Sustainable recycling of polyester wastes using a coordinatively unsaturated Zn catalyst

Domain: neuroscience
Published: 2026-04-10
Editorial outcome: minor_revision

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.

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 textiles1, 2. The widespread accumulation of polyester plastic waste has had profound impacts on the environment, marine ecosystems, and human health3–5. Chemical depolymerization of polyesters into monomeric constituents via hydrolysis6–9, alcoholysis10–16, aminolysis17, 18 and hydrogenolysis19–21 has been widely investigated. These processes offer a viable and scalable alternative to conventional end-of-life treatments, including mechanical recycling, incineration and landfilling22–25. 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 (Figure 1a), leading to reactor corrosion, complex wastewater treatment, and elevated energy demands26, 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 (Figure 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.

Homogeneous catalysts, such as ionic liquids and metal salts have demonstrated high activity in polyester depolymerization33–35. 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 mol –1 h–1 at 180 °C 36. In addition, Wang et al. reported a tetranuclear titanium catalyst that exhibits exceptionally high activity for PET methanolysis, reaching 1.9 ×107 g mol –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 (Figure 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 -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 (Figures 2a, b). Upon addition of -ascorbic acid, the sample color gradually changed from white to light yellow (insets of Figures 2a, b), suggesting structural changes. The strong interaction between -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 Figure 1), noticeable changes in the intensity of the (002) and (110) reflections suggest the introduction of a significant number of lattices vacancies38.

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 (Figure 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 (Figure 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 (Figure 2e). Electron spin resonance (ESR) spectroscopy was employed to assess unpaired electrons associated with defect sites (Figure 2f). The signal at g = 2.002 is assigned to electron trapping by oxygen vacancies (Ov), while the signal at g = 1.950 corresponds to electron trapping by zinc vacancies (Znv)39, 41. Compared to p–ZnO, d–ZnO displayed substantially enhanced Ov and Znv 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 (Mn) = 32 kDa, Supplementary Figure 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 (Mn =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 (Figure 3a, Supplementary Figure 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 (Figure 3a, Supplementary Figures 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.

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 (Figure 3b, Supplementary Figures 13, 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 > MnO > Fe2O3 > Fe3O4 > Al2O3 >TiO2 > ZrO2 > Nb2O5 > CeO2 > SiO2 (Figure 3c, Supplementary Figures 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 h–1 g cat–1, which is an order of magnitude higher than those reported for conventional hydrolysis catalysts (Figure 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 (PTET), were efficiently depolymerized into terephthalic acid (TPA) with yields exceeding 99%, while poly(butylene adipate-co-terephthalate) (PBAT) has a yield of 86%. (Figure 3e, Supplementary Figures 39–41, and Supplementary Table 5). Polycarbonate (PC) similarly depolymerized to bisphenol A (BPA) with yields >99% (Figure 3e, Supplementary Figure 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% (Figure 3e, Supplementary Figures 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 (Figure 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% (Figure 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% (Figure 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 (Figure 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 Figure 47) recovered TPA, which was subsequently repolymerized into recycled PET (r-PET). The r-PET was analyzed by ¹H NMR spectroscopy (Supplementary Figure 48), where the emergence of PET backbone signals at δ = 4.8 ppm confirmed the formation of r-PET. GPC revealed that the Mn =26 kDa of r-PET (Supplementary Figure 49), only slightly lower than that of the PET feedstock (Mn =32 kDa). The catalyst displayed excellent stability and recyclability, maintaining high activity and selectivity over five consecutive cycles (Supplementary Figure 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 Figure 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 (Figure 4a). The ESR signals at g = 2.002 and g = 1.950 are assigned to oxygen vacancies (Ov) and zinc vacancies (Znv), respectively, arising from the low-coordination environment induced by defect formation. As the temperature increased under air, the signal intensities of both Ov and Znv gradually increased, indicating that oxygen facilitates defect generation within d–ZnO42, 43. In contrast, under a N2 atmosphere, the Znv signal remained unchanged (Figure 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 (Figure 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.

To elucidate the activation mechanism of the solvent (H2O) on the surface of d–ZnO, in situ attenuated total reflectance fourier transform infrared (ATR-FTIRS) 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 (Figure 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–Ov), and d–ZnO with zinc vacancies (d–ZnO–ZnVC), 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–Znv is as low as 0.055 eV (Figure 4e, Supplementary Figure 52, and Supplementary Table 6), compared to 0.89 eV for d–ZnO–Ov and 1.24 eV for bulk ZnO (Figure 4e, Supplementary Figures 53, 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 (Figure 4f, Supplementary Figure 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 1,727 cm–1, ν(C–O–H) at 1406 cm−1, ν(C–O–C) at 1,338 cm–1, ν(C–O) at 1,259 cm–1, and ν(C–OH) at 1,083 cm–1 47, 48. With the presence of the d–ZnO sites, the ν(C=O) and ν(C–O–H) presented a red shift from 1,727 cm−1 to 1,708 cm−1 and 1,406 cm−1 to 1,390 cm−1, respectively (Figure 4f). This suggests that the carbonyl group produces a bond-weakening effect when coordinated to zinc sites, thereby enhancing the electrophilicity of the Mode31, 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 PECT 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 Figure 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 Figure 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–Ov), and d–ZnO with zinc vacancies (d–ZnO–Znv). During depolymerization, the adsorption of OOH* and OH* on d–ZnO–Ov surfaces facilitate their interaction with Mode to form the initial state (IS1*, ΔE = –0.94 eV) (Figure 4g, Supplementary Figure 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–Znv (Figure 4g, Supplementary Figures 58, 59, and Supplementary Tables 10, 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–Znv.

Based on the above findings, a schematic representation of the hydrolysis pathway catalyzed by d–ZnO is illustrated in Supplementary Figure 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 Figure 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 Figure 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 CO2-eq/kg and the NREU is 40.377 MJ/kg (Figure 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 CO2-eq/kg and the NREU is 38.125 MJ/kg (Figure 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 (Figure 5c, Supplementary Tables 17–26). Furthermore, we evaluated the environmental impact of d–ZnO across the full recycling process (Supplementary Figure 63 and Supplementary Tables 27, 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 (Figures 5d, 5e) 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.

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 (Figure 5f, 5g). 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 $1,049 per ton due to higher feedstock costs, slightly above that of conventional routes ($1,021 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.

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 -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. Afterward, 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) (2) (3).

Conversion = (m(PET input)−m(PET residue)) / m(PET input) ×100% (1)

Selectivity = n(product i) / ∑n(product) ×100% (2)

Yield = n(TPA) / theoretical n(TPA) ×100% (3)

STY = m(TPA) / (m(cat)×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 (EE500, 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 L) 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.

