Sustainable recycling of polyester wastes using a coordinatively unsaturated Zn catalyst

Abstract

Closed-loop recycling of polyester waste offers a promising route for addressing the environmental burden of plastic pollution and enabling resource circularity. However, current polyester plastic recycling methods face limitations in industrial scalability and environmental impact. Here, we report the development of a low-cost, earth-abundant, coordinatively unsaturated defect–zinc oxide (d–ZnO) catalyst for the efficient depolymerization of various polyester wastes, including polyethylene terephthalate, biodegradable polyesters, post-consumer polyesters, and mixed polyester waste. Mechanistic investigations reveal that defects structure in the catalyst enhance the activation of oxygen and water molecules, generating nucleophilic species that drive ester bond cleavage via carbonyl activation. The robustness and scalability of our strategy were further validated through large-scale closed-loop recycling of polyester waste plastics. Life cycle assessment (LCA) and techno-economic analysis (TEA) demonstrate the environmental sustainability and economic viability of this process across multiple metrics, including human health, ecosystem quality, resource consumption, and production cost. This work offers a broadly applicable, sustainable, and practical solution for management polyester plastic waste.

Hydrolytic polyethylene terephthalate (PET) depolymerization typically employs harsh acid or alkali conditions. Here, the authors develop a high efficiency defect-zinc oxide nanosheet catalyst for the hydrolytic depolymerisation of PET to terephthalic acid in water under neutral conditions.

Introduction

Polyesters are synthetic polymers featuring ester linkages and are produced at a scale exceeding 70 million tons per year for use in packaging, bottles, and textiles^1,2. The widespread accumulation of polyester plastic waste has had profound impacts on the environment, marine ecosystems, and human health^3–5. Chemical depolymerization of polyesters into monomeric constituents via hydrolysis^6–9, alcoholysis^10–16, aminolysis^17,18 and hydrogenolysis^19–21 has been widely investigated. These processes offer a viable and scalable alternative to conventional end-of-life treatments, including mechanical recycling, incineration and landfilling^22–25. However, pyrolysis and hydrogenolysis typically yield complex mixtures with low product selectivity. In contrast, solvent-assisted depolymerization of polyesters enables controlled cleavage of ester bonds to selectively yield targeted monomers. For instance, polyethylene terephthalate (PET) waste can be hydrolyzed in neutral aqueous media to produce terephthalic acid (TPA), which is suitable for repolymerization into recycled PET.

Nevertheless, conventional hydrolysis of polyesters typically requires harsh conditions, including strongly acidic or basic media or high-pressure systems (Fig. 1a), leading to reactor corrosion, complex wastewater treatment, and elevated energy demands^26,27. Recent emerging strategies (including in situ alkaline hydrolysis, solvent-assisted hydrolysis, binuclear Zn-catalyzed hydrolysis, and acetic acid-mediated depolymerization) have improved PET conversion efficiency (Fig. 1b)^28–32. However, these methods often involve alkaline environments that necessitate post-neutralization and the use of mixed solvents, which complicate product separation and increase process energy intensity. Therefore, there remains a critical need for catalytic systems that enable selective polyester depolymerization under neutral, organic solvent-free conditions.

Fig. 1. Chemical recycling routes of polyester.

a Traditionally hydrolysis routes and b emerging hydrolysis routes. c Coordinatively unsaturated Zn catalyst-driven hydrolysis routes in this work.

Homogeneous catalysts, such as ionic liquids and metal salts have demonstrated high activity in polyester depolymerization^33–35. Recently, metal-based heterogeneous catalysts supported by auxiliary ligands have also been reported. For example, Dove and co-workers developed a highly stable bimetallic “acid salt” catalyst system capable of depolymerizing PET and polylactic acid (PLA), achieving activities up to 1.32 × 10⁵ g_PET mol_LZnK^–1 h^–1 at 180 °C³⁶. In addition, Wang et al. reported a tetranuclear titanium catalyst that exhibits exceptionally high activity for PET methanolysis, reaching 1.9 × 10⁷ g_PET mol_Ti^–1 h^–1 at a catalyst loading of only 0.005 mol%³⁷. However, ionic liquids require tedious purification, and residual metal ions from metal salts catalysts can hinder repolymerization. In contrast, heterogeneous catalysts offer superior stability and ease of separation, making them attractive for polyester recycling. Nevertheless, several key challenges persist: (I) limited interfacial contact due to poor multiphase mass transfer among solid polyester, water, and catalyst. (II) Polyester hydrolysis relies on activation of water molecules by catalytic sites to facilitate ester bond cleavage, underscoring the importance of enhancing the affinity and activation of water on catalyst surfaces. (III) Conventional approaches to improve heterogeneous catalyst activity focus on modifying particle size, coupling with ionic liquids, or employing porous materials. These mainly tune surface-active sites or increase external surface area, but can compromise catalyst–substrate mass transfer efficiency and reduce overall reactivity. To address these limitations, innovative catalyst designs and deeper mechanistic insights are essential for enabling efficient and green chemical recycling of PET waste.

