Rhodium-acetate catalysts, oxidation

Finally, it should be mentioned that there is one important commercial application of the organic halide carbonylation. This is in the rhodium and methyl iodide-catalyzed conversion of methanol and carbon monoxide into acetic acid (25). The mechanism of the reaction appears to involve the oxidative addition of methyl iodide to the rhodium(I) catalyst followed by CO insertion and hydrolysis ... 

Acetic acid (CH3COOH) is a bulk commodity chemical with a world production of about 3.1 x 106 Mg/year, a demand increasing at a rate of +2.6% per year and a market price of US 0.44-0.47 per kg (Anon., 2001a). It is obtained primarily by the Monsanto or methanol carbonylation process, in which carbon monoxide reacts with methanol under the influence of a rhodium complex catalyst at 180°C and pressures of 30-40 bar, and secondarily by the oxidation of ethanol (Backus et al., 2003). The acetic fermentation route is limited to the food market and leads to vinegar production from several raw materials (e.g., apples, malt, grapes, grain, wines, and so on). 

The kinetics and mechanism of the carbonylation of methanol to acetic acid using Monsanto s rhodium complex catalyst has been extensively studied. The reaction is first order in both rhodium and CH3I promoter but zero order in CO pressure. It is believed that oxidative addition of CH3I is the rate-controlling step in this process. This is a unique example of designing a catalyst system with commercial viability in which the substrate (methanol) is first converted to CH3I... 

This reaction is rapidly replacing the former ethylene-based acetaldehyde oxidation route to acetic acid. The Monsanto process employs rhodium and methyl iodide, but soluble cobalt and iridium catalysts also have been found to be effective in the presence of iodide promoters. 

This process is one of the three commercially practiced processes for the production of acetic anhydride. The other two are the oxidation of acetaldehyde [75-07-0] and the carbonylation of methyl acetate [79-20-9] in the presence of a rhodium catalyst (coal gasification technology, Halcon process) (77). The latter process was put into operation by Tennessee Eastman in 1983. In the United States the total acetic anhydride production has been reported to be in the order of 1000 metric tons. 

Ca.ta.lysis, The readily accessible +1 and +3 oxidation states of rhodium make it a useful catalyst. There are several reviews of the catalytic properties of rhodium available (130—132). Rhodium-catalyzed methanol carbonylation (Monsanto process) accounted for 81% of worldwide acetic acid by 1988 (133). The Monsanto acetic acid process is carried out at 175°0 and 1.5 MPa (200 psi). Rhodium is introduced as RhCl3 but is likely reduced in a water... 

Alternatively, butadiene can be oxidized in the presence of acetic acid to produce butenediol diacetate, a precursor to butanediol. The latter process has been commercialized (102—104). This reaction is performed in the Hquid phase at 80°C with a Pd—Te—C catalyst. A different catalyst system based on PdCl2(NCCgH )2 has been reported (105). Copper- (106) and rhodium- (107) based catalysts have also been studied. 

With the exception of acetic, acryUc, and benzoic all other acids in Table 1 are primarily produced using oxo chemistry (see Oxo process). Propionic acid is made by the Hquid-phase oxidation of propionaldehyde, which in turn is made by appHcation of the oxo synthesis to ethylene. Propionic acid can also be made by oxidation of propane or by hydrocarboxylation of ethylene with CO and presence of a rhodium (2) or iridium (3) catalyst. 

Another metal that has attracted interest for use as electrode material is rhodium, inspired by its high activity in the catalytic oxidation of CO in automotive catalysis. It is found that Rh is a far less active catalyst for the ethanol electro-oxidation reaction than Pt [de Souza et al., 2002 Leung et al., 1989]. Similar to ethanol oxidation on Pt, the main reactions products were CO2, acetaldehyde, and acetic acid. Rh, however, presents a significant better CO2 yield relative to the C2 compounds than Pt, indicating a... 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

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)]. 

Meanwhile, Wacker Chemie developed the palladium-copper-catalyzed oxidative hydration of ethylene to acetaldehyde. In 1965 BASF described a high-pressure process for the carbonylation of methanol to acetic acid using an iodide-promoted cobalt catalyst (/, 2), and then in 1968, Paulik and Roth of Monsanto Company announced the discovery of a low-pressure carbonylation of methanol using an iodide-promoted rhodium or iridium catalyst (J). In 1970 Monsanto started up a large plant based on the rhodium catalyst. 

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