Styrene

Styrene
Preferred IUPAC name Styrene
Systematic name Ethenylbenzene
Other names Vinyl benzene; cinnamene; styrol; phenethylene; phenylethene; diarex HF 77; styrolene; styropol; vinylbenzene
Identifiers
CAS number	100-42-5 Y
PubChem	7501
ChemSpider	7220
UNII	44LJ2U959V
KEGG	C07083 Y
ChEBI	CHEBI:27452
ChEMBL	CHEMBL285235 N
RTECS number	WL3675000
Jmol-3D images	Image 1
SMILES c1ccccc1C=C
InChI InChI=1S/C8H8/c1-2-8-6-4-3-5-7-8/h2-7H,1H2 Key: PPBRXRYQALVLMV-UHFFFAOYSA-N
Properties
Molecular formula	C8H8
Molar mass	104.15 g/mol
Appearance	colorless oily liquid
Density	0.909 g/cm³
Melting point	-30 °C, 243 K, -22 °F
Boiling point	145 °C, 418 K, 293 °F
Solubility in water	< 1%
Refractive index (nD)	1.5469
Viscosity	0.762 cP at 20 °C
Structure
Dipole moment	0.13 D
Hazards
MSDS	MSDS
R-phrases	R10 R36
S-phrases	S38 S20 S23
Main hazards	flammable, toxic
NFPA 704	3 2 2
Flash point	31 °C
Related compounds
Related aromatic compounds	Ethylbenzene
N (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Styrene, also known as vinyl benzene, 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 15 billion pounds are produced annually.^[1] On 10 June 2011, the US National Toxicology Program has described styrene as "reasonably anticipated to be a human carcinogen".^[2][3] However, an academic panel recently reviewed the relevant scientific literature and concluded that "The available epidemiologic evidence does not support a causal relationship between styrene exposure and any type of human cancer."[15]

Occurrence, history, and use

Styrene is named for "styrax" (also called "storax Levant"), the resin from a Turkish tree, the Oriental sweetgum (Liquidambar orientalis), from which it was first isolated,^{[citation needed]} and not for the tropical Styrax trees from which benzoin resin is produced. Low levels of styrene occur naturally in many kinds of plants,^{[citation needed]} as well as a variety of foods such as fruits, vegetables, nuts, beverages, and meats.^{[citation needed]}

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. 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 on a large scale by alkylation of benzene with ethylene.^[1]

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.

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 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:

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.^[4] Styrene was first prepared by this method.^[5]

Other methods

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.^[6] 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^[7] that an approximately 9:1 mixture of styrene and ethylbenzene is obtained, with a total styrene yield of over 60%.^[8]

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.^[9]

Health effects

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.^{[10][11][12][1]} The US EPA has described styrene to be "a suspected toxin to the gastrointestinal tract, kidney, and respiratory system, among others."^[13][14] On 10 June 2011, the US National Toxicology Program has described styrene as "reasonably anticipated to be a human carcinogen".^[2][3] However, an academic panel, funded by the styrene industry^[15], recently reviewed the relevant scientific literature and concluded that "The available epidemiologic evidence does not support a causal relationship between styrene exposure and any type of human cancer."^[16]

The U.S. EPA does not have a cancer classification for styrene,^[17] but currently is evaluating styrene's cancer-causing potential through its Integrated Risk Information System (IRIS) program.^[18] The U.S. National Toxicology Program of the U.S. Department of Health and Human Services also currently is evaluating styrene's potential toxicity^[19] 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.".^[20] Chronic exposure to styrene leads to tiredness/lethargy, memory deficits, headaches and vertigo.^[21]

According to the Styrene Information and Research Center (an organization representing nearly all of the "North American styrene industry"),^[22] polystyrene plastic neither contains, nor breaks down into bisphenol A (BPA), a chemical used in plastic compounds that leads to developmental and reproductive problems in both adults and children.^[23]

