Quantification of the resonance stabilized C4H5 isomers and their reaction with acetylene

The resonance stabilized C4H5 radicals (CH3CCCH2, 2-C4H5; CH2CHCCH2, i-C4H5; CH3CHCCH, 12-C4H5) are among the most important precursors of benzene in hydrocarbon flames. Previous studies have revealed that i-C4H5 mainly reacts with acetylene to produce fulvene which promptly transforms to benzene, while the contribution from 2-C4H5 and 12-C4H5 remains unclear due to the obscure composition of these isomers in flames and the lack of accurate rate constants for related reactions. In the present work, we first calculated the cross sections of the resonance stabilized C4H5 radicals to quantify their composition in hydrocarbon flames (Hansen et al., 2006). The ratio of i-C4H5/(12-C4H5+ 2-C4H5) in the flame zone is deduced as 0.8-1.2 in fuel-rich allene, propyne, cyclopentene or benzene flames, and 2-C4H5 constitutes more than 70% in the sum of 2-C4H5 and 12-C4H5. We further studied the reaction kinetics of 2-/12-C4H5 with acetylene. Similar to i-C4H5, 2-/12-C4H5 tends to produce fulvene rather than directly form benzene when reacting with acetylene. However, the reaction rates of C2H2 + 2-/12-C4H5 are ∼one magnitude lower than that of i-C4H5 under combustion conditions. The role of the reaction between 2-/12-C4H5 and acetylene is controlled by the combined effect of concentration and reaction rates. By including above computational results into kinetic modeling, we finally conclude that although the concentration of 2-/12-C4H5 is comparable to that of i-C4H5, their contribution to the first aromatic ring in hydrocarbon flames from acetylene addition is limited. However, considering the noticeable concentration and the resonance stabilized structure, these species still have potential to generate aromatics.