Benzene precursors and formation routes in a stoichiometric cyclohexane flame

Benzene formation was found to be dominated by stepwise radical dehydrogenation of cyclohexane in a stoichiometric flat flame of cyclohexane/O2/32.5% Ar, 30.00 Torr pressure, and 35.0 cm s−1 feed velocity. This route, involving H-abstractions and β-scissions, is in contrast to conventional propargyl routes. Three types of analyses lead to this conclusion: identification of key flame species by mass and ionization energy; measurement and use of mole-fraction profiles in the flat flame; and mechanistic reactive-flow modeling of the flame, interpreted by analyzing the dominant reaction steps giving rise to the prediction. For relevant species, profiles of mole fraction were mapped by molecular-beam mass spectrometry in separate apparatuses with identical burners using electron ionization (UMass Amherst) and synchrotron VUV photoionization (LBNL ALS), respectively. In the latter, recently developed apparatus, ionization energies can be measured with greatly enhanced resolution, yielding improvements in species identification that include precise resolution of hydrocarbon isomers, crucial to the findings of this study.