Methanol carbonylation Cativa process

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) ... 

In 1996, BP Chemicals announced a new methanol carbonylation process, Cativa , based upon a promoted iridium/iodide catalyst which now operates on a number of plants worldwide [61-69]. Promoters, which enhance the catalytic activity, are key to the success of the iridium-based process. The mechanistic aspects of iridium-catalysed carbonylation and the role of promoters are discussed in the following sections. 

There has been a recent resurgence of interest in iridium catalysed methanol carbonylation, arising from the commercialisation by BP Chemicals of the Cativa process. This uses a promoted iridium catalyst and has now superseded the rhodium catalyst on a number of plants. Its success relies on the discovery of promoters which increase catalytic activity, particularly at commercially desirable low water concentrations. HP IR spectroscopy has been used to investigate the behavior of... 

Interest in iridium-catalyzed methanol carbonylation was rekindled in the 1990 s when BP Chemicals developed and commercialized the Cativa process, which utilizes an iridium/iodide catalyst and a ruthenium promoter. This process has the important advantage that the highest catalytic rates occur at significantly lower water concentration (ca. 5% wt) than for Monsanto s... 

For approximately 30 years, the most successful industrial process for the carbonylation of methanol relied on an iodide-promoted rhodium catalyst. This technology, originally developed by Monsanto and acquired by BP Chemicals in 1986, is responsible for the majority of the acetic acid synthesized industrially. Since then, the most important development in industrial carbonylation chemistry is the Cativa process, announced by BP Chemicals in 1996. ... 

The Cativa process is based on a promoted iridium catalyst, and offers a considerable improvement over the rhodium-based system as a result of increased catalyst stability at lower water concentrations, decreased by-product formation, higher rates of carbonylation, high selectivity (>99% based upon methanol), and improved yields on carbon monoxide. This is a more cost-effective process for methanol carbonylation owing to lower energy consumption and fewer purification requirements. Implementation of this new process has now been achieved in four plants worldwide. 

Although the carbonylation of methanol using an iodide-promoted iridium complex was first reported by Monsanto researchers Roth and Pauhk in 1968, and its mechanism studied by Forster and others, it was the rhodium system that was initially developed for commercialization. A more complex mechanism for iridium, involving both anionic and neutral intermediates was discovered, but it would take over twenty years to coimnercialize an iridium-based system for methanol carbonylation (Scheme 21). In the Cativa process, the iridium complex is promoted by two distinct... 

In 1996 BP announced the commercialization of their version of a low-water methanol carbonylation technology named Cativa based upon a promoted iridium catalyst. The Cativa process replaced the high-water Monsanto process which had been used by BP. 

In the 1990s, BP re-examined the iridium-catalyzed methanol carbonylation chemistry first discovered by Paulik and Roth and later defined in more detail by Forster [20]. The thrust of this research was to identify an improved methanol carbonylation process using Ir as an alternative to Rh. This re-examination by BP led to the development of a low-water iridium-catalyzed process called Cativa [20]. Several advantages were identified in this process over the Rh-catalyzed high-water Monsanto technology. In particular, the Ir catalyst provides high carbonylation rates at low water concentrations with excellent catalyst stability (less prone to precipitation). The catalyst system does not require high levels of iodide salts to stabilize the catalyst. Fewer by-products are formed, such as propionic acid and acetaldehyde condensation products which can lead to low levels of unsaturated aldehydes and heavy alkyl iodides. Also, CO efficiency is improved. 

The Ir-catalyzed methanol carbonylation reaction has been studied extensively by several groups 9f-h. The mechanism for the reaction is more complex than for the Rh reaction. The reaction involves a neutral and an anionic catalytic cycle. The extent of participation by each cycle depends on the reaction conditions. The anionic carbonylation pathway predominates in the Cativa process. The active Ir catalyst species is the iridium carbonyl iodide complex, [Ir(CO)2l2]. The carbonylation reaction proceeds through a series of reaction steps similar to the Rh catalyst process shown in Figure 1 however, the kinetics involve a different rate determining step. 

