The Monsanto Process

The role of the iodide promoter is to activate methanol and to produce iodo-methane, generally by direct reaction of HI. The organic reactions which take place in the medium are  

It is of interest to note that the carbonylation rate is independent of both the CO partial pressure and the methanol concentration. However, the rate is first-order in the rhodium and methyl iodide concentrations, consistent with the CH3I oxidative addition to [Rhl2(CO)2] being the rate-determining step. 

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This reaction is rapidly replacing the former ethylene-based acetaldehyde oxidation route to acetic acid. The Monsanto process employs rhodium and methyl iodide, but soluble cobalt and iridium catalysts also have been found to be effective in the presence of iodide promoters. 

Acetic acid from methanol by the Monsanto process, CH3OH -1-CO CH3COOH, rhodium iodide catalyst, 3 atm (44 psi), 150°C (302°F), 99 percent selectivity of methanol. 

Other catalyst systems such as iron V2O5-P2O5 over silica alumina are used for the oxidation. In the Monsanto process (Figure 6-4), n-butane and air are fed to a multitube fixed-bed reactor, which is cooled with molten salt. The catalyst used is a proprietary modified vanadium oxide. The exit gas stream is cooled, and crude maleic anhydride is absorbed then recovered from the solvent in the stripper. Maleic anhydride is further purified using a proprietary solvent purification system. ... 

Figure 6-4. The Monsanto process for producing maleic anhydride from butane (1) reactor, (2) absorber (3) stripper, (4) fractionator, (5) solvent purification.

An early success story in the field of catalytic asymmetric synthesis is the Monsanto Process for the commercial synthesis of l-DOPA (4) (see Scheme 1), a rare amino acid that is effective in the treatment of Parkinson s disease.57 The Monsanto Process, the first commercialized catalytic asymmetric synthesis employing a chiral transition metal complex, was introduced by W. S. Knowles and coworkers and has been in operation since 1974. This large-scale process for the synthesis of l-DOPA (4) is based on catalytic asymmetric hydrogenation, and its development can be... 

Such a complex, cw-Rh(CO)2I2, is the active species in the Monsanto process for low-pressure carbonylation of methanol to ethanoic acid. The reaction is first order in iodomethane and in the rhodium catalyst the rate-determining step is oxidative addition between these followed by... 

Only two distillation stages are needed for water and propanoic acid removal. This results in lower energy consumption which contributes to the overall reduction in CO2 emissions to 0.31 t/t product compared to 0.48 t/t product with the Monsanto process. 

The carbonylation of methanol was developed by Monsanto in the late 1960s. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. An older method involves the same carbonylation reaction carried out with a cobalt catalyst (see Section 9.3.2.4). For many years the Monsanto process has been the most attractive route for the preparation of acetic acid, but in recent years the iridium-based CATIVA process, developed by BP, has come on stream (see Section 9.3.2) ... 

Acetic acid is presently produced on a very large scale. World-wide production in 1977 was approximately 2,500,000 tons. The Monsanto process has now been licensed world-wide, and production from these plants when constructed will amount to more than 1,000,000 tons annually. 

UCB-MCI [Union Chimique—Chemische Bedrijven and Ministry of Chemical Industry for the USSR] An EHD process for making adiponitrile, differing from the Monsanto process in using an emulsion of acrylonitrile and in not using a membrane. 

One very important feature of the Monsanto process was the fact that the reaction was started with a slurry of reactants and ended with a slurry of the pure product with close to 100% ee, allowing easy separation of both the catalyst and the undesired racemate in one step. Critical issues for both the Monsanto and the VEB Isis processes were the quality of the starting material (enamide syntheses are often problematic) and especially the concentration of oxygen and peroxides in the reaction solution. 

Figure 6.2. NMR data for the methyl intermediate 2 in the Monsanto process [3]...

The rate equation for the Monsanto process under process conditions reads v = k.[RhI2(CO)2-].[CH3I]... 

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. 

The reaction scheme follows that of the Monsanto process except for the "organic" cycle, in which acetic acid replaces water, and methyl acetate replaces methanol (Figure 6.5) ... 

Re (ii). The "salt effect" is more intriguing. At low lithium concentrations (lithium is the most effective cation) the reaction is first order in the salt concentration and zero order in rhodium, methyl iodide, and carbon monoxide. The rate steeply increases with the lithium concentrations. At high lithium concentrations the rate dependencies equal the Monsanto process, i.e. first order in rhodium and methyl iodide, and zero order in CO. The metal salts are involved in two reactions ... 

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. 

It is interesting to note that, when operating at approximately 5% water content, and with good reaction rates, it is possible to save about 30% of the energy costs of the process when compared to the Monsanto process. In addition, the current price of rhodium is 20-fold that of iridium. Even with annual unit capacity productions as large as 500 000 tons, it is still possible to improve catalysis and to achieve significantly better performances. 

At about the same time, BP Chemicals, ivho ivere also licensors of the Monsanto process, developed their oivn process for carbonylation of a mixed MeOH/MeOAc/ H2O feed to AcOH and AC2O using a Rh and Mel catalysed process promoted with a quaternary ammonium iodide salt ([QAS]I), [7]. 

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). 

The Wacker process itself is becoming obsolete, largely because acetaldehyde is now more economically made by the reaction of methanol with CO via oxidative addition to the homogeneous catalyst RhI(CO)2I2 (the Monsanto process) ... 

Sadhu et al. [88] reported interesting observations when they processed their nanocomposites through a capillary using the Monsanto Processability Tester. They found that different morphologies were formed and more ordering took place because of the shearing of the nanocomposites through the capillary. 

One could call this type of electrocatalysis, which is due to the catalytic action of adsorbed species, electrocatalysis of the second kind. Most remarkably the selectivity and commercial success of the Monsanto process— the hydrodimerisation of arylonitrile to adipodinitrile—... 

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. 

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