Catalyst performance relationships

Catalyst Performance Relationships. Hexadecane cracking activity of AFS and USY zeolites, when corrected for deactivation effects, shows little or no dependence on framework composition. Rather, as shown in Figure 6, activity appears to be a function of total aluminum content independent of the method of dealumination. This result implies that hexadecane cracking occurs over both framework and extraframework acid sites and that it is the total number of such sites which determines catalytic activity. Hence, extraframework material in the USY samples makes a significant contribution to catalyst activity as reported by others(18.19). 

Selectivity parameters can be used to compare the catalytic performance of the different catalysts, and to find relationships between catalysts performance and physico-chemical features. Specifically, the following parameters were chosen (a) the O/C-methylation ratio, that is the ratio between the selectivity to 3-MA and that to 2,3-DMP+2,5-DMP+3,4-DMP (b) the ortho/para-C-alkylation ratio, that is the ratio between the selectivity to 2,3-DMP+2,5-DMP and the selectivity to 3,4-DMP (c) the 2,5-DMP/2,3-DMP selectivity ratio. Table 2 compares these parameters for MgO, Mg/Al/O and Mg/Fe/O catalysts. Data were reported at 30% m-cresol conversion, thus under conditions of negligible consecutive reactions. In this way it is possible to compare the ratio of the sole parallel... 

In order to control and monitor the coke deposition during heavy oil processing a quantitative model is mandatory. Often the pragmatic approach of a direct relationship between process parameters and the catalyst performance with time is used. It has been argued, however, that the "performance with time model beneficially starts from a description of the coke deposited followed by a relation between coke content and performance [2). The latter approach will be followed in this paper. 

In the applications described so far, catalytic data were not acquired along with the spectroscopic data, or the cells were unsuitable for correct measurements of the former. The determination of the catalyst structure and performance in a single experiment is not only of interest for catalysts but for any functional material. For instance, rather similar developments as in the field of catalysis have been reported in the fields of gas sensors and electrochemical devices. Many techniques allow for the simultaneous characterization of electrochemical materials and their performance (Luo and Weaver, 2001 Novak et al., 2000). Conductance cells provide a powerful approach to understanding of the structure-performance relationships at the molecular scale (Loridant et al., 1995). 

Fuel cell performance is affected by MEA composition, including catalyst loading, PTFE content in the gas diffusion layer, and Nafion content in the catalyst layer and membrane, each of which affects the performance in different ways, yielding distinct characteristics in the electrochemical impedance spectra. Even different fabrication methods may influence a cell s performance and electrochemical impedance spectra. With the help of the model described above, impedance spectra can provide us with a useful tool to probe structure-performance relationships and thereby optimize MEA structure and fabrication methods. 

In the field of isospecific propylene polymerization, systematic structure-activity relationship studies of metalloeenes have shown that the combination of 2-alkyl and 4-aryl substitution is cmcial for a technically suitable catalyst performance (high catalytic activity, excellent stereoselectivity, high melting point of the polymer, certain copolymer properties, etc.) [7, 8]. Consequently, there is a consider-... 

For catalyst design purposes it is first necessary to translate the catalyst performance parameters into a physical picture of catalyst structure. As we shall see, different performance parameters can give rise to different structural features and so a compromise is generally required. For example it is commonly found in industrial applications that initial catalyst activity may be sacrificed in favour of improved catalyst stability, since a lower activity and a prolonged operating catalyst life is in general preferable to a higher initial activity that decays rapidly. First, we should therefore discuss some of the relationships between the catalyst performance parameters and physical structure. 

Catalyst characterization tests include measurement of surface areas, chemisorption, pore-size distributions, crystal structure as determined by X-ray crystallography, reaction mechanisms as revealed by kinetics, and isotopic tracers and diagnostic catalytic reactions to test functional capabilities. These have been interpreted in terms of variation of catalyst preparation-structure-performance relationships. 

What has not been dealt with at nearly the same level of intensity is the relationship between adsorption and catalysis, and in particular between the energy terms in the parameters of the rate expression and catalyst performance. There are good reasons for this neglect, not the least being the lack of a clear and measurable relationship between adsorption and the rate of reaction - the activity - on a given catalyst. This is the problem that mechanistic rate expressions are well suited to illuminate by simulation, although the direct measurement of adsorption isotherms pertinent to the active species on the catalyst surface is not yet possible. At some future time the use of a TS-SSR (see Chapter 5) may give us a better appreciation of the adsorption properties of catalysts. 

Effective Area. - The catalyst consists of active metal dispersed on a substrate material. Therefore, there is a difference between the effective area used in mass transfer calculations of chemical reaction and the effective area used in heat transfer culculations which corresponds to the monolith surface area including surface roughness. The ratio of both effective areas must be defined based on experimental results, as they depend on catalyst type and manufacturing processes. The Thiele number is sometimes used for this same purpose. The relationship between effective area ratio and conversion efficiency is shown in Figure 3. This effective area ratio may be one of the characteristic values of the catalyst, which affects catalyst performance and catalyst temperature. The effective area ratio in the present study is estimated to be 0.3 for mass transfer and 1 for heat transfer based on the experimental data. 

The issue begins with an article by Ittel, Johnson, and Brookhart on late metal catalysts for ethylene homo- and copolymerization. They detail the newest generation of catalysts to be commercially licensed. Alt and Koppl then introduce ethylene and propylene polymerization by metallocene catalysts. Structure-performance relationships for unbridged and bridged... 

An important aim of theoretical catalysis is to develop the rules that relate catalyst performance to catalyst structure and composition. In this chapter, we introduce various general rules that concern this relationship. We have already referred to steric control which can be due to the interaction between the ligands in a homogeneous catalyst, organic overlayer on a heterogeneous surface, or the cavities within zeolites. The last will be extensively discussed in Chapter 4. 

Table 6.18.3 summarizes the composition, performance, and operating conditions of a typical three-way catalyst. Dimensional relationships of a washcoat are given in Figure 6.18.4. Figure 6.18.5 shows dose-up views of a monolith and of a single channel. 

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