Crude oil is the raw material from which alkanes are extracted via the process of fractional distillation. There is nothing more to this than you learnt at GCSE but you need to be able to briefly describe the process (and be clear that we are simply separating out molecules – intermolecular bonds are being broken when the oil is vaporised, not covalent bonds).
If you need a refresher, check this out:
Each fraction is a mixture of molecules with similar boiling points and numbers of carbon atoms. The gasoline fraction (bpt range 25-75°C; 5-7 carbon atoms) is used to make petrol but it has to further refined before it is fit for use. The other problem is that most crude oil is rich in high boiling fractions such as gas oil and residue, but not the more useful lighter fractions.
The blending of petrol is covered here, so let’s take a look at how the other fractions are dealt with …
| Isomerisation | Reforming | Steam cracking | Catalytic cracking | |
| Feedstock | C4 – C6 alkanes | C6 – C10 alkanes | C6 – C16 alkanes | C14 – C20 alkanes |
| Conditions | Pt / Al2O3 catalyst, 150°C | Pt / Al2O3 catalyst, 500°CHydrogen is added to the mix to reduce coking of the catalyst surface | No catalyst, 900°CSteam is added to the mix to reduce coking | Zeolite catalyst, 500°C |
| Products | Produces branched chain isomers | Produces alicyclic and aromatic molecules | Produces shorter straight chain alkanes and alkenes | Produces branched chain alkanes and alkenes |
Cracking
Cracking (either thermal cracking or catalytic cracking) breaks down long chain alkanes into short chain and branched chain alkanes, cycloalkanes and alkenes. The mixture of products is then fractionally distilled to separate them out from each other.
Industrially, the process involves passing vaporised hydrocarbons over a hot zeolite catalyst. The catalyst becomes coated with carbon from the decomposition of hydrocarbons over time and so must be regenerated by burning off the hot coke in a stream of air. The energy released from this reaction is used to heat the catalyst, helping to lower energy costs.
The basics of cracking are covered in the video below:
Cracking hydrocarbons can also be carried out in the lab using an aluminium oxide catalyst. You should be able to draw the equipment set-up and describe the method.
If you haven’t done this practical, you can watch it here:
Practice questions
- Zeolites can be used as heterogeneous catalysts in the cracking of crude oil.
(a) Explain the terms heterogeneous and catalyst
(b) Write an equation, using molecular formulae, for the cracking of decade to produce and propene and another product.
(c) Zeolites can also be used as molecular sieves. Two structural isomers with the molecular formula C8H10 are shown below:
If a mixture of these isomers is passed through a zeolite sieve they can be separated. Suggest how this separation occurs with reference to zeolite structure and the shape of these isomers.
- Which of these statements are true and which are false?
A. A zeolite catalyst is used in the fractional distillation of crude oil
B. Heavy crude oil fractions are cracked to produce small chain alkanes and alkenes
C. Each fraction in the fractional distillation of crude oil contains a mixture of hydrocarbons
D. The high temperatures in the fractionating column are needed to break the covalent bonds in the molecules in order to separate them
E. Isomerisation is a process that produces cyclic molecules from straight chain molecules
F. A winter blend of petrol needs to be more volatile than a summer blend
Answers
- (a) Heterogeneous: a catalyst in a different physical state to the reactants
Catalyst: a substance that speeds up a reaction and can be recovered unchanged at the end / can be regenerated / lowers the activation enthalpy for the reaction
(b) C10H22 ⇾ C3H6 + C7H16
(c) A zeolite sieve has a honeycomb structure with pores / channels that are a similar size to that of the molecules being separated. 1,4-dimethylbenzene will be able to slip through the pores / channels but 1,3-dimethylbenzene will get stuck due to the position of its branches.
2. Statements B, C, F are true; A, D, E are false.