Fuel Additives, Octane Numbers and Markovnikov Addition

Automotive fuels for internal combustion engines originate from the light distillates obtained from processing crude oil. Fuels obtained directly from distillation typically have an octane number below about 75, which is substantially below the range of 87 – 93 commonly found for gasoline. One method for improving the octane number of gasoline is through the use of fuel additives, often referred to as antiknock agents.

Antiknock agents were first investigated in the early part of this century¹ with ethanol (1917) and tetraethyllead (1921) being identified as candidates. Ethanol was apparently not pursued commercially at that time since it could not be patented. Consequently commercial production of tetraethyllead began in 1923. From that time until 1986, 7 million tons of the additive were produced and used in gasoline. Phase-out of tetraethyllead began in 1976 and was instituted as a consequence of the health risks associated with chronic lead exposure.

Methyl tert-butyl ether (MTBE) was first introduced as a commercial fuel additive as a consequence of the phaseout of tetraethyllead. Initially relatively small quantities were used. In the United States the situation changed dramatically with the introduction of the 1990 Clean Air Act². This Act required that reformulated gasoline used in areas with substantial ozone pollution (a major component of ground-level smog) have a minimum ‘oxygen content’ of 2%. Oxygen content in fuels is obtained through the addition of organic chemicals containing oxygen-based functional groups (e.g. alcohols and ethers), usually referred to as ‘oxygenates’. Oxygenates are intended to fulfil two objectives; the first is to raise the octane number while the second is to reduce ozone pollution.

At present MTBE is the major fuel oxygenate in use, with ethanol a distant second. Worldwide use is about 7 billion US gallons³. It is made by the Markovnikov addition of methanol to isobutene (2-methylpropene) in the liquid phase under pressure at 30-100°C; acid catalysis is required and is usually achieved through the use of an acidic ion exchange resin. The C₄ fraction of petroleum distillate (a mixture of isobutene (45-50%), 1-butene (25-30%), 2-butene (20%), butane and isobutane) is used for this reaction since isobutene is kinetically more reactive to Markovnikov addition than the other alkene components.

Very recently MTBE has come under intense scrutiny as a fuel additive due to its appearance in groundwater². When leaking fuel storage tanks release reformulated gasoline into the ground, MTBE rapidly contaminates groundwater as a consequence of its high water solubility (up to 5 parts per hundred which is very much higher than the hydrocarbon components in gasoline). When MTBE-contaminated groundwater enters municipal water supplies, the water becomes undrinkable due to a foul smell and taste. MTBE levels of 3 to 20 ppb are sufficient to cause this problem. As a consequence of groundwater contamination, phaseout of MTBE as a fuel additive is currently being considered in the United States. In Canada, substantially less MTBE is used as a fuel additive; an alternative additive, MMT (methylcyclopentadienyl manganese tricarbonyl), has been in use for about 15 years and has been the subject of a completely different controversy.

R. A. Whitney.

July 2000.

1. Chemical & Engineering News, April 17, 2000. p.29

2. Chemical & Engineering News, May 8, 2000. pp. 40-46

3. Chemical & Engineering News, April 17, 2000. p. 25