Yu
Zhou
a,
Jiaxing
Zhang
*ab,
Bowen
Shen
a,
Wenyan
Ba
f,
Shengping
You
*ac,
Mengfan
Wang
de,
Rongxin
Su
abf and
Wei
Qi
*ab
aChemical Engineering Research Centre, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China. E-mail: zhangjiaxing7137@tju.edu.cn; ysp@tju.edu.cn; qiwei@tju.edu.cn
bState Key Laboratory of Chemical Engineering and Low-Carbon Technology, Tianjin University, Tianjin 300072, PR China
cBeijing Meihao Biotechnology Co., Ltd, China
dSchool of Life Sciences, Faculty of Medicine, Tianjin University, Tianjin 300072, P. R. China
eYuantian Biotechnology (Tianjin) Co., Ltd, China
fTianjin Key Laboratory for Marine Environmental Research and Service, School of Marine Science and Technology, Tianjin University, Tianjin 300072, China
First published on 8th April 2025
Waste textile recycling is hampered by their complexity and high crystallinity, leading to substantial environmental pollution and resource wastage. Although enzymatic polyester recycling has promising substrate specificity, natural enzymes lack stability and activity on high-crystallinity substrates, and thus are limited in industrial viability. Herein, in light of the degradation mechanism of polyethylene terephthalate (PET) hydrolyase, we established a simple ethylene glycol (EG) selective catalytic system, possessing a nucleophilic attack mechanism on ester bonds similar to that of the catalytic serine residue in the natural enzyme. The EG catalytic system achieved efficient PET degradation, with a PET conversion rate of 99.63% and a terephthalic acid (TPA) yield of 95.46% under mild conditions (90 °C for 1 h). Meanwhile, the insolubility of the degradation products in EG catalytic system facilitates their separation, allowing the reaction system to be recycled at least five times. Additionally, first-principles molecular dynamics simulations revealed that the EG catalytic system generates active species EG−, which follows the same reaction mechanism as natural enzymes and has a lower energy barrier than alkaline hydrolysis. Notably, the process maintains effective hydrolysis capability at a 100 L scale, with selective degradation and decolorization, underscoring its industrial potential for PET degradation from colored composite textiles. Overall, this work offers a sustainable, efficient, and practical solution to the challenge of textile waste recycling.
Green foundation1. This study established a simple EG catalytic system for PET degradation, achieving the rapid decomposition of waste PET fibers under mild conditions (90 °C, 1 h) with a high PET conversion rate of 99.63% and a TPA yield of 95.46%. The reaction system can be directly recycled for subsequent PET degradation five times and enables selective degradation and decolorization of colored composite fibers.2. The process is green and scalable, free of toxic reagents or heavy metal ions, with its effectiveness comfirmed by both theoretical analysis (first-principles molecular dynamics simulation) and experimental validation. It exhibits excellent PET degradation performance at a 100 L scale and efficiently recycles real post-consumer PET waste, demonstrating significant potential for industrial applications. 3. EG produced during PET degradation is directly recycled within the EG catalytic reaction system, avoiding the high energy consumption of distillation separation. Meanwhile, TPA also has the potential to be upcycled into high-value products, and future research could focus on enhancing TPA valorization and expanding the applications of this process. |
Currently, the primary technologies for PET recovery include the following: (1) mechanical recycling, which involves PET melting and respinning processes, but results in relatively low and heterogeneous intrinsic viscosity values, thereby reducing the quality of recycled PET products.7,10,11 Additionally, mechanical recycling is limited to well-separated, high-purity waste plastics and faces challenges in recycling blended textiles.12 (2) Chemical recycling, typically encompassing hydrolysis, aminolysis, methanolysis, and glycolysis,13,14 which is ineffective for degrading complex mixtures and requires time-consuming and energy-intensive separation and purification processes.15 To address this issue, the metal-catalyzed oxidation process has been used to depolymerize mixed PET into oxygenated compounds, which are then biologically converted into value-added chemicals, thereby eliminating the need for mixed plastic waste separation.16 Moreover, chemical recycling often involves harsh conditions, such as high temperatures, high pressure, strong acids, strong bases, or expensive industrial catalysts,17 leading to high energy costs and the generation of hazardous waste. Further, the PET acetolysis process has been developed, which enables the continuous precipitation of terephthalic acid (TPA) from acetic acid, thereby avoiding the additional consumption of strong acids and bases required for TPA acidification.18 (3) Biological recovery utilizing PET hydrolase, which presents an eco-friendly solution characterized by mild reaction conditions and substrate specificity. However, biocatalysts suffer from poor stability during recycling and struggle to degrade highly crystalline substrates,19 thus hindering their industrialization.
