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Depolymerization of polyesters by a binuclear catalyst for plastic recycling

Nature Sustainability volume 6, pages 965–973 (2023)Cite this article

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Abstract

Plastics play an essential role in modern society; however, the relentless growth of their production is threatening both human health and ecosystems. As a result, there are intensive efforts in developing recycling technologies to repurpose waste plastics into the building blocks for valuable materials. Here we show a binuclear complex that can catalyse the degradation of poly(ethylene terephthalate) (PET)—the most widely used polyester globally—and a wide spectrum of other plastics including polylactic acid, polybutylene adipate terephthalate, polycaprolactone, polyurethane and Nylon 66. Inspired by hydrolases, the group of enzymes that catalyse bond cleavages with water, the present catalyst design features biomimetic Zn‒Zn sites that activate the plastic, stabilize the key intermediate and enable intramolecular hydrolysis. This synthetic catalyst delivers an activity of 36 mgPET d−1 gcatal−1 toward PET depolymerization at pH 8 and 40 °C and an activity of 577 gPET d−1 gcatal−1 at pH 13 and 90 °C for scalable PET recycling. We further demonstrate a closed-loop production of bottle-grade PET. This work presents a practical and viable solution for the sustainable management of plastics waste.

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Fig. 1: Binuclear catalyst design.
Fig. 2: Structural characterization of the binuclear catalyst.
Fig. 3: PET depolymerization under mild conditions.
Fig. 4: PET depolymerization mechanism over the binuclear catalyst.
Fig. 5: PET recycling over Zn2/C and wider applicability.

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Data availability

All relevant data that support the findings of this study are presented in the article and Supplementary Information. Crystallographic data for the structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre under deposition nos. CCDC 2082457 (Zn2L(NO3)), 2082456 (Zn2L(OH)2) and 2113556 ([Zn2L(BDC)]n). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. Source data are provided with this paper.

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Acknowledgements

This work was supported by the National Key R&D Program of China (grant no. 2019YFA0709200 to Z.N.), the National Natural Science Foundation of China (grant no. 22075162 to Z.N.), the Tsinghua University Initiative Scientific Research Program (grant no. 20221080067 to Z.N.) and the China Postdoctoral Science Foundation (grant no. 2021M691754 to S.Z.). The DFT calculations were performed using the Theory and Computation facility of the Centre for Functional Nanomaterials (CFN), which is a US Department of Energy Office of Science User Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704 (to P.L.). We acknowledge the BL14W1 station of the Shanghai Synchrotron Radiation Facility (SSRF) and the 4B9A station of the Beijing Synchrotron Radiation Facility (BSRF) for the collection of XAFS data.

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Author notes

  1. These authors contributed equally: Shengbo Zhang, Qikun Hu, Yu-Xiao Zhang.

Authors and Affiliations

  1. State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing, China

    Shengbo Zhang, Qikun Hu, Yu-Xiao Zhang, Yanfen Wu, Mingze Sun, Shuyan Gong & Zhiqiang Niu

  2. Chemistry Division, Brookhaven National Laboratory, Upton, NY, USA

    Haoyue Guo & Ping Liu

  3. Sinopec Yizheng Chemical Fibre Co., Ltd, Yangzhou, China

    Xingsong Zhu

  4. School of Life Sciences, Tsinghua University, Beijing, China

    Jiangang Zhang

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Contributions

Z.N. conceptualized and guided this work. Z.N. and S.Z. designed the experiments. S.Z., Q.H. and Y.-X.Z. performed the experiments and contributed equally to this work. H.G. and P.L. performed the DFT calculations. X.Z. carried out the production and characterization of rPET using recycled PTA. Y.W. and M.S. carried out the EXAFS measurements and analysis. S.G. and J.Z. performed the STEM measurements. Z.N., S.Z., Q.H., Y.-X.Z., H.G. and P.L. wrote the paper. All authors participated in the data analysis and commented on the manuscript.

Corresponding author

Correspondence to Zhiqiang Niu.

