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The Promoter


Effect of various additives on the rate for the iridium catalysed
carbonylation of methanol from batch autoclave data.


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













a Inspect the table of data. Why is run 1 included?
Why is run 9 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










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.





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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