CHEM3002-E1 CATALYSIS

SECTION A (50 marks)

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1. You are a scientist investigating the reaction A + B ⇌ C which is promoted using a heterogeneous Pd catalyst. Assume no catalyst deactivation occurs.

(a) Using Frenkel’s equations/assumptions, calculate ∆Hads for the molecules A, B and C on the surface of Pd catalyst using the information below. Show your working.

(b) Assuming the Langmuir-Hinshelwood model for the reaction above and using its rate equation:

write down the rate equation for the reaction above based on your answer in part (a). Justify all the assumptions that you make, and comment on their implications for catalysis. (5 marks)

(c) Experiments can be performed to corroborate the calculated ∆Hads values from part (a) and the assumptions/implications made in part (b). State one experimental technique that you would use, when you would perform it (before, during, or after reaction) and describe what you would expect to observe. (5 marks)

2. Explain which of the curves below (curve 1 and curve 2) best represents each of the Langmuir-Hinshelwood and Eley-Rideal mechanisms.

3. You are a scientist investigating the synthesis of ammonia using Ru clusters as a catalyst that have been deposited on five different supports (these are labelled: Ru/A, Ru/B, Ru/C, Ru/D and Ru/E). The samples are pre-treated in the reactor using a flow of H2 at 400 oC before the addition of N2.

You can assume that the reaction is performed using the same conditions for all samples, the mean diameter size of the synthesised supported Ru catalyst and catalyst loading are the same for all samples; and the reaction only takes place on the Ru catalyst surface. The results are shown in the graph below.

(a) Why is it beneficial to pre-treat the samples with H2 before the reaction? (2 marks)

(b) Why does a support with a relative low work function lead to an increase of rate of ammonia production (you can ignore Ru/C for this part)? (3 marks)

(c) Assuming that the relationship between work-function and rate of ammonia production is correct, give at least one reason why the supported catalyst Ru/C may have performed differently. Explain your answer. (4 marks)

(d) Based on your answer in part (c), describe an experimental technique to investigate the supported catalyst Ru/C. State what experimental technique you would use, when in the reaction you would perform it (before, during or after) and describe what you would expect to observe. (4 marks)

(e) Would you expect the same activation energy for the four supported catalysts Ru/A, Ru/B, Ru/D and Ru/E? Explain your answer. (4 marks)

4.As your Cu/ZnO catalyst is underperforming when compared to the competitor catalyst, you decided to use multimetallic CuPd/ZnO (Cu:Pd = 50:50 wt%) catalyst, as you know that the addition of Pd can improve the rate of CO2 conversion to methanol. You have characterised your new CuPd catalyst which showed that most of the Cu atoms are on the surface and Pd atoms are on the core of the CuPd catalyst, thus forming a core-shell nanostructure. Consider that the reaction only takes place on the metal catalyst surface.

(a) Calculate the TON at 24 hours and TOF (in units of h-1) for Cu/ZnO and CuPd/ZnO catalysts based on the information below. State any assumptions that you make.

Reaction conditions: Cu/ZnO and CuPd/ZnO (150 mg), 1 wt% metal loading (30% surface atoms; 70% core atoms), reaction volume 20 mL, total reactants = 0.5 M. (6 marks)

(b) Based on your results for part (a), is the CuPd catalyst more active for CO2 conversion compared to the Cu catalyst? Contrast the calculated TON and TOF for these catalysts to justify your answer. (5 marks)

(c) Using the results from part (a) and (b), contrast the sustainability of the use of CuPd instead of Cu catalyst in methanol synthesis. Justify your answer. (3 marks)

SECTION B (50 marks)

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5. This question refers to the Shell Higher Olefin Process (SHOP) for catalytic alkene oligomerisation. Shown below is an incomplete reaction mechanism for the formation of precatalyst E. Use this mechanism to answer question (a). Note: COD = 1,5- cyclooctadiene.

(a) Draw the structures for complexes D and E and determine the valence electron count and metal oxidation state for both complexes. (6 marks)

Shown below is an incomplete catalytic cycle for the oligomerisation of ethene by E. Use this mechanism to answer questions (b)–(d).

(b) Draw species H and determine the d-electron count for the metal. (2 marks)

(c) Classify the reactions involved in the transformation of G to H and of J to F. Identify how these reactions affect the oxidation state of the metal. (4 marks)

(d) SHOP produces α-alkenes with a range of different carbon chain lengths. Explain how the relative rates of transformations G to H and J to F influence the distribution of carbon chain lengths obtained. (3 marks)

(e) Precatalyst E is soluble in polar solvents (e.g. 1,4-butanediol), but has low solubility in non-polar solvents. Explain how this property can be exploited to reduce costs when SHOP is performed at an industrial scale. (3 marks)

6. An incomplete reaction scheme for a cobalt-catalysed hydroformylation process is shown below (R = alkyl).

(a) The Co2(CO)8 precatalyst generates the active catalyst in situ under the reaction conditions. Identify the active catalyst and provide a balanced equation for its formation. (2 marks)

(b) Identify the major product K and the minor product L, drawing their structures. (2 marks)

(c) The Shell-OXO hydroformylation process uses a phosphine-substituted cobalt catalyst, [HCo(PnBu3)(CO)3], instead of Co2(CO)8. This allows it to operate at lower pressures of H2 and CO.

i) Explain why [HCo(PnBu3)(CO)3] can operate at lower gas pressures than Co2(CO)8. (4 marks)

ii) How would you expect the product ratio of K:L to change if [HCo(PnBu3)(CO)3] were used instead of Co2(CO)8? Explain your reasoning. (4 marks)

7. (a) The zirconium and hafnium ansa-metallocenes N and P (pictured below) both polymerise propene to polypropylene in the presence of a methylaluminoxane (MAO) co-catalyst.

i) State the tacticity of polypropylene produced by N and P in the presence of MAO. (1 mark)

ii) Catalyst N (Zr) shows significantly higher activity for the polymerisation of propene than catalyst P (Hf). Suggest why this is the case and explain your reasoning. (3 marks)

(b) The reaction of bis(cyclopentadienyl)dimethylzirconium with 1 equivalent of B(C6F5)3 affords salt Q which is highly active in the polymerisation of ethene.

i) Identify species Q, drawing its structure. (2 marks)

ii) Draw a reaction mechanism for the reaction of one molecule of ethene with species Q. Show any relevant transition states. (4 marks)

(c) Treatment of two equivalents of B(C6F5)3 with one equivalent of KCN results in the formation of salt R, which displays two signals in its 11B NMR spectrum. Treatment of R with one equivalent of Ph3CCl results in the precipitation of KCl and the formation of salt S, which displays a similar 11B NMR spectrum to R. When bis(cyclopentadienyl)dimethylzirconium is treated with one equivalent of S, it results in the formation of salt T and Ph3CMe. T is significantly more active in the polymerisation of ethene than Q. An incomplete reaction scheme for the synthesis of T is shown below.

i) Identify species R, S, and T drawing their structures. (6 marks)

ii) Explain why species T is significantly more active than Q in the polymerisation of ethene. (4 marks)