| Effect of catalyst concentration on rate of desired
reaction, for Ir only, and for iridium-ruthenium mixture |
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The Promoter
Effect of various additives on the rate for the iridium
catalysed
carbonylation of methanol from batch autoclave data.
b Os, Re, W and Ir are all in the same period. Ru is next to rhodium. Zn and Cd are in the same group but further away from Rh and Ir. Li is far away in group 1.
c LiI reduces the rate (almost halves it).
d Because iodide is already involved in the reaction.
e Ru(CO)4I2 and Os(CO)4I2.
f Ru(CO)4I2 It puts the rate up 2.63 times.
g With no Ir the Ru compound has no catalytic activity (run 9). The promoter increases the rate of the catalysed reaction (run 3 compared to run 1).| Run Number | Additive | Additive:iridium molar ratio | Carbonylation rate mol dm-3 h-1 |
| 1 | none | - | 8.2 |
| 2 | LiI | 1:1 | 4.3 |
| 3 | Ru(CO)4I2 | 5:1 | 21.6 |
| 4 | Os(CO)4I2 | 5:1 | 18.6 |
| 5 | Re(CO)4Cl2 | 5:1 | 9.7 |
| 6 | W(CO)6 | 5:1 | 9.0 |
| 7 | ZnI2 | 5:1 | 11.5 |
| 8 | CdI2 | 5:1 | 14.7 |
| 9 | Ru(CO)4I2 | Control no iridium | 0 |
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Question a Inspect the table of data. Why is run 1 included?
b Look at a copy of the periodic table and note the positions of the metals tried. c What effect does the lithium iodide have on the rate? d Why do you think they tried iodides? e Which two compounds are most effective as promoters? f Os and Re compounds are very toxic. Which compound would you select as a promoter? By how much does it increase the rate? g Compare the data for runs 1, 3, and 9. They illustrate very clearly the role of a promoter. Comment
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| Effect of catalyst concentration on rate of desired
reaction, for Ir only, and for iridium-ruthenium mixture |
| Investigating the effect of the ruthenium promoter |
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| A
big advantage is how the system behaves at low water concentrations compared
to the rhodium system. The reaction rate increases as the water concentration falls. |
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Question The data is plotted in Excel in an X-Y scatter graph. Print a copy of the graph and draw two smooth curves through the sets of points. You should always draw your own lines like this, especially when you have obtained the data yourself. You can make judgements that Excel cannot about the significance of different points. a From your graph calculate the difference in rates at 2500 ppm of Iridium b Iridium is more soluble in the reaction medium then rhodium. Why is this an advantage
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b. Higher catalyst concentrations can be used and the rate increases as the
catalyst concentration increases
(for both Rh and Ir processes).