Abstract
Depolymerization is a promising strategy for recycling waste plastic into constituent monomers for subsequent repolymerization1. However, many commodity plastics cannot be selectively depolymerized using conventional thermochemical approaches, as it is difficult to control the reaction progress and pathway. Although catalysts can improve the selectivity, they are susceptible to performance degradation2. Here we present a catalyst-free, far-from-equilibrium thermochemical depolymerization method that can generate monomers from commodity plastics (polypropylene (PP) and poly(ethylene terephthalate) (PET)) by means of pyrolysis. This selective depolymerization process is realized by two features: (1) a spatial temperature gradient and (2) a temporal heating profile. The spatial temperature gradient is achieved using a bilayer structure of porous carbon felt, in which the top electrically heated layer generates and conducts heat down to the underlying reactor layer and plastic. The resulting temperature gradient promotes continuous melting, wicking, vaporization and reaction of the plastic as it encounters the increasing temperature traversing the bilayer, enabling a high degree of depolymerization. Meanwhile, pulsing the electrical current through the top heater layer generates a temporal heating profile that features periodic high peak temperatures (for example, about 600 °C) to enable depolymerization, yet the transient heating duration (for example, 0.11 s) can suppress unwanted side reactions. Using this approach, we depolymerized PP and PET to their monomers with yields of about 36% and about 43%, respectively. Overall, this electrified spatiotemporal heating (STH) approach potentially offers a solution to the global plastic waste problem.
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Data availability
Data supporting this study are available from the corresponding authors on reasonable request. Source data are provided with this paper.
Code availability
The code used in this study is available from the corresponding authors on reasonable request.
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Acknowledgements
This work is not directly funded. L.H. acknowledges support from the University of Maryland A. James Clark School of Engineering. Y.J. acknowledges US Department of Energy (DOE) grant support for the Plasma Science Center (DOE DE-SC0020233) and DOE Basic Energy Sciences (BES) grant DE-SC0021135. S.C. and D.L. acknowledge support from the DOE Office of Fossil Energy (DE-FE0031877). J.M. acknowledges the Richard and Judith Wien Professorship for unrestricted support. The authors acknowledge the Maryland NanoCenter, the Surface Analysis Center and the AIM Lab.
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Contributions
L.H. and Q.D. conceived the concept and designed the STH approach. L.H. and Q.D. designed the reactors, with input from S.C. and Y.L. Q.D., S.L., M.G., M.C., T.L. and A.Q. collected the experimental data. X.Z. conducted the temperature measurement, with help from Q.D. and S.L. A.D.L. and Y.J. conducted the molecular dynamics simulations. S.C. and D.L. analysed the gaseous products. Y.W. and X.P. analysed the liquid products. I.G.K. and J.M. helped with the data analysis and improved the manuscript. L.X. helped with the writing and figure design. L.H., Q.D. and A.H.B. collectively wrote the paper, with input from all authors. L.H. and Y.J. supervised the project. All authors discussed the results and contributed to the final manuscript.
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Q.D., S.C., D.L., Y.J. and L.H. report a PCT patent application of ‘High temperature, pulsed heating reactor and methods for polymer recycling’.
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Extended data figures and tables
Extended Data Fig. 1 The assembly of the STH system.
A thinner layer of carbon felt (about 2.3 mm) is used as the top heater layer. The two ends of the top heater layer are wrapped with Cu foil electrodes for Joule heating. The top and bottom layers are placed in soft contact (without external pressure) for the STH process. A quartz tube with 10.5 mm inner diameter was used to contain the carbon bilayer structure and the reactant reservoir. The bottom image shows the heater layer exhibiting a bright orange colour as we apply an electrical current through the top heater layer, demonstrating its Joule-heating capabilities.
Extended Data Fig. 2 Characterization of the wicked PP melt.
a, Schematic showing the position of the cross-sections cut from the reactor layer after STH operation for about 6 min (0.11 s power on at about 22 V, 0.99 s power off; small-scale reaction system). b–d, Representative optical microscopy images showing the wicking of PP in the reactor layer. The images correspond to the cross-sections shown in a. The white species are wicked plastic melt. The black objects are the cut pieces of the porous reactor layer with cross-sections facing out.
