procedures. This study has successfully developed an innovative catalyst-free and solvent-free carboxyl–amide exchange method based on the melt polycondensation process for the upcycling of waste PA66 into sustainable plastic. The plastic demonstrates a broad spectrum of functionalities tailored to meet the specific requirements of various fields. It also exhibits superior mechanical properties, enhanced thermal stability and closed-loop recyclability in comparison to commercial plastics. More significantly, the in situ formation of an anhydride linkage, serving as a labile bond, confers biodegradability to the plastic without necessitating the addition of extra monomers or additional processing steps. This study employs the conventional large-scale synthesis method, which is compatible with existing industrial equipment and holds promise for extensive industrial application. Additionally, the concept of generating easily breakable bonds in situ offers valuable insights for the design of novel degradable materials.

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Green foundation

1. Advancement in green chemistry: We have developed a one-step, catalyst-free, and solvent-free process for the upcycling of waste polyamide into sustainable, high-performance materials. By eliminating the reliance on solvents and traditional heavy metal catalysts, this approach provides notable environmental advantages.

2. Green chemistry achievement: By integrating a novel carboxyl–amide exchange mechanism into the large-scale melt polycondensation process, we have clarified the reaction pathway, which facilitates the complete recycling and reuse of waste polyamide. Moreover, in addition to recovering adipic acid, an important chemical feedstock, we have also developed a novel biodegradable polyamide.

3. Further research for a greener impact: Future research should focus on optimizing reaction conditions and integrating functional units to give recycled materials with specific properties that meet the needs of various applications. Meanwhile, developing closed-loop recycling capabilities will greatly enhance the environmental sustainability of these materials.

1. Introduction

Polyamide 66 (PA66), recognized as one of the earliest and most significant industrial synthetic plastics, has become a major contributor to marine pollution.^1–3 Its chemical stability and resistance to degradation pose substantial environmental challenges. Each year, an estimated 590 thousand tons of waste PA66 are generated worldwide, contributing to plastic pollution and impacting human health through the creation of microplastics under external influences.⁴ Recycling waste PA66 can mitigate the issue of environmental pollution, while the existing recycling methods are constrained by a complex process.^5–9

Given the relative stability of the amide bond, conventional chemical recycling processes for PA66 typically involve depolymerizing the polymer into monomers for subsequent utilization through a polymer–monomer–polymer approach, which usually necessitates the use of catalysts and large quantities of solvents.^10–12 This results in a complicated impurity removal procedure, raises environmental concerns, and increases the cost of recycled products. Traditional mechanical recycling methods for polymers typically involve polymer-to-polymer processes, which are noted for their simplicity, cost-effectiveness, and scalability.¹³ However, during prolonged use, the PA66 molecular chain undergoes partial degradation, resulting in diminished material performance and the necessity for downcycling. Furthermore, while this process can extend the service life of PA66, it does not address the fundamental issue of its non-biodegradability.

Inspired by the polymer-to-polymer recycling strategy in physical recycling, the direct chemical upcycling of waste PA66 into biodegradable materials with closed-loop recycling holds significant potential for scalable application in producing high-performance materials. This approach not only integrates the advantages of the mechanical recycling method, but also holds promise for addressing the non-degradability of polyamide by introducing specific degradable groups. Numerous approaches have been reported to address the non-degradability issue of polymers. Among them, a prominent strategy involves introducing weak bonds within the polymer chain to create easily breakable points, such as esters, acetals and ketones.^14–19 Furthermore, anhydride linkages have garnered significant attention as exchangeable linkages, frequently reported for their reversible characteristics in the realm of shape memory materials.^20,21 Simultaneously, due to the susceptibility of the anhydride linkage to hydrolysis, polymers that contain anhydride bonds in their main chains exhibit favorable degradation properties.²² The reversible formation and cleavage of the anhydride linkage in the main chain, along with its susceptibility to hydrolysis, may provide a novel approach for enhancing non-biodegradable polymer recyclability and biodegradation. While these methods provide a crucial theoretical framework for polymer biodegradation, the intentional incorporation of additional monomers to enhance the degradation performance may introduce greater intricacy in material synthesis and utilization. By refining the polymerization mechanism, it may become feasible to synthesize polymers with diverse chemical bond types in situ using simplified components, thereby endowing materials with a broad spectrum of properties, particularly degradability and closed-loop recyclability.

