Nature of the Catalyst

In spite of much effort, the nature of the active sites on acid—base inorganic catalysts is still not completely understood. However, the work on this problem has shown how complicated the surface structure may be and that several types of active centres may be simultaneously present on the surface the question is then which type plays the major role in a particular reaction. Also, the catalytic activity may be influenced to a large extent by impurities present in the feed (catalytic poisons) or by-products of the reaction. The last point is often not taken into account and it will be discussed specially in Sect. 1.2.6. First, the models of surface sites on the most important and best-studied catalysts will be described. 

The surface of silica (for detailed description of results see refs. 5, 11 and 12) contains a variable amount of hydroxyl groups and adsorbed wa- 

The surface of silica is highly reactive and hydroxyl groups exchange hydrogen for deuterium with D20 [14—16] but not with D2. They can be replaced by Cl from Cl2 or CC14 [16] and they react with silanes and aluminium chloride [15,19], Surface alcoholates are formed when silica is contacted with primary or secondary alcohols [20] either by the reaction with hydroxyl groups 

A number of other substances react with surface hydroxyl groups forming surface compounds [22], However, for catalysis, the hydrogen bonding seems to be more important. With alcohols, the hydrogen bonds are formed in such a way that surface hydroxyl groups act as donors of hydrogen [23], viz. 

Aluminium oxide exists in many crystalline modifications, usually designated by Greek letters, some with hexagonal and some with cubic lattices (cf. refs. 11 and 24). The best known and mostly used forms are a- and 7-alumina but practical catalysts are seldom pure crystallographic specimens. This makes the surface chemistry of aluminas rather complicated. Moreover, the catalytic activity of alumina depends very much on impurities. Small amounts of sodium (0.08—0.65%) poison the active centres for isomerisation but do not affect dehydration of alcohols [10]. On the other hand, traces of sulphates and silica may increase the number of strong acidic sites and change the activity pattern. 

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In many cases, however, well-designed catalysts provide intrinsically different reaction paths, and the specific nature of the catalyst surface can be quite important. This is clearly the case with unimolecular reactions for which the surface concentration effect is not applicable. 

Styrene. Styrene is readily polymerised to a glass-clear resin, polystyrene, but the exact nature of the polymer is influenced by the nature of the catalyst, the temperature, solvent, etc. 

At first glance it appears that these systems do conform fully to the discussion above this is an oversimplification, however. The ortho and para hydrogens in phenol are not equal in reactivity, for example. In addition, the technology associated with these polymers involves changing the reaction conditions as the polymerization progresses to shift the proportions of several possible reactions. Accordingly, the product formed depends on the nature of the catalyst used, the proportions of the monomers, and the temperature. Sometimes other additives or fillers are added as well. 

The importance of the nature of the catalyst on the hardening reaction must also be stressed. Strong acids will sufficiently catalyse a resol to cure thin films at room temperature, but as the pH rises there will be a reduction in activity which passes through a minimum at about pH 7. Under alkaline conditions the rate of reaction is related to the type of catalyst and to its concentration. The effect of pH value on the gelling time of a casting resin (phenol-formaldehyde ratio 1 2.25) is shown in Figure 23.15. 

The type of resin used, including the nature of the catalyst, the concentration of methylol groups and the average molecular weight. 

The rate of hydrolysis of sarin on Dowex-50 cation exchange resin is insensitive to the stirring rate. However, with a more active catalyst (Amberlite-IRA 400), the rate constant at 20°C was 5.3, 7.5, and 8.5 h at 60,800 and 1000 revolutions/min , respectively, suggesting that film diffusion was the rate-limiting. step. Thus, the mechanism of the rate-limiting step depends on the nature of the catalyst [34]. 

In this article we critically review most of the literature concerning non-catalyzed, proton-catalyzed and metal-catalyzed polyesterifications. Kinetic data relate both to model esterifications and polyeste-rificatiom. Using our own results we analyze the experimental studies, kinetic results and mechanisms which have been reported until now. In the case of Ti(OBu)f catalyzed reactions we show that most results were obtained under experimental conditions which modify the nature of the catalyst. In fact, the true nature of active sites in the case of metal catalysts remains largely unknown. 

The catalytic reaction can be subdivided into pore diffusion and chemisorption of reactants, chemical surface reaction, and desorption and pore diffusion of products, the number of steps depending upon the nature of the catalyst and the catalytic reaction. 

Although the regioselectivity of Friedel-Crafts acylations upon 1-phenylsulfonylpyrrole is ostensibly determined by the "hard-soft" nature of the catalyst <83JOC3214>, this paradigm may not be the controlling principle in determining the regioselectivity of acylations... 

It was demonstrated that these effects did not result from a permanent modification of the nature of the catalyst after recycling the catalyst used in styrene and reusing it in CH2CI2. Indeed, under these conditions the same stereoselectivities were obtained as with the use of the fresh catalyst in... 

These reactions are highly exothermic, for example one mole of -CH2- units generates 165 kj. The hydrocarbon distribution ranges from methane up to heavy waxes, depending on the nature of the catalyst and the reaction conditions. 

Mechanisms depending on carbanionic propagating centers for these polymerizations are indicated by various pieces of evidence (1) the nature of the catalysts which are effective, (2) the intense colors that often develop during polymerization, (3) the prompt cessation of sodium-catalyzed polymerization upon the introduction of carbon dioxide and the failure of -butylcatechol to cause inhibition, (4) the conversion of triphenylmethane to triphenylmethylsodium in the zone of polymerization of isoprene under the influence of metallic sodium, (5) the structures of the diene polymers obtained (see Chap. VI), which differ. both from the radical and the cationic polymers, and (6)... 

In 2003, Finke [6a] and Dyson [9] have reviewed tests commonly used to evaluate the nature of the catalyst starting with metallic molecular pre-catalysts, in order to estimate the in situ formation of heterogeneous active catalysts, in particular for hydrogenation processes. 

The exact procedure for entrapment will depend on the nature of the catalyst and the required sol-gel, but for enzyme entrapment it generally involves the hydrolysis or partial hydrolysis of an alkoxysilane precursor, giving a precursor sol-gel solution, followed by addition of more solvent and of the catalyst. Figure 5.7 provides a simple... 

The form in which the catalyst is employed is strongly dependent on the reaction involved, the scale of the process, the specific nature of the catalyst, and the type of reactor. Catalysts used for reactions carried out with a liquid feed are usually ground so as to pass through a 100 to... 

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