Polyoxometalate immobilization

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... 

In this context, much effort has also been invested in controlling the nuclearity of the catalyst ensemble through the selection of its precursor. One area in which considerable progress has been made involves the adsorption of polynuclear clusters onto supports [33]. Examples involving the immobilization of small, preformed polynuclear clusters on supports are the reactions of carbonyl clusters of the late metals [16, 34], the binding of polyoxometalates (POMs) and their neutral alkoxy analogues [35] and heteropolyacids such as the Keggin cluster [36, 37]. 

Figure 6.22. The immobilization of polyoxometalate POM catalyst within a polyethyleneglycol layer.

In conclusion, we have studied the supported polyoxometalate/H202/solvent system for the oxidation of sulphur containing compounds, and found that significant advantages in terms of product selectivity can be obtained when homogeneous species are immobilized. 

We thank the Organic Chemistry Group at Widnes R D. We also wish to thank our university collaborators, particularly Professor Bob Johnstone, and Dr. Preciosa Pires (Liverpool University, metal(IV) phosphates), Professor Bill Griffith, and Dr. Melanie Gresley (Imperial College, immobilized polyoxometalates). Professor Roger Sheldon, and Dr. Jihad Dakka (Delft University, chromium silicalite). 

The possibility of bioelectrochemical oxygen reduction was also explored. A bifunctional bioelectrocatalytic system was fabricated with the combination of Co-porphyrin supported on polyoxometalate-modified CNTs and peroxidase enzymes (horseradish and cabbage), deposited upon glassy carbon [203]. Later, the same research group used a cobalt porphyrin immobilized within the same peroxidases supported on MWCNTs [204]. 

Polyelectrolyte multilayers are able to very effectively immobilize various molecules, like enzymes, for example. These multilayers have been shown to exhibit useful biocatalytic (i.e., enzymatic) [219,220] activities hybrid multilayers containing polyoxometalate are promising for catalytic applications [164], ESA multilayers have also been used for molecular recognition [198,341], or, more specifically, as biosensors [75,166,250-252,342,343]. Electrocatalytic [195,253,256] and electrosensing [254] capabilities of multilayers deposited on electrodes have... 

Polyoxometalates (POMs) are transition metal oxygen clusters with well-defined atomic coordination structures. POMs are used as functional nano-colloidal materials and also as supports for catalysts via ion-pair interactions due to their acidic properties. Combinations of chiral diamines and POM 225 effectively catalyze enamine-based aldol reactions. Less than 1 mol% of chiral amine loading is suf-ficientto catalyze the reaction (Table 28.10, entries 1 and 2) [114]. Highly diastereo-and enantioselective cross-aldol reactions of aldehydes are accomplished using chiral diamine-POM 226 under emulsion conditions (entries 3 and 4) [115]. Sul-fonated polystyrene or fluoropolymer Nafion NR50 are also good supports for the immobilization of primary-tertiary diamines. The catalyst 227 can be recovered by filtration and reused for at least four cycles with no loss of stereoselectivity (entries 5 and 6) [116]. 

Immobilized NHC-Ir complexes 88 and 89 with carbon nanotubes or polyoxometalates were also successfully applied to TH of ketones (Figure 13.8). The carbon nanotube-supported NHC-Ir materials were active in the TH of cyclohexanone and TOF values < 5550 h" obtained. 

Further studies arc in progress with other catalysts belonging to the polyoxometalates group, for their hetcrogenizalion in iluorinated membranes (not only PVDF but also Hyflon) by the two techniques described immobilization in the membrane starting from homogeneous solution of the catalyst and the polymer and linking of the catalyst on the surface of plasma-modified membranes. 

Electrochromic devices have also utilized LbL assembled films for immobilizing the active electrochromic material that changes color at an applied voltage. Polyviologen (PV) was the first active material that was used for such purposes within PV-PSS multilayers, while the first LbL-based solid-state electrochromic device utilized poly(3,4-ethylene-dioxythiophene) (PEDOT) colloid as the electrochromic material within multilayers of PEDOT/PSS and polyaniline. Other nanoparticles were used for the development of electrochromic devices with LbL films, such as Pmssian blue, polyoxometalate, and tungsten oxide. A recent example demonstrated the use of coordination chemistry-based LbL assembly to fabricate fast-switching electrochromic films of zinc polyiminofiuorene and terpyridine. ... 

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