The Reactions of Hexyl Ions on USHY

Abstract

We have examined the behavior of C6 carbenium ions in the cracking of 2-methylpentane on USHY. We find that at 400°C, hexyl carbenium ions undergo hydride addition from the feed 10 times faster than proton release to the Brønsted base. This makes the isomerization of the feed a much faster reaction than the production of olefins with the same carbon number. We also find that proton release from a C6 ion to the Brønsted base requires a higher activation energy than a hydride transfer from the feed to the same ion. At high temperatures isomerization is therefore reduced with respect to olefin production. The presence of steam in the cracking mixture weakens the Brønsted bases, and reduces the rates of all reactions but encourages hydride transfer over proton release. This enhances the formation of paraffinic isomers of the feed. At the low steam dilution ratio of 0.07 mol/mol, hydride transfer in 2-methylpentane is as much as 18 times faster than proton release, resulting in a highly isomerized, highly saturated product. The full picture of individual ion fates is presented and gives an important insight into the causes underlying cracking selectivity and the possible methods for its control.