Transport rhodium complexes

This complex easily looses CO, which enables co-ordination of a molecule of alkene. As a result the complexes with bulky phosphite ligands are very reactive towards otherwise unreactive substrates such as internal or 2,2-dialkyl 1-alkenes. The rate of reaction reaches the same values as those found with the triphenylphosphine catalysts for monosubstituted 1-alkenes, i.e. up to 15,000 mol of product per mol of rhodium complex per hour at 90 °C and 10-30 bar. When 1-alkenes are subjected to hydroformylation with these monodentate bulky phosphite catalysts an extremely rapid hydroformylation takes place with turnover frequencies up to 170,000 mole of product per mol of rhodium per hour [65], A moderate linearity of 65% can be achieved. Due to the very fast consumption of CO the mass transport of CO can become rate determining and thus hydroformylation slows down or stops. The low CO concentration also results in highly unsaturated rhodium complexes giving a rapid isomerisation of terminal to internal alkenes. In the extreme situation this means that it makes no difference whether we start from terminal or internal alkenes. 

The biphasic SCCO2/IL separation concept was further extended to the hydro-formylahon of an aUcene in a continuous-flow system [35]. The active rhodium complex was immobilized in the IL [BMIM][PFg] with the help of the modified phosphine [l-propyl-3-methylimidazoliumj2[PhP(C5H4S03-3)2 and investigated for the hydroformylation of 1-octene. In this continuous-flow process the substrate, gases, and products were transported in and out of the reactor dissolved in SCCO2. The catalyst system exhibited a constant rate of more than 20 h with a rhodium metal content of less than 1 ppm in the collected product. 

We close this survey by pointing out that, in most cases described, the membrane serves as a transport barrier. Another possible role of a membrane was described by Schenning et al. [74], who dissolved manganese porphyrins in phospholipid membranes, into which lipidized rhodium complexes were incorporated. It was shown that this construct catalyzes the reaction HCOOH 4-02 O2 -F H2O2, and that the oxidation state of manganese oscillates in time, as detected by absorbance at 435 nm. Here the membrane colocahzes reactive centers and controls their reactions rates. These vesicle systems are therefore regarded as oscillatory enzyme mimics. 

The results of the study on the hydrogenation of different functional groups were summarized [194]. Here complexes of rhodium, palladium and nickel fixed on balls of densely cross-linked macroporous polystyrene of HAD-4 grade, on which an-thranilic acid residues were bonded, were used as catalysts. Special attention was paid to the kinetic study of cyclohexene hydrogenation by rhodium(+) derivatives and to elucidate the reaction mechanism using D2. The influence of diffusion restrictions on the reaction rate was discussed, in particular, that of the transport of hydrogen to pores and through the gas-liquid interface. 

The hydroformylation of olefins is a type of CO insertion reaction that is one of the most important industrial applications of homogeneous catalysis with transition metal complexes (208,209). Conventional industrial processes (e.g., the Oxo process) typically use either cobalt- or rhodium-based catalysts and conduct the reaction in two-phase gas-liquid reactors. Efficient transfer of the reactants from the gas phase into the liquid phase is of primary importance to minimize inherent mass transfer limitations (208). Reactor design thus focuses on optimizing this mass transfer rate by maximizing the interfacial area between phases. An SCE process eliminates this transport restriction since the hydrogen... 

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