Methyl tert-butyl ether is an organic compound with structural formula3COCH3. MTBE is a volatile,
flammable, and colorless liquid that is sparingly soluble in water. It has a minty odor vaguely
reminiscent of diethyl ether, leading to unpleasant taste and odor in water. MTBE is a gasoline
additive, used as an oxygenate to raise the octane number. Its use is controversial in
the US and declining in use in part because of its occurrence in groundwater and legislation
favoring ethanol. However, worldwide production of MTBE has been constant at about 18 million
tons/y owing to growth in Asian markets which are less subject to ethanol subsidies.
Production and properties MTBE is manufactured via the chemical reaction
of methanol and isobutylene. Methanol is derived from natural gas, and isobutylene is derived
from butane obtained from crude oil or natural gas, thus MTBE is derived from fossil fuels.
In the United States, it was produced in very large quantities per day in 1999) during its
use as a fuel additive. Uses
MTBE is almost exclusively used as a fuel component in fuel for gasoline engines. It
is one of a group of chemicals commonly known as oxygenates because they raise the oxygen
content of gasoline. As anti-knocking agent
In the US it has been used in gasoline at low levels since 1979 to replace tetraethyl
lead and to increase its octane rating helping prevent engine knocking. Oxygenates help gasoline
burn more completely, reducing tailpipe emissions from pre-1984 motor vehicles; dilutes or displaces
gasoline components such as aromatics and sulfur; and optimizes the oxidation during
combustion. Most refiners chose MTBE over other oxygenates primarily for its blending
characteristics and low cost. Alternatives to MTBE as an anti-knock agent
Other compounds are available as additives for gasoline including ethanol and some ethers
such as tert-amyl methyl ether. Ethanol has been advertised as a safe alternative
by the agricultural and other interest groups in the USA and Europe. In 2003, California
was the first U.S. state to start replacing MTBE with ethanol. Several other states started
switching soon thereafter. Advocates of both sides of the debate in the
United States sometimes claim that gasoline manufacturers have been forced to add MTBE
to gasoline by law. It might be more correct to say they have been induced to do so, although
any oxygenate would fulfill the law. An alternative to straight ethanol is the
related ether ETBE, which is manufactured from ethanol and isobutene. Its performance
as an additive is similar to MTBE, but due to the higher price of ethanol compared to
methanol, it is more expensive. Higher quality gasoline is also an alternative,
so that additives such as MTBE are unnecessary. Iso-octane itself is used. MTBE plants can
be retrofitted to produce iso-octane from isobutylene.
As a solvent As a solvent, MTBE possesses one distinct
advantage over most ethers - it has a reduced tendency to form explosive organic peroxides.
Opened bottles of diethyl ether or THF can build up dangerous levels of these peroxides
in months, whereas samples of MTBE are usually safe for years. For this reason, as well as
its higher boiling point, MTBE is extensively used as a solvent in industry, where safety
concerns and regulations make working with diethyl ether, THF, or other ethers much more
difficult and expensive. However, despite the popularity of MTBE in industrial settings,
it is rarely used as a solvent in academia with some exceptions.
MTBE forms azeotropes with water and methanol. Although an ether, MTBE is a poor Lewis base
and does not support formation of Grignard reagents. It is also unstable toward strong
acids. It reacts dangerously with bromine. Persistence and pervasiveness in the environment
MTBE gives water an unpleasant taste at very low concentrations, and thus can render large
quantities of groundwater non-potable. MTBE is often introduced into water-supply aquifers
by leaking underground storage tanks at gasoline stations or by gasoline containing MTBE spilled
onto the ground. The higher water solubility and persistence of MTBE cause it to travel
faster and farther than many other components of gasoline when released into an aquifer.
MTBE is biodegraded by the action of bacteria. In the proper type of bioreactor, such as
a fluidized bed bioreactor, MTBE can be rapidly and economically removed from water to undetectable
levels. Activated carbon produced from coconut shells and optimized for MTBE adsorption can
also reduce MTBE to undetectable levels.
According to the IARC, a cancer research agency of the World Health Organization, MTBE is
not classified as a human carcinogen. MTBE can be tasted in water at concentrations of
5 – 15 µg/l. As of 2007, researchers have limited data
about the health effects of ingestion of MTBE. The United States Environmental Protection
Agency has concluded that available data are inadequate to quantify health risks of MTBE
at low exposure levels in drinking water, but the data support the conclusion that MTBE
is a potential human carcinogen at high doses. Legislation and litigation in the U.S.
MTBE removal from groundwater and soil contamination in the U.S. is estimated to cost from $1 billion
to $30 billion, including removing the compound from aquifers and municipal water supplies
and replacing leaky underground oil tanks. In one case, the cost to oil companies to
clean up the MTBE in wells belonging to Santa Monica is estimated to exceed $200 million.
In another case, the City of New York estimated a $250 million cost for cleanup of a single
wellfield in Queens, NY. More recently, a jury awarded the State of New Hampshire $236
million in damages in order to treat groundwater contaminated by MTBE.
MTBE is banned in the US states of California and New York, starting January 1, 2004. As
of September 2005, twenty-five states had signed legislation banning MTBE.
In 2000, the U.S. EPA drafted plans to phase out the use of MTBE nationwide over four years.
As of fall 2006, hundreds of lawsuits are still pending regarding MTBE contamination
of public and private drinking water supplies. The Energy Policy Act of 2005, passed in the
U.S. House of Representatives, did not include a provision for shielding MTBE manufacturers
from water contamination lawsuits. This provision was first proposed in 2003 and had been thought
by some to be a priority of Tom DeLay and Rep. Joe Barton, then chairman of the Energy
and Commerce Committee. This bill did include a provision that gives MTBE makers, including
some major oil companies, $2 billion in transition assistance as MTBE is phased out over the
next nine years. Due to opposition in the Senate, the conference report dropped all
MTBE provisions. The final bill was passed by both houses and signed into law by President
Bush. The lack of MTBE liability protection is resulting in a switchover to the use of
ethanol as a gasoline additive. Some traders and consumer advocates are blaming this for
an increase in gasoline prices. The U.S. EPA currently lists MTBE as a candidate
for a maximum contaminant level in drinking water. MCLs are determined by the EPA using
toxicity data. See also
Ethyl tert-butyl ether tert-Amyl methyl ether
Tetraethyl lead References
External links MTBE in Europe
MTBE webpage from the EPA Summary of health effects data from IACR
Eliminating MTBE in Gasoline in 2006 An online resource about MTBE litigation and
the surrounding issues MTBE: What The Oil Companies Knew And When
They Knew It MTBE Resource Guide
European Union MTBE Risk Assessment Report Fuel Quality Directive Impact Assessment
Council Directive 85EEC of 5 December 1985 on crude-oil savings through the use of substitute
fuel components in petrol An assessment of the impact of ethanol-blended
petrol on the total NMVOC emission from road transport in selected countries