Catalysts supported, homogeneous

Although beyond the scope of this book, a vast amount of work has been directed to supporting homogeneous catalysts on solid supports including silica, alumina and zeolites, and functionalized dendrimers and polymers [19]. These give rise to so-called solid-liquid biphasic catalysis and in cases where the substrate and product are both liquids or gases then co-solvents are not always required. In many ways solvent-free synthesis represents the ideal method but currently solvent-free methods can only be applied to a limited number of reactions [20],... 

Silica-supported homogeneous catalysts, especially phosphino-iridium compounds, appear more promising in the hydrogenation of a,y9-unsaturated aldehydes, provided that their productivity can be improved and catalyst deactivation is avoided so that recycling of these materials could be meaningful [35]. 

This section has focused on liquid—liquid biphasic catalysis in which catalysts are supported in different solvents to the substrates and products. Considerable efforts have also been directed to supporting homogeneous catalysts on solid supports including silica, alumina, and zeolites as well as functionalized dendrimers and polymers.33 It has also been found that synergic effects sometimes prevail between particles embedded in the support and the tethered molecular catalyst, increasing the activity of the catalyst.34... 

We have been able to demonstrate fliat polymer supported homogeneous catalysts are proving to be a credible alternative to homogeneous catalysts in a wide range of industrially in rtant reactions. Similar reactivity and selectivity have been... 

C.J.F. Barnard, K.G. Griffin, J. Froelich, K. Ekman, M. Sundell and R. Peltonen. Fibre-based catalysts applications as heterogeneous and supported homogeneous catalysts for the fine chemicals industry. In Michael E. Ford, ed. Catalysis of Organic Reactions, Marcel Dekker, New York, 2001, pp 563-571. 

One reason that is often cited for studying supported homogeneous catalysts is the relative ease of separation of catalyst from reactants. However for this factor to have practical significance it is essential that metal elution is minimal and very few studies on elution have been carried out in the past. ° Elemental analyses on the catalyst before and after reaction are insufficiently accurate to detect small losses. Moreover, arguments for low Rh loss on the basis of no change in activity on re-use of catalyst are not valid as the reaction is normally under mass transfer control. As many catalyst-support systems contain residual halide which is a known catalyst poison, if simultaneous leaching of halide and metal occurs then little change in activity may result. 

Since supported homogeneous catalysts are fully described in Chapter 14, we only mention two particularly imaginative examples here, in both of which the positive charge of the catalyst system has a role to play. Whitesides et have bound a RhL2 species to avidin, a globular... 

The situation is quite different for supported homogeneous catalysts. They were briefly considered in Chapter 7, and will be treated at greater length in the section on triphase catalysis in Chapter 19. 

Hydroformylation catalysts can also be made by attaching the cobalt or rhodium carbonyl residues to a phosphine-functionahzed surface through phosphine-for-carbonyl substitution. The chemo- and regioselectivities observed for the supported homogeneous catalysts are typically quite different from those of their conventional analogues. 

Advances continue to be made in the synthesis and use of polymer-supported homogeneous catalysts. A rhodium(i) hydrogenation catalyst, prepared as outlined in Scheme 1, is much more versatile than most homogeneous rhodium catalysts. This catalyst will not only reduce a variety of olefinic and aromatic hydrocarbons under mild conditions but also carbonyl, nitrile, and nitro-groups. The catalyst is air stable, but its lifetime appears to be less than those of other polymer-supported catalysts. 

Fache, E. Mercier, C. Pagnier, N. Despeyroux, B. Panster, P. (1993) Selective hydrogenation of a,P-unsaturated aldehydes catalyzed by supported aqueous-phase catalysts and supported homogeneous catalysts,/. Mol. Catal.,19,117-31. 

Aresta, M. Dibenedetto, A Nocito, F Pastore, C. Comparison of the Behaviour of Supported Homogeneous Catalysts in the Synthesis of Dimethyl Carbonate from methanol and Carbon Dioxide PolyStyrene-Grafted Tin-MetaUoiganic Species Versus Silesquioxanes Linked Nb-Methoxo Species Inorg. Chim. Acta 2008, 361, 3215-3220. 

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