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.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The paper is an interesting one - I would say that there are some aspects that need clarification before publication can be considered.\nIn the introduction the authors do need to make a more balanced argument about some of the catalysts that are available for the depolymerization of polyesters. There are some much better ones out there than the authors allude to.\nDuring the mixed polymer recycling experiment why was the PLA not depolymerized - if the homopolymer was, would this not cause issues with a mixed feed? In the ESI table 5 needs more clarification - e.g. PLA/PET the yield of products in 100% but for PET/PE is is also 100% - the aspect of what is happening in the mixed polymer experiments needs more clarification/discussion.\nThe authors say this is scalable - but the largest experiment is 40g, whilst this is larger than most in the literature, I would not say this is scalable at this stage.\nThe mechanism section could be shorter - defects help and Zn acts as a Lewis acid are the more conclusions, which is what you might expect.", "reviewer_2": "This manuscript describes the use of defect-rich ZnO (d–ZnO) nanosheets for the hydrolytic depolymerization of polyesters, including PET, under neutral aqueous conditions. The authors report high terephthalic acid (TPA) yields, good catalyst recyclability, and supportive mechanistic studies involving in situ spectroscopy and DFT calculations. While the technical execution is generally sound, this work suffers from a severe lack of originality and represents little more than a minor variation of the authors’ previously published research. Therefore, I do not recommend its publication in Nature Communications.\nThe central claim of this manuscript—that defect-engineered ZnO enables efficient polyester depolymerization—is not new. The same research group recently published a nearly identical study in Nature Communications (2024, 15:6266), where they reported defect-rich Fe/ZnO for methanolysis of PET. Both reactions proceed via nucleophilic substitution at the ester bond, and both rely on defect sites (oxygen/zinc vacancies) to activate the nucleophile—methanol in the prior work, water in this submission. The mechanistic narrative is essentially identical, merely adapted from methanol to water.\nIt is a routine extension of an established catalytic platform to a different solvent system. The scientific community does not need separate high-profile publications for every solvent variation of the same core chemistry, especially when the underlying principles and catalyst design are unchanged.\nOther comments:\n1. Many previously reported systems operate under milder temperatures or ambient pressure, whereas this process requires 190 °C and high-pressure water—conditions that are energy-intensive and industrially challenging. The claim of “mild conditions” is therefore disingenuous.\n2. The current version of Figure 2 presents selectivity data for terephthalic acid (TPA), but yields are not clearly shown. Selectivity alone is insufficient to evaluate overall catalytic efficiency or to optimize reaction conditions, as it does not account for substrate conversion. For a meaningful comparison of catalytic activity across different catalysts or conditions, both conversion and yield must be reported. I recommend that the authors supplement Figure 2 (or its associated supplementary tables) with quantitative TPA yields to allow proper assessment of catalytic performance.\n3. Given that the catalyst is a metal oxide operating under hydrothermal conditions, there is a non-negligible risk of Zn leaching, especially in the presence of carboxylic acid products such as TPA, which can act as chelating agents. Even trace metal contamination can compromise the purity of recovered TPA and hinder its repolymerization into food-grade or high-performance r-PET. Therefore, I strongly recommend that the authors analyze the Zn content in the purified TPA product to assess product purity. This is critical for evaluating the practical viability of the process.\n4. On lines 229–231, the signal at g = 2.001 is assigned to surface-bound O₃⁻ species, while the signal at g = 2.002 is attributed to oxygen vacancies (OVs). These signals are very close in g-value, raising concerns about spectral resolution and assignment reliability. Please clarify how the authors deconvoluted these overlapping signals and whether simulations or power-dependent EPR measurements were performed to distinguish paramagnetic species. Additional justification or control experiments (e.g., isotopic labeling with ¹⁷O₂) would strengthen this assignment.\n5. The use of the dimer model compound (\"Mode\") assumes that its reactivity reflects that of PET. However, the structure appears to contain two distinct C–O–C linkages: –O–CH₂CH₂– (ethylene glycol unit) and –O–CH₃ (methyl ester end group). The disappearance of the ν(C–O–C) band at 1,338 cm⁻¹ is interpreted as cleavage of the C–O–CH₃ bond. However, without isotopic labeling or independent verification, it is ambiguous which ether linkage is being broken. If the methyl ester group is preferentially hydrolyzed over the glycolic linkage, this would suggest a bias in reactivity that may not reflect real PET chain scission. The authors should either justify the choice of this model compound more rigorously or consider using a more representative PET-like dimer (e.g., bis(2-hydroxyethyl) terephthalate).\n6. The claim that the merging of the ν(C–OH) doublet at 1,083 cm⁻¹ indicates “activation of interfacial water” (line 268) is speculative. This interpretation should be supported by references demonstrating similar behavior in well-defined systems, or by control experiments with deuterated water (H/D exchange).\n7. On line 19, the phrase “Closed-loop recycling of polyester wastes of polyester waste” contains a redundant repetition (“of polyester waste”) and should be corrected for clarity and grammatical accuracy.", "reviewer_3": "This manuscript reports the development of a coordinatively unsaturated defect-rich zinc oxide (d-ZnO) catalyst that enables highly efficient, neutral-condition hydrolysis of diverse polyester wastes, achieving >99% yield of TPA with a STY significantly exceeding previously reported heterogeneous catalysts. Mechanistic studies suggest that defect sites promote activation of O2 and H2O, generating nucleophilic species that cleave ester C-O bonds. The process is further validated at tens-gram scale and shown via LCA and TEA to provide environmental and economic advantages over petroleum-based TPA synthesis and alternative chemical recycling routes. The work is generally novel, comprehensive, and impactful, and addresses a critical challenge in sustainable plastic circularity. The catalytic performance is impressive, and the mechanistic exploration is extensive. However, to meet the standards of Nature Communications, several issues require clarification or further experimental support:\n1. The author should supplement the experiment with a blank control group without the catalyst to verify whether the catalyst has a catalytic effect.\n2. The author fails to mention the recovery of ethylene glycol, another important monomer derived from PET hydrolysis, throughout the entire text. Ethylene glycol is also a crucial chemical raw material. Could the author elaborate on their considerations regarding the recovery of ethylene glycol?\n3. Does the oxidation environment affect the chromaticity of the product TPA? Could the author provide a comparison of the chromaticity between the product TPA purified without activated carbon under optimal conditions and commercial TPA.\n4. It is currently unclear whether O2 is consumed during the reaction process. By measuring the partial pressure of O2 before and after the reaction, it can be determined whether O2 functions as a co-reactant or a promoter.\n5. Although recyclability has been verified in 5 runs, the leaching of Zn2+ was not confirmed through ICP-MS or XPS analysis, which is crucial for true heterogeneous catalysis.\n6. The author should briefly comment on the potential environmental issues related to d-ZnO nanoparticles."}, "author_response": "Response to referee comments, manuscript NCOMMS-25-75775\nREVIEWER #1\nBefore our response to the specific comments of the reviewer, we would like to highlight the key\nrevisions made:\n1. Revised the Abstract, Introduction, and Results sections for details and and provide more\ndetailed explanations regarding the significance and novelty of the work.\n2. Hydrolysis experiments using deuterated-Mode-d , DMT, and BHET have been added.\ncleavage of the O=C–O–CH and O=C–O–CH ester bonds and thereby gain deeper\n2 3\ninsight into the catalytic conversion mechanism.\n3. Additional control experiments, including reactions without catalyst and reactions under\nelevated oxygen pressure have been incorporated to confirm the essential roles of the\ncatalyst and oxygen.\n4. The zinc ion content in the recovered TPA was quantified using ICP-MS, confirming the\nabsence of Zn2+ leaching. Furthermore, the chromaticity of the TPA product was evaluated\nusing a colorimeter and compared with commercial TPA to verify that its purity meets the\nrequirements for producing high-performance r-PET.\nGeneral Comment: The paper is an interesting one - I would say that there are some aspects that\nneed clarification before publication can be considered.\nOur Response: We sincerely appreciate the reviewer’s compliments regarding the the novelty and\nsignificance of our work. A detailed response to the reviewer’s comments is presented below.\nComment 1: In the introduction the authors do need to make a more balanced argument about\nsome of the catalysts that are available for the depolymerization of polyesters. There are some\nmuch better ones out there than the authors allude to.\nOur Response: We sincerely thank the reviewers for their constructive comments. Following the\nsuggestion to provide a more comprehensive overview of recent advances in polyester\ndepolymerization catalysts, we have now incorporated relevant state-of-the-art studies into\nthe introduction. Recent research on catalytic chemical recycling of polyester plastics has\ndemonstrated significant progress. For example, Dove et.al. developed highly stable\nbimetallic “acid salt” complexes that integrate Zn(II)/Mg(II) with K(I)/Na(I), enabling\nefficient depolymerization of PET and PLA with activities up to 1.32×105 g mol –1 h–1\nPET LZnK\nat 180 °C (J. Am. Chem. Soc. 2025, 147, 36697–36705). In addition, Wang et al. reported a\ntetranuclear titanium catalyst exhibiting exceptional activity in the methanolysis of PET (1.9\n×107 g mol –1 h–1 at 0.005 mol% catalyst loading), which also enables the\nPET Ti\nrepolymerization of recovered monomers into high-molecular-weight polyesters, thereby\nadvancing closed-loop recycling (Angew. Chem. Int. Ed. 2025, e202505024).