High pressure auto-ignition and oxidation mechanisms of o-xylene, o-ethyltoluene, and n-butylbenzene between 600 and 900 K

A complex phenomenology of auto-ignition, similar to II-alkanes and n-alken es, has been revealed between 600-900 K and at pressures above 14 bar by st udying in a rapid compression machine stoichiometric mixtures of o-xylene ( o-methyltoluene), o-ethyltoluene, or n-butylbenzene in oxygen with lower co ncentrations than in air. Extensive chemical analyses of the reacting mixtu res before ignition were performed to elucidate the mechanisms of reaction. The classical low temperature scheme, modified for the reactivities of ben zylic-type hydrogen atoms and radicals, is valid. It appears that the addit ion of molecular oxygen to benzylic-type radicals leads to a double peroxid ation and low temperature branching only when the transfer of hydrogen in t he isomerization step occurs either from an ortho-alkyl group, or from anot her carbon atom of the same alkyl chain. The products observed are shown to be consistent with the proposed mechanism. The same complex pattern of aut o-ignition is found, not only for o-xylene, o-ethyltoluene, and n-butylbenz ene, but also for 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, n-propylb enzene, and ethylbenzene. When easily transferable hydrogen atoms are not a vailable for selective radicals such as peroxy radicals, branching occurs t hrough completely different pathways, which require higher temperatures and pressures. Then, the pattern of auto-ignition is much simpler, as already observed for toluene, m-xylene, p-xylene, and 1,3,5-trimethylbenzene [1], ( C) 2000 by The Combustion Institute.