Whole catalyst

Because catalyst surfaces are reactive and often sensitive to their environments, they may be irreversibly changed by exposure to undesired reactants. Upsets in plant operations can lead to catastrophic losses of whole catalyst charges. A large catalyst charge that is mined can cost hundreds of thousands of dollars as well as the cost of lost operation. 

With our optimized conditions in hand, the reaction was performed in a 100 litter flask on several occasions with good success. Even though we had React-IR data (p. 63, Figure 2.3) for monitoring the catalyst formation, the React-IR data did not provide additional information on the active catalyst. Therefore, a portion of activated catalyst solution was tested prior to addition of a whole catalyst solution to the real batch, to ensure success of the reaction. At the end of the reaction, the crude mixture was passed through a short silica gel pad to remove Mo and the mixture was used for the next reaction without further purification. 

If nonvolatile liquids are to be used to avoid the problems associated with volatile organic solvents, then it is very desirable that there is some convenient way of recovering the reaction products from the liquid. This approach is used in the biphasic systems described in Chapters 2-5. In the fluorous biphase (Chapter 3), reagents and catalysts are fine-tuned by adding perfluoroalkyl chains, known as ponytails , to ensure that only those chemicals will mix with the fluorous layer. Purification is simply a matter of separating the two phases. Transition metal catalysts with fluorous ligands will remain in the fluorous phase, and the whole catalyst-solvent mixture may be reused for another batch of reactions, as shown schematically in Figure 1.20b. 

The necessity to have more than one component in a catalyst arises from many needs those linked to the polyfunctionality often required for the different steps in a reaction, the need to enhance the rate of some reaction steps, inhibition of unwanted side reactions, provision of adequate thermal stability, to take advantage of observed synergetic effects. From a fundamental point of view, the presence of several metal elements in a common structure permits the adjustment of the local electronic properties, imposes well defined coordinations, limits the extent of oxidation-reduction phenomena, and may stabilize the whole catalyst by retarding sintering. Mixed oxide catalysts are used as such, or as precursors of active catalysts, for a whole range of important industrial processes, a representative selection of which is given in Table 1. 

The sequence of events for the growth of these filaments, which can destroy the whole catalyst bed, is now well understood [53-56]. Electron micrographs of the catalyst particles show that they are detached from... 

The plug-flow reactor may be operated in the differential or the integral mode. In the differential mode (small conversion) the whole catalyst can be considered to be exposed to the same concentration of reactants. The influence of products is generally weak, except when the catalyst is extremely sensitive to one particular product.Thc plug-flow reactor operating in the dif-... 

Finally, Hinderling and Chen used ESMS to screen the activity of a small library of eight Brookhart-type palladium(II) complexes in the solution-phase polymerization of ethylene [62]. The crude reaction mixture was quenched with DMSO, diluted, and electrosprayed in order to analyze the growing polymers chains. Upon CID in the gas phase, the polymer chain was fragmented from the catalyst by /J-hydride elimination, thus facilitating the identification of the most active catalysts in an otherwise dauntingly complex mass spectrum of a polymer mixture. Since this analysis can be performed simultaneously for a whole catalyst library, ESMS was hereby proven the method of choice for an assay of multiple, competitive and simultaneously occurring catalytic reactions. 

The discussion will also be restricted to reactors in which the range of temperature is too wide to permit the use of an average temperature to characterize the whole catalyst bed. No sharp line can be drawn following this restriction, but it clearly includes any case in which the rate of reaction cannot be described satisfactorily as a linear function of temperature over the whole range covered by the reactor. The only way to find out in doubtful cases whether the variation of temperature is significant is to make the calculation taking the variation into account. 

Tronconi et al. [44] have included Eq. (9) in a complete model for DeNO reaction and SO2 oxidation, showing that, opposite to the former reaction, the latter one occurs in a chemical regime due to its very low rate Dedicated experiments have indicated in fact that SO2 oxidation involves the whole catalyst volume also, application of literature cntena [45] have confirmed the absence of external and intraporous gradients of SO2 concentration [18. ... 

