Rhodium complex-catalyzed carbonylation methanol

Figure 8.7 Proposed mechanism of rhodium complex-catalyzed carbonylation of methanol (Forster, 1976).

Forster, D. On the mechanism of a rhodium-complex-catalyzed carbonylation of methanol to acetic acid. J. Am. Chem. Soc. 98, 846 (1976)... 

Forster, D. (1976) Mechanism of a rhodium-complex-catalyzed carbonylation of melhanol to acetic add. Journal of the American Chemical Society, 98, 846 Adamson, G.W., Daly, J.J. and Forster, D. (1974) Reaction of iodocarbonylrhodium ions with melhyl-iodide - structure of rhodium acetyl complex -[Me3PhN+]2[Rh2l6(MeCO)2(CO)2]2-. Journal of Organometallic Chemistry, 71, Cl7 Haynes, A., Mann, B.E., Gulliver, D.J., Morris, D.E. and Maillis, P.M. (1991) Direct observation of MeRh(CO)2l3 the key intermediate in rhodium catalyzed methanol carbonylation. Journal of the American Chemical Society 113, 8567 Haynes, A., Mann, B.E., Morris, G.E. and Maidis, P.M. (1993) Mechanistic studies on rhodium-catalyzed carbonylation reactions — spectroscopic detection and reactivity of a key intermediate, [MeRh(CO)2l3]. Journal of the American Chemical Society, 115, 4093. 

FIGURE 1 Process flow diagram for rhodium-complex-catalyzed methanol carbonylation. Adapted with permission from Figure 1 in reference [16], copyright 2006, Springer Science + Business Media. 

SCHEME 1 Catalytic cycle for the rhodium-complex-catalyzed methanol carbonylation. 

One approach that enables the use of lower water concentrations for rhodium-complex-catalyzed methanol carbonylation is the addition of iodide salts, as exemplified by the Celanese Acid Optimization (AO Plus) technology [11,33]. A lithium iodide promoter allows carbonylation rates to be achieved that are comparable with those in the conventional Monsanto process—but at significantly lower water concentrations. The AO technology has been implemented to increase productivity at the Celanese facility in Clear Lake, Texas, and in a new 500 kt/a plant in Singapore. 

It has been found that iodide salts can promote the oxidative addition of Mel to [Rh(CO)2I2], the rate-determining step in the cycle of the rhodium-complex-catalyzed methanol carbonylation reaction [20]. 

A process for the coproduction of acetic anhydride and acetic acid, which has been operated by BP Chemicals since 1988, uses a quaternary ammonium iodide salt in a role similar to that of Lil [8]. Beneficial effects on rhodium-complex-catalyzed methanol carbonylation have also been found for other additives. For example, phosphine oxides such as Ph3PO enable high catalyst rates at low water concentrations without compromising catalyst stability [40—42]. Similarly, iodocarbonyl complexes of ruthenium and osmium (as used to promote iridium systems, Section 3) are found to enhance the activity of a rhodium catalyst at low water concentrations [43,44]. Other compounds reported to have beneficial effects include phosphate salts [45], transition metal halide salts [46], and oxoacids and heteropolyacids and their salts [47]. 

Diphosphine ligands of the type Ar2P(CH2)nPAr2 (n = 2 4) were originally found by Moloy and Wegman [85,86] to be effective in the rhodium-complex-catalyzed reductive carbonylation of methanol to acetaldehyde (Equation (10)) when synthesis gas (CO + H2) was used instead of pure CO as the feed gas. With a ruthenium trichloride cocatalyst, in situ hydrogenation of the aldehyde to ethanol resulted in the overall homologation reaction shown in Equation (11). 

Adapted from Hjortkjaer, J. and Jensen, V. W., Rhodium complex catalyzed methanol carbonylation. Ind. Eng. (Jhem. Prod. Res. Dev., 15(1), 46-49 (1976). 

