Methanol, also known as methyl alcohol or wood alcohol, is a chemical compound with chemical formula CH3OH. It is the simplest alcohol, and is a light, volatile, colourless, flammable, poisonous liquid that is used as an antifreeze, solvent, fuel, and as a denaturant for ethyl alcohol.
Methanol is produced naturally in the anaerobic metabolism of many varieties of bacteria. As a result, there is a small fraction of methanol vapor in the atmosphere. Over the course of several days, atmospheric methanol is oxidized by oxygen by the help of sunlight to carbon dioxide and water.
Methanol burns in air forming carbon dioxide and water:
- 2CH3OH + 3 O2 --> 2CO2 + 4H2O
A methanol flame is almost colorless. Care should be exercised around burning methanol to avoid burning oneself on the almost invisible fire.
In their embalming process, the ancient Egyptians used a mixture of substances, including methanol, which they obtained from the pyrolysis of wood. Pure methanol, however, was first isolated in 1661 by Robert Boyle, who called it spirit of box, because he produced it via the distillation of boxwood. It later became known as pyroxylic spirit. In 1834, the French chemists Jean-Baptiste Dumas and Eugene Peligot determined its elemental composition. They also introduced the word methylene to organic chemistry, forming it from the Greek words methu, meaning "wine," and hyle, meaning "wood". The term methyl was derived in about 1840 by back-formation from methylene, and was then applied to describe methyl alcohol. This was shortened to methanol in 1892 by the International Conference on Chemical Nomenclature.
In 1923, the German chemist Matthias Pier , working for BASF developed a means to convert synthesis gas (a mixture of carbon monoxide and hydrogen derived from coke and used as the source of hydrogen in synthetic ammonia production) into methanol. This process used a zinc chromate catalyst, and required extremely vigorous conditions—pressures ranging from 300–1000 atm, and temperatures of about 400°C. Modern methanol production has been made more efficient through the use of catalysts capable of operating at lower pressures.
Today, synthesis gas is most commonly produced from the methane component in natural gas rather than from coal. Three processes are commercially practiced. At moderate pressures (10–20 atm) and high temperatures (around 850°C), methane reacts with steam on a nickel catalyst to produce syngas according to the chemical equation
CH4 + H2O --> CO + 3 H2
This reaction, commonly called steam-methane reforming or SMR, is endothermic and the heat transfer limitations place limits on the size of the catalytic reactors used. Methane can also undergo partial oxidation with molecular oxygen to produce syngas, as the following equation shows:
CH4 + 0.5 O2 --> CO + 2 H2
this reaction is exothermic and the heat given off can be used in-situ to drive the steam-methane reforming reaction. When the two processes are combined, it is referred to as autothermal reforming. The ratio of CO and H2 can be adjusted by using the water-gas shift reaction,
CO + H2O <--> CO2 + 3 H2,
to provide the appropriate stoichiometry for methanol synthesis.
The carbon monoxide and hydrogen then react on a second catalyst to produce methanol. Today, the most widely used catalyst is a mixture of copper, zinc oxide, and alumina first used by ICI in 1966. At 50–100 atm and 250°C, it can catylize the production of methanol from carbon monoxide and hydrogen with high selectivity
CO + 2 H2 --> CH3OH
It is worth noting that the production of synthesis gas from methane produces 3 moles of hydrogen for every mole of carbon monoxide, while the methanol synthesis consumes only 2 moles of hydrogen for every mole of carbon monoxide. One way of dealing with the excess hydrogen is to inject carbon dioxide into the methanol synthesis reactor, where it, too, reacts to form methanol according to the chemical equation
CO2 + 3 H2 --> CH3OH + H2O
Although natural gas is the most economical and widely used feedstock for methanol production, other feedstocks can be used. Where natural gas is unavailable, light petroleum products can be used in its place. The South African firm Sasol produces methanol using synthesis gas from coal.
Methanol is used on a limited basis to fuel internal combustion engines, mainly by virtue of the fact that it is not nearly as flammable as gasoline. Methanol blends are the fuel of choice in open wheel racing circuits like Champcars, as well as in radio controlled model airplanes. Drag racers and mud racers also use methanol as their primary fuel source. Methanol is required with a supercharged engine in Top Alcohol Dragster and all vehicles in Top Alcohol Funny Car have to run methanol. Mud racers have mixed methanol with gasoline and nitrous oxide to produce more power than gasoline and nitrous oxide alone.
When produced from wood or other organic materials, the resulting organic methanol (bioalcohol) has been suggested as renewable alternative to petroleum-based hydrocarbons. However, one cannot use BA100 (100% bioalcohol) in modern petroleum cars without modification.
Methanol is also used as a solvent and as an antifreeze in pipelines. The largest use of methanol by far, however, is in making other chemicals. About 40% of methanol is converted to formaldehyde, and from there into products as diverse as plastics, plywood, paints, explosives, and permanent press textiles.
In the 1990s, large amounts of methanol were used in the United States to produce the gasoline additive methyl tert-butyl ether (MTBE). The 1990 Clean Air Act required certain major cities to use MTBE in their gasoline to reduce photochemical smog. However, by the late 1990s, it was found that MTBE had leaked out of gasoline storage tanks and into the groundwater in sufficient amounts to affect the taste of municipal drinking water in many areas. Moreover, MTBE was found to be a carcinogen in animal studies. In the resulting backlash, several states banned the use of MTBE, and its future production remains uncertain.
Other chemical derivatives of methanol include dimethyl ether, which has replaced chlorofluorocarbons as the propellant in aerosol sprays, and acetic acid.
Health and safety
Methanol is toxic, as its metabolites formic acid and formaldehyde cause blindness and death. It enters the body by ingestion, inhalation, or absorption through the skin. Dangerous doses will build up if a person is regularly exposed to fumes or handles liquid without skin protection. If methanol has been ingested, a doctor should be contacted immediately. The usual fatal dose: 100–125 mL (4 fl oz). Toxic effects take hours to start, and effective antidotes can often prevent permanent damage. One treatment is the injection of ethanol: this slows down the breakdown of methanol by the liver, so that the toxic metabolites can not build up.
Symptoms of methanol ingestion are similar to those of intoxication: headache, dizziness, nausea, lack of coordination, confusion, drowsiness, followed by unconsciousness and death.
The ester derivatives of methanol do not share this toxicity.
Ethanol is sometimes denatured (adulterated) by the addition of methanol. The result is known as methylated spirits or meths. (The latter should not be confused with meth, a common abbreviation for methamphetamine.)
Pure methanol has been used in open wheel racing since the mid-1960's. Unlike petroleum fires, methanol fires can be extinguished with plain water. The decision was made shortly after the death of two drivers.
- Robert Boyle, The Sceptical Chemist (1661) – contains account of distillation of wood alcohol.
The Methanol Institute Industry trade group, lots of information on methanol's use in fuel cells and as an alternative fuel.