References

1. ^ ^{a b c} Denis H. James William M. Castor, “Styrene” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
2. ^ ^{a b} Harris, Gardiner (10 June 2011). "Government Says 2 Common Materials Pose Risk of Cancer". New York Times. http://www.nytimes.com/2011/06/11/health/11cancer.html. Retrieved 2011-06-11. 
3. ^ ^{a b} National Toxicology Program (10 June 2011). "12th Report on Carcinogens". National Toxicology Program. http://ntp.niehs.nih.gov/index.cfm?objectid=72016262-BDB7-CEBA-FA60E922B18C2540. Retrieved 2011-06-11. 
4. ^ Abbott, T. W.; Johnson, J. R. (1941), "Phenylethylene (Styrene)", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV1P0440 ; Coll. Vol. 1: 440 
5. ^ R. Fittig und F. Binder “Ueber die Additionsproducte der Zimmtssaure” in “Untersuchungen über die ungesättigten Säuren. I. Weitere Beiträge zur Kenntnifs der Fumarsäure und Maleïnsäure” by Rudolph Fittig, Camille Petri, Justus Liebigs Annalen der Chemie, 1879, volume 195, p 56-179. doi:10.1002/jlac.18791950103
6. ^ 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
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.)
8. ^ Stephen K. Ritter, Chemical & Engineering News, 19 March 2007, p.46.
9. ^ Styrene/Ethylbenzene 07/08-4 Report, ChemSystems, March 2009, p.64-73.
10. ^ MSDS (1 November 2010). "Material Safety Data Sheet Styrene (monomer) MSDS". MSDS. http://www.sciencelab.com/msds.php?msdsId=9925112. Retrieved 2011-06-11. 
11. ^ US EPA (December 1994). "OPPT Chemical Fact Sheets (Styrene) Fact Sheet: Support Document (CAS No. 100-42-5)". US EPA. http://www.epa.gov/chemfact/styre-sd.pdf. Retrieved 2011-06-11. 
12. ^ http://www.atsdr.cdc.gov/tfacts53.pdf
13. ^ "EPA settles case against Phoenix company for toxic chemical reporting violations". U.S. Environmental Protection Agency. http://yosemite.epa.gov/opa/admpress.nsf/6427a6b7538955c585257359003f0230/09b1cf3cd5813928852570d8005e1603!OpenDocument. Retrieved 2008-02-11. 
14. ^ "EPA Fines California Hot Tub Manufacturer for Toxic Chemical Release Reporting Violations". U.S. Environmental Protection Agency. http://yosemite.epa.gov/opa/admpress.nsf/6427a6b7538955c585257359003f0230/b3249e555c0ed02c852570d8005e1576!OpenDocument. Retrieved 2008-02-11. 
15. ^ http://stats.org/stories/2011/styrene_crosshairs_sept14_11.html
16. ^ Boffetta, P., et al., Epidemiologic Studies of Styrene and Cancer: A Review of the Literature, J. Occupational and Environmental Medicine, Nov.2009, V.51, N.11.
17. ^ [1] US environmental protection agency. Section I.B.4 relates to neurotoxicology.
18. ^ EPA IRIS track styrene page
19. ^ National Toxicology Program's Styrene Page
20. ^ Monographs on the Evaluation of Carcinogenic Risks to Humans, Volume 82 (2002), Some Traditional Herbal Medicines, Some Mycotoxins, Naphthalene and Styrene, pp. 436 - 550.
21. ^ US EPA (2006-06-28). "Styrene / Technology Transfer Network Air Toxics Web site". United States. http://www.epa.gov/ttnatw01/hlthef/styrene.html. Retrieved 2011-06-01. 
22. ^ "Styrene Information and Research Center". Styrene Information and Research Center (SIRC), Arlington, Va.. http://www.styrene.org/index.html. Retrieved 2011-06-12. 
23. ^ Jack Snyder (2009-08-24). "Styrene Not a Human Carcinogen, Does Not Contain BPA". Styrene Information and Research Center (SIRC), Arlington, Va.. http://www.styrene.org/news_styrene_human_health/not_carcinogen_no_bpa_statement.html. Retrieved 2010-07-26.