In this work, we development of coordinatively unsaturated defect–zinc oxide (d–ZnO) nanosheets as an efficient heterogeneous catalyst for polyester hydrolysis (Fig. 1c). Under neutral aqueous conditions at 190 °C, d–ZnO enabled complete depolymerization of PET, affording terephthalic acid (TPA) in >99% selectivity and achieving a space–time yield (STY) an order of magnitude higher than prior reports. The presence of oxygen molecules (O₂) was found to be essential for enhancing catalytic activity, while the combination of defects in d–ZnO played a pivotal role in selective C–O bond cleavage. In situ infrared spectroscopy and density functional theory (DFT) calculations revealed that defect sites facilitate the activation of water and O₂ to generate nucleophilic species, which selectively attack ester carbonyl groups and promote bond scission. The catalyst demonstrated remarkable tolerance to additives and impurities across a wide range of polyester feedstocks. Moreover, we demonstrate the successful repolymerization of recovered TPA into recycled PET, enabling a closed-loop recycling process. Life cycle assessment (LCA) and techno-economic analysis (TEA) comparisons against glycolysis, methanolysis, hydrolysis, and petroleum-based production systems confirm that this neutral hydrolysis approach offers superior environmental and economic performance. This work presents a viable and scalable approach for advancing circularity in polyester recycling.

Results

Characterization of catalysts

To enable catalytic polyester depolymerization, we synthesized coordinatively unsaturated defect–zinc oxide (d–ZnO) nanosheets via an _L-ascorbic acid-assisted etching strategy. Scanning electron microscopy (SEM) images demonstrate that the d–ZnO retain the overall morphology and grain size of the pristine ZnO (p–ZnO) nanosheets (Fig. 2a, b). Upon addition of _L-ascorbic acid, the sample color gradually changed from white to light yellow (insets of Fig. 2a, b), suggesting structural changes. The strong interaction between _L-ascorbic and ZnO during the etching process promotes the generation of lattice defects. Although the X-ray diffraction (XRD) patterns of d–ZnO and p–ZnO exhibit similar overall features (Supplementary Fig. 1), noticeable changes in the intensity of the (002) and (110) reflections suggest the introduction of a significant number of lattices vacancies³⁸.

Fig. 2. Structural characterizations of catalysts.

a, b SEM images of p-ZnO and d-ZnO structures. c Aberration-corrected scanning transmission electron microscopy image of d–ZnO. d Normalized X-ray absorption near-edge structure spectra of ZnO, Zn foil, and d-ZnO. e Normalized Fourier transform extended X-ray absorption fine structure spectra of ZnO, Zn foil, and d–ZnO. f ESR curves of p-ZnO and d–ZnO structures.

Aberration-corrected scanning transmission electron microscopy (AC-STEM) revealed locally disordered lattice regions within d–ZnO nanosheets, consistent with the presence of unsaturated Zn coordination sites (Fig. 2c)³⁹. Zn K-edge X-ray absorption near-edge structure (XANES) spectra show a shift to lower energy for d–ZnO compared to Zn foil and ZnO, indicative of a lower oxidation state of Zn in the defective lattice (Fig. 2d)⁴⁰. Moreover, Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) analysis confirmed that the local coordination environment around Zn atoms in d–ZnO remains close to that of Zn–O bonding (Fig. 2e). Electron spin resonance (ESR) spectroscopy was employed to assess unpaired electrons associated with defect sites (Fig. 2f). The signal at g = 2.002 is assigned to electron trapping by oxygen vacancies (O_V), while the signal at g = 1.950 corresponds to electron trapping by zinc vacancies (Zn_V)^39,41. Compared to p–ZnO, d–ZnO displayed substantially enhanced O_V and Zn_V signals, corroborating the L-ascorbic acid-induced formation of surface lattice defects.

Catalytic depolymerization of PET and real polyester waste over d–ZnO

To investigate the catalytic depolymerization of polyester, polyethylene terephthalate (PET; number-average molecular weight (M_n) = 32 kDa, Supplementary Fig. 2) was used as the model substrate. PET was reacted with the catalyst and water in a high-temperature and high-pressure reactor at 190 °C for 6 h. After cooling to room temperature, the products were collected and analyzed by nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC) to determine composition and purity.

