A Mechanistic Study of the Brønsted-Acid Catalysis ofn-Hexane → Propane + Propene, Featuring Carbonium Ions

Abstract

A hypothetical five-step catalytic cycle for Brønsted-mediated fission of an all-trans n-alkane was examined using density functional theory. Optimized geometries and transition states were determined for catalyst−reactant complexes, using three different monodentate catalyst ions (NH4+, H3O+, and H2F+). Despite the wide variety of catalyst acidities, protonated hexane appears as an intermediate (not a transition state) in each case. The protonated cyclopropane structure is the most likely initial form of the dissociated product ion. The predicted intermediates were seen to vary with catalyst acidity. The complete energy profiles of this model catalytic cycle are provided and fitted to a cosine expansion, which allows for generation of the energy profile for any Brønsted catalyst and any n-alkane only on the basis of the proton affinities of the n-alkane and the conjugate base of the catalyst. Remarks on the applicability to zeolites and ionic liquid catalysts are given.