The latest advance in carbonylation of methanol is the use of an Ir catalyst. Introduced as the Cativa process in 1996 by BP Chemicals, Ir-catalyzed production of acetic acid is now used worldwide.93 The catalytic cycle, called the... 

FIGURE 3 Schematic diagram of the main elements of a Cativa methanol carbonylation process. Adapted with permission from Figure 2 in reference [15], copyright 2006, Elsevier. 

A remarkable step change to existing technology has been 1996 the introduction of the Cativa technology by BP Chemicals (now BP Amoco). This process incorporated the first commercial use of iridimn (promoted by iodine, Ru-salt etc.), rather than rhodimn, as a catalyst for methanol carbonylation. The main improvements of the process are much higher reactivity (45 mol L h, Rh 10-15 mol L h ) coupled with low by-product formation and lower energy requirements for the purification of the product acid. 

In addition to rhodium-based catalysts, iridium-based catalysts have also been developed for carbonylation of methanol. The iridium system, known as the Cativa process, follows a cycle similar to the rhodium system in Figure 14.20, beginning with oxidative addition of... 

CH3I to [Ir(CO)2l2]. The first step in the iridium system is several hundred times faster than in the Monsanto process the second step, involving alkyl migration, is much slower, and it is rate determining for the Cativa process. In addition to the catalytic cycle involving the anion [Ir(CO)2l2] , an alternative neutral cycle involving Ir(CO)3l or Ir(CO)2l has been reported. Two excellent reviews of developments in catalytic methanol carbonylation are available. ... 

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 ]... 

Iridium-Catalyzed Carbonylation of Methanol BP s Cativa Process... 

Methanol carbonylation catalyzed by a combination of iridium-carbonyl compounds and iodide additives was first reported by Monsanto in the 1970s. The mechanism of this process was studied by Forster. ° In the 1990s, BP reported an improved catalyst system based on iridium and iodide that included a "promoter," such as [Ru(CO)jy j. These Ir-based Cativa catalysts are about five times more active than the Rh catalysts, more stable in the presence of low amounts of water (5 wt %), and more soluble. In addition, Lr is usually less expensive than Rh. BP not only built new Cativa plants, but were able to convert existing plants containing rhodium catalysts to plants containing iridium Cativa catalysts because of the similarity of the Ir and Rh systems. 

Scorpionate or pyrazole V(V) 1-4, 6, 10, and 15, and Re(in) 11, 12, 16, 17, 24-26 complexes have been used as catalysts for the carboxylation of gaseous alkanes via single-pot conversions [5a,fj. These syntheses of carboxylic acids are much simpler than those used in industry. For instance, in the case of the conversion of methane into acetic acid, the current industrial routes commonly involve three distinct stages and use more expensive catalysts and harder experimental conditions (e.g., the Mosanto and BP-Amoco Cativa processes of carbonylation of methanol, at the third stage, are based on Rh and Ir catalysts, respectively) [8b]. 

A. Haynes (2010) Adv. Catal., vol. 53, p. 1 — Catalytic methanol carbonylation An up-to-date review of the Monsanto and Cativa processes including background information. 

For a long time it was known that group VIII metal carbonyls are efficient catalysts for carbonylation reactions. In 1996, BP developed a new catalyst system for methanol carbonylation based on iridium (additionally promoted by iodine and Ru-salts), called the Cativa process. Fundamental studies had shown before that the oxidative addition of methyl iodide to iridium is 150-times faster than to rhodium. Thus, in the Cativa process this step is no longer rate determining (as in the case of Rh-based methanol carbonylation). The slowest step in the iridium-cyde is the insertion of CO. This step involves the elimination of iodide and coordination of an additional CO ligand to iridium (Figure 6.15.6). Accordingly, the reaction rate can be described by Eq. (6.15.8) ... 

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. 

Sunley, G.J. and Watson, D.J. (2000) High productivity methanol carbonylation catalysis using iridium. The Cativa process for the manufacture of acetic acid. Catal. Today, 58, 293-307. 

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