Parametric optimisation and mechanistic characterisation of polypropylene/HZSM-5 pyrolysis towards high-efficiency resource recycling

To achieve a plastic upcycling pyrolysis process with high conversion efficiency and low energy input, the pyrolysis conditions of polypropylene(PP)/HZSM-5 were optimised. Through reactive force field molecular dynamics (ReaxFF-MD) simulation techniques, it was discovered that the optimal pyrolysis temperature and catalyst loading of PP are 2000 K (748 K in the experiments) and 30 % respectively, with a conversion efficiency over 98 %. The catalytic efficiency of PP decreased by less than 2 % after 5 cycles for HZSM-5. Moreover, the catalytic effect can be breakdown down into two steps: Firstly, the HZSM-5 enhances the conversion of C₃H₅ and C₃H₇, and then these intermediate species have a higher tendency to form C₃H₆ as the final products. Furthermore, the deterioration mechanism of HZSM-5 was attributed to structural deformation on the active sites after numerous reactions with PP, and subsequently, a neutral hydroxyl group replaced the active site. Overall, this work demonstrated an in-depth characterisation approach using ReaxFF-MD in visualising the molecular breakdown process, revealing the catalytic and deactivation mechanism of HZSM-5 to PP.