Bulk and Supported Heteropolyacids

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. 

These materials are used industrially mainly in the reaction of hydration of alkanes (acid catalysis) and in the synthesis of methacrolein or isobutyric acid (redox catalysis). Because of the considerable number of available polyanion structures, it is possible to vary their acidic and redox properties according to the needs of a specific application. 

The number and strength of the acid centers of tungstic heteropolyacids have been determined by ammonia adsorption calorimetry. Ammonia is irreversibly absorbed, with the formation of the corresponding ammonium salts. An increase in the number of protons in Keggin heteropolyanions decreases the acidic strength. 

It is worth noting that differences in acid strength between anhydrous Keggin heteropolyacids (H3PW12O40 H4SiWi2O40 H3PM012O40 H4SiMoi204o) measured by ammonia adsorption calorimetry did not correlate simply with the catalytic activity in the reaction of rapeseed oil transesterification with methanol and ethanol [102]. 

Ru-HPA metal-acid bifunctional catalysts on various supports (siUca, graphite, KL zeolite) have also been characterized by NH3 adsorption calorimetry, which revealed heterogeneous acid site strength distributions varying in the order HPA-SiOj HPA-graphite HPA-KL [106]. 

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In catalysis, oxides with well defined acidic and basic properties are used in different forms that have found application in numerous catalytic applications in the gas-solid and liquid-solid heterogeneous catalysis [3, 46, 47], Among the most used oxide materials in catalysis, we And (i) bulk oxides (one component metal oxides) (ii) doped and moditied oxides (iii) supported metal oxides (dispersed active oxide component onto a support oxide component) (iv) bulk and supported binary metal oxides to quaternary metal oxides (mixed oxide compositions) (v) complex oxides (e.g., spinels, perovskites, hexa-aluminates, bulk and supported hydrotalcites, pillared clays, bulk and supported heteropolyacids, layered silicas, etc.). 

Complex oxides comprise a large variety of structures which gained importance in the catalysis field due to their acidity or basicity properties (e.g., spinels, perovskites, hexa-aluminates, bulk and supported hydrotalcites, pillared clays, bulk and supported heteropolyacids, etc.). 

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