Combined Experimental and Computational Study on the Unimolecular Decomposition of JP-8 Jet Fuel Surrogates. I. <i>n</i>-Decane (<i>n</i>-C₁₀H₂₂)

Abstract

Exploiting a high temperature chemical reactor, we explored the pyrolysis of helium-seeded n-decane as a surrogate of the n-alkane fraction of Jet Propellant-8 (JP-8) over a temperature range of 1100-1600 K at a pressure of 600 Torr. The nascent products were identified in situ in a supersonic molecular beam via single photon vacuum ultraviolet (VUV) photoionization coupled with a mass spectroscopic analysis of the ions in a reflectron time-of-flight mass spectrometer (ReTOF). Our studies probe, for the first time, the initial reaction products formed in the decomposition of n-decane-including radicals and thermally labile closed-shell species effectively excluding mass growth processes. The present study identified 18 products: molecular hydrogen (H₂), C2 to C7 1-alkenes [ethylene (C₂H₄) to 1-heptene (C₇H₁₄)], C1-C3 radicals [methyl (CH₃), vinyl (C₂H₃), ethyl (C₂H₅), propargyl (C₃H₃), allyl (C₃H₅)], small C1-C3 hydrocarbons [methane (CH₄), acetylene (C₂H₂), allene (C₃H₄), methylacetylene (C₃H₄)], along with higher-order reaction products [1,3-butadiene (C₄H₆), 2-butene (C₄H₈)]. On the basis of electronic structure calculations, n-decane decomposes initially by C-C bond cleavage (excluding the terminal C-C bonds) producing a mixture of alkyl radicals from ethyl to octyl. These alkyl radicals are unstable under the experimental conditions and rapidly dissociate by C-C bond β-scission to split ethylene (C₂H₄) plus a 1-alkyl radical with the number of carbon atoms reduced by two and 1,4-, 1,5-, 1,6-, or 1,7-H shifts followed by C-C β-scission producing alkenes from propene to 1-octene in combination with smaller 1-alkyl radicals. The higher alkenes become increasingly unstable with rising temperature. When the C-C β-scission continues all the way to the propyl radical (C₃H₇), it dissociates producing methyl (CH₃) plus ethylene (C₂H₄). Also, at higher temperatures, hydrogen atoms can abstract hydrogen from C₁₀H₂₂ to yield n-decyl radicals, while methyl (CH₃) can also abstract hydrogen or recombine with hydrogen to form methane. These n-decyl radicals can decompose via C-C-bond β-scission to C3 to C9 alkenes.