Zhongyue Zhou

Experimental Confirmation of the Low-Temperature Oxidation Scheme of Alkanes†

The control of auto-ignition can allow to increase the efficiency of internal combustion engines with clear potential positive effects on the problem of global warming.[1] The design of internal combustion engines,[2] as well as the improvement of safety in oxidation processes,[3] rely on a good understanding of the kinetic mechanism of the auto-ignition of organic compounds. Here we experimentally demonstrate a key assumption of this mechanism, which has been accepted for more than 20 years but never proven.[4-6] A detailed speciation of the hydroperoxides responsible for the gas-phase auto-ignition of organic compounds has been achieved for the first time, thanks to the development of a new system coupling a jet stirred reactor to a molecular-beam mass spectrometer combined with tunable synchrotron vacuum ultraviolet (SVUV) photoionization. The formation of alkylhydroperoxides (ROOH) and of carbonyl compounds including a hydroperoxide function (ketohydroperoxide) has been observed under conditions close to those actually observed before the auto-ignition. This result gives the experimental confirmation of an assumption made in all the detailed kinetic mechanisms developed to model auto-ignition phenomena.

Determination of absolute photoionization cross‐sections of aromatics and aromatic derivatives

The absolute photoionization cross-sections of aromatics and aromatic derivatives including toluene, ethylbenzene, n-propylbenzene, o-xylene, m-xylene, p-xylene, 1,3,5-trimethylbenzene, styrene, phenylacetylene, indene, indane, 1-methylnaphthalene, benzyl alcohol and benzaldehyde were measured at the photon energy range from ionization thresholds to 11.7 eV. The experiments were performed by tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Benzene was chosen as a calibration standard, since its photoionization cross-section is well known. Binary liquid mixtures of the investigated molecules and benzene were used in the measurements. Photo-induced fragments from the molecules were also observed, and their photoionization cross-sections are also presented.

Determination of absolute photoionization cross‐sections of alkanes and cyclo‐alkanes

Absolute photoionization and dissociative photoionization cross-sections of eleven n-alkanes (n-pentane, n-hexane, n-heptane, n-nonane, n-decane, n-undecane, n-dodecane, n-tridecane, n-tetradecane, n-pentadecane and n-hexadecane), three cyclo-alkanes (cyclopentane, methylcyclohexane and trans-decahydronaphthalene) and iso-octane were measured for photon energies from the ionization thresholds to 11.5 eV. The measurements were performed with the binary-liquid-mixture method utilizing the photoionization cross-sections of benzene as a calibration standard. The ionization energies of n-alkanes and cyclo-alkanes were also calculated at the B3P86/6-31 + +G(d,p) level and by the G3B3 method.

Pyrolysis of n-Heptane: Experimental and Theoretical Study

An experimental study of n-heptane pyrolysis (2.0% n-heptane in argon) has been performed at low pressure (400 Pa) within the temperature range from 780 to 1780 K. The pyrolysis products were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Photoionization mass spectra and photoionization efficiency spectra were measured to identify pyrolysis products, especially radicals and isomers. Mole fraction profiles of pyrolysis products versus temperature were also measured, indicating that H(2), CH(4), C(2)H(2), and C2-C6 alkenes are major pyrolysis products of n-heptane. Meanwhile, the thermal decomposition pathways of n-heptane have been investigated using theoretical calculation. The calculation results are in good agreement with the experimental measurement. On the basis of the experimental observation and theoretical calculation, the pyrolysis channels of unimolecular dissociation are proposed to understand the pyrolysis process of n-heptane.

