A crucial chain-branching step in autoignition of hydrocarbons is the decomposition of ketohydroperoxides (KHP) to form an oxy and a hydroxyl radical. Other pathways compete with chain-branching, such as “Korcek” dissociation of γ-KHP to a carbonyl and an acid. Here we characterize the formation of a γ-KHP and its decomposition to formic acid and acetone products from observations of n-butane oxidation. Two complementary experimental approaches that are based on molecular beam high-resolution mass spectrometry in conjunction with tunable synchrotron radiation for ionization have been used to characterize the formation of γ-KHP and its decomposition. In jet-stirred reactor measurements, KHP was detected above 590 K and its concentration was observed to decrease with increasing temperature, whereas the concentrations of formic acid and acetone products increased. Observation of characteristic isotopologs acetone-d3 and formic acid-d0 in the oxidation of CH3CD2CD2CH3 is consistent with a Korcek mechanism. In laser-initiated oxidation experiments of n-butane, formic acid and acetone are produced on the timescale of KHP removal. Modelling the time-resolved production of formic acid provides an estimated upper limit of 2 s-1 for the rate coefficient of KHP decomposition to formic acid and acetone. These results provide experimental and computational bounds that enable the construction of more realistic and accurate chemical kinetic mechanisms for low-temperature oxidation systems, including biofuel combustion and atmospheric oxidation.

Experimental characterization of a chain-terminating pathway in autoignition: Organic acid and carbonyl formation from γ-Ketohydroperoxide decomposition in n-Butane oxidation

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