Heteropolytungstic acids

The polyacid structure (Fig. 18.5) consists of an array of globular Keggin ions linked by protonated water layers. The structure of this layer of high water content is not known accurately and only oxygen atoms have been localized. They show a large thermal motion which is likely to be associated with a high dynamic and/or static disorder. The O. .. O distances are close to 0.25-0.26 nm. The structure of the heteropolytungstic acid hexahydrate can be represented by the formula... 

Anandan S, Ryu S, Cho W, Yoon M (2003) Heteropolytungstic acid (H3PWi204o)-encapsu-lated into the titanium-exchanged HY (TiHY) zeolite a novel photocatalyst for photoreduction of methyl orange. J Mol Catal A 195 201-208... 

The kinetics of the catalytic oxidation of cyclopentene to glutaraldehyde by aqueous hydrogen peroxide and tungstic acid have been studied and a compatible mechanism was proposed, which proceeds via cyclopentene oxide and /3-hydroxycyclopentenyl hydroperoxide. " Monosubstituted heteropolytungstate-catalysed oxidation of alkenes by t-butyl hydroperoxide, iodosobenzene, and dioxygen have been studied a radical mechanism was proved for the reaction of alkenes with t-BuOOH and O2, but alkene epoxidation by iodosobenzene proceeds via oxidant coordination to the catalyst and has a heterolytic mechanism. ... 

Hexacyanoferrates were immobilized on Au covered with SAM of 3,3 -thiodipropionic acid [86]. It has been found from voltammetric studies that the surface coverage of hexacyanoferrate is close to one monolayer and such an electrode exhibits very good surface redox behavior. Cheng et al. [87] have described the formation of an extremely thin multilayer film of polybasic lanthanide heteropolytungstate-molybdate complex and cationic polymer of quaternary poly(4-vinylpyridine), partially complexed with osmium bis(2,2 -bipyridine) on a gold electrode precoated with a cysteamine SAM. Consequently, adsorption of inorganic species might also be related to the properties of SAMs. This problem will be discussed in detail in a separate section later. 

Several other heteropolytungstates are known or presumed to have the Keggin structure (Table 6). Two 12-tungstoborates have been known for over a century, but these prove not to be simple a or (S isomers. The anion a -[BWi2O40]5 " seems to be an undoubted Keggin species based on isomorphism, vibrational spectrum and its behavior upon reduction. Tlie second isomer ( h ), crystallizing as the acid in hexagonal crystals, clearly is not a /3 isomer for its... 

The crystalline free acids and salts of heteropolymolybdates and heteropolytungstate anions are almost always highly hydrated. A given acid or salt will often form several solid hydrates. 

The Series A 12-acids and their salts are the best known of all heteropolymolybdates and heteropolytungstates and the only compounds of this type used thus far on a large commercial scale. 

The 6-heteropolymolybdates and 6-heteropolytungstates of tellurium are prepared by acidifying solutions containing a tellurate and a molybdate or tungstate, or by adding a base to a mixture of telluric and molybdic acids. 

In general, 12-heteropoly anions of Si+4 and P+s are more readily reduced than those of Ge+4. Furthermore, heteropolymolybdates are more readily reduced than corresponding heteropolytungstates. For example, in the phosphorus acids, the oxidizing power decreases in the order ... 

The 9-heteropolymolybdates and 9-heteropolytungstates are prepared at higher temperatures, higher concentrations and under a slightly less acid condition than the 12-anions7 23,21 28 The range of conditions necessary for formation is narrower than for other species. Once formed, however, the 1 9 anions remain undecomposed under conditions in which they would not form. 

A large number of other heteropolymolybdates or heteropolytungstates has been reported in the literature8,24 These are frequently single compounds and not members of any series. In many cases, the compounds are probably definite double salts or acid salts. Further work would be necessary to clarify the nature of these systems. 

H) The Wi20 o ligand in Keggin heteropolytungstate anions requires the presence of two non-acidic interior hydrogen atoms for stability, in the absence of the heteroatom (metatungstate, Section 2,3. the molybdenum analogue is unknown). 

When sodium tungstate is boiled with an excess of phosphoric acid the main heteropolytungstate products are two isomers of the Wells-Dawson anions and a smaller amount of another species (47). Crystallographic analysis of this species showed the composition of this anion to be [NaP5W3oOiio] ( P5W30, (47)) with the structure shown in Figure 39. ... 

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... 

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