Role of normal/cyclo-alkane in hydrocarbons pyrolysis process and product distribution

•n-Hexane, n-heptane, cyclohexane and methylcyclohexane pyrolysis are studied.•Difference of normal/cyclic alkane significantly affects the pyrolysis process.•A synergistic effect between normal and cyclic alkane is in the mixture pyrolysis.•n-Hexane promotes cyclohexane decomposition, while cyclohexane dose the opposite.•The synergistic effect is a function of temperature and feedstock composition. In order to reveal the effects of normal/cyclo-alkane on the pyrolysis process, n-hexane, n-heptane, cyclohexane, methylcyclohexane and n-hexane/cyclohexane mixtures were employed as model reactants, and a particular attention was paid to the measurement of the product distribution at 650−830 °C under atmosphere. Compared with the increase of chain length of normal alkane and the presence of methyl substituent on cycloalkane, the distinction between normal and cyclic alkane led to a significant difference on the pyrolysis process. Due to the normal structure, n-hexane directly generated alkyl radicals via the homolysis of CC bond; due to the cyclic structure, cyclohexane generated alkenyl and alkyl radicals via the successive reactions of ring-opening, isomerization and decomposition. As a result, cyclohexane pyrolysis obtained a lower conversion and selectivity to light alkanes and alkenes, while a higher selectivity to 1,3-butadiene, cyclohexene and aromatics compared to n-hexane pyrolysis. Furthermore, a synergistic effect between n-hexane and cyclohexane was observed in the mixture pyrolysis. The synergistic effect mainly depended on the hydrogen abstraction and was modulated as a function of reaction temperature and feedstock composition. In the mixture pyrolysis: (1) n-Hexane preferentially generated radicals and triggered the hydrogen abstraction, which promoted cyclohexane decomposition while was weakened by increasing cyclohexane content. (2) Cyclohexane competed for radicals with n-hexane in the hydrogen abstraction, which inhibited n-hexane decomposition and was enhanced by increasing cyclohexane content. (3) An increase of reaction temperature weakened both the inhibition effect of cyclohexane on n-hexane decomposition and the acceleration effect of n-hexane on cyclohexane decomposition, and it was attributed to the reduction of hydrogen abstraction at high temperature.