Upcycling polyolefins to methane-free liquid fuel by a Ru1-ZrO2 catalyst

Abstract

Upcycling waste plastics into liquid fuels presents significant potential for advancing the circular economy but is hindered by poor selectivity and low-value methane byproduct formation. In this work, we report that atomic Ru-doped ZrO₂ can selectively convert 100 grams of post-consumer polyethylene and polypropylene, yielding 85 mL of liquid in a solvent-free hydrocracking. The liquid (C₅-C₂₀) comprises ~70% jet-fuel-ranged branched hydrocarbons (C₈-C₁₆), while the gas product is liquefied-petroleum-gas (C₃-C₆) without methane and ethane. We found that the atomic Ru dopant in the Ru-O-Zr moiety functionalizes its neighboring O atom, originally inert, to create a Brønsted acid site. This Brønsted acid site, rather than the atomic Ru dopant itself, selectively governs the internal C-C bond cleavage in polyolefins through a carbonium ion mechanism, thereby enhancing the yield of jet-fuel-ranged hydrocarbons and suppressing methane formation. This oxide modulation strategy provides a paradigm shift in catalyst design for hydrocracking waste plastics and holds potential for a broad spectrum of applications.

Fig. 1. Solvent-free hydrocracking of PE and PP to methane-free fuels.

a The yields of liquid and gaseous products in solvent-free hydrocracking 4 g PE or PP over Ru₁-ZrO₂ at 250 °C under 3 MPa H₂ for 8 hours. b–f Hydrocarbon distribution (b), Photo (c), UV- Vis absorption spectra (d), ¹³C−¹H HSQC NMR (e) and relative amount of aromatics (f) calculated based on ¹H NMR (Supplementary Fig. 8) of the liquid product of hydrocracking PP on Ru₁-ZrO₂ at 300 °C under 1.5 MPa H₂ (e) and 1-4 MPa H₂ (f) for 8 hours.

Fig. 2. Relationship of Ru-O-Zr structure and performance of Ru₁-ZrO₂.

a–e TEM image (a), EDS element mappings (b), HAADF-STEM image (c), Fourier transform of EXAFS spectra (d), and Ru K-edge XANES spectra (e) of Ru₁-ZrO₂. Ru foil and RuO₂ were used as references. f H₂-TPR profiles of ZrO₂, Ru/SiO₂, Ru/ZrO₂, Ru₁-ZrO₂, and Ru₁-ZrO₂_500. These samples were not pretreated in H₂ at 275 °C to obtain their complete TPR profiles. g H₂/D₂ switch experiment tracking m/z = 3 (HD) by mass spectrometer: 10% H₂/Ar flow switched to 10% D₂/Ar flow with a constant rate of 20 mL/min at 250 °C at the time of 0 s. h O 1 s XPS spectra of Ru₁-ZrO₂ and Ru₁-ZrO₂_500. i FTIR spectra of pyridine adsorption at room temperature on ZrO₂ and Ru₁-ZrO₂ before and after (Ru₁-ZrO₂_500) in situ treatment in 20% H₂/Ar at 500 °C.

Fig. 3. β-scission mechanism of PO hydrocracking over Ru₁-ZrO₂.

a Reaction pathways and DFT calculated energy profiles of PP hydrocracking to gas products via β-scission mechanism with carbonium ions (energy unit: eV). The molecule structures in red or blue color are the same type of carbonium circularizing in the hydrocracking loop. b Schematic catalytic cycle of PO (PE or PP) hydrocracking on Ru₁-ZrO₂.

Fig. 4. Potential of Ru₁-ZrO₂ in practice.

a Liquid yields in the cycling experiments on Ru₁-ZrO₂. b, c Hydrocracking of 100 g post-consumer PP and PE substrates on Ru₁-ZrO₂ at 300 ^oC under 3 MPa H₂ for 8 hours: photos of the post-consumer plastics and the liquid product (b) and product selectivity (c).