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CHEM20711
CONTEMPORARY THEMES IN CHEMISTRY
1. Answer ALL parts.
(a) Answer BOTH parts (i) and (ii).
(i) Explain what is meant by the term sustainable development goal. Discuss the need to improve access to safe drinking water in many parts of the world. (6 marks)
(ii) Describe a technology that is currently used to produce drinking water by desalination of seawater. (6 marks)
(b) Answer BOTH parts (i) and (ii).
Circular nanofiltration membranes of effective diameter 3.00 cm consisting of MoS2 nanosheets on aporous support were prepared. They were then functionalised with either sunset yellow (SY) dye or crystal violet (CV) dye. Measurements were made of the volume (V) of water permeating through the membranes in a given time (t) under external pressure. Ion permeation experiments were also carried out and the % Na+ rejection evaluated. The results of these measurements are given in the following table.
|
Membrane |
V / cm3 |
t / min |
% Na+ rejection |
|
Pristine MoS2 |
39 |
45 |
27 |
|
MoS2 + SY |
26 |
30 |
97 |
|
MoS2 + CV |
48 |
15 |
96 |
(i) Calculate the water flux in units of L m−2 h− 1 for each membrane. (8 marks)
(ii) Discuss the potential of these membranes for water treatment applications. (4 marks)
(c) The 2008 Robeson upper bound for the CO2/ N2 gas pair is given by the equation:
P = kαn
where P is the CO2 permeability in barrer, α is the CO2/ N2 selectivity, and the constants k and n have the values 30,967,000 and −2.888, respectively. The following table gives experimental values of P and α for mixed matrix membranes of the polymer of intrinsic microporosity PIM-1 in combination with three different metal-organic frameworks (MOFs). Determine whether each membrane lies below or above the 2008 Robeson upper bound. Comment on your results.
|
MOF |
P / barrer |
α |
|
MOF-801 |
9,690 |
26.8 |
|
MUF-15 |
16,300 |
17.7 |
|
ZIF-8 |
4,820 |
19.3 |
(6 marks)
2. Answer ALL parts.
(a) Answer BOTH parts (i) and (ii).
Two zeolites have the respective chemical formulae:
Na{(C2H5)4N}x [Al5Si38O86]·20H2O
and
Rb2Caz [AlySi16O48]·14H2O
(i) Determine, with explanation, the values of x, y and z. (6 marks)
(ii) Describe the possible roles of the organic ion in Na{(C2H5)4N}x [Al5Si38O86]·20H2O during formation of the zeolite framework. (5 marks)
(b) Answer BOTH parts (i) and (ii).
(i) Describe, with examples, three different types of catalytically active species that can be introduced into zeolites and the method of formation of each type of catalytically active species. (7 marks)
(ii) State, with explanation, and using all the information provided, the type of shape selective catalysis occurring for the following zeolite catalysed reaction.
The product distribution of the reaction is not affected by varying the size of the zeolite particles.
(4 marks)
(c) Answer BOTH parts (i) and (ii).
(i) The unit cell of a zeolite viewed along the a axis is shown below in which the framework of the zeolite is shaded black and the pores are shaded white. Plot the pore size distribution graph for this zeolite. Assume that no pores run through the solid in any direction other than that shown.
(4 marks)
(ii) Discuss, with examples, how the catalytic functionality of zeolites may be applied to combat societal environmental concerns. (4 marks)
3. Answer ALL parts.
This question concerns the enzymatic synthesis of an amide from the two steps shown below.
(a) Answer ALL parts (i) - (iii).
(i) Identify biocatalysts A and B. (2 marks)
(ii) Both biocatalysts have been shown to work under the same reaction conditions. State whether the reactions should be run together in a single vessel or as separate sequential steps. Justify your answer in terms of process requirements. (6 marks)
(iii) Draw the structure of compounds 1, 2 and 3. (4 marks)
(b) Answer ALL parts (i) - (iii).
(i) Identify which of the following are involved in the mechanisms of
biocatalysis by A and B respectively.
(2 marks)
(ii) Suggest an arrow pushing mechanism for EITHER the production of compound 1 (mediated by biocatalyst A) OR the production of the final amide and compound 3 (mediated by biocatalyst B). There is no need to include recycling of cofactors. (8 marks)
(iii) Biocatalyst A was found to have low activity when tested with a ketone starting compound instead of the aldehyde shown above. Suggest why this might be the case and how the activity could be increased. (2 marks)
(c) The reaction catalysed by biocatalyst B can be performed at room temperature in water. A traditional route to the same products requires areflux reaction in benzene in the presence of a ruthenium complex. Compare and contrast the biocatalytic and traditional chemical routes in terms of environmental sustainability. (6 marks)