Direct measurement of ˙OH and HO2˙ formation in ˙R + O2 reactions of cyclohexane and tetrahydropyran.

Abstract

Formation of the key general radical chain carriers, ˙OH and HO2˙, during pulsed-photolytic ˙Cl-initiated oxidation of tetrahydropyran and cyclohexane are measured with time-resolved infrared absorption in a temperature-controlled Herriott multipass cell in the temperature range of 500-750 K at 20 Torr. The experiments show two distinct timescales for HO2˙ and ˙OH formation in the oxidation of both fuels. Analysis of the timescales reveals striking differences in behavior between the two fuels. In both cyclohexane and tetrahydropyran oxidation, a faster timescale is strongly related to the "well-skipping" (˙R + O2 → alkene + HO2˙ or cyclic ether + ˙OH) mechanism and is expected to have, at most, a weak temperature dependence. Indeed, the fast HO2˙ formation timescale is nearly temperature independent both for cyclohexyl + O2 and for tetrahydropyranyl + O2 below 700 K. A slower HO2˙ formation timescale in cyclohexane oxidation is shown to be linked to the sequential ˙R + O2 → ROO˙ → alkene + HO2˙ pathway, and displays a strong temperature dependence mainly from the final step (with energy barrier ∼32.5 kcal mol-1). In contrast, the slower HO2˙ formation timescale in tetrahydropyran oxidation is surprisingly temperature insensitive across all measured temperatures. Although the ˙OH formation timescales in tetrahydropyran oxidation show a temperature dependence similar to the cyclohexane oxidation, the temperature dependence of ˙OH yield is opposite in both cases. This significant difference of HO2˙ formation kinetics and ˙OH formation yield for the tetrahydropyran oxidation can arise from contributions related to ring-opening pathways in the tetrahydropyranyl + O2 system that compete with the typical ˙R + O2 reaction scheme. This comparison of two similar fuels demonstrates the consequences of differing chemical mechanisms on ˙OH and HO2˙ formation and shows that they can be highlighted by analysis of the eigenvalues of a system of simplified kinetic equations for the alkylperoxy-centered ˙R + O2 reaction pathways. We suggest that such analysis can be more generally applied to complex or poorly known oxidation systems.