Methylcyclopentadienyl manganese tricarbonyl
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Methylcyclopentadienyl manganese tricarbonyl (MMT or MCMT) is an organomanganese compound with the formula (C5H4CH3)Mn(CO)3. Initially marketed as a supplement for use in leaded
gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organi ...
, MMT was later used in unleaded gasoline to increase the octane rating. Following the implementation of the Clean Air Act (United States) (CAA) in 1970, MMT continued to be used alongside
tetraethyl lead Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb( C2H5)4. It is a fuel additive, first being mixed with gasoline beginning in the 1920s as a patented octane rating booster that al ...
(TEL) in the US as leaded gasoline was phased out (prior to TEL finally being banned from US gasoline in 1995), and was also used in unleaded gasoline until 1977. Ethyl Corporation obtained a waiver from the U.S. EPA (Environmental Protection Agency) in 1995, which allows the use of MMT in US unleaded gasoline (not including reformulated gasoline) at a treat rate equivalent to 8.3 mg Mn/L (manganese per liter). MMT has been used in Canadian gasoline since 1976 (and in numerous other countries for many years) at a concentration up to 8.3 mg Mn/L (though the importation and interprovincial trade of gasoline containing MMT was restricted briefly during the period 1997–1998) and was introduced into Australia in 2000. It has been sold under the tradenames HiTEC 3000, Cestoburn and Ecotane.> MMT is also used in China.


History of usage in the United States

Although initially marketed in 1958 as a smoke suppressant for gas turbines, MMT was further developed as an octane enhancer in 1974. When the
United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it ...
(EPA) ordered the phase out of TEL in gasoline in 1973, new fuel additives were sought. TEL has been used in certain countries as an additive to increase the octane rating of automotive gasoline but has been phased out in all countries since July 2021. In 1977, the US Congress amended the CAA to require advance approval by the EPA for the continued use of fuel additives such as MMT,
ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
, ethyl tert-butyl ether ( ETBE), etc. The new CAA amendment required a "waiver" to allow use of fuel additives made of any elements other than carbon, hydrogen, oxygen (within certain limits) and nitrogen. To obtain a waiver, the applicant was required to demonstrate that the fuel additive would not lead to a failure of vehicle emission control systems. Ethyl Corporation applied to the US EPA for a waiver for MMT in both 1978 and 1981; in both cases the applications were denied because of stated concerns that MMT might damage catalytic converters and increase hydrocarbon emissions. In 1988, Ethyl began a new series of discussions with the EPA to determine a program for developing the necessary data to support a waiver application. In 1990, Ethyl filed its third waiver application prompting an extensive four-year review process. In 1993, the U.S. EPA determined that use of MMT at 8.3 mg Mn/L would not cause, or contribute to, vehicle emission control system failures. Despite that finding, the EPA ultimately denied the waiver request in 1994 due to uncertainty related to health concerns regarding manganese emissions from the use of MMT. As a result of this ruling, Ethyl initiated a legal action claiming that the EPA had exceeded its authority by denying the waiver on these grounds. This was upheld by the US Court of Appeals and EPA subsequently granted a waiver which allows the use of MMT in US unleaded gasoline (not including reformulated gasoline) at a treat rate equivalent to 8.3 mg Mn/L. Implementation of this less-toxic alternative to TEL has been controversial. Opposition from automobile manufacturers and some areas of the scientific community has reportedly prompted oil companies to stop voluntarily the usage of MMT in some of their countries of operation. MMT is currently manufactured in the U.S. by the Afton Chemical Corporation, a subsidiary of New Market Corporation. It is also produced and marketed as Cestoburn by Cestoil Chemical Inc. in Canada.


Structure and synthesis

MMT is manufactured by reduction of bis(methylcyclopentadienyl) manganese using
triethylaluminium Triethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al2( C2H5)6 (abbreviated as Al2Et6 or TEA), as it exists as a dimer. This colorless liquid is pyrophoric. It is an industrially ...
. The reduction is conducted under an atmosphere of
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
. The reaction is exothermic, and without proper cooling, can lead to catastrophic
thermal runaway Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way t ...
. MMT is a so-called
half-sandwich complex Half sandwich compounds, also known as piano stool complexes, are organometallic complexes that feature a cyclic polyhapto ligand bound to an MLn center, where L is a unidentate ligand. Thousands of such complexes are known. Well-known examples i ...
, or more specifically a "piano-stool" complex (since the three CO ligands are like the legs of a stool). The
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
atom in MMT is coordinated with three carbonyl groups as well as to all five main carbon atoms of the methylcyclopentadienyl ring. These hydrophobic organic ligands make MMT highly lipophilic. A variety of related complexes are known, including ferrocene, which has also been used as an additive to gasoline. Many derivatives of MMT are known.


Safety

The human and environmental health impacts that may result from the use of MMT will be a function of exposure to either: (1) MMT in its original, unchanged, chemical form and/or (2) manganese combustion products emitted from vehicles operating on gasoline containing MMT as an octane improver.


