Synchrotron Photoionization Mass Spectrometry Measurements of Product Formation in Low-Temperature <i>n</i>‑Butane Oxidation: Toward a Fundamental Understanding of Autoignition Chemistry and <i>n</i>‑C<sub>4</sub>H<sub>9</sub> + O<sub>2</sub>/<i>s</i>‑C<sub>4</sub>H<sub>9</sub> + O<sub>2</sub> Reactions

Abstract

Product formation in the laser-initiated low-temperature (575–700 K) oxidation of <i>n</i>-butane was investigated by using tunable synchrotron photoionization time-of-flight mass spectrometry at low pressure (∼4 Torr). Oxidation was triggered either by 351 nm photolysis of Cl<sub>2</sub> and subsequent fast Cl + <i>n</i>-butane reaction or by 248 nm photolysis of 1-iodobutane or 2-iodobutane. Iodobutane photolysis allowed isomer-specific preparation of either <i>n</i>-C<sub>4</sub>H<sub>9</sub> or <i>s</i>-C<sub>4</sub>H<sub>9</sub> radicals. Experiments probed the time-resolved formation of products and identified isomeric species by their photoionization spectra. For stable primary products of butyl + O<sub>2</sub> reactions (e.g., butene or oxygen heterocycles) bimodal time behavior is observed; the initial prompt formation, primarily due to chemical activation, is followed by a slower component arising from the dissociation of thermalized butylperoxy or hydroperoxybutyl radicals. In addition, time-resolved formation of C<sub>4</sub>-ketohydroperoxides (C<sub>4</sub>H<sub>8</sub>O<sub>3</sub>, <i>m</i>/<i>z</i> = 104) was observed in the <i>n</i>-C<sub>4</sub>H<sub>9</sub> + O<sub>2</sub> and Cl-initiated oxidation experiments but not in the <i>s</i>-C<sub>4</sub>H<sub>9</sub> + O<sub>2</sub> measurements, suggesting isomeric selectivity in the combined process of the “second” oxygen addition to hydroperoxybutyl radicals and subsequent internal H-abstraction/dissociation leading to ketohydroperoxide + OH. To further constrain product isomer identification, Cl-initiated oxidation experiments were also performed with partially deuterated <i>n</i>-butanes (CD<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>CD<sub>3</sub> and CH<sub>3</sub>CD<sub>2</sub>CD<sub>2</sub>CH<sub>3</sub>). From these experiments, the relative yields of butene product isomers (<i>cis</i>-2-butene, <i>trans</i>-2-butene, and 1-butene) from C<sub>4</sub>H<sub>8</sub> + HO<sub>2</sub> reaction channels and oxygenated product isomers (2,3-dimethyloxirane, 2-methyloxetane, tetrahydrofuran, ethyloxirane, butanal, and butanone) associated with OH formation were determined. The current measurements show substantially different isomeric selectivity for oxygenated products than do recent jet-stirred reactor studies but are in reasonable agreement with measurements from butane addition to reacting H<sub>2</sub>/O<sub>2</sub> mixtures at 753 K

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