Temperature- and pressure-dependent rate coefficients for the HACA pathways from benzene to naphthalene

Abstract

RRKM-Master Equation calculations have been performed to evaluate temperature- and pressure-dependent rate coefficients for acetylene addition reactions to the C6H5, C6H4C2H, C6H5C2H2, and C6H4C2H3 radicals. These calculations indicate a strong pressure dependence for the role of various Hydrogen-Abstraction-C2H2-Addition (HACA) sequences for the formation of naphthalene from benzene. At atmospheric and lower pressures the C8H7 radicals, C6H4C2H3 and C6H5C2H2, cannot be stabilized above 1650K. As a result, both the Bittner–Howard HACA route, in which a second acetylene molecule adds to C6H5C2H2, and the modified Frenklach route, where a second C2H2 adds to the aromatic ring of C6H4C2H3 obtained by internal hydrogen abstraction, are unrealistic under low pressure flame conditions. At the higher pressures of some practical combustion devices (e.g., 100atm) these routes may be operative. Naphthalene is predicted to be the main product of the C6H5C2H2+C2H2 and C6H4C2H3+C2H2 reactions in the entire 500–2500K temperature range independent of pressure (ignoring the issues related to the instability of C8H7 species). Frenklach's original HACA route, where the second C2H2 molecule adds to the aromatic ring activated by intermolecular H abstraction from C8H6, involves the C6H4C2H+C2H2 reaction, which is shown to predominantly form dehydrogenated species with a naphthalene core (naphthyl radicals or naphthynes) at T <2000K and diethynylbenzene at higher temperatures. The temperature and pressure dependence of rate coefficients for the various reaction channels has been analyzed and the results clearly demonstrate the importance of pressure for the reaction outcome. Thus, one must use caution when using low-pressure flame studies to validate PAH mechanisms for use in broader ranges of pressure.