Isomerization Process

  Isomerization is a small-volume but important refinery process. Like alkylation, it is acid catalyzed and intended to produce highly-branched hydrocarbon mixtures. The low octane C₅/C₆ fraction obtained from natural gasoline or from a light naphtha fraction may be isomerized to a high octane product.

  Dual-function catalysts activated by either inorganic or organic chlorides are the preferred isomerization catalysts. A typical catalyst is platinum with a zeolite base. These catalysts serve the dual purpose of promoting carbonium ion formation and hydrogenation-dehydrogenation reactions. The reaction may start by forming a carbocation via abstraction of a hydride ion by a catalyst acid site. Alternatively, an olefin formed on the catalyst surface could be protonated to form the carbocation.

The carbocation isomerizes by a 1,2-hydride/methide shift as mentioned earlier (see this chapter, “Reforming Reactions”). Figure 1.1 shows the vapor phase equilibrium of hexane isomers.

Figure 1.1. Vapor phase equilibrium for hexanes.