Insight into the pyrolysis behavior of polyvinyl chloride using in situ pyrolysis time-of-flight mass spectrometry: Aromatization mechanism and Cl evolution

Abstract

Online characterization of primary volatiles is crucial for a better understanding of the aromatization mechanism and interaction between radical intermediates triggered by chlorine (Cl) evolution in pyrolysis of polyvinyl chloride (PVC). In this study, in situ pyrolysis time-of-flight mass spectrometry (in situ Py-TOF-MS) equipped with double ionization sources, vacuum ultraviolet photoionization (VUVPI, 10.6 eV), and electron ionization (EI, 70 eV) was applied to online capture primary volatiles released from PVC pyrolysis. According to the total ion currents of primary volatiles with temperature, the pyrolysis of PVC is divided into two stages: stage I (40–343 °C) and stage II (343–650 °C). The results indicated that benzene is the most abundant product that is formed through cyclization/aromatization of long-chain hydrocarbon radicals (LCRs) triggered by dechlorination in stage I, while naphthalene can be formed from interactions of cyclopentadienyls released by LCR cyclization/aromatization. Meanwhile, the hydrogen radicals (H·) extracted from the CH bonds will transfer and react with chlorine radicals (Cl·) to promote HCl release. Aromatics are also released in stage II, among which monoaromatic hydrocarbons (MAHs) are generated by cyclization of fragment-radicals and Diels-Alder (D-A) reactions, while polycyclic aromatic hydrocarbons (PAHs) are formed by cyclization/aromatization and rearrangement reactions. Simultaneously, the Cl-containing long-chain hydrocarbon radicals can fracture to form short-chain Cl-aliphatics as the temperature rises and can also be cyclized/aromatized to form Cl-MAHs. Additionally, other formation pathways, such as the interaction of released Cl· and aliphatic radicals to form Cl-aliphatics and the D-A reaction between dienes and Cl-aliphatics to form Cl-MAHs, were also found.