We found that the reaction atmosphere plays a crucial role in determining product selectivity. Depolymerization reactions were conducted under nitrogen and varying concentrations of O₂/N₂ atmospheres using PET (M_n = 32 kDa) as the substrate. Under a nitrogen atmosphere at 190 °C and neutral pH, p–ZnO exhibited minimal catalytic activity, yielding <5% PET conversion after 6 h. The product distribution included only 0.7% terephthalic acid (TPA), 4.3% oligomers (Fig. 3a, Supplementary Fig. 4, and Supplementary Table 1). Upon increasing the oxygen concentration from 1% to 10%, a marked decrease in byproduct formation was observed alongside a corresponding improvement in TPA selectivity, indicating the critical role of molecular oxygen in governing product distribution. At a 20%O₂/80% N₂ or under ambient air, complete PET depolymerization was achieved within 6 h, affording a TPA yield of 99%, and <1% yield of oligomers (Fig. 3a, Supplementary Figs. 5–12, and Supplementary Table 1). These findings unequivocally demonstrate that molecular oxygen is indispensable for promoting the selective hydrolysis of PET over p–ZnO.

Fig. 3. Catalytic depolymerization of PET and real polyester waste over d–ZnO.

a Selectivity of products from catalytic hydrolysis of PET in different reaction conditions. Conditions: PET (1.0 g), p–ZnO (10 mg), N₂/O₂ (1/0, 100/1, 10/1, 5/1, v/v, 0.1 MPa) or air (0.1 MPa), H₂O (8 mL), 190 °C. b, c Selectivity of products from catalytic hydrolysis of PET under different catalysts. Conditions: PET (1.0 g), catalyst (10 mg), air (0.1 MPa), H₂O (8 mL), 190 °C. d Comparison of activities of d–ZnO with other catalysts for PET hydrolysis. The activities values of other catalysts are calculated based on the amount of catalyst used reported in the literature. e Product selectivity in the hydrolysis of polyester waste. Conditions: polyester waste (PBT, PTT, PBAT, PBS, PBA, PCL, 1.0 g), d–ZnO (10 mg), H₂O (8 mL), 190 °C, 6 h. polyester waste (PLA, PC, 1.0 g), d–ZnO (10 mg), H₂O (8 mL), 170 °C, 6 h. HHA: 6-hydroxyhexanoic acid. BPA bisphenol A. LA: lactic acid. BDO: 1,4-butanediol. f Photographs and product selectivity in the hydrolysis of mixed polyester waste. Conditions: polyester waste (1.0 g), d–ZnO (10 mg), H₂O (8 mL), 190 °C, 6 h.

To further optimize the reaction, the effects of temperature, reaction time, and catalyst dosage were systematically investigated (Supplementary Table 2), achieving >99% TPA yield with 99% purity under optimal conditions. The d–ZnO achieved substantially higher PET conversion and TPA selectivity compared to the p–ZnO and commercial ZnO (Fig. 3b, Supplementary Figs. 13 and 14, and Supplementary Table 1), emphasizing the critical role of defect sites functionality in catalytic performance. Moreover, a series of defect–rich metal oxides were synthesized using analogous synthetic protocols. Their catalytic activities exhibited a strong correlation with the intrinsic Lewis acidity of the constituent metal ions, following the trend: d–ZnO > MgO > CaO > Co₃O₄ > MnO₂ > Fe₂O₃ > Fe₃O₄ > Al₂O₃ > TiO₂ > ZrO₂ > Nb₂O₅ > CeO₂ > SiO₂ (Fig. 3c, Supplementary Figs. 15–38, and Supplementary Table 3), which closely aligns with the relative Lewis acid strengths of the corresponding divalent metal ions.

Under optimized conditions, PET waste was completely depolymerized over d–ZnO via hydrolysis at 190 °C within 6 h. The process afforded a space-time yield (STY) of 56 g_TPA h^–1 g_cat^–1, which is an order of magnitude higher than those reported for conventional hydrolysis catalysts (Fig. 3d, Supplementary Table 4). STY was calculated as the mass of product obtained per gram of catalyst per hour. Considering variations in previous reports, hydrolysis conditions were classified into four categories: (1) supercritical CO₂, (2) acidic hydrolysis, (3) alkaline hydrolysis, (4) neutral hydrolysis, and (5) hydrogenolysis. Under neutral aqueous conditions (≥190 °C), d–ZnO exhibited significantly higher catalytic activity than previously reported systems. Even in comparison to supercritical CO₂ and acidic or basic conditions, d–ZnO outperformed in catalytic efficiency. Notably, while acidic and alkaline systems can enable PET degradation at lower temperatures, they typically operate under harsh pH conditions (pH <1 or pH > 10), raising environmental and operational concerns.

