The low-temperature autoignition of alkylaromatics: Experimental study and modeling of the oxidation of n-butylbenzene

The low-temperature oxidation of n-butylbenzene, an intermediate structure between alkanes and short-chain alkylaromatics. was studied between 640 and 840 K by rapid compression and by modeling. Delay times of one- and two-stage autoignitions were measured, and intermediate species after the cool flame were analyzed. First, a detailed mechanism for n-butane was developed with existing material. Then, an n-butylbenzene mechanism was built by taking into account the change of reactivity due to die introduction of the aromatic nucleus. Both mechanisms have been validated by simulations of the delays and the product concentrations. Finally, the n-butylbenzene mechanism was used to analyze the main low-temperature reaction pathways. The comparative calculation of the concentrations of alkyl, alkylperoxy. and hydroperoxyalkyl radicals in the cool flame of n-butane and n-butylbenzene illustrates the effects of the aromatic nucleus on the first steps of oxidation. A study of the competitive channels to the main molecular intermediate species shows that the internal transfer of a benzylic hydrogen to the peroxy sites is a major event in the development of reactions leading to branching and ignition. This can explain a precious observation that alkylaromatics with two ortho-alkyl groups or a long single lateral chain have die possibility of an internal transfer of a benzylic hydrogen and manifest a greater low-temperature reactivity than aromatics that have neither ortho-alkyl groups nor a long lateral chain.