Catalytically active sites oxidative coupling, methane

To promote both the conversion of reactants and the selectivity to partial oxidation products, many kinds of metal compounds are used to create catalytically active sites in different oxidation reaction processes [4]. The most well-known oxidation of lower alkanes is the selective oxidation of n-butane to maleic anhydride, which has been successfully demonstrated using crystalline V-P-O complex oxide catalysts [5] and the process has been commercialized. The selective conversions of methane to methanol, formaldehyde, and higher hydrocarbons (by oxidative coupling of methane [OCM]) are also widely investigated [6-8]. The oxidative dehydrogenation of ethane has also received attention [9,10],... 

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 issue of catalytic methane activation by active oxygen species associated with lattice oxygen has been the subject of many literature reports.Despite the fact that for a large number of oxidative coupling catalysts the involvement of lattice oxygen in the formation of Cj hydrocarbons is unquestionable, there is still a lack of convincing evidence that could resolve the issue of the nature of the active sites. 

Rare earth pyrochlores, possessing the general formula Ln2Sn207, where Ln denotes a rare earth, are active catalysts for the oxidative coupling of methane 150]. Enhanced conversion to useful hydrocarbons (e.g., ethene) is observed with pyrochlores containing rare earths with mixed valence behavior, particularly Sm, Eu, and Yb. Since the rare earth site thus appears to be crucially linked to the catalytic behavior, the distribution of such rare earth species within the lattice is of intrinsic interest. 

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