We further evaluated the depolymerization performance of d–ZnO across a variety of polyester substrates and mixed plastic waste. Structurally analogous polyesters to PET, such as polybutylene terephthalate (PBT) and polytrimethylene terephthalate (PTT), were efficiently depolymerized into terephthalic acid (TPA) with yields exceeding 99%, while poly(butylene adipate-co-terephthalate) (PBAT) has a yield of 86%. (Fig. 3e, Supplementary Figs. 39–41, and Supplementary Table 5). Polycarbonate (PC) similarly depolymerized to bisphenol A (BPA) with yields >99% (Fig. 3e, Supplementary Fig. 42, and Supplementary Table 5). Encouraged by these results, we further explored the depolymerization of bio-based polyesters. Polycaprolactone (PCL) was converted into 6-hydroxyhexanoic acid, polylactic acid (PLA) into lactic acid, poly(butylene adipate) (PBA) and poly(butylene succinate) (PBS) into 1,4-butanediol, all with yields above 96% (Fig. 3e, Supplementary Figs. 43–46, and Supplementary Table 5).

To assess the practical applicability of our catalytic system, we examined the depolymerization of post-consumer PET waste and complex plastic mixtures using d–ZnO. In model systems containing PET and common packaging plastics (PLA, PC, PE, PP, PS, and PVC), selective hydrolysis of PET proceeded efficiently, affording TPA in yields exceeding 98%, with negligible interference from mixed plastics (Fig. 3f, Supplementary Table 5). Remarkably, a wide range of real-world PET waste, including transparent and colored bottles was fully depolymerized under identical conditions, affording TPA in yields exceeding 99% (Fig. 3f, Supplementary Table 5). These findings indicate that typical additives such as plasticizers, stabilizers, and antioxidants do not compromise catalyst performance.

Depolymerization of polyester textiles and blended cotton/fabrics, despite the presence of complex additives like dyes, adhesives, and UV stabilizers, also affording TPA in yields exceeding 98% (Fig. 3f, Supplementary Table 5). While dye removal remains a challenge in aqueous systems, nearly complete decolorization was achieved through activated carbon adsorption, although the fate of dye-laden carbon requires further attention. Remarkably, d–ZnO retained high catalytic activity even for complex substrates such as carpets composed of synthetic/natural fiber blends and various additives, affording >90% yield of TPA under ambient air conditions (Fig. 3f, Supplementary Table 5).

Importantly, d–ZnO also demonstrated excellent performance in laboratory large-scale depolymerization of PET flakes (40 g), yielding over 34.5 g of high purity (>99%, Supplementary Fig. 47) recovered TPA, which was subsequently repolymerized into recycled PET (r-PET). The r-PET was analyzed by ¹H NMR spectroscopy (Supplementary Fig. 48), where the emergence of PET backbone signals at δ = 4.8 ppm confirmed the formation of r-PET. GPC revealed that the M_n = 26 kDa of r-PET (Supplementary Fig. 49), only slightly lower than that of the PET feedstock (M_n = 32 kDa). The catalyst displayed excellent stability and recyclability, maintaining high activity and selectivity over five consecutive cycles (Supplementary Fig. 50). Inductively coupled plasma mass spectrometry (ICP-MS) analysis of the digested TPA product confirmed that no Zn²⁺ leaching occurred. Following regeneration (see Supplementary Methods), the catalytic performance of d–ZnO was fully restored. XRD and SEM analyses (Supplementary Fig. 51) confirmed that the regenerated catalyst retained both the crystalline structure and morphology of the original material. These results collectively underscore the robustness and versatility of the d–ZnO catalyst in addressing diverse streams of polyester waste, offering a promising avenue for closed-loop chemical recycling under mild, scalable conditions.

Mechanistic studies for the depolymerization pathway

To elucidate the evolution of active sites on the catalyst surface in the presence of reactive gas atmospheres and water molecules, semi-in situ electron spin resonance (ESR) spectroscopy was employed to monitor signal variations of d–ZnO under air at various reaction temperatures (Fig. 4a). The ESR signals at g = 2.002 and g = 1.950 are assigned to oxygen vacancies (O_V) and zinc vacancies (Zn_V), respectively, arising from the low-coordination environment induced by defect formation. As the temperature increased under air, the signal intensities of both O_V and Zn_V gradually increased, indicating that oxygen facilitates defect generation within d–ZnO^42,43. In contrast, under a N₂ atmosphere, the Zn_V signal remained unchanged (Fig. 4b), suggesting that defect evolution is suppressed in the absence of oxygen. To further probe the nature of surface-adsorbed oxygen species, low-temperature electron paramagnetic resonance (EPR) spectroscopy was conducted at 110 K (Fig. 4c). d–ZnO exhibited a pronounced signal at g = 2.001, attributed to surface-bound O₃^– species, whereas commercial ZnO showed no detectable signal. Upon heating to 150 K, O₃^– species are known to dissociate into monoatomic O^– species, indicating that the active oxygen species stabilized on d–ZnO are predominantly O^– species⁴⁴.

