Molecular conversion of polypropylene pyrolysis revealed by Ag+ complexation ESI Orbitrap MS

• Ag + complexation Orbitrap MS resolved molecular conversion behavior of PP pyrolysis. • Low temperature and pressure favor mono-/diolefins with three-carbon repeating unit. • High temperature and pressure prefer to produce aromatics with short side chain. • Molecular-level conversion pathways of olefins and aromatics at varying temperatures were proposed. Pyrolysis has emerged as a key approach for converting plastic waste into valuable resources. However, this process is an extremely complex reaction system, and the molecular conversion behavior and pathways have not been fully understood. This study systematically investigated the thermal pyrolysis of polypropylene (PP) over a temperature range of 420–500 °C, and the composition of PP pyrolysis oils was analyzed using Ag + complexation electrospray ionization high-resolution Orbitrap mass spectrometry. The results indicate that the pyrolysis oils are primarily composed of mono-olefins and diolefins at lower temperature and pressure, with a three-carbon repeating unit. Secondary reactions intensified at elevated temperatures and pressure, progressively converting heavy olefins to polycyclic aromatics. The results of mass spectrometry analysis were compared with gas chromatography showing significant advantages, and were in good agreement with the molecular structure information provided by nuclear magnetic resonance hydrogen spectroscopy and Fourier transform infrared spectroscopy. Furthermore, the PP pyrolysis pathways at varying temperatures were proposed showing the full pathway of cracking and aromatization.