The Effects of Temperature on the Yields of Aliphatic and Aromatic Products from the Supercritical Pyrolysis of 1-Octene

Prior to their combustion, fuels for future high-speed aircraft are expected to experience supercritical conditions, leading to the production of polycyclic aromatic hydrocarbons (PAH), precursors to solid carbonaceous deposits, via pyrolytic reactions. These solid deposits can clog fuel-transfer lines, causing unsafe aircraft operation. To prevent the formation of fuel-line deposits, it is critical to understand the reaction pathways that lead to PAH formation in the supercritical fuel pyrolysis environment. To better understand the role of large 1-alkenes in PAH formation, supercritical pyrolysis experiments with model fuel 1-octene, a representative 1-alkene product from supercritical n-alkane pyrolysis, have been performed. The experiments are carried out in an isothermal, silica-lined stainless-steel flow reactor at 94.6 atm, 133 sec, and ten temperatures in the range 440-535 °C. Aliphatic products and one- and two-ring aromatics are analyzed by gas chromatography with flame-ionization detection and mass spectrometry. A two-dimensional high-pressure liquid-chromatographic technique establishes 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 also produced in the supercritical n-decane pyrolysis reaction environment. 1-Octene readily converts due to its weak allylic C—C bond, and its rapid conversion corresponds to the production of the highest-yield products at all temperatures, the straight-chain aliphatics—particularly n-alkanes, 1-alkenes, and 2-alkenes. At temperatures ≤ 500 °C, high yields of the C8 product 1-octene and the C16 products 6-hexadecene and 9-methyl-6-pentadecene provide evidence that hydrogen abstraction of 1-octene’s allylic C—H bond produces the resonance-stabilized radical 1-octen-3-yl, whose resonance structure 2-octen-1-yl abstracts hydrogen to form 2-octene or reacts with 1-octene at the first or second or to form, with high selectivity, 6-hexadecene or 9-methyl-6-pentadecene, respectively. Just like n-decane, 1-octene pyrolysis produces an abundance of methyl-substituted PAH, which are a ready source of arylmethyl and phenalenyl-type radicals. Reactions of these radicals with C2-C4 1-alkenes lead to PAH growth of successively higher ring number. PAH as large as nine rings are already produced at 500 °C, and the yields drastically increase as the temperature approaches 535 °C, 1-octene’s temperature of incipient solids formation