**1.1 Oxygenated fuels**

 Oxygenated fuels are oxygen-rich compounds such as alcoholic and ether fuels that act as gasoline additives in reformulated gasoline and oxyfuels. Oxygenates can be blended into gasoline in two forms: alcohols (such as methanol or ethanol) or ethers. They have potential to provide an alternative solution of environmental problems caused by fossil fuel use. The oxygenated ether fuels used more frequently are methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME), although the first fuel oxygenate used in reformulation was MTBE. The use of MTBE as an octane enhancer in the United States began in 1979 [1, 2].

MTBE is a compound (chemical formula C5H12O) that is synthetized by the chemical reaction of methanol and isobutylene, and it is almost exclusively used as a fuel additive in motor gasoline. Although ETBE and TAME have only secondary importance in production industry, recently they have become more prevalent, and they can be used instead of MTBE. In France, MTBE has been partially replaced by the ETBE since 1990 or as part of a "binary or ternary mixture" with MTBE [3–5].

ETBE (chemical formula C6H14O) is considered an attractive octane enhancer as it presents a lower reid vapor pressure (RVP) mixture and lower water solubility than MTBE and ethanol. The azeotropic mixture of ETBE with ethanol takes down the volatility of ethanol making it suitable as an additive for automatic gasoline [6, 7].

Finally, TAME (chemical formula C6H14O) was considered as an oxygenated fuel until the 1990s despite its octane rating lower than other oxygenated fuels; besides it is very soluble with other ethers, and it is highly soluble in water (12 g/l) [8, 9].

#### **1.2 Importance of oxygenated fuels in petroleum industry**

The leading produced fuel in the world is gasoline. Because of its widespread use and the fact that it is composed of that fraction of crude oil with lower boiling points, gasoline is the single largest source of volatile hydrocarbons to the environment. Motor gasoline comes in various blends with properties that affect engine performance. All motor gasolines are made of relatively volatile components of crude oil. Other fuels include distillate fuel oil (diesel fuel and heating oil), jet fuel, residual fuel oil, kerosene, aviation gasoline, and petroleum coke. In the petroleum refining process, heat distillation is used first to separate different hydrocarbon components. The lighter products are liquefied petroleum gases and gasoline, whereas the heavier products include heavy gas oils. Liquefied petroleum gases include ethane, ethylene, propane, propylene, n-butane, butylenes, and isobutane. Internal combustion engines of high compression ratio require gasoline with octane ratings that are sufficiently high to ensure efficient combustion [2, 10, 11].

Gasolines need additives that increase their octane rating so they can decrease their self-knock capacity, increasing their resistance to compression, and finally improve the quality of gasoline. An economical way of achieving these properties has been the use of anti-knock additives, such as tetraethyl and tetramethyl lead at concentrations up to 0.84 g/l. With the phasing out of lead from gasoline because it was increasingly recognized that lead is toxic and non-biodegradable, oxygenated fuel become a better alternative for gasoline additive, instead of lead. Oxygenated fuels act as octane enhancers, bringing the additional benefit of making gasoline burn almost completely. Actually, using oxygenated fuels in internal combustion engine leads to a reduction of greenhouse gas (GHG) emissions, compared to gasoline because they burn cleaner than regular gasoline and produce lesser carbon monoxide (CO) and nitrogen oxides (NOx) and they reduce emissions of unburned hydrocarbons. Furthermore, using oxygenated fuels in an internal combustion engine (ICE) provides an alternative to conventional fuels that can solve many environmental problems [6, 12, 13].

In the United States, air quality regulations placed on automobile exhaust gases have forced dramatic changes in gasoline formulations. By 1990, the Clean Air Act Amendments (CAAA) required additives, such as MTBE at 15% and ethanol, to be blended in gasoline in some metropolitan areas, heavily polluted by carbon monoxide, and to reduce carbon monoxide and ozone concentrations [14].
