|Jmol-3D images||Image 1|
|Molar mass||104.15 g/mol|
|Appearance||colorless oily liquid|
|Melting point||−30 °C (−22 °F; 243 K)|
|Boiling point||145 °C (293 °F; 418 K)|
|Solubility in water||< 1%|
|Refractive index (nD)||1.5469|
|Viscosity||0.762 cP at 20 °C|
|Dipole moment||0.13 D|
|S-phrases||S38 S20 S23|
|Main hazards||flammable, toxic|
|Flash point||31 °C (88 °F; 304 K)|
|Related aromatic compounds||Ethylbenzene|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
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 confer a less pleasant odor. Styrene is the precursor to polystyrene and several copolymers. Approximately 25 million tonnes (55 billion pounds) of styrene were produced in 2010.
Occurrence, history, and use
Styrene is named for styrax balsam, the resin of Liquidambar trees of the Hamamelidaceae 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 the nineteenth century, styrene was isolated by distillation of the natural storax balsam.
The manufacture of styrene by dehydrogenation of ethylbenzene was achieved in the 1930s. The production of styrene in the United States increased dramatically during the 1940s, when it was popularized as a feedstock for synthetic rubber.
The presence of the vinyl group allows styrene to polymerize. Commercially significant products include polystyrene, ABS, styrene-butadiene (SBR) rubber, styrene-butadiene latex, SIS (styrene-isoprene-styrene), S-EB-S (styrene-ethylene/butylene-styrene), styrene-divinylbenzene (S-DVB), styrene-acrylonitrile resin (SAN), and unsaturated polyesters used in resins and thermosetting compounds. These materials are used in rubber, plastic, insulation, fiberglass, pipes, automobile and boat parts, food containers, and carpet backing.
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.
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 inhibitors consisting of nitrated phenol-based retarders were developed. More recently, a number of additives have been developed that exhibit superior inhibition against polymerization. However, the nitrated phenols are still widely used because of their relatively low cost. 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.
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 treated with oxygen to form the ethylbenzene hydroperoxide. This hydroperoxide is then used to oxidize propylene to propylene oxide. The resulting 2-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. Historically, however, this process has suffered from low selectivity due to 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 developing route to styrene is via 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.
Styrene is regarded as a "hazardous chemical", especially in case of eye contact, but also in case of skin contact, of ingestion and of inhalation, according to several sources. Styrene is largely metabolized into styrene oxide in humans, resulting from oxidation by cytochrome P450. Styrene oxide is considered toxic, mutagenic, and possibly carcinogenic. Styrene oxide is subsequently hydrolyzed in vivo to styrene glycol by the enzyme epoxide hydrolase. The U.S. Environmental Protection Agency (EPA) has described styrene to be "a suspected toxin to the gastrointestinal tract, kidney, and respiratory system, among others." On 10 June 2011, the U.S. National Toxicology Program has described styrene as "reasonably anticipated to be a human carcinogen". However, a STATS author describes a review that was done on scientific literature and concluded that "The available epidemiologic evidence does not support a causal relationship between styrene exposure and any type of human cancer". Despite this claim, work has been done by Danish researchers to investigate the relationship between occupational exposure to styrene and cancer. They concluded, "The findings have to be interpreted with caution, due to the company based exposure assessment, but the possible association between exposures in the reinforced plastics industry, mainly styrene, and degenerative disorders of the nervous system and pancreatic cancer, deserves attention". The Danish EPA recently concluded that the styrene data do not support a cancer concern for styrene.
The U.S. EPA does not have a cancer classification for styrene, but currently is evaluating styrene's cancer-causing potential through its EPA|Integrated Risk Information System (IRIS) program. The U.S. National Toxicology Program of the U.S. Department of Health and Human Services also currently is evaluating styrene's potential toxicity To date, no regulatory body anywhere in the world has classified styrene as a known human carcinogen, although several refer to it in various contexts as a possible or potential human carcinogen. The International Agency for Research on Cancer considers styrene to be "possibly carcinogenic to humans". Chronic exposure to styrene leads to tiredness/lethargy, memory deficits, headaches and vertigo.
- New Process for Producing Styrene Cuts Costs, Saves Energy, and Reduces Greenhouse Gas Emissions, U.S. Department of Energy.
- Denis H. James; William M. Castor (2007), "Styrene", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley, p. 1
- >Denis H. James William M. Castor, “Styrene” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
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- Tatsuaki Yashima, Keiichi Sato, Tomoki Hayasaka and Nobuyoshi Hara "Alkylation on synthetic zeolites: III. Alkylation of toluene with methanol and formaldehyde on alkali cation exchanged zeolites" Journal of Catalysis, 1972, 26, 303. doi:10.1016/0021-9517(72)90088-7
- Peter Taffe, ICIS.com, 21 Jan 2008 (based on an paper given at The 6th European.Aromatics & Derivatives Conference – Antwerp, Belgium - 14-15 November, 2007.)
- Stephen K. Ritter, Chemical & Engineering News, 19 March 2007, p.46.
- Styrene/Ethylbenzene 07/08-4 Report, ChemSystems, March 2009, p.64-73.
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- Kenneth C. Liebman (1975). "Metabolism and toxicity of styrene". Environmental Health Perspectives 11: 115–119. doi:10.2307/3428333. JSTOR 3428333.
- "EPA settles case against Phoenix company for toxic chemical reporting violations". U.S. Environmental Protection Agency. Retrieved 2008-02-11.
- "EPA Fines California Hot Tub Manufacturer for Toxic Chemical Release Reporting Violations". U.S. Environmental Protection Agency. Retrieved 2008-02-11.
- Harris, Gardiner (10 June 2011). "Government Says 2 Common Materials Pose Risk of Cancer". New York Times. Retrieved 2011-06-11.
- National Toxicology Program (10 June 2011). "12th Report on Carcinogens". National Toxicology Program. Retrieved 2011-06-11.
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- Danish EPA 2011 review http://www.compositesworld.com/cdn/cms/uploadedFiles/danish_epa_styrene_review(2).pdf
-  US environmental protection agency. Section I.B.4 relates to neurotoxicology.
- EPA IRIS track styrene page
- National Toxicology Program's Styrene Page
- Monographs on the Evaluation of Carcinogenic Risks to Humans, Volume 82 (2002), Some Traditional Herbal Medicines, Some Mycotoxins, Naphthalene and Styrene, pp. 436 - 550.
- US EPA (2006-06-28). "Styrene / Technology Transfer Network Air Toxics Web site". United States. Retrieved 2011-06-01.
- title=Isolation and characterisation of fungi growing on volatile aromatic hydrocarbons as their sole carbon and energy source|author1=Francesc X. PRENAFETA-BOLDU|author2=Andrea KUHN|author3=Dion M. A. M. LUYKX|author4=Heidrun ANKE|author5=Johan W. van GROENESTIJN|author6=Jan A. M. de BONT|volume=105|issue=4|pages=477-484|date=April 2001|