Rhodium catalysts metal leach

An ionic liquid was fully immobilized, rather than merely supported, on the surface of silica through a multiple-step synthesis as shown in Fig. 15 (97). A ligand tri(m-sulfonyl)triphenyl phosphine tris(l-butyl-3-methyl-imidazolium) salt (tppti) was prepared so that the catalyst, formed from dicarbonylacetylacetonate rhodium and the ligand (P/Rh = 10), could be soluble in both [BMIMJBFq and [BMIM]PF6. The supported ionic liquid-catalyst systems showed nearly three times higher rate of reaction (rate constant = 65 min ) that a biphasic system for the hydroformylation of 1-hexene at 100°C and 1500 psi in a batch reactor, but the n/i selectivity was nearly constant the same for the two ( 2.4). Unfortunately, both the supported and the biphasic ionic liquid systems exhibited similar metal leaching behavior. 

Fell also described the hydroformylation of fatty acids with heterogenized cobalt carbonyl and rhodium carbonyl catalysts [37]. The products of the reaction with polyunsaturated fatty acids were, depending on the catalyst metal, poly- or monoformyl products. The catalyst carrier was a silicate matrix with tertiary phosphine ligands and cobalt or rhodium carbonyl precursors on the surface. The cobalt catalyst was applied at 160-180°C and gave mostly monofunctionalized fatty acid chains. With linoleic acid mixtures, the corresponding rhodium catalyst gave mono- and diformyl derivatives. Therefore, the rhodium catalyst was more feasible for polyfunctionalized oleocompounds. The reaction was completed in a batch experiment over 10 h at 100 bar and 140°C rhodium leaching was lower than 1 ppm. 

The positive situation with respect to catalyst life and metal leaching has been confirmed by a comparison of results obtained in a bydrosilylation reaction carried out with a homogeneous rhodium catalyst, a similar catalyst anchored to silica, and a corresponding organopolysiloxane-based system synthesized via route A [83]. 

In a further variation, the PVP-supported rhodium catalyst was used for methanol carbonylation in supercritical carbon dioxide [100]. This reaction medium has complete miscibility with CO and dissolves high concentrations of methanol and methyl iodide, while being a poor solvent for ionic metal complexes. Catalytic reaction rates up to half of those obtained in conventional liquid-phase catalysis were achieved with minimal catalyst leaching. 

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