Styrene

Styrene, also known as ethenylbenzene, vinylbenzene, and phenylethene, is an organic compound with the chemical formula C6H5CH=CH2. This derivative of benzene is a colorless oily liquid that evaporates easily and has a sweet smell, although high concentrations have a less pleasant odor. Styrene is the precursor to polystyrene and several copolymers. Approximately 25 million tonnes of styrene were produced in 2010. Styrene, also known as ethenylbenzene, vinylbenzene, and phenylethene, is an organic compound with the chemical formula C6H5CH=CH2. This derivative of benzene is a colorless oily liquid that evaporates easily and has a sweet smell, although high concentrations have a less pleasant odor. Styrene is the precursor to polystyrene and several copolymers. Approximately 25 million tonnes of styrene were produced in 2010. Styrene is named after storax balsam, the resin of Liquidambar trees of the Altingiaceae plant family. Styrene occurs naturally in small quantities in some plants and foods (cinnamon, coffee beans, and peanuts) and is also found in coal tar. In 1839, the German apothecary Eduard Simon isolated a volatile liquid from the resin (called storax or styrax (Latin)) of the American sweetgum tree (Liquidambar styraciflua). He called the liquid 'Styrol' (now: 'styrene'). He also noticed that when Styrol was exposed to air, light, or heat, it gradually transformed into a hard, rubber-like substance, which he called 'Styroloxyd' (styrol oxide, now: 'polystyrene'). By 1845, the German chemist August Hofmann and his student John Blyth (1814–1871) had determined Styrol's empirical formula: C8H8. They had also determined that Simon's 'Styroloxyd' — which they renamed 'Metastyrol' — had the same empirical formula as Styrol. Furthermore, they could obtain Styrol by dry-distilling Metastyrol. In 1865, the German chemist Emil Erlenmeyer found that Styrol could form a dimer, and in 1866 the French chemist Marcelin Berthelot stated that Metastyrol was a polymer of Styrol. Meanwhile, other chemists had been investigating another component of storax, namely, cinnamic acid. They had found that cinnamic acid could be decarboxylated to form cinnamène (or cinnamol), which appeared to be Styrol. In 1845, French chemist Emil Kopp suggested that the two compounds were identical, and in 1866, Erlenmeyer suggested that both cinnamol and Styrol might be vinyl benzene. However, the Styrol that was obtained from cinnamic acid seemed different from the Styrol that was obtained by distilling storax resin: the latter was optically active. Eventually, in 1876, the Dutch chemist van 't Hoff resolved the ambiguity: the optical activity of the Styrol that was obtained by distilling storax resin was due to a contaminant. Styrene plays an important role in chemical production to make latex, synthetic rubber, and other polystyrene resins. A common way to produce styrene is through the dehydrogenation of ethylbenzene.Benzene and ethylene are first compressed and sent to a reactor to produce ethylbenzenein the presence of a Friedel–Crafts catalyst (aluminum chloride) at approximately 95 °C. The reaction occurs as C6H6 + CH2CH2 → C6H5CH2CH3.The product mixture is then fed into a distillation column to be separated into ethylbenzene, benzene, and polyethylbenzenes. After the partial distillation, the ethylbenzene is fed out with a high purity of over 99% at 136 °C (boiling point). A benzene recycle stream (to the original entering benzene stream) and a mixture stream of polyethylbenzenes exit the reactor separately.The effluent mixture of polyethylbenzenes is heated via a heat exchanger and sent to a reactor at 200 °C to be dealkylated into benzene. The products, along with unreacted benzene, are then cooled via another heat exchanger and sent into the aforementioned benzene recycle stream.The fed-out ethylbenzene vapour stream is mixed with superheated steam and the resulting mixture is heated, then dehydrogenated to styrene mixture via an adiabatic reactor utilizing an iron (III) oxide catalyst. The reactor is run with added steam, with a typical yield of 88–94%, and the reaction occurs as C6H5CH2CH3 → C6H5CHCH2 + H2.The exit stream is condensed and sent to a distillation column, which separates a crude styrene stream from the vent gas (H2, other vapours), which exits from the top, and the liquid condensate, which exits from the column’s bottom. Via this process, the selectivity of styrene from ethylbenzene is approximately 90%. The crude styrene stream is purified via a polymerization inhibitor in a reactor. Styrene is also co-produced commercially in a process known as POSM (Lyondell Chemical Company) or SM/PO (Shell) for styrene monomer / propylene oxide. In this process ethylbenzene is treated with oxygen to form the ethylbenzene hydroperoxide. This hydroperoxide is then used to oxidize propylene to propylene oxide. The resulting 1-phenylethanol is dehydrated to give styrene: Styrene can be produced from toluene and methanol, which are cheaper raw materials than those in the conventional process. This process has suffered from low selectivity associated with the competing decomposition of methanol. Exelus Inc. claims to have developed this process with commercially viable selectivities, at 400–425 °C and atmospheric pressure, by forcing these components through a proprietary zeolitic catalyst. It is reported that an approximately 9:1 mixture of styrene and ethylbenzene is obtained, with a total styrene yield of over 60%. Another route to styrene involves the reaction of benzene and ethane. This process is being developed by Snamprogetti S.p.A. and Dow. Ethane, along with ethylbenzene, is fed to a dehydrogenation reactor with a catalyst capable of simultaneously producing styrene and ethylene. The dehydrogenation effluent is cooled and separated and the ethylene stream is recycled to the alkylation unit. The process attempts to overcome previous shortcomings in earlier attempts to develop production of styrene from ethane and benzene, such as inefficient recovery of aromatics, production of high levels of heavies and tars, and inefficient separation of hydrogen and ethane. Development of the process is ongoing. A laboratory synthesis of styrene entails the decarboxylation of cinnamic acid: