Oxidation and combustion of low alkylbenzenes at high pressure: Comparative reactivity and auto-ignition

The auto-ignition features of 11 alkylbenzenes in a rapid compression machine have been compared for stoichiometric mixtures in the lower temperature region (600-900 K), and at compressed pressures up to 25 bar, by following pressure traces and light emission. They are classified in two groups. Toluene, m-xylene, p-xylene, and 1,3,5-trimethylbenzene ignite only above 900 K and 16 bar. o-Xylene, ethylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, n-propylbenzene, 2-ethyltoluene, and n-butylbenzene ignite at much lower temperature and pressure. The second group shows a complex phenomenology similar to alkanes and alkenes when submitted to adapted conditions of reactant concentrations. Ignition in two steps and negative temperature dependence of ignition delays are observed in favorable cases. Some of them show a low-temperature luminescence. Ignition features of o-xylene and n-butylbenzene are similar, in spite of their dissimilar molecular structure. The higher degree of reactivity of the second group is ascribed to the close proximity and/or length of their alkyl chains. (C) 2000 by The Combustion Institute.