Iridium-Catalyzed Methanol Carbonylation

The diphosphine complexes [Ir(dppe)2]+ and [Ir(dppe)(cod)]+ reportedly catalyze the decar--bonylation of aldehydes.504,505 Methanol carbonylation is catalyzed by lr(Cl)4]-H20 to yield acetic acid. The active species is thought to be an acetyl iridium(III) species, wherein the rate-determining step involves electrophilic attack of this species on methanol. The effects of added iodide ion on reaction rates have also been investigated.506... 

Scheme 2. Catalytic cycles for the iridium-catalyzed carbonylation of methanol.

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.

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... 

Methane, C-H activation, via palladium carbenes, 8, 243 Methanol carbonylations to acetic acid, 1, 133 iridium-catalyzed... 

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

Scheme 3. Proposed mechanism for the iridium-catalyzed carbonylations of methanol.

Interest in iridium-catalyzed methanol carbonylation was rekindled in the 1990 s when BP Chemicals developed and commercialized the Cativa process, which utilizes an iridium/iodide catalyst and a ruthenium promoter. This process has the important advantage that the highest catalytic rates occur at significantly lower water concentration (ca. 5% wt) than for Monsanto s... 

Mechanism of the Iridium/Iodide Catalyzed Methanol Carbonylation... 

Discussion Point DPS hint Using the concepts behind the successful iridium catalyzed methanol carbonylation as guide which would you expect the difficult steps in a hydroformylation to be. 

Since 1979, numerous reviews have appeared on the kinetics, mechanisms, and process chemistry of the metal-catalyzed methanol carbonylation reaction [11, 14-20], especially the Monsanto rhodium-catalyzed process. In this section, the traditional process chemistry as patented by Monsanto is discussed, with emphasis on some of the significant improvements that Monsanto s licensee, Celanese Chemicals (CC) has contributed to the technology. The iridium-based methanol carbonylation process recently commercialized by BP Chemicals Ltd. (BP) will be discussed also. 

In the 1990s, BP re-examined the iridium-catalyzed methanol carbonylation chemistry first discovered by Paulik and Roth and later defined in more detail by Forster [20]. The thrust of this research was to identify an improved methanol carbonylation process using Ir as an alternative to Rh. This re-examination by BP led to the development of a low-water iridium-catalyzed process called Cativa [20]. Several advantages were identified in this process over the Rh-catalyzed high-water Monsanto technology. In particular, the Ir catalyst provides high carbonylation rates at low water concentrations with excellent catalyst stability (less prone to precipitation). The catalyst system does not require high levels of iodide salts to stabilize the catalyst. Fewer by-products are formed, such as propionic acid and acetaldehyde condensation products which can lead to low levels of unsaturated aldehydes and heavy alkyl iodides. Also, CO efficiency is improved. 

The Ir-catalyzed methanol carbonylation reaction has been studied extensively by several groups 9f-h. The mechanism for the reaction is more complex than for the Rh reaction. The reaction involves a neutral and an anionic catalytic cycle. The extent of participation by each cycle depends on the reaction conditions. The anionic carbonylation pathway predominates in the Cativa process. The active Ir catalyst species is the iridium carbonyl iodide complex, [Ir(CO)2l2]. The carbonylation reaction proceeds through a series of reaction steps similar to the Rh catalyst process shown in Figure 1 however, the kinetics involve a different rate determining step. 

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]. 

FIGURE 4 Effect of additive concentration on rate of iridium-catalyzed methanol carbonylation (190 °C, 22 bar, 1950 ppm Ir). Adapted with permission from Figure 2 in reference [125], copyright 2004, American Chemical Society. 

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