Monsanto process mechanism

In 1970, the first rhodium-based acetic acid production unit went on stream in Texas City, with an annual capacity of 150 000 tons. Since that time, the Monsanto process has formed the basis for most new capacities such that, in 1991, it was responsible for about 55% of the total acetic acid capacity worldwide. In 1986, B.P. Chemicals acquired the exclusive licensing rights to the Monsanto process, and 10 years later announced its own carbonylation iridium/ruthenium/iodide system [7, 8] (Cativa ). Details of this process, from the viewpoint of its reactivity and mechanism, are provided later in this chapter. A comparison will also be made between the iridium- and rhodium-based processes. Notably, as the iridium system is more stable than its rhodium counterpart, a lower water content can be adopted which, in turn, leads to higher reaction rates, a reduced formation of byproducts, and a better yield on CO. 

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. 

Assume that the basic mechanism of the Monsanto process is valid for the cobalt-based process. What would be the crucial hypothetical catalytic... 

The chemistry of acetyl-CoA synthesis is thought to resemble the Monsanto process for acetate synthesis in that a metal center binds a methyl group and CO and the CO undergoes a carbonyl insertion into the methyl-metal bond. Elimination of the acetyl group is catalyzed by a strong nucleophile, iodide in the industrial process and CoA in the biochemical one. Currently, there are two views of the catalytic mechanism. 

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. 

Similarly to the reaction with cobalt, the acetaldehyde intermediate formed will be further hydrogenated to ethanol. Overall, the Rh-catalized homologation mechanism resembles the Monsanto process with the exception that, as a result of the presence of hydrogen, acetaldehyde is now the main product and acetic acid definitely the only by-product. Some key catalyst components present at the end of the homologation reaction, such as Rh(diphosphine)COMe)l2 and [Ru(CO)l3 " have been isolated and identified by Moloy et al. [49]. It may be assumed that the Ru complex is responsible for the intermediate in-situ hydrogenation to the high ethanol selectivity obtained. 

The Monsanto process, one of the most successful industrial homogeneous catalytic processes, uses a Rh complex and catalytic HI to carbonylate MeOH to MeC02H. A Rh precatalyst (almost any Rh complex will do) is converted into Rh(CO)2l2, the active catalyst, under the reaction conditions. The mechanism of the reaction involves three steps. In the first step, MeOH and HI are converted to Mel and H20 by an Sn2 mechanism. In the second step, Mel and CO are converted to MeCOI under Rh catalysis. In the third step, H2O (generated in the first step) hydrolyzes MeCOI to afford MeC02H and to regenerate HI. 

The synthesis of acetic acid from methanol and CO is a process that has been used with commercial success by Monsanto. The mechanism of this process is complex a proposed outline is in Figure 14.20. The individual steps are the characteristic types of organometallic reactions described previously the intermediates are 18- or 16-electron species having the capability to lose or gain, respectively, 2 electrons. Solvent molecules may occupy empty coordination sites in the 4- and 5-coordinate 16-electron intermediates. The first step, oxidative addition of CH3I to [Rhl2(CO)2] , is rate determining. ... 

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]. 

As described previously, the first commercial application of asymmetric hydrogenation was the Monsanto process for the manufacture of L-Dopa, developed by Knowles (Equation 10.23). L-Dopa is used to treat Parkinson s disease. For these reasons, Halpem and Brown studied the mechanism of this enantioselective process, and the results of these studies were particularly enlightening about how physical organic principles apply to asymmetric catalysis. This process was used as a case study in Chapter 14 to present how enantioselectivity is controlled. The findings are reiterated briefly here. 

The Monsanto process has been thoroughly studied, and the relatively simple catalytic mechanism in Scheme 17.1, consisting of only anionic rhodium species, has been proposed. Virtually any source of rhodium or of iodidemay be introduced as a "precatalyst" to generate [Rh(CO)2lJ and Mel imder catalytic conditions. High-pressure IR spectroscopy has shown that the major rhodium species in the catalytic solutions is [Rh(CO)2l2]"(v Q = 2055 and 1985 cm" ). The overall rate law for the formation of acetic acid contains a linear dependence on rhodium and methyl iodide and no dependence on the concentrations of reactants (CO and methanol) and product (acetic acid). Consistent with the rate law, the turnover-limiting step has been proposed to be the oxidative addition of Mel to [Rh(CO)jIj]". 

