Methane oxidative dimerization

Figure 9 Schematic representation of the reaction pathway for methane oxidative dimerization on alkali (A) promoted oxide catalyst (BO). The pathway represents the most probable routes, including the coexistence of various phases in equilibrium with the reaction environment.

O2 + 4e" 20" through membrane to methane oxidative dimerization site... 

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. 

Ethane contained in natural gas may also contribute to methane coupling chemistry. In the temjjerature regime of interest in methane oxidative dimerization (-750 °C), ethane undergoes thermal cracking that increases with temperature. This generates an additional supply of methyl radicals via ... 

PREPARATION OF Li/MAGNESIUM BASIC SILICATES CATALYSTS FOR METHANE OXIDATIVE DIMERIZATION... 

Methane oxidation and partial oxidation, electrochemical promotion of, 308 dimerization, 470 reforming, 410 Methanol dehydrogenation electrochemical promotion of, 403 selectivity modification, 404 Methanol oxidation electrochemical promotion of 398 selectivity modification, 400 Microscopy... 

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]. 

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... 

Experimental test of this mechanism was conducted by performing a competition study with ethylene/methane mixtures in the tubular reactor. The results, summarized in Table 5, demonstrate that ethylene oxidation competes readily with methane oxidation under the experimental conditions of the electrocatalytic cell. The ratios of 2/ 1 calculated for these experiments are 4.0 and 4.6. This is in reasonable agreement with the ratio derived from the methane coupling experiments. Thus, the consecutive reaction mechanism can be applied successfully to systems of this type. The inescapable conclusion is that methane dimerization is limited by the relative rates of methane and ethylene activation. 

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. 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

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

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