The Formation of Polycyclic Aromatic Hydrocarbons from the Pyrolysis of Model 1-Alkene Fuels

To understand the role of 1-alkenes and allylic radicals in the reaction pathways leading to the formation and growth of polycyclic aromatic hydrocarbons (PAH), pyrolysis experiments have been performed with three 1-alkene fuels—propylene (CH₂=CH–CH₃), 1-butene (CH₂=CH–CH₂–CH₃), and 1-pentene (CH₂=CH–CH₂–CH₂–CH₃)—at temperatures of 600 – 1000 °C and a fixed residence time of 0.31 s. The experiments are carried out in an isothermal laminar-flow quartz-tube reactor. Analyses of the pyrolysis products by gas-chromatographic and high-pressure liquid-chromatographic techniques reveal that the three fuels differ in: 1) their conversion behavior, 2) the relative amounts of the major C₂ – C₄ species produced, and 3) the propensity for PAH formation. The propylene pyrolysis experiments reveal that propylene’s conversion becomes significant at temperatures ≥ 850 °C, where acetylene, propadiene, and propyne are produced in high yields and allyl and propargyl radicals are abundantly available for aromatic-growth reactions. In contrast, pyrolysis experiments with 1-pentene show that 1-pentene’s conversion is appreciable already above 600 °C, but that a large portion of the reacted carbon is “tied up” in 1-pentene’s highest-yield product ethylene. High yields of acetylene, propylene, propadiene, propyne, 1-butene, and 1,3-butadiene, however, and the readily formed allyl, propargyl, and butadienyl radicals result in increased formation of PAH from 1-pentene pyrolysis compared to propylene pyrolysis. The 1-butene pyrolysis experiments reveal that 1-butene’s conversion becomes substantial above 700 °C and that between 750 and 900 °C, 1-butene produces C₂ – C₄ products in higher yields compared to propylene or 1-pentene pyrolysis. The abundantly produced allyl, propargyl, methylallyl, and butadienyl radicals from 1-butene pyrolysis prove to be very effective aromatic-formation and -growth agents. Consequently, PAH products from 1-butene pyrolysis are both higher in number—69 two- to seven-ring product PAH having been identified, 67 of which for the first time from this fuel—and higher in yield (by factors of 2 – 6) than from the pyrolysis of the other two 1-alkene fuels. The findings of this experimental study unveil the importance of the molecular fuel structure and the critical role of allylic radicals in the formation and growth of PAH from 1-alkenes during fuel pyrolysis