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Unraveling the structure and chemical mechanisms of highly oxygenated intermediates in oxidation of organic compounds

  1. S. Mani Sarathya,1
  1. aClean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia;
  2. bDepartment of Chemistry, University of California, Berkeley, CA 94720;
  3. cDepartment of Physics, University of California, Berkeley, CA 94720;
  4. dChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720;
  5. eDepartment of Chemistry, University of Central Florida, Orlando, FL 32816-2450;
  6. fCombustion Research Facility, Sandia National Laboratories, Livermore, CA 94551;
  7. gPhysikalisch-Technische Bundesanstalt, 38116 Braunschweig, Germany;
  8. hAnalytical Core Laboratory, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia;
  9. iDepartment of Chemistry, Bielefeld University, D-33615 Bielefeld, Germany;
  10. jCNRS, Institut National des Sciences de l’Ingénierie et des Systèmes, Institut de Combustion, Aérothermique, Réactivité et Environnement, 45071, Orléans, Cedex 2, France
  1. Edited by Donald G. Truhlar, University of Minnesota, Minneapolis, MN, and approved November 6, 2017 (received for review May 6, 2017)

Significance

Highly oxygenated molecules are involved in autooxidation reactions leading to the formation of secondary organic aerosols (SOAs); they are also critical intermediates in autooxidation processes for liquid hydrogen degradation and the ignition of fuels in advanced combustion systems. However, these reactions are still poorly understood. In this study, we unveil a generalized reaction mechanism involving the autooxidation of peroxy radicals with at least three stages of sequential O2 addition. We elucidate important underlying kinetics and structural characteristics of autooxidation processes used for developing new technologies including those aimed at reducing climatically active SOAs and pollutants from fuel combustion. We show that advances can be made by bridging experimental and theoretical methods used by atmospheric and combustion scientists.

Abstract

Decades of research on the autooxidation of organic compounds have provided fundamental and practical insights into these processes; however, the structure of many key autooxidation intermediates and the reactions leading to their formation still remain unclear. This work provides additional experimental evidence that highly oxygenated intermediates with one or more hydroperoxy groups are prevalent in the autooxidation of various oxygenated (e.g., alcohol, aldehyde, keto compounds, ether, and ester) and nonoxygenated (e.g., normal alkane, branched alkane, and cycloalkane) organic compounds. These findings improve our understanding of autooxidation reaction mechanisms that are routinely used to predict fuel ignition and oxidative stability of liquid hydrocarbons, while also providing insights relevant to the formation mechanisms of tropospheric aerosol building blocks. The direct observation of highly oxygenated intermediates for the autooxidation of alkanes at 500–600 K builds upon prior observations made in atmospheric conditions for the autooxidation of terpenes and other unsaturated hydrocarbons; it shows that highly oxygenated intermediates are stable at conditions above room temperature. These results further reveal that highly oxygenated intermediates are not only accessible by chemical activation but also by thermal activation. Theoretical calculations on H-atom migration reactions are presented to rationalize the relationship between the organic compound’s molecular structure (n-alkane, branched alkane, and cycloalkane) and its propensity to produce highly oxygenated intermediates via extensive autooxidation of hydroperoxyalkylperoxy radicals. Finally, detailed chemical kinetic simulations demonstrate the influence of these additional reaction pathways on the ignition of practical fuels.

Footnotes

  • ?1To whom correspondence may be addressed. Email: zhandong.wang{at}kaust.edu.sa or Mani.Sarathy{at}kaust.edu.sa.
  • Author contributions: Z.W., N.H., P.D., and S.M.S. designed research; Z.W., D.M.P.-V., B.C., K.M., S.Y.M., H.W., S.S., M.A.R., N.H., P.D., and S.M.S. performed research; Z.W. analyzed data; and Z.W., D.M.P.-V., K.K.-H., N.H., P.D., S.R.L., and S.M.S. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.danielhellerman.com/lookup/suppl/doi:10.1073/pnas.1707564114/-/DCSupplemental.

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