Hexane isomerization side reactions

Whereas 2,2-diphenylhexamethyltrisilane (8) is a well-known photochemical precursor of diphenyl si ly Icnc72,155 156, a 1,3-silyl migration is usually a major side reaction in solution at room temperature. In the presence of excess ethanol, the irradiation of 8 in hexane gives 9 via diphenylsilylene extrusion and 10 (an isomeric mixture) via 1,3-silyl migration in 50 and 37%, respectively (Scheme 3)157. Since the product ratio does not depend on the solvent polarity, both reactions, silylene extrusion and 1,3-silyl migration, do not occur via the ICT state but via the nonpolar excited state158. However, the excited... 

The butanes show little tendency to crack or disproportionate (7) thus butane isomerization is fairly straightforward. However, the suppression of side reactions becomes more difficult as the molecular weight increases. With pentanes, disproportionation to isobutane and hexane is pronounced, amounting to as much as 63%. A typical composition of pentane disproportionation products is shown in Table III. Besides lowering the yield of isopentane, such side reactions shorten the life of the catalyst. Adding small amounts of cyclic hydrocarbons (7,15,18)... 

Pentane isomerization was used to increase the critical supply of aviation gasoline toward the end of the war. Two processes—one developed by Shell and one by Standard Oil Company (Indiana)—were commercialized. The pentane processes differ from butane isomerization mainly in the use of somewhat milder conditions and an inhibitor to suppress side reactions. In general, the problems of the butane processes are inherent also in pentane isomerization, but the quality of the feed stock is less important. Olefins can be as high as 0.2 %, although 0.05 % is preferable. The hexane content should not exceed about 5%, and sulfur and water contents should be as low as in the butane process. Catalyst life is much shorter than in the butane processes only about 30-50 gallons of isopentane are produced per pound of aluminum chloride. 

The side reactions occurring during the isomerization of hexanes have no effect on the chemical equilibrium. This is shown in Table XXXII. 

As we have seen, isomerization occurs as a side-reaction in all conversion processes. But in refining, when we say isomerization process, we are referring specifically to the on-purpose isomerization of n-butane, n-pentane, and n-hexane. The main purpose of n-paraffin isomerization is to produce isoparaffins with substantially higher octane numbers. An isomerization reaction for normal hexane was shown in Figure 17. 

In isomerizing butane with aluminum chloride, only a small amount of catalyst-hydrocarbon complex is formed, but pentane and especially hexane produce more complex. Both pentane and hexane tend to crack during treatment and this is the cause of the extensive side reactions. The formation of sludge is inhibited in pentane isomerization by the use of 0.3 to 1.3 weight per cent benzene, and in hexane isomerization, hydrogen is used as an inhibitor. 

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