While, there are many similarities between the mechanism of the Ir-catalyzed process and that of the Rh-catalyzed stystem, there are important differences. Unlike the dependence of the rate of the Monsanto process on only [Rh] and [CH I], the dependence of BP s iridium system on CO pressure, water, methyl acetate, methyl iodide, ruthenium promoter, and iridium are more complex and nonlinear. In situ IR spectroscopy of the iridium catalyst shows that the predominant species is the anionic Ir(III) methyl complex/flc,ds-[Ir(CH3)(CO)2y (2100 and 2047 cm" ). Instead of occurring by turnover-limiting oxidative addition of Mel, the iridium-catalyzed process occurs by turnover-limiting insertion of CO into the metal-methyl complex. 

The mechanism of the cobalt- (BASF), rhodium- (Monsanto), and iridium- (Cativa) catalyzed reaction is similar but the rate-determining steps differ and different intermediate catalyst complexes are involved. In all three processes two catalytic cycles occur. One cycle involves the metal carbonyl catalyst (II) and the other the iodide promoter (i). For a better overview only the catalytic cycle of the rhodium-catalyzed Monsanto process is presented in detail (Figure 6.15.4). Initially the rhodium iodide complex is activated with carbon monoxide by forming the catalytic active [Rhi2(CO)2] complex 4. Further the four-coordinated 16-electron complex 4 reacts in the rate-determining step with methyl iodide by oxidative addition to form the six-coordinated 18-electron transition methyl rhodium (I II)... 

Mechanism of the Monsanto process, but the oxidative addition of H2 occurs more rapidly than the reductive elimination of the acetyl and iodo ligands that is inhibited by the chelating diphosphine ligand. Finally, the cycle is closed by the reductive elimination of the acyl and hydride ligand (K.N.G. Moloy and R.W. Wegman in the general ref indicated above in 16.2, Chap. 22 of this ref). 

The presence of a lithium salt (Lil) as cocatalyst and hydrogen is very important for efficient production of acetic anhydride. The proposed reaction mechanism is shown in Figure 5 [42,43,47]. In this mechanism, there are two catalytic cycles for the formation of methyl acetate a rhodium-catalyzed cycle and a lithium-catalyzed cycle. The rhodium-catafyzed cycle is similar to the Monsanto process of methanol carbonylation (Fig. 1). The participation of the second cycle was discovered when it was found that the reaction rate was much enhanced when hydrogen and a lithium salt were added [43,44]. The role of hydrogen is to reduce the catalytically inactive Rh(CO)2l4 to the active Rh(CO)2l2. In the anhydrous medium used in the reaction, the formation of hydrogen by the reaction of carbon monoxide with water as in the water-gas shift reaction is not possible. Thus hydrogen must be added. 

Detailed mechanistic and theoretical analysis of the key mechanistic steps of the Cativa process, for the Cativa process See Ref 257,257a that is, the iridium-based catalytic carbonylation of methanol to acetic acid, have allowed several groups, " particularly Haynes and co-workers, to unravel the mechanism of the catalytic process. Ir(l) complexes [Ir(CO)(L-L)I] (LL = dppms, dppe, dppmo) provided important mechanistic information about the influence of stereoelectronic ligand effects on the organometallic reactivity of modified metal centers with Mel. The carbonylation of methanol promoted by iridium and rhodium complexes which is at the basis of both Cativa and Monsanto processes for the synthesis of acetic acid will be described in detail in a different chapter of this volume. 

Another fairly direct method for the synthesis of chiral amino acids involves the synthesis of a dehydroamino acid, 22.25, which is then hydrogenated in the presence of a chiral rhodium complex. The dehydroamino acid is prepared via an azlactone, with the full mechanism shown in Figure 22.21. The azlactone is then hydrolyzed to the dehydroamino acid. The dehydroamino acid derivative is readily reduced with a standard palladium catalyst, but the product is then racemic. If the catalyst used is [Rh(COD)(DIPAMP)] [BFJ, then the reaction proceeds with up to 99 % enantiomer excess (DIPAMP, 22.26). This is the basis of the Monsanto process for the production of the anti-Parkinson s drug, l-DOPA, 22.27, and William Knowles received the Nobel Prize for this work in 2001. 

It was discovered by Monsanto that methanol carbonylation could be promoted by an iridium/iodide catalyst [1]. However, Monsanto chose to commercialise the rhodium-based process due to its higher activity under the conditions used. Nevertheless, considerable mechanistic studies were conducted into the iridium-catalysed process, revealing a catalytic mechanism with similar key features but some important differences to the rhodium system [60]. 

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