A new apparatus for study of pressure-dependent laminar premixed flames with vacuum ultraviolet photoionization mass spectrometry

We report a home-made combustion apparatus for study of pressure-dependent laminar premixed flames with tunable vacuum ultraviolet photoionization mass spectrometry. The instrument consists of a flame chamber, a photoionization chamber with a single-stage sampling system, an ion transfer/storage system, and an orthogonal-acceleration reflectron time-of-flight mass spectrometer. Preliminary results of fuel-rich C(2)H(4)/O(2)/Ar flames at pressures of 30, 150, and 760 Torr have been obtained with this instrument. Compared to previous instruments [T. A. Cool, A. McIlroy, F. Qi, P. R. Westmoreland, L. Poisson, D. S. Peterka, and M. Ahmed, Rev. Sci. Instrum. 76, 094102 (2005); F. Qi, R. Yang, B. Yang, C. Q. Huang, L. X. Wei, J. Wang, L. S. Sheng, and Y. W. Zhang, Rev. Sci. Instrum. 77, 084101 (2006)], performances of the new apparatus have higher mass resolution (~3500 at m/z = 40), better detection limit (<1 ppm), and broader dynamic range (better than 5 order of magnitude).

On-line product analysis of pine wood pyrolysis using synchrotron vacuum ultraviolet photoionization mass spectrometry

AbstractThe pyrolysis process of pine wood, a promising biofuel feedstock, has been studied with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry. The mass spectra at different photon energies and temperatures as well as time-dependent profiles of several selected species during pine wood pyrolysis process were measured. Based on the relative contents of three lignin subunits, the data indicate that pine wood is typical of softwood. As pyrolysis temperature increased from 300 to 700xa0°C, some more details of pyrolysis chemistry were observed, including the decrease of oxygen content in high molecular weight species, the observation of high molecular weight products from cellulose chain and lignin polymer, and potential pyrolysis mechanisms for some key species. The formation of polycyclic aromatic hydrocarbons (PAHs) was also observed, as well as three series of pyrolysis products derived from PAHs with mass difference of 14xa0amu. The time-dependent profiles show that the earliest products are formed from lignin, followed by hemicellulose products, and then species from cellulose.n FigureThe pyrolysis study of pine wood based on synchrotron vacuum ultraviolet photoionization mass spectrometry.

Synchrotron vacuum ultraviolet (VUV) photo-induced fragmentation of cyclic dipeptides radical cations

Cyclic dipeptides, due to their chemical properties and various bioactivities, are very attractive for medicinal chemistry. Fragmentations of three simple cyclic dipeptides including cyclo(Gly–Gly), cyclo(Ala–Ala) and cyclo(Gly–Val) in the gas-phase are determined with synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (VUV PIMS) and theoretical calculations. Cyclo(Gly-Gly) and cyclo(Ala-Ala) show the similar fragmentation pathways. The primary decomposition reactions of cyclo(Gly-Gly) and cyclo(Ala-Ala) radical cations are found to be HNCO loss and CO elimination. The appearance energies (AEs) of fragment ions [CH2NHCOCH2]+• and [CH3CHNHCOCHCH3]+• are measured to be 10.21 and 9.66xa0±xa00.05xa0eV, respectively, which are formed from cyclo(Gly-Gly) and cyclo(Ala-Ala) radical cations with HNCO elimination. Due to the stabilization of the radical cation of cyclo(Gly-Val) with isopropyl side group, the dominant fragment ion m/z 114 assigned as [C4H6N2O2]+• is produced by γ-H migration and i cleavage to lose propylene. The ionization energies (IEs) of three cyclic dipeptides decrease in the order cyclo(Gly-Gly) (9.33xa0±xa00.05xa0eV)xa0>xa0cyclo(Ala-Ala) (9.21xa0±xa00.05xa0eV)xa0>xa0cyclo(Gly-Val) (9.09xa0±xa00.05xa0eV) from measurements of photoionization efficiency spectra. It implies that IEs of cyclic dipeptides are affected by substituent groups and symmetrical characterization of molecular structures. These observations of the chemical properties of cyclic dipeptides radical ion (M+•) may be important for understanding gas-phase molecular reactivity of 2,5-diketopiperazines and guiding diketopiperazine-based drug design.