Polyoxometalates catalytic oxidation

The development of catalysts for the oxidation of organic compounds by air under ambient conditions is of both academic and practical importance (1). Formaldehyde is an important intermediate in synthetic chemistry as well as one of the major pollutants in the human environment (2). While high temperature (> 120 °C) catalytic oxidations are well known (3), low temperature aerobic oxidations under mild conditions have yet to be reported. Polyoxometalates (POMs) are attractive oxidation catalysts because these extensively modifiable metal oxide-like structures have high thermal and hydrolytic stability, tunable acid and redox properties, solubility in various media, etc. (4). Moreover, they can be deposited on fabrics and porous materials to render these materials catalytically decontaminating (5). Here we report the aerobic oxidation of formaldehyde in water under mild conditions (20-40 °C, 1 atm of air or 02) in the presence of Ce-substituted POMs (Ce-POMs). 

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. 

Hiskia, A. Papaconstantinou, E. Thermal and Photochemical Catalysis by Polyoxometalates. Regeneration of Catalyst by Dioxygen. In Dioxygen Activation and Homogeneous Catalytic Catalytic Oxidation, Simandi, L. I., Ed. Elsevier Amsterdam, 1991 pp 429 435. 

Additional features of polyoxometalates impact some of the six limitations. First, polyoxometalates are oxidatively resistant as a great majority of them are composed of d° transition metal ions (most commonly W(VI), Mo(V), and/or V(V)) and oxide ions. Second, the unusually high stability of polyoxometalates coupled with the tunability of their catalytically relevant properties defines a considerable ability to avoid problems associated with intermediate oxidation states, p-oxo dimers, and product inhibition. Illustrations of two representative polyoxometalates, Wio0324 , an isopolyoxometalate (or isopolyanion) and (TM)XWii039 , a transition metal (TM) substituted heteropolyoxometalate (or heteropolyanion) in polyhedral notation are given in Figure 1. 

Hydrogen peroxide is a particularly attractive oxidant and holds a prominent position in the development of benign catalytic oxidation procedures. In recent years a number of highly versatile catalytic oxidation methods based on, for example, polyoxometalates [154], methyltrioxorhenium [155] or tungstate [4, 156] complexes in the presence of phase transfer catalysts, all using hydrogen peroxide as the terminal oxidant, have been introduced. Mn-catalyzed epoxidations, aldehyde... 

Gall RD, Hill CL, Walker JE (1996) Carbon powder and fiber-supported polyoxometalate catalytic materials. Preparation, characterization, and catalytic oxidation of dialkyl sulfides as mustard (HD) analogues. Chem Mater 8 2523-2527... 

Ti(IV) monosuhstituted Keggin-type polyoxometalates (Ti-POMs) are stable towards the hydrolysis of Ti-O-W bonds and oxidative degradation. The catalytic oxidation of 3 using H2O2 as the oxidant over Ti-POMs yields 2,3,5-TMHQ (1) and the biphenol (6) as main products (Fig. 16.6). The product distribution depends significantly on the TMP/Ti-POM molar ratio. ... 

Neumaim, R. (2010). Activation of Molecular Oxygen, Polyoxometalates, and Liquid-Phase Catalytic Oxidation, Inorg. Chem., 49, pp. 3594—3601. 

Abstract The catalytic oxidation is an area of the key technologies for converting petroleum-based feedstocks to useful chemicals such as diols, epoxides, alcohols, and carbonyl compounds. Many efficient homogeneous and heterogeneous oxidation systems based on polyoxometalates (POMs) with green oxidants such as H2O2 and O2 have been developed. This chapter summarizes the remarkable oxidation catalyses by POMs with multimetallic active sites. The multifunctionality of multimetallic active sites in POMs such as cooperative activation of oxidants, simultaneous activation of oxidants and substrates, stabilization of reaction intermediates, and multielectron transfer leads to their remarkable activities and selec-tivities in comparison with the conventional monometallic complexes. Finally, the future opportunities for the development of shape- and stereoselective oxidation by POM-based catalysts are described. 

The applications of polyoxometalates in catalytic dehalogenation of halocar-bons have been succinctly reviewed by Hill and coworkers [188]. This reaction involves the photocatalytic transformation of organic halides coupled with the oxidation of sacrificial organic reductants (secondary alcohols or tertiary amides) (Eq. (9)) [189, 190] ... 

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... 

Polyoxometalates undoubtedly have enormous catalytic potential in liquid phase selective oxidation of organic compounds. Various strategies for immobilization of POMs on solid matrices have been developed during the past two decades and opened new opportunities for practical applications. The most developed and widely used technique is electrostatic... 

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. 

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