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

A vacuum ultraviolet photoionization study on high-temperature decomposition of JP-10 (exo-tetrahydrodicyclopentadiene)

“…The detailed methodology is introduced in our previous works. 70,71 Briefly, the PIE curves were fitted by a linear combination of known PIE reference curves from an online PIE database 72 . Since the ion count normalized by the photon fluxes holds a direct proportional relationship with the mole fraction (concentration), the photoionization cross section, mass discrimination ( ( , ) ∝ ( ) • ( ) • ), 50, 57, 68, 69 and the ion counts measured in the experiment, the branching ratios between the concentrations of individual products can be calculated via (…”

Combined Experimental and Computational Study on the Unimolecular Decomposition of JP-8 Jet Fuel Surrogates. II: n-Dodecane (n-C12H26)

“…1) the initial concentration of n-dodecane is lower by 88% compared with that of n-decane due to the lower vapor pressure; 45 2) the branching ratios of these three species is much lower with respect to the other products.…”

“…The second possible mechanism to form higher alkenes involves H atom shifts in 1-alkyl radicals followed by C-C bond β-scission. We have shown 45…”

“…In our previous work considering the pyrolysis of n-decane 45 we have shown that the higher 1-alkyl radicals are unstable in the experimental temperature range and are subject to a rapid C-C bond β-scission producing ethylene C2H4 together with a smaller 1-alkyl. As shown in Fig.…”

“…Because of this difference, the computed branching ratios for the C-H β-scission channels in C4H9-C8H17 are very small and do not exceed 1-2% until the highest temperatures and pressures (2,500 K and 100 atm), where they reach 5-6%. 45 The relative yield of propene + H is higher from C3H7 and increases from 3-4% at 1,100-…”