It is crucial to emphasize that reversible chain exchange reactions are of paramount importance in the synthesis and depolymerization of polymers.²³ Specifically, these reactions are extensively utilized in large-scale polycondensation processes, including hydroxyl–ester exchange (HEE), ester–ester exchange, and carboxy–hydroxyl or amino group exchange reactions. For instance, in polyester synthesis, high molecular weight polyester can be produced via HEE reactions.^24–26 Conversely, the introduction of excess hydroxyl groups into the system can lead to the depolymerization of polyester into its monomeric constituents.²⁷ The fundamental principle underlying these reactions is the interchange of leaving groups while maintaining the overall group count. These reversible exchange reactions provide a wide array of opportunities for polymer diversification. Moreover, they can be seamlessly integrated into existing melt polycondensation equipment, enabling large-scale applications. However, the amide bond exhibits greater stability compared to the ester bond, making the exchange reaction more challenging. Additionally, introducing an excessive amount of exchanger will inevitably lead to depolymerization of the polymer into smaller molecules, thereby preventing the formation of useful polymeric materials. Our prior research has demonstrated that carboxylic acids can sublime within the melt polycondensation system and subsequently be removed from the system.^28,29 By employing an excess of carboxylic acid to react with PA66, an efficient exchange reaction may occur, enabling the synthesis of novel polymers while displacing adipic acid (AA), a critical industrial raw material. Currently, AA is predominantly produced via an energy-intensive and highly polluting oxidation process.^30–32 Utilizing a specific exchange reaction not only facilitates the preparation of high-performance new materials, but also enables the recovery of high-purity AA, thereby achieving the comprehensive utilization of waste polyamide.

Herein, we propose an innovative upcycling strategy for converting waste PA66 into sustainable plastic through conventional large-scale melt polycondensation without the use of any catalyst or solvent. This process results in a material whose performance exceeds that of commercial low-density polyethylene (LDPE), poly(butylene succinate) (PBS), and poly(butylene adipate-co-terephthalate) (PBAT), while enabling the recovery of high-purity AA (Fig. 1). This method integrates innovative carboxyl-amide exchange (CAE) during the melt polycondensation process, utilizing biobased long-chain dicarboxylic acids and waste PA66. Through precise modulation of the CAE mechanism, the resultant polymer forms an anhydride bond in situ without the need for additional monomers or catalysts. This anhydride-linked sustainable plastic demonstrates multifunctional and biodegradable properties. The study offers valuable insights into the development of novel materials in polymer science. Given that the melt polycondensation method boasts a well-established process and industrial equipment, this approach demonstrates considerable potential for large-scale applications.

Fig. 1 One-pot upcycling of waste PA66 into novel materials. Proposed routes of carboxyl–amide exchange (CAE) based on melt polycondensation to sustainable plastic. HDMA (green sphere): hydrogenated dimeric acid; AA (purple sphere): adipic acid; blue sphere: hexamethylendiamine.

2. Results and discussion

2.1. Synthesis procedure

Although the synthesis of polymers via CAE has not been reported, the presence of exclusive amide bonds in PA66 suggests that amides may potentially undergo exchange reactions with carboxylic acids, similar to HEE in melt polycondensation, leading to direct conversion into novel polymers. However, the careful selection of polymerizable dicarboxylic acid monomers is crucial as it directly influences the performance and sustainability of upcycled materials. Excitingly, hydrogenated dimeric acid (HDMA) emerges as an excellent candidate, derived from unsaturated fatty acids such as pine, soybean, and sunflower oil (Fig. 1). To determine the feasibility of CAE, a model reaction was carried out using N-methylacetamide and hexanoic acid (Fig. S1). The ¹H nuclear magnetic resonance (NMR) spectrum reveals the successful conversion into acetic acid and N-methylhexanamide, as evidenced by the presence of distinct peaks c and d. This suggests that the CAE reaction is feasible, and the extent of the reaction becomes increasingly significant as the temperature rises.

