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

Abstract

While there is consensus on the fact that OOQOOH radicals, produced by two oxygen additions from alkyl radicals, are the heart of the low-temperature oxidation of alkanes, the determination of the isomeric distribution and the quantification of their derived products (ketohydroperoxides and diones) are still a challenge. For the first time, heavy oxygenated products produced during alkane oxidation have been investigated using electron/ion coincidence mass spectrometry. The investigated prototype reaction is n-pentane oxidation carried out in a jet-stirred reactor (temperatures from 585 to 665 K, pressure of 1.1 bar, lean mixture). Identification attempts were made for m/z 100 and 118 species using coincident mass-tagged Slow PhotoElectron Spectra obtained by electron-ion coincidence mass spectrometry combined with first principle computations, consisting in the determination of their adiabatic ionization energies and the Franck-Condon envelope of the photoionization spectra. 4hydroperoxypentan-2-one has been confirmed as the dominant obtained ketohydroperoxide, as predicted by up-to-date kinetic models. However, difficulties due to fragmentation has made impossible the identification of the ketohydroperoxides present in lower amounts. In parallel, C5H8O2 isomers were identified, showing the possible formation, in addition to diones, of species with a ketone and an enol function. In addition, we provide new information on the first steps of the fragmentation pathways of C5 ketohydroperoxides. From the shape of their corresponding peaks on mass spectra and the energy and temperature dependence of their signal, ions at m/z 43, 57 and 85 have been identified as fragments from ketohydroperoxides. Taking into account these fragmentations lowers, by more than a factor of 10, the previously observed deviation between experiments and modeling for ketohydroperoxide mole fractions. The formation of the C1-C2 carboxylic acids, predicted from Korcek decomposition, was also observed, but with a favored formation of acetic acid versus formic acid that what was predicted for propane.

Identifying isomers of peroxy radicals in the gas phase: 1-C3H7O2vs. 2-C3H7O2

“…This work exemplifies the capabilities of i 2 PEPICO complemented by advanced computations, as a powerful method able to provide new insights into on-line analysis of complex gas-phase chemistry reactions, such as combustion and atmospheric oxidation, through isomer identification of elusive radical intermediates such as the propylperoxy radical presented here. Finally, the coincident ion kinetic energy distributions offer further analytical advantages to distinguish fragment (dissociative ionization) from parent (direct ionization) ions, as demonstrated here and recently applied to the quantification of ketohydroperoxides in low temperature combustion of alkanes 30 .…”

The identification and quantification of ketohydroperoxides and derived species produced during fuel low-temperature oxidation

“…This can well be seen in figure 4 for m/z 43, 57 and 87. This is due to the lower extraction field, which renders the experiment more sensitive to the Doppler broadening caused by the kinetic energy release in dissociative ionization (8). The fact that these fragments correspond well to KHP was confirmed by energy and temperature dependence of the signals at m/z 118, 85, 57 and 43.…”

“…The used set-up, based upon the undulator-based DESIRS VUV beamline of synchrotron SOLEIL, is described in details in refs. (7,8). This mass spectrometer is able not only to detect ions produced from the ionization events as that in Hefei, but also electrons in coincidence with the produced ions using the PhotoElectron PhotoIon COincidence (PEPICO) technique (9).…”

“…By combining PEPICO measurements and high-level ab initio calculations, an attempt of identification was made for ketohydroperoxides and diones obtained from n-pentane oxidation (8). At m/z 100, C5H8O2 isomers were identified and the analysis indicated the possible formation, in addition to diones, of species with a ketone and an enol function.…”

Photoelectron Photoion Coincidence Spectroscopy Provides Mechanistic Insights in Fuel Synthesis and Conversion

“…Nonetheless, the contribution of the isomer but-2-enyl-1-hydroperoxide could be ruled out as the calculated PES showed an intense electronic band between 10.3 and 10.4 eV, which was not observed experimentally. In their follow-up paper, 150 the group focused on the identification of the isomeric distribution and fragmentation patterns of ketohydroperoxides formed during the lowtemperature oxidation of n-pentane. The fragmentation patterns, i.e., the specific contributions of every fragment to the signals at the corresponding m/z, are needed to quantify KHPs using total ionization cross sections.…”