\nPage R1\nResponse to referee comments, manuscript NCOMMS-25-75775\nAccordingly, the following references have been added to the Introduction (lines 64–65) to\nstrengthen the discussion of catalytic depolymerization strategies: “In contrast, heterogeneous\ncatalysts offer superior stability and ease of separation, making them attractive for polyester\nrecycling. Recently, metal-based heterogeneous catalysts supported by auxiliary ligands have also\nbeen reported. For example, Dove and co-workers developed a highly stable bimetallic “acid salt”\ncatalyst system capable of depolymerizing PET and polylactic acid (PLA), achieving activities up to\n1.32×105 g mol –1 h–1 at 180 °C 36. In addition, Wang et al. reported a tetranuclear titanium\nPET LZnK\ncatalyst that exhibits exceptionally high activity for PET methanolysis, reaching 1.9 ×107 g mol –\nPET Ti\n1 h–1 at a catalyst loading of only 0.005 mol% 37.”\nComment 2: During the mixed polymer recycling experiment why was the PLA not depolymerized\n- if the homopolymer was, would this not cause issues with a mixed feed? In the ESI table 5 needs\nmore clarification - e.g. PLA/PET the yield of products in 100% but for PET/PE is is also 100% -\nthe aspect of what is happening in the mixed polymer experiments needs more\nclarification/discussion.\nOur Response: We appreciate the opportunity to clarify this point. In practical scenarios, polyester\nplastics frequently appear in blended forms (e.g., PET/PLA or PET/PE), which complicates\nconventional recycling. To address this challenge, we developed a stepwise selective\nchemical depolymerization strategy that leverages the distinct hydrolysis temperatures of\ndifferent polymers.\nFor PET/PLA blends, hydrolysis at 170 °C resulted in complete PLA conversion, affording\nlactic acid (LA) in >99% yield, while PET remained unreacted. This selective reactivity\nenabled clean separation of LA from the unconverted PET. Subsequent hydrolysis of the\nremaining PET at 190 °C led to complete depolymerization, producing terephthalic acid (TPA)\nin >99% yield (Figure 2f, Supplementary Table 5). To further evaluate selective recycling in\npolyolefin–polyester mixtures, we performed hydrolysis on PET/PE blends at 190 °C. Under\nthese conditions, PET was fully converted to TPA, whereas PE remained unreacted (Figure\n2f, Supplementary Table 5).\nIn summary, these results demonstrate that our temperature-programmed, stepwise\ndepolymerization method effectively overcomes the challenge of mechanically sorting mixed\nplastic waste. By exploiting intrinsic differences in polymer hydrolysis behavior, mixed\npolyester or polyester–polyolefin streams can be selectively converted into high-purity TPA\nmonomers, thereby avoiding the problematic co-production of inseparable mixed products.\nPage R2\nResponse to referee comments, manuscript NCOMMS-25-75775\nSupplementary Table 5. Catalytic performance of different polyester plastics and mixed plastics.\nEntry Plastic-type Plastic Products Oligomer Plastic\nconv. (%) yield (%) (%) (%)\n1a PBT 100 99 1 0\n2a PTT 100 99 1 0\n3b PC 100 99 1 0\n4b PLA 100 100 1 0\n5c PBS 100 99 1 0\n6c PBAT 100 84 16 0\n7c PBA 100 96 4 0\n8c PCL 100 99 1 0\n9c PET/PE 100 99 1 0\n10c PET/PP 100 99 1 0\n11c PET/PVC 100 99 1 0\n12c PET/PS 100 99 1 0\n13c PET/PLA 100 99 1 0\n14c PET/PC 100 99 1 0\n15c PET granule 100 99 1 0\n16c Water bottle 100 99 1 0\n17c Dyed bottle 100 99 1 0\n18c Textile 100 98 2 0\n19c Fiber/Cotton 100 98 2 0\n20c Carpet 100 96 4 0\naConditions: PET (1.0 g), d–ZnO (10 mg), 6 h, H2O (8 mL), 200°C.\nbConditions: PET (1.0 g), d–ZnO (10 mg), 6 h, H2O (8 mL), 160°C.\ncConditions: PET (1.0 g), d–ZnO (10 mg), 6 h, H2O (8 mL), 190°C.\nComment 3: The authors say this is scalable - but the largest experiment is 40g, whilst this is\nlarger than most in the literature, I would not say this is scalable at this stage.\nOur Response: We sincerely thank the reviewer for their insightful comments. We appreciate the\nreviewer’s valuable feedback. We agree that a 40 g batch does not represent true industrial\nscale. Our objective was to demonstrate that the process can be operated beyond conventional\nlaboratory-scale reactions while still achieving high conversion and product purity. As such,\nthe 40 g experiment was intended as a proof-of-concept to illustrate the scalability potential\nof the method.\nPage R3\nResponse to referee comments, manuscript NCOMMS-25-75775\nTo address this point, we have revised the manuscript to state more accurately that the process\nis amenable to scale-up rather than fully industrially scaled. The feasibility of further scale-\nup is supported by: (i) the relatively mild reaction temperature (190 °C), (ii) the use of\ninexpensive and readily recoverable reagents, and (iii) the high selectivity and product purity\nobserved even at tens-of-grams scale.\nWe made revisions to inappropriate statement and edited the corresponding description for\nclarification: “Importantly, d–ZnO also demonstrated excellent performance in laboratory large-\nscale depolymerization of PET flakes (40 g), yielding over 34.5 g of high purity (>99%,\nSupplementary Figure 49) recovered TPA, which was subsequently repolymerized into recycled PET\n(r-PET).”\nComment 4: The mechanism section could be shorter - defects help and Zn acts as a Lewis acid\nare the more conclusions, which is what you might expect.\nOur Response: We appreciate the reviewer’s valuable suggestions. We agree that the essential\nconclusions of our mechanistic study are: (i) defect sites significantly enhance catalytic\nactivity, and (ii) zinc functions as a Lewis acid to promote ester bond activation. However, to\navoid oversimplification and ensure scientific rigor, we also aimed to provide direct\nexperimental and theoretical evidence supporting these conclusions. By combining ESR/EPR\ncharacterization, in situ ATR-FTIR analysis, and DFT calculations, we were able to elucidate\nhow oxygen molecules regulate defect evolution and catalytic activity, how O and H O\n2 2\nactivation pathways depend on defect type (oxygen vacancies vs. zinc vacancies), and how\nthe activation barriers for O , H O, and PET differ among bulk ZnO, d–ZnO with oxygen\n2 2\nvacancies (d–ZnO–O ), and d–ZnO with zinc vacancies (d–ZnO–Zn ).\nV V\nIn response to the reviewer’s comments, we have streamlined the mechanistic discussion in\nthe revised manuscript to emphasize these key conclusions while retaining only the essential\nsupporting data needed to substantiate them.\nPage R4\nResponse to referee comments, manuscript NCOMMS-25-75775\nREVIEWER #2\nBefore our response to the specific comments of the reviewer, we would like to highlight the key\nrevisions made:\n1. Revised the Abstract, Introduction, and Results sections for details and and provide more\ndetailed explanations regarding the significance and novelty of the work.\n2. Hydrolysis experiments using deuterated-Mode-d , DMT, and BHET have been added.\ncleavage of the O=C–O–CH and O=C–O–CH ester bonds and thereby gain deeper\n2 3\ninsight into the catalytic conversion mechanism.\n3. Additional control experiments, including reactions without catalyst and reactions under\nelevated oxygen pressure have been incorporated to confirm the essential roles of the\ncatalyst and oxygen.\n4. The zinc ion content in the recovered TPA was quantified using ICP-MS, confirming the\nabsence of Zn2+ leaching. Furthermore, the chromaticity of the TPA product was evaluated\nusing a colorimeter and compared with commercial TPA to verify that its purity meets the\nrequirements for producing high-performance r-PET.\nGeneral Comment: This manuscript describes the use of defect-rich ZnO (d–ZnO) nanosheets for\nthe hydrolytic depolymerization of polyesters, including PET, under neutral aqueous\nconditions. The authors report high terephthalic acid (TPA) yields, good catalyst\nrecyclability, and supportive mechanistic studies involving in situ spectroscopy and DFT\ncalculations. While the technical execution is generally sound, this work suffers from a severe\nlack of originality and represents little more than a minor variation of the authors’ previously\npublished research. Therefore, I do not recommend its publication in Nature\nCommunications.\nThe central claim of this manuscript—that defect-engineered ZnO enables efficient polyester\ndepolymerization—is not new. The same research group recently published a nearly identical\nstudy in Nature Communications (2024, 15:6266), where they reported defect-rich Fe/ZnO\nfor methanolysis of PET. Both reactions proceed via nucleophilic substitution at the ester\nbond, and both rely on defect sites (oxygen/zinc vacancies) to activate the nucleophile—\nmethanol in the prior work, water in this submission. The mechanistic narrative is essentially\nidentical, merely adapted from methanol to water.\nIt is a routine extension of an established catalytic platform to a different solvent system. The\nscientific community does not need separate high-profile publications for every solvent\nvariation of the same core chemistry, especially when the underlying principles and catalyst\ndesign are unchanged.\nPage R5\nResponse to referee comments, manuscript NCOMMS-25-75775\nOur Response: We sincerely thank the reviewers for their valuable comments and the opportunity\nto clarify the novelty, scope, and broader significance of our work. While polyester\ndissolution and depolymerization fundamentally proceed via nucleophilic attack on ester\nbonds, our study introduces a general strategy for synthesizing defect-rich catalysts, leading\nto substantial improvements in the hydrolytic conversion of polyester waste. In this study, we\nuncovered a relay catalytic strategy for the hydrolytic recycling of mixed polyesters. We\nfound that molecular oxygen functions as a co-catalyst, enabling the activation of adsorbed\nO and H O at the interface of oxygen vacancies and zinc defects to generate nucleophilic\n2 2\noxygen species. These nucleophiles subsequently undergo ester-exchange reactions with the\npolyester chains at the defect sites, promoting selective cleavage of the ester bonds and\nproducing terephthalic acid. This relay-type synergistic catalysis arises from the simultaneous\nactivation of both the solvent and the polymer at a common defect interface, thereby\nsubstantially enhancing the efficiency of polyester hydrolysis. These findings provide a more\ncomprehensive mechanistic understanding of how catalyst defects promote polyester\ndepolymerization. Our catalytic system enables the selective recovery of mixed polyester\nwaste in practical applications, addressing a category of plastic waste that has not been\nsufficiently explored in closed-loop recycling strategies.\nInnovative catalyst design.\nThe defect-rich Fe/ZnO catalyst was prepared via heteroatom doping followed by high-\ntemperature reconstruction (Nature Communications, 2024, 15: 6266). In contrast to previous\napproaches, our unsaturated-coordinated Zn catalyst was synthesized using a mild, room-\ntemperature reduction method. This catalyst features a structure enriched in both oxygen\nvacancies and zinc defects, enabling a systematic study of the relationship between defect\ntype and enhanced hydrolysis performance (Figure R1a and Figure 1). We compared the\nhydrolytic activity of oxygen-poor zinc oxide (p–ZnO) and zinc oxide rich in oxygen\nvacancies and zinc defects (d–ZnO), finding that d–ZnO exhibits markedly higher\nperformance (Figure R1b and Figure 2). Notably, we are the first to demonstrate that PET\nhydrolysis catalyzed by this oxygen-vacancy and zinc-defect-rich catalyst is strongly\ndependent on oxygen concentration: increasing oxygen levels lead to progressively higher\nPET conversion, reaching complete conversion, whereas under a nitrogen atmosphere,\nconversion remains below 5%. Moreover, this synthesis strategy is readily extendable to other\nmetal oxides, significantly enhancing their hydrolysis activity (Figure R1c and Figure 2).