Heterogeneous Polyoxometalate-Based Systems

To reduce the environmental impact of POM-based catalysts, many different techniques of immobilization/heterogenization of these compounds have been developed. This final section outlines some relevant examples of the preparation of hybrid POM-based materials that could be useful to further expand the catalytic applications of POMs. 

Hybrid composite films enabling physical entrapment can be obtained by spin coating or dipping, starting from a suitable homogeneous solution. Water-soluble polymers, such as polyvinyl alcohol (PVA), polyacrylamide (PAA) and polyvinyl pyrrolidone (PVP), are generally employed for this type of immobilization. 

The limit in the preparation of hybrid materials by physical entrapment is the lack of an effective attachment between the organic and the inorganic domain, which affects the stability of the resulting hybrid material and can lead to phase separations between POM domains and the polymer matrix, severely limiting their applications. 

A different approach to increasing the stabihty of the hybrid material is to anchor the POM to the polymeric support via electrostatic interactions. Due to the cooperative effect of multiple ionic interactions, polycationic species allow an efficient 

A heterogeneous catalytic system was prepared upon grafting a cationic dihydroimidazolium-tagged silane on solid Si02. The resulting supported ionic liquid phase (SILP) has been used to immobilize [y-l,2-H2SiV2Wio04o] and was employed in a mixture of acetonitrile/t-butyl alcohol at 20°C for the oxidation of different substrates terminal olefins (66-82% yield), non-hindered internal olefins ( 70% yield) in 24 h, and sulfides (81-95% yield) in 4—lOh.  

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Monteleone, F., Cavani, F., Felloni, C., et al (2004). A Redox Process for the Production of Menadione and Use of Polyoxometalates as Oxidizing Agents, International Patent Application WO 2004014832 Strukul, G., Somma, F., BaUarini, N., et al. (2009). The Oxidation of 2-methyl-1-naphthol to Menadione with H2O2, Catalyzed by Nb-based Heterogeneous Systems, Ap/iZ. Catal. A Gen., 356, pp. 162-166 Zalomaeva, O., Ivanchikova, I., Kholdeeva, O., et al (2009). Kinetics and Mechanism of the Oxidation of Alkyl Substituted Phenols and Naph-thols with TBuOOH in the Presence of Supported Iron Phthalocyanine, New J. Chem., 33, pp. 1031-1037. 

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. 

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