Bulk-type catalysis

The redox properties can also be controlled by the formation of salts. However, the relationship between these properties and the catalytic activity for oxidation is not sufficiently clarified. For example, it was reported that the activity order for alkali salts was reversed by the reaction temperature [35]. Nevertheless, good correlations are obtained if the concept of surface- and bulk-type catalysis is appropriately considered [4, 36-38]. The reduction by H2 of free acid and group A salts proceeds both on the surface and in the... 

Fundamental correlations between redox properties and catalytic activity have successfully been established for the hydrogen form and alkali salts of 12-molybdophosphoric acid [1]. Provided that the contributions of surface- and bulk-type catalysis are properly taken into account, good monotonic relationships are obtained between the catalytic activity for oxidation and the reducibihty (or the oxidizing power) of the catalyst. The rate of oxidation of aldehydes, a surface-type reaction, correlates linearly with the surface reducibility of the catalyst, and the rate of oxidative dehydrogenation of cyclohexene, a bulk-type reaction, with the bulk reducibility [2]. 

While the acid strength of HPAs is high, they have a limitation in their use in catalysis, and this is due to their low surface area in the solid state (SlOm g- ), which corresponds to the external surface area of the crystal. When the reactants have a polar character, however, HPAs can take up polar molecules in amounts that correspond to more than 100 surface layer, and in this case their catalytic behaviour has been called bulk type catalysis [31"). Therefore, in the case of catalytic reactions involving polar molecules, they occur not only at the surface but also in the bulk solid of certain HPAs. The practical effect is that the catalytic system behaves like a highly concentrated solution, and this explains why these solids have been named pseudoliquids [31 j. Under the pseudoliquid conditions all acid sites are accessible to reactants, and the benefits of the system have been used commercially for reactions such as the hydration of propylene and n-butenc, separation of isobutene, and polymerization of tetrahydrofuran 20, 31". ... 

The bulk-type catalysis has been proved by several experiments such as i) a transient response analysis of the dehydration of 2-propanol, ii) a phase transition of the pseudo-liquid phase, and iii) the reactivity order of alcohols which was reversed depending on the partial pressure. Unusual pressure dependence as well as direct observation by MAS-NMR of reaction intermediates such as protonated alcohol and alkoxide have been reported for pseudo-liquid phase. ... 

Pseudoliquid and bulk type II behavior provide unique three-dimensional reaction environments for catalysis. 

Pseudoliquid-phase catalysis (bulk type I catalysis) was reported in 1979, and bulk type II behavior in 1983. In the 1980s, several new large-scale industrial processes started in Japan based on applications of heteropoly catalysts that had been described before (5, 6, 72) namely, oxidation of methacro-lein (1982), hydration of isobutylene (1984), hydration of n-butene (1985), and polymerization of tetrahydrofuran (1987). In addition, there are a few small- to medium-scale processes (9, 10). Thus the level of research activity in heteropoly catalysis is very high and growing rapidly. 

As will be described in more detail in later sections, in acid and oxidation catalysis by solid heteropoly compounds, that is, gas-solid and liquid-solid systems, there are three different classes of catalysis (1) surface catalysis, (2) bulk type 1 (pseudoliquid catalysis), and (3) bulk type II catalysis, as shown in Fig. 1. The latter two have been specifically demonstrated for heteropoly catalysts, and they could be found for other solid catalysts as well. 

Bulk type I catalysis was found in acid catalysis with the acid forms and some salts at relatively low temperatures. The reactant molecules are absorbed between the polyanions (not in a polyanion) in the ionic crystal by replacing water of crystallization or expanding the lattice, and reaction occurs there. The polyanion structure itself is usually intact. The solid behaves like a solution and the reaction medium is three-dimensional. This is called pseudoliquid catalysis (Sections l.A and VI). The reaction rate is proportional to the volume of the catalyst in the ideal case the rate of an acid-catalyzed reaction is proportional to the total number of acidic groups in the solid bulk. 

Bulk type II catalysis was discovered later for some oxidation reactions at high temperatures. Although the principal reaction may proceed on the surface, the whole solid bulk takes part in redox catalysis owing to the rapid migration into the bulk of redox carriers such as protons and electrons (Sections VII and IX). The rate is proportional to the volume of catalyst in the ideal case. 

These three classes of catalysis are distinctly different from each other in the ideal cases. But the extent of the contribution of the inner bulk of the catalyst depends on the rate of the catalytic reaction relative to the rate of diffusion of reactant and product molecules in bulk type I catalysis and on the rate of reaction relative to the rate of diffusion of redox carriers for the bulk type II catalysis. 

In this section, these influences will be described. Besides the acidic properties, the absorption properties of solid heteropolyacids for polar molecules are often critical in determining the catalytic function in pseudoliquid phase behavior. This is a new concept in heterogeneous catalysis by inorganic materials and is described separately in Section VI. With this behavior, reactions catalyzed by solid heteropoly compounds can be classified into three types surface type, bulk type I, and bulk type II (Sections VII and IX). Softness of the heteropolyanion is important for high catalytic activity, although the concept has not yet been sufficiently clarified. 

