Metal oxides, catalysts Metals, transition, substrates

The interaction and sorption of metal ions with metal oxide and clay surfaces has occupied the attention of chemists, soil scientists, and geochemists for decades (1-4). Transition metal oxides receiving particular emphasis have included various oxides of manganese and iron (5). Interest in sorption phenomena is promoted by the desire to better understand incorporation of metals into minerals, especially marine deposits ( ), the removal of trace metal pollutants and radionuclides from rivers and streams, via sorption and/or precipitation phenomena (1,6), and the deposition of metals on solid substrates in the preparation of catalysts (7,8). 

Very recently, Hu et al. claimed to have discovered a convenient procedure for the aerobic oxidation of primary and secondary alcohols utilizing a TEMPO based catalyst system free of any transition metal co-catalyst (21). These authors employed a mixture of TEMPO (1 mol%), sodium nitrite (4-8 mol%) and bromine (4 mol%) as an active catalyst system. The oxidation took place at temperatures between 80-100 °C and at air pressure of 4 bars. However, this process was only successful with activated alcohols. With benzyl alcohol, quantitative conversion to benzaldehyde was achieved after a 1-2 hour reaction. With non-activated aliphatic alcohols (such as 1-octanol) or cyclic alcohols (cyclohexanol), the air pressure needed to be raised to 9 bar and a 4-5 hour of reaction was necessary to reach complete conversion. Unfortunately, this new oxidation procedure also depends on the use of dichloromethane as a solvent. In addition, the elemental bromine used as a cocatalyst is rather difficult to handle on a technical scale because of its high vapor pressure, toxicity and severe corrosion problems. Other disadvantages of this system are the rather low substrate concentration in the solvent and the observed formation of bromination by-products. 

The four remaining papers all deal with the catalysis of liquid-phase oxidation processes by transition metal ions (6). A. T. Betts and N. Uri show in particular how metal complexes can either catalyze or inhibit oxidation according to their concentration. In this investigation, various hydrocarbons (especially 2,6,10,14-tetramethylpentadecane) were used as substrates, and metal ions were present either as salicylaldimine or di-isopropylsalicylate chelates. These compounds are considerably soluble in non-polar media, and this makes it possible to examine their effect over a much wider range of concentration than is usually accessible in this type of work. These studies show that catalyst-inhibitor conversion is always... 

The first successful catalytic animation of an olefin by transition-metal-catalysed N—H activation was reported for an Ir(I) catalyst and the substrates aniline and norbornene 365498. The reaction involves initial N—FI oxidative addition and olefin insertion 365 - 366, followed by C—FI reductive elimination, yielding the animation product 367. Labelling studies indicated an overall. vyw-addition of N—FI across the exo-face of the norbornene double bond498. In a related study, the animation of non-activated olefins was catalysed by lithium amides and rhodium complexes499. The results suggest different mechanisms, probably with /5-arninoethyl-metal species as intermediates. 

For efficient regeneration, the catalyst should form only labile intermediates with the substrate. This concept can be realized using transition metal complexes because metal-ligand bonds are generally weaker than covalent bonds. The transition metals often exist in different oxidation states with only moderate differences in their oxidation potentials, thus offering the possibility of switching reversibly between the different oxidation states by redox reactions. 

Selective oxidation of aromatic amines to nitroso-derivatives with hydrogen peroxide and a catalyst is also studied [68, 69], In kinetic systems with transition metal complexes substrate oxidation is accompanied by H202 dissociation to H20 and 02. Therefore, in this case, the occurrence of chemical induction would be expected. 

Transition metal nanoparticles supported on different substrates are used as catalysts for different reactions, such as hydrogenations and enantioselective-synthesis of organic compounds, oxidations and epoxidations, reduction, and decomposition [24,25], Among the supports that have been applied in the preparation of supported transition metal nanoparticles are active carbon, silica, titanium dioxide, and alumina. 

High volatility limits the selection of many otherwise useful materials. Use of silica, all alkali, and alkaline earth metals higher than magnesium as components in any substrate or supporting material cannot be recommended. Catalysts with the least amount of volatility include palladium and iron oxides. It may also be possible to form complex oxides with other transition metal oxides, such as Mn, Ni or Co, which could have an acceptable activity and stability. For example, the formation of transition metal aluminates could lower volatility and increase sintering resistance, but at the expense of decreased activity. 

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