Jack T. Fuller, III, Steven Butler, Deepa Devarajan, Austin Jacobs, Brian G. Hashiguchi, Michael M. Konnick, William A. Goddard, III, Jason Gonzales, Roy A. Periana, and Daniel H. Ess
ACS Catal. 2016, 6, 4312-4322
http://pubs.acs.org/doi/pdf/10.1021/acscatal.6b00226
TOC:
Abstract:
Methane conversion to methyl bisulfate by HgII(SO4) 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. HgII is also of significant interest because as a d10 transition metal it is similar to d10 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 HgII 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, [HgI(HSO4)]2, into Hg0 followed by Hg0 to HgII oxidation induced by SO3 from dehydration of sulfuric
acid. This catalytic cycle is efficient because in sulfuric acid the HgII/Hg0 potential results in a moderate free energy barrier for
oxidation (∼40 kcal/mol) and HgII is electrophilic enough to induce barriers of <40 kcal/mol for C−H activation and reductive
metal alkyl functionalization. Comparison of HgII to TlIII shows that while C−H activation and reductive metal alkyl
functionalization have reasonable barriers for TlIII, the oxidation of TlI to TlIII has a significantly larger barrier than Hg0 to HgII
oxidation and therefore TlIII is not catalytic in sulfuric acid. Comparison of HgII to CdII and ZnII reveals that while M0 to MII
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 (K2S2O8) methane oxidation to methanesulfonic acid in sulfuric acid.