Meanwhile, highly selective transformation of original building blocks is essential for effectively constructing target high-value products.20 However, textile fabrics are typically designed for enhanced performance by incorporating multiple fibre composites. For example, interwoven cotton–polyester blends are widely used in clothing and textile materials because they combine the comfort and breathability of cotton with the strength and abrasion resistance of PET.3 Consequently, the heterogeneity of blended fabrics and the lack of efficient sorting and separation techniques hinder textile recycling,21,22 ultimately leading to downcycled products.23 Notably, while PET hydrolases offer advantages such as mild reaction conditions and high substrate specificity, their industrial application in PET polyester recycling is hindered by low stability, short shelf life, high production costs, and limited activity on highly crystalline substrates.7,24–27 To solve this problem, numerous artificial biocatalysts have been developed as enzyme mimics, aiming to replicate the catalytic function of natural enzymes, while mitigating their limitations.28 Enzyme mimics exhibit simpler structures, enhanced stability, and greater efficiency than native enzymes, making them robust and potentially more suitable alternatives for enzymatic catalysis in industrial applications.28,29 However, the residual metal catalysts in PET waste can accelerate transesterification and polycondensation reactions, leading to chemical inhomogeneity in the recycled PET and altering its melt flow properties.30
Moreover, balancing cost and efficiency is a critical consideration for the industrial application of PET recycling. Before the valorisation of PET into high-value products, PET must be depolymerized into its monomers, ethylene glycol (EG) and TPA.31 The TPA generated from PET degradation can be recovered from the reaction solution via acidification.31 However, the separation of EG is hindered by its high viscosity, water solubility, and boiling point of 197.6 °C, which results in elevated energy consumption during the separation process and, consequently, increases the overall production costs in PET recycling.32
Herein, considering the PET hydrolysis mechanism of natural PET hydrolase, we aim to develop a recycling process for highly crystalline PET fibre that enables selective degradation and demonstrates potential for industrial application (Fig. 1). Firstly, due to the high energy consumption associated with separating the PET degradation product EG, we sought to utilize activated EG as a nucleophile in the PET degradation reaction, thereby eliminating the need for a separate EG recovery step. Importantly, this EG catalytic system demonstrated efficient degradation without the need for heavy-metal catalysts or toxic organic reagents and could be directly separated from the PET degradation products for reuse, thereby reducing the consumption of strong acids and bases. Further, to elucidate the PET degradation mechanism facilitated by the EG catalytic system, we employed proton nuclear magnetic resonance (1H NMR) spectroscopy and first principles metadynamics simulations to gain deeper insight into the PET degradation pathway. Finally, we assessed the industrial application potential of the EG catalytic system through a 100 L scale-up and the investigation of real textile waste. Additionally, the system's substrate specificity was validated across polyester fibres, cotton, and nylon, thereby confirming its selective catalytic properties. Thus, this study proposes a high-efficiency, recyclable, eco-friendly, and practical process for the hydrolysis of PET-based blended fabrics, with the potential to accelerate the industrialization of textile waste recycling.
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Fig. 1 PET hydrolysis mechanism of the EG catalytic system, in connection with the enzymatic catalytic mechanism. (a) Construction of the EG catalytic system based on the PET hydrolase mechanism, incorporating BhrPETase33 (gray, PDB ID: 7EOA), LCC-ICCG (an engineered leaf-branch compost cutinase,34 purple, PDB ID: 7VVE), IsPETase-PA35 (orange, PDB ID: 8J17), and TfCUT2![]() |
Considering the catalytic mechanism of PET hydrolase, we chose EG as the catalyst for two reasons: first, EG can function as a nucleophile after deprotonation, and second, it is both a component of PET and a degradation product, thus avoiding the introduction of impurities into the system. Therefore, we established an EG catalytic system with a NaOH component to explore the efficient degradation of PET. On the basis of the PET hydrolysis mechanism of the natural enzyme,40,41 the mechanism of PET degradation by the EG catalytic system is proposed in Fig. 1c. Initially, NaOH deprotonates EG to form EG−, which then interacts with the PET surface, forming substrate complexes (SC). The EG− then nucleophilically attacks the carbon atom of the carbonyl group in the PET unit (forming TI1), leading to the formation of bis(2-hydroxyethyl)terephthalate (BHET) and EG (AE). Subsequently, the free OH− nucleophilically attacks the carbonyl group in BHET (forming TI2) and degrades the ester bonds, thereby finally degrading BHET into product complexes (PC).
