Benzene is one of the important chemical compounds in the world, with the chemical formula C6H6. It is one of the natural constituents of crude oil and belongs to one of the elementary petrochemicals. As benzene has a cyclic ring which consists of continuous pi bond between the carbon atoms, it is categorised as an aromatic hydrocarbon. Benzene appears as clear, colourless liquid with a sweet smell. It is highly flammable and is used mainly as a precursor for the synthesis of more complex chemical molecules. However, due to its carcinogenic properties, its non-industrial applications are limited.
There are four main chemical processes which contributed to the industrial benzene production: (1) catalytic reforming, (2) toluene hydrodealkylation, (3) toluene disproportionation, and (4) steam cracking. The ATSDR Toxicological Profile for benzene has observed that the catalytic reforming contributed to 44-50% of the total U.S benzene production.
(1) Catalytic Reforming
This process involves adding hydrogen gas to a mixture of hydrocarbons (with boiling points 60–200 °C), with the aid of rhenium chloride catalyst at high temperature and pressure.
(2) Toluene hydrodealkylation
Toluene hydrodealkylation synthesise benzene by converting toluene to benzene. This involves the mixing of toluene with hydrogen, which is then passed over a chromium, molybdenum or platinum oxide catalyst at elevated temperature and pressure.
(3) Toluene disproportionation
Toluene disproportionation (TDP) serves as an alternative to toluene hydrodelakylation, when a chemical complex has similar demands for both benzene and xylene. Two toluene molecules are reacted and the methyl groups will rearrange from one toluene molecule to the other, giving one benzene molecule and one xylene molecule.
(4) Steam cracking
This process involves the production of ethylene and other alkenes from aliphatic hydrocarbons. Steam cracking can produce a benzene-rich liquid by-product known as pyrolysis gasoline. This liquid can be mixed with other hydrocarbons to yield aromatic hydrocarbons such as benzene.
Benzene is used as a component of gasoline to increase octane rating and reduces knocking. However, due to its negative health effects, the percentage of benzene found in gasoline has been decreasing through the years.
Benzene is mainly used as an intermediate to make other chemicals such as ethylbenzene, cumene, cyclohexane, nitrobenzene and alkylbenzene.
It is also used to make chemicals in the manufacture of industrial products such as dyes, detergents, explosives, pesticides, synthetic rubber, plastics, and pharmaceuticals.