Toluene Combustion:  Reaction Paths, Thermochemical Properties, and Kinetic Analysis for the Methylphenyl Radical + O₂ Reaction

Abstract

Aromatic compounds such as toluene and xylene are major components of many fuels. Accurate kinetic mechanisms for the combustion of toluene are, however, incomplete, as they do not accurately model experimental results such as strain rates and ignition times and consistently underpredict conversion. Current kinetic mechanisms for toluene combustion neglect the reactions of the methylphenyl radicals, and we believe that this is responsible, in part, for the shortcomings of these models. We also demonstrate how methylphenyl radical formation is important in the combustion and pyrolysis of other alkyl-substituted aromatic compounds such as xylene and trimethylbenzene. We have studied the oxidation reactions of the methylphenyl radicals with O2 using computational ab initio and density functional theory methods. A detailed reaction submechanism is presented for the 2-methylphenyl radical + O2 system, with 16 intermediates and products. For each species, enthalpies of formation are calculated using the computational methods G3 and G3B3, with isodesmic work reactions used to minimize computational errors. Transition states are calculated at the G3B3 level, yielding high-pressure limit elementary rate constants as a function of temperature. For the barrierless methylphenyl + O2 and methylphenoxy + O association reactions, rate constants are determined from variational transition state theory. Multichannel, multifrequency quantum Rice-Ramsperger-Kassel (qRRK) theory, with master equation analysis for falloff, provides rate constants as a function of temperature and pressure from 800 to 2400 K and 1 x 10(-4) to 1 x 10(3) atm. Analysis of our results shows that the dominant pathways for reaction of the three isomeric methylphenyl radicals is formation of methyloxepinoxy radicals and subsequent ring opening to methyl-dioxo-hexadienyl radicals. The next most important reaction pathway involves formation of methylphenoxy radicals + O in a chain branching process. At lower temperatures, the formation of stabilized methylphenylperoxy radicals becomes significant. A further important reaction channel is available only to the 2-methylphenyl isomer, where 6-methylene-2,4-cyclohexadiene-1-one (ortho-quinone methide, o-QM) is produced via an intramolecular hydrogen transfer from the methyl group to the peroxy radical in 2-methylphenylperoxy, with subsequent loss of OH. The decomposition of o-QM to benzene + CO reveals a potentially important new pathway for the conversion of toluene to benzene during combustion. A number of the important products of toluene combustion proposed in this study are known to be precursors of polyaromatic hydrocarbons that are involved in soot formation. Reactions leading to the important unsaturated oxygenated intermediates identified in this study, and the further reactions of these intermediates, are not included in current aromatic oxidation mechanisms.

A consistent chemical mechanism for oxidation of substituted aromatic species

“…Among the important pathways, the addition of molecular oxygen on methylphenyl radicals has been investigated in great detail by da Silva et al [36]. They report that the dominant reaction products are the cresoxy and methyloxepinoxy radicals (Fig.…”

Hydroxyl radical formation in the gas phase oxidation of distonic 2-methylphenyl radical cations

“…15 Despite the difference in bond dissociation energies, abstraction reactions by reactive free radicals are expected to provide significant yields of methylphenyl radicals; for instance at 1000 K the product branching ratio for methylphenyl and benzyl radicals by OH abstraction reactions is close to 1 : 3, increasing to 1 : 2 at 2000 K. 16 The toluene C-CH 3 bond scission, yielding the phenyl radical, is also significant during thermal decomposition: 20% at 1200 K, and 40% at 1500 K. 17 Likewise, decomposition of poly-methyl substituted benzenes, such as ortho-xylene, can produce methylphenyl radicals. 18,19 Isomerisation of the methylphenyl radical to benzyl radical is generally competitive under high-temperature combustion conditions, however lifetimes of methylphenyl species near autoignition temperatures (B1000 K) are such as to permit bimolecular reactions with O 2 . 19,20 Despite their plausible contribution in combustion environments, the methylphenyl + O 2 reaction has received relatively little attention.…”

“…One computational study has suggested that the 2-methylphenyl + O 2 reaction is substantially different from that of the unsubstituted phenyl radicals, in which formation of 2-quinone methide + OH is thought to compete (via a process shown in Scheme 1) with phenylperoxyltype isomerisation. 18 Experimental investigation of the methylphenyl + O 2 system can further the current understanding of toluene oxidation. However, identification and characterisation of reaction intermediates and products remain a challenge.…”

Theoretical considerations of secondary organic aerosol formation from H-abstraction of p-xylene

“…In daytime, the reaction of aromatic hydrocarbons with hydroxyl radicals is the major atmospheric loss process [14][15][16]. Several previous experimental [17][18][19] and theoretical [20][21][22][23][24][25][26] studies have unraveled the elementary reactions involved in aromatic oxidation.…”

“…Previous studies showed that in the presence of OH radical the main reaction path is OH addition to the aromatic ring to form a xylene-OH adduct (consuming $90% of OH radicals) with Habstraction from one methyl group to form a methylbenzyl radical (consuming $10% of OH radicals) being the minor route [18,[31][32][33][34]. In addition, theoretical investigations have mainly focused on OH-addition to xylene and the fate of xylene-OH adducts and intermediates [15,26,31,[34][35][36], whereas the H-abstraction mechanism for p-xylene oxidation following the initial OH attack and subsequent products have only been studied experimentally [22,25,37,38].…”