Oxidative methane activation

Activation and conversion of small molecules photocatalysis. Production of H2 from protons, N2 fixation for ammonia production, C02 fixation for organics, sulphide (H2S and Na2S) oxidation, methane activation for methanol production, etc. 

A topic of current interest is that of methane activation to give ethane or selected oxidation products such as methanol or formaldehyde. Oxide catalysts are used, and there may be mechanistic connections with the Fischer-Tropsch system (see Ref. 285). 

Homogeneous catalysts have been reported, which can oxidize methane to other functionalized products via C-H activation, involving an electrophilic substitution process. The conversion of methane into methyl bisulfate, using a platinum catalyst, in sulfuric acid, has been described. The researchers found that a bipyrimidine-based ligand could both stabilize and solubilize the cationic platinum species under the strong acidic conditions and TONs of >500 were observed (Equation (5)).13... 

Fig. 4. Relevant structures for the discussion of methane activation by (bipyrimi-dine)PtCl2 Methane complex of Pt(II) (A) methyl(hydrido)platinum(IV) complex, the product of the oxidative addition (B) transition state for intramolecular deprotonation of the methane complex ( cr-bond metathesis , sometimes also called electrophilic , C) intermolecular deprotonation of the methane complex ( electrophilic pathway , D).

Porous alumina tube externally coated with a MgO/PbO dense film (in double pipe configuration), tube thickness 2.5 mm, outer diameter 4 mm, mean pore diameter 50 nm, active film-coated length 30 mm. Feed enters the reactor at shell side, oxygen at tube side. Oxidative methane coupling, PbO/MgO catalyst in thin film form (see previous column). r-750X,Pr ed 1 bar. Conversion of methane <2%. Selectivity to Cj products > 97%. Omata et al. (1989). The methane conversion is not given. Reported results are calculated from permeability data. 

Chan and Wilson [52] tried to oxidize methane to methanol by oxygen on TMPcY and TMTPPY (TM= Co, Fe, Ru, Mn) in the temperature range of 548 K to 773 K. Only RuPcY, CoTPPY and MnTPPY are active towards alcohol formation, yields up to 0.5% being claimed (Table 5). All other catalysts give combustion of methane to carbon dioxide and water. In an attempt to repeat these experiments, the present authors only observed CO, CO2 and H2O formation, and rapid autoxidation of the catalyst. 

Two series of catalysts were synthesized for subsequent evaluation as methane dimerization catalysts. The first series was alkali modified zinc oxide (6) and magnesium oxide catalysts (7), which were reported to be active for methane activation, while the second series was ion modified perovskites described by Machida and Enyo (8). The objective of the present study was to determine whether the aerosol technique could provide a wide range of ion substitutions as homogeneous solid solutions, and to determine whether moderately high surface area catalysts could... 

The subject of heterogeneously catalyzed selective oxidation has been reviewed many times. Under the keyword combination selective catalytic oxidation the ISI database reports about 5400 papers. Over 100 reviews on the topic have been published. In the present discussion, the subjects of methane activation and model studies of unselective CO oxidation, which represent large fields, are excluded. Homogeneously or biologically catalyzed selective oxidation, a combined field that is about 10-fold larger in scientific coverage, is also excluded from this chapter. 

The field is defined here around the activation of butane, propane and ethane plus the oxidation of propene. The reason for this boundary is the similarity of the chemistry and the great need to understand the mechanism of selectivity of activated oxygen in these multi-step reactions. The processes cannot be conducted at high temperatures such as with methane activation as the target products are not stable under conditions where alkane activation is fast. The selective oxidation of... 

How the experimental panorama is influenced by parameters still to be defined was demonstrated by Shibata et al. [86]. Here, preliminary results obtained in aqueous media using a specific brand of high-purity commercial copper cathode were positive with regards to hydrocarbons C3+, provided that no electropolishing was performed before the electrochemical process. If electropolishing preceded the C02 reduction, the cathodes behaved similarly to any other copper cathode, leading essentially (besides hydrogen) only to methane end ethylene. A tentative explanation of this behavior was proposed which referred to the polycrystalline matrix of this brand of copper, which made it particularly adaptable to be covered by oxide layers active in the formation of C3+. However, further experimental evidence on the surface structure, composition and modifications with electrolysis time will be required to substantiate this hypothesis. 

Methane monooxygenase consists of a catalytically active diiron center. In the presence of oxygen this enzyme oxidizes methane and other hydrocarbons, i.e. molecules without any anchor-... 

Calculations show that for M = [CpIrin(PH3)(Me)]+ the oxidative addition mechanism A is the low-energy pathway, while Rhm may adopt path B.132 With complexes containing very labile ligands, such as the 171 -dichloromethane complex [Cp (PMe3)IrMe(ClCH2Cl)]+, methane activation takes place under very mild conditions and at temperatures as low as 10°C, while benzene adds rapidly at -30°C.133... 

An excellent short review of C-H bond activation including both thermal and photochemical mechanisms has been provided by Labinger and Bercaw. Lees has reviewed the photochemical and photophysical features of these reactions. Bergman has described the contributions of his group toward the understanding of the mechanisms of C-H activation up to 1992, and within the context of physical organic chemical methods. Schneider has examined the similarities between Si-H and C-H activation. Shilov and ShuFpin have provided extensive reviews of C-H activation. As with Si-H oxidative addition, the spin state of the photochemical intermediate plays a major role in the probability of insertion. This has been reviewed for the specific case of methane activation. 

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