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 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, 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 rubber, styrene-butadiene latex, SIS, S-EB-S,
styrene-divinylbenzene, styrene-acrylonitrile resin, 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.
Production Styrene is produced in industrial quantities
from ethylbenzene, which is in turn prepared on a large scale by alkylation of benzene
with 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.
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, 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 or SM/PO 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:
Laboratory synthesis A laboratory synthesis of styrene entails
the decarboxylation of cinnamic acid. Styrene was first prepared by this method.
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. 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.
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. 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 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 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.
Bioremediation The yeast like fungi Exophiala jeanselmei
can be used to treat air polluted with styrene. References
External links CDC - Styrene - NIOSH Workplace Safety and
Health Topic Safety and Health Topics | Styrene