Hydrogenation over rhodium catalysts

Hydrogenation Over Rhodium Catalysts Metal Crystallite Size and Catalyst Pore... 

TABLE 14.3. Product Selectivities for the Reaction of n-Hexane with Hydrogen over Rhodium Catalysts at 498 K. ... 

II. Low-Pressure Hydrogenation of Phenols over Rhodium Catalysts... 

Catalysts show remarkable product variation in hydrogenation of simple nitriles. Propionitrile, in neutral, nonreactive media, gives on hydrogenation over rhodium-on-carbon high yields of dipropylamine, whereas high yields of tripropylamine arise from palladium or platinum-catalyzed reductions (71). Parallel results were later found for butyronitrile (2S) and valeronitrile (74) but not for long-chain nitriles. Good yields of primary aliphatic amines can be obtained by use of cobalt, nickel, nickel boride, rhodium, or ruthenium in the presence of ammonia (4J 1,67,68,69). 

The hydrogenation of para-substituted anilines over rhodium catalysts has been investigated. An antipathetic metal crystallite size effect was observed for the hydrogenation of /Moluidinc suggesting that terrace sites favour the reaction. Limited evidence was found for catalyst deactivation by the product amines. Catalysts with pore diameters less than 13.2 nm showed evidence of diffusion control on the rate of reaction but not the cis trans ratio of the product. 

Figure 2.8 The surface reaction between adsorbed carbon monoxide and hydrogen to methane over rhodium catalysts occurs at lower temperatures in the presence of a vanadium oxide promoter, which is known to enhance the rate of CO dissociation (from Koerts el al. 113]). 

Hydrogenation over rhodium (5% on alumina) in acetic acid at room temperature and 3-4 atm of hydrogen was successfully used for reduction of benzene rings in compounds containing functions which would be hydrogen-olyzed over platinum or palladium catalyst [390], Palladium at low temperature usually does not reduce benzene rings and is therefore suited for hydro-genolysis of benzyl derivatives (pp. 150, 151). 

TABLE 5.8 Stereoselective Hydrogenation of Unhindered Cyclohexanones to Axial Alcohols over Rhodium Catalyst ... 

In general, carboxylic acids are hydrogenated with difficulty under mild conditions over usual metallic catalysts. However, it has been recognized that acetic acid may be reduced over platinum oxide under very mild conditions in the presence of perchloric acid.1 Kaplan observed that acetic acid was reduced rapidly over rhodium catalysts at room temperature and pressure, but the reduction stopped abruptly after a very small conversion.2 The occurrence of these reductions may lead to appreciable errors in the amounts of absorbed hydrogen when hydrogenations are carried out in acetic acid as solvent, although the reductions usually proceed only to limited extents, probably due to formation of some poisonous products. These undesired reductions of acetic acid solvent may be depressed in the presence of a substrate that may be adsorbed more strongly than acetic acid. 

In contrast to benzyl alcohol, a-substituted benzyl alcohols, benzyl ethers, and aryl ketones may be successfully hydrogenated over rhodium and rhodium-platinum catalysts to give the corresponding saturated products in high yields, as shown in eqs. 11.38-11.41. In the hydrogenations shown in eqs. 11.38 and 11.39, no racemization took place D-mandelic acid afforded D-hexahydromandelic acid in 94% yield and meso- and dl-2,3-dicyclohexyl-2,3-butanediol were obtained in 93 and 94% yields, respectively, by hydrogenation of the corresponding diphenyl compounds. 

Alkenes can be hydroformylated " by treatment with carbon monoxide and hydrogen over a catalyst. The most common catalysts are cobalt carbonyls (see below for a description of the mechanism) and rhodium complexes, " but other transition metal compounds have also been used. Cobalt catalysts are less active than the rhodium type, and catalysts of other metals are generally less active. " Commercially, this is called the 0x0 process, but it can be carried out in the laboratory in an ordinary hydrogenation apparatus. The order of reactivity is straight-chain terminal alkenes > straight-chain internal alkenes > branched-chain alkenes. With terminal alkenes, for example, the aldehyde unit is formed on both the primary and secondary carbon, but proper choice of catalyst and additive leads to selectivity for the secondary product " or primary... 

Hydrogenation of carbon dioxide over rhodium catalyst supported on silica... 

Solvents also influence the stereoselectivity of the phenol reduction, however, the solvent effect is also dependent on the catalyst used14-18. In contrast to ruthenium, which shows little dependence on the solvent, hydrogenations over rhodium are much more solvent sensitive. In this case the cis/trans ratio tends to decrease with increasing dielectric constant of the solvent. 

D. Y. Murzin, A. I. Allachverdiev, N. V. Kul kova. Kinetics of liquid phase stereoselective hydrogenation of 4-tert-butylphenol over rhodium catalysts. In Heterogeneous Catalysis and Fine Chemicals III. Elsevier 1993, pp 243-250... 

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