Methane oxidation by the ferryl ion

Abstract:

Previously, we have shown that the ferryl ion ([Fe$^\mathrm{IV}$O]$^{2+}$) is easily produced from Fenton's reagent (i.e. a mixture of Fe$^{2+}$ ions and H$_2$O$_2$ in aqueous solution), using DFT and Car-Parrinello MD calculations. In order to definitely conclude that the ferryl ion is indeed the active species in oxidation reactions with Fenton's reagent, we have in the present study studied the reactivity of the ferryl ion towards organic substrates, in particular the oxidation of methane to methanol. Our static DFT calculations on the [(H$_2$O)$_5$Fe$^\mathrm{IV}$O]$^{2+}$-CH$_4$ complex in vacuo show a strong prevalance for the oxygen-rebound mechanism over the methane-coordination mechanism, which is in agreement with the results for methane oxidation by bio-catalysts MMO and P450, but not with those for methane oxidation by bare metal-oxo ions. The highest energy barrier in the oxygen-rebound mechanism is only 3 kcal/mol, whereas in the methane coordination mechanism the highest barrier is 23 kcal/mol. Overall the oxidation reaction energy is downhill by 47 kcal/mol. We also have computed the free energy barrier of the H-abstraction reaction from methane by the ferryl ion (i.e. the first step in the rebound mechanism) in aqueous solution by the method of constrained (first principles) molecular dynamics. The free energy barrier of 22 kcal/mol in solution is significantly higher that it is in vacuo. Nevertheless, in combination with our previous work, we must conclude that the ferryl ion is indeed the active intermediate in Fenton chemistry.