Morphological and ReaxFF study on active site in zeolite structure for polypropylene upcycling towards high gas yield production

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Zeolites offer tunable acidity and porous frameworks, making them an ideal candidate for the plastic upcycling process. However, the efficiency of currently used zeolites has yet to satisfy needs in terms of industrial-scale applications. In-depth understanding is required to realise the catalytic effect of zeolite structures, particularly the active site distribution and Si/Al ratios of zeolites result in uncertainties towards their conversion efficiency and gas product of plastic recycling, leaving the quantitative relationship between zeolites' structural parameters and catalytic efficiency insufficiently understood. Herein, reactive molecular dynamics (ReaxFF-MD) simulations integrated with experimental validation are employed to elucidate how zeolite topology, aluminium distribution, and Si/Al ratio govern polypropylene (PP) upcycling behaviour. Comparative analyses of HZSM-5, HZSM-11, HZSM-23, and HZSM-35 reveal that HZSM-23 achieves the highest gas yield of 44% and complete conversion efficiency, corresponding to a 57.7% enhancement over non-catalytic PP pyrolysis. The strong acidity and derived activation energy (184.8 kJ/mol) confirmed its superior catalytic activity. Density functional theory (DFT) calculation identifies the T7 position with the lowest Fermi level (−3.488 eV), facilitating hydrogen transfer that converts ·C 3 H 5 intermediates into propylene (C 3 H 6 ). The optimal Si/Al = 30 ratio further balances acidity and desorption, maximising olefin selectivity. Orthogonal optimisation established HZSM-23@T7 (Si/Al = 30) as the most efficient configuration with good reusability and stability. This combined computational–experimental approach provides molecular-scale insights for rational zeolite design, offering a predictive pathway for industrial-scale production towards energy-efficient and sustainable plastic recycling.

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