Time-resolved measurements of product formation in the low-temperature (550–675 K) oxidation of neopentane: a probe to investigate chain-branching mechanism

Abstract

Product formation, in particular ketohydroperoxide formation and decomposition, were investigated in time-resolved, Cl-atom initiated neopentane oxidation experiments in the temperature range 550-675 K using a photoionization time-of-flight mass spectrometer. Ionization light was provided either by Advanced Light Source tunable synchrotron radiation or ∼10.2 eV fixed energy radiation from a H-discharge lamp. Experiments were performed both at 1-2 atm pressure using a high-pressure reactor and also at ∼9 Torr pressure employing a low-pressure reactor for comparison. Because of the highly symmetric structure of neopentane, ketohydroperoxide signal can be attributed to a 3-hydroperoxy-2,2-dimethylpropanal isomer, i.e. from a γ-ketohydroperoxide (γ-KHP). The photoionization spectra of the γ-KHP measured at low- and high pressures and varying oxygen concentrations agree well with each other, further supporting they originate from the single isomer. Measurements performed in this work also suggest that the "Korcek" mechanism may play an important role in the decomposition of 3-hydroperoxy-2,2-dimethylpropanal, especially at lower temperatures. However, at higher temperatures where γ-KHP decomposition to hydroxyl radical and oxy-radical dominates, oxidation of the oxy-radical yields a new important channel leading to acetone, carbon monoxide, and OH radical. Starting from the initial neopentyl + O reaction, this channel releases altogether three OH radicals. A strongly temperature-dependent reaction product is observed at m/z = 100, likely attributable to 2,2-dimethylpropanedial.

Chemistry deriving from OOQOOH radicals in alkane low-temperature oxidation: A first combined theoretical and electron-ion coincidence mass spectrometry study

“…an energy between 1 and 1.3 eV above the KHP ionization energies). This value is certainly too high when compared to values recently published for similar KHPs:  6.56 Mb for hydroperoxymethyl formate at 11.5 eV, an energy 1.5 eV above its ionization energy [8],  between 3.3 and 8.3 Mb for the different KHP isomers issued from tetrahydrofuran at an energy 1.5 eV above their ionization energies [10].…”

“…1 clearly shows the presence of three fragments that could be attributed to the fragmentation of C5KHP: m/z 43, 57 and 85. coincides with the fragmentation onset of acetone [22]. In addition, the temperature profiles shown in [8] and [10]. While Hansen et al [10] only found this fragmentation, Moshammer et al [8] also established the possibility of losing CO and an H atom.…”

“…In addition, the temperature profiles shown in [8] and [10]. While Hansen et al [10] only found this fragmentation, Moshammer et al [8] also established the possibility of losing CO and an H atom. In the present study, the mass spectra, such as that of Fig.…”

“…While their formation pathway is still not fully understood, diones are currently considered as directly formed from KHP or as products of chemically activated Q OOH + O2 reaction in parallel with the formation of KHP [2] [8]. Recent JSR works investigating the oxidation of oxygenated molecules (dimethyl ether [8], tetrahydrofuran [10]) have documented the possibility of better quantifying KHPs based on their fragmentation. The second purpose of this paper is shedding light on the first steps of the unimolecular decomposition pathways undertaken by the C5KHPs derived from n-pentane.…”

n-Heptane cool flame chemistry: Unraveling intermediate species measured in a stirred reactor and motored engine

“…intermediates with two double bonds and one hydroperoxy group, such as C7H12O2-6 in Scheme 4, may also be produced. Recent work on neopentane low-temperature oxidation performed by Eskola et al [60] suggested that a chemically activated QOOH+O2 reaction might be a source for 2,2dimehtylpropanedial in parallel to keto-hydroperoxide. This pathway may also lead to C7H12O2 species in n-heptane low-temperature oxidation.…”