According to CAE, the reaction between HDMA and PA66 can produce a new material and AA as a by-product (Fig. 2A). Expectedly, thermogravimetric analysis (TGA) reveals a decrease in the mass of the prepolymer at 152 °C (Fig. 2B), which coincides with the sublimation temperature of AA.³³ Meanwhile, a profusion of the white sublimation product was observed throughout the polycondensation process (Fig. 2C). The ¹H NMR spectrum confirms that the sublimation product is AA. Additionally, the formation of AA is fully supported by electrospray ionization mass spectrometry (ESI-MS) with the mass-to-charge ratio (m/z) of 145.0507 (Fig. S2) and high-performance liquid chromatography (HPLC, Fig. S3). Collectively, these results prove the successful formation of AA and the feasibility of CAE in melt polycondensation. Notably, high-purity AA was successfully recovered from waste PA66, thereby achieving greater economic and environmental benefits.

Fig. 2 Principles of upcycling via polycondensation. (A) Carboxyl–amide exchange reaction. (B) TGA of the PA66HDA0.7 prepolymer after the first-stage polycondensation, the PA66HDA0.7 polymer after the second-stage polycondensation and pure AA. (C) ¹H NMR spectrum of AA collected during the polycondensation. (D) MALDI-TOF-MS of the PA66HDA0.7 polymer. (E) DFT calculated energy profiles by using M06-2X/6-311++G(d, p).

2.2. Characterization of the copolymer

Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy and ¹H NMR spectroscopy reveal that HDMA was successfully incorporated into the PA66 chain (Fig. S4–S12 and Tables S1, S2). Intriguingly, it is evident that the final product (PA66HDAs) displays two distinct sets of peaks (peaks d and e) in the 2.5–4 ppm range, unlike PA66, which only shows peaks a and b. Peak d is attributed to the methylene groups adjacent to the anhydride linkage (Fig. S5–S12). Meanwhile, the PA66HDA0.7 and PA66 samples were analyzed using X-ray photoelectron spectroscopy (XPS, Fig. S13). The results indicate that PA66HDA0.7 contains relatively higher concentrations of carbon (C) and oxygen (O), while exhibiting lower levels of nitrogen (N) compared to PA66. Notably, XPS confirms a 0.35 eV shift of the O 2p peak observed in PA66HDA0.7 compared to PA66, attributed to the higher binding energy of –COO– (Fig. S14). The successful incorporation of HDMA into the PA66 backbone was further confirmed by the characteristic peaks P1 and P2 in the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) spectrum of PA66HDA0.7 (Fig. 2D). Surprisingly, the product obtained by melt polycondensation consists of cyclic components (P1 and P2) containing anhydride linkages (Fig. 2D). Moreover, the intrinsic viscosity ([η]) decreased significantly upon the introduction of HDMA (Table S1). This can be attributed to the inability of the long alkyl chains to effectively extend within the formic acid solvent, causing them to curl up and thereby reducing the hydrodynamic volume.³⁴ Advanced polymer chromatography (APC) using hexafluoroisopropanol as the mobile phase was employed to measure the molecular weight (M_n) and polydispersity index (PDI) of polymers. APC results for M_n and PDI showed that the M_n of the synthesized PA66HDAs is significantly higher than that of PA66, while the PDI is near 2, confirming the successful upcycling of PA66 (Fig. S15, Table S1).

More importantly, the above process eliminates the need for catalysts, reducing the introduction of extraneous components and avoiding toxic heavy metal catalysts commonly used in melt polycondensations. We proposed a possible reaction mechanism based on density functional theory (DFT, Fig. 2E). All the single-point energies obtained from the optimized structures were corrected using a frequency scale factor of 0.9700 via Shermo 2.6.³⁵ We identified two possible reaction pathways (path 1 and path 2, Fig. 2E), in which the proton from the carboxyl group can protonate either the carbonyl oxygen or the NH group. In both pathways, the carboxylate radical subsequently attacks the carbonyl carbon and forms an anhydride bond. However, the activation energy for path 2 (44.3 kcal mol⁻¹) is significantly lower than that for path 1 (53.9 kcal mol⁻¹). Therefore, the CAE reaction is more likely to proceed via path 2. Notably, previous reports demonstrating the involvement of carboxylic acid in amide hydrolysis leading to anhydride formation also support our proposed mechanism.³⁶ The CAE exchange reaction closely resembles the traditional HEE reaction (or transesterification), which is widely used in polyester synthesis (Fig. S16).³⁷ Calculations show that the activation energy for transesterification is 51.1 kcal mol⁻¹, similar to that of path 1 but much higher than path 2 in CAE. Given the broad use of HEE, CAE is also expected to be highly efficient and promising for wide application. The proposed reaction process, as illustrated in Fig. S17, involves the nucleophilic attack of the carboxyl group on the amide and then forms the anhydride in situ. The resulting amino group may then undergo further reaction with the acid anhydride to produce a new amide linkage, or alternatively, it can combine with the carboxylic acid present in the system to form an amide bond. This occurs due to the higher concentration of carboxylic acid compared to the amide bonds.

