Combined Experimental and Computational Study on the Unimolecular Decomposition of JP-8 Jet Fuel Surrogates. I. n-Decane (n-CH).

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 (H), C2 to C7 1-alkenes [ethylene (CH) to 1-heptene (CH)], C1-C3 radicals [methyl (CH), vinyl (CH), ethyl (CH), propargyl (CH), allyl (CH)], small C1-C3 hydrocarbons [methane (CH), acetylene (CH), allene (CH), methylacetylene (CH)], along with higher-order reaction products [1,3-butadiene (CH), 2-butene (CH)]. 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 (CH) 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 (CH), it dissociates producing methyl (CH) plus ethylene (CH). Also, at higher temperatures, hydrogen atoms can abstract hydrogen from CH to yield n-decyl radicals, while methyl (CH) 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.