Polyoxometalates, catalytically active

B. J.S. Johnson and A. Stein Surface Modification of Mesoporous, Macroporous, and Amorphous Silica with Catalytically Active Polyoxometalate Clusters. Inorg. Chem. 40, 801 (2001). 

The intercalation of anionic species into LDHs is an interesting alternative for the immobilization of catalytic complexes. Special attention is being paid to LDHs containing bulky and stable anions, e.g. polyoxometalates (POMs), since they can give rise to a wide range of microporous materials [4]. Many years ago, Pinnavaia et al. reported the intercalation of POM anions into LDHs, and found that the products showed significant photo-catalytic activity in the oxidation of isopropanol to acetone in the presence... 

Goals and five limitations in conjunction with the development of selective catalytic homogeneous oxidation systems are evaluated. Systems are presented that address several of the problems or goals. One involves oxidation of alkenes by hypochlorite catalyzed by oxidatively resistant d-electron-transition-metal-substituted (TMSP) complexes. A second involves oxidation of alkenes by H2O2 catalyzed by specific TMSP complexes, and a third addresses functionalization of redox active polyoxometalate complexes with organic groups. 

We report here three studies that address three separate but significant issues in the emerging area of selective catalytic oxidation by TMSP-type complexes. The first study establishes for the first time that some TMSP complexes are compatible with basic oxidants and basic conditions. The second study reports the first oxidation, in this case selective alkene epoxidation, by the economically and environmentally desirable oxidant, aqueous hydrogen peroxide, catalyzed by TMSP complexes. The third study demonstrates that redox active polyoxometalates can be derivatized with alcohols in a manner that should prove useful for fabricating future generations of more sophisticated and selective TMSP catalysts. 

The main type of inorganic-LDH hybrid system comes from intercalation of oxometalates into the interlayer space of LDHs because of the high catalytic activity of both oxometalates and LDHs. Typical synthetic routes are similar to those of pillared clays, as mentioned before. After pristine nitrate/chloride-LDHs are anion exchanged with polyoxometalates, usually at elevated temperatures, the pillared polyoxometalates are stabilized within the interlayer space by calcination. 

S. Roy Chowdhury, P. T. Witte, D. H. A. Blank, P. L. Alsters, J. E. ten Elshof, Recovery of homogeneous polyoxometalate catalysts from aqueous and organic media by a mesoporous ceramic membrane without loss of catalytic activity, Chem. Eur.. 12 (2006) 3061. 

Organometallic derivatives of polyoxometalates are perfectly characterized at the molecular level and in turn provide models for the grafting of an organometallic catalyst on an oxide surface, especially in the case of the supported derivatives. Beyond, they also display their own reactivity and catalytic activity.Some synergy between the organometallic and oxide parts has been reported in the literature, as well as bifunctional activity. The mobility of organometallic fragments at an oxide surface could also be reproduced on a polyanion. 

As described previously, the cations in LDHs are evenly distributed in thebrucite-like layers. Thus, in principle, the catalytic activity of LDHs can be well controlled by varying the cation ratio and incorporating different cations. Catalytically active constituents of LDH include the hydroxide groups and the metal ions themselves, especially if these are redox active. The introduction of catalytically active anions, such as polyoxometalates (POMs), can further modify the properties of LDHs. Thermal decomposition (calcination) of LDH gives mixed basic oxides of high surface area and catalytic activity. Finally, the reduction of LDH can give rise to finely divided catalytically active metal and to the prospect of metal/base bifunctional catalyst. 

Neumann R, Khenkin AM (1996) Anewdinuclearrhodium(III) sandwich polyoxometalate, [(WZnRH j)(ZnW,03,)J . Synthesis, characterization and catalytic activity. J Mol Catal A Chem 114 169-180... 

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