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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 Z.

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

Welz, Oliver W.; Savee, John D.; Eskola, Arkke J.; Sheps, Leonid S.; 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 Journal Article YEAR 2013
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New mechanistic insights to the O(3P) + propene reaction from multiplexed photoionization mass spectrometry

Physical Chemistry Chemical Physics

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

The reaction of O(3P) with propene (C3H6) has been examined using tunable vacuum ultraviolet radiation and time-resolved multiplexed photoionization mass spectrometry at 4 Torr and 298 K. The temporal and isomeric resolution of these experiments allow the separation of primary from secondary reaction products and determination of branching ratios of 1.00, 0.91 ± 0.30, and 0.05 ± 0.04 for the primary product channels CH3 + CH2CHO, C2H5 + HCO, and H2 + CH3CHCO, respectively. The H + CH3CHCHO product channel was not observable for technical reasons in these experiments, so literature values for the branching fraction of this channel were used to convert the measured product branching ratios to branching fractions. The results of the present study, in combination with past experimental and theoretical studies of O(3P) + C3H6, identify important pathways leading to products on the C3H6O potential energy surface (PES). The present results suggest that up to 40% of the total product yield may require intersystem crossing from the initial triplet C3H6O PES to the lower-lying singlet PES. © the Owner Societies.

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TYPE Conference YEAR 2012
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Synchrotron photoionization measurements of OH-initiated cyclohexene oxidation: Ring-preserving products in OH + cyclohexene and hydroxycyclohexyl + O 2 reactions

Journal of Physical Chemistry A

Ray, Amelia W.; Taatjes, Craig A.; Welz, Oliver W.; Osborn, David L.; Meloni, Giovanni

Earlier synchrotron photoionization mass spectrometry experiments suggested a prominent ring-opening channel in the OH-initiated oxidation of cyclohexene, based on comparison of product photoionization spectra with calculated spectra of possible isomers. The present work re-examines the OH + cyclohexene reaction, measuring the isomeric products of OH-initiated oxidation of partially and fully deuterated cyclohexene. In particular, the directly measured photoionization spectrum of 2-cyclohexen-1-ol differs substantially from the previously calculated Franck-Condon envelope, and the product spectrum can be fit with no contribution from ring-opening. Measurements of H 2O 2 photolysis in the presence of C 6D 10 establish that the addition-elimination product incorporates the hydrogen atom from the hydroxyl radical reactant and loses a hydrogen (a D atom in this case) from the ring. Investigation of OH + cyclohexene-4,4,5,5-d 4 confirms this result and allows mass discrimination of different abstraction pathways. Products of 2-hydroxycyclohexyl-d 10 reaction with O 2 are observed upon adding a large excess of O 2 to the OH + C 6D 10 system. © 2012 American Chemical Society.

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TYPE Journal Article YEAR 2012
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Absolute photoionization cross-section of the propargyl radical

Journal of Chemical Physics

Savee, John D.; Soorkia, Satchin; Welz, Oliver W.; Selby, Talitha M.; Taatjes, Craig A.; Osborn, David L.

Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoionization mass spectrometry we have measured the absolute photoionization cross-section for the propargyl (C 3H 3) radical, σ propargyl ion (E), relative to the known absolute cross-section of the methyl (CH 3) radical. We generated a stoichiometric 1:1 ratio of C 3H 3 : CH 3 from 193 nm photolysis of two different C 4H 6 isomers (1-butyne and 1,3-butadiene). Photolysis of 1-butyne yielded values of σ propargyl ion (10.213 eV)=(26.1±4.2) Mb and σ propargyl ion (10.413 eV)=(23.4±3.2) Mb, whereas photolysis of 1,3-butadiene yielded values of σ propargyl ion (10.213 eV)=(23.6±3.6) Mb and σ propargyl ion (10.413 eV)=(25.1±3.5) Mb. These measurements place our relative photoionization cross-section spectrum for propargyl on an absolute scale between 8.6 and 10.5 eV. The cross-section derived from our results is approximately a factor of three larger than previous determinations. © 2012 American Institute of Physics.

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TYPE Journal Article YEAR 2012
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Results 101–150 of 222
Results 101–150 of 222