Formation of Organic Acids and Carbonyl Compounds in n-Butane Oxidation via gamma-Ketohydroperoxide Decomposition

A crucial chain-branching step in autoignition is the decomposition of ketohydroperoxides (KHP) to form an oxy radical and OH. Other pathways compete with chain-branching, such as "Korcek" dissociation of gamma-KHP to a carbonyl and an acid. Here we characterize the formation of a gamma-KHP and its decomposition to formic acid+acetone products from observations of n-butane oxidation in two complementary experiments. In jet-stirred reactor measurements, KHP is observed above 590 K. The KHP concentration decreases with increasing temperature, whereas formic acid and acetone products increase. Observation of characteristic isotopologs acetone-d(3) and formic acid-d(0) 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+acetone.