\nThe resulting catalysts can selectively depolymerize and recover various mixed polyester\nwastes under practical conditions, exhibiting both high activity and excellent stability.\nPage R6\nResponse to referee comments, manuscript NCOMMS-25-75775\nFigure R1. a ESR curves of p-ZnO and d–ZnO structures. b Selectivity of products from catalytic\nhydrolysis of PET in different reaction conditions. Conditions: PET (1.0 g), p–ZnO (10 mg), N /O\n2 2\n(1/0, 100/1, 10/1, 5/1, v/v, 0.1 MPa) or air (0.1 MPa), H O (8 mL), 190°C. c Selectivity of products\n2\nfrom catalytic hydrolysis of PET under different catalysts. Conditions: PET (1.0 g), catalyst (10 mg),\nair (0.1 MPa), H O (8 mL), 190°C.\nparamagnetic resonance (EPR) spectroscopy to investigate the evolution of oxygen species\non the defective surface of d–ZnO. The ESR signals at g = 2.002 and g = 1.950 were assigned\nto oxygen vacancies (O ) and zinc vacancies (Zn ), respectively, arising from the low-\nV V\ncoordination environment induced by defect formation (Figure. R2a). As the temperature\nincreased under air, the signal intensities of both O and Zn gradually increased, indicating\nV V\nthat oxygen facilitates defect generation within d–ZnO. In contrast, under a N atmosphere,\n2\nthe Zn signal remained unchanged (Figure. R2b), suggesting that defect evolution is\nV\nsuppressed in the absence of oxygen. Low-temperature EPR spectra collected at 110 K were\nused to identify surface-adsorbed oxygen species (Figure. R2c). d–ZnO exhibited a distinct\nsignal at g = 2.001, attributed to surface O – species, whereas no corresponding signal was\n3\nobserved for commercial ZnO. Since O – species dissociate into atomic O– species upon\n3\nheating to 150 K, the reactive surface oxygen species on d–ZnO are mainly O–. In-situ\nattenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy was then\nused to probe solvent (H O) activation. The v(O–H) stretching signal observed in the 3500–\n2\n3150 cm–1 region corresponding to hydroxyl groups. A red shift of the v(O–H) stretching\nmode, along with an enhanced signal near 3150 cm–1, suggests that defect sites on d–ZnO\npromote interfacial water adsorption and O–H bond activation (Figure. R2d).\nTo further elucidate the activation mechanism, we calculated the energy barriers for O and\nzinc vacancies (d–ZnO–Zn ), focusing on the O → O * and H O + O * → OOH* + OH* steps\nV 2 2 2 2\n(Figure. R2e). The local defect structures of d–ZnO act as anchoring sites for both O and\n2\nby O to generate an intermediate species S2*, which further transforms into a transition state\n2\n(TS), eventually yielding nucleophilic species in the final state. Notably, the activation energy\nPage R7\nResponse to referee comments, manuscript NCOMMS-25-75775\nfor TS formation on d–ZnO–Zn is as low as 0.055 eV, compared to 0.89 eV for d–ZnO–O\nV V\nand 1.24 eV for bulk ZnO. These results highlight the superior activity of defect structures on\nd–ZnO in facilitating both O and H O activation pathways, which is critical for ester bond\n2 2\ncleavage in PET depolymerization.\nIn summary, the above analysis elucidates the defect-mediated reaction pathway of oxygen\nand water on d–ZnO (Figure. R3). Oxygen vacancies act as anchoring sites for O , while\n2\nzinc-coordinated sites activate H O to form species M1. Their interaction produces the M2\n2\nspecies through hydroxyl-hydrogen activation. Subsequently, O and H O in M2 interact with\n2 2\nthe defect sites of d–ZnO to form a transition state featuring a metal–alkoxy bond (TS),\nultimately generating the nucleophilic species in M3. This mechanism is fully consistent with\nEPR and in situ FTIR observations and highlights the critical role of defect engineering in\nenabling the cooperative activation of O and H O on d–ZnO.\n2 2\nFigure R2. a ESR profiles of d–ZnO in different temperature under air atmospheres. b ESR profiles\nof d–ZnO in 180°C under N and air atmospheres. c Low-temperature EPR spectra of commercial\n2 2 2\nbulk ZnO, d–ZnO–Zn , and d–ZnO–O .\nv v\nPage R8\nResponse to referee comments, manuscript NCOMMS-25-75775\nFigure R3. Schematic diagram for the conversion of O →O * and CH OH+ O *→OOH* +CH OH*\n2 2 3 2 3\non d–ZnO catalyst.\nMechanisms of polyester hydrolysis.\nTo elucidate the depolymerization mechanism of PET over the catalyst, synchrotron radiation\nin situ infrared spectroscopy was employed using a dimer model compound (Mode) to\nsimulate polyester conversion to monomers (Figure R4a). The evolution of characteristic\nvibrational modes during hydrolysis was monitored in real-time to reveal potential\nintermediates and reaction pathways. At the initial stage, characteristic IR bands for Mode in\nH O include the ν(C=O) stretching at 1,727 cm–1, ν(C–O–H) at 1406 cm−1, ν(C–O–C) at 1,338\n2\ncm–1, ν(C–O) at 1,259 cm–1, and ν(C–OH) at 1,083 cm–1. With the presence of the d–ZnO\nsites, the ν(C=O) and ν(C–O–H) presented a red shift from 1,727 cm−1 to 1,708 cm−1 and\n1,406 cm−1 to 1,390 cm−1, respectively. This suggests that the carbonyl group produces a\nbond-weakening effect when coordinated to zinc sites, thereby enhancing the electrophilicity\nof the Mode. Notably, the disappearance of the ν(C–O–C) signal at 1,338 cm–1, suggests\ncleavage of the C–O–CH or C–O–CH bond in Mode. These observations indicate that the\n3 2\nester C–O bond in Mode is cleaved, and methanol and ethylene glycol units are released and\nlinked to form TPA. Meanwhile, the terminal groups are capped by hydrogen atoms to yield\nmethanol and ethylene glycol, confirming that C–O bond cleavage within ester groups is the\nkey step during PET hydrolysis.\nTo gain further mechanistic insight into the role of catalyst surface defects in polyester\ndepolymerization, density functional theory (DFT) calculations were conducted using Mode\nas the model substrate. The activation and cleavage of the ester C–O bond was examined on\nbulk ZnO, d–ZnO with oxygen vacancies (d–ZnO–O ), and d–ZnO with zinc vacancies (d–\nV\nZnO–Zn ). During depolymerization, the adsorption of OOH* and OH* on d–ZnO–O\nV V\nsurfaces facilitate their interaction with Mode to form the initial state (IS1*, ΔE = –0.94 eV)\n(Figure R4b). Synergistic interactions between surface defects and OOH* species promote\ncleavage of the ester linkage (O=C–O–CH ), resulting in an intermediate state (IS3*, ΔE =\n2\n0.18 eV). Further transformation via the transition state TS1 (ΔE = 0.38 eV) drives scission\nof the C–O bond (O=C–O–CH ), sequentially generating TPA, 1-(2-hydroxyethyl)-4-methyl\n3\nterephthalate (HEMT), ethylene glycol and methanol. The corresponding activation energies\nfor this step are 0.56 eV on bulk ZnO and 0.52 eV on the d–ZnO–Zn (Figure R4b).\nV\nSimilarly, the ester bond in HEMT (O=C–O–CH ) is activated and cleaved in the presence of\nbarriers for C–O bond scission in HEMT is 0.49 eV on bulk ZnO and 0.52 eV on d–ZnO–\nZn .\nV\nPage R9\nResponse to referee comments, manuscript NCOMMS-25-75775\nFigure R4. a In situ high-temperature infrared spectroscopy to determine the Mode\ndepolymerization in H O under an air atmosphere. b Free energy profiles for Mode and EG\n2\non bulk ZnO, d–ZnO–Zn , and d–ZnO–O .\nv v\nHydrolysis experiments using model compounds (dimethyl terephthalate (DMT), bis(2-\nhydroxyethyl) terephthalate (BHET), and Mode-d ) were performed to elucidate ester bond-\n4\ncleavage pathways. 1H NMR analysis showed that DMT hydrolysis produced terephthalic\nacid (TPA), with characteristic aromatic (8.13 ppm) and carboxyl (13.4 ppm) signals, while\nthe presence of an O–CH signal at 3.89 ppm indicates cleavage at the O=C–O–CH bond\n3 3\n(Figure R5a). BHET hydrolysis similarly yielded TPA, with methylene resonances at 4.32\nand 3.72 ppm confirming cleavage at the O=C–O–CH linkage. For Mode-d , both O=C–O–\n2 4\nCH and O=C–O–CD bond cleavage pathways were observed. Infrared spectroscopy further\n3 2\nconfirmed TPA formation, with benzene ring vibrations at 1574 cm–1 and C=O stretching at\n1688 cm–1 (Figure R5b). Residual O–CH (1192 cm–1) and CH –OH (1065 cm–1) vibrations\n3 2\nindicate partially hydrolyzed intermediates.\nFigure R5. a, b 1H NMR and IR spectra of the hydrolysis products of DMT, BHET, and Mode-\nd , respectively.\nResponse to referee comments, manuscript NCOMMS-25-75775\nSuperior catalytic performance of PET and real polyester waste over d–ZnO.\nThe d–ZnO catalyst demonstrated significantly enhanced activity and selectivity compared\nwith previously reported systems. The process afforded a space-time yield (STY) of 56 g\nTPA\ng –1 h–1, which is an order of magnitude higher than those reported for conventional\ncat\nhydrolysis catalysts (Figure R6a). Considering variations in previous reports, hydrolysis\nconditions were classified into four categories: (1) supercritical CO , (2) acidic hydrolysis,\n2\n(3) alkaline hydrolysis, (4) neutral hydrolysis, and (5) hydrogenolysis. Under neutral aqueous\nconditions (≥190 °C), d–ZnO exhibited significantly higher catalytic activity than previously\nreported systems. Even in comparison to supercritical CO and acidic or basic conditions, d–\nWe systematically evaluated the depolymerization performance of d–ZnO across a broad\nrange of polyester substrates and mixed waste streams. Polyesters with structural motifs\nsimilar to PET, including polybutylene terephthalate (PBT) and polypropylene terephthalate\n(PTT), were quantitatively depolymerized to terephthalic acid (TPA) with yields exceeding\n99%, while the copolyester polybutylene adipate-terephthalate (PBAT) afforded an 86% TPA\nyield (Figure R6b). Polycarbonate (PC) was also efficiently converted to bisphenol A (BPA),\nachieving yields above 99% (Figure R6b). We further extended the catalytic system to bio-\nbased and aliphatic polyesters. Polycaprolactone (PCL) was transformed into 6-\nhydroxyhexanoic acid, polylactic acid (PLA) into lactic acid, and polybutylene adipate (PBA)\nand polybutylene succinate (PBS) into 1,4-butanediol, each with product yields above 96%\n(Figure R6b), highlighting the broad functional group compatibility of d–ZnO.\nTo assess selectivity in realistic waste streams, we examined mixed-plastic model systems\ncontaining PET and common packaging plastics (PLA, PC, PE, PP, PS, and PVC). PET was\nselectively and efficiently hydrolyzed, delivering TPA yields above 98%, and no significant\ninterference from the accompanying plastics was observed (Figure R6c). Remarkably, d–\nZnO also retained high catalytic activity toward complex post-consumer waste substrates,\nsuch as textiles composed of synthetic/natural fiber blends and materials containing diverse\nadditives, achieving TPA yields greater than 90% under ambient air (d–ZnO). Crucially, d–\nZnO demonstrated excellent scalability and robustness. In a 40 g batch depolymerization of\nPET flakes, more than 34.5 g of high-purity TPA (>99%) was recovered and successfully\nrepolymerized into recycled PET (r-PET). The catalyst exhibited outstanding durability,\nmaintaining high activity and selectivity over five consecutive cycles. These results\nunderscore the broad substrate scope, high selectivity, and operational stability of d–ZnO,\nestablishing it as a practical and versatile catalyst for closed-loop chemical recycling of\ndiverse polyester wastes under mild and scalable conditions.