Acetic acid is an important petrochemical that is currently produced from methane (or coal) in a three-step process based on carbonylation of methanol. We report a direct, selective, oxidative condensation of two methane molecules to acetic acid at 180 degrees C in liquid sulfuric acid. Carbon-13 isotopic labeling studies show that both carbons of acetic acid originate from methane. The reaction is catalyzed by palladium, and the results are consistent with the reaction occurring by tandem catalysis, involving methane C-H activation to generate Pd-CH3 species, followed by efficient oxidative carbonylation with methanol, generated in situ from methane, to produce acetic acid.

Some of the most efficient homogeneous catalysts for the lowtemperature, selective oxidation of methane to functionalized products employ a mechanism involving CÀH activation [1] with an electrophilic substitution mechanism. Several such systems have been reported based on the cations Hg II , [2] Pd II , [3] and Pt II . [4] These catalyst systems typically operate by two general steps that involve: A) CÀH activation by coordination of the methane to the inner sphere of the catalyst (E n+ ) followed by cleavage of the CÀH bond by overall electrophilic substitution to generate E n+ ÀCH 3 intermediates, and B) oxidative functionalization involving redox reactions of E n+ À CH 3 to generate the desired oxidized product CH 3 X.[4a]Consequently, efficient catalysts that follow this pathway would be expected to be "soft", highly electrophilic species that form relatively strong covalent bonds to carbon atoms and that are also good oxidants.We considered that gold cations could be uniquely efficient electrophilic catalysts for methane conversion because, as shown in the conceptual catalytic cycle (Scheme 1), [2, 4] This situation is not common, and in most catalytic systems based on "soft", redox-active electrophiles only one oxidation state of the redox couple is active for CÀH activation. Thus, we sought to explore the catalytic chemistry of gold cations for the oxidation of methane. To our knowledge, while gold complexes have been reported to facilitate free-radical reactions of alkanes with peroxides in low yields, [5] no homogeneous gold catalysts that operate by heterolytic CÀH activation and oxidative functionalization have been reported for the selective functionalization of alkanes. This is possibly because of the strong propensity for irreversible formation of gold metal, and any attempts to develop redox catalysis based on homogeneous Au cations must address this issue.In strong acid solvents such as triflic or sulfuric acid, Au III cations (generated by dissolution [6] of Au 2 O 3 ) react with methane at 180 8C to selectively generate methanol (as a mixture of the ester and methanol) in high yield (Table 1, entries 1 and 2). As expected, the irreversible formation of metallic gold is very evident after these reactions and, unlike reactions with Hg II , [2] Pt II , [4d] and Pd II [3a] that are catalytic in 96 % H 2 SO 4 , only stoichiometric reactions (turnover numbers (TONs) < 1) are observed with Au III [Eq. (1)]. Soluble cationic gold is essential for these reactions as no methanol is observed under identical conditions without added Au III ions (entry 3), or in the presence of metallic gold (entry 4) which is not dissolved in hot H 2 SO 4 .Consistent with the known nobility of gold metal, no methanol formed when SO 3 or persulfate (K 2 S 2 O 8 ) were added as possible oxidants of metallic gold (entries 5 and 6). We considered the use of Se VI ions as a more suitable oxidant. Se VI ions are a more powerful oxidizing agent than S VI ions (E o = 1.5 V SeO 4 2À /H 2 SeO 3 , E o = 0.17 V SO 4 2À /H 2 S...

Catalytic supremacy of Pt-single atoms achieved by CeOx–TiO2 interfaces.