Rhodium-catalyzed methanol carbonylation

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... 

Comparison of Cobalt- and Rhodium-Catalyzed Methanol Carbonylation Reactions... 

Figure 8.1 Catalytic cycle for the rhodium-catalyzed methanol carbonylation.

A substantial advance was made by discovery of the rhodium-catalyzed methanol carbonylation to acetic acid, developed by Monsanto. 

The mechanism of the rhodium-catalyzed methanol carbonylation has been extensively reviewed elsewhere (17,21,22). The reaction is characterized by kinetics which are independent of CO pressure and are first order both in rhodium and methyl iodide. Taken with the spectroscopic data, these are... 

A problem in all catalyzed processes is that the catalyst slowly loses its efficacy - it deactivates. In the rhodium catalyzed methanol carbonylation this occurs at high acid levels by oxidation by HI to a Rh(III) tetraiodide anion, [Rh(CO)2l4] (Equation 8). 

Discussion Point DPI Oxidative addition of methyl iodide to Rh(I) complexes can be greatly accelerated using electron donating phosphine ligands. Why has this effect not been taken advantage of in a commercial rhodium-catalyzed methanol carbonylation process, whereas phosphine ligands are employed in hydroformylation ... 

Rhodium catalyzed methanol carbonylation P. M. Maitlis, A. Haynes, G. J. Sunley, M. J. Howard, J. Chem. Soc., Dalton Trans., 1996, 2187. 

The reaction chemistry of the rhodium-catalyzed methanol carbonylation process under Monsanto conditions has been investigated extensively [6-8, 10, 12, 21, 26-29] (cf Section 2.1.2.1.1). The overall reaction kinetics are first order in both rhodium catalyst and methyl iodide promoter. The reaction is zero order in methanol and zero order in carbon monoxide partial pressure above 2 atm (eq. (6)) [27]. The kinetics agree well with the basic mechanism common to the three carbonylation reactions (see Section 2.1.2.1.1 and Tables 1 and 2). 

Scheme 1. Interrelated reaction paths for the rhodium-catalyzed methanol carbonylation process [5c] Me = CH,.

The continuous rhodium-catalyzed methanol carbonylation process consists of three major areas the reaction, flasher, and purification sections, as represented in Figure 1 [15]. 

Figure 1. Rhodium-catalyzed methanol carbonylation commercial process flow scheme [15].

INVOLVEMENT OF AN ACETATO LIGAND IN THE REDUCTIVE ELIMINATION STEP OF THE RHODIUM-CATALYZED METHANOL CARBONYLATION... 

Rhodium-Catalyzed Methanol Carbonylation the Ligand-Modified Monsanto Process... 

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 4.2 Rhodium-catalyzed methanol carbonylation. The organic cycle is the same as in cobalt-catalyzed methanol carbonylation.

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