Active catalyst, defined

Poisoning is operationally defined. Often catalysts beheved to be permanently poisoned can be regenerated (5) (see Catalysts, regeneration). A species may be a poison ia some reactions, but not ia others, depending on its adsorption strength relative to that of other species competing for catalytic sites (24), and the temperature of the system. Catalysis poisons have been classified according to chemical species, types of reactions poisoned, and selectivity for active catalyst sites (24). 

Homogeneous catalysis by transition metal clusters has been reviewed from the perspective of the specific transformations.Examples of very mixed-metal clusters catalyzing processes homogeneously are collected in Table IX. As is generally the case with homogeneous catalysis, the catalytic precursor is well defined, but the nature of the active catalyst is unclear. 

The catalyst activity is defined as the ratio of the reacted CO divided by input CO, and the catalyst selectivity towards CO in the H2-rich stream is defined as the ratio of the O2 consumed for CO oxidation over total O2 consumed for both H2 and CO oxidation. 

Along with these well-defined complexes, other protocols have been developed to directly involve imidazolinm salts with Pd sonrces and form the active catalysts in situ. One of the most popnlar consists of the nse of carbene precursors such as IMes HCl or IPr HCl with PdCOAc) or PdCdba) and a base [40]. A mixture of SIPr HCl and PdCOAc) in a 1 1 ratio was nsed for the synthesis of resveratrol analogues (MOM protected MOM = methoxymethylether) through decarbonylative Mizoroki-Heck coupling [41] (Scheme 6.9). 

The size of the cataly.st particle influences the observed rate of reaction the smaller the particle, the less time required for the reactants to move to the active catalyst sites and for the products to diffuse out of the particle. Furthermore, with relatively fast reactions in large particles the reactants may never reach the interior of the particle, thus decreasing the catalyst utilization. Catalyst utilization is expressed as the internal effectiveness factor //,. This factor is defined as follows ... 

The structure of the active catalyst and the mechanism of catalysis have not been completely defined. Several solid state complexes of BINOL and Ti(0-/-Pr)4 have been characterized by X-ray crystallography.158 Figure 2.4 shows the structures of complexes having the composition (BIN0Late)Ti2(0-/-Pr)6 and (BINOLate)Ti3(O-/-Pr)10. 

Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

The catalyst reported by Drent [48] was generated in situ by mixing a palladium source with the ligand. A palladium source is broadly defined as a complex or any form of palladium metal whereby upon mixing with the ligand an active catalyst is formed. Many palladium sources are possible, but the sources exemplified by Drent aretris(dibenzylideneacetone)dipalladium(0)(Pd2(dba)3),bis(dibenzylideneacetone) palladium(O) (Pd(dba)2), or palladium(II) acetate. 

Whether or not such electrophilic organometallic species can be identified, or indeed isolated, depends primarily on the stability of the counteranion. The per-fluorophenyl boron compounds B(C,sF5)3 and [B(C6F5)4] , first prepared by Stone and co-workers in 1963 [33], proved particularly useful in this respect. Their use in metallocene polymerisation catalysis [34, 35] led to significantly more active catalysts and well-defined catalyst systems that proved mechanistically informative. These results have then enabled similar species to be detected in the more complex MAO-activated catalyst systems (vide infra). 

Gas adsorption is the most commonly used method for characterizing the surface area of catalysts. Both physical adsorption and chemisorption may be used. Furthermore, EM can provide supplementary information. A large surface area is desirable since activity is defined as the rate per unit active surface area ((per metre) ), and this necessitates porous catalysts. Eor an idealized porous system. 

Dynamic atomic-resolution ETEM and diffraction studies provide fundamental insights into the catalyst precursor transformation mechanism. The studies reveal that the temperature regimes are critical to the formation of active catalysts. They show that the nature of the VHPO -> VPO transformation is topotac-tic. Topotaxy is defined as the conversion of a single crystal to a pseudomorph... 

Figure 9 shows more completely the relationship between activity and calcining temperature. Here activity is defined as the inverse of the time needed to make SOOOg of polymer per gram of catalyst. Activity increases with increasing calcining temperature up to a maximum at around 925°C, and then declines as sintering destroys the surface area and porosity of the silica base. Krauss has shown that the coordinative unsaturation of Cr(II)/ silica follows a similar trend (36). 

Chabanas, M., Baudouin, A., Coperet, C. and Basset, J.-M. (2001) A highly active well-defined rhenium heterogeneous catalyst for olefin metathesis prepared via surface organometallic chemistry. J. Am. Chem. Soc., 123, 2062. 

Figures 3 and 4 show the weight percent 850°F+ conversion as a function of the relative space velocity for PDU Run 2LCF-16 (SCT feedstock) and PDU Run 2LCF-17 (SRC-I feedstock), respectively. The 780°F check points used to measure catalyst activity are defined by the symbols C and CL, where the subscripts B and E refer to the beginning ana end ot the run. It was concluded that higher pressure was significantly contributing to a reduction in catalyst deactivation and both PDU Runs 2LCF-16 and 2LCF-17 were assumed to have a neglible catalyst deactivation.

Only a few years ago it appeared that only one case of homogeneous hydrogenation catalysis was known—the cuprous-acetate-in-quinoline system. The uniqueness of this system appeared to define a very special set of chemical and physical circumstances. However, recent searching for this type of catalyst has disclosed a number of active catalysts. It appears possible that many more will be found in the future and that the chemical reactivity of hydrogen at low temperatures has not been fully appreciated. As new and old systems become better defined, there is high hope that this scientific approach to hydrogenation catalysis will continue to provide critical information of theoretical and, ultimately, of great practical importance. 

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