On the role of resonantly stabilized radicals in polycyclic aromatic hydrocarbon (PAH) formation: pyrene and fluoranthene formation from benzyl–indenyl addition

Abstract

Resonantly stabilized radicals, such as propargyl, cyclopentadienyl, benzyl, and indenyl, play a vital role in the formation and growth of polycyclic aromatic hydrocarbons (PAHs) that are soot precursors in engines and flames. Pyrene is considered to be an important PAH, as it is thought to nucleate soot particles, but its formation pathways are not well known. This paper presents a reaction mechanism for the formation of four-ring aromatics, pyrene and fluoranthene, through the combination of benzyl and indenyl radicals. The intermediate species and transition structures involved in the elementary reactions of the mechanism were studied using density functional theory, and the reaction kinetics were evaluated using transition state theory. The barrierless addition of benzyl and indenyl to form the adduct, 1-benzyl-1H-indene, was found to be exothermic with a reaction energy of 204.2 kJ mol. The decomposition of this adduct through H-abstraction and H-loss was studied to determine the possible products. The rate-of-production analysis was conducted to determine the most favourable reactions for pyrene and fluoranthene formation. The premixed laminar flames of toluene, ethylbenzene, and benzene were simulated using a well-validated hydrocarbon fuel mechanism with detailed PAH chemistry after adding the proposed reactions to it. The computed and experimentally observed species profiles were compared to determine the effect of the new reactions for pyrene and fluoranthene formation on their concentration profiles. The role of benzyl and indenyl combination in PAH formation and growth is highlighted.

Polycyclic aromatic hydrocarbons in pyrolysis of gasoline surrogates (n-heptane/iso-octane/toluene)

“…Instead, the reaction of benzyl radical with indenyl radical (C6H5CH2+C9H7=A4+2H2) is the main suource for pyrene in pyrolysis of both TPRF70 and TPRF97.5. The reaction rate of C6H5CH2+C9H7=A4+2H2 in present model is calculated by Sinha et al [34], which has a lower reaction rate comparing to Slavinskaya's [37] work. However, it still over-predicts A4 in TPRF97.5…”

“…In the toluene pyrolysis mechanism by Yuan et al [33], the self-combination of two indenyl radicals leads to chrysene, instead of pyrene. To improve pyrene predictions, the original indenyl radicals addition reaction was replaced by reaction 2C9H7=CHRYSEN+2H and the reaction rate of C6H5CH2+C9H7=A4+2H2 was updated according to Sinha et al [34]. Furthermore, reactions of methylindene (C9H7CH3), estimated from Yuan et al [33],…”

A chemical kinetic modeling study of indene pyrolysis

“…Hansen et al [41] proposed repetitive addition of methyl and acetylene for PAH growth. A series of radical-involved PAH formation pathways have been proposed and calculated [42][43][44][45][46][47][48][49][50][51][52][53][54]. Finally, a number of PAH models [7][8][9]19,30,[55][56][57][58] have thus been developed and validated.…”

An experimental study of indene pyrolysis with synchrotron vacuum ultraviolet photoionization mass spectrometry

“…Recently, Sinha et al calculated the potential energy surface (PES) and rate constants for the reactions between indenyl and benzyl. 62 This pathway could be a necessary complement for the PAH growth from bicyclic to tetracyclic ones. As shown in Fig.…”