Recycling polyolefin plastic waste at short contact times via rapid joule heating

Abstract

The chemical deconstruction of polyolefins to fuels, lubricants, and waxes offers a promising strategy for mitigating their accumulation in landfills and the environment. Yet, achieving true recyclability of polyolefins into C₂-C₄ monomers with high yields, low energy demand, and low carbon dioxide emissions under realistic polymer-to-catalyst ratios remains elusive. Here, we demonstrate a single-step electrified approach utilizing Rapid Joule Heating over an H-ZSM-5 catalyst to efficiently deconstruct polyolefin plastic waste into light olefins (C₂-C₄) in milliseconds, with high productivity at much higher polymer-to-catalyst ratio than prior work. The catalyst is essential in producing a narrow distribution of light olefins. Pulsed operation and steam co-feeding enable highly selective deconstruction (product fraction of >90% towards C₂-C₄ hydrocarbons) with minimal catalyst deactivation compared to Continuous Joule Heating. This laboratory-scale approach demonstrates effective deconstruction of real-life waste materials, resilience to additives and impurities, and versatility for circular polyolefin plastic waste management.

Fig. 1. Chemical pathways of converting polyolefins to monomers.

Path 1 involves plastics hydrocracking at high hydrogen pressures to produce naphtha, which is then converted to C₂–C₄ olefins via steam cracking at high temperatures. Path 2 (this work) demonstrates the direct conversion of polyolefins via rapid pulse or continuous Joule heating over H-ZSM-5 catalysts.

Fig. 2. Performance of thermal and catalytic rapid pulse heating (RPH) on LDPE deconstruction at various operating conditions.

a Effect of DC voltage on the performance of thermal and catalytic RPH (60 pulses). b Effect of the number of pulses on the performance of RPH of LDPE over H-ZSM-5 catalyst (42 V). c Effect of polymer-to-catalyst ratio on LDPE conversion (42 V and 10 pulses). d Raman spectra of spent catalysts demonstrating increased coking at higher polymer-to-catalyst ratios. (e) Effect of the He gas flow rate on the performance of RPH of LDPE over H-ZSM-5 catalyst (42 V and 5 pulses).

Fig. 3. Pulsing effect on conversion.

Effect of (a) pulsing frequency (controlled by varying cooling times) and (b) heating time on the performance of rapid pulse heating (RPH) for LDPE deconstruction at constant energy consumption. The insets in (a) depict the pulses.

Fig. 4. Heating mode performance and catalyst reuse comparison.

a Effect of DC voltage on conversion and product fractions of extractable products for CJH of LDPE over H-ZSM-5 (reaction time = 500 ms). Reusability of CFP coated with H-ZSM-5 for (b) pulse heating (42 V and 10 pulses) and (c) CJH of LDPE (26 V and 500 ms). d Raman spectra of the spent H-ZSM-5 catalyst after 3 cycles of reuse for RPH and CJH of LDPE. e Weight % of coke obtained from TGA of spent catalysts after 4 reuse cycles for RPH and CJH of LDPE.

Fig. 5. Effect of Co-feeding Steam on Performance.

a Effect of co-feeding steam on RPH of LDPE over H-ZSM-5 catalyst (42 V, T_max = 730 °C). b Raman spectra of spent catalysts for RPH and CJH.

Fig. 6. Performance for various feedstocks and of other literature studies.

a Performance of rapid pulse heating (RPH) for the deconstruction of real-life plastics over H-ZSM-5 catalyst (42 V, 10 pulses and T_max = 730 °C). b Images of real-life grocery bags, fishnets, centrifuge tubes, Ziploc^TM bags, and PE bottles were used in this study. c Comparison of the product fractions of C₂–C₄ olefins vs. catalyst-to-polymer ratio of this work to literature for the catalytic deconstruction of PE^,,,–. Results are at high ( ~ 100%) conversion.