Low temperature chemistry of 1,2,4-trimethylbenzene in a blend with n-heptane

Abstract

As a representative aromatic component of practical fuels, understanding the low temperature chemistry of 1,2,4-trimethylbenzene under the reaction environment of fuel mixtures is necessary to improve the ignition and combustion process. In this work, low temperature oxidation of 1,2,4-trimethylbenzene was examined in a jet stirred reactor with reactants of n-heptane and 1,2,4-trimethybenzene in a fuel mixture (1:1 in mole, 500–800 K, ϕ = 0.5, τ = 2.0 s, p = 1 bar). Chemical species and their mole fraction evolution with reaction temperatures were recorded by the time of flight molecular beam mass spectrometer using synchrotron vacuum ultraviolet radiation as photon ionization source. Molecular structures of 56 species were identified by comparing the species ionization energies and the photonionization efficiency curves. 25 chemical formula of monocyclic oxygenated aromatics and oxygenated polycyclic aromatic hydrocarbons (OPAH) are detected, and 12 of them are identified. Some of the OPAHs are more toxic than the polycyclic aromatic hydrocarbons, indicating the importance of revealing their formation pathways in low temperature oxidation of aromatics. Numerical simulations using the latest kinetic model from Lawrence Livermore National Laboratory well predict the mole fractions of the reactants and most intermediates, but many monocyclic oxygenated aromatics and OPAHs are missing. The detection and identification of key monocyclic oxygenated aromatics (e.g., peroxides, aldehydes, dialdehydes, and ketohydroperoxides of 1,2,4-trimethylbenzene) provide important clues to unravel the low temperature oxidation pathways of alkylated aromatics like 1,2,4-trimethylbenzene. For monocyclic oxygenated aromatics and OPAHs that are missing in the kinetic model, potential formation pathways were proposed based on the molecular structure information, which might be useful for future kinetic model development.