Catalytic processes description

Process Description. Reactors used in the vapor-phase synthesis of thiophene and aLkylthiophenes are all multitubular, fixed-bed catalytic reactors operating at atmospheric pressure, or up to 10 kPa and with hot-air circulation on the shell, or salt bath heating, maintaining reaction temperatures in the range of 400—500°C. The feedstocks, in the appropriate molar ratio, are vaporized and passed through the catalyst bed. Condensation gives the cmde product mixture noncondensable vapors are vented to the incinerator. 

FCC process description adapted by permission from Fluid Catalytic Cracking Handbook, R. Sadeghbeigi, Gulf Publishing Company, Houston, Texas, 2000, pp. 3—17. 

Overall, catalytic processes in industry are more commonly described by simple power rate law kinetics, as discussed in Chapter 2. However, power rate laws are simply a parameterization of experimental data and provide little insight into the underlying processes. A micro-kinetic model may be less accurate as a description, but it enables the researcher to focus on those steps in the reaction that are critical for process optimization. 

The description of pure quantum mechanics (QM) methods presented in Section 3 has shown how in most cases they provide an accurate description of the electronic subtleties involved at the transition metal center of a catalytic process, but that they are unable to introduce the whole bulk of the catalyst substituents, which can be critical for selectivity issues. The description of pure molecular mechanics (MM) methods presented in subsection 4.1 has shown how these methods can easily introduce the steric bulk of the substituents, and accurately describe their steric interactions, but that they struggle badly when trying to describe properly the transition metal center and its immediate environment. The logical solution to this complementary limitations is to divide the chemical system in two regions, and to use a different description for each of them, QM for the metal and its environment, MM for the rest of the system. This is precisely the basic idea of hybrid quantum mechanics / molecular mechanics (QM/MM) methods. 

The description of bonding at transition metal surfaces presented here has been based on a combination of detailed experiments and quantitative theoretical treatments. Adsorption of simple molecules on transition metal surfaces has been extremely well characterized experimentally both in terms of geometrical structure, vibrational properties, electronic structure, kinetics, and thermo-chemistry [1-3]. The wealth of high-quality experimental data forms a unique basis for the testing of theoretical methods, and it has become clear that density functional theory calculations, using a semi-local description of exchange and correlation effects, can provide a semi-quantitative description of surface adsorption phenomena [4-6]. Given that the DFT calculations describe reality semi-quantitatively, we can use them as a basis for the analysis of catalytic processes at surfaces. 

X-ray diffraction and absorption techniques have rarely been used to investigate time-dependent changes. However, it will be shown that the techniques may also be used to characterize the dynamics of catalytic processes (Sections IV-VII), and in recent years the time resolution has improved by several orders of magnitude. This improvement has opened the door to new types of quantitative studies of the dynamical behavior of catalysts, and such information is often necessary for a detailed description and understanding of catalytic phenomena. Some possible future applications and advances in the techniques are summarized in Section VII. 

Description of catalytic processes at atomic/molecular levels... 

B. Flow Reactors. Laboratory-scale catalytic reactors and reactors for the reaction of solids with gases arc often constructed from metal. One of the principal objectives in the use of laboratory-scale catalytic reactors is the determination of rate data which can be associated with specific physical and chemical processes in a catalytic reaction. Descriptions are available for these kinetic analyses as they relate to reactor designs and reaction conditions. ... 

The electronic structure of a solid metal or semiconductor is described by the band theory that considers the possible energy states of delocalized electrons in the crystal lattice. An apparent difficulty for the application of band theory to solid state catalysis is that the theory describes the situation in an infinitely extended lattice whereas the catalytic process is located on an external crystal surface where the lattice ends. In attempting to develop a correlation between catalytic surface processes and the bulk electronic properties of catalysts as described by the band theory, the approach taken in the following pages will be to assume a correlation between bulk and surface electronic properties. For example, it is assumed that lack of electrons in the bulk results in empty orbitals in the surface conversely, excess electrons in the bulk should result in occupied orbitals in the surface (7). This principle gains strong support from the consistency of the description thus achieved. In the following, the principle will be applied to supported catalysts. 

Because there are many different ways to combine a catalyst with a membrane, there are numerous possible classifications of the CMRs. However, one of the most useful classifications is based on the role of the membrane in the catalytic process we have a catalytically active membrane if the membrane has itself catalytic properties (the membrane is functionalized with a catalyst inside or on the surface, or the material used to prepare the membrane is intrinsically catalytic) otherwise if the only function of the membrane is a separation process (retention of the catalyst in reactor and/or removal of products and/or dosing of reagents) we have a catalytically passive membrane. The process carried out with the second type of membrane is also known as membrane-assisted catalysis (a complete description of the different CMRs configurations will be presented in a specific chapter). 

In contrast to so-called microkinetic analyses, an important aspect of chemical reaction engineering involves the use of semiempirical rate expressions (e.g., power law rate expressions) to conduct detailed analyses of reactor performance, incorporating such effects as heat and mass transport, catalyst deactivation, and reactor stability. Accordingly, microkinetic analyses should not be considered to be more fundamental than analyses based on semiempirical rate expressions. Instead, microkinetic analyses are simply conducted for different purposes than analyses based on semiempirical rate expressions. In this review, we focus on reaction kinetics analyses based on molecular-level descriptions of catalytic processes. 

The work has largely focused on the coordination chemistry of transition metal ions (i.e., on the description of the nature and symmetry of their environments) (Section 2.1), in line with other spectroscopies, mainly optical (UV-vis), magnetic (EPR and NMR), which take advantage of partly filled d orbitals, and structural (EXAFS) (Sojka and Che, 2009). It has even become possible with PL via well-resolved fine structures to determine the extent of distortion of the environment of tetrahedral species (e.g., vanadium species in zeolites (Section 2.1.2)). It is likely that such information combined with that derived from other spectroscopies, vibrational on one hand, such as IR and Raman, and electronic on the other hand, such as EPR, will be applied by theoreticians to further improve the existing models and our understanding of the nature and role of surface species involved in catalytic processes. 

The forthcoming description of catalysts and catalytic processes should only serve as a primer towards understanding the basic principles with some examples of applications in the field of petroleum processing, chemical production, and environmental air purification. Table 7.1 gives a list of some of the many commercial catalytic applications. 

Description A one-step, fixed-bed catalytic process operates on a single-component or mixed feedstock to selectively produce diolefins. Feed is preheated, then contacted with catalyst in parallel fixed-... 

Georgii Boreskov, an eminent Soviet scientist in catalysis, devoted his research to the creation of an adequate physicochemical language for describing the phenomenon of heterogeneous catalysis, and he considered thermodynamics the fundamentals for the description. His analysis of the thermodynamic bases of catalytic processes produced the commonly accepted and experimentally proved Boreskov s rule on the approximate constancy of the specific catalytic activity ... 

As applied to catalysis, the microkinetic analysis of catalytic reactions is used most often. This is an instrument of an idealized description of com plex catalytic processes without consideration of the mass transfer that can affect considerably the observed kinetics of the catalytic transformations. The microkinetic analysis with the necessary consideration of the active sites balance for all types of active centers of the catalyst, even though it has several drawbacks, can provide important information about the potential influence of the very different thermodynamic factors. 

With this methodology, the problem is analyzed from the smallest to the largest scales, as appearing in the process description. As an example, in the case of a catalytic reactor, we consider the process on the following scales ... 

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