Aromatic hydrocarbons are hydrocarbon compounds with a ring structure, rather than the chain structure typically seen in olefins. In these compounds, a delocalized pi electron is shared amongst carbon atoms to form the typical ring structure. The most common aromatic products of petrochemical processing are benzene, toluene, and the xylene isomers, collectively known as BTX. Aromatics are created using reforming and come from crude oil or, more rarely, coal processing.
Benzene, Toluene, and Xylenes (BTX)
Benzene, C6H6, is the simplest of the aromatic hydrocarbons and its basic 6-carbon ring structure – the benzene ring – is the basis for other aromatic compounds. Benzene is a colorless, flammable liquid derived from petroleum. It has a sweet smell, which is the source of the name “aromatic” – you may be familiar with this sweet odor from gasoline (petrol), to which benzene is often added for its high octane number.
Benzene can be produced from crude oil feedstocks by using catalytic reforming, toluene hydrodealkylation, toluene disproportionation, or steam cracking. Benzene is an important intermediate for countless chemical products. For example, it can be used to produce ethylbenzene, a precursor to styrene and its polymer polystyrene, along with other high-use plastics and polymers. Other end products include nylon fibers, phenol and acetone for epoxy resins and phenolic resins, nylons, rubbers, lubricants, dyes, detergents, and more!
Benzene can also be used as an additive to gasoline to increase its octane rating, although toluene is often used instead, as it has fewer harmful potential side effects.
Toluene, C7H8, is comprised of a base benzene ring monosubstituted with a methyl group (CH3). This colorless, water-insoluble liquid is prized as a solvent. Its odor is most commonly associated with paint thinners. Toluene can be produced from crude oil or coal refining using catalytic reforming or ethylene cracking. Most of the toluene produced is converted to benzene via hydrodealkylation, the process of removing the methyl group to yield benzene (C6H6) and methane (CH4), due to the vast number of commercially useful chemicals that can be derived from benzene. However, toluene is also valuable in its own right, primarily as a solvent used in laboratory chemical processes as well as commercially in paints, paint thinners, permanent markers, lacquers, adhesives, and more.
In recent years, toluene has begun to be used more frequently as an octane booster in gasoline (petrol) fuels, in lieu of butene. Toluene has a similar octane rating but is considered much safer to use.
Xylenes (Dimethylbenzene Isomers)
The compound xylene (C8H10) come in the form of three different isomers – o-xylene, m-xylene, and p-xylene, depending on the location of the methyl group – collectively known as the xylenes.
Xylenes are colorless, highly flammable, water-insoluble liquids produced via the methylation of benzene and toluene during catalytic reforming. The isomer p-xylene is most commercially viable as it can be converted to terephthalic acid, the chemical used to make the polyester PET, via oxidation. The isomer o-xylene is a precursor to chemicals such as phthalate esters used as plasticizers and for the production of dyes. m-Xylene is primarily used as a precursor to chemicals used in the production of PET, the thermoplastic polymer fiber used fabrics, thermoformed food containers, and more.