Polycyclic aromatic hydrocarbons formation and growth during the supercritical pyrolysis of 1-octene

Abstract

To better understand the roles of larger 1-alkenes in reactions leading to the production of polycyclic aromatic hydrocarbons (PAH) and solids in the supercritical pre-combustion environments of future-high-speed-aircraft fuels, supercritical pyrolysis experiments have been conducted with the model fuel 1-octene, a representative 1-alkene product of supercritical n-alkane-fuel pyrolysis. The 1-octene pyrolysis experiments are performed in an isothermal, silica-lined stainless-steel flow reactor at 94.6 atm, 133 s, and ten temperatures in the range 440–535 °C. Straight-chain aliphatics—particularly n-alkanes, 1-alkenes, and 2-alkenes—are the highest-yield products at all temperatures. At temperatures ≤500 °C, high yields of the C8 product 2-octene and the C16 product 6-hexadecene evidence 1-octene's facile formation of the resonance-stabilized alkylallylic radical 1-octen-3-yl, whose resonance structure 2-octen-1-yl not only abstracts H to produce 2-octene but also reacts with 1-octene to produce, with high selectivity, 6-hexadecene—establishing that, at low temperatures, a long-chain alkene with an interior CC double bond can result from alkylallyl-radical addition to a 1-alkene. Gas-chromatographic analyses of 1-octene's aromatic products show that one- and two-ring aromatics are produced at temperatures ≥ 475 °C. High-pressure-liquid-chromatographic analyses reveal the identities of 180 three- to nine-ring PAH products—178 of which have never before been identified as pyrolysis products of a large 1-alkene and all of which are produced from supercritical n-decane pyrolysis as well. As in the case for n-decane, the high levels of methyl-substituted PAH in the 1-octene products provide a ready source for resonance-stabilized arylmethyl radicals and (after reaction with ethylene) phenalenyl-type radicals. Reactions of these arylmethyl and phenalenyl-type radicals with C2C4 1-alkenes lead to the sequential PAH growth that accounts for 1-octene's production of PAH as large as nine rings at temperatures as low as 500 °C and the dramatic increases in the PAH-product yields as temperature approaches 535°C, 1-octene's temperature of incipient solids formation.