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Unconventional peroxy chemistry in alcohol oxidation: The water elimination pathway

Journal of Physical Chemistry Letters

Welz, Oliver W.; Klippenstein, Stephen J.; Harding, Lawrence B.; Taatjes, Craig A.; Zador, Judit

Predictive simulation for designing efficient engines requires detailed modeling of combustion chemistry, for which the possibility of unknown pathways is a continual concern. Here, we characterize a low-lying water elimination pathway from key hydroperoxyalkyl (QOOH) radicals derived from alcohols. The corresponding saddle-point structure involves the interaction of radical and zwitterionic electronic states. This interaction presents extreme difficulties for electronic structure characterizations, but we demonstrate that these properties of this saddle point can be well captured by M06-2X and CCSD(T) methods. Experimental evidence for the existence and relevance of this pathway is shown in recently reported data on the low-temperature oxidation of isopentanol and isobutanol. In these systems, water elimination is a major pathway, and is likely ubiquitous in low-temperature alcohol oxidation. These findings will substantially alter current alcohol oxidation mechanisms. Moreover, the methods described will be useful for the more general phenomenon of interacting radical and zwitterionic states. © 2013 American Chemical Society.

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TYPE Conference YEAR 2013
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Low-temperature combustion chemistry of biofuels: Pathways in the low-temperature (550-700 K) oxidation chemistry of isobutanol and tert-butanol

Proceedings of the Combustion Institute

Savee, John D.; Eskola, Arkke J.; Sheps, Leonid; Osborn, David L.; Taatjes, Craig A.

Butanol isomers are promising next-generation biofuels. Their use in internal combustion applications, especially those relying on low-temperature autoignition, requires an understanding of their low-temperature combustion chemistry. Whereas the high-temperature oxidation chemistry of all four butanol isomers has been the subject of substantial experimental and theoretical efforts, their low-temperature oxidation chemistry remains underexplored. In this work we report an experimental study on the fundamental low-temperature oxidation chemistry of two butanol isomers, tert-butanol and isobutanol, in low-pressure (4-5.1 Torr) experiments at 550 and 700 K. We use pulsed-photolytic chlorine atom initiation to generate hydroxyalkyl radicals derived from tert-butanol and isobutanol, and probe the chemistry of these radicals in the presence of an excess of O2 by multiplexed time-resolved tunable synchrotron photoionization mass spectrometry. Isomer-resolved yields of stable products are determined, providing insight into the chemistry of the different hydroxyalkyl radicals. In isobutanol oxidation, we find that the reaction of the a-hydroxyalkyl radical with O2 is predominantly linked to chain-terminating formation of HO2. The Waddington mechanism, associated with chain-propagating formation of OH, is the main product channel in the reactions of O2 with b-hydroxyalkyl radicals derived from both tert-butanol and isobutanol. In the tert-butanol case, direct HO2 elimination is not possible in the b-hydroxyalkyl + O2 reaction because of the absence of a beta C-H bond; this channel is available in the b-hydroxyalkyl + O2 reaction for isobutanol, but we find that it is strongly suppressed. Observed evolution of the main products from 550 to 700 K can be qualitatively explained by an increasing role of hydroxyalkyl radical decomposition at 700 K. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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TYPE Conference YEAR 2013
OSTIScopus

Synchrotron photoionization measurements of fundamental autoignition reactions: Product formation in low-temperature isobutane oxidation

Proceedings of the Combustion Institute

Eskola, Arkke J.; Welz, Oliver W.; Savee, John D.; Osborn, David L.; Taatjes, Craig A.

Product formation in laser-photolytic Cl-initiated low-temperature (550-700 K) oxidation of isobutane in a slow-flow reactor was investigated by tunable synchrotron photoionization mass spectrometry. These experiments probed the time-resolved formation of products following photolytic initiation of the oxidation, and identify isomeric species by their photoionization spectra. The relative yields of oxygenated product isomers (2,2-dimethyloxirane, methylpropanal, and 3-methyloxetane) are in reasonable concord with measurements from Walker and co-workers (J. Chem. Soc. Faraday Trans. 74 (1) (1978) 2229-2251) at higher temperature. Oxidation of isotopically labeled isobutane, (CH3)3CD, suggests that methylpropanal formation can proceed from both (CH3)2CCH2OOH and CH 3CH(CH2)CH2OOH isomers. Bimodal time behavior is observed for product formation; the initial prompt formation reflects "formally direct" channels, principally chemical activation, and the longer-timescale "delayed" component arises from dissociation of thermalized ROO and QOOH radicals. The proportion of prompt to delayed signal is smaller for the oxygenated products than for the isobutene product. This channel-specific behavior can be qualitatively understood by considering the different energetic distributions of ROO and QOOH in formally direct vs. thermal channels and the fact that the transition states involved in the formation of oxygenated products are "tighter" than that for isobutene formation. © 2012 Published by Elsevier Inc. on behalf of The Combustion Institute.

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TYPE Conference YEAR 2013
OSTIScopus
Results 126–150 of 237
Results 126–150 of 237