Carbonylation of methanol to acetic acid

The advent of a large international trade in methanol as a chemical feedstock has prompted additional purchase specifications, depending on the end user. Chlorides, which would be potential contaminants from seawater during ocean transport, are common downstream catalyst poisons likely to be excluded. Limitations on iron and sulfur can similarly be expected. Some users are sensitive to specific by-products for a variety of reasons. Eor example, alkaline compounds neutralize MTBE catalysts, and ethanol causes objectionable propionic acid formation in the carbonylation of methanol to acetic acid. Very high purity methanol is available from reagent vendors for small-scale electronic and pharmaceutical appHcations. 

Carbonylation of methanol to acetic acid is fully discussed in Chapter 9. Another carbonylation process using a phosphine ligand to control the course of the reaction is a highly atom efficient route to the widely used monomer methyl methacrylate (Scheme 4.19). In this process the catalyst is based on palladium acetate and the phosphine ligand, bisphenyl(6-methyl-2-pyridyl) phosphine. This catalyst is remarkably (>99.5%) selective for the 2-carbonylation of propyne under the relatively mild conditions of <100 °C and 60 bar pressure. 

Industry uses a multitude of homogenous catalysts in all kinds of reactions to produce chemicals. The catalytic carbonylation of methanol to acetic acid... 

Historically, the rhodium catalyzed carbonylation of methanol to acetic acid required large quantities of methyl iodide co-catalyst (1) and the related hydrocarboxylation of olefins required the presence of an alkyl iodide or hydrogen iodide (2). Unfortunately, the alkyl halides pose several significant difficulties since they are highly toxic, lead to iodine contamination of the final product, are highly corrosive, and are expensive to purchase and handle. Attempts to eliminate alkyl halides or their precursors have proven futile to date (1). 

Recently, Eastman Chemical Company reported that ionic liquids can be successfully employed in a vapor take-off process for the carbonylation of methanol to acetic acid in the presence of rhodium and methyl iodide (3). While attempting to extend this earlier work to the carbonylation of ethylene to propionic acid, we discovered that, when using ionic liquids as a solvent, acceptable carbonylation rates could be attained in the absence of any added alkyl iodide or hydrogen iodide (4). We subsequently demonstrated that the carbonylation of methanol to acetic acid could also be operated in the absence of methyl iodide when using ionic liquids (5). 

Meanwhile, Wacker Chemie developed the palladium-copper-catalyzed oxidative hydration of ethylene to acetaldehyde. In 1965 BASF described a high-pressure process for the carbonylation of methanol to acetic acid using an iodide-promoted cobalt catalyst (/, 2), and then in 1968, Paulik and Roth of Monsanto Company announced the discovery of a low-pressure carbonylation of methanol using an iodide-promoted rhodium or iridium catalyst (J). In 1970 Monsanto started up a large plant based on the rhodium catalyst. 

Morris, G. (2005) Carbonylation of methanol to acetic acid and methyl acetate to acetic anhydride, in Mechanisms in Homogeneous Catalysis, A Spectroscopic Approach (ed. B. Heaton), Wiley-VGH Verlag GmbH, Weinheim, pp. 195-230. 

Carbonylation of Methanol to Acetic Acid and Methyl Acetate to Acetic Anhydride... 

As we learned in Chapters 3 and 4, many inorganic compounds, not just ammonia, are derived from synthesis gas, made from methane by steam-reforming. In the top 50 this would include carbon dioxide, ammonia, nitric acid, ammonium nitrate, and urea. No further mention need be made of these important processes. We discussed MTBE in Chapter 7, Section 4, and Chapter 10, Section 9, since it is an important gasoline additive and C4 derivative. In Chapter 10, Section 6, we presented -butyraldehyde, made by the 0x0 process with propylene and synthesis gas, which is made from methane. In Chapter 11, Section 8, we discussed dimethyl terephthalate. Review these pertinent sections. That leaves only two chemicals, methanol and formaldehyde, as derivatives of methane that have not been discussed. We will take up the carbonylation of methanol to acetic acid, now the most important process for making this acid. Vinyl acetate is made from acetic... 

Reppe reaction involves carbonylation of methanol to acetic acid and methyl acetate and subsequent carbonylation of the product methyl acetate to acetic anhydride. The reaction is carried out at 600 atm and 230°C in the presence of iodide-promoted cobalt catalyst to form acetic acid at over 90% yield. In the presence of rhodium catalyst the reaction occurs at milder conditions at 30 to 60 atm and 150-200°C. Carbon monoxide can combine with higher alcohols, however, at a much slower reaction rate. 

The most common oxidation states and corresponding electronic configurations of rhodium are +1 (tf8), which is usually square planar although some five coordinate complexes are known, and +3 (T) which is usually octahedral. Dimeric rhodium carboxylates are +2 (oxidation states —1 (industrial applications include rhodium-catalyzed carbonylation of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to tf-butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

Industrial Applications. Several large scale industrial processes are based on some of the reactions listed above, and more are under development. Most notable among those currently in use is the already mentioned Wacker process for acetaldehyde production. Similarly, the production of vinyl acetate from ethylene and acetic acid has been commercialized. Major processes nearing commercialization are hydroformylations catalyzed by phosphine-cobalt or phosphine-rhodium complexes and the carbonylation of methanol to acetic acid catalyzed by (< 3P) 2RhCOCl. 

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. 

Reaction (78) regenerates Mel from methanol and HI. Using a high-pressure IR cell at 0.6 MPa, complex (95) was found to be the main species present under catalytic conditions, and the oxidative addition of Mel was therefore assumed to be the rate determining step. The water-gas shift reaction (equation 70) also occurs during the process, causing a limited loss of carbon monoxide. A review of the cobalt-, rhodium- and iridium-catalyzed carbonylation of methanol to acetic acid is available.415... 

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 active species in the cobalt-catalyzed carbonylation of methanol to acetic acid are generally considered to be either the tetracarbonyl cobalt... 

The carbonylation of methanol to acetic acid and methyl acetate, and the carbonylation of the latter to acetic anhydride, was found by W. Reppe at BASF in the 1940s, using iodide-promoted cobalt salts as catalyst precursors. This process required very high pressure (600 bar) as well as high temperatures (230°C) and gave ca. 90% selectivity for acetic acid. 

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