In a study published in Nature Chemical Engineering, Prof. HAN Buxing and LIN Longfei’s group from the Institute of Chemistry of the Chinese Academy of Sciences, and collaborators from Beijing Normal University and Peking University, developed a kinetic decoupling-recoupling (KDRC) strategy that enables the conversion of polyethylene (PE) to ethylene and propylene with a yield of 79%.

Plastic waste, particularly PE, poses severe environmental challenges due to its large-scale accumulation and resistance to degradation. Chemical recycling that converts waste polyolefins back into monomers like ethylene and propylene is essential for establishing a circular plastic economy. However, conventional catalytic cracking methods suffer from low selectivity and high energy consumption.

In this study, the researchers designed a two-stage reaction system to address the issue of kinetic entanglement in single-stage reaction systems, which limits the improvement in yield of ethylene and propylene from PE cracking. They optimized the reaction condition in each reaction stage by developing a reaction kinetic model and implementing the KDRC strategy.

In Stage I, a layered self-pillared zeolite (LSP-Z100) selectively cracked PE into intermediate compounds (primarily butenes and pentenes) at 260 °C. In Stage II, a phosphorus-modified HZSM-5 (P-HZSM-5) zeolite converted these intermediates into ethylene and propylene via dimerization-β-scission at 540 °C.

To validate the reaction pathway, researchers employed synchrotron-based vacuum ultraviolet photoionization mass spectrometry to detect transient reaction, which provided direct experimental evidence of dimerization–β-scission pathway via capturing C8 intermediates.

Besides, they employed in situ neutron powder diffraction to reveal the precise locations of acid sites, which showed that phosphorus modification suppressed the occurrence of bimolecular side reactions by reducing acid site density, thereby enhancing selectivity of ethylene and propylene.

This catalytic reaction system operates without noble metals or external hydrogen. It demonstrates high catalytic performance in processing real-world plastic wastes, and the catalysts can be recycled. This work offers a practical and efficient route for plastic waste recycling.