C7 reaction mechanism and its self-imitation in the kinetic modeling of PAH formation

Polycyclic aromatic hydrocarbons (PAHs), serving as critical soot precursors, are formed via complicated chemical processes, such as the first aromatic ring formation and the aromatic ring growth, from small molecules. Benzene/phenyl (“C6”) is commonly considered as the critical first aromatic ring formed, which has attracted many studies on its formation and its further mass growth. Reactions of cyclopentadienyl (“C5”) are also recognized as PAH formation pathways without involving benzene. Compared to “C5” and “C6”, less attention has been paid to “C7” aromatics, such as vinyl-cyclopentadienyl, tropyl and fulvenallenyl, for their roles in PAH formation. Previous PAH models included very few reactions of “C7” sub-mechanism, and somewhat ignored the self-consistency of the reaction networks of aromatic species with characteristic molecular structures. In this study, we upgraded “C7” sub-mechanism and revealed the self-imitation between the reaction network of monocyclic and polycyclic aromatic hydrocarbons. Model validation with literature experimental data shows that: a) benzyl, vinyl-cyclopentadienyl, tropyl, and fulvenallenyl are crucial intermediates in “C7” chemistry; b) “C7” resonance stabilized radicals (RSRs) are mainly contributed by different entrance channels from small aromatic precursors; c) the reactions of “C7” species provide unique contribution to PAH formation according to their specific molecular moieties. This study also reveals the basic aromatic classes, “C5”, “C6”, “C7”, and “fC7”, for the hierarchical reaction network, and the self-imitation between the hierarchical reaction networks of monocyclic and polycyclic aromatic hydrocarbons. Future work will build a kinetic model up to very large PAHs, close to soot nanoparticle, using the self-imitation of the hierarchical reaction network proposed in this study.