Fig. 4. Mechanism study depolymerization PET over d–ZnO.

a ESR profiles of d–ZnO in different temperature under air atmospheres. b ESR profiles of d–ZnO in 180 °C under N₂ and air atmospheres. c Low-temperature EPR spectra of commercial ZnO and d–ZnO at 100 K. d In situ attenuated total reflectance (ATR) infrared spectra of the H₂O to H₂O^* process under an air atmosphere. e Free energy profiles for the conversion of H₂O to H₂O^* on bulk ZnO, d–ZnO–Zn_v, and d–ZnO–O_v. f In situ high-temperature infrared spectroscopy to determine the Mode depolymerization in H₂O under an air atmosphere. g Free energy profiles for Mode and EG on bulk ZnO, d–ZnO–Zn_v, and d–ZnO–O_v.

To elucidate the activation mechanism of the solvent (H₂O) on the surface of d–ZnO, in situ attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy was performed by gradually increasing the temperature from 25 °C to 190 °C. The v(O–H) stretching signal observed in the 3500–3150 cm^–1 region corresponding to hydroxyl groups. A red shift of the v(O–H) stretching mode, along with an enhanced signal near 3150 cm^–1, suggests that defect sites on d–ZnO promote interfacial water adsorption and O–H bond activation (Fig. 4d)^45,46. We further evaluated the energy barriers associated with O₂ and H₂O activation on bulk ZnO, d–ZnO with oxygen vacancies (d–ZnO–O_V), and d–ZnO with zinc vacancies (d–ZnO–Zn_V), focusing on the O₂ → O₂^* and H₂O + O₂^* → OOH^* + OH^* steps (Supplementary Data 1). The local defect structures of d–ZnO act as anchoring sites for both O₂ and H₂O molecules, forming the initial state (S1). Water molecules adsorbed at S1^* are activated by O₂ to generate an intermediate species S2^*, which further transforms into a transition state (TS), eventually yielding nucleophilic species in the final state. Notably, the activation energy for TS formation on d–ZnO–Zn_V is as low as 0.055 eV (Fig. 4e, Supplementary Fig. 52, and Supplementary Table 6), compared to 0.89 eV for d–ZnO–O_V and 1.24 eV for bulk ZnO (Fig. 4e, Supplementary Figs. 53 and 54, and Supplementary Tables 7, 8). These results highlight the superior activity of defect structures on d–ZnO in facilitating both O₂ and H₂O activation pathways, which is critical for ester bond cleavage in PET depolymerization. This mechanistic insight aligns well with the in situ FTIR observations, emphasizing the essential role of d–ZnO defects in catalytic O₂ and H₂O activation.

To elucidate the depolymerization mechanism of PET over the catalyst, synchrotron radiation in situ infrared (IR) spectroscopy was employed using a dimer model compound (Mode) to simulate polyester conversion to monomers (Fig. 4f, Supplementary Figs. 55). The evolution of characteristic vibrational modes during hydrolysis was monitored in real-time to reveal potential intermediates and reaction pathways. At the initial stage, characteristic IR bands for Mode in H₂O include the ν(C = O) stretching at 1727 cm^–1, ν(C–O–H) at 1406 cm⁻¹, ν(C–O–C) at 1,338 cm^–1, ν(C–O) at 1259 cm^–1, and ν(C–OH) at 1083 cm^–147,48. With the presence of the d–ZnO sites, the ν(C = O) and ν(C–O–H) presented a red shift from 1727 cm⁻¹ to 1708 cm⁻¹ and 1406 cm⁻¹ to 1390 cm⁻¹, respectively (Fig. 4f). This suggests that the carbonyl group produces a bond-weakening effect when coordinated to zinc sites, thereby enhancing the electrophilicity of the Mode^31,49. Notably, the ν(O = C–O) band at 1126 cm⁻¹ gradually disappears, accompanied by the emergence of a ν(C–OH) band at 1083 cm⁻¹, indicating cleavage of O = C–O–CH₃ and O = C–O–CH₂ ester bonds and the formation of methanol or ethylene glycol. Furthermore, the disappearance of the ν(C–O–C) signal at 1,338 cm^–1 suggests cleavage of the C–O–CH₃ or C–O–CH₂ bond in Mode. These observations indicate that the ester C–O bond in Mode is cleaved, and methanol and ethylene glycol units are released and linked to form TPA. Meanwhile, the terminal groups are capped by hydrogen atoms to yield methanol and ethylene glycol, confirming that C–O bond cleavage within ester groups is the key step during PET hydrolysis.

