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.
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
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
- 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.
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