Catalytically active sites properties

What is the most important property to consider when designing a catalyst for an industrial process (a) the rate per catalytically active site (b) the rate per unit weight of catalyst or (c) the rate per unit volume of catalyst Explain your answer. 

Zeolite molecular sieves are widely used as solid acid catalysts or catalyst components in areas ranging from petroleum refining to the synthesis of intermediates and fine chemicals (112,113). An important reason for their widespread use is the flexibility they oflFer regarding the tailoring of the concentration and nature of catalytically active sites and their immediate environments. We note that discrimination between chemical and structural aspects works well at a conceptual level, but one faces quite severe limitations as soon as one tries to separate the contributions of the two effects. The complexity arises because the chemical properties of a particular molecular sieve are connected with its framework density. 

Carbon nanotubes have nano-sized channels that have received significant attention in recent years, and there are rapid advances in techniques to produce them. However, currently researchers are still learning how to isolate, manipulate, and reduce the variations in the properties of these carbon nanotubes. Nonetheless, their potential in catalytic applications is evident. For example, it is possible to trap catalytic active sites inside a carbon nanotube.In other developments, methods have been discovered... 

For a specific reaction, the reaction rates, as defined in eqs. (3.2)-(3.4), depend on the nature of the catalytic active site, the surface arrangement of the catalyst, the temperanire, and the reactants concentration. Surface arrangement here denotes the macroscopic and measurable catalyst basic properties ... 

In the case of a porous catalyst, where the internal area contributes the most to the total area, Ss can be considered to be independent from the catalyst shape and size. Furthermore, the number of catalytic active sites per unit area can be considered a fixed property for a given catalyst. Consequently, the active sites concentration can (n/Ms) be also be... 

Complete understanding of the catalytic properties of M0S2 requires more knowledge of the edge planes that terminate the anisotropic layers and are the location of catalytically active sites. Further progress in developing this... 

To date, the composition of active sites is known for many enzymes, the most probable action mechanisms are suggested, and comparison data exist on catalytic group properties in enzymes and free molecules in solutions. Note also that the chemical composition of catalytic active sites of enzymes is independent of the presence of any specific compounds. Moreover, the majority of them are the well-known compounds for homogeneous catalysis histidine imidazole, carboxylic groups of aspartic and aminoglutaric acids, flavins, hemins, etc. However, as homogeneous catalysts, they possess rather moderate or even poor catalytic activity in appropriate reactions. 

In one of the first papers on the application of XAFS spectroscopy to catalysis (Lytle et al., 1974) is the statement "... these results demonstrate that the EXAFS technique can be a powerful tool for studying catalysis in order to determine the precise structural relationship between catalytically active sites and the surrounding atoms." It is exactly this precise structural relationship that is the critical kind of information needed if true structure-property relationships are to be developed. 

We mentioned that bonding in zeolites is dominated by covalent bonding. Therefore, zeolite properties can be described as being mainly locally dependent. For instance, it is known that Br0nsted acidic sites induce important distortions in the zeolite framework (see Figure 3). However, these distortions remain local. A direct application of this property is the validity of the use of small fragments to describe catalytic active sites (see Figure 5). 

The new mechanistic routes opened by the introduction of a catalyst to the reaction mixture typically start with the adsorption of the reactants. Because of the heterogeneous nature of the surface of the catalyst, both the adsorption and the subsequent conversion reactions may take place preferentially at particular ensembles of surface sites, often called active sites or active centers. An atomic ensemble may become active because of a specific structural arrangement of those atoms. Alternatively, the electronic properties of metal atoms may also influence the adsorption and activation of reactants. Typically, the performance of catalytic active sites depends on both structural and electronic effects. 

Protonic zeolites find industrial applications as acid catalysts in several hydrocarbon conversion reactions. The excellent activity of these materials is due to two main properties a strong Bronsted acidity of bridging Si—(OH)-Al sites (Scheme 3.4, right) generated by the presence of aluminum inside the silicate framework and shape selectivity effects due to the molecular sieving properties associated with the well defined crystal pore sizes, where at least some of the catalytically active sites are located. 

Furthermore, ruthenium-carbene complexes are highly tunable, well-defined, single-site, homogeneous catalysts. These characteristics provide the ability to access all catalytically active sites and thus to influence catalyst initiation, propagation, and stability properties. The relatively simple ligand environment of Ru-2, Ru-4, and... 

T ivalent and ammonium forms of Y zeolite possess catalytic activities for many acid-catalyzed reactions 11, 12, 16, 19). Although it is generally accepted that acidic hydroxyl groups are responsible for the activity of the calcined ammonium form 4, 6,19, 24), the reasons for the activity of the cation forms have been less well elucidated 6, 7, 9,15,19, 20, 25). Since the catalytic activity of the divalent cation forms usually is exhibited when suflBcient cations have been introduced so as to occupy accessible positions in the lattice, it has been suggested that various properties of the cations may be responsible for the introduction of active sites. Properties of the cations such as their electrostatic field and surface diffusion have been suggested 12, 15). Alternatively, it has been suggested that the divalent cations can introduce Bronsted acidity by dissociation of water 3, 5, 6,18,19, 20), thus... 

Another widely accepted viewpoint on the catalytically active sites in the silica-alumina is that they are aprotonic acids, viz., electrophilic A1 atoms with unfilled p-shell, the electron density being shifted from them towards the three surrounding 0 atoms (72). Such an electron deficiency confers on the A1 atom an afiinity towards an unshared electron pair of the basic adsorbed molecule, i.e., the properties of a Lewis acid. In fact, the addition compound, obtained on sorption of aniline vapor by a sublimed AICI3 film in a high vacuum exhibits the same shift towards the spectrum of benzene, as is the case with a protonic acid (73). 

The nature of the surface of the solid catalyst is ultimately vital to its performance and value. The number of catalytically active sites, their dispersion over the surface of the solid, their accessibility to substrate molecules and their activity or strength are fundamental properties that can be influenced by the nature of and any pretreatment of the support, the method of preparation of the... 

Knowledge of catalyst composition and structure is crucial to an understanding of the factors that affect catalyst activity and selectivity. Such information makes it possible to determine which portions of a catalyst are active and how changes in catalyst synthesis and pretreatment affect the properties of the catalytically active sites. Catalyst characterization is also vital to understanding the changes that occur in the structure and composition of a catalyst following both use under reaction conditions and regeneration to reactivate the catalyst. 

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