Formation of H-atoms in the pyrolysis of cyclohexane and 1-hexene: A shock tube and modeling study

Abstract

Cyclohexane (cC6H12) plays an important role in the combustion of practical liquid fuels, as a major component of naphthenic compounds. Therefore, the pyrolysis of cyclohexane was investigated by measuring the formation of H-atoms. The thermal decomposition of 1-hexene (1-C 6H12) was also studied, because of the assumption that 1-hexene is the sole initial product of cyclohexane decomposition. For cyclohexane, the measurements were performed over a temperature range of 1320-1550 K, at pressures ranging from 1.8 to 2.2 bar; 1-hexene experiments were done at temperatures between 1250 and 1380 K and pressures between 1.5 and 2.5 bar. For each experiment, the time-dependent formation of H-atoms was measured behind reflected shock waves by using the method of atomic resonance absorption spectrometry. For the dissociation of 1-hexene to n-propyl (C3H 7) and allyl (C3H5) radicals, the following Arrhenius expression was derived: kR2(T) = 2.3 × 10 16 exp(-36,672 K/T) s-1. For cyclohexane, overall rate coefficients (kov) were deduced for the global reaction cC 6H12→products + H from the H-atom time profiles; the following temperature dependency was obtained: kov(T) = 4.7 × - 1016 exp(-44,481 K/T) s-1. For both sets of rate coefficient values, an uncertainty of ±30% is estimated. Especially concerning the isomerization cC6H12→1-C 6H12, our experimental results are in excellent agreement with the rate coefficient values given by Tsang (Tsang, W. Int J Chem Kinet 1978, 10, 1119-1138). A reaction model was assembled that is able to reproduce the H-atom profiles measured for both sets of experiments. According to this model, H-atoms are mostly stemming from the thermal decomposition of allyl radicals (C3H5), which arise from the decomposition of 1-hexene. Furthermore, it will be shown that the recombination of allyl radicals with H-atoms to propene (C3H6) also represents a very important subsequent reaction. © 2010 Wiley Periodicals, Inc.