Search

International audience; The oxidation of a stoichiometric diethyl ether-oxygen-nitrogen mixture containing 5000 ppm of fuel was studied in a jet-stirred reactor at 10 atm and a residence times of 1 s. Experimental temperatures varied stepwise for 440 to 740 K and products were quantified by gas chromatography with TCD and FID, gas chromatography-mass spectrometry, and FTIR. Other experiments in the temperature range 480 to 570 K were performed for characterizing elusive cool flame products. To this end, gas samples were trapped in acetonitrile for flow injection analyses and liquid chromatography-mass spectrometry (Orbitrap Q-Exactive®). For ionization, we used positive and negative atmospheric pressure chemical ionization (APCI). Among fuel-specific products, hydroperoxides and diols (C4H10O3), carbonyl hydroperoxides (C4H8O4), acetic acid, di-keto ethers (C4H6O3), cyclic ethers (C4H8O2) and highly oxygenated molecules, i.e., keto-dihydroperoxides (C4H8O6), keto-trihydroperoxides (C4H8O8), di-keto-hydroperoxides (C4H6O5), and diketo-dihydroperoxides (C4H6O7), were detected. To confirm the presence of –OH or –OOH groups in oxidation products, H/D exchange with D2O was used. DNPH derivatization was used to identify carbonyls present in samples, especially those with a molecular weight below 50 amu which cannot be detected directly by the mass spectrometer. Chemical kinetic modeling using a mechanism taken from the literature was performed. Although reasonable agreement between the data and the simulations was observed for several species, some discrepancies between experimental and computed mole fractions were observed.

International audience; The oxidation of di-n-butyl-ether (DBE) was performed in a jet-stirred reactor (JSR) at 1 atm, 520 and 530 K, and from 480 to 670 K at 10 atm, at a residence time of 1 s, an equivalence ratio of 0.5, and an initial fuel concentration of 5000 ppm. Ignition experiments on DBE/air mixtures were also performed in a rapid compression machine (RCM) under stoichiometric conditions, 5 bar, and from 550 to 630 K. Low-temperature products formed in JSR and RCM experiments were characterized. To this end, high-resolution mass spectrometry analyses (HRMS) with flow injection analyses or ultra-high pressure liquid chromatography coupling were used to characterize hydroperoxides and diols (C8H18O3), ketohydroperoxides (C8H16O4), carboxylic acids (C2H4O2, C3H6O2, C4H8O2), di-keto ethers (C8H14O3), and highly oxygenated molecules (C8H14O5, C8H16O6, C8H14O6, C8H14O7, C8H16O8, and C8H14O9) resulting from up to five O2 additions on fuel's radicals. Whereas HOMs are of minor importance in combustion, they are considered key species for the formation of secondary organic aerosols. In addition, cyclic ethers and esters (C8H16O2) were observed. Heated electrospray and atmospheric pressure chemical ionizations (HESI and APCI) were used in positive and negative modes for MS analyses. H/D exchange with D2O was used to confirm the presence of –OH or –OOH groups in the products. The present results show that DBE oxidation proceeds similarly under JSR and RCM conditions. Whereas the CH2 groups neighboring the ether group are the most favorable sites for H-atom abstraction reactions, speciation indicated that other sites can react by metathesis forming a large pool of intermediates and products. Our kinetic reaction mechanism needs to be extended for simulating the formation of newly detected species.

International audience; Renewable feedstock such as biomass derivatives (hemicellulose, furfural) can be used to produce synthetic fuels, e.g., n-pentane, of interest for enhancing performance of diesel and gasoline engines. Whereas numerous studies of n-pentane have been published, its low temperature oxidation is not fully characterized and even recent kinetic models do not incorporate extended oxidation pathways which still need to be observed for n-pentane. In this context, a continuous flow fused-silica jet-stirred reactor (JSR) was used for studying the oxidation of 2500 ppm of n-pentane at 520‒800 K, 10 atm (representative of piston engines operating conditions), an equivalence ratio of 0.5, and a residence time of 1.5 s. Oxidation products were analyzed in the gas phase using gas chromatography (thermal conductivity, TCD and flame ionization, FID), Fourier-transform infrared spectrometry (FTIR), and electron impact ionization-quadrupole mass spectrometry (EI-qMS). Gaseous products were also dissolved in acetonitrile for characterization using flow injection analysis (FIA), high-pressure and ultra-high-pressure liquid chromatography (HPLC and UHPLC) coupled to atmospheric pressure chemical ionization (APCI) and Q-Exactive®-Orbitrap high resolution mass spectrometry (HRMS). This allowed detecting lower and higher mass oxygenated molecules such as methyl vinyl ketone (MVK) and 2,5-dihydrofuran (C4H6O), 2-butanone (C4H8O), 3-pentene-2-one (C5H8O), pentanediones (C5H8O2), cyclic ethers and pentanones (C5H10O), C3‒C5 alkylhydroperoxides (C3H8O2, C4H10O2, C5H12O2), C2‒C5 alkenylhydroperoxides (C2H4O2, C3H6O2, C4H8O2, C5H10O2), C3‒C5 keto-hydroperoxides (C3H6O3, C4H8O3, C5H10O3), and highly oxygenated molecules (C5H8O4, C5H12O4, C5H10O4, C5H10O5, C5H10O7) produced through multiply O2 addition on fuel’s radicals and internal H-shifts. Among these products 15 had not been reported before. Simulation of the present experiments showed discrepancies between experimental results and predictions.