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Fig. 2 Feasibility analysis of the EG catalytic system for PET degradation. (a) Comparison of the PET degradation efficiency between the EG catalytic system (100 mL reaction system, molar ratio of EG to NaOH 6![]() ![]() ![]() ![]() ![]() ![]() |
Considering the high energy consumption and cost associated with the separation of water-soluble EG, we investigated the feasibility of directly separating and recycling the EG catalytic system from the PET degradation product TPA (Fig. 2d). Interestingly, Na2TPA dissolved in EG (7.49 g L−1 at RT, Fig. S3b,† blue line) but was almost insoluble in the EG catalytic system (Fig. S3a†), thereby supporting the viability of direct separation of the reaction solution for reuse and TPA product recovery. This phenomenon could be attributed to the common-ion effect, in which the addition of a large amount of Na+ (from NaOH) substantially reduces the solubility of Na2TPA in EG catalytic system. Importantly, the excess NaOH in the EG catalytic system was directly recycled with EG, reducing the need for acid to neutralize the excess NaOH in the degradation system. This reduced the addition of strong acids and bases and minimized the generation of salt-containing wastewater (Fig. 2d). However, the traditional alkaline hydrolysis of PET presents difficulties in directly separating the reaction system from its degradation products, leading to excessive use of strong acids and bases and the generation of waste reaction solutions, which contribute to secondary pollution (Fig. 2d). As shown in Fig. 2e, the EG catalytic system reduced H2SO4 consumption by 12-fold, as the amount of H2SO4 required to neutralize excess alkali was minimized. Additionally, wastewater production was reduced by 10-fold compared to the alkaline hydrolysis system (see the calculation process in ESI†), thereby reducing environmental impact and lowering degradation costs. Furthermore, the Na2TPA separated from the EG reaction system must be dissolved in water to separate it from the unreacted substrate. Since Na2TPA has high solubility in water (Fig. S3b,† yellow line), only a small amount of water is required for complete dissolution. Simultaneously, the insolubility of TPA in water was confirmed, ensuring efficient recovery (Fig. S3a and S3b,† black line). Notably, the EG produced during PET degradation was directly reused as the reaction system, thereby avoiding the energy-intensive process of EG distillation and recovery. The EG catalytic system was successfully recycled at least five times (Fig. 2c), while maintaining excellent PET conversion (>94%) and TPA yield (>93%). However, since the reaction between EG and NaOH generates a small amount of water, continuous recycling of the EG catalytic system may lead to gradual water accumulation, which could potentially affect PET degradation efficiency and TPA separation over time. Therefore, it is worth further investigation into effective and low-cost dehydration methods in the future.
Furthermore, previous studies have indicated that a low solid-to-liquid ratio leads to higher energy consumption during PET treatment and increases carbon footprints,43 which is detrimental to the economy and the environment in the context of PET textile recycling. Therefore, we also explored the influence of substrate concentration on PET textile degradation. According to Fig. 2f, under lower substrate concentrations (30 and 50 g L−1), the EG catalytic system achieved over 95% PET conversion within 1 h. However, PET degradation efficiency decreased as the substrate concentration increased. Nevertheless, when the reaction time was extended to 1.5 h, the PET conversion at higher substrate concentrations (70 and 100 g L−1) rose to 95%, with a high TPA yield (>90%). Therefore, the EG catalytic system demonstrates efficient PET degradation at high substrate concentrations without the need for additional heavy-metal catalysts or toxic organic reagents (Table S1†), highlighting its sustainable and energy-saving features and making it suitable for industrial scale-up.
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Fig. 3 Mechanistic analysis of the EG catalytic system for PET degradation. (a) PET degradation process via the EG catalytic system; (b and c) NMR analysis of the (b) EG/NaOH system and (c) pure EG; (d) relative reaction energy pathway of the EG catalytic system compared to the alkaline hydrolysis system; (e and f) PET degradation mechanism in the EG catalytic system, explored via meta-FPMD. The free energy landscape is depicted using colour gradients, where darker shades indicate higher population densities. The reaction path is shown by scatter points, starting at blue points, passing through white points, and finally reaching red points at 3000 fs; (inset) conformation of the TI state during the reaction. Similar methods for reaction pathways visualization have also been applied by Gao and co-workers.47,48 |
To further understand the PET degradation mechanism catalysed by the EG catalytic system, we performed FPMD simulations to elucidate the precise details of the PET-EG− degradation process. As the catalytic process may be time-consuming, we introduced the metadynamics enhanced sampling method into FPMD (meta-FPMD) to accelerate the simulation progress. Utilizing two key distances d1 and d2 (Fig. 3e and f), as collective variables (CVs), we monitored the breaking of ester bond and formation of new bond during each reaction step. Specifically, d1 represents the distance between the carbonyl carbon and the oxygen of the ester bond in the substrate, reflecting bond cleavage, while d2 corresponds to the distance between the nucleophilic group (the hydroxyl oxygen of EG− in step 1 and hydroxide ion oxygen in step 2) and the carbonyl carbon of the ester bond in the substrate, indicating the formation of a new bond. Based on these CVs, we analysed the free energy landscape (FEL) of the catalytic systems within a 3000 fs simulation. The FEL of the ester exchange reaction from the SC to the AE intermediate, reconstructed from the meta-FPMD simulation (Fig. 3e and Movie S1†), revealed two energy minima and one key TI1 state. The energy path represents a reaction trajectory from SC to AE, passing through TI1 and terminating in the AE state. Similarly, the FEL transition from AE to PC proceeds through TI2, eventually forming the PC. This pathway aligns with that reported for natural enzymes degrading PET, such as LCC-ICCG41 and its mutant.49 The results demonstrate that the active species EG− degrades PET ester bonds through a reaction mechanism similar to that of natural enzymes, consistent with experimental results (Fig. 2a). We also conducted meta-FPMD on the alkaline hydrolysis system and calculated the FEL for the two-step reactions (Fig. S5†). Compared to the EG catalytic system, the alkaline hydrolysis system exhibited a higher energy barrier of the SC–AE reaction (Fig. 3d), resulting in lower PET degradation efficiency than that observed within the EG catalytic system.
