Catalytic cycles Monsanto acetic acid process

Organometallic compounds are used widely as homogeneous catalysts in the chemical industry. For example, if the alkene insertion reaction continues with further alkene inserting into the M C bond, it can form the basis for catalytic alkene polymerisation. Other catalytic cycles may include oxidative addition and reductive elimination steps. Figure above shows the steps involved in the Monsanto acetic acid process, which performs the conversion... 

The hydrocarboxylation reactions discussed above have been proposed to involve direct addition of water to the metal center prior to elimination of the product, analogous to the oxidative addition of hydrogen to a metal center at the end of a hydroformylation catalytic cycle. Another class of hydrocarboxylation reactions is more analogous to the haUde-promoted Monsanto acetic acid process, where one has a reductive elimination of an acyl halide species that is rapidly hydrolyzed with free water to generate the carboxylic acid and HX. 

Fig. 26.6 The Monsanto acetic acid process involves two interrelated catalytic cycles.

Reductive elimination is the product-forming step in some of the most important catalytic cycles, including hydrogenation, the Monsanto acetic acid process, and various types of cross-couplings. For this reason, detailed studies of this process have been conducted. Hrese studies have revealed examples of reductive eliminations to form H-H and C-H bonds, as well as reductive eliminations to form C-G and C-X bonds (in which X = halide, amide, alkoxide, thiolate, and phosphide). The mechanisms of these processes include the same pathways as have been deduced for oxidative addition (i.e., concerted, ionic, and radical), because reductive elimination is the same as oxidative addition, but in the reverse direction. 

FIGURE 12.3 The catalytic cycle proposed for the Monsanto acetic acid process. 

Mankind has produced acetic acid for many thousand years but the traditional and green fermentation methods cannot provide the large amounts of acetic acid that are required by today s society. As early as 1960 a 100% atom efficient cobalt-catalyzed industrial synthesis of acetic acid was introduced by BASF, shortly afterwards followed by the Monsanto rhodium-catalyzed low-pressure acetic acid process (Scheme 5.36) the name explains one of the advantages of the rhodium-catalyzed process over the cobalt-catalyzed one [61, 67]. These processes are rather similar and consist of two catalytic cycles. An activation of methanol as methyl iodide, which is catalytic, since the HI is recaptured by hydrolysis of acetyl iodide to the final product after its release from the transition metal catalyst, starts the process. The transition metal catalyst reacts with methyl iodide in an oxidative addition, then catalyzes the carbonylation via a migration of the methyl group, the "insertion reaction". Subsequent reductive elimination releases the acetyl iodide. While both processes are, on paper, 100%... 

As mentioned in the previous section, the carbonylation of methanol to acetic acid is an important industrial process. Whereas the [Co2(CO)s]-catalyzed, iodide-promoted reaction developed by BASF requires pressures of the order of 50 MPa, the Monsanto rhodium-catalyzed synthesis, which is also iodide promoted and which was discovered by Roth and co-workers, can be operated even at normal pressure, though somewhat higher pressures are used in the production units.4,1-413 The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster.414 The anionic complex m-[RhI2(CO)2]- (95) initiates the catalytic cycle, which is shown in Scheme 26. 

Figure 4.1 Monsanto process, (a) The organometallic catalytic cycle CH3I and CO react to give CH3COI. (b) The organic catalytic cycle water and HI act as catalysts to generate acetic acid and CH3I from CH3COI and methanol.

The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster. The anionic complex c -[Rhl2(CO)2] (95) initiates the catalytic cycle, which is shown in Scheme 26. 

The proposed reductive elimination of acetyl iodide from the Rh(III) coordination sphere is an important step in the Monsanto methanol carbonylation process (2) [73]. In the proposed catalytic cycle (Scheme 30), the oxidative addition of iodo-methane, formed from HI and methanol, is followed by the carbonyl insertion into the Rh-Me bond. The reductive elimination of acetyl iodide followed by its rapid hydrolysis furnishes the acetic acid and regenerates free HI. 

Scheme 30 Proposed catalytic cycle for methanol carbonylation to acetic acid (Monsanto process). Acyl-iodide reductive elimination from a Rh(ni) center is the key step toward the product formation...

Vinegar, or acetic acic, can be made simply through the fermentation of ethanol — yes, fermenting what was fermented However, this process is relatively slow. Acetic acid can be produced much quicker using a rhodium complex that causes the carbonylation of methanol. This process, known as the Monsanto process (named cifter the Monsanto company), accounts for over a million tons of acetic acid a year. The catalytic cycle is a six-step process, as shown in Figure 16-5. 

Rhodium-catalyzed carbonylation of methanol is known as the Monsanto process, which has been studied extensively. From the reaction mechanism aspect, the study of kinetics has proved that the oxidative addition of methyl iodide to the [Rh(CO)2l2] is the rate-determining step of the catalytic cycle. It was also observed that acetyl iodide readily adds to [Rh(CO)2l2], indicating that the acetyl iodide must be scavenged by hydrolysis in order to drive the overall catalytic reaction forward. An alternative to sequential reductive elimination and the hydrolysis of acetyl iodide is the nucleophilic attack of water on the Rh acetyl complex and the production of acetic acid. The relative importance of these two alternative pathways has not yet been fully determined, although the catalytic mechanism is normally depicted as proceeding via the reductive elimination of acetyl iodide from the rhodium center. The addition of iodide salts, especially lithium iodide, can realize the reaction run at lower water concentrations thus, byproduct formation via the water gas shift reaction is reduced, subsequently improving raw materials consumption and reducing downstream separation. In addition to the experimental studies of the catalytic mechanism, theoretical studies have also been carried out to understand the reaction mechanism [17-20]. 

Fig. 25.9 The two interrelated catalytic cycles in the Monsanto (M = Rh) and Cativa (M = Ir) acetic acid manufacturing processes.

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