Polyoxometalates catalysts, structure

Several functional ligninase models that do not contain key structural elements of LiP or MnP are worth mentioning since they do not use dihydrogen peroxide in the delignification process. Among them, robust polyoxometallate catalysts have been shown [64] to work well with dioxygen, thus making this chemistry commercially attractive. 

Figure 2.64 Structure of polyoxometalate catalysts used for the conversion of isobutane into methacrylic acid.

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). 

Polyoxometalates are important catalysts but they are also finding application in optical, electrical, and magnetic devices. Mixed-metal polyoxometalates with vanadium(V) in the polyoxoanion core confers enhanced properties to such structures, principally in their ability to form essentially infinite networks that can be utilized as coatings or as other thin film materials. Additionally, these materials have tunable electromagnetic and photochromic properties. In combination with organic polymers, so-called hybrid polymers, special electrochemical properties are conferred, making possible such electrical storage devices such as capacitors and batteries that utilize the redox properties of the polyoxometalate [7],... 

Polyoxometalates (POMs), also known as heteropolyacids (HPA),3 are a class of compounds formed from negatively charged inorganic metal-oxygen building blocks. When charge-balanced with cationic species, POMs self-assemble into unusual 3D structures with specific topological and electronic properties.214 POMs are commonly formed from polyanions of early transition metals such as W, Mo or V. These anions can be substituted with other transition metals. The diversities in POMs composition and structure make them attractive for many applications, particularly as Bronsted acid and redox catalysts. For example,... 

The layered double hydroxides have been of interest as anion exchangers, basic catalysts, and catalyst precursors. Pillared derivatives of the type discussed earlier for the clays have been synthesized by ion exchange with polyoxometallate anions but have lower thermal stability. Several authors have reported synthesis of layered double hydroxides pillared with the Vio028 ion by direct ion exchange or by techniques that involve preswelling the layer structure with an organic anion. For example, the terephthalate anion was used in the synthesis ofVio028 and Mo7 024 intercalated phases. 

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