Rhodium reduction

This process comprises passing synthesis gas over 5% rhodium on Si02 at 300°C and 2.0 MPa (20 atm). Principal coproducts are acetaldehyde, 24% acetic acid, 20% and ethanol, 16%. Although interest in new routes to acetaldehyde has fallen as a result of the reduced demand for this chemical, one possible new route to both acetaldehyde and ethanol is the reductive carbonylation of methanol (85). 

In contrast to triphenylphosphine-modified rhodium catalysis, a high aldehyde product isomer ratio via cobalt-catalyzed hydroformylation requires high CO partial pressures, eg, 9 MPa (1305 psi) and 110°C. Under such conditions alkyl isomerization is almost completely suppressed, and the 4.4 1 isomer ratio reflects the precursor mixture which contains principally the kinetically favored -butyryl to isobutyryl cobalt tetracarbonyl. At lower CO partial pressures, eg, 0.25 MPa (36.25 psi) and 110°C, the rate of isomerization of the -butyryl cobalt intermediate is competitive with butyryl reductive elimination to aldehyde. The product n/iso ratio of 1.6 1 obtained under these conditions reflects the equihbrium isomer ratio of the precursor butyryl cobalt tetracarbonyls (11). 

The PGM concentrate is attacked with aqua regia to dissolve gold, platinum, and palladium. The more insoluble metals, iridium, rhodium, mthenium, and osmium remain as a residue. Gold is recovered from the aqua regia solution either by reduction to the metallic form with ferrous salts or by solvent-extraction methods. The solution is then treated with ammonium chloride to produce a precipitate of ammonium hexachloroplatinate(IV),... 

The most common oxidatiou states and corresponding electronic configurations of rhodium are +1 which is usually square planar although some five coordinate complexes are known, and +3 (t7 ) which is usually octahedral. Dimeric rhodium carboxylates are +2 (t/) complexes. Compounds iu oxidatiou states —1 to +6 (t5 ) exist. Significant iudustrial appHcatious iuclude rhodium-catalyzed carbouylatiou of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to -butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

Sodium nitrite has been synthesized by a number of chemical reactions involving the reduction of sodium nitrate [7631-99-4] NaNO. These include exposure to heat, light, and ionizing radiation (2), addition of lead metal to fused sodium nitrate at 400—450°C (2), reaction of the nitrate in the presence of sodium ferrate and nitric oxide at - 400° C (2), contacting molten sodium nitrate with hydrogen (7), and electrolytic reduction of sodium nitrate in a cell having a cation-exchange membrane, rhodium-plated titanium anode, and lead cathode (8). 

In addition to platinum and related metals, the principal active component ia the multiflmctioaal systems is cerium oxide. Each catalytic coaverter coataias 50—100 g of finely divided ceria dispersed within the washcoat. Elucidatioa of the detailed behavior of cerium is difficult and compHcated by the presence of other additives, eg, lanthanum oxide, that perform related functions. Ceria acts as a stabilizer for the high surface area alumina, as a promoter of the water gas shift reaction, as an oxygen storage component, and as an enhancer of the NO reduction capability of rhodium. 

Precious Meta.1 Ca.ta.lysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role ia the reduction of NO, and is combiaed with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see Platinum-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal caimot directly participate ia the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

A TWC catalyst must be able to partition enough CO present in the exhaust for each of these reactions and provide a surface that has preference for NO adsorption. Rhodium has a slight preference for NO adsorption rather than O2 adsorption Pt prefers O2. Rh also does not cataly2e the unwanted NH reaction as does Pt, and Rh is more sinter-resistant than Pt (6). However, the concentrations of O2 and NO have to be balanced for the preferred maximum reduction of NO and oxidation of CO. This occurs at approximately the stoichiometric point with just enough oxidants (O2 and NO ) and reductants (CO, HC, and H2). If the mixture is too rich there is not enough O2 and no matter how active the catalyst, some CO and HC is not converted. If the mixture is too lean, there is too much O2 and the NO caimot effectively compete for the catalyst sites (53—58). 

Nonselective catalytic reduction systems are often referred to as three-way conversions. These systems reduce NO, unbumed hydrocarbon, and CO simultaneously. In the presence of the catalyst, the NO are reduced by the CO resulting in N2 and CO2 (37). A mixture of platinum and rhodium has been generally used to promote this reaction (37). It has also been reported that a catalyst using palladium has been used in this appHcation (1). The catalyst operation temperature limits are 350 to 800°C, and 425 to 650°C are the most desirable. Temperatures above 800°C result in catalyst sintering (37). Automotive exhaust control systems are generally NSCR systems, often shortened to NCR. 