International audience; The oxidation of a gasoline model-fuel (2500 ppm of n-heptane and 2500 ppm of iso-octane), called RON 50, was studied in a jet-stirred reactor (JSR) over the temperature range 560-700 K, at a total pressure of 10 atm, at a residence time of 1.5 s, and an equivalence ratio of 0.5. Gas samples were collected as a function temperature. Ignition of RON 50/air mixtures was also studied in a rapid compression machine (RCM) under the same fuel-lean conditions, 20 bar, 640 K. Gas samples were collected at variable reaction time. Products of low-T oxidation formed in a jet-stirred reactor and a rapid compression machine were dissolved in acetonitrile and analyzed by highresolution mass spectrometry. Flow injection analyses and ultrahigh-pressure liquid chromatography coupled to an Orbitrap® were used to characterize a wide range of species such as hydroperoxides, diols, ketohydroperoxides, carboxylic acids, diketones, cyclic ethers (C n H 2n O) formed by decomposition of alkyl hydroperoxy radicals, and highly oxidized species formed via up to six O 2 additions on alkyl radicals (C n H 2n O 11). Mass spectrometry analyses were conducted using atmospheric pressure chemical ionization running in negative and positive ionization modes. For confirming the presence of-OH or-OOH groups in the products, we performed H/D exchange by addition of D 2 O to samples. Under JSR conditions, we observed a wide range of n-heptane oxidation products: C 7 H 14 O x (x=1-11), C 7 H 12 O x (x=1-11), C 7 H 10 O x (x=1-9), C 7 H 8 O x (x=1-9), C 7 H 6 O x (x=1-8), and C 7 H 4 O x (x=1-6). Similarly, the following products of iso-octane oxidation were observed: C 8 H 16 O x (x=1-12), C 8 H 14 O x (x=1-11), C 8 H 12 O x (x=1-12), C 8 H 10 O x (x=2-10), C 8 H 8 O x (x=2-8), C 8 H 6 O x (x=1-7). Finally, C n H 2n (n=4-8), C n H 2n-2 (n=4-8), C n H 2n O (n=3-8), C n H 2n-2 O (n=3-8), C n H 2n-4 O (n=3-8), C n H 2n+2 O 2 (n=3-8), C n H 2n O 2 (n=2-8), C n H 2n-2 O 2 (n=3-8), C n H 2n-4 O 2 (n=3-8), and C n H 2n O 3 (n=2-8) were also observed. Most of these products were also detected in RCM samples. Products measurements indicated that RON 50 oxidation routes are similar under RCM and JSR experimental conditions. A kinetic reaction mechanism was used to compare the formation of products versus temperature in a JSR. New pathways need to be introduced in existing reaction schemes for predicting newly detected cool-flame products.

International audience; The oxidation of tetrahydrofuran (THF) was carried out in a continuously jet-stirred tank reactor (JSR) at a total pressure of 10 atm, in fuel-lean conditions (equivalence ratio = 0.5), an initial fuel mole fraction of 5000 ppm, at a mean residence time of 2 s, and for temperatures ranging from 550 to 620 K. High-resolution mass spectrometry analyses was used to characterize low-temperature oxidation products of THF. MS analyses were performed using atmospheric pressure chemical ionizations in positive and negative modes. Both flow injection analyses and ultra-high-pressure liquid chromatography-MS/MS allowed characterizing a large set of chemicals including hydroperoxides and diols (C 4 H 8 O 3), ketohydroperoxides (C 4 H 6 O 4), and more oxygenated molecules (up to C 4 H 8 O 7) resulting from up to three oxygen molecules addition of on α and β THF radicals. The existence of-OH or-OOH groups in the products was confirmed by hydrogen-deuterium exchange using D 2 O. We detected 24 products with molecular weight of 40-168, not reported in previous studies. Simulations using the latest THF oxidation chemical kinetic reaction mechanism available from the literature were compared to the present measurements of ketohydroperoxides and other products of THF cool flame. The kinetic scheme represented well the present qualitative data.

