Journal
COMBUSTION AND FLAME
Volume 164, Issue -, Pages 386-396Publisher
ELSEVIER SCIENCE INC
DOI: 10.1016/j.combustflame.2015.11.035
Keywords
Auto-oxidation; Chain-branching; Highly oxidized multifunctional molecules; Peroxides; Alternative isomerization; Synchrotron VUV photoionization mass spectrometry
Categories
Funding
- King Abdullah University of Science and Technology (KAUST)
- Saudi Aramco under the FUELCOM program
- KAUST
- National Key Scientific Instruments and Equipment Development Program of China [2012YQ22011305]
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences
- Department of Energy Gas Phase Chemical Physics Program at Lawrence Berkeley National Laboratory [DEAC02-05CH11231]
- Alexander von Humboldt Foundation
- DFG [SFB 686, TP B3]
- European Research Council under the European Community's Seventh Framework Programme (FP7)/ERC [291049-2G-CSafe]
- National Nuclear Security Administration [DE-AC04-94-AL85000]
- Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy [DEAC02-05CH11231]
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Chain-branching reactions represent a general motif in chemistry, encountered in atmospheric chemistry, combustion, polymerization, and photochemistry; the nature and amount of radicals generated by chain branching are decisive for the reaction progress, its energy signature, and the time towards its completion. In this study, experimental evidence for two new types of chain-branching reactions is presented, based upon detection of highly oxidized multifunctional molecules (HOM) formed during the gas-phase low-temperature oxidation of a branched alkane under conditions relevant to combustion. The oxidation of 2,5-dimethylhexane (DMH) in a jet-stirred reactor (JSR) was studied using synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). Specifically, species with four and five oxygen atoms were probed, having molecular formulas of C8H14O4 (e.g., diketo-hydroperoxide/keto-hydroperoxy cyclic ether) and C8H16O5 (e.g., keto-dihydroperoxide/dihydroperoxy cyclic ether), respectively. The formation of C8H16O5 species involves alternative isomerization of OOQOOH radicals via intramolecular H-atom migration, followed by third O-2 addition, intramolecular isomerization, and OH release; C8H14O4 species are proposed to result from subsequent reactions of C8H16O5 species. The mechanistic pathways involving these species are related to those proposed as a source of low-volatility highly oxygenated species in Earth's troposphere. At the higher temperatures relevant to auto-ignition, they can result in a net increase of hydroxyl radical production, so these are additional radical chain-branching pathways for ignition. The results presented herein extend the conceptual basis of reaction mechanisms used to predict the reaction behavior of ignition, and have implications on atmospheric gas-phase chemistry and the oxidative stability of organic substances. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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