Formation Mechanism of Polycyclic Aromatic Hydrocarbons beyond the Second Aromatic Ring

Abstract

The formation mechanism of polycyclic aromatic hydrocarbons (PAH) with three fused aromatic rings starting from naphthalene has been studied using accurate ab initio G3(MP2,CC)//B3LYP/6-311G** calculations followed by the kinetic analysis of various reaction pathways and computations of relative product yields. The results reveal new insights into the classical hydrogen abstraction-C2H2 addition (HACA) scheme of PAH growth. The HACA mechanism has been shown to produce mostly cyclopentafused PAHs instead of PAHs with six-member rings only, in contrast to the generally accepted view on this mechanism. Considering naphthalene as the initial reactant, the HACA-type synthesis of higher PAHs with all six-member rings, anthracene and phenanthrene, accounts only for 3-6% of the total product yield at temperatures relevant to combustion (1000-2000 K), whereas cyclopentafused PAHs, including acenaphthalene (41-48%), 4-ethynylacenaphthalene (∼14%), 3-ethynylacenaphthalene (∼7.5%), 1-methylene-1H-cyclopenta[b]naphthalene (∼6%), and 3-methylene-3H-cyclopenta[a]naphthalene (∼5%), account for another ∼75%. It has been shown that acetylene addition to the radical site adjacent to the bay region in naphthalene (as in 1-naphthyl radical) or other similar PAH with a bay region is highly unlikely to be followed by the addition of a second acetylene molecule; alternatively, the bay region closure with a buildup of a new five-member ring occurs. Acetylene addition to a nonbay carbon atom (as in 2-naphthyl radical) can be followed by the second acetylene addition only at T < 1000 K, producing anthracene and phenanthrene. However, at temperatures relevant to combustion, such pathways give negligible contributions to the total product yield, whereas the dominant reaction product, 2-ethynylnaphthalene, is formed by simple hydrogen atom elimination from the attached ethenyl group. An additional six-member ring buildup may occur only after intermolecular hydrogen abstraction from ethynyl-substituted PAH (2-ethynylnaphthalene), in particular, from the carbon atoms adjacent to the existing ethynyl (C2H) fragment, followed by C2H2 addition producing adducts with two ethynyl C2H and ethenyl C2H2 groups next to each other, which then undergo a fast six-member ring closure. Nevertheless, this process has been shown to be relatively minor (∼25%), whereas the major process is a five-member ring closure involving the same C2H and C2H2 groups and leading to a cyclopentafused PAH molecule. Although the computed product yields show a good agreement with experimentally observed concentrations of acenaphthalene and anthracene in various aliphatic and aromatic flames, the yield of phenanthrene, which exhibits an order of magnitude higher concentration than anthracene both in combustion flames and environmental mixtures, via the considered pathways is significantly underpredicted. This result points at the possible existence of another mechanism responsible for the formation of phenanthrene and other all-six-member-ring PAHs. The overall kinetic scheme for...

Polycyclic aromatic hydrocarbons (PAH) formation from the pyrolysis of different municipal solid waste fractions

“…In the tar produced from pyrolysis of different samples, the amount of phenanthrene was much higher than that of anthracene, which is consistent with the research of others, where the yield of phenanthrene exhibits an order of magnitude higher concentration than anthracene (Kislov et al, 2013). As shown in Fig.…”

“…It was reported that further growth of large PAH via HACA from acenaphthylene was possible (Shulka and Koshi, 2012). Wang and Violi (2006) reported that the 1,2 double bond in acenaphthylene is much more reactive than the 3,4 and 4,5 aromatic bonds, based on which it is easy to explain the formation of fluoranthene, as shown in The formation of a new six-member ring via this mechanism can account for ~25% of the total product yield, whereas the remaining ~75% of the products are CP-PAH molecules formed via a five-member ring closure involving the same ethynyl and ethenyl groups (Kislov et al, 2013). Richter and Howard (2000) and Marsh and Wornat (2000) also reported that it is unlikely to produce phenanthrene from naphthalene via HACA.…”

Critical Assessment of Photoionization Efficiency Measurements for Characterization of Soot-Precursor Species

“…showed that acetylene addition to naphthyl radicals in a high-temperature reactor led to formation of acenaphthylene, ethynylnaphthalene, and diethynylnaphthalene, but not anthracene or phenanthrene. 21 Their experimental results are in agreement with the theoretical study performed by Kislov et al,61 which showed that the HACA mechanism 23,62 preferentially leads to formation of five-membered rings in the molecular growth beyond naphthalene during combustion. In addition, the 3-ethynylcyclopenta[fg]acenaphthylene stabilomer species was recently predicted by a HACA-dominated mechanism to be among the most commonly produced soot-precursor species containing 16 carbon atoms in an acetylene counter-flow flame.…”

“…Based on recent literature, this stabilomer species appears to be a likely product of the HACA mechanism. 21,31,61 Hence, our PIE results at 200 u suggest that other chemical-growth mechanisms than HACA are important in this flame.…”

Determination of Polycyclic Aromatic Hydrocarbons in Cigarettes and Cigarette Smoke

“…Nine PAHs, as shown in The correlation coefficients among the detected PAHs in air samples after smoking are summarized in Table 5. The correlation coefficients between AN and other PAHs were not significant because AN can play an important role as a precursor to higher PAHs (Kislov et al 2013). However, the correlation coefficients for almost all of the other detected PAHs were greater than 0.643 (p < 0.01), showing that each PAH was strongly related to the other PAHs.…”