A highly efficient growth mechanism of polycyclic aromatic hydrocarbons

Abstract

A highly efficient growth mechanism of polycyclic aromatic hydrocarbons (PAHs) initiated and accelerated by phenyl radicals has been investigated on the basis of kinetic analysis of gas phase reaction products of pyrolysis of benzene with and without addition of acetylene and acetylene only. Pyrolytic reactions were performed in a flow tube reactor and the resulting products were detected by an in situ direct sampling mass spectrometric technique using a vacuum ultraviolet (VUV) single photon ionization (SPI) time of flight mass spectrometry (TOFMS). The detected species varies from smaller to larger PAHs up to m/z = 454 (C₃₆H₂₂) including primary PAHs, polyphenyl-PAHs and cyclopentafused-PAHs (CP-PAHs). The peculiarity of this result is an appearance of mass peaks at regular mass number intervals of ∼76 that correspond to phenyl-PAHs produced by phenyl radical addition (+C₆H₅, +77) followed by hydrogen elimination (−H, −1). All such mass peaks were found diminishing with appearance of −2 mass number peaks with increasing temperatures, certainly due to a conversion of thermally rather unstable phenyl-PAHs into stable condensed PAHs through a dehydrocyclization (−H₂, −2) process. In the same way, in the case of only acetylene pyrolysis, mass peaks at regular mass number intervals of 24 corresponding to the HACA (hydrogen abstraction/C₂H₂ addition) products, were observed. Kinetic analysis of formation pathways of those observed products showed the active role of PAC (phenyl addition/cyclization) because of its efficiency to continue the endless growth of PAHs, while the HACA was only found efficient for producing symmetrical PAHs by filling a triple fusing site (four carbon bay structure). Especially, acetylene was mixed with benzene to understand the impact of HACA on the PAC path ways that resulted in enhancement of phenyl-PAHs production in spite of trapping of active and chain carrier species phenyl radicals by C₂H₂ to form phenylacetylene. The comparison of HACA and PAC concluded that PAC is a highly efficient mechanism for the growth of PAHs and lastly their combined roles in combustion have been discussed. Hopefully, PAC will be useful to understand the process of aromatic growth, from furnaces to stellar atmospheres.