Catalyst porous solids, Chapter

In this chapter, we demonstrate the potential of such agents as catalysts/promoters in key steps for the derivatization of sugars. The most significant catalytic approaches in carbohydrate chemistry that use aluminosilicate porous materials, namely zeolites and montmorillonite clays, are reviewed and discussed. Silica gel is a porous solid silicate that has also been used for heterogeneous catalysis of organic reactions in general. We include here its usefulness as promoter and reagent support for the reactions under consideration. 

There are, however, two broad classes of exceptions to this conclusion. The first comes with the slow reaction of a gas with a very porous solid. Here reaction can occur throughout the solid, in which situation the continuous reaction model may be expected to better fit reality. An example of this is the slow poisoning of a catalyst pellet, a situation treated in Chapter 21. 

In gas-solid reactors when solid particles are held stationary (so-called fixed bed reactor), gas flows through a porous medium comprising macropores existing between pellets or packed solid particles and micropores within the catalyst pellets (or other porous solids). Issues such as isotropy of the porous medium, initial distribution of gases, characteristics of solid particles, ratio of characteristic length scale of solid particles and that of the reactor and so on, influence the flow within fixed bed reactors. Support screens are often used to cover the bed of solid particles to avoid fluidization and carry-over of bed particles. These reactors are extensively used in process industries. Some examples and illustrative flow simulations are discussed in Chapter 13. 

Chapter 3 dealt with the problem of the reaction kinetics for different gas-solid reactions, while chapter 5 dealt with the mass and heat transfer problems for porous as well as non-porous catalyst pellets. In chapter 5 different degrees of complexities and rigor were used. In chapter 5, the analysis started with the simplest case of non-porous catalyst pellets where the only mass and heat transfer Coefficients are those at the external surface which depend mainly on the flow conditions around the catalyst pellet and the properties of the reaction mixture. It was shown clearly that j-factor correlations are adequate for the estimation of the external mass and heat transfer coefficients (k, h) associated with these resistances. For the porous catalyst pellets different models with different degrees of rigor have been used, starting from the simplest case of Fickian diffusion with constant diffusivity, to the rigorous dusty gas model based on the Stefan-Maxwell equations for multicomp>onent diffusion. 

The principles of homogeneous reaction kinetics and the equations derived there remain valid for the kinetics of heterogeneous catalytic reactions, provided that the concentrations and temperatures substituted in the equations are really those prevailing at the point of reaction. The formation of a surface complex is an essential feature of reactions catalyzed by solids and the kinetic equation must account for this. In addition, transport processes may influence the overall rate heat and mass transfer between the fluid and the solid or inside the porous solid, > that the conditions over the local reation site do not correspond to those in the bulk fluid around the catalyst particle. Figure 2.1-1 shows the seven steps involved when a molecule moves into the catalyst, reacts, and the product moves back to the bulk fluid stream. To simplify the notation the index s, referring to concentrations inside the solid, will be dropped in this chapter. 

Porous carbons (e.g., activated carbons) are an important family of porous solids that have a wide spectrum of applications becanse of their remarkable properties, such as high specific surface area, chemical inertness, abundant repertory of surface functional groups, good thermal stability, and low cost of manufacture. Their chemistry and physics have been reviewed [22-25]. The most common way to produce activated carbons is to carbonize a carbon-containing precursor, followed by activation or posttreatment [26]. Becanse of practical requirements of various applications, techniques for control over the pore size of activated carbons have been the subject of research for several decades [18] for example, high burn-off activation, catalyst-assisted activation, and carbonization of polymer blends with thermally unstable components. For recent progress in the use of hydroxide activation, see Chapter 1 by Linares-Solano and coworkers in this volume [27]. However, none of these synthesis approaches is suitable for very precise control over pore structure, particle size, and morphology [19]. 

In this chapter, we summarize the recent advances in the development of nanaoreactors based on porous solid materials for chemical reactions, including the general methods for the fabrication of typical porous materials, (mesoporous silicas (MSs), carbon nanotubes (CNTs), and the MOFs), the assembly of the molecular catalysts in the cavities and pores of the porous materials, the chemical reactions in the porous-material-based nanoreactors, and some important issues concerning the porous-material-based nanoreactor, such as the pore confinement effect, the isolation effect, and the cooperative activation effect We close this chapter with an outlook of the future development of the nanoreactors. 

The zeolites are also known as molecular sieves because of their capacity to discriminate between molecules they find numerous uses in separation and catalytic processes. Although they appear to be solid particles to the naked eye, they are highly porous, with a typical specific surface area of about 1000 m2/g. Catalysis is discussed in Chapter 9, but the scope of that chapter does not permit detailed discussions of the various types of catalysts and the role of physisorption and chemisorption in catalysis this vignette provides a glimpse of the rationale used in the molecular design of new materials of interest in surface chemistry and how the concepts introduced in Chapter 1 and Chapter 9 fit into the larger scheme. 