\nPage R11\nResponse to referee comments, manuscript NCOMMS-25-75775\nFigure R6. a Comparison of activities of d–ZnO with other catalysts for PET hydrolysis. The\nactivities values of other catalysts are calculated based on the amount of catalyst used reported in the\nliterature. b Product selectivity in the hydrolysis of polyester waste. Conditions: polyester waste (PBT,\nPTT, PBAT, PBS, PBA, PCL, 1.0 g), d–ZnO (10 mg), H O (8 mL), 190°C, 6 h. polyester waste (PLA,\nmixed polyester waste. Conditions: polyester waste (1.0 g), d-ZnO (10 mg), H O (8 mL), 190°C, 6 h.\nmechanistic understanding for PET chemical depolymerization.\nComment 1: Many previously reported systems operate under milder temperatures or ambient\npressure, whereas this process requires 190 °C and high-pressure water—conditions that are\nenergy-intensive and industrially challenging. The claim of “mild conditions” is therefore\ndisingenuous.\nOur Response: We appreciate the reviewers’ comments regarding the use of the term “mild\nconditions” and agree that clarification is necessary. While many previously reported systems\nindeed operate at relatively low temperatures or under atmospheric pressure, these\nperformances were achieved exclusively in strongly alkaline or acidic media. In contrast, our\nsystem achieves efficient PET hydrolysis under neutral conditions at 190 °C. As shown in\nFigure R7, when compared with state-of-the-art neutral-condition depolymerization\nstrategies, this temperature is relatively mild and represents a significant reduction in\nPage R12\nResponse to referee comments, manuscript NCOMMS-25-75775\nchemical corrosiveness and operational hazard. We have revised the discussion to correct any\ninaccurate wording and to more accurately reflect the relative mildness of the reaction\nconditions within the proper context.\nTo address the reviewer’s comments, we revised the Discussion for clarification. The revised\ntext now reads: “Leveraging a d–ZnO catalyst, efficient depolymerization of polyester waste was\nachieved under mild conditions in air. Efficient depolymerization of polyester waste was achieved in\na neutral reaction medium and ambient air using the d–ZnO catalyst.”\nFigure R7. Comparison of activities of d–ZnO with other catalysts for PET hydrolysis. The activities\nvalues of other catalysts are calculated based on the amount of catalyst used reported in the literature.\nTo address the reviewer’s comments, we revised the scheme 1 for clarification.\nPage R13\nResponse to referee comments, manuscript NCOMMS-25-75775\nScheme 1. Chemical recycling routes of polyester. a Traditionally hydrolysis routes and b\nemerging hydrolysis routes. c Coordinatively unsaturated Zn catalyst-driven hydrolysis\nroutes in this work.\nComment 2: The current version of Figure 2 presents selectivity data for terephthalic acid (TPA),\nbut yields are not clearly shown. Selectivity alone is insufficient to evaluate overall catalytic\nefficiency or to optimize reaction conditions, as it does not account for substrate conversion.\nFor a meaningful comparison of catalytic activity across different catalysts or conditions,\nboth conversion and yield must be reported. I recommend that the authors supplement Figure\n2 (or its associated supplementary tables) with quantitative TPA yields to allow proper\nassessment of catalytic performance.\nOur Response: We thank the reviewers for their valuable comments regarding the yield data\npresented in our manuscript. We agree that both conversion and yield are essential for a\ncomprehensive assessment of catalytic performance. To clarify, detailed data on optimized\nreaction conditions, conversion, and yield are provided in the Supplementary Information\n(Supplementary Tables 1–5). These tables include the quantitative yields of TPA obtained\nPage R14\nResponse to referee comments, manuscript NCOMMS-25-75775\nunder various reaction conditions, enabling a complete evaluation of catalytic efficiency\n(Tables R1–R5).\nIn addition, we evaluated the space–time yield (STY) of the catalyst to further demonstrate\nits high performance. Given the diversity of conditions reported in the literature, we\ncategorized hydrolysis systems into five types for comparison: (1) supercritical CO\n2\nhydrolysis, (2) acidic hydrolysis, (3) alkaline hydrolysis, (4) neutral hydrolysis, and (5)\nhydrogenolysis. Under neutral aqueous conditions (≥190 °C), the catalytic activity of d–ZnO\nis markedly higher than that of previously reported systems. Moreover, even when compared\nwith supercritical CO , acidic, or alkaline systems, d–ZnO exhibits superior catalytic\n2\nefficiency (Figure R6). We believe that these additions address the reviewers’ concerns and\nprovide a more complete evaluation of the catalytic performance.\nTable R1. Selectivity of products from catalytic hydrolysis of PET in different reaction conditions.\nEntry Atmosphere PET TPA oligomer PET\nconv. (%) yield (%) (%) (%)\n1a N2/O2 (100%/0%) 3 0.7 2.3 97\n2a N2/O2 (99.9%/0.1%) 41.2 31.9 9.3 58.8\n3a N2/O2 (99%/1%) 61.5 56.2 5.3 38.5\n4a N2/O2 (90%/10%) 82 72.8 9.2 18\n5a N2/O2 (80%/20%) 88 78 10 12\n6a air 90 78.9 11.1 10\n7b air 100 99.9 0.1 0\n8c air 86 70 16 14\naConditions: PET (1.0 g), p–ZnO (10 mg), 6 h, H2O (8 mL), 190°C.\nbConditions: PET (1.0 g), d–ZnO (10 mg), 6 h, H2O (8 mL), 190°C.\ncConditions: PET (1.0 g), commercial–ZnO (10 mg), 6 h, H2O (8 mL), 190°C.\nPage R15\nResponse to referee comments, manuscript NCOMMS-25-75775\nTable R2. Optimization of catalytic hydrolysis of PET.\nEntry Temp. t mcat/mPET PET TPA\n(°C) (h) (mg/g) conv. (%) yield (%)\n1 190 0.5 10/1 0.5 -\n2 190 1 10/1 10 3.5\n4 190 3 10/1 64.5 46.3\n5 190 6 10/1 100 99\n6 190 6 2/1 71.5 56.8\n7 190 6 5/1 92 78.5\n8 190 6 10/1 100 99\n9 190 6 20/1 100 99\n10 170 6 10/1 13.5 3.6\n11 180 6 10/1 43.4 28.6\n12 190 6 10/1 100 99\n13 200 6 10/1 100 99\nTable R3. Selectivity of products from catalytic hydrolysis of PET in different reaction conditions.\nEntry Catalyst PET TPA oligomer PET\nconv. (%) yield (%) (%) (%)\n1 d–ZnO 100 99.9 0.1 0\nConditions: PET (1.0 g), catalyst (10 mg), 6 h, H2O (8 mL), 190°C.\nPage R16\nResponse to referee comments, manuscript NCOMMS-25-75775\nTable R4. Comparison of reported catalysts for PET hydrolysis.\nCatalyst Temp. pH TPA STY\n(°C) yield (%) gTPA gcat -1 h-1\nThis work d–ZnO 190 7 99 56\nd–ZnO 200 7 99 84.5\nSupercritical SO4 2–/TiO2 8 160 (15 MPa) - 99 0.71\nCO2 ZSM-59 205 (7.5 MPa) - 98 1.69\nWOx/TiO2 10 160 (15 MPa) - 98 0.56\nAcidic PTSA (80%)11 150 <1 96.2 0.01\nhydrolysis H2SO4 (3M)12 190 <1 100 0.11\nTsOH (GVL)13 170 <1 95 10.95\nTPA14 220 - 95.5 0.34\nGlacial acetic acid15 280 <1 99 0.08\nTfOH16 25 <1 91 0.05\nAlkaline hydrolysis NaOH /TBHDPB17 100 14 93.5 15.31\nKOH (DMC/EtOH) 18 35 14 99 3.41\nKOH (THF/EG)19 60 14 97.5 3.36\nK2CO3 20 150 14 99 0.21\nNaOH21 180 14 99 0.14\nNaOH (GVL) 21 90 14 99 2.9\nNaOH (DMI/EG) 22 100 14 99 1.03\nNaOH (TBAI)23 200 W 14 100 0.51\nZn2/C24 90 13 - 48.0 (gPET gcat -1 h-1)\nNaOH (ball milling)25 25 14 99 6.11\nNeutral hydrolysis D-MIL-10126 80 7 81.8 3.53\nt-BuNH2/NaCl27 197 7 50 0.22\nNb-HZSM-528 190 7 80 6.91\nNb-HZSM-528 200 7 94 8.12\nHZSM-529 200 7 82 28.3\nHZSM-529 230 7 99 34.2\nNi/γ‑Al2O3 30 220 7 20 1.67\nNi/γ‑Al2O3 30 267 7 97 8.12\nHydrogenolysis C/MoO2 31 260 - 87 1.25 (gTPA ncat -1 h-1)\nPage R17\nResponse to referee comments, manuscript NCOMMS-25-75775\nUiO-6632 260 - 72 0.32 (gTPA ncat -1 h-1)\nHf[OTf]4+Pd/C33 265 - 98 7.05 (gTPA ncat -1 h-1)\nTable R5. Catalytic performance of different polyester plastics and mixed plastics.\nEntry Plastic-type Plastic Products Oligomer Plastic\nconv. (%) yield (%) (%) (%)\n1a PBT 100 99 1 0\n2a PTT 100 99 1 0\n3b PC 100 99 1 0\n4b PLA 100 100 1 0\n5c PBS 100 99 1 0\n6c PBAT 100 84 16 0\n7c PBA 100 96 4 0\n8c PCL 100 99 1 0\n9c PET/PE 100 99 1 0\n10c PET/PP 100 99 1 0\n11c PET/PVC 100 99 1 0\n12c PET/PS 100 99 1 0\n13c PET/PLA 100 99 1 0\n14c PET/PC 100 99 1 0\n15c PET granule 100 99 1 0\n16c Water bottle 100 99 1 0\n17c Dyed bottle 100 99 1 0\n18c Textile 100 98 2 0\n19c Fiber/Cotton 100 98 2 0\n20c Carpet 100 96 4 0\naConditions: PET (1.0 g), d–ZnO (10 mg), 6 h, H2O (8 mL), 200°C.\nbConditions: PET (1.0 g), d–ZnO (10 mg), 6 h, H2O (8 mL), 160°C.\ncConditions: PET (1.0 g), d–ZnO (10 mg), 6 h, H2O (8 mL), 190°C.\nPage R18\nResponse to referee comments, manuscript NCOMMS-25-75775\nComment 3: Given that the catalyst is a metal oxide operating under hydrothermal conditions,\nthere is a non-negligible risk of Zn leaching, especially in the presence of carboxylic acid\nproducts such as TPA, which can act as chelating agents. Even trace metal contamination\ncan compromise the purity of recovered TPA and hinder its repolymerization into food-grade\nor high-performance r-PET. Therefore, I strongly recommend that the authors analyze the\nZn content in the purified TPA product to assess product purity. This is critical for evaluating\nthe practical viability of the process.\nOur Response: We sincerely thank the reviewer for the insightful comment. We appreciate the\nreviewer’s important point. We agree that Zn2+ leaching could potentially form complexes\nwith terephthalic acid, thereby reducing the purity of the recovered TPA and hindering its\nrepolymerization into high-performance r-PET. Following the reviewer’s suggestion, we\nconducted inductively coupled plasma mass spectrometry (ICP-MS) analysis on the digested\nTPA products obtained after five consecutive catalytic cycles. The results showed that no\nZn2+ was detected in the hydrolyzed TPA. These findings confirm that the catalyst remains\nstable during the PET hydrolysis process and that the resulting TPA is suitable for\nrepolymerization into high-performance r-PET.\nTo address the reviewer’s comments, we added the ICP-MS analysis for clarification. The added\ntext now reads: “Inductively coupled plasma mass spectrometry (ICP-MS) analysis of the digested\nTPA product confirmed that no Zn2+ leaching occurred.”\nComment 4: On lines 229–231, the signal at g = 2.001 is assigned to surface-bound O₃⁻ species,\nwhile the signal at g = 2.002 is attributed to oxygen vacancies (OVs). These signals are very\nclose in g-value, raising concerns about spectral resolution and assignment reliability.\nPlease clarify how the authors deconvoluted these overlapping signals and whether\nsimulations or power-dependent EPR measurements were performed to distinguish\nparamagnetic species. Additional justification or control experiments (e.g., isotopic labeling\nwith ¹⁷O₂) would strengthen this assignment.\nOur Response: We appreciate the reviewers’ attention to the spectral resolution and the attribution\nof the EPR signals at g = 2.001 and g = 2.002. We fully understand the concern regarding the\nproximity of these g-values and the possibility of signal overlap. We would like to clarify that\nthe two signals arise from distinct experiments conducted under different conditions, enabling\ntheir unambiguous assignment.