Typical solid catalysts

The typical solid catalyst used in technology consists of small catalytically active species, such as particles of metal, metal oxide, or metal sulfide, dispersed on a low-cost, high-area, nearly inert porous support such as a metal oxide or zeolite. The catalytic species are typically difficult to characterize in-... 

Surface area, pore volume, and mean pore radii for typical solid catalysts (Wheeler, 1950)... 

The third point is measuring more. Weckhuysen implied by this statement that the researcher should combine two or more spectroscopic techniques in one cell, to produce multitechnique characterizations. Given the complexity of a typical solid catalyst, the use of multiple characterization techniques has always been appreciated in catalyst characterization science. The novelty suggested here is to combine these techniques so that the spectroscopic measurement is made on the same catalyst at the same time. Other chapters in this volume address this point. 

Table 1.8 Surface Area, Pore Volume, and Average Pore Radius for Typical Solid Catalysts (Smith 1981)...

Efficient use of a catalyst requires high rates of reaction per unit volume and, since reaction takes place on the surface of a solid, catalysts have high surface areas per unit volume. Therefore, tlie typical catalyst is porous, witli... 

Separability. One of the greatest advantages of a solid catalyst is that it can be separated easily from the products of reaction. To do this successfully requires careful control of the process conditions so that exposure of the catalyst to nonreactant liquids capable of affecting or dissolving either the catalytic material or the support is prevented or rninimi2ed. Solid catalysts typically are used in axial or radial flow beds and multitubular reactors. Many successful commercial processes maintain the reactants and products in the gas phase while in contact with the catalyst to avoid catalyst degradation problems. 

A gas-liquid-particle process termed cold hydrogenation has been developed for this purpose. The hydrogenation is carried out in fixed-bed operation, the liquefied hydrocarbon feed trickling downwards in a hydrogen atmosphere over the solid catalyst, which may be a noble metal catalyst on an inert carrier. Typical process conditions are a temperature of 10°-20°C and a pressure of 2.5-7 atm gauge. The hourly throughput is as high as 20-kg hydrocarbon feed per liter of catalyst volume. 

Equations 4.31 and 4.32 also suggest another important fact regarding NEMCA on noble metal surfaces The rate limiting step for the backspillover of ions from the solid electrolyte over the entire gas exposed catalyst surface is not their surface diffusion, in which case the surfacediffusivity Ds would appear in Eq. 4.32, but rather their creation at the three-phase-boundaries (tpb). Since the surface diffusion length, L, in typical NEMCA catalyst-electrode film is of the order of 2 pm and the observed NEMCA time constants x are typically of the order of 1000 s, this suggests surface diffusivity values, Ds, of at least L2/t, i.e. of at least 4 10 11 cm2/s. Such values are reasonable, in view of the surface science literature for O on Pt(l 11).1314 For example this is exactly the value computed for the surface diffusivity of O on Pt(lll) and Pt(100) at 400°C from the experimental results of Lewis and Gomer14 which they described by the equation ... 

A very important part of such an undertaking is to be clear about what stages of a chemical process generate the most waste. Often this is found to be the separation stage, after the transformation of reactants to products, where all the various components of the final mixture are separated and purified. Approaches to chemical reactions which help to simplify this step are particularly powerful. Such an approach is exemplified by heterogeneous catalysis. This is an area of chemistry where the catalysts used are typically solids, and the reactants are all in the hquid or gas phase. The catalyst can speed up the reaction, increase the selectivity of the reaction, and then be easily recovered by filtration from the liquid, and reused. 

Solid catalysts can be subdivided further according to the reactor chosen. Dependent on the type of reactor the optimal dimensions and shapes of the catalyst particles differ. Catalysts applied in fixed beds are relatively large particles (typically several mm in diameter) in order to avoid excessive pressure drops. Extrudates, tablets, and rings are the common shapes. Figure 3.9 shows some commonly encountered particle shapes. 

