Selective oxidation of methane

Li/MgO(100) Mo(lOO) thin film (selective oxidation of methane to ethane) (7)... 

The selective oxidation of methane by a bis(/z-oxo)(Cu )2 species, sta-bihzed on various zeolites, was reported to proceed at high temperatures. Selective oxidation to methanol was observed [117]. 

Oxidation of Methane. A variety of new catalyst systems have been disclosed, and new reagents were developed with the aim to perform selective transformation of methane to methanol, methyl esters, and formaldehyde. Much work was carried out in strongly acidic solutions, which enhances the electrofilicity of the metal ion catalyst, and the ester formed is prevented from further oxidation. An important advance in the selective oxidation of methane to methanol is Periana s 70% one-pass yield with high selectivity in sulfuric acid solution under moderate conditions.1073 The most effective catalyst is a Pt-bipyrimidine complex. Pt(II) was shown to be the most active oxidation state generating a Pt-methyl intermediate that is oxidized to yield the product methyl ester. A density functional study... 

Under conditions where initiation is rate controlling, as it may be in selective oxidation of methane to yield higher value products, addition of NO2 may be favorable. The reaction... 

There are some arguments for the involvement of O radicals in the selective oxidations of methane. The 0 is registered by ESR upon adsorption of N20 on partially reduced surfaces of supported Mo and V oxides [28, 35]. The radical exhibits a very high reactivity. At room temperature, it readily interacts with methane to yield methanol as the main product, which can be registered by the IR in situ [36] or desorbed from the catalyst surface by heating [29, 30]. 

Beside V2Os and M0O3, many other catalytic systems were tested in selective oxidation of methane by N20, including various metal oxides [37, 38], zeolites [39], phosphates [40, 41] and polyoxometalates [42],... 

Gold has emerged as an effective catalyst for the selective oxidation of methane to methanol. Various possible pathways for the oxidation are discussed.29 Suitably substituted furans are transformed into phenols by the use of gold catalyst (1). It has been suggested, on the basis of kinetic isotope effect and trapping studies, that the key intermediate is an arene oxide. The postulation is also supported by DFT calculations.30... 

Formaldehyde production from natural gas is one of the most promising directions in modem chemical industry. However, the industrial realization of producing formaldehyde from methane is hampered by severe disadvantages, among which the most important are the low yields of the target product and multiple side reaction products. Therefore, the task in hand is the development of direct selective oxidation of methane to formaldehyde without formation of admixtures, which require additional thorough purification. 

Based on the above results, it may be concluded that selective oxidation of methane to formaldehyde with hydrogen peroxide is implemented by a mechanism different from that in which methanol is formed as the intermediate oxidation product [115]. 

The most widespread efforts made towards the achievement of selective oxidation of alkanes are targeted on methane, a principal constituent of natural gas f 6-8]. Activation of the very stable C-H bond of methane is a particularly demanding problem. One example in which this has been achieved on industrial scales is the Degussa process [9], Methane is coupled to ammonia by heterogeneous catalysis in order to produce HCN, an important fundamental material for industrial chemistry. An unsolved problem is the selective oxidation of methane to methanol a reaction that would convert the methane gas into a transportable liquid. In nature, monooxygenases have evolved. These are able to activate molecular oxygen and to... 

V.I. Sobolev, K.A. Dubkov, O.V. Panna, G.I. Panov, Catalysis Today (1995), 24, 251-252, Selective oxidation of methane to methanol on a FeZSM-5 surface... 

Magnesium oxide doped with Li is used as a catalyst for the oxidative conversion of methane to higher hydrocarbons [44] and for the selective oxidation of methane [25],... 

Short-contact-time reactions, defined as reactions occurring on a timescale of milliseconds, offer potential for conversion of hydrocarbons in one step into valuable products. Examples are the selective oxidation of methane in s)mgas without the formation of b)q5roducts (CO2, H2O, and coke) and the oxidative dehydrogenation of alkanes to give olefins or oxygenates. 

Electrophilic C-H activations can also be effected in water. At first glance, water would appear to be particularly unpromising as a solvent for such reactions. Because of their extremely poor coordinating ability alkanes should not be able to compete with water for coordination sites. Moreover, the intermediate metal-alkyl species would be prone to hydrolytic decomposition. In one respect, however, water is an almost ideal medium for C-H functionalization the O-H bond energy exceeds the corresponding C-H bond energy of even methane. Indeed, the selective oxidation of methane to methanol is carried out by methane monooxygenase in aqueous medium. 

Chu, W., Yang, W., and Lin, L. Selective oxidation of methane to syngas over NiO/barium hexaalu-minate. Catalysis Letters, 2001, 74, 139. 

Most of the current results on the selective oxidation of methane over metal oxide catalysts may be interpreted in terms of methyl radical chemistry. These radicals may either react with the oxides themselves to form methoxide ions or they may enter the gas phase. The methoxide ions on supported molybdena decompose to form formaldehyde or they react with water to yield methanol. On the basic oxides methoxide ions result in complete oxidation. Those radicals which enter the gas phase undergo typical free radical chemistry which includes coupling reactions to give ethane and chain branching reactions to give nonselective oxidation products. Secondary surface reactions, particularly with ethylene, also may result in complete oxidation. If further improvements in yields of partial oxidation products are to be achieved, ways must be found to more efficiently utilize the methyl radicals, both with respect to surface reactions and to gas phase reactions. In addition, if ethylene is the desired product, catalysts must be fine-tuned to the point where they will activate methane, but not ethylene. 

Possible mechanisms of alkane oxidation in the presence of heterogeneous catalysts are discussed in publications [44], The rate-determining step in the selective oxidation of methane is the rupture of the C-H bond. Methane activation can occur by a homolytic mechanism [44aji] ... 

Selective oxidation of methane is a subject of significant interest owing to the increased availability of natural gas as a feedstock. 

Groothaert MH, Smeets PJ, Sels BF, Jacobs PA, Schoonheydt RA (2005) Selective oxidation of methane by the Bis(/-oxo)dicopper core stabilized on ZSM-5 and mordenite zeolites. J Am Chem Soc 127 1394... 

---