\nThe signal at g = 2.001 (Figure 3c) is attributed to surface-bound O – species and was obtained\n3\nfrom low-temperature EPR measurements (110 K). Prior to measurement, the sample was\npretreated under N at 400 °C, followed by analysis under high vacuum. These conditions\n2\neffectively remove physisorbed oxygen, allowing the detection of intrinsic surface oxygen\nspecies. In contrast, the signal at g = 2.002 (Figures 3a and 3b) was recorded using room-\ntemperature ESR and is assigned to oxygen vacancies (O ), which exhibit characteristic\nV\nPage R19\nResponse to referee comments, manuscript NCOMMS-25-75775\nsignals in this g-value range. Because these measurements were performed at ambient\ntemperature and without vacuum removal of surface oxygen, the observed O signal does not\nV\noverlap with the low-temperature O – signal. Thus, the combination of different temperatures,\n3\ndifferent pretreatment atmospheres, and different measurement environments enables clear\ndiscrimination between the two species. This characterization approach is well established\nand has been described in detail in previous studies (ACS Catal. 2019, 9, 9751–9763).\nComment 5: The use of the dimer model compound (\"Mode\") assumes that its reactivity reflects\nthat of PET. However, the structure appears to contain two distinct C–O–C linkages: –O–\nCH₂CH₂– (ethylene glycol unit) and –O–CH₃ (methyl ester end group). The disappearance of\nthe ν(C–O–C) band at 1,338 cm⁻¹ is interpreted as cleavage of the C–O–CH₃ bond. However,\nwithout isotopic labeling or independent verification, it is ambiguous which ether linkage is\nbeing broken. If the methyl ester group is preferentially hydrolyzed over the glycolic linkage,\nthis would suggest a bias in reactivity that may not reflect real PET chain scission. The\nauthors should either justify the choice of this model compound more rigorously or consider\nusing a more representative PET-like dimer (e.g., bis(2-hydroxyethyl) terephthalate).\nOur Response: We appreciate the reviewer’s suggestion regarding the necessity of experimentally\nverified the C–O–C bond cleavage site. We conducted hydrolysis experiments using different\nmodel compounds (dimethyl terephthalate (DMT), bis(2-hydroxyethyl) terephthalate\n(BHET), and Mode-d ) to elucidate the bond-cleavage mechanisms during their conversion\n4\nto monomers. The chemical structures of the hydrolysis products were analyzed by 1H NMR\nspectroscopy (Figure R8a). For DMT, the main hydrolysis product was terephthalic acid\n(TPA), characterized by a singlet at 8.13 ppm corresponding to the four aromatic protons and\na peak at 13.4 ppm assigned to carboxylic acid protons, while a resonance at 3.89 ppm\ncorresponds to the O–CH group. These features indicate that bond cleavage occurs at the\n3\ncarboxyl proton signals appearing at 8.13 and 13.4 ppm, respectively. The signals at 4.32 and\n3.72 ppm correspond to methylene protons associated with COO–CH and CH –OH groups,\n2 2\nindicating cleavage at the O=C–O–CH bond. In the case of Mode-d , TPA was again the\n2 4\ndominant product, with characteristic signals at 8.13 and 13.4 ppm, along with a peak at 3.89\nppm corresponding to O–CH , indicating that bond cleavage occurs at both O=C–O–CH and\n3 3\nO=C–O–CH positions.\nskeletal vibration of the benzene ring was observed at 1574 cm–1 for the hydrolysis products\nof all model compounds, consistent with TPA formation. The C=O stretching vibration of\ncarboxylic acid groups appeared at 1688 cm–1. In addition, the hydrolysis products of DMT\nand Mode-d₄ exhibited characteristic O–CH vibrations at 1192 cm–1, whereas BHET-derived\nResponse to referee comments, manuscript NCOMMS-25-75775\nproducts showed CH –OH vibrations at 1065 cm–1, consistent with the partial retention of\n2\nmethyl ester or ethylene glycol ester fragments, in agreement with the 1H NMR results.\nFigure R8. a, b 1H NMR and IR spectra of the hydrolysis products of DMT, BHET, and Mode-\nd , respectively.\n“Furthermore, the disappearance of the ν(C–O–C) signal at 1,338 cm–1 suggests cleavage of the C–\nO–CH bond in Mode, leading to the formation of methanol. suggests cleavage of the C–O–CH or\n3 3\nC–O–CH bond in Mode. These observations indicate that the ester C–O bond in Mode is cleaved,\n2\nand ethylene glycol units are released and linked to form TPA. and methanol and ethylene glycol units\nare released and linked to form TPA. Meanwhile, the terminal groups are capped by hydrogen atoms\nto yield methanol and ethylene glycol, confirming that C–O bond cleavage within ester groups is the\nkey step during PET hydrolysis. Hydrolysis experiments using model compounds (dimethyl\nterephthalate (DMT), bis(2-hydroxyethyl) terephthalate (BHET), and Mode-d ) were performed to\n4\nelucidate ester bond-cleavage pathways. 1H NMR analysis showed that DMT hydrolysis produced\nterephthalic acid (TPA), with characteristic aromatic (8.13 ppm) and carboxyl (13.4 ppm) signals,\nwhile the presence of an O–CH signal at 3.89 ppm indicates cleavage at the O=C–O–CH bond\n3 3\n(Supplementary Figure 59a). BHET hydrolysis similarly yielded TPA, with methylene resonances at\n4.32 and 3.72 ppm confirming cleavage at the O=C–O–CH linkage. For Mode-d , both O=C–O–\n2 4\nCH and O=C–O–CD bond cleavage pathways were observed. Infrared spectroscopy further\n3 2\nconfirmed TPA formation, with benzene ring vibrations at 1574 cm–1 and C=O stretching at 1688 cm–\n1 (Supplementary Figure 59b). Residual O–CH (1192 cm–1) and CH –OH (1065 cm–1) vibrations\n3 2\nindicate partially hydrolyzed intermediates.”\nAdded a new Supplementary Figure 59 to the Supporting Information:\nPage R21\nResponse to referee comments, manuscript NCOMMS-25-75775\nSupplementary Figure 59. a, b 1H NMR and IR spectra of the hydrolysis products of DMT, BHET,\nand Mode-d , respectively.\n“activation of interfacial water” (line 268) is speculative. This interpretation should be\nsupported by references demonstrating similar behavior in well-defined systems, or by\ncontrol experiments with deuterated water (H/D exchange).\nOur Response: We appreciate the reviewer’s important suggestion. We agree that attributing the\nmerging of the ν(C–OH) doublet at 1083 cm⁻¹ to “interfacial water activation” is inaccurate.\nAs the reaction proceeds, the ν(O=C–O) absorption band at 1126 cm⁻¹ gradually disappears,\nwhile a new ν(C–OH) band emerges at 1083 cm⁻¹ (Figure 3f). This spectral evolution\nindicates cleavage of the O=C–O–CH and O=C–O–CH ester bonds, leading to the\n3 2\nformation of methanol or ethylene glycol, which are characterized by the ν(C–OH) vibration.\nAccordingly, we have revised the manuscript to correct this interpretation and to clarify that\nthe observed IR changes arise from ester bond cleavage and alcohol formation rather than\ndirect evidence of interfacial water activation.\nTo address the reviewer’s comments, we revised the line 268 for clarification. The revised text\nnow reads: “Notably, the ν(C–OH) doublet at 1,083 cm–1 is merged into a single peak, indicating\nactivation of interfacial water. the ν(O=C–O) band at 1126 cm⁻¹ gradually disappears, accompanied\nby the emergence of a ν(C–OH) band at 1083 cm⁻¹, indicating cleavage of O=C–O–CH and O=C–\ncontains a redundant repetition (“of polyester waste”) and should be corrected for clarity\nand grammatical accuracy.\nPage R22\nResponse to referee comments, manuscript NCOMMS-25-75775\nOur Response: We thank the reviewers for pointing out the repeated word in line 19. We agree\nthat the redundant repetition should be removed to improve clarity and grammatical accuracy.\nTo address the reviewer’s comments, we deleted the line 19 for clarification. The revised text\nnow reads: “Closed-loop recycling of polyester waste of polyester waste offers a promising route for\naddressing the environmental burden of plastic pollution and enabling resource circularity.”\nPage R23\nResponse to referee comments, manuscript NCOMMS-25-75775\nREVIEWER #3\nBefore our response to the specific comments of the reviewer, we would like to highlight the key\nrevisions made:\n1. Revised the Abstract, Introduction, and Results sections for details and and provide more\ndetailed explanations regarding the significance and novelty of the work.\n2. Hydrolysis experiments using deuterated-Mode-d , DMT, and BHET have been added.\ncleavage of the O=C–O–CH and O=C–O–CH ester bonds and thereby gain deeper\n2 3\ninsight into the catalytic conversion mechanism.\n3. Additional control experiments, including reactions without catalyst and reactions under\nelevated oxygen pressure have been incorporated to confirm the essential roles of the\ncatalyst and oxygen.\n4. The zinc ion content in the recovered TPA was quantified using ICP-MS, confirming the\nabsence of Zn2+ leaching. Furthermore, the chromaticity of the TPA product was evaluated\nusing a colorimeter and compared with commercial TPA to verify that its purity meets the\nrequirements for producing high-performance r-PET.\nGeneral Comment: This manuscript reports the development of a coordinatively unsaturated\ndefect-rich zinc oxide (d-ZnO) catalyst that enables highly efficient, neutral-condition hydrolysis\nof diverse polyester wastes, achieving >99% yield of TPA with a STY significantly exceeding\npreviously reported heterogeneous catalysts. Mechanistic studies suggest that defect sites promote\nactivation of O2 and H2O, generating nucleophilic species that cleave ester C-O bonds. The\nprocess is further validated at tens-gram scale and shown via LCA and TEA to provide\nenvironmental and economic advantages over petroleum-based TPA synthesis and alternative\nchemical recycling routes. The work is generally novel, comprehensive, and impactful, and\naddresses a critical challenge in sustainable plastic circularity. The catalytic performance is\nimpressive, and the mechanistic exploration is extensive. However, to meet the standards of Nature\nCommunications, several issues require clarification or further experimental support:\nOur Response: We sincerely appreciate the reviewer’s compliments regarding the novel,\ncomprehensive, and impactful of our work. We are grateful for the time and effort the\nreviewer has devoted to evaluating our manuscript and providing thoughtful feedback. We\nbelieve these revisions and additions address the reviewer’s concerns and significantly\nstrengthen the manuscript, making a compelling case for the contribution of our research to\nsustainable polyester plastic recycling technologies. Below, we provide a detailed point-by-\npoint response to the reviewer’s comments.\nComment 1: The author should supplement the experiment with a blank control group without\nthe catalyst to verify whether the catalyst has a catalytic effect.\nPage R24\nResponse to referee comments, manuscript NCOMMS-25-75775\nOur Response: We sincerely thank the reviewers for their valuable comments. To clearly\ndemonstrate the catalytic effect, we have added a blank control experiment without catalyst\nto the revised manuscript.\nOur results show that the d–ZnO catalyst enables complete conversion of PET under an air\natmosphere, affording a high yield of TPA (>99%). In contrast, in the absence of the catalyst,\nthe PET conversion remains below 5% and no TPA is detected. These additional experimental\nresults are now presented in Figure R1.\nFigure R1. The PET depolymerization reaction over catalyst-free.\nComment 2: The author fails to mention the recovery of ethylene glycol, another important\nmonomer derived from PET hydrolysis, throughout the entire text. Ethylene glycol is also a\ncrucial chemical raw material. Could the author elaborate on their considerations regarding\nthe recovery of ethylene glycol?\nOur Response: We appreciate the valuable comments from the reviewers, which have helped\nimprove the clarity and depth of our manuscript. We agree that ethylene glycol (EG) is an\nimportant monomer generated during PET hydrolysis and a key chemical feedstock.