Catalytic activity transition metal oxides, related

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

Heterogeneous catalysts which are active for the catalysis of the MPVO reactions include amorphous metal oxides and zeolites. Their activity is related to their surface basicity or Lewis acidity. Zeolites are only recently being developed as catalysts in the MPVO reactions. Their potential is related to the possibility of shape-selectivity as illustrated by an example showing absolute stereoselectivity as a result of restricted transition-state selectivity. In case of alkali or alkaline earth exchanged zeolites with a high aluminium content (X-type) the catalytic activity is most likely related to basic properties. For zeolite BEA (Si/Al=12), however, the dynamic character of those aluminium atoms which are only partially connected to the framework appear to play a role in the catalytic activity. Similarly, the Lewis acid character of the titanium atoms in aluminium free [Ti]-BEA explains its activity in the MPVO reactions. 

In 1964 a short investigation was made of EPR signals of platinum on alumina by F. Nozaki, D. Stamires and Turkevich.(70) The relation of catalytic activity of transition metal oxides to their EPR properties was studied. Thus in 1967 Kazanski investigated the chromium oxide on silica and its ability to carry out low temperature polymerization of ethylene. 

The oxide catalysts are microporous or mesoporous materials or materials containing both types of pores. In the latter case, the applicability is larger in terms of the molecular size of the reactants. Acid-base properties of these materials depend on the covalent/ionic character of the metal-oxygen bonds. These sites are involved in several steps of the catalytic oxidation reactions. The acid sites participate with the cation redox properties in determining the selective/unselective catalyst behavior [30,31]. Thus, many studies agree that partial oxidation of organic compounds almost exclusively involves redox cycles and acid-base properties of transition metal oxides and some authors have attempted to relate these properties with activity or selectivity in oxidation reactions [31,42]. The presence of both Bronsted and Lewis acid sites was evidenced, for example, in the case of the metal-modified mesoporous sihcas [30,39,43]. For the bimetallic (V-Ti, Nb-Ti) ions-modified MCM-41 mesoporous silica, the incorporation of the second metal led to the increase of the Lewis sites population [44]. This increased concentration of the acid sites was well correlated with the increased conversion in oxidation of unsaturated molecules such as cyclohexene or styrene [26,44] and functionalized compounds such as alcohols [31,42] or phenols [45]. 

Kher, K. Oxidation-reduction potentials and their relation to the catalytic activity of transition metal oxides. J. Catal 1967, 8,14—21. 

Transition metal hydroperoxo species are well established as important intermediates in the oxidation of hydrocarbons (8,70,71). As they relate to the active oxygenating reagent in cytochrome P-450 monooxygenase, (porphyrin)M-OOR complexes have come under recent scmtiny because of their importance in the process of (poiphyrin)M=0 formation via 0-0 cleavage processes (72-74). In copper biochemistry, a hydroperoxo copper species has been hypothesized as an important intermediate in the catalytic reaction of the copper monooxygenase, dopamine P-hydroxylase (75,76). A Cu-OOH moiety has also been proposed to be involved in the disproportionation of superoxide mediated by the copper-zinc superoxide dismutase (77-78). Thus, model Cun-OOR complexes may be of... 

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. 

The catalytic activity of transitional aluminas (y-, T)-, 5-, 6-AI2O3) are undoubtedly mostly related to the Lewis acidity of a small number of low coordination surface aluminum ions, as weU as to the high ionicity of the surface Al—O bond [101]. Alumina s Lewis sites have been well characterized by adsorption of several probes. They are the strongest among metal oxides. The number of such very strong Lewis sites present on transitional alumina surfaces depend on the dehydroxylation degree (depending on the activation temperature) and on the particular phase and preparation. 

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