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Competing interests

Z.N., S.Z., Q.H. and Y.-X.Z. have filed a PCT patent (China Patent application no. PCT/CN2021/124875). X.Z. is an employee of Sinopec Yizheng Chemical Fibre Co., Ltd. The remaining authors declare no competing interests.

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Nature Sustainability thanks Rey-Ting Guo and Thomas Maskow for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Structures of binuclear zinc complexes.

a‒c, Experimental (gray) and simulated (light gray) SCXRD patterns of Zn2L(NO3)2 (a), Zn2L(OH)2 (b), and [Zn2L(BDC)]n (c). The insets show the optical images of the single crystals and the corresponding simplified crystal structures (green: Zn; blue: N; red: O; gray: C). d, Hydrogen bonding network in Zn2L(OH)2 as resolved by SCXRD. Free solvent molecules and hydrogen atoms are omitted for clarity, except for the hydrogen atoms in the coordinated groups and hydrogen bonds.

Source data

Extended Data Fig. 2 The PET hydrolysis mechanism proposed based on experimental and DFT results.

The mechanism highlights the importance of co-operative substrate-binding on the adjacent zinc sites and the subsequent formation of six-membered (Zn-O-C-O-Zn-μ2-O) intermediate. Ball-and-stick models are given near the corresponding skeleton formulas (green: Zn; blue: N; red: O; gray: C; white: H). Ethylene glycol dibenzoate (EGD) is employed as the ester substrate for simplicity. EGM and BzO represent ethylene glycol monobenzoate and benzoate, respectively.

Extended Data Fig. 3 Performance evaluation of binuclear zinc catalysts.

a, The hydrolysis kinetics of crystalline PET granules (38%) over Zn2L(OH)2 with equivalent moles of free NO3‒ and Zn2L(NO3)2 with equivalent moles of free OH‒. Reaction conditions: 1.0 mM Zn2L(NO3)2 or Zn2L(OH)2, 2.0 mM NaOH or NaNO3, 10 mg crystalline PET granules, 60 °C. Note that the induction period was curtailed when the axial ligands were hydroxyl groups, in spite of the same feeding amount in the two cases. b, The performance of Zn2/C under different pH values and temperatures, where the areas of the circles represent the magnitude of the specific activity. The specific activity of the Zn2/C at pH 8 and 40 °C is 36 mgPET d−1 gcatal−1. The specific activities were calculated at 80% conversion of PET in 10 mL of NaOH aqueous solution containing 10 mg of PET and 4 mg of Zn2L(NO3)2 supported on carbon. c, Thermogravimetric analysis (TGA) of Zn2L(NO3)2, showing that the binuclear catalyst was stable below 340 °C. d, The hydrolysis kinetics of crystalline PET granules (38%) over Zn2/C, Zn(OAc)2, and blank control at pH 13 and 90 °C. Reaction conditions: 50 mL of NaOH aqueous solution (pH 13) containing 50 g of crystalline PET (38%) and 5 mg or 50 mg of Zn2L(NO3)2 supported on carbon or 35 mg of Zn(OAc)2 (Methods). All the error bars in a and d represent the standard deviations for three independent measurements and the hollow squares indicate mean values. The shading in the d represents the error range.

Source data

Extended Data Fig. 4 X-ray adsorption spectroscopy of Zn2L(OH)2 during reaction.

a‒c, XANES spectra (a), Fourier transforms of EXAFS spectra (b), and EXAFS in k-space spectra (c) of the binuclear Zn catalyst before and during the reaction. EXAFS spectra in R-space indicate an increased coordination number of zinc during the reaction.

