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Direct Partial Oxidation of Methane Catalyzed by an In Situ Generated Active Au(III) Complex at Low Temperature in Ionic Liquids

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posted on 2021-01-25, 21:45 authored by Tingyu Huang, Zhanwei Xu, Peifang Yan, Xiumei Liu, Hongjun Fan, Z. Conrad Zhang
An in situ generated AuIII catalyst is found to catalyze the direct oxidation of CH4 to C1 oxygenates in 1-ethylimidazolium bis-(trifluoromethylsulfonyl)­amide ([Eim]­[NTf2]) at 90 °C. The formation of 13CH3OH and H13COOH from 13CH4 as a feed verifies the CH4 oxidation to CH3OH and HCOOH. Ionic liquids (ILs) with a wide range of structural types as potential reaction media and a number of solid, liquid, and gaseous oxidants are screened in a temperature range of 90–200 °C. Among the ILs and the oxidants, [Eim]­[NTf2] and hydrogen peroxide (H2O2) are identified to be compatible as the stable solvent and the most efficient oxidant, respectively, for the selective oxidation of CH4 to C1 oxygenates, with CH3OH as the primary product. An AuIII-CH4 H-bonding structure, produced in situ by adding two molar equivalent of silver trifluoromethanesulfonate (AgOTf) to the AuCl3(phen) (phen=phenanthroline) precursor under high CH4 pressure, forms a resting state of the AuIII catalyst, which produces CH3OH in the presence of H2O. After each catalytic turnover, AuI is oxidized by H2O2 to regenerate the active AuIII state. In the absence of CH4, unstable AuCl­(OTf)2(phen) rapidly forms an orange-colored precipitate that shows no activity in CH4 activation. CH3OH overoxidation to HCOOH was dominantly catalyzed by potent Au0 species as a result of AuI disproportionation, which is the detrimental catalyst deactivation mechanism. Increasing CH4 pressure and H2O2 concentration successfully enhances the catalyst lifetime and significantly improves the CH4 oxidation efficiency with the improved CH3OH/HCOOH ratio. Density functional theory (DFT) calculations showed that (1) a C–H bond in CH4 was activated by forming AuIII-CH3 with a free energy barrier of 26.7 kcal/mol in a six-membered ring transition state and (2) AuIII-CH3 was functionalized to CH3OH by nucleophilic H2O with a free energy barrier of 29.1 kcal/mol or by MeOTf reductive elimination with a free energy barrier of 21.1 kcal/mol.

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