Investigation of cyclohexene thermal decomposition and cyclohexene + OH reactions

Abstract

Cyclohexene is a common intermediate in the oxidation of cyclohexane and alkyl-substituted cyclohexanes. It also plays an important role in the formation of polycyclic aromatic hydrocarbons (PAHs) by providing a route for the first aromatic ring. Two types of reactions are highly important in cyclohexene kinetics, i.e., H-abstraction by OH radicals and cyclohexene thermal decomposition. In this work, we have investigated these two reactions experimentally by employing sensitive UV diagnostics of OH radicals and 1,3-butadiene. We conducted rate coefficient measurements of cyclohexene + OH reaction (k1) over 933–1259 K and 1–4 bar using a narrow-linewidth UV absorption diagnostic of OH radicals near 306.67 nm. The carefully designed test conditions minimized the influence of secondary reactions such as cyclohexene thermal decomposition. Our determined OH + cyclohexene high-temperature rate coefficients show a positive temperature dependence and a negligible pressure dependence. Our rate values may be fitted with a two-parameter Arrhenius expression (units of cm3molecule−1s−1):k1=1.28×10−10e(−1225T)We also investigated channel-specific competition in OH + cyclohexene reaction. H-abstraction by OH from allylic (kc), alkylic (kb), and vinylic (ka) CH bonds contributes roughly 60%, 35%, and 5% over our temperature range. Our determined kc agrees excellently with literature rate coefficient of H-abstraction from the allylic site of 1-butene.We studied cyclohexene thermal decomposition reaction (k2) by tracing the product 1,3-butadiene near 212.5 nm over 1092–1361 K and 0.82–1 bar. Compared to literature works, our highly sensitive UV diagnostic of 1,3-butadiene enabled time-resolved measurements with low cyclohexene concentration, which guaranteed nearly isothermal conditions despite reaction endothermcity. Our measured rate coefficients may be expressed as (unit of s−1):k2=3.68×1014e(−31,562T)We implemented our determined rate coefficients (k1 and k2) in a literature cyclohexene kinetic model and also updated the 1,3-butadiene sub-mechanism. The updated model shows improved predictions of measured ignition delay times of cyclohexene.