Pseudoliquid 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 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. 

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. 

In ordinary heterogeneous catalysis of gas-solid and liquid-solid reactions, the reactions take place on the two-dimensional surfaces of solid catalysts (both on the outer surface and on the surfaces of pore walls). In contrast, the reactions of polar molecules in the presence of heteropoly catalysts often proceed not only on the surface but also in the bulk phase. We call this pseudoliquid phase behavior. The pseudoliquid phase is a unique reaction medium consisting of the three-dimensional solid bulk, as was first proposed in 1979 (17, 233, 234). 

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.

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". ... 

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... 

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