Growth network of PAH with 5-membered ring: Case study with acenaphthylene molecule

Numerous experimental and theoretical evidence indicates that polycyclic aromatic hydrocarbon (PAH) is the precursor of soot. However, the formation mechanism of PAH at flame conditions is not well understood. In this study, the growth of PAHs with a 5-membered ring via the hydrogen-abstraction-acetylene addition route was systematically investigated using acenaphthylene (A(2)R(5)) as a model molecule. Potential energy surfaces were searched using DFT/B3LYP/6-311+ G(d,p) and ROCCSD(T)/cc-pVDZ methods. The reaction rate coefficients of elementary reactions were evaluated using the transition state theory in the temperature range of 50 0-250 0 K. The results showed that H abstractions by H, OH, and CH3 radicals, and the subsequent C2H2 addition reactions, were highly sensitive to the 5-membered ring. H abstraction reaction from the C atom on a 5-membered ring is not favorable due to the relatively higher energy barrier among all competing reactions. In contrast, the reaction rate coefficient of subsequent C2H2 addition reaction on a 5-membered ring is the highest. The planar PAH with no substitution, planar PAH with C2H substitution, curved PAH with two adjacent 5-membered rings, and PAH with a C-CH2-C functional group, can be formed in the A(2)R(5)-H-C2H2 system investigated. The reactions investigated were merged into a detailed PAH mechanism to check the product distribution in a premixed C2H4 sooting flame. Owing to the existence of the 5-membered ring, the formation of planar PAHs with a C2H substitution was much more favorable than the formation of new 5-membered ring and 6-membered rings. The predicted mole fraction of all C14H8 species with one C2H substitution was as high as 2.8 x 10(-5), higher than that of phenanthrene with same C atom number (1.2 x 10(-5)). A similar phenomenon was observed for C16H8 species with two C2H substitutions. In the future, more attention should be given to PAHs with C2H substitution, due to their high concentrations and enhanced reactivity, as compared to the benzenoid PAH. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

This study systematically investigated the growth mechanism of PAHs with a 5-membered ring and their reaction characteristics under flame conditions. The results showed that H abstraction reactions on the 5-membered ring were unfavorable, while subsequent C2H2 addition reactions were most favorable. The formation of planar PAHs with C2H substitution was found to be more favorable compared to the formation of new 5-membered rings and 6-membered rings.