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Effect of Water Concentration On Rhodium and Iridium Catalysts
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| The maximum rate observed in the in
lab for the Ir/Ru process of 47 mol dm-3 h-1 has not yet been commercialised. |
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| The iridium catalyst proved to be more stable under a wide range of conditions, and more soluble so that it has no tendency to precipitate out of solution. This means that the catalyst can be continuously recycled within the plant. New catalyst does not need to be added. As well as this iridium is considerably cheaper. | Prices fluctuate daily and can be checked
at the website given below Johnson Matthey platinum prices
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Question Print a copy of the graph. Draw your own smooth curves, and then answer the questions a At what water concentration did the Ir/Ru process run fastest? b What happens to the rate of the Rh process when the water concentration drops c What is the lowest water concentration that could be used with Rh ?
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a at about 5%w/w
b It drops quite sharply
c About 8 % - 10 %. It is kept up to 10 % to prevent the rhodium complex from precipitating out of solution.
| There is a complex interdependence between variables in the iridium process. A pilot plant was used to optimise conditions. The pilot plant provided data on carbonylation rate, by-products, catalyst stability, corrosion rates and product quality under continuous operation. Laboratory and pilot plant work showed that the iridium catalyst is more selective, reducing the amount of ethanal from a few hundred ppm to only 30 ppm and propanoic acid concentrations from 1600 ppm to 400 ppm, simplifying the final purification. The overall yield rises to 98%. |
