Keggin-type compound

The two different mechanisms which have been proposed for the oxidation of isobutane over i) Mo-containing Keggin-type compounds and ii) W-containing Keggin-type and Wells-... 

The kinetics of adsorption and the amount of polar (alcohols) and nonpolar molecules, Keggin type compounds, was extensively studied through IR spectroscopy and thermogravimetric analysis [16,31,32,34,35]. Misono and coworkers [31] established that the rate of alcohol adsorption depends on the size of the probe molecule, and that the amount of adsorbed molecules in H3PW12O40 is an integral multiple of the number of protons. The observation of the diffusion coefficients and adsorption of polar molecules into the bulk structure of the HPAs proved unequivocally the pseudo-liquid phase behavior of those compounds. 

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). 

Niu, J., Zhao, J. Wang, J. et al. (2004) Synthesis, property and crystal structure of a novel two-dimensional network organic-inorganic hybrid compound based on the neodymium center and Keggin-type heteropolyanion of [a- N Journal of Molecular Structure, 699, 85-92. 

Keggin-type heteropoly compounds have attractive and important characteristics in terms of catalysis. They consist of heteropolyanions and counter-cations such as H, Cs or NHT When the counter-cations are protons, they are called heteropolyacids (HPA). An important characteristic of HPAs, such as 12-tungstophos-phoric acid (H3PW12O40), is the presence of very strong Bronsted acid sites. But the characteristics of HPAs strongly depend on temperature and relative humidity. When they are used in heterogeneous catalysis, it is often necessary to support them on high-surface-area oxides or activated carbons, in order to increase the surface contact with the reactants. 

A. Yoshida, S. Hikichi, N. Mizuno, Acid-base catalyses by dimeric disilicoicosatungstates and divacant y-Keggin-type silicodecatungstate parent Reactivity of the polyoxometalate compounds controlled by step-by-step protonation of lacunary W=O sites, J. Organomet. Chem. 692 (2007) 455. 

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. 

The development of catalysts obtained by means of heteropolyacids (HPA) and related compounds is a very important growing field. Since HPA are less corrosive and produce lower amount of wastes than conventional acid catalysts, they can be used as replacement in ecofriendly processes. Within the HPA, there is a special interest in those which present a Keggin type structure. 

This review analyzes the properties which are necessary for heterogeneous catalysts to promote the oxyfunctionalization of light paraffins to valuable chemicals. Three catalytic systems are discussed i) vanadium/phosphorus mixed oxide, the industrial catalyst for the oxidation of n-butane to maleic anhydride, which is here also examined for reactions aimed at the transformation of other hydrocarbons ii) Keggin-type heteropolycompounds, which are claimed for the oxidation of propane and isobutane, whose composition can be tuned in order to direct the reaction either to the formation of olefins or to the formation of oxygenated compounds iii) rutile-based mixed oxides, where rutile can act as the matrix for hosting transition metal ions or favour the dispersion of other metal oxides, thus promoting the different role of the various elements in the formation of acrylonitrile from propane. 

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. 

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