Antiknock additives, lead

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. 

The best known example is the electrosynthesis of tetraethyllead (TEL) Pb(C2Hj)4, which has been in wide use as an antiknock additive of gasoline, and still is in a number of countries. This substance is readily produced by reaction of ethyl radicals with the lead electrode ... 

Lead Tetraethyl (Tetraethyl lead, Lead Tetraethide, Tetraethyl Plumbane). Pb(C2Hs)4, mw 323.45, colorless liq, fr p -136°, bp about 200°, also stated as 227.7° with decompn, d 1.653g/cc at 20° insol in w si sol in ale, sol in benz and eth. May be prepd by die action of Pb chloride on Zn ethyl or on a Grignard reag. Used extensively as an antiknock addition to gasoline, and has been proposed by Fr investigators as a flash reducer in proplnts (Ref 2)... 

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. 

Lead tetraethyl Pb(C2H6)4 Antiknock additive in motor fluels... 

Large quantities of tetramethyl lead (TML) were used back in the 1960s as an antiknock additive in high-octane gasoline before they were banned because of air pollution problems. TML was produced in a batch reactor that had the interesting feature of requiring the removal of a byproduct during the batch. The main reaction involves a solid sodium-lead alloy and liquid methyl chloride ... 

Soulages, N. L. The analysis of lead antiknock additives by gas chromatography. 

The extensive use of lead antiknock additives in gasoline has made lead perhaps the most widely distributed toxic heavy metal in the urban environment and has greatly increased its availability for solution in natural waters. It is important for this reason to know whether its introduction into surface and ground waters by rainfall and runoff will make it available for solution or whether chemical processes will place a safe upper limit on its solubility. 

This metal becomes a pollutant when released into aqueous streams from metal treatment and recovery plants (e.g., lead-using battery manufacturers). In industrialized nations, until the 1970s, a major source of air pollution was lead emission from the tetraethyllead antiknock additives to gasoline this is. still a problem in many underdeveloped nations. Once ingested, lead readily forms complexes with enzymatic oxo groups in the organism, thus interfering with all the steps in haemo... 

This became one of the more commercially important lead compounds when its value as an antiknock additive to lower grade petrols was... 

Leaded fuels, containing antiknock additions such as tetra ethyl lead, have been abandoned nowadays because lead is a poison for human beings and for the three-way catalyst, especially for the removal of NO by rhodium. 

Since lead-containing antiknock additives of leaded gasolines rapidly destroy the activity of the catalytic emission control systems, unleaded gasolines have had to be used in automobiles fitted with this or related systems. However, since using this fuel modification also eliminates one of the main sources of lead emissions to the atmosphere, this trend also has a highly favorable additional impact [11, 12]. Before unleaded gasolines were on the market, it was common to find high lead levels in the core areas of busy cities... 

A number of other compounds have been tested as antiknock additives. Ferrocene and other related organometallics have shown antiknock activity, but are not as compatible in fuels as the lead alkyls. Many oxygen-containing compounds other than MTBE, such as Abutanol, Abutyl acetate, and ethanol also have antiknock capabilities [32], and like MTBE require addition at a higher rate to obtain equivalent antiknock activity. The direction to be taken for trouble-free antiknock additives remains to be seen. 

Figure 26. Speciation of lead in a soil affected by alkyllead compounds used as antiknock additives in gasoline. The soil EXAFS spectrum (solid) is compared to model spectra from a library of lead compounds. This comparison identifies salicylate and catechol functional groups as the predominant complexing chelates for divalent Pb. Adapted from Manceau et al. (1996). Figure continued on facing page.)...

---