Mechanistic insights into the Cr-Zr/ZSM-5 catalyzed cracking of waste polyethylene: Steering product distribution toward BTX aromatics and sustainable aviation fuel.

The catalytic conversion of polyethylene (PE) into a C6-C8 BTX-rich fraction and a C9-C16 SAF-range hydrocarbon fraction provides a potential route for plastic-carbon valorization, where "SAF-range" refers to the aviation-fuel-relevant carbon-number range rather than to specification-compliant finished SAF. However, many reported PE-upcycling routes require elevated temperatures, external H2, noble-metal catalysts, or high catalyst-to-feed ratios. Herein, we report a rationally engineered bifunctional Cr-Zr/ZSM-5 catalyst that converts PE at 280 °C and 0.5 MPa into targeted products with a liquid yield of 62 wt% and approximately 90% selectivity within the BTX-rich C6-C8 fraction and SAF-range C9-C16 hydrocarbons. Combined characterization, pyridine-adsorbed FTIR, online mass spectrometry, in situ FTIR, product analysis, and model-surface DFT calculations support a proposed bifunctional reaction network. A proposed reaction network is constructed, in which initial conversion may involve thermal and surface-assisted fragmentation at accessible external/near-external regions. Cr-Zr oxide-related species may subsequently facilitate adsorption, C-H activation, dehydrogenation, and olefin-intermediate formation. The resulting smaller olefinic intermediates can then undergo Brønsted-acid-initiated secondary conversion within the MFI channels, while product formation is accompanied by dynamic, competitive internal hydrogen redistribution without external H2. This work bridges atomic-scale engineering with catalytic function, providing a scalable, mechanism-guided platform for plastic waste valorization and establishing a fundamental mechanistic framework for the rational design of tandem catalytic systems in waste-to-fuel conversion.