Classification of catalysts

The chemical, thermal, and mechamcal stability of a catalyst determines its lifetime in industrial reactors. Catalyst stabUity is influenced by numerous factors, including decomposition, coking, and poisoning. Catalyst deactivation can be followed by measuring activity or selectivity as a function of time. 

Catalysts that lose activity during a process can often be regenerated before they ultimately have to be replaced. The total catalyst lifetime is of crucial importance for the economics of a process. 

Today the efficient use of raw materials and energy is of major importance, and it is preferable to optimize existing processes than to develop new ones. For various reasons, the target quantities should be given the following order of priority  

The numerous catalysts known today can be classified according to various criteria structure, composition, area of application, or state of aggregation. 

In supported catalysts the catalytically active substance is applied to a support material that has a large surface area and is usually porous. By far the most important catalysts are the heterogeneous catalysts. The market share of homogeneous catalysts is estimated to be only ca. 10-15 % [5, 6]. In the following, we shall briefly discuss the individual groups of catalysts. 

One common way to classify catalysts is in terms of the type of reaction they catalyze. 

Tabic 10-1 gives a list of representative reactions and their corresponding catalysts. Further discussion of each of these reaction classes and the materials that catalyze them can be found on the DVD-ROM/Web Pwfessioml Reference Shelf R10.1. 

for example, we were to form styrene from an equimolar mixture of ethylene and benzene, we could carry out an alkylation reaction to form ethyl 

The conductor catalysts are the metals (silver, platinum, vanadium, iron, etc.) and have the property of chemisorption by electron transfer. The semiconductor catalysts are the oxides, such as NiO, CU2O, and ZnO. These materials have the capability of interchanging electrons from the filled valence bands in a compound when sufficient energy is provided, 

It should be emphasized that the electronic theory is not without uncertainities and at present should be considered as a concept in transition. However, it does provide a convenient, and probably helpful, method of classifying solid catalysts. 

In connection with miscellaneous catalysts the work on polymerizing ethylene should be mentioned. It has been found that aluminum alkyl- 

This discussion of catalysts for reaction types is general and superficial. Much has and is being written on the subject, and helpful sources of summary information are available.  

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Kinds of Catalysts To a certain extent it is known what lands of reactions are speeded up by certain classes of catalysts, but individual members of the same class may differ greatly in activity, selectivity, resistance to deactivation, and cost. Since solid catalysts are not particularly selective, there is considerable crossing of lines in the classification of catalysts and the kinds of reactions they favor. Although some trade secrets are undoubtedly employed to obtain marginal improvements, the principal catalytic effects are known in many cases. 

Finally, a classification of catalysts by Matsuura [212] may be mentioned, in which the relation of adsorption entropy to heat of adsorption of butene-1 appears, surprisingly, to be linear. The conclusion can be drawn that moderate heats of adsorption (about 40—50 kcal mol 1) characterize suitable catalysts. Only here is the right combination of surface mobility and adsorption intensity found. Apparently, the oxygen is then tempered sufficiently to make a selective oxidation possible. Otherwise, the oxides are non-active (e.g. low heat of adsorption in FeP04 and low mobility) or active but non-selective because of high mobility coupled to a large heat of adsorption (e.g. Fe304). 

Intermediate processes of catalyzed organic reactions may involve neutral free radicals R , positive ions R+, or negative ions R as short-lived reactants. A classification of catalysts and processes from the point of view of elementary reactions between reagents and catalysts Is logically desirable but has not yet been worked out. However, there is a wealth of practice more or less completely documented, some proprietary but available at a price. The ensuing discussions are classified into kinds of catalysts and into kinds of processes. 

Because a catalyst affects the rate of reaction and not the ultimate equilibrium, it is not possible to give a general, kinetic description of catalyst behavior. Instead, a proper discussion of catalytic behavior can bo made only in terms of mechanism, which is, of course, unique for any given reaction. However, some general classification of catalysts is possible in terms of structure in relation to type of reaction mechanism involved. A useful classification of solids for this purpose is as follows ... 

It is re-emphasized that new Phillips-type catalysts continue to evolve, and this trend will likely continue. Even some of the new single-site catalysts are chromium catalysts. Many "hybrid" combinations of chromium oxide and single-site catalysts have been explored for film production and other applications. The classification of catalyst families as conventional or "new" is becoming ever more blurred, and many more important discoveries are expected. 

Varying acidic properties were obtained by changing the impregnation procedure and the phase composition of the titania support. The classification of catalysts in strong or weak acidity was done by pH... 

Table 2 Classification of catalysts according to tight and loose contact mode...

Recently, an attempt at going beyond the qualitative classification of catalysts through calculated enantioselectivities was made for a series of catalysts displaying varying stereospecificities. The computational approach was based on the simplest possible combination of quantum chemistry and molecular... 

The exact nature of the reasons for and the ease of formation of the surface complex are still not entirely known. One can visualize certain structural requirements of the underlying solid surface atoms in order to accomodate the reactants, and this has led to one important set of theories. Also, as will be seen, various electron transfer steps are involved in the formation of the complex bonds, and so the electronic nature of the catalyst is also undoubtedly important. This has led to other important considerations concerning the nature of catalysts. The classification of catalysts of Table 2.1-1 gives some specific examples (Innes see Moss [7]). Recent compilations also give very useful overviews of catalytic activity Thomas [8] and Wolfe [9]. Burwell [10] has discussed the analogy between catalytic and chain reactions ... 

Figure 19.4. Classification of catalyst ink and catalyst layers 19.3.1.1 Hydrophobic Ink...

Professional Reference Shelf R10.1. Classification of Catalysts RU).2,Hydrogen Adsorption... 

Arrhenius and Ostwald played very important roles in the early studies on add-base catalysis, one century ago. Arrhenius contributed to the definition of acids and bases, and established the dependence between the rate constants and the temperature. Additionally, he also formulated an electrolytic theory of dissociation that ultimately led to him receiving the 1903 Nobel Prize in Chemistry. Ostwald proposed useful definitions of catalysis and classifications of catalysts, but he was unable to develop a satisfactory theory of these effects. This is not surprising, in view of the very limited knowledge of the mechanisms of catalysis at his time, and of the lack of understanding of how molecular properties can influence the rates of reactions. Nevertheless, his seminal work on catalysis was rewarded by him receiving the 1909 Nobel Prize in Chemistry. 

Due to these disadvantages, research on the transesterification reaction using heterogeneous catalysts for biodiesel production has increased over the past decade (Lee and Wilson, 2014). Fig. 6.9 summarizes the classification of catalysts. Zhang et al. (2003) argued there is a considerable incentive for the substimtion of liquid bases by solid bases for the following reasons (1) energy intensive product/catalyst separation, (2) corrosiveness, and (3) the costs associated with the disposal of spent or neutralized caustics. 

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