Key Steps in the Mechanism of Carbonylation Processes

The principal reduced C2 by-product is EtCOOH. In Ir catalysed reactions, the carbonylation rate increases with [Ir], [MeOAc], [Mel] and [CO], The rate with respect to [H2O] passes through a maximum. In contrast to Rh systems, the water gas shift in Ir catalysed MeOH carbonylation tends to be a relatively constant fraction of carbonylation rate as the reactor composition is varied, though it tends to decrease with [Mel] and [MeOAc] but increase if ionic iodides are added [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]. 

In common with many catalysed reactions, the important features of carbonylation process chemistry may be associated with different aspects of the catalytic cyde. Broadly, process activity may vary either because (i) more of the catalyst is present in the active form, (ii) the activity of the catalyst in the active form is enhanced or inhibited or, less commonly, (iii) the rate controlling step does not involve the catalyst. The process selectivity may vary because of side reactions (i) occurring through the active catalyst cycle, (ii) involving inactive catalyst, or (iii) taking place because of the organic chemistry of the systems. Examples of all these contributions to overall process effidency are found in the various commerdal carbonylation processes. 

The main steps in the catalytic MeOH carbonylation cyde which were proposed for the Co catalysed process [2] have served, with some modification perhaps in the carbonylation of MeOAc to AC2O, to the present day and are familiar as a classic example of a metal catalysed reaction. These steps are shown in Eigure 5.1. They are of course, (i) the oxidative addition of Mel to a metal center to form a metal methyl species, (ii) the migratory insertion reaction which generates a metal acyl from the metal methyl and coordinated CO and (iii) reductive elimination or other evolution of the metal acyl spedes to products. Broadly, as will be discussed in more detail later, the other ligands in the metal environment are CO and iodide. To balance the overall chemistry a molecule of CO must also enter the cycle. 

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