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Study shows alkanes can rapidly acquire oxygen atoms in a free radical chain reaction; significant for fuel combustion and air pollution

An international team of researchers has shown that alkanes participate extensively in autoxidation reactions with oxygen molecules. The discovery, which overturns current chemical wisdom, has implications for air quality prediction and efficient fuel combustion in engines. An open-access paper on the work is published in the journal Communications Chemistry.

Oxidation chemistry controls both combustion processes and the atmospheric transformation of volatile emissions. In combustion engines, radical species undergo isomerization reactions that allow fast addition of O2. This chain reaction, termed autoxidation, is enabled by high engine temperatures, but has recently been also identified as an important source for highly oxygenated species in the atmosphere, forming organic aerosol.

Conventional knowledge suggests that atmospheric autoxidation requires suitable structural features, like double bonds or oxygen-containing moieties, in the precursors. With neither of these functionalities, alkanes, the primary fuel type in combustion engines and an important class of urban trace gases, are thought to have minor susceptibility to extensive autoxidation. Here, utilizing state-of-the-art mass spectrometry, measuring both radicals and oxidation products, we show that alkanes undergo autoxidation much more efficiently than previously thought, both under atmospheric and combustion conditions.

Even at high concentrations of NOx, which typically rapidly terminates autoxidation in urban areas, the studied C6–C10 alkanes produce considerable amounts of highly oxygenated products that can contribute to urban organic aerosol. The results of this inter-disciplinary effort provide crucial information on oxidation processes in both combustion engines and the atmosphere, with direct implications for engine efficiency and urban air quality.

—Wang et al.

Autoxidation is a chemical process in which oxygen molecules rapidly and sequentially add to organic molecules in a radical chain reaction. The process is critical for the timing of fuel combustion in engines and is a key step in the atmospheric conversion of volatile organic molecules into particulate matter.

Conventional knowledge suggests atmospheric autoxidation requires precursor molecules with features such as double bonds or oxygen-containing moieties, explains lead and co-corresponding author Zhandong Wang, now a professor at the University of Science and Technology of China, formerly a research scientist with Mani Sarathy at KAUST.

Alkanes—the primary component of combustion engine fuels and an important class of urban trace gases—do not have these structural features. As a result, alkanes were thought to have only minor susceptibility to extensive autoxidation, Wang says.

Wang and colleagues had earlier shown that alkanes do undergo extensive autoxidation under the hot high-pressure conditions of combustion. The team then set out to explore the possibility that alkane autoxidation also occurs under atmospheric conditions.

The team used chemical ionization atmospheric pressure interface time-of-flight mass spectrometry to detect products of atmospheric alkane autoxidation.

Strikingly, the yield of highly oxygenated organic molecules containing six or more oxygen atoms was much higher than expected.

—Zhandong Wang

Under combustion conditions, the team also observed alkanes that had undergone up to five sequential O2 additions, significantly higher than the three additions they observed previously.

These findings enrich our understanding of autoxidation processes and will allow us to better perform predictive simulations of combustion engines and atmospheric processes that impact air quality and climate.

—Mani Sarathy, co-corresponding author

Resources

  • Wang, Z., Ehn, M., Rissanen, M.P., Garmash, O., Quéléver, L., Xing, L., Monge-Palacios, M., Rantala, P., Donahue, N.M., Berndt, T., Sarathy, S.M. (2021) “Efficient alkane oxidation under combustion engine and atmospheric conditions.” Communications Chemistry 4, 18 doi: 10.1038/s42004-020-00445-3

  • Wang, Z., Popolan-Vaida, D. M., Chen, B., Moshammer, K., Mohamed, S.Y., Wang, H., Sioud, S., Raji, M.A., Kohose-Hoinghaus, K., Hansen, N., Dagaut, P., Leone, S. & Sarathy, S.M. (2017) “Unraveling the structure and chemical mechanisms of highly oxygenated intermediates in oxidation of organic compounds.” PNAS 114, 3102-13107 doi: 10.1073/pnas.1707564114

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