Pathway and Kinetic Analysis on the Propyl Radical + O2 Reaction System

Transcription

1 URL-J Rev. 1 PREPRINT Pathway and Kinetic Analysis on the Propyl Radical + 2 Reaction System J.W. Bozzelli W.J. Pitz This paper was prepared for submittal to the Fourth International onference on hemical Kinetics Gaithersburg, MD July 14-18, 1997 May 1997 Lawrence Livermore National Laboratory This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author.

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3 Pathway and Kinetic Analysis on the Propyl Radical + 2 Reaction System Joseph W. Bozzelli hemistry and hemical Engineering Department New Jersey Institute of Technology Newark, NJ bozzelli@tesla.njit.edu William J. Pitz Lawrence Livermore National Laboratories Livermore, A pitz@llnl.gov The reaction of alkyl radicals with molecular oxygen plays a key role in the oxidation of alkanes. In the negative temperature coefficient region of alkane oxidation, this reaction leads to the production of alkenes, oxygenates and chain branching. At higher temperatures, it leads primarily to the production of a conjugate alkene and the H2 radical. The reaction of alkyl radicals with 2 is important in atmospheric chemistry and in many practical combustion problems including automotive and gas turbine engines. In the case of 2H5 + 2, it was generally accepted that the reaction proceeds to 2H4 through the path: 2H5 + 2 <=TST1=>.# <= TST2=>.H# <=TST3=> 2H4 + H2 + M + M Mechanism (I) V V..H Recent work by Ignatyev et al. [1] using density functional theory has suggested a fourth transition state (TST4), where direct production of ethylene + H2 can result via concerted elimination of H2 from #. This postulated reaction path does not proceed through the hydroperoxy-ethyl radical (.H) intermediate. The existence of the cyclic transition state was first hypothesized by McAdam and Walker [2]. They reported that Mechanism (I) could not explain the observed pressure dependence of 2H5 + 2 = 2H4 + H2 at 298 K and the observed decrease in rate constant with increasing temperature from K. They suggested the existence of a long-lived, cyclic transition state and a concerted elimination of ethylene would explain these experimental observations. In subsequent work, Gutlati and Walker [3] postulated the existence of an analogous, long-lived, cyclic transition state for the i-3h7 + 2 system as well. However, Wagner [4], Bozzelli and Dean [5],

4 Kaiser [6], Koert et al. [7] have shown that Mechanism I can explain the observed temperature and pressure dependencies. In this study, we analyze the propyl + 2 reaction system using thermochemical kinetics, Transition State Theory (TST), molecular thermodynamic properties, quantum Kassel analysis (quantum RRK) for k(e) and modified strong collision analysis for fall off. yclic transition states for both hydrogen transfer and the H2 concerted elimination from propylperoxy are calculated using semi-empirical (MPA PM3) calculations [8] in addition to transition states for H2 elimination and epoxide formation from hydroperoxy-isopropyl. omputed rate constants for propyl + 2 are compared to the values of Gulati and Walker who measured the rate constants at 50 torr and over a temperature range of 653 to 773 K. omputed rate constants are also used in a detailed chemical kinetic mechanism and compared to the n-propyl + 2 data of Slagle et al. [9]. They measured the rate of disappearance of n-propyl by reaction with 2 over a temperature range of 297 to 635 K and a pressure range of 0.4 to 7 Torr, as well as the fall off data of the Kaiser and Wallington [10]. A potential energy diagram for the iso-propyl + 2 system is given in Fig. 1. The four transition states calculated from the semi-empirical molecular orbital calculations are sketched at the top of the figure. Note that the barrier for the H-atom shift channel of 2. is estimated to be similar to the barrier for H2 molecular elimination. This is in contrast to the work of Ignatyev et al. [1] who calculated that the barrier for H-atom shift is about 8 kcal/mole higher than the barrier for H2 elimination. The A-factor for the elimination channel is about a factor of two higher than for H-shift due to a looser transition state for elimination compared to H-atom shift. The calculated (chemical activation) rate constants of iso-propyl + 2 as a function of temperature and at one atmosphere are given in Fig. 2. Results from QRRK calculations are compared to rate constant measurements of Gulati and Walker [3], Fig. 3. Activation energies for H2 elimination and/or H-shift were adjusted to give the iso-propyl + 2 rate constant reported by Gulati and Walker. The activation energies for H2 elimination and H shift are both 29.8 kcal. The calculated curve exhibits a slightly positive activation energy compared to a slightly negative activation energy shown by the experimental results. The QRRK calculated curve includes contributions from both the H2 elimination channel and the H- shift channel. This work was performed under the auspices of the U.S. Dept. of Energy at LLNL under contract no. W-7405-Eng-48.

5 REFERENES 1. Ignatyev, I. S., Xie, Y., Allen, W. D., and Schaefer III, H. F., Mechanism of the 2H5 + 2 Reaction J. Phys. hem, In press, (1997). 2. McAdam, K. G., and Walker, R. W. J. hem. Soc., Faraday Trans. 83: (1987). 3. Gutlati, S. K., and Walker, R. W. J. hem. Soc., Faraday Trans. 2, 84: (1988). 4. Wagner, A. F., Slagle, I. R., Sarzynski, D., and Gutman, D. J. Phys. hem. 94: (1990). 5. Bozzelli, J.W., and Dean, A.M. J. Phys. hem. 94: (1990). 6. Kaiser, E. W. J. Phys. hem. 99: (1995). 7. Koert, D., Pitz, W. J., Bozzelli, J. W., and ernansky, N. P. The 26th Symposium (International) on ombustion, The ombustion Institute, Pittsburgh, PA, 1996, pp MPA: A General Molecular rbital Package (QPE 445): APE Bull., 3, 43, MPA 6.0: Frank J. Seiler Research Lab., U.S. Air Force Academy,, Slagle, I. R., Park, J.-Y., and Gutman, D. Twentieth Symposium (International) ombustion, The ombustion Institute, Pittsburgh, 1984, p Kaiser, E. W. J. Phys. hem. 99: (1995) (21.0) H H (18.0) (16.2) (16.2) (19.0) H H 10 (8.5) Enthalpy (kcal/mole) (-13.8) H (-0.9) = + H2 + H (-12.4)

6 Fig. 1 Potential energy diagram of isopropyl + 2 => products. 13 k [cm 3 /mole-sec] H6-E+H2 3H6-S+H2 2.H Y2+H 9 2*+H *H+H /T [K] Fig. 2 Results of QRRK calculation of isopropyl + 2 => products for a pressure of 1 atm. 3H6-E denotes propene via the molecular elimination channel and 3H6-S denotes propene formed via the H-atom shift channel.

7 10 12 k (cm 3 /mole-sec) Gulati and Walker (1988) QRRK calculation /T(K) Fig. 3 Rate constant of isopropyl + 2 => propene + H2 calculated by QRRK analysis and measured by Gulati and Walker [3]. The calculated rate constants include the contributions from internal H-atom transfer and H2 elimination. The product 2.H was assumed to react to propene + H2 which is its main fate. Presentation mode: We prefer to present this paper in a oral session ptional Information: Authors Personal URL on the World Wide Web: None available. URL s for the Author s Institutions on the World Wide Web: Bozzelli: Pitz:

8 Technical Information Department Lawrence Livermore National Laboratory University of alifornia Livermore, alifornia 94551