Catalyst features

Physisorption measurements showed that carbon nanomaterials exhibit rather meso- and macroporous structures (maximum micropore fraction, 15% see Table 2.1). The lowest specific surface area was measured with the platelet fiber catalyst exhibiting slightly more than 100 m2/g. The Co/HB material offers 120 m2/g of surface area, and the highest BET value was determined with the Co/ MW catalyst featuring nearly 290 m2/g. Carbon nanomaterials, though, are not really porous, as the space between the graphene layers is too small for nitrogen molecules to enter. The only location of adsorption is the external surface of the nanomaterials and the inner surface of the nanotubes. 

In the different volumes of this new series we will feature catalysts for oxidation and reduction reactions, hydrolysis protocols and catalytic systems for carbon-carbon bond formation inter alia. Many of the catalysts featured will be chiral, given the present day interest in the preparation of single-enantiomer fine chemicals. When appropriate, a catalyst type that is capable of a wide range of transformations will be featured. In these volumes the amount of practical data that is described will be proportionately less, and attention will be focused on the past uses of the system and its future potential. 

THE SELECTIVE OXIDATION OF ISOBUTANE CATALYZED BY POMs MAIN CATALYST FEATURES... 

The first class of amine-based nucleophilic catalysts to give acceptable levels of selectivity in the KR of aryl alkyl. yec-alcohols was a series of planar chiral pyrrole derivatives 13 and 14, initially disclosed by Fu in 1996 [25, 26]. Fu and co-workers had set out to develop a class of robust and tuneable catalysts that could be used for the acylative KR of various classes of. yec-alcohols. Planar-chiral azaferrocenes 13 and 14 seemed to meet their criteria. These catalysts feature of a reasonably nucleophilic nitrogen and constitute 18-electron metal complexes which are highly stable [54-58]. Moreover, by modifying the substitution pattern on the heteroaromatic ring, the steric demand and hence potentially the selectivity of these catalysts could be modulated. 

Since high current density at the maximum power density and the cost of the noble metals are important parameters for the commercialization of DMECs, H-CNE-supported Pt-Ru alloys maybe classified among the most efficient and cost-effective anode catalysts. It is also worth mentioning that the CNF-supported catalysts feature superior catalytic activity at the high temperatures where the mass transfer of methanol and oxygen is more favorable due to the fibrous network of CNEs. 

Further research has been performed and is continued to be reported, mostly with zeolites unloaded or loaded with Pt, and Ga- and Zn-promoted H-ZSM-5 or H-[Al]ZSM-5 catalysts to clarify the details of the complex transformations taking place and make further improvements. In addition, new catalysts were studied and reported. Reference should also be made to work addressing the problems of the modification of catalyst features of ZSM-5404 and the development of a new light naphtha aromatization process using a conventional fixed-bed unit.405 406... 

The [S]/[C] ratio is calculated on the basis of the molar mass of the catalyst. Typically, homogeneous chemical catalysts feature a molecular mass of about 1000 in comparison with that of an enzyme of 30-50 kDa. At equal turnover number (catalytic events per active site per unit time), such a view yields 30-50 times higher [S]/[C] ratios for homogeneous catalysts. As, however, the cost of a unit of activity or the ease of synthesis should be the guiding dimension and not the molar mass, such homogeneous catalysts are unduly favored over enzymes and antibodies. 

The Accelrys implementation of pharmacophore fingerprint descriptors is called 3D Keys. This application is based on standard Catalyst feature definitions and is a part of the Cerius2 software package [7]. 

GRID probe Catalyst feature GRID IEa) CiTest weight... 

Fig. 15.2 Percentage of the Thompson Derwent WDI database (2003) that maps common Catalyst features. Only entries with molecular weights between 100 and 600 were...

Synzymes may achieve rate enhancements via binding and proximity effects. Such effects can occur when two reactive partners are bound within the same nanospace, thus increasing their relative encounter frequencies (i.e., the effective concentrations) (Fig. 13.1a) [4]. Some synzymes participate as reagents, and these catalysts feature structurally distinct substrate-binding site(s), together with a cat-alytically effective site(s) (Fig. 13.1b). 

The wide range of desired functionalities on the surface of catalysts that includes, for example, strong acid sites for cracking reaction, mild acid-base pairs for the synthesis of fine chemicals, redox properties for oxidation reactions, or metal nanoparticles for hydrogenation or electrochemical applications seems to complicate the derivation of general rules with respect to catalyst synthesis. It is, however, accepted without controversy that good catalysts feature the following characteristics ... 

This new generation of catalysts demonstrated remarkable enantioselectivity at the level of the homogeneous catalysis57 as shown in the Abramson et al s contribution in this book. After removal of the soluble product, the catalyst featured the same activity and enantioselectivity without further regeneration. 

Conditions used in PE processes vary widely. Because the heat of polymerization for ethylene is quite high (variably reported to be between 22 and 26 kcal/mole), efficient heat removal is crucial for polyethylene processes. Selection of process must also accommodate catalyst features, such as its kinetic profile. Table 7.1... 

For the fiiture of zeolite catalysis, the important question is to what extent zeolites or their future derivatives display some of the desired catalyst characteristics envisioned for advanced catalytic processes. In addition, entirely new, unforeseen catalytic materials may emerge with catalytic properties that lead to step change. Following is a description of the relevance of zeolites to the desirable catalyst features already described ... 

As previously described, amphoteric catalysts featuring acidic-basic sites have appeared to be very effective. In this area, Buono s group has recently re-... 

As for the reduction of the ketones, the amphoteric catalysts featuring acidic-basic sites have been found to be very effective for the enantioselective catalysis of C-C bond formation. Thus, Soai et al. were the first to report the enantioselective addition of dialkylzincs to aldehydes using enantiomerically pure phosphin-amides and analogues as chiral catalysts in the presence of titanium tetraiso-propoxide. Numerous chiral organophosphorus compounds have been prepared and applied in a test reaction between benzaldehyde and diethylzinc [48, 49]. An important difference in terms of enantioselectivity was observed between the behavior of P=S (47-48) and P=0 (49) groups. Thus, the enan-... 

Fig. 4.2 Catalyst feature definition applied to a simple molecule, illustrating the inclusion of receptor site-points in the pharmacophore definition.

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