Antiknock fuel additives

Mixtures of 2-methylpentenes and 2,3-dimethylbutenes show good properties as antiknock fuel additives in that they raise the octane number in the low boiling ranges of gasoline. 

Ethylch [Albemarle], TM for antiknock fuel additive, tetraethyl lead. 

A recently marketed fuel additive is MMT (methyl cyclopentadienylmanganese tricarbonyl). MMT was first developed by the Ethyl Corporation in 1957 as an octane enhancing agent and has experienced a growth in demand in the 1990s. MMT was Ethyl Corporation s first major new antiknock compound since TEL. 

The desire to improve the octane rating of fuels after the refining process without the use of lead has intensified a search for other "antiknock compounds. For example, compounds of Mn (5) and Ce (9) have been explored for this purpose. Methylcyclopentadienyl manganese tricarbonyl, in particular, is already being marketed for octane improvement. At a recommended level of 0.125 g Mn/gal, 200-500 g of Mn can be expected to pass the exhaust system within 50,000 miles. The main criteria in accepting such fuel additives are their compatibility with catalytic systems, and, of course, health-safety considerations. 

Metal cyclopentadienyls have already proved to be of considerable technical interest. According to investigations carried out mainly in the United States, ferrocene is able to bring about smokeless combustion of fuel oil when added in amounts of 0.05% (1). Its use as an oil additive for jet motors and as an antiknock has also been proposed. Liquid ferrocene is said to have advantages as a thermally stable heat-transfer medium. Dicyclopentadienyl nickel also has possible applications, and dicyclopentadienyl titanium-dichloride has been studied in association with aluminum alkyls for the polymerization of ethylene (15, 133, 134). Finally, the carbonyl CHsC5H4Mn(CO)3 has been considered as a fuel additive in conjunction with tetraethyl lead (0). Other information is given in the patent literature (40). 

Ethyl Antiknock Compounds [Albemarle], TM for a series of fuel additives containing various percentages of tetraethyl lead, ethylene dibromide, ethylene dichloride, dye, kerosene, and antioxidant. All are used to improve the octane rating of motor fuels. 

The organic compounds that will be discussed are methylcyclopentadienyl manganese tricarbonyl (MMT), maneb and mancozeb, both fungicides, and mangafodipir (MnDPDP). MMT is a fuel additive developed in the 1950s to increase the octane level of gasoline and thus improve the antiknock properties of the fuel (Davis 1998 Lynam et al. 1999). Additional information concerning MMT is included in Chapter 4. 

From Lead in Gasoline. The use of gasoline containing lead as an antiknock agent has been considered the major anthropogenic lead source in Mexico, particularly in large cities and industrial areas. It is estimated that almost 30% of the total lead emitted in Mexico enters the air in and around Mexico City (see section, Fuel additives ), where more than 20% of the population of the country live (20 of an estimated 86 million for 1988). 

Pb(CH3)4 administered to mice by Intraperitoneal Injection Is converted Into trimethyllead species. The LD50 value In mice is given as 14.3 mg/kg [37]. In the muscle of mice, intoxicated subacutely or chronically with Pb(CH3)4, morphological anomalies concerning mostly the inner part of the blood vessels, sarcoplasmic reticulum, and mitochondria are observed [82]. The effect of microparticles from pyrolysis of Pb(CH3)4 and other antiknock compounds on lungs of mice was Investigated [62]. For studies of intoxication In mice by Pb(CH3)4, see also [8]. The toxicity of Pb(CH3)4 solutions to mice and other animals In combination with other compounds, mainly fuel additives, was studied [20, 25, 26]. 

The influence of Pb(C2H5)4 on effects of other fuel additives is examined in [360, 395]. A theoretical study of the distribution of gaseous lead compounds, such as PbO or lead(II) halides in the exhaust gas from the combustion of isooctane-air mixtures containing antiknock additives, appears in [576]. 

In addition to their antiknock properties, organic lead compounds possess bactericidal properties and motor fuels with lead are known to inhibit bacterial growth during storage in contact with water. With the disappearance of lead-based compounds, it is necessary to incorporate biocides from the cyclic imine family, (piperidine, pyrrolidine, hexamethyleneimine), alkylpropylene diamines or imidazolines (Figure 9.2). 

Fuels, Lubricants, and Transmission Fluids. Polyol esters of neopentanoic acid have been used as high vacuum pumping hquids that are stable in chemically aggressive environments (70). Esters such as 6- -ani1inophenoxy)hexy1 pivalate are used as antioxidants for synthetic ester lubricants (71). PivaUc anhydride [1538-75-6] has been claimed as an antiknock additive for gasoline (72). 

Unbumt gasoline and cracked hydrocarbons such as ethylene and propylene are also substantial constituents of exhaust. Gasoline contains additives such as benzene, toluene and branched hydrocarbons to achieve the necessary octane numbers. The direct emission of these volatile compounds, e.g. at gas stations, is a significant source of air pollution. Leaded fuels, containing antiknock additions such as tetra-ethyl-lead, have been abandoned because lead poisons both human beings and the three-way exhaust catalyst, especially for the removal of NO by rhodium. 

In practice, short-chain alkanes and alkenes are normally used as feedstock for shape-selective catalytic formation of isooctanes at relatively low temperatures. Until the 1980s, lead alkyls (Section 18.1) were added to most automotive fuels to help suppress engine knock, but they have been phased out in North America because of the chronic toxicity of lead and lead compounds. The most commonly used nonlead antiknock additive is now methyl tert-butyl ether [MTBE CH30C(CH3)3], which is made by the reaction of methanol with 2-methylpropene, (CHs C—CH2 (see Section 7.4). The latter is obtained by catalytic cracking of petroleum fractions to give 1-butene, which is then shape-selectively isomerized on zeolitic catalysts. 

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