Title : Mechanistic insights of PP/zeolites pyrolysis via ReaxFF MD and DFT simulations
Abstract:
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 and pyrolysis parameters. 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 and Density functional theory (DFT) calculation integrated with experimental validation are employed to elucidate how pyrolysis temperature, ratio of feedstock to zeolites, zeolite topology, aluminium distribution, and Si/Al ratio govern polypropylene (PP) upcycling behaviour. Through ReaxFF-MD simulation techniques, it was discovered that the optimal pyrolysis temperature and catalyst loading of PP are 2000 K (475 ℃ in the experiments) and 30 % respectively, with a conversion efficiency over 98 %. Moreover, the catalytic efficiency of PP decreased by less than 2 % after 5 cycles for HZSM-5. 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⁻¹) confirmed its superior catalytic activity. DFT calculation identifies the T7 position with the lowest Fermi level (-3.488 eV), facilitating hydrogen transfer that converts ·C3H5 intermediates into propylene (C3H6). 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 toward energy-efficient and sustainable plastic recycling.
