Carbonylation Monsanto process

Ca.ta.lysis, The readily accessible +1 and +3 oxidation states of rhodium make it a useful catalyst. There are several reviews of the catalytic properties of rhodium available (130—132). Rhodium-catalyzed methanol carbonylation (Monsanto process) accounted for 81% of worldwide acetic acid by 1988 (133). The Monsanto acetic acid process is carried out at 175°0 and 1.5 MPa (200 psi). Rhodium is introduced as RhCl3 but is likely reduced in a water... 

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... 

Fig. 22.4. Acetic acid via methanol carbonylation—Monsanto process. (Cheim Systems Report No. 8S-7. Copyr/ghr Chem Systems /nc,. Tarrytown. NY. and used by permission of copyright owner.)...

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... 

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) ... 

It is now nearly 40 years since the introduction by Monsanto of a rhodium-catalysed process for the production of acetic acid by carbonylation of methanol [1]. The so-called Monsanto process became the dominant method for manufacture of acetic acid and is one of the most successful examples of the commercial application of homogeneous catalysis. The rhodium-catalysed process was preceded by a cobalt-based system developed by BASF [2,3], which suffered from significantly lower selectivity and the necessity for much harsher conditions of temperature and pressure. Although the rhodium-catalysed system has much better activity and selectivity, the search has continued in recent years for new catalysts which improve efficiency even further. The strategies employed have involved either modifications to the rhodium-based system or the replacement of rhodium by another metal, in particular iridium. This chapter will describe some of the important recent advances in both rhodium- and iridium-catalysed methanol carbonylation. Particular emphasis will be placed on the fundamental organometallic chemistry and mechanistic understanding of these processes. 

One approach which enables lower water concentrations to be used for rhodium-catalysed methanol carbonylation is the addition of iodide salts, especially lithium iodide, as exemplified by the Hoechst-Celanese Acid Optimisation (AO) technology [30]. Iodide salt promoters allow carbonylation rates to be achieved at low (< 4 M) [H2O] that are comparable with those in the conventional Monsanto process (where [H20] > 10 M) while maintaining catalyst stability. In the absence of an iodide salt promoter, lowering the water concentration would result in a decrease in the proportion of Rh existing as [Rh(CO)2l2] . However, in the iodide-promoted process, a higher concentration of methyl acetate is also employed, which reacts with the other components as shown in Eqs. 3, 7 and 8 ... 

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. 

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 carbonylation of MeOH catalysed by Ir and Mel can also be operated at lower reactor ]H20] and higher ]MeOAc] than the original Monsanto process and without issues of catalyst stability. Commercially acceptable rates can be achieved at lower ]MeI] concentrations by using promoters such as carbonyl iodide complexes of Ru and Os or covalent iodides such as Inij or Znl2 ]9]. Ionic iodide salts are potent poisons for the Ir catalysed reaction ]11]. In contrast with the Rh catalysed systems, CH4 and not H2 is co-produced as a gaseous by-product (Eq. (8)). 

The third and now preferred method of acetic acid manufacture is the carbonylation of methanol (Monsanto process), involving reaction of methanol and carbon monoxide (both derived from methane). This is discussed in Chapter 12, Section 3. 

Another way to produce acetic acid is based on a carbonylation of methanol in the so called Monsanto process, which is the dominant technology for the production of acetic acid today [15]. Acetic acid then is converted to VAM by addition of ethylene to acetic acid in the gas phase using heterogeneous catalysts usually based on palladium, cadmium, gold and its alloys (vinylation reaction 3 in Fig. 2) [16] supported on silica structures. 

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. 

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. 

Later, in the 1970s, a new commerial process for AcOH by the Rh-catalyzed carbonylation of MeOH in the presence of HI, the Monsanto process, was developed (eq. 1.5) [5], It is significant that MeOH, which is a saturated compound, was shown to be carbonylated via Mel in this process, rather than unsaturated substrates such as alkenes and alkynes. 

Carbonylation of alkyl halides is rare. As an exception, AcOH is produced commercially by the Monsanto process from MeOH and CO using Rh as a catalyst in the presence of HI. In this process (Scheme 3.10), Mel is generated in situ from MeOH and HI and undergoes oxidative addition. Insertion of CO generates an acetylrhodium intermediate, and nucleophilic attack of water produces AcOH, regenerating the Rh catalyst and HI (or reductive elimination to give acetyl iodide and hydrolysis). 

In this chapter we discuss the mechanistic and other details of a few industrial carbonylation processes. These are carbonylation of methanol to acetic acid, methyl acetate to acetic anhydride, propyne to methyl methacrylate, and benzyl chloride to phenyl acetic acid. Both Monsanto and BASF manufacture acetic acid by methanol carbonylation, Reaction 4.1. The BASF process is older than the Monsanto process. The catalysts and the reaction conditions for the two processes are also different and are compared in the next section. Carbonylation of methyl acetate to acetic anhydride, according to reaction 4.2, is a successful industrial process that has been developed by Eastman Kodak. The carbonylation of propyne (methyl acetylene) in methanol to give methyl methacrylate has recently been commercialized by Shell. The Montedison carbonylation process for the manufacture of phenyl acetic acid from benzyl chloride is noteworthy for the clever combination of phase-transfer and organometallic catalyses. Hoechst has recently reported a novel carbonylation process for the drug ibuprofen. 

The CO used for the carbonylation reaction always contains some hydrogen. The side products in the BASF carbonylation process arise due to Fischer-Tropsch reaction catalyzed by the cobalt catalyst. The high temperatures and pressures used in the BASF process are conditions under which the Fischer-Tropsch reaction with soluble cobalt catalyst can take place. In the Monsanto process the reaction conditions are much milder, and the side-product-forming Fischer-Tropsch reaction is avoided. 

Do you expect differences between the rates of carbonylation of (a) CD3CDODCD3 and CH3CHOHCH3 by the Monsanto process (b) CH3OH and C2H5OH by the Monsanto and BASF processes (c) CD3OD and CH3OH by the Monsanto process If so, why ... 

The BASF cobalt/iodide catalyzed process for methanol carbonylation was quite quickly superseded by a rhodium/iodide catalyzed process discovered at Monsanto and first commercialized in 1970 at a plant in Texas City. The Monsanto process was a significant advance and became one of the few large tonnage processes to use a homogeneous transition metal catalyst. It was later... 

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. 

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