styrene n : a colorless oily liquid; the monomer for polystyrene [syn: cinnamene, phenylethylene, vinylbenzene]

English

Noun

styrene

1. an aromatic hydrocarbon, vinyl-benzene; a colourless, oily liquid, used in the manufacture of polymers such as polystyrene

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 high concentrations confer a less pleasant odor. Styrene is the precursor to polystyrene, an important synthetic material.

Occurrence, history, and use

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. The production of styrene in the United States increased dramatically during the 1940s to supply the war needs for synthetic rubber.

Because the styrene molecule has a vinyl group with a double bond, it can polymerize to give 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 and boat parts, food containers, and carpet backing.

Production

Styrene is produced in industrial quantities from ethylbenzene, which is in turn prepared from benzene and ethylene.

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 removes coke that tends to form on the iron oxide catalyst through the water gas shift reaction. The potassium promoter enhances this decoking reaction. The steam also dilutes the reactant and products, shifting the position of chemical equilibrium towards products. A typical styrene plant consists of two or three reactors in series, which operate under vacuum to enhance the conversion and selectivity. Typical per-pass conversions are ca. 65% for two reactors and 70-75% for three reactors. Selectivity to styrene is 93-97%. The main byproducts are benzene and toluene. Because styrene and ethylbenzene have similar boiling points (145 and 136 °C, respectively), their separation requires tall distillation towers and high return/reflux ratios. At its distillation temperatures, styrene tends to polymerize. To minimize this problem, early styrene plants added elemental sulfur to inhibit the polymerization. During the 1970s, new free radical inhibitor consisting of phenol-based retarders were developed. These reagents are added prior to the distillation.

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.

Other methods

Exelus Inc. (Livingston NJ, USA) produces styrene from toluene and methanol, at 425 °C and atmospheric pressure, by forcing these components through a proprietary zeolitic catalyst that affords a 9:1 mixture of styrene and ethylbenzene.

Health effects

Styrene is only weakly toxic, with an LD50 of 500-5000 mg/kg (rats)..

Styrene is classified as a possible human carcinogen by the International Agency for Research on Cancer (IARC). The U.S. Environmental Protection Agency (EPA) does not have a cancer classification for styrene, but is evaluating its potential carcinogenicity. The EPA has described styrene as "a suspected carcinogen" and "a suspected toxin to the gastrointestinal, kidney, and respiratory systems, among others."

References