Oxidative dimerization of methane

Numerous works on the oxidation of methane to methanol and/or formaldehyde as well as on the oxidative dimerization of methane were reviewed by many authors [22-27]. First, high selectivity of methane oxidation by N20 was reported by Lunsford et al. [28-30], Over a supported Mo oxide [30], the total selectivity to methanol and formaldehyde at low methane conversions attained 100%, although this rapidly dropped as the conversion increased (Table 7.4). High selectivity for this reaction was obtained also with supported vanadium oxide [31]. 

Ito T, Wang J, Lin CH, Lunsford JH. Oxidative dimerization of methane over a lithium-promoted magnesium oxide catalyst. J Am Chem Soc. 1985 107 5062-8. 

Kuchynka et al. [125] studied the electrochemical oxidative dimerization of methane to C2 hydrocarbon species using perovskite anode electrocatalysts. Three designs of solid oxide fuel cells were used, including tubular and flat plate solid electrolytes. The maximum current density for the dimerization reaction at these electrocatalysts was related to the oxygen binding energies on the catalyst surface. The anodic reaction was ... 

On the other hand, Ito et al. (99) found that the oxidative dimerization of methane to yield ethylene and ethane can be achieved with a high yield and good selectivity on Li-doped MgO catalysts. Since this pioneering work, many oxidic systems have been studied. Anpo et al. (100) found that surface sites of low coordination produced by the incorporation of Li into MgO play a vital role in the methane oxidative coupling reaction. Thus, although it was known that MgO acts as an acid-base catalyst, both the catalytic and photocatalytic activities of the MgO catalysts seem to be associated with the existence of surface ions in low coordination located on MgO microcrystals. 

The oxidative dimerization of methane in the presence of oxygen or air often gives substantial amounts of carbon oxides as byproducts [1]. Conversion of methane to methylchloride and further condensation to higher hydrocarbons and hydrogen chloride avoids this problem. The reoxidation of hydrogen chloride to chlorine, as well as the formation of methylchloride from methane is known technology. 

III. 6. C. Oxidative Dehydrogenation and Dehydrocyclization III. 6. D. Oxidative Dimerization of Methane... 

Later it was shown that sequential introduction of CH4 and O2 was not necessary to promote methane oxidative dimerization but this could be achieved directly by passing CH4/O2 mixtures over a metal oxide catalyst [57]. Since these early reports, work directed towards investigating the chemical oxidative dimerization of methane has increased with a significant number of papers [58-88] and reviews [89, 90] being published. 

Suzuki T, Wada K, Watanabe Y (1990) Effects of carbon dioxide and catalyst preparation on the oxidative dimerization of methane. Appl Catal 59 213-225... 

Tong Y, Rosynek MP, Lunsford JH (1990) The role of sodium carbonate and oxides supported on lanthanide oxides in the oxidative dimerization of methane. J Catal 126 291-298... 

Lin, C., Wang, J., and Lunsford, J.H. Oxidative dimerization of methane over sodium promoted calcium oxide. J. Catal 1988, 111, 302-316. 

Bartek, J.P., Hupp, J.M., Brazdil, J.F., and Grasselli, R.K. (1988) Oxidative Dimerization of Methane Over Lead-Magnesium Mixed Oxide Catalysts , Catalysis Today 3,117-26. 

As an illustration, the results of the application of the ESYCAD program to the oxidative coupling of methane are explained. For this reaction, methane may by activated at strong basic sites of the catalyst by hcterolytic chemisorption, resulting in methyl anions which may be oxidized to radicals. In the selective reaction, these radicals dimerize to ethane as the primary product. Acid sites or /j-conductivity should be avoided because they lead to total oxidation. Under reaction conditions the catalyst should be stable, i.e. not be oxidized or reduced or volatize, which can be checked by thermodynamics. 

Methane can be catalytically oxidized in the fuel cell mode to simultaneously generate electricity and C2 hydrocarbons by dimerization of methane using a yttria-stabilized zirconia membrane. A catalyst, used as the anode, is deposited on the side of the membrane that is exposed to methane and the cathode is coated on the other side of the membrane. When the catalyst Ag>Bi2C>3 is used as the anode for the reaction at 750> 900X and atmospheric total pressure, the selectivity to ethane and ethylene exceeds 90%. But this high selectivity is at the expense of low power output and low overall methane conversion (less than about 2%). 

A general mechanism of the oxidative coupling of methane over reducible oxide catalysts has been proposed by Lee and Oyama. Their reaction sequence is based on the cracking mechanism suggested by Kolts and Delzer which was adapted to the methane dimerization process. The similarities between these two processes as indicated by Lee and Oyama were as follows (1) the same materials (Mn/MgO, Fe/MgO, LajOj, Ce02) are active in both reactions,... 

Other steps used in the model assume that the heterogeneous conversion of methane is limited to the gas-phase availability of oxygen, O2 adsorption is fast relative to the rate of methane conversion, and heat and mass transports are fast relative to the reaction rates. Calculations for the above model were conducted for a batch reactor using some kinetic parameters available for the oxidative coupling of methane over sodium-promoted CaO. The results of the computer simulation performed for methane dimerization at 800 °C can be found in Figure 7. It is seen that the major products of the reaction are ethane, ethylene, and CO. The formation of methanol and formaldehyde decreases as the contact time increases. 

As an example we can consider methane dimerization, where the most important catalytic reaction in the oxidative coupling of methane is the production of methyl radicals ... 

It seems likely that in reaction 19, the interaction of 22 with methane would be similar to that found in the unsymmetrical methyllutetium dimer where a methyl group of one monomer donates electron density to the second monomer. On the other hand, orbital analysis of oxidative addition of methane on d ML and CpML fragments indicated that,... 

Keller and Bhasin were first to report in 1982 [1] on the catalytic one-step oxidative dimerization or coupling of methane (OCM) to C2 hydrocarbons, ethane and ethylene. Numerous investigations have followed this seminal work and a large number of catalysts have been found which give total selectivity to C2 hydrocarbons higher than 90% at low (<2%) methane conversion [2-6]. 

FIGURE 6.24 Redox behavior of the methano-dimer of a-tocopherol (bis(5-tocopheryl) methane, 28) temperature dependence of the oxidation with bromine. 

The dimer of the vanadyl silsesquioxane complex 148 was used by Mitsudo et al. to prepare catalysts with a characteristic pore structure and excellent activity toward the selective photoassisted catalytic oxidation of methane into methanal. " ... 

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