Pseudoliquid phase heteropoly compounds

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. 

The reaction of tert-butyl alcohol and methanol to form MTBE is also catalyzed by heteropoly compounds (221-223). A relationship was found between the amount of pyridine sorbed in or on heteropoly compounds and tert-butyl alcohol conversion (221). The dependence of the rate on methanol partial pressure resembles that for the absorption of methanol in the bulk, suggesting pseudoliquid phase behavior (223). 

As described above, heterogeneous catalytic reactions on heteropoly compounds are classified into three different types, surface, bulk type I (pseudoliquid phase), and bulk type II (Fig. 1). The surface reactions are typical of... 

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. 

Owing to the flexible nature of the secondary structure of the acid forms and group A salts, polar molecules such as alcohols and amines are readily absorbed into the solid bulk by substituting water mol ciiles and/or by expanding the interdistance between polyanions. Heteropoly acids which have absorbed a significant amount of polar molecules resemble in a sense a concentrated solution and are in a state between solid and solution. Therefore, this state is called the pseudo-liquid phase . Some reactions proceed mainly in this bulk phase. The tendency to form a pseudoliquid phase depends on the kind of heteropoly compound and the molecules to be absorbed and on the reaction conditions. 

---