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Autoxidation of Cyclohexane: Conventional Views Challenged by Theory and Experiment

Authors

  • Ive Hermans,

    1. Division of Quantum Chemistry and Physical Chemistry, Department of Chemistry, KULeuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium, Fax: (+33) 16-32-7992
    2. Centre for Surface Chemistry and Catalysis, Department of Interphase Chemistry, KULeuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium
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  • Thanh Lam Nguyen,

    1. Division of Quantum Chemistry and Physical Chemistry, Department of Chemistry, KULeuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium, Fax: (+33) 16-32-7992
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  • Pierre A. Jacobs Prof. Dr.,

    1. Centre for Surface Chemistry and Catalysis, Department of Interphase Chemistry, KULeuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium
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  • Jozef Peeters Prof. Dr.

    1. Division of Quantum Chemistry and Physical Chemistry, Department of Chemistry, KULeuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium, Fax: (+33) 16-32-7992
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Abstract

In spite of its industrial importance, the detailed reaction mechanism of cyclohexane autoxidation by O2 is still insufficiently known. Based on quantum chemical potential energy surfaces, rate coefficients of the primary and secondary chain propagation steps involving the cyclohexylperoxyl (CyOO.) radical were evaluated using multiconformer transition-state theory. Including tunneling and hindered-internal-rotation effects, the rate coefficient for hydrogen-atom abstraction from cyclohexane (CyH) by CyOO. was calculated to be k(T)=1.46×10−11×exp(17.8 kcal mol1/RT) cm3s−1(300–600 K), close to the experimental data. A “Franck–Rabinowitch cage” reaction between the nascent cyclohexylhydroperoxide (CyOOH) and cyclohexyl radical, products from CyOO.+CyH, is put forward as an initially important cyclohexanol (CyOH) formation channel. αH abstraction by CyOO. from cyclohexanone was calculated to be only about five times faster than that from CyH, too slow to explain all the observed side products. The α-hydrogen (αH) abstractions from CyOH and CyOOH by CyOO. are predicted to be about 10 and 40 times faster, respectively, than the CyOO.+CyH reaction. The very fast CyOO.+CyOOH reaction proceeds through the unstable Cy−αH.OOH radical that decomposes spontaneously into the ketone (Q[DOUBLE BOND]O) plus the .OH radical; the “hot” .OH is found to produce the bulk of the alcohol via a second, “activated cage” reaction analogous to that above. It is thus shown how the very reactive CyOOH intermediate is the predominant source of ketone and alcohol, while it also leads to some side products. The α-hydroxy-cyclohexylperoxyl radical formed during the moderately fast oxidation of CyOH is shown to decompose fast into HO2.+cyclohexanone in a rapidly equilibrated reaction, which constitutes a smaller, second ketone source. These two fast cyclohexanone forming routes avoid the need for unfavorable molecular routes hitherto invoked as ketone sources. The theoretical predictions are supported and complemented by experimental findings. The newly proposed scheme is also largely applicable to the oxidation of other hydrocarbons, such as toluene, xylene, and ethylbenzene.

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