Oxygen dependence, catalytic character

The kinetic experiments were not performed under true catalytic conditions, i.e. the pre-prepared [FeL(DTBC)] complexes were introduced into the reaction mixtures as reactants and excess substrate was not used. Nevertheless, the results are important in exploring the intimate details of the activation mechanisms of the metal ion catalyzed autoxida-tion reactions of catechols. In excess oxygen the reaction was first-order in the complex concentration and the first-order dependence in dioxygen concentration was also confirmed with the BPG complex. As shown in Table II, the rate constants clearly correlate with the Lewis character of the complex, i.e. the rate of the oxidation reaction increases by increasing the Lewis acidity of the metal center. 

For instance, two series of alkali-metal ion-exchanged zeoUtes have been investigated in order to analyze the possible correlations between the acidity and basicity of the X and Y zeolite structures and their catalytic properties [137]. The catalytic results for the 4-methylpentan-2-ol conversion show that activity and selectivity are both affected to some extent by the acid-base character of the catalysts. The activity was found to increase from Cs to li for both X and Y zeoUtes. The dehydrogenation reaction occurred only on Cs-X+CS2O, which presents very strong basicity. The product selectivity of the reaction was found to depend on both Lewis acidity and basicity Lewis basic or acidic sites of zeoUtes can be considered as acid-base pairs, in which both basic framework oxygens and neighboring cations are important [ 137]. 

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]. 

The active site in coordination catalysts for olefin polymerization is, therefore, a transition metal surrounded by ligands. The catalytic properties depend on the fine tuning between the transition metal and the ligands in terms of geometry and electronic character. In most cases the active site is produced by the activation of a complex called a precatalyst precursor. The pre[Pg.373]

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