Depending on the process requirements catalysts are produced in a variety of ways. Fig. 3.12 shows some typical processes used in catalyst manufacture. In all cases the process starts from a solution. The various process steps used arc explained in subsequent sections. Solid catalysts can be subdivided in bulk catalysts and supports and catalysts prepared by impregnation of shaped supports. 

In the vast majority of gas-solid reactions, gaseous or evaporated compounds react at the surface of a solid catalyst. These catalytic processes are very frequently used in the manufacture of bulk chemicals. They are much less popular in processing of the large molecules typical of fine chemistry. These molecules are usually thermally sensitive and as such they will at least partially decompose upon evaporation. Only thermally stable compounds can be dealt with in gas-solid catalytic processes. Examples in fine chemicals manufacture are gas-phase catalytic aminations of volatile aldehydes, alcohols, and ketones with ammonia, with hydrogen as... 

Considering the specific application of chemical synthesis, the presence of solid catalyst (particles/salts in a typical concentration range of 1-10% by weight of the reactants optimization is recommended in majority of the cases using laboratory... 

A selection of typical commercial and viable new solid catalysts... 

In view of the numerous advantages of POMs the development of strategies for converting them to solid catalysts is of primaiy interest. First, catalytically active POMs can be heterogenized in the form of insoluble salts using Cs, Ag, K, NH/ and some organic cations [37,49, 58-64]. Such salts possess micro/mesoporous structure and their smface area is typically in the range of 10-150 mVg. 

Catalytic epoxidation of olefins (typical procedure) Solid catalyst (1 g) prepared from XAD-2 resin is stirred with 20 mL of 1.0 M cyclohexane and 1.0 M H2O2 in t-BuOH or dioxane at 60°C for 24 h. Cyclohexenoxide is obtained in a quantitative yield. 

Whereas some knowledge has been obtained about the working mechanism of ammonia catalysts (51), this does not apply to the same extent to catalysts used for many other processes. However, a few typical cases of multicomponent catalysts have been investigated both in the author s laboratory and by others. The main conclusion to be drawn from these studies is that it would be wrong to seek one universal explanation for the promoter effects in solid catalysts. As outlined above, structural as well as chemical effects may cause the improvements which are observed after certain substances have been added to a given catalyst. 

Figure 11-7 Proposed mechanism for catalytic polymerization. An olefin and an alkyl group on an oiganometallic site react to add the olefin to the growing chain, Also shown are typical polymerization catalysts Ti/Si solid catalyst and organometallic metallocene catalysts.

MRI to characterize hydrodynamics within reactors is already established. The extent to which the potential of MR to study both hydrodynamics and chemical conversion is fully realized will depend on our ability to integrate the well-established MR spectroscopy techniques in liquid- and solid-state NMR into imaging pulse sequences, and still provide quantitative data in the magnetically heterogeneous environments typical of catalysts and reactors. 

Hydration means, in general, addition of the elements of water to a substance. Most of these reactions are non-catalytic or homogeneously catalysed processes. In this section, only hydration of olefins to alcohols, of acetylene to acetaldehyde, and of alkene oxides to glycols will be treated, since they are typical reactions where the application of solid catalysts has become important. 

Light oils are invariably hydroprocessed in gas-liquid-solid catalyst trickle-bed reactors (TBR). In these reactors, both the hydrogen and hydrocarbon streams flow down through one or more catalyst beds. A typical schematic diagram is shown in Figure 5.2—41 as an example of hydrodesulfurization process [60, 61]. 

In a slurry-batch reactor the hydrogen and the liquid are mixed intensively together with the solid catalyst. The catalyst concentration is normally a few wt%. The reaction time is typically a few hours. This means that the productivity becomes in the range of 100 - 1 000 kgproduct/m3reactorh. The transport resistance between the liquid and the catalyst ( C in Fig. 9.3-2) is normally the restricting factor. 

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