Catalytic Mechanism and Efficiency of Methane Oxidation by Hg(II) in Sulfuric Acid and Comparison to Radical Initiated Conditions

Abstract

Methane conversion to methyl bisulfate by Hg^(II)(SO_4) in sulfuric acid is an example of fast and selective alkane oxidation catalysis. Dichotomous mechanisms involving C–H activation and electron transfer have been proposed based on experiments. Radical oxidation pathways have also been proposed for some reaction conditions. Hg^(II) is also of significant interest because as a d^(10) transition metal it is similar to d^(10) main-group metals that also oxidize alkanes. Density-functional calculations are presented that use both implicit and a mixture of implicit/explicit solvent models for the complete Hg_(II) catalytic cycle of methane oxidation to methyl bisulfate. These calculations are consistent with experiment and reveal that methane is functionalized to methyl bisulfate by a C–H activation and reductive metal alkyl functionalization mechanism. This reaction pathway is lower in energy than both electron transfer and proton-coupled electron transfer pathways. After methane C–H functionalization, catalysis is completed by conversion of the proposed resting state, [Hg^I(HSO_4)]_2, into Hg^0 followed by Hg^0 to Hg^(II) oxidation induced by SO_3 from dehydration of sulfuric acid. This catalytic cycle is efficient because in sulfuric acid the Hg^(II)/Hg^0 potential results in a moderate free energy barrier for oxidation (∼40 kcal/mol) and Hg^(II) is electrophilic enough to induce barriers of <40 kcal/mol for C–H activation and reductive metal alkyl functionalization. Comparison of Hg^(II) to Tl^(III) shows that while C–H activation and reductive metal alkyl functionalization have reasonable barriers for Tl^(III), the oxidation of Tl^I to Tl^(III) has a significantly larger barrier than Hg^0 to Hg^(II) oxidation and therefore Tl^(III) is not catalytic in sulfuric acid. Comparison of Hg^(II) to Cd^(II) and Zn^(II) reveals that while M^0 to M^(II) oxidation and C–H activation are feasible for these first-row and second-row transition metals, reductive metal alkyl functionalization barriers are very large and catalysis is not feasible. Calculations are also presented that outline the mechanism and energy landscape for radical-initiated (K_2S_2O_8) methane oxidation to methanesulfonic acid in sulfuric acid.

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