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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

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
OSTIScopus

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
ScopusOSTI
Results 101–125 of 154
Results 101–125 of 154