Rhodium/iodide catalyst

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. 

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) ... 

Monsanto developed the rhodium-catalysed process for the carbonylation of methanol to produce acetic acid in the late sixties. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. At standard conditions the reaction is thermodynamically allowed,... 

The robust nature of the rhodium-iodide catalyst is also revealed in reactions with ortho-halo phenols that proved to be problematic with the first-generation catalyst system (Section 9.3.1). By employing the [Rh(PPF-P Bu2)I] catalyst, complete conversion is obtained with 2-bromophenol to give 6 in 94% yield, and with 95% enantiomeric excess after only 1.5 h of reaction time at 1 mol% catalyst loading (Scheme 9.3) [11]. The ready availability of these ring-opened compounds has been utilized to prepare enan-tiomerically enriched benzofurans 7. 

Employing protic and halide additives can effectively reverse the deleterious effect with aliphatic amines [8, 11]. The optimum results are obtained when ammonium iodide is employed as the addihve in combination with the second-generation rhodium-iodide catalyst. Under these conditions, a variety of aliphatic amines can be used to generate the aminotetrahn products in high yields and with excellent enantiomeric excess (Scheme 9.4). From a technical perspective, ammonium iodide benefits from being a combined proton and iodide source that is air-stable and nonhygroscopic. 

The catalytic activity of the methanol carbonylation is very dependent on the nature of the iodide promoter, and different chemistry appears to follow using HI or Nal in this regard (72). However, under otherwise identical conditions, the catalytic activity increased in the order Nal < CH3I < HI. Contrary to what is observed for the rhodium/iodide catalyst, Braca et al. did not consider CH3I to be directly involved in the catalytic carbonylation cycle (70-73). This conclusion is based on the observation that CH3I was not carbonylated under their reaction conditions. Instead, because of the necessity of a proton supplier and the promoting effect of Nal, these authors... 

The efficacy of an iridium/iodide catalyst for methanol carbonylation was discovered by Monsanto at the same time as their development of the process using the rhodium/iodide catalyst [5]. Mechanistic investigations by Forster employing in situ HPIR spectroscopy revealed additional complexity compared to the rhodium system [115]. In particular, the carbonylation rate and catalyst speciation were found to show a more complicated dependence on process variables, and three distinct regimes of catalyst behavior were identified. At relatively low concentrations of Mel, H20, and ionic iodide, a neutral iridium (I) complex [Ir(CO)sI] was found to dominate, and the catalytic reaction was inhibited by increasing the CO partial pressure. Addition of small amounts of a quaternary ammonium iodide salt caused the dominant iridium species to become an Ir(III) methyl complex, [Ir(CO)2l3Me]. Under these conditions, the rate... 

The carbonylation is carried out in the liquid phase with a rhodium-iodide catalyst. 

Methanol carbonylation with rhodium iodide catalysts gives acetic acid directly. Exercise 3.4... 

Nowadays, iodine is widely used for the manufacturing of X-ray contrast media, antimicrobial products, as tinctures of polyvinylpyrrolidone-iodine (Povidone-iodine), catalysts in chemical processes (e.g. for the production of acetic acid by carbonylation of methanol in the presence of a rhodium iodide-catalyst (Monsanto process) or an iridium iodide-catalyst (Cativa process)), and also on a smaller scale for the production of pharmaceuticals like thyroid hormones. [ 83 ]... 

The methyl acetate carbonylation process was successfully started and operated in the early 1980s as part of a coal-to-syngas-to-acetic anhydride complex. This new process introduction resulted in a major improvement in acetic anhydride production economics. In this process, methyl acetate, itself the product of a one-step esterification of acetic acid and methanol, is reacted with carbon monoxide in the presence of a promoted rhodium-iodide catalyst. Figure 22.20 illustrates this process... 

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