Heteropoly Keggin type

New acid catalyst comprising Keggin-type heteropoly acid supported on mesoporous silica for dehydration of acetic acid... 

The selectivity for acetic anhydride in the catalytic dehydration of acetic acid could be controlled by the pore size of pure mesoporous silica SBA-15. New acid catalyst comprising Keggin-type heteropoly acid supported on SBA-15 enhanced the activity etfectively when tungstophosphoric acid was highly dispersed on the silica substrate. 

I0M.S. Kaba, I.K. Song, and M.A. Barteau, Investigation of Framework and Cation Substitutions in Keggin-type Heteropoly acids Probed by Scanning Tunneling Microscopy and Tunneling Spectroscopy,/ Vac. Sci. Technol. A, 15,1299-1304(1997). 

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. 

Keggin type heteropoly anions and related species... 

Heteropoly Anions with Keggin-Type Structures... 

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. 

Firouzabadi, H., Iranpoor, N., Amani, K. 2003. Keggin-type heteropoly acids revealed high catalytic activity for swift and selective oxidation of various hydroxy functionalities to the corresponding carbonyl groups using ferric nitrate as an oxidant under mild and solvent-free conditions. Synthesis 408 12. 

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... 

This section will focus exclusively on self-exchange reactions between the iso-structural series of Keggin-type heteropoly tungstates, X +Wi2O40 " , X + = P + (anion 1), Si" + (anion 2), Al + (anion 3) and y = 8,9,10 for the fully oxidized, one-and two-electron reduced species, respectively. These electronic states are indicated... 

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

The reducibility of type-I polyoxometalates, notably the Keggin, Wells-Dawson, and Lindqvist (hexametalate) structures, to mixed-valence heteropoly blues was well established in the 1960s and... 

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