Industrial Catalysis

Producing petrol

Octane number

The tendency of a fuel to auto ignite and 'knock' is measured by its octane number. A fuel with a low octane number has a significant tendency to auto ignite while a fuel with an octane number over 100 has a much lower tendency to auto ignite.
The scale is based on the tendency to auto ignite of two hydrocarbons:

  • 2,2,4-trimethylpentane, which used to be called iso–octane (hence the name of the scale), is given an octane number of 100.
  • heptane is given an octane number of zero.

The octane number of any fuel is the percentage of 2,2,4-trimethylpentane in a mixture with heptane that has the same tendency to auto ignite. So petrol with an octane number of 97 has the same tendency to auto ignite as a mixture of 97% 2,2,4-trimethyl pentane and 3% heptane.

What affects octane number?

The octane number is affected by two main features of alkane molecules. Higher octane numbers occur with:

  • more branching: 2-methylbutane is less likely to autoignite than pentane.
  • shorter chains: pentane is less likely to autoignite than heptane

Isomerisation

Isomerisation is a process by which straight chain alkanes are converted to branch chain alkanes that can be blended in petrol to improve its octane rating.

isomerisation

 

isomerisation

pentane

2-methylbutane

octane no.

62

 

93

catalyst platinum finely dispersed on aluminium oxide

 

 

2-methylbutane and pentane are structural isomers. However, pentane is a straight chain and is more likely to autoignite.

An equilibrium mixture is formed. This is passed through a form of zeolite which acts as a molecular sieve to separate branched and straight chain molecules. The straight chain molecules can pass through the space in the 'sieve'. The unconverted reactants are then recyled over the catalyst again.

Structure of a zeolite crystal.
Electron micrograph of one type of zeolite crystal.