Heteropoly compounds heterogeneous

I4M. Misono, Heterogeneous Catalysis by Heteropoly Compounds of Molybdenum and Tungsten, Catalysis Reviews, 29,269-321 (1987). 

Heterogenous reactions, Sh/Nu ratio, 27 64 Heteroligand complex, 32 260-262 Heteropolyacids defined, 41 117 heteroatoms, 41 118, 120, 121 Prins reaction, 41 156 supported, 41 149-150 Heteropolyanions, 41 113, 117, 119-121 Heteropoly blues, 41 191 Heteropoly compounds absorption, 41 179-180, 190-191 acid-catalyzed reactions heterogeneous, 41 161-178 liquid phase, 41 150-161 acidic properties in solid state, 41 141-150 in solution, 41 139—14] catalysis, 41 114, 116-117, 190-191 as catalyst, 41 113-116, 117, 223-232... 

Early attempts to use heteropoly compounds as catalysts are summarized in reviews published in 1952 (//) and 1978 (7). The first industrial process using a heteropoly catalyst was started up in 1972 for the hydration of propylene in the liquid phase. The essential role of the Keggin structure in a solid heteropoly catalyst was explicitly shown in 1975 in a patent concerning catalytic oxidation of methacrolein. Systematic research in heterogeneous catalysis with these materials started in the mid-1970s and led to the recognition of quantitative relationships between the acid or redox properties and catalytic performance... 

One of the authors of this chapter has previously reviewed heterogeneous catalysis by heteropoly compounds (4-6). Catalysis in solution has also been described (7-10). In this chapter, we critically survey the literature and attempt to describe the essence of the catalytic chemistry of heteropoly compounds in solution and in the solid state. We have attempted to highlight the advantages of heteropoly catalysts as described in Table I. 

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. 

Heterogeneous Oxidation Reactions Catalyzed by Heteropoly Compounds... 

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... 

Soeda et al. (358) used Pd-supported heteropoly compounds for a heterogeneous Wacker-type reaction and found that Pd/Cs2 5H0 5PW6Mo6C)4o was active for oxidation of cyclohexene to produce cyclohexanone and cyclohex-enone. The active sites are assumed to be Pd2+ and Pd° for the two products, respectively. Homogeneous Wacker-type reactions are described in Section XI. 

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.

Keggin-type heteropoly compounds have attractive and important characteristics in terms of catalysis. They consist of heteropolyanions and counter-cations such as H, Cs or NHT When the counter-cations are protons, they are called heteropolyacids (HPA). An important characteristic of HPAs, such as 12-tungstophos-phoric acid (H3PW12O40), is the presence of very strong Bronsted acid sites. But the characteristics of HPAs strongly depend on temperature and relative humidity. When they are used in heterogeneous catalysis, it is often necessary to support them on high-surface-area oxides or activated carbons, in order to increase the surface contact with the reactants. 

M. Misono, Heterogeneous catalysis by heteropoly compounds of molybdenum and tungsten, Catal. 

Misono, M., Mizuno, N., Katamura, K., Kasai, A., Konishi, Y., Sakata, K., Okuhara, T., Yoneda, Y. 1982. Catalysis by heteropoly compounds. HI. The structure and properties of 12-heteropoly acids of molybdenum and tungsten and their salts pertinent to heterogeneous catalysis. Bulletin of the Chemical Society of Japan 55(2) 400-406. 

Mizuno N, Misono M (1998) Heterogeneous eatalysis. Chem Rev 98 199-218 Briand LE, Baronelti LE, Thomas HJ (2003) The state ofthe artonWells-Dawson heteropoly-compounds a review of their properties and applications. J Mol Catal A Gen 256 37-50 Hemender K, Reddy A, Pandurange Reddy V, Veeragaiah V (1992) Versatile synthesis of 6-alkyl/aryl-l/7-pyrazolo[3,4-d]pyrimidin-4[5//]-ones. Ind J Chem 316 163 166 Misono M, Okuhara T (1993) Solid superacid catalysts. Chem Tech 23(1I) 23 29 Kozhevnikov IV (1987) Advances in catalysis by heteropolyacids. Russ Chem Rev 56 811-825 Izumi Y, Urabe K, Onaka M (1992) Zeolite, elay and heteropoly acid in organic reaction, vol 99. Kodansha, Tokyo... 

Bauer Jr, W. (2002). Ullmann s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim. Misono, M. (1987). Heterogeneous Catalysis hy Heteropoly Compounds of Molybdenum and Tungsten, Catal. Rev., 29, pp. 269-321. 

A variety of polyanion structures are known. For example, the structure of the polyanion of so-called Keggin structure is shown in Fig. 3.74a. The heteropoly compounds having the Keggin structure are thermally more stable and rather e y obtained, so that investigations have been devoted mostly to this group. They are used as acid catalysts as well as oxidation catalysts in both hereto- and homogenous systems (cf. comprehensive review on heterogenous catalysis ). The principal merits of heteropoly compounds when they are used as solid acid catalysts are as follows. ... 

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