Pre-combustion storage and handling

The general public has minimal direct exposure to MMT. As stated by the US EPA in their risk assessment on MMT, "except for accidental or occupational contacts, exposure to MMT itself was not thought likely to pose a significant risk to the general population." Similarly, the Australian National Industrial Chemicals Notification and Assessment Scheme (NICNAS) stated that " nimal public exposure to MMT is likely as a result of spills and splashes of LRP ead replacement petroland aftermarket additives". The MMT dossier registered in the European Chemical Agency's webpage indicates that before combustion in gasoline, MMT is classified as an acute toxicant by the oral, dermal, and inhalation routes of exposure under the European Union's Classification, Labeling and Packaging Regulation (EC/1272/2008), implementing the Global Harmonized System (GHS) of Classification and Labeling. The US ATSDR (Agency for Toxic Substances and Disease Registry) notes that MMT is very unstable in light and degrades to a mixture of less harmful substances and inorganic manganese in less than 2 minutes. Therefore, human exposure to MMT prior to combustion in gasoline would not likely occur at significant levels. The US OSHA ( Occupational Health and Safety Administration) has not established a permissible exposure limit specifically for MMT. However, OSHA has set a
permissible exposure limit The permissible exposure limit (PEL or OSHA PEL) is a legal limit in the United States for exposure of an employee to a chemical substance or physical agent such as high level noise. Permissible exposure limits are established by the Occupational ...
at a ceiling of 5 mg/m3 for manganese and its compounds, while the
National Institute for Occupational Safety and Health The National Institute for Occupational Safety and Health (NIOSH, ) is the United States federal agency responsible for conducting research and making recommendations for the prevention of work-related injury and illness. NIOSH is part of the C ...
recommends workers not be exposed to more than 0.2 mg/m3, over an eight-hour time-weighted average. In Europe, the MMT DNELs (Derived No Effect Level) for workers by the inhalation and dermal routes of exposure are 0.6 mg/m3 and 0.11 mg/kg-day, respectively. The MMT DNELs for the general population by the inhalation and dermal routes of exposure are 0.11 mg/m3 and 0.062 mg/kg-day, respectively.


Combustion products

In 1994 (reaffirmed in 1998, 2001 and 2010), Health Canada concluded that "airborne manganese resulting from the combustion of MMT in gasoline powered vehicles is not entering the Canadian environment in quantities or under conditions that may constitute a health risk" and confirmed they were taking no action with respect to MMT. Similarly, the 2003 NICNAS report states that the airborne concentrations of manganese as a result of car emissions from vehicles using fuel containing MMT poses no health hazard. The assessment conducted by NICNAS asserts that " nganese, the principle degradation by-product from combustion of MMT, is naturally occurring and ubiquitous in the environment. It is an essential nutrient of plants and animals. Environmental exposure to Mn compounds will mostly arise through the gaseous phase. Eventually, these will deposit to land and waters. The emission of Mn into the environment from use of fuels containing MMT is unlikely to develop to levels of concern and therefore poses a low risk for terrestrial or aquatic environments." Additional health studies, overseen by the US EPA, were conducted to explain the transport of manganese in the body. In studies published from 2007 through 2011, no significant health effects are anticipated from the use of MMT in gasoline.


Overall combined risk assessment

Based on the low potential for the release of concentrated MMT (before its combustion in gasoline) under normal storage and use, as well as its rapid photo-degradation properties, it has been concluded in multiple technical and global regulatory assessments that significant impacts to human health or the environment from MMT use are not anticipated. NICNAS concluded that there is "low occupational risk associated with MMT" both "for workers involved in formulating and distributing LRP or aftermarket fuel additives and those involved in automotive maintenance". Further, they also concluded that there is a "low risk" to the public from the use of MMT. Significant human or environmental exposures associated with manganese compounds (manganese phosphate, manganese sulfate and manganese tetraoxide) from the combustion of MMT are not expected. In Health Canada's risk assessment on the health implications of the manganese combustion products of MMT, it was concluded that manganese exposures from MMT use are unlikely to pose a risk to health for any sub-group of the population. NICNAS similarly concluded that chronic Mn exposures (from all sources combined) are unlikely to be significantly changed by the use of MMT as a fuel additive. In 2013, a risk assessment on MMT was developed by ARCADIS Consulting and verified by an independent panel, according to the methodology provided by the European Commission in compliance with the requirements of the European Fuel Quality Directive (2009/30/EC). The conclusions of a risk assessment are that "for MMT and its transformation products, when MMT is used as a fuel additive in petrol, no significant human health or environmental concerns related to exposure to either MMT or its transformation ombustionproducts (manganese phosphate, manganese sulfate and manganese tetroxide) were identified at use at levels up to 18 mg Mn/L. Depending on the regional needs and the vehicle emission control technology available, an MMT treat rate in the range of 8.3 mg Mn/L to 18 mg Mn/L is scientifically justified and may deliver both environmental and economic benefits without significant adverse effects."conclusions of this risk assessment
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T2 Laboratories explosion and fire

On December 19, 2007 an explosion and fire occurred in the production of MMT, which killed four people and injured fourteen.


See also

*
List of gasoline additives


References

{{manganese compounds Organomanganese compounds Carbonyl complexes Antiknock agents Cyclopentadienyl complexes Half sandwich compounds