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Heteropoly compounds characterization

Keggin-type heteropoly compounds having Mo and V as addenda atoms are usually used for such oxidations. The catalysts reported in patents often contain several elements other than Mo, V, and P. An excess amount of P is added to stabilize the structure, and the presence of additional transition elements like Cu improves redox reversibility. Supported heteropoly catalysts are also important for industrial applications and have been characterized (69, 325, 326). [Pg.210]

Okuhara, Mizuno, and Misono report the catalytic properties of heteropoly compounds as exemplified by H,PWl3O40 and the anion [PW,2O40p. Some of these compounds are strongly acidic, and some have redox properties the large-scale applications involve acid-catalyzed reactions. The heteropoly compounds are metal oxide clusters, used as both soluble and solid catalysts. Their molecular character provides excellent opportunities for incisive structural characterization and for tailoring of the catalytic properties. Physical properties also affect catalytic performance. Catalysis sometimes occurs on the surface of the solid material, and sometimes it occurs in the swellable bulk. [Pg.446]

POMs are promising catalysts for acid, redox and bifunctional catalysis. In many structures, the transition metal addenda atoms such as Mo or W exist in two oxidation states, which results in different redox properties as determined by polarog-raphy. The exceptional ability of heteropolyanions to act as electron reservoirs has been demonstrated by the preparation and characterization of numerous reduced derivatives [32]. They also exhibit high solubility in polar solvents, which means that they can be used in homogeneous catalysis. The wide range of applications of heteropoly compounds are based on their unique properties which include size, mass, electron and proton transfer (and hence storage) abilities, thermal stability. [Pg.567]

The activity toward isopropanol dehydration is also used to characterize the acidity of HPA compounds since the product distribution depends on the nature of surface active sites (and bulk active sites considering the pseudo-liquid behavior of heteropoly compounds). Strong surface Brpnsted (H+) and Lewis acid sites catalyze the dehydration of isopropanol to propylene (di-isopropyl ether for weak Lewis acid sites) and redox/basic sites lead to the dehydrogenation of the alcohol to acetone. [Pg.360]

Heteropoly compounds are prepared in several ways. Solids are obtained by either precipitation, recrystallization or drying depending on the structure and composition. Caution must be used during preparation processes for hydrolytic decomposition of polyanions and nonhomogeneity of the metal cation to polyanion ratio in the precipitates. More elaborate preparation and characterization are necessary for the preparation of polyanions with mixed addenda atoms. [Pg.165]

Polyoxometalates are unique in their topological and electronic versatility and are important in analytical chemistry, catalysis and photocatalysis, hiochemistry, medicine and solid-state devices. " Thus, new classes of structures with often-unexpected reactivities and applications are still being characterized. A number of heteropolymolybdate and heteropoly tungstate anions, including more than 70 different heteroelements, are known. They exhibit dozens of stoichiometries and structures. Most of the known polyoxometalates are prepared in solution but a few compounds have been prepared by high-temperature solid-state reactions. " ... [Pg.217]

The first example of isomerism in the molybdic heteropoly acids was described by STRICKLAND for silicomolybdic acid. In order to compare the chemical properties of the isomers by polarography, the use of a dropping mercury electrode appeared inadequate, since mercury reacts with these compounds. A platinum electrode gives reproducible polarograms displaying perfect reversibility which makes possible the study and characterization of these compounds and of their reduction-products. [Pg.394]


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Heteropoly compounds

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