To understand oxidation catalysis by solid heteropoly compounds, the contrast between surface and bulk type II catalysis, and acid-redox bifunctionality... 

A. Concept of Surface and Bulk Type II Catalysis and Redox (Mars-Van Klevelen) Mechanism... 

A weak support effect, as shown in Fig. 59, is another indication of bulk type II behavior (327). With an increase in the loading of the heteropoly compound on the support, the rate of bulk type II catalysis increases to high loading levels, whereas the rate of surface catalysis shows saturation at relatively low loadings because of the decrease in the dispersion of the heteropoly compound on the support (327). 

Fig. 59. Catalytic oxidative dehydrogenation of cyclohexene (O, surface catalysis) and oxidation of acetaldehyde ( , bulk-type II) the catalyst was HjPMonO supported on Si02. Masses catalyst 0.2 g for cyclohexene and 0.1 g for acetaldehyde. (From Ref. 327.)...

Since the classification is essentially based on rates of catalytic reactions relative to rates of diffusion of redox carriers, there are oxidation reactions that are intermediate between the two limiting cases. We note that neither the molecular size nor the polarity of reactant molecules is the principal characteristic determining the type of catalysis. Although oxide ions migrate rapidly in the bulk, bulk type II catalysis is not observed for oxidation catalyzed by Bi-Mo oxides. In this case the rate-limiting step is a surface reaction. 

It is noteworthy that in the two industrial processes to produce methacrylic acid, both involving catalysis by H3PM012O40 and its alkali salts, one involves bulk type II catalysis and the other, surface type catalysis, as described in the following section. 

There are three prototypes of heterogeneous catalysis with heteropoly compounds as shown in Fig. 2 [4, 5]. Actual cases could be intermediate and vary by the kind of heteropoly compounds, reacting molecules, and reaction conditions. Ordinary heterogeneous catalysis is the surface type, where the catalytic reaction takes place on a two-dimensional surface. Bulk type I is the reaction in the pseudoliquid phase. The secondary structure (Fig. lb) of certain HPAs is flexible and polar molecules are readily absorbed in interstitial positions of the solid bulk to form the pseudoliquid phase. Bulk type II has been demonstrated for several catalytic oxidations at relatively high temperatures. The reaction fields for the bulk types are three-dimensional. 

For acid catalysis, the rates of bulk-type reactions show close correlations with the bulk acidity, while the catalytic activities for surface-type reactions are related to the surface acidity which is sensitive to the surface composition and often change randomly. Similarly, in the case of oxidation catalysis, good correlations exist between the oxidizing ability of catalyst and the catalytic activity for oxidation in both bulk-type and surface-type reactions. Acid and redox bifunctionality is another characteristic of HPAs. For example, the acidity and oxidizing ability work cooperatively for the oxidation of mcthacrolcin, whereas they function competitively for the oxidative dehydrogenation of isobutyric acid [5]. Interestingly, the former is of surface type and the latter of bulk type. 

Figure 2. Three types of heterogeneous catalysis for heteropoly compounds (a) surface type (b) bulk type I (pseudoliquid) (c) bulk type II.

It has been demonstrated that three different types of catalysis are possible for solid HPAs (211,212) (a) surface type, (6) pseudoliquid or bulk type (I), and (c) bulk type (II) catalysis (Fig. 19). In surface-type catalysis, the catalytic events occur, as for many other solid catalysts, on the outer surface and consequently, the reaction rate for acid-catalyzed reactions should be, in principle, proportional... 

The acid catalysis of heteropoly compounds in the solid state is classified into bulk-type and surface-type reations. The former type reactions proceed in the catalyst bulk and the latter only on the surface. Dehydration reactions of alcohols belong to the former and isomerization of butene to the latter. So the classification is closely related to the adsorption property of reactants. The activities for the surface-type reactions are more sensitive to pretreatment. 

Diffusivity and tortuosity affect resistance to diffusion caused by collision with other molecules (bulk diffusion) or by collision with the walls of the pore (Knudsen diffusion). Actual diffusivity in common porous catalysts is intermediate between the two types. Measurements and correlations of diffusivities of both types are Known. Diffusion is expressed per unit cross section and unit thickness of the pellet. Diffusion rate through the pellet then depends on the porosity d and a tortuosity faclor 1 that accounts for increased resistance of crooked and varied-diameter pores. Effective diffusion coefficient is D ff = Empirical porosities range from 0.3 to 0.7, tortuosities from 2 to 7. In the absence of other information, Satterfield Heterogeneous Catalysis in Practice, McGraw-HiU, 1991) recommends taking d = 0.5 and T = 4. In this area, clearly, precision is not a feature. 

There is not enough space here to give a detailed classification, but only to delineate the major families from which resins for industrial coatings may be selected. Resins may be divided into two groups according to their modes of film formation which may or may not involve a chemical reaction. In the first, the components must react together to form a crosslinked structure which may require heat, radiation or catalysis to effect the reaction. The bulk of resins used in industrial finishes are of this type. They are commonly referred to as chemically convertible or, simply, convertible. 

---