\nFollowing PET hydrolysis, filtration yields solid TPA and catalyst residues, while the filtrate\ncontains a mixture of water and EG. Due to the large difference in their boiling points, EG\ncan be recovered by distillation. At the industrial scale, this separation is typically achieved\nthrough rectification, which we have also incorporated into our Aspen simulation.\nWe acknowledge that the rectification of water and EG is the most energy intensive step in\nthe recovery process. In our simulation, we attempted to reduce this energy demand by\nintegrating heat recovered from other unit operations into the rectification section. Looking\nahead, the development of lower energy separation strategies such as membrane based\nseparation technologies or improved EG recovery processes could further reduce the energy\nfootprint of this step.\nPage R25\nResponse to referee comments, manuscript NCOMMS-25-75775\nComment 3: Does the oxidation environment affect the chromaticity of the product TPA?\nCould the author provide a comparison of the chromaticity between the product TPA\npurified without activated carbon under optimal conditions and commercial TPA.\nOur Response: Thank you for your valuable suggestions. The oxidizing environment is indeed a\nsignificant factor that can influence the color of recovered TPA. In our system, the hydrolysis\nreaction is performed under an air atmosphere, and the reaction conditions are sufficiently\nmild to avoid oxidative side reactions.\nFollowing the reviewers' comments, we have added a comparison of the color of two types\nof TPA: TPA obtained under optimal conditions without activated carbon purification and\ncommercially available TPA. Colorimetric values were measured using standard colorimetry\nand are now provided in the revised Figure R2. The recovered TPA exhibits a whiteness value\nof 91, comparable to that of commercial TPA (90.3), indicating that the air atmosphere used\nduring the reaction does not adversely affect product color.\nFigure R2. Color comparison between recycled TPA and commercially available TPA.\nComment 4: It is currently unclear whether O2 is consumed during the reaction process. By\nmeasuring the partial pressure of O2 before and after the reaction, it can be determined\nwhether O2 functions as a co-reactant or a promoter.\nOur Response: We appreciate the reviewers’ valuable comments. We agree that identifying\nwhether O is consumed during the reaction is essential for clarifying its mechanistic role. To\n2\naddress this point, we measured the partial pressure of O before and after the reaction.\n2 2\nthe reactor and heated at 190 °C for 6 h. Under these conditions, the internal pressure\nincreased to 2.25 MPa. After cooling to room temperature, the pressure returned to 0.5 MPa\n(Figure R3). These results demonstrate that the O partial pressure remains essentially\nResponse to referee comments, manuscript NCOMMS-25-75775\nunchanged, indicating that O is not consumed as a co-reactant. Instead, O acts as a promoter\n2 2\nthat modulates defect states and sustains catalyst activity.\nFigure R3. Pressure profile during PET hydrolysis under 0.5 MPa O .\nconfirmed through ICP-MS or XPS analysis, which is crucial for true heterogeneous catalysis.\nOur Response: We sincerely thank the reviewer for the insightful comment. We agree that\nconfirming the absence of Zn2+ leaching is crucial for establishing truly heterogeneous\ncatalysis. Although we had demonstrated that the catalyst retained its activity and selectivity\nover five consecutive cycles, direct evidence of Zn2+ leaching had not been provided in the\noriginal submission.\nFollowing the reviewers’ suggestion, we have now included quantitative analysis of Zn2+\nleaching in the revised manuscript. Inductively coupled plasma mass spectrometry (ICP-MS)\nmeasurements of the depolymerization products show that no Zn was detected in the\nhydrolyzed TPA. These results confirm that the catalyst remains stable during PET hydrolysis\nand support the conclusion that d–ZnO functions as a truly heterogeneous catalyst.\nTo address the reviewer’s comments, we added the ICP-MS analysis for clarification. The added\ntext now reads: “Inductively coupled plasma mass spectrometry (ICP-MS) analysis of the digested\nTPA product confirmed that no Zn2+ leaching occurred.”\nComment 6: The author should briefly comment on the potential environmental issues related to\nd-ZnO nanoparticles.\nOur Response: We thank the reviewer for their valuable suggestion. We appreciate the reviewer’s\nimportant point. Like other metal oxide nanomaterials, d–ZnO nanoparticles could pose\npotential environmental risks if released into natural ecosystems, including bioaccumulation,\ntoxicity to aquatic organisms, and disruption of microbial communities. However, in our\nsystem, the catalyst functions as a heterogeneous solid and can be readily recovered by simple\nfiltration. ICP-MS analysis confirmed that no Zn2+ leaching occurred during the reaction,\nPage R27\nResponse to referee comments, manuscript NCOMMS-25-75775\nindicating that the catalyst remains structurally stable and is unlikely to introduce dissolved\nzinc species into the environment.\nFurthermore, we performed a life cycle assessment (LCA) to evaluate the environmental\nimpact of zinc oxide in the overall recovery process (Figure R4 and Table R1, 2). The results\nshow that terrestrial ecotoxicity and human non-carcinogenic toxicity accounted for\napproximately 20% and 5% of the total impact, respectively, while other indicators\ncontributed only minor fractions. These findings are consistent with the known environmental\nprofiles of metal oxides.\nWe have added a brief discussion of these considerations in the revised manuscript to clarify the\npotential environmental impact and to emphasize the importance of catalyst recovery and reuse.\nThe added text now reads: “Furthermore, we evaluated the environmental impact of d–ZnO across the\nfull recycling process (Supplementary Figure 66 and Supplementary Table 27, 28). Terrestrial\necotoxicity and non-carcinogenic human toxicity accounted for approximately 20% and 5% of the\ntotal impact, respectively, whereas contributions from other categories were minimal. The d–ZnO\ncatalyst remained structurally stable throughout the reaction, and no Zn2+ leaching was detected,\nindicating that dissolved zinc species are not introduced into the environment.”\nFigure R4. Comparative life-cycle assessment results of case-CN or case-EU recycling route\nand ZnO-CN or ZnO-EU route.\nPage R28\nResponse to referee comments, manuscript NCOMMS-25-75775\nTable R1. Comparative LCA results of case-CN recycling route and ZnO-CN route.\nZnO-CN/r-\nCountry r-PET\nImpact category Reference unit PET(%)\nFine particulate matter formation kg PM2.5 eq 0.00211 0.51396\nFossil resource scarcity kg oil eq 0.86555 0.26129\nFreshwater ecotoxicity kg 1,4-DCB 0.04721 1.10859\nFreshwater eutrophication kg P eq 0.00028 0.77956\nCN\nGlobal warming kg CO eq 2.90954 0.25755\nHuman non-carcinogenic\nkg 1,4-DCB 0.81453 5.28237\ntoxicity\nIonizing radiation kBq Co-60 eq 0.04141 0.80668\nLand use m2a crop eq 0.01871 0.85222\nMarine ecotoxicity kg 1,4-DCB 0.06179 1.82566\nMarine eutrophication kg N eq 0.00003 0.58879\nMineral resource scarcity kg Cu eq 0.00264 1.25887\nOzone formation, Human health kg NO eq 0.00358 0.44194\nx\nOzone formation, Terrestrial\nkg NO eq 0.00368 0.43824\nx\necosystems\nStratospheric ozone depletion kg CFC11 eq 0.00000 0.37973\nTerrestrial acidification kg SO eq 0.00460 0.41520\nPage R29\nResponse to referee comments, manuscript NCOMMS-25-75775\nTable R2. Comparative LCA results of case-EU recycling route and ZnO-EU route.\nZnO-EU/r-\nCountry r-PET\nImpact category Reference unit PET(%)\nFine particulate matter formation kg PM2.5 eq 0.00102 0.50996\nFossil resource scarcity kg oil eq 0.76082 0.29916\nFreshwater ecotoxicity kg 1,4-DCB 0.04527 1.14360\nFreshwater eutrophication kg P eq 0.00038 0.65215\nEU\nGlobal warming kg CO eq 2.30234 0.28838\nHuman non-carcinogenic\nkg 1,4-DCB 0.73179 5.84102\ntoxicity\nIonizing radiation kBq Co-60 eq 0.18219 0.54739\nLand use m2a crop eq 0.01457 0.96645\nMarine ecotoxicity kg 1,4-DCB 0.05911 1.90449\nMarine eutrophication kg N eq 0.00003 0.52848\nMineral resource scarcity kg Cu eq 0.00207 1.58945\nOzone formation, Human health kg NO eq 0.00169 0.57301\nx\nOzone formation, Terrestrial\nkg NO eq 0.00178 0.55458\nx\necosystems\nStratospheric ozone depletion kg CFC11 eq 0.00000 0.39868\nTerrestrial acidification kg SO eq 0.00268 0.47105\nPage R30\nResponse to referee comments, manuscript NCOMMS-25-75775\nAdded a new Supplementary Figure 66 to the Supporting Information:\nSupplementary Figure 66. Comparative life-cycle assessment results of case-CN or case-EU\nrecycling route and ZnO-CN or ZnO-EU route.\nAdded a new Supplementary Tables 27, 28 to the Supporting Information:\nPage R31\nResponse to referee comments, manuscript NCOMMS-25-75775\nSupplementary Table 27. Comparative LCA results of case-CN recycling route and ZnO-CN route.\nZnO-CN/r-\nCountry r-PET\nImpact category Reference unit PET(%)\nFine particulate matter formation kg PM2.5 eq 0.00211 0.51396\nFossil resource scarcity kg oil eq 0.86555 0.26129\nFreshwater ecotoxicity kg 1,4-DCB 0.04721 1.10859\nFreshwater eutrophication kg P eq 0.00028 0.77956\nCN\nGlobal warming kg CO eq 2.90954 0.25755\nHuman non-carcinogenic\nkg 1,4-DCB 0.81453 5.28237\ntoxicity\nIonizing radiation kBq Co-60 eq 0.04141 0.80668\nLand use m2a crop eq 0.01871 0.85222\nMarine ecotoxicity kg 1,4-DCB 0.06179 1.82566\nMarine eutrophication kg N eq 0.00003 0.58879\nMineral resource scarcity kg Cu eq 0.00264 1.25887\nOzone formation, Human health kg NO eq 0.00358 0.44194\nx\nOzone formation, Terrestrial\nkg NO eq 0.00368 0.43824\nx\necosystems\nStratospheric ozone depletion kg CFC11 eq 0.00000 0.37973\nTerrestrial acidification kg SO eq 0.00460 0.41520\nPage R32\nResponse to referee comments, manuscript NCOMMS-25-75775\nSupplementary Table 28. Comparative LCA results of case-EU recycling route and ZnO-EU route.\nZnO-EU/r-\nCountry r-PET\nImpact category Reference unit PET(%)\nFine particulate matter formation kg PM2.5 eq 0.00102 0.50996\nFossil resource scarcity kg oil eq 0.76082 0.29916\nFreshwater ecotoxicity kg 1,4-DCB 0.04527 1.14360\nFreshwater eutrophication kg P eq 0.00038 0.65215\nEU\nGlobal warming kg CO eq 2.30234 0.28838\nHuman non-carcinogenic\nkg 1,4-DCB 0.73179 5.84102\ntoxicity\nIonizing radiation kBq Co-60 eq 0.18219 0.54739\nLand use m2a crop eq 0.01457 0.96645\nMarine ecotoxicity kg 1,4-DCB 0.05911 1.90449\nMarine eutrophication kg N eq 0.00003 0.52848\nMineral resource scarcity kg Cu eq 0.00207 1.58945\nOzone formation, Human health kg NO eq 0.00169 0.57301\nx\nOzone formation, Terrestrial\nkg NO eq 0.00178 0.55458\nx\necosystems\nStratospheric ozone depletion kg CFC11 eq 0.00000 0.39868\nTerrestrial acidification kg SO eq 0.00268 0.47105\nPage R33\nResponse to referee comments, manuscript NCOMMS-25-75775-A-Z\nREVIEWER #1\nGeneral Comment: The authors have done a very impressive job in responding to the reviewer\ncomments. From my side the authors have adequately addressed my concerns and added details\nto the paper.\nOur Response: We sincerely thank you for your meticulous review of our revised manuscript.