Lawrence Berkeley National Laboratory

We investigated temperature-dependent products in the pyrolysis of helium-seeded n-dodecane, which represents a surrogate of the n-alkane fraction of Jet Propellant-8 (JP-8) aviation fuel. The experiments were performed in a high temperature chemical reactor over a temperature range of 1200 K to 1600 K at a pressure of 600 Torr, with in situ identification of the nascent products in a supersonic molecular beam using single photon vacuum ultraviolet (VUV) photoionization coupled with the analysis of the ions in a reflectron time-of-flight mass spectrometer (ReTOF). For the first time, the initial decomposition products of n-dodecane-including radicals and thermally labile closed-shell species-were probed in experiments, which effectively exclude mass growth processes. A total of 15 different products were identified, such as molecular hydrogen (H₂), C2 to C7 1-alkenes [ethylene (C₂H₄) to 1-heptene (C₇H₁₄)], C1-C3 radicals [methyl (CH₃), ethyl (C₂H₅), allyl (C₃H₅)], small C1-C3 hydrocarbons [acetylene (C₂H₂), allene (C₃H₄), methylacetylene (C₃H₄)], as well as the reaction products [1,3-butadiene (C₄H₆), 2-butene (C₄H₈)] attributed to higher-order processes. Electronic structure calculations carried out at the G3(CCSD,MP2)//B3LYP/6-311G(d,p) level of theory combined with RRKM/master equation of rate constants for relevant reaction steps showed that n-dodecane decomposes initially by a nonterminal C-C bond cleavage and producing a mixture of alkyl radicals from ethyl to decyl with approximately equal branching ratios. The alkyl radicals appear to be unstable under the experimental conditions and to rapidly dissociate either directly by C-C bond β-scission to produce ethylene (C₂H₄) plus a smaller 1-alkyl radical with the number of carbon atoms diminished by two or via 1,5-, 1,6-, or 1,7- 1,4-, 1,9-, or 1,8-H shifts followed by C-C β-scission producing alkenes from propene to 1-nonene together with smaller 1-alkyl radicals. The stability and hence the branching ratios of higher alkenes decrease as temperature increases. The C-C β-scission continues all the way to the propyl radical (C₃H₇), which dissociates to methyl (CH₃) plus ethylene (C₂H₄). In addition, at higher temperatures, another mechanism can contribute, in which hydrogen atoms abstract hydrogen from C₁₂H₂₆ producing various n-dodecyl radicals and these radicals then decompose by C-C bond β-scission to C3 to C11 alkenes.