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

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‐C₆H₁₂) 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 (C₃H₇) and allyl (C₃H₅) radicals, the following Arrhenius expression was derived: k_R2(T) = 2.3 × 10¹⁶ exp(?36,672 K/T) s^?1. For cyclohexane, overall rate coefficients (k_ov) were deduced for the global reaction cC₆H₁₂ → products + H from the H‐atom time profiles; the following temperature dependency was obtained: k_ov(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‐C₆H₁₂, 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. Int J Chem Kinet 43: 107–119, 2011