Hydrolysis experiments using model compounds (dimethyl terephthalate (DMT), bis(2-hydroxyethyl) terephthalate (BHET), and Mode-d₄) were performed to elucidate ester bond-cleavage pathways. ¹H NMR analysis showed that DMT hydrolysis produced terephthalic acid (TPA), with characteristic aromatic (8.13 ppm) and carboxyl (13.4 ppm) signals, while the presence of an O–CH₃ signal at 3.89 ppm indicates cleavage at the O = C–O–CH₃ bond (Supplementary Fig. 56a). BHET hydrolysis similarly yielded TPA, with methylene resonances at 4.32 and 3.72 ppm confirming cleavage at the O = C–O–CH₂ linkage. For Mode-d₄, both O = C–O–CH₃ and O = C–O–CD₂ bond cleavage pathways were observed. Infrared spectroscopy further confirmed TPA formation, with benzene ring vibrations at 1574 cm^–1 and C = O stretching at 1688 cm^–1 (Supplementary Fig. 56b). Residual O–CH₃ (1192 cm^–1) and CH₂–OH (1065 cm^–1) vibrations indicate partially hydrolyzed intermediates.

To gain further mechanistic insight into the role of catalyst surface defects in polyester depolymerization, density functional theory (DFT) calculations were conducted using Mode as the model substrate (Supplementary Data 1). The activation and cleavage of the ester C–O bond was examined on bulk ZnO, d–ZnO with oxygen vacancies (d–ZnO–O_V), and d–ZnO with zinc vacancies (d–ZnO–Zn_V). During depolymerization, the adsorption of OOH^* and OH^* on d–ZnO–O_V surfaces facilitate their interaction with Mode to form the initial state (IS1^*, ΔE = –0.94 eV) (Fig. 4g, Supplementary Fig. 57, and Supplementary Table 9). Synergistic interactions between surface defects and OOH^* species promote cleavage of the ester linkage (O = C–O–CH₂), resulting in an intermediate state (IS3^*, ΔE = 0.18 eV). Further transformation via the transition state TS1 (ΔE = 0.38 eV) drives scission of the C–O bond (O = C–O–CH₃), sequentially generating TPA, 1-(2-hydroxyethyl)−4-methyl terephthalate (HEMT), ethylene glycol and methanol. The corresponding activation energies for this step are 0.56 eV on bulk ZnO and 0.52 eV on the d–ZnO–Zn_V (Fig. 4g, Supplementary Figs. 58 and 59, and Supplementary Tables 10 and 11). Similarly, the ester bond in HEMT (O = C–O–CH₃) is activated and cleaved in the presence of OOH^* and OH^*, producing TPA and methanol via TS2 (ΔE = 0.46 eV). The activation energy barriers for C–O bond scission in HEMT is 0.49 eV on bulk ZnO and 0.52 eV on d–ZnO–Zn_V.

Based on the above findings, a schematic representation of the hydrolysis pathway catalyzed by d–ZnO is illustrated in Supplementary Fig. 60. Molecular oxygen and water are first adsorbed and subsequently activated on the surface of d–ZnO, generating reactive OOH^* and OH^*species. These species nucleophilic attack the carbonyl carbon of PET, while the carbonyl oxygen is simultaneously activated by the d–ZnO of Zn sites. This synergistic effect enhances the cleavage of C–O bonds, leading to the formation of oligomers. Through a series of subsequent steps, comprising O₂ and H₂O activation, further nucleophilic attacks, carbonyl activation, and C–O bond scission.

LCA and TEA for closed-loop recycling PET

Sustainability assessments are crucial for evaluating the environmental, social, and economic impacts of recycling processes. To assess the environmental impact of the staged polyester recycling process, a cradle-to-gate life cycle assessment (LCA) was conducted using a cutoff approach⁵⁰. Activity data for the process were obtained by constructing a conceptual factory model using Aspen software (Supplementary Fig. 61). The model begins with polyester waste as the input stage and concludes with the recovered recycled polyester (Supplementary Table 12). The conceptual factory’s annual processing capacity is set at 250,000 tons, based on polyester waste statistics. The modeling process for staged polyester waste recycling includes several phases (Supplementary Fig. 62, Supplementary Tables 13–15): (1) collection, transportation, and pretreatment of polyester waste; (2) catalyst synthesis, polyester depolymerization, and product separation; (3) repolymerization, and (4) melt extrusion and pelletizing. To improve modeling accuracy, experimental, literature, and industry data were incorporated into the comprehensive simulation of the entire recycling process. Background data on chemicals and electricity production were sourced from the ecoinvent v3.7 database.