Source data

Extended Data Fig. 5 The variation of coordination geometries in response to the change of coordination number of the metal sites.

a, Ball-and-stick model of Zn2L(NO3)2 as resolved by SCXRD, where hydrogen atoms are omitted for clarity. b, The relative position of disordered Zn atoms and N4O2 plane. Lavender: pentacoordinated; green: hexacoordinated. Atom–plane distances: Zn1, 0.123 Å; Zn1A, 0.391 Å; Zn2, 0.090 Å; Zn2A, 0.508 Å. c‒d, Pentacoordinated (c) and hexacoordinated (d) configuration of Zn atoms in Zn2L(NO3)2. In the case of pentacoordinate, the axial positions of Zn atoms are occupied by nitrate ions and form distorted square pyramidal geometry. The two Zn atoms distinctly lie out of N4O2 plane, with atom–plane distances of 0.391 Å and 0.508 Å, and elongated Zn···Zn distance of 3.249(5) Å, respectively. In the case of hexacoordinate, the two Zn atoms take distorted octahedral configuration, with additional methanol molecules coordinated to the Zn atoms. The Zn–plane distances are negligible 0.123 Å and 0.090 Å, while Zn···Zn distance is shortened to 3.153(3) Å. This statistical distribution in the crystal structure of Zn2L(NO3)2 indicates that the pentacoordinated Zn atoms could form a bond to another molecule and turn into hexacoordinate structure.

Extended Data Fig. 6 Post-reaction characterization of Zn2/C.

a‒d, HADDF-STEM image (scale bar: 5 nm) (a), XANES spectra (b), Fourier transforms of EXAFS spectra (c), and EXAFS spectra in k space (d) of the Zn K-edge of spent Zn2/C. The XAFS spectra of the fresh Zn2/C were also presented in b‒d for references. These results indicate the structure of the spent Zn2/C remained the same to its original form.

Source data

Extended Data Fig. 7 Crystallinity of different PET materials.

a‒b, XRD patterns (a) and differential scanning calorimetry (DSC) analysis (b) of amorphous film (Goodfellow Ltd, ES301445), crystalline granule (Macklin, P875573), dyed water bottle (Coca-cola, USA), and cloth fiber (online purchase). Insets in a show the images of different PET materials. The corresponding crystallinity was listed in parentheses in b.

Source data

Extended Data Fig. 8 PET recycling using Zn2/C.

a‒b, Flow diagram (a) and representative photos (b) of the PET recycling process, where TPA-Na2 and EG represent disodium terephthalate and ethylene glycol, respectively. c‒d, The purity of the recycled pure terephthalic acid (PTA) was checked by 1H NMR (c) and high-performance liquid chromatography (d). Both confirmed that the purity of terephthalic acid (TPA) was above 99% according to the national standard GB/T 30921.1. e, Comparison of rPET made from the recycled PTA with the PET made from virgin PTA.

Source data

Extended Data Fig. 9 Techno-economic analysis based on a capability of 100 thousand tons of PET waste per year.

a, Histogram analysis of the cost and revenue for Zn2/C-catalysed PET recycling, showing an annual profit of 26.0 million USD on 0.1 megatons of PET waste. b, Simplified flow diagram of the PET recycling process. The raw material inputs are shown in blue, and the products are shown in red. c, Simplified economic summary. More detailed analysis is provided in the Supplementary Information.

Extended Data Fig. 10 The depolymerization of different PET sources and various polyesters and polyamide with and without Zn2/C at pH 13 and 60 °C.

Reaction conditions: 10 mL of NaOH aqueous solution (pH 13), 0.32 g of polyesters or polyamide and 2 mg of Zn2L(NO3)2 supported on carbon. Reaction time: Carpet (60 h), fiber (50 h), dyed bottle (50 h), waste bottle (50 h), amorphous (38 h), crystalline granule (50 h), Nylon 66 (6 day), PBT (12 day), PC (14 day), PBA (38 h), P3/4HB (18 h), PHB (27 h), PGA (10 h), PBS (12 h), PEF (7 h), PU (2 h), PCL (16 h), PLA (18 h), and PBAT (16 h). All the error bars in this figure represent the standard deviations for three independent measurements and the bars indicate mean values.

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Supplementary information

Supplementary Information

Characterizations, economic analysis, DFT calculations, Supplementary Tables 1‒5 and References.

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Zhang, S., Hu, Q., Zhang, YX. et al. Depolymerization of polyesters by a binuclear catalyst for plastic recycling. Nat Sustain 6, 965–973 (2023). https://doi.org/10.1038/s41893-023-01118-4

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