Iridium-Catalyzed Carbonylation of Methanol

Monsanto also discovered significant catalytic activity for iridium/iodide catalysts however, they chose to commercialize the rhodium-based process due to its higher activity under conventional high water conditions. Despite this, detailed mechanistic studies by Forster and his colleagues were undertaken at Monsanto and revealed a catalytic mechanism for iridium which is similar to the rhodium system in many respects, but with additional complexity due to participation of both anionic and neutral complexes (see below). 

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 

The Cativa process was first commercialized in 1995, with the retro-fitting to an existing rhodium-based plant in Texas City (USA), and several other acetic acid plants now use the Cativa technology. 

A range of compounds enhance the activity of an iridium catalyst. The promoters fall into two categories (i) carbonyl or halocarbonyl complexes of W, Re, Ru, Os and Pt and (ii) simple iodides of Zn, Cd, Hg, Ga and In. The preferred ruthenium promoter is effective over a range of water concentrations the maximum rate being attained at ca. 5% wt H2O, as in the absence of promoter. By contrast, ionic iodides such as Lil and BU4NI are strong catalyst poisons. 

7 Mechanism of the Iridium/Iodide Catalyzed Methanol Carbonylation 

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

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... 

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

The iridium-catalyzed carbonylation of methanol known as the Cotiva process was aimounced by BP Chemicals in 1996 it now operates on a number of plants worldwide [43—46]. The advantages of iridium catalysts are better stability because of stronger metal—ligand bonding, broad reaction conditions tolerability, and others. 

The analogous anionic iridium complex reacts with methyl iodide 150 times faster than the rhodium complex. The iridium complex also reacts 140-200 times faster than the rhodium analog with higher alkyl iodides/ but competing radical mechanisms appear to occur during the addition of the higher alkyl iodides. More details on the mechanism of rhodium and iridium-catalyzed carbonylation of methanol are provided in Chapter 17. 

Iridium-Catalyzed Carbonylation of Methanol BP s Cativa Process... 

Figure 4.3 Iridium-catalyzed carbonylation of methanol. Hydrolysis of 4.14 and 4.15 is not shown and the organic cycle is on the left.

Figure 8.5 Catalytic cycle for the metal-catalyzed carbonylation of methanol, with the reductive elimination step highlighted. In the case of iridium, the diiodotricarbonyl species has also been suggested as a possible precursor to reductive elimination. What aie the issues of stereochemistry associated with the intermediates What special basis-set requirements will be involved in modeling this system ...

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. 

If ethylene is present during the carbonylation of methanol catalyzed by IrCl4, once again with Mel as promoter, methyl propionate is formed.416 The reaction depends on the presence of iridium hydride species in solution, and a rhodium analogue of the reaction exists. The full details of the mechanism are not known but the basic steps are shown in Scheme 34. The intermediates are all believed to be complexes of iridium(IIl). 

Iridium supported on active carbon was found to be less active than rhodium but more active than nickel (33). Methyl chloride is less efficient but still a moderate promoter in the Ir/C-catalyzed vapor-phase carbonylation of methanol... 

In the mid-1960s, Paulik and Roth at Monsanto Co discovered that rhodium and an iodide promoter were more efficient than cobalt, with selectivities of 99% and 85%, with regard to methanol and CO, respectively. Moreover, the reaction is operated under significantly milder conditions such as 40-50 bar pressure and around 190 °C [8]. Even though rhodium was 1000 times more costly than cobalt at this time, Monsanto decided to develop the rhodium-based catalyst system mainly for the selectivity concerns, and thus for the reduction of the process cost induced by the acetic acid purification, even if it was necessary to maintain a 14% w/w level of water in the reactor to keep the stability of the rhodium catalyst. In addition, Paulik et al. [9] demonstrated that iridium can also catalyze the carbonylation of methanol although at a lower rate. However, it is noteworthy that the catalytic system is more stable, especially in the low partial pressure zones of the industrial unit. 

The conditions employed for iridium-catalyzed carbonylation (ca. 180-190 °C, 20-40 bar) are comparable to those of the rhodium-based process. A variety of iridium compounds (e.g., I1CI3, IrU, H2I1CI6, Ir4(CO)i2) can be used as catalyst precursors, as conversion into the active iodocarbonyl species occurs rapidly under process conditions. In a working catalytic system, the principal solvent component is acetic acid, so the methanol feedstock is substantially converted into its acetate ester (Equation (2)). Methyl acetate is then activated by reaction with the iodide co-catalyst (Equation (3)). Catalytic carbonylation of methyl iodide formally gives acetyl iodide (Equation (4)) prior to rapid hydrolysis to the product acetic acid (Equation (5)). However, it is difficult to establish the true intermediacy of acetyl... 

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

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. 

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. 

The attainment of optimum rate at relatively low [H2O] is a significant benefit for the iridium system, since it results in less costly product purification. A typical configuration for an iridium-catalyzed methanol carbonylation plant is shown in Figure 2. The feedstocks (MeOH and CO) are fed to the reactor vessel on a continuous basis. In the initial product separation step, the reaction mixture is passed from the reactor into a flash tank where the pressure is reduced to induce vaporization of most of the volatiles. The catalyst remains dissolved in the liquid phase and is recycled back to the reactor vessel. The vapor from the flash tank is directed into a distillation train, which removes methyl iodide, water, and heavier byproducts (e.g., propionic acid) from the acetic acid product. At the relatively high water levels used in the rhodium-catalyzed Monsanto process, three distillation columns are typically required. In the Cativa process, a lower water concentration means that the necessary product purification can be achieved with only two columns. 

Scheme 1 Catalytic cycles for iridium-catalyzed methanol carbonylation and WGS reaction. Adapted from Forster, D. J. Chem. Soc., Dalton Trans. 1979, 1639-1645, with permission from The Royal Society of Chemistry.

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