Benzene: properties, production and applications
Benzene (C₆H₆) is the simplest and most important representative of aromatic hydrocarbons and one of the fundamental feedstock building blocks of the modern petrochemical industry. It is a colourless liquid with a characteristic sweetish odour, lighter than water and insoluble in it, but miscible with most organic solvents. Benzene is the starting compound for the production of styrene, phenol, acetone, nylon and many other products, and its global consumption is comparable to that of ethylene and propylene.
Physicochemical Properties
The molecular formula of benzene is C₆H₆, with a relative molecular mass of 78.11. Density at 20 °C is 0.878–0.880 g/cm³. Boiling point — 80.1 °C, melting point — 5.5 °C. Benzene is highly flammable, and its vapours form explosive mixtures with air. Flash point — minus 11 °C, autoignition temperature — about 560 °C.
Benzene is highly toxic and is a Group 1 carcinogen according to the International Agency for Research on Cancer (IARC). It affects the haematopoietic system and causes leukaemia and other severe diseases. The maximum allowable concentration of benzene vapours in the working area air in Russia is 5 mg/m³ (for comparison, the US OSHA has set a limit of 1 ppm as a time-weighted average). When working with benzene, strict safety measures are required, including equipment sealing, effective ventilation and personal protective equipment.
Production Technology
The main industrial sources of benzene are catalytic reforming of gasoline fractions, pyrolysis of petroleum feedstock (in pyrolysis tar) and toluene hydrodealkylation. Significant quantities of benzene are also obtained from coal tar in coke-chemical production. Modern technologies allow benzene of high purity suitable for polymerisation and fine organic synthesis to be obtained.
The quality of petroleum benzene in Russia and CIS countries is regulated by GOST 9572-93 (superseded by GOST 31909-2013), which establishes requirements for the product intended for the production of synthetic fibres, rubbers, plastics and other products. For coal benzene, GOST 8448-78 applies, and for reagent-grade benzene — GOST 5955-75. Key quality indicators are the mass fraction of the main substance, density, distillation limits, sulfur, thiophene and non-volatile residue content.
Applications
Production of ethylbenzene and styrene. The largest consumption area for benzene is alkylation with ethylene to form ethylbenzene, which is then dehydrogenated to styrene — the monomer for polystyrene, ABS plastics and synthetic rubbers.
Production of cumene and phenol. Benzene is alkylated with propylene to form cumene (isopropylbenzene), which is further processed into phenol and acetone — key products for the production of polycarbonates, epoxy resins and other materials.
Production of cyclohexane. Hydrogenation of benzene yields cyclohexane — a feedstock for caprolactam and adipic acid, used in the production of nylon-6 and nylon-66.
Other directions. Benzene is used to produce aniline, maleic anhydride, chlorobenzene, nitrobenzene, alkylbenzenes and other intermediates. It is also used as a solvent in laboratory practice and as a component of high-octane gasolines (limited due to toxicity).
Safety and Storage
Benzene is stored in hermetically sealed steel containers, in well-ventilated areas, away from ignition sources and oxidisers. Transportation is carried out in specialised tank cars and containers in compliance with the rules for the transport of dangerous goods. When working with benzene, personal protective equipment, including respirators and protective clothing, must be used, and regular air monitoring must be carried out.