Using recycled polyester pellets as the endpoint for assessment, the environmental impact of polyester closed-loop recycling process was compared with that of petroleum-based synthetic processes, including human health, resource consumption, and ecosystem impact⁹. Notably, the process demonstrated favorable performance in two critical metrics: Global Warming Potential (GWP) and non-renewable energy use (NREU). In China, the GWP is 2.830 kg CO_2-eq/kg and the NREU is 40.377 MJ/kg (Fig. 5a, Supplementary Table 16), which are approximately 47.3% and 55.1% lower than the petroleum-based production route, respectively. In Europe, the GWP is 2.236 kg CO_2-eq/kg and the NREU is 38.125 MJ/kg (Fig. 5b, Supplementary Table 16), which are approximately 7.2% and 45.5% lower than the petroleum-based production route, respectively. The key factors contributing to these impacts were the consumption of energy (electricity and steam) and chemicals. The polyester closed-loop recycling process significantly reduced environmental impact, primarily due to its integrated energy application and high recycling efficiency (Fig. 5c, Supplementary Tables 17–26). Furthermore, we evaluated the environmental impact of d–ZnO across the full recycling process (Supplementary Fig. 63 and Supplementary Tables 27 and 28). Terrestrial ecotoxicity and non-carcinogenic human toxicity accounted for approximately 20% and 5% of the total impact, respectively, whereas contributions from other categories were minimal. The d–ZnO catalyst remained structurally stable throughout the reaction, and no Zn²⁺ leaching was detected, indicating that dissolved zinc species are not introduced into the environment. Additionally, a comparison with methanolysis, hydrolysis, and petroleum-based production processes (Fig. 5d, e) identified core comparison metrics: global warming potential, and non-renewable energy use. With the exception of wastewater generation, which was similar to the petroleum-based production process, the hydrolysis recycling process outperformed the alternatives in all other metrics.

Fig. 5. Life-cycle assessment (LCA) and techno-economic analysis (TEA) of closed-loop PET recycling.

Comparison of a non-renewable energy use (NREU) and b global warming potential (GWP). c Comparative life-cycle assessment results of case-CN or case-EU recycling route and virgin PET-CN or virgin PET-EU route. d, e Comparison of NREU and GWP in different routes for recycling PET. EU: Europe, CN: China, JP: Japan. f Analysis of cost contribution based on clean PET, waste PET, and textile waste recycling processes. g Comparison of minimum selling prices based on clean PET, waste PET, and textile waste.

Through techno-economic analysis, we compared the total capital investment and unit production costs of the staged polyester closed-loop recycling process with those of the petroleum-based synthetic process (Fig. 5f, g). The minimum selling price (MSP) of TPA was assessed for different production routes based on PET feedstocks (Supplementary Tables 29–35). Using clean PET sources results in a relatively high MSP of $1049 per ton due to higher feedstock costs, slightly above that of conventional routes ($1021 per ton)^51,52. In contrast, PET waste and PET textile waste yield substantially lower MSPs of $770 and $470 per ton, respectively. Overall, the hydrolysis recycling process represents an economically viable, environmentally sustainable technology for polyester waste disposal.

Discussion

Taken together, we have developed a scalable recycling strategy for polyesters and polyester-based mixtures that addresses key challenges in sustainable plastic waste management. Efficient depolymerization of polyester waste was achieved in a neutral reaction medium and ambient air using the d–ZnO catalyst. This approach delivers a significantly enhanced space–time yield of up to 56 g_TPA h^–1 g cat^–1 an order of magnitude greater than previously reported catalysts. The method demonstrated broad applicability across diverse post-consumer polyesters and mixed plastic waste, effectively converting them into valuable monomers. Mechanistic studies revealed that molecular oxygen and water are preferentially adsorbed and activated at surface defect sites on d–ZnO, generating nucleophilic species. These species subsequently promote ester C–O bond cleavage in polyesters, leading to the formation of terephthalic acid. Moreover, life cycle assessments have shown that this method has significantly lower environmental impacts on human health, ecosystems, and resource depletion than petroleum-based TPA production. Technoeconomic analysis further supports the feasibility of this approach for recycling polyester into r-TPA for closed-loop plastic reuse. Overall, this work presents an energy-efficient and scalable solution to recycling polyester plastic waste, offering a promising pathway toward a circular plastic economy.

