Heteropoly ions

Tetrahedral Octahedral Icosahedral PW12O40 etc. TeMo O n MnMo OaV MnNb 12O3I CeMoi2042 PaWisO J H4C02M010O38 

The PW12O40 ion. An X-ray study of the pentahydrate of 12-phosphotungstic acid showed that the complex ion has the structure shown in Fig. 11.6(b). The five water molecules are arranged between these large negative ions so that the 

6 (a) Arrangement of one group of three WOe octahedra relative to the central PO4 

It is noted in MSIC (pp. 90, 144) that there are three ways of joining together four edge-sharing groups of 3 octahedra (as in Fig. 11.5(b)) to form complexes with cubic symmetry  

Shape of central cavity Octahedral Tetrahedral Tetrahedral 

---

Another example of steric selectivity involves the homopoly and heteropoly ions of molybdenum, tungsten, etc. Each molybdenum(VI) and tungsten(VI) ion is octahedraHy coordinated to six oxygen (0x0) ligands. Chromium (VT) is too small and forms only the weU-known chromate-type species having four 0x0 ligands. The abiUty of other cations to participate in stable heteropoly ion formation is also size related. 

These appHcations are mosdy examples of homogeneous catalysis. Coordination catalysts that are attached to polymers via phosphine, siloxy, or other side chains have also shown promise. The catalytic specificity is often modified by such immobilization. Metal enzymes are, from this point of view, anchored coordination catalysts immobilized by the protein chains. Even multistep syntheses are possible using alternating catalysts along polymer chains. Other polynuclear coordination species, such as the homopoly and heteropoly ions, also have appHcations in reaction catalysis. 

Various kinds of oxide materials, including single oxides, mixed oxides, molybdates, heteropoly-ions, clays, and zeolites, are used in catalysis they can be amorphous or crystalline, acid or basic. Furthermore the oxides can be the actual catalysts or they can act as supports on which the active catalysts have been deposited. Silica and alumina are commonly used to support both metals and other metal oxide species. Amorphous silica/alumina is a solid acid catalyst, it is also used as a support for metals, when bifunctional (acid and metal) catalysis is required, e.g., in the cracking of hydrocarbons. Other acid catalysts are those obtained by the deposition of a soluble acid on an inert support, such as phosphoric acid on silica (SPA, used in the alkylation of benzene to cumene. Section 5.2.3). They show similar properties to those of the soluble parent acids, while allowing easier handling and fixed bed operation in commercial units. 

In contrast, hexavalent transition metals, such as molybdenum and tungsten, polymerize in acid by aggregation to specifically favored geometries containing oxide octahedra of MoOg and W06 (the iso poly acids) (5a). These oxide structures have cavities which accept foreign ions, to form heteropoly ions (5b). The central cavity is a tetrahedral site, often occupied by P0, Si0, etc, a fact utilized in phosphate precipitation ... 

Post-column generation and detection of the AsO molybdate heteropoly ion Use of Ag nanostructured multilayer films LOD = 5 pg/L Simultaneous determination of six As ions LOD < 4 pg/L... 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

It is shown that both Sb(III) and Bi(III) can speed reduction of 12-molybdophosphate (12-MPC) to the corresponding heteropoly blue (12-MPB) by ascorbic acid (AA). It is found that mixed polyoxometalates can be formed in solution which reduce considerably more rapidly than 12-MPC. Complete formation of mixed POM is observed only if significant excess of Me(III) ions is used in the reaction. POM responsible for blue color was synthesized by selective extraction. Chemical analysis of tetrabutyl-ammonium salt is in accordance with formula of (TBAl PMeflllfMo O j (Me = Sb, Bi). IR spectmm of mixed POM is identical to 12-MPC. 

