Kinetic analysis of the pathways to naphthalene formation from phenyl + 1,3-Butadiyne reaction

Having a better understanding of polycyclic aromatic hydrocarbon (PAH) formation under flame conditions contributes to optimizing the fuel reforming process, where soot poisons the downstream catalyst. In this work, the phenyl + 1,3-Butadiyne reaction is systematically investigated to examine its contribution to naphthalene formation. The reaction potential energy surfaces were calculated using DFT/M06–2X/cc-pvtz and G4 methods. The temperature- and pressure-dependent reaction rate constants were calculated using RRKM theory with solving master equation. The results revealed that 2-naphthyl could be directly formed by phenyl + 1,3-Butadiyne reaction. With H assistance, naphthalene could be formed by the pathway of phenyl + 1,3-Butadiyne → C₆H₅CHCCCH (+H) → C₆H₅CHCHCCH (+H) →naphthalene +H. The proposed pathway is kinetically favorable, and featured by relatively low energy barrier. The importance of the proposed pathway reaction was confirmed in a premixed and a diffusion C₂H₄/O₂/Ar flame simulations, where the enhancement of naphthalene by the investigated reactions is notable. The mole fraction of A₂ is promoted by a factor of 10% in premix C₂H₄/O₂/Ar flame and 30% in C₂H₄/O₂/Ar counterflow flame, bringing the prediction results closer to the experimental results. The relative contribution of different reaction route to A₂ formation is evaluated for HACA, cyclopentadienyl radical-cyclopentadienyl radical, phenyl-vinylacetylene[1], benzyl radical-propargyl radical, indene-CH₂ and phenyl-1,3-Butadiyne routes in premixed and diffusion C₂H₄/O₂/Ar flames. This work suggests that the PAH growth by 1,3-Butadiyne addition reaction is an effective pathway for A₂ formation, which should be considered in future PAH mechanism.