Addenda atoms

These two important properties for catalysis can be controlled by choosing appropriate constituent elements (type of polyanion, addenda atom, heteroatom, countercation, etc.). 

Nonoxygen elements in the inner part of polyanions (usually P, Si, As, etc.) are called heteroatoms (in some cases, central atoms) and those in the peripheral part (usually Mo, W, V, Nb, etc.) are called addenda atoms or polyatoms (Section II.A). We use more or less conventional terminology here. 

Mixed addenda heteropolyanions with regiospecific substitution need careful preparation by use of lacunary heteropolyanions. If they are prepared from aqueous solutions of corresponding oxoanions, the products are usually mixtures of heteropolyanions having different compositions of addenda atoms. General procedures for the syntheses of various kinds of heteropolyacids are described in the literature (51-54). 

The extent of reduction of H3PM012O40 during the oxidation of methacrolein has also been investigated by application of ESR spectroscopy for detection of Mo5+ (103). The states of V in the mixed-valence Keggin anion and Cu countercation were also investigated. The results show that more reducible countercations or addenda atoms such as Cu2+ and V5+ are reduced first, and an electron is localized on them (104-106). 

The mixed-addenda atoms affect the redox properties mixed-addenda heteropoly compounds are used as industrial oxidation catalysts. For example, the rate of reduction by H2 is slower and less reversible for solid PMO 2-,VJto m+, than for solid PM012O40, although the former are stronger oxidants than the latter in solution (279, 280). The effects of substituting V for Mo on the catalytic activity are controversial (279, 281-284). Differences in redox processes between solutions and solids, the thermal or chemical stability of the heteropoly compounds, and the effects of countercations in solids have been suggested to account for the discrepancies. 

Co2 +-Substitution at the addenda atoms gives catalysts for the epoxidation of olefins in the presence of aldehyde [293). PWM-Co is the most active among the mono-transition-metal-substituted polyanions the order of activity is PWn-Co > -Mn 2= -Fe 2= -Cu > -Ni. Here, PWll(M + )0(379", ) (M = Co2 +, Cu2+, Fe3 +, Ni2 +, Mn2 + ) is denoted by PWn M. The same order was observed for the oxidation of isobutyraldehyde, suggesting that the oxidation of aldehyde to give peracid is an important step in the reaction. It has been reported that substitution of V5+ for Mo6+ in PMo O3 gives a good catalyst for epoxidation and the Baeyer-Villiger reaction [294). 

Fig. 56. Effects of addenda atoms on the catalytic oxidation of cyclopentene by H202 at 303 K.. ( ) H3PMoi2O40 + H3PW 2O40. Reaction time, 3 h. (From Ref. 304.)...

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). 

Isopolyoxometalates and heteropolyoxometalates are formulated as [MmOy]p and [X MmOy] (xc to), respectively, where M is the addenda atom and X is the heteroatom. The most common addenda atoms are the d°-early-transition-metal cations such as W6 +, Mo6+ and Vs+. The heteroatoms can be p-, d- or f-block elements such as P5 +, As5 +, Si4 +, Ge4 + and B3 +. Among a wide variety of heteropolyoxometalates, the Keggin structures are the most stable and more easily available. Keggin anions, typically represented by the formula [X + M O ] 8- , contain one central heteroatom and twelve addenda atoms (four M3013 triads). 

The lacunary POMs can be obtained by the removal of one or more addenda atoms from fully occupied POMs. The most stable lacunary compounds are obtained with Si4 + as the heteroatom. While the dodecatungstosilicates are stable in acidic solution, hydrolytic cleavages ofW—O bonds occur and well-defined lacunary POMs with eleven, ten and nine tungsten atoms are produced when the pH increases. 

Table 1. Known addenda atoms ( ) and heteroatoms (O) incorporated in heteropolyanions.

The typical method for the preparation of heteropolyacids is the acidification of aqueous solutions of oxo-anions of addenda atoms and heteroatoms [lb] ... 

Lacunary Keggin anions, a-XMnO, have a defect Keggin structure, in which one addenda atom and one terminal oxygen are missing. These lacunary hetero-... 

Heteropolyanions with W or Mo atoms as a polyatom (addenda atom) are fisted in Table 9. The thermal stability increases in the sequence of H4SiMoi2O40 < H3PM012O40 < H4SiWi2O40 < H3PW12O40, where H3PW12O40 decomposes when heated higher than 673 K. ... 

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. 

NMR has also been widely used for structural elucidation of vanadium-containing mixed addenda heteropoly compounds, owing to the large natural abundance of nuclei. The number of structural isomers increases with the increase in the number of addenda atoms and their position in the Keggin structure is confirmed by NMR spectroscopy [138]. 

Polyoxometalates encompass complexes known historically as heteropoly- and isopolyanions (or acids). Other labels, e.g., heteropoly electrolytes, metal-oxygen cluster anions, have also been applied from time to time. The present chapter will discuss only those complexes containing V, Nb, Ta, Mo, and/or W as the major component (the addenda atoms). The relatively small number of polyoxochromates is excluded, since Crvl is restricted to tetrahedral four-coordination by oxide, and these complexes are if anything more closely related to the nonmetal polyoxoanions like sulfates. 

Several salts of Keggin anions, especially those with mixed addenda (Mo, V) or capped species, crystallize with the anion on an inversion center, i.e., with the central (tetrahedral) atom appearing to be eight-coordinate. This type of disorder, unless carefully analyzed,132 leads to erroneous conclusions regarding bond lengths and coordination environments of the addenda atoms. 

Figure 2 Cation-polyanion interactions and loci of reactivity in a typical POM. The black addenda atom (frequently a J-electron-containing metal) on the right of the POM has the terminal H20 ligand.

When polycondensation of these addenda atoms is carried out in the presence of another oxoanion containing an heteroatom X, an Heteropolyanion is obtained, formulated as [XxMmOy] l , X belongs usually to the main group elements (B, Si, Ge, P, As,. ..) but can also be a metal. 

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