Carbon dioxide rhodium

In one patent (31), a filtered, heated mixture of air, methane, and ammonia ia a volume ratio of 5 1 1 was passed over a 90% platinum—10% rhodium gauze catalyst at 200 kPa (2 atm). The unreacted ammonia was absorbed from the off-gas ia a phosphate solution that was subsequently stripped and refined to 90% ammonia—10% water and recycled to the converter. The yield of hydrogen cyanide from ammonia was about 80%. On the basis of these data, the converter off-gas mol % composition can be estimated nitrogen, 49.9% water, 21.7% hydrogen, 13.5% hydrogen cyanide, 8.1% carbon monoxide, 3.7% carbon dioxide, 0.2% methane, 0.6% and ammonia, 2.3%. 

GP 8] [R 7] Ignition occurs at a rhodium catalyst at catalyst temperatures between 550 and 700 °C, depending on the process parameters [3]. Total oxidation to water and carbon dioxide is favored at low conversion (< 10%) prior to ignition. Once ignited, the methane conversion increases and hence the catalyst temperature increases abruptly. 

The photocatalyzed reduction of carbon dioxide at elevated pressure was also investigated. Porous glass beads were used to obtain efficient gas-liquid contact. With isopropanol as the solvent and 2-propyl formate as the reducing agent,the reaction products were carbon monoxide and hydrogen. The catalyst, chloro(tetraphenyl-porphinato)rhodium(III), was irradiated with visible light /21/. 

The system is not limited to the use of synthesis gas as feed. Mixtures of carbon dioxide and hydrogen also give rise to the formation of polyhydric alcohols, and it is also claimed that the reaction mixture can consist of steam and carbon monoxide (62). This latter claim is consistent with the presence of C02 in the reaction mixture when CO/H2 is used as feed [infrared data (62)], and suggests that these ionic rhodium systems are also active catalysts for the water gas-shift reaction (vide infra). 

With reference to the homogeneous catalyst systems thus far reported for the synthesis of hydrocarbons/chemicals from carbon monoxide and hydrogen, only the anionic rhodium systems of Union Carbide show any appreciable shift activity. With neutral species of the type M3(CO)12 (M = Ru or Os), only small quantities of carbon dioxide are produced under the synthesis conditions (57). 

In addition to the polymeric rhodium catalysts previously discussed, monomeric rhodium systems prepared from [Rh(CO)2Cl]2 by addition of strong acid (HC1 or HBF4) and Nal in glacial acetic acid have also been shown to be active homogeneous shift catalysts (80). The active species is thought to be an anionic iodorhodium carbonyl species, dihydrogen being produced by the reduction of protons with concomitant oxidation of Rh(I) to Rh(III) [Eq. (18)], and carbon dioxide by nucleophilic attack of water on a Rh(III)-coordinated carbonyl [Eq. (19)]. 

D. Koch, W. Leitner, Rhodium-Catalyzed Hydroformylation in Supercritical Carbon Dioxide ,/ Am. Chem. Soc 1998,120,13398. 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

Supercritical fluids (e.g. supercritical carbon dioxide, scCCb) are regarded as benign alternatives to organic solvents and there are many examples of their use in chemical synthesis, but usually under homogeneous conditions without the need for other solvents. However, SCCO2 has been combined with ionic liquids for the hydroformylation of 1-octene [16]. Since ionic liquids have no vapour pressure and are essentially insoluble in SCCO2, the product can be extracted from the reaction using CO2 virtually uncontaminated by the rhodium catalyst. This process is not a true biphasic process, as the reaction is carried out in the ionic liquid and the supercritical phase is only added once reaction is complete. 

Another way of getting around the problem of the separation of the catalyst from the substrate is via use of a flow reactor [38], Supercritical carbon dioxide has been used successfully as a medium for the hydroformylation of 1-octene using an immobilized rhodium catalyst. The catalyst is covalently fixed to silica through the modifying ligand A-(3-trimethoxysilyl-n-propyl)-4,5-bis(diphenylphosphino)phenoxazine (Figure 8.13). Selectivity was found to be... 

Rh (NHo) 5C1] (OH) 2, is produced by mixing the chloride with moist silver oxide. Silver chloride is precipitated, and a strongly alkaline liquid produced containing the hydroxide. The base absorbs carbon dioxide from the air, removes ammonia from ammonium salts, and precipitates metallic hydroxides from solutions of the metallic salts. It is only known in solution, and on evaporation of the liquid it is slowly transformed in the cold, more rapidly on heating, into a mixture of aquo-pentammino-rhodium chloride and aquo-pentammino-rhodium hydroxide, thus ... 

Dichloro-tetrapyridino-rhodium Hydroxide, [Rh py4Cl2](OH), may be obtained from the chloride by grinding it with freshly precipitated silver oxide. The liquid so produced absorbs carbon dioxide from the air and liberates ammonia from ammonium salts. 

Japanese chemists succeeded in obtaining good yields of methane by reaction of H2 with a mixture of carbon monoxide and carbon dioxide, at temperatures as low as 270 °C, by use of a special mixed catalyst containing nickel as the most important metallic constituent. Why is nickel used In the same vein, why is platinum or platinum-rhodium alloy (but not nickel) used in catalytic converters for automobile exhausts (See also Section 17.4.)... 

Rasmussen, S.C., Richter, M.M., Yi, E., Place, H. and Brewer, KJ. (1990) Synthesis and characterization of a series of novel rhodium and iridium complexes containing polypyridyl bridging ligands Potential uses in the development of multimetal catalysts for carbon dioxide reduction. Inorg. Chem., 29, 3926—3932. 

Carbon dioxide insertion into a rhodium-carbon bond has been found in the reaction of Rh(Ph)(PPh3)3 with C02 under 20 atm of pressure at room... 

The hydrogenation of vinylnaphthalene 1 was performed by mixing solid chloro-tris(triphenylphosphine)rhodium catalyst (7.0 mg, 7.6 pmol) with solid 2-vinyl-naphthalene (350 mg, 2.27 mmol, substrate Rh=300 l), both fine powders. The mixture was placed, with a stirring bar, into a 22 mm diameter flat-bottomed glass finer in a 160-mL high-pressure vessel, which was then sealed and warmed to 33 °C in a water bath. The vessel was flushed and pressurized with H2 to 10 bar. This was considered the start of the reaction. Carbon dioxide was then added to a total pressure of 67 bar. After 30 min, the vessel was removed from the water bath and vented. The product mixture was dissolved in CDCI3 and characterized by H NMR spectroscopy. 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

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