Isooctane, catalytic cracking

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. 

Catalyst deactivation during catalytic cracking of n-octane, isooctane and 1-octene over USHY zeolite at mild conditions and short times on stream... 

The process of catalytic reforming introduces chains, and catalytic cracking introduces double bonds. Not only do these two processes increase the amount of gasoline that s produced, but they also improve the quality of the gasoline s burning characteristics. Also notice that benzene, an aromatic compound, has an octane value of 106. Its burning characteristics are better than isooctane. Other substituted aromatic compounds have octane ratings of almost 120. However, benzene and some related compounds are health hazards, so they re not used. 

The thermal stability of NH4Y zeolite in which ammonium ions have been exchanged at various levels with La3+ ions was studied. The catalytic activity of these La zeolites in isooctane cracking was measured as a function of pretreatment temperature, and an IR study of the chemisorption of pyridine was used to determine the numbers of Bronsted and Lewis sites. The structural damage resulting from high temperature calcination was examined qualitatively. 

Cracking of isooctane is used to check catalytic activity of progressively exchanged Na-zeolites and to try to correlate acid and catalytic properties. 

Catalytic Activity. The cracking of isooctane was studied. For catalytic activity measurements, the samples were heated in flowing helium or hydrogen from room temperature to 465°C for 15 hours. Then the gas saturated with isooctane passed through the catalyst in the microreactor and was analyzed by gas chromatography. 

Catalytic activity is influenced also by ions in (or near) the supercage. As long as these ions are not exchanged, the cracking of isooctane is not important. In Na-H-Y zeolites, only the exchange of the last ions noticeably increases it. 

Similar study was performed on decationated mordenites [107], which have narrower pores and a higher Si/Al ratio than Y zeolites. From microcalorimetric measurements it was found that the number of acid sites increases nearly linearly with an increase of decationation level. Growth of the exchange level increased the acid strength, and removal of the last NH ions resulted in the formation of strong sites (Odiff > 130 kJ moP ). This increased concentration of strong acid sites explained that mordenites are more active in the cracking of n-octane than HY zeolites. However with bulkier reactants such as isooctane and ethylbenzene, the difference in catalytic activity decreases due to shape selectivity effects. 

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