Reactions of methyl radicals with isobutane at temperatures between 800 and 950 kelvin

Abstract

The methyl radical induced pyrolysis of isobutane has been studied in the temperature range of 800-950 K and at pressures of 80 and 130 mbar. We used CO 2 -laser induced heating by fast vibrational-translational (V-T) energy transfer from SF 6 to the reaction mixture. This is a convenient method to study homogeneous high temperature kinetics since the reactor walls remain cold. The radial temperature distribution in the reactor has been investigated by four different methods: stationary heat balance, optical absorption, pressure rise, and the temperature dependence of the rate of an isomerization reaction. Methyl radicals were produced in the presence of isobutane via the fast thermal decomposition of di-tert-butyl-peroxide and the products were analyzed using gas chromatography and mass spectroscopy. The main products of the overall reaction were propene (C 3 H 6 ) and isobutene (i -C 4 H 8 ), whereas ethene (C 2 H 4 ), propane (C 3 H 8 ), neo-pentane (neo-C 5 H 12 ), and isopentane (i -C 5 H 12 ) were minor products. The product distribution showed strong temperature dependence. The product yields and their temperature dependence were analyzed by a kinetic model developed for the high temperature oxidation of n-butane and the low temperature oxidation of n-pentane, and isopentane. We included a few missing reactions and slightly adjusted some rate constants to obtain a better agreement between the modeling and the experiments. The extended mechanism consists of 63 species and 171 reactions to be evaluated further in forthcoming experiments.

Evaluated Site-Specific Rate Constants for Reaction of Isobutane with H and CH3: Shock Tube Experiments Combined with Bayesian Model Optimization

“…The activation energies for reactions R656 and R657 were increased by less than 1.8 kJ/mol, and the A-factor for R656 was increased by 26%. The posterior model well-describes the experimental data from Goos et al 24 and from Konar et al; 8 it predicts Konar's ratios to within 27% and Goos' ratios to 15%. At temperatures above 900 K, it predicts our experimental data to 25% but overpredicts the ratio at low temperatures by 20−40%, meaning that our data disagree slightly with the other literature values at temperatures below 900 K, or that our model does not fully describe the chemistry in our shock tube.…”

β-Bond Scission and the Yields of H and CH3 in the Decomposition of Isobutyl Radicals

“…Relative rates of C–H and C–C β-scission from the literature ,, and used in kinetics models. ,,,,,, Branching ratios for compounds other than isobutyl have been scaled for the number of H and CH 3 as to be directly comparable to isobutyl.…”

“…Since β-scissions are endothermic, their rate constants are often calculated from the reverse reaction–radical addition to an alkene − – and have additional uncertainties associated with the thermodynamics. While there have been some direct high quality measurements of decomposition rate constants and relative rate constants for C–H and C–C scission, − current combustion kinetics models use a large range of values, ,− most based on the reverse reactions. ,,− There is continuing debate about the influence of 1,2 H-shift isomerizations in small alkyl radicals, as reflected by conflicting assumptions and rates constants in the above referenced combustion models. (Note: an x,y H-shift refers to a shift in the radical center from the x position to the y position via a hydrogen transfer …”

Laser powered homogeneous pyrolysis of ethyne, propyne, and propadiene initiated by methyl radicals: formation and degradation of hydrocarbons at 800–950 K

“…The overall procedure has been described in more detail in our previous papers of the methyl initiated pyrolysis of butane, propene and isobutane. 8,12,13 In the fol-lowing we will discuss our results with respect to the temporal evolution of the main products during the 50-100 ms heating period, to the sensitivity analysis of the reactions for the main products and for benzene, and to the extraction of the main reaction routes in order to derive the central part of the mechanism for our special conditions of the CH 3 initiated C 2 H 2 , p-C 3 H 4 , and a-C 3 H 4 pyrolysis.…”