The production of benzene in the low-temperature oxidation of cyclohexane,cyclohexene, and cyclohexa-1,3-diene

The oxidation and auto-ignition of cyclohexane, cyclohexene, and cyclohexa- 1,3-diene have been studied by rapid compression between 600 K to 900 K and 0.7 MPa to 1.4 MPa to identify the low-temperature pathways leading to ben zene from cyclohexane. Auto-ignition delay times were measured and concentr ation-time profiles of the C-6 intermediate products of oxidation were meas ured during the auto-ignition delays. Cyclohexane showed two-stage ignition at low temperatures, but single-stage ignition at higher temperatures, and a well-marked negative-temperature coefficient. By contrast there was neit her a cool flame, nor a negative-temperature coefficient for cyclohexa-1,3- diene. Cyclohexene behaved in an intermediate way without a cool flame, but with a narrow, not very marked negative-temperature coefficient. The ident ified C-6 products belong to three families: the bicyclic epoxides and cycl ic ketones, the unsaturated aliphatic aldehydes, and the conjugated alkenes , which are always the major products. The formation of C-6 products from c yclohexane is explained by the classical scheme for low-temperature oxidati on, taking into account addition of O-2 to cyclohexyl radicals and the vari ous isomerizations of the resulting peroxy radicals. Most of the C-6 produc ts from cyclohexene are predicted by the same scheme, beginning with the fo rmation of the allylic cyclohexenyl radical. However, addition of HO2 to th e double bond has to be included to predict the formation of 1,2-epoxycyclo hexane. For cyclohexa-1,3-diene, the classical scheme is not valid: the C-6 oxygenated products are only formed by addition of HO2, to the double bond . For all three hydrocarbons, the pathways to benzene are those leading to conjugated alkenes, and they are always more efficient than those producing oxygenated products, either by adding HO2 to double bonds, or by addition of O-2 to the initial cyclic radical. (C) 2001 by The Combustion Institute.