\nYour insightful comments and constructive suggestions have enhanced the\ncomprehensiveness, rigor, and overall persuasiveness of our work.\nPage R1\nResponse to referee comments, manuscript NCOMMS-25-75775-A-Z\nREVIEWER #2\nGeneral Comment: Although the study is supported by a large volume of data, the fundamental\nissue of its incremental contribution persists, and I doubt that further modifications will\nresolve this core concern. I do not recommend its publication in Nature Communications. I\nhave the following reservations regarding the authors' justification of novelty:\nOur Response: We sincerely appreciate the time and efforts the reviewer has invested in\nevaluating our manuscript and providing detailed feedback. We believe these revisions and\nadditions address the reviewer’s concerns and significantly strengthen the manuscript. Below,\nwe provide a detailed point-by-point response to the reviewer’s comments.\nComment 1: The authors emphasize the \"room-temperature reduction\" synthesis of d-ZnO as a\n\"milder\" approach compared to previous high-temperature methods. It does not constitute a\nfundamental shift in heterogeneous catalysis. Many different catalysts can be prepared by\nroom-temperature reduction. Meanwhile The active sites (Zn/O vacancies) and the support\nmaterial remain essentially the same. For a high-impact journal, the expectation is a new\ncatalytic principle rather than a process optimization of an existing catalyst family.\nOur Response: We We thank the reviewer for this insightful and constructive comment. We\nrespectfully clarify that the significance of our work does not lie solely in replacing a high-\ntemperature process with a milder one, but rather in establishing a distinct defect-generation\npathway that leads to fundamentally different defect states, spatial distributions, and catalytic\nmicroenvironments.\nFirst, although d–ZnO has indeed been reported using high-temperature hydrogenation or\nvacuum annealing, these approaches are governed by thermodynamic equilibration at\nelevated temperatures. In contrast, our room-temperature strategy employs ascorbic acid–\nmediated chemical reduction, which proceeds under kinetically controlled conditions. This\ndifference is crucial: High-temperature reduction typically produces bulk-distributed oxygen\nvacancies accompanied by lattice reconstruction and partial sintering. Our room-temperature\nreduction selectively generates surface-confined, highly dispersed oxygen vacancies coupled\nwith low-coordinated Zn species, while preserving the crystalline framework and\nmorphology.\nSecond, the ascorbic acid reduction pathway introduces a controlled electron-transfer process\nat room temperature. Unlike hydrogenation, which removes lattice oxygen via thermal\nactivation, ascorbic acid acts as a mild reductant that selectively extracts surface oxygen\natoms through interfacial redox chemistry. This produces: A higher proportion of shallow\ndonor-type oxygen vacancies. Stronger electronic coupling between Zn sites and vacancy\ndefects. Suppressed defect clustering and phase transformation. These characteristics are not\nPage R2\nResponse to referee comments, manuscript NCOMMS-25-75775-A-Z\nattainable under conventional thermodynamic reduction conditions and directly influence\nadsorption energetics and reaction pathways.\nThird, we respectfully emphasize that the conceptual advance lies in decoupling defect\ngeneration from high-temperature thermodynamic constraints, enabling defect engineering\nunder kinetic control. This provides: A strategy for tailoring vacancy depth (surface vs. bulk).\nTunable defect density without crystallographic degradation. A generalizable framework for\nroom-temperature defect modulation in metal oxides. Therefore, our contribution extends\nbeyond synthetic convenience. It demonstrates that defect formation pathways under\nthermodynamic and kinetic control lead to distinct catalytic states even within the same\nmaterial family, which in turn govern catalytic behavior. In this sense, the work contributes\nto a broader catalytic principle: the pathway of defect creation determines the nature of active\nsites and their catalytic function.\nWe hope this clarification addresses the reviewer’s concern and better articulates the\nfundamental implications of our work beyond synthetic mildness.\nComment 2: The authors argue that the \"relay catalytic strategy\" involving molecular oxygen\nand water differentiates this work from their previous publication in Nature Communications\n(2024, 15:6266). However, the fundamental catalytic logic appears identical to the previous\nFe/ZnO system. Both relies on vacancies to activate oxygen and ester bonds via nucleophilic\npathways. Moreover, ATR-IR only shows water is activated, but the proposed “zinc-\ncoordinated sites” activate H2O is not supported.\nOur Response: We thank the reviewers for their valuable comments regarding the yield data\npresented in our manuscript. We sincerely thank the reviewer for this careful and insightful\nevaluation. We respectfully clarify that, although both studies involve defect-mediated\nactivation, the catalytic logic and reaction network in the present work are fundamentally\ndistinct from our previously reported Fe/ZnO system.\n1. Distinction from the previously reported Fe/ZnO mechanism\nIn our previous study published in Nature Communications (2024, 15: 6266), the catalytic\npathway was centered on a V –Zn2+–O–Fe3+ asymmetric interfacial site, in which O and\no 2\nCH OH were co-adsorbed and activated to form CH OH* and OOH* intermediates. These\n3 3\nspecies cooperatively promoted PET ester bond activation and C–O bond cleavage. The key\nfeature of that system was the Fe–Zn interfacial redox coupling that mediated oxidative\nmethanolysis.\nBy contrast, the present work does not rely on Fe incorporation or asymmetric Fe–Zn dual\nsites. Instead, we find that molecular oxygen functions as a co-catalyst that dynamically\nactivates O and H O adsorbed at the O –Zn -defect interface of defective ZnO (Figure R1).\n2 2 v v\nPage R3\nResponse to referee comments, manuscript NCOMMS-25-75775-A-Z\nSpecifically: O is activated at oxygen vacancy sites. H O is simultaneously activated at\n2 2\nadjacent Zn-defect coordination environments. The activated oxygen species generate\nnucleophilic oxygen intermediates at the same defective interface. These nucleophiles\nsubsequently attack the polyester ester bonds via a transesterification-like pathway,\nselectively cleaving the ester linkage to yield terephthalic acid. Importantly, this process\nconstitutes a relay-type synergistic catalysis, in which solvent (H O), molecular oxygen,\n2\nand polymer chains are concurrently activated at a single defective interface. The catalysis is\ntherefore not merely vacancy-assisted bond cleavage, but rather a spatially coupled activation\nsequence that links O reduction, H O activation, and ester bond scission in a continuous\n2 2\ninterfacial cycle.\nThus, while both systems involve defect participation, the fundamental catalytic principle\ndiffers: The previous Fe/ZnO system operates via redox-active Fe–Zn asymmetric dual sites\nand oxidative methanolysis intermediates. The present system establishes a defect-confined\nO /H O relay activation mechanism without transition-metal redox centers.\n2 2\nFigure R1. Schematic diagram for the conversion of O →O * and H O+ O *→OOH* +OH* on d–\n2 2 2 2\nZnO catalyst.\n2. On the activation of H O at Zn defect sites\ncoordination defect sites activate water molecules. The ATR-IR data primarily support the\nformation of activated hydroxyl species.\nDensity functional theory calculations further confirm the proposed mechanism. The results\ndemonstrate that the energy barrier for H\nv\nand d–ZnO–Zn\nv\nis\nsignificantly lower than that on pristine bulk ZnO (Figure R2). These results indicate that\ndefect formation substantially facilitates H O activation and stabilizes nucleophilic\n2\nintermediates. Although spectroscopic techniques cannot isolate the exact coordination\nconfiguration of the Zn site during activation, the combined experimental and theoretical\nevidence consistently supports the conclusion that d–ZnO lowers the kinetic barrier for water\nactivation. In addition, the experimental results show that d–ZnO exhibits significantly\nenhanced catalytic performance compared with pristine ZnO, further supporting the critical\nrole of defects in promoting water activation.\nPage R4\nResponse to referee comments, manuscript NCOMMS-25-75775-A-Z\nFigure R2. Free energy profiles for the conversion of H O to H O* on bulk ZnO, d–ZnO–Zn , and d–\n2 2 v\nZnO–O .\nv\nComment 3: The revised Scheme 1 is somewhat misleading to the readers. Hydrothermal\ndegradation of PET has been extensively reported, yet the authors intentionally portray the\n'traditional hydrolysis route' as requiring excessively harsh conditions. In reality, neutral\nhydrothermal PET degradation has already been validated at a continuous and large-scale\nlevel, achieving efficient conversion to TPA monomers in just 5 minutes. This is significantly\nmore efficient than the results reported in this manuscript.\nOur Response: We sincerely thank the reviewer for this important comment and agree that neutral\nhydrothermal PET depolymerization has been extensively investigated and industrially\ndemonstrated. To clarify, industrial neutral hydrolysis is typically conducted in reactors under\nhigh-temperature and high-pressure conditions, using water as the sole reactant. The reaction\ntemperature is generally maintained between 200 and 300 °C. Below 200 °C, the hydrolysis\nrate becomes extremely slow, whereas temperatures exceeding 300 °C tend to promote\nundesirable side reactions, including excessive formation of diethylene glycol and other by-\nproducts.\nOur intention was not to challenge the technical maturity or efficiency of industrial\nhydrothermal hydrolysis. Rather, the distinction of the present work lies in demonstrating that,\nunder neutral conditions at a comparatively lower temperature of 190 °C, defect-engineered\nZnO enables complete PET hydrolysis with 100% conversion and without detectable side\nreactions. In this system, d–ZnO serves as a catalyst that lowers the activation barrier for ester\nbond cleavage, allowing quantitative depolymerization below the conventional industrial\ntemperature. Thus, the conceptual advance is not the replacement of hydrothermal processing\nitself, nor a claim of superior throughput compared to optimized industrial continuous\nsystems. Instead, it lies in revealing how defect-mediated catalysis enables selective and\ncomplete PET hydrolysis under milder thermal conditions while suppressing side reactions.\nPage R5\nResponse to referee comments, manuscript NCOMMS-25-75775-A-Z\nREVIEWER #3\nGeneral Comment: These revisions have clarified issues related to catalyst control, EG recovery,\noxygen consumption, and Zn2+ leaching, and have provided a detailed description of potential\nenvironmental issues associated with d-ZnO nanoparticles through a life cycle assessment. The\ncontent is comprehensive, the logic is clear, and the reporting is thorough. The manuscript is ready\nfor publication.:\nOur Response: We sincerely appreciate the time and efforts the reviewer has invested in\nevaluating our manuscript and providing detailed feedback.\nPage R6"}]

Decision letter