The Monsanto Acetic Acid Synthesis

The Monsanto acetic acid synthesis is a classic example of an industrially useful transformation based upon organometallic reactions. In this synthetic procedure, carbon monoxide and methanol are coupled to form acetic acid in water. Two catalysts are used. The first is HI, playing the role of a strong acid with a nucleophilic counterion, and the second is the 

The catalytic cycle involved in the Monsanto acetic acid synthesis. A. An equilibrium established in solution that supplies CH J. 

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Scheme 1. Proposed mechanism for the Monsanto acetic acid synthesis.

Reaction of [Mn(R)(CO)j] with neutral nucleophiles is by far the most widely studied type of reaction for [Mn(R)(CO)s] compounds. The reaction usually involves addition of the neutral neucleophile, L, and is accompanied by CO insertion/alkyl migration to form an acyl species [Eq. (29)]. L is usually a tertiary phosphine (PR3), an alkylated amine (RNH2), or free carbon monoxide. Besides being a carbon-carbon bond forming reaction of fundamental importance, alkyl migration reactions of transition metal alkyl species have direct relevance to catalysis, especially for the 0X0 or hydroformylation process (2), the Monsanto acetic acid synthesis (2), and the synthesis of ethylene glycol (94). 

Acetic Acid and Anhydride. Synthesis of acetic acid by carbonylation of methanol is another important homogeneous catalytic reaction. The Monsanto acetic acid process developed in the late 1960s is the best known variant of the process. 

Monsanto acetic acid synthesis 4), and the hydroformylation or 0X0 reaction (5). A key mechanistic step in catalytic carbonylation reactions is the migration of an alkyl group onto an adjacent carbonyl ligand. This reaction involves the formation of a new carbon-carbon bond and has been termed a carbonyl insertion reaction since a CO ligand has been formally inserted into the transition metal-carbon (r-bond. Because of the industrial and commercial importance of these catalytic reactions, the search for stoichiometric systems in which this step can be observed directly has been, and still is, one of great endeavor. 

All known ACS enzymes are bifunctional in that they possess a C cluster with COdFI activity in addition to an A cluster (the ACS active site. Scheme 9). In the enzymes, a CO tunnel is described through which GO can pass directly from the C cluster, where it is generated from CO2, to the A cluster, where acetyl GoA synthesis takes place. Again, two mechanisms were proposed that differ in the order of binding events and redox states involved. In essence, however, GO binds to an Ni-GHs species, followed by insertion and generation of an Ni-acetyl species, which upon reaction with GoA liberates the acetyl GoA product. It is interesting to note that methylation of Ni occurs by reaction with methyl cobalamin (Scheme 7). In M. thermoacetica, the cobalamin is the cofactor for a rather unique protein called the corrinoid iron sulfur protein (GFeSP). The above process, even if mechanistic details still remain in question, resembles the industrial Monsanto acetic acid synthesis process (Scheme 9, bottom). In this case, however, the reaction is catalyzed by a low-valent Rh catalyst. 

The period from 1970 to 1985 saw radical changes in the production of acetic acid and acetic anhydride. By 1985, both products would be generated not from ethylene, but from synthesis gas which in turn could be generated fi om abundant resources such as coal, natural gas, and in the future, biomass. At the end of this period, acetaldehyde became a very small contributor to the total acetyl product stream since it was no longer required to make acetic acid or acetic anhydride and ethylene would only be required to produce vinyl acetate and to meet a much diminished acetaldehyde market. These advances were the result of two significant process breakthroughs - the Monsanto Acetic Acid Process and the Eastman Chemical Company Acetic Anhydride Process which will be discussed below. 

CODH is unusual in that it can bring about two reactions (e.g., Eqs. 16.37 and 16.39) that are particularly interesting to the organometallic chemist the reduction of atmospheric CO2 to CO (CODH reaction, Eq. 16.37) and the synthesis of acetyl coenzyme A (ACS reaction, Eq. 16.39) from CO, a CH3 group taken from a corrinoid iron-sulfur protein (denoted CoFeSP in the equation), and coenzyme A, a thiol. These are analogous to reactions we have seen earlier the water-gas shift reaction (Eq. 16.36) and the Monsanto acetic acid process (Eq. 16.38). 

This process may be competitive with butane oxidation (see Hydrocarbon oxidation) which produces a spectmm of products (138), but neither process is competitive with the process from synthesis gas practiced by Monsanto (139) and BASF (140) which have been used in 90% of the new acetic acid capacity added since 1975. 

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. 

