Neohexane isomerization

Fig. 4. 3C complexes, the existence of which can be seen in experimental evidence from exchange [neopentane Rh (Pt)] and from hydrogenolysis and isomerization (neopentane, neohexane). As in Fig. 3, the known ( ) and the speculative aspects ( ) of the 3C complex formation are indicated. 

Equilibria. The equilibrium distributions of butane, pentane, and hexane isomers have been experimentally determined (5, 16) and are diagrammed in Figure 2. In each case, lower temperatures favor the more highly branched structures. At the approximately 200° F. temperature usually employed for isomerization, the butane equilibrium mixture contains about 75% isobutane. That for pentane contains about 85% isopentane.. In the case of hexane, the equilibrium product contains about 50% neohexane and has a Motor octane rating of about 82. In all cases, of course, the yield of the desired isomers can be increased by fractionation and recycle. 

Napththa Isomerization. The only commercial isomerization of light naphtha was carried out in two plants employing the isomate process developed by the Standard Oil Co. (Indiana) (20). In this process, a feed containing normal pentane and low octane number hexanes is converted to isopentane and to hexanes of higher octane number. Pentanes and hexanes in any ratio may be processed. By recycle of selected fractions of the product, concentrates of isopentane or of neohexane and diisopropyl can be obtained as the ultimate products. 

The rates at which the above conversions take place vary considerably as shown in Table II. The interconversion of 2-methylpentane and 3-methylpentane is extremely rapid while the isomerization of 2,3-dimethylbutane to 2,2-dimethylbutane is very slow. Thus, the ratedetermining step in the formation of 2,2-dimethylbutane (neohexane) from n-hexane is the slow isomerization of 2,3-dimethylbutane to 2,2-dimethylbutane. In the isomerization of w-hexane to 2,3-dimethylbutane, the rate-determining step is the isomerization of n-hexane to the methylpentanes. 

Table VI shows a hydrocarbon material balance for operation of a neohexane unit. This operation is typical in that pentane and hexane are isomerized together and the recycle ratio is set by the amount of feedstock available and the capacity of the equipment.

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