Effect of methyl substituent on adamantane pyrolysis at atmospheric and high pressure: A combined experimental and theoretical study

Adamantane derivatives, as promising high-density fuels, have been widely concerned in fields of volume-limited aerospace vehicles due to high density, heat of combustion and thermal oxidation stability. To reveal insights into their high-temperature decomposition behaviors, an experimental and theoretical investigation of adamantane (A) and 1-methyl adamantane (1-MA) pyrolysis was performed in a flow tube reactor at atmospheric to high pressure. Over twenty species were detected and identified, including C1-C5 light hydrocarbons, C5-C7 cyclic olefins as well as C6-C12 aromatics. The favorable decomposition channels of A and 1-MA are proposed and the effects of fuel structure characteristics and pressures on product formation behaviors are revealed. For the primary decomposition of A and 1-MA, the contributions of reaction types exhibit the obvious dependence on the pressures and temperatures, involving the unimolecular C-C dissociations by ring-opening, CH 3 -elimination as well as H-abstractions by H and CH 3 attacking. Considering methyl-substitution weakens dissociation energies of C-C and C-H bonds and leads more H-abstraction sites, 1-MA shows a higher pyrolysis reactivity than A. The effects of structure characteristics are also embodied in the formation of newly chain-branched unsaturated species and the concentrations of common small hydrocarbons and aromatics. Compared to A decomposition, the methyl substituent in 1-MA promotes the emergence of 1-butene, 2-methyl-1-butene, mesitylene, methylindane and methylnaphthalene, and increases the concentrations of methane, 2-methylpropene and xylene isomers, formed via β-scission, hydrogen transfer, isomerization and dehydrogenation routes of fuel radicals. Meanwhile, more methyl-substitutions accelerate the decomposition of alkyl-adamantanes and pyrolysis activity follows by a sequence of dimethyl-adamantane > methyl-adamantane > adamantane, boosting the production of methyl-specific species.