Molecular-growth pathways in premixed flames of benzene and toluene doped with propyne

Abstract

In a combined experimental and modeling effort, we investigated the molecular-growth pathways in propyne-doped low-pressure premixed flames of benzene and toluene. We determined the chemical structures of these two flames with flame-sampling molecular-beam mass spectrometry. The mole fraction profiles of the aromatic intermediates served as validation targets for two chemically detailed mechanisms that were independently developed at the German Aerospace Center (DLR) and at the Lawrence Livermore National Laboratory (LLNL). Reaction path analyses reveal the important pathways for indene, naphthalene, and phenanthrene. There is no appreciable fuel-structure effect and molecular growth was observed to be driven by radical-radical recombination reactions, and ring-closure and ring-enlargement reactions with little contribution from the classical HACA mechanism. Indene is formed in both flames through the reactions of the phenyl and propargyl radicals. The benzyl radical plays only a very minor role in the formation of indene through the reaction with acetylene. Reactions of the propargyl radical with fulvenallenyl, a C7H5 isomer, contribute significantly to naphthalene formation in both flames investigated here. Benzyl radicals contribute to naphthalene formation via reactions with propargyl radical through formation of phenyl-substituted butadienyl and vinylacetylene isomers. Ring-enlargement reactions converting indene's five-membered ring into naphthalene's six-membered ring also contribute in small amounts to naphthalene. Fulvenallenyl radicals also contribute substantially to phenanthrene formation.