Polyoxometalate catalysis

Harrup MK, Hill CL (1994) Polyoxometalate catalysis ofthe aerobic oxidation of hydrogen sulfide to sulfur. Inoig Chem 33 5448-5455... 

Nakato T, Toyoshi Y, Kimura M, Okuhara T (1999) Unique catalysis of an acidic salt of heteropolyacid and consisting of microciystallites. Catal Today 52 23-28 Rong C, Anson FC (1994) Simplified preparations and electrochemical behavior of two chromium-substituted heteropolytungstate anions. Inorg Chem 33 1064-1070 Harrup MK, Hill CL (1994) polyoxometalate catalysis of the aerobic oxidation of hydrogen-sulfide to sulfur. Inorg Chem 33 5448-5455... 

Catalysts for Fine Chemical Synthesis Volume 2 Catalysis by Polyoxometalates... 

The versatility and accessibility of polyoxometalates (POMs) have led to the various applications in the fields of structural chemistry, analytical chemistry, surface science, medicine, electrochemistry, photochemistry, and catalysis. Especially, POMs have received much attention in the area of oxidation and acid catalysis [25-31]. Several categories of POMs are formed by proper selection of the starting components and by the adjustment of pH and temperature. Typical examples are shown in Figure 13.1 (i) isopolyoxometalates of the general formula, MxO> 1, produced by condensation... 

Liquid-phase oxidations catalyzed by polyoxometalates are covered by Mizuno, Kamata, and Yamaguchi (Chapter 13). Finally, the developing field of enantioselective heterogeneous catalysis is presented by Coman, Poncelet, and Parvulescu (Chapter 14). 

Supported Polyoxometalate-Based Heterogeneous Catalysis for Liquid Phase Selective Oxidations... 

Supported Polyoxometalate-Based Heterogeneous Catalysis for Liquid. 

Acknowledgments. Support for the research was provided by the National Science Foundation (oxometal complexes and hydrocarbon chemistry), the U.S. Army Research Office (decontamination catalysis and derivatized polyoxometalates). 

Subsequently, Backvall and coworkers developed triple-catalysis systems to enable the use of dioxygen as the stoichiometric oxidant (Scheme 3) [30-32]. Macrocyclic metal complexes (Chart 1) serve as cocatalysts to mediate the dioxygen-coupled oxidation of hydroquinone. Polyoxometallates have also been used as cocatalysts [33]. The researchers propose that the cocatalyst/BQ systems are effective because certain thermodynamically favored redox reactions between reagents in solution (including the reaction of Pd° with O2) possess high kinetic barriers, and the cocatalytic mixture exhibits highly selective kinetic control for the redox couples shown in Scheme 3 [27]. 

The interest in the redox, catalytic, and electrocatalytic properties of unsubstituted and substituted polyoxometalates arouses much attention [2-15] because they are a versatile family of molecular metal-oxide clusters with applications in catalysis as well as in medicine and material science. Such versatility must be traced to at least two main characteristics. First, the size and mass of these unique molecular oxides place their solution chemistry in an intermediate position between small molecule solution chemistry and infinite lattice solid-state chemistry. Second, their redox behaviors may be very flexible and finely tuned on purpose, by changing smoothly their composition, with a... 

Heteropolyanions and isopolyanions are polymeric oxoanions (polyoxometalates) (2, 3, 5, 6). The structure of a heteropolyanion or polyoxoanion molecule itself is called a primary structure (5, 6, 77). There are various kinds of polyoxoanion structure (Section II.A. 1). In solution, heteropoly anions are present in the unit of the primary structure, being coordinated with solvent molecules and/ or protonated. Most heteropolyanions tend to hydrolyze readily at high pH (Section 1I.C). Protonation and hydrolysis of the primary structure may be major structural concerns in solution catalysis. Heteropoly compounds in the solid state are ionic crystals (sometimes amorphous) consisting of large polyanions, cations, water of crystallization, and other molecules. This three-dimensional arrangement is called the secondary structure. For understanding catalysis by solid heteropoly compounds, it is important to distinguish between the primary structure and the secondary structure (5, 6, 17). Recently, it has been realized that, in addition... 

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