Methods

Materials synthesis

Synthesis of pristine ZnO (p–ZnO). 0.01 M ZnCl₂ (200 mL) solution was mixed with 100 mL deionized water. Next, 0.02 M NaOH (200 mL) solution was drop adding to form white flocculent precipitates under stirring for 1 h. The product was then harvested by centrifugation, washed several times with ethanol and water, and dried in a vacuum oven.

Synthesis of defect–rich ZnO (d–ZnO). 0.01 M ZnCl₂ (200 mL) solution was mixed with 100 mL deionized water. Next, 0.02 M NaOH (200 mL) solution was drop adding to form white flocculent precipitates under stirring. After another 5 min, 0.01 M _L-ascorbic acid (200 mL) solution was added drop to the above solution, and the solution was stirred for another 1 h. The product was then harvested by centrifugation, washed several times with ethanol and water, and dried in a vacuum oven.

Catalytic performance of PET hydrolysis

PET hydrolysis was performed in a 20 mL reactor. 1.0 g of PET particles, 8 mL H₂O, and the appropriate catalyst were added to a 20 mL high-temperature and high-pressure reactor under a stirring speed of 300 rpm. The mass fraction of PET and catalyst was 0.2–2%. Then the hydrothermal reactor was heated to 190 °C for 6 h. Afterwards, the reactor was cooled down to room temperature, and the products were separated from the solution by filtration to obtain terephthalic acid (TPA), without further purification. The purity of recycled TPA was according to national standard GB/T 30921.1. Conversion, yield, and space-time yield (STY) data were calculated by Eq. (1–3).

$${\mathrm{Conversion}}_{\mathrm{PET}}=\frac{\mathrm{m}\left(\mathrm{PET\; input}\right)-\mathrm{m}\left(\mathrm{PET\; residue}\right)}{\mathrm{m}\left(\mathrm{PET\; input}\right)}\times 100\%$$ 1

$${\mathrm{Selectivity}}_i=\frac{n\left({product}_i\right)}{\sum n\left(product\right)}\times 100\%$$ 2

$${\mathrm{Yield}}_{TPA}=\frac{n\left(\mathrm{TPA}\right)}{theoreticaln\left(\mathrm{TPA}\right)}\times 100\%$$ 3

$$\mathrm{STY}=\frac{m\left(TPA\right)}{m\left(cat\right)\times t}$$ 4

Characterization

XRD patterns were recorded on a Rigaku D/MAX-2500 diffractometer using a filtered Cu-Kα radiation source (λ  =  1.54056 Å). High-angle angular dark field-scanning transmission electron microscopy (HAADF-STEM) images were acquired by a JEM-ARM200F transmission electron microscope operated at 200 kV and equipped with a probe spherical aberration corrector. The XAFS spectra at Zn K-edge were acquired at the 4B9A station of the Beijing Synchrotron Radiation Facility (BSRF), operated at 2.5 GeV with a maximum current of 250 mA. Electron paramagnetic resonance (EPR) spectra were obtained on a JEOL JES-FA200. Low-temperature EPR spectroscopy measurements were conducted a spectrometer operating at the X-band frequency (E500, Bruker). ¹H-NMR spectra of TPA, Mode, and r-PET were collected by an AVI 400 MHz NMR spectrometer (Bruker). Scanning electron microscopy (SEM) images of the samples were conducted using GeminiSEM 500.

In situ high-temperature and high-pressure infrared spectrometric experiments

To elucidate the depolymerization processes of Mode, in situ FTIR measurements were carried out using a Bruker Vextex 70 spectroscope equipped with a mercury-cadmium-telluride (MCT) detector. A background measurement was recorded to establish a baseline for subsequent measurements. Following this, H₂O (10 μL) was introduced into the reactor, the temperature was then incrementally increased from room temperature (~25 °C) to 190 °C at a controlled ramp rate of 10 °C min^–1. Upon reaching 190 °C, this temperature was sustained for 60 minutes. Typical signals indicative of intermediates formed during the depolymerization processes were diligently captured and analyzed using FTIR spectroscopy.

Author contributions

J.C. conceived the idea. J.C. designed experiments. J.C., H.L., and W.C. performed experiments. J.C., H. L, W.C., and S.F. contributed to the data interpretation. X.L. contributed to the DFT calculations and analysis. J.C. co-wrote the first draft. J.C. and S.F. edited the manuscript. All the authors discussed the results.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewers for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data that supports the findings of the study are included in the main text and supplementary information files. Source data are provided with this paper. All data are available from the corresponding author upon request. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-71862-6.