Formation of H-Atoms in the Pyrolysis of Cyclohexane and 1-Hexene: A Shock Tube and Modeling Study

摘要

Cyclohexane (cC6H_(12)) 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 -C6H_(12)) 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 x 10~(16)exp(—36,672 K/T) s~(-1). For cyclohexane, overall rate coefficients (k_(OV)) were deduced for the global reaction cC6H_(12) → products + H from the H-atom time profiles; the following temperature dependency was obtained: k_(OV)(T) = 4.7 x 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 cC6H_(12)→ 1-C6H_(12), our experimental results are in excellent agreement with the rate coefficient values given by Tsang (Tsang, W. Int I 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.