Rhodium iodide

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) ... 

An anionic rhodium iodide carbonyl complex was supported on polyvinylpyrrolidone for the carbonylation of methanol in the presence of scC02 [98], Depending on the reaction conditions and method of extraction, less than 0.08% rhodium leaching was observed. Saturation of the support with methyl iodide was found to be vital to enhance the longevity and recyclability of the catalyst. 

Monsanto acetic acid A process for making acetic acid by carbonylation of methanol, catalyzed by rhodium iodide. Operated by BP. 

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 two catalyst components are rhodium and iodide, which can be added in many forms. A large excess of iodide may be present. Rhodium is present as the anionic species RhI2(CO)2. Typically the rhodium concentration is 10 mM and the iodide concentration is 1.5 M, of which 20% occurs in the form of salts. The temperature is about 180 °C and the pressure is 50 bar. The methyl iodide formation from methanol is almost complete, which makes the reaction rate also practically independent of the methanol concentration. In other words, at any conversion level (except for very low methanol levels) the production rate is the same. For a continuous reactor this has the advantage that it can be operated at a high conversion level. As a result the required separation of methanol, methyl acetate, methyl iodide, and rhodium iodide from the product acetic acid is much easier. 

This protonation may, besides the desired CO insertion, also form inactive trivalent rhodium iodides. In the Monsanto acetic acid process the addition of the reducing agent H2 is not required for two reasons ... 

In SILP carbonylation we have introduced a new methanol carbonylation SILP Monsanto catalyst, which is different from present catalytic alcohol carbonylation technologies, by using an ionic liquid as reaction medium and by offering an efficient use of the dispersed ionic liquid-based rhodium-iodide complex catalyst phase. In perspective the introduced fixed-bed SILP carbonylation process design requires a smaller reactor size than existing technology in order to obtain the same productivity, which makes the SILP carbonylation concept potentially interesting for technical applications. 

The halide exchange protocol also allows the use of other nucleophiles such as activated methylenes. The rhodium iodide complex was found to be the most... 

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. 

Triethylenediamino - rhodium Iodide, [Rh en3]I3. H20, is obtained by rubbing the 1-camphor nitronate to a thin paste with water and solid sodium iodide. The iodide so formed is extracted with water and then recrystallised. It crystallises in small glistening cubes, and has rotation of [a]o—50°, [M]i>—336-5°. 

Iodo-pentammino-rhodium Iodide, [Rh(NH3)jI]I2, is best prepared by warming aquo-pentammino-rhodium hydroxide at 100° C. with excess of hydriodic acid. It forms rhombic crystals wiiich are bright orange-yellow in colour, sparingly soluble in water and insoluble in hydriodic acid and in alcohol.2... 

Flash distillation of the product where very high vacuums are applied at moderate temperatures so the solvents and products vaporize, which are collected and condensed in a condenser, leaving the catalyst behind in the vessel. In the Monsanto acetic acid process, the catalyst rhodium iodide is left behind in the reboiler once the products are flashed off (see Section 4.9). 

Figure 1 Cycle for rhodium/iodide catalysed methanol carbonylation...

In 1968 Monsanto reported a chemically related process based on rhodium iodide complexes. Due to its high reaction rates, high selectivity and different kinetics, the process differs substantially from the cobalt process. Commercialization was achieved in 1970. Operating conditions are remarkably mild 30 bar, 180°C. 

Activation Parameters for the Carbonylation of Linear Primary Alcohols with the Rhodium-Iodide Systenf... 

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... 

Monsanto acetic acid A process for making acetic acid by carbonylation of methanol, catalyzed by rhodium iodide. Operated by BP. A variation of this process, the low water process, used added Group 1 metal iodides such as lithium iodide to enhance the productivity this was practiced by Celenese and by Daicel. 

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... 

As well as the water produced by esterification, quite a high concentration of water (ca. 10 M) is required to maintain high rates and prevent deactivation by precipitation of the rhodium catalyst (see Box 3). Separation of water from the acetic acid product by distillation incurs substantial costs. In addition, high water levels increase the rate of the water gas shift reaction (Section 4.1.3), catalyzed in competition with carbonylation by the rhodium/iodide system... 

Mechanism of Rhodium/Iodide Catalyzed Methanol Carbonylation... 

These kinetic and model studies support a promotional effect contributed primarily by the formation of acetate salts [5], The iodide functions as a catalyst stabilizer to preclude the formation of insoluble rhodium iodides [5c, 23], Under process conditions the majority of the inorganic salt is in the Lil form (eq. (11)) [23],... 

Suitable catalysts for this type of process must be capable of hydrogenating both carboxylic acids and their esters to alcohols, but also of carbonylating these compounds to their homologous acids. The best catalytic systems known contain either Rh or Ru in the presence of iodide. Ruthenium iodide systems are the most active ones in the hydrogenation reaction, but suffer from low activity in the carbonylation step, whereas rhodium iodide systems are very active when carbonylating alcohols to their acids (cf. Section 2.1.2.1). 

---