2.3. Properties of the copolymer

The melting temperatures (T_m) of all PA66HDAs gradually decreased with the increase of HDMA content by differential scanning calorimetry (DSC), indicating a more flexible molecular chain structure (Fig. S18). The gradual disappearance of the crystal peak at 935 cm⁻¹ in the FTIR spectrum further confirms that the copolymer has transitioned to an amorphous state (Fig. S4). Thermogravimetric analysis (TGA) indicates an increase in the decomposition temperatures of PA66HDAs (Fig. 3A, Table S3), which indirectly confirms the presence of cyclic polymer components within the polymer matrix. Specifically, T_d,10% values for PA66 and PA66HDA1.0 were found to be 384 °C and 426 °C, respectively. Given that cyclic polymers lack end groups, they exhibit superior thermal stability compared to linear polymers.^38–40 Furthermore, PA66HDAs demonstrate exceptional toughness properties with the increased introduction of HDMA (Fig. 3B). Specifically, this was evidenced by a high level of ductility as indicated by an elongation at break of 722.6% and a breaking stress of approximately 34.5 MPa for PA66HDA1.0. The breaking strength of the PA66HDAs synthesized in this study surpasses that of commercially available PBAT, LDPE, and PBS (Fig. 3C). Meanwhile, the toughness of PA66HDAs far exceeds that of LDPE, PBT, PBS, and PBAT. Moreover, PA66HDAs also exhibit excellent processability, allowing for the creation of a wide range of intricate shapes. We present a straightforward processing technique (Fig. 3D, SI Movie S1). Initially, the material solution was applied onto various substrates such as glass dishes and beakers. Subsequently, water was introduced to facilitate uniform shape formation and then the material could be effortlessly removed in diverse configurations. Notably, water serves as an environmentally friendly, readily available, and cost-effective solvent in these processing methods that hold great potential for large-scale applications. Due to its excellent plasticity, PA66HDAs can be easily processed into various shapes, such as thin cups and windmills (Fig. 3D and E). PA66HDAs also exhibit a wide melt processing temperature range, which results from their relatively high thermal decomposition temperature compared to conventional polyamides (e.g. PA66, PA6) and polyesters (Tables S3 and S4). Meanwhile, the light transmittance of PA66HDA1.0 is significantly superior to that of the PA66 film and comparable to that of glass in the visible wavelength range (364–750 nm, Fig. S19).

Fig. 3 Characterization of material properties. (A) TGA of PA66 and PA66HDAs. (B) Mechanical properties of PA66HDAs. (C) Comparison of the mechanical properties of PA66HDAs with several commercial polymers. LDPE, PBT, PBS and PBAT as reported in ref. 25. (D) The film cup of PA66HDA0.7 prepared via solvent casting. The process involved casting the solution onto the interior surface of a beaker, followed by demolding using water. The resulting film cup exhibited a certain water-carrying capacity. (E) Shape memory behavior of the windmill prepared using PA66HDA0.7 reverts to a flat shape through heating. (F) Mechanism diagram of thermal response shape memory behavior through heating and cooling. (G) Images of the PA66HDA0.7 fusion-spinning fiber, a PA66HDA0.7 single fiber with a diameter of 0.2 mm supporting a 100 g weight, and a SEM image of the PA66HDA0.7 fiber, scale bar = 50 μm. (H) Three-dimensional cross-linked networks and their self-deformability in solvents. A dimeric acid (DA) containing double bonds was utilized as an alternative to HDMA and PA66 in the polycondensation process to synthesize PA66DA0.7, which was subsequently cross-linked via a click reaction.

