Optimum catalytic activity

By coprecipitating the catalytically active component and the support to give a mixture that is subsequently dried, calcined (heated in air), and reduced to yield a porous material with a high surface area. This procedure is followed when materials are cheap and obtaining the optimum catalytic activity per unit volume of catalyst is the main consideration. 

A freshly prepared catalyst may not exhibit optimum catalytic activity upon its first introduction into the reactant stream. There may be efficient but undesirable side reactions that need to be eliminated. For this purpose a small amount of poison is often added to the reaction mixture or introduced in the form of pretreatment. Thus deactivating impurities may also be used, in small quantities, to improve the selectivity of the working catalyst. 

For example, the dried silica gel can be treated with a precious metal organometallic solution which upon reduction leaves highly dispersed metal particles of controlled size for optimum catalytic activity. 

Alkylation of isobutane with 2-butene using various l-alkyl-3-methylimidazolium halide-aluminum chloride catalysts has been successfully demonstrated [53]. Among theses ionic liquids, [OMIM]Br-AlCl3 displayed the best performance in terms of activity and selectivity for this reaction. From various parametric studies, such as anion compositions and temperature, optimum catalytic activity was observed at 80 °C and X = 0.52. This reaction has also been used to study the Lewis acidity of ionic liquids [54]. An interesting alkylation of an aUcene can also be found in the cyclization of dodecene in a [BMIM]Q-AlCl3 (X = 0.67)/ethanol mixture at 6 MPa pressure (Scheme 5.2-14). The authors claim 27% yield and 93% selectivity for the formation of cyclododecane [55]. 

When the distribution of short sequences of acrylic acid in the copolymers is predominant, and the short sequence distribution is approximately equal to that of the long sequences for styrene, the complexes show optimum catalytic activity. This regularity was observed not only in the iron system, which produces polybutadiene with equibinary structure (cis-1,4 and 1,2 about 1 1), but also in the neodymium system producing high cis-1,4 polybutadiene. 

The oxidation of p-xylene to terephthalic acid occurs by the same general mechanism as described above for alkylaromatics. Bromide ions and nitrogen-containing ligands provide synergetic mixtures with metal salts [56-58,61]. Pyridine is one of the most efficient additives [59,60j. Oxo-centered cobalt(III) clusters have been isolated from the reaction mixtures, which can be regarded as active intermediates [34,62,63]. The optimum catalytic activity was found to be associated with the composition corresponding to Co Br py = 1 2 (1-2) [64]. 

Optimum catalytic activity was obtained with 3 p Ni pre-sonicated in ethanol for 1 h at 10°C using 34 W total power. Under these conditions 65% octane was obtained after 60 min hydrogenation at 25°C compared with no octane using untreated catalyst (Table 10.4). The increase in... 

However, the cyclic mechanism of Scheme 1 carries an inherent problem, namely the contradictory pH demands of two key steps in alkaline aqueous solutions. Step (a), the activation of coordinated CO by reaction with OH , would of course be favored by increasing the pH. However, doing so may well prove counterproductive, since step (c), the protonation of the relatively weak base My(CO)xiH, would be suppressed. Thus, the conflicting demands of two key steps in the cycle may limit the optimum catalytic activity. This has been nicely demonstrated in the catalytic system based on the simple iron carbonyl Fe(CO)s in alkaline solution (37). The somewhat greater success seen with certain related catalysts in aqueous amine solutions may be due to the role of general base/general acid catalysis of these steps by the amines and their conjugate acids (19). 

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