Bulk type II catalysis

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 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. 

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). 

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. 

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... 

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. 

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. 

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.)...

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. 

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

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. ... 

Selectivity in oxidation catalysis has been reviewed for conventional catalysts used for the production of bulk chemicals and epoxidations. The point of activation of the substrate is identified as a key factor identifying three mechanistic features. These are (i) activation of the weakest C-H bond in a substrate, (ii) activation of the strongest C-H bond and (iii) electrophilic attack in olefins. Key features of each type of reaction are identified and new catalyst types needed to break through existing selectivity barriers are discussed. 

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