No abstract

This work provides new temperature-dependent mole fractions of elusive intermediates relevant to the lowtemperature oxidation of dimethyl ether (DME). It extends the previous study of Moshammer et al. [J. Phys. Chem. A 2015, 119, 7361−7374] in which a combination of a jet-stirred reactor and molecular beam mass spectrometry with singlephoton ionization via tunable synchrotron-generated vacuumultraviolet radiation was used to identify (but not quantify) several highly oxygenated species. Here, temperature-dependent concentration profiles of 17 components were determined in the range of 450−1000 K and compared to up-to-date kinetic modeling results. Special emphasis is paid toward the validation and application of a theoretical method for predicting photoionization cross sections that are hard to obtain experimentally but essential to turn mass spectral data into mole fraction profiles. The presented approach enabled the quantification of the hydroperoxymethyl formate (HOOCH 2 OCH 2 O), which is a key intermediate in the low-temperature oxidation of DME. The quantification of this ketohydroperoxide together with the temperature-dependent concentration profiles of other intermediates including H 2 O 2 , HCOOH, CH 3 OCHO, and CH 3 OOH reveals new opportunities for the development of a next-generation DME combustion chemistry mechanism.

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.

S, et al. (2018) n-Heptane cool flame chemistry: Unraveling intermediate species measured in a stirred reactor and motored engine. Combustion and Flame 187: 199-216.

In this work, we studied the low-temperature oxidation of a stoichiometric 2-methylhexane/O 2 /Ar mixture in a jet-stirred reactor coupled with synchrotron vacuum ultraviolet photoionization molecular-beam mass spectrometry. The initial gas mixture was composed of 2% 2-methyhexane, 22% O 2 and 76% Ar and the pressure of the reactor was kept at 780 Torr. Low-temperature oxidation intermediates with two to five oxygen atoms were observed. The detection of C 7 H 14 O 5 and C 7 H 12 O 4 species suggests that a third O 2 addition process occurs in 2-methylhexane low-temperature oxidation. A detailed kinetic model was developed that describes the third O 2 addition and subsequent reactions leading to C 7 H 14 O 5 (keto-dihydroperoxide and dihydroperoxy cyclic ether) and C 7 H 12 O 4 (diketo-hydroperoxide and keto-hydroperoxy cyclic ether) species. The kinetics of the third O 2 addition reactions are discussed and model calculations were performed that reveal that third O 2 addition reactions promote 2-methylhexane auto-ignition at low temperatures.