Rhodium (Rh)

Rhodium is used in the following sectors of homogeneous catalysis:

Hydroformylation (oxo synthesis) is the conversion of alkenes into aldehydes with a mixture of carbon monoxide (CO) and hydrogen (H2), which is called "Synthesis Gas". Here, low-valent rhodium represents the established catalyst metal, which is applied via various compounds or precursors according to the processing systems (one phase or two phases, high or low pressure). It mainly replaced the former cobalt processes.

Carbonylations are conversions with carbon monoxide (CO) under formation of aldehydes, ketones, or carboxylic acids by inserting a C=O group.

The most important process on a large scale is the carbonylation of methanol to acetic acid ("Monsanto Process"). Besides, there are other product routes, e.g. the manufacture of acetic anhydride from methyl acetate. All these processes are based on Rh in the presence of iodine-containing compounds. Recent methods are also based on iridium with co-catalysts ("Cativa Process").

For the production of fine or pharmaceutical chemicals and specialties, some functional groups can be carbonylated by using various phosphorus-containing palladium compounds.

Hydrogenations are reactions of unsaturated molecules with hydrogen (H2). Precious metal compounds of rhodium, ruthenium, or iridium rank with the most effective homogeneous hydrogenation catalysts because of their outstanding selectivities.

Catalysts for hydrogenations are often equivalent to those for hydroformylations, where the reactants H2 and CO are applied.

Well-known is especially the “Wilkinson's Catalyst”: chlorotris(triphenylphosphane)rhodium(I) for the cis-hydrogenation of C=C's.

Hydrogenations lead to chiral centres in prochiral molecules. Like right and left hands, "chiral" molecules of the same constitution (empirical formula, structure) can coincide only with their mirror-images. Chirality is extraordinarily important to modern agents in the field of life science, where almost just one chiral variant is active to a specific purpose. Such "asymmetric hydrogenations" are feasible in a very effective way by using precious metal precursors along with chiral ligands. Heraeus offers a broad range of such precursors relevant to industry, mostly organometallics.

Hydrosilations represent reactions to build up higher-molecular organosilicon compounds (silanes, siloxanes), and thus they are the basis of manufacturing silicones. Today silicone products are found all over the dental, automotive, building, polymer, or paper industries. Platinum is the most effective metal for this homogeneously catalytic reaction. In special cases rhodium as well as palladium may serve as hydrosilation catalysts.

Moreover, Heraeus manufactures new catalysts on a commercial scale and in strict confidence according to customers’ specifications.

Compound CAS Number Formula Metal ­Cont. ­approx. Color
(Acetylacetonato)carbonyl(triphenylphosphane)rhodium(I) “ROPAC” * 25470-96-6 [Rh(acac)(CO)(PPh3)] 21 % yellow
Chlorotris(triphenylphosphane)rhodium(I) “Wilkinson’s Catalyst” 14694-95-2 [RhCl(PPh3)3] 11 % red
Di-μ-chloro-bis[(cycloocta-1.5-diene)rhodium(I)] 12092-47-6 [{Rh(cod)}2(μ-Cl)2] 42 % yellow
Rhodium acetate 42204-14-8 "Rh(OAc)x" 39 % brown
Rhodium acetate solution 42204-14-8 "Rh(OAc)x" up to 8 % brown
Rhodium 2-ethylhexanoate solution in 2-ethyl hexanol “Rh 2-EH” 20845-92-5 Rh(C8H15O2)3 up to 2 % reddish brown
Rhodium(III) iodide * 15492-38-3 RhI3 21 % black

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