Exploring the pyrolysis chemistry of 1,3,5-trimethylcyclohexane with insight into fuel isomeric and multiple substitution effects

Abstract

To reveal insights into the combustion mechanism of multiple alkyl substituent cycloparaffins, this work reports an experimental and modeling study of 1,3,5-trimethylcyclohexane (T135MCH) pyrolysis in an extended flow reactor at low and atmospheric pressures. More than 30 species were detected and quantified employing synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry, and a detailed kinetic model developed based on reaction classes and update kinetic data was validated against the measured species profiles with a reasonable agreement. The reaction flux analyses were performed to reveal the key pathways of the fuel decomposition, intermediates production and aromatics formation. For the primary decomposition, the branching ratios of reaction types show strong dependence on changes of pressures and temperatures, including unimolecular methyl elimination, unimolecular ring-opening isomerization and H-abstraction. Besides the direct dissociation channels, major intermediate hydrocarbons are formed via stepwise dehydrogenation, recombination with ĊH3 radical or “formally direct” chemically activated reactions triggered by Ḣ atom addition. Monocyclic aromatic hydrocarbons such as benzene and toluene can be produced by traditional H-abstraction/β-C-H scission sequence, cyclopentadiene-related pathways, or recombination mechanism from small linear products. The formations of indene and naphthalene are controlled by C5+C5 and C5+C4 mechanism respectively. The comparison work of species profiles combined with theoretical calculations of bond dissociation enthalpies (BDEs) was performed to reveal the multiple CH3-group substituent and isomeric effects of methylcyclohexane (MCH), 1,2,4-trimethylcyclohexane (T124MCH) and T135MCH on pyrolysis activity and ethylene/benzene formation. Besides the increased reaction active sites, the added CH3-group and ortho-substitution can both weaken the strength of CC and CH bonds, leading to the promoting decomposition activity. The different formation tendencies of products are caused by different BDEs, length of carbon skeleton, as well as complex fuel-specific pathways.