Keggin-type heteropoly 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). 

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. 

Rafiee, E. Fakhri, A. Joshaghani, M. Coumarins Facile and expeditious synthesis via Keggin-type heteropoly compounds under solvent-free... 

A variety of methods to study the acidity of heteropoly compounds are reported in the literature in line with the study of the catalytic applications of the HPAs. These methods, mainly applied on Keggin type structures, evaluate the acid strength and the amount of molecules adsorbed with probe molecules (ammonia, pyridine) different from actual reactants. The studies that used actual reactant molecules, such as alcohols, determine the total amount of adsorption sites rather than the number of active acid sites at a certain temperature of reaction. In fact, the studies reported in the literature focused on the intermediate adsorbed species rather than on the in situ identification and quantification of the products of... 

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