Alkanes over solid superacids

The difficulties encountered in handling liquid superacids and the need for product separation from catalyst in batch processes have stimulated research in the isomerization of alkanes over solid superacids. The isomerization of 2-methyl- and 3-methylpentane and 2,3-dimethylbutane, using SbF5-intercalated graphite as a catalyst, has been studied in a continuous flow system. ... 

Halogenation. Fluorination, chlorination, and bromination of alkanes catalyzed by superacids have been reported.1,2 Reactions may be carried out in the liquid phase, or in the gas phase over solid superacids or supported noble metal catalysts. High selectivity and relatively mild reaction conditions are the main features of these transformations. 

Subsequently, selective ionic chlorination of methane to methyl chloride was achieved in the gas phase over solid superacid catalysts.526 For example, chlorination of methane in excess chlorine over Nafion-H and SbF5-graphite gave methyl chloride with 88% and 98% selectivity, respectively (185°C and 180°C, 18% and 7% conversion).527 Similarly, electrophilic bromination of alkanes has also been carried out528 (Scheme 5.53). 

Sommer and coworkers have made important observations with respect to the activation of alkanes over sulfated zirconia, a new type of solid superacid. Whereas isotope exchange of small alkanes occurs with the involvement of the corresponding pentacoordinate ions, a classical carbenium ion-type mechanism was found to be operative for larger homologs (propane, isobutane). The exception is the isomerization of n-butane over sulfated zirconia promoted by Pt and alumina, where the initiation step for isomerization was suggested to be the protolysis of the C-H bond. ... 

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