Ethanoc Acid: Production method 3: The Cativa process

In 1986, BP acquired the Monsanto ethanoic acid technology, and developed existing research using iridium as a catalyst. The result was the Cativa process, described as the most important development in ethanoic acid production since the Monsanto Process.

The mechanism involving iridium is different to that of rhodium as catalyst, as iridium works best under different conditions. The Cativa process also uses ruthenium compounds as promoters in the reaction. These increase the reaction rate by three times, even though ruthenium on its own has negligible catalytic activity in this system.

Reaction mechanism
The mechanism of the reaction has been studied in great detail, in particular the role of the catalyst. For more detailed information on the mechanism visit the catalysis site.

diagram: iridium catalyst

Benefits

Like the Monsanto process, the reactions are theoretically 100% atom efficient. The use of iridium/ iodide as a catalyst has numerous benefits compared with rhodium/ iodide:

Iridium costs only about one fifth as much as rhodium (current prices can be found at the Johnson Matthey web site
The process is faster and more effective, requiring less catalyst to be used.
The catalyst has a much higher turnover number (TON). The TON is a measure of how many cycles the catalyst can go through before it will need replacing.
Iridium is even more selective for methanol, which increases the overall yield and reduces by- products, resulting in lower purification costs and reduced waste.
Iridium complexes are more soluble in the reaction mixture than rhodium complexes. This means that the catalyst is not lost by precipitation and does not need replacing so frequently. The water content in the reaction vessel can also be reduced, speeding up the process and reducing the energy needed at the distillation and purifying stages.
Existing plant can be modified to run the Cativa process at half the cost of building a new plant. This is referred to as retrofitting.
Cativa plants have a higher throughput - a single plant can produce up to 75% more ethanoic acid than was previously possible using the Monsanto process.