Rhodium catalyzed carbonylations of olefins and methanol can be operated in the absence of an alkyl iodide or hydrogen iodide if the carbonylation is operated in the presence of iodide-based ionic liquids. In this chapter, we will describe the historical development of these non-alkyl halide containing processes beginning with the carbonylation of ethylene to propionic acid in which the omission of alkyl hahde led to an improvement in the selectivity. We will further describe extension of the nonalkyl halide based carbonylation to the carbonylation of MeOH (producing acetic acid) in both a batch and continuous mode of operation. In the continuous mode, the best ionic liquids for carbonylation of MeOH were based on pyridinium and polyalkylated pyridinium iodide derivatives. Removing the highly toxic alkyl halide represents safer, potentially lower cost, process with less complex product purification. 

THE CARBONYLATION OF METHANOL CATALYZED BY RHODIUM COMPLEXES IN SOLUTION... 

The most common oxidation states and corresponding electronic configurations of rhodium are +1 (tf8), which is usually square planar although some five coordinate complexes are known, and +3 (T) which is usually octahedral. Dimeric rhodium carboxylates are +2 (oxidation states —1 (industrial applications include rhodium-catalyzed carbonylation of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to tf-butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

Industrial Applications. Several large scale industrial processes are based on some of the reactions listed above, and more are under development. Most notable among those currently in use is the already mentioned Wacker process for acetaldehyde production. Similarly, the production of vinyl acetate from ethylene and acetic acid has been commercialized. Major processes nearing commercialization are hydroformylations catalyzed by phosphine-cobalt or phosphine-rhodium complexes and the carbonylation of methanol to acetic acid catalyzed by (< 3P) 2RhCOCl. 

Reaction (78) regenerates Mel from methanol and HI. Using a high-pressure IR cell at 0.6 MPa, complex (95) was found to be the main species present under catalytic conditions, and the oxidative addition of Mel was therefore assumed to be the rate determining step. The water-gas shift reaction (equation 70) also occurs during the process, causing a limited loss of carbon monoxide. A review of the cobalt-, rhodium- and iridium-catalyzed carbonylation of methanol to acetic acid is available.415... 

Rhodium compounds and complexes are also commercially important catalysts. The hydroformylation of propene to butanal (a precursor of hfr(2-ethyUiexyl) phthalate, the PVC plasticizer) is catalyzed by hydridocarbonylrhodium(I) complexes. Iodo(carbonyl)rhodium(I) species catalyze the production of acetic acid from methanol. In the flne chemical industry, rhodium complexes with chiral ligands catalyze the production of L-DOPA, used in the treatment of Parkinson s disease. Rhodium(II) carboxylates are increasingly important as catalysts in the synthesis of cyclopropyl compounds from diazo compounds. Many of the products are used as synthetic, pyrethroid insecticides. Hexacyanorhodate(III) salts are used to dope silver halides in photographic emulsions to reduce grain size and improve gradation. 

Several further experimental and theoretical studies of the oxidative addition of methyl iodide to Rh(I) complexes have been reported, in part because of its importance in the rhodium-catalyzed carbonylation of methanol (see Carbonylation Processes by Homogeneous Catalysis) to... 

The reaction of divalent metals, such as copper, nickel, and so on, with dioxetanes in methanol leads to clean catalytic decomposition into carbonyl fragments/ The reaction rates increase with increasing Lewis acidity of the divalent metal and indicate, therefore, typical electrophilic cleavage of the dioxetane. On the other hand, univalent rhodium and iridium complexes catalyze the decomposition of dioxetanes into carbonyl fragments via oxidative addition. 

Ditertiary phosphines such as (86), (92), and (98) (100) (Scheme 6) have found important uses as ligands for metal-catalyzed transformations, including e.g., palladium-catalyzed Grignard cross couplings,194,205 rhodium-catalyzed Michael additions,2 hydrocyanations,206 copolymerizations,20 and palladium-catalyzed animations.208 Rhodium complexes of (86) are catalysts for the carbonylation of methanol.188 More recently the ligand bite angle of ditertiary phosphines such as (100) has been shown to influence catalytic activity/selectivity in several important catalytic processes.209-213... 

In the rhodium-catalyzed carbonylation of methanol via methyl iodide, acetyl iodide is formed by reductive elimination from an anionic rhodium acyl complex [T14] ... 

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