Reaction pathways for the growth of polycyclic aromatic hydrocarbons during the supercritical pyrolysis of n-decane, as determined from doping experiments with 1- and 2-methylnaphthalene

In order to investigate growth mechanisms of polycyclic aromatic hydrocarbons (PAH) that can lead to solids formation in the pre-combustion environments of future high-speed aircraft, we have performed supercritical pyrolysis experiments with the model alkane fuel n-decane (critical temperature, 344.5 °C; critical pressure, 20.7 atm), to which 2-methylnaphthalene and 1-methylnaphthalene, two two-ring products of n-decane pyrolysis, have each been added as a dopant (each dopant concentration, 8.8 mg/g n-decane). The experiments are conducted in an isothermal, silica-lined stainless-steel flow reactor at 570 °C, 94.6 atm, and 133 s, conditions of incipient solids formation. Analyses of the PAH products by high-pressure liquid chromatography with diode-array ultraviolet-visible absorbance and mass-spectrometric detection reveal vast differences in the growth behavior of the two dopants. In the case of 2-methylnaphthalene, growth is virtually limited to the two- and three-ring PAH that result from 2-naphthylmethyl's reactions with methyl, ethylene, propene, and 1-butene-all abundant in the n-decane pyrolysis environment. In the case of 1-methylnaphthalene, the methyl group's attachment to a carbon that is just-adjacent to a “valley” carbon of the naphthalene structure permits 1-naphthylmethyl's reaction with ethylene to form phenalene, an unstable C13H10 PAH that readily loses hydrogen to form the resonance-stabilized phenalenyl radical. This radical sets in motion a sequence for PAH growth that involves reactions of the C2-C4 1-alkenes with both phenalenyl-type radicals and arylmethyl radicals (whose methyl group is attached to a carbon that is just-adjacent to a valley carbon)-producing unsubstituted PAH and methylated PAH of successively higher ring number. As a consequence of these reactions, even though the dopant 1-methylnaphthalene undergoes only 4.5% conversion, it selectively increases (by up to a factor of four) the yields of many of n-decane's favored four- to eight-ring PAH products-demonstrating the prevalence of the arylmethyl/alkene/phenalenyl reaction mechanism in the supercritical n-decane pyrolysis environment.