Investigation on primary decomposition of ethylcyclohexane at atmospheric pressure

To get a better understanding of the combustion chemistry of cycloalkanes with long side chain, the pyrolysis of ethylcyclohexane (ECH) was studied in a flow reactor at atmospheric pressure. The pyrolysis species were analyzed by two methods, synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography. Dozens of species were identified and quantified, including lots of isomers. The emphasis of this study is to investigate the primary decomposition of ECH, including its initial decomposition, isomerization, and further reactions of the cyclic C8H15 radicals formed from the H-abstraction of ECH. The observation of C8H16 alkene indicates the existence of ring-opening isomerization reaction of ECH. The ring-opening isomerization reaction of cyclic C8H15 radicals produces alkenyl radicals, whose further decomposition constitutes the various chain and branched intermediates in ECH pyrolysis. The formation of isoprene and vinylcyclopentane is discussed, which highlights isomerization reactions of radical addition on the double bond of alkenyl radicals, such as oct-6-en-1-yl and oct-5-en-1-yl radicals. The theoretical calculation on the reaction pathways of oct-5-en-1-yl radical also shows that its internal H-migration pathway via eight-membered ring might be competitive to the one via five-membered ring. On the other hand, the decomposition of cyclic C8H15 radicals causes the formation of cyclic intermediates, i.e. C8H14 alkenes, methylenecyclohexane and cyclohexene, which are potential aromatic precursors.