In 1826 J. J. Berzelius found that acidification of solutions containing both molybdate and phosphate produced a yellow crystalline precipitate. This was the first example of a heteropolyanion and it actually contains the phos-phomolybdate ion, [PMoi204o] , which can be used in the quantitative estimation of phosphate. Since its discovery a host of other heteropolyanions have been prepared, mostly with molybdenum and tungsten but with more than 50 different heteroatoms, which include many non-metals and most transition metals — often in more than one oxidation state. Unless the heteroatom contributes to the colour, the heteropoly-molybdates and -tungstates are generally of varying shades of yellow. The free acids and the salts of small cations are extremely soluble in water but the salts of large cations such as Cs, Ba" and Pb" are usually insoluble. The solid salts are noticeably more stable thermally than are the salts of isopolyanions. Heteropoly compounds have been applied extensively as catalysts in the petrochemicals industry, as precipitants for numerous dyes with which they form lakes and, in the case of the Mo compounds, as flame retardants. 

A. Molybdenum blue method Discussion. Orthophosphate and molybdate ions condense in acidic solution to give molybdophosphoric acid (phosphomolybdic acid), which upon selective reduction (say, with hydrazinium sulphate) produces a blue colour, due to molybdenum blue of uncertain composition. The intensity of the blue colour is proportional to the amount of phosphate initially incorporated in the heteropoly acid. If the acidity at the time of reduction is 0.5M in sulphuric acid and hydrazinium sulphate is the reductant, the resulting blue complex exhibits maximum absorption at 820-830 nm. 

Previously, Osaka and coworkers [29-31] employed ion-transfer voltammetry to determine the standard ion-transfer potentials (Aq 4> ) of heteropoly- and isopolyoxome-talate anions (in short, polyanions) at the nitrobenzene (NB)/W and 1,2-dichloroethane (1,2-DCE)/W interfaces is directly related to the transfer energy by... 

A standard Lowry-based protein assay has been adjusted to the special conditions encountered with skin [126], Basically, proteins reduce an alkaline solution of Cu(II)-tartrate to Cu(I) in a concentration-dependent manner. Then, the formation of a blue complex between Folin-Ciocalteau reagent (a solution of complex polymeric ions formed from phosphomolybdic and phosphotungstic heteropoly acids) and Cu(I) can be measured spectrophotometrically at 750 nm. A calibration curve can be obtained by dissolving known amounts of stratum corneum in 1 M sodium hydroxide. A piece of tape that has not been in contact with skin is subjected to an identical procedure and serves as negative control. The method was recently adapted to a 96-well plate format, notably reducing analysis times [129],... 

Acidity, 27 284, 285 catalytic performance, 30 121 crystalline titanium silicates, 41 319-320 estimating, 37 166 heteropoly compounds, 41 139-150 ion exchange and, zeolites, 31 5-6 sulfate-supported metal oxides, 37 186-187 surface, monolayer dispersion, 37 34-35 tin-antimony oxide, 30 114-115, 125-1256 Acids, see also specific compounds adsorption of, on oxide surfaces, 25 243-245... 

Dealkylation, heteropoly compunds, 41 160-161, 170-174 Deamination, 27 259 Pd(NH3>/ ions, 39 142 Debye-Waller factor, 21 177 Decahydronaphthalenes conformation of, 18 17-19 isomeric, formation of in hydrogenation of naphthalenes, 18 23-20 rrans-Decalin... 

Under acid conditions, molybdate reacts with orthophosphate, P04 to form a blue heteropoly acid, molybdophosphoric acid. A similar reaction occurs with arsenate ion, As04. In the presence of vanadium, the product is yellow vanadomolybdophosphoric acid. These reactions are used for colorimetric analyses of phosphate, arsenate, and many other substances. Colloidal molybdenum blue has limited apphcations such as dyeing silk. It readily absorbs onto surface-active materials. 

In our experiments we screened zeolites, ion-exchange resins, heteropoly compounds and mixed metal oxides. Several alcohols were used to show the range of applicability. The selectivity was assessed by testing the formation of side products in a suspension of catalyst in alcohol (e.g. SZ in 2-ethylhexanol) under reflux for 24 hours. Under the reaction conditions, no by-products were detected by GC analysis. 

---