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X-Ray photoelectron spectroscopic studies of lanthanum oxide based oxidative coupling of methane catalysts, indicate a high binding energy O(1s) peak at 532.6–533.2 eV, which is strong evidence for surface superoxide (O2−) or peroxycarbonate (CO4=).

Considerable disagreement often exists regarding the conversion and activity of many oxidative coupling catalysts. The wide discrepancy in results, obtained on a given catalyst, is essentially due to differences in temperature and space velocity as well as the use of inert gas diluents. We have thus used three dimensional graphics in order to locate the region of maximum C2+ yield as a function of catalyst bed hot spot temperature and linear space velocity. The conclusion of our study, involving pure La2O3, is that much of the research in this field is being carried out in the wrong region of catalyst activity.

Die Pd(II)-aoetate (I) reagieren mit tert.-Butylhydroperoxid (II) zu den tetrameren Titelkomplexen (III).

The influence of reaction temperature, space velocity and methane and oxygen partial pressures were studied for the oxidative coupling of methane reaction (OCM) over a lithium doped lanthanum oxide catalyst. The kinetic data obtained with this catalyst, at temperatures between 650 and 750°C, indicate that oxygen is adsorbed non-dissociatively and non-competitively. Product selectivities extrapolated to zero percent methane conversion are similar to those obtained with Li/MgO, suggesting that both rare earth and alkaline earth based catalysts involve similar mechanisms. Carbon monoxide and ethylene were found to be secondary products exclusively.

In this progress report an attempt is made to rationalize, from a mechanistic point of view, the different ways in which oxygen is transferred from inorganic and organic peroxides to nucleophilic substrates, particularly olefins. Oxygen transfer from transition-metal peroxides, which is relevant to catalytic oxidations using O2, H2O2 or ROOH, occurs via a cyclic or “pseudocyclic” peroxymetalation in which a dioxametallacycle is formed. Owing to the wide discrepancy between peroxymetalation and the conventional oxidation mechanism, i.e. nucleophilic attack of the substrate at the electrophilic “active oxygen”, we propose an alternative mechanism involving dioxiranes as the reactive species. The generation of dioxiranes appears to be a common denominator in the reactions of most organic peroxides e.g. peroxy acids, the reaction of electrophilic ketones with H2O2, or ozonizations. Oxygen transfer from dioxirane reagents probably involves the formation of a charge-transfer π-complex between the substrate and the carbon atom of the dioxirane, and the subsequent formation of a cyclic peroxidic intermediate.

When mixtures of methane, ethane and oxygen are cofed over oxidative coupling of methane catalysts, oxygen is preferentially consumed for the oxidative dehydrogenation of ethane, resulting in low m...

The role of d o metal peroxides as intermediates in catalytic oxidations involving H 2 O 2 and ROOH as the oxygen source is studied through the stoichiometric oxidizing properties of the isolated d o metal peroxidic complexes. Whereas heterolytic epoxidation of olefins requires the coordination of olefins as a prerequisite for a cyclic or a pseudocyclic peroxymetalation mechanism, homolytic oxidation of hydrocarbons involves a bimolecular mechanism with radical intermediates, and is less selective. A complete elucidation of the mechanism of the Halcon epoxidation process has been made possible by studying the reactivity of the complexes VO(OPhsal)(OOtBu) towards olefins.

An attempt is made to correlate the catalytic activity of d0 metal complexes in the oxidation of hydrocarbons by H2O2 and ROOH with the oxidizing properties of their peroxidic derivatives. The triangular structure of bound peroxo and alkylperoxidic groups is a common characteristic of d0 metal peroxides. The heterolytic or homolytic nature of the catalytic oxidation is governed by the heterolytic or homolytic cleavage of the triangular peroxidic moiety. The mechanism of selective heterolytic epoxidation involves the peroxymetalation of the coordinated olefin. Homolytic oxidations are less selective and involve the non-stereospecific epoxidation of olefins and the hydroxylation of alkanes and arenes by biradical peroxidic species.