Pyrolytic growth of polycyclic aromatic hydrocarbons by cyclopentadienyl moieties

Polycyclic aromatic hydrocarbon (PAH) growth from cyclopentadiene (CPD) and indene (C9H8), which contains the five-member ring cyclopentadienyl moiety, was investigated experimentally in a four-second flow reactor over a temperature range of 550 to 850 degreesC. Major products observed were grouped into two categories: ortho-fused six-member ring compounds formed from combinations of two reactants, and PAHs with one less carbon atom than the two reactants and containing one cyclopentadienyl moiety. Benzene and naphthalene were also formed by addition of one carbon atom to the reactants. Reaction pathways to these products were proposed. Addition of the cyclopentadienyl moiety to pi bond produced a resonance-stabilized radical, which further reacted by one of two unimolecular channels. Decomposition by beta scission produced a biaryl intermediate, which then underwent two ling expansion sequences that were proposed for CPD-to-naphthalene conversion. In the case of indene and indene/CPD combinations, branching in the ring expansion steps led to three C18H12 isomers (chrysene, benz[a]anthracene, and benzo[c] phenanthrene) and two C14H10 isomers (anthracene and phenanthrene), respectively. Intramolecular addition competed with the beta scission pathway to produce a 7-norbornenyl radical, which then decomposed to benzofluorenes (C17H12), fluorene, and benandenes (C13H10), and indene from combinations of indenes, indene/CPD, and CPDs, respectively. This second reaction channel, in which one of the cyclopentadienyl moieties was retained, was observed at all temperatures and was most important at temperatures below 700 degreesC. Reaction pathway analysis using the semiempirical PM3 method was used to explain both the preferential formation of chrysene and anthracene from combinations of two indenes and CPD/indene, respectively, by the first reaction channel, and the temperature dependence in the relative rates of the two reaction channels.