Extended Data Fig. 3 Temperature measurement using the customized quartz tube with a small opening.
a, Temperature maps of the heater and reactor layers at various time points during one heating cycle of 1.10 s (power on time: 0.11 s at about 22 V; power off time: 0.99 s), measured in the customized quartz tube with a small opening (about 7 × 14 mm) on the tube wall. The temperature scale is the same that as shown in Fig. 2g. This temperature measurement features better accuracy, as the bilayer structure is housed in a quartz tube setup similar to the real STH process but has lower spatial resolution because of the limited size of the opening on the quartz tube wall and background noise from the quartz tube for differentiating the position of the heater and reactor layers compared with the measurement using the environmental chamber. b, Comparing the temporal temperature profiles measured using the lab-built environmental chamber and the customized quartz tube at the position labelled in red as shown in Fig. 2c. c, Comparing the temporal temperature profiles measured using the lab-built environmental chamber and the customized quartz tube at the position labelled in pink as shown in Fig. 2c.
Extended Data Fig. 4 A typical gas chromatograph of the PP pyrolysis products by STH (0.11 s power on at about 22 V and 0.99 s power off).
Inset, the gas bag used to collect the reaction products in the gas phase, which is connected downstream in the STH system.
Extended Data Fig. 5 Reactive molecular dynamics simulations of PP pyrolysis at 1,527 °C.
a–d, A series of representative images of the simulated space, showing the transition from PP (0 ns; a) to a large number of C3H6 monomers (about 5.5 ns; b), and then to various aromatic species (about 28–100 ns; c and d) owing to dehydrogenation and aromatization under extended continuous heating. e, The counts of C3H6 and H2 molecules throughout the simulated reaction time frame. The number of C3H6 monomers reaches a peak concentration within a short period of time (about 5.5 ns), followed by a gradual drop owing to dehydrogenation, C–C bond coupling, aromatization etc., whereas the number of H2 molecules continues increasing as the reaction approaches chemical equilibrium.
Extended Data Fig. 6 The larger-scale STH system.
Schematic (a) and images (b) of the larger-scale STH system. A quartz tube with 34 mm inner diameter was used to contain the carbon bilayer structure and reactant reservoir. This system allowed us to react up to 1.0 g of PP material within 35 min in a batch reaction mode.
Extended Data Fig. 7 Temperature measurement during the PET depolymerization by STH.
a, Temperature maps of the heater and reactor layers at various time points during one heating cycle of 1.10 s (power on time: 0.11 s at about 26 V; power off time: 0.99 s). b, The temporal heating profiles of the four representative positions shown in Fig. 2c. The orange region indicates power on, whereas the blue region indicates power off.
Extended Data Fig. 8 Gas chromatograph by GC-MS of the liquid products from PET pyrolysis by means of STH using a heating duration of 0.11 s at about 26 V in a period of 1.10 s.
Inset, digital image showing the collected products (about 2 wt% in acetone) after three batches of the STH process with PET. A fraction of the products was collected downstream with the carrier gas. The rest of the products condensed in the quartz tube, which was rinsed off using acetone and combined with the downstream products.
Extended Data Fig. 9 A typical gas chromatograph by GC-FID of the gaseous products from the STH process with PET using a heating duration of 0.11 s at about 26 V in a period of 1.10 s.
Acetylene is observed as one of the gaseous products, which is consistent with the β-CH hydrogen-transfer mechanism35. Note that acetylene is prone to secondary reactions, therefore the measured content does not directly reflect the total amount of acetylene produced along with the formation of 1,4-benzenedicarboxylic acid.
Extended Data Fig. 10 A prototype design of a continuous STH process that involves melting, wicking, vaporization and reaction, along with pulsed electrical heating for recycling commodity plastics such as polyolefins and polyesters.
The recycling system integrates commercially available tools and materials, demonstrating the potential practicality of the STH approach.
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Dong, Q., Lele, A.D., Zhao, X. et al. Depolymerization of plastics by means of electrified spatiotemporal heating. Nature 616, 488–494 (2023). https://doi.org/10.1038/s41586-023-05845-8
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DOI: https://doi.org/10.1038/s41586-023-05845-8
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