Reaction pathways of molecular growth for bay-region methyl-substituted polycyclic aromatic hydrocarbons during supercritical pyrolysis of n-decane, as determined from experiments with 4-methylchrysene dopant

Abstract

To investigate the molecular-growth reactions of bay-region methyl-substituted polycyclic aromatic hydrocarbons (PAH) in the supercritical n-alkane-fuel pyrolysis environment, we have performed supercritical n-decane pyrolysis experiments with and without the dopant 4-methylchrysene (dopant concentration, 0.684 mg/g n-decane), in an isothermal, silica-lined stainless-steel flow reactor at 568 °C, 94.6 atm, and 133 s, conditions of rapid PAH growth. Analyses of the reaction products by high-pressure liquid chromatography with diode-array ultraviolet-visible absorbance and mass-spectrometric detection reveal that in the supercritical n-decane pyrolysis environment—where the aliphatic growth species methyl, ethylene, propene, and 1-butene are abundant—molecular growth of the four-ring 4-methylchrysene to high-ring-number PAH occurs chiefly through two routes, both of which take advantage of the bay-region position of 4-methylchrysene's methyl substituent. In the first route, 4-methylchrysene's resonance-stabilized arylmethyl radical 4-chrysenylmethyl attacks the aryl carbon just across the bay of 4-methylchrysene's structure to form the C19H12 methylene-bridged PAH 4H-cyclopenta[def]chrysene, whose resonance-stabilized radical 4-cyclopenta[def]chrysenyl reacts with methyl and ethylene in ring-expansion reactions that selectively produce the five-ring C20H12 benzo[a]pyrene and its derivatives 4- and 5-methylbenzo[a]pyrene. The arylmethyl radicals 4- and 5-benzo[a]pyrenylmethyl then undergo a series of reactions with n-decane's high-yield C2C4 1-alkenes to selectively produce particular six- to nine-ring peri‑condensed benzenoid PAH such as dibenzo[a,e]pyrene, benzo[a]naphtho[2,3-e]pyrene, dibenzo[e,ghi]perylene, benzo[a]coronene, and naphtho[8,1,2-abc]coronene—all of whose yields increase dramatically with 4-methylchrysene doping. In 4-methylchrysene's second major route of molecular growth, 4-chrysenylmethyl's attack of the aryl carbon across the bay is followed by an intramolecular rearrangement that produces 4H-benzo[cd]fluoranthene, a non-fully aromatic C19H12 PAH with an internal five-membered ring. Reactions of the resonance-stabilized benzo[cd]fluoranthenyl radicals with methyl, propene, and 1-butene selectively produce the six-ring C22H12 indeno[1,2,3-cd]pyrene and four particular seven-ring C26H14 benzindeno[1,2,3-cd]pyrenes—each of which contains an internal five-membered ring, evidencing the distinctive molecular-growth pathways of bay-region methyl-substituted PAH in the supercritical n-alkane-fuel pyrolysis environment.