Iridium catalyst, acetic acid production

Commissioning of first plants with metallocenes as catalysts for PP production (1995) BP(1992) introduction of iridium in acetic acid... 

The high activity of a ruthenium-promoted iridium catalyst has improved productivity in plants that previously used rhodium catalysts [123], For example, a 75% increase in throughput was achieved at the Samsung-BP plant in Ulsan, South Korea. Another benefit of the iridium catalyst is higher selectivity, with smaller amounts of both gaseous and liquid by-products. The WGS reaction does occur, but at a lower rate than for rhodium, resulting in reduced formation of C02 and CH4. Since the process is less sensitive to CO partial pressure, the reactor can operate with a lower rate of bleed of recycle gas which, in combination with the secondary reactor, results in an increase in CO conversion from 85% (Rh) to >94% (Ir). Selectivity to acetic acid is >99% based on methanol with reduced propionic acid by-product formation relative to the process with the rhodium catalyst. This, along with the lower water... 

In the BASF process, methanol and CO are converted in the liquid phase by a homogeneous Co-based catalyst. The reaction takes place in a high-pressure Hastelloy reactor. In recent decades the BASF process has been increasingly replaced by low-pressure alternatives mainly due to lower investment and operating costs. In the low-pressure Monsanto process methanol and CO react continuously in liquid phase in the presence of a Rhl2 catalyst. In 1996, BP developed a new attractive catalyst based on iridium (Cativa process) the oxidative addition of methyl iodide to iridium is 150-times faster than to rhodium. The search for acetic acid production processes with even lower raw material costs has led to attempts to produce acetic acid by ethane oxidation. In the near future ethane oxidation will most likely not compete with methanol carbonylation (even though ethane is a very cheap and attractive raw material) because of the low ethane conversions, product inhibition problems, and a large variety of by-products. 

The attainment of optimum rate at relatively low [H2O] is a significant benefit for the iridium system, since it results in less costly product purification. A typical configuration for an iridium-catalyzed methanol carbonylation plant is shown in Figure 2. The feedstocks (MeOH and CO) are fed to the reactor vessel on a continuous basis. In the initial product separation step, the reaction mixture is passed from the reactor into a flash tank where the pressure is reduced to induce vaporization of most of the volatiles. The catalyst remains dissolved in the liquid phase and is recycled back to the reactor vessel. The vapor from the flash tank is directed into a distillation train, which removes methyl iodide, water, and heavier byproducts (e.g., propionic acid) from the acetic acid product. At the relatively high water levels used in the rhodium-catalyzed Monsanto process, three distillation columns are typically required. In the Cativa process, a lower water concentration means that the necessary product purification can be achieved with only two columns. 

In 1996, consumption in the western world was 14.2 tonnes of rhodium and 3.8 tonnes of iridium. Unquestionably the main uses of rhodium (over 90%) are now catalytic, e.g. for the control of exhaust emissions in the car (automobile) industry and, in the form of phosphine complexes, in hydrogenation and hydroformylation reactions where it is frequently more efficient than the more commonly used cobalt catalysts. Iridium is used in the coating of anodes in chloralkali plant and as a catalyst in the production of acetic acid. It also finds small-scale applications in specialist hard alloys. 

The formation of C-C bonds is of key importance in organic synthesis. An important catalytic methodology for generating C-C bonds is provided by carbonylation. In the bulk chemicals arena this is used for the production of acetic acid by methanol carbonylation (Eqn. (9)) in the presence of rhodium- or, more recently, iridium-based catalysts (Maitlis et al, 1998). 

Extensive investigations in our laboratories on the deactivation of rhodium and iridium catalysts has shown there to be a number of different mechanisms involved. Both, rhodium and iridium catalysts are generally less stable at higher temperatures, and have more labile ligands than their ruthenium counterparts. All of the catalysts are affected by pH, but the ruthenium catalysts seem to be more readily deactivated by acid. Indeed, these reactions are often quenched with acetic acid, whilst stronger acids are used to quench the rhodium reactions. Each of the catalysts can be deactivated by product inhibition, the ruthenium catalyst with aromatic substrates such as phenylethanol, and the rhodium and iridium ones by bidentate chelating products. 

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. 

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 ]... 

Jones, J.H. (2000) The Cativa process for the manufacture of acetic acid iridium catalyst improves productivity in an established industrial process. Platinum Met. Rev., 44, 94-105. 

Recently, the Cativa process for liquid-phase carbonylation to produce acetic acid has been com-mercialized.2 This process uses an iridium-based catalyst instead of rhodium, produces less propionic acid and acetaldehyde, and uses a much lower water concentration in the reaction mixture. The last aspect results in a reaction product stream that contains less water, so much less energy is needed for distillation to separate water from acetic acid. 

The conditions employed for iridium-catalyzed carbonylation (ca. 180-190 °C, 20-40 bar) are comparable to those of the rhodium-based process. A variety of iridium compounds (e.g., I1CI3, IrU, H2I1CI6, Ir4(CO)i2) can be used as catalyst precursors, as conversion into the active iodocarbonyl species occurs rapidly under process conditions. In a working catalytic system, the principal solvent component is acetic acid, so the methanol feedstock is substantially converted into its acetate ester (Equation (2)). Methyl acetate is then activated by reaction with the iodide co-catalyst (Equation (3)). Catalytic carbonylation of methyl iodide formally gives acetyl iodide (Equation (4)) prior to rapid hydrolysis to the product acetic acid (Equation (5)). However, it is difficult to establish the true intermediacy of acetyl... 

The worldwide production of acetic acid is more than 10 million tons per year of which about 80% is based on methanol carbonylation technology. Methanol can be carbonylated to give acetic acid by using metal complexes of cobalt or rhodium or iridium as catalysts. All the three processes require the presence of some water and methyl iodide in the... 

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