Catalyzed Methanol Carbonylation

The chemistry of [Ir(CO)3I], [Ir(CO)2I2 ], and [CH3Ir(CO)2I3 ] has been explored independent of the carbonylation reaction. Some of the perti- 

The kinetic dependencies together with the chemical observations above lead to the following scheme for the catalytic reaction  

The ionic cycle is important under reaction conditions where iodide ion can exist, e.g., higher water levels (CH3OH + HI CH3I + H20) or with salt additives. However, while higher ionic iodide levels give an iridium species capable of very rapid reaction with methyl iodide, they also serve to inhibit the formation of an acyl species. The relatively slow conversion of [CH3Ir(CO)2I3] to an acyl species is almost certainly not 

A further complication is evident in the spectroscopic studies of the reacting iridium solutions, namely, a competing catalytic water gas shift reaction involving hydrido-iridium(III) species. Choice of reaction conditions determines the proportion of the iridium occupied in this catalytic cycle. 

The rate of the methanol carbonylation reaction in the presence of iridium catalysts is very similar to that observed in the presence of rhodium catalysts under comparable conditions (29). This is perhaps initially surprising in view of the well-recognized greater nucleophilicity of iridium(I) complexes as compared to their rhodium(I) analogues. It can be seen from the above studies that the difference in the chemistry of the metals at the trivalent stage of the catalytic cycle serves to produce faster rates of alkyl migration with the rhodium system thus, overall the two metal catalysts give comparable rates. 

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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... 

Traditional rhodinm catalyzed methanol carbonylation to acehc acid (Eqn. 14) requires 15-20 wt.% Mel as co-catalyst (MeOH MeI normally 10 1) and proceeds via an Sn2 reachon between Mel and Rh(CO)2l2" (8). Consistent with the Sn2 behavior, the reachon is T order in both Mel and Rh. (See Scheme 37.2 for the mechanism.)... 

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

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

Figure 8.3 (a) Catalytic cycle for the iridium-catalyzed methanol carbonylation (b) catalytic cycle for the iridium-catalyzed water gas shift (WGS) reaction. Both as originally proposed by D. Forster (adapted from Ref [25]). 

Figure 8.4 Methanol-assisted migratory CO insertion in the iridium-catalyzed methanol carbonylation (adapted from Refs [37, 38]).

Figure 8.6 Main step showing the role of the cocatalyst in the iridium-catalyzed methanol carbonylation reaction.

Acetic Acid Production via Low-Pressure, Nickel-Catalyzed Methanol Carbonylation... 

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

Acetic Acid. Although at the time of this writing Monsanto s Rh-catalyzed methanol carbonylation (see Section 7.2.4) is the predominant process in the manufacture of acetic acid, providing about 95% of the world s production, some acetic acid is still produced by the air oxidation of n-butane or light naphtha. n-Butane is used mainly in the United States, whereas light naphtha fractions from petroleum refining are the main feedstock in Europe. 

Mel, in Rh-catalyzed methanol carbonylation, 7, 256 to monocarbonyl iridium complexes, 7, 284 in mononuclear ruthenium and osmium alkynyl formations,... 

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