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New Insights into Low-Temperature Oxidation of Propane from Synchrotron Photoionization Mass Spectrometry and Multiscale Informatics Modeling

Journal of Physical Chemistry A

Welz, Oliver; Burke, Michael P.; Antonov, Ivan O.; Goldsmith, C.F.; Savee, John D.; Osborn, David L.; Taatjes, Craig A.; Klippenstein, Stephen J.; Sheps, Leonid S.

Low-temperature propane oxidation was studied at P = 4 Torr and T = 530, 600, and 670 K by time-resolved multiplexed photoionization mass spectrometry (MPIMS), which probes the reactants, intermediates, and products with isomeric selectivity using tunable synchrotron vacuum UV ionizing radiation. The oxidation is initiated by pulsed laser photolysis of oxalyl chloride, (COCl)2, at 248 nm, which rapidly generates a ∼1:1 mixture of 1-propyl (n-propyl) and 2-propyl (i-propyl) radicals via the fast Cl + propane reaction. At all three temperatures, the major stable product species is propene, formed in the propyl + O2 reactions by direct HO2 elimination from both n- and i-propyl peroxy radicals. The experimentally derived propene yields relative to the initial concentration of Cl atoms are (20 ± 4)% at 530 K, (55 ± 11)% at 600 K, and (86 ± 17)% at 670 K at a reaction time of 20 ms. The lower yield of propene at low temperature reflects substantial formation of propyl peroxy radicals, which do not completely decompose on the experimental time scale. In addition, C3H6O isomers methyloxirane, oxetane, acetone, and propanal are detected as minor products. Our measured yields of oxetane and methyloxirane, which are coproducts of OH radicals, suggest a revision of the OH formation pathways in models of low-temperature propane oxidation. The experimental results are modeled and interpreted using a multiscale informatics approach, presented in detail in a separate publication (Burke, M. P.; Goldsmith, C. F.; Klippenstein, S. J.; Welz, O.; Huang H.; Antonov I. O.; Savee J. D.; Osborn D. L.; Zádor, J.; Taatjes, C. A.; Sheps, L. Multiscale Informatics for Low-Temperature Propane Oxidation: Further Complexities in Studies of Complex Reactions. J. Phys. Chem A. 2015, DOI: 10.1021/acs.jpca.5b01003). The model predicts the time profiles and yields of the experimentally observed primary products well, and shows satisfactory agreement for products formed mostly via secondary radical-radical reactions.

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TYPE Journal Article YEAR 2015
DOIOSTIScopus

Direct observation and kinetics of a hydroperoxyalkyl radical (QOOH)

Science

Savee, John D.; Papajak, Ewa P.; Rotavera, Brandon R.; Huang, Haifeng; Eskola, Arkke J.; Welz, Oliver; Sheps, Leonid S.; Taatjes, Craig A.; Zador, Judit Z.; Osborn, David L.

Oxidation of organic compounds in combustion and in Earth's troposphere is mediated by reactive species formed by the addition of molecular oxygen (O2) to organic radicals. Among the most crucial and elusive of these intermediates are hydroperoxyalkyl radicals, often denoted "QOOH." These species and their reactions with O2 are responsible for the radical chain branching that sustains autoignition and are implicated in tropospheric autoxidation that can form low-volatility, highly oxygenated organic aerosol precursors. We report direct observation and kinetics measurements of a QOOH intermediate in the oxidation of 1,3-cycloheptadiene, a molecule that offers insight into both resonance-stabilized and nonstabilized radical intermediates. The results establish that resonance stabilization dramatically changes QOOH reactivity and, hence, that oxidation of unsaturated organics can produce exceptionally long-lived QOOH intermediates.

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TYPE Conference Poster YEAR 2015
OSTIScopus

Photoionization mass spectrometric measurements of initial reaction pathways in low-temperature oxidation of 2,5-dimethylhexane

Journal of Physical Chemistry A

Rotavera, Brandon R.; Zador, Judit Z.; Welz, Oliver; Sheps, Leonid S.; Scheer, Adam M.; Savee, John D.; Akbar Ali, Mohamad; Lee, Taek S.; Simmons, Blake S.; Osborn, David L.; Violi, Angela; Taatjes, Craig A.

Product formation from R + O2 reactions relevant to low-temperature autoignition chemistry was studied for 2,5-dimethylhexane, a symmetrically branched octane isomer, at 550 and 650 K using Cl-atom initiated oxidation and multiplexed photoionization mass spectrometry (MPIMS). Interpretation of time- and photon-energy-resolved mass spectra led to three specific results important to characterizing the initial oxidation steps: (1) quantified isomer-resolved branching ratios for HO2 + alkene channels; (2) 2,2,5,5-tetramethyltetrahydrofuran is formed in substantial yield from addition of O2 to tertiary 2,5-dimethylhex-2-yl followed by isomerization of the resulting ROO adduct to tertiary hydroperoxyalkyl (QOOH) and exhibits a positive dependence on temperature over the range covered leading to a higher flux relative to aggregate cyclic ether yield. The higher relative flux is explained by a 1,5-hydrogen atom shift reaction that converts the initial primary alkyl radical (2,5-dimethylhex-1-yl) to the tertiary alkyl radical 2,5-dimethylhex-2-yl, providing an additional source of tertiary alkyl radicals. Quantum-chemical and master-equation calculations of the unimolecular decomposition of the primary alkyl radical reveal that isomerization to the tertiary alkyl radical is the most favorable pathway, and is favored over O2-addition at 650 K under the conditions herein. The isomerization pathway to tertiary alkyl radicals therefore contributes an additional mechanism to 2,2,5,5-tetramethyltetrahydrofuran formation; (3) carbonyl species (acetone, propanal, and methylpropanal) consistent with β-scission of QOOH radicals were formed in significant yield, indicating unimolecular QOOH decomposition into carbonyl + alkene + OH. (Chemical Equation Pesented).

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TYPE Journal Article YEAR 2014
DOIOSTIScopus
Results 51–75 of 90
Results 51–75 of 90