Active sites alkenes

One example has used a manganese porphyrin and iodobenzene encapsulated within a dendrimer to bring about shape-selective epox-idation of alkenes. The important aspect of catalysts is that the reactants can move rapidly to the active site, and that the products can be removed rapidly from the active site and expelled from the dendrimer. 

The fluorination of CF3CH2CI into CF3CH2F over chromium oxides is accompanied by a dehydrofluorination reaction (formation mainly of CF2=CHC1). This dehydrofluorination is responsible for the deactivation of the catalyst. A study of the dehydrofluorination reaction of CF3CH2CI proves that the reaction is favoured when the degree of fluorination of chromium oxide increases. Consequently it would be favoured on strong acid sites. Adding nickel to chromium oxide decreases the formation of alkenes and increases the selectivity for fluorination while the total activity decreases. Two kinds of active sites would be present at the catalyst surface. The one would be active for both the reactions of dehydrofluorination and of fluorination, the other only for the fluorination reaction. 

Although the complete mechanism for each of the previously described reactions is not known, substantial details have been worked out. First, it is clear that Ti is incorporated into the framework of the silicalite structure. Too much Ti (more than about 2.5%) in the preparation steps forms nonframework TiOz crystallites, which decompose H202. Second, the rate enhancement due to methanol suggests a tight association at the Ti active site as shown in Fig. 6.8.37,38 This is supported by the fact that methanol oxidizes much more slowly than other alcohols.47 This tight coordination of methanol is proposed to increase the electrophilicity of the Ti-coordinated H202 and facilitate oxygen transfer to the alkene.31... 

Alkalization of iron catalysts causes two different effects. The selectivities of 1-alkenes are raised and both the growth probability a2 and the fraction f2 are markedly increased, as already shown in Figure 11.2. Detailed studies on the promoter effect of alkali have revealed the effect on 1-alkene selectivity to saturates at 1 mass% of K2C03, while the effect on f2 already begins at 0.2 mass% of K2C03.1213 This difference points to specific active sites in Fischer-Tropsch syn-... 

Epoxidation of alkeneic reactants is faster on titanium-grafted silicates (such as A, B and C) than on the coprecipitated titanosilicates (such as D and E). This difference was attributed to the fact that on extra-framework titanium-grafted silicates, the catalytically active sites are virtually all exposed and accessible, whereas on the coprecipitated material some of them may be buried within the silicate walls and, thus, cannot adsorb reactant molecules. 

If the tetra- and tripodal Ti structures and the titanium oxo species derived from these structures in the presence of ROOH (R = H, alkyl) are involved as active sites and reaction intermediates, the next step beyond their identification is to seek correlations between the structure and concentrations of these titanium oxo species and catalytic activity and selectivity. Clerici and Ingallina (204) were the first to propose the Ti(02H) group as the active site of alkene epoxidation by... 

Landis and coworkers [140] have developed an active-site counting method based on H-labelling, for the metallocene-catalyzed alkene polymerization. After quenching the reaction by addition of methanol, the polymer is analyzed by NMR, which allows the quantification of Zr-alkyl sites. A typical NMR of quenched polymer is shown in Scheme 1.7 (label is found at terminal positions only). This technique has been applied to the polymerization of 1-hexene catalyzed by [Zr(rac-C2H4(l-indenyl)2)Me][MeB(QF5)3], 91. As shown in Scheme 1.7, there are two possible approaches ... 

A Stern-Volmer plot obtained in the presence of donors for the stilbene isomerization has both curved and linear components. Two minimal mechanistic schemes were proposed to explain this unforeseen complexity they differ as to whether the adsorption of the quencher on the surface competes with that of the reactant or whether each species has a preferred site and is adsorbed independently. In either mechanism, quenching of a surface adsorbed radical cation by a quencher in solution is required In an analogous study on ZnS with simple alkenes, high turnover numbers were observed at active sites where trapped holes derived from surface states (sulfur radicals from zinc vacancies or interstitial sulfur) play a decisive role... 

The major problem of the application of zeolites in alkane-alkene alkylation is their rapid deactivation by carbonaceous deposits. These either strongly adsorb on acidic sites or block the pores preventing the access of the reactants to the active sites. A further problem is that in addition to activity loss, the selectivity of the zeolite-catalyzed alkylation also decreases severely. Specifically, alkene formation through oligomerization becomes the dominant reaction. This is explained by decreasing ability of the aging catalyst to promote intermolecular hydride transfer. These are the main reasons why the developments of several commercial processes reached only the pilot plant stage.356 New observations with Y zeolites reconfirm the problems found in earlier studies.358,359... 

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