Such a different conclusion can be understood by considering the difficulties connected to the experimental determination and the definition of Thiele modulus parameters, such as So and D. According to Chien, S means the catalyst primary particle size with a value of about 10 cm for a-TiCIj instead, in the Multigrain model, Sp seems to correspond to the size of the whole catalyst granule. 

Easy chemical and thermal manipulation of the active sites in a catalyst is desirable to carry out nearly uniform changes throughout the whole catalyst during catalyst synthesis and activation steps. This attribute is particularly important in the formulation of bifiinctional and multifunctional catalysts because ineffective interaction between relevant active sites is critical in multifunctional catalysis. The growth of multifunctional catalysis adds further emphasis to the need for uniform catalyst sites both in chemistry and in difihisivity. 

Physically this means that the whole catalyst surface is covered with A (0a -> 1, 0B 0). Therefore, varying the partial pressure of A does not influence the reaction rate. The reaction is said to be zero order in A (and B). The overall activation energy is 3° = 32 (see Fig. 3.6), provided the concentration of active sites Nt is temperature independent. 

In conclusion, bimetallic Pt-Sn/alumina catalysts prepared by successive impregnations with an intermediary reduction step and introduction of the tin salt (SnCU) under hydrogen are less sensitive to coke deactivation than catalysts prepared by coimpregnation. This behavior probably results from a more effective interaction between the two metals, leading to smaller platinum ensembles, as evidenced by the low hydrogenolysis activity. However, the amount of coke deposited on the whole catalyst depends on the nature of the feed and therefore on the nature of the dehydrogenated species which are more or less active precursors for coke deposition on the support. 

The very detrimental effects of contaminants such as Fe and Ca on the accessibility and performance of catalysts has been reported. Apparently, these contaminants can result in (liquid) eutectic melts on the surface of the catalyst particles, which can block their entrance pores and even cover the whole catalyst surface. 

Similarly to the zirconocene/MAO catalysts, the activity of the Idemitsu catalyst systems is poor when the whole catalyst system (catalyst-i-co-catalyst) is taken into account. In the very best example (in terms of catalyst activity and polymer yield) nickel bis (acetylacetonate) was used in combination with MAO (molar ratio 1 200) to homopolymerize norbornene in toluene at 50 °C for 4 h affording a 70% conversion into high molecular weight (M 2.2x10 ), toluene-soluble poly-(norbornene). The ratio of norbornene to nickel was 20000 1 and the ratio of norbornene to aluminum was only 100 1. Thus, while the yield of polymer is high based on the nickel catalyst (25 700 g per g nickel), the yield based on aluminum is very poor (260 g per g aluminum or about 120 g per g MAO). It can readily be estimated that the cost of the MAO activator alone would add significantly to the cost of the polymer, as well as requiring costly removal of catalyst residues from the polymer. 

Equation (11-92) is also applicable for the whole catalyst pellet when diffusion resistance is negligible. Furthermore, it gives the selectivity at any location within the pellet. The selectivity will vary with position in the pellet as Cg/C changes. Diffusion resistance causes to decrease going from the outer surface toward the center of the pellet. Since B is formed within the pellet and must diffuse outward in order to enter the bulk stream, Cg increases toward the pellet center. Equation (11-92) shows qualitatively that these variations in and Cg both act to reduce the global, or pellet, selectivity for B. 

For laboratory tests only part of a honeycomb (4, 9 or 36 channels) or powdered material is required. For bench-scale and pilot-scale testing a whole catalyst element will be needed. These testing methods are discussed in this section. 

The amount of coke deposited on the whole catalyst is nearly... 

The theory of complex reactions with additional balance equation also gives the possibility of computing the rate law of catalytic reactions with ionic intermediates where it is important to take into account participation of the bulk of the catalyst. In this case, from the viewpoint of the theory of complex reactions, it means that besides the balance equation which corresponds to the total electroneutrality of the surface species, there will be another one which describes electroneutrality of the whole catalyst. 

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