Simultaneous production of ketohydroperoxides from low temperature oxidation of a gasoline primary reference fuel mixture

Ketohydroperoxides (KHPs) are oxygenated hydrocarbons that include carbonyl and peroxide functional groups. The decomposition pathways of KHPs are chain branching pathways in low temperature chemistry of alkanes and oxygenated biofuels, which makes KHPs critical intermediates in ignition processes in advanced combustion engines. In this work, KHP formation was investigated in the low temperature oxidation of a primary reference fuel (PRF 70), a binary mixture of 30 vol% n-heptane and 70 vol% iso-octane to represent a low octane number gasoline surrogate. Species were detected and measured in a jet-stirred reactor (JSR), coupled with time-of-flight molecular beam mass spectrometer using synchrotron vacuum-ultraviolet radiation as photon ionization source (SVUV-PI-TOF-MBMS) which provides in-situ measurement of unstable KHPs. Simultaneous KHPs production from higher reactivity n-heptane and lower reactivity iso-octane at similar temperatures was observed. Kinetic modeling was used to study the reactivity across a wide temperature range and to examine KHP formation chemistry. Specifically, signals vs. temperature profiles in experiments and mole fraction vs. temperature profiles in simulations, were compared to indicate the most possible saturated KHP isomers. Possible formation pathways for olefinic KHPs and di-olefinic KHPs are discussed. This work reports simultaneous formation of KHPs from two fuel molecules in a gasoline surrogate mixture, discusses important reaction pathways in low temperature oxidation, and explains simultaneous KHP production in n-heptane/iso-octane mixtures, i.e., low temperature oxidation of less reactive iso-octane is initiated by OH radicals produced from high reactivity n-heptane oxidation.