Active site atomic arrangement, selectivity

This paper is a summary of our current understanding of this system. In particular, we will be discussing the observations in terms of selectivity with respect to the availability of reactive lattice oxygen. The organization of the paper is as follows. First, the general features of the reaction scheme for alkane oxidation on vanadate catalysts will be presented. This is followed by a discussion of results on the effect of ease of removal of oxygen from the lattice on the selectivity, and then a discussion on the importance of the atomic arrangement of the active sites. 

Comparison of the selectivity toward alcohols in the FT reaction of W03, WC, W2C and the XPS and CO adsorption results provide information on the specificity of active sites on each catalyst. W2C favors CO dissociation and formation of hydrocarbons with excess methane and C02 without oxygenate production. It has a more pronounced metallic character than WC perhaps because its metal atoms are in a close-packed arrangement. After pretreatment in flowing H2 at 673 K before FT reaction, part of the surface oxygens (abundant after passivation) probably reacts with carbidic carbon and are removed from the uppermost surface layer. Moreover,... 

The oxygen atoms must be distributed on the surface of a selective oxidation catalyst in an arrangement that provides for limitation of the number of active oxygen atoms in various isolated groups (site isolation). 

The prochiral face selectivity of the catalytic sites is due to the unique spatial arrangement of the stereorigid Me2C(Cp)(Flu) ligand encompassing the Zr atom and its active coordination positions [Figure 3.42 (R = H)]. 

Figure 3 Structural alignments with discrete properties. Methods are based on discrete properties using the DG algorithm (1) or clique-detection (11) as implemented in distance comparisons (DISCO), and Apex-3D. The structure representation, based on discrete properties, resorts to one atomic descriptor (I), usually the atom type, or multiple atomic or site descriptors (II). In the first method (I), the conformational analysis is restricted to the generation of molecular geometries which allow a common arrangement of selected phaimacophoric moieties present in a rigid compound used as template. In the second method (II), the conformational analysis procedure may involve a systematic enumeration of all the possible conformadons for each ligand. The search similarity is directed towards the confirmation of a predefined pharmacophore postulated by the modeler or from some classical SAR in the case of the active analog approach (1), or the automated identification of pharmacophores and bioacdve conformations (II)...

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