Analysis of Some Reaction Pathways Active during Cyclopentadiene Pyrolysis

The cyclopentadienyl radical (cC₅H₅) is among the most stable radical species that can be generated during the combustion and pyrolysis of hydrocarbons and it is generally agreed that its contribution to the gas phase reactivity is significant. In this study the kinetics of one key cC₅H₅ reaction channel, namely the reaction between cC₅H₅ and cyclopentadiene (cC₅H₆), was investigated using ab initio calculations and RRKM/Master Equation theory. It was found that most of the excited C₅H₅_C₅H₆ adducts formed by the addition of cC₅H₅ to cC₅H₆ decompose back to reactants and that the major reaction products are, in order of importance, indene, vinylfulvene (a most probable styrene precursor), phenylbutadiene, and benzene. The preferred reaction pathway of the C₅H₅_C₅H₆ adduct is started by the migration of the tertiary hydrogen of the C₅H₅ ring to a vicinal carbon and followed by the β-opening of the C₅H₆ ring, which is the rate determining step. Successive molecular rearrangements lead to decomposition to the four possible products. The kinetic constants for the four reaction channels, calculated at atmospheric pressure and interpolated in cm³ mol^–1 s^–1 between 900 and 2000 K, are k_indene = 10^25.197T^–3.935 exp­(−11630/T(K)), k_vinylfulvene = 10^65.077T^–14.20 exp­(−37567/T(K)), k_benzene = 10^29.172T^–4.515 exp­(−20570/T(K)), and k_{phenylbutadiene} = 10^16.743T^–1.407 exp­(−11804/T(K)). The predictive capability of the reaction set so determined was tested through the simulations of recent cC₅H₆ pyrolysis and combustion experiments using a detailed kinetic mechanism. A quantitative agreement with experimental data was obtained by assuming that vinylfulvene converts rapidly to stryrene, increasing its reaction channel by a factor of 2, and assuming that phenylbutadiene rapidly decomposes with equal probability to styrene and benzene.