Issue 16, 2018

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.

Graphical abstract: Direct measurement of ˙OH and HO2˙ formation in ˙R + O2 reactions of cyclohexane and tetrahydropyran

Supplementary files

Article information

Article type
Paper
Submitted
05 Dez 2017
Accepted
25 Jan 2018
First published
08 Feb 2018

Phys. Chem. Chem. Phys., 2018,20, 10815-10825

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

M. Chen, B. Rotavera, W. Chao, J. Zádor and C. A. Taatjes, Phys. Chem. Chem. Phys., 2018, 20, 10815 DOI: 10.1039/C7CP08164B

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