Formation mechanisms of aromatic compounds in aliphatic flames

Possible mechanisms for the formation of aromatic compounds in flames of aliphatic fuels were assessed by comparing predicted formation rates against experimental values calculated from mole-fraction profiles of compounds in near-sooting, premixed flat flames of 1,3-butadiene at a pressure of 2.67 kPa. The data were obtained by molecular beam sampling with on-line mass spectrometry. Postulated mechanisms, consisting of rate-controlling addition of 1,3-butadienyl radical to acetylenes followed by cyclization and H-atom elimination, and having the following overall expression and rate parameters, are consistent with the data: ▪ Acetylenic Species Aromatic product E, kcal/mol Acetylene, C 2H 2 Benzene, C 6H 6 3.7 Methylacetylene, C 3H 4 Toluene, C 6H 5CH 3 3.7 Diacetylene, C 4H 2 Phenylacetylene, C 6H 5C 2H 1.8 Vinylacetylene, C 4H 4 Styrene, C 6H 5C 2H 3 0.6 where k is l/mol s. However, the measured rate of toluene formation differs from the prediction in a manner that may indicate an important role for another toluene-forming reaction and toluene conversion to benzyl radical. The postulated role of C 4H 5 in aromatics formation is consistent with previous flame data in which the formation behavior of another product of C 4H 5, namely, C 4H 4, mimics that of aromatics. Diels-Alder reactions of 1,3-butadiene and free-radical mechanisms involving 1,3-butadiene are shown to be too slow to account for the formation of aromatic species in this flame.