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Fig. 4 Practical application of the EG catalytic system. (a) 1 L and (b) 100 L scale-up tests of the EG catalytic system for PET fibres degradation. |
Furthermore, the EG catalytic system was applied to degrade waste curtains and scarves recovered from waste stations, which mainly consist of PET polyester fibres with attached dye. As shown in Fig. 5a, the system achieved efficient degradation of PET textiles, with curtains and scarves reaching PET conversion rates of over 90%. However, the degradation efficiency for real textiles was slightly lower than that of pure PET polyester, prompting further analysis of scarf degradation residues using FTIR (Fig. S6a†). Notably, the FTIR spectrum of the scarf degradation residues revealed distinct characteristic peaks compared to those of pure PET polyester, indicating that the undegraded components belonged to other materials present in the blended fabric. This result demonstrated the potential selective degradation capability of the EG catalytic system. Meanwhile, as a safe organic solvent, EG exhibits good affinity for organic dyes, allowing them to be dissolved and removed from the degradation products (Fig. 5b). Activated carbon can further eliminate residual colour, thereby enhancing the quality of the recovered TPA. Additionally, the chemical structure of the obtained TPA was verified using FTIR spectroscopy and XRD (Fig. 5c and d), which matched that of commercially purified TPA. Moreover, HPLC analysis (Fig. S6b†) confirmed that the recovered TPA possessed high purity (>99%) and was free of oligomers. These results strongly demonstrate the efficiency of the EG catalytic system in degrading real waste textiles. However, as the EG catalytic system is continuously recycled, dyes may also accumulate over time; thus, more efficient and cost-effective decolorization methods should be considered in the future.
Given that textiles often consist of mixed fibres, selective degradation plays a crucial role in obtaining a single degradation product, thereby simplifying further recycling. Therefore, the substrate specificity of the EG catalytic system was further explored using nylon, cotton, and mixtures of nylon/cotton with PET polyester as the degradation substrates (Fig. S7†). Notably, Fig. 5e shows that the degradation of cotton and cotton/polyester fibre mixtures resulted in the recovery of more than 95% of cotton, while achieving 100% PET conversion. Similarly, the degradation of nylon and nylon/polyester fibre blends recovered 100% of the nylon and also achieved 100% PET conversion. Furthermore, FTIR analysis confirmed that raw and residual nylon/cotton fibres exhibited the same characteristic peaks (Fig. 5f). The degradation of nylon involves the cleavage of amide bonds, which exhibit low reactivity toward nucleophilic attack on the carbon due to the partial double bond character of the C–N bond, caused by delocalization between the nitrogen lone pair and the CO bond.50 SEM images in Fig. 5g also showed that raw and degraded nylon/cotton fibres and their residues exhibited similarly smooth surfaces, further confirming the EG catalytic system's remarkable selective degradation ability. Meanwhile, the degradation of glycosidic bonds in cotton requires a proton donor and a nucleophile/base group;51 however, the EG catalytic system lacks a proton donor, which explains why nylon and cotton were selectively resistant to degradation under mild reaction conditions. Moreover, the saline wastewater generated during TPA acidification was ingeniously repurposed as a buffer solution for cellulases that were used to degrade the remaining cotton after PET degradation in the mixed fabrics. After seven days of enzymatic hydrolysis of waste cotton, a glucose yield of 77.5% was achieved (Fig. S8†), thereby further facilitating the optimal utilization of waste resources.
Overall, the EG catalytic system proposed in this study combines the dual functionalities of PET degradation and dye decolorization, enabling the selective degradation of PET fibres from blended textiles. Furthermore, efforts to reduce the use of strong acids and bases and to optimize the decolorization of products will help enhance the environmental sustainability and efficiency of the process. This process facilitates the effective recovery and value-added utilization of waste textiles, making it a promising candidate for industrial-scale textile recycling.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5gc00248f |
This journal is © The Royal Society of Chemistry 2025 |