The oxidation of di-n-propyl-ether (DPE) was performed in a jet-stirred reactor at 1 and 10 atm, at residence times of 1 and 0.7 s, respectively, and initial fuel concentrations of 5000 and 1000 ppm at 1 and 10 atm, respectively. Atmospheric pressure experiments were used for characterization of cool flame products. The 10 atm experiment provided KHPs profile vs. temperature and mole fraction profiles of stable species which were obtained through sonic probe sampling, gas chromatography, Fourier transform infrared spectrometry analyses. High resolution mass spectrometry analyses (HRMS) with syringe direct injection or ultra-high-pressure liquid chromatography coupling was used to characterize hydroperoxides (C3H8O2, C6H14O3), diols (C6H14O3), ketohydroperoxides (C6H12O4), carboxylic acids, and highly oxygenated molecules (C6H12O6, C6H12O8) resulting from up to four O2 additions on fuel's radicals. Heated electrospray and atmospheric pressure chemical ionizations (HESI and APCI) were used in positive and negative mode. Whereas the CH2 groups neighboring the ether function are the most favorable sites for H-atom abstraction reactions, speciation indicated that other sites can react by metathesis forming a large pool of intermediates. Our kinetic reaction mechanism represents the experimental data for most of the stable species but need to be expended for simulating the formation of newly detected species.

International audience; With the growing interest for biomass-derived fuels the understanding of the combustion chemistry of terpenes becomes of major scientific importance. Terpenes have been proposed as biofuels for aviation because of their high energy density. They usually develop cool flames below 800 K. Very complex processes occur there, with the formation of peroxides intermediates such as ketohydroperoxides and highly oxidized molecules (HOMs) containing both hydroperoxy and carbonyl groups. Such chemicals are relatively unstable and difficult to analyze.We studied the low-temperature oxidation of alpha-pinene, beta-pinene, and limonene (C10H16) in a jet-stirred reactor. The experimental conditions were selected to maximize the production of ketohydroperoxides. We oxidized 5000 ppm of these three terpenes at 1 bar, T = 590 K, an equivalence ratio of 0.5, and at a residence time of 1 s. High-resolution mass spectrometry analyses were performed on solubilized products of terpenes oxidation in cooled acetonitrile. The samples were analyzed using soft HESI electrospray ionization (+/-) and an Orbitrap® mass spectrometer (resolution: 140,000, mass accuracy RO2 QOOH; QOOH + O2 OOQOOH HOOQ’OOH followed by HOOQ’OOH + O2 (HOO)2Q’OO (i) (HOO)2POOH → OH + (HOO)2P=O (i.e., C10H14O5) and (ii) (HOO)2POOH + O2 → (HOO)3POO (HOO)3P’OOH → OH + (HOO)3P=O (i.e., C10H14O7). Fourth oxygen addition yielding C10H14O9 was also observed in the present work. Hydrogen–Deuterium exchange reactions using D2O were used to confirm the presence of –OH groups in the products.

International audience; With the growing interest for biomass-derived fuels the understanding of the combustion chemistry of ethers becomes of major scientific importance. Ethers usually develop strong cool flames at relatively low temperatures. Very complex processes occur there, with the formation of peroxidized intermediates such as ketohydroperoxides and highly oxidized molecules. Such chemicals are relatively unstable and difficult to analyze.We studied the low-temperature oxidation of dipropyl ether in a jet-stirred reactor. The experimental conditions were selected to maximize the production of ketohydroperoxides, based on the kinetic model of Serinyel et al. (2019). We oxidized 5000 ppm of dipropyl ether at 1 bar, T = 520–530 K, an equivalence ratio of 0.5, and at a residence time of 1 s. Analyses were performed on solubilized products of dipropyl ether oxidation in cooled acetonitrile. The samples were analyzed using soft HESI electrospray ionization (+/-) and an Orbitrap mass spectrometer (resolution: 140,000, mass accuracy RO2 QOOH; QOOH + O2 OOQOOH HOOQ’OOH followed by HOOQ’OOH + O2 (HOO)2Q’OO (i) (HOO)2POOH → OH + (HOO)2P=O (i.e., C6H12O6) and (ii) (HOO)2POOH + O2 → (HOO)3POO (HOO)3P’OOH → OH + (HOO)3P=O (i.e., C6H12O8).The so-called Korcek mechanism through which ketohydroperoxides are transformed into stable products, namely propanoic acid here, was also observed. Hydrogen–Deuterium exchange reactions using D2O served to confirm the presence of –OH groups in the products.

© 2016 American Chemical Society. This work provides new temperature-dependent mole fractions of elusive intermediates relevant to the low-temperature 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 single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet 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 (HOOCH2OCH2O), which is a key intermediate in the low-temperature oxidation of DME. The quantification of this keto-hydroperoxide together with the temperature-dependent concentration profiles of other intermediates including H2O2, HCOOH, CH3OCHO, and CH3OOH reveals new opportunities for the development of a next-generation DME combustion chemistry mechanism.

International audience; 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. 4-hydroperoxypentan-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.