Two other crucial factors are mass transfer and heat transfer. In Chapter 3 we assumed that the reactions were homogeneous and well stirred, so that every substrate molecule had an equal chance of getting to the catalytic intermediates. Here the situation is different. When a molecule reaches the macroscopic catalyst particle, there is no guarantee that it will react further. In porous materials, the reactant must first diffuse into the pores. Once adsorbed, the molecule may need to travel on the surface, in order to reach the active site. The same holds for the exit of the product molecule, as well as for the transfer of heat to and from the reaction site. In many gas/solid systems, the product is hot as it leaves the catalyst, and carries the excess energy out with it. This energy must dissipate through the catalyst particles and the reactor wall. Uneven heat transfer can lead to hotspots, sintering, and runaway reactions. 

In Chapters 4 and 6, several methods of characterization of solids that are normally used for catalyst testing were described. In particular, the parameters which characterize the surface morphology of a porous catalyst are the same that characterize a porous adsorbent, that is, the specific surface area, S [m2/g], the micropore volume, W1 [cm3/g], the sum of the micropore andmesopore volumes, that is, the pore volume, W [cm3/g], and the pore size distribution (PSD), AVp/ADp (see Chapter 6). 

At its operating temperature, 700-900 K, S02 oxidation catalyst consists of a molten film of V, K, Na, (Cs) pyrosulfate salt on a solid porous Si02 substrate. The molten film rapidly absorbs S02(g) and 02(g) - and rapidly produces and desorbs SC>3(g), Chapters 7 and 8. 

Fig. 7.1. Bed of catalyst pieces for oxidizing S02 to S03. It is circular, 7 to 17 m diameter. Industrial S02 oxidation is done in a converter of 3 to 5 such beds, Figs. 7.6 and 7.7. Downward gas flows are 25 Nm3/minute per m2 of top surface. Active catalyst consists of a molten V, K, Na, Cs, S, O phase supported on a solid porous silica substrate, Chapter 8. A top layer of silica rock holds the catalyst in place. A bottom layer prevents the catalyst from sticking to the stainless steel support grid.

The question remains as to when the various diffusion effects really influence the conversion rate in fluid-solid reactions. Many criteria have been developed in the past for the determination of the absence of diffusion resistance. In using the many criteria no more information is required than the diffusion coefficient DA for fluid phase diffusion and for internal diffusion in a porous pellet, the heat of reaction and the physical properties of the gas and the solid or catalyst, together with an experimental value of the observed global reaction rate (R ) per unit volume or weight of solid or catalyst. For the time being the following criteria are recommended. Note that intraparticle criteria are discussed in much greater detail in Chapter 6. 

The specific reaction rate is independent of the velocity of fluid and for the solid sphere considered here, independent of particle size. However, for porous catalyst pellets, may depend on particle size for certain situations, as shown in Chapter 12. 

Microporous and, more recently, mesoporous solids comprise a class of materials with great relevance to catalysis (cf. Chapters 2 and 4). Because of the well-defined porous systems active sites can now be built in with molecular precision. The most important catalysts derived from these materials are the acid zeolites. The acid site is defined by the crystalline structure and exhibits great chemical and steric selectivities for catalytic conversions, such as fluid catalytic cracking and alkane isomerization (cf. Chapter 2). In Section 9.5 we discuss the synthesis of zeolites and, briefly, of mesoporous solids. 

Our objective here is to study quantitatively how these external physical processes affect the rate. Such processes are designated as external to signify that they are completely separated from, and in series with, the chemical reaction on the catalyst surface. For porous catalysts both reaction and heat and mass transfer occur at the same internal location within the catalyst pellet. The quantitative analysis in this case requires simultaneous treatment of the physical and chemical steps. The effect of these internal physical processes will be considered in Chap, 11. It should be noted that such internal effects significantly affect the global rate only for comparatively large catalyst pellets. Hence they may be important only for fixed-bed catalytic reactors or gas-solid noncatalytic reactors (see Chap. 14), where large solid particles are employed. In contrast, external physical processes may be important for all types of fluid-solid heterogeneous reactions. In this chapter we shall consider first the gas-solid fixed-bed reactor, then the fluidized-bed case, and finally the slurry reactor. 

Although the surface phenomena associated with gas solid catalytic reactions and their implications on the behaviour of the system was briefly introduced earlier (chapter 2), it is discussed here with more details because of the importance of these phenomena for porous catalyst pellets. Also, more emphasis will be given to the surface phenomena and their effect on both steady state and dynamic characteristics of the porous catalyst pellet. 

The porous titanium silicate TS-1 represents one of the great commercial successes of recent years. Despite only being reported for the first time in the last decade, it is already established as an oxidation catalyst in the manufacture of hydroquinone, and processes based on its use as a catalyst in the epoxidation of propene and the ammoxidation of cyclohexanone are near the production stage.14 The use of the increasingly diverse range of molecular sieve solid catalysts is also described in Chapter 2. 

Catalysts used in industrial processes, whether made from the active substance directly or deposited on carriers, are often porous cylindrical pellets produced by medium pressure extrusion (Fig. 5-10bl-b6, Chapter 5). While almost all methods of size enlargement by agglomeration can be used to produce porous bodies from solid powders and post-treatment processes can yield strong pieces with a high accessible internal porosity [B.23, B.78, B.97] that fulfill the previously discussed requirements, pelleting offers a number of advantages. 

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