Most ring syntheses of this type are of modern origin. The cobalt or rhodium carbonyl catalyzed hydrocarboxylation of unsaturated alcohols, amines or amides provides access to tetrahydrofuranones, pyrrolidones or succinimides, although appreciable amounts of the corresponding six-membered heterocycle may also be formed (Scheme 55a) (73JOM(47)28l). Hydrocarboxylation of 4-pentyn-2-ol with nickel carbonyl yields 3-methylenetetrahy-drofuranone (Scheme 55b). Carbonylation of Schiff bases yields 2-arylphthalimidines (Scheme 55c). The hydroformylation of o-nitrostyrene, subsequent reduction of the nitro group and cyclization leads to the formation of skatole (Scheme 55d) (81CC82). 

Scheme 2.12. Enantioselective Reduction of 2-Acetamidoacrylic Acids by Chiral Phosphine Complexes of Rhodium...

The 17-ethylene ketal of androsta-l,4-diene-3,17-dione is reduced to the 17-ethylene ketal of androst-4-en-3,17-dione in about 75% yield (66% if the product is recrystallized) under the conditions of Procedure 8a (section V). However, metal-ammonia reduction probably is no longer the method of choice for converting 1,4-dien-3-ones to 4-en-3-ones or for preparing 5-en-3-ones (from 4,6-dien-3-ones). The reduction of 1,4-dien-3-ones to 4-en-3-ones appears to be effected most conveniently by hydrogenation in the presence of triphenylphosphine rhodium halide catalysts. Steroidal 5-en-3-ones are best prepared by base catalyzed deconjugation of 4-en-3-ones. ... 

The reduction of keto steroids by treatment with chloroiridic acid, or sodium chloroiridate, and trimethyl phosphite has been studied in some detail.Ketones at the 2- and 3-positions are reduced predominantly to the corresponding axial alcohols, while ketones at 4,6,7,11,12,17 and 20 are not affected. The rate of reaction is increased by addition of aqueous sodium hydroxide. Replacement of sodium chloroiridate by tris(triphenylphos-phine)rhodium chloride gives a system which reduces a 3-keto steroid to the... 

Reduction of the A" -double bond with the rhodium complex is a very slow reaction, but it has been accomplished in 17)S-hydroxyandrost-4-en-3-one (140)d The product, 4a, 5a-d2-androstan-17j3-ol-3-one (141), is a further example of the preferential a-side deuteration in homogeneous solution as contrasted with the )S-face attack with heterogeneous catalysts. [For a more convenient preparation of compound (141) see section V-C.]... 

Despite the above similarities, many differences between the members of this triad are also to be noted. Reduction of a trivalent compound, which yields a divalent compound in the case of cobalt, rarely does so for the heavier elements where the metal, univalent compounds, or hydrido complexes are the more usual products. Rhodium forms the quite stable, yellow [Rh(H20)6] " ion when hydrous Rh203 is dissolved in mineral acid, and it occurs in the solid state in salts such as the perchlorate, sulfate and alums. [Ir(H20)6] + is less readily obtained but has been shown to occur in solutions of in cone HCIO4. 

A solution of resorcinol (11 g) in sodium hydroxide solution (4.8 g of sodium hydroxide in 20 ml of water) is hydrogenated in the presence of 1.1 g of 5 % rhodium on alumina for 16-18 hours at 50 psi initial pressure in a Parr apparatus. The reduction stops after the absorption of 1 equivalent of hydrogen. The catalyst is removed by filtration through celite, and the aqueous solution is carefully acidified with concentrated hydrochloric acid at 0°. The crude product is collected by filtration, dried in air, and recrystallized from benzene to give 1,3-cyclohexanedione, mp 105-107. ... 

Closely related to the use of rhodium catalysts for the hydrogenation of phenols is their use in the reduction of anilines. The procedure gives details for the preparation of the catalyst and its use to carry out the low-pressure reduction of /j-aminobenzoic acid. Then, as in the preceding experiment, advantage is taken of the formation of a cyclic product to carry out the separation of a mixture of cis and trans cyclohexyl isomers. 

A solution of 76 g (S)-( + )-mandelic acid in 400 ml methanol and 5 ml acetic acid was reduced over 5% rhodium-on-alumina under 100 psig for 10 h. The catalyst was removed by filtration through Celite, and the methanol was removed in a rotary evaporator. The white, solid residue was dissolved in I 1 of hot diethyl ether and filtered while hot. After reduction of the volume to 400 ml, 250 ml cyclohexane was added. The remainder of the ether was removed, and the cyclohexane solution was stored for several hours in a refrigerator. The white crystals were filtered and dried in vacuo at 40 C the yield of (S)-( + )-hexahydromandelic acid was 71%. 

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