Additional chain-branching pathways in the low-temperature oxidation of branched alkanes

Abstract

a b s t r a c tChain-branching reactions represent a general motif in chemistry, encountered in atmospheric chemistry, combustion, polymerization, and photochemistry; the nature and amount of radicals generated by chainbranching are decisive for the reaction progress, its energy signature, and the time towards its completion. In this study, experimental evidence for two new types of chain-branching reactions is presented, based upon detection of highly oxidized multifunctional molecules (HOM) formed during the gas-phase low-temperature oxidation of a branched alkane under conditions relevant to combustion. The oxidation of 2,5-dimethylhexane (DMH) in a jet-stirred reactor (JSR) was studied using synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). Specifically, species with four and five oxygen atoms were probed, having molecular formulas of C 8 H 14 O 4 (e.g., diketo-hydroperoxide/ketohydroperoxy cyclic ether) and C 8 H 16 O 5 (e.g., keto-dihydroperoxide/dihydroperoxy cyclic ether), respectively. The formation of C 8 H 16 O 5 species involves alternative isomerization of OOQOOH radicals via intramolecular H-atom migration, followed by third O 2 addition, intramolecular isomerization, and OH release; C 8 H 14 O 4 species are proposed to result from subsequent reactions of C 8 H 16 O 5 species. The mechanistic pathways involving these species are related to those proposed as a source of low-volatility highly oxygenated species in Earth's troposphere. At the higher temperatures relevant to auto-ignition, they can result in a net increase of hydroxyl radical production, so these are additional radical chain-branching pathways for ignition. The results presented herein extend the conceptual basis of reaction mechanisms used to predict the reaction behavior of ignition, and have implications on atmospheric gas-phase chemistry and the oxidative stability of organic substances.

New insights into the low-temperature oxidation of 2-methylhexane

“…The reactor was kept at a quasi-atmospheric pressure of 780 Torr and temperatures between 530 and 740 K. Gas flows were adjusted to achieve a residence time of 0.5 s. The temperature was measured by a K-type thermocouple located in the vicinity of the sampling position. The uncertainty in reactor temperature is estimated to be ±20 K [4] . The JSR product gases are analyzed by a custom-built reflectron time-of-flight system with a sensitivity of 1 ppm, a dynamic range of several orders of magnitude, and a mass resolution of m/ m ∼ 2500.…”

“…This study is concerned with the auto-oxidation of 2-methylhexane, which is an important component for surrogate fuels [ 1 -3 ]. It is a continuation of the earlier work of Wang et al [4] in which the oxidation of 2,5-dimethylhexane was studied. This work is of particular interest because detailed studies on heptane isomers have revealed that low-temperature auto-oxidation and auto-ignition are particularly affected by the number and location of methyl branches [5,6] .…”

“…In this work, the third O 2 addition to P(OOH) 2 , and subsequent pathways of OOP(OOH) 2 were added to the 2MHX kinetic model of Mohamed et al [13] following a similar methodology proposed by Wang et al [4,12] . The subsequent pathways of OOP(OOH) 2 include the following three classes: (i) H-atom migrations of the C-H attached to the -OOH group and CO β-scission to KDHP + OH; (ii) intramolecular H-atom migrations of the C-H attached to the non -OOH group to form T(OOH) 3 , which cyclizes to DHPCE + OH and/or C-O β-scission to olefinic dihydroperoxides (ODHP) + HO 2 ; and (iii) concerted eliminations of HO 2 to form ODHP.…”

“…In their reaction schemes, the P(OOH) 2 radicals decompose into hydroperoxy cyclic ethers and/or olefinic hydroperoxides. Wang et al [4] detected highly oxidized multifunctional (HOM) molecules with five and four oxygen atoms during the autooxidation of 2,5-dimethylhexane in a jet-stirred reactor (JSR). The authors proposed a third O 2 addition reaction scheme to P(OOH) 2 radicals, which predicts additional radical chain branching intermediates.…”

Chemical Kinetic Insights into the Octane Number and Octane Sensitivity of Gasoline Surrogate Mixtures

“…At the MON related conditions, the mixtures do not display prominent two-stage ignition characteristics. Instead, a distinguishing feature is a broad intermediate temperature heat release (ITHR) 69 that transitions to HTHR, causing the main ignition event. For all of the fuel mixtures studied, the magnitude of the ITHR in 825 K simulations is lower than the magnitude of the LTHR observed in 750 K simulations.…”