Multiple benzene-formation paths in a fuel-rich cyclohexane flame

Detailed data and modeling of cyclohexane flames establish that a mixture of pathways contributes to benzene formation and that this mixture changes with stoichiometry. Mole-fraction profiles are mapped for more than 40 species in a fuel-rich, premixed flat flame (/ = 2.0, cyclohexane/O2/30% Ar, 30 Torr, 50.0 cm/s) using molecular-beam mass spectrometry with VUV-photoionization at the Advanced Light Source of the Lawrence Berkeley National Laboratory. The use of a newly constructed set of reactions leads to an excellent simulation of this flame and an earlier stoichiometric flame (M.E. Law et al., Proc. Combust. Inst. 31 (2007) 565–573), permitting analysis of the contributing mechanistic pathways. Under stoichiometric conditions, benzene formation is found to be dominated by stepwise dehydrogenation of the six-membered ring with cyclohexadienyl¢ benzene + H being the final step. This finding is in accordance with recent literature. Dehydrogenation of the six-membered ring is also found to be a dominant benzene-formation route under fuel-rich conditions, at which H2 elimination from 1,3-cyclohexadiene contributes even more than cyclohexadienyl decomposition. Furthermore, at the fuel-rich condition, additional reactions make contributions, including the direct route via 2C3H3 ¢ benzene and more importantly the H-assisted isomerization of fulvene formed from i-/n-C4H5 + C2H2, C3H3 + allyl, and C3H3 + C3H3. Smaller contributions towards benzene formation arise from C4H3 + C2H3, 1,3-C4H6 + C2H3, and potentially via n-C4H5 + C2H2. This diversity of pathways is shown to result nominally from the temperature and the concentrations of benzene precursors present in the benzene-formation zone, which are ultimately due to the feed stoichiometry. 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.