Styrene

Styrene, also known as vinyl benzene as well as many other names (see table), is an organic compound with the chemical formula C₆H₅CH=CH₂. Under normal conditions, this aromatic hydrocarbon is an oily liquid. It evaporates easily and has a sweet smell, although common impurities confer a less pleasant odor. Styrene is an important precursor to polystyrene, an important synthetic material.

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Occurrence and history

Styrene is named after the styrax trees from whose sap (benzoin resin) it can be extracted. Low levels of styrene occur naturally in plants as well as a variety of foods such as fruits, vegetables, nuts, beverages, and meats. It is produced in industrial quantities from benzene and ethylene via the intermediate ethylbenzene. The production of styrene in the United States was increased dramatically during the 1940's to supply the war needs for synthetic rubber. Because the styrene molecule has a vinyl group with a double bond, it can polymerize. It is used as a monomer to make plastics such as polystyrene, ABS, styrene-butadiene (SBR) rubber, styrene-butadiene latex, SIS (styrene-isoprene-styrene), S-EB-S (styrene-ethylene/butylene-styrene), styrene-divinylbenzene (S-DVB), and unsaturated polyesters. These materials are used in rubber, plastic, insulation, fiberglass, pipes, automobile parts, food containers, and carpet backing.

Styrene is classified as a possible human carcinogen by the United States Environmental Protection Agency (EPA) and by the International Agency for Research on Cancer (IARC).^[1]

Production

Dehydrogenation of ethylbenzene

Styrene is most commonly produced by the catalytic dehydrogenation of ethylbenzene. Ethylbenzene is mixed in the gas phase with 10–15 times its volume in high-temperature steam, and passed over a solid catalyst bed. Most ethylbenzene dehydrogenation catalysts are based on iron(III) oxide, promoted by several percent potassium oxide or potassium carbonate. On this catalyst, an endothermic, reversible chemical reaction takes place.

Steam serves several roles in this reaction. It is the source of heat for powering the endothermic reaction and it continuously removes coke that tends to form on the iron oxide catalyst through the water/gas shift reaction C + 2H₂O → CO₂ + 2H₂. The potassium promoter on the catalyst is present to enhance this decoking reaction. The steam injected with the reactor feed also dilutes the concentration of the reactant and products in the reaction mixture, shifting the position of chemical equilibrium towards products. A typical styrene plant operates two or three reactors in series and operates under vacuum conditions to enhance the conversion and selectivity of the reaction. Typical per-pass conversions are on the order of 65% if two reactors are used and 70% to 75% if three reactors are used. Selectivity to styrene is 93% to as high as 97% depending upon reactor operating pressure, catalyst and conversion. The main byproducts of the reaction are benzene and toluene, these are somewhat easily removed by distillation. The separation of styrene from the remaining ethylbenzene requires tall distillation towers and high reflux ratios, because styrene and ethylbenzene have similar boiling points (145 °C for styrene, 136 °C for ethylbenzene). Distillation and separation of the crude styrene into product styrene is also complicated by the fact that the temperatures involved in the distillation of styrene initiate the polymerization of the styrene. To combat this, early styrene plants added elemental sulfur to inhibit the rate of polymerization. During the 1970s additive chemicals consisting of phenol based retarders were developed. These and the more recently developed free radical inhibitor chemicals are now added prior to distillation. These additives limit the rate of polymerization and allow for the separation and purification of the product styrene.

Improving conversion and so reducing the amount of ethylbenzene that must be separated is the chief impetus for researching alternative routes to styrene. Other than the POSM process, none of these routes like obtaining styrene from butadiene have been commercially demonstrated.

Via ethylbenzenehydroperoxide

Commercially styrene is also co-produced with propylene oxide in a process known as POSM (Lyondell Chemical Company) or SM/PO (Shell) for Styrene Monomer / Propylene Oxide. In this process ethylbenzene is reacted with oxygen to form the hydroperoxide of ethylbenzene. This hydroperoxide is then used to oxidize propylene to propylene oxide. The resulting phenylethanol is dehydrated to give styrene:

C₆H₅CH₂CH₃ + O₂ → C₆H₅CH₂CH₂O₂H

C₆H₅CH₂CH₂O₂H + CH₃CH=CH₂ → C₆H₅CH₂CH₂OH + CH₃CHCH₂O

C₆H₅CH₂CH₂OH → C₆H₅CH=CH₂ + H₂O

Laboratory synthesis

A laboratory synthesis of styrene entails the decarboxylation of cinnamic acid.^[2]

Via engineered catalyst

Researchers at Exelus Inc. (Livingston NJ, USA) produced styrene directly from toluene and methanol in a single step, at 425 degrees C and atmospheric pressure, by forcing these components through a proprietary zeolitic material which catalyzes them into styrene and ethylbenzene in a 9:1 mixture.^[3]

Health effects

Styrene is a toxin, an irritant, and a potential carcinogen. See MSDS.

References

1. ^ Styrene. Occupational Safety and Health Administration. Retrieved on 2007-12-02.
2. ^ Abbott, T. W.; Johnson, J. R. "Phenylethylene [Styrene]" Organic Syntheses, Coll. Vol. 1, p.440 (1941).
3. ^ Stephen K. Ritter, Chemical & Engineering News, 19 March 2007, p.46