Catalysts high activity sites

In all the examples of exodendrally functionalized enantioselective den-drimer catalysts, the active sites in the periphery of the support were well-defined immobilized molecular catalysts. An alternative is provided by the possibility of attaching chiral multi-functional molecules to the end groups of dendrimers which, due to their high local concentrations, may interact more or less strongly with an achiral reagent and thus induce enantioselectivity in a transformation of a prochiral substrate. Asymmetric induction thus occurs by way of a chiral functionalized microenvironment for a given reaction. 

Although there is dispute about the exact oxidation state of titanium in the active species [Ti(III) or Ti(IV)], it was suggested, from the results of ESR measurements, that Ti(III) species form highly active sites for producing syndiotactic polystyrene in styrene polymerisation systems with the TiBz4—[Al(Me)0]x catalyst [50]. The moderately low catalyst activity is attributable to the stability of the benzyl transition metal derivatives towards reduction. 

Catalytic superactivity of electron-deficient Pd for neopentane conversion was recently verified for Pd/NaHY (157, 170). The reaction rate was positively correlated with the proton content of the catalyst. Samples that contained all the protons generated during H2 reduction of the catalysts were two orders of magnitude more active than silica-supported Pd. Samples prepared by reduction of Pd(NH3)2+NaY displayed on intermediate activity. It was suggested that Pd-proton adducts are highly active sites in neopentane conversion. With methylcyclopentane as a catalytic probe, all Pd/NaY samples deactivated rapidly and coke was deposited. Two types of coke were found (by temperature-programmed oxidation), one of... 

It is well known that the catalytic activity and selectivity of a metal for a certain reaction may be substantially altered by the presence of a second metal in the system (25). In the present work, although Pt has been the metal of primary interest, two experiments were done with the bimetallic Pt-Ru/Al-A system in order to get an idea about the effect of a second metal on the activity and selectivity of Pt. From Table 3 it is observed that an equimolar amount of Ru, when combined with 1.5% Pt, does not affect the activity and selectivity of the Pt-alone catalyst. When the amount of Ru is doubled (see item 2 in Table 3), however, the activity increases by a factor of about 5, while the selectivity for various products remains virtually unchanged. This dramatic increase in activity is probably due to the formation of new, highly active sites at the bimetallic interface. The selectivity patterns of the two bimetallic catalysts are very similar to that of the Pt-alone catalyst. This implies that the new sites on the bimetallic and those on Pt-alone catalysts are of same nature as far as the selectivity is concerned. 

Reaction control throngh complexation of substrate by supramolecular host is a relatively new idea compared to the conventional approaches that involve simple coUisional attack or coordination of snbstrate to metal. Multiple non-covalent interactions in supramolecular assembly bind and locate a site-specific substrate in the right position, orientation and conformation near the catalyst or active site, stabilize the high-energy transition state, and eventually make the reaction faster and more selective. Typical examples are found in enzymatic reactions, which proceed with high specificity and efficiency in aqueous solutions under mild conditions. These observations in natural systems have inspired... 

What we have seen is, that enzymes are highly specific catalysts in biological systems. Enzymes are catalytic proteins, they represent the most efficient class of catalysts. Their active site can, for example, be a carboxylic or an amino group, embedded in a specific geometry. Several weak interactions (electrostatic, H-bonds, van der Waals) help in establishing the highly specific manner in which a substrate mulecule binds to the active site. 

The system shows high activity, but differs frran Ti- and V-based catalysts as they produce HDPE with broad MWD. The catalyst contains active sites with different geometries and activities... 

Thus, to summarize the results obtained by Mossbauer spectroscopy, it is clear from the similarity between the changes in ORR activity of these Fe/N/C catalysts illustrated in Fig. 10.21a and the changes in the amount of N-FeN2 + 2- NH C highly active sites illustrated in Fig. 10.21d in the same catalysts that Fe-Nx catalytic sites are indeed active toward ORR in these Fe/N/C catalysts. 

Currently, this process is employed to produce propylene (lower than 2% of world production), butenes, butadiene, isobutene, and styrene, operating at high temperatures (550-600°C). Nevertheless, this reaction carries the drawback of being an endothermic process (positive formation enthalpy, AG, of 132,119, and 116 kJ/mol, for ethane, propane, and -butane, respectively). Moreover, undesired reactions occur such as cracking of both paraffins and olefins and especially coke formation which blocks the catalyst s active sites, causing deactivation. 

Nanocomposite oxide, W03-Zr02, as a heterogeneous catalyst with highly active sites was prepared for the synthesis of variety of coumarins under solvent-free systems combined with MWI. The presence ofW03 in the Zr02 matrix is understood to alter the physicochemical characteristics of Zr02 (Scheme 27) (08MI234).This is another example of dually modified catalyzed Pechmann reaction. 

Of the noble metals, palladium exhibits the highest activity for CO oxidation.As already mentioned for Pd-promoted CeCoO catalysts, the reaction over a noble metal supported on ceria involves a cooperative effect between the metals and the oxide, the so-called dual site mechanism. A recent study by temporal analysis of products (TAP) experiments for Pt supported over ceria showed two independent sites with different activities. The high activity site was associated with the metal/support interface and the low activity one was located on the support. The different sites were characterized by two different activation energies. Moreover, at variance with the stable number of high activity sites, the number of the low active sites increased with reaction temperature. 

The Anderson group pursued the research and found out that at 400°C propylene was readily converted to aromatic products, while ethylene generally exhibited low reactivity [132], However, it was observed that certain catalyst preparation showed considerable activity in promoting ethylene conversion [132], It was speculated that these particular catalysts may possess a few highly active sites, which are easily poisoned by coke formation. 

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