Stability, heteropoly compounds

Greater durability of the colloidal Pd/C catalysts was also observed in this case. The catalytic activity was found to have declined much less than a conventionally manufactured Pd/C catalyst after recycling both catalysts 25 times under similar conditions. Obviously, the lipophilic (Oct)4NCl surfactant layer prevents the colloid particles from coagulating and being poisoned in the alkaline aqueous reaction medium. Shape-selective hydrocarbon oxidation catalysts have been described, where active Pt colloid particles are present exclusively in the pores of ultramicroscopic tungsten heteropoly compounds [162], Phosphine-free Suzuki and Heck reactions involving iodo-, bromo-or activated chloroatoms were performed catalytically with ammonium salt- or poly(vinylpyrroli-done)-stabilized palladium or palladium nickel colloids (Equation 3.9) [162, 163],... 

The mixed-addenda atoms affect the redox properties mixed-addenda heteropoly compounds are used as industrial oxidation catalysts. For example, the rate of reduction by H2 is slower and less reversible for solid PMO 2-,VJto m+, than for solid PM012O40, although the former are stronger oxidants than the latter in solution (279, 280). The effects of substituting V for Mo on the catalytic activity are controversial (279, 281-284). Differences in redox processes between solutions and solids, the thermal or chemical stability of the heteropoly compounds, and the effects of countercations in solids have been suggested to account for the discrepancies. 

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). 

Meaningful thermal stability studies on heteropoly compounds must be accompanied by solubility studies of the heated materials to ascertain actual decomposition. In addition, the length of heating must also be specified since such decomposition may be rate dependent in view of the fact that both 12-molybdophosphoric and 12-molybdosilicic acids decompose slowly with time even at room temperature5. ... 

A considerable amount of literature has appeared which describes the use of heteropolymolybdates and their tungsten analogs in various applications. Of these, the main applications have centered on catalysis. Of particular importance to the catalytic behavior of heteropoly compounds is the solubility and solvolytic behavior in both aqueous and organic media and the thermal stability and oxidation-reduction behavior. 

Supported hcteropoly compounds, where heteropoly compounds are dispersed on the surface of porous solids, are important for applications, since the surface area of heteropolyacids is usually low. The structure, stabil-... 

POMs are promising catalysts for acid, redox and bifunctional catalysis. In many structures, the transition metal addenda atoms such as Mo or W exist in two oxidation states, which results in different redox properties as determined by polarog-raphy. The exceptional ability of heteropolyanions to act as electron reservoirs has been demonstrated by the preparation and characterization of numerous reduced derivatives [32]. They also exhibit high solubility in polar solvents, which means that they can be used in homogeneous catalysis. The wide range of applications of heteropoly compounds are based on their unique properties which include size, mass, electron and proton transfer (and hence storage) abilities, thermal stability. 

The order of stability also depends on the nature of the central atom (Si > Zr, Ti > Ge > P > As) and on the nature of the surrounding anion groups (W > Mo > V). This classification has some relevance to hydrotreating catalysts if, for example, introduction of Si into alumina as a support (giving silica-alumina, zeolite, etc.) leads to the formation of Mo —Si or W —Si heteropoly compounds, they will not be degraded during calcination. 

The stability of the Mo—P heteropoly compounds also depends on the P/Mo ratio in the impregnation solution. For P/Mo ratios lower than 0.4, M05P2 decomposes into M09P and MouP as follows (66) ... 

It has been reported in the literature that water accelerates the desorption of IBA and MAA (ref. 11,12). This is demonstrated in Fig, 4 When water is added into the feed (filled symbols) no decrease due to adsorption phenomena can be observed. In fact, during the first few hours of time on stream even a slight increase to a constant level in the formation of MAA is observed. It has been reported in the literature (ref. 13) that the presence of water stabilizes the Keggin unit and even the formation of new heteropoly compounds out of oxides is possible. Therefore, it can be assumed that the observed initial increase is related to these effects. After 2Et hours tirrre on strearn the addition of water... 

However, attempts to find similar relationships for mixed-metal heteropoly compounds such as molybdovanadophosphates have not been successful. This has been due to the low thermal stability of these compounds. For example, PMohVO40 and PM010V2O40 decomposed to PM012O40 and VOx above 200 lC [4]. We attempted to stabilize the heteropolyanions by forming their cesium salts. Although the possibility of slight decomposition could not be excluded, high yields were obtained for the conversion of isobutyric acid to methacrylic acid (MAA) as shown in Fig. 1 [5]. 

Comuzzi, C., Primavera, A., Trovarelli, A., Bini, G., and Cavani, F. Thermal stability and catalytic properties of the Wells-Dawson K P2Wi8062 IOH2O heteropoly compound in the oxidative dehydrogenation of isobutane to isobutene. Top. Catal 1999, 9, 251. 

Heteropoly compounds are efficient catalysts for various reactions in solution, e.g. hydration, etheration and esterification. They usually exhibit much higher catalytic activities than mineral acids. The hi activities of heteropoly compounds are principally due to the strong acidity and the stabilization of reaction intermediates by complex formation. 

Typical TD results for the samples prepared as described above are shown in Fig. 3.75. Those data demonstrate that heteropoly compounds in the acid form are strong protonic acids and that all the protons contribute to the acidity. For quantitative discussion it is necessary to conflrm the establishment of equilibrium and the stability of the polyanion structure. The acid strength estimated by the TD of NH3 for Si02-supported heteropoly acids paralleled that in solution. ... 

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