Decomposition of methylbenzyl radicals in the oxidation and pyrolysis of xylenes

Document Type

Conference Proceeding

Publication Date

1-1-2009

Abstract

Alkyl benzyl radicals are important initial products in thermal and combustion reactions of substituted aromatic fuels. The decomposition reactions of the three isomeric methylbenzyl radicals, formed as primary products in xylene combustion, are studied by use of computational chemistry. Results show that these reactions are significantly more complex than previously reported. Thermochemical properties are calculated using the G3X and G3SX model chemistries, with isodesmic and atomization work reactions. G3X atomization calculations reproduce heats of formation for the 14 reference species in the work reactions to a mean unsigned error of 0.23 kcal mol-1, and maximum error of 0.70 kcal mol-1, slightly outperforming the G3SX method. For the target species the isodesmic and atomization heats of formation agree to within 0.20 kcal mol-1, on average. We posit that this study approaches the crossover point at which atomization calculations offer improved accuracy over isodesmic ones for these species. We propose that m-xylylene is not the decomposition product of m-methylbenzyl, as is previously reported. Instead, it is hypothesized that the m-methylbenzyl radical decomposes to p-xylylene (and perhaps some of the lessstable o-xylylene) via a ring-contraction/methylene-migration (RCMM) mechanism, with activation energy of around 70 kcal mol-1. At higher temperatures m-methylbenzyl is expected to also decompose to 2- and 3- methylfulvenallene + H, with activation energy of around 84 kcal mol-1. The o-methylbenzyl radical is suggested to primarily decompose to o-xylylene + H with bond dissociation energy of 67.3 kcal mol-1, with fulvenallene + CH3 proposed as a minor product set. Finally, the p-methylbenzyl radical decomposes solely to p-xylylene + H with bond dissociation energy 61.5 kcal mol-1. Rate expressions are estimated for all reported reactions, based on thermochemical kinetic considerations, with further modeling along with detailed experiments needed to better refine rate constants and branching ratios for methylbenzyl radical decomposition. These calculated reaction paths (mechanisms) and the kinetics for methylbenzyl radical decomposition are consistent with the experimental data.

Identifier

84946574375 (Scopus)

ISBN

[9781615676682]

Publication Title

Fall Meeting of the Eastern States Section of the Combustion Institute 2009

First Page

221

Last Page

236

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