Rhodium as catalysts

Rylander and Himelstein studied the hydrogenation of resorcinol over supported platinum, palladium, rhodium, and palladium-rhodium as catalysts in ethanol at 65°C and an initial hydrogen pressure of 0.34 MPa. Extensive hydrogenolysis to give cyclohex -anol took place over 5% Pt-C, 5% Pd-C, and 2.5% Pd-2.5% Rh-C, and the yields of cyclohexane-1,3-diol were only 18.7-26.2%. Rhodium catalysts caused hydro-... 

Looking at the hydroformylation of propene, the technology of the cobalt- catalyzed processes has remained basically unchanged over the years, whereas the introduction of rhodium as catalyst has founded a new generation of hydroformylation processes. 

In principle other halogens apart from iodide may also be used as a promoter. For bromide, Monsanto reports the synthesis of propionic acid from ethylene with selectivities of more than 99 % when a large excess of bromide in the form of hy-drobromic acid is added to a rhodium catalyst [21]. The carbonylation is catalytic both with respect to the rhodium and the bromide components. Nonhalogen promoters in combination with rhodium as catalyst are phenols, thiophenols, thiocar-boxylic acids, and sulfonic acids [22]. 

The highly developed enantioselective hydrogenation of prochiral functionalized alkenes using chiral phosphine complexes of ruthenium or rhodium as catalysts has become very common in academic laboratories as well as in industry... 

As a consequence of classical homogeneous hydroformylation systems, the study of this reaction in ILs has essentially been developed around rhodium as catalyst. Only a few exceptions deal with the use of other metals such as platinum, cobalt, or ruthenium. 

Using supported rhodium as catalyst, 2-nitrostyrene is converted in benzene into skatole in 70y. yield, with CO/H2 (160 atm) at 160"CD2ll. However this reaction, conducted under hydrotormi1 ation conditions, involves -formation o-f 2-homogeneous catalysis, reduction o-f the nitro group by heterogeneous catalysis, then ring closure and thermal dehydration ... 

Other ligands have been used with palladium and rhodium as catalysts in the carbonylation under pressure of nitroarenes. However, with the exception of phenanthroline and related derivatives vide infra), these catalytic systems appear to be less active than those previously described. 

A comparative study was made of noble metals other than rhodium as catalysts for this bulk glass ceramic type of crystallization. None of those tried, which included platinum, palladium, gold, silver, and iridium, were as effective as rhodium in causing uniform crystallization throughout the volume of the glass. This is perhaps associated with the fact that rhodium, with a lattice constant of 3.796 A, has a smaller unit cell than any of the other metals tried, which have constants ranging from 3.831 for iridium to 4.086 for silver. Ease of oxidation to an ionic form may also enter as a factor in the poor catalytic activity of the other metals. 

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