Single-site active centers

When combined with the isolation and reactivity studies of the patterned aminosilica (7), the increased activity of the patterned catalysts provide further evidence that the patterning technique developed allows for the synthesis of aminosilicas which behave like isolated, single-site materials (although a true single site nature has not been proven). As the olefin polymerization catalysts supported by the patterned materials show a marked improvement over those materials supported on traditional aminosilicas, these patterned materials should be able to improve supported small molecular catalysis as well. Future improvements in catalysis with immobilized molecular active sites could be realized if this methodology is adopted to prepare new catalysts with isolated, well-defined, single-site active centers. 

Highly dispersed surface species, with the limiting form of single-site active centers, play a primary role in a number of catalytic materials because of their peculiar features in terms of activity and selectivity. Both oxo-species and transition metal ions supported on oxides (or in zeotype materials) belonging to these types... 

By single-site catalysts we mean catalysts where the breaking and formation of chemical bonds occurs at isolated active centers whose chemical activity is dominated by the electronic properties of a single atomic species or of a small cluster of atoms that can act in an independent way with respect to others. 

Homogeneous catalysts are very often known as examples of single-site catalysts characterized by complete structural definition and (presumably) complete knowledge of the chemical processes occurring at their catalytic centers. It is a matter of fact that the homogeneous catalysts are molecular complexes constituted by an active core containing a single active atom (of-... 

True examples of single-site catalysts are enzymes, where active sites are made mainly by metalhc centers (mono- or polynuclear species) whose coordination sphere is completely defined by ligands [1-4]. The strength of enzymes is the combined effect of metal center activity with the specific behavior of metal coordination sphere hgands. These species play a key role, being optimized to create an environment suitable for (i) metal centers approaching and coordinating by reactants (ii) product removal from the catalytic centers at the end of the reaction in order to avoid further reactions. 

The general formula LnMR of a single site catalyst, where L is a ligand set, M the active metal center and R a group that may initiate polymerization can also... 

The mixture of metallocene and co-catalyst is soluble. Its active center, which is chiral, induces with a very low rate of defects only one type of monomer linkage ( single site catalysts )- That is why high activities (some 1,000 kg polymer/g... 

Four of the chlorines are in the crystal lattice and bridge-bonded with neighboring titaniums, while the fifth single-bonded chlorine (Cl) protrudes from the crystal surface. The active center, which is a hexacoordinated transition metal with one vacant octahedral site, is created by replacing the single-bonded chlorine by alkylation with the alkylaluminum ... 

The function of the electron-mediating proteins which contain a single redox active site (e.g., rubredoxin, azurins, flavodoxins, plasto-cyanins) is mainly related to the first aspect. Still, the pronounced specificity encountered in their function in biological energy conversion processes indicates that their redox center, often a transition metal ion, is embedded in an evolutionarily optimized polypeptide envelope. The... 

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