Cativa™ process

To achieve a high reaction rate at low concentrations of ionic iodide, promoters assisting the removal of free iodide are applied in the process. Possible promoters are simple iodide complexes of zinc, cadmium, and gallium or carbonyl-iodide complexes of osmium, tungsten, and ruthenium (the latter being used preferably). 

Interestingly, the Cativa process can be realized as a drop-in replacement in plants that have been used beforehand for the rhodium-based Monsanto process, a fact that makes the technology change very attractive for the plant owner. Today, very efficient high capacity production plants based on the Cativa process with capacities of up to 500 000 ta are in commercial operation worldwide. 

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The CATIVA process uses an iridium catalyst promoted by ruthenium... 

As well as increasing the reaction rate and catalyst stability, at all-important low water concentrations and low CO partial pressures, the iridium system also produces lower levels of by-products. These improvements combine to give the CATIVA process the following advantages ... 

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) ... 

Cationomycin, 20 135 Cations, catalytic effect of, 15 849-850 Cativa process, 16 74 Cativa technology, 19 621 Catla, common and scientific names, 3 187t Cat litter... 

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 This uses a promoted iridium catalyst and has now superseded the rhodium catalyst on a number of plants.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... 

As described above (Section 3.3.1.1), in situ HP IR measurements under catalytic conditions identified the anion [MeIr(CO)2l3] as the catalyst resting state in the Cativa process. The rate controlling step in the catalytic cycle was proposed to be carbonylation of [MeIr(CO)2l3] (Eq. (7)). 

In 1986, BP Chemicals became the owners of the Monsanto technology. They subsequently also developed their own Cativa process, aimounced in 1996, carbonylation of MeOH to AcOH catalysed by Ir and Mel and promoted with specific metal iodides [8]. As with the improvements in the original Monsanto Rh process, Cativa had benefits such as improved catalyst stability and more favorable operating conditions [9]. 

The major Ir species observed in MeOH carbonylation to AcOH in the Cativa process under commercial operating conditions are [IrMe(CO)2l3] and [Ir(CO)2l4] ... 

Many other modifications, particularly of the Rh and Mel catalysed carbonylation of MeOH, have been proposed and some of these have been operated commercially or may have been tested at significant pilot plant scale. These include, for example, the use of phosphine oxide species such as PPh30 [20] as promoters and systems involving immobilizing the Rh on ion exchange resins [21]. Numerous examples of ligand modified catalysts have been described, particularly for Rh, though relatively few complexes have been shown to have any extended lifetime at typical process conditions and none are reported in commercial use [22, 23]. The carbonyl iodides of Ru and Os mentioned above in the context of the Cativa process are also promoters for Rh catalysed carbonylation of MeOH to AcOH [24]. 

This iridium reaction is the basis of BP s new Cativa process launched in 1996,73 which offers significant operational efficiency gains and is expected to eventually replace the current rhodium catalysts. 

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... 

The Cativa process was first commercialized in 1995, with the retro-fitting to an existing rhodium-based plant in Texas City (USA), and several other acetic acid plants now use the Cativa technology. 

Although the studies at Monsanto indicated that an iodide promoted iridium catalyzed reaction was feasible, much more basic research was needed before a viable commercial procedure (the Cativa process) became possible. 

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 order to more fully understand the mechanism for the promotion of carbonylation in the Cativa process, a recent comprehensive study utilizing spectroscopic methods and ab initio calculations was carried out to explore the role of the promoters. The acceleration of the migratory insertion step is the result of iodide abstraction from the iridium center in c,m-[fr(CO)2l3Me] by the promoters to yield the neutral species [Ir(CO)3l2Me] in the presence of CO. It was determined that the migratory insertion of carbon monoxide in [fr(CO)3l2Me] is 700 times faster than for the anionic species [fr(CO)2l3Me]. ... 

In addition to rhodium-based catalysts, iridium-based eatalysts have also been developed in a process known as the Cativa process. The iridium system follows a cycle similar to the rhodium system in Figure 14-16, beginning with oxidative addition of j CH3I to [Ir(CO)2l2] The first step in the iridium system is much more rapid than in the Monsanto process and the second step is much slower the second step, involving alkyl . migration, is rate determining for the Cativa process. ... 

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. 

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. 

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