A combined experimental and numerical investigation focusing on the effects of CH3 substituent on the PAH chemistry in the pyrolysis of cyclopentane and methylcyclopentane

Abstract

The combustion behavior of cycloalkanes has long fascinated researchers because their cyclic unique structural properties may significantly influence their reactivity and combustion kinetics. In order to achieve a better understanding of the kinetics of cycloalkanes during intermediate to high temperature combustion chemistry, the pyrolysis of cyclopentane (CP) and methylcyclopentane (MCP) was studied in a jet-stirred reactor (869–1210 K, 1 atm) to explore the impact of methyl substituents on polycyclic aromatic hydrocarbon (PAH) formation and combustion kinetics. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) identified monocyclic to polycyclic aromatics, including acenaphthylene, phenanthrene, and pyrene. A newly developed kinetic model, validated against experimental data, revealed distinct decomposition behaviors: MCP exhibited lower initial decomposition temperatures and faster formation of C1–C4 hydrocarbons and aromatics compared to CP. This is attributed to the methyl group’s lower energy barrier, enhancing MCP’s reactivity. While CP pyrolysis generated higher concentrations of 1,3-cyclopentadiene and resonance-stabilized C5 cyclic radicals, these intermediates did not notably elevate PAH levels. In contrast, MCP’s methyl side chain promoted earlier fuel breakdown and accelerated PAH/soot precursor formation via enhanced radical production and alkylation pathways. These findings highlight that methyl substitution in cycloalkanes lowers thermal stability, accelerates decomposition, and amplifies aromatic growth, emphasizing structural effects on combustion chemistry and pollutant formation. The study provides critical insights into fuel design and emission control strategies for cyclic hydrocarbons.