Keggin unit

The same is known to occur in the case of V-substituted POMs, in which the ions, originally located in the primary anion, are then transferred during the reaction into the cationic position of the POM framework, with generation of lacunary or decomposed Keggin units ° this phenomenon leads to a considerable increase in catalytic activity. 

H3PW12O40 is one of the most common heteropoly compounds. A distinct X-ray diffraction pattern is seen for H3PWi2O40.6H2O, where the Keggin units are linked by H+(H20)2 bridges, resulting in a body-centered cubic structure and hence the X-ray diffraction pattern. The water molecules can be easily replaced by a number of polar molecules such as alcohols and amines (Misono, 1987). 

Many one-dimensional chain complexes constructed by monovacant Keggin units [a-XMii039l" (X = Si, P, Ge M = Mo, W = 4, 5, 6) and RE linkers have been prepared and structurally characterized. These one-dimensional chain complexes can be separated into two series. The first series include [Yb(SiWn039)(H20)2] , [Y(GeWn039)(H20)2] ,... 

The IR spectra of polyoxometallates have already been comprehensively discussed in the literature. The IR bands have been assigned previously [124—127]. The four distinct oxygen sites in a Keggin unit are represented in Figure 14.2(b) and correspond to the following description ... 

Figure 14.10 Example of Te capping a Keggin unit and helping to stabilize the anion. The Te cation is five coordinated, involving four oxygens from the same Keggin unit. (Taken from Ref [146]).

Desorption of amines from acid sites should occur with heat uptake (endothermic). Since the measurements were performed in presence of air, oxidation of the amine should be expected to occur on catalytically active surface sites accompanied by heat release. This is obviously the case when the external surface is covered by Keggin units [W12040] where... 

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... 

Several solid-state polyoxometalate structures that incorporate PW9MII3 Keggin units (with edge-shared MIIOfi octahedra, and thus formally derived from the A-PW9034 anion) have been described and their magnetic properties have been reported. These include [Ni3( I FOpPWioCbgl I2C)]7, 194... 

Studies of the effects of loading and the temperature and duration of calcination have demonstrated that the Keggin Unit is primarily responsible for the catalytic activity in the partial oxidation of methane (Figs. 13-14). The linear increase in the rate of formation of various products with increase... 

As is evident from Fig. 13 the optimum loading of HPMo on SiO2 is approximately 120 micromole KU per gram of support which is equivalent to approximately 1000 A2/ku, as compared with the value of 100 A2 estimated for the cross-sectional area of a Keggin Unit. Consequently it is highly probable that each KU is isolated on the support surface. 

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