Li/MgO is one of the most frequently investigated catalysts for the oxidative coupling of methane. Besides catalytic testing, it is also a suitable system to perform surface science experiments and quantum chemical calculations, which is not possible for many other active catalysts. However, the real structure of Li/MgO, the nature of the active center and the structure - activity relationship remain unclear, despite all the research that has been done. The aim of this review is to summarize the available knowledge on Li/MgO to structure and accelerate and improve the ongoing work on this catalytic system.

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A Critical Assessment of Li/MgO-Based Catalysts for the Oxidative Coupling of Methane

MgO crystals intentionally doped with Al were characterized by analytical electron microscopic examinations and positron lifetime measurements. Large spinel (MgO Al2O3) precipitates were observed in samples with high contents of Al. A well‐defined crystallographic relationship between the precipitates and the matrix was found. The characteristics of positron lifetime spectra appear to depend on the valence state of the different impurities in the MgO lattice suggesting that positrons are trapped by vacancy impurity complexes.

Analytical electron microscopic studies and positron lifetime measurements in Al‐doped MgO crystals

We apply quantum chemical methods to the study of active site structures and reaction mechanisms in mesoporous silica and metal oxide catalysts. Our approach is based on the use of both molecular cluster and embedded cluster (QM/MM) techniques, where the active site and molecular complex are described using density functional theory (DFT) and the embedding matrix simulated by shell model potentials. We consider three case studies: alkene epoxidation over the microporous TS-1 catalyst; methanol synthesis on ZnO and Cu/ZnO and C–H bond activation over Li-doped MgO.

Computational approaches to the determination of active site structures and reaction mechanisms in heterogeneous catalysts

The relationship between catalyst morphology and performance in the oxidative coupling of methane

Publisher Summary This chapter discusses the formation, detection, and role in catalysis of surface-generated gas-phase radicals. The presence of surface-generated gas-phase radicals is well established for many catalytic decomposition and oxidation reactions. The radicals generally are observed at reaction temperatures in excess of 400° C, and in some cases temperatures of approximately 800°C are required. The surface-generated radicals may be significant as (1) initiators of chain reactions, (2) intermediates in gas-phase coupling reactions, (3) a means for the transport of intermediates from one surface to another, (4) experimental evidence for a radical mechanism, and ( 5 ) a means of determining the rate limiting step of a catalytic cycle. Even qualitative results are useful in several of these applications; however, more quantitative data are needed to establish whether gas-phase radical coupling is indeed a primary pathway for product formation. The practical utilization of this source of gas-phase radicals, in many cases, require the design of reactors and catalyst-bed configurations so as to maximize the desired reactions. It may, for example, be important to operate at short contact times, with the catalyst at much higher temperatures than the gas phase, or, in order to achieve high selectivity for the partial oxidation of alkanes, it may be necessary to operate in a pulse mode so that the alkyl radicals do not react with gas-phase molecular oxygen.

Surface-Generated Gas-Phase Radicals: Formation, Detection, and Role in Catalysis

Transmission electron microscope investigations of as-grown Li-doped MgO have revealed the presence of precipitates with an average size of 220 A. Detailed micro-diffraction studies and moire pattern analyses have identified these precipitates as Li2O with the cubic fluorite structure. Exposures of Li-doped MgO crystals either to high temperatures in air for 10 min followed by quenching or to a large dose of ionizing radiation at ambient temperature leads to a reduction in the precipitate size. These results lend support to our previous conjecture from optical and magnetic resonance studies that localized lithium-rich environments are responsible for the stable [Li]0 centres and that the source of these lithium ions are the precipitates.

H. T. Tohver

Papers

Transmission electron microscope studies on Li-doped MgO