Additionally, PA66HDAs exhibit remarkable shape memory capability (Fig. 3E, Fig. S20, SI Movie S2). Once heated to shape and rapidly cooled to room temperature, the alkyl chains from the HDMA units quickly aligned accordingly. As the temperature increased again, these alkyl chains coiled back into a thermodynamically stable, spring-like conformation, enabling the material to exhibit macroscopic shape memory behavior (Fig. 3F). Dynamic mechanical analysis (DMA) reveals the precise controllability of T_g in PA66HDAs, ranging from −1 to 30 °C (Fig. S21–S27, Table S3). This tunability ensures that the material can satisfy the stringent requirements of diverse applications. Expect that, PA66HDA0.7 can undergo fusion spinning to obtain micron-sized silk (200 μm) with exceptional mechanical properties, capable of effortlessly supporting weights up to 100 g (Fig. 3G, Fig. S28). These experiments demonstrate that the polymers have achieved superior thermal and mechanical properties, highlighting the extensive potential applications. Meanwhile, the incorporation of double bonds in recycled materials enables further modifications, thereby expanding their potential applications (Fig. 3H, Fig. S29). For example, PA66DA0.7 containing double bonds was synthesized via the melt polycondensation of long-chain unsaturated dicarboxylic acid (DA) with PA66. Subsequently, a covalently crosslinked PA66DA0.7-HDT was generated through a click reaction between the double bond and thiol.⁴¹ PA66DA0.7 was soluble in chloroform prior to crosslinking, yet it demonstrated swelling behavior exclusively after the crosslinking process. Notably, the PA66DA0.7-HDT film can be self-manipulated and self-rolled within the solvent through solvent-induced stimuli (SI Movie S3). Upon gradual removal of the solvent, the film autonomously reverted back to its original state, showcasing self-propelling characteristics akin to a robotic system (SI Movie S4).

2.4. Closed-loop recycling approach and biodegradation

The resulting PA66HDAs can undergo quantitative chemical recycling under acidic conditions or degrade in compost, utilizing anhydride bonds as cleavage points (Fig. 4A). A closed-loop recycling approach was demonstrated through the polymer-to-polymer recycling of 10.0 g of PA66HDA0.5 in 15.0 g of HDMA. Complete depolymerization occurred at 200 °C within 3 hours without extra solvents or catalysts. Moreover, the depolymerization products exhibit excellent solubility in a range of conventional solvents, particularly ethanol (ET), which is both a safe and environmentally benign solvent (Fig. 4B). This characteristic facilitates a straightforward method for separating mixed plastics through dissolution. MALDI-TOF-MS confirmed the existence of HDMA or AA as the terminal dicarboxylate-monomer (P1, P2, P3) as the main depolymerization product (Fig. 4C). PA66HDA0.5 was resynthesized through melt polycondensation using these dicarboxylate-terminated monomers to verify the effectiveness of closed-loop recycling. The results demonstrated that PA66HDA0.5 maintained a consistent chemical structure and mechanical and thermal properties over all three cycles (Fig. S30–S33). Moreover, this monomer can serve as a precursor for repolymerization to form the primary oligomer or other novel polymers. For example, the dicarboxylate-terminated monomer can undergo polycondensation with diols to form polyester-amide. When the polymer reaches a non-recyclable state, it can be designed to degrade under specific conditions. Notably, although the hydrophobicity and water absorption of PA66HDAs are lower than those of PA66, they retain compostable degradability due to the presence of anhydride bonds in the polymer chain (Fig. S34 and S35, Table S4, Fig. 4D). Composting experiments further demonstrate that a higher HDMA content leads to significantly enhanced degradation rates, with complete decomposition achieved under certain conditions (Fig. 4D). For instance, PA66HDA1.0 completely decomposed under composting conditions within approximately 20 weeks. In contrast, PA66 exhibited minimal changes during the composting process. SEM reveals signs of degradation on the surface of PA66HDA0.7, including holes and corrosion, while the surface morphology of PA66 remained largely unchanged (Fig. S36). Meanwhile, PA66HDAs with increasing HDMA content showed more pronounced changes in [η] and weight, providing further evidence for degradation (Fig. 4E, S37). The occurrence of degradation was also confirmed by MALDI-TOF-MS analysis, which showed a molecular weight of approximately 500 Da, including HDMA as a degradation product and a mixed product consisting of a single PA66 unit attached to HDMA (Fig. S38A). Based on the above analysis, we propose a potential degradation pathway. First, the anhydride bonds in PA66HDAs undergo hydrolysis under composting conditions, forming a carboxyl-terminated low-molecular-weight oligomer. These carboxyl groups then release hydrogen ions, which catalyze the hydrolysis of amide bonds, producing even lower-molecular-weight compounds that can be further decomposed by microorganisms (Fig. S38B). These findings demonstrate the biodegradation of PA66HDAs, in contrast to the non-degradable characteristics of PA66. This aligns with existing literature, which suggests that the degradation of polyamides, particularly PA66, is nearly impossible in natural environments.⁴² Meanwhile, PA66HDAs can serve not only as an effective substitute for commercial non-degradable polymers, but also address the environmental pollution issues stemming from non-degradable polymers and their low recycling efficiency. This work can serve as a valuable reference for the design of a degradable polymer by modifying chemical bonds, thereby making polyamide-based products biodegradable.

