Understanding benzene formation pathways in pyrolysis of two C6H10 isomers: Cyclohexene and 1,5-hexadiene

To explore the fuel isomeric effects on the benzene formation pathways, the pyrolysis of two C6H10 isomers, cyclohexene (cC6H10) and 1,5-hexadiene (C6H10-15), was investigated by using molecular-beam mass spectrometry with tunable synchrotron radiation as the ionization source. The isomer-resolved pyrolysis intermediates, including some key radicals, were clearly identified and quantified at different temperatures for both fuels. A new kinetic model was developed and validated against the experimental results. The fuel-specific intermediates pools, the dominant fuel destruction pathways, as well as specific reactions channels leading towards benzene formations under pyrolysis conditions were revealed through experimental and modeling efforts. The elimination reaction (cC6H10 = C2H4+ C4H6) and the bond fission (C6H10-15 = C3H5-A + C3H5-A) dominate the consumption of cC6H10 and C6H10-15, respectively. Although the fuel structures of cC6H10 and C6H10-15 and their corresponding intermediate pools are quite different, the stepwise dehydrogenation reactions via cyclohexadiene isomers contribute to the majority of the benzene formation in the pyrolysis of both fuels. The recombination between the propargyl radical (C3H3) and allyl radical (C3H5-A) also contributes to benzene formation in the case of C6H10-15, while the C4+ C2 pathway provides a small amount of benzene in the case of cC6H10.