Keggin-type anion

Venturello and co-workers reported that in the presence of aqueous H2O2 a phosphotungstate of the Keggin type gives phosphotungstate anions possessing four W atoms, each one displaying 2 peroxo moieties. 

According to Mioc et al. [13], Keggin s anions are transformed at about 600 °C in a new monophosphate bronze type compound PWgOae. The TG diagram shows that the decomposition takes place without appreciable weight loss. 

In recent years heteropolycompounds have been studied for the oxidation of propane to acrylic acid and of isobutane to methacrylie aeid. Rohm Haas Company was the first in 1981 to claim the one-step oxidation of isobutane to methacrolein and methacrylie acid (55). Even though no reference is made to heteropolycompounds, the claimed catalyst compositions correspond to Keggin-type structures. In the patents later issued by Sumitomo (56,57) an important role was claimed to be played by vanadium (in an anionic position), by cesium (in a cationic position), as well as by an excess of phosphorus with respect to the stoichiometric composition. These catalysts gave selectivities to methacrylie acid plus methacrolein close to 70 %, with isobutane conversions in the 10 to 13 % range. Besides carbon oxides, acetic acid was the main by-product. 

Some new structural polyoxometalates were obtained, which are listed in the Table 1. In Table 1, the compound [Bu4N]6[H30]2[Moi304o]2 synthesized by Professor K.L. Tang was a new Keggin-type isopolyoxomolybdate, in which, the [BuaN] cations, [H3O] cations, [Moi304o] anions have different symmetry. Both anions have Keggin-type structures. The structures are based on a central Mo(A)04 slightly distorted tetrahedron surrounded by 12 Mo(B)Oe octahedra. 

Figure 2e. A shoulder at 932 cm is attributed to either monotungstate anions or Keggin-type aluminotimgstic species deposited on alumina surface... 

Keggin type heteropoly anions and related species... 

The study of such reaction types will be a challenge for the future. In any case, one example for a growth process according to this model scheme has been discovered A cluster anion containing 37 molybdenum atoms ([Hi4Mo370ii2] ) is formed via intermediates [27], one of which, the cluster of the type (containing an e-Keggin-type nucleus capped by four... 

Heteropoly Anions with Keggin-Type Structures... 

Prolonged standing of solutions of 1 12 anions (Keggin type) leads to the formation of 2 18 anions (Dawson type) (5.366). This process is accelerated by boiling or by adding phosphoric acid. 

Keggin-type structure [17]. The infrared signals at 569, 871, and 994 cirr indicate that the heteropoly anion collapses completely towards M0O3 at 873 K. 

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