Fuel oxygenates are oxygen-containing compounds used as gasoline additives to increase octane ratings and produce cleaner burning fuel. The common oxygenates fall into two major chemical groups: ether compounds, consisting of organic compounds characterized by an oxygen atom linking two hydrocarbon groups, or alcohols, consisting of an alkyl group (such as methyl, ethyl, or isopropyl) bonded to a hydroxyl (oxygen-hydrogen) group. In addition to MTBE and ethanol, other common oxygenates include TBA, TAME, ETBE, and DIPE. TAEE, TAA, and methanol have also been used to a lesser extent. Figure 24.1 shows the molecular structure of commonly used oxygenates. Some of these oxygenates could also be present in commercial formulations of other oxygenates as by-products or degradation products. For example, TBA is often found in commercial formulations of MTBE.
Oxygenates came into widespread use in the United States in the late 1970s as an octane booster, replacing alkyl lead additives, which were being phased out in an effort to reduce lead emissions from vehicles. The use of oxygenates in gasoline was increased after the passage of the 1990 Clean Air Act Amendments that included requirements to increase the oxygen content of fuel to reduce air emissions. The amendments required the use of oxygenated fuel (Oxyfuel) with a minimum of 2.7% by weight oxygen in 39 carbon monoxide nonattainment areas during wintertime and reformulated gasoline (RFG) with a minimum of 2.0% by weight during the remainder of the year.5
In 1998, approximately 30% of all gasoline in the United States contained oxygenates. At that time, MTBE was the most common fuel oxygenate, present in more than 80% of oxygenated fuels. However, due to increasing restrictions on the use of MTBE, this percentage has decreased over the past several years. In 1998, ethanol was the second most common fuel oxygenate, present in about
Methyl tert-butyl ether HC_
Ethyl tert-butyl ether (ETBE)
Diisopropyl ether (DIPE)
Ethanol H3C _ CH2OH
Tert-butyl alcohol h c_C_OH
FIGURE 24.1 Molecular structures of common fuel oxygenates. (Adapted from U.S. EPA, Technologies for Treating MTBE and Other Fuel Oxygenates, EPA 542-R-04-009, United States Environmental Protection Agency, Washington, DC, May 2004.)
15% of oxygenated fuels. Other oxygenates were used in the remaining 5% of oxygenated fuels. As of 2002, 17 states and the District of Columbia were required to use gasoline that contains MTBE or other oxygenates to reduce air pollution. Recent surveys have found that MTBE is present in states that did and did not use RFG. MTBE has been found in gasoline, as well as heating oil and diesel fuel. Sources of MTBE included areas used for storage, transportation, and use.5 The Refs [5-7] contain additional information on the historic use of oxygenates.
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