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...

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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...

Energetic ionic liquids (EILs) are of great interest. [1][2][3] They offer enhanced stability, higher densities, no vapor pressure, and, hence, no vapor toxicity. As a general principle, the stability of energetic ionic compounds can be greatly enhanced by making the cation the fuel and the anion the oxidizer. The formal positive charge increases the ionization potential of the fuel cation, and the formal negative charge decreases the electron affinity of the anion. In this manner, the fuel cation becomes more oxidizer-resistant, and the oxidizer anion is protected against premature reduction by the cation. For environmental reasons, it is also desirable to avoid halogen-containing ingredients, such as perchlorates.The previously known EILs consist of small oxidizing anions, such as ClO 4 À , NO 3 À , or N(NO 2 ) 2 À , and large fuel cations containing quaternary nitrogen heterocycles with long, asymmetric, poorly packing side chains. The most serious drawback of these EILs is that they are underoxidized. The small anions do not carry sufficient oxygen for complete oxidation of the large fuel cations to carbon monoxide, resulting in poor performance. In rocket propulsion, a low molecular weight of the exhaust products is very important. [4,5] Furthermore, at high flame temperatures CO 2 is dissociated almost completely to CO and O 2 (Boudouard equilibrium).[6] Therefore, it is often sufficient to oxidize the carbon content only to CO and not to CO 2 to achieve nearmaximum performance.[5] The aim of this study was the preparation of halogen-free, CO-balanced, EILs.In 1998, the concept of oxidizer-balanced EILs was proposed, and in 2002, its practicability was shown by the preparation of 1-ethyl-3-methylimidazolium tetranitratoborate, [7] a compound that turned out to be indeed an ionic liquid with a freezing point of À25 8C. However, its energy content and thermal stability were marginal. Herein, we report on a significantly improved compound using the tetranitratoaluminate anion as a thermally more stable highoxygen carrier and the 1-ethyl-4,5-dimethyltetrazolium cation as a more energetic counterion (imidazole, DH o f = + 49.8 kJ mol À1 ; [8] tetrazole, DH o f = + 237.1 kJ mol À1 [9] ). These are the first CO-balanced EILs. Although an oxygenbalanced tetrazolium salt, 5-aminotetrazolium nitrate, was recently reported, [10] its melting point of 173 8C does not classify it as an ionic liquid.Polynitratoaluminates were first studied in the 1960s in the USA [11] and, subsequently, during the 1970s in the USSR. [12][13][14][15][16][17][18][19][20][21][22][23] Several examples of alkali metal, [12][13][14][15][16][17][18][19][20][21] NO 2 + , [22,23] and ethylammonium salts [24] of tetra-, penta-, and hexanitratoaluminate anions are known. The tetranitratoaluminate anion contains 12 oxygen atoms; of these, 10.5 are available to oxidize a fuel cation.Alkylated tetrazolium cations were used in this work because of their large positive heats of formation and their potential to form ionic liquids. Ionic salts of the t...