The recent dramatic increase in the price of petroleum feedstocks has made the search for high selectivities more urgent. Several new processes based on carbon monoxide sources are currently competing with older oxidation processes.103,104 The more straightforward synthesis of acetic acid from methanol carbonylation (Monsanto process) has made the Wacker process obsolete for the manufacture of acetaldehyde, which used to be one of the main acetic acid precursors. Several new methods for the synthesis of ethylene glycol have also recently emerged and will compete with the epoxidation of ethylene, which is not sufficiently selective. The direct synthesis of ethylene... 

Using methanol, a variety of fine diemicals can be made by conventional and new ways. Homogeneous catalysts have already contributed here. In tills connection the acetic acid synthesis by Monsanto must be mentioned [28]. Scheme 7 summarizes processes based on methanol, which arc under consideration or already at the development stage. 

In many applications acetic acid is used as the anhydride and the synthesis of the latter is therefore equally important. In the 1970 s Halcon (now Eastman) and Hoechst (now Celanese) developed a process for the conversion of methyl acetate and carbon monoxide to acetic anhydride. The process has been on stream since 1983 and with an annual production of several 100,000 tons, together with some 10-20% acetic acid. The reaction is carried out under similar conditions as the Monsanto process, and also uses methyl iodide as the "activator" for the methyl group. 

Methanol process. BASF introduced high-pressure technology way back in I960 to make acetic acid out of methanol and carbon monoxide instead of ethylene. Monsanto subsequently improved the process by catalysis, using an iodide-promoted rhodium catalyst. This permits operations at much lower pressures and temperatures. The methanol and carbon monoxide, of course, come from a synthesis gas plant. 

Also of great attractiveness is the direct synthesis of acetic acid from syngas, which would circumvent the two step process of Monsanto. Selectivities of up to 50 % are claimed. An economic analysis by Hoechst A.G. indicates (1/7) that this process is already economically feasible at a 80 % C2 Oxygenate selectivity. 

The direct carbonylation of methanol yielding acetic acid, the Monsanto process, represents the best route for acetic acid. Carbonylation of methyl acetate, obtained from methanol and acetic acid, gives acetic anhydride, a technology commercialized by Tennessee Eastman (22). It is noteworthy that this process is based on coal derived synthesis gas to give as the final product cellulose acetate. A combination of Monsanto and Tennessee Eastman technology opens the door for the combined synthesis of acetic acid and acetic anhydride. 

An interesting reaction of methyl formate is its isomerization to give acetic acid. Based on patent literature, a number of companies have recently reinvestigated this isomerization which has been known for over 30 years ( ). It is unlikely that it can compete with the Monsanto process however, since it doesn t need pure CO and may be operable at milder reaction conditions, some potential may be seen. Combining isomerization to acetic acid and decarbonylation to methanol and CO, could provide a direct synthesis for acetic anhydride starting directly from methyl formate (Equation 13). 

Whatever the source of synthesis gas, it is the starting point for many industrial chemicals. Some examples to be discussed are the hydroformylation process for converting alkenes to aldehydes and alcohols, the Monsanto process for the production of acetic acid from methanol, the synthesis of methanol from methane, and the preparation of gasoline by the Mobil and Fischer-Tropsch methods. 

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%... 

The synthesis of acetic acid from methanol and CO is a process that has been used with great commercial success by Monsanto since 1971. The mechanism of this process is complex a proposed outline is shown in Figure 14-16. As in the hydroformylation process, the individual steps of this mechanism are the characteristic types of... 

The low-pressure acetic acid process was developed by Monsanto in the late 1960s and proved successful with commercialization of a plant producing 140 X 10 metric tons per year in 1970 at the Texas City (TX, USA) site [21]. The development of this technology occurred after the commercial implementation by BASF of the cobalt-catalyzed high-pressure methanol carbonylation process [22]. Both carbonylation processes were developed to utilize carbon monoxide and methanol as alternative raw materials, derived from synthesis gas, to compete with the ethylene-based acetaldehyde oxidation and saturated hydrocarbon oxidation processes (cf. Sections 2.4.1 and 2.8.1.1). Once the Monsanto process was commercialized, the cobalt-catalyzed process became noncom-... 

Using a Monsanto catalyst system (Rh/Mel/CO) the carbonylation of methyl acetate has a long induction period in the absence of water and a low reaction rate in comparison with the synthesis of acetic acid under typical reaction conditions. 

The synthesis of acetic acid is one of the most rapidly growing chemical applications for methanol. The process for manufacture of acetic acid was developed by Monsanto. The reaction runs at a temperature of 150-200°C and a pressure of 30.62 atm. The catalyst used is rhodium salts with certain ligands and in the presence of an iodine compound. The reaction is ... 

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