Fig. 4 Sustainable properties of the material. (A) Illustration of closed-loop PA66HDA recycling and environmental degradation. (B) Schematic representation illustrating the separation of depolymerized products from a mixture of plastics. ET: ethanol; DCM: dichloromethane; TCM: trichloromethane; DMF: N,N-dimethylformamide. (C) MALDI-TOF-MS of the depolymerization products. (D) Photographs of composting degradation. (E) Changes in the intrinsic viscosity ([η]) of PA66 and PA66HDAs under composting conditions.

3. Conclusion

A sustainable polymer with a controllable structure has been successfully synthesized from waste PA66 via a facile catalyst-free and solvent-free melt polycondensation process. By incorporating a novel carboxyl–amide exchange mechanism during melt polycondensation, the resultant polymer features anhydride linkages without requiring any additional catalysts or solvents. This copolymer exhibits multifunctionality, markedly enhanced thermal and mechanical properties, significantly surpassing those of commercial plastics. Furthermore, subsequent functionalization via the introduction of double bonds imparts a range of unique characteristics, such as the ability to autonomously move in solvents akin to robotic behavior. Additionally, the copolymers exhibit excellent degradation performance and closed-loop recycling capability, which can alleviate existing environmental pollution concerns. This work not only holds immense potential for wide-ranging applications, but also introduces an innovative approach to polymer recycling, thereby significantly contributing towards future design of advanced organic materials.

Author contributions

Conceptualization: H. J. Z., Q. Q. C, and W. Y. L.; methodology: H. J. Z., Q. Q. C., W. Y. L., X. W. C., and S. H. N.; investigation: H. J. Z., S. H. N. and M. Y. G.; funding acquisition: H. J. Z., Q. Q. C., W. Y. L., and X. W. C; project administration: H. J. Z., and Q. Q. C; supervision: H. J. Z., Q. Q. C., W. Y. L., and X. W. C.; writing – original draft: H. J. Z., Q. Q. C., S. H. N., and M. Y. G.; writing – review & editing: H. J. Z., Q. Q. C., W. Y. L., and X. W. C.

Conflicts of interest

The authors declare that they have no competing interests.

Data availability

All data are available in the main text or the supplementary information (SI). Supplementary information is available. Supplementary information includes the raw materials, synthesis procedures, characterization data, and DFT calculation results, along with tables (4) and figures (38). See DOI: https://doi.org/10.1039/d5gc03424h.

Acknowledgements

This work was supported by the Zhejiang Provincial Natural Science Foundation of China (No. LQN25E030006), the National Natural Science Foundation of China (No. 22405242, 52403130), Zhejiang Sci-Tech University (No. 23212157-Y, 25212140-Y), the Guangdong Basic and Applied Basic Research Foundation (No. 2023A1515110526), the Special Fund for the Sci-Tech Innovation Strategy of Guangdong Province (No. STKJ202209079), and Chemistry and Chemical Engineering Guangdong Laboratory (No. 2111017).

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Footnote

† These authors are considered as co-first authors.

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