Alkane Cracking and Isomerization on Solid Acid Catalysts

6 Alkane Cracking and Isomerization on Solid Acid Catalysts 

The basic concept underlying alkylation reactions of aromatics is the formation of a stabilized carbocation able to attack nucleophilic substrates. Hydrocarbon cracking and hydrocracking, alkane isomerization, and olefin alkylation are important processes based on related alkane carbocation chemistry in the production of various types of hydrocarbons such as the branched ones for high octane gasolines. Zeolites and metal oxides are the preferred catalysts. 

We shall not treat cracking processes here due to the complexity of these high-temperature (usually around 500°C) reactions. However, cycloalkane dehydrogenation to aromatics (Appendix A2.4.4), alkane isomerization and olefin alkylation (leading to branched alkanes from linear ones) occur via such carbocation rearrangements. 

Alkane isomerization is carried out under hydrogen pressure on zeohte-type catalysts, which are particularly robust, being insensitive to poisons such as sulfur-containing compounds and water at 200-280°C. The use of so-called dual catalysts, containing both a solid acid species and a transition metal such as 

Olefin alkylation, long carried out in solution with Bronsted acids, is now beginning to turn to the use of zeolites or other solid acid reagents under mild conditions. The main problem is removal of both the heat generated by the reaction and the carbon deposits. While the former problem can be solved by slurry reactor techniques, the latter requires a chemical treatment which is best carried out by metal-catalyzed hydrogenation at high temperature. 

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